The Basis for a Renewable Energy Cycle - ACS Publications

May 19, 2009 - Electrolyzers operated on 25% H2, 25% CO2, and 50% H2O at 800 °C and 1.3 V yielded a syngas production rate of ∼7 sccm/cm2. The use ...
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Energy & Fuels 2009, 23, 3089–3096

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Syngas Production By Coelectrolysis of CO2/H2O: The Basis for a Renewable Energy Cycle Zhongliang Zhan, Worawarit Kobsiriphat,† James R. Wilson,† Manoj Pillai,‡ Ilwon Kim,‡ and Scott A. Barnett*,‡,† Department of Materials Science and Engineering, Northwestern UniVersity, 2220 Campus DriVe, EVanston, Illinois 60208 and Functional Coating Technology LLC, 1801 Maple AVe., EVanston, Illinois 60201 ReceiVed February 5, 2009. ReVised Manuscript ReceiVed April 28, 2009

Electrolysis was carried out at 700-800 °C using solid oxide electrochemical cells with H2O-CO2-H2 mixtures at the Ni-YSZ cathode and air at the LSCF-GDC anode. (YSZ ) 8 mol %, Y2O3-stabilized ZrO2, GDC ) Ce0.9Gd0.1O1.95, and LSCF ) La0.6Sr0.4Co0.2Fe0.8O3). The cell electrolysis performance decreased only slightly for H2O-CO2 mixtures compared to H2O electrolysis and was much better than for pure CO2 electrolysis. Mass spectrometer measurements showed increasing consumption of H2O and CO2 and production of H2 and CO with increasing electrolysis current density. Electrolyzers operated on 25% H2, 25% CO2, and 50% H2O at 800 °C and 1.3 V yielded a syngas production rate of ∼7 sccm/cm2. The use of electrolytically produced syngas for producing renewable liquid fuels is discussed; an energy-storage cycle based on such liquid fuels is CO2-neutral, similar to hydrogen, and has the potential to be more efficient and easier to implement.

1. Introduction Recently, there has been renewed interest in solid oxide electrolysis cells (SOECs) for producing hydrogen from electrical energy sources.1-12 Compared to the more conventional water electrolysis technology, SOECs operating at ∼800 °C require less electrical energy because ∼25% of the energy required to split H2O is thermal.13 SOEC technology has the advantage that it can build on a solid oxide fuel cell (SOFC) technology that has seen a significant research and development * To whom correspondence should be addressed. E-mail: s-barnett@ northwestern.edu. † Northwestern University. ‡ Functional Coating Technology LLC. (1) Brisse, A.; Schefold, J.; Zahid, M. Int. J. Hydrogen Energy 2008, 33 (20), 5375–5382. (2) Gopalan, S.; Mosleh, M.; Hartvigsen, J. J.; McConnell, R. D. J. Power Sources 2008, 185 (2), 1328–1333. (3) Hauch, A.; Ebbesen, S. D.; Jensen, S. H.; Mogensen, M. J. Mater. Chem. 2008, 18 (20), 2331–2340. (4) Hauch, A.; Jensen, S. H.; Ramousse, S.; Mogensen, M. J. Electrochem. Soc. 2006, 153 (9), A1741-A1747. (5) Herring, J. S.; O’Brien, J. E.; Stoots, C. M.; Hawkes, G. L.; Hartvigsen, J. J.; Shahnam, M. Int. J. Hydrogen Energy 2007, 32 (4), 440– 450. (6) Jensen, S. r. H.; Larsen, P. H.; Mogensen, M. Int. J. Hydrogen Energy 2007, 32 (15), 3253–3257. (7) Mingyi, L.; Bo, Y.; Jingming, X.; Jing, C. J. Power Sources 2008, 177 (2), 493–499. (8) Mogensen, M.; Jensen, S. H.; Hauch, A.; Chorkendorff, I.; Jacobsen, T. Ceramic Eng. Sci. Proc. 2008, 28 (4), 91–101. (9) Ni, M.; Leung, M. K. H.; Leung, D. Y. C. Int. J. Hydrogen Energy 2008, 33 (9), 2337–2354. (10) Udagawa, J.; Aguiar, P.; Brandon, N. P. J. Power Sources 2007, 166 (1), 127–136. (11) O’Brien, J. E.; Stoots, C. M.; Herring, J. S.; Hartvigsen, J. J. Nucl. Technol. 2007, 158, 118–131. (12) Hawkes, G. L.; O’Brien, J. E.; Stoots, C. M.; Herring, J. S. Nucl. Technol. 2007, 158, 132–144. (13) Quandt, K. H.; Streicher, R. Int. J. Hydrogen Energy 1986, 11, 309–315.

