thumb|Demonstration model of a direct methanol fuel cell (black layered cube) in its enclosure
thumb|Scheme of a proton-conducting fuel cell
A fuel cell is an electrochemical cell that converts the chemical energy of a fuel (often hydrogen) and an oxidizing agent (often oxygen) into electricity through a pair of redox reactions. Fuel cells are different from most batteries in requiring a continuous source of fuel and oxygen (usually from air) to sustain the chemical reaction, whereas in a battery the chemical energy usually comes from substances that are already present in the battery. Fuel cells can produce electricity continuously for as long as fuel and oxygen are supplied.
The first fuel cells were invented by Sir William Grove in 1838. The first commercial use of fuel cells came almost a century later following the invention of the hydrogen–oxygen fuel cell by Francis Thomas Bacon in 1932. The alkaline fuel cell, also known as the Bacon fuel cell after its inventor, has been used in NASA space programs since the mid-1960s to generate power for 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, trains, 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 ions, often positively charged hydrogen ions (protons), to move between the two sides of the fuel cell. At the anode, a catalyst causes the fuel to undergo oxidation reactions that generate ions (often positively charged hydrogen ions) and electrons. The ions move from the anode to the cathode through the electrolyte. At the same time, electrons flow from the anode to the cathode through an external circuit, producing direct current electricity. At the cathode, another catalyst causes ions, electrons, and oxygen to react, forming water and possibly other products. Fuel cells are classified by the type of electrolyte they use and by the difference in start-up time ranging from 1 second for proton-exchange membrane fuel cells (PEM fuel cells, or PEMFC) to 10 minutes for solid oxide fuel cells (SOFC). A related technology is flow batteries, in which the fuel can be regenerated by recharging. Individual fuel cells produce relatively small electrical potentials, about 0.7 volts, so cells are "stacked", or placed in series, to create sufficient voltage to meet an application's requirements. In addition to electricity, fuel cells produce water vapor, heat and, depending on the fuel source, very small amounts of nitrogen dioxide and other emissions. PEMFC cells generally produce fewer nitrogen oxides than SOFC cells: they operate at lower temperatures, use hydrogen as fuel, and limit the diffusion of nitrogen into the anode via the proton exchange membrane, which forms NOx. The energy efficiency of a fuel cell is generally between 40 and 60%; however, if waste heat is captured in a cogeneration scheme, efficiencies of up to 85% can be obtained. 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 the 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 1932, English 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 (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 a 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 & 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).
thumb|800x800px|The number of patent families (solid lines) and non-patent publications about different electrochemical powersources by year. Also shown as the magenta line is the inflation-adjusted oil price in US$/liter in linear scale.|center
UTC Power was the first company to manufacture and commercialize a large, stationary fuel cell system for use as a cogeneration power plant in hospitals, universities and large office buildings.
In recognition of the fuel cell industry and America's role in fuel cell development, the United States Senate recognized October 8, 2015 as National Hydrogen and Fuel Cell Day, passing S. RES 217. The date was chosen in recognition of the atomic weight of hydrogen (1.008).
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 ionizes the fuel, 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 an 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.
thumb|upright=1.35|A block diagram of a fuel cell
Design features in a fuel cell include:
- The electrolyte substance, which usually defines the type of fuel cell, and can be made from a number of substances like potassium hydroxide, salt carbonates, and phosphoric acid.
- The most common fuel that is used is hydrogen.
- The anode catalyst, usually fine platinum powder, breaks down the fuel into electrons and ions.
- The cathode catalyst, often nickel, converts ions into waste chemicals, with water being the most common type of waste.
- Gas diffusion layers that are designed to resist oxidization.
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.
Proton-exchange membrane fuel cells
thumb|upright=1.75|Construction of a high-temperature [[proton-exchange membrane fuel cell|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); Porous carbon papers; reactive layer, usually on the polymer membrane applied; polymer membrane.]]
upright=1.55|thumb|Condensation of water produced by a PEMFC on the air channel wall. The gold wire around the cell ensures the collection of electric current.
thumb|SEM micrograph of a PEMFC MEA cross-section with a non-precious metal catalyst cathode and Pt/C anode. False colors applied for clarity.
