- Email: [email protected]
(La,Sr)(Fe,Co)O3-based cathode contact materials for intermediate-temperature solid oxide fuel cells
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 1
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
(La,Sr)(Fe,Co)O3-based cathode contact materials for intermediate-temperature solid oxide fuel cells Eugene N. Naumovich a, Kiryl Zakharchuk b, Szymon Obre˛bowski a, Aleksey Yaremchenko b,* a
Department of High Temperature Electrochemical Processes, Institute of Power Engineering e Research Institute, August owka 36, 02-981, Warsaw, Poland b CICECO e Aveiro Institute of Materials, Department of Materials and Ceramic Engineering, University of Aveiro, 3810-193 Aveiro, Portugal
article info
abstract
Article history:
Development of the cathode contact materials (CCMs) for intermediate-temperature solid
Received 18 July 2017
oxide fuel cells (IT-SOFCs) is a relatively new task promoted by the appearance of full-sized
Received in revised form
cells, which may operate below 700 C with sufficient performance, on the market. CCM
19 September 2017
must ensure a low-resistive interface between interconnect and cathode and is supposed
Accepted 22 September 2017
to be sintered at temperatures below 800 C, as imposed by the specifications of IT-SOFCs
Available online xxx
and corresponding sealants. The present work is focused on the elaboration of CCMs derived from perovskite-like La0.6Sr0.4Co0.2Fe0.8O3-d employing two approaches: introduc-
Keywords:
tion of the A-site cation vacancies and partial substitution by copper in B sublattice. Both
SOFC
approaches were found to result in a higher electrical conductivity below 800
Interconnect
compared to the parent material. All studied materials exhibit acceptable coefficients of
C if
Area-specific resistance
thermal expansion, 13.5e14.8 ppm K1 at 25e700 C. Area-specific resistance (ASR) of CCM/
Cathode contact material
chromium barrier (Mn1.5Co1.5O4)/interconnect (Crofer 22APU) assemblies prepared by the
Crofer 22 APU
screen-printing was measured in air at 660e750 C. The studies revealed that morphology
Chromium barrier
of CCM powder should be considered as a key parameter in the formation of interfaces with a low resistivity. The best ASR values, below 4 mU cm2 at 660e700 C, were obtained for La0.6Sr0.4Co0.15Cu0.10Fe0.75O3-d, La0.6Sr0.4Co0.15Cu0.05Fe0.80O3-d and (La0.60Sr0.40)0.995Co0.20 Fe0.80O3-d as CCMs. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction High-temperature electrochemical devices, solid oxide fuel/ electrolysis cells (SOFC/SOEC), offer prospective solutions for sustainable energy production [1e3]. The progress in design, reached during the last 15 years, pushed the SOFC technology close to the commercialization, whereas SOECs are considered
as one of the feasible options for chemical conservation of green energy produced from renewable sources [4e6]. Attainable performance and efficiency of the SOFC-based generators is constrained, among other factors, by internal resistivity of the cell components. In addition to polarization of the electrodes and conductivity of the solid electrolyte, particular role in SOFC with contemporary planar design is
* Corresponding author. E-mail address: [email protected] (A. Yaremchenko). https://doi.org/10.1016/j.ijhydene.2017.09.122 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Naumovich EN, et al., (La,Sr)(Fe,Co)O3-based cathode contact materials for intermediate-temperature solid oxide fuel cells, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.09.122
2
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 1
played by the resistivity of the interface between cathode and metal interconnect (Fig. 1). A need for a special layer to ensure low resistivity of cathode-interconnect interface became evident in the middle of 1990s [7,8]. The first generation of the metal interconnects (e.g. Ducrolloy® from Plansee [9]) had a high chromium content, and issues of the interfacial resistivity were considered in conjugation with unfavorable chemical reactivity between perovskite oxide cathode and chromia from a scale growing on the interconnect surface [8,10e12]. It was clearly demonstrated that chromium may interact with cathodes, directly in a contact zone and via the gas phase in the form of volatile compounds, and that stable and low resistivity of the cathode-interconnect junction may be reached if interconnect is covered with so-called chromium barrier and a special contact layer made of high-conducting oxides (for example, La0.8Sr0.2Co0.5Mn0.5O3 or La0.8Sr0.2CoO3 [11]). Utilization of the metal interconnects became feasible after introduction of special ferritic steels, such as Crofer 22H® and Crofer 22APU® from TyssenKrupp [13,14] and Santergy HT441® from Sandvik [15], to the market. These steels have thermal expansion matching that of yttria-stabilized zirconia (YSZ) solid electrolyte, and also are designed to form highconductive dense scale in oxidative conditions [16,17]. However, formation of the chromia-based scale still leads to a problem of catalytic poisoning of the cathodes by volatile CrOx compounds [18e20]. Novel cathode materials with expected stability against CrOx contamination are under development (e.g. Refs. [21,22]), however the confirmation of their applicability requires long-term tests (>5000 h) which are still to be done. Spinel-type (Mn,Co)3O4 (MCO) solid solutions demonstrated high efficiency as chromium barriers: their thermal
expansion matches that of chromium-doped ferritic steels, electrical conductivity is high enough, and these materials bind chromium as a dopant without structural changes and with a low impact on the conductivity. MCO layer with thickness ~10e15 mm was found to be sufficient to avoid evaporation of volatile chromium species [23]. While the interface between anode and interconnect is de facto metal-to-metal junction, the cathode-side counterpart is a contact between ceramics layers, which must be properly distributed along the surface of approximately 100 100 mm2. This means not only a low local resistivity, but also a uniformity which must be achieved regardless of imperfections in geometry of the cells and interconnects, as expected for mass production. This target can be reached by application of interlayer of so-called cathode contact material, or CCM, which is made of high-conductive ceramics or composite to ensure good electrical contact between the cathode and interconnect (Fig. 1). A reasonable choice for CCM seems to be perovskite ceramics closely related to the phases used as cathode materials, in particular, (La,Sr)CoO3 (Larring and Norby, [11]), (La,Sr)(Fe,M)O3 with M ¼ Co or Cu (Montero et al. [24,25]), some manganites [25e28], La(Ni,M)O3 with M ¼ Fe or n-Ruiz et al. [29,30]), etc.. Other Co (Montero et al. [25], Mora complex oxides of different structural types were also considered including some high-temperature superconductors [31] and rock-salt-like (Ni,Co)O solid solutions [32]. Composite CCMs may comprise metal particles and inorganic binders [33,34] or metal net [35,36] as a structural component and perovskite phase as a conductive filler. Generally, CCM should meet the following requirements: high electrical conductivity, chemical inertness, match in thermal expansion
Fig. 1 e Schematic drawing of electrical interfaces in the repeating unit of a planar SOFC stack: (A) state-of-the-art planar design with a ceramic interconnect and electrolyte-supported cells; (B) contemporary planar design with a stamped metallic interconnect and anode-supported cells. Please cite this article in press as: Naumovich EN, et al., (La,Sr)(Fe,Co)O3-based cathode contact materials for intermediate-temperature solid oxide fuel cells, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.09.122
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 1
with other parts, and sufficient sinterability [37]. The latter means that ceramic layer should be sintered at least partially, i.e. it should form a kind of mechanically-sustained body with stable electric junctions between the grains. One of the criteria for detection of earlier stage of sintering is so-called incipient sintering point observed in dilatometric curves, i.e. temperature in heating profile where onset of densification of the green body becomes evident (for example, Fig. 11 in Tucker et al. [37]). It is necessary to note that incipient sintering point cannot be considered as unambiguous criterion for selection of the CCM. Resistivity of a green body on early stage of the densification may strongly depend on the factors such as powder morphology (see, for example, Haviar et al. [38]). Fabrication of the intermediate-temperature SOFC imposes even more stringent requirements to the sinterability of CCM since the commercially-available glasses [39] and cells [40] are designed for the stack-assembly temperature below 750 C and safe operation zone below 700 C. Taking into account that relatively short duration of the sealing procedure by melted glass, CCM layer should be sinterable at regular operation temperatures, e.g. below 675 C. One type of materials that meet this condition is LSCF-based perovskites [37]. An important complementary factor is the chemical similarity of LSCF to lanthanum-strontium cobaltites (LSC) which are considered as most likely cathodes for IT-SOFC; full-sized cells with such cathodes are already present on the market [40]. Note that, according to producers datasheets [40], LSC cathodes offers high performance, but thermal expansion limitations did not allows heating cell beyond 750 C. Thus, LSCF can be considered as a starting point for elaboration of CCM appropriate for IT-SOFC. The present work is focused therefore on the assessment of the La0.6Sr0.4Co0.2 Fe0.8O3-based perovskite-like solid solutions as prospective CCMs. Two approaches were employed to attain the level of sinterability required for IT-SOFC: introduction of A-site vacancies into the perovskite lattice (see, for example, Mai et al. [41]), and partial substitution by a dopant (copper in the present case) that tends to form oxides with a lower melting point and can be expected to promote earlier densification (for instance, Fig. 5 in Tietz et al. [27]). Perovskite-type LaNi0.6Fe0.4O3-d (LNF) was tested in this work for comparison as an example of CCM proposed for regular SOFC [25,37]. This oxide has high electrical conductivity and coefficient of thermal expansion (11.4 ppm K1) well-matching that of YSZ [42,43], and initially was considered as SOFC cathode material [44]. The incipient sintering point of LNF (932 C) is however substantially higher compared to that of LSCF(637 C [37]). It is also necessary to note also that the conductivity of LNF may vary depending on preparation route [45]. While dense LNF samples demonstrates conductivity above 600 S cm1 in IT-SOFC thermal zone [44] poorlysintered samples may have an order of magnitude lower conductivity with clear impact of synthesis route.
