PPT of Fuel Cell Fundamentals and Types of Fuel Cells

Fuel Cell

1. Presentation on Fuel Cell and Mathematical Modeling of PEMFC Fuel Cell

2. Contents Introduction Working Principle Advantages, Disadvantages, Applications of Fuel cell Different Types of Fuel Cell Performance of Fuel Cell Mathematical Modeling of PEMFC Fuel Cell

3. Introduction Fuel Cell A fuel cell is an electrochemical device that produces electricity without combustion by combining hydrogen and oxygen to produce water and heat. Brief History of Fuel cell First developed by William Grove In 1839 Grove was experimenting on electrolysis (the process by which water is split into hydrogen and oxygen by an electric current), when he observed that combining the same elements could also produce an electric current 1930s -1950s Francis Thomas Bacon, a British scientist, worked on developing alkaline fuel cells. Demonstrated a working stack in 1958. The technology was licensed to Pratt and Whitney where it was utilized for the Apollo spacecraft fuel cells. Fig.1: Fuel Cell Fig.2: Fuel Cell used in Appolo Mission

4. Working Principle A fuel cell is a device that uses hydrogen (or hydrogen-rich fuel) and oxygen to create electricity by an electrochemical process. A single fuel cell consists of an electrolyte sandwiched between two thin electrodes (a porous anode and cathode) Hydrogen, or a hydrogen-rich fuel, is fed to the anode where a catalyst separates hydrogen's negatively charged electrons from positively charged ions (protons) . At the cathode, oxygen combines with electrons and, in some cases, with species such as protons or water, resulting in water or hydroxide ions, respectively Fig.3: Working of Fuel Cell

5. Advantages Environmental Friendliness Versatility in Fuel Sources Quiet Operation Cogeneration Capabilities Reduced Dependency on Fossil Fuels Disadvantages Cost Hydrogen Infrastructure Fuel Availability and Storage Sensitivity to Contaminants Applications Transportation  Portable Power Stationary Power Generation Backup Power Remote and Off-Grid Power Marine and Maritime Applications Military and Defense Microgrids and Smart Grids Off-Grid Telecommunications Types of Fuel Cell Alkaline fuel cells (AFC) Phosphoric acid fuel cell (PAFC) Molten carbonate fuel cell (MCFC)  Polymer electrolyte membrane fuel cell (PEMFC) Solid oxide fuel cell (SOFC)

6. Alkaline Fuel Cells (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 onboard spacecraft. The alkaline fuel cell uses an alkaline electrolyte such as 40% aqueous potassium hydroxide. In alkaline fuel cells, negative ions travel through the electrolyte to the anode where they combine with hydrogen to generate water and electrons. These fuel cells use a solution of potassium hydroxide in water as the electrolyte and can use a variety of nonprecious metals as a catalyst at the anode and cathode. AFCs are high-performance fuel cells due to the rate at which chemical reactions take place in the cell, 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). Fig.4: Alkaline Fuel Cell

7. Molten Carbonate Fuel Cells (MCFC) The molten carbonate fuel cell uses a molten carbonate salt as the electrolyte. It has the potential to be fueled with coal- derived fuel gases, methane or natural gas. These fuel cells can work at up to 60% efficiency In molten carbonate fuel cells, negative ions travel through the electrolyte to the anode where they combine with hydrogen to generate water and electrons. Molten carbonate fuel cells (MCFCs) are currently being developed for natural gas and coal-based power plants for electrical utility, industrial, and military applications. 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. Fig.5: Molten Carbonate Fuel Cell

8. Phosphoric Acid Fuel Cells (PAFC) The phosphoric acid fuel cell (PAFC) is considered the "first generation" of modern fuel cells. 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. In phosphoric acid fuel cells, protons move through the electrolyte to the cathode to combine with oxygen and electrons, producing water and heat. 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 PAFCs are more tolerant of impurities They are 85 percent efficient when used for the co-generation of electricity and heat, but less efficient at generating electricity alone (37 to 42 percent). 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. Like PEM fuel cells, PAFCs require an expensive platinum catalyst, which raises the cost of the fuel cell. Fig.6: Phosphoric Acid Fuel Cell

