Protective coatings for Cr2O3-forming interconnects of solid oxide fuel cells

international journal of hydrogen energy 34 (2009) 9220–9226

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Protective coatings for Cr2O3-forming interconnects of solid oxide fuel cells Y. Liu*, D.Y. Chen Materials Science and Engineering Program, Department of Chemical, Materials and Biomolecular Engineering and Institute of Materials Science, University of Connecticut, Storrs, CT 06269, USA

article info

abstract

Article history:

Cr2O3 evaporation from Cr2O3-forming metallic interconnects during operation of the solid

Received 28 June 2009

oxide fuel cells (SOFC) can poison other cell components and cause degradation. Protective

Received in revised form

NiFe2O4 spinel coatings on interconnect alloys were developed by electroplating and screen

24 August 2009

printing, respectively. Results indicate that NiFe2O4 coatings can significantly improve the

Accepted 9 September 2009

oxidation resistance of the alloy while providing effective conducting path with inherent

Available online 2 October 2009

low resistance, and also are expected to serve as a diffusion barrier to effectively reduce the

Keywords:

the coatings. A very interesting and smart coating structure was reported.

Cr2O3 evaporation. Two coating techniques were evaluated in terms of the performances of ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

SOFC Interconnect Spinel Coating Layered structures

1.

Introduction

Recent progress in development of the solid oxide fuel cells (SOFC) has led to the reduction of its operating temperature to the range of 600–800  C, which makes metallic alloys feasible for SOFC stack component application such as interconnects[1–5]. Metallic interconnects have attracted a great deal of attention due to their low cost, high electronic and thermal conductivity, good manufacturability, and improved mechanical strength, etc., compared to traditional ceramic interconnects [1,6–10]. The current interest of metallic interconnects are the Fe-based and low CTE (the coefficient of thermal expansion) Ni-based Cr2O3-forming alloys due to their easy availability, low cost, appropriate CTE value and the electrically conductive nature of Cr2O3 compared to other protective scales such as Al2O3 and SiO2 [11–14]. Numerous investigations have been conducted on the performances of

the Cr2O3-forming metallic interconnects [15–22]. In our previous work [23], the behavior of several potential alloys, i.e., Ebrite, Crofer 22 APU, Haynes 230 and Haynes 242, was systematically studied and evaluated in both the oxidizing (cathode) and reducing (anode) environments. Due to the highly negative energy of formation, Cr2O3 was selectively formed on the alloy surface in both atmospheres and functioned as a barrier layer to prevent direct corrosion attack to inner alloys. However, Cr (III) species in Cr2O3 layer is inclined to transform into volatile Cr (VI) species in the SOFC cathode environment [24,25] and migrate to poison other fuel cell components and cause severe cell degradation, thus severely hindered the application of many Cr2O3-forming alloys. Developing a protective coating for the metallic interconnects, which is electronically conductive, nonvolatile, and chemically compatible with other cell components, is one of the most straightforward and economical solutions for above

* Corresponding author. Tel.: þ1 860 486 8960; fax: þ1 860 486 4745. E-mail addresses: [email protected], [email protected] (Y. Liu). 0360-3199/$ – see front matter ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.09.022

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problem. Recently, conductive and protective layers based on spinels, like Mn–Co, Mn–Cu spinels, etc. [26–35], are demonstrated to be successful for effectively suppressing the release of Cr species by separating Cr2O3 from direct contact with the environment. However, a Cr2O3 or Cr-rich layer, which has much lower conductivity compared to spinel, still inevitably forms at the interface between the spinel coating and the substrate, indicating that limiting the growth of the Cr-based oxide scale is as a significant issue as the choice of conductive spinel coating in order to obtain low electronic resistance for the SOFC interconnects. NiFe2O4 spinel coating, which is nonvolatile, conductive and has better chemical compatibility to the Fe-based and Ni-based metallic interconnects, can be employed not only to effectively reduce the Cr2O3 evaporation but also significantly enhance the hot corrosion resistance of the Cr2O3-forming interconnects, i.e., limiting the growth of the Cr-based oxide scale, by either smartly controlling the Ni content in its precursor material or improving the coating density and thickness. To the best of our knowledge, the NiFe2O4 spinel as protective coating on SOFC interconnects has not been reported so far. In this paper, we successfully developed two scenarios of NiFe2O4 spinel coatings with various Ni content by screen printing and electroplating, respectively. The coatings and corresponding coating techniques are evaluated in terms of the performances of the coatings.

