Local pH Effects on the Temperature Dependence of Product Formation in CO₂ Electrolyzers

Large-scale CO₂ electrolyzers will likely operate at elevated temperatures, but the effect of temperature on the microenvironment near copper catalysts is largely unknown. In this work, we use confocal Raman spectroscopy to reveal that the local pH is a critical parameter controlling product formation during CO₂ reduction at elevated temperatures. We found that higher temperatures lead to stronger pH gradients consisting of greater surface to bulk pH differences over shorter boundary layers. At -0.6 V, the surface to bulk pH difference at 75 °C was 1.8 units higher than that at 25 °C just from the effect of temperature alone. These results imply that most CO₂ electrolyzers operating at elevated temperatures were evaluated under much more alkaline microenvironment conditions than previously conjectured at 25 °C. Correlation between surface pH and product analysis shows that a high surface pH (9.9) is beneficial for multicarbon products formation below 45 °C. However, above 55 °C when the surface pH (10.3) becomes too high due to increased surface-bound hydrogen coverage, hydrogenation of C₁ intermediates is favored, thus compromising carbon-carbon coupling toward C₂₊ products.