Water Reactivity on the LaCoO₃ (001) Surface: An Ambient Pressure X‑ray Photoelectron Spectroscopy Study

The reactivity of water with the (001)_pc surface of epitaxial LaCoO₃ (LCO) thin films was investigated as a function of relative humidity (RH) by ambient pressure X-ray photoelectron spectroscopy. Specifically, water isobars (pH₂O = 100 mTorr) were performed cooling from 300 to 25 °C, reaching a final RH of ∼0.3%. Significant changes were found in the O 1s and C 1s core-level spectra at different RHs, which were deconvoluted to yield new insights into the hydroxylation and hydration of the LCO surface. Surface hydroxyl groups were found dominant, which were accompanied by minor components including (bi)­carbonates, adsorbed water, and undercoordinated/surface-dipole-influenced oxygen sites on the perovskite surface. A multilayer model was used to quantify the coverage of each species, from which the LCO (001)_pc surface was found to exhibit three different regimes upon increasing RH. The water reactivity with the LCO surface proceeded by surface hydroxylatation to reach saturation (up to ∼0.5 ML), after which carbonates were found to displace hydroxyl groups, and then adsorption of water molecules

Dehydration Pathway for the Dissociation of Gas-Phase Formic Acid on Pt(111) Surface Observed via Ambient-Pressure XPS

While model studies of surface science under ultrahigh vacuum (UHV) have made significant contributions to understanding electrochemistry, many issues related to electrochemical phenomena still remain unanswered due to the extreme environmental differences between UHV and liquid conditions. Electrochemical formic acid (HCOOH) oxidation is one such example. While the dehydration step in the indirect oxidation pathway (HCOOH → H₂O + CO_ad → 2H⁺ + 2e^– + CO₂) is observed in the electrochemical oxidation of formic acid on Pt(111) surface, the surface science studies conducted in UHV condition reported the complete HCOOH dissociation to H₂ and CO₂ on Pt(111) surface with no adsorbed CO at room temperature. A dehydration mechanism may also exist in gas-phase HCOOH dissociation in some conditions different from UHV, but it has not been demonstrated with a surface science method due to pressure limitations. Using ambient pressure X-ray photoelectron spectroscopy (AP-XPS), we observed the dehydration mechanism of gas-phase HCOOH in unprecedented high pressure environment for the first time. This study is a demonstration of reconciling the disagreement between electrocatalysis and surface science by bridging the environment gap

An Operando Investigation of (Ni–Fe–Co–Ce)O_<i>x</i> System as Highly Efficient Electrocatalyst for Oxygen Evolution Reaction

The oxygen evolution reaction (OER) is a critical component of industrial processes such as electrowinning of metals and the chlor-alkali process. It also plays a central role in the development of a renewable energy field for generation a solar fuels by providing both the protons and electrons needed to generate fuels such as H₂ or reduced hydrocarbons from CO₂. To improve these processes, it is necessary to expand the fundamental understanding of catalytically active species at low overpotential, which will further the development of electrocatalysts with high activity and durability. In this context, performing experimental investigations of the electrocatalysts under realistic working regimes (i.e., under operando conditions) is of crucial importance. Here, we study a highly active quinary transition-metal-oxide-based OER electrocatalyst by means of operando ambient-pressure X-ray photoelectron spectroscopy and X-ray absorption spectroscopy performed at the solid/liquid interface. We observe that the catalyst undergoes a clear chemical-structural evolution as a function of the applied potential with Ni, Fe, and Co oxyhydroxides comprising the active catalytic species. While CeO₂ is redox inactive under catalytic conditions, its influence on the redox processes of the transition metals boosts the catalytic activity at low overpotentials, introducing an important design principle for the optimization of electrocatalysts and tailoring of high-performance materials

Understanding the Oxygen Evolution Reaction Mechanism on CoO_<i>x</i> using <i>Operando</i> Ambient-Pressure X‑ray Photoelectron Spectroscopy

