Water crossover

In DMFCs, the water balance analytical method has been used as a tool to study the fuel (methanol) and water crossover from the anode toward the cathode. Xu, Zhao, and He [120] and Xu and Zhao [180] performed a thorough investigation of how different cathode DLs and MPLs affected the total water crossover from the anode side. In order to be able to perform the water balance equations, they also collected the water at both outlets of the cell. This analysis technique was vital for them to be able to observe how different characteristics for fhe cafhode DL affect not only the overall performance of the fuel cell buf also fhe nef wafer drag coefficient and water crossover in DMFCs. 

C. Xu and T. S. Zhao. In situ measurements of water crossover through the membrane for direct methanol fuel cells. Journal of Power Sources 168 (2007) 143-153. 

Material balance analysis proves to be a critical diagnostic tool for the development of portable DMFC systems. In this analysis methanol balance on the anode side along with the methanol crossover rate typically measured by an infrared CO2 sensor is conducted. In addition, water balance on both anode and cathode sides is performed in which the cathode water amount is carefully collected by a moisture trap and measured. From such analyses Muller et al. " revealed that the water balance on the DMFC anode is highly negative, thus calling for membrane development with low water crossover in addition to low methanol crossover. 

It appears, 2 mil membrane experiences a serious mass-transport limitation as a result of high methanol and water crossover to the cathode particularly when operating with air. 

It is favorable for fuel cell operation when reduced methanol transport across the membrane is accompanied by proper water management. In particular, a low water crossover from the anode to the cathode is necessary to avoid flooding of the cathode. The dependence of water permeation on the membrane thickness is weak. Only a small decrease in water permeation is observed for the commercial Nafion membranes, whereas the thickness of the recast membranes has no significant influence on the water transport rate. In contrast, the effect of temperature on water permeation is strong. At 65°C, the rates are higher by a factor of 5 compared to those at 25°C. 

DMFCs have potential near-term applications mainly in the portable power source market, as they are smaller, lighter, simpler, and cleaner than conventional batteries. Liquid methanol is consumed directly in a DMFC, which implies a higher energy density of the fuel cell system. But the power densities achievable with state-of-the-art DMFCs are still very small in comparison to hydrogen-fuelled PEMFCs. One of the major problems lies in the use of liquid methanol solution on the anode of the DMFC, which, on the one hand, keeps the ionomeric membrane water saturated (and thus no humidification is needed) but, on the other hand, does not keep fuel (methanol or any other organic fuel, e.g., formic acid, ethanol) and water from permeating to the cathode side, since the basic PFSA membranes are permeable to both methanol and water. - The fuel and water crossover from anode to cathode hampers the performance of the air cathode. 

When water is confined in a hydrophobic substrate, it exhibits a lower 7p than water confined in a hydrophilic substrate, and the protein hydration water crossover temperature decreases with pressure. This P effect reflects the increase in the protein-water interaction and the increase in the water s ability to access the protein hydrophobic core [32]. Using UV spectroscopy, similar P effects on the dynamic properties of biomolecules have been studied in p iactoglobulin [33], which is also a sensitive food protein. These results suggest that these pressure effects on proteins have universality. 

We see that under realistic operating conditions (when < 0.5) the velocity variation does not exceed 15% (Figure 4.1). Thus, to a good approximation we may simply set w = in model equations this relation will be extensively used below. The effect of large water crossover on the cell performance will be considered in Section 4.2.3. 

Equations (4.34) and (4.35) are obtained assuming that the flow velocity in the channel is constant. However, large water crossover may induce variation of the flow velocity. The results of Section 4.1 allow us to rationalize the effect of water crossover on cell performance. 

Recent in rovement in their membrane technology produced peak power densities of 230 mW cm (Figure 2.11) with Pt cathodes and pure O2 for the thinnest of the membranes (17 pm fiilly hydrated) [13]. It is thought that the improved performance on decreasing membrane thickness is due to the increased water crossover from the anode to the cathode, where it is consumed (Equation 229). Au and Ag cathodes were again tested in this study, giving reduced performance compared with Pt... 

Although the use of Nation membranes doped with phosphotungstic and silicotungstic acids has been considered for DMFC, the effective application of HPA/polymer hybrid membranes for DMFC face additional problems. The main inconvenience is the high methanol and water crossover. Furthermore, in many... 

Use of Thicker Electrolyte Similar to the water crossover, a thick electrolyte can restrict crossover but also limits performance via increased ohmic losses through the electrolyte. 

For DAFCs using a liquid feed, like the DMFC, the water balance and fuel crossover problem are more acute than the hydrogen fuel cell. Dilute Uquid solutions, thicker membranes, and capillary pressure management are used to control these two issues. As a result of the high methanol and water crossover in the DMFC, the open-circuit potential is very low, and performance is also low compared to the H2 PEFC. However, the use of diffusion barriers in the anode and capillary pressure management eliminates the need for highly dilute methanol solutions, and these systems may ultimately be more appropriate than their H2 PEFC counterparts for portable applications. 

For a net drag coefficient of 4.0, no hydraulic permeation effects, and an electro-osmotic drag coefficient of 3.0, calculate the water crossover and equivalent f)ower lost per day for a 10 M methanol solution at idle in the anode of a 10-cell, 10-cm /cell DMFC stack. 

On the other hand, reducing membrane thickness can avoid water drag or water crossover. With lower membrane resistance (an enhancement in membrane... 

---