Anion-Containing Noble-Metal-Free Bifunctional Electrocatalysts for

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Anion-Containing Noble-Metal-Free Bifunctional Electrocatalysts for Overall Water Splitting Bingyan Xiong, Lisong Chen, and Jianlin Shi ACS Catal., Just Accepted Manuscript • Publication Date (Web): 06 Feb 2018 Downloaded from http://pubs.acs.org on February 6, 2018

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ACS Catalysis

Anion-Containing Noble-Metal-Free Bifunctional Electrocatalysts for Overall Water Splitting Bingyan Xiong†, Lisong Chen †*, Jianlin Shi†, ‡*

† Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering, East China Normal University, Shanghai 200062, P. R. China ‡ State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, P. R. China

ABSTRACT

With ever-increasingly severe environment pollutions and energy demand, it is becoming greatly important to develop sustainable energy resources as alternatives to traditional fossil fuels. As one of the ultimate clean energy sources, hydrogen (H2) energy produced by water splitting using electricity from solar radiation, wind, tide, nuclear fusion, etc., has attracted more and more public attention since the late 18th century. Although noble metal-based electrocatalysts perform well, they suffer severely from the high cost and rarity from the viewpoint of practical applications. Recently, various kinds of non-noble metal electrocatalysts based on low cost and earth abundant transition metals have been developed for both hydrogen evolution reaction (HER) and oxygen (O2) evolution reaction (OER). Among them, non-noble

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metal bifunctional electrocatalysts (NHOBEs), showing high performances in both HER and OER, are therefore of great significance and importance in future applications. With NHOBEs, one will be able to significantly simplify the fabrication procedures of electrolyzers, and more importantly, elevate the efficiency of “all in one” electrocatalytic water splitting systems, which will greatly favor their industrial applications by demanding substantially lowered the production costs. While the laws of NHOBEs from the same main group anionic elements such as oxygen, nitrogen, carbon, and boron families have not been well-reported so far. Herein, recent significant progresses of NHOBEs classified by different main group anionic elements are summarized, and emphases are placed to the designs, syntheses, electrocatalytic performances in water-splitting and future possible applications of NHOBEs. Moreover, the prospects of a number of vigorously investigated NHOBE catalysts and the possible trends of future developments are also out-looked. In addition, current challenges facing the researches in the electrocatalysts for water-splitting are discussed.

KEYWORDS

anion-containing, noble-metal-free, bifunctional electrocatalyst, overall water splitting, energy conversion

1 INTRODUCTION As global warming and rapid depletion of fossil fuels become a rapidly growing concern, it is urgent and important to develop renewable alternative energy sources.1-3 H2 is a kind of green energy with high energy density without carbon dioxide (CO2) releasing in energy conversion processes so it has been widely considered to be the most promising clean and renewable


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alternative to fossil fuels. However, currently, H2 is mainly obtained from the steam reforming of natural gas and/or methanol at elevated temperatures.4 Moreover, it is difficult to purify the obtained H2 by eliminating carbon monoxide unavoidably produced in the reforming process, and trace carbon monoxide will make great damages to catalysts, especially the noble-metalinvolved electrocatalysts, during the H2 utilization. Electrochemical water splitting driven by renewable energy sources (e.g., solar, geothermal) is a promising approach for environmental friendly production of high-purity hydrogen fuels.5,6 Therefore, it has been widely considered to be a “core clean energy technology” that is essential for the hydrogen economy.7,8 Although it requires only 1.23 V potential in theory for the electrochemical water splitting, but in practice a much higher overlarge (usually > 1.8 V) is necessary to overcome the activation barrier of the reaction. The large overpotential comes from two half-cell reactions: a sluggish four-electron transfer kinetic for anodic OER and a fairly facile two-electron for cathodic HER.9-15 To minimize the overpotentials at both electrodes and enhance the energy conversion efficiency, the effective electrocatalysts must be implemented to lower the activation energies of the above half-cell reactions.16,17 Noble metal (e.g., Pt, Pd) and noble metal oxide (e.g., IrO2, RuO2) catalysts show good HER and OER activities, respectively.18-22 Nevertheless, the high cost, rarity, and poor stability of these noble metal-based catalysts are the great obstacles in their large-scale applications.12,23 Hence, it is desirable to develop earth-abundant, economical electrocatalysts with high HER and/or OER performances as substitutes for noble metal-based catalysts.24 Paradoxically, electrocatalysts for HER and OER generally perform well separately in different electrolytes. The incompatible integration of two kinds of catalysts in the same media often leads to inferior overall performance due to the mismatch of pH ranges in which the


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catalysts are stable and the most active.25 Nowadays, available electrocatalysts always show high OER performance but inferior HER performance in alkaline solution, whilst superior HER activity but inferior OER activity in acidic media. Above all, high-performance bifunctional electrocatalysts that simultaneously catalyze HER and OER in the same media can largely simplify the fabrication procedures of electrolyzers and the equipments needed to facilitate the industrial application, and substantially lower the production costs.26-30 Thus, the design and preparation NHOBEs with excellent activity and in the meantime favorable cost for overall water splitting is of key importance yet remains a great challenge. As a booming field in the energy sector in recent years, there have been numbers of excellent reviews on either HER or OER.31-36 However, until recently, reviews for NHOBEs can be rarely found, several groups have summarized main types of NHOBEs such as cobalt-, nickel- and iron-based materials et al., in detail, and the challenges and perspectives are also proposed.37-39 Recently, the important role of anions in NHOBEs has been revealed. However, there is still no relevant reviews on this object. Therefore, unlike the other reviews available in literatures, it is noteworthy that this article aims to not only give a comprehensive review of recent progresses on NHOBEs classified by different main group anionic elements, but also summarizes the state-ofthe-art studies of approaches to achieve high-activity NHOBEs. Moreover, we will try to reveal the relationship between the electrochemical performance and different-anions-involved NHOBEs, as well as provide the challenges and perspectives for these NHOBEs. Firstly, we will focus on the electrochemistry of overall water splitting and some vital parameters for evaluating the catalytic performances of water electrolysis catalysts. Then, we summarize recent meaningful advances in the design, syntheses, and applications of NHOBEs. Finally, we will present the main remaining challenges and perspectives in this fast growing field


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briefly. We believe that this timely and comprehensive review will stimulate more extensive studies and attract ever larger attention from researchers worldwide, and even non-specialists as well, further promoting the design and fabrication of NHOBEs with high performance, and outlooking further research developments for energy conversion.

2 ELECTROCHEMISTRY OF OVERALL WATER SPLITTING REACTION Since the first report on electrochemical water splitting with a cathode and an anode in an electrolyzer in 1789,30,40 enormous efforts have been made to develop multifarious water splitting electrocatalysts derived from earth-abundant elements. The overall water electrolysis reaction is rather simple: H2O → H2 + 1/2 O2. Significantly, the two half reactions depend strongly on the pH value of the electrolyte.36 The standard-state free energy change (∆ Go) for converting 1 mol of water molecules into hydrogen and oxygen gases is +237.2 kJ mol–1. It is clear that this reaction is an energy uphill process and the enthalpy change (∆ Ho) required is +286 kJ mol–1 for the formation of 1 mol of H2. The performance of electrocatalyst is often assessed by the current density at a certain potential.41 The thermodynamic electrical potential for a reversible water electrolysis cell voltage is 1.23 V, 0 V for HER and 1.23 V for OER, respectively. An extra higher potential above the equilibrium potential, named overpotential (η), is needed to obtain a reasonable reaction rate, which is a key parameter to evaluate electrocatalysts’ activity. There are also a number of other essential parameters to pay attention to, namely the total electrode activity, Tafel plot, Faradaic efficiency, turnover frequency (TOF), as well as electrochemical stability.

2.1 Overpotential


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The water splitting activity can be generally evaluated by linear sweep voltammetry (LSV) or cyclic voltammetry (CV). Considering that the non-Faradaic capacitive current will be a part of total current, LSV or CV results are only the preliminary assessments. When electrocatalyst exhibits excellent performance for both HER and OER in the same media, a full-water electrolyzer can be assembled using the catalysts at both cathode and anode in a two-electrode system with KOH, H2SO4 et al., as the electrolyte. Generally, the real potential (overpotential) at the current density of 10 mA cm-2 or 20 mA cm-2 is compared and smaller potential voltage (overpotential) value means a higher electrochemical activity.42

2.2 Tafel Plot The Tafel plot, which is derived from LSV result and recorded with the linear portion at relatively low overpotentials being fitted to the Tafel equation (η = a + b log j, where η is the overpotential, j is the current density, and b is the Tafel slope), demonstrates the relationship between the overpotential (η) and the logarithm of the current density (log j). It also provides significant kinetic information of the reaction mechanism for a given electrocatalyst. Besides, another important parameter, which can be a criterion of the intrinsic electrocatalytic activity of the materials at the reversible potential, called the exchange-current density (j0). It can be obtained from the Tafel equation when η is assumed to be zero. One of the standards for a high peformance electrocatalyst in water electrolysis is a low Tafel slope and a high exchange current density.

