Direct Energy Conversion: Fuel Cells

Transcription

1 Direct Energy Conversion: Fuel Cells Section.. in the Text Book References: Direct Energy Conversion by Stanley W. Angrist, Allyn and Beacon, 8. Fuel Cell Systems, Explained by James Larminie and Andrew Dicks, Wiley, 00. Fuel Cell Technology Hand Book, Edited by Gregor Hoogers, CRC Press, 00

2 Fuel Cells Introduction: 0 Hydrocarbon Fuels Energy stored in chemical bonds Combustion Useful power Bypass the conversion-to-heat and mechanical-to-electrical processes A fuel cell is an electrochemical device in which the chemical energy of a conventional fuel is converted directly and efficiently into low voltage, direct current electrical energy. Since the conversion can be carried out isothermally (at least in theory), the Carnot limitation on efficiency does not apply.

3 Fuel Cell Efficiency 0

4 PEM Fuel Cell Performance

5 William Grove 8 Fuel Cells 0 Grove noted with interest that this device, which used platinum electrodes in contact with dilute sulfuric acid would cause permanent deflection of a galvanometer connected to the cell. He also noted the difficulty of producing high current densities in a fuel cell that uses gases.

6 Fuel Cells Mond & Langer (88) - Gas battery 0

7 Daniell Cell We 0 will use the term anode to mean the electrode at which oxidation takes place - losing of electrons Cathode is the electrode at which reduction takes place - electrons are gained from the external circuit

8 Sustainable Energy Science and Engineering Center Hydrogen Fuel Cell The Fuel Cell is a device which converts hydrogen and oxygen into electricity. It achieves this using a process which is the reverse of electrolysis of water first identified by William Grove in 8. 0 The common types of fuel cells are phosphoric acid (PAFC), molten carbonate (MCFC), proton exchange membrane (PEMFC), and solid oxide (SOFC), all named after their electrolytes. Because of their different materials and operating temperatures, they have varying benefits, applications and challenges, but all share the potential for high electrical efficiency and low emissions. Because they operate at sufficiently low temperatures they produce essentially no NOx, and because they cannot tolerate sulfur and use desulfurized fuel they produce no SOx.

9 Historical Development 0

10 Fuel Cell Types 0

11 Hydrogen - Oxygen Fuel Cell 0 At the anode the hydrogen gas ionizes releasing electrons and creating H + ions (or protons). This reaction releases energy. H H + + e At the cathode, oxygen reacts with electrons taken from the electrode, and H + ions from the electrolyte, to form water O + e + H + H O An acid with free H + ions. Certain polymers can also be made to contain mobile H + ions - proton exchange membranes (PEM)

12 Membrane Electrode Assembly 0 The MEA consists of two electrodes, the anode and the cathode, which are each coated on one side with a thin catalyst layer and separated by a proton exchange membrane (PEM). The flow-field plates direct hydrogen to the anode and oxygen (from air) to the cathode. When hydrogen reaches the catalyst layer, it separates into protons (hydrogen ions) and electrons. The free electrons, produced at the anode, are conducted in the form of a usable electric current through the external circuit. At the cathode, oxygen from the air, electrons from the external circuit and protons combine to form water and heat.

13 Fuel Cell Stack Hydrogen Hydrogen flows through channels in flow field plates to the anode where the platinum catalyst promotes its separation into protons and electrons. Hydrogen can be supplied to a fuel cell directly or may be obtained from 0 natural gas, methanol or petroleum using a fuel processor, which converts the hydrocarbons into hydrogen and carbon dioxide through a catalytic chemical reaction. Membrane Electrode Assembly Each membrane electrode assembly consists of two electrodes (the anode and the cathode) with a very thin layer of catalyst, bonded to either side of a proton exchange membrane. Air Air flows through the channels in flow field plates to the cathode. The hydrogen protons that migrate through the proton exchange membrane combine with oxygen in air and electrons returning from the external circuit to form pure water and heat. The air stream also removes the water created as a by-product of the electrochemical process. Flow Field Plates Gases (hydrogen and air) are supplied to the electrodes of the membrane electrode assembly through channels formed in flow field plates. Fuel Cell Stack To obtain the desired amount of electrical power, individual fuel cells are combined to form a fuel cell stack. Increasing the number of cells in a stack increases the voltage, while increasing the surface area of the cells increases the current.

14 Micro Fuel Cell The fuel cells are mm and generate up to 00 mwatts. CWRU 0 (Case Western Reserve University) researchers have miniaturized this process through the use of micro fabrication technology, which is used to print multiple layers of fuel cell components onto a substrate. Inks were created to replicate the components of the fuel cell, which means that the anode, cathode, catalyst and electrolyte are all made of ink, rather than traditional fuel cell materials. Researchers screen printed those inks onto a ceramic or silicon structure to form a functioning fuel cell.

15 Proton Exchange Membrane Fuel Cells (PEMFC) PEM fuel cells use a solid polymer membrane (a thin plastic 0 film) as the electrolyte. This polymer is permeable to protons when it is saturated with water, but it does not conduct electrons. The reactions at the electrodes are as follows: Anode Reactions: H => H+ + e- Cathode Reactions: O + H+ + e- => H O Overall Cell Reactions: H + O => H O Compared to other types of fuel cells, PEMFCs generate more power for a given volume or weight of fuel cell. This high-power density characteristic makes them compact and lightweight. In addition, the operating temperature is less than 00ºC, which allows rapid start-up. These traits and the ability to rapidly change power output are some of the characteristics that make the PEMFC the top candidate for automotive power applications.

16 Alkaline Fuel Cell Alkaline fuel cells (AFC) are one of the most developed technologies and have been used since the mid-0s by NASA in the Apollo and Space Shuttle programs. The fuel cells on board these spacecraft provide electrical power for on-board systems, as well as drinking water. AFCs are among the most efficient in generating electricity 0 at nearly 0%. Alkaline fuel cells use an electrolyte that is an aqueous (water-based) solution of potassium hydroxide (KOH) retained in a porous stabilized matrix. The concentration of KOH can be varied with the fuel cell operating temperature, which ranges from C to 0 C. The charge carrier for an AFC is the hydroxyl ion (OH-) that migrates from the cathode to the anode where they react with hydrogen to produce water and electrons. Water formed at the anode migrates back to the cathode to regenerate hydroxyl ions. The chemical reactions at the anode and cathode in an AFC are shown below. This set of reactions in the fuel cell produces electricity and by-product heat. Anode Reaction: H + OH- => H O + e- Cathode Reaction: O + H O + e- => OH- Overall Net Reaction: H + O => H O One characteristic of AFCs is that they are very sensitive to CO that may be present in the fuel or air. The CO reacts with the electrolyte, poisoning it rapidly, and severely degrading the fuel cell performance. Therefore, AFCs are limited to closed environments, such as space and undersea vehicles, and must be run on pure hydrogen and oxygen. Furthermore, molecules such as CO, H O and CH, which are harmless or even work as fuels to other fuel cells, are poisons to an AFC.

