A Neutral pH Aqueous Organic–Organometallic Redox Flow Battery with Extremely High Capacity Retention

TLDR

The study demonstrates a neutral‑pH aqueous organic–organometallic redox flow battery using only earth‑abundant elements and outlines future avenues for performance enhancement. The battery employs bis((3‑trimethylammonio)propyl)ferrocene dichloride as the positive electrolyte and bis(3‑trimethylammonio)propyl viologen tetrachloride as the negative electrolyte, separated by an anion‑conducting chloride‑permeable membrane, with trimethylammoniopropyl functionalization enabling ~2 M solubility, suppressing decomposition, and reducing crossover. The battery achieved unprecedented cycling stability, retaining 99.9943 % per cycle and 99.90 % per day at 1.3 M, and 99.9989 % per cycle and 99.967 % per day at 0.75–1.00 M, representing the highest reported retention, and this performance could enable decadal lifetimes for cost‑effective grid‑scale renewable storage.

Abstract

We demonstrate an aqueous organic and organometallic redox flow battery utilizing reactants composed of only earth-abundant elements and operating at neutral pH. The positive electrolyte contains bis((3-trimethylammonio)propyl)ferrocene dichloride, and the negative electrolyte contains bis(3-trimethylammonio)propyl viologen tetrachloride; these are separated by an anion-conducting membrane passing chloride ions. Bis(trimethylammoniopropyl) functionalization leads to ∼2 M solubility for both reactants, suppresses higher-order chemical decomposition pathways, and reduces reactant crossover rates through the membrane. Unprecedented cycling stability was achieved with capacity retention of 99.9943%/cycle and 99.90%/day at a 1.3 M reactant concentration, increasing to 99.9989%/cycle and 99.967%/day at 0.75–1.00 M; these represent the highest capacity retention rates reported to date versus time and versus cycle number. We discuss opportunities for future performance improvement, including chemical modification of a ferrocene center and reducing the membrane resistance without unacceptable increases in reactant crossover. This approach may provide the decadal lifetimes that enable organic–organometallic redox flow batteries to be cost-effective for grid-scale electricity storage, thereby enabling massive penetration of intermittent renewable electricity.