Nafion/HPA composite membranes

Figure 1.24. Sample of a linear sweep voltammogram on MEAs containing Nafion/PTA membranes of Types I-III (25% PTA loading). Scan rate 4 mV/s room temperature ambient pressure operation 200 cm H2 on anode 200 cm N2 on cathode [127]. (Reprinted from Journal of Membrane Science, 232, Ramani V, Kunz HR, Fenton JM, Investigation of Nafion /HPA composite membranes for high temperature/low relative humidity PEMFC operation, 31-44 2004 with permission from Elsevier.)...

Ramani V, Kunz HR, Fenton JM. Investigation of Nafion /HPA composite membranes for high temperature/low relative humidity PEMFC operation. J Membr Sci2004 232 31-44. 

The extraction of PTA from the composite membranes, HPA/BPSH-40, was carefully examined using tapping mode AFM, as shown in Fig. 7.10. PTA/Nafion 117 composite membrane was used as a control experiment. After immersion in liquid water, PTA/Nafion 117 showed irregular holes (0.2 pm in diameter) on the membrane surface, which was supposed to be traces of PTA extraction (Fig. 10a). In contrast, HPA/BPSH-40 composite membrane did not show any holes after liquid water treatment, indicative of a good retention of PTA in the composite, as shown in Fig. 7.10b. This could be partly attributed to the strong hydrogen bonding interaction between BPSH and PTA shown in Scheme 7.3. 

Trogadas and Ramani summarized the modification of PEM membranes, including Nafion modified by zirconium phosphates, heteropolyacids, hydrogen sulfates, metal oxides, and silica. Membranes with sulfonated non-fluorinated backbones were also described. The base polymers polysulfone, poly(ether sulfone), poly(ether ether ketone), polybenzimidazole, and polyimide. Another interesting category is acid-base polymer blend membranes. This review also paid special attention to electrode designs based on catalyst particles bound by a hydrophobic poly-tetrafluoroethylene (PTFE) structure or hydrophilic Nafion, vacuum deposition, and electrodeposition method. Issues related to the MEA were presented. In then-study on composite membranes, the effects of particle sizes, cation sizes, number of protons, etc., of HPA were correlated with the fuel cell performance. To promote stability of the PTA within the membrane matrix, the investigators have employed PTA supported on metal oxides such as silicon dioxide as additives to Nafion. 

Since the particle size of the HPA additive (1-10 pm) in the type I composite membranes was much larger than the cluster size, little improvement in conductivity was seen over pure Nafion at low humidities. The conductivity of the composite membranes at 120°C and 35% relative humidity were of the order of... 

Both the approaches were combined together to evaluate the performance of these HPA-loaded membranes with small particles [17] and MEA stabilization techniques to yield a stabiUzed MEA for operation at 120°C and 35% relative humidity (RH). MEAs were prepared using Nafion /phosphotungstic acid composite membranes with phosphotungstic acid (HPW) particle size of 30-50 nm and the HPW additive stabilized by substituting its protons with cesium counter ions. The Nafion in the membrane and electrodes were simultaneously converted to the Cs form by an imi-exchange process. The melt processability of the Nafion in the Cs form permitted the MEA to be heat treated at 200°C and 30 atm, promoting the development of a durable membrane/electrode interface. The prior stabilization of the HPW permitted MEA reprotonation with minimal additive loss which was confirmed by FTIR, TGA, and in situ electrochemical impedance spectroscopy (EIS). 

Importantly, for elevated temperature PEM fuel cell operation, the HPAs may be structurally stable to >600°C, although under anhydrous conditions their stability may be limited to 200°C, and they incorporate some water molecules and protons up to >300°C depending on the system [15]. Because of their structural diversity, these materials are particularly suitable for incorporation into a wide variety of membrane materials for which they can be specifically tailored. They have been studied in four composite systems HPAs infused into perfluorinated sulfonic acid (PFSA) polymers such as Nafion [16,17], HPA cast in inert matrices such as poly (vinyl alcohol) (PVA) [18], HPA immobilized in polymer/silicate nanocomposites via sol-gel methods [19], and HPA directly incorporated into polymer films via functionalization to monomers [20]. Here after a discussion of fundamental studies, we review the various HPA-based materials used for fuel cells. 

X 10 S/cm at 120°C (see Table 3), whereas the widely used commercial membrane Nafion 117 exhibited a room temperature conductivity of 10 S/cm that increased to only 1.2 x 10 S/cm at 120°C. hi contrast, the composite of HPA O/PSF exhibited a proton conductivity of 2.0 x 10 S/cm at room temperature that increased only to 7.0 x 10 S/cm at a temperature of 100°C. The incorporation of HPA into SPSF not only rendered the membranes suitable for elevated temperature operation of PEMFC but also provides an inexpensive alternative compared to Nafion . 

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