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Application of PVD methods to solid oxide fuel cells
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Application of PVD methods to solid oxide fuel cells A.A. Solovyev a,∗ , N.S. Sochugov b , S.V. Rabotkin b , A.V. Shipilova b , I.V. Ionov b , A.N. Kovalchuk a , A.O. Borduleva a a b
Tomsk Polytechnic University, 30 Lenin Avenue, Tomsk 634050, Russia Institute of High Current Electronics, Siberian Branch of the Russian Academy of Sciences, 2/3 Akademichesky Avenue, Tomsk 634055, Russia
a r t i c l e
i n f o
Article history: Received 10 December 2013 Received in revised form 17 March 2014 Accepted 22 March 2014 Available online xxx Keywords: Solid oxide fuel cell Metal support Magnetron sputtering Thin film YSZ electrolyte NiO/YSZ anode
a b s t r a c t In this paper, attention is paid to the application of such a method of vacuum physical vapor deposition (PVD) as magnetron sputtering for fabrication of a solid oxide fuel cell (SOFC) materials and structures. It is shown that the YSZ (yttria-stabilized zirconia) electrolyte and Ni–YSZ anode layers with required thickness, structure and composition can be effectively formed by PVD methods. The influence of parameters of pulsed power magnetron discharge on the deposition rate and the microstructure of the obtained YSZ electrolyte films were investigated. It is shown that the deposition rate of the oxide layers by magnetron sputtering can be significantly increased by using asymmetric bipolar power magnetrons, which creates serious prerequisites for applying this method on the industrial scale. Porous Ni–YSZ anode films were obtained by reactive co-sputtering of Ni and Zr–Y targets and subsequent reduction in the H2 atmosphere at a temperature of 800 ◦ C. The Ni–YSZ films comprised small grains and pores of tens of nanometers. © 2014 Elsevier B.V. All rights reserved.
1. Introduction A solid oxide fuel cell is an electrochemical device which converts free energy of a chemical reaction directly into electrical energy [1]. Therefore, the device has a very high (60%) electrical efficiency. A solid oxide fuel cell is a high temperature fuel cell and its range of operating temperature is 700–1000 ◦ C. A single SOFC comprises an electrolyte, a porous anode, and a porous cathode, which are made of solid ceramic materials. Air and hydrogen are supplied to cathode and anode sides of a fuel cell, correspondingly, and chemical reaction starts in the fuel cell. Oxygen ions from the cathode side move through an ion conducting electrolyte to the anode side, where they combine with hydrogen to produce electricity, water, and heat. Yttria-stabilized zirconia (YSZ) is the most commonly used material for a solid electrolyte, while lanthanum strontium manganite (LSM) and Ni–YSZ are conventional cathode and anode materials, respectively. The conventional techniques of SOFC manufacturing are powder methods comprising a raw material preparation (powder production and preparation, sizing), fabrication of fuel cell layers by tape casting, slip casting, pressing, extrusion, screen printing, dip coating, spraying, etc. The final step is drying and high-temperature sintering of fabricated layers. In general, these techniques enable
∗ Corresponding author. Tel.: +7 3822 491 651; fax: +7 3822 491 651. E-mail addresses: [email protected], [email protected] (A.A. Solovyev).
fabrication of only thick layers (tens of microns). Therefore, new techniques allowing fabrication of thinner layers (1–10 m) are needed. Despite being extensively used to modify surface of materials, plasma-beam technologies are not so much used in SOFC manufacturing because of the high cost of the equipment and relatively low deposition rates. Despite these challenges, vacuum methods offer a number of unique advantages. Very thin dense layers can be deposited on either porous or dense substrates, which can enable higher power densities. The films can be fabricated at temperatures much lower than required in the traditional ceramic processing. That will avoid the unwanted interfacial reactions, such as zirconia and lanthanum manganite to form a resistive pyrochlore phase. Vacuum techniques are also well-suited for fabrication of interlayers, where small grain sizes and thin layers are required. The unique aspect of vacuum deposition is the ability to fabricate structures that are not otherwise achievable [2]. There are many examples of the use of sputtering [3–5], laser ablation [6,7] and electron beam evaporation [8] to prepare thin, fully dense YSZ electrolyte layers on porous and dense substrates, to produce a variety of air and fuel electrode compositions, electrical interconnects, and corrosion-resistant barriers. Application of vacuum deposition techniques to solid oxide fuel cells was discussed in a recent review [2]. Magnetron sputtering (MS) is a form of physical vapor deposition (PVD) that is widely applied to deposit a variety of coatings, many of which are of industrial importance. Interestingly, W.R.
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Grove, who is considered to be the inventor of the fuel cell, in 1852, first described the phenomenon known today as sputtering. There are numerous examples of using magnetron sputtering to fabricate components for SOFC due to the versatility of this technique as well as the ability to control composition and morphology. Most often, the aim was to prepare thin, dense electrolyte layers on porous supports. This approach reduces the internal resistance of the fuel cell and the operating temperatures, thus, giving a greater flexibility in the choice of materials for a stack construction [9,10]. However, a significant drawback of the method of magnetron sputtering is low deposition rates of the electrolyte layers. It is considered that relatively low deposition rates (<5 m/h) limit the applicability of magnetron sputtering to a large-scale fuel cell manufacturing [11]. Deposition rates of less than 3 m/h were reported for reactive DC sputtered YSZ films [2], about 3 m/h for reactive pulsed DC sputtered YSZ (at a power of 1.5 kW) [12] and 0.50–0.75 m/h for RF sputtered YSZ films (at a power of 300 W) [13]. Magnetron sputtering is also used for fabrication of electrode layers. Sputter-deposited electrodes include lanthanum manganite [14], lanthanum cobaltite [15], silver [16], platinum [17], and nickel [18], as well as composites of the above materials with zirconia. Electrode performance depends critically on its microstructure. Sputtering methods are good for controlling the length of the triple phase boundary (gas–electrode–electrolyte). The purpose of this paper is to show the new opportunities of magnetron sputtering concerning SOFC technology, such as an increase in deposition rate of YSZ electrolyte and formation of nanostructured anode.
2. Experimental details Substrates used for ZrO2 :Y2 O3 film deposition were reduced anodes composed of 60 vol.% Ni and 40 vol.% Zr0.9 Y0.1 O1.95 , made of a tape produced by ESL ElectroScience, USA. The anode substrates were about 0.5 mm thick and 20 mm in diameter. Thin films of YSZ were deposited by reactive magnetron sputtering of a composite Zr0.86 Y0.14 cathode with a diameter of 100 mm. Sputtering was carried out in the oxygen–argon atmosphere at a working pressure of 0.2 Pa. Power supplies produced by Applied Electronics. Ltd. (Russia) were used for direct current (DC), unipolar pulsed and bipolar pulsed sputtering at a frequency of 20–100 kHz. The average discharge power was 1.5–2 kW. The distance between the magnetron target and the substrate was about 80 mm. Prior to the film deposition the substrates were heated up to 550–600 ◦ C. Anode supported fuel cells were manufactured by coating and drying of a La0.7 Sr0.3 MnO3 paste (Fuelcellmaterials, USA) at a temperature of 100 ◦ C. Fuel cell testing was carried out on the device ProboStatTM (NorECs, Norway) according to the scheme and the research methodology described in our previous work [19]. NiO–YSZ coatings were deposited by reactive magnetron cosputtering of two cathodes, which were placed at the angle of 45◦ to the plane of the substrate. Both cathodes were 75 mm in diameter and located at a distance of 90 mm from the substrate-holder. The Zr0.86 Y0.14 cathode was pulsed sputtered at a power of 1250 W and a frequency of 100 kHz. Ni (99.995% pure) cathode was DC sputtered. At a constant discharge power applied to the Zr0.86 Y0.14 cathode the power applied to the Ni cathode was varied in the range of 300–700 W to control the Ni content in the deposited film. The coatings were deposited on YSZ electrolyte substrate being 200 m thick and 20 mm in diameter, made of a tape produced by ESL ElectroScience. They were used for fabrication of electrolyte-supported fuel cells. Several samples of coating were reduced to Ni/YSZ at 800 ◦ C for 2 h in the H2 atmosphere. The argon flow rate was maintained constant (50 ml/min) and the oxygen flow varied from 3 to 40 ml/min to deposit stoichiometric coatings.
