EIS in Fuel Cell Science

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www.dlr.de Chart 1 EIS in Fuel Cell Science Dr.

1 Chart 1 EIS in Fuel Cell Science Dr. Norbert Wagner German Aerospace Center (DLR) Institute for Engineering Thermodynamics Pfaffenwaldring 38-49, 7569 Stuttgart, Germany Electrochemical Impedance Spectroscopy Fundamentals and Applications November 215 Frankfurt am Main Chart 2 Presentation outline Introduction Motivation Types of Fuel Cells Experimental set-up for different types of FCs Microstructure of fuel cells and modeling of fuel cells with equivalent circuits Impedance models of porous electrodes Different applications of EIS in FC research Contributions to performance loss of PEFC Degradation mechanism of PEFC Time dependent EIS CO poisoning of PEFC-anodes Flooding of PEFC cathodes EIS measured on Ag-Gas Diffusion Electrodes (GDE) during Oxygen Reduction in alkaline electrolyte (AFC) EIS measured at fuel cell stack (SOFC) Conclusion

2 Chart 3 Motivation Characterization of Fuel Cells by Electrochemical Impedance Spectroscopy: Determination of electrode structure and reactivity, separation of electrode structure from electrocatalytically activity Determination of reaction mechanism (kinetic) and separation of different overvoltage contributions to the fuel cell performance loss Determination of degradation mechanism of electrodes, electrolyte and other fuel cell components (bipolar plates, end plates, sealings, etc.) Determination of optimum operation condition (e.g. gas composition, temperature, partial pressure), cell design (flow field) and stack design Chart 4 Schematic representation of main types of fuel cells Temperature AFC 8 C PEM 8 C PAFC 2 C MCFC 65 C SOFC 1 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 3-2 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

half cell with liquid electrolyte Test cell for SOFC (short stack) (Solid Oxide Electrolyte) de Chart 6 Fuel cell

Oxygen Reduction Reaction (ORR): O 2/air = RT/[(1- )2F] [ln i - ln i * ] Cathode Cathode, Cathode ct,c Ohmic loss

3 Chart 5 Experimental set up and cells used for EIS Segmented and single PEFC cell (polymer electrolyte) Fuel half cell with liquid electrolyte Test cell for SOFC (short stack) (Solid Oxide Electrolyte) Chart 6 Fuel cell overvoltage and current density / voltage characteristic Hydrogen Oxidation Reaction (HOR): H = RT/2F i/i* 2 Oxygen Reduction Reaction (ORR): O 2/air = RT/[(1- )2F] [ln i - ln i * ] Cathode Cathode, Cathode ct,c Ohmic loss = ir Transport limitation (diffusion) Potential U Cell Voltage (U C ) d +( r ) d = - RT/2F ln (1 - i/i lim ) d +( r ) Fuel cell voltage U C = U - ct,h 2 - ct,o2/air - d - Anode ct,a Current density (Current/Surface?)

4 Chart 7 Electrochemical Impedance Spectroscopy: Application to Fuel Cells Chart 8 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

distance from the electrode surface N. Wagner, K.A.

), ISBN-978--444-5293-7, Elsevier Amsterdam, Vol.2, pp.

5 Chart 9 Schematic representation of the different steps and their location during the electrochemical reactions as a function of distance from the electrode surface N. Wagner, K.A. Friedrich, Dynamic Response of Polymer Electrolyte Fuel Cells in Encyclopedia of Electrochemical Power Sources (Ed. J. Garche et al.), ISBN , Elsevier Amsterdam, Vol.2, pp , 29 Chart 1 Overview of the wide range of dynamic processes in FC Electric double layer charging Membrane humidification Charge transfer fuel cell Liquid water reactions transport Gas diffusion processes Degradation and ageing effects Changes in catalytic properties / poisoning Temperature effects microseconds milliseconds seconds minutes hours days months Time / s

2 at 2 bar 5 Impedance / m Diffusion Charge transfer of ORR Charge transfer of HOR Phase o R M 8 3 6 2 4 15 1 O

Frequency / Hz 2 N. Wagner in Pem Fuel Cell Diagnostic Tools, Haijaing Wang, Xiao-Zi Yuan, Hui Li (Eds.

treatment (Bio)-Organic synthesis Auxiliary supply Fuel Cells GDE anode ee l ctrical power cah t ode Electrolysis

