Quantum confinement vs. mesoporosity in SnO2: oxygen vacancies dictate dominant formate selectivity in hybrid CO2 electrolysis with unprecedented 480 mV energy savings

Abstract

The electrochemical reduction of CO₂ (CO₂RR) to formate offers a sustainable pathway for carbon utilization, yet its practical implementation is hindered by high energy demands and competing reactions. Here, we present a hybrid CO₂ electrolysis system that synergistically couples cathodic CO₂RR with anodic ethylene glycol oxidation (EgOR), achieving unprecedented energy efficiency and formate selectivity. Hierarchically structured SnO₂ nanoflowers (NFs) are engineered on Sn foil to serve as the cathode, rich in oxygen vacancies, which result in the in situ formation of SnO₂/SnO heterostructures. This design enables a faradaic efficiency (FE) of ∼90% for formate production at −1.034 V vs. RHE, with a high partial current density of 26 mA cm⁻² outperforming SnO₂ quantum dots (QDs) and pristine Sn foil. Density functional theory (DFT) calculations reveal that oxygen vacancies and SnO₂/SnO interfaces stabilize key intermediates (e.g., *OCHO), lowering the activation barrier for the CO₂RR. Meanwhile, Ni-decorated SnO₂ NF anodes replace energy-intensive oxygen evolution with kinetically favourable EgOR, reducing the anodic overpotential by 210 mV compared to the OER. In a divided H-type cell, the hybrid CO₂RR‖EgOR system delivers a formate FE of 178% at a 430 mV lower cell voltage than conventional CO₂RR‖OER electrolysis, while simultaneously valorizing ethylene glycol. Notably, the GDL-based flow cell configuration further enhances performance, achieving a 480 mV lower cell voltage (at 20 mA cm⁻²) with an overall formate FE of 183%. This work demonstrates a scalable strategy to enhance CO₂ to formate conversion through defect engineering, heterostructure design, and hybrid electrolysis, offering a viable route toward sustainable carbon-neutral technologies.