Nico Holmberg

Theoretical Insight into the Hydrogen Evolution Activity of Open-Ended Carbon Nanotubes

Carbon nanotubes (CNTs), while inactive by themselves, are often used as a platform in the search of new catalysts for the hydrogen evolution reaction (HER) by introducing metal nanoparticles or other dopants. Here, we examine the HER activity of pristine open-ended CNTs considering both the effects of chirality and hydrogen coverage using electronic structure calculations. The results indicate that the formation of different 5-ring structures at the end of the CNT introduces surface sites that are highly active toward HER, whereas the activity of traditional 6-ring sites is not greatly altered by tube termination. At fixed hydrogen coverage, the enhanced activity of these sites was attributed to valence orbitals residing close to the highest occupied molecular level facilitating electron transfer to protons.

Efficient Constrained Density Functional Theory Implementation for Simulation of Condensed Phase Electron Transfer Reactions

Constrained density functional theory (CDFT) is a versatile tool for probing the kinetics of electron transfer (ET) reactions. In this work, we present a well-scaling parallel CDFT implementation relying on a mixed basis set of Gaussian functions and plane waves, which has been specifically tailored to investigate condensed phase ET reactions using an explicit, quantum chemical representation of the solvent. The accuracy of our implementation is validated against previous theoretical results for predicting electronic couplings and charge transfer energies. Subsequently, we demonstrate the efficiency of our method by studying the intramolecular ET reaction of an organic mixed-valence compound in water using a CDFT based molecular dynamics simulation.

Ab initio Kinetic Monte Carlo simulations of dissolution at the NaCl–water interface

We have used ab initio molecular dynamics (AIMD) simulations to study the interaction of water with the NaCl surface. As expected, we find that water forms several ordered hydration layers, with the first hydration layer having water molecules aligned so that oxygen atoms are on average situated above Na sites. In an attempt to understand the dissolution of NaCl in water, we have then combined AIMD with constrained barrier searches, to calculate the dissolution energetics of Na(+) and Cl(-) ions from terraces, steps, corners and kinks of the (100) surface. We find that the barrier heights show a systematic reduction from the most stable flat terrace sites, through steps to the smallest barriers for corner and kink sites. Generally, the barriers for removal of Na(+) ions are slightly lower than for Cl(-) ions. Finally, we use our calculated barriers in a Kinetic Monte Carlo as a first order model of the dissolution process.

Ion Transport through a Water–Organic Solvent Liquid–Liquid Interface: A Simulation Study

Ion interactions and partitioning at the water-organic solvent interface and the solvation characteristics have been characterized by molecular dynamics simulations. More precisely, we study sodium cation transport through water-cyclohexane, water-1,2-dichloroethane, and water-pentanol interfaces, providing a systematic characterization of the ion interfacial behavior including barriers against entering the organic phase as well as characterization of the interfaces in the presence of the ions. We find a sodium depletion zone at the liquid-liquid interface and persistent hydration of the cation when entering the organic phase. The barrier against the cation entering the organic phase and ion hydration depend strongly on specific characteristics of the organic solvent. The strength of both barrier and hydration shell binding (persistence of the cation hydration) go with the polarity and the surface tension at the interface, that is, both decrease in order cyclohexane-water > 1,2-dichloroethane-water > pentanol-water. However, the size of the hydration shell measured in water molecules bound by the cation entering the less polar phase behaves oppositely, with the cation carrying most water to the pentanol phase and a much smaller in size, but very tightly bound water shell to cyclohexane. We discuss the implications of the observations for ion transport through the interface of immiscible or poorly miscible liquids and for materials of confined ion transport such as ion conduction membranes or biological ion channel activity.

Diabatic model for electrochemical hydrogen evolution based on constrained DFT configuration interaction

The accuracy of density functional theory (DFT) based kinetic models for electrocatalysis is diminished by spurious electron delocalization effects, which manifest as uncertainties in the predicted values of reaction and activation energies. In this work, we present a constrained DFT (CDFT) approach to alleviate overdelocalization effects in the Volmer-Heyrovsky mechanism of the hydrogen evolution reaction (HER). This method is applied a posteriori to configurations sampled along a reaction path to correct their relative stabilities. Concretely, the first step of this approach involves describing the reaction in terms of a set of diabatic states that are constructed by imposing suitable density constraints on the system. Refined reaction energy profiles are then recovered by performing a configuration interaction (CDFT-CI) calculation within the basis spanned by the diabatic states. After a careful validation of the proposed method, we examined HER catalysis on open-ended carbon nanotubes and discovered that CDFT-CI increased activation energies and decreased reaction energies relative to DFT predictions. We believe that a similar approach could also be adopted to treat overdelocalization effects in other electrocatalytic proton-coupled electron transfer reactions, e.g., in the oxygen reduction reaction.