Plasma-Sprayed LSM Protective Coating on Metallic Interconnect of SOFC

Abstract

In this study, a (La_0.8Sr_0.2)_0.98MnO₃ protective layer was prepared on the C276, Crofer22 APU, SUS304, and SUS430 alloys by the atmospheric plasma spraying technique (APS). The oxidation behavior and electrical property of these metal alloys have been investigated isothermally at 800 °C in air for up to 300 h. Results showed that the ferritic steels transform into MnCr₂O₄ spinels and a Cr₂O₃ layer during isothermal oxidation. The C276 alloy formed NiCr₂O₄ and FeCr₂O₄ layers; these are protective and act as an effective barrier against chromium migration into the outer oxide layer, and the alloy demonstrated good oxidation resistance and a reasonable match to the coefficient of thermal expansion of the substrate and a low-oxide scale area-specific resistance. The ASR effects on the formation of oxide scale have been investigated, and the ASR of coated samples was below 0.024 Ω·cm². It has good electrical conductivity for SOFC in long-term use.

1. Introduction

Solid oxide fuel cells (SOFCs) have recently attracted interest because of their high energy conversion efficiency, low pollution emission, effective waste-heat recovery, and relatively flexible fuel choice. The interconnection is an important component in the SOFCs, which provides electrical connection between contiguous cells as well as separation of the oxygen (cathode side) and fuel gases (anode side). Interconnected materials must possess the following characteristics: excellent electrical conductivity, corrosion resistance in oxidizing and reducing environment, and matched coefficient of thermal expansion (CTE) with other SOFC components. In recent years, metallic material systems have been investigated for interconnected applications including Fe-based [1,2,3], ZMG 232, Crofer22 APU; Cr-based [4], Cr₅FeY₂O₃; Ni-based alloys [5,6], Haynes 230, Haynes 242, Hastelloy S; Ni–Cr alloy systems; and so on. However, the Cr₂O₃ scale is known to release volatile oxides in the formation of CrO₃ or CrO₂(OH)₂, which takes place in the oxidizing environment of the SOFC cathode side [7]. The chromia-forming alloys have some inherent weakness, i.e., poisoning the cathode’s electrochemical activity and drastically deteriorating SOFC performance under operating environments of SOFC due to Cr₂O₃ evaporation over long-term operation.

In order to reduce the rate of oxide scale growth and prevent chromium volatilization to the cathode side, coating a conductive ceramic/metal layer on alloys is an effective method. Many materials have been used to decrease oxide growth, increase oxide scale conductivity, improve adhesion between oxide scale and alloys, and inhibit Cr migration from substrate to the oxide surface. The materials adopted as coatings include reactive element oxides [8], perovskites [9], and spinels [10]. The coating techniques of the mentioned materials on stainless steels include sol–gel techniques [11], screen printing slurry coating [12,13], and magnetron sputtering [14]. The coating materials are the perovskite materials with a chemical structure of ABO3, of which A and B are metallic cations: A is a large trivalent rare earth cation (e.g., La or Y) and B is usually a trivalent transition metal cation (e.g., Cr, Ni, Fe, Co, Cu or Mn). La_1−xSr_xMnO₃ (LSM) is a widely used cathode material for SOFCs because of its excellent compatibility with other cell components and good performance at high temperatures.

LSM ceramic is also employed as a barrier on metallic interconnect by various coating processes. Lee et al. [12] reported that LSCF and LSM were coated on Crofer22 APU by screen printing and sintered over temperatures ranging from 1000 to 1100 °C in N₂. The ASR at 800 °C for the LSCF coated metal was less than the one coated with LSM. Hwang et al. [15] coated LSM on STS444 by aerosol deposition and thermal spray. They indicated that LSM coating by these two methods was not enough to prevent Cr evaporation from the interconnect to the cathode. Choi et al. [16] also used aerosol deposition to form a protective coating on the STS414 interconnect. The barrier materials employed were LSM–YSZ composite ceramics. The LSM–YSZ composite ceramics showed higher stability and electrical conductivity than the sole LSM layer at 800 °C, possibly due to the high microstructural stability of YSZ against grain growth at 800 °C. Palcut et al. [17] employed the plasma spray method to fabricate the LSM coating on Ceofer22 APU and four other kinds of metallic interconnect. The results show that the LSM coating acts as an oxygen transport barrier that can significantly reduce the corrosion rate. Base on the above research, a thermal sprayed LSM coating was shown to be denser and better at Cr prevention. In this study, we used the plasma spraying technique to deposit an LSM coating on the iron-based and nickel-based interconnect and investigate the phase transformation, area-specific resistance and microstructures for each metallic interconnect in detail.

