Identification of K⁺-determined reaction pathway for facilitated kinetics of CO₂ electroreduction.

Abstract

Cations such as K⁺ play a key part in the CO₂ electroreduction reaction, but their role in the reaction mechanism is still in debate. Here, we use a highly symmetric Ni-N₄ structure to selectively probe the mechanistic influence of K⁺ and identify its interaction with chemisorbed CO₂^-. Our electrochemical kinetics study finds a shift in the rate-determining step in the presence of K⁺. Spectral evidence of chemisorbed CO₂^- from in-situ X-ray absorption spectroscopy and in-situ Raman spectroscopy pinpoints the origin of this rate-determining step shift. Grand canonical potential kinetics simulations - consistent with experimental results - further complement these findings. We thereby identify a long proposed non-covalent interaction between K⁺ and chemisorbed CO₂^-. This interaction stabilizes chemisorbed CO₂^- and thus switches the rate-determining step from concerted proton electron transfer to independent proton transfer. Consequently, this rate-determining step shift lowers the reaction barrier by eliminating the contribution of the electron transfer step. This K⁺-determined reaction pathway enables a lower energy barrier for CO₂ electroreduction reaction than the competing hydrogen evolution reaction, leading to an exclusive selectivity for CO₂ electroreduction reaction.