Photoelectrochemical water splitting is a promising approach for renewable production of hydrogen from solar energy and requires interfacing advanced water-splitting catalysts with semiconductors. Understanding the mechanism of function of such electrocatalysts at the atomic scale and under realistic working conditions is a challenging, yet important, task for advancing efficient and stable function. This is particularly true for the case of oxygen evolution catalysts and, here, we study a highly active Co₃O₄/Co­(OH)₂ biphasic electrocatalyst on Si by means of <i>operando</i> ambient-pressure X-ray photoelectron spectroscopy performed at the solid/liquid electrified interface. Spectral simulation and multiplet fitting reveal that the catalyst undergoes chemical-structural transformations as a function of the applied anodic potential, with complete conversion of the Co­(OH)₂ and partial conversion of the spinel Co₃O₄ phases to CoO­(OH) under precatalytic electrochemical conditions. Furthermore, we observe new spectral features in both Co 2p and O 1s core-level regions to emerge under oxygen evolution reaction conditions on CoO­(OH). The <i>operando</i> photoelectron spectra support assignment of these newly observed features to highly active Co⁴⁺ centers under catalytic conditions. Comparison of these results to those from a pure phase spinel Co₃O₄ catalyst supports this interpretation and reveals that the presence of Co­(OH)₂ enhances catalytic activity by promoting transformations to CoO­(OH). The direct investigation of electrified interfaces presented in this work can be extended to different materials under realistic catalytic conditions, thereby providing a powerful tool for mechanism discovery and an enabling capability for catalyst design

Electrochemical Activation of CO₂ through Atomic Ordering Transformations of AuCu Nanoparticles

Precise control of elemental configurations within multimetallic nanoparticles (NPs) could enable access to functional nanomaterials with significant performance benefits. This can be achieved down to the atomic level by the disorder-to-order transformation of individual NPs. Here, by systematically controlling the ordering degree, we show that the atomic ordering transformation, applied to AuCu NPs, activates them to perform as selective electrocatalysts for CO₂ reduction. In contrast to the disordered alloy NP, which is catalytically active for hydrogen evolution, ordered AuCu NPs selectively converted CO₂ to CO at faradaic efficiency reaching 80%. CO formation could be achieved with a reduction in overpotential of ∼200 mV, and catalytic turnover was enhanced by 3.2-fold. In comparison to those obtained with a pure gold catalyst, mass activities could be improved as well. Atomic-level structural investigations revealed three atomic gold layers over the intermetallic core to be sufficient for enhanced catalytic behavior, which is further supported by DFT analysis

Instability at the Electrode/Electrolyte Interface Induced by Hard Cation Chelation and Nucleophilic Attack

Electrochemistry is necessarily a science of interfacial processes, and understanding electrode/electrolyte interfaces is essential to controlling electrochemical performance and stability. Undesirable interfacial interactions hinder discovery and development of rational materials combinations. By example, we examine an electrolyte, magnesium­(II) bis­(trifluoromethanesulfonyl)­imide (Mg­(TFSI)₂) dissolved in diglyme, next to the Mg metal anode, which is purported to have a wide window of electrochemical stability. However, even in the absence of any bias, using in situ tender X-ray photoelectron spectroscopy, we discovered an intrinsic interfacial chemical instability of both the solvent and salt, further explained using first-principles calculations as driven by Mg²⁺ dication chelation and nucleophilic attack by hydroxide ions. The proposed mechanism appears general to the chemistry near or on metal surfaces in hygroscopic environments with chelation of hard cations and indicates possible synthetic strategies to overcome chemical instability within this class of electrolytes

Synthesis of Pt₃Y and Other Early–Late Intermetallic Nanoparticles by Way of a Molten Reducing Agent

Early–late intermetallic phases have garnered increased attention recently for their catalytic properties. To achieve the high surface areas needed for industrially relevant applications, these phases must be synthesized as nanoparticles in a scalable fashion. Herein, Pt₃Ytargeted as a prototypical example of an early–late intermetallichas been synthesized as nanoparticles approximately 5–20 nm in diameter via a solution process and characterized by XRD, TEM, EDS, and XPS. The key development is the use of a molten borohydride (MEt₃BH, M = Na, K) as both the reducing agent and reaction medium. Readily available halide precursors of the two metals are used. Accordingly, no organic ligands are necessary, as the resulting halide salt byproduct prevents sintering, which further permits dispersion of the nanoscale intermetallic onto a support. The versatility of this approach was validated by the synthesis of other intermetallic phases such as Pt₃Sc, Pt₃Lu, Pt₂Na, and Au₂Y

