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  • 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.

  • Hydrogen produced from the steam methane reforming of natural gas is the most common fuel, but

  • for greater efficiency hydrocarbons can be used directly such as natural gas and alcohols

  • like methanol. Fuel cells are different from batteries in that they require a continuous

  • source of fuel and oxygen/air to sustain the chemical reaction whereas in a battery the

  • chemicals present in the battery react with each other to generate an electromotive force.

  • Fuel cells can produce electricity continuously for as long as these inputs are supplied.

  • The first fuel cells were invented in 1838. The first commercial use of fuel cells came

  • more than a century later 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 also used to power fuel-cell vehicles,

  • including forklifts, automobiles, buses, boats, motorcycles and submarines.

  • There are many types of fuel cells, but they all consist of an anode, a cathode 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 followed by the difference in startup

  • time ranging from 1 sec for PEMFC to 10 min for SOFC. Fuel cells come in a variety of

  • sizes. Individual fuel cells produce relatively small electrical potentials, about 0.7 volts,

  • so cells are "stacked", or placed in series, to increase the voltage and meet an application's

  • requirements. 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 in

  • cogeneration if waste heat is captured for use.

  • The fuel cell market is growing, and Pike Research has estimated that the stationary

  • fuel cell market will reach 50 GW by 2020.

  • History

  • The first references to hydrogen fuel cells appeared in 1838. In a letter dated October

  • 1838 but published in the December 1838 edition of The London and Edinburgh Philosophical

  • Magazine and Journal of Science, Welsh physicist and barrister William Grove wrote about the

  • development of his first crude fuel cells. He used a combination of sheet iron, copper

  • and porcelain plates, and a solution of sulphate of copper and dilute acid. In a letter to

  • the same publication written in December 1838 but published in June 1839, German physicist

  • Christian Friedrich Schönbein discussed the first crude fuel cell that he had invented.

  • His letter discussed current generated from hydrogen and oxygen dissolved in water. Grove

  • later sketched his design, in 1842, in the same journal. The fuel cell he made used similar

  • materials to today's phosphoric-acid fuel cell.

  • In 1939, British engineer Francis Thomas Bacon successfully developed a 5 kW stationary

  • fuel cell. In 1955, W. Thomas Grubb, a chemist working for the General Electric Company,

  • 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 and McDonnell Aircraft, leading

  • to its use during Project Gemini. This was the first commercial use of a 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 U.S. 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. In 1991, the first hydrogen fuel cell

  • automobile was developed by Roger Billings. UTC Power 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 be

  • the sole supplier of fuel cells to NASA for use in space vehicles, having supplied fuel

  • cells for the Apollo missions, and the Space Shuttle program, and is developing fuel cells

  • for cell phone towers and other applications. Types of fuel cells; design

  • 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:

  • 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 breaks down the fuel into

  • electrons and ions. The anode catalyst is usually made up of very fine platinum powder.

  • The cathode catalyst 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

  • Mass transport loss. To deliver the desired amount of energy, the

  • fuel cells can be combined in series to yield higher voltage, and in parallel to allow a

  • higher current to be supplied. Such a design is called a fuel cell stack. The cell surface

  • area can also be increased, to allow higher current from each cell. Within the stack,

  • reactant gases must be distributed uniformly over each of the cells to maximize the power

  • output. Proton exchange membrane fuel cells

  • In the archetypical hydrogenoxide proton exchange membrane fuel cell design, a proton-conducting

  • polymer membrane separates the anode and cathode sides. This was called a "solid polymer electrolyte

  • fuel cell" in the early 1970s, before the proton exchange mechanism was well-understood.

  • 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 are 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 because the membrane is electrically insulating. On the cathode catalyst, oxygen

  • molecules react with the electrons and protons to form water.

  • In addition to this pure hydrogen type, there are hydrocarbon fuels for fuel cells, including

  • diesel, methanol and chemical hydrides. The waste products with these types of fuel are

  • carbon dioxide and water, when hydrogen is used the CO2 is released when methane from

  • natural gas is combined with steam in a process called steam methane reforming to produce

  • the hydrogen, this can take place in a different location to the fuel cell potentially allowing

  • the hydrogen fuel cell to be used indoors for example in fork lifts.

