Impact of Catalysis on Clean Energy in Road Transportation

Energy & Fuels 1998, 12, 1121-1129

1121

Impact of Catalysis on Clean Energy in Road Transportation Peter G. Gray* and Jonathan C. Frost Johnson Matthey Technology Centre, Blount’s Court, Sonning Common, Reading, RG4 9NH, U.K. Received May 5, 1998. Revised Manuscript Received August 12, 1998

The fastest growing sector for the utilization of energy is that of transportation, the bulk of this being road vehicles. From an environmental and resource viewpoint, the major issues are how to cleanly and efficiently use energy for road vehicles. Over the last 20 years, major savings in emissions from cars have been achieved by the use of automotive catalysts. However, localized pollution (in major population centers) continues to be an issue, as does the need for improved fuel efficiency (to reduce CO2 emissions and conserve energy resources). These are the main drivers for the development of advanced fuel cell technology, which, in turn, relies upon compact and efficient onboard fuel processing to enable a commercially viable use of the technology in cars. Current trends indicate that these new technologies will reach the market in the next decade. Success in all three clean-energy technologies, catalytic convertors, PEM fuel cells, and onboard fuel processors, has been and continues to be dependent on advances in catalysis.

Introduction Developing and developed countries are experiencing a major growth in energy usage in the transport sector. Within this sector, most energy is used by, and pollution produced by, road vehicles. Over the last 20 years, major savings in emissions from cars have been achieved by the use of automotive catalysts. However, localized pollution (in major population centers) continues to be an issue, as does the need for improved fuel efficiency (to reduce CO2 emissions and conserve energy resources). Because of thermodynamic limitations, internal combustion engines will not be able to achieve both high efficiency and zero emissions, with current technology being close to the best that can be expected. Major improvements in the future will be obtained by moving away from heat engines, i.e., by using electrochemical rather than chemical combustion. In this area, fuel cells offer major gains in fuel efficiency and emissions for mobile power sources. The most promising fuel cell technology for transport applications is the PEMFC (polymer electrolyte membrane fuel cell). Rapid advances in this technology (both in catalytic materials and engineering) have been made over the last 10 years, with prototype buses (using hydrogen fuel) currently being tested. The dual constraints of limited onboard vehicle space (for hydrogen storage) and existing transport fuel infrastructure mean that catalytic onboard fuel processing (to produce hydrogen) of liquid fuels, such as methanol or gasoline, is essential for the introduction of fuel cells on cars. The recent UNFCCC Kyoto conference (United Nations Framework Convention on Climate Change, Kyoto Conference, December 1997) highlighted the disparate energy usage needs and aspirations of many countries; however, what is unquestionable is the imperative for

clean and efficient use of energy within all countries. It is now clearly recognized, in both mature and fastgrowing economies, that the legitimate desire to maintain, or improve, the quality of life should be consistent with the responsible use of energy, responsible meaning energy use that is clean locally (e.g., in cities) and efficient globally (to conserve resources and reduce CO2 emissions). To meet this challenge, catalyst technology has been employed to control pollutant emissions from combustion processes as well as to enable technologies that more efficiently convert fuel into electrical power. While this article is primarily based on work done in the United Kingdom and on U.K. statistics, the general trends and conclusions are applicable to other developed countries as well as developing countries. Overall energy usage, both locally and globally, is usually broken down into four sectors: transport, industry, domestic, and service. Within the U.K., transport is the fastest growing sector and accounts for 33.5% of the total energy consumption1 (Figure 1). Energy consumption by industry has decreased dramatically since the 1970s, due to restructuring and modernization of plants. Domestic- and service-sector energy consumption has been growing but not at a rapid rate. All of these trends can be seen in most mature and fast-growing economies, apart from the additional growth in industrial energy consumption in developing countries. As transportation accounts for the most energy consumption and growth, this is the sector that is focused on in this article. A breakdown of transportation into individual sectors shows that road transport accounts for, by far, the major use of energy (80%), followed by air, then sea and rail1 (1) An Appraisal of U.K. Energy Research, Development, Demonstration and Dissemination; ETSU, HMSO: London, 1994.

