Real-time investigations of Pt(111) surface transformations in sulfuric acid solutions

A Happy Get-Together - Probing Electrochemical Interfaces by Non-Linear Vibrational Spectroscopy

Electrochemical interfaces are key structures in energy storage and catalysis. Hence, a molecular understanding of the active sites at these interfaces, their solvation, the structure of adsorbates, and the formation of solid-electrolyte interfaces are crucial for an in-depth mechanistic understanding of their function. Vibrational sum-frequency generation (VSFG) spectroscopy has emerged as an operando spectroscopic technique to monitor complex electrochemical interfaces due to its intrinsic interface sensitivity and chemical specificity. Thus, this review discusses the happy get-together between VSFG spectroscopy and electrochemical interfaces. Methodological approaches for answering core issues associated with the behavior of adsorbates on electrodes, the structure of solvent adlayers, the transient formation of reaction intermediates, and the emergence of solid electrolyte interphase in battery research are assessed to provide a critical inventory of highly promising avenues to bring optical spectroscopy to use in modern material research in energy conversion and storage.

Probing the electrode-solution interfaces in rechargeable batteries by sum-frequency generation spectroscopy

An in-depth understanding of the electrode-electrolyte interaction and electrochemical reactions at the electrode-solution interfaces in rechargeable batteries is essential to develop novel electrolytes and electrode materials with high performance. In this perspective, we highlight the advantages of the interface-specific sum-frequency generation (SFG) spectroscopy on the studies of the electrode-solution interface for the Li-ion and Li-O₂ batteries. The SFG studies in probing solvent adsorption structures and solid-electrolyte interphase formation for the Li-ion battery are briefly reviewed. Recent progress on the SFG study of the oxygen reaction mechanisms and stability of the electrolyte in the Li-O₂ battery is also discussed. Finally, we present the current perspective and future directions in the SFG studies on the electrode-electrolyte interfaces toward providing deeper insight into the mechanisms of discharging/charging and parasitic reactions in novel rechargeable battery systems.

Sulfate, Bisulfate, and Hydrogen Co-adsorption on Pt(111) and Au(111) in an Electrochemical Environment

The co-adsorption of sulfate, bisulfate and hydrogen on Pt(111) and Au(111) electrodes was studied based on periodic density functional calculations with the aqueous electrolyte represented by both explicit and implicit solvent models. The influence of the electrochemical control parameters such as the electrode potential and pH was taken into account in a grand-canonical approach. Thus, phase diagrams of the stable coadsorption phases as a function of the electrochemical potential and Pourbaix diagrams have been derived which well reproduce experimental findings. We demonstrate that it is necessary to include explicit water molecules in order to determine the stable adsorbate phases as the (bi)sulfate adsorbates rows become significantly stabilized by bridging water molecules.

In-situ Raman analysis of hydrogenation in well-defined ultrathin molybdenum diselenide deposits synthesized through vapor phase deposition

We report on the synthesis, characterization and in-situ Raman spectroscopy analysis of hydrogenation in ultrathin crystalline MoSe₂ deposits. We use a controllable vapor phase synthesis method using MoSe₂ powder as the only precursor, to fabricate nano- to micro-size few layer thick MoSe₂ deposits with tunable number densities on SiO₂/Si substrates. We employ this controllable synthesis method to correlate characteristic Raman spectroscopy response of MoSe₂ at ca. 242 cm⁻¹ (A_1g) and ca. 280 cm⁻¹ (E_2g¹) with the thickness of the deposits acquired from atomic force microscopy (AFM). We also use this array of well-defined atomically thin MoSe₂ deposits to study possible hydrogenation effects on select architectures using in-situ Raman spectroscopy. Interestingly, our analysis indicates that ultrathin MoSe₂ deposits with exposed edges show a blue shift of 1–2 cm⁻¹ when exposed to H₂ flow at 150–250 sccm for 2–4 hours in a sealed reaction cell. Exposure to Ar flow under same condition reverses the observed shift in the A_1g mode of the select MoSe₂ deposits. Our measurements provide in-situ evidence for hydrogen adsorption on MoSe₂ deposits at room temperature and insight into the possible active sites for hydrogen reactions on layered dichalcogenides at lower dimensions.

