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A fuel cell is an electrochemical energy conversion device. It produces electricity from various external quantities of fuel (on the anode side) and oxidant (on the cathode side). These react in the presence of an electrolyte. Generally, the reactants flow in and reaction products flow out while the electrolyte remains in the cell. Fuel cells can operate virtually continuously as long as the necessary flows are maintained.
Fuel cells are different from batteries in that they consume reactant, which must be replenished, while batteries store electrical energy chemically in a closed system. Additionally, while the electrodes within a battery react and change as a battery is charged or discharged, a fuel cell's electrodes are catalytic and relatively stable.
Many combinations of fuel and oxidant are possible. A hydrogen cell uses hydrogen as fuel and oxygen as oxidant. Other fuels include hydrocarbons and alcohols. Other oxidants include air, chlorine and chlorine dioxide.
Fuel cell design
In essence, a fuel cell works by catalysis, separating the component electrons and protons of the reactant fuel, and forcing the electrons to travel through a circuit, hence converting them to electrical power. The catalyst is typically comprised of a platinum group metal or alloy. Another catalytic process takes the electrons back in, combining them with the protons and the oxidant to form waste products (typically simple compounds like water and carbon dioxide).
In the archetypal hydrogen–oxygen proton exchange membrane fuel cell (PEMFC) design, a proton-conducting polymer membrane, (the electrolyte), separates the anode and cathode sides. This was called a "solid polymer electrolyte fuel cell" (SPEFC) in the early 1970s, before the proton exchange mechanism was well-understood. (Notice that "polymer electrolyte membrane" and "proton exchange membrane" result in the same acronym.)
On the anode side, hydrogen diffuses to the anode catalyst where it later dissociates into protons and electrons. The protons are conducted through the membrane to the cathode, but the electrons are forced to travel in an external circuit (supplying power) because the membrane is electrically insulating. On the cathode catalyst, oxygen molecules react with the electrons (which have traveled through the external circuit) and protons to form water — in this example, the only waste product, either liquid or vapor.
In addition to this pure hydrogen type, there are hydrocarbon fuels for fuel cells, including diesel, methanol (see: direct-methanol fuel cells) and chemical hydrides. The waste products with these types of fuel are carbon dioxide and water.
The materials used in fuel cells differ by type. The electrode–bipolar plates are usually made of metal, nickel or carbon nanotubes, and are coated with a catalyst (like platinum, nano iron powders or palladium) for higher efficiency. Carbon paper separates them from the electrolyte. The electrolyte could be ceramic or a membrane.
A typical PEM fuel cell produces a voltage from 0.6 V to 0.7 V at full rated load. Voltage decreases as current increases, due to several factors:
To deliver the desired amount of energy, the fuel cells can be combined in series and parallel circuits, where series yield higher voltage, and parallel allows a stronger current to be drawn. Such a design is called a fuel cell stack. Further, the cell surface area can be increased, to allow stronger current from each cell.
Fuel cell design issues
The principle of the fuel cell was discovered by German scientist Christian Friedrich Schönbein in 1838 and published in the January 1839 edition of the "Philosophical Magazine". Based on this work, the first fuel cell was developed by Welsh scientist Sir William Robert Grove in 1843. The fuel cell he made used similar materials to today's phosphoric-acid fuel cell. In 1955, W. Thomas Grubb, a chemist working for the General Electric Company (GE), further modified the original fuel cell design by using a sulphonated polystyrene ion-exchange membrane as the electrolyte. Three years later another GE chemist, Leonard Niedrach, devised a way of depositing platinum onto the membrane, which served as catalyst for the necessary hydrogen oxidation and oxygen reduction reactions. This became known as the 'Grubb-Niedrach fuel cell'. GE went on to develop this technology with NASA, leading to it being used on the Gemini space project. This was the first commercial use of a fuel cell. It wasn't until 1959 that British engineer Francis Thomas Bacon successfully developed a 5 kW stationary fuel cell. In 1959, a team led by Harry Ihrig built a 15 kW fuel cell tractor for Allis-Chalmers which was demonstrated across the US at state fairs. This system used potassium hydroxide as the electrolyte and compressed hydrogen and oxygen as the reactants. Later in 1959, Bacon and his colleagues demonstrated a practical five-kilowatt unit capable of powering a welding machine. In the 1960s, Pratt and Whitney licensed Bacon's U.S. patents for use in the U.S. space program to supply electricity and drinking water (hydrogen and oxygen being readily available from the spacecraft tanks).