For comparison testing two SOFCs with YSZ electrolyte and various anodes were fabricated. In fuel cell 1 NiO/YSZ anode 5 m in thickness, produced by magnetron sputtering, was used. Fuel cell 2 had a pasted NiO/YSZ anode produced by applying the anode paste (50% NiO and 50% YSZ) (ESL ElectroScience), followed by sintering at 1350 ◦ C. The cathode of the fuel cell was formed by coating the back side of the YSZ substrate with La0.7 Sr0.3 MnO3 paste (Fuelcellmaterials, USA) and drying it at a temperature of 100 ◦ C. The structure of the deposited layers was investigated by the scanning electron microscopes JEOL JSM-7500FA and Hitachi TM3000 (Japan). X-ray structure analysis of the films was carried out with the diffractometer XRD 7000S (Shimadzu, Japan), where CuK␣ radiating was used. Films thickness was measured by the interferometer MII-4.
3. Results and discussion 3.1. Magnetron deposition of the YSZ electrolyte The microstructure of the YSZ coatings deposited by reactive DC and pulsed magnetron sputtering (at a frequency of 50 kHz) on Ni/YSZ anode substrates is shown in Fig. 1a and b. It can be interpreted in terms of the structure zone model of Thornton [20]. The model describes three different zones and a transition zone. The higher the ratio between a substrate temperature during the deposition and a melting point of the material is, the denser the layer is. In the present study, the ratio is low due to the high melting point of YSZ (about 2780 ◦ C). The maximum substrate temperature during the deposition process is limited to 600 ◦ C by the equipment parameters. Thus, the microstructure of the YSZ layers belongs to the transition zone and, therefore, the microstructure exhibits densely packed columnar grains. The surface diffusion of adatoms is insufficient to overcome the effects of shadowing resulting from the pores and growing grains. The same microstructure of as-deposited YSZ films was observed by Wang et al. [21]. It was shown that the sputtered YSZ films could be considerably densified by a high-temperature sintering in the air. It was suggested that an optimized sintering temperature for the sputtered YSZ films should be around 1250 ◦ C. At that temperature the columnar structures merged together and became totally indistinguishable. Nédélec et al. [22] used an RF bias of a substrate with a power varied from 0 to 0.5 W cm−2 to suppress the columnar structure of an YSZ coating. It was shown that the use of the bias allows changing the layer growth morphology from columnar to a denser structure. In particular, it was demonstrated that the gas-tight electrolyte layers could be deposited by PVD on anode substrates without any prior surface treatment or coating. Annealing in air sometimes could not be used to improve electrolyte microstructure. Thus, for example, it cannot be used for metal-supported fuel cell fabrication because of irreversible oxidation of a metal support. In our previous work [12] another method of improvement of the YSZ coating microstructure was shown. Magnetron sputtering combined with a pulsed electronbeam treatment (EBT) led to the formation of dense YSZ films with a fine microstructure. Fig. 1c shows a SEM image of the anode substrate with a thin YSZ sublayer about 1.3 m thick after electron beam treatment. It can be seen that the columnar structure of the YSZ film becomes blurred. It was shown that 4–5 m thick films deposited by magnetron sputtering combined with a pulsed EBT had twice much higher gas permeability than twice thicker films fabricated without EBT on porous supports. Hence, the problem of fabrication of thin YSZ electrolyte layers on porous supports with a controlled microstructure could be solved in one or another way, but there still is a problem of the low
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Fig. 1. The SEM pictures of the cross-sections of the sputtered YSZ electrolyte films deposited by reactive magnetron sputtering on the Ni/YSZ anode substrate in different modes: (a) DC MS, (b) unipolar pulsed MS (50 kHz), (c) unipolar pulsed MS (50 kHz) combined with electron beam treatment, (d) bipolar pulsed MS (80 kHz).
deposition rate by PVD methods. A way to increase the deposition rate of dielectric films from metallic targets is the elimination of target poisoning. Poisoning is the build-up of insulating layers on the target surface. Asymmetric bipolar pulsed MS is probably an optimal solution of the target poisoning problem because it sets up conditions which cause the insulators on the target to be sputtered first and with a higher sputter yield than the base material (a mechanism called preferential sputtering) [23]. Thus, the advantage of bipolar pulsed MS is not only an effective arc control during reactive magnetron sputtering leading to a high deposition stability, but also an increase in the deposition rate due to the “preferential sputtering” mode. As the charge is built up on the target during a negative voltage pulse, the ions are actually repelled by electrostatic repulsion of the Ar+ ions and the positive capacitor voltage. When the polarity is rapidly reversed to about +100 V the plasma-facing surface of the dielectric film is charged up to the opposite polarity (−100 V). As the reverse pulse ends and the voltage returns to the sputter mode (−400 V) the effective voltage on the plasma side of the target reaches −500 V. Thus, the argon ions are drawn by electrostatic attraction to the insulators and strike with an extra energy (500 V versus 400 V), thereby sputtering the insulators off the target first, eliminating target poisoning [23]. In this study, we investigated the deposition rate and the properties of YSZ films fabricated by pulsed unipolar and asymmetric bipolar pulsed MS. The films were obtained at a pressure of Ar 0.2 Pa. Such oxygen flow rate was chosen so that the system was in the hysteresis transition region but close to the metallic mode yielding the highest deposition rate while obtaining stoichiometric films. In order to run the system in the transition region between the poisoned and the metallic state of the targets, the cathode voltage was stabilized by the oxygen flow rate.
