Electrochemical X-ray Photoelectron Spectroscopy Research Articles

A laboratory-based electrochemical NAP-XPS system for operando electrocatalysis studies

During electrocatalytic reactions, the electrode, adsorbates, electrolyte ions, and solvent molecules at the electrode-electrolyte interface each play an important role. Electrochemical X-ray photoelectron spectroscopy (XPS) holds great promise for deciphering these roles, providing the oxidation state or bonding environment of every element present at the interface. However, combining the vacuum required for XPS with the wet environment needed for electrochemistry constitutes a technical challenge, requiring purpose-built instrumentation and spectro-electrochemical cell design. Here, we present a laboratory-based electrochemical XPS instrument optimized for operando studies on nano-structured electrocatalysts. The core of the system is a 3D printed spectro-electrochemical cell containing a membrane-electrode-graphene assembly. We show that this design enables us to probe the electrode surface, interfacial water, and interfacial ions under well-defined potential control. Meanwhile, the introduction of a mesoporous membrane into the assembly enables the transport of any molecular or ionic reactant towards the working electrode, opening the way to study any aqueous phase electrocatalytic system using laboratory-based electrochemical XPS. We exemplify this for the oxygen reduction reaction.

Origin of O2 Generation in Sulfide-Based All-Solid-State Batteries and its Impact on High Energy Density.

The cathode surface of sulfide-based all-solid-state batteries (SBs) is commonly coated with amorphous-LiNbO3 in order to stabilize charge-discharge reactions. However, high-voltage charging diminishes the advantages, which is caused by problems with the amorphous-LiNbO3 coating layer. This study has investigated the degradation of amorphous-LiNbO3 coating layer directly during the high-voltage charging of SBs. O2 generation via Li extraction from the amorphous-LiNbO3 coating layer is observed using electrochemical gas analysis and electrochemical X-ray photoelectron spectroscopy. This O2 leads to the formation of an oxidative solid electrolyte (SE) around the coating layer and degrades the battery performance. On the other hand, elemental substitution (i.e., amorphous-LiNbxP1- xO3) reduces O2 release, leading to stable high-voltage charge-discharge reactions of SBs. The results have emphasized that the suppression of O2 generation is a key factor in improving the energy density of SBs.

Monitoring the Behavior of Na Ions and Solid Electrolyte Interphase Formation at an Aluminum/Ionic Liquid Electrode/Electrolyte Interface via Operando Electrochemical X-ray Photoelectron Spectroscopy.

In electrochemical energy storage devices, the interface between the electrode and the electrolyte plays a crucial role. A solid electrolyte interphase (SEI) is formed on the electrode surface due to spontaneous decomposition of the electrolyte, which in turn controls the dynamics of ion migration during charge and discharge cycles. However, the dynamic nature of the SEI means that its chemical structure evolves over time and as a function of the applied bias; thus, a true operando study is extremely valuable. X-ray photoelectron spectroscopy (XPS) is a widely used technique to understand the surface electronic and chemical properties, but the use of ultrahigh vacuum in standard instruments is a major hurdle for their utilization in measuring wet electrochemical processes. Herein, we introduce a 3-electrode electrochemical cell to probe the behavior of Na ions and the formation of SEI at the interface of an ionic liquid (IL) electrolyte and an aluminum electrode under operando conditions. A system containing 0.5 molar NaTFSI dissolved in the IL [BMIM][TFSI] was investigated using an Al working electrode and Pt counter and reference electrodes. By optimizing the scan rate of both XPS and cyclic voltammetry (CV) techniques, we captured the formation and evolution of SEI chemistry using real-time spectra acquisition techniques. A CV scan rate of 2 mVs-1 was coupled with XPS snapshot spectra collected at 10 s per core level. The technique demonstrated here provides a platform for the chemical analysis of materials beyond batteries.

