Electrolyte Additives and 3D X‑ray Tomography Study of All Iron Redox Flow Batteries in a Full-Cell Configuration for High Capacity Retention

All-iron redox flow batteries (Fe-RFBs) can be the grid’s most cost-effective and environmentally friendly electrochemical energy storage solution. However, low-capacity retention due to electrolyte degradation is the major hurdle in its progress. This study examines the effects of different electrolyte additives and their effects on the Fe-RFBs system in a full-cell configuration. Adding hydrochloric acid (HCl) results in a slow aging of the iron chloride electrolyte compared to that without HCl, leading to improved electrochemical performance. This improvement is evident in terms of both capacity retention and Coulombic efficiency (CE). HCl (0.2 M) addition in the electrolyte results in a discharge capacity retention of around 98% compared with 73% without HCl after 150 cycles. Moreover, the plating patterns of Fe at the negative electrode are also greatly affected due to HCl. Ex situ 3D X-ray tomography of Fe-plated electrodes in the presence and absence of HCl in an all-iron chloride electrolyte within the full-cell system is conducted, and the iron deposition pattern on the electrodes is obtained by FE-SEM. This study reveals the critical role of additives in the reversibility of iron deposition and stripping and its overall effect on cycling life to all iron redox flow batteries.