Supplementary Information

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1 Supplementary Information Ni 2 P(O)/Fe 2 P(O) Interface Can Boost Oxygen Evolution Electrocatalysis Peng Fei Liu, Xu Li, Shuang Yang, Meng Yang Zu, Porun Liu, Bo Zhang, Li Rong Zheng, # Huijun Zhao, and Hua Gui Yang *, Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai , China Centre for Clean Environment and Energy, Gold Coast Campus, Griffith University, Queensland 4222, Australia State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Fudan University, Shanghai , China # Institute of High Energy Physics, Chinese Academy of Sciences, Beijing , China * Correspondence and requests for materials should be addressed to H. G. Y. ( hgyang@ecust.edu.cn) (H.G.Y).

2 Experimental Section Chemicals. Ammonium fluoride (NH 4 F), nickel (II) nitrate hexahydrate (Ni(NO 3 ) 2 6H 2 O) and ferric (III) nitrate nonahydrate (Fe(NO 3 ) 3 9H 2 O) were obtained from Sinopharm Chemical Regent Co., Ltd. Urea was purchased from Shanghai Lingfeng Chemical Regent Co., Ltd. Sodium hypophosphite (NaH 2 PO 2 ) was obtained from Aladdin. Nafion (5 wt%) was obtained from Sigma-Aldrich. Iridium (IV) oxide (IrO 2 ) were obtained from Strem Chemicals, Inc. All chemicals were of analytical grade and used without any further purification. Ni foam was washed in 6 M HCl, ethanol and deionized water several times to ensure the surface of the Ni foam was well cleaned before use. Synthesis of Ni(OH) x precursor. The hydroxide precursors were synthesized by a hydrothermal method. For the preparation of Ni(OH) x precursor, urea (10 mmol) and Ni(NO 3 ) 2 6H 2 O (2 mmol) was added into deionized water (40 ml) for continuous stirring of 15 min, and then injected into a 50 ml Teflon-lined stainless steel autoclave. Afterwards, 3 pieces of Ni foam (with area of 3 cm * 0.5 cm) were placed in the autoclave and kept at 100 o C for 10 h. After the autoclave cooled down slowly at room temperature, the samples were collected and washed with water and ethanol several times and then dried at 60 o C for 12 h. Synthesis of S-Ni(OH) x /Fe(OH) x precursor. The synthetic procedure was similar to the synthesis of Ni(OH) x precursor except that the precursors containing urea (10 mmol), NH 4 F (8 mmol) and Fe(NO 3 ) 3 9H 2 O (2 mmol) and being kept at 120 o C for 10 h. Synthesis of M-Ni(OH) x /Fe(OH) x precursor. After synthesis of S-Ni(OH) x /Fe(OH) x precursor, 3 pieces of S-Ni(OH) x /Fe(OH) x decorated Ni foam were placed in the autoclave which containing urea (10 mmol) and Ni(NO 3 ) 2 6H 2 O (2 mmol) dissolved in the deionized water (40 ml). Then the autoclave was kept at 100 o C for 10 h. After the autoclave cooled down slowly at room temperature, the samples were collected and washed with water and ethanol several times and then dried at 60 o C for 12 h. Synthesis of corresponding phosphides. To synthesize phosphides, NaH 2 PO 2 (1.5 g) was placed at the upstream side of the tube furnace and the corresponding hydroxide precursors were placed at the downstream side. Subsequently, the samples were heated to 400 C, and then stayed for 120 min under Ar gas flowing at 75 s.c.c.m. We