Soft X-ray Absorption Spectroscopy of Liquids for Understanding Chemical Processes in Solution

Soft X-ray absorption spectroscopy (XAS) involving excitation processes of a core electron to unoccupied states is an effective method to study local structures around excited C, N, and O atoms in liquid samples. Since soft X-rays are strongly absorbed by air and liquid itself, we have developed transmission-type liquid flow cells, where the absorbance of liquid samples can be easily reduced and optimized by controlling the liquid thickness. By using the transmission-mode XAS techniques, we have investigated local structures of several liquid samples such as concentration dependence of aqueous pyridine solutions and unexpected temperature-dependent structural changes in liquid benzene from the precise energy shift measurements in XAS spectra with the help of molecular dynamics simulation and inner-shell calculations. These XAS techniques are also applied to in situ/operando observation of chemical processes in solutions such as catalytic and electrochemical reactions.

Resolving the chemical identity of H2SO4 derived anions on Pt(111) electrodes: they're sulfate

Understanding how electrolyte composition controls electrocatalytic reactions requires molecular-level insight into electrode/electrolyte interaction. Perhaps the most basic aspect of this interaction, the speciation of the interfacial ion, is often controversial for even relatively simple systems. For example, for Pt(111) in 0.5 M H₂SO₄ it has long been debated whether the adsorbed anion is SO₄^2-, HSO₄^- or an H₃O⁺SO₄^2- ion pair. Here we apply interface-specific vibrational sum frequency (VSF) spectroscopy and theory to this problem and perform an isotope exchange study: we collect VSF spectra of Pt(111) in H₂SO₄(H₂O) and D₂SO₄(D₂O) as a function of bias and show that at all potentials they are identical. This is the most direct spectroscopic evidence to date that SO₄^2- is the dominant adsorbate, despite the fact that at 0.5 M H₂SO₄ bulk solution is dominated by HSO₄^-. This approach is based on the unique selection rule of the VSF spectroscopy and thus offers a new way of accessing general electrode/electrolyte interaction in electrocatalysis.

Specific Ion Effects of Dodecyl Sulfate Surfactants with Alkali Ions at the Air-Water Interface

The influence of Li⁺, Na⁺ and Cs⁺ cations on the surface excess and structure of dodecyl sulfate (DS⁻) anions at the air–water interface was investigated with the vibrational sum-frequency generation (SFG) and surface tensiometry. Particularly, we have addressed the change in amplitude and frequency of the symmetric S-O stretching vibrations as a function of electrolyte and DS⁻ concentration in the presence of Li⁺, Na⁺ and Cs⁺ cations. For the Li⁺ and Na⁺ ions, we show that the resonance frequency is shifted noticeably from 1055 cm⁻¹ to 1063 cm⁻¹ as a function of the surfactants’ surfaces excess, which we attribute to the vibrational Stark effect within the static electric field at the air–water interface. For Cs⁺ ions the resonance frequency is independent of the surfactant concentration with the S-O stretching band centered at 1063 cm⁻¹. This frequency is identical to the frequency at the maximum surface excess when Li⁺ and Na⁺ ions are present and points to the ion pair formation between the sulfate headgroup and Cs⁺ counterions, which reduces the local electric field. In addition, SFG experiments of the O-H stretching bands of interfacial H₂O molecules are used in order to calculate the apparent double layer potential and the degree of dissociation between the surfactant head group and the investigated cations. The latter was found to be 12.0%, 10.4% and 7.7% for lithium dodecyl sulfate (LiDS), sodium dodecyl sulfate (SDS) and cesium dodecyl sulfate (CsDS) surfactants, which is in agreement with Collins ‘rule of matching water affinities’.

Heterogenized Molecular Catalysts: Vibrational Sum-Frequency Spectroscopic, Electrochemical, and Theoretical Investigations

