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Battery and Fuel Cell

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BATTERY &  FUEL CELL

       The fuel cell is an electrical energy producer. It takes a fuel ,say methanol and leads into the oxidizing anode and at the counter-cathode , oxygen in air is reduced. The free energy of oxidation directly comes out as electrical energy. So fuel cell may be called electrochemical electricity producer.

       Batteries, on the other hand, must have electricity produced elsewhere ( during  charging by  an external electricity produced by  fossil fuels in thermal power). The battery receives this electricity which drives a reaction on each of the two electrodes , up a free energy gradient for the overall reaction. The "charged" battery can then deliver the stored electricity to release this energy downhill in a spontaneous reaction.

   So batteries store electricity and fuel cell produce it. Fuel cells produce electricity with no polluting effluents whatsoever. But with batteries , the problems of pollution are there from the combustion of coal or oil used to make electricity with which to charge them. Fuel cells can give uninterrupted energy as long as fuel is there. Batteries give energy during discharging and the cycles of charging and discharging are limited to a life period.

Types of fuel cells

Fuel cells are classified primarily by the kind of electrolyte they employ. This determines the kind of chemical reactions that take place in the cell, the kind of catalysts required, the temperature range in which the cell operates, the fuel required, and other factors. These characteristics, in turn, affect the applications for which these cells are most suitable

Direct Methanol Fuel Cell

Most fuel cells are powered by hydrogen, which can be fed to the fuel cell system directly or can be generated within the fuel cell system by reforming hydrogen-rich fuels such as methanol, ethanol, and hydrocarbon fuels. Direct methanol fuel cells (DMFCs), however, are powered by pure methanol, which is mixed with steam and fed directly to the fuel cell anode.

Direct methanol fuel cells do not have many of the fuel storage problems typical of some fuel cells since methanol has a higher energy density than hydrogen—though less than gasoline or diesel fuel. Methanol is also easier to transport and supply to the public using our current infrastructure since it is a liquid, like gasoline.

Methanol Fuel cell (DMFC)

Anode Reaction :    CH3OH(l) + H2O(l) .= CO2(g) + 6H+ + 6e-,                 

Ea = -0.016V (1)

Cathode Reaction : 1.5O2(g) + 6H+ + 6e- = 3H2O(l),                              

 Ec = 1.229V (2)

Overall Reaction :    CH3OH(l) + 1.5O2(g) . 2H2O(l) + CO2(g)             

  E. = 1.213V (3)

 Ethanol Fuel cell (DEFC)

Anode reaction:    CH3CH2OH + 3H2O ®. 2CO2 +12H+ +12e-    

(Ea = 0.084Vversus SHE)

Cathode reaction:   3O2 +12H+ +12e-.® 3H2O                           

(Ec = 1.229Vversus SHE)

Overall reaction: CH3CH2OH + 3O2.® 2CO2 +3H2O                  (Ecell = 1.145 V)

Notes Fuel cell

Direct methanol fuel cells require a solution of methanol in water, if the concentration of methanol is too high you will damage the membrane in the fuel cell. Manufacturers recommend that you work with concentrations that do not exceed 3%. In practice, this is often a little overcautious, and concentrations around 4% can work successfully. However, note that concentrations much higher than this will certainly damage your fuel cell, and concentrations over 3% are not recommended for prolonged use.

 

Microbial Fuel cell

When micro-organisms consume a substrate such as sugar in aerobic conditions they produce carbon dioxide and water. However when oxygen is not present they produce carbon dioxide, protons and electrons

Anode Reaction :    C12H22O11 + 13H2O ---> 12CO2 + 48H+ + 48e-

Cathode Reaction : 12O2(g) + 48H+ + 48e- = 3H2O(l),                    Ec = 1.229V

 Glucose Fuel cell

The electrochemical oxidation of glucose fuel is as follows:

C6H12O6 + 2OH → C6H12O7 + 2e− + H2O, E0an = 0.853 V… (1)

12O2+2e− + H2O → 2OH, E0cath = 0.403 V…. (2)    

Overall:

C6H12O6 + 12 O2 → C6H12O7, E0cell = 1.256 V…. (3)

Energy ΔG= nFE;

= 2 X 23061 X 1.256 = 57929.23 cal= 243.30 K joules/mole=1.35KJ/gm. 

Where n=No of Electrons ,F=Faraday’s constant. E=Cell Potential.

