Surface layer expansion due to proton insertion during reduction of RuO{sub 2}(100) single crystal electrodes.

Ruthenium dioxide has received much interest due to its nearly ideal properties for use as the active material in ultracapacitors. Ultracapacitors,a new type of energy storage device, share similarities to both conventional capacitors in that energy is stored by separation of charge and to batteries where charge is stored in chemical reactions. The chemical charge storage mechanism is referred to as pseudo-capacitance, as the Faradaic charge is distributed relatively even across a large portion of the operating voltage range. This property, which is unlike most reversible electrochemical reactions, results in charge-discharge curves for these materials being very similar to a capacitor. Despite many conventional studies of these materials, much is not known about the electrochemical reactions and the structural aspects of the reactions. The goal of our study is to investigate the modification of the surface structure accompanied by the pseudo-capacitance reactions on RuO{sub 2} electrodes. In order to study the surface structure, we performed x-ray diffraction rod measurements on single crystal surfaces. RuO{sub 2} single crystals were grown by open-flow chemical transport method with the evaporation temperature of 1350 C and the oxygen flow of 60 cc/min. These samples were analyzed in-situ in a solution of 0.5 M H{sub 2}SO{sub 4} under potential control using a conventional three-electrode potentiostat configuration. X-ray measurements were carried out at National Synchrotron Light Source (X6B, X10A, and X25A) and Advanced Photon Source (12BM) using focused optics