Fuel cells incorporating nanotubes in fuel feed

Details Fuel cells incorporating nanotubes in fuel feed Fuel cells incorporating nanotubes in fuel feed

2000-08-18

2003-07-08

Chaney, Carol (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S006000, C429S006000, C429S047000

Reexamination Certificate

active

06589682

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
In one aspect, the present invention relates to novel, improved proton exchange membrane (“PEM”) fuel cells.
In a second aspect, the present invention relates to novel, improved components for PEM fuel cells and fuel cell stacks and to processes for manufacturing and assembling those components.
In still other aspects, the present invention relates to novel stacking configurations, humidification features, and gas distribution designs for PEMs.
DEFINITIONS
In the interest of clarity and brevity, abbreviations will be employed extensively in this specification. These abbreviations are listed below:
Term
Abbreviation
Proton Exchange Membrane
PEM
Membrane Electrode Assembly
MEA
Solid Polymer Electrolyte
SPE
Gas Diffusion Layer
GDL
Flow Field Plate
FFP
Nanoscale Gas Distribution System
NGDS
Single Walled Nanotube
SWNT
Chemical Vapor Deposition
CVD
Physical Vapor Deposition
PVD
Single Walled Carbon Nanotubes
SWNT
Programmable Logic Device
PLD
BACKGROUND OF THE INVENTION
Sir William Grove in 1839 showed that he could create electrical energy from chemical energy, the reverse of electrolysis of water, by using platinum electrodes. More recently, major efforts have been directed to the development of PEM fuel cells. PEM fuel cells have been used in NASA's Space Programs for over 20 years, and are currently of greater interest as a means of addressing the growing concerns of pollution related to the use of internal combustion engines in our society.
The basic components of a fuel cell include: an anode, a cathode, an electrolyte, and delivery systems for fuel and oxygen. When the cell is in operation, the electrodes are connected to an external load by conducting wires. In a PEM fuel cell, the electrolyte is comprised of a thin membrane made of a polymer similar to polytetrafluoroethylene (commonly known under the trade name TEFLON), but incorporating sulfonic acid groups within the polymer's molecular structure. NAFION® 117, NAFION® 112, AND NAFION® 115 are typical. These are solid polymer electrolytes (“SPE”) available from E.I. DuPont de Nemours & Co. The sulfonic acid groups are acid ions and constitute the active electrolyte. The membrane functions to conduct hydrogen nuclei (H
+
ions or protons) from one face through the membrane to the opposite face while effectively blocking the flow of diatomic hydrogen molecules through the membrane. The electrodes, catalyst, and membrane electrolyte together form the MEA.
Hydrogen is oxidized at the anode as it comes into contact with a catalyst (typically platinum), and is disassociated into protons and electrons. The protons are solvated by water in the membrane, and travel through the membrane by passing from one sulfonic acid group to the next. As the protons migrate across the SPE the electrons travel through the external load to the cathode. Reduction occurs at the cathode where oxygen reacts with the protons and electrons to form water and heat, the sole byproducts. In the PEM fuel cell, the two reactions are:
2H
2
→4H
+
+4e
−
at the anode side of the cell, and
O
2
+4H
+
+4e
−
→2H
2
O at the cathode side of the cell.
Since the maximum electrochemical potential for the reaction of water is 1.23 volts with an electrical efficiency of about 0.5-0.8 for a single cell at room temperature, a stacking arrangement of single cells in series is needed to deliver currents at various desired voltages for most practical applications.
Typical problems which inhibit the efficiency of heretofore proposed PEM fuel cells are gas distribution, current collection, and membrane hydration, which affects hydrogen/oxygen conversions and internal resistances. Other problems associated with the currently existing PEM fuel cells focus on economic issues such as the ability to mass produce the fuel cells, and how to monitor and maintain them once they have been delivered to the consumer.
One of the more significant problems posed by currently existing PEM fuel cells is the reactant gas efficiency. The efficiency of converting the reactant gases to the product is related, in large part, to the hydrogen/oxygen gas distribution within the cell at the anode and cathode respectively. Because not all reactant gas introduced to the fuel cell is converted from chemical to electrical energy, a greater supply of fuel is needed to produce a desired output than is suggested by the foregoing equations, thereby lessening the otherwise beneficial attributes of the system.
The poor reactant gas efficiency is primarily due to the crossover of unreacted gases through the SPE. In traditional PEM fuel cells, the electrodes are comprised of a carbon/platinum/polymer-based slurry that is deposited onto the SPE. When a fuel gas (i.e. hydrogen) locates the catalyst, it must bind to the catalyst site, yield an electron, and be immediately solvated into the electrolyte. A problem arises because the reactant gases do not always find a catalyst reaction site covered or enveloped by the electrolyte with which to bind and react. The lack of transport control of reactant gases to the catalyst reaction sites therefore limits the kinetics of the reaction and produces an inefficient result.
One solution to this problem is to provide a higher concentration of catalyst in the electrode slurry. This, however, has disadvantages because the catalyst is often a precious metal, the increased use of which significantly adds to the overall cost of the fuel cell.
An additional problem with currently existing PEM fuel cells is the low conductivity of the traditional carbon/metal-catalyst electrode's gas diffusion layer (GDL) and flow field plate (FFP) interface surfaces. In order to direct the electrical energy produced by the fuel cell to the external load, a means must be provided to: (a) collect the electron flow over the entire area of the membrane; and (b) ensure an uninterrupted electrically conductive flow path from the catalyzed surfaces (electrodes) of the membrane to these current-collector devices. Conductivity of these layers is usually limited to the conductivity or resistivity of the collection plate material (e.g., graphite has a resistivity of 1100 &mgr;&OHgr;·cm). Some collector plates and/or GDLs are made of metals such as corrosion resistant stainless steel. Although use of this material will increase conductivity of the collector plate, such use will also undesirably increase the weight of the fuel cell, particularly when it is incorporated into a stacked configuration to produce the desired output. Other materials, incorporating metals in varying concentrations, are available to increase the conductivity of the collector plate, but their use is not cost-effective for the mass production of PEM fuel cells. Moreover, the manufacture of traditional carbon graphite FFPs is costly and also results in a heavy collection plate (carbon graphite has a bulk density of 1.77 g/cm
3
), making the stacking needed for a desired voltage range less effective in terms of power-to-weight ratio.
A third efficiency problem associated with currently existing PEM fuel cells relates to the issue of hydration. Because the SPE membrane's conductivity is coupled to the amount of water present, particularly in relation to the anode, a means of keeping the membrane moist to a controlled degree is necessary.
SUMMARY OF THE INVENTION
There have now been invented and disclosed herein certain new and novel PEM fuel cells incorporating stacking configurations which provide for the delivery of current at voltages consistent with the practical application of the fuel cells, which is not true of the prior art PEM fuel cells. Confining the reactant gases to the desired areas of flow in a membrane-minimizing configuration is optimized for PEM fuel cells in accord with the principles of the present invention by effecting the uniform distribution of a force on the fuel cell components that increases the conductivity of the carbon/metal-catalyst electrode, GDL, FFP interface surfaces, thereby increasing fuel cell perform

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