Hydrogen Fuel of the Future

Hydrogen gas can be used as a fuel in the same way as carbon-containing compounds; some futurists believe that the world will eventually have a hydrogen-based economy. Hydrogen gas combines with oxygen gas to produce water, and in the process it releases a substantial quantity of energy:

The idea that hydrogen would be the ultimate fuel of the future goes back at least as far as 1874, when it was mentioned by a character in the novel Mysterious Island by Jules Verne. Indeed, hydrogen has already found use as fuel in applications for which lightness is an important factor, namely, in powering the Saturn rockets to the moon and the U.S. space shuttles.

Hydrogen is superior even to electricity in some ways, since its transmission by pipelines over long distances consumes less energy than the transmission through wires of the same amount of energy as electricity and since batteries are not required for local storage of energy.

However, as we shall see in rhe material that follows, the substantial technical problems in the production, storage, transportation, and usage of hydrogen—the need to create a new infrastructure for it—mean that a hydrogen economy is probably still many decades away.

Combusting Hydrogen

Hydrogen gas can be combined with oxygen to produce heat by conventional flame combustion or by low-temperature combustion in catalytic heaters. The combustion efficiency, i.e., the fraction of energy converted to useful energy rather than to waste heat, is approximately 25%, about the same as for gasoline. The main advantages to using hydrogen as a combustion fuel are its low mass per unit of energy produced and the smaller (but not zero) quantity of polluting gases its combustion produces, when compared to other fuels. BMW, Ford, and Mazda may begin marketing hydrogen-fueled combustion-engine cars by 2010.

Although it is sometimes stated that hydrogen combustion produces only water vapor and no pollutants, this in fact is not true. Of course, no carbon-containing pollutants, including carbon dioxide, are emitted. Since combustion involves a flame, however, some of the nitrogen from the air that is used as the source of oxygen reacts to form nitrogen oxides, NOx, Some hydrogen peroxide, H2Oz , is released as well. Thus hydrogen-burning vehicles are not really zero-emission systems. It is true that the lower flame temperature for the H2 + 02 combustion, compared to that for fossil fuels with oxygen, inherently produces less NOx, perhaps two-thirds less. The nitrogen oxide release can be eliminated by using pure oxygen rather than air to burn the hydrogen; alternatively, it can be reduced even further by passing the emission gases over a catalytic converter or by lowering the flame temperature as much as possible, e.g., by reducing the H2/02 ratio to half the stoichiometric amount.

Generating Electricity by Powering Fuel Cells with Hydrogen

Hydrogen and oxygen can be combined in fuel cells in order to produce electricity (a hydrogen technology also used in space vehicles). Fuel cells are similar in operation to batteries except that the reactants are supplied continuously. In the hydrogen-oxygen fuel cell, the two gases are passed over separate electrodes that are connected by an external electrical connection through which electrons travel and also by an electrolyte through which ions travel.

The components of a fuel cell, then, are the same as those of an electrolysis operation in which water would be split into hydrogen and oxygen, but the chemical reaction that occurs is exactly the opposite. Instead of electricity being used to drive the electrolysis reaction, it is produced. Fuel cells have the advantage over combustion since a more useful form of energy is produced (electricity rather than heat) and the process produces no polluting gases as by-products. In principle, the only product of the reaction is water. At the catalytic surface of the first electrode, the H2 gas produces H+ ions and electrons, which travel around the external circuit to the second electrode, across which 02 gas is bubbled (see Figure 8-13). Meanwhile, the H+ ions

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FIGURE 8-13 Schematic diagram of a hydrogen -oxygen fuel cell (PEMFC version).

travel through the electrolyte and recombine with the electrons and 02 to produce water at the electrode. Although some of the reaction energy is necessarily released as heat—about 20%, due to requirements of the second law of thermodynamics— most of it is converted to electrical energy-associated with the current that flows between the electrodes. Electric motors, whether in a fuel-cell or battery vehicle, are 80-85% efficient in converting electrical to mechanical energy. Real fuel cells overall are now about 50-55% efficient; 70% efficiency may be attained eventually. By contrast, internal combustion engines using gasoline are 15-25% efficient, diesels 30-35%.

Many auto manufacturers are currently pursuing the development of electric cars that use fuel cells. Prototype buses running in Vancouver and Chicago use innovative fuel cells for their power source. The electrolyte used in the fuel cells of these vehicles is a hair-thin (about 100 ¡¿m) synthetic polymer that acts as a proton exchange membrane. The membrane, when moist, conducts protons well since it incorporates sulfonate groups. It also keeps the hydrogen and oxygen gases from mixing. The electrodes of such polymeric'electrolyte-membrane fuel cells, labeled PEMFC in Figure 8-14, are graphite with a small amount of platinum dispersed as nanoparticles in thin layers (about 50 ¡jlm thick) on its surface. Each cell, which operates at about 80°C, generates about 0.8 V of electricity, so many must be stacked together in order to provide sufficient power for the vehicle. In the current versions of the buses, compressed hydrogen is stored in tanks under the roof of the bus.

Some incentives to develop vehicles that use fuel cells powered by hydrogen are

• to reduce urban smog, which is partially produced from emissions from gasoline and diesel engines;

• to reduce energy consumption, since fuel cells are much more efficient in producing motive power than are combustion engines; and

• to reduce carbon dioxide emissions, since fuel cells powered by hydrogen are carbon-free.

