Sir William Robert Grove
O Fuel Cell, Where Art Thou?
August / September 2004
Volume 2 Number 4
Okay, so we've yet to re-visit those long gas station lines reminiscent of the 70s, but with gasoline prices soaring many are wondering just how high energy costs will climb. In the thirty years since the Carter-era energy crisis, our consumption of energy has expanded at alarming rate. Fossil fuels—€”crude oil, coal, natural gas—€”are the source of more than 85 percent of the energy consumed in the United States. Keeping up with the growing demand for fossil energy has become an increasingly complex issue, fraught with innumerable political and technical factors. Although conservative estimates of the known reserves of these resources suggest that our supplies will last at least another 100 to 150 years at the current rate of consumption, the environmental impact and geopolitical consequences of our reliance on fossil fuels will continue to be conspicuous and problematic.
With the exception of reducing energy use—€”which is one viable energy management strategy, but not one that's been met with great enthusiasm—€”the most direct way to make resources last longer is to use them more efficiently; i.e., wring more energy out of a given amount of fuel. Fuel cell technology is one means of doing this. Not only do fuel cells offer greater efficiency than gas turbines or combustion engines; in fact, they are three times more efficient and they are also noiseless, low-maintenance, virtually pollution-free and can be scaled from pocket portable to megawatt size. What they currently are not, however, is commercially viable. But this is beginning to change. Researchers at Pacific Northwest National Laboratory (PNNL) have been developing fuel cell technologies that will bridge the chasm from bench-scale demonstration to commercial application.
The early 1800s was a banner period for electrochemists and witnessed the invention of both the battery by Alessandro Volta and the fuel cell by William Groves, two devices that operate similarly. Both rely on the transport of ionic species to convert chemical energy directly into electrical energy. However, while a battery is a self-contained device, a fuel cell uses an external source of fuel and oxidizing reagent (see figure 1). As long as these are continually supplied to the fuel cell, it will operate indefinitely, at least in theory. The actual operational lifetime of these devices has been one of the stumbling blocks to commercialization. Other barriers include cost, and depending on the application, size, weight and the type of fuel that can be used. While Volta's discovery has blossomed into a ubiquitous product, Sir William's fuel cell has remained a novelty item, finding use only in niche applications (for example, space vehicles) where issues such as cost are of secondary concern.
Recognizing both the tremendous promise of fuel cell technology and the shortcomings that have kept it from the marketplace, the Department of Energy embarked on an ambitious program to push fuel cells closer to commercialization.
Fostering strong collaboration between the national labs, industry and academia, the DOE has made substantial investments in two leading fuel cell technologies: polymer electrolyte membrane fuel cells (PEMFCs) and solid oxide fuel cells (SOFCs). Among the various types of fuel cells currently in development—€”which can be differentiated by the electrolyte employed and/or the nominal operating temperature—€”PEMFCs are the most technologically mature and therefore closest to widespread commercial use. These devices operate at low temperature (~80 —€“ 95°C), offer relatively high power densities and are portable, making them ideal as the primary power source for electric cars.
In order to function properly, PEMFCs require a source of clean hydrogen gas.
Although there has been much discussion on possibilities and merits of hydrogen as a fuel source, the bottom line is the realization of a hydrogen economy is conservatively two decades away. For PEMFCs to become viable, they must operate on a widely available hydrocarbon fuel such as gasoline, diesel or methanol.
This means that the PEMFC must be equipped with a reformer, which is essentially a chemical reactor that breaks hydrocarbon fuels into a hydrogen-rich gas stream. However, by incorporating a reformer at the front-end of our PEMFC stack, we increase the complexity of the overall system as well as add to its weight, volume and cost. Researchers at PNNL are using microchannel miniaturization technology developed at the lab to decrease the weight and volume of a standard PEMFC reformer by over ten times, accompanied by a commensurate drop in the cost of the device. Employing fine-scale channels cut into a series of thin plates, the microchannel reformer is designed to enhance the breakdown of hydrocarbon fuels at faster rates and improve hydrogen production efficiencies in a much smaller size package without sacrificing gas throughput.
A second aspect critical to gasoline-powered PEMFCs is the ability to produce hydrogen gas that is free of sulfur compounds and carbon monoxide, both which form during the fuel reforming process and can poison the fuel cell stack.
Combining microchannel technology with recent advances in fuel processing made at the lab, including catalytic desulfurization and the controlled conversion of carbon monoxide to carbon dioxide, PNNL scientists are designing a small-scale post-reformation processor that essentially cleans up the reformed fuel gas stream for direct use in the PEMFC stack. When completed, the entire fuel processing system, including the reformer and a gas vaporizer, will be able to support a 50kW PEMFC stack—€”enough power to operate an electric car—€”and will measure only slightly larger than a shoe box.
