Chemistry: electrical current producing apparatus – product – and – Having earth feature
Reexamination Certificate
2001-04-30
2004-10-12
Bell, Bruce F. (Department: 1746)
Chemistry: electrical current producing apparatus, product, and
Having earth feature
C429S047000, C429S047000, C429S047000, C204S283000, C204S290010, C204S294000, C204S296000
Reexamination Certificate
active
06803143
ABSTRACT:
INTRODUCTION AND BACKGROUND
The present invention relates to gas diffusion structures and gas diffusion electrodes for fuel cells, in particular for PEM fuel cells in which a solid polymer is used as electrolyte.
Fuel cells convert a fuel and an oxidizing agent, spatially separated from each other, to electricity, heat and water at two electrodes. Hydrogen or a hydrogen-rich gas is used as the fuel and oxygen or air as the oxidizing agent. The process for energy conversion in the fuel cell is characterized by particularly high efficiency. For this reason, fuel cells in combination with electric motors are gaining increasing importance as an alternative to traditional internal combustion engines.
So-called polymer electrolyte fuel cells (PEM fuel cells) are suitable for use as energy converters in electric cars due to their compact structural design, their power density and their high efficiency.
PEM fuel cells consist of a stacked arrangement (“stack”) of membrane electrode assemblies (MEA), between which are arranged bipolar plates to supply gas and conduct electricity. A membrane electrode assembly consists of a polymer electrolyte membrane which is provided with reaction layers on both faces, namely the electrodes. One of the reaction layers is designed as an anode for the oxidation of hydrogen and the second reaction layer is designed as a cathode for the reduction of oxygen. So-called gas diffusion structures made of carbon fiber paper or carbon fiber cloth which facilitate good access by the reaction gases to the electrodes and efficient conduction of the cell current are applied to the electrodes. The anodes and cathodes contain so-called electrocatalysts which catalytically support the relevant reaction (oxidation of hydrogen or reduction of oxygen). Metals from the platinum group in the periodic table of elements are preferably used as catalytically active components.
In most cases, so-called supported catalysts are used in which the catalytically active platinum group metals have been applied in a highly dispersed form on the surface of a conductive support material. The average crystallite size of the platinum group metals is between about 1 and 10 nm. Finely divided carbon blacks have proved to be suitable support materials.
The polymer electrolyte membrane consists of polymer materials which conduct protons. These materials are also called ionomers for short in the following. A tetrafluoro-ethylene/fluorovinylether copolymer with acid functions, in particular sulfonic acid groups is preferably used. Such a material is sold by E.I. DuPont under the tradename Nafion®. However, other, in particular fluorine-free, ionomer materials such as sulfonated polyether ketones or aryl ketones or polybenzimidazoles can also be used.
A further improvement in the electrochemical cell power and a clear reduction in the costs of the system are required for the wide commercial use of PEM fuel cells in motor vehicles.
An essential prerequisite for an increase in cell power is the optimal supply and removal of the particular reaction gas mixtures to and from the catalytically active centers in the catalyst layers. In addition to supplying hydrogen to the anodes, the ionomer material in the anodes must be constantly moistened with water vapour (moistening water) in order to ensure optimum proton conductivity. The water generated at the cathodes (reaction water) must be removed continuously in order to avoid flooding of the pore system in the cathodes and thus obstruction of the oxygen supply.
U.S. Pat. No. 4,293,396 describes a gas diffusion electrode which consists of an open-pore electrically conducting carbon cloth. A uniform mixture of catalytic carbon particles and hydrophobic binder particles is evenly deposited on said cloth, with said mixture of particles adhered within the cloth pores and to the yarns of the cloth. The hydrophobic binder particles are made from Teflon® (PTFE). According to an. article from Wilson, Valerio and Gottesfeld (Electrochimica Acta Vol. 40, No 3., pp. 355 to 363, 1995) the impregnation of a carbon cloth with a carbon black/PTFE mixture results in a porous body of which the majority of pores is on the scale of the carbon black particles, or in the neighbourhood of 10 nm. Thus, the pores of the gas diffusion electrodes of U.S. Pat. No. 4,293,296 are filled with a microporous material.
