OH formation and H2 adsorption at the liquid water-Pt(111) interface

Water orientation on platinum surfaces controlled by step sites

In this work, the adsorption structure of deuterated water on the stepped platinum surface is studied under an ultra-high vacuum by using heterodyne-detected sum-frequency generation spectroscopy. On a pristine Pt(553), D2O molecules adsorbed at the step sites act as hydrogen bond (H-bond) donors to the adjacent terrace sites. This ensures the net D-down orientation at the terrace sites away from the steps. In particular, the pre-adsorption of oxygen atoms at the step sites significantly alters the D-down configuration. The oxygen pre-adsorption leads to a spontaneous dissociation of the post-adsorbed water molecules at the step to form hydroxyl (OD) species. Since the hydroxyl at the step acts as a strong H-bond acceptor, D2O at the terrace no longer maintains the D-down configuration and adopts flat-lying configurations, significantly reducing the number of D-down molecules at the terrace. Density-functional theoretical calculations support these pictures. This work demonstrates the critical role of steps in controlling the net orientation of the interfacial water and provides an important reference for future considerations of the reactions at electrochemical interfaces.

Ab initio molecular dynamics investigation of the Pt(111)-water interface structure in an alkaline environment with high surface OH-coverages

In this study, we investigate the structure of the Pt(111)-water interface in an alkaline environment with large OH coverages of 1/3, 2/3 and 1 monolayer using a large well-equilibrated system. We observe that the OH coverage influences both the orientational distribution of the water molecules and their density, with more structure associated with higher coverage. At the same time, there is evidence of a highly dynamic hydrogen bond network on the lower coverage systems with substantial exchange of water between the surface and the solvent. In addition to OH and H2O species, which are preferentially located at the top sites, the 1/3 and 2/3 monolayer surfaces also contain O atoms, which are relatively stable and prefer the hollow sites. In contrast, the 1 monolayer surface shows none of these dynamics, and is unlikely to be active. The dynamic coexistence of O, OH and H2O on Pt(111) electrodes in alkaline conditions necessitates the investigation of several possible reaction paths for processess like ORR and water splitting. Finally, the exchange processes observed between the solvent and the interface underscore the need to explicitly include liquid water in simulations of systems similar to Pt(111).

From 2e- to 4e- pathway in the alkaline oxygen reduction reaction on Au(100): Kinetic circumvention of the volcano curve

We report the free energy barriers for the elementary reactions in the 2e- and 4e- oxygen reduction reaction (ORR) steps on Au(100) in an alkaline solution. Due to the weak adsorption energy of O2 on Au(100), the barrier for the association channel is very low, and the 2e- pathway is clearly favored, while the barrier for the O-O dissociation channel is significantly higher at 0.5 eV. Above 0.7 V reversible hydrogen electrode (RHE), the association channel becomes thermodynamically unfavorable, which opens up the O-O dissociation channel, leading to the 4e- pathway. The low adsorption energy of oxygenated species on Au is now an advantage, and residue ORR current can be observed up to the 1.0-1.2 V region (RHE). In contrast, the O-O dissociation barrier on Au(111) is significantly higher, at close to 0.9 eV, due to coupling with surface reorganization, which explains the lower ORR activity on Au(111) than that on Au(100). In combination with the previously suggested outer sphere electron transfer to O2 for its initial adsorption, these results provide a consistent explanation for the features in the experimentally measured polarization curve for the alkaline ORR on Au(100) and demonstrate an ORR mechanism distinct from that on Pt(111). It also highlights the importance to consider the spin state of O2 in ORR and to understand the activation barriers, in addition to the adsorption energies, to account for the features observed in electrochemical measurements.

