Nickel-Based Electrocatalysts for Energy-Related Applications

Subscriber access provided by University of Glasgow Library

Review

Nickel–Based Electrocatalysts for Energy Related Applications: Oxygen Reduction, Oxygen Evolution, and Hydrogen Evolution Reactions Varun Vij, Siraj Sultan, Ahmad M. Harzandi, Abhishek Meena, Jitendra N. Tiwari, Wang Geun Lee, Taeseung Yoon, and Kwang S. Kim ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b01800 • Publication Date (Web): 06 Sep 2017 Downloaded from http://pubs.acs.org on September 6, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Catalysis is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 74

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

ACS Catalysis

Nickel–Based Electrocatalysts for Energy Related Applications: Oxygen Reduction, Oxygen Evolution, and Hydrogen Evolution Reactions Varun Vij, Siraj Sultan, Ahmad Harzandi, Abhishek Meena, Jitendra N. Tiwari,* Wang–Geun Lee, Taeseung Yoon, Kwang S. Kim*

Center for Superfunctional Materials, Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST), 50, UNIST–gil, Ulsan 44919, Republic of Korea

ABSTRACT. The persistently increasing energy consumption and the low abundance of conventional fuels have raised serious concerns all over the world. Thus, the development of technology for clean energy production has become the major research priority worldwide. The globalization of advanced energy conversion technologies like rechargeable metal–air batteries, regenerated fuel cells, and water splitting devices has been majorly benefitted by the development of apposite catalytic materials that can proficiently carry out the pertinent electrochemical processes like oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and water hydrolysis. Despite a handful of superbly performing commercial catalysts, the high cost and low electrochemical stability of precursors have consistently discouraged their long term viability. As a promising substitute of conventional platinum, palladium, iridium, gold, silver, and ruthenium based catalysts, various transition metal (TM) ions (for example, Fe, Co, Mo, Ni, V, Cu, etc.) have been exploited to develop advanced electroactive materials to outperform the state–of–art catalytic properties. Among these TMs, nickel has emerged as one of the most hopeful constituent due to its exciting electronic properties and anticipated synergistic effect to dramatically alter surface properties of materials to favor electrocatalysis. This review article will broadly confer about recent reports on nickel–

ACS Paragon Plus Environment

1

ACS Catalysis

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

Page 2 of 74

based nano-architectured materials and their applications toward ORR, OER, HER, and whole water splitting. Based on these applications and properties of nickel derivatives, futuristic outlook of these materials have also been presented. KEYWORDS. nickel–based electrocatalysts, oxygen reduction reaction, oxygen evolution reaction, hydrogen evolution reaction, water splitting, bifunctional electrocatalysts.

Table of Content


2

Page 3 of 74

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

ACS Catalysis

1. INTRODUCTION Constantly increasing energy demands as well as rising global warming by CO2 emissions and existing nature of fossil fuels have led us to focus on developing technologies for clean energy production. High performing metal-based catalysts for ORR, OER, and HER are of supreme importance for critical renewable energy technologies like fuel cells, batteries, and electrochemical water splitting (Figure 1).1-7 The ORR, OER, and HER are ORR: 2H+ + 1/2O2 + 2e– → H2O OER: H2O → 1/2O2 + 2H+ + 2e– HER: 2H+ + 2e– → H2 The detailed mechanisms of ORR, OER, and HER in acidic and basic electrolyte are as follows.8-10 ORR in acidic electrolyte:

ORR in basic electrolyte:

OER in acidic electrolyte:

OER in basic electrolyte:

where, * indicates the active sites of catalyst, and OH*, O* and OOH* represents adsorbed intermediates species. HER in acidic and basic electrolyte


3

ACS Catalysis

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

Page 4 of 74

Currently, ruthenium (Ru) and iridium (Ir) oxides are considered as the most efficient OER electrocatalysts,11-12 whereas carbon based platinum (Pt) has been regarded as best performing electrocatalyst for ORR and HER.13-16 It is known that among electrocatalysts Pt is best for HER in acidic solution for cathodic half reaction, while Ru and Ir are best for OER in acidic solution for anodic half reaction. On the other side, the metal oxides and nanostructured carbon materials perform well in alkaline solution for OER. Although these catalysts with aforementioned metal sources are known to decrease the activation energy barriers for above mentioned sluggish electrochemical reactions, their high cost and scarcity become the most disappointing factors to rule out their commercial viability as renewable energy technologies for long run.


4

Page 5 of 74

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

ACS Catalysis

Figure 1. Schematic representation of energy related applications i.e. oxygen reduction reaction (ORR) in fuel cell, oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) by water hydrolysis. Recently as an economical and efficient replacement to these expensive metal precursors, nickel–based electrocatalysts in the shape of nickel foam (NF),17 alloys,18 nitrides,19 phosphides,20 oxides,21 metal organic frameworks (MOFs),22 etc. have exhibited very promising electrocatalytic activity and stability toward ORR, OER and HER, as monofunctional or bifunctional materials. The high synergistic effect between Ni and neighboring hetero-atoms results in much better surface adsorption properties which can be attributed to the enhancement in the electrocatalytic properties of resulting materials. Due to low price, high elemental abundance, strong strength, better ductility, high corrosion resistance, good heat conduction, and high electrical conductivity, Ni–based materials have been widely studied for their electrochemical applications.

Figure 2. Diagrammatic representation of exploitation of nickel-based electrocatalysts.


5

ACS Catalysis

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

Page 6 of 74

Although most of the commonly known Ni compounds are bivalent, it is capable of acquiring other valences (–1 to +4) also, which makes it highly susceptible toward undergoing various electronic transitions. Owing to these interesting electronic properties, high conductivity and thermal stability, Ni has been a very frequent choice for designing electrocatalytic materials. Keeping this in consideration, the idea of this review article is to cover recent reports based on Ni–based electrocatalysts for ORR, OER, HER, and bifunctional reactions which are schematically presented in Figure 2 to encourage the development of catalytic materials from non–precious and more abundant metal precursors like Ni. Recently, Gong et al.23 published a mini review which covered recent reports based on Ni–based electrocatalyst for only alkaline HER in a very concise manner. Therefore, the role of Ni–based materials in ORR, OER, acidic HER, and bifunctional applications has not been enclosed in that review. In fact, to best of our knowledge, there is so far no review article which covers these electrocatalytic applications of Ni–based materials in a broad manner. In this regard, this review will discuss (1) the relative electrocatalytic efficiency of Ni–based materials for ORR, OER, HER, and bifunctional reactions, (2) the role of chemical structure and morphology of metallic Ni nanostructures on catalytic activities, (3) the function of other metals in Ni alloys or vice–versa, and their derivatives having the surface adsorption capability to affect the overall catalytic activity, and (4) the challenges and futuristic scope of Ni–based materials in the electrocatalytic applications.

2. ORR ELECTROCATALYSTS 2.1. Nickel–Based ORR. The complete switching to renewable resources demands more advancement in electrochemical technologies for energy conversion and storage like fuel cells and metal–air batteries. With approaching commercialization of proton exchange membrane fuel cells (PEMFCs) as clean energy generator, the biggest challenge is to achieve high electrocatalytic performance by non–expensive electrocatalytic electrodes.24-26 Pt–based anodic and cathodic electrodes have been well established to efficiently carry out hydrogen oxidation and oxygen reduction, respectively, in PEMFCs. However, due to the high overpotential and sluggish kinetics, ORR has been considered as the limiting step over hydrogen oxidation in fuel cell mechanism. Therefore, seeking appropriate cathodic material for ORR is a bigger challenge than


6

Page 7 of 74

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

ACS Catalysis

for anodic half reaction. Pt–based materials have been known to possess excellent electrochemical activity for ORR in PEMFCs.27-28 However, the biggest drawback of using Pt– based PEMFCs is their high production cost due to less abundant and highly expensive metal precursor. On the other hand, Ni, being from same group but much more naturally abundant, has been used as a very good replacement for these metals for past some years. Among various Ni– based electrocatalytic materials, Ni–based bimetallic or trimetallic alloys, Ni–based heterostructures, and Ni nanoparticles (NPs) have been reported to exhibit high susceptibility toward surface absorption of molecular oxygen to facilitate ORR. In this section, we will highlight most recent reports on ORR active Ni–based materials with various chemical compositions with respective physical morphologies. Stamenkovic et al.29 headed with the study of synergy between surface chemistry of nickel incorporated Pt alloy on single-crystal surfaces at electrochemical interfaces and their electronic structure for the ORR. The surface electronic structural study showed that the d-orbital density of state is significantly affected by the structure (–2.70 eV on Pt3Ni (110) to –3.10 eV on Pt3Ni(111) to –3.14 eV on Pt3Ni(100)). The substantial role of nickel in oxygen reduction mechanism was inferred from density functional theory (DFT) calculations on the adsorption of OH and H2O at 0.25 monolayer coverage on modeled Pt(111) surfaces, with and without 50% Ni atoms. It exhibited that only in the presence of nickel sublayer, OH reacted with H+ to yield H2O on the catalyst surface with a positive shift in reversible potential on Pt(111) of 0.10 V in multielectron oxygen reduction reaction (½O2 + 2H+ + 2e– = H2O). This report not only encouraged alloying of Pt to develop bimetallic systems for electrocatalysis but also brought up the potential of nickel to offer high synergistic effect in multicomponent system to enhance catalytic activity. 2.2. Ni Alloys Based ORR. Encouraged by the work of Stamenkovic et al.29 on the enhanced ORR performance of Pt by incorporating Ni in Pt–Ni bimetallic materials, there has been a constant ongoing research to modify this bimetallic system in terms of shapes and morphologies like octahedron, truncated octahedron, cuboctahedron, icosahedron, and cube.30-34 Alloying Ni with Pt not only decreases the relative amount of naturally scarce Pt metal but also enhances the catalytic mass activity and durability of otherwise less stable pure Pt.35-39 Huang et al.40 adopted a surface engineering approach to improve the catalytic performance of well explored Pt3Ni(111) bulk surface. This approach involved controlled doping of TMs like V, Cr, Mn, Fe, Co, Mo, W,


7

ACS Catalysis

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

Page 8 of 74

and Re on the surface of octahedral nanocrystal of Pt3Ni(111). It was concluded that Mo–doped Pt3Ni/C catalyst showed best ORR performance with specific activity of 10.3 mA cm-2, mass activity of 6.98 A mg-1Pt, significantly better stability for 8000 potential cycles than non–doped surface. The Monte Carlo simulations indicated that Mo–oxide may crowd out surface Ni and Pt of the nanocrystal to form strong Mo–Ni/Pt bonds towards improved catalysis and stability. a

d

b

c

e

f

Figure 3. ORR properties of octahedral Pt–Ni/C. (a) ORR polarization curves, (b,c) Active area and mass specific ORR current densities (jarea and jmass) of PtNi/C, Pt1.5Ni/C, Pt2Ni/C, Pt3Ni/C, Pt4Ni/C, and commercial Pt/C in O2–saturated 0.1 M HClO4 with a scan rate of 10 mV s-1 at 1600 rpm. (d) Cyclic voltammograms, (e) ORR, and (f) jarea & jmass of Pt1.5Ni/C and Pt/C after accelerated stability test. Reproduced with permission from ref 41 Although octahedral PtNi alloy NPs are known to exhibit ORR capacity in PEMFCs, the economical mass production of these materials has always been a challenge. Zhang et al.41 reported a low–cost synthetic strategy for mass production of carbon supported octahedral Pt−Ni alloy NPs with promising ORR activity. The material exhibited a significant shift of 64 mV toward the positive direction in reference to commercial Pt/C attributing to improved reaction kinetics. Among the five octahedral Pt–Ni/C catalysts (PtNi/C, Pt1.5Ni/C, Pt2Ni/C, Pt3Ni/C, and Pt4Ni/C) which depend on Pt/Ni ratio, Pt1.5Ni/C exhibited current densities of 19.31 and 3.99 mA cm–2 Pt at 0.85 and 0.90 V vs. reversible hydrogen electrode (RHE) which were 32 and 20 times larger than commercial Pt/C (Figure 3a-c), and their mass–specific kinetic current densities of


8

Page 9 of 74

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

ACS Catalysis

the material were 16 and 10 times larger than commercial Pt/C, respectively. In addition, Pt1.5Ni/C exhibited greater electrochemical stability for 4000 voltammetric cycles in comparison with the Pt/C catalyst (Figure 3d-f). Both experimental and theoretical studies suggested that Pt−Ni tends to grow into octahedron (111) surfaces when Ni content is sufficiently high in Pt1.5Ni/C to dominate CO adsorption, whereas pure Pt tends to grow into cubic (100) surfaces. Since the adsorbed CO is desorbed after exposure to air by Pt−Ni through CO oxidation, the clean particle surface can actively catalyze the reaction even at room temperature. In another very interesting example of surface engineering, He et al.18 used chemical etching of octahedral PtNi3 alloy with sodium borohydride solution to give Pt–Ni/Ni–B hybrid system with in situ resulting amorphous nickel–boron membrane. This hybrid system was experimentally achieved by dealloying the Pt–Ni alloy accompanied with interfacial fabrication of amorphous nickel boride membrane as an electron acceptor to induce synergistic effect with crystalline octahedral alloy. This structural evolution resulted in much higher electrocatalytic activity toward ORR with 27 times better mass activity than commercial Pt/C at 0.9 V. However, the specific activity of PtNi/Ni–B/C at 9.32 mA cm–2 was 32 times higher than that of Pt/C at 0.28 mA cm–2 and 4 times higher than that of undoped PtNi/C catalyst at 0.9 V. A shift of about 52 mV in half wave potential with enhanced stability during 5000 cycles was observed due to NiB membrane doping which has been attributed to the electronic effect of Ni–B membrane on the Pt–O and Pt–OH chemical bonding by boron oxide controlled electron transfer. Alloying Pt with Ni results in d–band broadening and thus lowering the binding energy of oxygen containing species to enhance ORR capacity. However, at the same time, unalloyed part and oxides of alloying metal can practically reduce the ORR activity and stability of Pt based functioning batteries by blocking sites of sulfonic groups in proton exchange membranes. Jeon et al.42 used hydroquinone (HQ) dissolved ethanol at high temperature as a new chemical method instead of commonly used acid treatment for leaching out Ni from PtNi alloy NPs. They intricately compared the effect of conventional leaching agent, sulfuric acid and HQ, on the extent and type of Ni leached from pristine PtNi alloys, Pt3Ni and Pt3Ni2. Since Ni is known to be dissolved in strong acids, sulfuric acid showed much higher dissolution of unalloyed Ni in comparison with HQ, however the degree of dealloying of Ni from Pt was higher in the case of HQ as compared to sulfuric acid. Hence, despite the higher content of Ni in PtNi–HQ, the higher upshift in d–band center was observed with high ORR activity and durability. These results