SOFCs are the least mature of all fuel cell technologies, but they offer distinct advantages over their brethren. Specifically, the SOFC is the only fuel cell constructed entirely from solid materials; all others employ a liquid electrolyte and require a means of maintaining that liquid so the fuel cell will not dry out and lose function. Additionally, because an SOFC stack operates at high temperature (700°C or higher), fuel reformation can occur directly within the system, eliminating the need for an external fuel reformer. The heat generated during operation is not only sufficient to carry out internal fuel reformation and keep the stack at its proper operating temperature, but can also be utilized to increase the overall efficiency of the system. Even more intriguing, due to the temperature of operation and the type of electrochemical reactions taking place in an SOFC, carbon monoxide will not poison the stack, but instead serves as an additional source of fuel. However, a number of advancements in SOFC technology are still required to bring down costs and improve performance and reliability to the point that commercialization becomes feasible. Research being conducted at PNNL is bringing these devices closer to this realization.
Researchers have focused their efforts on four main areas of SOFC research: materials development, processing and manufacturing, stack and system modeling, and system validation and technology transfer. Materials researchers are manipulating the chemical composition of the ceramic cathode and engineering the interface that it makes with the zirconia electrolyte, not only to derive more power from the stack, but to do so at lower temperatures. Lowering the temperature a hundred or so degrees is essential to reducing the overall cost of the system. Cooler operating temperatures mean that inexpensive metal components —€“ as opposed to far more costly and less reliable ceramic support materials —€“ can be used both within the stack and the rest of the power plant system.
Similarly, new metal and composite seals invented at the lab allow the stack to operate longer and offer greater reliability particularly under rugged conditions, like those encountered in automotive, trucking, and aerospace applications.
The new materials will have little value in a commercial stack if they cannot be cost- effectively manufactured into components. Using a synthesis technique known as the glycine nitrate process, originally developed at PNNL in the late 80s but now employed commercially by several manufacturers, lab staff has demonstrated the ability to control ceramic powder composition to very fine levels in large batch sizes, with excellent batch-to-batch consistency. By tape casting and sintering manufacturing processes commonly employed in the capacitors and automotive electronics industries—€”the powders are fabricated into full-size solid cells. Manufacturing trials and statistical analysis have been applied to identify the "sweet spot" for the numerous combinations of process variables, ensuring that the cells can be optimally manufactured and processing can eventually be scaled to production. PNNL metallurgists have applied the same methodology to develop inexpensive stamped interconnects and current collectors —€“ the metallic components used in the SOFC stack.
A team of solid mechanics, fluid dynamics, materials, electrochemistry, and computer software experts at the lab have been at work developing software tools for modeling not only the electrochemical reactions that occur within SOFCs but how these reactions are dependent on stack design —€“ for example, gas flow patterns and rates inside the stack, pressure drops across the cells, local fuel utilization, and thermal profiles dependent on component and stack geometry.
SOFC manufacturers are employing these simulation tools to build virtual SOFC stacks and systems, run them through their paces, and make design changes accordingly. The models are validated using data taken from actual cell, stack, and system testing done both at the lab and with collaborating SOFC system manufacturers. This is where the rubber meets the road—€”where all of the effort carried out in materials research, process and manufacturing development, and system design simulation is brought together into one package—€”the fabrication and testing of a complete, working system. Partnering with manufacturers such as Delphi Corporation, Fuel Cell Energy, SOFCo, and Siemens-Westinghouse, PNNL is forging ahead to make Sir William Grove's discovery a commercial reality.
K. S. Weil, Ph.D., is a senior scientist at the Pacific Northwest National Laboratory in Seattle.
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Father of the Fuel Cell
In 1839 the foundation for today's fuel cell technology has already been laid. It was the Welsh justice and physician Sir William Robert Grove (1811-1896) who developed the first working prototype. This prototype consisted of two platinum electrodes which were separately surrounded by a glass cylinder. One of the cylinders was filled with hydrogen, the other with oxygen. Both electrodes were immersed in diluted sulphuric acid—€”which was the electrolyte—€”and created the electric connection. At the electrodes voltage was produced. This voltage was very low and therefore Grove linked several of these fuel cells to get a higher voltage.
Grove's contemporaries underestimated the importance of his discovery and the fuel cell was forgotten. Only in the 1950s, against the background of the Cold War, his idea was taken up again. Space travel and military technology required compact and powerful energy sources.
Spacecraft and submarines require electric power and it is not possible to work with internal combustion engines. Because of batteries being too heavy for spacecrafts, NASA (e.g. in the Apollo program) decided in favor of the direct chemical generation of electric power by fuel cells.
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