Supporting Information

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1 Supporting Information Designing hybrid NiP 2/NiO nanorod arrays for efficient alkaline hydrogen evolution Meng-Ying Wu, Peng-Fei Da, Tong Zhang, Jing Mao,*, Hui Liu,*, and Tao Ling,*, Key Laboratory for Advanced Ceramics and Machining Technology of Ministry of Education, School of Materials Science and Engineering, Tianjin University, Tianjin , China. S-1

2 Supplementary Figures Figure S1. (a) TEM image of NiO NRs. (b) XRD spectra of as-prepared NiO NRs and NRs after phosphorization. S-2

3 0.4 Hydrogen oxidation Current (ma) V RHE = V SCE V Hydrogen evolution Potential (V vs. SCE) Figure S2. Calibration of the reference saturated calomel electrode (SCE). The calibration was performed in hydrogen saturated electrolyte with a Pt sheet as the working electrode. Cyclic voltammetry run at a scan rate of 1 mv s, and the average of the two potentials at which the -1 current value was zero was taken as the thermodynamic potential. Therefore, in 1 M KOH, VRHE = VSCE V. S-3

4 Figure S3. Illustration diagram of cation exchange process. S-4

5 Figure S4. SEM characterizations of hybrid NiP /NiO NRs with varied molar ratios of NiP and NiO NRs. (a1)-(a3) 1:4 NiP /NiO NRs. (b1)-(b3) 1:2 NiP /NiO NRs. (c1)-(c3) 3:2 NiP /NiO NRs. (d1)-(d3) NiP NRs S-5 2

6 Figure S5. O 1s spectrum of 1:2 NiP 2/NiO NRs after argon sputtering. The large area of peak II suggests that O-vacancies on NiO are present after the phosphorization treatment. Note that this sample was etched with an argon beam for 30 s to remove the surface NiP 2. S-6

7 NiP 2 /NiO NRs graphite rod Figure S6. Photograph of a home-made electrochemical cell for the hydrogen evolution reaction (HER) measurements. The three electrode system consists of a SCE reference electrode, a graphite rod as the counter electrode and the synthesized catalyst on CFP as the working electrode. S-7

8 Figure S7. Polarization curves of hybrid NiP 2/NiO NRs with varied NiP 2:NiO composition ratios, NiO NRs, NiP 2 NRs and Pt/C. S-8

9 Figure S8. EIS spectra of hybrid 3:2 NiP2/NiO NRs and NiP2 NRs at (a) high and (b) low frequencies. It shows that the transfer resistance of 3:2 NiP2/NiO NRs is higher than NiP2 NRs, which may leads to the relatively worse performance of 3:2 NiP2/NiO NRs. S-9

10 Figure S9. SEM characterization of hybrid 1:2 NiP2/NiO NRs after durability test 10 ma cm - 2 ). (a) and (b) Low magnification. (c) High magnification. S-10

11 Figure S10. XRD spectrum of hybrid 1:2 NiP2/NiO NRs after durability test 10 ma cm -2 ) compared with that of fresh sample. S-11

12 Figure S11. Long term durability of 1:2 NiP 2/NiO NRs at a current density of 50 ma cm -2. S-12

13 Figure S12. Characterization of 1:2 NiP 2/NiO NRs after long-term durability test 50 ma cm - 2 ). (a) and (b) Low and high magnification SEM characterization. (c) XRD spectra of 1:2 NiP 2/NiO NRs. before and after durability test. (d) EDS spectrum. S-13

14 Figure S13. Characterization of Pt catalysts directly deposited on CFP substrate. It shows that Pt nanoparticles with sizes of about 5 nm are distributed uniformly on CFP substrate. S-14

15 Supplementary Tables Table S1. Quantitative EDS analysis of hybrid NiP2/NiO NRs in TEM. The average NiP2:NiO data were analyzed based on five nanorods for each phosphorization temperature. Phosphorization temperature ( o C) Element (at %) Ni P O Average NiP2: NiO ratio Nanorod Nanorod Nanorod Nanorod Nanorod Nanorod Nanorod Nanorod Nanorod Nanorod Nanorod Nanorod Nanorod Nanorod Nanorod :4 1:2 3:2 Table S2. Summary of the recently reported highly active HER catalysts in alkaline solution. S-15

16 Catalyst Substrate Loading (mg cm ) -2 Electrolyte 10 ma cm -2 (mv ) Tafel slope (mv dec -1 ) Reference NiP2/NiO NRs CFP ~ M KOH This work NixPy CFP ~ M KOH ~ [1] NiP2 nanosheet CC M KOH [2] Ni-P CFP M KOH [3] Ni5P4 Films Ni foil -- 1 M KOH [4] Ni2P GCE ~ M KOH ~ [5] NiCoP/rGO CFP M KOH [6] Co2P nanorods Ti foil 1 1 M KOH ~ [7] Ni-Co-P GCE M KOH [8] CoP nanowire CC M KOH [9] CoP@NC GCE M KOH [10] HNDCM Co/CoP CM -- 1 M KOH [11] N@MoPCx GCE M KOH [12] Co4Ni1P NTs RDE M KOH [13] Abbreviations: NRs=Nanorods; CFP = Carbon fiber paper; CC=Carbon cloth; GCE=Glassy carbon e l e c t r o d e ; rgo = Reduced graphene oxide; NC = N-doped carbon; HNDCM=Nitrogen-doped nanoporous graphitic carbon membranes; CM = Carbon membrane; NTs=Nanotubes;RDE=Rotating disk electrode Supplementary References S-16

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