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CHEMICAL PROBLEMS 2019 no. 2 (17) ISSN 2221-8688
UDC 544.65
MAGNETRON DEPOSITION OF ANODE FUNCTIONAL LAYERS FOR SOLID
OXIDE FUEL CELLS
A.A. Solovyev1, I.V. Ionov1, A.V. Shipilova1, V.A. Semenov1, E.A. Smolyanskiy2
institute of High Current Electronics SB RAS, 2/3 Akademichesky Ave., Tomsk, 634055, Russia, e-mail: andrewsol@mail.ru
2Tomsk Polytechnic University, 30 Lenina Ave., Tomsk, 634050, Russia Received 26.05.2019
Abstract: The proposed review is dedicated to the latest achievements in the field of magnetron deposition of thin-film anode functional layers (AFLs)for solid oxide fuel cells (SOFCs) of various designs (electrolyte-supported, anode-supported and micro-SOFC). Manufacturing of SOFCs components remains nowadays a key point for the industrial development of this electrochemical conversion device. Formation of thin nanostructured anodes by physical vapor deposition methodsallowsincreasing efficiency of SOFCs due to the reduction of polarization losses at the anode. The review discusses the use of thin-film anodes for different SOFC configurationsintended to work in low, intermediate and hightemperature ranges. The influence of the deposition parameters, microstructure and chemical composition of AFLs on SOFCperformance is analyzed.
Keywords: SOFC, anode functional layer, thin film, magnetron sputtering, polarization losses
DOI:10.32737/2221-8688-2019-2-252-266
Introduction
Solid oxide fuel cells (SOFCs) are electrochemical devices that allow directly converting the chemical energy of fuel into electrical and thermal[1]. Due to the numerous advantages of SOFC, they are actively developing throughout the world. Within this topic, an urgent task is the development of medium-temperature, i.e. operating at 600-800 °C, anode-supported SOFC. The latter makes it possible to reduce internal losses in SOFC and increase the service life of power plants. Internal losses in SOFC consist mainly of ohmic losses in the electrolyte and activation losses in the electrodes.The former can be minimized by reducing the thickness of the electrolyte. Formation of nanostructured thin-film anodes with a developed three-phase boundary (TPB) "metal-ion conductor-fuel gas" allows reducing of activation losses in the anode [2,3]. SOFC anodes are metal ceramic composites consisting of a phase with oxygenion conductivity (for example, yttria-
stabilized zirconia (YSZ) or gadolinium doped ceria (GDC)) and a phase with electronic conductivity (metal or alloy). In anode-supported SOFC, the anode consists of at least two layers: a current collector layer (about 500 microns thick) and a thin functional electrochemically active layer (about 15 microns thick). The current collector layer should have a porous structure sufficient to supply the fuel to the TPB and remove the reaction products. Also it should ensure high mechanical strength and electrical conductivity.An electrochemical reaction takes place in the functional layer. Its intensity depends on the operating temperature and the density of the three-phase boundary. As the operating temperature decreases, the activation losses in the anode and their contribution to the total resistance of the SOFC increase.
For the manufacture of SOFC anodes, methods such as slip-casting and screen printing are widely used. However, they
require long stages of high-temperature sintering, which can cause undesirable reactions between the SOFC layers, deformations, cracks. These methods make it difficult to manufacture anode functional layers (AFLs) with a particle size of less than 1 micron even when using nanopowders, due to the coarsening of particles during high-temperature sintering [4]. A high surface energy of nanopowders contributes to their excessive agglomeration. The methods of physical vapor deposition (PVD) do not have these disadvantages [5]. They make it possible to reduce the temperature of the formation of functional layers and to implement structures that, in principle, cannot be created using powder sintering methods. PVD includes methods based on target evaporation, ion-beam sputtering and ion-plasma sputtering. For the manufacture of SOFC layers, the most frequently used methods are electron-beam [6, 7] and laser evaporation, magnetron sputtering. The laser evaporation method is used to form electrolytes [8, 9] and anode functional layers of the anode-supported SOFC [10].However, electron beam and laser evaporation do not allow the deposition of coatings on large area substrates. Among PVD methods, magnetron sputtering is the most promising method for SOFC fabrication.The advantages of this method are the possibility of independent regulation of the main parameters of the sputtering process, formation of uniform coatings with required characteristics, and the possibility of processing of large-area surfaces [11]. Regulating the process parameters such as particle energy, degree of ionization, substrate temperature, substrate bias voltage, deposition rate, gas pressure, discharge power and operating mode of the magnetron sputtering system makes it possible to produce dense or nanoporous coatings.
Deposition of anode functional layers for electrolyte-supported SOFC
When NiO/YSZ or NiO/GDC AFLsare formed by the reactive magnetron sputtering in the atmosphere of Ar + O2, their microstructure, including porosity, can be regulated by the substrate temperature, deposition rate, particle angle of incidence,
working pressure [12]. The substrate temperature has the greatest influence on the structure of the material (amorphous or crystalline) and its phase composition. While the deposition rate, particle angle of incidence, working pressure, mainly affect the density of the coating (the presence of porosity).An important parameter is the ratio of Ar/O2 in a vacuum chamber during reactive magnetron sputtering. The Ar/O2 ratio of argon and oxygen affects the oxidation of the target during the sputtering process and, consequently, the target sputtering rate and film deposition rate. On the one hand, at low oxygen partial pressure, the film deposition rate is higher. However, the resulting film may have a lack of oxygen and then it will require high-temperature annealing in air. On the other hand, at high oxygen partial pressure, sputtering is carried out in the so-called "poisoned" mode. In this mode, the deposition rate of the film is minimal, but it has a stoichiometric composition.
