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Highly efficient and robust nickel phosphides as bifunctional electrocatalysts for overall water-splitting Jiayuan Li, Jing Li, Xuemei Zhou, Zhaoming Xia, Wei Gao, Yuanyuan Ma, and Yongquan Qu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b00731 • Publication Date (Web): 11 Apr 2016 Downloaded from http://pubs.acs.org on April 16, 2016

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ACS Applied Materials & Interfaces

Highly efficient and robust nickel phosphides as bifunctional electrocatalysts for overall watersplitting Jiayuan Li, †,# Jing Li, †,# Xuemei Zhou, † Zhaoming Xia, † Wei Gao, † Yuanyuan Ma, †,* and Yongquan Qu†, ‡,* †

Center for Applied Chemical Research, Frontier Institute of Science and Technology, and State

Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an Jiaotong University, Xi’an, 710049, China. ‡

MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed

Matter, Xi’an Jiaotong University, Xi’an, 710049, China.

KEYWORDS: Hydrogen Evolution Reaction; Bifunctional Catalysts; Overall Water Splitting; Nickel Phosphide; Electrocatalysis

ABSTRACT: To search for the efficient non-noble metal based and/or earth-abundant electrocatalysts for overall water-splitting is critical to promote the clean-energy technologies for hydrogen economy. Herein, we report nickel phosphides (NixPy) catalysts with the controllable phases as the efficient bifunctional catalysts for water electrolysis. The phases of NixPy were

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determined by the temperatures of the solid phase reaction between the ultrathin Ni(OH)2 plates and NaH2PO2·H2O. The NixPy with the richest Ni5P4 phase synthesized at 325 ºC (NixPy-325) delivered the efficient and robust catalytic performance for hydrogen evolution reaction (HER) in the electrolytes with a wide pH range. The NixPy-325 catalysts also exhibited a remarkable performance for oxygen evolution reaction (OER) in a strong alkaline electrolyte (1.0 M KOH) due to the formation of surface NiOOH species. Furthermore, the bifunctional NixPy-325 catalysts enabled a highly performed overall water-splitting with ~ 100 % Faradaic efficiency in 1.0 M KOH electrolyte, in which a low applied external potential of 1.57 V led to a stabilized catalytic current density of 10 mA/cm2 over 60 h.


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1. INTRODUCTION Nowadays, great attentions have been focused on the clean energy resources owing to the depletion of fossil energy and the constantly deteriorating ecological issues induced by fossil fuels burning.1-4 As a renewable clean energy, hydrogen possesses a super high calorific value and thus plays a critical role in clean-energy technologies.5 Hydrogen production from water-gas shift reaction containing CO impurity may cause the catalyst poisoning for the subsequent hydrogen utilization processes. Water electrolysis assisted by the catalysts potentially provides an effective approach to acquire high-purity hydrogen.6 However, the high overpotentials for the two half reactions of water electrolysis including hydrogen and oxygen evolution reactions (HER and OER) are generally required to obtain expected reaction rates, which leads to an overlarge required potential (> 1.8 V) instead of ~ 1.23V in theory for the practical overall water-splitting.2, 7 The effective HER and OER catalysts can reduce the overpotentials for electrolysis, thereby making the overall reaction more energy-saving.8 Currently, the noble metal based catalysts (Pt, Ru, Ir, et. al.) deliver the best catalytic performance for watersplitting.9-10 However, the global reserve scarcity and the exorbitant price of noble metals limit their extensive usage. Thus, the replacement of noble metals with earth-abundant materials is desired for the clean-energy technologies. Recently, nickel phosphides (NixPy) have been considered as the acid-stable, costeffective and efficient electrocatalysts for HER.11-16 NixPy shares the similar catalytic mechanism of the hydrogenases happened in nature, suggesting their excellent HER catalytic performance.11,

17

Besides, NixPy have also been proven to possess the

application prospect in OER, which paves the way for developing the bifunctional NixPy


