Sustainable Energy Science and Engineering Center
Direct Energy Conversion: Fuel Cells
Section 4.7.1 in the Text Book 1982. References: Direct Energy Conversion by Stanley W. Angrist, Allyn and Beacon,
Fuel Cell Systems, Explained by James Larminie and Andrew Dicks, Wiley, 2003. Fuel Cell Technology Hand Book, Edited by Gregor Hoogers, CRC Press, 2002 Sustainable Energy Science and Engineering Center Fuel Cells
Introduction:
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Hydrocarbon Fuels 1 Useful power 2 6 9 Combustion Energy stored3 in chemical bonds 7 4 11
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. Sustainable Energy Science and Engineering Center Fuel Cell Efficiency
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5 2 6 9 1 3 7 4 11 Sustainable Energy Science and Engineering Center PEM Fuel Cell Performance Sustainable Energy Science and Engineering Center Fuel Cells William Grove 1839
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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. Sustainable Energy Science and Engineering Center Fuel Cells Mond & Langer (1889) - Gas battery
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5 2 6 9 1 3 7 4 11 Sustainable Energy Science and Engineering Center Daniell Cell
1 We10 will use the term anode to mean the electrode at which
5 oxidation takes place - losing of 2 6 9 1 3 electrons 4 7 Cathode is the electrode at which 11 reduction takes place - electrons are gained from the external circuit 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 1863. 1 10
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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. Sustainable Energy Science and Engineering Center Historical Development
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5 2 6 9 1 3 7 4 11 Sustainable Energy Science and Engineering Center Fuel Cell Types
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5 2 6 9 1 3 7 4 11 Sustainable Energy Science and Engineering Center Hydrogen - Oxygen Fuel Cell
At the anode the hydrogen gas ionizes 1 10 releasing electrons and creating H+ ions (or
5 protons). This reaction 1 releases energy. 2 6 9 3 + − 2H2 → 4H + 4e 7 4 11 At the cathode, oxygen reacts with electrons taken from the electrode, and H+ ions from the electrolyte, to form water
− + O2 + 4e + 4H → 2H2O
An acid with free H+ ions. Certain polymers can also be made to contain mobile H+ ions - proton exchange membranes (PEM) Sustainable Energy Science and Engineering Center Membrane Electrode Assembly
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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. Sustainable Energy Science and Engineering Center 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 1 10 natural gas, methanol or petroleum using a fuel processor, which converts the hydrocarbons into hydrogen and carbon dioxide through a catalytic
5 chemical reaction. Membrane Electrode Assembly 1 2 6 9 3 Each membrane electrode assembly consists of two electrodes (the anode and the4 cathode) with a very thin layer of catalyst, bonded to either side of a proton exchange membrane. Air 11 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. Sustainable Energy Science and Engineering Center Micro Fuel Cell
The fuel cells are 5 mm3 and generate up to 100 mWatts.
1 CWRU10 (Case Western Reserve University) researchers have miniaturized this process
5 through the use of micro fabrication technology, which1 is used to print multiple 2 6 9 3 layers of fuel cell components onto a 7 4 substrate. Inks were created to replicate 11 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. Sustainable Energy Science and Engineering Center Proton Exchange Membrane Fuel Cells (PEMFC)
PEM fuel cells use a solid polymer membrane (a 1 thin plastic10 film) as the electrolyte. This polymer is permeable to protons when it is saturated with water, but it does not conduct electrons.
5 6 1 2 9 The reactions at the electrodes are as follows: 3 4 7 Anode Reactions: 2H2 => 4H+ + 4e- Cathode Reactions: O + 4H+ + 4e- => 2 H O 11 2 2 Overall Cell Reactions: 2H2 + O2 => 2 H2O
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 100ºC, 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. Sustainable Energy Science and Engineering Center Alkaline Fuel Cell
Alkaline fuel cells (AFC) are one of the most developed technologies and have been used since the mid-1960s 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 1 efficient in generating electricity10 at nearly 70%. Alkaline fuel cells use an electrolyte that is an aqueous (water-based) solution
5 of potassium hydroxide (KOH) retained in a porous stabilized matrix. The 2 6 concentration9 of KOH can be varied wi1 th the fuel cell operating temperature, 3 which ranges from 65°C to 220°C. The charge carrier for an AFC is the 7 hydroxyl4 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 back11 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: 2 H2 + 4 OH- => 4 H2O + 4 e- Cathode Reaction: O2 + 2 H2O + 4 e- => 4 OH- Overall Net Reaction: 2 H2 + O2 => 2 H2O
One characteristic of AFCs is that they are very sensitive to CO2 that may be present in the fuel or air. The CO2 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, H2O and CH4, which are harmless or even work as fuels to other fuel cells, are poisons to an AFC. Sustainable Energy Science and Engineering Center Alkaline Fuel Cell System
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5 2 6 9 1 3 7 4 11 Sustainable Energy Science and Engineering Center Solid Oxide Fuel Cell
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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 H2O and the CO to CO2. 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. Sustainable Energy Science and Engineering Center Solid Oxide Fuel Cell
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5 2 6 9 1 3 7 4 11 Sustainable Energy Science and Engineering Center Molten-carbonate Fuel Cell
The diaphragm between the anode and the cathode consists of a matrix filled with a 1 10 2- carbonate electrolyte. Carbonate ions (CO3 ) pass through the diaphragm and reach the anode. Here
5 they discharge an oxygen atom, which combines 2 6 9 with the hydrogen1 flowing past to form water 3 4 (H2O). This sets carbon dioxide (CO2) and two 7 electrons free. The electrons flow over an 11 electronic conductor to the cathode: current flows. Similarly, the remaining carbon dioxide
(CO2) 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. Sustainable Energy Science and Engineering Center Carbon Conversion Fuel Cell
Carbon (C) and oxygen (O2) can react in a high- 1 temperature10 fuel cell with the carbon, delivering electrons (e) to an external circuit that can power a motor. The net
5 electrochemical reaction— carbon and oxygen forming 2 6 carbon9 dioxide—is the same1 as the chemical reaction for 3 carbon combustion, but it allows greater efficiency for 4 7 electricity production. The pure carbon dioxide (CO2) product11 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 (10- to 1,000-nanometer-diameter) carbon particles distributed in a mixture of molten lithium, sodium, or potassium carbonate at a temperature of 750 to 850°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 1 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. Sustainable Energy Science and Engineering Center 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 1 external circuit10 to the cathode electrocatalyst where they are consumed together with oxygen in a reduction reaction. The
5 circuit is maintained within the cell by the conduction of 2 6 protons9 in the electrolyte1 . 3 4 In modern cells, electrolytes based on proton conducting 7 polymer electrolyte membranes (e.g., Nafion™) are often used,11 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,
CH3OH + 3/2O2 CO2 + 2H2O 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. Sustainable Energy Science and Engineering Center Direct Methanol Fuel Cell
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5 2 6 9 1 3 7 4 11 Sustainable Energy Science and Engineering Center Direct Methanol Fuel Cell
1 10 Condenser
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11 Fuel cell stack
Load Sustainable Energy Science and Engineering Center Fuel Cell Applications
1Stationary power generation10 ~ 5 - 250 kW
Portable applications5 ~ 1 kW or lower 2 6 9 1 3 Automotive applications7 4 ~ 5 - 100 kW Airplane Applications ~11 10 - 250 kW
1kW = 1.3404826 horsepower Sustainable Energy Science and Engineering Center Stationary Power Generation
Important factors: The hours of operation per year 1 10 The electric efficiency of the electricity generation process
5 The2 capital6 investment 9 1 3 Fuel cells are particularly7 4 suitable for on-site power generation. Utilizing the heat generated 11by the fuel cell improves the overall efficiency - Combined Heat and Power generation (CHP). Sustainable Energy Science and Engineering Center PEMPC Power Plant
Process Flow Diagram for a Ballard 250 kW PEMFC Plant
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5 2 6 9 1 3 7 4 11 Sustainable Energy Science and Engineering Center Fuel Cell Powered Automobile - 1967
GM Electrovan
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5 2 6 9 1 3 Alkaline fuel cell modules 7 4 supplying 32 kW 11 Sustainable Energy Science and Engineering Center Hydrogen Fuel Cell Car
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5 2 6 9 1 3 7 4 11 Sustainable Energy Science and Engineering Center Automotive Applications
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5 2 6 9 1 3 7 4 11 Sustainable Energy Science and Engineering Center Fuel Cell Performance
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5 2 6 9 1 3 7 4 11 Sustainable Energy Science and Engineering Center Fuel Cell Powered Automobile - Progress
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5 2 6 9 1 3 7 4 11 Sustainable Energy Science and Engineering Center Fuel Cell Powered Automobile - Progress
Daimler-Chrysler NeCar: 1 10
5 2 6 9 1 3 7 4 11 Sustainable Energy Science and Engineering Center Fuel Cell Powered Automobile - Progress Ford Focus Hydrogen -powered fuel cell vehicle
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5 2 6 9 1 3 7 4 11 Sustainable Energy Science and Engineering Center Fuel Cell Powered Automobile - Progress Honda fuel cell car
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5 2 6 9 1 3 7 4 11 Sustainable Energy Science and Engineering Center Methanol Fuel Cell Powered Automobile Toyota’s Methanol-powered Fuel cell Electric Vehicle
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5 2 6 9 1 3 7 4 11 Sustainable Energy Science and Engineering Center Fuel Cell Powered Automobile
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An x-ray view of Mitsubishi's new fuel cell Grandis minivan. Sustainable Energy Science and Engineering Center Portable Application
Typically well under 100W of power with significantly higher power densities or larger energy storage capacity than those of advanced batteries. Power1 generation on a larger scale , say 1 kw 10continuous output to replace gasoline or diesel generators or supply quiet electric power on boats, caravans or trucks.
