Amorphous Nickel-Based Thin Film As a Janus Electrocatalyst for

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Amorphous Nickel-Based Thin Film As a Janus Electrocatalyst for Water Splitting Chengyu He,† Xinglong Wu,*,‡,§ and Zhiqiang He† †

National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, People’s Republic of China ‡ Key Laboratory of Modern Acoustics of MOE and Institute of Acoustics, Department of Physics, Nanjing University, Nanjing 210093, People’s Republic of China § Department of Physics, Ningbo University, Ningbo 315211, People’s Republic of China S Supporting Information *

ABSTRACT: Hydrogen generated by water splitting provides a renewable energy source, but development of materials with efficient electrocatalytic water splitting capability is challenging. Thin-film electrocatalytic material (H2−NiCat) with robust water reduction properties, which can be readily prepared by a reduction-induced electrodeposition method from nickel salts in a borate-buffered electrolyte (pH 9.2), is reported. The material consists of nanoparticles with nickel oxide or hydroxide species located at the surface and metallic nickel in the bulk. The catalyst mediates H2 evolution in a near-neutral aqueous buffer at low overpotential. The catalyst requires a subsequent oxidative pretreatment in order to attain a well-defined hydrogen evolution reaction (HER) activity, and the 1.5 h anodized catalyst film exhibits a HER current density of about 1.50 mA cm−2 at 0.452 V overpotential over a period of 24 h with no observable corrosion. In addition, it can be converted by anodic equilibration into an amorphous Ni-based oxide film (O2−NiCat) to catalyze O2 evolution, and the switch between the two catalytic forms is fully reversible. The robust, bifunctional, switchable, and noble-metal-free catalytic material has immense potential in artificial solar water-splitting devices.

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INTRODUCTION Hydrogen produced by solar water splitting provides a clean, sustainable, and abundant energy source to address the global energy problem.1−3 Inspired by photochemical water splitting in natural photosynthesis, artificial solar water-splitting devices are now being designed and tested to generate H2 by using inorganic materials composed of gas-evolving catalysts and light-harvesting semiconductors.4−7 Platinum and other noble metals are efficient electrocatalysts for the hydrogen evolution reaction (HER),8 but they are too scarce and expensive thus hampering large-scale commercial energy production. Hence, it is of both scientific and commercial interests to develop alternative catalysts that are more abundant and economical. The alternative materials typically include Raney Ni and Ni alloys,9−13 which are generally used as the HER catalysts in alkaline electrolytes. MoS2 nanoparticles14,15 and their analogues16−19 have been suggested as promising HER catalysts, but they often operate in an acidic environment. There have been recent breakthroughs in water oxidation [i.e., oxygen evolution reaction (OER)] involving cobalt/phosphate catalysts20−24 and nickel-based oxide catalysts,25,26 which can function safely under mild conditions while exhibiting high activity and self-healing. As compared to molecular catalysts, the primary advantage of cobalt phosphate and nickel borate is that they can easily be integrated with semiconductor electrodes.27−30 Cobo et al.31 recently reported a Janus © 2014 American Chemical Society

cobalt-based catalytic material prepared electrochemically from cobalt salts in a phosphate buffer based on the earlier work by Kanan and Nocera.20−24 They demonstrated that the material could catalyze both H2 evolution and water oxidation by altering the applied electrochemical potential. However, despite recent progress, there are still substantial challenges in producing an efficient, sustainable, and low-cost electrocatalyst. A review of the literature suggests that nickel oxide or hydroxide films exhibit promising catalytic properties under alkaline conditions for the electrochemical oxidation of water.32−34 Indeed, in recent years, the nickel-based oxide catalyst films developed by Nocera and co-workers have attracted much attention because of its efficiency at nearneutral conditions, self-assembly from low cost materials, and stability to corrosion.25,26 Nickel oxide or hydroxide films deposited on conducting substrates can be prepared by several physical and chemical methods such as sol−gel processes,35 chemical precipitation,36 vacuum evaporation,37 electron beam deposition,38 and sputtering deposition techniques.39 Among the various synthesis techniques, electrochemical deposition is possibly the simplest and most versatile approach. For instance, nickel hydroxide films are commonly prepared galvanostatically Received: August 14, 2013 Revised: February 12, 2014 Published: February 12, 2014 4578

