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des catalyst membrane and the necessary hardware such as current collectors and gaskets 26 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 27 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 28 the electrolyte could be a polymer membrane proton exchange membrane fuel cell design issues edit cost 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 29 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 2 in platinum usage without reduction in performance 30 monash university melbourne uses pedot as a cathode 31 a 2011 published study 32 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 33 water and air management 34 35 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 2 o 2 2h 2 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 35 to 40 c 31 to 104 f while automotive fuel cells require a 5 000 hour lifespan the equivalent of 240 000 km or 150 000 miles under extreme temperatures current service life is 2 500 hours about 120 000 km or 75 000 mi 36 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 28 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 37 using this heat in cogeneration can enhance the efficiency of phosphoric acid fuel cells from 40 to 50 to about 80 37 since the proton production rate on the anode is small platinum is used as a 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 38 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 cshso 4 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 2 po 4 and have demonstrated lifetimes in the thousands of hours 39 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 40 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 2 gas and o 2 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 70 140 c and provides a potential of about 0 9 v 41 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 6 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 42 the chemical reactions for the sofc system can be expressed as follows 43 anode reaction 2h 2 2o 2 2h 2 o 4e cathode reaction o 2 4e 2o 2 overall cell reaction 2h 2 o 2 2h 2 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 44 propane and butane 45 these fuel cells are at an early stage of development 46 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 47 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 6 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 48 molten carbonate fuel cell edit main article molten carbonate fuel cell 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 this salt liquefies at high temperatures allowing for the movement of charge within the cell in this case negative carbonate ions 49 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 49 the chemical reactions for an mcfc system can be expressed as follows 50 anode reaction co 3 2 h 2 h 2 o co 2 2e cathode reaction co 2 ½o 2 2e co 3 2 overall cell reaction h 2 ½o 2 h 2 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 6 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 united states 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 6 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 49 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 51 electric storage fuel cell edit 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 52 biofuel cell edit main articles microbial fuel cell and enzymatic 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 53 applications include wastewater treatment and renewable energy production 54 conductive polymers may be used to improve electron transfer between enzymes and electrodes 55 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 56 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 57 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 58 comparison of fuel cell types edit fuel cell name electrolyte qualified power w working temperature c efficiency status cost usd w ce...
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