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catalysts toward overall water-splitting reaction.18-20 An nanostructured Ni5P4 films as the bifunctional catalysts enabled a catalytic current density of 10 mA/cm2 at 1.7 V in alkaline media for overall water splitting.18 Afterwards, the Janus Ni2P catalysts realized 10 mA/cm2 toward overall water splitting at 1.63 V under the similar conditions.19 Very recently, an three-dimensional (3D) hierarchically Ni2P/Ni/nickel foam system with a high Ni2P loading was found to possess a superior overall water-splitting performance.20 Despite the significant interests and great efforts on the bifunctional electrocatalysts, the performance of the majority of the reported bifunctional electrocatalysts is still below that of the noble metal-based catalysts (e.g. Pt║RuO2 benchmarks).21 Significant challenges remain on developing novel catalysts with robust and efficient overall water-splitting performance. In this work, we reported the efficient and robust bifunctional nickel phosphides catalysts for both HER and OER by a facile solid-phase conversion of the thin Ni(OH)2 plates into the NixPy catalysts with controllable phases. At the low reaction temperature of 275 ºC, pure Ni2P phase is obtained. With the increase of the reaction temperatures, Ni5P4 and NiP2 become the major phases and then Ni2P becomes the dominated phase again until reaching the high temperature (> 475 ºC). NixPy synthesized at the reaction temperature of 325 ºC (NixPy-325) possesses the richest Ni5P4 phase, which correspondingly exhibits the superior HER performance in electrolytes with wide pH range, especially in acid media. The NixPy-325 catalysts also delivered a high and robust activity for OER in alkaline electrolyte. Thus, the bifunctional NixPy-325 catalysts for water electrolysis achieved a stabilized catalytic current density of 10 mA/cm2 at 1.57 V over 60 hours with a ~ 100 % Faradaic efficiency in 1.0 M KOH.


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2. EXPERIMENTAL SECTION 2.1 Preparation of β-Ni(OH)2 nanoplate precursor. β-Ni(OH)2 nanoplate precursor was synthesized according to our previous report.22 Typically, 20 mL of Ni(NO3)2·6H2O aqueous solution (0.1 M) was slowly added into 20 mL of NaOH solution (8.0 M). After aging for 30 min at room temperature, the reaction solution was kept at 100 °C for 24 h. After centrifugation, the products were thoroughly washed by water and ethanol. Finally, the samples were dried under vacuum at 50 °C and stored for future use. 2.2 Preparation of NixPy catalysts. A series of NixPy catalysts were prepared by the solid phase reactions between β-Ni(OH)2 and NaH2PO2·H2O (molar ratio of 1:5) at various temperatures (from 275 to 475 ºC) under the protection of Ar. The reactions were raised to the desired temperature with a rate of 5 ºC/min and hold at that temperature for 2h. After naturally cooled down, the products were washed by water to remove the unreacted salts and dried at 50 ºC. The series of NixPy catalysts are named as NixPy-T, where T presents the reaction temperature. 2.3 Characterization. The phases of various catalysts were determined by powder X-ray diffraction (XRD) performed on Shimadzu X-ray diffractometer (Cu Kα radiation). The surface properties of as-synthesized NixPy were probed by X-ray photoelectron spectroscopy (XPS, Thermo Electron Model with Al Kα as the excitation source). Transmission electron microscopy (TEM) images were obtained from Hatchie HT-7700. High resolution TEM (HRTEM) images were acquired on the Tecnai G2 F20 s-Twin under 200 kV. 2.4 Electrochemical tests. Catalytic performance of the NixPy catalysts were evaluated on CHI 660D in a three-electrode configuration for HER and OER and a two-electrode configuration for overall water splitting. The mixture containing 4 mg of NixPy catalysts,


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0.768 mL of water, 0.2 mL of ethanol and 0.032 mL of 5% Nafion was prepared as the catalyst slurry. The catalytic electrodes were fabricated by drop-casting of the slurry onto carbon fiber papers (CFP) with a loading of ~ 0.15 mg/cm2. In the three-electrode system for HER and OER measurements, we used the catalytic electrode, Pt wire and saturated calomel electrode (SCE) as the work, counter and reference electrodes, respectively. In the two-electrode system for overall water splitting, the NixPy325 electrodes were used as both negative and positive electrodes for HER and OER, respectively. The catalytic behavior of the Pt (10 wt%)/C was recorded for a benchmark comparison. The durability of the NixPy for HER, OER and overall water splitting was studied by the time-dependent current density profiles in various pH environments.

