Carbon and Sulfur Tolerant anodes for SOFCs Stylianos G. Neophytides FORTH Institute of Chemical Engineering Sciences Hydrogen days 2014, Prague 2-4 April, 2014
ΙΤΕ/ΕΙΧΗΜΥΘ
Outline Introduction to SOFCs and and the Internal steam reforming process Carbon tolerance NiAu/YSZ NiAu/GDC, NiAuMo/GDC Physicochemical characterization, catalytic, electrocatalytic and Abient Pressure Photoelectron spectroscopy experiments Sulfur tolerance NiAu/GDC, NiAuMo/GDC
Basic Operational principles Cathode reaction: ½ O 2 + 2e - = O 2- Electrical Energy (e - ) Anode reactions: H 2 + O 2- = H 2 O + 2e - CO +O 2- = CO 2 + 2e - C n H 2n+2 + (3n+1)O 2 = nco 2 + (n+1)h 2 O + (6n+2)e -
Internal reforming proceeds through the water produced by the fuel at the anode
ADVANTAGES CH 4 +H 2 O CO+3H 2 H 2 +O 2- H 2 O+2e - H 2 is directly produced in the SOFC H 2 is readily oxidized for the production of electricity DISADVANTAGES The exposure of the anode in high CH 4 /H 2 O may result in C deposition Low CH 4 /H 2 O ratios cause a decrease in the cell s Nerst potential Objectives Internal Methane steam reforming reaction Development of anode electrocatalysts active for catalytic CH 4 steam reforming and resistive to graphitic carbon formation and sulphur poisoning
Anode Ni based cermets, Ni/GDC, Ni/YSZ Cathode Sr-doped LaMnO 3 (LSM) La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 (LSCF) Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3 δ (BSCF) Sm 0.5 Sr 0.5 CoO 3 Electrolyte: ZrO 2 (Y 2 O 3 ) (YSZ) La 0.9 Sr 0.1 Ga 0.8 Mg 0.2 O 2.85 (LSGM) Samaria doped Ceria (SmDC) Scandia doped ceria (ScDC) Scandia stabilized zirconia (ScSZ) Common SOFC materials[1] [1] C. Sun and U. Stimming, Journal of Power Sources 171 (2007) 247. H 2 electrooxidation CH 4 steam reforming dissociate O 2 high electronic and ionic cond. thermal expansion coefficient dense high ionic conductivity electronic insulators
Basic SOFC designs-configurations Planar: Anode-Supported
Basic SOFC designs-configurations Cathode-Supported Electrolyte-Supported Metal-Supported Flat design Tubular design
Basic SOFC designs-configurations Planar: Electrolyte-Supported
Outlet Experimental electrochemical reactor Pt wire connected to Working electrode Inlet YSZ tube Counter electrode contact Quartz tube
Ni-Au/YSZ anode Carbon Tolerant Ni-Au SOFC Electrodes operating under Internal Steam Reforming Conditions, Ilias Gavrielatos, Vasilis Drakopoulos and Stylianos G. Neophytides, Journal of Catalysis 259 (2008) 75 84
The combustion synthesis method Precursors Ni (NO 3 ) 2, ΗAuCl 4, ZrO(NO 3 ) 2 Y(NO 3 ) 3, CH 4 N 2 O 363 K 873 K combustion
Cross section images of reduced NiAu/YSZ The electrode-electrolyte three phase boundary Electrode/gas phase interface
