NaHSO3 as a Key Component in Developing Enhanced Performance Electrolytes for All-Iron Redox Flow Batteries

Redox flow batteries (RFBs) are attractive electrochemical systems for large-scale energy storage. Despite the most developed ones being those based on vanadium, the search for new chemistries is essential to overcome several problems associated with this metal identified as a critical raw material. All-iron redox flow battery (A-IRFB) is an interesting device due to iron abundance and worldwide distribution. However, the poor performance of its negative half-cell, due to the sluggish plating/stripping processes related to the Fe²⁺/Fe⁰ redox pair, negatively impacts its energy efficiency and long-term performance. Here, it is demonstrated that the addition of a low concentration of NaHSO₃ (10 mM), as a novel additive, to an electrolyte formulation based on 0.5 M FeCl₂, 3 M NaCl, and 10 mM citric acid (H₃Cit) remarkably improves the electrochemical behavior of the negative half-cell. The enhanced performance can be explained as the additive guarantees a low oxygen solution content (reductant agent), promotes the plating/stripping reactions (improving the kinetics of the Fe⁰ deposit through the formation of a FeHSO₃⁺ complex), and diminishes the contribution of the competitive hydrogen evolution reaction. The use of this key additive opens up a promising scenario for the development of A-IRFBs with significantly enhanced electrochemical performance, thus boosting their potential commercial development.