9. Polymer electrolyte membrane fuel cell (PEMFC) PEMFC uses a proton-conducting polymer membrane as the electrolyte and porous carbon electrodes containing a platinum catalyst. Hydrogen is typically used as the fuel. These cells operate at relatively low temperatures and can quickly vary their output to meet shifting power demands. Deliver high power density and offer the advantages of low weight and volume, compared to other fuel cells. They only hydrogen, oxygen from the air, and water to operate and do not require corrosive fluids like some fuel cells. Best candidates for cars, buildings and smaller applications. Fig.7: Polymer Electrolyte membrane Fuel Cell

10. Solid Oxide Fuel Cells (SOFC) SOFC uses a solid ceramic electrolyte, such as zirconium oxide stabilized with yttrium oxide, instead of a liquid and operate at 800 to 1,000°C. In solid oxide fuel cells, negative ions travel through the electrolyte to the anode where they combine with hydrogen to generate water and electrons. Efficiencies of around 60 per cent and are expected to be used for generating electricity and heat in industry and potentially for providing auxiliary power in vehicles. 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. High temperature operation removes the need for precious-metal catalyst, thereby reducing cost. They are not poisoned by carbon monoxide (CO), which can even be used as fuel. Sulphur resistant which allows SOFCs to use gases made from coal. 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 Fig.8: Solid Oxide Fuel Cell

11. Reactions in different types of fuel cell

12. Comparison of different types of fuel cell

13. Performance of Fuel Cell Cell Efficiency The thermal efficiency of a fuel conversion device is defined as the amount of useful energy produced relative to the change in enthalpy, ∆H, between the product and feed streams. Conventionally, chemical (fuel) energy is first converted to heat, which is then converted to mechanical energy, which can then be converted to electrical energy. Fuel cells convert chemical energy directly into electrical energy. In the ideal case the change in Gibbs free energy, ∆G, of the reaction is available as useful electrical energy . The ideal efficiency of a fuel cell is given as:  

14. At standard conditions of 25°C (298°K) and 1 atmosphere, the thermal energy ( ∆H ) in the hydrogen/oxygen reaction is 285.8 kJ/mole, and the free energy available for useful work is 237.1 kJ/mole. Thus, the thermal efficiency of an ideal fuel cell operating on pure hydrogen and oxygen at standard conditions is: The efficiency of an actual fuel cell is often expressed in terms of the ratio of the operating cell voltage to the ideal cell voltage. As the ideal voltage of a cell operating on pure hydrogen and oxygen at 1 atm pressure and 25ºC is 1.229 V. Thus, the thermal efficiency of an actual fuel cell operating at a voltage of V cell , based on the higher heating value of hydrogen, is given by  

15. Actual Performance of Fuel Cell The actual cell potential is decreased from its ideal potential because of several types of irreversible losses as below: Activation-related losses: These stem from the activation energy of the electrochemical reactions at the electrodes. These losses depend on the reactions at hand, the electro-catalyst material and microstructure, reactant activities (and hence utilization), and weakly on current density. Ohmic losses: Ohmic losses are caused by ionic resistance in the electrolyte and electrodes, electronic resistance in the electrodes, current collectors and interconnects, and contact resistances. Ohmic losses are proportional to the current density, depend on materials selection and stack geometry, and on temperature. Mass-transport-related losses: These are a result of finite mass transport limitations rates of the reactants and depend strongly on the current density, reactant activity, and electrode structure. Fig.9: Polarization characteristics of Fuel Cell

16. Mathematical Modeling of PEMFC Stack voltage output of a Fuel cell is given as (1) Where N is number of cells connected in series and V cell is single cell voltage. Single fuel cell output voltage is defined as follows [9]: = - - - (2) where, E nernst is the irreversible voltage, V act are the activation voltage loss, V ohmic is the ohmic voltage drop and V conc . is the concentration voltage losses. = 1.22 - 8.5 (T- 298.15) + 4.3085 T(ln[ ] + 0.5 (3) Where T is operating temperature, is pressure of hydrogen at anode and is pressure of oxygen at cathode.  

17. (4) (5) Where, are relative humidity at anode and cathode respectively, is saturated pressure of water. are inlet pressure at anode and cathode and is the cell current. Activation Voltage: ] (6) Where, ζ 1, ζ 2, ζ 3, and ζ 4 are parametric coefficients. is concentration of oxygen.  

18. Ohmic voltage drop: (7) are electronic and ionic resistances. (8) Where (9) Where is parametric coefficient. Concentration Voltage loss: (10) Where, is parametric coefficient, is actual current density and maximum current density By using this mathematical model the performance of PEMFC can be obtained and can be seen from polarization characteristics given in Fig.9.