Applications Of Molecular Simulations In Nano-Electrocatalysis

Understanding the structure and property of nanoparticles and electrode-electrolyte interface, and the nature of electrochemical reactions are among the key problems in modern electrocatalysis. In this thesis, molecular simulations are applied to investigate these problems. First we have constructed density functional theory (DFT) based modified embedded atom method (MEAM) potentials, based on which the structue and stability of nanoparticles are investigaed by using classical molecular dynamics (MD). Next, Microkinetic modeling and DFT calculations are combined to unravel the particle size effect of Pt for hydrogen electrode reactions (HERs) and its surface structure origin. Finally, we have performed DFT calculations on the phenomen of anion specific adsorption, which is vital to the electrochemical reaction. The main contents and results are summarized as follows:1. DFT based MEAMPresently, the direct DFT calculations on nanoparticulate systems (typically comprising a few hundreds to several tens of thousands of atoms) remain difficult. An alternative is to use molecular simulations based on empirical or semi-empirical potentials, however, the reliability and accuracy is doubted. To solve the dilema, we combined DFT calculations with MEAM which can effectively describe the many body interactions in metallic systems by using DFT calculations to gain the quantities for fitting of MEAM potential parameters. Particularly, a reliable approcah for DFT surface energy calculation is introdeced, which leads to a significant improvement of MEAM in pridicting the properties of surface and nanoparticles. On this basis, we have constructed MEAM potentials for various face-centered cubic (fcc) metals.In addition, the influence of two exchange-correlation functionals, PBE and PBEsol, in constructing the MEAM potentials are compared. It is found that PBEsol would be more appropriate in describing the bulk (lattice constant) and surface properties (surface energy) for Ag and Au. After obtaining the MEAM parameters for Ag, Au and their alloys, we conducted MD simulations on the surface reconctruction of (110) planes, the segregation energy of substitutional impurities to (100) surfaces, the structures of adatom clusters on the (100) surfaces, the cohesive engery of Au nanoclusters and the stability of Ag-Au core-shell structures. These results agree well with both the experiment and DFT calculations results.2. The particle size effects of Pt in HERsThe particle size effects of Pt for HERs are investigated using combined microkinetic modeling and DFT calculations. A volcano relation between the exchange current density (j0) and the surface coverage of the reactive H atom at the equilibrium potential (θ0) has been established, which allow the activity trend of various Pt-based catalysts for HERs to be predicted. From the DFT-calculated adsorption isotherms, it is concluded that the reactive H atoms at the equilibrium potential of HERs are those adsorbed at the fcc sites on Pt(111) surface, the long-bridge sites on Pt(110) surface and those pair-adsorbed at the bridge sites on Pt(100) surface. The corresponging θ0are0.95ML,0.20ML and0.25ML, respectively. These results suggest thatj0of HERs for Pt(110) and Pt(100) surfaces are similar to each other but much higher than that for Pt(111) surfaces.Although the individual edge atom rows on cubo-octahedron nanoparticles of fee metals are similar in structures to the top atom rows on the corresponding (110) surfaces, the catalytic properties are not simply equivalent due to that the long-bridge sites are absent on nanoparticle edge. Thus, the reactive H atoms on the edge of Pt nanoparticles are those adsorbed at the top sites with a θ0of0.07ML, which makes the edge and (111) facet atoms the less active surface atoms than (100) facet atoms on Pt nanoparticles. On this basis, we can conclude that there is a negative particle size effect of Pt for HERs, namely, j0decreases with decreasing particle size, which is concordant with our experiment results.3. DFT study on specific adsorptionSpecific adsorption of anion can significatly affect the electrode reactions. The previous theoretical studies on this regard either applied cluster model which can not reveal the property of extend electrode, or didn’t take into account solvent effect. We perform DFT calculations on the halogen adsorbing on Pt(111) surface in the presence and absence of water, the geometry and electronic structures are also analyzed. It is indicated that the Pt-F bond is ionic nature, while the Pt-Cl, Pt-Br and Pt-I bonds are convalent nature, and the extent of convalent increases. In vacuum condition, the adsorption energy follows the order:Br<Cl<I<F, which is different from that observed on the specific adsorption on Pt electrode. This suggests the importance of solvent. If water is added, there is a partial charge transfer between the Pt surface and halides. And the extent of the charge transfer follows the order:F-<Cl-<Br-<I-, which is in accordance with experiment.