It is seen from above reaction 243KJ energy is produced from 1 mole of glucose oxidation

Alkaline

Alkaline fuel cells (AFCs) were one of the first fuel cell technologies developed, and they were the first type widely used in the U.S. space program to produce electrical energy and water onboard spacecraft. These fuel cells use a solution of potassium hydroxide in water as the electrolyte and can use a variety of non-precious metals as a catalyst at the anode and cathode. High-temperature AFCs operate at temperatures between 100ºC and 250ºC (212ºF and 482ºF). However, more-recent AFC designs operate at lower temperatures of roughly 23ºC to 70ºC (74ºF to 158ºF). 

AFCs are high-performance fuel cells due to the rate at which chemical reactions take place in the cell. They are also very efficient, reaching efficiencies of 60 percent in space applications. 

The disadvantage of this fuel cell type is that it is easily poisoned by carbon dioxide (CO2). In fact, even the small amount of CO2 in the air can affect the cell's operation, making it necessary to purify both the hydrogen and oxygen used in the cell. This purification process is costly. Susceptibility to poisoning also affects the cell's lifetime (the amount of time before it must be replaced), further adding to cost.

Polymer Electrolyte Membrane

Polymer electrolyte membrane (PEM) fuel cells—also called proton exchange membrane fuel cells—deliver high power density and offer the advantages of low weight and volume, compared to other fuel cells. PEM fuel cells use a solid polymer as an electrolyte and porous carbon electrodes containing a platinum catalyst. They need only hydrogen, oxygen from the air, and water to operate and do not require corrosive fluids like some fuel cells. They are typically fueled with pure hydrogen supplied from storage tanks or onboard reformers.

Polymer electrolyte membrane fuel cells operate at relatively low temperatures, around 80°C (176°F). Low temperature operation allows them to start quickly (less warm-up time) and results in less wear on system components, resulting in better durability. However, it requires that a noble-metal catalyst (typically platinum) be used to separate the hydrogen's electrons and protons, adding to system cost. The platinum catalyst is also extremely sensitive to CO poisoning, making it necessary to employ an additional reactor to reduce CO in the fuel gas if the hydrogen is derived from an alcohol or hydrocarbon fuel. This also adds cost. Developers are currently exploring platinum/ruthenium catalysts that are more resistant to CO.

Phosphoric Acid

Phosphoric acid fuel cells use liquid phosphoric acid as an electrolyte—the acid is contained in a Teflon-bonded silicon carbide matrix—and porous carbon electrodes containing a platinum catalyst. The chemical reactions that take place in the cell are shown in the diagram to the right 

The phosphoric acid fuel cell (PAFC) is considered the "first generation" of modern fuel cells. It is one of the most mature cell

types and the first to be used commercially, with over 200 units currently in use. This type of fuel cell is typically used for stationary power generation, but some PAFCs have been used to power large vehicles such as city buses

Molten Carbonate

Molten carbonate fuel cells (MCFCs) are currently being developed for natural gas and coal-based power plants for electrical utility, industrial, and military applications. MCFCs are high-temperature fuel cells that use an electrolyte composed of a molten carbonate salt mixture suspended in a porous, chemically inert ceramic lithium aluminum oxide (LiAlO2) matrix. Since they operate at extremely high temperatures of 650ºC (roughly 1,200ºF) and above, non-precious metals can be used as catalysts at the anode and cathode, reducing costs.Unlike alkaline, phosphoric acid, and polymer electrolyte membrane fuel cells, MCFCs don't require an external reformer to convert more energy-dense fuels to hydrogen. Due to the high temperatures at which they operate, these fuels are converted to hydrogen within the fuel cell itself by a process called internal reforming, which also reduces cost. The high temperatures at which these cells operate and the corrosive electrolyte used accelerate component breakdown and corrosion, decreasing cell life. Scientists are currently exploring corrosion-resistant materials for components as well as fuel cell designs that increase cell life without decreasing performance. 

Solid Oxide

Solid oxide fuel cells (SOFCs) use a hard, non-porous ceramic compound as the electrolyte. Since the electrolyte is a solid, the cells do not have to be constructed in the plate-like configuration typical of other fuel cell types. SOFCs are expected to be around 50-60 percent efficient at converting fuel to electricity. In applications designed to capture and utilize the system's waste heat (co-generation), overall fuel use efficiencies could top 80-85 percent.Solid oxide fuel cells operate at very high temperatures—around 1,000ºC (1,830ºF). High temperature operation removes the need for precious-metal catalyst, thereby reducing cost. It also allows SOFCs to reform fuels internally, which enables the use of a variety of fuels and reduces the cost associated with adding a reformer to the system.

