Untersuchung der elektrochemischen Oxidation von Borhydrid unter Verwendung einer stark basischen Aniontauscher Membran Brennstoffzelle

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1 Untersuchung der elektrochemischen Oxidation von Borhydrid unter Verwendung einer stark basischen Aniontauscher Membran Brennstoffzelle Project No. 117 Z. Jusys, S. Sun, and R.J. Behm Institut für Oberflächenchemie und Katalyse (IOK), Universität Ulm V. Gogel, U. Riek, and L.Jörissen Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg

2 Outline Motivation Experimental: - Pt/C, Au/C and PtAu/C catalysts - thin-film electrodes - hydrodynamic voltammetry Hydrogen / Borohydride oxidation: - rotating disk electrode (RDE) - online mass spectrometry (DEMS) - high temperature / pressure (HT/HP) Half- and full cell studies in alkaline polymer membrane fuel cell hardware Summary 1

3 Outline Motivation Experimental: - Pt/C, Au/C and PtAu/C catalysts - thin-film electrodes - hydrodynamic voltammetry Hydrogen / Borohydride oxidation: - rotating disk electrode (RDE) - online mass spectrometry (DEMS) - high temperature / pressure (HT/HP) Half- and full cell studies in alkaline polymer membrane fuel cell hardware Summary 2

4 Known: Motivation Borohydride a promising fuel for alkaline fuel cells Catalysis Today 17 (211) - high theoretical voltage - no CO poisoning - high energy density - easier storage and distribution - hydrogen generation - Unknown: mechanistic and kinetic details complete (x = 8 e - ) vs. incomplete (x < 8 e - ) - BH 4 + xoh - - = BO 2 + (x - 2)H 2 + (4 -.5x)H 2 + xe - Present work: Systematic model studies of borohydride oxidation in alkaline medium (UU) half- and full cell experiments in fuel cell hardware (ZSW) 3

5 Outline Motivation Experimental: - Pt/C, Au/C and PtAu/C catalysts - thin-film electrodes - hydrodynamic voltammetry Hydrogen / Borohydride oxidation: - rotating disk electrode (RDE) - online mass spectrometry (DEMS) - high temperature / pressure (HT/HP) Half- and full cell studies in alkaline polymer membrane fuel cell hardware Summary 4

6 Experimental: techniques / methods Thin-film electrode Catalysts: Pt/C (2 wt%, E-Tek), Au/C (7 wt%, ZSW Ulm), PtAu/C (25 wt%, ZSW, Ulm) 5

7 Experimental: techniques / methods Thin-film electrode Catalysts: Pt/C (2 wt%, E-Tek), Au/C (7 wt%, ZSW Ulm), PtAu/C (25 wt%, ZSW, Ulm) Rotating disk electrode Online mass spectrometry High temperature/pressure (RDE) (DEMS) (HT/HP) flow cell rotation / convection 6 dual thin-layer flow cell pressurized channel flow cell

8 Experimental: liquid nitrogen trap for DEMS.2.1 N 2 cooling m/z = 2.1 I MS / na water vapor decrease by factor >1 2 improved signal-to-noise ratio.1 N 2 cooling m/z = 2 isotopic analysis of hydrogen evolved time / s

9 Outline Motivation Experimental: - Pt/C, Au/C and PtAu/C catalysts - thin-film electrodes - hydrodynamic voltammetry Hydrogen / Borohydride oxidation: - rotating disk electrode (RDE) - online mass spectrometry (DEMS) - high temperature / pressure (HT/HP) Half- and full cell studies in alkaline polymer membrane fuel cell hardware Summary 8

10 1 mm H 2 oxidation on Pt/C well-defined mass transport HOR hindrance by PtO 9

11 1 mm H 2 oxidation on PtAu/C lower HOR activity inactive Au component 1

12 Outline Motivation Experimental: - Pt/C, Au/C and PtAu/C catalysts - thin-film electrodes - hydrodynamic voltammetry Hydrogen / Borohydride oxidation: - rotating disk electrode (RDE) - online mass spectrometry (DEMS) - high temperature / pressure (HT/HP) Half- and full cell studies in alkaline polymer membrane fuel cell hardware Summary 11