3 treated all the hydroxide precursors following the above step, to finally obtain Ni 2 P, S-Ni 2 P/Fe 2 P and M-Ni 2 P/Fe 2 P samples from Ni(OH) x, S-Ni(OH) x /Fe(OH) x and M-Ni(OH) x /Fe(OH) x precursors, respectively. Synthesis of one-pot derived NiFeP. To prepare the hydroxide precursor, the synthetic procedure was similar to the synthesis of Ni(OH) x precursor except that the precursors containing urea (10 mmol), NH 4 F (8 mmol), Ni(NO 3 ) 2 6H 2 O (1 mmol) and Fe(NO 3 ) 3 9H 2 O (1 mmol) and being kept at 120 o C for 10 h. After that, the phosphidation treatment was similar to synthesis of Ni 2 P and other phosphides. Synthesis of corresponding annealed oxides. To obtain oxide samples, the heat treatment was directly performed under air environment and corresponding hydroxide precursors were annealed at 500 o C for 2 h with an increment of 2 o C/min. We treated all the hydroxide precursors following the above step, to finally obtain Ni(OH) x -DO, S-Ni(OH) x /Fe(OH) x -DO and M-Ni(OH) x /Fe(OH) x -DO samples from Ni(OH) x, S-Ni(OH) x /Fe(OH) x and M-Ni(OH) x /Fe(OH) x precursors, respectively. Preparation of IrO 2 decorated electrodes. We firstly prepared the ink solution containing IrO 2 (10 mg), Nafion (40 µl) and ethanol (1 ml) which were under ultrasonic treatment for 30 min. Then the Ni foam (with exposure area of 0.5 cm * 0.5 cm) was immersed into the ink solution for 30 s and dried with evaporation of ethanol. The immersing loading treatment was repeated for 8 times, and the final loading of IrO 2 was about 3.2 mg/cm 2. Electrochemical measurements. All the OER electrochemical tests were performed in a conventional three-electrode system at an electrochemical station (CHI 660E), using Ag/AgCl (3.5 M KCl solution) electrode as the reference electrode, a graphite rod (spectral purity, 3 mm in diameter) as the counter electrode and as-synthesized catalysts on Ni foam as the working electrode. The catalyst geometric mass loadings were calculated from the weight increment after catalyst preparation. The mass loadings of Ni 2 P, S-Ni 2 P/Fe 2 P, M-Ni 2 P/Fe 2 P and NiFeP are around 3.0, 3.5, 4.0 and 3.4 mg/cm 2, respectively. Linear sweep voltammetry (LSV) with the backward scan rate of 1 mv/s was conducted in 1 M KOH (ph 13.6) purged by Ar for 30 min at room temperature, to avoid the oxidative peak and possible capacitance current. All potentials were referenced to reversible hydrogen electrode (RHE) by following calculations: E RHE = E Ag/AgCl ph All the LSV curves were not ir-corrected. To obtain the onset overpotential, LSV curves were conducted with the

4 backward scan rate of 1 mv/s, and identify the point of overpotential across the X axis. AC impedance measurements were carried out in the same configuration when the working electrode was biased at a certain overpotential from 10 5 Hz to 0.1 Hz with an AC voltage of 5 mv. The chronopotentiometric curves were corrected with ir-compensation, and the equivalent series resistance (R s ) can be obtained from the EIS Nyquist plot as the first intercept of the main arc with the real axis. The stability test was conducted at the constant current density of 40 ma/cm 2 for motivate OER process. The ECSA was determined by measuring the capacitive current associated with double-layer charging from the scan-rate dependence of cyclic voltammograms (CVs). The potential window of cyclic voltammograms was 1.15 to 1.25 V vs. RHE. The double layer capacitance (C dl ) was estimated by plotting the j = (j a - j c ) 1.25 V against the scan rate, respectively. The liner slope is twice of the double layer capacitance C dl. The ECSA of each sample was calculated according the equation: ECSA = S C dl /C s, in which S stands for the real geometric area of the Ni foam electrode and C s represents the specific capacitance (assuming the value of the catalysts for OER in 1 M KOH is mf/cm 2 ). Electrochemical oxidation treatments. For OER tests, the catalysts were cycled 100 times using cyclic voltammetry (CV) from 1.2 to 1.8 V (vs. RHE) with the scan rate of 50 mv/s, to fully activate the catalyst and generate active oxidized layers at OER condition. For different characterizations, such as FESEM, TEM, and XAFS analysis, the M-Ni 2 P/Fe 2 P-O sample and controlled samples were obtained after treated at the potential of 1.6 V (vs. RHE) for about 2 hours. For XRD and XPS analysis, the M-Ni 2 P/Fe 2 P-O sample and controlled samples were obtained after chronopotentiometric test at the current density of 40 ma/cm 2 for different time. Characterizations. The morphologies and structures of the samples were characterized by scanning electron microscopy (SEM, Hitachi S4800) and transmission electron microscopy (TEM, JEM-2100F and Philips Tecnai F20). The crystal structure was determined by X-ray diffraction (XRD, D/max2550V). Furthermore, the chemical states of the elements in catalysts were studied by X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD), and the binding energy of C 1s peak at ev was taken as an internal standard.

5 Figure S1. Structure schematic for M-Ni 2 P/Fe 2 P precatalysts in-situ transferring to M-Ni 2 P/Fe 2 P-O postcatalysts, with active oxidized surface layers and conductive bulk phosphide.

6 Figure S2. FESEM images of M-Ni(OH) x /Fe(OH) x at different magnifications.

7 Figure S3. The visual difference between S-Ni 2 P/Fe 2 P and M-Ni 2 P/Fe 2 P, revealing that M-Ni 2 P/Fe 2 P owns more active units, with more Ni 2 P/Fe 2 P interfaces as precatalysts to boost OER.

