水传递实例-2016解读.ppt

2.3 Electro-osmotic drag (EOD) in PEM by diffusion by electro-osmotic drag (EOD) Anode Cathode H2 O2 H2O excess O2 H2O excess H2 Electrical Circuit Electro-osmotic drag Water diffusion Dragging of water molecules by protons Membrane dehydration, especially at anode side, causing ohmic losses EOD coefficient, α (mol water/ mol proton) Anode Cathode H2 O2 H2O excess O2 H2O excess H2 Electrical Circuit Electro-osmotic drag Water diffusion For fuel cell operated with dry reactant feeds, maximum back-diffusion of water = Dry feed operation of fuel cells requires α 0.5 Scatter in EOD coefficient values Ye Wang; J. of Electrochem. Soc. (2007). Small values equilibrium with vapor Large values equilibrium with liquid 阳极 生成水 阴极 正常工作 水浓度梯度 电渗作用 反扩散 H2O/H+ H2O 质子交换膜 质子交换膜燃料电池中的水迁移 * M. J. Cheah, J. Bengizer. et al., J. Phys. Chem. B (2011) 10239-10250. Steady State Proton and Water Transport Model Water flux decreases while current is constant M. J. Cheah, J. Bengizer. et al., J. Phys. Chem. B (2011) 10239-10250. Electro-osmotic Drag Interfacial transport and membrane conductivity are self-regulating in keeping the membrane hydrated The effective EOD is substantially reduced due to interfacial transport The water content in the membrane can be far removed from the vapor phase activity * * 1、水少了不行: 膜、接触电阻，2、水多了也不行，使反应物分压降低，尤其高温；堵塞气体通道，传质受阻，阴极水淹阳极干涸，3、分布不均，膜局部溶胀，反应不均匀 * 膜内水传递：建模意义：水管理调节，自增湿操作指导。最简化模型：传质通用，困难和创新都在于如何针对具体过程，修正模型参数 * 不加电流环境。忽略两侧边界层传质阻力，因为流速较快。特点：加上膜与气、液界面阻力。 * * * * 讲此模型的特点 * * * * * * * 模型特点：考虑膜界面上的气态水传质阻力 * For Nafion with 12H2O, C=5.5*10-4(mol/ml) * * 结论：气、液态水进料阻力控制因素不同，膜的厚度影响膜内水梯度，这是实验难以测定的 * * 大电流密度下，携带水数量增多，容易造成阳极脱水。课题：自增湿操作，电渗系数？ Electro-osmotic drag is quantified as the EOD coefficient, alpha multiplied by the protonic current. In a fuel cell operated with dry reactant feeds, the only source of water is the water generated at the cathode catalyst layer by the electrochemical reaction. In the hypothetical case that all the water produced at t