Theoretical Study to Improve O → OH Reactions in the Fuel Cell ORR

Using density functional theory (DFT), Pt sandwich catalysts¹ are studied to improve the oxygen reduction reaction (ORR) in fuel cells.  The ORR can occur through six different reactions, in which the oxygen hydration² (O_ad + H₂O_ad → 2OH_ad) reaction was determined to be the rate determining step.  A sandwich catalyst consists of Pt on the surface layer, a different metal in the second, and Pt in the bulk.  The catalysts have been shown to display unique properties that improve the sluggish ORR in fuel cells. 

     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 O_ad and OH_ad.  This value is related to the exothermicity of a reaction with O_ad as reactant and OH_ad as product.  Thus, Pt sandwich catalysts with large d-band centers calculated using nudged elastic band (NEB) perform exceptionally well for reactions that convert O_ad to OH_ad.

     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.

References:

1.         C. A. Menning, H. H. Hwu and J. G. G. Chen, J Phys Chem B 110(31), 15471-15477 (2006).

2.         Y. Sha, T. H. Yu, B. V. Merinov, P. Shirvanian and W. A. Goddard, Journal of Physical Chemistry Letters 2 (6), 572-576 (2011).