Emerging Materials in Heterogeneous Electrocatalysis Involving

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Emerging materials in heterogeneous electrocatalysis involving oxygen for energy harvesting Moumita Rana, Sanjit Mondal, Lipipuspa Sahoo, Kaustav Chatterjee, Pitchiah E. Karthik, and Ujjal K. Gautam ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09024 • Publication Date (Web): 17 Sep 2018 Downloaded from http://pubs.acs.org on September 18, 2018

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ACS Applied Materials & Interfaces

Emerging Materials in Heterogeneous Electrocatalysis Involving Oxygen for Energy Harvesting Moumita Rana,‡ Sanjit Mondal,† Lipipuspa Sahoo, † Kaustav Chatterjee, † Pitchiah E. Karthik† and Ujjal K. Gautam*,† †Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER)-Mohali, Sector

81, Mohali, SAS Nagar, Punjab 140306, India ‡ IMDEA Materials

Institute, C/Eric Kandel 2, Parque de Tecnogetafe, Getafe-28906, Spain

Corresponding author:*[email protected] (All authors contributed equally)

Keywords: Bifunctional catalysts; Electrocatalysis; Heterogeneous catalysis, Oxygen reduction reaction; Oxygen evolution reaction;

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Abstract: Water-based renewable energy cycle involved in water splitting, fuel cells and metal-air batteries has been gaining increasing attention for sustainable generation and storage of energy. The major challenges in these technologies arise due to the poor kinetics of the oxygen reduction reaction (ORR) and the oxygen evolution reactions (OER), besides the high cost of the catalysts. Attempts to address these issues have led to the development of many novel and inexpensive catalysts as well as newer mechanistic insights, particularly so in the last three-four years when more catalysts have been investigated than ever before. With the growing emphasis on bifunctionality i.e. materials that can facilitate both oxidation and evolution of oxygen, this review is intended to discuss all major families of ORR, OER and bifunctional catalysts such as metals, alloys, oxides, other chalcogenides, pnictides and metalfree materials developed during this period in a single platform, while also directing the readers to specific and detailed review articles dealing with each family. In addition, each section highlights the latest theoretical and experimental insights that may further improve ORR/OER performances. The bifunctional catalysts being sufficiently new, no consensus appear to have emerged about the efficiencies. Therefore a statistical analysis of their performances by considering nearly all literature reports that have appeared in this period is presented. The current challenges in rational design of these catalysts as well as probable strategies to improve their performances have been presented.


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1. Introduction The continuous rise in global energy demand is leading to an escalated consumption of fossil fuels and increase in greenhouse gases in the atmosphere. Therefore the development of complementary energy resources is gaining attention.1–6 The target set by the Department Of Energy-USA is to produce 80% of electricity from renewable resources by 2050.7 The major renewable energy cycles are based on water, carbon and nitrogen, among which the waterenergy cycle is considered to the most promising one.1 In this cycle, energy harvested by solar cell, wind and sea current can be stored in the form of chemical energy by the electrochemical splitting of water to produce hydrogen and oxygen. These molecules with high energy density can later be recombined by redox processes in a fuel cell for power generation.8 Water cycle also plays an important role in hybrid electrical devices such as regenerative fuel cells and metal-air batteries where water splitting and oxygen reduction take place in one of the electrodes respectively (Figure 1).9–11 Among the reactions involved in the water-energy cycle, the sluggish kinetics of oxygen reduction reaction (ORR, O2 + 4H+ + 4e− → 2H2O in acidic media or O2 + 2H2O + 4e− → 4OH− in alkaline media) and oxygen evolution reaction (OER, 2H2O → O2 + 4H+ + 4e− in acidic media or 4OH− → O2 + 2H2O + 4e− in alkaline media) is the primary limiting factor, compromising device efficiencies.5,12 Therefore, significant efforts have been made to develop highly efficient electrocatalysts for ORR and OER. Benefiting from these findings, generating of bifunctional catalysts that can simultaneously perform ORR and OER has emerged as the latest research hotspot. Both molecular as well as heterogeneous catalysts have been found to be promising, even though recovery and recyclability of the molecular catalysts remains a bigger challenge.13–17 Heterogeneous catalysts are advantageous in this regard, despite suffering from loss of some catalytic activity under harsh electrochemical conditions.18 So far, carbon supported Pt nanocrystals and precious metal oxides (e.g. IrO2, RuO2) have been used as highly efficient benchmarking catalysts for ORR and OER



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respectively.19,20 Thus the bottleneck for commercialization of these technologies has been the high cost these catalysts, leading to an extensive search for economically affordable materials with sufficiently high activity and stability.21

Figure 1: Schematic showing the scope of ORR, OER and bifunctional electrocatalysts in water cycle associated with renewable energy, where oxygen is either generated or consumed in one of the electrode processes. In an electrolyzer cell, water converts to O2 by splitting at the anode (OER) whereas in a fuel cell, oxygen gets reduced at the cathode forming water (ORR). The same reactions also occur in a metal-air battery during discharging and charging respectively.

Understanding of the material surface, surface-reactant interaction and associated reaction mechanism is necessary to overcome the challenges. In aqueous medium ORR occurs mainly through two pathways:(i) direct 4-electron reduction pathway from O2 to H2O, and (ii) 2electron reduction pathway forming hydrogen peroxide.22 The 4-electron processes are preferred for fuel cell applications while 2 electron pathway can also be useful for industrial production of H2O2. It was found that the 4-electron reduction process can occur by two mechanisms: (i) dissociative mechanism and (ii) associative mechanism. In both, O2 first gets adsorbed on the catalyst surface (Eq. 1, 4). In the first case, the adsorbed O2 molecule splits to form adsorbed atomic oxygen, which is further reduced by 2 more electrons to form H2O. 4 ACS Paragon Plus Environment

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O2 + * = 2O*

(1)

O* + H+ + e- = *OH

(2)

*OH + H+ + e- = H2O + *

(3)

Here * denotes the active sites on the catalyst surface and placed near the atom which is adsorbed. The affinity between such an active site and reactant species is crucial in determining the ease of the reaction. Density functional theory (DFT) studies has established that a Sabatier volcano relationship exists between the binding affinity of OH on the catalyst surface and its ORR activity.23,24 In dissociative ORR pathway, if the catalyst surface does not stabilize O2, H2O2

cannot be produced. Alternatively, in associative mechanism, the O-O bond does not cleave initially and instead, O2 + * = O2*

(4)

O2* + H+ + e- = *OOH

(5)

*OOH + * = HO* + O*

(6)

HO2* + H+ + e- = H2O + O*

(7)

O* + H+ + e- = *OH

(8)

2*OH + 2H+ + 2e- = 2H2O + 2*

(9)

Considering this mechanism, if the O-O bond in the adsorbed species (Eq. 7) is not broken, it results in the formation of H2O2, which can be further reduced to H2O or can be desorbed as the final product. It is desirable to have the O2 reduction occurring at potentials close to the thermodynamic ORR potential of 1.23 V of (vs. NHE) with a satisfactory reaction rate. Kinetics of ORR is obstructed by the first electron transfer from the cathode to oxygen, the hydration of oxygen and desorption of the intermediates. Therefore, estimation of the binding energy of the intermediates on catalyst surface is necessary. Initiated by pioneering work of Bockris on oxygen evolution reaction, researchers have carried out extensive work in the last five decades or so and elucidate reaction mechanisms associated with OER considering both acidic or alkaline reaction conditions.25,26 Despite some disagreement, most of the proposed mechanisms are similar and include a surface adsorbed –OH or a surface-bound –O group.27, 28 A general mechanism can be outlined in the 5 ACS Paragon Plus Environment


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following equations (Eq. 10-14). The primary difference between the various pathways is the participation of a surface hydroperoxy group (-OOH), which is generated from a surface hydroxyl group. (Eq. 13-14 and Eq. 18-19). In acidic conditions, OER occurs with the adsorption was H2O on the catalyst surface involving the steps described below (either following Eq (10-12) of Eqs (10, 11, 13, 14) or a combination of both the pathways): * + H2O → *OH + H+ + e-

(10)

* OH → O* + H+ + e-

(11)

O* → * + O2

(12)

*O + H2O → *OOH + H+ + e-

(13)

*OOH → * + O2 + H+ + e-

(14)

On other hand in alkaline condition, OER occurs involving surface adsorbed -OH groups: * + OH- → OH* + e-

(15)

*OH + OH- → O* + H2O + e-

(16)

O* → * + ½ O2

(17)

O*+ OH- → *OOH + e-

(18)

*OOH + OH-→ * + O2 + H2O + e-

(19)

In a neutral medium, OER is expected to follow either of mechanisms depending on the nature of the catalyst surface charge. Water molecules will be adsorbed on a negative surfaces and hydroxyl group will be involved on positive surfaces. Even though investigations on ORR and OER date back to several decades, it gained momentum only recently.22 The last three-four years have been particularly significant when more number of catalysts has been investigated than ever before and novel materials have been developed. In-operando spectroscopic and other measurement techniques such as ambient pressure X-ray photoelectron spectroscopy and X-ray absorption near-edge spectroscopy have been frequently employed to gain insights into the reaction mechanism and catalyst deactivation. In addition, several new families of bifunctional ORR-OER catalysts have also


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been discovered. Figure 2 represents the most widely investigated ORR and OER active as well as bifunctional materials. ORR catalysts Pt NC (spheres, tetrahedra, octahedra, wires, thinfilms); Pt NC on various catalyst support (e.g. C and N/S/P doped C); noble metal alloy NC (PtNi, PtAg, Pd@Pt CS, PdCuPt, [email protected] CS, TRM coatedPt3Ni/C, Au doped Pd6CoCu etc.); Pnictide-carbideschalcogenide NC (Fe2N/N-doped C, Fe−Fe3C/C, FeW-C, Fe−P/C, Fe3C-CNT,CoxN-CNTs, MoS2/C, Fe or Co doped C etc.); metal free C (N- or N,S-doped G or high surface area amorphous C etc.)

ORR-OER bifunctional catalysts Free and supported spinels (Co3O4, Co3O4/N-CNT, CoMn2O4/C, ZnCo2O4/N-CNT, Co@Co3O4/N-doped C, NixCoyO4@Co/N-doped G, Co3O4/N-doped G etc.); Free and supported Perovskites (primarily BaNiO3 related phases e.g. Nd doped Ba0.5Sr0.5Co1.5Fe0.5O5+δ, La0.5Sr0.5CoO3-x); other free and supported oxides (La2O3, CoO, CoO/C, NiO/CoN, MnOx/N-dopedC, CoO@Co/N-doped GO etc.); layered hydroxides (NiFe-LDH, NiFe DH/ C, CoOOH); pnictide-carbideschalcogenides (Fe/Co-N doped C, Fe-Co alloy NC/CNT, WS2@WC/CNT, Co1-xS/N,S-doped GO, CoxSe/C, (NiFe)xN/C, Co4N/C, Ni3Fe/N doped C); Metal free C (Mesoporous C, P- or P,S-doped C, Ndoped GO, C)

OER catalysts Spinels (Mn3O4, NiFe2O4, MnFe2O4, LiMn2O4); Perovskites (BaNiO3, (Lax Sr1-x)(CoyFe1-y)O3-δ, (Bax Sr1x)(CoyFe1-y)O3-δ, Ca doped (Pr,Ba)CoO3-δ); layered Hydroxide (NiCu DH, NiOOH, Ni(OH)2); Other oxides (CoO, α-(FexCo1-x)Oy, LiCoO2, Fe/Al doped LiNiO2); pnictide-carbide-boride-phosphide-chalcogenide (Fe doped Ni-B, NiS, Ni3Se2, Ni3S2, CoSe2, (NixFe1-x)Se2, Co-Pi, Ni-Pi, Co-Pi/Ti, Black P, Co3N, Ni3N, Ni2P, CoMnP, g-C3N4/ Ti3C2, (Ni,Co)Se0.85–NiCo LDH, NiSe@NiOOH, Fe(PO3)2/Ni2P foam).

Figure 2: The most widely investigated ORR, OER and ORR-OER bifunctional electrocatalysts. [C: Carbon, CNT: carbon nanotube, CS: core-shell, DH: Double hydroxide, G: graphene (many authors loosely refer to graphene oxide and reduced graphene oxide as graphene too) LDH: Layered Double hydroxide, NC: nanocrystal, TM: Transition metal.] A number of critical review articles describing the progress in the fields ORR and OER activities have been published recently.1,5,29–31 Most of these are devoted to developments in a single family of compounds and some, illustrating the emerging insights on the reaction



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mechanisms in-depth.1,32 It is clear that the underlying principles responsible for improved ORR and OER efficiencies in a material form the basis for development of bifunctional catalysts. However, the bifunctional catalysts have not been discussed in light of the developments in the field of ORR and OER. Besides, the literature efficiencies of the bifunctional catalysts appear to be widely and alarmingly varying, probably due to the need for tuning two functionalities simultaneously unlike in ORR and OER, and thus without setting any clear guidelines for further studies. In this review, we summarize the recent progress in the performances of the catalysts for oxygen reduction, oxygen evolution and bifunctional ORROER reactions by considering nearly all important families of materials viz. of oxides, hydroxides, chalcogenides, pnictides, and metal free and heteroatom doped carbon-based materials. It is not intended to provide an overview that is in-depth in nature but to create a single-platform for the readers to access the important findings pertaining to ORR, OER and bifunctionality in the last 3-4 years. We highlight the recently developed key-strategies for improving ORR and OER functionality and also point out the few critical investigations that have led to in-depth understanding of the reaction mechanism or newer insights. In the case of bifunctional catalysts, because these are relatively new and difficult to compare, we have taken into consideration a statistically significant number of recent reports and compared their performances in terms of the most, the least and mean activities for the different families of catalysts. In the concluding section, we point out the current challenges and opportunities in catalyst design and benchmarking their performances that could help in creating more efficient materials in near future.

2. Measurements and performances: The performance of a catalyst for ORR and OER is usually evaluated using a three-electrode half-cell set-up at room temperature, wherein the catalyst is loaded on the working electrode. Under the experimental conditions, the reactant


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species are adsorbed on the electrode surface thereby facilitating the electron transfer from the electrode to an intermediate species followed by the release of the product. The performance of a catalyst is examined by linearly varying the applied voltage and measuring the current (Linear Sweep Voltammetry, LSV). The efficiency of a catalyst depends on both thermodynamic and kinetic considerations and can be benchmarked by measuring several parameters of which half-wave potential (E1/2), overpotential (η) and Tafel slope are most widely used. In an ideal condition, an electrochemical reaction should take place at zero overpotential i.e. at equilibrium potential (Eeq). However, this does not happen due to activation barriers associated with electron transfer and intrinsic electrical resistance of the measurement set-up. Thus η can be considered as a measure of the excess applied potential (E) over the equilibrium potential (E- Eeq) required for the particular reaction. In case of OER, η is usually reported at a current density of 10 mAcm-2. The total current (i) in an electrochemical experiment originate from the anodic and the cathodic current (Eq. 20).33 𝑖 = 𝑖𝑎𝑛𝑜𝑑𝑒 + 𝑖𝑐𝑎𝑡ℎ𝑜𝑑𝑒

(20)

The relationship between  and i can be expressed using the Butler-Volmer (B-V) equation (Eq. 21).

𝑖 = 𝑖0 (𝑒

𝛼𝑎 𝑛𝐹  𝑅𝑇

−𝑒

−𝛼𝑐 𝑛𝐹  𝑅𝑇 )

(21)

where i0 is the exchange current, 𝛼𝑎 𝑎𝑛𝑑 𝛼𝑐 denote transfer coefficients for the anodic and cathodic reactions respectively, and n is the number of electrons involved. At high field 𝑅𝑇

conditions (high  > 𝛼 𝐹), only one of the reactions dominate. Assuming that to be the anodic 𝑎

conditions and T = 300 K, the B-V equation can be rearranged to give the Tafel equation (Eq. 22). 9 ACS Paragon Plus Environment


 =

0.059 0.059 log 𝑖 − 𝑙𝑜𝑔𝑖0 𝛼𝑎 𝑛 𝛼𝑎 𝑛

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(22)

The slope of the Tafel plot (η vs. log i) is defined as [0.059/αan] and is denoted in the form of mV/decade current. This is a kinetic parameter for that particular electrochemical reaction which is affected by electron transfer coefficient, rate constants and electron transfer numbers during or before rate determining step (RDS); and though not straightforward, can be used to derive information about the RDS. In those special cases where there is no significant difference in concentration of reactant in electrode surface and bulk, and where there is only one RDS which is much slower than the rest of the elementary steps, Tafel slope can ideally be used to estimate the number of electrons transferred from the electrode to a reactive species in the RDS. However such situations are expected to be extremely rare (the absolute rate theory of electron transfer theory of Marcus suggests that the same is impossible because energy associated with reorientation of the medium prefer a step-wise electron transfer process) and thus over-simplified interpretation may lead to hypothesizing inaccurate reaction pathways. It may however be pointed out here that a number of researchers loosely assume a transfer coefficient of 0.5 while considering Tafel slopes for ORR and OER, and thereby arrive at an electron transfer number, n = 1, 2, 3, or 4 when the slopes are found to be 120 mV, 60 mV, 40 mV or 30 mV /decade respectively. Electron transfer coefficient is dependent on the surface coverage of the electrode with reaction intermediates which in turn is dependent on the applied potential and therefore cannot be constant over the potential range the Tafel analysis is carried out. Detailed discussions on this key issue of utilization of Tafel slope for analysis of electrochemical ORR and OER mechanisms may be found in several articles.34, 35 A recent 'Technical Report' prepared by IUPAC recommends that instead of nα, α alone be used in Butler-Volmer and Tafel relations, giving α the status of an experimentally determined quantity that is devoid of any mechanistic conclusions.36


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As dissolved oxygen is the reactive species in ORR, the electrochemical measurements are carried out in an O2 saturated electrolyte. In an OER experiment, if the generated oxygen has to be further quantified by another technique, an inert atmosphere is used. The choice of the electrolyte is made on the basis of pH requirement, adsorption affinity of the electrolyte on the catalyst surface, the stability of the catalyst and the catalyst binder. Thus acidic media is preferred for the noble metal based catalysts since these tend to undergo surface oxidation in basic media. Acids with larger anions, such as HClO4 are suitable electrolyte to yield higher current densities. On the other hand, HCl is not a suitable due to the high polarizability of Clion and its stronger affinity to the catalyst surface. Transition metal oxides tend to leach out in acidic media and therefore basic electrolytes are usually employed. Nafion, a common proton exchanger and catalyst binder is not stable in basic media due to salt formation and many catalysts have been tested in the binder-free form.37 Finally, it is important to use a suitable reference electrode compatible with the electrolyte for accurate and stable measurements. One may also note a fundamental difference in ORR and OER kinetics originating at the supply of reactants to the catalyst surface. ORR is a diffusion limited process wherein diffusion of dissolved O2 to the electrode can limit the current. OER on the other hand is not diffusion limited as it involves water or OH- abundantly present near the electrode. Therefore, ORR LSV plots should be acquired in same scan rate to compare the performance of various catalysts. 3. Emerging materials for ORR Among various electrocatalytic processes involving oxygen, ORR has been more extensively studied in the last decade. A diverse range of materials has been explored as catalysts including noble metals, metal oxides and carbon-based materials. Even though their performances have been constantly improving with design strategies, significantly aided by theoretical studies, further progress is needed in finding economically viable catalyst systems.



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Pt exhibits excellent ORR activity and has been considered as the benchmark ORR catalyst for Polymer Electrolyte Membrane (PEM) fuel cells. Taking due consideration of its high activity but low earth abundance, Department of Energy (DOE), USA has estimated a minimum attainable current value of 0.44A per g of Pt in a full cell configuration, besides Co> Fe>Mn> Cr due to variation of electron population in the eg orbitals.136 Two characteristics features are important for high OER activity- a) eg electrons in surface cation should be near to unity, and b) high covalent character in transition metal-oxygen bond. Out of nearly ten perovskite materials, Ba0.5Sr0.5Co0.8Fe0.2O3–δ (BSCF) was found to have the best OER activity, which aced the volcano plot (Figure 6a) exhibiting one order higher performance than that of the state-ofthe-art iridium oxide catalysts.137 Recently, Guo et al. developed Ca0.9Yb0.1MnO3-δ

by

hydrogen treatment where the perovskite attains an optimum Mn eg electron filling of ~ 0.81 and exhibits higher electrical conductivity and OER activity over the pristine sample.138 Similar observation was reported in the case of Fe based perovskite also.139 In addition, when the


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oxygen p-band is close to the Fermi level of transition metals, OER activity shows marked enhancement (shown in Figure 6b, c).140 Figure 5: (a) Schematic illustration of structural transformation within single-crystal and polycrystalline Co3O4 showing the formation of more active/oxidized phases that generate structural stress and/or voids and weaken the stability of materials. (b) OER polarization (a)

(b)

curves of Co3O4@CoO SC, Co3O4 SC, IrO2, and RuO2 catalysts. Reproduced with permission from Ref. 132. Copyright 2015 Nature Publishing Group. Diaz-Morales et al. synthesized Ir-based double perovskites (Ir-DP) having general formula A2BB′O6, where A is a large cation and B, B’ are smaller cations.141 It was inferred that 3D network of corner-sharing octahedron is an important prerequisite for the enhanced OER activity as well as high chemical stability of Ir-DPs under harsh anodic electrochemical



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conditions. With extensive details, Han et al. have recently reviewed Fe-based perovskite materials for OER applications recently.142

(a)

(b)

(c)

Figure 6. (a) The relation between the OER catalytic activity, defined by the overpotentials @ 50 mA cm−2 of OER current, and the occupancy of the eg-symmetry electron of the transition metal (B in ABO3). Reproduced from Ref. 137, copyright 2011 AAAS. (b) Schematic representation of the O p-band for transition metal oxides and (c) potential at 0.5 mAcm-2 versus the O p-band centre relative to EF (eV) of (Ln0.5Ba0.5)CoO3-δ with Ln = Pr, Sm, Gd and Ho. The O p-band centre relative to the Fermi level was computed by DFT. Reproduced with permission from Ref. 140. Copyright 2015 Nature Publishing Group.

