Multiscale Modelling of Structural Transformation in Metal Nanocatalysts for CO2 Electroreduction
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The precise control of nanostructure and surface atomic arrangement can be used to tune the electrocatalytic properties of materials and improve their performance. Unfortunately, the long-term structural stability of electrocatalysts with complex nanoscale morphology, a necessary requirement for industrial implementation, often remains elusive. This work explores structural changes in complex cathodic nanoscale electrocatalysts with and without oxidation state changes during CO2 reduction reaction (CO2RR) or hydrogen evolution reaction (HER). Based on the experimentally obtained structural and surface analyses, density functional theory (DFT) calculations and finite element method (FEM) simulations, I elucidate the mechanisms of the structural dynamics in electrocatalysts under bias, demonstrating that the reaction intermediates (RIs) of electrolysis reactions, the electrocatalyst material and the current density distribution are the crucial factors in influencing the structural transformations in electrocatalysts. In chapter 3, I focus on the structural behaviour of hydroxide-derived copper during CO2RR and well-defined gold and palladium core cages and branched nanoparticles in the course of CO2RR and HER conditions using ex situ high-resolution scanning electron microscopy and electrochemical surface analysis via underpotential deposition of lead. First, the structural transformation with the oxidation state change were observed in Cu(II)-nanoparticle-derived copper electrodes when producing them by electrochemically reducing the precursor of Cu(OH)2 under CO2RR conditions, with the structural changes in this case directed by the interplay between facet stabilization by CO2RR intermediates, electrochemical Ostwald ripening and field-induced reagent concentration effect. Moreover, the structural behavior of well-defined gold and palladium nanoparticles are explored under electrolysis reactions of CO2RR and HER. The morphological changes were also observed in these well-defined nanoparticles without oxidation state change under both electrolysis conditions, with more pronounced morphological changes observed in specific localities of complex nanoscale electrocatalysts (e.g., narrow constrictions). More importantly, the structural changes of the studied nanoparticles are accelerated by the RIs of the electrolysis reactions, with the extent and rates of structural transformations depending on the material of the nanostructure and the nature of the environment and electrocatalytic reaction at its surface. The experimental observations discussed in Chapter 3 revealed a significant influence of the surface reactions on the restructuring of nanoparticles, which motivated me to perform a DFT analysis of the RIs on the mobility of different metals. Thus, in chapter 4, I explore the parameters affecting the mobility of the surface metal atoms using DFT and MD calculations. Specifically, I consider several electrocatalyst compositions of common interest in CO2RR electrocatalyst design: Cu, Ag, Au, Pd, and Cu3Pd, and the key RI associated with CO2 reduction and accompanying reactions, i.e., HER and oxygen reduction reaction: *COOH, *H, and *OOH. Using DFT calculations, I demonstrate that that RIs may promote the thermodynamic process of vacancy formation (VF) and accelerate the migration kinetics of the adatom on crystallographic facets of Au and Pd. In addition to the thermodynamic VF assessment, I also expand the atomic mobility assessment method to enable the evaluation of both thermodynamics and kinetics of VF via DFT and to probe the stability of a complex surface structure (compared to low-index facets) mimicking nanostructured catalysts using ab initio molecular dynamic (AIMD) simulations. Based on the atomic mobility assessment for a series of metal and metal-alloy CO2RR catalysts, this chapter provides a more comprehensive description of atomic mobility induced by RIs during CO2RR electrolysis. The DFT calculations performed in Chapter 4 demonstrated the influence of the RIs and material on the atomic mobility in electrocatalysts, however, which cannot explain the locality of the structural changes in the morphologically complex nanoparticles observed in Chapter 3. To understand the origins of the locality and directionality of atomic mobility in nanocatalysts, I decide to investigate the electrochemical physics effects of the studied nanostructures. Therefore, in chapter 5, I summarize the current status and recent advances in the theoretical models used for the electric field (E-field), reaction current density (J_electrolyte), electrode current density (J_electrode) simulations using FEM, propose the protocols for reliable simulations of these electrochemical effects, and study the electrochemical performances various nanostructured electrodes with complex morphologies. In section 2 of this chapter, I discuss the setup of the FEM simulation domains and the interfaces of Comsol Multiphysics required for the simulations performed in this chapter. In section 3, I introduce the fundamentals relevant to the essential electrochemical phenomena at the electrode-electrolyte interface, the classical theories for modelling these electrochemical effects, and the advanced modifications to these classical theories to account for the steric effect of the solution species and field-dependent dielectric function of the electrolyte. In section 4-6, I describe a protocol for simulating the E-field, J_electrolyte and J_electrode under different electrolysis conditions. In section 7, I demonstrate simulations on the specific electrode models with complex shapes that represent nanoparticle-based electrodes. In section 8, I compare the electrochemical performances, especially the current density, of various nanostructured electrodes including anchored nanostar, nanostar, core-cages and frames. In section 9, I discuss the application scope of the models involved in this work as well as their limitations. This chapter provides a useful departure point for electrocatalysis researchers to begin implementing FEM in their work and will facilitate a wider adoption of computational studies and rational design of nanoscale effects in electrocatalysts to further improve the performance of the electrocatalysts for CO2RR and other clean energy conversion reactions. In summary, using a series of complex nanostructured electrodes, this thesis demonstrates that RIs of electrolysis reactions, the nature of the electrocatalyst material and the current density distribution are the important factors determining the extent of and trends in structural transformation of electrocatalysts under electrolysis conditions. Furthermore, the mechanistic descriptions of the impact of the factors mentioned above on the structural transformation are revealed using DFT and FEM simulations. Moreover, this thesis establishes a general framework for evaluating the structural transformations in cathodic metal nanocatalysts and explain specific qualitative trends. In chapter 6, I provide an outlook for future work on both improving the predictive power of the framework and expanding the scope of nanoscale electrocatalysts and reactions it is applied to. In conjunction with catalyst design rules, this mechanistic framework will facilitate the development of nanostructured electrocatalysts with sufficient stability for sustainable applications.
Cite this version of the work
Feng Li (2022). Multiscale Modelling of Structural Transformation in Metal Nanocatalysts for CO2 Electroreduction. UWSpace. http://hdl.handle.net/10012/18540