10.1021/ef980110f CCC: $15.00 © 1998 American Chemical Society Published on Web 09/24/1998

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Gray and Frost

tion that the world car population could double from 400 million to 800 million in the next 20 years.3 For these reasons, this paper will concentrate on clean and efficient energy usage in the road-transport sector. This article discusses the leading role of catalysis in the development of clean energy in the transport sector, covering (i) conventional technology developed and used over the last 20 years for use with internal combustion engines and (ii) advanced fuel cell and associated fuelprocessing technologies for clean and efficient mobile power sources for the next century. In this context, specific technologies developed, or under investigation, by Johnson Matthey will be used as examples. This paper is not a review of the technical literature; it is intended to give a general overview of both progress over the last 20 years, likely future advances in the next 5-10 years, and the impact that this has had and will continue to have. By its nature, the scope of this article is very broad, so all of the specific individual advances cannot be cited. For further detail, the reader is referred to reviews of the technical literature specifically on automotive catalysis, fuel cells, and fuel processing, which are published from time to time; some examples are given in refs 4-6.

Figure 1. Energy in the U.K.schanging demands.

Impact of Existing Catalyst Technology

Figure 2. Energy in the U.K.stransport sectors. Table 1. U.K. Air Pollutant Emissions (%) by Source (1994) road transport other transport domestic power generation industry service total (ktonne)

NOx

CO

SO2

CO2

PM10

49 7 3 24 15 2 2218

89 0.8 6 0.5 3.5 0.2 4833

2 2 3 65 25 3 2719

21 2 15 29 28 5 150 000

25 3 14 15 41 2 263

(Figure 2). Efforts to improve energy efficiency, therefore, need to focus on road transport. From the perspective of promoting clean energy usage, a breakdown of major pollutant emissions (NOx, CO, SO2, CO2, and PM10) by source provides a useful guide to identifying the sector(s) most responsible for producing pollutants. Recent environmental statistics2 have shown that the road-transport sector produces the most NOx and CO, as well as significant quantities of CO2 and PM10 (Table 1). The power generation and industrial sectors produce the most SO2, however, these are usually located outside the center of major population areas. From the above points it can been seen that from the perspective of clean and efficient energy usage, road transport is the sector where most improvements need to be made. This is further emphasized by the expecta(2) Digest of Environmental Statistics; Report No. 18; Department of the Environment, Government Statistical Service, HMSO; London, 1996.

Apart from better design and control of internal combustion engines used for transportation applications, the principal technology for reducing pollutant emissions is the catalytic convertor. The most common of these devices is the three-way catalytic convertor, which oxidizes carbon monoxide and hydrocarbons to carbon dioxide and water while simultaneously reducing nitrogen oxides to nitrogen.7 Physically, the convertor is an open monolithic structure to allow easy flow of gas, with the active catalyst being coated on the channel walls of the monolith. The catalyst is typically platinum, although palladium, rhodium, and alloys thereof are often used. While primarily designed to eliminate HC, CO, and NOx, these catalysts can also partially reduce particulates (PM10, sub-10 µm particulate matter), although there are other catalytic devices, such as the CRT (continuously regenerating trap), specifically designed for this function. The first catalytic convertors were introduced in the United States (California) to combat smog problems in the mid to late 1970s; these performed as oxidation-only catalysts for CO and HC. The control of NOx as well became possible with the introduction of three-way catalysts and stoichiometric engine control in the early 1980s. Other countries, notably Japan, Australia, and some European countries, soon followed suit. In 1993, catalytic convertors became mandatory across the whole European Union for all new cars (including old model ranges), although new model ranges were equipped earlier. (3) Riley, R. Q. Alternative Cars in the 21st Century; SAE: Warrendale, 1994. (4) Gottesfeld, S.; Zawodzinski, T. A. Polymer Electrolyte Fuel Cells. Adv. Electrochem. Sci. Eng. 1997, 5, 195-301. (5) Ralph, T. R.; Hards, G. A. Powering the Cars and Homes of Tomorrow, Chemistry and Industry (U.K.); May 4, 1998, 337-342. (6) Hoogers, G. Fuel Cells: Power for the Future. Phys. World 1998, August, 31-36. (7) Gulati, S. T. et al, Advanced Three-Way Converter System for High-Temperature Exhaust Aftertreatment, SAE 970265, 1997.

