Electrochemical Characterization of Silver Gas Diffusion Electrodes during Oxygen Reduction in Alkaline Solution

Transcription

1 Chart 1 Electrochemical Characterization of Silver Gas Diffusion Electrodes during Oxygen Reduction in Alkaline Solution Norbert Wagner German Aerospace Center (DLR) Institute of Technical Thermodynamics, Stuttgart, Germany International Workshop on Impedance Spectroscopy (IWIS) Chemnitz, September 30-October 2, 2013

2 Chart 2 Presentation outline Introduction and motivation Examples of porous electrodes and some technical applications of the ORR in alkaline solution Metal-(Li)-Air Battery Cathode of the Alkaline Fuel Cell (AFC) Silver-based gas-diffusion electrodes for chlor-alkali electrolysis with oxygen depolarized cathodes (ODC) Electrode production techniques at the DLR Theory and model of the electrochemical impedance spectra (EIS) of porous electrodes Evaluation of EIS measured during oxygen reduction at Ag based GDE in 10 M NaOH at 80 C Conclusion

3 Chart 3 Motivation Why Li-air batteries? Highest theoretical specific energy density ( Wh/kg). Cathodic reactant, O 2 from ambient air, does not have to be stored Environmental friendliness Higher safety than Li-ion batteries (only one of the reactants contained in the battery) Potentially longer cycle and shelf lives

4 Chart 4 Motivation Why Li-air batteries? Highest theoretical specific energy density ( Wh/kg). Cathodic reactant, O 2 from ambient air, does not have to be stored Environmental friendliness Higher safety than Li-ion batteries (only one of the reactants contained in the battery) Potentially longer cycle and shelf lives G. Girishkumar et al., J. Phys. Chem. Lett., 2010, 1,

5 Chart 5 Architectures of Li-Air Batteries Non-aqueous electrolyte: 2Li + + O 2 + 2e - Li 2 O 2 E rev = 2,959 V 2Li + +2e - + (1/2) O 2 Li 2 O E rev = 2,913 V Aqueous electrolyte: 4Li + O 2 + 2H 2 O 4LiOH (alkaline media) E rev = 3,446 V 4Li + O 2 + 4H + 2H 2 O + 4Li + (acidic media) E rev = 4,274 V

6 Chart 6 Schematic architecture of Lithium-air battery with aqueous alkaline electrolyte and GDE (OCR and OER) Interlayer Lithium Festkörper Solid Li + -conductor Li + -Leiter Reaktions Reaction - produkte products Aqueous Welectroyte ässrige Elektrolyt solution - lösung O 2 -Reduktion -Reduction Reaction equation (alkaline electrolyte): 4Li + O 2 + 2H 2 O 4LiOH; E = 3,45 V

7 Chart 7 Alkaline Fuel Cell (AFC) with Gas Diffusion Electrodes (Anode: Ni-GDE, Cathode: Ag-GDE) Schematically representation of an AFC DLR-Bipolar AFC stack

8 Chart 8 Schematically representation of cell voltage and potentials in an alkaline fuel cell, ph=14 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

9 Chart 9 Cell voltage and potentials in an electrolyzer for chlorine production with ODC (Oxygen Depolarised Cathode) Anode 4 Cl - 2 Cl e - Cathode with ORR O H 2 O + 4 e - 4 OH - Cathode (conventional) 4 H 2 O + 4 e - 4 OH H 2 Cell Voltage [V] ΔE 0 = 1.23V with ODC E 0 = 0.96 V without ODC E 0 = 2.19V Current density

10 Chart 10 Schemtically representation of an electrolyzer for chlorine production with ODC (Oxygen Depolarised Cathode) Catalyst Current collector Gas Phase Liquid Phase

11 Chart 11 Schemtically representation of an electrolyzer for chlorine production with ODC (Oxygen Depolarised Cathode) Catalyst Gas Phase Current collector Liquid Phase O 2 O 2 O O Electrical Circuit e - O O e - e - e - O e - Net O H O -2 OH - Ag H NaOH 30% (H 2 O) OH - Silver GDE (ODC) NaO H 2 H 2 O Na + Na + Na + Na + 4 NaOH NaOH Membrane NaOH 32% Cl - Na + Cl 2 Anode e - NaCl solution (Brine)

12 Chart 12 Chlorine production with ODC (Oxygen Depolarized Cathode) Chlorine production unit with ODC technique at Bayer in Ürdingen (20,000 t/y) since May 2011 European Chlorine Production 2012 : 12.6 mio. t

13 Chart 13 Overview of production techniques for electrodes at DLR-TT Small scale production Mid scale production Large scale production membrane roller coating nozzle powder s uppo rter Colloidal Suspension Spraying catalyst additive Nitrog en Dry Powder Spraying Vacuum or Atmospheric Plasma Spraying Pressing Reactive Rolling and Mixing (RMR)

