A Successful Transition from a Vanadium Redox Flow Battery Stack to an Energy Efficient Electrochemical Desalination Module

Abstract Vanadium Redox Flow Batteries (VRFB) are considered prime candidates for stationary energy storage due to their unique capability to decouple power and energy. With the aim of making electrochemical water desalination more energy efficient when scaling up the technology, potential benefits from repurposing a commercial VRFB stack as a Capacitive Deionization (CDI) module were studied. Accordingly, a 5-cell CDI Stack equipped with 1250 cm2 electrodes was assembled by revamping a 5kWh-VRFB. Electrosorption experiments showed a great versatility with a maximum salt adsorption capacity of 14.2 mg g-1 and an average salt adsorption rate of 0.321 mg g-1min-1. Furthermore, a long-term test performed over 100 cycles validated the robustness of the system by achieving salt concentration reductions of 10-12 mM at a constant production rate of 4 L h-1m-2. Moreover, adapting a VRFB stack specifically designed for energy optimization allowed a substantial reduction of energy consumption of 75% (from 0.84 kWh m-3 to 0.21 kWh m-3) by effectively recovering the energy employed in the desalination step. These results demonstrated the successful transition from VRFB to CDI showcasing the relevance of the Water-Energy Nexus and opening the path for exploring a second life of exhausted VRFB stacks.