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Frontiers of Engineering: Reports on Leading-Edge Engineering from the 2005 Symposium (2006)

Chapter:Fuel Cells: Current Status and Future Challenges

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Suggested Citation:"Fuel Cells: Current Status and Future Challenges." National Academy of Engineering. 2006. Frontiers of Engineering: Reports on Leading-Edge Engineering from the 2005 Symposium. Washington, DC: The National Academies Press. doi: 10.17226/11577.
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Fuel Cells: Current Status and Future Challenges

STUART B. ADLER

University of Washington

Seattle, Washington


Fuel cells, which convert chemical energy directly to electricity, are more efficient than current means of energy conversion. The question is where they might fit in the broad spectrum of energy choices. This paper briefly reviews and compares polymer-electrolyte fuels cells (PEFCs) and solid-oxide fuel cells (SOFCs) and then describes significant scientific challenges that must be overcome before these technologies can become commercially competitive.

Fuel cells are not a new idea. Sir William Grove first demonstrated the conversion of hydrogen to electricity using an acid-electrolyte fuel cell in 1839. However, turning this idea into a practical means of energy conversion has proved to be elusive. A major technical and cost barrier has been implementation of liquid electrolytes, the basis for most commercial fuel cells (e.g., alkaline fuel cells, molten-carbonate fuel cells). In contrast, the fuel cells of greatest commercial interest today are based on solid electrolytes, which have benefited from recent advances in materials and manufacturing.

For the purposes of discussion, we can divide solid-electrolyte fuel cells into two types: (1) PEFCs, often referred to as proton-exchange-membrane (or PEM) fuel cells; and (2) SOFCs. Figure 1 illustrates how these types of fuel cells function.

A common justification for fuel cells has been environmental protection—the idea that fuel cells produce only water as a combustion by-product and thus are “zero emission” devices. However, it is difficult to make the case for fuel cells based on this argument alone. Although fuel cells themselves produce only

Suggested Citation:"Fuel Cells: Current Status and Future Challenges." National Academy of Engineering. 2006. Frontiers of Engineering: Reports on Leading-Edge Engineering from the 2005 Symposium. Washington, DC: The National Academies Press. doi: 10.17226/11577.
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FIGURE 1 Two types of solid electrolyte fuel cells. a. In a PEFC, a proton-conducting polymer membrane is exposed on one side to fuel (hydrogen) and on the other to air. On the hydrogen side (anode), H2 gas is oxidized, and the protons thus created migrate to the other side of the membrane (cathode), where O2 gas in the air is reduced to water. Some portion of the reversible work of the net reaction is recovered as a voltage difference between cathode and anode, delivered to an external circuit by the flow of electrons. PEFCs typically operate at 80~100°C. b. In an SOFC, a ceramic oxygen ion conductor at elevated temperatures (500~1,000°C) serves as the electrolyte membrane. In this case, the fuel (which can be a mixture of H2, CO, and/or hydrocarbons) is oxidized to H2O and CO2 at the anode, while O2 is reduced to O2– at the cathode. In both types of fuel cells, cells are normally assembled into multicell stacks to increase system voltage and provide a means of distributing gases (fuel and air) evenly.

water, the production of hydrogen from hydrocarbons, such as oil or coal, involves the production of carbon dioxide (CO2) and requires the suppression of sulfur dioxide (SO2). Thus fuel cells merely transfer the environmental problem elsewhere.

In addition, numerous technologies are already available that can eliminate SO2 and nitrogen oxides (NOX) from combustion. Widespread implementation of these technologies is simply a matter of cost and political will. Thus, one can

Suggested Citation:"Fuel Cells: Current Status and Future Challenges." National Academy of Engineering. 2006. Frontiers of Engineering: Reports on Leading-Edge Engineering from the 2005 Symposium. Washington, DC: The National Academies Press. doi: 10.17226/11577.
×

easily imagine an energy economy based entirely on clean combustion of hydrogen or other multisource fuels that does not include fuel cells.

To understand the potential role of fuel cells, we must instead consider their primary advantage—efficiency. In this regard, fuel cells are an enabling (rather than a displacing) technology. They recover energy that is normally lost by the irreversible process of combustion. Thus, fuel cells offer a potential path toward overall reduction of fuel consumption that combustion simply cannot provide, even after many years of incremental improvements.