3. RESULTS AND DISCUSSION The phase evolution and morphological features of as-prepared catalysts were monitored by XRD and TEM. Synthesis of the NixPy-T nanocatalysts started from the preparation of the ultrathin β-Ni(OH)2 nanoplates (Figure 1a), as reported in our previous study.22 The corresponding XRD pattern (Figure S1) reveals the as-prepared nanoplate precursor is consisted of high-purity β-Ni(OH)2 phase (PDF No. 14-0117). The β-Ni(OH)2 nanoplate precursor was converted into the corresponded phosphide products at various reaction temperatures via a facile solid-state reaction under the protection of Ar. The phase controllability on asprepared NixPy catalysts was determined by the reaction temperatures. The crystalline phase structures of as-prepared NixPy at various reaction temperatures were monitored by XRD (Figure 1b). XRD pattern (black curve, Figure 1b) revealed that the phosphidation reaction happened at 275 ºC with the disappearance of β-Ni(OH)2 phase and appearance


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of a new hexagonal Ni2P phase (PDF No. 03-0953). When the reaction temperature was raised to 300 ºC, the dominated phase of NixPy was the hexagonal Ni2P phase along with appearance of the hexagonal Ni5P4 (PDF No. 18-0883) and cubic NiP2 (PDF No. 21-0590) phases, suggesting a temperature-dependent phase evolution of NixPy started from 300 ºC. Elevating the reaction temperature to 325 ºC, the hexagonal Ni5P4 and cubic NiP2 phases were increased accompanied with the decrease of Ni2P phase. Upon further increasing the reaction temperature to 375 ºC, it was found that the content of the Ni5P4 and NiP2 phases began to decrease as shown their XRD pattern (green curve in Figure 1b). When the reaction temperature reached 475 ºC (blue curve in Fig 1b), Ni2P became the major phase again due to the decomposition of Ni5P4 and NiP2 into Ni2P at high reaction temperatures (> 325 ºC). The molar content of various phases in the NixPy-T catalysts are estimated via the quantitative phase composition analysis using the Rietveld method (Figure S2).23 Figure 1c shows a morphological transformation of NixPy-325 from the nanoplate of βNi(OH)2 into a porous network with a crystalline size of ~ 10 nm after phosphidation. Such a phenomenon can be attributed to the PH3 generation by the decomposing of NaH2PO2 and the temperature-induced structural collapse of the plate-like precursor at high reacting temperature, similar to previous reports.24-25 From HRTEM image of NixPy-325 (Figure 1d), the lattice fringe spacings of 0.203, 0.284 and 0.244 nm were indexed to the (201) crystal surface of the hexagonal Ni2P phase, (201) crystal surface of the hexagonal Ni5P4 phase and (210) crystal surface of the cubic NiP2 phase respectively. It further supports the phase evloution of NixPy-325, consistent with the XRD results (Figure 1b). The similar morphological deformation and features of the NixPy catalysts at other reaction temperatures (NixPy-275, NixPy-375 and NixPy-475) were also observed (Figure S3).


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XPS technique was utilized to ascertain the atomic ratio and electronic structures of the asprepared NixPy-T nanocatalysts. Figure 2a and 2b give the XPS fine spectra of NixPy-T at Ni 2p and P 2p region. The calculated atomic ratios for Ni to P of NixPy-275, NixPy-325, NixPy-375 and NixPy-475 are 2.05 : 1, 1.29 : 1, 1.45 : 1 and 1.78 : 1 respectively, which are consistent with the XRD quantitative analysis (Figure S2). When the reaction temperature is 275 ºC (black curve in Figure 2a and 2b), two apparent peaks at 852.4 eV and 129.6 eV are ascribed to the positively charged Ni and negatively charged P in Ni2P, respectively.26 With the further increase of the reaction temperatures, the Ni 2p3/2 peaks of NixPy-325, -375 and -475 (red, green and blue curves in Figure 2a) shift to 853.0, 852.7 and 852.5 eV, respectively. While, the corresponded P 2p3/2 peaks of NixPy-325, -375 and -475 (red, green and blue curves in Figure 2b) shift to 129.1, 129.3 and 129.5 eV, respectively. Compared with the energy level of zero valence state Ni and P,27 the more shift towards high energy of Ni 2p3/2 suggests more positively charged Ni centers (δ+) in the series of NixPy-T catalysts. On the other hand, the energy levels of P 2p3/2 show more shift to low binding energy, indicating more nagetively charged nature (δ － ) of P species in NixPy-T catalysts. Thus, an efficient charge-transfer between metal and P is expected for the nickel phosphide catalysts with a larger value of δ.28-29 Herein, XPS results indicate the amount of charge transfer (δ) in the order of δ(NixPy-325) > δ(NixPy-375) > δ(NixPy-475) > δ(NixPy-275), which is highly relevant to their intrinsic HER catalytic activity and desired for high performance HER electrocatalysis.13, 30 Since the catalysts were stored in air, the slight surface oxidation of the catalysts were also observed with weak peaks at ~856 eV in Ni 2p region and ~134.5 eV in P 2p region (Figure 2b).15 The electrocatalytic activity of the NixPy-T catalysts for HER was evaluated in 0.5 M H2SO4 and 1.0 M KOH electrolytes (Figure 3a). The commercial 10 wt% Pt/C catalysts


