Sneha A. Akhade

Theoretical insight on reactivity trends in CO2 electroreduction across transition metals

Density Functional Theory (DFT) based models have been widely applied towards investigating and correlating the reaction mechanism of CO2 electroreduction (ER) to the activity and selectivity of potential electrocatalysts. Herein, we examine the implications of the theoretical choices used in DFT models that impact the stability of the reaction intermediates and the limiting potential (UL) of the activity/selectivity determining steps in CO2 ER across transition metals. Three theoretical choices are considered: (i) the type of exchange-correlation (XC) functional, (ii) the surface facet of the metal electrocatalyst, and (iii) the effect of solvation. The impact of the theoretical choices is also studied in the context of deriving scaling relationships for electrocatalyst screening. The analyses reveal that the choice of XC functional (PBE versus RPBE) can alter binding energies of CO2 ER intermediates by 0.30 eV, but have little impact on surface reaction energetics. Surface termination has greater impact, as OH*-terminated adsorbates bind weaker on average by 0.26 eV on stepped facets. Including explicit local solvation stabilizes the OH*-terminated adsorbates, preferentially decreasing the UL for CO* → COH* reduction. Trends in CO2 ER selectivity across metals predicted using scaling correlations differ signficantly from explicitly calculated values due to deviations from the linear binding energy correlations. The difference is most pronounced when the effect of explicit solvation is considered.

Charge optimized many body (COMB) potentials for Pt and Au

Interatomic potentials for Pt and Au are developed within the third generation charge optimized many-body (COMB3) formalism. The potentials are capable of reproducing phase order, lattice constants, and elastic constants of Pt and Au systems as experimentally measured or calculated by density functional theory. We also fit defect formation energies, surface energies and stacking fault energies for Pt and Au metals. The resulting potentials are used to map a 2D contour of the gamma surface and simulate the tensile test of 16-grain polycrystalline Pt and Au structures at 300 K. The stress-strain behaviour is investigated and the primary slip systems {1 1 1}〈1 [Formula: see text] 0〉 are identified. In addition, we perform high temperature (1800 K for Au and 2300 K for Pt) molecular dynamics simulations of 30 nm Pt and Au truncated octahedron nanoparticles and examine morphological changes of each particle. We further calculate the activation energy barrier for surface diffusion during simulations of several nanoseconds and report energies of [Formula: see text] eV for Pt and [Formula: see text] eV for Au. This initial parameterization and application of the Pt and Au potentials demonstrates a starting point for the extension of these potentials to multicomponent systems within the COMB3 framework.

Effect of Surface Chemistry on Water Interaction with Cu(111).

The interfacial dynamics of water in contact with bare, oxidized, and hydroxylated copper surfaces are examined using classical molecular dynamics (MD) simulations. A third-generation charge-optimized many-body (COMB3) potential is used in the MD simulations to investigate the adsorption of water molecules on Cu(111), and the results are compared to the findings of density functional theory (DFT) calculations. The adsorption energies and structures predicted by COMB3 are generally consistent with those determined with DFT. The COMB3 potential is then used to investigate the wetting behavior of water nanodroplets on Cu(111) at 20, 130, and 300 K. At room temperature, the simulations predict that the spreading rate of the base radius, R0, of a water droplet with a diameter of about 1.5 nm exhibits a spreading rate of R0 ≈ t(0.16) and a final base radius of 3.5 nm. At 20 and 130 K, water droplets are predicted to retain their structure after adsorption on Cu(111) and to undergo minimal spreading in agreement with scanning tunneling microscopy data. When the same water droplet encounters a reconstructed, oxidized Cu(111) surface, the classical MD simulations predict wetting with a spreading rate of R ≈ t(0.14) and a final base radius of 3.0 nm. Similarly, our MD simulations predict a spreading rate of R ≈ t(0.14) and a final base radius of 2.5 nm when water encounters OH-covered Cu(111). These results indicate that oxidation and hydroxylation cause a reduction in the degree of spreading and final base radius that is directly associated with a decreased spreading rate for water nanodroplets on copper.

Applied Potentials in Variable-Charge Reactive Force Fields for Electrochemical Systems

An atomic description of water dynamics and electrochemical properties at electrode-electrolyte interfaces is presented using molecular dynamics with the third generation of the charge-optimized many-body (COMB3) potential framework. Externally applied potentials in electrochemical applications were simulated by offsetting electronegativity on electrode atoms. This approach is incorporated into the variable charge scheme within COMB3 and is used to investigate electrochemical systems consisting of two Cu electrodes and a water electrolyte with varying concentrations of hydroxyls (OH-) and protons (H+). The interactions between the electronegativity offset method and the charge equilibration method in a variable charge scheme are analyzed. In addition, a charge equilibration method for electrochemical applications is proposed, where the externally applied potentials are treated by the electronegativity offset on the electrodes thus enforcing charge neutrality on the electrolyte. This method is able to qualitatively capture the relevant electrochemistry and predict consistently correct voltages with precalibration.