NiO/YSZ AFL, for example, can be formed from two metal Ni and Zr/Y (86/14 at.%) targets by their joint sputtering, and one Ni/Zr/Y (68.8/26.6 /4.6 wt.%) target. The first method allows adjusting the content of Ni in the formed layer. But the second method is easier to implement.
In [13] Ni-YSZ films are deposited at room temperature by reactive magnetron sputtering from a single Ni/Zr/Y metallic target at rates ashigh as 4 p,m h-1. The bipolar pulsedvoltage with the pulse frequency 100 kHzwas used. The as-deposited film surface (Fig. 1,a,b) has granular morphology with small grains of 10 nm, and a cauliflower-like structure. The cross section shows that structure similar to cauliflower arises from a columnar structure, which is typical for magnetronsputteredfilms [14].
After annealingat 900 °C for 90 min NiO/YSZ film has a denser structure due to atomic reorganisation duringheat
treatment(Fig. 1,c,d). The cross-section is no longer columnar and grainsand boundaries are less distinguished. According to X-Ray diffraction analysis, before annealing, the film exhibited a poorcrystalline quality. But after annealing at 900 °C, diffraction peaks
correspondingto the 8YSZ, Ni0(200) and Ni(111) phases were detected. The microstructure of reduced thin-film anodes and their electrocatalytic properties were not studied.
Garcia-Garcia et al. [15] prepared
highly porous, columnar NiO/YSZ thin films by reactive pulsed DC magnetron sputtering (200 W, 80 kHz) with deposition at glancing incidence. The pressure during deposition was maintained at 5 x 10-3 mbar in a process gas that consisted of 40 sccmAr and 2.5 sccm O2.
b)
Fig. 1. SEM images of NiO/YSZ film as deposited (a, b) and after annealing at 900 °C for 90 min in air (c, d) [13].
The deposited at room temperature films were also essentially amorphous. Annealing such films at 850°C in hydrogen resulted in film crystallization withpreservation of the columnar
structure.However, during the NiO reduction to metallic Ni latter was partially segregated to the film surface and formation of Ni agglomerates with a size of 0.7-2 |im (Fig. 2) was occurred.
Fig. 2.Cross section of the as-deposited thin film(a) and after reduction in hydrogen (b). Siliconsubstrate [15].
Thus, themaindisadvantageofnickel- desurfaceduringreductioninhydrogen [15]. The basedanodefunctionallayersformedbyPVDmet amount of agglomerates depends on the nickel hodsistheformationofnickelagglomeratesonano content in the film. This phenomenon was also
observed for anodes formed by laser evaporation [10]. Ni agglomeration leads to a decrease in the three-phase boundary area at the anode [16] and can lead to the formation of cracks in the electrolyte film.
Agglomeration of particles of the Ni phase is usually related to the Ostwald ripening process [17]. This process may be based on thetransport of Ni particles through the gas phase (evaporation and deposition) [18]. It is known that Ni(OH)2 has a six-fold evaporation rate as compared to that of Ni. Transport of Ni atoms could occur from small
granules to large ones byevaporation of Ni(OH)2.
To solve the problem of nickel agglomeration, the influence of annealing in an air atmosphere at temperatures of 800-1200 °C on the microstructure of NiO/YSZ films formed by laser evaporation was studied in [10]. Annealing at 1200 °C has been shown to prevent nickel agglomeration, due to the formation of a strong YSZ matrix, which inhibits the enlargement of nickel granules (Fig. 3).
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MiijSl . 1 F 1U1
9u'*M 1 1 urn
Fig. 3.Cross section of the PLD deposited NiO/YSZ films after reductionat 600 °C in 4% H2. a) without post-annealing, b)annealed in air at1200 °C for 1 h [10].
In another work ofGarcia-Garcia et and substrate normal. A 50 mm diameter
al. [19] ~5
^mNi/YSZanodewaspreparedonYSZelectrolyt ebyreactivepulsedmagnetronsputteringatanobli queangleofincidence.Target-substrate distance was 5 cm with an angle of 80° betweentarget
Ni/Zr/Y alloy (68.8/26.6/4.6 wt.%) target was used to produce Ni(50 vol.%)/YSZ films at a discharge power of 200 W, 80 kHz frequency, working pressure of 5-10-3 mbar and room temperature.
Fig. 4.SEMimages of the cross section (a) and the surface (b) of the Ni/YSZ anode, deposited by magnetron sputtering at an angle to the substrate, after preliminary annealing at a temperature of 1250 °C in air and reduction in hydrogen at a temperature of 850 °C [19].
Deposition rate of 1.5 |mh- was achieved. It was shown that pre-annealing of the amorphous as-deposited NiO/YSZ film at 1250 °Cwith the subsequent reduction produces a film consisting of uniformly distributed tiltedcolumnar grains having extensive three-phase boundaries (Fig. 4). The performance of thin film anode was comparable to thatof the commercial anode 50 |m thick (FuelCellMaterials (USA)), which was 10 times thicker and prepared by the doctor blade procedure. Single cell based on commercially available solid oxide half-cell comprising YSZ electrolyteand cathode (20 mm Single ElectrodeCell - Cathode Only, FuelCellMaterials, USA) with magnetron sputtered NiO/YSZ anode had maximum power density of about 450 mW cm-2 at 850 °C.
