Designing interfacial structures for selective electrocatalysis

Abstract

Selectivity of electrocatalysis in fuel cells stresses on the tolerance of electrode catalysts to cross-over species. A typical polymer electrolyte membrane fuel cell (PEMFC) employs a Nafion-based membrane to separate the Pt-based cathode and anode. Nevertheless, the intrinsic porosity of Nafion membranes allows reactants and products to cross over from one electrode chamber to the other. As a result, the poor selectivity of Pt leads to mixed reactivity at both electrodes and thus, decreased output voltage and power density. In principle, the membrane can be eliminated if cathode and anode catalysts are selective to the desired half reaction. Herein, the thesis aims to develop selective cathode and anode catalysts. For the cathode, transition metal chalcogenides are selective catalysts for the oxygen reduction reaction. We investigate the structure, activity, surface dynamics, and mechanism of a nickel sulfide catalyst during the oxygen reduction catalysis. By employing the selective nickel sulfide catalyst as the cathode, we construct a proof-of- concept membrane-free fuel cell which significantly outperforms an unselective Pt-cathode congener. For the anode, there is still a paucity of intrinsically selective fuel oxidation catalysts. To achieve high 02 tolerance, we design a new configuration of electrocatalysis by employing a solid oxide-based mixed electron-proton conductor (MEPC) as a condensed membrane to segregate the catalyst and electrolyte while only transporting H-atom equivalents, thus blocking 02 and impurities dissolved in the electrolyte from reaching the catalyst surfaces. We investigate the activity, selectivity, and mechanism of the catalyst/MEPC membrane composite electrode.