Fundamental Consderatons of Fuel Cells for Moblty Applcatons Davd E. Foster Engne Research Center Unversty of Wsconsn - Madson Future Engnes and Ther Fuels ERC 2011 Symposum June 9, 2011
Motvaton Reducng the fuel consumptons of our moblty systems s of paramount mportance Necesstates a fundamental evaluaton of what are the practcal lmts of effcency for our moblty systems Ths has been an actve area wthn the ERC wth a focus on IC Engnes Need to supply a smlar, fundamental analyss of practcal lmts to other canddate power plants and fuels Fuels Cells
Acknowledgements Professor Mark Lnne, Professor and CERC Drector, Appled Mechancs, Combuston, Chalmers Unversty of Technology Professors Ryan P. O Hayre, Suk-Won Cha, Whtney G. Colella and Frtz B. Prnz, Fuel Cell Fundamentals 2 nd edton, John Wley & Sons, 2009
Maxmum Theoretcal Work from a Thermodynamc Process W = G max IC Engnes and Fuel Cells are smlar energy converters Both convert the chemcal nternal energy n the fuel nto work and exhaust the spent fuel and oxdzer (products) Both are governed by the same thermodynamc relatonshp for maxmum theoretcal work Both are chemcal engnes n whch the energy transformaton s accomplshed va a chemcal reacton n a thermodynamc process
Dfferences Between IC Engnes and Fuel Cells IC Engne The energy converson of the fuel s accomplshed va a chemcal reacton The charge transfer occurs drectly between two chemcal speces wthout the lberaton of free electrons Fuel Cell The energy converson of the fuel s accomplshed va an electrochemcal reacton The charge transfer of the reacton s physcally separated and takes place between electrodes and the respectve chemcal speces
Implcatons of ths on the Thermodynamc Analyss IC Engne Wmax = G rxn W max Fuel Cell ( H H ) + T * ( S S ) = Q + T ( S S ) = prod react 0* prod react HV 0 * W elect = where : nfe n F E = = = G rxn number of moles of Faraday's constant = potental dfference prod electrons react
Smple Fuel Cell Example At standard temperature and pressure consder: 1 H 2+ O2 H 2O 2 To obtan hgher voltages one uses a stack of many ndvdual cells
Further Development of the Fuel Cell Maxmum Work Expresson Nernst Equaton W elect = nfe = G rxn Grxn E = realze : E = f ( T, P, µ ) nf wth : µ = µ ( P, T ) + RT ln a where : a E E o = E o 0 = actvty = RT nf ln o Πa Πa o v products v reactants (deal gas) = Standard state reversble voltage f f = P P o
Electrochemstry Includes Movement n a Potental Feld ~ µ = wth : µ + z φ z = Fφ = µ o + RT charge number, ln F a = z Fφ = electrcal potental experences by speces + Faraday's constant, Now we must also consder electrcal potental wth the chemcal potental electrochemcal potental There s a change n the electrochemcal potental experenced by the electrons as they move from the anode to the cathode (a source of entropy generaton)
Practcal Fuel Cell Effcency Need to account for: Voltage losses Chemcal actvaton losses Ohmc losses Mass transport dffusve (concentraton) losses Fuel Utlzaton losses Accountng for the fact that not all of the fuel provded to the fuel cell partcpates n the electrochemcal reacton Ths performance s gven n currentvoltage curves
Current Voltage Curve Ref: Ryan P. O Hayre, Suk-Won Cha, Whtney G. Colella and Frtz B. Prnz, Fuel Cell Fundamentals 2nd edton, John Wley & Sons, 2009
Electrochemcal Processes are Heterogeneous Steps n an Electrochemcal Reacton: 1. Mass transport to the electrode 2. Absorpton of the H 2 onto the electrode surface 3. Separaton of H 2 nto two chemsorbed H atoms on electrode surface 4. Transfer of electrons of chemsorbed H atoms to electrode releasng H + 5. Mass transport of H + away from the electrode Schematc of Electrochemcal Process Ref: Ryan P. O Hayre, Suk-Won Cha, Whtney G. Colella and Frtz B. Prnz, Fuel Cell Fundamentals 2nd edton, John Wley & Sons, 2009
Chemcal Actvaton Losses Electrochemcal reactons take place at an nterface between the electrode and electrolyte Current densty s a more fundamental metrc than current whch then causes a focus on per-unt-area reacton rates Reacton rate s determned by the actvaton energy (barrer) In electrochemcal reactons the actvaton energy can be manpulated by varyng the cell potental The free energy of a charged speces s senstve to voltage changng the voltage of the cell changes the sze of the actvaton barrer The current produced wth a fuel cell ncreases exponentally wth actvaton overvoltage (voltage whch s sacrfced to overcome the actvaton barrer) We sacrfce part of the thermodynamcally avalable cell voltage to produce a net current Actvaton overvoltage occurs at both the anode and cathode
