Active Sites for ORR on Pt in PEM Fuel Cells: A DFT Perspective

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For ORR on Pt, the classic model categorizes active sites based on surface motifs, such as terraces and steps. However, this simplistic approach often leads to orders of magnitude errors in catalyst activity predictions and qualitative uncertainties of active sites, thus limiting opportunities for catalyst design. Using stepped Pt(111) surfaces and ORR as examples, we will illustrate that the root cause of larger errors and uncertainties is such a simplified categorization overlooks atomic site-specific reactivity driven by surface stress release. Specifically, we will show how surface stress release at steps introduces inhomogeneous strain fields, resulting in distinct electronic structures and reactivity for terrace atoms with identical local coordination. This phenomenon leads to a cluster of active sites flanking both sides of the step edge. We will demonstrate strategies to enhance ORR activity in low temperature (<100 °C) PEM fuel cells by leveraging this effect, such as varying terrace widths,[1] adjusting the thickness of 2D nanosheets[2] or controlling external stress.[3] For ORR in high tempatature (>100°C) PEM fuel cells, we found that, in contrast to ORR under hydrous conditions, (111) terrace sites are not active for ORR under anhydrous conditions, because of weakened binding of ORR intermediates induced by O* accumulation on the surface. On the other hand, step edges, which are generally not active for ORR under hydrous conditions, are predicted to be the active sites for ORR under anhydrous conditions. Among them, (110) type step edge with a unique configuration of accumulated O stabilizes O2 adsorption and facilitates O2 dissociation, which lead an overpotential <0.4 V. To improve ORR catalysts in high‐temperature PEMFCs, it is desirable to maximize (110) step edge sites that present between two (111) facets of nanoparticles.[4] [1] G. Liu, A. J. Shih, H. Deng, K. Ojha, X. Chen, M. Luo, I. T. McCrum, M. T. M. Koper, J. Greeley, Z. Zeng, Nature 2024, 626, 1005-1010.[2] L. Wang, Z. Zeng, W. Gao, T. Maxson, D. Raciti, M. Giroux, X. Pan, C. Wang, J. Greeley, Science 2019, 363, 870-874.[3] K. J. Sawant, Z. Zeng, J. P. Greeley, Angew. Chem., Int. Ed. 2024, 63, e202312747.[4] G. Liu, H. Deng, J. Greeley, Z. Zeng, Chinese Journal of Catalysis 2022, 43, 3126-3133.