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Energy & Fuels 2007, 21, 1098-1101
Experimental Studies on the Influence of H2S on Solid Oxide Fuel Cell Performance at 800 °C Arnstein Norheim,*,† Ivar Wærnhus,‡ Markus Brostro¨m,§ Johan E. Hustad,† and Arild Vik‡ Department of Energy and Process Engineering, Norwegian UniVersity of Science and Technology, Kolbjørn Hejes Vei 1A, 7491 Trondheim, Norway, Prototech AS, Postboks 6034, Postterminalen, 5892 Bergen, Norway, and Energy Technology and Thermal Process Chemistry, Umeå UniVersity, 901 87 Umeå, Sweden ReceiVed October 24, 2006. ReVised Manuscript ReceiVed January 9, 2007
Short-term tests showing the influence sulfur has on solid oxide fuel cell (SOFC) performance have been performed. The experiments were performed using two single-cell SOFC setups operated at 800 °C. In setup I, sulfur (H2S) was mixed into the fuel gas in concentrations ranging from 20 to 100 ppm. It was found that the performance decreased with increasing sulfur concentration up to 80 ppm. The performance loss at 80 and 100 ppm sulfur was equal. At a current density of 200 mA cm-2, the operating voltage was reduced from 0.810 V at 0 ppm H2S to 0.790 V at 100 ppm H2S, corresponding to an increase in the area-specific cell resistivity (ASR) of 0.10 Ω cm2. In setup II, sulfur levels of 80, 120, and 240 ppm were introduced. In all these three cases the ASR increased by around 0.13 Ω cm2. Removing the sulfur impurity when the 240 ppm H2S exposure test was finished the cell performance fully recovered, indicating no irreversible changes in the cell structure.
Introduction The solid oxide fuel cell (SOFC) may be fuelled by a variety of fuels such as natural gas,1-4 oil-derived gases and liquids,5-8 pure ammonia,9 and synthesis gases from coal and biomass gasification.7,10-12 This is possible mainly due to the high operating temperature (600-1000 °C) enabling the SOFC to both internally reform light hydrocarbons and electrochemically oxidize CO in addition to hydrogen. The BioSOFC project aims at demonstrating integration of biomass steam gasification, high-temperature gas cleaning, and a SOFC stack.13 Biomass gasification producer gases do, * Corresponding author. Telephone: +47 73550369. E-mail:
[email protected]. † Norwegian University of Science and Technology. ‡ Prototech AS. § Umeå University. (1) Peters, R.; Dahl, R.; Klu¨ttgen, U.; Palm, C.; Stolten, D. J. Power Sources 2002, 106, 238-244. (2) Peters, R.; Riensche, E.; Cremer, P. J. Power Sources 2000, 86, 432441. (3) Meusinger, J.; Riensche, E.; Stimming, U. J. Power Sources 1998, 71, 315-320. (4) Singhal, S. C. Solid State Ionics 2000, 135, 305-313. (5) Ahmed, K.; Gamman, J.; Fo¨ger, K. Solid State Ionics 2002, 152153, 485-492. (6) Schuler, A.; Za¨hringer, T.; Doggwiler, B.; Ru¨egge, A. Proc. Fourth Eur. Solid Oxide Fuel Cell Forum 2000, 1, 107-114. (7) Dicks, A.; Larminie, J. Proc. Fourth Eur. Solid Oxide Fuel Cell Forum 2000, 2, 927-936. (8) Petrik, M. A.; Milliken, C. E.; Ruhl, R. C.; Lee, B. P. Proc. Fuel Cell Semin. 1998, 124-127. (9) Wojcik, A.; Middleton, H.; Damopoulos, I.; Van herle, J. J. Power Sources 2003, 118, 342-348. (10) Van herle, J.; Mare´chal, F.; Leuenberger, S.; Favrat, D. J. Power Sources 2003, 118, 375-383. (11) Proell, T.; Rauch, R.; Aichernig, C.; Hofbauer, H. Proc. ASME Turbo Expo 2004, GT2004-53900. (12) Staniforth, J.; Ormerod, R. M. Catal. Lett. 2002, 81, 19-23. (13) Hustad, J. E.; et al. Proceedings of the 14th European Biomass Conference and Exibition, Paris, 2005; pp 763-766.
