Facet-Dependent Oxygen Evolution Reaction Activity of IrO from Quantum Mechanics and Experiments.

The diversity of chemical environments present on unique crystallographic facets can drive dramatic differences in catalytic activity and the reaction mechanism. By coupling experimental investigations of five different IrO facets and theory, we characterize the detailed elemental steps of the surface redox processes and the rate-limiting processes for the oxygen evolution reaction (OER). The predicted complex evolution of surface adsorbates and the associated charge transfer as a function of applied potential matches well with the distinct redox features observed experimentally for the five facets.

Interplay between Facets and Defects during the Dissociative and Molecular Adsorption of Water on Metal Oxide Surfaces.

Surface terminations and defects play a central role in determining how water interacts with metal oxides, thereby setting important properties of the interface that govern reactivity such as the type and distribution of hydroxyl groups. However, the interconnections between facets and defects remain poorly understood. This limits the usefulness of conventional notions such as that hydroxylation is controlled by metal cation exposure at the surface.

Correlation between oxygen evolution reaction activity and surface compositional evolution in epitaxial LaSrNiFeO thin films.

Water electrolysis can use renewable electricity to produce green hydrogen, a portable fuel and sustainable chemical precursor. Improving electrolyzer efficiency hinges on the activity of the oxygen evolution reaction (OER) catalyst. Earth-abundant, ABO-type perovskite oxides offer great compositional, structural, and electronic tunability, with previous studies showing compositional substitution can increase the OER activity drastically.

Role of Electronic Structure on Nitrate Reduction to Ammonium: A Periodic Journey.

Electrocatalysis is a promising approach to convert waste nitrate to ammonia and help close the nitrogen cycle. This renewably powered ammonia production process sources hydrogen from water (as opposed to methane in the thermal Haber-Bosch process) but requires a delicate balance between a catalyst's activity for the hydrogen evolution reaction (HER) and the nitrate reduction reaction (NORR), influencing the Faradaic efficiency (FE) and selectivity to ammonia/ammonium over other nitrogen-containing products. We measure ammonium FEs ranging from 3.

Understanding methanol dissociative adsorption and oxidation on amorphous oxide films.

Interactions between a transition metal (oxide) catalyst and a support can tailor the number and nature of active sites, for instance in the methanol oxidation reaction. We here use ambient pressure X-ray photoelectron spectroscopy (AP-XPS) to identify and compare the surface adsorbates that form on amorphous metal oxide films that maximize such interactions. Considering AlMO (M = Fe or Mn) films at a range of methanol : oxygen gas ratios and temperatures, we find that the redox-active transition metal site (characterized by methoxy formation) dominates dissociative methanol adsorption, while basic oxygen sites (characterized by carbonate formation) play a lesser role.

Contribution of the Sub-Surface to Electrocatalytic Activity in Atomically Precise La Sr MnO Heterostructures.

Electrocatalytic reactions are known to take place at the catalyst/electrolyte interface. Whereas recent studies of size-dependent activity in nanoparticles and thickness-dependent activity of thin films imply that the sub-surface layers of a catalyst can contribute to the catalytic activity as well, most of these studies consider actual modification of the surfaces. In this study, the role of catalytically active sub-surface layers was investigated by employing atomic-scale thickness control of the La Sr MnO (LSMO) films and heterostructures, without altering the catalyst/electrolyte interface.

Understanding the Electronic Structure Evolution of Epitaxial LaNiFeO Thin Films for Water Oxidation.

Rare earth nickelates including LaNiO are promising catalysts for water electrolysis to produce oxygen gas. Recent studies report that Fe substitution for Ni can significantly enhance the oxygen evolution reaction (OER) activity of LaNiO. However, the role of Fe in increasing the activity remains ambiguous, with potential origins that are both structural and electronic in nature.

Understanding the Role of Surface Heterogeneities in Electrosynthesis Reactions.