5 2 6 9 1 3 7 4 11 Sustainable Energy Science and Engineering Center Solar Powered Airplane Helios
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5 2 6 9 1 3 7 4 Instead of jet fuel, Helios has about 62,00011 solar cells across the wing. The solar cells collect energy from the Sun and convert it to electricity, which runs the 14 small motors, which turn the 14 propellers. The propellers are specially designed to pull the aircraft aloft even in the very thin air that's 18 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 11, 2002 DIGITAL PHOTOS FROM SOLAR AIRPLANE TO IMPROVE COFFEE HARVEST
Funded by NASA Sustainable Energy Science and Engineering Center Electric Powered Airplane
The new Electric Plane, or E-Plane, is a high-speed, all- carbon French1 DynAero Lafayette III, built and donated by American Ghiles Aircraft. The E-Plane is being converted10 from a combustion engine to electric propulsion in three
stages. The first flights, planne5 d for next year, will be on 6 1 lithium2 ion batteries. The next flights will be9 powered by a 3 combination of lithium ion batteries augmented by a fuel 7 4 cell. Finally, the aircraft will be powered totally by a hydrogen fuel cell, with a range of more than11 500 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. Sustainable Energy Science and Engineering Center Fuel Cell Based Aircraft Propulsion
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Source: NASA TM-2003-212393 Sustainable Energy Science and Engineering Center Fuel Cell Powered Aircraft
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5 2 6 9 1 3 7 4 11 Sustainable Energy Science and Engineering Center Fuel Cell Motorbike to Hit US Streets
Top Speed: 50 mph (80 kmh) Range: 100 miles (160 km) Hydrogen Storage tank capacity: 1 kg Cost: $6,000 - 8000 Manufacturer: Intelligent Energy, London, UK ENY: Emission Neutral Vehicle 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. $1,500 and could produce enough hyd rogen to fill up the ENV for about 25 cents per tank,. Eggleston said. National Geographic News, August 2, 2005 Sustainable Energy Science and Engineering Center Fuel Cell System Sustainable Energy Science and Engineering Center FC Implementation Requirements Sustainable Energy Science and Engineering Center
Direct Energy Conversion: Fuel Cells
Section 4.7.1 in the Text Book 1982. References: Direct Energy Conversion by Stanley W. Angrist, Allyn and Beacon,
Fuel Cell Systems, Explained by James Larminie and Andrew Dicks, Wiley, 2003. Fuel Cell Technology Hand Book, Edited by Gregor Hoogers, CRC Press, 2002 Sustainable Energy Science and Engineering Center Hydrogen - Oxygen Fuel Cell
At the anode the hydrogen gas ionizes 1 10 releasing electrons and creating H+ ions (or
5 protons). This reaction 1 releases energy. 2 6 9 3 + − 2H2 → 4H + 4e 7 4 11 At the cathode, oxygen reacts with electrons taken from the electrode, and H+ ions from the electrolyte, to form water
− + O2 + 4e + 4H → 2H2O An acid with free H+ ions. Certain polymers can also be made to contain mobile H+ ions - proton exchange membranes (PEM) Sustainable Energy Science and Engineering Center Fuel Cell Input and Output
Hydrogen Energy Electricity Energy = VIt
Fuel Cell Heat
Water Oxygen Energy 11 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. Sustainable Energy Science and Engineering Center Hydrogen-oxygen Fuel Cell
The basic reaction:
2H2 + O2 → 2H2O 1 10 1 H2 + O2 → H2O
5 2 2 6 9 1 The product3 is one mole of H2O ( 18g = 1 gmole) and the reactants are 4 one mole of H2 (2g7 = 1 gmole) and a half a mole of O2 (32g = 1 gmole). The molar specific Gibbs free11 energy, g in ‘per mole’ form is commonly used.
∆g = g products − greac tan ts 1 ∆g = (g ) − (g ) − (g ) H 2O H 2 2 O2
∆g =−237kJ /mole Liquid water product at 298K ∆g =−199.6kJ /mole Gaseous water product at 873K
Negative sign indicates that the energy is released 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 each1 molecule of hydrogen used. 10 + − H2 → 2H + 2e − +
5 O2 + 2e + 2H → H2O 2 6 9 1 For one mole3 of hydrogen used, 2N electrons pass round the external circuit- where 4 N is the Avogadro’s number.7 If -e is the charge of one electron, then the charge that flows is 11 -2Ne = -2F 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) Electrical work done = charge × voltage = -2FE joules With no losses, we have ∆g = −2FE −∆g E = 2F Sustainable Energy Science and Engineering Center Thermodynamic Potentials
Four quantities called "thermodynamic potentials" are useful in the chemical ∆U = Q - W 1 thermodynamics of reactions and non- H =10 U + PV cyclic processes. They are internal F = U - TS Helmoltz free energy
energy, the enthalpy, the Helmholtz5 free G = U - TS + PV Gibbs free energy 2 6 9 1 energy and the Gibbs3 free energy. The four thermodynamic 7potentials4 are Q: heat added to the system related by offsets of the "energy from11 W: work done by the system the environment" term TS (energy you U: internal energy can get from the system’s environment T: absolute temperature by heating and the "expansion work" term PV (work to give the system final S: final entropy volume V at constant pressure. V: final volume Sustainable Energy Science and Engineering Center Second Law of Thermodynamics
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5 2 6 9 1 3 A fuel cell represented7 4as a control volume. E stands for electrical potential, measured in volts. 11 For any isolated system, the 2nd law states that
∆Sisolated ≥ 0
Sgen = ∆Stotal = ∆Ssys + ∆Ssurr ≥ 0
Total change in entropy of both entropy change in the entropy change in the the system and surroundings components of the system surroundings Sustainable Energy Science and Engineering Center 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 1 entropy change for the system is the10 difference between the entropy of
products, SP and the reactants, SR with N representing the number of moles
of each component5 in the reaction. 2 6 9 1 3 4 7 ∆Ssys = Sp − SR = ∑NP sP − ∑NR sR 11 Any heat produced or consumed in the reaction is included in the
expression for the surroundings, where Qsurr is the heat transferred from the system to the surroundings and To is the temperature of the surroundings.
