Composite solid polymer electrolyte membranes

Details Composite solid polymer electrolyte membranes Composite solid polymer electrolyte membranes

1999-03-03

2001-06-19

Weiner, Laura (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Having earth feature

C429S047000, C429S300000, C429S303000, C429S305000, C429S306000, C429S309000, C521S027000, C521S133000

Reexamination Certificate

active

06248469

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to novel composite solid polymer electrolyte membranes (SPEMs) for use in electrochemical applications. Methods for producing the composite membranes of the invention are also disclosed.
BACKGROUND OF THE INVENTION
There is a considerable need in both the military and commercial sectors for quiet, efficient and lightweight power sources that have improved power density. Military applications include, but are not limited to, submersibles, surface ships, portable/mobile field generating units, and low power units (i.e., battery replacements). For example, the military has a strong interest in developing low range power sources (a few watts to a few kilowatts) that can function as replacements for batteries. Commercial applications include transportation (i.e., automotive, bus, truck and railway), communications, on-site cogeneration and stationary power generation.
Other interest exists for household applications, such as radios, camcorders and laptop computers. Additional interest exists in larger power sources or sources of higher power density that can be used in operating clean, efficient vehicles. In general, there is a need for quiet, efficient and lightweight power sources anywhere stationary power generation is needed.
Additionally, the use of gasoline-powered internal combustion engines has created several environmental, exhaust gas-related problems. One possible solution to these environmental problems is the use of fuel cells. Fuel cells are highly efficient electrochemical energy conversion devices that directly convert the chemical energy derived from renewable fuel into electrical energy.
Significant research and development activity has focused on the development of proton-exchange membrane fuel cells. Proton-exchange membrane fuel cells have a polymer electrolyte membrane disposed between a positive electrode (cathode) and a negative electrode (anode). The polymer electrolyte membrane is composed of an ion-exchange polymer (i.e., ionomer). Its role is to provide a means for ionic transport and prevent mixing of the molecular forms of the fuel and the oxidant.
Solid polymer electrolyte fuel cells (SPEFCs) are an ideal source of quiet, efficient, and lightweight power. While batteries have reactants contained within their structure which eventually are used up, fuel cells use air and hydrogen to operate continuously. Their fuel efficiency is high (45 to 50 percent), they do not produce noise, operate over a wide power range (10 watts to several hundred kilowatts), and are relatively simple to design, manufacture and operate. Further, SPEFCs currently have the highest power density of all fuel cell types. In addition, SPEFCs do not produce any environmentally hazardous emissions such as NO
x
and SO
x
(typical combustion by-products).
The traditional SPEFC contains a solid polymer ion-exchange membrane that lies between two gas diffusion electrodes, an anode and a cathode, each commonly containing a metal catalyst supported by an electrically conductive material. The gas diffusion electrodes are exposed to the respective reactant gases, the reductant gas and the oxidant gas. An electrochemical reaction occurs at each of the two junctions (three phase boundaries) where one of the electrodes, electrolyte polymer membrane and reactant gas interface.
During fuel cell operation, hydrogen permeates through the anode and interacts with the metal catalyst, producing electrons and protons. The electrons are conducted via an electrically conductive material through an external circuit to the cathode, while the protons are simultaneously transferred via an ionic route through the polymer electrolyte membrane to the cathode. Oxygen permeates to the catalyst sites of the cathode, where it gains electrons and reacts with protons to form water. Consequently, the products of the SPEFC's reactions are water, electricity and heat. In the SPEFC, current is conducted simultaneously through ionic and electronic routes. Efficiency of the SPEFC is largely dependent on its ability to minimize both ionic and electronic resistivity to these currents.
Ion exchange membranes play a vital role in SPEFCs. In SPEFCs, the ion-exchange membrane has two functions: (1) it acts as the electrolyte that provides ionic communication between the anode and cathode; and (2) it serves as a separator for the two reactant gases (e.g., O
2
and H
2
). In other words, the ion-exchange membrane, while serving as a good proton transfer membrane, must also have low permeability for the reactant gases to avoid cross-over phenomena that reduce performance of the fuel cell. This is especially important in fuel cell applications in which the reactant gases are under pressure and the fuel cell is operated at elevated temperatures.
Fuel cell reactants are classified as oxidants and reductants on the basis of their electron acceptor or electron donor characteristics. Oxidants include pure oxygen, oxygen-containing gases (e.g., air) and halogens (e.g., chlorine). Reductants include hydrogen, carbon monoxide, natural gas, methane, ethane, formaldehyde and methanol.
Optimized proton and water transports of the membrane and proper water management are also crucial for efficient fuel cell application. Dehydration of the membrane reduces proton conductivity, and excess water can lead to swelling of the membranes. Inefficient removal of by-product water can cause flooding of the electrodes hindering gas access. Both of these conditions lead to poor cell performance.
Despite their potential for many applications, SPEFCs have not yet been commercialized due to unresolved technical problems and high overall cost. One major deficiency impacting the commercialization of the SPEFC is the inherent limitations of today's leading membrane and electrode assemblies. To make the SPEFC commercially viable (especially in automotive applications), the membranes employed must operate at elevated/high temperatures (>120° C.) so as to provide increased power density, and limit catalyst sensitivity to fuel impurities. This would also allow for applications such as on-site cogeneration (high quality waste heat in addition to electrical power). Current membranes also allow excessive methanol crossover in liquid feed direct methanol fuel cells (dependent on actual operating conditions, but is typically equivalent to a current density loss of about 50 to 200 mA/cm
2
@ 0.5V). This crossover results in poor fuel efficiency as well as limited performance levels.
Several polymer electrolyte membranes have been developed over the years for application as solid polymer electrolytes in fuel cells. However, these membranes have significant limitations when applied to liquid-feed direct methanol fuel cells and to hydrogen fuel cells. The membranes in today's most advanced SPEFCs do not possess the required combination of ionic conductivity, mechanical strength, dehydration resistance, chemical stability and fuel impermeability (e.g., methanol crossover) to operate at elevated temperatures.
DuPont developed a series of perfluorinated sulfonic acid membranes known as Nafion® membranes. The Nafion® membrane technology is well known in the art and is described in U.S. Pat. Nos. 3,282,875 and 4,330,654. Unreinforced Nafion® membranes are used almost exclusively as the ion exchange membrane in present SPEFC applications. This membrane is fabricated from a copolymer of tetrafluoroethylene (TFE) and a perfluorovinyl ethersulfonyl fluoride. The vinyl ether comonomer is copolymerized with TFE to form a melt-processable polymer. Once in the desired shape, the sulfonyl fluoride group is hydrolyzed into the ionic sulfonate form.
The fluorocarbon component and the ionic groups are incompatible or immiscible (the former is hydrophobic, the latter is hydrophilic). This causes a phase separation, which leads to the formation of interconnected hydrated ionic “clusters”. The properties of these clusters determine the electrochemical characteristics of the polymer, since protons are conducted through the mem

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