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Applying Low-Pressure Plasma Spray (LPPS) for coatings in low-temperature SOFC
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 e7
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Applying Low-Pressure Plasma Spray (LPPS) for coatings in low-temperature SOFC Kang Yuan a,*,1, Jing Zhu b,1, Wenjing Dong b,**, Yueguang Yu a, Xiaoliang Lu a, Xiaojuan Ji a, Xunying Wang b a
Beijing General Research Institute of Mining and Metallurgy, Beijing 100160, China Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Key Laboratory of Ferro & Piezoelectric Materials and Devices of Hubei Province, Faculty of Physics and Electronic Science, Hubei University, Wuhan, Hubei 430062, China
b
article info
abstract
Article history:
Low-temperature solid oxide fuel cell (LTSOFC) has shown great potentials for commercial
Received 30 December 2016
applications in clean energy generation. Seeking for low cost and easy fabrication method
Received in revised form
is one of the most important issues for LTSOFC investigations. This paper introduces a new
1 April 2017
coating spray technology, namely Low-Pressure Plasma Spray (LPPS), for efficiently
Accepted 20 April 2017
manufacturing different functional coatings of LTSOFC. By applying the LPPS technique,
Available online xxx
uniform and dense Ni0.8Co0.15Al0.05LiO2d (NCAL) coatings were made on both solid bipolar plates and porous nickel foams to perform as protecting coatings and electrode catalyst
Keywords:
coatings respectively. Microstructure study showed that multi phases were formed and in-
LPPS
situ nano-micro crystallization occurred in the coatings during the LPPS process. Around
Coating
30 W output was achieved in a 4-cell stack indicating that the LPPS sprayed NCAL coatings
Low-temperature SOFC
on bipolar plates worked well. A fuel cell based on the NCAL-coated Ni foam reached an
In-situ nano-micro crystallization
open circuit voltage (OCV) at 1.08 V and a maximum power density of 717 mW cm2 at 550 C. This study reveals that LPPS is a promising technology for fabricating coatings of LTSOFC. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The solid oxide fuel cell (SOFC) has been developed extensively for several decades for the application of clean-energy power generation [1,2]. Some rare-earth oxides like yttriastabilized zircoina (YSZ) become good ionic conductor at high temperatures (800e1000 C) and have been widely studied and used as high temperature SOFCs (HTSOFCs). However, many
problems occurred for the HTSOFCs working at high temperatures like corrosion of the metallic interconnectors, sealing issues, and thermal stresses among different functional layers in fuel cells, which limit the commercialization of SOFCs. In the recent decade, the development of SOFC has moved to low-medium temperature range, and low temperature SOFCs (LTSOFCs) are widely investigated to achieve high power output at the temperature ranges of 300e600 C [3e5].
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (K. Yuan), [email protected] (W. Dong). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.ijhydene.2017.04.215 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Yuan K, et al., Applying Low-Pressure Plasma Spray (LPPS) for coatings in low-temperature SOFC, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.215
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The development of LTSOFC brings about reduced manufacturing costs and makes possible the application of inexpensive ferritic stainless steel bipolar plates as interconnectors. However, Stainless steel bipolar plate interconnectors suffer from oxidation and chromium poisoning, resulting in reduced cell performance and stability. Applying oxide coatings on interconnectors can avoid the corrosion of metallic bipolar plates and also provide high electronic conductivity. Oxidation protective coatings like lanthanum ferrite (LSF) and lanthanum cobalt ferrite (LSCF) can be coated on the interconnectors to provide anti-oxidation protection and decrease chromium poisoning content to the cathode [6e8]. Recent studies showed that some lithium battery electrode materials exhibited good performance as electrodes [9,10], therefore are potential to be coatings on fuel cells and interconnectors. To coat oxides on fuel cells and interconnectors, coating technologies can be used with low cost materials and high manufacture efficiency. Various technologies have been applied for making coatings in SOFCs, for example, technologies of thermal spray [6], spin coating [7], and dip coating [11]. As SOFC materials are typical oxide, thermal spray is the most time-saving technique to form coatings. Plasma spray technique has been used to make NiOeYSZ/YSZ/LaSrMnO sandwich layers functioning as anodeeelectrolyteecathode materials for HTSOFC [12,13]. For higher requirement of coating uniformity and large scale coating, plasma spray in vacuum is needed as larger heating zone can be generated so that powders can be more uniformly dispersed and heated in the plasma stream [14,15]. Recently a new technology, namely Low-Pressure Plasma Spray (LPPS), has been developed for coating spray by the company Oerlikon Metco. LPPS is capable to make uniform and dense coatings in short time, and has been applied to make coatings for the HTSOFC systems, like LaSrMnO (LSM) anti-corrosion electrode coating and yttriastabilized zirconia (YSZ) electrolyte coating [16]. However, LPPS technique has not been applied on LTSOFC so far. This paper will show some results of the cutting-edge research of LTSOFC manufacture using the LPPS technique by taking spraying NieCoeAleLi oxides (NCAL) coatings as the example. NCAL is a good charge collector with high electronic conductivity and shows good catalytic activity for both H2 and O2 [10]. In this paper, NCAL, as an oxide coating on electrodes and interconnectors, is investigated while the potential of the LPPS technology for making SOFCs is also discussed.