Rhenium and manganese bipyridyl tricarbonyl complexes have attracted intense interest for their promising applications in photocatalytic and electrocatalytic CO₂ reduction in both homogeneous and heterogenized systems. To date, there have been extensive studies on immobilizing Re catalysts on solid surfaces for higher catalytic efficiency, reduced catalyst loading, and convenient product separation. However, in order for the heterogenized molecular catalysts to achieve the combination of the best aspects of homogeneous and heterogeneous catalysts, it is essential to understand the fundamental physicochemical properties of such heterogeneous systems, such as surface-bound structures of Re/Mn catalysts, substrate-adsorbate interactions, and photoinduced or electric-field-induced effects on Re/Mn catalysts. For example, the surface may act to (un)block substrates, (un)trap charges, (de)stabilize particular intermediates (and thus affect scaling relations), and shift potentials in different directions, just as protein environments do. The close collaboration between the Lian, Batista, and Kubiak groups has resulted in an integrated approach to investigate how the semiconductor or metal surface affects the properties of the attached catalyst. Synthetic strategies to achieve stable and controlled attachment of Re/Mn molecular catalysts have been developed. Steady-state, time-resolved, and electrochemical vibrational sum-frequency generation (SFG) spectroscopic studies have provided insight into the effects of interfacial structures, ultrafast vibrational energy relaxation, and electric field on the Re/Mn catalysts, respectively. Various computational methods utilizing density functional theory (DFT) have been developed and applied to determine the molecular orientation by direct comparison to spectroscopy, unravel vibrational energy relaxation mechanisms, and quantify the interfacial electric field strength of the Re/Mn catalyst systems. This Account starts with a discussion of the recent progress in determining the surface-bound structures of Re catalysts on semiconductor and Au surfaces by a combined vibrational SFG and DFT study. The effects of crystal facet, length of anchoring ligands, and doping of the semiconductor on the bound structures of Re catalysts and of the substrate itself are discussed. This is followed by a summary of the progress in understanding the vibrational relaxation (VR) dynamics of Re catalysts covalently adsorbed on semiconductor and metal surfaces. The VR processes of Re catalysts on TiO₂ films and TiO₂ single crystals and a Re catalyst tethered on Au, particularly the role of electron-hole pair (EHP)-induced coupling on the VR of the Re catalyst bound on Au, are discussed. The Account also summarizes recent studies in quantifying the electric field strength experienced by the catalytically active site of the Re/Mn catalyst bound on a Au electrode based on a combined electrochemical SFG and DFT study of the Stark tuning of the CO stretching modes of these catalysts. Finally, future research directions on surface-immobilized molecular catalyst systems are discussed.

Molecular-Scale Structure of Electrode-Electrolyte Interfaces: The Case of Platinum in Aqueous Sulfuric Acid

Knowledge of the molecular composition and electronic structure of electrified solid-liquid interfaces is key to understanding elemental processes in heterogeneous reactions. Using X-ray absorption spectroscopy in the interface-sensitive electron yield mode (EY-XAS), first-principles electronic structure calculations, and multiscale simulations, we determined the chemical composition of the interfacial region of a polycrystalline platinum electrode in contact with aqueous sulfuric acid solution at potentials between the hydrogen and oxygen evolution reactions. We found that between 0.7 and 1.3 V vs Ag/AgCl the electrical double layer (EDL) region comprises adsorbed sulfate ions with hydrated hydronium ions in the next layer. No evidence was found for bisulfate or Pt-O/Pt-OH species, which have very distinctive spectral signatures. In addition to resolving the long-standing issue of the EDL structure, our work establishes interface- and element-sensitive EY-XAS as a powerful spectroscopic tool for studying condensed phase, buried solid-liquid interfaces relevant to various electrochemical processes and devices.

Preparation of complex model electrocatalysts in ultra-high vacuum and transfer into the electrolyte for electrochemical IR spectroscopy and other techniques

Model studies at complex, yet well-defined electrodes can provide a better understanding of electrocatalytic reactions. New experimental devices are required to prepare such model electrocatalysts with atomic-level control. In this work, we discuss the design of a new setup, which enables the preparation of well-defined electrocatalysts in ultra-high vacuum (UHV) using the full portfolio of surface science techniques. The setup allows for direct transfer of samples from UHV and the immersion into the electrolyte without contact to air. As a special feature, the single crystal sample is transferred without any sample holder, which makes the system easily compatible with most electrochemical in situ methods, specifically with electrochemical infrared reflection absorption spectroscopy, but also with other characterization methods such as single-crystal cyclic voltammetry, differential electrochemical mass spectrometry, or electrochemical scanning tunneling microscopy. We demonstrate the preparation in UHV, the transfer in inert atmosphere, and the immersion into the electrolyte for a complex model catalyst that requires surface science methods for preparation. Specifically, we study Pt nanoparticles supported on well-ordered Co₃O₄(111) films which are grown on an Ir(100) single crystal. In comparison with reference experiments on Pt(111), the model catalyst shows a remarkably different adsorption and reaction behavior during CO electrooxidation in alkaline environments.