In the archetypical hydrogen–oxide proton-exchange membrane fuel cell (PEMFC) design, a proton-conducting polymer membrane (typically nafion) contains the electrolyte solution that 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 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 (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 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. When hydrogen is used, the CO 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 forklifts.
The different components of a PEMFC are
- bipolar plates,
- electrodes,
- catalyst,
- membrane, and
- the necessary hardware such as current collectors and gaskets.
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. The membrane electrode assembly (MEA) is referred to 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 types of noble metals are usually used as the catalyst for PEMFC, and these can be contaminated by carbon monoxide, necessitating a relatively pure hydrogen fuel. The electrolyte could be a polymer membrane.
Proton-exchange membrane fuel cell design issues
; Cost: In 2013, the Department of Energy estimated that 80 kW automotive fuel cell system costs of per kilowatt could be achieved, assuming volume production of 100,000 automotive units per year and per kilowatt could be achieved, assuming volume production of 500,000 units per year. 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 (1.0–0.7 mg/cm<sup>2</sup>) 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. A recently published article demonstrated how the environmental burdens change when using carbon nanotubes as carbon substrate for platinum.
; Water and air management (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, the 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 2H<sub>2</sub> + O<sub>2</sub> → 2H<sub>2</sub>O 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 , while automotive fuel cells require a 5,000-hour lifespan (the equivalent of ) under extreme temperatures. Current service life is 2,500 hours (about ). Automotive engines must also be able to start reliably at and have a high power-to-volume ratio (typically 2.5 kW/L).
; Limited carbon monoxide tolerance of some (non-PEDOT) cathodes. Using this heat in cogeneration can enhance the efficiency of phosphoric acid fuel cells from 40 to 50% to about 80%.
Solid acid fuel cell
Solid acid fuel cells (SAFCs) are characterized by the use of a solid acid material as the electrolyte. At low temperatures, solid acids have an ordered molecular structure like most salts. At warmer temperatures (between 140 and 150°C for CsHSO<sub>4</sub>), some solid acids undergo a phase transition to become highly disordered "superprotonic" structures, which increases conductivity by several orders of magnitude. SAFC systems use cesium dihydrogen phosphate (CsH<sub>2</sub>PO<sub>4</sub>) and have demonstrated lifetimes in the thousands of hours.
Alkaline fuel cell
The alkaline fuel cell (AFC) or hydrogen-oxygen fuel cell was designed and first demonstrated publicly by Francis Thomas Bacon in 1959. It was used as a primary source of electrical energy in the Apollo space program. The cell consists of two porous carbon electrodes impregnated with a suitable catalyst such as Pt, Ag, CoO, etc. The space between the two electrodes is filled with a concentrated solution of KOH or NaOH which serves as an electrolyte. H<sub>2</sub> gas and O<sub>2</sub> gas are bubbled into the electrolyte through the porous carbon electrodes. Thus the overall reaction involves the combination of hydrogen gas and oxygen gas to form water. The cell runs continuously until the reactant's supply is exhausted. This type of cell operates efficiently in the temperature range 343–413K (70 -140 °C) and provides a potential of about 0.9V. Alkaline anion exchange membrane fuel cell (AAEMFC) is a type of AFC which employs a solid polymer electrolyte instead of aqueous potassium hydroxide (KOH) and it is superior to aqueous AFC.
High-temperature fuel cells
Solid oxide fuel cell
Solid oxide fuel cells (SOFCs) 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 (800–1000 °C) and can be run on a variety of fuels including natural gas.
SOFCs are unique because negatively charged oxygen ions travel from the cathode (positive side of the fuel cell) to the anode (negative side of the fuel cell) instead of protons travelling vice versa (i.e., 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 a 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: 2H<sub>2</sub> + 2O<sup>2−</sup> → 2H<sub>2</sub>O + 4e<sup>−</sup>
:Cathode reaction: O<sub>2</sub> + 4e<sup>−</sup> → 2O<sup>2−</sup>
:Overall cell reaction: 2H<sub>2</sub> + O<sub>2</sub> → 2H<sub>2</sub>O
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 (natural gas), 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 (heat-resistant materials made of ceramic and metal) can reduce coking and the loss of performance. Another disadvantage of SOFC systems is the long start-up, 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%.
