Chapter: Fuel Cell Technology

1. Chapter: Fuel Cell Technology

2. Table of Contents • Introduction to the historical background of fuel cells. • Fuel Cell Basics. • Fundamentals of Electrochemistry. • Fundamentals of Thermodynamics. • High and Low Temperature Fuel Cells. • Fuel Cell System Integration. • Operations of Fuel Cells. • Health and Safety Aspects.

3. Historical background Sir W.R.Grove B

4. Sir Grove‘s Battery B The principle of an electroliser, shown left; of a fuel cell, shown right (Source: Larminie, 2000).

5. Historical Overview 1838/39 Discovery of the fuel cell effect: • 1838 C.F. Schönbein “On the Voltaic Polarization of Certain Solid and Fluid Substances”. • 1839 Sir W. Grove “On the Voltaic Series and the Combination of Gasses by Platinum”. 1843 Construction of a “gas battery” by Grove. 1889 Work by L. Mond and C. Langer led to the first alkaline fuel cell. They also discovered the high polarization losses at the oxygen electrode. 1896 W.W. Jaques used molten sodium hydroxide as an electrolyte in order to directly convert coal into electricity. 1900 W. Nernst carried out conceptual work on solid electrolyte fuel cells (SOFC). 1905 F. Haber carried out systematic thermodynamic investigations regarding hydrogen consuming fuel cells. 1932 F.T. Bacon started a long term fuel cell development program. 1935 W. Schottky developed the theoretical fundamentals of the SOFC. 1938 E. Baur and H. Preis first reported on experimental SOFC work. 1959 F.T. Bacon constructed the first working 5 kW alkaline fuel cell stack. 1964 Diaphragm gas cell supplied in Gemini spacecraft. 1967 Concept of the phosphorus-sour gas cell by UTC. 60/80ies alkaline fuel cells are used for Apollo and space shuttle missions. 1984 “Rediscovery” of the Polymer. Sir W. Grove Niche applications B Pre-batch production

6. Historical Fuel Cell Applications US Space Programme Conventional batteries too large, heavy and toxic. Photovoltaics not yet practical. Spacecrafts already carrying H2 and O2. Water by-product. Historical fuel cell applications B

7. Fuel cells for NASA space programme Nasa Space Shuttle Orbiter fuel cell. One of three fuel cells aboard the Space Shuttle. These fuel cells provide all of the electricity as well as drinking water when Space Shuttle is in flight. It produces 12 kilowatts electricity adn occupies 154 litres (Source: NASA). B

8. Fuel Cell Basics Why do we need Fuel Cells? Diminishing oil supplies. Reduce greenhouse gasses. Reduce toxic emissions. B

9. Overview of Fuel Cell Technology Direct conversion of chemical to electrical energy. Efficient conversion. Minimal pollution because of no combustion. Unlike batteries, reductant (hydrogen) and oxidant (air) must be replenished. B

10. Cold and Warm Combustion H O Heat Electricity Flow Fuel electricity Turbine Generator H fuel • Warm Combustion: • uncontrolled reaction process • The released heat will be transmitted to a working medium (e.g. water, steam) • The working medium runs through a cycle powering a turbine with generator • Cold Combustion (Fuel cells): • controlled reaction (no flame) • Direct transformation from chemical to electrical energy • Indirect transformation via working medium not necessary! B Source: WBZU

11. Efficiencies in Theory • Higher Efficiency of electrochemical process vs.Carnot process • Saves Energy • Reduces CO2- Emissions  FC Source: WBZU I Especially at low temperature fuel cells works very efficient !

