Orientational Geometry, Surface Density, and Binding Free Energy of Intermediates as Full Descriptors for Electrochemical CO₂ Reduction at Metal Surfaces

Metal catalysts for the electrochemical CO₂ reduction reaction (CO₂RR) have attracted widespread attention due to their high catalytic efficiency, stability, broad product diversity, and ease of preparation. Studies show that the product distribution and yield of the electrochemical CO₂RR on metal surfaces result from the metal's binding energy of an intermediate adsorbed CO (*CO). However, reaction pathways could be manipulated by other thermodynamic parameters, such as orientation and surface density. In this work, the CO₂RR on Au electrode surfaces was comprehensively analyzed using high-performance <i>in situ</i> electrochemical sum-frequency generation (EC-SFG) spectroscopy. The improved signal intensities allowed the reaction to be monitored with a fast time resolution, extracting key thermodynamic and kinetic features from the experiments. Our EC-SFG spectrometer allowed the comprehensive analysis of the potential-dependent polarized SFG signal, allowing us to quantify *CO orientation at the Au electrode surface as a function of applied potential. These experimental results were then used to determine the maximum surface density and binding energies of the *CO intermediate in a self-contained analysis. These EC-SFG experiments enabled us to quantify the reaction rate constant for the system. We then discuss how the binding energy, orientation angle, and absolute surface density of an intermediate should be fully considered in understanding its thermodynamic behaviors in the CO₂RR. This work demonstrates the potential of high-efficiency EC-SFG spectroscopy to provide an all-inclusive analysis of the CO₂RR on metal surfaces and opens the door for other catalysts to be investigated using this technique to determine the best system for efficient electrocatalysis.