Tuning the oxygen evolution reaction activity of Co3O4 nanocubes via controlled metal doping and surface faceting

Water electrolysis is a critical component of green hydrogen production and energy storage and transformation. However, the sluggish kinetics of the four-electron oxygen evolution reaction (OER) hinder this process, which therefore requires effective catalysts. In recent years, there has been increasing interest in tuning the OER activity of spinel-type mixed-transition metal oxide catalysts by controlling their electronic structure (via doping) and surface structure (morphology) as well as cation diffusion (oxygen vacancy). In this study, we report the controlled synthesis of M 0.1 Co 2.9 O 4 nanocubes of similar size and comparable dopant concentrations with redox-active and -inactive divalent (M = Mn, Zn) and trivalent (M = Cr) metal cations via a robust hydrothermal process. This allows for a direct structure-activity correlation for OER in 1.0 M KOH electrolyte. Cr doping was found to promote the OER activity by lowering the overpotential for the Cr 0.1 Co 2.9 O 4 nanocubes by 115 mV compared to undoped Co 3 O 4 nanocubes, thus achieving a comparable overpotential to benchmark catalysts, whereas Mn doping showed a detrimental catalytic effect. Calcining Cr-doped nanocubes (Cr 0.1 Co 2.9 O 4 ) resulted in Cr segregation to the outer 20 nm surface of the resulting Cr 0.1 Co 2.9 O 4 -calc nanocubes as proven by XPS depth profiling and HR-TEM imaging. Cr 0.1 Co 2.9 O 4 -calc nanocubes exhibit lower OER activity than the untreated Cr 0.1 Co 2.9 O 4 nanocubes; however, they are still more active than the undoped Co 3 O 4 nanocubes. While no morphological or particle size changes were observed in the particles after OER, electronic surface transformations were detected post-catalysis using XPS. This work demonstrates the crucial role of the bulk and surface composition, metal segregation, and electronic structure of cobalt spinel electrocatalysts in OER and therefore contributes to the rational design of nanocatalysts for water electrolysis. • Facile synthesis of size- and composition-controlled M₀.₁Co₂.₉O₄ nanocubes without organic capping agent. • Distinct dopant-dependent OER performance is revealed, with Cr doping significantly boosting activity. • OER activity tuning is directly linked to electronic structure modulation, particularly surface Co 2+ content • Low-temperature calcination-induced Cr segregation alters surface composition and electronic structure. • Revealed critical roles of bulk/surface composition, metal segregation, and electronic structure in governing OER activity