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1 Electronic Supplementary Material (ESI) for Energy & Environmental Science. This journal is The Royal Society of Chemistry 2015 Supplementary Information Ni 2 P as a Janus catalyst for water splitting: the oxygen evolution activity of Ni 2 P nanoparticles Lucas-Alexandre Stern, Ligang Feng, Fang Song, and Xile Hu* Laboratory of Inorganic Synthesis and Catalysis, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne (EPFL), ISIC-LSCI, BCH 3305, 1015 Lausanne (Switzerland), xile.hu@epfl.ch S1

2 Experimental details X-ray diffraction (XRD) measurements were carried out on a X'Pert Philips diffractometer in Bragg- Brentano geometry with Cu K α1 radiation (λ = nm) and a fast Si-PIN multi-strip detector. The tube source was operated at 45 kv and 40 ma. The diffraction patterns were analysed and compared with references in the international center of diffraction data (ICDD). HRTEM images were taken on a FEI Tecnai Osiris equipped with an 11 Megapixel Gatan Orius CCD camera. TEM images were taken on a Philips FEI CM12 with a LaB 6 source operated at 120 kv accelerating voltage. The samples were prepared by ultrasonic dispersion of the materials in absolute ethanol. The slurries were mixed with a micropipette by several suction-release cycles to ensure representative and reproducible TEM samples. A few drops of the mixed suspensions were deposited onto the carbon-coated grids. XPS measurements were performed at the Molecular and Hybrid Materials Characterization Centre (MHMC) at the École Polytechnique Fédérale de Lausanne (EPFL). The instrument used was a PHI5000 VersaProbe II XPS system by Physical Electronics (PHI) with a detection limit of 1 atomic percent. Monochromatic X-rays were generated by an Al K source (1,4867 ev). The diameter of the analyszed area is 10 m. To perform depth profiling measurement, an Ar + ion gun was used to sputter the sample surface. X-ray photoelectron spectroscopy measurements were performed on two different Ni 2 P samples: as-synthesized nanoparticles and the nanoparticles electrochemically pretreated at 1.5 V vs. RHE for one hour. The pretreated sample was deposited onto a gold and chrome coated fluorine-doped tin oxide substrate. The thickness of the gold layer is 150 m while the chrome layer has a thickness of 10 m. The procedure employed for the synthesis of Ni 2 P nanoparticles has already been reported by our group. [1] Briefly, 0.66 g of H 2 NaO 2 P H 2 O (for analysis, Acros) and 0.3 g of NiCl 2 6H 2 O (ReagentPlus, Aldrich) were mixed and ground together at room temperature and ambient atmosphere. The resulting solid was deposited on a quartz boat, which was transferred into a tubular furnace. The mixture was heated at 250 C for one hour under a constant flow of N 2. The colour of the solid mixture changed from green/white to black during the heating process. The resulting solid is then cooled down to room temperature and passivated under a flux of N 2 /O 2 for one hour. The black solid is collected and ground. Side products and impurities are washed off from the product using distilled water (Millipore Milli-Q Integral water purification system, 18.2 MΩ cm resistivity). The nanoparticles are finally dried overnight in an oven at 50 C. The synthesis of Ni 2 P nanowires is an air-sensitive reaction that requires careful precautions. The reaction was carried out under inert atmosphere using glove-box and Schlenk line techniques. The Ni 2 P nanowires synthesis was performed following a previous report. [2] In short, a stock solution composed of 0.5 mmol of Ni(acac) 2 (for synthesis, VWR), 2 mmol of oleic acid (suitable for cell culture, BioReagent, Aldrich) in 10 ml trioctylamine (TOA, 98%, Aldrich) was heated to 120 C. Then, the stock solution was slowly injected using a syringe pump at a rate of 0.05 ml min -1 into a stirred mixture consisting of 5 ml of TOA and 3 mmol of trioctylphosphine (TOP 90%, technical grade, AcroSeal, Acros) heated at 320 C. During injection of the stock solution, the colour of the reaction mixture changed from transparent to yellow, orange and finally black. Once the stock solution was consumed, the mixture was allowed to cool down to room temperature. The obtained products were then washed using a mixture of hexane (HiPerSolv CHROMANORM for HPLC 97%, VWR) and ethanol (absolute alcohol without additive 99.8%, Aldrich), centrifuged and the supernatant was decanted. The washing step was repeated twice, then the nanowires were dispersed in ethanol, collected on a glass watch and dried in an oven at 50 C overnight. Part of the nanowires sample was annealed at S2

