Fuel Cell Technology - DOC

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					Fuel Cell Technology


As world needs today, is a fuel which is having more efficiency and can be available easily at low cost. So the best solution is a fuel cell ,as it is having a vast scope and it can be constructed by many ways . In this paper we are going to focus out attention of the developments that are done by using fuel cell and more importance over the development that can be done. We have included the history of the fuel cell as it is important to get the knowledge, when it was developed. Along with the history we are also including the both aspects of cell, that is positive and negative. The negative aspects are the one in which the fuel cells are lagging. We are also giving the latest developments of fuel cell. Fuel cell are developed by many ways, but the world is ignoring the use of fuel cells as they are unknown to fuel cells. So special attention towards various ways for development of fuel cell is included in this paper. This paper also includes the various number of fuel cells with their special features and applications in various fields of technology. The major advantage of using a fuel cell is its reduced pollution. Today the need is pollution free environment even though it is not possible, but using fuel cell technology we can do so since fuel cell gives byproduct as water. We have given more stress on the pollution aspect in this paper. In this paper we also include the operation and principle of fuel cell.


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The principle of the fuel cell was discovered by German scientist Christian Friedrich Schönbein in 1838 and published in the January 1839 edition of the "Philosophical Magazine".[5] Based on this work, the first fuel cell was developed by Welsh scientist Sir William Robert Grove in 1843. The fuel cell he made, used similar materials to today's phosphoric-acid fuel cell. It wasn't until 1959 that British engineer Francis Thomas Bacon successfully developed a 5 kW stationary 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 US 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. UTC's Power subsidiary was the first company to manufacture and commercialize a large, stationary fuel cell system for use as a co-generation power plant in hospitals, universities and large office buildings. UTC Power continues to market this fuel cell as the PureCell 200, a 200 kW system. UTC Power continues to be the sole supplier of fuel cells to NASA for use in space vehicles, having supplied the Apollo missions and currently the Space Shuttle program.

Research and development

August 2005: Georgia Institute of Technology researchers use triazole to raise the operating temperature of PEM fuel cells from below 100 °C to over 120 °C, claiming this will require less carbon-monoxide purification of the hydrogen fuel.[17]


September 2005: Technical University of Denmark (DTU) scientists announced in September 2005 a method of storing hydrogen in the form of ammonia saturated into a salt tablet. They claim it will be an inexpensive and safe storage method.[18]


January 2006: Virent Energy Systems is working on developing a low cost method[19] for producing hydrogen on demand - from certain sugar/water mixtures (using one of glycerol, sorbitol, or hydrogenated glucose derivatives). Such a technology, if successful would solve many of the infrastructure (hydrogen storage) issues associated with the hydrogen economy[20].

In the archetypal example of a hydrogen/oxygen proton exchange membrane fuel cell (PEMFC), a protonconducting polymer membrane, (the electrolyte), separates the anode and cathode sides. On the anode side, hydrogen diffuses to the anode catalyst where it later dissociates into protons and electrons. The protons are conducted through the membrane to the cathode, but the electrons are forced to 3

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 this example, the only waste product is water vapor and/or liquid water. In addition to pure hydrogen, there are hydrocarbon fuels for fuel cells, including diesel, methanol (see: direct-methanol fuel cells) and chemical hydrides. The waste products with these types of fuel are carbon dioxide and water. The materials used in fuel cells differ by type. The electrode/bipolar plates are usually made of metal, nickel or carbon nanotubes, and are coated with a catalyst (like platinum, nano iron powders or palladium) for higher efficiency. Carbon paper separates them from the electrolyte. The electrolyte could be ceramic or a membrane. A typical fuel cell produces about 0.86 volt. To create enough voltage, the cells are layered and combined in series and parallel circuits to form a fuel cell stack. The number of cells used is usually greater than 45 but varies with design.

The efficiency of a fuel is very dependent on the current through the fuel cell: as a general rule, the more current drawn, the lower the efficiency(This is because as current increases I2R loses increases). A cell running at 0.6V has an efficiency of about 50%, meaning that 50% of the available energy content of the hydrogen is converted into electrical energy; the remaining 50% will be converted into heat. For a hydrogen cell the second law efficiency is equal to cell voltage divided by 1.23, when operating at standard conditions. This voltage varies with fuel used, and quality and temperature of the cell. The difference between enthalpy and Gibbs free energy (that cannot be recovered) will also appear as heat.

