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trolyte 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 14 15 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 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 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 16 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 17 types of fuel cells design edit 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 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 18 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 19 gas diffusion layers that are designed to resist oxidization 19 a typical fuel cell produces a voltage from 0 6 to 0 7 v at a 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 interconnections mass transport loss depletion of reactants at catalyst sites under high loads causing rapid loss of voltage 20 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 edit main article proton exchange membrane fuel cell 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 21 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 22 sem micrograph of a pemfc mea cross section with a non precious metal catalyst cathode and pt c anode 23 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 24 25 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 2 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 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...
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