Impact of Catalysis on Clean Energy

Energy & Fuels, Vol. 12, No. 6, 1998 1123

Figure 3. (a) U.K. CO emissions savings due to autocatalysts. (b) U.K. NOx emissions savings due to autocatalysts. (c) U.K. HC emissions savings due to autocatalysts. (d) U.K. PM10 emissions savings due to autocatalysts.

Since the introduction of this technology, dramatic reductions in the major pollutants (HC, CO, NOx, and PM10) have been achieved2,8 (Figure 3). In the United Kingdom alone, CO emissions savings from 1990 to 1995 were 1.3 million tonne while NOx savings were 0.3 million tonne. Major savings in HC and PM10 emissions were also achieved. By the year 2025, projected NOx savings will be over 1.7 million tonne, CO savings over 6 million tonne, HC savings over 1.2 million tonne, and PM10 savings over 0.1 million tonne. While existing automotive catalyst technology has been successful in achieving a major reduction in pollutant emissions in the transportation sector at a national/countrywide level, pollutant emissions at the local level are still a major challenge. Taking particulates as an example, while vehicles are the source of approximately 25% of the emissions at a national level, locally, within a city (e.g., London), they are responsible for approximately 90% of the emissions1 (Figure 4). While the picture is improving nationally, vehicle pollutant emissions are still a major issue to be addressed locally. This is one of the driving forces for even further improvements in engine and auto-catalyst technology. The two other main factors that are driving improvements in auto-catalyst technology are evermore stringent emissions legislation and more efficient engine technology (lean burn engines). Taking the United States as an example, despite major savings in emissions from vehicles over the past 2 decades, ULEV9 (8) Murrells, T. Vehicle Emissions Projection Model; AEA Technology: 1997. (9) www.epa.gov/omswww.

(Ultralow Emission Vehicle) legislation sets limits on gaseous pollutant emissions that are one-half, or even less, of the current LEV (low emission vehicle) values; this is to be phased in from 1998 to 2000 with all new vehicles complying by 2002/3 (Table 2). This mandate is primarily aimed at addressing local emissions problems. Lean burning gasoline engine technology has recently been introduced into cars in order to improve the fuel economy and reduce CO2 emissions. These engines have the potential to provide a 25-40% improvement in fuel economy, a 5-10% improvement in peak power, and significantly reduced CO2 emissions. However, conventional auto catalysts cannot reduce NOx emissions under lean conditions, so a new type of lean-NOx auto catalyst has been developed. Rather than continuously reducing NOx during operation, as in a conventional three-way auto catalyst, the new lean-NOx catalyst functions by storing NOx as nitrate within the catalyst support (Figure 5). When the storage capacity of the support is exhausted, the engine is momentarily switched to rich operation to generate an excess of CO and HC in the exhaust stream, where this rich pulse acts to decompose the stored nitrate and reduce the released NOx to N2. Thus, the overall reduction in NOx in the emissions is achieved by operating the engine management system in a lean-rich cycle (Figure 6). This lean-NOx auto-catalyst technology is particularly effective in controlling NOx emissions but is very susceptible to poisoning by sulfur in the fuel. Fuel sulfur also affects the efficiency of conventional gasoline and diesel catalysts but not as severely as lean-NOx auto


Gray and Frost

Figure 4. Locally, emissions are a major challenge. Table 2. U.S. Emissions Limits (g/mi) none 1975 1977 1980 1983 1994 1997 2003 JM ULEV

