Dense Y-doped ion conducting perovskite films of BaZrO3, BaSnO3, and BaCeO3 for SOFC applications produced by powder aerosol deposition at room temperature

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Dense Y-doped ion conducting perovskite films of BaZrO3, BaSnO3, and BaCeO3 for SOFC applications produced by powder aerosol deposition at room temperature €rg Exner*, Tobias Nazarenus, Jaroslaw Kita, Ralf Moos Jo €tsstraße University of Bayreuth, Department of Functional Materials, Center of Energy Technology (ZET), Universita 30, 95440 Bayreuth, Germany

highlights
Dense proton conducting thick-films were produced by Powder Aerosol Deposition.
Densification achieved by room temperature spray-coating - no sintering required.
Comprehensive study on three doped materials classes: BaZrO3, BaSnO3, and BaCeO3.
Annealed films show bulk-like electrical conductivities and ionic transfer numbers.
Film deposition on porous NiO-cermet electrode as required for SOFC membranes.

article info

abstract

Article history:

State-of-the-art solid oxide fuel cells (SOFC) are based on oxide ion conducting zirconia

Received 24 July 2019

electrolytes, typically doped by yttria or scandia. Major drawback of these systems are their

Received in revised form

high operation temperatures of 800  C and above. These are necessary for a sufficient ionic

17 January 2020

conductivity. Instead of oxide ions, also protonic charge carriers could be used in SOFC. The

Accepted 23 January 2020

material classes of barium zirconates (BaZrO3), barium stannates (BaSnO3), and barium

Available online xxx

cerates (BaCeO3) are described as good proton conductors, especially when the B-site of the ABO3 perovskite structure is aliovalently doped by yttria. Their protonic conductivity

Keywords:

values in the moderate temperature regime up to 600  C are comparable to YSZ at 800  C,

Proton conducting ceramics

making these compounds ideal candidates for a usage in future SOFC. Unfortunately, very

Dense solid electrolyte membrane

high sintering temperatures up to 1800  C are required to process dense and therefore gas-

Post-deposition annealing

tight solid electrolyte membranes. However, a novel spray coating method called powder

Room temperature impact

aerosol deposition (PAD, also known as AD) enables to form fully dense ceramic films

consolidation (RTIC)

directly at room temperature without any necessary sintering processes. Films are

Aerosol deposition (AD)

deposited from ceramic powders that are accelerated by a dry carrier gas flow under

Vacuum kinetic spraying (VKS)

vacuum conditions. In this work, we investigated the film formation of three different barium based perovskite ceramics, namely yttria doped barium zirconate, barium stannate, and barium cerate by powder aerosol deposition. The optical and mechanical quality of films was evaluated using scanning electron microscopy and microhardness indentation and their crystallographic properties were characterized by X-ray diffraction. The electrical behavior

* Corresponding author. E-mail address: [email protected] (J. Exner). https://doi.org/10.1016/j.ijhydene.2020.01.164 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Exner J et al., Dense Y-doped ion conducting perovskite films of BaZrO3, BaSnO3, and BaCeO3 for SOFC applications produced by powder aerosol deposition at room temperature, International Journal of Hydrogen Energy, https://doi.org/ 10.1016/j.ijhydene.2020.01.164

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was analyzed by electrochemical impedance spectroscopy and DC polarization methods up to temperatures of 1000  C and 800  C, respectively. Furthermore, a preliminary study about the film formation on porous electrodes was conducted. © 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Fuel cells are electrochemical devices that directly convert the chemical energy of various fuels like hydrogen, CH4 or CO to electrical power. They are considered environmentally friendly with low to zero chemical and particulate matter emissions and feature high energy densities and conversion efficiencies [1]. Typical applications range from stationary combined power and heat generation in domestic usage with up to 200 kW output [1] to mobile devices like auxiliary power units (APU) for cars or trucks [2]. Commercially available solid oxide fuel cells (SOFC) are typically based on Y3þ or Sc3þ doped zirconia solid electrolyte membranes due to their high mechanical and chemical stability and their high oxide ionic conductivity of 102 S/cm to 101 S/cm [3]. However, very high operation temperatures of 800  C and above are necessary, leading to strong requirements for the interconnects of the cell. They also limit the cell life time due to demanding thermal cycling as well as degradation of the involved materials (interdiffusion and formation of electrical insulating layers) [4]. Furthermore, the transport of oxide ions results in the formation of water at the anode. This successively dilutes the circulating fuel (hydrogen) with water vapor. New classes of functional ceramic materials allow to transport protons instead of oxide ions [5]. Their key advantage are the high protonic conductivities of 102 S/cm, which are already achieved at lower temperatures of 600  C [5]. The transport of protons takes place by the Grotthuss- or by vehicle-mechanism, or a combination of both [6]. Additionally, in case of proton-conducting membranes, the water is formed at the cathode (air side). This could considerably simplify the fuel circulation. A wide range of ceramic compounds were found with a predominant protonic conductivity, based on ABO3-perovskites (barium cerate BaCeO3d [7], barium zirconate BaZrO3d [8], barium stannate BaSnO3d [9], and mixtures of them, like BaCe1-xZrxO3d [10]), A2B2O5 brownmillerite phases (Ba2In2O5 [11]), ortho-niobates (LaNbO4 [12]), or rare-earth tungstates (LnW1/6O2 with Ln ¼ Na, Nd, Gd, Er [13]). Almost all compounds need doping to achieve or enhance the protonic conductivity [5]. Especially the first group with the mostly cubic ABO3 perovskite structure features a high chemical and thermal stability in conjunction with high protonic conductivities up to 102 S/cm at 600  C [14]. Although a large number of doped perovskite compounds have already been investigated in regard to their properties and protonic conductivities, such as BaHfO3d [15], SrHfO3d [16], KTaO3d [17], SrZrO3d [18,19], SrCeO3d [20], as well as complex structures like Sr0.5La0.5CoO3d [20] and SrCe1xZrxO3d [21], in particular BaCeO3d and BaZrO3d are considered to be the most promising solid membrane materials for medium-to high-temperature SOFC application [5]. For the latter two compounds,

