H. Eugene McBrayer Professor of Chemical Engineering
Professor of Chemical and Biomolecular Engineering
New Proton-Exchange Membranes for Fuel Cells
Composite Nanofiber Network Membranes:
We are developing a completely new approach to ion-exchange membrane preparation, where a three-dimensional interconnected network of electrospun polyelectrolyte nanofibers is created and then the interfiber voids are filled with an inert/uncharged polymer. Such a phase-separated nanomorphology is not unlike the desired/ideal structure in a block copolymer system, but there is more flexibility in the fabrication approach. Thus, the polyelectrolyte, fiber diameter, fiber volume fraction, and inert polymer can be chosen/fixed independently. The uncharged (hydrophobic) polymer which surrounds each nanofiber restricts fiber swelling in water and provides mechanical strength to the membrane, thus permitting the fibers to have a fixed-charge concentration much greater than that which is practical in a homogenous ion-exchange membrane. Also, the interconnecting nanofiber structure ensures optimum utilization of percolation pathways of the ionomer.
Pre-Stretched Recast Nafion®:
A new Nafion membrane micromorphology has been generated by uniaxially stretching and then annealing a gelled (10% solvent content)cast film. The proton conductivity of uniaxially pre-stretched recast Nafion (0.1 S/cm at 25oC for membranes in water) was independent of draw ratio, whereas the methanol permeability decreased by a factor of 1.4-2.0, as compared to commercial Nafion, for films elongated to a draw ratio of 4. A pre-stretched recast Nafion film in the 130-180 μm thickness range, when used in a direct methanol fuel cell, generated a high power density at 0.4 V (88 mW/cm2 at 60oC, 1.0 M methanol, and ambient pressure air), as compared to 58 mW/cm2 for Nafion 117. Very high power was generated with pre-stretched recast Nafion (180 μm) by increasing the cell temperature to 80oC, increasing the anode/cathode catalyst loading to 8.0 mg/cm2, and increasing the cathode air pressure to 25 psig. Under these conditions the power density at 0.4 V for a 1.0 M methanol feed solution was 240 mW/cm2 and the maximum power density was 252 mW/cm2 (at 0.35 V).
Alkaline Fuel Cell Membranes:
Our approach is to fabricate a phase-separated segmented copolymer anion-exchange membrane with hydrophobic (non-polar) blocks to limit membrane swelling and impart mechanical strength to the membrane and hydrophilic (polyelectrolyte) blocks with backbone dimethyl ammonium anion-exchange groups for OH- conduction. We are targeting membranes with the following properties: (i) a hydroxyl ion conductivity of between 0.08 and 0.15 S/cm for a water-equilibrated membrane at 25oC, (ii) a water swelling at 25oC of 25-75%, (iii) a thickness of 50 μm, and (iv) good mechanical properties when immersed in water or an aqueous methanol solution.
Polyphosphazenes, a class of hybrid organic/inorganic polymers, are being investigated for use as the membrane material in direct liquid methanol and hydrogen/air PEM (proton exchange membrane) fuel cells. Selected polyphosphazenes are sulfonated, crosslinked and/or blended with an inert polymer, cast into thin films, characterized, and then tested in a fuel cell. Work in this area includes fundamental polymer chemistry (sulfonation of polymers, polymer blending, and solid-state crosslinking), membrane characterization (measurement of solvent permeability, proton conductivity, solvent swelling, and mechanical/thermal properties), and fuel cell evaluation (preparation of membrane-electrode-assemblies and fuel cell performance testing).
Development of Space-Charge Models for Ion Uptake and Transport in Ion-Exchange Membranes
Theories for multicomponent ion uptake and transport are being developed and tested. The models consider solvation free energy changes that occur during ion sorption, the orientation of solvent dipoles inside a membrane pore (due to the strong electric field generated by the membrane's fixed charge groups), and ion-pair formation between multivalent counterions and membrane fixed-charge sites. Models have been applied to sorption and transport of monovalent and monovalent/divalent cation salt mixtures in Nafion® cation-exchange membranes and in commercial anion-exchange membranes. Future work is focused on weak electrolyte uptake in cation-exchange membranes, membrane transport with non-aqueous and mixed solvents, and amino acid uptake and transport in ion-exchange membranes.
Organic Electrochemical Synthesis
Organic electrochemical reactions are being carried out in PEM (proton-exchange membrane) fuel cell reactors that do not require a supporting electrolyte salt and can accommodate gaseous feed streams. Fuel cell reactors are currently being investigated for organic electro-hydrogenation reactions, using precious metal catalyst powder electrodes with either water or H2 gas as the source of hydrogen. In one project, edible oils (e.g., soybean and canola oils) have been partially hydrogenated electrochemically in a PEM reactor. Partially hydrogenated oil products were characterized by a low percentage of trans fatty acid isomers (which are known contributors to coronary heart disease) and a moderately/sufficiently high concentration of saturated stearic acid for different food applications.
J. B. Ballengee and P. N. Pintauro, “Morphological Control of Electrospun Nafion Nanofiber Mats”.Journal of the Electrochemical Society, 158, B568-B572 (2011).
J. Choi, K.M. Lee, R. Wycisk, P.N. Pintauro and P. T. Mather, “Nanofiber Composite Membranes With Low Equivalent Weight Perfluorosulfonic Acid Polymers”, Journal of Materials Chemistry, 20, 6282-6290 (2010).
J. Choi, W. Zhang, R.Wycisk, P. N. Pintauro, K. M. Lee and P. T. Mather, “High Conductivity Perfluorosulfonic Acid Nanofiber Composite Fuel Cell Membranes”, ChemSusChem, 3, 1245-1248 (2010).
J. Muldoon, J. Lin, R. Wycisk, N. Takeuchi, H. Hamaguchi, T. Saito, K. Hase, F. F. Stewart, and P. N. Pintauro, “High Performance Fuel Cell Operation with a Non-fluorinated Polyphosphazene Electrode Binder”, Fuel Cells, 9, 518-521 (2009).
R. Tandon and P.N. Pintauro, “Solvent Effects During Multicomponent Ion Uptake Into a Nafion Cation-Exchange Membrane,” Journal of Membrane Science, 341, 21-29 (2009).
J. Lin, P.-H. Wu, R. Wycisk, P. N. Pintauro, and Z. Shi. “Properties of Water in Pre-Stretched Nafion,”Macromolecules, 41, 4284-4289 (2008).
J. Lin, P.-H.Wu, R.Wycisk, A. Trivisonno, Peter N. Pintauro, “Direct Methanol Fuel Cell Operation with Pre-stretched Recast Nafion,” Journal of Power Sources, 183, 491–497 (2008).