Improving the voltage and lifetime of aqueous redox flow batteries utilizing the organometallic catholyte iron (II/III) Tris-2,2’-Bipyridine

Renewable energy, such as solar and wind, helps move away from the reliance on fossil fuels, which causes the significant issue of atmospheric carbon dioxide and greenhouse gas emissions. One of the biggest challenges of renewable energy is the lack of storage options so it can be readily available when it is inaccessible (i.e., when environmental conditions change, and solar and wind energy are not accessible). Of the potential energy storage options, electrochemical storage, specifically battery storage, has become popular because of the potential for long-term storage they provide. Some examples of battery storage include lithium-ion and redox flow. While redox flow batteries (RFBs) are an attractive option for long-term energy storage, a lack of suitable high-potential catholyte species hinders the development. Hydrolysis of the charged (oxidized) form typically occurs when the catholyte’s redox potential approaches that of water, leading to performance degradation. Here we show that hydrolysis of an oxidized (charged) organometallic catholyte, which normally leads to severe voltage losses, can be curtailed through interactions with oxidized carbon surfaces. We discovered that the addition of activated carbon cloth (ACC) to the reservoir of low-cost, high-potential iron (II/III) tris-2,2’-bipyridine ([Fe(bpy)3]2+/3+) catholyte-limited aqueous redox flow batteries extends their lifetime and boosts discharge voltage. A similar effect is observed when the cathode is electrochemically oxidized (overcharged) on the first cycle and by modifying electrolyte pH. Oxidized carbons appear to modify the structure of the charged catholyte’s hydrolysis product, the dimer µ-O-[FeIII(bpy)2(H2O)]24+, and/or change the solution pH, permitting its reduction at a more favorable redox potential than in the ACC-free catholyte. Near-neutral-pH RFBs employing 1,1?-bis(3-sulfonatopropyl)-4,4?- bipyridinium ((SPr)2V) anolyte in excess and the with the [Fe(bpy)3]2+/3+ catholyte containing ACC exhibited high-voltage discharge for 600 cycles (41 days) with no discernable capacity fade. We also demonstrate that a zero-pot, solvent-free synthetic method yields long-lived [Fe(bpy)3]2+/3+-based RFBs when employing the novel voltage-boosting methods.