Ionic Blockades Control the Efficiency of Energy Recovery in Forward Bias Bipolar Membranes

Limited understanding exists about the operation of bipolar membranes (BPMs) in forward bias to convert protonic gradients into electrical work, despite its emerging role in many electrochemical devices. In these device contexts, the BPM is typically exposed to complex electrolyte mixtures, but their impact on polarization remains poorly understood. Herein, we develop a mechanistic model explaining the forward bias polarization behavior of BPMs in mixed electrolytes with different acidities/basicities. This model invokes that weak acids/bases accumulate in the BPM and impose an ionic blockade that inhibits the recombination of stronger acids/bases, resulting in a substantial neutralization overpotential. We demonstrate the utility of our model to fuel cells and redox flow batteries, and introduce two materials design strategies for mitigating this inhibition. Lastly, we apply our findings to enhance the energy efficiency of carbonate management in CO2 electrolyzers. This work highlights how non-equilibrium local environments at membrane-membrane interfaces can define the efficiency of protonic-to-electrical energy conversion.