4.2 LDH and other non-noble metal-based catalysts: Transition metal oxides and hydroxides such as CoO, NiO, LiCoO2, Ni-Fe LDH, Co-Ni LDH etc. offer fewer possibilities of tuning the electrical properties when compared with perovskites and spinels.5,143 However the ease of preparation and nanostructure formation leading to high surface area makes them attractive candidates for OER. Besides, these oxides and hydroxides 28 ACS Paragon Plus Environment

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were envisioned to be good OER catalysts since the oxyhydroxide intermediates formed on spinel surfaces during OER has similarities with their structure. Furthermore, oxygen defects in these oxides create unusual metal oxidation states that play the most crucial role in determining their efficiencies. Oxygen deficient phases enable fast charge transfer along with optimal adsorption energies for OER intermediates.144 Gardner et al. examined two polymorphs of LiCoO2 under OER conditions to understand the above-mentioned mechanism in detail.145 They showed that both, layered and cubic phases of LiCoO2 forms same type of structure during catalysis, i.e. the cubic spinel, LiCo2O4, though only the cubic LiCoO2 nanoparticles exhibited excellent OER performance. The activity of cubic LiCoO2 was ascribed to the existence of [Co4O4]n+ cubane structural units that provided not only a lesser oxidation potential to Co4+ formation, but also decreased inter cubane hole mobility which was not observed in the layered case. Another interesting recent study showed that the formation of cubane can be made facile by electrochemical delithiation when the formation of cubane becomes facile.146 The overpotential of the LDH phase diminished from 510 mV to 390 mV after delithiation. Weng et al. reported a new layered Na1xNiyFe1-yO2

double-oxide deposited on Ni foam exhibiting activity and stability surpassing

those of IrO2 and RuO2.147 The Na ion de-intercalation resulted in decreased potential from 1.52 V to 1.49 V @ 10 mAcm-2 with a Tafel slope of 40 mV/dec. Here, the increased partial oxidation state of Fe and Ni in de-intercalated Na1-xNiyFe1-yO2 accelarate the OER kinetics. OER active LDH (Figure 7a) mostly includes cobalt and nickel-based compounds.148–150 Subbaraman et al. carried out an extensive study on 3d transition metal (Ni, Co, Fe, Mn) hydr(oxy)oxide and inferred a reactivity trend similar to that of perovskite systems following an order of Ni > Co > Fe >Mn, which is reverse to the M2+δ-OH bond strength.151 It appears that higher OER activity for NiOOH can be attributed to the optimum bonding strength



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between Ni and OH species.152 The OER activity of LDH mounted on 3D Ni foam results in further improvements due to better dispersion, faster mass transport and synergism between

(a)

(b)

Figure 7: (a) Schematic illustration of layered double hydroxide structure and chemical components Reproduced with permission from Ref. 150. Copyright 2015 Royal Society of Chemistry. (b) OER performance of exfoliated NiFe-, NiCo and CoCo-NS (nanosheet) and non-exfoliated NiFe-, NiCo and CoCo-B (bulk). Reproduced with permission from Ref. 143. Copyright 2014 Nature Publishing Group.

the catalysts with the support. In this regard, Song et al. have demonstrated Ni-Fe LDH through exfoliation process, which exhibited an excellent OER activity having lower overpotential of 300 mV with Tafel slope of 40 mVdec-1 (Figure 7b).143 Interestingly, Klaus et al. demonstrated that the OER performance of Ni(OH)2 would increase in presence of trace amount of Fe.153 Similarly, Ni improves the activity of CoOOH, while Mn does not.149 Their electrochemical impedance spectroscopic studies revealed that Ni incorporation improves the ability of the catalysts to stabilize surface intermediates, whereas Mn incorporation impedes intermediate stabilization. Kundu and co-workers have recently


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demonstrated petal-like 3D hierarchical array of β-Ni(OH)2 nanosheets and nanoburls activated by faceting.154 The formation of oxyhydroxide exhibited a lower overpotential 300 mV at a current value of 10 mAcm–2 and an exceptional TOF = 47.14 s-1(@ 1.53V vs. RHE). For further information, a concise review article by Hunter et al. may be referred to.155

4.3. Other transition metal chalcogenides and pnictides: Metal chalcogenides, layered transition-metal chalcogenides (TMCs) and pnictides have interesting electrochemical properties which depend on their elemental composition, crystal structure, size, and defects. Compared to the metal oxides with high OER activities, as discussed earlier, the heavier congeners of the chalcogen family exhibit lower band-gap and higher electrical conductivity, thus expected to exhibit further improved OER performance. The chalcogenides have been widely investigated for hydrogen evolution reaction, HER156,157 and ORR158,159 in the past due to their fast charge transfer ability and stability in both acidic as well as alkaline media. Recent studies have shown that the TMCs and pnictides also exhibit robust and comparable OER activities as that of the oxide materials.5 Interestingly though, as a few careful studies have pointed out, that these compounds potentially undergo chemical transformation on the surface under the anodic conditions forming thin layers of oxygencontaining species such as oxide or hydroxide layers that are responsible for their high OER activities.160,161 Therefore chalcogenides and pnictides appear to act as unique precursors, assisting in the facile in-situ formation of OER active layers on the catalyst surface. Majority of the OER active chalcogenides and pnictides consists of Ni and Co based compounds, and even though the origin of their high activity is not clear, it is perhaps related to the ease of formation of oxide layers on the surface. For example, NiS nanosheet arrays supported on a stainless steel mesh have shown efficient OER activity with an overpotential of ~300 mV at 10 mAcm-2 current density.162 In comparison to oxides, other metal-chalcogen bonds are weaker



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and more covalent in nature. Besides, doping at the metal site becomes more convenient due to the larger size of the anions and metal-polyhedra. Therefore the catalytic activity can be conveniently tuned by controlling the metal to chalcogen ratio and by doping at the metal site. For the first time Swesi et al. demonstrated a metal rich form of Nickel subselenide, Ni3S2 as a potential candidate for OER in alkaline condition. The potential they observed was 1.53V @ 10 mAcm-2, with a stability performance of 18 hours conducted by chronoamperometry.163 The performance can be further improved by faceting the catalyst particles and exposing a more suitable crystal facet. While demonstrating this, Zou and co-workers induced in-situ growth of {2̅10} high-index faceted nanosheet of Ni3S2 arrays on nickel foam that exhibited ∼100% faradaic yield toward OER and remarkable catalytic stability for 200 h. The authors illustrated with the help of DFT calculations that orientation of the Ni atoms in this facet is significantly more favourable for OER as compared to conventional facets.164 Further by incorporating Ni in Co0.85Se, enhancement in the electrical conductivity was observed that resulted in better OER activity. Co0.85Se and (Ni,Co)0.85Se exhibited overpotential of 324 mV and 255 mV respectively.165 The latter catalyst also showed durability for 24 hours with a minimal increase in overpotential to 276 mV. In addition to the incorporation of Ni to Co0.85Se, intentional coating with layered double hydroxide on the surface of (Ni,Co)0.85Se by electrodeposition enhanced the OER activity with an overpotential to 216 mV with a Tafel slope of 77 mV/dec.165 The scope of these materials for OER have been elegantly reviewed by Suen et al and Kuang et al. 5,166 Akin to metal chalcogenides, metal pnictides, especially metal nitrides and metal phosphides (M2N, M3N, M4N, MP and M2P; M= Co, Ni) has also been extensive studies for OER activities.167–170 Here too, a thin layer of metal oxide or hydroxide is believed to be formed on the surface of these catalysts due to surface oxidation under alkaline conditions which is responsible for OER activities.125,171 For example, Stern et al. have demonstrated for the first


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time a nickel pnictides, ~Ni2P undergoes surface oxidation under the electrochemical conditions and transforms to Ni2P/NiOx core-shell assembly which led to an OER overpotential of 290 mV @ 10 mAcm-2 and Tafel slope of 59 mVdec-1, besides long-term stability up to 10 hours.172 Among phosphorus-containing materials, the Co-based compounds such as CoP, Li2CoP2O7, Na2CoP2O7, NaCoPO4 and LiCoPO4 exhibited appreciable OER activity and durability comparable to nitrides.173,174 As example, Zhu et al. used a simple electrodeposition technique to fabricate arrays of mesoporous CoP nanorods on Ni foam which was used as working electrode for OER.175 Hydrothermally synthesized CoP nanorods has been shown to exhibit consistent OER current density of 10 mAcm–2 at an overpotential of 320 mV and Tafel slope of 71 mV dec–1 in 1 M KOH for over 12 h. 143 The surface oxidation was found to be quite severe in this case giving rise to significantly crystalline cobalt oxide coating on the catalyst surface. By doping metal pnictides with another metal has been shown to further improve the OER activity.167 Recently, Xiao et al. fabricated a new family of Co-based bimetallic phosphide ultrathin nanosheets (CoM-P-NS, M = Ni, Mn, Cu, Zn) with homogeneous composition and unique porous architecture using ultrathin metal-organic framework nanosheets as precursor. 176 They reported a remarkably low overpotential for OER ~ 209 mV in an alkaline solution for the optimized nickel foam supported- CoNi (20:1) phosphide with stability for 11 hours. They reported a remarkably low overpotential for OER ~ 209 mV in alkaline solution for the optimized nickel foam supported- CoNi (20:1) phosphide with stability for 11 hours. Overall, investigations on pnictides mediated OER is limited in number when compared with oxides and chalcogenides, probably because the attribution of the activities becomes complicated due to oxidation of the surface. However of late, increasing number of studies has been carried out in the context of bifunctional catalysis, as discussed in the next section, since their high activity



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and stability cannot be neglected despite complex surface chemistry. Very recently, to investigate how the OER activity of transition metal pnictide (TMPs) varies systematically

Figure 8: (a) TEM image of FeCoNiP nanostructures showing the hollow tubular structure. The panels on the side are STEM-HAADF image and corresponding elemental maps of C, Fe, Co, Ni and P. (b) OER polarization curves of the various catalysts and (c) the corresponding overpotentials required to achieve anodic current densities of 10, 20, 50 and 100 mA cm-2. Reproduced with permission from Ref. 177. Copyright 2018 Royal Society of Chemistry.

with the catalyst composition, Liu and coworkers synthesized a series of mixed TMP nanoparticles supported on carbon nanofibers.177 They found that presence of a secondary transition metal in pnictides indeed significantly increase their OER activity in the order of FeP


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< NiP < CoP < FeNiP < FeCoP < CoNiP < FeCoNiP (Figure 8). In particular, the trimetallic pnictide FeCoNiP has exhibited one of the lowest OER overpotential of 200 mV with TOF over 0.94 s-1 at the overpotential of 350 mV. The Tafel slopes of the mixed TMPs are found to be in the range of 60 – 120 mV/dec, suggesting that the adsorption of OH− on the TMP nanoparticles is probably the rate limiting step. These materials have exhibited ‘no degradation’ stability for 24 hours. Overall, investigations on pnictides mediated OER is limited in number when compared with oxides and chalcogenides, probably because the attribution of the activities becomes complicated due to oxidation of the surface. However of late, increasing number of studies has been carried out in the context of bifunctional catalysis, as discussed in the next section, since their high activity and stability cannot be neglected despite complex surface chemistry. 4.4 Carbonaceous materials: The recent progress in using carbon-based materials as electrocatalysts have been remarkable. During the last decade carbon-based materials such a graphene, meso/microporous carbons, CNTs and other nanocarbons have been found to be highly active for OER, just as in the case of ORR. These materials show promising OER activity due to huge surface area and excellent electrical conductivity. Moreover here too, akin to ORR, the catalytic performances have been further enhanced by doping these with N, S, P and O.178–180 N doped carbon having pyridinic and quaternary-N acted as the active sites for OER as revealed by Hashimoto and co-workers.181 Furthermore, these workers concluded that OER performance improves with the increasing amount of pyridinic and quaternary N sites on carbon. The findings gave strong grounds to establish that N species are the active sites for OER. Moreover, edge dangling bonds, surface functional groups or defects in carbon materials contribute to electrocatalytic performance in alkaline medium. Conversely, metal-doped carbon materials show appreciable activity in acidic medium too. Recently Yang et al.



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deposited Co3O4 nanosheets on carbon paper (Co3O4-CP) through electroplating followed by a two-step calcination procedure.182 During this process, a layer of amorphous carbon formed insitu on the top of Co3O4. The authors postulated that due to this layer, Co3O4-CP exhibited excellent OER activity in 0.5 M H2SO4 with excellent overpotential 370 mV, and also the commendable stability for ~90 hours @100 mAcm-2 with a Tafel slope of 87 mV/dec. Due to high N content and excellent chemical stability along with graphene-like structures, g-C3N4has also gained significant attention for electrocatalytic OER.183 Tahir et al. has developed strongly coupled hybrid cobalt hydroxide nanowires by adding graphitic carbon nitride nanosheets.184 They demonstrated the presence of mere 5% g-C3N4 resulted in coupled Co(OH)2@g-C3N4 nanosheets with an overall diameter of 110 nm and a coating C3N4 layer of 10 nm, which exhibits a remarkable overpotential of 320 mV as well as high stability up to 10 hours having Tafel slope of 59 mV/dec. Subsequently it was found that by adding tiny bit of transition metals in the form of nanoparticles, OER performance of carbon-based materials can be further improved. Very recently, for example, Zheng et al. fabricated M-C3N4/CNT (M=Cr, Mn, Fe, Co, Ni, Cu, Zn) and found that the cobalt analogue exhibited the best OER performance.185 Physicochemical and electrochemical characterization combined with DFT computation was employed to identify the active sites. A Co−N coordination moiety in the gC3N4 matrix is suggested to be responsible for such activity. Such investigations are few and very recent in nature. A more detailed understanding of this reaction active sites will be desirable and interesting. When we document here the most recent findings, reviews detailing the use of carbonaceous materials for OER have been published elsewhere.186,187 4.5 Emerging insights on performance: We have so far discussed some of the highly efficient OER catalysts. Table 2 lists the performance and stabilities such catalysts reported of late. One cannot fail to notice in these reports that the attribution of high catalytic activities involves some experimental evidence and some arbitrariness, such as synergistic effect. However, there has been increasing number of dedicated studies elucidating the fine details of 36 ACS Paragon Plus Environment

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reaction mechanism, which should assist rational design of highly active OER catalysts. As in the case of ORR, the oxygen evolution reaction is also a four electron process. Herein hydroxyl anion gets adsorbed on the catalyst surface, followed by electron transfer. Alternatively, two OH- adsorbed at two consecutive sites may undergo association to form H2O2, which may dissociate to O2 and H2O by disproportionation. In a recent study using an in-situ spectrometry, Koper and co-workers showed the formation of various intermediates such as trihydroxide, peroxide and oxide layers on Au surface at different applied potential, before the released molecular oxygen.28 In order to understand the OER mechanism on IrO2 surface, Casalongue et al. employed in-situ ambient pressure XPS, and showed that under OER condition, the population of Ir4+ was reduced with the evolution of Ir5+.188 Such change in the oxidation state of surface Ir from +4 to +5 is coupled to a decrease in surface concentration of adsorbed hydroxyl species, supporting a peroxy species (OOH) mediated deprotonation mechanism. The OER activity of spinel and perovskite oxides also relies on the switchable oxidation state of the metal ions, where the catalytic process can involve the lattice oxygen too. Using in-situ18O isotope labelling and on-line electrochemical mass spectrometry, Grimaud et al. in a very careful study showed the evolution of labelled molecular oxygen (34O2 and 36O2) where LaCoO3 and related perovskite was employed as active materials.189 The degree of covalency in the M-O bonds is important, since the number of surface oxide layers (few to few tens) participating in the reaction depends on this parameter. There is another similarity with the ORR reaction, i.e. here too the presence of a single electron in the eg orbital of the metal ion at B site of perovskite have shown superior OER activity. Deng et al. reported on the presence of Cu3+ species as OER active site for the catalysts Cu, Cu2O, Cu(OH)2, and CuO using in-situ Raman and X-ray absorption near-edge structure spectroscopy (XANES).190 From several other such in-situ studies, it is emerging that a higher oxidation state of the metal ions acts as OER active centre in the metal oxide catalysts.



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Unlike the case of metal ions, investigations on the role of the lattice oxygen (LO) in the OER mechanism is rather limited. In a very recent DFT study, the participation of LO and consequent variation in activity have been carefully examined by Kolpak and coworkers.191 They argued that the previous studies point to two competing OER mechanisms operating

Figure 9: (a) Schematic illustration of the Adsorbate Evolution Mechanism (AEM) and LatticeOxygen Participation Mechanism (LOM) operating simultaneously in perovskite oxides. (b, c) Plots highlighting the differences in the reaction free energies of the four charge transfer steps in AEM and LOM respectively for lanthanum-based perovskites, which are plotted as functions of oxygen adsorption energy, ΔGO. The shaded region shows the theoretical OER activity volcano and the dashed line indicates the equilibrium OER potential. The difference between the dashed line and the boundary of the activity volcano depicts the theoretical overpotential for OER. Reproduced with permission from Ref. 191. Copyright 2018 American Chemical Society. simultaneously on perovskites viz. Adsorbate Evolution Mechanism (AEM) and LatticeOxygen Participation Mechanism (LOM) that can be clearly distinguished on the basis of participation of LO (Figure 9a). AEM can be represented by Eq 10, 11, 13, 14, while they gets modified in LOM as follows: OH* → (VO + OO*) + H+ + e-

(23) 38

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(VO + OO*) + H2O → O2 + (VO + *OH) + H+ + e-

(24)

(VO + *OH) + H2O → (HO-site* + *OH) + H+ + e-

(25)

(HO-site* + *OH) → *OH + H+ + e-

(26)

Where VO defines surface oxygen vacancy. The study showed that (1) AEM based OER activity volcano is universal for all perovskites while (2) LOM can induce higher OER activity depending on the A site cation which is related to the different behaviour of the reaction free energies of the four OER charge transfer steps and lowering of the thermodynamic overpotential (Figure 9b, c). Relying on the two mechanism approach, the authors went on to predict new materials with potentially higher OER activity, which would be worth testing. Similar studies involving other OER active materials is expected to help developing superior electrocatalysts. Table 2: Overview of recently developed efficient OER catalysts Sl no.

Materials

EOER@10mA cm-2

Stability

Tafel Slope

Electrolyte

Ref.

(mVdec-1)

vs RHE (V) 1.

IrO2 NP @ DNA

1.542

900 min †,NL

32

0.1 M NaOH

123

2.

IrO2/Au2O3

1.60

-

-

0.1 M NaOH

122

3.

IrOx/SrIrO3 film

1.57

30 h ¶@ 10 mAcm-2, Δη=+20

50

0.5 M H2SO4

121

mV 4.

Fenton-treated gold surface

1.8

-

35

0.1M NaOH

192

5.

BaNi0.83O2.5

1.57

20 h¶, NL

-

0.1 M KOH

193

6.

La0.6Sr0.4Co0.6Fe0.4O3–δ

1.619

> 3 h ¶@ 10 mAcm-2,NL

62.5

0.1 M KOH

79

7.

Pr0.5Ba0.5CoO3-δby Ca doping

1.67

1000 cycles₡,86.4 %

73

0.1M KOH

185

8.

Ba0.5Sr0.5Co0.8Fe0.2O3–d

1.6

-

-

0.1 M KOH

137

9.

La0.95FeO3-δ

1.63

-

48

0.1 M KOH

139

10.

Co3O4 (Plasma engraved)

1.53

2000 cycles₡, 87 %

68

0.1 M KOH

130

11.

CoFe2O4/C NRAs

1.47

60 h¶@ 50 mAcm-2, NL

45

1 M KOH

127

12.

NiFe2O4

1.67

-

98

0.1 M KOH

128

13.

CuFe2O4

1.64

-

94

0.1 M KOH

128

14.

Na0.08Ni0.9Fe0.1O2

1.49

30 h ¶@ 10 mAcm-2, NL

40

1.0 M KOH

147

15.

De-LiCoO2

1.62

10 h †,NL

57

0.1M KOH

146

16.

Ni3Se2

1.53

18 h †,NL

122.0

0.3 M KOH

163



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17.

CoMnP

1.56

500 cycles₡, Δη= +40 mV

61

1M KOH

78

18.

Ni-Fe LDH

1.53

-

40

1 M KOH

143

19.

Ni-CoOOH

1.516

25 h¶@ 10 mAcm-2,

36

0.1M KOH

149

NL 20.

Co-Pi NA/Ti

1.68

20 h†, NL

187

0.1M PBA

194

21.

Mn-Co oxyphosphide

1.55

8 h †, NL

52

1.0M KOH

195

22.

g-C3N4/ Ti3C2

1.65

5000 cycles₡,

74.6

0.1M KOH

183

Δη= +10 mV 23.

CoS-DNA hybrids

1.58

1000 min†, NL

61

1M KOH

124

24.

Black Phosphorous

1.6

10000 s †, NL

72.9

0.1 M KOH

196

25.

Fe(PO3)2/Ni2P 3D foam

1.407

20 h †, NL

51.9

1 M KOH

197

26.

cobalt–vanadium

1.445

_

44

1 M KOH

198

hydr(oxy)oxide 27.

CeO2 in NiO

1.60

10000 s †, 77%

118.7

1 M KOH

199

28.