A Structural Model for Transient Pt Oxidation during Fuel Cell Start-up Using Electrochemical X-Ray Photoelectron Spectroscopy

Potential spikes during the start-up (SU) and shutdown (SD) of fuel cells are a major cause of platinum (Pt) electrocatalyst degradation, which limits the lifetime of the device. The electrochemical oxidation of Pt that occurs on the cathode during the potential spikes plays a key role in this degradation process. However, the composition of the oxide species formed, as well as their role in catalyst dissolution remains unclear. In this study, we employ a special arrangement of XPS (X-ray Photoelectron Spectroscopy), in which the Pt electrocatalyst is covered by graphene, making the in situ examination of the Pt oxidation/reduction under wet conditions possible. We use this assembly to investigate oxidation state changes of Pt within fuel cell relevant potential window. We show that above 1.1 VRHE, a mixed Ptδ+/Pt2+/Pt4+ surface oxide is formed, with an average oxidation state that gradually increases as the potential is increased. By comparing a model based on the XPS data to the oxidation charge measured during potential spikes, we show that our description of Pt oxidation is also valid during the transient conditions of fuel cell SU/SD. This is due to the rapid Pt oxidation kinetics during the pulses. As a result of the irreversibility of Pt oxidation, some remnants of oxidized Pt remain at typical fuel cell operating potentials after a pulse. Figure 1

Boron-Doped Diamond Electrodes: Fundamentals for Electrochemical Applications.

Boron-doped diamond (BDD) electrodes have emerged as next-generation electrode materials for various applications in electrochemistry such as electrochemical sensors, electrochemical organic synthesis, CO2 reduction, ozone water generation, electrochemiluminescence, etc. An optimal BDD electrode design is necessary to realize these applications. The electrochemical properties of BDD electrodes are determined by important parameters such as (1) surface termination, (2) surface orientation, and (3) boron doping level.In this Account, we discuss how these parameters contribute to the function of BDD electrodes. First, control of the surface termination (hydrogen/oxygen) is described. The electrochemical conditions such as the solution pH and the application potential were studied precisely. It was confirmed that an acidic solution and the application of negative potential accelerate hydrogenation, and the mechanism behind this is discussed. For oxygenation, we directly observed changes in surface functional groups by in situ attenuated total reflection infrared spectroscopy and electrochemical X-ray photoelectron spectroscopy measurements.Next, the difference in surface orientation was examined. We prepared homoepitaxial single-crystal diamond electrodes comprising (100) and (111) facets with similar boron concentrations and resistivities and evaluated their electrochemical properties. Experimental results and theoretical calculations revealed that (100)-oriented single-crystal BDD has a wider space charge layer than (111)-oriented BDD, resulting in a slower response. Furthermore, isolated single-crystal microparticles of BDD with exposed (100) and (111) crystal facets were grown, and we studied the electrochemical properties of the respective facets by combination with hopping-mode scanning electrochemical cell microscopy.We also systematically investigated how the boron concentration and sp2 species affect the electrochemical properties. The results showed that the appropriate composition (boron and sp2 species level) is dependent on the application. The transmission electron microscopy images and electron energy loss spectra of highly boron-doped BDD are shown, and the relationship between the composition and electrochemical properties is discussed. Finally, we investigated in detail the effect of the sp2 component on low-doped BDD. Surprisingly, although the sp2 component is usually expected to induce a narrowing of the potential window and an increase in the charging current, low-doped BDD showed the opposite trend depending on the degree of sp2 carbon.The results and discussion presented in this Account will hopefully promote a better understanding of the fundamentals of BDD electrodes and be useful for the optimal development of electrodes for future applications.

Investigations on the electrochemistry and reactivity of tantalum species in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide using X-ray photoelectron spectroscopy (in situ and ex-situ XPS)

Tantalum is a metal whose properties enable it to be used in a wide range of technological applications. In the present work, the underlying electrochemical processes of tantalum species in ionic liquids (ILs) are investigated using in situ X-ray photoelectron spectroscopy (XPS) in ultra-high vacuum. For this purpose electrochemical deposition of tantalum on a gold wire from 0.1 mol/L TaF5 in a 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([Py1,4]TFSI) electrolyte was studied by in situ electrochemical XPS. The cyclic voltammogram shows two reduction peaks and one oxidation peak, indicating a complex electrochemical behaviour. The electrochemical in situ XPS gives evidence of a reduction reaction, although no metallic tantalum could be detected on the gold wire. In order to investigate the reactivity of tantalum in more detail, thin IL films were prepared on tantalum by physical vapor deposition and these were examined by XPS. Decomposition reactions were observed with the formation of subhalides, oxyfluorides, or tantalum sulfides, showing the decomposition of the ionic liquid. The decomposition products indicate the splitting of the C-F bond of the TFSI anion.

