Non-woven fiber webs

Details Non-woven fiber webs Non-woven fiber webs

2000-06-12

2002-11-26

Weiner, Laura (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S251000, C429S304000, C429S142000, C429S145000, C442S417000, C442S331000, C442S332000, C442S348000, C442S367000

Reexamination Certificate

active

06485856

ABSTRACT:

The present invention relates to a novel non-woven fibre web with continuous reinforcing strands, a membrane comprising said web and a membrane electrode assembly comprising said membrane, all of which have application in electrochemical devices, for example for use in a fuel cell. The invention further describes a process for the manufacture of the web, membrane and membrane electrode assembly.
Electrochemical cells invariably comprise at their fundamental level a solid or liquid electrolyte and two electrodes, the anode and cathode, at which the desired electrochemical reactions take place. A fuel cell is an energy conversion device that efficiently converts the stored chemical energy of its fuel into electrical energy by combining either hydrogen, stored as a gas, or methanol stored as a liquid or gas, with oxygen to generate electrical power. The hydrogen or methanol is oxidised at the anode and the oxygen is reduced at the cathode of the electrochemical cell. In these cells gaseous reactants and/or products have to be diffused into and/or out of the cell electrode structures. The electrodes therefore are specifically designed to be porous to gas diffusion in order to optimise the contact between the reactants and the reaction sites in the electrode to maximise the reaction rate. The electrolyte which has to be in contact with both electrodes to maintain electrical contact in the fuel cell may be acidic or alkaline, liquid or solid, in nature. The proton exchange membrane fuel cell (PEMFC) is the most likely type of fuel cell to find wide application as a more efficient and lower emission power generation technology in a range of markets including stationary and portable power devices and as alternative to the internal combustion engine in transportation. In the PEMFC, whether hydrogen or methanol fuelled, the electrolyte is a solid proton conducting polymer membrane, commonly based on perfluorosulphonic acid materials.
In the PEMFC the combined laminate structure formed from the membrane and the two electrodes is known as a membrane electrode assembly (MEA). The MEA will typically comprise several layers, but can in general be considered, at its basic level, to have five layers, which are defined principally by their function. On either side of the membrane an anode and cathode electrocatalyst is incorporated to increase the rates of the desired electrode reactions. In contact with the electrocatalyst containing layers, on the opposite face to that in contact with the membrane, are the anode and cathode gas diffusion substrate layers. The anode gas diffusion substrate is designed to be porous and to allow the reactant hydrogen or methanol to enter from the face of the substrate exposed to the reactant fuel supply, and then to diffuse through the thickness of the substrate to the layer which contains the electrocatalyst, usually platinum metal based, to maximise the electrochemical oxidation of hydrogen or methanol. The anode electrocatalyst layer is also designed to comprise some level of the proton conducting electrolyte in contact with the same electrocatalyst reaction sites. With acidic electrolyte types the product of the anode reaction are protons and these can then be efficiently transported from the anode reaction sites through the electrolyte to the cathode layers. The cathode gas diffusion substrate is also designed to be porous and to allow oxygen or air to enter the substrate and diffuse through to the electrocatalyst layer reaction sites. The cathode electrocatalyst combines the protons with oxygen to produce water and is also designed to comprise some level of the proton conducting electrolyte in contact with the same electrocatalyst reaction sites. Product water then has to diffuse out of the cathode structure. The structure of the cathode has to be designed such that it enables the efficient removal of the product water. If water builds up in the cathode, it becomes more difficult for the reactant oxygen to diffuse to the reaction sites, and thus the performance of the fuel cell decreases. In the case of methanol fuelled PEMFCs, additional water is present due to the water contained in the methanol, which can be transported through the membrane from the anode to the cathode side. The increased quantity of water at the cathode requires removal. However, it is also the case with proton conducting membrane electrolytes, that if too much water is removed from the cathode structure, the membrane can dry out and the performance of the fuel cell also decreases.
The complete MEA can be constructed by several methods. The electrocatalyst layers can be bonded to one surface of the gas diffusion substrates to form what is known as a gas diffusion electrode. The MEA is then formed by combining two gas diffusion electrodes with the solid proton-conducting membrane. Alternatively, the MEA may be formed from two porous gas diffusion substrates and a solid proton-conducting polymer membrane catalysed on both sides (hereinafter referred to as a catalyst coated membrane or CCM); or indeed the MEA may be formed from one gas diffusion electrode and one gas diffusion substrate and a solid proton-conducting polymer catalysed on the side facing the gas diffusion substrate.
Conventionally, the solid proton conducting membrane electrolytes used in the PEMFC and other devices are selected from commercially available membranes, for example perfluorinated membranes sold under the trade names Nafion® (E.I. DuPont de Nemours and Co.), Aciplex® (Asahi Chemical Industry) and Flemion® (Asahi Glass KK). For application in the PEMFC the membranes are typically below 200 &mgr;m in thickness to provide a high level of ionic conductivity. However, for the advanced, high power density fuel cells, these need to have membranes less than 100 &mgr;m thick and preferably less than 50 &mgr;m thick. It is also necessary with these membranes that a high level of water is present within the membrane to provide efficient proton hydration and a high proton conductivity. The dimensional changes that occur as the level of water content (hydration) of the membrane changes are a particular problem during fabrication of the MEA as the stresses set up by changes in hydration during the conventionally employed thermal bonding process can be so large as to break the bond between the catalyst and the membrane, or the catalyst and the substrate. Furthermore, the dimensional changes that occur due to the changes in the level of hydration of the membrane lead to considerable difficulties in handling membranes during the fabrication of large area MEAs (for example greater than 500 cm
2
). The thinner the membrane, the more difficult the handling becomes.
To address these problems composite membrane structures have been prepared. With thicker types of membrane (e.g. >350 &mgr;m) developed for other applications, it has been possible to incorporate ‘macro’ reinforcing materials such as woven polytetrafluoroethylene (PTFE) to minimise such dimensional changes. However, these thicker materials have too low an ionic conductivity to be of use in the PEMFC. U.S. Pat. No. 5,547,551 assigned to W.L. Gore & Associates Inc. describe the fabrication of ultra-thin composite membranes below 25 &mgr;m in thickness which comprise incorporating proton conducting polymer material into an expanded porous PTFE membrane. According to Kolde et al., Electrochemical Society Proceedings Vol. 95-23, p193-201 (1995), the composite membrane shows a considerably lower reduction tensile strength on hydration and much improved dimensional stability compared to the conventional non-reinforced membranes. The material has, however, a higher specific resistance (lower ionic conductivity) than an unmodified pure proton conducting membrane such as Nafion® 117 by a factor of at least two.
The higher specific resistance of the above composite membrane means that in practice it has to be much thinner than the equivalent pure proton conducting membrane to maintain the same overall conductivity and thus cell performance. However, reducing the thickness of

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