Towards an atomic-scale understanding of electrochemical interface

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Towards an atomic-scale understanding of electrochemical interface structure and dynamics Olaf M. Magnussen, and Axel Gross J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 15 Feb 2019 Downloaded from http://pubs.acs.org on February 15, 2019

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Journal of the American Chemical Society

Towards an atomic-scale understanding of electrochemical interface structure and dynamics Olaf M. Magnussen† and Axel Groß‡ †Institute of Experimental and Applied Physics, Kiel University, Olshausenstr. 40, 24098 Kiel/Germany ‡Institute of Theoretical Chemistry, Ulm University, Albert-Einstein-Allee 11, 89081 Ulm/Germany, and Helmholtz Institute Ulm, Helmholtzstr. 11, 89081 Ulm/Germany E-mail: Abstract For the knowledged-based development of electrochemical processes, a better fundamental understanding of the interfaces between electrodes and electrolytes is necessary. This requires insight into the interface structure and dynamics on the atomic-scale, including that of the liquid electrolyte in the near-surface region, i.e., in the inner and outer part of the electrochemical double layer. This perspective describes current studies of simple and well-defined electrochemical interfaces by first-principles electronic structure calculations and in situ structure-sensitive methods. It is shown that these experimental and theoretical studies are now approaching a level, where they can operate on the same footing, making direct comparison of the obtained results feasible. Using selected examples, progress in clarifying the structure and dynamics of the double layer, of adsorbed species on electrode surfaces, and of initial steps in electrochemical phase formation processes is discussed.

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Introduction The recent two decades have seen a renaissance of electrochemistry, a discipline that is at the heart of many key technologies for the 21th century. Electrochemical processes are central to sustainable energy conversion and storage, offer promising solutions to the reduction of pollutants and the environmental benign processing of chemicals, play an important role in material processing, e.g. in the microelectronics industry, and are important in a plethora of other fields, ranging from sensors to biochemistry and corrosion. Further developments in these areas require a better understanding of the corresponding electrode-electrolyte interfaces, specifically of their structure on the molecular scale and of the elementary steps of the interface reactions. This is a challenging task, even for simple electrochemical systems, as illustrated in Figure 1 by a cartoon-type picture. Already the structure of bulk electrolytes, which typically are multicomponent liquid phases, is complex and even more so that of their interfaces to other materials. Further complexity arises from the potential gradient across the interface, which varies with the electrode potential, and the associated strong electric fields. In addition, these condensed matter interfaces are only to a limited extent accessible to experimental surface analytic methods, especially under electrochemical reaction conditions. Although first models of the structure of the electric double layer (EDL) at electrode surfaces were formulated more than a century ago, 1–4 detailed data on the atomic-scale interface structure emerged only in the 1980ies and 1990ies, when researchers began applying techniques from ultrahigh vacuum (UHV) surface science to electrochemical systems. 5,6 Starting from in situ optical spectroscopy and ex situ approaches, that involve controlled emersion and subsequent analysis in UHV, a wide range of experimental methods have been developed since then (table 1). These include in particular methods which allow determination of the atomic composition and structure of the electrode in situ in electrochemical environment or even operando during electrochemical reactions. Recently, even in situ techniques that provide insight into the electronic structure have become available. Simultaneously, 2


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Figure 1: Aspects of the microscopic structure and dynamics of electrochemical interfaces. Schematically illustrated are the interface structure in (a) the absence and (b) presence of chemisorbed adlayers, (c) the surface transport and the interactions of adsorbed species, and (d) the early stages of electrochemical phase formation processes. major advances in simulation and first-principles theory have made an increasingly better description of electrochemical interfaces and electrochemical reactions possible. In this perspective, we will give an overview of current progress and remaining challenges in this area, which often is named electrochemical surface science. Combining an experimentalist’s and a theoretician’s point of view, we will specifically focus on steps towards the development of a true atomistic picture of these complex interfaces. The latter will be achieved once that experimental data on the interface structure and interface processes for a specific system can be directly compared, without any additional assumptions, with results from first-principles models. This state has been reached (at least for simple systems) in UHV surface science, but for electrochemical interfaces still represents a considerable challenges, both from the viewpoint of experiment and theory. We will focus on a few central aspects, shown schematically in Figure 1, which will be discussed for selected well-defined cases. First, we consider the structure of the interface, in particular the highly dynamic state of the electrolyte side. Key questions here are the

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Table 1: Methods used for atomic-scale studies of electrochemical interface structure. in situ / operando experimental methods AFM EXAFS GISAXS IRAS SERS SFVS STM SXRD XPS XRR

atomic force microscopy 7 extended X-ray absorption fine structure grazing incidence small angle X-ray scattering infrared reflection absorption spectroscopy 8,9 surface-enhanced Raman spectroscopy 10,11 sum-frequency vibrational spectroscopy 12 scanning tunneling microscopy 7,13 surface X-ray diffraction 14,15 X-ray photoelectron spectroscopy X-ray reflectivity 14 first-principles theoretical methods

AIMD CHE DFT

ab initio molecular dynamics computational hydrogen electrode density functional theory

structure and dynamics of interface water and counter ions in the outer part of the double layer. Furthermore, we will address how chemisorbed adsorbates are affected by the electrochemical environment. Such adsorbates are involved in most technologically relevant electrochemical processes and their surface transport and interactions with the substrate and other adsorbates follow in principle similar rules as those governing their counterparts on surfaces exposed to vacuum. However, it is becoming increasingly clear that electric fields and coadsorbed electrolyte species at the interface can lead to strong modifications of the surface dynamic behavior. Finally, we will also discuss elementary steps in electrochemical phase formation processes for the specific case of the initial stages of oxide formation. Obviously, the field of electrochemical interface science is broad and the selection of topics and examples given here is necessarily subjective. Nevertheless, we consider the discussed problems as key issues for the better understanding of electrochemical interfaces in the coming decades.

