Ethanol electrooxidation on the Pt(111) electrode has been studied with computational theory. Using a solvation model and a modified Poison-Boltzmann theory for electrolyte polarization, standard reversible potentials for forming 17 reaction intermediates in solution were calculated with density functional theory. Reversible potentials for adsorbed intermediates were then determined by inputting calculated adsorption energies into a linear Gibbs energy relationship. A path to CO 2 was found where surface potentials were low and close to the calculated 0.004 V reversible potential for the 12 electron oxidation of ethanol. An exception was the 0.49 V potential for forming the OH(ads) from H 2 O(l), this being required for oxidation of CO(ads) and RH(ads) intermediates. The surface potentials show that acetyl, OCCH 3 (ads) forms at small positive potentials and decomposes to CH(ads), CH 3 (ads), and CO(ads), which poison the surface at these potentials. Energy losses due to non-electron transfer reaction steps are small and cause a small shift in the reversible potential for the 12 electron oxidation. Values for adsorption bond strengths over a perfect catalyst were determined. It is concluded that on an ideal catalyst most intermediates will adsorb more weakly and OH more strongly than on Pt(111). Unlike a fossil fuel, ethanol is renewable and can be produced from biomass. Compared to methanol, ethanol is less toxic. The specific energy density of ethanol is high (8.0 kWh/kg). It is liquid, which makes it easy to store and transport. These advantages make the direct ethanol fuel cell (DEFC) a promising green energy source. However, commercialization of DEFC is hindered by the slow inefficient electrooxidation reaction of ethanol on platinum. Platinum is active as the anode electrocatalyst in hydrogen fuel cells but is much less active for ethanol oxidation. Theoretical understanding of the failures of platinum will establish the specific steps during the electrooxidation which present the challenges. As shown in this paper, some of these challenges can be overcome by using electrocatalysts where reaction intermediates have specific adsorption energies to the active site. Designing materials with these properties is the goal for electrocatalyst development.The complete electrochemical oxidation reaction which takes place at the anode surface produces twelve electrons, twelve protons, and carbon dioxide:The 4 Thus, the overpotential for oxidation is about 300 mV-400 mV under typically employed conditions of study.However, the oxidation is not complete, and several final oxidation products have been observed over platinum electrodes. Iwasita's on-line differential electrochemical mass spectroscopy (DEMS) mea- * Electrochemical Society Active Member.z E-mail: aba@po.cwru.edu surements identified acetaldehyde, OCHCH 3 , forming at potentials greater than about 0.3 V and identified CO 2 forming at potentials greater than about 0.5 V.2 Fourier transform infrared (FTIR) spectroscopy provided evidence for the functional g...
Quantum chemical theory is used to identify the reasons for platinum's limitations as an electrocatalyst for oxidizing methanol at fuel cell anodes. The linear Gibbs energy relation (LGER) method is employed to predict reversible potentials for reaction steps for intermediates on the electrode surface. In this procedure, standard reversible potentials are calculated for the reactions in bulk solution phase and then they are perturbed using calculated adsorption bond strengths to the electrode surface, yielding the equilibrium potentials for each electron transfer step for adsorbed intermediates. Adsorption properties of ideal electrocatalysts for the methanol oxidation are found by imposing the condition that the reversible potential of each electron transfer step equals that for the overall reaction. The adsorption bond strengths that provide the ideal properties also apply to formic acid oxidation and carbon dioxide reduction. It is instructive to think of the ideal electrocatalyst as a lens that focusses the reversible potentials for the n individual electron transfer steps to the reversible potential for the n-electron process. It is found that the ideal catalyst will adsorb many intermediates, including HOOC, CO, OCH, HOC, HOCH, HOCH2, and OCH3 more weakly than platinum, and OOCH and OH more strongly. For example, for one possible pathway it is necessary to weaken adsorption bond strengths for HOCH2, HOCH, OCH, HOOC by about 0.5 eV, weaken adsorption CO by about 1.1 eV and strengthen OH adsorption by about 0.6 eV. These results imply a need for developing new multi-component catalysts.
The cyclic voltammogram for hydrogen on Pt(111) has been calculated using potential-dependent Gibbs reduction energies obtained by the Interface 1.0 code. The reversible potentials, U rev , are predicted by the equilibrium condition where the Gibbs energy of the oxidized reactant plus an electron and the Gibbs energy of the reduced product, when graphed as functions of electrode potential, cross and are equal at the reversible potential. Reversible potentials are calculated for 12 different coverages of H(ads), and a third-order analytic function is fit to the results. Using the derivative of this function, the experimental voltage scan rate, and the experimentally observed maximum H(ads) coverage, the cyclic voltammogram can be calculated. With the Langmuir isotherm contribution −TΔS added to the Gibbs energies, the width of the predicted voltammogram and its maximum current density compare favorably with measurements from the literature. In detailed shape, the predicted current densities are curved more than the experimental ones near the maximum values, which is a feature ensured by the addition of the Langmuir term, which has an inflection at 0.5 ML coverage. This suggests the need for modification of the Langmuir isotherm near 0.5 ML coverage and possibly subtle improvements to the surface models used.
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