effort.14 In most cases, the electrolyzers have utilized designs and materials similar to SOFCs: Ni-YSZ fuel electrodes, YSZ electrolytes, and LSM-YSZ air electrodes. These devices have yielded good performance, for example, electrolysis current density of 3.6 A/cm2 at 950 °C and 1.48 V for nickel electrode supported electrochemical cells operating on 70% H2O + 30% H2 with a steam utilization of 37%.6 The long-term stability of SOECs remains an open issue, as a number of degradation/ failure mechanisms have been reported, including anode delamination.4,15-22 Alternative fuel-electrode materials have been investigated to enhance performance or stability, such as Ni-infiltrated samarium-doped ceria23 and lanthanum-substituted strontium titanate/ceria.24 However, the present paper focuses on SOEC operation using conventional solid oxide fuel cell materials. A potential advantage of SOECs is their good chemical flexibility, such that the same basic cells can also do electrolysis (14) Singhal, S. C.; Kendall, K., High-Temperature Solid Oxide Fuel Cells: Fundamentals, Design, and Application; Elsevier: New York, 2003. (15) Hauch, A.; Jensen, S. H.; Bilde-Sorensen, J. B.; Mogensen, M. J. Electrochem. Soc. 2007, 154 (7), A619-A626. (16) Accorsi, R.; Bergmann, E. J. Electrochem. Soc. 1980, 127 (4), 804– 811. (17) Yamashita, A.; et al., Anodic and Cathodic Electrode Reaction at La1-xSrxMnO3; Bossel, U., Ed.; European Fuel Cells Group: Lucerne, Switzerland, 1994; pp 661-669. (18) Eguchi, K.; Hatagishi, T.; Arai, H. Solid State Ionics 1996, 1245, 86–88,. (19) Heneka, M. J.; Ivers-Tiffe´e, E. In Degradation of SOFC Single Cells Under SeVere Current Cycles, SOFC IX, Quebec, 2005; Singhal, S. C.; Mizusaki, J., Eds. Electrochemical Society: Pennington, NJ, 2005; pp 534543. (20) Jiang, S. P.; Love, J. G.; Zhang, J. P.; Hoang, M.; Ramprakash, Y. Solid State Ionics 1999, 121, 1–10. (21) Jiang, S. P.; Love, J. G. Solid State Ionics 2003, 158, 45. (22) Jiang, S. P.; Wang, W. Electrochem. Solid-State Lett. 2005, 8, A115-A118. (23) Uchida, H.; Osada, N.; Watanabe, M. Electrochem. Solid State Lett. 2004, 7 (12)). A500–A502. (24) Marina, O. A.; Pederson, L. R.; Williams, M. C. J. Electrochem. Soc. 2007, 154 (5), B452-B459.

10.1021/ef900111f CCC: $40.75  2009 American Chemical Society Published on Web 05/19/2009

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Figure 1. A cross-sectional SEM micrograph that shows the active layers in a typical solid oxide cell and also indicates schematically the coelectrolysis process.

of CO2 to CO.25-29 However, the only study published on the coelectrolysis of H2O/CO2 to H2/CO (syngas) was an exploratory work at Idaho National Laboratory, with the application of chemically storing electrical energy from nuclear power plants.11 The rationale for doing coelectrolysis of H2O/CO2 is to produce syngas, which can in turn be converted into liquid fuels. This can be used as the basis for a renewable liquid-fuel energy storage cycle, as discussed in more detail in Section 2. Figure 1 is a cross-sectional SEM micrograph that shows the active layers in a typical solid oxide cell and also indicates schematically the coelectrolysis process. Here we show that H2O/CO2 coelectrolysis can be carried out in solid oxide electrolyzers similar to the current state-ofthe-art Ni-YSZ-supported solid oxide fuel cells, operating at temperatures of 700-800 °C. A preliminary analysis of the round-trip energy efficiency expected from the liquid fuel energy storage cycle, compared with that of a hydrogen storage cycle, is presented. 2. Renewable Energy Storage The highest efficiency for use of renewable electricity (e.g., wind, solar, geothermal, and hydroelectric) can be achieved by direct electricity use. However, there are three situations where this is not feasible or only partially feasible: (1) When the renewable electricity supply does not match demand, energy should be stored in large quantities, typically at the same power generation site, for later conversion back to electricity; (2) When the electrical source is remote from the demandsat a distance greater than practical for electricity transmissionsenergy storage and transport would be required; (3) When the limited range of electric battery vehicles makes grid recharging only a partial solution, such that chemical fuels are still required for long-haul vehicles. A number of methods are available for such large-scale energy storage,30-32 but many are not readily applicable to all (25) Sridhar, K. R.; Vaniman, B. T. Solid State Ionics 1997, 93, 321– 328. (26) Tao, G.; Sridhar, K. R.; Chan, C. L. Solid State Ionics 2004, 175, 615–619. (27) Tao, G.; Sridhar, K. R.; Chan, C. L. Solid State Ionics 2004, 175, 621–624. (28) Bidrawn, F.; Kim, G.; Corre, G.; Irvine, J. T. S.; Vohs, J. M.; Gorte, R. J. Electrochem. Solid-State Lett. 2008, 11 (9), B167-B170. (29) Hartvigsen, J.; Elangovan, S.; Frost, L.; Nickens, A.; Stoots, C. M.; O’Brien, J. E.; Herring, J. S. ECS Trans. 2008, 12 (1), 625–637. (30) Baker, J. Energy Policy 2008, 36, 4368–4373. (31) Walawalkar, R.; Apt, J.; Mancini, R. Energy Policy 2007, 35 (4), 2558–2568.

energy carrier

methanol

iso-octane

liquid H2

LiH

CaH2

energy density (MJ/L)