Qsurr ∆Ssurr = To Q S = S − S + surr gen ()P R sys To Sustainable Energy Science and Engineering Center 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 other1 measurable quantities. 10 The thermodynamic property relations are
5 2 6 9 1 3 du = Tds − Pdv 7 dh 4= Tds − vdP 11 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 da = du - Tds - sdT = -sdT - Pdv Gibbs function: G = H - TS; g = h - Ts dg = -sdT + vdP 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 1 can perform is equal to the change in G as reactants10 go to products. This work is done by the movement of electrical charge through a voltage, and at equilibrium
5 2 6 9 1 3Wmax = −∆G cell 4 dG = dH7 − TdS − SdT = d(U + PV ) − Tds − SdT 11 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 Sustainable Energy Science and Engineering Center Chemical Thermodynamics The effect of temperature and pressure on ∆G
For a reversible process: δQ = TdS 1 10 If the system is restricted to doing expansion work then
5δW = PdV 2 6 9 1 3 dG = VdP − SdT n is the number of moles 7 4 PV = nRT 11 Ideal gas law For isothermal process dP dG = nRT P P2 G2 − G1 = nRT ln P1 o P2 G2 = G + nRT ln P o stands for standard reference state Sustainable Energy Science and Engineering Center Chemical Thermodynamics Equilibrium of a gas mixture:
For a chemical reaction occurring at constant pressure and temperature, the 1 reactant gases A and B form products M10and N.
5 aA + bB ⇔ mM + nN 2 6 9 1 Where a,3 b, m and n are stoichiometric coefficients. 4 The change in the7 Gibbs energy 11
∆G = mgM + ngN − agA − bgB Where g is in molar quantity (kJ/mol)
o PM o PN o PA o PB ∆G = mg M + RT ln + ng N + RT ln − ag A + RT ln − bg B + RT ln P o P o P o P o
o o o o o ∆G = mg M + ng N − ag A − bgB In terms of standard Gibbs energy. The reference pressure is usually taken as 1 atm. Sustainable Energy Science and Engineering Center Chemical Thermodynamics
Equilibrium of a gas mixture:
1 m n o PM PN 10 ∆G =∆G + RT ln a b PA PB 5
∆G =∆Go + RT lnQ 2 6 9 Q: Reaction coefficient1 for the pressures P m3P n Q = M N 7 4 P a P b A B 11 The change in Gibbs energy of a reaction involving gases is:
∆G =∆Go + RTlnQ Sustainable Energy Science and Engineering Center Chemical Thermodynamics Maximum work: The maximum work that a system can perform is related to the change in Gibbs energy. For a reversible process (δQ = TdS) 1 10 dG = −δW + PdV + VdP − SdT
At constant pressure5 and temperature 2 6 9 1 3 dG = −δW + PdV = −(δW − PdV ) 7 4 Since there is only electrochemical work, W , in which electrical charge moves 11 e through a voltage, we have
dG = −δWe
The Gibbs energy change is negative of the electrochemical work, we can then write as
∆G = δWe Sustainable Energy Science and Engineering Center Chemical Thermodynamics 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 or W . In electrical terms, the 1 10 e cell work done by electrons with the charge neF (ne 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
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-1 constant - 96,485C/mole ) moving through a potential1 difference, E ( voltage 2 6 9 difference across3 electrodes) is 7 4 o RT We = neFE 11 E = E − lnQ neF ∆G =−neFE o o o E is the standard electrode potential ∆G =−neFE We also assume here that a complete reversible o ∆G =∆G + RT lnQ oxidation of a mole of fuel o ∆G RT Electrical work done = charge × voltage E = − lnQ neF neF Sustainable Energy Science and Engineering Center Hydrogen - Oxygen Fuel Cell
For a hydrogen-oxygen fuel cell, the overall reaction stoichiometry is 1 1 10 H + O → H O 2 2 2 2
5 2 6 9 1 3 The electrons7 transferred4 in this reaction, ne = 2. Using the partial pressures of water, hydrogen11 and oxygen in the reaction coefficient, we then have RT P E = E o − ln H 2O n F P P1/2 e H 2 O2
Diluting the reactant gases will lower the maximum voltage that the cell can produce. Sustainable Energy Science and Engineering Center 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 1 0.1 MPa, the partial pressures of nitrogen10 and oxygen are 0.07809
MPa and 0.02095 MPa5 are respectively. 2 6 9 1 The product 3gas stream contains two parts of H2 and one part of O2 by moles and volume 7and 4the reaction takes place at 0.1 MPa, we will have 11 2 P = × 0.1= 0.0667Mpa H 2 3 1 P = × 0.1= 0.0333MPa O2 3 Sustainable Energy Science and Engineering Center Thermal Efficiency
Thermal efficiency of a reversible heat engine is determined by
1 W T η = net =1− 10L th Q T
5 in H 2 6 9 1 Electrochemical3 cells such as storage batteries and fuel cells, operate at constant 4 temperatures with the7 products of the reaction leaving at the same temperature as reactants (isothermal reaction). The11 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
Wmax,cell = −∆G
Change in the Gibbs function between products and reactants Sustainable Energy Science and Engineering Center Thermal Efficiency
The work, which is done by the movement of electrons through a difference in electrical potential is
1 10 Wcell = ne FE
5 With2 the6 higher value of the fuel 9replaces Q the maximum1 thermal efficiency 3 in at the open circuit voltage Eo , of an electrochemical cell is given by 7 4 11 n FE o η = e th,cell,max HHV
For example, Eo = 1.23 V for a hydrogen-oxygen fuel cell
n FE o 2 × 96485 ×1.23 η = e = = 0.83 th,cell,max HHV 285840
HHV of H2 = 285.84 kJ/mol; LHV = 241 kJ/mole Sustainable Energy Science and Engineering Center Thermal Efficiency
The Gibbs energy of the formation of water vapor is -228.582 kJ/mol at 1 10 285.15K and 1 atm)
It decreases to -164.429 kJ/mol at
5 1500K.1 2 6 9 3 7 4 The current efficiency, a measure of fuel utilization (or fuel consumed to 11 produce an electrical current ) is given by
I ηI = −ne FN fuel
Fuel flow rate mol/s Sustainable Energy Science and Engineering Center Maximum Efficiency
Pressure: 1 atm
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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. Sustainable Energy Science and Engineering Center Heating Values for Selected Fuels
Fuel HHV (MJ/kg) LHV (MJ/kg) HHV/LHV LHV/HHV Coal 1 34.1 33.3 10 1.024 0.977
CO 10.9 5 10.9 1.000 1.000 2 6 9 1 Methane 3 55.5 50.1 1.108 0.903 4 Natural gas 42.5 7 38.1 1.115 0.896 11 Propane 48.9 45.8 1.068 0.937 Gasoline 46.7 42.5 1.099 0.910 Diesel 45.9 43.0 1.067 0.937 Hydrogen 141.9 120.1 1.182 0.84 Sustainable Energy Science and Engineering Center Reversibility
Actual work against maximum work potential: For a heat engine this is equivalent to 1 10 W η = act
5 heatengine W 2 6 9 Carnot 1 3 For fuel cells, this7 can4 be written as 11 ηact neFE E ηR = = o = o ηrev neFE E
For a hydrogen-oxygen fuel cell, the value for Eo is 1.23V at 295K and 1 atm.