2.3 Turnover Frequency (TOF) The value of TOF, defined as the amount of reactant that a catalyst can convert to a desired product per catalytic site per unit of time, can reflect the intrinsic catalytic activity of each


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ACS Catalysis

catalytic site. The TOF (s-1) value can be calculated according to the equation: TOF = (jA)/(αFn), where j (mA cm−2) represents the current density for the sample at a given overpotential during the LSV measurement; A is the surface area of the working electrode; α is the electron number of the target product (electrons/mol); F is Faraday's constant (96485.3 C mol−1), and n is the moles (mol) of coated metal atoms on the electrode calculated from m (g) divided by the molecular weight M (g/mol) of the catalysts. Besides, not all atoms in a catalyst own catalytic activity and can be equally accessible. Therefore it is difficult to get the accurate value of TOF. Nevertheless, this parameter will be relevant and useful for comparing the performances of similar catalytic materials.

2.4 Faradaic Efficiency The Faradaic efficiency is the parameter of electrochemical activity which can reflect the utilization efficiency of electrons. For the water electrolysis reaction, the Faradaic efficiency can be obtained by calculating the ratio of the experimentally produced gas amount to the theoretical produced gas amount. To be specific, the amount of O2 and H2 produced by a water electrolyzer is detected by gas chromatography in gas-tight electrochemical cells of two-electrode configuration. The amount of evolved O2 and H2 gases by water splitting reaction under a certain current density in a certain period of time should be measured. By calculating the ratio between the theoretical (nt) and actual gas (na) productions with following equation: Faradaic efficiency = nt /na = bFna /(It), we can obtain the Faradaic efficiency (b is the electron number of the target product (electrons/mol)).

2.5 Stability


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Besides the overall water splitting activity, the long-term durability is another critical parameter that determines the possibility of practical applications for energy conversion and storage. To evaluate this, there are two common methods: the first is chronoamperometry (i.e., the I – t curve) or chronopotentiometry (i.e., the E – t curve). A certain current density larger than 10 mA cm-2 or a potential on which the current density is larger than 10 mA cm-2 is set for a long period, and the corresponding potential or current density variation is recorded vs time. The second way is to perform a recycling CV or LSV experiment. The number of cycles should be more than 1000 at an accelerated scanning rate of 100 mV s−1, and the loss of current density measured by LSV after a certain cycle number is used to evaluate the electrocatalytic stability performance. An electrocatalyst is demonstrated to be of excellent stability for water splitting if negligible variation of potential or current density in the first approach, or a negligible loss of current density in the second approach is obtained.

3

NON

PRECIOUS

METAL

BIFUNCTIONAL

WATER

SPLITTING

ELECTROCATALYSTS Generally, the NHOBEs which own the same main group anion elements such as oxygen, nitrogen, carbon, and boron families always exhibit similar chemical properties and show the approximately the same reaction mechanism during the elctrochemical processes. Based on their anions species, NHOBEs can be classified into oxygen family, nitrogen family, boron family, and carbon family transition metal compounds. All these four kinds of NHOBEs will be discussed in detail. Besides, this paper reviews the performances of HER, OER, overall watersplitting, and the stabilities in alkaline electrolyte for the NHOBEs.


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3.1 Oxygen Family Transition Metal Compounds On account of the consistency of the outermost six electrons, oxygen species in the oxygen family transition metal compounds has a valence of minus two and the compounds display particular chemical and physical properties. As a rule, the metal chalcogenides usually follow the reaction steps of their corresponding metal oxide analogues.43,44 The adsorption of hydroxide and in situ generated oxyhydroxide species on the oxygen family transition metal catalyst surface can provide abundant exposed active sites and large contact area with the electrolyte, thus influencing the OER rate seriously.45,46 3.1.1 Transitional Metal Oxides Owing to advantages such as low cost, abundance, rich redox reactions, relatively good anticorrosion performance within a wide range of electrochemical window in base, transitional metal oxides (TMOs) are becoming one of the most promising candidates to substitute for noblemetal based bifunctional electrocatalysts for overall water splitting. Cui and co-workers developed a simple wet-chemical route followed by an annealing treatment to fabricate porous MoO2 nanosheets (Figure 1).47 In the synthetic route, molybdenum source is ammonium molybdate tetrahydrate ((NH4)6Mo7O24·4H2O) and nickel foam substrate itself acts as a reductant. The introduction of sodium dodecyl sulfate (SDS) into water leads to the formation of an oil– water interface. During the hydrothermal process, the precursor nanosheets are assembled at the interface above. The MoO2 nanosheets display excellent electro-activity in both HER and OER with relatively low onset overpotentials, considerably high current densities, and small Tafel slopes. A current density of 10 mA cm−2 is achieved a rather low cell voltage of about 1.53 V. The catalytic activity can be maintained for at least 24 h in a two-electrode configuration (1 M


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KOH). The use of polymer binders is avoided by the direct growth of active electrocatalysts on conductive substrates. This approach not only simplifies the synthetic procedures of electrodes, but also shows decreased cost of electrocatalysts. Moreover, the features such as high surface areas and hierarchical porosity are responsible for the formation of abundant active sites and thus enhanced mass transport rate in the electrocatalytic processes. Among unprecious-metal-based electro-catalyst materials, mixed oxides, especially nickel cobaltites (NiCo2O4) with special structure, have received considerable interest as they have shown electrocatalytically active for bifunctional water electrolysis. Lin’s group employed a simple thermal driven conversion process to prepare hierarchical NiCo2O4 hollow microcuboids. Ni–Co based precursor with uniform hollow microcuboid morphology is obtained through a solvothermal methodand, which later acts as sacrificial template for the hierarchical NiCo2O4 hollow microcuboids. The synthesized materials show excellent activity toward overall water splitting, with the 10 mA cm-2 water-splitting current being reached by applying just 1.65 V (vs RHE) (Figure 2).48 The high performance is ascribed to the unique 1D nanowire mesh and unique 3D hierarchical hollow structure, which provide a large active surface area and the enlarged number of exposed active sites, facilitate the diffusion of active species across the electrolyte and the release of evolved gas bubbles. As a result, surface electrochemical reactions are largely accelerated. Wang et al. also described the improvement of catalytic activity when transition metal Ni-Fe oxide nanoparticles (~20 nm) were electrochemically transformed into ultra-small (2–5 nm) NiFeOx nanoparticles through lithium-induced conversion reactions, which showed rather large surface areas and high density of catalytically active sites. NiFeOx nanoparticles exhibit high activity and stability for overall water splitting in basic medium,


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ACS Catalysis

achieving 10 mA cm-2 water-splitting current density at 1.51 V for over 200 h. This catalyst demonstrates great potential in the future industrial applications for overall water splitting.49

Figure 1. a) Preparation schematics of porous MoO2/Ni foam; b,c) SEM top-view images of porous MoO2 nanosheets; d) SEM side-view image of porous MoO2 nanosheets; e) activities for overall water splitting of Ni foam, commercial Pt/C, compact MoO2, and porous MoO2; f) photo of water-splitting device powered by an AA battery at a nominal voltage of ≈1.5 V; g) long-term controlled current water splitting at 10 mA cm-2 for 24 h for porous MoO2. Reprinted from Figure 1 and Figure 4 of ref47with permission from Wiley-VCH.


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Perovskite oxides are also attractive candidates as catalysts for the electrolysis of water in alkaline energy storage and conversion systems, and present similar members with metallic oxides for bifunctional water electrolysis.50,51 Shao’s group synthesized SrNb0.1Co0.7Fe0.2O3–δ perovskite nanorod (SNCF-NR) by a facile electrospinning approach. The synthesized sample exhibits both highly efficient OER and HER activities as well as good durability in alkaline solution.51 The high performance can be attributed to the larger amount of surface oxygen vacancies. When the catalysts are used as both cathode and anode in an alkaline water electrolyte, a current density of 10 mA cm−2 at a rather low voltage of ≈1.68 V and good durability of 30 h continuous electrolysis operation have be achieved. It’s a promising candidate of NHOBEs in alkaline environment.