Laterally Resolved Free Energy Profiles and Vibrational Spectra of Chemisorbed H Atoms on Pt(111)

A scheme to compute laterally resolved free energy surfaces and spectral signatures of specifically adsorbed ions on electrode surfaces from their ab initio molecular dynamics (AIMD) trajectories is proposed. Considering H-covered Pt(111) electrodes, both in contact with water and vacuum and for various H coverages, we systematically explore the impact of explicit water and H-coverage on site occupancy, providing direct insight into the proportion of underpotential and overpotential deposited hydrogen adsorbates. Extending this approach further, we can obtain laterally resolved vibrational spectra of the Pt-H stretch modes. We discuss how the difference between the free energy basins of the on-top and fcc-hollow adsorption sites explains the features of the experimentally observed spectral fingerprints in this system. These fingerprints do not contain only information about the stable and metastable adsorption sites but also about intermediate short-lived adsorbate configurations. Our results also show that for these properties chemisorbed H2O acts as a spectator and does not qualitatively influence the relative stabilities of the adsorption sites and their spectral fingerprints.

Toward a Realistic Surface State of Ru in Aqueous and Gaseous Environments

Identifying the surface species is critical in developing a realistic understanding of supported metal catalysts working in water. To this end, we have characterized the surface species present at a Ru/water interface by employing a hybrid computational approach involving an explicit description of the liquid water and a possible pressure of H₂. On the close-packed, most stable Ru(0001) facet, the solvation tends to favor the full dissociation of water into atomic O and H in contrast with the partially dissociated water layer reported for ultra-high-vacuum conditions. The solvation stabilization was found to reach -0.279 J m², which results in stable O and H species on Ru(0001) in the presence of liquid water even at room temperature. Conversely, introducing even a small H₂ pressure (10^-2 bar) results in a monolayer of chemisorbed H at the interface, a general trend found on the three most exposed facets of Ru nanoparticles. While hydroxyls were often hypothesized as possible surface species at the Ru/water interface, this computational study clearly demonstrates that they are not stabilized by liquid water and are not found under realistic reductive catalytic conditions.

Neural network potentials for accelerated metadynamics of oxygen reduction kinetics at Au-water interfaces

The application of ab initio molecular dynamics (AIMD) for the explicit modeling of reactions at solid-liquid interfaces in electrochemical energy conversion systems like batteries and fuel cells can provide new understandings towards reaction mechanisms. However, its prohibitive computational cost severely restricts the time- and length-scales of AIMD. Equivariant graph neural network (GNN) based accurate surrogate potentials can accelerate the speed of performing molecular dynamics after learning on representative structures in a data efficient manner. In this study, we combined uncertainty-aware GNN potentials and enhanced sampling to investigate the reactive process of the oxygen reduction reaction (ORR) at an Au(100)-water interface. By using a well-established active learning framework based on CUR matrix decomposition, we can evenly sample equilibrium structures from MD simulations and non-equilibrium reaction intermediates that are rarely visited during the reaction. The trained GNNs have shown exceptional performance in terms of force prediction accuracy, the ability to reproduce structural properties, and low uncertainties when performing MD and metadynamics simulations. Furthermore, the collective variables employed in this work enabled the automatic search of reaction pathways and provide a detailed understanding towards the ORR reaction mechanism on Au(100). Our simulations identified the associative reaction mechanism without the presence of *O and a low reaction barrier of 0.3 eV, which is in agreement with experimental findings. The methodology employed in this study can pave the way for modeling complex chemical reactions at electrochemical interfaces with an explicit solvent under ambient conditions.

Machine Learning Assisted Simulations of Electrochemical Interfaces: Recent Progress and Challenges

The electrochemical interface, where the adsorption of reactants and electrocatalytic reactions take place, has long been a focus of attention. Some of the important processes on it tend to possess relatively slow kinetic characteristics, which are usually beyond the scope of ab initio molecular dynamics. The newly emerging technique, machine learning methods, provides an alternative approach to achieve thousands of atoms and nanosecond time scale while ensuring precision and efficiency. In this Perspective, we summarize in detail the recent progress and achievements made by the introduction of machine learning to simulate electrochemical interfaces, and focus on the limitations of current machine learning models, such as accurate descriptions of long-range electrostatic interactions and the kinetics of the electrochemical reactions occurring at the interface. Finally, we further point out the future directions for machine learning to expand in the field of electrochemical interfaces.