Structural Model for Transient Pt Oxidation during Fuel Cell Start-up Using Electrochemical X-ray Photoelectron Spectroscopy.

Potential spikes during the start-up and shutdown of fuel cells are a major cause of platinum electrocatalyst degradation, which limits the lifetime of the device. The electrochemical oxidation of platinum (Pt) that occurs on the cathode during the potential spikes plays a key role in this degradation process. However, the composition of the oxide species formed as well as their role in catalyst dissolution remains unclear. In this study, we employ a special arrangement of XPS (X-ray photoelectron spectroscopy), in which the platinum electrocatalyst is covered by a graphene spectroscopy window, making the in situ examination of the oxidation/reduction reaction under wet conditions possible. We use this assembly to investigate the change in the oxidation states of Pt within the potential window relevant to fuel cell operation. We show that above 1.1 VRHE (potential vs reversible hydrogen electrode), a mixed Ptδ+/Pt2+/Pt4+ surface oxide is formed, with an average oxidation state that gradually increases as the potential is increased. By comparing a model based on the XPS data to the oxidation charge measured during potential spikes, we show that our description of Pt oxidation is also valid during the transient conditions of fuel cell start-up and shutdown. This is due to the rapid Pt oxidation kinetics during the pulses. As a result of the irreversibility of Pt oxidation, some remnants of oxidized Pt remain at typical fuel cell operating potentials after a pulse.

Novel covalent immobilization of cobalt (II) octa acyl chloride phthalocyanines onto phenylethylamine pre-grafted gold via spontaneous amidation

In this work, a gold electrode surface was pre-grafted with phenylethylamine (PEA) thin film for spontaneous covalent immobilization of cobalt (II) octa acyl chloride (CoOAClPc) via the nucleophilic addition reaction. Pre-grafting of PEA thin film was achieved by electrochemical reduction of 4-(2-aminoethyl) benzene diazonium (AEBD) salt. Cobalt (II) octa acyl chloride phthalocyanine (CoOAClPc) was successfully synthesized and covalently immobilized onto the pre-grafted gold electrode surface to form a thin monolayer of Au-PEA-CoOAClPc. Upon hydrolysis of the thin monolayer, the –COCl terminal functional groups were converted to -COOH to form Au-PEA-CoOCAPc. The presence of PEA and PEA-CoOCAPc was confirmed by electrochemical X-ray photoelectron spectroscopy surface characterization. The pH sensitivity of -COOH terminal groups was studied using a negatively charged [Fe(CN)6]3−/4− and a positively charged [Ru(NH3)6]2+/3+ redox probes. The Au-PEA-CoOCAPc was investigated for electrocatalytic and electroanalytical properties towards the detection of catecholamine neurotransmitters (dopamine, epinephrine, and norepinephrine). The electrocatalytic oxidation of the catecholamine neurotransmitters in PBS solution (pH 7.4) was studied using differential pulse voltammetry (DPV) in a linear range up to 50 μM. The limit of detection (LOD) was determined to be 64 nM, 0.22 μM and 0.17 μM for dopamine (DA), epinephrine (EP) and norepinephrine (NOR) respectively. The limit of quantification (LOQ) was determined to be 0.21 μM, 0.73 μM and 0.56 μM for DA, EP and NOR respectively. Stable and reproducible novel method of electrode fabrication to form Au-PEA-CoOCAPc was achieved. Excellent analytical properties (sensitivity, low detection limits and limit of quantification) were obtained by using the fabricated electrochemical sensors, Au-PEA-CoOCAPc, for the determination of catecholamine neurotransmitters and screening of ascorbic acid (AA).