Mo & Fe modified

1.51

50 h †, NL

47

1 M KOH

200

Ni(OH)2/NiOOH Legends : ¶- Chronopotentiometry; †- Chronoamperometry ; ₡- CV cycles ;NL- Negligible loss

5. Emerging bifunctional ORR-OER catalysts Development of efficient bifunctional catalysts for oxygen redox reaction is essential for improving the efficiency and durability of energy conversion devices such as regenerative fuel cells or metal-air batteries. Pt/C is considered as benchmark catalyst for ORR, while IrO2 and RuO2are taken as benchmark catalyst for OER. However, neither of them are good for both OER/ORR bifunctional activity.201 Recently, many non-noble metal (e.g. Co, Fe, Ni, Mn, Cr) based oxides, perovskites, spinels and layered double hydroxides based materials have been found to be highly active bifunctional electrocatalysts.166,202–205 Combining efficient individual OER and ORR catalyst can also serve as bifunctional behaviour. Besides, metal-free dopedcarbon based materials are also emerging as an attractive class of compounds in this area.205,206 Investigations on bifunctional catalysts are relatively newer as compared to ORR and OER catalysts and the reports are scattered. In the following, we have reviewed the most successful strategies to obtain efficient bifunctional catalysts.


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5.1 Spinels and perovskites: High bifunctional activity in spinels such as Co3O4 is originated from the mixed valences of cobalt (Co2+/Co3+) which facilitate the electron transfer through donor-acceptor interaction thereby providing sites for reversible electron transfer in anodic OER and cathodic ORR.166 However, surface area and electrical conductivity of the metal oxides are usually low.207,208 Recently, several research groups have attempted to improve their surface area employing innovative approaches. In one such approach, Park et al. developed high surface area 3D ordered mesoporous Co3O4 using polystyrene beads as a template and observed significantly enhanced bifunctional activity.209 Using 10 nm Co3xMnxO4,

Li et al. demonstrated that nano-crystallization of the material reduces the path length

of the electrons and is a promising way to improve the conductivity and thereby catalytic activity.208 However such approaches do not lead to significant improvements as the nanoparticles tend to agglomerate on the electrode surface and the interparticle junctions lead to high electrical resistance. Alternatively, mounting the catalyst particles on a conducting catalyst support such as CNT or graphene can also improve their activities.61,207,210–212 Liu et al. demonstrated this by integrating ZnCo2O4 quantum dots on conductive N-CNT that exhibited superior bifunctional behaviour (Figure 10a-c).207 N doping makes carbon atom more electropositive and oxygen-philic, which promotes ORR. In such a report, Zhao et al. obtained Cu@N-CNT/CoxOy composites in which Cu is embedded inside the CNT and CoxOy decorated outside the CNT as shown in the inset of Figure 10d).211 Detailed investigation using DFT calculations revealed that the work function (Փ) of CNT surface can be further reduced by encapsulating Cu metal inside CNT, making the surfaces more active towards ORR and favouring the adsorption of hydroxyl ion on the SW-CNT in the vicinity of CoxOy. This material has displayed 60 mV more positive onset potential compared to Cu@NCNTs for ORR and explicits an overpotential of 370 mV @ 10 mAcm-2 for OER (Figure 10 d).211 Such strategies has been extended to many other spinel systems.61,213 Notably, cobalt oxide present



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on N-doped carbon-based material exhibits a series of Faradaic peak in voltammogram as compared to the pristine samples, probably due to the formation of Co-N bond.214

(a)

(b)

(d)

(c)

Figure 10: (a) ORR and (b) OER activities of ZnCo2O4/N-CNT, Co3O4/N-CNT, ZnCo2O4, and Comparison with benchmark catalysts. (c) Schematic illustration of ORR and OER mechanism on the surface of ZnCo2O4/N-CNT. Reproduced with permission from Ref 207. Copyright 2016 Wiley-VCH. (d) ORR and OER activities of pure CNTs, Cu@CNTs, Cu@NCNTs, Cu@NCNT/CoxOy composites, Pt/C and IrO2 in 0.1 M aq. KOH solution, Inset is a schematic diagram of a Cu@NCNT/CoxOy composite. Reproduced with permission from Ref 211. Copyright 2017 Wiley-VCH. Apart from surface area and conductivity, the ease of charge transfer between the catalyst surface and the reactant is the key to the facile bifunctional activity. From this perspective, enhanced ORR/OER activity of the perovskite can be related to the number of transition metal eg electrons, the position of p-band centre of oxygen and the d-band centre of transition metal ion with respect to Fermi energy level and the population of oxygen vacancies.2,215 Close proximity of O 2p-band center and Fermi level decreases the energy of oxygen vacancy formation facilitating the oxygen surface exchange coefficient and promotes OER/ORR activity.216 Increasing the number of oxygen vacancies favours both ORR and OER catalysis.


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For example, in the case of La1−xFeO3-δ, it was shown that along with surface oxygen vacancy, formation of trace quantity of Fe4+ species having d-orbital filling (t2g3 eg1) facilitates the bifunctional behaviour.139 Detailed investigations on the electronic configuration of perovskites (ABO3) revealed that electronic configuration of B site cation having more than 1 eg electron favours OER while less than 1 promotes ORR.99,137,217,218 In a similar way, other perovskite oxides like Lan+1NinO3n+1, La0.8Sr0.2Mn1−xNixO3, La0.5Sr0.5CoO3-x also exhibit high ORR/OER catalytic performances where higher oxygen stoichiometry promotes higher oxidation state of cobalt due to increased hybridization between Co-3d and O-2p orbital.219–222 Yagi and co-workers compared the bifunctional behaviour of simple (AMnO3) and quadruple (AMn7O12) perovskite where A=Ca or La.4 They observed that the AMn7O12 exhibited bifunctional behaviour while AMnO3 displayed only ORR activity as shown in Figure 11 a, b. The authors attributed the exceptional OER activity of the quadruple perovskite (LaMn7O12) to its unique structure (Figure 11 c) that allows an average Mn-Mn distance (~3.2 Å) favouring O-O bond formation. The reason for enhanced ORR activity of LaMn7O12 has not clearly been understood. As seen in Figure 11 d, the reaction intermediate superoxides O22− is easily formed due to optimum Mn-Mn distance facilitating OER kinetics. On the other hand, Yan et al showed that doping of a large cation (e.g. Ir) at the B site of LaMnO3 (La0.8Sr0.2)1-xMn1-xIrxO3 (x=0.5) leads to one of best perovskites based bifunctional catalyst in terms of activity and durability.223 Herein, the creation of A site deficiency as well as doping of a larger cation at B site resulted in a larger lattice structure and reduced valence state of Mn, consequently weakening the bonding with adsorbed oxygenated intermediate, which eventually facilitated both ORR and OER kinetics. Cho and co-workers have synthesized a series of doped BSCF perovskite materials to impart bifunctional behaviour.205,224 Nanocrystals of La-doped BSCF perovskite catalysts exhibit outstanding bifunctional performance due to high surface area and presence of La cation in A site which increases the oxidation state of cobalt cation at the B site.205



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Further improvement in the catalytic activity can be achieved by mixing the perovskite material with nitrogen-doped carbon material as the former is good for OER and later can improve the ORR activity.225–228

(a)

(c)

(b)

(d)

Figure 11: a) Comparison of OER activities of LaMnO3, CaMnO3, LaMn7O12, CaMn7O12 and RuO2. b) Comparison of ring/disk current density for ORR of LaMnO3, CaMnO3, LaMn7O12, CaMn7O12, Pt/C. c) Crystal structure LaMn7O12, shown by following colors: orange polyhedra stand for MnO6 octahedra, the green square stands for MnO4 square, and blue spheres stand for O atoms of the OH − adsorbates, respectively. d) Schematic of OER mechanism on LaMn7O12. Inset (center): MnO4 square and MnO6 octahedra of AMn7O12 (A=La). Reproduced with permission Ref 4. Copyright 2017 Wiley-VCH Similarly, Lee et al. demonstrated that addition of polypyrrole to perovskite oxide resulted in remarkable improvement in bifunctional behaviour.225 Superior activity therein arises due to preferential adsorption of oxygen on polypyrrole forming charge transfer complex (Py+O2-) that fastens the O2 redox kinetics. Following this, very recently, it was found that the double 44 ACS Paragon Plus Environment

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perovskite NdBa0.5Sr0.5Co1.5Fe0.5O5+δ (NBSCF) incorporated in N doped reduced GO also exhibits excellent bifunctional behaviour.229 NBSCF exhibited very low potential gap of 0.766 V between ORR and OER. DFT analysis inferred that oxygen p-band centre moves away from the fermi level of Nd after its doping in double perovskite, leading to high activity and stability. These findings have demonstrated that tuning the electronic structure of perovskites through different doping and combining with doped carbon can be a promising route towards imparting excellent bifunctionality. To be noted that Zhu et al. and Cai et al. have reviewed electrochemical performances of these family of materials recently.1,230

5.2 LDH and other metal oxides: Apart from spinels and perovskites, other transition metal oxides such as CoxOy, NixOy, MnxOy have also shown attractive bifunctional activity.231–233 MnxOy generally enhances ORR kinetics but has poor OER activity.234 Combining MnxOy with efficient OER catalysts such as RuO2 or CoxOy can improve their bifunctional behaviour. Recently Yoon et al. reported a facile synthesis of double-walled RuO2/Mn2O3composite fibers which exhibited efficient bifunctional activity.232 On the other hand, controlling the morphology to expose a suitable crystal facet that favours oxygen adsorption should result in further enhancement of the bifunctional behaviour. With this motivation, single-crystal CoO nanorods (SC CoO NRs) prepared by Ling et al. exhibited impressive catalytic performance and stability for ORR/OER.144 These nanorods have desired oxygen vacancies generated by surface structure engineering, as shown in Figure 12 a. Oxygen vacancies localized on the {111}-O facets improves the conductivity of the material as well as adsorption of ORR/OER intermediates suggesting enhanced bifunctional catalytic activity. For ORR SC CoO NRs presented half-wave potential of 0.85V, similar to that of Pt/C with specific kinetic current density >7 times than that of polycrystalline PC-CoO NRs (Figure 12 b,c). On the other hand, 2.7 fold increase in specific current density than PC-CoO NRs was observed (Figure 12 d,e). 45 ACS Paragon Plus Environment


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Employing another popular approach very recently, oxygen vacancies have been created by doping oxides with N in order to enhance oxyphilicity of the catalyst surface. Guo and coworkers have demonstrated that nitrogenization of NiCo2O4 nanowires resulted in the formation of NiO/CoN porous interface NW arrays, giving rise to outstanding bifunctional activity.233 Herein, increased oxygen vacancies due to the incorporation of nitrogen resulted in decreased coordination of cobalt and coupling between NiO and CoN interface facilitate bifunctional activity, this was experimentally verified using EXAFS and ESR studies. Similar to the perovskite samples discussed earlier, it is emerging that combining the oxide materials with a heteroatom doped (usually nitrogen) carbon network is a useful strategy to improve the electrocatalytic performance of low dimensional material.235–238 5.3 Transition metal chalcogenides, pnictides and carbides: Due to their similarities in electronic structure with the metal oxides, metal chalcogenides and nitrides are also expected to exhibit appreciable bifunctional activity.239–243 However low conductivity of these materials in pure form limit their performance. From this point of view, hybrid structure of transition metal chalcogenides (TMCs) with doped carbon material have been created that showed excellent bifunctionality.243 In such an approach, Tiwari et al. reported a simple strategy to synthesize stacked WS2 nanosheets on the surface of conductive CNTs which effectively changes the charge density and spin density of carbon in CNT leading to enhanced bifunctional activity for ORR and OER.243 The proposed mechanism explains that H+ ions are connected to sulphur atoms via hydrodesulfurization process, while oxygen gets adsorbed on tungsten facilitate ORR. Metallic WC enhances the hydrodesulfurization by fast electron transfer from W to CNT.


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(a)

(b)

(d)

(c)

(e)

Figure 12: a) Schematic representation of surface engineering of SC CoO NRs. b) Comparison of ORR activities of SC, PC CoO NRs and Pt catalysts in 1M KOH solution with inset of chronoamperometric test for ORR after methanol addition. c) Specific activity of ORR at 0.6 V RHE. d) Comparison of OER activities of SC, PC CoO NRs and commercial RuO2 catalyst in 1M KOH solution with inset of Tafel plot for OER. e) Specific activity of OER at 1.65 V RHE. Reproduced with permission from ref 144. Copyright 2016 Nature Publishing Group. All these processes simultaneously add up to give promising catalytic activity towards ORR/OER. Similarly, hybrid structure of cobalt sulfide with N doped carbon network (N-C) also drawn a lot of attention due to its outstanding bifunctional activity for oxygen redox



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reaction.239–242 In these materials also Co-S acts as active site for both ORR and OER and carbon improves the conductivity of the system. Metal nitrides are emerging as promising electrode materials for oxygen redox reactions.125,244,245 Chen et al. synthesized atomically dispersed Fe-Nx on N and S co-decorated on carbon network which exhibited superior bifunctional activity.246 High catalytic activity of this material can be ascribed to (i) N and S doping creates positive polarization on neighbouring carbon center facilitating oxygen adsorption, (ii) atomically dispersed Fe-Nx species on carbon network provides more number of active site, (iii) heteroatom doping increases the conductivity of the system causing faster electron transfer from Fe to carbon network. Similarly, Zhang and co-workers245 first theoretically predicted that generation of Co4N and Co-N-C as active ORR and OER site respectively on a composite material should facilitate the redox reactions. 245 A proof-of-concept experiment revealed the same in the ORR and OER activity of Co4N, Ndoped carbon fibers network, and carbon cloth (Co4N/CNW/CC). However such examples are scarce and further investigation is required for understanding and enhancing the catalytic activity of metal nitrides. Metal carbides are unique due to a small difference in electronegativity between the constituting atoms, which makes their Fermi level behave somewhat similar to noble metals. Kou et al. have developed a precise thermal-chlorination strategy for transformation of metal carbide to metal-self doped graphene.247 They observed that among the various 2D metal carbides (MxCy e.g., Cr3C2, Mo2C, NbC, and VC) that have been examined as bifunctional catalysts, Cr self-doped graphene displayed considerably enhanced bifunctional activity towards ORR/OER. Cr being less electronegative than carbon acquires slightly positive charge facilitating the adsorption of oxygen molecules. Incorporation of Cr creates defect in the graphene sheet which enhances the catalytic activity.


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5.4 Carbon-based materials: 5.4.1 Metal-free catalysts: As discussed in the earlier sections that heteroatom-doped carbon exhibits useful ORR and OER activities, one would envision them having excellent bifunctional activity too. Indeed investigations in the last couple of years have shown that a number of metal-free carbon-based materials have met such expectations. However, their performance depends mainly on the microstructures (therefore all efficient ORR and OER catalysts are not necessarily efficient bifunctional catalysts too), percentage and nature of heteroatom doping and active surface area. Nitrogen is the most widely used heteroatom, while increasing number of heteroatoms by this precursor alone is outdated.248,249 Hu et al. synthesized N and S co-doped graphitic sheet (SHG) as an active bifunctional catalyst which also catalyzes hydrogen evolution reaction.

Figure 13: Comparison of bifunctional activity of SHG, GC, GS, Pt/C and RuO2 nanoparticles. Inset: ΔE (Ej=10 –E1/2) of ORR and OER activities for various catalyst reported catalyst. Reproduced with permission from Ref: 250. Copyright 2017 Wiley-VCH.

SHG exhibited ORR activity comparable to commercial Pt/C, retaining ~93% stability up to 100 h, and also displayed comparable OER activity as that of benchmarked RuO2 as displayed



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in Figure 13.250 Authors have established that co-doping of both N and S is responsible for exceptional high activity as it generates positive C centres which facilitate oxygen adsorption. Shinde et al. have used phosphoric acid, methane sulphonic acid precursors for co-doping P and S on carbon nitride sponges (P,S-CNS) respectively, it has shown excellent bifunctional activity such as ƞ = 330 mV for OER and ORR E1/2 of 870 mV.251 Positive carbon sites adjacent to nitrogen and oxygen in these samples are active for both ORR and OER while P and S promote its improved efficiency through modification of electronic structure. In general, heteroatom doping increases the conductivity of carbon framework thereby improving the rate of charge transfer that results in enhanced catalytic activity.251,252 On the other hand, large surface area materials possessing a number of pores and dangling atoms tremendously increase the accessibility of the active sites which promote fast electron transfer as well as ion diffusion at electrochemical interfaces. In a theoretical investigation, Guo and co-workers demonstrated the bifunctional activity of P and N co-doped graphene framework. The authors elucidated the P atom being larger than C and N, always move out of the planar surface in the graphitic framework and occupy the edge sites. These P sites get readily oxidized in presence of oxygen hence become inactive towards ORR/OER. While in basal plane, P sites adjacent to N in a graphitic framework triggers the adjacent carbon sites for OER, the N-doped carbon sites are efficient for ORR.253 This was further supported by the experimental results as this material showed superior ORR and OER activity and stability than the noble metal-based benchmarked catalysts such as Pt, and Ru. Wang and coworkers demonstrated in-situ exfoliation of carbon cloth using Ar plasma which resulted in oxygen functionalized graphene sheet (P-CC).252 The synergistic effect between defects formed by in-situ exfoliation and O-doping enhances the catalytic activity. One should however note that though the carbon-based materials possess excellent bifunctional activity comparable with transition metal oxides, they suffer from poor stability. Further, a compiled and illustrative 50 ACS Paragon Plus Environment

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review article on carbon-based materials for OER/ORR performances by Chen et al. can also be referred by readers.31 5.4.2 Metal atom loaded carbonaceous materials: Insertion of transition metals into carbon network was found to enhance the bifunctional activity by several folds than the un-doped ones. NiFe-N/C and MnCo-N/C, Ni3Fe/N-C sheets, CoxSe@NC bimetallic nitrogen-doped carbon framework outperforms most other well-known bifunctional catalyst families with ΔE (EOER 226,254 Enhanced OER activity can be attributed @10 mAcm-2-E1/2,ORR) in the range of ~0.73- 0.76 V.

to the bimetallic centre while superior ORR performance can be related to the N doped carbon. However, detailed investigation is required to appreciate the origin of bifunctional behaviour in M-N-C systems. Metal-nitrogen doped carbon (M-N-C) systems have been realized as promising materials for ORR overcoming commercial Pt/C due to their low cost, durability and high catalytic activity.255,256 However, these materials have moderate OER activity. In a recent report, Hu et al. explained the bifunctionality of nanocarbon-intercalated graphene material doped with nitrogen and iron (Fe–N-CIG).257 The catalytically active sites are N, Fe3O4, and possibly FeNx developed during doping on the graphene surface. However, Fe-N-C exhibit inferior OER activity compared to other state-of-the-art materials. The sluggish OER kinetics is because of formation of semiconductive FeOOH species during OER which reduces electron transfer rate. Hence doping with OER active material in Fe-N-C system can further enhance the efficiency. Many reports are available in the literature describing bimetallic nitrogen-doped carbon as an efficient material for Oxygen redox reaction.258–261 Numerous studies found that cobalt doping in Fe-N-C system is highly beneficial to improve OER kinetics.261–263 Su et al. synthesized bimetal FeCo nanoparticles encapsulated on N-doped graphitic carbon nanotubes (N-GCNT) serves as an excellent oxygen redox reaction catalyst.258 Acid treatment of the material so as to remove the metal nanoparticle does not show any deterioration in catalytic 51 ACS Paragon Plus Environment


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activity implying pyridinic N–C structure as the key electroactive sites. However, FeCo nanoparticles are crucial for structuring of carbon nanotube. Contrary, DFT calculation explained a synergistic effect present between FeCo nanoparticles and N-CNT helps in surface modification favouring both ORR and OER. Other studies revealed that addition of a bimetal to M-N-C system increases the amount of pyridine nitrogen and strong coupling between metal nanoparticle and nitrogen doped carbon considered as the prime factor for bifunctional activity.260 Yang et al. extensively studied the catalytically active sites for ORR, OER and HER on FeCo-N-C species.261 By performing many control experiments are such as monometal doping and thiocyanate poisoning of the metal site, it was found that Fe–N–C is the active site for both ORR and OER while Co–N–C is the active site for HER. However, the origin of the bifunctional behaviour of Fe-N-C site is still not well-understood. Enhanced OER activity is possibly due to the bimetallic centre while ORR performance can be related to the N-doped carbon as discussed earlier. 5.5 Emerging insights on performance: We have so far discussed some of the most interesting ORR/OER bifunctional catalysts that have emerged recently. In Table 3, we depict the most efficient catalysts reported in the last couple of years. However while sorting the recent publications, one can easily notice that the performances of these catalysts vary widely among the different reports. Even for the closely related systems, their activities seem to differ vastly. This can be potentially confusing in screening an appropriate catalyst and thus evaluating their performances on an overall basis rather than being selective. In Figure 14, we plot the activity for various families of bifunctional catalysts in alkaline electrolyte using box-whisker plot and denoting the maximum, minimum, median and mean values for a range of data obtained from over 150 literature reports in last two-three years (references in Supporting Information). We neglected few reports where stability has not been examined. In the figure, we compare the E1/2 and η for ORR and OER 52 ACS Paragon Plus Environment

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respectively. A higher value of E1/2 (i.e. a higher position for a catalyst in panel (i) in Figure 14)) and smaller η (i.e. a lower position in penal (ii)) is a good indicator for a promising bifunctional catalyst. To assist the readers with a concise and systematic approach towards screening an effective materials, we have classified the catalysts into nine different families: spinel, perovskite, metal oxide, metal hydroxide, transition metal chalcogenide (TMC), pnictide (M-N), carbide (M-C), metal-N-C (M-N-C) and metal-free carbon. The same for the benchmarking catalysts are also marked. While the plot can be easily understood, we point out the key observations in the following. It is apparent from the figure that the same family may have widely varying activity as depicted by the spread in the highest and lowest activity values. It will be more useful to compare them on the basis of the median of the data set. The families can be further subdivided on the basis of the two crucial approaches, adopted to improve them, viz. (i) doping a catalyst with other metals, and (ii) combining them with carbon-based materials as catalyst supports. Investigations on the spinel and the perovskite families, in particular, are extensive as compared to the others. Therefore the spinels are further divided into pure spinel, mixed spinel and carbon supported spinel. Similarly, the perovskite family consists of binary (pure), ternary (either A site or B site doped), quaternary (both A and B sites doped) systems and their carbon supported analogues. Investigations on metal oxides and hydroxides are limited and hence classified as with and without C support. Investigations on M-N, M-C, M-N-C families are even more limited and all such catalysts are analyzed together. In the case of ORR, one can observe that the activities of the best catalysts are approaching that of commercial Pt/C in alkaline medium, even surpassing them occasionally. Within a family, generally put, the pure compounds exhibit minimal activities while using a carbon- based support improves them towards highest activity. In case of the spinel family for instance, the ORR activities of binary spinel members are at least 150 mV lower than the C supported one. Such distinct trends are observed within the perovskite, metal oxide and metal hydroxide



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family as well. Furthermore, the activity of the pure compound improves significantly upon doping with another suitable metal. It appears that more the number of different metal atoms

Figure 14: Box-whisker plot comparing performances for various families of ORR-OER bifunctional catalysts in alkaline media, developed in the last 3-4 years. Based on a large number of literature reports, the maximum, minimum, median and mean for each set of data set has been plotted (see references in Supporting Information). The upper panel compares the half-wave potential (E1/2) depicting the ORR activities. The lower panel represents the overpotential corresponding to 10 mAcm-2 OER current. The filled square represents the mean of the data set. For convenience, the corresponding values for the benchmark catalysts have been plotted on the extreme right of the plot.


