Patent Application
20080063914
For a fuller understanding of the nature and desired objects of the present invention, reference is made to the following detailed description taken in conjunction with the accompanying drawing figures wherein:
The following definitions are for convenient reference with respect to the following description and are not to be construed in a limiting manner.
The term Gel Permeation Chromatography (“GPC”) shall be understood to mean or refer to a method or technique used in order to determine the molecular weight (Mn and Mw) and dispersity of the polymers.
The term Nuclear Magnetic Resonance (“NMR”) shall be understood to mean or refer to a method or technique used in order to identify the chemical and molecular structure of the polymers and the proportion of the monomers in the copolymers.
The term Fourier-Transform Infrared Spectroscopy (“FT-IR”) shall be understood to mean or refer to a method or technique used in order to identify the chemical and molecular structure of the polymers in association with NMR data.
The term Dynamic Mechanical Analysis (“DMA”) shall be understood to mean or refer to a method or technique used in order to identify the Tg (glass transition temperature) of the polymers, homopolymers, copolymers and blends.
The term Thermogravimetric Analysis (“TGA”) shall be understood to mean or refer to a method or technique used in order to study the thermal, and oxidative stability before and after Fenton's test as well as before and after doping with acids.
The term Fenton's Test shall be understood to mean or refer to a method or technique used in order to study and determine the oxidative stability of the polymers and their blends.
The term Scanning Electron Microscope (“SEM”) shall be understood to mean or refer to the equipment or instrument used in order to obtain data of the structure in the nano level of the membranes and the degree of compatibility and dispersion of the polymer blends.
The term Single Fuel Cell Test shall be understood to mean or refer to a method or technique used in order to obtain real practical data of the proton conductivity of the polymer electrolyte membranes, and test their fuel cell performance. The single cell test is a quite complex procedure and one important aspect is the MEA preparation. Designing the MEA, several parameters should be taken into account in order to have satisfying performance of the cell. The selection of the catalyst, the specimen of the ionomer, the proportion of catalyst/ionomer, conditions like pressure and temperature, the thickness of the membrane, the doping level and the testing surface of the membrane are some of such parameters. Diversions of these parameters are carried out so as to optimize the cell test for each polymer or polymer blend.
On each side of the doped membrane a gas diffusion electrode is deposited. The catalyst ink consists of the electrolyte and platinum supported on active carbon with varying Pt loading. The GDE consists of “flat type” gas diffusion layer or carbon paper provided by ETEK on which the catalyst ink was deposited by using screen printing or aerography. The membrane electrode assembly is mounted into the 5×5 cm single cell by applying a torque of 9 N*m at each nut of the cell (number of nuts: 8). Each electrode's area is 25 cm2. Using a system of a galvanostat/potensiostat, I-V plots are acquired, and using this system in cooperation with a frequency response detector, A.C. impedance measurements are also acquired.
The present invention relates to the development of polymer electrolyte membranes based on blends of aromatic polyethers containing pyridine units in the main chain. These polymeric blends are easily doped with inorganic acids such as phosphoric acid, resulting in ionically conducting membranes. The membranes show excellent mechanical properties and exceptional thermal and oxidative stability. Combination of these polymeric membranes with electrodes that contain the desired amount of the noble metal supported on carbon and the polymeric binder of the same composition as the membrane results in membrane electrode assemblies that can operate at temperatures between 120° C. and 180° C.
The polymer blends are composed of a homopolymer of the structure 1 which is easily doped with phosphoric acid and a copolymer of the structure 2 which provide excellent mechanical properties and the good dimensional stability.
Homopolymer 1 and copolymer 2 are synthesized according to published procedures (Chemistry of Materials 2003, 15(46), 5044, Journal of the Membrane Science 2003, 252, 115) properly modified for the various copolymer compositions. Polymer 1 has high glass transition temperature up to 280° C., while copolymer 2 has glass transition temperature in the range of 160-220° C. depending on the copolymer composition. When the Fenton's test is applied, these polymers and copolymers show exceptional oxidative stability. With this test the membranes are exposed to a strongly oxidative environment. More specifically, membranes are treated with hydrogen peroxide in the presence of ferrous ions at 80° C. for 48 hours. Hydroxyl and hydroperoxy radicals are created which can cause structural changes to polymers through addition to the benzene rings. This could lead to either chain scission or to ring opening. Such structural changes could further affect some physical properties of the materials. However, these polymers and copolymers retain their flexibility and mechanical integrity. This is shown in
Blends of homopolymer 1 with copolymer 2 at blend compositions ranging from 10:90 to 90:10 and preferably between 20:80 and 80:20 are prepared by mixing dimethylacetamide solution of the respective polymers in the proper ratio. The resulting solutions are stirred at room temperature for 3 hours and then casted on a glass dish. The solvent is evaporated in an oven at 70-120° C., for 24 hours. The membranes are washed with distilled water and dried under vacuum at 170° C. for 72 hours. The miscibility behavior of the blends is then examined through dynamic mechanical analysis using the single glass transition criterion. In most cases, polymer blends are not miscible. However, some polymer pairs are miscible due to some type of specific interactions between the two components. In the present invention, the blends are miscible in most cases. An example is given in
The membranes are doped with phosphoric acid at different temperatures and for different doping times, depending on the membrane composition. The doping level is defined as the weight percent of the acid per gram of the polymer, copolymer or blend. An example of the doping behavior of the membrane of polymer 1/copolymer 2 at a 50/50 blend composition is shown in
The carbon backing where the gas diffusion electrodes are constructed, is either carbon cloth or carbon paper. The hydrophobic sublayer, which is applied onto the carbon substrate, consists of carbon black (20-70 wt %) containing a hydrophobic polymer (30-80 wt %). More specifically, an emulsion of polytetrafluoroethylene (PTFE) is mechanically mixed with carbon black for 30 minutes. The bulk of the solvent is removed by filtration. The slurry is applied onto the carbon paper or cloth. After sintering at a temperature of 350° C. for 30 minutes, this layer is rendered hydrophobic.
