ScienceWeek

ENERGY TECHNOLOGY: HYDROGEN AND FUEL CELLS

The following points are made by G.W. Crabtree et al (Physics Today 2004 December):

1) A major attraction of hydrogen as a fuel is its natural compatibility with fuel cells. The higher efficiency of fuel cells -- currently 60% compared to 22% for gasoline or 45% for diesel internal combustion engines -- would dramatically improve the efficiency of future energy use. Coupling fuel cells to electric motors, which are more than 90% efficient, converts the chemical energy of hydrogen to mechanical work without heat as an intermediary. This attractive new approach for energy conversion could replace many traditional heat engines. The broad reach of that efficiency advantage is a strong driver for deploying hydrogen fuel cells widely.

2) Although fuel cells are more efficient, there are also good reasons for burning hydrogen in heat engines for transportation. Jet engines and internal combustion engines can be rather easily modified to run on hydrogen instead of hydrocarbons. Internal combustion engines run as much as 25% more efficiently on hydrogen compared to gasoline and produce no carbon emissions. The US and Russia have test-flown commercial airliners with jet engines modified to burn hydrogen. Similarly, BMW, Ford, and Mazda are road-testing cars powered by hydrogen internal combustion engines that achieve a range of 300 kilometers, and networks of hydrogen filling stations are being implemented in some areas of the US, Europe, and Japan. Such cars and filling stations could provide an early start and a transitional bridge to hydrogen fuel-cell transportation.

3) The versatility of fuel cells makes them workable in nearly any application where electricity is useful. Stationary plants providing 200 kilowatts of neighborhood electrical power are practical and operating efficiently. Such plants can connect to the electrical grid to share power but are independent of the grid in case of failure. Fuel-cell power for consumer electronics like laptop computers, cell phones, digital cameras, and audio players provide more hours of operation than batteries at the same volume and weight. Although the cost per kilowatt is high for these small units, the unit cost can soon be within an acceptable consumer range. Electronics applications may be the first to widely reach the consumer market, establish public visibility, and advance the learning curve for hydrogen technology.

4) The large homogeneous transportation market offers enormous potential for hydrogen fuel cells to dramatically reduce fossil fuel use, lower harmful emissions, and improve energy efficiency. Fuel cells can be used not only in cars, trucks, and buses, but also can replace the diesel electric generators in locomotives and power all-electric ships. Europe already has a demonstration fleet of 30 fuel-cell buses running regular routes in 10 cities, and Japan is poised to offer fuel-cell cars for sale.

5) However, a host of fundamental performance problems remain to be solved before hydrogen in fuel cells can compete with gasoline. The heart of the fuel cell is the ionic conducting membrane that transmits protons or oxygen ions between electrodes while electrons go through an external load to do their electrical work. Each of the half reactions at work in the circuit requires catalysts interacting with electrons, ions, and gases traveling in different media. Designing nanoscale architectures for these triple percolation networks that effectively coordinate the interaction of reactants with nanostructured catalysts is a major opportunity for improving fuel-cell performance. The trick is to get intimate contact of the three phases that coexist in the cell -- the incoming hydrogen or incoming oxygen gas phase, an electrolytic proton-conducting phase, and a metallic phase in which electrons flow into or from the external circuit.[1-5]

References (abridged):

1. M. I. Hoffert et al., Nature 395, 891 (1998); Energy Information Administration, International Energy Outlook 2004, rep. no. DOE/EIA-0484 (2004)

2. J. A. Turner, Science 285, 687 (1999); Special Report: Toward a Hydrogen Economy, Science 305, 957 (2004)

3. US Department of Energy, Office of Basic Energy Sciences, Basic Research Needs for the Hydrogen Economy, US DOE, Washington, DC (2004); Basic Energy Sciences Advisory Committee, Basic Research Needs to Assure a Secure Energy Future, US DOE, Washington, DC (2003); Committee on Alternatives and Strategies for Future Hydrogen Production and Use, The Hydrogen Economy: Opportunities, Costs, Barriers, and R&D Needs, National Research Council, National Academies Press, Washington, DC (2004)

4. O. Khasalev, J. A. Turner, Science 280, 425 (1998); S. U. M. Khan, M. Al-Shahry, W. B. Ingler, Science 297, 2189 (2002); N. S. Lewis, Nature 414, 589 (2001)

5. C. Perkins, A. W. Weimar, Int. J. Hydrogen Energy (in press).

Physics Today http://www.physicstoday.org

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Related Material:

MATERIALS SCIENCE: ON SOLID ACID FUEL CELLS

The following points are made by D.A. Boysen et al (Science 2004 303:68):

1) Solid acid compounds comprise hydrogen-bonded oxyanions, often tetrahedral (such as SO4, SeO4, PO4, and AsO4), and metal cations, which provide overall charge balance to the hydrogen bond network. Several solid acids exhibit an ordered arrangement of hydrogen bonds at room temperature and, upon slight heating, become structurally disordered. A typical example is CsHSO4, which transforms from a monoclinic to a tetragonal structure at 141 deg C (1). Accompanying this transformation is an increase in proton conductivity of two to three orders of magnitude, reaching values as high as 10^(-2)/ohm.cm. Both the transition and the ion transport are commonly referred to as "superprotonic".

