Temperature Effects on the Local pH of the CO2 Reduction Reaction over Copper

The electrochemical reduction of CO₂ (CO₂RR) presents a pathway to counteract carbon emissions, supply feedstocks to the chemical industry, and thus facilitate a circular CO₂ economy to meet net-zero emissions goals by 2050. Among all CO₂RR electrocatalysts, copper is the only pure metal that produces valuable multi-carbon products, such as ethanol and ethylene, with reasonable Faradaic efficiencies. Although most copper-based CO₂RR studies are performed at room temperature, heat transfer limitations in industrial CO₂ electrolyzers will lead to temperature buildup near the copper cathode surface. Higher temperatures impact the distribution of species in the reaction microenvironment, which in turn affects product formation and reaction selectivity over the competing hydrogen evolution reaction (HER). The objective of this study, therefore, was to probe how temperature affects the concentration of local species and the reaction microenvironment pH at various potentials.

To achieve this objective, surface-enhanced Raman spectroscopy (SERS) experiments were performed in a custom-built temperature-controlled electrolytic cell charged with a CO₂-sparged KHCO₃ electrolyte. This cell modulated temperature between 25 and 75°C, and potential between 0.1 and -0.6 V (vs. RHE). We show that the local pH varies dramatically from its bulk value as a function of both potential and temperature. The local H⁺ concentration dependence on temperature suggests that local pH is a key parameter controlling reaction selectivity at higher temperatures. These results have significant implications for the types of controllers and heat exchangers necessary for thermal management of the CO₂RR, which is a crucial consideration when designing large-scale industrial CO₂ electrolyzers.