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Figure 2. (a) Scheme of the sythesis of NiCo2O4 hollow microcuboids; (b) HER result of hierarchical NiCo2O4 hollow microcuboids by the method of polarization curve; (c) OER result of hierarchical NiCo2O4 hollow microcuboids by the method of polarization curve; (d) chronopotentiometry results of OER and HER by NiCo2O4 hollow microcuboids in 1 M NaOH at 10 and -10 mA cm-2, respectively; (e) stability test of the electrolyzer at 10 and 20 mA cm-2; inset in (e) shows image of NiCo2O4 electrode and the device of overall water-splitting reaction. Reprinted from Figure 1, Figure 4 and Figure 5 of ref 48 with permission from WileyVCH.


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Table 1. Summary of the HER, OER, and Water Splitting Activities of Recently Reported Bifunctional Water Splitting Metal Oxide Electrocatalysts

η for HER

Tafel

η for OER

Tafel

@

slope

@

slope

Overall

correspondi

(mV

correspondi

(mV

voltage

ng j

dec-1)

ng j

dec-1)

(V@mA

Stability of water Bifuntional Electrolyte Catalyst

(mV@mA

for

(mV@mA

for

cm-2)

HER

cm-2)

OER

splitting(V or mA

Ref.

cm-2 @ h)

-2

cm )

10 mA cm-2/20mA NiCo2O4

1.0 M NaOH

110@10

49.7

290@10

53.0

1.65@10

MoO2

1.0 M KOH

27@10

41

260@10

54

1.53@10

10 mA cm-2@24

47

CoMnO@CN

1.0 M KOH

-a

-

308@165

97

1.5@11

1.7 V @55

52

WO2 HN/NF

1.0 M KOH

48@10

43

300@10

71

1.59@10

10 mA cm-2@190

53

NiFeOx

1.0 M KOH

88@10

150

250@10

31.5

1.51@10

10 mA cm-2@200

49

1.0 M KOH

-

-

340@1200

38.8

1.67@10

20 mA cm-2@10

54

1 M KOH

232@10

103

370@10

48

1.68@10

10 mA cm-2@30

39

NiFe/NiCo2O4/

cm-2@36

48

NF SNCF-NRs a

No mentioned in the article or used Pt as counter electrode when measure the performance of HER.

3.1.2 Transitional Metal-Based Hydroxides and Oxyhydroxides Transitional metal-based hydroxides (TMOHs) and oxyhydroxides (TMOOHs) have sparked considerable attention as water splitting electrocatalysts owing to their low cost and high activity in alkaline media. Very recently, several research groups have reported TMOHs and TMOOHs, especially Ni(OH)255, NiFe hydroxides56, VOOH11. These materials exhibit high activities toward bifunctional water splitting and excellent durabilities. Among 3d metal based catalysts, Ni-based complexes have gained tremendous attention due to their earth-abundant nature and their good


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ACS Catalysis

catalytic performances towards the HER and OER.57-62 Xing et al. reported their recent efforts toward full water splitting by growing nanoporous Ni(OH)2@Ni films on conductive carbon cloth (Ni(OH)2@Ni/CC) by a two-step electrodeposition technique, which approached a current density of 100 mA cm-2 and overpotentials of 190 mV for the HER and 458 mV for the OER with outstanding durability in alkaline media.55 When the Ni(OH)2@Ni/CC is employed as catalysts on both the anode and cathode for overall water splitting, it shows superior long-term stability even surpassing that of the integrated Pt/C and IrO2 catalysts. The density functional theory (DFT) calculations suggest that the synergistic electrocatalytic effects between Ni(OH)2 and Ni in Ni(OH)2@Ni films accelerate the overall electrochemical water splitting activity. Hang’s group also reported a general approach for the controlled synthesis of a class of NiM (M = Fe, Co, Mn) hydroxide nanosheets (HNSs) with thicknesses of 2 nm.56 Such unique structures and chemical composition enable the NiFe HNSs to be both the cathodic and the anodic catalysts for overall water splitting with a current density of 10 mA cm-2 at 1.67 V and a remarkable durability for at least 12 h. By contrast, Friebel et al. reported that Ni sites in Ni1−xFexOOH were not active sites for the OER process.63 Operando X-ray absorption spectroscopy (XAS) using high energy resolution fluorescence detection (HERFD) reveals that Fe3+ in Ni1−xFexOOH occupies octahedral sites with unusually short Fe−O bond distances, induced by its edge-sharing with surrounding [NiO6] octahedra. By a computational approach they demonstrate that the structural motifs result in near optimal adsorption energy of OER intermediates and low overpotentials at Fe sites. Lepidocrocite VOOH with a special composition and hollow nanosphere morphology was synthesized and applied as an electrocatalyst for water splitting for the first time by Wang et al.11 By tuning the surface area of the nanospheres, the optimal performance can be achieved with low


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overpotentials of 270 mV for OER and 164 mV for HER at 10 mA cm-2 in 1.0 M KOH. When used as both the anode and cathode catalysts for overall water splitting, the VOOH requires as low as 1.62 V to reach the current density of 10 mA cm-2. All the test results reveal that the bulk composition, chemical state and the structure of the VOOH remain unchanged during the HER catalysis. V5+ is observed from the high resolution XPS V 2p spectrum, which is proposed as the origin of the high OER performance. Therefore, VOOH is indeed a highly active and stable catalyst towards the water splitting under alkaline condition. Owing to the excellent properties such as large surface area, prominent electrical conductivity, high mechanical strength, and structural flexibility, carbon materials have been being used in various electrical and electrochemical areas, for example, as conductive supports for water splitting. In addition, nitrogen doping in carbon can indeed intensity the coupling between the metal atoms and C atoms, which leads to enhanced chemical/thermal stability. Moreover, selecting different precursors of varied components and molecular-level structures can result in the regulated interactions among metal atoms, N and C atoms among their components, thus enabling the optimization of both activity and durability of the catalytic materials. Resultantly, excellent catalytic activities have been obtained for the water splitting.45,64-66 Ren et al. reported a facile impregnation/pyrolysis procedure to synthesize nitrogen-doped carbon supported nickel catalysts with enhanced activity and appealing stability.65 The well-dispersed nickel/nickel (oxyhydr)oxide nanoparticles, which strongly bound to a porous nitrogen doped carbon matrix, could effectively alter the electronic structure and resultantly enhance the activities and stabilities of the nickel (oxyhydr)oxides towards OER in alkaline condition.


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Table 2. Summary of the HER, OER, and Water Splitting Activities of Recently Reported Bifunctional Water Splitting Transitional Metal-Based Hydroxide and Oxyhydroxide Electrocatalysts

η for HER

Tafel

η for OER

Tafel

@

slope

@

slope

Overall

correspondi

(mV

correspondi

(mV

voltage

ng j

dec-1)

ng j

dec-1)

(V@mA

Stability of Bifuntional Electrolyte Catalyst

(mV@mA

for

(mV@mA

for

cm-2)

HER

cm-2)

OER

water splitting(V -2

Ref.

or mA cm-2 @ h)

cm )

VOOH

1.0 M KOH

164@10

104

270@100

68

1.62@10

50 mA cm-2@50

11

NiFe HNSs

1.0 M KOH

189@10

87.2

220@10

40.7

1.67@10

10 mA cm-2@12

56

1.0 M KOH

68@10

97

350@100

168

1.58@10

20 mA cm-2@16

55

Ni(OH)2@Ni/ CC

3.1.3 Transitional Metal-Based Sulphides Transition metal sulfides (TMSs), with relatively low intrinsic electrical resistivity facilitating charge transfer during the electrocatalytic HER and OER processes, have also provoked substantial attention in the past few years.9,18,35,43,45,49,67 Feng et al. adopted a hydrothermal method by the direct sulfidization of NF using thiourea at 150 ºC for 5 h, to prepare Ni3S2 nanosheet arrays on Ni foams (Figure 3).68 The material affords a current density of 10 mA/cm2 at small η = 223 and 260 mV for HER and OER, respectively, and outstanding durability in basic solution. The excellent activities are primarily attributed to the synergistic effect between its nanosheet array architecture and ሼ2ത 10ሽ high-index facets. The introduction of anions can significantly modify the morphologies and electronic structure of TMSs, resulting in the increased surface active site densities and the electrical conductivity.35,69,70 A notable N-anion


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Page 18 of 60

decorated Ni3S2 bifunctional electrocatalyst for water splitting synthesized through a facile onestep calcination route was put forward for the first time by Xie’s group.71 The less compact and porous structure of N-Ni3S2 electrodes can be derived under the assistance of low concentrations of N, S sources from the slow decomposition of thiourea powder. The N-Ni3S2/NF 3D electrode exhibits overpotentials of 330 and 110 mV to reach current densities of 100 and 10 mA cm−2 for OER and HER in 1.0 M KOH, respectively. Moreover, an overall water-splitting device comprised of this electrode delivers a current density of 10 mA cm−2 at the cell voltage of 1.48 V, owing to the accelerated electron transfer, increased surface area and active site density, optimal (∆GH*) and (∆GH2O*) as well as unique 3D configuration. Owing to their low costs, outstanding redox capabilities, favorable electrical conductivities, Ni- and Co-based sulfides are presently among the most promising candidates as NHOBEs. To combine the fascinating advantages of them, hierarchical NiCo2S4 nanowire arrays on a Ni foam substrate using a two-step hydrothermal method was demonstrated by Sivanantham et al.8 The water electrolyzer enabled by this kind of electrocatalyst delivers a current density of 10 mA cm– 2

under a cell voltage of 1.63 V. Because of the existing of Co(OH)2 and NiOOH active phases

during OER, the electrocatalyst exhibits high activity and stability.


