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Nickel substituted CuMn2O4 spinel coatings for solid oxide fuel cell interconnects
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 6 ) 1 e6
Available online at www.sciencedirect.com
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Nickel substituted CuMn2O4 spinel coatings for solid oxide fuel cell interconnects Simon Joshi*, Anthony Petric Department of Materials Science and Engineering, McMaster University, 1280 Main Street West, Hamilton, Ontario L8S 4L7, Canada
article info
abstract
Article history:
Protective CueNieMneO spinel coatings on UNS430 solid oxide fuel cells interconnects
Received 21 February 2016
were formed by oxidizing electroplated nickel, copper, and manganese in air at 800 C.
Received in revised form
Nickel prevented delamination and buckling damage of the coatings. Substituting copper
8 June 2016
with nickel up to 40% in CuMn2O4 spinel decreased conductivity slightly from ~110 to ~95 S/
Accepted 17 August 2016
cm at 800 C. XRD showed Cu0.77Ni0.45Mn1.78O4 exhibited predominantly spinel phase
Available online xxx
whereas nickel free Cu1.18Mn1.82O4 exhibited mixed oxide phase. Nickel promotes adhesion and formation of uniform protective coatings as <1 mm Cr-oxide scale formed after 120 h of
Keywords: SOFC
oxidation and it also stabilizes the spinel phase at lower temperatures. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Spinel coatings Ni-Cu-Mn-O Interconnects Chromium poisoning Electrical conductivity
Introduction Solid oxide fuel cells (SOFC) operate as a stack of many cells in series to generate high voltage [1]. At 800 C, ferritic stainless steel or chromium alloys are the preferred choice for the interconnect between cells because its coefficient of thermal expansion (CTE) matches the rest of the cell, is economical, and has high resistance to oxidation [2]. However, on the air side, these alloys grow thick chromium oxide scale that has low conductivity of ~5 mS/cm [3]. Further, chromium can evaporate from the oxide and precipitate at active sites of the cathode, which causes degradation of cell performance, a phenomenon known as chromium poisoning. To counteract
this, CoeMneO spinel and/or LaMnO3 perovskite coatings are applied to limit the flux of oxygen and impede the growth of chromium oxide. Another potential material with much greater economic benefit than CoeMneO and LaMnO3 oxides that can impede Cr2O3 formation is CueMneO spinel [4]. CueMneO spinel coatings can be formed on stainless steel by electroplating followed by oxidation in air at ~800 C [2]. However, it is difficult to plate adherent copper coatings on stainless steel using acidic sulfate solutions [5]. A nickel strike is generally applied to promote adhesion [5]. During oxidation, the inward diffusion of oxygen causes manganese and copper to oxidize and react to form spinel [2]. In addition, oxygen will also diffuse to the substrate to form a thin Cr-oxide scale. A similar growth of Cr-oxide scale has also been reported for
* Corresponding author. McMaster University, Department of Materials Science and Engineering, JHE 258 e 1280 Main Street West, Hamilton, Ontario L8S 4L7, Canada. E-mail address: [email protected] (S. Joshi). http://dx.doi.org/10.1016/j.ijhydene.2016.08.075 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Joshi S, Petric A, Nickel substituted CuMn2O4 spinel coatings for solid oxide fuel cell interconnects, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.075
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other types of coatings [6,7]. However, it is not clear the role nickel will play on the CueMneO spinel formation reaction. Thus, one of the objectives of this research is to study the effect of electroplated nickel on the formation of CueMneO spinel. Spinels are generally cubic and belong to the space group Fd3m. The oxygen anions form a close-packed lattice, which results in poor oxygen conduction [3,6,2]. Electrical conductivity is believed to occur mainly by electron hopping between octahedral sites, for example, between Mn(III) and Mn(IV) in manganese spinels [8e10]. Many models have been proposed for the CuMn2O4 spinel cation distribution [11]. The general consensus is that Mn(III) and Mn(IV) cations occupy octahedral sites with some Mn(II) cations on both tetrahedral and octahedral sites whereas Cu(I) and/or Cu(II) cations occupy tetrahedral sites with some Cu(II) cations on octahedral sites [11]. This type of configuration allows for high conductivity of >100 S/cm at 800 C and spinel stability between 200 and 1600 C, depending on composition [11]. In the case of NieMne O spinel, nickel has a strong octahedral site preference resulting in replacement of aliovalent manganese cations from the octahedral site. This causes the NiMn2O4 spinel to transform below 750 C to ilmenite (NiMnO3) which has a higher oxidation state of manganese [12,13]. The strong octahedral site preference of nickel may be responsible for the low electrical conductivity of 1.4 S/cm at 800 C [8]. This study explores the optimal amount of nickel that should be electroplated to form stable and conductive CueNieMneO spinel.