however, contain sulfur, mainly as H2S, in concentrations in the range of 100-200 ppm.14 By applying a high-temperature sulfur filter, the H2S level is expected to be reduced to well below 50 ppm. Complete sulfur removal is, however, not expected, and the SOFC must therefore operate on a fuel gas containing significant amounts of sulfur in this system. In the present work short-term data on the SOFC performance degradation at varying sulfur concentrations up to 100 ppm (setup I) and 240 ppm (setup II) are presented. The main fuel gas compositions were mixtures of 60/40% (v) and 67/33% (v) H2/CO2 in setup I and setup II, respectively. These gas compositions are similar, at thermodynamic equilibrium at the fuel cells’ operating temperature, to producer gases from biomass steam gasification. Several researchers have studied the detrimental effect sulfur has on SOFC performance using mainly the state-of-the-art anode/electrolyte materials selection (Ni-YSZ/YSZ). Most of the systematic work so far has been done on SOFC half-cells, i.e., investigation of the sulfur effect on the anode/electrolyte interface only. Examples of this approach are the works of Matsuzaki and Yasuda,16 Dees et al.17 and Primdahl and Mogensen.18 The available data on the performance of a complete SOFC cell or stack fuelled by gases containing sulfur (14) Hustad, J. E.; et al. Proceedings of the Second World Biomass Conference, Rome, 2004; pp 1094-1097. (15) Hofbauer, H.; Rauch, R.; Bosch, K.; Koch, R.; Aichernig, C. In Pyrolysis and Gasification of Biomass and Waste; Bridgewater, A.V., Ed.; CPL Press: Newbury, UK, 2003; pp 527-536. (16) Matsuzaki, Y.; Yasuda, I. Solid State Ionics 2000, 132, 261-269. (17) Dees, D. W.; Balachandran, U.; Dorris, S. E.; Heiberger, J. J.; McPheeters, C. C.; Picciolo, J. J. In Proceedings of the 1st International Symposium on Solid Oxide Fuel Cells; Singhal, S. C., Ed.; The Electrochemical Society: Pennington, NJ, 1989; Vol. 89-11, pp 317-321. (18) Primdahl, S.; Mogensen, M. In Proceedings of the 6th International Symposium on Solid Oxide Fuel Cells; Dokiya, M., Singhal, S. C., Eds.; The Electrochemical Society: Pennington, NJ, 1999; Vol. 99-19, pp 530540.
10.1021/ef060532m CCC: $37.00 © 2007 American Chemical Society Published on Web 02/09/2007
H2S Influence on Solid Oxide Fuel Cell Performance
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Table 1. Summary of SOFC Performance Loss Data at Varying H2S Levels operating temp (°C) 1000
fuel mixture (vol %) 79/21 H2/H2O
900
a
1000 1000 850 1000 950
97/3 H2/H2O 97/3 H2/H2O
950
78/22 CGa/H2O
89/11 H2/H2O 67/33 H2/H2O
H2S level (ppm)
voltage drop at max current density (V)
max current density (A cm-2)
increase in ASR from no H2S (Ω cm2)
2 4 6 8 10 15 2 4 8 105 35 35 1 0.5 1.5 2 3 10 1 4
0.002 0.035 0.045 0.060 0.062 0.074 0.080 0.090 0.110
0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3
0.053 0.010 0.015 0.020 0.020 0.025 0.010 0.015
0.1 0.1 0.5 0.15 0.15 0.15 0.15 0.15 0.15 0.15
0.01 0.12 0.15 0.20 0.21 0.25 0.27 0.30 0.37 0.18 0.17 0.90 0.11 0.07 0.10 0.13 0.13 0.17 0.07 0.10
ref 16
17 18 4 19
CG, coal gas, unspecified composition.