Oxygen Reduction Kinetics Enhancement on a Heterostructured Oxide Surface for Solid Oxide Fuel Cells

Heterostructured interfaces of oxides, which can exhibit transport and reactivity characteristics remarkably different from those of bulk oxides, are interesting systems to explore in search of highly active cathodes for the oxygen reduction reaction (ORR). Here, we show that the ORR of ∼85 nm thick La_0.8Sr_0.2CoO_3−δ (LSC₁₁₃) films prepared by pulsed laser deposition on (001)-oriented yttria-stabilized zirconia (YSZ) substrates is dramatically enhanced (∼3−4 orders of magnitude above bulk LSC₁₁₃) by surface decorations of (La_0.5Sr_0.5)₂CoO_4±δ (LSC₂₁₄) with coverage in the range from ∼0.1 to ∼15 nm. Their surface and atomic structures were characterized by atomic force, scanning electron, and scanning transmission electron microscopy, and the ORR kinetics were determined by electrochemical impedance spectroscopy. Although the mechanism for ORR enhancement is not yet fully understood, our results to date show that the observed ORR enhancement can be attributed to highly active interfacial LSC₁₁₃/LSC₂₁₄ regions, which were shown to be atomically sharp

Influence of Strain on the Surface–Oxygen Interaction and the Oxygen Evolution Reaction of SrIrO₃

Understanding how physicochemical properties of materials affect the oxygen evolution reaction (OER) has enormous scientific and technological implications for the OER electrocatalyst design. We present our investigation on the role of strain on the surface–oxygen interaction and the OER on well-defined single-termination SrIrO₃ films. Our approach employs a combination of molecular-beam epitaxy, electrochemical characterizations, ambient-pressure X-ray photoelectron spectroscopy, and density functional theory (DFT). We find that inplane compressive strain weakens the surface oxygen binding strength on SrIrO₃; however, it has a negligible effect on the surface oxygen electroadsorption and the OER. We explain this observation, which goes against a commonly held intuition that a change in the surface oxygen binding strength should influence surface oxygen electroadsorption and OER by recognizing that the trend in surface oxygen adsorption measured in the gas phase does not account for the presence of water in the surface oxygen electroadsorption. Inclusions of surface water molecules allow DFT to qualitatively reproduce the electroadsorption trend, highlighting the importance of surface water in the surface–oxygen interaction. Our finding suggests that a commonly held assumption between surface oxygen binding strength (in vacuum, no water) and electroadsorption (requiring water) is not always a simple one-to-one description and calls for a more in-depth investigation on the structure of water at electrochemical interfaces

Ambient-Pressure XPS Study of a Ni–Fe Electrocatalyst for the Oxygen Evolution Reaction

Chemical analysis of solid–liquid interfaces under electrochemical conditions has recently become feasible due to the development of new synchrotron radiation techniques. Here we report the use of “tender” X-ray ambient-pressure X-ray photoelectron spectroscopy (APXPS) to characterize a thin film of Ni–Fe oxyhydroxide electrodeposited on Au as the working electrode at different applied potentials in 0.1 M KOH as the electrolyte. Our results show that the as-prepared 7 nm thick Ni–Fe (50% Fe) film contains Fe and Ni in both their metallic as well as oxidized states, and undergoes further oxidation when the sample is subjected to electrochemical oxidation–reduction cycles. Metallic Fe is oxidized to Fe³⁺ and metallic Ni to Ni^2+/3+. This work shows that it is possible to monitor the chemical nature of the Ni–Fe catalyst as a function of potential when the corresponding current densities are small. This allows for <i>operando</i> measurements just above the onset of OER; however, current densities as they are desired in photoelectrochemical devices (∼1–10 mA cm^–2) could not be achieved in this work, due to ohmic losses in the thin electrolyte film. We use a two-dimensional model to describe the spatial distribution of the electrochemical potential, current density, and pH as a function of the position above the electrolyte meniscus, to provide guidance toward enabling the acquisition of <i>operando</i> APXPS at high current density. The shifts in binding energy of water with applied potential predicted by the model are in good agreement with the experimental values