A Survey of Additives to Improve the Solubility, Capacity, and Cycle Life of Emerging Aqueous Redox Flow Battery Electrolytes
Authors: Holubowitch Nicolas , Bui Huyen , Afzal Maira , Burghoff Alexis , Jabbar Ayesha 
Published: July 2022

Low carbon intensity wind and solar renewable energies are gaining ground on the entrenched fossil fuel-based energy market; however, energy storage is still an unsolved problem to making renewable energy economically competitive for global-scale adoption. Aqueous redox flow batteries (ARFBs) offer the characteristics necessary to store renewable energy at large scales: customizable energy capacity and power delivery, inflammable/non-toxic components, diverse chemistry options with ultra-long calendar/cycle lifetimes, and rapidly dropping system costs. There is an ever-broadening array of inorganic, organic, and organometallic active species under study for ARFBs. Much research has focused on novel anolyte or catholyte development. Despite this progress, the stringent cost targets set by the DOE (<$150 kWh-1 by 2023) virtually necessitate that all battery electrolytes be earth-abundant, off-the-shelf chemicals. Few of the so-called “next generation” ARFB electrolytes based on organic or organometallic species meet this requirement. Here, we explore opportunities to improve widely available ARFB electrolytes with low-cost additives that improve their solubility, cycle lifetime, and/or energy density.

In this work we survey the impact of several additives on the battery performance of two major catholytes, iron tris-bipyridine (Fe(bpy)3
2+/3+) and 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO-OH). Fe(bpy)3
2+/3+ has been of interest for energy storage for decades but not yet realized in a practical device. It is a low-cost, non-toxic, easily synthesized complex that operates at neutral pH with one of the highest achievable redox potentials among ARFB catholytes. We introduce the shortcomings of Fe(bpy)3
2+/3+, i.e. low aqueous solubility and dimerization after charging, and present means of addressing these issues. Isopropyl alcohol as an cosolvent additive boosts the solubility of Fe(bpy)3
2+/3+ to permit achieved capacities double that of the additive-free solution. To mitigate dimerization, copper hexacyanoferrate (CuHCF) added to the catholyte reservoir improves Fe(bpy)3
2+/3+’s discharge voltage through an apparent catalytic effect. Furthermore, we have discovered that alternative supporting electrolytes, i.e. LiNO3 vs. KCl, improve discharge voltage as well as cycle lifetime in Fe(bpy)3
2+/3+ catholyte-limited ARFBs.

TEMPO-OH is another widely studied, low-cost ARFB catholyte with a high redox potential. It has moderate aqueous solubility and is susceptible to side reactions in both acidic and basic chemical environments. The TEMPO core is often covalently derivatized via synthetic organic approaches in order to improve its properties (solubility, stability, redox potential) in ARFBs. Despite these enhancements, the synthetic approaches are likely not practical for global scale deployment of TEMPOs in ARFBs. Thus, here we seek to enhance the commercially available TEMPO-OH complex via electrolyte/additive engineering. Two recent studies demonstrate strategies to improve the solubility of TEMPO-OH using sodium xylenesulfonate and ionic liquid additives. We report on the full battery performance of these modified electrolytes against the control solutions. To improve energy density, we push the TEMPO-OH catholytes towards their solubility limits to quantify the degradation rates of catholyte-limited ARFBs as a function of TEMPO-OH and additive concentrations.

The work presented here showcases several specific examples of novel additives, supporting electrolytes, and their combinations applied to the low-cost Fe(bpy)3
2+/3+ and TEMPO-OH ARFB catholytes. We quantify significant metrics, i.e. capacity retention, achievable energy density, and voltage/energy efficiency, in the modified electrolytes. We hope the work inspires further studies on engineering the support system of ARFB catholytes and anolytes using low-cost, off-the-shelf additives.