(79c) Effects of Temperature and Gas-Liquid Mass Transfer on the Operation of Small Electrochemical Cells for the Quantitative Evaluation of CO2 Reduction Electrocatalysts

In the last few years, there has been increased interest in electrochemical CO₂ reduction (CO2R). Many experimental studies employ a membrane separated, electrochemical cell with a mini H-cell geometry to characterize CO2R catalysts in aqueous solution. Gas products are measured periodically by sampling the effluent CO_2, which is continuously sparging the cell, while liquid products are measured by batch sampling of the electrolyte. The sensitivity of this approach for the detection of liquid products can be increased by increasing the catalyst surface area to electrolyte volume ratio (S/V) of the cell. However, we show that operating cells with high S/V at high current densities can have subtle consequences, especially for the cell temperature and electrolyte CO₂ concentration.

Both effects were evaluated quantitatively in high S/V cells using Cu electrodes and a bicarbonate buffer electrolyte. Electrolyte temperature is a function of the current and total voltage passed through the cell as well as the cell geometry. Even at very high current density, 20 mA cm^-2, the measured temperature rise was less than 4 °C and a decrease of <10% in the dissolved CO₂ concentration is predicted. In contrast, limits on the CO₂ gas-liquid mass transfer into the cells produce much larger effects. By using the pH in the cell to measure the dissolved CO₂ concentration, significant undersaturation of CO₂ is observed, even at more modest current densities of 10 mA cm^-2.

Importantly, undersaturation of CO₂ produces large changes in the faradaic efficiency product distribution observed on Cu electrodes, with H₂ production becoming increasingly favored as the CO₂ undersaturation worsens. We show that the size of the CO₂ bubbles being introduced into the cell is critical for maintaining the equilibrium CO₂ concentration in the electrolyte, and we have designed a high S/V cell that is able to maintain the equilibrium CO₂ concentration at current densities up to 15 mA cm^-2.