In order to increase the efficiency of thin-film anodes Garcia-Garcia et al. [20] suggested producing Ni-containing anodes comprising alternating layers of GDC and YSZ of 200 nm thickness.Also hybrid anode produced by co-deposition of Ni/YSZ and GDC were tested.The conditions for the deposition of layers did not change as compared with the previous works of the authors [15, 19]. In the as-deposited state, multi-layer anodes exhibited the highly porous columnar architecture, with well-definedboundaries between the NiO/YSZ and NiO/GDC layers (Fig. 5,a). The columnar structure was preserved after annealing in air (Fig. 5,b). As shown in Fig. 5,c,after the final reduction, highlyporous thin anodes with pronounced columnar architecture were obtained.
wüh 2tjm 2 pm 2 Mm
Fig. 5. SEM cross-section images of multi-layer thin-film anodes: a)as-deposited, b) annealed in air, c) reduced in hydrogen [20].
After subsequent fuelcell testing, the porous columnar architecture of the two-component layered thin film anodes wasmaintained and their resistance to delamination from the underlying YSZ electrolyte was superior to thatof corresponding single component Ni-YSZ and Ni-GDC thin films. The performances of the multi-layer and hybridanodes were almost the same and far superior to the single component YSZ and GDCanodesand also superior to theconventional commercially available thick anodes. Maximum power density of about 600 mW cm-2 at 850 °C is achievedwith hydrogen as fuel.
In another work Garcia-Garcia et al. [21] showed that thin Ni-GDC anodes exhibit excellent transport properties, robust under thermal cycling, and have good adhesion to the
YSZ electrolyte. Similarly prepared Au-doped Ni-GDC anodesexhibited the same morphology, porosity, durability and electrochemical performance (power density of about 500 mW cm-2 at 850 °C in hydrogen). However, under steamreforming conditions with CH4/H2O mixtures the behaviour of the Au-doped Ni-GDC anodeswere far superior, exhibiting retention of good power density and dramaticallyimproved resistance to deactivation by carbon deposition (Fig. 6).
Tanveeret al. [22] suggested using thin nickel-samaria-doped-ceria (Ni-SDC) anodes prepared by magnetron sputtering forintermediate temperature electrolyte-supported solid oxide fuel cells. The anode was deposited by radio-frequencysputtering on a 150p,mscandia-stabilized-zirconia
(SSZ)substrate at room temperature.
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operation vcltage 0.7 V
S/C=0.3
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Au/Ni-GDC Ni GDC
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24 4S 72 96 120 14* 168 192
operation time (hours)
Fig. 6.The V-I and P-I characteristics of single cells with Ni-GDC and Au/Ni-GDC anodes obtained at 850°C withhydrogen as fuel (a).I-t behavior of single cells with Ni-GDC and Au/Ni-GDC anodes operated at 0.7 V polarization voltage, 800°C andsteam to carbon ratio S/C = 0.3 [21].
The deposition process lasted for 5 hrs at a constant power of 100 W, and pressure of 50 mTorr. To analyze the effect of reactive O2 gas on the anode manufacturing process, two different background gases were used during deposition, mixture of Ar/O2:8/2 and Ar. Both methods resulted in producing nano-porous Ni-SDC films after reduction in H2. But the reactively sputtered anodes have a 20% slower growth rate than the non-reactively sputtered ones. Also, the presence of this amount of oxygen in the sputtering chamber resulted in producing lesser catalytic Ni in the anodes. This resulted in a poorer performance for the cells containing reactively sputtered anodes when compared to nonreactive ones. Anodes were not annealed in air after deposition; therefore, agglomeration of Ni was observed on the anode surface after cell testing.LSM/YSZ cathode was screenprinted on the other side of the electrolyte. Peak powerdensities for such cells were 240, 140and50mW cm-2 at 800, 700 and 600°C.
Jou and Wu [23] investigated thin Ni/YSZ anodes fabricated by reactive co-sputtering of a Ni and a Zr-Y target, followed by sequentially annealing in air at 900°C, as part of an YSZ electrolyte-supported SOFC.A distinctive feature of the study was conducting test inasingle-chamberconfiguration,inamixtureofCH4 and air(CH4:O2 volume ratio = 2:1) at700 °C.Themaximumpowerdensityofthe SOFCusingthe triple-layered Ni-YSZ thin film astheanodewas1.14mWcm-2, whichwaslarger thanthatof0.76mWcm-2, obtainedusingascreen-printed20-mm-thick Ni/YSZanode.
Fig. 7.Surface SEM image of Ru/CGO film deposited on YSZ/Si3N4/Si substrate (nominal volumetric ratio of Ru/CGO = 7/3) [24].
Deposition of anode functional layers formicro-SOFC
Takagiei al. [24] demonstrated that magnetron sputtered anodes could be used in micro-solid oxide fuel cells (^SOFCs) also. Rutheniumand gadolinia-doped ceria (Ru/CGO) composite nano-crystalline thin film anodes were synthesized by co-sputtering from Rumetal (DC power) and CGO oxide (RF power) targets without substrate heating. Ru/CGO composite films on YSZ/SisWSi substrate showed highly granular morphology with primary grain sizes of ~10 nm andsecondary particles or pillars as large as 70-100 nm as shown inFig. 7.