If the voltage were 0.7 V, the efficiency ηR would be 0.57, indicating that 43% 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. Sustainable Energy Science and Engineering Center 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
∆G T∆S n FE ItE 1 η = =1− = e10 = i ∆H ∆H ∆H ∆H
Where I is the current5 and t the for which the current flows. 2 6 9 1 In a fuel3 cell under load, the actual electromotive force that drives the electrons 7 4 through the external circuit will fall below E to some lower value, we will call 11 Eac. 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 −n FE η = e ac ac ∆H Sustainable Energy Science and Engineering Center Hydrogen - Oxygen Fuel Cell
Quantity H2 0.5O2 H2O Change
1 Enthalpy 010 0 -285.83 kJ ∆H = -285.83 kJ
5
Entropy 130.68 J/K 0.5 x 205.14 J/K 69.91 J/K T∆S = -48.7 kJ 2 6 9 1 3 7 4 Pressure: 1 atmosphere Temperature:11 298K
W = P∆V = (101.3 kPa)(1.5 moles)(-22.4 x 10-3 m3/mol)(298K/273K) = -3715 J
∆U = ∆H - P∆V = -285.83 kJ - 3.72 kJ = -282.1 kJ η= −237.1/−285.83 = 0.83 (83%) ∆G = ∆H - T∆S = -285.83 kJ + 48.7 kJ = -237.1 kJ Sustainable Energy Science and Engineering Center Homework Problem
Due: November 3, 2005 Examine how the ideal efficiency of a simple hydrogen-oxygen fuel cell changes1 as its operating temperature is 10raised from 298K to 1000K. Also calculate the actual and ideal efficiencies for the cell operating in the standard
conditions of 298K 5and 1 atm. The actual cell voltage is 0.75V while the cell 6 1 2 9 3 delivers 1.53 amps. The hydrogen flow is 0.25 cm /sec. Independent 3 measurements reveal7 that4 0.06 cm /sec of hydrogen is escaping through electrolyte unreacted. 11 Sustainable Energy Science and Engineering Center Basic Fuel Cell Reactions
1 The overall reaction of a PEM fuel cell is: H2 + O2 ⇒ H2O 2 This reaction is the same as the reaction of hydrogen combustion, which is an exothermic process (energy is released):
1 H + O ⇒ H O + heat 2 2 2 2
The heat, typically given in terms of enthalpy, of a chemical reaction is the difference between the heats of formation of products and reactants:
1 kJ ∆H = ()h f − ()h f − ()h f =−286kJ /g − 0 − 0 =−286 H 2O H 2 2 O2 mol
Heat of formation of liquid water: -286 kJ/mol at 25oC and at atmospheric pressure. 1 H + O ⇒ H Ol()+ 286kJ /mol 2 2 2 2
Reference: PEM fuel cells: theory and Practice, Frano Barber, Elsevier Academic Press, 2005 Sustainable Energy Science and Engineering Center 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 1 mol of hydrogen = the enthalpy of hydrogen combustion reaction = 286 kJ/mol The result of combustion is liquid water at 25oC and the value of 286 kJ/mol is considered as Higher Heating Value (HHV). If the combustion is done with excess oxygen and allowed to cool down to 25oC, the product will be in the form of vapor mixed with unburned oxygen. The resulting heat release is measured to be 241 kJ/mol, known as Lower Heating Value (LHV). 1 H + O ⇒ H Og()+ 241kJ /mol 2 2 2 2
The difference between HHV and LHV is the heat of evaporation of water at 25oC: H fg = 286 − 241= 45kJ /mol Sustainable Energy Science and Engineering Center 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: 1 ∆S = ()S f − ()S f − ()S f H 2O H 2 2 O2 At 25oC and at one atmosphere
hf (kJ/mol) sf (kJ/mol) 48.68 kJ/mol is converted Hydrogen 0 0.13066 into heat. Oxygen 0 0.20517 Water (liquid) -286.02 0.06996 Water (Vapor) -241.98 0.18884
∆G =−286.02 − 298 × ()0.06996 − 0.13066 − (0.5 × 0.20517) = −237.36kJ /mol Sustainable Energy Science and Engineering Center Theoretical Fuel Cell Potential
Electrical work: We = neFE = −∆G
ne = 2 (two electron per molecule); F =96,485 Coulombs/electron- mol. The theoretical potential of fuel cell at 25oC and at one atmosphere: −∆G 237,340J /mol E = = =1.23Volts neF 2 × 96,485As/mol
Temperature Effect:
a, b and c are empirical Coefficients, different for each gas Sustainable Energy Science and Engineering Center Theoretical Fuel Cell Potential
a b c (T − 298.15)2 (T − 298.15)3 ∆HT =∆H298.15 +∆aT()− 298.15 +∆b +∆c H 28.91404 -0.00084 2.01E-06 2 3 2 2 T ()T − 298.15 O 25.84512 0.012987 -3.9E-06 ∆S =∆S +∆aln +∆bT()− 298.15 +∆c 2 T 298.15 298.15 2
H2O (g) 30.62644 0.009621 1.18E-06 1 ∆a = a − a − a ∆H (kJ/mol) ∆S (kJ/mol) ∆G (kJ/mol) H 2O H 2 2 O2
1 1 H2 + O2 ⇒ H2Ol() -286.02 -0.1633 -237.34 ∆b = bH O − bH − bO 2 2 2 2 2 1 H + O ⇒ H Og() -241.98 -0.0444 -228.74 1 2 2 2 2 ∆c = c − c − c H 2O H 2 2 O2
For T=298.15, E = 1.23 Volts For T=373.15, E = 1.167 Volts Sustainable Energy Science and Engineering Center Theoretical Fuel Cell Efficiency
∆G 237.34 η = = = 0.83 ∆H 286.02 ∆G 228.74 η = = = 0.945 ∆HLHV 241.98
−∆G −∆G n F 1.23 η = = e = = 0.83 −∆H −∆H 1.482
ne F
Potential corresponding to hydrogen’s higher heating value Sustainable Energy Science and Engineering Center Theoretical Fuel Cell Potential
For anEffect ideal of gas: Pressure:
The change in Gibbs free energy may be shown to be: ∆G = Vm dP Therefore: 3 Where Vm= molar volume, m /mol and P = pressure, Pa
PVm = RT
dP dG = RT P o o P G : Gibbs free energy at G = G + RT ln standard temperature, 25oC Po and at one atmosphere Sustainable Energy Science and Engineering Center Theoretical Fuel Cell Potential Equilibrium of a gas mixture: For a chemical reaction occurring at constant pressure and temperature, the 1 reactant gases A and B form products M10and N. aA + bB ⇔ mM + nN
5 2 Where6 a, b, m and n are stoichiometric9 coefficients.1 3 The change in the7 Gibbs4 energy 11 ∆G = mGM + nGN − aGA − bGB Where g is in molar quantity (kJ/mol) m n P P M N P P ∆G =∆Go + RT ln o o a b P P A B Po Po In terms of standard Gibbs energy. The reference pressure is usually taken as 1 atm. Sustainable Energy Science and Engineering Center Theoretical Fuel Cell Potential
Equilibrium of a gas mixture:
1 m n o PM PN 10 ∆G =∆G + RT ln a b PA PB 5
∆G =∆Go + RT lnQ 2 6 9 Q: Reaction coefficient1 for the pressures P m3P n Q = M N 7 4 P a P b A B 11 The change in Gibbs energy of a reaction involving gases is:
∆G =∆Go + RTlnQ Sustainable Energy Science and Engineering Center Theoretical Fuel Cell Potential
For a hydrogen-oxygen fuel cell, the overall reaction stoichiometry is 1 1 10 H + O → H O 2 2 2 2
5 2 6 9 1 3 The Nernst equation7 4 becomes: 11 RT P ∆G =∆Go + ln H 2O n F P P1/2 e H 2 O2
RT P P1/2 E = E o + ln H 2 O2 n F P e H 2O
When liquid water is produced in a fuel cell: P =1 H 2O For higher reactant pressures the cell potential is higher Sustainable Energy Science and Engineering Center Theoretical Fuel Cell Potential