typically the B-site (Zr4þ or Ce4þ) is doped by trivalent rare earth cations such as La3þ [22], Gd3þ [7,22,23], Sc3þ [24,25], Yb3þ [25], and Y3þ [8,23e28] to increase the protonic conductivity. Mostly, large dopant concentrations (or additions) between 5 mol-% and 20 mol-% are used, whereas even higher doping levels decrease the conductivities as caused by reduced charge carrier concentrations and mobilities [5,29]. When directly comparing BaZr1-xYxO3d and BaCe1-xYxO3d, barium cerate features a higher protonic conductivity in the medium temperature range [30], while barium zirconate exhibits an enhanced chemical stability, especially towards H2O and CO2 [31]. While these material properties promote a potential use in SOFC, a major drawback still exists in form of very high sintering temperatures up to 1800  C, especially for yttria doped barium zirconate [32]. Sintering aids may lower the required sintering temperature to 1350  C, however they also negatively influence the protonic conductivity [5]. The issue of very high sintering temperatures may be resolved by a novel spray coating method called Powder Aerosol Deposition Method (PAD; often also abbreviated as ADM or AD). Here, dense and gas-tight ceramic thick films are formed directly at room temperature [33], with film thicknesses from sub-micrometer to several hundred micrometers. This dry spray coating technique combines a variety of advantages over conventional coating methods. Key feature is the film deposition at room temperature, without the necessity of a heat treatment either during or subsequent to the deposition. This means that dense ceramic films can be formed directly at room temperature just by spraying a sub-micrometer to micrometer-sized raw ceramic powder onto the surface to be coated. Furthermore, a broad range of coating materials including oxide [34e37], non-oxide ceramics [38e40], and a variety of functional ceramics [41] can be processed by PAD onto a wide variety of substrate materials like metals [42,43], ceramics [44], glasses [45], or even polymers [46]. While many publications describe the usage of functional PAD films for applications in gas sensors, superconductors, batteries or solar cells, only a small number is dedicated to PAD films in solid oxide fuel cells [47]. Nevertheless, PAD films were already realized as solid electrolyte (e.g. YSZ [35,48,49], (La,Sr)(Ga,Mg,Co)O3d [34,50e52], barium zirconate [53], Sm0.2Ce0.8O2d [54]), as cathode (e.g. LSCF [48e50], LSM [55,56]), as diffusion barrier layer (e.g. (Gd,Ce)O2d) [57e59]), and as oxidation protective coating for metallic interconnects (e.g. MnCo2O4 [60], LaNiO3 [61], La0.67Sr0.33MnO3d [62]). Furthermore, PAD was already used to produce both, the solid electrolyte as well as the porous cathode of certain SOFC. Here, Choi et al. successfully applied PAD film combinations of (La,Sr)(Ga,Mg,Co)O3d | LSCF-GDC [34], and YSZ | LSCF [48] to anode-supported fuel cells. However, a SOFC completely prepared by powder aerosol deposition has not been reported yet.

Please cite this article as: Exner J et al., Dense Y-doped ion conducting perovskite films of BaZrO3, BaSnO3, and BaCeO3 for SOFC applications produced by powder aerosol deposition at room temperature, International Journal of Hydrogen Energy, https://doi.org/ 10.1016/j.ijhydene.2020.01.164

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The deposition mechanism of PAD, typically referred as Room Temperature Impact Consolidation (RTIC), involves fracturing of impacting particles in combination with plastic deformation of the newly formed fragments [63]. Subsequently, impacting particles additionally consolidate the already deposited film (hammering effect) [64]. A more detailed description of the aerosol deposition process, its deposition mechanism, and the resulting film properties can be found in overview articles of Akedo's group from 2008 [65] and more recently of the authors' group from 2015 [66]. In this work, we investigate the film deposition of three different perovskite compounds that already showed a high protonic conductivity and therefore high potential for a future use in SOFC. Film formation of the yttria doped barium zirconate, stannate, and cerate is achieved at room temperature by powder aerosol deposition. Mechanical and microstructural analysis of the produced functional films is conducted in combination with temperature dependent ac and dc electrical characterization for conductivity determination.

Experimental details Mixed oxide synthesis Three different barium based perovskites, each doped with 20 mol-% yttrium, were synthesized and prepared for powder aerosol deposition, namely barium cerate (BaCe0.8Y0.2O3d, abbreviated as BCY20), barium zirconate (BaZr0.8Y0.2O3d, abbreviated as BZY20) and barium stannate (BaSn0.8Y0.2O3d, abbreviated as BSY20). A conventional mixed oxide synthesis with commercially available starting materials was used for all three powders. Stoichiometric amounts of barium carbonate (BaCO3, Alfa Aesar Barium carbonate, 99%), Yttria (Y2O3, Alfa Aesar Yttrium(III) oxide, REacton, 99.99%) as well as ceria (CeO2, Alfa Aesar Cerium(IV) oxide, 99.5%), zirconia (ZrO2, Alfa Aesar Zirconium(IV) oxide, 99.5%) or stannic oxide (SnO2, Alfa Aesar Tin(IV) oxide, 99.9%), respectively were mixed and wet-milled in a planetary ball mill (Fritsch Pulverisette 5, Idar-Oberstein, Germany) for 15 min. Milling took place in a zirconia milling jar (which consists of ZrO2, stabilized with 3.5 wt% MgO) and zirconia milling media (ZrO2, stabilized with 5 mol-% Y2O3) using cyclohexane as milling fluid. Afterwards, the mixtures were dried and subsequently calcined in a muffle furnace in air atmosphere. While for BCY20 and BZY20 a calcination temperature Tcalcination of 1250  C was suitable, as suggested by literature [67,68], a significantly higher temperature of 1600  C was required for BSY20 (at least 1450  C according to Ref. [69]). Each peak calcination temperature was held for 12 h, with heating and cooling rates set to 4 K/min. All information concerning

the synthesis of the perovskite powders are summarized in Table 1. In order to further reduce the particle size and enhance the aerosol deposition [57,70], all synthesized powders were ground again using the identical milling setup, however for an extended time of 30 min. Subsequently, cyclohexane was removed by rotary evaporator (Heidolph Hei-VAP Advantage, Schwabach, Germany) and furnace-dried at 200  C for at least 96 h. Prior to deposition, the powders were sieved (mesh size 90 mm) to mechanically break large, soft agglomerates that could negatively affect the aerosol generation and film building process [71]. The powders were again dried at 200  C for at least 48 h to obtain a free-flowing powder.

Powder aerosol deposition Films were produced using a custom-made PAD apparatus as previously reported in Ref. [72]. An oxygen carrier gas flow of 6e8 l/min was utilized, resulting in an absolute pressure of 1 mbar in the deposition chamber and 250e300 mbar in the aerosol generator. All substrates were cleaned with ethanol prior to deposition. The nozzle, with an outlet slit-orifice size of 10 mm by 0.5 mm, was held at a distance of 2 mm from the substrate. The substrate was moved horizontally at a speed of 2 mm/s to produce 10 mm  10 mm films. PAD films were deposited onto three different substrates:
Single crystalline silicon (boron doped, orientation (911), 525 mm thickness; Crystec, Berlin, Germany) for XRD analysis
Alumina (635 mm thickness; Ceramtec Rubalit 708S, Plochingen, Germany) to identify the quality of PAD films by scanning electron microscopy (SEM)
Screen printed and sintered platinum interdigital electrodes (IDE) on alumina substrates for electrical characterization (conductivity and ionic transfer number) The exact geometry of the interdigital electrodes and their electrical field line distribution are described in detail in Refs. [73,74]. The used dimensions are also stated in section Electrical conductivity and ionic transfer number. Images of a variety of produced films on different substrates are displayed in Fig. 1. All films appear homogeneous, independently of the used substrate, yet slightly darker than their sintered counterparts. BZY20 films show a strong grey color on the alumina substrate, whereas BSY20 and especially BCY20 appear in a light grey tone. For all PAD films on platinum interdigital electrodes, the underlying electrodes are clearly visible, even when buried below a 5 mme10 mm thick PAD film. This is related to the semi-transparent PAD film properties [75] that

Table 1 e Abbreviation, composition, used starting materials, calcination temperatures (Tcalcination), and mean particle diameter (d50) of all three synthesized perovskite powders. Abbreviation BZY20 BSY20 BCY20

Composition BaZr0.8Y0.2O3d BaSn0.8Y0.2O3d BaCe0.8Y0.2O3d

Starting materials including molar ratios BaCO3, 0.8∙ZrO2 and 0.1∙Y2O3 BaCO3, 0.8∙SnO2 and 0.1∙Y2O3 BaCO3, 0.8∙CeO2 and 0.1∙Y2O3

Tcalcination (for 12 h) 

1250 C 1600  C 1250  C

d50 1.7 mm 1.0 mm 1.3 mm

Please cite this article as: Exner J et al., Dense Y-doped ion conducting perovskite films of BaZrO3, BaSnO3, and BaCeO3 for SOFC applications produced by powder aerosol deposition at room temperature, International Journal of Hydrogen Energy, https://doi.org/ 10.1016/j.ijhydene.2020.01.164

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scanning electron microscope (Leo 1530 VP, Zeiss, Oberkochen, Germany).