12. Efficiencies in the real world! Fuel Cells  Diesel Steam- and Gasturbines  Gasoline Electric Power Efficiency I

13. Fuel Cell Components Most fuel cell power systems comprise a number of components: Unit cells, in which the electrochemical reactions take place. Stacks, in which individual cells are combined by electrically connecting the cells to form units with the desired output capacity. Balance of plant which comprises components that provide feedstream conditioning (including a fuel processor if needed), thermal management, and electric power conditioning among other interface functions. B

14. Main Cell Components • Main cell components B

15. Battery vs Fuel Cell Battery stores energy within the battery’s reductant. Battery stops when chemical reactants are consumed. Fuel cell converts energy from fuel and oxidant that are continuously supplied. B

16. Functioning of a PEM FC I

17. Though the direct use of conventional fuels in fuel cells would be desirable, most fuel cells under development today use gaseous hydrogen, or a synthesis gas rich in hydrogen, as a fuel. Hydrogen has a high reactivity for anode reactions, and can be produced chemically from a wide range of fossil and renewable fuels, as well as via electrolysis. For similar practical reasons, the most common oxidant is gaseous oxygen, which is readily available from air. Fuel cells are classified according to the choice of electrolyte and fuel, which in turn determines the electrode reactions and the type of ions that carry the current across the electrolyte. B

18. Critical Functions of Cell Components ‘Three-phase interface’. Microscopic regions. Electrode in contact with electrolyte. Improved performance: Reduced thickness of electrolyte. Better materials used in electrode and electrolyte. Wider temperature ranges. B

19. Other Critical Functions of the Unit Cell Components Electrolyte: Transports dissolved reactants to electrode. Conducts ionic charge between electrodes. Physical barrier between fuel and oxidant. Electrodes: Conduct electrons to and from three-phase interface. Ensure even distribution of gasses over the cell. Ensure reaction produces are led away. B

20. Electrodes Porous material. Electrically conductive material. Catalysts needed at low temperatures. Most cells under development are planar (rectangular or circular) or tubular. B

21. Fuel Cell Applications Stationary – power plants. Mobile – cars, scooters, bicycles. Portable power – replacement for batteries. Various – locomotive, airplanes, boats, submarines. B

22. Fundamentals of Electrochemistry Electrochemical reactions involve both a transfer of electrical charge and a change in Gibbs energy, which is very important in the case of fuel cell. Gibbs free energy = energy available to do external work, neglecting any work done by changes in pressure and/or volume. In a fuel cell, the ‘external work’ involves moving electrons around an external circuit – any work done by a change in volume between the input and output is not harnessed by the fuel cell. B

23. When working with chemical reactions, the zero energy point is normally defined as pure elements, in the normal state, at standard temperature and pressure (25°C, 0.1MPa). The term ‘Gibbs free energy of formation’, Gf , rather than the ‘Gibbs free energy’ is used when adopting this convention. In a fuel cell, it is the change in this Gibbs free energy of formation, Gf, that gives the energy released. This change is the difference between the Gibbs free energy of the products and the Gibbs free energy of the inputs or reactants. Gf = Gfof products − Gfof reactants If there are no losses in the fuel cell, all Gibbs free energy is converted into electrical energy. B

24. Theoretical fuel cell potential In general, electrical work is a product of charge and potential Wel = q·E where Wel = electrical work (Jmol-1) ;q = charge (Coulombs mol-1); E = potential (Volts). The total charge transferred in a reaction per mol of fuel consumed is equal to: q = -nNAvgqel where n = number of electrons transferred per molecule of fuel; Navg= number of molecules per mole (Avogadro’s number) = 6.022·1023 molecules/mol; qel = charge of 1electron = 1.602 10-19 Coulomb. B

25. The product of Avogadro’s number and charge of 1 electron is known as Faraday’s constant: F = 96,485 Coulombs/electron-mol. -nNAvgqel = -nF Electrical work is therefore: Wel = -nFE The maximum amount of electrical energy generated in a fuel cell corresponds to Gibbs free energy, ΔG: Wel = ΔG The theoretical potential of fuel cell is then E = -ΔG/(nF) This equation gives the electromotive force (EMF) or reversible open circuit voltage (OCV) of the fuel cell. B