By reducing the overall amount of CO2 produced per kilowatt (kW) of usable power, increased efficiency may, ultimately, have environmental benefits as well. In addition, the required retooling of the fuel infrastructure toward more generic, small-molecule fuels (e.g., H2, CO, CH4) might also lead to centralization of CO2 production, which would facilitate carbon sequestration and reduce the vulnerability of particular energy sectors to fluctuations in the supply of particular fuel sources (e.g., the dependence of gasoline prices on the availability of oil from the Middle East).

COMPARISONS BETWEEN PEFCs AND SOFCs

A primary factor influencing the trade-off between capital and efficiency in fuel cells is operating temperature. SOFC stacks, which operate at temperatures ranging from 550°C to 900°C, produce high-quality waste heat that can be captured for increased efficiency, combined heat and power, or reformation of hydrocarbons (HCs). SOFC stacks tend to operate adiabatically wherein excess air is used as the primary coolant, and thus heat can be recovered from the SOFC exhaust. This feature has made SOFCs very attractive for the production of stationary power, where efficiency is of high importance relative to capital cost, and operation on reformed HCs is an advantage. Allowable capital costs for stationary power ($400/kW) are about 10 times higher than for PEFCs in automotive applications (DOE, 2004b).

By using thin-film ceramics supported on low-cost metal alloys, SOFC developers have reduced material and manufacturing costs, lowered operating temperatures, and significantly mitigated cell-degradation problems. Figure 2 shows an example of a metal-supported cell based on a thin ceria electrolyte, capable of stable power densities of ~500 mW/cm2 at 570°C (Brandon, 2005). Systems based on this type of cell are nearing efficiency and cost targets for use in homes (combined heat and power) and auxiliary power units for trucks and aircraft.

In contrast, PEFCs have historically been designed to operate isothermally, at or below 80°C. Low operating temperatures have made them more suitable for small or mobile applications, for which capital cost requirements are much more stringent, pure hydrogen (H2) is assumed to be available, and the efficiencies of heat integration are less important. The most challenging market from a capital-cost perspective is motive power (cars), for which allowable capital costs are

Suggested Citation:"Fuel Cells: Current Status and Future Challenges." National Academy of Engineering. 2006. Frontiers of Engineering: Reports on Leading-Edge Engineering from the 2005 Symposium. Washington, DC: The National Academies Press. doi: 10.17226/11577.
×

FIGURE 2 Example of a metal-supported, thin-film solid-oxide fuel cell capable of operation below 600°C. Photo courtesy of Ceres Power, Ltd., reproduced with permission.

estimated to be on the order of $35/kW (Garman, 2003). PEFCs are also generally thought to match the size, weight, and start-up constraints for primary power in automobiles.

Substantial progress has been made in increasing the power density of PEFCs (>1 kW/kg) (Gasteiger et al., 2005), as well as reducing the amount of platinum (Pt) catalyst to a level that is reasonable to recycle (<15g/vehicle, three to four times the catalyst in a catalytic converter) (Cooper, 2004; Gasteiger et al., 2005). Based on these successes, several of the world’s biggest automakers, including General Motors, Ford, Daimler, and Honda (Figure 3), have built demonstration cars.

Despite these significant advances, solid-electrolyte fuel cells have not yet achieved widespread penetration into the energy market for many reasons. In particular, fuel cell systems are still too costly to be competitive with existing technology at current energy prices. Although this situation may change as fuel prices rise and capital costs come down with manufacturing improvements and economies of scale, fundamental technological barriers must also be overcome before cost reductions are likely. Many of these technological hurdles have been described in detail elsewhere (DOE, 2004a). The discussion below focuses on areas of fundamental research where breakthroughs might lead to significant technological advancements.

Suggested Citation:"Fuel Cells: Current Status and Future Challenges." National Academy of Engineering. 2006. Frontiers of Engineering: Reports on Leading-Edge Engineering from the 2005 Symposium. Washington, DC: The National Academies Press. doi: 10.17226/11577.
×

FIGURE 3 Sandy Spallino, first individual customer to purchase a PEFC-powered car, fills up her Honda FCX at one of many H2 refueling stations planned for California. Source: Honda press release, June 2005.