Pulsed unipolar sputtering (Fig. 2a) was at a frequency of 50 kHz [24]. Bipolar sputtering was carried out at 80 kHz with reverse pulses of 4 s and the value of 15% of the nominal negative sputtering voltage (Fig. 2b). We measured the current–voltage characteristics of the magnetron discharge and the discharge voltage depending on the oxygen flow rate using the unipolar and bipolar power supply (Figs. 3 and 4). This correlation allows judging about the processes occurring in the reactive discharge and properties of the deposited coatings. The current–voltage characteristics (CVC) of the discharge in argon have an ordinary shape common to an abnormal glow discharge: the discharge current increases monotonically with increasing voltage. However, for bipolar power the steepness of CVC is significantly lower which simplifies controlling discharge at a high power level. At low currents in reactive discharge the target surface is covered with a dielectric film and therefore the CVC vary linearly. However, after the transition from the oxide mode to metallic the CVC increases linearly as in pure argon. In general, when the bipolar power supply is used the influence of the oxygen flow rate on the CVC shape is less pronounced than in the case of pulsed power. The correlation of the discharge voltage on the oxygen flow rate measured at a stabilized discharge power of 1 and 2 kW (Fig. 4) have a common shape with a pronounced transition mode separating the metal mode from the oxide mode. The dependence was not obtained in the unipolar mode at a pulsed power of 2 kW because of an intensive arcing at the cathode. Only a slight dependence can be seen on the configuration of the power pulses. At low gas flow rates by addition of oxygen while the target surface is not covered with oxide the voltage increases slightly (by 5–10 V). Then, at higher oxygen flows the voltage drops to the values typical of the oxide mode. The same behaviour of the discharge voltage with a zirconium
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Fig. 5. X-ray diffraction pattern of the magnetron sputtered YSZ films: 1 – unipolar pulsed MS (50 kHz) combined with electron beam treatment, 2 – bipolar pulsed MS (80 kHz).
Fig. 2. The discharge current and voltage waveforms at: (a) unipolar pulsed (50 kHz), (b) bipolar pulsed (80 kHz) reactive magnetron sputtering of ZtY target.
Fig. 3. The CVC of the magnetron discharge with unipolar (dashed lines) and bipolar (solid graphs) power supply in the Ar and Ar + O2 atmosphere at the Ar flow of 180 sccm and O2 flow of (sccm): 1–0; 2–30; 3–45; 4–60.
Fig. 4. The dependence of the discharge voltage from the oxygen flow: 1 – unipolar (P = 0.8 kW), 2 – bipolar (P = 1 kW) and 3 – bipolar (P = 2 kW) sputtering of ZrY target.
target was observed in [25]. With increasing discharge power the dependence is shifted to higher voltages and gas flow rates. The maximal deposition rate of stoichiometric YSZ films in a pulsed bipolar mode was 12 m/h, which is several times higher than those achieved by DC MS or unipolar pulsed MS. The deposition rate was about 70% of the film deposition rate in pure argon at the same power (17 m/h). It is attributed to higher effective compound erosion rate in the bipolar pulsed MS than in dc and pulsed MS, for the same average target power. Reactive bipolar pulsed magnetron sputtering of YSZ allows deposition of transparent films at lower target coverage (the chemisorption of reactive gas species on the target surface). In addition to that, a lower deposition rate in the unipolar pulsed MS is explained by frequent tripping of the arc protection system and the temporary removal of the voltage from the magnetron cathode, which, in turn, accelerates the process of target poisoning. YSZ films fabricated by the unipolar or bipolar sputtering were studied and compared by X-ray diffractometry and electron microscopy. The objective was to determine if an increase in the film growth rate affects negatively the microstructure and the phase composition of the films. Fig. 5 shows the X-ray diffraction patterns of the YSZ film deposited by the unipolar pulsed MS on a porous anode substrate with a YSZ sublayer treated by an electron beam and a YSZ film deposited by bipolar pulsed MS on porous anode substrate. The Miller indices and peak positions intensities for the 8 mol% Y2 O3 -doped ZrO2 powder reference sample (ICDD powder diffraction database entry # 030-1468) with a cubic fluorite structure are included for comparison. In all cases, the cubic YSZ film phase and metallic Ni were generally detected. No peaks of monoclinic, tetragonal or amorphous phases were detected. It should be noted that films deposited with a bipolar powered magnetron have narrower peaks on the diffractograms indicating greater grain size. The considerably lower density of grain boundaries per volume unit in the coarse grained microstructure affects the physical properties of the films in a favourable fashion for solid electrolytes. In particular, the electrical resistivity is lowered due to the blocking nature of grain boundaries in YSZ, while the resistance to chemical degradation by interdiffusion, which occurs usually along the grain boundaries, increases [26]. The microstructure of the YSZ coatings deposited by the bipolar pulsed MS on the Ni/YSZ anode substrate is shown in Fig. 1d. Despite the fact that the columnar structure cannot be completely eliminated by the use of a bipolar power supply, the resulting film has a relatively dense structure and a good adhesion to the substrate. That fact, coupled with a greater deposition rate and increased grain size, suggests that it is preferable to use the asymmetric bipolar power supply of the magnetron in the industrial scale technology.
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Current–voltage and power characteristics of the fuel cell with YSZ electrolyte, deposited by the bipolar pulsed MS are shown in Fig. 6. 1–1.06 V open circuit voltage was achieved at 600–800 ◦ C, which is close to the theoretically possible in the air (1.08 V). This indicates good impermeability of the electrolyte, which correlates with the image of its structure obtained by scanning electron microscopy. Power density generated by the fuel cell reaches 360 mW/cm2 at 800 ◦ C and 0.7 V. 3.2. Magnetron deposition of the NiO/YSZ anode NiO/YSZ films with a required composition (Ni 40–50 wt.%) were fabricated at a discharge power of 1250 W and 700 W of ZrY and Ni magnetron, correspondingly. The argon and oxygen flow rate were 50 and 15–22 ml/min, respectively. The film deposition rate in this mode was 12.5 m/h, which is higher than that of films fabricated by RF sputtering (0.12–0.25 m/h) [27] and sputtering of a Ni/Zr/Y alloy cathode (4 m/h) [28]. Fig. 7 shows a secondary electron image of the surface of a Ni/YSZ film consisting of 42 vol.% Ni after reduction in hydrogen (for 2 h at T = 800◦ C). It is a nanoporous film with a grain size of 50 nm. Pore formation in a Ni/YSZ film mainly occurs because of the reduction of the volume taken by Ni during NiO reduction in hydrogen. Molar volumes of NiO and Ni calculated from their density and molar/atomic mass differs by 40% and have a value of 10.96 and 6.587 cm3 /mol, correspondingly. The voids formed during the
Fig. 8. Diffraction pattern of the Ni/YSZ film, reduced in the hydrogen atmosphere at T = 800 ◦ C (2 h). The standard Ni and YSZ peaks positions have been shown by the dashed lines. The film has been obtained at Ni magnetron at the power of 630 W and 15 ml/min oxygen flow.
reduction of NiO form a nanoporous structure mainly on the border of the solid electrolyte due to the increased triple-phase boundary on the anode side. It is possible to adjust the porosity of Ni–YSZ films, including thickness, by altering the ratio of NiO and YSZ in the coating during the deposition. Fig. 8 shows a typical diffractogram of the 23.5-m-thick Ni/YSZ film reduced in hydrogen. The film has a polycrystalline structure consisting of a Ni phase (55.3 wt.%) represented mainly by the peaks of Ni (1 1 1) and (2 0 0) and of the cubic phase of YSZ (44.7 wt.%). 2 values of the phases coincide with nominal angles corresponding to the reflections from the bulk materials. The average size of the coherent scattering for YSZ calculated by the Scherrer formula was 18 nm. Nanocomposite structure of Ni–YSZ films is prerequisite to increase the area of three-phase boundary at the surface of the electrolyte. However, a thin film anode may be too thin to provide sufficient electrical conductivity. Thick nanoporous Ni–YSZ film can have high conductivity, but at the same time, may cause loss of concentration in the fuel cell due to elongation of a narrow gas transportation path. Therefore, while carrying out comparison tests, a further collector layer of Ni paste about 100 m thick was applied to the anode of fuel cell 1, deposited by magnetron sputtering. Fig. 9 shows a comparison of the current–voltage and power characteristics of fuel cells 1 and 2, different from each other only in the formation of Ni–YSZ anode. The maximum power density of fuel cells number 1 and 2 at 800 ◦ C reaches 210 and 90 mW/cm2 ,
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Fig. 7. Secondary electron image of the Ni/YSZ film surface after reduction in the hydrogen atmosphere (2 h at T = 800 ◦ C). Ni containing in the coating is about 42 vol.%. Scanning electron microscopy.