Experimental Solid solutions La0.60Sr0.35Co0.20Fe0.80O3-d, La0.55Sr0.40Co0.20 Fe0.80O3-d, La0.60Sr0.40Co0.15Cu0.10Fe0.75O3-d and La0.60Sr0.40 Co0.15Cu0.05Fe0.80O3-d were synthesized by glycine-nitrate self-
3
combustion process (GNP, [46]) using La(NO3)3$6H2O (99.9%, Alfa Aesar), Sr(NO3)2 (99.0%, Sigma-Aldrich), Co(NO3)2$6H2O (98%, Sigma-Aldrich), Fe(NO3)3$9H2O (98%, Sigma-Aldrich), Cu(NO3)2$2.5H2O (98%, Sigma-Aldrich) and glycine (99%, Sigma-Aldrich) as starting reagents. After combustion, the powders were calcined in air at 950 C for 6 h and ball-milled. Ceramic samples for characterization (including commercial LSCF-based materials) were shaped by uniaxial pressing at 40 MPa followed by isostatic pressing at 200 MPa, and sintered to a gas-tight state at 1250e1300 C for 5 h. Sintered dense ceramic pellets were cut into rectangular bars and polished for electrical and dilatometric measurements. XRD patterns were recorded at room temperature using PANalytical X'Pert PRO diffractometer (CuKa radiation). Crystal lattice parameters were calculated using FullProf software [47]. The dilatometric measurements (vertical Linseis L70/2001 instrument) were carried out in flowing air between room temperature and 1100 C with a constant heating/cooling rate of 3 C/min. The electrical conductivity (s) was studied as function of temperature in air by 4-probe DC technique. Microstructural characterization was performed by scanning electron microscopy (SEM, Hitachi SU-70 microscope) coupled with energy dispersive analysis (EDS, Bruker Quantax 400 detector). Analysis of SEM images was done using ImageJ software [48]. Fine powders for preparation of the CCM layers were prepared by milling in ethanol using zirconia vessel and balls during at least 2 h in Fritsch Pulverisette 6 planetary mill. The inks for brush-painting and screen printing were prepared using a-terpineol (96%, Sigma-Aldrich) as solvent, polyvinyl butyral (ABCR) as binder, dibutyl sebacate (97%, Aldrich) as plasticizer and KD-1 (CRODA) as surfactant in weight ratio 10:(0.3…3):(0…1):0.3, correspondingly. Quantity of the ceramic component in the ink was equal to the mass of a-terpineol. Similar inks were also prepared using commercial perovskitetype La0.60Sr0.35Co0.20Fe0.80O3-d, (La0.60Sr0.40)0.995Co0.20Fe0.80O3-d and LaNi0.6Fe0.4O3-d, and spinel-type Mn1.5Co1.5O4 powders; the latter was used as a chromium barrier. For convenience, the abbreviations used for the studied materials hereafter are listed in Table 1. The inks were screen-printed or brush-painted (LSCF only) on metal coupons produced from Crofer 22APU®, a special chromium-alloyed steel designed for application as SOFC interconnect [14]. One side of the coupon disk was precovered with MCO layer following to reductive sintering procedure [23,49] in dry 4% H2e 96% Ar mixture. Then CCM ink was applied on MCO-covered area and dried at 60e70 C for at least 1 h. In the course of experiments, two prepared coupons were placed simultaneously in a special cell (Fig. 2), where areaspecific resistance (ASR) of the interfaces sandwiched between Crofer 22APU® body and platinum counter-electrode was measured by quasi-4-probe technique using bidirectional DC. Design of the experiment differs slightly from well-established techniques, where a metal paste is applied as current-collecting layer on CCM [35,37] or ceramic surrogates of cathode layer are used [11,25]. Whilst the usage of painted contacts grants higher reproducibility, the approach of “dry” plane-to-plane contacts, used in this work, allows to test the ability of CCM layer to compensate minor
Please cite this article in press as: Naumovich EN, et al., (La,Sr)(Fe,Co)O3-based cathode contact materials for intermediate-temperature solid oxide fuel cells, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.09.122
4
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 1
Table 1 e Abbreviations used for the materials. Composition
Abbreviation
Supplier
La0.60Sr0.35Co0.20Fe0.80O3-d La0.55Sr0.40Co0.20Fe0.80O3-d La0.60Sr0.40Co0.15Cu0.10Fe0.75O3-d La0.60Sr0.40Co0.15Cu0.05Fe0.80O3-d
LSCF1 LSCF2 LSCCF1 LSCCF2
Synthesized in this work by GNP
La0.60Sr0.35Co0.20Fe0.80O3-d (La0.60Sr0.40)0.995Co0.20Fe0.80O3-d
LSCF LSCF995
Fuel cell materials
Mn1.5Co1.5O4 LaNi0.6Fe0.4O3-d
MCO LNF
KCeracell
misfits in geometry and to follow more strictly the actual procedure of the stack fabrication, although may lead to a higher scattering in experimental results. The usage of ceramic cathode surrogates gives a closer reproduction of the CCM/cathode interface, but also brings some uncertainty related to the microstructural differences and bulk resistivity of the surrogate. Temperature profile for these measurements included two main stages (Fig. 3): the first stage emulated the heating plan and sealing routine of the IT-SOFC assembly, and the second stage corresponded to acquisition of the conductivity data at the IT-SOFC operation temperatures.
Results and discussion General characterization of LSCF-based perovskites
assigned to trace amounts of iron and cobalt oxide impurities. The calculated lattice parameters and the density of prepared ceramics are given in Table 2. GNP synthesis produced powders consisting of sponge-like particles agglomerates; this initial microstructure is partly maintained after ink preparation procedure (Supplementary data, Fig. S2: LSCF1). Finer commercial powders with more uniform particle size did not show any changes in the course of ink preparation (Supplementary data, Fig. S2: LSCF and LSCF995). Another comment is that LSCF powder has substantially smaller grain size compared to other CCMs; this may result in a lower electrical conductivity of ceramics due to impact of the grain boundaries.
Table 2 e Lattice parameters and density of LSCF-based perovskite ceramics. Composition
Lattice parameters a, A
LSCF1 LSCF2 LSCCF1 LSCCF2 LSCF LSCF995
5.5065 5.4985 5.4939 5.4990 5.5119 5.5012
c, A (2) (2) (2) (2) (2) (2)
13.3800 13.3759 13.3719 13.3763 13.3870 13.3753
(6) (6) (7) (7) (6) (6)
Density, g/cm3
Relative density, %
6.17 6.04 6.13 6.08 6.22 6.17
97.6 97.4 96.2 95.8 98.7 97.2
Note: theoretical density was calculated neglecting oxygen deficiency, in agreement with the experimental value of oxygen stoichiometry for La0.60Sr0.40Co0.20Fe0.80O3-d in air at room temperature [50].
XRD analysis confirmed that all synthesized and commercial LSCF-based ceramic powders have perovskite-type structure with rhombohedral distortion (space group R3c), in agreement with ICDD PDF #82-2911. Minor additional peaks (2Q ¼ 36.0e36.8 ) on the background level in XRD patterns of LSCF1, LSCF2 and LSCF (see Supplementary data, Fig. S1) were
Fig. 2 e Scheme of the sample holder for the measurements of ASR. The image in upper right corner shows a coupon made from Crofer 22APU® (∅10 mm, thickness 0.2 mm), with upper LSCCF1 and intermediate MCO layers, as it looks after tests.
Fig. 3 e Typical temperature profile for ASR measurements experiment and corresponding variations of ASR for two LSCCF1-based samples measured simultaneously.
Please cite this article in press as: Naumovich EN, et al., (La,Sr)(Fe,Co)O3-based cathode contact materials for intermediate-temperature solid oxide fuel cells, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.09.122
5
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 1
Table 3 e Properties of LSFC-based ceramics. Average CTE, ppm K1
Composition
RT-700 C
700e1100 C
13.8 14.1 13.9 14.3 13.5 14.8
22.6 24.5 24.1 24.3 21.7 24.4
LSCF1 LSCF2 LSCCF1 LSCCF2 LSCF LSCF995
s650C, S cm1
400 470 460 455 345 440
transition metal cations [52,53]. This is not crucial for IT-SOFC, but is strongly unfavorable for conventional high-temperature SOFC where the presence of layers with CTE above 15 ppm K1 leads to a mismatch of thermal expansivity with respect to other cell components and, consequently, thermomechanical instability. The electrical properties of La1-xSrxCo0.20Fe0.80O3-d perovskites strongly depend on the strontium content [50,54]. Acceptor-type substitution of lanthanum by strontium is compensated by the formation of electron-holes and oxygen € gervacancies (assuming AIIIBIIIO3 host lattice and using Kro Vink notation): 0 SrLa ¼ ½h, þ 2 VO,,
(1)
While the formation of oxygen vacancies dominates at higher temperatures, increasing strontium content results in the increase of electron-hole concentrations and electrical conductivity in the intermediate temperature range [50,54]. Incorporation of cation vacancies into the A-sublattice and acceptor-type doping by copper into the B-sublattice can be expected to contribute to the generation of chargecompensating electron-holes and oxygen vacancies:
Fig. 4 e Dilatometric curves (A) and temperature dependence of electrical conductivity (B) of dense LSCFbased ceramics in air. Literature data on LNF [45] are shown for comparison.
Dilatometric curves collected in air demonstrated a nonlinear behavior typical for perovskite-type cobaltites and ferrites [51,52] with a change of slope on heating (Fig. 4A). The coefficients of thermal expansion (CTE) vary in the range 13.5e15.0 ppm K1 in the low-temperature range, but increase up to 24.4 ppm K1 at temperatures above 700 C (Table 3). Such behavior is associated with increasing role of the chemical contribution to overall expansion originating from the oxygen losses from perovskite lattice on heating and conjugated variations of average oxidation state and radii of
h 000 i 0 SrLa þ 3 VLa ¼ ½h, þ 2 VO,,
(2)
0 0 SrLa þ CuB ¼ ½h, þ 2 VO,,
(3)
Indeed, both the approaches e a moderate introduction of the A-site vacancies and B-site substitution by copper e were found to results in an enhancement of electrical transport in the intermediate temperature range (Fig. 4B and Table 3). The conductivity reasonably increases with increasing Cu content and with decreasing La:Sr ratio in A-site deficient CCM. The results of electrical measurements demonstrate the advantage of copper doping in temperature range of 620e675 C relevant for IT-SOFC operation. This type of substitution allows to gain similar or higher electrical conductivity compared to A-site deficient LSCF995 (Fig. 4B and Table 3), but without introduction of cation vacancies. The latter is known to decrease the thermodynamic stability of perovskites (see, for example, Marinescu et al. [55]) and, in the case of LSCF-based solid solutions, may cause a segregation of secondary phases [56] leading to a poor reproducibility of properties and accelerated degradation.
Protective layer MCO layer, fabricated by the screen-printing with reductive sintering, demonstrated a uniform microstructure of the
Please cite this article in press as: Naumovich EN, et al., (La,Sr)(Fe,Co)O3-based cathode contact materials for intermediate-temperature solid oxide fuel cells, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.09.122
6
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 1
Fig. 5 e Top-view SEM micrographs of MCO layers on Crofer 22APU® coupons after ASR measurements: A, B and C e surface with different magnification; D e the edge of the MCO protecting layer near the current collector. surface (Fig. 5A and B), but variable thickness with wave-like profile (Supplementary data, Fig. S3(left)). A square pattern can be observed when viewed from the top (Fig. 5C), apparently inherited from the screen in the course of fabrication. SEM analysis demonstrated that the thickness of MCO layer is typically 20e30 mm on the top of the “wave” and 7e15 mm in a “valley”, giving averaged value of ~20 mm that in general fits in the acceptable range sufficient for protection from chromia evaporation [23,49,57]. It is important to note that, while in general the adhesion of MCO layer may be considered as very good, deformation of the coupons, propagated from the bended current collectors, can generate small cracks (Fig. 5D). These cracks are potential source of the irreproducibility, and any deformation of the coated surface should to be avoided.