2.

Fe(NO3)3

Ni(NO3)2

Water Solution 1

Glycine

Solution 2 GNP Process NiFe2O4 Powder Screen Printing NiFe2O4 Coating Reducing Ni, Fe Metals Oxidizing Dense NiFe2O4 Coating Fig. 1 – Schematic of the preparation procedures for screen printed NiFe2O4 coating.

Experimental

The alloy of Ebrite, a typical Cr2O3-forming metallic interconnect material, was selected as a substrate. The chemical composition of Ebrite is Fe-26Cr-1Mo-0.01 Mn-0.025Si-0.001C0.02S (wt.%). Rectangular samples (about 12 mm  11 mm  1 mm) were cut from the alloy sheets by electric discharge machining (EDM). Each sample was drilled a hole with a diameter of 1 mm on the upper center and then polished to 800 grits using SiC sand paper, ultrasonically cleaned in acetone and dried immediately before coating experiments. The procedure for screen printing was illustrated in Fig. 1. NiFe2O4 powder was synthesized first by Glycine Nitrate Combustion Process (GNP). Precursor materials, i.e., Ni(NO3)2, Fe(NO3)3 and Glycine, were mixed together with mole ratio of 1:2:4.7 in a big beaker to form uniform solution, then heated on a hot plate. After all water totally evaporated out, Glycine started burning as a fuel for reaction of Ni(NO3)2 and Fe(NO3)3. Finally NiFe2O4 spinel powder was synthesized. The synthesized powder was heated at 700  C for 2 h in order to eliminate any extra Glycine, then ball milled in the liquid of Iso-propanal for 30 mins. The ball-milled fine powder was collected and mixed with binding material (V-6 Binder, etc.) to make the paste, which is to be screen printed on the as-prepared Ebrite. The as-printed spinel coating was dried at 80  C for 30 min and then reduced in the reducing environment (5%H2 þ 95%Ar, vol.%) at 900  C for 3 h. Then oxidization at 800  C in air for 150 h follows to re-form the spinel coating. The above procedure is very critical to get a much denser spinel coating compared to the as-printed one. In order to get NiFe2O4 spinel coating by electroplating, the deposition of Ni-21Fe (wt.%) compound was carried out first

following the experimental procedure reported by Li [36]. Compared to the screen printed coating, significant amount of Ni element was added in order to increase the hot corrosion resistance of the electroplated Ebrite but without formation of too much NiO in the coating. The surface of the as-prepared Ebrite was grit-blasted before transferring to the deposition cell. The anode was a 10  10 cm2 square platinum foil. The electrolyte used was a nickel sulfamate based solution composed of 185.7 g L1 nickel sulfamate, 1.95 g L1 ferrous sulfate hepta hydrate, 31 g L1 boric acid, 2 g L1 L-ascorbic acid and 0.5 g L1 SNAP (Sulfamate Nickel Anti-pit). Deposition was carried out at room temperature of 25  C and a pH value of 2. The applied current density and the rotation speed were DC 20 mA cm2 and 500 rpm, respectively. The deposition time was approximately 40 min. After deposition of the Ni–Fe compound, the sample was thermally exposed at 800  C in air for 150 h. The oxide scale with NiFe2O4 spinel was expected to form. The phase structure of as-synthesized NiFe2O4 powder by GNP process and the oxide scales with thermally grown NiFe2O4 spinel was identified with X-ray diffraction (XRD). The surface morphologies and crosssections of the oxide scales were observed using scanning electron microscopy (SEM) with a JEOL 6335F field-emission instrument (Tokyo, Japan) and an energy-dispersive X-ray analysis (EDX). Electrical resistance of the oxide scale was measured using a 2-probe 4-point method in air from 500  C to 800  C with a step size of 50  C. Fig. 2 shows the schematic of the experimental setup for electrical resistance measurement. The upper and lower oxide surfaces were covered with Pt paste and Pt meshes with four Pt leads for current supply and voltage drop measurement. A constant

