Computer modeling of electrochemical systems;  DFT-based simulations of electrified interfaces;  Electrochemical energy storage and conversion;  Electrodics from first principles;  Modeling of batteries and fuel cells;  Parameter-free study of electrochemical interfaces

Under equilibrium conditions, a bulk electrolyte is an electroneutral, isotropic, and homogeneous solution, and there are no electric fields along any preferential direction. However, at the frontier with another material with mobile free charges (ions or electrons) and a different chemical potential – e.g., the electrode – some anisotropy in the forces experienced by the particles arises, and charge redistribution occurs. Although the interfacial region remains overall neutral, either side of the boundary becomes charged to an equal and opposite extent, forming the so-called electrical double layer or electrified interface. This gives rise to a potential difference across the interface. This potential difference is usually not large (~1 V); however, the dimension of the interphase region is very small (~10⁻⁷ cm) with the consequence that the field gradient is enormous (~10⁷ V/cm). The potential difference across the electrified interface and the structure of the double layer are intimately related. The fundamental problem in electrochemical studies is to unravel this functional relationship and the effect of the field at the electrode–electrolyte interface.

There is a long history of theoretical, DFT-based surface science [3] – often in combination with experiments – resulting in a deep insight into the structure and reactions on metal and oxide surfaces as well as comprehensive studies on electronic, structural, and dynamical properties in energy materials [4].

An established and very popular model to predict reactivity and catalytic performance of different electrodes by means of simple descriptors was proposed by Hammer and Nørskov in 1995 [5]. In this model, adsorption energies are related to d-band position in the metal and to barriers in catalytic reactions. For instance, oxygen reduction activity – which can be calculated in terms of activation energies, using the concept of Sabatier analysis – was related in a volcano plot to oxygen binding energy on different metal surfaces. The position of the center of d-electrons density with respect to metal Fermi level in some cases correlates linearly with oxygen atom chemisorption energy. However, this is only true if the metals under consideration are very similar. Even with this caveat, this approach has proven to be extremely useful, especially when oxygen binding energy is used as a parameter in describing the catalytic performance. However, it has to be said that using this approach, the complicated processes governing EC conversion are oversimplified and electrolyte presence is not considered explicitly.

Unraveling the atomistic structure and dynamics of metal–water interfaces is of crucial interest from both a fundamental and an applied point of view, in phenomena and technological processes such as corrosion, lubrication, heterogeneous catalysis, and electrochemistry. The established picture for water at this interface is of an icelike structure with enhanced density close to the electrode. These metal–water interfaces have been traditionally modeled statically, using the water bilayer model, with every second water molecule oriented parallel to the surface and the other molecules pointing one H atom either toward (H-down configuration) or away from the surface (H-up configuration). More recent studies [6] have shown that a remarkably rich variety of equilibrium structures, including dissociated or not hexagonal water rings, can form at the interface with even simple, flat surfaces, depending on the strain acting on the water layer and, in general, on the nature of the substrate. To address the open issues in electrode/electrolyte interface modeling, the theoretical surface science community has followed a stepwise approach based on first principles by considering systems with increasing complexity [7]. For instance, chemical trends in adsorbed halides- and water-induced work function changes have been correlated with water–metal interaction strength, charge redistribution at the interface, and metal electronegativity.

While an important progress has been achieved due to these static approaches, the equilibrium structure of metal–electrolyte interfaces in ambient condition is yet to be unraveled.

To take into account the effect of temperature in the modeling of the interface between electrode and water, numerous molecular dynamics (MD) studies [7, 8] used empirical and classical force fields models. The main limitation of most of these studies is that they do not explicitly include charge polarization effects as well as do not describe breaking and formation of chemical bonds and charge transfer at interfaces and in bulk electrolytes.

The only approaches suitable to describe the subtle interplay of electronic, ionic, and thermal effects are those based on first principles, especially when coupled with MD. Among those, DFT represents a very good compromise between accuracy and numerical efficiency. Recently, the increased availability of computational resources allowed a few first-principles MD studies to treat explicit water [8].