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Double layer structure and dynamics Electrochemistry is concerned with the structure and processes at the interface between an electron conductor – the electrode – and an ion conductor – the electrolyte . 16 At the electrified interface between the electrode and the electrolyte, a so-called electric double layer forms. A charged electrode will attract oppositely charged ions from the electrolyte resulting in a capacitor-like arrangement, as was first realized by Helmholtz. 1 This is illustrated in Figure 1a for an aqueous electrolyte where (solvated) anions adsorb on the positively charged electrode surface. In addition, there is also a region with a diffuse thermal distribution of ions present, as first described by Gouy 2 and Chapman. 3 A combination of the Helmholtz and Gouy-Chapman models was proposed by Stern, 4 leading to a sequence of linearly and exponentially decreasing potentials within the double layer region. Although this model has its limitations , 17 it was used for a long time as the standard model of the EDL, as there were no experimental and computational tools available to reliably resolve the EDL’s microscopic structure. In early first-principles studies addressing the structure of the interface between a metallic electrode and an aqueous electrolyte, typically ice-like structures of the water were assumed for the sake of numerical convenience. 18–20 However, work function measurements of water films on metal surfaces could only be reconciled with ab initio molecular dynamics (AIMD) simulations assuming that the water layers directly at the electrodes are already liquid-like. 21 Furthermore, density functional theory (DFT) calculations showed that there is a strong polarization of the water molecules close to the metal electrodes involving some charge transfer from the water towards the metal. 21,22 This means that a proper modelling of electrode/electrolytes interfaces not only necessitates to take the liquid nature of the electrolyte into account through averaging over molecular dynamics simulations, but also demands proper electronic structure calculations that reliably reproduce electronic polarization effects. This requires to perform AIMD simulations in which in every time step the geometric and electronic structure is determined by firstprinciples electronic structure calculations. Hence, only recently it has become possible, due 5



to the ever-increasing computer power, to realistically model the atomistic structure of the EDL by running the computationally demanding AIMD simulations for sufficiently large systems and sufficiently long run times. 22–28 An example of such an AIMD study with 144 water molecules on Pt(111) per 6 × 6 surface unit cell corresponding to six water layers 22 is shown in Figure 2. A snapshot of the system’s trajectory together with the time evolution of the work function and the electrostatic potential of Pt(111) with and without a water film is plotted. From the averaged work function of the electrode covered by an ion-free water film the potential of zero charge (pzc) can be derived. 17,29 A value of 4.96 V with respect to the Fermi energy of Pt is obtained which is in good agreement with the absolute pzc of 4.9 V measured using the CO adsorption technique. 17,30 Note that there is still a variation of ±0.23 eV of the work function along the trajectory (Figure 2b). This is a consequence of the finite size of the unit cell used in the simulation. AIMD simulations with only 36 water molecules per unit cell even yielded a much larger variation of ±0.61 eV. 24 It can furthermore be clearly seen that there is an oscillatory dependence of the potential on the distance from the interface close to the electrode (Figure 2c). In addition, also in the first layers of the Pt electrode the presence of the water film leads leads to a rather strong modification of the electrostatic potential. The continuum description, which is the basis for the Stern model described above, can not yield such an behavior, neither in the electrolyte nor in the electrode. An analysis of the electronic charge distribution along the AIMD simulations reveals that close to the electrode the electrostatic potential is not only governed by the layered structure of the water, but also influenced by a significant polarization of the water molecules. 21,22 Furthermore, there is a partial charge transfer from the first water layer towards the Pt electrode. This demonstrates that the proper description of the electric double layer indeed requires a quantum chemical treatment. It also shows that the electric double layer should be regarded as an internal electric field, determined by the charge distribution of the atomic structure at the interface. 31 On the other hand, the par-

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a) Snapshot of trj.pzc Unitcell

Pt slab

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Figure 2: AIMD simulation of a water layer on Pt(111) with 144 water molecules per 6 × 6 surface unit cell, corresponding to six water layers equilibrated at 298 K. (a) Snapshot of the simulation together with the cell dimensions; (b) time evolution of the work function along the trajectory; (c) comparison of the electrostatic potential V(z) of Pt(111) in vacuum (dashed line) with the averaged potential of the water-covered Pt(111) electrode (solid line). The difference is plotted in the lower panel.(reprinted from Ref. 22 with the permission of AIP Publishing).

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ticular atomistic structure of the electrode surface under equilibrium conditions, considering also adsorbates, is a consequence of the electrochemical environment. This environment is characterized by the corresponding electrochemical potentials of the species present in the electrolyte, as will be shown below. Figure 2c also indicates that the averaged electrostatic potential becomes rather flat beyond the first water layer. For H-covered Pt(111) 22,24 and OH-covered Pt(111), 25 similar results have been found as far as the atomic distributions of the hydrogen and oxygen atoms as a function of the distance from the electrode are concerned, i.e., from the second water layer on rather bulk-like structure have been obtained in the AIMD simulations. This suggests that apart from the diffuse layer the width of the electric double layer in aqueous electrolytes is lower than 1 nm. Experimentally, the precise molecular structure of the EDL has been an ongoing topic of in situ studies for more than 20 years. In particular, synchrotron-based X-ray surface diffraction (SXRD) and vibrational spectroscopy methods have been employed. 32,33 The latter include infrared reflection adsorption spectroscopy (IRAS), 8,9 surface enhanced Raman spectroscopy (SERS), 10,11 and sum-frequency vibrational spectroscopy (SFVS). 12 All these techniques can provide information on the local bonding and orientation of molecular species near the interface. Only SFVS is intrinsically surface-sensitive, IRAS and SERS require careful subtraction of the signal resulting from the bulk electrolyte or amplification of the contribution of surface species by plasmonic enhancement. Such enhancement can be achieved even on single crystal surfaces, if one deposits on the surface Au or Ag nanoparticles that are surrounded by a chemically inert shell. 10,11 Quantitative interpretation of all these spectroscopic methods is difficult and often controversial, however. Surface X-ray diffraction measures the scattered intensities at the reciprocal space positions of the crystal truncation rods (CTRs) of the substrate and, if present, of superstructure rods, resulting from two-dimensional ordered adlayers (e.g., of adsorbed species and surface reconstructions). In principle the full three-dimensional arrangement of the atoms in the interface region can be obtained from these data. 14,15 Superstructure rods and non-specular

