Nafion composite membranes DMFC performance

Some PVA-based membranes showing modest selectivities, like sulfonated PVA [339] and PVA/PSSA/mordenite [361] exhibit unexpected high MPD (81 and 74 mW.cm , respectively) at 70 °C. PVA/poly(ether sulfone)/PWA membranes [354], with very low selectivity, show MPD in the range 95-117 mW.cm at 80 °C. In summary, the performance of DMFC with PVA-based membranes could not yet overpass the results obtained with Nafion composite membranes, although some promising results have been obtained during the last years. Indeed, the results shown in Table 6.3 forNafi(Mi/PVA blends [114] and PVA-coated Nafion membranes are superior to those obtained with PVA-based membranes. 

Bauer F, Willert-Porada M (2005) Characterization of zirconium and titanium phosphates and direct methanol fuel cell (DMFC) performance of functionally graded Nafion composite membranes prepared out of them. J Power Sources 145 101-107... 

Yang HN, Lee JY, Jeong JY, Na Y, Kim WJ (2011) Cell performance of DMFC fabricated with H H—ETS-lO/Nafion composite membrane. Micropor Mesopor Mat 143 215-220... 

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. 

Some authors superficially modified Nafion membranes in order to reduce methanol crossover. For instance, hydrocarbon films have been deposited on Nafion membranes by plasma deposition [163], and trilayer membranes were prepared by casting Nafion layers on a central Nafion/PVdF composite layer [164]. Nafion treated with supercritical CO2 seems to increase crystallinity, as well as proton conductivity and DMFC performance [165, 166]. 

Paradoxically, the efforts to reduce the methanol permeabilities of Nalion with inorganic or organic fillers in most cases yield composite membranes with permeabilities similar to that obtained by optimizing the cast procedure of pure Nafion [302]. Nevertheless, the reduction of methanol permeability by itself is not a criterion for improving DMFC performance because it is usually associated to a reduction of the proton conductivity. We will analyze this property in Sect. 6.5.5 as a previous step to discuss the behavior of the proton-conducting membranes in terms of alcohol selectivity defined by Eq. 6.2. 

The analysis of overall efficiency for Nafion 112, 115 and 117 membranes in DMFC at 80 °C [4] shows (see Fig. 6.26) that Nafion thicker membranes (Nafion 117) exhibits better performance at low power densities, while thinner membranes (Nafion 112) give higher performance at high power density as a consequence of a balance between methanol crossover and ohmic losses, in agreement with experimental and simulation results [327, 328]. The extension of this analysis to Nafion composites is not simple because methanol crossover current was not determined in most of the composite membranes investigated. 

It can be concluded that the MPD reported for DMFC with pristine or composite Nafion membranes at temperatures above 100 °C are similar or lower than those reported by Ren et al. [327] in 1996, using Nafion 112 (250 mW.cm at 110 °C and 380 mW.cm at 130 °C) meaning that, in spite of the efforts to improve the performance of Nafion-based DMFC, little progress has been achieved. The situation at room and intermediate temperatures seems to be promissory, with several Nafion composites showing very good performances in DMFC. 

Watanabe and coworkers [5, 454, 455] studied the DMFC performance of sPI and crosslinked sPI membranes obtained by polycondensation of NTDA, bis (3-sulfo propoxy)benzidine (BSPB), and alkaline diamine [5]. Other studies on polyimide membranes focused on methanol selectivity [456], DMFC performance [457], and the effect of crosslinking on the methanol selectivity [458]. Hybrid sPI membranes were prepared, including composites with PAMPS [459], PTA [460], Nafion infiltrated sPI membranes [461], sPI membranes coated with crosslinkable poly(ethylene glycol) dimethylacrilate (PEGDMA) [462], and mesoporous silica [463, 464],... 

All the sPI composite membranes are located in flie A quadrant, that is have higher proton conductivities and lower methanol permeability than Nafion. The sPl composite with PTA [460] has a poor DMFC performance, but sPBl layered with PEGDMA reached MPD of 120 mW.cm at 60 °C [462]. The behavior of the sPI/Nafion composite is exceptional, exhibiting =79 and a MPD of 130 mW. cm at 70 °C with 5 M methanol. 

Blended membranes of PVDF and either sulfonated poly[(3-methylphenoxy) (4-ethylphenoxy)phosphazene] (SP3MP4EPP) or sulfonated poly[(4-ethylphenoxy) (phenoxy)phosphazene] (SP4EPPP) were also investigated for their performance in DMFCs [87,88]. Three-layer composite MEAs were fabricated, in which two electrode-pressed Nafion 112 membranes were attached to both sides of blended... 

The DMFC performances for a series of Nafion-based hybrid manbranes compared with that of pristine Nafion are shown in Fignre 9.22a through c, respectively. Most of the hybrid membranes displayed better cell performance compared with that of Nafion. For Nafion-MZP and Nafion-MTP composite monbranes. 

FIGURE 9.22 Performance curves for DMFCs with Nation 117 and (a) Nafion-silica, (b) Nafion-MZP, and (c) Nafion-MTP composited membranes at 70°C using aqueous methanol and oxygen reactants at atmospheric pressure. Anode, 2 M aqueous methanol with a flow rate of 2 seem cathode, oxygen with a flow rate of 200 seem. (From Sahu, A.K. et al., J. Membr. Sci., 345, 305, 2009.)... 

---