18.0

33.7

9.9

2.2

0.82

of the situations noted above. For example, water pumping is only useful for case 1, storing energy for later conversion back to electricity at the same site. The most generally useful method is conversion of electrical energy into a chemical fuel, because of the good storability and transportability of most chemical fuels. Hydrogen has been the most widely studied candidate for chemical storage and has considerable merit for case 1, because hydrogen could be simply compressed and then converted back to electricity in a fuel cell. However, in cases 2 and 3 involving fuel transport, there are well-known difficulties with hydrogen storage (requiring, e.g., compression, cryogenic liquification, or formation of a metal hydride compound) and transport compared with liquid fuels.33,34 Overcoming these significant barriers would require extensive technological development and investment, not only the storage and transport issues but also the requirement to replace the incumbent liquid fuel infrastructure with a hydrogen infrastructure.33 On the other hand, the key advantage of hydrogen is that it works well with regenerative fuel cells, where energy is stored using water/steam electrolysis and subsequently converted back to electricity in fuel cell mode.35-39 Synthetic liquid fuels have been proposed as alternatives to hydrogen,34,40 primarily because of their much better storage attributes. In particular, the synthetic liquid fuel is similar to currently used fossil fuels (although much cleaner with regard to contaminants such as sulfur) and hence is easily stored and transported using the existing infrastructure and methodss no new hydrogen infrastructure is needed. The volume energy densities of methanol and iso-octane, given in Table 1, are much higher than compressed hydrogen, liquefied cryogenic hydrogen, metal hydrides, etc.;41 thus, fuel storage volumes will be lower. Furthermore, liquid fuels are easily transferred at high rates; note that a typical vehicle gasoline fuelling rate of ∼0.5 L/s corresponds to an energy transfer rate of 17 MW, or 27 mols H2 per second. Transferring hydrogen at such high rates may be problematic, especially if metal hydride storage is used. Recent advances in fuel cells increasingly allow them to operate effectively with liquid alcohol and hydrocarbon fuels.42-46 (32) Dell, R. M.; Rand, D. A. J. J. Power Sources 2001, 100 (1-2), 2–17. (33) Petrovic, J.; Milliken, J.; Devlin, P.; Read, C. In Challenges to Hydrogen Production, DeliVery, and Storage: USDOE’s program response, 2003 Fuel Cell Seminar, 2003; Committee, F. C. S., Ed.; Courtesy Associates: 2003; pp 988-991. (34) Bossel, U.; Eliasson, B.; Taylor, G. In The Future of the Hydrogen Economy: Bright or Bleak, European Fuel Cell Forum, Lucerne, Switzerland, 2003; Bossel, U., Ed.; European Fuel Cell Forum: Lucerne, Switzerland, 2003. (35) Smith, W. J. Power Sources 2000, 86 (1-2), 74–83. (36) Barbir, F.; Molter, T.; Dalton, L. Int. J. Hydrogen Energy 2005, 30 (4), 351–357. (37) Maclay, J. D.; Brouwer, J.; Scott Samuelsen, G. Int. J. Hydrogen Energy 2006, 31 (8), 994–1009. (38) Mitlitsky, F.; Myers, B.; Weisberg, A. H. Energy Fuels 1998, 12 (1), 56–71. (39) Jiang, Y.; Gostovic, D.; Von Dollen, P.; Mansourian, N.; Pillai, M.; Kim, I.; Barnett, S. A. Electrochem. Solid State Lett. 2009, In press. (40) Olah, G. A.; Goeppert, A.; Prakash, G. K. S. Beyond Oil and Gas: The Methanol Economy; Wiley-VCH: Weinheim, 2006. (41) US Department of Energy, Hydrogen Storage, http://www1.eere. energy.gov/hydrogenandfuelcells/storage/index.html. (42) Jiang, Y.; Virkar, A. V. J. Electrochem. Soc. 2001, 148 (7), A706A709. (43) Zhan, Z.; Barnett, S. A. Science 2005, 308, 844–846. (44) Pillai, M. R.; Kim, I.; Bierschenk, D. M.; Barnett, S. A. J. Power Sources 2008, 185, 1086-1093.

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Figure 2. Schematic illustration of liquid-fuel energy cycles.

Indeed, solid oxide fuel cells utilizing endothermic internal reforming of liquid fuels may have efficiency and system advantages.43 Thus, hydrogen is not the only fuel that can be used in a renewable energy storage cycle with fuel cells. Figure 2 shows schematically a generic liquid-fuel energy cycle utilizing a renewable electrical source. The feedstock chemicals are H2O and CO2, which are converted to H2 and CO by electrolysis (Figure 1), for example: CO2 + 2H2O + 6e- f CO + 2H2 + 3O2-

(1)

The combination H2 + CO is termed “synthesis gas”, or “syngas.” The next step is conversion of syngas to chemicals; for example, catalytic processes are widely used to produce methanol or hydrocarbons (Fischer-Tropsch synthesis).47-49 Desired H2/CO ratios vary from ∼1.4 to 2.1, and this can be varied by the H2O/CO2 ratio input to the electrolyzer. Finally, the liquid fuel is oxidized to produce electricity in a fuel cell or mechanical energy in a heat engine. Note that the cycle is CO2 neutral, that is, the same amount of CO2 is produced in oxidizing the hydrocarbon as is consumed in electrolysis. An important difference between the cycle in Figure 2 and hydrogen energy storage is that CO2 must be supplied to the electrolyzer, in addition to H2O. Thus, the CO2 produced upon fuel oxidation must be captured, stored, and transported. CO2 capture methods envisioned for use in sequestration could be employed.50 Although this might seem to be a significant disadvantage, it is really little different than the hydrogen cycle. Since there are relatively few geographical sites where there is both an abundant supply of fresh water and a renewable energy source available, in most cases it will be necessary to capture, store, and transport water. CO2 is easily compressed into liquid form at a relatively low pressure of 830 psi at 20 °C, or cooled into solid form at -78.5 °C at ambient pressure (this is much easier and requires much less energy than compression or liquefication of hydrogen). Thus, it is only slightly more difficult to transport H2O + CO2 than H2O alone. If vehicles are used to transport fuel from the production site, then the same vehicles can transport H2O and CO2 on their return trips. Note that production of the liquid fuel from syngas is currently practiced in “gas-to-liquids” (GTL) processes, where natural gas is the feedstock used to produce syngas, and “coal(45) Atkinson, A.; Barnett, S. A.; Gorte, R. J.; Irvine, J. T. S.; McEvoy, A. J.; Mogensen, M. B.; Singhal, S.; Vohs, J. Nat. Mater. 2004, 3, 17–27. (46) McIntosh, S.; Gorte, R. J. Chem. ReV. 2004, 104, 4845–4865. (47) Wilhelm, D. J.; Simbeck, D. R.; Karp, A. D.; Dickenson, R. L. Fuel Process. Technol. 2001, 71, 139–148. (48) Iglesia, E. Appl. Catal. A: Gen. 1997, 161, 59–78. (49) Yang, R. Q.; Fu, Y. L.; Zhang, Y.; Tsubaki, N. J. Catal. 2004, 228 (1), 23–35. (50) Song, C. Catal. Today 2006, 115 (1-4), 2–32.

