Critical Roles of pH and Activated Carbon on the Speciation and Performance of an Archetypal Organometallic Complex for Aqueous Redox Flow Batteries

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 voltage─two 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)]24+, converting it to a higher-potential μ-dihydroxo form, μ-[FeIII(bpy)2(H2O)(OH)]24+, at lower pH. The existence of free bpyH22+ 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.