Supplementary Figure 1. Supplementary Figure 2.

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1 Supplementary Figure 1. STEM annular dark field (ADF) image of NiO/Ni-CNT showing non-uniform coating of NiO nanoparticles on Ni cores (the red circles show individual NiO nanoparticles with different orientations, and the red arrow points out a NiO/Ni heterostructure with obvious morphology of Ni core nonuniformly coated with small NiO nanoparticles. Supplementary Figure 2. a) Ni K edge XANES spectrum of NiO/Ni-CNT (black), standard pattern of Ni (blue) and NiO (purple), linear fitting spectrum (red). b) XPS survey spectrum of NiO/Ni-CNT and sputtered NiO/Ni-CNT by Ar ion gun. 1

2 Supplementary Figure 3. Tafel plots of the HER catalytic activity of NiO/Ni-CNT loaded a) on RDE at a loading of 0.28 mg cm -2 and b) on Ni foam at a loading of 8 mg cm -2. Supplementary Figure 4. a-c) STEM BF images of a) NiO/Ni-CNT b) NiO/CNT and c) Ni/CNT, d-f) STEM ADF images of d) NiO/Ni-CNT e) NiO/CNT and f) Ni/CNT, g-i) Electron diffraction (ED) patterns of g) NiO/Ni-CNT h) NiO/CNT and i) Ni/CNT 2

3 Supplementary Figure 5. a) STEM ADF image of NiO/CNT hybrid. b-d) Chemical maps for the spatial distribution of O, Ni and C, from the whole area shown in Figure a. e) Reconstructed chemical map from the whole area shown in Figure b, with Ni in red, O in green and C in blue. Supplementary Figure 6. a) STEM ADF image of Ni/CNT hybrid. b-d) Chemical maps for the spatial distribution of O, Ni and C, from the whole area shown in Figure a. e) Reconstructed chemical map from the whole area shown in Figure a, with Ni in red, O in green and C in blue. 3

4 Supplementary Figure 7. Linear sweep voltametry of the CNT alone compared to other three hybrid materials in 1 M KOH at a scan rate of 1 mv s -1 under the loading of 0.28 mg cm -2 on RDE. Supplementary Figure 8. a) BET surface area analysis of NiO/Ni-CNT and Ni/CNT b) Linear sweep voltametry of NiO/Ni-CNT and Ni/CNT in 1 M KOH under the loading of 0.28 mg cm -2 on RDE with normalization to surface area. Supplementary Figure 9. Linear sweepment voltametry of Ni/CNT hybrid and physical mixture of Ni/CNT and NiO/CNT in 1 M KOH at a scan rate of 1 mv/s under the loading of 0.28 mg cm -2 on RDE. 4

5 Supplementary Figure 10. Ni L edge XANES spectra of CNT hybrid and CNT-free nanoparticle. Supplementary Figure 11. a) Linear sweep voltametry of NiO/Ni-CNT with differently oxidized CNT precursors (1x CNT, 2x CNT and 4.5x CNT, x refers to the mass ratio of KMnO 4 to C used in the modified Hummer s method) deposited on Ni foam at a scan rate of 1 mv s -1 under the loading of 8 mg cm -2 in 1 M KOH Supplementary Figure 12. a) Uncompensated linear sweep voltametry of NiO/Ni-CNT deposited on Ni foam at a scan rate of 1 mv s -1 under the loading of 8 mg cm -2 in 1 M KOH (resistance=~1.0 ohm). b) Uncompensated linear sweepment voltametry of water electrolysis using NiO/Ni-CNT as HER catalyst and NiFe LDH as OER catalyst (both loaded into Ni foam at a loading of 8 mg cm -2 ) in 1 M KOH under different temperature (resistance =~1.6 ohm at 23 o C and ~1.1 ohm at 60 o C) 5

6 Supplementary Figure 13. Mean value and standard deviation of 6 linear sweep voltammetry curves of NiO/Ni-CNT loaded on Ni foam at a loading of 8 mg cm -2 in 1 M KOH Supplementary Figure 14. (a-b) Linear sweepment voltametry of NiO/Ni-CNT, Pt/C deposited on Ni foam and pure Ni foam at a scan rate of 1 mv s -1 under the loading of 8 mg cm -2 in (a) NaHCO 3 -Na 2 CO 3 buffer (ph=10.0) and (b) K-borate buffer (ph=9.5). (c) Chronopotentiometry of NiO/Ni-CNT in three electrolytes under a constant current density of 20 Ma cm -2. Supplementary Figure 15. Linear sweepment voltametry of water electrolysis using NiO/Ni-CNT as HER catalyst and NiFe LDH as OER catalyst (both loaded into Ni foam at a loading of 8 mg cm -2 ) before (solid line) and after 24 h stability test (dashed line) in 1 M KOH under different temperature. 6

7 Supplementary Table 1 Summary of the HER catalytic activity of representative catalysts Catalyst Loading Electrolyte Overpotential Current density Reference (mg cm -2 ) (mv) (ma cm -2 ) NiO/Ni-CNT M KOH This work NiO/Ni-CNT 8 1 M KOH This work Ni-Mo nanopowder M KOH McKone, et al. 1 Ni-Mo nanopowder M H 2 SO McKone, et al. 1 Ni-Mo nanopowder M KOH McKone, et al. 1 CoP on Ti M H 2 SO 4 ~85 20 Popczun, et al. 2 Ni 2 P M H 2 SO Popczun, et al. 3 MoS 2 /RGO M H 2 SO Li, et al. 4 Ni-Mo on Ni 40 1 M KOH Xiao, et al. 5 Ni-Mo-N nanosheet M HClO Chen, et al. 6 Mo 2 C/CNT M HClO 4 ~ Chen, et al. 7 1 McKone, J. R., Sadtler, B. F., Werlang, C. A., Lewis, N. S. & Gray, H. B. Ni-Mo Nanopowders for Efficient Electrochemical Hydrogen Evolution. Acs Catalysis 3, (2013). 2 Popczun, E. J., Read, C. G., Roske, C. W., Lewis, N. S. & Schaak, R. E. Highly active electrocatalysis of the hydrogen evolution reaction by cobalt phosphide nanoparticles. Angewandte Chemie (International ed. in English) 53, (2014). 3 Popczun, E. J. et al. Nanostructured Nickel Phosphide as an Electrocatalyst for the Hydrogen Evolution Reaction. Journal of the American Chemical Society 135, (2013). 4 Li, Y. et al. MoS2 Nanoparticles Grown on Graphene: An Advanced Catalyst for the Hydrogen Evolution Reaction. Journal of the American Chemical Society 133, (2011). 5 Xiao, L. et al. First implementation of alkaline polymer electrolyte water electrolysis working only with pure water. Energy & Environmental Science 5, (2012). 6 Chen, W.-F. et al. Hydrogen-Evolution Catalysts Based on Non-Noble Metal Nickel-Molybdenum Nitride Nanosheets. Angewandte Chemie-International Edition 51, (2012). 7 Chen, W. F. et al. Highly active and durable nanostructured molybdenum carbide electrocatalysts for hydrogen production. Energy & Environmental Science 6, (2013). 7