Demonstration model of a direct-methanol fuel cell. The actual fuel cell stack is the layered cube shape in the center of the image

Scheme of a proton conducting fuel cell

A fuel cell is a device that converts the chemical energy from a fuel into electricity through a chemical reaction with oxygen or another oxidizing agent.^[1] Hydrogen is the most common fuel, but hydrocarbons such as natural gas and alcohols like methanol are sometimes used. Fuel cells are different from batteries in that they require a constant source of fuel and oxygen to run, but they can produce electricity continually for as long as these inputs are supplied.

Welsh Physicist William Grove developed the first crude fuel cells in 1839. The first commercial use of fuel cells was in NASA space programs to generate power for probes, satellites and space capsules. Since then, fuel cells have been used in many other applications. Fuel cells are used for primary and backup power for commercial, industrial and residential buildings and in remote or inaccessible areas. They are used to power fuel cell vehicles, including automobiles, buses, forklifts, airplanes, boats, motorcycles and submarines.

There are many types of fuel cells, but they all consist of an anode (negative side), a cathode (positive side) and an electrolyte that allows charges to move between the two sides of the fuel cell. Electrons are drawn from the anode to the cathode through an external circuit, producing direct current electricity. As the main difference among fuel cell types is the electrolyte, fuel cells are classified by the type of electrolyte they use. Fuel cells come in a variety of sizes. Individual fuel cells produce very small amounts of electricity, about 0.7 volts, so cells are "stacked", or placed in series or parallel circuits, to increase the voltage and current output to meet an application’s power generation requirements.^[2] In addition to electricity, fuel cells produce water, heat and, depending on the fuel source, very small amounts of nitrogen dioxide and other emissions. The energy efficiency of a fuel cell is generally between 40-60%, or up to 85% efficient if waste heat is captured for use.

History[link]

Sketch of William Grove's 1839 fuel cell

The principle of the fuel cell was discovered by German scientist Christian Friedrich Schönbein in 1838 and published in one of the scientific magazines of the time.^[3] Based on this work, the first fuel cell was demonstrated by Welsh scientist and barrister Sir William Robert Grove in the February 1839 edition of the Philosophical Magazine and Journal of Science^[4] and later sketched, in 1842, in the same journal.^[5] 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'.^{[citation needed]} GE went on to develop this technology with NASA and McDonnell Aircraft, leading to its use during Project Gemini. This was the first commercial use of a fuel cell. In 1959, 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). In 1991, the first hydrogen fuel cell automobile was developed by Roger Billings.^[6]

United Technologies Corporation's UTC 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 marketed their fuel cell, the PureCell 200, a 200 kW system, now replaced by a 400 kW version, the PureCell Model 400.^[7] UTC Power continues to be the sole supplier of fuel cells to NASA for use in space vehicles, having supplied fuel cells for the Apollo missions,^[8] and 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.

Types of fuel cells; design[link]

Fuel cells come in many varieties; however, they all work in the same general manner. They are made up of three adjacent segments: the anode, the electrolyte, and the cathode. Two chemical reactions occur at the interfaces of the three different segments. The net result of the two reactions is that fuel is consumed, water or carbon dioxide is created, and an electric current is created, which can be used to power electrical devices, normally referred to as the load.

At the anode a catalyst oxidizes the fuel, usually hydrogen, turning the fuel into a positively charged ion and a negatively charged electron. The electrolyte is a substance specifically designed so ions can pass through it, but the electrons cannot. The freed electrons travel through a wire creating the electric current. The ions travel through the electrolyte to the cathode. Once reaching the cathode, the ions are reunited with the electrons and the two react with a third chemical, usually oxygen, to create water or carbon dioxide.

The most important design features in a fuel cell are^{[citation needed]}:

• The electrolyte substance. The electrolyte substance usually defines the type of fuel cell.
• The fuel that is used. The most common fuel is hydrogen.
• The anode catalyst, which breaks down the fuel into electrons and ions. The anode catalyst is usually made up of very fine platinum powder.
• The cathode catalyst, which turns the ions into the waste chemicals like water or carbon dioxide. The cathode catalyst is often made up of nickel but it can also be a nanomaterial-based catalyst.

A typical 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:

• Activation loss
• Ohmic loss (voltage drop due to resistance of the cell components and interconnects)
• Mass transport loss (depletion of reactants at catalyst sites under high loads, causing rapid loss of voltage).^[9]

To deliver the desired amount of energy, the fuel cells can be combined in series and parallel circuits, where series yields higher voltage, and parallel allows a higher current to be supplied. Such a design is called a fuel cell stack. The cell surface area can be increased, to allow stronger current from each cell.

Proton exchange membrane fuel cells[link]

In the archetypical hydrogen–oxygen proton exchange membrane fuel cell^[10] (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 the synonyms "polymer electrolyte membrane" and "proton exchange mechanism" result in the same acronym.)

On the anode side, hydrogen diffuses to the anode catalyst where it later dissociates into protons and electrons. These protons often react with oxidants causing them to become what is commonly referred to as multi-facilitated proton membranes. 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 indirect methanol fuel cells) and chemical hydrides. The waste products with these types of fuel are carbon dioxide and water.

Construction of a high temperature PEMFC: Bipolar plate as electrode with in-milled gas channel structure, fabricated from conductive composites (enhanced with graphite, carbon black, carbon fiber, and/or carbon nanotubes for more conductivity);^[11] Porous carbon papers; reactive layer, usually on the polymer membrane applied; polymer membrane.

Condensation of water produced by a PEMFC on the air channel wall. The gold wire around the cell ensures the collection of electric current.^[12]

The different components of a PEMFC are (i) bipolar plates, (ii) electrodes, (iii) catalyst, (iv) membrane, and (v) the necessary hardwares.^[13] The materials used for different parts of the fuel cells differ by type. The bipolar plates may be made of different types of materials, such as, metal, coated metal, graphite, flexible graphite, C–C composite, carbon–polymer composites etc.^[14] The membrane electrode assembly (MEA), is referred as the heart of the PEMFC and usually made of a proton exchange membrane sandwiched between two catalyst coated carbon papers. Platinum and/or similar type of noble metals are usually used as the catalyst for PEMFC. The electrolyte could be a polymer membrane.