rates, μmol/sec Electrokinetic measurements under internal steam reforming conditions on Ni/YSZ -1100-1000 -900 T=850 0 C V WC r H2 7 6 V WC, mv -800-700 -600-500 -400 r CO r CO2 y CH4 = 20 % y H2 O = 6.2% 5 4 3-300 2-200 -100 1 0 0 0 200 400 600 800 1000 I, ma Carbon Tolerant Ni-Au SOFC Electrodes operating under Internal Steam Reforming Conditions, Ilias Gavrielatos, Vasilis Drakopoulos and Stylianos G. Neophytides, Journal of Catalysis 259 (2008) 75 84
Short term stability test Electrocatalytic performance under 10%H 2 before and after the stability test 390 360 330 300 270-1000 -800 T= 850 C y H2 = 10% y H2 O = 4.2% Current, ma 240 210 180 150 120 90 60 T=850 C y CH4 =22% y H2O =6.5% V cell =-500mV F T =109cc/min V WC, mv -600-400 -200 before after F T = 80cc/min 30 0 0 10 20 30 40 50 60 Time, hrs 0-100 0 100 200 300 400 500 600 700 800 I, ma
TPO following CH 4 dissociative adsorption Ni(1%at Au)-YSZ CO 2, nmoles/sec CO, nmoles/sec 100 75 50 25 0 10 8 6 4 2 0 T=673 K T=723 K T=773 K T=923 K Pulse of O 2 at T< 450 K, O 2 /C=2 20 o /min, F t =26 μmoles/sec Simultaneous evolution of CO and H 2 Decomposition of CH x O species at elevated Temperature H 2, nmoles/sec 8 6 4 2 0 500 600 700 800 900 1000 Temperature, K N.C.Triantafyllopoulos,S.G.Neophytides, J. Catalysis 239, 187-199 (April 2006)
REACTION MECHANISM r C CH 4 CH 2ad + 2H ad CH 2ad C c + 2H ad H 2 O, O 2, O 2- r OX CH 2ad H 2 CO ad C c + 2O ad CO 2 H 2 CO ad CO + H 2 To avoid carbon deposition r OX >r C
Ni/GDC NiAu/GDC NiAuMo/GDC Study of the synergistic interaction between nickel, gold and molybdenum in novel modified NiO/GDC cermets, possible anode materials for CH4 fueled SOFCs, Niakolas, D.K., Athanasiou, M., Dracopoulos, V., Tsiaoussis, I., Bebelis, S., Neophytides, S.G. Applied Catalysis A: General 456, pp. 223-232 (2013)
Deposition Co-Precipitation Commercial NiO/GDC powder as the support, hydrogen tetrachloroaurate (HAuCl 4 ) and ammonium heptamolybdate [(NH4)6Mo7O24 4H2O] Adjustment of ph and Temp. of the suspension NH 3 as precipitant agent Filtering, drying and final calcination at 850 & 1100 C
SEM-BSE on Au-Mo-NiO/GDC (a) (b) Au 0 varies 10-50 nm Au 0 varies 10-150 nm Calcined at 850 C Calcined at 1100 C MoO x species could not be detected
H 2 -TPR on Au-Mo-NiO/GDC d(δwt.%)/dtemp 0,3 0,2 0,1 Ramp from room temp up to 850 0 C with 5 0 C/min and 10% H 2 /Ar Ni/GDC 3wt% Au-Ni/GDC 10wt% Mo-Ni/GDC 3wt% Au-10wt% Mo-Ni/GDC 3wt% Au-30wt% Mo-Ni/GDC 0,0 0 100 200 300 400 500 600 700 800 Temp, 0 C Study of the synergistic interaction between nickel, gold and molybdenum in novel modified NiO/GDC cermets, possible anode materials for CH4 fueled SOFCs, Niakolas, D.K., Athanasiou, M., Dracopoulos, V., Tsiaoussis, I., Bebelis, S., Neophytides, S.G. Applied Catalysis A: General 456, pp. 223-232 (2013)
