Efficient Hydrogen Evolution of Oxidized Ni-N₃ Defective Sites for Alkaline Freshwater and Seawater Electrolysis

For mass production of high-purity hydrogen fuel by electrochemical water splitting, seawater electrolysis is an attractive alternative to the traditional freshwater electrolysis due to the abundance and low cost of seawater in nature. However, the undesirable chlorine ion oxidation reactions occurring simultaneously with seawater electrolysis greatly hinder the overall performance of seawater electrolysis. To tackle this problem, electrocatalysts of high activity and selectivity with purposely modulated coordination and an alkaline environment are urgently required. Herein, it is demonstrated that atomically dispersed Ni with triple nitrogen coordination (Ni-N₃ ) can achieve efficient hydrogen evolution reaction (HER) performance in alkaline media. The atomically dispersed Ni electrocatalysts exhibit overpotentials as low as 102 and 139 mV at 10 mA cm^-2 in alkaline freshwater and seawater electrolytes, respectively, which compare favorably with those previously reported. They also deliver large current densities beyond 200 mA cm^-2 at lower overpotentials than Pt/C, as well as show negligible current attenuation over 14 h. The X-ray absorption fine structure (XAFS) experimental analysis and density functional theory (DFT) calculations verify that the Ni-N₃ coordination, which exhibits a lower coordination number than Ni-N₄ , facilitates water dissociation and hydrogen adsorption, and hence enhances the HER activity.