Electrolyte-Dependent Roles of Metallic Nanoparticles in Electrochemical CO2 Reduction to CO

The electrochemical reduction of carbon dioxide (CO2) to carbon monoxide (CO) is a promising strategy for converting a greenhouse gas into a valuable chemical feedstock. Metallic nanoparticles provide a versatile platform for tuning catalytic activity and selectivity through control of size, morphology, composition, and surface electronic structure. However, their performance is strongly dependent on the electrolyte environment, which governs reaction pathways, intermediate stabilization, mass transport, and competing side reactions. This review examines metallic nanoparticle catalysts for CO2-to-CO electroreduction across aqueous electrolytes, ionic liquids, and molten salts. Emphasis is placed on how each medium modifies nanoparticle behavior: aqueous systems are governed by proton-coupled electron transfer and competition with hydrogen evolution; ionic liquids can stabilize activated CO2-derived species and modify the electric double layer; and molten salts operate through high-temperature carbonate- and oxide-ion chemistry. The review further compares nanoparticle design strategies, transport and stability limitations, and the extent to which catalyst and electrolyte effects can be distinguished. Nanoparticle performance cannot be separated from the electrolyte environment. Catalyst composition and structure must therefore be considered together with electrolyte-dependent effects on reactant availability, intermediate stabilization, and competing reactions.