Experimental UNS 430 stainless steel (McMaster Carr) was cut into ~15 mm 30 mm 0.9 mm coupons and polished using 400 SiC grit paper. The samples were masked on both sides by Fisher Scientific brand tape (catalog # 15-951) on top of duct tape. On one side, a circular 5 cm2 opening was left in the mask. The samples were activated in 10% v/v H2SO4 solution at 50 C for 5 min before electroplating. An EG&G 273A potentiostat was used to electroplate nickel from a plating solution of (NiSO4$6H2O, EMD 98%; Na2SO4$10H2O; Alfa 99%; NH4Cl, Caledon 99%; H3BO3, Caledon 99%), copper from a solution of (CuSO4$5H2O, Alfa, 99%; H2SO4, Caledon 98%), and manganese from its solution (MnSO4$5H2O, Alfa, ACS 98% and (NH4)2SO4, Alfa, 99%). A saturated calomel electrode was used as reference electrode and platinum plate ~25 cm2 was used as a counter electrode. Nickel was first electroplated followed by copper and finally manganese. Before each step, the substrate was rinsed with DI water. The solution was constantly agitated by a magnetic stirrer for
copper and manganese deposition. The deposition parameters along with cathode efficiencies [4,14,15] are listed in Table 1. After deposition, the coatings were oxidized in air at 800 C for 120 h. The heating/cooling rates were 5 C/min. Electrical conductivity testing was performed on sintered oxide rods made from NiO (Sigma Aldrich, 99%), CuO (Sigma Aldrich, 98%), and MnO2 (Alfa, 99.9%). The powders were weighed, ball milled in anhydrous ethanol using 5 mm diameter yttria-stabilized zirconia balls (TOSOH USA, Inc., Grove City, OH, USA), dried, pelletized, and annealed at 800 C for 24 h at 5 C/min heating rate. The annealed samples were cooled and ground to a fine powder. Again, the powders were ball milled in anhydrous ethanol for 24 h. About 5 mL of PVB binder (1 wt% in ethanol) was added to the ethanol solution before ball milling. The solution was dried and the powder pressed into a parallelepiped shape. Cracked samples were recycled. Crack-free samples were sintered at 1100 C. The heating/cooling rates were 0.5 C/min up to 600 C and 5 C/ min from 600 to 1100 C and down to room temperature. The dwell time was 1 h at 200, 400, and 600 C, 72 h at 1100 C, and 24 h at 800 C. After cooling, surface irregularities were polished off. Samples with visible cracks that could not be removed were rejected. Conductivity was measured by the four point DC method [8]. A constant current was applied through Pt electrodes across the sample. Two voltage probes about 10 mm apart were used to measure the potential drop. The sample was equilibrated in the furnace until a stable conductivity value was measured for at least 30 min. Conductivity was measured in both directions (heating and cooling) between 600 and 925 C. XRD analyses were performed using a Brucker D8 Advance X-ray powder diffractometer with Cu Ka1 radiation at a 2q angle range of 15 e60 . The peaks were identified using standards from the JCPDS database. Microstructure was examined by JEOL 6610LV scanning electron microscopy (SEM) fitted with EDAX (Oxford instruments).
Results and discussion Conductivity isotherms (average value of heating and cooling) of CuxNi1xMn2O4 are shown in Fig. 1a. For 0.65 x 1.00 in the CuxNi1xMn2O4 spinel the conductivity values in the temperature range of 750 e925 C increases slightly from 92 to 108 S/cm for x ¼ 0.65 to 96e118 S/cm for x ¼ 1.00. However; below 750 C, the conductivity decreases as x (Cu content) increases and the decrease in conductivity is more dominant at 600 C. For x < 0.65, decrease in conductivity is observed for all isotherm plots. Conductivity testing of Ni1Mn2O4 for which
Table 1 e Electroplating parameters for nickel, copper, and manganese.
Solution Solution pH Current density Cathode efficiencies
Ni
Cu
Mn
0.3 M NiSO4 þ 0.5 M Na2SO4 þ 0.3 M NH4Cl þ 0.24 M H3BO3 3.2e3.8 25 mA/cm2 90%
0.8 M CuSO4 þ 0.4 M H2SO4
0.3e0.6 M MnSO4 þ 1.0 M (NH4)2SO4 þ 0.03 M NH2OH$HCl 3.0 with H2SO4 100 mA/cm2 55%
0.7 48 mA/cm2 100%
Please cite this article in press as: Joshi S, Petric A, Nickel substituted CuMn2O4 spinel coatings for solid oxide fuel cell interconnects, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.075
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Fig. 1 e (a) Isothermal conductivity plot of CueNieMneO spinel and (b) conductivity testing of Cu0.77Ni0.45Mn1.78O4 and Cu1.18Mn1.82O4.
x ¼ 0 (Cu free) in CuxNi1xMn2O4 at 800 C is reported in literature to be 1.4 S/cm and the literature data supports the decrease in conductivity observed for all isotherm plots in Fig. 1. Thus, conductivity testing for Ni1Mn2O4was not performed for this experiment. To investigate if conductivity was dependent on manganese concentration in the spinel, conductivity testing was performed on Cu0.77Ni0.45Mn1.78O4 and Cu1.18Mn1.82O4 and Fig. 1b shows when copper was substituted with nickel, improvement in conductivity was still observed indicating nickel addition to the spinel is the important part. The low conductivity of 20 S/cm at 600 C for the Ni-free sample is due to the spinel phase decomposing to bixbyite þ spinel [11] as only the spinel oxide is conductive due to mixed valent manganese cations at octahedral sites. The high conductivity obtained by adding nickel to the spinel over a wide temperature and composition range for CuxNi1xMn2O4 and even when the manganese content is altered from 1.78 to 2.0 correlates with the existence of a dominant stable spinel phase over a wide composition, which is not seen for only CueMneO spinel [8]. This may indicate NieCueMneO spinel has a wide spinel stability zone similar to that observed in CoeMn binary phase diagram in air [8]. Further study is warranted on developing a ternary phase diagram for MneNie Cu system from ~500 to 900 C temperature range but this type of study is out of the scope of this paper. Based on the isotherm plots, the stoichiometry near Cu0.65Ni0.35Mn2O4 is believed to be the critical composition for nickel content beyond which, some of the spinel will transform to another non-conductive oxide phase. Thus, substituting copper or even manganese by nickel to a certain extent can increase the conductivity of the spinel. Post-conductivity testing XRD analyses were performed on two samples (Fig. 2a,b), one nickel rich (Cu0.77Ni0.45Mn1.78O4) and another nickel-free (Cu1.18Mn1.82O4). After conductivity testing finished with the last data point at 600 C, the samples were left to furnace cool for structural characterization. XRD data showed that addition of nickel to the CueMneO spinel (Fig. 2a) can stabilize the dominant spinel phase. CueMneO has a very narrow spinel stability region which is generally not retained on cooling (Fig. 2b) but addition of nickel seems to stabilize the spinel phase and the formation of secondary
Fig. 2 e XRD results of spinel oxides heated in the range of 600-925 C and then furnace cooled: (a) Cu0.77Ni0.45Mn1.78O4 and (b) Cu1.18Mn1.82O4.