Experimental Material preparation A commercial Ni0.8Co0.15Al0.05LiO2d (NCAL) powder from Tianjin Bamo company (China) was used to manufacture coatings on stainless steel bipolar plates and nickel foams. The as-received powders had good flowability in the powder feeders of LPPS equipment, and were therefore directly used for the coating manufacture including interconnector coatings on bipolar plates and electrode coatings on Ni foams. The NCAL coatings were manufactured in Sulzer Metco LPPS-TF system. The main components of the system and the
working process are schematically drawn in Fig. 1. In the upright combustion chamber (around 3 m, setting as 1.5 mbar atmospheric pressure), a long, wide and stable plasma stream can be generated (core heating zone > Ø20 cm). Via the sweeping of the plasma gun, the NCAL powders were heated and sprayed onto the workpieces to form uniform coatings. The workpieces can be exchanged via the transfer chamber without stopping the vacuum pump of the combustion chamber so that a successive manufacture process can be achieved with high working efficiency. The spray plasma power was about 70 kW and the spray distance was 900 mm. The surface temperature of the workpieces was in-situ monitored by an infrared pyrometer, and could reach 500e900 C during the spray. The coatings are usually denser than those made in air atmosphere because the powder were finer for easier melting and can be more sufficiently heated in the wider plasma stream with high energy. Large scale coating is allowed because of the widened plasma stream. In the LPPS spray process, 50 um thick coating can be obtained in less than 30 s, while at the same time high coating quality can be maintained as required. No other thin film technologies like CVD/PVD or sputtering can compete with it by producing efficiency. The coatings made by the LPPS technique also give high bonding strength with substrates not only because of the well-melting status of powders due to high plasma power (higher than that of APS which is usually less than 45 kW) and small size of the powders (easier for melting) but also due to that some powders are evaporated to promote atomic bonding with the substrate.
Electrochemical measurement The performance of NCAL protective coating (also as a charge collector) on bipolar plates was investigated through a stack test. The structure of the stack was illustrated in Fig. 2a. The stack used here was 6*6 cm2 with an active area of 25 cm2, and fuel cell was sandwiched in two NCAL-coated bipolar plates with the NCAL coating facing at the fuel cell. Following such structure, a 4-piece stack was made and measured at 550 C. To evaluate the electronic conductivity of NCAL, NCAL powders were pressed into a pellet using a dry-press method under a uniaxial pressure of 280 MPa. A thin layer of Ag was pasted on both sides of the pellet as electrode. Then, electrochemical impedance spectra (EIS) measurements were performed on the sample using an electrochemical workstation (Gamry Reference 3000, USA) in frequency range from 1 MHz to 0.1 Hz under an open circuit voltage (OCV) mode, with an amplitude of 10 mV at the temperature range from 350 to 600 C. In the test, hydrogen was used as fuel and ambient air as oxidant with flow rates in the range of 160e180 mL min1. The performance of NCAL coating on Ni foam was studied on device structured as Fig. 2b. The function layer of the fuel cell was made of samaria-doped ceria (Sm0.2Ce0.8O2, SDC) and NCAL (70 wt.% SDC mixing with 30 wt.% NCAL, named as SDCeNCAL in the following text) (Fig. 2b). The cell layer was fabricated by loading the SDCeNCAL composite powders between two LPPS spraying NCAL coated Ni foams under a uniaxial pressure of 280 MPa. The resultant fuel cell displayed a symmetric configuration, NCALeNi/SDCeNCAL/NCALeNi, with an active area of 0.64 cm2 and a thickness of 1 mm. The
Please cite this article in press as: Yuan K, et al., Applying Low-Pressure Plasma Spray (LPPS) for coatings in low-temperature SOFC, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.215
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 e7
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Fig. 1 e Schematic drawing of the LPPS equipment for coating spray.
electrochemical performances (IeV and IeP characteristics) of the fuel cell (Fig. 2b) were tested at 550 C by a computer integrated instrument (IT8500), where pure (99.999%) and dry hydrogen was used as the fuel while air as oxidant. The flow rates for both gases were in the range of 160e180 mL min1.