2. Experiments

2.1. Materials

The investigated metallic materials were nickel-based alloys C276 and iron-based alloys: Crofer22 APU, SUS304, and SUS430 (YoungSing Metal Enterprise Co., Ltd., Tainan, Taiwan) were used for comparison. The chemical compositions of these alloys are listed in

Table 1. Compositions of alloys selected for interconnect (wt %).

Alloys	Compositions
Fe-Based Alloys	Fe	Cr	Mn	Ni	Si	Al		Total
SUS 304	Bal.	19.16	1.41	8.38	0.64	0.18		100
SUS 430	Bal.	16.14	0.17	0.16	0.26	–		100
Crofer22 APU	Bal.	23.48	0.49	0.03	0.12	–		100
Ni-Based Alloys	Ni	Cr	Mo	Fe	Mn	W	Co	Total
C 276	Bal.	16.02	17.72	5.72	0.46	3.98	0.03	100

. The alloys were cut into rectangular samples of 10 × 10 × 1 mm³ and each sample was ground to #1000 with SiC sand paper, cleaned in acetone, and dried.

2.2. Plasma Spraying Process

Spray-dried lanthanum strontium manganese oxide (LSM) powder (Oerlikon Metco, Pfäffikon, Switzerland) with the composition (La_0.8Sr_0.2)_0.98MnO₃ was used as the plasma spray feedstock. In experiments with high-power plasmas, the torch sprayed the feedstock onto substrates that were mounted on rotating machinery. For experiments, a robot moved the torch along a path to coat stationary substrates at a standoff distance. The substrates were cooled during spraying by air gun. The plasma spraying operating parameters were varied to determine the envelope of conditions in which it is possible to produce coatings of LSM. These parameters, including the plasma gas flow rate, the plasma gas composition, the arc current, and the nozzle size, are listed in

Table 2. Parameters of plasma spraying for (La_0.8Sr_0.2)_0.98MnO₃ (LSM) coating.

Spraying Parameters	LSM
Power (kW)	50
Torch current (A)	660
Primary gas (Ar) (L/min)	50
Secondary gas (H2) (L/min)	14
Stand-off distance (cm)	10
Feedstock carrier gas (L/min)	3
Powder feed rate (rpm)	5
Surface speed, vs (rpm)	150
Transverse speed, vt (mm/s)	5
Spraying loop	3

.

2.3. Oxidation Test

Oxidation testing was performed on plasma-sprayed LSM-coated interconnects treated at 800 °C in air for 300 h; interconnects without coating were used as comparisons. The phase constituents of the precipitated oxide were identified by X-ray diffraction (XRD) (Bruker D2 Phaser, Bruker, Bremen, Germany). The surface morphologies and cross sections of the oxidized interlayer were observed by using scanning electron microscopy (SEM) (Hitachi S-4700, Hitachi, Ltd., Tokyo, Japan) with an energy dispersive spectroscopy (EDS) (Oxford Instruments, Oxfordshire, UK).

2.4. Area-Specific Resistance Measurement

The electrical resistance of oxidized specimens was measured by using a four-point method at 800 °C in air for 300 h. As shown in Figure 1, two of the oxidized surfaces were covered in Ag–Pd paste; moreover, Ag–Pd meshes attached with four Ag leads were placed on top of the paste for current collection. A constant current of 0.3 A was applied across two Ag leads using a power source; the corresponding voltage drop across the other two Ag leads was measured using a multimeter. A widely accepted parameter for scaling the electrical resistance of thermally grown oxide is area specific resistance (ASR). At each temperature, the resistance (R) was calculated according to Ohm’s law, R = (V/I)/2. The ASR was equal to the R-value multiplied by the platinum electrode screen printing electrodes on the specimen area.

