Water back diffusion

To reduce methanol crossover and improve water back diffusion through the membrane in passive DMFCs, Kim et al. [179] designed an MEA with... 

Peled ef al. [177] also designed a novel MEA in order to improve the water back diffusion from fhe cathode to the anode side. They used a liquid-water barrier layer (LWBL), which consisted of a paste, made out of PTFE and carbon black particles, fhat was inserted in the pores of fhe CFP to form a layer inside fhe paper. Up to seven layers were necessary in order to achieve a uniform layer of 20-50 pm in thickness. Testing showed that the LWBL on the cathode DL creates a hydraulic pressure that forces (or pushes) the water back from fhe cafhode toward the anode, thus improving the cell s water management at different operating conditions. 

Here, we have used a low frequency electric field in order to minimize the effect of loose water back diffusion under a step voltage or a DC electric field. Other parameters have been experimentally measured to be K 10 m /CP, a lA/mV or S/m. Figure 2.3 depicts a more detailed set of data pertaining to the Onsager coefficient L as a function of electric field E. 

Due to the high water activity in the cathode, a driving force for water back diffusion through the electrolyte membrane to the anode is forming causing water to diffuse back to the anode side. 

Second, the two major mechanisms of water transport through the membrane, the electroosmotic drag and back-diffusion, can create complex transient behavior involving different time scales. For example, during a step change in current density, the electroosmotic drag wiU immediately remove water from the anode side of the membrane before water back-diffusion from the cathode to anode takes effect. The time constant of water diffusion across a membrane can be evaluated by... 

This can cause a temporary dryout on the anode side of the membrane and hence a jump in membrane resistance or a sharp decrease in cell voltage. This voltage drop is, however, recoverable within a period of the time constant that is characteristic of water back-diffusion through the membrane. 

This condition holds for sufficiently large water concentrations at the inlet and/or large parameter r. Physically, large r means a high rate of liquid water back diffusion in the membrane and/or a low rate of water vapour leakage to the cathode channel. In both these cases, the upper limit of the current density which satisfies (4.99) increases. 

The conductivity of the membrane in a PEMFC is directly related to its water content, which depends on (1) the water carried by the humidified reactant gases (2) the water generated at the cathode as a result of electrochemical reaction (3) the electroosmotic drag— that is, the water carried by the protons from the anode to the cathode and (4) the water back diffusion from the cathode to the anode. Therefore, it is obvious that water management is a complex issue. 

Ren et al. reported that the electroosmotic drag numbers increased with temperature for a fully hydrated Nafion 117 membrane [44]. The drag numbers were reported to be 2 and 5 H20/H at 15 and 130 °C, respectively. They obtained the drag numbers by balancing the water collected at the cathode in an operating DMFC, where water back-diffusion from the cathode to the anode is negligible. 

Similarly, if the anode RH is very low, for example, 10%, membrane performance will increase with anode humidification. If the cathode RH is high, the membrane can be humidified through water back diffusion from cathode to anode when the hydrogen humidification is low. On the other hand, if the hydrogen humidity increases, the hydrogen partial pressure drops, and under this situation the hydrogen humidity has little effect on cell performance (see Figure 10.31). 

For a thin membrane, water back diffusion may be sufficient to counteract the anode-drying effect due to the electroosmotic drag. However, for a thicker membrane, drying may occur on the anode side. This was very vividly demonstrated by Buchi and Scherer [19], who created thick membranes by combining several layers of Nafion membranes. They showed that the membrane resistance is independent of current density for the membranes up to 120 pm, but it does increase for thicker membranes (Figure 4-7). 

Water content in the cathode exhaust is equal to the amount of water brought in the cell by humid air at the inlet, plus water generated in the cell and plus the net water transport across the membrane, that is, the difference between electroosmotic drag and water back diffusion ... 

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