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CTRs, i.e., those in which the scattering vector has an in-plane component, provide information on interface atoms with a defined lateral position relative to the surface lattice. The specular CTR, which corresponds to the intensity of the reflected X-ray beam as a function of incident angle, is determined by the total-electron-density distribution along the surfacenormal direction, averaged within the surface plane. It thus also contains contributions from species without defined in-plane positions, for example, water layers and hydrated ions. However, for low-Z species such as water the X-ray scattering cross section is low, making quantitative studies challenging. Furthermore, reliable fitting of structural models of the interface to the data often requires time-consuming measurement of a larger number of CTRs. Only in the last 10 years technical advances at synchrotron sources, especially the widespread use of photon-counting 2D detectors, made in situ SXRD studies,where more than two or three CTRs are recorded, more common. Early in situ SXRD measurements detected the presence of water layers on Ag(111) surfaces and observed potential-dependent changes, which could be assigned to an inversion in the water orientation upon changing the polarity of the electrode. 34 However, very high surface densities of the water adlayer were obtained, which have been debated controversially. Evidence for the reorientation of adsorbed water near the potential of zero charge was also found in IRAS and SFVS studies of noble metal surfaces. 35–37 While these results are in qualitative agreement with the simulations, they do not yet provide quantitative data that would enable an in-depth comparison with theory. Such data would be necessary to establish trends in the potential-dependent interaction of water with different electrode surfaces, rather than to rely on comparison with gas phase data, i.e., ice-like adlayers on uncharged surfaces. Experimental tests of the potential distribution in the double layer are another largely unresolved issue. First promising attempts to determine this distribution directly have been recently published. Here, in situ photoelectron spectroscopy 38,39 or SFG 40 were use to probe the Stark shifts in the interface region, which allow measuring the local electric field. These first studies could only show compatibility with the classic Gouy-Chapman-Stern theory and

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did not have sufficient resolution to investigate the deviations due to the molecular structure, that are predicted by the AIMD simulations. Making this possible would be an important milestone in the further improvement of these techniques. As demonstrated above, AIMD simulations can yield detailed insights into the atomistic and electronic structure of electrochemical interfaces. Still, because of the liquid nature of water, the AIMD simulations have to be run for at least 30-40 ps for a given interface structure in order to yield statistically significant results. 22,25 It also means that every AIMD run is still associated with a tremendous computational cost. This prohibits the routine quantum chemical determination of structures and processes at electrochemical interfaces with a sufficient number of explicit water molecules, for example for the evaluation of the energetics of possible reaction intermediates in electrocatalytic reactions at electrode surfaces or in the determination of reaction barriers and paths. A computationally attractive alternative is to take the presence of liquid electrolytes into account using implicit solvent models, 41 i.e., through a polarizable dielectricum. Whereas this approach has been used routinely in solvation quantum chemistry, 42 it has only recently become available for surface studies. 43,44 There is an increasing number of applications of this approach to study structures and processes at electrochemical electrode-electrolyte interfaces. 45–49 These studies reproduce, for example, the stabilization of adsorbates containing the hydrophilic OH group in an aqueous environment, thus explaining the different selectivity in the first dehydrogenation step of catalytic methanol oxidation on Pt(111) in the gas phase and in electrochemical environment. 46,47 Nevertheless, a thorough validation of the reliability of the implicit solvent approach for the description of electrochemical interfaces admittedly is still missing. This is due to the fact that quantum chemical benchmark calculations employing explicit solvent molecules are still rather expensive. The AIMD simulations discussed above 22,24,25 indicate that the water structure is rather dynamic, with significant local variations which can not be reflected in a continuum model. There are further unsolved issues associated with implicit solvent models such as the correct placement of charges in the

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electrolyte. 50 Hence it is not clear yet whether implicit solvent model can trustfully capture the mean chemical interaction of such a strongly fluctuating aqueous environment.

Chemisorbed adsorbates The chemisorption of electrolyte species on electrode surfaces is central to almost any electrochemical reaction and consequently has been studied in great detail. A particularly important case here is the adsorption of anions, which are ubiquitous in electrochemical systems, almost always chemically bound, and known to affect a wide range of electrochemical reactions. For example, anion effects have been found in many electrocatalytic reactions and strongly affect electrodeposition and dissolution processes, because of their role as complexing agents. Fundamental studies have revealed a rich potential-dependent surface phase behavior for anions on single crystal metal electrodes, leading to ordered adlayer phases of high packing density (see figure 1b). 51 This complex potential-dependent interface structure in turn can determine the metal surface structure and reactivity.

In the following, we focus on the

adsorption of halides, the most simple type of anions, on the unreconstructed surfaces of fcc metals. On the (100) surfaces of coinage and transition metals chloride, bromide, and iodide preferably adsorb in the fourfold hollow sites of the metal substrate, increase in coverage towards more positive potentials, and, at saturation coverage, form a dense adlayer, most commonly in form of a c(2×2) superstructure (Figure 3a). 51–59 The formation of the c(2×2) structure occurs in a second-order phase transition 53,54 and can be described by 2D lattice gas models, assuming suitable repulsive dipole-dipole interactions between the adsorbates. 60,61 This characteristic adsorption behavior is found, e.g., for chloride and bromide on Cu(100) and Ag(100). On a first glance, it strongly resembles the gas phase adsorption of the corresponding halogen species. However, recent in-depth SXRD studies and corresponding DFT calculations revealed subtle but distinct differences to the latter, caused by the adjacent

