The electrolysis of water and carbon dioxide require the cathode and anode to operate under different conditions of optimal pH. We have recently demonstrated that bipolar membranes (BPM) provide a continuous source of H+ and OH- to maintain the cathode and anode pH. However, the mechanism of water dissociation in the BPM under reverse bias conditions and the relative contribution of membrane bulk and interfacial regions to the overall impedance remain unclear. We have made BPMs with layer-by-layer assembled graphite oxide (GO) sandwiched between anion- and cation- exchange membrane layers. The best homemade BPM performs comparably to the commercial one in terms of steady-state polarization and long term stability during water electrolysis. Scanning electron microscopy (SEM) images indicate the presence of a ~200 μm anion exchange layer and a ~20 μm cation exchange layer, whereas the GO layer is too thin to be imaged by SEM. Electrochemical impedance spectroscopy (EIS) was employed to decouple the various impedance processes across the BPM. Increasing the current density drives water dissociation, thus resulting in multiple time constants in the spectrum. A high frequency arc is correlated to the interfacial region of BPM, where water dissociation occurs. Commercial BPMs exhibit a two orders of magnitude longer time constant than the homemade BPM, indicating a thinner interfacial region with a larger electric field. In contrast, the steady state polarization curve does not show a dramatic difference, implying that factors other than the electric field may promote water dissociation. The protonation-deprotonation reaction between water and the catalyst has been previously suggested to enhance the rate of water dissociation. To test this hypothesis, we compared the high frequency time constants of BPMs with and without the catalyst. Ion transport is greatly modified if there is no catalyst present. In the low frequency range, the impedance shows a transition from Warburg type to Gerischer type, in which a chemical reaction couples to pure diffusion. However, quantitative information about the water dissociation rate will only be obtained if we can decouple the transport of water dissociation products from that of electrolyte ions. Improving the long term efficiency of electrolysis systems requires an understanding of the relationship between membrane structure and series impedances, which will be the subject of future research in this project.

Electrochemical impedance spectroscopy of bipolar membranes for water and CO2 electrolysis

253rd National Meeting (2017)

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