CO

NOx

HC

34 15 15 7 3.4 3.4 3.4 1.7

4 3 2 2 1 0.4 0.2 0.2

4.1 1.5 1.5 0.41 0.41 0.25 0.075 0.04

0.15

0.016

0.27

catalysts. In addition, any sulfur in the fuel ultimately becomes acid rain or sulfate particulates. For these reasons, reductions in fuel sulfur are highly desirable. Future Impact of New Catalyst Technologies Because of thermodynamic and materials limitations, heat (internal combustion) engines will never be able to achieve a very high efficiency and zero emissions. Engines and auto catalysts currently on, or near to, the market are close to being the best that can be expected from existing technology. Major improvements in the future will only be obtained by moving away from heat cycle engines, i.e., by using electrochemical rather than chemical combustion. In this area, fuel cells offer major gains in fuel efficiency and emissions for mobile power sources. The most promising fuel cell technology for transport applications is the PEMFC (polymer electrolyte membrane fuel cell) running on hydrogen fuel. Rapid advances in this technology (both in engineering and catalytic materials) have been made over the last 10 years, with prototype buses and cars currently being tested. The dual constraints of limited onboard vehicle space (for hydrogen storage) and existing transport fuel infrastructure mean that vehicle onboard fuel processing (to produce the hydrogen used by fuel cells) is essential for the introduction of fuel cells on cars. The success of both of these technologies, the PEM fuel cell and the onboard fuel processor, relies heavily on recent developments in catalyst technology. PEM Fuel Cells. It now seems probable that internal-combustion-powered vehicles will be superseded by

Figure 5. Mechanism of lean-NOx catalysis.

electric vehicles during the course of the first half of the next century. The incentives for this change come from stringent legislation for controlling local pollution (e.g., the U.S. ZEV, zero emission vehicle, mandate9), and from the global need for responsible use of fuel. Batteries were originally seen as the technology of choice for zero emission vehicles. However, in the period from



Figure 6. Operating a lean-NOx catalyst.

Figure 8. Section through the membrane electrode assembly.

Figure 7. PEM fuel cell membrane electrode assembly and stack.

1990 to 1996, when the ZEV mandate was deferred, only moderate improvements in battery performance were reported. In contrast, during the same period for the PEMFC, a five-fold increase in specific power was achieved while cheaper materials were adopted without compromising performance. Therefore, there is the growing view that in the design of vehicles, the conflicting demands of high performance and environmental protection can be overcome by using a fuel cell to generate onboard electric power. An individual PEM fuel cell consists of a membraneelectrode assembly (MEA) sandwiched between two bipolar gas-flow plates (Figure 7). The gas-flow plates, typically made of graphite or metal, serve two functions: to provide gas distribution to the electrodes in the MEA and to collect electric current from the surface of the MEA (in a bipolar arrangement, the cells are connected in series rather than in parallel). The membrane-electrode assembly consists of two electrodes, the anode and the cathode, separated by a polymer membrane electrolyte, which is an electronic insulator but conducts protons. A typical MEA, e.g., in a Ballard MkV stack, is flat and square with an active (catalyzed) area of 250 cm2. The electrodes are made of gas-permeable carbon-fiber paper or cloth coated on the inside (the side in contact with the membrane) with a catalyst, usually platinum on a carbon support. Fuel (hydrogen) is fed to the anode, while oxidant (air) is fed to the cathode. The individual cells, which generate approximately 0.6-0.7 V, are assembled together into a stack to give the overall voltage and current required. The current produced by the stack is the same as that generated by an individual cell (as the cells are connected in a simple bipolar arrangement) and is usually specified as amps per cm2 geometric area of electrode (as the electrodes are flat). A typical high-performance

PEMFC electrode will typically produce up to, or greater than, 1 A/cm2, depending on the operating conditions. Hydrogen in the anode, by action of the catalyst, is ionized and broken down into protons and electrons. The protons migrate through the polymer electrolyte membrane to the cathode, so that the anode acquires a negative charge from the electrons left behind (Figure 8). At the same time, oxygen molecules in the cathode, with the help of the catalyst, take on, or consume, electrons to generate oxygen ions, causing the cathode to have a positive charge (relative to the anode). This potential difference is exploited by passing the electrons through an external circuit (from anode to cathode), thus generating electric power. To complete the reaction, the protons that have migrated through the membrane combine with the oxygen ions to form water. Some heat is also generated but considerably less than in a combustion process. It can be seen that the oxidation of hydrogen (at the anode) and the reduction of oxygen (at the cathode), combining together to form water, is exactly the same overall process as in a chemical combustion process. The principal difference is that, in a fuel cell, the combustion process occurs electrochemically rather than chemically (as in a heat engine). Consequently, electric current is produced directly, without having to go through a heat engine cycle, hence thermodynamically, the electrochemical process is much more efficient. Furthermore, the only reaction product is pure water, thus it is, on its own, a true zero emission device.10 Apart from the inherent characteristics of high efficiency and zero emissions, the PEM fuel cell has other advantages: high power density (1kW/L for the latest Ballard-Daimler Benz Mk 7 stack), low temperature (80 °C), no moving parts, no liquid electrolyte, and virtually no noise. All of these characteristics make the PEM fuel cell ideal for transportation and small stationary power applications. The theoretical (open circuit) voltage for a H2-O2 fuel cell is 1.2 V. In practical operation, various losses accrue which decrease the overall voltage available as more current is drawn from the cell (Figure 9). There are four processes responsible for loss in voltage (and hence efficiency): cathode activation, cell resistance, (10) Ralph, T. R. Platinum Met. Rev. 1997, 41 (3), 102.