17 Alkaline Fuel Cell System 0

18 Solid Oxide Fuel Cell 0 Solid oxide fuel cells (SOFC) can also utilize carbon monoxide (CO). This makes them more fuel flexible and also generally more efficient with available fuels, such as natural gas or propane. Hydrogen and CO can be produced from natural gas and other fuels by steam reforming, for example. Fuel cells like SOFCs that can reform natural gas internally have significant advantages in efficiency and simplicity when using natural gas because they do not need an external reformer. When the ions reach the fuel at the anode they oxidize the hydrogen to H O and the CO to CO. In doing so they release electrons, and if the anode and cathode are connected to an external circuit this flow of electrons is seen as a dc current. This process continues as long as fuel and air are supplied to the cell.

19 Solid Oxide Fuel Cell 0

20 Molten-carbonate Fuel Cell The diaphragm between the anode and the cathode consists of a matrix filled with a carbonate 0 electrolyte. Carbonate ions (CO - ) pass through the diaphragm and reach the anode. Here they discharge an oxygen atom, which combines with the hydrogen flowing past to form water (H O). This sets carbon dioxide (CO ) and two electrons free. The electrons flow over an electronic conductor to the cathode: current flows. Similarly, the remaining carbon dioxide (CO ) is fed to the cathode side, where it absorbs the electrons and an oxygen atom from the air that is flowing past. It then re-enters the process as a carbonate ion.

21 Carbon Conversion Fuel Cell Carbon (C) and oxygen (O ) can react in a hightemperature 0fuel cell with the carbon, delivering electrons (e) to an external circuit that can power a motor. The net electrochemical reaction carbon and oxygen forming carbon dioxide is the same as the chemical reaction for carbon combustion, but it allows greater efficiency for electricity production. The pure carbon dioxide (CO ) product can be sequestered in an underground reservoir or used to displace underground deposits of oil and gas. Instead of using gaseous fuels, as is typically done, the new technology uses aggregates of extremely fine (0- to,000-nanometer-diameter) carbon particles distributed in a mixture of molten lithium, sodium, or potassium carbonate at a temperature of 0 to 80 C. The overall cell reaction is carbon and oxygen (from ambient air) forming carbon dioxide and electricity. The reaction yields 80 percent of the carbon oxygen combustion energy as electricity. It provides up to kilowatt of power per square meter of cell surface area a rate sufficiently high for practical applications. Yet no burning of the carbon takes place.

22 Direct Methanol Fuel Cell Fuel cell that utilizes methanol as fuel. When providing current, methanol is electrochemically oxidized at the anode electrocatalyst to produce electrons which travel through the external circuit 0 to the cathode electrocatalyst where they are consumed together with oxygen in a reduction reaction. The circuit is maintained within the cell by the conduction of protons in the electrolyte. In modern cells, electrolytes based on proton conducting polymer electrolyte membranes (e.g., Nafion ) are often used, since these allow for convenient cell design and for high temperature and pressure operation. The overall reaction occurring in the DMFC is the same as that for the direct combustion of methanol, CH OH + /O CO + H O Since the fuel cell operates isothermally, all the free energy associated with this reaction should in principle be converted to electrical energy. However, kinetic constraints within both electrode reactions together with the net resistive components of the cell means that this is never achieved. As a result, the working voltage of the cell falls with increasing current drain. These losses are known as polarization and minimizing the factors that give rise to them is a major aim in fuel cell research.

23 Direct Methanol Fuel Cell 0

24 Direct Methanol Fuel Cell Condenser 0 Fuel cell stack Load

25 Fuel Cell Applications Stationary power generation 0~ - 0 kw Portable applications ~ kw or lower Automotive applications ~ - 00 kw Airplane Applications ~ 0-0 kw kw =.08 horsepower

26 Sustainable Energy Science and Engineering Center Stationary Power Generation Important factors: The hours of operation per year The electric efficiency of the electricity generation process The capital investment Fuel cells are particularly suitable for on-site power generation. Utilizing the heat generated by the fuel cell improves the overall efficiency - Combined Heat and Power generation (CHP). 0

27 PEMPC Power Plant Process Flow Diagram for a Ballard 0 kw PEMFC Plant 0

28 Fuel Cell Powered Automobile - GM Electrovan 0 Alkaline fuel cell modules supplying kw

29 Hydrogen Fuel Cell Car 0

30 Automotive Applications 0

31 Fuel Cell Performance 0

32 Fuel Cell Powered Automobile - Progress 0

33 Fuel Cell Powered Automobile - Progress 0 Daimler-Chrysler NeCar:

34 Fuel Cell Powered Automobile - Progress Ford Focus Hydrogen -powered fuel cell vehicle 0

35 Fuel Cell Powered Automobile - Progress Honda fuel cell car 0

36 Methanol Fuel Cell Powered Automobile Toyota s Methanol-powered Fuel cell Electric Vehicle 0

37 Fuel Cell Powered Automobile 0 An x-ray view of Mitsubishi's new fuel cell Grandis minivan.

38 Portable Application Typically well under 00W of power with significantly higher power densities or larger energy storage capacity than those of advanced batteries. Power generation on a larger scale, say kw continuous output to replace gasoline or diesel generators or supply quiet electric power on boats, caravans or trucks. 0

39 Helios Solar Powered Airplane 0 Instead of jet fuel, Helios has about,000 solar cells across the wing. The solar cells collect energy from the Sun and convert it to electricity, which runs the small motors, which turn the propellers. The propellers are specially designed to pull the aircraft aloft even in the very thin air that's 8 miles high. The next project for the Helios is to use fuel cells to store enough of the sun's energy during the day to continue flying through the night. When this happens, Helios will be able to stay up for weeks and months at a time. The Helios, developed by Paul McCready, CEO of Aerovironment Corp., March, 00 DIGITAL PHOTOS FROM SOLAR AIRPLANE TO IMPROVE COFFEE HARVEST Funded by NASA

40 Electric Powered Airplane The new Electric Plane, or E-Plane, is a high-speed, allcarbon French DynAero Lafayette III, built and donated by 0 American Ghiles Aircraft. The E-Plane is being converted from a combustion engine to electric propulsion in three stages. The first flights, planned for next year, will be on lithium ion batteries. The next flights will be powered by a combination of lithium ion batteries augmented by a fuel cell. Finally, the aircraft will be powered totally by a hydrogen fuel cell, with a range of more than 00 miles. Supported by Foundation for Advancing Science and Technology Education (FASTec) showed off the plane it is developing as the world s first piloted fuel-cell-powered aircraft.

41 Fuel Cell Based Aircraft Propulsion 0 Source: NASA TM-00-

42 Fuel Cell Powered Aircraft 0

43 Fuel Cell Motorbike to Hit US Streets ENY: Emission Neutral Vehicle Top Speed: 0 mph (80 kmh) Range: 00 miles (0 km) Hydrogen Storage tank capacity: kg Cost: $, Manufacturer: Intelligent Energy, London, UK Intelligent Energy is currently developing devices called reformers that extract hydrogen from biodiesel fuels (typically made from vegetable oils or animal fats) and ethanol (generally made from grain or corn). The units would sell for around U.S. $,00 and could produce enough hyd rogen to fill up the ENV for about cents per tank,. Eggleston said. National Geographic News, August, 00

44 Fuel Cell System

45 FC Implementation Requirements

46 Direct Energy Conversion: Fuel Cells Section.. in the Text Book References: Direct Energy Conversion by Stanley W. Angrist, Allyn and Beacon, 8. Fuel Cell Systems, Explained by James Larminie and Andrew Dicks, Wiley, 00. Fuel Cell Technology Hand Book, Edited by Gregor Hoogers, CRC Press, 00

47 Hydrogen - Oxygen Fuel Cell 0 At the anode the hydrogen gas ionizes releasing electrons and creating H + ions (or protons). This reaction releases energy. H H + + e At the cathode, oxygen reacts with electrons taken from the electrode, and H + ions from the electrolyte, to form water An acid with free H + ions. Certain polymers can also be made to contain mobile H + ions - proton exchange membranes (PEM) O + e + H + H O

48 Fuel Cell Input and Output Hydrogen Energy Electricity Energy = VIt Fuel Cell Heat Water Oxygen Energy Power = VI; Energy = VIt 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 round an external circuit. It is the change in Gibbs free energy G, difference between the Gibbs free energy of the products and the Gibbs free energy of the reactants or inputs is important.