UTC's Power subsidiary was the first company to manufacture and commercialize a large, stationary fuel cell system for use as a co-generation power plant in hospitals, universities and large office buildings. UTC Power continues to market this fuel cell as the PureCell 200, a 200 kW system. UTC Power continues to be the sole supplier of fuel cells to NASA for use in space vehicles, having supplied the Apollo missions and currently the Space Shuttle program, and is developing fuel cells for automobiles, buses, and cell phone towers; the company has demonstrated the first fuel cell capable of starting under freezing conditions with its proton exchange membrane automotive fuel cell.
Types of fuel cells
Fuel cell efficiency
The efficiency of a fuel cell is dependent on the amount of power drawn from it. Drawing more power means drawing more current, which increases the losses in the fuel cell. As a general rule, the more power (current) drawn, the lower the efficiency. Most losses manifest themselves as a voltage drop in the cell, so the efficiency of a cell is almost proportional to its voltage. For this reason, it is common to show graphs of voltage versus current (so-called polarization curves) for fuel cells. A typical cell running at 0.7 V has an efficiency of about 50%, meaning that 50% of the energy content of the hydrogen is converted into electrical energy; the remaining 50% will be converted into heat. (Depending on the fuel cell system design, some fuel might leave the system unreacted, constituting an additional loss.)
For a hydrogen cell operating at standard conditions with no reactant leaks, the efficiency is equal to the cell voltage divided by 1.48 V, based on the enthalpy, or heating value, of the reaction. For the same cell, the second law efficiency is equal to cell voltage divided by 1.23 V. (This voltage varies with fuel used, and quality and temperature of the cell.) The difference between these number represents the difference between the reaction's enthalpy and Gibbs free energy. This difference always appears as heat, along with any losses in electrical conversion efficiency.
Fuel cells are not constrained by the maximum Carnot cycle efficiency as combustion engines are, because they do not operate with a thermal cycle. At times this is misrepresented by saying that fuel cells are exempt from the laws of thermodynamics, because most people think of thermodynamics in terms of combustion processes (enthalpy of formation). The Laws of Thermodynamics hold for chemical processes (Gibb's free energy), like fuel cells, also but the maximum theoretical efficiency is higher (83% efficient at 298K ) than the Otto cycle thermal efficiency (60% for compression ration of 10 and specific heat ratio of 1.4). Of course, comparing limits imposed by thermodynamics is not a good predictor of practically achievable efficiencies. Also if propulsion is the goal, electrical output of the fuel cell has to still be converted into mechanical power with the corresponding inefficiency. In reference to the exemption claim, the correct claim is that the "limitations imposed by the second law of thermodynamics on the operation of fuel cells are much less severe than the limitations imposed on conventional energy conversion systems". Consequently, they can have very high efficiencies in converting chemical energy to electrical energy, especially when they are operated at low power density, and using pure hydrogen and oxygen as reactants.
For a fuel cell operated on air (rather than bottled oxygen), losses due to the air supply system must also be taken into account. This refers to the pressurization of the air and adding moisture to it. This reduces the efficiency significantly and brings it near to the efficiency of a compression ignition engine. Furthermore fuel cells have lower efficiencies at higher loads.
The tank-to-wheel efficiency of a fuel cell vehicle is about 45% at low loads and shows average values of about 36% when a driving cycle like the NEDC (New European Driving Cycle) is used as test procedure. The comparable NEDC value for a Diesel vehicle is 22%.
It is also important to take losses due to production, transportation, and storage into account. Fuel cell vehicles running on compressed hydrogen may have a power-plant-to-wheel efficiency of 22% if the hydrogen is stored as high-pressure gas, and 17% if it is stored as liquid hydrogen.
Fuel cells cannot store energy like a battery, but in some applications, such as stand-alone power plants based on discontinuous sources such as solar or wind power, they are combined with electrolyzers and storage systems to form an energy storage system. The overall efficiency (electricity to hydrogen and back to electricity) of such plants (known as round-trip efficiency) is between 30 and 50%, depending on conditions. While a much cheaper lead-acid battery might return about 90%, the electrolyzer/fuel cell system can store indefinite quantities of hydrogen, and is therefore better suited for long-term storage.
Solid-oxide fuel cells produce exothermic heat from the recombination of the oxygen and hydrogen. The ceramic can run as hot as 800 degrees Celsius. This heat can be captured and used to heat water in a combined heat and power (CHP) application. When the heat is captured, total efficiency can reach 80-90%. CHP units are being developed today for the European home market.
Fuel cell applications
Fuel cells are very useful as power sources in remote locations, such as spacecraft, remote weather stations, large parks, rural locations, and in certain military applications. A fuel cell system running on hydrogen can be compact, lightweight and has no major moving parts. Because fuel cells have no moving parts, and do not involve combustion, in ideal conditions they can achieve up to 99.9999% reliability. This equates to less than one minute of down time in a six year period.
