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[1]) into electricity through a pair of?redox?reactions.[2]?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.[3]?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 startup 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.[4]?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 less 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 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.[31]?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/cm2) in platinum usage without reduction in performance.[32]?Monash University,?Melbourne?uses?PEDOT?as a?cathode.[33]?A 2011-published study[34]?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.[35]

Water and air management[36][37]?(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 or 150,000?mi) under extreme temperatures. Current?service life?is 2,500 hours (about 120,000?km or 75,000?mi).[38]?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/L).

Limited?carbon monoxide?tolerance of some (non-PEDOT) cathodes.[30]

Phosphoric acid fuel cell[edit]

Main article:?Phosphoric acid fuel cell

Phosphoric acid fuel cells (PAFCs) 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 protons from the anode to the cathode and to force electrons to travel from anode to cathode through an external electrical circuit. These cells commonly work in temperatures of 150 to 200°C. 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.[39]?Using this heat in?cogeneration?can enhance the efficiency of phosphoric acid fuel cells from 40 to 50% to about 80%.[39]?Since the proton 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.[40]

Solid acid fuel cell[edit]

Main article:?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 CsHSO4), some solid acids undergo a phase transition to become highly disordered "superprotonic" structures, which increases conductivity by several orders of magnitude. The first proof-of-concept SAFCs were developed in 2000 using cesium hydrogen sulfate (CsHSO4).[41]?Current SAFC systems use cesium dihydrogen phosphate (CsH2PO4) and have demonstrated lifetimes in the thousands of hours.[42]

Alkaline fuel cell[edit]

Main articles:?Alkaline fuel cell?and?Alkaline anion exchange membrane 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.[43]?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. H2?gas and O2?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 343–413?K and provides a potential of about 0.9?V.[44]?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[edit]

Solid oxide fuel cell[edit]

Main article:?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.[5]

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.[45]?The chemical reactions for the SOFC system can be expressed as follows:[46]

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?(natural gas),[47]?propane, and butane.[48]?These fuel cells are at an early stage of development.[49]

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.[50]?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%.[5]

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 the 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