non-spinel phases are significantly reduced. Only the spinel phase has significant conductivity and the stabilization of the spinel phase by the addition of nickel is believed to contribute to the high conductivity observed for 0.8 x 0.6 in the CuxNi1xMn2O4 system shown in Fig. 1a. The cation distribution based on literature data for CuMn2O4 is given by equation (1) [11]. Substituting copper with nickel in the spinel will initially cause displacement of the copper cations from octahedral sites due to the strong
Please cite this article in press as: Joshi S, Petric A, Nickel substituted CuMn2O4 spinel coatings for solid oxide fuel cell interconnects, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.075
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octahedral site preference of nickel. This conclusion is reflected by the conductivity data as replacing copper with nickel did not significantly alter conductivity values above 750 C. Electrical conductivity is believed to be due to electrons hopping between the multivalent manganese cations present at the octahedral site [8e10] and small additions of nickel does not appear to affect the concentration of manganese cations at the octahedral site. As the nickel content increases to x ¼ 0.4 in CuxNi1xMn2O4, the conductivity data indicate that most copper cations at the octahedral sites will be replaced by nickel. Above x ¼ 0.4, the decrease in
conductivity data indicates manganese cations at octahedral sites begin to be replaced by nickel and may precipitate a new phase.
Cux 2þ Mn1yþz 2þ
h i Cu1x 2þ Mn2yþz 3þ Mny 4þ
(1)
Fig. 3aec are SEM micrographs of Cu0.8Ni0.2Mn2.0O4 coating oxidized at 800 C for 120 h. The plating times and thicknesses for each metallic layer are listed in Table 2. The metallic layer thicknesses were calculated from plating time, current density, and cathode efficiency (equations (2)e(4)) [4,15].
Fig. 3 e Cu0.8Ni0.2Mn2O4 coatings oxidized in air at 800 C for 120 h: (a) uniform defect free section, (b) a section with thick Croxide scale, and (c) buckling damage section. Please cite this article in press as: Joshi S, Petric A, Nickel substituted CuMn2O4 spinel coatings for solid oxide fuel cell interconnects, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.075
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Table 2 e Electroplating time and coating thickness for coatings shown in Figs. 3 and 4. Stoichiometry
Cu0.8Ni0.2Mn2.0O4 Cu0.60Ni0.45Mn1.95O4
Ni
Cu
Mn
Time (s)
Thickness (mm)
Time (s)
Thickness (mm)
Time (s)
Thickness (mm)
120 200
0.9 1.5
180 130
3.2 2.3
420 365
9.1 7.9
x ¼ WA =WT 100%
(2)
WT ¼ I A t=ðn FÞ
(3)
Coating Thickness ¼ ðWT xÞ=r
(4)
The cathode efficiency “x” is the ratio of actual coating weight per unit area “WA” and theoretical coating weight per unit area “WT”. The theoretical weight was calculated based on Faraday law where “I” is current, “A” is the atomic weight, t is the time, “n” is the number of electrons, and “F” is the Faraday constant. The coatings were deposited for a specific time period as noted in Table 2 such that complete oxidation in air at 800 C would form a uniform phase of composition Cu0.8Ni0.2Mn2.0O4 with thickness in the range of 25 ± 2 mm [14,15]. Based on the study by Wei et al., reaction of the metallic layers to form spinel should initiate after reaching 750 C and the coatings should be fully converted into a spinel layer within 24 h without significant oxidation of the substrate [2]. Depositing nickel for at least 120 s ensured the coatings did not spall off. The coatings in Fig. 3a were adherent and protective as <1 mm Cr2O3 scale was detected at most locations. In a few locations, ~10 mm thick Cr-oxide scale was detected (Fig. 3b). Buckling damage was still present in a few locations but the size (length of separation from substrate, which is ~55 mm in Fig. 3c) and was significantly less than that of Cue MneO spinel oxide coatings (~200 mm separation) observed in our previous studies [4,15]. The frequency of buckling damage present in Cu0.8Ni0.2Mn2.0O4 coatings was also less than that observed on CueMneO coatings. Thus, the micrographs reveal Cu0.8Ni0.2Mn2.0O4 coating performed better than Cue MneO spinel coatings but still some areas exhibited nonuniform features such as buckling damage and growth of Cr2O3 scale. When greater amount of copper was substituted with nickel to form coatings with stoichiometry Cu0.60Ni0.45Mn1.95O4 and oxidized in air for 120 h at 800 C, the coatings exhibited uniform features throughout (Fig. 4a). The metallic layers were electroplated for the specified amount of time stated in Table 2 so that after oxidation, the coatings would transform within 24 h into a spinel with uniform phase of composition approximately Cu0.60Ni0.45Mn1.95O4 [14,15] and have a thickness of about 25 ± 2 mm. The high nickel content prevented the coatings from spalling or buckling. The mean coefficient of thermal expansion (CTE) of metallic nickel from ambient temperature to 550 C is ~14.7 ppm/ C [16], which is less than that for Cu (18.1 ppm/ C) [16] but greater than UNS 430 (11.4 ppm/ C) [17]. When a significant amount of copper with high CTE compared to the substrate is replaced by a nickel layer with lower CTE, it can significantly reduce the thermal stress produced by CTE
Fig. 4 e Cu0.6Ni0.4Mn2O4 coatings oxidized in air at 800 C for 120 h: (a,b) micrograph of coatings at both high and low magnification, (c) EDS point analysis of the high-magnification coatings in (b).