is sparse and at least not as systematic as the references cited above. Singhal4 has reported data on the performance drop in the tubular Siemens Westinghouse SOFCs by addition of 1 ppm H2S. Stolten et al.19 published data on SOFC performance degradation as H2S in the range from 0.5 to 10 ppm was added to their planar SOFC. Petrik et al.8 reported that their planar SOFC was able to tolerate high-sulfur containing military fuels referred to as JP-8 and F-76. The SOFC was reported to be stable at a 300 ppm H2S spiked composition. Furthermore, it is in this work stated that similar results were obtained for H2S concentrations up to 2000 ppm. No operational data were given in this work. Table 1 gives a summary of the available performance data from the literature cited above. Most data are read from the figures in the respective papers. The data in the table giving the changes in area specific resistivity (ASR) are, if not explicitly stated in the papers, calculated as the voltage drop at the actual H2S level (V) divided by the corresponding current density (A cm-2) giving the ASR (in Ω cm2). One may conclude, based on the presented literature that the ASR increases nonlinearly with increasing sulfur concentration (i.e., as the H2S level increases the additional performance loss decreases). Furthermore, the SOFC’s tolerance toward sulfur seems to increase with increasing operating temperature. The objective of the present study is to systematically investigate the influence of H2S on SOFC performance at sulfur levels relevant for biomass gasification producer gases. This is motivated by the diversion in the previous published data on the effect of sulfur impurities and the lack of systematic data on complete SOFC performance at varying sulfur levels. Since the fuel gases in the present work do not contain methane, the effect sulfur has on methane reforming in the SOFC is not revealed here. Sulfur is known to reduce the performance of nickel-based reforming catalysts.20 Thus, a SOFC operating on fuel gases containing hydrocarbon compounds could therefore experience additional performance losses from sulfur impurities by a reduction in the reaction rates of the reforming reactions. (19) Stolten, D.; Spa¨h, R.; Schamm, R. in Stimming, U.; Singhal, S. C.; Tagawa, H.; Lehnert, W. Proceedings of the 5th International Symposium on Solid Oxide Fuel Cells; The Electrochemical Society: Pennington, NJ, 1997; Vol. 97-40, pp 88-93. (20) Alstrup, I.; Rostrup-Nielsen, J. R.; Røen, S. Appl. Catal. 1 1981, 5, 303-314.
Figure 1. Schematic detail of the single-cell SOFC setup used in the present work.
Experimental Section Planar, anode-supported SOFCs delivered by Forschungszentrum Ju¨lich were cut into a circular shape with a diameter of 31 mm giving an active cell area of 7.5 cm2. The cells have an approximately 1.6 mm thick porous Ni-Y2O3-doped ZrO2 (Ni-YSZ) anode, a 5 µm thick dense YSZ electrolyte, and a 55 µm thick porous Sr-doped LaMnO3 (LSM)-YSZ cathode.21 The cells were further modified in-house by application of a screen-printed LSM layer. Figure 1 shows schematically details of the single-cell setup used in the present work. On each electrode, LaCrO3-based interconnects were used for gas distribution and current collection. The two interconnects were connected via Pt films and Pt wires to an ampere meter, a resistance, and a current control unit. Cell voltage was measured by connecting Pt wires from the electrodes to a voltmeter. Finally, cell temperature was measured by an S-type thermocouple placed close to the anode in the fuel inlet tube. Data acquisition was done via a LabView FieldPoint system. The cell perimeter was sealed by a sheet gasket in order to minimize gas leakages. Finally, the setup was placed in an electrically heated oven and heated to 800 °C at a rate of 60 °C h-1. After reaching the operating temperature of 800 °C, a fuel gas mixture of H2/CO2 at 60/40% (v) at a total flow rate of 250 mL min-1 was fed to the anode in setup I. Setup II was fed by H2/CO2 at 67/33% (v) at a total rate of 300 mL min-1. Both cells were thus operated at high excess fuel ratios. At the maximum loads reported here (200 mA cm-2), the fuel utilization ratios were (21) Basu, R. N.; Blass, G.; Buchkremer, H. P.; Sto¨ver, D.; Tietz, F.; Wessel, E.; Vinke, I. C. In Proceedings of the 7th International Symposium on Solid Oxide Fuel Cells; Yokokawa, H., Singhal, S. C., Eds.; The Electrochemical Society: Pennington, NJ, 2001; Vol. 2001-16, pp 9951001.
