We present a novel concept for coupling energy storage and water desalination using an acid–base flow battery architecture. In this device, electrical energy is stored through the reversible generation of acid and base, while salt is simultaneously removed from a central salt chamber. The device operates with non-toxic, earth-abundant electrolytes - NaOH and HCl - and utilizes hydrogen as an efficient redox mediator, avoiding crossover of redox active species and enabling high reversibility. We demonstrate that the degree of desalination directly impacts the desalination flow battery's open-circuit voltage and internal resistance, with high efficiency achieved at partial desalination. At 7 mA cm⁻², the device desalinates 0.5 M NaCl by 31% with 90–97% ion removal efficiency and 50% water recovery. Modelling of specific energy consumption indicates values as low as 14–18 kJ mol(NaCl)⁻¹ are achievable using state-of-the-art membranes and compartment designs. This places the device performance in line with leading desalination flow batteries while unlocking additional value through energy storage using abundant chemicals. We propose its use in decentralized coastal grids powered by intermittent renewables, where it can balance energy supply for downstream processes while at the same time desalinating seawater. This work outlines a scalable sustainable approach to address the water-energy nexus using benign and abundant chemicals.

Salt-ion crossover through bipolar membranes (BPMs) is a major source of capacity fade and efficiency loss in acid-base flow batteries (ABFBs), yet its mechanisms remain poorly understood. Here, we quantify Na+ and Cl− crossover in a five-cell ABFB stack under varying electrolyte composition, state of charge (SoC), current density, and temperature. The results show that crossover occurs predominantly through BPMs and is governed by diffusion assisted by H+/OH− neutralization. At ambient conditions, Cl− crossover exceeds Na+ by roughly twofold, with apparent activation energies of 15 kJ mol−1 (Cl−) and 33 kJ mol−1 (Na+), reflecting asymmetric co-ion diffusion within the cation- and anion-exchange layers. Increasing current density reduces the relative contribution of crossover, whereas higher electrolyte concentration, SoC, and temperature increase it. Based on the combined voltage efficiency and crossover analysis, we recommend operating ABFBs under conditions corresponding to BPM efficiencies above 80% - that is, with dilute electrolytes (≤0.25 M), near-equimolar acid and base (SoC ≈ 50%), and cycling current densities exceeding 5 mA cm−2. These insights clarify the interplay between salt-ion transport and BPM operation, and provide design guidelines for next-generation ABFBs and other BPM-based electrochemical systems.

Bipolar membranes (BPMs) emerge as a valuable component in novel energy conversion devices utilizing a water-splitting reaction within BPMs. However, the opposite process, proton and hydroxide recombination (forward bias), remains challenging to control due to its strong dependence on the electrolyte composition. Even minor contamination of acid and base solutions by salt can significantly compromise the BPM performance. This study examines the impact of salt contamination on the BPM performance under forward bias. The results reveal that, during neutralization, salt ions accumulate near the BPM junction, hindering H⁺and OH^–transport toward the catalytic interface. Notably, the anion-exchange layer exhibits a high sensitivity to salt contamination in the base solution, with active site swapping between OH^–and anions emerging as the rate-determining step. The extent of this transport limitation depends on the acid/base-to-salt ratio. To address this issue, mitigation strategies are explored, including asymmetric BPMs. Reducing the thickness of the anion-exchange layer significantly enhances OH^–mobility, thereby increasing the limiting current density of neutralization in salt-contaminated electrolytes. These insights offer a deeper understanding of mass-transport limitations in BPMs and highlight pathways to optimize performance in energy conversion applications.