Understanding the H3PO4 effect
on the catalyst’s
activity under a relevant condition is important for high-temperature
polymer electrolyte membrane fuel cell (HT-PEMFC) catalyst research.
Here, we report a high-temperature rotating disk electrode (HT-RDE)
study of oxygen reduction reaction (ORR) in H3PO4. With the regular electrochemical protocol, we found that H3PO4 reduction could occur during cyclic voltammetry
study and form a reductive speciesphosphorus acid (H3PO3). To obtain reliable ORR measurement, we optimized
the protocol to avoid the H3PO3 generation.
The ORR activity of carbon-supported PtM (M = Fe, Co, Ni, Ru, Pd,
and Ir) bimetallic alloy catalysts measured with this HT-RDE method
showed higher ORR activity than Pt. To understand the alloying effect,
we combine experiments in diluted solutions to distinguish the alloying
effect on Pt–O binding and Pt–H3PO4 binding. The results indicate that H3PO4 mainly
reduces available sites for ORR, with little effect on neighboring
site’s Pt–O binding via Pt–H3PO4 interaction, which is also supported by the density functional
theory calculation of the Pt–O binding energy with/without
H2PO4. Further study in a phosphoric acid-doped
quaternary ammonium-biphosphate ion pair coordinated polyphenylene
(PA-QAPOH) membrane electrode assembly (MEA) shows that the active
alloy catalyst has better performance in both the HT-RDE and MEA.
Also, the MEA gives higher ORR activity than the HT-RDE because of
the higher pressure and less phosphoric acid content of the MEA. Yet,
the gap between the HT-RDE and MEA is significantly smaller than that
between the room temperature (RT)-RDE and MEA, suggesting the importance
of temperature and H3PO4 concentration in understanding
ORR in HT-PEMFCs.
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