Stabilization of a Co(ii) site via a Co–O–Fe bridging motif for enhanced electro-oxidation of methanol to formate

Abstract

Cobalt-based materials are promising electrocatalysts for the methanol electro-oxidation reaction (MOR). However, the Co site easily undergoes over-oxidation, resulting in a decreased selectivity toward formate. The Co–O–Fe bridging motif in FeCoO_x reported in this work could inhibit the over-oxidation of Co³⁺ to Co⁴⁺, thereby reducing the occurrence of the competing surface oxygen evolution reaction taking place on the Co⁴⁺ site, leading to high catalytic performance in the MOR. The optimized FeCoO_x-2 achieves an industrial-level current density of 100 mA cm⁻² at a relatively low voltage of 1.38 V vs. RHE and maintains stable operation for 80 hours with 100% faradaic efficiency toward formate. In addition, FeCoO_x-2 exhibits excellent activity (η₁₀₀ = 288 mV) toward the hydrogen evolution reaction. By employing FeCoO_x-2 as a bifunctional catalyst, coupling of the MOR with the HER results in a hydrogen production rate of 52.91 μL s⁻¹, representing a 25% energy savings compared to the OER || HER system. These results outperform the previously reported data in this field. Operando EIS, in situ Raman, and XPS analyses revealed that the Fe site suppresses over-oxidation of Co³⁺ species by donating electrons to the Co site via the Co–O–Fe bridging motif. The dual Co³⁺ and Fe³⁺ active sites enhanced the selectivity and long-term stability of the MOR.