2) Fuel cell operation at the slightly elevated temperatures enabled by superprotonic solid acids relaxes the purity requirements on the hydrogen fuel in a hydrogen/air fuel cell, increases the activity of the catalysts, provides useful waste heat for cogeneration configurations, and, particularly important for automotive applications, reduces the size of the radiator. Moreover, unlike polymers, solid acids exhibit anhydrous proton transport, and thus, careful water management to remove water from the cathode and replenish it at the anode is not necessary. For direct methanol fuel cell (DMFC) applications, the solid, water-free nature of the electrolyte raises the possibility of fuel-impermeable membranes.

3) The implementation of known superprotonic solid acids in fuel cells has been hindered by their water solubility and poor mechanical behavior. Despite these properties, successful operation of CsHSO4-based fuel cells has been demonstrated (2). The key to this success was operating the fuel cells above 100 deg C; at these temperatures, H2O is present in the form of steam and is thus harmless to the otherwise water-soluble electrolyte. A greater obstacle, recently recognized, arises from the catalyzed reduction of sulfate and selenate solid acids under hydrogen atmospheres (3).

4) In summary: Although they hold the promise of clean energy, state-of-the-art fuel cells based on polymer electrolyte membrane fuel cells are inoperable above 100 deg C, require cumbersome humidification systems, and suffer from fuel permeation. These difficulties all arise from the hydrated nature of the electrolyte. In contrast, "solid acids" exhibit anhydrous proton transport and high-temperature stability. The authors demonstrate continuous stable power generation for both H2/O2 and direct methanol fuel cells operated at 250 deg C using a humidity-stabilized solid acid CsH2PO4 electrolyte.(4,5)

References (abridged):

1. A. I. Baranov, L. A. Shuvalov, N. M. Shchagina, JETP Lett. 36, 459 (1982)

2. S. M. Haile, D. A. Boysen, C. R. I. Chisholm, R. B. Merle, Nature 410, 910 (2001)

3. R. B. Merle, C. R. I. Chisholm, D. A. Boysen, S. M. Haile, Energy Fuels 17, 210 (2003)

4. B. M. Nirsha et al., Russ. J. Inorg. Chem. 27, 770 (1980).

5. D. A. Boysen, S. M. Haile, H. Liu, R. A. Secco, Chem. Mater. 15, 727 (2003)

Science http://www.sciencemag.org

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HISTORY OF APPLIED PHYSICS: FUEL CELLS

In general, the term "fuel cell" refers to a system (cell) in which the chemical energy of a fuel is converted directly into electrical energy. The simplest fuel cell is one in which hydrogen is oxidized to form water over porous sintered nickel electrodes, the nickel acting as a catalyst, and the process producing an electric current via a flow of ions between an anode and a cathode.

The following points are made by Henry Petroski (American Scientist 2003 91:398):

1) Many proposed applications of the fuel cell may seem futuristic, but the device itself dates from 1839, when the Welsh-born British jurist and physicist William Robert Grove (1811-1896) devised a "gas battery". Unlike the now familiar dry cell of Alessandro Volta (1745-1827) -- whose casing contained all its energy-producing ingredients and which produced electricity only as long as it could sustain the chemical reaction -- Grove's gas battery produced electricity as long as it was fueled by an external source. Volta's simple voltaic pile preceded the fuel cell by 39 years, however, so the more complicated device was shelved for a century or so, during which time great strides were made in motive power.

2) Within a couple of decades of the invention of the dry-cell battery, Michael Faraday (1791-1867) demonstrated the principle of the electric motor and soon thereafter that of electromagnetic induction, which led to the electric generator. By the early 1830s working electric motors were being made, and well before the end of the decade electric-driven road vehicles and paddle boats were the subjects of experiments. By 1859, an early version of the lead-acid battery used in today's automobiles had been developed -- with the most important capability of being repeatedly discharged and recharged. As early as 1873, storage batteries were powering electric motors and driving vehicles, and by 1882 these electric "cars" could reach speeds of almost 10 miles per hour and travel distances as great as 25 miles. The first demonstration of a vehicle powered by an internal combustion engine was still a couple of years away.

3) At the end of the 19th century, an electric vehicle held the world speed record of 61 miles per hour, and in the US in 1900 almost as many electric-driven cars (1575) were being manufactured as steam-driven ones (1684). Combined, they outnumbered by more than three to one gasoline-engined cars. The electric vehicle had the clear advantage of quietness over its then-unmuffled competitors, and it did not need to be hand-cranked to be started. However, after the introduction of the Ford Model K in 1906 and the Model T in 1909, the internal-combustion engine became the power source of choice. By 1912, there were 900,000 gasoline-powered vehicles in America, outnumbering the 30,000 electrics thirty-to-one. At about the same time, the self-starter and silencer were introduced, thus making the internal combustion engine more user friendly and desirable. It was much faster to add gasoline to a tank than to recharge heavy batteries, and the gasoline provided greater range, even if extra canisters of fuel had to be carried.

4) Different energy sources are now usually compared by means of a measure known as energy density, which is the ratio of power to weight. Today, a conventional lead-acid battery has an energy density of about 35 watt-hours per kilogram compared with gasoline's 2000. Although more exotic types of batteries have higher energy densities than do lead-acid cells, they still rank an order of magnitude less than gasoline and are expensive to manufacture.

5) The use of gasoline-powered vehicles in World War I conditioned many young veterans to favor the internal-combustion engine. The last new model of an electric car to be built in America during that era was introduced in 1921 -- at a price four times that of a Model T. The electric vehicle essentially went into forced hibernation for decades, until environmental and energy crises reawakened interest in a nonpolluting alternative to the internal combustion engine.

American Scientist http://www.americanscientist.org

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