# Tackling Faradaic Imbalance in Redox Flow Batteries by the Use of a Solid Reducing Agent

## TL;DR

This paper introduces a new method to fix capacity loss in redox flow batteries by using a solid reducing agent to reverse chemical imbalances.

## Contribution

The first use of a solid reducing agent, LiFePO4, to address faradaic imbalance in redox flow batteries.

## Key findings

- Adding LiFePO4 to the catholyte reverses the accumulation of K3Fe(CN)6, restoring battery capacity.
- XRD analysis confirms the chemical reduction of K3Fe(CN)6 to K4Fe(CN)6 via LFP oxidation.
- The method significantly recovers lost capacity in flow cells with catholyte limitations.

## Abstract

Redox flow batteries (RFBs) represent a promising technology
for
large-scale energy storage. However, they suffer from capacity fading
due to various factors, including desynchronization in the state of
charge of the anolyte and catholyte, often caused by irreversible
electrochemical side reactions. This study proposes a novel strategy
to mitigate and reverse the effects of the faradaic imbalance by,
for the first time to the best of our knowledge, introducing a solid
reducing agent, LiFePO4 (LFP), in the catholyte compartment.
The use of a heterogeneous reaction facilitates the removal of the
reaction product, in contrast to homogeneous reducing agents. The
strategy is implemented in a battery comprising K4Fe­(CN)6 as the catholyte and a viologen, 1,1′-bis­(3-sulfonatopropyl)-4,4′-bipyridinium
(BSPV), as the anolyte in 1M KCl supporting electrolyte at neutral
pH. The presence of trace oxygen in the anolyte leads to the accumulation
of K3Fe­(CN)6 in the catholyte, resulting in
a faradaic imbalanceused here as a case study. Introducing
LFP pellets into the catholyte chemically reduces the accumulated
K3Fe­(CN)6 back to K4Fe­(CN)6 via a spontaneous redox process, accompanied by the oxidation of
LFP to FePO4, as confirmed by XRD analysis. Implementation
of this method in a flow cell with a capacity-limiting catholyte results
in a significant recovery of the lost capacity, which is attributed
to the reduction of accumulated K3Fe­(CN)6 by
the LFP pellets. This study presents a promising approach to addressing
the faradaic imbalance in RFBs, potentially leading to improved performance
and extended operational lifetime of these systems.