Karen Chan

Designing an improved transition metal phosphide catalyst for hydrogen evolution using experimental and theoretical trends

Transition metal phosphides (TMPs) have emerged as highly active catalysts for the hydrogen evolution reaction (HER). However, insights into the trends and limitations in their activity are lacking, and there are presently no guidelines for systematically improving their intrinsic activity. The complexity and variations in their structures further pose challenges in theoretically estimating their activity. Herein, we demonstrate a combined experimental–theoretical approach: by synthesizing different TMPs and comparing experimentally determined HER activities with the hydrogen adsorption free energies, ΔGH, calculated by density functional theory, we determine the level of detail needed in the simulations to bring out useful trends in the experimental data. In particular, we show that the TMPs follow the HER volcano relationship. Using our combined experimental–theoretical model, we predict that the mixed metal TMP, Fe0.5Co0.5P, should have a near-optimal ΔGH. We synthesized several mixtures of Co and Fe phosphides alloys and confirmed that Fe0.5Co0.5P exhibits the highest HER activity of the investigated TMPs. Furthermore, our results suggest that there could be inherent limitations in the intrinsic HER activity of TMPs that prevent them from performing as well as Pt-group metals. Our work demonstrates that it is possible to generate and verify a model of activity trends with predictive capabilities even for new transition metal compounds with varied structures and surface terminations. The identification of an improved mixed metal TMP based on theoretical predictions and subsequent synthesis and testing demonstrates the need for an approach that combines theory and experiment to understand and ultimately design advanced catalysts.

Transition-metal doped edge sites in vertically aligned MoS2 catalysts for enhanced hydrogen evolution

Highly active and low-cost catalysts for electrochemical reactions such as the hydrogen evolution reaction (HER) are crucial for the development of efficient energy conversion and storage technologies. Theoretical simulations have been instrumental in revealing the correlations between the electronic structure of materials and their catalytic activity, and guide the prediction and development of improved catalysts. However, difficulties in accurately engineering the desired atomic sites lead to challenges in making direct comparisons between experimental and theoretical results. In MoS2, the Mo-edge has been demonstrated to be active for HER whereas the S-edge is inert. Using a computational descriptor-based approach, we predict that by incorporating transition metal atoms (Fe, Co, Ni, or Cu) the S-edge site should also become HER active. Vertically standing, edge-terminated MoS2 nanofilms provide a well-defined model system for verifying these predictions. The transition metal doped MoS2 nanofilms show an increase in exchange current densities by at least two-fold, in agreement with the theoretical calculations. This work opens up further opportunities for improving electrochemical catalysts by incorporating promoters into particular atomic sites, and for using well-defined systems in order to understand the origin of the promotion effects.

Theoretical Insights into a CO Dimerization Mechanism in CO2 Electroreduction

In this work, we present DFT simulations that demonstrate the ability of Cu to catalyze CO dimerization in CO2 and CO electroreduction. We describe a previously unreported CO dimer configuration that is uniquely stabilized by a charged water layer on both Cu(111) and Cu(100). Without this charged water layer at the metal surface, the formation of the CO dimer is prohibitively endergonic. Our calculations also demonstrate that dimerization should have a lower activation barrier on Cu(100) than Cu(111), which, along with a more exergonic adsorption energy and a corresponding higher coverage of *CO, is consistent with experimental observations that Cu(100) has a high activity for C-C coupling at low overpotentials. We also demonstrate that this effect is present with cations other than H(+), a finding that is consistent with the experimentally observed pH independence of C2 formation on Cu.

Molybdenum Sulfides and Selenides as Possible Electrocatalysts for CO2 Reduction

Linear scaling relations between reaction intermediates pose a fundamental limitation to the CO2 reduction activity of transition‐metal catalysts. To design improved catalysts, we propose to break these scaling relations by binding key reaction intermediates to different sites. Using density functional theory, we demonstrate this principle in the active edge sites in MoS2, MoSe2, and Ni‐doped MoS2. These edges show the unique property of selectively binding COOH and CHO to bridging S or Se atoms and CO to the metal atom. DFT calculations suggest a significant improvement in CO2 reduction activity over the transition metals. Our results point to the broader application of the active edge sites of transition‐metal dichalcogenides in complex electrochemical processes.

Rational design of MoS2 catalysts: tuning the structure and activity via transition metal doping

Density functional theory was used to study how transition metal doping could be used as a method for systematically fine-tuning the structure and activity of MoS2 catalysts. Through studying the hydrogen evolution reaction (HER) on the edge sites, the role of the metal dopant was determined to be in modifying the strength of sulfur binding on the edge, which determines hydrogen binding onto sulfur atoms on the edge through a negative linear scaling. A simple thermodynamic quantity, ΔGS, is thus identified, which allows for a description of both the stable structure and adsorption at the edge sites. This provides a descriptor-based framework for the rational design of new MoS2-type catalysts, where a metal dopant can be chosen to either strengthen or weaken the binding of key intermediates as desired. We also elucidate the unique coverage dependence of hydrogen binding, which explains why MoS2-type catalysts tend to have near-optimal hydrogen binding. These results are expected to be more general and easily extended to other reactions on other layered transition metal dichalcogenides. Besides confirming the high HER activity of previously studied MoS2 catalysts, we find 6 additional candidates that show marked improvement in hydrogen adsorption free energies over pristine MoS2.