9

ACS Catalysis

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

Page 10 of 74

concluded that the ORR activity of the bimetallic alloy system is directly dependent on the chemical and structural aspect of surface layer irrespective of the concentration of Pt. Emphasizing on the structural factor in another work by Choi et al.,43 one–pot synthesis was used to develop shape controlled urchin-like Pt2Ni alloy based ORR catalyst with mass activity of 1.58 A mg–1, which is 12.2 times higher than that of commercial Pt/C NPs and specific activity 20.7 times higher than that of Pt. The urchin-like nanostructure played a vital role in inhibiting the specific anion adsorption on the surface of electrocatalyst which can poison the material to hamper the stability of ORR in hydrogen fuel cells. Unlike the thermodynamically stable nanoarchitectures which tend to expose (111) facets of Pt, the urchin-like arrangement is assumed to expose high energy facets (100) which played a vital role in inhibiting specific anion adsorption on the surface of electrocatalyst which can poison the material to hamper the stability of ORR in hydrogen fuel cells. Cyclic voltammetric and X-ray absorption near edge structure (XANES) spectrum analysis exhibited that the higher ORR activity of the system was attributed to the electronic perturbation due to alloying which resulted in higher electron population in the 5d band in urchin-like structure to lower the adsorption energy of OH which unblocked the active surface area for better electrocatalysis. It has now been a well-established fact that modification of electronic structure of Pt layer adjacent to Pt–metal alloy can be achieved by varying the energy of electrons of bulk Pt which can be achieved by electrochemical or chemical incorporation of other TMs. Li et al.44 compared the electrocatalytic activity of binary alloys PtNi and PtCo with ternary alloys Pt–Ni–Co to study the effect of relative amount of Pt in a system. Reducing the amount of Pt in the material by incorporating more metals in the catalyst system enhanced the ORR activity. Similar to the alloys performing better toward ORR in comparison to the pure Pt metal, ternary alloys turned out to perform even better than binary alloys. The ternary alloy PtNiCo exhibited kinetic current density of 15.89 mA cm–2 which was 8.8 and 9.7 times higher than that of binary alloys PtCo and PtNi, respectively. Also, the mass activity of ternary alloy (4.23 A mg–1) is 9.6 times larger than that of PtCo and 10.8 times larger than that of PtNi. Besides the synergistic intetractions between Pt and other transition metals, different coeletrodeposition charge densities of Ni and Co lead to different compositions in ternary alloys following Pt replacement and later electrochemical dealloying, resulting in optimal Pt/Ni/Co composition with more surface active Pt which bears more d-band vacancies of the ternary alloy catalyst to enhance the ORR activities.


10

Page 11 of 74

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

ACS Catalysis

A very big challenge for practical viability of direct methanol fuel cell as potentially clean energy conversion system is the low methanol tolerance of Pt based commercial electrocatalysts, which leads to a significant drop in catalytic activity with every voltammetric step. A lot of research has been focused on reducing the amount of Pt by incorporating other TMs to enhance the durability and methanol tolerance of the ORR active material.45-46 Zou et al.47 used heat treatment under H2/N2 atmosphere to carry out the conversion of disordered PtNi alloy into ordered PtNi intermetallic compound which is known to undergo structural transformation to serve better ORR activity with higher methanol tolerance than disordered alloy analogs. The XRD analysis suggested that the ordered-PtNi/C exhibited NiPt bimetallic phase resulting in intermetallic alloy NPs. However, in the disordered analogue, no peak corresponding to Ni was observed, which means Ni is intercalated in single phase Pt fcc structure to form an alloy phase that easily undergo leaching to suppress the catalytic activity. These electrochemically durable (5000 potential cycles) ordered PtNi intermetallic NPs also exhibited specific activity of 0.85 V which is 6 times better than Pt/C and 3 times better than disordered PtNi alloy. Liu et al.48 incorporated Ni on terminal surface in ternary metal system of 5–fold twinned Pd2NiAg nanocrystals due to excellent ORR activity of Ni. The reason of using nanocrystals over ternary alloys was due to significantly high stacking fault energy and lattice mismatch due to variation in sizes of metals involved. The 5–fold twinned nanocrystals can be described as a pentagonal prism with (100) facets capped by two pentagonal pyramids with (111) facets which can be achieved by controlling the molar ratio of metal precursors. These Pd2NiAg nanocrystals exhibited better minimum onset potential of −0.05 V (vs Ag/ AgCl) and minimum half–wave potentials of −0.131 V than spherical Pd2NiAg, 5–fold twinned Pd2Ag nanocrystals, and Pt/C, due to a surface valence electron effect and superior geometrical arrangement. Yang et al.49 reported Pt–Ni–Ir based ternary metal nanocrystals with respective atomic ratio of 55:29:16 which possessed truncated octahedral shape with an average size of 10 nm, and exhibited high ORR catalytic activity and durability in acidic media. This ternary catalyst showed mass activity and specific activity of 511 mA mg–1 (Pt) and 1.03 mA cm–2 at 0.9 V (vs. RHE), respectively, which were about 4.8 and 6.0 times better than that of the Pt/C catalyst. The high ORR activity of Pt–Ni–Ir catalytic material has been attributed to a compressive strain formed on Ni scarce surface of the nanocrystal which was achieved by etching low–coordinated surface Ni by the trace oxygen present in the synthesis solution.


11

ACS Catalysis

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

Page 12 of 74

Zhao et al.50 synthesized ∼28 nm octahedral [email protected] core–shell nanocrystals by depositing about four atomic layers of PtNi alloy on the surface of Pd nanocrystals. This approach was the combination of two well established techniques: (1) replacing bulk of Pt with a less expensive metal i.e. core–shell synthesis and (2) generation of alloy nanocrystals with certain facets by employing TMs into Pt. The octahedral Pd@Pt–Ni/C nanocrystals with an ultrathin PtNi alloy shell possess the mass activity of 0.79 A mg-1Pt which was 4.9 times higher than that of Pt/C (0.16 A mg-1Pt) at 0.9 V. The better ORR polarization curve and mass activity of this catalyst was due to more Pt active sites arising from alloying Pt with Ni, and modified electronic and geometric structures of shell atoms in core–shell structure. Instead of involving noble metals like Pt, Zhang et al.51 used Co to develop hybrid nanosheets by thermally growing NiCo2O4 nanocrystals on reduced graphene oxide (rGO) which resulted in very high ORR activity with high current density and low over–potential. The NiCo2O4–rGO hybrid material exhibited an ORR onset potential of about 88 mV (vs. Ag/AgCl) that is only about 72 mV more negative than that of commercial Pt/C catalyst at 2500 rpm. Also, this ~4 emechanism of ORR exhibited the current density of 2.0 mA cm–2 at the potential of 0.8 V. Besides, the material showed much better methanol tolerance in reference to the Pt/C catalysts due to coupling effect between NiCo2O4 and rGO. Zeng et al.52 reported a pyrolyzed system bearing bimetallic nanocrystal alloy of Ni and Co over N-doped carbon nanotubes (NCNTs) as ORR electrocatalyst NiCo@NCNT-700 with catalytic performace comparable to commercial Pt/C catalyst under alkaline conditions. Amongst various atomic Ni:Co ratios, the catalyst with 3:7 Ni:Co ratio exhibited best ORR properties with high onset and half-wave potentials of 0.93 V and 0.82 V (vs RHE) with current density around 5 mA cm-2. The K-L plots of NiCo@NCNT-700 suggested approximately 4-electron transfer (3.7-3.9) per O2 molecule. The competent catalytic performance could be attributed to the cocontributions of uniformly adsorbed Ni-Co alloy NPs, graphitic nitrogen doped carbon, Co-N bond formation. To generate oxygen reduction catalyst with high tolerance toward methanol, ethanol and ethylene glycol, Yu et al.53 used a solvothermal method to dope nickel and iron, separately over CoSe2. Although doping of Fe in CoSe2 resulted in slightly more positive onset and half wave potential due to higher active surface area in comparison to Ni doped analogue, they both exhibited high durability in acidic media. A study by Lehtimäki et al.54 brought up another


12

Page 13 of 74

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

ACS Catalysis

aspect of nickel doping in non-precious ORR catalysts. The nickel doping in MnO2 based ORR catalyst by microwave assisted hydrothermal synthesis resulted in significant involvement of the disc support to increase the number of electron transfer from 3.6 (non-doped MnO2) to 3.9 (Ni doped MnO2). To investigate ORR activities and mechanisms on pristine and doped Si60C60 nanocages, Chen et al.55 carried out DFT calculations. The study exhibited that doping of otherwise inefficient electrocatalysts Si60C60 with Ni or Co resulted in increase in catalytic activity towards oxygen reduction. Dual mechanism for ORR involved (i) end-on adsorption of O2 over doped Ni/Co by H2OO dissociation in which the O−O bond is fully broken after the second H transfer, and (ii) bridge adsorption of O2 completed via an OOH dissociation pathway in which the O−O bond is directly broken after the first H transfer. Doping of Ni resulted in decrease in adsorption energy Eads of OOH intermediate from -2.18 to -1.44 eV whereas in the case of OH it reduced from 3.74 to -2.63 eV which enhances the adsorption of reaction intermediates and spontaneity of ORR. Amongst Ni and Co as dopants, Ni doped nanocages showed much superior catalytic properties. These reports suggest that the exploration of highly efficient, economical and sustainable ORR electrocatalysts are of great significance in energy conversion devices to fully or partially replace rare and expensive noble metals. 2.3. Nickel Oxide NPs Based ORR. The transition metal oxides have a number of advantages to offer as electrocatalytic components i.e. high availability and low cost, high activity, thermodynamic stability, and low electrical resistance. Besides, these metal oxides exhibit very strong synergistic effect in binary materials to support electrocatalytic processes like oxygen reduction reaction. Yung et al.56 used hydrothermal approach to grow Ni–NiO NPs on poly– (diallyldimethylammonium chloride) (PDDA) to help in formation of a layered structured hybrid graphene by surface modification (PDDA–G) on carbon supported materials. The electrocatalytic investigation of Ni–NiO/PDDAG hybrid toward ORR was done in acidic medium which showed good stability of catalyst toward a large number of electrochemical cycles. Besides, the material exhibited selective catalysis toward ORR than methanol oxidation5759

to enhance its practical application. Porous tubular nanostructures of NiO/NiCo2O4 hybrid were synthesized via a coaxial

electrospinning technique by Cui et al.60 Interestingly, two discrete components NiO and


13

ACS Catalysis

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

Page 14 of 74

NiCo2O4 exhibited a unique homogeneous interfacial chemical distribution which results in comparable onset potential of −0.05 V (vs Ag/ AgCl) and higher current density (~3 mA cm-2) and durability in comparison with commercial Pt/C catalyst. Similar Tafel plot of NiO/NiCo2O4 and Pt/C indicated 4 e- process involved in ORR carried out by NiO/NiCo2O4 as well, which results in formation of water as its only byproduct. The unique porous NiO/NiCo2O4 nanotubes with BET (Brunauer, Emmett and Teller) surface area of 28.60 m2g–1 showed good dispersion of its components which resulted in improved electrocatalytic performance with 87 % retention of relative current even after 40000 s. Ensafi et al.61 used diazonium reaction to functionalize Nile blue over reduced graphene to introduce the adsorption of nitrogen containing molecules on NiO for heterogeneous electrochemical catalysis. They partially replaced NiO with Pd to bring up the capability of bimetallic system to enhance the electrochemical activity and stability. The electro reduction analysis of Pd/NiO@Nile–rGO/CPE electrode showed shift of oxygen reduction peak toward negative potential with increasing scan rates, confirming the kinetic limitation of the electrochemical reaction. It also suggested that at sufficient overpotential, the process is diffusion controlled instead of surface controlled. 2.4. Ni-MOFs based ORR. Although the inconsistent stability of electrocatalysts toward ORR can be alleviated by thermal treatment, it introduces new concerns like unspecified catalytic active sites, and non-optimized structure–function relationships in catalyst. MOFs are one of the compelling choices to succeed these challenges due to their high surface area, active site density and porous tunable chemical structures for controlled reactions in microenvironment. Miner et al.62 introduced Ni3(HITP)2 (HITP = 2, 3, 6, 7, 10, 11-hexaiminotriphenylene) as a conductive 2D layered MOF as a leading new class of highly ordered materials for ORR electrocatalysis. The activation controlled Tafel plot revealed a Tafel slope of -128 mVdec-1 (Fig. 3) at 0.787 V belonging to the formation of the superoxide anion which points toward the formation of H2O2 (87%). However, broadening the overpotential range to 0.82–0.54V resulted in percentage decrease in H2O2 production which enhances the production of water as byproduct. Other than high surface area and great active sites density, direct strong adhesion of material over the electrode surface resulted in highly competitive onset potential of 0.18 V relative to Pt in basic media.


14

Page 15 of 74

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

ACS Catalysis

Tang et al.63 reported synthesis of a class of Co-based dual metal and nitrogen co-doped MOF catalysts with high porosity with metal and nitrogen uniformly distributed within the graphite carbon matrix through a pyrolysis procedure. These characteristics resulted in the superior ORR performance of the material in both alkaline and neutral media. Among various Co-M based dual metal systems (Zn, Cu, Fe, Ni), the Ni/Co N-doped carbon framework exhibited the best ORR performance with a low onset potential of 0.347 V at pH 7 and power density of 4335.6 mW m−2 in microbial fuel cells with excellent stability. In alkaline media, an onset potential of −0.085 V with high diffusion limit current of 6.66 mA cm−2 and a desired four-electron process was observed. The high stability of the catalytic material was attributed to the unique microstructures consisted of metal species closely wrapped with in the graphite carbon. The well-defined structures of MOFs enable systematical examination of the number of included variables, valency and coordination setting. The structure–function investigation and mechanistic studies of these materials can inspire the development and broadening of MOFs towards the targeted design of other ORR electrocatalysts. Evidently, the targeted higher Faradaic efficiency of 4-electron ORR process for maximizing energy density in industrial settings may be promisingly achievable with MOFs. 2.5. Other Nickel NPs Based ORR. Nickel NPs based materials not only provide the stronger anchoring surface to the catalysts but also increase the active surface areas of material to expose maximum possible catalytic sites. Kodera et al.64 reported synthesis of graphene–covered Ni NPs from methane gas by microwave–assisted catalytic decomposition. The PEMFC application of this material was studied in reference to commercial Pt/C catalyst at variable pHs. The electroreduction of oxygen at neutral pH was recorded to be almost same for both graphencovered Ni NPs and Pt/C, whereas graphene coating introduce higher corrosion resistance in acidic solution than non supported Ni NPs. Farjami and Deiner reported the kinetics of the ORR of α–Ni(OH)2 and the ORR of α–Ni(OH)2 supported on graphene oxide (GO) using rotating disk linear sweep voltammetry (LSV) in alkaline solutions of varying oxygen and hydroxyl concentrations.65 The α–Ni(OH)2/GO catalyst shows lower onset potential and higher current density than α–Ni(OH)2 or GO due to the synergistic effect of α–Ni(OH)2 and GO. However, with decrease in concentration of hydroxyl groups in the solution from 0.5 to 0.1 M and 0.05 M, a slight inclination in current between −0.45 and −0.55 V, and increased sloped current at potentials beyond −0.55 V was observed.


