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 (NF) catalyst are engineered on Sn foil to serve as the cathode, rich in oxygen vacancies which results 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 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 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.