(662b) Novel Fuel Cell Membranes

Introduction The aim of this project is synthesis of novel nanostructured composite proton exchange membranes (PEM). The PEM membranes that have been synthesized are based on polyether sulfone (PES) and heteropolyacids. Blends of polyether sulfone with other polymers like polybenzimidazole have also been synthesized. Several blends have been synthesized by various research groups [1-3]. Nanocomposites of Nafion and silica are being made using sol-gel process [4]. Heteropolyacids (HPAs) are used as inorganic proton conductors because they exhibit high proton conductivity but also are stable at high temperatures [5]. Surface coating techniques are applied to the HPA particles. Polymer coating of heteropolyacids (HPA) is done in order to prevent the HPAs from being washed out of the membrane. In addition, functional groups attached to the polymer can provide enhancement of proton conductivity because they can be reacted with sulfonic acid. Atom transfer radical polymerization (ATRP) was used as a surface polymerization technique. The surface initiator is grafted onto the HPA surface and is initiated by electrons from the redox reaction of metal halide (CuBr) [6-8]. Then, the monomer is initiated and followed by propagation and termination. Conductivity tests were done on the surface functionalized HPA- PES composite membranes and the non- functionalized HPA-PES membranes. Tests were done with various HPAs like, silicotungstic acid (SWA), phosphotungstic acid (PWA), silicomolybdic acid and phosphomolybdic acid. Sulfonation of the vulcanized ethylene-propylene-diene terpolymer membranes using acetyl sulfate has been carried out [9]. The sulfonation of polymer when performed using acetyl sulfate, the temperature of the system has to be below 0 0C. Another method to prepare a sulfonated polymer is by reacting with chlorosulfonic acid. This was used for sulfonated PSf [11]. Polyether sulfone (PES) was sulfonated using chlorosulfonic acid and the membrane characteristics were studied. It has been observed that for PES, only three sulfonating agents can be used ? oleum, chlorosulfonic acid, SO3 or its complexes [12]. New aromatic polymers bearing pyridine ring have been synthesized [13]. The polymer synthesized by them was tested at 160 0C for conductivity. PES was sulfonated by employing chlorosulfonic acid and acetyl sulfate. The composite membranes of sulfonated polymer and HPA were synthesized. Characterization of the membranes for their proton conductivity is being done by using Electrochemical Impedance Spectroscopy (EIS). The Nyquist plots obtained from it can be used to represent the proton conductivity of the membranes. Tg of a polymer is determined by using Differential Scanning Calorimetry (DSC). The performance of the membrane is examined by creating a membrane electrode assembly and testing it on a single membrane fuel cell testing device. The sulfonated polymers were tested by determining the Ion exchange capacity (IEC) of the polymer. This gave an estimate of the sulfonation extent of the polymer. High Tg membranes are needed for fuel cells running at high temperatures; around 130- 150 0C. Operating fuel cells at high temperature will enable the reaction kinetics to proceed faster; at approximately an exponential pace and also reduces the catalyst poisoning of the MEA. Novel crosslinked poly(ether sulfone) using bisazide was synthesized and it displayed a conductivity of almost 0.8 S/cm at 1000C [14]. Pyridine based PES copolymers were synthesized and were tested for conductivity over 1300C. They displayed conductivity on the order of 10-2 S/cm after acid doping with phosphoric acid [15]. The synthesis procedure usually involves the reaction of two diols with an aryl halide dissolved in a polar aprotic solvent using a weak base a catalyst [16]. Our attempt is to create a novel polymer that possesses a high Tg and that can allow the fuel cell to operate at temperature range of 120 0C. Novel polymer based on PES and polyquinoxalines are being synthesized using nucleophilic substitution reactions. Apart from being resistant to high temperature, sulfonic groups are being attached to the polymer covalently by sulfonating one of the monomers and then polymerizing it with other monomers at 180 0C. MALDI-MS technique helped us to determine the molecular weights of our novel polymers. Fuel Cell Device: The purpose of creating a fuel cell with the most commonly used components in PEM fuel cells today is to record the