Air vs oxygen: 1 10
5
0.5 6 9 P 1 2 RT O2 RT 1 3 ∆E = EO − Eair = ln = ln 2 4 7 neF Pair neF 0.21 11 At 80oC , the voltage loss becomes 0.012V. In practice, this is much higher. Sustainable Energy Science and Engineering Center Home work
1. For a hydrogen/air fuel cell operating at 60oC 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 ∆HT and ∆ST). 2. Calculate the expected difference in theoretical cell potential between a hydrogen/oxygen fuel cell operating 80oC and 5 bar for reactant gases and the same fuel cell operating at atmospheric pressure. What if the pressure is increased to 10 bar? Sustainable Energy Science and Engineering Center 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. 1 10 Electrolyte Backing layer
5 2 6 9 1 3 4 Catalyst7 layer 11
Gas diffusion layer
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. Sustainable Energy Science and Engineering Center Electrochemical Kinetics
The general half-reaction expression for the oxidation of a reactant is:
1
Where reactant R loses electrons and becomes Ox, the product of oxidation, and 2 6 9 1 n is the number3 of electrons that are transferred in the reaction. For the opposite direction, Ox gains7 electrons,4 undergoing reduction to form R in the half- reaction. 11
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 3. Sustainable Energy Science and Engineering Center Electrochemical Kinetics
Single-step Electrode reactions Considering only one direction of the reaction, the current produced is 1 10
Where I is the current5 (in amperes), A is the active area of the electrode (cm2), F 6 9 1 is the Faraday’s3 constant (the charge per mole of electrons= 96,485 - i = nF.j (coulombs/mole e ) 7and j is4 the flux of reactants reaching the surface (mole/sec). The current density (per unit area)11 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
Surface concentration Forward rate coefficient of the reactant Sustainable Energy Science and Engineering Center Electrochemical Kinetics
Single-step Electrode reactions For the backward reaction (subscript b), the flux produced by the oxidation of R is 1 10
5 jb = kb [R]o 2 6 9 1 3 Backward rate coefficient 7 4 11 The net flux is
j = j f − jb
The net current density that appears on the electrode when the current is produced is given by
i = nFk( f [Ox]o − Fkb []R o ) Sustainable Energy Science and Engineering Center Electrochemical Kinetics
Butler-Volmer Equation From the Transition state theory (refer to any physical chemistry book), the heterogeneous1 rate coefficient, k. is a 10function of the Gibbs energy of activation and is given by 5 2 6 k 9T −∆G ≠ 3 k = B exp Boltzmann’s constant 4 h RT7 (1.38049x10-23 j/oK) Gibbs energy of activation Plank’s constant (6.621x10-34 Js) (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 ∆G ≠ =∆G ≠ + nF∆φ Reduction c Change in electrical potential ≠ ≠ Oxidation ∆G =∆G c − n()1− β F∆φ
Transfer coefficient (0.5) Sustainable Energy Science and Engineering Center Electrochemical Kinetics
For a reduction reaction
≠ 1 kBT −∆G c, f 10 −nβF∆φ k f = exp exp h RT RT
5 2 6 9 1 3 Chemical Component 4 Electrical Component
11 With the over potential is defined as η =∆φ − ∆φrev
For a hydrogen-oxygen fuel cell, the reversible potential of the anode is 0 V; at the cathode it is +1.23 V at 25oC.
≠ kBT −∆G c, f −nβF∆φrev −nβFη k f = exp exp exp Reduction h RT RT RT ≠ kBT −∆G c, f −n(1− β)F∆φrev −n(1− β)Fη kb = exp exp exp Oxidation h RT RT RT Sustainable Energy Science and Engineering Center Electrochemical Kinetics 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 1 10 exchange current density, io is defined as
5 2
nF[ Ox] ko, f = nF[ R] ko,b ≡ io(A /cm ) 6 o 9 o 1 The current3 density is given by 7 4 −nβFη11 −n(1− β)Fη i = ioexp − exp η = ∆φ − ∆φ RT RT rev 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 1.23 V respectively. Sustainable Energy Science and Engineering Center Fuel Cell Components Fuel Cell Components Impact on Performance
The proton exchange membrane fuel cell (PEMFC) 1 10
5 2 6 9 1 3 7 4 11
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 0,7V and power densities up to about 1 W/cm2 electrode area. Sustainable Energy Science and Engineering Center The Proton Exchange Membrane Fuel Cell (PEMFC)
1 10
5 H → 2H + + 2e−
2 Anode (Er = 0 V) 2 6 9 1 3 1 + − O2 + 2H + 2e →1H2O Cathode (E = 1.23 V) 7 4 2 r 11
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 0.2 mg/cm2. The basic raw material cost of platinum for a 1- kW PEMPC cell is about $10 - a small portion of the total cost. Sustainable Energy Science and Engineering Center The PEM Fuel Cell The gas diffusion layer and backing layer (substrate) at the anode allows hydrogen to reach the reactive zone Electrolyte Backing layer within the electrode. Upon reaching, protons migrate through the ion conducting membrane, and electrons 1 10 are conducted through the gas diffusion layer and Catalyst layer ultimately to the electric terminals of the fuel stack.
The substrate therefore has to 5be porous to allow gas 2 6 9 1 and electrically conducting.3 Not all of the chemical Gas diffusion layer energy supplied to the MEA by7 reactants4 is converted into electric power. Heat will also be generated and the 11 substrate also acts as a heat conductor to remove heat Membrane Electrode from the reactive zones of the MEA. Assembly (MEA)
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. Sustainable Energy Science and Engineering Center The PEM Fuel Cell
MEA Component Task/effect Anode Substrate Fuel supply and distribution, Electron conduction, heat removal from reaction zone, 1 water10 supply into electrocatalyst Anode catalyst layer Catalysis of anode reaction, proton conduction 5
into membrane, electron conduction into 2 6 9 1 3 substrate, water and heat transport 4 Proton exchange membrane7 Proton conduction, water transport, electronic 11 insulation Cathode catalyst layer 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 4 Sustainable Energy Science and Engineering Center 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 difference1 between the theoretical voltage of 10 a given electrode and the experimental
voltage when the current is5 drawn from the 2 6 9 1 cell. The losses 3are classified in three categories: Chemical 7 polarization.4 Concentration polarization and resistance 11 polarization. The theoretical value of the open circuit voltage of a hydrogen-oxygen fuel cell is given by −∆g V = E = f 2F This gives a value of about 1.23 V for a cell operating at 298K.