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Fig. 6. Current voltage and power characteristics of fuel cell with YSZ electrolyte, deposited by the bipolar pulsed magnetron sputtered at temperatures of 600, 700 and 800 ◦ C.
Fig. 9. Current–voltage and power characteristics of YSZ electrolyte-supported fuel cells and NiO/YSZ anode deposited by magnetron sputtering (1) and pasted NiO/YSZ anode (2). The measurements were performed at a temperature of 800 ◦ C, a hydrogen flow rate of 110 ml/min and an air flow of 300 ml/min.
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respectively. Thus, the fuel cell with the nanocomposite anode formed by magnetron sputtering is more than twice as efficient as a traditional cell with an anode made of a mixture of sub-micron thickness, with subsequent high-temperature sintering. This is not only due to the three-phase boundary area increase and higher electrocatalytic activity of Ni nanoparticles, but also due to decrease in the total internal resistance of fuel cells, which is largely due to the resistance of the electrode/electrolyte boundary and the contact between the anode and current collectors. 4. Conclusion Both the data in literature and the experimental results presented in this paper show that magnetron sputtering can be used for deposition of dense YSZ electrolyte films, with a thickness of several microns, on a porous support. Depending on the deposition and post-treatment parameters the film microstructure can be controlled from a porous one, consisting of canonical crystallites separated by interstices, to a structure with densely packed grains. It was shown that the deposition rate of magnetron sputtered oxide layers (<5 m/h at RF, DC or unipolar pulsed sputtering) can be increased up to 12 m/h using the asymmetric bipolar power supply. In this paper reactive deposition rate of 70% of the metallic deposition rate was achieved using the asymmetric bipolar pulsed power supply. There are two reasons for the high deposition rate and, namely, a lower target coverage during the deposition and the elimination of the arcing process on the sputtered target. Porous nanocomposite Ni–YSZ anode layers with the structure and composition corresponding to modern requirements to SOFC electrodes can also be formed by reactive magnetron sputtering. Comparison testing of electrolyte-supported fuel cells demonstrated that when NiO–YSZ anode formed by magnetron sputtering was used, power density reached 210 mW/cm2 at 800 ◦ C, which is twice higher than the values obtained using a NiO–YSZ anode formed by the traditional method of high-temperature sintering. Most likely, this is due to the more advanced three-phase boundary in the nanocomposite anode. The developed techniques can be used to fabricate SOFC anode and electrolyte layers. Acknowledgements The authors thank the Nano-Center TPU for carrying out SEM and XRD analysis and gratefully acknowledge the Russian Foundation for Basic Research (grant no. 12-08-31164) for financial support. This research was carried out within the State assignment of the Institute of High Current Electronics SB RAS. References [1] J. Larminie, A. Dicks, Fuel Cell Systems Explained, 2nd ed., Wiley, 2003. [2] L.R. Pederson, P. Singh, X.-D. Zhou, Application of vacuum deposition methods to solid oxide fuel cells, Vacuum 80 (2006) 1066–1083. [3] S. Mahieu, P. Ghekiere, G. De Winter, D. Depla, R. De Gryse, O.I. Lebedev, G. Van Tendeloo, Influence of the Ar/O2 ratio on the growth and biaxial alignment of yttria stabilized zirconia layers during reactive unbalanced magnetron sputtering, Thin Solid Films 484 (2005) 18–25.
[4] A. Nagata, H. Okayama, Characterization of solid oxide fuel cell device having a three-layer film structure grown by RF magnetron sputtering, Vacuum 66 (2002) 523–529. [5] E. Wanzenberg, F. Tietz, P. Panjan, D. Stöver, Influence of pre- and post-heat treatment of anode substrates on the properties of DC-sputtered YSZ electrolyte films, Solid State Ionics 159 (2003) 1–8. [6] H. Hidalgo, E. Reguzina, E. Millon, A.-L. Thomann, J. Mathias, C. BoulmerLeborgne, T. Sauvage, P. Brault, Yttria-stabilized zirconia thin films deposited by pulsed-laser deposition and magnetron sputtering, Surf. Coat. Technol. 205 (2011) 4495–4499. [7] F. Kokai, K. Amano, H. Ota, F. Umemura, XeCl laser ablation of yttria stabilized zirconia, Appl. Phys. A – Mater. Sci. Process. 54 (1992) 340–342. [8] S. Uhlenbruck, N. Jordan, J.M. Serra, H.P. Buchkremer, D. Stöve, Application of electrolyte layers for solid oxide fuel cells by electron beam evaporation, Solid State Ionics 181 (2010) 447–452. [9] J. Will, A. Mitterdorfer, C. Kleinlogel, D. Perednis, L.J. Gauckler, Fabrication of thin electrolytes for second-generation solid oxide fuel cells, Solid State Ionics 131 (2000) 79–96. [10] L.C. De Jonghe, C.P. Jacobson, S.J. Visco, Supported electrolyte thin film synthesis of solid oxide fuel cells, Annu. Rev. Mater. Res. 33 (2003) 169–182. [11] A.O. Isenberg, Cost Reduction in Fabrication Processes for SOFC Cell Components, European Fuel Cell Forum 2, Lucern, Switzerland, 2000, pp. 745. [12] N.S. Sochugov, A.A. Soloviev, A.V. Shipilova, S.V. Rabotkin, V.P. Rotshtein, I.T. Sigfusson, The effect of pulsed electron beam pretreatment of magnetron sputtered ZrO2 :Y2 O3 films on the performance of IT-SOFC, Solid State Ionics 231 (2013) 11–17. [13] A.L. Shaula, J.C. Oliveira, V.A. Kolotygin, V.V. Kharton, A.A. Cavaleiro, Sputtered YSZ based protective thin films for SOFCs, Surf. Eng. 26 (8) (2010) 584–589. [14] L.S. Wang, S.A. Barnett, Ag-perovskite cermets for thin film solid oxide fuel cell air-electrode applications, Solid State Ionics 76 (1995) 103–113. [15] A. Ringuede, Fouletier, Oxygen reaction on strontium-doped lanthanum cobaltite dense electrodes at intermediate temperatures, J. Solid State Ionics 139 (2001) 167–177. [16] L.S. Wang, E.S. Thiele, S.A. Barnett, Sputter deposition of yttria-stabilized zirconia and silver cermet electrodes for SOFC applications, Solid State Ionics 52 (1992) 261–267. [17] N. Nakagawa, H. Yoshioka, C. Kuroda, M. Ishida, Electrode performance of a thin-film YSZ cell set on a porous ceramic substrate by rf sputtering technique, Solid State Ionics 35 (1989) 249–255. [18] G.J.O. La, J. Hertz, H. Tuller, Y. Shao-Horn, Microstructural features of RFsputtered SOFC anode and electrolyte materials, J. Electroceram. 13 (2004) 691–695. [19] A.A. Solov’ev, N.S. Sochugov, A.V. Shipilova, K.B. Efimova, A.E. Tumashevskaya, Mid-temperature solid oxide fuel cells with thin film ZrO2 :Y2 O3 electrolyte, Russ. J. Electrochem. 