3 450 C for 4 hours under 5% H 2 /N 2 gas in order to remove the amorphous surfactant layer present on the as-synthesized nanowires surface. The synthesis of Ni nanoparticles followed reported procedures. [3] Briefly, 1 mmol of Ni(acac) 2 was added to 7 ml of oleylamine and the mixture was stirred under nitrogen atmosphere at room temperature for 20 minutes. The temperature was risen to 130 C and maintained for 20 minutes. The mixture was then rapidly heated to 240 C and kept at this temperature for 30 minutes. The mixture was then cooled down to room temperature and 20 ml of ethanol was then added to precipitate the black nanoparticles. The product was separated from the solution by centrifugation and washed by repeated precipitation of the mixture of ethanol and hexane. The nanoparticles were annealed at 450 C for 4 hours under 5% H 2 /N 2 gas in order to remove the amorphous surfactant layer present on the as-synthesized nanoparticles surface. Bulk Ni(OH) 2 sample (film) was electrochemically deposited onto the glassy carbon surface following a literature procedure. [4] To be specific, the pretreated electrode was rinsed with absolute ethanol, dried with compressed air, and dipped in a 0.1 M nickel nitrate (Ni(NO 3 ) 2 6H 2 O, purum p.a. 97%, Aldrich) aqueous solution. Then a cathodic current of 16 ma cm -2 was applied for 10 seconds. IrO 2 (99.9% Ir) powder was purchased from abcr and used as received. NiO nanoparticles (nanopowder, <50 nm particle size (TEM), 99.8% trace metals basis, Aldrich) was used as received. Electrochemical measurements were recorded using a Gamry Instruments Reference 3000 potentiostat. A traditional three-electrode configuration was used. For polarization and electrolysis measurements, a platinum wire was used as the auxiliary electrode and a double-junction Ag/AgCl (KCl saturated) electrode was used as the reference electrode. Both the counter and reference electrode were rinsed with distilled water and dried with compressed air prior measurements. Glassy carbon electrode (~0.071 cm 2 ) was used as the working electrode unless stated otherwise. Potentials were referenced to a reversible hydrogen electrode (RHE) by adding a value of ( *pH) V. All potentials were converted and referred to the RHE unless stated otherwise. The current density is normalised over the geometric surface area of the electrode. The value j u, used to obtain a dimensionless logarithm, corresponds to a unit current density of 1 ma cm -2. Ohmic drop was corrected using the current interrupt method. A total electrolyte volume of ~50 ml was used to fill the glass cell. The basic electrolyte, 1 M KOH (ph 13.62), was prepared from a standard solution (Titrisol, Merck Millipore) using distilled water. During electrochemical experiments, the electrolyte was agitated using a magnetic stirrer rotating at 300 rpm. The cyclic voltammetry experiments for the oxygen evolution reaction were conducted in 1 M KOH at 25 C using a pretreated working electrode and a scan rate of 5 mv s -1 across a potential window of V vs. RHE. Electrochemical pretreatment of the working electrode was previously described by our group. [5] For this purpose, the bare working electrode underwent 20 CV cycles at a potential window of 0.5 to 1.9 V vs. RHE at a scan rate of 100 mv s -1 in 1 M KOH at 25 C. Finally, the pretreated electrode was rinsed with absolute ethanol and dried with compressed air. The catalyst powders were loaded on a pretreated working electrode via drop-casting of 10 µl of catalyst ink, equivalent to a loading of 0.14 mg cm -2. The catalyst suspension, used throughout most electrochemical experiments, was a 1 ml solution consisting of 1 mg of catalyst, 4 µl of Nafion (117 solution, Aldrich), 200 µl of 2-propanol (HPLC grade, 99.8%, Fischer Scientific), and 796 µl of distilled water. Prior recording the OER activity of Ni 2 P, the catalysts were activated by 25 CV scans along the potential window of V vs. RHE in 1 M KOH at a scan rate of 5 mv s -1. The electrochemical surface area (ESCA) was evaluated in terms of doubler layer capacitance. To determine the capacitance value, catalysts deposited onto pretreated glassy carbon electrode were first activated by 25 cyclic voltammetry scans. Then 10 cyclic voltammetry scans were performed at S3