More recently, a method was devised to investigate coupled proton–electron transfer reactions [9]. This approach is based on ab initio MD but relies on foundations that might be seen as intermediate between simulation and theory. The half-reactions consisting of the addition/removal of protons/electrons are simulated in a homogeneous solution environment. Quantitative contact with electrochemistry is established by measuring free energies with respect to a common reference point, corresponding to the free energy of an aqueous hydronium ion H₃O⁺. Once the thermochemical corrections are applied, free energy differences obtained by this method do correspond to the ΔG₀ measured with respect to the normal hydrogen electrode (NHE). This method is sophisticated and powerful in describing the thermodynamic bulk properties, but it lacks a comprehensive description of the metal/electrolyte interface that is central to electrochemistry. Perhaps more importantly, it does not provide a description of the overpotential related to the transfer of species to, from, or across the interface, which generally could be useful in determining the efficiency of the electrode and catalyst. Finally, the method cannot be extended easily to real-time, real-space formulations of time-dependent DFT, which will be eventually required to describe the full kinetics of the system.

First-principles methods and MD are crucial for describing fundamental aspects of EC conversion, where chemical and electronic structure transformations occur. However, the complexity of these approaches and their high computational cost still restrict most of the existing studies to short simulation times and relatively small simulation cells, reducing the statistical validity of their conclusions.

A primary example of the currently limited ability to describe atomic and electronic scale processes at EI is our limited knowledge of the structure of the equilibrium DL and the effect of an external bias applied to the electrolyte through the electrodes even for the most basic metal–electrolyte interfaces. Secondly, the computed activation barriers for charge transfer at interfaces are usually not accurate (see however Ref. [4]) due to neglecting the effect of the environment.

In the following, we will present in some detail the methodological open issue of modeling the effect of the application of an external potential on the electrode and some of the proposed solutions.

In the final part of this entry, we will speak about modeling EC devices such as batteries and fuel cells, presenting some recent examples of the state of the art first-principles modeling of these devices.

The interest and the challenge in modern simulation of EC processes consist in representing the effect of variation of the electrostatic potential and thus the strong surface charge density redistribution and the fluctuating electric field at the electrode–electrolyte interface.

In principle, to describe an EC process a non-equilibrium approach should be used (such as the non-equilibrium Green’s function formalism used in molecular electronics) and the open boundary conditions for describing the electrode, electrolyte, and chemical transformations occurring therein. Moreover, different timescales, ranging from that of slow solvent rearrangements to that of electron transfer up to fast solvent polarization effects, should be treated at the same level. These difficulties are the reason for the delay in advance of theoretical material science in this area, and only recently, the growth of computer power enabled treating such systems using DFT. The extensive computational power when using such approach, however, is still not available for the wide scientific community to be used in ordinary calculations.

The simplest models of half-cell reactive processes use atomic clusters in gas phase and external parameters to simulate experimental equilibrium potential. To take into account the effect of an electric field on adsorption behavior at electrodes, a homogeneous electric field is sometimes applied perpendicularly to the cluster surface. The electrode potential is tuned by varying the external applied field; yet the relation of the EC potential to a known reference remains unclear.

Most of the standard simulation methods resort to periodic boundary conditions (PBC) to speed up the computation and minimize the effect of finite sample sizes, thus mimicking an infinite perfect crystal. Adopting periodic slabs provides a good description of the electronic and geometric structures, crucial for describing adsorption processes and relevant EC quantities at EI. However, the use of PBC makes it difficult to apply an electric field across the system, as required to simulate a system under bias. The supercell geometry, coupled with current methods to account for long-range Coulomb interactions (Ewald and variations thereof), implies, in turn, that the system must be globally neutral. Previous models described the charge on the electrode by including additional occupied KS orbitals and/or resorting to fractional occupation numbers. In other cases, neutrality is enforced by adding a judicious distribution of fixed countercharges [7]. These could take the form of a homogeneous background charge uniformly distributed over the simulation cell [10], charged planes [11], a dipole layer into the vacuum region [12], or hydronium ions [13] placed at a fixed distance from the electrode surface in order to charge the electrode.

The conceptual evolution of these approaches will be now critically highlighted in some detail.

Subsequently, a dipole layer was introduced into the vacuum region to induce electrode polarization – thus accounting for the effect of an external field. The main challenge in this case is to know the strength of the electric field induced by the applied potential. The authors evaluated this contribution assuming plate capacitor behavior, |E| = U/d, and guesstimating the Helmholtz layer thickness.

