Quantification of the Oxidation State Composition of the Cu Surface for the Electrochemical CO2 Reduction Reaction

Understanding the dynamic oxidation state of copper electrocatalysts is essential for unraveling the mechanisms governing the electrochemical CO2 reduction reaction (CO2RR) and for rationally designing catalysts with improved selectivity toward multicarbon products. Copper is unique among monometallic catalysts in its ability to reduce CO2 beyond CO to form valuable hydrocarbons. However, its surface undergoes continuous reconstruction under reaction conditions, generating mixtures of Cu, Cu2O, and CuO whose relative abundance remains difficult to quantify in situ. Here, we report a surface-specific, in-situ spectroscopic strategy that combines nonresonant electrochemical sum frequency generation (NR EC-SFG) spectroscopy with two-photon fluorescence (TPF) to quantify the composition of Cu electrodes during cyclic voltammetry from -0.5 V to 1.2 V. A distinct NR EC-SFG response was observed for each Cu species, enabling identification of copper oxide separate from metallic Cu. TPF selectively probed Cu2O, offering an internally consistent metric for tracking oxide growth. These identifying features allowed us to develop a calibration procedure to quantify the accumulation rate of partial disperse monolayer equivalents of copper oxides, from which we extracted Cu2O and CuO accumulation rates of 0.17 and 0.07 monolayer equivalents per cycle, respectively. As a result, we can determine the percent copper oxide composition of a Cu electrode after any number of cycles, revealing rapid formation of Cu2O and slower accumulation of CuO. This heterogeneous, dynamically reconstructed surface persisted even under cathodic conditions relevant to CO2RR, highlighting the importance of accounting for oxide species when interpreting catalytic activity and selectivity. Beyond Cu, this work established a foundation for quantifying metal-semiconductor compositions at electrochemical interfaces.