Membrane-electrode-assembly with solid polymer electrolyte

Details Membrane-electrode-assembly with solid polymer electrolyte Membrane-electrode-assembly with solid polymer electrolyte

2001-05-31

2004-04-20

Weiner, Laura (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Having earth feature

C429S047000, C429S047000, C429S006000, C427S115000

Reexamination Certificate

active

06723464

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a membrane-electrode-assembly with a solid polymer electrolyte and to a manufacturing method thereof, and more particularly to an improved catalyst layer structure for use in a membrane-electrode-assembly with a solid polymer electrolyte.
FIG.
1
(
a
) depicts the basic structure of a fuel cell using a membrane-electrode-assembly with a solid polymer electrolyte. A solid polymer electrolyte
1
is sandwiched between an anode
2
and a cathode
3
, and gas diffusion layers
4
and
5
are formed on the outside of the anode
2
and cathode
3
. On the anode side, hydrogen ions (protons) and electrons are produced by the catalyst constituting the anode
2
from a hydrogen gas fed to the anode
2
through the gas diffusion layer
4
, and the resulting hydrogen ions pass through the solid polymer electrolyte
1
and form water by reacting with an oxygen gas fed to the cathode
3
via the gas diffusion layer
5
on the side of the cathode
3
and with electrons fed to the cathode
3
through outside circuitry.
Anode: H
2
→2H
+
+2e
−
Cathode: ½O
2
+2H
+
+e
−
→H
2
O
The solid polymer electrolyte
1
may, for example, be a solid polymer electrolyte membrane composed of a membrane based on perfluorosulfonic acid such as an ion-conducting resin typified by a Nafion® polymer. The ability to form this polymer into a membrane is well known in the art, described, for example, in “Procedure for Preparing Solution Cast Perfluorosulfonate lonomer Films and Membranes,” R. B. Moore and C. R. Martin, Anal. Chem., 58, 2569 (1986), and in “Ion Exchange Selectivity of NAFION® Films on Electrode Surfaces,” M. N. Szentirmay and C. R. Martin, Anal. Chem., 56, 1898 (1984).
It is also known to form stronger and thinner ion conducting membranes by reinforcing the ion-conducting polymer. In U.S. Pat. No. 5,547,551 and U.S. Pat. No. 5,599,614 to Bahar et al a composite structure of an ion conducting material contained in a base material characterized by the presence of nodes interconnected by fibrils is described. This membrane can be prepared much thinner than the ion-conducting polymer alone while still retaining enough strength for handling and use. These thinner membranes can offer improved cell performance because there is reduced cell resistance, and therefore less power loss during fuel cell operation.
The anode
2
and cathode
3
should preferably be composed of a catalyst capable of promoting the necessary electrode reactions. The composition of the catalyst used in the anode and cathode are well known in the art. Typically, some form of dispersed Pt is used in the anode, often in the form of Pt on carbon particles, while the catalyst in the cathode is typically also a Pt or Pt alloy, again often dispersed on finely grained carbon particles. Often, the catalyst is combined with an ion-conducting material or other binders and subsequently applied to the SPE membrane. Additionally, it is known in the art that one can also provide a catalyst-containing layer on the gas diffusion media.
The use of bi-layer electrodes have been described by Wilkinson in U.S. Pat. No. 5,795,669. Wilkinson's teachings are directed toward improved poisoning resistance. He disclosed the use of a two layer electrode, where one is specifically tailored to be electrochemically active, i.e., includes the presence of an ionomer, and one of which is specifically tailored to be active only in the gas phase, i.e., does not contain an ionomer. Wilkinson specifically teaches the advantage of this electrode arrangement for reducing the concentration of poisoning species in the gas phase. The layers are formed sequentially one on top of the other. The catalyst in each of the two layers is also different. The presence of the gas-active catalyst is taught as being capable or reducing the effect poisons present in the gas phase on the electrochemical reaction catalyst
The gas diffusion layers
4
and
5
are composed of a material having electric conductivity and gas permeability, such as carbon paper, woven fabric, nonwoven fabric, or another material consisting of carbon fibers.
A membrane-electrode-assembly with a solid polymer electrolyte can be easily manufactured by a method in which a solution containing catalyst particles and an ion-conducting resin is applied to the surface of a gas diffusion layer obtained using carbon paper, woven fabric, nonwoven fabric, or another material consisting of carbon fibers; and the coated catalyst diffusion layer is dried, yielding a catalyst layer. A product (gas diffusion layer/catalyst layer conjugate) in which catalyst layers
2
and
3
containing catalyst particles and ion-conducting resins are formed on the surfaces of the gas diffusion layers
4
and
5
is commonly bonded by hot pressing or another technique on both sides of the solid polymer electrolyte membrane
1
, as shown in FIG.
2
. In preferred practice, a layer
6
(
7
) composed of carbon-based particles and a fluorine-based resin (or ion-conducting resin) is disposed between the gas diffusion layer
4
(
5
) and catalyst layer
2
(
3
), as shown in FIG.
1
(
b
) (fragmentary expanded view of section B in FIG.
1
(
a
)). The same manufacturing method is used in this case. Referring again to
FIG. 2
, hot pressing or another technique is employed in this particular case to bond the gas diffusion layer/catalyst layer conjugate
8
,
9
to the solid polymer electrolyte membrane
1
because the solid polymer electrolyte membrane
1
and the catalyst layers
2
and
3
need to be joined together with minimal contact resistance. It has therefore been proposed to use methods in which ion-conducting resin solutions are used as adhesives, methods in which the components are joined using solvents capable of dissolving solid polymer electrolyte membrane materials, and other methods in addition to the hot pressing, roll pressing, and other thermocompression methods typically employed as prior art (JP (Kokai) 7-220741, 8-148167, etc.).
The catalyst layers
2
and
3
are sometimes formed directly on the surfaces of the solid polymer electrolyte membrane
1
by spraying, screen printing, decal transfer (in which the catalyst layers are thermally transferred after being formed on PTFE sheets or the like), and other methods, as shown in FIG.
3
. In such cases a membrane-electrode-assembly with a solid polymer electrolyte is constructed by combining a membrane/catalyst layer conjugate
10
with the gas diffusion layers
4
and
5
.
When, however, the gas diffusion layer/catalyst layer conjugate is bonded under heat and pressure to a solid polymer electrolyte membrane after being formed, physical or chemical damage may sometimes occur as a result of heating in the membrane itself or in the gas diffusion layers during hot pressing or another type of thermocompression bonding due to the recent trend for using thinner solid polymer electrolyte membranes. Another drawback is that because this method joins the solid polymer electrolyte membrane and the catalyst layers only slightly and yields a two-dimensional contact, the high contact resistance and physical or chemical damage to the membrane itself result in the poor performance and reduced durability of the membrane-electrode-assembly. In addition, methods in which the gas diffusion layer/catalyst layer conjugate thus formed is bonded to the solid polymer electrolyte membrane with the aid of a solution or solvent are disadvantageous in that the solid polymer electrolyte membrane is dissolved in the solution or solvent and is thus more likely to be damaged, yielding a membrane-electrode-assembly whose performance is compromised in the same manner as above.
On the other hand, methods in which catalyst layers are directly formed on the surfaces of a solid polymer electrolyte membrane are expected to provide good bonding between the solid polymer electrolyte membrane and the catalyst layers and to allow membrane-electrode-assemblies to perform better than when a gas diffusion layer/cat

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