Inverse kinetic isotope effects in the oxygen reduction reaction at platinum single crystals

Although the oxygen reduction reaction (ORR) involves multiple proton-coupled electron transfer processes, early studies reported the absence of kinetic isotope effects (KIEs) on polycrystalline platinum, probably due to the use of unpurified D₂O. Here we developed a methodology to prepare ultra-pure D₂O, which is indispensable for reliably investigating extremely surface-sensitive platinum single crystals. We find that Pt(111) exhibits much higher ORR activity in D₂O than in H₂O, with potential-dependent inverse KIEs of ~0.5, whereas Pt(100) and Pt(110) exhibit potential-independent inverse KIEs of ~0.8. Such inverse KIEs are closely correlated to the lower *OD coverage and weakened *OD binding strength relative to *OH, which, based on theoretical calculations, are attributed to the differences in their zero-point energies. This study suggests that the competing adsorption between *OH/*OD and *O₂ probably plays an important role in the ORR rate-determining steps that involve a chemical step preceding an electrochemical step (CE mechanism).

Potential-Dependent Pt(111)/Water Interface: Tackling the Challenge of a Consistent Treatment of Electrochemical Interfaces

The interface between an electrode and an electrolyte is where electrochemical processes take place for countless technologically important applications. Despite its high relevance and intense efforts to elucidate it, a description of the interfacial structure and, in particular, the dynamics of the electric double layer at the atomic level is still lacking. Here we present reactive force-field molecular dynamics simulations of electrified Pt(111)/water interfaces, shedding light on the orientation of water molecules in the vicinity of the Pt(111) surface, taking into account the influence of potential, adsorbates, and ions simultaneously. We obtain a shift in the preferred orientation of water in the surface oxidation potential region, which breaks with the previously proclaimed strict correlation to the free charge density. Moreover, the characterization is complemented by course of the entropy and the intermolecular ordering in the interfacial region complements the characterization. Our work contributes to the ongoing process of understanding electric double layers and, in particular, the structure of the electrified Pt(111)/water interface, and aims to provide insights into the electrochemical processes occurring there.

Platinum-Water Interaction Induced Interfacial Water Orientation That Governs the pH-Dependent Hydrogen Oxidation Reaction

Understanding the electrode-water interface structure in acid and alkali is crucial to unveiling the underlying mechanism of pH-dependent hydrogen oxidation reaction (HOR) kinetics. In this work, we construct the explicit Pt(111)-H₂O interface models in both acid and alkali to investigate the relationship between the HOR mechanism and electrode-electrolyte interface structure using ab initio molecular dynamics and density functional theory. We find that the interfacial water orientation in the outer Helmholtz layer (OHP) induced by the Pt-water interaction governs the pH-dependent HOR kinetics on Pt(111). In alkali, the strong Pt-interfacial water electrostatic interaction behaves as a narrow OHP, which increases the proportion of "H-down" interfacial water and leads to less adsorbed water entering the inner Helmholtz plane (IHP), decreasing the work function of Pt(111). Furthermore, the more "H-down" interfacial water stabilizes the H_ad adsorption, prevents H_ad desorption, and suppresses the Volmer step of HOR by forming the solvated [H_ad···H₂O···H₂O] complex. Our work provided a visualized molecular-level mechanism to understand the nature of pH-dependent HOR kinetics.

Ion Solvation Free Energy Calculation Based on Ab Initio Molecular Dynamics Using a Hybrid Solvent Model

Free energy calculation of small molecules or ion species in aqueous solvent is one of the most important problems in electrochemistry study. Although there are many previous approaches to calculate such free energies, they are based on ab initio methods and suffer from various limitations and approximations. In the current work, we developed a hybrid approach based on ab initio molecular dynamics (AIMD) simulations to calculate the ion solvation energy. In this approach, a small water cluster surrounding the central ion is used, and implicit solvent model is applied outside the water cluster. A dynamic potential well is used during AIMD to keep the water cluster together. Quasi-harmonic approximation is used to calculate the entropy contribution, while the total energy average is used to calculate the enthalpy term. The obtained solvation voltages of the bulk metal agree with experiments within 0.3 eV, and the simulation results for the solvation energies of gaseous ions are close to the experimental observations. Besides the free energies, radial pair distribution functions and coordination numbers of hydrated cations are also obtained. The remaining challenges of this method are also discussed.