mismatch during heating that is responsible for buckling damage. EDS analysis in Fig. 4b showed the chromium oxide scale to be less than 1 mm thick. Thus, substituting ~40% copper with nickel not only stabilized the spinel phase, but also improved the quality and performance of the coatings. All data suggest addition of nickel not only improves the adhesion of the electroplated copper coatings but also improves the performance of the CueMneO spinel oxide. One hypothesis to the improved performance is that nickel accelerates formation of spinel solid solution even at lower temperatures and also increases the spinel phase stability region. Conductivity testing at 600 C showed that NieCueMneO based oxide had conductivity greater than CueMneO oxide
Please cite this article in press as: Joshi S, Petric A, Nickel substituted CuMn2O4 spinel coatings for solid oxide fuel cell interconnects, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.075
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due to the dominant phase being the cubic spinel phase. This is verified by XRD in which NieCueMneO oxides exhibited dominant spinel phase. If nickel can accelerate formation of spinel phase at 600 C or lower, then the electroplated NieCue Mn metallic layers may react to form a spinel phase at 600 C or less and at a much greater rate than CueMn metallic layers. Spinel CTE matches the substrate and is even lower than that of nickel. Early formation of spinel phase can prevent the buckling damage associated with CueMneO oxide coatings thereby forming a dense, adherent, and uniform defect free coating that can provide adequate corrosion protection.
Conclusion Protective NieCueMneO spinel coatings can be formed on stainless steel by electroplating followed by oxidation. Substituting up to 40% copper with nickel in the CuMn2O4 spinel is beneficial as it helps maintain a conductivity of ~70 S/ cm even at 600 C and XRD shows a dominant cubic spinel phase is maintained for slowly cooled samples. Further substitution of copper by nickel is not beneficial as conductivity drops significantly, indicating different phase formation. When nickel is added to the CuMn2O4 spinel, it initially replaces copper at octahedral sites. Based on XRD and high conductivity values obtained at lower temperatures, substitution of copper by nickel may also expand the spinel stability region at lower temperatures. Spinel formation at lower temperature is advantageous as it can reduce the CTE of the coatings and prevent buckling damage. CTE of metallic nickel is slightly higher than that of UNS 430 but much lower than copper so substituting copper with nickel can prevent/reduce buckling damage during the initial heating stage before spinel phase forms. The resulting Cu0.6Ni0.4Mn2O4 coatings are dense, adherent, uniform, and provide good corrosion protection throughout. The native chromium oxide scale was less than 1 mm thick after 120 h of oxidation. Another advantage of nickel is that it prevented spallation. Thus, substituting copper with nickel in the CuMn2O4 spinel is beneficial as it makes electroplating easier, maintains high conductivity to lower temperatures, and improves oxidation resistance by stabilizing the spinel phase.
Acknowledgments The authors wish to acknowledge the Natural Sciences and Engineering Research Council of Canada and Stackpole Canada for financial support. We are grateful to the Canadian Center for Electron Microscopy and the Brockhouse Institute
for Materials Research at McMaster for assistance with sample preparation and analysis.
references
[1] Singhal SC, Kendall Kevin. High temperature solid oxide fuel cells, fundamentals, design and applications. Oxford: Elsevier; 2003. p. 174. [2] Wei P, Bateni MR, Deng X, Petric A. Conversion of copper and manganese metallic films to spinel coating. J Mater Sci 2012;47:5205e15. [3] Kurokawa H, Jacobson CP, DeJonghe LC, Visco SJ. Chromium vaporization of bare and of coated ironechromium alloys at 1073 K. Solid State Ion 2007;178:287e96. [4] Joshi S, Silvia C, Wang P, Mozharivskyj Y, Petric A. Coppermagnesium-manganese spinel coatings for solid oxide fuel cell interconnects. J Electrochem Soc 2014;161:F233e8. [5] Lowenheim FA. Electroplating. McGraw-Hill Book Company; 1978. [6] Magdefrau NJ, Chen L, 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 Sources 2013;227:318e26. [7] Armstrong TJ, Smith M, Virkar AV. In: Singhal SC, Mizusaki J, editors. Solid oxide fuel cells (SOFC), vol. IX. Pennington, NJ: The Electrochemical Society; 2005. p. 1795. PV 2005e07. [8] Petric A, Ling H. Electrical conductivity and thermal expansion of spinels at elevated temperatures. J Am Ceram Soc 2007;90:1515e20. [9] Fang D, Chen C, Winnubst AJA. Preparation and electrical properties of FexCu0.10Ni0.66Mn2.24xO4 (0x0.90) NTC ceramics. J Alloy Comp 2008;454:286e91. [10] Gillot B, Karroubi M, Metz R, Rousset A. Thermal stability, crystallographic and electrical properties in undoped and Badoped Cu-Ni manganite spinels. Solid State Ion 1991;48:93e9. [11] Wei P, Bieringer M, Cranswick LMD, Petric A. In situ hightemperature X-ray and neutron diffraction of CueMn oxide phases. J Mater Sci 2010;45:1056e64. [12] Bhandage GT, Keer HV. A correlation of the physical properties of the NixCu1-xMn2O4 system. J Phys C Solid State Phys 1976;9:1325e30. [13] Tang XX, Manthiram A, Goodenough JB. NiMn2O4 revisited. J Less-Comm Met 1989;156:357e68. [14] Michaud XD, MSc Thesis, McMaster University, Hamilton, Canada. [15] Joshi S, Silvia C, Petric A. Magnesium-manganese spinel coatings for solid oxide fuel cell interconnects. ECS Trans 2013;50:153e60. [16] Nix FC, MacMair D. The thermal expansion of pure metals: copper, gold, aluminum, nickel, and iron. Phys Rev 1941;60:597e605. [17] Mills KC, Su Y, Li Z, Brooks RF. Equations for the calculation of the thermo-physical properties of stainless steel. ISIJ Int 2003;44:1661e8.
