In this contribution, we will review the concepts and principles used to characterize and discuss the structure, stability, adsorption properties and catalytic reactivity of bimetallic surfaces in an atomic scale picture. Starting from early stages, we will emphasize recent experimental and theoretical findings that resulted in a rapidly improving atomic-scale understanding of adsorption and catalytic surface reactions on these surfaces. While examples are often taken from our own work, the resulting insights are of general validity.

Cobalt-free LiNi0.5Mn1.5O4 (LNMO) is a promising alternative to the commonly used cobalt-containing positive electrode active materials in lithium-ion batteries (LIBs), owing to its high redox potential, relatively low cost, and low environmental impact. The high cell voltage, however, comes along with several challenges that need to be overcome before the material can be successfully used in commercial cells. Herein, these challenges are addressed by introducing three additives into the liquid organic carbonate-based electrolyte, namely tris (trimethylsilyl)-phosphite (TTSPi), lithium bis(oxalato) borate (LiBOB), and ethyl-(2,2,2-trifluoroethyl) carbonate (TFEC). The optimized electrolyte composition enables superior performance of Li & Vert;LNMO and graphite & Vert;LNMO cells because of the stabilized interphases at both the negative and the positive electrode and, thus, suppressed electrolyte decomposition. This is demonstrated by the substantially reduced gassing upon cycling and shelf-storage. These results are anticipated to contribute to the successful commercialization of LNMO in more sustainable LIBs.

We report results of a systematic study of the borohydride oxidation reaction (BOR) in borohydride containing 0.5 M NaOH electrolyte over Pt/C, Au/C and AuPt/C thin-film catalyst electrodes, performed under enforced mass transport conditions. Employing rotating disk electrode (RDE) and thin-layer flow cell differential electrochemical mass spectrometry (DEMS) measurements, we identify kinetic limitations over a wide range of transport conditions. Together with the highly sensitive detection of evolved hydrogen as a function of potential, due to the use of a cold trap at the mass spectrometer inlet, this allows us to separate changes in the reaction selectivity, from complete to incomplete borohydride oxidation, from other kinetic limitations. Evaluation of the (apparent) number of electrons transferred per borohydride ion, both from the RDE measurements via the Koutecky-Levich formalism and from the DEMS measurements via the H2 formation current, further supports the identification of complete borohydride oxidation (8 electron transfer) and incomplete oxidation (< 8 electrons transfer) reaction conditions. Using data on isotope labeled BD4- oxidation that we had published earlier, we identify weak secondary kinetic isotope effects for all catalysts, which indicate that B-H bond breaking does not represent the rate limiting step.

Continuing our investigation of electronic metal−support interactions (EMSIs) in heterogeneous catalysis, we have investigated the influence of the position and the number of O-vacancies on their stabilization by the Ru nanorod, on the charge transfer from the support to the metal, and on CO adsorption on the Ru nanorod. Employing density functional theory based calculations and using a model system consisting of a ZrO2(111) support and a three-layer Ru nanorod, we find that O-vacancies are significantly stabilized only if they are in direct contact to the Ru nanorod, with the extent of stabilization depending on the distance between vacancy and the nearest Ru atom at the interface. Vacancy formation aside the Ru nanorod or in deeper layers of the support is not enhanced by the metal. The Ru induced stabilization of the O-vacancies is closely coupled with the charge transfer from the support to the metal upon vacancy formation, which is true also in the presence of neighbored O-vacancies. The CO adsorption energy can be substantially modified by four characteristic effects, including charge transfer from the support to the metal, coordination effects, a combination of COad induced deformation energies and changes in the interface energy and direct interactions between CO and partly reduced Zr surface ions directly neighbored to an O-vacancy, depending on the adsorption site and on the number and positions of the O-vacancies. Thus, it is not possible to completely describe the adsorption properties by using the d-band model, in particular not for adsorption on the interface sites. The general relevance of this findings for adsorption and catalytic reactions is discussed.