On-Chip in Situ Monitoring of Competitive Interfacial Anionic Chemisorption as a Descriptor for Oxygen Reduction Kinetics

The development of future sustainable energy technologies relies critically on our understanding of electrocatalytic reactions occurring at the electrode-electrolyte interfaces, and the identification of key reaction promoters and inhibitors. Here we present a systematic in situ nanoelectronic measurement of anionic surface adsorptions (sulfates, halides, and cyanides) on ultrathin platinum nanowires during active electrochemical processes, probing their competitive adsorption behavior with oxygenated species and correlating them to the electrokinetics of the oxygen reduction reaction (ORR). The competitive anionic adsorption features obtained from our studies provide fundamental insight into the surface poisoning of Pt-catalyzed ORR kinetics by various anionic species. Particularly, the unique nanoelectronic approach enables highly sensitive characterization of anionic adsorption and opens an efficient pathway to address the practical poisoning issue (at trace level contaminations) from a fundamental perspective. Through the identified nanoelectronic indicators, we further demonstrate that rationally designed competitive anionic adsorption may provide improved poisoning resistance, leading to performance (activity and lifetime) enhancement of energy conversion devices.

In operando observation system for electrochemical reaction by soft X-ray absorption spectroscopy with potential modulation method

In order to investigate local structures of electrolytes in electrochemical reactions under the same scan rate as a typical value 100 mV/s in cyclic voltammetry (CV), we have developed an in operando observation system for electrochemical reactions by soft X-ray absorption spectroscopy (XAS) with a potential modulation method. XAS spectra of electrolytes are measured by using a transmission-type liquid flow cell with built-in electrodes. The electrode potential is swept with a scan rate of 100 mV/s at a fixed photon energy, and soft X-ray absorption coefficients at different potentials are measured at the same time. By repeating the potential modulation at each fixed photon energy, it is possible to measure XAS of electrochemical reaction at the same scan rate as in CV. We have demonstrated successful measurement of the Fe L-edge XAS spectra of aqueous iron sulfate solutions and of the change in valence of Fe ions at different potentials in the Fe redox reaction. The mechanism of these Fe redox processes is discussed by correlating the XAS results with those at different scan rates.

Quantitative aspects of normalized differential reflectance spectroscopy: Pt(111) in aqueous electrolytes

A theoretical model is herein proposed to account for changes in the normalized differential reflectance, ΔR/R, of well-defined single crystal Pt(111) surfaces|aqueous electrolyte interfaces. It assumes that ΔR/R is proportional to the area of the electrode either bare or covered by neutral and/or nominally charged species and, for a specific type of site, is modulated by the applied potential, E. Correlations between the coverage of the various species and E were obtained from data reported in the literature or by coulometric analysis of linear voltammetric scans. Excellent agreement was found for the adsorption/desorption of hydrogen and that of bisulfate from acidic electrolytes both on bare, and cyanide-modified Pt(111). Also discussed are extensions of this technique in the transient mode involving the reduction of adsorbed nitric oxide, NO, on Pt(111).

Tailoring the catalytic activity of electrodes with monolayer amounts of foreign metals

During the past decade, electrocatalysis has attracted significant attention primarily due to the increased interest in the development of new generations of devices for electrochemical energy conversion. This has resulted in a progress in both fundamental understanding of the complex electrocatalytic systems and in the development of efficient synthetic schemes to tailor the surface precisely at the atomic level. One of the viable concepts in electrocatalysis is to optimise the activity through the direct engineering of the properties of the topmost layers of the surface, where the reactions take place, with monolayer and sub-monolayer amounts of metals. This forms (bi)metallic systems where the electronic structure of the active sites is optimised using the interplay between the nature and position of the atoms of solute metals at the surface. In this review, we focus on recent theoretical and experimental achievements in designing efficient (bi)metallic electrocatalysts with selective positioning of foreign atoms to form a variety of active catalytic sites at the electrode surface. We summarize recent results published in the literature and outline challenges for computational and experimental electrocatalysis to engineer active and selective catalysts using atomic layers.