1100 Energy & Fuels, Vol. 21, No. 2, 2007
Norheim et al.
Figure 2. Operating cell voltage at a constant load of 200 mA cm-2 vs H2S concentration. Table 2. Area-Specific Cell Resistivity (ASR) versus H2S Concentration H2S (ppm)
ASR (Ω cm2)
increase in ASR from no H2S (Ω cm2)
0 20 40 50 60 80 100
0.630 0.670 0.690 0.705 0.720 0.730 0.730
0 0.04 0.06 0.08 0.09 0.10 0.10
approximately 7.5% and 4% for setup I and setup II, respectively. Air was fed to the cathode at a rate of 300 mL min-1 in both setups. After reducing the anode, the cell in setup I was operated at a constant load of 70 mA cm-2 for 300 h, and the cell in setup II was operated at a constant load of 50 mA cm-2 for 400 h in order to achieve stable operation. H2 and CO2 were supplied from gas bottles of 2.5-quality. A premixed certified gas bottle containing 1000 ppm H2S in CO2 was used as H2S supply. Thus, by adjusting the flow of the pure CO2 and that of the H2S containing CO2, a range of H2S concentrations could be obtained. All gas flows were controlled by Bronkhorst mass flow controllers. For setup I each sulfur exposure lasted for 1 h. During this period the cell was operated at a current density of 200 mA cm-2 until stable operation was achieved. The cell in setup II was operated at a current density of 50 mA cm-2 for 24 h at each sulfur exposure level. After establishing IV curves at the given sulfur level, the cell was operated at clean gas conditions for 24 h. When switching from one sulfur concentration to another, both cells were operated at open circuit conditions.
Results and Discussion Setup I. The operating cell voltage at a constant load of 200 mA cm-2 at each sulfur concentration is shown in Figure 2. It can be seen from the data that the cell voltage at constant load decreases more or less linearly up to 60 ppm H2S. From 60 to 80 ppm H2S there is a small additional voltage drop only; finally, the last step from 80 to 100 ppm H2S causes no additional voltage drop. This means that the deteriorating effect sulfur has on the SOFC performance seems to reach its final level at around 80 ppm H2S. The open circuit voltage (OCV) at 0 ppm H2S was 0.936 V giving an area specific cell resistance of 0.63 Ω cm2 based on the cell voltage at 200 mA cm-2. Assuming the same OCV when sulfur is introduced, the area specific cell resistance at the different sulfur concentrations can be summarized as shown in Table 2. The increase in ASR presented in this work is smaller at a given H2S level as compared to the literature cited above.
Figure 3. Phase diagram of nickel in an atmosphere of H2 and H2O at 800 °C and an H2S concentration of 300 ppm. The SOFC operating interval is indicated in the upper right corner of the diagram (arrow).
Figure 4. Cell performance at varying sulfur (H2S) concentrations for the cell in setup II. Main gas composition: 67/33% (v) H2/CO2.
