Electrochemistry of Ce(IV)/Ce(III) redox couples in mixed solutions for aqueous flow battery: Experimental and molecular modelling study

Cerium redox couple is widely investigated in aqueous flow batteries due to its high redox potential. While mixed supporting electrolytes of methanesulfonic acid (MSA) and sulfuric acid have been adopted to increase the solubility and reversibility of the cerium redox reaction, the influence of such mixed ligands on the electron transfer is absent. Herein, the density functional theory is applied to study the activation energy of the electron transfer, which is in agreement with the reaction kinetics measured by electrochemical impedance spectroscopy. Cerium complexes with different ratios of methanesulfonate and sulfate ligands are assumed with optimized geometries. The standard redox potentials of these cerium complexes are calculated, which follows the same trend as the open circuit potentials of the cerium electrolyte. The exchange current densities of cerium redox couples in the electrolytes with different ratios of MSA and sulfuric acid are 0.16–0.28 mA cm⁻² on glassy carbon electrode, qualitatively conforming the small variation of activation energies.