4 different scan rate in a potential window where no Faradaic process occurs. Typically this window ranged from 1 to 1.1 V vs. RHE in 1 M KOH. The cyclic voltammetry sequence was performed at the following scan rates: 20, 40, 80, 100, 150, 200, 400, 600, 800, 1000, 1200, 1500 and 2000 mv s -1. The capacitance was determined from the tenth cyclic voltammetry curve of each scan rate. All catalysts followed the before-mentioned procedure except Ni 2 P nanowires. For this material deposited and activated on a pretreated GC electrode, the potential was cycled 5 times between 0.9 to 1.1 V vs. RHE at each of the following scan rate: 20, 40, 60, 80, 100 and 120 mv s -1. The fifth cyclic voltammetry curve served as basis for the ESCA calculation of Ni 2 P nanowires. To characterize Ni, NiO and Ni 2 P nanoparticles during the OER process, nanoparticles samples were electrochemically pretreated at 1.5 V vs. RHE for one hour. For the stability and the oxygen yield measurements of Ni 2 P, galvanostatic experiments were performed in 1 M KOH at 25 C applying a fixed current density (10 ma cm -2 and 14 ma cm -2, respectively) to the pretreated working electrode for several hours (10 hours and 4 hours, respectively). Prior to the galvanostatic measurements, the materials were cycled 20 times using cyclic voltammetry (CV) to fully activate the catalyst at OER conditions. To remove the NiO x layer after OER, the electrode was dipped in acid solutions. The electrode was then rinsed with distilled water and subjected to CV scans in 1 M H 2 SO 4 with a scanrate of 20 mv s -1. Prior to the use of Ni foam (GoodFellow, porosity 95%, purity 95%) as catalyst support, the foam pieces were cut to leave a determined square surface area. The remaining surface was isolated from the electrolyte using hot glue as insulator. Then, the foam pieces were pretreated in HCl (12 M) for 10 minutes to increase the support s hydrophilicity and were rinsed with deionized water. To measure the HER activity of Ni 2 P in 1 M KOH, 1.8 mg of catalyst dispersed in i-proh, was loaded on 1 centimeter square of pretreated Ni foam. A linear sweep voltammetry experiment was then performed across a potential window of -0.1 to -0.3 V vs. RHE. The electrolyte was agitated using a magnetic stirrer rotating at 300 rpm. To evaluate the bifunctionality of Ni 2 P in alkaline solutions, the catalyst was first dispersed in i-proh, and the suspension was loaded on two Ni foam pieces. The Ni foam support was pretreated and fabricated so that the exposed surface area to the electrolyte corresponded to 1 centimeter square. The loading of catalyst was of 5 mg cm -2. To ensure the catalyst binds to the Ni foam support, 45 L of Nafion was then dispersed on each electrode. The alkaline electrolyzer operated using a twoelectrode settings in a simple glass beaker containing 80 ml of 1 M KOH electrolyte solution. One Ni 2 P loaded Ni foam served as cathode, a second one served as anode. LSV experiments were performed across a potential window of 1.1 to 1.75 V at a scan rate of 10 mv s -1 with vigorous stirring (above 500 rpm) of the electrolyte to ensure full activation of the catalyst on the anode. After oxidation and activation, a final LSV was recorded to measure the catalytic efficiency of Ni 2 P operating as alkaline electrolyzer. Then, the stability of the electrolyzer was examined using galvanostatic experiments in the same electrolyte. The current density was kept constant at 10 ma cm -2 over 10 hours of electrolysis. To evaluate the Faraday yield of our alkaline electrolyzer, two pretreated pieces of Ni foam (exposing a surface area of 1 cm 2 ) were loaded with Ni 2 P up to 10 mg cm -2. The i-proh slurries containing the Ni 2 P catalysts also contained 25 l of Nafion. Prior to the electrolysis, Ni 2 P loaded Ni foams underwent supplementary Nafion solution addition. In total to ensure the catalyst binding to the Ni foam support, a total of 75 l of Nafion was used per electrode. Then the Ni foam electrodes were inserted in a gas tight electrochemical cell, connected to a SensorTechnics DSDX0500D4R differential pressure transducer. The pressure data was recorded using an A/D Labjack U12 interface with a S4