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within the crystal lattice of the catalyst, better is the performance. Metal hydroxides generally are good for OER, but there are at least few instances where meaningful bifunctional behaviour has also been observed. Among other materials, M-N-Cs exhibit the highest E1/5 value of > 0.9 V and in average, marginally better ORR activity when compared with pnictide and metal carbide. Overall, the performances of the metal-free carbon-based materials appear better than the perovskites and metal hydroxides. In the case of OER performance, a trend is recognizable in moving from left to right of the figure where the overpotential reaches a minimum in the family of TMC. According to our literature survey, the spread of OER activity was found to be the least in case of pure spinels and highest in case of TMCs. On a closer look at the first 4 families, it is evident that use of a carbon-based support enhances the OER activity, greatly at times, as in the case of ORR. LDHbased materials are known to exhibit excellent water oxidation ability in both metal-air batteries and standalone OER half-cell reactions, but the incorporation of carbon drastically decreases the overpotential median from ~0.4 to ~0.3 V. The activity of an efficient bifunctional catalyst is a trade-off between a good OER and ORR activities, so it is impertinent to improve the individual half-cell reactions independently. From Figure 14, it appears that carbon supported metal hydroxides, M-N and TMC families are more promising bifunctional catalysts. From the large difference in the applied potential of ORR and OER, it occurs that the same surface site can’t act as active site for both the reactions and thus both sites ought to be independently engineered, if possible, to attain superior bifunctionality. This notion has been augmented from a recent observation by using in-situ operando X-ray absorption spectroscopy (XAS) study, which shows that the ORR and OER reaction conditions stabilize markedly different oxidation state of the active metal.264 In the case of N doped carbon, the ratio of pyridinic and quaternary N-atom was found to be crucial in improving their efficiency.265 Since



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such investigations are relatively recent and sparse, and the origin of bifunctionality is expectedly more complex than ORR or OER activity alone, more detailed understanding of the nature of the catalyst surface would be necessary to make rational progress. Table 3: Overview of recently developed efficient ORR/OER bifunctional catalysts. No.

Materials

E1/2,ORR vs. RHE (V)

EOER @10 mA -2

cm vs. RHE

Tafel Slope

EOER @10

for OER

mAcm-2-

Stability ( ORR/OER)

Ref.

-1

(V)

(mVdec ) E1/2,ORR (V)

1

3DOM Co3O4

0.65

1.66

58

1.01

1000 cycle, ΔE = 28 mV @ 0.1 mA cm-2₡ /

209

1000 cycle, ΔE = 13 mV @ 10 mA cm-2₡ 2

c-CoMn2 /C

0.83

~ 1.78

-

0.95

50 h, 91.5%, @ 0.8V† / -

208

3

ZnCo2O4 /N-CNT

0.87

1.66

70.6

0.79

10 h, 90 % @ 0.5 V† / 10 h, 95 % @ 1.58 V†

207

4

Co@Co3O4/NC-1

0.80

1.65

91.5

0.85

25 h, NL @0.77 V† / 45 h, NL @10mA cm-2⁋

213

5

NixCoyO4 /Co–NG

0.804

1.629

77.2

0.825

20000 s, 97.6 % @0.6 V†/ 1000 s, ΔE = 0.1 V

266

@20 mA cm−2⁋ 6

Co3O4 /N-CNTA

0.79

1.662

79.65

0.877

10000 s, 85.6%† / 10000 s, 112%†

210

7

Co3O4 /NRGO*

0.82

1.65

80

0.83

- / 22 h, ΔE = ~ 3.7 mV @ 10 mA cm−2⁋

214

8

NBSCF

0.869

1.635

40

0.766

10 h, ΔE = ~0.08 V @ -3 mA cm-2⁋ / 10 h, ΔE

229

= ~0.04 V @ 5 mA cm-2⁋ 9

LSMI

0.78

1.67

103

0.89

5h @ 1.56, 97.6 % / 5h@ 1.56V, 90%

223

10

LO-NF-NCNTs

0.772

1.74

-

0.968

10 h, 83 % @ 0.8 V† / 11 h, 95.8% @1.87 V†

227

11

SC-CoO-NRs*

0.85

1.56

44

0.71

10 h, 97% @ 0.60 V† / -

144

12

NiO/CoN PINWs*

0.68

1.53

35

0.85

10000 cycles, NL₡ / 30000 cycles, 92.44 % ₡

233

13

CoO@Co/N-rGO

0.81

1.65

-

0.84

5000 cycle, NL₡/ 1000 cycle NL₡

235

14

NC-CoO/C

0.793

1.592

45.2

0.799

20000s, 97%@0.65V† / 6000s, 90%@ 1.6 V†

231

15

NiFe-LDH – Fe–

0.793

1.539 ± 0.006

-

0.747 ± 0.009

24 h, ΔE = +0.4 @ -3 mA cm-2¶ / 24 h, NL @

267

4 mA cm-2¶

N–C 16

N-GCNT/FeCo-3

0.92

1.73

99.5

0.81

20 h, 87.58% @ 0.3 V† / 20 h, ΔE = +0.06

258

V@10 mA cm−2¶ 17

FeNO-CNT-

0.87

1.66

63

0.786

55000 s, ~92% @ 0.65 V† / 25000 s, ~ 75%

259

@ 1.65 V†

CNFF-800 18

Co0.85Se@NC*

0.82

1.55

75

0.73

10h, 82.5% @0.67 V† / 12h, 93.4% @1.55 V†

226

19

Co1-xS/N-S-G

0.862

1.601

63

0.739

30000 s, 80% @ 0.682 V† / 200 cycle, ΔE =

239

+20 mV @ 10 mA cm−2¶ 20

Co4N/CNW/CC*

0.8

1.54

81

0.74

20 h, 98% @ 0.5 V† / 20 h, 98% @ 1.54 V†

245

21

PPy/FeTCPP/Co

0.86

1.61

61

0.75

10 h, 95 % @ 0.86 V† / 10 h, ~84% @ 1.57V†

261

22

Ni3Fe/N-C sheets

~0.76

1.60

77

0.84

12000s, 96% @ .7 V† / 12000s, 50% @1.6 V†

254

23

BNPC-1100

0.793

1.55

201

0.757

8000 cycle , 75% ₡ / 10 h, NL

268


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24

N-GRW*

0.84

1.59

52

0.75

12 h, 90% @ 0.7 V† / 24 h, 91% @ 1.59 V†

205

25

PS-CNS

0.87

1.56

64

0.69

8000 s, 97.4 % † / -

251

26

SHG

0.87

1.60

71

0.73

100h, ~93%@ −0.3V† / 20h, ~91%@ 1.55 V†

250

27

S,N-Fe/N/C-CNT

0.85

1.60

82

0.75

10000 cycle, ~98 %₡ / NA

246

28

Fe–N-CIG

0.84

1.67

96

0.80

1000 s, 96% @ 0.7 V† / 1000 s, NL @ 1.67V†

257

29

C@Co-NGR

0.83

1.68

98.8

0.85

3000 cycles, NL₡ / NA

269

Legends : ¶- Chronopotentiometry; †- Chronoamperometry ;₡ - CV cycles ;Electrolyte – 0.1 M KOH * - 1 M KOH as e NL- Negligible loss.

6. Conclusions and Outlook: The key to the success of the water-based renewable cycle energy was deemed to be overcoming the challenge of replacing the expensive benchmarking catalysts with inexpensive earth-abundant materials and improving the kinetics of the sluggish electrocatalytic reactions involving oxygen. Led by this, a large number of electroactive materials have been developed in the recent years. This review summarizes the most significant research findings on oxygen reduction and oxygen evolution reactions discovered recently, including the most efficient catalysts for ORR, OER and bifunctional activities as well as the key mechanistic insights that may help further improvement. It is apparent that a number of factors such as size, shape, composition, porosity, surface structure, crystal defect, surface strain, preparation methods and post-treatment play crucial roles in determining the activity of a catalyst. This has given rise to vastly different efficiencies even for the closely related systems, of which many are indeed excellent and matches benchmarked performances. Despite notable progress, there are several challenges yet to be overcome before viable utilization of water renewable energy cycle. The first would be to ascertain the actual reaction centers and to elucidate the mechanisms occurring on the surface of inexpensive catalysts in greater detail. There are considerable differences in opinion regarding the active catalytic centers in complex materials. As an example, Kim et al. proposed that ORR activity is generated from the compound containing only Fe−Fe3C@C sites76 whereas Strickland et al. attributed the same activity to synergistic effect existing between Fe−Fe3C@C sites and the Fe−Nx sites.270 It is particularly intriguing in the case of chalcogenides, nitrides, phosphides and carbides where a stronger 57 ACS Paragon Plus Environment


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metal-oxygen bond than the existing metal anion bond in the catalyst lattice is shown to form under the reaction condition which is responsible for oxygen evolution and besides, which must be cleaved in order to regenerate the catalyst surface. It appears that the catalyst surface undergoes oxidation forming an oxide and hydroxide layer which is the actual active center for the reaction. An interesting editorial by Prof. Jin of ACS Energy Letters correctly pointed out that the accurate identification of the catalytic center is necessary to claim a non-oxide earth abundant catalyst as an efficient one.271 A multipronged approach using in-situ spectroscopic studies, isotope labelling studies and theoretical investigations will be highly beneficial. Second, even though some of the catalysts are extremely good, their stability under the harsh electrochemical conditions is an important concern. For sustainability, the renewable energy devices are expected to run for years without the need for a change in the catalysts, suggesting that long-term stability of the catalysts must be evaluated with priority. However, envisioning stability of these materials is still difficult due to the absence of any credible long duration stability test (at the moment, such tests are carried out for a few hours or few thousand cycles at best as seen for from the tables). Careful monitoring for prolonged period may lead to findings of profound interest for electrocatalysis or otherwise. Considering the carbon-based supports, even though these have been widely used in conjunction with most inorganic catalysts, these would eventually be exposed to high electrode potential during start-up and shutdown conditions in a device, that could lead to strong corrosion.251 Particularly during fuel starvation, the support would potentially undergo carbon oxidation reaction, (COR, ECO2/C = 0.207 V vs. NHE at T = 298 K), aggravated at defect sites.250,272 Therefore, due attention must be paid and alternatives have to be developed.273–278 In addition, a considerable amount of catalyst active surface area gets masked while using a catalyst support and self-supported catalysts could be beneficial for higher surface area.279,280 In this context, use of transition metal foams where the surface is chemically converted to the corresponding catalyst (while the 58 ACS Paragon Plus Environment

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core provides higher conductivity) is noteworthy and ought to be investigated in greater detail.281 In order to control size and shape, surfactant and capping agents have been widely used. However, the effect of their presence on the catalytic properties has been rarely probed. For example, Bao et al. have shown recently that PVP-free octahedral Pd@Pt3−4L/C catalyst have specific activity 50% higher than that of the PVP-stabilized materials C.109 Besides, even though the smaller particles show high mass activity, there is a possibility of activity loss during prolonged usage due to changes in their shape or coalescence triggered by the presence of a large number of under coordinated atoms. In regards to the production of catalyst materials, even though shape controlled nanomaterials have shown promising activities, their utilization in large-scale device fabrication requires scalability of their synthesis procedure, which is rather non-trivial. On the other hand, catalyst decorated carbon materials are equally facile and can be easily produced in large scale. However, these are mostly investigated in alkaline medium, which is undesirable due to the poor chemical stability of anion exchange membranes. Therefore, the budgetary focus of US DOE (2016) has been shifted to development of noble metal-based ORR active materials usable in acidic electrolyte.7 This underlines the need to search for electrocatalysts that are inexpensive and yet exhibit high performance in acidic medium. Given the fact that the active catalytic sites are often different from the crystallographic sites (and many catalysts such as carbon based ones etc. are barely crystalline), and identification of the active site involves indirect set of measurements, detailed theoretical investigations and developing models on reaction pathways would be extremely helpful in developing robust ORR-OER catalysts and designing experiments towards discerning reaction mechanisms. A few of recent theoretical studies have been carried out in this regards, though their numbers are rather limited.282-287 For example, Ag is preferable ORR catalyst over benchmark Pt due to their



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higher abundance. However, despite a lower onset potential, their performance is not at per with Pt due to the non-optimal adsorption strength of the intermediates. The theoretical studies on Ag surfaces have been aimed at calculating the binding energies of the intermediates. However, these calculations do not take in to account the neighbouring surface adsorbed species. Therefore Liu and co-workers have recently examined the ORR mechanism on Ag(111) using a combined DFT and Kinetic Monte Carlo method to mimic real electrodes. 282 In the process, they identified an interesting new chemisorbed water (*H2O)‐mediated 4 e− associative pathway. They showed that despite a lower ORR onset, the current change is slower than Pt because as the electrode potential increases, coverage of the Ag electrode by H2O and OH significantly different than in Pt leading to lower current. Such studies clearly demonstrate the need to conclusively examine the reaction mechanisms on various electrode surfaces and also calls for different approaches to enhance the reaction rates by focusing of adsorption energies of solvent for example. Similarly, the electrocatalytic activities of bulk-immiscible alloys are poorly explored as compared to the bulk-miscible counterparts owing to the difficulties in their synthesis. Henkelman and co-workers have computationally screened Agand Au-based alloys for the ORR and further demonstrated experimentally that alloy nanocrystals of classically immiscible Au and Rh i.e. Au2Rh indeed exhibit excellent ORR activity.288 Such reports should inspire development of novel synthetic strategies to realize these metastable phases at the nanoscale. Understanding functionality of some of the most active catalysts such as poorly crystalline metal hydroxides and recently developed catalysts containing transition metal atoms in N-doped carbon network are more challenging. In regard to the hydroxides, Goddard III and co-workers investigations using grand canonical quantum mechanics including kinetics approach underlines the need for uniform distribution of Fe and Ni in (Fe,Ni)OOH since Fe can dramatically reduce the reaction overpotential while neighbouring Ni can efficiently catalyse O-O coupling.289 It is encouraging to observe an


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increase in detailed theoretical studies dedicated to screening of novel catalysts and elucidating reaction mechanisms that would certain to inspire development of more improved catalysts. Finally, in order to access these catalysts in a practical sense, one must develop prototype devices and test the performance for long hours. With this, and considering the rapid progress that has been made in the last few years, a clear picture on the viability of water-cycle based renewable devices should energy in the next few years.

Acknowledgements: SM and KC thank IISER-Mohali for fellowship. LS thanks UGC for senior research fellowship. PEK thanks SERB-India for DST-NPD fellowship. Supporting Information: references used to prepare Figure 14.



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References (1) (2) (3) (4)

(5)

(6) (7)

(8)

(9)

(10)

(11) (12)

(13)

(14) (15)

(16)

(17)

(18)

Zhu, Y.; Zhou, W.; Shao, Z. Perovskite/Carbon Composites: Applications in Oxygen Electrocatalysis. Small 2017, 13, 1603793. Sivula, K.; Van De Krol, R. Semiconducting Materials for Photoelectrochemical Energy Conversion. Nat. Rev. Mater. 2016, 1, 1–16. Ma, T. Y.; Dai, S.; Qiao, S. Z. Self-Supported Electrocatalysts for Advanced Energy Conversion Processes. Mater. Today 2016, 19, 265–273. Yamada, I.; Fujii, H.; Takamatsu, A.; Ikeno, H.; Wada, K.; Tsukasaki, H.; Kawaguchi, S.; Mori, S.; Yagi, S. Bifunctional Oxygen Reaction Catalysis of Quadruple Manganese Perovskites. Adv. Mater. 2017, 29, 1603004. Suen, N.-T.; Hung, S.-F.; Quan, Q.; Zhang, N.; Xu, Y.-J.; Chen, H. M. Electrocatalysis for the Oxygen Evolution Reaction: Recent Development and Future Perspectives. Chem. Soc. Rev. 2017, 46, 337–365. Arbib, J.; Seba, T. Rethinking Transportation. 2017, 70, 2020-2030. Mai, T.; Sandor, D.; Wiser, R.; Schneider, T. Renewable Electricity Futures Study: Executive Summary. NREL/TP-6A20-52409-ES, National Renewable Energy Laboratory (NREL), , Golden, CO. 2012, 2-3. Das, V.; Padmanaban, S.; Venkitusamy, K.; Selvamuthukumaran, R.; Blaabjerg, F.; Siano, P. Recent Advances and Challenges of Fuel Cell Based Power System Architectures and Control – A Review. Renew. Sustain. Energy Rev. 2017, 73, 10–18. Wang, Y.; Leung, D. Y. C.; Xuan, J.; Wang, H. A Review on Unitized Regenerative Fuel Cell Technologies, Part-A: Unitized Regenerative Proton Exchange Membrane Fuel Cells. Renew. Sustain. Energy Rev. 2016, 65, 961–977. Truong, T. T.; Liu, Y.; Ren, Y.; Trahey, L.; Sun, Y. Morphological and Crystalline Evolution of Nanostructured MnO2 and Its Application in Lithium-Air Batteries. ACS Nano 2012, 6, 8067–8077. Sumboja, A.; Ge, X.; Zong, Y.; Liu, Z. Progress in Development of Flexible Metal– Air Batteries. Funct. Mater. Lett. 2016, 9, 1630001. Li, D.; Lv, H.; Kang, Y.; Markovic, N. M.; Stamenkovic, V. R. Progress in the Development of Oxygen Reduction Reaction Catalysts for Low-Temperature Fuel Cells. Annu. Rev. Chem. Biomol. Eng. 2016, 7, 509–532. Oldacre, A. N.; Friedman, A. E.; Cook, T. R. A Self-Assembled Cofacial Cobalt Porphyrin Prism for Oxygen Reduction Catalysis. J. Am. Chem. Soc. 2017, 139, 1424– 1427. Costentin, C.; Robert, M.; Savéant, J.-M. Molecular Catal. Electrochem. Reactions. Curr. Opin. Electrochem. 2017, 2, 26–31. Zoladek, S.; Rutkowska, I. A.; Blicharska, M.; Skorupska, K.; Kulesza, P. J. Enhancement of Oxygen Reduction at Co-Porphyrin Catalyst by Supporting onto Hybrid Multi-Layered Film of Polypyrrole and Polyoxometalate-Modified Gold Nanoparticles. J. Solid State Electrochem. 2016, 20, 1199–1208. Nunes, M.; Fernandes, D. M.; Rocha, I. M.; Pereira, M. F. R.; Mbomekalle, I.-M.; De Oliveira, P.; Freire, C. Phosphomolybdate@Carbon-Based Nanocomposites as Electrocatalysts for Oxygen Reduction Reaction. ChemistrySelect 2016, 1, 6257–6266. Dong, R.; Zheng, Z.; Tranca, D. C.; Zhang, J.; Chandrasekhar, N.; Liu, S.; Zhuang, X.; Seifert, G.; Feng, X. Immobilizing Molecular Metal Dithiolene–Diamine Complexes on 2D Metal–Organic Frameworks for Electrocatalytic H2Production. Chem. - A Eur. J. 2017, 23, 2255–2260. Maganas, D.; Trunschke, A.; Schlögl, R.; Neese, F. A Unified View on Heterogeneous 62 ACS Paragon Plus Environment

Page 63 of 81 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(19)

(20)

(21)

(22)

(23)

(24) (25) (26)

(27) (28)

(29)

(30) (31)

(32)

(33)

(34)

