The acid-base flow battery: Tradeoffs between energy density, efficiency, and stability

The deployment of renewable energy inevitably relies on environmentally friendly energy storage systems. An acid-base flow battery (ABFB) uses the principle of bipolar membrane (BPM) (reverse) electrodialysis to store excess electrical energy in abundant and benign materials (sodium chloride and water). However, this technology suffers from high energy losses due to undesired ion crossover through the membranes. In this study, we investigate the contribution of the membrane type as well as the different ions in the battery, and identify the major causes for this crossover and thus capacity losses. We also study the ion crossover under different ABFB operating conditions (pH gradient, current density, and used ions). The main losses are due to the diffusion-driven acid crossover through the BPM, facilitated by the small size and high mobility of protons. Operating at higher pH gradients leads to a higher energy density, but the ABFB stability is compromised due to the higher driving force for co-ion crossover. Running the ABFB at higher current densities does not alter the diffusion-driven crossover, but does make it relatively less important than the desired electromigration. However, this goes at the expense of higher ohmic losses and thus lower voltaic efficiency. Switching to divalent ions (such as sulfate) successfully prevents acid crossover through the BPM, but increases the membranes resistance, thus decreasing the voltaic efficiency.