Title

Chemical and Electrochemical Investigation of a Carbonate Selective Catalyst and a Room Temperature Carbonate Fuel Cell

Date of Completion

January 2011

Keywords

Engineering, Chemical|Energy

Degree

Ph.D.

Abstract

In this work, the feasibility of operating an anion exchange membrane fuel cell on the carbonate cycle at low temperature (< 50°C) was demonstrated by investigating the effect of carbonate on hydrogen oxidation, oxygen reduction and anion exchange membrane stability. A Ca2Ru 2O7-y pyrochlore was synthesized and designed for the preferential electrochemical formation of CO3-2, over OH -. ^ Introduction of carbon dioxide to the cathode stream of an anion exchange membrane fuel cell caused a slight increase in performance and evolution of CO2 from the anode effluent when using Pt/C as the cathode catalyst. This result showed some formation of CO3-2 at the cathode on and partial operation on the carbonate cycle. Using a rotating disk electrode, it was shown that low concentrations of carbonate had a minimal effect on the oxygen reduction kinetics.^ Hydrogen oxidation was investigated with hydroxide or carbonate anions in alkaline media. Cyclic voltammetry showed carbonate participation in hydrogen oxidation through a 2-electron pathway. It was shown that oxidation with carbonate anions is kinetically favored over oxidation with hydroxide anions.^ It is suggested that lower reaction enthalpies and formation of less stable reaction intermediates is the primary cause for the increase in kinetics in carbonate environments.^ The chemical stability of commercial anion exchange membranes in concentrated carbonate and concentrated and dilute hydroxide was investigated. An ionic conductivity decrease with time was observed when exposed to concentrated hydroxide. Constant conductivity with time was observed when the membranes were exposed to carbonate. Enhanced stability in the presence of carbonate was attributed to its weak nucleophilic character compared to hydroxide, decreasing chemical interactions with the membrane and preventing the chemical degradation mechanisms.^ A Ca2Ru2O7-y pyrochlore was synthesized through a novel hydrothermal route. The use of a strong oxidizing atmosphere created the conditions required to obtain the high oxidation states necessary for crystal formation. Using temperature programmed desorption, preferential adsorption of carbon dioxide over water was confirmed. Fuel cell experiments showed the preferential formation of carbonate anions and an increase in performance when operating on the carbonate cycle. Thin-film electrodes were used to prove the electrochemical stability and activity of the catalyst. ^