Introduction to Basic Electrochemistry for Fuel Cells and Electrolysis

Transcription

1 Introducton to Basc Electrochemstry for Fuel Cells and Electrolyss Claude LAMY, Professor Insttut Européen des Membranes, CNRS - GDR n 3339 (PACS), Unversty of Montpeller 2, 2 Place Eugène Batallon, Montpeller, France, E-mal: claude.lamy@emm.unv-montp2.fr 1 2 nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

2 Introducton Thermodynamcs and theoretcal energy effcency Reacton mechansms and knetcs lmtatons The Polymer Electrolyte Fuel Cell Prncple of a Polymer Electrolyte Fuel Cell Cell components (membrane, catalysts, etc.) Survey of varous applcatons of PEFC The Drect Methanol Fuel Cell Reacton mechansms and knetcs lmtatons The fuel crossover problem Conclusons 2 2 nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

3 The Hydrogen/Oxygen Fuel Cell H 2 + ½ O 2 H 2 O + electrcty (+ heat) wth G r = kj/mole ; H r = kj/mole It looks lke the reverse of water electrolyss : H 2 O + electrcty H 2 + ½ O 2 Electrcal Energy : W e = - G 118 MJ/kg 33 kwh/kg Standard e.m.f. : E = - G/nF = V Theoretcal effcency : ε = G/ H = 83% (25 C) C.F. Schönben, W.R. Grove, Phlosophcal Magazne, (1839) 1801 : Dscovery of the Volta Cell 1839 : Brth of Georges Leclanché 1859 : Gaston Planté dscovered the Pb/PbO 2 battery 1867 : Inventon of the Leclanché cell 3 2 nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

4 Practcal energy effcency of water electrolyss and fuel cells j = I/S = n F v (A/cm 2 ) Water electrolyss H 2 2 H + E electrolyss = 2,1 V E FC = 0,7 V 2 H + H 2 O 2 H 2 O H 2 O O 2 E / V(SHE) Electrolyss effcency 1.23/2.1 59% Fuel Cell effcency 0.7/ % Overall effcency (1.23/2.1)x(0.7/1.23 ) = 0.7/2.1 33% H 2 /O 2 Fuel Cell 4 2 nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

5 How does work a fuel cell : t s an electrochemcal devce whch transforms drectly the combuston energy of a fuel (- G) nto electrcty (and heat) Comparson wth other electrc source and Internal Combuston Engne (ICE) Leclanché Cell Zn/MnO 2 Electrcty (W e ) Energetc Effcency Electrcty Fuel Tank Oxdant Battery Pb/PbO 2 Fuel Cell H 2 /O 2 (ar) Electrcty (W e ) Electrcty (W e ) Heat (Q) Electrochemcal Source ε = W e /(- H) = G/ H 80 to 97 % Practcal effcency 40 to 60 % Thermal Engne (Carnot) Fuel Tank Oxdant Internal Combuston Engne: Heat (Q 1, T 1 ) Mechancal work (W m ) Heat (Q 2, T 2 ) ε r = W m /(- H) = (Q 1 -Q 2 )/Q 1 = 1 Q 2 /Q 1 = 1 T 2 /T 1 e.g. ε = 1 T 2 /T 1 30 to 45 % Practcal effcency 18 to 30 % 5 2 nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

6 Thermodynamc Relatons A key varable n Electrochemstry: the Electrochemcal Potental Defnton Thermodynamc relatons Equlbrum between 2 charged phases α and β Nernst potental The Standard Hydrogen Electrode (SHE) Theoretcal energy effcency Under standard condtons (j = 0) As a functon of temperature 6 2 nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

7 Defnton of the Electrochemcal potental Consder a charged phase α contanng dfferent speces () : solvent molecules (H 2 O), chemcal speces A of charge z (z = 0 for a neutral speces, z > 0 for a caton, < 0 for an anon). The Electrochemcal Potental of one mole of speces () n phase α s the total work to brng speces () from µ α a standard reference pont (wth no nteracton), usually Infnty ( ), to the bulk phase α : Ψ = Χ = q 4πεoε r r r N p.r 4πε 3 oεr r + + A µ α + Phase α - - χ Ψ α + α µ α α α µ = G / N ) T,P,N j ( = µ α + z µ α = chemcal potental; F = Faraday = Ne o = C Φ = nner potental; Ψ = outer potental; Χ = surface potental F Φ 7 2 nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012 α wth Φ α = Ψ α + χ α