15

ACS Catalysis

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

Page 16 of 74

Increase in alkalinity also assisted in increase in the number of electrons involved in oxygen reduction from 3.4 (at 0.05 M hydroxyl) to ~3.9 (at 0.5 M hydroxyl). The α–Ni(OH)2/GO catalyst has a greater chemical reaction rate constant and kinetic rate constant in comparison to unsupported Ni(OH)2 as lower activation energy required for adsorption of O2 on the supported catalyst. The orders of reaction with respect to hydroxyl for α–Ni(OH)2/GO and α–Ni(OH)2 are 0 and 0.1, respectively, whereas that with respect to oxygen is 1. Unlike most catalysts in which higher solution concentration of hydroxyl results in a higher equilibrium surface concentration to block the catalytic sites, OH-probably does not retard O2 adsorption on the α-Ni(OH)2 catalysts to retain the available sites on catalytic surface. Falkowski et al.66 reported thin film (on gold electrode) of Ni3S2 NPs as an efficient oxygen reduction catalyst at neutral pH with an onset potential of 0.8 V (vs. RHE) and high Faradaic efficiency for 4 e– reduction of O2 to water. In comparison to mostly used metal oxide NPs in literature, interaction in TM sulfides is more covalent, which suppresses the corrosion under similar conditions. This work has brought up the Ni3S2 NPs as a promising economical cell component and biological oxidation catalyst for ORR under neutral pH conditions in functional devices. The electrocatalytic activity of the Ni coordinated thiophosphorylated calix[4]resorcinols for ORR in a PEMFC was studied by Kadirov et al.67 Calixarene derivatives have been well explored in catalytic reactions due to their resistance toward hydrolysis. The purpose of thiophosphorylation of calixresorcinol is the incorporation of additional coordination centers represented by four P=S fragments in the macromolecules, which could enhance the binding ability toward various metal ions through hydrolytically stable tetracoordinate phosphorus atom. However, ORR at the rotating glassy carbon disk electrode modified by this unprecedented scaffold based catalyst in acidic media suggested it to be 2.1 e- process. Up to now, we have emphasized the role of Ni-based or Ni incorporated bimetallic or trimetallic catalytic systems in modifying oxygen reduction properties. The alterations in composition, electrochemical active surface area, surface morphology, bulk microporosity, surface polarization, and overall core–shell structure by the introduction of Ni in respective form play a pivotal role in uplifting the catalytic capabilities of the material toward reduction of oxygen at cathodic end. Table 1 compares the catalytic parameters for ORR of nickel based nonprecious electrpocatalysts. These research advancements can effectively assist in the


16

Page 17 of 74

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

ACS Catalysis

development of sustainable and commercially viable PEMFCs to replace conventional energy generating systems. Besides enhancing the native electrochemical performances of noble metal based monometallic systems, Ni can reduce the overall production value of the active materials by significantly decreasing the ratio of metals like Pt.

Table 1. Comparison of ORR performances of nickel based non-precious electrocatalysts. Catalyst

Onset potential

Half-wave potential

Current density

Ref

(mA cm-2) Pd2NiAg

−0.05 V (vs Ag/ AgCl) in

-0.131 V (vs Ag/

~4.0

48

nanocrystals

alkaline media

AgCl)

NiCo@NCNT-

0.93 V (vs RHE) in acidic

0.82 V (vs RHE)

~5.0

52

700*a

media

NiO/NiCo2O4

−0.05 V (vs Ag/ AgCl) in

---

~3.0

60

---

2.5 - 3.0

62

---

6.66

63

0.61 V (vs RHE)

1.0-1.5

66

alkaline media Ni3(HITP)2

0.82 V (vs RHE) in acidic

framework

media

Ni/CoNC

−0.085 V (vs RHE) in

framework

alkaline media

Ni3S2 NPs

0.80 V (vs RHE) in acidic media

3. OER ELECTROCATALYSTS 3.1. Nickel-Based OER. Environmental and economic concerns raised by the excessive use of fossil fuels have made renewable energy resources more attractive. Electrochemical splitting of water, a combination of two half–cell reactions being HER at a cathode and OER at an anode to generate hydrogen and oxygen is a secure and promising renewable alternative to fossil fuel.68 In water splitting the HER is a simple reaction that occurs very easily on many metals at low overpotential.69 In contrast, the OER is more complex and has sluggish kinetics, because the generation of only one oxygen molecule involves transfer of 4 e- and removal of 4 H+ from


17

ACS Catalysis

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

Page 18 of 74

water, which leads to high overpotential for whole water splitting.69,70 The high overpotential for OER significantly hinders the whole efficiency of the process, which impedes the production of hydrogen from splitting of water at the industrial level.69

Moreover, the OER is also an

important half–cell reaction in many rechargeable batteries and fuel cells, which possess high energy density, specific energy, and low cost.71 For instance, zinc–air and lithium–air batteries have enormously high energy densities and light weight, while they are cost effective.72 However, the performance of these batteries is largely hampered by the slow and sluggish kinetics of the OER at the cathode.73

Therefore, the development of stable and effective

electrocatalyst for OER is required in order to promote the slow evolution kinetics, facilitate the reaction, and improve the overall energy efficiency.71 Currently, some noble metal and their compounds, such as Ir, Ru, IrO2, and RuO2, exhibit the benchmark catalytic activity toward OER in aqueous alkaline and acidic solutions.74 However, the widespread practical applications of these noble metal base compounds are restricted due to their exorbitant price and scarcity. Thus, the development of cost effective catalysts with high electrocatalytic activity, durability, and stability for OER is highly desirable. Recently, earth abundant TM based materials (Fe, Ni, Mn, Co, etc.) have been extensively explored and some of them exhibit considerable electrocatalytic activity for OER.69, 75 The resulting performances in water splitting devices or metal air batteries can be considerably influenced by changing the crystal structure, chemical composition, size distribution, electrical conductivity, hierarchical porosity, and surface chemistry of the metal. In the last decade, Ni-based materials have long been considered for electrocatalytic OER activities in basic solution due to their low cost, superior catalytic activity and stability. Being from the same group of highly electrochemically active Pt, the structural and electronic parameters of nickel based materials and their correlation with catalytic properties have gathered a lot of interest by many researchers. For example, in a study of nickel borates Ni-Bi films, Bediako et al.76 performed coulometric measurements correlated with X-ray absorption nearedge structure spectra to understand the role of oxidation states and structural variation on catalytic rate of OER. Since nickel borates have unique property to undergo a prerequisite subsequent oxidative pretreatment or anodization to attain a well-defined OER activity, these are appropriate materials to set up the correlation between oxidation state/structure and OER efficiency of nickel based catalysts. The study exhibited that the catalyst with high OER activity (after anodization) showed that the activated catalysts


18

Page 19 of 74

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

ACS Catalysis

possess an average oxidation state of +3.6 indicating considerable degree of Ni(IV) centers, whereas the nickel centers in nonactivated films mainly possess the Ni(III) oxidation state. On the other hand, the structural effect on OER studied by XAS spectra revealed that the short-range structure of anodized Ni−Bi belongs to γ-NiOOH, and that of nonanodized Ni−Bi is mostly βNiOOH. The results for Ni−Bi suggested that γ-NiOOH carries more active catalytic sites than βNiOOH, which makes γ-NiOOH intrinsically more active toward the OER in alkaline media. In contrast to many previous reports, this work proposed the +4 oxidation state and the γ-NiOOH structural parameters to be superior for high OER. This section further presents several recent progresses in synthetic methods, structural properties and breakthroughs in Ni-based OER catalyst. It also provides the basic knowledge in designing outstanding catalysts for water oxidation with further optimization of Ni-based hybrids and the novel strategies in developing advanced functional materials. Then, the summary and future prospects are provided.

3.2. NiFe Layered Double Hydroxides Based OER. Layered double hydroxides (LDHs) are a class of 2D ionic materials consisted of monovalent, divalent, or trivalent metal cationic layers with charge balancing anions and molecules of solvation in the inter–layer region.77 The common monovalent, divalent or trivalent metal cations in LDH are Li+, Mg+2,Ni+2, Ca+2, Zn+2, Co+2, Cu+2, Co+3, Al+3, Cr+3, and Fe+3.78,79 The incorporated metal ions, solvation molecules, and intercalated anions between the layers lead to a higher inter–layer space and inimitable redox features in LDHs, endowing their prominent electro–chemical performances. Currently, LDHs have been extensively applied in chemical sensors, supercapacitors, dye–sensitized solar cells, and photocatalytic water oxidation due to their chemical versatility and flexible open structures.80-82 In addition, different types of LDHs have been widely prepared as OER electro– catalysts.78-79, 83 Among them, NiFe LDHs as catalytic materials have drawn extensive attention because of its remarkable catalytic activities and stability in electro–chemical water oxidation. Candelaria et al.77 prepared bimetallic Fe–Ni NPs by simple multistep based aqueous synthesis procedure. For comparison, they also synthesized monometallic Ni and Fe NPs. From analysis they found that the bimetallic Fe–Ni NPs have a core shell-like structure, with an iron hydroxide core surrounded by nickel hydroxide shell with incorporated iron. In bimetallic system, the iron was found in the form of iron hydroxides and iron oxides nanoparticles, whereas Ni was found in the form α–Ni(OH)2 shells with a small amount of β–NiOOH. In monometallic Ni, the nickel


19

ACS Catalysis

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

Page 20 of 74

hydroxide was in the form of β–Ni(OH)2 and α–Ni(OH)2 with significant amount of metallic Ni. From electro–chemical measurements they found that the Fe–Ni NPs exhibit considerably higher catalytic activity toward the OER as compared to the monometallic Ni and Fe NPs. This result reveals that the incorporation of Fe in Ni(OH)2/NiOOH has a noticeable impact on OER activity as compared to monometallic Fe or Ni NP. Similarly Zhang et al.78 prepared highly effective OER NiFe film in two step electro– deposition method using cathodic electrolysis of aqueous solution of a Ni(NO3)2 and anodic deposition of FeSO4. The OER performance of this NiFe film was surprisingly outstanding, only a potential of 240 mV was required for a current of 10 mA cm−2. The outstanding OER performance of NiFe film was due to the following reasons. First, this material incorporated with FeIII is intrinsically very active for oxidation of water.

In electrochemical processes the

incorporated FeIII is the modulator for the valence of Ni species. On one hand the incorporated FeIII elevates the Ni valence state in Ni(OH)2 which shifts the NiII oxidation to the anodic site, and enhances the driving force for the formation of

intermediate oxidized Ni species for

efficient water oxidation. On the other hand, incorporated FeIII decreases the Ni valence state in NiOOH and thus decreases the required number of electrons for Ni oxidation, which in turn lowers the energy barrier to make active intermediate species of Ni for effective water oxidation. The doping of FeIII also generates the defects surface for the coordination of catalyst. All these features are favorable for the effective water oxidation. Second, the catalyst obtained from stepwise electrodeposition approach has an inter-connected reticular mesoporous structure that allows effective mass transportation during water oxidation. In addition, transfer of charge in the interconnected structure is more efficient than isolated catalyst. To examine the active site of FexNi1–x OOH, Swierk et al. studied systematically the effect of Fe+3 incorporation into NiOOH.79 From activation energy and impedance spectroscopy measurements they found that the catalysis of OER was more energetically favorable on FexNi1−xOOH as compared to FeOOH or NiOOH. The Faradaic resistances of the monometallic catalysts were typically 2 orders of magnitude greater than FexNi1−xOOH, while activation energy of monometallic catalysts for OER was 3 fold higher than FexNi1−xOOH catalysts. Similarly, Trotochaud et al.83 also investigated the effect of Fe in NiFe LDH structure for OER activity. Upon the Fe addition into the Ni(OH)2/NiOOH, more than 30 fold increase in electrical


20

Page 21 of 74

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

ACS Catalysis

conductivity was observed. They further studied to find out that crystallinity of films increases with aging, while the Ni(OH)2 with iron drastically increases the OER activity. Engineering more active sites and larger surface area of LDH is an effective way to enhance the catalytic performance and to reduce the overpotential for OER. For example, Zhang et al. designed Ni–Fe LDH hollow microsphere (HMS) via one step in situ growth technique using SiO2 as a template.84 The resulting Ni–Fe LDH–HMS catalyst possessed high surface area (155.4 m2g−1) and large mesoporous superb hydrophilicity, which can facilitate a full exposure of efficient active sites toward OER. Consequently, the Ni–Fe LDH–HMS exhibited superior OER performance (small overpotential of 239 mV at 10 mA cm−2), excellent stability, and large anodic current (71.69 mA cm−2 at 0.3 V).

Figure 4. (a) Schematic diagram of the exfoliation process for monolayers LDHs. Each single layer is composed of edge sharing octahedral MO6 moieties (M denotes a metal element). Metal atoms: purple spheres; Oxygen atoms: red spheres; Inter-layer anions and water molecules: grey spheres. Hydrogen atoms are omitted. (b) OER activity of several exfoliated LDHs nanosheets, including NiFe and NiCO LDH sheets. The inset shows the zoom-in plot of OER activity for


21

ACS Catalysis

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

Page 22 of 74

better insight of overpotential at current density of 10 mA cm-2. Reproduced with permission from ref 85.

