Computational modeling of electrochemical systems has gained significant interest over the past decade. In this talk, we will focus upon how experimental data can be used in continuum modeling, best-practices and lessons learned, as well as emergent continuum-scale approaches that resolve pore-level phenomena and provide macroscopic design and operation recommendations.

     To provide context for some of these ideas, some of our past models that utilize experimental/computational collaborations and/or novel computational approaches will be highlighted. In particular, we will discuss our recently developed pore-scale model of an ion-exchange membrane. Ion exchange membranes (IEMs) are crucial to the efficient operation of many electrochemical devices but detailed understanding of the microscopic transport mechanisms within an IEM remain elusive. Volume-averaged continuum modeling approaches have typically been applied to the entire IEM domain and are useful for macroscopic properties, however the water domains thought to be responsible for the bulk of ionic transport have rarely been modeled explicitly. In this contribution, we build upon previous modeling efforts and assume that water domains can be modeled as cylindrical, charged pores. We develop a generalizable, two-dimensional continuum model of a water domain through an ion exchange membrane using a modified Poisson-Nernst-Planck framework. Our model incorporates solvent transport, migration, diffusion, adaptive permittivity and viscosity models, and the finite-size effects of co- and counter-ions. By using our model to simulate transport under different operating conditions, we can visualize resultant spatial profiles of concentration and potential within a nanopore.  Additionally, we quantify the relative contribution of each transport mechanism to the flux of co- and counter-ions through pores of varying properties and demonstrate the utility of this model through its adaptability to its many applications for electrosynthesis, carbon removal, and fuel cell technologies.

     Finally, as electrochemical processes are inherently multi-scale, we will provide our perspective and upcoming work on opportunities for coupling continuum modeling with other time- and length-scales for a deeper understanding of electrochemical transport phenomena.

To become more CO₂ neutral, Europe’s energy supply system and its chemical industry are getting more and more electrified. H₂ and products from CO₂ are envisioned as a suitable long-term energy storage option. They can be produced with high efficiencies by electrolysis. Yet, electrolysis processes account for only a small fraction of all chemical production processes. Over the next few decades, we expect to see a systematic and large-scale ramp-up of electrolysis for the production of various e-Fuels and chemicals.

A requirement for the establishing competitive electrolysis processes is a thorough, quantitative understanding and optimization of catalyst and electrode processes. This includes the identification of experimentally validated electrolysis models and, in particular, the model-based analysis and optimization of the electrodes. To date, there are few continuum-level models that include adequate reaction kinetics to describe electrochemical performance and product selectivity.

This presentation will show how suitable kinetic models and model parameters can be determined for a wide range of electrosynthesis processes: PEM water electrolysis [1-3], CO₂ reduction in aqueous [4], and organic electrolyte [5]. In some of these applications, electrochemical reaction kinetics play a major role, whereas others suffer from slow sorption processes carbonation reactions in the electrolyte, gas/liquid phase equilibria or slow transport processes.

Dynamic measurements, such as cyclic voltammetry and chronoamperometry/potentiometry, are well reproduced by the models, and the underlying processes that cause a characteristic dynamic response are revealed. Crucially, the models are then used to identify the performance-limiting processes among the various reaction and transport processes, and measures are proposed to improve the performance of the electrodes. The kinetic models are essential tools and building blocks for the model-based design and optimization and condition diagnosis of electrolysis cells.

Zero-gap membrane electrode-based CO/CO₂ electrolysis, powered by renewable energy sources, presents a promising avenue for achieving sustainable production of key building-block chemicals, including ethanol and ethylene. Nevertheless, achieving the capability to operate at industrially relevant current densities exceeding 200 mA/cm² and maintaining stable performance for extended periods up to 1000 hours demands substantial further development and understanding.

This presentation will focus on the application of in-operando X-ray technology to elucidate degradation mechanisms within a zero-gap membrane electrode CO/CO₂ electrolysis system. ^[1-3] In the context of CO₂ electrolysis, the transport mechanisms of cations and water inside a membrane electrode assembly during operation will be discussed. The dynamic behavior of cations and water is linked to flooding issues and salt precipitation in the gas diffusion layer (GDL), which leads to performance degradation. ^[1-2] For CO electrolysis, our findings reveal that GDL flooding, the potential presence of metal contaminants at the cathode, and the anodic oxidation of liquid products at the anode cause changes in selectivity during long-term stability tests.^[3] Some appropriate strategies are demonstrated to mitigate some of these issues.