8 Equlbrum at the electrode-electrolyte Interface Inner potental Φ Φ M α E = Φ M -Φ S β Φ S 0 dstance to electrode x µ µ E α β α β = µ.e. µ = µ - µ = = M S S M µ + z F Φ = µ = µ 8 2 nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012 M = Φ M - Φ S = (µ S - µ M + z ) / z F Φ E: electrode potental; [A ]: actvty z A A + n e - z B B wth µ = µ o + RT Ln[A ] and n = z A - z B µ S A µ M µ S + n = E = Φ M Φ S = (µ AS -µ BS + µ em )/nf e B E = (µ Ao -µ Bo )/nf +RT/nF Ln([A]/[B]) + µ em /F E = E o A/B +RT/nF Ln([A]/[B]) + µ em /F whch looks lke a Nernst law, but E s not drectly measurable F S 0

9 Only the potental dfference between 2 electrodes, the potental of each s defned by 2 electrochemcal systems n equlbrum, s measurable : z A A1 1 + n 1 e - z B B1 z 1 and A A2 2 + n 2 e - z B B2 2 E eq = E 2 E 1 = E o A2/B2 E o A1/B1 + RT/nF Ln([A 2 ] [B 1 ]/[B 2 ] [A 1 ]) snce the electron chemcal potental (µ em /F) cancels out wth the same metal (Cu) as connectng leads at the extremty of the measurng devce. Ths corresponds to the overall chemcal reacton : z a 2 A A2 z 2 + b 1 B B1 z 1 a 1 A A1 z 1 + b 2 B B2 2 wth equlbrum constant K e = ([A 1 ] a1 [B 2 ] b2 / [A 2 ] a2 [B 1 ] b1 ) = exp(- G r /RT) where the reacton free energy s related to the chemcal potentals: so that: E eq = E 2 E 1 = - G r / nf or Gr = Njµ j - Nµ j (j = products, = reactants) Gr + nf Eeq = Gr = nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

10 Important remarks : Actvty and Fugacty For an electrolytc soluton the actvty [A ] of speces () s related to ts concentraton C = N /V (N moles n soluton of volume V) by: [A ] = γ C where γ s the actvty coeffcent (γ 0 f C 0 deal soluton) For a gaseous phase the fugacty of a gaseous speces () s related to ts partal pressure through ts chemcal potental µ = µ o + RT Ln (f /P o ) where P o s the pressure of a reference state (P o = 1 bar usually) and f s proportonal to the partal pressure p,.e. f = γ p wth an actvty coeffcent γ 0 f p 0 deal gas) In the followng, actvty and fugacty wll be assmlated to concentraton and partal pressure, respectvely nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

11 Thermodynamc Relatons G < 0 for spontaneous reacton ; E cell = E c E a G = - S T + V P + Σ µ N wth µ = µ o + RT Ln[A ] Then : W e = - nf E cell = F T + P V = G T,P = H T S For an half cell whose electrode potental s controlled by an electrochemcal reacton : A + n e - B wth µ = µ o + RT Ln[A ] E eq = E o A/B + (RT/nF) Ln ([A ] / [B ]) wth E o A/B = (µ Ao - µ Bo ) / nf = - G o /nf (standard state) one may defne a standard Redox potental E o A/B n the potental scale of the Standard Hydrogen Electrode (SHE). The Redox potental of all electrochemcal systems can thus be classfed n the SHE scale (electrochemcal seres) nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

12 Alkal whch gves: GDR n H e - H 2 Pt/H 2 (p=1 bar) / ([H + ]=1), 25 C A + n e - B A + n /2 H 2 B + n H + wth G r < 0 or > 0 o o = N µ - Njµ = µ + n µ - (µ + n / 2µ ) - n F E G j + E Reference electrode: the Standard Hydrogen Electrode (SHE) r B H A H 2 = j eq = E snce E A/B SHE - E SHE = (µ o H + = (µ A -µ - 1/2µ B o H 2 ) /nf - (µ ) /F = 0-1/2µ by defnton Thus E eq = E o A/B + (RT/nF) Ln ([A ] / [B ]) Nernst law Zn 2+ /Zn SHE 12 2 nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012 o H + Cu 2+ /Cu T o H 2 Fe 3+ /Fe 2+ ) /F = (µ A -µ B O 2 /H 2 O eq ) /nf E/V Halogen

13 Reference electrodes: Pt wre Platnum wre H 2 gas p H2 = 1 bar Hg Hg 2 Cl 2 KCl saturated soluton Pt/Pt fol 1 M H 2 SO 4 Frt KCl crystals Porous Glass Frt Standard Hydrogen Electrode (SHE) Saturated Calomel Electrode (SCE) 13 2 nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

14 Workng electrode Reference electrode Scheme of a 3-electrode cell for electrochemcal measurements wth the potental control of the workng electrode by a potentostat Salt brdge Luggn capllary Coolng water 14 2 nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