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corresponding data analysis was performed by using Athena software. Electrocatalytic Performance Measurements. The electrocatalytic HER performance of the H2−NiCat modified FTO electrode was measured in a Ni-free 0.1 mol L−1 NaBi electrolyte (pH = 9.2). During the current−time measurement, the evolved H2 was detected by gas chromatography (Shimadzu GC-8A, Ar carrier gas). To get a deeper insight into the electrochemical behavior of the deposited material, the H2− NiCat modified substrate was transferred to a nickel-free 0.1 mol L−1 NaBi electrolyte (pH = 9.2) with potential switching between oxidative (+1.1 V versus Ag/AgCl) and reductive conditions (−1.2 V versus Ag/AgCl). Remarkably, when the same electrode is operated at a positive potential, a stable anodic current density is achieved and oxygen evolution occurs. This means that the single material can catalyze both H2 evolution and water oxidation under negative and positive potentials, respectively. In order to further explore this issue, similar experiments were also conducted for the O2−NiCat film obtained by electrodeposition at +1.1 V (versus Ag/AgCl) for 3 h in 0.1 mol L−1 NaBi electrolyte (pH = 9.2) containing 1.0 mmol L−1 Ni2+.

or potentiostatically from an aqueous solution containing nickel nitrate.40−43 The redox chemistry and the OER activity of nickel-based electrodes prepared by electrochemical deposition have been discussed in a number of studies.32 However, to our knowledge, there are few reports on the electrocatalytic H2 production over nickel oxide or hydroxide thin films up to now. In this study, we develop a novel hydrogen evolution catalyst (H2−NiCat) that can be conveniently prepared by a reductioninduced electrodeposition method from nickel salts in a boratebuffered electrolyte. The catalyst mediates H2 evolution in a near-neutral aqueous buffer at low overpotential. The catalyst requires a subsequent oxidative pretreatment in order to attain a well-defined HER activity, and the 1.5 h anodized catalyst film exhibits a HER current density of about 1.50 mA cm−2 at 0.452 V overpotential over a period of 24 h with no observable corrosion. Furthermore, it can be converted by anodic equilibration to the previously reported Ni-based oxide film25,26 (O2−NiCat) to catalyze O2 evolution, and the switch between the two catalytic forms is fully reversible. The robust, bifunctional, switchable, and noble-metal-free catalytic material has immense potential in artificial solar water-splitting devices.

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EXPERIMENTAL SECTION Materials. Ni(NO3)2·6H2O 99.999%, H3BO3 99.999%, and NaOH were used as received from Sigma-Aldrich and Ni(BO2)2·xH2O was purchased from Alfa Aesar. Fluorine− tin−oxide (FTO) coated glass slides with a 14 Ω/sq surface resistivity and coating thickness of 350 nm were purchased from Nippon Sheet Glass (Japan). Electrochemical Methods. The electrochemical measurements were performed at ambient temperature in a threeelectrode cell connected to a CHI 660D workstation (CH Instrument). The FTO substrates were used as the working electrode, whereas Ag/AgCl (3.5 mol L−1 KCl-filled) and platinum mesh served as the reference and auxiliary electrodes, respectively. The electrolyte was 0.1 mol L−1 H2BO3−/H3BO3 solution (pH = 9.2) (NaBi electrolyte). The potentials were reported as measured versus the Ag/AgCl electrode and as calculated versus reversible hydrogen electrode (RHE) using the following formula: E(RHE) = E(Ag/AgCl) + 0.205 V + 0.059pH. Electrodeposition and in Situ Catalyst Formation. The catalyst films were produced by controlled-potential electrolysis in a two-compartment cell separated by a Nafion 117 film. In the electrodeposition, the auxiliary compartment held 50 mL of NaBi electrolyte, and the working side held 50 mL of NaBi electrolyte containing 1.0 mmol L−1 Ni(NO3)2·6H2O. The working electrode was a 2 cm × 1 cm FTO-coated glass and rinsed with acetone and deionized water prior to use. Typically, 1 cm2 area of the electrode was immersed in the solution. Electrolysis was carried out at −1.2 V (versus Ag/AgCl) and a catalyst film was formed on the working electrode surface after about three hours. Characterization. The electrodeposited products were characterized by scanning electron microscopy (SEM; Hitachi S-4800) equipped with an energy dispersive X-ray spectroscopy (EDS) detector, X-ray diffraction (XRD; Philips X’pert Pro Xray diffractometer), and X-ray photoelectron spectroscopy (XPS; Thermo-VG Scientific, ESCALAB 250). X-ray absorption spectra (XAS) measurements at the nickel K-edge were performed on beamline BL14W1 of the Shanghai Synchrotron Radiation Facility. XAS spectra were collected at ambient temperature in fluorescence and absorption mode and the