13 1 mm NaBH 4 oxidation on Pt/C 12 early onset (ca. -.2 V, RHE) well-defined mass transport ( 3 HOR) negligible BOR suppresion by PtO

14 1 mm NaBH 4 oxidation on Au/C 13 later onset (ca..2 V, RHE) ill-defined mass transport certain BOR suppresion by AuO

15 1 mm NaBH 4 oxidation on PtAu/C 14 early onset (ca. -.1 V, RHE) less-defined mass transport (cf. HOR) lower activity at lower and higher potentials

16 Apparent number of electrons in 1 mm NaBH 4 oxidation 9 6 negative-going negative-going number of electrons 8 7 Pt/C number of electrons 5 Au/C number of electrons 9 8,,2,4,6,8 1, 1,2 E / V (RHE) negative-going PtAu/C,,2,4,6,8 1, 1,2 1,4 E / V (RHE) 15,4,6,8 1, 1,2 1,4 E / V (RHE) Levich-Koutecky kinetic analysis: Pt/C n e, app ~ 8 PtAu/C Au/C n e, app ~ 6 n e, app < 8 incomplete oxidation

17 Outline Motivation Experimental: - Pt/C, Au/C and PtAu/C catalysts - thin-film electrodes - hydrodynamic voltammetry Hydrogen / Borohydride oxidation: - rotating disk electrode (RDE) - online mass spectrometry (DEMS) - high temperature / pressure (HT/HP) Half- and full cell studies in alkaline polymer membrane fuel cell hardware Summary 16

18 Base CV (left) and 1 mm H 2 (right) oxidation on Pt/C,1,3,,2 / ma / ma,1 -,1, 4, m/z = 2 m/z = 2 I MS / na I MS / na -5 2, -1, 17,,2,4,6,8 1, 1,2 1,4 E / V (RHE) well-resolved H 2 formation in HER,,2,4,6,8 1, 1,2 1,4 E / V (RHE) H 2 consumption mirrors HOR

19 Base CV (left) and 1 mm H 2 (right) oxidation on Au/C,5,5 / ma, / ma, -,5 -,5 1, m/z = 2 1, m/z = 2 I MS / na 5, I MS / na,, 18 -,4 -,2,,2,4,6,8 1, 1,2 1,4 E / V (RHE) ca..2 V overvoltage for the HER -,4 -,2,,2,4,6,8 1, 1,2 1,4 E / V (RHE) poor catalyst for the HOR

20 Base CV (left) and 1 mm H 2 (right) oxidation on PtAu/C,5,2 / ma, / ma,1 -,5 -,1, 3, m/z = 2 m/z = 2 I MS / na 2, I MS / na -5 1,, 19,,2,4,6,8 1, 1,2 1,4 E / V (RHE) early onset of the HER -1,,2,4,6,8 1, 1,2 1,4 E / V (RHE) H 2 consumption mirrors HOR

21 Outline Motivation Experimental: - Pt/C, Au/C and PtAu/C catalysts - thin-film electrodes - hydrodynamic voltammetry Hydrogen / Borohydride oxidation: - rotating disk electrode (RDE) - online mass spectrometry (DEMS) - high temperature / pressure (HT/HP) Half- and full cell studies in alkaline polymer membrane fuel cell hardware Summary 2

22 1 mm NaBH 4 (left) or NaBD 4 (right) oxidation on Pt/C,3,2 / ma,1, 4 m/z = 2 I MS / na 2,,2,4,6,8 1, 1,2 1,4 E / V (RHE) 21

23 1 mm NaBH 4 (left) or NaBD 4 (right) oxidation on Pt/C,3,2 / ma,1, 4 m/z = 2 I MS / na 2 22,,2,4,6,8 1, 1,2 1,4 E / V (RHE) H 2 consumption or formation?