Electrodeposition of Indium from an Ionic Liquid Investigated by In Situ Electrochemical XPS

The electrochemical behavior and electrodeposition of indium in an electrolyte composed of 0.1 mol/L InCl3 in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([Py1,4]TFSI) on a gold electrode were investigated. The cyclic voltammogram revealed several reduction and oxidation peaks, indicating a complex electrochemical behavior. In the cathodic regime, with the formation of an In-Au alloy, the reduction of In(III) to In(I) and of In(I) to In(0) takes place. In situ electrochemical X-ray photoelectron spectroscopy (XPS) was employed to investigate the reduction process by monitoring the oxidation states of the components during the cathodic polarization of 0.1 mol/L InCl3/[Py1,4]TFSI on a gold working electrode under ultra-high vacuum (UHV) conditions. The core electron binding energies of the IL components (C 1s, O 1s, F 1s, N 1s, and S 2p) shift almost linearly to more negative values as a function of the applied cell voltage. At −2.0 V versus Pt-quasi reference, In(I) was identified as the intermediate species during the reduction process. In the anodic regime, a strong increase in the pressure in the XPS chamber was recorded at a cell voltage of more than −0.5 V versus Pt quasi reference, which indicated, in addition to the oxidation reactions of In species, that the oxidation of Cl− occurs. Ex situ XPS and XRD results revealed the formation of metallic In and of an In-Au alloy.

The Oxidation of Platinum under Wet Conditions Observed by Electrochemical X-ray Photoelectron Spectroscopy.

During the electrochemical reduction of oxygen, platinum catalysts are often (partially) oxidized. While these platinum oxides are thought to play a crucial role in fuel cell degradation, their nature remains unclear. Here, we studied the electrochemical oxidation of Pt nanoparticles using in situ XPS. When the particles were sandwiched between a graphene sheet and a proton exchange membrane that is wetted from the back, a confined electrolyte layer was formed, allowing us to probe the electrocatalyst under wet conditions. We show that the surface oxide formed at the onset of Pt oxidation has a mixed Ptδ+/Pt2+/Pt4+ composition. The formation of this surface oxide is suppressed when a Br-containing membrane is chosen due to adsorption of Br on Pt. Time-resolved measurements show that oxidation is fast for nanoparticles: even bulk PtO2·nH2O growth occurs on the subminute time scale. The fast formation of Pt4+ species in both surface and bulk oxide form suggests that Pt4+-oxides are likely formed (or reduced) even in the transient processes that dominate Pt electrode degradation.

In Situ Spectroscopic Study on the Surface Hydroxylation of Diamond Electrodes.

Carbon-based materials are regarded as an environmentally benign alternative to the conventional metal electrode used in electrochemistry from the viewpoint of sustainable chemistry. Among various carbon electrode materials, boron-doped diamond (BDD) exhibits superior electrochemical properties. However, it is still uncertain how surface chemical species of BDD influence the electrochemical performance, because of the difficulty in characterizing the surface species. Here, we have developed in situ spectroscopic measurement systems on BDD electrodes, i.e., in situ attenuated total reflection infrared spectroscopy (ATR-IR) and electrochemical X-ray photoelectron spectroscopy (EC-XPS). ATR-IR studies at a controlled electrode potential confirmed selective surface hydroxylation. EC-XPS studies confirmed deprotonation of C-OH groups at the BDD/electrolyte interface. These findings should be important not only for better understanding of BDD's fundamentals but also for a variety of applications.

Zirconium Oxide-Based Compounds as Non-Platinum Cathodes for Polymer Electrolyte Fuel Cells

In order to clarify the active sites of zirconium oxide-based compounds as non-platinum cathodes for PEFCs, we investigated the active sites of the zirconium oxide-based compounds prepared from oxy-zirconium phthalocyanine using MWCNT as a support (ZrCxNyOz/MWCNT) and the variation of the surface chemical state under electrochemical reactions by using in-situ X-ray absorption spectroscopy and electrochemical X-ray photoelectron spectroscopy. The oxygen vacancies of the zirconium oxide phase in the ZrCxNyOz/MWCNT could work as active sites for the electrochemical oxidation and the oxygen reduction reaction providing oxygen species adsorption sites. In addition, the zirconium oxide-based compounds supported by MWCNT (20wt% oxide) prepared by arc-plasma deposition method showed high oxygen reduction current density (220 mA g-1 @ 0.8 V), indicating that the zirconium oxide-based compounds which contained no iron and nitrogen-doped carbon had active sites for the oxygen reduction reaction.

