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A fuel cell is an electrochemical energy conversion device which converts the chemical energy of a continuous supply of reactants such as hydrogen and oxygen into water and in the process it produces electricity. Every fuel cell has two electrodes, one positive and one negative called the anode and cathode respectively. The reactions that produce electricity occur at the electrodes. An electrolyte is also present in every fuel cell. It carries electrically charged particles from one electrode to the other. Furthermore, there is a presence of a catalyst which speeds the reactions at the electrodes.
The main purpose of a fuel cell is to produce electricity that can be directed outside the cell to do work, such as powering an electric motor etc. In use today, most fuel cells use hydrogen and oxygen as the chemicals. There are many types of fuel cells and usually each operates slightly different. Generally, a stream of hydrogen fuel is fed into the anode where the splitting of the hydrogen atom into a proton and an electron is encouraged by a catalyst. Since the hydrogen atoms become ionized hence, they are positively charged and pass through the electrolyte. However, the negatively charged electrons provide the current through wires to do work. If alternating current (AC) is needed, the direct current output of the fuel cell will be routed through a conversion device called an inverter. Oxygen enters the fuel cell through the cathode and it will combine with electrons returning from the electrical circuit and hydrogen ions that have travelled through the electrolyte from the anode. The electrolyte plays a vital role. It must only allow appropriate ions to pass between the anode and cathode. If free electrons or other substances are allowed to travel through the electrolyte they would affect the chemical reactions whether they combine at anode or cathode, much of the hydrogen and oxygen used in generating electricity and ultimately combine to form a harmless byproduct, water, which is drain from the cell.
As long as a fuel cell is supplied with hydrogen and oxygen, it will generate electricity.
In summary, the electrode reactions are:-
Anode reaction (oxidation): H2 (g) + 2OH-(aq) 2H2O(l) + 2e-
Cathode reaction (reduction): O2(g) + 2H2O (l) + 4e- 4OH-(aq)
The overall reaction is obtained by adding the two equations above:
2H2(g) + O2(g) 2H2O(l)
The above information was taken from:-
Author unknown, Fuel cells [Online] (Update Unknown) Available at: - www.fuels.si.edu/basics.html [Accessed 5th April, 2010].
Types of Fuel Cell Technologies
There are many different types of fuel cells, each using a different chemistry. Fuel cells are usually classified according to their operating temperature and the type of electrolyte they use. The main types of electrolyte used today are alkali, molten carbonate, phosphoric acid which are all liquid electrolytes and proton exchange membrane(PEM) and solid oxide are both solid. The type of fuel also depends on the electrolyte. Each type of fuel cell has advantages and disadvantages compared to each other. The following list describes the ten main types of fuel cells.
Alkaline Fuel Cells (AFC)
Alkaline fuel cells (AFC) are one of the most developed technologies that have been used since the mid-1960s by NASA in Apollo and space shuttle programs. Alkaline fuel cells operate on compressed hydrogen and oxygen. They use an electrolyte that is an aqueous solution of potassium hydroxide (KOH) retained in a porous stabilized matrix. The concentration of potassium hydroxide can be varied with the fuel cell operating temperature, which usually ranges from 650c to 2200c. The efficiency is about 70%. The charge carrier for an AFC is the hydroxyl ion (OH-) that moves from the cathode to the anode where they will eventually react with hydrogen to produce water and electrons. Water formed at the anode moves back to the cathode to regenerate hydroxyl ions. The chemical reactions at the anode and cathode are as follows:-
Anode reaction: - 2H2(g) + 4OH-(aq) 4H2O (l) + 4e-
Cathode reaction: - O2(g) + 2H2O(l) + 4e- 4OH-(aq)
Overall reaction: - 2H2(g) + O2(g) 2H2O (l)
The above reactions in the fuel cell produce electricity and by-product heat. The cell output ranges from 300 watts to 5 kilowatts. A major characteristic of AFCs is that they are sensitive to carbon dioxide which may be present in the fuel or air. The carbon dioxide reacts with the electrolyte, poisoning it rapidly, and degrades its performance. As a result, AFCs are limited to closed environments, such as space and undersea vehicles and they require pure hydrogen and oxygen. In addition molecules such as carbon monoxide (CO), water (H2O) and methane (CH4), which are harmless or even work as fuels to other fuel cells, are poisons to an AFC. On the other hand, AFCs are the cheapest fuel cells to manufacture. This is because the catalyst that is required on the electrodes can be any of a number of different materials that are inexpensive compared to the other catalysts required for the other types of fuel cells. AFCs are not considered for automobile applications because of their sensitivity to poisoning. AFCs operate at low temperatures and are among the most efficient fuel cells.
