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Microstructure and Performance Investigation of a Solid Oxide Fuel Cells Based on Highly Asymmetric YSZ Microtubular Electrolytes Krzysztof Kanawka, Francesco Dal Grande,‡ Zhentao Wu,† Alan Thursfield,‡ Douglas Ivey,§ Ian Metcalfe,‡ Geoffrey Kelsall,† and Kang Li*,† Department of Chemical Engineering, Imperial College London, South Kensington Campus, London, SW7 2AZ, United Kingdom, School of Chemical Engineering and AdVanced Materials, Newcastle UniVersity, Newcastle Upon Tyne NE1 7RU, United Kingdom, and Department of Chemical and Materials Engineering, UniVersity of Alberta, Edmonton, Alberta, Canada T6G 2 V4
A combined phase inversion and sintering technique was used to fabricate highly asymmetric yttria-stabilized zirconia (YSZ) hollow fibers with a 1.9 mm external diameter and 210 µm total wall thickness. These consisted of a thin (∼10 µm), dense outer layer and a much thicker (∼200 µm) inner layer with finger-like voids. Such highly asymmetric structures formed gas-tight electrolytes and mechanical supports of a microtubular solid oxide fuel cell (SOFC). Nickel oxide-yttria-stabilized zirconia (NiO-YSZ) particles were infused into the pores from alcoholic dispersions and then sintered at 1300 °C, prior to electroless Ni layers being deposited, forming composite anodes. The Ni layer deposited by electroless plating had the function of improving the anodic current collection. Cathodes were deposited by slurry-coating lanthanum strontium manganite particles onto the outer surfaces of YSZ hollow fibers, followed by sintering at 1000 °C and reduction of the anode at 800 °C in a hydrogen environment, completing the fabrication of the microtubular SOFCs. Mechanical strengths of ∼226 MPa were derived from three-point bending tests on hollow fibers sintered at 1450 °C. Initial SOFC performance measurements with 5% H2 as the fuel and air as the oxidant resulted in maximum power densities of 18 mW cm-2 at 800 °C. This low value resulted largely from the discontinuous nature of the electrolessly deposited nickel anode; present work aims to improve its structure and hence SOFC performance. 1. Introduction Solid oxide fuel cells (SOFCs) are energy conversion devices that have recently gained much interest because of their environmentally friendly operation, fuel flexibility, and high chemical to electrical conversion rate.1,2 Among several electrolyte materials currently investigated for SOFC applications, yttria-stabilized zirconia (YSZ) is commonly used because of its considerable ionic conductivity at elevated temperatures, excellent mechanical properties, and low cost. In contrast to flat sheet or tubular SOFCs, the microtubular geometry possesses several advantages, such as increased surface area available for electrodes and minimized sealing problems. Up to now, the most common techniques used for the fabrication of microtubular SOFCs are cold isostatic pressing,3 extrusion,4-6 and gelcasting.7 A combined phase inversion and sintering method has been recently employed by Wei and Li8 and Jin et al9 for the fabrication of YSZ hollow fibers, providing a rapid fabrication process and better control of the microstructure if compared with the extrusion technique. Nickel/yttria-stabilized zirconia/lanthanum strontium manganite (Ni/YSZ/LSM) microtubular electrolyte-supported SOFCs were developed in the early 90s by Singhal and Kendall10 and by Kilbride,11 but this design was abandoned due to the thick, dense electrolyte layer which resulted in high ohmic losses. Wei and Li8 recently revisited the electrolyte-supported design, employing phase inversion in order to fabricate a highly porous hollow fiber with a thin, dense layer to reduce the ohmic losses. However, the thickness of the electrolyte was still virtually the * To whom correspondence should be addressed. Tel: 44 (0) 2075945676. Fax: 44 (0) 207-5945629. E-mail: Kang ·
[email protected]. ‡ Newcastle University. † Imperial College London. § University of Alberta.
