Theoretical Study to Improve O → OH Reactions in the Fuel Cell ORR
Twenty-eight different transition metals were tested as the middle sandwich layer. We found that the correlation of these special catalysts with the electronic d-band center is different from what was previously thought. For the 28 catalysts tested, the d-band center fits best linearly with a new index that measures non-correlated binding energy of Oad and OHad. This value is related to the exothermicity of a reaction with Oad as reactant and OHad as product. Thus, Pt sandwich catalysts with large d-band centers calculated using nudged elastic band (NEB) perform exceptionally well for reactions that convert Oad to OHad.
Breaking down the ORR into three basic steps, we categorize the catalytic property that can best improve each step:
Step I) O2Activation: High O binding Energy.
Step II) OH Formation: High Index
Step III) OH Consumption: Low OH binding Energy.
While previous models based on binding energy can predict relatively well a catalystic reaction for Step I and Step III, improving Step II of the ORR was difficult to characterize due to the correlated binding energy of O and OH. Improving this new index was found to be a new strategy to improve Step II ORR reactions. Studying the density of state, we explain why the d-band center is related to this index in Pt sandwich catalysts to give theoretical insights.
Figure 1: Binding energy of 28 different Pt sandwich catalysts. The line in the middle describes the general correlated binding energy of pure metal catalysts between O and OH. The perpendicular distance for each catalyst from the middle line is the new index, in which, Pt-W-Pt is the highest, while Pt-Ag-Pt is the lowest.
1. C. A. Menning, H. H. Hwu and J. G. G. Chen, J Phys Chem B 110(31), 15471-15477 (2006).