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Mechanistic Insights into the Hydrogen Oxidation Reaction on PtNi Alloys in Alkaline Media: A First-Principles Investigation
journal contribution
posted on 2020-08-29, 15:43 authored by Lianming Zhao, Haijun Liu, Yonghui Liu, Xiaonan Han, Jing Xu, Wei Xing, Wenyue GuoThe promising alkaline
anion exchange membrane fuel cell suffers
from sluggish kinetics of the hydrogen oxidation reaction (HOR). However,
the puzzling HOR mechanism hinders the further development of highly
active catalysts in alkaline media. In this work, we conducted detailed
first-principles calculations to acquire a deep understanding of the
alkaline HOR mechanism on PtNi bulk alloys [Pt3Ni(111),
Pt2Ni2(111), and PtNi3(111)] and
its surface alloy [PtNisurf(111)]. The full free energy
profiles suggest that the HOR on PtNi alloys proceeds via the Tafel–Volmer mechanism, that is, the direct decomposition
of H2 into two adsorbed H, followed by its reaction with
OH– in the electrolyte, as the rate-determining
step, to form H2O. Therefore, the HOR activity of PtNi
alloys is solely impacted by the adsorption of hydrogen, rather than
hydroxyl species, though the oxophilicity is also enhanced by alloying
Pt with Ni. Thermodynamically, a moderate H adsorption free energy,
ΔGH* ≈ 0.414 eV, is calculated
to be an optimal candidate for the HOR at pH = 13. Alloying Pt with
Ni can elevate the d-band center (εd), push the value
of ΔGH* closer to 0.414 eV, and
thus lower the free energy barrier (Ea) of the rate-determining Volmer reaction, leading to the highest
HOR activity of PtNi3(111) among all considered PtNi alloys.
This situation is further confirmed by both the microkinetic model
and the Tafel plot, where PtNi3(111) exhibits the highest
reaction rate (r = 9.42 × 103 s–1 site–1) and the largest exchange
current density (i0 = 1.42 mA cm–2) for HOR in alkaline media. This work provides a fundamental understanding
of the HOR mechanism and theoretical guidance for rational design
of electrocatalysts for HOR in alkaline media.