1. Dramatically improved oxygen reduction cathodes using polyoxometalate co-catalysts
Curtis Shannon, Department of Chemistry and Biochemistry, Auburn University, Auburn, AL, 36849
Thermodynamic analyses suggest that the oxygen reduction reaction (ORR) at a metal (M) electrode can be viewed as consisting of an initial dissociative chemisorption step (1) followed by the four-electron reduction of the oxide to water (2).
2M + O2 ==== 2MO (1)
2MO + 4e– + 4H+ ==== 2H2O + 2M (2)
Elements which form stable bonds to O, perform well for O2 bond scission and poorly for O atom reduction. Conversely, metals which reduce adsorbed O atoms efficiently, are ineffective with respect to O2 bond scission. Thus, a simple guideline for the development of bimetallic catalysts has been proposed: combine a metal that is good at O-O bond scission with a second metal that reduces adsorbed O atoms efficiently. Polyoxometalates (POMs) are nanometer sized metal oxygen containing anions that have a wide range of catalytic applications. We believe that transition metal substituted POMs adsorbed on the surface of an electrode that is catalytically active for the reduction of adsorbed O atoms (i.e., Au, Pd, Pt) should function in much the same way as a typical bimetallic ORR catalyst. We set out to design POM co-catalysts for Au, Pd and Pt electrodes using simple thermohemial guidelines.
Positive shifts in ORR potential in the presence of POM catalyst correlates with ∆H°f of the bulk metal oxide in agreement with simple thermochemical models.
Co-substituted Keggin POM catalysts shift the ORR potential on Pt electrodes by over 50 mV positive of bare Pt.