26. Consider the hydrogen/oxygen fuel cell. The basic reaction is H2 → 2H+ + 2e-(anode) ½ O2 + 2H+ + 2e- → H2O (cathode) H2 + ½ O2 → H2O (overall) For the hydrogen fuel cell, two electrons pass round the external circuit for each water molecule produced and each molecule of hydrogen used. So, the reversible open circuit voltage (OCV) of the hydrogen fuel cell is: E = -ΔG/(2F) Because G, n and F are known, the theoretical fuel cell potential (OCV) of hydrogen/oxygen at T = 298.15K is E= 1.23 Volts . B

27. Operational Fuel Cell Voltages • Voltage losses Voltage for a typical low temperature, air pressure, fuel cell. B This graph is called Polarisation curve

28. The characteristic shape of the voltage/current density graph or polarization curve results from four major irreversibilities. • Activation losses. • 2.Fuel crossover and internal currents. • 3. Ohmic losses. • 4. Mass transport or concentration losses. B

29. Combining all these irreversibilities, the operating voltage of a fuel cell can be given by the following equation: E = Eocv-ΔVact-ΔVohm-ΔVtrans B

30. Activation losses -The Tafel equation B • Tafel plots for slow and fast electrochemical reactions.

31. The activation overvoltage profile is given by the Tafel equation: ΔVact = Bln( i ∕ i0 ) The constant B is called Tafel slope and it is given by: B = RT ∕ (2aF) The current density i0 is called the exchange current density. B

32. Fundamentals of Thermodynamics • Heat of reaction Consider the hydrogen/oxygen fuel cell. The basic reaction is H2 → 2H+ + 2e-(anode) ½ O2 + 2H+ + 2e- → H2O (cathode) H2 + ½ O2 → H2O (overall) The overall reaction is the same as the reaction of hydrogen combustion. Combustion is an exothermic process, which means that there is energy released in the process. H2 + ½ O2 → H2O + heat The heat or enthalpy (ΔH) of a chemical reaction is the difference between the heats of formation of products and reactants. This means: ΔH = (hf)H2O-(hf)H2- ½ (hf)O2 B

33. Heat of formation of liquid water is -286kjmol-1 at 25°C and heat of formation of elements is by definition equal to zero. Therefore ΔH = (hf)H2O-(hf)H2- ½ (hf)O2 = -286 KJ/mol -0 -0 = -286KJ/mol The negative sign means that heat is being released in the reaction, that is, this is an exothermic reaction. So equation is H2 + ½ O2 → H2O + 286 KJ/mol The enthalpy of hydrogen combustion reaction is also called the hydrogen’s heating value. It is the amount of heat that may be generated by a complete combustion of 1 mol of hydrogen. B

34. Theoretical electrical work Hydrogen heating value is used as a measure of energy input in a fuel cell. This is the maximum amount of energy that may be extracted from hydrogen. In a fuel cell a portion of the energy input(ΔH) is converted into electricity and it corresponds to Gibbs free energy (ΔG). ΔG = ΔH -TΔS There are some irreversible losses in energy conversion due to creation of entropy (ΔS). ΔS is the difference between entropies of products and reactants. ΔS = (sf)H20 – (sf)H2 – ½ (sf)O2 Therefore, at 25°C, out of 286.02KJmol-1 of available energy, 237.34KJmol-1 can be converted into electrical energy and the remaining 48.68KJmol-1 is converted into heat. At temperatures other than 25°C, these values are different. B

35. Effect of the temperature and pressure • Temperature The theoretical cell potential E = -ΔG/(nF) changes with temperature. E = - [ΔH/(nF)-TΔS/(nF)] B

