Surface Chemical Structure of Carbon Electrodes and Electrochemical Behavior of Redox Couples in Flow Batteries
Authors: Jaimes Raciel , Miranda-Hernández Margarita 
Published: February 2020

Due to their decoupling from electrical power and energy storage capacity, redox flow batteries represent a high-capacity, versatile and low-cost energy storage system. However, the low energy density (related to the potential difference, the low solubility and the solvent electrochemical window of stability), and the steadiness to the oxidation-reduction cycle of the species used (long-term spice instability) are two factors that have prevented its massive use [1, 2]. Among the species that have been explored for these purposes are those with metallic centers: VIV/VV, VII/VIII, Ti3+/TiO2+, Fe2+/Fe3+, Cr2+/Cr3+, Ce4+/Ce3+; halogens (Br−/ClBr2 [3-6], redox pairs of coordination metals [7], in addition to multiple quinones [8], sometimes having an acceptable performance a commercial level. Moderately successful devices use transition metal species (vanadium) with less accessibility and greater restriction in optimization possibilities than synthesizable organic species, so the need to get proper substitutes continues. A factor that is fundamental to the electrochemical performance and long-term stability of the species is the chemical nature of the electrode used. In redox flow cells, the most commonly used electrode material is carbon such as graphite, felt, carbon paper, glassy carbon, fiber, and carbon polymer composites. These materials are highly sensitive to the chemical treatment they are subjected to activate them. It has been reported that the current response varies up to an order of magnitude for the same redox species when the electrode has been activated [4]. It is widely known that exist an influence of the electrodes on the electrochemical response of the redox species, when they are used in flow batteries, however, a profound ignorance of the chemical factors that direct this effect persists. The lack of knowledge of the effect of different electrode surface conditions makes the results reported in the literature incomparable in many cases. The evaluation of new electrode candidates must consider the physicochemical structure of the electrode material and its relationship with the redox couples. In this work, we present the ab-initio computational modeling of the chemical interactions between the redox molecule/carbon electrode. The chemical composition of the surface of the electrodes and different redox couples used in flow batteries are considered such as VIV/VV, VII/VIII, (Br−/ClBr2, [Ru(bpy)3]2+, 2-hydroxynaphtoquinone. The theoretical results are compared with those obtained by electrochemical characterization of carbon paper, and carbon felt, previously subjected to chemical treatments for their activation. The electrochemical responses of redox species are related to molecule-electrode chemical interactions. The objective is to show that the pretreatment-structure-reactivity relationships of the electrodes influence a more effective investigation and allow a better selection of the redox pairs for the flow batteries.
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