ASR studies of cathode contact materials Area specific resistance (ASR) of MCO-covered coupons and coupons with perovskite-type LSCF-based or LNF CCM layer applied over MCO was measured using selected thermal profile (Fig. 3). The results are presented in Fig. 6A. As expected, MCO layer alone was found to result in highest ASR values. Despite its high incipient sintering temperature, LNF layer provided relatively low ASR, but also demonstrated scattering in ASR values up to 20% for two samples from the same batch measured simultaneously (Fig. 6B). In contrast, in the case of LSCCF1, a moderate scattering in ASR was observed between the data sets collected in different measurements rather than between two samples studied simultaneously in one experiment (Fig. 6B). The lowest ASR was
obtained for CCM layers made of LSCCF1, LSCCF2 and commercial LSCF995 powders. SEM studies revealed that fine microstructure of the CCM surface (Fig. 7) is only a secondary factor that may impact ASR. Comparison of cross-sectional micrographs of the coupons with different CCMs demonstrates that low ASR can be reached for CCM with comparatively loose microstructure such as LSCCF1 (Fig. 7, and Fig. S3(right) in Supplementary data). Apparently, a favorable morphology of CCM is a loose layer, formed by agglomerated particles of irregular shape and non-uniform size, which may support higher density of the packaging and, correspondingly, higher conductivity. As one can note, higher packaging density allows to overrun lower conductivity (see Fig. 7: LSCCF1 vs. LSCF2). It is likely that a coarse and spongy surface of CCM layer can ensure better electric contact at the interface of two solid bodies under minor mechanical load. In such conditions, the bodies themselves cannot be deformed to fit each other's surface, and resistivity of the interface will be determined by few point junctions. On the contrary, for a loose layer of CCM, such load is enough to compress it against the opposite surface and to yield evenly distributed junctions. Loose morphology of the CCM layer in some way contradicts the requirements to cathodic layer of the planar SOFC: the cathode must have a low tangential resistivity to support current collection on interconnect ribs. These tangential currents have a relatively long path (1 mm or more). Thus, the powders fabricated for cathodes must form relatively dense and tight-bonded layers; this may be reached by usage of the stiff well-formed particles of a regular shape (as observed for LNF, Fig. 7). However, the
Please cite this article in press as: Naumovich EN, et al., (La,Sr)(Fe,Co)O3-based cathode contact materials for intermediate-temperature solid oxide fuel cells, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.09.122
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 1
Fig. 6 e (A) Temperature dependence of the ASR for selected CCM samples. (B) Reproducibility of ASR measurements for assemblies with LNF and LSCCF1 contact materials. For each CCM, the plot (B) shows the data collected in two different experiments for two samples (top and bottom) simultaneously (see Fig. 2 for details).
role of CCM layer is to provide the current flow orthogonal to cathode layer with expected path length shorter than 0.1 mm, and in that case a necessary level of resistivity may be reached due to better surface-to-surface conformity. One can expect that a compressive deformation of the loose layer of flake-like CCM particles compensates lower binding and density and leads to a better performance. Due to some uncertain reasons, the quality of screenprinted layers made of commercial LSCF powder was
7
unsatisfactory: after drying, the surface was non-smooth, with macro-defects such as thinning or delamination. Therefore, LSCF layer was applied onto the coupon by brush-painting. Whilst visual quality of thus-prepared layer looked acceptable, this technique cannot ensure full morphological conformity of the LSCF layer contact surface with that of other materials prepared by screen-printing. Commercial LSCF powder had small grain size and formed a dense but very thin layer on the MCO interlayer surface, as confirmed by the microscopic studies in combination with EDS elemental mapping (Fig. 8). Image analysis gave averaged thickness of LSCF layer around 1.7 mm, which is more than one order of magnitude thinner compared to screen-printed LSCCF1 (~65 mm), LSCF995 (~43 mm) and LNF (~40 mm) layers (Fig. 7). The thin LSCF layer demonstrated only a minor advantage compared to a sole MCO layer (Fig. 6A). LSCF1 layer (with thickness of ~40 mm), with nearly identical cation composition but prepared by GNP route, showed substantially better performance, which barely can be explained by the difference in electrical conductivity (Fig. 4B). Preliminary assessment of long-term stability for the best CCMs, LSCCF1 and LSCF995, demonstrated that the degradation rate of ASR values at 650 C grows from 0.03% per h to 0.05% per h after 900 h of testing. Note that the acquisition of more accurate kinetic data requires more precise and steady temperature control and should be developed as a separate study. After almost 1000 h of testing, degradation of ASR corresponded to approximately 30% (Fig. 9). Taking into account starting ASR values of 3.5e4.0 mU cm2, an extrapolation suggest that this rate of augment in CCM/MCO/ interconnect assembly resistivity should not be critical for the cells with 0.2e1.0 U cm2 electrochemical ASR. At the same time, thermal shock introduced by unintended electricity cutoff at ~980 h led to a sharp jump of resistivity. Similar (but not so prominent) effect was induced by the short-term jump of temperature up to 662 C at ~575 h; this also proves a vulnerability of CCM/MCO/interconnect assembly to abrupt alteration of temperature. It is necessary to note that up-todate published information [58] includes claims of particular sensibility of the interconnect-CCM-cathode interface to the variations in temperature due to mismatch in thermal expansion coefficients. Post-mortem SEM/EDS analysis (see Supplementary data, Fig.S4) demonstrated that both LSCF995 and LSCCF1 layers are contaminated with chromium, which may be considered as a key cause of the degradation. However, the fact that chromium concentration in the upper CCM layer is higher than in MCO interlayer implies that contamination of CCM occurs by volatile Cr species originating from the uncovered surface of coupons rather than due to solid state chemical diffusion trough MCO. The degradation can be possibly contributed by a surface strontium segregation characteristic for LSCF-based materials in the course of long-term thermal treatments; in particular, Cr deposition was reported to occur preferentially on the segregated SrO particles on the surface of LSCF cathodes [59,60]. Possible reactivity at the interface between the layers seems to be less critical due to comparatively low temperature of tests (650 C).
Please cite this article in press as: Naumovich EN, et al., (La,Sr)(Fe,Co)O3-based cathode contact materials for intermediate-temperature solid oxide fuel cells, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.09.122
8
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 1
Fig. 7 e Microstructure of the CCM layers after tests. Left column: top-view SEM micrographs of the CCM surface; right column: cross-section of the Crofer 22APU coupons with screen-printed MCO chromium barrier interlayer and CCM top layer. Micrographs are collected after ASR measurements, and observed thickness of CCM layers may differ slightly from the actual thickness in operandi.
Please cite this article in press as: Naumovich EN, et al., (La,Sr)(Fe,Co)O3-based cathode contact materials for intermediate-temperature solid oxide fuel cells, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.09.122
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 1
9
Fig. 8 e SEM micrographs and a micrograph with overlaid EDS elemental mapping (bottom right) of cross-section of the Crofer 22APU coupon with screen-printed MCO chromium barrier interlayer and brush-painted LSCF top layer.
performed at 660e750 C in contact with Crofer 22APU interconnect and MCO chromium barrier interlayer. All studied materials exhibit acceptable coefficients of thermal expansion, 13.5e14.8 ppm K1, in low-temperature range. Minor substitution by copper in B sublattice or introduction of cation vacancies in A sublattice results in an increase of electrical conductivity at temperatures below 800 C if compared to the parent La0.60Sr0.35Co0.20Fe0.80O3-d. It was found that comparatively lax sponge-like microstructure of CCM layer is rather favorable for minimizing the resistivity by smoothing out the irregularity of screen-printed chromium barrier interlayer and thus providing higher electrical contact area. The lowest ASR values were obtained for CCM/MCO/interconnect assemblies with LSCCF1, LSCCF2 and LSCF995 as cathode contact materials. Both cation-deficient and copper-substituted LSCFbased CCM demonstrated sensitivity to thermal shock.
Acknowledgements Fig. 9 e Long-term degradation of ASR of Pt/CCM/MCO/ interconnect interfaces at 650 C.
Conclusions A series of GNP-synthesized and commercial LSCF-based perovskites and commercial LNF were evaluated as IT-SOFC cathode contact materials. The comparative studies were
E.N. Naumovich and S. Obre˛bowski would like to acknowledge financial support from the Polish Ministry of Higher Education and Science statutory program, grant no. CPC/4/STAT/2016. K. Zakharchuk and A. Yaremchenko would like to acknowledge financial support from the FCT, Portugal (project IF/01072/ 2013/CP1162/CT0001 and project CICECO e Aveiro Institute of Materials POCI-01-0145-FEDER-007679 (FCT ref. UID/CTM/ 50011/2013), financed by national funds through the FCT/MEC and when appropriate co-financed by FEDER under the PT2020 Partnership Agreement).
Please cite this article in press as: Naumovich EN, et al., (La,Sr)(Fe,Co)O3-based cathode contact materials for intermediate-temperature solid oxide fuel cells, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.09.122
10
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 1
Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.ijhydene.2017.09.122.
references
[1] Williams MC, Strakey JP, Surdoval WA. The U.S. Department of Energy, Office of Fossil Energy Stationary Fuel Cell Program. J Power Sour 2005;143:191e6. [2] Zhang X, Chan SH, Ho HK, Tan SC, Li M, Li G, et al. Towards a smart energy network: the roles of fuel/electrolysis cells and technological perspectives. Int J Hydrog Energy 2015;40:6866e919. [3] Wilberforce T, Alaswad A, Palumbo A, Dassisti M, Olabi AG. Advances in stationary and portable fuel cell applications. Int J Hydrog Energy 2016;41:16509e22. [4] Laguna-Bercero MA. Recent advances in high temperature electrolysis using solid oxide fuel cells: a review. J Power Sour 2012;203:4e16. [5] Jensen SH, Larsen PH, Mogensen M. Hydrogen and synthetic fuel production from renewable energy sources. Int J Hydrog Energy 2007;32:3253e7. [6] Ferrero D, Lanzini A, Santarelli M, Leone P. A comparative assessment on hydrogen production from low-and hightemperature electrolysis. Int J Hydrog Energy 2013;38:3523e36. [7] Schmidt H, Bru¨kner B, Fischer K. Interfacial functional layers between the metallic bipolar plate and the ceramic electrodes in the high temperature solid oxide fuel cells. In: Dokiya M, Yamamoto O, Tagawa H, Singhal SC, editors. Solid oxide fuel cells IV, Proceedings of Fourth International Symposium on Solid Oxide Fuel Cells (SOFC-IV), Yokohama, Japan, 18-23 June, 1995. USA: The Electrochemical Society, Inc.; 1995. p. 869e78. € nsel M, Pattanaik A, Khanna AS, [8] Quadakker WJ, Greiner H, Ha ner W. Compatibility of perovskite contact layers Malle between cathode and metallic interconnector plates of SOFCs. Solid State Ionics 1996;91:55e67. € ck W, Sakaki T. Ducropur, Ducrolloy e new [9] Martinz HP, Ko chromium materials. J Phys IV 1993;03(C9):205e13. [10] Hilpert K, Dos D, Miller M, Peck DH, Weiß R. Chromium vapor species over solid oxide fuel cell interconnect materials and their potential for degradation processes. J Electrochem Soc 1996;143:3642e7. [11] Larring Y, Norby T. Spinel and perovskite functional layers between plansee metallic interconnect (Cr-5 wt % Fe-1 wt % Y2O3) and ceramic (La0.85Sr0.15)0.91MnO3 cathode materials for solid oxide fuel cells. J Electrochem Soc 2000;147:3251e6. [12] Badwal SPS, Deller R, Foger K, Ramprakash Y, Zhang JP. Interaction between chromia forming alloy interconnects and air electrode of solid oxide fuel cells. Solid State Ionics 1997;99:297e310. [13] Crofer 22 H e material data sheet no. 4050. ThyssenKrupp VDM; 2010. [14] Crofer 22 APU e material data sheet no. 4046. ThyssenKrupp VDM; 2010. [15] Sandvik Sanergy HT 441dSandvik Materials Technology http://smt.sandvik.com/en/products/strip-steel/stripproducts/coated-strip-steel/sandvik-sanergy-ht/[Accessed 21 June 2017]. [16] Wu J, Liu X. Recent development of SOFC metallic interconnect. J Mater Sci Technol 2010;26:293e305.