Material characterization The X-ray diffraction (XRD) patterns of the powder and coatings were measured by a diffractometer (Bruker D8 Advance) with Cu-Ka radiation (l ¼ 1.54060 A). Phases in the materials were identified by analyzing the XRD spectrums with the species in standard database. The microstructures of the samples were detected in a scanning electron microscope (SEM, SUPRA55) equipped with an energy dispersive X-ray (EDX) spectrometer.
Results and discussion NCAL powder and LPPS spray
Fig. 2 e Fuel cell device illustration: (a) coating on bipolar from the device to stack, (b) structures of a single fuel cell.
The NCAL powder had typical agglomeration morphology, as shown in Fig. 3, with a diameter ranging from several micrometers to dozens of micrometers. The XRD pattern of the powder (Fig. 4) was identified as Li(Ni0.9Co0.1)O2 (PDF No. 704311). The LPPS spray was processed in the vacuum chamber where the atmospheric pressure was set as ~1.5 mbar. A wide plasma stream was generated in the chamber and the sprays can be uniformly coated on the substrates. Fig. 5 shows the typical working status in the LPPS chamber. When the stream was swept onto the workpieces, the molten powders struck onto the surface of the workpieces with high speed (hundred meters per sec) to form dense coatings. The turbulence at the back side of the fixture was a typical low pressure-induced fluid phenomenon. A coating of 50 mm thickness was achieved with 30 s spray process.
Please cite this article in press as: Yuan K, et al., Applying Low-Pressure Plasma Spray (LPPS) for coatings in low-temperature SOFC, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.215
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Fig. 3 e SEM image of the morphology of the NCAL powder.
Fig. 4 e XRD spectrum of NCAL powder. The phase is identified as Li(Ni0.9Co0.1)O2 (PDF No. 70-4311).
Fig. 5 e The working status of NCAL coating spray in the LPPS vacuum chamber.
NCAL coating on bipolar electrode plate The XRD result of the coated NCAL on bipolar electrode plate was presented in Fig. 6. The coating contains three phases, i.e. 49.6 vol.% Li0.3Ni0.7O2 (PDF No. 75-0543), 41.4 vol.% Li2CO3 (PDF No. 87-0729), and 9.0 vol.% Li(Ni0.9Co0.1)O2 (PDF No. 70-4311). As the NCAL powder only had single Li(Ni0.9Co0.1)O2 phase, the phases of Li0.3Ni0.7O and Li2CO3 were evidently formed during the coating spray process. Li0.3Ni0.7O was probably formed due
Fig. 6 e XRD spectrum of NCAL coating made by LPPS. Three main phases were identified: blue lines for Li(Ni0.9Co0.1)O2 (PDF No. 70-4311), green lines for Li0.3Ni0.7O (PDF No. 75-0543), and red lines for Li2CO3 (PDF No. 870729). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
to high-temperature phase transformation from Li(Ni0.9Co0.1) O2; while the formation of Li2CO3 could be due to a reaction of the powder with CO2 [2]. The carbon source should be further ascertained, but can be possibly from the spray environment or the metal substrate or impurities in the powder. The multiphase formed could bring abundant grain and phase boundaries for the coating. As shown in Fig. 7a, the NCAL coating was quite dense comparing with a lab-made membrane via powder metallurgy. Furthermore, the coating showed the occurrence of in-situ nano-micro crystallization as presented by Fig. 7b (see the “ridge” morphology). The “ridges” were homogeneously distributed through the whole coating. The dense coating morphology was also identically confirmed by the cross section image in Fig. 8. Pores can be found locally in the coating but the coating's porosity was quite low. A quick image analysis by using ImageJ software showed less than 5% porosity. Furthermore, in the coating matrix, as presented in Fig. 8b, melting status between the powder particles has been observed. Such melting-induced strong bonding among the powders would result in high gas tightness of the coating, which is benefit for avoiding the oxidation of bipolar plates. EDX mapping and composition analyses showed no heterogeneous distribution of the elements such as carbon, nickel and cobalt. That indicated the new phases, i.e. Li0.3Ni0.7O and Li2(CO3), could be in-situ formed at Li(Ni0.9Co0.1)O2 phase. If so, the nano-micro crystals may be the mixtures of all the phases. A 4 cell stack was made using NCAL coated bipolar plates as interconnectors. An electrical power output of around 30 W was obtained with an active area of 25 cm2 for each cell. It demonstrates that NCAL coating on bipolar plates by LPPS technique works well for collecting and transport electrons. After the stack performance test, NCAL coating spallation from the steel bipolar plate occurred due to thermal stress induced by temperature change. Further work is needed to increase the bonding strength by, for example, introducing a metallic bond layer under the oxide coating to improve thermal stress resistance. Fig. 9 displays the temperature-
Please cite this article in press as: Yuan K, et al., Applying Low-Pressure Plasma Spray (LPPS) for coatings in low-temperature SOFC, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.215
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Fig. 8 e SEM images showing (a) cross section of NCAL coating on bipolar plate, (b) melting-bonding morphology in the coating. Fig. 7 e Surface morphology of NCAL coating in SEM: (a) an overview, (b) “ridge”-like structures.
dependent electrical properties of powder-pressed NCAL. The conductivity of the NCAL was 1e3 S cm1 at the temperature range between 350 and 600 C.
presented in Fig. 11. As can be seen, a maximum power density (Pmax) of 717 mW cm2 and OCV of 1.08 V was achieved, indicating the adaptation of the LPPS-made NCAL coatings with good electrochemical property for the LTSOFC devices.
