Mn ²⁺ -assisted high-capacity retention in iron–chromium flow batteries <i>via</i> hydrogen evolution inhibition and Cr ³⁺ /Cr ²⁺ activation

The iron-chromium redox flow battery (ICRFB) has emerged as one of the most promising technologies for large-scale energy storage systems. At the same time, the parasitic hydrogen evolution reaction (HER) during the negative process remains a challenge for the long-term operation. To solve this issue, Mn²⁺ is used as an additive to enhance the stability and performance of ICRFBs. The results demonstrate that Mn²⁺ not only effectively inhibits HER but also accelerates the kinetics of Cr³⁺/Cr²⁺ and Fe³⁺/Fe²⁺ to some extent. Notably, the introduction of 0.002 M Mn²⁺ into the electrolyte significantly enhanced the coulombic efficiency (CE) and capacity retention of the battery. Specifically, at a current density of 40 mA cm^-2, the CE increased from 96.34% to 97.15%. Furthermore, after 100 long-term cycles, the Mn²⁺-modified electrolyte retained 73% of its initial discharge capacity, whereas the pristine electrolyte exhibited only 39% retention. This performance enhancement can be attributed to two key mechanisms. The negative shift of the hydrogenation potential induced by Mn²⁺ doping effectively inhibits the hydrogenation side reaction, and this result is also verified in the DFT (Density Functional Theory) calculations. The Mn²⁺ in the solution increases the electrochemical activity of the reaction system, thereby reducing the degradation of the electrolyte. These findings provide critical insights into the design of effective electrolyte additives for high-performance ICRFBs, highlighting Mn²⁺ as a promising candidate for mitigating capacity fade and improving overall electrochemical efficiency.