same as the thickness of the hollow fiber wall as the anode material was coated on the lumen surface of the fibers. Novel highly asymmetric YSZ hollow fibers were recently prepared via a combined phase inversion and sintering method by Yin et al.12 These YSZ hollow fibers have the potential to provide an attractive structure for electrolyte-supported microtubular SOFCs due to the reduced thickness of the gas-tight dense electrolyte layer and increased inner surface area while maintaining good mechanical properties. Despite these advantages, this kind of hollow fiber has not yet been used in SOFC technology. Mechanical and performance durability are required for the technical and economical success of SOFCs. The SOFC based on Ni/YSZ/LSM requires a high temperature of work (800-1000 °C), which results in a high thermal stress and fuel cell degradation. The so-developed electrolyte-supported design possesses superior stability with respect to the electrodesupported designs. For example, the electrolyte-supported design assures higher mechanical stability under Ni anode oxidationreduction than the anode-supported counterpart, where the volume expansion-contraction results in small cracks and fractures which compromise the SOFC gas-tightness and Ni continuity. In this article, we report an initial investigation of the performance of highly asymmetric YSZ hollow fibers as electrolytes for SOFCs. In contrast to previous investigations, the anode layer is deposited into the finger-like voids of the highly asymmetric YSZ hollow fiber. A design of this type potentially extends the anode surface area in a microtubular SOFC and therefore can contribute to an increase in the efficiency of operation. The microtubular SOFC fabrication route described in this paper can be also employed with other electrolyte materials such as gadolinia-doped ceria (CGO) and cathode materials such as lanthanum strontium cobalt ferrite
10.1021/ie1002558 2010 American Chemical Society Published on Web 05/28/2010
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(LSCF), which allows one to decrease the working temperature to the intermediate range of 450-600 °C with beneficial effects on SOFC mechanical stability and performance durability because of the lower thermal stress, with also a significant decrease of the fabrication costs as stainless steel interconnections can be used instead of the much more expensive lanthanum and yttrium chromite interconnections (required at high temperatures). 2. Experimental Section 2.1. Materials. Yttria-stabilized zirconia (YSZ) (Nextech Materials Ltd., 8 mol % yttrium, 0.3 µm) was used as the hollow fiber starting material. Polyethersulfone (PESf) (Ameco Performance Radel A-300) and N-methyl-2-pyrrolidone (NMP) (Sigma Aldrich, HPLC grade) were used as the polymer binder and the solvent for the spinning of the hollow fibers, repsectively. Ethanol (VWR International Ltd. 99.7, 100% v/v), polyethyleneglycol 400 (Merck Schuchardt OHG PEG-400), and arlacel P135 (Uniqema, Spain) were used as additives in the spinning suspension. The anode suspension was prepared by mixing nanoscale nickel oxide (NiO) (99.8% purity, average particle size of 20 nm from PI-KEM Ltd., UK) and YSZ (Nextech Materials Ltd., 8 mol% yttrium, surface area of 155 m2 g-1) powders in ethanol. Microcrystalline cellulose powder (Sigma Aldrich, CAS Number 9004-34-6) was added as a pore former. Epoxy glue was used to cover the outer surface of the hollow fiber to avoid its contamination. Hydrochloric acid (HCl) (Sigma Aldrich, concentration 10 g L-1) was used for cleaning the hollow fiber prior to nickel plating. Palladium(II) chloride (PdCl2) (Alfa Aesar, concentration 0.4 g L-1) and hydrochloric acid (HCl) (Sigma Aldrich, concentration 3.3 g L-1) were used for the catalytic step in the nickel electroless bath. Nickel(II) acetate (Alfa Aeasar, concentration 40 g L-1), sodium hydroxide (NaOH) (Alfa Aesar, concentration 7.5 g L-1), ethylenediaminetetraacetic acid (EDTA) (Alfa Aesar, concentration 4 g L-1), and hydrazine (N2H4) (Alfa Aesar, concentration 20 g L-1) were used for the plating step. More detailed information about the nickel electroless deposition is reported elsewhere.13 La0.8Sr0.2MnO3 powder (LSM) (Nextech Materials Ltd., surface area 5.0 m2 g-1), YSZ powder (Daiichi Kigenso, Japan, 8 mol % yttrium, 25-35 nm mean particle size), and ethylene glycol (Fluka, purity 98.0%) were used for cathode preparation. 