Charge/discharge properties of activated carbon/ruthenocene hybrid electrodes in an ionic liquid electrolyte

Abstract

The electrochemical behavior of ruthenocene (RuCp2) in organic electrolytes is complicated by disproportionation or dimerization reactions owing to the reactive nature of the RuCp2+ cation upon oxidation. In this study, RuCp2 is hybridized inside the micropores of microporous activated carbon (AC) via gas-phase adsorption. The resulting AC/RuCp2 hybrids can be used as electrode materials without dissolution of the adsorbed RuCp2 into electrolyte solution by combining with ionic liquid electrolytes. The charge storage of the hybrids is based on the reversible redox reaction of RuCp2 inside the micropores of AC. This hybridization is not accompanied by the volumetric expansion of the AC particles regardless of the RuCp2 contents. In addition, the AC/RuCp2 hybrids exhibit high redox potential as high as ca. 3.9–4.3 ​V (vs. Li/Li+). These features are advantageous for enhancing the volumetric energy density, which is more important than the enhancement of the gravimetric one, in terms of practical applications of electrical energy storage devices. In addition, the hybridized RuCp2 molecules have a large contact area with a conductive carbon surface, and therefore, enable fast charge transfer at the contact interface. RuCp2 is oxidized to RuCp2+ during charging, and RuCp2+ is pulled to the oppositely (i.e., negatively) charged counter electrode. This attractive force incurs desorption of RuCp2, but ionic liquids shield the attractive force. Consequently, the hybridization realizes a volumetric capacity enhancement with balancing high power density and long cycle lifetimes.