Advanced Ceramics

SPECIAL SECTION/ADVANCED CERAMICS: Redefining Ceramic Fuel Cells

April 2, 2008
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The future of alternative energy lies in the ability to deliver portable power to market.

Figure 1. Propane-powered solid oxide fuel cell system weighing less than 1.5 kilograms and generating 600 watt-hours per day (equivalent to 20 D-cell batteries per day).


Wikipedia’s community of experts can’t overcome incorrect assumptions on the emerging alternative energy space. Wikipedia’s solid oxide fuel cell (SOFC) entry, which describes SOFCs as devices that use a solid (ceramic) electrolyte to facilitate the generation of energy, notes that these fuel cells are “intended mainly for stationary applications.” In fact, leading innovators are proving that ceramic fuel cells can include small and light energy devices. Indeed, the future of alternative energy lies in the ability to deliver this portable power to market.

Portable power is the holy grail of alternative energy research. Small, lightweight energy devices bridge the gap between the current market leaders, batteries and gas generators. Batteries become heavier as power output rises, and gas generators become noisier and produce ever-increasing pollution in tandem with increases in power output. In contrast, SOFCs deliver a wide range of energy output, between 20 and 250 watts, making them incredibly energy-dense for their weight and size.

Besides providing portable power, SOFCs offer a huge advantage over typical proton exchange membrane (PEM) fuel cells. PEM requires very pure hydrogen fuel, which is costly and difficult to obtain. In addition, PEM fuel cells require expensive platinum catalysts that are poisoned by many low-level contaminants such as carbon monoxide and sulfur. In contrast, SOFCs are fueled by readily available bottled propane or butane gas. Propane and butane are not expensive, and they are trusted and available at over 25,000 retailers in the U.S. alone.

Along with the market shift to ceramic SOFCs is a shift from the traditional power generator manufacturing model, which was based on a limited number of very large custom-built utility installations. Portable power devices, including SOFCs, represent a new breed of mass-produced generators that are compact and lightweight.

A New Type of SOFC

Portable SOFCs based on a small tube design were originally developed by Professor Kevin Kendall of the University of Birmingham in the UK. Kendall realized that the disadvantages of the traditional SOFC design could be avoided if thermal-shock-sensitive planar stacks or large tubes were replaced with small tubes of a few millimeters in diameter.

Small tubes relax the design constraints of SOFCs’ high operating temperature (600-900ºC). The active part of a small tube can be hot, while a short distance away the cold end of the tube can be sealed with simple rubber. The small tube design eliminated two of the major disadvantages of SOFCs-problems with hot seals and thermal cycles.

Figure 1 shows a 20-watt generator. Although the active membranes are at SOFC temperatures of around 700ºC, the generator is only slightly warm to the touch and the exhaust is cooler than body temperature (see Figure 2). These generators operate in a range of field conditions from artic cold to desert heat (- 40 to 50ºC).

Figure 2. Thermal image of an engineer outdoors on a snowy day holding an operating SOFC and a cup of hot coffee. It is evident that the warmest areas of the operating fuel cell are similar to body temperature.

The Future of SOFCs

Complete SOFC power systems, fueled by ordinary bottled gas, are being manufactured to produce complete hybridized power just seconds after pressing a simple “on” button. Intended for rugged, all-weather field use, these generators are designed for construction sites, military use or off-road/off-grid power. The market for this type of SOFC product is limited only by current demand vs. its unlimited potential.

The small tube design was used as the basis of these portable SOFCs, which were developed for the U.S. Defense Department under the Defense Advanced Research Projects Agency (DARPA) Palm Power program. Starting around the size of a lunch box, the generators are small, lightweight and reliable.

These durable portable generators may be dropped, shaken or otherwise handled roughly and remain intact. In fact, one of the most promising applications for SOFCs is use in small, unmanned aerial vehicles (UAVs). Combining a low-mass, shock-proof design with the inherent durability of a small ceramic tube, SOFCs can deliver a powerpod for UAVs that can not only survive the rigors of take-off, flight and successful landing, but also emergency landings.

Figure 3. Multilayer anode/electrolyte/cathode in a finished solid oxide fuel cell. The 14-micron-thick electrolyte layer can be easily produced through microfabrication by coextrusion.

A New Type of Ceramic Manufacturing

Instead of conventional ceramic fabrication methods, these generators are produced through ceramic powder that is loaded in thermoplastics to undergo hot extrusion. Ordinary wet ceramic extrusion produces a soft tube, which forms a weak and brittle green tube after slow drying. In contrast, thermoplastic extrusion provides fast cooling to produce a strong and flexible green tube that can be manufactured with thinner walls. Thermoplastics do require binder burnout, which can be a problem with large sections but is no problem for thin-walled tubes.

The major advantage to this new thinking on ceramic fabrication is the combination of anode and electrolyte by extruding several of these two different materials at the same time (co-extrusion). Combined with the size reduction as the plastic materials are pushed through an extrusion die, it is possible to produce complex micro-scale features (microfabrication). In fact, a microfabrication by coextrusion process was developed to manufacture nearly complete tubular cells with electrolyte layers only 10-20 microns thick.*

Figure 3 shows a 10-micron-thick zirconia electrolyte on a multilayer anode. Significant size reduction occurs during coextrusion.

Subsequent fabrication involves the usual ceramic sintering steps, followed by assembly operations to produce completely wired-up individual cells. Combining everything into a small package, a ceramic microreactor is placed in each cell so that simple propane fuel can be used directly in the cells.

To make a generator, the appropriate number of cells are collected into stacks, assembled inside an efficient ceramic thermal insulation package, and fitted to a cold manifold so that fuel and air can be easily introduced. This produces the SOFC stack, a ready-to-use unit. A compete generator system is made by combining the stack with the “balance of plant,” including pumps, values, control circuits, displays and a storage battery. The battery is important for hybrid power, supplying the dynamic variable power required by the users’ various duty cycles while being continuously charged by the fuel cell.

*Developed and patented by Adaptive Materials, Inc.

A Ni-zirconia anode tube used in ceramic fuel cell systems.

The Road to Mass Production

The markets that adopted SOFCs early are well established. Some are motivated by environmental concerns, others by the need for more power than batteries can supply. Some are frustrated with inefficient generators, while others want to be the first to take advantage of an innovative new technology.

Applications for SOFCs exist in the leisure, medical, military and industrial markets, and demand in emerging commercial markets is strong. As the need for portable power continues to expand, so does the market potential-fueled by readily available fuel sources and made possible by ceramics.

For more information, contact Adaptive Materials, Inc. at 5500 South State St., Ann Arbor, MI 48108; (734) 302-7632; fax (734) 222-9283; e-mail jeff.basch@adaptivematerials.com; or visit www.adaptivematerials.com.

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