However, the nonlinear increase in cell resistance at increasing H2S levels is comparable to the cited literature. Setup II. By further increasing the sulfur concentration above 100 ppm H2S, no additional performance loss is expected based on the results from setup I. This is valid up to a sulfur concentration where the sulfur starts to react chemically with the nickel in the SOFC anode and NiS(s) or NiSO4(s) is being formed. However, as shown in Figure 3, even at an H2S concentration of 300 ppm no chemical reaction is expected between sulfur and nickel at 800 °C. The calculations shown are performed by using the program FactSage.22 The cell in setup II was exposed to 80, 120, and 240 ppm H2S. As shown in Figure 4, the cell performance is close to equal at all three levels of sulfur concentration. The increase in ASR from the value at no sulfur impurities was around 0.13 Ω cm2 for all three levels of sulfur exposure. After removing the sulfur impurities from the fuel gas, the cell performance recovered to its initial level. The chemical stability of the cell was verified by postexperimental ESEM/EDS and XRD analysis. No sulfur (ESEM/ EDS) or sulfur-containing crystalline compounds (XRD) were found in the anode structure. This gives further reason to conclude that the cell performance degradation caused by adding up to 240 ppm H2S to the fuel gas is reversible and that sulfur does not, at these H2S concentration levels, react chemically with the nickel in the SOFC anode. (22) Bale, C. W.; Chartrand, P.; Degterov, S. A.; Eriksson, G.; Hack, K.; Ben Mahfoud, R.; Melancon, J.; Pelton, A. D.; Petersen, S. Calphad 2002, 26 (2), 189-228.
H2S Influence on Solid Oxide Fuel Cell Performance
Conclusions SOFC performance losses at 800 °C have been investigated as H2S in the range from 20 to 100 ppm and from 80 to 240 ppm was mixed into the anode fuel gas. The sulfur concentrations investigated here are in the range of expected levels in biomass gasification producer gases.14 The lower levels investigated are expected to be achieved only after sulfur removal by means of a high-temperature sulfur filter. It has been shown that the operating cell voltage at a constant load of 200 mA cm-2 decreases as H2S is introduced to the fuel gas. The cell in setup I experienced a close to linear voltage drop with increasing sulfur concentration from 0 to 60 ppm H2S. The additional voltage loss from 60 to 80 ppm H2S was smaller, and from 80 to 100 ppm H2S no additional voltage drop was observed. At 80 and 100 ppm H2S the total voltage drop from 0 ppm H2S was found to be 20 mV at a constant load of 200 mA cm-2. This corresponds to an increase in ASR of 0.10 Ω cm2. For the higher sulfur levels investigated in setup II, it was found that the SOFC performance was close to equal at 80, 120, and 240 ppm H2S. The cell in setup II experienced an increase in ASR of 0.13 Ω cm2 at all three levels of sulfur impurities investigated and at a constant load of 200 mA cm-2.
Energy & Fuels, Vol. 21, No. 2, 2007 1101
These results show that the FZ Ju¨lich SOFCs may provide high power densities even when fuelled by gases containing high amounts of sulfur. From the viewpoint of using biomass gasification producer gases as fuel for SOFCs, these results are positive. However, the duration of each sulfur exposure period was short, only 1 h for the cell in setup I and 24 h for the cell in setup II. Extended operation on sulfur containing fuel gases should be conducted in order to reveal any long-term effects. Acknowledgment. Alf Berland (Prototech) is greatly acknowledged for his assistance in assembling the single cell setups and for his help during the experiments. Dan Bostro¨m (Energy Technology and Thermal Process Chemsitry, Umeå University) is acknowledged for his work with the XRD analysis. This work is part of the ongoing BioSOFC project aimed at demonstrating utilization of biomass gasification producer gases as fuel for SOFCs by integration of biomass steam gasification, high-temperature gas filtration, and a SOFC stack. The project is financed by the Research Council of Norway and the Norwegian power companies Agder Energi, Hadeland Energi, Trondheim Energiverk, and Vardar. EF060532M