to-liquids” (CTL), where coal is the feedstock. These processes are expected to become increasingly important as petroleum resources dwindle. The process proposed here, renewable-toliquids (RTL), could be increasingly adopted in the future as natural gas and coal also become increasingly depleted. It should be a significant advantage that the RTL technology and its infrastructure would be quite similar to the preceding GTL and CTL technologies. Most of the major steps of the liquid fuel cyclescatalytic fuel production from syngas, storage, transport, and operation of fuel cells on methanol42 or liquid hydrocarbons43,46,51,52s are either already in widespread use or have been demonstrated. The only step not yet extensively investigated is the electrolytic production of syngas (eq 1), which is the topic of the present paper. 3. Experimental Procedures 3.1. SOEC Preparation. The SOECs used in this study consisted of Ni-YSZ negative electrode supports, thin YSZ electrolytes, and LSCF-GDC positive electrodes (YSZ ) 8 mol % Y2O3-stabilized ZrO2, GDC ) Ce0.9Gd0.1O1.95, and LSCF ) La0.6Sr0.4Co0.2Fe0.8O3). The supports were pressed from NiO and YSZ (50:50 wt %) powders with 10% starch filler and then bisque fired at 1100 °C. Negative electrode active layers of NiO-YSZ and thin dense YSZ electrolyte layers were deposited on the NiO-YSZ supports using a colloidal deposition technique as described previously.51 The structures were cofired in air at 1400 °C for 4 h. LSCF-GDC electrodes were prepared by screen printing. The ink was prepared by mixing LSCF powder with GDC powder in a weight ratio of 70:30 and then mixing with a vehicle (Heraeus) in a three-roll mill. The ink was screen printed onto the YSZ electrolyte and fired at 900 °C for 4 h. A layer of pure LSCF was then screen printed from an ink (prepared similarly to the LSCF-GDC ink) and fired at 900 °C for 4 h. In some cases, LSM-YSZ cathodes were screen printed and fired at 1250 °C for 1 h, along with a second pure layer of LSM subsequently fired at 1250 °C (LSM ) La0.8Sr0.2MnO3). The final fuel cells were 1.5∼2.5 cm in diameter, with negative electrode thickness of ∼0.6 mm, electrolyte thickness of ∼10 µm, and positive electrode thickness of 20-30 µm. Estimated negative electrode porosity was ≈40%, and positive electrode porosity was ≈30%. The positive electrode area, which defined the cell active area, was 0.5∼2.4 cm2. 3.2. Electrochemical Testing and Mass Spectrometer Measurement. The SOECs were tested in a tube furnace at temperatures from 700 to 800 °C, using a standard testing geometry as reported previously.53 At the beginning of each test, the Ni-based electrode was fully reduced in humidified H2 at 800 °C. The cells were then tested with ambient air maintained at the positive electrode, while the fuel at the negative electrode was H2 mixed with CO2-H2O. The CO2 and H2 were flowed through a bubbler containing H2O in order to entrain a known partial pressure. Current density versus voltage and electrochemical impedance spectra (EIS) were obtained using an IM6 Electrochemical Workstation (ZAHNER, Germany). The frequency range for the impedance measurement was 0.1 Hz to 100 kHz. The fuel exhaust gas was sampled via a capillary tube with inlet placed near the Ni-YSZ electrode, and was analyzed using a Transpector 2 Gas Analysis System (Inficon L100, electron impact ionization using 40 eV electrons) that was differentially pumped, using a turbomolecular pump, to a measurement pressure of ∼5 × 10-5 Torr. Note that H2O was removed from the products using a desiccant in order to avoid poisoning of the mass spectrometer. In order to obtain the actual exhaust compositions via quantitative analysis of these spectra, the mass spectrometer system was (51) Zhan, Z.; Barnett, S. A. J. Power Sources 2006, 155, 353–357. (52) Zhan, Z.; Barnett, S. A. J. Power Sources 2006, 157, 422–429. (53) Zhan, Z.; Lin, Y.; Pillai, M.; Kim, I.; Barnett, S. A. J. Power Sources 2006, 161 (1), 460–465.