(35)

and Homogeneous Catalysts through a Combination of Spectroscopy and Quantum Chemistry. Faraday Discuss. 2016, 188, 181–197. Jung, S.; McCrory, C. C. L.; Ferrer, I. M.; Peters, J. C.; Jaramillo, T. F. Benchmarking Nanoparticulate Metal Oxide Electrocatalysts for the Alkaline Water Oxidation Reaction. J. Mater. Chem. A 2016, 4, 3068–3076. Mamtani, K.; Singh, D.; Tian, J.; Millet, J. M. M.; Miller, J. T.; Co, A. C.; Ozkan, U. S. Evolution of N-Coordinated Iron–Carbon (FeNC) Catalysts and Their Oxygen Reduction (ORR) Performance in Acidic Media at Various Stages of Catalyst Synthesis: An Attempt at Benchmarking. Catal. Letters 2016, 146, 1749–1770. Gao, X.; Zhang, H.; Li, Q.; Yu, X.; Hong, Z.; Zhang, X.; Liang, C.; Lin, Z. Hierarchical NiCo2O4 hollow Microcuboids as Bifunctional Electrocatalysts for Overall Water-Splitting. Angew. Chemie - Int. Ed. 2016, 55, 6290–6294. Song, C.; Zhang, J. Electrocatalytic Oxygen Reduction Reaction. In PEM Fuel Cell Electrocatalysts and Catalyst Layers: Fundamentals and Applications; Zhang, J., Ed.; Springer London: London, 2008, 89–134. Nørskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.; Jónsson, H. Origin of the Overpotential for Oxygen Reduction at a Fuel-Cell Cathode. J. Phys. Chem. B 2004, 108, 17886–17892. Mukerjee, S. Role of Structural and Electronic Properties of Pt and Pt Alloys on Electrocatalysis of Oxygen Reduction. J. Electrochem. Soc. 1995, 142, 1409. Bockris, J. O. M. Kinetics of Activation Controlled Consecutive Electrochemical Reactions: Anodic Evolution of Oxygen. J. Chem. Phys. 1956, 24, 817–827. Symes, M. D.; Surendranath, Y.; Lutterman, D. A.; Nocera, D. G. Bidirectional and Unidirectional PCET in a Molecular Model of a Cobalt-Based Oxygen-Evolving Catalyst. J. Am. Chem. Soc. 2011, 133, 5174–5177. Rossmeisl, J.; Qu, Z. W.; Zhu, H.; Kroes, G. J.; Nørskov, J. K. Electrolysis of Water on Oxide Surfaces. J. Electroanal. Chem. 2007, 607, 83–89. Diaz-Morales, O.; Calle-Vallejo, F.; de Munck, C.; Koper, M. T. M. Electrochemical Water Splitting by Gold: Evidence for an Oxide Decomposition Mechanism. Chem. Sci. 2013, 4, 2334. Yan, Y.; Xia, B. Y.; Zhao, B.; Wang, X. A Review on Noble-Metal-Free Bifunctional Heterogeneous Catalysts for Overall Electrochemical Water Splitting. J. Mater. Chem. A 2016, 4, 17587–17603. Shao, M.; Chang, Q.; Dodelet, J.-P.; Chenitz, R. Recent Advances in Electrocatalysts for Oxygen Reduction Reaction. Chem. Rev. 2016, 116, 3594–3657. Chen, M.; Wang, L.; Yang, H.; Zhao, S.; Xu, H.; Wu, G. Nanocarbon / Oxide Composite Catalysts for Bifunctional Oxygen Reduction and Evolution in Reversible Alkaline Fuel Cells : A Mini Review. J. Power Sources 2018, 375, 277–290. Gong, M.; Wang, D. Y.; Chen, C. C.; Hwang, B. J.; Dai, H. A Mini Review on NickelBased Electrocatalysts for Alkaline Hydrogen Evolution Reaction. Nano Res. 2016, 9, 28–46. S, A.; Ede, S. R. R.; Kannimuthu, K.; Sam Sankar, S.; Sangeetha, K.; Pitchiah, E. K.; kundu, subrata. Precision and Correctness in the Evaluation of Electrocatalytic Water Splitting: Revisiting Activity Parameters with a Critical Assessment. Energy Environ. Sci. 2018, 11, 744–771. Shinagawa, T.; Garcia-Esparza, A. T.; Takanabe, K. Insight on Tafel Slopes from a Microkinetic Analysis of Aqueous Electrocatalysis for Energy Conversion. Sci. Rep. 2015, 5, 13801. Holewinski, A.; Linic, S. Elementary Mechanisms in Electrocatalysis: Revisiting the ORR Tafel Slope. J. Electrochem. Soc. 2012, 159, 864–870. 63 ACS Paragon Plus Environment


(36)

(37)

(38)

(39)

(40)

(41)

(42)

(43)

(44)

(45)

(46)

(47)

(48)

(49)

(50)

(51)

(52)

Page 64 of 81

Guidelli, Rolando, Richard G. Compton, Juan M. Feliu, E. G.; Lipkowski, J.; Schmickler, W.; Trasatti, S. Defining the Transfer Coefficient in Electrochemistry: An Assessment (IUPAC Technical Report). Pure Appl. Chem. 2014, 86, 245–248. Shui, J.; Du, F.; Xue, C.; Li, Q.; Dai, L. Vertically Aligned N-Doped Coral-like Carbon Fiber Arrays as Efficient Air Electrodes for High-Performance Nonaqueous Li-O2 batteries. ACS Nano 2014, 8, 3015–3022. Jr., P. N. R. Structure Sensitivity in the Electrocatalytic Properties of Pt I . Hydrogen Adsorption on Low Index Single Crystals and the Role of Steps. J. Electrochem. Soc. 1979, 126, 67–77. Xia, B. Y.; Wu, H. Bin; Yan, Y.; Wang, H. B.; Wang, X. One-Pot Synthesis of Platinum Nanocubes on Reduced Graphene Oxide with Enhanced Electrocatalytic Activity. Small 2014, 10, 2336–2339. Kibsgaard, J.; Gorlin, Y.; Chen, Z.; Jaramillo, T. F. Meso-Structured Platinum Thin Films: Active and Stable Electrocatalysts for the Oxygen Reduction Reaction. J. Am. Chem. Soc. 2012, 134, 7758–7765. Tripković, V.; Cerri, I.; Bligaard, T.; Rossmeisl, J. The Influence of Particle Shape and Size on the Activity of Platinum Nanoparticles for Oxygen Reduction Reaction: A Density Functional Theory Study. Catal. Letters 2014, 144, 380–388. Lv, H.; Li, D.; Strmcnik, D.; Paulikas, A. P.; Markovic, N. M.; Stamenkovic, V. R. Recent Advances in the Design of Tailored Nanomaterials for Efficient Oxygen Reduction Reaction. Nano Energy 2016, 29, 149–165. Tian, N.; Zhou, Z.; Sun, S.-G.; Ding, Y.; Wang, Z. L. Synthesis of Tetrahexahedral Platinum Nanocrystals with High Index Facets and High Electro Oxidation Activity. Handb. Chem. Phys. 2007, 316, 732–735. Xia, B. Y.; Wu, H. Bin; Wang, X.; Lou, X. W. Highly Concave Platinum Nanoframes with High-Index Facets and Enhanced Electrocatalytic Properties. Angew. Chemie Int. Ed. 2013, 52, 12337–12340. Yu, T.; Kim, D. Y.; Zhang, H.; Xia, Y. Platinum Concave Nanocubes with High-Index Facets and Their Enhanced Activity for Oxygen Reduction Reaction. Angew. Chemie Int. Ed. 2011, 50, 2773–2777. Liang, H. W.; Cao, X.; Zhou, F.; Cui, C. H.; Zhang, W. J.; Yu, S. H. A Free-Standing Pt-Nanowire Membrane as a Highly Stable Electrocatalyst for the Oxygen Reduction Reaction. Adv. Mater. 2011, 23, 1467–1471. Xia, B. Y.; Wu, H. Bin; Yan, Y.; Lou, X. W.; Wang, X. Ultrathin and Ultralong Single-Crystal Platinum Nanowire Assemblies with Highly Stable Electrocatalytic Activity. J. Am. Chem. Soc. 2013, 135, 9480–9485. Kim, K. W.; Kim, S. M.; Choi, S.; Kim, J.; Lee, I. S. Electroless Pt Deposition on Mn3O4 Nanoparticles via the Galvanic Replacement Process: Electrocatalytic Nanocomposite with Enhanced Performance for Oxygen Reduction Reaction. ACS Nano 2012, 6, 5122–5129. Lim, B.; Lu, X.; Jiang, M.; Camargo, P. H. C.; Cho, E. C.; Lee, E. P.; Xia, Y. Facile Synthesis of Highly Faceted Multioctahedral Pt Nanocrystals through Controlled Overgrowth. Nano Lett. 2008, 8, 4043–4047. Rana, M.; Chhetri, M.; Loukya, B.; Patil, P. K.; Datta, R.; Gautam, U. K. High-Yield Synthesis of Sub-10 Nm Pt Nanotetrahedra with Bare (111) Facets for Efficient Electrocatalytic Applications. ACS Appl. Mater. Interfaces 2015, 7, 4998–5005. Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G.; Ross, P. N.; Lucas, C. A.; Markovic, N. M. Improved Oxygen Reduction Activity on Pt3Ni(111) via Increased Surface Site Availability. Science. 2007, 315, 493–497. Choi, S. ; Xie, S.; Shao, M.; Odell, J. H.; Lu, N.; Peng, H. C.; Protsailo, L.; Guerrero, 64 ACS Paragon Plus Environment

Page 65 of 81 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(53)

(54)

(55)

(56) (57)

(58)

(59)

(60)

(61)

(62)

(63)

(64)

(65)

(66) (67)

S.; Park, J.; Xia, X. Synthesis and Characterization of 9 nm Pt-Ni Octahedra with a Record High Activity of 3.3 A/MgPtfor the Oxygen Reduction Reaction. Nano Lett. 2013, 13, 3420–3425. Carpenter, M. K.; Moylan, T. E.; Kukreja, R. S.; Atwan, M. H.; Tessema, M. M. Solvothermal Synthesis of Platinum Alloy Nanoparticles for Oxygen Reduction Electrocatalysis. J. Am. Chem. Soc. 2012, 134, 8535–8542. Wu, J.; Zhang, J.; Peng, Z.; Yang, S.; Wagner, F. T.; Yang, H. Truncated Octahedral Pt3Ni Oxygen Reduction Reaction Electrocatalysts. J. Am. Chem. Soc. 2010, 132, 4984–4985. Wu, J.; Qi, L.; You, H.; Gross, A.; Li, J.; Yang, H. Icosahedral Platinum Alloy Nanocrystals with Enhanced Electrocatalytic Activities. J. Am. Chem. Soc. 2012, 134, 11880–11883. Zhang, J.; Yang, H.; Fang, J.; Zou, S. Synthesis and Oxygen Reduction Activity of Shape-Controlled Pt 3Ni Nanopolyhedra. Nano Lett. 2010, 10, 638–644. Zhao, X.; Chen, S.; Fang, Z.; Ding, J.; Sang, W.; Wang, Y.; Zhao, J.; Peng, Z.; Zeng, J. Octahedral [email protected] Core-Shell Nanocrystals with Ultrathin PtNi Alloy Shells as Active Catalysts for Oxygen Reduction Reaction. J. Am. Chem. Soc. 2015, 137, 2804– 2807. Wang, X.; Vara, M.; Luo, M.; Huang, H.; Ruditskiy, A.; Park, J.; Bao, S.; Liu, J.; Howe, J.; Chi, M. Pd@Pt Core-Shell Concave Decahedra: A Class of Catalysts for the Oxygen Reduction Reaction with Enhanced Activity and Durability. J. Am. Chem. Soc. 2015, 137, 15036–15042. Huang, X.; Zhao, Z.; Cao, L.; Chen, Y.; Zhu, E.; Lin, Z.; Li, M.; Yan, A.; Zettl, A.; Wang, Y. M. High-Performance Transition Metal-Doped Pt3Ni Octahedra for Oxygen Reduction Reaction. Science. 2015, 348, 1230–1234. Yang, X.; Roling, L. T.; Vara, M.; Elnabawy, A. O.; Zhao, M.; Hood, Z. D.; Bao, S.; Mavrikakis, M.; Xia, Y. Synthesis and Characterization of Pt-Ag Alloy Nanocages with Enhanced Activity and Durability toward Oxygen Reduction. Nano Lett. 2016, 16, 6644–6649. Fu, S.; Zhu, C.; Song, J.; Zhang, P.; Engelhard, M. H.; Xia, H.; Du, D.; Lin, Y. Low Pt-Content Ternary PdCuPt Nanodendrites: An Efficient Electrocatalyst for Oxygen Reduction Reaction. Nanoscale 2017, 9, 1279–1284. Kobayashi, S.; Aoki, M.; Wakisaka, M.; Kawamoto, T.; Shirasaka, R.; Suda, K.; Tryk, D. A.; Inukai, J.; Kondo, T.; Uchida, H. Atomically Flat Pt Skin and Striking Enrichment of Co in Underlying Alloy at Pt3Co(111) Single Crystal with Unprecedented Activity for the Oxygen Reduction Reaction. ACS Omega 2018, 3, 154–158. Li, J.; Xi, Z.; Pan, Y. T.; Spendelow, J. S.; Duchesne, P. N.; Su, D.; Li, Q.; Yu, C.; Yin, Z.; Shen, B. Fe Stabilization by Intermetallic L10-FePt and Pt Catalysis Enhancement in L10-FePt/Pt Nanoparticles for Efficient Oxygen Reduction Reaction in Fuel Cells. J. Am. Chem. Soc. 2018, 140, 2926–2932. Zhu, C.; Li, H.; Fu, S.; Du, D.; Lin, Y. Highly Efficient Nonprecious Metal Catalysts towards Oxygen Reduction Reaction Based on Three-Dimensional Porous Carbon Nanostructures. Chem. Soc. Rev. 2016, 45, 517–531. Kulkarni, A.; Siahrostami, S.; Patel, A.; Nørskov, J. K. Understanding Catalytic Activity Trends in the Oxygen Reduction Reaction. Chem. Rev. 2018, 118, 2302– 2312. Dai, L.; Xue, Y.; Qu, L.; Choi, H.-J.; Baek, J.-B. Metal-Free Catalysts for Oxygen Reduction Reaction. Chem. Rev. 2015, 115, 4823–4892. Gong, K.; Du, F.; Xia, Z.; Durstock, M.; Dai, L. Nitrogen-Doped Carbon Nanotube 65 ACS Paragon Plus Environment


(68)

(69)

(70)

(71)

(72)

(73)

(74) (75)

(76)

(77)

(78)

(79)

(80)

(81)

(82)

Page 66 of 81

Arrays with High Electrocatalytic Activity for Oxygen Reduction. Science. 2009, 323, 760–764. Lu, L.; Hao, Q.; Lei, W.; Xia, X.; Liu, P.; Sun, D.; Wang, X.; Yang, X. WellCombined Magnetically Separable Hybrid Cobalt Ferrite/Nitrogen-Doped Graphene as Efficient Catalyst with Superior Performance for Oxygen Reduction Reaction. Small 2015, 11, 5833–5843. Unni, S. M.; Bhange, S. N.; Illathvalappil, R.; Mutneja, N.; Patil, K. R.; Kurungot, S. Nitrogen-Induced Surface Area and Conductivity Modulation of Carbon Nanohorn and Its Function as an Efficient Metal-Free Oxygen Reduction Electrocatalyst for AnionExchange Membrane Fuel Cells. Small 2015, 11, 352–362. Liu, Z.; Zhang, G.; Lu, Z.; Jin, X.; Chang, Z.; Sun, X. One-Step Scalable Preparation of N-Doped Nanoporous Carbon as a High-Performance Electrocatalyst for the Oxygen Reduction Reaction. Nano Res. 2013, 6, 293–301. Ferrero, G. A.; Preuss, K.; Fuertes, A. B.; Sevilla, M.; Titirici, M.-M. The Influence of Pore Size Distribution on the Oxygen Reduction Reaction Performance in Nitrogen Doped Carbon Microspheres. J. Mater. Chem. A 2016, 4, 2581–2589. Tao, H.; Yan, C.; Robertson, A. W.; Gao, Y.; Ding, J.; Zhang, Y.; Ma, T.; Sun, Z. NDoping of Graphene Oxide at Low Temperature for the Oxygen Reduction Reaction. Chem. Commun. 2017, 53, 873–876. Xu, J.; Gao, Q.; Zhang, Y.; Tan, Y.; Tian, W.; Zhu, L.; Jiang, L. Preparing TwoDimensional Microporous Carbon from Pistachio Nutshell with High Areal Capacitance as Supercapacitor Materials. Sci. Rep. 2014, 4, 5545. Chaudhari, K. N.; Song, M. Y.; Yu, J. S. Transforming Hair into Heteroatom-Doped Carbon with High Surface Area. Small 2014, 10, 2625–2636. Rana, M.; Arora, G.; Gautam, U. K. N- and S-Doped High Surface Area Carbon Derived from Soya Chunks as Scalable and Efficient Electrocatalysts for Oxygen Reduction. Sci. Technol. Adv. Mater. 2015, 16, 14803. Kim, J. H.; Sa, Y. J.; Jeong, H. Y.; Joo, S. H. Roles of Fe−Nx and Fe−Fe3C@C Species in Fe−N/C Electrocatalysts for Oxygen Reduction Reaction. ACS Appl. Mater. Interfaces 2017, 9, 9567–9575. Fu, X.; Choi, J. Y.; Zamani, P.; Jiang, G.; Hoque, M. A.; Hassan, F. M.; Chen, Z. CoN Decorated Hierarchically Porous Graphene Aerogel for Efficient Oxygen Reduction Reaction in Acid. ACS Appl. Mater. Interfaces 2016, 8, 6488–6495. Ren, G.; Lu, X.; Li, Y.; Zhu, Y.; Dai, L.; Jiang, L. Porous Core-Shell Fe3C Embedded N-Doped Carbon Nanofibers as an Effective Electrocatalysts for Oxygen Reduction Reaction. ACS Appl. Mater. Interfaces 2016, 8, 4118–4125. Zhang, Z.; Yang, S.; Dou, M.; Ji, J.; Wang, F. One-Step Preparation of N-Doped Graphitic Layer-Encased Cobalt/Iron Carbide Nanoparticles Derived from CrossLinked Polyphthalocyanines as Highly Active Electrocatalysts towards the Oxygen Reduction Reaction. Catal. Sci. Technol. 2017, 7, 1529–1536. Lu, Z.; Xu, W.; Ma, J.; Li, Y.; Sun, X.; Jiang, L. Superaerophilic Carbon-NanotubeArray Electrode for High-Performance Oxygen Reduction Reaction. Adv. Mater. 2016, 28, 7155–7161. Song, L.; Wang, T.; Wang, Y.; Xue, H.; Fan, X.; Guo, H.; Xia, W.; Gong, H.; He, J. Porous Iron-Tungsten Carbide Electrocatalyst with High Activity and Stability toward Oxygen Reduction Reaction: From the Self-Assisted Synthetic Mechanism to Its Active-Species Probing. ACS Appl. Mater. Interfaces 2017, 9, 3713–3722. Liang, Y.; Li, Y.; Wang, H.; Zhou, J.; Wang, J.; Regier, T.; Dai, H. Co3O4 Nanocrystals on Graphene as a Synergistic Catalyst for Oxygen Reduction Reaction. Nat. Mater. 2011, 10, 780–786. 66 ACS Paragon Plus Environment

Page 67 of 81 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(83)

(84) (85)

(86) (87)

(88)

(89)

(90)

(91)

(92)

(93)

(94) (95)

(96)

(97)

(98)