2.2. Fabrication of Highly Asymmetric YSZ Hollow Fibers. NMP, ethanol, PEG-400, and dispersant were mixed together with YSZ powder (0.3 µm) in a planetary ball miller (MTI Corporation model SFM-1 desktop planetary ball miller) for 24 h. PESf was then added to the mixture prior to a further mixing for 24-48 h, forming a uniform particle suspension. The suspension was fully degassed in a vacuum chamber with stirring, transferred into a 200 mL stainless steel syringe, and extruded through a tube-in-orifice spinneret (orifice diameter/ inner diameter of 3.0/1.2 mm) into an external coagulant bath containing tap water. The internal coagulant was a mixture of 90 wt % NMP balanced with deionized water. The extrusion rate of the spinning suspension and the flow rate of the internal coagulant were controlled by Harvard syringe pumps (Harvard Aparatus, model PHD 2000). The air gap was set to zero during the spinning (the spinneret was immersed in the external coagulation bath). To complete the solidification, the hollow fiber precursors were kept in a water bath for at least 24 h. Table 1 summarizes the composition of the spinning suspension and spinning conditions used in the fabrication of the YSZ
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Table 1. Compounds and Quantities Used for Fabrication of Highly Asymmetric YSZ Hollow Fibers compound
quantity (wt %)
YSZ powder NMP ethanol PEG-400 dispersant PESf internal coagulant external coagulant spinneret i.d/o.d air gap
65.0% 18.38% 7.00% 3.00% 0.12% 6.50% 90%/10% NMP/DI water tap water 1.2 mm/3.0 mm 0 mm
Table 2. Ni Electroless Plating Recipe (based on N2H4) step
compound
concentration
conditions
cleaning catalysis
HCl PdCl2 HCl Ni(II) acetate NaOH EDTA N2H4
10 g L-1 0.4 g L-1 3.3 g L-1 40 g L-1 7.5 g L-1 4 g L-1 20 g L-1
20 °C, 5 min 20 °C, 1 min
plating
85 °C pH 8-9
hollow fibers. The hollow fiber precursors were heated in a tubular furnace (Elite, Model TSH 17/75/450) at 600 °C for 2 h to remove the organic polymer binder and then sintered at 1450 °C for 4 h in air. 2.3. Fabrication of microtubular SOFC. Sintered, highly asymmetric YSZ hollow fibers were cut to a length of approximately 5 cm and weighed. An epoxy glue layer was applied to protect the outer surface from contamination. The hollow fiber samples were then soaked under vacuum in an ethanol solution in which NiO and YSZ (10-20 nm particle size) were uniformly dispersed for 20 min. The ratio between NiO and YSZ was set to such a value that after NiO to Ni reduction, the Ni/YSZ volume ratio was 60/40. After soaking, the samples were cleaned, dried, and weighed. This procedure was repeated five times to deposit the NiO-YSZ mixture into the finger-like voids. In order to improve the continuity of the anode, a NiO-YSZ layer was wash-coated onto the lumen surface of the YSZ fibers by immersing samples in a NiO-YSZ ethanol suspension followed by drying at 150 °C for 1 h. After 7 washcoating cycles, the samples were sintered at 1300 °C for 4 h in air. The wash-coating was repeated an additional 10 times to improve the continuity of the layer followed by sintering at 1300 °C for 4 h in air to complete this stage of the anode deposition process. A Ni layer was then coated onto the deposited wash-coated layer by electroless plating. Sensitization and catalysis steps were carried out by dipping the samples into a beaker containing the sensitization or catalyzing solution, while pure Ni was coated using a plating solution based on hydrazine (N2H4) as the reducing agent. Electroless plating bath compositions and conditions are summarized in Table 2. The LSM/YSZ cathode was slurry-coated onto the outer surface of the hollow fiber. The LSM/YSZ weight ratio was set to 50/50, while the concentration of ethylene glycol in the slurry was set to 50 wt %. After coating, the cathode was sintered at 1000 °C for 6 h in air. The active length of the cathode was 1 cm. 2.4. Characterization. The mechanical strength of the prepared hollow fibers was measured by the three-point bending test using an Instron model 5544 tensile tester with a load cell of 1 kN. The 30 mm long, highly asymmetric YSZ hollow fibers sintered 1450 °C were fixed onto the sample holder. The
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Figure 1. Schematic of the SOFC setup.
