CO₂ Reduction: From Homogeneous to Heterogeneous Electrocatalysis

Due to increasing worldwide fossil fuel consumption, carbon dioxide levels have increased in the atmosphere with increasingly important impacts on the environment. Renewable and clean sources of energy have been proposed, including wind and solar, but they are intermittent and require efficient and scalable energy storage technologies. Electrochemical CO2 reduction reaction (CO2RR) provides a valuable approach in this area. It combines solar- or wind-generated electrical production with energy storage in the chemical bonds of carbon-based fuels. It can provide ways to integrate carbon capture, utilization, and storage in energy cycles while maintaining controlled levels of atmospheric CO2. Electrochemistry allows for the utilization of an electrical input to drive chemical reactions. Because CO2 is kinetically inert, highly active catalysts are required to decrease reaction barriers sufficiently so that reaction rates can be achieved that are sufficient for electrochemical CO2 reduction. Given the reaction barriers associated with multiple electron-proton reduction of CO2 to CO, formaldehyde (HC(O)H), formic acid, or formate (HC(O)OH, HC(O)O-), or more highly reduced forms of carbon, there is also a demand for high selectivity in catalysis. Catalysts that have been explored include homogeneous catalysts in solution, catalysts immobilized on surfaces, and heterogeneous catalysts. In homogeneous catalysis, reduction occurs following diffusion of the catalyst to an electrode where multiple proton coupled electron transfer reduc...