MATERIAL PROPERTIES BY DESIGN

Many of the materials used in SOFCs and PEFCs today are similar to the ones used 25 years ago. Examples include the nickel (Ni)-cermet anode used in most SOFCs and the perfluorosulfonic acid (PFSA) membrane used as the electrolyte in most PEFCs (Dupont Nafion®). Despite numerous difficulties with these materials, they are still considered state of the art because their unique combination of properties is still unmatched. However, they also introduce fundamental problems (Figure 4). In SOFCs, Ni-cermet has very poor sulfur tolerance, especially below 800°C, which makes it unsuitable as a long-term SOFC anode (DOE, 2004a). PEFC developers have concluded, that to be successful in cars, the system must operate at 110~120°C, which introduces severe performance and degradation problems for PFSA (Gasteiger and Mathias, 2003). To date, a trial and error approach has been used to search for new materials. However, further advances are likely to require a directed design approach (Hickner et al., 2004) and/or combinatorial methods (Kilner et al., 2005).

PROBING AND CONTROLLING MICROSTRUCTURE/NANOSTRUCTURE

Despite the technological advances in SOFC and PEFC technology in the last ten years, our understanding and design capability are mostly at the macro-

Suggested Citation:"Fuel Cells: Current Status and Future Challenges." National Academy of Engineering. 2006. Frontiers of Engineering: Reports on Leading-Edge Engineering from the 2005 Symposium. Washington, DC: The National Academies Press. doi: 10.17226/11577.
×

FIGURE 4 Relationship between proton conductivity and relative humidity in the adjoining gas at various temperatures for PFSA and phosphoric-acid-doped polybenzimidazole (PBI). Curves are also shown for materials that would enable and be ideal for system simplification. Source: Gasteiger and Mathias, 2003.

scopic/empirical level. The microstructure of a PEFC electrode, for example, is still understood only in a very general sense; exactly how the catalyst, ionomer, and gas come together and affect performance is generally not well understood and thus not amenable to intelligent design. For example, one proposed strategy for improving the catalyst in PEFC cathodes is to concentrate Pt particles near the opening of the aqueous flow channel in the PFSA ionomer; at present they are distributed randomly throughout the electrode matrix. However, this type of nanostructural analysis, let alone control, is not possible today.

As shown in Figure 5, one possible technique on the horizon for SOFCs is focused-ion beam milling coupled with electron microscopy or other surface analytical techniques, which may make it possible to analyze and direct electrode microstructures in new ways (J. Wilson et al., 2005). Researchers have also recently demonstrated solution impregnation of materials into an electrolyte host matrix to obtain SOFC electrodes with improved hydrocarbon activity or O2 reduction performance (Huang et al., 2005; McIntosh and Gorte, 2004).

Suggested Citation:"Fuel Cells: Current Status and Future Challenges." National Academy of Engineering. 2006. Frontiers of Engineering: Reports on Leading-Edge Engineering from the 2005 Symposium. Washington, DC: The National Academies Press. doi: 10.17226/11577.
×

FIGURE 5 3-D reconstruction of pores inside a porous Ni-YSZ SOFC anode, based on 2-D FIB-SEM image slices. At left are cross-sectional image slices corresponding to the reconstructed 3-D image.

UNDERSTANDING ELECTRODE DEGRADATION AND OTHER DEGRADATION PROCESSES

The vast majority of work in the last 10 years has been focused on improving fuel cell performance. However, as the technology has now reached some performance targets, and as more cells and stacks have been tested for longer periods of time, long-term durability has risen to the top of the list of performance targets. For example, SOFC electrodes can be very sensitive to chromia (Cr) poisoning (Simner and Stevenson, 2004). Although electrode degradation has been positively linked to Cr contamination from metal interconnects, it is not clear why some electrode materials are more sensitive than others or why seemingly similar electrodes tested by different groups degrade at different rates. The answers to these questions require a much deeper mechanistic and scientific understanding of electrode processes than we currently possess.

Recent advances in microfabrication and diagnostics may significantly improve our ability to control and analyze electrode reactions (Adler, 2004; J.R. Wilson et al., in press). Recent work using nonlinear electrochemical impedance spectroscopy to resolve SOFC cathode reaction mechanisms may eventually improve our ability to diagnose how and why electrodes degrade and guide the selection of new materials and fabrications to mitigate degradation.