Electrochemical Activation of CO2 through Atomic Ordering Transformations of AuCu Nanoparticles

Precise control of elemental configurations within multimetallic nanoparticles (NPs) could enable access to functional nanomaterials with significant performance benefits. This can be achieved down to the atomic level by the disorder-to-order transformation of individual NPs. Here, by systematically controlling the ordering degree, we show that the atomic ordering transformation, applied to AuCu NPs, activates them to perform as selective electrocatalysts for CO2 reduction. In contrast to the disordered alloy NP, which is catalytically active for hydrogen evolution, ordered AuCu NPs selectively converted CO2 to CO at faradaic efficiency reaching 80%. CO formation could be achieved with a reduction in overpotential of ∼200 mV, and catalytic turnover was enhanced by 3.2-fold. In comparison to those obtained with a pure gold catalyst, mass activities could be improved as well. Atomic-level structural investigations revealed three atomic gold layers over the intermetallic core to be sufficient for enhanced catalytic behavior, which is further supported by DFT analysis.

Electrochemical Barriers Made Simple.

A major challenge in the theoretical treatment of electrochemical charge transfer barriers is that simulations are performed at constant charge, which leads to dramatic potential shifts along the reaction path. Real electrochemical systems, however, operate at constant potential, which corresponds to a hypothetical model system of infinite size. Previous studies of hydrogen evolution have relied on a computationally costly scheme that extrapolates the barriers calculated on increasingly larger cells, and extension of this scheme to more complex reactions would be prohibitively costly. We present a new method to determine constant potential reaction energetics for simple charge transfer reactions that requires only (1) a single barrier calculation in an electrochemical environment and (2) the corresponding surface charge at the initial, transition, and final states. This method allows for a tremendous reduction in the computational resources required to determine electrochemical barriers and paves the way for a rigorous DFT-based kinetic analysis of electrochemical reactions beyond hydrogen evolution.

Understanding trends in electrochemical carbon dioxide reduction rates

Electrochemical carbon dioxide reduction to fuels presents one of the great challenges in chemistry. Herein we present an understanding of trends in electrocatalytic activity for carbon dioxide reduction over different metal catalysts that rationalize a number of experimental observations including the selectivity with respect to the competing hydrogen evolution reaction. We also identify two design criteria for more active catalysts. The understanding is based on density functional theory calculations of activation energies for electrochemical carbon monoxide reduction as a basis for an electrochemical kinetic model of the process. We develop scaling relations relating transition state energies to the carbon monoxide adsorption energy and determine the optimal value of this descriptor to be very close to that of copper.

Promoter Effects of Alkali Metal Cations on the Electrochemical Reduction of Carbon Dioxide

The electrochemical reduction of CO2 is known to be influenced by the identity of the alkali metal cation in the electrolyte; however, a satisfactory explanation for this phenomenon has not been developed. Here we present the results of experimental and theoretical studies aimed at elucidating the effects of electrolyte cation size on the intrinsic activity and selectivity of metal catalysts for the reduction of CO2. Experiments were conducted under conditions where the influence of electrolyte polarization is minimal in order to show that cation size affects the intrinsic rates of formation of certain reaction products, most notably for HCOO-, C2H4, and C2H5OH over Cu(100)- and Cu(111)-oriented thin films, and for CO and HCOO- over polycrystalline Ag and Sn. Interpretation of the findings for CO2 reduction was informed by studies of the reduction of glyoxal and CO, key intermediates along the reaction pathway to final products. Density functional theory calculations show that the alkali metal cations influence the distribution of products formed as a consequence of electrostatic interactions between solvated cations present at the outer Helmholtz plane and adsorbed species having large dipole moments. The observed trends in activity with cation size are attributed to an increase in the concentration of cations at the outer Helmholtz plane with increasing cation size.

Potential Dependence of Electrochemical Barriers from ab Initio Calculations.

We present a simple and computationally efficient method to determine the potential dependence of the activation energies for proton-electron transfer from a single ab initio barrier calculation. We show that the potential dependence of the activation energy is given by the partial charge transferred at the transition state. The method is evaluated against the potential dependence determined explicitly through multiple calculations at varying potential. We show that the transfer coefficient is given by the charge transferred from the initial to transition state, which has significant implications for electrochemical kinetics.