[17] Geng S, Zhu J. Promising alloys for intermediate-temperature solid oxide fuel cell interconnect application. J Power Sour 2006;160:1009e16. [18] Yokokawa H, Horita T, Sakai N, Yamaji K, Brito ME, Xiong YP, et al. Thermodynamic considerations on Cr poisoning in SOFC cathodes. Solid State Ionics 2006;177:3193e8. [19] Schuler JA, Wuillemin Z, Hessler-Wyser A, Comminges C, Yousfi Steiner N, Van herle J. Cr-poisoning in (La,Sr)(Co,Fe)O3 cathodes after 10,000 h SOFC stack testing. J Power Sour 2012;211:177e83. [20] Jiang SP, Chen X. Chromium deposition and poisoning of cathodes of solid oxide fuel cells e a review. Int J Hydrog Energy 2014;39:505e31. [21] Zhao L, Cheng Y, Jiang SP. A new, high electrochemical activity and chromium tolerant cathode for solid oxide fuel cells. Int J Hydrog Energy 2015;40:15622e31. [22] Kolisetty A, Fu Z, Koc R. Development of La(Cr,Co,Fe,Ni)O3 system perovskites as interconnect and cathode materials for solid oxide fuel cells. Ceram Int 2017;43:7647e52. [23] Yang Zhenguo, Xia Guan-Guang, Li Xiao-Hong, Stevenson JW. (Mn,Co)3O4 spinel coatings on ferritic stainless steels for SOFC interconnect applications. Int J Hydrog Energy 2007;32:3648e54. € ver D, Cassir M, Villarreal I. [24] Montero X, Fischer W, Tietz F, Sto Development and characterization of a quasi-ternary diagram based on La0.8Sr0.2(Co,Cu,Fe)O3 oxides in view of application as a cathode contact material for solid oxide fuel cells. Solid State Ionics 2009;180:731e7. € ver D, Cassir M, Villarreal I. [25] Montero X, Tietz F, Sto Comparative study of perovskites as cathode contact materials between an La0.8Sr0.2FeO3 cathode and a Crofer22APU interconnect in solid oxide fuel cells. J Power Sour 2009;188:148e55. [26] McCarthy BP, Pederson LR, Chou YS, Zhou XD, Surdoval WA, Wilson LC. Low-temperature sintering of lanthanum strontium manganite-based contact pastes for SOFCs. J Power Sour 2008;180:294e300. [27] Tietz F, Schmidt A, Zahid M. Investigation of the quasiternary system LaMnO3eLaCoO3e‘‘LaCuO3’’dI: the series La(Mn0.5Co0.5)1xCuxO3d. J Solid State Chem 2004;177:745e51. [28] Konysheva E, Laatsch J, Wessel E, Tietz F, Christiansen N, Singheiser L, et al. Influence of different perovskite interlayers on the electrical conductivity between La0.65Sr0.3MnO3 and Fe/Cr-based steels. Solid State Ionics 2006;177:923e30. n-Ruiz A, Vidal K, Larran ~ aga A, Arriortua MI. Chemical [29] Mora compatibility and electrical contact of LaNi0.6Co0.4O3ed (LNC) between Crofer22APU interconnect and La0.6Sr0.4FeO3 (LSF) cathode for IT-SOFC. Fuel Cells 2013;13:398e403. n-Ruiz A, Vidal K, Larran ~ aga A, Arriortua MI. Evaluation [30] Mora of using LaNi0.6Fe0.4O3ed contact layer between La0.6Sr0.4FeO3 cathode and Crofer22APU interconnect for solid oxide fuel cells. Fuel Cells 2016;16:330e9. € ver D. High-temperature [31] Tietz F, Arul Raj I, Jungen W, Sto superconductor materials for contact layers in solid oxide fuel cells: I. Sintering behavior and physical properties at operating temperatures. Acta mater 2001;49:803e10. [32] Lu Zigui, Xia Guanguang, Templeton JD, Li Xiaohong, Nie Zimin, Yang Zhenguo, et al. Development of Ni1-xCoxO as the cathode/interconnect contact for solid oxide fuel cells. Electrochem Commun 2011;13:642e5. [33] Tucker MC, Cheng Lei, DeJonghe LC. Inorganic bindercontaining composite cathode contact materials for solid oxide fuel cells. J Power Sour 2013;224:174e9. n V, Trejo R, Lara[34] Tucker MC, DeJonghe LC, Garcı´a-Negro Curzio E. Mechanical and electrochemical performance of
Please cite this article in press as: Naumovich EN, et al., (La,Sr)(Fe,Co)O3-based cathode contact materials for intermediate-temperature solid oxide fuel cells, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.09.122
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 1
[35]
[36]
[37]
[38]
[39]
[40]
[41]
[42]
[43]
[44]
[45]
[46]
composite cathode contact materials for solid oxide fuel cells. J Power Sour 2013;239:315e20. n-Ruiz A, Vidal K, Larran ~ aga A, Laguna-Bercero MA, Mora zquez JM, Slater PR, et al. LaNi0.6Co0.4O3d dipPorras-Va coated on FeeCr mesh as a composite cathode contact material on intermediate solid oxide fuel cells. J Power Sour 2014;269:509e19. n-Ruiz A, Vidal K, Larran ~ aga A, Porras-Va zquez JM, Mora Slater PR, Arriortua MI. Evaluation of using protective/ conductive coating on Fe-22Cr mesh as a composite cathode contact material for intermediate solid oxide fuel cells. Int J Hydrog Energy 2015;40:4804e18. Tucker MC, Cheng Lei, DeJonghe LC. Selection of cathode contact materials for solid oxide fuel cells. J Power Sour 2011;196:8313e22. nek Z, Sajgalik Haviar M, Pa P. The use of electrical conductivity measurements to study sintering mechanisms. Ceram Int 1985;11:13e6. Glass and glass-ceramic sealants for solid oxide fuel cells”, http://www.schott.com/epackaging/english/glass/powder/ fuel_cells.html [Accessed 27 February 2017]. SOFC single cells technical data”, http://www.elcogen.com/ en/sofc-single-cells/technical-data [Accessed 27 February 2017]. € ver D. Mai A, Haanappel VAC, Uhlenbruck S, Tietz F, Sto Ferrite-based perovskites as cathode materials for anodesupported solid oxide fuel cells: Part I. Variation of composition. Solid State Ionics 2005;176:1341e50. Basu RN, Tietz F, Teller O, Wessel E, Buchkremer HP, € ver D. LaNi0.6Fe0.4O3 as a cathode contact material for Sto solid oxide fuel cells. J Solid State Electrochem 2003;7:416e20. Tucker MC, DeJonghe LC. Electrical contact material in hightemperature electrochemical devices. US20120258241 A1, April 07 2011. Chiba R, Yoshimura F, Sakurai Y. An investigation of LaNi1-xFexO3 as a cathode material for solid oxide fuel cells. Solid State Ionics 1999;124:281e8. Bevilacqua M, Montini T, Tavagnacco C, Vicario G, Fornasiero P, Graziani M. Influence of synthesis route on morphology and electrical properties of LaNi0.6Fe0.4O3. Solid State Ionics 2006;177:2957e65. Chick LA, Pederson LR, Maupin GD, Bates JL, Thomas LE, Exarhos GJ. Glycine-nitrate combustion synthesis of oxide ceramic powders. Mater Lett 1990;10:6e12.
11
[47] Rodrı´guez-Carvajal J. Recent advances in magnetic structure determination by neutron powder diffraction. Phys B Condens Matter 1993;192:55e69. [48] Abramoff MD, Magalhaes PJ, Ram SJ. Image processing with ImageJ. Biophot Int 2004;11:36e42. [49] Magdefrau NJ, Chen Lei, Sun EY, Yamanis J, Aindow M. Formation of spinel reaction layers in manganese cobaltite e coated Crofer22 APU for solid oxide fuel cell interconnects. J Power Sour 2013;227:318e26. [50] Stevenson JW, Armstrong TR, Carneim RD, Pederson LR, Weber WJ. Electrochemical properties of mixed conducting perovskites La1 - xM xCo1- yFeyO3 d (M ¼ Sr, Ba, Ca). J Electrochem Soc 1996;143:2722e9. [51] Lein HL, Wiik K, Grande T. Thermal and chemical expansion of mixed conducting La0.5Sr0.5Fe1xCoxO3d materials. Solid State Ionics 2006;177:1795e8. [52] Yaremchenko AA, Mikhalev SM, Kravchenko ES, Frade JR. Thermochemical expansion of mixed-conducting (Ba,Sr) Co0.8Fe0.2O3-d ceramics. J Eur Ceram Soc 2014;34:703e15. [53] Adler SB. Chemical expansivity of electrochemical ceramics. J Am Ceram Soc 2001;84:2117e9. [54] Tai LW, Nasrallah MM, Anderson HU, Sparlin DM, Sehlin SR. Structure and electrical properties of La1-xSrxCo1-yFeyO3. Part 2. The system La1-xSrxCo0.2Fe0.8O3. Solid State Ionics 1995;76:273e83. [55] Marinescu C, Vradman L, Tanasescu S, Navrotsky A. Thermochemistry of perovskites in the lanthanumestrontiumemanganeseeiron oxide system. J Solid State Chem 2015;230:411e7. [56] Konysheva EYu, Xu Xiaoxiang, Irvine JTS. On the existence of a-site deficiency in perovskites and its relation to the electrochemical performance. Adv Mater 2012;24:528e32. [57] Bateni MR, Wei Ping, Deng Xiaohua, petric A. Spinel coatings for UNS 430 stainless steel interconnects. Surf Coat Technol 2007;201:4677e84. [58] Blum L. An analysis of contact problems in solid oxide fuel cell stacks arising from differences in thermal expansion coefficients. Electrochim Acta 2017;223:100e8. [59] Jiang SP, Zhang S, Zhen YD. Deposition of Cr species at (La,Sr)(Co,Fe)O3 cathodes of solid oxide fuel cells. J Electrochem Soc 2006;153:A127e34. [60] Zhao L, Drennan J, Kong C, Amarasinghe S, Jiang SP. Surface segregation and chromium deposition and poisoning on La0.6Sr0.4Co0.2Fe0.8O3-d cathodes of solid oxide fuel cells. ECS Trans 2013;57:599e604.