Suggested Citation:"Fuel Cells: Current Status and Future Challenges." National Academy of Engineering. 2006. Frontiers of Engineering: Reports on Leading-Edge Engineering from the 2005 Symposium. Washington, DC: The National Academies Press. doi: 10.17226/11577.
×

OUTLOOK

Fuel cells continue to face major technological hurdles that may require many years of research and development before they can be overcome. In addition, fuel cells are not likely to be implemented in isolation. They must be part of a larger shift in fuel infrastructure and efficiency standards, which will require sustained political and economic pressure—and time. Finally, like any technology, economy of scale will require a natural maturation process over many years or decades (DeCicco, 2001).

Taken together, these hurdles suggest that the widespread adoption of fuel cell technology is not likely in the short term. Successful advancement of fuel cell technology will require a sustained, long-term commitment to fundamental research, commercial development, and incremental market entry.

REFERENCES

Adler, S.B. 2004. Factors governing oxygen reduction in solid oxide fuel cell cathodes. Chemical Reviews 104(10): 4791–4844.


Brandon, N.P. 2005. Metal Supported Solid Oxide Fuel Cells for Operation at 500–600°C. Paper presented at SSI-15, the International Conference on Solid State Ionics, Baden-Baden, Germany, July 17–22, 2005.


Cooper, J.S. 2004. Recyclability of Fuel Cell Power Trains. Pp. 792–801 in Proceedings of the SAE 2004 World Congress. SAE Technical Paper 2004-01-1136. Warrendale, Pa.: SAE International.


DeCicco, J. 2001. Fuel Cell Commercialization Perspectives: Market Context and Competing Technologies. Paper presented at the 2001 Gordon Conference on Fuel Cells, Bristol, R.I., July 29– August 3, 2001.

DOE (U.S. Department of Energy). 2004a. Fuel Cell Handbook, 7th ed. National Energy Technology Laboratory, Morgantown, West Virginia. Washington, D.C.: Office of Fossil Energy, U.S. Department of Energy.

DOE. 2004b. Fuel Cell Program Annual Report. National Energy Technology Laboratory, Morgantown, West Virginia. Washington, D.C.: Office of Fossil Energy, U.S. Department of Energy.


Garman, D. 2003. Testimony by David K. Garmen, Assistant Secretary, Energy Efficiency and Renewable Energy, U.S. Department of Commerce. Hearing before the Committee on Science, 108th Congress, Serial Number 108-4, March 5, 2003. Available online at: http://commdocs.house.gov/committees/science/hsy85417.000/hsy85417_0f.htm.

Gasteiger, H.A., and M.F. Mathias. 2003. Fundamental Research and Development Challenges in Polymer Electrolyte Fuel Cell Technology. Paper presented at the Workshop on High Temperature PEM Fuel Cells, Pennsylvania State University, December 2003.

Gasteiger, H.A., S.S. Kocha, B. Sompalli, and F.T. Wagner. 2005. Activity benchmarks and requirements for Pt, Pt-alloy, and non-Pt oxygen reduction catalysts for PEMFCs. Applied Catalysis B: Environmental 56(1–2): 9–35.


Hickner, M.A., H. Ghassemi, Y.S. Kim, B.R. Einsla, and J.E. McGrath. 2004. Alternative polymer systems for proton exchange membranes (PEMs). Chemical Reviews 104(10): 4637–4678.

Huang, Y.Y., J.M. Vohs, and R.J. Gorte. 2005. Characterization of LSM-YSZ composites prepared by impregnation methods. Journal of the Electrochemical Society 152(7): A1347–A1353.

Suggested Citation:"Fuel Cells: Current Status and Future Challenges." National Academy of Engineering. 2006. Frontiers of Engineering: Reports on Leading-Edge Engineering from the 2005 Symposium. Washington, DC: The National Academies Press. doi: 10.17226/11577.
×

Kilner, J.A., J.C.H. Rossiny, and S. Fearn. 2005. Combinatorial Searching for Novel Mixed Conductors. Paper presented at SSI-15 International Conference on Solid State Ionics, Baden-Baden, Germany, July 17–22, 2005.


McIntosh, S., and R.J. Gorte. 2004. Direct hydrocarbon solid oxide fuel cells. Chemical Reviews 104(10): 4845–4865.


Simner, S.P., and J.W. Stevenson. 2004. Cathode-Chromia Interactions. Paper presented as part of the SECA 2004 Annual Meeting and Core Program Review, Boston, Massachusetts 2004.