Proton exchange membrane fuel cell design issues[link]

• Costs. In 2009, the Department of Energy reported that 80-kW automotive fuel cell system costs in volume production (projected to 500,000 units per year) are US$61 per kilowatt.^[15] The goal is US$35 per kilowatt. Cost reduction over a ramp-up period of about 20 years is needed in order for PEM fuel cells to compete with current market technologies, including gasoline internal combustion engines.^[16] Many companies are working on techniques to reduce cost in a variety of ways including reducing the amount of platinum needed in each individual cell. Ballard Power Systems has experiments with a catalyst enhanced with carbon silk which allows a 30% reduction (1 mg/cm² to 0.7 mg/cm²) in platinum usage without reduction in performance.^[17] Monash University, Melbourne uses PEDOT as a cathode.^[18] A 2011 published study^[19] documented the first metal-free electrocatalyst using relatively inexpensive doped carbon nanotubes that are less than 1% the cost of platinum and are of equal or superior performance.
• Water and air management^[20] (in PEMFCs). In this type of fuel cell, the membrane must be hydrated, requiring water to be evaporated at precisely the same rate that it is produced. If water is evaporated too quickly, the membrane dries, resistance across it increases, and eventually it will crack, creating a gas "short circuit" where hydrogen and oxygen combine directly, generating heat that will damage the fuel cell. If the water is evaporated too slowly, the electrodes will flood, preventing the reactants from reaching the catalyst and stopping the reaction. Methods to manage water in cells are being developed like electroosmotic pumps focusing on flow control. Just as in a combustion engine, a steady ratio between the reactant and oxygen is necessary to keep the fuel cell operating efficiently.
• Temperature management. The same temperature must be maintained throughout the cell in order to prevent destruction of the cell through thermal loading. This is particularly challenging as the 2H₂ + O₂ -> 2H₂O reaction is highly exothermic, so a large quantity of heat is generated within the fuel cell.
• Durability, service life, and special requirements for some type of cells. Stationary fuel cell applications typically require more than 40,000 hours of reliable operation at a temperature of -35 °C to 40 °C (-31 °F to 104 °F), while automotive fuel cells require a 5,000 hour lifespan (the equivalent of 240,000 km (150,000 mi)) under extreme temperatures. Current service life is 7,300 hours under cycling conditions.^[21] Automotive engines must also be able to start reliably at -30 °C (-22 °F) and have a high power to volume ratio (typically 2.5 kW per liter).
• Limited carbon monoxide tolerance of some (non-PEDOT) cathodes.

High temperature fuel cells[link]

SOFC[link]

Solid oxide fuel cells use a solid material, most commonly a ceramic material called yttria-stabilized zirconia (YSZ), as the electrolyte. Because SOFCs are made entirely of solid materials, they are not limited to the flat plane configuration of other types of fuel cells and are often designed as rolled tubes. They require high operating temperatures (800°C to 1000°C) and can be run on a variety of fuels including natural gas.^[22]

SOFCs are unique in that negatively charged oxygen ions travel from the cathode (negative side of the fuel cell) to the anode (positive side of the fuel cell) instead of positively charged hydrogen ions travelling from the anode to the cathode, as is the case in all other types of fuel cells. Oxygen gas is fed through the cathode, where it reacts with electrons to create oxygen ions. The oxygen ions then travel through the electrolyte to react with hydrogen gas at the anode. The reaction at the anode produces electricity and water as by-products. Carbon dioxide may also be a by-product depending on the fuel, but the carbon emissions from an SOFC system are less than those from a fossil fuel combustion plant.^[23] The chemical reactions for the SOFC system can be expressed as follows:^[24]

Anode Reaction: 2H₂ + 2O^–2 → 2H₂O + 4e^–

Cathode Reaction: O₂ + 4e^– → 2O^–2

Overall Cell Reaction: 2H₂ + O₂ → 2H₂O

SOFC systems can run on fuels other than pure hydrogen gas. However, since hydrogen is necessary for the reactions listed above, the fuel selected must contain hydrogen atoms. In order for the fuel cell to operate, the fuel must be converted into pure hydrogen gas. SOFCs are capable of internally reforming light hydrocarbons such as methane (natural gas), propane and butane. Heavier hydrocarbons including gasoline, diesel, jet fuel and biofuels can serve as fuels in a SOFC system, but an external reformer is required.

Challenges exist in SOFC systems due to their high operating temperatures. One such challenge is the potential for carbon dust to build up on the anode, which slows down the internal reforming process. Research to address this “carbon coking” issue at the University of Pennsylvania has shown that the use of copper-based cermet (heat-resistant materials made of ceramic and metal) can reduce coking and the loss of performance.^[25] Another disadvantage of SOFC systems is slow start-up time, making SOFCs less useful for mobile applications. Despite these disadvantages, a high operating temperature provides an advantage by removing the need for a precious metal catalyst like platinum, thereby reducing cost. Additionally, waste heat from SOFC systems may be captured and reused, increasing the theoretical overall efficiency to as high as 80%-85%.^[22]

The high operating temperature is largely due to the physical properties of the YSZ electrolyte. As temperature decreases, so does the ionic conductivity of YSZ. Therefore, to obtain optimum performance of the fuel cell, a high operating temperature is required. According to their website, Ceres Power, a UK SOFC fuel cell manufacturer, has developed a method of reducing the operating temperature of their SOFC system to 500-600 degrees Celsius. They replaced the commonly used YSZ electrolyte with a CGO (cerium gadolinium oxide) electrolyte. The lower operating temperature allows them to use stainless steel instead of ceramic as the cell substrate, which reduces cost and start-up time of the system.^[26]