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Figure 3. Comparison of the electrolysis characteristics for a typical SOEC, fuel, Ni-YSZ | YSZ | LSCF-GDC, LSCF, air, operated at 800 °C on 50% H2 and 50% H2O; 25% H2 and 75% CO2; and 25% H2, 25% CO2, and 50% H2O at 100 sccm: (a) voltage versus current density and (b) electrochemical impedance spectra.

calibrated using a gas mixture of 20% H2 and 80% CO2 at room temperature. This calibration is critical in achieving more-accurate H2 contents since the presence of significant quantities of highermass ions interferes with observation of hydrogen in the mass spectrometer, a phenomenon known as zero blast.54 Note that the cracking pattern of gaseous CO overlaps with that of CO2 at m/e ) 28. Nevertheless, the contribution to the peak at m/e ) 28 from gaseous CO could be obtained by subtracting that due to CO2 cracking, which is always a fraction of the peak at m/e ) 44. Thus, H2, CO, and CO2 were measured directly by the mass spectrometer, while the steam content was calculated from mass conservation.

4. Results 4.1. Electrochemical Measurement. Figure 3A shows the typical voltage V versus current density J dependence in both fuel cell and electrolysis modes for H2O-H2, CO2-H2, and CO2-H2O-H2 gas mixtures. The H2O-H2 result, provided as a baseline for comparison, was similar to that reported previously for steam electrolysis.13 Lower open-circuit voltages were observed for CO2-H2 and CO2-H2O-H2 than for H2-H2O, due to the lower H2 content in the former mixtures. Open-circuit voltages were within 70 mV of the values predicted for these gas compositions from the Nernst equation.14 The J-V curves were approximately linear at low J, and the low-current (54) Operating manual of Transpector 2 gas analysis system; Inficon; I., IPN 074-276.

resistance increased with increasing CO2 content in the gas phase. At higher J values there was a “current limitation,” that is, a more rapid change in cell voltage with increasing current density; this indicated that concentration polarization, a mass transport limitation in the electrodes usually related to gas diffusion,55 was significant. In fuel cell mode, the concentration polarization was higher for the lower-H2 mixtures, since the lower H2 content made it more difficult to transport sufficient H2 to the fuel cell.55 In electrolysis mode, the concentration polarization was generally higher for lower total H2O + CO2 concentration (50% vs 75% H2O + CO2), since the lower concentration made it more difficult to transport these reactants to the electrolyzer. The concentration polarization was higher for 75% CO2 than for 25% CO2 + 50% H2O; this can be explained by the lower gas diffusivity in the CO2-rich gas due to the higher molecular weight of CO2 compared to H2O (see more detailed discussion below). Electrochemical impedance measurements, performed at 800 °C and open circuit in these same three gas compositions, are shown in Figure 3B. The total area-specific resistance (ASR), taken as the difference between the real-axis intercepts of the arcs in the plot, increased from 0.21 Ω cm2 for H2/H2O to 0.22 Ω cm2 for H2/CO2/H2O to 0.27 Ω cm2 for H2/CO2. These changes, though small, were deemed significant because they occurred reproducibly upon switching gases for a given cell. (55) Jiang, Y.; Virkar, A. V. J. Electrochem. Soc. 2003, 150, A942.

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Table 2. Measured Widths of Polarization Arcs Measured in Air with Various Fuel Mixtures at 800 °C, Compared with Calculated Diffusion Coefficients for the Same Gas Mixtures gas composition

H2/H2O

H2/CO2/H2O

H2/CO2

low-freq arc (Ω cm2) high-freq arc (Ω cm2) DH2 (cm2/s) DH2O (cm2/s) DCO (cm2/s) DCO2 (cm2/s)

0.075 0.047 5.74 3.80

0.079 0.052 5.39 2.86 1.67 1.43

0.106 0.074 4.86 1.95 1.29 1.19

This trend is consistent with the changes in slopes of the J-V curves (Figure 3A) near open-circuit. The high-frequency intercept was ∼0.08 Ω cm2, consistent with prior reports for thin-YSZ-electrolyte SOFCs56 and mainly due to the YSZ ohmic resistance. Prior studies of LSCF-GDC electrodes, similar to those used in the present study, indicated a negligibly small polarization resistance at 800 °C of ∼0.01 Ω cm2.57 Thus, the polarization arcs in Figure 3B, which showed two overlapping arcs, were primarily associated with the fuel (Ni-YSZ) electrode. Prior EIS studies of SOFCs with similar electrodes showed similar results,58,59 and it was found that the higherfrequency arc followed an Arrhenius dependence typical of electrochemical processes such as charge transfer or surface diffusion, whereas the low-frequency arc showed a very weak temperature dependence often associated with a gas diffusion process. Table 2 gives the widths of the polarization arcs for each gas mixture. The resistances associated with both arcs increased with increasing CO2 content. The above observation of a larger high-frequency arc with increasing CO2 content suggests that the charge transfer processes associated with CO-CO2 oxidation/reduction are slower than the corresponding H2-H2O processes. This is consistent with prior results showing that solid oxide fuel cell anodes typically have more difficulty oxidizing CO to CO2, compared to oxidizing H2 to H2O.60 Indeed, the kinetically fast and reversible water-gas shift reaction, with the Ni-YSZ electrode acting as a catalyst, combined with the H2/H2O electrochemical reaction, likely provides an important alternative pathway to direct CO/CO2 electrochemical reaction. Table 2 also shows that the low-frequency arcs increased with increasing CO2 content. This is consistent with the larger concentration polarization,55 indicated by the more pronounced current limitation in Figure 3A for the 25% H2 and 75% CO2 mixture compared to the 25% H2, 25% CO2, and 50% H2O mixture. The diffusion coefficients for H2, H2O, CO, and CO2 at 800 °C, calculated for the three gas compositions, are given in Table 2. The diffusivities generally decreased with increasing CO2 content. From H2/H2O to H2/CO2/H2O to H2/CO2, the diffusion coefficient of hydrogen dropped by 6 and 10%, and the low-frequency arc increased by 5 and 34%, respectively. The relatively large increase in the low-frequency arc from H2/ CO2/H2O to H2/CO2 might be related to lower gas diffusivities in the CO2-rich mixture. In summary, the above results suggest that the higher performance for H2O/CO2 coelectrolysis than for CO2 electrolysis can be attributed to a higher H2/H2O (56) VonDollen, P.; Barnett, S. A. J. Am. Ceram. Soc. 2005, 88, 3361– 3368. (57) Murray, E. P.; Sever, M. J.; Barnett, S. A. Solid State Ionics 2002, 148, 27–34. (58) Lin, Y.; Zhan, Z.; Liu, J.; Barnett, S. A. Solid State Ionics 2005, 176, 1827–1835. (59) Zhan, Z.; et al. In Propane Fueled Solid Oxide Fuel Cells, SOFC VIII, Pennington, NJ, 2003; Singhal, S. C.; Dokiya, M., Eds. Electrochem. Soc.: Pennington, NJ, 2003; pp1286-1294. (60) Eguchi, K.; Kojo, H.; Takeguchi, T.; Kikuchi, R.; Sasaki, K. Solid State Ionics 2002, 152-153, 411–416.

