Electrode substrate for electrochemical cells based on...

Details Electrode substrate for electrochemical cells based on... Electrode substrate for electrochemical cells based on...

2001-03-01

2003-01-28

Bell, Bruce F. (Department: 1741)

Chemistry: electrical current producing apparatus, product, and

Having earth feature

C204S283000, C204S284000, C204S294000, C502S101000, C423S447100, C423S447200, C423S448000

Reexamination Certificate

active

06511768

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to electrode substrates for electrochemical cells, particularly polymer electrolyte membrane fuel cells (PEMFC) and Phosphoric Acid Fuel Cells (PAFC), and processes for their production.
BACKGROUND OF THE INVENTION
A fuel cell converts fuel, such as hydrogen, and an oxidant, typically oxygen, to electricity and reaction products. This electrochemical reaction is facilitated by electrocatalysts, typically from the platinum group.
Fuel cells typically are constituted of units, as shown in
FIG. 1
, called single cells
1
, comprising an electrode assembly
1
′ where a membrane or electrolyte layer
2
is sandwiched between two electrodes
3
and
4
, individually referred to as anode
3
and cathode
4
. These electrodes are typically flat and have at least two parallel surfaces, the membrane or electrolyte layer
2
being positioned between these surfaces of the two electrodes.
Each of the electrodes
3
and
4
is composed of a porous conductive electrode substrate
3
′ and
4
′, usually made of carbon fiber paper or carbon cloth, and a thin electrocatalyst layer
3
″ and
4
″, preferably comprising finely divided platinum or other noble metal catalysts.
When using hydrogen as fuel, the fuel gas is oxidised at the anode
3
yielding protons and electrons. The former migrate through the membrane layer
2
from the anode to the cathode
4
, while the electrons are transported through an external circuit to the cathode
4
. At the cathode
4
, oxygen is reduced by consumption of two electrons per atom, to form oxide anions which enter the electrolyte layer and react with the protons that have crossed the electrolyte layer to form water. As shown in this
FIG. 1
, separator plates
5
and
6
which are adjacent to the electrodes
3
and
4
, may incorporate grooves
8
and
9
on the surfaces opposite to the electrodes providing access for the fuel and oxidant to the electrodes. The separator plates
5
and
6
can be covered with current collector plates
7
and
7
′ usually made of metal which also act as conductive connection between two adjacent single cells.
PEMFC generally employ a membrane electrode assembly (MEA,
1
′) as single cell comprising a thin polymer membrane
2
with high proton conductivity placed between two electrode sheets
3
and
4
. PAFC single cells are typically constituted of a thin phosphoric acid containing matrix layer
2
sandwiched between the two electrodes
3
and
4
.
The electrodes
3
and
4
mainly comprise of an electrically conductive and chemically inert electrode substrate (ES)
3
′ and
4
′ and an electrocatalyst layer (
3
″ and
4
″) facing the membrane or electrolyte
2
. The ES has a porous structure to provide an efficient entry passage and planar distribution for the fuel and oxidant to the catalyst layers
3
″ and
4
″ as well as an exit for the reaction products away from the catalyst layer. It also features other important properties such as high electrical conductivity, chemical stability, mechanical strength, and homogeneity.
As is shown in
FIG. 1
, it is advantageous to separate the functions of providing access and distributing fuel and oxidant (established by the grooves
8
and
9
in the separator or distributor plates
5
and
6
in
FIG. 1
) and the support of the catalyst layer
3
″ and
4
″ by the electrode substrates
3
′ and
4
′. The separator or distributor plates
5
and
6
are usually made of metal or other conductive materials as they shall also serve to collect the current. They incorporate grooves
8
and
9
or other means of distribution of liquids or gases. These separator plates are stacked on the electrode substrates on the side opposite the electrolyte layer
2
.
Current can be collected in the distributor or separator plates (as mentioned above), or in separate current collector plates which can be a solid metal sheet if they form the outer part of the assembly, or can be a mesh or porous conductive plate if they are stacked between the fuel feed and the electrodes (between
4
and
6
, or between
3
and
5
, in an assembly as otherwise shown in FIG.
1
). It is also possible to combine the separator plates and current collector plates.
Since various gases and liquids have to permeate through the ES, high porosity is a preferable feature of an ES. At the same time, the pore size distribution needs to be adjusted to the general characteristics of practical fuel cells. The grooves in the electrode substrates provide a very coarse distribution of fuel and oxidant. These need to be evenly transported and finely distributed to the catalyst layer through the ES. Furthermore, various types of gases and liquids have to be transported through the ES which requires fine-tuning and adaptation of the ES porous network. Hence, adjusting the degree of porosity as well as pore size and its distribution of an ES is important for the performance of a fuel cell.
Equally important is the through-plane (perpendicular to the large surface) electrical conductivity of the ES since they provide a conductive path between the catalyst layer and the separator or current collector plates. A low electrical conductivity can result in substantial power losses of the fuel cell. Usually, high porosity of an ES has to be balanced against improved through-plane conductivity or vice versa.
Mechanical properties of ES play an increasingly important role for the production of commercial fuel cells since the ES are being handled by automatic equipment, and product integrity determines the commercial success of fuel cells.
In the light of fuel cell commercialisation efforts, ES are also required to be processable as a continuous roll material. This allows the application of industrial scale processes for the catalyst layer deposition and other required manufacturing steps.
Furthermore, a continuous roll ES provides high homogeneity and product uniformity in comparison with ES produced in a batch-mode.
Commonly used ES materials for fuel cells include carbon fibers (papers, felt, and woven cloth), metal fibers (mesh or gauze), and polymers (gauze filled with carbon materials).
A carbon fiber paper ES is usually made in such way that the carbon fibers are aligned mainly in planar direction. Due to the high anisotropy of carbon fibers, the in-plane conductivity of such carbon fiber paper is high but through-plane conductivity is poor. Such carbon fiber paper can be rendered suitable as ES for fuel cells if it is manufactured using a carbonisable binder followed by carbonising this product at high temperatures to achieve satisfactory through-plane conductivity (cf. U.S. Pat. No. 4,851,304). This type of ES is shown as a cross-section in FIG.
2
. Carbon fibers
10
are aligned mainly in planar direction; carbonised binder particles
11
contribute to the mechanical stability of the ES. Carbonisable binder in this context means a binder, usually a binder resin which cross-links under the action of heat, that can be converted to elemental carbon in a high yield when heated for a prolonged time, i. e. more than 5 minutes up to several hours, above the decomposition temperature with the exclusion of oxygen or oxidising gases. This expensive batch-process yields ES with poor mechanical properties. WO 98/27606 relates to a process for filling carbon fiber papers and polymer substrates having low through-plane conductivity with conductive materials. The ES resulting from this procedure still lack a high through-plane conductivity and have a low porosity because the pores of the starting materials have to be filled with a high fraction of conductive material to achieve a sufficient level of through-plane conductivity.
Woven carbon cloth can be utilised as ES base material, but it is expensive and restricts the options to reduce the ES thickness. Metal fibers suitable for fuel cell ES are expensive since they need to be oxidation and corrosion resistant, and therefore must be selected from the noble metals

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