47 (2011) 494–502. [20] J.A. Thornton, Influence of apparatus geometry and deposition conditions on the structure and topography of thick sputtered coatings, J. Vacuum Sci. Technol. 11 (4) (1974) 666–670. [21] H. Wang, W. Ji, L. Zhang, Y. Gong, B. Xie, Y. Jiang, Y. Song, Preparation of YSZ films by magnetron sputtering for anode-supported SOFC, Solid State Ionics 192 (1) (2011) 413–418. [22] R. Nédélec, S. Uhlenbruck, D. Sebold, V.A.C. Haanappel, H.-P. Buchkremer, D. Stöver, Dense yttria-stabilised zirconia electrolyte layers for SOFC by reactive magnetron sputtering, J. Power Sources 205 (2012) 157–163. [23] J. Sellers, Asymmetric bipolar pulsed DC: the enabling technology for reactive PVD, Surf. Coat. Technol. 98 (1998) 1245–1250. [24] N.S. Sochugov, V.O. Oskirko, R.E. Spirin, A power supply for magnetron sputtering systems, Instrum. Exp. Tech. 56 (2) (2013) 178–184. [25] K. Sarakinos, J. Alami, C. Klever, M. Wuttig, Process stabilization and enhancement of deposition rate during reactive high power pulsed magnetron sputtering of zirconium oxide, Surf. Coat. Technol. 202 (20) (2008) 5033–5035. [26] S. Heiroth, R. Frison, J.L.M. Rupp, T. Lippert, E.J.B. Meier, E.M. Gubler, M. Döbeli, K. Conder, A. Wokaun, L.J. Gauckler, Crystallization and grain growth characteristics of yttria-stabilized zirconia thin films grown by pulsed laser deposition, Solid State Ionics 191 (2011) 12–23. [27] H. Jou Sh Wu Tz, Thin porous Ni–YSZ films as anodes for a solid oxide fuel cell, J. Phys. Chem. Solids 69 (2008) 2804–2812. [28] E. Rezugina, A.L. Thomann, H. Hidalgo, P. Brault, V. Dolique, Y. Tessier, Ni–YSZ films deposited by reactive magnetron sputtering for SOFC applications, Surf. Coat. Technol. 204 (2010) 2376–2380.
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Application of PVD methods to solid oxide fuel cells A.A. Solovyev a,∗ , N.S. Sochugov b , S.V. Rabotkin b , A.V. Shipilova b , I.V. Ionov b , A.N. Kovalchuk a , A.O. Borduleva a a b
Tomsk Polytechnic University, 30 Lenin Avenue, Tomsk 634050, Russia Institute of High Current Electronics, Siberian Branch of the Russian Academy of Sciences, 2/3 Akademichesky Avenue, Tomsk 634055, Russia
a r t i c l e
i n f o
Article history: Received 10 December 2013 Received in revised form 17 March 2014 Accepted 22 March 2014 Available online xxx Keywords: Solid oxide fuel cell Metal support Magnetron sputtering Thin film YSZ electrolyte NiO/YSZ anode
a b s t r a c t In this paper, attention is paid to the application of such a method of vacuum physical vapor deposition (PVD) as magnetron sputtering for fabrication of a solid oxide fuel cell (SOFC) materials and structures. It is shown that the YSZ (yttria-stabilized zirconia) electrolyte and Ni–YSZ anode layers with required thickness, structure and composition can be effectively formed by PVD methods. The influence of parameters of pulsed power magnetron discharge on the deposition rate and the microstructure of the obtained YSZ electrolyte films were investigated. It is shown that the deposition rate of the oxide layers by magnetron sputtering can be significantly increased by using asymmetric bipolar power magnetrons, which creates serious prerequisites for applying this method on the industrial scale. Porous Ni–YSZ anode films were obtained by reactive co-sputtering of Ni and Zr–Y targets and subsequent reduction in the H2 atmosphere at a temperature of 800 ◦ C. The Ni–YSZ films comprised small grains and pores of tens of nanometers. © 2014 Elsevier B.V. All rights reserved.
1. Introduction A solid oxide fuel cell is an electrochemical device which converts free energy of a chemical reaction directly into electrical energy [1]. Therefore, the device has a very high (60%) electrical efficiency. A solid oxide fuel cell is a high temperature fuel cell and its range of operating temperature is 700–1000 ◦ C. A single SOFC comprises an electrolyte, a porous anode, and a porous cathode, which are made of solid ceramic materials. Air and hydrogen are supplied to cathode and anode sides of a fuel cell, correspondingly, and chemical reaction starts in the fuel cell. Oxygen ions from the cathode side move through an ion conducting electrolyte to the anode side, where they combine with hydrogen to produce electricity, water, and heat. Yttria-stabilized zirconia (YSZ) is the most commonly used material for a solid electrolyte, while lanthanum strontium manganite (LSM) and Ni–YSZ are conventional cathode and anode materials, respectively. The conventional techniques of SOFC manufacturing are powder methods comprising a raw material preparation (powder production and preparation, sizing), fabrication of fuel cell layers by tape casting, slip casting, pressing, extrusion, screen printing, dip coating, spraying, etc. The final step is drying and high-temperature sintering of fabricated layers. In general, these techniques enable
∗ Corresponding author. Tel.: +7 3822 491 651; fax: +7 3822 491 651. E-mail addresses: [email protected], [email protected] (A.A. Solovyev).
fabrication of only thick layers (tens of microns). Therefore, new techniques allowing fabrication of thinner layers (1–10 m) are needed. Despite being extensively used to modify surface of materials, plasma-beam technologies are not so much used in SOFC manufacturing because of the high cost of the equipment and relatively low deposition rates. Despite these challenges, vacuum methods offer a number of unique advantages. Very thin dense layers can be deposited on either porous or dense substrates, which can enable higher power densities. The films can be fabricated at temperatures much lower than required in the traditional ceramic processing. That will avoid the unwanted interfacial reactions, such as zirconia and lanthanum manganite to form a resistive pyrochlore phase. Vacuum techniques are also well-suited for fabrication of interlayers, where small grain sizes and thin layers are required. The unique aspect of vacuum deposition is the ability to fabricate structures that are not otherwise achievable [2]. There are many examples of the use of sputtering [3–5], laser ablation [6,7] and electron beam evaporation [8] to prepare thin, fully dense YSZ electrolyte layers on porous and dense substrates, to produce a variety of air and fuel electrode compositions, electrical interconnects, and corrosion-resistant barriers. Application of vacuum deposition techniques to solid oxide fuel cells was discussed in a recent review [2]. Magnetron sputtering (MS) is a form of physical vapor deposition (PVD) that is widely applied to deposit a variety of coatings, many of which are of industrial importance. Interestingly, W.R.
