PEM Fuel Cells for Transportation

For transportation, PEM fuel cells are at present the most promising option to replace the current internal combustion engine. They operate like a refuelable battery, generating electricity for as long as they are supplied with hydrogen fuel and oxygen (from air). At the core of these fuel cells, a thin polymer film constitutes the electrolyte. Until now, this PEM has been based predominantly on a fluorocar-bon polymer produced by DuPont and known under the commercial name of Nafion®; this polymer is permeable to protons when saturated with water, but does not conduct electrons. It is sandwiched between two platinum impregnated porous carbon (graphite) electrodes. PEM fuel cells can, in theory, convert more than 50% of the fuel into usable energy. They have a high power density, which translates into low weight and volume requirements. In addition, they operate at low temperature (generally 80 °C), which allows for fast start-ups, and they can very rapidly change their power output as a function ofdemand. Furthermore, they are safe, quiet, easy to operate, and of low maintenance. For all these reasons, PEM fuel cells are seen as the most suitable candidate for automotive applications, but they are also considered and being developed for small power applications. However, before PEM fuel cell-powered cars leave the assembly lines in the millions, numerous problems will have to be solved. The price of a prototype PEM fuel cell is today in the order of hundreds if not thousands of dollars per kW. A fuel cell with about 65-70 kW output (equivalent to ca. 90 horse power) is needed to power a small car. Thus, dramatic cost reductions are necessary to reach a price of less than $50 kW-1, which would begin to make the PEM fuel cell competitive with ICEs.

The presently used perfluorinated membranes are way too expensive, representing about one-third of the cost of a fuel cell stack. New and inexpensive efficient materials, such as hydrocarbon membranes, will have to be developed to replace them. At the same time, they should be chemically and mechanically stable, have a great durability and a high tolerance to fuel impurities or reaction byproducts, such as carbon monoxide (CO).

Another key element for proper PEM fuel cell operation is the thin layer of platinum catalyst coated with electrically conductive graphite on both sides of the membrane. The use of platinum is necessary because of the low temperature of operation. Platinum however, is an expensive precious metal and extensive research is being conducted in order to reduce its content. These efforts include ways of increasing the catalysts activity (e.g., by using nano-sized dispersed metals) so that less can be used for a same power output, or to seek alternative and cheaper catalysts. At a low operating temperature, platinum has also the disadvantage of binding strongly with CO, a common impurity in hydrogen obtained by reforming of fossil fuels. This reduces the platinum's availability for hydrogen chemisorption and electro-oxidation. At 80 hC, the platinum catalyst can tolerate only a few ppm of CO in hydrogen before its activity begins to subside. Thus, the hydrogen fuel used must be of very high purity, requiring additional purification steps such as oxidation of CO over gold or other catalysts - all of which will add to the cost of hydrogen obtained from hydrocarbons. In order to increase the catalysts tolerance to CO, new bi-catalysts such as those based on platinum/ ruthenium are currently being developed for use in fuel cells. PEM fuel cells that can potentially operate at temperatures 120 hC or higher will also alleviate the CO poisoning issue. However, high-temperature membranes with adequate water content and proton conductivity need to be developed to achieve this lofty goal.

Today, numerous PEM fuel cell-powered prototype vehicles are being tested worldwide by the major automobile companies, including Ford, General Motors, Honda, Toyota, Renault, and Volkswagen. DaimlerChrysler for example, has been working intensively on fuel cell technology since the early 1990s, and is now preparing a group of 60 of its latest hydrogen fuel cell car, the F-Cell, to be tested for their performance under everyday driving conditions. In addition, the company has also developed both a light duty vehicle and a bus which run on hydrogen. Small fleets of buses are presently driven on a daily basis for public transportation in Amsterdam, Luxembourg, and Reykjavik. Fleets of transit buses are ideal for the introduction of alternative fuels because they travel only short distances and generally refuel in a central depot. Furthermore, the installation of volumi nous fuel tanks, which can be easily accommodated in these larger vehicles, is less of a problem than in a passenger car. In fact, transit buses is one area where alternative fuels have had the most success. Diesel buses in many cities have been progressively replaced by much less-polluting compressed natural gas (CNG) buses. To make fuel cell-powered vehicles affordable, however, major technological breakthroughs are clearly needed in order to make a significantly lowering of production costs and also to mitigate safety issues. If hydrogen fuel has to be used to fuel these cars, the onboard hydrogen storage capacity must also to be substantially improved, and a massive and very expensive infrastructure for delivering needed hydrogen to the users must be built from scratch. Therefore, we need to look more carefully at other options. ICEs, for instance, have been continuously improved for more than a century, and are becoming increasingly efficient and ever less pollutant-emitting. Their combination with batteries and electric motors in hybrid vehicles increases their efficiencies even further. In order to compare various vehicles with different fuels, engines, electric motors, and drive trains, a well-to-wheel (WTW) analysis, representing the overall efficiency of an energy source from the time it is extracted from Earth or any other resource, to when it actually turns the wheels of the vehicle, is generally conducted. In the case of a gasoline vehicle operating with a regular ICE, the WTW gives an overall efficiency of only 14%. A gasoline ICE hybrid vehicle, however, can achieve a WTW efficiency of 28%, comparable to the present day 29% obtained with a fuel cell vehicle using compressed hydrogen generated from natural gas. The efficiency of a diesel hybrid ICE vehicle was found to be even higher than that for fuel cell motor car [93]. Fuel cells may be more efficient in converting hydrogen into electricity than through mechanical conversion, but the energy required to produce, handle and store hydrogen lowers the overall efficiency considerably. Considering greenhouse gas emissions, WTW analysis shows that hydrogen fuel cell vehicles offer only a slight advantage over hydrocarbon-based hybrid vehicles if hydrogen used in the fuel cell is derived from fossil sources.

The ICE is a proven and reliable technology, and its combination with a battery/ electric motor has already been applied today in hybrid vehicles (Toyota Prius, Honda Insight, etc.). These are slightly more expensive than regular cars, but they allow a considerable reduction in fuel consumption and lower emissions. Fuel cell vehicles, on the other hand, are still in the developmental phase, with developers striving to bring down manufacturing costs and improve the lifetime and reliability of the fuel cell stacks. To reduce the consumption of petroleum and reduce CO2 emissions, deploying hybrid cars on a large scale seems to be a better and more reasonable solution in the short term, than to rely on the still uncertain future advances in fuel cell technology, which however eventually might become economically viable for the transportation sector in the long term.

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