Charging up the slabs leads effectively to a variation in the electrode potential. Since in periodic calculations the unit cell has to be neutral, a countercharge is needed to maintain cell neutrality. We will illustrate this issue in more detail in the following. For the moment, we note that once the slab is allowed to take on a net charge, it is possible to set the electrode charge also implicitly, by fixing the chemical potential of the electrons associated with the slab. Along this line, Lozovoi [11] presented a grand-canonical formulation of charged slabs, where a chemical potential, μ, is defined and kept fixed relative to a reference electrode at finite distance from the slab. However, restraining μ is often less efficient than varying the charge in the slab until the value corresponding to a desired μ is found.

The most important contribution along this line was provided in 2006 by Filhol [10]. In Filhol’s approach, a net surface charge q is introduced onto the electrode via the variation of the total number of electrons present in the slab. Cell neutrality is enforced adding a constant countercharge background –q/W, where W is the cell volume (this countercharge is by default present in standard DFT codes in case of charged calculations in order to avoid electrostatic divergence). The potential drop at the EI induced by the presence of the charged electrode is quantified by comparing the Fermi level of the system to a reference potential at a defined distance from the metal/water interface.

The presence in this model of a constant countercharge background, mimicking the effect of the counterions, introduces a parabolic contribution to the potential outside the electrode. Interestingly, the artifacts arising from this distribution are reduced at the presence of polarizable water layers. However, since the counterions are modeled as uniformly spread in the electrolyte, rather than localized in the vicinity of the electrode surface, the electric field just outside the surface is too small.

An alternative solution was developed in 2007 by Rossmeisl and Norskov [13], who proposed a model where counterions are explicitly considered and the electrode potential is varied by varying the number of protons/electrons in the DL. Excess H atoms are thus included at fixed positions in the water layer, which should result in formation of solvated protons and transfer of electrons to the metal.

A different approach, proposed by Otani and Sugino [14], relies on performing nonperiodic slab calculations with effective screening medium. Green’s function technique is used to solve the Poisson equation with these boundary conditions. The slab is sandwiched by a continuum medium characterized by dielectric constant ε. For instance, to model the structure of a Pt/water interface in acidic conditions, vacuum (ε = 1) is used for the Pt side of the slab and a perfect conductor (ε = ∞) beyond the water side. Changing the number of electrons produces a charge on the slab surface and induces a countercharge in the perfect conductor. A vacuum region is present between water and the perfect conductor. In this model, the countercharge is placed in a layer beyond the water, 15–20 Å far from the electrode. This again represents a fictitious condition because in a real interface, the countercharge is very close to the electrode surface.

A clever usage and tuning of the methodologies highlighted above [4] led to different applications, simulating half-reactions in electro-catalysis – sometimes including the modeling of kinetic effects [2, 4, 7]. For instance, the thermodynamic approach of Ref. [13] has been used to study equilibrium coverage of metal halides as a function of the electrode potential. H₂ dissociation barrier at water/metal interfaces at room temperature as well as structure and stability of water layers as a function of electrode coverage due to hydrogen and other anions has also been recently addressed. In these cases, the effect of the electrode potential has been included using the approach described in Ref. [7, 8].

Other times, different levels of theory have been combined, for instance, by possible inclusion of the presence of the electrolyte using a polarizable medium [15].

The validity of the highlighted theoretical approaches, which charge the electrode to simulate the effect of the application of an external potential, relies on a correct description of the single-electron energies. Charging the electrode surface will occur only provided the KS density of states reflects the ordering shown in Fig. 1. The DFT approach determines self-consistently the charge of each species, filling KS electron states according to their relative energy with respect to the system Fermi energy, determined in this case by the Fermi level of the electrode.

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This points out an important methodological open issue to be addressed in view of a realistic description of EC interfaces: the well-known inadequacy of standard DFT in predicting single-electron energies for ions in solution, and for materials currently used as electrodes – such as metals and metal oxides.

In current DFT approximations, the energy of eigenstates depends on the occupation [9], and, as a result, the highest occupied level of cations may fall below the global Fermi energy upon discharging, and the lowest unoccupied level of anions might rise above E_F when accepting the extra negative charge. As a result, the DFT charge on ions in electrolytic solutions might turn out to be a fraction of the electron charge |e|, instead of reaching the integer value ± n as occurs in reality.

To prevent the artificial charge delocalization and improve the description of energy levels, the adoption of exact exchange schemes is required [9]. Such schemes are computationally extremely demanding. The system size and the simulation time become again an obstacle. Alternative, computationally viable solutions would be highly beneficial.