MCFC[link]

Molten carbonate fuel cells (MCFCs) require a high operating temperature, 650 °C (1,200 °F), similar to SOFCs. MCFCs use lithium potassium carbonate salt as an electrolyte, and at high temperatures, this salt melts into a molten state that allows for the movement of charge (in this case, negative carbonate ions) within the cell.^[27]

Like SOFCs, MCFCs are capable of converting fossil fuel to a hydrogen-rich gas in the anode, eliminating the need to produce hydrogen externally. The reforming process creates CO₂ emissions. MCFC-compatible fuels include natural gas, biogas and gas produced from coal. The hydrogen in the gas reacts with carbonate ions from the electrolyte to produce water, carbon dioxide, electrons and small amounts of other chemicals. The electrons travel through an external circuit creating electricity and return to the cathode. There, oxygen from the air and carbon dioxide recycled from the anode react with the electrons to form carbonate ions that replenish the electrolyte, completing the circuit.^[27] The chemical reactions for an MCFC system can be expressed as follows:^[28]

Anode Reaction: CO₃^-2 + H₂ → H₂O + CO₂ + 2e^-

Cathode Reaction: CO₂ + ½O₂ + 2e^- → CO₃^-2

Overall Cell Reaction: H₂ + ½O₂ → H₂O

As with SOFCs, MCFC disadvantages include slow start-up times because of their high operating temperature. This makes MCFC systems not suitable for mobile applications, and this technology will most likely be used for stationary fuel cell purposes. The main challenge of MCFC technology is the cells' short life span. The high temperature and carbonate electrolyte lead to corrosion of the anode and cathode. These factors accelerate the degradation of MCFC components, decreasing the durability and cell life. Researchers are addressing this problem by exploring corrosion-resistant materials for components as well as fuel cell designs that may increase cell life without decreasing performance.^[22]

MCFCs hold several advantages over other fuel cell technologies, including their resistance to impurities. They are not prone to “carbon coking”, which refers to carbon build-up on the anode that results in reduced performance by slowing down the internal fuel reforming process. Therefore, carbon-rich fuels like gases made from coal are compatible with the system. The Department of Energy claims that coal, itself, might even be a fuel option in the future, assuming the system can be made resistant to impurities such as sulfur and particulates that result from converting coal into hydrogen.^[22] MCFCs also have relatively high efficiencies. They can reach a fuel-to-electricity efficiency of 50%, considerably higher than the 37-42% efficiency of a phosphoric acid fuel cell plant. Efficiencies can be as high as 65% when the fuel cell is paired with a turbine, and 85% if heat is captured and used in a Combined Heat and Power (CHP) system.^[27]

FuelCell Energy, a Connecticut-based fuel cell manufacturer, develops and sells MCFC fuel cells. The company says that their MCFC products range from 300 kW to 2.8 MW systems that achieve 47% electrical efficiency and can utilize CHP technology to obtain higher overall efficiencies. One product, the DFC-ERG, is combined with a gas turbine and, according to the company, it achieves an electrical efficiency of 65%.^[29]

Comparison of fuel cell types[link]

Efficiency of leading fuel cell types[link]

File:EERE Fuel Cell Comparison Chart.pdf

Glossary of Terms in table:

• Anode: The electrode at which oxidation (a loss of electrons) takes place. For fuel cells and other galvanic cells, the anode is the negative terminal; for electrolytic cells (where electrolysis occurs), the anode is the positive terminal.^[30]
• Aqueous solution: a: of, relating to, or resembling water b : made from, with, or by water.^[31]
• Catalyst: A chemical substance that increases the rate of a reaction without being consumed; after the reaction, it can potentially be recovered from the reaction mixture and is chemically unchanged. The catalyst lowers the activation energy required, allowing the reaction to proceed more quickly or at a lower temperature. In a fuel cell, the catalyst facilitates the reaction of oxygen and hydrogen. It is usually made of platinum powder very thinly coated onto carbon paper or cloth. The catalyst is rough and porous so the maximum surface area of the platinum can be exposed to the hydrogen or oxygen. The platinum-coated side of the catalyst faces the membrane in the fuel cell.^[30]
• Cathode: The electrode at which reduction (a gain of electrons) occurs. For fuel cells and other galvanic cells, the cathode is the positive terminal; for electrolytic cells (where electrolysis occurs), the cathode is the negative terminal.^[30]
• Electrolyte: A substance that conducts charged ions from one electrode to the other in a fuel cell, battery, or electrolyzer.^[30]
• Fuel Cell Stack: Individual fuel cells connected in a series. Fuel cells are stacked to increase voltage.^[30]
• Matrix: something within or from which something else originates, develops, or takes form.^[32]
• Membrane: The separating layer in a fuel cell that acts as electrolyte (an ion-exchanger) as well as a barrier film separating the gases in the anode and cathode compartments of the fuel cell.^[30]
• Molten Carbonate Fuel Cell (MCFC): A type of fuel cell that contains a molten carbonate electrolyte. Carbonate ions (CO3-2) are transported from the cathode to the anode. Operating temperatures are typically near 650°C.^[30]
• Phosphoric Acid Fuel Cell (PAFC): A type of fuel cell in which the electrolyte consists of concentrated phosphoric acid (H3PO4). Protons (H+) are transported from the anode to the cathode. The operating temperature range is generally 160°C–220°C.^[30]
• Polymer Electrolyte Membrane (PEM): A fuel cell incorporating a solid polymer membrane used as its electrolyte. Protons (H+) are transported from the anode to the cathode. The operating temperature range is generally 60°C–100°C.^[30]
• Solid Oxide Fuel Cell (SOFC): A type of fuel cell in which the electrolyte is a solid, nonporous metal oxide, typically zirconium oxide (ZrO2) treated with Y2O3, and O-2 is transported from the cathode to the anode. Any CO in the reformate gas is oxidized to CO₂ at the anode. Temperatures of operation are typically 800°C–1,000°C.^[30]
• Solution: a: an act or the process by which a solid, liquid, or gaseous substance is homogeneously mixed with a liquid or sometimes a gas or solid, b : a homogeneous mixture formed by this process; especially : a single-phase liquid system, c : the condition of being dissolved^[33]