Electrical conductivity and ionic transfer number

Fig. 1 e Images of various films produced by powder aerosol deposition on different substrates: (a) alumina (for SEM) and platinum interdigital electrodes on alumina (for electrical measurements), (b) silicon (for crystallographic analysis). The scale bar is identical for all samples. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

are attributed to the high density, small grain size, and relatively small thickness of the PAD film [76]. Therefore, the BSY20 film on silicon appears considerably darker, because the dark color of the Si substrate shines through the PAD film.

Powder and film analysis (ceramography methods) The powder particle size distribution was measured by laser scattering (Mastersizer 2000, Malvern Instruments Ltd, Malvern, UK). All powders exhibited a mean particle size d50 between 1 mm and 2 mm (Table 1). Therefore, the synthesized powders fall within the required particle size range of 0.5 mme5 mm, which is typically stated for a successful powder aerosol deposition [66,72]. X-ray powder diffraction (XRD) analyses (D8 Advance, Bruker, Billerica, MA, USA) were conducted between 20 and 80 (2q) in steps of 0.02 for a hold time of 1 s, on all powders and the thereof sprayed films on silicon. This XRD system consisted of a 2.2 kW copper anode featuring a Ka1 wavelength of 1.5406  A with an included Ge-Ka1 monochromator and an energy-dispersive 1-D LYNXEYE detector. X'Pert HighScore Plus software was used for structure verification. In order to determine the crystallite size and the internal strain, Rietveld refinement (with a combination of Gaussian and Lorentzian shape functions) was conducted using the TOPASAcademic software. A differentiation of size and strain is possible in a single measurement, since both effects depend differently on the Bragg angle q of the reflection [77]. The widening due to the crystallite size changes with 1/cos(q), whereas the microstrain depends on tan(q). The thickness of the sprayed films was measured by a € ttingen, Germany) while stylus profilometer (PGK/S2, Mahr, Go hardness measurements were taken by a microindentation hardness tester (Fischerscope H100, Fischer Technology Inc., Windsor, CT, USA) in accordance to DIN EN ISO 14577 1e3. Here, ten measurements with an applied indentation force of 100 mN were performed for each sample and the average hardness values were calculated. To investigate the film morphology, samples on alumina and IDE structures were prepared for cross-sectional imaging (fractured cross-sections or embedded in resin followed by grinding and polishing) on a

The electrical conductivity was determined in an alumina tube furnace upon heating between from 400  C to 900  C (BCY20)/1000  C (BSY20 and BZY20) with 100  C steps in a humidified air atmosphere. Furnace and samples were heated at a rate of 5 K/min and were allowed to equilibrate for 15 min before each measurement. A precision impedance analyzer (Novocontrol Alpha-A, Germany) was used to measure impedance characteristics in a frequency range from 10 MHz down to 0.1 Hz at 50 mV rms amplitude of the ac signal. The high and medium frequency range of the EIS data was fitted using ZView software with a parallel resistor - capacitor (R||C) circuit or a parallel resistor - constant phase element (R||CPE) as equivalent electrical circuit components to determine the resistance R. If a low frequency contribution occurred, which is caused by the electrode (typically in form of a Warburg impedance or another high-ohmic R||CPE), it was excluded from fitting. The total conductivity s (including all possible charger carriers like protons, oxide ions and electrons) was calculated by equation (1): s¼

1 R$F

(1)

Here, F in the unit of a length (typically given in cm or mm) describes the geometry of the electrodes and the solid electrolyte film. F was then calculated in accordance to Refs. [71,74] by measured film thickness t, and the IDE-geometry (finger length l ¼ 4.5 mm, finger width w ¼ 100 mm, spacing between fingers d ¼ 100 mm and number of fingers n ¼ 15): F¼

ð2n  1Þ$l$t þ 2$n$w$t d

(2)

For the 5 mm, 7 mm, and 9 mm thick PAD films used for electrical characterization, values F of 6.6 mm, 9.2 mm, and 12 mm, respectively were used in equation (1) to calculate the electrical conductivity s. Please note that for interdigital electrode structures predominantly the in-plane conductivity of films is measured [73]. A further differentiation of s into a grain and an interface contribution is not possible due to the nanocrystalline PAD film morphology [71] and the used interdigital electrode setup [73]. While this conductivity s ¼ stotal contains all possible charge carriers, especially ionic charge carriers like protons and oxide ions are desired for solid electrolyte membranes, while the parasitic electronic conductivity selectronic shall be minimized. The ionic transfer number tionic describes the ratio of ionic s ionic to total conductivity stotal, as calculated in equation (3): tionic ¼

sionic stotal  selectronic Rtotal ¼ ¼1 stotal stotal Relectronic

(3)

Here, the resistance Rtotal is determined by impedance spectroscopy, whereas Relectronic is measured using the DC polarization technique. Different voltage steps up to 100 mV were applied by a Keithley Sourcemeter 2400 to the PAD films on interdigital electrodes and the resulting current was

Please cite this article as: Exner J et al., Dense Y-doped ion conducting perovskite films of BaZrO3, BaSnO3, and BaCeO3 for SOFC applications produced by powder aerosol deposition at room temperature, International Journal of Hydrogen Energy, https://doi.org/ 10.1016/j.ijhydene.2020.01.164

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measured. Samples were placed in the identical alumina tube furnace as well as sample holder while temperatures were adjusted in the range of 400  C to 800  C in 100  C steps. Further details about the DC polarization cycle and the principle of used ion blocking electrodes are described in section ionic transfer number tionic and summary.

Results and discussion Powder aerosol deposition of all three barium-based perovskite powders resulted in the formation of homogeneous films, as visible in Fig. 1. Thicknesses between 2 and 10 mm were easily achieved during initial parameter screening. For a detailed and comparable electrical investigation, the number of scans for each powder was adjusted in order to build films between 5 and 9 mm in thickness on the interdigital electrodes. The “results and discussion” section is subdivided in respect to different film properties like morphology, hardness, as well as crystalline and electrical properties. Each subsection describes the three different materials side-by-side for better comparability.

Film morphology The film morphology of PAD films on alumina substrates was investigated by SEM. In case of BCY20, an additional investigation of a film on platinum interdigital electrodes was conducted.

Barium zirconate (BZY20) The fractured cross-section of the yttria doped barium zirconate film (BZY20) is displayed in Fig. 2. Subfigures (a) to (c) show different magnifications of an untreated (as-deposited) film, while (d) depicts the film after a thermal annealing at

5

1000  C (identical scale to subfigure c). The low-magnification overview reveals the formation of a uniform film with a thickness of about 10 mm on the alumina substrate. Higher magnifications reveal the typical undulation of the film surfaces, which is the result of the continuous particle impact during deposition combined with a plastic deformation (particle flattening) [65,66] and consolidation through subsequent impacting particles (hammering effect) [64]. While the sintered alumina substrates consists of large grains, the PAD film comprises a highly consolidated, nanocrystalline structure, especially visible within the highest-magnification image (c). Furthermore, a tight bonding to the substrate is apparent. Only on this scale, a few nanopores can be detected. Nevertheless, the film exhibits a superior integrity with an almost dense microstructure, also free of cracks or delamination. Since thermal annealing is conducted on films during electrical characterization, the film morphology was investigated after the highest used annealing temperature of 1000  C (Fig. 2 d). Even after this heat treatment procedure, the films still feature the nanocrystalline film morphology with the identical amount of nanopores. It can be stated that for these high melting point ceramics, conventional sintering effects do not have to be considered even for relatively high temperature of 1000  C. This fact becomes important for the discussion concerning the electrical properties of untreated and thermally annealed PAD films (see below).

