Characterization of Bipolar Membranes for Electrochemical Applications

Abstract

Greenhouse gas concentrations are ever increasing in the Earth's atmosphere, causing the global temperature to have reached over a 1.0 degree Celsius increase relative to pre-industrial levels. A further increase in this temperature can have disastrous effects on our society, thus the increase in greenhouse gases must be attenuated. Electrochemical water splitting and CO2 reduction have been identified as promising routes to generate carbon neutral fuels. Efforts are made to increase the performance of the electrochemical configuration, however, a stable system that is applicable for industrial applications has not yet been achieved. The research field is ever changing and thus the choice of electrolytes and catalysts fluctuates. There is a desire for a stable configuration that can cope with these changes. A bipolar membrane (BPM) has the unique ability to pair two different electrolytes which can be optimized for their respective oxidation and reduction reactions. A BPM configuration is applicable for high performance rates, since it splits water at the interface layer, therefore accelerating the rate of ion transport compared to a conventional monopolar membrane. Despite the advantages of the BPM, there are still knowledge gaps on the exact mechanism behind the BPM and its electrochemical response. It is known that co-ion permeation through the BPM lowers the efficiency of the water dissociation reaction, thus decreasing the stability of the system in the long run. Therefore, it is relevant to further explore the effect of ion characteristics on ion cross-over. This research focuses on the degree of ion cross-over for a variety of electrolytes, with different pKa values. Multiple buffer solutions were tested at the cathode in a flow cell configuration for a current density range of 0-150 mA/cm2. The anolyte was a 0.5M NaOH solution for every tested catholyte. All experiments were performed for a constant period of time, after which the electrolytes were analysed for ion cross-over. It has been observed that the pKa value, the mobility and the valence of an ion influence the ion cross-over through the bipolar membrane. A large ion size decreases the ion cross-over to negligible amounts, for permeation through the membrane is limited. However, high BPM potentials have been detected for large molecules due to an assumed diffusion boundary layer, which increases the resistance at the cation exchange layer. For all tested catholytes low co-ion permeation (%) is measured at high current densities. Thereupon, the age of the BPM significantly affects the degree of ion cross-over. This observation is stronger for the anion exchange layer, for its permselectivity is lower. Additionally, the catholyte choice appears to influence the opposing Na+ flow from the anolyte to the catholyte.
This thesis demonstrates that for high current densities co-ion permeation reaches negligible values. In addition, the results indicate that an ideal ion size region exists, where ions cannot permeate the membrane easily, yet do not form a diffusion boundary layer (thus do not increase the BPM potential). This knowledge is relevant for optimizing the BPM configuration for industrial applications, since a clever choice of electrolytes can further stabilize the system. Future research should focus on up-scaling the set-up and testing the commercial BPM for stability and permselectivity.