One of the greatest scientific challenges of our time is to provide the solution to the dramatic increase in energy demand and costs. Such a solution should be of minimal environmental impact. Energy policies based on capture and sequestration of CO₂ as well as those providing the setting for hydrogen production, storage, and distribution require massive infrastructures and are, at present, beyond the market capacity. In the short to intermediate future, EC devices such as batteries and fuel cells have a great potential for solving key aspects of the energy challenge.

Fuel cells (FCs) are EC devices in which a hydrogen-rich fuel reacts with an oxidizing agent, normally O₂. Fuel and oxygen are split at the opposite sides of an electrolyte, through which, an ion diffuses to combine with an ion with opposing charge, at the opposite side. This process allows the flow of electrons, i.e., electric power (see Fig. 2, which shows how a solid oxide fuel cells works). Like a combustion engine, FCs use a chemical fuel to generate power, but like in Batteries (Bs), the chemical energy is converted directly to electricity by means of an EC process, excluding the combustion step. As a consequence, FCs operate at much higher efficiency and generate fewer pollutants than traditional combustion methods. Bs convert chemical energy directly into electrical power, exploiting the difference in potential between electrodes. Nowadays, Bs power billions of portable electronic devices and are likely to represent the storage technology of choice in the near term for several other applications. The leading Li-based Bs technology represents a $50 billion dollar industry in the USA [16]. Energy storage devices can be utilized together with renewable but intermittent energy sources (like solar or wind) and/or conversion devices. Bs fulfills the need for readily available energy and could provide the means for the vehicle electrification, leading to a reduction of the carbon fingerprint.

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Among the various types of FCs, solid oxide fuel cells (SOFCs) are of great interest, because of their ability to utilize, except from hydrogen, a wide variety of commonly available fuels at high efficiency, including waste [17]. Thus, SOFCs do not rely on the development of hydrogen infrastructure and represent a midterm solution to lower both the energy demand and the CO₂ emissions. Typically SOFCs find a natural application as combined heat and power devices for residential applications. They are convenient because they work with the present energy distribution system and potentially could drive the transition toward hydrogen-based economy.

Major factors responsible for the growth of fuel cell market include the ability to integrate fuel cells in stationary, portable, and transportation applications to function as an off-grid power source, with zero emissions. The key concerns in the industry pertain to the high cost of interconnection (mainly in SOFCs, due to their high operating temperature), catalyst for both the cathode and anode compartments, their commercialization, and establishment of fuel cell infrastructure [17].

A broad scientific effort is devoted by both the academic and industrial communities for improving this energy-related technology; however, the gap separating the actual state of the art of the energy storage and conversion technology from that required for widespread commercialization is still large. On the one hand, increasing the specific energy capacity, the power, the safety, and the life cycle and lowering the cost of various devices remain a challenge, especially when one wishes to power hybrid electric vehicles. Improved electrode materials based on new ideas and advanced electrolytes with higher ionic conductivities and increased voltage stability must be developed. On the other hand, performances, durability, and cost of FCs must be improved. In this area, the major challenges consist of developing new electrolytes, electrodes, and catalyst materials/structures. For instance, while precious catalysts received a great attention and their performance reach the commercialization limit, the non-precious catalysts perform worse, and realization of their full potential is subject to extensive exploratory work.

Unraveling microscopic processes at the electrode surface remains challenging for both experimental and computational communities [18]. As an example, fuel oxidation at the triple phase boundary (TPB) between Ni electrode, yttria-stabilized zirconia (YSZ) electrolyte, and fuel gas phase in commercial SOFCs was subject to numerous experimental and computational studies. However, in spite of these studies, it was still not clear where water was formed. Impedance spectroscopy measurements identified the rate-limiting step (RLS) with a charge-transfer reaction with an activation barrier in the range 1.04–1.68 eV [19].

Kinetic modeling of impedance spectra had proposed three different assignments of the nature of the RLS, identified either with H⁺, OH⁻, or O²⁻ spillover.

