Acid-Base Flow Batteries for Sustainable Energy Storage: Balancing Energy Recovery and Efficiency

Acid-base flow batteries (ABFBs) represent a novel approach to addressing the needs for advanced energy storage solutions to overcome the stochastic nature of renewable generation and ensure a consistent matching of energy supply with demand. ABFB relies on bipolar membrane electrodialysis (BMED) and reverse electrodialysis (BMRED) to generate pH gradient in the charging phase (BMED) through external electric power and then convert it into electricity during the discharging phase (BMRED). Although ABFB could be an ecofriendly and affordable alternative to other commercial batteries, it demands research to increase its current low maturity. This study examines the performance of ABFBs by evaluating operating conditions to maximize the Round-Trip Efficiency (RTE) and Gross Energy Density (GED). Furthermore, the trade-offs between RTE and GED indicators are analyzed for the first time in the literature. Experimental results ascertain that the RTE is maximized with low charging and high discharging current densities, attaining a maximum RTE of 12.6%. The maximum GED is achieved with high current densities for both phases, reaching a maximum of 4.2 Wh/L. Thus, RTE and GED cannot be maximized simultaneously. The indicator to be prioritized should be based on the specific needs of the desired application.