Interfacial Metal Oxides Stabilize Cu Oxidation States for Electrocatalytical CO₂ Reduction

Modulating the oxidation state of copper (Cu) is crucial for enhancing the electrocatalytic CO₂ reduction reaction (CO₂RR), particularly for facilitating deep reductions to produce methane (CH₄) or multi-carbon (C₂₊) products. However, Cu^δ+ sites are thermodynamically unstable, fluctuating their oxidation states under reaction conditions, which complicates their functionality. Incorporating interfacial metal oxides has emerged as an effective strategy for stabilizing these oxidation states. This review provides an in-depth examination of the reaction mechanisms occurring at oxide-modified Cu^δ+ sites, offering a comprehensive understanding of their behavior. We explore how Cu/metal oxide interfaces stabilize Cu oxidation states, showing that oxides-modified Cu catalysts often enhance selectivity for C₂₊ or CH₄ products by stabilizing Cu⁺ or Cu²⁺ sites. In addition, we discuss innovative strategies for the rational design of efficient Cu catalytic sites tailored for specific deep CO₂RR products. The review concludes with an outlook on current challenges and future directions, offering new insights into the rational design of selective and efficient CO₂RR catalysts.