Fuel Cells: Energy Conversion through Chemical Reactions, Study notes of Public Policy

Download Fuel Cells: Energy Conversion through Chemical Reactions and more Study notes Public Policy in PDF only on Docsity! Introduction to Fuel Cell Science and Engineering Lecture #5 - Fuel Cell Fundamentals Chris Yang Hydrogen Economy TTP 289-002 2 Topics • Electrochemistry • Thermodynamics • Kinetics • Single Cells • Stacks • Fuel Cell Systems • Applications 3 What is a fuel cell? • An electro-chemical energy conversion device ! Similar to a battery • Difference is mostly a matter of strict definitions • Uses fluid fuels • Energy storage vs energy conversion ! Converts chemical energy ! electricity without intermediate heat step ! There is heat release that is required but for a different reasons • Can be a low temperature device ! All chemical reactions involve electrons so it’s a more precise form of energy conversion than • burning a fuel to liberate the energy as heat (lower quality energy) and then trying to scavenge as much of the heat energy back to high quality energy (mechanical) 4 Motivation for Fuel Cells • Direct Chemical Energy Conversion ! High Efficiency - part load ! Low-to-zero Emissions (depends on fuel) ! Cogeneration / Distributed Generation ! Fuel Flexibility - possibly carbon neutral • Uncertainties ! Economics ! Fuel Source • Emissions • Efficiency 9 Fuel Cell Power and Efficiency 0 0.2 0.4 0.6 0.8 1 1.2 0 200 400 600 800 1000 1200 1400 current density [mA/cm2] c e ll p o te n ti a l [V ] 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 p o w e r [W /c m 2 ] E = E0 – b ln i –Ri –me (ni) 10 Fuel Cell Thermodynamics • Potential electricity output of a chemical reaction #G is the the free energy change for the reaction #H is the enthalpy change for the reaction #S is the entropy change for the reaction • H2 + 1/2 O2 " H2O + work + heat • #G is the maximum useful work associated with a chemical reaction • #H is the maximum heat associated with a chemical reaction • Entropy loss for the overall reaction • 1.5 gas molecules " 0.5 or 0 gas molecules ! "G = "H #T"S ! "G = #nFErev ! "H = #nFEtherm 11 Redox fuel reactions • Oxidation states - amount of oxygen and hydrogen ! CH4 - #Hf = -74.9 kJ/mol ! CH2O - #Hf = -115.9 kJ/mol ! CO - #Hf = -110.5 kJ/mol ! CH3OH - #Hf = -201.1 kJ/mol ! CH2OOH - #Hf = -387.5 kJ/mol ! CO2 - #Hf = -393.5 kJ/mol ! H - #Hf = 218.0 kJ/mol ! HO - #Hf = 39.0 kJ/mol ! H2 - #Hf = 0 kJ/mol ! H2O - #Hf = -242.8 kJ/mol 12 Fuel cell thermodynamics • #G = #H -T#S • Entropy reduction (#S <0) for fuel cell reaction ! #G < 0 because #H is very negative, so reaction is spontaneous ! Heat must be liberated to counteract entropy loss H2 + 1/2O2 ! H2O 1 bar reactant pressure (standard conditions) Temp product H2O phase ! Hrxn (kJ/mol) ! Grxn (kJ/mol) " theoretical Etn (V) Erev (V) 25°C LHV (vapor) -241.8 -228.6 94.5% 1.253 1.185 HHV (liquid) -286 - 237.3 83% 1.482 1.229 80°C LHV -242.3 -226.2 93.3% 1.256 1.172 HHV -283.8 -233.7 82.3% 1.471 1.212 130°C LHV -242.8 -223.9 92.2% 1.258 1.160 HHV -282.1 -230.4 81.7% 1.462 1.195 200°C LHV -243.8 -219.1 89.8% 1.259 1.131 600°C LHV -247.2 -198.1 80.1% 1.277 1.023 1000°C LHV -249.4 -175.8 70.5% 1.288 0.908 13 Voltage and energy • The Nernst equation ! #G = #G0+RTlnQ ! Chemical reaction: aA + bB " cC + dD ! #G0=-nF#Erev ! #E = #Erev - (RT/nF)lnQ • As Q increases, reaction goes towards completion, voltage drops • The potential energy associated with less reactants is lower than the potential energy associated with more reactants • #G=-nF#E - an analogous equation can be used to convert #H into a potential ! #H=-nF#Etn ! Q = C[ ] c D[ ] d [A] a [B] b ! "theoretical = maximum work output total energy input = #G #H = Erev Etn ! "real = actual work output total energy input = E Etn 14 Fuel cell efficiency (2) • Components of cell efficiency • Reversible efficiency - theoretical efficiency • Voltage efficiency- efficiency based upon operating voltage • Faradaic efficiency • Utilization efficiency - reactant utilization • Auxiliary loads - parasitic power consumption • Total Efficiency is multiplicative ! " v = E E rev ! " rev = #G #H = E rev E thermal ! " f = i i f ! " u = nreacted ntotal ! " a =1# PPL Ptotal ! "total = "rev"v" f"u"a 19 Mixed potential • Platinum can be oxidized which can be a problem because it competes with hydrogen oxidation. ! Pt + H2O $ PtOH + H + + e- ! Pt + H2O $ PtO + 2H + + 2e- ! 