The Oxidation of Platinum under Wet Conditions Observed by Electrochemical X-ray Photoelectron Spectroscopy

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 Pt δ+ /Pt 2+ /Pt 4+ 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 PtO 2 · n H 2 O growth occurs on the subminute time scale. The fast formation of Pt 4+ species in both surface and bulk oxide form suggests that Pt 4+ -oxides are likely formed (or reduced) even in the transient processes that dominate Pt electrode degradation.

Al2Pt for Oxygen Evolution in Water Splitting: A Strategy for Creating Multifunctionality in Electrocatalysis

“…Thesuperior activity,obtained after first 24 hofoperation and lasting for another 72 h, is followed by as hape change indicative of enhanced transport limitations.T ransformation of the initially gel-like active layer [48] into ac rystalline Pt compound [49,50] and/or ar e-precipitated aluminum oxide surface phase represents ascenario of formation and gradual passivation of an active OER termination layer. It is important that the degradation does not lead to the failure of the active layer over long times and whole complex structure (cf.details below) still possesses sufficient electrical conductivity to warranty electrochemical performance.T he activity response during the long-term stability experiment is summarized in Figure 5c using 20 mA cm À2 as am arker for electrochemical activity.T he slowly decreasing activity after 100 ho fo peration (exceptional duration of stability test in fundamental studies!)…”

Shining light on the solid–liquid interface:in situ/operandomonitoring of surface catalysis

“…26), 148 wherein the high mechanical strength of ultra-thin graphene sheets/nanobubbles offer good signal quality using soft X-rays at pressures reaching 1 bar, even under continuous liquid flow. 149,150 Such systems have exhibited excellent sensitivity even at photon energies as low as 275 eV. 149 The use of graphene in the cell architecture renders such cells ideal for in situ studies of metal-graphene catalysts for OER.…”

“…149 The use of graphene in the cell architecture renders such cells ideal for in situ studies of metal-graphene catalysts for OER. 150,151 For example, the observation of carbonyl-like hybrid interfaces forming in situ between Co and graphene has helped identify the electrocatalytic active site. 151 Deposited nanoparticle catalysts can also be interrogated in the liquid phase by this approach, unlocking diverse applications across heterogeneous catalysis.…”

Al2Pt für die Sauerstoffentwicklungsreaktion bei der Wasserspaltung: eine Strategie zur Erzeugung von Multifunktionalität in der Elektrokatalyse

“…Aufdie hçhere Aktivität, die nach den ersten 24 herzielt wurde und weitere 72 ha ndauert, folgt eine ¾nderung, die auf eine verstärkte Tr ansporteinschränkung hinweist. Die Umwandlung der anfänglich Gel-artigen, aktiven Schicht [48] in eine kristalline Platinverbindung [49,50] und/oder eine wieder ausgefällte Aluminiumoxid-Oberflächenphases tellt ein Szenario der Bildung und allmählichen Passivierung einer aktiven OER-Terminierungsschicht dar.W ichtig ist, dass die Degradation nicht zur Deaktivierung der Schicht über lange Zeiträume führt und die gesamte komplexe Struktur (siehe Details unten) immer noch eine ausreichende elektrische Leitfähigkeit aufweist, um die elektrochemische Leistung zu gewährleisten. Das Aktivitätsverhalten während des Langzeitstabilitätsexperiments ist in Abbildung 5c zusammengefasst, wobei Potentialwerte bei der Stromdichte von 20 mA cm À2 als Marker füre ine erhçhte elektrochemische Aktivitätv erwendet werden.…”