Iridium oxide-based
catalysts are uniquely active and stable in
the oxygen evolution reaction. Theoretical work attributes their activity
to oxyl or μ1-O species. Verifying this intermediate
experimentally has, however, been challenging. In the present study,
these challenges were overcome by combining theory with new experimental
strategies. Ab initio molecular dynamics of the solid–liquid
interface were used to predict spectroscopic features, whereas sample
architecture, developed for surface-sensitive X-ray spectroscopy of
electrocatalysts in confined liquid, was used to search for these
species under realistic conditions. Through this approach, we have
identified μ1-O species during oxygen evolution.
Potentiodynamic X-ray absorption additionally shows that these μ1-O species are created by electrochemical oxidation currents
in a deprotonation reaction.
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.
An electrode for the oxygen evolution reaction based on a conductive bi-layered free standing graphene support functionalized with iridium nanoparticles was fabricated and characterized by means of potentiometric and advanced X-ray spectroscopic techniques. It was found that the electrocatalytic activity of iridium nanoparticles is associated to the formation of Ir 5d electron holes. Strong Ir 5d and O 2p hybridization, however, leads to a concomitant increase O 2p hole character, making oxygen electron deficient and susceptible to nucleophilic attack by water. Consequently, more efficient electrocatalysts can be synthesized by increasing the number of electron-holes shared between the metal d and oxygen 2p.
The usage of iridium as an oxygen-evolution-reaction (OER) electrocatalyst requires very high atom efficiencies paired with high activity and stability. Our efforts during the past 6 years in the Priority Program 1613 funded by the Deutsche Forschungsgemeinschaft (DFG) were focused to mitigate the molecular origin of kinetic overpotentials of Ir-based OER catalysts and to design new materials to achieve that Ir-based catalysts are more atom and energy efficient, as well as stable. Approaches involved are: (1) use of bimetallic mixed metal oxide materials where Ir is combined with cheaper transition metals as starting materials, (2) use of dealloying concepts of nanometer sized core-shell particle with a thin noble metal oxide shell combined with a hollow or cheap transition metal-rich alloy core, and (3) use of corrosion-resistant high-surface-area oxide support materials. In this mini review, we have highlighted selected advances in our understanding of Ir–Ni bimetallic oxide electrocatalysts for the OER in acidic environments.
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