Usually, LDHs adopt a platy or lamellar morphology and can be delaminated or exfoliated directly into single layer nanosheets through numerous soft chemical techniques, leading to overall more active edge sites and high surface area.85 To enhance the water oxidations activity of LDHs, Song group for the first time converted bulk LDHs to single layer nanosheets by liquid phase exfoliation approach (Figure 4a).85 The exfoliated NiFe LDH single layer nanosheets exhibited outstanding OER performance and produced higher current density than their bulk counterparts as well as the commercial IrO2 catalyst (Figure 4b) because a number of active edge sites are produced during exfoliation approach. Though liquid phase exfoliation approach is an effective way to produce single layered LDHs, main drawback of this process is its sophistication, because it requires many steps for more than two days. Therefore, an exfoliation free approach is desirable to make monolayered LDH nanosheets. In this regard, Han et al. prepared monolayer NiFe LDH nanosheets with incorporation of molybdate ion (charge balancing species) within interlayers by simple one step exfoliation free hydrothermal method.86 During the reaction, Fe+3 and Ni+2 in the presence of a molybdate salt were hydrolyzed upon the hydrothermal decomposition of urea. Molybdate ions were expected to be confined within the interlayers of the resulting LDH nanosheets. Compared to their bulk NiFe–LDHs counterparts, these monolayers nanosheets were four times greater in electrochemically active sites and exhibited almost three times better OER performance with small Tafel slope (40 mV dec–1) and long term stability. The authors proposed that the function of Ni species within this electrocatalyst is providing active sites during catalytic reaction, and neighbouring Fe species induce transfer of partial charge to Ni sites, and trigger them for efficient water oxidation. They also determined the number of active sites by approximate redox active method, assuming that the only Ni active sites within this catalyst participate in the water oxidation process. Although some NiFe LDHs have shown satisfactory OER activity, one major problem associated with LDH is its poor conductivity which hampers their practical applications. Researchers have tried to solve this issue by hybridizing it on some conductive materials such as rGO and carbon nanotubes (CNTs). Long and coworkers designed an OER active electro–


22

Page 23 of 74

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

ACS Catalysis

catalyst (FeNi–GO LDHs) from FeNi LDHs cation and negatively charged GO layers.87 The synthesized FeNi/GO LDHs performed outstanding OER activity with low overpotential of 210 mV and small Tafel slope of 40 mV dec–1. When the FeNi/GO LDH was reduced to FeNi/rGO LDH, the OER overpotential further reduced to 195 mV with high TOF (0.98 s–1 at 0.3 V). The superior OER properties of FeNi/rGO LDH resulted from the synergy between the high electro– catalytic activity of double hydroxide and transport of electron arising from the conductive sheets of rGO. Ma et al88 synthesized Ni+2Fe+3/GO nanosheets by homogeneous precipitation technique using hexamethylenetetramine (HMT) and anthraquinone–2–sulfonate (AQS) as hydrolyzing and oxidizing agents, respectively. The resulting Ni+2Fe+3/GO manifested remarkable OER performance (10 mA cm−2 = 0.23V) in 1 M KOH with small Tafel slope (42 mV dec–1). The OER activity was further improved (η10 mA cm−2 = 0.21V) by integrating Ni+2Fe+3 with rGO which has been ascribed to the following two factors: (i) Incorporation of GO prevents the LDHs aggregation and segregates the nanosheets, improving the active electrochemical surface areas, and (ii) The interfacial and direct contact between GO and Ni+2Fe+3 LDH at a molecular level produces a synergistic effect for the water oxidation process. Tang and coworkers89 fabricated a hybrid of NiFe–LDH and framework of nitrogen doped graphene (NGF) via defect anchored nucleation and confined growth approach. NGF was employed as a conductive mesoporous substrate for the growth of nanometer sized NiFe LDHs. The dopant nitrogen and induced defects topology of graphene framework contributed to the anchoring of metal cations and growth of nanometer sized NiFe–LDHs; thereby, NiFe LDHs were uniformly decorated in the framework of mesoporous graphene. As a result, the hierarchical structure with conductive mesoporous networks, intimate inter–facial coupling, suppressed particle aggregation, interconnected electron highway, and entirely exposed active catalytic sites, were responsible to outperform a commercial catalyst (Ir/C), with considerably higher catalytic activities, enhanced kinetics and durability for OER in basic conditions. Meanwhile, for the electrodes preparation, binding polymers (e.g., nafion or polytetrafluoroethylene) are required to hold the OER catalysts material on GC electrodes to increase the number of active buried sites and dead volume.90-91 Moreover, the addition of binding polymer increases the contact resistance at the current collector and the catalyst interface, thereby retarding the transfer of electron.91 Therefore, the fabrication of catalytic electrodes by conventional method has usually led to inadequacies of active materials loading level, poor


23

ACS Catalysis

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

Page 24 of 74

electron transfer between current collector and catalysts and reduction of active sites.90 These problems can adversely influence the catalytic performance, durability and power density during practical applicability. To overcome such problems, it is essential to find a strategy for fabrication of catalytic electrode directly by integrating catalytic materials for OER and current collectors. The design of binder free and self–supported 3D electrodes is a promising alternative.90 In practice, the ink with binder has been loaded directly on metal foam, which exhibits excellent durability and activity.91 However, the in situ growth of catalyst on 3D conductive substrates is expected to be more stable and attractive. This issue has been intensively addressed and studied by many expert groups of this field. For example, Chen and co–workers reported in situ growth of N doped NiFe LDH nanolayers (N–NiFe LDH) on a 3D Ni substrate.90 This electrode exhibited remarkable performance toward OER, affording a very small overpotential of 1.46 mV at 10 mA cm−2, durability operation for >60 h and high Faradaic efficiency of about 98%. Interestingly, the electrode can change its color dynamically to dark black from gray silver during OER operations, and the coloration processes persist for 5000 cyclic voltammetry (CV) cycles, rendering it a valuable tool to monitor the OER catalytic process. The excellent catalytic performance of N–NiFe LDH nanolayer contributed to advanced structural properties of electrode materials such as ultra-thin catalyst, highly doping of N content, and 3D conductive NF framework. Similarly, Lu and Zhao prepared a highly efficient, stable and freestanding OER electrode by electro–deposition of amorphous Ni–Fe hydroxide nanosheets composite on macroporous 3D Ni substrates without binding materials.91 The NiFe/NF electrode exhibits outstanding catalytic activity towards OER; the onset potential was only 200 mV, and a current density of 1,000 mA cm–2 was delivered at 270 mV. The high catalytic performance of the NiFe/NF electrodes was ascribed to several factors: (i) intrinsically high OER performance of the NiFe catalysts, (ii) hierarchically nanoscale porous configuration of the catalyst, which enables huge active working surface area and superb bubble dissipation capability, (iii) binder free one step electro–deposition approach which offers low electrical resistance for water oxidation. The intrinsic activities of NiFe catalyst for OER were further evaluated from TOF. The TOF of NiFe at a potential of 400 mV was 0.075 s–1, which is almost three times higher than that obtained with commercial Ir/C catalyst (0.027 s–1).


24

Page 25 of 74

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

ACS Catalysis

3.3. Nickel Oxide and Hydroxide Based OER. Nickel oxides and hydroxides have been extensively employed in different fields, such as supercapacitors, batteries, catalysis, and so on.92-93 Recently, it has been studied as a catalyst for OER due to its promising catalytic activity and low cost. The electronic, optical, surface and catalytic properties of the nanomaterials are highly dependent on their morphology, porosity, size and surface microstructure. Thus, different techniques have been established to tune the morphology, surface microstructure, and size of nickel oxides/hydroxide for improving their activities for OER.93-94 For instance, ultra–small crystalline NiO NPs were fabricated by a solvothermal reaction in the tert–butanol solvent.92 It was observed that the decrease in particle size led to Ni+3 ions formation on the NiO surface, which is normally associated to the electro–catalytic activities of the Ni-based compounds. Moreover, the increase in surface area was also observed with decreasing dimension of particle, which is favorable for improving the OER activity. Cheng group demonstrated the formation of NiO nanosheets array on carbon cloth (NiO NA/CC) by annealing its precursor.93 This NiO NA/CC 3D electrode was highly active for OER with onset overpotential of 295 mV and Tafel slope of 116 mV dec–1. It afforded a current density of 10 mA/cm2 at over–potential of 422 mV and preserved the durability for 10 h. Similarly, Ng et al.94 examined the combined effects of dopant cerium and supported gold on the OER activity of electrodeposited NiOx films. From optimizing conditions they found that Ni:Ce solutions in the ratio of 95:5 were better in the OER performance. The remarkable enhancement in the OER activity of NiCeOx (95:5 Ni:Ce) was observed when it was supported on a thin layer of gold as compared to that supported on GC. The NiCeOx–Au delivered a current density of 10 mA cm–2 at overpotential of 279 mV. Furthermore, when NiCeOx–Au catalyst was paired with molybdenum catalyst (HER catalyst) in a water electrolyser, it delivered a current density of 50 mA at very small overpotential of 1.5V for continuous operation of 24h. To find the intrinsic activity of NiCeOx–Au catalyst they also estimated the TOF by normalizing the generation of O2 rate to the whole number of existing metal ions in the catalyst, which was 0.080 s–1 at overpotential of 280 mV. The TOF of NiCeOx–Au catalyst was almost double that of NiCeOx–GC catalyst (0.046 s–1). The outstanding performance of NiCeOx–Au was attributed to electronic and geometric effects between Ce dopant and Ni oxide host, which facilitates the local environment of electron, resulting in favorable and moderate binding energies to the OER


25

ACS Catalysis

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

Page 26 of 74

intermediates on the catalyst surface, and also to the strong interactions between the more conductive substrate (Au) and catalyst. Kang et al.95 synthesized monodisperse mesoporous NiO/NiFe2O4 multicomposite hollow nanocages catalyst from simple calcination of Ni3[Fe(CN)6]2 precursors. The mesoporous and hollow structures catalyst delivered a current density of 10 mA cm–2 at overpotential of 303 mV with Tafel slope of 58.5 mV dec–1. Additionally this catalyst showed excellent stability at current 60 mA cm–2 for 12 h. The excellent OER performance of the NiO/NiFe2O4 can be attributed to its unique cage-like mesoporous structure and high surface area, which provides an efficient path for electrolyte diffusion and large contact areas for electrode–electrolyte during application and thus facilitate a huge number of active sites for water oxidation. Similarly Li and Zeng96 reported sandwich-like rGO@CoNiOx (composed of CoNiOx nanosheets on rGO nanosheets) nanocomposite by simple two step strategy. This nanocomposite efficiently combined the immense OER activity of the CoNiOx with high conductive rGO nanosheets. As a result, this catalyst exhibited outstanding performance and durability for OER in both 1.0 M and 0.1 M KOH solution with low overpotentials and high current densities (280 and 320 mV at 10 mA cm−2, respectively), strong stability for about 20 h and small Tafel slopes (42.0 and 45.0 mV dec−1, respectively). Furthermore, they found that the OER activity of this catalyst can be promoted significantly by anodic conditioning which can be credited to in–situ generation of more active catalytic species

(such as metal hydroxide and oxyhydroxides) and oxygen

vacancies on the catalyst surface. The high OER activity of rGO@CoNiOx was assigned to its unique physico–chemical structural properties, such as large surface area, sandwich-like sheet on sheet and thin porous nanosheets of CoNiOx particles and charge conducting rGO. Zhou and coworkers synthesized nanoplates of ultrathin β–Ni(OH)2 and its composites with multi walled CNTs (MWCNTs)

by simple one step hydrothermal method without the use of any

surfactants.97 The composite of β–Ni(OH)2 with MWCNTs exhibited better OER activity in term of over–potential, Tafel slope, exchange current density and high current densities at fixed applied potentials than Ni(OH)2 nanoplates and physical mixture of MWCNTs + Ni(OH)2. The Tafel slope of MWCNTs + Ni(OH)2 composite (87 mV dec–1) was smaller than nanoplates of β– Ni(OH)2 (165 mV dec–1) and their physical mixture(140 mV dec–1). 3.4. Nickel Sulfides Based OER. Recently, Nickel sulfides (NiSx) have drawn significant attention due to its superb electrocatalytic/electrochemical application, and therefore, it has been


26

Page 27 of 74

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

ACS Catalysis

applied widely in energy storage and conversion devices such as batteries, supercapacitors, solar cells, HER, and ORR.98-99 In addition, Ni-based sulfides also have excellent catalytic activity for OER. For example Zou group reported nanosheets of Ni3S2 catalyst on NF with outstanding OER activity and stability.

98

Similarly, Sun group has fabricated iron–doped Ni3S2 on NF with

superior stability and very small over–potential of 253 mV at 100 mA cm–2.99 Fang et al.100 fabricated Ni promoted cobalt disulfide nanowire (Ni2.3%–CoS2) supported on carbon cloth by two step hydrothermal protocols. The Ni2.3%–CoS2/CC was efficient catalyst toward OER with small overpotential of 370 mV at high current density of 100 mA cm–2. Ouyang et al.101 prepared porous Ni3S2 nanorods supported on NF by simple hydrothermal method. The direct growth of Ni3S2 nanorods on NF offered strong interactions between Ni3S2 and NF, leading to better electron transport and durability during long time operation. The catalyst exhibited high catalytic activity toward OER which generated a current density of 10 mA cm–2 at overpotential of 217 mV. The outstanding OER activity of Ni3S2/AT-NF can be attributed to the following reasons. Firstly, Ni3S2 nanorods are standing very closely to each other with a length of 1 to 2 mm, due to which it provide fast and facile channels for electrolyte diffusion and electron transport . Secondly, the unique porous structure of catalyst provides large electrochemical active surface area, which can expose large catalytic sites for water oxidation process. Thirdly, the growth of Ni3S2 nanorods on NF provides a strong interaction between the Ni3S2 and NF, which can boost the OER activity and long term durability. Chen et al. developed a facile hydrothermal approach to grow nickel sulfide (NiS) nanosheets directly on stainless steel mesh.102 These nanosheets form a homogeneous film with thickness of ∼280 nm on the steel. The as synthesized catalyst demonstrated higher activity for OER with an overpotential of 297 mV and small Tafel slope of 47 mV dec−1. This promising activity was attributed to the direct contact between the active NiS nanosheets and the extremely conductive stainless steel, which provided an efficient pathway for electron transfer. The high OER activity of the catalyst can also be attributed to NiS nanostructure which can provide a correct exposure of catalytic Ni centers. Such a consequence was also supported by electrical impedance spectroscopy (EIS) studied, which suggests that the NiS@SLS exhibits smallest charge transfer resistance and largest number of catalytic sites during water oxidation process compared to other reference samples. Moreover, during water oxidation process the NiS undergoes a transition from oxidation state of Ni(II) to Ni(III) which could probably work as active metal center on the