14 Chart 14 Overview of production techniques for electrodes at DLR-TT membrane roller coating nozzle powder supporter catalyst additive Nitrogen + Solvent-free production techniques + Almost all kind of catalysts and conductive agents can be processed + Possible wide-range electrode thickness Production technique dependend from for example density or particle size Processing at room temperature + Very thin layers Processing at room temperature + Solvent-free production techniques + Catalysts can be sythesized from nitrate solutions + Metal substrates can be coated Heat resistant materials required

15 Chart 15 Production Techniques -Dry Spraying Technique -Wet Spraying Techniquen - Reactive Mixing and Rolling (RMR) -Screen printing - Reactive Mixing Addi t ives Ca t al yst s M etal net GDE

16 Chart 16 Dry Powder Spraying Technique membrane roller RMR (Substrat) Dry sprayed layer coating nozzle powder supporter catalyst additive Top view Nitrog en

17 Chart 17 Multi-layer Gas Diffusion Electrodes with different porous layers Dry sprayed C/PTFE Reactive Mixing and Rolling Ag-PTFE N. Wagner, T. Kaz, DE A1, 2002

18 Chart 18 3D reconstruction (TEM+FIB) of Ag-PTFE based Gas Diffusion Electrode 10 μm P. Balasubramanian, S. Eswara Moorthy, J. Bernhard, U. Kaiser, Uni Ulm (unpublished result).

19 Chart 19 3D reconstruction of CT pictures of the reduced Ag-GDE in the xz-plane

20 Chart 20 SEM pictures of Ag-GDE, produced by the RMR technique (Ag 2 O+PTFE) Ag-GDE, unused part Ag-GDE, used

21 Chart 21 Schematically representation of measuring cell and experimental conditions CE = Pt or Ni RE = Hg/HgO or Hydroflex WE = ODC Electrolyte = MOH (M: Li, K, Na) T = 80 C (60, 40 and 20 C) O 2 O 2 -mixtures with N 2 or He

22 Chart 22 Current density / potential characteristic 0 Cathode Cathode Hg/HgO 0, Cathode k,c CE RE ODC Potential U 0 d Reference (NHE) Current density

23 Chart 23 Electrochemical Impedance Spectroscopy

24 Chart 24 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 /

25 Chart 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 Z m Pore Electrode, porous layer Porous layer Z n I I H. Göhr in Electrochemical Applications/97,

26 Chart 26 Electrode Model with cylindrical, homogeneous pores and complex Faraday-impedance Zq=

27 Chart 27 CV of a polished Ag electrode, 25% KOH, O 2 sat.

28 Chart 28 Model of the oxygen reduction Simulated Impedance Spectra Data for -20 macm -2, 6 N KOH, 25 C 5 Z / phase / o mf 990 m m 4 4 mf 950 m m 6 50 mf 950 m 7 50 m 8 5 mf 950 m m O 2 -diffusion -heterogeneous reaction OH- diffusion (into solution) Charge transfer resistance Double layer capacity Electrolyte resistance m 0 10m 100m K 10K 100K frequency / Hz D.W. Wabner, Metalloberfläche Angew. Elektrochemie, Band 28 (1974) 21-25

29 Chart 29 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)

30 Chart 30 Impedance Measurements during ORR in 10 N NaOH, on Silver Electrodes at Different Current Densities, i< -50 macm m Z / ma ma ma ma ma ma ma ma ma ma phase / o Z' / 15 ma 50 ma 25 ma 20mA 30 ma 35 ma 40 ma 45 ma 10 ma 5 ma Z'' / 0 100m K 3K 10K 100K frequency / Hz Bode representation Nyquist representation

31 Chart 31 2 Z / Impedance Measurements during ORR in 10 N NaOH, on Silver Electrodes at Different Current Densities, i> -50 macm ma phase / o 90-1 Z' / ma m 600m ma ma ma ma ma ma ma ma 0 100m K 3K 10K 100K frequency / Hz Bode representation ma 150 ma 100 ma 50 ma 200 ma 250 ma 300 ma 350 ma 400 ma 450 ma Nyquist representation Z'' /

32 Chart 32 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

33 Chart 33 Current density dependency of the charge transfer resitance R ct 1.6 Rct / current/ma

34 Chart 34 Current density dependency of electrolyte resistance inside the pore 2.5 pore electrolyte res. / current/ma

35 Chart 35 U-i characteristic and current density dependency of impedance elements R ad and R ct ir-corr. Potential vs. NHE / V 1,1 1,08 1,06 1,04 1,02 1 0,98 0,96 0,94 0,92 8,00 7,00 6,00 5,00 4,00 3,00 2,00 1,00 Rad; Rct / Ohm 0,9 0,00-0,10-0,08-0,06-0,04-0,02 0,00 Current density / Acm -2