Vac = V −∆Vconc(c) −∆Vchem(c) −∆Vconc(a ) −∆Vchem(a) − ∑IR Sustainable Energy Science and Engineering Center 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
1 ∆Vchem = a′ + b ′ ln10J −RT
5 a′ = ln( jo ) 2 6 αnF9 1 3 −RT 7 b4′ = αnF 11
Where J is the apparent current density at the electrode, α and jo are kinetic parameters, the former being a constant that represents the fraction of ∆Vchem that aids a reaction in proceeding (For a hydrogen electrode, its value is about 0.5 for a great variety of electrode materials and for the oxygen electrode it is between 0.1 and 0.5. 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. Sustainable Energy Science and Engineering Center Chemical Polarization
The effect of current density and the exchange current density on chemical polarization loss 1 10
5 2 6 9 1 3 7 4 11
Potential loss Sustainable Energy Science and Engineering Center 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 voltage1 that a fuel cell would deliver under10 completely reversible conditions. The concentration of electrolyte in the vicinity of an electrode during reaction should be
maintained at the desirable5 condition. 2 6 9 1 3 RT cif 7 4 ∆Vconc(c) = ln nF cb 11
cb : average concentration in bulk electrolyte
cif : concentration at the interface RT JL ∆Vconc(c) = ln nFD nF JL − J J = c L δ ′ b RT JL + J ∆Vconc(a) = ln D: diffusion coefficient nF JL Sustainable Energy Science and Engineering Center Concentration Polarization
An empirical equation better describes the concentration polarization losses:
10 1 j d
5 ∆Vconc = c × e 2 6 9 1 3 -5 Where c and d are empirical7 4 coefficients with values of c= 3 x10 V and d = 0.125 A/cm2. 11 For less than 0.5 A/cm2, the potential loss due to concentration polarization is quite small and increases rapidly to about 0.2 V at 1.5 A/cm2. Sustainable Energy Science and Engineering Center Resistance Polarization
When an electrochemical reaction occurs at an electrode there is generally a significant change in the specific conductivity of electrolyte which involves an1 additional loss of potential. 10 Hydrogen-oxygen fuel cells employing concentrated solutions of potassium 5
or sodium hydroxide as electrolytes show that resistance polarization is 2 6 9 1 negligibly low3 even at fairly high current densities. 7 4 Ohmic Losses: V= IR 11
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 0.1 and 0.2 Ωcm2. Sustainable Energy Science and Engineering Center Heat Transfer
In cells with high current densities, it is often important to calculate the heat transfer within a fuel cell.
10 1)1 The electrochemical reaction producing the current in the cell is not adiabatic which gives rise to a reversible heat transfer whose
magnitude is 5T∆S. 2 6 9 1 2) Some3 of the fuel reacts chemically with the oxidizer rather than 4 electrochemically7 to generate an irreversible heat transfer. 11 3) The cell operates at some voltage less than the theoretical open circuit voltage with the difference manifesting itself as I2R and I ∆V heat in the cell (I is the current drawn and R and ∆V represent irreversible resistances and voltage drops).
Ý Ý Ý Ý 1 Qt = Qrev + Qchem (irr) + Q∆ V = []T∆S ++nF() Vac −V nF Generally small Sustainable Energy Science and Engineering Center Fuel Cell Voltage Losses
0.5
Activation polarization loss 0.4
)
V
(
s 0.3
s
Lo
l
a
i
t
n 0.2
e t Ohmic losses
o
P
0.1
Concentration polarization loss 0 0 500 1000 1500
Current density (mA/cm2) Sustainable Energy Science and Engineering Center Polarization Curve
10 1
5 2 6 9 1 3 7 4 11
Source: http://www.h2net.org.uk/PDFs/EndUse/H2NET-2.pdf Sustainable Energy Science and Engineering Center
Direct Energy Conversion: Fuel Cells
Section 4.7.1 in the Text Book 1982. References and Sources: Direct Energy Conversion by Stanley W. Angrist, Allyn and Beacon,
Fuel Cell Systems, Explained by James Larminie and Andrew Dicks, Wiley, 2003. Fuel Cell Technology Hand Book, Edited by Gregor Hoogers, CRC Press, 2002 Fuel Cell Hand Book, US DOE - available on the web. Sustainable Energy Science and Engineering Center Fuel Cell Design Calculation
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 30% solution1 of KOH (potassium hydroxide). The10 cell operates at a temperature of 298K and the fuel and air are supplied at one atmosphere.
The faradic efficiency (the5 fraction of the reaction which is occurring electrochemically to 2 6 9 1 give a current is 3called faradic or current efficiency, ηF = 1/(nFNfu); where Nfu is the total number of moles of fuel reacted7 4 electrochemically per second) for the cell is estimated to be 95% 11 20% of the fuel supplied to the cell will escape through the electrolyte unreacted.
Separation between electrodes, w= 0.25cm Height of the cell, l = 12 cm w =0.25cm Depth of the cell, d = 6cm
l =12cm Average electrolyte velocity, u = 5cm/s (Supplied by an external pump) Sustainable Energy Science and Engineering Center Electrolyte Properties
The physical properties of the electrolyte at 298K area as follows: 1 10 -3 3 Concentration: 30% KOH (wt) or cb= 6.9 x 10 mole/cm
Density: ρ = 1.294 gm/cm5 3 2 6 9 1 Dynamic Viscosity:3 µ = 2.43 x 10-2 poise 7 4 -2 2 Kinematic Viscosity: ν = 1.887 x 1011 cm /s Conductivity: σ = 0.625 (ohm-cm)-1 Diffusion Coefficient for OH- ions: D = 1.5 x 10-7 cm2/s Sustainable Energy Science and Engineering Center Open Circuit Voltage
We now calculate the open circuit voltage. We assume that each electrode reaction follows the half-cell reactions 1 10 + − Anode E = 0 H2(g) → 2H + 2e o 0.5O (g) + 2H + + 2e− → H O(l)
5 2 2 Cathode Eo = 1.23v 2 6 9 1 Yielding a cell3 reaction of 7 4 H2(g) + 0.5O2(g) → H2O(l) 11 We may now apply the Nernst equation to our cell potential RT P E = E o − ln H 2O n F P P1/2 e H 2 O2
ni Pi = Pt nt th where Pt is the total pressure of the mixture and ni/nt is the mole fraction of the i constituent. Sustainable Energy Science and Engineering Center Open Circuit Voltage 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 from1 a solution. It is equal to partial pressure10 only when the vapor behaves like o an ideal gas. We also define a quantity called the activity aA = fA/fA . We can
then write the Nernst 5equation as 2 6 9 1 3 RT a E 4= E o − ln H 2O 7 n F a a1/2 e H 2 O2 11
The activity of H2O to be used in this equation should be that of water in 30% 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 0.21. Then we have (1.99)(298) ()1 E = V =1.23− ln ()2 ()23060 ()1 ()0.21 0.5 E = V =1.23− 0.013ln(2.18) =1.22volts Sustainable Energy Science and Engineering Center Chemical Polarization
We initially assume an operating current density for our fuel cell of 0.5 amp/cm2 Use1 Tafel equation to calculate losses due to10 chemical polarization.
5 ∆Vchem = a ′ + b ′ lnJ 2 6 −RT9 3 a ′ = ln( j ) αnF o 7 4 −RT b ′ = αnF 11
∆Vchem(a) = 0.14 + 0.005lnJ
∆Vchem(c) = 0.20 + 0.007lnJ
Where the current density is expressed in milliamperes per square centimeter.
∆Vchem(a) = 0.14 + 0.005ln(500) = 0.17volt
∆Vchem(c) = 0.20 + 0.007ln(500) = 0.24volt Sustainable Energy Science and Engineering Center Concentration Polarization We now calculate the polarization due to concentration gradients in the electrolyte near the electrodes. RT JL 1 ∆Vconc(c) = ln 10 nF JL − J nFD JL = cb δ ′ 5 RT JL + J
∆Vconc(a) = ln 6 9 1 2 nF JL 3 Calculation of the limiting7 current4 density: Velocity of the electrolyte = 5 cm/s;11 For a fully established flow between the electrodes, we have −2/3 −2/3 −1/3 JL,av =1.62nFcb u(Rew ) (Sc) (w /l) uw Re = = 66.2 w ν ν Sc = =1.26x105 D w = 0.0208 l 2 JL,av = 0.956amp/cm Sustainable Energy Science and Engineering Center Concentration Polarization
RT JL ∆Vconc(c) = ln ≈ 0.01volt 1 nF JL − J 10 RT JL + J 5∆Vconc(a) = ln ≈ 0.01volt
nF JL 2 6 9 1 3 4 The gas side concentration7 polarization at the anode is ignored. We will calculate the gas side polarization at11 the air electrode using the partial pressure of oxygen in the air for pg.
