Typesof Fuel Cells1.PolymerElectrolyteFuel Cells (PEFC)alsocalled Polymer ElectrolyteMembrane(PEM)FuelCells:These"burn"H,&O,onlyproductisMEAGraphiteplateGraphitH,O-CLEAN ENERGY (ifH,renewablyMnflow-felcsourced!)NeedPtparticlessupported oncarbonelectrodes-PROBLEMSSS!SeparateH,oxidationandO,reductionwithaproton-permeablemembrane,usuallyNafion-PROBLEM(seelater)！PEM fuel cell.OxygenReductionReaction(ORR)isH,ONegativeelectrode:1/2O2+2e-+2H+sluggish-rate determining step (rds)!Highpower output, canbeminiaturisedPositive electrode:H2★2H++2e-(mobileandstationarygeneration),★H,OOverall:H,+1/20,goodpower:weightratio(spaceapplications,transportetc)
Types of Fuel Cells 1. Polymer Electrolyte Fuel Cells (PEFC) also called Polymer Electrolyte Membrane (PEM) Fuel Cells: • These “burn” H2 & O2, only product is H2O – CLEAN ENERGY (if H2 renewably sourced!) • Need Pt parGcles supported on carbon electrodes – PROBLEM $$$! • Separate H2 oxidaGon and O2 reducGon with a proton-permeable membrane, usually Nafion – PROBLEM (see later)! • Oxygen ReducGon ReacGon (ORR) is sluggish – rate determining step (rds)! • High power output, can be miniaturised (mobile and staGonary generaGon), good power:weight raGo (space applicaGons, transport etc). PEM fuel cell. Negative electrode: 1/2 O2 + 2e– + 2H+ H2O Positive electrode: H2 2H+ + 2e– Overall: H2 + 1/2O2 H2O
Typesof Fuel CellsCH,OH + H,O022.DirectMethanol Fuel Cells(DMFC):n具CO2eThese"burn"liquid methanol CH,OHinair/oxygen-liberatesCO,!MNeed Pt particles supported on carbon2H+3electrodes-PROBLEMSSS!2HSeparatehalf-reactionswithaproton-HcO,2H20permeablemembrane,usuallyNafion-PROBLEM(seelater)!PtPURucathanodeelectrolyteOxidationofmethanolnowrds-slow!Liquid fuel storageeasier than H,(gas))DMFCcanbeminiaturised (mobileandstationarygeneration),lowerpowerCO2+6H++6e-Anode:CHOH+H,OoutputthanPEMduetomethanolCathode:3/20,+6H++6e-￥3H,Ooxidation overpotential.Liguidmethanolpermeates membraneand is directly oxidised by O2-PROBLEM!
Types of Fuel Cells 2. Direct Methanol Fuel Cells (DMFC): • These “burn” liquid methanol CH3OH in air/oxygen – liberates CO2! • Need Pt parGcles supported on carbon electrodes – PROBLEM $$$! • Separate half-reacGons with a protonpermeable membrane, usually Nafion – PROBLEM (see later)! • OxidaGon of methanol now rds – slow! • Liquid fuel storage easier than H2 (gas), can be miniaturised (mobile and staGonary generaGon), lower power output than PEM due to methanol oxidaGon overpotenGal. • Liquid methanol permeates membrane and is directly oxidised by O2 – PROBLEM! Anode: CH3OH + H2O CO2 + 6H+ + 6e– Cathode: 3/2O2 + 6H+ + 6e– 3H2O DMFC
Types of Fuel Cells3.SolidOxideFuelCells(SOFCs):stackcellrepeating unitOperateathightemperaturesnodesubstratanode layer(600-900oC)cf30-90°CforPEM/DMFCectrolytelavecathode layetUsemethaneorotherhydrocarboninterconnectactlayeelectrolyteriastabilized(YSzfuels - reformation to produce H2cathode contact layerM/YSZ.cermcellfrancathode(La,Sr)MnO,(La.SrCoFe)O,sealincoccurs in situ.CurrenttransportedthroughaCERAMIC02：T88membrane(e.g.yttria-stabilizedzirconia,YSZ)byO2-ionsnotHtcathanodeelectrolyteTheceramicelectrolyteisthereforeanionic-conductorabove60o°cSOFCProblems withlocalisedoverheating-materialsmust havesimilar coefficientsAnode:H2+202-H,0+4eof thermal expansion to avoid cracking→202-Cathode: O, + 4e--instacks
Types of Fuel Cells 3. Solid Oxide Fuel Cells (SOFCs): • Operate at high temperatures (600-900oC) cf 30-900C for PEM/DMFC • Use methane or other hydrocarbon fuels – reformaGon to produce H2 occurs in situ. • Current transported through a CERAMIC membrane (e.g. ybria-stabilized zirconia, YSZ) by O2- ions not H+ • The ceramic electrolyte is therefore an ionic-conductor above 6000C • Problems with localised overheaGng – materials must have similar coefficients of thermal expansion to avoid cracking in stacks. Anode: H2 + 2O2– H2O + 4e– Cathode: O2 + 4e– 2O2– SOFC
