The all-vanadium redox flow batteries (VRFB) are an attractive candidate among electrochemical energy storage and conversion technologies for few kW- to MW- scale applications. Being comprised of the four vanadium species as redox couples, the separation of energy density from power density are one of its salient features. However, along with other issues, the problems of charge / discharge at faster rates (due to the increase of overpotentials) and cycling stability (due to the cross-over of vanadium species as well as irreversibility at the electrodes) hinder the broader market penetration of VRFB systems. One of the strategies to overcome these may be the introduction of electrocatalysts to the electrode surface to improve the reaction kinetics of the positive (VO2+ / VO2+) and negative (V3+ / V2+) redox couples. Tin-based materials are found to be highly effective for various catalytic applications, particularly for the energy storage technologies. Thus, electrocatalytic effects of tin for VRFB are evaluated in this study. Based on the deposition potential of tin, the in-situ electrodeposition of tin is a convenient and feasible strategy being employed in this study.
The electrocatalytic activity of tin species by introducing in the electrolyte are investigated. Cyclic voltammetry revealed remarkable improvement in the reaction kinetics of vanadium redox couples, in particular V3+ / V2+, owing to the coupling of redox activities of tin and vanadium. The peak potential separation for anode redox couple were reduced from 1011 to 589 mV in the presence of Sn2+ in the electrolyte. VRFB performance is evaluated in terms of various concentrations of Sn2+, presence of Sn2+ in either anolyte or catholyte and comparison with Sn4+ species. VRFB with Sn2+ in the electrolyte exhibited significant enhancement of key performance parameters of energy efficiency, voltage efficiency, specific discharge capacity, discharge energy density and cycling stability. At a high current density of 150 mA cm-2, VRFB with Sn2+ containing electrolyte exhibited an energy efficiency of 77.3%, an increase by 3.7% with the corresponding increase in discharge capacity and discharge energy density by 26.2 and 32%, respectively over the pristine system. This helps in increasing the electrolyte utilization at higher current densities by the lowering the overpotentials for charge / discharge reactions. Post-cycling analysis by SEM revealed the presence of Sn nanoparticles on the carbon felt electrode which was further confirmed by X-ray photoelectron spectroscopy. Electrochemical impedance spectroscopy reflected the acceleration of charge transfer process for the negative redox couple. Therefore, electrocatalytic impact through in-situ electrodeposition of Sn can be a feasible, cost effective and potential phenomenon for the performance improvement of VRFB.