49 Sustainable Energy Science and Engineering Center The basic reaction: Hydrogen-oxygen Fuel Cell H + O H O H + O H O The product is one mole of H O ( 8g = gmole) and the reactants are one mole of H (g = gmole) and a half a mole of O (g = gmole). The molar specific Gibbs free energy, g used. 0 in per mole form is commonly g = g products g reac tan ts g = (g ) H O (g ) H (g ) O g = kj /mole g =.kj /mole Liquid water product at 8K Gaseous water product at 8K Negative sign indicates that the energy is released

50 Sustainable Energy Science and Engineering Center Fuel Cell Input and Output If there are no losses, then all the Gibbs free energy is converted into electrical energy. Two electrons pass round the external circuit for each water molecule produced and each molecule of hydrogen used. 0 H H + + e O + e + H + H O For one mole of hydrogen used, N electrons pass round the external circuit- where N is the Avogadro s number. If -e is the charge of one electron, then the charge that flows is -Ne = -F coulombs F is the faraday constant, or the charge on one mole of electrons. The electrical work done moving this charge round the circuit is (E is the voltage of the fuel cell) With no losses, we have Electrical work done = charge voltage = -FE joules g = FE E = g F

51 Thermodynamic Potentials Four quantities called "thermodynamic potentials" are useful in the chemical thermodynamics of reactions and noncyclic processes. They are internal energy, the enthalpy, the Helmholtz free energy and the Gibbs free energy. The four thermodynamic potentials are related by offsets of the "energy from the environment" term TS (energy you can get from the system s environment by heating and the "expansion work" term PV (work to give the system final volume V at constant pressure. U = Q - W H = 0U + PV F = U - TS Helmoltz free energy G = U - TS + PV Gibbs free energy Q: heat added to the system W: work done by the system U: internal energy T: absolute temperature S: final entropy V: final volume

52 Second Law of Thermodynamics 0 A fuel cell represented as a control volume. E stands for electrical potential, measured in volts. For any isolated system, the nd law states that S isolated 0 S gen = S total = S sys + S surr 0 Total change in entropy of both the system and surroundings entropy change in the components of the system entropy change in the surroundings

53 Second Law of Thermodynamics Considering the chamber in which chemical reaction takes place, the system is a control volume with mass flowing across its boundaries. The entropy change for the system is the 0difference between the entropy of products, S P and the reactants, S R with N representing the number of moles of each component in the reaction. S sys = S p S R = Any heat produced or consumed in the reaction is included in the expression for the surroundings, where Q surr is the heat transferred from the system to the surroundings and T o is the temperature of the surroundings. N P s P N R s R S surr = Q surr T o S gen = S P S R ( ) sys + Q surr T o

54 other measurable quantities. The Maxwell Relations Consider a simple compressible control mass of fixed chemical composition. The following relations are found to be useful in the calculation of entropy in terms of The thermodynamic property relations are du = Tds Pdv dh = Tds vdp 0 Entropy can not be measured We eliminate entropy from these equation by introducing two new forms of the thermodynamic property relation. Helmholtz function: A = U - TS ; a = u - Ts Gibbs function: G = H - TS; g = h - Ts da = du - Tds - sdt = -sdt - Pdv dg = -sdt + vdp

55 Sustainable Energy Science and Engineering Center Chemical Thermodynamics Chemical reactions proceed in the direction that minimizes the Gibbs energy G. The change in G is negative as the reaction approaches equilibrium and at chemical equilibrium the change in G is zero. The maximum work that an electrochemical cell 0 can perform is equal to the change in G as reactants go to products. This work is done by the movement of electrical charge through a voltage, and at equilibrium W max cell = G dg = dh TdS SdT = d(u + PV ) Tds SdT dg = du + PdV + VdP TdS SdT dg = δq δw + PdV + VdP TdS SdT For a spontaneous reaction at constant temperature and pressure in a closed system and doing only expansion-type work, we will get dg = δq TdS 0

56 The effect of temperature and pressure on G For a reversible process: Chemical Thermodynamics δq = TdS If the system is restricted to doing expansion work then 0 For isothermal process δw = PdV dg = VdP SdT PV = nrt Ideal gas law n is the number of moles dg = nrt dp P G G = nrt ln P P G = G o + nrt ln P P o stands for standard reference state

57 Equilibrium of a gas mixture: Chemical Thermodynamics For a chemical reaction occurring at constant pressure and temperature, the reactant gases A and B form products M0 and N. Where a, b, m and n are stoichiometric coefficients. The change in the Gibbs energy aa + bb mm + nn G = mg M + ng N ag A bg B Where g is in molar quantity (kj/mol) G = m g o M + RT ln P M P o + n g o N + RT ln P N P o a g o A + RT ln P A P o b g o B + RT ln P B P o G o = mg M o + ng N o ag A o bg B o In terms of standard Gibbs energy. The reference pressure is usually taken as atm.

58 Chemical Thermodynamics Equilibrium of a gas mixture: G = G o + RT ln P m n M P N P a b A P B G = G o + RT lnq m P N n Q = P M P a b A P B 0 Q: Reaction coefficient for the pressures The change in Gibbs energy of a reaction involving gases is: G = G o + RTlnQ

59 Sustainable Energy Science and Engineering Center Maximum work: The maximum work that a system can perform is related to the change in Gibbs energy. For a reversible process (δq = TdS) At constant pressure and temperature Chemical Thermodynamics Since there is only electrochemical work, W e, in which electrical charge moves through a voltage, we have 0 dg = δw + PdV + VdP SdT dg = δw + PdV = (δw PdV ) dg = δw e The Gibbs energy change is negative of the electrochemical work, we can then write as G = δw e

60 The Nernst equation and open circuit: The electrochemical work, which is done by the movement of electrons through a difference in a electrical potential, is denoted as W 0 e or W cell. In electrical terms, the work done by electrons with the charge n e F (n e is the number of electrons transferred per mole of fuel and F is the charge carried by a mole of electrons, which is Faraday s constant -,8C/mole - ) moving through a potential difference, E ( voltage difference across electrodes) is W e = n e FE G = n e FE G o = n e FE o G = G o + RT lnq E = Go n e F RT n e F lnq Chemical Thermodynamics E = E o RT n e F lnq E o is the standard electrode potential We also assume here that a complete reversible oxidation of a mole of fuel Electrical work done = charge voltage

61 Hydrogen - Oxygen Fuel Cell For a hydrogen-oxygen fuel cell, the overall reaction stoichiometry is 0 H + O H O The electrons transferred in this reaction, n e =. Using the partial pressures of water, hydrogen and oxygen in the reaction coefficient, we then have E = E o RT n e F ln P H O / P H P O Diluting the reactant gases will lower the maximum voltage that the cell can produce.