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electrolyte. 55–59,62 For a c(2×2) adlayer of Cl on Cu(100), comparison of detailed SXRD data, obtained under UHV conditions 63 and in electrochemical environment, 55–58 indicates characteristic differences. First, in the presence of the electrolyte the Cl-Cu bond length is noticable increased and the spacing between the two topmost Cu layers is contracted as compared to UHV. DFT calculations could only reproduce this effect by including a layer of water molecules on top of the Cl adlayer. Thus, these bond relaxations can be attributed to solvation of the halide, resulting in partial charge screening. A second difference in comparison to UHV is a characteristic inversion of the oscillatory intensity distribution along the diffraction rods of the c(2×2) superlattice (Figure 3b), which indicates that the buckling of the Cu atoms in the second layer is reversed. On the basis of DFT calculations, also this effect could be assigned to the influence of water and cations in the outer Helmholtz plane of the EDL. Using a similar approach, also the influence of cations that interact more strongly with the adlayer (i.e., are “specifically adsorbing”) has been assessed. On metal surfaces that are fully covered by a c(2×2) halide adlayer a partial coverage of coadsorbed (hydrated) K+ and Cs+ cations was found by SXRD. 58,59,62,64 These coadsorbed cations were proposed to be located in fourfold coordinated sites with respect to the anions underneath, bound to those via bridging hydration water, and gradually desorbed towards more positive potentials (see Figure 3e). Furthermore, for Ag(100) in Cs+ and Br− containing electrolyte, detailed potential-dependent studies revealed cooperative adsorption, where the anion and cation mutually promoted their adsorption in an intermediate coverage range. This manifested in an enhanced Br adsorption as compared to electrolytes with non-specifically adsorbing Li+ cations and a corresponding maximum in Cs+ coverage near the potential of the 2D phase transition between disordered and c(2×2) adlayer (Figure 3c). 59 The structural data indicated that at very negative potentials the (hydrated) cations adsorb on the metal surface, probably next to anions, but with increasing potential are gradually displaced vertically to positions on top of the Br adlayer. More recently, similar adsorption of hydrated cations was

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Figure 3: (a) Structural arrangement in the c(2×2) halide adlayer on (100)-oriented fcc metal surfaces. (b) X-ray intensity distribution along the (01) rod of c(2×2)-Cl covered Cu(100) in 10 mM HCl and UHV (reprinted from Ref. 57 with the permission of the APS). (c) Influence of alkali cations on the Br adlayer coverage on Ag(100) (reprinted from Ref. 59 with the permission of the APS). (d) Temporal changes of the specular crystal truncation rod of c(2×2)-Br covered Ag(100) in Cs-containing solution during a potential step from -0.1 to -0.6 V and (e) corresponding adlayer structure (reprinted from Ref. 62 with the permission of the ACS).

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also reported in halide-free KOH solution. 65 As illustrated by these studies, also the outer parts of the EDL are nowadays accessible to in situ structure-sensitive techniques, allowing to probe the subtle interplay of the molecular interactions in this part of the interface region. These in turn can noticeably affect the electrochemical reactivity, as shown explicitly for electrocatalytic reactions in electrolytes containing alkali metal ions. 66 A problem in comparing the electrochemical adlayer structure with that under UHV conditions is to chose the correct reference state. For c(2×2)-Cl on Cu(100), a rough estimation based on work functions and the potential of zero charge suggests that in the electrochemical case the metal surface is substantially more negatively charged than in UHV. 57 On the experimental side, better understanding should be obtainable from in situ techniques that provide insights into the electronic structure of the metal and adsorbate. Here, resonant X-ray scattering methods have recently emerged as an interesting new tool, as they allow probing the charge distribution at the electrochemical interface. Data obtained for halide adlayers on Cu(100) and Au(100) suggest a significant potential drop, extending up to the second layer of metal atoms. 67 The latter qualitatively resembles the potential dependence in the AIMD simulation of the Pt(111) / water interface, shown in Figure 2c. Although these first results are promising, their quantitative interpretation is difficult and will require detailed first-principles simulations. 68 From the perspective of theory, the equilibrium adlayer structure of electrochemical interfaces can be conveniently derived using the approach of the Computational Hydrogen Electrode (CHE). 69,70 The CHE is based on the combination of statistical mechanics with an atomistic description of interfaces and provides a numerically very attractive way of deriving how the reference state in the adsorption of ions on electrochemical interfaces depends on electrochemical control parameters. It does so by taking advantage of the redox potential of solvated ionic species with their corresponding gas-phase species. This method now is the standard approach of estimating the equilibrium structure of electrochemical electrode/electrolyte interfaces as a function of electrode potential and the concentration of the

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involved species in the electrolyte. It is based on the notion that in thermal equilibrium the most stable adsorbate structure is associated with the lowest Gibbs free energy of adsorption ∆γ, ∆γ =

X 1 Gsurf,ads − Gsurf,0 − ni µ ˜i (T, p, U ) , AS i

(1)

where Gsurf,ads and Gsurf,0 are the Gibbs free energies of the adsorbate-covered and the clean surface, µ ˜i is the electrochemical potential of the ion species i that adsorbs with ni ions per surface area AS . Using the redox couple 12 A2 + e− * ) A− , 71 where A− is a halide such as Cl− , Br− , and I− , their electrochemical potential can be expressed as 1 µ ˜ A− − µ ˜e− = µA2 + e(USHE − U 0 ) + kB T ln(aA− ), 2

(2)

where U 0 is the reduction potential of the corresponding halide and a is the thermodynamic activity of the anion A− . Up to here everything is exact. However, as we saw in the preceding section, the firstprinciples determination of Gibbs free energies of interface structures is still prohibitively expensive on a routine basis. Therefore typically not free energies G are calculated, but rather total energies E, i.e. entropic effects upon adsorption are neglected. In addition, due to the good screening properties of metal electrodes, hydrogen adsorption energies have been found to depend only rather weakly on any applied electric field

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and are therefore

assumed to be independent of the potential. Furthermore, solvation effects are ignored. This is justified by DFT calculations that show the adsorption energies of hydrogen and CO on a metal electrode to be hardly influenced by the presence of water layers, 18 which reflects the fact that water is relatively weakly interacting with metal electrodes. 19,21 On the other hand, experimental results, such as the different bond length of adsorbed Cl to the Cu(100) surface in UHV and in electrolyte solution (see above), indicate significant electrolyte effects. Hence, it is still an open question how much the presence of the electrochemical environment influences adsorption properties. Despite of the uncertainties, these simplifications have

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nonetheless been employed in many of the current theoretical studies based on concept of the CHE because of their computational efficiency. And still, many meaningful results have thus been obtained, as will be illustrated in the following example.