Some facts
Fuel cells cannot store energy like a battery, but in some applications, such as stand-alone power plants based on discontinuous sources such as solar or wind power, they are combined with electrolyzers and storage systems to form an energy storage system. The overall efficiency (electricity to hydrogen and back to electricity) of such plants (known as round-trip efficiency) is between 30 and 50%, depending on conditions.[9] While a much cheaper lead-acid battery might return about 90%, the electrolyzer/fuel cell system can store indefinite quantities of hydrogen, and is therefore better suited for long-term storage. Solid-oxide fuel cells produce exothermic heat from the recombination of the oxygen and hydrogen. The ceramic can run as hot as 800 degrees Celsius. This heat can be captured and used to heat water in a 4

combined heat and power (CHP) application. When the heat is captured, total efficiency can reach 80-90%. CHP units are being developed today for the European home market.

Fuel cell applications
Fuel cells are very useful as power sources in remote locations, such as spacecraft, remote weather stations, large parks, rural locations, and in certain military applications. A fuel cell system running on hydrogen can be compact, lightweight and has no major moving parts. Because fuel cells have no moving parts, and do not involve combustion, in ideal conditions they can achieve up to 99.9999% reliability[10]. This equates to less than one minute of down time in a six year period. A new application is micro combined heat and power, which is cogeneration for family home, office buildings and factories. This type of system generates constant electric power (selling excess power back to the grid when it is not consumed), and at the same time produce hot air and water from the waste heat. A lower fuel-to-electricity conversion efficiency is tolerated (typically 15-20%), because most of the energy not converted into electricity is utilized as heat. Some heat is lost with the exhaust gas just as in a normal furnace, so the combined heat and power efficiency is still lower than 100%, typically around 80%. In terms of energy however, the process is inefficient, and one could do better by maximizing the electricity generated and then using the electricity to drive a heat pump. Phosphoric-acid fuel cells (PAFC) comprise the largest segment of existing CHP products worldwide and can provide combined efficiencies close to 80% (45-50% electric + remainder as thermal). UTC Power is currently the world's largest manufacturer of PAFC fuel cells. Molten-carbonate fuel cells have also been installed in these applications, and solid-oxide fuel cell prototypes exist. However, since electrolyzer systems do not store fuel in themselves, but rather rely on external storage units, they can be successfully applied in large-scale energy storage, rural areas being one example. In this application, batteries would have to be largely oversized to meet the storage demand, but fuel cells only need a larger storage unit (typically cheaper than an electrochemical device).


Types of fuel cells
Fuel Name Cell Electrolyte Qualified Working Power (W) Metal Aqueous alkaline ? Temperature (°C) above -20 ? Commercial/Research Electrical Status efficiency

hydride fuel solution cell (e.g.potassium hydroxide) Electrogalvanic fuel cell Aqueous alkaline ? solution potassium hydroxide) Zinc-air battery Aqueous alkaline ? solution potassium hydroxide) Microbial fuel cell Polymer membrane humic acid Reversible fuel cell Polymer membrane (ionomer) Direct Aqueous alkaline ? (e.g., ? or ? (e.g., (e.g.,

50%Ppeak @ 0

under 40



under 40


Mass production

under 40



under 50






borohydride solution fuel cell Alkaline fuel cell

sodium hydroxide) Aqueous alkaline 10 kW to under 80 solution potassium hydroxide) (e.g., 100 kW Cell: 60– Commercial/Research 70% System: 62% 1mW 100 kW to 90–120 Cell: 20– Commercial/Research 30% System: 10–20%

Direct methanol fuel cell

Polymer membrane (ionomer)


Reformed methanol fuel cell

Polymer membrane (ionomer)

5W 100 kW

to (Reformer)250– Cell: 50– Commercial/Research 300 (PBI)125–200 60% System: 25–40%



up to 140 Above mW/cm² ? 90–120

25 ?


ethanol fuel membrane cell (ionomer)

Formic acid Polymer fuel cell membrane (ionomer) Proton exchange membrane fuel cell Polymer membrane (ionomer) Nafion® (e.g., or





100W to (Nafion)70–120 Cell: 50– Commercial/Research 500 kW (PBI)125–200 70% System: 30–50%

Polybenzimidazole fiber) RFC Redox - Liquid electrolytes 1 kW to ? with redox shuttle 10MW & membrane (Ionomer) Phosphoric Molten phosphoric up 10MW to 150-200 Cell: 55% Commercial/Research System: 40% Molten carbonate fuel cell Molten carbonate sodium bicarbonate NaHCO3) Protonic H+-conducting ? 700 ? Research alkaline 100MW (e.g., 600-650 Cell: 55% Commercial/Research System: 47% polymer ? Research

acid fuel cell acid (H3PO4)

ceramic fuel ceramic oxide cell Direct carbon fuel cell Several different ? 700-850 Cell: 80% Commercial/Research System: 70%


Solid oxide O2--conducting fuel cell ceramic (e.g.,


to 700–1000

Cell: 60– Commercial/Research 65% System: 55–60%

oxide 100MW zirconium

dioxide, ZrO2)

Description of some of the above Fuel Cell:Zinc-air battery
Zinc-air batteries, also called "zinc-air fuel cells" are a non-rechargeable electro-chemical battery powered by the oxidation of zinc with oxygen from the air. These batteries have very high energy densities and are relatively inexpensive to produce. These batteries are used in hearing aids and in experimental electric vehicles. They may be an important part of a future zinc economy. Zinc-air batteries have properties of fuel cells as well as batteries: the zinc is the fuel; the rate of the reaction can be controlled by controlling the air flow; and used zinc/electrolyte paste can be removed from the cell and replaced with fresh paste. Research is being conducted in powering electric vehicles with zinc-air batteries.

