Studies in heterogeneous electrocatalysis have largely focused on controlling the active sites and understanding how it influences reaction pathways through techniques such as surface spectroscopy (e.g., in situ Raman and infrared spectroscopy) combined with theory ( 6– 8). 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. 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 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. Specifically, the C 2+ turnover increases only after reaching a density of ∼100 CO molecules per surface Cu atom. 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. 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. 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. 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. Despite the importance of the microenvironment in heterogeneous electrocatalysis, its role remains unclear due to a lack of suitable characterization techniques.
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