Electrochemical Reconstruction Leading to Stable Borate-linked [Co(Fe)]₈B₂O₁₆H₂ Crystalline Phase for Efficient Oxygen Evolution

Abstract

Recent development of oxygen evolution reaction (OER) catalysts shifted to multiple-element, even high-entropy materials for high activity and high stability, but the clarification of synergistic mechanism between elements has been challenging. Here by utilizing state-of-the-art machine-learning based global structure exploration and electrochemical experiments, we show that an iron, boron and cobalt ternary oxide catalysts, Co_xFe_yB_z with x:y:z=4:1:1, initially being amorphous, reconstructs into a crystalline phase with the formula [Co(Fe)]₈B₂O₁₆H₂, featuring Borate (BO₃)-linked CoO₂ layers exposing Fe cation sites. This borate-linked structure delivers rational performance metrics, and more importantly, exhibits exceptional and multifaceted stability: retaining 97% of its initial stability after a 200 hours chronoamperometric test with almost no voltage change after multi-current step testing, and also maintaining a stable potential throughout a 200‑h intermittent test (12‑h start–shutdown cycles at 10 mA cm⁻²) in 1M KOH electrolyte. While boron acts as the structural skeleton dopant, iron not only helps to stabilize the Co(Fe)O₂ layer surfaces but also enhances the OER activity by stabilizing markedly the key terminal O* and OOH* intermediates by ~0.50 eV, leading to a characteristic Raman peak at 940 cm^-1.