The molecular structure of the electrical double layer determines the chemistry in all electrochemical processes. Using x-ray absorption spectroscopy (XAS), we probed the structure of water near gold electrodes and its bias dependence. Electron yield XAS detected at the gold electrode revealed that the interfacial water molecules have a different structure from those in the bulk. First principles calculations revealed that ~50% of the molecules lie flat on the surface with saturated hydrogen bonds and another substantial fraction with broken hydrogen bonds that do not contribute to the XAS spectrum because their core-excited states are delocalized by coupling with the gold substrate. At negative bias, the population of flat-lying molecules with broken hydrogen bonds increases, producing a spectrum similar to that of bulk water.

The X-ray absorption spectra (XAS) of lithium polysulfides (Li2Sx) of various chain lengths (x) dissolved in a model solvent are obtained from first-principles calculations. The spectra exhibit two main absorption features near the sulfur K-edge, which are unambiguously interpreted as a pre-edge near 2471 eV due to the terminal sulfur atoms at either end of the linear polysulfide dianions and a main-edge near 2473 eV due to the (x - 2) internal atoms in the chain, except in the case of Li2S2, which only has a low-energy feature. We find an almost linear dependence between the ratio of the peaks and chain length, although the linear dependence is modified by the delocalized, molecular nature of the core-excited states that can span up to six neighboring sulfur atoms. Thus, our results indicate that the ratio of the peak area, and not the peak intensities, should be used when attempting to differentiate the polysulfides from XAS.

The carbon–carbon coupling via electrochemical reduction of carbon dioxide represents the biggest challenge for using this route as platform for chemicals synthesis. Here we show that nanostructured iron (III) oxyhydroxide on nitrogen-doped carbon enables high Faraday efficiency (97.4%) and selectivity to acetic acid (61%) at very-low potential (−0.5 V vs silver/silver chloride). Using a combination of electron microscopy, operando X-ray spectroscopy techniques and density functional theory simulations, we correlate the activity to acetic acid at this potential to the formation of nitrogen-coordinated iron (II) sites as single atoms or polyatomic species at the interface between iron oxyhydroxide and the nitrogen-doped carbon. The evolution of hydrogen is correlated to the formation of metallic iron and observed as dominant reaction path over iron oxyhydroxide on oxygen-doped carbon in the overall range of negative potential investigated, whereas over iron oxyhydroxide on nitrogen-doped carbon it becomes important only at more negative potentials.

Electrochemically grown cobalt on graphene exhibits exceptional performance as a catalyst for the oxygen evolution reaction (OER) and provides the possibility of controlling the morphology and the chemical properties during deposition. However, the detailed atomic structure of this hybrid material is not well understood. To elucidate the Co/graphene electronic structure, we have developed a flow cell closed by a graphene membrane that provides electronic and chemical information on the active surfaces under atmospheric pressure and in the presence of liquids by means of X-ray photoelectron spectroscopy (XPS). We found that cobalt is anchored on graphene via carbonyl-like species, namely Co(CO)x , promoting the reduction of Co(3+) to Co(2+), which is believed to be the active site of the catalyst.

Photons and electrons are two common relaxation products upon X-ray absorption, enabling fluorescence yield and electron yield detections for X-ray absorption spectroscopy (XAS).The ions that are created during the electron yield process are relaxation products too, which are exploited in this study to produce ion yield for XA detection. The ionic currents measured in a liquid cell filled with water or iron(III) nitrate aqueous solutions exhibit characteristic O K-edge and Fe L-edge absorption profiles as a function of excitation energy. Application of two electrodes installed in the cell is crucial for obtaining the XA spectra of the liquids behind membranes. Using a single electrode can only probe the species adsorbed on the membrane surface. The ionic-current detection, termed as total ion yield (TIY) in this study, also produces an undistorted Fe L-edge XA spectrum, indicating its promising role as a novel detection method for XAS studies in liquid cells.Key words: total ion yield (TIY), ionic current, liquid flow-cell, total electron yield (TEY), X-ray absorption spectroscopy (XAS) TOC GraphicPage 2 of 17 ACS Paragon Plus EnvironmentThe Journal of Physical Chemistry Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 3 Studying the electronic structure of liquid water and aqueous solutions by soft X-rays has attracted much attention in recent years, 1-6 and continues to be a vital research field. Resonant excitation by X-rays is highly element-specific, which makes X-ray absorption spectroscopy (XAS) a widely used tool in many scientific disciplines. Due to the vacuum requirement for soft X-ray propagation, detection of XA spectra for liquid (volatile) samples in vacuum is very challenging. One of the most applied techniques to introduce liquid samples into a vacuum chamber is a liquid flow-cell with an ultra-thin membrane separating the liquid from the vacuum. 7,8 When equipped with multiple electrodes, such a liquid cell can act as a standard electrochemical cell. It is therefore of great interest to combine XAS and the liquid cell technique for in situ/operando investigations on liquid-based materials. [9][10][11][12] When a sample's thickness exceeds the penetration depth of soft X-rays, which is the case for the liquid cell adopted in this study, detection of XA spectra in the transmission mode is not applicable. Therefore, fluorescence yield (FY) or electron yield (EY) must be employed to acquire XA spectra. Due to the significant thickness of typical membranes, e.g. Si 3 N 4 and SiC membranes (~ 100 nm), the electrons created within liquid solutions cannot penetrate the membrane and escape into vacuum. The FY was thus considered the only feasible way to probe the liquid-phase species behind membranes. However, EY has been recently realized in liquid cell studies, thanks to the newly developed graphene membra...

