Effects of operating temperature on kinetics and performance of iron/iron redox flow batteries

The all-iron redox flow battery (IRFB) offers a promising low-cost solution for large-scale energy storage; however, its performance is compromised by parasitic reactions, particularly hydrogen evolution. The impact of operating temperature on lab-scale cell performance is a critical yet underexplored area. This study systematically examines the influence of temperature on an IRFB utilizing a 1.5 M FeCl$_2$, 2 M NH$_4$Cl, and 0.2 M HCl electrolyte. Electrochemical characterization was performed over the temperature range of 20–80 °C, while lab-scale cell cycling stability was assessed over 20–50 °C. Electrochemical analysis indicated that elevated temperatures significantly enhance reaction kinetics, as evidenced by a nearly fivefold increase in the diffusion coefficient of the Fe$^{2+}$/Fe$^{3+}$ redox couple, rising from 1.89 × 10$^{−6}$ cm$^2$ s$^{−1}$ at 20 °C to 8.93 × 10$^{−6}$ cm$^2$ s$^{−1}$ at 80 °C. Morphological studies further revealed improved and more uniform iron deposition at higher temperatures. Nonetheless, initial battery cycling revealed that while kinetics improved, maintaining a temperature of 50 °C resulted in rapid performance degradation and electrolyte precipitation, driven by accelerated hydrogen evolution and subsequent pH shifts. An optimal operating temperature of 40 °C was identified, effectively balancing kinetic advantages with manageable side reactions. To enhance long-term stability, a soft-start cycling protocol was introduced, beginning cycles at 20 °C before ramping the temperature to 40 °C. This approach successfully reduced early-stage hydrogen losses and more than doubled the stable operational lifetime to over 80 cycles. Extended validation under high-capacity constant current constant voltage (CCCV) conditions, with in situ monitoring, confirmed that this protocol induces a self-stabilizing effect, characterized by a progressive reduction in parasitic hydrogen evolution currents and robust tolerance to negolyte pH excursions beyond the critical precipitation threshold at pH 3. Under these optimized conditions, the battery achieved a coulombic efficiency of 94%, a voltaic efficiency of 63%, and an energy efficiency of 60% at a current density of 25 mA cm$^{−2}$. These findings highlight the crucial role of thermal management in IRFB systems and present a viable strategy for enhancing their efficiency and long-term cyclability.