15 Nernst equaton for the H 2 /O 2 system Electrochemcal half-cell reactons: H 2 > 2 H e - 1/2 O H e - > H 2 O Overall reacton : H 2 + ½ O 2 > H 2 O Then : E eq = - G r /nf = - G o /2F + (RT/2F) Ln[(p H2 ) (p O2 )½ / p H 2 O] E eq = E o + (RT/2F) Ln[(p H2 ) (p O2 )½ / p H 2 O] wth E o = - G o /2F s the standard cell voltage,.e. E o = V at 25 C, where p H2, p O2, and p H 2 O are the partal pressure of hydrogen, oxygen and water, respectvely. Therefore by ncreasng the pressure of reactants (H 2 and O 2 ) and decreasng the pressure of the product (water) one can ncrease the cell voltage. The varaton of E eq wth T s contaned manly n the entropc term,.e. S ro = nf (de o /dt) : temperature coeffcent nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

16 Thermodynamcs of Hydrogen Oxdaton Anode H 2 > 2 H e - H OH - > 2 H 2 O + 2 e - acd medum alkalne medum 1/2 O H e - > H 2 O acd medum Cathode 1/2 O2 + H 2 O + 2 e - > 2 OH - alkalne medum H 2 + 1/2 O 2 > H 2 O overall reacton wth G = kj and H = kj/mole H 2 (standard state) Energy effcency under standard condtons: ε r cell = G / H = G 237x 10 E eq = E + c - E a = = nf 2x96500 = 1,23 Volt G 237x 10 W e = = = xM 3600x2 33 kwhr/kg 16 2 nd Jont European Summer School for FC & H2 Technology-Heraklon-September

17 Reversble energy effcency (I = 0) For a Fuel Cell workng under standard condtons (25 C, 1 bar) : cell We n F Eeq G T S 237 εr = = = = 1 = = ( H) ( H) H H % For a Thermal Engne, such as an ICE, workng between an hot source Q 1 at 350 C and a cold source Q 2 at 80 C : thermal Wr Q 353 (80 C) T2 εr = = = = 1 - = ( H) Q T 623 (350 C) % 17 2 nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

18 Thermodynamc data for the H 2 /O 2 combuston reacton (kj/mol) as a functon of temperature Physcal state of water T/K H G E eq Effcency (ε= G/ H) Standard state Lqud Lqud Lqud Gaseous Gaseous Gaseous Gaseous Gaseous Gaseous Gaseous Gaseous nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

19 Thermodynamc data for the H 2 /O 2 combuston reacton (kj/mol): H 2 + 1/2 O 2 > H 2 O In the lqud state of water (273 K < T < 373 K) : H = G + T S = H HHV = H LHV + Q Cond so that n the standard state (T = K, p =1 bar) : Q Cond = H HHV H LHV = = - 44 kj/mol ε r HHV = G / H HHV = 237/286 = 0.83 In the gaseous state of water (T > 373 K) : H = G + T S = H LHV so that at e.g. T = 400 K ( 127 C, p =1 bar) : H = H LHV = kj/mol ε r LHV = G / H LHV = 224/243 = 0.92 HHV = Hgh Heatng Value ; LHV = Low Heatng Value ; Q Cond = Heat of Condensaton 19 2 nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

20 1,00 0,90 GDR n 3339 Theoretcal energy effcency (FC vs. Carnot) ε Cell ε Carnot (T cold = 25 C) ε Carnot (T cold = 80 C) Lnear (ε Cell) Polynomal (ε Carnot (T cold = 25 C) Polynomal (ε Carnot (T cold = 80 C) ε cell = ε Carnot 25 C 0,74 at T 1150 K (877 C) 0,80 0,70 FC effcency Theoretcal effcency 0,60 0,50 0,40 Carnot effcency ε cell = ε Carnot 80 C 0,72 at T 1250 K (977 C) 0,30 0,20 0,10 0, Hot source temperature / K 20 2 nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

21 Knetcs Relatons A typcal knetcs law n Electrochemstry: the Butler-Volmer equaton Knetcs of a smple electrochemcal reacton The Butler-Volmer law Establshment of the Butler-Volmer law Mass transfer lmtatons Cell voltage vs. current densty curves Energy effcency under workng condtons (j 0) 21 2 nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

22 Knetc relatons for a smple electrochemcal reacton : A + n e - B I = dq/dt = nf ( dn A /dt) = nf ( dn B /dt) I = nf v wth the reacton rate v = ( dn A /dt) I < 0 for a reducton reacton ; I > 0 for an oxdaton reacton mass transfer I electron transfer mass transfer Electrolyte I ons Reacton mechansm 22 2 nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

23 Knetcs of a charge transfer reacton z (heterogeneous reacton) : A A + n e - z B B A z A B z B Actvaton barrer for an electrochemcal reacton (K s the decrease n actvaton energy due to the electrode catalyst) G = G + nf E 0 I 0 G 0 I 0 : reducton reacton (cathodc) I = - c v c G 0 I 0 : oxdaton reacton (anodc) I = nf nf a v a 23 2 nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