24 1 mm NaBH 4 (left) or NaBD 4 (right) oxidation on Pt/C,3,3,2,2 / ma,1 / ma,1,, m/z = 2 4 m/z = 2 4 I MS / na 2 I MS / na 2 m/z = 4 m/z = 3 23,,2,4,6,8 1, 1,2 1,4,,2,4,6,8 1, 1,2 1,4 E / V (RHE) E / V (RHE) H 2 consumption or formation? H 2 from H 2 O D 2 from BD 4 -

25 1 mm NaBH 4 (left) or NaBD 4 (right) oxidation on Au/C,2,2 / ma,1 / ma,1,, 1 m/z = 2 1 I MS / na 5 I MS / na 5 m/z = 2 m/z = 4 m/z = 3 -,4 -,2,,2,4,6,8 1, 1,2 1,4 E / V (RHE) -,4 -,2,,2,4,6,8 1, 1,2 1,4 E / V (RHE) 24

26 1 mm NaBH 4 (left) or NaBD 4 (right) oxidation on Au/C,2,2 / ma,1 / ma,1,, 1 m/z = 2 1 I MS / na 5 I MS / na 5 m/z = 2 m/z = 4 m/z = ,4 -,2,,2,4,6,8 1, 1,2 1,4 -,4 -,2,,2,4,6,8 1, 1,2 1,4 E / V (RHE) E / V (RHE) higher BOR selectivity for hydrogen evolution at.4 < E <.8 V

27 1 mm NaBH 4 (left) or NaBD 4 (right) oxidation on PtAu/C,2,2 / ma,1 / ma,1,, m/z = 2 m/z = I MS / na I MS / na 1 1 m/z = 3 m/z = 4 26,,2,4,6,8 1, 1,2 1,4,,2,4,6,8 1, 1,2 1,4 E / V (RHE) E / V (RHE) H 2 consumption or formation? H 2 from H 2 O D 2 from BD 4 -

28 Open-circuit potential and deuterium/hydrogen formation 1, Pt/C 1, AuPt/C 1, Au/C,8,8,8 E / V (RHE),6,4,6,4,6,4,2,2,2,,, 6 m/z = 2 3 m/z = 2 6, m/z = 4 I MS / na 4 2 4, 2 m/z = 4 m/z = 3 m/z = 4 1 m/z = 4 m/z = 3 m/z = 4 2,, m/z = 3 m/z = time / s time / s D 2 formation prevails at 1.1 V H 2 formation at E m (-2 mv, Pt/C & PtAu/C) time / s D 2 formation open-circuiting Au/C no hydrogen at E m (22 mv, Au/C) 27

29 Mixed-potential in borohydride oxidation BOR BD 4 - H + ////////////////////////////////////////////////////// e - current net HER E M potential H 2 formation from water at E m < V (Pt/C & PtAu/C) D 2 formation from BD - 4 at E > 1. V (no HOR on oxide covered catalyst) E m only for partial reactions in a common E region (HER overvoltage on Au/C) 28

30 Outline Motivation Experimental: - Pt/C, Au/C and PtAu/C catalysts - thin-film electrodes - hydrodynamic voltammetry Hydrogen / Borohydride oxidation: - rotating disk electrode (RDE) - online mass spectrometry (DEMS) - high temperature / pressure (HT/HP) Half- and full cell studies in alkaline polymer membrane fuel cell hardware Summary 29

31 1 mm NaBH 4 oxidation on Pt/C at elevated temperature Positive-going scan (a) Negative-going scan (b) Positive-going scan / ma o C 4 o C 6 o C 8 o V 1 o C Negative-going scan E / V (RHE).8 only 2 increase of BOR with temperature linearity of Arrhenius plots no change in r.d.s..6 apparent activation energy ~ 6.5 kj mol -1 (similar to that for the HOR) T -1 / K -1 3

32 1 mm NaBH 4 oxidation on Au/C at elevated temperature / ma Positive-going scan (a) Negative-going scan (b) 23 o C 4 o C 6 o C 8 o C 1 oc V Positive-going scan Negative-going scan E / V (RHE) about 2 increase of BOR with temperature no thermal activation of BOR at E <.2 V linearity of Arrhenius plots no change in r.d.s. apparent activation energy ~ 11 2 kj mol T -1 / K -1 31

33 1 mm NaBH 4 oxidation on PtAu/C at elevated temperature Positive-going scan (a) Negative-going scan (b) Positive-going scan / ma o C 4 o C 6 o C 8 o C 1 o C E / V (RHE) F V Negative-going scan 1 about 2 increase of BOR with temperature linearity of Arrhenius plots no change in r.d.s. apparent activation energy ~ 6.5 kj mol -1 (similar to that for Pt/C) T -1 / K -1 32