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I

Pt mesh

V

Alloy

Oxide Scale

I

Pt mesh

Pt Paste

V

Fig. 2 – Schematic of the ASR measurement setup.

current of 10 mA supplied by a Keithley 2400 current source meter was used in all measurements and the voltage drop was recorded accordingly by a Keithley 2100 volt meter. A widely accepted parameter for scaling the electrical resistance of the oxide scales, area specific resistance (ASR), was reported here. ASR reflected both the electrical conductivity and the thickness of the oxide scale. At each temperature, the resistance (R) was calculated according to the Ohm’s law, R ¼ V/I. The ASR was then equal to R multiplied by the area that the Pt paste covered.

3.

Results

Fig. 3 shows the X-ray diffraction pattern from NiFe2O4 powder synthesized by GNP process. All peaks are sharp and consistent with a typical NiFe2O4 powder sample [index No.:10-0325], indicating that pure and fine NiFe2O4 powders was obtained by the GNP process. The in situ reduction of the screen printed NiFe2O4 spinel at high temperature removed the binding material in the paste and reduced the spinel oxide into metals, as follows is the in situ re-oxidation of the Ni, Fe metals with thermal exposure at 800  C, and consequently, a relatively dense coating was obtained, as shown in Fig. 4a. XRD pattern (Fig. 3) of the coating together with EDX analysis verified that the coating by screen printing was composed of NiFe2O4 spinel phase. No observable Cr2O3 sublayer formed under NiFe2O4 coating with a typical thickness of around w17 mm during the thermal exposure time of 150 h,

Fig. 3 – XRD results of the NiFe2O4 powder by GNP process and coatings developed by screening printing and electroplating.

except under the area where the substrate incidentally peeled off during the sample preparation, as indicated in Fig. 4b, Cr2O3 forms. The peeled-off substrate might result from the incident flake-off during the polishing process of sample preparation. In the following reducing and especially oxidation procedure, the oxygen diffused into the area under the flake-off region to develop Cr2O3. In contrast, no observable Cr2O3 layer was detected under non-peeling surface areas. Above difference indicated that the oxygen that diffused into the coating were ‘‘consumed’’ by preferentially reacting with the Fe and Ni to form the NiFe2O4 spinel, while Cr2O3 formation only took place in the area under flake-off region without Fe and Ni during the exposure time. As exposure time goes on after the NiFe2O4 spinel phase completely forms, the Cr2O3 layer may start to develop under it. The development of the Cr2O3 layer is expected to be slow with the coating serving as the barrier for the inward diffusion of the oxygen, and the NiFe2O4 spinel coating by screen printing is expected to function as a protective layer to effectively reduce the evaporation of the inner Cr species during service time by preventing it from direct contact with the outer atmosphere. The X-ray diffraction pattern and structure of the coating by electroplating are shown in Figs. 3 and 5, respectively. XRD pattern together with EDX analysis verified that the coating was composed of NiFe2O4 spinel, NiO and Cr2O3. NiFe2O4 spinel coating with thickness of w9 mm formed simultaneously with Cr2O3 sub-layer. The Cr2O3 layer has a thickness of w1 mm. NiO was non-continuous and embedded in NiFe2O4 on the bottom. Though the conductivity of NiO is times higher than Cr2O3 at the high temperature [37], the structure of noncontinuous NiO without separating more conductive NiFe2O4 spinel from sub-layer is desirable for coating’s better conducting performance. The thickness of the coating by electroplating is pretty uniform. The NiFe2O4 spinel coating is expected to reduce the evaporation of the inner Cr2O3 layer by separating it from direct exposure to the environment, and meanwhile also provide conducting path to sub-layer for its inherent high conductivity at fuel cell working temperature. The Cr2O3 thickness formed in the coating was significantly less than that (w2.5 mm) of uncoated Ebrite in similar situation [22,23], in which only Cr2O3 layer forms in the oxide scale at 800  C in the oxidizing environment. Fig. 6 shows the electrical property of Ebrite with coating by screen printing and electroplating in comparison to uncoated Ebrite. The area specific resistance of the coated samples is significantly lower than uncoated ones. Without continuous Cr2O3 layer, the Ebrite with coating by screen printing displayed much lower resistance compared to the one by electroplating. Correspondingly, the activation energy (w0.35 eV) for coating by screen printing, calculated by the slope of the ASR plot, is significantly lower than the activation energy (w0.88 eV) of the coating by electroplating, which has similar value with the uncoated Ebrite sample. Among all constitutes of coatings, Cr2O3 is the least conductive phase. So energy of activation of the coating is primarily determined by the Cr2O3 layer and the smaller the thickness of Cr2O3 layer, the higher the conductivity the coating has. The coating by electroplating has continuous but thinner Cr2O3 layer compared to uncoated Ebrite, thus it possesses similar

