Department of Chemical and Biological Engineering, Princeton University, Princeton, New Jersey 08544, USA.
J Phys Chem B. 2011 Sep 1;115(34):10239-50. doi: 10.1021/jp204785t. Epub 2011 Aug 5.
Dynamic and steady-state water flux, current density, and resistance across a Nafion 115 membrane-electrode-assembly (MEA) were measured as functions of temperature, water activity, and applied potential. After step changes in applied potential, the current, MEA resistance, and water flux evolved to new values over 3000-5000 s, indicating a slow redistribution of water in the membrane. Steady-state current density initially increased linearly with increasing potential and then saturated at higher applied potentials. Water flux increases in the direction of current flow resulting from electro-osmotic drag. The coupled transport of water and protons was modeled with an explicit accounting for electro-osmotic drag, water diffusion, and interfacial water transport resistance across the vapor/membrane interface. The model shows that water is dragged inside the membrane by the proton current, but the net water flux into and out of the membrane is controlled by interfacial water transport at the membrane/vapor interface. The coupling of electro-osmotic drag and interfacial water transport redistributes the water in the membrane. Because water entering the membrane is limited by interfacial transport, an increase in current depletes water from the anode side of the membrane, increasing the membrane resistance there, which in turn limits the current. This feedback loop between current density and membrane resistance determines the stable steady-state operation at a fixed applied potential that results in current saturation. We show that interfacial water transport resistance substantially reduces the impact of electro-osmotic drag on polymer electrolyte membrane fuel cell operation.
动态和稳态水通量、电流密度和 Nafion 115 膜电极组件（MEA）的电阻作为温度、水活度和施加电势的函数进行了测量。在施加电势发生阶跃变化后，电流、MEA 电阻和水通量在 3000-5000 s 内演变为新值，表明膜内的水缓慢再分配。稳态电流密度最初随施加电势线性增加，然后在较高的施加电势下饱和。由于电渗拖曳，水通量沿电流流动方向增加。水和质子的耦合传输通过显式考虑电渗拖曳、水扩散和界面水在蒸汽/膜界面处的传输阻力进行建模。该模型表明，质子电流将水拖入膜内，但膜内和膜外的净水流由膜/蒸汽界面处的界面水传输控制。电渗拖曳和界面水传输的耦合重新分配了膜内的水。由于进入膜内的水受到界面传输的限制，电流的增加会使膜的阳极侧缺水，从而增加膜的电阻，这反过来又会限制电流。这种电流密度和膜电阻之间的反馈环决定了在固定施加电势下稳定的稳态操作，导致电流饱和。我们表明，界面水传输阻力大大降低了电渗拖曳对聚合物电解质膜燃料电池运行的影响。