Barium stannate (BSY20) The cross-sectional SEM-images of the yttria-doped barium stannate (BSY20) film are shown in Fig. 3 (a) to (c) with increasing magnification. This film features the seamless bonding to the alumina substrate, too, unaffected of the substrate roughness. In this case, the BSY20 film's surface appears even flatter compared to the interface to the substrate. This may be related to

Fig. 2 e SEM images (fractured cross-section) of a powder aerosol deposited BZY20 film on an alumina substrate: (a) lowmagnification overview to (c) high-magnification film morphology and (d) after a thermal annealing treatment at 1000  C. Please cite this article as: Exner J et al., Dense Y-doped ion conducting perovskite films of BaZrO3, BaSnO3, and BaCeO3 for SOFC applications produced by powder aerosol deposition at room temperature, International Journal of Hydrogen Energy, https://doi.org/ 10.1016/j.ijhydene.2020.01.164

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Fig. 3 e Cross-sectional SEM images of a powder aerosol deposited BSY20 film on an alumina substrate: (a) lowmagnification overview to (c) high-magnification film morphology. Magnification of images increases from left to right. particle size of d50 ¼ 1.0 mm that is significantly smaller than for the other investigated powders with values of d50 ¼ 1.3e1.7 mm (see Table 1). As a result, the film thickness is not constant but in the range of 5e10 mm. This BSY20 film is fully dense with no visible defects like pores or cracks, indicating that the RTIC mechanism successfully took place and the film is completely consolidated.

quality of the corresponding interface on alumina. Furthermore, the microstructure is identical independently of the used substrate. To summarize, the powder aerosol deposition of all three perovskite powders yielded well consolidated films at room temperature with a tight bonding to all investigated substrates. Films show a nearly dense, predominant nanocrystalline microstructure.

Barium cerate (BCY20) SEM images of the yttria-doped barium cerate (BCY20) film are shown in Fig. 4 when alumina (a)-(c) and interdigital electrodes on alumina (d)-(f), respectively, were chosen as substrate. On the low-magnification image (a), the typical surface undulation as for BSY20 films becomes apparent, possibly caused again by the larger particle size (d50 ¼ 1.3 mm). The 7 mm thick BCY20 film exhibits an equally good interface to the substrate as for the BZY20 and BSY20 films and therefore also features a high adhesion. However, at the highest magnification, the microstructure of the film shows a deviation from the previous PAD films with an increased amount of micropores. Presumably, the RTIC mechanism took place, since the majority of visible grains within the film are below 200 nm and therefore significantly smaller than the d50 of the used BCY20 particles. This points out, that the film consolidation occurred, yet possibly to a slightly smaller extend. The BCY20 film on platinum interdigital electrodes exhibits an identical appearance as on the alumina substrate. The interface between film and platinum electrode matches the

Film hardness The hardness of powder aerosol deposited films is an additional indicator for the success of the consolidation, especially when compared to hardness values of sintered bulk samples. As already shown for powder aerosol deposited ceria films [78], the film hardness is strongly dependent on whether the Room Temperature Impact Consolidation mechanism occurs or not. There, a binary outcome was observed: if RTIC took place, films feature a very high, near bulk-like hardness caused by the highly consolidated film; otherwise, films possess a chalklike, non-consolidated microstructure with a very low hardness. In order to avoid any influence of the substrate to the indentation hardness measurement, a small indentation force of 100 mN was set, resulting in an indentation depth not larger than 1 mm. For direct comparison, sintered bulk samples were produced using the identical powders as for PAD. However, a binder (2 wt.-% BASF Luviskol VA37 and 3 wt.-% 1,2-

Fig. 4 e Cross-sectional SEM images of a powder aerosol deposited BCY20 film on an alumina substrate (a)e(c) and platinum interdigital electrodes (d)e(f). Magnification of images increases from left to right. Please cite this article as: Exner J et al., Dense Y-doped ion conducting perovskite films of BaZrO3, BaSnO3, and BaCeO3 for SOFC applications produced by powder aerosol deposition at room temperature, International Journal of Hydrogen Energy, https://doi.org/ 10.1016/j.ijhydene.2020.01.164

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Butanediol) was added before uniaxial pressing at 110 MPa. Sintering was conducted in a muffle furnace in air atmosphere for 10 h at a peak temperature of 1600  C for BSY20 and BZY20 and 1500  C for BCY20, respectively. Fig. 5 shows the microhardness values of all films and sintered bulk samples. Error bars denote the corresponding standard deviation. For all investigated materials, the film hardness exceeds those of the sintered samples. While for BZY20, the hardness of the film is already about a factor of 2.4 larger than bulk (PAD: 3.33 GPa vs bulk: 1.38 GPa), the difference is even larger for BSY20 (PAD: 4.05 GPa vs bulk: 0.54 GPa) and BCY20 (PAD: 2.36 GPa vs bulk: 0.12 GPa) with factors of 7.5 and 20, respectively. This imposingly underlines the advantage of powder aerosol deposition to produce high quality ceramic films directly from a raw powder. In contrast to conventional sintering, no additional binders nor a high temperature heat treatment are necessary to achieve well-consolidated, hard films. Since only a standard, non-optimized binder system was used for bulk sample production, hardness values are relatively small, possibly due to a still high porosity. In case of optimized sintering profiles and binder systems, higher Vickers hardness values were reported: 4.95 GPa for barium zirconate [79], 5.0 GPa for barium stannate [80] and 2.34 GPa for BaCeO3 [79]. In terms of mechanical properties, PAD films are close to literature values, especially in case of BCY20 and BSY20.

Crystalline properties

micro hardness / GPa

XRD was used for structure verification of all powders and thereof sprayed films on (911) silicon substrates. Fig. 6 shows the XRD pattern of powders and films of (a) BZY20, (b) BSY20, and (c) BCY20. The left-hand side displays the complete measured 2q range, while each (110) reflection close to 2q ¼ 30 is shown in an additional inset on the right-hand side. All synthesized powders are single-phased cubic perovskites, matching published powder diffraction files within the ICDD database (BaZrO3: 00-006-0399, BaSnO3: 00-015-0780, and BaCeO3: 00-022-0074). No residues of the starting oxides nor of the yttria dopant were detected by XRD (assuming typical detection limits of 1% or less). Therefore, the mixed oxide synthesis proves to be suitable to synthesize the investigated yttria-doped perovskites materials. The pattern of the powder aerosol deposited films coincide with the respective used powder in terms of 2q-position of all reflections. This indicates that the crystalline phase of the powder is retained during film formation. It is a common feature of PAD related to the room 7 6 5 4 3 2 1 0