Structure and energetics of liquid water-hydroxyl layers on Pt(111)

The interactions between liquid water and hydroxyl species on Pt(111) surfaces have been intensely investigated due to their importance to fuel cell electrocatalysis. Here we present a molecular dynamics study of their structure and energetics using an ensemble of neural network potentials, which allow us to obtain unprecedented statistical sampling. We first study the energetics of hydroxyl formation, where we find a near-linear adsorption energy profile, which exhibits a soft and gradual increase in the differential adsorption energy at high hydroxyl coverages. This is strikingly different from the predictions of the conventional bilayer model, which displays a kink at 1/3ML OH coverage indicating a sizeable jump in differential adsorption energy, but within the statistical uncertainty of previously reported ab initio molecular dynamics studies. We then analyze the structure of the interface, where we provide evidence for the water-OH/Pt(111) interface being hydrophobic at high hydroxyl coverages. We furthermore explain the observed adsorption energetics by analyzing the hydrogen bonding in the water-hydroxyl adlayers, where we argue that the increase in differential adsorption energy at high OH coverage can be explained by a reduction in the number of hydrogen bonds from the adsorbed water molecules to the hydroxyls.

Addressing Dynamics at Catalytic Heterogeneous Interfaces with DFT-MD: Anomalous Temperature Distributions from Commonly Used Thermostats

Density functional theory-based molecular dynamics (DFT-MD) has been widely used for studying the chemistry of heterogeneous interfacial systems under operational conditions. We report frequently overlooked errors in thermostated or constant-temperature DFT-MD simulations applied to study (electro)catalytic chemistry. Our results demonstrate that commonly used thermostats such as Nosé-Hoover, Berendsen, and simple velocity-rescaling methods fail to provide a reliable temperature description for systems considered. Instead, nonconstant temperatures and large temperature gradients within the different parts of the system are observed. The errors are not a "feature" of any particular code but are present in several ab initio molecular dynamics implementations. This uneven temperature distribution, due to inadequate thermostatting, is well-known in the classical MD community, where it is ascribed to the failure in kinetic energy equipartition among different degrees of freedom in heterogeneous systems (Harvey et al. J. Comput. Chem. 1998, 726-740) and termed the flying ice cube effect. We provide tantamount evidence that interfacial systems are susceptible to substantial flying ice cube effects and demonstrate that the traditional Nosé-Hoover and Berendsen thermostats should be applied with care when simulating, for example, catalytic properties or structures of solvated interfaces and supported clusters. We conclude that the flying ice cube effect in these systems can be conveniently avoided using Langevin dynamics.

Ab Initio Simulations of Water/Metal Interfaces

Structures and processes at water/metal interfaces play an important technological role in electrochemical energy conversion and storage, photoconversion, sensors, and corrosion, just to name a few. However, they are also of fundamental significance as a model system for the study of solid-liquid interfaces, which requires combining concepts from the chemistry and physics of crystalline materials and liquids. Particularly interesting is the fact that the water-water and water-metal interactions are of similar strength so that the structures at water/metal interfaces result from a competition between these comparable interactions. Because water is a polar molecule and water and metal surfaces are both polarizable, explicit consideration of the electronic degrees of freedom at water/metal interfaces is mandatory. In principle, ab initio molecular dynamics simulations are thus the method of choice to model water/metal interfaces, but they are computationally still rather demanding. Here, ab initio simulations of water/metal interfaces will be reviewed, starting from static systems such as the adsorption of single water molecules, water clusters, and icelike layers, followed by the properties of liquid water layers at metal surfaces. Technical issues such as the appropriate first-principles description of the water-water and water-metal interactions will be discussed, and electrochemical aspects will be addressed. Finally, more approximate but numerically less demanding approaches to treat water at metal surfaces from first-principles will be briefly discussed.