62 Partial Pressures In a mixture of gases, the total pressure is the sum of all the partial pressures of the components of the mixture. For example, in air at 0 0. MPa, the partial pressures of nitrogen and oxygen are MPa and 0.00 MPa are respectively. The product gas stream contains two parts of H and one part of O by moles and volume and the reaction takes place at 0. MPa, we will have P H = 0.= 0.0Mpa P O = 0. = 0.0MPa

63 Thermal Efficiency Thermal efficiency of a reversible heat engine is determined by η th = W net 0 = T L Q in T H Electrochemical cells such as storage batteries and fuel cells, operate at constant temperatures with the products of the reaction leaving at the same temperature as reactants (isothermal reaction). The chemical energy of the reactants is converted to electrical energy instead of being consumed to raise the temperature of the products, like in heat engines. Therefore this conversion process is less irreversible than the combustion reaction. The maximum work for electrochemical cell is given simply by W max,cell = G Change in the Gibbs function between products and reactants

64 Thermal Efficiency The work, which is done by the movement of electrons through a difference in electrical potential is W cell = n e FE 0 With the higher value of the fuel replaces Q in the maximum thermal efficiency at the open circuit voltage E o, of an electrochemical cell is given by η th,cell,max = n FE o e HHV For example, E o =. V for a hydrogen-oxygen fuel cell η th,cell,max = n e FE o HHV = = 0.8 HHV of H = 8.8 kj/mol; LHV = kj/mole

65 Thermal Efficiency 0 The Gibbs energy of the formation of water vapor is -8.8 kj/mol at 8.K and atm) It decreases to -. kj/mol at 00K. The current efficiency, a measure of fuel utilization (or fuel consumed to produce an electrical current ) is given by η I = I n e FN fuel Fuel flow rate mol/s

66 Maximum Efficiency Pressure: atm 0 Note: Voltage losses are nearly always less at higher temperatures, so in practice fuel cell voltages are usually higher at higher temperatures. The waste heat from higher temperature fuel cells is more useful Fuel cells do not always have a higher efficiency limit than heat engine.

67 Heating Values for Selected Fuels Fuel HHV (MJ/kg) LHV (MJ/kg) HHV/LHV LHV/HHV Coal CO Methane Natural gas Propane Gasoline Diesel Hydrogen

68 Reversibility Actual work against maximum work potential: For a heat engine this is equivalent to For fuel cells, this can be written as 0 η heatengine = W act W Carnot η R = η act η rev = n efe n e FE o = E E o For a hydrogen-oxygen fuel cell, the value for E o is.v at K and atm. If the voltage were 0. V, the efficiency η R would be 0., indicating that % of the available energy was not converted to work. This work potential (exergy) is lost, dissipated as heat because of the inefficiencies or polarizations within the fuel cell.

69 Actual Efficiency The ideal efficiency is simply the change in free energy, which is the maximum useful work we can obtain from any system, divided by the heat of reaction η i = G H = T S 0 H = n efe H = ItE H Where I is the current and t the for which the current flows. In a fuel cell under load, the actual electromotive force that drives the electrons through the external circuit will fall below E to some lower value, we will call E ac. The reasons for this drop are: a) An undesirable reaction may be taking place at the electrodes or else where in the cell; b) Something may be hindering the reaction at anode or cathode; c) a concentration gradient may become established in the electrolyte or in the reactants; d) Joule heating associated with the IR drop occurs in the electrolyte. The actual efficiency is η ac = n e FE ac H

70 Hydrogen - Oxygen Fuel Cell Quantity H 0.O H O Change Enthalpy kj H = -8.8 kj Entropy 0.8 J/K 0. x 0. J/K. J/K T S = -8. kj Pressure: atmosphere Temperature: 8K W = P V = (0. kpa)(. moles)(-. x 0 - m /mol)(8k/k) = - J U = H - P V = -8.8 kj -. kj = -8. kj G = H - T S = -8.8 kj + 8. kj = -. kj η=./ 8.8 = 0.8 (8%)

71 Homework Problem Due: November, 00 Examine how the ideal efficiency of a simple hydrogen-oxygen fuel cell changes as its operating temperature is 0 raised from 8K to 000K. Also calculate the actual and ideal efficiencies for the cell operating in the standard conditions of 8K and atm. The actual cell voltage is 0.V while the cell delivers. amps. The hydrogen flow is 0. cm /sec. Independent measurements reveal that 0.0 cm /sec of hydrogen is escaping through electrolyte unreacted.

72 Basic Fuel Cell Reactions The overall reaction of a PEM fuel cell is: H + O H O This reaction is the same as the reaction of hydrogen combustion, which is an exothermic process (energy is released): H + O H O + heat The heat, typically given in terms of enthalpy, of a chemical reaction is the difference between the heats of formation of products and reactants: H = ( h f ) H ( h O f ) H ( h f ) = 8kJ /g 0 0 = 8 kj O mol Heat of formation of liquid water: -8 kj/mol at o C and at atmospheric pressure. H + O H Ol ()+ 8kJ /mol Reference: PEM fuel cells: theory and Practice, Frano Barber, Elsevier Academic Press, 00

73 Hydrogen HHV and LHV Hydrogen heating value is used as a measure of energy input in a fuel cell. Hydrogen heating value: the amount of heat that may be generated by a complete combustion of mol of hydrogen = the enthalpy of hydrogen combustion reaction = 8 kj/mol The result of combustion is liquid water at o C and the value of 8 kj/mol is considered as Higher Heating Value (HHV). If the combustion is done with excess oxygen and allowed to cool down to o C, the product will be in the form of vapor mixed with unburned oxygen. The resulting heat release is measured to be kj/mol, known as Lower Heating Value (LHV). H + O H Og ()+ kj /mol The difference between HHV and LHV is the heat of evaporation of water at o C: H fg = 8 = kj /mol

74 Theoretical Electrical Work Not all the hydrogen s energy can be converted into electricity. The portion of the reaction enthalpy that can be converted to electricity corresponds to Gibbs free energy: G = H T S S is the difference between entropies of products and reactants: At o C and at one atmosphere h f (kj/mol) Hydrogen 0 Oxygen 0 Water (liquid) -8.0 Water (Vapor) -.8 S = ( S f ) H ( S O f ) H ( S f ) O s f (kj/mol) kj/mol is converted into heat. G = ( ( ) )=.kj /mol

75 Electrical work: Theoretical Fuel Cell Potential W e = n e FE = G n e = (two electron per molecule); F =,8 Coulombs/electronmol. The theoretical potential of fuel cell at o C and at one atmosphere: E = G n e F =,0J /mol,8as/mol =.Volts Temperature Effect: a, b and c are empirical Coefficients, different for each gas