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was used as the reference catalysts, which exhibited a high HER performance, similar in previous reports.11,

31

Significantly, the NixPy-T catalysts also possessed good HER

catalytic activities characterized by their large HER current densities at the specific potentials in acid media. Among them, the highest HER catalytic activity was obtained when T was 325 ºC. Increasing or decreasing the phosphating temperature induces a decreased activity for the NixPy-T catalysts, which suggests a phosphating temperature-HER catalytic activity dependence for the NixPy-T catalysts. The relative smaller overpotentials of 62 and 160 mV (η20) are required to reach a HER current density of 20 mA/cm2 for NixPy-325 in acid and alkaline media, respectively. The derived Tafel slopes are 46.1 and 107.3 mV/dec for NixPy-325 in acid (0.5 M H2SO4) and alkaline (1.0 M KOH) media (Figure 3b). The values of 46.1 mV/dec (Tafel slope) and 62 mV (η20) for the NixPy-325 catalyst are comparable to the best reported values for Ni5P4 nanocrystals (Tafel slope=33 mV/dec and

η20=35 mV)14 and superior to those of the majority of the precious metal free HER catalysts (Figure 3c, Table S1 and S2),9,

12, 15, 25, 31-40

which indicates a superior HER activity of the

NixPy-325 catalyst in acid media. Also, the NixPy-325 exhibits the HER activity comparable to those of many reported noble metal free HER catalysts in alkaline media9, 11, 18-19, 41-45 (Table S3). Derived from Figure 3a, the HER exchange current densities for NixPy-325 is 0.275 and 0.172 mA/cm2 in acid and alkaline media (Figure S4). These values are larger than those for the majority of the previously reported noble metal free HER catalysts9, 12, 15, 25, 31-40 (Table S1 and S2). Turnover frequency (TOF) normalized to each active site was used to evaluate the intrinsic catalytic activity of NixPy-325. The description in Supporting Information and Figure S5 and S6 show the details to calculate TOF for each catalyst. The previous works


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reported that Pt was the most active catalyst toward HER with a high TOF of 0.8 s-1 at 0 mV (vs. RHE).46 The active MoS2 catalysts reach a TOF of 0.75 s-1 at an overpotential of 300 mV.31 In contrast, the overpotential of 72 mV is needed for NixPy-325 to reach the TOF of 0.75 s-1, further revealing its superior intrinsic catalytic activity. To achieve a TOF of 4 s-1, the NixPy-325 catalysts require a low overpotential of 189 mV, which is smaller than those of the active CoP nanowire catalyst9 (240 mV) and those of core−shell MoS2@MoO3 nanowire catalyst47 (272 mV). Also, the NixPy-325 catalysts can reach a high TOF of 1.6 s-1 at 100 mV(vs. RHE) in 0.5 M H2SO4 electrolyte, which is much larger than CoP nanowires9 (TOFη=100 mV=1.15 s-1), CoP nanocrystals48 (TOFη=100 mV=1.10 s-1), CoS nanocatalyst48 (TOFη=100

mV=0.39

s-1) and Co2P nanorods34 (TOFη=100 mV=0.22 s-1) under the

similar conditions. Significantly, the obtained TOF values of the NixPy-325 catalysts were generally over those of other NixPy-T catalysts at each specific overpotential (Figure S6), revealing the highest intrinsic catalytic activity of NixPy-325. The high HER performance of the NixPyT catalysts can be attributed to the similar catalytic mechanism to the hydrogenases.11, 17 Analogous to hydrogenases,11,