Figure 9. PEM fuel cell single-cell performance limitations.

anode activation, and mass transport resistance. These losses manifest themselves in different current density ranges: cathode activation losses mainly occur in the low-current density region (up to 0.1 A/cm2), cell resistance has its largest effect in the medium-current density range (0.3-0.7 A/cm2), anode activation occurs in the low- to medium-current range, and mass transport losses occur in the high-current density range (from 1 A/cm2 upward, for operation on air). Cathode activation loss is caused by the relatively slow nature of the oxygen reduction reaction that occurs on the cathode. There is more than one reaction pathway for the reduction of O2 to H2O (by the addition of electrons and protons), and each has a significant activation barrier to overcome for the reaction to proceed at a useful rate (i.e., produce a useful current). The oxygen reduction reaction kinetics are usually described by the Butler-Volmer equation and exhibit an exponential relationship between the cell potential and current. Thus, the decrease in cell potential is most marked over the first few decades of increasing current, up to approximately 100 mA/cm2, with a relatively minimal loss in voltage (compared to other processes) above this point. Cell resistance losses are due to an Ohmic drop in both the polymer membrane (related to proton conduction) and the bipolar plates and external circuit (electronic conduction), with the major losses being in the membrane. The membrane is commonly based on a polymeric TFE backbone with sulfonic acid groups acting as the immobilized counterions for the protons. The membrane needs to be highly humidified in order to conduct protons, as the protons migrate as hydrated species. The most commonly used membrane material in PEM fuel cells is Nafion, originally manufactured by DuPont for the chlor-alkali industry. Membrane thickness is typically 100 µm, although more recent MEA’s utilize much thinner membranes (down to 10-20 µm) to minimize cell voltage drop due to Ohmic resistance losses. Obviously, cell resistance causes a linear loss in voltage as cell current increases, therefore becoming more important at medium to higher currents.

Gray and Frost

Like cathode activation, anode activation is due to a reaction activation barrier. However, hydrogen oxidation on the anode is an extremely facile reaction (much more rapid than oxygen reduction), so the associated voltage loss is very small. Anode losses are minimal if the fuel used is pure hydrogen; however, more commonly, fuel cells are supplied with reformate which contains components that affect anode performance. For example, the product from partial oxidation/steam reforming of methanol or gasoline will generally contain, on a dry basis, 40-60% H2, 1-3% CO, ∼ 20% CO2, and N2 as the remainder. The species mainly responsible for anode losses are CO and CO2. CO poisons the anode catalyst by strongly adsorbing on the active sites, thus blocking H2 access to the sites where the dissociation/ oxidation reaction occurs. Only relatively small concentrations of CO will cause severe poisoning of the anode, e.g., for a simple, older style, platinum/carbon anode catalyst, 20 ppm of CO will cause at least a 50% loss in efficiency, primarily in the medium-current density range (Figure 10a).11 CO2 poisons the anode catalyst indirectly by two (proposed) mechanisms: reverse water-gas shift reaction on the catalyst produces strongly adsorbed CO and reaction of CO2 with adsorbed (dissociated) H2 forms COH-type residues which also adsorb strongly to the active sites. This has a similar effect to CO poisoning but is not as severe and occurs only at high concentrations of CO2 (Figure 10b).12 Various techniques, such as alloying of Pt with other metals or bleeding small amounts of air into the anode, have been used to greatly improve the tolerance of anodes to CO and CO2 poisoning, with the latest technology anodes typically being tolerant to 20-100 ppm of CO. Voltage (efficiency) loss due to mass transport resistance is primarily a problem in the cathode. Oxygen (in the air feed) diffuses through the carbon-fiber paper or cloth substrate (usually ∼300 µm thick) and through the catalyst layer structure itself to reach the catalytically active sites (Figure 11). The driving force for this process is a concentration drop from the outside (flowfield gas distributor) to the inside of the electrode; thus the greater the current drawn from the cell, the greater the amount of oxygen consumed and, hence, the greater the drop in concentration. The oxygen reduction reaction is positive order in O2 concentration (usually first order), so the drop in O2 concentration at the active sites decreases the reaction efficiency, which is manifested as a loss in cell potential. Furthermore, the product of the oxygen reduction reaction, liquid water, must also be transported out of the electrode and stripped from the system. The driving force for the flow of liquid water out of the cathode (catalyst layer and carbon-fiber substrate) is hydraulic pressure, or more precisely, a gradient in the capillary pressure in the water-filled pores. At high-current density, the cathode structure becomes increasingly flooded with water (due to increasing capillary pressure), which blocks off gas channels, thus further increasing gas diffusional resistance (11) Wilson, M. S.; Derouin, C. R.; Valerio, J. A.; Gottesfeld, S. Electrocatalyst Issues in Polymer Electrolyte Fuel Cells; Proceedings of the 28th Intersoc. Energy Conversion Engineering Conference, 1993; Vol. 1, pp 1203-1208. (12) Swathirajan, S. General Motors Program on Fuel Cell R&D for Vehicles; Proceedings of the 1994 Fuel Cell Seminar, San Diego, California, 1994; pp 204-207.