24 Expresson of the reacton rates wth the Arrhenus law elec o elec a a B o a B where α s the charge transfer coeffcent (0 < α < 1),.e. the fracton of electrcal energy whch actvates the reducton reacton (A B), and (1- α) nfe actvatng the oxdaton reacton (B A). Thus: j = I/ S = nf 24 2 nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012 a + a B a At equlbrum v a = v c and j = 0, so that j a = -j c, E = E eq and C elec = C o : vc = kc(e,t) S CA = kc S CA exp(- Gc /RT) wth Gc = Gc + α nf E elec elec v = k (E,T) S C = k S C exp(- G /RT) wth G = G - (1-α) nf E o elec + o elec + [ k C exp(- G /RT) exp[(1-α)nfe /RT] - k C exp(- G /RT) exp[-αnfe /RT]] o o + o o o + j a = -j c = j o = nfk a CB exp(- Ga RT) exp[(1-α)nfe eq /RT] = nfkcc A exp(- Gc Then E eq =E o A/B + RT/nF Ln(C Ao /C Bo ) wth E o A/B = exp(- G o /RT) Nernst law and j o = nf (k co ) 1- α (k ao ) α (C Ao ) 1- α (C Bo ) α = nf k s o (C Ao ) 1- α (C Bo ) α Out of equlbrum j = j a - j c 0. Dvdng j by j o one obtans: j(η) = j o [(C B elec /C Bo ) exp{(1-α)(nf/rt)η} - (C A elec /C Ao ) exp{-α(nf/rt)η}] or j(η) = j o [exp{(1-α)(nf/rt)η} exp{-α(nf/rt)η}] (no mass transfer lmtaton) Butler-Volmer law wth η = E - E eq the overvoltage and j o the exchange current densty c A a + c o a + /RT) exp(-αnfe eq /RT)

25 Expresson of the Butler-Volmer law n some lmtng cases Very often α = ½ so that j(η) = 2 j o sh[(nf/2rt) η] hyperbolc snus curve If α = 0 or 1 then j(η) = j o [exp(nf/rt η) - 1] or j(η) = j o [1 - exp(nf/rt) η] For η << RT/nF ( 25/n mv at 25 C) then j(η) = j o (nf/rt) η = η/r t or η = R t j wth R t = RT/nFj o the charge transfer resstance ( Ohm s law) For η >> RT/nF then j(η) = j o exp[(1- α )nf/rt η] or η = RT/(1-α)nF Ln(j/j o ) and for η << - RT/nF then j(η) = - j o exp[- α nf/rt η] or η = -RT/αnF Ln(-j/j o ).e. η = a ± b log j Tafel law wth the Tafel slopes b = 2.3 RT/α nf n V dec -1 For the Hydrogen Oxdaton Reacton (HOR) the exchange current densty s very hgh (j o 1 ma cm -2 ) so that the reacton s reversble and the Ohm s law does apply (lnear relatonshp between overvoltage and current densty) For the Oxygen Reducton Reacton (ORR) the exchange current densty s small (j o 1 µa cm -2 ) so that the reacton s rreversble (one may neglect the reverse reacton,.e. water oxdaton) and one may wrte: j(η) = - j o exp(-α nf/rt η) or η c act = - RT/α c nf Ln( j /j o ) actvaton overvoltage 25 2 nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

26 Mass transfer lmtatons : concentraton overvoltage The reacton knetcs can be controlled by the mass transfer (dffuson, mgraton, convecton) of the reactng speces nsde the electrolytc phase, so that the current densty s proportonal to the mass flux densty J of speces (): j = z F J wth J = 1/S N / t = C v (v s the velocty). J can be expressed by the Nernst-Planck equaton: r CD r r r ext zf r r ext J = - µ + Cv = - D C - DC Φ + Cv RT RT In the followng we wll neglect the contrbuton of the mgraton current j m = z Fu C E = z λ C E = κ E and the convecton current j cv = z F C v ext so that we wll only consder the dffuson process (Fck s laws) of speces (): r r r r d st d d J = - D C (Fck' s 1 law) and j = z F J C r d nd = - dvj = D C (Fck' s 2 law mass conservaton) t wth D = dffuson coeffcent; u = z FD /RT = electrc moblty; λ = Fu = molar conductvty; Κ = z λ C = onc conductvty; E = - Φ = electrcal feld 26 2 nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