7

temperature processing combined with the short impact times in the nanosecond range. According to XRD, all PAD films are single-phased cubic perovskites, too. While reflection positions of powder and film agree, the reflection shapes are distinctly different. All films exhibit a significantly broadened reflection, clearly visible in the magnified inset of the (110) reflection. Please note that for a better comparability, the data of the inset was adjusted with an FFT filter (fast Fourier transformation, Origin 2018G software, cutoff frequency of (0.2 )1, Fourier components with frequencies higher than the cutoff frequency were removed) and that the 2q reflection positions were matched compensating the displacement error of the measurement (changes in 2q < 0.05 ). The altered shape of the reflection is attributed to a change in crystallite size and microstrain during deposition. Through Rietveld refinement, both parameters can be determined independently and are summarized in Table 2. All powders exhibit only little to no microstrain with crystallite sizes between 160 nm and 260 nm. While BSY20 powders have the smallest crystallite size of 162 nm, BZY20 and BCY20 show increased sizes of 221 nm and 256 nm, respectively. In one of our previous publications, we demonstrated that for ceria powders a crystallite size between 150 nm and 300 nm is preferable for powder aerosol deposition [78]. By using the mixed-oxide synthesis, a suitable crystallite size is directly achieved during the calcination without any further effort. During powder aerosol deposition, crystallite sizes considerably decrease due to particle fracturing in accordance to the theory of the RTIC mechanism. In case of BSY20, crystallites within the film are about a factor of 12 smaller than within the powder. In contrast, for BZY20 and BCY20 only a decrease by a factor of 9 and 7, respectively, occurs. Combining information from film morphology (SEM) and particle fracturing (XRD), there is a tendency that increased fracturing factors may lead to a more consolidated film with less nanopores. A microstrain increase during film formation was observed for all materials. This is a side effect of the RTIC mechanism and was evoked by the plastic deformation during particle impact. Hence, this increase in uniform microstrain [81] is also a sign of a successful deposition to dense films [78]. Before we continue with the electrical characterization of the deposited films, the general results of the film deposition are summarized shortly. All films formed by powder aerosol deposition feature a nearly dense film morphology with a high hardness and exhibit a predominant nanocrystalline, yet single-phased microstructure. Therefore, the films are well suited for the subsequent electrical characterization, since any contributions of impurities or secondary phases on the electrical results can be neglected.

Electrical conductivity

PAD

bulk BZY20

bulk

PAD BSY20

PAD bulk BCY20

Fig. 5 e Vickers microhardness data of powder aerosol deposited films and sintered bulk samples (determined according to DIN EN ISO 14577e1).

All samples were measured using electrochemical impedance spectroscopy (EIS) in air atmosphere upon heating and subsequent cooling. For each of the three perovskite film materials, impedance spectroscopy data is shown in the Nyquist representation in subfigures at temperatures of 400  C, 600  C, and 800  C, respectively. Each subfigure contains the EIS data at a particular measuring temperature upon heating (hollow

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8

30

40

50 2θ / °

60

20

70

80

BSY20

PAD film powder 30

40

50 2θ / °

60

relative intensity / (a.u.)

20 (c)

70

80

BCY20

PAD film powder 30

40

50 2θ / °

60

70

80

relative intensity / (a.u.)

powder

BZY20 PAD film powder

29 relative intensity / (a.u.)

(b)

PAD film

relative intensity / (a.u.)

20

BZY20

30 2θ / °

31 BSY20

PAD film

powder

29 relative intensity / (a.u.)

(a)

relative intensity / (a.u.)

international journal of hydrogen energy xxx (xxxx) xxx

28

30 31 2θ / ° BCY20

PAD film powder

29 2θ / °

30

Fig. 6 e X-ray diffraction (XRD) patterns of the different films on silicon substrates formed by powder aerosol deposition and the therefore used (mixed oxide synthesized) powders in the complete 2q range from 20 to 80 : (a) BZY20, (b) BSY20, and (c) BCY20. The dashed area marks the (110) reflection and is shown in detail in the inset on the right-hand side of each subfigure. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Table 2 e Crystallite sizes and microstrain values of all investigated perovskite powders and thereof sprayed PAD films, as determined by Rietveld method. Material BZY20 BSY20 BCY20

Type

Crystallite size

Microstrain

powder film powder film powder film

221 nm 25 nm 162 nm 14 nm 256 nm 34 nm

0.02% 0.25% 0.09% 0.30% 0.03% 0.20%

circles) and cooling (filled circles). Because of the used interdigital electrode structure, total resistance values (given in U) are used instead of area specific resistances (ASR, typically given in U/cm2). In cases of largely differing electrical resistances, e.g. one of both measurement is hardly visible anymore, an additional inset with a magnified section of the figure is provided in the respective subfigure. Please note that the inset is plotted with the same axis units (kU or MU), however with a different axis scaling. Electrical conductivities

s are calculated as described in section Electrical conductivity and ionic transfer number and equation (1). This quantity contains all charge carriers, like protons, oxide ions, electrons, or defect electrons, and therefore represents the total conductivity.

Barium zirconate (BZY20) The EIS data of the 5 mm thick BZY20 film are displayed in Fig. 7. For all measured temperatures, only a single contribution is apparent. While at 400  C and 600  C a fully developed (partially flattened) semicircle exists, at higher temperatures only the high frequency part of this semicircle is still visible. The electrical resistance of the BZY20 film upon heating is 70 MU, 125 kU, and 0.8 kU at 400  C, 600  C, and 800  C, respectively. As a consequence of the thermal annealing occurring during the measurement up to 1000  C, the electrical resistances differ when measured during cooling with values of 85 kU, 2.6 kU, and 0.3 kU at 400  C, 600  C and 800  C, respectively. Especially at the lowest measured temperature of 400  C, the resistance dropped by nearly three orders of magnitude due to annealing step. This behavior was already

Please cite this article as: Exner J et al., Dense Y-doped ion conducting perovskite films of BaZrO3, BaSnO3, and BaCeO3 for SOFC applications produced by powder aerosol deposition at room temperature, International Journal of Hydrogen Energy, https://doi.org/ 10.1016/j.ijhydene.2020.01.164

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20

0

0.1

0

0.1

0.2

20 40 Re{Z} / MΩ

60

-Im{Z} / kΩ

Heating 2

0

Cooling

0

0

1

2

3

150

(c) T=800°C Heating

-Im{Z} / kΩ

0.2 0.0 0.0

Cooling 0.2

0.4 0.6 Re{Z} / kΩ

40 0

50 0

60 0

10-6

He atin g

0.8

0.9

g

1.0 1.1 1.2 1.3 1000/T / 1/K

PAD film

1.4

1.5

4

50 100 Re{Z} / kΩ

0.4

Cool in

Fig. 8 e Electrical conductivity s of the BZY20 film (with a thickness of 5 mm) in air atmosphere upon heating and subsequent cooling. A sintered bulk sample, produced from the identical powder, is given as reference.

1 0

10-4

10-8

(b) T=600°C 50

BZY20 sintered bulk

10-2

Cooling

0.0 0.0

T / °C 70 0

Heating

10 00 90 0 80 0

BZY

σ / 1/Ωcm

-Im{Z} / MΩ

(a) T=400°C

0.8

1.0

Fig. 7 e Impedance spectroscopy data of a BZY20 film (with a thickness of 5 mm) on platinum interdigital electrodes in air, upon initial heating and upon subsequent cooling (after a maximum temperature of 1000  C). Shown is the Nyquist representation of the spectra at (a) 400  C, (b) 600  C, and (c) 800  C. The insets use the same axis labels and units as their corresponding subfigure and describe the sample upon cooling in detail.

film exhibits a significantly increased activation energy Ea of 1.73 eV compared to 0.82 eV of the sintered sample. At the highest measured temperature of 1000  C, the conductivity s ¼ 1.4∙102 S/cm of the film is close to the sintered bulk (s ¼ 3.6∙102 S/cm). Upon cooling, both BZY20 samples show an almost identical behavior with a similar activation energy (film: Ea ¼ 0.88 eV and bulk: Ea ¼ 0.82 eV). The results indicate that a minimum annealing temperature of 900-1000  C is required for BZY20 films to recover the high electrical conductivities that are typical for this material composition. Compared to conventional sintering with required temperatures of 1600  C to 1750  C [29,89], powder aerosol deposition is still beneficial since highly dense films with high electrical conductivities are already achieved at moderate annealing temperatures up to 1000  C. This increase in conductivity during initial heating is a remanent one. All conductivity values after the first heating (consecutive measurements) match the data upon cooling, as already demonstrated for ionic [71] and mixed ionic and electronic [57] conductors.