Stress-relaxed composite thin film anodes were developed on self-supported liSOFCs with YSZ thin film electrolytes and porous platinum cathodes. |iSOFCs were
tested with roomtemperature humidified methane as fuel and air as the oxidant. They exhibited an open circuit voltage of0.97 V and a peak power density of 275 mWcm-2 at 485 °C.
Deposition of anode functional layers for anode-supportedSOFC
In the works described above, thin-film anodes were mainly deposited on the electrolyte discs for the use in electrolyte-supported SOFCs. However, as was shown by us, thin-film anodes can also be used in anode-supported SOFC to reduce the polarization resistance at the anode [25]. The idea is to increase the characteristics of the anode-supported cells (ASC) by forming a thin-film anode functional layer between the thin-film electrolyte and commercial tape casted anode, as shown in Fig. 8.
Fig. 8.The schematicof anode-supported cell c functional layer.
To study the influence of thin AFL on SOFCs'performance, single cells with and without AFL werefabricated. A 2 cm in diameter and 0.4-mm-thick NiO/YSZanode supports cut out by laser from commercial anodesheets (Ningbo SOFCMAN Energy Technology, China) were usedas substrates for magnetron deposition of both NiO/GDC and NiO/YSZ anode functional layers. AFLs were depositedby reactive co-sputtering of Ni,Zr0.86Y0.14 and Ce0.9Gd0.1 targets with diameter of 75mm. The distance between the magnetron target center andsubstrate was 80 mm. Prior to film deposition, the substrateswere heated to 400 °C.The deposited AFL samples were annealed in air atmosphereat different temperatures (6001200 °C) for 1 h. For a good thermo-
figuration: a) conventional, b) with thin anode
mechanical compatibility with theelectrolyte, the cells with NiO/GDC AFL were tested withGDC electrolyte, while cells with NiO/YSZ AFLs weretested with YSZ electrolyte. Electrolyte layers4-^m-thick were also deposited by reactive magnetron sputtering. Thedetails of this process were described in our previous work [26].The LSCF/GDC cathode layer was prepared on all cellsby a screen-printing method using La0.6Sr0.4Co0.2Fe0.8O3/Ce0.9Gd0.1O2 paste (CERA-FC, Korea). The cathode area was 1 cm2.Cathode sintering was performed at a temperature of 800 °Cfor 1 h before the fuel cell electrochemical test.
Analysis of cross-sections of Ni/YSZ and Ni/GDC AFLs (Fig. 9) showed that the composite films acquire a finely
porousstructure after reduction in hydrogen. both with an anode substrate andwith an For all samples, a good contact of the AFL electrolyte layer was observed.
Fig. 9. Cross sectional SEM images of the SOFCs with nanostructuredanode functional layers: a)Ni/YSZ AFL (NiO 60 vol.%.),b)Ni/GDC AFL (Ni 40 at. %). SEM images were obtainedafter cells testing [25].
The grain size and pore size of AFLsare hundreds of nanometers, in contrast to the substratehaving micron-sized grains and pores. AFLs have arelatively smooth and uniform surface that facilitates depositionof a high quality dense and thin electrolyte layer leading to reduction in resistive losses.
Theelectrochemical activity of a cermet anode with the givenmicrostructure and materials depends on the volume and onthe grain size [27]. Smaller grains increase the triple phaseboundary length and, as a result, the polarization resistancedecreases.
Fig. 10.Cross-sectional SEM images focused on the interface anode-electrolyte of the YSZ-based SOFCs without AFL (a), with AFL (c), andGDC-based SOFCs without AFL (b), with AFL (d) [25].
Figure 10 shows cross-sectional SEM images focused onthe anode-electrolyte interface of the YSZ- and GDC-basedSOFCs without and with anode functional layers. The fineand homogeneous microstructure of the AFLs enhances thecontinuity of the interface that should reduce interfacepolarization resistance.In order to estimate the impact of the NiO/YSZ and Ni/GDC AFL anodefunctional layers on the fuel cell performance, the current-voltage (I-V) and current-power (I-P) curves of the anode-supportedcells with and without AFL were compared atdifferent temperatures, as shown in Figs. 11 and 12.
The open circuit voltages (OCV) of cellswith YSZ electrolytewere 1.08-1.17 V, depending on the temperature, that is closeto the theoretical value of the SOFC.The peak power densities of the cell without AFL were 132, 469, and950mWcm-2 at 600, 700, and 800 °C, respectively. Singlecell with magnetron sputtered NiO/YSZ AFL (NiO 60 vol. %)has peak power densities of 222, 675, and 1240mWcm-2 atthe same temperatures. Thus, the peak power density of fuelcells with the AFL is 30% higher at 800 °C and 68% higherat 600 °C than that of the cell without AFL. That is, the effectof AFL on the fuel cell performance is more pronounced atlow temperatures.
Fig. 11.I-V and I-P characteristics of the anode-supported SOFC withYSZ electrolyte and LSCF/GDC cathode without (a) and with (b) magnetronsputtered NiO/YSZ AFL (NiO 60 vol. %) measured at temperatures of600-800 °C [25].