Molten- Carbonate Fuel Cells (MCFCs)
Molten carbonate fuel cells (MCFCs) are high temperature fuel cells. Its electrolyte composed of a molten mixture of carbonate salts. Two mixtures that are currently being used are lithium carbonate and sodium carbonate suspended in a porous, chemically inert ceramic lithium aluminium oxide ( LiAlO2) matrix. Since they operate at extremely high temperatures of 6500c and above they melt the carbonate salts and achieve high ion mobility through the electrolyte. Being heated to a temperature of approximately 6500c, these salts melt and become conductive to carbonate ions which flow from the cathode to the anode where they combine with hydrogen to give carbon dioxide, water and electrons. These electrons are routed through an external circuit back to the cathode producing electricity and by- product heat on the way.
Anode reaction:- CO32- + H2 H2O + CO2 +2e-
Cathode reaction:- CO2(g) + 1\2 O2(g) + 2e- CO32-
Overall reaction:- H2(g) + 1\2 O2(g) + CO2 (cathode) H2O(g) + CO2(g) (anode)
The efficiency of MCFCs ranges from 60% and this could rise to 80% if the waste heat is captured and used. The nickel electrodes catalysts are used by MCFCs are inexpensive compared to the platinum used in other fuel cells. The high temperature at which these cells operate has both advantages and disadvantages. At the higher temperature they are able to internally reform hydrocarbons, such as natural gas and petroleum, to generate hydrogen within the fuel cell structure. The high temperature of MCFCs usually limits damage from carbon monoxide poisoning but sulphur remains a problem. On the other hand, the cells take significant time to reach the operating temperature, making them unsuitable for transport applications and the high temperature and corrosive nature of the electrolyte means that they would probably be too hot for home power generation.
Proton Exchange Membrane Fuel Cells (PEMFC)
Proton exchange membrane fuel cells also called polymer electrolyte membrane (PEM) fuel cells. Polymer electrolyte membrane fuel cells has a high power density and a relatively low operating temperatures about 800c (1760F) which allows them to start up rapidly from cold and as a result there is less wearing on system component resulting in better durability. The PEM fuel cells work with a thin polymer membrane as the electrolyte which is permeable to protons but it does not conduct electrons. The electrodes are porous and made from carbon containing a platinum catalyst. PEMFC uses hydrogen as the fuel and the charge carrier is the hydrogen ion. The hydrogen ions will permeate across the electrolyte to the cathode and the electron will therefore flow through an external circuit and provide power. Oxygen is provided to the cathode and it combines with the electrons the hydrogen ions to produce water. The following are the reactions at the electrodes:-
Anode reaction:- 2H2(g) 4H+ + 4e-
Cathode reaction:- O2(g) + 4H+ + 4e- 2H2O
Overall reaction:- 2H2 + O2 2H2O + energy
PEMFCs generate more power for a given volume or weight of fuel cell compared to other types of fuel cells. As a result of its high power density characteristic makes them compact and lightweight. The efficiency is about 40% to 50%. Since the electrolyte used in PEMFCs is solid and not liquid the sealing of the anode and cathode gases is simpler with a solid electrolyte hence, it is less expensive to manufacture. On the other hand, a disadvantage of the PEMFC for some application is its low operating temperature. Temperatures near 1000c seem not high enough to perform useful cogeneration. Furthermore, since it is required for the electrolyte to be saturated with water to operate optimally, careful control of the moisture of the anode and cathode stream is important. Finally, since PEMFC require pure hydrogen to operate as they are very susceptible to poisoning by carbon monoxide and other impurities this is another drawback of PEMFC. This is as a result of low operating temperature of the cell which necessitates the use of a highly sensitive catalyst. PEMFC are basically used for transportation applications and some stationary applications.