34 Outline Motivation Experimental: - Pt/C, Au/C and PtAu/C catalysts - thin-film electrodes - hydrodynamic voltammetry Hydrogen / Borohydride oxidation: - rotating disk electrode (RDE) - online mass spectrometry (DEMS) - high temperature / pressure (HT/HP) Half- and full cell studies in alkaline polymer membrane fuel cell hardware Summary 33

35 Studies in Polymer Electrolyte Membrane Fuel Cell Hardware Evaluation of catalyst materials for the oxidation of borohydride (BOR) in alkaline electrolytes Preparation of porous electrodes Variation of substrate Variation of catalyst layer thickness Variation of noble metal loading Half- and full cell tests to investigate activity and mass transport properties -34-

36 Equipment for the half cell tests (2M EtOH + 3M KOH)- Zufuhr Reference- Electrode Referenzelektrode O 2 --Zufuhr Supply Haber-Luggin-Capillary Haber-Liggin-Kapillare Counterelectrode (Cathode) Gegenelektrode (Kathode) 3M KOH 2M NaOH Spacer Distanzstück Arbeitselektrode (Anode) Working elektrode (Anode) (x mm NaBH 4 + 2M NaOH)- Supply Anion-Exchange Membrane -35-

37 Electrodes Investigated with Respect to Electrochemical Oxidation of Borhydride (BOR) 2% Pt/C (E-Tek) on GDL Sigracet 1BB, 1, mg Pt /cm², 3 wt.% Nafion (Reference) 1% Pt/C (E-Tek) on GDL Sigracet 1BB, 1, mg Pt /cm², 3 wt.% Nafion 4% Pt/C (E-Tek) on GDL Sigracet 1BB 1, mg Pt /cm², 3 wt.% Nafion 2% Pt/C (E-Tek) on GDL Graphite Paper Toray 1 mg Pt /cm², 3 wt.% Nafion 2% Pt/C (E-Tek) on GDL Sigracet 1BB,6 mg Pt /cm², 3 wt.% Nafion 1% Pt/C (E-Tek) on GDL Sigracet,28 mg Pt /cm², 3 wt.% 25% PtAu/C (ZSW) on GDL Sigracet 1BB 1, mg Noble metal /cm², 3 wt.% Nafion 8% Au/C (ZSW) on GDL Sigracet 1BB,2 mg Au /cm², 3 wt.% Nafion Variation of catalyst layer thickness Variation of catalyst loading Variation of catalyst -36-

38 SEM pictures for the BOR anodes (x1,) 1% Pt/C 25% Pt/Au/C 2% Pt/C 8% Au/C -37-

39 Catalyst Characterization Particle Size (XRD), Electrochemically Active Area (CV) Catalyst E-Tek 1% Pt on Vulcan XC 72 Crystallite Size nm Electrochemicall y Active Area (cm 2 /cm 2 geom) 75 E-Tek 2% Pt on Vulcan XC E-Tek 4% Pt on Vulcan XC ZSW 25% Pt/Au on Vulcan XC ZSW 8% Au on Vulcan XC

40 DBFC vs. ADMFC 6 5 T Zelle = 8 C ADMFC, PtRu-Black, 5 mg PtRu/cm² DBFC, 2% Pt/C, 1 mg Pt/cm² j/ ma*cm HOR + BH3OH- OR* MOR Indirect BOR* E=,3V E vs. RHE/ mv BH H 2 O = H 2 + BH 3 OH - Direct BOR* E =,7 V Anodic reaction already at low potentials: Hydrolytic decomposition of BH 4- to H 2 Saturation of current observed in porous electrodes -39- *H.Dong et al. J. Phys.Chem., 19(25) 1896

41 Temperature Dependence of BOR using 2%Pt/C (5 mm NaBH 4 + 2M NaOH) 9 j/ ma*cm C 4 C 6 C 8 C,,1,2,3,4,5,6 E vs. RHE/ V - Increasing temperature leads to increasing anode OCP - Initial steep increase of current followed by limitation increase of limiting current with temperature (kinetics of BH 4- decomposition?) -4-