Fig. 12.I-V and I-P characteristics of the anode-supported SOFC withGDC electrolyte and LSCF/GDC cathode without (a) and with (b)magnetron sputtered NiO/GDC AFL (Ni 40 at. %) measured at temperaturesof 400-600 °C [25].
The OCV values of the cells with GDC 400, 500, and 600 °C, respectively. electrolyte were 1.134, 0.920,and 0.811V at Thereduction of ceria from Ce4+ to Ce3+ in a
reducingatmosphere increases electronic conductivity, which decreasesthe open-circuit voltage of the cell. But at atemperature below 500 °C the electronic conductivity ofGDC becomes negligible and OCV reaches high valuescomparable with YSZ electrolyte. The maximum powerdensities of cell without AFL were 25, 125, and 305mWcm-2 at 400, 500, and600 °C, respectively. The OCV and maximum powerdensities of the cell with AFL were 1.037, 0.940, 0.861 Vand 40, 200, 460mWcm-2 at 400, 500, and 600 °C,respectively. The increase in power density due to theformation of AFL is about 60% depending on the temperature.
The total area-specific resistance (ASR) of YSZ-based cellwithout AFL calculated from linear fit of I-V curve measuredat 800 °C was 0.27ficm2. When AFL is applied, the ASRdecreased to 0.2ficm2. The same dependence is observedwhen using NiO/GDC AFL. At 600 °C AFL depositionresults in reduction of ASR from 0.54 to 0.38ficm2. Thisimplies that a deposited NiO/YSZ and NiO/GDC AFLsreduced the total cell ASR by 26-30%.
Deposition of Cu-based SOFC anode functional layers
Ni-based anodes are usually used in solid oxide fuel cells when fuel is hydrogen. In this case anodes experience relatively low degradation rates and fast electrochemical kinetics. But if synthesis gas is used as fuel, Ni could be easily poisoned by contaminants like sulfur or carbon [28, 29]. Carbon deposition reduces the cell performance by blocking the anode reaction sites and damaging the microstructure of Ni in the anode. In recent
studies [30, 31], Cu has been suggested as an alternative to Ni as the electronic conductor in SOFC anodes. Recently, Cu/yttria stabilized zirconia (YSZ), Cu/samaria-doped ceria (SDC) and Cu/gadolinia-doped ceria (GDC) anodes [32-36] have been developed and tested, showing good electronic conductivity and tolerance towards sulfur-containing fuels. However, Cu has a low catalytic activity for hydrogen or hydrocarbon electrochemical oxidation, and so doped ceria should be used instead of YSZ in order to improve the cell performance.
Despite the advantages of Cu-based composite anodes, it is difficult to apply conventional ceramic processing methods to fabricate Cu-containing SOFCs due to the low melting point of Cu (that is 1085 °C). Presently they are manufactured using a multistep wet ceramic technique that requires more processing and firing steps and not attractive for mass production. Furthermore, high sintering temperatures increase interdiffusion of elements between adjacent cell layers.To avoid the sintering of copper oxide at high temperatures, we proposed using magnetron sputtering for thin CuO/GDC anodes deposition.
CuO/GDC thin films were deposited by reactive magnetron co-sputtering of Cu and Ce09Gd01 targets with diameter of 75 mm. Sputtering was carried out in oxygen-argon atmosphere at working pressure of 0.2 Pa in pulse bipolar mode at pulse repetition frequency of 80 kHz and positive pulse duration of 4 p,s. The argon and oxygen flow rates were fixed at 26 and 33 sccm, respectively.
Fig. 13. Surface morphology of Cu/GDC film reduction in hydrogen).
Targets were positioned at 45° to the fixed substrate holder. The distance between the magnetron target center and substrate was 80 mm. Prior to film deposition, the substrates were heated to 450 °C. The porous commercial NiO/YSZ anodes (S OF CM AN, China) with a thickness of 0.4 mm were used as substrates. The discharge power of a Ce-Gd magnetron was 1 kW. The discharge power of the Cu magnetron ranged from 100 to 700 W to control the volume content of CuO in the film. The copper content in the films was determined after films reduction in hydrogen. For example, at 500 W power of the Cu magnetron, the film contained 42 at. % of Cu.
Surface morphology of 2 |im thick Cu/GDC film (42 at.% of Cu) after reduction in H2at a temperature of 750 °C for 2 hours is shown in Fig. 13. As can be seen from the SEM image, Cu agglomeration occurs on the surface of the film, in analogy with what happens when nickel-containing thin films are
deposited on porous NiO/YSZ substrate (after
reduced at high temperature. In order to solve this problem, preliminary annealing of the films in air at 800and 1000 °C was carried out, by analogy with Ni-based anodes.
Figures 14 and 15 show surface and cross-sectional images of thin-film CuO/GDC anodes deposited at power of Cu magnetron 500 W and annealed at a temperature of 800 and 1000 °C, respectively. After annealing, the samples were reduced in hydrogen. Both films formed intimate contact across the interface between the film and the substrate. Moreover, film/substrate interfaces are homogeneous. At an annealing temperature of 800 °C, agglomeration of copper into isolated particles having characteristic dimensions of less than 1 p,m occurs on the surface of the film (Fig. 14,a). After annealing at 1000 °C,the absence of Cu agglomerates on the surface of the film and formation of a more porous structure of the film with local cracks of micron width on the surface is observed (Fig. 15,a).