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Grove, who is considered to be the inventor of the fuel cell, in 1852, first described the phenomenon known today as sputtering. There are numerous examples of using magnetron sputtering to fabricate components for SOFC due to the versatility of this technique as well as the ability to control composition and morphology. Most often, the aim was to prepare thin, dense electrolyte layers on porous supports. This approach reduces the internal resistance of the fuel cell and the operating temperatures, thus, giving a greater flexibility in the choice of materials for a stack construction [9,10]. However, a significant drawback of the method of magnetron sputtering is low deposition rates of the electrolyte layers. It is considered that relatively low deposition rates (<5 m/h) limit the applicability of magnetron sputtering to a large-scale fuel cell manufacturing [11]. Deposition rates of less than 3 m/h were reported for reactive DC sputtered YSZ films [2], about 3 m/h for reactive pulsed DC sputtered YSZ (at a power of 1.5 kW) [12] and 0.50–0.75 m/h for RF sputtered YSZ films (at a power of 300 W) [13]. Magnetron sputtering is also used for fabrication of electrode layers. Sputter-deposited electrodes include lanthanum manganite [14], lanthanum cobaltite [15], silver [16], platinum [17], and nickel [18], as well as composites of the above materials with zirconia. Electrode performance depends critically on its microstructure. Sputtering methods are good for controlling the length of the triple phase boundary (gas–electrode–electrolyte). The purpose of this paper is to show the new opportunities of magnetron sputtering concerning SOFC technology, such as an increase in deposition rate of YSZ electrolyte and formation of nanostructured anode.
2. Experimental details Substrates used for ZrO2 :Y2 O3 film deposition were reduced anodes composed of 60 vol.% Ni and 40 vol.% Zr0.9 Y0.1 O1.95 , made of a tape produced by ESL ElectroScience, USA. The anode substrates were about 0.5 mm thick and 20 mm in diameter. Thin films of YSZ were deposited by reactive magnetron sputtering of a composite Zr0.86 Y0.14 cathode with a diameter of 100 mm. Sputtering was carried out in the oxygen–argon atmosphere at a working pressure of 0.2 Pa. Power supplies produced by Applied Electronics. Ltd. (Russia) were used for direct current (DC), unipolar pulsed and bipolar pulsed sputtering at a frequency of 20–100 kHz. The average discharge power was 1.5–2 kW. The distance between the magnetron target and the substrate was about 80 mm. Prior to the film deposition the substrates were heated up to 550–600 ◦ C. Anode supported fuel cells were manufactured by coating and drying of a La0.7 Sr0.3 MnO3 paste (Fuelcellmaterials, USA) at a temperature of 100 ◦ C. Fuel cell testing was carried out on the device ProboStatTM (NorECs, Norway) according to the scheme and the research methodology described in our previous work [19]. NiO–YSZ coatings were deposited by reactive magnetron cosputtering of two cathodes, which were placed at the angle of 45◦ to the plane of the substrate. Both cathodes were 75 mm in diameter and located at a distance of 90 mm from the substrate-holder. The Zr0.86 Y0.14 cathode was pulsed sputtered at a power of 1250 W and a frequency of 100 kHz. Ni (99.995% pure) cathode was DC sputtered. At a constant discharge power applied to the Zr0.86 Y0.14 cathode the power applied to the Ni cathode was varied in the range of 300–700 W to control the Ni content in the deposited film. The coatings were deposited on YSZ electrolyte substrate being 200 m thick and 20 mm in diameter, made of a tape produced by ESL ElectroScience. They were used for fabrication of electrolyte-supported fuel cells. Several samples of coating were reduced to Ni/YSZ at 800 ◦ C for 2 h in the H2 atmosphere. The argon flow rate was maintained constant (50 ml/min) and the oxygen flow varied from 3 to 40 ml/min to deposit stoichiometric coatings.
For comparison testing two SOFCs with YSZ electrolyte and various anodes were fabricated. In fuel cell 1 NiO/YSZ anode 5 m in thickness, produced by magnetron sputtering, was used. Fuel cell 2 had a pasted NiO/YSZ anode produced by applying the anode paste (50% NiO and 50% YSZ) (ESL ElectroScience), followed by sintering at 1350 ◦ C. The cathode of the fuel cell was formed by coating the back side of the YSZ substrate with La0.7 Sr0.3 MnO3 paste (Fuelcellmaterials, USA) and drying it at a temperature of 100 ◦ C. The structure of the deposited layers was investigated by the scanning electron microscopes JEOL JSM-7500FA and Hitachi TM3000 (Japan). X-ray structure analysis of the films was carried out with the diffractometer XRD 7000S (Shimadzu, Japan), where CuK␣ radiating was used. Films thickness was measured by the interferometer MII-4.
3. Results and discussion 3.1. Magnetron deposition of the YSZ electrolyte The microstructure of the YSZ coatings deposited by reactive DC and pulsed magnetron sputtering (at a frequency of 50 kHz) on Ni/YSZ anode substrates is shown in Fig. 1a and b. It can be interpreted in terms of the structure zone model of Thornton [20]. The model describes three different zones and a transition zone. The higher the ratio between a substrate temperature during the deposition and a melting point of the material is, the denser the layer is. In the present study, the ratio is low due to the high melting point of YSZ (about 2780 ◦ C). The maximum substrate temperature during the deposition process is limited to 600 ◦ C by the equipment parameters. Thus, the microstructure of the YSZ layers belongs to the transition zone and, therefore, the microstructure exhibits densely packed columnar grains. The surface diffusion of adatoms is insufficient to overcome the effects of shadowing resulting from the pores and growing grains. The same microstructure of as-deposited YSZ films was observed by Wang et al. [21]. It was shown that the sputtered YSZ films could be considerably densified by a high-temperature sintering in the air. It was suggested that an optimized sintering temperature for the sputtered YSZ films should be around 1250 ◦ C. At that temperature the columnar structures merged together and became totally indistinguishable. Nédélec et al. [22] used an RF bias of a substrate with a power varied from 0 to 0.5 W cm−2 to suppress the columnar structure of an YSZ coating. It was shown that the use of the bias allows changing the layer growth morphology from columnar to a denser structure. In particular, it was demonstrated that the gas-tight electrolyte layers could be deposited by PVD on anode substrates without any prior surface treatment or coating. Annealing in air sometimes could not be used to improve electrolyte microstructure. Thus, for example, it cannot be used for metal-supported fuel cell fabrication because of irreversible oxidation of a metal support. In our previous work [12] another method of improvement of the YSZ coating microstructure was shown. Magnetron sputtering combined with a pulsed electronbeam treatment (EBT) led to the formation of dense YSZ films with a fine microstructure. Fig. 1c shows a SEM image of the anode substrate with a thin YSZ sublayer about 1.3 m thick after electron beam treatment. It can be seen that the columnar structure of the YSZ film becomes blurred. It was shown that 4–5 m thick films deposited by magnetron sputtering combined with a pulsed EBT had twice much higher gas permeability than twice thicker films fabricated without EBT on porous supports. Hence, the problem of fabrication of thin YSZ electrolyte layers on porous supports with a controlled microstructure could be solved in one or another way, but there still is a problem of the low
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Fig. 1. The SEM pictures of the cross-sections of the sputtered YSZ electrolyte films deposited by reactive magnetron sputtering on the Ni/YSZ anode substrate in different modes: (a) DC MS, (b) unipolar pulsed MS (50 kHz), (c) unipolar pulsed MS (50 kHz) combined with electron beam treatment, (d) bipolar pulsed MS (80 kHz).
