PNNL

Abstract

All-soluble iron redox flow batteries (Fe-RFBs) are emerging as scalable, durable, and safe energy-storage systems, by leverage the abundance and low cost of iron while enabling complete decoupling of power and energy. Recent advances highlight the critical role of ligand design in tuning Fe redox chemistry, stability, and battery performance. In this study, we systematically compare monoamine and diamine carboxylate ligands (NTA, EDTA) with their phosphonate analogues (NTMPA, EDTMP) to elucidate structure–property relationships governing Fe-complex performance as anolytes. Cyclic voltammetry and full-cell testing reveal that phosphonate-based complexes, Fe(NTMPA)2 and Fe-EDTMP, exhibit redox potentials 0.12 and 0.20 V lower than their carboxylate counterparts, respectively, consistent with the higher basicity and stronger coordination of phosphonate ligands. Fe-EDTMP anolyte also provide enhanced stability,