For more information see Glossary of fuel cell terms

Theoretical maximum efficiency[link]

The energy efficiency of a system or device that converts energy is measured by the ratio of the amount of useful energy put out by the system ("output energy") to the total amount of energy that is put in ("input energy") or by useful output energy as a percentage of the total input energy. In the case of fuel cells, useful output energy is measured in electrical energy produced by the system. Input energy is the energy stored in the fuel. According to the U.S. Department of Energy, fuel cells are generally between 40–60% energy efficient.^[34] This is higher than some other systems for energy generation. For example, the typical internal combustion engine of a car is about 25% energy efficient.^[35] In combined heat and power (CHP) systems, the heat produced by the fuel cell is captured and put to use, increasing the efficiency of the system to up to 85–90%.^[22]

The theoretical maximum efficiency of any type of power generation system is rarely reached in practice, and it does not consider other steps in power generation, such as production, transportation and storage of fuel and conversion of the electricity into mechanical power. However, this calculation allows the comparison of different types of power generation. The maximum theoretical energy efficiency of a fuel cell is 83%, operating at low power density and using pure hydrogen and oxygen as reactants (assuming no heat recapture)^[36] According to the World Energy Council, this compares with a maximum theoretical efficiency of 58% for internal combustion engines.^[36] While these efficiencies are not approached in most real world applications, high temperature fuel cells (solid oxide fuel cells or molten carbonate fuel cells) can theoretically be combined with gas turbines to allow stationary fuel cells to come closer to the theoretical limit. A gas turbine would capture heat from the fuel cell and turn it into mechanical energy to increase the fuel cell’s operational efficiency. This solution has been predicted to increase total efficiency to as much as 70%.^[37]

In practice[link]

The tank-to-wheel efficiency of a fuel cell vehicle is greater than 45% at low loads^[38] and shows average values of about 36% when a driving cycle like the NEDC (New European Driving Cycle) is used as test procedure.^[39] The comparable NEDC value for a Diesel vehicle is 22%. In 2008 Honda released a demonstration fuel cell electric vehicle (the Honda FCX Clarity) with fuel stack claiming a 60% tank-to-wheel efficiency.^[40]

It is also important to take losses due to fuel 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.^[41] Fuel cells cannot store energy like a battery,^[42] except as hydrogen, 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. Most hydrogen, however, is produced by steam methane reforming, and so most hydrogen production emits carbon dioxide.^[43] The overall efficiency (electricity to hydrogen and back to electricity) of such plants (known as round-trip efficiency), using pure hydrogen and pure oxygen can be "from 35 up to 50 percent", depending on gas density and other conditions.^[44] 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 micro combined heat and power (m-CHP) application. When the heat is captured, total efficiency can reach 80-90% at the unit, but does not consider production and distribution losses. CHP units are being developed today for the European home market.

Professor Jeremy P. Meyers, in the Electrochemical Society journal Interface in 2008, wrote, "While fuel cells are efficient relative to combustion engines, they are not as efficient as batteries, due primarily to the inefficiency of the oxygen reduction reaction (and ... the oxygen evolution reaction, should the hydrogen be formed by electrolysis of water). ... [T]hey make the most sense for operation disconnected from the grid, or when fuel can be provided continuously. For applications that require frequent and relatively rapid start-ups ... where zero emissions are a requirement, as in enclosed spaces such as warehouses, and where hydrogen is considered an acceptable reactant, a [PEM fuel cell] is becoming an increasingly attractive choice [if exchanging batteries is inconvenient]".^[16]

Applications[link]

Type 212 submarine with fuel cell propulsion of the German Navy in dry dock

Power[link]

Stationary fuel cells are used for commercial, industrial and residential primary and backup power generation. Fuel cells are very useful as power sources in remote locations, such as spacecraft, remote weather stations, large parks, communications centers, rural locations including research stations, and in certain military applications. A fuel cell system running on hydrogen can be compact and lightweight, and have 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.^[45] This equates to less than one minute of downtime in a six year period.^[46]

Since fuel cellelectrolyzer 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.^[47] There are many different types of stationary fuel cells so efficiencies vary, but most are between 40% and 60% energy efficient.^[22] However, when the fuel cell’s waste heat is used to heat a building in a cogeneration system this efficiency can increase to 85%.^[22] This is significantly more efficient than traditional coal power plants, which are only about one third energy efficient.^[48] Assuming production at scale, fuel cells could save 20-40% on energy costs when used in cogeneration systems.^[49] Fuel cells are also much cleaner than traditional power generation; a fuel cell power plant using natural gas as a hydrogen source would create less than one ounce of pollution (other than CO₂) for every 1,000 kW produced, compared to 25 pounds of pollutants generated by conventional combustion systems.^[50] Fuel Cells also produce 97% less nitrogen oxide emissions then conventional coal-fired power plants.

