Electrocatalytic Reactivity of Hydrocarbons on a Zirconia Electrolyte Surface

Abstract

An experimental survey of the electrochemical reactivity of five common fuel species was made employing a solid oxide electrolyte galvanic cell with porous Au and Pt electrodes in the temperature range 700°–850°C. The electrolyte used was (SSZ). The fuel species electro‐oxidized at the anode were: , , , , and . Rates of reaction were determined coulometrically, so that species other than could have undergone an undetermined amount of thermal dissociation during electro‐oxidation. The concomitant reactivity of , which is reduced at the cathode, was also investigated. The current‐overpotential behavior at both the cathode and anode was found to be similar whether Au or Pt was used to form the porous electrodes. In the low overpotential range, the rate of charge transfer is found to be rate determining for both the cathodic and anodic reactions. Activation enthalpies obtained from an analysis of the data in this low overpotential range are also found to be similarly independent of the electrode materials. Reduction of the electrolyte by current blackening leads to two to three orders of magnitude increase in both the cathodic and anodic current density at a given overpotential. Again, the current overpotential characteristics obtained with Au and Pt electrodes are very similar. Activation enthalpies obtained with the electrolyte in the blackened state do not deviate significantly from those obtained with an unblackened electrolyte. These experimental observations are consistent with a reaction mechanism in which the major electrochemical steps occur at active sites on the electrolyte surface rather than on the metal electrodes. The electrochemical reaction sites are hypothesized to be oxygen vacancies with electrons migrating along the electrolyte surface to or away from these active sites. Thus, an effective localized electronic conductivity of the electrolyte surface is postulated.