Figure 4. Voltage versus current density results at various temperatures for a typical SOEC (Ni-YSZ | YSZ | LSCF-GDC, LSCF) operated at 800 °C with a 25% H2, 25% CO2, and 50% H2O mixture at 100 sccm.

Figure 5. Voltage vs time for a typical SOECs (Ni-YSZ | YSZ | LSM-YSZ, LSM) operated at 800 °C on 25% H2, 25% CO2, and 50% H2O at 50 sccm, J ) 1.05 A/cm2.

diffusion rate in the porous electrode, higher charge transfer reaction rates for H2/H2O than CO/CO2, and H2 production via the water-gas shift reaction due to the presence of H2O. Figure 4 shows the J-V curves at different temperatures for a 25% H2, 25% CO2, and 50% H2O gas mixture. Open-circuit voltages ranged from 0.9 to 0.94 V and were consistent with values predicted using the Nernst equation,14 of 0.89 to 0.92 V. J values at a given V generally increased with increasing temperature, as typically observed for SOECs. The electrolysis current densities increased from 0.7 to 1.4 A/cm2 at 1.3 V with increasing temperature from 700 to 800 °C. The J-V dependence was generally linear near open circuit but showed increasing slope at sufficiently large J, suggesting concentration polarization. Most of the above electrical tests were done over 10-20 h, but a few tests were done for longer times. Figure 5 shows an example of such a test done in the electrolysis mode for ≈100 h. The test was carried out using a 25% H2, 25% CO2, and 50% H2O mixture at 50 sccm, J ) 1.05 A/cm2, and 800 °C. There was an increase in voltage by ∼2% during the 100 h test. The slight degradation in the electrolysis performance is not surprising as other reports have also noted this effect.4,15 A number of mechanisms have been proposed to explain the degradation, including segregation of silica impurities to interfaces in Ni-YSZ;15 Ni migration in Ni-YSZ electrodes when exposed to high steam content;16 instability in (La,Sr)MnO3 (LSM) air electrodes on YSZ electrolytes,17,20-22

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Figure 7. Exhaust gas composition versus current density at 800 °C for a SOEC (fuel, Ni-YSZ | YSZ | LSM-YSZ, LSM, air) fed with a 25% H2, 25% CO2, and 50% H2O fuel mixture at 50 sccm. Figure 6. Typical mass spectra of the electrolysis products at 800 °C and two different current densities for 25% H2, 25% CO2, and 50% H2O inlet gas mixture at 50 sccm.

density of -1 A/cm2 was measured, corresponding to a syngas production rate of ∼7 sccm cm-2. 5. Discussion

including changes in the chemical state of La,61,62 air electrode delamination,18 zirconate phase formation at LSM-YSZ interfaces;19 and morphological changes at the LSM/YSZ interface.21 SOEC stability is clearly a critical issue, but it is not the main topic of the present work. 4.2. Mass Spectrometer Measurements. Differentially pumped mass spectrometer measurements of the SOEC exhaust gases were carried out for operation with inlet gas mixtures of 25% H2, 25% CO2, and 50% H2O. The spectrum at open circuit (Figure 6A) shows main peaks at atomic mass 2 (H2+), 28 (CO+), and 44 (CO2+). Note that H2O was intentionally removed from the product gas stream to avoid irreversible contamination of vacuum chamber surfaces with H2O. Peaks at 12 (C+) and 16 (O+ or O2+2) were present due to the fragmentation of CO2 by electron bombardment in the mass spectrometer. Figure 6B shows the mass spectrum obtained with the SOEC operated at J ) 0.6 A/cm2. Relative to the J ) 0 case (Figure 6A), the H2+ and CO+ peaks increased while the CO2+ peak decreased, indicating that coelectrolysis of CO2/H2O to CO/H2 was occurring. Quantitative analyses of the mass spectra were performed using a mass spectrometer measurement on a calibration gas mixture of 20% H2 and 80% CO2. Figure 7 summarizes the results for product compositions measured at different current densities. Note that the H2O content was calculated via conservation of hydrogen. The measured gas composition at 800 °C and open circuit was substantially different than the inlet compositions; this was explained by the water-gas shift reaction catalyzed by the Ni-YSZ electrode. The measured composition at OCV was closer to the calculated equilibrium gas composition for these conditionss19% H2, 56% H2O, 6% CO and 19% CO2sthan the inlet composition. There was significant deviation for the H2 and H2O contents that can be explained by the “zeroblast” errors in mass spectrometer measurements54 of H2 and the errors that accumulate in calculating the H2O content. With increasing J, both the CO and H2 contents increased, whereas the H2O and CO2 contents dropped. These results demonstrate that these SOECs are effective for producing syngas in the coelectrolysis. At ∼1.3 V and 800 °C, an electrolysis current (61) Yildiz, B.; Chang, K. C.; Myers, D.; Carter, J. D.; You, H. In 7th European Solid Oxide Fuel Cell Forum, Lucerne, 3-7 July 2006, 2006; Bossel, U., Ed. Lucerne, 2006. (62) Wang, W.; Huang, Y.; Jung, S.; Vohs, J. M.; Gorte, R. J. J. Electrochem. Soc. 2007, 153 (11), A2066-A2070.