Properties of Zinc-Air Battery

Zinc-air batteries have very high specific energy compared to other batteries (110 to 200 W·h/kg or 400 to 720 kJ/kg)


Zinc-air batteries put out continuous energy as they dissipate their energy, and the voltage does not drop until the battery is over 80-85% depleted.

 

Zinc-air batteries have very long shelf lives, as long as they are sealed (no oxygen is let in) Zinc-air batteries have a very high self-discharge rate when exposed to air, as the zinc will spontaneously react with oxygen, and the water catalyst in the battery will tend to dry out.


To prevent self-discharging the battery has to be resealed when not in use. Moisture in the battery can be maintained with use of a humidified environment.

o  

Zinc-air batteries must not be over saturated with water, though. Avoid immersing in water! Zinc-air batteries use cheap materials and can be produced in mass quantities inexpensively. Zinc-air batteries are not electrically rechargeable, but the zinc can be recycled or “mechanically recharged” in which the zinc oxide from the used batteries can be smelted back into zinc metal (and thus reduced) and remixed with recycled electrolyte.


Microbial fuel cell
A microbial fuel cell (MFC) or biological fuel cell is a device in which micro-organisms oxidize compounds such as glucose, acetate or wastewater. The electrons gained from this oxidation are transferred towards an electrode, called the anode. From the anode, the electrons depart through an electrical circuit towards a second electrode, the cathode. At the cathode, the electrons are transferred towards a high potential electron acceptor, preferrably oxygen. As current now flows over a potential difference, power is generated as a result of bacterial activity. The power outputs reported thus far are usually small, in the order of magnitude of about a milliwatt. While no commercially available applications exist at the moment, this is expected to change in the coming years. Mainly electricity generation out of wastewater is one of the main focuses at the moment. Also glucosepowered pacemakers that would need no other power supply than the glucose present in the bloodstream, bio-sensors, nutrient removal systems, ... are under development.

A microbial fuel cell is a device that converts chemical energy to electrical energy by the catalytic reaction of microorganisms. A typical microbial fuel cell consists of anode and cathode compartments separated by a cation specific membrane. In the anode compartment, fuel is oxidized by microorganisms, generating electrons and protons. Electrons are transferred to the cathode compartment through an external electric circuit, and the protons are transferred to the cathode compartment through the membrane. Electrons and protons are consumed in the cathode compartment, combining with oxygen to form water. In general, there are two types of microbial fuel cell, mediator and mediator-less microbial fuel cell. Biological fuel cells take glucose and methanol from food scraps and convert it into hydrogen and food for the bacteria.

Further Uses
The electricity from the fuel cells could be harnessed for use by applications such as EcoBots, Gastrobots and Biosensors Since the current generated from a microbial fuel cell is directly proportional to the strength of wastewater used as the fuel, an MFC can be used to measure the strength of wastewater [Kim, B. H., Chang, I. S., Gil, G. C., Park, H. S. and Kim, H. J. (2003) Novel BOD (biological oxygen demand) sensor using mediator-less microbial fuel cell. Biotechnology Letters, 25, 541-545.] The strength of wastewater is commonly evaluated as biochemical oxygen demand (BOD) values. BOD values are determiined incubating samples for 5 days with proper source of microbes, usually activate sludge collected from sewage works. When BOD values are used as a real time control parameter, 5 days' incubation is too long. An MFC-type BOD sensor can be used to measure real time BOD values. Oxygen and nitrate are preferred electron acceptors over the electrode reducing current generation from an MFC. An MFC-type BOD sensors underestimate BOD values in the presence of these electron acceptors. This can be avoided inhibiting aerobic and nitrate respirations in the MFC using terminal oxydase inhibitors such as cyanide and azide [Chang, I. S., Moon, H., Jang, J. K. and 9

Kim, B. H. (2005) Improvement of a microbial fuel cell performance as a BOD sensor using respiratory inhibitors. Biosensors and Bioelectronics 20, 1856-1859.] This type of BOD sensor is commercially available.