The operation of all electrochemical energy-related systems, such as supercapacitors, batteries, fuel cells, etc. depends largely on the processes occurring at electrochemical interfaces at which charge separation and chemical reactions occur. Evolution of structure and composition at the interface between electrodes and electrolytes affects all the device′s functional parameters including power and long-term performance stability. The analytical techniques capable of exploring the interfaces are still very limited, and more often only ex situ studies are performed. This sometimes leads to a loss of important pieces of the puzzle, hindering the development of novel technologies, as in many cases intermediates and electrochemical reaction products cannot be “quenched” for post-process analyses. Techniques capable of operando probing of electrochemical interfaces by photons and neutrons have become an extensively growing field of research. This review aims at highlighting approaches and developing ideas on the adaptation of photoelectron, X-ray absorption, vibrational spectroscopy, nuclear magnetic resonance, and X-ray and neutron reflectometry in electrochemical studies

The structure and growth of water films on surfaces is reviewed, starting from single molecules, two-dimensional wetting layers, and liquid interfaces. This progression follows the increase in temperature and vapor pressure environmental conditions from a few degrees Kelvin in ultra-high vacuum, where Scanning Tunneling and Atomic Force Microscopies (STM and AFM) provide crystallographic information at the molecular level, to ambient conditions where surface sensitive spectroscopic techniques provide electronic structure information. We show how single molecules bind to metal and non-metal surfaces, their diffusion and aggregation. We examine how water molecules can be manipulated by the STM tip via excitation of vibrational and electronic modes, which trigger molecular diffusion and dissociation. We review also the adsorption and structure of water on non-metal substrates including mica, alkali halides, and others under ambient humid conditions. We finally review recent progress in the exploration of the molecular level structure of solid-liquid interfaces, which impact our fundamental understanding of corrosion and electrochemical processes. variety of names: atomic force microscopy (AFM) and its variants of contact and non-contact), lateral friction, and attractive electrostatic force microscopies, known as scanning polarization force microscopy (SPFM) and Kelvin Probe Force Microscopy (KPFM). The non-contact attractive AFM modes allow us to observe water films formed on solids with minimal disruption. We will first discuss adsorption configurations, diffusion, aggregation, and dissociation of water on metal surfaces induced thermally, vibrationally and electronically using STM. These topics were reviewed also by Hodgson and Haq in 2009 [3] from the theoretical point of view. We then move to formation of larger clusters and monolayers. The discussion of the "ice" layers is followed by liquid water films minerals (salts, oxides) in ambient conditions. In the last section, we also review recent studies of liquid water near a solid, electrified surface. Here the knowledge obtained from X-ray absorption spectroscopy (XAS) has proved useful for better understanding of atomistic pictures of electrochemical processes. Single molecules: diffusion, aggregation, dissociation Water on metal surfaces at low coverage-monomers and small clusters STM studies of adsorption of water on metal surfaces started in the early 2000's, with noble metals, including Au, Ag, Cu, and more reactive transition metals, such as Pt, Pd, Rh, Ru have been used as substrates. The first observation of individual water molecules was reported in 2001 by Lauhon and Ho [4] on Cu(001) using low-temperature STM. They pointed out several phenomena occurring during imaging of adsorbed water: 1) stable clusters form at relatively low coverage, 2) imaging of small clusters (less than 6 molecules) often leads to the tip inducing molecular motions, and 3) large clusters have internal structures that are often difficult to resolve. Morgenstern et al. [5-9] st...

Revealing the active nature of oxide-derived copper is of key importance to understand its remarkable catalytic performance during the cathodic CO 2 reduction reaction (CO 2 RR) to produce valuable hydrocarbons. Using advanced spectroscopy, electron microscopy, and electrochemically active surface area characterization techniques, the electronic structure and the changes in the morphology/roughness of thermally oxidized copper thin films were revealed during CO 2 RR. For this purpose, we developed an in situ cell for X-ray spectroscopy that could be operated accurately in the presence of gases or liquids to clarify the role of the initial thermal oxide phase and its active phase during the electrocatalytic reduction of CO 2 . It was found that the Cu(I) species formed during the thermal treatment are readily reduced to Cu 0 during the CO 2 RR, whereas Cu(II) species are hardly reduced. In addition, Cu(II) oxide electrode dissolution was found to yield a porous/void structure, where the lack of electrical connection between isolated islands prohibits the CO 2 RR. Therefore, the active/stable phase for CO 2 RR is metallic copper, independent of its initial phase, with a significant change in its morphology upon its reduction yielding the formation of a rougher surface with a higher number of underco-ordinated sites. Thus, the initial thermal oxidation of copper in air controls the reaction activity/selectivity because of the changes induced in the electrode surface morphology/roughness and the presence of more undercoordinated sites during the CO 2 RR.