Please cite this article in press as: Joshi S, Petric A, Nickel substituted CuMn2O4 spinel coatings for solid oxide fuel cell interconnects, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.075
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Nickel substituted CuMn2O4 spinel coatings for solid oxide fuel cell interconnects Simon Joshi*, Anthony Petric Department of Materials Science and Engineering, McMaster University, 1280 Main Street West, Hamilton, Ontario L8S 4L7, Canada
article info
abstract
Article history:
Protective CueNieMneO spinel coatings on UNS430 solid oxide fuel cells interconnects
Received 21 February 2016
were formed by oxidizing electroplated nickel, copper, and manganese in air at 800 C.
Received in revised form
Nickel prevented delamination and buckling damage of the coatings. Substituting copper
8 June 2016
with nickel up to 40% in CuMn2O4 spinel decreased conductivity slightly from ~110 to ~95 S/
Accepted 17 August 2016
cm at 800 C. XRD showed Cu0.77Ni0.45Mn1.78O4 exhibited predominantly spinel phase
Available online xxx
whereas nickel free Cu1.18Mn1.82O4 exhibited mixed oxide phase. Nickel promotes adhesion and formation of uniform protective coatings as <1 mm Cr-oxide scale formed after 120 h of
Keywords: SOFC
oxidation and it also stabilizes the spinel phase at lower temperatures. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Spinel coatings Ni-Cu-Mn-O Interconnects Chromium poisoning Electrical conductivity
Introduction Solid oxide fuel cells (SOFC) operate as a stack of many cells in series to generate high voltage [1]. At 800 C, ferritic stainless steel or chromium alloys are the preferred choice for the interconnect between cells because its coefficient of thermal expansion (CTE) matches the rest of the cell, is economical, and has high resistance to oxidation [2]. However, on the air side, these alloys grow thick chromium oxide scale that has low conductivity of ~5 mS/cm [3]. Further, chromium can evaporate from the oxide and precipitate at active sites of the cathode, which causes degradation of cell performance, a phenomenon known as chromium poisoning. To counteract
this, CoeMneO spinel and/or LaMnO3 perovskite coatings are applied to limit the flux of oxygen and impede the growth of chromium oxide. Another potential material with much greater economic benefit than CoeMneO and LaMnO3 oxides that can impede Cr2O3 formation is CueMneO spinel [4]. CueMneO spinel coatings can be formed on stainless steel by electroplating followed by oxidation in air at ~800 C [2]. However, it is difficult to plate adherent copper coatings on stainless steel using acidic sulfate solutions [5]. A nickel strike is generally applied to promote adhesion [5]. During oxidation, the inward diffusion of oxygen causes manganese and copper to oxidize and react to form spinel [2]. In addition, oxygen will also diffuse to the substrate to form a thin Cr-oxide scale. A similar growth of Cr-oxide scale has also been reported for
* Corresponding author. McMaster University, Department of Materials Science and Engineering, JHE 258 e 1280 Main Street West, Hamilton, Ontario L8S 4L7, Canada. E-mail address: [email protected] (S. Joshi). http://dx.doi.org/10.1016/j.ijhydene.2016.08.075 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Joshi S, Petric A, Nickel substituted CuMn2O4 spinel coatings for solid oxide fuel cell interconnects, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.075
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other types of coatings [6,7]. However, it is not clear the role nickel will play on the CueMneO spinel formation reaction. Thus, one of the objectives of this research is to study the effect of electroplated nickel on the formation of CueMneO spinel. Spinels are generally cubic and belong to the space group Fd3m. The oxygen anions form a close-packed lattice, which results in poor oxygen conduction [3,6,2]. Electrical conductivity is believed to occur mainly by electron hopping between octahedral sites, for example, between Mn(III) and Mn(IV) in manganese spinels [8e10]. Many models have been proposed for the CuMn2O4 spinel cation distribution [11]. The general consensus is that Mn(III) and Mn(IV) cations occupy octahedral sites with some Mn(II) cations on both tetrahedral and octahedral sites whereas Cu(I) and/or Cu(II) cations occupy tetrahedral sites with some Cu(II) cations on octahedral sites [11]. This type of configuration allows for high conductivity of >100 S/cm at 800 C and spinel stability between 200 and 1600 C, depending on composition [11]. In the case of NieMne O spinel, nickel has a strong octahedral site preference resulting in replacement of aliovalent manganese cations from the octahedral site. This causes the NiMn2O4 spinel to transform below 750 C to ilmenite (NiMnO3) which has a higher oxidation state of manganese [12,13]. The strong octahedral site preference of nickel may be responsible for the low electrical conductivity of 1.4 S/cm at 800 C [8]. This study explores the optimal amount of nickel that should be electroplated to form stable and conductive CueNieMneO spinel.
