Tuning Silver Nanostructures for Electrochemical CO2 Reduction to Value-Added Fuels

Abstract

When combined with the use of renewable energies, electrochemical CO2 reduction reaction (CO2RR) is an attractive way to alleviate the greenhouse gas effects and the related environmental issues.[1] However, the inertness of CO2 itself, the sluggish multi-electron transfer kinetics and the competitive hydrogen evolution reaction (HER) during CO2RR render the simultaneous achievement of desirable catalytic activity and product selectivity on most electrocatalysts difficult.[2-3] CO2RR on most electrode surfaces requires large overpotential due to the poor catalytic activity. The rising costs of metals, in particular, noble metals such as Ag, Au and Pd, are the main hindrance toward large-scale application. Thus, the enhanced performance is desirable upon loading a certain or even lower level of noble metal nanostructures as compared to their counterparts for CO2RR. To this end, substantial experimental and theoretical efforts have been devoted to surface engineering by introducing grain boundary, oxide-reduction or oxygen plasma treatments. These have all previously been recognized to considerably improve CO2RR catalytic activity. As a critical structural feature, the morphology (e.g., unique architecture and/or size effects), can also greatly affect the catalytic activities of metal nanostructures, which can be achieved through purposefully tailoring energetically favourable low-coordinated atoms over various morphologies. Morphology control of size effect has been confirmed to greatly influence the catalytic activity for CO2RR over Au,[4] Ag[5] and Pd[6] NPs. However, there have been limited experimental and theoretical investigations of morphology control on the effects of both unique architecture and size, especially the nanostructures which are wholly enclosed by energetically favourable specific facets. With these new materials, the quantities of metals and the associated costs required to achieve a certain level of catalytic efficiency may be reduced significantly.To this end, various Ag nanostructures were successfully synthesized, and it is found that these Ag nanostructures enclosed by energetically favourable facets were impressively efficient and stable for CO2RR toward CO formation, accompanied with high CO selectivity in a broad potential window. The considerably enhanced catalytic activity and selectivity toward CO2RR were systematically rationalized through analyzing the density functional theory (DFT) calculations, the percentages of various catalytically active sites and how these specific Ag nanostructures affecting the active sites as well as the partial density of states (PDOS).