6 Chart 11 Bode representation of EIS measured at different current densities, PEFC operated at 8 C with H 2 and O 2 at 2 bar 5 Impedance / m Diffusion Charge transfer of ORR Charge transfer of HOR Phase o R M O V=597 mv; i=4 macm -2 V=497 mv; i=53 macm -2 V=397 mv; i=66 macm -2 + V=317 mv; i=76 macm -2 1m 1m K 1K 1K Frequency / Hz 2 N. Wagner in Pem Fuel Cell Diagnostic Tools, Haijaing Wang, Xiao-Zi Yuan, Hui Li (Eds.), Chart 12 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 OH + ClṈa+ - - membrane reaction layer diffusion layer flow field/current collector HO, 2 O 2 NaCl H 2 O

7 Chart 13 PEFC: Schematic Diagram (cross section) Chart 14 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

8 Chart 15 SEM micrograph of PEFC elctrode (Pt/C+PTFE) Chart 16 TEM micrograph of Carbon Supported Platinum Catalyst

(Bio)-Organic synthesis Batteries and supercaps Auxiliary supply GDE Electrolysis (Water, NaCl, HCl, etc.

9 Chart 17 SEM picture of PTFE/C powder Chart 18 Field of application of porous electrodes Water purification and treatment (Bio)-Organic synthesis Batteries and supercaps Auxiliary supply GDE Electrolysis (Water, NaCl, HCl, etc.) Hydrogen Current collector Fuel Cells anode ee l ctrical power cah t ode Cl 2 NaOH Membrane Process fluids Packed bed cathode H 2 electrons poo r t ns O 2 Na + + OH - Cl - - O 2 membrane reaction layer diffusion layer flow field/current collector HO, 2 O 2 NaCl H 2 O

www.dlr.de Chart 19 Nyquist representation of Impedance of RCtransmission line, model of a flooded pore R imaginary part / -3-2.5-2 -1.5-1 -.5 C=5mF Pore 1 mhz -1 -.5.5 1 1.

10 Chart 19 Nyquist representation of Impedance of RCtransmission line, model of a flooded pore R imaginary part / C=5mF Pore 1 mhz real part / R = 3 Ω C =.5 F Z( i ) C R coth i C R R = R/3 = δl/3πr 2 r L i RC δ = specific electrolyte resistance r = pore radius L = pore lenght Chart 2 Simple pore model with faradaic processes in pores RC-transmission line of a flooded pore R. De Levie, Electrochim. Acta, 8(1963) 751 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

element r -3 c r ct imaginary part / -2.5-2 -1.5-1 C=5mF C+Rpor(3 Ohm) C//R(1.

11 Chart 21 Nyquist representation of porous electrode impedance with faradaic impedance element r -3 c r ct imaginary part / C=5mF C+Rpor(3 Ohm) C//R(1.5 Ohm) r = 3 c = 5 mf r ct = real part / Chart 22 Thin-film model and agglomerate plus thin-film model of a porous electrode I.D. Raistrick, Electrochim. Acta, 35(199) 1579

12 Chart 23 Agglomerated Electrodes Hierarchical model (Cantor-block model) side side Gasmetal (backing) n=2 lz/az n=1 l/a l/a lz n= ionic l electrolyte side l current M. Eikerling, A.A. Kornyshev, E. Lust J. Electrochem. Soc., 152 (25) E24 S.H. Liu, Phys. Rev. Letters, 55(1985) 5289 T.Kaplan, L.J.Gray, and S.H.Liu, Phys. Rev. B 35 (1987) 5379 Cylindrical homogeneous porous electrode model (H. Göhr) Ions (H+, OH -,..) Current (e-) pores Zp1 Zpi Zpn Current collector GDL Zo Electrolyte Zq ZS Zq1 Zqi Zqn Zm Pore Ze Zp Electrode, porous layer electrolyte Zs1 Zsi Zsn porous layer Zn I I H. Göhr in Electrochemical Applications/97,

13 Chart 25 Bode diagram of EIS at different cell voltages measured at PEFC (H 2 /O 2 ) at 8 C, Impedance / Phase o 3 1 3m 1m 3m O E=124 mv; I= ma E=841 mv; I=125 ma E=597 mv; I=923 ma + E=317 mv; I=1751 ma m 1m 1m K 1K 1K Frequency / Hz Chart 26 EIS at Polymer Fuel Cells (PEFC): Contributions to the cell impedance at different current densities 1 Cell impedance /Ohm Cell impedance / Ohm 1,1,1, Current density / macm Current density /macm -2 R diffusion R membrane R anode R cathode

14 Chart 27 Evaluation of the U-i characteristics from EIS Cell voltage /mv U 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 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 Chart 28 EIS at Polymer Fuel Cells (PEFC): Contributions to the overal U-i characteristic determined by EIS Cell voltage / mv R N C N R K C dl,c R M R A C dl,a Current density / macm -2 E E C E A E M E Diff.