Direct methanol fuel cell
Direct-methanol fuel cells or DMFCs are a subcategory of Proton-exchange fuel cells where, the fuel, methanol, is not reformed, but fed directly to the fuel cell.

Because methanol is fed directly into the fuel cell, complicated catalytic reforming is unneeded, and storage of methanol is much easier than that of hydrogen because it does not need to be done at high pressures and (or) low temperatures, as methanol is a liquid. The energy density of methanol (the amount of energy released by using a given volume of methanol) is orders of magnitude greater than even highly compressed hydrogen.

Direct-ethanol fuel cells
Direct-ethanol fuel cells or DEFCs are a subcategory of Proton-exchange fuel cells where, the fuel, ethanol, is not reformed, but fed directly to the fuel cell.


DEFC uses Ethanol in the fuel cell instead of the more toxic methanol. Ethanol is an attractive alternative to methanol because it comes with a supply chain that's already in place. Ethanol also remains the easier fuel to work with for widespread use by consumers. Because ethanol is fed directly into the fuel cell, complicated catalytic reforming is unneeded, and storage of ethanol is much easier than that of hydrogen because it does not need to be done at high pressures, as ethanol is a liquid. The energy density of ethanol (the amount of energy released by using a given volume of ethanol) is orders of magnitude greater than even highly compressed hydrogen. Ethanol is a hydrogen-rich liquid and it has a higher energy density (8.0 kWh/kg) compared to methanol (6.1 kWh/kg). Ethanol can be obtained in great quantity from biomass through a fermentation process from renewable resources like from sugar cane, wheat, corn, or even straw. Bio-generated ethanol (or bio-ethanol) is thus attractive since it will not change the natural balance of carbon dioxide in the atmosphere. This is in sharp contrast to the use of fossil fuels. The use of ethanol would also overcome both the storage and infrastructure challenge of hydrogen for fuel cell applications. In a fuel cell, the oxidation of any fuel requires the use of a catalyst in order to achieve the current densities required for commercially viable fuel cells, and platinum-based catalysts are some of the most efficient materials for the oxidation of small organic molecules.

Proton exchange membrane fuel cell PEM Fuel Cell


Polymer electrolyte membrane
To function, the membrane must conduct hydrogen ions (protons) but not electrons as this would in effect "short circuit" the fuel cell. The membrane must also not allow either gas to pass to the other side of the cell, a problem known as gas crossover. Finally, the membrane must be resistant to the reducing environment at the anode as well as the harsh oxidative environment at the cathode. Unfortunately, while the splitting of the hydrogen molecule is relatively easy by using a platinum catalyst, splitting the stronger oxygen molecule is more difficult, and this causes significant electric losses. An appropriate catalyst material for this process has not been discovered, and platinum is the best option. Another significant source of losses is the resistance of the membrane to proton flow, which is minimized by making it as thin as possible, on the order of 50 μm. The PEMFC is a prime candidate for vehicle and other mobile applications of all sizes down to mobile phones, because of its compactness. However, the water management is crucial to performance: too much water will flood the membrane, too little will dry it; in both cases, power output will drop. Water management is a very difficult subject in PEM systems. A wide variety of solutions for the managing water exsist including integration of electroosmotic pumps. Furthermore, the platinum catalyst on the membrane is easily poisoned by carbon monoxide (no more than one part per million is usually acceptable) and the membrane is sensitive to things like metal ions, which can be introduced by corrosion of metallic bipolar plates. PEM systems that use reformed methanol were proposed, as in Daimler Chrysler Necar 5; reforming methanol, i.e. making it react to obtain hydrogen, is however a very complicated process, that requires also purification from the carbon monoxide the reaction produces. A platinum-ruthenium catalyst is necessary as some carbon monoxide will unavoidably reach the membrane. The level should not exceed 10 parts per million. Furthermore, the start-up times of such a reformer reactor are of about half an hour. Alternatively, methanol, and some other biofuels can be fed to a PEM fuel cell directly without being reformed, thus making a direct methanol fuel cell (DMFC). These devices operate with limited success. The most commonly used membrane is Nafion® by DuPont®, which relies on liquid water humidification of the membrane to transport protons. This implies that it is not feasible to use temperatures above 80–90˚C, since the membrane would dry. Other, more recent membrane types, based on Polybenzimidazole (PBI) OR phosphoric acid, can reach up to 220˚C without using any water management: higher temperature allow for better efficiencies, power densities, ease of cooling (because of larger allowable temperature differences), reduced sensitivity to carbon monoxide poisoning and better controllability (because of absence of water management issues in the membrane); however, these recent types are not as common and most research


labs and papers still use Nafion. Companies producing PBI membranes include Celanese and PEMEAS, and there is an EU research project regarding these membranes. Efficiencies of PEMs are in the range of 40-50%.


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References:1) 2) 3) 4) 5)