Types of fuel cells

FUEL CELLS- TYPES

1. TYPES OF FUEL CELLS G.SURAJ 17IS1D0207

2. WHAT IS A FUEL CELL ?? A fuel cell is an electrochemical cell that converts the chemical energy from a fuel into electricity through an electrochemical reaction of hydrogen fuel with oxygen or another oxidizing agent.

3. Ordinary Combustion Process Of Fuel

4. The Process Of Fuel Cell

5. Parts of a Fuel Cell Anode Negative post of the fuel cell. Conducts the electrons that are freed from the hydrogen molecules so that they can be used in an external circuit. Etched channels disperse hydrogen gas over the surface of catalyst . Cathode Positive post of the fuel cell. Etched channels distribute oxygen to the surface of the catalyst. Conducts electrons back from the external circuit to the catalyst. Recombine with the hydrogen ions and oxygen to form water . Electrolyte Proton exchange membrane. Specially treated material, only conducts positively charged ions. Membrane blocks electrons. Catalyst Special material that facilitates reaction of oxygen and hydrogen. Usually platinum powder very thinly coated onto carbon paper or cloth. Rough & porous maximizes surface area exposed to hydrogen or oxygen. The platinum-coated side of the catalyst faces the PEM.

6. Principle Of Operation Pressurized hydrogen gas (H 2 ) enters cell on anode side. Gas is forced through catalyst by pressure. When H 2 molecule comes contacts platinum catalyst, it splits into two H+ ions and two electrons (e-). Electrons are conducted through the anode Make their way through the external circuit (doing useful work such as turning a motor) and return to the cathode side of the fuel cell. On the cathode side, oxygen gas (O 2 ) is forced through the catalyst Forms two oxygen atoms, each with a strong negative charge. Negative charge attracts the two H+ ions through the membrane, Combine with an oxygen atom and two electrons from the external circuit to form a water molecule (H 2 O).

7. Reactions In Fuel Cell Can Be Represented As

8. Fuel Cell Can Be Represented As

9. Types Of Fuel Cells Fuel cells are classified primarily by the kind of electrolyte they employ. This classification determines the kind of electro-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. There are several types of fuel cells currently under development, each with its own advantages, limitations, and potential applications.

10. Prominent Types Polymer Electrolyte Membrane fuel cell(PEMFC) Direct Methanol fuel cell (DMFC) Alkaline fuel cell (AFC) Phosphoric acid fuel cell (PAFC) Molten-carbonate fuel cell (MCFC) Solid-oxide fuel cell (SOFC ) Reversible Fuel Cells

11. PEM 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 with other fuel cells. PEM fuel cells use a solid polymer as an electrolyte and porous carbon electrodes containing a platinum or platinum alloy catalyst. They need only hydrogen, oxygen from the air, and water to operate. T PEM 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. PEM fuel cells have a practical efficiency of 60%. Power output is in the range of 5-200 kW.  They are ideal for transportation and portable power . PEM fuel cells are particularly suitable for use in passenger vehicles, such as cars and buses.

12. Diagram Of PEM

13. Electrochemical Reactions Occuring in PEMFC At the anode: H 2 = 2H+ + 2e- At the cathode: 1/2O 2 + 2H+ + 2e- = H 2 O Overall cell reaction: l/2O 2 + H 2 = H 2

14. Direct Methanol Fuel Cells Direct methanol fuel cells (DMFCs), however, are powered by pure methanol, which is usually mixed with water 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 cell systems because methanol has a higher energy density than hydrogen—though less than gasoline or diesel fuel. DMFCs are often used to provide power for portable fuel cell applications such as cell phones or laptop computers.

15. DMFC ELECTRO CHEMICAL EQUATION : Anode (Oxidation) CH 3OH + 6OH − →5H2O + 6 e − + C O 2 Cathode (Reduction) 3/ 2 O 2 + 3H2O + 6 e − → 6OH- Overall reaction C H 3 O H + 3/ 2 O 2 → 2   H 2 O + C O2

16. AFC 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 on-board 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. 3. A key challenge for this fuel cell type is that it is susceptible to poisoning by carbon dioxide (CO2). The operating temperature of AFCs is about 70°C and their power output is 10-100 kW. They have been widely used for space and defense applications, where pure hydrogen is used. Their excessive cost and sensitivity to CO 2 , have restricted their research and development, no matter their high efficiency and power density.

