Ab initio computational modeling of the electrochemical reactivity of quinones on gold and glassy carbon electrodes

We propose electronic structure modeling, in accordance with the Gerischer conceptualization, to explain the electrochemical response of the benzoquinone/hydroquinone (BQ/HQ) redox couple on gold and glassy carbon (GC) electrodes. Specifically, coupling differed to gold compared to GC; this is of interest because BQ/HQ are representative of a promising class of molecules used in redox flow batteries (RFBs). We calculated the energy difference between the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO), Fermi levels (E-F), and density of states (DOS) of the electrode materials via density functional theory in periodic systems. The Au (111) surface was considered for gold modeling, whereas graphite and graphene were used for GC. We compared these results with experimental data to explain the differences in the reversible behavior of the BQ/HQ couple in both electrode materials, setting the pH, electrolyte composition, and electrode geometry equal in both electrodes. The loss of reversibility in GC can be attributed to the anodic branch of the cyclic voltammetry response, which is consistent with the larger energetic distance found for the HQ-HOMO from the EF of the electrode compared to BQ-LUMO. In contrast, the energetic distances in gold are similar in both cases and agree with the symmetry of the experimental current-potential response for both the anodic and cathodic branch. To validate the model, we calculated the total DOS and projected DOS for different quinone molecules-such as 2,5-dichloro-1,4-benzoquinone, 2-hydroxy-1,4-naphthoquinone, and 1,2,4-trihydroxynaphtalene-adsorbed on gold and GC. The experimental findings support our hypothesis. Additionally, graphite and graphene models equivalently described GC electrodes; both models showed a similar trend in adsorption energy for quinone molecules [due to van der Waals (VDW) interactions], DOS, and partial charge density. For the gold electrode, we found a similar trend in adsorption energy to the graphite and graphene, also attributable to VDW interactions. Our results indicate that theoretical modeling can explain electrochemical principles which underpin quinone-based energy storage systems and RFBs. (C) 2018 Elsevier Ltd. All rights reserved.