Theory and Application of Porous Electrodes in Fuel Cell Characterization. Dr. Norbert Wagner DLR, Institut für Technische Thermodynamik, Stuttgart

Size: px

Start display at page:

Download "Theory and Application of Porous Electrodes in Fuel Cell Characterization. Dr. Norbert Wagner DLR, Institut für Technische Thermodynamik, Stuttgart"

Christal McCormick
5 years ago
Views:

1 Theory and Application of Porous Electrodes in Fuel Cell Characterization Dr. Norbert Wagner DLR, Institut für Technische Thermodynamik, Stuttgart Kronach Impedance Days 2009 "Hermann Göhr" Kloster Banz, May, 27 th 29 th, 2009

2 Presentation outline Introduction Examples of porous (technical) electrodes Theory and models of porous electrodes Impedance models Application of Göhr s porous electrode model EIS measured at PEFC EIS measured during oxygen reduction on silver in alkaline solution Outlook Experimental set up for EIS applied for stack measurements

3 Why porous electrodes? Current density / macm -2 i = 100 macm -2 i 0 = 1 macm -2 i 0 = 10 macm -2 b = 25 mv/decade HER η 1 η 2 HOR Overvoltage / mv Butler-Volmer equation for hydrogen oxydation (HOR) and hydrogen evolution reaction (HER) Enlargement of active electrode surface Lowering of overvoltage at same current input (electrolyzer) or output (fuel cell) Increasing of power density (galvanic cells) Increasing of storage capacity (supercaps) Lowering catalyst loading by increasing active surface

4 Fuel cell overvoltage and current density / voltage characteristic Hydrogen Oxidation Reaction (HOR): H 2 = RT/2F i/i * 0 Cathode Oxygen Reduction Reaction (ORR): Cathode 0, Cathode O 2/air = RT/[(1-)2F] [ln i - ln i * ] ct,c Ohmic loss = ir Transport limitation (diffusion) Potential U 0 Cell Voltage (U C ) d +( r ) d = - RT/2F ln (1 - i/i lim ) d +( r ) Fuelcellvoltage U C = U 0 - ct,h 2 - ct,o2/air - d - Anode ct,a Current density (Current/Surface)

Field of application of porous electrodes Batteries and supercaps Water purification and treatment (Bio)-Organic synthesis Auxiliary supply Fuel Cells GDE anode ee l ctrical power cah t ode

5 Field of application of porous electrodes Batteries and supercaps Water purification and treatment (Bio)-Organic synthesis Auxiliary supply Fuel Cells GDE anode ee l ctrical power cah t ode Electrolysis (Water, NaCl, etc.) Hydrogen Current collector H 2 O 2 Cl 2 NaOH, H 2 Membrane Process fluids Packed bed cathode electrons poo r t ns ClṈa+ OH membrane reaction layer diffusion layer flow field/current collector HO, 2 O 2 NaCl H 2 O

6 Electrochemical Impedance Spectroscopy: Application to Fuel Cells Potential excitation signal - E(t) Cell voltage U U/I - Characteristic of a Fuel Cell Current I Current response signal- I(t)

7 Schematic diagram of the U-i characteristic of PEFC and Electrochemical Impedance Measurements Ruhespannung (ohne Stromfluß) R ac An U ( Anode ) i U = ir M Anodic Overvoltage Cell voltage U n U(Cell) R ac Cath R dc Cell U ( Cathode ) i U ( Cell ) i i Cathodic Overvoltage U-i measured i n Current density

8 PEFC: Schematic Diagram (cross section)

9 SEM picture of PTFE/C powder

10 Multi-layer Gas Diffusion Electrodes with different porous layers Carbon-PTFE Layer (Dry sprayed) Ag-PTFE Layer (Rolled Layer)

11 Brief Overview of Porous electrode models and Applications Authors Reference Model and system J. -P Candy, P Fouilloux, M. Keddam, H. Electrochim. Acta, 26(1981) 1029 Ni in alkaline solution Takenouti R. De Levie Electrochim. Acta, 8(1963) 751 Transmission line model, J.S. Newman and C.W. Tobias J. Electrochem. Soc., 109(1962) 1183 Steady-state J. Giner, C. Hunter J. Electrochem. Soc., 116(1969) 1124 Flooded-agglomerate model, Pt-GDE, OCR in alkaline solution K. Mund, F.v. Sturm Electrochim. Acta, 20(1975) 463 HOR on Ni in alkaline solution S. Sunde, Electrochim. Acta, 42(1997) 2637 Composites, SOFC P. Björnbom Electrochim. Acta, 32(1987) 115 Steady state model R. Holze, W. Vielstich J. Electrochem. Soc., 131(1984) 2298 OCR in alkaline solution T.E. Springer, I.D. Raistrick J. Electrochem. Soc., 136(1989) 1594 Flooded-agglomerate and thin film model, differential element of a pore wall H. Göhr Poster ISE Erlangen, 1983 Homogeneous porous model, Pb in sulfphuric acid G. Paasch, K. Micka, P. Gersdorf Electrochim. Acta, 38(1993) 2653 Macrohomogeneous porous electrode model W. Scheider J. Phys. Chem., 79(1975) 127 Model with pore branching S. Srinivasan, H. D. Hurwitz, J. O'M Bockris J. Chem. Phys., 46(1967) 3108 Thin film model M. Kramer, M. Tomkiewicz J. Electrochem. Soc. 131(1984) Stochastic approach with interpenetrating network A. Winsel, E. Bashtavelova J. Power Sources, 73(1998) 242 Agglomerate-of-spheres model M. Tomkiewicz, B. Aurian-Blajeni J. Electrochem. Soc. 135(1988) 2743 True effective medium approach H. Keiser, K.D. Beccu, M.A. Gutjahr Electrochim. Acta, 21(1976) 539 Various geometries of single pore, Ni-GDE