Yuan, K.; Zhuang, X.; Fu, H.; Brunklaus, G.; Forster, M.; Chen, Y.; Feng, X.; Scherf, U. Two-Dimensional Core-Shelled Porous Hybrids as Highly Efficient Catalysts for the Oxygen Reduction Reaction. Angew. Chemie - Int. Ed. 2016, 55, 6858–6863. Wang, D.-W.; Su, D. Heterogeneous Nanocarbon Materials for Oxygen Reduction Reaction. Energy Environ. Sci. 2014, 7, 576. Zhou, X.; Qiao, J.; Yang, L.; Zhang, J. A Review of Graphene-Based Nanostructural Materials for Both Catalyst Supports and Metal-Free Catalysts in PEM Fuel Cell Oxygen Reduction Reactions. Adv. Energy Mater. 2014, 4, 1301523. Dey, S.; Mondal, B.; Chatterjee, S.; Rana, A.; Amanullah, S.; Dey, A. Molecular Electrocatalysts for the Oxygen Reduction Reaction. Nat. Rev. Chem. 2017, 1, 98. Zitolo, A.; Goellner, V.; Armel, V.; Sougrati, M.-T.; Mineva, T.; Stievano, L.; Fonda, E.; Jaouen, F. Identification of Catalytic Sites for Oxygen Reduction in Iron- and Nitrogen-Doped Graphene Materials. Nat. Mater. 2015, 14, 937. Huo, L.; Liu, B.; Zhang, G.; Si, R.; Liu, J.; Zhang, J. 2D Layered Non-Precious Metal Mesoporous Electrocatalysts for Enhanced Oxygen Reduction Reaction. J. Mater. Chem. A 2017, 5, 4868–4878. Zitolo, A.; Ranjbar-Sahraie, N.; Mineva, T.; Li, J.; Jia, Q.; Stamatin, S.; Harrington, G. F.; Lyth, S. M.; Krtil, P.; Mukerjee, S. Identification of Catalytic Sites in CobaltNitrogen-Carbon Materials for the Oxygen Reduction Reaction. Nat. Commun. 2017, 8, 957. Yang, L.; Cheng, D.; Xu, H.; Zeng, X.; Wan, X.; Shui, J.; Xiang, Z.; Cao, D. Unveiling the High-Activity Origin of Single-Atom Iron Catalysts for Oxygen Reduction Reaction. Proc. Natl. Acad. Sci. 2018, 115, 6626-6631. Kramm, U. I.; Lefèvre, M.; Larouche, N.; Schmeisser, D.; Dodelet, J.-P. Correlations between Mass Activity and Physicochemical Properties of Fe/N/C Catalysts for the ORR in PEM Fuel Cell via 57Fe Mössbauer Spectroscopy and Other Techniques. J. Am. Chem. Soc. 2014, 136, 978–985. Sa, Y. J.; Seo, D.-J.; Woo, J.; Lim, J. T.; Cheon, J. Y.; Yang, S. Y.; Lee, J. M.; Kang, D.; Shin, T. J.; Shin, H. S. A General Approach to Preferential Formation of Active Fe–Nx Sites in Fe–N/C Electrocatalysts for Efficient Oxygen Reduction Reaction. J. Am. Chem. Soc. 2016, 138, 15046–15056. Chen, Y.; Ji, S.; Wang, Y.; Dong, J.; Chen, W.; Li, Z.; Shen, R.; Zheng, L.; Zhuang, Z.; Wang, D. Isolated Single Iron Atoms Anchored on N-Doped Porous Carbon as an Efficient Electrocatalyst for the Oxygen Reduction Reaction. Angew. Chemie Int. Ed. 2017, 56, 6937–6941. Indra, A.; Song, T.; Paik, U. Metal Organic Framework Derived Materials: Progress and Prospects for the Energy Conversion and Storage. Adv. Mater. 2018, 0, 1705146. Zhang, H.; Osgood, H.; Xie, X.; Shao, Y.; Wu, G. Engineering Nanostructures of PGM-Free Oxygen-Reduction Catalysts Using Metal-Organic Frameworks. Nano Energy 2017, 31, 331–350. Li, Z.; Sun, H.; Wei, L.; Jiang, W.-J.; Wu, M.; Hu, J.-S. Lamellar Metal Organic Framework-Derived Fe–N–C Non-Noble Electrocatalysts with Bimodal Porosity for Efficient Oxygen Reduction. ACS Appl. Mater. Interfaces 2017, 9, 5272–5278. Lai, Q.; Zheng, L.; Liang, Y.; He, J.; Zhao, J.; Chen, J. Metal–Organic-FrameworkDerived Fe-N/C Electrocatalyst with Five-Coordinated Fe-Nx Sites for Advanced Oxygen Reduction in Acid Media. ACS Catal. 2017, 7, 1655–1663. Mukerjee, S.; Srinivasan, S.; Soriaga, M. P.; McBreen, J. Effect of Preparation Conditions of Pt Alloys on Their Electronic, Structural, and Electrocatalytic Activities for Oxygen Reduction. XRD, XAS, and Electrochemical Studies. J. Phys. Chem. 1995, 99, 4577–4589. 67 ACS Paragon Plus Environment


(99)

(100) (101)

(102) (103)

(104)

(105)

(106)

(107)

(108)

(109)

(110)

(111)

(112)

(113)

(114) (115)

Page 68 of 81

Suntivich, J.; Gasteiger, H. A.; Yabuuchi, N.; Nakanishi, H.; Goodenough, J. B.; ShaoHorn, Y. Design Principles for Oxygen-Reduction Activity on Perovskite Oxide Catalysts for Fuel Cells and Metal-Air Batteries. Nat. Chem. 2011, 3, 546–550. Zhang, L.; Xia, Z. Mechanisms of Oxygen Reduction Reaction on Nitrogen-Doped Graphene for Fuel Cells. J. Phys. Chem. C 2011, 115, 11170–11176. Yu, D.; Zhang, Q.; Dai, L. Highly Efficient Metal-Free Growth of Nitrogen-Doped Single-Walled Carbon Nanotubes on Plasma-Etched Substrates for Oxygen Reduction. J. Am. Chem. Soc. 2010, 132, 15127–15129. Qu, L.; Liu, Y.; Baek, J. B.; Dai, L. Nitrogen-Doped Graphene as Efficient Metal-Free Electrocatalyst for Oxygen Reduction in Fuel Cells. ACS Nano 2010, 4, 1321–1326. Chen, S.; Bi, J.; Zhao, Y.; Yang, L.; Zhang, C.; Ma, Y.; Wu, Q.; Wang, X.; Hu, Z. Nitrogen-Doped Carbon Nanocages as Efficient Metal-Free Electrocatalysts for Oxygen Reduction Reaction. Adv. Mater. 2012, 24, 5593–5597. Guo, D.; Shibuya, R.; Akiba, C.; Saji, S.; Kondo, T.; Nakamura, J. Active Sites of Nitrogen-Doped Carbon Materials for Oxygen Reduction Reaction Clarified Using Model Catalysts. Science. 2016, 351, 361–365. Xing, T.; Zheng, Y.; Li, L. H.; Cowie, B. C. C.; Gunzelmann, D.; Qiao, S. Z.; Huang, S.; Chen, Y. Observation of Active Sites for Oxygen Reduction Reaction on NitrogenDoped Multilayer Graphene. ACS Nano 2014, 8, 6856–6862. Malko, D.; Kucernak, A.; Lopes, T. In Situ Electrochemical Quantification of Active Sites in Fe–N/C Non-Precious Metal Catalysts - Supporting Information. Nat. Commun. 2016, 7, 1–47. Kuttiyiel, K. A.; Sasaki, K.; Park, G.-G.; Vukmirovic, M. B.; Wu, L.; Zhu, Y.; Chen, J. G.; Adzic, R. R. Janus Structured Pt–FeNC Nanoparticles as a Catalyst for the Oxygen Reduction Reaction. Chem. Commun. 2017, 53, 1660–1663. Huang, X.; Yang, Z.; Dong, B.; Wang, Y.; Tang, T.; Hou, Y. In Situ Fe2N@N-Doped Porous Carbon Hybrids as Superior Catalysts for Oxygen Reduction Reaction. Nanoscale 2017, 9, 8102–8106. Bao, S.; Vara, M.; Yang, X.; Zhou, S.; Figueroa-Cosme, L.; Park, J.; Luo, M.; Xie, Z.; Xia, Y. Facile Synthesis of Pd@Pt3 - 4L Core-Shell Octahedra with a Clean Surface and Thus Enhanced Activity toward Oxygen Reduction. ChemCatChem 2017, 9, 414–419. Li, M.; Zhao, Z.; Cheng, T.; Fortunelli, A.; Chen, C. Y.; Yu, R.; Zhang, Q.; Gu, L.; Merinov, B. V.; Lin, Z. Ultrafine Jagged Platinum Nanowires Enable Ultrahigh Mass Activity for the Oxygen Reduction Reaction. Science 2016, 354, 1414–1419. Choi, K.-H.; Jang, Y.; Chung, D. Y.; Seo, P.; Jun, S. W.; Lee, J. E.; Oh, M. H.; Shokouhimehr, M.; Jung, N.; Yoo, S. J. A Simple Synthesis of Urchin-like Pt–Ni Bimetallic Nanostructures as Enhanced Electrocatalysts for the Oxygen Reduction Reaction. Chem. Commun. 2016, 52, 597–600. Yu, H.; Shang, L.; Bian, T.; Shi, R.; Waterhouse, G. I. N.; Zhao, Y.; Zhou, C.; Wu, L. Z.; Tung, C. H.; Zhang, T. Nitrogen-Doped Porous Carbon Nanosheets Templated from g-C3N4 as Metal-Free Electrocatalysts for Efficient Oxygen Reduction Reaction. Adv. Mater. 2016, 28, 5080–5086. Niu, W.; Li, L.; Liu, X.; Wang, N.; Liu, J.; Zhou, W.; Tang, Z.; Chen, S. Mesoporous N-Doped Carbons Prepared with Thermally Removable Nanoparticle Templates: An Efficient Electrocatalyst for Oxygen Reduction Reaction. J. Am. Chem. Soc. 2015, 137, 5555–5562. Singh, K. P.; Bae, E. J.; Yu, J. S. Fe-P: A New Class of Electroactive Catalyst for Oxygen Reduction Reaction. J. Am. Chem. Soc. 2015, 137, 3165–3168. Yang, W.; Liu, X.; Yue, X.; Jia, J.; Guo, S. Bamboo-like Carbon Nanotube/Fe3C Nanoparticle Hybrids and Their Highly Efficient Catalysis for Oxygen Reduction. J. 68 ACS Paragon Plus Environment

Page 69 of 81 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(116)

(117)

(118) (119) (120) (121)

(122)

(123)

(124)

(125)

(126) (127)

(128)

(129) (130)

(131)

(132)

Am. Chem. Soc. 2015, 137, 1436–1439. Kwon, T.; Jun, M.; Kim, H. Y.; Oh, A.; Park, J.; Baik, H.; Joo, S. H.; Lee, K. VertexReinforced PtCuCo Ternary Nanoframes as Efficient and Stable Electrocatalysts for the Oxygen Reduction Reaction and the Methanol Oxidation Reaction. Adv. Funct. Mater. 2018, 28, 1706440. Thanh, T. D.; Chuong, N. D.; Hien, H. Van; Kim, N. H.; Lee, J. H. CuAg@Ag CoreShell Nanostructure Encapsulated by N-Doped Graphene as a High-Performance Catalyst for Oxygen Reduction Reaction. ACS Appl. Mater. Interfaces 2018, 10, 4672– 4681. Kötz, R. XPS Studies of Oxygen Evolution on Ru and RuO2 Anodes. J. Electrochem. Soc. 1983, 130, 825. Kötz, R.; Neff, H.; Stucki, S. Anodic Iridium Oxide Films: XPS‐Studies of Oxidation State Changes And. J. Electrochem. Soc. 1984, 131, 72–77. Marshall, A. T.; Haverkamp, R. G. Electrocatalytic Activity of IrO2-RuO2 Supported on Sb-Doped SnO2 Nanoparticles. Electrochim. Acta 2010, 55, 1978–1984. Seitz, L. C.; Dickens, C. F.; Nishio, K.; Hikita, Y.; Montoya, J.; Doyle, A.; Kirk, C.; Vojvodic, A.; Hwang, H. Y.; Norskov, J. K. A Highly Active and Stable IrOx /SrIrO3 Catalyst for the Oxygen Evolution Reaction. Science. 2016, 353, 1011–1014. Karthik, P. E.; Raja, K. A.; Kumar, S. S.; Phani, K. L. N.; Liu, Y.; Guo, S.-X.; Zhang, J.; Bond, A. M. Electroless Deposition of Iridium Oxide Nanoparticles Promoted by Condensation of [Ir(OH)6]2− on an Anodized Au Surface: Application to Electrocatalysis of the Oxygen Evolution Reaction. RSC Adv. 2015, 5, 3196–3199. Anantharaj, S.; Karthik, P. E.; Kundu, S. Self-Assembled IrO2 Nanoparticles on a DNA Scaffold with Enhanced Catalytic and Oxygen Evolution Reaction (OER) Activities. J. Mater. Chem. A 2015, 3, 24463–24478. Karthick, K.; Anantharaj, S.; Karthik, P. E.; Subramanian, B.; Kundu, S. SelfAssembled Molecular Hybrids of CoS-DNA for Enhanced Water Oxidation with Low Cobalt Content. Inorg. Chem. 2017, 56, 6734–6745. Chen, P.; Xu, K.; Fang, Z.; Tong, Y.; Wu, J.; Lu, X.; Peng, X.; Ding, H.; Wu, C.; Xie, Y. Metallic Co4N Porous Nanowire Arrays Activated by Surface Oxidation as Electrocatalysts for the Oxygen Evolution Reaction. Angew. Chemie - Int. Ed. 2015, 54, 14710–14714. Koza, J. A.; He, Z.; Miller, A. S.; Switzer, J. A. Electrodeposition of Crystalline Co3O4 -A Catalyst for the Oxygen Evolution Reaction. Chem. Mater. 2012, 24, 3567–3573. Lu, X. F.; Gu, L. F.; Wang, J. W.; Wu, J. X.; Liao, P. Q.; Li, G. R. Bimetal-Organic Framework Derived CoFe2O4/C Porous Hybrid Nanorod Arrays as High-Performance Electrocatalysts for Oxygen Evolution Reaction. Adv. Mater. 2017, 29, 1604437. Li, M.; Xiong, Y.; Liu, X.; Bo, X.; Zhang, Y.; Han, C.; Guo, L. Facile Synthesis of Electrospun MFe2O4 (M = Co, Ni, Cu, Mn) Spinel Nanofibers with Excellent Electrocatalytic Properties for Oxygen Evolution and Hydrogen Peroxide Reduction. Nanoscale 2015, 7, 8920–8930. Li, Y.; Hasin, P.; Wu, Y. NixCo3-xO4 Nanowire Arrays for Electrocatalytic Oxygen Evolution. Adv. Mater. 2010, 22, 1926–1929. Xu, L.; Jiang, Q.; Xiao, Z.; Li, X.; Huo, J.; Wang, S.; Dai, L. Plasma-Engraved Co3O4 Nanosheets with Oxygen Vacancies and High Surface Area for the Oxygen Evolution Reaction. Angew. Chemie - Int. Ed. 2016, 55, 5277–5281. Wang, H. Y.; Hsu, Y. Y.; Chen, R.; Chan, T. S.; Chen, H. M.; Liu, B. Ni3+-Induced Formation of Active NiOOH on the Spinel Ni-Co Oxide Surface for Efficient Oxygen Evolution Reaction. Adv. Energy Mater. 2015, 5, 1500091. Tung, C. W.; Hsu, Y. Y.; Shen, Y. P.; Zheng, Y.; Chan, T. S.; Sheu, H. S.; Cheng, Y. 69 ACS Paragon Plus Environment


(133)

(134)

(135) (136) (137)

(138)

(139)

(140)

(141)

(142)

(143) (144)

(145)

(146)

(147)

(148) (149)

Page 70 of 81

C.; Chen, H. M. Reversible Adapting Layer Produces Robust Single-Crystal Electrocatalyst for Oxygen Evolution. Nat. Commun. 2015, 6, 8106. Wang, H. Y.; Hung, S. F.; Chen, H. Y.; Chan, T. S.; Chen, H. M.; Liu, B. In Operando Identification of Geometrical-Site-Dependent Water Oxidation Activity of Spinel Co3O4. J. Am. Chem. Soc. 2016, 138, 36–39. Zhao, Q.; Yan, Z.; Chen, C.; Chen, J. Spinels: Controlled Preparation, Oxygen Reduction/Evolution Reaction Application, and Beyond. Chem. Rev. 2017, 117, 10121–10211. Matsumoto, Y.; Sato, E. Oxygen Evolution on La1-xSrxMnO3 electrodes in Alkaline Solutions. Electrochim. Acta 1979, 24, 421–423. Bockris, J. O.; Otagawa, T. Mechanism of Oxygen Evolution on Perovskites. J. Phys. Chem. 1983, 87, 2960–2971. Suntivich, J.; May, K. J.; Gasteiger, H. A.; Goodenough, J. B.; Shao-Horn, Y. A Perovskite Oxide Optimized for Oxygen Evolution Catalysis from Molecular Orbital Principles. Science. 2011, 334, 1383–1385. Guo, Y.; Tong, Y.; Chen, P.; Xu, K.; Zhao, J.; Lin, Y.; Chu, W.; Peng, Z.; Wu, C.; Xie, Y. Engineering the Electronic State of a Perovskite Electrocatalyst for Synergistically Enhanced Oxygen Evolution Reaction. Adv. Mater. 2015, 27, 5989– 5994. Zhu, Y.; Zhou, W.; Yu, J.; Chen, Y.; Liu, M.; Shao, Z. Enhancing Electrocatalytic Activity of Perovskite Oxides by Tuning Cation Deficiency for Oxygen Reduction and Evolution Reactions. Chem. Mater. 2016, 28, 1691–1697. Grimaud, A.; May, K. J.; Carlton, C. E.; Lee, Y.-L.; Risch, M.; Hong, W. T.; Zhou, J.; Shao-Horn, Y. Double Perovskites as a Family of Highly Active Catalysts for Oxygen Evolution in Alkaline Solution. Nat. Commun. 2013, 4, 2439. Diaz-Morales, O.; Raaijman, S.; Kortlever, R.; Kooyman, P. J.; Wezendonk, T.; Gascon, J.; Fu, W. T.; Koper, M. T. M. Iridium-Based Double Perovskites for Efficient Water Oxidation in Acid Media. Nat. Commun. 2016, 7, 12363. Han, B.; Grimaud, A.; Giordano, L.; Hong, W. T.; Diaz-Morales, O.; Lee, Y.-L.; Hwang, J.; Charles, N.; Stoerzinger, K. A.; Yang, W. Iron-Based Perovskites for Catalyzing Oxygen Evolution Reaction. J. Phys. Chem. C 2018, 122,8445-8454. Song, F.; Hu, X. L. Exfoliation of Layered Double Hydroxides for Enhanced Oxygen Evolution Catalysis. Nat. Commun. 2014, 5, 4477. Ling, T.; Yan, D. Y.; Jiao, Y.; Wang, H.; Zheng, Y.; Zheng, X.; Mao, J.; Du, X. W.; Hu, Z.; Jaroniec, M. Engineering Surface Atomic Structure of Single-Crystal Cobalt (II) Oxide Nanorods for Superior Electrocatalysis. Nat. Commun. 2016, 7, 12876. Gardner, G.; Al-Sharab, J.; Danilovic, N.; Go, Y. B.; Ayers, K.; Greenblatt, M.; Charles Dismukes, G. Structural Basis for Differing Electrocatalytic Water Oxidation by the Cubic, Layered and Spinel Forms of Lithium Cobalt Oxides. Energy Environ. Sci. 2016, 9, 184–192. Lu, Z.; Chen, G.; Li, Y.; Wang, H.; Xie, J.; Liao, L.; Liu, C. Identifying the Active Surfaces of Electrochemically Tuned LiCoO2 for Oxygen Evolution Reaction. J. Am. Chem. Soc. 2017, 139, 6270-6276. Weng, B.; Xu, F.; Wang, C.; Meng, W.; Grice, C. R.; Yan, Y. A Layered Na1−x Niy Fe1−yO2 Double Oxide Oxygen Evolution Reaction Electrocatalyst for Highly Efficient Water-Splitting. Energy Environ. Sci. 2017, 10, 121–128. Bode, H.; Dehmelt, K.; Witte, J. Zur Kenntnis Der Nickelhydroxidelektrode—I.Über Das Nickel (II)-Hydroxidhydrat. Electrochim. Acta 1966, 11, 1079. Chen, Z.; Kronawitter, C. X.; Yeh, Y.-W.; Yang, X.; Zhao, P.; Yao, N.; Koel, B. E. Activity of Pure and Transition Metal-Modified CoOOH for the Oxygen Evolution 70 ACS Paragon Plus Environment

Page 71 of 81 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(150)

(151)

(152) (153)

(154)

(155) (156)

(157)

(158)

(159)

(160)

(161)

(162)

(163) (164)

(165)

(166)

Reaction in an Alkaline Medium. J. Mater. Chem. A 2017, 5, 842–850. Gu, Z.; Atherton, J. J.; Xu, Z. P. Hierarchical Layered Double Hydroxide Nanocomposites: Structure, Synthesis and Applications. Chem. Commun. 2015, 51, 3024–3036. Subbaraman, R.; Tripkovic, D.; Chang, K. C.; Strmcnik, D.; Paulikas, A. P.; Hirunsit, P.; Chan, M.; Greeley, J.; Stamenkovic, V.; Markovic, N. M. Trends in Activity for the Water Electrolyser Reactions on 3d M(Ni,Co,Fe,Mn) Hydr(Oxy)Oxide Catalysts. Nat. Mater. 2012, 11, 550–557. Sabatier, P. Hydrogenations et Dehydrogenations Par Catalyse. Ber. Dtsch. Chem. Ges. 1911, 44, 1984–2001. Klaus, S.; Cai, Y.; Louie, M. W.; Trotochaud, L.; Bell, A. T. Effects of Fe Electrolyte Impurities on Ni(OH)2/NiOOH Structure and Oxygen Evolution Activity. J. Phys. Chem. C 2015, 119, 7243–7254. Anantharaj, S.; Karthik, P. E.; Kundu, S. Petal-like Hierarchical Array of Ultrathin Ni(OH)2 Nanosheets Decorated with Ni(OH)2 Nanoburls: A Highly Efficient OER Electrocatalyst. Catal. Sci. Technol. 2017, 7, 882–893. Hunter, B. M.; Gray, H. B.; Müller, A. M. Earth-Abundant Heterogeneous Water Oxidation Catalysts. Chem. Rev. 2016, 116, 14120–14136. Chhowalla, M.; Shin, H. S.; Eda, G.; Li, L. J.; Loh, K.; Zhang, H. The Chemistry of Two-Dimensional Transition Metal Dichalcogenide Nanosheets. Nat. Chem. 2013, 5, 263. Maitra, U.; Gupta, U.; De, M.; Datta, R.; Govindaraj, A.; Rao, C. N. R. Highly Effective Visible-Light-Induced H2 Generation by Single-Layer 1T-MoS2 and a Nanocomposite of Few-Layer 2H-MoS2 with Heavily Nitrogenated Graphene. Angew. Chemie Int. Ed. 2013, 52, 13057–13061. Feng, Y.; He, T.; Alonso-Vante, N. In Situ Free-Surfactant Synthesis and ORRElectrochemistry of Carbon-Supported Co3S4 and CoSe2 Nanoparticles. Chem. Mater. 2008, 20, 26–28. Alonso-vante, N.; Cattarin, S.; Musiani, M. Electrocatalysis of O2 Reduction at Polyaniline + Molybdenum-Doped Ruthenium Selenide Composite Electrodes. J. Electroanal. Chem. 2000, 481, 200–207. Liao, M.; Zeng, G.; Luo, T.; Jin, Z.; Wang, Y.; Kou, X.; Xiao, D. Three-Dimensional Coral-like Cobalt Selenide as an Advanced Electrocatalyst for Highly Efficient Oxygen Evolution Reaction. Electrochim. Acta 2016, 194, 59–66. Mabayoje, O.; Shoola, A.; Wygant, B. R.; Mullins, C. B. The Role of Anions in Metal Chalcogenide Oxygen Evolution Catalysis: Electrodeposited Thin Films of Nickel Sulfide as “Pre-Catalysts.” ACS Energy Lett. 2016, 1, 195–201. Chen, J. S.; Ren, J.; Shalom, M.; Fellinger, T.; Antonietti, M. Stainless Steel MeshSupported NiS Nanosheet Array as Highly Efficient Catalyst for Oxygen Evolution Reaction. ACS Appl. Mater. Interfaces 2016, 8, 5509–5516. Swesi, A. T.; Masud, J.; Nath, M. Nickel Selenide as a High-Efficiency Catalyst for Oxygen Evolution Reaction. Energy Environ. Sci. 2016, 9, 1771–1782. Feng, L. L.; Yu, G.; Wu, Y.; Li, G. D.; Li, H.; Sun, Y.; Asefa, T.; Chen, W.; Zou, X. High-Index Faceted Ni3S2 Nanosheet Arrays as Highly Active and Ultrastable Electrocatalysts for Water Splitting. J. Am. Chem. Soc. 2015, 137, 14023–14026. Xia, C.; Jiang, Q.; Zhao, C.; Hedhili, M. N.; Alshareef, H. N. Selenide-Based Electrocatalysts and Scaffolds for Water Oxidation Applications. Adv. Mater. 2016, 28, 77–85. Kuang, M.; Zheng, G. Nanostructured Bifunctional Redox Electrocatalysts. Small 2016, 12, 5656–5675. 71 ACS Paragon Plus Environment