Carbon supported noble metal (Pt) catalyst, is mechanically mixed with a dimethylacetamide polymer solution until the catalyst is uniformly distributed. The weight ratio of the noble metal to carbon is about 1:9 to 4:6, most preferably 3:7, while the concentration of the polymer solution is 1-4 wt %. The selected polymer is either a pure polymer or a blend. The weight ratio of the noble metal to the ionomer is 7:3 to 1:1. The Pt/C/ionomer slurry is obtained after evaporation of the solvent. The slurry is then applied onto the gas diffusion layer. Sufficient fluidity of the slurry ensures that the resulting surface of the catalyst layer is appropriate. The electrodes are finally dried under vacuum in an oven at temperature of 190° C. for 1-2 hours and the final noble metal loading is 0.5-1 mg/cm2.
The membrane electrode assembly is prepared by pressing the gas diffusion anode, the gas diffusion cathode and the doped polymer membrane into a 5×5 cm single cell by applying a torque of 9 N*m at each nut of the cell (total of 8 nuts). The electrodes area is therefore 25 cm2. The membrane is then doped with phosphoric acid at a doping level of 150 to 250 wt % and its thickness after doping varies from 90-200 μm.
The following non-limiting examples are illustrative of the invention. All documents mentioned herein are incorporated herein by reference.
0.5 g of polymer 1 is dissolved in 10 ml dimethylacetamide and 0.5 g of copolymer 2 is also dissolved in 10 ml dimethylacetamide at room temperature. The two solutions are mixed and stirred at room temperature for 3 hours. The solution is filtrated through glasswool and poured into a 100 mm diameter glass dish. The solvent is slowly evaporated in an oven at 80° C. for 24 hours and the membrane is washed with water and dried at 170° C. for 48 hours under vacuum. The membrane is immersed in 85 wt % phosphoric acid at 80° C. for 2 hours in order to reach a doping level of 200 wt %.
0.7 g of polymer 1 is dissolved in 14 ml dimethylacetamide and 0.3 g of copolymer 2 is also dissolved in 6 ml dimethylacetamide at room temperature. The two solutions are mixed and stirred at room temperature for 3 hours. The solution is filtrated through glasswool and poured into 100 mm diameter glass dish. The solvent is slowly evaporated in an oven at 80° C. for 24 hours and the membrane is washed with water and dried at 170° C. for 48 hours under vacuum. The membrane is immersed in 85 wt % phosphoric acid at 80° C. for 5 hours in order to reach a doping level of 320 wt %.
A slurry of 70 wt % carbon powder and 30 wt % PTFE is applied onto carbon cloth. The resulting hydrophobic layer is dried and sintered at 350° C. for 30 minutes. A mixture of 50 wt % Pt from a Pt/C 28.6% catalyst powder and 50 wt % polymer 1 from a 3 wt % polymer solution in dimethylacetamide, is well mixed and applied onto the supporting layer of the carbon cloth. The electrodes are first dried at 80° C. for 20 hours and at 190° C. for 1 hour under vacuum. The platinum loading in the catalyst layer of the anode and the cathode is 1 mg/cm2.
For the preparation of the MEA, a blend of polymer 1/copolymer 2 50/50 is doped with phosphoric acid at a doping level of 150 wt %. The membrane thickness is 80 μm after the doping. The membrane electrode assembly is mounted into the 5×5 cm single cell by applying a torque of 9 N*m at each nut of the cell (total of 8 nuts). The electrodes' area is therefore 25 cm2.
As mentioned above, the assembly is mounted in the 5×5 cm single cell. Current density versus cell voltage curves are measured at each temperature after the cell performance reaches a steady state. Dry hydrogen and oxygen are supplied under atmospheric pressure.
This application claims priority to and the benefit of Provisional U.S. Application Ser. No. 60/843,800, filed Sep. 11, 2006, the entire contents of which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60843800 | Sep 2006 | US |