27

ACS Catalysis

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

Page 28 of 74

catalyst surface for catalytic applications. Furthermore, the water molecules adsorbed on the NiS surface would probably lead to the hydrated NiS formation, which could also act as additional catalytic active sites for oxidation of water and will boost the OER performance of the catalyst. Dong et al.103 synthesized nickel–iron sulfides supported on NF (NiFeS/NF) as an OER electro– catalysts by a facile two–step process involving electrochemical and hydrothermal deposition. The NiFeS/NF was investigated for OER in basic media, which showed high activity and stability with Tafel slope of 45.4 mV dec–1. You and Sun104 synthesized hierarchically porous nickel sulfide (h–NiSx) supported on NF by template free electro–deposition and low temperature sulfurization method. The hierarchically porous structure of NiSx with 3D arrangement offered enormous interfacial area, appropriate charge separation and gas transport. Moreover, mesopores and macropores of the h–NiSx provided highly active and efficient sites for the catalytic reaction. With such a desirable properties, this h–NiSx displayed outstanding OER activity (180 mV at 10 mA cm−2) and durability in basic electrolyte (1.0 M KOH). To understand the outstanding OER activity of h–NiSx catalyst, its electrochemically active surface area (ECSA) was evaluated from the electro-chemical double layer capacitance (C dl) by checking its voltammograms in the potential range of 1.0 to 1.1 V (non-Faradaic region) versus RHE. The obtained C dl of h–NiSx catalyst was ≈54 mF cm−2, which suggested that with complementary mesopores and macropores it also possessed much higher ECSA that permitted more dynamic accessibility of the active catalytic sites toward water oxidation. Furthermore, 3D hierarchically inter-connected macroporous structure of h-NiSx would also facilitate the mass transport and gas diffusion, collectively improve the OER activity. 3.5. Nickel Selenides Based OER. In recent times, the effective OER activity was attained with Ni oxide and hydroxide. However, the electronegativity of selenium (2.55) is much smaller than that of oxygen (3.44). On the basis of this basic information, it is possible that nickel selenides would have higher electrical conductivity and catalytic activity than nickel oxide. For example, Xu et al.105 compared the OER activity of four Ni-based material (Ni, NiO, Ni3Se2 and NiSe). From the experimental results they demonstrated that Ni3Se2 displays better catalytic activity for water oxidation as compared to other selected catalyst. The experimental OER activity of Ni3Se2 was much better than that of Ni, although the results of DFT calculation suggested that the Ni conductivity is larger than that of Ni3Se2. This anomaly resulted from the diverse oxidation surface between Ni3Se2 and Ni. In case of pure Ni metal, the Se incorporation


28

Page 29 of 74

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

ACS Catalysis

in to the framework of Ni will cause some degree of atomic displacement and alteration of bond length and coordination numbers. Thus from oxidation surface reaction viewpoint, longer bond lengths and lower coordination numbers would weaken the Ni−Ni bond strengths in Ni3Se2 and will facilitate the partial oxidation surface (forming catalytically active OER species) during the water oxidation process. Consequently, for Ni3Se2, it is more facile to generate active species on the surface of catalyst in comparison to pure metallic Ni, thereby possessing higher capability for the catalysis of OER as compared to other synthesized catalyst. Similarly, Wu et al. grew hexagonal Ni0.85Se on graphite substrate by simple electro– deposition method.106 The Ni0.85Se catalyst had superior durability and catalytic activity for OER in basic media. It achieved a 10 mA cm–2 of current density at overpotentials of about 302 mV. The excellent OER activity of Ni0.85Se catalyst can be attributed to the vacancies of metal in their hexagonal structure. Shi et al. investigated the OER performance of Ni3Se2 film deposited on Cu foam (Ni3Se2/CF).107 This electrode showed high OER activity of 50 mA cm−2 at only 340 mV of potential.

Figure 5. (a) The powder X-ray diffraction patterns of Ni3Se2 electrodeposited on Au substrate for different time periods. The inset shows the crystal structure of Ni3Se2 (Ni in blue; Se in red). (b) XPS spectra of the catalyst showing the Ni 2p peaks. The inset shows peaks corresponding to Se 3d. (c) LSV for catalysts electrodeposited at different time periods in N2 saturated 0.3M KOH solution at a scan rate of 10 V s–1. The inset shows the Tafel slope. Reproduced with permission from ref 108. Swesi and coworkers investigated the nickel selenide (Ni3Se2) as a potential OER electro– catalyst in basic solution for the first time.108 Ni3Se2 was electrochemically deposited on the conductive Au substrate from the solutions containing 10 mM Ni(CH3CO2)2.4H2O, 25 mM LiCl


29

ACS Catalysis

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

Page 30 of 74

and 10 mM SeO2 at constant potential of –0.80 V (vs. Ag/AgCl) in different time of period (40, 130, 300, and 600 s). It was found that the crystallinity of the film was enhanced with the increasing of deposition time as revealed by the increasing peak intensity of the powder X-ray diffraction (PXRD) (Figure 5a). Elemental compositions of the catalyst were also examined through X-ray photoelectron spectroscopy (XPS), which contains two main peaks at 852.6 and 869.8 eV, assigned to Ni 2p3/2 and Ni 2p1/2 in Ni3Se2. The spectra of the film deposited in 600 s also contain two weak intensity satellite peaks of Ni 2p3/2 and Ni 2p1/2 for NiSe at 856.1 and 873.6 eV (Figure 5b). HRTEM image of the film showed its crystalline nature with lattice spacing of 110 corresponding to Ni3Se2. To study the OER activity of Ni3Se2, LSV measurements were performed in N2 saturated KOH (0.3 M) at a sweep rate of 10 mV s–1. The OER activity of the Ni3Se2 (Figure 5c) increased with the increase in loading amount of catalyst. However, after some limit of the film thickness the decrease in OER activity of Ni3Se2 was observed, because some of the active sites were blocked with the overgrowth and multilayers of the catalyst, thereby, decreasing the access of the water molecules to the active catalytic sites.

Li et al. synthesized the NiSe@NiOOH core-shell by solvothermal selenization and electrochemical oxidation (ISEO).109 The OER performance of NiSe@NiOOH/NF was investigated, which generated a current of 50 mA cm−2 at potential of only 332 mV with long time stability. The high performance of OER can be ascribed to a large number of electrochemical active sites derived from the NiOOH shell and the stability and good conductivity obtained from NiSe core. Also the synergistic effect between NiOOH shell and NiSe core can be contributed to high OER activity. Similarly, Wang et al.110 fabricated nickel– iron selenide nanosheets ((Ni0.75Fe0.25)Se2) on carbon cloth as an electrocatalyst for OER using solvothermal and hydrothermal approaches. The catalyst exhibited high OER activity with very low overpotential of only 255 mV at 35 mA cm−2 with Tafel slope of 47.2 mV dec−1 and high durability during a 28 h. The high performance of the prepared nanosheets for OER can be assigned to the porous structure, high conductivity, and large active electrochemically surface area of the catalyst derived from incorporation of Fe and Se. The porous nanosheets like structure of the synthesized catalyst contains enormous number of small size particles, which can be exposed fully to the water oxidation with high catalytic surface area. Meanwhile, it can induce the release of oxygen bubbles from the surface of electrode and prevent them from harming and


30

Page 31 of 74

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

ACS Catalysis

clustering the catalyst. During the water oxidation process, the Fe2+ and Ni2+ in the synthesized catalyst are oxidized to higher valence oxidation state. It probably produces Ni1−xFexOOH on the catalyst surface, which assists as the active material for water oxidation. Also the high conductivity of (Ni0.75Fe0.25)-Se2 has been attributed to the high ability of electron transport among reactive sites of the catalyst and electrode. Chi et al.111 also designed a nickel-iron diselenide ((Ni0.5Fe0.5)Se2) catalyst supported on carbon fiber cloth (CFC) by two step process. Firstly, Ni0.5Fe0.5 nanosheets were

electrochemically deposited on CFC and converted to

(Ni0.5Fe0.5)Se2 by a direct selenization process. (Ni0.5Fe0.5)Se2 was investigated for OER activity, which outperformed the commercial Ir/C catalysts. They also checked the OER activity of pure FeSe2 and NiSe2 and were found that they exhibited very low activity for water oxidation as compare to Ni0.5Fe0.5Se2. The high OER activity of Ni0.5Fe0.5Se2 can be attributed to the synergistic effect between Fe and Ni in the synthesized catalyst. The Ni0.5Fe0.5Se2 have an apparent oxidation peak at potential of 1.43 V vs. RHE, which can be ascribed to surface oxidation reactions of Ni from low valance state to high oxidation state (Ni2+ to Ni3+). The obtained high oxidation Ni3+ can be regarded as the active center for water oxidation. 3.6. Nickel Phosphates and Phosphides Based OER. Phosphorus base materials with TM (TM–PS) represent a wide range of class from TM phosphorus carbon (TM–P–C) to TM phosphate (TM–Pi), phosphonate and phosphide (TM–P).112-113 These TM–PS materials have attracted considerable interest for energy storage and conversion technologies due to their high electro–catalytic efficiency and stability.113 TM–Pi and TM–P are explored as efficient catalysts for HER with high stability and activity at a wide range of pH 0–14.114-116 Several studies have also proved that these catalysts (TM–Pi and TM–P) display prominent OER performance in basic media.117-121 For example, Yu et al. synthesized carbon coated nickel phosphide (Ni–P) by facile co–precipitation and phosphidation technique.112 In a typical synthesis protocol, first Prussian blue analogue (PBA) Ni(H2O)2[Ni(CN)4] were synthesized by co–precipitation method. In the second step, PBA was converted to porous Ni–P plates by low temperature phosphidation using NaH2PO2 as a phosphide source. The morphology of the as synthesized catalyst were checked by HR-TEM, and found that the nickel phosphides were covered with 2 to 3 nm thick amorphous carbon layer. The carbon coated Ni–P nanoplates exhibited BET surface area of 35 m2 g-1. To get insight in to the water oxidation mechanism of Ni-P, the XPS analysis of Ni-P before and after stability experiment were performed. The XPS spectrum of Ni2p after water


31

ACS Catalysis

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

Page 32 of 74

oxidation exhibited shift from 855.6 eV to 856.4 eV, which can be assigned to Ni oxidized species. The binding energy peaks of P2p were located at 129.1 eV (lower binding energy peaks) and 133.6 eV (can be assigned to the oxidized phosphate species), which further confirm the oxidation of Ni-P. The Ni–P exhibited higher OER activity in 1.0 M KOH, yielding a current density of 10 mA cm–2 at low overpotential of 300 mV. The outstanding OER performance of the Ni-P nanoplates can be ascribed to the following aspects. First the amorphous carbon incorporated in to Ni-P obtained by the MOF-assisted method, boost the charge transfer capability and conductivity. Second, the porous structure and uniform morphology of the catalyst can maximize the exposure of active catalytic sites for electrocatalytic reactions and facilitate the reactants diffusion in the electrolyte. Third, the in situ oxidized Ni species on the Ni-P surface can act as active center for catalytic reactions of OER. Ledendecker et al.113 investigated nickel phosphide nanosheets (Ni5P4–NS) as an electro–catalyst for OER in alkaline environment. Ni5P4–NS was synthesized by simply heating red phosphorus and NF at 550 °C for 1h under continuous flow of inert gas. The OER catalytic activity of Ni5P4–NS was evaluated in 1M KOH, which was better in performance and stability than the commercial catalyst with Tafel slope 40 mV dec–1.

The OER activity of Ni5P4–Ni2P–NS and their morphology, composition and

crystallographic alteration during OER process were reported by Wang group.122 They demonstrated that NiP NS would convert into NiO/Ni(OH)x, thus making a Ni–P/NiO (Ni(OH)x) core shell that could improve the OER performance during water oxidation. Highly ordered mesoporous cobalt–nickel phosphides (CoNiP) catalyst was synthesized using hard templating method by Fu et al.121 The OER activity of CoNiP was examined and compared with RuO2 catalysts in 1 M KOH solution by LSV. The Co3Ni1P (3 and 1 indicating the molar ratio of Co and Ni during synthesis process) exhibited efficient OER performance with low overpotential of 220 MV, which was much lower than the commercial RuO2 catalyst. The superior OER activity of Co3Ni1P can be attributed to the porous structure and phosphates group which facilitate the transport of electron and proton during the catalytic application. Similarly, He et al. designed nanocomposite nanoboxes of Ni–Co mixed phosphides and carbon (NiCoP/C nanoboxes) from MOF.118 The NiCoP/C exhibited enhanced OER performance with overpotential of 330 mV at 10 mA cm−2 with Tafel slope of 96 mV dec–1 and durability of about10 h. The OER activity of NiCoP/C nanoboxes can be attributed to the following factors. First, a synergistic effect of bimetallic phosphides in the composite may enhance catalytic


32

Page 33 of 74

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

ACS Catalysis

activity for OER and the in situ derived carbon is expected to promote the efficiency of charge transfer. Second, these nanoboxes with high porous structure can offer more catalytic active sites during applications. Moreover, the unique hollow structure with large surface area can endow more contact area between electrode and electrolyte. Zhan et al.117 investigated the origin of Ni(II) phosphate (NiPi) based OER catalytic activity. From XPS measurements and elemental mapping they suggested that with potential CV cycling, NiII of NiPi was oxidized with the loss of phosphate ion, due to which the NiPi was progressively converted to a more OER active Ni(OH)2/NiOOH form. Another important conclusion of this study was the effect of Fe incorporation into NiPi on the OER activity. From the results they found that with the incorporation of Fe the OER activity was enhanced without any adverse effect on durability, thereby making it better candidate than nickel–iron oxide and hydroxide. Li et al. studied the synergistic effect of iron on the OER activity of nickel phosphate.120 They used two–step approach involving hydrothermal and electrochemical deposition for the synthesis of iron doped nickel phosphate on NF (Fe–NiPi/NF). For comparison, they also synthesized NiPi electrode (NiPi/NF) without incorporation of iron. The OER performances of as prepared electrodes were evaluated in 1 M KOH solution, and found that the Fe–NiPi/NF electrode exhibited outstanding OER activity and stability, delivering a current density of 10 mA cm−2 at 220 mV and extremely high current of 500 mA at 290 mV. The OER performance of Fe– NiPi/NF electrode was much better than the NiPi/NF and commercial catalyst. The outstanding activity of Fe–NiPi /NF electrode for OER can be ascribed to (i) the synergistic effect among incorporated Fe and NiPi, (ii) excellent transport of charge due to proper design of catalyst, and (iii) the macroporous structure of 3D NF which affords a large number of active sites, transport of effective mass, and dissipation of gas bubbles at high current density. A series of NiPi single walled nanotubes with different lengths and diameters were fabricated by controlling the ions of alkali metal such as Na+, K+, Li+ and Cs+.119 The tubes were named as: nickel phosphate Li– nanotubes (NiPLi–NTs), nickel phosphate K–nanotubes (NiPK–NTs), nickel phosphate Na– nanotubes (NiPNa–NTs), and nickel phosphate Cs–nanotubes (NiPCs–NTs). From the TEM images, they observed narrower and longer nanotubes of Ni phosphate by using ions of alkali metal with a bigger radius. The electrostatic interaction among initial stage formed building blocks and the ions of alkali metal is confirmed to determine the lengths and diameters of the resulting nanotubes. Besides, Fe and Co dopants can also alter the composition and diameter of


