45026-AC5
Electron-Transfer Reactions of Transition Metal-Substituted 'Sandwich' Polyoxometalates on Electrode Surfaces
Narrative Report
The oxygen reduction reaction (ORR) plays an important role in many man made energy conversion processes as well as in aerobic pathways in biological systems. A promising strategy for improving the ORR kinetics of Pt based systems is the use of multimetallic catalysts. In this research, transition metal substituted POMs adsorbed on the surface of electrodes that are catalytically active for the reduction of adsorbed O atoms are used to model bimetallic ORR catalysts.
We studied the influence of a series of transition metal substituted Wells-Dawson (P2W17MnO62(12-n)–; M = WVI, FeII, CoII, RuII) and Keggin (PW12O403– and PCoW11O395–) anions on the oxygen reduction reaction (ORR) at Au, Pd and Pt. In one study, we carried out a series of cyclic voltammetry experiments for Au electrodes immersed in electrolytes containing a series of transition metal substituted Wells-Dawson (P2W17MnO62(12-n)–; M = WVI, FeII, CoII, RuII) and Keggin (PW12O403– and PCoW11O395–) anions. The Wells-Dawson and Keggin series were chosen because these compounds are extremely well characterized and because they are known to be stable at low pH. Au was chosen as the cathode for these measurements because it efficiently reduces adsorbed O atoms and is not catalytically active for O2 bond. Wells-Dawson POMs adsorbed on Au lead to large positive shifts of the ORR potential. The magnitude of the shift depends on the transition metal and correlates with the free enthalpy of formation of the corresponding metal oxide. POMs with stronger M-O bonds exhibit a weaker influence on oxygen reduction at Au surfaces (i.e., a less positive shift of the potential) than POMs with weaker M-O bonds.
Current research is focused on integrating POM co-catalysts with a variety of metal nanoparticles (NPs). Because our results using macro-scale electrodes clearly indicate the existence of a critical surface activity of POM for optimum catalyst performance, we think it is necessary to employ a thin film architecture in which intimate contact between the NPs and POMs (such as POM-stabilized NPs) can be avoided and/or controlled. We also sought an approach that would allow parameters related to the POM and metal NPs to be changed and optimized independently. For these reasons, we decided to use layer-by-layer (LbL) assembly techniques to prepare hybrid POM-NP electrodes in which one layer contains polyelectrolyte stabilized NPs and the second the desired POM or POMs. Our initial results demonstrate that synergy between the POM and metal NP leads to a roughly four-fold improvement in specific activity SA over currently existing ORR catalysts.
Two Ph.D. students were supported on this grant this year. One student, Anand Sankarraj completed his dissertation research in the fall of 2007 and defended his dissertation in July 2008.
