Specific Capacitance of RuO2(110) Depends Sensitively on Surface Order

We report the specific capacitance, Cs, of variously ordered RuO2(110) surfaces measured in a 1 M H2SO4 electrolyte with unprecedented precision. Employing similar to 10 nm thick, atomically flat epitaxial RuO2 films along with a geometric surface area-controlled electrochemical cell, we determine an upper limit of Cs = (35.35 +/- 0.53) mu Fcm-2 of the idealized, i.e., well-ordered and stoichiometric RuO2(110) surface by electrochemical impedance spectroscopy (EIS) and scan-rate-dependent cyclic voltammetry. We demonstrate, however, that a slight decrease in surface quality immediately translates into pronounced cyclovoltammetric differences and a significant increase of Cs. This needs to be considered when determining electrochemically active surface areas (ECSAs) of geometrically ill-defined RuO2 catalysts -a benchmark to measure electrochemical performance -from their measured double-layer capacitance.