5 sampling interval of 0.1 point per second. Prior electrolysis, the pressure sensor apparatus was calibrated. Calibration of the pressure sensor apparatus was performed by injecting known amount of gas volume within the cell and measuring the corresponding pressure observed. The collected data allows the generation of a calibration curve that will yield the amount of gas evolved in our catalytic system. The faradaic yield was calculated from the total amount of charge passed through the cell during electrolysis and the total amount of gas evolved recorded by the pressure sensor. The anode was as well activated prior pressure sensing experiment. The electrolysis experiment that was monitored via pressure sensor was a galvanostatic experiment at constant current density of 10 ma cm -2. To prevent gas mixture during water splitting experiments, an alkaline electrolysis system was fabricated using an electrochemical cell where the cathode and anode components are separated by a glass frit. The experiments tested the activity of Ni 2 P loaded Ni foams and the activity of the Ni foams without catalyst. Under these conditions, Ni foams were cut and isolated to expose only a certain surface area (0.5 cm 2 for Ni 2 P loaded electrodes, 1 cm 2 for Ni foams without catalyst). The catalyst loading for Ni 2 P was 10 mg cm -2. Both experiments required an activation process to fully activate the anodes. This activation process consisted of a series of LSV voltammetry at a potential window of 1.1 to 1.8 V. After activation a final LSV scan was performed to evaluate the electrochemical activity of the two systems. Subsequent to the LSV, a galvanostatic experiment was performed to probe the stability of the alkaline electrolyzer using Ni 2 P as electrocatalyst. The amount of oxygen produced in electrolysis was measured by an Ocean Optics Multifrequency Phase Fluorimeter (MFPF-100) with a FOXY-OR 125 probe. A linear two point calibration curve was created using air (20.9% O 2 ) and a sealed glass flask that had been purged with N 2 for several hours (0% O 2 ). The electrochemical cell was filled with 1 M KOH until 7.7 ml of headspace remained. Then the oxygen probe was inserted in the airtight H-shaped glass cell. The headspace was purged by nitrogen for one hour to establish a baseline in oxygen concentration (0%). A constant current density of 14 ma cm -2 was then passed for 210 min. The quantity of oxygen produced during this time was determined by converting its volume to mol using the ideal gas law. The faradaic yield was calculated from the total amount of charge passed through the cell and the total amount of oxygen produced, assuming that four electrons are needed to produce one O 2 molecule. S5

