Application of Electrochemical Impedance Spectroscopy for Fuel Cell Characterization

Transcription

1 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, 14 th 16 th, 2010

2 Presentation outline Introduction and motivation Examples of porous (technical) electrodes Theory and models of porous gas diffusion 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 Electrochemical kinetic and electrode structure Current density / macm -2 i = 100 macm i 0 = 1 macm -2 i 0 = 10 macm -2 b = 25 mv/decade η 1 η 2 HOR Increasing power output (P=I U) at constant cell voltage (overvoltage) by: enlargement of active electrode surface using porous electrodes (electrode structure) increasing i 0 (electrode material with high catalytic activity) HER Overvoltage / mv Butler-Volmer equation for hydrogen oxydation (HOR) and hydrogen evolution reaction (HER) I= Surface i= Surface i 0 exp{(αrt/zf)η}

4 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

5 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)

6 Schematic representation of main types of fuel cells Temperature AFC 80 C PEM 80 C PAFC 200 C MCFC 650 C SOFC 1000 C Oxidant O 2 O HO 2 2 O HO 2 2 CO O 2 2 O 2 Current Cathode Charge carrier in electrolyte OH - H + H + CO 3 - O 2 - Anode Fuel gas H HO 2 2 H 2 H 2 H 2 HO 2 CO CO 2 H 2 HO 2 CO CO 2 Load Alkaline FC Polymer Electrolyt Membrane FC Phosphoric Acid FC Molten Carbonate FC Solid Oxide FC

7 Measuring methods used for fuel cell and fuel cell components characterization : in-situ und ex-situ methods In-situ measuring methods Current-voltage characteristic (U(i)) Electrochemical Impedance Spectroscopy (EIS) Local and time resolved Cyclic Voltammetry (CV) Current interruption (CI)) Chronopotentiometry (CP) und Chronoamperometry (CA) Current density distribution

8 Ex-situ measuring methods used for fuel cell and fuel cell components characterization Scanning electron microscopy (SEM) and Transmission electron microscopy (TEM) Energy dispersive X-ray spectroscopy (EDS) X-ray photoelectron spectroscopy (XPS) X-Ray Diffraction (XRD) Thermal gravimetric analysis (TGA) Porosimetry (Hg-Porosimetry) Measurement of the specific surface area (BET-measurement) Determination of gas permeability

9 Electrochemical Impedance Spectroscopy: Application to Fuel Cells

10 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)

11 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

12 The Metal-Electrolyte Interface

13 The Metal-Electrolyte Interface + - Double layer capacity (C dl )

14 The Metal-Electrolyte Interface Double layer capacity (C dl ) Faraday-Impedance (Z F )

15 Impedance spectra of a simple electrochemical system (Z F =R ct ): Nyquist representation -8 Imaginary part / 2f max = max =1/C dl R ct -6 R ct =10-4 R el =1 f max =15.9 Hz -2 0 C dl =1 mf 2 R el R ct R el +R ct Real part /

16 Impedance spectra of a simple electrochemical system (Z F =R ct ): Bode representation Impedance / Phase 10 R el +R ct f max =52.8 Hz 80 R ct =10 R el = R ct 40 C dl =1 mf 2f max =(1/R ct C dl )(1+R ct /R el ) 1/2 at =2f=1: Z C =1/C dl (------) 2 R el R el 1 10m 100m K 10K Frequency / Hz 20 0

17 Schematic representation of different steps during electrochemical reaction as a function of distance from electrode surface Ox + ne - Red Adsorption Ox* Ox* Ox bulk Chem. Mass Des. reaction transport n e - Ox ad * Charge transfer Red ad * Des. Ads. Chem. reaction Red* Red* Electrode Double layer Reaction layer Mass transport Red bulk Diffusion layer

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

19 SEM micrograph of a cross section of SOFC Cathode Electrolyte Anode

20 SEM micrograph of a porous silver membrane

21 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

22 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

23 Nyqusit representation of porous electrode impedance with faradaic impedance element -3 Simple pore model with faradaic processes in pores RC-transmission line of a flooded pore imaginary part / C=500mF C+Rpor(3 Ohm) C//R(1.5 Ohm) r c r ct r = 3 c = 500 mf r ct = real part /

24 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

25 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,

26 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

27 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

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

29 Bode diagram of measured EIS at different cell voltages (current densities) I 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

30 Bode diagram of measured EIS at different cell voltages (current densities) II 50 Impedance / m Diffusion Charge transfer of ORR Phase o Charge transfer of HOR 60 R M O V=597 mv; i=400 macm -2 V=497 mv; i=530 macm -2 V=397 mv; i=660 macm -2 + V=317 mv; i=760 macm m 100m K 10K 100K Frequency / Hz N. Wagner, K.A. Friedrich, Dynamic Operational Conditions. In: J. Garche, C. Dyer, P. Moseley, Z. Ogumi, D. Rand and B. Scrosati, editors. Encyclopedia of Electrochemical Power Sources, Vol. 2. Amsterdam: Elsevier, 2009, pp

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 C C dl,c R M R A C dl,a E 0 E C 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 Evaluation of EIS measured during ORR Equivalent circuit and R ct = f(i) R / m m ms -1/ s mf m m 3.18 m m nh N current/ma 1 3

41 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

42 Experimental EIS set-up for stack measurements