Electrochemical reconstruction leading to a borate-linked [Co(Fe)]8B2O16H2 crystalline phase for the efficient and stable OER

Abstract

Recent developments relating to oxygen evolution reaction (OER) catalysts have shifted to multiple-element or even high-entropy materials for achieving high activity and high stability, but clarification of the 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 in iron, boron and cobalt ternary oxide catalysts, Co_xFe_yB_z with x : y : z = 4 : 1 : 1, the initially amorphous material 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: it retains 97% of its initial stability after 200 h of chronoamperometric testing, shows almost no voltage change after multi-current step testing, and maintains a stable potential throughout 200 h of intermittent testing (12 h start–shutdown cycles at 10 mA cm⁻²) in 1 M KOH electrolyte. While boron acts as the structural skeleton dopant, iron not only stabilizes the Co(Fe)O₂ layer surfaces but also enhances the OER activity by markedly stabilizing the key terminal O* and OOH* intermediates by ∼0.50 eV, giving rise to a characteristic Raman peak at 940 cm⁻¹.