All-iron redox flow battery in flow-through and flow-over set-ups: the critical role of cell configuration

Significant differences in performance between the two prevalent cell configurations in all-soluble, all-iron redox flow batteries are presented, demonstrating the critical role of cell architecture in the pursuit of novel chemistries in non-vanadium systems. Using a ferrocyanide-based posolyte, and a negolyte containing a hydroxylamine-based iron complex, higher maximum power density, energy efficiency, and electrolyte utilisation were observed with a flow-over cell that incorporated a carbon paper, compared with a flow-through configuration that used a graphite felt. Capacity fade was lower in the flow-over case, likely the result of a set-up with lower overpotentials, as indicated by polarisation curve analysis. Capacity fade in the flow-through case increased upon lowering current density, suggesting a different degradation pathway, dominated instead by electrolyte cross-over. These findings highlight the potential of novel non-vanadium chemistries in both flow-through and flow-over cells, prompting further research exploration of cell architectures.