1/2O2 + H + + e - " H2O • Reactions occur on one electrode and lead to reduced potential ! Hydrogen and methanol crossover is another reason for a mixed or corrosion potential i V 20 Oxidation kinetics • The current potential (iV) curves will depend strongly only the kinetics of the fuel being oxidized 0 0.2 0.4 0.6 0.8 1 0 250 500 750 1000 1250 1500 current density [mA/cm2] c e ll v o lt a g e [ V ] 2M Methanol Pure Hydrogen Hydrogen + 100 ppm CO 21 Ion conduction • Electric Field ! ' = #V/d • Force on Ion ! Felectric = z e ' = [z e #V]/d • Balance between Forces - electric field and viscous drag ! Fdrag = 6()v& ()-cross sectional area, v-velocity, &-viscosity) ! Velocity = [ze']/[6()&] • Conduction speed (velocity) is related to current and conductivity ! increased current/conductivity • greater electric field (thinner electrolyte, larger voltage) • increased charge on ion • smaller cross sectional area • lower electrolyte viscosity (increased temperature) 22 Ion conduction (2) • Liquid Electrolytes ! Ionic migration in presence of electric field ! Grotthus Mechanism • Positive ion conduction ! Acid electrolyte fuel cells ! PAFC, PEMFC, DMFC • Negative ion conduction ! basic/salt electrolyte ! AFC, MCFC • Solid Electrolyte ! defect conduction ! Solid Oxide Fuel Cell • Ohm’s Law ! #V = IR _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ + + + + + + + + + + + + + + + + electrolyte double layers anode cathode _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ + + + + + + + + + + + + + + + + electrolyte double layers anode cathode ideal fuel cell real fuel cell 23 Mass transport (concentration) overpotential • If the reaction is proceeding at the electrodes, there will be a reduction in the concentration of reactants. • Reactants are supplied to the electrodes via diffusion • At very high current densities (reaction rate) the rate of diffusion may be slower than the rate of reactant utilization • The Nernst equation describes effects of reduction in reactant concentration ! #E = #Erev - (RT/nF)lnQ ! Q = C[ ] c D[ ] d [A] a [B] b 24 Gas diffusion electrodes • 3 phase contact area ! Gas, liquid and solid • Reaction on platinum catalyst particles (35 nm) on carbon powder ! Reactant diffusion ! Production of H+ and e- ! Need to conduct protons away into membrane ! Conduct e- to current collector Catalyst layer impregnated with polymer electrolyte Electrocatalyst particle on carbon support Substrate Diffusion layer Active layer Polymer electrolyte 29 Polymer membranes • Water is key ! Membrane reorganization ! Conductivity Nafion 115 0.001 0.01 0.1 1 0 2 4 6 8 10 12 ! [H2O/SO3H] " [ S /c m ] Nafion 115 30 CO poisoning • CO poisoning of anode electrocatalyst ! CO + Pt ! Pt-CO ! H2 + 2Pt ! 2 Pt-H 0.00 0.20 0.40 0.60 0.80 1.00 50 100 150 200 250 Temperature [oC] C O F ra c ti o n a l C o v e ra g e ! C O . 1 ppm CO 10 ppm CO 100 ppm CO 31 Direct Methanol Fuel Cell (DMFC) • Electrolyte ! Same as PEMFC, Perfluorosulfonic Acid Membrane - (i.e. Nafion) • Electrode reactions ! Anode: CH3OH + H2O " CO2 + 6H + + 6e- ! Cathode: 1/2 O2 + 2H + + 2e- " H2O ! Overall: CH3OH + 3/2 O2 " CO2 + 2H2O • Operating Temperature and Pressure ! 60°C to 100°C • Fuel introduced as liquid feed 1 M methanol (1 mole methanol/liter) on anode side • Platinum ruthenium catalyst • Advantages ! Easier transportation/storage of methanol ! water and thermal management issues easier • Disadvantages ! Low electro-activity for methanol ! Reduced efficiency and power • Issues/Challenges ! Develop electrocatalysts for electro-oxidation of methanol to improve kinetics and current density ! Minimize crossover of methanol, which is very soluble in polymer membrane ! Reduce poisoning of catalysts by reaction intermediates 32 Solid oxide fuel cells (SOFC) • Solid electrolyte ! Nonporous Y2O3 stabilized ZrO2 (30-40 µm thickness) • Electrode Reactions ! Anode Reaction: H2 + O 2- " H2O + 2e - ! Cathode Reaction: O2 + 2e - " 2O2- • High Operating Temperature ! Internal reforming ! Rapid kinetics without precious metals ! High quality heat for cogeneration, bottoming cycle ! Materials requirements • Advantages of solid state system ! 2 phase interface ! reduced corrosion, electrolyte management issues • Oxide Ion Conduction • Cell Configurations ! Tubular Seal-less Design • Siemens Westinghouse Tubular Cell ! Bipolar (Flat Plate) Configuration • Complexity and Seals 33 Solid Oxide Fuel Cells 34 Operation and system considerations • Fuel utilization ! fuel and air leave in exhaust stream ! low utilization " high H2 concentration in cell and exhaust ! high utilization " low H2 concentration in cell and exhaust • Optimization ! Efficiency, operating cost, reliability, fuel cell life, operating flexibility • Tradeoffs: ! efficiency vs power density • Operating Parameters: ! Load ! Temperature ! Pressurization ! Gas concentration ! Reactant utilization • Integration of secondary systems ! fuel processing ! cogeneration/bottoming cycle 60 70 80 90 E ff ic ie n c y H2 Utilization factor current density mA/cm2 voltage V Higher Efficiency Lower Efficiency