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Fuel Cells Come Down to Earth
Institute engineers develop new designs for an energy source first used in space.
by Edward A. Bass, P.E.
The search for clean and efficient alternatives to fossil fuel combustion for power generation has led to several new approaches to energy production; one of the most attractive of these is electrochemical fuel cell technology. Fuel cells have several advantages that make them attractive power sources for vehicle propulsion including high thermal efficiency, extremely low or zero emissions, and low noise and vibration in comparison to conventional powertrains. Institute scientists and engineers, supported by the SwRI Advisory Committee for Research, are carrying out specific research into the structural design, thermodynamics, and heat transfer processes of one class of these cells, known as proton exchange membrane (PEM) or polymer electrolyte membrane fuel cells. PEM fuel cells have found application in a number of critical markets, including the automotive industry.
For example, SwRI is conducting computer simulations to identify technologies that will contribute to the national goal set by the Partnership for a New Generation of Vehicles (PNGV) -- a prototype passenger car by the year 2004 that can achieve 80 miles per gallon. PEM fuel cells have made the short list of technologies for focused research by the principal PNGV participants -- the U.S. government and the U.S. Council for Automotive Research (USCAR), which represents Chrysler, Ford, and General Motors. As part of the PNGV program, the Institute is developing a fuel cell model that will include the performance characteristics of a PEM fuel cell system.
In a related project, the Institute is developing and testing components and systems for electric and hybrid-electric commercial vehicles, such as auxiliary power units (APU) that operate on natural gas, thus extending the range of purely electric vehicles while retaining their low emissions levels. Fuel cells, combined with fuel reformers for the production of hydrogen (H2), are excellent candidates for future use in APUs.
Fuel cells combine H2 and oxygen (O2) without combustion to deliver a direct current in a continuous process that can be described as electrolysis in reverse. Fuel cells are unlike batteries in that they do not need recharging. The H2 and O2 are combined with the help of a catalyst incorporated into an electrode; electricity, heat, and water are the by-products. Cells can be added together into modular units that are extremely efficient, capable of converting more than 40 percent of the fuel's energy into usable form. The cells require little maintenance and can produce electricity in residential and environmentally sensitive areas.
Although the physical principles behind power generation from fuel cells were discovered in 1839 by the British scientist Sir William Grove, successful research, development, and use of the cells occurred only recently, in the context of the U.S. space program. Fuel cells were first used to supply electric power for spacecraft during the Gemini-Titan V mission in 1965, and their use has continued through the Apollo and space shuttle programs to the present.
Fuel cells are employed in the space program because they produce the highest amount of energy for a given weight and volume. In addition, because liquid hydrogen and oxygen are used as rocket fuel, the same combination can be used to generate electric power. Fuel cells can even provide drinking water for the crew.
Commercial success with a broader use of fuel cells in vehicles, where ambient air is used instead of oxygen to eliminate the need for extra onboard storage tanks, has been handicapped by a number of problems that include cell size, weight, durability, and cost. For example, some existing PEM fuel cells, which are composed of channeled carbon plates that serve as electrodes and flow path manifolds, have suffered from uneven air distribution that can result in inefficient operation. The Institute goal is to address these problems by investigating alternatives and developing new cell architectures.
Several prototype PEM fuel cells using novel cylindrical designs have been assembled and tested at SwRI, based on the hypothesis that cylindrical cells can overcome some of the weaknesses caused by the flat-plate geometry of existing PEM fuel cell designs. Other important characteristics that can affect cell efficiency, including electrical and thermal resistance, reactant pressures, temperature, catalyst loading, and surface area, are being studied to evaluate their effects on the mass and energy balances in the cell. A proton exchange material called Nafion®, manufactured by DuPont, is being used as the electrolyte.
Institute staff members have designed and assembled three types of cylindrical fuel cells using proton exchange materials -- rolled sheet, solution cast, and sputter-coated fuel cells. A patent application has been filed that describes these techniques.
The rolled sheet technique proved simplest for constructing a cylindrical fuel cell. Carbon cloth, treated with Teflon® and a platinum-on-carbon catalyst, was rolled onto a porous, tubular, stainless steel or carbon dowel (serving as a current conductor) and bonded into place to provide the inner electrode, called the hydrogen bearing anode, of the fuel cell. Wrapping the Nafion® membrane around the anode proved problematical, as the membrane is dimensionally unstable and can grow some 20 percent in size when saturated with water. If the membrane becomes too large, contact with the electrodes is poor; if too small, the membrane will not completely separate the hydrogen and oxygen. To form the outer electrode, which is the air or oxygen cathode of the cell, another layer of treated carbon cloth was rolled onto the membrane and bonded in place. A current conductor for the cathode was supplied by winding carbon fiber or stainless steel wire around the assembly. Testing and validation of the design showed that performance was greatly improved by increasing clamping pressure.
The solution casting approach eliminates the problem of clamping pressure by forming three layers from solutions -- one containing an anode catalyst, one containing Nafion®, and one containing a cathode catalyst -- into one composite structure. However, after experiments with several prototypes, it has become clear that more work is necessary to eliminate leaking and to fabricate a stronger membrane layer.
The third technique consists of applying a promising catalyst-electrode treatment to a robust sheet-form membrane. The Institute's expertise in materials surface modification was employed to replace the catalyzed carbon cloth of the rolled sheet cell with layers containing carbon and platinum deposited onto the membrane sheet material by direct ion sputtering. Extensive surface analyses have been conducted on a sample, and a fuel cell based on this technique will be tested next.
In a newly awarded project, Institute scientists and engineers will attempt ion beam-assisted deposition of platinum and carbon on large membrane-electrode assemblies such as those used in flat, conventional fuel cells. The technique is expected to result in reduced catalyst loads.
Though the potential of the fuel cell, particularly the PEM fuel cell, for use in vehicle propulsion is undisputed, issues such as cost, size, and a refueling infrastructure remain unresolved. Work is under way to solve these problems at a number of research and industrial organizations, driven to a certain degree by PNGV goals. For example, USCAR reports that improvements to the PEM fuel cell electrode structure since 1984 have greatly reduced the amount of platinum used in the catalyst, bringing associated platinum costs down significantly. Advances in fuel cell power density and modular flexibility are addressing size and weight concerns. Research is also being conducted to perfect a technique to reform gasoline and alternative fuels in the vehicle to produce hydrogen, without reducing efficiency or producing emissions.
If the challenges facing fuel cell development are successfully met, this space age power plant may be driving the vehicles of the 21st century, providing a serious alternative to the internal combustion engine.
The research reported here is being conducted by a multidisciplinary Institute team that includes Craig M. Wall of the Chemistry and Chemical Engineering Division, John B. Campbell and James H. Arps of the Materials and Structures Division, and Robert K. Howard, Patrick M. Merritt, and Christopher A. Sharp of the Automotive Products and Emissions Research Division.
Published in the Summer 1996 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.