33

ACS Catalysis

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

Page 34 of 74

the nanotubes and boosting its OER performance. After Fe and Co doping, the overpotential of NiPNa –NTs (best sample) at current density of 10 mA was 300 mV with Tafel slope of 56.3 mV dec–1. In this part we summarize the key developments and progress of recently reported Ni-based OER catalyst including NiFe LDHs, nickel oxide/hydroxide, nickel phosphates/phosphides, NiSe and NiS. There has been immense curiosity in Ni-based catalyst for OER, since it generally exhibits superb catalytic abilities and durability for oxygen evolution. However, the main drawback of the most OER electrocatalyst is usually their poor conductivity and long term durability. Designing of highly stable, cost–effective, efficient and environmentally friendly catalyst for OER electrocatalysis is desirable. In the light of this review, effective OER electrocatalyst should be excellent electroconductor and have moderate energy of adsorption toward oxygen intermediates on the electrocatalyst surface. Better redox centers (ability to gain or lose to electrons) can be critical for the effective OER catalysis and for high catalytic ability more active electrocatalytic sites would be desirable. Furthermore, it is essential for electrocatalysts to possess the capability to resist corrosion during application particularly under large anodic potential. 3.7. Ni-MOF based OER. Attributing to the generally high specific surface areas and the coupling units (metal ions and metal-oxo units) in MOFs, they serve as both homogeneous and heterogeneous catalytic sites. Besides, being highly crystalline, MOFs are easily recyclable and robust under various chemical and physical conditions. Unfortunately, the low mass permeability, low conductivity, and blockage of active metal centers by organic ligands, are the biggest limitation of MOFs based electrocatalysts. Zhao et al.123 reported the synthesis of ultrathin nanosheets of NiCo bimetal organic framework from a mixed solution of Ni2+, Co2+ and benzenedicarboxylic acid. Thus generated nanosheets exhibited high electrocatalytic activity for the OER after being deposited over copper foam with a low onset potential of 1.39 V and an overpotential of -189 mV at current density of 10 mA cm-2 in alkaline medium. The higher OER efficiency of bimetallic MOF system in comparison to the monometallic systems were ascribed to the ultrathin surface metal state bearing coordinatively unsaturated metal sites and synergistic interactions between two metals. The deposition of bimetallic NiCo system on Cu foam further enhance the catalytic activity owing to excellent conductivity and electron transfer ability of the copper substrates. In another similar example, He et al.124 obtained 2D Co-based monometallic


34

Page 35 of 74

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

ACS Catalysis

and bimetallic hydroxides as ultrathin nanosheets from Co-based MOFs using simple liquid decomposition method at room temperature. Amongst unltra nanosheets of monometallic Co, Co/Zn, and Co/Ni hydroxides, Co/Ni based nanosheets exhibited the best catalytic activity towards OER with overpotentials of 324 and 372 mV at current densities of 10 and 100 mA cm– 2

. Besides, it showed large turnover frequency of 0.16 s–1 at η = 380 mV and a small Tafel slope

of 33 mV dec–1. Other than synergistic interactions between Co and Ni, the exposure of a larger number of unsaturated active catalytic sites on the edges of nanosheets of Co-Ni hydroxide could be responsible for its higher intrinsic OER activity. In another example based on bimetallic system driven from MOF, Jiang et al.125 reported synthesis of Fe-Ni oxide architectures from corresponding Fe-Ni aminoterephthalate MOFs as self-sacrificial templates with various Fe/Ni ratios. The OER active composites have strong dependency on the initial ratio of Fe and Ni, and exhibited best catalytic properties corresponding to Fe0.5Ni0.5Ox. XRD analysis showed the presence of coexistence of the spinel NiFe2O4 and NiO in highest perfomring catalyst Fe0.5Ni0.5Ox. Thus optimized catalyst provided the lowest onset potential of -1.63 V (vs. the RHE) and smallest overpotential of -584 mV at a current density of 10 mA cm_2, in which NiFe2O4 served a dominating role in increasing overall OER activity. Han et al.126 pyrolized Ni-MOF at 600 ºC under nitrogen atmosphere to get Ni NPs with diameter of 5-8 nm embedded on nitrogen doped CNTs. Thus, the obtained meso/microporous catalyst exhibited excellent OER catalysis with Tafel slope of 106 mV dec-1 for overpotential of 0.46 V at current density of 10 mA cm-2 which was reportedly 160 mV lower than that of commercial RuO2. Yang et al. also used similar pyrolysis approach in which MOF of Ni-vanillic thio-semicarbazone (NiL2, L = C9H10N3O2S) was decomposed at 500 ºC in a NH3 atmosphere to yield the NiS@N/S-C hybrid. The obtained hybrid demonstrated the excellent OER electrocatalysis in alkaline medium, showing a low onset potential of 417 mV to deliver a current density of 10 mA cm-2 with high durability of 10h. 4. HER ELECTROCATALYSTS 4.1. Nickel-Based HER. Currently, the researchers have been consistently inspired to utilize the Ni-based electrocatalysts in clean energy producing reactions as an alternative to Pt due to (i) similar chemical properties, (ii) same group number in periodic table, and (iii) cheap and abundant quantity of Ni. Hydrogen evolution is a half challenging part of the full water splitting,


35

ACS Catalysis

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

Page 36 of 74

a well–known reaction for clean energy production, which is mostly achieved commercially by well standardized but very expensive noble metal (e.g. Pt) based catalysts.127-128 Here, we will cover the most recent researches on HER by using Ni-based electrocatalysts in the alkaline and acid media. It has been found that the HER proceeds via either Volmer–Tafel or Volmer–Heyrovsky mechanism (eqs. 1–5) depending on the electrolyte’s pH.129-131 The Volmer step in acidic electrolyte involves reduction of the proton, whereas the reduction of a water molecule is the key step of mechanism for hydrogen evolution in alkaline solution (eqs. 1, 2). In acid solution there are two possibilities for the second step following either Tafel or Heyrovsky (eqs. 3, 4). Following Heyrovsky mechanism, one more OH– and one H2 molecule will be formed in alkaline electrolyte. H+ + e– → Had

(Volmer step in acid solution)

……....(1)

H2O + e– → Had + OH–

(Volmer step in alkaline solution)

……....(2)

Had + Had → H2

(Tafel)

……....(3)

H+ + Had + e– → H2

(Heyrovsky)

……....(4)

H2O + Had + e– → H2 + OH– (Heyrovsky)

………(5)

The Tafel mechanism is same in alkaline and acidic media. However, aforementioned equations show that in alkaline electrolyte, the HER process involves an HO–H dissociation reaction (Volmer step) prior to joining two adsorbed hydrogen atoms which proceeds two orders of magnitude slower than in acidic media.132-134 Finding the optimized chemisorption energy of hydrogen Had on the surface of electrode materials can give a guidance on selecting proper HER catalyst. For all HER TM catalyst, the volcano plot (Figure 6a) is a feasible way to predict the possible activity of a new composition.135 The incorporation of nanostructured Ni derivatives into state of the art materials can effectively increase the electrochemically active surface area of the binary or ternary alloy materials to enhance the HER catalytic activity in alkaline as well as acidic media. Specifically, well-established Pt, metallic dopants like molybdenum and iron have a considerable role in reducing the overpotential and enhancing catalytic activity of Ni-based materials by tuning the native electronic structure of catalytic surface. Beside metal oxides and hydroxides, many researchers are focusing on using nonmetals to manipulate metal’s structure to achieve higher surface area. C, O, N, P and S are among most studied nonmetal elements that have ever been used for HER. For example, Gong et al.136 used NiO/Ni–CNT composite as a


36

Page 37 of 74

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

ACS Catalysis

HER catalyst in alkaline media. The as synthesized catalyst exhibited 10 mA cm–2 current density at ~81 mV overpotential. The role of Ni has been described as an efficient species to expedite the H adsorption step, whereas NiO played the role of preferential absorbing region of the OH– produced by Ni sites. The positive charge as well as higher number of empty d orbitals of Ni2+ as compared to metallic Ni is the origin of the stronger electrostatic affinity of NiO for OH– ions. The oxidized CNT is a substantial substrate for growing NiO/Ni heterostructure. The synthesis of NiO/Ni without CNT support led to an aggregated Ni particles in a plate–like morphology with very poor HER performance (Figure 6b–d).136 a

c

b

d

Figure 6. (a) Trasatti’s HER volcano plot (Reproduced with permission from ref

135

). The

exchange current in y axis plotted vs. the energy of the intermediate metal–hydrogen bond strength on the x axis. (b) A schematic illustration of the synthesis process of NiO/Ni with and without CNT support (c) Bright-field scanning transmission electron microscopy (BF-STEM) image showing the typical morphology of the CNT-free Ni nanoplate and the corresponding reconstructed maps from the highlighted area with Ni in red and O in green. Only a very small amount of NiO particles is observed on the surface. Scale bar, 20 nm. (d) Linear sweep voltametry of NiO/Ni–CNT hybrid and CNT–free Ni nanoplate in 1M KOH at a scan rate of 1 mV s–1 under the loading of 0.28 mg cm–2 on RDE. Reproduced with permission from ref 136. 4.2. Nickel-Platinum Alloys Based HER. As Pt based catalysts have been established to show the highest exchange current density with the average chemisorption energy of hydrogen on its surface, the efforts to achieve the high surface areas of these materials can significantly reduce


37

ACS Catalysis

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

Page 38 of 74

the consumption of Pt. The use of Ni in the Pt based structure to improve its surface area and active sites is another practical method which has been used by many researchers.137 Qian et al. successfully enhanced the surface area of Pt by incorporating 3D porous Ni which enhanced the HER activity of Pt in comparison to that of commercial Pt/C. Higher surface area acquired by Pt through Ni porous structure improved the HER performance to reach 9.47 mA cm–1 at an overpotential of 45 mV, whereas loading of same quantity of Pt on the smooth Ni surface with lower surface area did not show any significant improvment in its HER activity. This confirmed the role of small sized Pt particles with greater surface area in providing better electrocatalytic activity toward HER.137 Oh et al.138 synthesized Cartesian–coordinate–like hexapod shaped Pt structure. Initially they synthesized core–dual shell Pt@Ni@Co nanostructures followed by removal of Ni@Co shell. The active nature of the synthesized catalyst (22 mV overpotential to drive current density of 5 mAcm−2) was attributed to hexapod structure of the catalyst and high percentage of Pt. The role of Ni:Co ratio in the Pt@Ni@Co nanostructure was explained by its template effect on the shape as well as size control of the catalyst. The increase in percentage of Co can favor the formation of hexapod structure by simultaneously reducing its size. The ratio of Ni is another major influencing parameter in HER activity to accelerate water dissociation step.139 Although Ni has been known to have a similar H atom binding energy as that of Pt, Ni has never been counted as a good HER electrocatalyst in alkaline media due to its weak ability to desorp OH– species from the surface of catalyst which blocks the active sites to obstruct further water dissociation.140 Neverthless, it is well established that the oxides and hydroxides of TMs are effective materials for breaking the HO−H bond and desorbing OH– group.136, 141-146 Kavian et al.147 reported that the high HER performance of Pt–Ni octahedral nanocrystals with Ni(OH)2 (7.68 mA cmPt–2 at 50 mV overpotential in 0.1 M KOH) was due to Ni(OH)2 in the Volmer step to facilitate dissociation of water and generation of Had on the surface of Pt (111). The reported mass activity of the catalyst (1.81 A mgPt–1) was 4.6 times higher than that of commercial Pt/C (0.39 A mgPt–1).147 Wang et al.148 reported an analogous work about HER activity of a Ni(OH)2 assisted Pt/C catalysis. Multilayered (ML) Ni(OH)2–(50 wt %)–Pt/C reduced HER overpotential by 39 mV in comparison to Pt/C. Further increasing ML Ni(OH)2 facilitated the water dissociation step in Volmer step, however contrarily blocked the active sites of the Pt surface and decelerated Had recombination. On the other hand, only 20 wt% of single layered (SL) Ni(OH)2


38

Page 39 of 74

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

ACS Catalysis

on Pt/C achieved 5 mA cm–2 current density at about 42 mV lower overpotential as compared with Pt/C. This indicates that the influence of SL Ni(OH)2 is greater than that of ML Ni(OH)2. However, the further increase in amount of SL Ni(OH)2 reduced the total HER activity for same reasons as for ML Ni(OH)2. The electrochemical surface area vs. current density plot showed that in both catalysts, the best current density has been achieved at 60% of Electron Spectroscopy for Chemical Analysis (ESCA), which was achieved by the lower amount of SL Ni(OH)2 (20 wt %), as compared to ML Ni(OH)2 (50 wt %). With the same amount of both SL and ML Ni(OH)2, SL Ni(OH)2 can boost the speed of Volmer step more than ML Ni(OH)2. Further increase in the amount of Ni(OH)2 started to show adverse effects on HER by contaminating catalytic sites. Wang et al. synthesized Pt3Ni/NiS heterostructured catalyst which showed a current density of 37.2 mAcm–2 at an overpotential of 70 mV, 9.7 times higher than that of the commercial Pt/C.149 Similar to Ni(OH)2, the role of NiS was explained to facilitate Volmer step in the dissociation of H–OH bond. DFT calculations have shown that the energy barrier for the dissociation of H–OH in water molecule is about 0.89 and 0.32 eV on the surface of Pt (111) and NiS (100), respectively. Furthermore, the calculated Gibbs free energies of hydrogen adsorption on the surface of different compositions indicated that NiS cannot be a good surface for combining Had to form H2 because Had strongly binds to the NiS surface and hinders H2 production. Since the calculated free energy for the surface of Pt3Ni was very similar to the surface of Pt (111), the Tafel or Heyrovsky step can easily proceed on the surface of Pt3Ni. It has also been experimentally shown that high concentration of K+ in solution plays an important role in stimulating hydrogen production since the presence of hydrated cations can enhance the production of Had by destabilizing H–OH bond.134, 149 The alkaline solution is an appropriate electrolyte for HER process due to better stability of the catalyst at higher pH. However, the slower reaction in alkaline medium as compared to the acidic media should be improved by different strategies like combining the compensating TMs to boost the reaction speed in different directions. Oxides of TMs are efficient components for removing the excess hydroxyl ions from the catalyst surface to liberate the blocked active sites. TMs exist in a different slope of the Volcano plot, which has influenced the activity of emerged catalyst. Increase in surface area of the active catalyst is another assured method to gain better catalytic activity, whereas NF network is known to assist in enhancing the electroactive surface area to improve HER activity.


