(733e) Non-Equilibrium Continuum Modeling of Energy Recovery in Forward-Biased Bipolar Membranes | AIChE

(733e) Non-Equilibrium Continuum Modeling of Energy Recovery in Forward-Biased Bipolar Membranes

Authors 

Bell, A. T., UC Berkeley
Weber, A., Lawrence Berkeley National Laboratory
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 for applications where sustainable acid and base generation is crucial. However, forward-biased BPMs (FB-BPMs), in which electrical work is extracted from pH gradients through H+/OH– recombination, are still poorly understood. This is unfortunate, because FB-BPMs attractive for a variety of applications. In CO2 reduction, FB-BPMs furnish optimal alkaline cathode environments for CO2 electrolysis, mitigate the crossover of (bi)carbonate species, and enable recovery of energy from acid-base recombination to reduce energy requirements for electrolysis. FB-BPMs also overcome challenges with membrane dehydration in hydrogen fuel cells by generating water at the CL to continuously humidify the cell. Finally, FB-BPMs can be used increase the energy density of redox flow batteries (RFBs) by providing an additional source of voltage (beyond the faradaic reactions at the electrodes), enabling operation outside the water stability region. Enabling these applications requires significantly higher current and power densities for FB-BPMs than the current state-of-the-art. Hence, an improved understanding of the physical phenomena that limit current density and energy recovery in these materials is needed.

In this work, an experimentally-validated continuum modeling framework is developed to elucidate mechanisms of transport and recombination in FB-BPMs. Simulations reveal that open-circuit voltage (OCV) in FB-BPMs is a dynamic property controlled by the intricate balance of ion-recombination and crossover, and that the presence of competitive counter-ion impurities significantly attenuates limiting current density of FB-BPMs through selective competition with H+ and OH– for sites within the ion-conducting polymer layers. Additionally, fixed-charge neutralization reduces the concentration of available sites that mediate the transport of H+ and OH–, further diminishing limiting current densities. Collectively, the established theory underscores the importance of non-equilibrium transport and kinetic phenomena in dictating the efficiency and performance of FB-BPMs for energy recovery. The implications of the simulations are discussed in the context of CO2 electrolysis systems and energy storage systems that employ FB-BPMs, and guidelines for the development of future FB-BPM materials and devices are presented.