First-principles study of RuO2 as supercapacitor electrode

performance electrode material for electrochemical supercapacitors [1,2]. The specific capacitance of RuO2·0.5H2O can be 800 f/g [2,3] , which is five times higher than that of its anhydrous counterpart. The origin of such high specific capacitance along with the charge storage mechanisms of RuO2-based materials have been experimentally investigated for a long time, but is still a topic of debate [3-5]. We have studied structural, thermal and kinetic properties of ruthenium dioxide with first-principles calculations. Fig. 1: schematics of the stability of different structure models for hydrous RuO2. We begin with the structural model of hydrous ruthenia. Hydrogen may be incorporated in metal vacancies inside the oxide host ("bulk model") [6]. In another proposed structure, water occupies the region between rutile nano-grains ("core +grain-boundary model") [7]. Our theory optimized hydrogen positions within RuO2 with proton-compensated Ru vacancies using a combination of a systematic search algorithm based on electrostatics, database searching and first-principles calculations. We find that all the considered bulk model structures are unstable by 0.3-0.4 eV per H2O with respect to phase separation into anhydrous RuO2 and water (Fig. 1). Structures with hydroxyl groups or aggregate H2O are significantly lower in energy (though still unstable with respect to phase separation), demonstrating that the water prefers to agglomerate outside RuO2. Our results strongly disfavor the bulk model with hydrogen inside RuO2 and support the nano-grain model of hydrous ruthenia