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ACS Catalysis

Figure 3. Linear sweep voltammetry results of (a) HER and (b) OER in an alkaline medium (pH 14) over Ni3S2/NF; (c) current density vs time (I−t) curves of HER and OER with Ni3S2/NF recorded for over 200 h; (d) stability test of electrolyzer whose anode and cathode are both Ni3S2/N at an applied potential of 1.76 V (pH 14). Inset: an image exhibiting generation of H2 and O2 bubbles on the Ni3S2/NF electrodes. Reproduced with 68

permission. Reprinted from Figure 3 of ref

with permission from American Chemical Society.

There are also several other ternary and quaternary TMS bifunctional water splitting electrocatalysts reported in recent years, such as Mo-doped Ni3S2 on Ni foams,67 NiFeS/NF,43,72 CoMoS3,15 Cu@CoSx CF45 and CoS2/GF.9 Table 3 lists a detailed comparison of the electrochemical performances of those TMS bifunctional water splitting electrocatalysts.


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Page 20 of 60

Table 3. Summary of the HER, OER, and Water Splitting Activities of Recently Reported Bifunctional Water Splitting Transitional Metal-Based Sulfide Electrocatalysts

η for HER

Tafel

η for OER

Tafel

@

slope

@

slope

Overall

correspondi

(mV

correspondi

(mV

voltage

ng j

dec-1)

ng j

dec-1)

(V@mA


(mV@mA

for

(mV@mA

for

cm-2)

HER

cm-2)

OER

splitting(V or mA -2

Ref.

cm-2@h)

cm )

N-Ni3S2/NF

1.0 M KOH

110@10

-

330@100

70

1.48@10

1.55 V@8

71

200-SMN/N

1.0 M KOH

-

-

180@100

45.5

1.53@10

10 mA cm-2@15

67

Ni3S2/NF

1.0 M KOH

223@10

-

260@10

-

1.76@13

1.76 V@150

68

CoMoS3

1.0 M KOH

143@10

105

320@10

-

-

-

15

1.0 M KOH

-

-

340@10

68

1.60@10

1.85 V@10

69

1.0 M KOH

134@10

-

160@10

-

1.50@10

1.80 V @200

45

1.0 M KOH

-

-

198@10

56

1.625@10

10 mA [email protected]

72

1.0 M KOH

-

-

210@10

40.1

1.63@10

1.63 V@50

8

1.0 M KOH

-

-

1.528@20

82.6

1.74@20

1.80 V@18

9

1.0 M KOH

-

-

136@10

50

1.45 @10

1.45 V@100

29

1.0 M KOH

136@10

-

160@10

95

1.53@10

10 mA cm-2@10

73

Co9S8@NO SC Cu@CoSx/ CF Ni0.7Fe0.3S2 NiCo2S4 NW/NF CoS2/GF MoOx/Ni3S2 /NF NixCo3−xS4/ Ni3S2/NF

3.1.4 Transitional Metal-Based Selenide Materials At present, to the best of our knowledge, there have been only a few reports on the synthesis of transitional metal-based selenide (TMSe) bifunctional electrocatalysts for overall water


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ACS Catalysis

splitting, including FeSe2,74 CoSe,75,76 Ni3Se2,77 et al. Liu et al. reported the synthesis of monoclinic Co3Se4 thin nanowires on cobalt foam via a facile one-pot hydrothermal process using selenourea as the Se source for the first time.75 The synthesized sample shows extraordinarily high operational stability and activity for the electrocatalytic splitting of water.11 The remarkable electrocatalytic activity and stability of the Co3Se4/CF can be attributed to a nonstoichiometric cobalt selenide with mixed valences of Co2+/Co3+, and the incorporation of Fe impurities from reagent-grade KOH solution into Co3Se4. Moreover, it has a unique “core–shell” electrode structure with dense thin NWs array firmly grown on the surface of 3D porous cobalt foam. The CoOOH nanosheets in situ converted from Co3Se4 NWs during continuous OER operation adhere to the underlying CF and are of good structural robustness and integrity, ensuring the exceptional electrocatalytic stability for long-time water splitting. Xu et al. emphasized that the construction of hydrophilic and aerophobic surface of Ni3Se2 nanoforest on Ni was of vital importance to the high electrocatalytic activities and stabilities for both HER and OER.77 Such an excellent performance results from the intrinsically metallic behavior, and the usage of highly conductive Ni foam.


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Page 22 of 60

Table 4. Summary of the HER, OER, and Water Splitting Activities of Recently Reported Bifunctional Water Splitting Transitional Metal-Based Selenide Materials

η for HER

Tafel

η for OER

Tafel

@

slope

@

slope

Overall

correspondi

(mV

correspon

(mV

voltage

ng j

dec-1)

ding j

dec-1)

(V@mA


(mV@mA

for

(mV@mA

for

cm-2)

HER

cm-2)

OER


Ref.

cm-2 @ h)

cm )

FeSe2

1.0 M KOH

-

-

245@10

-

1.73@10

10 mA cm-2@24

74

Co3Se4

1.0 M KOH

262@100

72

320@397

44

1.59@10

100 mA cm-2@28

75

CoSe

1.0 M KOH

-

-

510@150

74.7

1.75@10

1.85 V @24

76

NF-Ni3Se2/Ni

1.0 M KOH

97@100

79

353@100

-

1.62@10

1.70 V@140

77

3.2 Nitrogen Family Transition Metal Compounds Nitrogen family transition metal compounds mainly include transitional metal-based nitrides (TMNs) and transitional metal-based phosphatides (TMPs). Density functional theory calculations have already predicted that TMNs allow for faster charge-carrier transportation, better electrical conductivity than their oxides and hydroxides, on account of the electron-rich structure.78 Metal nitrides such as Ni3N nanosheets, Co4N nanowires, and MoN nanosheets have metallic characters.79-82 Moreover, the nonmetal and isolated metal atoms that function as protonacceptor and hydride-acceptor sites on the surface of nitrogen family transition metal compounds benefit their electrocatalytic performance.36 The elongation of the M-P bond can accelerate the oxidation process of metal atoms at high overpotential due to the rapid, one-electron, one-proton equilibrium between transition metal (III)-OH and transition metal (IV)-O in which a phosphate species is the proton acceptor, followed by a chemical turnover-limiting process involving


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ACS Catalysis

oxygen-oxygen bond coupling.83 They are of the similar chemical properties and reaction mechanisms of electrocatalytic water splitting. 3.2.1 Transitional Metal-Based Nitrides According to density functional theory calculations, some TMNs have metallic characters, which can facilitate electron transfer involved in water splitting.84 Yin et al. reported a simple approach for synthesizing bimetallic Ni–Mo nitride nanotubes as bifunctional electrocatalysts for full water splitting.85 To drive a current density of 10 mA cm-2 requires overpotentials of 295 mV for the OER and 89 mV for the HER. The alkaline water electrolyzer with the materials as both cathode and anode catalysts requires a rather low cell voltage of 1.596 V to achieve a current density of 10 mA cm-2, and shows excellent stability even at a high current density of 370 mA cm-2 over 30 h. The NiOOH and -NH groups in situ formed at the catalyst surface during the OER process are demonstrated to be active species for the OER, while the Ni(OH)2, -NH and Mo species at the catalyst surface play a key role in the HER process. Further experimental and theoretical results confirm that proper heteroatoms doping can enhance the electro-conductivity and increase the number density of active sites, and thus resulting in higher activities. Zhang et al. reported an in situ approach for the growth of hierarchical FeNi3N/NF nanostructures on surface-redox-etching Ni foam as a bifunctional electrocatalyst for overall water splitting.86 It exhibits extraordinarily high 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, respectively, and more than 400 h of consistent galvanostatic electrolysis at a constant current density of 10 mA cm−2 without any visible voltage elevation for water splitting. The outstanding water electrocatalytic performance of the FeNi3N/NF can be attributed to the strong interaction between electrode patches and active substances, as well as the intrinsic metallic characters and special electronic structure of FeNi3N.