Adsorption of ions and hydrophobic solutes are key processes in electrochemistry. The former regulates the Electric Double Layer and dictates important quantities, e.g. capacitance. The latter has major implications for heterogeneous catalysis, where small hydrophobic molecules are commonly involved as reactants and products. Despite solvation of charged and hydrophobic species are remarkably different, I will show in this presentation that the exotic way hydrophobicity arises at electrified metal-water interfaces influences both of them with a common underlying molecular mechanism.1-3 

I will first show from classical molecular dynamics1 that the peculiar molecular arrangement of electrified gold/water interfaces induces atypical fluctuations of the liquid water density, resulting in a hydrophobic water-water interface formed close to the metal. I will then illustrate how such hydrophobicity dictates solvation free energy and regulates the accumulation of hydrophobic solutes (e.g. CO)1,3, as well as some ions (e.g. Cl-)2, at the interface. I will finally discuss some implication of these findings for electrochemical reactions involving hydrophobic molecules, with examples for CO2 and N2 reduction, as well as acid-base chemistry.1,5

[1] A. Serva, M. Salanne, M. Havenith, S. Pezzotti. PNAS 2021,118, e2023867118.

[2] S. R. Alfarano, S. Pezzotti, C. Stein, et al. PNAS 2021, 118, e2108568118.

[3] A. Serva, S. Pezzotti, J. Chem. Phys. 2024, 160, 9.

[4] A. Serva, M. Havenith, S. Pezzotti. J. Chem. Phys. 2021, 155, 204706.

[5] S. Murke, W. Chen, S. Pezzotti, M. Havenith. J. Am. Chem. Soc 2024, 146, 12423.

Ammonia (NH₃) is a crucial feedstock used across many sectors, ranging from the production of fertilizers to fine chemicals, and it is also a promising hydrogen carrier for decarbonizing hard-to-abate sectors. An alternative to its main production route, the Haber-Bosch process, is the electrochemical reduction of nitrate (NO₃⁻), which is a pervasive water pollutant. Cu-based electrodes have demonstrated excellent activity and selectivity towards ammonia, preventing undesired pathways such as dinitrogen (N₂) production. However, the cathodic potentials applied during operation induce significant reconstructions on the electrode, which are exacerbated by nitrate's strong oxidizing nature [1,2], thus adding substantial challenges to their accurate modelling and understanding. In this presentation, I will discuss our advances in elucidating the reaction mechanism of nitrate reduction to ammonia on Cu-based catalysts. To this end, we first mapped the complete reaction network, including key adsorbed intermediates such as nitrite (NO₂*), nitrogen (N*), and hydroxylamine (NH₂OH*). Then, by using ab-initio methods based on Density Functional Theory, we obtained the full energy profile including all intermediates and relevant transition states. These elements were wrapped up into a transient-state microkinetic model to identify the dominant reaction pathways and assess the influence of both the reaction environment (including solvent, pH, and electric potential) and the catalyst's history. This study paves the way for a comprehensive understanding of complex reaction networks and their interactions with the reaction environment.

Molecular dynamics can be a powerful tool to study the properties of electrochemical interfaces.

In this talk, I will take you on a walk through the atomic scale world that unfolds at the surface of metallic electrodes. We will start by visiting a forest of adsorbates, including H and OH. In this context, I will discuss why even adsorbed hydrogen on platinum surfaces, which is probably one of the most studied electrochemical interfaces, still bares its secrets. We will then go further and view the influence of adsorbed OH on the operando electrochemical interface. Here, I will ask the question why the presence of OH can (not) change everything. Finally, we will look further up from the surface and scrutinize the behavior of ions at the interface. Can molecular dynamics help us unravelling at which height the ions reside?  What happens to the solvation structure of the ions as they approach the interface? And how closely can ions pack at charged metal surfaces?