Coca-Cola, Google, Walmart, Sysco, FedEx, UPS, Ikea, Staples, Whole Foods, Gills Onions, Nestle Waters, Pepperidge Farm, Sierra Nevada Brewery, Super Store Industries, Brigestone-Firestone, Nissan North America, Kimberly-Clark, Michelin and more have installed fuel cells to help meet their power needs.^[51][52] One such pilot program is operating on Stuart Island in Washington State. There the Stuart Island Energy Initiative^[53] has built a complete, closed-loop system: Solar panels power an electrolyzer which makes hydrogen. The hydrogen is stored in a 500 US gallons (1,900 L) at 200 pounds per square inch (1,400 kPa), and runs a ReliOn fuel cell to provide full electric back-up to the off-the-grid residence. Another closed system loop was unveiled in late 2011 in Hempstead, NY.^[54]

Cogeneration[link]

Combined heat and power (CHP) fuel cell systems, including Micro combined heat and power (MicroCHP) systems are used to generate both electricity and heat for homes (see home fuel cell), office building and factories. The 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. MicroCHP is usually less than 5 kWe for a home fuel cell or small business.^[55]

The waste heat from fuel cells can be diverted during the summer directly into the ground providing further cooling while the waste heat during winter can be pumped directly into the building. The University of Minnesota owns the patent rights to this type of system^[56][57]

Co-generation systems can reach 85% efficiency (40-60% electric + remainder as thermal).^[22] Phosphoric-acid fuel cells (PAFC) comprise the largest segment of existing CHP products worldwide and can provide combined efficiencies close to 90%.^[58] Molten Carbonate (MCFC) and Solid Oxide Fuel Cells (SOFC) are also used for combined heat and power generation and have electrical energy effciences around 60%.^[59]

Fuel Cell Electric Vehicles (FCEVs)[link]

Configuration of components in a fuel cell car.

Toyota FCHV PEM FC fuel cell vehicle.

Mercedes-Benz (Daimler AG) Citaro fuel cell bus on Aldwych, London.

Toyota FCHV-BUS at the Expo 2005.

Element One fuel cell vehicle.

The world's first certified Fuel Cell Boat (HYDRA), in Leipzig/Germany

Automobiles[link]

Although there are currently no Fuel cell vehicles available for commercial sale, over 20 FCEVs prototypes and demonstration cars have been released since 2009. Demonstration models include the Honda FCX Clarity, Toyota FCHV-adv, and Mercedes-Benz F-Cell.^[60] As of June 2011 demonstration FCEVs had driven more than 4,800,000 km (3,000,000 mi), with more than 27,000 refuelings.^[61] Demonstration fuel cell vehicles have been produced with "a driving range of more than 400 km (250 mi) between refueling".^[62] They can be refueled in less than 5 minutes.^[63] The U.S. Department of Energy's Fuel Cell Technology Program claims that, as of 2011, fuel cells achieved 53–59% efficiency at ¼ power and 42–53% vehicle efficiency at full power,^[64] and a durability of over 120,000 km (75,000 mi) with less than 10% degradation, double that achieved in 2006.^[62] In a Well-to-Wheels simulation analysis, that "did not address the economics and market constraints", General Motors and its partners estimated that per mile traveled, a fuel cell electric vehicle running on compressed gaseous hydrogen produced from natural gas could use about 40% less energy and emit 45% less greenhouse gasses than an internal combustion vehicle.^[65] A lead engineer from the Department of Energy whose team is testing fuel cell cars said in 2011 that the potential appeal is that "these are full-function vehicles with no limitations on range or refueling rate so they are a direct replacement for any vehicle. For instance, if you drive a full sized SUV and pull a boat up into the mountains, you can do that with this technology and you can't with current battery-only vehicles, which are more geared toward city driving."^[66]

Some experts believe that fuel cell cars will never become economically competitive with other technologies^[67][68] or that it will take decades for them to become profitable.^[69][70] In July 2011, the Chairman and CEO of General Motors, Daniel Akerson, stated that while the cost of hydrogen fuel cell cars is decreasing: "The car is still too expensive and probably won't be practical until the 2020-plus period, I don't know."^[71] Analyses cite the lack of an extensive hydrogen infrastructure in the U.S. as an ongoing challenge to Fuel Cell Electric Vehicle commercialization. In 2006, a study for the IEEE showed that for hydrogen produced via electrolysis of water: "Only about 25% of the power generated from wind, water, or sun is converted to practical use." The study further noted that "Electricity obtained from hydrogen fuel cells appears to be four times as expensive as electricity drawn from the electrical transmission grid. ... Because of the high energy losses [hydrogen] cannot compete with electricity."^[72] Furthermore, the study found: "Natural gas reforming is not a sustainable solution".^[72] "The large amount of energy required to isolate hydrogen from natural compounds (water, natural gas, biomass), package the light gas by compression or liquefaction, transfer the energy carrier to the user, plus the energy lost when it is converted to useful electricity with fuel cells, leaves around 25% for practical use."^[16][38][73] Despite this, several major car manufacturers have announced plans to introduce a production model of a fuel cell car in 2015. Toyota has stated that it plans to introduce such a vehicle at a price of around US$50,000.^[74] In June 2011, Mercedes-Benz announced that they would move the scheduled production date of their fuel cell car from 2015 up to 2014, asserting that "The product is ready for the market technically. ... The issue is infrastructure."^[75] Other manufacturers planning to sell fuel cell electric vehicles commercially by 2016 or earlier include General Motors (2015),^[76] Honda (2015 in Japan),^[77] Hyundai (2015)^[78] and Nissan (2016).^[79]

In 2003 US President George W. Bush proposed the Hydrogen Fuel Initiative (HFI). This aimed at further developing hydrogen fuel cells and infrastructure technologies with the goal of producing commercial fuel cell vehicles. By 2008, the U.S. had contributed 1 billion dollars to this project.^[80] The Obama Administration has sought to reduce funding for the development of fuel cell vehicles, concluding that other vehicle technologies will lead to quicker reduction in emissions in a shorter time.^[81] Steven Chu, the US Secretary of Energy, stated that hydrogen vehicles "will not be practical over the next 10 to 20 years".^[82] He told MIT's Technology Review that he is skeptical about hydrogen's use in transportation because of four problems: "the way we get hydrogen primarily is from reforming [natural] gas. ... You're giving away some of the energy content of natural gas. ... [For] transportation, we don't have a good storage mechanism yet. ... The fuel cells aren't there yet, and the distribution infrastructure isn't there yet. ... In order to get significant deployment, you need four significant technological breakthroughs."^[83] Critics disagree. Mary Nichols, Chairwoman of California's Air Resources Board, said: "Secretary Chu has firmly set his mind against hydrogen as a passenger-car fuel. Frankly, his explanations don’t make sense to me. They are not based on the facts as we know them."^[84]