A first principle study of water adsorbed on flat and stepped silver surfaces

The structural, electronic and vibrational properties of a water layer on Ag(100) and Ag(511) have been studied by first principles calculations and ab initio molecular dynamics simulations. The most stable...

Implicit Solvation Methods for Catalysis at Electrified Interfaces

Implicit solvation is an effective, highly coarse-grained approach in atomic-scale simulations to account for a surrounding liquid electrolyte on the level of a continuous polarizable medium. Originating in molecular chemistry with finite solutes, implicit solvation techniques are now increasingly used in the context of first-principles modeling of electrochemistry and electrocatalysis at extended (often metallic) electrodes. The prevalent ansatz to model the latter electrodes and the reactive surface chemistry at them through slabs in periodic boundary condition supercells brings its specific challenges. Foremost this concerns the difficulty of describing the entire double layer forming at the electrified solid–liquid interface (SLI) within supercell sizes tractable by commonly employed density functional theory (DFT). We review liquid solvation methodology from this specific application angle, highlighting in particular its use in the widespread ab initio thermodynamics approach to surface catalysis. Notably, implicit solvation can be employed to mimic a polarization of the electrode’s electronic density under the applied potential and the concomitant capacitive charging of the entire double layer beyond the limitations of the employed DFT supercell. Most critical for continuing advances of this effective methodology for the SLI context is the lack of pertinent (experimental or high-level theoretical) reference data needed for parametrization.

Is the water/Pt(111) interface ordered at room temperature?

The structure of the water/Pt(111) interface has been a subject of debate over the past decades. Here, we report the results of a room temperature molecular dynamics study based on neural network potentials, which allow us to access long time scale simulations while retaining ab initio accuracy. We find that the water/Pt(111) interface is characterized by a double layer composed of a primary, strongly bound adsorption layer with a coverage of ∼0.15 ML, which is coupled to a secondary, weakly bound adsorption layer with a coverage of ∼0.58 ML. By studying the order of the primary adsorption layer, we find that there is an effective repulsion between the adsorbed water molecules, which gives rise to a dynamically changing, semi-ordered interfacial structure, where the water molecules in the primary adsorption layer are distributed homogeneously across the interface, forming frequent hydrogen bonds to water molecules in the secondary adsorption layer. We further show that these conclusions are beyond the time scales accessible to ab initio molecular dynamics.

Static and dynamic water structures at interfaces: A case study with focus on Pt(111)

An accurate atomistic treatment of aqueous solid-liquid interfaces necessitates the explicit description of interfacial water ideally via ab initio molecular dynamics simulations. Many applications, however, still rely on static interfacial water models, e.g., for the computation of (electro)chemical reaction barriers and focus on a single, prototypical structure. In this work, we systematically study the relation between density functional theory-derived static and dynamic interfacial water models with specific focus on the water-Pt(111) interface. We first introduce a general construction protocol for static 2D water layers on any substrate, which we apply to the low index surfaces of Pt. Subsequently, we compare these with structures from a broad selection of reference works based on the Smooth Overlap of Atomic Positions descriptor. The analysis reveals some structural overlap between static and dynamic water ensembles; however, static structures tend to overemphasize the in-plane hydrogen bonding network. This feature is especially pronounced for the widely used low-temperature hexagonal ice-like structure. In addition, a complex relation between structure, work function, and adsorption energy is observed, which suggests that the concentration on single, static water models might introduce systematic biases that are likely reduced by averaging over consistently created structural ensembles, as introduced here.

