Chemistry: electrical current producing apparatus – product – and – Having earth feature
Reexamination Certificate
2000-06-15
2004-01-13
Ryan, Patrick (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Having earth feature
C429S047000, C442S341000, C442S342000, C442S344000
Reexamination Certificate
active
06677073
ABSTRACT:
The present invention relates to a novel non-woven fibre web with continuous reinforcing strands, a porous gas diffusion substrate, a porous gas diffusion electrode and a membrane electrode assembly, all of which may have application in electrochemical devices, for use for example in a fuel cell. The invention further relates to a process for the manufacture of the web, substrate, electrode 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 oxygen is reduced at the cathode. 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 also has to be in contact with both electrodes and in fuel cell devices may be acidic or alkaline, liquid or solid, in nature. In the proton exchange membrane fuel cell (PEMFC), whether hydrogen or methanol fuelled, the electrolyte is a solid proton-conducting polymer membrane, commonly based on perfluorosulphonic acid materials. The 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 generation devices and as alternative engines to the internal combustion engine in transportation.
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; 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.
Gas diffusion substrates or electrodes are employed in many different electrochemical devices in addition to fuel cells, including metal-air batteries, electrochemical gas sensors, and electrochemical reactors for the electrosynthesis of useful chemical compounds.
Traditionally, the gas porous substrates used in the PEMFC are based on high density materials such as rigid carbon fibre paper (e.g. Toray TGP-H-60 or TGP-H-90 from Toray Industries Inc., Japan) or woven carbon cloths, such as Zoltek PWB-3 (Zoltek Corporation, 3101 McKelvey Road, St. Louis, Mo. 63044). These materials present a number of problems in terms of cost, compatibility with high volume manufacturing processes and adaptability to the characteristics required for a wide range of cell designs and operating regimes. Current estimates suggest that these types of material are at least an order of magnitude too expensive for many applications, particularly for transportation, and their physical structure cannot be readily modified to ensure compatibility with the range of operating conditions envisaged. With existing carbon fibre papers the rigidity of the material precludes the development of high volume reel to reel processes, which offer the most attractive route for the manufacture of the quantities of MEAs required. Carbon cloths could be used in reel to reel processes but their dimensional instability and tendency to fray at cut edges impose significant additional difficulties in scaling up to high volume processes. For PEMFC's to become commercially viable power sources over a range of applications the gas porous substrate will require a manufacturing process capable of producing millions of square metres of material at very low cost and able to impart specific structural properties to the material for each application.
More recently, flexible electrode structures based on a porous substrate comprising a non-woven web of carbon fibres, with a particulate material embedded within the fibre web as disclosed in EP 0 791 974 have shown comparable performances to structures based on carbon fibre paper or cloth, without the drawbacks previously outlined. Such materials fulfil the requirement for a process capable of delivering a range of materials with specific structural properties as disclosed in PCT patent applications PCT/GB00/00390 and PCT/GB00/00382 giving a physically strong, dimensionally stable and handleable web at a cost compatible with motive power applications. One outstanding issue with webs based on a non-woven fibre web with an embedded particulate material is that the structure has a significant degree of compressibility. This characteristic can pose problems when there are a large number of MEAs in a fuel cell stack, due to the large movement on compression of the MEAs in the stack during assembly. Also in stack designs with particularly wide flow field channels in the fuel and oxidant plates th
Brown Karen Leanne
Cooper Susan Joy
Gascoyne John Malcolm
Ralph Thomas Robertson
Johnson Matthey Public Limited Company
Mercado Julian
RatnerPrestia
Ryan Patrick
LandOfFree
Non-woven fiber webs does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Non-woven fiber webs, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Non-woven fiber webs will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3205171