In H⁺ spillover (H_Ni → H_YSZ + e_Ni), H atoms adsorbed on Ni hop either to a hydroxyl ion or an oxygen ion on the YSZ surface and here combine with oxygen to produce water. In O²⁻ spillover (O²⁻_YSZ → O_Ni + 2e_Ni), oxygen ions hop from the YSZ surface to the Ni surface, undergoing two charge-transfer reactions that take place before, after, or during the hop. In OH⁻ spillover, the OH⁻ formed on YSZ surface after H⁺ spillover jumps back on the Ni anode. Following O²⁻ and OH⁻ spillover, water formation occurs on Ni surface. It has been also shown that the polarization resistance is reduced by a high water partial pressure.

In this respect, ab initio simulations provided crucial insight on the nature of the RLS and the role of adsorbed water. A recent perspective on DFT modeling of three-phase boundary SOFC electrode has been provided by Shishkin and Ziegler [20]. A realistic modeling of these processes implies that electrode dimension and chemical activity allow charge transfer from O²⁻ to Ni, as it results in the potential drop at the DL, during SOFCs operation.

A state of the art model for the Ni/YSZ interface [21] is shown in Fig. 3. This realistic model allowed to study explicitly the different processes which lead to water desorption from the TPB and their dependence on the local defective structure of the YSZ surface moiety. In this study, it has been proven that the most favorable oxidation pathway involves hydrogen spillover from Ni to YSZ surface, which represents the RLS of the process. This study has shown that water formation and desorption take place on the YSZ surface and are mediated by the diffusion of a hydroxyl group on a Zr site close to the Ni/YSZ interface. The process continues with formation of water by the direct transfer from Ni cluster of a second H atom and its subsequent release. The smaller polarization resistance experimentally observed when water partial pressure increases in the anodic chamber is also explained in terms of enlargement of the active triple phase boundary. In the article, it is shown how this is made possible by the presence of water in the environment, in the form of adsorbed hydroxyl groups on the YSZ surface.

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Such a detailed mechanistic insight was made possible due to the combination of a variety of DFT-based techniques such as MD (to explore the configurational space), metadynamics (often used as an exploratory tool to guess possible reaction pathways), and nudged elastic band approach (to quantify potential energy profiles) for the optimization of those reaction trajectories. It is worth mentioning that this study required extensive computational time. Therefore, the limitation of the computational facilities can play a critical role in mechanistic studies at electrode surfaces.

An alternative approach for studying surface reactivity related to EC interphases is to use a cluster model of the active catalysts site, instead of periodic cell calculations. This approach has been particularly useful in modeling non-precious oxygen reduction reaction (ORR) catalysts, which were demonstrated to have comparable catalytic activity to that of Pt-based catalysts [22]. In such studies, the reaction paths and barriers are found by locating the all relevant minimum energy structures which represent reactants, products, or stable intermediates, as well as confirmed transition state species (i.e., structures having one imaginary frequency) on the potential energy surface. Usually such computational schemes are not prohibitively computationally expensive for using hybrid DFT potentials as well as post-Hartree–Fock electron correlation schemes, as opposed to slab periodic models. The favorable agreement between the computational data and experimental outcome may be partially attributed to the well-tuned DFT functionals, accounting for various types of interactions, typically found in metallo-organic catalysis [23].

Understanding of the EI at the battery electrodes is particularly important due to the formation of the solid electrolyte interphase SEI [24, 25] which represents the actual EC surface during the stable operation of the battery. This intermediate layer could be formed due to the reactivity of the surface of the electrode with the electrolyte in the presence of chemically active species formed during battery operation. Therefore, the modeling of surface reactivity should take into account the non-perfect electrode covered with this additional layer. Only in recent years, DFT studies appeared tackling the formation of this layer or its possible precursors.

The importance of modeling the SEI or its precursors cannot be underestimated in metal–air batteries. Metal–air batteries have the highest energy density because most of the cell volume is occupied by the anode while the cathode active material, i.e., the O₂, is not stored in the battery, rather adsorbed constantly from air into the porous conductive carbon black (Fig. 4a). If Li metal is used as an anode, the specific capacity can be as high as 3,842 mA h g⁻¹. During the discharge of the cell, an oxidation reaction occurs at the anode, \( \mathrm{L}\mathrm{i}\ \to\ {\mathrm{Li}}^{+} + {\mathrm{e}}^{-}. \)

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Li⁺ ion is moving across the electrolyte while the electrons flow in the external cycle to the cathode where a reduction reaction occurs, 2Li⁺ + 2e⁻ + O₂ → Li₂O₂ (or Li⁺ + e⁻ + O₂ → LiO₂).