RT Pr 1 ∆Vconc(g) = ln = 0.013ln = 0.02volts nF Pg 0.21
Where Pr is the gas pressure in the pores of the electrode Sustainable Energy Science and Engineering Center Resistance Polarization
Jw 0.5 × 0.25 1 IR = = 10= 0.2volts σ 0.625
5 2 6 9 1 We now compute3 the actual operating voltage of the fuel cell by subtracting from the open circuit7 voltage4 1.22 volts, the various polarization losses and IR drop across the cell: 11
Vac =1.22 − 0.17 − 0.24 − 0.01− 0.01− 0.02 − 0.02 = 0.57Volt The power out put of the cell:
Po = VacI = VacJA = 0.57 × 0.5 × 72 = 20.5watts Sustainable Energy Science and Engineering Center Efficiency The thermal efficiency is given by
−nFVac −2(mole − elec /mole) × 23.06(kcal /volt − mole) × 0.57Volt ηac = = = 38.5% 1 ∆H −68.32kcal10 /mole ∆H =−68.32kcal /mole
5 F2 = 23.066 kcal/volt-mole 9 1 3 The efficiency based 7on the 4actual voltage is V 0.57 11 η = ac = = 46.7% v V 1.22
Assuming that the product of the reaction is liquid water, the faradaic efficiency is given by. JA 0.5 × 72 ηF = = = 0.95 nFN fu 2 × 96500 × N fu F = 96,500 (amp-sec)/(mole-sec) −4 N fu =1.96 ×10 moleH2 /s Sustainable Energy Science and Engineering Center Heat Transfer
Ý Qrev = ηF N fu ()∆H − ∆G = 0.95 × N fu × (−68.32 − (−56.69))= −9.1watts 1 ∆H =−68.32kcal /mole 10 ∆G =−56.69kcal /mole
5 2 6 9 1 3 QÝ = (1−η )N (∆H)= −2.8watts chem(irr) 4 F fu Ý 7 Q∆ V = IV()ac −V =−23.4watts 11
Total heat that must be removed from the cell for it to stay steady state is 35.3 watts, 50% larger than net power output of the cell. Sustainable Energy Science and Engineering Center Home Work
Design calculation for a fuel cell: Home work: Using your own calculations reproduce the curve below for the 1 fuel discussed in the class today. 10
5 2 6 9 1 3 7 4 11 Sustainable Energy Science and Engineering Center Heat Generation
Area = 100 cm2; Operating pressure = 1 atm; Operating Temperature = 80oC; E = 0.7 V; Current generation = 0.6 A/cm2.
Power due to heat = Total power generated10 - electrical power 1 P = P -P
heat 5 total electrical 2 6 = (V x I 9) - (V x I ) 1 3 ideal cell cell cell =7 (1.2V4 -0.7V) x 60A 11 = 0.5 V x 60A = 30 J/s = 108 KJ/hr = 30 W While generating about 42 W of electrical energy. Sustainable Energy Science and Engineering Center Operating Variables
Pressure, Temperature, Gas composition, Reactant utilization and Current density
1 10
5 2 6 9 1 3 7 4 11 Sustainable Energy Science and Engineering Center Operating Pressure
1 10
5 2 6 9 1 3 7 4 11
P2 ∆Vgain = Cln P1 0.286 −4 T1 P2 C ≈ 0.03− 0.06volts ∆Vloss = 3.58 ×10 −1λ ηmηc P1
Motor and drive system Compressor Stoichiometry (~ 2.0) efficiency~ 0.95 efficiency ~ 0.75 Sustainable Energy Science and Engineering Center Operating Pressure
Net voltage change resulting 1 10 from operating at high pressure for two different PEM
5 fuel cell models 2 6 9 1 3 7 4 11 Sustainable Energy Science and Engineering Center Temperature Effect
1 10
5 2 6 9 1 3 7 4 11 Sustainable Energy Science and Engineering Center Polarization Contribution
1 10
5 2 6 9 1 3 7 4 11 Sustainable Energy Science and Engineering Center Effect of Oxygen Pressure
1 10
5 2 6 9 1 3 7 4 11 Sustainable Energy Science and Engineering Center Polarization Curve
10 1
5 2 6 9 1 3 7 4 11
Source: http://www.h2net.org.uk/PDFs/EndUse/H2NET-2.pdf Sustainable Energy Science and Engineering Center Polymer Electrolyte Membrane Fuel Cell
Load Fuel Cell stack 1
5 2 6 9 1 3 7 4 11 Sustainable Energy Science and Engineering Center The Backing Layer
Effective diffusion of each reactant gas to the catalyst on the Membrane
Equivalent concentration Electrode Assembly (MEA). The diffusion profile takes place from a region of high
concentration, outer side of the backing layer, 1 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 True concentration contact with the entire surface area of profile the catalyzed membrane. Assist in water management during the cell operation. Allows the right amount Diffusion layer δ 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. Porous carbon cloth or carbon paper, typically 100 - 300µm thick. About 4 - 12 sheets are used. Sustainable Energy Science and Engineering Center The Bipolar Plate
Main tasks: Current conduction; Heat conduction; control of gas flow and product water removal 10 1 The first main task is to provide reactant gases evenly across the active area of the
5 MEA. Current designs have channels to 6 1 2 9 carry reactant gas from the point at which it 3 enters the cell to the point at which the gas 7 4 exits. The flow field in the channels has a 11 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. Sustainable Energy Science and Engineering Center Water Management
1 10
5 2 6 9 1 3 7 4 11 Use the water leaving the cell to do the humidification.
