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Ab Initio Study of the Charge-Storage Mechanisms in RuO2-Based Electrochemical Ultracapacitors

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journal contribution
posted on 12.01.2012, 00:00 by Yongduo Liu, Fei Zhou, Vidvuds Ozolins
The charge-storage mechanisms in ruthenium dioxide are investigated with first-principles density-functional theory (DFT) calculations. Both proton-intercalated bulk and proton-adsorbed RuO2(110) surfaces have been studied to obtain insight into the energetics of discharging processes in RuO2-based aqueous supercapacitors. We predict the existence of a stable ruthenium oxyhydroxide (RuOOH) and demonstrate that the RuO2(110) surface as well as a significant fraction of the available subsurface sites remain completely protonated during cyclic voltammetry measurements in aqueous electrolytes. DFT calculations also show that the bulk proton insertion not only is kinetically inhibited by a high energy barrier of proton diffusion in bulk RuO2 (1.62 eV), but also is thermodynamically inactive under the experimental operating conditions due to a low intercalation voltage (0.08 V in the dilute limit). These results exclude simple bulk or surface protonation as the reason for excellent electrochemical performance of hydrous nanocrystalline RuO2 supercapacitors and point to the importance of subsurface and grain boundary effects.