76 Theoretical Fuel Cell Potential H O H O (g) a b c.0e-0 -.E-0.8E-0 H T = H 8. + at ( 8.)+ b ( T 8.) + c ( T 8.) T ( S T = S 8. + aln + bt ( 8.)+ c T 8. ) 8. H (kj/mol) S (kj/mol) G (kj/mol) a = a H O a H a O H + O H Ol () b = b H O b H b O H + O H Og () c = c H O c H c O For T=8., E =. Volts For T=., E =. Volts

77 Theoretical Fuel Cell Efficiency η = G H =. 8.0 = 0.8 η = G = 8. H LHV.8 = 0. η = G H = G n e F H n e F =..8 = 0.8 Potential corresponding to hydrogen s higher heating value

78 Effect of Pressure: Theoretical Fuel Cell Potential The change in Gibbs free energy may be shown to be: G = V m dp Where V m = molar volume, m /mol and P = pressure, Pa For an ideal gas: PV m = RT Therefore: dg = RT dp P G = G o + RT ln P P o G o : Gibbs free energy at standard temperature, o C and at one atmosphere

79 Sustainable Energy Science and Engineering Center Equilibrium of a gas mixture: For a chemical reaction occurring at constant pressure and temperature, the reactant gases A and B form products M and N. The change in the Gibbs energy Theoretical Fuel Cell Potential Where a, b, m and n are stoichiometric coefficients. 0 aa + bb mm + nn G = mg M + ng N ag A bg B Where g is in molar quantity (kj/mol) P M P G = G o + RT ln o P A P o In terms of standard Gibbs energy. The reference pressure is usually taken as atm. m a PN P o P B P o b n

80 Theoretical Fuel Cell Potential Equilibrium of a gas mixture: G = G o + RT ln P m n M P N P a b A P B G = G o + RT lnq m P N n Q = P M P a b A P B 0 Q: Reaction coefficient for the pressures The change in Gibbs energy of a reaction involving gases is: G = G o + RTlnQ

81 Theoretical Fuel Cell Potential For a hydrogen-oxygen fuel cell, the overall reaction stoichiometry is The Nernst equation becomes: 0 H + O H O G = G o + RT n e F ln P H O / P H P O / E = E o + RT n e F ln P H P O P H O When liquid water is produced in a fuel cell: P H O = For higher reactant pressures the cell potential is higher

82 Theoretical Fuel Cell Potential Air vs oxygen: 0 E = E O E air = RT n e F ln P O = RT P air n e F ln 0. At 80 o C, the voltage loss becomes 0.0V. In practice, this is much higher. 0.

83 Home work. For a hydrogen/air fuel cell operating at 0 o C with reactant gases at atmospheric pressure and with liquid water as a product, calculate the theoretical cell potential taking into account the changes of reaction enthalpy and entropy with temperature (equations for H T and S T ).. Calculate the expected difference in theoretical cell potential between a hydrogen/oxygen fuel cell operating 80 o C and bar for reactant gases and the same fuel cell operating at atmospheric pressure. What if the pressure is increased to 0 bar?

84 Sustainable Energy Science and Engineering Center Backing layer Catalyst layer Electrochemical Kinetics A chemical reaction involves both a transfer of electrical charge and a charge in Gibbs energy. The electrochemical reaction occurs at the interface between the electrode and electrolyte. Gas diffusion layer 0 Electrolyte Typical Electrode Design The charge must overcome an activation energy barrier in moving from electrolyte to an electrode. The magnitude of the barrier determines the rate of reaction. The Butler-Volmer equation gives the current density, that is derived from transition state theory of electrochemistry.

85 Electrochemical Kinetics The general half-reaction expression for the oxidation of a reactant is: Where reactant R loses electrons and becomes Ox, the product of oxidation, and n is the number of electrons that are transferred in the reaction. For the opposite direction, Ox gains electrons, undergoing reduction to form R in the halfreaction. On an electrode at equilibrium conditions, both processes occur at equal rates and the currents produced by two reactions balance each other, giving no net current from the electrode Source: Fuel cell technology Handbook - Chapter.

86 Sustainable Energy Science and Engineering Center Single-step Electrode reactions Electrochemical Kinetics Considering only one direction of the reaction, the current produced is 0 Where I is the current (in amperes), A is the active area of the electrode (cm ), F is the Faraday s constant (the charge per mole of electrons=,8 (coulombs/mole e - i = nf.j ) and j is the flux of reactants reaching the surface (mole/sec). The current density (per unit area) is i = nf j The current is produced from the reactants that reach the surface of the electrode and lose or gain electrons. The flux is determined by the rate of conversion of the surface concentration of the reactant. For the forward reaction (subscript f), the flux arising from the reduction of Ox is j f = k f [ Ox] o Forward rate coefficient Surface concentration of the reactant

87 Electrochemical Kinetics Single-step Electrode reactions For the backward reaction (subscript b), the flux produced by the oxidation of R is 0 j b = k b [ R] o Backward rate coefficient The net flux is j = j f j b The net current density that appears on the electrode when the current is produced is given by ( ) i = nfk f [ Ox] o Fk b R [] o

88 Sustainable Energy Science and Engineering Center Butler-Volmer Equation From the Transition state theory (refer to any physical chemistry book), the heterogeneous rate coefficient, k. is a 0 function of the Gibbs energy of activation and is given by Boltzmann s constant k = k BT h G exp RT (.80x0 - j/ o K) Plank s constant (.x0 - Js) Electrochemical Kinetics Gibbs energy of activation (kj/mol) Because an electrochemical reaction occurs in the presence of an electrical field, the Gibbs energy of activation consists of both chemical and electrical terms Reduction G = G c + nf φ Oxidation G = G c n( β)f φ Transfer coefficient (0.) Change in electrical potential

89 For a reduction reaction Electrochemical Kinetics k f = k BT h exp G c, f RT Chemical Component 0 exp nβf φ RT Electrical Component With the over potential is defined as η = φ φ rev For a hydrogen-oxygen fuel cell, the reversible potential of the anode is 0 V; at the cathode it is +. V at o C. k f = k T B h exp G c, f exp nβf φ rev exp nβfη RT RT RT k b = k T B h exp G c, f n( β)f φ exp rev n( β)fη exp RT RT RT Reduction Oxidation

90 Butler-Volmer Equation when the electrode is in equilibrium and at its reversible potential, the overpotential and external current are both zero. In this case, the 0 exchange current density, i o is defined as [ ] o k o, f = nf[ R] o k o,b i o (A /cm ) nf Ox The current density is given by i = i o exp nβfη n( β)fη exp RT RT Electrochemical Kinetics Reduction term Oxidation term If η > 0 then the oxidation component becomes large and the reduction reaction on the on the electrode becomes small. The net current density is negative, which corresponds to a net oxidation reaction where electrons leave the anode of the fuel cell. In operating fuel cells, because of the cathode reaction of oxygen reduction requires a more significant overpotential (η) than the anode reaction. For hydrogen-oxygen fuel cell, the reversible potentials at the anode and cathode are 0V and. V respectively. η = φ φ rev

91 Fuel Cell Components Fuel Cell Components Impact on Performance The proton exchange membrane fuel cell (PEMFC) 0 Special plastic membrane used as the electrolyte + electrodes (anode & cathode) is called the membrane electrode assembly (MEA). It is not thicker than a few hundred microns. When supplied with fuel and air, generates electric power at cell voltages around 0,V and power densities up to about W/cm electrode area.

