Electro-osmotic circulation

First, the solute transport in a porous filter (membrane) separating two solutions at different concentrations or electric potentials is likely to be dominated by electro-osmotic circulation as compared to molecular electrodiffusion in the pores. An accurate calculation of the circulation seems desirable. In particular, the observations upon the highly performing composite heterogeneous ion-exchange membrane [13], formed by casting a thin... 

No mechanical equilibrium is possible in a single pore with a thin double layer. The equilibrium of previous sections actually corresponds to an electro-osmotic circulation confined to a pore. In this case moving away from equilibrium corresponds to this circulation s breaking out of the capillary. 

Solution-. The observed effect is the sum of two contributions, one of which is the electro-osmotic flow of the medium through the cell. The latter has its maximum value at the center since the layer of fluid adjacent to the walls is stationary. The particles tracked at the center of the cell therefore possess the maximum increment in velocity due to electroosmotic flow. Since the cell is a closed compartment, the liquid displaced by electroosmosis along the walls must circulate by a backflow down the center of the tube. Since the total liquid flow in a closed cell must be zero, the appropriate value from Figure 12.10a to use for the velocity is the average of observations made at all depths. ... 

Dutta et al. [54] used the unified approach to study mass transport between the channels of a PEM fuel cell with a serpentine flow field. Their model is three-dimensional and allows for multi-species transport. They studied the effect of flow channel width in the serpentine flow field on velocity distribution, gas mixture distribution and reactant consumption. Serpentine flow fields allow for a greater area for diffusion of the supply gases. Their results showed that for low humidity conditions, water transport is dominated by electro-osmotic effects, i.e., water flows from anode to cathode at the side of the cell closer to the gas channel inlet. At the outlet side of the cell, water transport is dominated by back diffusion, and it flows in the opposite direction. Thus the serpentine flow field allows for circulation of the water within the cell. 

The key to running a fuel cell without external humidification is to set the air stoichiometry so that the relative humidity of the exit air is about 100% and to ensure that the cell design is such that the water is balanced within the cell. One way of doing this is described by BUchi and Srinivasan (1997) and is shown in Figure 4.13. The air and hydrogen flows are in opposite directions across the MEA. The water flow from anode to cathode is the same in all parts, as it is the electro-osmotic drag , and is directly proportional to the current. The back diffusion from cathode to anode varies, but is compensated for by the gas circulation. Other aids to an even spread of humidity are narrow electrodes and thicker gas diffusion layers, which hold more water. 

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