XPS-evidence for in-situ electrochemically-generated carbene formation

Stable N-heterocyclic carbenes (NHC) are a class of compounds that has attracted a huge amount of interest in the last decade. One way to prepare NHCs is through chemical or electrochemical reduction of 1,3-disubstituted imidazolium cations. We are presenting an in-situ electrochemical X-ray Photoelectron Spectroscopy (XPS) study where electrochemically reduced imidazolium cations lead to production of stable NHC. The electroactive imidazolium species is not only the reactant, but also part of the ionic liquid which serves as the electrolyte, the medium and the electroactive material. This allows us to directly probe the difference between the parent imidazolium ion and the NHC through the use of XPS. The interpretation of the results is supported by both observation of reversible redox peaks in the voltammogram and the density functional theory calculations.

Oxygen Reduction Reaction Mechanism of Connected Platinum-Iron Nanoparticle Catalysts Probed By EC-XPS

The widespread use of polymer-electrolyte fuel cells (PEFCs) requires more active, stable, and low cost electrocatalysts than platinum. Bimetallic electrocatalysts such as platinum-iron (PtFe) have been attracting a great deal of attention as an oxygen reduction reaction (ORR) catalyst since they have demonstrated both higher oxygen reduction activity and improved stability with much smaller amounts of platinum. Connected PtFe-nanoparticle catalyst with a beaded network by connected PtFe nanoparticles and a superlattice structure (a chemically-ordered face-centered tetragonal (fct) structure) exhibit specific ORR activity that is higher than those of the commercial Pt catalyst supported on carbon black (Pt/C) and even fct-PtFe nanoparticles (without any beaded networks) supported on carbon black. [1] Thus, the unique structures of connected PtFe-nanoparticle catalysts, such as connected beaded network and hollow structure, enhance an ORR activity. Electrochemical x-ray photoelectron spectroscopy (EC-XPS) applied for the reaction mechanism analyses of PEFC catalysts. Electrochemical reactions occur at catalyst electrode surface, and then, the electrode is transferred to an UHV chamber immediately. Finally the electrode surface involving reaction intermediates is analyzed by using XPS without exposing to the air. Since structure of electrochemical-double layers is maintained during the measurement process, we can identify the intermediate species, and evaluate adsorption strength, etc., namely, we can analyze reaction mechanism. In this study, we performed electrochemical x-ray photoelectron spectroscopy (EC-XPS) measurements in order to understand the factors that influence the high ORR activity of the connected PtFe-nanoparticle catalyst. The EC-XPS results for connected PtFe-nanoparticle catalysts under ORR conditions showed the adsorption of oxygen species on the catalyst surfaces, indicating that ORR intermediates were detected. We could observe ORR processes of connected PtFe-nanoparticle catalysts. In this presentation, we will also show the EC-XPS measuring data for other platinum catalysts and discuss the factors that influence the high ORR activity of the connected PtFe-nanoparticle catalyst in detail. [1] T. Tamaki, H. Kuroki, S. Ogura, T. Fuchigami, Y. Kitamoto, and T. Yamaguchi, “Connected nanoparticle catalysts possessing a porous, hollow capsule structure as carbon-free electrocatalysts for oxygen reduction in polymer electrolyte fuel cells”, Energy Environ. Sci., 2015, 8, 3545-3549.

In Situ X-ray Photoelectron Spectroscopy for Electrochemical Reactions at Solid/Liquid Interfaces

