Theory of Electronic and Ionic Perturbations at Supported Electrocatalyst Nanoparticles

The shift toward a defossilized economy and sustainable society depends on the development of high-performance and cost-effective energy storage and conversion technologies, such as fuel cells and electrolyzers. Electrochemical processes in these devices generally require the use of electrocatalysts to accelerate reactions towards certain products, yet current materials require improvements in activity, selectivity, and stability while remaining sufficiently cheap. Achieving these advancements necessitates rational design and knowledge-based optimization, which hinge on a fundamental understanding of the underlying physical and chemical processes. Theory and computation offer a strong complement of experimental studies, allowing for indepth understanding of catalytic mechanisms and structure–property relationships. Since electrocatalytic reactions occur at the electrical double layer (EDL) at the interface between a solid electrode and a liquid electrolyte, extensive simulation studies have focused on the structure and dynamics of the EDL. However, most of these studies have been limited to idealized homogeneous planar electrodes, whereas real electrocatalysts typically consist of nanoparticles (NPs) dispersed on support materials to maximize active surface area and thereby minimize loading. Though insights from planar electrodes provide a basic understanding, they overlook key features of supported NP electrocatalysts, including nanoscopic heterogeneities in composition and structure. At the NP–support interface, electron redistribution equilibrates Fermi levels, while at the NP–electrolyte and support– electrolyte interfaces, two EDLs overlap and codetermine the electric potential and ion distribution. Notably, the intricate interplay between electronic interactions and ionic interactions introduces complexities beyond the capabilities of existing methodologies. This thesis addresses these challenges by developing a semiclassical continuum model within the framework of density–potential functional theory. The model captures correlated electronic and ionic equilibration across NPs, support, and electrolyte, and allows highly efficient simulations under both constant-charge and constant-potential conditions. Due to the availability of reliable experimental data on the differential capacitance of gold (Au) and silver (Ag) electrodes, they are chosen as the model system, with Ag NPs supported on an Au surface. Simulation results reveal that Fermi-level equilibration induces electron redistribution not only at the NP–support interface but also at their respective external surfaces in contact with the electrolyte. The electric field in electrolyte surrounding the supported NP leads to ionic charge separation. This peculiar behavior of external-surface charging and ionic separation seen in supported NP electrodes, validated through firstprinciples calculations, can be no longer described by the classical concept of the potential of zero charge (PZC) for planar electrodes. To address this, I define a global and two local characteristic electrode potential. The global PZC characterizes the overall charge-neutral state of the supported NP, while at the local level, support-induced perturbations in electronic and ionic charge densities at the NP’s active surface give rise to two new characteristic potentials: the potential of zero local electronic charge (PZLeC) and the potential of zero local ionic charge (PZLiC). PZLeC and PZLiC differ by more than 0.5 V in dilute electrolytes but converge to the PZC in concentrated solution. Furthermore, I demonstrate that the differential capacitance curve can exhibit either one minimum or multiple minima, depending on the NP size. Local ion concentration, pH, electric potential and field, subsumed as local reaction conditions, or local reaction environment, are crucial to catalyst activity, selectivity and stability. To better rationalize the local reaction conditions in supported NP electrodes, I introduce a descriptor, the effective reactant concentration, defined as the average reactant concentration over the reaction plane around supported NPs. Using Au-supported Ag NPs as a model system, I investigate how support material, NP size, NP coverage on support, bulk ion concentration and electrode potential affect this descriptor. The methodology presented in this thesis enables accurate modelling of local reaction conditions at mesoscopic electrochemical interfaces at low computational cost. By providing a continuum-level description, this framework captures essential features of heterogeneous, multicomponent mesostructures while avoiding the complexity and computational cost of atomistic simulations. The insights gained contribute to the development of enhanced supported electrocatalyst systems and offer general relevance for understanding electrochemical properties in such systems. Follow-up collaborative work with experimentalists aims to visually demonstrate our simulation results on charge redistribution using electron holography. However, open questions remain regarding how to integrate actual catalytic processes into this framework, particularly chemical bonding phenomena that require atomistic-level descriptions. Future work will focus on bridging the gap between continuum modeling and atomistic simulations to achieve a more comprehensive understanding of the interplay between mesoscopic reaction conditions and molecular-scale catalytic mechanisms.