36. Pressure Pressure causes a change in Gibbs free energy which can be expressed in the following way. dG = VmdP where Vm = molar volume (m3 mol-1); P= pressure (Pa). For an ideal gas : PVm = RT Therefore: dG = RTdP/P After integration: G = G0 + RTln(P/P0) G0 is Gibbs free energy at standard temperature and pressure (25°C and 1atm), and P0 is the reference or standard pressure(1atm). For any chemical reaction: jA + kB  mC + nD ΔG = mGC + nGD - jGA - kGB B

37. ΔG = ΔG0 + RT ln {[ (PC/P0)m (PD/P0)n] / [(PA/P0)j (PB/P0)k]} This is the Nernst equation, where P is the partial pressure of the reactant or product species and P0 is the reference pressure (i.e. 1atm). For the hydrogen/oxygen fuel cell reaction, the Nernst equation becomes: ΔG = ΔG0 + RT ln [PH2O /(PH2PO20.5)] E = E0 + RT/(nF) ln[PH2PO20.5/PH20] B

38. Theoretical fuel cell efficiency The efficiency of any energy conversion device is defined as the ratio between useful energy output and energy input. In the case of fuel cell, the useful energy output is the electrical energy produced and energy input is the enthalpy of hydrogen. Assuming that all of the Gibbs free energy can be converted into electrical energy, the maximum possible efficiency of a fuel cell is: h = ΔG / ΔH = 237.34 /286.02 = 83% B

39. High and low temperature fuel cells Currently, six classes of fuel cell have emerged as viable systems for the present and near future. Distinctions are made based on temperature, electrolyte and application. B

40. On the basis of their temperature of operation, fuel cells are classified as: • High temperature FC • Molten Carbonate Fuel Cell (MCFC). • Solid Oxide Fuel Cell (SOFC). • Medium temperature FC • Alkaline Electrolyte Fuel Cell. • Phosphoric Acid Fuel Cell(PAFC). • Low temperature FC • Proton Exchange Membrane Fuel Cell(PEMFC). • Direct methanol Fuel Cell(DMFC). B

41. Overview Fuel Cell Types B

42. High temperature Fuel Cell • Molten Carbonate Fuel Cell (MCFC) B

43. Advantages of MCFCs: • No expensive electro-catalysts needed, thanks to high temperature. • Disadvantages of MCFCs: • Very corrosive and mobile electrolyte requires expensive material for cell hardware. • High temperature promote material problems. • High internal resistance limits power density. B

44. Solid Oxide Fuel Cell (SOFC) • The electrolyte is a solid, nonporous metal oxide, usually Y2O3-stabilized ZrO2. It functions as a conductor of oxide ions. • Typically: • the anode is Co-ZrO2 or Ni-ZrO2 cermet (ceramic-metallic materials), • the cathode is Sr-doped LaMnO3. B

45. Schematic cross-section of cylindrical Siemes Westinghouse SOFC tube B

46. Gas manifold design for a tubular SOFC and cell-to-cell connections in a tubular SOFC B

47. Advantages: • Cell can be cast into various shapes, thanks to solid electrolytes. • Solid ceramic construction alleviates corrosion problems. • Solid electrolyte also allows precise engineering and avoids electrolyte movement. • Disadvantages: • Thermal expansion mismatches among materials. • Sealing between cells is difficult in flat plate configuration. • High operating temperature places severe constraints on materials. • Difficult fabrication processes. B

48. Medium temperature Fuel Cell • Alkaline Electrolyte Fuel Cell (AFC) B

49. Advantages: • Excellent performance on H2 and O2 compared to other candidate fuel cells. • Disadvantages: • Sensitivity of the electrolyte to CO2 requires the use of highly pure H2 as a fuel. • If ambient air is used as the oxidant, the CO2 in the air must be removed. B

50. Phosphoric Acid Fuel Cell (PAFC) The electrochemical reactions are: Anode: H22H++2e- Cathode: ½ O2+2H++2e- H2O total reaction: ½ O2+H2 H2O Principles of operation of Phosphoric Acid Fuel Cell (source: UTC Fuel Cells) B