Gregory M. Haugen

Interplay between Structure and Relaxations in Perfluorosulfonic Acid Proton Conducting Membranes

This study focuses on changes in the structure of ionomer membranes, provided by the 3M Fuel Cells Component Group, as a function of the equivalent weight (EW) and the relationship between the structure and the properties of the membrane. Wide-angle X-ray diffraction results showed evidence of both non-crystalline and crystalline ordered hydrophobic regions in all the EW membranes except the 700 EW membrane. The spectral changes evident in the vibrational spectra of the 3M membranes can be associated with two major phenomena: (1) dissociation of the proton from the sulfonic acid groups even in the presence of small amounts of water; and (2) changes in the conformation or the degree of crystallinity of the poly(tetrafluoroethylene) hydrophobic domains both as a function of EW and membrane water content. All the membranes, regardless of EW, are thermally stable up to 360 °C. The wet membranes have conductivities between 7 and 20 mS/cm at 125 °C. In this condition, the conductivity values follow VTF behavior, which suggests that the proton migration occurs via proton exchange processes between delocalization bodies (DBs) that are facilitated by the dynamics of the host polymer. The conductivity along the interface between the hydrophobic and hydrophilic domains makes a larger contribution in the smaller EW membranes likely due to the existence of a greater number of interfaces in the membrane. The larger crystalline domains present in the higher EW membranes provide percolation pathways for charge migration between DBs, which reduces the probability of charge transfer along the interface. Therefore, at higher EWs although there is charge migration along the interface within the hydrophobic-hydrophilic domains, the exchange of protons between different DBs is likely the rate-limiting step of the overall conduction process.

The Effect of Heteropoly Acids on Stability of PFSA PEMs under Fuel Cell Operation

Membranes were cast from mixtures of the 3M perfluorinated sulfonic acid ionomer [side chain -O-(CF 2 ) 4 -SO 3 H] and various heteropoly acids (HPAs) at a 10 or 20 wt % doping level. Membrane electrode assemblies (MEAs) fabricated from these membranes were subjected to a fuel cell testing protocol from 70 to 100°C under relatively dry conditions, dew point of 70°C, to avoid leaching of the HPA. The most significant finding was that the more stable HPAs, H 4 SiW 12 O 40 , α-H 3 P 2 W 18 O 62 , and H 6 P 2 W 21 O 71 , reduce the rate of F - by over half and improve the power of the MEA by 9% under these conditions.

Copolymerization of Divinylsilyl-11-silicotungstic Acid with Butyl Acrylate and Hexanediol Diacrylate: Synthesis of a Highly Proton-Conductive Membrane for Fuel-Cell Applications

Highly conducive to high conductivity: Polyoxometalates were incorporated in the backbone of a hydrocarbon polymer to produce proton-conducting films. These first-generation materials contain large, dispersed clusters of polyoxometalates. Although the morphology of these films is not yet optimal, they already demonstrate practical proton conductivities and proton diffusion within the clusters appears to be very high.

Investigation of Heteropolyacid Based Composite Membranes

Proton exchange membrane fuel cells (PEMFCs), the clean energy technology finds wide applications in portable energy applications, stationary power and electronics. They have several advantages such as high power density, high energy conversion efficiency and low emission. When operated at high temperatures, several problems faced at low temperature such as CO poisoning can be overcome. Unfortunately, the current per fluorinated sulfonic acid (PFSA) ionomers, such as Nafion faces serious problem when operated at high temperature. As a result of water loss at high temperatures, the proton conductivity drops resulting in a poor overall efficiency. One way to solve this problem is to support the water loss by introduction of external humidifiers but at the cost of making the fuel cell system more complex. Alternately, PFSA ionomers can be modified with inorganic materials with high water retention capacity even at high temperatures. Several classes of inorganic materials such as zirconia, zirconium phosphates, silica, titania and heteropolyacids are widely used as additives for the PFSA ionomers. (1-3) Heteropolyacids, a class of inorganic oxides, are of particular interest due to their interesting properties. They possess enough water of hydration and can keep the membrane hydrated. Their high proton conductivity, good water retention capacity and thermal stability (within the fuel cell operating conditions) make them very attractive to modify the PFSA ionomers. However, water solubility of heteropolyacids is a serious drawback which requires careful attention. There have been always constant efforts to develop an additive based on heteropolyacid with good stability and immobilization in the PFSA ionomers. In our approach, we have prepared an inorganic composite containing the lacunary heteropolyacid and zirconia by a sol-gel method. Lacunary heteropolyacid was prepared from the parent heteropolyacid purchased from commercial sources. Using the zirconium propoxide precursor, a sol-gel method was adopted to prepare the composite of zirconia and heteropolyacid. The PFSA ionomer (20% by weight) was provided by 3M Corporation and the membranes were recast by adding required amount of the additive to get a set of membranes with varying loadings. Proton conductivity was measured using ac impedance spectroscopy in a four electrode mode. Conductivity was measured in a test equity oven at controlled temperature and humidity. The data were collected from 30 to 110 °C at varying % RH’s such as 100, 80 and 50. High proton conductivity was observed for certain loadings of the additive doped PFSA membranes over the control undoped PFSA membrane. Other techniques were also employed to characterize the membranes. Infrared spectrum and EDX analysis were done to see the presence of heteropolyacid in the membrane. Characteristic bands of the heteropolyacid were observed in the range, 1000 to 700 cm for the membranes containing the additive. EDX spectrum showed the characteristic peaks of the constituent elements of the heteropolyacid in the membrane. XRD analysis of the membranes showed the peaks of additive in addition to the characteristic peak of the PTFE in the PFSA ionomer. Acknowledgements. This research was supported by the U.S. Department of Energy, EERE Cooperative Agreement No. DEFG36-07G017006.