Adaptation of a Vanadium Redox Flow Battery for Thermal Applications Using a Solid Capacity Booster

Abstract

In this work a novel approach for the use of the Vanadium Redox Flow Batteries (VRFB) is made by viewing it as a dual system with the ability to store both electrical and thermal energy. Electrical energy is thanks to the electrochemical reactions of the vanadium ions dissolved in the electrolyte, while thermal energy will use the sensible heat of the electrolyte mass.The performance of VRFB has been widely studied over the last 40 years and there is an extensive bibliography on the subject regarding the electrochemical performance attending only to the conventional energy storage requirements, but the hybrid electrical-thermal function is a novel subject that has been scarcely explored. One of the most critical aspects is the narrow operating temperature range of these batteries (15-40°C) that strongly limits their thermal energy storage capacity.Practical vanadium electrolytes operate with electrolytes containing a vanadium concentration between 1.6 and 2.0 M, and usually also employ cooling and heating systems to maintain the temperature between 15 and 40ºC. In particular, at temperatures below 15ºC the slower reaction kinetics, high viscosity and reduced conductivity of the concentrated electrolytes introduce remarkable energy losses in the battery. On the other hand, at temperatures over 35ºC the positive electrolyte is not stable due to the limited solubility of VO2 + species. High temperatures also boost the kinetics of parasitic reactions such as hydrogen and oxygen evolution, thus increasing energy losses.In this study, we address these limitations aiming to open up new possibilities of a successful integration of vanadium redox flow batteries into thermal applications. For this, the functionality and stability limits of a diluted vanadium electrolyte are analyzed for an extended operational temperature window from 5ºC to 50ºC and compared to commercial electrolyte formulations. In a second step, the use of a solid capacity booster is investigated to compensate the loss of energy density due to electrolyte dilution.[1] S. Berling, S. Mavrikis, N. Patil, E. García – Quismondo, J. Palma, C. Ponce de León “Lignin as redox-targeted catalyst for the positive vanadium electrolyte”, Electrochemistry Communications 142 (2022) 107339.[2] S. Berling, J. M. Hidalgo, N. Patil, E. García - Quismondo, J. Palma, C.Ponce de León, “A mediated vanadium flow battery: Lignin as redox-targeting active material in the vanadium catholyte”. Submitted