Supplementary Material

Growth rate and resistivity of Ni thin films as a function of substrate temperature (Figure S1), XPS spectra of Ni thin films (Figure S2), Optimization of ALD process for Ni catalyst (Figure S3), Equivalent Ni thickness and Ni atom density on carbon black (Figure S4), Magnified XRD (Figure S5), XPS O 1s spectra (Figure S6), Chronoamperometric stability test (Figure S7), Relationship between HER overpotential and the Ni0/Ni2+ ratio of Ni/C catalysts (Figure S8), Average particle size and size dispersion (Table S1), Relative % of binding energy (Table S2), Comparison of HER performance (Table S3)