Figure 4: Pourbaix diagram, derived from density functional theory calculations employing the computational hydrogen electrode approach, showing the stable phases for co adsorbed chlorine and hydrogen on Pt(111) as a function of the electrode potential and the pH value (reprinted from Ref. 26 with permission from Elsevier). Based on the CHE approach, phase diagrams of the thermodynamical stable adsorbate phases as a function of the electrochemical potential of the involved species can be derived. As the electrochemical potentials depend on the ion concentration and the electrode potential, these phase diagram can also be converted into diagrams showing the thermodynamical stable phases as a function of electrode potential and pH value, so-called surface Pourbaix diagrams. In one of the first applications, surface Pourbaix diagrams relevant for the oxygen reduction activity of Ag, Pt, and Ni(111) surfaces were evaluated. 72 Figure 4 represents a more recent example of a Pourbaix diagram derived from DFT calculations, showing the stable phases of coadsorbed chlorine and hydrogen on Pt(111) for a fixed chloride activity of aCl− = 0.1 in aqueous electrolyte that would correspond to an 0.1 molar concentration for 16


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an ideal solution. 26,73 The boundary between clean metal electrode and Cl-covered phases exhibits a horizontal slope indicating that protons are not involved in the transition whereas a boundary slope of -59 meV per pH reflects the involvement of hydrogen in the formation of these adlayer phases. Interestingly, there is only a small pocket around pH 4 in which a coadsorbate phase of hydrogen and chlorine is stable, otherwise only purely hydrogen-covered or chlorine-covered surfaces are stable. This is at first sight surprising as one would expect that protons as cations and chlorides as anions would exhibit an attractive interaction on the surface. However, except for fluoride, adsorbed halides exhibit a strong polarization upon adsorption on metal surfaces. This leads to a dipole moment that is opposite to what one would expect for the adsorption of an anion, 74 resulting in a repulsive electrostatic interaction with adsorbed hydrogen. A closer analysis of the stability of the adsorbate phases reveals that without the presence of chloride in the aqueous electrolyte the hydrogen adlayer phases would be stable up to higher electrode potentials. 73 Thus, there is a competitive adsorption of hydrogen and chlorine, i.e., upon increasing the electrode potential adsorbed hydrogen will be replaced by chlorine. This result is in nice agreement with corresponding experimental observations. 75 It shows that indeed one obtains reasonable results for adsorbate phases of atoms and small molecules at metal electrodes by employing the concept of the computational hydrogen electrode, even if the electrochemical environment is only taken into account with respect to the reference energy of the species in solution. Still, in spite of the success of this computationally very convenient approximation, further method development is required. In particular, it has currently not been systematically addressed whether the approximation of neglecting the electrochemical environment in the adsorption on metal electrodes is really justified. Furthermore, semiconductor electrodes even pose a more severe problem 76 as they do not screen electric fields as effectively as metals. It should also be admitted that up to now mostly specifically adsorbed atoms and molecules have been considered in first-principles studies of electrochemical interfaces. How non-specifically adsorbed species, that are char-

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acterized by an intact solvation shell, influence interface properties has hardly been studied yet, although they are a very common phenomenon at electrochemical interfaces. In fact, as the examples given in the section on the double layer structure demonstrate, increased theoretical and numerical efforts are nowadays undertaken for a more realistic consideration of the electrochemical environment.

Adsorbate dynamics on electrode surfaces As for surfaces under UHV conditions, many processes at electrochemical interfaces are governed by the surface dynamics of adsorbed species. This involves diffusion of adsorbates on atomically smooth terraces as well as across and along steps, adsorbate-adsorbate interactions, leading e.g. to bond formation and nucleation of adlayer phases, and interactions of adsorbates with surface defects, such as steps, substrate adatoms, and vacancies (figure 1c). Detailed understanding of these elementary reaction steps, i.e., their mechanisms and associated activation energies, is still in its infancy. In an electrochemical environment, all these steps occur in the presence of the EDL, which can lead to pronounced modifications as compared to solid-vacuum interfaces. Clear evidence of the latter has been found in studies of well defined model systems, some of which are discussed here. For experimental studies of these localized events, direct observations by high resolution in situ microscopy methods, such as scanning tunneling microscopy (STM) or atomic force microscopy (AFM), 7,13 are most suitable.

For the case of surface diffusion, already

earlier studies of metal electrode self-diffusion, based on morphological equilibrium fluctuations, decay of surface structures, or the nucleation and growth behavior in homoepitaxial electrodeposition, found a strong influence on the potential and the anion species in the electrolyte. 77–80 Similar results could be obtained in a series of studies by in situ high-speed scanning tunneling microscopy (video-STM), in which high-resolution video sequences of the electrode surface were recorded at rates of 10 to 20 images per second. 81–85 Here the surface

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dynamics of individual adsorbates could be directly observed on the atomic scale (Figure 5). These measurements employed (100)-oriented electrode surfaces of Cu and Ag that were covered by c(2×2) adlayers of Cl and Br, in which low coverages of sulfur, methyl thiolate, or lead adsorbates were embedded. The adsorbates moved on the surface by thermally activated jumps between neighboring sites of the c(2×2) lattice, with jump rates between 1 and 103 s−1 (Figure 5a). Quantitative analysis of the tracer diffusion revealed in all cases a strong exponential potential dependence, indicating a linear change of the diffusion barrier with potential (Figure 5b). This behavior can be rationalized by the interaction of the partially charged adsorbate with the double layer’s electric field. In a first order approximation, the transient change in the adsorbate’s dipole moment during the jump leads to an electrostatic energy contribution to the diffusion barrier that (for a constant double layer capacity) is proportional to the potential. 78,81 Interestingly, the change of the diffusion barrier with potential, which is a direct measure of the involved dipole moment change, is the same for the anionic Sad , the cationic Pbad , and the molecular CH3 Sad species on Cl-covered Cu(100), despite the rather different chemical nature of these adsorbates. 83 This suggested that the potential dependence is governed by a factor that is common to all three systems, namely the coadsorbed anions. Clear evidence for this was found in a recent study of Sad on c(2×2)-Br covered Cu(100), where an inverted potential dependence as compared to the Cl-covered Cu surface was found. 84 Complementary DFT calculations indicated that this surprising behavior has to be attributed to a change in the diffusion mechanism, induced by the halide adlayer (Figure 5c). For diffusion on the metal surface, always a negative dipole moment change corresponding to an increase in the diffusion barrier with potential is found, independent of the halide species. This is illustrated in the left panel of Figure 5c for a diffusion pathway, where the Sad and neighboring halide coadsorbates exchange their positions on the surface via a rotational motion. However, in the case of coadsorbed Br, diffusion on top of the surface does not provide the lowest energy barriers. Instead, another mechanism is found to be preferable, which involves transient exchange

