Tuneable oxygen evolution in CoFe electrocatalysts via phosphate-mediated phase engineering

Designing robust and earth-abundant oxygen evolution reaction (OER) catalysts for alkaline water electrolysis requires precise control of crystal phase, surface chemistry, and electronic structure. Here, we report a simple hydrothermal strategy to tune Co-Fe catalysts through phosphate-mediated phase engineering, evolving from layered double hydroxides (low P) to potassium cobalt phosphate frameworks (intermediate P) and hydrated cobalt phosphates (high P). Structural evolution reveals the role of alkali cations in stabilizing phosphate phases under alkaline conditions. Electrochemical evaluation in 1M KOH shows that phosphate incorporation significantly enhances OER activity, with intermediate P contents achieving the best balance of low overpotential (257-265 mV at 10 mA cm −2 ), small Tafel slopes (42-47 mV dec −1 ), and improved durability. Fe-containing catalysts outperform the Co-only analogue, demonstrating that phosphate incorporation must be coupled with Fe to enable efficient charge transfer and mixed-valence redox. Post-stability analysis indicates that the phosphate frameworks act as structure-directing precatalysts that reconstruct into the active oxyhydroxide phase during OER. These trends are validated in a single-cell anion exchange membrane water electrolyzer, where phosphate-containing CoFe anodes achieve lower cell voltages. Overall, this work establishes phosphate-Fe synergy and scalable hydrothermal synthesis as key design strategy for high-performance Co-based OER catalysts. • Hydrothermal synthesis tunes CoFe catalysts via phosphate phase control. • Fe incorporation modulates electronic structure of phosphate frameworks. • Optimized catalysts deliver ∼260 mV at 10 mA cm −2 for alkaline OER. • Phosphate phases reconstruct into active CoFe oxyhydroxides. • Performance validated in single-cell AEM water electrolysis.