BIOELECTROCHEMICAL FUEL CELL

A bioelectrochemical fuel cell is a device that realizes the direct conversion of biochemical energy into electricity. The driving force of  bioelectrochemical fuel cells is the redox reaction of substrate by microorganisms. There are two types of what can be called a microbial fuel cell. The first type utilizes electroactive metabolites, e.g., hydrogen, converted by microbial metabolism or enzyme reaction in a chemical fuel cell, and the other type utilizes mediators as electron transporters from a certain metabolic pathway of the microorganism or enzyme directly to electrodes

General characteristics of chemical and biological fuel cell

  Chemical Fuel Cell  Biological Fuel Cell
Catalyst Noble Metals  Microorganism / enzyme
pH Acidic Solution (pH<1) Neutral Solution pH 7.0-9.0
Temperature  over 200 ° C Room Temperature 22-25 ° C
Electrolyte Phosphoric-acid Phosphate Solution 
Capacity High Low
Efficiency 40 - 60 % over 40 %
Fuel Type Natural gas, H2, etc. Any Carbohydrates and hydrocarbons

 

 Electrochemical Cell reactions of  different Fuel cells

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Development of High Energetic Electrodes Fuel Cell

The main science and engineering for fuel cell development is to produce high electro catalytic energy materials. Because fuels gets oxidized electrochemically  on the substrate of the these electrode materials, releasing electron at a fast rate that gives rise to current and energy. details that Precious group metals and alloys such as Pt,Pt/C, Pt-Ru, Pd, Pt-Ru-Pd, Pt–Sn/SnO2, act as very good electrodes for fuel cell. But these precious metals are very expensive that hinders the developments of fuel cell to the next generation for producing energy at an economical rate at domestic and industrial sectors. So quantity of the amount of precious materials needs to be reduced or non Pt based inexpensive materials need to be developed.

Paul produced nano porous anodized aluminum (Al) where pore are filled up by precious metal through electrodeposition. Among the non Pt based electrode materials, Ni based alloys are being investigated.  They show good electrocatalytic properties. MnO2 with nano carbon is also reported to exhibit catalytic activities for alcoholic and glucose fuels. have been found in Ni-Co alloys, These electrodes materials have been investigated in some alternative fuel such as methanol, ethanol, and glucose solution respectively.

Laboratory Experiment of Fuel  Cell

MnO2-Nano Carbon Electrode for Glucose fuel Cell

Scanning Electron microscope image. Developed by electro deposition on 304 steel

 

Working Principle

A fuel cell works similar to a battery. In a battery there are two electrodes, which are separated by an electrolyte. At least one of the electrodes is generally made of a solid metal. This metal is converted to another chemical compound during the production of electricity in the battery. The energy that the battery can produce in one cycle is limited by the amount of this solid metal that can be converted. In the fuel cell an electrode that is not consumed and a fuel that continuously replenishes the fuel cell replace the solid metal. This fuel reacts with an oxidant such as oxygen from the other electrode. A fuel cell can produce electricity as long as more fuel and oxidant are pumped through it. At the anode the hydrogen molecules give up electrons and form hydrogen ions, a process which is made possible by the platinum catalyst. The electrons travel to the cathode through an external circuit, producing electrical current. The proton exchange membrane allows protons to flow through, but stops electrons from passing through it. As a result, while the electrons flow through an external circuit, the hydrogen ions flow directly through the proton exchange membrane to the cathode, where they combine with oxygen molecules and the electrons to form water.

                                                  Anode: 2H2 --> 4H+ + 4e-

       Cathode: 4e- + 4H+ + O2 --> 2H2O

           Overall: 2H2 + O2 --> 2H2O

The ideal available electrical work (assuming no losses by heat) from the electrons flowing through the circuit isWmax= -n F E

where n is the number of equivalents, or electrons per molecule of fuel, F is the Faraday (96,493 Coulombs per equivalent) and E is  the thermodynamic reversible voltage of the reaction (1.229 for this reaction) Thus, Wmax  comes out as 548 W.h. So theoretically 548 W.h of enrgy can be obtained out of a liter tank of hydrogen.