39

ACS Catalysis

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

Page 40 of 74

4.3. Nickel-Molybdenum Alloys Based HER. The volcano type plot, where the current densities measured for cathodic release of hydrogen at a given overpotential are plotted with respect to the metal-H binding energy, introduced a new possibility of metal based catalytic composition (Figure 6a). Based on hydrogen binding energy, the optimal composition of Mo vs. Ni could show enhanced HER performance. Wang et al. electrochemically deposited Ni–Mo on the Cu foam to make 3D nanoporous electrocatalyst. The fabricated composite on the surface of copper foam showed high current density of 20 mA cm–2 at a small overpotential of 34 mV.150 Nickel–Molybdenum nitride nanosheets were synthesized by Chen et al. to study the HER activity in acidic medium. The overpotential was 78 mV (vs. RHE) better than Ni free analog, i.e., Mo appended nitrogen doped carbon. The relatively better activity of the catalyst was explained by the synergetic effect between Ni and Mo. The near edge X-ray absorption fine structure (NEXAFS) analysis clarified that the Ni–Ni/Ni–Mo distance was more/less than normal bond distance, respectively. The decrease of the Ni–Mo distance downshifted the d–band center of the Mo atoms in terms of Fermi level and thus reduced the hydrogen binding energy which was facilitating the Had recombination step. Moreover, the relatively higher stability of NiMoNx catalysts (a similar polarization curve after 2000 voltammetric cycles) as compared to NiMo is referred to the stabilizing effect of nitride. 151 Despite the high electrochemical activity and tendency to increase active surface area, nickel suffers from instability in acidic solutions, and thus is usually not considered suitable for the electrocatalytic HER in acidic media. However, Chang et al.152 grew graphene over nickel foam to protect it from oxidative corrosion, and utilize it as substrate to enhance the weaker catalytic activity of MoSx by developing strong 3D electrocatalytic architecture, where the MoSx is effectively grown through thermolysis. The current density for the synthesized catalyst operated at 0.2 V is around 45 mA cm−2. The advantage of 3D Ni foam as an electrode is (i) the higher MoSx catalyst loading over Ni foam in comparison to other carbon-based electrodes such as carbon paper, carbon cloth, and graphite mats, (ii) the highest current density exhibited by MoSx grown on graphene covered Ni foam, and (iii) the low resistance of electrode to result in higher electrochemical performance. Incorporating of Ni in Mo2C nano–rods, Xiao et al. successfully enhanced the HER onset potential.153 Wang et al. explained the role of Ni in the Ni–Mo2C hollow structure, with the synergetic effects between Ni NPs and molybdenum carbide.132 As it is predictable from


40

Page 41 of 74

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

ACS Catalysis

Volcano plot, the produced hydrogen atoms in the Volmer step binds strongly to the surface of molybdenum carbide, which prompts the first step reaction and in contrary decelerates either the Heyrovsky or Tafel step. Introducing Ni with weaker hydrogen binding energy can reduce the binding strength of Had to the surface of Mo atoms and improve HER activity of the material. This observation was explained by the electron transfer from molybdenum carbide to neighboring Ni atoms. The synergistic effect contributed more positive charge to Mo atoms and more negative charge to Ni atoms, which in turn decreased the Gibbs free energy of hydrogen bound to the surface of molybdenum carbide to facilitate its desorption The later studies by Popczun et al. showed that despite the lower initial overpotential of NiMo nanopowder than that of Ni2P, the activity of Ni-Mo nanopowder drops relatively much faster in acidic medium.154 The factor behind the reported 130 and 180 mV overpotential at 20 and 100 mA cm–2 current density of the Ni2P catalyst was the enclosed catalyst on (100) surface in which the presence of both proton and hydride–acceptor centers in the surface facilitates the HER activity. It has been implicated that the increase in the amount of phosphorous in the NiP composition can improve the HER activity as well as stability. The less positive charge of Ni as well as the reduced number of exposed Ni active sites in the presence of P justifies the better HER activity and stability.155-156 In addition, the doping of a catalyst with different cations or anions customized its electronic structure and optimized the hydrogen adsorption energy, which eventually improved its HER activity.157-158 Long et al. improved the HER activity of NiS catalyst by adding Fe which facilitates the Heyrovsky step by decreasing the energy barrier and increases the released energy for H2 production on the surface of NiFeS electrocatalyst.159 a

b


41

ACS Catalysis

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

Page 42 of 74

Figure 7. (a) The polarization curves recorded on MoS2(1–x)Se2x/NiSe2 foam hybrid, MoS2/NiSe2 foam hybrid and pure NiSe2 foam electrodes compared with a Pt wire. (b) LSV curves of HCl– Ni@C, A–Ni–C and Pt/C. Reproduced with permission from ref 160. Zhuo et al.160 enhanced the HER activity of Ni2P catalyst with high current density of 100 mA cm–2 at high overpotential of 84 mV by adding a very small amount of Se. They used Se in the structure of MoS2 to synthesize MoS2(1–x)Se2x/NiSe2 hybrid catalysts and observed a reduction in Gibbs free energy for hydrogen adsorption on the surface of MoS2(1–x)Se2x edges (Figure 7a, b). However poor electric conductivity and inefficient electrical contact of Mo based materials make their catalysts inefficient towards HER target. As a solution, 3D porous structure of the underlying metallic NiSe2 was an effective additive to provide fast electron transfer from less– conductive MoS2(1–x)Se2x to the electrode and finally enhance HER activity.161 It has also been shown that porous carbon materials with TMs based catalysts had an excellent effect on picking up the HER activity, attributed to high porosity, low energy barrier for hydrogen adsorption,162163

density of state comparable to noble metals,164 good electric conductivity, and high corrosion

resistance.165 Wang et al. showed this effect along with the influence of incorporation of Ni metal in MoxC– Ni at nitrogen–doped carbon vesicle. The role of nitrogen doping is to tune the electron density of graphene layers with lone pair electrons of N. The finally improved HER activity of the synthesized catalyst achieves 10 mA cm–2 current density at 68 mV overpotential in 0.5 M H2SO4.165 In a similar strategy, Yin et al. synthesized Ni−C−N nanosheets containing NiC and Ni3N, and reported the current density of 10 mA cm–2 at 60.9 mV overpotential in which its current density was reduced by 10% after 70 h testing time. The speed barrier in the HER process by Pt catalyst is controlled by Tafel mechanism with Tafel slope of 30 mV dec–1. The reported Tafel slope of 32 mV/dec for the Ni–C–N nanosheet suggested the Tafel mechanism to be dominant in HER process.166 Besides, the introduction of new metallic as well as nonmetallic elements for improving the electronic structure of the catalyst has been reported as a new strategy to isolate the metallic base of the catalyst in atomic scale. Electrochemical activation methods have also been reported to be an acceptable way for activating elecrocatalyst. 4.4. Ni-MOF Based HER. Recently, metal organic frameworks have emerged as a promising class for generating evenly distributed metal nanostructures over ordered carbon matrices as


42

Page 43 of 74

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

ACS Catalysis

efficient electrocatalyst for HER. Xu et al.135 reported the synthesis of robust molybdenum carbide-based hollow structure through dissolution−regrowth process, in which a Mo based polyoxometalate-anion-incorporated Ni-MOF hollow structure is prepared. Further, carbonlayer-coated Ni decorated hollow molybdenum carbide structures was obtained by carbon nitride coating followed by annealing at high temperature. The carbon coated catalyst generated after annealing at 800 °C resulted in best HER activity with overpotential of 0.123 V for current density of 10 mA cm-2. The role of Ni NPs is to undergo etching in acidic media to produce hollow Mo carbide structures and introduce synergistic function with Mo carbide. Besides, carbon coating of hollow strcutures played a vital role in uplifting catalytic activity by (i) avoiding aggregation or degradation of particles, (ii) acting as mechnanical reinforcement to protect the catalyst under extreme alkaline conditions, (iii) facilitating charge transfer through its porous structure, and (iv) enhancing the electronic conductivity. As it is predictable from Volcano plot, the produced hydrogen atoms in the Volmer step binds strongly to the surface of molybdenum carbide, which prompts the first step reaction and in contrary, decelerates either the Heyrovsky or Tafel step. The synergistic effect contributed more positive charge to Mo atoms and more negative charge to Ni atoms, which in turn decreased the Gibbs free energy of hydrogen bound to the surface of molybdenum carbide to facilitate its desorption. To establish the role of nickel based MOFs in enhancing the catalytic activities of well-known catalysts, Kumar et al.167 reported another cost effective synthesis of Ni2P NPs from a Ni-BTC MOF and NaH2PO2. The catalytic system exhibited photoexcited hydrogen evolution by water splitting under simulated solar radiation. MOF-derived Ni2P NPs interact with CdS nanocrystals to give Ni2P/CdS composite, which proved to be 62 time more efficient than only CdS toward H2 production. The enhanced catalysis has been attributed to the tendency of Ni2P to separate the photoexcited charge carriers formed in CdS by avoiding their recombination in Ni2P/CdS composite resulting in higher electric conductivity. Wang et al.168 surface-modified Ni nanoparticles derived from Ni-MOF Ni2-(bdc)2ted (bdc = 1,4-benzenedicarboxylic acid; ted = triethylene-diamine) by pyrolysis in the presence of NH3 which results in Ni nanoparticles with surface nitridation accompanied with a thin carbon layer coating. Thus, generated NPs exhibited excellent performance toward HER with a very low overpotential of only 61 mV and 88 mV at a current density 10 and 20 mA cm-2, respectively. The high catalytic activity has been attributed to the surface modification which involved surface


43

ACS Catalysis

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

Page 44 of 74

nitridation and the strong carbon removal effect of NH3 during the pyrolysis of the Ni-MOF delivering the desired surface state for HER. The use of sulfide in the structure of HER electrocatalyst has been to improve the catalytic activity.98, 169 Using MOF [Ni2(L-asp)2(bpy)].CH3OH.H2O, Fan et al. reported a synthesis of onion-like nickel–carbon (Ni–C)-based nanoshells which are atomically isolated Ni species anchored on graphitized carbon. These nanoshells serve high activity for HER with overpotentials of -34, -48, and -112 mV for the current densities of 10, 20, and 100 mA cm-2, respectively, in acidic media. A low Tafel slope of 41 mV per decade demonstrated that the Volmer–Heyrovsky mechanism is dominant and a large exchange current density of 1.2 mA cm-2 with high surability of >25h makes it a promising replacement for commercial Pt based catalysts. Low charge transfer resistance and strong chemical and electronic coupling between graphitized carbon and atomically discrete Ni moieties have been attributed to high HER activity of catalyst.170 Yu et al.171 used Ni-Co based Prussian blue analogue (PBA) MOF as a template for the synthesis of NiS cubic hollow frameworks via a structure-induced anisotropic chemical etching/anion exchange reaction of PBA nanocubes in the presence of Na2S at elevated temperature. The NiS with 100, 300, and 600 nm hollow nanoframes sizes required overpotential of 94, 115, and 148 mV for the evolution of hydrogen in alkaline medium, respectively, to drive a current density of 10 mA cm-2, which can be attributed to the higher surface area of frameworks with 100 nm (155 m2 g-1) in comparison to the that with 300 nm (143 m2 g-1) or 600 nm (82 m2 g-1) sizes. This synthetic strategy might inspire a new way to generate nanoframe-like hollow structures with variable sizes and dimensions. Table 2. Comparison of some of the recent Ni-based HER electrocatalysts in alkaline media. Overpotential (mV vs. RHE) SL Ni(OH)2–Pt/C 157 Catalyst

Current density Tafel slope (mA cm–2) (mV dec-1) 5 NA

Electrolyte

Ref.

1 M KOH

148

Ni–Mo2C nanorod

140

10

49

1 M KOH

153

NiMoC

123

10

84

1 M KOH

135

Ni/NiO–CNT

80

10

82

1 M KOH

136

Ni–0.2NH3

61

10

71

1 M KOH

168

Pt–Ni/C

60

10

60

0.1 M KOH

147

Pt/3D nickel

45

9.47

136

1 M KOH

137

Pt/Ni/Co

22

5

NA

0.1 M KOH

138


44

Page 45 of 74

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

ACS Catalysis

NiMo

34

20

NA

1 M KOH

150

Table 3. Compaison of some of the recent Ni-based HER electrocatalysts in acid media.

Catalyst

Overpotential (mV vs. RHE)

Current density (mAcm–2)

Tafel slope (mV dec-1)

Electrolyte

Ref.

NiMoNx

225

5

35.9

0.1m HClO4

151

Ni2P

130

20

46

0.5M H2SO4

154

FeNiS

105

10

40

0.5M H2SO4

159

NiP1.93Se0.07

84

10

41

0.5M H2SO4

160

MoS2(1x)Se2x/NiSe2

69

10

42.1

0.5M H2SO4

161

Mo0.27Ni2.4@900

68

10

45

0.5M H2SO4

165

Ni−C−N

60.9

10

32

0.5M H2SO4

166

Here, we emphasized the applications of electrochemically active catalysis for HER in alkaline media and acidic media through two general Volmer–Tafel and Volmer–Heyrovsky mechanisms. The incorporation of nanostructured Ni derivatives into state of the art materials can effectively increase the electrochemically active surface area of the binary or ternary alloy materials to enhance the HER catalytic activity in alkaline as well as acidic media. Specifically, besides well-established Pt, metallic dopants like molybdenum and iron, and non–metallic dopants like phosphides and sulfides have a considerable role in reducing the overpotential and enhancing catalytic activity of Ni-based materials by tuning the native electronic structure of catalytic surface. Reproducing high efficiency by using Ni in its different forms and in combination with metals and non-metals has been a hot research topic (Table 2–3).

5. BIFUNCTIONAL ELECTROCATALYSTS FOR HER/OER 5.1. Nickel-Based Bifunctional HER/OER. To accomplish full water splitting, the coupling of HER and OER catalysts in the same electrolytic system is desirable from the viewpoint of simplification, cost reduction and practical application of the system. In this regard, Ni is the most efficient bifunctional element used in catalyzing both cathodic HER and anodic OER in basic media.172-183 Figure 8 illustrates the favorable bifunctional process on surface of


45

ACS Catalysis

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

Page 46 of 74

electrocatalysts. Since nickel is not stable (due to leaching of Ni atom) in acidic media, we do not discuss nickel based acidic electrolyzer.