measured force at the breaking point was later used to calculate the bending strength (σF) from the following equation8 σF )
8FLD π(D4 - di4)
where F is the measured force at breakup and L, D, and di are the length (30 mm) and outer and inner diameters of the fiber, respectively. Gas-tightness of the hollow fiber was measured using a method developed by Tan et. al,13 while hollow fiber porosity was measured by mercury porosimetry using a Micrometrics Auto Pore IV (Micromeritics Instrument Corporation). Scanning electron microscopy (JEOL, JSM-5610 LV or JSM-840A models) was used to investigate the morphology of highly asymmetric YSZ hollow fibers and the electrodes. 2.5. Performance Test of the Microtubular SOFC. The SOFC module (cathode/electrolyte/anode and current collectors) based on the highly asymmetric YSZ hollow fiber was inserted into a silica tube (210 mm long, 22 mm external diameter) closed with two ceramic end-caps (Precision Ceramics from Macor machinable glass ceramic) with three through-holes to accommodate the gas inlet-outlets and thermocouple sleeve. A schematic of the SOFC module is shown in Figure 1. The central holes were connected to the highly asymmetric YSZ hollow fiber through an alumina tube where the fuel was fed (lumen side). At each side, one of the external holes was used for the oxidant flow inlet/outlet (shell side), while the remaining holes were used for the insertion of the thermocouple and for the cathodic current collector path. The anodic current collector was drawn out from a hole near the fuel inlet. Ag wires (Sigma Aldrich, 0.25 mm diameter, purity 99.9%) were used as the electrode current collector, and contact with the electrode was secured with Pt ink (Metalor). The alumina tubes of the lumen side had a mechanical external support that also assures straight positioning of the fiber. Internal components were sealed with the high-temperature resistant cement (Aremco Products ceramabond powder and activator 571). The SOFC module based on a single highly asymmetric YSZ hollow fiber was placed inside of the bore of a tube furnace (Vecstar model VCTF2) and heated to the operating temperature of 800 °C with heating/cooling rates of 2 °C min-1. Fuel (5% H2, 95% Ar) and oxidant (air) were both supplied at 30 mL (STP) min-1. Mass flow controllers (Hastings Instruments model 400) were used to set the desired flow rate. Anodic and cathodic current collectors were connected to a potentiostat (Amel model 7050) for the electrical measurements. Impedance spectroscopy was used to measure the ohmic resistance of the fuel cell and current collector terminals. The current collector on the anodic side was in the form of a coiled wire inside of the lumen and was inserted after anode deposition. 3. Results and Discussion Figure 2a-c shows the cross section and the inner and outer surfaces of the hollow fiber precursor spun under the conditions presented in Table 1. From a micrograph of the cross section of a typical hollow fiber (Figure 2a), two distinct domains are
Figure 2. SEM secondary electron (SE) images of the (a) cross section, (b) inner surface, and (c) outer surface of a highly asymmetric YSZ hollow fiber precursor.
visible, a thin outer part exhibiting a sponge-like structure and the rest of the fiber exhibiting extended finger-like structures. The mechanism of formation of this structure can be explained according to Kingsbury and Li,14 who suggested that the creation of the finger-like structures initiates from the instabilities at the interface between the suspension and the coagulant. The created finger-like structures are then formed as a result of solvent/ nonsolvent exchange and corresponding precipitation of the invertible polymer binder. The use of the internal coagulant made of 90 wt % NMP and 10 wt % water contributes to the formation of the fingers, which penetrate through the hollow fiber, creating macropores on the inner surface. Figure 3a-d shows the cross section and the inner and outer and the dense electrolyte layer of the hollow fiber sintered at 1450 °C for 4 h in air. As a result of sintering, the top spongelike layer developed into a separation layer, while the finger-
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Figure 3. SEM SE images of the (a) cross section, (b) inner surface, (c) outer surface, and (d) dense electrolyte layer (at higher magnification) of a highly asymmetric YSZ hollow fiber sintered in air at 1450 °C for 4 h.
Figure 4. Result of a mercury porosimetry test of a highly asymmetric YSZ hollow fiber sintered at 1450 °C for 4 h.
