Iron complex with multiple negative charges ligand for ultrahigh stability and high energy density alkaline all-iron flow battery

Abstract

Alkaline all-iron flow batteries (AIFBs) are highly attractive for large-scale and long-term energy storage due to the abundant availability of raw materials, low cost, inherent safety, and decoupling of capacity and power. However, a stable iron anolyte is still being explored to address complex decomposition, ligand crossover, and energy density to improve battery performance. Herein, a promising metal-organic complex, Fe(NTHPS), consisting of FeCl₃ and 3,3′,3″-nitrilotris (2-hydroxypropane-1-sulfonate) (NTHPS), is specifically designed for alkaline all-iron flow battery. The NTHPS exhibits strong binding strength with iron ions, resulting in ultrahigh stability during the charge-discharge process. AIFB based on the [Fe(CN)₆]^4- catholyte and Fe(NTHPS) showcases an exceptionally high capacity retention of 97.8 % after 2000 cycles (0.0011 % per cycle), maintaining high coulombic efficiency near 100 %. Furthermore, with a solubility as high as 1.82 mol⁻¹, the Fe(NTHPS) anolyte demonstrates an ultra-high theoretical capacity of 47.23 Ah L⁻¹. This multiple negative charges ligand not only resolves existing barrier associated with AIFBs, but also provides valuable insight for their commercial application.