Theory and Application of Electrochemical Impedance Spectroscopy for Fuel Cell Characterization Wagner N., Schiller G., Friedrich K.A.

Transcription

1 Theory and Application of Electrochemical Impedance Spectroscopy for Fuel Cell Characterization Wagner N., Schiller G., Friedrich K.A. Deutsches Zentrum für Luft- und Raumfahrt e.v. (DLR) Institut für Technische Thermodynamik Pfaffenwaldring 38-40, Stuttgart

2 Presentation outline Introduction Motivation Types of Fuel Cells Experimental set-up for different types of FCs Modeling of fuel cells with equivalent circuits and microstructure of fuel cells electrodes Impedance models of porous electrodes Different applications of EIS in FC research Contributions to performance loss of PEFC EIS on segmented SOFC EIS measured on Ag-gas diffusion electrodes Conclusion and Outlook

3 Motivation Characterization of Fuel Cells by Electrochemical Impedance Spectroscopy: Determination of electrode structure and reactivity, separation of electrode structure from electrocatalytical activity Determination of electrochemical active surface (locally resolved) Determination of reaction mechanism 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

4 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 OH - CO 3-2 O -2 Charge carrier in electrolyte H + H + 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

5 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 OH - CO 3-2 O -2 Charge carrier in electrolyte H + H + 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

6 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)

7 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?)

8 Electrochemical Impedance Spectroscopy: Application to Fuel Cells

9 Electrochemical Impedance Spectroscopy: Application to Fuel Cells

10 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 , 2009

11 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

12 Bode representation of EIS measured at different current densities, PEFC operated at 80 C with H 2 and O 2 at 2 bar 50 Impedance / m Diffusion Charge transfer of ORR Charge transfer of HOR Phase o 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

13 PEFC: Schematic Diagram (cross section)

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

15 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

16 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?

17 SEM micrograph of PEFC elctrode (Pt/C+PTFE)

18 TEM micrograph of Carbon Supported Platinum Catalyst

19 SEM-picture of Silver-Gas Diffusion Cathode

20 SEM picture of PTFE/C powder

21 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 O 2 Na + + OH - Cl - - poo r t ns O 2 membrane reaction layer diffusion layer flow field/current collector HO, 2 O 2 NaCl H 2 O

22 Nyquist 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 i RC δ = specific electrolyte resistance r = pore radius L = pore lenght

23 Nyquist representation of porous electrode impedance with faradaic impedance element r -3 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 /

24 Agglomerated Electrodes Hierarchical model (Cantor-block model) Gas metal (backing) side side n = 2 l z /a z n = 1 l/a l/a l z n = 0 ionic current electrolyte side l l M. Eikerling, A.A. Kornyshev, E. Lust J. Electrochem. Soc., 152 (2005) 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

25 Cylindrical homogeneous porous electrode model (H. Göhr) Ions (H +, OH -,..) Current (e - ) electrolyte Z p1 pores Z pi Z pn Current collector GDL Z o Z q Electrolyte Z S Z p Z e Z q1 Z qi Z qn Z s1 Z si Z sn porous layer Z m Pore Electrode, porous layer Z n I I H. Göhr in Electrochemical Applications/97,

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

27 Bode diagram of measured EIS at different cell voltages 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

28 EIS at Polymer Fuel Cells (PEFC): Contributions to the cell impedance at different current densities 10 Cell impedance /Ohm Cell impedance / Ohm 1 0,1 0,01 0, Current density / macm R diffusion R membrane R anode R cathode Current density /macm -2

29 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 C E A E M E Diff Current density / macm -2

30 Evaluation of EIS with the porous electrode model Summary of current density dependency of pore resistance elements Pore electrolyte resistance / mohm Pore Electrolyte Resistance Anode Pore Electrolyte Resistance Cathode Cell voltage / mv Current / A 0

31 Segmented SOFC cell design with segmented bipolar plates 16 Segments with air flow channels Voltage probe Thermocouple Current probe SOFC Metallic housing 16 Segments with fuel gas channels Capillary for gas chromatography

32 Fuel gas OCV distribution of ASC at 800 C and simulated reformate (50% H % N 2 + 3% H 2 O, 0.08 SlpM/cm² air) Voltage U Air rev U fuel gas Nernst equation: 0 rev RT ln zf p p O2 air H 2O p H 2 Produced water: S4: 0.61%, S8: 0.72%, S12: 0.78%, S16: 3.30%

33 EIS at OCV, ASC with segmented cathode, cm 2 Dry hydrogen Hydrogen+ 3% H 2 O

34 Bode Diagram of EIS, measured at PEFC, 75 C, 0.5 Acm -2 Variation of gas flow rates Impedance / m Phase / O 1.5H; Air = 1.2; E = 644 mv + 1.5H; Air = 1.5; E = 675 mv 1.1H; Air = 2; E = 653 mv 1.5H; Air = 2; E = 654 mv active surface area 50 cm o K 10K 0

35 EIS on PEFC, 80 C, 5 A, cathode fed with different gas composition, λ=1.5, N111 IP CCM (Ion Power Inc.) Z / m phase / o Air 5 A 50% N2+50% O2 5 A 50% He+50% O2 5 A Oxygen 5 A Z' / m Air 5 A Z'' / m % He+50% O2 5 A 50% N2+50% O2 5 A Oxygen 5 A 100m K 3K frequency / Hz

36 Reactive Mixing and Rolling (RMR) GDE Production Technique for AFC Electrodes

37 Schematically representation of cell voltage and potentials in an alkaline fuel cell Cell Voltage [V] Cathode with ORR O H 2 O + 4 e - 4 OH Anode 2 H 2 +4 OH - 4 H 2 O + 4 e ΔE 0 = 1.23V AFC Current density

38 Current density / potential characteristic 0 Cathode Cathode Hg/HgO 0, Cathode k,c CE RE ODC Potential U 0 d Reference Current density

39 SEM picture of cross section of silver membrane with 0.2µm pores diameter

40 CV s (1 mv/s) from -700 mv to 450 mv vs. Hg/HgO bei 80 C in 10 M NaOH, O 2 Current / ma um 5.0um 0.45um um 1.2um 0.8um Potential / mv

41 Vergleich Impedanzspektren, aufgenommen in 10 M NaOH bei 80 C, -700 mv vs. Hg/HgO nach 60 Minuten 1.5 Z / 0.8um 0.2um phase / o Z' / um 5.0um 3.0um 1.2um m Z'' / m 500m um 3.0um 0.8um 0.45um 5.0um 0.2um K 3K 10K 30K frequency / Hz

42 Equivalent circuit with relaxation impedance and measurement at -700 mv, 1.2 µm membrane Z / phase / o m s m m mf m m mf m m nh m 550m 100m K 3K 10K frequency / Hz 0

43 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

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