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Workshop research field and scientific challenge

Electrocatalytic reactions are nowadays of paramount importance to realize the green energy transition and meet the goals of the Paris agreement, since many of the technological solutions which are considered key to achieve carbon neutrality are based on the use of electrochemical devices. An electrochemical cell is a device that is either (i) capable of obtaining energy from a chemical reaction or (ii) converting electrical energy into chemical fuel. In the first type of devices, the energy generated by a spontaneous chemical reaction is converted into electrical energy. This mechanism, for example, is at the heart of fuel-cell technology, wherein molecular hydrogen and oxygen are combined to form water, generating an electric current. Fuel-cells represent attractive technology for zero emission mobility since they can power electric vehicles using hydrogen as an energy source.   The second type of devices, known as electrolyzers, are electrochemical cells where an electrical energy input is supplied to drive chemical reactions which are not spontaneous. As an example, water electrolysis (water splitting) coupled to renewable energy sources is a promising method to produce green hydrogen with zero carbon emissions and even out fluctuations in the output of intermittent power generation (such as solar and wind). In the last years, significant efforts have been devoted to deepen the understanding of both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) occurring at the cathode and anode, respectively, in water electrolyzers. Other examples of reactions which have gained considerable attention in recent years include the electrochemical reduction of CO2 and N2. The first reaction typically aims to convert CO2 into chemical fuels or value-added products, while the second one strives to generate ammonia for the sustainable production of fertilizers.

In all electrochemical devices, catalysts are required to speed up the reaction kinetics while increasing the selectivity of the desired products. Efficiency and selectivity of an electrocatalytic process are key features to make electrocatalysis economically competitive and industrially relevant. Therefore, in recent years, much effort has been devoted to gain understanding of what occurs at the catalyst surface with the aim to control and improve electrochemical processes. In this context, computational electrocatalysis has emerged as an important field since it can provide fundamental mechanistic understanding of complex reactions occurring at an electrode surface. This field not only provides support for interpreting experimental observation but can be used to guide the design of new materials and predict the activity of electrocatalysts which have not been yet synthesized in the laboratory. Hence, computer simulations have the power to shed light on chemical processes at the nanometric and sub-nanometric scale, and as such represents a unique tool to boost the understanding and the optimization of electrochemical systems.  

A breakthrough in the simulation of electrocatalysts processes was the development of the so-called computational hydrogen electrode (CHE) approach in 2004 [2] (nearly 8,000 citations in Google Scholar), with the name CHE coined later by Peterson et al. [3]. The CHE method is a way to assess the Gibbs energy profile of an electrochemical reaction at a given catalyst surface. At the core of this approach is a specified reaction mechanism and the evaluation of the Gibbs energy of the corresponding elementary steps, ∆G = ∆H − T∆S. In this approach the Gibbs energy of individual component, Gi = Hi − TSi, of each of the elementary step (reaction intermediate) are calculated by electronic structure calculations (DFT or wavefunction-based methods). The evaluation of reaction energies via quantum mechanical calculations gives the approach predictive capabilities. Thanks to its simplicity and computational efficiency, the CHE model has enabled both in-depth studies of reaction pathways and large-scale computational screening studies, dedicated to identifying optimal catalyst formulations and structures. Yet, practical implementations of the CHE rely on a number of simplifications and approximations which are not necessarily always justified. First, the common characteristic of electrochemical processes described above is the importance of the interface between the electrode surfaces and the liquid electrolyte in which reagents (H2, O2, H2O, CO2, N2 … depending on the specific process) are dissolved. Most often the liquid phase is treated in a simplified way and solvation effects are typically evaluated either by inclusion of bilayer of “frozen” water molecules or through implicit solvation models. Such approaches do not grasp the complexity of electrode/liquid interface and limit the comprehension of the surface processes. Moreover, the presence of electrolyte species dissolved in the liquid phase is often neglected although they have recently been shown experimentally to play a major role in some reaction mechanisms. Another limitation of the CHE approach is that the external applied potential is not treated explicitly, and the influence of electrode voltage is included a posteriori to reaction Gibbs energy differences which involve electron transfer with either a dielectric continuum or a simplified atomistic representation of the solvent. The sequence of Gibbs energy differences is then used to estimate the minimum electrical potential Umin required to drive the reaction.  

Finally, the CHE approach does not consider the activation energy needed for the various elementary steps, only the Gibbs energy of metastable intermediates enters in the estimate of the electrical potential needed. It has become clear from recent calculations that energy barriers representing the activation energy which needs to be overcome can in fact determine the required potential.    

To address the above limitations in the field of computational electrocatalysis, we aim to organize of a Lorentz workshop on “Atomistic modelling of solid-liquid interfaces in electrocatalysis” wherein the main leaders in the field will discuss how to go beyond the state-of-the-art in the simulation of electrochemical processes at electrode surfaces by devising new approaches to simulate liquid-solid interfaces and improve the description and understanding of phenomena which are still poorly understood. This is a significant challenge as liquid-solid interfaces in the presence of an applied voltage calls for a major step forward in the current methods to describe electrocatalytic processes.

Workshop aim

This workshop aims to tackle the complexity of electrode-electrolyte interfaces for relevant applications in electrocatalysis by bringing together physicists, chemists, electrochemists, materials scientists, and engineers with complementary expertise in computational modelling of: (i) solid surfaces and solid-liquid interfaces; (ii) complex liquid phases (dielectric solutions); (iii) electrocatalysis and (iv) machine learning. The main focus of the event will be to discuss the state-of-the-art and beyond in the development of approaches and methods for the accurate modelling of electrochemical processes. Some of the biggest challenges in the field, that we aim at addressing during the workshop are: 

1)    Improvement of implicit solvation models
2)    The inclusion of the explicit liquid phase in the simulations
3)    Study of the effect of the electrolyte solution on the catalysis
4)    The inclusion of an external applied potential

These aspects are important to achieve an accurate, quantitative description of electrochemical processes which can be directly correlated with experimental investigations. Indeed, if successful, this methodological development would have important implications for the development of energy storage and conversion devices, including fuel cells, batteries, supercapacitors, and electrolyzers for green H2 production, and CO2/N2 conversion into value-added chemicals.  

This workshop will be considered a success if it produces a clear path towards a hierarchy of computational approaches which go beyond the CHE approach, where the effect of applied potential is explicitly included in calculations of Gibbs reaction activation energies for the various possible reaction mechanisms, ranging from faster and more approximate approaches useful for screening purposes to more detailed and computationally demanding approaches that can reach the level of accuracy needed for reliable predictions of electrocatalytic activity.