Design of active, stable, and earth-abundant acidic oxygen evolution catalysts

Abstract

Solar-driven electrochemical splitting of water to hydrogen and oxygen is crucial for the production of inexpensive, sustainable, and carbon-neutral fuels for future global energy requirements. Of the two half-reactions for water splitting, developing catalysts for the oxygen evolution reaction (OER) is a challenge because orchestrating the coupled transfer of four protons and four electrons is kinetically demanding. Currently, inexpensive and active OER catalysts comprised of earth-abundant elements from cobalt and nickel oxides operate in neutral and alkaline pH. These same systems corrode under acidic conditions, which is a regime important for electrolyzers and photoelectrochemical devices as well as for fundamental mechanistic studies. In this thesis, we iteratively design an active, stable, and earth-abundant acidic oxygen evolution catalyst through multiple generations. The first version focused on an electrodeposited manganese oxide (MnOx) catalyst that is stable in acid but exhibits low OER activity. Kinetic and mechanistic analysis on deposition and oxygen evolution mechanisms show that its stability manifests from strong manganese-oxygen bonds and self-healing. The second generation system improved activity while retaining acid stability by activating MnOx through a voltage cycling protocol. Structural studies show that activation induces a lower bulk manganese oxidation state and turbostratic disorder. This catalyst architecture was redesigned in the third iteration as a mixed metal oxide where functionality is decoupled into separate metals: Co was employed as the catalytic element while Mn served as the structural component. These CoMnOx films demonstrate the facile OER kinetics of Co and the acid corrosion resistance of Mn. However, these films cannot operate at high potentials since Mn dissolves as permanganate. Thus in our final fourth generation catalyst, we replaced the structural component with lead (doped with iron), an optimization discovered by potential-pH analysis. These CoFePbOx films exhibit ~70 mV/decade Tafel slopes and long-term stability at 1 mA/cm² in pH 2.5, operating at only 200 mV higher overpotential than iridium oxide. Overall, we demonstrate the ability to rationally modify and design an active, stable, and earth-abundant oxygen evolution catalyst.