UC Berkeley

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. This type of electrochemical cell is a mini-chemical reactor and it is important to monitor the reaction conditions within the reactor to ensure that they are constant throughout the study. We show that operating cells with high catalyst surface area to electrolyte volume ratios (S/V) at high current densities can have subtle consequences due to the complexity of the physical phenomena taking place on electrode surfaces during CO2R, particularly as they relate to the cell temperature and bulk 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/total voltage passed through the cell and the cell geometry. Even at a very high current density, 20 mA cm^-2, the temperature increase 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 CO₂ concentration, significant undersaturation of CO₂ is observed in the bulk electrolyte, even at more modest current densities of 10 mA cm^-2. Undersaturation of CO₂ produces large changes in the faradaic efficiency observed on Cu electrodes, with H₂ production becoming increasingly favored. 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 near-equilibrium CO₂ concentration at current densities up to 15 mA cm^-2.