3. Results and Discussion

The cross-sectional microstructures of these four specimens are shown in Figure 2. As shown in Figure 2, oxide scale on the alloy without plasma-sprayed LSM displayed various types and thickness under the 800 °C heat treatment. It is obvious that the three iron-based metals, Crofer22 APU, SUS304 and SUS430, were oxidized with the formation of MnCr₂O₄ spinel and Cr₂O₃ phases (Supplementary Materials, Figure S1, full profile refinements are necessary to obtain more information about the XRD measurements). These metals oxidized at 800 °C include not only the Cr₂O₃ but also the MnCr₂O₄ spinel phase as the major phase; the manganese element in the substrate can be completely dissolved in Cr₂O₃, so often the Cr/Mn spinel structure appears in the outer oxide layer [18]. Crofer22 APU shows a thinner oxide scale than the other two, with a thickness of 2 to 5 μm (Figure 2). This is due to the high Cr content in the Crofer22 to form dense Cr₂O₃ and inhibit the growth of oxide scale. In addition to the Cr₂O₃, the MnCr₂O₄ also appeared in the oxide and formed on the outside of Cr₂O₃. For SUS304 and SUS430, the obvious MnCr₂O₄ spinel structure (part 2 in Figure 2c,d) could present on the outside of the Cr₂O₃ oxide (part 1 in Figure 2c and Figure 3d). The total thickness of the oxide scale in SUS430 is 30 to 40 μm and 10 to 12 μm thick in SUS304. MnCr₂O₄ has a lower resistance than the other oxides and has the effect of preventing further oxidation of Cr₂O₃. This is because MnCr₂O₄ can prevent Cr from diffusing outwards and can inhibit Cr₂O₃ from contacting with oxygen. On the C276 alloy, in addition to the substrate phase of Ni–Cr–Mo, Cr₂O₃ NiCr₂O₄ and NiFe₂O₄ are the major phases in the oxide layer (Supplementary Materials, Figure S1). Fe₂O₃ and Cr₂O₃ dissolved to form a spinel structure of FeCr₂O₄ oxide. In C276 alloy NiCr₂O₄ spinel is also found, which is generated under high-temperature oxidation by the Cr₂O₃ and NiO reaction. The electrical conductivity of both FeCr₂O₄ and NiCr₂O₄ at higher temperatures are higher than that of Cr₂O₃ and precipitated on the outer layer, which helps reduce the oxidation and hinder the diffusion of chromium. The oxide scale of C276 in Figure 2a is very thin, around 1 to 3 μm.

Cross-sectional views of the as-sprayed LSM coating on the metallic substrate are shown in Figure 3. The LSM was well deposited on the substrate, with 30–50 μm thickness. The CTE of four different substrates is similar to LSM and they mostly match. The thickness of the coating by APS is pretty uniform. There was no crack between the oxide layer and the substrate. Figure 4 showed cross-sectional views of the LSM/interconnect specimens after oxidation for 300 h in air at 800 °C. The structure of the LSM coating was stable and similar to the as-sprayed one. We can see the obvious oxidation formed in the interface between the coating and the substrate. There was a single phase of perovskite structure in the LSM coating (Supplementary Materials, Figure S2; full profile refinements are necessary to obtain more information about the XRD measurements). No oxide was found in the LSM coating, which could indicate that the LSM coating can block Cr₂O₃ and MnCr₂O₄ from the metallic interconnect and result in the chromium elements finding it difficult to diffuse through the LSM coating to the LSM coating surface. The plasma-sprayed LSM layer can indeed effectively reduce the high-temperature oxidation of chromium oxide formed and decrease the outward diffusion of cathode poisoning.

The oxidation layer was evaluated. Figure 5 displayed the EDS line scan to analyze cross-sectional element distribution. Figure 5a shows that the oxide interlayer on C276 matches the substrate well and has an average thickness of 1–2 μm. From the element distribution in Figure 5a, the Cr₂O₃ layer has a thickness of 1 μm in the inner part of the oxide. Also, MnCr₂O₄ (ad maybe some NiCr₂O₄) existed on the outside of Cr₂O₃. The MnCr₂O₄ and NiCr₂O₄ spinel is desirable for better conducting performance. The MnCr₂O₄ spinel reduces the Cr diffusion into the LSM coating and provides higher conductivity at the SOFC working temperature.

Figure 5b,c reveal the oxide and element distribution of ferrite alloy Crofer 22 APU and SUS430. The oxide thickness formed (average thickness of 2–3 μm) in the coating was significantly less than that of the uncoated substrate in a similar situation, in which Cr₂O₃ and MnCr₂O₄ formed in the oxide scale at 800 °C. From the Cr distribution in Figure 5b,c, the Cr₂O₃ grew first in the substrate side and then Mn became enriched and diffused quickly outwards to Cr₂O₃, which caused the formation and rapid growth of a MnCr₂O₄ spinel in the LSM side. Mn/Cr/spinel could effectively inhibit the evaporation of Cr and reduce the growth rate of Cr₂O₃. Finally, chromia and spinel were major phases in its layer grown in air.

Figure 5d shows that there is less Mn in the oxide scale of SUS304 than in that of SUS430 and Crofer22 APU. The Cr₂O₃ was the main part of the oxide compared to the two kinds of substrate above. This would be confirmed by the ASR data due to the lower conductivity of Cr₂O₃ than MnCr₂O₄. In the substrate side, Fe diffused outwards to Cr₂O₃, which caused the formation of the FeCr₂O₄ spinel. On the other hand, a thin layer of MnCr₂O₄ was formed in the LSM side of the oxide. This dense layer was very important to limit the growth of the Cr₂O₃ and reduce its evaporation. LSM coating on the substrate blocks its contact with the oxygen in air; the oxidation behavior between the LSM coating and the substrate was due to a thermally grown oxide layer (TGO), causing a relatively dense and thin oxide layer on the surface.