15 Chart 29 Evaluation of EIS with the porous electrode model Summary of current density dependency of pore resistance elements 4 12 Pore electrolyte resistance / mohm Pore Electrolyte Resistance Anode Pore Electrolyte Resistance Cathode Cell voltage / mv Current / A Chart 3 Improved Evaluation Techniques for Time Resolved Electrochemical Impedance Spectroscopy (TREIS) Real time drift compensation Time course interpolation Z-HIT compensation

16 impedance Chart 31 Improved evaluation technique: Time course interpolation drift affected data interpolated data time course of the impedance at single frequencies Requirements time Series measurement Time for each measured frequency AND for each spectrum frequency Chart 32 Reforming of Methane CH 4 + 2H 2 O => 4H 2 + CO 2 (CO) Methane Compres. Reformer Shiftreactor HT a) Reformer-heating b) Cat-Burner 9% CO Residual Gas 3% CO Shiftreactor LT COcleaning PEM- Fuel Cell Heat,5% CO H 2,CO 2 <,5%CO Air (O 2 ) E-Energy

17 Chart 33 Appearance of voltage oscillations during galvanostatic operation of PEFC with H 2 + 1ppm CO Chart 34 Time resolved EIS - CO poisoning of Pt-anode Imaginary part / m 8 a 6 Cell voltage / mv b c d e f Overv. CO / mv a b c d e f Time / s Real part / m Time progression of cell voltage and overvoltage in galvanostatic mode of PEFC operation (217 macm -2 ) Pt-anode, H ppm CO at 8 C Nyquist plot of EIS measured at different times during poisoning of the Pt-anode with CO

18 Chart 35 Measured and simulated time resolved EIS - CO poisoning Imaginary Part / m Time Real Part / m Nyquist plot of EIS during CO poisoning of the Pt-anode at 217 macm 2, H 2 +1 ppm CO Chart 36 Time resolved EIS - CO poisoning Phase Impedance / m m 1 1 1K Frequency / Hz 5 1 Time / ks 3 1 1m 1 1 1K Frequency / Hz 5 1 Time / ks Bode plot of EIS during CO poisoning of the Pt-anode at 217 macm 2, H 2 +1 ppm CO

2 4 6 8 Real part / m 1 = 1 s; R k = 1 m ; R = 1 m R M = 5 m ; C dl = 1 mf Z F ( ) = R F = 9,1 m C dl Faraday-impedance: Z F

19 Chart 37 Simulated EIS with Relaxation Impedance: Theory Imaginary part / m Z F R M khz C dl R k L k 2 4 1mHz R M R Real part / m 1 = 1 s; R k = 1 m ; R = 1 m R M = 5 m ; C dl = 1 mf Z F ( ) = R F = 9,1 m C dl Faraday-impedance: Z F = R /(1+ R /Z k ) Relaxation imp.: Z k = (1+ i ) / I F dlnk/de Time constant: = R k L k Relaxation resistance: R k = 1 / I F dlnk/de Relaxation inductivity: L k = R k Chart 38 Time resolved EIS Flooding the cathode at 2 A, dead end

20 Chart 39 Time resolved EIS, flooding the cathode at 2 A, 8 C; time dependency of the cell voltage and impedance elements Cell voltage [ mv ] 85 8 R M C dl,c Vol. produced water [ ml ] W / m s -1/2 R ct / m 6 W R ct, C Time / Ks Time / Ks Chart 4 Change of cell voltage during constant load at 5 ma cm -2 Current density / ma cm -2, Cell voltage / mv µv/h 12 Current density Cell voltage 13 Elapsed time / h

21 Chart 41 Bode Plot of EIS measured during PEFC operation over 1 h at 5 ma cm -2 Impedance / m Phase Time m K 1K 1K Frequency / Hz Chart 42 Variation of RC during PEFC operation at 5 ma cm R N R C R A 15 R M Impedance / mohm C N C dl,c C dl,a New start up after 24 h Cathode Elapsed time / h