27 C (x,t) / t = D and J = - D C (x) = ( C GDR n 3339 Resoluton of Fck s equatons (n the approxmaton of a sem-nfnte lnear dffuson, under steady state condtons): d C (x) C o C / x, 2 C (x,t) / x / x) C x= 0 / t = 2 = 0 0 x + C (0) = and j leadng to C o d = ± z F D ( C C / x) = - C (0) x + C (0) δ / x C x= 0 o - C (0) δ (0 x δ) C (0) Electrode Electrolyte δ = thckness of the dffuson layer 0 0 Electrode dstance 27 2 nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012 δ x

28 Mass transfer lmtatons : concentraton overvoltage For the electrochemcal reacton, A + n e - B, the rate of whch s controlled only by dffuson, the current densty can be wrtten as: ljl = j d = nf D c o c (0) / δ, wth n = z A z B wth c (0) the concentraton at the electrode surface. For hgh rate,.e. hgh current denstes, all the reactng speces arrvng at the electrode surface are mmedately consumed and ther actvty (concentraton) becomes 0. Thus the concentraton gradent reaches a maxmum value, leadng to a lmtng current densty: j l = nf D c o / δ wth D (n m 2 /s or cm 2 /s) the dffuson coeffcent of speces (), c o ts concentraton n the bulk of electrolyte and δ the thckness of the dffuson layer (n the approxmaton of a sem-nfnte lnear dffuson) nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

29 Mass Mass transfer lmtatons: : concentraton overvoltage By dong the rato of j wth j l one may obtan: ljl/j l = [c o c (0)] / c o = 1 - c (0)/c o = 1 - p (0)/p o The superfcal concentraton (or partal pressure for gaseous speces) s : c (0)/c o = p (0)/p o = 1 - ljl / j l For a very fast transfer reacton, only lmted by mass transfer, the Nernst equaton does apply at the surface, leadng to concentraton overvoltages: E eq = E o A/B + (RT/nF) Ln (c A (0)/c Ao / c B (0)/c Bo ) = E 1/2 + (RT/nF) Ln[(j-j lc )/(j la -j)] (equaton of a Polarographc Wave assumng the same dffuson coeffcents) η η conc a conc c = = RT n F a RT n F c ln 1 ln 1 j j j l a j l c at the at the anode cathode 29 2 nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

30 E(j) curves lmted by mass transfer (dffuson) Current densty / A cm ,2 0,4 0,6 0,8 1 1,2 1,4 1,4 1,2 1,0 0,8 0,6 0,4 Cell voltage / V 0,2 0,0 Cell voltage / V 1,4 1,2 1,0 0,8 0,6 0,4 0,2 0,0 0 0,2 0,4 0,6 0,8 1 1,2 1,4 Current densty / A cm -2 Tafel slope b c = 60 mv/dec. Lmtng current j lc = 1.4 A/cm nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

31 Cell voltage vs. current densty E(j) curves The cell voltage vs. current densty characterstcs for the H 2 /O 2 fuel cell, takng nto account charge transfer overvoltages, concentraton overvoltages and ohmc drop can thus be wrtten as: E(j) = E c (j) E a (j) R e j = E eq ( η a act (j) + η a conc (j) + η c act (j) + η C conc (j) ) R e j E(j) = E eq + RT αnf ln j o a j 2 j o c + RT nf ln 1 j l j a 1 j l j c R e j 31 2 nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

32 IjI = n F v = n F(1/S) dn /dt j/ma cm GDR n 3339 Current densty vs. electrode potental for electrochemcal reactons nvolved n fuel cells H 2 H 2 CH 3 OH CH 3 OH O 2 E a Pt Pt -Ru -- -X X X E = E c 0.8 E a V 0.8 V Pt Pt -Ru -Ru Pt Pt -Ru -Ru Pt E c O η a 0.3 V η c E eq (CH 3 OH) 1.21 V E eq ( CH 3 OH) 1.21 V 0.5 E/V vs. RHE nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

33 Cell voltage vs. current densty E(j) curves j 0 = 10-6 ; R e = 0,10 ; j l = 2,2 j 0 = 10-6 ; R e = 0,15 ; j l = 1,4 j 0 = 10-8 ; R e = 0,15 ; j l = 1,3 j 0 = 10-8 ; R e = 0,30 ; j l = 1,2 1,200 E eq = 1.23 V 1,000 0,800 Charge transfer overvoltage Ohmc losses R e j Mass transfer overvoltage E/ V 0,7 0,600 0,400 0,200 0, ,25 0,4 0,5 0,75 0,9 1 1,25 1,5 j/a cm -2 P o = 0,175 W/cm 2 P/P o =1,6 P/P o = 3,6 P/P o = 6 E(j) = E c (j) E a (j) R e j = E eq ( η c (j) + η a (j) + R e j ) 33 2 nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