The technical application of bimetallic core-shell particles, which are highly attractive because of their high electrocatalytic activity, depends crucially on their long-term stability under operating conditions. In the present multi-method study, we explored the stability of structurally well-defined Ru-core Pt-shell model systems during the CO oxidation (COOR) and methanol oxidation (MOR) reactions. These electrodes consist of a single-crystalline Ru(0001) substrate covered by epitaxial Pt films of one to three atomic layers. The reaction-induced modifications in the surface morphology were identified by scanning tunneling microscopy (STM) measurements performed before and after the electrocatalytic measurement, which reveal a higher stability for electrodes with around three layers of Pt (up to 1.4 V vs. the reversible hydrogen electrode) than for those with fewer layers. Differential electrochemical mass spectrometry (DEMS) measurements carried out during the COOR allow separation of the COOR currents from surface redox processes, providing insight into the role of surface oxidation / reduction processes during the COOR. Operando surface X-ray diffraction (SXRD) measurements performed during electro-oxidation of methanol confirm the much higher stability of the electrodes with three Pt layers. The main conclusion of this work is that during the electro-oxidation of organic molecules, the stability of the electrodes is, in general, improved due to the reactive removal of OH/O species from the surface.

Using periodic density functional theory calculations we have investigated the stability, electronic properties and CO adsorption properties of bimetallic Pt Ag surfaces, including pseudomorphic Ag film covered Pt(111) surfaces and PtxAg1-x/Pt(111) monolayer surface alloys. The data provide detailed insights into the relative stabilities of different surface configurations, as indicated by their formation enthalpies and surface energies, and changes in their electronic properties, i.e., in the projected local densities of states and shifts in the d-band center. The adsorption properties of different Ptn ensembles were systematically tested using CO as probe molecule. In addition to electronic ligand and strain effects, we were particularly interested in the role of different adsorption sites and of the local COad coverage, given by the number of CO molecules per Pt surface atom in the Ptn ensemble. Different from PdAg surfaces, variations in the adsorption energy with adsorption site and with increasing local coverage are small up to one COad per Pt surface atom. Finally, formation of multicarbonyl species with more than one COad per Pt surface atom was tested for separated Pt1 monomers and can be excluded at finite temperatures. General trends and aspects are derived by comparison with comparable data for PdAg bimetallic surfaces.

We report differential electrochemical mass spectrometry (DEMS) measurements on blocking effects induced by adsorbed CO or surface oxide on the hydrogen oxidation reaction (HOR) on a polycrystalline Pt electrode, comparing also with earlier results obtained on a thin-film Pt/Vulcan supported catalyst electrode.

For decades, it has been recognized that alloying platinum (Pt) with a secondary metal can enhance the catalytic activity of the oxygen reduction reaction (ORR) compared to pristine Pt catalysts. However, the mechanisms underlying this phenomenon vary significantly from one alloy to another. Here, we report the results of a computational study on the origin of the experimentally observed enhanced ORR activity of AgxPt1-x/Pt(1 1 1) monolayer surface alloy with 7 %-50 % Ag contents. A phase-separation model was developed and able to generate 2D phase-separation distributions of Ag and Pt atoms in AgxPt1-x/Pt(1 1 1) surfaces in line with atomic resolution scanning tunneling microscopy. We employed DFT-calculated *OH adsorption energy as a descriptor to obtain the activity of those surfaces, which reveals the ORR activity dominated by the reaction on Pt(Pt6) heptamers and also gives evidence of long-range self-induced surface strain as the source of the enhanced activity of binary AgxPt1-x/Pt(1 1 1) surfaces, i.e., the slightly larger surface Ag atoms induce a compressive strain of Pt-Pt bonds of the Pt(Pt6) heptamers, which increases the activity of binary surfaces compared to the pristine Pt(1 1 1) surface. Moreover, the excellent simulated-experimental agreement for the polarization curves shows the high quality of this approach and its more general potential for an improved understanding of the catalytic properties of inhomogeneous binary surfaces as the basis for a rational design of binary catalysts.