Channels in bipolar plates Sustainable Energy Science and Engineering Center PEM Fuel Cells (PEMFC)
Some times referred to as Solid Polymer fuel cell (SPFC)
The1 electrolyte: Ion conduction polymer10 (works at low temperatures, hence start quickly)
5 Electrodes:2 6 Catalyzed porous electrodes9 1 3 The anode-electrolyte-cathode7 4 assembly (membrane electrode assemblies (MEA)) is one item and is very thin.11 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
Source: Fuel Cell systems Explained by James Larminie & Andrew Dicks, Wiley, 2003, Chapter 4. Sustainable Energy Science and Engineering Center Polymer Electrolyte Most commonly used: sulphonated fluoropolymers - fluoroethylene
1 10
5 sulphonated fluoroethylene 2 6 9 1 3 (PTFE) 7 4 11
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 (~50µm). It is strongly hydrophobic, which helps to drive the product water out of the electrode. Sustainable Energy Science and Engineering Center Electrodes
Platinum is used generally as the catalyst (0.2 mg/cm2) 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. 1 10 The carbon-supported catalyst is fixed to a porous and conductive
5 material2 such6 as carbon cloth or carbon paper.9 The carbon paper1 3 also diffuses the gas on to catalyst7 4-the gas diffusion layer. Alternatively, the platinum on carbon 11catalyst is fixed directly to the electrolyte, thus manufacturing the electrode directly on to the membrane. Then a gas diffusion layer - carbon cloth or paper (200 to 500 µm thick) is added. It also forms an electrical connection between the carbon-supported catalyst and the bipolar plate. Sustainable Energy Science and Engineering Center Platinum Loading Effect
1 10
5 2 6 9 1 3 7 4 11 Sustainable Energy Science and Engineering Center Water Management
Proton conductivity is proportional to the water content. The H+ ions1 moving from the anode to the cathode 10pull water
Molecules with them (up to five5 H2O molecules are dragged 2 6 9 1 for each proton - electro-osmotic3 drag). At high current 4 densities, the anode side of the7 electrolyte can be dried out. 11 At temperatures over 335K, the electrodes are typically dry. Solution: Air and hydrogen humidification before they enter the cell. Sustainable Energy Science and Engineering Center Heat Production
2 kW fuel cell system 12 W fuel cell 1 10
5 2 6 9 1 3 7 4 11 Sustainable Energy Science and Engineering Center PEM Fuel Cell Stack
Load
1
5 2 6 9 1 3 7 4 11 Sustainable Energy Science and Engineering Center Multi-cell Stack Performance
Simulation Results - 1 10 PEMPC stack
5 Hydrogen produced from 2 6 9 1 gasoline 3 7 4 11
Source: Simulation study of a PEM Fuel cell system fed by Hydrogen produced by partial oxidation by Ozdogan S., et al. Sustainable Energy Science and Engineering Center Multi-cell stack Performance
1 10
5 2 6 9 1 3 7 4 11 Sustainable Energy Science and Engineering Center
PEMFC System
Hydrogen tank
10 1 2 6
5 9 1 3 7 4 11 Sustainable Energy Science and Engineering Center Cooling Air Supply
1 10
5 2 6 9 1 3 7 4 11
Ballard Nexa PEM Fuel cell. Sustainable Energy Science and Engineering Center
Fuel Cell Systems and Hydrogen Production Sustainable Energy Science and Engineering Center Fuel Cell Type
10 1
5 < 5kW 2 6 9 1 3 7 4 5 - 250kW 11
< 100W
250kW
250kW - MW
2kW - MW Sustainable Energy Science and Engineering Center Electrochemical Reactions
10 1
5 2 6 9 1 3 7 4 11 Sustainable Energy Science and Engineering Center Efficiency
10 1
5 2 6 9 1 3 7 4 11 Sustainable Energy Science and Engineering Center Efficiency
Source: Hazem Tawfik, Sept 2003 Sustainable Energy Science and Engineering Center Pressure Effects
Hydrogen pressure
Oxygen pressure
Source: Hazem Tawfik, Sept 2003 Sustainable Energy Science and Engineering Center Temperature Effect
Source: Hazem Tawfik, Sept 2003 Sustainable Energy Science and Engineering Center Humidity effect at Room Temperature
Source: Hazem Tawfik, Sept 2003 Sustainable Energy Science and Engineering Center
Parametric Effects: Temperature has most effect
Source: Hazem Tawfik, Sept 2003 Sustainable Energy Science and Engineering Center
Air Vs O2
Source: Hazem Tawfik, Sept 2003 Sustainable Energy Science and Engineering Center PEMFC Emissions
PC25 Fuel cell: 200 kW Fuel: Natural gas
Source: Hazem Tawfik, Sept 2003 Sustainable Energy Science and Engineering Center Fuel Cell System
Fuel Cell Stack 1 10 Control System
5 Fuel Delivery 2 6 9 1 3 Air Delivery 7 4 11 Thermal Management Water Management Power Conditioning Sustainable Energy Science and Engineering Center Critical Materials and Costs
Example: Polymer Electrolyte Fuel Cell Stack (1 kW) -Polymer membrane 1 10 - Catalyst (precious metals)
5 - Bipolar plate 1 2 6 9 3 7 4
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 Sustainable Energy Science and Engineering Center PEMFC Challenges
• MEA tolerance for CO in reformed H2 • High temperature operation (~120oC)
• MEA Durability - 40,000 hrs with < 10% degradation, 1% cross over, area resistance <0.1 ohm.cm2 • Cost - $1500/kW, $10/kW for MEA • Efficiency - 30 ~ 50% • Fixed cost of Graphite bipolar plate: $130/kW • Running cost of hydrogen per kWh : $0.405
Source: Hazem Tawfik, Sept 2003 Sustainable Energy Science and Engineering Center Fuel Cell Types
Alkaline (AFC) 1 10 Solid Polymer (SPFC, PEM or PEFC)
5 2 6 9 1 Direct3 Methanol (DMFC) 7 4 Phosphoric Acid (PAFC)11 Molten Carbonate (MCFC) Solid Oxide (SOFC) Sustainable Energy Science and Engineering Center Fuels
Type Application 1 Hydrogen Transport,10 stationary & Portable
Methanol5 Transport & Portable 2 6 9 1 Natural3 Gas Stationary 7 4 Gasoline Transport11 Diesel Transport Jet Fuels Military Sustainable Energy Science and Engineering Center Fuel Reforming
Hydrogen is produced from fuel reforming system such as methane and steam. 1 10 CH + H O → 3H + CO
4 5 2 2 water gas shift reaction 2 6 CO + H O → H +9 CO 1 3 2 2 2 7 4 Carbon monoxide has a tendency11 to occupy platinum catalyst sites, hence must be removed. Other fuels:
C8H18 + 8H2O →17H2 + 8CO Sustainable Energy Science and Engineering Center 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 H are 1 10 n m
5 2 6 9 1 CH4 + H2O3→ CO + 3H2;∆H = 206kJ /mol 7 m 4 Cn Hm + nH2O → nCO + + n H2 2 11
CO + H2O → CO2 + H2;∆H =−41kJ /mol Sustainable Energy Science and Engineering Center DMFC System
1 10
5 2 6 9 1 3 7 4 11 Sustainable Energy Science and Engineering Center Liquid-Feed DMFC Reactions
1 10
5 2 6 9 1 3 7 4 11 Sustainable Energy Science and Engineering Center Direct Methanol Fuel Cell
1 10 Operating at ambient
5 conditions 2 6 9 1 3 7 4 11 Sustainable Energy Science and Engineering Center Micro-scale Methanol Fuel Processor
1 10
5 2 6 9 1 3 7 4 11 Sustainable Energy Science and Engineering Center Hydrogen Production
Source: Sustainable Energy Science and Engineering Center Hydrogen Production
Source: Sustainable Energy Science and Engineering Center Hydrogen From Water
There is enough water to sustain hydrogen! Sustainable Energy Science and Engineering Center Electrolysis Sustainable Energy Science and Engineering Center Electrolysis Sustainable Energy Science and Engineering Center Photoelectrolysis Sustainable Energy Science and Engineering Center Hydrogen Production Sustainable Energy Science and Engineering Center Photoelectrochemical Conversion System Sustainable Energy Science and Engineering Center Electrolysis Efficiency
Systems that claim 85 % Sustainable Energy Science and Engineering Center Photoelectrolysis Sustainable Energy Science and Engineering Center Photoelectrolysis Sustainable Energy Science and Engineering Center Photoelectrolysis Sustainable Energy Science and Engineering Center Artificial Photosynthesis Sustainable Energy Science and Engineering Center Thermochemical Water Splitting Sustainable Energy Science and Engineering Center Thermochemical Production Sustainable Energy Science and Engineering Center Thermochemical Production
Thermal-to-hydrogen energy efficiency
Solar-thermal heat source is a logical choice Sustainable Energy Science and Engineering Center Thermochemical Production
Solar-thermal heat source Sustainable Energy Science and Engineering Center Thermochemical Cycle Efficiency
Process Temperature (oC) Heat-to-Hydrogen Efficiency (%)
Electrolysis 20-25 Sulfur-iodine thermochemical cycle 850 45-49 Calcium-bromine thermochemical cycle 760 36-40 Copper-chlorine thermochemical cycle 550 41*
* Energy efficiency calculated based on thermodynamics Sustainable Energy Science and Engineering Center Solar Thermochemical System Sustainable Energy Science and Engineering Center Thermochemical Process Sustainable Energy Science and Engineering Center Hydrogen Production Sustainable Energy Science and Engineering Center Fossil Fuel Use Sustainable Energy Science and Engineering Center Solar Heat Generation Sustainable Energy Science and Engineering Center Solar Heat Generation Sustainable Energy Science and Engineering Center 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 H2) 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. http://www.energy.csiro.au/