A new application is micro combined heat and power, which is cogeneration for family homes, office buildings and factories. This type of system generates constant electric power (selling excess power back to the grid when it is not consumed), and at the same time produces hot air and water from the waste heat. A lower fuel-to-electricity conversion efficiency is tolerated (typically 15-20%), because most of the energy not converted into electricity is utilized as heat. Some heat is lost with the exhaust gas just as in a normal furnace, so the combined heat and power efficiency is still lower than 100%, typically around 80%. In terms of exergy however, the process is inefficient, and one could do better by maximizing the electricity generated and then using the electricity to drive a heat pump. Phosphoric-acid fuel cells (PAFC) comprise the largest segment of existing CHP products worldwide and can provide combined efficiencies close to 90% (35-50% electric + remainder as thermal). UTC Power is currently the world's largest manufacturer of PAFC fuel cells. Molten-carbonate fuel cells have also been installed in these applications, and solid-oxide fuel cell prototypes exist.
However, since electrolyzer systems do not store fuel in themselves, but rather rely on external storage units, they can be successfully applied in large-scale energy storage, rural areas being one example. In this application, batteries would have to be largely oversized to meet the storage demand, but fuel cells only need a larger storage unit (typically cheaper than an electrochemical device).
One such pilot program is operating on Stuart Island in Washington State. There the Stuart Island Energy Initiative has built a complete, closed-loop system: Solar panels power an electrolyzer which makes hydrogen. The hydrogen is stored in a 500 gallon tank at 200 PSI, and runs a ReliOn fuel cell to provide full electric back-up to the off-the-grid residence. The SIEI website gives extensive technical details.
The world's first Fuel Cell operated and certified passenger ship was the "HYDRA" (see picture). Mr. Christian Machens was the founder of the company "etaing GmbH" and realised this project with a small team of young engineers in Leipzig. It was christened in June 2000 in Bonn. The Fuel Cell System (AFC type, 6,5 kWel net output) was built in Wurzen near Leipzig, the hull was built in Hamburg and it was certified by the Germanischer Lloyd (Hamburg). The boat has transported around 2.000 persons without any major technical problems. The main advantages of the AFC technology are that the system can start at freezing temperatures (-10°C) and is not sensitive to a salty environment.
Hydrogen transportation and refueling
The first public hydrogen refueling station was opened in Reykjavík, Iceland in April 2003. This station serves three buses built by DaimlerChrysler that are in service in the public transport net of Reykjavík. The station produces the hydrogen it needs by itself, with an electrolyzing unit (produced by Norsk Hydro), and does not need refilling: all that enters is electricity and water. Royal Dutch Shell is also a partner in the project. The station has no roof, in order to allow any leaked hydrogen to escape to the atmosphere.
The GM 1966 Electrovan was the automotive industry's first attempt at an automobile powered by a hydrogen fuel cell. The Electrovan, which weighed more than twice as much as a normal van, could travel up to 70 miles an hour.
The 2001 Chrysler Natrium used its own on-board hydrogen processor. It produces hydrogen for the fuel cell by reacting sodium borohydride fuel with Borax, both of which Chrysler claimed was naturally occurring in great quantity in the United States. The hydrogen produces electric power in the fuel cell for near-silent operation and a range of 300 miles without impinging on passenger space. Chrysler also developed vehicles which separated hydrogen from gasoline in the vehicle, the purpose being to reduce emissions without relying on a nonexistent hydrogen infrastructure and to avoid large storage tanks.
In 2005 the British firm Intelligent Energy produced the first ever working hydrogen run motorcycle called the ENV (Emission Neutral Vehicle). The motorcycle holds enough fuel to run for four hours, and to travel 100 miles in an urban area. Its top speed is 50 miles per hour. Honda is also going to offer fuel-cell motorcycles.
There are numerous prototype or production cars and buses based on fuel cell technology being researched or manufactured. Research is ongoing at a variety of motor car manufacturers. Honda has announced the release of a hydrogen vehicle in 2008.
Currently, a team of college students called Energy-Quest is planning to take a hydrogen fuel cell powered boat around the world (as well as other projects using efficient or renewable fuels). Their venture is called the Triton.
Type 212 submarines use fuel cells to remain submerged for weeks without the need to surface.
Boeing researchers and industry partners throughout Europe are planning to conduct experimental flight tests in 2007 of a manned airplane powered only by a fuel cell and lightweight batteries. The Fuel Cell Demonstrator Airplane research project was completed recently and thorough systems integration testing is now under way in preparation for upcoming ground and flight testing. The Boeing demonstrator uses a Proton Exchange Membrane (PEM) fuel cell/lithium-ion battery hybrid system to power an electric motor, which is coupled to a conventional propeller.