Figure 5. Gas-tightness test of the highly asymmetric YSZ hollow fiber sintered at 1450 °C for 4 h in air.
like macrovoids (as seen in Figure 3a and d), although smaller in size (as a result of shrinkage during sintering), retained their shape created during phase inversion. Figure 4 shows the result of the mercury porosimetry test carried out on the highly asymmetric YSZ hollow fibers sintered at 1450 °C for 4 h. Two domains of pores are visible, one related to finger-like macrovoids (pore size entrance around 10 µm) and the second related to smaller pores of size between 0.1 and 1 µm. The microstructure of the highly asymmetric YSZ hollow fiber exhibits several features suited for electrolyte-supported SOFCs. As suggested by Yin et al.,12 the presence of a densified, thin separation layer on the top of a fiber and extended fingerlike structures might be ideal for various membrane-related processes including SOFC. The highly asymmetric YSZ hollow fibers described in this work have an approximate thickness of 210 µm. Moreover, hollow fibers were observed to have the
high breaking points during the three-point bending test; an average value of 8.53 ((1.84) N was recorded. The measured average mechanical strength for hollow fibers sintered at 1450 °C was calculated to be 226 ((49) MPa. This result is comparable to data reported by Wei and Li,8 whose fibers had similar wall thickness and diameter. However, Wei and Li’s asymmetric YSZ hollow fiber microstructure was divided into two parts, a dense separation part and a porous part with a relatively high density of macrovoids. In this work, the density of the macrovoids is lower, and the microstructure between them appears to be fully densified, which resulted in good mechanical properties for SOFC application. Figure 5 shows typical gas-tight properties of a highly asymmetric YSZ hollow fiber, where the pressure in the test module is plotted against time. The slight pressure decrease is probably a sum of possible leakages from joints in the system
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Table 3. Mass Change during the NiO-YSZ Particle Coating Procedurea weight increase (wt %)
cycle number
0 1.93 ( 0.17 7.79 ( 0.42 6.77 ( 0.34 15.84 ( 0.36 13.55 ( 0.27
0 5 12 13 23 24
additional information state before coating after first sintering after second sintering
a
Cycles 1-5: Ethanol particle solution soaking in vacuum. Cycles 6-12: first series of wash-coat depositions. Cycle 13: first sintering. Cycle 14-23: second series of wash-coat depositions. Cycle 24: second sintering.
Figure 7. Composite SEM SE image of macrovoid filled with NiO and YSZ particles from the coating procedure and oxidized nickel from electroless plating. Smaller NiO particles from Ni plating are visible deeper within the macrovoid. Deposition of nanoparticles and electroless plating were done from the left side of this image (open end of the macrovoid).
Figure 6. (a) Composite SEM SE image of a macrovoid filled with NiO and YSZ nanoparticles. NiO and YSZ nanoparticles were deposited from the left side of the image (open end of macrovoid). (b) Composite Ni X-ray (KR) map (400s collection time) of a macrovoid filled with NiO and YSZ nanoparticles from Figure 6a. Regions containing Ni are false colored a green color. NiO and YSZ nanoparticles were deposited from the left side of the picture (open end of the macrovoid).
and gas permeation through the hollow fiber. The measured value, 2.39 × 10-8 mol m-2 Pa-1 s-1, can be considered acceptable for SOFC application, although it is slightly lower than that in previously reported research on hollow fibers prepared by the phase inversion/sintering process by Wei and Li.8 However in these cases, the dense separation layer was much thicker in comparison to the thin, dense layer of the fabricated highly asymmetric YSZ hollow fibers. 3.2. Deposition of the Composite Anode. Table 3 lists the weight gain of highly asymmetric YSZ hollow fibers during NiO and YSZ particle deposition. The maximum recorded weight increases were significantly over 10% and were achieved after two different stages of coating cycles. The purpose of the first stage was to impregnate the macrovoids with anode particles. This stage was achieved by soaking the hollow fibers in an ethanol suspension of particles with vacuum suction achieved by the use of vacuum pump. The purpose of the second stage was to create a continuous anode layer on the lumen surface of the hollow fibers. This stage was achieved by a washcoating method by drying at 150 °C. Figure 6a and b shows a typical finger-like structure impregnated with NiO and YSZ particles. Some degree of penetration (with infiltration of about 80-90 µm from the open end of the macrovoid) is clearly visible. However, with increasing distance from the open end of the macrovoid, NiO particles become less and less interconnected. At the surface of the anode, it was observed that the continuous NiO-YSZ layer developed cracks, probably due to shrinkage of particles during sintering, and revealed bare areas of the lumen surface. This lack of uniformity led to poor axial electrical conductivity of the anode.
