Modeling Of Kinetics Of Methanol Oxidation Based On Dual Site Mechanism In Porous Pt-Ru Anode For DMFC

The kinetics of methanol oxidation on Pt-Ru anode can not be simply described by exponential type equations as B-V or Tafel since the coverages of intermediate species are directly related to rate expression according to dual site mechanism. The theoretical models of intrinsic and macro kinetics for methanol oxidation in the porous anode of DMFC has been developed.First, an intrinsic kinetics expression including the coverage ratios of CO and OH is derived from the dual set mechanism of methanol hydroxylation on Pt-Ru catalyst surface. Then the kinetic parameters reported by Scott etc. is modified and the active energies are regressed by using experimental polarization data and the coverages of intermediate species involving adsorbed methanol (M_ad), CO_ads and OH_ads are quantificationally analyzed . Furthermore, the relationship between coverage and overpotential is discussed under different operation temperature and methanol concentration, then the theoretical polarization curve is calculated based on adsorption-reaction kinetics.Second, balances for both material and charge in a differential volume of the porous anode are carried out to educe the model equations which are coupled for describing concentration and potential distributions in the anode. Then the generalized macro kinetics model is obtained with dimensionless variables and parameters. The physical characters of the porous anode, including the thickness of catalyst layer, special area, effective diffusion and conductive coefficients, as well as the catalytic characters related to CO and OH coverage ratios as functions of the variable of thickness in particular, are involved in this model. Furthermore, the expressions of effectiveness and polarization curve for the porous electrode are presented. The model is a boundary problem of a set of nonlinear second order differential equations, from which two equivalent differential equations can be obtained separately by decoupling. The model equation is solved by using Newman's BAND (J) program while inserting a subroutine to calculate the coverage ratios at each note.The result of intrinsic kinetics shows that the small coverage of CO_ads has the significant influence on kinetic behaviors. The rate determining step will be the electro-decomposition of adsorbed methanol at low overpotential and it will change to methanol activation adsorption on Pt site at high overpotential. It is a transition area without rate determine step at middle overpotential. And the result of macro kinetics shows that the macro model predicted values give good correspondence with experimental polarization data reported under lower current densities, and departed from the data explicably under higher current densities as CO₂ bubbling was considered. The current density was almost zero under low overpotential with different methanol concentration because of the electro-decomposition rate of adsorbed methanol on Pt site was low and the crucial issues for improving performance of the porous anode should be to increase the activity of Pt set of the catalyst for methanol electro decomposition, and to optimize both micro- and macro-structures of the anode for mitigating the influence of two phase flow.