Energy devices and methods for the fabrication of energy...

Details Energy devices and methods for the fabrication of energy... Energy devices and methods for the fabrication of energy...

2000-06-08

2004-06-22

Ryan, Patrick (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Having earth feature

C429S006000, C429S047000, C429S047000

Reexamination Certificate

active

06753108

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to energy devices such as batteries and fuel cells and also relates to methods for the fabrication of such devices. Specifically, the present invention is directed to energy devices having a reduced thickness that can be fabricated using traditional or non-traditional methods to form thin layers within the device.
2. Description of Related Art
With the advent of portable and hand-held electronic devices and an increasing demand for electric automobiles due to the increased strain on non-renewable natural resources, there is a need for the rapid development of high performance, economical power systems, for example batteries and fuel cells, that have a reduced size and weight.
The further size reduction of portable electronic devices is limited by the inability to provide sufficient power without adding substantial bulk to the device. For example, most of the volume and weight of a typical cellular telephone resides not in the telephone electronics but in the battery required to power the telephone. Small computing devices such as laptop computers and personal digital assistants (PDA's) would also benefit from smaller, lighter batteries. Other potential uses for small, lightweight batteries include global positioning system (GPS) transceivers.
Batteries can be divided into primary (non-rechargeable) and secondary (rechargeable) batteries. Common types of primary batteries include metal-air batteries such as Zn-air, Li-air and Al-air, alkaline batteries and lithium batteries. Common types of secondary batteries include nickel-cadmium, nickel metal hydride and lithium ion batteries.
One type of metal-air battery which offers many competitive advantages is the zinc-air battery, which relies upon the redox couples of oxygen and zinc to produce energy. Zinc-air batteries operate by adsorbing oxygen from the surrounding air and reducing the oxygen using an oxygen reduction catalyst at the air electrode (cathode). As the oxygen is reduced, zinc metal is oxidized. The reactions of a zinc-air alkaline battery during discharge are:
Cathode: O
2
+2H
2
O+4e
−
→4OH
−
Anode: 2Zn→2Zn
2+
+4e
−
Overall: 2Zn+O
2
+2H
2
O→2Zn(OH)
2
Typically, air electrodes are alternatively stacked with zinc electrodes and are packaged in a container that is open to the air. When the battery cell discharges, oxygen is reduced to O
2−
at the cathode while zinc metal is oxidized to Zn
2+
at the anode. Since Zn can be electrodeposited from aqueous electrolytes to replenish the anode, zinc-air batteries can be secondary batteries as well as primary batteries.
Among the advantages of secondary zinc-air batteries over other rechargeable battery systems are safety, long run time and light weight. The batteries contain no toxic materials and operate at one atmosphere of pressure. They can operate as long as 10 to 14 hours, compared to 2 to 4 hours for most rechargeable lithium-ion batteries and can be stored for long periods of time without losing their charge. The light weight of zinc-air batteries leads to good power density (power per unit of weight or volume), which is ideal for portable applications.
The two major problems associated with secondary zinc-air batteries, however, are limited total power and poor rechargeability/cycle lifetime. Increased power is becoming a major area of attention for battery manufacturers trying to meet the increased demands of modern electronics. Current zinc-air batteries can deliver from about 200 to 450 W/kg which may enable the batteries to be used in certain low-power laptops and other portable devices that have relatively low power requirements. Most laptops and other portable electronic devices, however, require batteries that are able to provide a level of power that is higher than the capabilities of current zinc-air batteries. The main reason for the low power of zinc-air batteries is believed to be related to the inefficiency of the catalytic reaction to reduce oxygen in the air electrodes. Poor accessibility of the catalyst and the local microstructural environment around the catalyst and adjoining carbon reduces the efficiency of the oxygen reduction. See, for example, P. N. Ross et al.,
Journal of the Electrochemical Society
, Vol. 131, pg. 1742 (1984).
Rechargeability is also a problem with zinc-air batteries. The batteries have a short cycle life, degrading significantly in performance after about 200 recharging cycles or less. The short cycle life of zinc-air batteries is also believed to be related to the catalyst used in the air electrodes. Specifically, it is believed that corrosion of the carbon used for the electrocatalyst in these systems leads to a loss in capacity and a decreased discharge time.
Primary (non-rechargeable) alkaline zinc-air batteries are currently used to power hearing aids and other devices that require low current densities over long periods of time. Zinc-air hearing aid batteries also include an air cathode and a zinc-based anode. The electrocatalyst powder is formed into a layer for the air cathode which catalytically converts oxygen in the air into hydroxide ion. The hydroxide ion is then transported in an alkaline electrolyte through a separator to the anode where it reacts with zinc to form zincate ion (Zn(OH)
4
2−
) and zinc ion (Zn
2+
) and liberates electrons. Improved electrocatalytic layers at the air cathode would advantageously extend the life of such primary batteries.
In addition to improvements in energy storage, there is a need for improvements in environmentally friendly and economical energy production. Fuel cells are electrochemical devices which are capable of converting the energy of a chemical reaction into electrical energy without combustion and with virtually no pollution. Fuel cells are unlike batteries because fuel cells convert chemical energy to electrical energy as the chemical reactants are continuously delivered to the fuel cell. When the fuel cell is off, it has zero electrical potential. As a result, fuel cells are typically used to produce a continuous source of electrical energy and compete with other forms of continuous electrical energy production such as the combustion engine, nuclear power stations and coal-fired power stations. The different types of fuel cells are categorized by the electrolyte used in the fuel cell. The five main types of fuel cells are alkaline, molten carbonate, phosphoric acid, solid oxide and proton exchange membrane (PEM) or solid polymer.
One of the critical requirements for these energy devices is the efficient catalytic conversion of the reactants and a significant obstacle to the wide-scale commercialization of such devices is the need for highly efficient electrocatalytic layers for this conversion process.
One example of a fuel cell utilizing electrocatalytic layers for the chemical reactions is a PEM fuel cell. A PEM fuel cell stack includes hundreds of membrane electrode assemblies (MEA's) each including a cathode and anode constructed from, for example, carbon cloth. The anode and cathode sandwich a proton exchange membrane which has a catalyst layer on each side of the membrane. Power is generated when hydrogen is fed into the anode and oxygen (air) is fed into the cathode. In a reaction typically catalyzed by a platinum-based catalyst in the catalyst layer of the anode, the hydrogen ionizes to form protons and electrons. The protons are transported through the proton exchange membrane to a catalyst layer on the opposite side of the membrane (the cathode) where another catalyst, typically platinum or a platinum alloy, catalyzes the reaction of the protons with oxygen to form water. The reactions can be written as follows:
Anode: 2H
2
→4H
+
+4e
−
Cathode: 4H
+
+4e
−
+O
2
→2H
2
O
Overall: 2H
2
+O
2
→2H
2
O
The electrons formed at the anode are routed to the cathode through an electrical circuit which provides the electrical power.
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