Buses[link]

In total there are over 100 fuel cell buses deployed around the world today. Most buses are produced by UTC Power, Toyota, Ballard, Hydrogenics, and Proton Motor. UTC Buses have already accumulated over 970,000 km (600,000 mi) of driving.^[85] Fuel cell buses have a 39-141% higher fuel economy than diesel buses and natural gas buses.^[86] Fuel cell buses have been deployed around the world including in Whistler Canada, San Francisco USA, Hamburg Germany, Shanghai China, London England, São Paulo Brazil as well as several others.^[87] The Fuel Cell Bus Club is a global cooperative effort in trial fuel cell buses. Notable Projects Include:

• 12 Fuel cell buses are being deployed in the Oakland and San Francisco Bay area of California.^[87]
• Daimler AG, with thirty-six experimental buses powered by Ballard Power Systems fuel cells completed a successful three-year trial, in eleven cities, in January 2007.^[88][89]
• A fleet of Thor buses with UTC Power fuel cells was deployed in California, operated by SunLine Transit Agency.^[90]

The first Brazilian hydrogen fuel cell bus prototype in Brazil was deployed in São Paulo. The bus was manufactured in Caxias do Sul and the hydrogen fuel will be produced in São Bernardo do Campo from water through electrolysis. The program, called "Ônibus Brasileiro a Hidrogênio" (Brazilian Hydrogen Autobus), includes three additional buses.^[91][92]

Forklifts[link]

Fuel cell powered forklifts are one of the largest sectors of fuel cell applications in the industry.^[93] Most fuel cells used for material handling purposes are powered by PEM fuel cells, although some direct methanol fuel forklifts are coming onto the market. Fuel cell fleets are currently being operated by a large number of companies, including Sysco Foods, FedEx Freight, GENCO (at Wegmans, Coca-Cola, Kimberly Clark, and Whole Foods), and H-E-B Grocers.^[94]

Fuel cell powered forklifts provide significant benefits over both petroleum and battery powered forklifts as they produce no local emissions, can work for a full 8 hour shift on a single tank of hydrogen, can be refueled in 3 minutes and have a lifetime of 8–10 years. Fuel cell powered forklifts are often used in refrigerated warehouses as their performance is not degraded by lower temperatures. Many companies do not use petroleum powered forklifts, as these vehicles work indoors where emissions must be controlled and instead are turning towards electric forklifts. Fuel cell forklifts offer green house gas, product lifetime, maintenance cost, refueling and labor cost benefits over battery operated fork lifts.^[95]

Motorcycles and bicycles[link]

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 160 km (100 mi) in an urban area, at a top speed of 80 km/h (50 mph).^[96] In 2004 Honda developed a fuel-cell motorcycle which utilized the Honda FC Stack.^[97][98] There are other examples of bikes^[99] and bicycles^[100] with a hydrogen fuel cell engine.

Airplanes[link]

Boeing researchers and industry partners throughout Europe conducted experimental flight tests in February 2008 of a manned airplane powered only by a fuel cell and lightweight batteries. The Fuel Cell Demonstrator Airplane, as it was called, used a Proton Exchange Membrane (PEM) fuel cell/lithium-ion battery hybrid system to power an electric motor, which was coupled to a conventional propeller.^[101] In 2003, the world's first propeller driven airplane to be powered entirely by a fuel cell was flown. The fuel cell was a unique FlatStack^TM stack design which allowed the fuel cell to be integrated with the aerodynamic surfaces of the plane.^[102]

There have been several fuel cell powered unmanned aerial vehicles (UAV). A Horizen fuel cell UAV set the record distance flow for a small UAV in 2007.^[103] The military is especially interested in this application because of the low noise, low thermal signature and ability to attain high altitude. In 2009 the Naval Research Laboratory’s (NRL’s) Ion Tiger utilized a hydrogen-powered fuel cell and flew for 23 hours and 17 minutes.^[104] Boeing is completing tests on the Phantom Eye, a high-altitude, long endurance (HALE) to be used to conduct research and surveillance flying at 20,000 m (65,000 ft) for up to four days at a time.^[105] Fuel cells are also being used to provide auxiliary power in aircraft, replacing fossil fuel generators that were previously used to start the engines and power on board electrical needs.^{[105][not in citation given]} Fuel cells can help airplanes reduce CO₂ and other pollutant emissions and noise.

Boats[link]

The world's first Fuel Cell Boat HYDRA used an AFC system with 6.5 kW net output. Iceland has committed to converting its vast fishing fleet to use fuel cells to provide auxiliary power by 2015 and, eventually, to provide primary power in its boats. Amsterdam recently introduced its first fuel cell powered boat that ferries people around the city's famous and beautiful canals.^[106]

Submarines[link]

The Type 212 submarines of the German and Italian navies use fuel cells to remain submerged for weeks without the need to surface.

The latest in fuel cell submarines is the U212A—an ultra-advanced non-nuclear sub developed by German naval shipyard Howaldtswerke Deutsche Werft, who claim it to be "the peak of German submarine technology."^[107] The system consists of nine PEM (Proton Exchange Membrane) fuel cells, providing between 30 kW and 50 kW each. The ship is totally silent giving it a distinct advantage in the detection of other submarines.^[108] Fuel cells offer other advantages to submarines in addition to being completely silent; they can be distributed throughout a ship to improve balance and require far less air to run, allowing ships to be submerged for longer periods of time. Fuel cells offer a good alternative to nuclear fuels.