RESULTS AND DISCUSSION Reduction-induced electrodeposition is suitable for the preparation of electrocatalysts if the precipitated materials possess catalytic activity. The possibility for Ni-catalyzed water reduction is explored using electrochemical reduction of an aqueous solution containing borate and Ni 2+ . Cyclic voltammetry performed using a FTO electrode immersed in a 0.1 mol L−1 H2BO3−/H3BO3 solution (pH = 9.2) (NaBi electrolyte) containing 1.0 mmol L−1 Ni2+ exhibits a sharp catalytic wave with an onset potential of −0.25 V versus RHE during the cathodic sweep (Figure 1a, lines 2). The anodic return scan displays a broad and relatively weak oxidation wave centered at 0.35 V versus RHE, which can be attributed to oxidation of the surface electrodeposited species formed during the sweep through the catalytic wave. Successive scans show an increase of the cathodic and anodic peak currents associated with the redox events, suggesting deposition of materials on the electrode surface. Neither film formation nor catalytic wave is observed in the absence of Ni2+ (Figure 1a, line 1). Furthermore, cyclic voltammetry of 1.0 mmol L−1 Ni(NO3)2· 6H2O in 0.1 mol L−1 NaNO3 electrolyte (pH adjusted to 9.2 by NaOH) is indistinguishable from the background of the FTO electrode in the absence of Ni2+. This suggests that a protonaccepting electrolyte such as borate is crucial to electrodeposition and catalysis. The presence of the cathodic catalytic wave prompts us to examine the electrode activity during controlled-potential electrolysis. During electrolysis at −1.2 V (versus Ag/AgCl) without stirring in 0.1 mol L−1 NaBi electrolyte (pH = 9.2) containing 1.0 mmol L−1 Ni2+, a rising current density that reaches a plateau (>0.60 mA cm−2) over a period of 2 h (Figure 1b) is observed. During this time, a dark coating forms on the FTO surface (inset of Figure 1b), and bubbles can be seen nucleating from the coating. These results are indicative of in situ formation of a hydrogen evolution catalyst (hereafter named H2−NiCat) on the FTO electrode. The morphology of the electrode coating formed during electrolysis in the presence of Ni2+ is examined by SEM. The electrodeposited materials consist of particles that have coalesced into a thin film and the individual particles have a 4579

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Figure 3a shows the entire XPS spectra of the electrodeposited H2−NiCat on FTO together with the data obtained

Figure 1. (a) Cyclic voltammetry curves of FTO substrates in 0.1 mol L−1 NaBi electrolyte (pH = 9.2) with no Ni2+ ion present (line 1) and with 1.0 mmol L−1 Ni2+ present (lines 2). The curves are recorded at different times in consecutive measurements at a scanning rate of 50 mV s−1. The potentials are measured against a Ag/AgCl reference electrode and converted versus RHE by using E(RHE) = E(Ag/AgCl) + 0.205 V + 0.059pH. (b) Current density profile in controlledpotential electrolysis at −1.2 V (versus Ag/AgCl) in 0.1 mol L−1 NaBi electrolyte (pH = 9.2) containing 1.0 mmol L−1 Ni2+ using a FTO cathode. The inset shows a photograph of a film obtained by electrolysis at −1.2 V (versus Ag/AgCl) for 3 h.

Figure 3. XPS survey (a), Ni 2p (b), O 1s (c), and B 1s (d) core level spectra acquired from the H2−NiCat film electrodeposited on the FTO substrate (upper line) and commercial Ni(BO2)2·xH2O (lower line).

