Methanol crossover in DMFC

The impact of membrane thickness and equivalent weight of a number of Nafion-type membranes upon the methanol crossover in DMFCs has been investigated by Narayanan et al. It was demonstrated that that the membrane thickness has a significant impact on crossover, corresponding to a reduction of 40-50% observed from 5 mil to 14 mil. When the extent of crossover is compared under operating conditions of 300 mA/cm the reduction observed corresponds to approximately 10.5 mA/cm per mil thickness. However, the... 

Several studies have characterized methanol crossover in DMFCs [20, 21]. Due to the hydroxyl group and its hydrophilic properties, methanol interacts with the ion exchange sites and is dragged by hydronium ions in addition to diffusion as a result of concentration gradient between anode and cathode. 

In spite of the similar electronic properties to Pt, the electrocatalytic activity of Pd for the HOR is lower than that of Pt [85] this is presumably due to the stronger interaction of Pd-Had than that of Pt-Had [86]. However, by changing the Had binding energy through alloying Pd with Pt, the HOR rate on the catalytic surfaces can be increased, which may be comparable to that of Pt [87]. On the other hand, some Pd alloys (e.g., Pd-Fe, Pd-Co) were found to have little activity to MOR these Pd-M alloys are however, considered to be the excellent methanol-toletant materials for solving the methanol crossover in DMFCs [88]. 

Z. Qi and A. Kaufman. Open-circuit voltage and methanol crossover in DMFC. J. Power Sources, 110 177-185, 2002. 

Schaffer, T., Hacker, V., Tschinder, T., Besenhand, J. O., and Prenninger, P., 2005, Introduction of an improved gas chromatographic analysis and comparison of methods to determine methanol crossover in DMFCs , J. Power Sources 145 (2) 188-198. 

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. 

The membrane in a membrane fuel cell fulfils several important functions as stated in the introduction. Nafion was the first commercially available membrane, which lead to a breakthrough in fuel cell technology. Today, various companies are engaged in membrane development especially for this purpose, aiming at improved material properties. The goals are less sensitivity towards elevated temperature and dry operation, better chemical and mechanical stability and reduced methanol crossover for DMFC operation. A significant improvement of the mechanical stability was achieved by incorporation of a PTFE porous sheet as mechanical support for the membrane material [13,14]. 

According to the literature [63-64], for the measurement of methanol crossover in a running DMFC single cell, the equivalent current (ie, mA/cm ) of methanol crossover rate can be approximately expressed as a linear equation,... 

Figure 23. Equivalent current of methanol crossover in a DMFC using l.OM methanol at various operating temperatures. The points and the lines are experimental and calculated results, respectively.

The most conventional method to determine methanol crossover in a DMFC is to monitor the CO2 content in the cathode exhaust gas flux by using an optical infrared sensor, by gas chromatographic analysis, or by mass spectrometry [132], However, these measurements are based on the assumptions that flie crossed over methanol at the cathode is completely oxidized and that there is no CO2 permeation from the anode to the cathode. In reality, in particular for operation at high current density, a large amount of CO2 permeates from the anode to the cafliode in the DMFC. So far, no reliable method is available to measure the methanol crossover through the membrane from the anode to the cathode at the operating status. 

Cobalt is used to promote CO oxidation in reformers [284, 285], suggesting PtCo alloys may be useful catalysts for H2 oxidation in the presence of CO. PtCo alloys have been proposed as improved methanol oxidation catalysts [286] because cobalt may assist with CO removal (CO is an intermediate in meflianol electrooxidation) through a mechanism analogous to the PtRu bifunctional mechanism. PtCo alloys have also been studied as improved ORR catalysts [200, 287, 288]. In addition to their improved ORR kinetics, these alloys have been shown to be more tolerant to methanol crossover in direct methanol fuel cells (DMFCs), again possibly through improved CO removal kinetics [289]. However, Stevens et al. [235] observed no impact on CO-stripping with the addition of eobalt to Pt, and explained this as due to surface cobalt dissolving away. 

The main problem with DMFCs is the high methanol crossover of the conventional PEMs, which results in a lower OCV and poor fuel utilization. To reduce the methanol crossover in the PEMs, most researchers, instead of using thinner membranes such as Nafion-115,... 

Nafion-112, and Nafion-212, use the thicker membrane Nafion-117 in DMFCs. The use of crosslinked PVA electrospun nano-fiber film supported Nafion composite membranes (Nafion/ PVA-fiber, thickness 50 pm) in DMFCs has been reported to exhibit a much better DMFC performance than Nafion-117 and Nafion/PVA blended PEMs [26-31]. Several researchers blended the Nafion PEMs with low methanol compatible PVA to reduce the methanol crossover in the PEMs [32-35]. However, these modified Nafion membranes had thicknesses greater than 175 pm, which were similar to (or higher than) that of the neat Nafion-117 membrane. Although there was a decrease in the methanol crossover from these Nafion/PVA blended membranes, the proton transfer resistance of these membranes increased, resulting in a lower DMFC performance. The advantage of applying the thin Nafion/PVA-fiber PEMs to the DMFCs is that the methanol crossover can be reduced without increasing the area specific resistance (i.e., Lla) because of low membrane thickness. Table 12.1 summarizes the thickness, proton conductivity, and Lja of the fiber reinforced Nafion composite membranes obtained from literature reports. The mechanical properties of the composite membranes reported in literature are also listed in Table 12.2. 

The basic concept to use block co-polymer for the application to the DMFC is that ordered hydrophilic/hydrophobic phase separations offer a route for the selective transport of proton ions with reduced methanol crossover in the hydrophilic domains, because block co-polymers can be selectively sulfonated using post-sulfonation methods, and the block co-polymers can be verified over a wide range of structures during anionic polymerization. For example, methanol transport behaviors of a triblock co-polymer ionomer, sulfonated poly(styrene-isobutylene-styrene) (S-SIBS), were compared with Nafion to determine whether the sulfonated block co-polymer could serve as a viable alternative membrane for application to the DMFC [62]. The S-SIBS membranes showed approximately 5-10 times more methanol selectivity than that of Nafionll , although the S-SIBS membranes exhibited low conductivity compared with Nafion 117. 

External humidification is not needed in the DMFC, due to the liquid anode solution, but prevention of cathode flooding is critical to ensure adequate performance. To combat flooding and methanol crossover in the DMFC, several approaches have been used, as schematically shown in Figure 6.53 ... 

Semi-interpenetrating polymer network composite membranes consisting of poly (styrene-sulfonic acid) and poly(vinylidene fluoride) (PVDF) were developed jointly at the University of Southern CaUfomia and the Jet Propulsion Laboratory (Prakash et al., 2004). The methodology made use of the thermally initiated radical interpolymerization of styrene that was absorbed into a PVDF film with subsequent sulfonation of the resultant polystyrene. Membranes had a proton conductivity of 0.06-0.09 S/cm with three times lower methanol crossover in a DMFC at 55°C and 0.5 M methanol. 

Nafion is currently the most widely used PEM in PEMFC applications and has been the technology standard materials of the PEM and the electrolyte binder of the CL of PEMFCs. Lots of researchers devoted efforts to enhance the mechanical strength to reduce membrane thickness, improve the moisture retention capability to enhance conductivity, and reduce the methanol crossover for DMFCs. Two of the major modifications are (1) fiber-reinforced Nafion composite membranes and (2) Inor-NP hybridizing Nafion composite membranes, which are discussed in the following two sections. 

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