Not all geographic markets are ready for SOFC powered m-CHP appliances. Currently, the regions that lead the race in Distributed Generation and deployment of fuel cell m-CHP units are the EU and Japan.
Electrochemical extraction of energy from hydrogen via fuel cells is an especially clean method of meeting our power needs, but not an efficient one, due to the necessity of adding large amounts of energy to either water or hydrocarbon fuels in order to produce the hydrogen. It must however be noted that regarding the concept of the hydrogen vehicle, burning/combustion of hydrogen in an internal combustion engine (IC/ICE) is often confused with the electrochemical process of generating electricity via fuel cells (FC) in which there is no combustion (though there is a small byproduct of heat in the reaction). Both processes require the establishment of a hydrogen economy before they may be considered commercially viable, and even then, the aforementioned energy costs make a hydrogen economy of questionable environmental value. Hydrogen combustion is similar to petroleum combustion, and like petroleum combustion, still results in nitrogen oxides as a by-product of the combustion, which lead to smog. Hydrogen combustion, like that of petroleum, is limited by the Carnot efficiency, but is completely different from the hydrogen fuel cell's chemical conversion process of hydrogen to electricity and water without combustion. Hydrogen fuel cells emit only water during use, while producing carbon dioxide emissions during the majority of hydrogen production, which comes from natural gas. Direct methane or natural gas conversion (whether IC or FC) also generate carbon dioxide emissions, but direct hydrocarbon conversion in high-temperature fuel cells produces lower carbon dioxide emissions than either combustion of the same fuel (due to the higher efficiency of the fuel cell process compared to combustion), and also lower carbon dioxide emissions than hydrogen fuel cells, which use methane less efficiently than high-temperature fuel cells by first converting it to high-purity hydrogen by steam reforming. Although hydrogen can also be produced by electrolysis of water using renewable energy, at present less than 3% of hydrogen is produced in this way.
Hydrogen is an energy carrier, and not an energy source, because it is usually produced from other energy sources via petroleum combustion, wind power, or solar photovoltaic cells. Hydrogen may be produced from subsurface reservoirs of methane and natural gas by a combination of steam reforming with the water gas shift reaction, from coal by coal gasification, or from oil shale by oil shale gasification. Electrolysis, which requires electricity, and high-temperature electrolysis/thermochemical production, which requires high temperatures (ideal for nuclear reactors), are two primary methods for the extraction of hydrogen from water.
As of 2005, 49.7% of the electricity produced in the United States comes from coal, 19.3% comes from nuclear, 18.7% comes from natural gas, 6.5% from hydroelectricity, 3% from petroleum and the remaining 2.8% mostly coming from geothermal, solar and biomass.  When hydrogen is produced through electrolysis, the energy comes from these sources. Though the fuel cell itself will only emit heat and water as waste, pollution is often produced to make the hydrogen that it runs on; unless it is either mined, or generated by solar, wind or other clean power sources. If fusion power were to become a viable energy source then this would provide a clean method of producing abundant electricity. Hydrogen production is only as clean as the energy sources used to produce it. A holistic approach has to take into consideration the impacts of an extended hydrogen scenario. This refers to the production, the use and the disposal of infrastructure and energy converters.
Nowadays low temperature fuel cell stacks proton exchange membrane fuel cell (PEMFC), direct methanol fuel cell (DMFC) and phosphoric acid fuel cell (PAFC) make extensive use of catalysts. Impurities poison or foul the catalysts (reducing activity and efficiency), thus higher catalyst densities are required. Limited reserves of platinum quicken the synthesis of an inorganic complex very similar to the catalytic iron-sulfur core of bacterial hydrogenase to step in. Although platinum is seen by some as one of the major "showstoppers" to mass market fuel cell commercialization companies, most predictions of platinum running out and/or platinum prices soaring do not take into account effects of thrifting (reduction in catalyst loading) and recycling. Recent research at Brookhaven National Laboratory could lead to the replacement of platinum by a gold-palladium coating which may be less susceptible to poisoning and thereby improve fuel cell lifetime considerably. Current targets for a transport PEM fuel cells are 0.2 g/kW Pt – which is a factor of 5 decrease over current loadings – and recent comments from major original equipment manufacturers (OEMs) indicate that this is possible. Also it is fully anticipated that recycling of fuel cells components, including platinum, will kick-in. High-temperature fuel cells, including molten carbonate fuel cells (MCFC's) and solid oxide fuel cells (SOFC's), do not use platinum as catalysts, but instead use cheapter materials such as nickel and nickel oxide, which are considerably more abundant (for example, nickel is used in fairly large quantities in common stainless steel).
Research and development
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Fuel_cell". A list of authors is available in Wikipedia.|