METHOD OF MAKING A MEMBRANE ELECTRODE ASSEMBLY COMPRISING A VAPOR BARRIER LAYER, A GAS DIFFUSION LAYER, OR BOTH
Methods of producing an electrochemical conversion assembly comprising an electrochemical conversion cell are provided. The electrochemical conversion cells which comprises a membrane electrode assembly, first and second flowfield portions defined on opposite sides of the membrane electrode assembly, and, at least one vapor barrier layer disposed between the membrane electrode assembly and at least one of the flowfield portions. The method further includes selecting a desired mass transfer coefficient MTC for the at least one vapor barrier layer of at least about 0.05 cm and optimizing one or more of the porosity, the tortuosity, and the thickness of the vapor barrier layer to produce the desired MTC in the vapor barrier layer.
1 . A method of producing an electrochemical conversion assembly comprising:
providing an electrochemical conversion assembly comprising at least one electrochemical conversion cell, the electrochemical conversion cell comprising a membrane electrode assembly, first and second flowfield portions defined on opposite sides of the membrane electrode assembly, and at least one vapor barrier layer disposed between the membrane electrode assembly and at least one of the flowfield portions;
selecting a desired mass transfer coefficient (MTC) for the at least one vapor barrier layer of at least about 0.05 cm, wherein the
MTC
=
D
D
eff
h
,
where h is a thickness of the vapor barrier layer, D is the free gas-phase diffusivity through the vapor barrier layer, and D eff is the effective diffusivity through the vapor barrier layer; and
optimizing one or more of the porosity, the tortuosity, and the thickness of the vapor barrier layer to produce the desired MTC in the vapor barrier layer.
2 . A method according to claim 1 wherein the optimized vapor barrier layer comprises a thickness of up to 100 μm.
3 . A method according to claim 1 wherein the optimized vapor barrier layer comprises a porosity of between about 20% to about 70%.
4 . A method according to claim 1 wherein the optimized vapor barrier layer comprises a tortuosity of between about 4 to about 10.
5 . A method according to claim 1 further comprising adhesively bonding the vapor barrier layer to the membrane electrode assembly in order to reduce electrical resistance across an interface therebetween.
6 . A method according to claim 1 further comprising hot pressing the vapor barrier layer to the membrane electrode assembly in order to reduce electrical resistance across an interface therebetween.
7 . A method according to claim 1 wherein the vapor barrier layer comprises metal, or graphite.
8 . A method according to claim 1 wherein the vapor barrier layer comprises carbon fibers, carbon sheets, or combinations thereof
9 . A method according to claim 1 wherein the vapor barrier layer comprises filler material comprising fluoropolymers, carbon particles, carbonizable thermoset resin, ionomers, or combinations thereof.
10 . A method of producing an electrochemical conversion assembly comprising:
providing an electrochemical conversion assembly comprising at least one electrochemical conversion cell, the electrochemical conversion cell comprising a membrane electrode assembly, first and second flowfield portions defined on opposite sides of the membrane electrode assembly, and a gas diffusion layer between the membrane electrode assembly and the flowfield portions;
selecting a desired mass transfer coefficient (MTC) for the gas diffusion layer of at least about 0.10 cm, wherein the
MTC
=
D
D
eff
h
,
where h is a thickness of the gas diffusion layer, D is the free gas-phase diffusivity through the gas diffusion layer, and D eff is the effective diffusivity through the gas diffusion layer; and
optimizing one or more of the porosity, the tortuosity, and the thickness of the gas diffusion layer to produce the desired MTC in the gas diffusion layer.
11 . A method according to claim 10 wherein the gas diffusion layer comprises a substrate layer and a microporous layer arranged thereon.
12 . A method according to claim 10 wherein the gas diffusion layer comprises carbon paper, carbon fibers, fluoropolymers, and combinations thereof.
13 . A method according to claim 10 wherein the optimized gas diffusion layer comprises a thickness of between about 150 μm to about 250 μm.
14 . A method according to claim 10 wherein the optimized gas diffusion layer comprises a porosity of between about 20% to about 70%.
15 . A method according to claim 10 wherein the optimized gas diffusion layer comprises a porosity of between about 4 to about 10.
16 . A method of producing an electrochemical conversion assembly comprising:
providing an electrochemical conversion assembly comprising at least one electrochemical conversion cell, the electrochemical conversion cell comprising a membrane electrode assembly, first and second flowfield portions defined on opposite sides of the membrane electrode assembly, respective gas diffusion layers between the membrane electrode assembly and the flowfield portions on opposite sides of the membrane electrode assembly, and respective vapor barrier layers between the respective gas diffusion layers and the membrane electrode assembly;
selecting a desired overall mass transfer coefficient (MTC overall ) wherein the gas diffusion layers and vapor barrier layers define an overall mass transfer coefficient (MTC overall ) of at least about 0.15 cm, wherein
MTC
overall
=
(
D
gdl
D
gdleff
h
gdl
)
GDL
+
(
D
vbl
D
vbleff
h
vbl
)
VBL
,
where h gdl is a thickness of the gas diffusion layer, D gdl is the free gas-phase diffusivity through the gas diffusion layer, D gdleff is the effective diffusivity through the gas diffusion layer, h vbl is a thickness of the vapor barrier layer, D vbl is the free gas-phase diffusivity through the vapor barrier layer, D vbleff is the effective diffusivity through the vapor barrier layer; and
optimizing one or more of the porosity, the tortuosity, and the thickness of the gas diffusion layer, the vapor barrier layer, or both, to produce the desired MTC overall .