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Page 24 of 60

Table 5. Summary of the HER, OER, and Water Splitting Activities of Recently Reported Bifunctional Water Splitting Transitional Metal-Based Nitride

η for HER

Tafel

η for OER

Tafel

@

slope

@

slope

Overall

correspondi

(mV

correspondi

(mV

voltage

ng j

dec-1)

ng j

dec-1)

(V@mA



Ref.

cm-2 @ h)

(mV@mA

for

(mV@mA

for

cm-2)

HER

cm-2)

OER

1.0 M KOH

115@10

52.1

270@10

41.54

1.652@20

1.6 V@40

64

FeNi3N/NF

1.0 M KOH

75@10

98

202@100

40

1.62@10

10 mA cm-2@400

86

Co-PNCNF

1.0 M KOH

-

-

285@10

73

1.66@10

1.66 V@10

66

Ni3N/CMFs/

cm )

Ni3N

3.2.2 Transitional Metal-Based Phosphatides Since the first report on Nickel phosphide in 2013,87 TMPs have been widely studied as a kind of novel earth-abundant electrocatalysts which is of great promising in substituting for precious metal based catalysts to catalyze HER and OER.88-94 Since then, considerable efforts have been made on TMP materials for full water splitting.88,95 Several research groups have explored different approaches to engineer TMP for overall water splitting. Representatively, Liu’s group reported a fast and convenient one-step method by exposing Ni foam in phosphorus vapor at an elevated temperature for a short time. The obtained Ni-P shows efficient bifunctional electrocatalytic activities when serving as an overall water splitting catalyst (Figure 4).96 A catalytic current density of 10 mA cm-2 has been obtained at an applied potential of 1.64 V for overall water splitting with an Faradic efficiency as high as 90.2%. When the current density is 20 mA cm-2, the cell voltage slowly increases in the initial 60 h, and becomes stabilized up to


24

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ACS Catalysis

300 h, and then increases again at a slow rate to 1000 h. Notably, as confirmed by SEM, TEM and XRD results, the Ni–P nanosheets on the ligament surface are transformed into NiO/Ni(OH)x, thus forming a Ni–P/NiO(Ni(OH)x) heterojunction. These components can potentially provide more active sites to participate in the reactions. Generally, noble metal-free bifunctional electrocatalysts for overall water splitting can be classified into two categories: molecular catalysts and multicomponent catalysts. The abovementioned reports also demonstrate the fact that true active species of molecular catalysts are probably the ones in situ generated. Alloying is a vitally important strategy to improve catalytic activities of multicomponent catalysts,97 and the syntheses of alloyed transition metal compounds are becoming one of the most important approaches in developing highly efficient catalysts for water splitting. Several research groups have explored different approaches to engineer alloyed TMPs (e.g., NiFeP/NF,98 NiCoP,7,99 NiMoP,100 NiCuP101) for overall water splitting. Nanoporous (Co0.52Fe0.48)2P bifunctional electrocatalyst towarded HER and OER in basic electrolytes was synthesized by Tan and Wang, using the method of metallurgical alloy designing and electrochemical etching.102 As both the cathode and anode of an electrolyzer, nanoporous (Co0.52Fe0.48)2P shows an outstanding performance in water electrolysis: the operation voltage at the current density of 10 mA cm-2 is as low as 1.53 V in 1.0 M KOH (Figure 5), which is comparable to the commercial electrolyzer with paired Pt/C and IrO2 catalysts. XPS data of (Co0.52Fe0.48)2P reveals that Fe incorporation leads to the electronic structural changes in Co and P atoms, and thus enhances the water splitting performance. As suggested by DFT calculations, the substitution of Fe for Co in the Co-rich alloys can largely reduce the free energy of hydrogen adsorption to the level of Pt at the optimal composition. Moreover, Fe doping can improve the OER activities of Co and Fe oxides (hydroxides) at oxidative potentials by significantly


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Page 26 of 60

increasing its ECSA. Further XPS analysis on the post-OER-test of nanoporous (Co0.52Fe0.48)2P indicates the formation of catalytically active transitional metal oxides on the surface of the nanoporous

alloy.

The

synergistic

effect

among

the

Fe-doped

cobalt

iron

oxide/hydroxide/phosphate species leads to a greatly enhanced OER performance.

Figure 4. (a) Low- and (b) high-magnification top-view SEM images of porous Ni–P foam; (c) steady-state current density as a function of applied voltage (scan rate 5 mV s-1) over porous Ni–P foam electrolyzer; (d) E−t curves of the porous Ni–P foam and bare Ni foam electrolyzers recorded at varied current densities; (e) electrolysis efficiency as a function of current density; (f) E−t curves obtained at 10 and 20 mA cm-2. Inset: image showing the productions of H2 and O2 from the electrodes at 10 mA cm-2. The tests above were carried 96

out at room temperature in 1.0 M KOH. Reprinted from Figure 1 and Figure 5 of ref

with permission from

Royal Society of Chemistry.


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ACS Catalysis

Figure 5. (Co1-xFex)2P for two-electrode water splitting: (a) schematic image of an electrolyzer with nanoporous (Co1-xFex)2P as both anode and cathode in alkaline media; (b) polarization curve of np(Co0.52Fe0.48)2P as a bifunctional water-splitting catalyst in 1.0 M KOH; (c) chronoamperometry of the np(Co0.52Fe0.48)2P as both anode and cathode for water splitting at a voltage of 1.55 V for 50 hours. Reprinted 102

from Figure 4 of ref

with permission from Royal Society of Chemistry.

To date, various methods have been investigated to obtain TMP with high water splitting activity, and among them, electronic structure regulation by incorporating different elements is one of the most promising general TMP synthesis strategies.88 Qiao’s group proposed a novel approach for fabricating a bifunctional catalyst electrode (Fe- and O-doped Co2P grown on nickel foam, CoFePO@Ni Foam) by annealing CoFeOH in the presence of triphenylphosphine


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Page 28 of 60

(TPP) 600 ℃ for 3 h in N2 atmosphere. The high temperature pyrolysis converts the anion of the materials from OH to P, and the synthesized Co2P shows much enhanced water electrolysis activity with a current density of 10 mA cm−2 at a lower overpotential of 335.5 mV than that of CoFeOH in 1.0 M KOH electrolyte (Figure 6).103 This reveals that the atomic modulation between cation and anion plays an important role in optimizing the electrocatalytic activity. Furthermore, the intrinsic catalytic activity and stability of Co2P nanoparticles can be enhanced when fortified with heteroatom doped carbon materials.12,91,104 For instance, Das et al.10 developed a one-step calcination strategy to synthesize pure phase Co2P nanoparticles encapsulated in N, P dual-doped carbon nanotubes (denoted as Co2P/CNT). The electrolyzer using Co2P/CNT as catalysts on both anode and cathode delivers a current density of 10 mA/cm2 at around 1.53 V. It even rivals the state-of-art combination of commercial Pt/C and RuO2 (Figure 7). The size and morphology of electrocatalysts have large influences on the activity and selectivity of water splitting reaction, so are the TMP materials. Various morphologies of TMP for water electrolysis have been reported, such as NiMoP2 nanowires,100 Ni/NiP nanoparticles,105 NiP nanosheets,96 NiCoP nanocone arrays,99 Ni-Fe-P porous nanorods,26 CoP hollow polyhedrons14, et al. The most preferred structure for electrocatalysts should offer sufficient and accessible surface active sites, high conductivity and free diffusion pathways benefiting electron and mass (electrolyte and resultants) transfer, respectively, etc. A general and scalable method for the fabrication of TMP was recently presented by Wang et al.28 They synthesize nickel phosphide (CP@Ni-P) bifunctional electrocatalysts integrated on carbon fiber paper by electrodepositing Ni on functionalized CP, followed by a convenient one-step phosphorization


28

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ACS Catalysis

treatment in phosphorus vapor at 500 °C. The CP@Ni-P electrode exhibits favored overpotentials of 162 and 250 mV at a cathodic current density of 100 mA cm−2 towards H2 evolution in acidic and alkaline solutions, respectively. Moreover, the CP@Ni-P electrode also demonstrates superior catalytic performance towards OER. A current density of 50.4 mA cm−2 is achieved at an overpotential of as low as 0.3 V in 1.0 M KOH. The electrode sustains at 10 mA cm−2 for as long as 180 h with negligible degradation. During the OER in alkaline solution, the surface Ni-P is transformed to a Ni-P/NiO/Ni(OH)x heterojunction, which is suggested to be partially responsible for the enhanced electrocatalytic performance for OER. Moreover, the macroporous configuration of the electrode facilitates the mass transport of reactants/products as well as the release of H2/O2 gas (Figure 8).28 When using two identical CP@Ni-P electrodes at both cathode and anode, the electrolyzer can split water at an efficiency of as high as 91.0 % at 10 mA cm−2 for 100 h.