34 Energy effcency under workng condtons (j 0) E(j) = + Ec (j) Ea (j) R e j = Eeq (η a (j) + η c (j) + R e j) Eeq = + Eeq Eeq η a = E - (j) E - eq η c = E + (j) E + eq R e = electrolyte and nterface resstance wth η a > 0 (fuel oxdaton) wth η c < 0 (oxygen reducton) E(j) (η a (j) + η c (j) + R e j) 0.7 j εe = = 1 = 57% andεf = = E eq E eq 1.23 j m nexp F E(j) n F Eeq E(j) nexp cell ε cell = = = εr εe ε ( H) ( H) E n ε cell = 0.83 x 0.57 x % and ε CHP 85 to 95% eq F nexp n 34 2 nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

35 The Polymer Electrolyte Fuel Cell (PEFC) Prncple of a Polymer Electrolyte Fuel Cell Cell components (membrane, catalysts, etc.) 35 2 nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

36 Schematc representaton of a PEFC elementary cell 2e - + W e H 2 2e - Anodc catalyst 2 H + I PEM 2e - 4 OH ½ O 2 H 2 O Cathodc catalyst Overall reacton: H 2 + ½ O 2 H 2 O wth G < nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

37 Polymer Electrolyte Fuel Cell (PEFC) MEA End Plate Bpolar Plate Teflon gasket Bpolar plate End Plate Schematc representaton of a PEFC elementary cell 37 2 nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

38 Schematc representaton of the Membrane Electrode Assembly (MEA) Anode Vent Cathode Vent Proton Exchange Membrane ( 100 µm) Gas Dffuson Backng (200 µm) H + Catalyst layer (10 µm) 0.5 to 0.9 V Thckness e 500 µm Anode Feed, H 2, Alcohol Cathode Feed, O 2 Thckness e 5 mm Membrane Electrode Assembly (MEA) 38 2 nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

39 Influence of the membrane specfc resstance (R e = e/σ) on the cell voltage-current densty characterstcs E(j) Cell voltage E/V 1,200 1,000 0,800 0,7 0,600 0,400 E eq = 1,23 V Charge transfer actvaton Cell voltage E(j) = E + (j) E (j) - R e j Ohmc drop R e j 0,200 Mass transfer actvaton 0, ,25 0,4 0,5 0,75 1,0 1,25 1,5 Current densty j/a cm -2 j o = 10-8 A cm -2 ; R e = 0,15 Ω cm 2 ; j l = 1,3 A cm -2 j o = 10-8 A cm -2 ; R e = 0,30 Ω cm 2 ; j l = 1,2 A cm nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

40 E(j) characterstcs of a PEFC elementary cell wth dfferent membranes : Dow (e = 125 µm) ; o Nafon 115 (e = 125 µm) ; Nafon 117 (e = 175 µm) Cell voltage (E/V) Specfc resstance (R e = e / σ) e.g. for Nafon 115 : cm / S cm Ω cm 2 DUPONT NAFION CF 2 CF 2 CF 2 CFCF 2 CF 2 O Dow CF 2 CF CF 3 >< O Current densty (j/a cm -2 ) After K. Prater (Ballard), J. Power Sources, 29 (1990) 239 CF 2 CF 2 O=S=O O H nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

41 Influence of the mass transfer lmtatons (lmtng current densty j l ) on the cell voltage-current densty characterstcs E(j) Cell voltage E/ V 1,200 1,000 0,800 0,7 0,600 0,400 E eq = 1.23 V Charge transfer overvoltage Cell voltage E(j) = E + (j) E (j) - R e j Ohmc losses R e j Mass transfer overvoltage 0,200 0, ,25 0,4 0,5 0,75 0,9 1,0 1,25 1,5 Current densty j/a cm -2 j 0 = 10-6 A cm -2 ; R e = 0,10 Ω cm 2 ; j l = 2,2 A cm -2 j 0 = 10-6 A cm -2 ; R e = 0,15 Ω cm 2 ; j l = 1,4 A cm nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

42 Cell voltage vs. current densty curves for an H 2 /ar FC wth MEAs usng three dfferent types of carbon cathode GDL (80 C, 0.22 mg cm 2 Pt loadng) : ( ) mcroporous layer-coated and 30 wt.% FEP-mpregnated; ( ) 30 wt.% FEP-mpregnated; ( ) 10 wt.% FEP-mpregnated. After C. Lm, C.Y. Wang, Electrochm. Acta 49 (2004) Current densty / ma cm nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