12 Simple pore model of interface charging RC-transmission line of a flooded pore R = electrolyte resistance inside the pore per unit length C = interface capacitance per unit length Z( i) R ic coth irc

Nyqusit representation of Impedance of RCtransmission line, model of a flooded pore R imaginary part / -3-2.5-2 -1.5-1 -0.5 0 C=500mF Pore 100 mhz -1-0.5 0 0.

13 Nyqusit representation of Impedance of RCtransmission line, model of a flooded pore R imaginary part / C=500mF Pore 100 mhz real part / R = 3 Ω C = 0.5 F Z( i) C R i C coth R 0 R 0 = R/3 = δl/3πr 2 r L irc δ = specific electrolyte resistance r = pore radius L = pore lenght

14 Simple pore model with faradaic processes in pores RC-transmission line of a flooded pore ct r = electrolyte resistance inside the pore per unit length c = interface capacitance per unit length r ct = interface charge transfer resistance per unit lenght (Faraday impedance y 2 )

15 Nyqusit representation of porous electrode impedance with faradaic impedance element -3 r c r ct imaginary part / C=500mF C+Rpor(3 Ohm) C//R(1.5 Ohm) r = 3 c = 500 mf r ct = real part /

16 Generalization of RC-transmission line of a flooded pore

17 Thin-film model of a porous electrode

18 Thin-film model and agglomerate plus thin-film model of a porous electrode I.D. Raistrick, Electrochim. Acta, 35(1990) 1579

19 Theory of Agglomerated Electrodes Gas metal (backing) side side ionic current electrolyte side M. Eikerling, A.A. Kornyshev, E. Lust, J. Electrochem. Soc., 152 (2005) E24

20 Hierarchical model (Cantor-block model) n = 1 n = 1 n = 2 n = 1 n = 2 (a 2 /a z ) 2 R C aa z ) 2 (a 2 /a z )R C aa z ) (a 2 /a z ) 2 R n = 2 l z /a z n = 1 n = 0 n = 0 R n = 0 l l/a l/a l l z Z R j μ 1 j s μ L μ 2 2 Naz/ a S.H. Liu, Phys. Rev. Letters, 55(1985) 5289 T.Kaplan, L.J.Gray, and S.H.Liu, Phys. Rev. B 35 (1987) 5379 (1) μ 1 R Naz/ a Naz/ a j (2) 1 μ 1 j... (3) μ C

21 Cylindrical homogeneous porous electrode model (H. Göhr) I electrolyte pores metal Z p1 Z pi Z pn Z e Z q1 Z qi Z qn Z m Z s1 Z si porous layer Z sn H. Göhr in Electrochemical Applications/97,

22 Cylindrical homogeneous porous electrode model (H. Göhr) II Z o Z S Ions (H +, OH -,..) Electrolyte Z q Z p Pore Current (e - ) Electrode, porous layer Z* = ( Z C = cosh p Zs) Zp Z Z * Z s q Z # = Zp Zs ( Zp Zs) Zp Zs S = sinh Z * p Z * Zp Z P = q 0 = v = Z o Z * Z p Z Z s q n = Z Zs s = = 1-p Zn* Zp Zs s Z n 2 2 C( 1C) 2 pss( p qns qo) Z = Z # +Z* S( 1qnqo) C( qnqo) I I

23 Porous electrode model with faradaic impedance

24 Simulation of Impedance Spectra using Porous Electrode Model Z / m phase / 0 90 imaginary part / m pore 0m pore 10m pore 20m pore 30m pore 0m pore 10m pore 20m pore 30m pore 0m pore 10m pore 20m pore 30m real part / m 100m K 10K 100K frequency / Hz (R ct C dl + R el,por )+R M +L R ct = 50 m C dl = 50 mf R M = 3m L = 20 nh 0 100m K 10K 100K frequency / Hz Rel,por = 0 m Ω 10mΩ 20 mω 30mΩ

25 Electrochemical Impedance Spectroscopy: Experimental Set-up Flow contoller Pressure regulator Electrochemical workstation Humidifier PEFC