Experimental UNS 430 stainless steel (McMaster Carr) was cut into ~15 mm 30 mm 0.9 mm coupons and polished using 400 SiC grit paper. The samples were masked on both sides by Fisher Scientific brand tape (catalog # 15-951) on top of duct tape. On one side, a circular 5 cm2 opening was left in the mask. The samples were activated in 10% v/v H2SO4 solution at 50 C for 5 min before electroplating. An EG&G 273A potentiostat was used to electroplate nickel from a plating solution of (NiSO4$6H2O, EMD 98%; Na2SO4$10H2O; Alfa 99%; NH4Cl, Caledon 99%; H3BO3, Caledon 99%), copper from a solution of (CuSO4$5H2O, Alfa, 99%; H2SO4, Caledon 98%), and manganese from its solution (MnSO4$5H2O, Alfa, ACS 98% and (NH4)2SO4, Alfa, 99%). A saturated calomel electrode was used as reference electrode and platinum plate ~25 cm2 was used as a counter electrode. Nickel was first electroplated followed by copper and finally manganese. Before each step, the substrate was rinsed with DI water. The solution was constantly agitated by a magnetic stirrer for
copper and manganese deposition. The deposition parameters along with cathode efficiencies [4,14,15] are listed in Table 1. After deposition, the coatings were oxidized in air at 800 C for 120 h. The heating/cooling rates were 5 C/min. Electrical conductivity testing was performed on sintered oxide rods made from NiO (Sigma Aldrich, 99%), CuO (Sigma Aldrich, 98%), and MnO2 (Alfa, 99.9%). The powders were weighed, ball milled in anhydrous ethanol using 5 mm diameter yttria-stabilized zirconia balls (TOSOH USA, Inc., Grove City, OH, USA), dried, pelletized, and annealed at 800 C for 24 h at 5 C/min heating rate. The annealed samples were cooled and ground to a fine powder. Again, the powders were ball milled in anhydrous ethanol for 24 h. About 5 mL of PVB binder (1 wt% in ethanol) was added to the ethanol solution before ball milling. The solution was dried and the powder pressed into a parallelepiped shape. Cracked samples were recycled. Crack-free samples were sintered at 1100 C. The heating/cooling rates were 0.5 C/min up to 600 C and 5 C/ min from 600 to 1100 C and down to room temperature. The dwell time was 1 h at 200, 400, and 600 C, 72 h at 1100 C, and 24 h at 800 C. After cooling, surface irregularities were polished off. Samples with visible cracks that could not be removed were rejected. Conductivity was measured by the four point DC method [8]. A constant current was applied through Pt electrodes across the sample. Two voltage probes about 10 mm apart were used to measure the potential drop. The sample was equilibrated in the furnace until a stable conductivity value was measured for at least 30 min. Conductivity was measured in both directions (heating and cooling) between 600 and 925 C. XRD analyses were performed using a Brucker D8 Advance X-ray powder diffractometer with Cu Ka1 radiation at a 2q angle range of 15 e60 . The peaks were identified using standards from the JCPDS database. Microstructure was examined by JEOL 6610LV scanning electron microscopy (SEM) fitted with EDAX (Oxford instruments).
Results and discussion Conductivity isotherms (average value of heating and cooling) of CuxNi1xMn2O4 are shown in Fig. 1a. For 0.65 x 1.00 in the CuxNi1xMn2O4 spinel the conductivity values in the temperature range of 750 e925 C increases slightly from 92 to 108 S/cm for x ¼ 0.65 to 96e118 S/cm for x ¼ 1.00. However; below 750 C, the conductivity decreases as x (Cu content) increases and the decrease in conductivity is more dominant at 600 C. For x < 0.65, decrease in conductivity is observed for all isotherm plots. Conductivity testing of Ni1Mn2O4 for which
Table 1 e Electroplating parameters for nickel, copper, and manganese.
Solution Solution pH Current density Cathode efficiencies
Ni
Cu
Mn
0.3 M NiSO4 þ 0.5 M Na2SO4 þ 0.3 M NH4Cl þ 0.24 M H3BO3 3.2e3.8 25 mA/cm2 90%
0.8 M CuSO4 þ 0.4 M H2SO4
0.3e0.6 M MnSO4 þ 1.0 M (NH4)2SO4 þ 0.03 M NH2OH$HCl 3.0 with H2SO4 100 mA/cm2 55%
0.7 48 mA/cm2 100%
Please cite this article in press as: Joshi S, Petric A, Nickel substituted CuMn2O4 spinel coatings for solid oxide fuel cell interconnects, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.075
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Fig. 1 e (a) Isothermal conductivity plot of CueNieMneO spinel and (b) conductivity testing of Cu0.77Ni0.45Mn1.78O4 and Cu1.18Mn1.82O4.