Thermodynamicsof Fuel Cells:Enthalpy,GibbsEnergyandOpenCircuit VoltageConsiderH2/O,combustionenthalpy:H2+O,>HOAH=-296kJmol-1Burninghydrogen in thisway converts all of theenthalpyintoheat H=AQ;wecannot extract useful work as AG,unless webuild a heat-engine(turbine)-veryinefficient!Afuelcell "cold"combusts H,by spatially separating twohalf-cell reactions:ihaneceelectrolyte*HO E,°=-1.2 VNegativeelectrode:1/2O2+2e-+2H+O.Positiveelectrode:H2→2H++2e-E,°=0.0 VOverall:H,+1/20,→H,O2H,OEoc(cell)=E,°-E,=+1.2VVoltage (Eoc)CdevelopedH,is oxidised into protons and electrons at theanode,theprotons travelthrough apermeable polymermembrane electrolyte and are consumed inthe reduction ofOat the cathode.Thus thetwo electrodesgraduallybecome charged (like capacitors)andavoltage developsacrossthecell.if wesupplyenoughH,and O,intothe cell,this"OpenCircuitVoltage",Eac,canreachatheoreticalvalueof1.2V
Thermodynamics of Fuel Cells: Enthalpy, Gibbs Energy and Open Circuit Voltage • Consider H2/O2 combusGon enthalpy: H2 + ½ O2 à H2O ΔH = -296 kJmol-1. • Burning hydrogen in this way converts all of the enthalpy into heat ΔH = ΔQ ; we cannot extract useful work as ΔG, unless we build a heat-engine (turbine) – very inefficient! • A fuel cell “cold” combusts H2 by spaGally separaGng two half–cell reacGons: • H2 is oxidised into protons and electrons at the anode, the protons travel through a permeable polymer membrane electrolyte and are consumed in the reducGon of O2 at the cathode. Thus the two electrodes gradually become charged (like capacitors) and a voltage develops across the cell. If we supply enough H2 and O2 into the cell, this “Open Circuit Voltage”, EOC , can reach a theoreGcal value of 1.2 V. Negative electrode: 1/2 O2 + 2e– + 2H+ H2O Positive electrode: H2 2H+ + 2e– Overall: H2 + 1/2O2 H2O Voltage (EOC) developed E1 o = 0.0 V E2 o = -1.2 V EOC (cell) = E1 0 – E2 0 = +1.2 V
ThermodynamicsofFuelCells:Enthalpy,GibbsEnergy and Open Circuit VoltageCan we use this open circuit voltage,Eoc,to generate electrical power?ThermodynamicssaysYEs!Changesinenthalpy,AH,entropy,AS,andGibbsEnergy,AG(theenergyavailableto do useful work)areall thermodynamicStateFunctions.That is,theirvaluesforagivenreaction arethesameregardlessof thepathtaken to getbetween initial andfinalstates.Inotherwords,theGibbsenergy,△G,isrelatedtotheenthalpyofcombustionby△H=△G+T△S(whereTistemperature/K)and it isthesamewhetherwecombustH,bychemical combustion (burning),orbyelectrochemicalreactions(coldcombustion)△G=-nFEoc(Eq 1.1)EocissimplyrelatedtoGibbsenergyas:Wheren=the numberof electrons transferred (n=2forPEM,6for DMFC)F=Faraday'sconstant,96485Cmol-1(thechargeinCoulombscarriedby1moleof e)InotherwordsAGofthefuelcell istheelectrostaticenergystored ina capacitorwith a charge,-nF, at a voltage, Eoc!
• Can we use this open circuit voltage, EOC , to generate electrical power? Thermodynamics says YES! • Changes in enthalpy, ΔH, entropy, ΔS, and Gibbs Energy, ΔG (the energy available to do useful work) are all thermodynamic State Func:ons. That is, their values for a given reacGon are the same regardless of the path taken to get between ini:al and final states. • In other words, the Gibbs energy, ΔG, is related to the enthalpy of combusGon by: ΔH = ΔG + TΔS (where T is temperature / K) and it is the same whether we combust H2 by chemical combusGon (burning), or by electrochemical reacGons (cold combusGon). • EOC is simply related to Gibbs energy as: ΔG = –nFEOC (Eq 1.1) Where n = the number of electrons transferred (n=2 for PEM, 6 for DMFC) F = Faraday’s constant, 96485 Cmol-1 (the charge in Coulombs carried by 1 mole of e–) • In other words ΔG of the fuel cell is the electrostaGc energy stored in a capacitor with a charge, –nF, at a voltage, EOC! Thermodynamics of Fuel Cells: Enthalpy, Gibbs Energy and Open Circuit Voltage