A High Potential, Low Capacity Fade Rate Iron Complex Posolyte for Aqueous Organic Flow Batteries

摘要

An iron complex, tris(4,4′-bis(hydroxymethyl)-2,2′-bipyridine) iron dichlorideis reported, which operates at near-neutral pH with a redox potentialof 0.985 V versus SHE. This high potential compound is employed in theposolyte of an aqueous flow battery, paired with bis(3-trimethylammonio)propyl viologen tetrachloride in the negolyte, exhibiting an open-circuitvoltage of 1.3 V at near-neutral pH. It demonstrates excellent cycling performancewith a low temporal capacity fade rate of 0.07% per day over 35 daysof cycling. The extended cycling lifetime is the result of low permeability andimproved structural stability of the newly developed iron complex comparedto that of the iron tris(bipyridine) complex. The combination of high redoxpotential and low capacity fade rate compares favorably with those of allpreviously demonstrated organic and organometallic aqueous posolytes.Extensive investigation into the possible degradation mechanisms, includingpost-mortem chemical and electrochemical analyses, indicates that stepwiseligand dissociations of the iron complex are responsible for the reportedcapacity loss during cell cycling. This investigation provides unprecedentedinsight to guide further improvements of such metalorganic compounds forenergy storage and conversion applications.