As an example for bimetallic surfaces in general, we have systematically investigated the thermodynamic surface properties of bimetallic Ag/Pt(111) and Ag/Pd(111) surfaces, including pseudomorphic Ag film covered surfaces and M1Ag3/M(111) (M = Pt, Pd) monolayer surface alloys, by periodic density functional theory calculations. Employing larger, symmetric unit cells and slabs, we could determine the surface energy of the asymmetric surface region without interference with contributions from the bottom side of the slab used in these calculations. In the calculation of formation energies, we distinguish between bulk and slab formation energies. Interface energies are derived from appropriately structured bulk unit cells, and corrected for contributions arising from the compression of pseudomorphic film layers (compression energy). While the general trends for the Pt(111)- and Pd(111)-based systems are rather similar, we also find specific differences. Possible reasons for these trends and the specific discrepancies will be addressed. We propose that the procedures presented here are of general validity and can be applied also to other complex surfaces.

The anode/electrolyte interface behavior, and by extension, the overall cell performance of sodium-ion batteries is determined by a complex interaction of processes that occur at all components of the electrochemical cell across a wide range of size- and timescales. Single-scale studies may provide incomplete insights, as they cannot capture the full picture of this complex and intertwined behavior. Broad, multiscale studies are essential to elucidate these processes. Within this perspectives article, several analytical and theoretical techniques are introduced, and described how they can be combined to provide a more complete and comprehensive understanding of sodium-ion battery (SIB) performance throughout its lifetime, with a special focus on the interfaces of hard carbon anodes. These methods target various length- and time scales, ranging from micro to nano, from cell level to atomistic structures, and account for a broad spectrum of physical and (electro)chemical characteristics. Specifically, how mass spectrometric, microscopic, spectroscopic, electrochemical, thermodynamic, and physical methods can be employed to obtain the various types of information required to understand battery behavior will be explored. Ways are then discussed how these methods can be coupled together in order to elucidate the multiscale phenomena at the anode interface and develop a holistic understanding of their relationship to overall sodium-ion battery function. Here, several analytical methods across multiple time and length scales are discussed, covering a wide range of physical and (electro)chemical properties. To fully grasp the complexity of sodium-ion battery anodes, integrated studies on the same battery system, ranging from the cellular level to the atomic level, are required.image

We have systematically investigated mechanistic details of the CO methanation reaction over a supported Ru catalyst in an idealized H-2-rich reformate gas mixture with low CO:H-2 ratio, employing isotope labelling techniques. From a comprehensive set of transient FTIR spectroscopy measurements, following the buildup / disappearance of different adsorbed species upon exchange of isotope marked reactants during reaction or during desorption, we derive that under these conditions the reaction is dominated by an associative reaction pathway, involving first the formation of adsorbed formyl intermediates, followed by hydrogenation of these intermediates and C-O bond breaking. Formate formation can be excluded. Measurements of kinetic isotope effects (KIEs) revealed a weak secondary inverse KIE with values of r(CH4)/r(CD4) between 0.6 (175 degrees C) and 1.0 (230 degrees C), indicating that at lower temperatures C-H bond formation, most likely HCOad hydrogenation, is rate-determining, while at higher temperatures the reaction is increasingly controlled by C-O bond breaking.

Advances in lithium-ion battery (LIB) research have strived for high-energy, safe, and sustainable materials. Li-rich Li1.2Ni0.2Mn0.6O2 (LRNM) cathodes have shown great promise with high voltages and excellent specific capacities. Obstacles preventing the viability and commercialization of LRNM are the initial capacity loss of roughly 30% and rapid voltage decay upon cycling. Herein, lithium oxalate (Li2C2O4) is investigated as a pre-lithiation additive to utilize the first-cycle lithium re-intercalation losses of LRNM to compensate the irreversible lithium consumption in graphite and high-capacity a-Si on copper silicide nanowire (a-Si/CuSi NW) anodes. Specifically, the decomposition process of Li2C2O4 as well as the interaction between the electrode components inside the cell are comprehensively examined. This concept allows us to extend the cycle life of graphite||LRNM cells at 1 C from less than 500 cycles (142 mAh g(-1), 78%) to more than 900 cycles (143 mAh g(-1), 82%) before reaching the 80% capacity retention threshold. Finally, LRNM electrodes with a precisely balanced concentration of Li2C2O4 are prepared in order to compensate the high irreversible losses of a-Si/CuSi NW anodes, achieving a capacity retention of almost 50% with a remaining specific capacity of 82 mAh g(-1) after 400 cycles in high-energy a-Si/CuSi NW||LRNM lithium-ion cells.