` Figure 8. Schematic reaction of the HER and OER over the bifunctional electrocatalysts surface. 5.2. Nickel Alloys Based Bifunctional HER/OER. An easy fabrication of Ni–Fe/C hybrids was developed by Liang et al. as electrocatalysts for whole water splitting.183 The symmetric two–electrode cell was fabricated with Ni–Fe/C electrocatalysts and used for full water splitting with the applying voltage of 1.58 V at the current density of 10 mA cm–2. These catalysts were also found highly stable over the period of 24 h. The excellent bifunctional activities and superior stability could be due to high conductivity, better charge transfer, and induced structural transformations of Ni–based catalysts into more active species via Fe doping.183 Wang et al. reported a NiCoFe layered triple hydroxides (LTHs)/carbon fiber cloth (CFC) as bifunctional catalysts for full water splitting in 1.0 M KOH solution.184 The NiCoFe LTHs/CFC electrodes showed high electrocatalytic performances (such as low onset potential, less Tafel slope, and better durability) for both HER and OER in the alkaline media. In addition, it exhibited an overall water splitting with a low onset potential (∼1.51 V), a small splitting potential (∼1.55 V) at current density of 10 mA cm–2. The excellent catalytic performance of the optimal NiCoFe LTHs/CFC attributed to the large specific surface area, with porous structured sheet, and triple hydroxides, which provided efficient exposure of the electrocatalytic active sites, free diffusion of gas (H2 and O2) with rapid solution penetration and diffusion, and strongly electronic effects between the triple hydroxides, respectively.184 The NiCoFe ternary alloys encapsulated in graphene layers were reported by Chen et al.185 From DFT calculations they found that the electronic properties of multi-metals can be optimized by changing the number of transferred electrons between alloys and graphene. The tuned NiCoFe ternary alloys exhibited good activities toward HER (overpotential 149 mV at 10


46

Page 47 of 74

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

ACS Catalysis

mA cm−2) and OER (overpotential 288 mV at 10 mA cm−2). Nevertheless, hindered by the complex and tedious fabricating approaches, less studies have been focused on the multimetallic alloys, particularly the elaborated tuning of electronic properties and a proper study of the relationship between metal composition and full water splitting activity. Overall, we can conclude that significant progress in catalytic activity has been achieved in alkaline media. However, enormous challenges remain for nonprecious electrocatalysts to achieve activity below the 1.5 V (in two electrodes system) with high stability (cyclic stability test up to 10,000 CV cycles and chronoamperometric durability up to 7 days). To further reduce the overall overpotential for full electrolyzer, the multimetallic components have gained a considerable attention due to the higher degree of freedom of alloys based systems as compared to those with pure metals. In this context, Li et al.186 synthesized a rationally structured multimetallic Ni–Mo/Cu nanowire based free–standing electrode which was prepared by scalable electrochemistry. The as obtained Ni–Mo/Cu nanowires showed excellent HER (overpotential 152 mV at 20 mA cm−2) and OER (overpotential 280 mV at 20 mA cm−2) due to the combination of conductive structured scaffold and highly active outer layers. When assembled for the overall water splitting, the Ni– Mo/Cu nanowires showed a cell voltage of 1.69 V at 20 mA cm−2 with good stability (12 h) and Faradic efficiency (100%). 5.3. Nickel Nitrides, Oxides, Phosphides, Sulfides based HER/OER. For the utility of Nialloy nitrides, Wang et al. obtained NP–stacked porous Ni3FeN nanosheets, which displayed the best OER activity in alkaline solutions with low overpotential of 223 mV at 10 mA cm–2 and HER property with a very low overpotential of 45 mV at 10 mA cm−2.187 Working as both anode and cathode catalysts in a two–electrode overall water splitting, the fabricated electrode only required a cell voltage of 1.495 V to deliver a current density of 10 mA cm−2. The catalytic activity might be due to the higher specific area and more active sites for OER and HER. A high–resolution TEM (HRTEM) image of Ni3FeN nanorods showed a lattice fringe with the spacing of ~0.215 nm, in agreement with the (111) plane of the FeNi3N crystal. The Ni3FeN/NF electrode exhibited superior activities for both OER and HER with low overpotentials of 202 and 75 mV at 10 mA cm–2, Tafel slopes of 40 and 98 mV dec–1, with excellent durability of 400 h. More importantly, it displayed a stable current density of 10 mA cm–2 with the applying voltage of 1.62 V for overall water splitting in a two–electrode system. The remarkable catalytic result of


47

ACS Catalysis

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

Page 48 of 74

the Ni3FeN/NF could be credited to the strong interaction between electrode patches and active sites along with the intrinsic metallic behavior and unique electronic structure of Ni3FeN hybrid. A study on the effect of P in NiFe alloy on the electrocatalytic OER and HER has been reported by Ng et al.178 This catalyst can be used as both anode and cathode for whole water splitting with an overpotential as low as 326 mV to deliver 10 mA cm−2 with 100% faradic efficiency. The high bifunctional activity has been attributed to the iron ions trapped into Ni–P, which produces a large number of active sites and promote electron transport and thereby enhance its whole water splitting performance in basic electrolytes. Alshareef et al. reported a novel PH3 plasma–assisted approach to modified NiCo hydroxides into ternary NiCoP which could also simultaneously enhance its HER and OER activities.188 The ternary NiCoP nanostructure material coated on NF displayed high electrocatalytic activity toward the HER with a small overpotential (32 mV) at 10 mA cm–2 in alkaline media. This nanostructure material also showed better OER catalytic activity in terms of overpotential (280 mV) at current density of 10 mA cm−2. Furthermore, when used as both the cathode and anode electrodes for whole water splitting, a cell voltage of 1.58 V was required to achieve a current density of 10 mA cm–2. The high HER and OER activities were attributed to more accessible active sites, unique electronic structure which further tuned the hydrogen (or water) adsorption energy. The ternary metal phosphide NiMoP2 nanowire as an electrocatalyst were reported for HER and OE by Kuang et al.189 This catalyst was fabricated by in situ phosphidation of the hydrothermally treated NiMoO4 nanowire/carbon cloth. The resulting product exhibited a high electrocatalytic activity with a low overpotential of 199 and 330 mV at a current density of 10 mA cm–2 for HER and OER in basic solution, respectively. Furthermore when this catalyst was used as both anode and cathode electrodes for full electrolyzer, it delivered a current density of 10 mA cm–2 at a potential of 1.67 V. This high catalytic activity is attributed to large active area and maximum number of electroactive surface/active sites. Using a facile solvothermal procedure, Yu et al.190 reported a 3D carbon–coated Ni8P3 nanosheet fabricated as a bifunctional electrocatalyst for both HER and OER. According to their results, this hybrid catalysts show excellent HER activity and extraordinary durability at a wide pH range. In addition, the OER with low overpotential (267 mV) at 10 mA cm–2 and a small Tafel slope (51 mV dec–1) were also reported. Interestingly, the cell voltage of 1.65 V was


48

Page 49 of 74

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

ACS Catalysis

required to deliver 10 mA cm–2 of current density for full water splitting. This remarkable performance for OER/HER can be ascribed to the synergistic effect at the Ni8P3/NiOx heterojunction. However, in a reverse approach, Yu et al.191 improved the catalytic activity of a nickel-based bifunctional material by electrodepositing thin film of nickel oxide onto multiwalled CNTs by reduction-induced electrodeposition method. At various potentials from 0.65 to 0.93 V, as a function of the overpotential for water oxidation, the catalyst exhibited Tafel’s slope of 137 mV dec-1, and overpotential of 0.33 V for current density of 0.5 mA cm-2. The catalyst also exhibited very low onset potential of -0.98 V for HER using ITO as substrate. The stable current density was observed to be -1.0 mA cm-2 at the −1.2 V for HER, and 1.5 mA cm-2 at +1.1 V for OER. The microscopic study indicated the close and ordered interactions between MWCNTs and O2-NiOx, which can possibly enhance the transport of electrons effectively during the OER. As for the utility of Ni oxides, Xi et al.192 utilized solvothermal approach for assembling nanosheets of partially reduced Ni and NiO NPs wrapped in thin graphitic carbon over Ni foam as substrate as highly effective bifunctional electrocatalysts of OER and HER. The synergistic inetractions between Ni and NiO in dual valent system can generate a net effective positive charge on the metallic Ni surface to serve more catalytic sites for the OER to enhance catalytic efficiency by showing minimum overpotential of 310 mV for 10 mA cm-2. On the other hand, the high affinity of the generated OH- toward NiO would motivate the adsorption of H over the metallic nickel surface, thus facilitating HER by exhibiting minimum overpotential of 110 mV for 10 mA cm-2. The catalytic electrode achieved 10 mA cm-2 of current density at a 1.64 V cell voltage for full water splitting in alkaline media. Shen et al. reported the Mo–doped Ni3S2 nanorods as a bifunctional electrocatalyst for water splitting.173 In alkaline full water splitting, this electorcatalyst showed a cell voltage of 1.53 V at 10 mA cm−2 and maintained the cyclic durability for 15 h. Ni3S2 was often used with NF for better water splitting. Tang et al. reported in situ hydrothermal growth of NiSe nanowires,193 which however was also used with NF for better water splitting. 5.4. Nickel Foam Based Bifunctional HER/OER. As for the utility of NF (working electrode) as biofunctional HER/OER, Ni-metal(Fe,Co) alloys and Ni-(N,P/OH/O,S.Se)x were widely exploited. For the use of Ni-alloy, Luo et al.73 reported the utility of NF (working


49

ACS Catalysis

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

Page 50 of 74

electrode) for the loading of NiFe LDH (NiFe LDH/NF) to generate a bifunctional catalyst in alkaline solution. The water electrolyzer supplies current density of 10 mA cm–2 at a cell voltage of ~1.7 V, which is lower than that of Ni(OH)2/NF (~1.82V). WE now discuss the use of Ni-(N,P/OH/O,S.Se)/NF. Introducing nitrogen into the graphene layer was optimized by Gibbs adsorption free energy (∆G) of the reaction intermediates, which is responsible for HER and OER activity. During OER process, the Ni NPs surface was partially oxidized which may be because of the structural changes in the graphene layers during the long term stability test. These oxidized species on the Ni NPs surface in Ni@NC could also work as active center for OER process. Zhang et al. grew a Fe–Ni nitride structures on surface–redox–etching NF.19 The pore size of NF is ∼250 µm, which was used as substrate material for the deposition of large amount of NiFe(OH)x nanosheets with a thickness of ∼20 nm. The SEM images of FeNi3N/NF showed that the sheets of NiFe hydroxide changed into macroporous structures in the presence of nanorods (diameter ~50 nm). Chen et al.194 designed a 3D porous Ni/Ni8P3 electrode through acid treatment of NF, followed by phosphorization. When this material was used as both anode and cathode for full water splitting, a cell voltage of 1.61 V was required to deliver a current density of 10 mA cm−2. The excellent catalytic activity of the NiCoP/NF was attributed to the lowest chemisorption free energy (∆GH) of hydrogen and unique morphologies. Although it indicates that Ni–based sulfides are better nonprecious catalysts for full water splitting, there is still a large room to improve the bifunctional catalytic properties. In this issue, Ni–based phosphide (P) might be more active catalysts for water splitting because the P–terminated surface attracts protons as a base and make their discharge easier, facilitating the HER and OER easily. Using the hydrothermal and electrodeposition technique, Zhao et al.195 reported the synthesis of porous NiFe/NiCo2O4/NF and performed full water splitting for hydrogen and oxygen production. Due to formation of mesoporous surface on NiFe double hydroxide sheets, the NiFe/NiCo2O4/NF shows a lower Tafel value of 38.8 mV dec−1 than that of the commercial catalysts (RuO2 and Ir/C). They showed the Faraday efficiency of NiFe/NiCo2O4/NF hybrid to be 99.8%, indicating that all charge is used up for the oxidation of water to oxygen. Furthermore, this hybrid interconnected between layers of the nanostructures also exhibited the excellent durability (OER activity stable upto 10 h) due to the binder free catalysts. This superior water


50

Page 51 of 74

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

ACS Catalysis

oxidation performance of NiFe/NiCo2O4/NF can be explained from the following important OER features: first NiCo2O4 nanoflakes layer deposition increases the active surface area of the catalyst, offering more active sites for catalytic reactions. Secondly, with NiCo2O4 nano-flakes layer deposition, the characteristic oxidation peak shifts toward more negative potential (1.37 V vs. RHE), which suggest that NiCo2O4 facilitated the oxidation of Ni3+ to Ni4+ species and drastically improve the OER performance of the catalyst (Ni3+/Ni4+ species are the active sites for OER in NiFe double hydroxides). In addition, NiFe/NiCo2O4/NF electrode displayed better HER activity with low overpotential of 105 mV at current density of 10 mA cm−2. In two– electrode water electrolysis system, this hybrid required a 1.67 V to deliver a current density of 10 mA cm−2. The best performance obtained for whole water splitting with NiFe/NiCo2O4/NF catalyst can be contributed to the unique hierarchical architecture of the electrode which consists of a 3D porous NF, vertically aligned NiCo2O4 nanoporous nano-flakes and NiFe double hydroxide mesoporous nanosheets. This hierarchical architecture catalyst offers significantly large number of catalytic active sites and enhanced available surface area for whole water splitting. It is generally known that the high–index planes are more catalytically favorable surface atomic structures (e.g., more atomic steps) than that of low–index planes. In this regard, Zou group reported high OER and HER performance of [2̅ 1 0] high–index faceted Ni3S2 nanosheet arrays on NF (Ni3S2/NF).98 This material exhibited good catalytic activities in terms of Faradaic yield (~100%) and stable electrocatalytic activity (for >200 h), which is due to the synergistic effect between its nanosheet array architecture and [2̅ 1 0] high–index facets. The Ni3S2/NF– based full water splitting showed a splitting potential 1.76 V at a current density of 13 mA cm-2. Kim and coworkers reported one step fabrication of binder–free CoS–doped β– Co(OH)2@amorphous MoS2+x on NF for overall water splitting at room temperature with high efficiency (Figure 9a).181 The OER active CoS–doped β–Co(OH)2 and HER active MoS2+x are strongly bound to each other due to CoSx bridging, forming three dimensional networks for better water splitting activity. The HER catalytic activity of the hybrid catalyst showed low overpotential of 143 mV at 10 mA cm−2, due to the unsaturated sulfur site of amorphous MoS2+x. Meanwhile, this hybrid used as OER electrocatalyst required an overpotential of 380 mV at 10 mA cm−2 in alkaline media, attributed to the conversion of β–Co(OH)2 to conductive CoOOH. More importantly, as both anode and cathode electrocatalysts in a two–electrode water splitting,


51

ACS Catalysis

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

Page 52 of 74

it showed stable current density of 10 mA cm-2 with the applying voltage of 1.58 V. The catalyst CoS–Co(OH)2@aMoS2+x exhibited high electrochemical performance and durability with 14 mA cm–2, 50 mA cm–2, and 105 mA cm–2 at 1.6, 1.7, and 1.8 V, respectively, for 100,000 s (Figure 9b).

Figure 9. (a) The schematic illustration for one step synthesis of CoS–Co(OH)2@aMoS2+x/NF and high–resolution TEM images. (b) Chronopotentiometric measurements for two-electrodes system used by CoS–Co(OH)2@aMoS2+x at 1.6, 1.7, and 1.8 V, respectively, in alkaline media. Reproduced with permission from ref 181 Wu et al.177 used ultrathin nanosheet–based hollow MoOx/Ni3S2 composite microsphere catalysts as bifunctional electrocatalysts for water splitting as well as their symmetric two– electrode water–splitting electrolyzer. The MoOx/Ni3S2/NF as OER electrocatalysts showed excellent catalytic activity with almost 100% Faraday efficiency, low overpotential of ~136 mV at 10 mA cm–2, small Tafel slope (50 mV dec–1), and high stability under basic media for >200 h. As a HER electrocatalyst, MoOx/Ni3S2/NF also exhibited small overpotential (

Nickel-Based Electrocatalysts for Energy-Related Applications

Recommend Documents