6 Counts (arb. unit) Counts (arb. unit) a (111) (201) (210) (002) (300) (211) Ni 2 P Ni(PH 2 O 2 ) 2 (H 2 O) (degree) b (111) (201) (210) (300) (002) (211) NiO 2 Ni 2 P Ni 12 P (degree) Figure S1 a) XRD pattern of the as-synthesized Ni 2P nanoparticles. The stick patterns correspond to diffraction peaks of standard samples in the ICDD database, i.e., Ni 2P (ICDD ) and Ni(PH 2O 2) 2 (H 2O) 6 (ICDD ). b) Powder X-ray diffraction pattern of Ni 2P nanowires. The stick patterns correspond to diffraction peaks of standard samples in the ICDD database, i.e., Ni 2P (ICDD ), Ni 12P 5 (ICDD ) and NiO 2 (ICDD ). In both cases, the most prominent features correspond to Ni 2P, indicating high purity of the materials. S6

7 a b c d Figure S2. a) TEM image of polydispersed Ni 2P nanoparticles. b) Corresponding selected area electron diffraction (SAED) pattern of polydispersed Ni 2P nanoparticles. The SAED pattern is indicative of polydispered crystalline material. c) TEM image of a single crystalline Ni 2P nanowire. d) Corresponding SAED pattern of Ni 2P nanowires. The SAED pattern of the nanowire confirms the monodispersity and the crystallinity of the material. S7

8 Counts (arb. unit) a (111) (200) Ni (220) (degree) b c d e f g Figure S3. a) XRD pattern of the Ni nanoparticles. The stick patterns correspond to diffraction peaks of standard sample in the ICDD database, i.e., Ni (ICDD ) b) TEM image Ni nanocrystals prior OER catalysis. c) TEM image of Ni nanocrystals after OER catalysis. Inset: SAED pattern of a single Ni nanocrystal. d) HAADF- STEM image of Ni sample after 1.5 V vs. RHE for one hour. The contrast increases with the atomic number Z and sample thickness. e - g) Elemental mapping images obtained from EDX in STEM mode on the annealed Ni sample after catalysis. e) Elemental mapping of nickel. f) Elemental mapping of oxygen. g) Combined elemental mapping of oxygen and nickel. The particles shows an oxide layer with low relative content of Ni after potentiostatic conditions at 1.5 V vs. RHE for one hour. This suggests that a similar behavior to Ni 2P is observed on Ni after OER, i.e., the formation of a core-shell heterostructure. S8

9 Counts (arb. unit) a (012) (003) (104) (021) (006) (024) NiO (degree) b c Figure S4. a) XRD pattern of the commercial NiO nanoparticles. The stick patterns correspond to diffraction peaks of standard sample in the ICDD database, i.e., NiO (ICDD ) b) TEM image of polydispersed NiO nanoparticles prior OER catalysis. c) TEM image of polydispersed NiO nanoparticles after OER catalysis. Inset: SAED pattern of NiO nanopowder. The particles are not structurally modified and remain crystalline after potentiostatic conditions at 1.5 V vs. RHE for one hour. S9

10 Current density (ma cm -2 ) Ni 2 P nanowires Ni 2 P nanowires (annealed) Potential (V vs. RHE) Figure S5. Cyclic voltammograms comparing the oxygen evolution activity of nickel phosphide nanowires. The as-synthesized nanowires generates current density of 10 ma cm -2 at 332 mv of overpotential (red curve). Upon annealing, the coating layer of surfactant is removed, allowing higher conductivity and higher active sites exposure. Consequently, the oxidation current prior OER is significantly increased. Thus, the oxygen evolving capabilities of the Ni 2P nanowires are improved. Current density of 10 ma cm -2 are now obtained at 291 mv of overpotential (orange curve), similar to the polydispersed Ni 2P nanoparticles. S10