Several groups have investigated the shrinkage properties of YSZ and NiO particles.15,16 The conclusion from their research is that presintering of particles can reduce total shrinkage. Moreover, presintering of particles can also reduce the difference between NiO and YSZ shrinkage behavior and total shrinkage. On the basis of these reports, it may be possible to use presintered NiO and YSZ particles, which would exhibit reduced shrinkage during sintering at the target temperature for the anode. However, a dedicated study would be required in order to choose the optimum presintering temperature and presintering time for the NiO and YSZ mixture of particles in order to obtain shrinkage parameters such as rate and total volume change of both NiO and YSZ. In order to improve Ni continuity and electrical conductivity of the anode, Ni electroless plating was carried out on the NiO-YSZ surface. After plating, the resultant structure was cosintered in air at 1000 °C (cathode sintering), which oxidized deposited Ni into NiO. Figure 7 shows the image of a macrovoid filled with NiO particles from both NiO-YSZ coating and oxidized Ni from the electroless bath. In this case, fine NiO particles (produced by oxidation during cathode sintering) were observed deeper inside of the macrovoids. Finally, after cathode sintering, a reduction at 800 °C was performed. The resultant anode conductivity was 3 × 104 S m-1 (considering the whole anode thickness of 150 µm). Inside of the finger-like structures, two types of Ni particles are visible, very fine Ni particles from the electroless plating step and larger particles from the NiO-YSZ particle coating, as shown in Figure 8. Fine Ni particles were found again deeper inside of the macrovoids but were not well-connected to each other. This suggests that the increased inner surface area of the highly asymmetric hollow fiber was not fully used, and with a more efficient technique, the impregnation could be improved. At the anode surface, as shown in Figure 9, all of the area seems to be covered by Ni from the electroless plating procedure, which significantly reduced the total length of triphase boundaries and open porosity. Moreover, the relatively dense anode top layer would very likely limit the flow of reaction gases to and from triphase boundaries located deeper inside of the anode (e.g., in the macrovoids). 3.3. Fuel Cell Tests. The cross section of the full SOFC based on a highly asymmetric YSZ hollow fiber is presented in Figure 10. The anode can be seen on the inner lumen of the hollow fiber, and the cathode can be seen on the outer surface
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Figure 8. SEM SE images and Ni X-ray (KR) maps (600s collection time) of a macrovoid pore filled with YSZ and reduced Ni. Ni regions are false colored green. Deposition of nanoparticles and electroless plating were made from the left side of this image (open end of the macrovoid).
Figure 10. SEM SE cross section image of partially assembled SOFC. The Ni anode is located on the inner side of the fiber and the LSM-YSZ cathode is deposited on the outer side of the fiber.
Figure 9. (a) SEM SE image of the anode surface after final reduction. Thin cracks are likely a result of sample preparation (breaking of ceramic fiber in order to expose the inner surface). (b) SEM image of the anode surface after final reduction. Thin cracks are likely a result of sample preparation (breaking of the ceramic fiber in order to expose the inner surface).
of the hollow fiber. Figure 11 presents the recorded performance of the solid oxide fuel cell as a function of the current density.
A maximum power density of 18 mW cm-2 was obtained at 800 °C with an OCV of 0.93 V, close to the theoretical value of 0.96 V (for a 5% H2, 95% Ar fuel mixture). The thin, dense electrolyte YSZ layer of 20 µm should theoretically result in an ohmic resistance at 800 °C of 0.1 Ω cm2, whereas a much higher value of 6.7 Ω cm2 was measured by impedance spectroscopy (at OCV conditions). As the ohmic resistance of the electrodes can be considered negligible, the contact resistance between the anode and anodic current collector is believed to be the responsible of such high ohmic resistance. The data from the measured OCV, ohmic resistance, and power density values suggest that the low performance of the assembled SOFC is mainly a result of an unoptimized anode microstructure, that is, discontinuity of the electrode, low porosity of the top layer, and reduced penetration of particles into the macrovoids. 4. Conclusions Highly asymmetric YSZ hollow fibers for electrolyte support were successfully fabricated, characterized, and tested in a solid
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would like to thank Nicolas Droushiotis for his help at the early stages of experiments. Literature Cited
Figure 11. Performance of a SOFC based on a highly asymmetric YSZ hollow fiber showing voltage and power density as a function of current density.