from a commercially available powdery Ni(BO2)2·xH2O sample for comparison. The Ni 2p, O 1s, and B 1s core level XPS spectra of the H2−NiCat sample disclose the presence of Ni, B, and O elements, consistent with the EDS analysis. In the Ni region of the electrodeposited sample (Figure 3b), two sets of broad signals corresponding to the 2p3/2 (855.4 eV) and 2p1/2 (872.9 eV) core levels are observed. They are in a typical range of Ni2+ or Ni3+ bound to oxygen.44 The O 1s signal is located at 531.0 eV (Figure 3c). The B 1s peak is weak, and the binding energy of 191.6 eV corresponds to the core levels of central boron atoms in the borate structure (Figure 3d). The elemental contents in the H2−NiCat determined by XPS are 19.7 at % Ni, 39.7 at % O, and 0.9 at % B, corresponding to a 1:2 atomic ratio of Ni/O and a small boron content. As the Ni 2p and O 1s core level binding energies of nickel oxides and hydroxides are in the same range as the sample, the surface of the electrodeposited

diameter of around 20−100 nm (Figure 2a). Electrolysis at −1.2 V (versus Ag/AgCl) for 3 h yields a film of about 0.6 μm in thickness. The XRD pattern acquired from the electrodeposited catalyst reveals amorphous features, and no peaks related to crystalline phases are observed except those associated with the substrate FTO layer (Supporting Information, Figure S1). EDS examination suggests that the principal elements in the materials are Ni and O together with a small amount of B (Figure 2b). The Sn and Si signals arise from the FTO-coated glass substrate.

Figure 2. (a) SEM image of the electrodeposited catalyst prepared by electrolysis at −1.2 V (versus Ag/AgCl) for 3 h in 0.1 mol L−1 NaBi electrolyte (pH = 9.2) containing 1.0 mmol L−1 Ni2+. The inset shows the local enlargement. (b) Typical EDS spectrum of the FTO electrode modified by the electrodeposited H2−NiCat. 4580

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H2−NiCat can be described as an amorphous Ni oxide or hydroxide incorporated with a small amount of borate anions. To provide insight into the atomic structure, XAS are collected at the Ni K-edge for the H2−NiCat films deposited on FTO substrates. Figure 4a presents the X-ray absorption near-

Figure 5. (a) Linear sweep voltammetry curve of the as-prepared FTO electrode modified with H2−NiCat (1 cm2) (line 1) together with the data obtained from a blank FTO substrate (1 cm2) (line 2) for comparison. The corresponding Tafel plot of the H2−NiCat modified FTO electrode is shown in the inset. These curves are recorded from an aqueous 0.1 mol L−1 NaBi electrolyte (pH = 9.2) with no Ni2+ present at a scanning rate of 5 mV s−1. (b) Hydrogen production measured by gas chromatograph (circle dots) as a function of time and corresponding current (line 3) over the H2−NiCat modified FTO electrode (1 cm2) at −1.2 V (versus Ag/AgCl) applied potential. The black dashed line illustrates the theoretical amount of H2 produced assuming a Faradaic efficiency of 100%.

Figure 4. (a) Ni K-edge XANES spectra of the H2−NiCat film (line 1), the 1.5 h anodized H2−NiCat film (line 2), and the O2−NiCat film (line 3). For comparison, Ni metal foil (line 4) and NiO (line 5) reference spectra are shown. (b) FT EXAFS spectra of Ni metal foil (line 4), the H2−NiCat film (line 1), and the 1.5 h anodized H2− NiCat film (line 2).

edge structure (XANES) spectra of a H2−NiCat film together with the data obtained from Ni foil and NiO powder for comparison. It is known that the edge position of a XANES spectrum reflects the oxidation state of Ni and shifts to higher energies with increasing oxidation state. Here the edge shape and position of the H2−NiCat film are similar to that of Ni foil, which indicates a mean Ni oxidation state of 0 for the H2− NiCat film. Figure 4b presents the Fourier-transformed extended X-ray absorption fine structure (FT EXAFS) spectra recorded for the H2−NiCat film together with that of Ni foil. The FT EXAFS spectrum of the H2−NiCat film exhibits peaks at distances similar to those of Ni foil, suggesting a dominating contribution of the cubic close-packed phase of metallic nickel. However, the magnitude of EXAFS oscillation is lower than that observed for a Ni foil, which suggests a certain amount of nonmetallic contribution. On the basis of the preceding characterizations, we infer that the electrodeposited H2− NiCat films are made of nanoparticles with nickel oxide or hydroxide species located at the surface and metallic nickel in the bulk. The modified electrode is transferred to a Ni-free 0.1 mol L−1 NaBi electrolyte (pH = 9.2) to study the electrocatalytic HER activity. Figure 5a (line 1) depicts the linear sweep voltammetry curve of the electrode prepared freshly by passing 5.75 C/cm2 for 3 h. Overpotential values of 0.183 and 0.484 V are required to reach current densities of 0.5 and 2 mA cm−2, respectively. The Tafel plot analysis (inset in Figure 5a) gives a Tafel slope of 226 mV/decade and an exchange current density of 10−4.1 A cm−2. As compared with the recently reported H2−CoCat material,31 the H2−NiCat film has lower onset overpotential but larger Tafel slope. The exchange current density is