Barium stannate (BSY20) observed for different electrical conducting functional materials when processed to films by aerosol deposition [71,73,82e85]. As-deposited PAD films bear a high compressive strain [86] in combination with a distorted crystalline lattice [87,88], both caused by the RTIC mechanism during film formation. This strain, the dislocations, and even amorphous regions impede the movement of charge carriers in asdeposited, untreated films. During the first heating, however way below typical sintering temperatures, the strain is released and the local distortion of the crystalline lattice diminishes [71]. Consequently, close to bulk-like conductivities can be regained. The described annealing behavior is clearly visible in the Arrhenius-like representation of the total electrical conductivity s of the BZY20 film in Fig. 8. The electrical conductivity s of the untreated BZY20 film at the lowest measured temperature of 400  C is 2.5∙108 S/cm and therefore 3.5 orders of magnitude smaller than for the sintered bulk sample produced by the identical powder. During initial heating, the difference in s between both sample types continuously diminishes, because the deposited BZY20

The impedance spectroscopy data of 7 mm thick BSY20 film at temperatures of 400  C, 600  C, and 800  C, respectively are shown in Fig. 9. The data consist of two contributions with a semicircle at high frequencies and a flattened semicircle at low frequencies. At higher measuring temperatures, both contributions merge progressively. The electrical resistance of the BSY20 film upon heating is 700 kU, 6.4 kU, and 0.84 kU at 400  C, 600  C and 800  C, respectively. Upon cooling after annealing at 1000  C, these resistances are also lowered with values of 38 kU, 3.1 kU, and 0.59 kU at 400  C, 600  C. and 800  C, respectively. Compared to the BZY20 film, the annealing effect is smaller. At a measuring temperature of 400  C, R decreased by a factor of 18 by thermal annealing, compared to a higher factor of 823 for the BZY20 film. The calculated total conductivity s of the BSY20 film is shown in Fig. 10 in comparison to a sintered bulk sample from Ref. [9]. Upon initial heating, the conductivity s of the BSY20 film at 400  C is 2.2∙106 S/cm and therefore again below s of a sintered bulk sample [9], yet only by 1.5 orders of magnitude. During heating, s of the film rises to 4.5∙103 S/cm at 1000  C

Please cite this article as: Exner J et al., Dense Y-doped ion conducting perovskite films of BaZrO3, BaSnO3, and BaCeO3 for SOFC applications produced by powder aerosol deposition at room temperature, International Journal of Hydrogen Energy, https://doi.org/ 10.1016/j.ijhydene.2020.01.164

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international journal of hydrogen energy xxx (xxxx) xxx

(a) T=400 °C

Heating

Cooling

0.00 0.00

0.02

0.04

Cooling 0.2

0.4 0.6 0.8 Re{Z} / MΩ

1.0

Barium cerate (BCY20) The measured EIS data of the 9 mm thick BCY20 film on interdigital electrodes are shown in Fig. 11 in the Nyquist representation. In general, two contributions are visible: a pronounced semicircle at high frequencies and a less distinctive, flattened component (especially upon heating) at low frequencies. Since the second component is not well defined during initial heating, possibly due to undergoing annealing effects, the resistance of the film is determined as the value of Re{Z} at the lowest measured frequency. Therefore, the electrical resistance of the BCY20 film upon heating is 200 kU, 2.9 kU, and 270 U at 400  C, 600  C, and 800  C, respectively. After annealing at 900  C, the resistances drop to 4.7 kU, 398 U, and

4 Heating 2 0

Cooling 0

2

4 6 Re{Z} / kΩ

8

10

(c) T=800°C Heating

-Im{Z} / kΩ

0.4

(a) T=400°C

0.2

0.2

0.4 0.6 Re{Z} / kΩ

0.8

1.0

σ / 1/Ωcm

sintered bulk

10-4 Hea ting

10

10-8

0.8

1.0

1.2 1.4 1000/T / 1/K

PAD film

1.6

1.8

Fig. 10 e Electrical conductivity s of the BSY20 film (with a thickness of 7 mm) in air atmosphere upon heating and subsequent cooling. A sintered BSY20 bulk sample is given as reference [9]. with a high activation energy of Ea ¼ 0.98 eV (bulk sample: Ea ¼ 0.60 eV). Upon subsequent cooling, the film features the bulk-like behavior with an identical activation energy of 0.60 eV. Also, the total conductivity of both samples closely

Cooling 0

50

100 150 Re{Z} / k:

200

(b) T=600°C 2 0.2

1

0

30 0

40 0

50 0

BSY20

10-2

3 Cooling 2 1 0 0 1 2 3 4 5 6

Heating

50 0

T / °C 60 0

10 0 90 0 0 80 0 70 0

Fig. 9 e Impedance spectroscopy data of a BSY20 film (with a thickness of 7 mm) on platinum interdigital electrodes in air, upon initial heating and upon subsequent cooling (after a maximum temperature of 1000  C). Shown is the Nyquist representation of the spectra at (a) 400  C, (b) 600  C, and (c) 800  C. The insets use the same axis labels and units as their corresponding subfigure and describe the sample upon cooling in detail.

-6

BCY

100

Cooling

0.0 0.0

-Im{Z} / k:

-Im{Z} / kΩ

(b) T=600°C

-Im{Z} / k:

0.0 0.0

Heating

0.0 0.0

Cooling 0.2

0.4

Cooling 0

1

2 3 Re{Z} / k:

4

(c) T=800 °C 0.2 -Im{Z} / k:

-Im{Z} / MΩ

0.02

0.4 0.2

matches with only a minor derivation about a factor of two. Therefore, the annealing behavior of the deposited BSY20 film generally follows that of the BZY20 film. However, it seems that for BSY20 already lower annealing temperatures may be sufficient. At 900  C, conductivities upon heating and cooling coincide, indicating that the annealing procedure is completely finished.

BSY

Heating

0.1

0.0 0.0

Cooling 0.1

0.2 0.3 Re{Z} / k:

0.4

Fig. 11 e Impedance spectroscopy data of a BCY20 film (with a thickness of 9 mm) on platinum interdigital electrodes in air, upon initial heating and upon subsequent cooling (after a maximum temperature of 900  C). Shown is the Nyquist representation of the spectra at (a) 400  C, (b) 600  C, and (c) 800  C. The insets use the same axis labels and units as their corresponding subfigure and describe the sample upon cooling in detail.

Please cite this article as: Exner J et al., Dense Y-doped ion conducting perovskite films of BaZrO3, BaSnO3, and BaCeO3 for SOFC applications produced by powder aerosol deposition at room temperature, International Journal of Hydrogen Energy, https://doi.org/ 10.1016/j.ijhydene.2020.01.164

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international journal of hydrogen energy xxx (xxxx) xxx

The previously shown total electrical conductivities s, determined by electrochemical impedance spectroscopy, contain contributions of all possible charge carriers like protons, oxide ions, defect electrons, or electrons. To investigate the percentage of the desired ionic conductivity compared to the parasitic electronic conductivity, a further differentiation is required. For this purpose, the ionic transfer number tionic is

V / 1/:cm

Cooli ng Hea ting

10-4 10-6

1.0

1.2 1.4 1000/T / 1/K

30 0

40 0

50 0

sintered bulk

10-2

10-8 0.8

60 0

90 0 80 0 70 0

T / °C

BCY20

PAD film

1.6

1.8

Fig. 12 e Electrical conductivity s of the BCY20 film (with a thickness of 9 mm) in air atmosphere upon heating and subsequent cooling. A sintered BCY20 bulk sample is given as reference [90].

