Membrane electrode assembly and method of its production

Details Membrane electrode assembly and method of its production Membrane electrode assembly and method of its production

2001-06-21

2004-02-03

Bell, Bruce F. (Department: 1746)

Chemistry: electrical and wave energy

Apparatus

Electrolytic

C204S296000, C204S252000, C429S047000, C429S047000, C429S047000, C429S006000, C429S006000

Reexamination Certificate

active

06685806

ABSTRACT:

TECHNICAL FIELD
The present invention relates to the electrochemical industry in general, and more particularly to a membrane-electrode assembly (“MEA”) based on fluoro-containing ion-exchange membranes and to a method for its production. Such MEAs are widely used in fuel cells, in water electrolyzers, and in other electrochemical processes.
BACKGROUND ART
MEA consisting of the fluoro-containing ion exchange membrane M174-SK™ (Russian trademark [RTM]) and of layers of an electrode material (electrode composition) situated on both sides of the membrane are already known. The electrode composition consists of a mixture of an electrocatalyst and an ion-exchange polymer [USSR patent 1,258,095 IPC S25V 11/10, 1990]. As an ion-exchange polymer in the electrode composition, an inorganic proton-conducting electrolyte (polyantimonic acid, acidic zirconium phosphate) is used. The electrocatalyst is platinum, palladium, or rhodium black.
MF-4 SK membrane is a 300 micron thick cation-exchange membrane (“CEM”) made from a hydrolyzed copolymer of tetrafluoroethylene and a perfluorosulfur group-containing vinyl ether, having the following structural formula:
Our experiments showed that this copolymer has an equivalent weight (EW) of 1200 and a degree of crystallinity of 12%, as shown in control Example 1.
The MEA is produced by applying an electrode composition on both sides of the CEM. The sedimentation method is used. The electrode composition consists of a mixture of an electrocatalyst and an ion-exchange polymer (polyantimonic acid) powder. The composition is fixed by electric current treatment in water at 90° C., where the current density is 0.5-1 A/cm
2
.
A MEA is produced that consists of, for example, CEM MF-4SK and layers of the electrode composition on both sides with an electrocatalyst (platinum black, size of particles 0.01 micron) on both the cathode and the anode sides. The MEA deionized water electrolysis has the following characteristics: the voltage is 2.2V when the current density is 1 A/cm
2
, and the temperature is 100° C. The voltage does not change over a period of time of 1000 hours.
Advantageously, the MEA is stable over a period of time of 1000 hours.
The disadvantage of the MEA (USSR patent 1,258,095) is the impossibility to achieve high adhesion between layers of the electrode composition and CEM because the ion-exchange polymer (polyantimonic acid) is gradually dissolved after a long period of time (more than 1000 hours) of electrolysis of water. Consequently, over a long period of working with MEA, one can observe a tendency for exfoliation of the electrode composition. The method of the MEA production does not permit precise regulation of the composition and the amount of the electrode material applied to CEM. The method is complicated by the fact that layers of electrode composition (electrocatalytic layers) are applied by a method of sedimentation, which requires the following electric current characteristic to fix layers on CEM: a rather high electrocatalyst loading of 1-2 mg/cm
2
on the cathode and 4-6 mg/cm
2
on the anode.
A MEA with a porous cathode is also known. Such a MEA consists of a polymeric ion exchange membrane of Nafion® (trademark of CEM by Du Pont) type and a porous layer of an electrode material, a mixture of electrocatalyst particles and a binder (Russian Federation [RF] patent 2,015,207, IPC S25V 11/20, 1994), settled on the cathode side of CEM. The porous cathode layer of the electrode composition is made of a mixture of electrocatalytic particles and the binder, polytetrafluorethylene. The membrane (Nafion®) is produced from a hydrolyzed copolymer of tetrafluoroethylene and perfluorinated vinyl ethers containing ion-exchange groups. For water electrolysis of CEM (Du Pont) Nafion® 120 with —SO
3
