A lack
of suitable high-potential catholytes hinders the development
of aqueous redox flow batteries (RFBs) for large-scale energy storage.
Hydrolysis of the charged (oxidized) catholyte typically occurs when
its redox potential approaches that of water, with a negative impact
on battery performance. Here, we elucidate and address such behavior
for a representative iron-based organometallic complex, showing that
the associated voltage and capacity losses can be curtailed by several
simple means. We discovered that addition of activated carbon cloth
(ACC) to the reservoir of low-cost, high-potential [Fe(bpy)3]2+/3+ catholyte-limited aqueous redox flow batteries
extends their lifetime and boosts discharge voltagetwo typically
orthogonal performance metrics. Similar effects are observed when
the catholyte’s graphite felt electrode is electrochemically
oxidized (overcharged) and by modifying the catholyte solution’s
pH, which was monitored in situ for all flow batteries.
Modulation of solution pH alters hydrolytic speciation of the charged
catholyte from the typical dimeric species μ-O-[FeIII(bpy)2(H2O)]2
4+, converting
it to a higher-potential μ-dihydroxo form, μ-[FeIII(bpy)2(H2O)(OH)]2
4+,
at lower pH. The existence of free bpyH2
2+ at
low pH is found to strongly correlate with battery degradation. Near-neutral-pH
RFBs employing a viologen anolyte, (SPr)2V, in excess with
the [Fe(bpy)3]2+/3+ catholyte containing ACC
exhibited high-voltage discharge for up to 600 cycles (41 days) with
no discernible capacity fade. Correlating pH and voltage data offers
powerful fundamental insight into organometallic (electro)chemistry
with potential utility beyond battery applications. The findings,
with implications toward a host of other “near-neutral”
active species, illuminate the critical and underappreciated role
of electrolyte pH on intracycle and long-term aqueous flow battery
performance.