Potential-Dependent Pt(111)/Water Interface: Tackling the Challenge of a Consistent Treatment of Electrochemical Interfaces.

The interface between an electrode and an electrolyte is where electrochemical processes take place for countless technologically important applications. Despite its high relevance and intense efforts to elucidate it, a description of the interfacial structure and, in particular, the dynamics of the electric double layer at the atomic level is still lacking. Here we present reactive force-field molecular dynamics simulations of electrified Pt(111)/water interfaces, shedding light on the orientation of water molecules in the vicinity of the Pt(111) surface, taking into account the influence of potential, adsorbates, and ions simultaneously. We obtain a shift in the preferred orientation of water in the surface oxidation potential region, which breaks with the previously proclaimed strict correlation to the free charge density. Moreover, the characterization is complemented by course of the entropy and the intermolecular ordering in the interfacial region complements the characterization. Our work contributes to the ongoing process of understanding electric double layers and, in particular, the structure of the electrified Pt(111)/water interface, and aims to provide insights into the electrochemical processes occurring there.

In silico characterization of nanoparticles.

Nanoparticles (NPs) make for intriguing heterogeneous catalysts due to their large active surface area and excellent and often size-dependent catalytic properties that emerge from a multitude of chemically different surface reaction sites. NP catalysts are, in principle, also highly tunable: even small changes to the NP size or surface facet composition, doping with heteroatoms, or changes of the supporting material can significantly alter their physicochemical properties. Because synthesis of size- and shape-controlled NP catalysts is challenging, the ability to computationally predict the most favorable NP structures for a catalytic reaction of interest is an in-demand skill that can help accelerate and streamline the material optimization process. Fundamentally, simulations of NP model systems present unique challenges to computational scientists. Not only must considerable methodological hurdles be overcome in performing calculations with hundreds to thousands of atoms while retaining appropriate accuracy to be able to probe the desired properties. Also, the data generated by simulations of NPs are typically more complex than data from simulations of, for example, single crystal surface models, and therefore often require different data analysis strategies. To this end, the present work aims to review analytical methods and data analysis strategies that have proven useful in extracting thermodynamic trends from NP simulations.

Simulations of the Oxidation and Degradation of Platinum Electrocatalysts.

Improved understanding of the fundamental processes leading to degradation of platinum nanoparticle electrocatalysts is essential to the continued advancement of their catalytic activity and stability. To this end, the oxidation of platinum nanoparticles is simulated using a ReaxFF reactive force field within a grand-canonical Monte Carlo scheme. 2-4 nm cuboctahedral particles serve as model systems, for which electrochemical potential-dependent phase diagrams are constructed from the thermodynamically most stable oxide structures, including solvation and thermochemical contributions. Calculations in this study suggest that surface oxide structures should become thermodynamically stable at voltages around 0.80-0.85 V versus standard hydrogen electrode, which corresponds to typical fuel cell operating conditions. The potential presence of a surface oxide during catalysis is usually not accounted for in theoretical studies of Pt electrocatalysts. Beyond 1.1 V, fragmentation of the catalyst particles into [Pt6 O8 ]4- clusters is observed. Density functional theory calculations confirm that [Pt6 O8 ]4- is indeed stable and hydrophilic. These results suggest that the formation of [Pt6 O8 ]4- may play an important role in platinum catalyst degradation as well as the electromotoric transport of Pt2+/4+ ions in fuel cells.

Grand Canonical ReaxFF Molecular Dynamics Simulations for Catalytic Reactions.

In order to study the time-dependent behavior of catalytic systems during operation, we have developed a grand canonical molecular dynamics approach based on the ReaxFF reactive force-field framework. After describing the details of the implementation, the capabilities of this method are demonstrated by studying the gas-phase water formation from oxygen and hydrogen on platinum catalysts during the steady state where we discuss the effects of the surface structure as well as the importance of kinetics. The approach presented here can be extended to other dynamic (catalytic) systems, providing a framework for exploring catalytic and electrocatalytic processes, in particular, allowing studies on the effects of reaction conditions on a system's behavior, characteristics, and stability.