11 a b c d e f Figure S6. (a) HRTEM image of the Ni2P nanoparticles after electrochemical pretreatment at 1.5 V vs. RHE for one hour. Inset (lower left): FFT of the framed area (middle). The spots observed on the FFT are indicative of registry order and so of crystallinity. The lattice fringes spacing of the materials were determined using FFT. They correspond to the characteristic <100> facet of Ni 2P and the specific facet of NiO x compounds. (b) HAADF-STEM image of the Ni 2P nanoparticles. The contrast increases with the atomic number Z and sample thickness. The small spots observed within the outer layer delimit ultrafine nanoparticles (c) (f) Corresponding EDX maps of the elements on the sample region shown in (a). (b) Nickel elemental mapping. (c) Oxygen elemental mapping. (d) Phosphorus elemental mapping. (e) Combined elemental mapping of Ni, O, and P. This shows the presence of a surface oxide layer around metallic nickel phosphide core.. S11

12 Counts (arb. unit) Counts (arb. unit) a Binding energy (ev) b Binding energy (ev) S12

13 Counts (arb. unit) Counts (arb. unit) Counts (arb. unit) c Ni 2p d O 1s Binding energy (ev) e P 2p Binding energy (ev) Binding energy (ev) Figure S7. a) XPS survey spectrum of the nickel phosphide nanoparticles after electrochemical pretreatment one hour at 1.5 V vs. RHE. The presence of gold and tin comes from the substrate onto which the particles were deposited. b) XPS survey spectrum of the as-prepared nickel phosphide nanoparticles after argon ion sputtering for 5 minutes corresponding to a sputtering depth of 50 nm. Sputtering after 5 minutes indicated vacillating concentrations of oxygen and phosphorus, most likely due to inhomogeneous sputtering. The arrows direction indicate the increase or decrease of atomic content as the sputtering depth increases. c and d) High-resolution XPS depth-profiling spectra of oxygen and phosphorus, respectively. The binding energies correspond to previous report for Ni 2P. [6] c) High-resolution depth-profiling XPS spectra of the Ni 2p area. As the profiling depth increases (arrow direction), the FWHM decreases, indicative of stronger metallic Ni content. This confirms the presence of a surface oxide layer around metallic nickel phosphide core. d) Oxygen 1s area high-resolution XPS spectra. Deeper scans show lower oxygen content. e) High-resolution XPS spectra of the P 2p area. S13

14 Figure S8. LSV of Ni 2P to pre-activate the catalyst on glassy carbon electrode in 1 M KOH. As the number of scans increases (arrow direction), the catalytic activity increases. Conditions: 5 mv s -1, loading 0.14 mg cm -2. S14

15 a b c d e f Figure S9. a) HRTEM image of the as-synthesized Ni 2P nanoparticles. The absence of lattice fringes in the oxide layer surrounding the particles is indicative of amorphous material. b) HAADF-STEM image whose contrast increases with the atomic number Z and sample thickness. c - f) Corresponding distribution maps of the elements on the sample region shown in a). c) Nickel elemental mapping. d) Oxygen elemental mapping. e) Phosphorus elemental mapping. f) Combined elemental mapping of Ni, O, and P. The shell layer being composed notably of O and P indicates residual P 2O 5 present. S15

16 Counts (arb. unit) Counts (arb. unit) a Binding energy (ev) b Binding energy (ev) S16

17 Counts (arb. unit) Counts (arb. unit) Counts (arb. unit) c Ni 2p d O 1s Binding energy (ev) e P 2p Binding energy (ev) Binding energy (ev) Figure S10. a) XPS survey spectrum of the as-prepared nickel phosphide nanoparticles. b) XPS survey spectrum of the as-prepared nickel phosphide nanoparticles after argon ion sputtering for 25 minutes corresponding to a sputtering depth of 250 nm. c - e) High-resolution XPS depth-profiling spectra of nickel, oxygen and phosphorus. The binding energies correspond to previous report for Ni 2P. [6] The arrows direction indicate the increase or decrease of atomic content as the sputtering depth increases. c) High-resolution XPS spectra of the Ni 2p area. As the profiling depth increases, the half-width at full maximum decreases indicative of stronger metallic Ni content. This confirms the presence of a coating nickel oxide layer around metallic nickel phosphide core. d) Oxygen 1s high-resolution XPS spectra. Deeper scan indicates lower oxygen content validating the limited thickness of the oxide layer. e) High-resolution XPS spectra of the P 2p region. The higher-content of phosphorus in the bulk material supports the presence of metallic core beneath the oxidation layer. S17