All this information can be used to better understand electrochemical processes at the interface.

Under electrochemical conditions the catalyst might change composition, phase and deactivate. Although all these processes are crucial to the understanding of the materials and processes and the success of the technologies little is known of the detailed processes taking place. In the presentation I will revise a few cases of rearrangement of materials under reaction conditions and electrochemical environment. These will include the activation of materials for the oxygen evolution reaction (OER) and the rearragement copper based catalysts in CO2 reduction (eCO2RR). The changes in the properties and the pretreatment of the materials can control the selectivity thus calling for more advanced simulations that can take into account these structural modifications. This opens the way to a new, more complex view on the catalytic activity under true reaction conditions and also provides information on how to avoid deactivation paths that might lead to catastrophic failure in the actual devices.

Oxide-water interfaces host many chemical reactions in nature and industry. There, reaction free energies markedly differ from bulk. While we can experimentally and theoretically measure these changes, we are often unable to address the fundamental question: what catalyses these reactions? Recent studies suggest that surface and electrostatics contributions are insufficient to answer. The interface modulates chemistry in subtle ways. Revealing them is essential to understanding interfacial reactions, hence improving industrial processes. Here, we introduce a thermodynamic approach combined with cavitation free energy analysis to disentangle the driving forces at play. We find water dictates chemistry via large variations of cavitation free energies across the interface. The resulting driving forces are both large enough to determine reaction output and highly tunable by adjusting interface composition, as showcased for silica-water interfaces. These findings shift the focus from common interpretations based on surface and electrostatics, and open exciting perspectives for regulating interfacial chemistry.

Electrochemical CO₂ reduction (eCO₂R) is a promising technology to store renewable energy into chemical bonds and close the carbon cycle. However, to date its industrial exploitation is limited to the production of CO and HCOO^–, whilst CO₂ conversion to C₂₊ chemicals is possible only on the laboratory scale.¹ Low C₂₊ activity and selectivity are due as well to poor understanding of the CO₂ reduction pathways beyond C₁ products, and consequently of the active sites responsible for multi-carbon products, mainly ethanol, ethylene, and n-propanol.² Spectroscopy study can significantly enhance this understanding, by allowing the detection of reaction intermediates on the surface, and density functional theory (DFT) can support the assignment of these spectroscopic signals.³

Here, by means of DFT simulations, we unveil reaction mechanisms and active sites to produce ethylene and ethanol during the electrochemical reduction of CO₂ by supporting the assignment of Raman spectroscopic signals.^4,5 In the first study,⁴ we assessed CO coverage effects on Cu(100). At high CO surface coverage (> 0.5 ML), average CO binding energy decreases due to CO-CO repulsive interactions and the abundance of weakly adsorbed CO on atop sites. Besides, vibrational analysis shows that the Cu-CO stretching peak increases to the detriment of the C=O rotation peak at high CO coverages, in line with analogous experimental observations within the ethylene selectivity window. By employing the Hammer’s decomposition scheme, we confirmed that surface atop CO intermediates are the most reactive species to form the CO-CO dimer, having a very endothermic rebond energy (i.e. proxy of dimer dissociation). In the second work,⁵ we focused on the selectivity switch between ethanol and ethylene occurring during eCO₂R at around –0.8 V vs RHE on oxide-derive copper nanocubes. At this applied potential, four vibrational fingerprints are detected via surface-enhanced Raman spectroscopy (SERS), namely 1182, 1318, 1453, and 1595 cm^–1. Density functional theory simulations and further reduction studies of selective precursors attribute these vibrations to the *OCHCH₂ intermediate, the first selective precursor to ethanol and n-propanol. The formation of this intermediate is favored by distorted Cu active sites with low s-band states, which stabilize the terminal oxygen.

Based on the insights here proposed, we call for an updated reaction mechanism for CO₂ reduction to ethylene and ethanol. Both these products require high CO coverage and undercoordinated sites, which facilitates the CO-CO dimerization step, while distorted Cu atoms stabilize *OCHCH₂, opening the selective route to ethanol and n-propanol. These guidelines support the rational design of electrocatalysts to maximize ethanol and ethylene selectivity independently.