The U.S. Pat. No. 4,564,427 describes gas depolarized cathodes having hydrophobic barrier layers for circulation electrolyte electrochemical cells. The cathode consists of a porous carbonized fiber substrate having the barrier layer comprising fluorocarbon polymer and carbon deposited thereon and a catalytic layer on top of the barrier layer. The barrier layer has a mean pore diameter size of about 0.2 to about 0.4 &mgr;m. Thus, the barrier layer is a microporous layer, too.
German patent document DE 195 44 323 A1 discloses a gas diffusion electrode for polymer electrolyte fuel cells which contains a carbon fiber fleece which is coated with a mixture of a carbon black suspension and a polytetrafluoroethylene suspension and is then sintered.
In EP 0 869 568, a gas diffusion layer is described which consists of a carbon fiber woven cloth which has been provided with a coating of carbon black and a fluorinated polymer on the face turned towards the catalyst layer.
The gas diffusion electrodes and gas diffusion structures or layers described in U.S. Pat. No. 4,293,396, DE 195 44 323 A1 and EP 0 869 568 are strongly hydrophobic and are not flooded by moistening or reaction water. However, free access by the reactive mixture to the catalyst layer is clearly obstructed by filling the macropores in the entire layer with a microporous material made from carbon black and polytetrafluoroethylene (PTFE). This leads to low power values when operating with dilute gases such as air and reformate gas, in particular at low stoichiometry, that is at high gas utilization.
In Canadian patent application CA 2,052,221, a hydrophobic, porous and simultaneously electrically conductive material is described which consists of a porous and electrically conductive sheet-like material and a hydrophobic polymer which is introduced into the sheet-like material by impregnation. The proportion of polymer present is 2 to 14 wt. %. WO 97/13287 describes an electrochemical cell which contains a gas diffusion structure which consists of two layers, wherein the pores in the first layer are smaller than those in the second layer and the second layer has a porosity of at least 82% and an average pore size of at least 10 &mgr;m. Also described is a non-woven gas diffusion structure in which the second layer has a porosity of at least 50% and an average pore width of 35 &mgr;m. The pore size in the fine-pored layer is cited as 0.1 to 10 &mgr;m and its porosity as at least 10%.
The gas diffusion structures described in WO 97/13287, in particular when operating with moistened reactive gases at high current density, low reactive gas pressure and low stoichiometry, suffer from flooding of the gas diffusion structures with reaction or moistening water.
EP 0 928 036 A1 describes a carbon cloth based electrocatalytic gas diffusion electrode comprising on one or both of its surfaces one or more microporous layers manufactured from different mixtures of carbon black and PTFE.
WO 99/56335 describes a structure which is intended to facilitate the simultaneous transport of gases and liquids. It consists of a porous carbon fiber substrate which has 75-95% of hydrophobic pore volume and 5-25% hydrophilic pores. The two fractions are blended in one layer and are in direct contact. Here again, pores with a pore width greater than a few 10 &mgr;m are present which are flooded when there is a great deal of product water produced or with intense moistening. This leads in particular at high current density to gas transport problems.
U.S. Pat. Nos. 4,927,514 and 5,441,823 describe air cathodes and processes for their production. They consist of a porous support layer made of carbon and hydrophobic polymers which make contact with an
Bänisch Volker
Bayer Armin
Fehl Knut
Lehmann Thomas
Zuber Ralf
Bell Bruce F.
Kalow & Springut LLP
Umicore AG & Co. KG
Wills M.
LandOfFree
Gas diffusion structures and gas diffusion electrodes for... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Gas diffusion structures and gas diffusion electrodes for..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Gas diffusion structures and gas diffusion electrodes for... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3262192