17, 49

the surface positively charged Ni centers and

negatively charged P species in nickel phosphides serve as hydride and proton acceptors, respectively. The NixPy-T catalysts synthesized at various reaction temperatures exhibit the similar morphology with different phases, leading to the differentiation in their intrinsic HER catalytic activity. Specifically, when the reaction temperature is 325 ºC, the XRD patterns (Figure 1b and Figure S2) reveal the richest Ni5P4 phase in NixPy-325 compared to other NixPy-T catalysts. It brings the smallest atomic ratio for Ni to P of NixPy-325 (1.29 : 1). The least atomic ratio for Ni to P brings the highest average valence state


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of the metal centers Ni in the NixPy-325 catalysts, which correspond to their intrinsically charged nature of the Ni sites (δ+).13-14 The XPS results also present the most positive shift of Ni 2p3/2 for NixPy-325 (Figure 2a), indicating the most positively charged surface Ni sites. The more positively charged metal centers in hydrogenases-like catalysts deliver the higher HER intrinsic catalytic activity.13, 30, 48 Thus, the largest δ+ of Ni metal active sites for NixPy-325 can be employed to explain its best intrinsic catalytic activity among the NixPy-T nanocatalysts. The catalytic performance of the series of NixPy catalysts was further explored by electrochemical impedance spectroscopy (EIS, Figure S7). The EIS spectra are simulated by a two-time constant parallel (2TP) equivalent circuit model (inset, Figure S7), which has been widely employed to simulate the HER kinetics of the Ni-based phosphide and alloy catalysts.50-54 The simulated data well matches the experimental results in this model. Parameters derived from the model were summarized in Table S5. In 2TP model, Rs represents the uncompensated solution resistance. All electrodes deliver the similar solution resistance. The first time constants of CPE1 and R1 describe the charge-transfer kinetics for HER, in which CPE1 is related to the double layer capacitance and R1 represents catalytic charge-transfer resistance.50, 54 The values of R1 for the NixPy-T catalysts are in the order: NixPy-325 (17.9 Ω) < NixPy-375 (21.8 Ω) < NixPy-475 (24.1 Ω) < NixPy-275 (29.9 Ω), revealing the fastest charge-transfer process of NixPy325 for HER. CPE2 (hydrogen adsorption pseudo-capacitance) and R2 (hydrogen adsorption resistance) in the parallel circuit describe hydrogen adsorption behavior on the surface of the electroactive materials.50, 54 The smallest hydrogen absorption resistance of NixPy-325 (Table S5 and Figure S7) indicates an easier absorption of the hydrogen intermediates on the surface of NixPy-325, which benefits to the HER process.52, 54 Additionally, EIS data can reflect the actual


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surface area of electrocatalytic layers by calculating the double layer capacitance (CDL), CDL = [CPE1 / (Rs-1 + R1-1)1-n](1/n).55-56 The calculated CDL values for the NixPy-T catalysts (Table S5) indicate the similar real active surface areas of NixPy-325, NixPy-375 and NixPy-475 and slightly smaller surface area of NixPy-275, consistent with the results of the electrochemical cyclic voltammetry testing (Figure S5). We further investigated the HER catalytic durability of the NixPy-325 catalysts via a chronoamperometric method (i-t) at the static overpotentials of 100 mV and 200 mV in acidic (0.5 M H2SO4) and basic (1.0 M KOH) electrolytes, respectively. The experiments were performed in an air-tight system under continuous stirring at 1500 rpm to get rid of the produced H2 bubbles. The catalytic current densities of NixPy-325 can stabilize at least 18 hours (Figure 3d), suggesting the robust activity of the NixPy-325 catalysts in both the strong acidic and alkaline aqueous electrolytes. The pH monitoring of the both acidic and basic electrolyte solutions revealed near constant respective pH values (Figure S8 and S9) within the testing time window for HER durability of NixPy-325, indicating no obvious alkalization or acidification happened. After HER in both acidic and basic media, the XRD and XPS spectra of the spent HER NixPy-325 catalyst were also characterized. As shown in Figure S10, the phase of the NixPy-325 catalysts was preserved as a mixture of Ni2P, Ni5P4 and NiP2 without any apparent phase evolution or change. The very similar Ni 2p XPS spectra of the NixPy-325 before and after HER (Figure S10) demonstrated the structure stability of the catalysts for HER reaction in both acid and basic solutions. Figure 4a shows the OER activity of the NixPy-325 catalysts under various pH environments. Very weak current densities of the NixPy-325 catalysts in acidic (0.5 M H2SO4) and neutral (1.0 M PBS, pH=7.0) media indicate their inactive nature toward