Molten-carbonate fuel cell
Molten carbonate fuel cells (MCFCs) require a high operating temperature, , 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 cell – in 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 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.
:Anode reaction: CO<sub>3</sub><sup>2−</sup> + H<sub>2</sub> → H<sub>2</sub>O + CO<sub>2</sub> + 2e<sup>−</sup>
:Cathode reaction: CO<sub>2</sub> + ½O<sub>2</sub> + 2e<sup>−</sup> → CO<sub>3</sub><sup>2−</sup>
:Overall cell reaction: H<sub>2</sub> + ½O<sub>2</sub> → H<sub>2</sub>O
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.
Electric storage fuel cell
The electric storage fuel cell is a conventional battery chargeable by electric power input, using the conventional electro-chemical effect. However, the battery further includes hydrogen (and oxygen) inputs for alternatively charging the battery chemically.
Biofuel cell
A biofuel cell converts chemical energy from biological substances into electrical energy using biological catalysts, such as enzymes or microorganisms. The process involves the oxidation of a fuel, like glucose, at the anode, releasing electrons and protons. The electrons travel through an external circuit to generate electrical current, while at the cathode, oxygen is typically reduced to water or hydrogen peroxide, completing the circuit. Applications include wastewater treatment and renewable energy production. Conductive polymers may be used to improve electron transfer between enzymes and electrodes.
The integration of nanomaterials, such as carbon nanotubes and metal nanoparticles, are used to enhance the performance of BFCs. These materials increase the surface area of electrodes and facilitate better electron transfer, resulting in higher power densities. Three-dimensional porous structures and graphene-based materials, have been used to improve conductivity and stability, and hybrid biofuel cells that combine BFCs with supercapacitors or secondary batteries are being developed to provide stable and continuous energy output. BFCs are being explored as power sources for implantable devices like pacemakers and biosensors.to potentially eliminate the need for traditional batteries, and fiber-type EBFCs show potential in implantable applications. The power density of BFCs, however, is generally lower than that of conventional energy sources, the stability of enzymes and microorganisms over extended periods is another concern, and scalability and commercial viability also pose hurdles.
Comparison of fuel cell types
{| class="wikitable sortable plainrowheaders" style="text-align:center;"
! rowspan=2 | Fuel cell name
! rowspan=2 | Electrolyte
! rowspan=2 | Qualified power (W)
! rowspan=2 | Working temperature (°C)
! colspan=2 | Efficiency
! rowspan=2 | Status
! rowspan=2 | Cost (USD/W)
|-
! Cell
! System
|-
! scope="row" |Electro-galvanic fuel cell
|Aqueous alkaline solution<!--e.g., potassium hydroxide-->
|
|
| ||
|
|3-7
|-
! scope="row" |Direct formic acid fuel cell (DFAFC)
|Polymer membrane (ionomer)
|
|
| ||
|
|10-20
|-
! scope="row" |Alkaline fuel cell
|Aqueous alkaline solution <!--e.g., potassium hydroxide-->
|
|
| ||
|
|50-100
|-
! scope="row" |Proton-exchange membrane fuel cell
|Polymer membrane (ionomer) <!--e.g. Nafion or Polybenzimidazole fiber-->
|
|
| ||
|
|125
|-
!scope=row|Reformed methanol fuel cell
|Polymer membrane (ionomer)
|
|
| ||
|
|8.50
|-
!scope=row|Direct-ethanol fuel cell
|Polymer membrane (ionomer)
|
|
| ||
|
|12
|-
!scope=row|Redox fuel cell (RFC)
|Liquid electrolytes with redox shuttle and polymer membrane (ionomer)
|
| ||
|
|
|12.50
|-
!scope=row|Phosphoric acid fuel cell
|Molten phosphoric acid (H<sub>3</sub>PO<sub>4</sub>)
|
|
| ||