Stimulated by the increasing interest in ion adsorption effects on electrocatalytic reactions and by recent more detailed reports on the potential dependent adlayer structures formed on Ru(0001) in pure HClO4 and H2SO4 electrolytes, we revisited the oxygen reduction reaction (ORR) on structurally well-defined Ru(0001) single crystal surfaces prepared under ultrahigh vacuum conditions. We demonstrate that the complex, potential-dependent activity both for the ORR and for H2O2 formation is closely related to potential-dependent changes in the composition and structure of the adlayer. Our results demonstrate the enormous effects adsorbed species can have on the ORR reaction characteristics, either by surface blocking, e.g., by (co-)adsorbed bisulfate species, or by participation in the reaction, e.g., by *H transfer from adsorbed H or OH to O2. The comparison with results obtained on polycrystalline Ru, which differ significantly from Ru(0001) data, furthermore underlines the importance of using structurally well-defined surfaces as a reference system for future theoretical studies.

As part of a comprehensive study on the reduction of COx on supported Ru catalysts we systematically investigated the dynamic interaction of CO2 with a Ru/γ-Al2O3 catalyst in a temporal analysis of products (TAP) reactor, focusing on the redox properties of the catalyst, specifically on the deposition of active oxygen (Oact) from CO2. This was investigated by exposing the pre-reduced catalyst to CO2 pulses and subsequent titration of the deposited Oact by CO pulses. CO2 is much less active for Oact deposition than O2. Reductive co-reactants reduce (CO) or increase (H2) the activity of CO2 for Oact deposition. 18O-marked CO2 pulses show facile oxygen exchange of CO2 with the catalyst, despite the inertness of the support. Based on excess effluent CO2 we conclude that surface carbonates can both build up on the surface upon interaction with CO or CO2 and also decompose upon O2 or CO2 pulsing, releasing additional CO2.

Reversible Mg deposition/stripping and O-2 reduction/evolution on a Pt film electrode in neat and O-2-saturated 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl) imide (BMP-TFSI) electrolytes, containing Mg(TFSI)(2) and/or Mg(BH4)(2) as Mg source as well as Mg(BH4)(2) and/or the crown ether 18-c-6 as additive, were investigated by online differential electrochemical mass spectrometry (DEMS) and by scanning electron microscopy/energy dispersed X-ray spectroscopy. Combined cyclic voltammetry and DEMS measurements reveal a complex network of partial reactions, including borohydride electro-oxidation by reaction with water or O-2, chemical bulk reaction of these components, as well as electro-oxidation of H-2, and electrolyte decomposition, in addition to the primary reactions Mg deposition/stripping and ORR/OER. They provide detailed insights into the potential dependent reactions occurring under these conditions, demonstrating that also the additive 18-c-6 undergoes decomposition upon reduction of Mg2+. Contributions from chemical bulk reactions are resolved by DEMS measurements in borohydride containing solution without a Pt electrode. Electrocatalytic borohydride oxidation, explored by similar measurements with a Pt electrode, can lead to H-2 or H+ formation. Under open circuit potential conditions, charge compensation by the ORR results in the formation of a mixed potential. Consequences of these findings for applications in Mg-air batteries are discussed.