36 Chart 36 Current density dependency of k ad, R ad and R ct, determined from EIS evaluation k ad =1/C ad R ad 70 Rad; Rct / Ohm Reaction rate constante / s Current density / Acm -2

37 Chart 37 CVs measured from OCP+10 mv, 1 mvs -1 in different concentrated NaOH solutions, 80, O 2 Current / A N N N Potential / mv

38 Chart 38 CVs measured from OCP+10 mv, 1 mvs -1 in 10 N NaOH, at different temperatures, O 2 Current / A C 40 C 60 C C Potential / mv

39 ddd Chart 39 EIS measured at different temperatures during OCR 10 M NaOH, 20 macm m 500m Z / a aa aaa aaa phase / o a b a bb bbb bbb a 20 ma aa b b a aa c c b aa c ccc b a aa c aaa cc b aaa 20 C b aa d c aaa bb aaa dd c aa aaa aaa aa aaa aaa aaa c d dd cc d dd b b bbb c bbb cc bb bbb 40 C bbb c bbb bb bbb c bbb bb bbb bbb bbb c d d d d d d c c d cc d c cc dd ccc 60 C d ccc dd ccc cc ddd ccc ccc cc ccc ccc ccc dd ddd a aa a b b b bb b bb ca c aaa c cc ccc cc bb ccc aa b bb a bbb d dd ddd ddd dd ddd ddd d ccc a a d d ddd aaa ddd dd ddd ddd dd ddd ddd ddd a a a aaa aaa aaa b b b bbb bbb c c c ccc dd ccc 80 C d dd bb cc dd bbb ccc ddd aa aaa bbb ccc ddd aaa bbb ccc ddd aa bb cc dd aaa bbb ccc ddd aaa bbb ccc ddd aa bb cc dd aaa bbb ccc ddd aaa bbb ccc ddd aaa ddd bbb ccc 0 100m K 3K 10K frequency / Hz C ddd b b b b b b c c cb c c c cc a c cc b c a c aa a bbbbb aaa c c ddd dd d ccc dddddddd bbb aaa ccccc aaa Z' / a a a 60 C 20 ma b a a b a a b b b b b bb b 40 C a a a a a a a a a 20 C Z'' /

40 aaaa cc Chart 40 EIS measured at different temperatures during OCR 10 M NaOH, 200 macm Z / a aa aaa aaa aa aaa a a a aa a aa a a a aa 200mA phase / o Z' / 2 20 C 200mA m a aa aaa aaa aa aaa aaa aa aaa aaa aaa bbbbbbbbbbb bbb bbb bb b bb bbb b bb bb b bb 40 C c cc ccc ccc cc ccc ccc b cc bb bb bbb dddd c bbb bb dcc bbb bbb bbb ddd c d cc ddd c dd c dd ccc 60 C d ddddddddddddddddddddddddddddddddddddd ccc 80 C ccc cc ccc ccc cc ccc ccc ccc ddd dddddd bbbbbbbbbbbbbbbb ccc cc cccccc d cc b cc bbb b b b cc 40 C 60 C 80 C a aa a aaa aa a a a aa a a a a a a a a a a 20 C aa Z'' / aa aaa aaa aa aaa aaa aaa aaa a aa aaa aaa aa aaa aa aaa aaa b bb bbb bbb bb bbb bbb bb bbb bbb bbb bb bbb bbb bb bbb c cc ccc ccc cc ccc ccc c c ccc ccc cc ccc ccc ccc cc ddddddddddddddddddddddddddddddddddddddddddddddd ccc bbb ccc aa bb cc aaa bbb ccc dddddddd aaa bbb ccc bbb aaa ccc 0 100m K 3K 10K frequency / Hz

41 Chart 41 Determination of kinetic parameters from CV measurements : ir-correction measured curve ir-corrected curve Potential vs. RHE / mv Current density / macm -2

42 Chart 42 Determination of kinetic parameters from CV measurements : Tafel plot Potential vs. RHE / mv E 0 = 1114 mv i 0 = Acm -2 b = 80 mv/decade Current density / macm -2

43 Chart 43 Conclusion From the evaluation of the measured impedance spectra one can propose a reaction mechanism for the ORR: Adsorptions- / heterogeneous reactions and charge transfer reaction are consecutive reactions Reaction mechanism and rate determining step is changing at higher current densities at ca. 20 macm -2 Production parameters, composition and structure have a strong influence on electrode reactivity Change of reaction zone with current density

44 Chart 44 Thank you for your attention! Acknowledgment