Fig. 14. Surface (a) and cross-sectional (b) SEM images of thin-film CuO/GDC AFLs annealed at a temperature of 800 °C in air and reduced in hydrogen (power of Cu magnetron - 500 W).
Fig. 15.Surface (a) and cross-sectional (b) SEM images of thin-film CuO/GDC AFLs annealed at a temperature of 1000 °C in air and reduced in hydrogen (power of Cu magnetron - 500 W).
These results show that magnetron sputtering is also a good technique for preparation of the thin-film copper-cermet anode material for SOFC. However,
electrochemical properties of magnetron sputtered CuO/GDC thin films are under investigation.
CONCLUSIONS
Uniform and porouscompositethin films consisting of oxygen ionic conductivephase (YSZ or GDC) and electronic conductive phase (Ni or Cu) may be prepared by reactive magnetron sputtering. Annealing the as-deposited films in hydrogen resulted in NiOor CuOreduction to metallic Ni or Cu, which then segregated to the film surface with particles formation. However in the case of nickel based films, pre-calcination of the as-deposited films in air at 1200 °C followed by reduction produces films consisting of uniformly distributed nano-grains having extensive three-phase boundaries. The exceptionally high degree of dispersion andintermixing of the anode components promotes generation of veryextensive three phase boundaries.Moreover, under thermalcycling in both oxidizing and reducing atmospheres, nickel based anode functional layersare robust against morphologicalchanges and delamination from an YSZ substrate. Accordingly, because their thinnessrenders
them robust against strain-induced separation from the substrate, they are also promisingcandidates for use as highly conducting, stabilizing buffer layers for accommodating strain between conventional anodes and the YSZ electrolyte, and for fabrication of graded-compositionNi-YSZ anodes for use in SOFC applications. The SOFC performance is significantlyincreased using a nanostructured Ni/YSZ or Ni/GDC AFL as compared to the performance of cells without it. The increasing the TPB density near the electrolyte and anode interface increases the number of sites available for the charge transfer bythe AFL, leading to an enhancement in the cell performance.
Magnetron sputtered AFLs can be used to improve the efficiency of various types of cells, such as electrolyte-supported, anode-supported, and micro-SOFC. The only exceptions are metal-supported SOFC, because as-deposited AFLs require annealing in air after deposition.
Acknowledgements
This work was supported by the Russian Science Foundation (grant No. 17-79-30071).
REFERENCES
1. MahatoN., BanerjeeA., Gupta A., OmarS., Balani K. Progress in material selection for solid oxide fuel cell technology: A review. Prog. Mater. Sci.2015, vol. 72, pp. 141337.
2. Kennouche D., Hong J., Noh H.S., Son J.W., Barnett S.A. Three-dimensional microstructure of high-performance pulsedlaser deposited Ni-YSZ SOFC anodes.
Phys. Chem. Chem. Phys.2014, vol.16, no. 29, pp. 15249-15255.
3. Noh H.S., Yoon K.J., Kim B.K., Je H.J., Lee H.W., Lee J.H., Son J.W. The potential and challenges of thin-film electrolyte and nanostructured electrode for yttria-stabilized zirconia-base anode-supported solid oxide fuel cells. J. Power Sources. 2014, vol. 247, pp. 105-111.
4. Park J.H., Han S.M., Yoon K.J., Kim H., Hong J., Kim B.K., LeeJ.H., Son J.W. Impact of nanostructured anode on low-temperature performance of thin-film-based anode-supported solid oxide fuel cells. J. Power Sources. 2016, vol. 315, pp. 324330.
5. Coddet P., Liao H.,Coddet C.A review on high power SOFC electrolyte layer manufacturing using thermal spray and physical vapour deposition technologies. Adv. Manuf. 2014, vol. 2, pp. 212-221.
6. Tanhaei M., Mozammel M. Yttria-stabilized zirconia thin film electrolyte deposited by EB-PVD on porous anode support for SOFC applications. Ceram. Int. 2017, vol. 43, no. 3, pp. 3035-3042.
7. Hong Y.S., Kim S.H., Kim W.J., Yoon H.H. Fabrication and characterization GDC electrolyte thin films by e-beam technique for IT-SOFC. Curr. Appl. Phys.2011. vol. 11, no. 5, pp. 163-168.
8. Hidalgo H., Reguzina E., MillonE., ThomannA.L., Mathias J., Boulmer-Leborgne C., Sauvage T., Brault P. Yttria-stabilized zirconia thin films deposited by pulsed-laser deposition and magnetron sputtering. Surf. Coat. Technol.2011, vol. 205, no. 19, pp. 4495-4499.
9. De Vero J.C., Develos-Bagarinao K., Matsuda H., Kishimoto H., Ishiyama T., Yamaji K., HoritaT.,Yokokawa H. Sr and Zr transport in PLD-grown Gd-doped ceria interlayers. Solid State Ion.2018, vol. 314, pp. 165-171.
10. Noh H.S., Son J.W., Lee H., Ji H.I., Lee J.H., Lee H.W.Suppression of Ni agglomeration in PLD fabricated Ni-YSZ compositefor surface modification of SOFC anode. J. Eur. Ceram. Soc. 2010, vol. 30, pp. 3415-3423.