AbstractIn der Heterogenen Katalyse ist die Aufklärung des Reaktionsmechanismus essentiell für die systematische Optimierung der Katalysatoren, aber aufgrund der unbekannten Natur der aktiven Zentren herausfordernd. Für einen molekular‐definierten, auf einer metallorganischen Gerüstverbindung (UiO‐66) geträgerten Kupfer‐Einzelatom‐Katalysators konnten wir ein detailliertes mechanistisches Bild der CO Oxidation erarbeiten. Basierend auf einer Kombination von in situ/operando Spektroskopien, kinetischen Messungen inklusive kinetischer Isotopeneffekte sowie Dichtefunktional‐Theorie basierten Rechnungen konnten wir den aktiven Platz, Reaktionsintermediate und Übergangszustände des dominierenden Reaktionszyklus sowie Veränderungen in den Cu Oxidations‐ und Spinzuständen während der Reaktion identifizieren. Die Reaktion verläuft über die reaktive Dissoziation von adsorbiertem O2, durch Reaktion von O2,ad mit koadsorbiertem COad, als ratenlimitierendem Schritt. Das verbleibende O Atom, das das Cu Zentrum mit einem benachbarten Zr4+ Ion verbindet, wird in einem zweiten aktivierten Schritt abreagiert.

Elucidating the reaction mechanism in heterogeneous catalysis is critically important for catalyst development, yet remains challenging because of the often unclear nature of the active sites. Using a molecularly defined copper single-atom catalyst supported by a UiO-66 metal-organic framework (Cu/UiO-66) allows a detailed mechanistic elucidation of the CO oxidation reaction. Based on a combination of in situ/operando spectroscopies, kinetic measurements including kinetic isotope effects, and density-functional-theory-based calculations, we identified the active site, reaction intermediates, and transition states of the dominant reaction cycle as well as the changes in oxidation/spin state during reaction. The reaction involves the continuous reactive dissociation of adsorbed O2 , by reaction of O2,ad with COad , leading to the formation of an O atom connecting the Cu center with a neighboring Zr4+ ion as the rate limiting step. This is removed in a second activated step.

The practical utilization of ether electrolytes has long been restricted due to the concern on its electrochemical oxidation stability. Recently, it has been demonstrated that ethers are compatible with a series of polyanionic cathodes for sodium batteries. However, the specific cathode-electrolyte interface is still poorly understood. In this work, via the use of highly complementary surface and interfacial characterization techniques, we identify that the use of an 1 M NaPF6-diglyme solution allows the formation of a unique polymer-inorganic cathode-electrolyte-interphase (CEI) on high-voltage Na3V2(PO4)(2)F-3 polyanionic cathodes, contributing to excellent cyclability (capacity retention of 96.2% after 300 cycles at 0.5C, 1C = 128 mAh g(-1)) and outstanding rate capability (124, 120 and 112 mAh g(-1), at 5C, 10C and 20C, respectively). The peculiar interfacial chemistry disclosed here may open up new opportunities for building high performance sodium batteries.

A seawater-electrolyte Mg/H2O battery, converting chemical energy of Mg into electricity and hydrogen via anodic/chemical oxidation of Mg and cathodic reduction of water, combines electricity generation and H2 production, thus is not only ideal power source for marine equipment, but also intriguing on-line/offshore H2 generator for fuel cells. Nonetheless, it substantially suffers from the slow kinetics of hydrogen evolution reaction at the cathode. Herein, we propose a proof-of-concept design of a photo-assisted Mg/H2O battery with simulated seawater (0.5 M NaCl, pH = 6.8) as the electrolyte, and a CuSCN/Cu2O photocathode is applied to promote the hydrogen evolution reaction kinetics and battery performance. Under illumination, the CuSCN/Cu2O reveals a photocurrent density of up to 4.32 mA cm-2 at 0 V vs reversible hydrogen electrode, over 1.7 times higher than that of Cu2O. A photo-assisted Mg/H2O battery is demonstrated with a peak power density of 1.18 mW cm-2 and a H2 production rate of 0.56 mL cm-2 min- 1 under illumination, which is 3.6 and 9.6 times higher than those obtained in the dark. This novel strategy of a photo-assisted Mg/H2O battery enables the simultaneous con-version of photo-and chemical energy into electric energy and hydrogen from seawater.