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Author: Stuart B. Adler
Fuel cells of the future will be based on solid electrolytes.
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 chal-lenges 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 (see PDF version for figures) 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 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 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 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.
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 prob-lems 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 macroscopic/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).
Understanding Electrode Degradation and Other Degradation Processes
The vast majority of work in the last ten 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.
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.
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