In this perspective, we highlight the role of surface heterogeneity in electrosynthesis reactions. Heterogeneities may come in the form of distinct crystallographic facets, boundaries between facets or grains, or point defects. We approach this topic from a foundation of surface science, where signatures from model systems provide understanding of observations on more complex and higher-surface-area materials.

Tuning proton-coupled electron transfer by crystal orientation for efficient water oxidization on double perovskite oxides.

Developing highly efficient and cost-effective oxygen evolution reaction (OER) electrocatalysts is critical for many energy devices. While regulating the proton-coupled electron transfer (PCET) process via introducing additive into the system has been reported effective in promoting OER activity, controlling the PCET process by tuning the intrinsic material properties remains a challenging task. In this work, we take double perovskite oxide PrBaSrCoFeO (PBSCF) as a model system to demonstrate enhancing OER activity through the promotion of PCET by tuning the crystal orientation and correlated proton diffusion.

Hydrogen Bonding Enhances the Electrochemical Hydrogenation of Benzaldehyde in the Aqueous Phase.

The hydrogenation of benzaldehyde to benzyl alcohol on carbon-supported metals in water, enabled by an external potential, is markedly promoted by polarization of the functional groups. The presence of polar co-adsorbates, such as substituted phenols, enhances the hydrogenation rate of the aldehyde by two effects, that is, polarizing the carbonyl group and increasing the probability of forming a transition state for H addition. These two effects enable a hydrogenation route, in which phenol acts as a conduit for proton addition, with a higher rate than the direct proton transfer from hydronium ions.

Tuning the Electronic Structure of LaNiO through Alloying with Strontium to Enhance Oxygen Evolution Activity.

The perovskite oxide LaNiO is a promising oxygen electrocatalyst for renewable energy storage and conversion technologies. Here, it is shown that strontium substitution for lanthanum in coherently strained, epitaxial LaNiO films (La Sr NiO) significantly enhances the oxygen evolution reaction (OER) activity, resulting in performance at = 0.5 comparable to the state-of-the-art catalyst BaSrCoFeO .

Structure, Magnetism, and the Interaction of Water with Ti-Doped FeO Surfaces.

The functionality of magnetite, FeO, for catalysis and spintronics applications is dependent on the molar ratio of Fe and Fe and their distribution at the surface. In turn, this depends on a poorly understood interplay between crystallographic orientation, dopants, and the reactive adsorption of atmospheric species such as water. Here, (100)-, (110)-, and (111)-oriented films of titano-magnetite, FeTiO, were grown by pulsed laser deposition and their composition, valence distribution, magnetism, and interaction with water were studied by ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and X-ray magnetic circular dichroism.

Strain Effect on Oxygen Evolution Reaction Activity of Epitaxial NdNiO Thin Films.

Epitaxial strain can cause both lattice distortion and oxygen nonstoichiometry, effects that are strongly coupled at heterojunctions of complex nickelate oxides. Here we decouple these structural and chemical effects on the oxygen evolution reaction (OER) by using a set of coherently strained epitaxial NdNiO films. We show that within the regime where oxygen vacancies (V) are negligible, compressive strain is favorable for the OER whereas tensile strain is unfavorable; the former induces orbital splitting, resulting in a higher occupancy in the d orbital and weaker Ni-O chemisorption.

Erratum: Activate lattice oxygen redox reactions in metal oxides to catalyse oxygen evolution.

This corrects the article DOI: 10.1038/nchem.2695.

Probing the Surface of Platinum during the Hydrogen Evolution Reaction in Alkaline Electrolyte.

Understanding the surface chemistry of electrocatalysts in operando can bring insight into the reaction mechanism, and ultimately the design of more efficient materials for sustainable energy storage and conversion. Recent progress in synchrotron based X-ray spectroscopies for in operando characterization allows us to probe the solid/liquid interface directly while applying an external potential, applied here to the model system of Pt in alkaline electrolyte for the hydrogen evolution reaction (HER). We employ ambient pressure X-ray photoelectron spectroscopy (AP-XPS) to identify the oxidation and reduction of Pt-oxides and hydroxides on the surface as a function of applied potential, and further assess the potential for hydrogen adsorption and absorption (hydride formation) during and after the HER.