X-ray photoelectron spectroscopy (XPS) is the most powerful technique for the determination of surface compositions, oxidation states and electronic structures of materials of interest with very high surface sensitivity. Since surface structure and oxidation states of materials in vacuum can be significantly different from those under operation, in situ analysis is strongly desired. However, because XPS requires vacuum, it is difficult to conduct in situ surface analysis at solid/liquid interface during electrochemical processes.“Electrochemical photoelectron spectroscopy” has been developed1 and utilized for practical applications such as electrocatalysts for fuel cells.2 In this system, samples can be transferred from an electrochemical chamber in an ambient atmosphere to an analysis chamber kept in vacuum immediately after the electrochemical treatments without being exposed to air, so that any contamination of the surface from exposure to open air can be avoided. However, structure and oxidation state of the electrode surface may be changed during the transfer from the solution, where the electrode is under electrochemically controlled conditions, to vacuum, where potential control is not possible. Here, we constructed an in situ electrochemical XPS apparatus for the measurements at solid/liquid interface under electrochemical condition using hard X-rays and a micro-volume cell equipped with an ultrathin Si membrane.3 The conceptual design of the present in situ electrochemical XPS arrangement is illustrated in Figure 1. A micro-volume cell filled with a liquid is sealed by an ultra-thin Si membrane,4 which serves as a separator between vacuum and liquid, a window for X-rays and photoelectrons, and a working electrode for electrochemical reactions. The cell is placed in an analysis chamber for XPS and synchrotron radiation hard X-ray is irradiated to the vacuum side of the membrane. X-rays penetrate to the solution side through the membrane and photoelectrons emitted from Si exposed to the solution, i.e., at solid/liquid interface, can pass through the Si membrane, and can then be collected and analyzed by a hemispherical photoelectron analyzer. Its operation was demonstrated for potential-induced Si oxide growth in water.3 Effect of potential and time on the thickness of Si and Si oxide layers was quantitatively determined at sub-nanometer resolution.3 References(1) Wieckowski, A. Interfacial Electrochemistry: Experimental, Theory and Applications; Marcel Dekker: New York, 1999.(2) Wakisaka, M.; Udagawa, Y.; Suzuki, H.; Uchida, H.; Watanabe, M. Energy Environ. Sci. 2011, 4, 1662.(3) Masuda, T.; Yoshikawa, H.; Noguchi, H.; Kawasaki, T.; Kobata, M.; Kobayashi, K.; Uosaki, K. Appl. Phys. Lett. 2013, 103.(4) Kolmakov, A.; Dikin, D. A.; Cote, L. J.; Huang, J. X.; Abyaneh, M. K.; Amati, M.; Gregoratti, L.; Gunther, S.; Kiskinova, M. Nat. Nanotechnol. 2011, 6, 651.

In situ x-ray photoelectron spectroscopy for electrochemical reactions in ordinary solvents

In situ electrochemical X-ray photoelectron spectroscopy (XPS) apparatus, which allows XPS at solid/liquid interfaces under potential control, was constructed utilizing a microcell with an ultra-thin Si membrane, which separates vacuum and a solution. Hard X-rays from a synchrotron source penetrate into the Si membrane surface exposed to the solution. Electrons emitted at the Si/solution interface can pass through the membrane and be analyzed by an analyzer placed in vacuum. Its operation was demonstrated for potential-induced Si oxide growth in water. Effect of potential and time on the thickness of Si and Si oxide layers was quantitatively determined at sub-nanometer resolution.

Core Level Data of Ionic Liquids: Monitoring Charging by In Situ Electrochemical X-ray Photoelectron Spectroscopy

Ionic liquids (ILs) are reported to provide large electrochemical stabilitywindows(4‐6V)makingthemattractiveelectrolytesforelectrochemical processes and applications. 1 Most ILs are also ultra high vacuum (UHV) compatible. Therefore, these compounds allow for application of analytical tools to, for instance, study electrochemical processes in situ in the UHV. 2 From fundamental X-ray photoelectron spectroscopy (XPS) studies of ILs, sample charging is known to be a major challenge. 3,4 Recently, we showed that ILs prepared on an activated carbon (AC) support show significantly improved XPS data with respect to charging. This result suggested that such a preparation method may provide a straightforward opportunity for the determination of reliable binding energies (BE) of ILs in this research field. 5 The lack of charging observed on the AC support was referred to the high surface area and, therefore, high double layer capacitance of AC when compared to common supports such as metal foils. In the present letter we further investigate charging using our recently designed in situ electrochemical XPS cell. 6 This setup allows recording shifts of core level spectra of the IL and measuring the potential of the WE simultaneously. In particular, the OCP can be measured as a function of the irradiation time. In agreement with the fundamental concept of electrochemically shifted binding energies, 6 the XPS lines shift by −1.0 eV/V with the X-ray induced OCP shift. Furthermore, the in situ investigations confirm that the lack of charging previously observed on AC support can indeed be explained by the high double layer capacitance of the support compensating the X-ray induced OCP and BE shift observed on smooth substrates.