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OER in acidic and neutral environments. In contrast, the NixPy-325 catalysts delivered a superior catalytic activity for OER with a low required overpotential η10 of 320 mV and a small Tafel slope of 72.2 mV/dec. These values are comparable to or even better than those of the reported earth-abundant OER catalysts (Table S4).57 The catalytic performance of the NixPy-325 catalysts was even better than that of the commercially available 10 wt% Ir/C catalyst, which delivered a catalytic OER performance with a required η10 of 370 mV and Tafel slope of 70.0 mV/dec (Figure 4a). The redox peak at ~ 1.4 V (vs. RHE) in the polarization plot (Figure 4a) corresponds to the formation of surface NiOOH species, consistent with previous reports of other nickel-based bifunctional electrocatalysts.18-19, 50, 58 The catalytic stability of the NixPy-325 catalyst was evidenced by the un-degraded OER current density at a constant overpotential of 430 mV over 18 h (Figure 4b). The close pH values of the electrolyte before and after OER (Figure S9) indicate no obvious acidification during the testing of catalytic stability by NixPy-325. To investigate the origin of the OER catalytic capability of NixPy-325, the XRD and XPS analysis of the NixPy-325 catalysts before and after OER reaction were performed. In Figure S11, their XRD patterns revealed the formation of the NiOOH phase (PDF No. 06-0141) for the spent NixPy-325 catalysts. High-resolution XPS spectra of the used NixPy325 catalyst (Figure S11b and S11c) clearly present a new shoulder peak at 856.1 eV (Ni 2p) and a much stronger XPS peak of the oxidized phosphorus species at ~ 134 eV in P 2p region, which are relative to the formation of the NiOOH.50, 59-61 Taken all together, the formation of the surface NiOOH species contributes to the high catalytic capability of NixPy-325 for OER under a strong alkaline media.18, 62-64 Moreover, the nickel phosphides can effectively tune the electronic structures of the surface catalytic NiOOH by the formation of the NixPy/NiOOH heterojunction and subsequently improve their OER performance.18, 65-66


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Inspired by the active and stable catalytic activity of the NixPy-325 catalysts for two half reactions of water electrolysis in 1.0 M KOH, the catalytic capability of the NixPy-325 catalyst for the overall water splitting was explored. The photograph of the electrolyzer is shown in Figure S8. In this case, a low external bias of 1.57 V was high enough to drive an overall water-splitting current density of 10 mA/cm2 (Figure 5a). Such a voltage is slightly larger than those of the bifunctional lithium-induced NiFeOx (1.51 V)67, 3D hierarchically Ni2P/Ni/NF system (1.49 V)20 and CoMnO@CN (1.50 V),68 but comparable to that of the combination of Pt║RuO2 benchmarks (1.57 V),21 and even smaller than that of the combination of Ni0.33Co0.67S2║NiCo2O4 (1.72 V)41 and those of the majority of the state-of-the-art bifunctional catalysts (NiMo hollow nanorods: 1.64 V,21 NiFe layered double hydroxide: 1.70 V,44 Ni(OH)2: 1.82 V,44 NiSe nanowire: 1.63 V,58 Co-P films: 1.65 V,69 NiCo2S4: 1.68 V70) at the same catalytic current density, as shown in Figure 5b. The catalytic performance of NixPy-325 is also better than the single-phase Ni5P4 and Ni2P nanostructures as the overall water splitting catalysts with the external voltages at 1.7 and 1.63 V to achieve 10 mA/cm2, respectively (Figure 5b).1819

Additionally, the NixPy-325 also exhibited an impressive durability toward overall water

splitting reaction operated in alkaline electrolyte. As demonstrated in Figure 5c, the current density was stabilized at ~10 mA/cm2 (V=1.57 V) over 60 h, suggesting their promise to replace precious metal catalysts for the production of clean hydrogen. The generated H2 and O2 were further measured quantitatively by gas chromatography. The good agreement of the experimentally generated and theoretically calculated amount of H2 and O2 (Figure 5d) reveals the Faradic efficiency of ~ 100 % for both HER and OER in this electrolyzer, with the ratio of H2 to O2 being nearly 2.


