Hyman Matthew P, Medlin J Will
Department of Chemical and Biological Engineering, University of Colorado, Boulder, Colorado 80309, USA.
J Phys Chem B. 2005 Apr 7;109(13):6304-10. doi: 10.1021/jp045155y.
The effect of homogeneous electric fields on the adsorption energies of atomic and molecular oxygen and the dissociation activation energy of molecular oxygen on Pt(111) were studied by density functional theory (DFT). Positive electric fields, corresponding to positively charged surfaces, reduce the adsorption energies of the oxygen species on Pt(111), whereas negative fields increase the adsorption energies. The magnitude of the energy change for a given field is primarily determined by the static surface dipole moment induced by adsorption. On 10-atom Pt(111) clusters, the adsorption energy of atomic oxygen decreased by ca. 0.25 eV in the presence of a 0.51 V/A (0.01 au) electric field. This energy change, however, is heavily dependent on the number of atoms in the Pt(111) cluster, as the static dipole moment decreases with cluster size. Similar calculations with periodic slab models revealed a change in energy smaller by roughly an order of magnitude relative to the 10-atom cluster results. Calculations with adsorbed molecular oxygen and its transition state for dissociation showed similar behavior. Additionally, substrate relaxation in periodic slab models lowers the static dipole moment and, therefore, the effect of electric field on binding energy. The results presented in this paper indicate that the electrostatic effect of electric fields at fuel cell cathodes may be sufficiently large to influence the oxygen reduction reaction kinetics by increasing the activation energy for dissociation.
采用密度泛函理论（DFT）研究了均匀电场对Pt(111)表面原子氧和分子氧吸附能以及分子氧解离活化能的影响。对应于带正电表面的正电场会降低Pt(111)表面氧物种的吸附能，而负电场则会增加吸附能。给定电场下能量变化的幅度主要由吸附诱导的静态表面偶极矩决定。在10原子的Pt(111)团簇上，在0.51 V/A（0.01 au）的电场存在下，原子氧的吸附能降低了约0.25 eV。然而，这种能量变化很大程度上取决于Pt(111)团簇中的原子数，因为静态偶极矩会随着团簇尺寸减小。使用周期性平板模型进行的类似计算表明，相对于10原子团簇的结果，能量变化小了约一个数量级。对吸附的分子氧及其解离过渡态的计算显示出类似的行为。此外，周期性平板模型中的底物弛豫会降低静态偶极矩，因此电场对结合能的影响也会降低。本文给出的结果表明，燃料电池阴极电场的静电效应可能足够大，通过增加解离活化能来影响氧还原反应动力学。