H ion exchanging groups, see Russian Federation patent H; see RE patent describing tetrafluoroethylene and perfluorinated vinyl ethers containing ion-exchange groups for water electrolysis of CEM (Du Pont) Nafion® 120 with SO
3
tetrafluoroethylene ion exchanging groups. The membrane (Nafion®) is produced from hydrol248, 5 American Chemical Society, Washington D.C.
The MEA mentioned above is produced by applying a mixture of electrocatalytic particles and inactive conducting material with the binder (polytetrafluorethylene) and aluminum powder on an aluminum sheet by technique A. After drying at 105° C., for example, sintering at 325° C. is carried out for 10 minutes. Then, the aluminum sheet with the layer of electrode material is placed on the cathode surface of CEM and pressed at 175° C. and a pressure of 50-60 kg/cm
2
. After pressing, the MEA is dipped in a caustic soda solution to dissolve the aluminum sheet and aluminum powder (the latter is used as a promoter of porosity). Then, the layer of electrode material become porous. The advantage of the MEA of Russian Federation (RF) patent No. 2,015,207 is that MEA's lifetime increases because the binder (polytetrafluorethylene) does not dissolve during the electrolysis. When such MEA is used for water electrolysis, the cell voltage is 1.8-1.9 V.
The disadvantage of the MEA described above is that the adhesion between the applied porous layer of electrode material and the CEM surface is not as strong as required. During MEA tests of long duration, exfoliation of the porous layer of the electrode material occurs and the evolving gases are deposited at the surface between the CEM and the porous layer. This results in an increase of the MEA voltage. Moreover, as is shown in the Russian Federation patent (example 4), the disadvantage of the described MEA is a comparatively high electrocatalyst loading because of its particular capsulation by polytetrafluorethylene during production (pressing at 325° C. and a pressure of 50-60 kg/cm
2
).
Moreover, the volume porosity of the electrocatalytic layer of the electrode material is uncontrollable, so the transport of gases and liquids in the reaction zone is impeded and the electrochemical properties of the MEA worsen.
The manner of producing the described MEA is rather complicated, because high temperature (>300° C.) sintering and aluminium lixiviation to form the porous layer of an electrode material are required. Moreover, production of the MEA by pressing at 175° C. leads to the particular destruction of cation-exchanging groups that worsens CEM electrochemical characteristics and may destroy the whole MEA.
The art having an essential set of attributes closest to the claimed MEA and its method of production is the MEA, and the method of its production, described in U.S. Pat. No. 5,399,184 HOIM 8/10, 1995. U.S. Pat. No. 5,399,184 discloses an MEA that consists of a fluoro-containing cation-exchange membrane made of a tetrafluoroethylene hydrolyzed copolymer and a fluorosulfur group-containing vinyl ether, with an exchange capacity of 0.83-1.43~meq/g (as described in the specification) or 1.12-1.43 meq/g (as described in the patent's Examples and Claims), which corresponds to an EW of 900-1300, and discloses porous layers of an electrode material situated on both of its surfaces. These layers are made of a mixture of an electrocatalyst with an inactive electroconducting material and a fluoropolymer binder. The fluoropolymer binder is a cation-exchange fluoropolymer with a composition identical to the membrane polymer, or may be polytetrafluorethylene. The CEM is made of a hydrolyzed copolymer of tetrafluoroethylene with a perfluorsulfur-containing vinyl ether. Its structural formula is:
For example, it may be CEM produced by Du Pont—Nafion® 117. This membrane is made of a copolymer with a degree of crystallinity of 12%. [ACS Symposium Perfluorinated lonomer Membranes, Lake Buena Vista, Fla. Feb. 23-26, 1982, Series 180, pp 217-248, American Chemical Society, Washington D.C.]
The MEA specified by prototype (U.S. Pat. No. 5,399,184) is produced by application of the paste of electrode material on the both surfaces of the CEM. The latter consists of the hydrolyzed copolymer of tetrafluoroethylene with th

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