18 Current density (ma cm -2 ) a b Figure S11. (a) EDX spectrum of Ni 2P nanoparticles after one hour electrolysis at 1.5 V vs. RHE. Qualitative X- ray analysis detected the presence of iron on the surface of the catalytic particles (cyan sticks). (b) Cyclic voltammogram probing the activity of Ni 2P under iron free conditions. The removal of iron impurities from the electrolyte lead to an increase of 120 mv in the overpotential required to reach 10 ma cm -2. Conditions: 5 mv s -1, iron-free electrolyte, 0.14 mg cm -2, pretreated GC. S18

19 Current density (ma cm -2 ) Potential (V vs. RHE) Figure S12. LSV of Ni 2P on Ni foam in 1 M KOH for HER. The catalyst generates 10 ma cm -2 of current density at 221 mv of overpotential. Conditions: 5 mv s -1, loading 1.8 mg cm -2. S19

20 Figure S13. Faraday yield measurement of the alkaline electrolyser fabricated from Ni 2P loaded Ni foams (loading 10 mg cm -2 ). The quantity of gas evolved was determined by a pressure sensor. The overlapping lines between the theoretical and experimental values indicate a quantitative Faraday yield of the system, after an induction period of about 250 s. During this induction period, the gas generated are dissolved in the solution to reach an equilibrium. After the induction period, the gas generated can be measured by the pressure sensor. The galvanostatic experiment was performed over 1 hour at a constant current density of 10 ma cm -2. The anode was activated prior the galvanostatic experiment. The cathode was used as it is. S20

21 Current density (ma cm -2 ) Potential (V) Ni 2 P Time (s) Ni foam Potential (V) Figure S14. LSV to evaluate the alkaline electrolyzer activity under gas separating conditions. The Ni 2P system (loading 10 mg cm -2, activated anode) necessitates only 360 mv to generate 10 ma cm -2. This result is similar to our previous conditions without gas separating equipment. The Ni foam support was evaluated under similar conditions (gas separation, activated anode but without catalyst) and required 560 mv to generated 10 ma cm -2. A galvanostatic experiment on the Ni 2P alkaline electrolyzer with glass frit separation was performed (Inset) and indicates the good stability of the system over 10 hour of electrolysis at 10 ma cm -2. An optical photograph illustrates the electrochemical cell used for these experiments. S21

22 Current density (ma cm -2 ) Scan # Potential (V vs. RHE) Figure S15. LSV of Ni 2P in 1 M H 2SO 4 after OER in alkaline media. High HER activity is obtained after one thousand scans, indicating the removal of NiO x shell. Conditions: 20 mv s -1, 0.14 mg cm -2. S22

23 References [1] L. Feng, H. Vrubel, M. Bensimon, X. Hu, Phys. Chem. Chem. Phys. 2014, 16, [2] Y. Chen, H. She, X. Luo, G.-H. Yue, D.-L. Peng, J. Cryst. Growth 2009, 311, [3] Y. Chen, D.-L. Peng, D. Lin, X. Luo, nanotechnology 2007, 18, [4] D. A. Corrigan, R. M. Bendert, J. Electrochem. Soc. 1989, 136, [5] L.-A. Stern, X. Hu, Faraday Discuss. 2014, 176, [6] P. E. R. Blanchard, A. P. Grosvenor, R. G. Cavell, A. Mar, J. Mater. Chem. 2009, 19, S23