Next-generation redox flow batteries will benefit from the progress of macroscopic continuum models that enable the optimization of new architectures without the need of expensive fabrication and experimentation. Despite previous attempts, there is still need for robust and thoroughly validated models. Here, a steady-state two-dimensional unit-cell model of an all-vanadium redox flow battery is presented. The model integrates state-of-the-art descriptions of the fundamental physical phenomena, along with new features such as local mass transfer coefficients for each active species, precise sulfuric acid dissociation kinetics, and experimental data of the electrochemical parameters and electrolyte properties. The model is validated at different states of charge and flow rates using polarization, conductivity and open circuit voltage measurements. Then, the contribution of operating conditions on battery performance is studied by analyzing its separate effect on the various phenomena that affect cell performance, such as local pore mass transfer limitations, parasitic hydrogen evolution reactions, crossover and self-discharge fluxes. The resulting model is a reliable tool that can be used to assess the relevance of these coupled phenomena that take place simultaneously within the reaction cell. This important information is critical to optimize cell components, reactor design and to select optimal operating conditions.

Redox Flow Batteries (RFBs) are an evolved electrochemical energy storage technology crucial for the transition into a renewable energy future. Mainstream adoption of RFBs is subject to reduction of the capital and operation costs. In that sense, of particular importance is the optimization of the electrochemical stack, which affects the overall efficiency of the battery. Several studies have investigated the importance of thermal effects on the performance of vanadium redox flow batteries through transient non-isothermal models [1-2]. However, due to the simplified assumptions made by these authors some important temperature-dependent features are not included. Therefore, reliable and validated continuum models are also crucial to study the impact of the system temperature on all the fundamental physics in the electrochemical cells as well as to find the key aspects to optimize the stack. In this work, we present a 2D stationary model of a vanadium redox flow battery cell with a comprehensive and updated multiphysics description. The model was validated at a constant temperature of 25 ºC by polarization, conductivity, and open circuit voltage measurements. In a second step, the temperature was also included as a key operating variable in the model. Electrolyte properties, electrochemical rate constants, and H2SO4 dissociation equilibrium were described as a function of temperature as well as the state of charge (SoC) and the total vanadium concentration. The model is currently being validated through a second experimental campaign (see Fig. 1) conducted inside a climatic chamber imitating different environmental scenarios. The model can be used to explore the relevance of each phenomenon or element in the electrochemical stack and the influence of the operating conditions on them, e.g. temperature, state of charge and volumetric flow rate. By using the proposed model, we can elucidate the best temperature strategy to increase the performance of vanadium redox flow batteries in diverse operating scenarios. Figure 1. Open circuit voltage – SoC relation at different operating temperatures. References Al-Fetlawi, A.A. Shah, F.C. Walsh, Electrochimica Acta, 55, 78-89 (2009). Tang, S. Ting, J. Bao, M. Skyllas-Kazacos, Journal of Power Sources, 203, 165-176 (2012). Acknowledgments This work has been partially funded by the Agencia Estatal de Investigación (PID2019-106740RB-I00 and RTC-2017-5955-3/AEI/10.13039/501100011033). Figure 1