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Figure 5: (a) Images from an STM video sequence of Cu(100) in 0.01 M HCl at 0.32 VSCE , showing the hopping diffusion of Sad on the (2×2) covered surface. (b) Potential-dependent tracer diffusion rates for Sad on Cl and Br covered Cu(100) electrode surface. (c) DFT calculations for two different Sad diffusion pathways, diffusion by rotation of Sad and halide coadsorbates (left panel) and by Sad exchange with a Cu surface atom (right panel). Shown are selected position along the first half of the diffusion pathway, the adsorption energy along the path, and the corresponding dipole moment changes (results adopted from Ref. 84 with permission by Wiley-VCH).

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of the Sad with a Cu surface atom (Figure 5c, right panel). Along a part of the calculated pathway for this diffusion mechanism indeed a small positive dipole moment change occurred, as required for the observed potential-dependence. However, at the position of the energy maximum the dipole moment change was negative and the effect thus could not be rigorously reproduced. The reason for this discrepancy is probably that the outer part of the EDL had to be neglected, because of the large in-plane unit cells required for DFT calculations of such diffusion pathways. This is particularly problematic for configurations with pronounced vertical modulation, for example the state in the exchange diffusion path where the Sad is fully embedded (Figure 5d, E8 ). Here, the fact that the solvation of the Brad bound on top of the transient Cu adatom is not taken into account may noticeably affect the calculated energy and dipole moment. Including such solvation effects in first-principles calculations will be an important task for the full understanding of these phenomena. As consideration of explicit water layers is still too computationally expensive for the determination of barriers, a first step towards this goal would be inclusion of solvation via a homogeneous polarizable medium (see the section on double layer structure and dynamics ). The described studies of model systems that are covered by a saturation coverage c(2×2) adlayer benefit experimentally from reasonably low diffusion rates and, with regard to firstprinciples theory, from the well-ordered structure in the ”‘inner Helmholtz plane”’ of the EDL. Similar data for adsorbate systems where coadsorbed species are present only as lowcoverage disordered adlayers, as adlayers with more complex (e.g., incommensurate) structure, or as weakly adsorbed species (e.g., ”‘non-specifically”’ adsorbing anions) are missing so far. Such studies will be needed for a better understanding of anion effects and the role of in-plane order in electrode surface dynamics. A further problem is the limitation of these measurements to species that are strongly chemisorbed in the fourfold-hollow adsorption sites on (100) surfaces of fcc metals, which experience a particularly strongly modulated surface energy landscape. More weakly bound adsorbates or adsorbates on other surface orientations, especially (111) surfaces, have a too high surface mobility for direct Video-STM

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studies. For those cases, new experimental in situ techniques have to be developed, that can quantitatively determine several orders of magnitude higher diffusion rates. Possible candidates for this task are methods that create concentration gradients on the surface and measure their decay, e.g., via optical reflectance. Adsorbate-adsorbate interactions on electrode surfaces have been determined in the past by comparison of lattice gas models with experimental adsorption isotherms. 86 For example, this approach provided interaction parameters for Cl and Br on Ag(100), which were in agreement with the results of DFT calculations for various discrete coverages. 87,88 Here, the isotherms for Br could be fully explained by long-range electrostatic repulsion, whereas for Cl additional short-range attractive interactions were necessary. More direct experimental data were again obtained from atomic-scale in situ Video-STM observations of Sad , Pbad , and CH3 Sad on Cu(100) and Ag(100) surfaces, embedded in a c(2×2) lattice of coadsorbed halide. 82,83,89,90 By a quantitative analysis of local adsorbate configurations in the videos, the effective interactions between these adsorbates as well as the diffusion barriers in the neighborhood of another adsorbate could be determined. Interaction energies and diffusion barrier changes on the order of a few ten meV were obtained, extending typically over a rather short distance of ≤ 1 nm. This is similar to results found in UHV-STM studies of simple adsorbate systems. Furthermore, in some electrochemical systems evidence for the interaction of adsorbates with metal adatoms 83 and vacancies 85 was found. First-principles studies of these interactions currently do not exist. They would require even larger in-plane unit cells than employed for investigations of surface diffusion, leading to prohibitively large calculation costs. Taking the continuous progress in numerical simulation of such systems into account, also these effects should be addressable within the next years. Finally, we briefly discuss electrocatalytic processes, in which the molecular dynamics of adsorbates and adsorbate-adsorbate interactions arguably are particularly important. As far as their first-principles treatment is concerned, many studies have addressed the hydrogen, oxygen or methanol oxidation and evolution reactions. 46–48,69–72,91–93 These studies mostly

