Controllable Carbon Felt Etching by Binary Nickel Bismuth Cluster for Vanadium–Manganese Redox Flow Batteries

Abstract

Various redox couples have been reported to increase the energy density and reduce the price of redox flow batteries (RFBs). Among them, the vanadium electrolyte is mainly used due to its high solubility, but electrode modification is still necessary due to its low reversibility and sluggish kinetics. Also, an incompatible ion exchange membrane with redox-active species leads to self-discharge referred to as crossover. Here, we report a V/Mn RFB using an anion exchange membrane (AEM) for crossover mitigation and etched carbon felt by nickel–bismuth (NB-ECF) for the vanadium anolyte. The NB-ECF significantly enhances the reversibility and kinetics of the V2+/V3+ redox reaction, attributed to inhibited irreversible hydrogen evolution by the Bi catalyst and increased carboxyl groups by nickel (etching and NiO catalyst). Notably, the V/Mn cell employed in the NB-ECF maintains a high energy efficiency of 85.7% during 50 cycles without capacity degradation at a current density of 20 mA cm–2, which is attributed to a synergistic effect of crossover mitigation and facilitated V2+/V3+ redox reaction. This study demonstrates the novel electrocatalyst design of carbon felt using two metal species.

Investigating Manganese–Vanadium Redox Flow Batteries for Energy Storage and Subsequent Hydrogen Generation

“…Perhaps when EIS results for Nafion 212 are read in conjunction with EDS results in Figure a, this suggests that the carbon-felt materials with a certain degree of passivation with the oxide layer could be more efficient. Substantial efforts have been made to improve the kinetic activity of carbon-based gas diffusion layers by surface functionalization–acid treatment, thermal treatment, heteroatom doping, and metal oxide coating. − From this point of view, recently, MnOx-based oxide layers have enhanced the electrochemical performance of carbon-based electrodes to accelerate reaction kinetics for vanadium redox species. − Likewise, from Figure c, we observe that the combined resistances for Regime II (∼95% CE) and Regime III (∼96% CE) with FAP 450 are higher than that of Regime I. In a similar line of thought, we speculate that when the full cell is assembled with FAP 450, it can potentially curb active species crossover but aggravate higher MnO 2 passivation.…”