Trimetallic spinel NiCo_2−xMo_xO₄ oxygen evolution catalyst enabling bias-free solar water splitting with inverted perovskite solar cells

Efficient spinel oxide catalysts are pivotal for driving the oxygen evolution reaction (OER) for hydrogen production via solar-driven water splitting. Designing trimetallic spinel oxides with high-valence metal ions which leach out to promote surface reconstruction is an effective strategy to maximize active sites for the OER. Herein, we report a trimetallic spinel oxide, NiCo_2−xMo_xO₄ as an efficient OER catalyst, generating 10 mA cm⁻² at a low overpotential of 250 mV, and demonstrating stability for over 25 h. Experimental and spectroscopic results indicate that the partial leaching of Mo ions from tetrahedral sites in the electrolyte facilitates Ni²⁺ oxidation to Ni³⁺, leading to the formation of an active nickel (oxy)hydroxide with numerous catalytic sites. Furthermore, integrating this spinel oxide in a 2-electrode water electrolyzer coupled with an inverted p-i-n perovskite solar cell enables bias-free solar water splitting with a solar-to-hydrogen efficiency of 8.8%. This work underscores the efficacy of using high-valence metal ions as effective dopants in activating spinel oxide pre-catalysts for the OER, thereby broadening their application in solar-driven water splitting technologies.