43 Influence of the catalytc propertes of electrodes (exchange current densty j o ) on the cell voltage E(j) Cell voltage E/V 1,200 1,000 0,800 0,7 0,600 0,400 0,200 E eq = 1,23 V E(j) = E + (j) - E (j) - R e j = E eq ( η c (j) + η a (j) ) - R e j Charge transfer overvoltage Ohmc drop R e j Mass transfer overvoltage 0, ,25 0,4 0,5 0,75 0,91,0 1,25 1,5 Current densty j/a cm -2 j o = 10-6 A cm -2 ; R e = 0,15 Ω cm 2 ; j l = 1,4 A cm -2 j o = 10-8 A cm -2 ; R e = 0,15 Ω cm 2 ; j l = 1,3 A cm nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

44 Cell voltage / mv GDR n 3339 Fuel cell characterstcs of a DEFC recorded at 110 C. Influence of the nature of the bmetallc catalyst (30% loadng) Pt Pt-Sn(80:20) Pt-Ru(80:20) Pt-Mo(80:20) Current densty / ma cm -2 Polarzaton curves 30 Pt 25 Pt-Sn(80:20) Pt-Ru(80:20) Pt-Mo(80:20) Current densty / ma cm -2 Power densty curves Anode catalyst : 1.5 mg.cm -2 ; Cathode catalyst : 2 mg.cm -2 (40% Pt/XC72 E-TEK) Membrane : Nafon 117; Ethanol concentraton : 1 M After C. Lamy et al., n Catalyss for Sustanable Energy Producton, edted by P. Barbaro Power densty / mw cm -2 and C. Banchn, Wley-VCH, Wenhem, 2009, Chap.1, pp nd Jont European Summer School for FC & H2 Technology-Heraklon-September

45 Survey of varous applcatons of PEM Fuel Cells What are ther uses?? Producton of electrc energy (and eventually heat n CHP systems) n a wde range of power (1 W to about 10 MW) wth a smlar energy effcency: Electrc Power Plant (1 to 10 MW) Electrc Vehcle (10 to 200 kw) Auxlary Power Unt (APU from 1 to 100 kw) Aerospace power supply (1 to 50 kw) Household Applcaton (1 to 10 kw) Power supply for portable electroncs (1 to 100 W) 45 2 nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

46 Cty Bus equpped wth a 200 kw FC for the 2008 Olympc Games n Bejng 46 2 nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

47 APU for arcrafts and spacecrafts Fuel Cell System (SAFRAN Group) 47 2 nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

48 Arbus 320 equpped wth a PEFC APU as demonstrated by DLR (Germany) 48 2 nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

49 Applcaton to portable electroncs (1 to 100 W) Laptop Computer ~ W Cell Phone ~ 1-10 W 49 2 nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

50 The Drect Methanol Fuel Cell (DMFC) Prncple of a Drect Methanol Fuel Cell Reacton mechansms and knetcs problems 50 2 nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

51 W e = 6.1 kwhr/kg A 6e - I 6e - + Anode Electrolyte Cathode CH 3 OH + H 2 O Electronc conductor + catalyst Ionc conductor e.g. a PEM CO 2 + 6H + + 6e - 6H + Electronc conductor + catalyst 6e - + 6H + + 3/2O 2 Oxygen (Ar) I CO 2 Drect Methanol Fuel Cell 3H 2 O 51 2 nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

52 Reacton mechansm of the electrocatalytc oxdaton of methanol at platnum-based electrodes Pt + CH 3 OH Pt - CH 3 OH ads Pt - CH 3 OH ads Pt - CHO ads + 3 H e - Pt - CHO ads Pt - CO ads + H + + e - Ru + H 2 O Ru OH ads + H + + e - Pt - CO ads + Ru OH ads Pt + Ru + CO 2 + H + + e - Overall reacton CH 3 OH + H 2 O CO H e nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

53 Ths a complex 6-electron reacton mechansm, whch needs the preparaton of bmetallc and plurmetallc electrocatalysts for methanol oxdaton catalysts carbon Pt-Ru Ru Pt Pt carbone Pt Pt carbone Pt Ru Ru carbone Ru Pt-Ru alloy Pt-Ru/XC72 Pt and Ru on the same support Pt+Ru/XC72 Pt and Ru on dfferent supports Pt/XC72+Ru/XC72 Modfcaton of platnum by Ruthenum usng the collodal method (after Bönnemann et al.) 53 2 nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

54 Evaluaton of the electrocatalytc actvty of PtRu / C electrodes by cyclc voltammetry (room temperature) I/ ma mg Pt Pt/XC72 Pt+Ru/XC72 Pt/XC72+Ru/XC72 Pt-Ru/XC72 I/mA mg Pt Pt/XC72 Pt+Ru/XC72 Pt/XC72+Ru/XC72 Pt-Ru/XC72 0 0,0 0,2 0,4 0,6 0,8 1,0 1,2 0 0,3 0,4 0,5 E/V vs. RHE E/V vs. RHE Oxdaton of preadsorbed CO (v=50mv s -1 ) Oxdaton of 0.1M methanol (v=5mv s -1 ) On same C partcles On dfferent > Alloy > C partcles > Platnum On same C partcles > > Alloy Platnum > On dfferent C partcles 54 2 nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