Figure 10. (a) Anode poisoning by CO in reformate. (b) Anode poisoning by CO2 in reformate. Table 3. Commercial PEMFC Systems Target Costs target systems costs ($U.S./kW) stationary power systems heavy duty vehicles (e.g., buses) light duty vehicles (e.g., cars) component costs

1993

1997

1566

191

1500 200 50-75

Figure 11. Mass transport at the cathode.

and reducing the oxygen concentration. In this region, research and development work has focused on optimizing the structure of the catalyst layer to enable favorable transport of both oxygen and liquid water. So far, the discussion has focused on technical aspects of fuel-cell design and operation and the function of the catalysts. Within this context, all of the technological targets (set within companies and by the US PNGV, Partnership for a New Generation of Vehicles13), such as power density and efficiency, for implementation of PEM fuel cells in buses and cars have been met, due to dramatic progress over the last 5-10 years. Consequently, the remaining targets to be met, in the near future, are of a commercial nature. The projected costs for various applications, based on vehicle companies’ and government estimates of what the consumer is prepared to pay for PEMFC systems, are shown in Table 3 along with estimated component costs (in mass production). Hence, PEM fuel cells are considered commercially feasible (from a cost-only point of view) for stationary power and bus applications, with further cost reductions required for car applications. Historically, the major cost item in the PEM fuel cell has been the precious metal (Pt) catalyst. However, (13) Review of the Research Program of the Partnership for a New Generation of Vehicles; Third Report; National Academy Press: Washington, DC, 1997.

Figure 12. PEM fuel cellsloading/performance targets.

major improvements in fuel-cell catalyst technology over the last 5 years have enabled the Pt loading (thus cost) to be reduced by over an order of magnitude, while ongoing developments will give a further order of magnitude reduction (Figure 12). Consequently, catalysts are no longer the major expense in a fuel cell, with other components, such as membrane materials and fabrication, contributing a greater proportion to the total cost. With these impressive achievements in meeting technical and commercial targets, as well as their inherently clean (zero emission) and efficient operation, PEM fuel cells are now being regarded as serious contenders for producing stationary and mobile (electric) power early next century. In this respect, 1997 can be regarded as a watershed year: prototype buses (Ballard and Daimler-Benz) have gone into fleet service, a number of


Gray and Frost

Figure 13. (a) Hydrogen storage systemssEnergy Density. (b) Energy storage systemssrelative sizes.