92 The Proton Exchange Membrane Fuel Cell (PEMFC) 0 H H + + e O + H + + e H O Anode (E r = 0 V) Cathode (E r =. V) The electrochemical reactions take place at the anode and the cathode catalyst layer respectively. The best catalyst is platinum. The catalyst is used at the rate of about 0. mg/cm. The basic raw material cost of platinum for a - kw PEMPC cell is about $0 - a small portion of the total cost.

93 The gas diffusion layer and backing layer (substrate) at the anode allows hydrogen to reach the reactive zone within the electrode. Upon reaching, protons migrate through the ion conducting membrane, and electrons are conducted through the gas diffusion layer and ultimately to the electric terminals of the fuel stack. The substrate therefore has to be porous to allow gas and electrically conducting. Not all of the chemical energy supplied to the MEA by reactants is converted into electric power. Heat will also be generated and the substrate also acts as a heat conductor to remove heat from the reactive zones of the MEA. 0 The PEM Fuel Cell Backing layer Catalyst layer Gas diffusion layer Membrane Electrode Assembly (MEA) Electrolyte Water is formed at the cathode. If the water is in liquid form, there is a risk of liquid blocking the pores within the substrate and consequently gas access to the reactive zone. The oxidant used in most applications is air, therefore, 80% of the gas present is inert. Fuel cell operation will result in depletion of oxygen towards the active cathode catalyst. The membrane acts as a proton conductor, thus requires it to be well humidified. Because the proton conduction process relies on membrane water. As a consequence, an additional water flux from anode to cathode is present and is associated with the migration of protons. Humidity is often provided with the anode gas by pre-humidifying the reactant.

94 The PEM Fuel Cell MEA Component Task/effect Anode Substrate Fuel supply and distribution, Electron conduction, heat removal from reaction zone, water 0 supply into electrocatalyst Anode catalyst layer Proton exchange membrane Cathode catalyst layer Catalysis of anode reaction, proton conduction into membrane, electron conduction into substrate, water and heat transport Proton conduction, water transport, electronic insulation Catalysis of cathode reaction, oxygen transport to reaction sites, proton conduction from membrane to reaction sites, electron conduction from substrate to reaction zone, water removal from reaction zone into substrate and heat removal. Cathode substrate Oxidant supply and distribution, electron conduction towards reaction zone, heat removal and water transport Source: Fuel cell technology handbook, Chapter

95 Factors Limiting Fuel Cell Performance The losses that takes place at electrodes are generally attributed to some form of polarization - a term used to denote the difference between the theoretical voltage of a given electrode and the experimental voltage when the current is drawn from the cell. The losses are classified in three categories: Chemical polarization. Concentration polarization and resistance polarization. The theoretical value of the open circuit voltage of a hydrogen-oxygen fuel cell is given by V = E = g f F This gives a value of about. V for a cell operating at 8K. V ac = V V conc(c) V chem(c ) V conc(a ) V chem(a ) 0 IR

96 Chemical or Activation Polarization It is customary to express the voltage drop due to chemical polarization by strictly empirical equation, called the Tafel equation as V chem = a + a = RT αnf ln( j ) o b = RT αnf b lnj 0 Where J is the apparent current density at the electrode, α and j o are kinetic parameters, the former being a constant that represents the fraction of V chem that aids a reaction in proceeding (For a hydrogen electrode, its value is about 0. for a great variety of electrode materials and for the oxygen electrode it is between 0. and 0.. The best possible value of b` will have little impact), the later being the exchange current density, intimately related to the height of the activation energy barrier. Gas diffusion electrode reduces the chemical polarization by maximizing the three-phase interface of gas-electrode-electrolyte. The small pores create large reactive surface areas per unit geometrical area and allow free entrance to reactants and exit to products. Increases in pressure and temperature will also generally decrease chemical polarization.

97 0 Chemical Polarization The effect of current density and the exchange current density on chemical polarization loss Potential loss

98 Concentration Polarization After current begins to flow in an electrochemical cell, there is a loss of potential due to inability of the surrounding material to maintain the initial concentration of the bulk fluid. This uneven concentration produces a back EMF which opposes the voltage that a fuel cell would deliver under 0 completely reversible conditions. The concentration of electrolyte in the vicinity of an electrode during reaction should be maintained at the desirable condition. V conc(c) = RT nf ln c if c b : average concentration in bulk electrolyte c b c if : concentration at the interface V conc(c) = RT nf ln J L J L J V conc(a) = RT nf ln J L + J J L J L = nfd δ c b D: diffusion coefficient

99 Concentration Polarization An empirical equation better describes the concentration polarization losses: 0 j d V conc = c e Where c and d are empirical coefficients with values of c= x0 - V and d = 0. A/cm. For less than 0. A/cm, the potential loss due to concentration polarization is quite small and increases rapidly to about 0. V at. A/cm.

100 an additional loss of potential. negligibly low even at fairly high current densities. Resistance Polarization When an electrochemical reaction occurs at an electrode there is generally a significant change in the specific conductivity of electrolyte which involves Hydrogen-oxygen fuel cells employing concentrated solutions of potassium or sodium hydroxide as electrolytes show that resistance polarization is Ohmic Losses: V= IR In most fuel cells the resistance is mainly caused by the electrolyte, through the cell interconnects or bipolar plates. Reducing these internal resistances can be accomplished by the use of electrodes with the highest possible conductivity, good design and use of appropriate materials for the bipolar plates and cell interconnects and making electrolyte as thin as possible. Typical values for R are between 0. and 0. Ωcm. 0

101 Heat Transfer In cells with high current densities, it is often important to calculate the heat transfer within a fuel cell. ) The electrochemical reaction producing 0 the current in the cell is not adiabatic which gives rise to a reversible heat transfer whose magnitude is T S. ) Some of the fuel reacts chemically with the oxidizer rather than electrochemically to generate an irreversible heat transfer. ) The cell operates at some voltage less than the theoretical open circuit voltage with the difference manifesting itself as I R and I V heat in the cell (I is the current drawn and R and V represent irreversible resistances and voltage drops). [ ( )] Q Ý t = Q Ý rev + Q Ý chem(irr) + Q Ý V = nf T S ++nf V V ac Generally small

102 0. Fuel Cell Voltage Losses 0. Activation polarization loss Potential Loss (V) Ohmic losses Current density (ma/cm ) Concentration polarization loss

103 Polarization Curve 0 Source:

104 Direct Energy Conversion: Fuel Cells Section.. in the Text Book References and Sources: Direct Energy Conversion by Stanley W. Angrist, Allyn and Beacon, 8. Fuel Cell Systems, Explained by James Larminie and Andrew Dicks, Wiley, 00. Fuel Cell Technology Hand Book, Edited by Gregor Hoogers, CRC Press, 00 Fuel Cell Hand Book, US DOE - available on the web.