55 Effect of the crossover of the fuel through the polymer membrane MeOH / mmol Tme / hours Nafon 117 NEM 1 NEM 6 NEM 20 NEM 21 Permeablty to methanol of dfferent Solvay membranes n comparson wth Nafon 117 at room temperature nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

56 As a result of the crossover of the fuel to the cathode, the electrode potental E m wll be determned by a mxed reacton resultng from the reducton of oxygen and the oxdaton of methanol at the same potental E m. As both reactons are qute rreversble, a Tafel behavor s practcally always observed. Under these condtons, the current densty (j m ) and mxed potental (E m ) are gven by the equatons : and : where j o and b are the exchange current densty and the Tafel slopes, respectvely, for both half-cell reactons nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012 o ) (E m E c E o (E m a ) - b c m = joa b a = joc 10 j E m b = c 10 E b o a a + b + b a c E o c babc + b + b a c j log j Ths wll result n a negatve shft, E m, of the mxed potental at the oxygen cathode, as gven by the followng equaton : E m babc = b + b a c j log j oa o'a 120 log10 2 wth j o a = 10 3 j oa and b a b c 120 mv/decade for both the oxygen reducton (at hgh current denstes) and the methanol oxdaton reactons. 3 oc oa 180 mv

57 Effect of methanol crossover through the membrane 1,0 0,8 0,6 E /V 0,4 0,2 emf MeOH/O 2 E an MeOH. E cat MeOH cross-over E an H 2. E cat H 2. 0, j / ma cm -2 Polarzaton curves of an H 2 /O 2 PEFC and a DMFC n the presence of methanol at the cathodc compartment : ( ) potental of the PEFC cathode; (O) potental of the DMFC cathode ; ( ) DMFC cell voltage 57 2 nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

58 E(j) and P(j) curves of a sngle DMFC wth Pt-Ru/C electrodes of dfferent Pt-Ru atomc ratos E / V P / mw.cm j / ma.cm -2 Pt-Ru (1:1) Pt-Ru (2:1) Pt-Ru (4:1) Anode 2 mg.cm -2 Pt-Ru / C, Nafon 117, Cathode 2 mg.cm -2 Pt / C, [MeOH] = 2 M, 2 ml/mn., P MeOH = 2 bar ; T cell = 110 C, O 2 = 120 ml/mn., P O2 = 2.5 bar ; T MeOH = T O2 = 95 C nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

59 Applcatons of DMFC to portable electroncs: cell phone, laptop computer, cam recorder, etc. TOSHIBA DMFC for portable applcatons 59 2 nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

60 Dfferent knds of applcatons of FC GdR ITSOFC MCFC Bo-fuel Cells GdR PACEM GdR PACEM Aerospace SOFC AFC PAFC Statonary Portable PEMFC PEFC Mn PAC Transportaton 1mW 0.1 W 1W 10 W 100W 1 kw 10 kw 100kW 1 MW GdR PACEM GdR PACEM 60 2 nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

61 References [1] P.W. ATKINS, Physcal Chemstry, Oxford Unversty Press, Oxford, [2] A.J. BARD, L.R. FAULKNER, Electrochemcal Methods - Fundamentals and Applcatons, John Wley & Sons, New York, [3] J. O M BOCKRIS, S. SRINIVASAN, Fuel Cells : ther electrochemstry, McGraw Hll Book Co., New York, [4] A.J. APPLEBY, F.R. FOULKES, Fuel Cell Handbook, Van Nostrand Rhenhold, New York, [5] K. KORDESH, G. SIMADER, Fuel cells and ther applcatons, VCH, Wenhem, [6] K. PRATER, J. Power Sources, 29 (1990) 239. [7] C. LAMY, J.-M. LEGER, J. Phys.IV, Colloque C1, Vol.4 (1994) C [8] C. LAMY, J.M. LEGER, S. SRINIVASAN, Drect Methanol Fuel Cells : From Fundamental Aspects to Technology Development, n "Modern Aspects of Electrochemstry", J. O'M. Bockrs, B.E. Conway (Eds.), Plenum Press, New York, volume 34 (2001) pp [9] Handbook of Fuel Cells, W. Velstch, H. Gasteger, A Lamm (Eds.), Wley, Chchester (UK), Volume 1, "Fundamentals and Survey of Systems", (2003) pp Volume 2 to 6, [10] F. BARBIR, PEM Fuel Cells: Theory and Practce, Elsever/Academc Press, nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012

62 Thank you for your knd attenton 62 2 nd Jont European Summer School for FC & H2 Technology-Heraklon-September 2012