Please cite this article in press as: Naumovich EN, et al., (La,Sr)(Fe,Co)O3-based cathode contact materials for intermediate-temperature solid oxide fuel cells, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.09.122
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
(La,Sr)(Fe,Co)O3-based cathode contact materials for intermediate-temperature solid oxide fuel cells Eugene N. Naumovich a, Kiryl Zakharchuk b, Szymon Obre˛bowski a, Aleksey Yaremchenko b,* a
Department of High Temperature Electrochemical Processes, Institute of Power Engineering e Research Institute, August owka 36, 02-981, Warsaw, Poland b CICECO e Aveiro Institute of Materials, Department of Materials and Ceramic Engineering, University of Aveiro, 3810-193 Aveiro, Portugal
article info
abstract
Article history:
Development of the cathode contact materials (CCMs) for intermediate-temperature solid
Received 18 July 2017
oxide fuel cells (IT-SOFCs) is a relatively new task promoted by the appearance of full-sized
Received in revised form
cells, which may operate below 700 C with sufficient performance, on the market. CCM
19 September 2017
must ensure a low-resistive interface between interconnect and cathode and is supposed
Accepted 22 September 2017
to be sintered at temperatures below 800 C, as imposed by the specifications of IT-SOFCs
Available online xxx
and corresponding sealants. The present work is focused on the elaboration of CCMs derived from perovskite-like La0.6Sr0.4Co0.2Fe0.8O3-d employing two approaches: introduc-
Keywords:
tion of the A-site cation vacancies and partial substitution by copper in B sublattice. Both
SOFC
approaches were found to result in a higher electrical conductivity below 800
Interconnect
compared to the parent material. All studied materials exhibit acceptable coefficients of
C if
Area-specific resistance
thermal expansion, 13.5e14.8 ppm K1 at 25e700 C. Area-specific resistance (ASR) of CCM/
Cathode contact material
chromium barrier (Mn1.5Co1.5O4)/interconnect (Crofer 22APU) assemblies prepared by the
Crofer 22 APU
screen-printing was measured in air at 660e750 C. The studies revealed that morphology
Chromium barrier
of CCM powder should be considered as a key parameter in the formation of interfaces with a low resistivity. The best ASR values, below 4 mU cm2 at 660e700 C, were obtained for La0.6Sr0.4Co0.15Cu0.10Fe0.75O3-d, La0.6Sr0.4Co0.15Cu0.05Fe0.80O3-d and (La0.60Sr0.40)0.995Co0.20 Fe0.80O3-d as CCMs. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction High-temperature electrochemical devices, solid oxide fuel/ electrolysis cells (SOFC/SOEC), offer prospective solutions for sustainable energy production [1e3]. The progress in design, reached during the last 15 years, pushed the SOFC technology close to the commercialization, whereas SOECs are considered
as one of the feasible options for chemical conservation of green energy produced from renewable sources [4e6]. Attainable performance and efficiency of the SOFC-based generators is constrained, among other factors, by internal resistivity of the cell components. In addition to polarization of the electrodes and conductivity of the solid electrolyte, particular role in SOFC with contemporary planar design is
* Corresponding author. E-mail address: [email protected] (A. Yaremchenko). https://doi.org/10.1016/j.ijhydene.2017.09.122 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Naumovich EN, et al., (La,Sr)(Fe,Co)O3-based cathode contact materials for intermediate-temperature solid oxide fuel cells, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.09.122
2
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 1
played by the resistivity of the interface between cathode and metal interconnect (Fig. 1). A need for a special layer to ensure low resistivity of cathode-interconnect interface became evident in the middle of 1990s [7,8]. The first generation of the metal interconnects (e.g. Ducrolloy® from Plansee [9]) had a high chromium content, and issues of the interfacial resistivity were considered in conjugation with unfavorable chemical reactivity between perovskite oxide cathode and chromia from a scale growing on the interconnect surface [8,10e12]. It was clearly demonstrated that chromium may interact with cathodes, directly in a contact zone and via the gas phase in the form of volatile compounds, and that stable and low resistivity of the cathode-interconnect junction may be reached if interconnect is covered with so-called chromium barrier and a special contact layer made of high-conducting oxides (for example, La0.8Sr0.2Co0.5Mn0.5O3 or La0.8Sr0.2CoO3 [11]). Utilization of the metal interconnects became feasible after introduction of special ferritic steels, such as Crofer 22H® and Crofer 22APU® from TyssenKrupp [13,14] and Santergy HT441® from Sandvik [15], to the market. These steels have thermal expansion matching that of yttria-stabilized zirconia (YSZ) solid electrolyte, and also are designed to form highconductive dense scale in oxidative conditions [16,17]. However, formation of the chromia-based scale still leads to a problem of catalytic poisoning of the cathodes by volatile CrOx compounds [18e20]. Novel cathode materials with expected stability against CrOx contamination are under development (e.g. Refs. [21,22]), however the confirmation of their applicability requires long-term tests (>5000 h) which are still to be done. Spinel-type (Mn,Co)3O4 (MCO) solid solutions demonstrated high efficiency as chromium barriers: their thermal
expansion matches that of chromium-doped ferritic steels, electrical conductivity is high enough, and these materials bind chromium as a dopant without structural changes and with a low impact on the conductivity. MCO layer with thickness ~10e15 mm was found to be sufficient to avoid evaporation of volatile chromium species [23]. While the interface between anode and interconnect is de facto metal-to-metal junction, the cathode-side counterpart is a contact between ceramics layers, which must be properly distributed along the surface of approximately 100 100 mm2. This means not only a low local resistivity, but also a uniformity which must be achieved regardless of imperfections in geometry of the cells and interconnects, as expected for mass production. This target can be reached by application of interlayer of so-called cathode contact material, or CCM, which is made of high-conductive ceramics or composite to ensure good electrical contact between the cathode and interconnect (Fig. 1). A reasonable choice for CCM seems to be perovskite ceramics closely related to the phases used as cathode materials, in particular, (La,Sr)CoO3 (Larring and Norby, [11]), (La,Sr)(Fe,M)O3 with M ¼ Co or Cu (Montero et al. [24,25]), some manganites [25e28], La(Ni,M)O3 with M ¼ Fe or n-Ruiz et al. [29,30]), etc.. Other Co (Montero et al. [25], Mora complex oxides of different structural types were also considered including some high-temperature superconductors [31] and rock-salt-like (Ni,Co)O solid solutions [32]. Composite CCMs may comprise metal particles and inorganic binders [33,34] or metal net [35,36] as a structural component and perovskite phase as a conductive filler. Generally, CCM should meet the following requirements: high electrical conductivity, chemical inertness, match in thermal expansion
Fig. 1 e Schematic drawing of electrical interfaces in the repeating unit of a planar SOFC stack: (A) state-of-the-art planar design with a ceramic interconnect and electrolyte-supported cells; (B) contemporary planar design with a stamped metallic interconnect and anode-supported cells. Please cite this article in press as: Naumovich EN, et al., (La,Sr)(Fe,Co)O3-based cathode contact materials for intermediate-temperature solid oxide fuel cells, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.09.122
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 1
with other parts, and sufficient sinterability [37]. The latter means that ceramic layer should be sintered at least partially, i.e. it should form a kind of mechanically-sustained body with stable electric junctions between the grains. One of the criteria for detection of earlier stage of sintering is so-called incipient sintering point observed in dilatometric curves, i.e. temperature in heating profile where onset of densification of the green body becomes evident (for example, Fig. 11 in Tucker et al. [37]). It is necessary to note that incipient sintering point cannot be considered as unambiguous criterion for selection of the CCM. Resistivity of a green body on early stage of the densification may strongly depend on the factors such as powder morphology (see, for example, Haviar et al. [38]). Fabrication of the intermediate-temperature SOFC imposes even more stringent requirements to the sinterability of CCM since the commercially-available glasses [39] and cells [40] are designed for the stack-assembly temperature below 750 C and safe operation zone below 700 C. Taking into account that relatively short duration of the sealing procedure by melted glass, CCM layer should be sinterable at regular operation temperatures, e.g. below 675 C. One type of materials that meet this condition is LSCF-based perovskites [37]. An important complementary factor is the chemical similarity of LSCF to lanthanum-strontium cobaltites (LSC) which are considered as most likely cathodes for IT-SOFC; full-sized cells with such cathodes are already present on the market [40]. Note that, according to producers datasheets [40], LSC cathodes offers high performance, but thermal expansion limitations did not allows heating cell beyond 750 C. Thus, LSCF can be considered as a starting point for elaboration of CCM appropriate for IT-SOFC. The present work is focused therefore on the assessment of the La0.6Sr0.4Co0.2 Fe0.8O3-based perovskite-like solid solutions as prospective CCMs. Two approaches were employed to attain the level of sinterability required for IT-SOFC: introduction of A-site vacancies into the perovskite lattice (see, for example, Mai et al. [41]), and partial substitution by a dopant (copper in the present case) that tends to form oxides with a lower melting point and can be expected to promote earlier densification (for instance, Fig. 5 in Tietz et al. [27]). Perovskite-type LaNi0.6Fe0.4O3-d (LNF) was tested in this work for comparison as an example of CCM proposed for regular SOFC [25,37]. This oxide has high electrical conductivity and coefficient of thermal expansion (11.4 ppm K1) well-matching that of YSZ [42,43], and initially was considered as SOFC cathode material [44]. The incipient sintering point of LNF (932 C) is however substantially higher compared to that of LSCF(637 C [37]). It is also necessary to note also that the conductivity of LNF may vary depending on preparation route [45]. While dense LNF samples demonstrates conductivity above 600 S cm1 in IT-SOFC thermal zone [44] poorlysintered samples may have an order of magnitude lower conductivity with clear impact of synthesis route.
