Abstract
During the electrochemical reduction of oxygen, platinum catalysts are often (partially) oxidized. While these platinum oxides are thought to play a crucial role in fuel cell degradation, their nature remains unclear. Here, we studied the electrochemical oxidation of Pt nanoparticles using in situ XPS. When the particles were sandwiched between a graphene sheet and a proton exchange membrane that is wetted from the back, a confined electrolyte layer was formed, allowing us to probe the electrocatalyst under wet conditions. We show that the surface oxide formed at the onset of Pt oxidation has a mixed Ptδ+/Pt2+/Pt4+ composition. The formation of this surface oxide is suppressed when a Br-containing membrane is chosen due to adsorption of Br on Pt. Time-resolved measurements show that oxidation is fast for nanoparticles: even bulk PtO2·nH2O growth occurs on the subminute time scale. The fast formation of Pt4+ species in both surface and bulk oxide form suggests that Pt4+-oxides are likely formed (or reduced) even in the transient processes that dominate Pt electrode degradation.
Highlights
When the particles were sandwiched between a graphene sheet and a proton exchange membrane that is wetted from the back, a confined electrolyte layer was formed, allowing us to probe the electrocatalyst under wet conditions
Fuel cells play a central role in the transition to a sustainable society, allowing us to use the energy stored in renewable carrier molecules such as hydrogen, methanol, or ammonia
We have studied the electrochemical oxidation of platinum nanoparticles using in situ X-ray photoelectron spectroscopy (XPS)
Summary
Fuel cells play a central role in the transition to a sustainable society, allowing us to use the energy stored in renewable carrier molecules such as hydrogen, methanol, or ammonia. Compact design and modest operating temperature, the proton exchange membrane fuel cell (PEMFC) is of particular interest.[1] As a catalyst for the multielectron reactions involved in the electrochemical fuel combustion, platinum has shown unique activity. Under the corrosive conditions present on the cathode side of PEMFCs, where the oxygen reduction reaction (ORR) takes place, platinum is prone to dissolution.[2] In particular, transient electrode potentials[3−8] and the combination of low pH and high oxygen pressure[3,9] are detrimental. It was shown that the surface of Pt cathodes is (partially) oxidized at ORR potentials, in the presence of oxygen.[3,10−15]
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