Rechargeable soluble iron-air flow batteries based on organometallic complexes

This is the authors' abstract. We don't add key points for this paper.

Soluble anolytes combining highly negative redox potentials with reversible operation in concentrated alkaline media are required to couple the scalable capacity of flow batteries with the high specific energy of metal-air systems. A rechargeable soluble iron-air flow battery is demonstrated using an organometallic Fe(III)/Fe(II) anolyte based on 2,2-bis-(hydroxyethyl)-(iminotris)-(hydroxymethyl)-methane (MET), coupled with alkaline oxygen electrodes operated separately for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Density functional theory screening, ab-initio molecular dynamics and classical molecular dynamics identify MET as the preferred ligand, providing a persistent Fe coordination environment in 5 M NaOH and favouring a strongly negative Fe(III)/Fe(II) potential. Voltammetric measurements reveal a reversible Fe(MET) couple at ca. –1.10 V vs. Ag|AgCl in 5 M NaOH, placing it among the more negative soluble Fe-based redox couples reported in aqueous electrolytes. Galvanostatic negative-half-cell cycling further demonstrates repeatable charge-discharge behaviour under oxygen-free conditions. In a parallel-plate flow cell incorporating an m-PBI separator, a Ni OER electrode and a MnO 2 air-breathing ORR electrode, the battery delivers an open-circuit voltage of ca. 1.2 V and remains operational for more than 50 cycles at 10 mA cm −2 , with average coulombic and energy efficiencies of ca. 99.9% and 40%, respectively. The 20 μm m-PBI membrane exhibits an apparent Fe(MET) permeability coefficient of 2.9 × 10 −8 cm 2 min −1 . Progressive divergence of the charge and discharge voltages indicates accumulating polarisation, while complementary oxygen-electrode stability characterisation reveals morphological, interfacial and surface evolution consistent with their contribution to the increasing voltage losses. The study establishes a highly negative-potential soluble organometallic iron anolyte for iron-air flow-battery operation and identifies oxygen-electrode polarisation and cell-level resistance as the principal limitations of the present configuration.