Experimental Solid solutions La0.60Sr0.35Co0.20Fe0.80O3-d, La0.55Sr0.40Co0.20 Fe0.80O3-d, La0.60Sr0.40Co0.15Cu0.10Fe0.75O3-d and La0.60Sr0.40 Co0.15Cu0.05Fe0.80O3-d were synthesized by glycine-nitrate self-
3
combustion process (GNP, [46]) using La(NO3)3$6H2O (99.9%, Alfa Aesar), Sr(NO3)2 (99.0%, Sigma-Aldrich), Co(NO3)2$6H2O (98%, Sigma-Aldrich), Fe(NO3)3$9H2O (98%, Sigma-Aldrich), Cu(NO3)2$2.5H2O (98%, Sigma-Aldrich) and glycine (99%, Sigma-Aldrich) as starting reagents. After combustion, the powders were calcined in air at 950 C for 6 h and ball-milled. Ceramic samples for characterization (including commercial LSCF-based materials) were shaped by uniaxial pressing at 40 MPa followed by isostatic pressing at 200 MPa, and sintered to a gas-tight state at 1250e1300 C for 5 h. Sintered dense ceramic pellets were cut into rectangular bars and polished for electrical and dilatometric measurements. XRD patterns were recorded at room temperature using PANalytical X'Pert PRO diffractometer (CuKa radiation). Crystal lattice parameters were calculated using FullProf software [47]. The dilatometric measurements (vertical Linseis L70/2001 instrument) were carried out in flowing air between room temperature and 1100 C with a constant heating/cooling rate of 3 C/min. The electrical conductivity (s) was studied as function of temperature in air by 4-probe DC technique. Microstructural characterization was performed by scanning electron microscopy (SEM, Hitachi SU-70 microscope) coupled with energy dispersive analysis (EDS, Bruker Quantax 400 detector). Analysis of SEM images was done using ImageJ software [48]. Fine powders for preparation of the CCM layers were prepared by milling in ethanol using zirconia vessel and balls during at least 2 h in Fritsch Pulverisette 6 planetary mill. The inks for brush-painting and screen printing were prepared using a-terpineol (96%, Sigma-Aldrich) as solvent, polyvinyl butyral (ABCR) as binder, dibutyl sebacate (97%, Aldrich) as plasticizer and KD-1 (CRODA) as surfactant in weight ratio 10:(0.3…3):(0…1):0.3, correspondingly. Quantity of the ceramic component in the ink was equal to the mass of a-terpineol. Similar inks were also prepared using commercial perovskitetype La0.60Sr0.35Co0.20Fe0.80O3-d, (La0.60Sr0.40)0.995Co0.20Fe0.80O3-d and LaNi0.6Fe0.4O3-d, and spinel-type Mn1.5Co1.5O4 powders; the latter was used as a chromium barrier. For convenience, the abbreviations used for the studied materials hereafter are listed in Table 1. The inks were screen-printed or brush-painted (LSCF only) on metal coupons produced from Crofer 22APU®, a special chromium-alloyed steel designed for application as SOFC interconnect [14]. One side of the coupon disk was precovered with MCO layer following to reductive sintering procedure [23,49] in dry 4% H2e 96% Ar mixture. Then CCM ink was applied on MCO-covered area and dried at 60e70 C for at least 1 h. In the course of experiments, two prepared coupons were placed simultaneously in a special cell (Fig. 2), where areaspecific resistance (ASR) of the interfaces sandwiched between Crofer 22APU® body and platinum counter-electrode was measured by quasi-4-probe technique using bidirectional DC. Design of the experiment differs slightly from well-established techniques, where a metal paste is applied as current-collecting layer on CCM [35,37] or ceramic surrogates of cathode layer are used [11,25]. Whilst the usage of painted contacts grants higher reproducibility, the approach of “dry” plane-to-plane contacts, used in this work, allows to test the ability of CCM layer to compensate minor
Please cite this article in press as: Naumovich EN, et al., (La,Sr)(Fe,Co)O3-based cathode contact materials for intermediate-temperature solid oxide fuel cells, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.09.122
4
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 1
Table 1 e Abbreviations used for the materials. Composition
Abbreviation
Supplier
La0.60Sr0.35Co0.20Fe0.80O3-d La0.55Sr0.40Co0.20Fe0.80O3-d La0.60Sr0.40Co0.15Cu0.10Fe0.75O3-d La0.60Sr0.40Co0.15Cu0.05Fe0.80O3-d
LSCF1 LSCF2 LSCCF1 LSCCF2
Synthesized in this work by GNP
La0.60Sr0.35Co0.20Fe0.80O3-d (La0.60Sr0.40)0.995Co0.20Fe0.80O3-d
LSCF LSCF995
Fuel cell materials
Mn1.5Co1.5O4 LaNi0.6Fe0.4O3-d
MCO LNF
KCeracell
misfits in geometry and to follow more strictly the actual procedure of the stack fabrication, although may lead to a higher scattering in experimental results. The usage of ceramic cathode surrogates gives a closer reproduction of the CCM/cathode interface, but also brings some uncertainty related to the microstructural differences and bulk resistivity of the surrogate. Temperature profile for these measurements included two main stages (Fig. 3): the first stage emulated the heating plan and sealing routine of the IT-SOFC assembly, and the second stage corresponded to acquisition of the conductivity data at the IT-SOFC operation temperatures.
Results and discussion General characterization of LSCF-based perovskites
assigned to trace amounts of iron and cobalt oxide impurities. The calculated lattice parameters and the density of prepared ceramics are given in Table 2. GNP synthesis produced powders consisting of sponge-like particles agglomerates; this initial microstructure is partly maintained after ink preparation procedure (Supplementary data, Fig. S2: LSCF1). Finer commercial powders with more uniform particle size did not show any changes in the course of ink preparation (Supplementary data, Fig. S2: LSCF and LSCF995). Another comment is that LSCF powder has substantially smaller grain size compared to other CCMs; this may result in a lower electrical conductivity of ceramics due to impact of the grain boundaries.
Table 2 e Lattice parameters and density of LSCF-based perovskite ceramics. Composition
Lattice parameters a, A
LSCF1 LSCF2 LSCCF1 LSCCF2 LSCF LSCF995
5.5065 5.4985 5.4939 5.4990 5.5119 5.5012
c, A (2) (2) (2) (2) (2) (2)
13.3800 13.3759 13.3719 13.3763 13.3870 13.3753
(6) (6) (7) (7) (6) (6)
Density, g/cm3
Relative density, %
6.17 6.04 6.13 6.08 6.22 6.17
97.6 97.4 96.2 95.8 98.7 97.2
Note: theoretical density was calculated neglecting oxygen deficiency, in agreement with the experimental value of oxygen stoichiometry for La0.60Sr0.40Co0.20Fe0.80O3-d in air at room temperature [50].
XRD analysis confirmed that all synthesized and commercial LSCF-based ceramic powders have perovskite-type structure with rhombohedral distortion (space group R3c), in agreement with ICDD PDF #82-2911. Minor additional peaks (2Q ¼ 36.0e36.8 ) on the background level in XRD patterns of LSCF1, LSCF2 and LSCF (see Supplementary data, Fig. S1) were
Fig. 2 e Scheme of the sample holder for the measurements of ASR. The image in upper right corner shows a coupon made from Crofer 22APU® (∅10 mm, thickness 0.2 mm), with upper LSCCF1 and intermediate MCO layers, as it looks after tests.
Fig. 3 e Typical temperature profile for ASR measurements experiment and corresponding variations of ASR for two LSCCF1-based samples measured simultaneously.
Please cite this article in press as: Naumovich EN, et al., (La,Sr)(Fe,Co)O3-based cathode contact materials for intermediate-temperature solid oxide fuel cells, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.09.122
5
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 1
Table 3 e Properties of LSFC-based ceramics. Average CTE, ppm K1
Composition
RT-700 C
700e1100 C
13.8 14.1 13.9 14.3 13.5 14.8
22.6 24.5 24.1 24.3 21.7 24.4
LSCF1 LSCF2 LSCCF1 LSCCF2 LSCF LSCF995
s650C, S cm1
400 470 460 455 345 440
transition metal cations [52,53]. This is not crucial for IT-SOFC, but is strongly unfavorable for conventional high-temperature SOFC where the presence of layers with CTE above 15 ppm K1 leads to a mismatch of thermal expansivity with respect to other cell components and, consequently, thermomechanical instability. The electrical properties of La1-xSrxCo0.20Fe0.80O3-d perovskites strongly depend on the strontium content [50,54]. Acceptor-type substitution of lanthanum by strontium is compensated by the formation of electron-holes and oxygen € gervacancies (assuming AIIIBIIIO3 host lattice and using Kro Vink notation): 0 SrLa ¼ ½h, þ 2 VO,,
(1)
While the formation of oxygen vacancies dominates at higher temperatures, increasing strontium content results in the increase of electron-hole concentrations and electrical conductivity in the intermediate temperature range [50,54]. Incorporation of cation vacancies into the A-sublattice and acceptor-type doping by copper into the B-sublattice can be expected to contribute to the generation of chargecompensating electron-holes and oxygen vacancies:
Fig. 4 e Dilatometric curves (A) and temperature dependence of electrical conductivity (B) of dense LSCFbased ceramics in air. Literature data on LNF [45] are shown for comparison.
Dilatometric curves collected in air demonstrated a nonlinear behavior typical for perovskite-type cobaltites and ferrites [51,52] with a change of slope on heating (Fig. 4A). The coefficients of thermal expansion (CTE) vary in the range 13.5e15.0 ppm K1 in the low-temperature range, but increase up to 24.4 ppm K1 at temperatures above 700 C (Table 3). Such behavior is associated with increasing role of the chemical contribution to overall expansion originating from the oxygen losses from perovskite lattice on heating and conjugated variations of average oxidation state and radii of
h 000 i 0 SrLa þ 3 VLa ¼ ½h, þ 2 VO,,
(2)
0 0 SrLa þ CuB ¼ ½h, þ 2 VO,,
(3)
Indeed, both the approaches e a moderate introduction of the A-site vacancies and B-site substitution by copper e were found to results in an enhancement of electrical transport in the intermediate temperature range (Fig. 4B and Table 3). The conductivity reasonably increases with increasing Cu content and with decreasing La:Sr ratio in A-site deficient CCM. The results of electrical measurements demonstrate the advantage of copper doping in temperature range of 620e675 C relevant for IT-SOFC operation. This type of substitution allows to gain similar or higher electrical conductivity compared to A-site deficient LSCF995 (Fig. 4B and Table 3), but without introduction of cation vacancies. The latter is known to decrease the thermodynamic stability of perovskites (see, for example, Marinescu et al. [55]) and, in the case of LSCF-based solid solutions, may cause a segregation of secondary phases [56] leading to a poor reproducibility of properties and accelerated degradation.
Protective layer MCO layer, fabricated by the screen-printing with reductive sintering, demonstrated a uniform microstructure of the
Please cite this article in press as: Naumovich EN, et al., (La,Sr)(Fe,Co)O3-based cathode contact materials for intermediate-temperature solid oxide fuel cells, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.09.122
6
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 1
Fig. 5 e Top-view SEM micrographs of MCO layers on Crofer 22APU® coupons after ASR measurements: A, B and C e surface with different magnification; D e the edge of the MCO protecting layer near the current collector. surface (Fig. 5A and B), but variable thickness with wave-like profile (Supplementary data, Fig. S3(left)). A square pattern can be observed when viewed from the top (Fig. 5C), apparently inherited from the screen in the course of fabrication. SEM analysis demonstrated that the thickness of MCO layer is typically 20e30 mm on the top of the “wave” and 7e15 mm in a “valley”, giving averaged value of ~20 mm that in general fits in the acceptable range sufficient for protection from chromia evaporation [23,49,57]. It is important to note that, while in general the adhesion of MCO layer may be considered as very good, deformation of the coupons, propagated from the bended current collectors, can generate small cracks (Fig. 5D). These cracks are potential source of the irreproducibility, and any deformation of the coated surface should to be avoided.