105 Performance of a single cell operating on hydrogen as the fuel and oxygen in air as the oxidizer. The porous electrodes of either carbon or nickel employed are separated by a 0% solution of KOH (potassium hydroxide). The 0 cell operates at a temperature of 8K and the fuel and air are supplied at one atmosphere. The faradic efficiency (the fraction of the reaction which is occurring electrochemically to give a current is called faradic or current efficiency, η F = /(nfn fu ); where N fu is the total number of moles of fuel reacted electrochemically per second) for the cell is estimated to be % Fuel Cell Design Calculation 0% of the fuel supplied to the cell will escape through the electrolyte unreacted. Separation between electrodes, w= 0.cm w =0.cm l =cm Height of the cell, l = cm Depth of the cell, d = cm Average electrolyte velocity, u = cm/s (Supplied by an external pump)

106 Electrolyte Properties The physical properties of the electrolyte at 8K area as follows: Concentration: 0% KOH (wt) or c b =. x 0 - mole/cm Density: ρ =. gm/cm Dynamic Viscosity: µ =. x 0 - poise Kinematic Viscosity: ν =.88 x 0 - cm /s Conductivity: σ = 0. (ohm-cm) - Diffusion Coefficient for OH - ions: D =. x 0 - cm /s 0

107 Open Circuit Voltage We now calculate the open circuit voltage. We assume that each electrode reaction follows the half-cell reactions Yielding a cell reaction of H (g) H + + e 0.O (g) + H + + e H O(l) 0 H (g) + 0.O (g) H O(l) Anode E o = 0 We may now apply the Nernst equation to our cell potential E = E o RT n e F ln P H O / P H P O Cathode E o =.v P i = n i n t P t where P t is the total pressure of the mixture and n i /n t is the mole fraction of the i th constituent.

108 Sustainable Energy Science and Engineering Center The partial pressures in the Nernst equation are often eliminated in favor of a function that is derived from general equation of state, generally denoted by f, the fugacity, which is a measure of the tendency of a component to escape from a solution. It is equal to partial pressure 0only when the vapor behaves like an ideal gas. We also define a quantity called the activity a A = f A /f Ao. We can then write the Nernst equation as E = E o RT n e F ln a H O a H a O The activity of H O to be used in this equation should be that of water in 0% KOH solution. This value is somewhat less than one, but we may take it as one to make our answer conservative. The hydrogen is supplied at one atm (has an activity of one since the activity is equal to the partial pressure of an ideal gas), the activity of oxygen is 0.. Then we have E = V =.. ()00 / ( )( 8) ( ) ln () ()0. ( ) 0. E = V =. 0.0ln(.8) =.volts Open Circuit Voltage

109 Sustainable Energy Science and Engineering Center We initially assume an operating current density for our fuel cell of 0. amp/cm Use Tafel equation to calculate losses due to chemical polarization. V chem = a + b lnj a = RT αnf ln( j ) o b = RT αnf Chemical Polarization 0 V chem(a ) = ln J V chem(c) = ln J Where the current density is expressed in milliamperes per square centimeter. V chem(a ) = ln(00) = 0.volt V chem(c) = ln(00) = 0.volt

110 We now calculate the polarization due to concentration gradients in the electrolyte near the electrodes. V conc(c) = RT 0 nf ln J L J L J J L = nfd c V conc(a) = RT nf ln J L + J δ b J L Calculation of the limiting current density: Velocity of the electrolyte = cm/s; For a fully established flow between the electrodes, we have J L,av =.nfc b u Re w Re w = uw ν =. Concentration Polarization ( ) / ( Sc) / ( w /l) / Sc = ν D =.x0 w l = J L,av = 0.amp/cm

111 Concentration Polarization V conc(c) = RT nf ln J L 0.0volt J L J 0 V conc(a) = RT nf ln J L + J 0.0volt J L The gas side concentration polarization at the anode is ignored. We will calculate the gas side polarization at the air electrode using the partial pressure of oxygen in the air for p g. V conc(g ) = RT nf ln P r P = 0.0ln = 0.0volts g 0. Where P r is the gas pressure in the pores of the electrode

112 Resistance Polarization We now compute the actual operating voltage of the fuel cell by subtracting from the open circuit voltage. volts, the various polarization losses and IR drop across the cell: 0 IR = Jw = = 0.volts σ 0. V ac = = 0.Volt The power out put of the cell: P o = V ac I = V ac JA = = 0.watts

113 The thermal efficiency is given by Efficiency η ac = nfv ac (mole elec /mole).0(kcal /volt mole) 0.Volt = H 8.kcal 0 /mole H = 8.kcal /mole F =.0 kcal/volt-mole The efficiency based on the actual voltage is η v = V ac V = 0.. =.% = 8.% Assuming that the product of the reaction is liquid water, the faradaic efficiency is given by. η F = JA nfn fu = N fu = 0. N fu =. 0 moleh /s F =,00 (amp-sec)/(mole-sec)

114 Heat Transfer Q Ý rev = η F N fu ( H G)= 0. N fu ( 8. (.))=.watts H = 8.kcal /mole G =.kcal /mole 0 Q Ý chem(irr) = ( η F )N fu ( H)=.8watts Q Ý V = IV ( ac V)=.watts Total heat that must be removed from the cell for it to stay steady state is. watts, 0% larger than net power output of the cell.

115 Home Work Design calculation for a fuel cell: Home work: Using your own calculations reproduce the curve below for the fuel discussed in the class today. 0

116 Heat Generation Area = 00 cm ; Operating pressure = atm; Operating Temperature = 80 o C; E = 0. V; Current generation = 0. A/cm. Power due to heat = Total power generated 0 - electrical power P heat = P total -P electrical = (.V -0.V) x 0A = (V ideal x I cell ) - (V cell x I cell ) = 0. V x 0A = 0 J/s = 08 KJ/hr = 0 W While generating about W of electrical energy.

117 Operating Variables Pressure, Temperature, Gas composition, Reactant utilization and Current density 0

118 Operating Pressure 0 T V loss =.8 0 P η m η c P 0.8 λ Stoichiometry (~.0) V gain = Cln P P C volts Motor and drive system efficiency~ 0. Compressor efficiency ~ 0.

119 Operating Pressure 0 Net voltage change resulting from operating at high pressure for two different PEM fuel cell models

120 Temperature Effect 0

121 Polarization Contribution 0

122 Effect of Oxygen Pressure 0

123 Polarization Curve 0 Source:

124 Polymer Electrolyte Membrane Fuel Cell Load Fuel Cell stack

125 The Backing Layer Equivalent concentration profile Porous carbon cloth or carbon paper, typically 00-00µm thick. About - sheets are used. δ True concentration profile Diffusion layer Effective diffusion of each reactant gas to the catalyst on the Membrane Electrode Assembly (MEA). The diffusion takes place from a region of high concentration, outer side of the backing layer, to a region of low concentration, the inner side of the backing layer next to catalyst layer where the gas is consumed by the reaction. The gas is in contact with the entire surface area of the catalyzed membrane. Assist in water management during the cell operation. Allows the right amount of water vapor to reach the MEA to keep the membrane humidified. It also allows the liquid water produced at the cathode to leave the cell.

126 The Bipolar Plate Main tasks: Current conduction; Heat conduction; control of gas flow and product water removal 0 The first main task is to provide reactant gases evenly across the active area of the MEA. Current designs have channels to carry reactant gas from the point at which it enters the cell to the point at which the gas exits. The flow field in the channels has a large impact on the distribution of gases. The design also affects water supply to the membrane and water removal from the cathode. The second main task is that of current collector. Usually made of graphite into which channels are machined. These plates have high electronic and good thermal conductivity and stable in the chemical environment inside the cell.