11. Solovyev A.A., Shipilova A.V.,Rabotkin S.V., Ionov I.V., Smolyanskiy E.A.Magnetron deposition of yttria-stabilisedzirconia electrolytefor solid oxide fuel cells. Eurasian Journal of Physics and Functional Materials. 2018, vol. 2, no. 3, pp. 206-218.
12. Barranco A., Borras A., Gonzalez-Elipe A.R., Palmero A. Perspectives on oblique angle deposition of thin films: From
fundamentals to devices. Prog. Mater. Sci.2016, vol. 76, pp. 59-153.
13. RezuginaE., Thomann A.L., Hidalgo H., Brault P., Dolique V., Tessier Y.Ni-YSZ films deposited by reactive magnetron sputtering for SOFC applications. Surf. Coat. Technol. 2010, vol. 204, pp. 23762380.
14. Chawla V., JayaganthanR., Chandra R. Structural characterizations of magnetron sputterednanocrystallineTiN thin films.Mater. Charact. 2008, vol. 59, pp. 1015-1020.
15. Garcia-Garcia F.J., Yubero F., Gonzalez-Elipe A.R., Balomenou S.P., Tsiplakides D., Petrakopoulou I., Lamber, R.M. Porous, robust highly conducting Ni-YSZ thin film anodes prepared by magnetron sputtering at oblique angles for application as anodes and buffer layers in solid oxide fuel cells. Int. J. Hydrog. Energy.2015, vol.40, no. 23, pp. 7382-7387.
16. Muecke U.P., Akiba K., Infortuna A., Salkus T.,Stus N.V., Gauckler L.J.Electrochemical performance of nanocrystalline nickel/gadolinia-doped ceriathin film anodes for solid oxide fuel cells. Solid State Ion. 2008, vol. 178, pp. 1762-1768.
17. Holzera L., Iwanschitz B., Hocker T., Munch B.,Prestat M., Wiedenmann D., Vogt U., Holtappels P.,Sfeir J., Mai A., Graule T. Microstructure degradation of cermet anodes for solid oxide fuel cells: quantification of nickel grain growth in dry and in humid atmospheres./. Power Sources. 2011, vol. 196, pp. 1279-1294.
18. Gubner A., Landes H., Metzger J., Seeg H., Stubner, R. Proceedings of theFifth International Symposium on Solid Oxide Fuel Cells (SOFC-V), StimmingU.,Singhal S.C., TagawaH., LehnertW., Editors, PV 97-40, pp. 884850, The Electrochemical Society Proceedings Series, Pennington, NJ 1997.
19. Garcia-Garcia F.J., Yubero F., Espinos J.P., Gonzalez-Elipe A.R., Lambert R.M. Synthesis, characterization and performance of robust poison-resistant ultrathin film yttria stabilized zirconia -nickel anodes for application in solid
electrolyte fuel cells. J. Power Sources. 2016, vol. 324, pp. 679-686.
20. Garcia-Garcia F.J., Beltrân A.M., YuberoF., Gonzâlez-ElipeA.R., Lambert R.M.High performance novel gadolinium doped ceria/yttria stabilizedzirconia/nickel layered and hybrid thin film anodes for application insolid oxide fuel cells.J. Power Sources. 2017, vol. 363, pp. 251-259.
21. Garcia-Garcia F.J., YuberoF., Gonzâlez-ElipeA.R., Lambert R.M.Microstructural engineering and use of efficientpoison resistant Au-doped Ni-GDC ultrathinanodes in methane-fed solid oxide fuel cells. Int. J. Hydrog. Energy. 2018, vol. 43, pp. 885893.
22. Tanveer W.H., Ji S., Yu W., Cho G.Y., Lee Y.H., Park T., Lee Y., Kim Y.,Cha S.W. Effect of 20%02 reactive gas on RF-sputtered Ni-SDC cermet anodes for intermediatetemperature solid oxide fuel cells.Curr. Appl. Phys.2016, vol. 16, no. 12, pp. 1680-1686.
23. Jou S., Wu T.H. Thin porousNi-YSZfilmsasanodesforasolidoxidefuelcell. J. Phys. Chem. Solids2008, vol. 69, pp. 28042812.
24. Takagi Y., Adam S., Ramanathan S. Nanostructured ruthenium -gadolinia-doped ceria composite anodes for thinfilm solid oxide fuel cells. J. Power Sources. 2012, vol. 217, pp. 543-553.
25. Ionov I.V., Solovyev A.A., ShipilovaA.V., LebedynskiyA.M.,SmolyanskiyE.A., LaukA.L., SemenovV.A. Reactive co-sputter deposition of nanostructured cermet anodesfor solid oxide fuel cells. Jpn. J. Appl. Phys. 2018, vol. 57, article number 01AF07.
26. SolovyevA.A., ShipilovaA.V., IonovI.V., KovalchukA.N., RabotkinS.V., OskirkoV.O. Magnetron-sputtered YSZ and CGO electrolytes for SOFC. J. Electron. Mater. 2016, vol. 45, pp. 3921-3928.
27. Fukunaga H., Ishino M., Yamada K. Effective thickness of Ni-Sm-doped ceria cermet anode for solid oxide fuel
fell.Electrochem. Solid-State Lett.2007, vol. 10,pp. B16-B18.
28. Flytzani-Stephanopoulos M., Sakbodin M., Wang Z. Regenerative adsorption and removal
of H2S from hot fuel gas streams by rare earth oxides.Science. 2006, vol. 312, pp. 1508-1510.