26 Bode diagram of measured EIS at different cell voltages (current densities) I m Z / phase / 0 OCV 1024 mv 952 mv 901 mv 871 mv 841 mv 807 mv 777 mv 747 mv 717 mv m 30 30m 15 10m 0 10m 100m K 10K 100K frequency / Hz

27 Bode diagram of measured EIS at different cell voltages (current densities) II 50 Impedance / m Phase o O E=597 mv; i=400 macm -2 E=497 mv; i=530 macm -2 E=397 mv; i=660 macm -2 + E=317 mv; i=760 macm m 100m K 10K 100K Frequency / Hz

28 Bode diagram of measured EIS at different cell voltages (current densities) III Impedance / Phase o m 100m 30m O E=1024 mv; I=0 ma E=841 mv; I=1025 ma E=597 mv; I=9023 ma + E=317 mv; I=17510 ma m 0 10m 100m K 10K 100K Frequency / Hz

29 Common Equivalent Circuit for Fuel Cells R ct,c R ct,a R M C dl,c C dl,a

30 Common Equivalent Circuit for Fuel Cells Diffusion of O 2 Z diff R ct,c R ct,a R M C dl,c C dl,a

31 Common Equivalent Circuit for Fuel Cells Diffusion of H 2 Z diff R ct,c R ct,a Z diff R M C dl,c C dl,a

32 EIS at Polymer Fuel Cells (PEFC): Common equivalent circuit and boundary case R N R ct,c R ct,a R M Porous electrode with pore electrolyte resistance (R por ) and surface layer resistance (R S ) C N C dl,c C dl,a R ct,c R ct,a R M C dl,c C dl,a Equivalent circuit of the PEFC: anode and cathode simulated without pores, without diffusion (valid for example at lower current densities)

33 Bode diagramm of the EIS, measured at the PEFC at 80 C, symmetrical gas supply of the cell Impedance / Phase o 10 O 2 /O 2 H 2 /H m 20 10m 10m 100m K 10K 100K Frequency / Hz 0

34 EIS at Polymer Fuel Cells (PEFC): Contributions to the cell impedance at different current densities 0.2 Cell impedance /Ohm Cell impedance /Ohm Current density /macm Current density /macm -2

35 Evaluation of the U-i characteristics from EIS Cell voltage /mv Current density /macm -2 measured curve: U n = f(i n ) calculated curve: U n = i n R n (without integration) calculated curve using method II: U n = a n i 2 n +b n i n +c n x calculated curve using method I: U n = a n i n +b n R U n U I n Integration method I: U U U 1 ( ) ( I I ) I I n n n n n n Integration method II: 2 U a I b I c with: n n n n n n R R n 1 n a n 2 ( I I ) n 1 b R 2 a I n n 1 n n 1 c U 2 a I b I n n 1 n n 1 n n 1 n

36 EIS at Polymer Fuel Cells (PEFC): Contributions to the overal U-i characteristic determined by EIS 1100 Cell voltage / mv R N C N R K C dl,c R M R A C dl,a E 0 E K E A E M E Diff Current density / macm -2

37 Evaluation of EIS with the porous electrode model I I Rp,a; Rct,a; Rpor,a /Ohm R p, a ( Rpor, a R Rpor tanh Rct, ct, a , a Current density /macm -2 a ) I I Porous electrode resistance (R p, a ), charge transfer resistance (R ct, a ) and electrolyte resistance (R por, a ) in the pore of the anode at different current densities

38 Evaluation of EIS with the porous electrode model i-v characteristic and current dependency of pore electrolyte resistance of the anode and cathode Pore electrolyte resistance / mohm R el,por,anode R el,por,kathode Cell voltage / mv Current / A

39 Z / 5 ma Impedance Measurements during Oxygen Reduction Reaction (ORR) in 10 N NaOH, on Silver Electrodes at Different Current Densities 10 ma 15 ma 20 ma 25 ma 30 ma 35 ma 40 ma 45 ma phase / o 100 ma 95 ma 90 ma 85 ma 80 ma 75 ma 70 ma 65 ma 60 ma 55 ma 50 ma Z' / 15 ma 20 ma 10 ma 5 ma Z'' / 500m 0 10m 100m K 10K 100K frequency / Hz ma i / ma

40 Outlook Further improvement of porous electrode models Combination and extension of existent and new models Application of EiS to segmented cells Experimental validation of models using PEFC and DMFC electrodes with different porous structure Gas Diffusion Electrodes (GDE) for Oxygen Consumption Reaction (OCR) in alkaline solution using different gas compositions

Application of Electrochemical Impedance Spectroscopy for Fuel Cell Characterization

Application of Electrochemical Impedance Spectroscopy for Fuel Cell Characterization Dr. Norbert Wagner DLR, Institut für Technische Thermodynamik, Stuttgart Kronach Impedance Days 2010 Kloster Banz, April,