CO2 activation at Au(110)-water interfaces: An ab initio molecular dynamics study

The electrochemical reduction of CO₂ into valuable chemicals under mild conditions has become a promising technology for energy storage and conversion in the past few years, receiving much attention from theoretical researchers investigating the reaction mechanisms. However, most of the previous simulations are related to the key intermediates of *COOH and *CO using the computational hydrogen electrode approach under vacuum conditions, and the details of the CO₂ activation are usually ignored due to the model simplicity. Here, we study the CO₂ activation at the Au-water interfaces by considering the dynamics of an explicit water solvent, where both regular ab initio molecular dynamics and constrained ab initio molecular dynamics simulations are carried out to explore the CO₂ adsorption/desorption reactions from the atomic level. By introducing K⁺ cations into Au(110)-water interfacial models, an electrochemical environment under reducing potentials is constructed, where the reaction free energy (0.26 eV) and activation energy (0.61 eV) are obtained for CO₂ adsorption based on the thermodynamic integration. Moreover, the Bader charge analysis demonstrates that CO₂ adsorption is activated by the first-electron transfer, forming the adsorbed CO₂ ^- anion initiating the overall catalytic reaction.

Properties of the Pt(111)/electrolyte electrochemical interface studied with a hybrid DFT-solvation approach

Self-consistent modeling of the interface between solid metal electrode and liquid electrolyte is a crucial challenge in computational electrochemistry. In this contribution, we adopt the effective screening medium reference interaction site method (ESM-RISM) to study the charged interface between a Pt(111) surface that is partially covered with chemisorbed oxygen and an aqueous acidic electrolyte. This method proves to be well suited to describe the chemisorption and charging state of the interface at controlled electrode potential. We present an in-depth assessment of the ESM-RISM parameterization and of the importance of computing near-surface water molecules explicitly at the quantum mechanical level. We found that ESM-RISM is able to reproduce some key interface properties, including the peculiar, non-monotonic charging relation of the Pt(111)/electrolyte interface. The comparison with independent theoretical models and explicit simulations of the interface reveals strengths and limitations of ESM-RISM for modeling electrochemical interfaces.

Cross-Sphere Electrode Reaction: The Case of Hydroxyl Desorption during the Oxygen Reduction Reaction on Pt(111) in Alkaline Media

Hydroxide ion is a common electrolyte when electrode reactions take place in alkaline media. In the case of oxygen reduction reaction on Pt(111), we demonstrate by ab initio molecular dynamics calculations that the desorption of hydroxyl (OH*) from the electrode surface to form a solvated OH^- is a cross-sphere process, with the OH* reactant in the inner sphere and the OH^- product directly generated in the aqueous outer sphere. Such a mechanism is distinct from the typical inner sphere and outer sphere reactions. It is dictated by the strong hydrogen bonding interactions between a hydroxide ion and water molecules and is facilitated by proton transfer through solvation layers. It should play a significant role whenever OH* desorption, or its reverse, OH^- adsorption, is involved in an electrochemical reaction.

Thermodynamic cyclic voltammograms: peak positions and shapes

Based on a mean-field description of thermodynamic cyclic voltammograms (CVs), we analyze here in full generality, how CV peak positions and shapes are related to the underlying interface energetics, in particular when also including electrostatic double layer (DL) effects. We show in particular, how non-Nernstian behaviour is related to capacitive DL charging, and how this relates to common adsorbate-centered interpretations such as a changed adsorption energetics due to dipole-field interactions and the electrosorption valency - the number of exchanged electrons upon electrosorption per adsorbate. Using Ag(111) in halide-containing solutions as test case, we demonstrate that DL effects can introduce peak shifts that are already explained by rationalizing the interaction of isolated adsorbates with the interfacial fields, while alterations of the peak shape are mainly driven by the coverage-dependence of the adsorbate dipoles. In addition, we analyze in detail how changing the experimental conditions such as the ion concentrations in the solvent but also of the background electrolyte can affect the CV peaks via their impact on the potential drop in the DL and the DL capacitance, respectively. These results suggest new routes to analyze experimental CVs and use of those for a detailed assessment of the accuracy of atomistic models of electrified interfaces e.g. with and without explicitly treated interfacial solvent and/or approximate implicit solvent models.

