Abstract
Discovering new materials for energy storage requires reliable and efficient protocols for predicting key properties of unknown compounds. In the context of the search for new organic electrolytes for redox flow batteries, we present and validate a robust procedure to calculate the redox potentials of organic molecules at any pH value, using widely available quantum chemistry and cheminformatics methods. Using a consistent experimental data set for validation, we explore and compare a few different methods for calculating reaction free energies, the treatment of solvation, and the effect of pH on redox potentials. We find that the B3LYP hybrid functional with the COSMO solvation method, in conjunction with thermal contributions evaluated from BLYP gas-phase harmonic frequencies, yields a good prediction of pH = 0 redox potentials at a moderate computational cost. To predict how the potentials are affected by pH, we propose an improved version of the Alberty-Legendre transform that allows the construction of a more realistic Pourbaix diagram by taking into account how the protonation state changes with pH.
Highlights
The main goal of this contribution is the establishment and validation of a standard procedure for the computational prediction of redox potentials of organic molecules undergoing a proton-coupled electron transfer
The procedure we present is designed to be integrated into a computational discovery workflow: it takes as input the molecular structures of the oxidized and reduced forms (Ox and Red), typically in a text string format such as SMILES, and outputs redox potentials as a function of pH
Taking as the input the SMILES strings of the reduced and oxidized forms, we show that widely available cheminformatics tools can be used to determine the main protonation state at pH = 0 and the major conformer
Summary
The main goal of this contribution is the establishment and validation of a standard procedure for the computational prediction of redox potentials of organic molecules undergoing a proton-coupled electron transfer. The experimental redox potentials at pH 0, 7, and 13 reported by Wedege et al [17] for a set of 28 molecules of the quinone family have been chosen as a consistent data set for validating our computational protocol. It includes molecules with a different number of aromatic rings (i.e., benzo-, naphtho- and anthraquinones) and a diverse range of substituents and span a wide range of redox potentials.
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