Addendum: Activating lattice oxygen redox reactions in metal oxides to catalyse oxygen evolution.

This corrects the article DOI: 10.1038/nchem.2695.

Activating lattice oxygen redox reactions in metal oxides to catalyse oxygen evolution.

Understanding how materials that catalyse the oxygen evolution reaction (OER) function is essential for the development of efficient energy-storage technologies. The traditional understanding of the OER mechanism on metal oxides involves four concerted proton-electron transfer steps on metal-ion centres at their surface and product oxygen molecules derived from water. Here, using in situ O isotope labelling mass spectrometry, we provide direct experimental evidence that the O generated during the OER on some highly active oxides can come from lattice oxygen.

Influence of LaFeO Surface Termination on Water Reactivity.

The polarity of oxide surfaces can dramatically impact their surface reactivity, in particular, with polar molecules such as water. The surface species that result from this interaction change the oxide electronic structure and chemical reactivity in applications such as photoelectrochemistry but are challenging to probe experimentally. Here, we report a detailed study of the surface chemistry and electronic structure of the perovskite LaFeO in humid conditions using ambient-pressure X-ray photoelectron spectroscopy.

Nanoscale structural oscillations in perovskite oxides induced by oxygen evolution.

Understanding the interaction between water and oxides is critical for many technological applications, including energy storage, surface wetting/self-cleaning, photocatalysis and sensors. Here, we report observations of strong structural oscillations of BaSrCoFeO (BSCF) in the presence of both HO vapour and electron irradiation using environmental transmission electron microscopy. These oscillations are related to the formation and collapse of gaseous bubbles.

Correlation of nanoscale behaviour of forces and macroscale surface wettability.

In this manuscript, we demonstrate a method based on atomic force microscopy which enables local probing of surface wettability. The maximum pull-off force, obtained from force spectroscopy shows a remarkable correlation with the macroscopically observed water contact angle, measured over a wide variety of surfaces starting from hydrophilic, all the way through to hydrophobic ones. This relationship, consequently, facilitates the establishment of a universal behaviour.

Insights into electrochemical reactions from ambient pressure photoelectron spectroscopy.

The understanding of fundamental processes in the bulk and at the interfaces of electrochemical devices is a prerequisite for the development of new technologies with higher efficiency and improved performance. One energy storage scheme of great interest is splitting water to form hydrogen and oxygen gas and converting back to electrical energy by their subsequent recombination with only water as a byproduct. However, kinetic limitations to the rate of oxygen-based electrochemical reactions hamper the efficiency in technologies such as solar fuels, fuel cells, and electrolyzers.

Highly Active Epitaxial La(1-x)Sr(x)MnO3 Surfaces for the Oxygen Reduction Reaction: Role of Charge Transfer.

Most studies of oxide catalysts for the oxygen reduction reaction (ORR) use oxide powder, where the heterogeneity of exposed surfaces and the composite nature of electrodes limit fundamental understanding of the reaction mechanism. We present the ORR activity of epitaxially oriented La(1-x)Sr(x)MnO3 surfaces and investigate, by varying Sr substitution, the relationship between the role of charge transfer and catalytic activity in an alkaline environment. The activity is greatest for La(1-x)Sr(x)MnO3 with 33% Sr, containing mixed Mn(3+/4+), and the (110) and (111) orientations display comparable activities to that of the (001).

Thickness-Dependent Photoelectrochemical Water Splitting on Ultrathin LaFeO3 Films Grown on Nb:SrTiO3.

The performance of photoelectrodes can be modified by changing the material chemistry, geometry, and interface engineering. Specifically, nanoscale active layers can facilitate the collection of charge carriers. In heterostructure devices, the multiple material interfaces are particularly important, which at present are not well understood for oxides.