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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - SPECTROCHEM (First-Principles Spectroscopies in Realistic Electrochemical Environments)

Teaser

Summary

The SPECTROCHEM project focused on the development and the application of computational tools to simulate spectroscopies in the presence of a realistic electrochemical environment. The interest in electrochemistry is motivated by the role that this discipline is expected to play in converting energy from renewable source into easily-storable chemical fuels. Understanding the microscopic details of working catalysts is crucial since this knowledge can boost the development of novel catalyst materials. Recent advances in various spectroscopic techniques have enabled the characterization of electrocatalysts in working electrochemical cells. Thus, operando surface-enhanced infrared and Raman spectroscopies and X-ray absorption spectroscopy are nowadays employed to investigate the catalystsâ€™ evolution under realistic conditions of applied potential.

On the theory side, first-principles methods have the potential to significantly contribute to the interpretation of the measured spectra. However, technical limitations hamper the use of fully-atomistic simulations in the presence of wet environments. The goal of this project has been two-fold. First, I have worked on the development and the implementation of continuum models to account for the presence of the solvent and the ions at electrochemical interfaces. This class of models enables an accurate description of electrolyte solutions without sacrificing the accuracy of first-principles methods for the electrode surface and its adsorbates. Second, we have combined these continuum models with computational spectroscopic tools to study relevant electrocatalytic processes. By comparing the results of our simulations to experimental data we have validated the accuracy of the proposed methodology and assessed the sensitivity of the results on the parameters of the model. Results of the calculations have proved useful to interpret measurements performed under operando conditions, enabling the identification of reaction intermediates in the reactions considered.

Work performed

The development of continuum solvation models entailed two sub-projects. First, I have worked on the implementation and the validation of a hierarchy of electrolyte models. I have tested their ability to reproduce a representative experimental observable that is very sensitive on the details of the diffuse layer. Results suggest that only the most elaborate among the tested models is able to qualitatively reproduce all the experimentally-observed features. Second, I have collaborated in the development and implementation of a continuum-solvation technique that allows to deal with porous and open structures, enabling the treatment of a wide class of extended systems in combination with implicit solvation. This strategy guarantees that pockets that are smaller than a solvent molecule remain dielectric free.

The application of the developed continuum-solvation models entailed four sub-projects. The first sub-project involved a collaboration with the experimental group of Dr Katrin Domke (Max Planck Institute for Polymer Research, Mainz). In the collaboration, we targeted a prototypical electrochemical reaction. From the comparison between computed vibrational frequencies and Raman shifts obtained through operando experiments, we have identified some intermediates on the metal electrode surface that are likely to take part in the reduction process. In a second sub-project, we have worked in close collaboration with the group of Prof. Yang Shao-Horn (MIT) on a highly-relevant electrocatalytic process, i.e. the CO2 reduction reaction. Three catalyst materials have been considered and theoretically-computed surface-enhanced infrared absorption cross-sections compared to corresponding operando experiments. CO adsorbates have been identified on the Pt and Au surfaces, while various adsorbates have been observed on Cu. The third subproject focused on another highly-relevant process, i.e. the oxygen evolution reaction. Iridium oxide (IrO2) is the reference catalyst for this process, and for this reason extensive experimental work has been conducted on this system. Simulations of operando X-ray absorption near-edge structure (XANES) spectra have been carried out and compared to available experimental data. The comparison supports the formation of electron-deficient surface oxygen species in the OER-relevant voltage regime. Furthermore, surface hydroxyl groups are suggested to be progressively oxidized at larger potentials, giving rise to a shift in the Ir absorption cross-section that qualitatively agrees with measurements. In a final sub-project, an additional application of the continuum solvation models that is not directly related to electrochemistry has been developed. In particular, the use of a continuum embedding has been found effective in stabilizing negatively-charged configurations of isolated systems. Note that such configurations are typically predicted to be unbound by standard functionals in DFT.

The main form of dissemination has been represented by publications in scientific journals. In order to make the results of the project broadly available to any interested researcher the articlesâ€™ preprints have been made available through a public repository (arXiv). I have participated to the spring meeting of the German Physical Society (DPG), presenting the results of this project in the focus session â€œFrontiers of Electronic Structure: Focus on the Interface Challengeâ€. I have also been invited to give a MARVEL Junior Seminar at EPFL, where I had the chance to disseminate results to young researchers from diverse scientific backgrounds. The slides of the presentations have been made publicly available through the ResearchGate platform. All the scientific software implementations have been made publicly available through the GitHub and GitLab ENVIRON repositories, and corresponding updates published on the ENVIRON website (www.quantum-environment.org).

Final results

Continuum solvation models have been traditionally employed as a computationally inexpensive approach to account for solvation effects in isolated systems. This class of methods have been recently reconsidered in the context of materials science, but in very few studies the performance of continuum models has been thoroughly investigated. We have performed a systematic investigation of the accuracy of a hierarchy of continuum electrolyte models and validated them against a representative observable that is very sensitive on the details of the diffuse layer (i.e. the differential capacitance).

The solvent-aware interface that has been developed in this project represents a novel strategy that could enable the automatic high-throughput screening of materials in implicit solvent without problematic behaviours with porous materials.

The coupling of the implemented continuum solvation models with the XSpectra software to calculate XANES cross-sections also represents a novel element that paves the way towards a more direct simulation of operando X-ray absorption measurements.

All the software implementations have been made public, so that interested researchers will be able to exploit and further develop them according to their needs. Our findings and the future investigations that our software implementations will make possible can contribute to the development of a deeper understanding of electrocatalysis, with relevant implications in energy conversion and storage.