Predicting Redox Potentials of Flow Battery-Relevant Model Electrolytes
Authors: Holubowitch Nicolas 
Published: February 2024

Redox flow batteries (RFBs) are rapidly gaining traction among large-scale storage solutions for intermittent renewable energy. Given the anticipated global scale of this technology, there is an intensive ongoing search for active electrolyte species containing all earth-abundant elements. Potential RFB actives must satisfy several criteria to maximize battery performance, namely, high solubility in the RFB solvent, optimum reduction potential, and chemical stability in all redox states. Given the near infinite possible molecular combinations of the primary atoms C, H, N, O, S, and Fe, computational chemistry is an invaluable tool to accelerate the discovery of promising new RFB active compounds.

Towards that end, we performed DFT calculations on two major systems: iron(III/II) tris-2,2’-bipyridine, [Fe(bpy)3]3+/2+, as a model organometallic species, and azoles as a class of small, aromatic organic compounds. For [Fe(bpy)3]3+/2+, after extensive benchmarking of various functionals, basis sets, and other parameters, we accurately predict this complex’s one-electron reduction potential. The computational protocols employed on this system serve to open up the entire class or organometallics to predictive DFT calculations for future high-throughput screening. We also performed frequency calculations in the construction of a potential energy surface of [Fe(bpy)3]3+/2+’s degradation products, with consideration of all possible spin states, during this species’ hydrolysis-initiated dimerization to µ-O-[Fe(III)(bpy)2(H2O)]2
4+. The PES is discussed in light of literature-reported mechanisms for this system.

Our study on functionalized azole compounds is motivated by recent demonstrations of azo compounds for RFBs in both aqueous and non-aqueous configurations. The azo group and its related nitro to amino group-containing monomeric species offers a plethora of potentially reversible redox transformations. Here, we investigate the redox potential and solvation energies of six azole-based compounds via DFT. Amino/nitro-functionalized azoles are small, aromatic compounds with the possibility to store up to six electrons per core, which would afford unprecedented energy densities in RFBs. The findings identify several species along the nitro-azo-amino pathway that have favorable redox potentials as anolyte or catholyte species (Fig. 1), as well as potentially high water solubility. Such coupling of anolyte-catholyte species from the same class of compounds would afford a crossover-proof RFB configuration analogous to vanadium systems that has not been demonstrated for aqueous organic flow batteries. Moreover, redox potentials between cis and trans isomers of the azo species were quantified, implicating further utility of these compounds in photo-mediated cells. These results will guide experimental investigations into the feasibility of implementing azole compounds for RFBs.

Figure 1. Calculated redox potentials (Latimer diagram) for various azoles, including monomeric (nitro-nitroso-hydroxylamine-amino) and dimeric (azo-type) electron transfer events. Species falling outside the -0.5 to 0.5 V range (dotted lines) implicate their potential viability as RFB anolyte (V<-0.5) or catholyte (V>0.5) redox couples. Potentials were derived from DFT-level frequency calculations (free energies) utilizing the TPSS functional, def2-TZVPP basis set, and implicit solvation (CPCM H2O, vacuum- and solvent-optimized geometries).

Figure 1