oxide fuel cell. Results from bending strength, gas-tightness, and mercury porosimetry tests as well as SEM photomicrographs indicate that this design can be considered suitable for robust, electrolyte-supported solid oxide fuel cell systems. On the other hand, highly asymmetric YSZ hollow fibers might have several disadvantages associated with the deposition process of the internal electrode. For example, NiO-YSZ particle impregnation requires numerous deposition cycles and two sintering procedures, which may prove to be an ineffective and uneconomical method when more than a few cells must be assembled. In addition, even if problems currently associated with anode deposition are solved, the resulting performance and conductivity of that electrode might be still lower than in the case of anode-support designs due to lower thickness, higher porosity, and limited control of microstructure. Anode deposition represents the next challenging step to be optimized for the fabrication of high-performance microtubular electrolyte-supported SOFCs. Acknowledgment The authors thank the U.K. Engineering and Physical Science Research Council for Grants EP/E00136X and EP/E00220X providing project studentships for K.K. and F.D.G. The authors
(1) Yamamoto, O. Solid oxide fuel cells: fundamental aspects and prospects. Electrochim. Acta 2000, 45, 2423–2435. (2) Droushiotis, N.; et al. Characterization of NiO-yttria stabilised zirconia (YSZ) hollow fibers for use as SOFC anodes. Solid State Ionics 2009, 180, 1091–1099. (3) Campana, R.; et al. Ni-YSZ cermet micro-tubes with textured surface. J. Eur. Ceram. Soc. 2009, 29, 85–90. (4) Du, Y.; Sammes, N. M. Fabrication and properties of anodesupported tubular solid oxide fuel cells. J. Power Sources 2004, 136, 66– 71. (5) Suzuki, T.; et al. Fabrication and characterization of micro tubular SOFCs for operation in the intermediate temperature. J. Power Sources 2006, 160, 73–77. (6) Liu, Y.; et al. Fabrication and characterization of micro-tubular cathode-supported SOFC for intermediate temperature operation. J. Power Sources 2007, 174, 95–102. (7) Dong, D.; et al. Improvement of cathode-electrolyte interfaces of tubular solid oxide fuel cells by fabricating dense YSZ electrolyte membranes with indented surfaces. J. Power Sources 2008, 175, 201– 205. (8) Wei, C. C.; Li, K. Yttria-stabilized zirconia (YSZ)-based hollow fiber solid oxide fuel cells. Ind. Eng. Chem. Res. 2008, 47, 1506–1512. (9) Jin, C.; et al. Electrochemical properties analysis of tubular NiOYSZ anode-supported SOFCs fabricated by the phase-inversion method. J. Membr. Sci. 2009, 341, 233–237. (10) Singhal, S. C.; Kendall, K. High Temperature Solid Oxide Fuel Cells: fundamentals, design and applications; Elsevier: New York, 2003. (11) Kilbride, I. P. Preparation and properties of small diameter tubular solid oxide fuel cells for rapid start-up. Journal of Power Sources. 1996, 61, 167–171. (12) Yin, W.; et al. Highly asymmetric yttria stabilized zirconia hollow fibre membranes. J. Alloys Compd. 2009, 476, 566–570. (13) Grande, F. D.; et al. Microstructure and performance of novel Ni anode for hollow fibre solid oxide fuel cells. Solid State Ionics 2009, 180, 800–804. (14) Kingsbury, B. F. K.; Li, K. A morphological study of ceramic hollow fibre membranes. J. Membr. Sci. 2009, 328, 134-140. (15) Bao, W.; Chang, Q.; Meng, G. Effect of NiO/YSZ compositions on the co-sintering process of anode-supported fuel cell. J. Membr. Sci. 2005, 259, 103–109. (16) Matsushima, T.; Ohrui, H.; Hirai, T. Effects of sinterability of YSZ powder and NiO content on characteristics of Ni-YSZ cermets. Solid State Ionics 1998, 111, 315–321.
ReceiVed for reView February 2, 2010 ReVised manuscript receiVed April 19, 2010 Accepted April 28, 2010 IE1002558