x ¼ 0 (Cu free) in CuxNi1xMn2O4 at 800 C is reported in literature to be 1.4 S/cm and the literature data supports the decrease in conductivity observed for all isotherm plots in Fig. 1. Thus, conductivity testing for Ni1Mn2O4was not performed for this experiment. To investigate if conductivity was dependent on manganese concentration in the spinel, conductivity testing was performed on Cu0.77Ni0.45Mn1.78O4 and Cu1.18Mn1.82O4 and Fig. 1b shows when copper was substituted with nickel, improvement in conductivity was still observed indicating nickel addition to the spinel is the important part. The low conductivity of 20 S/cm at 600 C for the Ni-free sample is due to the spinel phase decomposing to bixbyite þ spinel [11] as only the spinel oxide is conductive due to mixed valent manganese cations at octahedral sites. The high conductivity obtained by adding nickel to the spinel over a wide temperature and composition range for CuxNi1xMn2O4 and even when the manganese content is altered from 1.78 to 2.0 correlates with the existence of a dominant stable spinel phase over a wide composition, which is not seen for only CueMneO spinel [8]. This may indicate NieCueMneO spinel has a wide spinel stability zone similar to that observed in CoeMn binary phase diagram in air [8]. Further study is warranted on developing a ternary phase diagram for MneNie Cu system from ~500 to 900 C temperature range but this type of study is out of the scope of this paper. Based on the isotherm plots, the stoichiometry near Cu0.65Ni0.35Mn2O4 is believed to be the critical composition for nickel content beyond which, some of the spinel will transform to another non-conductive oxide phase. Thus, substituting copper or even manganese by nickel to a certain extent can increase the conductivity of the spinel. Post-conductivity testing XRD analyses were performed on two samples (Fig. 2a,b), one nickel rich (Cu0.77Ni0.45Mn1.78O4) and another nickel-free (Cu1.18Mn1.82O4). After conductivity testing finished with the last data point at 600 C, the samples were left to furnace cool for structural characterization. XRD data showed that addition of nickel to the CueMneO spinel (Fig. 2a) can stabilize the dominant spinel phase. CueMneO has a very narrow spinel stability region which is generally not retained on cooling (Fig. 2b) but addition of nickel seems to stabilize the spinel phase and the formation of secondary
Fig. 2 e XRD results of spinel oxides heated in the range of 600-925 C and then furnace cooled: (a) Cu0.77Ni0.45Mn1.78O4 and (b) Cu1.18Mn1.82O4.
non-spinel phases are significantly reduced. Only the spinel phase has significant conductivity and the stabilization of the spinel phase by the addition of nickel is believed to contribute to the high conductivity observed for 0.8 x 0.6 in the CuxNi1xMn2O4 system shown in Fig. 1a. The cation distribution based on literature data for CuMn2O4 is given by equation (1) [11]. Substituting copper with nickel in the spinel will initially cause displacement of the copper cations from octahedral sites due to the strong
Please cite this article in press as: Joshi S, Petric A, Nickel substituted CuMn2O4 spinel coatings for solid oxide fuel cell interconnects, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.075
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octahedral site preference of nickel. This conclusion is reflected by the conductivity data as replacing copper with nickel did not significantly alter conductivity values above 750 C. Electrical conductivity is believed to be due to electrons hopping between the multivalent manganese cations present at the octahedral site [8e10] and small additions of nickel does not appear to affect the concentration of manganese cations at the octahedral site. As the nickel content increases to x ¼ 0.4 in CuxNi1xMn2O4, the conductivity data indicate that most copper cations at the octahedral sites will be replaced by nickel. Above x ¼ 0.4, the decrease in
conductivity data indicates manganese cations at octahedral sites begin to be replaced by nickel and may precipitate a new phase.
Cux 2þ Mn1yþz 2þ
h i Cu1x 2þ Mn2yþz 3þ Mny 4þ
(1)
Fig. 3aec are SEM micrographs of Cu0.8Ni0.2Mn2.0O4 coating oxidized at 800 C for 120 h. The plating times and thicknesses for each metallic layer are listed in Table 2. The metallic layer thicknesses were calculated from plating time, current density, and cathode efficiency (equations (2)e(4)) [4,15].