The round-trip efficiency η, that is, the fraction of the original electrical energy obtained after the storage and utilization cycle, is a key figure of merit for a renewable energy cycle. The efficiency of the liquid fuel cycle should be at least comparable to that of a hydrogen cycle. Otherwise, the advantages noted in Section 2 are moot. In the following sections, we discuss and compare estimated efficiencies for hydrogen and liquid fuel cycles considering electrolysis and fuel cell processes (Section 5.1), and then considering storage/transport losses (Section 5.2). The cost of a technology can also provide a major barrier to commercial success, as is well-known in the fuel cell field. Thus, a cost estimation for the proposed renewable liquid fuel cycle is given in Section 5.3. It must be emphasized that the discussion below provides only estimates, meant to indicate the initial feasibility of the proposed approach, and that more complete experimental and modeling studies will be required to accurately establish costs and efficiencies. 5.1. Round-trip Efficiency: Electrolysis and Fuel Cell Operation. The ideal round-trip efficiency ηID of a regenerative fuel cell, for electrolysis operation at voltage VE and fuel cell operation at VFC, can be written:35,36 ηID ) VFCQFC/VEQE

(2)

where QFC is the charge transferred during fuel cell operation, and QE is the charge transferred during electrolysis operation. This is an ideal efficiency in the sense that parasitic losses, electrical losses external to the stack, and the energy required for preheating reactants are not included. In a closed-loop H2/ H2O storage system, QFC ) QE, such that the efficiency is VFC/ VE. A typical VFC value is 0.7 V; although higher efficiency can normally be achieved at higher voltage, power densities can become prohibitively small. VE ≈ 1.3 V is commonly used, since at this value the current density is usually reasonably large and the heat produced by the SOEC is sufficient to maintain stack temperature, exactly compensating for the endothermic electrolysis reaction. Note that if heat from the stack must be used to boil the water for H2O electrolysis, then VE ≈ 1.5 V is required.8 Thus, the ideal round-trip efficiency is either ∼43 or 37.5%. If the storage system is not a closed loop, that is, if the electrolytically produced fuel is transported for use at a different location, then the fuel cell fuel utilization, ≈80%, must be considered yielding QFC/QE ≈ 0.8 in eq 2.

Syngas Production By Co-Electrolysis of CO2/H2O

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Table 3. Comparison of the Total Synthesis and Transport Efficiencies of Different Types of Fuelsa H2 gas

H2 liquid

91 94 93 97 77

65 90

syngas68

synthesis from packaging34,69 transport (truck)34,69 transport (pipeline)34,69 gas tank transfer34,69 total distribution (from packaging to transfer)

59

methanol

FT gasoline

99

75

98 98

99 74

a

Figures are taken as high-end values averaged between references cited.

The results in Figure 3 indicate that electrolysis of 33% CO2 + 67% H2O is similar to H2O electrolysis (ASR values of 0.22 and 0.21 Ω cm2, respectively, at 800 °C), so there is no need to employ higher VE or lower VFC, that would decrease ηID, to achieve reasonable current densities. However, ηID may be significantly higher for a liquid fuel cycle, relative to a hydrogen cycle, due to thermal synergies between the electrochemical and catalytic reactions.63 In the endothermic electrolysis process, the problem is to provide the heat required without reducing ηID. The CO2-H2O electrolysis process has the significant advantage that heat from the exothermic liquid-fuel production process can be used to help boil liquid water and preheat the gases entering the electrolyzer. For example, the methanol production reaction CO + 2H2 f CH3OH has an enthalpy change of -91 kJ,64 whereas a typical Fischer-Tropsch synthesis reaction CO + 2H2 f sCH2s + H2O has an enthalpy change of -165 kJ.65 Thus, less heat from stack operation losses will be required compared to H2 electrolysis, allowing lower VE. In addition, the heat input for water boiling is reduced by ≈33% for electrolysis of 2H2O + CO2 compared to pure H2O. The above discussion shows that there should be an efficiency advantage during electrolysis when using liquid hydrocarbon fuels instead of hydrogen. Using eq 2, the estimated ideal roundtrip efficiency for a liquid energy carrier is ≈43% (VFC ) 0.7 V, VE ) 1.3 V, and QFC/QE ) 1), compared to ≈37.5% for hydrogen (VFC ) 0.7 V, VE ) 1.5 V, and QFC/QE ) 1). Actual values will of course be lower, due to fuel-cell fuel efficiency and losses not considered in eq 2, but these losses should be similar for H2 and liquid fuels. 5.2. Round-trip Efficiency: Transport and Storage. The impact of fuel transport and storage steps on efficiency must also be considered. Table 3 provides a comparison of literature values of these efficiencies for compressed H2, cryogenic liquid H2, methanol, and Fischer-Tropsch gasoline. Although cryogenic liquid hydrogen has a reasonably high energy density (Table 1), it requires even more energy than pressurization and is susceptible to substantial losses due to leakage and evaporation, yielding relatively low total distribution efficiency. Multiplying the H2 efficiency values in Table 3 with the round-trip efficiency from the previous section yields reasonable agreement with prior estimates of overall cycle efficiency, ∼20 to 25%.34,66 Fischer-Tropsch synthesis (FTS) can have a carbon conversion efficiency of about 75%.67 One drawback of FTS is that it produces a wide range of carbon compounds rather than a single fuel. However, the various species can be separated and used (63) Gottmann, M.; Mcelroy, J. F.; Mitlitsky, F.; Sridhar, K. R. SORFC power and oxygen generation method and system. 2006. (64) Rostrup-Nielsen, J. R. Catal. Today 2000, 63 (2-4), 159–164. (65) O. Haid, M.; Schubert, P. F.; Bayens, C. A. Synthetic fuel and lubricants production using gas-to-liquids technology. DGMK Tagungsbericht 2000, 2000-3, 205–212. (66) Bossel, U. On the way to a sustainable energy future (presented at Intelec ’05, Sept. 2005Berlin). (67) Lutz, B. Hydrocarbon Engineering 2001, 6 (11), 23–26.