8 Figure S4. FESEM images of (a),(b) Ni(OH) x and (c),(d) Ni 2 P.

9 Figure S5. FESEM images of (a-c) S-Ni(OH)x/Fe(OH)x and (d-f) S-Ni2P/Fe2P.

10 Figure S6. SEM images of the precursor of NiFeP (NiFe(OH) x ), NiFeP and NiFeP after stability test (NiFeP-O).

11 Figure S7. Cross-section TEM image of M-Ni 2 P/Fe 2 P and corresponding elemental mapping images of Ni, Fe and P.

12 Figure S8. XRD patterns of Ni 2 P, S-Ni 2 P/Fe 2 P and M-Ni 2 P/Fe 2 P samples before OER test, respectively.

13 Figure S9. XRD patterns of (a) Ni(OH) x annealing derived oxides (Ni(OH) x -DO), (b) S-Ni(OH) x /Fe(OH) x annealing derived oxides (S-Ni(OH) x /Fe(OH) x -DO) and (c) M-Ni(OH) x /Fe(OH) x annealing derived oxides (M-Ni(OH) x /Fe(OH) x -DO), respectively.

14 Figure S10. XRD patterns of NiFe(OH) x and M-Ni(OH) x /Fe(OH) x precursors. Notes: The precursor of NiFeP (NiFe(OH) x ) kept the layered double hydroxide structure (PDF# ), in which Ni and Fe element were homogeneously dispersed. For comparison, the precursor of M-Ni 2 P/Fe 2 P (M-Ni(OH) x /Fe(OH) x ) showed relatively bad crystallinity, with diffraction peaks which might correspond to FeOOH (PDF# ). Although we couldn t clearly detect the crystalline structure of M-Ni(OH) x /Fe(OH) x, we conclude that the structure of M-Ni(OH) x /Fe(OH) x is different from NiFe(OH) x.

15 Figure S11. XRD patterns of NiFeP and M-Ni 2 P/Fe 2 P samples, revealing no obvious difference between these two catalysts.

16 Figure S12. XPS spectra of Ni and Fe 2p region for NiFe(OH) x and M-Ni(OH) x /Fe(OH) x precursors.

17 Figure 13. XPS spectra of Ni and Fe 2p region for NiFeP and M-Ni 2 P/Fe 2 P samples.

18 Figure S14. XPS spectra of Ni and Fe 2p region for NiFeP-O-2 and M-Ni 2 P/Fe 2 P-O-2 samples (at j of 40 ma/cm 2 for 2 h). Notes: In Figure S12, we observed that Ni element of NiFe(OH) x underwent a positive shift compared with M-Ni(OH) x /Fe(OH) x, while the Fe element of NiFe(OH) x displayed a negative shift compared with M-Ni(OH) x /Fe(OH) x. In Figure S 13 and 14, XPS spectra for NiFeP, M-Ni 2 P/Fe 2 P, NiFeP-O and M-Ni 2 P/Fe 2 P-O indicated the same tendency for Ni and Fe elements. In recent reports, 1, 2 researchers have proved that the incorporated Fe would synergistically improve the Ni valance because of the strong electronic interaction in the NiFe based catalysts. In our work, the Ni and Fe elements were homogeneously dispersed in the whole for the sample of NiFeP, its precursor and its post-oer samples on the surface, while Ni and Fe elements only interacted on the interfaces for M-Ni 2 P/Fe 2 P, its precursor and its post-oer samples. Based on these results, we would like to think the surface oxidized layers might be trace amount (Fe)Ni doped (Ni)FeOOH, not homogeneously dispersed NiFe based (oxy)hydroxides.

19 Figure S15. The survey XPS spectra of M-Ni 2 P/Fe 2 P before and after OER test for 70 h, illustrating that the content of P element became lower after stability test.

20 Figure S16. XPS spectra of P 2p region for M-Ni 2 P/Fe 2 P sample after different time of stability test (at j of 40 ma/cm 2 for 0 h, 2 h, 70 h and 100 h). Notes: Recently, researchers have systematically studied the influence of P leaching phenomenon of phosphide OER precatalysts. 3, 4 Lee, J. Y. et al. have proved that surface phosphates would be oxidized during the OER and the surface formed oxyhydroxides are real active materials. Excellent and systematic work by Alshareef, H. N. et al. has clearly demonstrated that the surface phosphates would be oxidized at a high j of 30 ma/cm 2 for a longer stability test. We also collected the M-Ni 2 P/Fe 2 P-O samples after different time of stability test (0 h, 2 h, 70 h and 100 h, named as M-Ni 2 P/Fe 2 P-O-0, M-Ni 2 P/Fe 2 P-O-2, M-Ni 2 P/Fe 2 P-O-70 and M-Ni 2 P/Fe 2 P-O-100) and conducted the XPS analysis. In Figure S16, we observed the P leaching phenomenon and negligible phosphide signals on the surface. However, P signals could still be detected as PO 3-4 species after 100 hours stability test. We also found that the OER activity slightly degraded for long time stability test, which might result from the reduction of PO 3-4 species.

