Abstract:

A theoretical model of macro kinetics for methanol oxidation in the porous anode of DMFC was developed.First, an intrinsic kinetics expression including the coverage ratios of CO and OH was derived from the dual site mechanism of methanol hydroxylation on Pt-Ru catalyst surface.Second, the balances for both material and charge in a differential volume of the porous anode were calculated to the model equations coupled for describing concentration and potential distributions in the anode.Then the generalized macro kinetics model was obtained with dimensionless variables and parameters.The physical characteristics of the porous anode, including the thickness of catalyst layer, specific area, effective diffusion and conductive coefficients, as well as the catalytic characteristics related to CO and OH coverage ratios as functions of the variable of thickness in particular, were included in this model.Furthermore, the expressions of effectiveness and polarization curve for the porous electrode were presented.The model was 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 was solved by using Newman’s BAND (J) program while inserting a subroutine to calculate the coverage ratios at each note.Through comparing the model predicted values with experimental polarization data reported，it was found that the predictions agreed with the data satisfactorily under lower current densities, and were regularly lower than the data under higher current densities as CO₂ bubbling became considerable.Detailed analysis of macro kinetics showed that 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.This work may also provide an analysis tool for the performance of anodes in DEFC as well as DBFC.

Key words:

直接甲醇燃料电池, 多孔阳极, 甲醇氧化, 双位机理, 宏观动力学, 数学模型

摘要：

本文旨在建立和求解DMFC多孔阳极甲醇氧化宏观动力学理论数模。根据Pt-Ru催化剂上双位机理，得到包含CO和OH覆盖率的甲醇水解本征动力学表达式;通过对该多孔阳极微元体积中的物料和电（荷）量衡算，导出了描述电极中浓度和超电势分布的两个耦联的模型方程。经量纲1化后，获得以量纲1变量和准数表示的普遍化宏观动力学数模。该模型包括催化层厚度l、比表面积a、有效扩散系数D_e和有效液相电导率κ_e等工程参数，特别是包括了与动力学参数相关且作为厚度变量函数的CO和OH覆盖率。进而，文中还给出了DMFC多孔阳极效率因子和极化曲线的计算公式。该数模为一非线性二阶微分方程组边值问题，经解耦，可得到两个同解的微分方程。用Newman的BAND（J）程序对方程进行数值求解，并在每一节点计算中嵌入一个计算CO和OH覆盖率的子程序。将模型预测值与甲醇氧化多孔阳极的极化实验数据进行对比发现：当宏观电流密度较低时二者能很好相符;当宏观电流密度较高、CO₂形成气泡的影响变大时，实验值有规律地偏低于预测值。详细的宏观动力学分析表明：提高催化剂Pt位甲醇电分解活性以减少功率损失、优化多孔电极微观和宏观结构以削弱两相流影响，应是改善该阳极性能的重要课题。本工作也可为直接乙醇燃料电池（DEFC）、直接硼氢化物燃料电池（DBFC）阳极的理论分析提供参考。

关键词:

直接甲醇燃料电池, 多孔阳极, 甲醇氧化, 双位机理, 宏观动力学, 数学模型