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Fig. 4 – Surface (a) and cross section (b) of the protective coating developed by screen printing.

activation energy but smaller area specific resistance. Due to the direct contact of the highly conductive NiFe2O4 spinel phase to the metal substrate, the coating by screen printing exhibits significantly lower activation energy and area specific resistance.

4.

Discussion

NiFe2O4 coatings are not only expected to serve as an impactful barrier to effectively reduce the Cr evaporation, but also can significantly improve the oxidation resistance of the alloy. A Cr2O3 layer forms at the interface between the spinel coating and the substrate due to its highly negative energy of formation, but its growth can be effectively suppressed or reduced by the proposed coatings. The effect of limiting the Cr2O3 growth was evaluated by the effective Cr2O3 layer thickness, i.e., the equivalent thickness to Cr2O3 in terms of the same area specific resistance value, as shown in Fig. 7. The effective Cr2O3 layer thickness of the coated Ebrite by screen printing is over 12 times smaller than the uncoated one, while that of the coated Ebrite by electroplating reduced to less than half of the uncoated sample. For the screen printed Ebrite, the

NiFe2O4 spinel coating functions as a blocking barrier to limit the growth of the Cr2O3 layer by consuming oxygen that diffused in and preventing the substrate from the direct exposure to the oxygen in the environment. As for the coated Ebrite by electroplating, the un-oxidized Ni–Fe layer exists between the Cr2O3 and the substrate. Ni-based alloys are well recognized for its lower oxidation kinetics than ferritic stainless steels [22]. The Ni–Fe layer in the developed coating is expected to have excellent oxidation resistance as a Ni-based alloy with intentionally introduced significant amount of Ni (79 wt.%). Moreover, the Ni–Fe layer separates the Cr2O3 layer from the substrate, and works as a blocking barrier by delaying the transportation of Cr species to Cr2O3 layer. The dense and thick coating was found to be very important to limit the growth of the Cr2O3 and reduce its evaporation. During the coating preparation, the appropriate ball milling and reducing procedures are of great significance in order to obtain a dense coating by screen printing. The oxidation following reducing step is a reaction sintering process to form the spinel coating and make the coating dense at the same time. The ball milling procedure makes the powder much finer to have more surface energy and the reduction of the total amount of surface energy contributes to the driving force for

Fig. 5 – Cross sections of the protective coating developed by electroplating at low magnification (a) and high magnification (b), respectively.

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Fig. 6 – Scale ASR and linear fitting for Ebrite with and without coating after thermal exposure at 800 8C for 150 h.

sintering [38]. As shown in Fig. 8, the coating developed without appropriate reducing or ball milling procedures is porous, and a continuous Cr2O3 layer was formed under the porous coating (even though much thicker coating) and can deteriorate the conduction performance of the interconnect, whereas under the dense coating Cr2O3 layer formation can be significantly suppressed (see Fig. 4). On the other hand, the oxygen diffusion rate in the dense coating is much lower than that in the porous coating and oxygen can be better and more adequately ‘‘consumed’’ by reacting with Ni–Fe before reaching the interface to form the Cr2O3 layer between the coating and the substrate. The denser and the thicker the coating is, the more effective of the coating to reduce the growth of Cr2O3 layer, the better performance the coating has in terms of both conduction and oxidation resistance for both coating techniques. By smartly tailoring the composition, a very interesting and smart coating structure can be obtained. Fig. 9 is a schematic structure model of such a tailored structure shown in Fig. 5.