Page 72 of 81

(167) Li, D.; Baydoun, H.; Verani, C. N.; Brock, S. L. Efficient Water Oxidation Using CoMnP Nanoparticles. J. Am. Chem. Soc. 2016, 138, 4006–4009. (168) Read, C. G.; Callejas, J. F.; Holder, C. F.; Schaak, R. E. General Strategy for the Synthesis of Transition Metal Phosphide Films for Electrocatalytic Hydrogen and Oxygen Evolution. ACS Appl. Mater. Interfaces 2016, 8, 12798–12803. (169) Liu, M.; Li, J. Cobalt Phosphide Hollow Polyhedron as Efficient Bifunctional Electrocatalysts for the Evolution Reaction of Hydrogen and Oxygen. ACS Appl. Mater. Interfaces 2016, 8, 2158–2165. (170) Shalom, M.; Ressnig, D.; Yang, X.; Clavel, G.; Fellinger, T. P.; Antonietti, M. Nickel Nitride as an Efficient Electrocatalyst for Water Splitting. J. Mater. Chem. A 2015, 3, 8171–8177. (171) Chang, J.; Xiao, Y.; Xiao, M.; Ge, J.; Liu, C.; Xing, W. Surface Oxidized CobaltPhosphide Nanorods As an Advanced Oxygen Evolution Catalyst in Alkaline Solution. ACS Catal. 2015, 5, 6874–6878. (172) Stern, L.-A.; Feng, L.; Song, F.; Hu, X. Ni2P as a Janus Catalyst for Water Splitting: The Oxygen Evolution Activity of Ni2P Nanoparticles. Energy Environ. Sci. 2015, 8, 2347–2351. (173) Bai, Y.; Zhang, H.; Feng, Y.; Fang, L.; Wang, Y. Sandwich-like CoP/C Nanocomposites as Efficient and Stable Oxygen Evolution Catalysts. J. Mater. Chem. A 2016, 4, 9072–9079. (174) Kim, H.; Park, J.; Park, I.; Jin, K.; Jerng, S. E.; Kim, S. H.; Nam, K. T.; Kang, K. Coordination Tuning of Cobalt Phosphates towards Efficient Water Oxidation Catalyst. Nat. Commun. 2015, 6, 8253. (175) Zhu, Y. P.; Liu, Y. P.; Ren, T. Z.; Yuan, Z. Y. Self-Supported Cobalt Phosphide Mesoporous Nanorod Arrays: A Flexible and Bifunctional Electrode for Highly Active Electrocatalytic Water Reduction and Oxidation. Adv. Funct. Mater. 2015, 25, 7337– 7347. (176) Xiao, X.; He, C.-T.; Zhao, S.; Li, J.; Lin, W.; Yuan, Z.; Zhang, Q.; Wang, S.; Dai, L.; Yu, D. A General Approach to Cobalt-Based Homobimetallic Phosphide Ultrathin Nanosheets for Highly Efficient Oxygen Evolution in Alkaline Media. Energy Environ. Sci. 2017, 10, 893–899. (177) Xu, J.; Li, J.; Xiong, D.; Zhang, B.; Liu, Y.; Wu, K.-H.; Amorim, I.; Li, W.; Liu, L. Trends in Activity for the Oxygen Evolution Reaction on Transition Metal (M = Fe, Co, Ni) Phosphide Pre-Catalysts. Chem. Sci. 2018, 9, 3470–3476. (178) Gadipelli, S.; Zhao, T.; Shevlin, S. A.; Guo, Z. Switching Effective Oxygen Reduction and Evolution Performance by Controlled Graphitization of a Cobalt–nitrogen–carbon Framework System. Energy Environ. Sci. 2016, 9, 1661–1667. (179) Ma, T. Y.; Ran, J.; Dai, S.; Jaroniec, M.; Qiao, S. Z. Phosphorus-Doped Graphitic Carbon Nitrides Grown In Situ on Carbon-Fiber Paper: Flexible and Reversible Oxygen Electrodes. Angew. Chemie - Int. Ed. 2015, 54, 4646–4650. (180) Yu, X.; Zhang, M.; Chen, J.; Li, Y.; Shi, G. Nitrogen and Sulfur Codoped Graphite Foam as a Self-Supported Metal-Free Electrocatalytic Electrode for Water Oxidation. Adv. Energy Mater. 2016, 6, 1501492. (181) Zhao, Y.; Nakamura, R.; Kamiya, K.; Nakanishi, S.; Hashimoto, K. Nitrogen-Doped Carbon Nanomaterials as Non-Metal Electrocatalysts for Water Oxidation. Nat. Commun. 2013, 4, 1–7. (182) Yang, X.; Li, H.; Lu, A. Y.; Min, S.; Idriss, Z.; Hedhili, M. N.; Huang, K. W.; Idriss, H.; Li, L. J. Highly Acid-Durable Carbon Coated Co3O4 nanoarrays as Efficient Oxygen Evolution Electrocatalysts. Nano Energy 2016, 25, 42–50. (183) Ma, T. Y.; Cao, J. L.; Jaroniec, M.; Qiao, S. Z. Interacting Carbon Nitride and 72 ACS Paragon Plus Environment

Page 73 of 81 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(184)

(185)

(186) (187) (188)

(189)

(190)

(191)

(192)

(193)

(194)

(195)

(196)

(197)

(198)

(199)

Titanium Carbide Nanosheets for High-Performance Oxygen Evolution. Angew. Chemie - Int. Ed. 2016, 55, 1138–1142. Tahir, M.; Mahmood, N.; Pan, L.; Huang, Z.-F.; Lv, Z.; Zhang, J.; Butt, F. K.; Shen, G.; Zhang, X.; Dou, S. X. Efficient Water Oxidation through Strongly Coupled Graphitic C3N4 Coated Cobalt Hydroxide Nanowires. J. Mater. Chem. A 2016, 4, 12940–12946. Zheng, Y.; Jiao, Y.; Zhu, Y.; Cai, Q.; Vasileff, A.; Li, L. H.; Han, Y.; Chen, Y.; Qiao, S.-Z. Molecule-Level g-C3N4 Coordinated Transition Metals as a New Class of Electrocatalysts for Oxygen Electrode Reactions. J. Am. Chem. Soc. 2017, 139, 3336– 3339. Xu, Y.; Kraft, M.; Xu, R. Metal-Free Carbonaceous Electrocatalysts and Photocatalysts for Water Splitting. Chem. Soc. Rev. 2016, 45, 3039–3052. Lai, J.; Nsabimana, A.; Luque, R.; Xu, G. 3D Porous Carbonaceous Electrodes for Electrocatalytic Applications. Joule 2018, 2, 76–93. Sanchez Casalongue, H. G.; Ng, M. L.; Kaya, S.; Friebel, D.; Ogasawara, H.; Nilsson, A. In Situ Observation of Surface Species on Iridium Oxide Nanoparticles during the Oxygen Evolution Reaction. Angew. Chemie Int. Ed. 2014, 53, 7169–7172. Zhang, Y.; Chang, T. R.; Zhou, B.; Cui, Y. T.; Yan, H.; Liu, Z.; Schmitt, F.; Lee, J.; Moore, R.; Chen, Y. Direct Observation of the Transition from Indirect to Direct Bandgap in Atomically Thin Epitaxial MoSe2. Nat. Nanotechnol. 2014, 9, 111–115. Deng, Y.; Handoko, A. D.; Du, Y.; Xi, S.; Yeo, B. S. In Situ Raman Spectroscopy of Copper and Copper Oxide Surfaces during Electrochemical Oxygen Evolution Reaction: Identification of Cu III Oxides as Catalytically Active Species. ACS Catal. 2016, 6, 2473–2481. Yoo, J. S.; Rong, X.; Liu, Y.; Kolpak, A. M. Role of Lattice Oxygen Participation in Understanding Trends in the Oxygen Evolution Reaction on Perovskites. ACS Catal. 2018, 8, 4628–4636. Karthik, P. E.; Jeyabharathi, C.; Phani, K. L. Oxygen Evolution Reaction Electrocatalyzed on a Fenton-Treated Gold Surface. Chem. Commun 2014, 50, 2787– 2790. Lee, J. G.; Hwang, J.; Hwang, H. J.; Jeon, O. S.; Jang, J.; Kwon, O.; Lee, Y.; Han, B.; Shul, Y.-G. A New Family of Perovskite Catalysts for Oxygen-Evolution Reaction in Alkaline Media: BaNiO3 and BaNi0.83O2.5. J. Am. Chem. Soc. 2016, 138, 3541–3547. Xie, L.; Zhang, R.; Cui, L.; Liu, D.; Hao, S.; Ma, Y.; Du, G.; Asiri, A. M.; Sun, X. High-Performance Electrolytic Oxygen Evolution in Neutral Media Catalyzed by a Cobalt Phosphate Nanoarray. Angew. Chemie - Int. Ed. 2017, 56, 1064–1068. Guan, B. Y.; Yu, L.; Lou, X. W. D. General Synthesis of Multishell Mixed-Metal Oxyphosphide Particles with Enhanced Electrocatalytic Activity in the Oxygen Evolution Reaction. Angew. Chemie 2017, 129, 2426–2429. Jiang, Q.; Xu, L.; Chen, N.; Zhang, H.; Dai, L.; Wang, S. Facile Synthesis of Black Phosphorus: An Efficient Electrocatalyst for the Oxygen Evolving Reaction. Angew. Chemie - Int. Ed. 2016, 55, 13849–13853. Zhou, H.; Yu, F.; Sun, J.; He, R.; Chen, S.; Chu, C.-W.; Ren, Z. Highly Active Catalyst Derived from a 3D Foam of Fe(PO3)2/Ni2P for Extremely Efficient Water Oxidation. Proc. Natl. Acad. Sci. 2017, 114, 5607–5611. Liu, J.; Ji, Y.; Nai, J.; Niu, X.; Luo, Y.; Guo, L.; Yang, S. Ultrathin Amorphous Cobalt-Vanadium Hydr(Oxy)Oxide Catalysts for Oxygen Evolution Reaction. Energy Environ. Sci. 2018, 11, 1736-1741. Gao, W.; Xia, Z.; Cao, F.; Ho, J. C.; Jiang, Z.; Qu, Y. Comprehensive Understanding of the Spatial Configurations of CeO2 in NiO for the Electrocatalytic Oxygen 73 ACS Paragon Plus Environment


(200)

(201)

(202)

(203)

(204)

(205)

(206)

(207)

(208)

(209)

(210)

(211)

(212)

(213)

(214)

Page 74 of 81

Evolution Reaction: Embedded or Surface-Loaded. Adv. Funct. Mater. 2018, 28, 1706056. Jin, Y.; Huang, S.; Yue, X.; Du, H.; Shen, P. K. Mo- and Fe-Modified Ni(OH)2/NiOOH Nanosheets as Highly Active and Stable Electrocatalysts for Oxygen Evolution Reaction. ACS Catal. 2018, 8, 2359–2363. Zhao, A.; Masa, J.; Xia, W.; Maljusch, A.; Willinger, M. G.; Clavel, G.; Xie, K.; Schlögl, R.; Schuhmann, W.; Muhler, M. Spinel Mn-Co Oxide in N-Doped Carbon Nanotubes as a Bifunctional Electrocatalyst Synthesized by Oxidative Cutting. J. Am. Chem. Soc. 2014, 136, 7551–7554. Liu, X.; Park, M.; Kim, M. G.; Gupta, S.; Wu, G.; Cho, J. Integrating NiCo Alloys with Their Oxides as Efficient Bifunctional Cathode Catalysts for Rechargeable ZincAir Batteries. Angew. Chemie - Int. Ed. 2015, 54, 9654–9658. Jung, J. Il; Jeong, H. Y.; Kim, M. G.; Nam, G.; Park, J.; Cho, J. Fabrication of Ba0.5Sr0.5Co0.8Fe0.2O3-δ Catalysts with Enhanced Electrochemical Performance by Removing an Inherent Heterogeneous Surface Film Layer. Adv. Mater. 2015, 27, 266– 271. Zhan, Y.; Du, G.; Yang, S.; Xu, C.; Lu, M.; Liu, Z.; Lee, J. Y. Development of Cobalt Hydroxide as a Bifunctional Catalyst for Oxygen Electrocatalysis in Alkaline Solution. ACS Appl. Mater. Interfaces 2015, 7, 12930–12936. Jung, J.-I.; Risch, M.; Park, S.; Kim, M. G.; Nam, G.; Jeong, H.-Y.; Shao-Horn, Y.; Cho, J. Optimizing Nanoparticle Perovskite for Bifunctional Oxygen Electrocatalysis. Energy Environ. Sci. 2016, 9, 176–183. Li, R.; Wei, Z.; Gou, X. Nitrogen and Phosphorus Dual-Doped Graphene/Carbon Nanosheets as Bifunctional Electrocatalysts for Oxygen Reduction and Evolution. ACS Catal. 2015, 5, 4133–4142. Liu, Z. Q.; Cheng, H.; Li, N.; Ma, T. Y.; Su, Y. Z. ZnCo2O4 Quantum Dots Anchored on Nitrogen-Doped Carbon Nanotubes as Reversible Oxygen Reduction/Evolution Electrocatalysts. Adv. Mater. 2016, 28, 3777–3784. Li, C.; Han, X.; Cheng, F.; Hu, Y.; Chen, C.; Chen, J. Phase and Composition Controllable Synthesis of Cobalt Manganese Spinel Nanoparticles towards Efficient Oxygen Electrocatalysis. Nat. Commun. 2015, 6, 7345. Park, M. G.; Lee, D. U.; Seo, M. H.; Cano, Z. P.; Chen, Z. 3D Ordered Mesoporous Bifunctional Oxygen Catalyst for Electrically Rechargeable Zinc-Air Batteries. Small 2016, 12, 2707–2714. Tian, W.; Li, H.; Qin, B.; Xu, Y.; Hao, Y.; Li, Y.; Zhang, G.; Liu, J.; Sun, X.; Duan, X. Tuning the Wettability of Carbon Nanotube Arrays for Efficient Bifunctional Catalysts and Zn–air Batteries. J. Mater. Chem. A 2017, 5, 7103–7110. Zhao, X.; Li, F.; Wang, R.; Seo, J. M.; Choi, H. J.; Jung, S. M.; Mahmood, J.; Jeon, I. Y.; Baek, J. B. Controlled Fabrication of Hierarchically Structured Nitrogen-Doped Carbon Nanotubes as a Highly Active Bifunctional Oxygen Electrocatalyst. Adv. Funct. Mater. 2017, 27, 1605717. Yan, W.; Cao, X.; Tian, J.; Jin, C.; Ke, K.; Yang, R. Nitrogen/Sulfur Dual-Doped 3D Reduced Graphene Oxide Networks-Supported CoFe2O4 with Enhanced Electrocatalytic Activities for Oxygen Reduction and Evolution Reactions. Carbon N. Y. 2016, 99, 195–202. Aijaz, A.; Masa, J.; Rosler, C.; Xia, W.; Weide, P.; Botz, A. J. R.; Fischer, R. A.; Schuhmann, W.; Muhler, M. Co@Co3O4 Encapsulated in Carbon Nanotube-Grafted Nitrogen-Doped Carbon Polyhedra as an Advanced Bifunctional Oxygen Electrode. Angew. Chemie - Int. Ed. 2016, 55, 4087–4091. Kumar, K.; Canaff, C.; Rousseau, J.; Arrii-Clacens, S.; Napporn, T. W.; Habrioux, A.; 74 ACS Paragon Plus Environment

Page 75 of 81 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(215)

(216)

(217)

(218)

(219)

(220)

(221)

(222)

(223)

(224) (225)

(226)

(227)

(228)

Kokoh, K. B. Effect of the Oxide-Carbon Heterointerface on the Activity of Co3O4/NRGO Nanocomposites toward ORR and OER. J. Phys. Chem. C 2016, 120, 7949–7958. Jung, J. Il; Jeong, H. Y.; Lee, J. S.; Kim, M. G.; Cho, J. A Bifunctional Perovskite Catalyst for Oxygen Reduction and Evolution. Angew. Chemie - Int. Ed. 2014, 53, 4582–4586. Gupta, S.; Kellogg, W.; Xu, H.; Liu, X.; Cho, J.; Wu, G. Bifunctional Perovskite Oxide Catalysts for Oxygen Reduction and Evolution in Alkaline Media. Chem. - An Asian J. 2016, 11, 10–21. Risch, M.; Stoerzinger, K. A.; Maruyama, S.; Hong, W. T.; Takeuchi, I.; Shao-Horn, Y. La0.8Sr0.2MnO3- δ decorated with Ba0.5Sr0.5Co0.8Fe0.2O3-δ: A Bifunctional Surface for Oxygen Electrocatalysis with Enhanced Stability and Activity. J. Am. Chem. Soc. 2014, 136, 5229–5232. Matsumoto, Y.; Yoneyama, H.; Tamura, H. Influence of the Nature of the Conduction Band of Transition Metal Oxides on Catalytic Activity for Oxygen Reduction. J. Electroanal. Chem. 1977, 83, 237–243. Yu, J.; Sunarso, J.; Zhu, Y.; Xu, X.; Ran, R.; Zhou, W.; Shao, Z. Activity and Stability of Ruddlesden-Popper-Type La n +1 Nin O3n +1 (n =1,2,3 and ∞) Electrocatalysts for Oxygen Reduction and Evolution Reactions in Alkaline Media. Chem. - A Eur. J. 2016, 22, 2719–2727. Wang, Z.; You, Y.; Yuan, J.; Yin, Y. X.; Li, Y. T.; Xin, S.; Zhang, D. Nickel-Doped La0.8Sr0.2Mn1-XNiXO3Nanoparticles Containing Abundant Oxygen Vacancies as an Optimized Bifunctional Catalyst for Oxygen Cathode in Rechargeable Lithium-Air Batteries. ACS Appl. Mater. Interfaces 2016, 8, 6520–6528. Liu, G.; Chen, H.; Xia, L.; Wang, S.; Ding, L. X.; Li, D.; Xiao, K.; Dai, S.; Wang, H. Hierarchical Mesoporous/Macroporous Perovskite La0.5Sr0.5CoO3-XNanotubes: A Bifunctional Catalyst with Enhanced Activity and Cycle Stability for Rechargeable Lithium Oxygen Batteries. ACS Appl. Mater. Interfaces 2015, 7, 22478–22486. Wang, J.; Yang, T.; Lei, L.; Huang, K. Ta-Doped SrCoO3−δ as a Promising Bifunctional Oxygen Electrode for Reversible Solid Oxide Fuel Cells: A Focused Study on Stability. J. Mater. Chem. A 2017, 5, 8989–9002. Yan, L.; Lin, Y.; Yu, X.; Xu, W.; Salas, T.; Smallidge, H.; Zhou, M.; Luo, H. La0.8Sr0.2MnO3-Based Perovskite Nanoparticles with the A-Site Deficiency as High Performance Bifunctional Oxygen Catalyst in Alkaline Solution. ACS Appl. Mater. Interfaces 2017, 9, 23820–23827. Jung, J. Il; Park, S.; Kim, M. G.; Cho, J. Tunable Internal and Surface Structures of the Bifunctional Oxygen Perovskite Catalysts. Adv. Energy Mater. 2015, 5, 1501560. Lee, D.-G.; Kim, S. H.; Joo, S. H.; Ji, H.-I.; Tavassol, H.; Jeon, Y.; Choi, S.; Lee, M.H.; Kim, C.; Kwak, S. K. Polypyrrole-Assisted Oxygen Electrocatalysis on Perovskite Oxides. Energy Environ. Sci. 2017, 10, 523–527. Meng, T.; Qin, J.; Wang, S.; Zhao, D.; Mao, B.; Cao, M. In Situ Coupling of Co0.85 Se and N-Doped Carbon via One-Step Selenization of Metal–organic Frameworks as a Trifunctional Catalyst for Overall Water Splitting and Zn–air Batteries. J. Mater. Chem. A 2017, 5, 7001–7014. Wang, Z.; Zhang, F.; Jin, C.; Luo, Y.; Sui, J.; Gong, H.; Yang, R. La2O3-NCNTs Hybrids in-Situ Derived from LaNi0.9Fe0.1O3-C Composites as Novel Robust Bifunctional Oxygen Electrocatalysts. Carbon N. Y. 2017, 115, 261–270. Park, H. W.; Lee, D. U.; Park, M. G.; Ahmed, R.; Seo, M. H.; Nazar, L. F.; Chen, Z. Perovskite-Nitrogen-Doped Carbon Nanotube Composite as Bifunctional Catalysts for Rechargeable Lithium-Air Batteries. ChemSusChem 2015, 8, 1058–1065. 75 ACS Paragon Plus Environment