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Page 30 of 60

Figure 6. (a) Schematic representation of the formation of CoFePO; (b) FESEM of CoFeOH; (c) FESEM result; LSV curves of (d) HER and (e) OER; insets are the LSV curves under 10 mA cm−2 for HER and OER, respectively; (f) turnover frequency results of HER at 300 mV and OER at 400 mV; (g) I-t results of CoFePO tested for 100 h HER and OER process; (h) overall water-splitting characteristics in a homemade two-electrode configuration; (i) I-t results of CoFePO−CoFePO; inset is the LSV curves before and after 100 h stability testing. All the measurements are completed in 1 M KOH electrolyte. Reprinted from Figure 1 and Figure 3 of 103

ref

with permission from American Chemical Society.


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ACS Catalysis

Figure 7. (a) TOC graph; (b) TEM and HRTEM (inset) results; (c) steady-state current density as a function of applied voltage in an alkaline medium (1 M KOH) over RuO2/RuO2, RuO2/Pt-C, Pt-C/Pt-C and Co2PCNT/Co2P-CNT at a scan rate of 2 mV/s; (d) I-t result of the electrolyzer at 10 mA/cm2 (background graph exhibits the production of O2 and H2 bubbles at the respective carbon paper electrodes). Reprinted from TOC, Figure 2 and Figure 6 of ref10 with permission from Elsevier Ltd.


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Page 32 of 60

Figure 8. (a) Low- and (b) high-magnification SEM results of CP@Ni-P; (c) LSV results of CP@Ni-P toward both HER and OER; (d) the LSV result recorded at a scan rate of 5 mV s−1 in a two-electrode configuration; (e) chronopotentiometry results at 10 and 20 mA cm−2; (f) a photograph showing generation of H2 and O2 bubbles on the electrodes at 20 mA cm−2. All the results were measured in 1.0 M KOH. Reprinted from Figure 1 and 28

Figure 5 of ref

with permission from WILEY-VCH.


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ACS Catalysis

Table 6. Summary of the HER, OER, and Water Splitting Activities of Recently Reported Bifunctional Water Splitting Transitional Metal-Based Phosphatide Electrocatalysts

η for HER

Tafel

η for OER

Tafel

@

slope

@

slope

Overall

correspondi

(mV

correspondi

(mV

voltage

Stability of water Bifuntional Electrolyte

-1

ng j

dec )

ng j

dec )

(V@mA

(mV@mA

for

(mV@mA

for

cm-2)

cm-2)

HER

cm-2)

OER

1.0 M KOH

-

-

292@10

68

1.0 M KOH

87.5@10

38.1

274.5@10

51.7

Catalyst

Co2P/CNT

-1

CoFePO@Ni

Ref.

@ h)

10 mA [email protected]

10

1.75 V@100

103

1.5635@

Foam Co2P@N, P-

1.53@10

splitting(V or mA cm-2

10 1.0 M KOH

-

-

280@10

72

1.64@10

1.64 V@25

104

NiCoP

1.0 M KOH

32@10

37

280@10

87

1.58@10

10/20/50 mA cm-2@24

7

(Co1-xFex)2P

1.0 M KOH

64@10

45

270@10

30

1.53@10

1.55 V@50

102

Ni–P foam

1.0 M KOH

-

-

350@191

23

1.64@10

20 mA cm-2@1000

96

CP@Ni-P

1.0 M KOH

250@100

85.4

180@20

73

10 mA cm-2@100

28

20 mA cm-2@12

98

PCN/CNTs

1.53@50 .4 1.556@1 NiFe–P

1.0 M KOH

-

-

204@20

88 0

Ni-Fe-P

1.0 M KOH

-

-

256@10

40

1.52@10

20 mA cm-2@24

26

Cu3P@NF

1.0 M KOH

-

-

320@10

54

1.67@10

1.70 V@12

106

NiCoP

1.0 M KOH

197@100

54

370@100

116

1.64@20

20 mA cm-2@28

99

1.0 M KOH

199@100

112

320@20

90.6

1.65@10

10 mA cm-2@24

100

1.0 M KOH

-

-

300@50

49

1.60 V@10

101

1.71 V@15

107

NiMoP2 NW/CC (NiCuP)

1.6@20.

nano-foam Co2P/Co-foil

8 1.0 M KOH

-

-

319@10

59

1.71@10


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3.3 Boron Family Transition Metal Compounds Transition metal borides (TMBs) have been investigated in terms of OER activities since the 80s of the last century, while the HER activities of TMBs were rarely reported.108 Recently, TMBs have been developed to be alternative NHOBEs because they are generally very stable and can generate abundant active sites for both HER and OER in a wide pH range. By now, a few TMBs bifunctional electrocatalysts for overall water splitting have been reported, such as FeB2,109 Co2B,110 Co–Ni–B@NF111 et al. A highly efficient bifunctional active FeB2 electrocatalyst in alkaline media was prepared from a facile chemical reduction method.109 The excellent OER activity of FeB2 is ascribed to the formation of FeOOH/FeB2 active sites. DFT calculations demonstrate that the B-rich surface possesses appropriate binding energy for chemisorption and desorption of hydrogen-containing intermediates, thus favoring the HER process. Schuhmann’s group also proposed that amorphous Co2B prepared by the chemical reduction of CoCl2 using NaBH4 was an exceptionally efficient electrocatalyst for water electrolysis.110 During OER, Co2B can be irreversibly oxidized to CoOOH surface layer, and the formed heterogeneous junctions of CoOOH/Co2B serve as active sites. EXAFS observations indicate that B induced lattice strain in the crystal structure of the catalysts, and this kind of lattice strain potentially diminishes the thermodynamic and kinetic barriers of the hydroxylation reaction (formation of the OOH* intermediate), the key limiting step in OER.


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Table 7. Summary of the HER, OER, and Water Splitting Activities of Recently Reported Bifunctional Water Splitting Transitional Metal-Based Boride Materials

η for HER

Tafel

η for OER

Tafel

@

slope

@

slope

Overall

correspondi

(mV

correspon

(mV

voltage


-1

-1

ng j

dec )

ding j

dec )

(V@mA

(mV@mA

for

(mV@mA

for

cm-2)

cm-2)

HER

cm-2)

OER

splitting(V or mA cm-2

Ref.

@ h)

FeB2

1.0 M KOH

-

-

296@10

52.4

1.57@10

10/20/50 mA cm-2@24

109

Co2B-500

1.0 M KOH

-

-

370@10

45.0

1.81@10

1/10/30 mA cm-2@60

110

1.0 M KOH

-

-

313@10

120

1.72@10

10 mA cm-2@12

111

Co–Ni– B@NF

3.4 Carbon Family Transition Metal/Metal-Free Compounds 3.4.1 Transitional Metal-Based Carbides Recently, transitional metal-based carbides (TMCs, TM = Mo,112-114 W,115-117 and Fe84) have been widely explored as classical materials for the HER and OER in certain environments. However, there are only a few reports about the bifunctional TMCs for water splitting. Nickel carbides own the advantages of cost-effectiveness, superior electric conductivity, and relative earth-abundance. Moreover, because of their Pt-like d-band electronic structure, good chemical stability as well as low cost, molybdenum carbide materials hold a special promise for bifunctional HER and OER. Ai et al. reported that Co4Mo2@NC presented excellent performance in an alkaline water electrolyzer. Cell voltages of 1.67 and 1.74 V stimulate current densities of 5 and 10 mA cm-2, respectively. The prepared Co4Mo2@NC/Ti shows a maintained cell current density of 5 mA cm-2 for more than 6 h, exhibting a reasonbaly high durability when


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used in water splitting system.118 Ma’s group reported a facile electrodeposition approach for fully exposed { 1ത 20} high-index faceted dendritic hexagonal NiCx nanosheets on Ni-coated copper foil, with an optimized carbon content of 16.7 at% (denoted as d-NiC0.2NS/Ni/CF). The synthesized d-NiC0.2NS/Ni/CF possesses enhanced mass/electron transport capability and fully exposed active sites, therefore shows remarkable catalytic activity (A current density of 15 mA cm-2 can be obtained at a cell voltage of 1.61 V), a nearly 100% faradaic yield and affording superior catalytic stability (water spliting with 15 mA cm-2 for more than 100 h) in basic media.59

Table 8. Summary of the HER, OER, and Water Splitting Activities of Recently Reported Bifunctional Water Splitting Transitional Metal-Based Carbides

η for HER

Tafel

η for OER

Tafel

@

slope

@

slope

Overall

correspondi

(mV

correspon

(mV

voltage

ng j

dec-1)

ding j

dec-1)

(V@mA

(mV@mA

for

(mV@mA

for

cm-2)

cm-2)

HER

cm-2)

OER

1.0 M KOH

-

-

330@10

48.7

1.74@10

5 mA cm-2@6

118

1.0 M KOH

-

-

228@10

55

1.61@15

15 mA cm-2@100

59

1.0 M KOH

179@10

101

368@10

-

1.66@10

1.74 V@10

119


Co4Mo2@NC

splitting(V or mA

Ref.