Fig. 7 – Comparison of the effective Cr2O3 layer thickness of coated and uncoated Ebrite.

A NiFe2O4 top layer functioned as the protective and highly conductive layer. For the area where NiO forms, the NiFe2O4 conducting veins exist to guarantee the high electronic conduction from top to the sub-layer. The sub-layer of Nibased Ni–Fe is expected to be oxidation resistant itself and also a desirable functional barrier layer for reducing the Cr2O3 formation at the interface as well as conducting electricity to the metallic interconnect. Above conducting vein structure could be achieved by smartly designing and tailoring the composition of the electroplated alloy where oxides grow naturally during oxidation. The conducting vein structure forms thermodynamically due to the large difference of Gibbs free energy of formation between the vein oxide (NiFe2O4) and the matrix oxide (NiO). Vein oxide phase separated to form a network of conducting veins for maximum conducting effect in cases the less conductive matrix oxide inevitably forms simultaneously. Kinetically, the NiFe2O4 phase prefers to form and grows out to top of the oxide layers, extra Ni formed non-continuous NiO phase on the bottom of the NiFe2O4 oxide layer where it acts as matrix for NiFe2O4 conducting vein. Adding too much amount of Ni will make NiO continuous to block the electricity conduction, while too small amount of Ni will worsen the oxidation resistance of the coating layer and finally ruin the smart structure of the coating. Moreover, a dense Ni–Fe layer with significant amount of Ni is needed to suppress the transportation of Cr species to form oxide at the interface. Such a conducting vein structure, as a naturally formed conductive layered structure during oxidation, could find application where the oxidation is a big concern either at elevated temperatures or ambient temperatures that deteriorates the electronic conduction of the metal substrate. Such applications include the bipolar plates in Proton Exchange Membranes (PEM) fuel cells and the electrical contacts broadly used in electronic devices. Such layered structure is self-healing as it is naturally formed on the substrate alloy and will form again after damage. This trait is especially meaningful for electrical contact application with inevitable wear during service. Similar advantage applies for bipolar plates of PEM fuel cells. Both coatings developed by the screen printing and electroplating are expected to effectively reduce the Cr evaporation and limit the growth of Cr2O3 at the same time. The screen printing technique is relatively easy and efficient compared to the electroplating. Without Cr2O3 layer, the screen printed coating displayed better electronic conduction for the thermal exposure time in our experiment. However, the suppression of forming Cr2O3 layer is expected to be temporary and as the service time goes on, a Cr2O3 layer is expected to form essentially at the interface. The density of the coating is limited compared to that by the electroplating technique. While for the smart structure of the coating by electroplating, it is denser and can more efficiently delay oxygen diffusion. In a long run, this coating structure is more stable and more advantageous to limit the growth of the Cr2O3 layer because the dense Ni–Fe layer resists oxidation and separates the oxygen that diffused into the interface from the source of Cr species of the substrate. The thickness of the coating by electroplating is better controllable and the coating has better adhesion to the substrate as well as more flexibility for the geometry of substrate.

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Fig. 8 – Surface (a) and cross section (b) of the porous coating developed by screen printing without appropriate reducing or ball milling procedure.

Oak Ridge National Lab (ORNL) and Tennessee Technological University for their technical support and facility access.

references

Fig. 9 – Structure of oxide scale with conducting vein and functional layers.

5.

Conclusions

NiFe2O4 coatings prepared by both screen printing and electroplating processes can significantly improve the oxidation resistance of the metallic interconnect while providing effective conducting path to substrate, and also are expected to serve as an impactful barrier to reduce the Cr2O3 evaporation by separating Cr species from direct exposure to the environment and limiting the growth of Cr2O3. Thick, dense and stable coating is desirable for above functions. The smart layered coating structure by electroplating is more stable and advantageous in a long run as a protective coating for SOFC interconnects and the novel conducting vein structure could find applications such as PEM bipolar plates and electrical contacts.

Acknowledgement This research was supported by the Institute of Materials Science, University of Connecticut. Special thanks are given to

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