Page 76 of 81

(229) Kim, N.-I.; Afzal, R. A.; Choi, S. R.; Lee, S. W.; Ahn, D.; Bhattacharjee, S.; Lee, S.C.; Kim, J. H.; Park, J.-Y. Highly Active and Durable Nitrogen Doped-Reduced Graphene Oxide/Double Perovskite Bifunctional Hybrid Catalysts. J. Mater. Chem. A 2017, 5, 13019–13031. (230) Cai, X.; Lai, L.; Lin, J.; Shen, Z. Recent Advances in Air Electrode for Zn-Air Batteries: Electrocatalysis and Structural Design. Mater. Horiz. 2017, 4, 945–976. (231) Kim, H.; Kim, Y.; Noh, Y.; Lee, S.; Sung, J.; Kim, W. B. Thermally Converted CoO Nanoparticles Embedded into N-Doped Carbon Layers as Highly Efficient Bifunctional Electrocatalysts for Oxygen Reduction and Oxygen Evolution Reactions. ChemCatChem 2017, 9, 1503–1510. (232) Yoon, K. R.; Lee, G. Y.; Jung, J.-W.; Kim, N.-H.; Kim, S. O.; Kim, I.-D. OneDimensional RuO2/Mn2O3 Hollow Architectures as Efficient Bifunctional Catalysts for Lithium–Oxygen Batteries. Nano Lett. 2016, 16, 2076–2083. (233) Yin, J.; Li, Y.; Lv, F.; Fan, Q.; Zhao, Y. Q.; Zhang, Q.; Wang, W.; Cheng, F.; Xi, P.; Guo, S. NiO/CoN Porous Nanowires as Efficient Bifunctional Catalysts for Zn-Air Batteries. ACS Nano 2017, 11, 2275–2283. (234) Cao YL; Yang HX; Ai XP; Xiao LF. The Mechanism of Oxygen Reduction on MnO2Catalyzed Air Cathodin Alkaline Solution. J Electroanal Chem 2003, 557, 127–134. (235) Liu, X. X.; Zang, J. B.; Chen, L.; Chen, L. B.; Chen, X.; Wu, P.; Zhou, S. Y.; Wang, Y. H. A Microwave-Assisted Synthesis of CoO@Co Core–shell Structures Coupled with N-Doped Reduced Graphene Oxide Used as a Superior Multi-Functional Electrocatalyst for Hydrogen Evolution, Oxygen Reduction and Oxygen Evolution Reactions. J. Mater. Chem. A 2017, 5, 5865–5872. (236) Yin, J.; Fan, Q.; Li, Y.; Cheng, F.; Zhou, P.; Xi, P.; Sun, S. Ni-C-N Nanosheets as Catalyst for Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2016, 138, 14546– 14549. (237) Zhong, Y.; Xia, X.; Shi, F.; Zhan, J.; Tu, J.; Fan, H. J. Transition Metal Carbides and Nitrides in Energy Storage and Conversion. Adv. Sci. 2016, 3, 1500286. (238) Pandey, J.; Hua, B.; Ng, W.; Yang, Y.; van der Veen, K.; Chen, J.; Geels, N. J.; Luo, J.-L.; Rothenberg, G.; Yan, N. Developing Hierarchically Porous MnO x /NC Hybrid Nanorods for Oxygen Reduction and Evolution Catalysis. Green Chem. 2017, 19, 2793–2797. (239) Qiao, X.; Jin, J.; Fan, H.; Li, Y.; Liao, S. In Situ Growth of Cobalt Sulfide Hollow Nanospheres Embedded in Nitrogen and Sulfur Co-Doped Graphene Nanoholes as a Highly Active Electrocatalyst for Oxygen Reduction and Evolution. J. Mater. Chem. A 2017, 5, 12354–12360. (240) Cao, X.; Zheng, X.; Tian, J.; Jin, C.; Ke, K.; Yang, R. Cobalt Sulfide Embedded in Porous Nitrogen-Doped Carbon as a Bifunctional Electrocatalyst for Oxygen Reduction and Evolution Reactions. Electrochim. Acta 2016, 191, 776–783. (241) Wang, Z.; Xiao, S.; An, Y.; Long, X.; Zheng, X.; Lu, X.; Tong, Y.; Yang, S. Co(II)1XCo(0)x/3Mn(III)2x/3S Nanoparticles Supported on B/N-Codoped Mesoporous Nanocarbon as a Bifunctional Electrocatalyst of Oxygen Reduction/Evolution for High-Performance Zinc-Air Batteries. ACS Appl. Mater. Interfaces 2016, 8, 13348– 13359. (242) Tang, Y.; Jing, F.; Xu, Z.; Zhang, F.; Mai, Y.; Wu, D. Highly Crumpled Hybrids of Nitrogen/Sulfur Dual-Doped Graphene and Co9S8 Nanoplates as Efficient Bifunctional Oxygen Electrocatalysts. ACS Appl. Mater. Interfaces 2017, 9, 12340–12347. (243) Tiwari, A. P.; Kim, D.; Kim, Y.; Lee, H. Bifunctional Oxygen Electrocatalysis through Chemical Bonding of Transition Metal Chalcogenides on Conductive Carbons. Adv. Energy Mater. 2017, 7, 1602217. 76 ACS Paragon Plus Environment

Page 77 of 81 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(244) Xu, K.; Chen, P.; Li, X.; Tong, Y.; Ding, H.; Wu, X.; Chu, W.; Peng, Z.; Wu, C.; Xie, Y. Metallic Nickel Nitride Nanosheets Realizing Enhanced Electrochemical Water Oxidation. J. Am. Chem. Soc. 2015, 137, 4119–4125. (245) Meng, F.; Zhong, H.; Bao, D.; Yan, J.; Zhang, X. In Situ Coupling of Strung Co4N and Intertwined N-C Fibers toward Free-Standing Bifunctional Cathode for Robust, Efficient, and Flexible Zn-Air Batteries. J. Am. Chem. Soc. 2016, 138, 10226–10231. (246) Chen, P.; Zhou, T.; Xing, L.; Xu, K.; Tong, Y.; Xie, H.; Zhang, L.; Yan, W.; Chu, W.; Wu, C. Atomically Dispersed Iron–Nitrogen Species as Electrocatalysts for Bifunctional Oxygen Evolution and Reduction Reactions. Angew. Chemie - Int. Ed. 2017, 56, 610–614. (247) Kou, Z.; Meng, T.; Guo, B.; Amiinu, I. S.; Li, W.; Zhang, J.; Mu, S. A Generic Conversion Strategy: From 2D Metal Carbides (MxCy) to M-Self-Doped Graphene toward High-Efficiency Energy Applications. Adv. Funct. Mater. 2017, 27, 1604904. (248) Tian, G. L.; Zhao, M. Q.; Yu, D.; Kong, X. Y.; Huang, J. Q.; Zhang, Q.; Wei, F. Nitrogen-Doped Graphene/Carbon Nanotube Hybrids: In Situ Formation on Bifunctional Catalysts and Their Superior Electrocatalytic Activity for Oxygen Evolution/Reduction Reaction. Small 2014, 10, 2251–2259. (249) Li, M. T.; Zhang, L. P.; Xu, Q.; Niu, J. B.; Xia, Z. H. N-Doped Graphene as Catalysts for Oxygen Reduction and Oxygen Evolution Reactions: Theoretical Considerations. J. Catal. 2014, 314, 66–72. (250) Hu, C.; Dai, L. Multifunctional Carbon-Based Metal-Free Electrocatalysts for Simultaneous Oxygen Reduction, Oxygen Evolution, and Hydrogen Evolution. Adv. Mater. 2017, 29, 1604942. (251) Shinde, S. S.; Lee, C. H.; Sami, A.; Kim, D. H.; Lee, S. U.; Lee, J. H. Scalable 3-D Carbon Nitride Sponge as an Efficient Metal-Free Bifunctional Oxygen Electrocatalyst for Rechargeable Zn-Air Batteries. ACS Nano 2017, 11, 347–357. (252) Liu, Z.; Zhao, Z.; Wang, Y.; Dou, S.; Yan, D.; Liu, D.; Xia, Z.; Wang, S. In Situ Exfoliated, Edge-Rich, Oxygen-Functionalized Graphene from Carbon Fibers for Oxygen Electrocatalysis. Adv. Mater. 2017, 29, 1606207. (253) Chai, G.-L.; Qiu, K.; Qiao, M.; Titirici, M.-M.; Shang, C.; Guo, Z. Active Sites Engineering Leads to Exceptional ORR and OER Bifunctionality in P,N Co-Doped Graphene Frameworks. Energy Environ. Sci. 2017, 10, 1186–1195. (254) Fu, G.; Cui, Z.; Chen, Y.; Li, Y.; Tang, Y.; Goodenough, J. B. Ni3Fe-N Doped Carbon Sheets as a Bifunctional Electrocatalyst for Air Cathodes. Adv. Energy Mater. 2017, 7, 1601172. (255) Li, Z.; Li, G.; Jiang, L.; Li, J.; Sun, G.; Xia, C.; Li, F. Ionic Liquids as Precursors for Efficient Mesoporous Iron-Nitrogen-Doped Oxygen Reduction Electrocatalysts. Angew. Chemie - Int. Ed. 2015, 54, 1494–1498. (256) Yasuda, S.; Furuya, A.; Uchibori, Y.; Kim, J.; Murakoshi, K. Iron-Nitrogen-Doped Vertically Aligned Carbon Nanotube Electrocatalyst for the Oxygen Reduction Reaction. Adv. Funct. Mater. 2016, 26, 738–744. (257) He, D.; Xiong, Y.; Yang, J.; Chen, X.; Deng, Z.; Pan, M.; Li, Y.; Mu, S. NanocarbonIntercalated and Fe–N-Codoped Graphene as a Highly Active Noble-Metal-Free Bifunctional Electrocatalyst for Oxygen Reduction and Evolution. J. Mater. Chem. A 2017, 5, 1930–1934. (258) Su, C. Y.; Cheng, H.; Li, W.; Liu, Z. Q.; Li, N.; Hou, Z.; Bai, F. Q.; Zhang, H. X.; Ma, T. Y. Atomic Modulation of FeCo–Nitrogen–Carbon Bifunctional Oxygen Electrodes for Rechargeable and Flexible All-Solid-State Zinc–Air Battery. Adv. Energy Mater. 2017, 7, 1602420. (259) Ji, D.; Peng, S.; Safanama, D.; Yu, H.; Li, L.; Yang, G.; Qin, X.; Srinivasan, M.; 77 ACS Paragon Plus Environment


(260) (261)

(262)

(263)

(264)

(265)

(266)

(267)

(268)

(269)

(270)

(271) (272)

(273)

Page 78 of 81

Adams, S.; Ramakrishna, S. Design of 3-Dimensional Hierarchical Architectures of Carbon and Highly Active Transition Metals (Fe, Co, Ni) as Bifunctional Oxygen Catalysts for Hybrid Lithium-Air Batteries. Chem. Mater. 2017, 29, 1665–1675. Wang, J.; Ciucci, F. Boosting Bifunctional Oxygen Electrolysis for N-Doped Carbon via Bimetal Addition. Small 2017, 13, 1604103. Yang, J.; Wang, X.; Li, B.; Ma, L.; Shi, L.; Xiong, Y.; Xu, H. Novel Iron/CobaltContaining Polypyrrole Hydrogel-Derived Trifunctional Electrocatalyst for SelfPowered Overall Water Splitting. Adv. Funct. Mater. 2017, 27, 1606497. Bates, M. K.; Jia, Q.; Doan, H.; Liang, W.; Mukerjee, S. Charge-Transfer Effects in Ni–Fe and Ni–Fe–Co Mixed-Metal Oxides for the Alkaline Oxygen Evolution Reaction. ACS Catal. 2015, 6, 155–161. Zeng, M.; Liu, Y.; Zhao, F.; Nie, K.; Han, N.; Wang, X.; Huang, W.; Song, X.; Zhong, J.; Li, Y. Metallic Cobalt Nanoparticles Encapsulated in Nitrogen-Enriched Graphene Shells: Its Bifunctional Electrocatalysis and Application in Zinc–Air Batteries. Adv. Funct. Mater. 2016, 26, 4397–4404. Gorlin, Y.; Lassalle-Kaiser, B.; Benck, J. D.; Gul, S.; Webb, S. M.; Yachandra, V. K.; Yano, J.; Jaramillo, T. F. In Situ X-Ray Absorption Spectroscopy Investigation of a Bifunctional Manganese Oxide Catalyst with High Activity for Electrochemical Water Oxidation and Oxygen Reduction. J. Am. Chem. Soc. 2013, 135, 8525–8534. Yang, H. Bin; Miao, J.; Hung, S. F.; Chen, J.; Tao, H. B.; Wang, X.; Zhang, L.; Chen, R.; Gao, J.; Chen, H. M. Identification of Catalytic Sites for Oxygen Reduction and Oxygen Evolution in N-Doped Graphene Materials: Development of Highly Efficient Metal-Free Bifunctional Electrocatalyst. Sci. Adv. 2016, 2, e1501122–e1501122. Hao, Y.; Xu, Y.; Liu, J.; Sun, X. Nickel–cobalt Oxides Supported on Co/N Decorated Graphene as an Excellent Bifunctional Oxygen Catalyst. J. Mater. Chem. A 2017, 5, 5594–5600. Dresp, S.; Luo, F.; Schmack, R.; Kühl, S.; Gliech, M.; Strasser, P. An Efficient Bifunctional Two-Component Catalyst for Oxygen Reduction and Oxygen Evolution in Reversible Fuel Cells, Electrolyzers and Rechargeable Air Electrodes. Energy Environ. Sci. 2016, 9, 2020–2024. Qian, Y.; Hu, Z.; Ge, X.; Yang, S.; Peng, Y.; Kang, Z.; Liu, Z.; Lee, J. Y.; Zhao, D. A Metal-Free ORR/OER Bifunctional Electrocatalyst Derived from Metal-Organic Frameworks for Rechargeable Zn-Air Batteries. Carbon N. Y. 2017, 111, 641–650. Yang, W.; Chen, L.; Liu, X.; Jia, J.; Guo, S. A New Method for Developing DefectRich Graphene Nanoribbons/Onion-like Carbon@Co Nanoparticles Hybrid Materials as an Excellent Catalyst for Oxygen Reactions. Nanoscale 2017, 9, 1738–1744. Strickland, K.; Miner, E.; Jia, Q.; Tylus, U.; Ramaswamy, N.; Liang, W.; Sougrati, M. T.; Jaouen, F.; Mukerjee, S. Highly Active Oxygen Reduction Non-Platinum Group Metal Electrocatalyst without Direct Metal-Nitrogen Coordination. Nat. Commun. 2015, 6, 7343. Jin, S. Are Metal Chalcogenides, Nitrides, and Phosphides Oxygen Evolution Catalysts or Bifunctional Catalysts? ACS Energy Lett. 2017, 2, 1937–1938. Castanheira, L.; Silva, W. O.; Lima, F. H. B.; Crisci, A.; Dubau, L.; Maillard, F. Carbon Corrosion in Proton-Exchange Membrane Fuel Cells: Effect of the Carbon Structure, the Degradation Protocol, and the Gas Atmosphere. ACS Catal. 2015, 5, 2184–2194. Castanheira, L.; Dubau, L.; Mermoux, M.; Berthome, G.; Caque, N.; Rossinot, E.; Chatenet, M.; Maillard, F. Carbon Corrosion in Proton-Exchange Membrane Fuel Cells: From Model Experiments to Real-Life Operation in Membrane Electrode Assemblies. ACS Catal. 2014, 4, 2258–2267. 78 ACS Paragon Plus Environment

Page 79 of 81 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(274) Borup, R.; Meyers, J.; Pivovar, B.; Kim, Y. S.; Mukundan, R.; Garland, N.; Myers, D.; Wilson, M.; Garzon, F.; Wood, D. Scientific Aspects of Polymer Electrolyte Fuel Cell Durability and Degradation. Chem. Rev. 2007, 107, 3904–3951. (275) Zhang, R.-Q.; Lee, T.-H.; Yu, B.-D.; Stampfl, C.; Soon, A. The Role of Titanium Nitride Supports for Single-Atom Platinum-Based Catalysts in Fuel Cell Technology. Phys. Chem. Chem. Phys. 2012, 14, 16552. (276) Okatsu, H.; Morrill, M. R.; Shou, H.; Barton, D. G.; Ferrari, D.; Davis, R. J.; Agrawal, P. K.; Jones, C. W. Supported K/MoS2 and K/Mo2C Catalysts for Higher Alcohol Synthesis from Synthesis Gas: Impact of Molybdenum Precursor and Metal Oxide Support on Activity and Selectivity. Catal. Letters 2014, 144, 825–830. (277) Ge, P.; Scanlon, M. D.; Peljo, P.; Bian, X.; Vubrel, H.; O’Neill, A.; Coleman, J. N.; Cantoni, M.; Hu, X.; Kontturi, K. Hydrogen Evolution across Nano-Schottky Junctions at Carbon Supported MoS2 Catalysts in Biphasic Liquid Systems. Chem. Commun. 2012, 48, 6484. (278) Lin, F.; Boettcher, S. W. Adaptive Semiconductor/Electrocatalyst Junctions in WaterSplitting Photoanodes. Nat. Mater. 2014, 13, 81–86. (279) Rana, M.; Patil, P. K.; Chhetri, M.; Dileep, K.; Datta, R.; Gautam, U. K. Pd-Pt Alloys Nanowires as Support-Less Electrocatalyst with High Synergistic Enhancement in Efficiency for Methanol Oxidation in Acidic Medium. J. Colloid Interface Sci. 2016, 463, 99–106. (280) Chhetri, M.; Rana, M.; Loukya, B.; Patil, P. K.; Datta, R.; Gautam, U. K. Mechanochemical Synthesis of Free-Standing Platinum Nanosheets and Their Electrocatalytic Properties. Adv. Mater. 2015, 27, 4430–4437. (281) Huan, T. N.; Rousse, G.; Zanna, S.; Lucas, I. T.; Xu, X.; Menguy, N.; Mougel, V.; Fontecave, M. A Dendritic Nanostructured Copper Oxide Electrocatalyst for the Oxygen Evolution Reaction. Angew. Chemie - Int. Ed. 2017, 56, 4792–4796. (282) Liu, S.; White, M. G.; Liu, P. Oxygen Reduction Reaction on Ag(111) in Alkaline Solution: A Combined Density Functional Theory and Kinetic Monte Carlo Study. ChemCatChem 2017, 10, 540–549. (283) Bhatt, M. D.; Lee, G.; Lee, J. S. Density Functional Theory (DFT) Calculations for Oxygen Reduction Reaction Mechanisms on Metal-, Nitrogen- Co-Doped Graphene (M-N2-G (M=Ti, Cu, Mo, Nb and Ru)) Electrocatalysts. Electrochim. Acta 2017, 228, 619–627. (284) Shin, H.; Xiao, H.; Goddard, W. A. In Silico Discovery of New Dopants for Fe-Doped Ni Oxyhydroxide (Ni1–xFexOOH) Catalysts for Oxygen Evolution Reaction. J. Am. Chem. Soc. 2018, 140, 6745–6748. (285) Sun, X.; Li, K.; Yin, C.; Wang, Y.; He, F.; Bai, X.; Tang, H.; Wu, Z. The Oxygen Reduction Reaction Mechanism on Sn Doped Graphene as an Electrocatalyst in Fuel Cells: A DFT Study. RSC Adv. 2017, 7, 729–734. (286) Chen, X.; Hu, R.; Bai, F. DFT Study of the Oxygen Reduction Reaction Activity on Fe−N4-Patched Carbon Nanotubes: The Influence of the Diameter and Length. Materials (Basel). 2017, 10, 549. (287) Zhao, S.; Yan, L.; Luo, H.; Mustain, W.; Xu, H. Recent Progress and Perspectives of Bifunctional Oxygen Reduction/Evolution Catalyst Development for Regenerative Anion Exchange Membrane Fuel Cells. Nano Energy 2018, 47, 172–198. (288) Li, H.; Luo, L.; Kunal, P.; Bonifacio, C. S.; Duan, Z.; Yang, J. C.; Humphrey, S. M.; Crooks, R. M.; Henkelman, G. Oxygen Reduction Reaction on Classically Immiscible Bimetallics: A Case Study of RhAu. J. Phys. Chem. C 2018, 122, 2712–2716. (289) Xiao, H.; Shin, H.; Goddard, W. A. Synergy between Fe and Ni in the Optimal Performance of (Ni,Fe)OOH Catalysts for the Oxygen Evolution Reaction. Proc. Natl. 79 ACS Paragon Plus Environment


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Emerging Materials in Heterogeneous Electrocatalysis Involving

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