Solid Oxide Fuel Cell
Solid oxide fuel cell (SOFCs) use a hard, non- porous ceramic electrolyte such as zirconium oxide stabilized with yttrium oxide. Solid oxides fuel cells usually operate at very high temperatures about 800-10000c. In order to operate at such high temperatures, the electrolyte is thin, solid ceramic material that is quite conductive to oxygen ions (O2-). Since its operation is at high temperatures it does not need a precious metal catalyst hence, reducing cost. It also allow SOFCs to reform fuels internally so that they can use a variety of fuels and thereby reducing cost associated with adding a reformer to the system. The energy is produce by the migration of oxygen anions from the cathode to the anode to oxidize the fuel gas, which is a mixture of hydrogen and carbon monoxide. The oxygen ion (O2-) is the charge carrier in the SOFC. At the cathode, the oxygen molecules from the air are split into oxygen ions and four electrons. The oxygen ions are then conducted through the electrolyte and combine with hydrogen at the anode, releasing four electrons. The electrons that are generated at the anode move via an external circuit back to the cathode where they reduce the incoming oxygen hence, completing the cycle and producing electric power and by- product heat. The reactions are as follows:-
Anode reaction:- 2H2(g) +2O2- 2H2O + 4e-
Cathode reaction:- O2(g) + 4e- 2O2-
Overall reaction:- 2H2(g) + O2(g) 2H2O
Efficiency is around 50%-60% converting fuel to electricity. However, in application designed to capture and utilized the waste heat the overall fuel use efficiencies could be between the range 80%- 85%. SOFCs are sulphur resistant fuel type and are able to cope with several orders of magnitude more of sulphur than other cell types. Furthermore, they are not poisoned by carbon monoxide(CO), as a result carbon monoxide can be used as fuel. This property allows SOFCs to use gases made from coal. These cells are more stable than MCFCs due to the presence of the solid electrolyte but the construction materials needed to contain the high temperatures generated tend to be more expensive. SOFCs have both advantages and disadvantages. The high temperature allow them to cope with impure fuels, such as these obtained from the gasification of coal or gases from industrial process.
On other hand, the high temperatures usually require more expensive materials of construction. Also, the high temperature limits applications of SOFC units and they tend to be rather large. While solid electrolytes cannot leak, but they can crack. Furthermore, the high temperature results in a slow start up and requires significant thermal shielding to retain heat and protect personnel, which may be acceptable for utility applications but not for transportation and small portable application. SOFCs are expected to be used for generating electricity and heat in industry and providing auxiliary power in vehicles.
Diagram of Solid Oxide Fuel Cell taken from:- Author unknown, FCT Fuel Cells- Types of fuel cells [Online] (Update Unknown) Available at: - http://www.eere.energy.gov/hydrogen and fuel cells/ fuel cells/ fc_types.html [Accessed 23rd March, 2010].
Phosphoric- Acid Fuel Cell (PAFC)
Phosphoric- acid fuels were developed in the mid-1960s and field- tested since the 1970s, they have improved significantly in stability, performance and cost. Phosphoric acid fuel cells use liquid phosphoric acid as an electrolyte. The acid is usually contained in a Teflon- bonded silicon carbide matrix. The carbon electrodes which are porous contain a platinum catalyst. Phosphoric acid cells work at slightly higher temperature range than PEMFCs or AFCs approximately 1500c- 2000c. Hydrogen ion (H+) is the charge carrier in this type of fuel cell. The procedure is quite similar to the PEMFC where the hydrogen introduced at the anode is split into its protons and electrons. The protons will then migrate through the electrolyte and combine with the oxygen at the cathode to form water. The electrons are routed through an external circuit where they can perform useful work. Electricity and by- product heat are produced. The reactions are as follows:-
Anode reaction:- 2H2(g) 4H+ + 4e-
Cathode reaction:- O2(g) + 4H+ + 4e- 2H2O
Overall reaction:- 2H2 + O2 2H2O
PAFCs are more tolerant of impurities in fossil fuels that have been reformed into hydrogen than PEMFC, which are poisoned easily by carbon monoxide since carbon monoxide binds to the platinum catalyst at the anode thereby decreasing the fuel cellâ€™s efficiency.