Other applications[link]

• Providing power for base stations or cell sites^[109][110]
• Distributed generation
• Emergency power systems are a type of fuel cell system, which may include lighting, generators and other apparatus, to provide backup resources in a crisis or when regular systems fail. They find uses in a wide variety of settings from residential homes to hospitals, scientific laboratories, data centers,^[111]
• telecommunication^[112] equipment and modern naval ships.
• An uninterrupted power supply (UPS) provides emergency power and, depending on the topology, provide line regulation as well to connected equipment by supplying power from a separate source when utility power is not available. Unlike a standby generator, it can provide instant protection from a momentary power interruption.
• Base load power plants
• Fuel cell APU for Refuse Collection Vehicle
• Hybrid vehicles, pairing the fuel cell with either an ICE or a battery.
• Notebook computers for applications where AC charging may not be readily available.
• Portable charging docks for small electronics (e.g. a belt clip that charges your cell phone or PDA).
• Smartphones, laptops and tablets.
• Small heating appliances^[113]

Fueling stations[link]

Hydrogen fueling station.

There are already over 85 hydrogen refueling stations in the U.S.^[114] The National Research Council estimated that creating the infrastructure to supply fuel for 10 million FCVs through 2025 would cost the government US$8 billion over 16 years.^[115]

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.^{[citation needed]}

As part of the California Hydrogen Highway initiative California has the most extensive hydrogen refueling infrastructure in the U.S.A. As of June 2011 California had 22 hydrogen refueling stations in operation.^[114] Honda announced plans in March 2011 to open the first station that would generate hydrogen through solar-powered renewable electrolysis.^{[citation needed]} South Carolina also has two hydrogen fueling stations, in Aiken and Columbia, SC. According to the South Carolina Hydrogen & Fuel Cell Alliance, the Columbia station has a current capacity of 120 kg a day, with future plans to develop on-site hydrogen production from electrolysis and reformation. The Aiken station has a current capacity of 80 kg. The University of South Carolina, a founding member of the South Carolina Hydrogen & Fuel Cell Alliance, received 12.5 million dollars from the United States Department of Energy for its Future Fuels Program.^[116]

Japan also has a hydrogen highway, as part of the Japan hydrogen fuel cell project. Twelve hydrogen fueling stations have been built in 11 cities in Japan. Canada, Sweden and Norway also have hydrogen highways implemented.

Markets and economics[link]

In 2010, fuel cell industry revenues exceeded a $750 million market value worldwide,^[117] although, as of 2010, no public company in the industry had yet become profitable.^[118] There were 140,000 fuel cell stacks shipped globally in 2010, up from 11 thousand shipments in 2007, and in 2010 worldwide fuel cell shipments had an annual growth rate of 115%.^[119] Approximately 50% of fuel cell shipments in 2010 were stationary fuel cells, up from about a third in 2009, and the four dominant producers in the Fuel Cell Industry remain the United States, Germany, Japan and South Korea.^[120] The Department of Energy Solid State Energy Conversion Alliance found that, as of January 2011, stationary fuel cells generated power at approximately $724 to $775 per kilowatt installed.^[121] Bloom Energy, a major fuel cell supplier, says its fuel cells will meet a return on investment in 3–5 years, as its fuel cells generate power at 9-11 cents per kilowatt-hour, including the price of fuel, maintenance, and hardware.^[122][123]

Low temperature fuel cell stacks proton exchange membrane fuel cell (PEMFC), direct methanol fuel cell (DMFC) and phosphoric acid fuel cell (PAFC) use a platinum catalyst. Impurities create catalyst poisoning (reducing activity and efficiency) in these low-temperature fuel cells, thus high hydrogen purity or higher catalyst densities are required.^[124] Although there are sufficient platinum resources for future demand,^[125] most predictions of platinum running out and/or platinum prices soaring do not take into account effects of 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.^[126] Another method would use iron and sulphur instead of platinum. This is possible through an intermediate conversion by bacteria. This would lower the cost of a fuel cell substantially (as the platinum in a regular fuel cell costs around US$1,500, and the same amount of iron costs only around US$1.50). The concept is being developed by a coalition of the John Innes Centre and the University of Milan-Bicocca.^[127] PEDOT cathodes are immune to monoxide poisoning.^[128]

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. Recycling of fuel cells components, including platinum, will conserve supplies. 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 cheaper materials such as nickel and nickel oxide. They also do not experience catalyst poisoning by carbon monoxide, and so they do not require high-purity hydrogen to operate. They can use fuels with an existing and extensive infrastructure, such as natural gas, directly, without having to first reform it externally to hydrogen and CO followed by CO removal.

Research and development[link]

• August 2005: Georgia Institute of Technology researchers use triazole to raise the operating temperature of PEM fuel cells from below 100 °C to over 125 °C, claiming this will require less carbon-monoxide purification of the hydrogen fuel.^[129]
• 2008 Monash University, Melbourne uses PEDOT as a cathode.^[18]
• 2009 Researchers at the University of Dayton, in Ohio, show that arrays of vertically grown carbon nanotubes could be used as the catalyst in fuel cells.^[130]
• 2009: Y-Carbon has begun to develop a carbide-derived-carbon-based ultracapacitor with high energy density which may lead to improvements in fuel cell technology.^[131][132]
• 2009: A nickel bisdiphosphine-based catalyst for fuel cells is demonstrated.^[133]

See also[link]

	Electronics portal
	Energy portal
	Sustainable development portal

References[link]

Further reading[link]

• Vielstich, W., et al. (eds.) (2009). Handbook of fuel cells: advances in electrocatalysis, materials, diagnostics and durability. 6 vol. Hoboken: Wiley, 2009.
• Gregor Hoogers (2003). Fuel Cell Technology – Hand Book. CRC Press. 
• James Larminie and Andrew Dicks (2003). Fuel Cell Systems Explained, 2nd Edition. John Wiley and Sons. 
• High Temperature Solid Oxide Fuel Cells-Fundamentals, Design and Applications. Elsevier. 2003. 
• Frano Barbir (2005). PEM Fuel Cells-Theory and Practice. Elsevier Academic Press. 
• EG&G Technical Services, Inc. (2004). Fuel Cell Technology-Hand book, 7th Edition. U.S. Department of Energy. 
• Matthew M. Mench (2008). Fuel Cell Engines. John Wiley & Sons, Inc.. 