Fig. 3 e Cu0.8Ni0.2Mn2O4 coatings oxidized in air at 800 C for 120 h: (a) uniform defect free section, (b) a section with thick Croxide scale, and (c) buckling damage section. Please cite this article in press as: Joshi S, Petric A, Nickel substituted CuMn2O4 spinel coatings for solid oxide fuel cell interconnects, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.075
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Table 2 e Electroplating time and coating thickness for coatings shown in Figs. 3 and 4. Stoichiometry
Cu0.8Ni0.2Mn2.0O4 Cu0.60Ni0.45Mn1.95O4
Ni
Cu
Mn
Time (s)
Thickness (mm)
Time (s)
Thickness (mm)
Time (s)
Thickness (mm)
120 200
0.9 1.5
180 130
3.2 2.3
420 365
9.1 7.9
x ¼ WA =WT 100%
(2)
WT ¼ I A t=ðn FÞ
(3)
Coating Thickness ¼ ðWT xÞ=r
(4)
The cathode efficiency “x” is the ratio of actual coating weight per unit area “WA” and theoretical coating weight per unit area “WT”. The theoretical weight was calculated based on Faraday law where “I” is current, “A” is the atomic weight, t is the time, “n” is the number of electrons, and “F” is the Faraday constant. The coatings were deposited for a specific time period as noted in Table 2 such that complete oxidation in air at 800 C would form a uniform phase of composition Cu0.8Ni0.2Mn2.0O4 with thickness in the range of 25 ± 2 mm [14,15]. Based on the study by Wei et al., reaction of the metallic layers to form spinel should initiate after reaching 750 C and the coatings should be fully converted into a spinel layer within 24 h without significant oxidation of the substrate [2]. Depositing nickel for at least 120 s ensured the coatings did not spall off. The coatings in Fig. 3a were adherent and protective as <1 mm Cr2O3 scale was detected at most locations. In a few locations, ~10 mm thick Cr-oxide scale was detected (Fig. 3b). Buckling damage was still present in a few locations but the size (length of separation from substrate, which is ~55 mm in Fig. 3c) and was significantly less than that of Cue MneO spinel oxide coatings (~200 mm separation) observed in our previous studies [4,15]. The frequency of buckling damage present in Cu0.8Ni0.2Mn2.0O4 coatings was also less than that observed on CueMneO coatings. Thus, the micrographs reveal Cu0.8Ni0.2Mn2.0O4 coating performed better than Cue MneO spinel coatings but still some areas exhibited nonuniform features such as buckling damage and growth of Cr2O3 scale. When greater amount of copper was substituted with nickel to form coatings with stoichiometry Cu0.60Ni0.45Mn1.95O4 and oxidized in air for 120 h at 800 C, the coatings exhibited uniform features throughout (Fig. 4a). The metallic layers were electroplated for the specified amount of time stated in Table 2 so that after oxidation, the coatings would transform within 24 h into a spinel with uniform phase of composition approximately Cu0.60Ni0.45Mn1.95O4 [14,15] and have a thickness of about 25 ± 2 mm. The high nickel content prevented the coatings from spalling or buckling. The mean coefficient of thermal expansion (CTE) of metallic nickel from ambient temperature to 550 C is ~14.7 ppm/ C [16], which is less than that for Cu (18.1 ppm/ C) [16] but greater than UNS 430 (11.4 ppm/ C) [17]. When a significant amount of copper with high CTE compared to the substrate is replaced by a nickel layer with lower CTE, it can significantly reduce the thermal stress produced by CTE
Fig. 4 e Cu0.6Ni0.4Mn2O4 coatings oxidized in air at 800 C for 120 h: (a,b) micrograph of coatings at both high and low magnification, (c) EDS point analysis of the high-magnification coatings in (b).
mismatch during heating that is responsible for buckling damage. EDS analysis in Fig. 4b showed the chromium oxide scale to be less than 1 mm thick. Thus, substituting ~40% copper with nickel not only stabilized the spinel phase, but also improved the quality and performance of the coatings. All data suggest addition of nickel not only improves the adhesion of the electroplated copper coatings but also improves the performance of the CueMneO spinel oxide. One hypothesis to the improved performance is that nickel accelerates formation of spinel solid solution even at lower temperatures and also increases the spinel phase stability region. Conductivity testing at 600 C showed that NieCueMneO based oxide had conductivity greater than CueMneO oxide
Please cite this article in press as: Joshi S, Petric A, Nickel substituted CuMn2O4 spinel coatings for solid oxide fuel cell interconnects, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.075
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due to the dominant phase being the cubic spinel phase. This is verified by XRD in which NieCueMneO oxides exhibited dominant spinel phase. If nickel can accelerate formation of spinel phase at 600 C or lower, then the electroplated NieCue Mn metallic layers may react to form a spinel phase at 600 C or less and at a much greater rate than CueMn metallic layers. Spinel CTE matches the substrate and is even lower than that of nickel. Early formation of spinel phase can prevent the buckling damage associated with CueMneO oxide coatings thereby forming a dense, adherent, and uniform defect free coating that can provide adequate corrosion protection.
Conclusion Protective NieCueMneO spinel coatings can be formed on stainless steel by electroplating followed by oxidation. Substituting up to 40% copper with nickel in the CuMn2O4 spinel is beneficial as it helps maintain a conductivity of ~70 S/ cm even at 600 C and XRD shows a dominant cubic spinel phase is maintained for slowly cooled samples. Further substitution of copper by nickel is not beneficial as conductivity drops significantly, indicating different phase formation. When nickel is added to the CuMn2O4 spinel, it initially replaces copper at octahedral sites. Based on XRD and high conductivity values obtained at lower temperatures, substitution of copper by nickel may also expand the spinel stability region at lower temperatures. Spinel formation at lower temperature is advantageous as it can reduce the CTE of the coatings and prevent buckling damage. CTE of metallic nickel is slightly higher than that of UNS 430 but much lower than copper so substituting copper with nickel can prevent/reduce buckling damage during the initial heating stage before spinel phase forms. The resulting Cu0.6Ni0.4Mn2O4 coatings are dense, adherent, uniform, and provide good corrosion protection throughout. The native chromium oxide scale was less than 1 mm thick after 120 h of oxidation. Another advantage of nickel is that it prevented spallation. Thus, substituting copper with nickel in the CuMn2O4 spinel is beneficial as it makes electroplating easier, maintains high conductivity to lower temperatures, and improves oxidation resistance by stabilizing the spinel phase.
Acknowledgments The authors wish to acknowledge the Natural Sciences and Engineering Research Council of Canada and Stackpole Canada for financial support. We are grateful to the Canadian Center for Electron Microscopy and the Brockhouse Institute
for Materials Research at McMaster for assistance with sample preparation and analysis.
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Please cite this article in press as: Joshi S, Petric A, Nickel substituted CuMn2O4 spinel coatings for solid oxide fuel cell interconnects, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.075