There is no oxide detected in the LSM protective coating due to the thickness (Supplementary Materials, Figure S2). Based on this, we can say that the Cr did not reach the surface of the LSM coating for 300 h in air at 800 °C. The LSM coating indeed prevents Cr from diffusing to the cathode. However, the LSM could react with Cr diffused from the substrate to form LaCrO₃, (Cr, Mn)₃O₄, or Cr₂O₃. The Cr evaporation is non-negligible. This was confirmed by the point scan in Figure 6. Even Cr could be inhibited in the oxide scale, but little of it could pass into the LSM coating. In the case of SUS304, Cr with 3.0 at % was detected in the position of LSM coating 3 μm from the oxide. No Cr was detected when the diffusion length was 3 μm from the oxide. The case was similar in SUS430: Cr with 0.49 at % was detected at the 2 μm position and no Cr was obtained in diffusion lengths of more than 6 μm. The diffusion length of Cr is dependent on the oxidation time and oxide thickness; the Cr will disperse further with a longer oxidation time.

The ASR of four kinds of interconnect and plasma-sprayed LSM-coated alloys after oxidation for 300 h in air at 800 °C are shown in Figure 7. The ASRs for all LSM-coated alloys of interest in the present study are as follows: SUS 304 > SUS 430 > Crofer22 APU > C 276. The same rankings for the metal interconnects without LSM coating are also shown. Metal alloys without plasma-sprayed LSM coating displayed much higher resistance than the LSM protective interconnect. The higher resistance was contributed by the oxide thickness. Results from Figure 2 and Figure 4: the oxide scale on the substrate without LSM coating is 5–10 times thicker than the LSM protective one. The decreased ASR of an LSM-coated interconnect indicates that the plasma-sprayed LSM coating indeed functions as a protective barrier. An iron-based alloy, SUS304, showed thinner oxide scale than SUS430 (Figure 2), but it displayed the highest resistance. This was due to the Cr diffusing quickly to form a thicker Cr₂O₃ layer than SUS430 (part 1 in Figure 2). The ASR of the SS304 was 0.024 Ω·cm², which was the worst conductivity. Crofer22 APU had a low impedance and its oxidation resistance properties were better than those of other iron-based alloys; the ASR of the Crofer22 APU was 0.01 Ω·cm², which proves the effectiveness of the LSM coating as a barrier to oxidation by inhibiting Cr diffusion and oxygen inside diffusion to thicken Cr₂O₃ and form MnCr₂O₄. For the nickel-based alloy C276, the ASR was significantly lower than the others; the ASR of the C276 was 0.0042 Ω·cm², which meets the requirements of interconnects for SOFCs. C276 was characterized by the formation of a NiCr₂O₄ spinel and a NiFe₂O₄ layer on top of the inner Cr₂O₃ layer, such that a low ASR is attributed to the higher electrical conductivity of both NiCr₂O₄ and NiFe₂O₄ than of Cr₂O₃ and MnCr₂O₄; these also indicated the potentially low Cr vaporization of the oxide scale of the alloy. Thus, it is interesting to investigate the Cr poisoning and deposition effects of the interconnect materials on cathodes. As this ASR test demonstrates, LSM is an excellent protective coating material for metallic interconnects of SOFCs.

4. Conclusions

The effects of plasma-sprayed LSM coating were evaluated in air at 800 °C as a protective barrier to oxidation for the C276, Crofer22 APU, SUS430 and SS304 alloys as metallic interconnect materials, and the following conclusions can be made based on the obtained results.

• The plasma spraying method is a convenient method to generate a LSM coating on C276, Crofer22 APU, SUS430 and SUS304 alloys. An LSM coating has a thickness of 30–50 μm. The LSM layer is found to be quite dense, with few closed pores and without cracking. The XRD results indicated that the LSM protective layer could inhibit or delay Cr diffusing to the cathode.
• The oxide scale of the LSM-coated Crofer22 APU, SUS304 and SUS430 iron-based alloys after oxidation for 300 h in air at 800 °C consists of an inner layer of Fe/Mn-doped Cr₂O₃ and an outer layer of Fe-doped MnCr₂O₄. The growth of Cr₂O₃ and formation of MnCr₂O₄ potentially alleviate the Cr poisoning and deposition. The oxide scale of the LSM-coated C276 alloy after oxidation for 300 h in air at 800 °C consists of an inner layer of dense Cr₂O₃ and an outer layer of NiCr₂O₄ and few NiFe₂O₄. The formation of NiCr₂O₄ reduces Cr evaporation by separating the inner Cr₂O₃ layer from direct exposure to the environment.
• LSM-coated C276, Crofer22 APU, SUS430 and SUS304 alloys had lower resistances between 0.0042 and 0.024 Ω·cm² at 800 °C. The LSM layer is an excellent coating material for metallic interconnections, and has the potential to be used in SOFC applications.