-2

cm @ h)

dNiC0.2NS/Ni/ CF Ni/Mo2C-PC

3.4.2 Metal-Free Heteroatom-Doped Cabon Materials Carbon materials doped with various heteroatoms feature unique advantages for water splitting electrocatalysis due to their low cost, excellent electrical conductivity, tunable


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molecular structures, abundance, high catalytic efficiency, multifunctionality, and strong tolerance to acid/alkaline environments. Recently, water splitting reactions over doped carbon materials have been intensively studied, with the bottleneck for HER catalytic efficiency. In particular, the co-doping of carbon atoms with multiple heteroatoms (such as, N, P, S or O) can result in different electronegativities from that of C, enhancing the electrocatalytic water splitting activities by a synergic effect with respect to single heteroatom-doped counterparts. But more importantly, these catalysts show tailorable catalytic capabilities for specific electrocatalytic reactions by altering doping types, sites, and levels. Generally, the electrocatalytic activitis of metal-free heteroatom-doped cabon materials depend on following three aspects: (1) chemical composition and the interactions between different components determine the intrinsic nature of active sites; (2) the specific surface area and the presence of hierarchically porous structure can greatly affect the accessibility of active sites and transport properties of reaction-relevant species; (3) the electrical conductivity of the catalyst and its binder-free structure can determine the the electron transfer energy. In summary, carbon-based full water splitting electrocatalysts which exhibit the above three features will achieve high performances for water electrolyse.30 Lai et al. first reported a metal-free, N, P, and O tri-doped porous graphite carbon@oxidized carbon cloth (ONPPGC/OCC) water splitting electrocatalyst by a simple cost-effective method using aniline, phytic acid and OCC as precursors.30 It enables a high-performance basic water electrolyzer with a cell voltage of 1.66 V at 10 mA cm-2. Additionally, this electrode presents excellent catalytic performance and durability under both neutral and acidic conditions, which is supposed to derive from the 3D porous structure of the material, a high electrical conductivity of graphitized carbon and binder-free electrode. According to the high frequency region of the


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Nyquist plot, ONPPGC/OCC provides a low diffusion transfer resistance, indicative of favorable diffusion transport kinetics. At the same time, the presence of O, N and P in nanocarbon provide abundant active sites for both HER and OER. Besides, Dai’s group reported a simple pyrolysis route of polyaniline (PANi)-coated graphene oxide (GO-PANi) in the presence of ammonium hexafluorophosphate (AHF) to fabricate metal-free N, P, and F tri-doped graphene electrocatalysts for ORR, OER, HER, Zn–air battery and water splitting with high performances (Figure 9).120 The heteroatom-doping can change the surface properties, introduce the generation of defects and tune their distributions in the carbon matrix and graphene sheets, hence enhancing electrocatalytic activities and the electrocatalytic performances.

Table 9. Summary of the HER, OER, and Water Splitting Activities of Recently Reported Bifunctional Water Splitting Metal-Free Heteroatom-Doped Carbon Materials

η for HER

Tafel

η for OER

Tafel

@

slope

@

slope

Overall

correspondi

(mV

correspondi

(mV

voltage

ng j

dec-1)

ng j

dec-1)

(V@mA


(mV@mA -2

for

(mV@mA -2

for


Ref.

cm-2 @ h)

cm )

cm )

HER

cm )

OER

0.1 M KOH

310@10

112

230@10

71

1.68@10

10 mA cm-2@30

121

1 M KOH

-

-

410@10

83

1.66@10

10 mA cm-2@10

30

N,S-CNT

1.0 M KOH

170@5

133

360@10

56

2.03@10

2.1 V@20

42

NFPGN

1.0 M KOH

330@10

109

340@10

78

1.90@10

1.91 V @12

122

GO-PANi-FP

1.0 M KOH

520@10

-

~520@10

136

-

-

120

SHG ONPPGC/ OCC


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Figure 9. SEM results of (a) GO-PANi; (b) GO-PANi31-FP, and; (c) GO-PANi51-FP. TEM of (d) GOPANi31-FP and the corresponding (e) elemental mappings of C, N, P, and F; Optical images of electrodes (f) before and (g) after water electrolysis powered by Zn–air batteries; (h) generated O2 and H2 volumes versus water-splitting time. Reprinted from Figure 1 and Figure 4 of ref

120

with permission from Wiley-VCH.


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4 CONCLUSIONS AND PERSPECTIVES Among the ongoing efforts in searching for sustainable, clean, and highly efficient energy systems to meet the ever-increasing energy demands of modern society, the development of NHOBEs with excellent performances for overall water splitting has attracted tremendous attentions. This review highlights recent breakthroughs of NHOBEs classified by different anionic elements, summarizes the specialities and the underlying composition-structureperformance relationships of every anionic family. The electrochemistry of overall water splitting and a number of vital parameters for evaluating the catalytic performance of water electrolysis are first introduced and discussed. Then, the recent advances in the design, synthesis, and applications of NHOBEs classified by different anionic elements are summarized. By comparing the water splitting performances among various NHOBEs (Table 1 to 9), several interesting phenomena can be observed. First, from the standpoint of composition, the water splitting activities of TMSs, TMOs, TMPs, TMCs are superior to that of non-metallic heteroatom-doped carbon catalysts. The possible mechanisms for this issue are that they intrinsically possess high metallic conductivity and more active bifunctional catalytic sites suitable for both HER and OER. Second, the catalysis materials with relatively large active area, high amounts of accessible catalytic sites, and hydrophilic, aerophobic micro/nano structures directly grown on the conducting substrates facilitate both the charge carrier transports and bubble releases from the electrode surfaces, thus benefitting mass and charge transfer in the HER, OER and water electrolysis processes.77 Into the bargain, the heterostructures with abundant interfaces manifest excellent chemisorption capabilities for both hydrogen and oxygen-containing intermediates by the altered electronic properties of the active sites, thus leading to the outstanding OER and HER electrocatalytic activities and stabilities in


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alkaline media. For instance, the constructed interfaces between Ni3S2 and MoS2 as well as the between NiO and MoS2 have the advantages of the H-chemisorption on MoS2 and the HOchemisorption on Ni3S2 and NiO, therefore, the Gibbs free energies of the corresponding intermediates will decrease effectively and thus favoring the dissociation of the O-H bonds in H2O molecules, OH and OOH intermediates.104 Last but not least, since these electrocatalysts are self-supported, where their catalytically active phases are bound to the conductive substrates without any polymer binder, intimate and firm contacts between the catalyst and the substrate can be formed. The strong interaction between the active species and the substrates accerlate the electron transfer in the electrocatalytic processes and the elimination of the interfacial overpotential between the two components.29 Although great efforts have been made and enormous progresses have been achieved, NHOBEs are still far from satisfaction, therefore more attentions should be paid to the electrocatalysts themselves before commercialization, as outlined below. 4.1 Mechanism Investigation From the viewpoint of materials design, achieving a desirable electrocatalyst is based closely on the thoughtful understanding of the underlying mechanisms toward reactions. For this reason, the catalytic mechanisms of both HER and OER on various bifunctional catalysts should be well understood at the molecular levels by combining experimental design and theoretical analysis. To be specific, experimental design may rely on the rapid developments of advanced ex situ and in situ techniques. For example, in situ high-resolution TEM technique under ambient conditions may solve the problems that how the composition and structure of a catalyst evolve under reaction-relevant conditions in the real-time of electrocatalytic processes. In situ Raman, X-ray photoelectron spectroscopy, and X-ray absorption spectra, have been adopted to detect the