The EC performance of this type of cells strongly depends on the electrolyte. By an example, initially, organic carbonates were used; however, it was found that they are unstable toward the nucleophilic attack by the ORR products. Therefore, the understanding and optimization of the EC cathode electrolyte interface are crucial for battery performance.

Very recent studies tackled the reactivity of the electrode surface by modeling each possible reaction of the surface with electrolyte molecules. As an example, the reactivity of propylene carbonate with basic species derived from the Li₂O₂ molecule in solution (Li₂O₂, LiO₂⁻, and O²⁻) and the (100) surface of Li₂O₂ in the presence of the propylene carbonate solvent was recently studied [26]. A wealth of information is provided on the chemical reactivity of the ORR products with the electrolyte. As much as 15 individual reaction paths were examined, combining classical and DFT potentials together with free and potential energy methods.

The complexity of such studies is increased when taking into account particular electronic features of the bulk and surfaces. Again, for Li–air batteries, it was demonstrated that the stable surfaces of Li₂O₂ could be oxygen rich and have a half-metallic character, while the bulk is an insulator [27].

In rechargeable lithium-ion batteries (LIB), in which both the anode and the cathode are intercalation electrodes, the conductivity depends on the positive and negative charge conduction rate both in the bulk of the electrode and on their surface. Schematic representation of a Li-ion cell, which is typically used in portable devices, is shown in Fig. 4b. Interestingly, LiFePO₄ was successfully commercialized as a cathode in rechargeable LIBs. However, its surface characterization and the impact of surface composition on Li transport properties have been almost ignored. Recently, this trend has changed. For instance, NEB calculations on a vacuum slab model have shown that the diffusion of Li on the surface has higher activation barrier (0.52–0.77 eV) than for diffusion barrier in the bulk (0.19–0.29 eV) [28].

Understanding the basic principles of operation of novel materials and devices for energy storage and conversion opens new horizons within theoretical material science. It allows designing new strategies – going beyond a mere trial and error procedure – for improving actual technology, developing new materials, and establishing basic relationships in the electrochemistry of interfaces and electrolytes.

The constant development of computational power and new theoretical methods enabled the modeling of EC phenomena purely on the basis of microscopic information of the atomistic structure and our knowledge of electronic phenomena. In particular, parameter-free approaches such as DFT are unique in providing a comprehensive description of the subtle interplay between thermal, electronic, and ionic effects at EI.

The most important progresses in the simulation of realistic interfaces for energy storage and conversion was achieved by the theoretical surface science community. The studies summarized above demonstrated how DFT simulations can provide crucial insight on the nature of key mechanisms at EC bulk and interfaces in batteries and fuel cells. However, many important methodological issues are still open in electrochemistry. For instance, the disclosure of the functional relationship between DL structure, potential drop at EI, and effect of an applied potential is only in its infancy, especially for complex EI.

In this entry, we have highlighted the recent conceptual evolution leading to the development of the most popular methodologies to describe EC phenomena and processes. We have shown how standard simulation methods – which use periodic boundary conditions and supercells – resort to charging the electrode to simulate the effect of an applied potential to an electrolytic cell and how this approach allowed achieving relevant progress in the last 10–20 years. We illustrated how all these models reach global charge neutrality and how most of them reproduce the localization of the electric field and potential energy drop within a microscopic distance from the metal surface.

However, none of these methods currently provide a fully realistic and self-consistent view of the electrostatic DL characterizing an EI, due to the small size of the samples, the artificial assumptions on charge distribution, the lack of description of the open nature of EC systems, and how their dynamical behavior is affected by an applied external potential. Importantly, the kinetics of redox processes at the electrode fully including the effect of the solvent environment is still mostly disregarded.

More realistic models for EIs are needed, possibly including explicit dynamical description of electrolyte solution at the interface with the electrode and its effects on the complex activated phenomena occurring within the DL.

A combination of classical electrochemistry, electronic structure, and classical surface science knowledge will be essential for addressing the still numerous open issues in this intrinsically multidisciplinary research area. Crucially, a substantial progress will be enabled by the increasing availability of computational power.

Finally, the understanding of atomistic and molecular scale processes that occur in EC devices and their interplay can only be achieved by addressing the relevant questions using theoretical models, experimental techniques, computational methodology, and associated simulations as complementary tools.