External links[link]

• Make your own hydrogen fuel cell
• Fuel starvation in a hydrogen fuel cell animation
• Animation how a fuel cell works and applications
• Fuel Cell Origins: 1840-1890
• TC 105 IEC Technical standard for Fuel Cells
• EERE: Hydrogen, Fuel Cells and Infrastructure Technologies Program
• Thermodynamics of electrolysis of water and hydrogen fuel cells
• 2002-PORTABLE POWER APPLICATIONS OF FUEL CELLS
• US Fuel Cell Council
• DoITPoMS Teaching and Learning Package- "Fuel Cells"
• Animation how a fuel cell works

v t e Galvanic cells Non-rechargeable: primary cells Alkaline battery Aluminium battery Bunsen cell Chromic acid cell Clark cell Daniell cell Dry cell Grove cell Leclanché cell Lithium battery Mercury battery Nickel oxyhydroxide battery Silicon–air battery Silver-oxide battery Weston cell Zamboni pile Zinc–air battery Zinc–carbon battery Zinc–chloride battery Rechargeable: secondary cells Automotive battery Lead–acid battery Lead–acid battery (gel) Lithium–air battery Lithium-ion battery Beltway battery Lithium-ion polymer battery Lithium iron phosphate battery Lithium–sulfur battery Lithium–titanate battery Nickel–cadmium battery Nickel–hydrogen battery Nickel–iron battery Nickel–lithium battery Nickel–metal hydride battery Low self-discharge NiMH battery Nickel–zinc battery Potassium-ion battery Rechargeable alkaline battery Sodium-ion battery Sodium–sulfur battery Vanadium redox battery Zinc–bromine battery Zinc–cerium battery Kinds of cells Battery Wet cell Dry cell Concentration cell Flow battery Fuel cell Trough battery Voltaic pile Parts of cells Anode Catalyst Cathode Electrolyte Half-cell Ions Salt bridge Semipermeable membrane

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Wood was one of the first fuels used by humans and is still the primary energy source in much of the world.^{[citation needed]}

Fuel is any material that stores energy that can later be extracted to perform mechanical work in a controlled manner. Most fuels used by humans undergo combustion, a redox reaction in which a combustible substance releases energy after it ignites and reacts with the oxygen in the air. Other processes used to convert fuel into energy include various other exothermic chemical reactions and nuclear reactions, such as nuclear fission or nuclear fusion. Fuels are also used in the cells of organisms in a process known as cellular respiration, where organic molecules are oxidized to release usable energy. Hydrocarbons are by far the most common source of fuel used by humans, but many other substances, such as radioactive metals, are currently used as well .

Chemical[link]

Chemical fuels are substances that release energy by reacting with substances around them, most notably by the process of oxidation.

Biofuels[link]

Biofuel can be broadly defined as solid, liquid, or gas fuel consisting of, or derived from biomass. Biomass can also be used directly for heating or power—known as biomass fuel. Biofuel can be produced from any carbon source that can be replenished rapidly e.g. plants. Many different plants and plant-derived materials are used for biofuel manufacture.

Perhaps the earliest fuel employed by humans is wood. Evidence shows controlled fire was used up to 1.5 million years ago at Swartkrans, South Africa. It is unknown which hominid species first used fire, as both Australopithecus and an early species of Homo were present at the sites.^[1] As a fuel, wood has remained in use up until the present day, although it has been superseded for many purposes by other sources. Wood has an energy density of 10–20 MJ/kg.^[2]

Recently biofuels have been developed for use in automotive transport (for example Bioethanol and Biodiesel), but there is widespread public debate about how carbon efficient these fuels are.

Fossil fuels[link]

Fossil fuels are hydrocarbons, primarily coal and petroleum (liquid petroleum or natural gas), formed from the fossilized remains of ancient plants and animals^[3] by exposure to high heat and pressure in the absence of oxygen in the Earth's crust over hundreds of millions of years.^[4] Commonly, the term fossil fuel also includes hydrocarbon-containing natural resources that are not derived entirely from biological sources, such as tar sands. These latter sources are properly known as mineral fuels.

Nuclear[link]

Nuclear fuel is any material that is consumed to derive nuclear energy. Technically speaking this definition includes all matter because any element will under the right conditions release nuclear energy, the only materials that are commonly referred to as nuclear fuels though are those that will produce energy without being placed under extreme duress.

Fission[link]

Nuclear fuel pellets are used to create nuclear energy.

The most common type of nuclear fuel used by humans is heavy fissile elements that can be made to undergo nuclear fission chain reactions in a nuclear fission reactor; nuclear fuel can refer to the material or to physical objects (for example fuel bundles composed of fuel rods) composed of the fuel material, perhaps mixed with structural, neutron moderating, or neutron reflecting materials. The most common fissile nuclear fuels are ²³⁵U and ²³⁹Pu, and the actions of mining, refining, purifying, using, and ultimately disposing of these elements together make up the nuclear fuel cycle, which is important for its relevance to nuclear power generation and nuclear weapons. In addition there are various types of fuels.

Fusion[link]

Fuels that produce energy by the process of nuclear fusion are currently not utilized by man but are the main source of fuel for stars, the most powerful energy sources in nature. Fusion fuels tend to be light elements such as hydrogen which will combine easily.

In stars that undergo nuclear fusion, fuel consists of atomic nuclei that can release energy by the absorption of a proton or neutron. In most stars the fuel is provided by hydrogen, which can combine together to form helium through the proton-proton chain reaction or by the CNO cycle. When the hydrogen fuel is exhausted, nuclear fusion can continue with progressively heavier elements, although the net energy released is lower because of the smaller difference in nuclear binding energy. Once iron-56 or nickel-56 nuclei are produced, no further energy can be obtained by nuclear fusion as these have the highest nuclear binding energies.^[5]