Temperature enhances the ohmic and mass transport behaviour in membrane electrode assembly carbon dioxide electrolyzers

Abstract

The operating temperature is a critical parameter for improving the performance of a carbon dioxide electrolyzer. Specifically, the power density reduced by up to 35% (at 215 mA/cm2) when increasing the operating temperature from 25 °C to 60 °C, and increasing the cell temperature led to significantly lower ohmic resistances and mass transport limitations. A 5–fold reduction in ohmic resistance (from 3.35 Ω·cm2 to 0.64 Ω·cm2) was achieved by increasing the cell temperature from 25 °C to 60 °C at 215 mA/cm2. These reductions in ohmic overvoltages were attributed to higher cation exchange membrane water content at higher operating temperatures observed via synchrotron X-ray radiography. The higher water content at higher temperatures was attributed to the relaxation of the polymer backbone of the membrane. The dominating mechanisms for mass transport limitations at high current densities (≥305 mA/cm2) were temperature dependent. Specifically, at 40 °C, liquid water in the electrode inhibited reactant carbon dioxide transport to the reaction sites; whereas, at 60 °C, the product gas saturation in the electrode inhibited mass transport. However, a higher temperature led to consistently lower mass transport losses at each current density (e.g., a reduction from 0.75 V to 0.37 V at 395 mA/cm2 when increasing the temperature from 40 °C to 60 °C) since less water accumulated in the cathode gas diffusion layer.