Size-dependent selectivity of iron-based electrocatalysts for electrochemical CO2 reduction

Abstract

Applying non-precious metal catalysts to electrocatalytic CO₂ reduction (CO₂R) to CO efficiently, rather than gold-based nanomaterials, is of practical significance. Here, we adopt zeolitic imidazolate frameworks (ZIFs) to assist the synthesis of Fe metal catalysts with various sizes, ranging from single atoms to over 100 nm. As a result, the CO₂ reduction performance of Fe metal catalysts with different particle sizes is different. Dominantly, Fe single-atomic sites exhibit optimum activity in producing CO, presenting a current density of 46.5 mA cm⁻², with nearly 100% faradaic efficiency (FE) for CO (at −0.9 V vs. RHE), while the result for 7.6 nm Fe nanoparticles is slightly lower (ca. 92%). With further increasing the Fe particle size, the CO₂RR performance weakens and allows the H₂ evolution reaction (HER), which demonstrates the size-dependent selectivity of iron-based electrocatalysts for electrochemical CO₂ reduction. Operando X-ray absorption spectroscopy revealed the active sites to be discrete Fe²⁺ ions, coordinated to the pyrrolic nitrogen (N) atoms of the N-doped carbon support.