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interfacial structure and composition during the electrochemical reactions. Nevertheless, the real active species of transition metal sulﬁdes, selenides, nitrides, and phosphides are highly questionable when utilized as bifunctional electrocatalysts for water splitting. The advantages of these electrocatalysts are supposed to be as follows: Firstly, they are facile to obtain of special structures by using conventional synthetic methods; Secondly, they are actually the precursors to the real active OER catalysts, such as oxide/hydroxides, which are generated during operation by the oxidation from the precusors. The derived metal oxides/hydroxides could be formed with high surface area, unusually amorphous or metastable, and more catalytically active nanostructures, that could boost the overall performance; Thirdly, they are more conductive than the corresponding metal oxides/hydroxides and thus can serve as the conductive scaffolds/supoprts for the in-situ generated active metal oxide/hydroxide active species. In addition, the synergistic electronic interactions between/among the different components can make the composite electrocatalysts more active than the simple oxides. Correspondingly, the main disadvantage of the electrocatalysts of metal sulﬁdes, selenides, nitrides, phosphides, etc. is that they are thermodynamically less stable than metal oxides under oxidizing potentials. So, in fact, they are actually the “precatalyst” when used as OER catalysts in acid or alkaline media. Therefore, it is of great importance to characterize the surface sensitive structural after the OER reactions, to understand what are the true catalytically active species on the surface for water splitting. Significantly, Jin indicated that, unless the catalytic active species on the surface are still the original compounds by rigorous postcatalysis structural analysis, researchers cannot and should not refer to these unstable materials as OER catalysts or bifunctional catalysts, so the detections of on-line structure and composition changes during the electrochemical reactions are necessary.123 However, untill now, in situ spectroscopic measurements have been very limited in


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probing active sites and the interfacial structure and composition evolutions of catalysts during the reaction processes. Much more than this, researchers in the electrocatalytic, photocatalytic and many other catalytic fields can take the advantages of sophisticated ex situ and in situ techniques because of their similar operating concepts to HER and OER. Water splitting is acknowledged as one of the simplest reactions, but its underlying fundamental mechanisms at the molecular level are still unclear. The DFT calculations are capable of providing a reasonably high level of accuracy in evaluating experimental data and predict potential pathways of the reactions, and thus confirming the active sites and expediting the material screening processes as well, which will favor the searching for better-composed electrocatalysts and their future applications. 4.2 Designing Novel Electrode Materials The development of “all-in-one” NHOBE is still in its infancy, and more work is needed. This review introduces NHOBEs classified by four anionic main groups, however, potentially more NHOBEs with other anionic groups can be developed for electrocatalysis. Presently, the electrocatalyst design still relies heavily on pre-acquired knowledge of existing materials and by trial-and-errors, but very few materials have been developed based on the rational design and fundamental understanding of the catalytic mechanisms of the targeted electrochemical reactions. This situation hinders the further development of high-performance water electrolysis catalysts. For example, a perfect morphology control of catalyst will enable its specific surface area to be further increased and more catalytic active sites to be exposed, thus shortening the distances for charge/mass transport and elevating the conductivity and electrochemical performances. Thus the innovative design and synthese of different unique nanostructures for significantly improving the electrocatalytic performance is still a great challenge for water splitting. Moreover, the total cost


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of the whole synthetic and electrochemical processes, including the control of waste and acid/base pollution, should be carefully taken into consideration and well optimized, in order to realize efficient and robust deployment of large-scale water splitting for sustainable energy requirement globally. Above all, electronic and phase structure design and tunnation, active site creation and exposure, elaborate controls of morphology and dimension, and the lattice strain regulation of the electro-catalysts by means of alloying, compounding, heteroatom-doping and synthetic procedure innovation/contol, etc., are the most common ways for designing the novel NHOBEs to date. Table 10 summarises the useful strategies for enhancing the electrocatalytic performance of NHOBEs with the same anions. In conclusion, a perfect water splitting electrocatalyst should meet several standards: 1) high catalytic efficiency comparable to or even surpassing the noble catalysts; 2) excellent durability in a wide pH value range; 3) low cost to ensure cost-effective hydrogen production of water electrolysis; 4) scalability to ensure a wide range of commercial uses.


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Table 10. Summary of the Potentially Useful Strategies for Enhancing the Electrocatalytic Performance of NHOBEs With the Same Anions

Fortifying Electronic NHOBE

Alloying

with

Control of

Lattice

carbon

morphology

strain

structure regulation materials

TMPs

102

103

124

14

-

TMSs

8

71

9

125

-

TMOs

126

50

52

47

-

TMOH/TMOOH

56

-

-

11

-

TMNs

86

127

64

64

-

TMSes

-

-

76

75

-

TMBs

111

109

-

-

110

TMCs

-

-

119

-

-

-

30

-

121

-

Metal-free heteroatom-doped cabon materials

4.3 Operating Media and Equipments To date, the activities of NHOBEs are commonly examined in alkaline media. Nevertheless, alkaline water electrolysis also faces the challenges of relatively high energy consumption, costy installation and maintenance, safety and durability, etc. By contrast, neutral medium better suits


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the mass production due to the intrinsic benign nature, low cost and weak causticity, is therefore highly imperative, which, however, still remains a big challenge for the development of water electrolysis catalysts effective and stable under neutral conditions.128 In addition, due to the extremely abundant seawater resource, developing and utilizing seawater as the medium is another important means to solve the issue of fresh water resources presently in short. Exploring the catalysts with high tolerance against the harsh situation of seawater should be actively pursued. At the same time, it will be of great significance to integrate binder-free 3D electrodes into solar PVs or wind generator devices for sharply cutting the total cost and bringing blossom to the field of environmentally friendly hydrogen production. 4.4 Stability Issues The composition, structure, morphology are all the factors responsible for the stabilities of water electrolysis catalysts, and thus, advanced catalysts with outstanding stabilities can be obtained only if all these factors are fully identified and carefully gotten under control. Unfortunately, enhancing the electrocatalytic activity often encounters the stability degradation from time to time for water electrolysis, most probably due to the enhanced sensitivity of the highly active electrocatalysts to the medium and operation environment. However, the origin of such an undesirable relationship between the stability and activity is not yet fully understood in some cases, to which more attention should be paid for a balanced performance. The understanding of catalyst corrosion in certain media via theoretical prediction and/or experimental verification, may contribute to the design of excellent catalysts with the best tradeoff between activity and stability. 4.5 Standardized Testing


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Up to now, the fair comparisons among various electrocatalysts reported for water splitting are still difficult, resulting from the non-standardized measurement and evaluation methods employed. To be specific, the electrochemical activities are only normalized to the apparent geometric electrode area in many studies. However, more important factors that essentially affecting electrocatalytic activity, such as accessible active size density and distribution, properties of current collector and electrolyte, size and morphology of catalytic materials, the specific surface states, effective surface area as well as the mass loadings of catalysts on the electrode are either ignored or not aligned to make effective comparisons. Such issues lead to unfair performance comparisons among different materials by only evaluating the overall currents. Other important performance indexes, typically the overpotential, and others such as Tafel slope, TOF value (i.e., catalytic activity normalized by mass and electrode area), exchange current density, Faradic efficiency and stability can be used for more accurate comparison. Additionally, we must pay attention to the choice of counter-electrode. Pt has been frequently chosen as the counter electrode without applying an ion-exchange membrane between the working and counter electrodes during the electrochemical measurements of HER in large numbers of published reports. It is worthy to point out that a number of researchers have not realized that both electrochemical and chemical dissolutions of Pt in acidic or alkaline electrolyte will prompt the re-deposition of Pt on the working electrode, which it will result in largely enhanced HER activity due to re-deposited Pt but the actual catalysts used. As a consequence, in the measurement of HER, Pt should not be used as the counter-electrode, unless an ion-exchange membrane is used in between the work electrode and counter electrode. The suggestions for choosing a proper counter-electrode as follows: Firstly, selecting a chemically and electrochemically stable counter-electrode. For instance, glassy carbon or graphite can be used as


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counter electrode for HER or ORR; Pt can be used as counter electrode for the OER (Pt tends to redeposit onto cathode, and very slight Pt dissolution in media will have negligible effect on the OER performance). Secondly, the half-reaction taking place at the auxiliary electrode should be fast enough to support the reaction at the working electrode. To achieve this, we can choose a counter electrode with high catalytic activity, or we can use a larger-surface-area counter electrode than that of the working electrode to obtain a large energetic barrier.129 Moreover, standardized testing of catalysts by third-parties is strongly encouraged, so that the comparation between various catalysts will be fairer, more accurate and easier than before. In this regard, Jaramillo’s group have contributed a lot, though more efforts should be made.130,131

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was financially supported by National Natural Science Foundation of China (51702099), Shanghai Sailing Program (17YF1403800), China Postdoctoral Science Foundation funded project (2017M611500), Opening Project of State Key Laboratory of High Performance Ceramics and Superfine Microstructure (SKL201702SIC).


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