TPR-XRD on Au-Mo-Ni/GDC (c) (1') 635 o C (c) (1) (2) Ni GDC Ni (c) (2') Ni Intensity (a.u.) 588 o C 539 o C 490 o C Au NiO 440 o C Mo (b) 472 o C 420 o C 370 o C Au NiO 325 o C 546 o C (a) 534 o C 468 o C NiO Au NiO Au (b) Ni GDC Ni NiO Au NiO Au (a) Ni GDC Ni 418 o C 366 o C NiO 315 o C NiO NiO 36 37 38 39 40 41 38 40 42 44 46 48 50 52 2Φ 2θ (deg) (b) (a) NiO NiO NiO Ni Au Au Ni 42 43 44 45 800 0 C 635 0 C 588 0 C 539 0 C 490 0 C 440 0 C 384 0 C 25 0 C 800 0 C 598 0 C 534 0 C 472 0 C 420 0 C 370 0 C 325 0 C 25 0 C 800 0 C 596 0 C 546 0 C 468 0 C 418 0 C 366 0 C 315 0 C 28 0 C NiAuMo/GDC NiAu/GDC Ni/GDC
0,12 0,10 TGA on Au-Mo-NiO/GDC Ramp from room temp up to 850 0 C 11% CH 4 /Ar g carbon /g cat. 0,08 0,06 0,04 Ni/GDC 3wt.% Au-Ni/GDC 10wt.% Mo-Ni/GDC 3wt.% Au-10wt.% Mo-Ni/GDC 0,02 0,00 0 100 200 300 400 500 600 700 800 Temp, 0 C
Activation energies of CH 4 dissociation on various NiAu/GDC powders The activation energy of CH 4 dissociation increases with increasing Au content
TGA-MS on Au-Mo-NiO/GDC Sample Ni/GDC 10wt.%Mo 3wt.% Au 3wt.Au -10wt.% Mo r H2 (mmol m -2 s -1 ) 1.2 1.0 0.5 0.3 r Carbon (mmol m -2 s -1 ) 0.116 0.127 0.041 0.039 0,25 0,20 T = 750 0 C 20% CH 4, 10% H 2 O Binary and ternary cermets are active. g carbon /g cat. 0,15 0,10 0,05 Ni/GDC 10wt.% Mo-Ni/GDC 3wt.% Au-Ni/GDC 3wt.% Au - 10wt.% Mo-Ni/GDC Less active for H 2 production and carbon deposition, compared to Ni/GDC. Synergy between Ni, Au and Mo for decrease of carbon deposits. 0,00 0 1 2 3 Time, min
Stability of the Ni/GDC and NiAu/GDC under S/C=0.5 at 0.5A/cm 2 1200 O.C. Pure H 2 feed Cell Voltage (mv) 1100 1000 900 800 700 600 (1a) (2a) O.C. (1b) (2b) (1c) T=850 o C (1d) 500 90 100 110 200 250 300 350 400 450 Time (hrs) Au doped Ni/GDC as a new anode for SOFCs operating under rich CH4 internal steam reforming, Niakolas D.K., Ouweltjes J.P., Rietvelt G., Dracopoulos V., Neophytides S.G., International Journal of Hydrogen energy, 35(15), 7898-7904, (2010) (2c) (1e) S/C=0.5
rates, μmol/sec Electrokinetic behavior of NiAu/YSZ and NiAu/GDC NiAu/GDC NiAu/YSZ -1100-1000 -900 T=850 0 C V WC r H2 7 6 V WC, mv -800-700 -600-500 -400 r CO r CO2 y CH4 = 20 % y H2 O = 6.2% 5 4 3-300 2-200 -100 1 0 0 0 200 400 600 800 1000 I, ma
During current application CO is being oxidized into CO 2 only through WGS reaction Mathematical modeling of Ni/GDC and Au-Ni/GDC SOFC anodes performance under internal methane steam reforming conditions, Souentie, S., Athanasiou, M., Niakolas, D.K., Katsaounis, A., Neophytides, S.G., Vayenas, C.G. Journal of Catalysis 306, pp. 116-128 (2013)
Ambient pressure X-ray photoelectron spectroscopy shows massive reduction of CeO 2 into Ce 2 O 3 and enhancement in the current 0,14 0,12 TGA 20% CH4, 10% H2O T = 750 0 C g carbon /g cat. 0,10 0,08 0,06 0,04 0,02 20kPa CH 4 10kPa H 2 O NiAu/GDC Ni/GDC 0 5 10 15 20 25 30 35 40 On the active surface state of nickel-ceria solid oxide fuel cell anodes during methane electrooxidation, Papaefthimiou, V., Shishkin, M., Niakolas, D.K., Athanasiou, M., Law, Y.T., Arrigo, R., Teschner, D., (...), Zafeiratos, S. Advanced Energy Materials 3 (6), pp. 762-769 (2013) 0,00 Time, min