prototype cars have been announced with two (Daimler-Benz and Toyota) demonstrated and planned for market release in 2004/5, and most significantly, from a commercial viewpoint, many major company alliances and partnerships have been formed and very large amounts of capital committed to set up manufacturing plants. These recent advances in PEM fuel cells have relied heavily on the success in developing advanced catalyst technology. On-Board Fuel Processing for Fuel Cells. One of the remaining issues in fuel-cell development concerns the device’s fuel-hydrogen. Storage and distribution is currently difficult and costly and the space required for onboard storage (while available on buses) is prohibitive for small cars. One solution is to develop processors, onboard the vehicle, to convert more amenable fuels, such as gasoline or methanol, to hydrogen as and when it is required. This technology is complex and requires further processes to purify the processed hydrogen. Onboard processing of conventional liquid fuel is presently regarded as the most promising method of supplying a fuel cell in a small vehicle, as this offers the best energy density and occupies the smallest volume. Other methods of storing fuel onboard the vehicle, such as liquid hydrogen, compressed hydrogen gas, or metal hydride, have been proposed. However, for these means of storing energy, the volume required for a fixed total amount of energy (kWh) is considerably greater than for methanol or gasoline (Figure 13). Consequently, much research and development effort has focused on reforming hydrocarbon fuels to produce hydrogen. Several routes for reforming hydrocarbons have been investigated: direct thermal reforming, steam reforming, noncatalytic partial oxidation, catalytic partial oxidation, and autothermal reforming. The optimum choice of processing technology and fuel for the purpose of onboard hydrogen generation is, however, a contentious issue. Daimler-Benz and Toyota have demonstrated a commitment to using methanol and converting it to hydrogen by reaction with water (steam reforming). Other car companies would prefer to use existing infrastructure fuels (gasoline or diesel). With this aim, Epyx (Arthur D. Little) has recently demonstrated a compact fuel-processing system which produces hydrogen by reacting gasoline with a controlled amount of air (partial oxidation). In fact, most of the fuel-processing technologies being looked at or developed for mobile use are based either

Figure 14. HotSpot reactorsdesign improvements.

on partial oxidation or steam reforming. Partial oxidation is a fast process, resulting in small reactor size, fast start-up, and rapid response to changes in load. Being exothermic, however, it can lead to low-vehicle efficiency if the heat generated is wasted. By contrast, steam reforming is potentially more efficient, producing hydrogen from both the fuel and the water feed. Being endothermic, “waste” energy from other parts of the system can be usefully recycled. Unfortunately, steam reforming is a slow process, requiring a large reactor and long response times. Thus, to achieve compactness and a fast response requires compromise. Usually, some methanol conversion, and hence efficiency, is sacrificed in an onboard steam reformer. Novel catalyst and reactor technology, such as the Johnson Matthey HotSpot reactor, provides a useful alternative as it combines the best features of both processing methods.14 It can be started up cold by partial oxidation, thus generating enough heat to drive the endothermic steam reforming process. Since both processes occur on the same HotSpot catalyst particles, heat transfer occurs over microscopic distances, avoiding the need for complex heat-exchange engineering. When surplus energy becomes available from other parts of the system (for example hydrogen rejected from the fuel cell), it can be diverted to the HotSpot reactor(s) to increase the amount of steam reforming. Research and development efforts over the last 10 years have resulted in an exponential increase in the specific hydrogen production rate of this reactor (Figure 14). (14) Jenkins, J. W.; Schutt, E. Platinum Met. Rev. 1989, 33 (3), 118.


Figure 15. hotspotscharacteristics and scale-up.

An individual HotSpot reactor, or module, has a volume of 245 cm3 (slightly smaller than a Coke can) and produces 750 L/h of hydrogen from methanol fuel. This is equivalent to a power density of 3 kW/L for each reactor, assuming that 1000 L of hydrogen per hour will produce 1 kWe of PEM fuel-cell power. The reactors


can be combined in parallel to produce a range of modular fuel processors with different maximum outputs but identical response times (Figure 15). The relatively low concentration of carbon monoxide produced (1-3%) is removed catalytically by a small, multistage, selective oxidation reactor (the Johnson Matthey Demonox CO Cleanup system, reduces CO to less than 10 ppm). As with the recent progress in PEM fuel-cell technology, it can be seen that advances in catalyst technology have played a central role in the development of successful onboard fuel processors. This is particularly critical given that technically feasible and commercially viable onboard fuel processing is an enabling technology for fuel cells in cars, the biggest potential market for fuel-cell technology. EF980110F

Impact of Catalysis on Clean Energy in Road Transportation

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