ASR studies of cathode contact materials Area specific resistance (ASR) of MCO-covered coupons and coupons with perovskite-type LSCF-based or LNF CCM layer applied over MCO was measured using selected thermal profile (Fig. 3). The results are presented in Fig. 6A. As expected, MCO layer alone was found to result in highest ASR values. Despite its high incipient sintering temperature, LNF layer provided relatively low ASR, but also demonstrated scattering in ASR values up to 20% for two samples from the same batch measured simultaneously (Fig. 6B). In contrast, in the case of LSCCF1, a moderate scattering in ASR was observed between the data sets collected in different measurements rather than between two samples studied simultaneously in one experiment (Fig. 6B). The lowest ASR was
obtained for CCM layers made of LSCCF1, LSCCF2 and commercial LSCF995 powders. SEM studies revealed that fine microstructure of the CCM surface (Fig. 7) is only a secondary factor that may impact ASR. Comparison of cross-sectional micrographs of the coupons with different CCMs demonstrates that low ASR can be reached for CCM with comparatively loose microstructure such as LSCCF1 (Fig. 7, and Fig. S3(right) in Supplementary data). Apparently, a favorable morphology of CCM is a loose layer, formed by agglomerated particles of irregular shape and non-uniform size, which may support higher density of the packaging and, correspondingly, higher conductivity. As one can note, higher packaging density allows to overrun lower conductivity (see Fig. 7: LSCCF1 vs. LSCF2). It is likely that a coarse and spongy surface of CCM layer can ensure better electric contact at the interface of two solid bodies under minor mechanical load. In such conditions, the bodies themselves cannot be deformed to fit each other's surface, and resistivity of the interface will be determined by few point junctions. On the contrary, for a loose layer of CCM, such load is enough to compress it against the opposite surface and to yield evenly distributed junctions. Loose morphology of the CCM layer in some way contradicts the requirements to cathodic layer of the planar SOFC: the cathode must have a low tangential resistivity to support current collection on interconnect ribs. These tangential currents have a relatively long path (1 mm or more). Thus, the powders fabricated for cathodes must form relatively dense and tight-bonded layers; this may be reached by usage of the stiff well-formed particles of a regular shape (as observed for LNF, Fig. 7). However, the
Please cite this article in press as: Naumovich EN, et al., (La,Sr)(Fe,Co)O3-based cathode contact materials for intermediate-temperature solid oxide fuel cells, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.09.122
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 1
Fig. 6 e (A) Temperature dependence of the ASR for selected CCM samples. (B) Reproducibility of ASR measurements for assemblies with LNF and LSCCF1 contact materials. For each CCM, the plot (B) shows the data collected in two different experiments for two samples (top and bottom) simultaneously (see Fig. 2 for details).
role of CCM layer is to provide the current flow orthogonal to cathode layer with expected path length shorter than 0.1 mm, and in that case a necessary level of resistivity may be reached due to better surface-to-surface conformity. One can expect that a compressive deformation of the loose layer of flake-like CCM particles compensates lower binding and density and leads to a better performance. Due to some uncertain reasons, the quality of screenprinted layers made of commercial LSCF powder was
7
unsatisfactory: after drying, the surface was non-smooth, with macro-defects such as thinning or delamination. Therefore, LSCF layer was applied onto the coupon by brush-painting. Whilst visual quality of thus-prepared layer looked acceptable, this technique cannot ensure full morphological conformity of the LSCF layer contact surface with that of other materials prepared by screen-printing. Commercial LSCF powder had small grain size and formed a dense but very thin layer on the MCO interlayer surface, as confirmed by the microscopic studies in combination with EDS elemental mapping (Fig. 8). Image analysis gave averaged thickness of LSCF layer around 1.7 mm, which is more than one order of magnitude thinner compared to screen-printed LSCCF1 (~65 mm), LSCF995 (~43 mm) and LNF (~40 mm) layers (Fig. 7). The thin LSCF layer demonstrated only a minor advantage compared to a sole MCO layer (Fig. 6A). LSCF1 layer (with thickness of ~40 mm), with nearly identical cation composition but prepared by GNP route, showed substantially better performance, which barely can be explained by the difference in electrical conductivity (Fig. 4B). Preliminary assessment of long-term stability for the best CCMs, LSCCF1 and LSCF995, demonstrated that the degradation rate of ASR values at 650 C grows from 0.03% per h to 0.05% per h after 900 h of testing. Note that the acquisition of more accurate kinetic data requires more precise and steady temperature control and should be developed as a separate study. After almost 1000 h of testing, degradation of ASR corresponded to approximately 30% (Fig. 9). Taking into account starting ASR values of 3.5e4.0 mU cm2, an extrapolation suggest that this rate of augment in CCM/MCO/ interconnect assembly resistivity should not be critical for the cells with 0.2e1.0 U cm2 electrochemical ASR. At the same time, thermal shock introduced by unintended electricity cutoff at ~980 h led to a sharp jump of resistivity. Similar (but not so prominent) effect was induced by the short-term jump of temperature up to 662 C at ~575 h; this also proves a vulnerability of CCM/MCO/interconnect assembly to abrupt alteration of temperature. It is necessary to note that up-todate published information [58] includes claims of particular sensibility of the interconnect-CCM-cathode interface to the variations in temperature due to mismatch in thermal expansion coefficients. Post-mortem SEM/EDS analysis (see Supplementary data, Fig.S4) demonstrated that both LSCF995 and LSCCF1 layers are contaminated with chromium, which may be considered as a key cause of the degradation. However, the fact that chromium concentration in the upper CCM layer is higher than in MCO interlayer implies that contamination of CCM occurs by volatile Cr species originating from the uncovered surface of coupons rather than due to solid state chemical diffusion trough MCO. The degradation can be possibly contributed by a surface strontium segregation characteristic for LSCF-based materials in the course of long-term thermal treatments; in particular, Cr deposition was reported to occur preferentially on the segregated SrO particles on the surface of LSCF cathodes [59,60]. Possible reactivity at the interface between the layers seems to be less critical due to comparatively low temperature of tests (650 C).
Please cite this article in press as: Naumovich EN, et al., (La,Sr)(Fe,Co)O3-based cathode contact materials for intermediate-temperature solid oxide fuel cells, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.09.122
8
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 1
Fig. 7 e Microstructure of the CCM layers after tests. Left column: top-view SEM micrographs of the CCM surface; right column: cross-section of the Crofer 22APU coupons with screen-printed MCO chromium barrier interlayer and CCM top layer. Micrographs are collected after ASR measurements, and observed thickness of CCM layers may differ slightly from the actual thickness in operandi.
Please cite this article in press as: Naumovich EN, et al., (La,Sr)(Fe,Co)O3-based cathode contact materials for intermediate-temperature solid oxide fuel cells, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.09.122
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 1
9
Fig. 8 e SEM micrographs and a micrograph with overlaid EDS elemental mapping (bottom right) of cross-section of the Crofer 22APU coupon with screen-printed MCO chromium barrier interlayer and brush-painted LSCF top layer.
performed at 660e750 C in contact with Crofer 22APU interconnect and MCO chromium barrier interlayer. All studied materials exhibit acceptable coefficients of thermal expansion, 13.5e14.8 ppm K1, in low-temperature range. Minor substitution by copper in B sublattice or introduction of cation vacancies in A sublattice results in an increase of electrical conductivity at temperatures below 800 C if compared to the parent La0.60Sr0.35Co0.20Fe0.80O3-d. It was found that comparatively lax sponge-like microstructure of CCM layer is rather favorable for minimizing the resistivity by smoothing out the irregularity of screen-printed chromium barrier interlayer and thus providing higher electrical contact area. The lowest ASR values were obtained for CCM/MCO/interconnect assemblies with LSCCF1, LSCCF2 and LSCF995 as cathode contact materials. Both cation-deficient and copper-substituted LSCFbased CCM demonstrated sensitivity to thermal shock.
Acknowledgements Fig. 9 e Long-term degradation of ASR of Pt/CCM/MCO/ interconnect interfaces at 650 C.
Conclusions A series of GNP-synthesized and commercial LSCF-based perovskites and commercial LNF were evaluated as IT-SOFC cathode contact materials. The comparative studies were
E.N. Naumovich and S. Obre˛bowski would like to acknowledge financial support from the Polish Ministry of Higher Education and Science statutory program, grant no. CPC/4/STAT/2016. K. Zakharchuk and A. Yaremchenko would like to acknowledge financial support from the FCT, Portugal (project IF/01072/ 2013/CP1162/CT0001 and project CICECO e Aveiro Institute of Materials POCI-01-0145-FEDER-007679 (FCT ref. UID/CTM/ 50011/2013), financed by national funds through the FCT/MEC and when appropriate co-financed by FEDER under the PT2020 Partnership Agreement).
Please cite this article in press as: Naumovich EN, et al., (La,Sr)(Fe,Co)O3-based cathode contact materials for intermediate-temperature solid oxide fuel cells, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.09.122
10
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 1
Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.ijhydene.2017.09.122.
references
[1] Williams MC, Strakey JP, Surdoval WA. The U.S. Department of Energy, Office of Fossil Energy Stationary Fuel Cell Program. J Power Sour 2005;143:191e6. [2] Zhang X, Chan SH, Ho HK, Tan SC, Li M, Li G, et al. Towards a smart energy network: the roles of fuel/electrolysis cells and technological perspectives. Int J Hydrog Energy 2015;40:6866e919. [3] Wilberforce T, Alaswad A, Palumbo A, Dassisti M, Olabi AG. Advances in stationary and portable fuel cell applications. Int J Hydrog Energy 2016;41:16509e22. [4] Laguna-Bercero MA. Recent advances in high temperature electrolysis using solid oxide fuel cells: a review. J Power Sour 2012;203:4e16. [5] Jensen SH, Larsen PH, Mogensen M. Hydrogen and synthetic fuel production from renewable energy sources. Int J Hydrog Energy 2007;32:3253e7. [6] Ferrero D, Lanzini A, Santarelli M, Leone P. A comparative assessment on hydrogen production from low-and hightemperature electrolysis. Int J Hydrog Energy 2013;38:3523e36. [7] Schmidt H, Bru¨kner B, Fischer K. Interfacial functional layers between the metallic bipolar plate and the ceramic electrodes in the high temperature solid oxide fuel cells. In: Dokiya M, Yamamoto O, Tagawa H, Singhal SC, editors. Solid oxide fuel cells IV, Proceedings of Fourth International Symposium on Solid Oxide Fuel Cells (SOFC-IV), Yokohama, Japan, 18-23 June, 1995. USA: The Electrochemical Society, Inc.; 1995. p. 869e78. € nsel M, Pattanaik A, Khanna AS, [8] Quadakker WJ, Greiner H, Ha ner W. Compatibility of perovskite contact layers Malle between cathode and metallic interconnector plates of SOFCs. Solid State Ionics 1996;91:55e67. € ck W, Sakaki T. Ducropur, Ducrolloy e new [9] Martinz HP, Ko chromium materials. J Phys IV 1993;03(C9):205e13. [10] Hilpert K, Dos D, Miller M, Peck DH, Weiß R. Chromium vapor species over solid oxide fuel cell interconnect materials and their potential for degradation processes. J Electrochem Soc 1996;143:3642e7. [11] Larring Y, Norby T. Spinel and perovskite functional layers between plansee metallic interconnect (Cr-5 wt % Fe-1 wt % Y2O3) and ceramic (La0.85Sr0.15)0.91MnO3 cathode materials for solid oxide fuel cells. J Electrochem Soc 2000;147:3251e6. [12] Badwal SPS, Deller R, Foger K, Ramprakash Y, Zhang JP. Interaction between chromia forming alloy interconnects and air electrode of solid oxide fuel cells. Solid State Ionics 1997;99:297e310. [13] Crofer 22 H e material data sheet no. 4050. ThyssenKrupp VDM; 2010. [14] Crofer 22 APU e material data sheet no. 4046. ThyssenKrupp VDM; 2010. [15] Sandvik Sanergy HT 441dSandvik Materials Technology http://smt.sandvik.com/en/products/strip-steel/stripproducts/coated-strip-steel/sandvik-sanergy-ht/[Accessed 21 June 2017]. [16] Wu J, Liu X. Recent development of SOFC metallic interconnect. J Mater Sci Technol 2010;26:293e305.