  • The different components of a PEMFC are; bipolar plates,

  • electrodes, catalyst,

  • membrane, and the necessary hardware.

  • 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, carbonpolymer composites etc. The membrane electrode assembly

  • is referred as the heart of the PEMFC and is 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 Costs. In 2013, the Department of Energy estimated

  • that 80-kW automotive fuel cell system costs of US$67 per kilowatt could be achieved, assuming

  • volume production of 100,000 automotive units per year and US$55 per kilowatt could be achieved,

  • assuming volume production of 500,000 units per year. In 2008, professor Jeremy P. Meyers

  • estimated that cost reductions over a production ramp-up period will take about 20 years after

  • fuel-cell cars are introduced before they will be able to compete commercially with

  • current market technologies, including gasoline internal combustion engines. 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 experimented with

  • a catalyst enhanced with carbon silk, which allows a 30% reduction in platinum usage without

  • reduction in performance. Monash University, Melbourne uses PEDOT as a cathode. A 2011

  • published study documented the first metal-free electrocatalyst using relatively inexpensive

  • doped carbon nanotubes, which are less than 1% the cost of platinum and are of equal or

  • superior performance. Water and air management. 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, while automotive fuel cells require a 5,000-hour lifespan)

  • under extreme temperatures. Current service life is 7,300 hours under cycling conditions.

  • Automotive engines must also be able to start reliably at −30 °C and have a high power-to-volume

  • ratio. Limited carbon monoxide tolerance of some

  • cathodes. Phosphoric acid fuel cell

  • Phosphoric acid fuel cells were first designed and introduced in 1961 by G. V. Elmore and

  • H. A. Tanner. In these cells phosphoric acid is used as a non-conductive electrolyte to

  • pass positive hydrogen ions from the anode to the cathode. These cells commonly work

  • in temperatures of 150 to 200 degrees Celsius. This high temperature will cause heat and

  • energy loss if the heat is not removed and used properly. This heat can be used to produce

  • steam for air conditioning systems or any other thermal energy consuming system. Using

  • this heat in cogeneration can enhance the efficiency of phosphoric acid fuel cells from

  • 40–50% to about 80%. Phosphoric acid, the electrolyte used in PAFCs, is a non-conductive

  • liquid acid which forces electrons to travel from anode to cathode through an external

  • electrical circuit. Since the hydrogen ion production rate on the anode is small, platinum

  • is used as catalyst to increase this ionization rate. A key disadvantage of these cells is

  • the use of an acidic electrolyte. This increases the corrosion or oxidation of components exposed

  • to phosphoric acid. High-temperature fuel cells

  • SOFC

  • Solid oxide fuel cells use a solid material, most commonly a ceramic material called yttria-stabilized

  • zirconia, 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 and can be run on a

  • variety of fuels including natural gas. SOFCs are unique in that negatively charged

  • oxygen ions travel from the cathode to the anode 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 absorbs 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. The chemical

  • reactions for the SOFC system can be expressed as follows:

  • Anode Reaction: 2H2 + 2O2− → 2H2O + 4e− Cathode Reaction: O2 + 4e– → 2O2−

  • 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. 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, propane and butane. These fuel cells are at an early stage of development.

  • 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 can reduce coking and the loss of performance.

  • 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%.

  • 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 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.

  • MCFC

  • Molten carbonate fuel cells require a high operating temperature, 650 °C, similar to

  • SOFCs. MCFCs use lithium potassium carbonate salt as an electrolyte, and this salt liquefies

  • at high temperatures, allowing for the movement of charge within the cellin this case,

  • negative carbonate ions. 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 CO

  • 2 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. The chemical reactions

  • for an MCFC system can be expressed as follows: Anode Reaction: CO32− + H2 → H2O + CO2

  • + 2e− Cathode Reaction: CO2 + ½O2 + 2e− → CO32−

  • 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. 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. 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