Achieving a molecular-level understanding of interfacial (photo)electrochemical processes is essential in order to tailor novel and highly-performing catalytic systems. The corresponding recent development of in situ and operando tools has posed new challenges on experimental architectures. In this study, we use ambient pressure X-ray photoelectron spectroscopy (AP-XPS) to probe the solid/liquid electrified interface of a polycrystalline Pt sample in contact with an alkaline electrolyte during hydrogen and oxygen evolution reactions. Using the “dip-and-pull” technique to probe the interface through a thin liquid layer generated on the sample surface, we observe that the electrolyte meniscus becomes unstable under sustained driving of an electrolyte-consuming reaction (such as water oxidation). The addition of an electrochemically inert supporting electrolyte mitigates this issue, maintaining a stable meniscus layer for prolonged reaction times. In contrast, for processes in which the electrolyte is replenished in the reaction pathway (i.e. water reduction in alkaline conditions), we find that the solid/liquid interface remains stable without addition of a secondary supporting electrolyte. The approach described in this work allows the extension of operando AP-XPS capabilities using the “dip-and-pull” method to a broader class of reactions consuming ionic species during complex interfacial faradaic processes.

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. This new window into the surface chemistry of Pt in alkaline electrolyte brings insight into the nature of the rate limiting step, the extent of H ad/absorption, and its persistence at more anodic potentials.

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 LaFeO₃ in humid conditions using ambient-pressure X-ray photoelectron spectroscopy. Comparing the two possible terminations of the polar (001)-oriented surface, we find that the LaO-terminated surface is more reactive toward water, forming hydroxyl species and adsorbing molecular water at lower relative humidity than its FeO₂-terminated counterpart. However, the FeO₂-terminated surface forms more hydroxyl species during water adsorption at higher humidity, suggesting that adsorbate-adsorbate interactions may impact reactivity. Our results demonstrate how the termination of a complex oxide can dramatically impact its reactivity, providing insight that can aid in the design of catalyst materials.

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 Al_(1-x)M_xO_y (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. Product detection, however, indicates complete oxidation to carbon dioxide and water with partial oxidation products (dimethyl ether) comprising a minor species. Comparing the intensity of methoxy and hydroxyl features at a fixed XPS chemical shift suggests methanol deprotonation during adsorption in oxygen rich conditions for high transition metal content. However, increasing methanol partial pressure and lower metal site density may promote oxygen vacancy formation and the dehydroxylation pathway, supported by a nominal reduction in the oxidation state of iron sites. These findings illustrate that AP-XPS and mass spectrometry together are powerful tools in understanding metal-support interactions, quantifying and probing the nature of catalytic active sites, and considering the link between electronic structure of materials and their catalytic activity.

Transition metal perovskite oxides are promising electrocatalysts for the oxygen reduction reaction (ORR) in fuel cells, but a lack of fundamental understanding of oxide surfaces impedes the rational design of novel catalysts with improved device efficiencies. In particular, understanding the surface chemistry of oxides is essential for controlling both catalytic activity and long-term stability. Thus, elucidating the physical nature of species on perovskite surfaces and their catalytic enhancement would generate new insights in developing oxide electrocatalysts. In this article, we perform near-ambient pressure XPS of model brownmillerite Sr2Co2O5 (SCO) epitaxial thin films with different crystallographic orientations. Detailed analysis of the Co 2p spectra suggests that the films lose oxygen as a function of temperature. Moreover, deconvolution of the O 1s spectra shows distinct behavior for (114)-oriented SCO films compared to (001)-oriented SCO films, where an additional bulk oxygen species is observed. These findings indicate a change to a perovskite-like oxygen chemistry that occurs more easily in (114) SCO than (001) SCO, likely due to the orientation of oxygen vacancy channels out-of-plane with respect to the film surface. This difference in surface chemistry is responsible for the anisotropy of the oxygen surface exchange coefficient of SCO and may contribute to the enhanced ORR kinetics of La0.8Sr0.2CoO3-δ thin films by SCO surface particles observed previously.

Cobalt-containing perovskite oxides are promising electrocatalysts for the oxygen evolution reaction (OER) in alkaline electrolyzers. However, a lack of fundamental understanding of oxide surfaces impedes rational catalyst design for improved activity and stability. We couple electrochemical studies of epitaxial La1−xSrxCoO3−δ films with in situ and operando ambient pressure X-ray photoelectron spectroscopy to investigate the surface stoichiometry, adsorbates, and electronic structure. In situ investigations spanning electrode compositions in a humid environment indicate that hydroxyl and carbonate affinity increase with Sr content, leading to an increase in binding energy of metal core levels and the valence band edge from the formation of a surface dipole. The maximum in hydroxylation at 40% Sr is commensurate with the highest OER activity, where activity scales with greater hole carrier concentration and mobility. Operando measurements of the 20% Sr-doped oxide in alkaline electrolyte indicate that the surface stoichiometry remains constant during OER, supporting the idea that the oxide electrocatalyst is stable and behaves as a metal, with the voltage drop confined to the electrolyte. Furthermore, hydroxyl and carbonate species are present on the electrode surface even under oxidizing conditions, and may impact the availability of active sites or the binding strength of adsorbed intermediates via adsorbate–adsorbate interactions. For covalent oxides with facile charge transfer kinetics, the accumulation of hydroxyl species with oxidative potentials suggests the rate of reaction could be limited by proton transfer kinetics. This operando insight will help guide modeling of self-consistent oxide electrocatalysts, and highlights the potential importance of carbonates in oxygen electrocatalysis.