[17] Geng S, Zhu J. Promising alloys for intermediate-temperature solid oxide fuel cell interconnect application. J Power Sour 2006;160:1009e16. [18] Yokokawa H, Horita T, Sakai N, Yamaji K, Brito ME, Xiong YP, et al. Thermodynamic considerations on Cr poisoning in SOFC cathodes. Solid State Ionics 2006;177:3193e8. [19] Schuler JA, Wuillemin Z, Hessler-Wyser A, Comminges C, Yousfi Steiner N, Van herle J. Cr-poisoning in (La,Sr)(Co,Fe)O3 cathodes after 10,000 h SOFC stack testing. J Power Sour 2012;211:177e83. [20] Jiang SP, Chen X. Chromium deposition and poisoning of cathodes of solid oxide fuel cells e a review. Int J Hydrog Energy 2014;39:505e31. [21] Zhao L, Cheng Y, Jiang SP. A new, high electrochemical activity and chromium tolerant cathode for solid oxide fuel cells. Int J Hydrog Energy 2015;40:15622e31. [22] Kolisetty A, Fu Z, Koc R. Development of La(Cr,Co,Fe,Ni)O3 system perovskites as interconnect and cathode materials for solid oxide fuel cells. Ceram Int 2017;43:7647e52. [23] Yang Zhenguo, Xia Guan-Guang, Li Xiao-Hong, Stevenson JW. (Mn,Co)3O4 spinel coatings on ferritic stainless steels for SOFC interconnect applications. Int J Hydrog Energy 2007;32:3648e54. € ver D, Cassir M, Villarreal I. [24] Montero X, Fischer W, Tietz F, Sto Development and characterization of a quasi-ternary diagram based on La0.8Sr0.2(Co,Cu,Fe)O3 oxides in view of application as a cathode contact material for solid oxide fuel cells. Solid State Ionics 2009;180:731e7. € ver D, Cassir M, Villarreal I. [25] Montero X, Tietz F, Sto Comparative study of perovskites as cathode contact materials between an La0.8Sr0.2FeO3 cathode and a Crofer22APU interconnect in solid oxide fuel cells. J Power Sour 2009;188:148e55. [26] McCarthy BP, Pederson LR, Chou YS, Zhou XD, Surdoval WA, Wilson LC. Low-temperature sintering of lanthanum strontium manganite-based contact pastes for SOFCs. J Power Sour 2008;180:294e300. [27] Tietz F, Schmidt A, Zahid M. Investigation of the quasiternary system LaMnO3eLaCoO3e‘‘LaCuO3’’dI: the series La(Mn0.5Co0.5)1xCuxO3d. J Solid State Chem 2004;177:745e51. [28] Konysheva E, Laatsch J, Wessel E, Tietz F, Christiansen N, Singheiser L, et al. Influence of different perovskite interlayers on the electrical conductivity between La0.65Sr0.3MnO3 and Fe/Cr-based steels. Solid State Ionics 2006;177:923e30. n-Ruiz A, Vidal K, Larran ~ aga A, Arriortua MI. Chemical [29] Mora compatibility and electrical contact of LaNi0.6Co0.4O3ed (LNC) between Crofer22APU interconnect and La0.6Sr0.4FeO3 (LSF) cathode for IT-SOFC. Fuel Cells 2013;13:398e403. n-Ruiz A, Vidal K, Larran ~ aga A, Arriortua MI. Evaluation [30] Mora of using LaNi0.6Fe0.4O3ed contact layer between La0.6Sr0.4FeO3 cathode and Crofer22APU interconnect for solid oxide fuel cells. Fuel Cells 2016;16:330e9. € ver D. High-temperature [31] Tietz F, Arul Raj I, Jungen W, Sto superconductor materials for contact layers in solid oxide fuel cells: I. Sintering behavior and physical properties at operating temperatures. Acta mater 2001;49:803e10. [32] Lu Zigui, Xia Guanguang, Templeton JD, Li Xiaohong, Nie Zimin, Yang Zhenguo, et al. Development of Ni1-xCoxO as the cathode/interconnect contact for solid oxide fuel cells. Electrochem Commun 2011;13:642e5. [33] Tucker MC, Cheng Lei, DeJonghe LC. Inorganic bindercontaining composite cathode contact materials for solid oxide fuel cells. J Power Sour 2013;224:174e9. n V, Trejo R, Lara[34] Tucker MC, DeJonghe LC, Garcı´a-Negro Curzio E. Mechanical and electrochemical performance of
Please cite this article in press as: Naumovich EN, et al., (La,Sr)(Fe,Co)O3-based cathode contact materials for intermediate-temperature solid oxide fuel cells, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.09.122
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 1
[35]
[36]
[37]
[38]
[39]
[40]
[41]
[42]
[43]
[44]
[45]
[46]
composite cathode contact materials for solid oxide fuel cells. J Power Sour 2013;239:315e20. n-Ruiz A, Vidal K, Larran ~ aga A, Laguna-Bercero MA, Mora zquez JM, Slater PR, et al. LaNi0.6Co0.4O3d dipPorras-Va coated on FeeCr mesh as a composite cathode contact material on intermediate solid oxide fuel cells. J Power Sour 2014;269:509e19. n-Ruiz A, Vidal K, Larran ~ aga A, Porras-Va zquez JM, Mora Slater PR, Arriortua MI. Evaluation of using protective/ conductive coating on Fe-22Cr mesh as a composite cathode contact material for intermediate solid oxide fuel cells. Int J Hydrog Energy 2015;40:4804e18. Tucker MC, Cheng Lei, DeJonghe LC. Selection of cathode contact materials for solid oxide fuel cells. J Power Sour 2011;196:8313e22. nek Z, Sajgalik Haviar M, Pa P. The use of electrical conductivity measurements to study sintering mechanisms. Ceram Int 1985;11:13e6. Glass and glass-ceramic sealants for solid oxide fuel cells”, http://www.schott.com/epackaging/english/glass/powder/ fuel_cells.html [Accessed 27 February 2017]. SOFC single cells technical data”, http://www.elcogen.com/ en/sofc-single-cells/technical-data [Accessed 27 February 2017]. € ver D. Mai A, Haanappel VAC, Uhlenbruck S, Tietz F, Sto Ferrite-based perovskites as cathode materials for anodesupported solid oxide fuel cells: Part I. Variation of composition. Solid State Ionics 2005;176:1341e50. Basu RN, Tietz F, Teller O, Wessel E, Buchkremer HP, € ver D. LaNi0.6Fe0.4O3 as a cathode contact material for Sto solid oxide fuel cells. J Solid State Electrochem 2003;7:416e20. Tucker MC, DeJonghe LC. Electrical contact material in hightemperature electrochemical devices. US20120258241 A1, April 07 2011. Chiba R, Yoshimura F, Sakurai Y. An investigation of LaNi1-xFexO3 as a cathode material for solid oxide fuel cells. Solid State Ionics 1999;124:281e8. Bevilacqua M, Montini T, Tavagnacco C, Vicario G, Fornasiero P, Graziani M. Influence of synthesis route on morphology and electrical properties of LaNi0.6Fe0.4O3. Solid State Ionics 2006;177:2957e65. Chick LA, Pederson LR, Maupin GD, Bates JL, Thomas LE, Exarhos GJ. Glycine-nitrate combustion synthesis of oxide ceramic powders. Mater Lett 1990;10:6e12.
11
[47] Rodrı´guez-Carvajal J. Recent advances in magnetic structure determination by neutron powder diffraction. Phys B Condens Matter 1993;192:55e69. [48] Abramoff MD, Magalhaes PJ, Ram SJ. Image processing with ImageJ. Biophot Int 2004;11:36e42. [49] Magdefrau NJ, Chen Lei, Sun EY, Yamanis J, Aindow M. Formation of spinel reaction layers in manganese cobaltite e coated Crofer22 APU for solid oxide fuel cell interconnects. J Power Sour 2013;227:318e26. [50] Stevenson JW, Armstrong TR, Carneim RD, Pederson LR, Weber WJ. Electrochemical properties of mixed conducting perovskites La1 - xM xCo1- yFeyO3 d (M ¼ Sr, Ba, Ca). J Electrochem Soc 1996;143:2722e9. [51] Lein HL, Wiik K, Grande T. Thermal and chemical expansion of mixed conducting La0.5Sr0.5Fe1xCoxO3d materials. Solid State Ionics 2006;177:1795e8. [52] Yaremchenko AA, Mikhalev SM, Kravchenko ES, Frade JR. Thermochemical expansion of mixed-conducting (Ba,Sr) Co0.8Fe0.2O3-d ceramics. J Eur Ceram Soc 2014;34:703e15. [53] Adler SB. Chemical expansivity of electrochemical ceramics. J Am Ceram Soc 2001;84:2117e9. [54] Tai LW, Nasrallah MM, Anderson HU, Sparlin DM, Sehlin SR. Structure and electrical properties of La1-xSrxCo1-yFeyO3. Part 2. The system La1-xSrxCo0.2Fe0.8O3. Solid State Ionics 1995;76:273e83. [55] Marinescu C, Vradman L, Tanasescu S, Navrotsky A. Thermochemistry of perovskites in the lanthanumestrontiumemanganeseeiron oxide system. J Solid State Chem 2015;230:411e7. [56] Konysheva EYu, Xu Xiaoxiang, Irvine JTS. On the existence of a-site deficiency in perovskites and its relation to the electrochemical performance. Adv Mater 2012;24:528e32. [57] Bateni MR, Wei Ping, Deng Xiaohua, petric A. Spinel coatings for UNS 430 stainless steel interconnects. Surf Coat Technol 2007;201:4677e84. [58] Blum L. An analysis of contact problems in solid oxide fuel cell stacks arising from differences in thermal expansion coefficients. Electrochim Acta 2017;223:100e8. [59] Jiang SP, Zhang S, Zhen YD. Deposition of Cr species at (La,Sr)(Co,Fe)O3 cathodes of solid oxide fuel cells. J Electrochem Soc 2006;153:A127e34. [60] Zhao L, Drennan J, Kong C, Amarasinghe S, Jiang SP. Surface segregation and chromium deposition and poisoning on La0.6Sr0.4Co0.2Fe0.8O3-d cathodes of solid oxide fuel cells. ECS Trans 2013;57:599e604.
Please cite this article in press as: Naumovich EN, et al., (La,Sr)(Fe,Co)O3-based cathode contact materials for intermediate-temperature solid oxide fuel cells, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.09.122