Bio-Inspired Electroactive Organic Molecules for Aqueous Redox Flow Batteries. 1. Thiophenoquinones

Redox
flow batteries (RFB) utilizing water-soluble organic redox
couples are a new strategy for low-cost, eco-friendly, and durable
stationary electrical energy storage. Previous studies have focused
on benzoquinones, napthoquinones, and anthraquinones as the electroactive
species. Here, we explore a new class of moleculesthiophenoquinonesspecifically
focusing on the caldariellaquinone-, sulfolobusquinone-, and benzodithio­phenoquinone-like
frameworks that are used for metabolic processes in thermophilic aerobic Sulfolobus archaebacteria. We demonstrated that B3LYP/6-311+G­(d,p)
thermochemical calculations (using the SMD solvation model) reproduce
experimental reduction potentials to within ±0.04 V. We then
studied the effect of amine, hydroxyl, methyl, fluoro, phosphonic
acid, sulfonic acid, carboxylic acid, and nitro functional group modifications
on the reduction potential and Gibbs energy of solvation in water
(using density functional theory) and aqueous solubility (using cheminformatics).
Next we enumerated all of the 10 611 possible combinations
of functional group substitutions on these frameworks and identified
1056 potential molecules with solubilities exceeding 2 mol/L; of these,
36 molecules have reduction potentials below 0.25 V and 15 molecules
above 0.95 V (versus the standard hydrogen electrode (SHE)). The combination
of high solubility and wide voltage range makes these molecules promising
candidates for high performance aqueous RFB applications. Finally,
using our data set of ab initio reduction potentials,
we developed a cheminformatics model that predicts ab initio reduction potentials to within ±0.09 V based solely on molecular
connectivity. We found that a model trained with as few as 200 examples
generates rank-ordered predictions allowed us to identify the highest
performance candidates with half the number of ab initio calculations. This offers a strategy for improving the tractability
of future computational searches for high performance RFB molecules.