significantly higher than that found for the NiMo-based materials (between 10−6 and 10−4 A cm−2)45 and Ni-MoS3 film (2.8 × 10−4 A cm−2).46 To further verify that the measured current is from water splitting and not from any undesired side reactions, the current−time measurement of the H2−NiCat modified FTO electrode is performed for 6 h at −1.2 V applied potential (versus Ag/AgCl) with the gas chromatographic product quantification (Figure 5b). Electrolysis is performed in a Ni-free 0.1 mol L−1 NaBi electrolyte (pH = 9.2) in a gastight electrochemical cell under an Ar atmosphere. During the span of 6 h, the quantity of electrons passing through the outer circuit is 21.16 C, corresponding to 219.3 μmol. The amount of evolved H2 as a function of time measured by gas chromatographic analysis of the headspace is represented with the circle dots in Figure 5b. The amount of H2 measured rises in accordance with what is predicted by assuming that all the current is caused by two-electron reduction of water to produce H2 (black dashed line). This demonstrates that the observed current is indeed due to electrocatalytic hydrogen evolution with unity Faradaic efficiency. To see the electrochemical behavior of the deposited material, the H2−NiCat modified substrate is transferred to a nickel-free 0.1 mol L−1 NaBi electrolyte (pH = 9.2) with the potential switched to the oxidative condition. Remarkably, when the same electrode is operated at a positive potential (+1.1 V versus Ag/AgCl), a stable anodic current density of 0.60 mA cm−2 is achieved, and concomitantly, oxygen evolution occurs (Supporting Information, Figure S2 and S3). This means that the single material can catalyze both H2 evolution 4581

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and water oxidation under negative and positive potentials, respectively. By performing the oxidative treatment at +1.1 V versus Ag/AgCl for 1.5 h, an anodized H2−NiCat film is obtained. Over the course of this anodization, the efficiency of the 1.5 h anodized film in mediating the HER is enhanced drastically, and the stable current density of about 1.50 mA cm−2 at 0.452 V overpotential is obtained over a period of 24 h (Figure 6). In contrast, the nonanodized H2−NiCat film exhibits a decaying current density to 0.64 mA cm−2 after 24 h under the same conditions.

Figure 6. Current−time plots of the H2−NiCat film modified FTO electrode (line 1) and the electrode of the 1.5 h anodized catalyst film (line 2). These curves are obtained from nickel-free 0.1 mol L−1 NaBi electrolyte (pH = 9.2) at an applied potential of −1.2 V (versus Ag/ AgCl).

The Janus-type behavior, which is rarely exhibited in nonnoble metal catalysts,31 possibly stems from a change in structure of the catalyst under negative and positive potentials. Thus, a detailed analysis by SEM, EDS, XPS, and XAS is carried out to characterize the films of the H2−NiCat anodized at +1.1 V versus Ag/AgCl for 1.5 h. The SEM result shows that the films consist of homogeneous-distributed nanoparticles (Supporting Information, Figure S4). The EDS and XPS analyses illustrate that the elemental components are similar to those of H2−NiCat, but there is larger oxygen content in the 1.5 h anodized films (Supporting Information, Figures S5 and S6). The FT EXAFS spectrum of the 1.5 h anodized H2−NiCat film exhibits peaks at the similar distances and much lower intensities to those of Ni foil and nonanodized H2−NiCat film, suggesting a sizable nonmetallic contribution (Figure 4). In order to further explore the Janus-type feature, alternate potential switches between oxidative (+1.848 V versus RHE, upper lines in Figure 7a,b) and reductive conditions (−0.452 V versus RHE, lower lines in Figure 7a,b) are performed for the H2−NiCat film prepared by cathodic deposition for 3 h (line 1 in Figure 7a,b). The current densities for H2 and O2 evolution reach up to 1.52 and 0.60 mA cm−2 at 0.452 and 0.618 V overpotentials, respectively. No decrease in the activity for both H2 and O2 evolution is observed after several cycles. After the polarization switching measurement, the catalyst film on the electrode is characterized by XAS (Supporting Information, Figure S7). The Ni K-edge XANES spectra and FT EXAFS spectra of the film almost completely overlap with those of the 1.5 h anodized H2−NiCat film, suggesting the stability upon repeated switch between anodic and cathodic potential applications. Similar results are also observed for the O2− NiCat film prepared by electrodeposition at +1.1 V (versus Ag/ AgCl) for 3 h in 0.1 mol L−1 NaBi electrolyte (pH = 9.2) containing 1.0 mmol L−1 Ni2+ (Figure 7c,d). It can be concluded that the material can perform fast redox-dependent

