Mass Transport in a Gas-Fed CO2 Electrolyzer with a Forward-Bias Bipolar Membrane

Abstract

In the context of synthetic fuel and chemical production, CO2 electrolyzers using bipolar membranes (BPM) in forward bias have proven to overcome current limitations in state-of-the-art alkaline CO2 electrolyzers. These limitations are CO2 cross-over and carbonate deposition in the cathode catalyst layer, which limit the large-scale development of alkaline CO2 electrolyzers. Devices with a BPM in forward bias (Figure 1) solve these issues by setting the anode under acidic conditions while keeping the cathode alkaline. This configuration prevents carbonate ions to reach the anode and thus suppresses CO2 cross-over. Moreover, it allows pure water as an anolyte. Pure water eradicates unwanted cation cross-over and thus prevents carbonate deposition at the cathode. However, the use of a BPM for CO2 also has limitations. The most prominent ones are low stability and high over-voltage. These are partly caused by the complex and unexplored water transport.This work presents efforts to characterize and optimize different water transport phenomena. For this, we study cells of different complexity. It implies understanding first the water diffusion across the membrane, by setting a water concentration gradient between the electrodes and analyzing water transport by dew point measurement. The membrane hydration state can be determined with the simultaneously measured ion conductivity. Then, the water transport of a full cell is measured in parallel to its electrochemical performance. In this setup, a gas chromatograph and a mass spectrometer measure the selectivity of the cathode reaction.The optimal conditions at the cathode are a subtle balance between excess and scarcity of water. Water is a reactant in the CO2 reduction reaction and helps to humidify the membrane, ensuring good ionic conductivity. Additionally, water is formed by ion recombination at the BPM junction affecting the membrane hydration state. Still, we observe that the water supply at the cathode can be insufficient, especially at high current density. However, too much water can flood the cathode and block CO2 transport to the catalyst layer.In summary, we deliver a clearer picture of the water paths and demonstrate that optimized material and operating conditions help to overcome current limitations. Figure 1