0.10

BCY20 PAD 0.08 T = 400 °C I 3 I2

0.06 0.04

I1

0.02

U1

0.00

0

60

1.5

I4 U4

1.0

U3 I5

U2

I / µA

Ionic transfer number tionic and summary

determined using direct current polarization measurements on the previously annealed films. Since the electrodes are completely covered between the dense, gas-tight solid electrolyte and the alumina substrate, no three-phase boundaries exist due to the absence of gaseous oxygen, hydrogen or water. As a consequence, ionic charge carriers are blocked at the interface of the solid electrolyte to the metal electrode and cannot contribute to the steady-state DC current. Hence, only the electronic current is measured. Fig. 13 depicts a typical measurement where defined voltage steps Ux are applied and the resulting currents Ix in the equilibrium state are measured. Directly after a voltage increase, the current I overshoots temporarily until the ionic charge carriers are transported to the electrode interface and are blocked again. In order to verify the electrochemical stability during the DC polarization, after four consecutive voltage increases (U1 to U4) a fifth step with the lowest applied voltage U5 ¼ U1 is conducted. It is validated that the respective currents I5 and I1 also coincide and therefore any electrochemical change of the sample can be ruled out. When switching from U5 to U1, the current I temporarily undershoots, meaning that ionic charge carriers move in the opposite direction due to the lowered potential U. The electronic resistance Relectronic of the samples, which represents only the transportation of defect electrons and electrons, is calculated as the slope of applied voltages Ux over measured Ix for all five measured points. The ionic transfer number is determined in accordance to equation (3) using the electronic resistance Relectronic from DC polarization measurements and the total resistance Rtotal obtained from EIS data (section Electrical conductivity, values upon cooling after thermal annealing). Ionic transfer numbers close to 1 are desirable, since in this case the parasitic electrical losses are comparatively low. Fig. 14 shows the ionic transfer numbers tionic of aerosol deposited BZY20, BSY20 and BCY20 films within the temperature range of 400  C and 800  C. Barium zirconate (BZY20) produced by PAD has the highest ionic transfer number larger than 0.9 within the complete measured temperature range, however it decreases slightly with increasing temperature, in accordance with [92]. At 400  C, barium cerate (BCY20) initially also exhibits a very high transfer number, which however drops continuously to 0.7 at 800  C. The measured transfer numbers of the BCY20 film are thus slightly above the values from the literature (tionic ¼ 0.8 at 500  C [93]). Barium stannate (BSY20) falls far behind the other two materials and, even at 400  C, has only a transition number of 0.75, which increasingly falls to 0.50 at 800  C.

U/V

158 U at 400  C, 600  C, and 800  C, respectively. Fig. 12 shows the calculated total conductivity of the BCY20 film in comparison to a sintered sample. The conductivity upon heating increases from 5.0∙106 S/ cm at 400  C to 8.7∙103 S/cm at 900  C with an averaged activation energy of 0.99 eV. After annealing at 900  C, the activation energy Ea ¼ 0.54 eV is smaller, yet slightly larger than for sintered BCY20 bulk samples with an Ea of 0.45 eV. After annealing, the conductivity s ¼ 1.9∙104 S/cm at 400  C is about a factor of 38 higher compared to the untreated film upon heating, yet by a factor of 3 below the sintered sample. It is possible, that a higher annealing temperature could further increase the electrical conductivity, since there is still a difference of 40% at the second highest measured temperature of 800  C between heating and cooling direction. For all measured functional perovskites films produced by powder aerosol deposition, the following general behavior can be found: The electrical conductivity of the untreated films after initial powder aerosol deposition is significantly reduced, typically by one to three orders of magnitude. However, by a moderate thermal annealing of the films upto 900-1000  C, the electrical conductivities are enhanced permanently in all cases and nearly reach bulk-like conductivities. Conducted post-annealing XRD measurements showed a release of microstrain, while the crystallite size was nearly unaffected. The microstrain was identified to impede the movement of charge carriers in powder aerosol deposited films and therefore a moderate thermal annealing is reasonable to regain the high functional properties [71]. The small remaining gap between s of PAD films and sintered samples may be related to the microstructure of the films with a dense, yet flake-like morphology of grains caused by flattening of impacting particles during RTIC, as described in Refs. [73,91].

0.5

U5 120 t/s

180

0.0 240

Fig. 13 e DC polarization measurement of the BCY20 film (with a thickness of 9 mm) on platinum interdigital electrodes in air.

Please cite this article as: Exner J et al., Dense Y-doped ion conducting perovskite films of BaZrO3, BaSnO3, and BaCeO3 for SOFC applications produced by powder aerosol deposition at room temperature, International Journal of Hydrogen Energy, https://doi.org/ 10.1016/j.ijhydene.2020.01.164

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international journal of hydrogen energy xxx (xxxx) xxx

ionic transfer number τionic

1.0 BZY20 0.8 BCY20

0.6

BSY20

0.4 0.2

PAD films 0.0

400

500

600 T / °C

700

800

Fig. 14 e Comparison of the ionic transfer number tion of BZY20, BSY20, and BCY20 films produced by powder aerosol deposition; measured in air atmosphere upon cooling.

40 0

50 0

60 0

T / °C 70 0

10-1

10 00 90 0 80 0

A further separation of the ionic transfer number into a protonic and oxide ionic part was not possible yet, however is intended for future investigations. Here, proton and electron blocking electrodes made of scandium doped zirconia (ScSZ) electrodes that predominantly transport oxide ions shall be used. Up to now, the internal resistance of the ScSZ electrodes was significantly higher than the resistances of the perovskite solid electrolytes, which is why the measurement was dominated by the electrodes. Even if no differentiation into a protonic and an oxide ionic fraction is possible at the moment, the low activation energies Ea of 0.5 eVe0.6 eV, especially in case of BCY20 and BSY20, suggest a dominant protonic conductivity [94]. The total conductivities of all three materials upon cooling are compared in Fig. 15. The information gained from tionic and s enable to rank the suitability of these powder aerosol deposited films for a future application in electrochemical devices like SOFC. The statement on the evaluated films of this work depends strongly on the desired operating temperature. In the low to moderate temperature range from 400  C to just over 500  C, the yttrium-doped barium cerate (BCY20) film seems favorable, as it features the highest conductivity between 2∙104 S/ cm and 103 S/cm and a high ionic transition number between 0.87 and 0.94. Barium zirconate (BZY20) may be preferred for the high temperature range above 700  C, since it has a

σ / 1/Ωcm

10-2 10-3 BCY20 BSY20 BZY20

10-4 10-5

10-6

0.8

1.0

1.2 1000/T / 1/K

1.4

1.6

Fig. 15 e Comparison of the electrical conductivity s of BZY20, BSY20, and BCY20 dense films produced by powder aerosol deposition, measured in air atmosphere upon cooling.

comparable conductivity to BCY20, yet still owning a high ionic transfer number of around 0.9 that is significantly higher than BCY20 (0.7). In direct comparison to BCY20 and BZY20, the yttrium-doped barium stannate (BSY20) is subject to both low conductivities and lower ionic transition numbers. BCY20 and BZY20 PAD films would be potential candidates for later use in ceramic fuel cells due to their good functional properties and dense films. Since PAD films are dense and gas-tight even at film thicknesses of 5 mm and below, a very thin solid electrolyte membrane could further decrease the internal resistance of a SOFC.