Wilson, J., W. Kobsiriphat, R. Mendoza, J. Hiller, D. Miller, K. Thornton, P. Voorhees, S. Adler, and S. Barnett. 2005. Three Dimensional Reconstruction of Solid Oxide Fuel Cell Electrodes. Unpublished paper.

Wilson, J.R., D.T. Schwartz, and S.B. Adler. In press. Nonlinear electrochemical impedance spectroscopy for mixed-conducting SOFC cathodes. Electrochemica Acta. Available online at: http://dx.doi.org/doi:10.1016/j.electacta.2005.02.109.

Suggested Citation:"Fuel Cells: Current Status and Future Challenges." National Academy of Engineering. 2006. Frontiers of Engineering: Reports on Leading-Edge Engineering from the 2005 Symposium. Washington, DC: The National Academies Press. doi: 10.17226/11577.
×

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Suggested Citation:"Fuel Cells: Current Status and Future Challenges." National Academy of Engineering. 2006. Frontiers of Engineering: Reports on Leading-Edge Engineering from the 2005 Symposium. Washington, DC: The National Academies Press. doi: 10.17226/11577.
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Suggested Citation:"Fuel Cells: Current Status and Future Challenges." National Academy of Engineering. 2006. Frontiers of Engineering: Reports on Leading-Edge Engineering from the 2005 Symposium. Washington, DC: The National Academies Press. doi: 10.17226/11577.
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Suggested Citation:"Fuel Cells: Current Status and Future Challenges." National Academy of Engineering. 2006. Frontiers of Engineering: Reports on Leading-Edge Engineering from the 2005 Symposium. Washington, DC: The National Academies Press. doi: 10.17226/11577.
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Suggested Citation:"Fuel Cells: Current Status and Future Challenges." National Academy of Engineering. 2006. Frontiers of Engineering: Reports on Leading-Edge Engineering from the 2005 Symposium. Washington, DC: The National Academies Press. doi: 10.17226/11577.
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Suggested Citation:"Fuel Cells: Current Status and Future Challenges." National Academy of Engineering. 2006. Frontiers of Engineering: Reports on Leading-Edge Engineering from the 2005 Symposium. Washington, DC: The National Academies Press. doi: 10.17226/11577.
×
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Suggested Citation:"Fuel Cells: Current Status and Future Challenges." National Academy of Engineering. 2006. Frontiers of Engineering: Reports on Leading-Edge Engineering from the 2005 Symposium. Washington, DC: The National Academies Press. doi: 10.17226/11577.
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Suggested Citation:"Fuel Cells: Current Status and Future Challenges." National Academy of Engineering. 2006. Frontiers of Engineering: Reports on Leading-Edge Engineering from the 2005 Symposium. Washington, DC: The National Academies Press. doi: 10.17226/11577.
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Suggested Citation:"Fuel Cells: Current Status and Future Challenges." National Academy of Engineering. 2006. Frontiers of Engineering: Reports on Leading-Edge Engineering from the 2005 Symposium. Washington, DC: The National Academies Press. doi: 10.17226/11577.
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Suggested Citation:"Fuel Cells: Current Status and Future Challenges." National Academy of Engineering. 2006. Frontiers of Engineering: Reports on Leading-Edge Engineering from the 2005 Symposium. Washington, DC: The National Academies Press. doi: 10.17226/11577.
×
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Suggested Citation:"Fuel Cells: Current Status and Future Challenges." National Academy of Engineering. 2006. Frontiers of Engineering: Reports on Leading-Edge Engineering from the 2005 Symposium. Washington, DC: The National Academies Press. doi: 10.17226/11577.
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This volume includes 16 papers from the National Academy of Engineering's 2005 U.S. Frontiers of Engineering (USFOE) Symposium held in September 2005. USFOE meetings bring together 100 outstanding engineers (ages 30 to 45) to exchange information about leading-edge technologies in a range of engineering fields. The 2005 symposium covered four topic areas: ID and verification technologies, engineering for developing communities, engineering complex systems, and energy resources for the future. A paper by dinner speaker Dr. Shirley Ann Jackson, president of Rensselaer Polytechnic Institute, is also included. The papers describe leading-edge research on face and human activity recognition, challenges in implementing appropriate technology projects in developing countries, complex networks, engineering bacteria for drug production, organic-based solar cells, and current status and future challenges in fuel cells, among other topics. Appendixes include information about contributors, the symposium program, and a list of meeting participants. This is the eleventh volume in the USFOE series.

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