Density functional theory study of active sites and reaction mechanism of ORR on Pt surfaces under anhydrous conditions

Identifying active sites and catalytic mechanism of the oxygen reduction reaction under anhydrous conditions are crucial for the development of next generation proton exchange membrane fuel cells (PEMFCs) operated at a temperature>100 °C. Here, by employing density functional theory calculations, we studied ORR on flat and stepped Pt(111) surfaces with both (110) and (100) type of steps. 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. (110) type step edge can stabilize O2 adsorption and decrease O2 dissociation barrier because of a unique configuration of accumulated O, which is likely the active site for the oxygen reduction reaction under anhydrous conditions.