Jung-Chou Lin

Study of blend membranes consisting of NafionR and vinylidene fluoride–hexafluoropropylene copolymer

An attempt to modify membranes for direct methanol fuel cells by blending NafionR with a (vinylidene fluoride)–hexafluoropropylene copolymer (VDF–HFP copolymer) from their solutions is reported. The purpose of this work was to reduce the methanol transport while still retaining the essential proton conductivity in a water-containing environment. The apparent conductivity, methanol barrier property, and equilibrium contact angle as a function of the membrane compositions are discussed. The blend membranes were also investigated using X-ray diffraction (XRD) and differential scanning calorimetry (DSC). Compared with the pure NafionR membrane, the NafionR/VDF–HFP copolymer blend membrane with 62.5 vol % of the VDF–HFP copolymer shows a decrease in the apparent conductivity by about 2 orders of magnitude, and the methanol barrier properties increase substantially when only 25 vol % of the VDF–HFP-copolymer is incorporated. The equilibrium contact angles of water drops on the NafionR/VDF–HFP copolymer blend membranes as a function of the VDF–HFP copolymer content are rather similar to the plot of the advancing angle versus the percentage of the lower-surface-energy phase. X-ray diffraction studies indicate that these two polymers crystallize separately when blended and cast from their solutions, and the crystallization behavior is equivalent to that of the unblended state. DSC reveals that when the VDF–HFP copolymer is mixed with NafionR in their solution forms, an interdiffusion or other interaction takes place at the interfaces between their noncrystalline regions.

Trilayer Membranes with a Methanol-Barrier Layer for DMFCs

Trilayer membranes (50 μm thick) composed of one central methanol-barrier layer and two conductive layers were developed to suppress methanol crossover in liquid-fed direct methanol fuel cells (DMFCs). Nafion-(poly)vinylidene fluoride (PVDF) was used as a central methanol-barrier layer with two outside Nafion layers. Both proton conduction and methanol diffusion were controlled by the properties of the Nafion-PVDF layer. The thickness of the barrier layer and the PVDF content affected methanol crossover and proton conductivity of the trilayer membrane. A 55 wt % PVDF, 45 wt % Nafion, 10 μm thick barrier layer with two outside 20 μm Nafion layers reduced methanol crossover by 64% compared to a 50 μm thick Nation membrane. Membrane electrode assembly performance with the trilayer membrane was better than a 50 μm thick Nafion membrane because of lower methanol crossover and the presence of a good membrane/electrode interface for proton transfer.

CO Tolerance of Carbon-Supported Platinum-Ruthenium Catalyst at Elevated Temperature and Atmospheric Pressure in a PEM Fuel Cell

Experiments were performed to determine the carbon monoxide (CO) tolerance of an atmospheric pressure, hydrogen-oxygen, proton-exchange-membrane (PEM) fuel cell at elevated temperatures (80 to 120°C). A Nafion-Teflon-Zr(HPO 4 ) 2 composite membrane was used to avoid the high resistance normally encountered with a Nafion membrane due to dehydration at the low reactant relative humidities encountered under these conditions. A Pt-Ru/C (40 wt % precious metal, Pt/Ru = 1/1 atomic ratio) anode catalyst of 0.4 mg Pt-Ru/cm 2 loading was used. The anode polarization decreased significantly as the cell temperature was increased from 80 to 105 to 120°C due to both smaller CO coverage on the anode catalyst and improved activity of the catalyst for CO and hydrogen oxidation. At elevated temperatures, the loss of water at the anode resulted in an increase in the measured cell resistance with increasing CO concentration. This effect is presumed to be due to an increase in the anode polarization that was partially measured along with the membrane resistance, when determined using the current interruption technique. The cell performance did not increase monotonically with temperature. When the CO concentration in the hydrogen at the anode was above 100 ppm, the fuel cell performance at 105°C was higher than that at 80°C. Dehydration of the membrane and catalyst layers tended to offset improved CO tolerance at elevated temperatures from 105 to 120°C at the same reactant water vapor content.