Some analysts point out that the cost of improving air quality and C02 emissions by switching the transportation system to such vehicles over the next few decades is much higher than alternative strategies, such as scrapping old cars, improving vehicle fuel efficiency, reducing NOx emissions from power plants, and capturing and sequestering C02 emissions from power plants. In addition, hydrogen leaked into the air acts indirectly as a greenhouse gas, since it reacts with and decreases the concentration of OH; this result will slightly increase the atmospheric lifetime of methane and hence its concentration, since the main sink for CH4 is its reaction with OH, as discussed in Chapters 5 and 6.

Anode

Electrolyte

Cathode

Anode

Electrolyte

Cathode

FIGURE 8-14 Operating characteristics of various fuel cells. ISource: B. C. H. Steele and A. Heinzel, "Materials for Fuel Cell Technologies/' Nature 414 (2001): 345.1

Obtaining Fuel-Cell Hydrogen from Liquid Fuels

Because of the limited practicality of _

transporting hydrogen in individual cars and trucks, there is active research in designing systems that allow it to be extracted as needed from liquid fuels, which are much more convenient to transport. For example, in the near future, the hydrogen may instead be obtained as needed from liquid methanol by onboard decomposition of the latter to hydrogen gas using the reverse of the methanol-formation reaction discussed earlier:

FIGURE 8-14 Operating characteristics of various fuel cells. ISource: B. C. H. Steele and A. Heinzel, "Materials for Fuel Cell Technologies/' Nature 414 (2001): 345.1

In the General Motors version of this process, the re-former unit operates at 275°C and uses a copper oxide/zinc oxide catalyst to promote the reaction. The water-gas shift reaction is subsequently used to react the CO in the synthesis gas with steam and provide additional H2 gas, giving the overall reaction

Similar processes have been developed that convert gasoline, diesel fuel, octane, or aqueous ethanol into carbon dioxide and hydrogen. Unfortunately, the current PEMFC and alkaline fuel cells (see Problem 8-9), as well as one based on phosphoric acid, all require relatively pure hydrogen, free especially of carbon monoxide—a gas that is formed in the re-former process and is difficult to eliminate completely. Carbon monoxide bonds to the sites of the catalyst (e.g., platinum) intended to promote the fuel-cell reaction and blocks the catalytic activity there. Concentrations of CO greater than 20 ppm in the hydrogen gas slow down most fuel cells appreciably. Perhaps a CO-tolerant electrode catalyst, possibly one incorporating a second metal or a metal oxide as well as platinum, will be developed in the future to overcome this problem by oxidizing the adsorbed CO to carbon dioxide. Hydrogen that is virtually free of carbon monoxide could be produced from methanol by an oxidative steam re-forming process at 230°C:

Since oxygen is involved, however, not all the fuel value of methanol is captured in the hydrogen product.

PROBLEM 8-9

An alkaline electrolyte can be used in the H2-02 fuel cell to replace the acidic environment mentioned previously (see Figure 8-13). Assuming that the reaction of 02 with water and electrons produces hydroxide ions, OI L , and that these ions travel to the other electrode, where they react with hydrogen to give up electrons and produce more water, deduce the two balanced half-reactions and the balanced overall reaction for such a fuel cell. (The alkaline fuel cell, labeled AFC in Figure 8-14, is used in space shuttles and Apollo spacecraft to provide electricity.)

Fuel cells may also be used in the near future in small electric power plants, partly because pollutant emissions from them are so small compared to those from fossil-fuel combustion (e.g., only about 1% of the NOx). Indeed, phosphoric acid-based fuel cells (PAFC in Figure 8-14) have been operating since the early 1990s in some hospitals and hotels to generate power. The most promising fuel cells for power plants involve a molten carbonate salt—e.g., potassium and lithium carbonates, plus additives, at 650°C—as the electrolyte. (The tnoten-carbonate fuel ceU is labeled MCFC in Figure 8-14.) The hydrogen gas reacts with carbonate ions, C03z~~, to produce carbon dioxide, water, and electrons at the anode, while the carbon dioxide reacts with electrons and oxygen from air to re-form the carbonate ions at the cathode. The carbon dioxide must be recycled from the anode back to the cathode during the cell's operation. The hydrogen is produced on-site by reaction of methane with steam, so CH4 is the actual source of the fuel energy here. By-product waste heat can be recovered and used in a cogeneration mode, and the dc electricity produced by the fuel cell is converted to ac for distribution. MCFC efficiency approaches 50%.

Like the molten-carbonate fuel cell, a cell based on solid oxides (see SOFC, Figure 8-14) is much more tolerant of carbon monoxide impurities in the fuel hydrogen gas than are the other fuel cells. The electrolyte in the solid-oxide fuel cell is a ceramic mixture of oxides of zirconium and yttrium. The oxide anion, O"^, produced from oxygen gas carries the charge between the electrodes, travels through the solid from cathode to anode as the fuel is consumed, and forms water at the anode. The high operating temperature (up to 1000°C) of the solid-oxide cell allows fuel to be re-formed internally and to form hydrogen ions without the use of expensive catalysts, so methane or other hydrocarbons can be used as the fuel instead of hydrogen. However, carbon deposits tend to form at the anode and stick to it at the high operating temperatures that are involved with these units.

The solid-oxide fuel cell, like the molten 'Csrbonste one, is practical for central power plants, and its fuel efficiency is also high. Both types suffer from practical problems with electrodes, such as carbon deposition. The latter problem for solid-oxide fuel cells can be overcome by operating at a lower temperature.

PROBLEM 8-10

For the molten-carbonate fuel cell, obtain balanced half-reactions for the processes at the two electrodes and add them together to determine the overall reaction.

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