REACTION MECHANISM NiAu/GDC r C activated on Ni CH 4 CH 2ad + 2H ad CH 2ad C c + 2H ad H 2 O, O 2, O 2- r OX activated on Ce 2 O 3 CH 2ad H 2 CO ad C c + 2O ad CO 2 Ni activated decompomposition H 2 CO ad CO + H 2 To avoid carbon deposition r OX >r C
Comparison of NiAu/YSZ and NiAu/GDC CH 4 activation takes place on Ni CH x O formed on Ni is an intermediate and decomposes into CO and H 2 Au hinders CH 4 dehydrogenation Ni into C deposites Under lean S/C NiAu/YSZ is selectively electrooxidizing H 2 due to the low coverage of CH x O CH 4 activation takes place on Ce 2 O 3 CH x O is formed on Ce 2 O 3 and decomposes on Ni into CO and H 2 Au hinders CH4 dehydrogenation Ni into C deposites Under lean S/C NiAu/GDC is selectively catalyzing CH 4 partial electrooxidation due to the abundant formation of CH x O
Sulfur Tolerance
Effect of H 2 S on Ni/GDC under H 2 and reforming conditions 0.8 0.7 10ppm H2S in H2 0.6 Potential, V 0.5 0.4 0.3 0.2 T = 850 0 C, I = 40mA H 2 O = 5%, H 2 S = 10ppm CH 4 = 2.5% S/C = 2 0.1 0 20 40 60 80 100 120 140 160 180 200 220 240 Time, min
Stability in the presence of H 2 S 10ppm H 2 S/H 2-0.9-0.8-0.7 Potential (V) -0.6-0.5-0.4-0.3-0.2-0.1 0.0 Ni/GDC 3Au-Ni/GDC 3Mo-Ni/GDC 3Au-3Mo-Ni/GDC T = 850 0 C, I = 40mA F total = 100cc/min 10ppm H 2 S in H 2 0 20 40 60 80 100 120 140 160 180 200 220
Stability in the presence of H 2 S CH 4 SR, S/C=2 + 10ppm H 2 S Potential (V) -0.9-0.8-0.7-0.6-0.5-0.4-0.3-0.2-0.1 0.0 0 20 40 60 80 100 120 140 Time (minutes) Ni/GDC 3Au-Ni/GDC 3Mo-Ni/GDC 3Au3Mo-Ni/GDC T = 850 0 C, I = 40mA F total = 100cc/min H 2 O = 5%, H 2 S = 10ppm CH 4 = 2.5% S/C = 2
Stability in the presence of H 2 S CH 4 SR, S/C=0.13 + 10ppm H 2 S Potential (V) -0.9-0.8-0.7-0.6-0.5-0.4-0.3-0.2-0.1 0.0 T = 850 0 C, I = 40mA F total = 100cc/min H 2 O = 5%, H 2 S = 10ppm CH 4 = 38% S/C = 0.13 0 20 40 60 80 100 120 140 160 180 200 Time (minutes) Ni/GDC 3Au-Ni/GDC 3Mo-Ni/GDC 3Au3Mo-Ni/GDC
Conclusions Current status The introduction of Au modified Ni/YSZ into a carbon tolerant catalyst by the formation of NiAu1%at/YSZ syrface alloy Modification of commercial NiO/GDC with D.P. and/or D. CP. of Au and/or Mo resulted in new binary and ternary materials, possible anodes in CH 4 plus H 2 S fueled SOFCs Binary and ternary samples show catalytic activity though lower than the undoped Ni/YSZ Synergistic interaction between Ni, Au and Mo towards the decrease of carbon deposition for the catalytic CH 4 dissociation and steam reforming reactions Synergy is attributed to the formation of Ni-Au-Mo solid solution
Research Team Dr Dimitris Niakolas Dr Nikos Triantafyllopoulos Dr Ilias Gavrielatos Michalis Athanasiou
Acknowledgements FCH-JU project Understanding and minimizing anode degradation in hydrogen and natural gas fuelled SOFCs, Acronym:ROBANODE FCH-JU project Innovative SOFC Architecture based on Triode Operation, Acronym:T-cell