Modeling Electrified Pt(111)-Had/Water Interfaces from Ab Initio Molecular Dynamics

Unraveling the atomistic structures of electric double layers (EDL) at electrified interfaces is of paramount importance for understanding the mechanisms of electrocatalytic reactions and rationally designing electrode materials with better performance. Despite numerous efforts dedicated in the past, a molecular level understanding of the EDL is still lacking. Combining the state-of-the-art ab initio molecular dynamics (AIMD) and recently developed computational standard hydrogen electrode (cSHE) method, it is possible to realistically simulate the EDL under well-defined electrochemical conditions. In this work, we report extensive AIMD calculation of the electrified Pt(111)-Had/water interfaces at the saturation coverage of adsorbed hydrogen (Had) corresponding to the typical hydrogen evolution reaction conditions. We calculate the electrode potentials of a series of EDL models with various surface charge densities using the cSHE method and further obtain the Helmholtz capacitance that agrees with experiment. Furthermore, the AIMD simulations allow for detailed structural analyses of the electrified interfaces, such as the distribution of adsorbate Had and the structures of interface water and counterions, which can in turn explain the computed dielectric property of interface water. Our calculation provides valuable molecular insight into the electrified interfaces and a solid basis for understanding a variety of electrochemical processes occurring inside the EDL.

Impact of Water Coadsorption on the Electrode Potential of H-Pt(1 1 1)-Liquid Water Interfaces

Density functional theory molecular dynamics simulations of H-covered Pt(111)-H_{2}O interfaces reveal that, in contrast to common understanding, H_{2}O coadsorption has a significant impact on the electrode potential of and plays a major role in determining the stability of H adsorbates under electrochemical conditions. Based on these insights, we explain the origin behind the experimentally observed upper limit of H coverage well below one monolayer and derive a chemically intuitive model for metal-water bonding that explains an unexpectedly large interaction between coadsorbed water and adsorbates.

Size dependence of hydrophobic hydration at electrified gold/water interfaces

Hydrophobic hydration at metal/water interfaces actively contributes to the energetics of electrochemical reactions, e.g. [Formula: see text] and [Formula: see text] reduction, where small hydrophobic molecules are involved. In this work, constant applied potential molecular dynamics is employed to study hydrophobic hydration at a gold/water interface. We propose an adaptation of the Lum-Chandler-Weeks (LCW) theory to describe the free energy of hydrophobic hydration at the interface as a function of solute size and applied voltage. Based on this model we are able to predict the free energy cost of cavity formation at the interface directly from the free energy cost in the bulk plus an interface-dependent correction term. The interfacial water network contributes significantly to the free energy, yielding a preference for outer-sphere adsorption at the gold surface for ideal hydrophobes. We predict an accumulation of small hydrophobic solutes of sizes comparable to CO or [Formula: see text], while the free energy cost to hydrate larger hydrophobes, above 2.5-Å radius, is shown to be greater at the interface than in the bulk. Interestingly, the transition from the volume dominated to the surface dominated regimes predicted by the LCW theory in the bulk is also found to take place for hydrophobes at the Au/water interface but occurs at smaller cavity radii. By applying the adapted LCW theory to a simple model addition reaction, we illustrate some implications of our findings for electrochemical reactions.

Thermodynamic Cyclic Voltammograms Based on Ab Initio Calculations: Ag(111) in Halide-Containing Solutions

Cyclic voltammograms (CVs) are a central experimental tool for assessing the structure and activity of electrochemical interfaces. Based on a mean-field ansatz for the interface energetics under applied potential conditions, we here derive an ab initio thermodynamics approach to efficiently simulate thermodynamic CVs. All unknown parameters are determined from density functional theory (DFT) calculations coupled to an implicit solvent model. For the showcased CVs of Ag(111) electrodes in halide-anion-containing solutions, these simulations demonstrate the relevance of double-layer contributions to explain experimentally observed differences in peak shapes over the halide series. Only the appropriate account of interfacial charging allows us to capture the differences in equilibrium coverage and total electronic surface charge that cause the varying peak shapes. As a case in point, this analysis demonstrates that prominent features in CVs do not only derive from changes in adsorbate structure or coverage but can also be related to variations of the electrosorption valency. Such double-layer effects are proportional to adsorbate-induced changes in the work function and/or interfacial capacitance. They are thus especially pronounced for electronegative halides and other adsorbates that affect these interface properties. In addition, the analysis allows us to draw conclusions on how the possible inaccuracy of implicit solvation models can indirectly affect the accuracy of other predicted quantities such as CVs.