42 Influence of BH 4- Concentration on the E-lgj- Curves (2%Pt/C) 6 5 T Zelle = RT Elektrolyt: x mm NaBH 4 + 2M NaOH Anode: 1mg Pt/cm²(2% Pt/C) E vs. RHE/ mv c(nabh4)= 5 mm c(nabh4)= 5 mm 1,,5 1, 1,5 2, 2,5 lgj/ ma*cm -2 Two distinctly different regions observed in the Tafel plot at elevated concentration -41-

43 Potential Dependence of Activation Energy of the BOR (Pt/C- Anode at 1 mg/cm²) Katalysator Schichtdicke, µ E a / kjmol -1,3 V RHE,4 V RHE,5 V RHE 1% Pt/ Vulkan 35 6,2 3,2 1,2 2% Pt/ Vulkan 15 6,2 5,1 5,3 4% Pt/ Vulkan 1 5,4 5, 4,5 Variation of Activation Energy with BH 4- Concentration (Pt/C Anode using 2% Pt/Vulcan) Elektrolyt E a / kjmol -1,3 V RHE,4 V RHE,5 V RHE 5 mm NaBH 4 + 2M NaOH 6,2 5,1 5,3 5 mm NaBH 4 + 2M KOH 5,1 4,1 3,7-42-

44 Investigation of BOR Activity as a Function of Electrode Thickness Pt/C- Anode (1 mg Pt/cm²) in (,5M NaBH 4 +2M NaOH)- Electrolyte Tz=25 C Membran: A6 (Tokuyama) Anode: Pt/C (E-Tek), 1mg Pt/cm²; GDL: 1BB Kathode: Pt-Black (E-Tek), 5 mg Pt/cm²; GDL: TGP Tz=8 C Membran: A6 (Tokuyama) Anode: Pt/C (E-Tek), 1mg Pt/cm²; GDL: 1BB Kathode: Pt-Black (E-Tek), 5 mg Pt/cm²; GDL: TGP % Pt/C, d= 32-35µ 8 1% Pt/C, d=32-35µ U/ mv 6 4 2% Pt/C, d= 13-15µ 4% Pt/C, d= 8-1µ U/mV 6 4 2%Pt/C, d=13-15µ 4%Pt/C, d=8-1µ 2 Elektrolyte:,5M NaBH 4 + 2M NaOH j/ ma*cm -2 2 Elektrolyte:,5M NaBH4 + 2M NaOH j/ma*cm -2 - No significant dependence observed - Electrode microstructure might have large influence -43-

45 Full Cell Experiments, Variation of Electrode Substrate Toray TGP 12 vs. SGL 1BB U/ mv Tz= 25 C Membran: A6 (Tokuyama) Anode: 2% Pt/C (E-Tek), 1 mg Pt/cm², GDL: var. Kathode: Pt- Black (E-Tek), 5 mg Pt/cm², GDL: TGP-12 U, SGL 1BB U, TGP-12 E, TGP-12 E, SGL 1BB E vs RHE/ V U/ mv Tz= 8 C U, SGL 1BB U, TGP-12 E, SGL 1BB E, TGP E vs. RHE/ mv j/ ma*cm j/ ma*cm -2 - Higher open porosity of TGP-12 allows higher current density - Higher BH 4- permeation observed - Electrode microstructure has a significant influence on achievable current density -44-

46 Parasitic H 2 -Evolution for Pt/C- Anodes 1mg Pt/cm² at 6 C in.5m NaBH 4 + 2M KOH under OCP Katalysator Anoden-Dicke, µm H 2 - Entwicklung, ml/min*cm² 1% Pt/C, E-Tek ,3 2% Pt/C, E-Tek ,2 4% Pt/C, E-Tek 8-1 3,2 - Minimum H 2 -Evolution in thickest anode having the largest Electrochemically Active Surface Area Katalysator Anoden-Dicke, µm Pt- Belegung, mg Pt/cm² H 2 - Entwicklung, ml/min*cm² 1% Pt/C, E-Tek 1,28 1,6 2% Pt/C, E-Tek 1,6 1,4 4% Pt/C, E-Tek 1 1, 3,2 - Low Pt-loading and larger Electrochemically Active Surface Area are leading to reduction of the Hydrogen Evolution reaction -45-