Table 4. Cost Evaluation of Methanol and Octane Produced from Different Energy Sources cost to produce electricity source

methanol

cost (cents/kWh) wind 5a solar 21b US industrial avg. 6c

octane

$/gallon cents/kWh $/gallon cents/kWh 1.57 6.59 1.88

8.29 34.80 9.93

3.85 16.17 4.62

10.86 45.60 13.03

a http://www.awea.org/faq/cost.html. b http://www.solarbuzz.com/SolarPrices. htm. c http://www.eia.doe.gov/cneaf/electricity/epm/epm_sum.html.

as fuels or chemical feedstocks for different applications, just as is done currently in fuel refining. Furthermore, solid oxide fuel cells can exhibit considerable fuel flexibility, as indicated by reports of operation on various fuels including methanol,42 methane,58 propane,59 butane,46 iso-octane,43,51,52 and dodecane.46 Nonetheless, the actual useful fuel converted will be less than the carbon conversion efficiency. Catalytic methanol synthesis from syngas, utilizing a number of passes to improve conversion, is an exothermic and equilibrium limited synthesis reaction with overall conversion efficiency of over 99%. It is clear from Table 3 that production and distribution of liquid methanol and higher hydrocarbons (such as octane) is more efficient than storage and transport of hydrogen. Summarizing the above efficiency discussion, higher roundtrip efficiency should be achievable for either a methanol or a liquid hydrocarbon cycle, compared to a hydrogen cycle. The methanol and Fischer-Tropsch synthesis cycles appear to have similar efficiency potential; methanol synthesis is more efficient than hydrocarbon synthesis, but this is offset by the efficiency advantages afforded by hydrocarbon internal reforming discussed above. 5.3. Cost Estimation. Based on the energy efficiencies cited above, an electricity-only cost to produce a gallon of fuel was estimated using current (2008) costs for wind and solar renewable electricity (Table 4). Although the prices are higher than those for fossil fuels, the cost from wind power is within reason, especially considering that this is a renewable CO2-free energy source. For example, a 1 MW wind turbine can generate on average 2.7 million kWh per year, corresponding to 35 000 gallons of octane or 86 000 gallons of methanol per year. The high cost of photovoltaics leads to a relatively high cost for fuel in Table 4. Nonetheless, as the cost of renewable energy drops and of fossil fuels increases, renewable fuels will become cost competitive even without accounting for elimination of CO2 production. 6. Conclusions The above results have demonstrated that state-of-the-art Ni-YSZ electrode supported SOFCs can work reversibly as the solid oxide electrolysis cells (SOECs) to efficiently produce syngas via coelectrolysis of H2O/CO2. For a typical feedstock composition of 2H2O/CO2, the electrolysis performance decreased only slightly compared to H2O electrolysis, and was much better than for pure CO2 electrolysis. SOECs operated on 25% H2, 25% CO2, and 50% H2O at T ≈ 800 °C and V ≈ 1.3 V yielded reasonably stable syngas production rates of ∼7 sccm/cm2. Subsequent catalytic processes to form methanol or other liquid hydrocarbon fuels (68) Spath, P. L.; Dayton, D. C. National Renewable Energy Laboratory Technical Report NREL TP-510-34929 2003. (69) Hekkert, M. P.; Hendriks, F. H. J. F.; Faaij, A. P. C.; Neelis, M. L. Energy Policy 2005, 33 (5), 579–594.

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from the syngas, that are widely used industrially, could then be used in a renewable energy cycle. Solid oxide fuel cells, which have been shown to work effectively with these fuels, would be a preferred method to convert the fuel back to electricity. This proposed energy cycle is CO2 neutral but has the significant advantage that the liquid fuels are very similar to current fossil fuels; the technological and infrastructure barriers to introducing this energy cycle are thus

Zhan et al.

significantly less than for a hydrogen energy cycle, allowing a easier transition to a carbon-neutral and cost-competitive liquid fuel economy using renewable electricity. Furthermore, thermal synergies in the liquid fuel cell should allow a substantially higher round-trip energy efficiency than a hydrogen cycle. EF900111F