Metal Oxide-Based Materials for Electrochemical CO2 Reduction

The CO2 level in the atmosphere has been increasing since the industrial revolution owing to anthropogenic activities. The increased CO2 level has led to global warming and also has detrimental effects on human beings. Reducing the CO2 level in the atmosphere is urgent for balancing the carbon cycle. In this regard, reduction in CO2 emission and CO2 storage and usage are the main strategies. Among these, CO2 usage has been extensively explored, because it can reduce the CO2 level and simultaneously provide opportunities for the development in catalysts and industries to convert CO2 as a carbon source for preparing valuable products. However, transformation of CO2 to other chemicals is challenging owing to its thermodynamic and kinetic stabilities. Among the CO2 utilization techniques, electrochemical CO2 reduction (ECR) is a promising alternative because it is generally conducted under ambient conditions, and water is used as the economical hydrogen source. Moreover, ECR offers a potential route to store electrical energy from renewable sources in the form of chemical energy, through generation of CO2 reduction products. To improve the energy efficiency and viability of ECR, it is important to decrease the operational overpotential and maintain large current densities and high product selectivities; the development of efficient electrocatalysts is a critical aspect in this regard. To date, many kinds of materials have been designed and studied for application in ECR. Among these materials, metal oxide-based materials exhibit excellent performance as electrocatalysts for ECR and are attracting increasing attention in recent years. Investigation of the mechanism of reactions that involve metallic electrocatalysts has revealed the function of trace amount of oxidized metal species-it has been suggested that the presence of metal oxides and metal-oxygen bonds facilitates the activation of CO2 and the subsequent formation and stabilization of the reaction intermediates, thereby resulting in high efficiency and selectivity of the ECR. Although the stability of metal oxides is a concern as they are prone to reduction under a cathodic potential, the catalytic performance of metal oxide-based catalysts can be maintained through careful designing of the morphology and structure of the materials. In addition, introducing other metal species to metal oxides and fabricating composites of metal oxides and other materials are effective strategies to achieve enhanced performance in ECR. In this review, we summarize the recent progress in the use of metal oxide-based materials as electrocatalysts and their application in ECR. The critical role, stability, and structure-performance relationship of the metal oxide-based materials for ECR are highlighted in the discussion. In the final part, we propose the future prospects for the development of metal oxide-based electrocatalysts for ECR.