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focused on the identification of reaction intermediates in these electrocatalytic processes using the concept of the computational hydrogen electrode. Many important aspects, such as the importance of scaling relations 69 for the electrocatalytic activity, have been elucidated in these studies. Barriers in electrocatalytic processes such as the oxygen reduction reaction, 94 CO2 electro-reduction, 95 or methanol electro-oxidation 46 have been derived from DFT calculations. However, the dependence of the barriers on the electrode potential was either neglected in such studies or taken into account only in a simplified approach, by invoking an electron transfer coefficient in a CHE approach. 46,95 Yet, along a reaction path, typically the atomic rearrangement changes in such a way that locally the dipole moment and thus also the work function changes. Therefore, in principle a constant potential method, 96 preferentially in combination with an explicit consideration of the aqueous electrolyte, is required to reliably determine reaction barriers as a function of the electrochemical control parameters. Although there are promising ideas, 97 a generally accepted constant potential method is currently not yet available. Hence, our atomistic understanding of electrocatalytic reaction mechanisms at electrode-electrolyte interfaces is still rather limited. On the experimental side, almost all knowledge on reaction mechanisms is still obtained from kinetic studies by electrochemical and spectroscopic methods. One of the biggest challenge here may be the development of approaches that provide direct insights into the molecular dynamics on the true timescales of the reaction. Similar as in ultrafast studies of homogeneous reaction dynamics, this may be feasible by pump-probe techniques, applied to photoelectrocatalytical model systems.

Elementary processes in electrochemical phase formation Electrochemical reactions are not restricted to processes on the electrode surface, but can involve substantial restructuring of the electrode material in the near-surface region or even 23



in the bulk (figure 1d). Examples for this include electrochemical intercalation processes, as in Li ion batteries, and phase formation reactions, such as electrooxidation. Such processes too have been addressed in recent years, as will be discussed in the following for the case of Pt(111) surface oxidation – an important and extensively studied model system. It has been known for a long time that Pt oxidation starts with a place exchange, where Pt atoms within the topmost atomic layer are extracted and oxygen atoms are inserted into this layer. The detailed atomic arrangement near the place exchange site and the extraction mechanism were clarified only recently, however. Several groups performed calculations of Pt(111) surface oxidation, employing DFT or reactive force fields. 98–100 These studies indicated a characteristic phase sequence where with increasing oxygen coverage first buckling of the Pt surface, then extraction of a Pt surface atom, and finally insertion of oxygen into the subsurface site underneath the extracted Pt atom was found (Figure 6a). The extraction of the Pt atom occurred at Oad coverages ≥ 0.5 ML and the extracted atom was stabilized by three chemisorbed oxygen atoms, located on the neighboring surface around the place exchange site. It was suggested that the driving force for this extraction is the strong induced dipole moment. 98 Furthermore, it was shown that the presence of water molecules significantly lowers the energies associated with extraction and oxygen insertion, i.e., that these processes are promoted by the electrochemical environment (Figure 6b). Further atomistic insight into this place exchange mechanism came from in situ SXRD studies of Pt(111) electrodes in the potential range of oxidation. Detailed crystal truncation rod measurements allowed to unambiguously determine the interface structure in this regime. They showed that at low coverages the extracted Pt atoms are vertically displaced by ≈ 2 ˚ A, but located at the same in-plane lattice positions as the Pt surface atoms, confirming the geometry found in the DFT studies. 102 Furthermore, time-resolved SXRD measurements during cyclic voltammograms and potential step experiments allowed to directly probe the place exchange kinetics. 101,103,104 Specifically, it was shown that place exchange commences at the anodic peak at 1.05 VRHE , which traditionally had been assigned to the formation

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Figure 6: Initial steps in the surface oxidation of Pt(111). (a) Minimum energy pathway for Pt place exchange with an oxygen atom, obtained from DFT calculations. The corresponding energy diagram in (b) illustrates the important role of hydration water around the extracted Pt atom (reprinted from Ref. 98 with permission of Springer). (c) In situ SXRD studies of Pt(111) in 0.1 M HClO4 (c) during a cyclic voltammogram, showing that the place exchange initially is highly reversible, and (d) place exchange kinetics, obtained from potential-step SXRD experiments (from Ref. 101 ).

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of an oxygen adsorbate layer (Figure 6c). However, this place exchange is fully reversible as long as the coverage of extracted Pt atoms remains sufficiently low (≤ 0.15 ML), as demonstrated by the complete recovery of the SXRD intensity at potentials ≤ 0.8 VRHE in the reverse potential sweep. This explains why despite place exchange repeated cycling over this range does not result in changes in the voltammogram. Only after exceeding the critical coverage of extracted Pt atoms, when significant overlap of the place exchange sites starts, irreversible structural changes occur. Similar experiments also provided quantitative data on the exchange kinetics at fixed potential, demonstrating that the coverage of extracted atoms increases logarithmic with time (Figure 6d). This result is in excellent agreement with the measured electrochemical charge transfer kinetics, which exhibits a similar growth law, 105 indicating that place exchange and the electrochemical formation of surface oxygen species are directly linked. At present, atomistic details of such electrochemical material conversion processes are available only for few systems. Even for Pt, only oxidation of the (111) surface has been studied at length by SXRD or first-principles theory, and only in non-specifically adsorbing electrolyte. More systematic experimental and theoretical studies should in the future allow to assess the influence of the surface structure and electrolyte composition as well as help clarifying how exactly the atomic-scale oxidation and oxide reduction mechanisms affect Pt dissolution and surface restructuring. Likewise, these appraoches are equally useful for obtaining atomistic insights into more complex phase formation processes at electrode surfaces, e.g., in battery materials.