17. AFC Electro chemical Equation: Anode: H 2 + 2(OH) -  2H 2 O + 2 e - Cathode: ½ O 2 + HO 2 + 2e -  2(OH) - Over all Cell Reaction: H 2 + ½ O 2 + CO 2  H 2 O Diagram of an Alkaline Fuel Cell . 1:Hydrogen 2:Electron flow 3:Load 4:Oxygen 5:Cathode 6:Electrolyte 7:Anode 8:Water 9:Hydroxyl Ions

18. PAFC Phosphoric acid fuel cells (PAFCs) 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 PAFC is considered the "first generation" of modern fuel cells. PAFCs have an operating temperature of 200 °C. The power output varies from 200 kW to 20 MW. The main disadvantage is that it has no self-starting capability, because at lower temperatures (40-50 °C) freezing of concentrated Phosphoric Acid occurs. PAFCs are more than 85% efficient when used for the co-generation of electricity and heat but they are less efficient at generating electricity alone (37%–42%). PAFCs are also less powerful than other fuel cells, given the same weight and volume. As a result, these fuel cells are typically large and heavy. PAFCs are also expensive.

19. PAFC Electro Chemical Equation : Anode reaction: 2H 2 (g) → 4H + + 4e‾ Cathode reaction: O 2 (g) + 4H + + 4e‾ → 2H 2 O Overall cell reaction: 2 H 2 + O 2 → 2H 2 O

20. MCFC Molten carbonate fuel cells (MCFCs) are currently being developed for natural gas and coal-based power plants for electrical utility, industrial, and military applications. 2. 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 matrix. As they operate at high temperatures of 650°C (roughly 1,200°F), non-precious metals can be used as catalysts at the anode and cathode, reducing costs . Molten carbonate fuel cells, when coupled with a turbine, can reach efficiencies approaching 65%, considerably higher than the 37%–42% efficiencies of a phosphoric acid fuel cell plant. When the waste heat is captured and used, overall fuel efficiencies can be over 85 %. 6 The primary disadvantage of current MCFC technology is durability. The high temperatures at which these cells operate and the corrosive electrolyte used accelerate component breakdown and corrosion, decreasing cell life.

21. MCFC Electrochemical Equation :              Anode: H 2 + CO 3 2-  H 2 O +CO 2 + 2 e -               Cathode : ½ O 2 + CO 2 + 2e -  CO 3 2-               Cell: H 2 + ½ O 2 + CO 2  H 2 O + CO 2  

22. SOFC Solid oxide fuel cells (SOFCs) use a hard, non-porous ceramic compound as the electrolyte . SOFCs are around 60% efficient at converting fuel to electricity and operate at very high temperatures—as high as 1,000°C (1,830°F). 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 . In addition, they are not poisoned by carbon monoxide, which can even be used as fuel. This property allows SOFCs to use natural gas, biogas, and gases made from coal. High-temperature operation has disadvantages. It results in a slow startup and requires significant thermal shielding to retain heat and protect personnel, which may be acceptable for utility applications but not for transportation.

23. SOFC Electrochemical Equation :                      Anode: H 2 + O 2  H 2 O + 2 e -                      Cathode: ½ O 2 + 2e -  O 2-                Cell : H 2 + ½ O 2  H 2 O

24. Reversible Fuel Cells Reversible fuel cells produce electricity from hydrogen and oxygen and generate heat and water as byproducts, just like other fuel cells. However, reversible fuel cell systems can also use electricity from solar power, wind power, or other sources to split water into oxygen and hydrogen fuel through a process called electrolysis. Reversible fuel cells can provide power when needed, but during times of high power production from other technologies (such as when high winds lead to an excess of available wind power), reversible fuel cells can store the excess energy in the form of hydrogen. This energy storage capability could be a key enabler for intermittent renewable energy technologies.

25. RFC Reversible fuel cell concept : RFC system integrated into the home :

26. RFC

27. Fuel cell COMPARISON CHART

28. Advantages of Fuel Cells High Efficiency- when utilizing co-generation, fuel cells can attain over 80% energy efficiency. Good reliability- quality of power provided does not degrade over time . Noise- offers a much more silent and smooth alternative to conventional energy production . Environmentally beneficial- greatly reduces CO2 and harmful pollutant emissions . Size reduction- fuel cells are significantly lighter and more compact.

29. Disadvantages of F uel C ells Expensive to manufacture due the high cost of catalysts (platinum ). Lack of infrastructure to support the distribution of hydrogen. A lot of the currently available fuel cell technology is in the prototype stage and not yet validated . Hydrogen is expensive to produce and not widely available .