Intrinsic bipolar membrane characteristics dominate the effects of flow orientation and external pH-profile on the membrane voltage

Abstract

The practical energy required for water dissociation reaction in bipolar membrane (BPM) is still substantially higher compared to the thermodynamic equivalent. This required energy is determined by the bipolar membrane voltage, consisting of (1) thermodynamic potential and (2) undesired voltage losses. Since the pH gradient over the BPM affects both voltage components, in this work, pH gradient is leveraged to decrease the BPM-voltage. We investigate the effect of four flow orientations: 1) co-flow, 2) counter-flow, 3) co-recirculation, and 4) counter-recirculation, on the pH gradient and BPM-voltage, using an analytical model and chronopotentiometry experiments. The analytical model predicts the experimentally obtained pH accurately and confirms the importance of the flow orientation in determining the longitudinal pH gradient profile over the BPM in the bulk solution. However, in contrast to the simulated results, our observations show the effect of flow orientations on the BPM-voltage to be insignificant under practical operating conditions. When the water dissociation reaction in the BPM is dominant, the internal local pH inside of the membrane determines its final voltage, shadowing the effect of the external pH-gradient in the bulk solution. Therefore, although changing the flow orientation affects the bulk pH, it does not influence the local pH at the BPM junction layer and hence the BPM-voltage. Instead, opportunities for reducing the membrane voltage are in the realm of improved catalysts and ion exchange layers of the BPM.