Revealing the limits of energy recovery in forward-bias bipolar membranes

The ability for bipolar membranes (BPMs) to interconvert voltage and pH makes them attractive materials for use in energy conversion and storage. Reverse-biased BPMs, which use electrical voltage to dissociate water into acid and base, have become increasingly well-studied. However, forward-biased BPMs (FB-BPMs), in which voltage is extracted from pH gradients through recombination, are poorly understood. In this work, physics-based modeling elucidates how complex coupling of transport and kinetics dictates the performance of FB-BPMs in electrochemical devices. Simulations reveal that the open-circuit potential (OCP) of FB-BPMs is dictated by the balance of ion recombination and crossover, where recombination of buffering counter-ions attenuates OCP. Uptake of ionic impurities and fixed-charge neutralization limit achievable current densities by reducing the fraction of fixed-charge sites that mediate recombination. The model highlights the importance and nuances of selective ion management in mitigating energy losses and provides insight into the engineering of FB-BPMs for energy applications.