127 Water Management 0 Use the water leaving the cell to do the humidification. Channels in bipolar plates

128 Some times referred to as Solid Polymer fuel cell (SPFC) The electrolyte: Ion conduction polymer 0(works at low temperatures, hence start quickly) Electrodes: Catalyzed porous electrodes The anode-electrolyte-cathode assembly (membrane electrode assemblies (MEA)) is one item and is very thin. The MEA s are connected in series using bipolar plates. No corrosive fluid hazards - suitable for use in portable applications and widely used in cars and buses The most commonly used polymer membrane: Nafion PEM Fuel Cells (PEMFC) Source: Fuel Cell systems Explained by James Larminie & Andrew Dicks, Wiley, 00, Chapter.

129 Polymer Electrolyte Most commonly used: sulphonated fluoropolymers - fluoroethylene 0 sulphonated fluoroethylene (PTFE) The strong bonds between the fluorine and the carbon make it durable and resistant to chemical attack and can be made into very thin films (~0µm). It is strongly hydrophobic, which helps to drive the product water out of the electrode.

130 Electrodes Platinum is used generally as the catalyst (0. mg/cm ) Anode and cathode are essentially the same. The platinum is formed into very small particles on the surface of larger particles of finely divided carbon powders. The carbon-supported catalyst is fixed to a porous and conductive also diffuses the gas on to catalyst -the gas diffusion layer. material such as carbon cloth or carbon paper. The carbon paper Alternatively, the platinum on carbon catalyst is fixed directly to the electrolyte, thus manufacturing the electrode directly on to the membrane. Then a gas diffusion layer - carbon cloth or paper (00 to 00 µm thick) is added. It also forms an electrical connection between the carbon-supported catalyst and the bipolar plate. 0

131 Platinum Loading Effect 0

132 Water Management Proton conductivity is proportional to the water content. The H + ions moving from the anode to the cathode 0 pull water Molecules with them (up to five H O molecules are dragged for each proton - electro-osmotic drag). At high current densities, the anode side of the electrolyte can be dried out. At temperatures over K, the electrodes are typically dry. Solution: Air and hydrogen humidification before they enter the cell.

133 Heat Production W fuel cell 0 kw fuel cell system

134 PEM Fuel Cell Stack Load

135 Multi-cell Stack Performance 0 Simulation Results - PEMPC stack Hydrogen produced from gasoline Source: Simulation study of a PEM Fuel cell system fed by Hydrogen produced by partial oxidation by Ozdogan S., et al.

136 Multi-cell stack Performance 0

137 PEMFC System Hydrogen tank 0

138 Cooling Air Supply 0 Ballard Nexa PEM Fuel cell.

139 Fuel Cell Systems and Hydrogen Production

140 Fuel Cell Type 0 < kw - 0kW < 00W 0kW 0kW - MW kw - MW

141 Electrochemical Reactions 0

142 Efficiency 0

143 Efficiency Source: Hazem Tawfik, Sept 00

144 Pressure Effects Hydrogen pressure Oxygen pressure Source: Hazem Tawfik, Sept 00

145 Temperature Effect Source: Hazem Tawfik, Sept 00

146 Humidity effect at Room Temperature Source: Hazem Tawfik, Sept 00

147 Parametric Effects: Temperature has most effect Source: Hazem Tawfik, Sept 00

148 Air Vs O Source: Hazem Tawfik, Sept 00

149 PEMFC Emissions PC Fuel cell: 00 kw Fuel: Natural gas Source: Hazem Tawfik, Sept 00

150 Fuel Cell System Fuel Cell Stack 0 Control System Fuel Delivery Air Delivery Thermal Management Water Management Power Conditioning

151 Critical Materials and Costs Example: Polymer Electrolyte Fuel Cell Stack ( kw) -Polymer membrane - Catalyst (precious metals) - Bipolar plate 0 Source: Material development for cost reduction of PEFC by J. Garche, L. Jorissen & K.A. Friedrich, Center for Solar energy and hydrogen research, Baden-Wuerttemberg (ZSW), Germany

152 PEMFC Challenges MEA tolerance for CO in reformed H High temperature operation (~0 o C) MEA Durability - 0,000 hrs with < 0% degradation, % cross over, area resistance <0. ohm.cm Cost - $00/kW, $0/kW for MEA Efficiency - 0 ~ 0% Fixed cost of Graphite bipolar plate: $0/kW Running cost of hydrogen per kwh : $0.0 Source: Hazem Tawfik, Sept 00

153 Fuel Cell Types Alkaline (AFC) Solid Polymer (SPFC, PEM or PEFC) Direct Methanol (DMFC) Phosphoric Acid (PAFC) 0 Molten Carbonate (MCFC) Solid Oxide (SOFC)

154 Fuels Type Application Hydrogen 0 Transport, stationary & Portable Methanol Natural Gas Gasoline Transport & Portable Stationary Transport Diesel Transport Jet Fuels Military

155 Fuel Reforming Hydrogen is produced from fuel reforming system such as methane and steam. CH + H O H + CO CO + H O H + CO Carbon monoxide has a tendency to occupy platinum catalyst sites, hence must be removed. Other fuels: C 8 H 8 + 8H O H + 8CO 0 water gas shift reaction

156 Fuel Reformer Steam reforming: It is mature technology, practiced industrially on a large scale for hydrogen production. The basic reforming reactions for methane and a generic hydrocarbon C n H m are 0 CH + H O CO + H ; H = 0kJ /mol C n H m + nh O nco + m + n H CO + H O CO + H ; H = kj /mol

157 DMFC System 0

158 Liquid-Feed DMFC Reactions 0

159 Direct Methanol Fuel Cell 0 Operating at ambient conditions

160 Micro-scale Methanol Fuel Processor 0

161 Hydrogen Production Source:

162 Hydrogen Production Source:

163 Hydrogen From Water There is enough water to sustain hydrogen!

164 Electrolysis

165 Electrolysis

166 Photoelectrolysis

167 Hydrogen Production

168 Photoelectrochemical Conversion System

169 Electrolysis Efficiency Systems that claim 8 %

170 Photoelectrolysis

171 Photoelectrolysis

172 Photoelectrolysis

173 Artificial Photosynthesis

174 Thermochemical Water Splitting

175 Thermochemical Production

176 Thermochemical Production Thermal-to-hydrogen energy efficiency Solar-thermal heat source is a logical choice

177 Thermochemical Production Solar-thermal heat source

178 Thermochemical Cycle Efficiency Process Temperature ( o C) Heat-to-Hydrogen Efficiency (%) Electrolysis 0- Sulfur-iodine thermochemical cycle 80 - Calcium-bromine thermochemical cycle 0-0 Copper-chlorine thermochemical cycle 0 * * Energy efficiency calculated based on thermodynamics

179 Solar Thermochemical System

180 Thermochemical Process

181 Hydrogen Production

182 Fossil Fuel Use

183 Solar Heat Generation

184 Solar Heat Generation

185 Solar Thermal Hydrogen Production A concept for integrating solar thermal energy and methane gas to produce a range of solar-enriched fuels and synthesis gas (CO and H ) that can be used as a power generation fuel gas, as a metallurgical reducing gas or as chemical feed stock e.g. in methanol production.