The presence and role of the intermediary CO reservoir in heterogeneous electroreduction of CO(2)

Despite the importance of the microenvironment in heterogeneous electrocatalysis, its role remains unclear due to a lack of suitable characterization techniques. Multistep reactions like the electroconversion of CO(2) to multicarbons (C(2+)) are especially relevant considering the potential creation of a unique microenvironment as part of the reaction pathway. To elucidate the significance of the microenvironment during CO(2) reduction, we develop on-stream substitution of reactant isotope (OSRI), a method that relies on the subsequent introduction of CO(2) isotopes. Combining electrolytic experiments with a numerical model, this method reveals the presence of a reservoir of CO molecules concentrated near the catalyst surface that influences C(2+) formation. Application of OSRI on a Cu nanoparticle (NP) ensemble and an electropolished Cu foil demonstrates that a CO monolayer covering the surface does not provide the amount of CO intermediates necessary to facilitate C-C coupling. Specifically, the C(2+) turnover increases only after reaching a density of ∼100 CO molecules per surface Cu atom. The Cu NP ensemble satisfies this criterion at an overpotential 100 mV lower than the foil, making it a better candidate for efficient C(2+) formation. Furthermore, given the same reservoir size, the ensemble’s intrinsically higher C-C coupling ability is highlighted by the fourfold higher C(2+) turnover it achieves at a more positive potential. The OSRI method provides an improved understanding of how the presence of CO intermediates in the microenvironment impacts C(2+) formation during the electroreduction of CO(2) on Cu surfaces.