Figure 7. (a) Current−time plot for a FTO electrode (1 cm2) during controlled-potential coulometry initially at −1.2 V versus Ag/AgCl (3 h, H2−NiCat deposition, line 1) in 0.1 mol L−1 NaBi electrolyte (pH = 9.2) containing 1.0 mmol L−1 Ni(NO3)2·6H2O and after transferring to a nickel-free 0.1 mol L−1 NaBi electrolyte (pH = 9.2), with the potential switched between oxidative (+1.1 V versus Ag/AgCl, upper lines) and reductive conditions (−1.2 V versus Ag/AgCl, lower lines). (b) Charges passed through the FTO electrode during the experiment. (c) Current−time plot for a FTO electrode (1 cm2) during controlledpotential coulometry initially at +1.1 versus Ag/AgCl (3 h, O2−NiCat deposition, line 1) in 0.1 mol L−1 NaBi electrolyte (pH = 9.2) containing 1.0 mmol L−1 Ni(NO3)2·6H2O and after transferring to a nickel-free 0.1 mol L−1 NaBi electrolyte (pH = 9.2), with the potential switched between reductive (−1.2 V versus Ag/AgCl, lower lines) and oxidative conditions (+1.1 V versus Ag/AgCl, upper lines). (d) Charges passed through the FTO electrode during the experiment.

interconversion between the H2−NiCat and O2−NiCat forms, catalytically competent for H2 and O2 evolution, respectively. The fully reversible structural transformation without loss of activity is possible because the materials on the electrode exist in equilibrium with metal ions in solution. The H2−NiCat material is unique for the following reasons. First of all, it can be readily prepared by electroreduction of simple Ni2+ solutions in the presence of borate, and all the chemicals are inexpensive. Second, H2−NiCat is an active electrocatalytic material for HER and operates safely at low overpotentials and under near-neutral conditions (room temperature and pH = 9.2). Third, it can be formed on diverse conducting substrates with varying geometry and easily interfaced with a variety of light-harvesting and chargeseparating semiconductors. Fourth, it requires a subsequent anodic pretreatment to attain a well-defined HER current density of about 1.50 mA cm−2 at 0.452 V overpotential over a period of 24 h. Fifth, it can be converted by anodic 4582

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equilibration to an O2−NiCat film to catalyze O2 evolution, and the switch between the two catalytic forms is fully reversible. As a result, H2−NiCat constitutes a robust, bifunctional, and switchable catalyst for solar energy conversion and storage.

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CONCLUSIONS A nickel-based thin film with robust water reduction properties is prepared by reduction-induced electrodeposition in diluted Ni2+ solutions in the presence of borate. The catalyst exhibits favorable activity in H2 evolution from a near-neutral aqueous buffer at low overpotential and long-time stability with no observable corrosion. The materials can be converted by anodic equilibration to the Ni-based oxide film (O2−NiCat) to catalyze O2 evolution, and the switch between the two catalytic forms is fully reversible. The robust, bifunctional, switchable, and noble-metal-free catalytic materials have immense potential in artificial solar water-splitting devices.

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ASSOCIATED CONTENT

S Supporting Information *

XRD analysis of the H2−NiCat film; electrodeposition processes of the H2−NiCat and 1.5 h anodized H2−NiCat films; characterizations and properties of the 1.5 h anodized H2−NiCat material; XAS characterization of the catalyst film under polarization switching measurement. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(X.L.W.) E-mail: [email protected]. Tel: +86-25-83686303. Fax: +86-25-83595535. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was jointly supported by National Basic Research Programs of China (2011CB922102 and 2013CB932901), PAPD, and China Postdoctoral Science Foundation (2012M511238). Partial support was also provided by Natural Science Foundation of Jiangsu Province (BK20130555) and National Natural Science Foundation of China (11374141). The authors thank the assistance from Shanghai Synchrotron Radiation Facility during XAS studies.

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