22 Chart Variation of R A during PEFC operation at 5 ma cm -2 Anode 8 7 Impedance / mohm R N R C R A 2 1 R M New start up after 24 h C N C dl,c C dl,a Elapsed time / h Evaluation and Analyse of Voltage Loss by EIS 14 Cathode 2 mv 12 1 Anode 68.8 mv Reversibl e voltage loss Voltage loss / mv Cathode 43.2 mv Irreversible voltage loss 48.6 mv 2 Anode 5.4 Irreversible voltage loss Reversible voltage loss Overall cell voltage loss M. Schulze, N. Wagner, T. Kaz, K.A. Friedrich, Electrochim. Acta 52 (27)

23 Chart 45 AFC: Impedance Measurements during ORR in 1 N NaOH, on Silver Electrodes at Different Current Densities, i< -5 macm m Z / ma ma ma ma ma ma ma ma ma ma phase / o Z' / 15 ma 5 ma 25 ma 2mA 3 ma 35 ma 4 ma 45 ma 1 ma 5 ma Z'' / 1m K 3K 1K 1K frequency / Hz Bode representation Nyquist representation Chart 46 2 Z / Impedance Measurements during ORR in 1 N NaOH, on Silver Electrodes at Different Current Densities, i> -5 macm ma phase / o 9-1 Z' / ma ma m 6m ma ma ma ma ma ma ma 1m K 3K 1K 1K frequency / Hz Bode representation ma 15 ma 1 ma 5 ma 2 ma 25 ma 3 ma 35 ma 4 ma 45 ma Nyquist representation Z'' /

ad/het =RT(k- i)/n 2 F 2 c s A(k 2 + 2 ) With A=electrode surface, k= first order

The heterogenous reaction impedance can be converted into a parallel combination of R

24 Chart 47 Adsorptions- and heterogeous reaction impedance Definition of Z ad/het : Z ad/het =RT(k- i)/n 2 F 2 c s A(k ) With A=electrode surface, k= first order reactions rate, F=Faraday constant, c s =surface concentration and angular frequency =2 f. The heterogenous reaction impedance can be converted into a parallel combination of R ad/het and C ad/het : R ad/het =RT/(n 2 F 2 c s Ak) C ad/het =n 2 F 2 c s A/(RT) Chart 48 Electrode Model with cylindrical, homogeneous pores and complex Faraday-impedance Zq=

25 Chart 49 Evaluation of EIS measured during ORR Equivalent circuit and R ad = f(i) R / 6 L Rel 4 Rct Rpor Rad Cad Cdl current/ma Chart 5 U-i characteristic and current density dependency of impedance elements R ad and R ct ir-corr. Potential vs. NHE / V 1,1 1,8 1,6 1,4 1,2 1,98,96,94,92 8, 7, 6, 5, 4, 3, 2, 1, Rad; Rct / Ohm,9, -,1 -,8 -,6 -,4 -,2, Current density / Acm -2

26 Chart 51 Current density dependency of k ad, R ad and R ct, determined from EIS evaluation k ad =1/C ad R ad 7 Rad; Rct / Ohm Reaction rate constante / s Current density / Acm -2 M. Lang Impedance Spectroscopy at SOFC short stack U5 U4 Cell 5 Voltage probes U3 I Cell 4 U2 Cell 3 Potentiostat/ Electronic Load Current probes U1 Cell 2 Cell 1

www.dlr.de Chart 53 Comparison of the sequential measurement principle for impedance spectra data acquisition with the synchronous parallel approach http://www.zahner.de/products/addon-cards/pad4.

27 Chart 53 Comparison of the sequential measurement principle for impedance spectra data acquisition with the synchronous parallel approach Chart 54 Nyquist impedance diagram of the five individual cells within the SOFC short stack at OCP (1.19 V), measured synchronously Operation under dry fuel gas (5 % H % N 2 and air at 75 Cº. Symbols: measurement data, solid lines: model fit

28 Chart 55 Conclusion Determination of the individual potential losses during fuel cell operation Determination of degradation mechanism and performance loss Improvement of fuel cell performance and stability by understanding instead of trial and error Determination of critical operation conditions of fuel cells Chart 56 Norbert Wagner, Electrochemical power sources Fuel cells, in Impedance Spectroscopy: Theory, Experiment, and Applications, 2 nd Edition, Edited by Evgenij Barsoukov and J. Ross Macdonald, John Wiley&Sons, Inc., 25, pp , ISBN:

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

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,