29. Papurello D., Lanzini A., Fiorill S.,Smeacetto F., Singh R., Santarelli M. Sulfur poisoning in Ni-anode solid oxide fuel cells (SOFCs): deactivation in single cells and a stack.Chem. Eng. J. 2016, vol. 283, pp. 1224-1233.
30. Gross M.D., Vohs J.M., Gorte R.J. Recent progress in SOFC anodes for direct utilization of hydrocarbons. J. Mater. Chem.2007, vol. 17, pp. 3071-3077.
31. Lu C., Worrell W.L., Gorte R.J., Vohs J.M. SOFCs for direct oxidation of hydrocarbon fuels with samaria-doped ceria electrolyte. J. Electrochem. Soc.2003, vol. 150, pp. A354-A358.
32. Lee S., Hong H.S., Lee M.J., Woo S.K. Characterization of CuO/YSZ high temperature electrolysis cathode material fabricated by high energy ball-milling: 900 °C reduced by hydrogen exposure..Mater. Test. 2014, vol. 56, pp. 40-46.
33. Park S., Gorte R.J., Vohs J.M. Tape cast solid oxide fuel cells for the direct oxidation of hydrocarbons.J. Electrochem. Soc. 2001, vol. 148, pp. A443-A447.
34. Benoved N., Kesler O. Air plasma spray processing and electrochemical characterization of Cu-SDC coatings for use in solid oxide fuel cell anodes.J. Power Sources. 2009, vol. 193, pp. 454-641.
35. De Marco V., Grazioli A., Sglavo V.M. Production of planar copper-based anode supported intermediate temperature solid oxide fuel cells cosintered at 950 °C.J. Power Sources. 2016, vol. 328, pp. 235240.
36.Azzolini A., Sglavo V.M., Downs J.A. Production and performance of copper-based anode-supported SOFCs.ECSTrans.2015, vol. 68, pp. 25832596.
МАГНЕТРОННОЕ ОСАЖДЕНИЕ АНОДНЫХ ФУНКЦИОНАЛЬНЫХ СЛОЕВ ДЛЯ ТВЕРДООКСИДНЫХ ТОПЛИВНЫХ ЭЛЕМЕНТОВ
А.А. Соловьев1, И.В. Ионов1, А.В. Шипилова1, В.А. Семенов1, Е.А. Смолянский2
1 Институт сильноточной электроники СО РАН, 634055, г. Томск, пр. Академический, 2/3, e-mail: andrewsol@mail.ru 2 Томский политехнический университет, 634050, г. Томск, пр. Ленина, 30
Предлагаемый обзор посвящен последним достижениям в области магнетронного осаждения тонкопленочных анодных функциональных слоев(АФС) для твердооксидных топливных элементов (ТОТЭ) различных конструкций (на несущем электролите, на несущем аноде и микро-ТОТЭ). Производство компонентов ТОТЭ остается в настоящее время важнымфактором для промышленного развития этого электрохимического устройства. Формирование тонких наноструктурированных анодных слоев методами физического газофазногоосаждения позволяет повысить эффективность ТОТЭ за счет уменьшения поляризационных потерь на аноде. В обзоре обсуждается использование тонкопленочных анодов для различных конструкций ТОТЭ, работающих в диапазонах низких, средних и высоких температур. Проанализировано влияние параметров осаждения, микроструктуры и химического состава АФС на характеристики ТОТЭ. Ключевые слова: ТОТЭ, анодный функциональный слой, тонкая пленка, магнетронное распыление, поляризационные потери.
BORK OKSiD YANACAQ ELEMENTLdRi ÜQÜNANOD FUNKSiONAL ÖRTÜKLdRiN MAQNETRON QÖKDÜRÜLMdSi
A.A. Solovyov1, I.V. Ionov1, A.V. Shipilova1, V.A. Semenov1, E.A. Smolyanskiy2
1Rusiya EA Sibir Bölmdsinin Ddqiq elektronika institutu 634055, Rusiya, Tomsk §dh., pr. Akademigeskiy pr., 2/3, e-mail: andrewsol@mail.ru
2Tomskpolitexnik universiteti 634050, Rusiya, Tomsk §dh., Lenin pr., 30
Tdqdim olunan icmal müxtdlif konstruksiyali bark oksid yanacaq elementldri ügün anod funksional tdbdqdldrinin maqnetron gökdürmd yolu ild alinmasina hdsr olunub. Nazik nanostrukturlu anod tdbdqdldrinin qaz fazadan gökdürmd usulu ild formala§masi bdrk oksid yanacaq elementldrinin effektivliyini anodda polyarizasiya itkisinin azalmasi hesabina artirir. icmalda a§agi, orta vd yüksdk temperatur intervalinda i§ldydn müxtdlif konstruksiyali bdrk oksid yanacaq elementldri ügün nazik tdbdqdli anodlarin istifaddsi müzakird olunur. Anod funksional tdbdqdldrinin mikrostrukturu vd kimydvi tdkkibinin, eldcd dd gökdürmd parametrldrinin bdrk oksid yanacaq elementldrinin xarakteristikalarina tdsiri analiz edilib.
Agar sözlw. anod funksional örtükldri, bdrk oksid yanacaq elementldri, maqnetron sdpilmd metodu