Feasibility investigation e powder aerosol deposition on porous surfaces For an application of powder aerosol deposited functional films in any membrane device, the formation of dense films on porous electrodes is a crucial point. Three-phase boundaries are necessary to enable the permeation of ionized gaseous species (hydrogen, oxygen, water etc.) into a solid electrolyte through direct contact with a catalytic electrode. The electrode needs to be porous to enable the access of gaseous species, yet the solid electrolyte must be fully dense and gas-tight to prevent any gas leakage. Since powder aerosol deposition is based on the impaction and deposition of ceramic particles using the complex Room Temperature Impact Consolidation mechanism, the formation of dense films on porous surfaces may prove difficult. Here, the surface morphology may influence the momentum transfer of the impacting particle or the lowered mechanical strength of the porous electrode could lead to its destruction (as seen for sand-blasting). In order to investigate, whether or not dense solid electrolyte films can be formed on porous, gaspermeable electrodes by powder aerosol deposition, BCY20 was sprayed onto a sintered NiO-cermet anode of a commercially available SOFC. Cross-sectional SEM images of this sample are shown in Fig. 16. As visible in the low-magnification image (Fig. 16 a), a homogeneous BCY20 film was successfully applied to the porous NiO-cermet anode by powder aerosol deposition. Even larger pores with several micrometer in size were filled during deposition or are bridged by the electrolyte film (Fig. 16 b). The PAD films feature the identical morphology as the films on alumina or platinum electrodes (see Fig. 4). The highest magnification reveals that also round NiO-particles were neatly coated, proving that a certain amount of surface undulation and roughness is tolerable for PAD. The general possibility to coat porous structures may be the key to utilize the promising coating method of powder aerosol deposition for the broad field of membrane applications, making use of the highly dense and well adhering solid electrolyte films in combination with near bulk-like functional properties. Furthermore, the required temperatures, e.g. for thermal annealing, remain comparably moderate at 800  C to 1000  C compared to the otherwise much higher required sintering temperature. Typically, during cell production, at least one sintering step, e.g. for the cathode, is still necessary. Hence, an additional annealing step may even become obsolete, since annealing of the powder aerosol deposited solid electrolyte film is achieved in situ during SOFC production.

Please cite this article as: Exner J et al., Dense Y-doped ion conducting perovskite films of BaZrO3, BaSnO3, and BaCeO3 for SOFC applications produced by powder aerosol deposition at room temperature, International Journal of Hydrogen Energy, https://doi.org/ 10.1016/j.ijhydene.2020.01.164

international journal of hydrogen energy xxx (xxxx) xxx

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Fig. 16 e Cross-sectional SEM images of an aerosol deposited BCY20 film on a porous NiO-cermet structure as used in commercially available SOFC: (a) low-magnification overview to (c) high-magnification film morphology.

Another advantage of powder aerosol deposition is the possibility to build dense composite films if powder mixtures are used instead of single-phased ceramic powders [82,95,96]. Mixtures of solid electrolyte (e.g. BCY20) and anode materials (NiO) would enable a well-defined composite structure between the porous anode and the dense solid electrolyte. This could improve the adhesion of both components while lowering the thermal stress caused by different thermal expansion coefficients and furthermore enables to additional adjust the three-phase boundaries (microstructure and material composition). The demonstrated features of powder aerosol deposition underline the high potential of this coating technique for a future usage in SOFC production, that is not limited to the three investigated perovskite materials, but may also be expanded to conventional zirconia based solid electrolytes.

Conclusion In this work, we demonstrated the film deposition of three different barium-based perovskite ceramics, namely barium zirconate, barium stannate and barium cerate, using the powder aerosol deposition method. These materials feature a high ionic conductivity and hence are promising to be applied in electrochemical devices like solid oxide fuel cells. All materials doped with 20 mol-% yttrium were synthesized by the solid-state synthesis (mixed oxide route). The deposition of the dry ceramic powders by PAD led to phase-pure, highly dense films with thicknesses of 5 mme10 mm directly at room temperature without sintering. The high hardness of 2.3 GPae4.1 GPa of the untreated films is an additional indicator for the high film quality, especially since hardness values of sintered bulk samples are significantly surpassed by PAD films. XRD and Rietveld refinement confirmed the nanocrystalline structure of all films with crystallite sizes between 14 nm and 34 nm in combination with a high microstrain of 0.2%e0.3%. Both properties are a result of the underlying RTIC mechanism of the powder aerosol deposition. Electrochemical impedance spectroscopy and dc polarization measurements were conducted to investigate the temperature dependent electrical conductivity s as well as the ionic transfer number tionic. As-deposited, non-treated PAD films exhibited a significantly decreased conductivity, between 1.5 and 3 decades below their corresponding sintered bulk samples. However, upon initial heating, this difference continuously diminished in all cases and nearly bulk-like conductivities were achieved at 900e1000  C. Upon cooling, the behavior of PAD

films was identical to bulk with closely matching activation energies. The described enhancement of the electrical conductivity is attributed to the release of internal microstrain through thermal annealing that impeded the movement of charge carriers. After annealing, high conductivities of up to 103 S/cm were observed for barium cerate (BCY20) in the moderate temperature range between 400  C and 500  C and even up to 102 S/ cm for barium zirconate (BZY20) in the high temperature regime above 800  C. While BZY20 features high ionic transfer numbers larger 0.9 even up to 800  C, for BCY20 this was only true for temperatures between 400  C and 500  C. The third investigated material, barium stannate (BSY20), falls below BZY20 and BCY20 in both, conductivity and ionic transfer number. In a feasibility study, powder aerosol deposition of functional ceramics was also conducted on porous NiO electrodes. This is a preliminary yet important test for the production of dense solid electrolytes on gas permeable electrodes as necessary for the production of SOFC. The deposition was successful, showing the identical dense morphology as for non-porous alumina substrates or platinum electrodes. To summarize, the method of powder aerosol deposition demonstrated a high potential to be utilized to produce dense solid electrolytes in a variety of applications, without the otherwise necessary high sintering temperatures while still exhibiting good functional properties. In future work, we intend to investigate a working solid oxide fuel cell with a powder aerosol deposited solid electrolyte.

Acknowledgement Funding from the Bayerische Forschungsstiftung (BFS) in the framework of the collaborative research project ForOxiE2 is gratefully acknowledged (Grant AZ-1143-14). The authors are indebted to Mrs. Angelika Mergner and the Bavarian Polymer Institute (BPI) for SEM imaging, as well as to the chair of metals and alloys (Prof. Uwe Glatzel) for XRD and microhardness indentation measurements.

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Please cite this article as: Exner J et al., Dense Y-doped ion conducting perovskite films of BaZrO3, BaSnO3, and BaCeO3 for SOFC applications produced by powder aerosol deposition at room temperature, International Journal of Hydrogen Energy, https://doi.org/ 10.1016/j.ijhydene.2020.01.164

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Please cite this article as: Exner J et al., Dense Y-doped ion conducting perovskite films of BaZrO3, BaSnO3, and BaCeO3 for SOFC applications produced by powder aerosol deposition at room temperature, International Journal of Hydrogen Energy, https://doi.org/ 10.1016/j.ijhydene.2020.01.164