However, they are 85% efficient when used for the cogeneration of electricity and heat but it is less efficient at generating electricity alone (37%- 42%) which is lower than that of other fuel cell systems and these systems also take longer time to warm up than PEMFC. Despite these disadvantages, there are also many advantages of PAFC which include simple construction, stability and low electrolyte volatility. In addition, PAFCs are less powerful than other fuel cells, given the same weight and volume. As a result these fuel cells are large and heavy. PAFCs are also expensive PAFCs require an expensive platinum catalyst which raises the cost of the fuel cell like PEMFC. This type of fuel cell is used for stationary power generator, but some PAFCs have been used to power hospitals, schools, small power stations and large vehicles such as city buses.
Direct- Methanol Fuel Cell (DMFC)
Direct methanol fuel cell is very similar to the polymer exchange membrane fuel cell in that the electrolyte is a polymer and hydrogen ion is the charge carrier. The liquid methanol (CH3OH) is oxidized in the presence of water at the anode generating carbon dioxide, hydrogen ions and the electrons travel through the external circuit as the electric output of the fuel cell. Through the electrolyte the hydrogen ions travel and react with oxygen and the electrons from the external circuit to form water at the anode completing the circuit. The reactions are as follows:-
Anode reaction: - CH3OH + H2O CO2 + 6H +6e-
Cathode reaction: - 3\2 O2 + 6H+ + 6e- 3H2O
Overall reaction: - CH3Oh + 3\2O2 CO2 + 2H2O
These cells are operated at around 1200c and usually give an efficiency of around 40%. The low temperature and no requirement of a fuel reformer make the DMFC an excellent fuel cell for very small to mid-sized applications, such as cellular phones, laptop computers and other consumer products, up to automobile power plants. One of the drawbacks of the DMFC is that the low- temperature oxidation of methanol to hydrogen ions and carbon dioxide needs a large quantity of platinum catalyst than in conventional PEMFCs. Therefore the cost is increased, however, expected to be more than outweighed by the convenience of using a liquid fuel and the ability to function without a reforming unit.
Zinc Air Fuel Cell (ZAFC)
Zinc- air fuel cells have similar characteristics with a number of the other types of fuel cells as well as some characteristics of batteries. The electrolyte for a ZAFC is a ceramic solid and the charge carrier is the hydroxyl ion, (OH-). The ZAFC operates at 7000c to achieve high electrical fuel efficiency with hydrocarbon fuels and high electrolyte conductivity for the charge carrier. The anode which is made of zinc and is supplied with hydrogen or even hydrocarbons. The cathode electrode is separated from the air supply with a gas diffusion electrode (GDE), which is a permeable membrane that allows atmospheric oxygen to pass through. The oxygen reacts with hydrogen to form hydroxyl ions and water at the cathode. The reactions are as follows:-
Anode reaction: - CH4 + H2O CO2 + 6H+ +6e-
Zn + OH- ZnO + H+ + e-
Cathode reaction: - O2 +2H+ + 2e- 2OH-
O2+ 4H+ + 4e- 2H2O
Overall reaction: - CH4 + 2O2 CO2 + 2H2O
ZAFCs usually contain a zinc fuel tank and a zinc refrigerator that automatically and silently regenerates the fuel. ZAFC operates at high temperatures which enables internal reforming of hydrocarbons, eliminating the need for an external reformer to generate hydrogen. Another advantage of the high operating temperature is that the by- product heat can be used to produce high- pressure steam that is useful in many industrial and commercial applications.