21 Figure S17. LSV curves of hydroxides, annealed oxides and phosphides catalysts. The polarization curves were obtained with a backward scan rate of 1 mv/s, which were not ir-corrected.

22 Figure S18. EIS spectra of M-Ni(OH) x /Fe(OH) x, S-Ni 2 P/Fe 2 P-O, and M-Ni 2 P/Fe 2 P-O catalysts, respectively.

23 Figure S19. CVs at different scan rates in a potential window where no Faradaic processes occur (1.15 to 1.25 V vs. RHE) for (a) M-Ni 2 P/Fe 2 P-O, (b) S-Ni 2 P/Fe 2 P-O and (c) Ni 2 P-O, respectively, with the scan rates from 10 to 60 mv/s.

24 Figure S20. Enlarged chronopotentiometric curves in the narrow potential region obtained with the M-Ni 2 P/Fe 2 P-O catalyst with constant current density of 40 ma/cm 2 for about 120 h.

25 Figure S21. Multi-current process of M-Ni 2 P/Fe 2 P-O catalyst in 1 M KOH. The catalyst was stabilized and then started at the current density of 40 ma/cm 2 and ended at 240 ma/cm 2, with 40 ma/cm 2 increment every 500 seconds.

26 Table S1. Comparison of OER performances for Ni 2 P/Fe 2 P-O catalysts and recently reported active catalysts in 1 M KOH solutions. Catalysts η 10 (mv) Tafel slope (mv/dec) Substrates References M-Ni 2 P/Fe 2 P-O a Ni foam this work S-Ni 2 P/Fe 2 P-O a Ni foam this work G-FeCoW a 191 / gold coated Ni foam Science 352, (2016) NiFe LDH b Ni foam Nature Commun. 6, 6616 (2015) CoSn(OH) / GCE Energy Environ. Sci. 9, (2016) b Na 0.08 Ni 0.9 Fe 0.1 O GCE Energy Environ. Sci. 10, (2017) NiCeO x -Au b 271 / GCE Nature Energy 1, (2016) NiTi-MMO b GCE J. Am. Chem. Soc (2016) NiCo-UMOFNs b 189 / Cu foam Nature Energy 1, (2016) b Ni 3 S / Ni foam J. Am. Chem. Soc (2015) Co-S b carbon paper/carbon tubes ACS Nano 10, (2016) b Ni x Fe 1-x Se Ni foam Nature Commun. 7, 12324, (2016) (Ni, Co) 0.85 Se b carbon fabric collector Adv. Mater. 28, (2016) CoN b Ni foam Angew. Chem., Int. Ed. 55, (2016) Co 4 N b GCE Angew. Chem., Int. Ed. 54, (2016) Ni 3 N-COF b GCE Adv. Energy Mater. 6, (2016) Ni 3 FeN b Ni foam Adv. Energy Mater. 10, (2016) Ni 2 P GCE Energy Environ. Sci. 8, (2015) CoNi-P b Ni foam Energy Environ. Sci. 10, 893 (2017) b MnCoPO x GCE Angew. Chem., Int. Ed. 56, (2017) Notes: a the potential was not ir-corrected; b the potential was ir-corrected; η 10 means the overpotential needed to drive the current density of 10 ma/cm 2 ; GCE was abbreviation for glassy carbon electrode.

27 References (1) Zhu, K.; Wu, T.; Zhu, Y.; Li, X.; Li, M.; Lu, R.; Wang, J.; Zhu, X.; Yang, W. Layered Fe-Substituted LiNiO 2 Electrocatalysts for High-Efficiency Oxygen Evolution Reaction. ACS Energy Lett. 2017, 2, (2) Li, Y. B.; Zhao, C. Iron-Doped Nickel Phosphate as Synergistic Electrocatalyst for Water Oxidation. Chem. Mater. 2016, 28, (3) Zhan, Y.; Lu, M.; Yang, S.; Liu, Z.; Lee, J. Y. The Origin of Catalytic Activity of Nickel Phosphate for Oxygen Evolution in Alkaline Solution and its Further Enhancement by Iron Substitution. ChemElectroChem 2016, 3, (4) Liang, H.; Gandi, A. N.; Xia, C.; Hedhili, M. N.; Anjum, D. H.; Schwingenschlögl, U.; Alshareef, H. N. Amorphous NiFe-OH/NiFeP Electrocatalyst Fabricated at Low Temperature for Water Oxidation Applications. ACS Energy Lett. 2017, 2,