Nanorod Array-Based Hierarchical NiO Microspheres as a Bifunctional Electrocatalyst for a Selective and Corrosion-Resistance Seawater Photo/Electrolysis System

Utilizing earth-abundant seawater over scarce freshwater for hydrogen fuel production via water electrolysis is a promising/ sustainable strategy. However, the serious anodic corrosion due to the competing chloride oxidation reaction significantly hampers the overall stability of the electrolyzer. Therefore, it demands an efficient and robust catalyst with high selectivity and corrosion resistance for direct seawater splitting. Here, we present a bifunctional catalyst developed by morphology engineering to form nanorod array-based hierarchical NiO microspheres (NRAHM-NiO) as a three-dimensional (3D) hierarchical oxide/hydroxide urchin-like structure material for highly selective seawater splitting against chloride oxidation. Benefitting from highly intrinsic electroactive sites, good charge transferability, fast-releasing gas bubbles, and corrosion resistance as well as hydrophilic surface, the NRAHM-NiO exhibits outstanding bifunctional hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) catalytic activity toward selective and durable overall seawater splitting. The system requires small cell voltages of 1.66 and 2.01 V to drive current densities of 100 and 500 mA cm-2 at room temperature, respectively. Such catalytic activity is superior compared to the benchmark Pt/C(-)||Pt/C(+) and Pt/C(-)||IrO2(+) pair systems. Importantly, this device demonstrates specific stability as well as selectivity toward the OER in seawater with 99% faradaic efficiency without forming any chlorine species. The experimental results are well supported by density functional theory (DFT) calculations. Powered by a single solar cell, the integrated photolysis system shows 9.9% solar-to-hydrogen (STH) efficiency.