The electrolyte that ZAFC uses has some advantages over other electrolytes such as it does not require water saturation as does the polymer membrane of the PEMFFs and hence, cannot dry out, eliminating the need to carefully control anode and cathode moisture levels. Since it is a solid there is no leakage unlike the liquid electrolytes. On the other hand, due to the consumption of the zinc anode, it requires replacing at intervals.
Regenerative Fuel Cells (RFC)
The regenerative fuel cell concept is new but it is being research by a number of groups worldwide. Regenerative fuel cell (RFC) is a system that can operate in a closed loop. It produces electricity from hydrogen and oxygen and thereby generating heat and water as by- products similar to other fuel cells. The only difference is that the regenerative cell also performs the reverse reaction, electrolysis. Solar- powered electrolyser separate water into hydrogen and oxygen. The hydrogen and oxygen then enter the fuel cell which generates electricity, heat and water. The water is then recirculated back to the solar powered electrolyser and the process begin all over again. In addition, regenerative fuel cell systems can also use electricity from solar power or other sources to divide the excess water into oxygen and hydrogen fuel by the process of electrolysis.
Taken from:- Disque, Anna and Harris, Rebecca; Regenerative Fuel Cells [Online] (Update Unknown). Available at: www.btownccs.k12.in.us/bchs/science/physical/titp/â€¦ [Accessed 13th April, 2010].
Protonic Ceramic Fuel cell (PCFC)
Protonic ceramic fuel cells are new type of fuel cells. It uses a ceramic electrolyte material that possesses a high protonic conductivity at very high temperatures. Protonic ceramic fuel cells operate at high temperature of 7000c and share both the thermal and kinetic advantages of high temperatures like with molten carbonate and solid oxide fuel cells and it also demonstrate excellent benefits of proton conduction as in proton exchange membrane fuel cells and phosphoric acid fuel cells. To achieve very high electrical fuel efficiency with hydrocarbon fuels the operating temperature is necessary. Protonic ceramic fuel cells can operate at high temperatures and electrochemically oxidize fossil fuels directly to the anode. This eliminates the intermediate step of producing hydrogen through the costly reforming process. The surface of anode in the presence of water vapour absorb gaseous molecules of the hydrogen fuel and hydrogen atoms are efficiently stripped off to be absorbed into electrolyte, with carbon dioxide as the primary reaction product. The solid electrolyte of protonic ceramic fuel cells cannot dry out like the electrolyte present in proton exchange membrane fuel cells or it cannot leak out like the electrolyte present in phosphoric acid fuel cell.
Microbial Fuel Cell
Microbial fuel cell or biological fuels use the catalytic reaction of microorganisms such as bacteria to convert virtually any organic material into fuel. Glucose, acetate and wastewater are some of the common compounds used. MFC is a recent development which was due to the factors that affect optimum operation such as the bacteria used in the system and the type of ion membrane etc. Bacteria in mediator- less MFCs have electrochemically active redox enzymes such as cytochromes on their outer membrane that can transfer electrons to external materials. Microbial fuel cell usually consists of anode and cathode compartment which is separated by a cation specific membrane. At the anode compartment, fuel is oxidized by microorganisms, generating electrons and protons. Electrons move to the cathode compartment through an external electric circuit and the proton are usually transferred to the cathode compartment through the membrane. Electrons and protons are consumed in the cathode compartment, combining with oxygen to form water. There are two types of microbial fuel cells: mediator and mediator- less microbial fuel cells. Microbial cells are electrochemically inactive. The electron transfer from microbial cells to the electrode is facilitated by mediators such as thionine, methyl blue etc. A mediator is not required by mediator- less microbial fuel cell but it uses electrochemically active bacteria to transfer electrons to the electrode.
The above information was taken from the following sources:-
Author unknown, FCT Fuel Cells- Types of fuel cells [Online] (Update Unknown) Available at: - http://www.eere.energy.gov/hydrogen and fuel cells/ fuel cells/ fc_types.html [Accessed 23rd March, 2010].
Author unknown, Types- Alkaline Fuel Cells [Online] (Update Unknown) Available at: - www.fctec.com/fctec_types_afc.asp [Accessed 23rd March, 2010].