deposition rate by PVD methods. A way to increase the deposition rate of dielectric films from metallic targets is the elimination of target poisoning. Poisoning is the build-up of insulating layers on the target surface. Asymmetric bipolar pulsed MS is probably an optimal solution of the target poisoning problem because it sets up conditions which cause the insulators on the target to be sputtered first and with a higher sputter yield than the base material (a mechanism called preferential sputtering) [23]. Thus, the advantage of bipolar pulsed MS is not only an effective arc control during reactive magnetron sputtering leading to a high deposition stability, but also an increase in the deposition rate due to the “preferential sputtering” mode. As the charge is built up on the target during a negative voltage pulse, the ions are actually repelled by electrostatic repulsion of the Ar+ ions and the positive capacitor voltage. When the polarity is rapidly reversed to about +100 V the plasma-facing surface of the dielectric film is charged up to the opposite polarity (−100 V). As the reverse pulse ends and the voltage returns to the sputter mode (−400 V) the effective voltage on the plasma side of the target reaches −500 V. Thus, the argon ions are drawn by electrostatic attraction to the insulators and strike with an extra energy (500 V versus 400 V), thereby sputtering the insulators off the target first, eliminating target poisoning [23]. In this study, we investigated the deposition rate and the properties of YSZ films fabricated by pulsed unipolar and asymmetric bipolar pulsed MS. The films were obtained at a pressure of Ar 0.2 Pa. Such oxygen flow rate was chosen so that the system was in the hysteresis transition region but close to the metallic mode yielding the highest deposition rate while obtaining stoichiometric films. In order to run the system in the transition region between the poisoned and the metallic state of the targets, the cathode voltage was stabilized by the oxygen flow rate.
Pulsed unipolar sputtering (Fig. 2a) was at a frequency of 50 kHz [24]. Bipolar sputtering was carried out at 80 kHz with reverse pulses of 4 s and the value of 15% of the nominal negative sputtering voltage (Fig. 2b). We measured the current–voltage characteristics of the magnetron discharge and the discharge voltage depending on the oxygen flow rate using the unipolar and bipolar power supply (Figs. 3 and 4). This correlation allows judging about the processes occurring in the reactive discharge and properties of the deposited coatings. The current–voltage characteristics (CVC) of the discharge in argon have an ordinary shape common to an abnormal glow discharge: the discharge current increases monotonically with increasing voltage. However, for bipolar power the steepness of CVC is significantly lower which simplifies controlling discharge at a high power level. At low currents in reactive discharge the target surface is covered with a dielectric film and therefore the CVC vary linearly. However, after the transition from the oxide mode to metallic the CVC increases linearly as in pure argon. In general, when the bipolar power supply is used the influence of the oxygen flow rate on the CVC shape is less pronounced than in the case of pulsed power. The correlation of the discharge voltage on the oxygen flow rate measured at a stabilized discharge power of 1 and 2 kW (Fig. 4) have a common shape with a pronounced transition mode separating the metal mode from the oxide mode. The dependence was not obtained in the unipolar mode at a pulsed power of 2 kW because of an intensive arcing at the cathode. Only a slight dependence can be seen on the configuration of the power pulses. At low gas flow rates by addition of oxygen while the target surface is not covered with oxide the voltage increases slightly (by 5–10 V). Then, at higher oxygen flows the voltage drops to the values typical of the oxide mode. The same behaviour of the discharge voltage with a zirconium
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Fig. 5. X-ray diffraction pattern of the magnetron sputtered YSZ films: 1 – unipolar pulsed MS (50 kHz) combined with electron beam treatment, 2 – bipolar pulsed MS (80 kHz).
Fig. 2. The discharge current and voltage waveforms at: (a) unipolar pulsed (50 kHz), (b) bipolar pulsed (80 kHz) reactive magnetron sputtering of ZtY target.
Fig. 3. The CVC of the magnetron discharge with unipolar (dashed lines) and bipolar (solid graphs) power supply in the Ar and Ar + O2 atmosphere at the Ar flow of 180 sccm and O2 flow of (sccm): 1–0; 2–30; 3–45; 4–60.
Fig. 4. The dependence of the discharge voltage from the oxygen flow: 1 – unipolar (P = 0.8 kW), 2 – bipolar (P = 1 kW) and 3 – bipolar (P = 2 kW) sputtering of ZrY target.
target was observed in [25]. With increasing discharge power the dependence is shifted to higher voltages and gas flow rates. The maximal deposition rate of stoichiometric YSZ films in a pulsed bipolar mode was 12 m/h, which is several times higher than those achieved by DC MS or unipolar pulsed MS. The deposition rate was about 70% of the film deposition rate in pure argon at the same power (17 m/h). It is attributed to higher effective compound erosion rate in the bipolar pulsed MS than in dc and pulsed MS, for the same average target power. Reactive bipolar pulsed magnetron sputtering of YSZ allows deposition of transparent films at lower target coverage (the chemisorption of reactive gas species on the target surface). In addition to that, a lower deposition rate in the unipolar pulsed MS is explained by frequent tripping of the arc protection system and the temporary removal of the voltage from the magnetron cathode, which, in turn, accelerates the process of target poisoning. YSZ films fabricated by the unipolar or bipolar sputtering were studied and compared by X-ray diffractometry and electron microscopy. The objective was to determine if an increase in the film growth rate affects negatively the microstructure and the phase composition of the films. Fig. 5 shows the X-ray diffraction patterns of the YSZ film deposited by the unipolar pulsed MS on a porous anode substrate with a YSZ sublayer treated by an electron beam and a YSZ film deposited by bipolar pulsed MS on porous anode substrate. The Miller indices and peak positions intensities for the 8 mol% Y2 O3 -doped ZrO2 powder reference sample (ICDD powder diffraction database entry # 030-1468) with a cubic fluorite structure are included for comparison. In all cases, the cubic YSZ film phase and metallic Ni were generally detected. No peaks of monoclinic, tetragonal or amorphous phases were detected. It should be noted that films deposited with a bipolar powered magnetron have narrower peaks on the diffractograms indicating greater grain size. The considerably lower density of grain boundaries per volume unit in the coarse grained microstructure affects the physical properties of the films in a favourable fashion for solid electrolytes. In particular, the electrical resistivity is lowered due to the blocking nature of grain boundaries in YSZ, while the resistance to chemical degradation by interdiffusion, which occurs usually along the grain boundaries, increases [26]. The microstructure of the YSZ coatings deposited by the bipolar pulsed MS on the Ni/YSZ anode substrate is shown in Fig. 1d. Despite the fact that the columnar structure cannot be completely eliminated by the use of a bipolar power supply, the resulting film has a relatively dense structure and a good adhesion to the substrate. That fact, coupled with a greater deposition rate and increased grain size, suggests that it is preferable to use the asymmetric bipolar power supply of the magnetron in the industrial scale technology.