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4. CONCLUSIONS In summary, NixPy nanocatalysts with controllable phases were prepared via a facile solid-phase phosphorization between β-Ni(OH)2 naoplates and NaH2PO2·H2O. The modulation of the reaction temperatures could bring the similar morphology with different phases for NixPy. NixPy-325 with the richest Ni5P4 phase delivered high-efficient HER performance in both acidic and alkaline electrolytes. The superior HER catalytic activity of NixPy-325 is originated from the more positively charged Ni sites, which is relative to its higher valence state of the metal center Ni determined by its phase structures. The remarkable OER catalytic activity of NixPy-325 enables the catalysts as the bifunctional electrocatalysts for the overall water splitting. NixPy-325 as both the positive and negative electrodes delivers an efficient and robust catalytic behavior for overall water-splitting with a ~ 100 % Faradaic efficiency in 1.0 KOH.

ASSOCIATED CONTENT Supporting Information. XRD, TEM and CV data of NixPy-T, TOF and exchange current density calculations of NixPy-325, Nyquist plots for NixPy-T, optical photograph of NixPy325║NixPy-325 water electrolyzer and supplementary tables. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author


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E-mail: [email protected], [email protected]. Author Contributions #These authors contributed equally. Notes The authors declare no competing ﬁnancial interest. ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China under Grant No 21401148 and 21201138, National 1000 plan, partially funded by the Ministry of Science and Technology of China through a 973-program (No. 2012CB619401) and by the Fundamental Research Funds for the Central Universities (No. xjj2013102 and xjj2013043).


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Figure 1. (a) TEM image of β-Ni(OH)2 precursor. (b) Powder XRD patterns of as-prepared NixPy-T catalysts. (c) TEM image of NixPy at reaction temperature of 325 ºC (NixPy-325). (d) HRTEM image of NixPy-325.


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Figure 2. The XPS fine spectra of (a) Ni 2p and (b) P 2p region for the NixPy-T catalysts.


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Figure 3. (a) Polarization curves for the 10 wt% Pt/C and NixPy-T catalysts in acid (0.5 M H2SO4) and alkaline (1.0 M KOH) media at a scan rate of 5 mV/s. (b) The Tafel plots for the 10 wt% Pt/C and NixPy-T catalysts. (c) Comparison of the Tafel slope and the required overpotential at the catalytic current density of 20 mA/cm2 (η20) for NixPy-325 with other noble metal free HER electrocatalysts in acidic media. (Detailed comparison with other state-of-the-art HER catalysts in acidic media can be found in Table S2 and S3) (d) Time-dependent current density curves for the NixPy-325 catalyst under static overpotential of 100 and 200 mV in acid and alkaline media for 18 hours.


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Figure 4. (a) The OER performance of the 10 wt% Ir/C and NixPy-325 catalysts in various pH environments. The scan rate is 5 mV/s (b) The OER catalytic stability of the NixPy-325 catalysts in 1.0 M KOH for 18 hours.


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Figure 5. Overall water-splitting performance of the bifunctional NixPy-325 catalysts. (a) Polarization curve for the NixPy-325 catalyst in 1.0 M KOH at a scan rate of 5 mV/s. The inset is the schematic diagram for overall water-splitting reaction in a two-electrode configuration. (b) Comparison of the required voltage at a current density of 10 mA/cm2 for the NixPy-325 catalyst with other state-of-the-art noble metal free bifunctional catalysts. (c) The catalytic stability of the NixPy-325 catalysts in 1.0 M KOH for 60 hours. (d) The amount of the theoretically calculated and experimentally measured gas versus time for overall water-splitting of NixPy-325 with constant current density of 30 mA/cm2.


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Table of Contents Graphic


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