Conclusions and outlook The examples discussed here illustrate that electrochemistry is approaching a point where a full description of electrochemical interfaces on the atomic scale gets into reach. On the one hand, application of modern experimental in situ methods allows to probe the interface

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structure and dynamics in great detail. On the other hand, first-principles theory increasingly succeeds in capturing the complexity of these interfaces, including the liquid electrolyte and the presence of potential gradients. In simple, well-defined systems, experimental data and computational results can be directly compared, indicating convergence of experiment and theory, a hallmark of a successful scientific discipline. Nevertheless, considerable progress in this field is still necessary. Partly, this can be made by consequential further development of existing methods, as outlined already in the previous chapters. However, there also exist grand challenges, which will require novel approaches. Some of these we will now discuss in this final chapter. From a theoretical point of view, there are two main challenges with respect to the more reliable atomistic modeling of electrochemical interfaces that need to be addressed in order to make even closer contact with the experiment: (i) the liquid nature of the aqueous electrolyte should to be appropriately taken into account, and (ii) the electrode potential needs to be controlled along a simulation. In principle we know the appropriate scheme to meet these challenges: to run AIMD simulations at constant potential with a sufficiently large number of explicit water molecules and adequately long simulation times. This corresponds to the Holy Grail of computational electrochemistry. However, AIMD simulations are still computationally rather expensive. Yet, the numerically attractive alternative to represent the liquid electrolyte within implicit solvent models 24,106 has its limitations, in particular when significant charge reorganization occurs in the liquid electrolyte. 41 As an alternative, a combination of explicit and implicit solvent 107 may be used for example using interaction potentials derived from machine-learning techniques. 108 Still, in order to validate approaches involving implicit solvent models, reliable references are required. Hence, there is certainly a need for explicite large-scale AIMD simulations of structures and processes at electrochemical interfaces. As far as constant potential calculations are concerned, Lozovoi et al. 96 presented in a seminal work a scheme to run constant potential calculations in which charge neutrality

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is achieved by introducing a compensating charge. However, this introduces convergence problems as the number of electrons may exhibit large fluctuations in the self-consistent field cycles. 96,109 Later, Bonnet et al. 109 introduced an alternative scheme in which electronic charge is exchanged through fictitious interactions with a potentiostat, so that the electrode potential oscillates around a desired value along a molecular dynamics run. Still, at every time step the charge is kept constant in the electronic structure calculations. Recently, this approach has been extended. 110,111 Still, a critical issue is the choice of a proper reference for the absolute value of the electrode potential. 109–112 There have been further promising approaches, 113 but it is certainly fair to say that there is no generally accepted approach to perform constant potential calculations yet. Hence, there is still a considerable need for the development and implementation of more sophisticated models to better describe electrochemical electrode/electrolyte interfaces on the molecular scale. The combination of theoretical and numerical challenges together with a fundamentally and technologically very important and interesting research field makes the theoretical study of electrochemical interfaces very demanding, but at the same time also very exciting and gratifying. Hence we anticipate that there will be significant progress in this field in the years to come. In terms of experiments, further development of in situ methods is needed, especially for electronic structure determination and structure-sensitive studies with high time resolution. Techniques for studying the electronic structure of the electrode surface and species at the interface in situ and operando are currently a hot topic and of great interest in electrocatalysis. 114 Most of those rely on the detection of photoelectrons, which requires vacuum conditions and can be done from both sides of the interface, i.e., by employing either ultrathin electrodes or thin wetting layers of the electrolyte. Problems of these approaches are ohmic potential drops due to the limited conductivity in the thin layer and difficulties in applying these methods to well-defined single crystal electrodes. In view of the significant ongoing work in this area, we expect these methods to become more routinely available, also

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for more fundamental studies of double layer structure. Alternatively, information on the electronic structure might also be obtained by photon-based methods, such as the resonant X-ray scattering technique described above. Regarding improved temporal resolution, the ultimate goal would be studies of nonequilibrium interface dynamics on femto- to nanosecond time scales. This would allow direct comparison with the dynamics found in AIMD simulations. Optical pump-probe techniques for such ultrafast studies are in principle well-established and were already applied to electrochemical systems. 9,115–117 Specifically, ultrafast vibrational spectroscopy methods have made great progress in recent years, allowing to study equilibrium and non-equilibrium dynamics on picosecond and sub-picosecond time scales (see Ref. 9 for a recent review). With further developments, these methods should allow to probe in detail the dynamics of the EDL, including that of interfacial water. Furthermore, similar pump-probe studies with probe pulses in the hard X-ray regime have become possible with the advent of X-ray free-electron lasers (XFELs), enabling ultrafast X-ray scattering and spectroscopy studies. However, these experiments focus up to now almost exclusively on the dynamics of molecular species in solution or bulk materials. Even studies of solid-vacuum interfaces are very scarce and only employed X-ray spectroscopy. 118,119 X-ray pump-probe experiments that probe the surface structure by diffraction techniques or study solid-liquid interfaces have not been reported yet. For such studies, several problems have to be overcome: First, the extremely bright XFEL beam most likely results in extensive damage of the sample and the electrolyte. This can be overcome by electrolyte exchange and shifting the spot of the beam on the sample to another location, but the latter requires large samples with very homogeneous surface structure. Secondly, fast triggering of the dynamic process of interest is necessary. This rules out excitation by potential changes, which is limited by the time constant associated with double layer charging and is usually restricted to the milli- to microsecond regime for larger planar samples. Ideally, an optical trigger should be used, i.e., a photoelectrochemical process. For example, this could induce a (temporary) charge separation across the interface

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and thus allow studying the non-equilibrium structural dynamics of electrochemical charge transfer. Most likely, this is easiest to realize at a semiconductor-electrolyte interface, e.g., in model systems for electrochemical solar cells. For both experiment and theory, a further important objective are similar detailed studies of the interface structure and electrochemical reactions on more complex electrode materials. These would assist the knowledge-based development of electrode materials for electrochemical energy conversion and storage, where nonmetallic electrode materials, such as oxides and modified graphite, become increasingly important. For many of these materials, structurallydefined interfaces to the electrolyte have been studied only marginally or not at all. The preparation of the latter may require UHV methods, as already successfully employed for bimetallic alloys 120 and epitaxial Co oxides. 121 Apart from overcoming this “material gap”, the structure of these interfaces also needs to be clarified under highly oxidizing or reducing conditions and during electrochemical reactions at substantial current densities. This will require operando methods that can be employed during high mass transport in the electrolyte and adverse processes, such as gas evolution. Considering that such studies are even rare for simple single crystal metal surfaces, considerable progress is still required for a true atomistic picture of electrochemical interfaces under realistic reaction conditions.

Acknowledgement The work of AG contributes to the research performed at CELEST (Center for Electrochemical Energy Storage Ulm-Karlsruhe).

Author section Email: [email protected], [email protected]

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Towards an atomic-scale understanding of electrochemical interface

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