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Current–voltage and power characteristics of the fuel cell with YSZ electrolyte, deposited by the bipolar pulsed MS are shown in Fig. 6. 1–1.06 V open circuit voltage was achieved at 600–800 ◦ C, which is close to the theoretically possible in the air (1.08 V). This indicates good impermeability of the electrolyte, which correlates with the image of its structure obtained by scanning electron microscopy. Power density generated by the fuel cell reaches 360 mW/cm2 at 800 ◦ C and 0.7 V. 3.2. Magnetron deposition of the NiO/YSZ anode NiO/YSZ films with a required composition (Ni 40–50 wt.%) were fabricated at a discharge power of 1250 W and 700 W of ZrY and Ni magnetron, correspondingly. The argon and oxygen flow rate were 50 and 15–22 ml/min, respectively. The film deposition rate in this mode was 12.5 m/h, which is higher than that of films fabricated by RF sputtering (0.12–0.25 m/h) [27] and sputtering of a Ni/Zr/Y alloy cathode (4 m/h) [28]. Fig. 7 shows a secondary electron image of the surface of a Ni/YSZ film consisting of 42 vol.% Ni after reduction in hydrogen (for 2 h at T = 800◦ C). It is a nanoporous film with a grain size of 50 nm. Pore formation in a Ni/YSZ film mainly occurs because of the reduction of the volume taken by Ni during NiO reduction in hydrogen. Molar volumes of NiO and Ni calculated from their density and molar/atomic mass differs by 40% and have a value of 10.96 and 6.587 cm3 /mol, correspondingly. The voids formed during the
Fig. 8. Diffraction pattern of the Ni/YSZ film, reduced in the hydrogen atmosphere at T = 800 ◦ C (2 h). The standard Ni and YSZ peaks positions have been shown by the dashed lines. The film has been obtained at Ni magnetron at the power of 630 W and 15 ml/min oxygen flow.
reduction of NiO form a nanoporous structure mainly on the border of the solid electrolyte due to the increased triple-phase boundary on the anode side. It is possible to adjust the porosity of Ni–YSZ films, including thickness, by altering the ratio of NiO and YSZ in the coating during the deposition. Fig. 8 shows a typical diffractogram of the 23.5-m-thick Ni/YSZ film reduced in hydrogen. The film has a polycrystalline structure consisting of a Ni phase (55.3 wt.%) represented mainly by the peaks of Ni (1 1 1) and (2 0 0) and of the cubic phase of YSZ (44.7 wt.%). 2 values of the phases coincide with nominal angles corresponding to the reflections from the bulk materials. The average size of the coherent scattering for YSZ calculated by the Scherrer formula was 18 nm. Nanocomposite structure of Ni–YSZ films is prerequisite to increase the area of three-phase boundary at the surface of the electrolyte. However, a thin film anode may be too thin to provide sufficient electrical conductivity. Thick nanoporous Ni–YSZ film can have high conductivity, but at the same time, may cause loss of concentration in the fuel cell due to elongation of a narrow gas transportation path. Therefore, while carrying out comparison tests, a further collector layer of Ni paste about 100 m thick was applied to the anode of fuel cell 1, deposited by magnetron sputtering. Fig. 9 shows a comparison of the current–voltage and power characteristics of fuel cells 1 and 2, different from each other only in the formation of Ni–YSZ anode. The maximum power density of fuel cells number 1 and 2 at 800 ◦ C reaches 210 and 90 mW/cm2 ,
250
Voltage (V)
200 1
2
150 100 50 0
Fig. 7. Secondary electron image of the Ni/YSZ film surface after reduction in the hydrogen atmosphere (2 h at T = 800 ◦ C). Ni containing in the coating is about 42 vol.%. Scanning electron microscopy.
Power density (mW/cm 2)
Fig. 6. Current voltage and power characteristics of fuel cell with YSZ electrolyte, deposited by the bipolar pulsed magnetron sputtered at temperatures of 600, 700 and 800 ◦ C.
Fig. 9. Current–voltage and power characteristics of YSZ electrolyte-supported fuel cells and NiO/YSZ anode deposited by magnetron sputtering (1) and pasted NiO/YSZ anode (2). The measurements were performed at a temperature of 800 ◦ C, a hydrogen flow rate of 110 ml/min and an air flow of 300 ml/min.
Please cite this article in press as: A.A. Solovyev, et al., Application of PVD methods to solid oxide fuel cells, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.03.163
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respectively. Thus, the fuel cell with the nanocomposite anode formed by magnetron sputtering is more than twice as efficient as a traditional cell with an anode made of a mixture of sub-micron thickness, with subsequent high-temperature sintering. This is not only due to the three-phase boundary area increase and higher electrocatalytic activity of Ni nanoparticles, but also due to decrease in the total internal resistance of fuel cells, which is largely due to the resistance of the electrode/electrolyte boundary and the contact between the anode and current collectors. 4. Conclusion Both the data in literature and the experimental results presented in this paper show that magnetron sputtering can be used for deposition of dense YSZ electrolyte films, with a thickness of several microns, on a porous support. Depending on the deposition and post-treatment parameters the film microstructure can be controlled from a porous one, consisting of canonical crystallites separated by interstices, to a structure with densely packed grains. It was shown that the deposition rate of magnetron sputtered oxide layers (<5 m/h at RF, DC or unipolar pulsed sputtering) can be increased up to 12 m/h using the asymmetric bipolar power supply. In this paper reactive deposition rate of 70% of the metallic deposition rate was achieved using the asymmetric bipolar pulsed power supply. There are two reasons for the high deposition rate and, namely, a lower target coverage during the deposition and the elimination of the arcing process on the sputtered target. Porous nanocomposite Ni–YSZ anode layers with the structure and composition corresponding to modern requirements to SOFC electrodes can also be formed by reactive magnetron sputtering. Comparison testing of electrolyte-supported fuel cells demonstrated that when NiO–YSZ anode formed by magnetron sputtering was used, power density reached 210 mW/cm2 at 800 ◦ C, which is twice higher than the values obtained using a NiO–YSZ anode formed by the traditional method of high-temperature sintering. Most likely, this is due to the more advanced three-phase boundary in the nanocomposite anode. The developed techniques can be used to fabricate SOFC anode and electrolyte layers. Acknowledgements The authors thank the Nano-Center TPU for carrying out SEM and XRD analysis and gratefully acknowledge the Russian Foundation for Basic Research (grant no. 12-08-31164) for financial support. This research was carried out within the State assignment of the Institute of High Current Electronics SB RAS. References [1] J. Larminie, A. Dicks, Fuel Cell Systems Explained, 2nd ed., Wiley, 2003. [2] L.R. Pederson, P. Singh, X.-D. Zhou, Application of vacuum deposition methods to solid oxide fuel cells, Vacuum 80 (2006) 1066–1083. [3] S. Mahieu, P. Ghekiere, G. De Winter, D. Depla, R. De Gryse, O.I. Lebedev, G. Van Tendeloo, Influence of the Ar/O2 ratio on the growth and biaxial alignment of yttria stabilized zirconia layers during reactive unbalanced magnetron sputtering, Thin Solid Films 484 (2005) 18–25.
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Please cite this article in press as: A.A. Solovyev, et al., Application of PVD methods to solid oxide fuel cells, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.03.163