Author unknown, Fuel cells [Online] (Update Unknown) Available at: - www.fuel cells.org/basics/how.html [Accessed 5th April, 2010].
Author unknown, Protonic Ceramic Fuel Cells [Online] (Last Updated 6th March, 2006) Available at: - www.cogeneration.net/protonic_ceramics_fuel_cells.html. [Accessed 13th April, 2010].
Explanation of the process involved in the Polymer Exchange Membrane Fuel Cell (PEMFC)
In the diagram above there are four basic elements of a PEMFC. These include anode, cathode, electrolyte and catalyst. The work of the anode is to conduct the electrons that are freed from the hydrogen molecules so that they can be used in an external circuit. There are channels etched into it that simply disperse the hydrogen gas equally over the surface of the catalyst.
The cathode also contains channels etched into it that easily distribute the oxygen to the surface of the catalyst. Another duty of the cathode is that it conducts the electrons back from the external circuit to the catalyst, where they can recombine with the hydrogen ions and oxygen to form water. The electrolyte which is a proton exchange membrane conducts positively charged ions and it usually stops electrons. The membrane is usually hydrated in order to function and remain stable. The catalyst is a special material that facilitates the reaction of oxygen and hydrogen. The catalyst is made from platinum nanoparticles very thinly coated onto carbon paper. It is rough and porous so that the maximum surface area of the platinum can be exposed directly to the hydrogen or oxygen. The platinum- coated side of the catalyst faces the polymer exchange membrane. However, the pressurized hydrogen gas enters the fuel cell on anode side. The pressure forces the gas through the catalyst. When a hydrogen molecule comes in contact with the platinum on the catalyst, it splits into two H+ ions and two electrons. The electrons are therefore conducted through the anode, where they make their way through the external circuit and return to the cathode side of the fuel cell. However, on the cathode side of the fuel cell, oxygen gas is forced through the catalyst, where it forms two oxygen atoms. Both atoms have a strong negative charge. As a result of the negative charge it attracts the two H+ ions through the membrane, where they combine with an oxygen atom and two of the electrons from the external circuit to form water molecule. This reaction in a single fuel cell produces approximately 0.7 volts. To increase this voltage to reasonable level separate fuel cells must be combined to form a fuel- cell stack thereby producing a cell output range from 50 to 250 kilowatts.
Present Applications of Polymer Exchange Membrane Fuel Cells
The polymer exchange membrane system are used in many different applications however, it can be placed into three main groups: Transportation (including niche applications, light duty markets and buses), stationary (large and small applications) and portable. The majority of PEM units are being used for portable applications. Since the portable markets tend to be the most developed than the others. Secondly, it is widely used in the small stationary units than in the transportation applications.
In summary, PEMFCs are used in transportation, powered buses, electric powered bicycle and lightweight vehicles, powered leisure yachts, stationary applications, stationary power system, UPS system in mobile phone station, portable communication and portable computers.
Projected Trends of Polymer Exchange Membrane Fuel Cells
The level of interest in PEM technology is likely to grow and strengthen in future as more high profile companies begin to provide hydrogen storage solutions. Based on studies, in terms of stationary applications the outlook for PEM technology is also promising. The power sectors which favour the use of a PEM system are enjoying a certain amount of growth currently. The transportation market is likely to provide a tremendous long term potential for PEM fuel cell units. The future for PEM fuel systems is quite promising as the technology can be utilized in several key market sectors which expand growth opportunities over the short, medium and long term time span. There are still challenges to overcome with the new and evolving marketplace. Like the prices of PEM units are very expensive and it arise challenges for many producers. In the future for PEM fuel cells is dynamic and evolving.
The above information was taken from the following sources:-
Author unknown, How fuel cell work [Online] (Update Unknown) Available at: - www.auto.howstuff works.com [Accessed 5th April, 2010].
Crawley Gemma, Proton Exchange Membrane (PEM) fuel cells [Online] (Last Updated March, 2006) Available at: - www.fuelcelltoday.com [Accessed 13th April, 2010].