Molecular origin of negative component of Helmholtz capacitance at electrified Pt(111)/water interface

Electrified solid/liquid interfaces are the key to many physicochemical processes in a myriad of areas including electrochemistry and colloid science. With tremendous efforts devoted to this topic, it is unexpected that molecular-level understanding of electric double layers is still lacking. Particularly, it is perplexing why compact Helmholtz layers often show bell-shaped differential capacitances on metal electrodes, as this would suggest a negative capacitance in some layer of interface water. Here, we report state-of-the-art ab initio molecular dynamics simulations of electrified Pt(111)/water interfaces, aiming at unraveling the structure and capacitive behavior of interface water. Our calculation reproduces the bell-shaped differential Helmholtz capacitance and shows that the interface water follows the Frumkin adsorption isotherm when varying the electrode potential, leading to a peculiar negative capacitive response. Our work provides valuable insight into the structure and capacitance of interface water, which can help understand important processes in electrocatalysis and energy storage in supercapacitors.

Energy–entropy competition in cation–hydroxyl interactions at the liquid water–Pt(111) interface

At water–Pt(111) interfaces, cation–*OH interactions are found to consist of both internal energy stabilizations and entropy costs emphasizing the complexity of such systems.

Water structures on a Pt(111) electrode from ab initio molecular dynamic simulations for a variety of electrochemical conditions

A structural analysis of solvating water layers on a Pt(111) electrode has been performed based on extensive ab initio molecular dynamics simulations. We have emulated different electrochemical conditions by varying the concentration of hydrogen ions in the water layers, which effectively corresponds to a variation in the electrode potential. We present a detailed analysis of the arrangement and orientation of the water molecules and also address their mobility in the solvation layer.

Synergistic adsorptions of Na2CO3 and Na2SiO3 on calcium minerals revealed by spectroscopic and ab initio molecular dynamics studies

FTIR, XPS, and ab initio molecular dynamics studies demonstrated that sodium silicate (Na₂SiO₃) adsorbs on fluorite with a higher affinity when they are treated beforehand by sodium carbonate (Na₂CO₃) due to proton exchange(s).

The synergistic effects between sodium silicate (Na₂SiO₃) and sodium carbonate (Na₂CO₃) adsorbed on mineral surfaces are not yet understood, making it impossible to finely tune their respective amounts in various industrial processes. In order to unravel this phenomenon, diffuse reflectance infrared Fourier transform and X-ray photoelectron spectroscopies were combined with ab initio molecular dynamics to investigate the adsorption of Na₂SiO₃ onto bare and carbonated fluorite (CaF₂), an archetypal calcium mineral. Both experimental and theoretical results proved that Na₂CO₃ adsorbs onto CaF₂ with a high affinity and forms a layer of Na₂CO₃ on the surface. Besides, at low Na₂SiO₃ concentration, silica mainly physisorbs in a monomeric protonated form, Si(OH)₄, while at larger concentration, significant amounts of polymerised and deprotonated forms are identified. Prior surface carbonation induces an acid–base reaction on the surface, which results in the formation of the basic forms of the monomers and the dimers, i.e. SiO(OH)₃^– and Si₂O₃(OH)₄^2–, even at low coverage. Their adsorption is highly favoured compared to the acid forms, which explains the synergistic effects observed when Na₂SiO₃ is used after Na₂CO₃. The formation of the basic form on the bare surface is observed only by increasing the surface coverage to 100%. Hence, when Na₂CO₃ is used during a separation process, lower Na₂SiO₃ concentrations are needed to obtain the same effect as with lone Na₂SiO₃ in the separation process.