47 Variation of Pt-Loading at Constant Catalyst Layer Thickness of 1 µm Tz=25 C Membran: A6 (Tokuyama) Anode: Pt/C (E-Tek), x mg Pt/cm²;d=1µ, GDL: 1BB Kathode: Pt-Black (E-Tek), 5 mg Pt/cm²; GDL: TGP Tz=8 C Membran: A6 (Tokuyama) Anode: Pt/C (E-Tek), x mg Pt/cm²;d=1µ, GDL: 1BB Kathode: Pt-Black (E-Tek), 5 mg Pt/cm²; GDL: TGP-12 U/ mv Elektrolyt:,5M NaBH 4 + 2M NaOH x=,28 mg Pt/cm² x=,6 mg Pt/cm² x= 1, mg Pt/cm² U/ mv Elektrolyt:,5M NaBH 4 + 2M NaOH x=,28 mg Pt/cm² x=,6 mg Pt/cm² x=1, mg Pt/cm² j/ ma*cm j/ ma*cm -2 - No significant influence of catalyst loading observed -46-

48 Variation of Catalyst Composition 25% room temperature 8% room temperature % Pt/C 8% Au/C j/ ma*cm % Pt/C 25% PtAu/C j/ma*cm ,,1,2,3,4,5,6 E vs RHE/ V,,2,4,6,8 1, E vs. RHE/ mv - General shape of i/v-curves are similar - Gold containing electrodes show lower activity -47-

49 Activation Energies of Gold Catalyzed Electrodes 25% Pt/Au Elektrolyt on Vulcan XC 72 Ea/kJmol -1,3 V RHE,4 V RHE,5 V RHE 5 mm NaBH4 + 2M NaOH 8,3 6,4 5,1 5 mm NaBH4 + 2M NaOH 8,8 8,6 7,7 Elektrolyt 25% Pt/Au on Vulcan XC 72 Ea/kJmol -1,3 V RHE,4 V RHE,5 V RHE 5 mm NaBH4 + 2M NaOH 12,6 11,9 9,9 - Gold containing electrodes show larger activation energies -48-

50 Outline Motivation Experimental: - Pt/C, Au/C and PtAu/C catalysts - thin-film electrodes - hydrodynamic voltammetry Hydrogen / Borohydride oxidation: - rotating disk electrode (RDE) - online mass spectrometry (DEMS) - high temperature / pressure (HT/HP) Half- and full cell studies in alkaline polymer membrane fuel cell hardware Summary

51 Summary of Model Studies RDE: apparent electron number of - ca. 8 (Pt/C: V; PtAu/C: V) complete oxidation of borohydride without hydrogen release - ca. 6 (Au/C: V) hydrogen release during borohydride oxidation DEMS: deuterium labeled borohydride - discriminating the hydrogen source - deuterium release in borohydride oxidation in oxide region - hydrogen release from water under open-circuit conditions - mixed potential formation (Pt/C and PtAu/C: H 2 generation from water) HT-HP: linearity of Arrhenius plots no change in the r.d.s. low apparent activation energy negligible kinetic limitations 5

52 Summary Cell Experiments No significant dependence of BOR electrode activity observed on Anode thickness - Anode Pt-loading Strong dependence of BOR-rate from concentration and temperature BOR activation energy - Decreasing for electrodes with larger electrochemically active surface area (thicker electrodes) - Decreasing with increasing BH4- concentration Minimum Hydrogen evolution rate in anodes with high electrochemically active surface area Indications for strong influence of electrode microstructure found Significantly lower performance of Au-containing electrodes observed - Reason could be lower electrochemically active surface area due to larger noble metal particle size

53 Publications Publications resulting from the present project: Invited talk at 222 nd ECS Meeting, PRiME 212, Honolulu, HI, USA Symposium E-2.2 Materials and Fuels for Alkaline Systems (Abstract #1571) S. Sun, Z. Jusys, and R.J. Behm Borohydride electrooxidation: new insights based on an old paradigm Publications resulting from previous project (3SF311C): S. Sun, Z. Jusys, and R.J. Behm Electrooxidation of ethanol on Pt-based and Pd-based catalysts in alkaline electrolyte under fuel cell relevant reaction and transport conditions Journal of Power Sources 231 (213)

54 Acknowledgments Financial support: Brennstoffzellen- und Batterie-Allianz Baden-Württemberg (BBA-BW) Thank you for your attention! 53