Schematc of Actvaton Losses Actvaton losses from H 2 O 2 fuel cell anode versus cathode Relatonshp between the current densty output and the actvaton overvoltage s exponental and s know as the Bulter Volmer equaton Ref: Ryan P. O Hayre, Suk-Won Cha, Whtney G. Colella and Frtz B. Prnz, Fuel Cell Fundamentals 2nd edton, John Wley & Sons, 2009
Mnmzng Actvaton Overvoltage Losses 1. Increase reactant concentraton Has an mpact on cell voltage Nernst Eq. 2. Increase reacton temperature Nernst Eq. - ths slghtly reduces voltage 3. Decrease actvaton barrer catalyst 4. Increase the number of reacton stes (hgh surface area electrodes)
Ohmc Losses (Charge Transport) Accumulaton/depleton of electrons at the two electrodes creates a voltage gradent whch drves the transport of the electrons In the electrolyte the accumulaton/depleton of protons creates both a voltage gradent and a concentraton gradent whch drves the transport of the protons from the anode to the cathode Charge Flux = σ dv ; σ = conductvty, V = dx PEMFC schematc Ref: Ryan P. O Hayre, Suk-Won Cha, Whtney G. Colella and Frtz B. Prnz, Fuel Cell Fundamentals 2nd edton, John Wley & Sons, 2009 voltage
Ohmc Losses Charge transport contrbutes to a lnear decrease n operatng voltage Ionc charge transport tends to be more dffcult than electronc charge transport onc transport resstance domnates Fuel cell resstance scales wth electrolyte thckness Resstances wthn the fuel cell are addtve η ohmc = R = ( R + R ) onc ohmc elect Ref: Ryan P. O Hayre, Suk-Won Cha, Whtney G. Colella and Frtz B. Prnz, Fuel Cell Fundamentals 2nd edton, John Wley & Sons, 2009
Ohmc Losses Summary Voltage that s expended to drve conductve charge transport s a loss Ohms law Resstance s domnated by onc charge transport Thckness s an mportant parameter Electrolyte choce s very mportant Lqud Polymer Ceramc
Mass transport governs supply and removal of reactants and products Mass transport to the fuel cell electrodes s typcally domnated by dffuson Dffusve transport lmtatons n the electrode lead to a lmtng current densty (reactant concentraton falls to zero at the fuel cell catalyst layer) Reactant deleton affects both the Nernstan cell voltage and the knetc reacton rate Mass transport n fuel cell flow structures s typcally domnated by convecton Vscosty and pressure drop (desgn) mpact these losses Mass Transport Schematc of dffuson layer at the anode Ref: Ryan P. O Hayre, Suk-Won Cha, Whtney G. Colella and Frtz B. Prnz, Fuel Cell Fundamentals 2nd edton, John Wley & Sons, 2009
Mass Transport Couples wth the Nernst Equaton Electrochemcal reacton drves dffuson by reducng the concentraton of the reactant at the electrode surface Concentraton depleton at the surface reduces the voltage Nernst Eq Reactant concentraton at the catalyst surface decreases, and product concentraton at the catalyst surface ncreases, relatve to bulk concentraton ncreases actvaton losses (not ncluded n the fgure) Concentraton loss due to Nernstan effects Ref: Ryan P. O Hayre, Suk-Won Cha, Whtney G. Colella and Frtz B. Prnz, Fuel Cell Fundamentals 2nd edton, John Wley & Sons, 2009
Summary of Thermodynamc Losses n a Fuel Cell V = E η η η thermo act ohmc conc Ref: Ryan P. O Hayre, Suk-Won Cha, Whtney G. Colella and Frtz B. Prnz, Fuel Cell Fundamentals 2nd edton, John Wley & Sons, 2009
Performance Summary The performance curve gven for a partcular fuel cell s the result of unavodable losses Ref: Ryan P. O Hayre, Suk-Won Cha, Whtney G. Colella and Frtz B. Prnz, Fuel Cell Fundamentals 2nd edton, John Wley & Sons, 2009
Effcency at Peak Power Voltage at peak power Peak Power Effcency at peak power ~ 48% Does not nclude converson of electrcty to rotatonal moton
Closng Comments Fuel Cell Thermodynamc Challenges Rasng the effcency at hgh load Issues to be addressed: Voltage losses - (thermodynamcally assocated wth power generaton), Ohmc losses, and Dffusve losses Hgh load effcency s nferor to IC Engne IC Engne Thermodynamc Challenges Rasng effcency at low load Issues to be addressed Pumpng work, Combuston gas γ control (temperature and composton), exhaust energy Low load effcency s nferor to Fuel Cell Hybrdzaton, etc
Socal or External Constrants Cost Perceved Utlty (moblty paradgm): Range, performance, avalablty of an acceptable energy carrer, local emsson sgnature, nose, convenence of use.. Lfe cycle ecologcal footprnt Etc. (how easly they connect wth Cloud).
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