Unveiling
the composition-dependent catalytic mechanism of Pt-based
alloy cathodes for the oxygen reduction reaction (ORR) helps improve
the proton exchange membrane fuel cells. Using density functional
theory calculations, this study investigates the ORR catalytic performance
of the Pt–Ni system with various compositions (1.00, ∼0.99,
0.75, 0.50, 0.25, ∼0.01, and 0.00). The ordered solid solution
PtNi3(111) system shows activity comparable to Pt(111)
and is cost-effective. The Ni1/Pt(111) system, featuring
a single Ni atom on the Pt(111) surface as a surface single-atom alloy
(SSAA), demonstrates the highest activity with an overpotential of
only 0.28, which could be further reduced to 0.21 V by decreasing
the surface Ni concentration to 1/16 monolayer coverage. The predicted
high activity of Ni1/Pt(111) is confirmed when considering
factors such as the implicit solution environment, constant potential
conditions, and protonation capability. Moreover, surface-adsorbed
oxygen species driven by reaction conditions stabilize these single
Ni atoms of Ni1/Pt(111) by preventing segregation and dissolution
processes, thereby exhibiting a dual functionality. This study reveals
the composition dependence of Pt-based alloys and highlights the stability
mechanisms of SSAA catalysts during the ORR.