baseline power output with the new membrane installed versus a Nafion membrane. The optimization of the device was done with the monopolar plates. A prototype fuel cell was modeled using SolidWorks, based on a 25cm2 membrane electrode assembly. The fuel cell was then manufactured using standard commercially available materials. The materials used were copper for the current collector plates, 6061 aluminum for the rigid frame, and graphite for the monopolar plates. These materials were selected so that a baseline of performance could be recorded with the most common materials used in fuel cell design today. Machining of the prototype fuel cell was completed and assembled. Thin Teflon gaskets were placed between the copper and the frame to avoid shorting the fuel cell and more gaskets were placed in the center for supporting the membrane. The fuel cell was connected to the fuel cell testing system and pressurized with hydrogen, a hydrogen sensor was placed on the outside to detect leaks and none were detected. The fuel cell was then initially tested with a high flow rate and amperage just to see if it would function. The fuel cell performed as expected. Future objectives for research are to modify the monopolar plate material to decrease cost and increase strength due to higher temperatures and pressure. The other materials in the fuel cell may also need to be changed due to these higher temperatures and pressures. The monopolar plates and flow patterns can be optimized for non-humidified operation and to run at different flow rates. Fuel Cell Modeling: The initial test cell geometry for the fuel cell is complete and zone assignments and continuum assignments have been declared. Work on the computer cluster for computational purposes is being completed. Research into turbulence in fuel cells is being conducted. A basic straight channel PEM fuel cell model is being conducted with viscous friction involved for a variety of turbulence models. Simulated/artificial turbulence is desired to be added if base geometry will not alone create some turbulence, or, more specifically, mixing. This will be done after conclusive results from initial turbulence modeling are determined. It is desired to show conclusively that turbulence, or rather mixing, does or does not improve the performance of a PEM fuel cell and if it can indeed, be created. Optional methods for creating a channel which may facilitate turbulence are: wider channel, roughness modeling, non-laminar fluid entry, geometrical changes during flow. Key Results: The composite membranes based on sulfonated PES (SPES) and HPAs were synthesized. Conductivity tests were carried out on them and IEC was calculated. The results are listed in Table 1. A composite blend of 30% HPA, sulfonated polyimide (PI) & SPES was synthesized. An initial polarization curve was obtained by making a MEA and testing it in single membrane fuel cell testing device. Polarization curves have been obtained from the device fabricated by our group. Future work includes synthesis of novel polymers that possesses high conductivity and obtaining performance data for more composite blended membranes. Tests will be done on our membranes in the fuel cell device built and performance results will be compared with commercial fuel cell tests. References: 1. A. Rahimpour et al, Journal of membrane science, 311(2008) 349-359. 2. Han- Lang Wu et al, European Polymer Journal, 42, (2006), 1688-1695. 3. F. Schonberger et al, , Solid State Ionics, 178, 2007, 547-554 4. H. Jiang et al., ?Infinite chains constructed from flexible bis(benzimidazole)-based ligands', Polyhedron (2008), in press. 5. He et al, ?Proton conductivity of phosphoric acid doped polybenzimidazole and its composites with inorganic proton conductors', Journal of membrane sciences, 226,169-184, 2003. 6. G. Odian, John Willey and Sons, Inc., 2004. 7. A. Fuchs, et. al., Polymer, Vol. 47, pp. 7653 ? 7663, 2006. 8. J. Sutrisno, Chemical Eng. Master Thesis UNR, 2008. 9. F. B. Bujans et al, Acta mater, Article in press, (2008). 10. O'Farrell et al, US Patent 4,303,766, (1981). 11. C. Manea et el, Journal of Membrane Science, 206, 443, (2002). 12. R. Guan et al, European Polymer Journal, 41, 1554, (2005). 13. M. L. Di Vona et al, Solid State Ionics, 179, 1161, (2008). 14. M. K. Daleteou et al, Journal of Membrane Science, 252, 115, (2002). 15. Young- Seok Oh et al, Journal of Membrane Science, 323, 309 (2008). 16. E.K. Pefkianakis et al, Macromolecular Rapid communications, 26, 1724, (2005).