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NiFe and CoFe (MFe) layered double hydroxides (LDHs) are among the most active electrocatalysts for the alkaline oxygen evolution reaction (OER). Herein, we combine electrochemical measurements, operando X-ray scattering and absorption spectroscopy, and density functional theory (DFT) calculations to elucidate the catalytically active phase, reaction center and the OER mechanism. We provide the first direct atomic-scale evidence that, under applied anodic potentials, MFe LDHs oxidize from as-prepared α-phases to activated γ-phases. The OER-active γ-phases are characterized by about 8% contraction of the lattice spacing and switching of the intercalated ions. DFT calculations reveal that the OER proceeds via a Mars van Krevelen mechanism. The flexible electronic structure of the surface Fe sites, and their synergy with nearest-neighbor M sites through formation of O-bridged Fe-M reaction centers, stabilize OER intermediates that are unfavorable on pure M-M centers and single Fe sites, fundamentally accounting for the high catalytic activity of MFe LDHs. NiFe and CoFe layered double hydroxides are among the most active electrocatalysts for the alkaline oxygen evolution reaction. Here, by combining operando experiments and rigorous DFT calculations, the authors unravel their active phase, the reaction center and the catalytic mechanism.

The reduction of Pt content in the cathode for proton exchange membrane fuel cells is highly desirable to lower their costs. However, lowering the Pt loading of the cathodic electrode leads to high voltage losses. These voltage losses are known to originate from the mass transport resistance of O 2 through the platinum–ionomer interface, the location of the Pt particle with respect to the carbon support and the supports’ structures. In this study, we present a new Pt catalyst/support design that substantially reduces local oxygen-related mass transport resistance. The use of chemically modified carbon supports with tailored porosity enabled controlled deposition of Pt nanoparticles on the outer and inner surface of the support particles. This resulted in an unprecedented uniform coverage of the ionomer over the high surface-area carbon supports, especially under dry operating conditions. Consequently, the present catalyst design exhibits previously unachieved fuel cell power densities in addition to high stability under voltage cycling. Thanks to the Coulombic interaction between the ionomer and N groups on the carbon support, homogeneous ionomer distribution and reproducibility during ink manufacturing process is ensured. Reducing Pt content in cathodes for proton exchange membrane fuel cells is crucial to lower costs but results in high voltage losses. A Pt catalyst/support design that substantially reduces local oxygen-related mass transport resistance is reported.

Water splitting catalysed by earth-abundant materials is pivotal for global-scale production of non-fossil fuels, yet our understanding of the active catalyst structure and reactivity is still insufficient. Here we report on the structurally reversible evolution of crystalline Co 3 O 4 electrocatalysts during oxygen evolution reaction identified using advanced in situ X-ray techniques. At electrode potentials facilitating oxygen evolution, a sub-nanometre shell of the Co 3 O 4 is transformed into an X-ray amorphous CoO x (OH) y which comprises di-μ-oxo-bridged Co 3+/4+ ions. Unlike irreversible amorphizations, here, the formation of the catalytically-active layer is reversed by re-crystallization upon return to non-catalytic electrode conditions. The Co 3 O 4 material thus combines the stability advantages of a controlled, stable crystalline material with high catalytic activity, thanks to the structural flexibility of its active amorphous oxides. We propose that crystalline oxides may be tailored for generating reactive amorphous surface layers at catalytic potentials, just to return to their stable crystalline state under rest conditions. Understanding of catalyst structure and reactivity is important for the development of water splitting catalysts. Here, the authors report reversible structural transformation of the near-surface of crystalline Co 3 O 4 electrocatalysts to an amorphous CoO x (OH) y during oxygen evolution.

Efficient catalysts for the anodic oxygen evolution reaction (OER) are critical for electrochemical H 2 production. Their design requires structural knowledge of their catalytically active sites and state. Here, we track the atomic-scale structural evolution of well-defined CoO x (OH) y compounds into their catalytically active state during electrocatalytic operation through operando and surface-sensitive X-ray spectroscopy and surface voltammetry, supported by theoretical calculations. We find clear voltammetric evidence that electrochemically reducible near-surface Co 3+ –O sites play an organizing role for high OER activity. These sites invariably emerge independent of initial metal valency and coordination under catalytic OER conditions. Combining experiments and theory reveals the unified chemical structure motif as µ 2 -OH-bridged Co 2+/3+ ion clusters formed on all three-dimensional cross-linked and layered CoO x (OH) y precursors and present in an oxidized form during the OER, as shown by operando X-ray spectroscopy. Together, the spectroscopic and electrochemical fingerprints offer a unified picture of our molecular understanding of the structure of catalytically active metal oxide OER sites. Knowledge of the active sites in catalysts—including the sites that form under working conditions—is vital for future design and development. Here, the authors track the atomic-scale changes in a series of well-defined cobalt-based oxide electrocatalysts, showing that the structurally distinct catalysts develop a similar structural motif as they transform into the catalytically active state.

The electro-oxidation of water to oxygen is expected to play a major role in the development of future electrochemical energy conversion and storage technologies. However, the slow rate of the oxygen evolution reaction remains a key challenge that requires fundamental understanding to facilitate the design of more active and stable electrocatalysts. Here, we probe the local geometric ligand environment and electronic metal states of oxygen-coordinated iridium centres in nickel-leached IrNi@IrO x metal oxide core–shell nanoparticles under catalytic oxygen evolution conditions using operando X-ray absorption spectroscopy, resonant high-energy X-ray diffraction and differential atomic pair correlation analysis. Nickel leaching during catalyst activation generates lattice vacancies, which in turn produce uniquely shortened Ir–O metal ligand bonds and an unusually large number of d -band holes in the iridium oxide shell. Density functional theory calculations show that this increase in the formal iridium oxidation state drives the formation of holes on the oxygen ligands in direct proximity to lattice vacancies. We argue that their electrophilic character renders these oxygen ligands susceptible to nucleophilic acid–base-type O–O bond formation at reduced kinetic barriers, resulting in strongly enhanced reactivities. The precise understanding of the active phase under reaction conditions at the molecular level is crucial for the design of improved catalysts. Now, Strasser, Jones and colleagues correlate the high activity of IrNi@IrO x core–shell nanoparticles with the amount of lattice vacancies produced by the nickel leaching process that takes place before and during water oxidation, and elucidate the underlying structural-electronic effects.