PNNL

Abstract

Redox flow batteries (RFB) for use as a stationary energy storage system require redox-active materials with high capacity, stability, and sustainability. Aqueous soluble organic (ASO) redox-active materials have recently attracted significant attention as alternatives to traditional transition metal ions. However, reported reversible capacities are substantially lower than their theoretical values based on solubility. In this paper, we describe a phenazine-based ASO compound with an exceptionally high reversible capacity that exceeds 90% of its theoretical solubility value. In our combined nuclear magnetic resonance (NMR) and density functional theory (DFT) study, we discovered that by strategically modifying the phenazine molecular structure, preferential solvation is enabled, thereby increasing the solubility of the phenazine from near-zero to as much as 1.8 M, while its redox potential can be shifted by more than 400 mV. An RFB based on the highly alkaline-soluble and two-electron transfer phenazine derivative, 7,8-dihydroxyphenazine-2-sulfonic acid exhibits an operating voltage at 1.4 V and capacity retention of 99.98% per cycle over 500 cycles at a reversible capability of 67 Ah L-1. Development of this phenazine compound represents a new approach for realizing energy-dense, cost-effective ASO flow batteries for stationary energy storage applications.