NCAL coating on Ni foams In our devices, the NCAL coating on the Ni foam functions as electrode catalysts (both anode and cathode) and current collectors. The morphology of the as-brought foam is presented in Fig. 10a. Large pores and high porosity (99%) can be observed. The macro structure of the foams was not changed after LPPS spray (Fig. 10b), indicating that the Ni networks contains a good creep resistance during the high-temperature spray process (~800 C). Fig. 10c shows that the NCAL coatings on the Ni networks had similar microstructure as the coatings made on the bipolar electrodes (Fig. 7a). The cross sections of the SDC-NCAL fuel cells with the LPPS spraying NCAL coated Ni foams were observed in SEM before and after the IeV/IeP measurement, showing no obvious pores in the fuel cell layer. Furthermore, the fuel cell worked well during the electrochemical test, demonstrating that the cell was robust. The typical electrochemical performances of LTSOFC using the plasma sprayed NCAL on porous Ni foam (not suppressed) as electrode is
Fig. 9 e Arrhenius curve showing the conductivity of NCAL at different temperatures.
Please cite this article in press as: Yuan K, et al., Applying Low-Pressure Plasma Spray (LPPS) for coatings in low-temperature SOFC, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.215
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Fig. 12 e Schematic structure of a proposed EFFC made by LPPS technology.
demonstrated the essential function of boundaries and interfaces inside the functional layer in fuel cells [4,17]. Therefore, the LPPS technology could be not only used for making electrode catalyst layer but also very potential for making ionic conductive layer for LTSOFC. As a result, in our incoming research we aim to fabricate electrolyte (layer)-free fuel cells (EFFCs) by LPPS. The schematic structure of such EFFC is presented in Fig. 12. The substrate can be made via a powder-metallurgy approach for the standing of the coatings [13,16], while all the layers (porous catalyst layers and dense function layer) will be made via LPPS coating technology. The key difference of this EFFC from conventional SOFC is that the functional layer is the mixture of semiconductor NCAL and ionic conductor SDC.
Conclusions Fig. 10 e SEM images showing (a) nickel foam before coating, (b) nickel foam after coating, (c) the morphology of the NCAL coating on the nickel foam.
As LPPS is powerful especially for rapid preparation of dense and uniform coatings, it is expected to be applied for the fuel cell layer manufacture as well. This study has demonstrated that LPPS can make coatings with multiphase and insitu nano-micro crystalline structures which may improve fuel cell properties. For LTSOFC, many investigations have
In this study, LPPS technology was applied to manufacture NCAL coatings as interconnector coatings on bipolar plates and also as electrode coatings on Ni foam for low-temperature SOFC (LTSOFC). The LPPS made NCAL coating on bipolar plates presented a dense structure which helps to protect the bipolar plates from oxidation or corrosion. The LPPS sprayed NCAL coatings on bipolar plates (6*6 cm2) demonstrated potential use in stack since a power of about 30 W was generated by a 4cell stack at 550 C. The same LPPS coated NCAL has been proved to be active on Ni foam when used as the electrode catalyst layer of LTSOFC, which has achieved the fuel cell performances at Pmax of 717 mW cm2 and OCV of 1.08 V. The results demonstrated that the LPPS technique provides a new fast and effective way to manufacture continuous, uniform and dense coatings for LTSOFCs. Phase transformation occurred during the coating spray process due to the partial melting of the powders. Nano-micro crystallization also took place in the coatings which can be beneficial for improving fuel cell properties since interfaces like grain/phase boundaries in the functional layers of LTSOFC are important.
Acknowledgments
Fig. 11 e IeV/IeP characteristics of cell based on spray coated NCAL on Ni foam at 550 C.
This work was supported by the funding from the project in Beijing General Research Institute of Mining and Metallurgy (Grant Nos. QC-201702 and YJ-2016-04), the International Cooperation project (Grant No. 2015DFA51530), the Natural
Please cite this article in press as: Yuan K, et al., Applying Low-Pressure Plasma Spray (LPPS) for coatings in low-temperature SOFC, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.215
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Science Foundation of Hubei Province (Grant No. 2015CFA120) and the National Natural Science Foundation of China (NSFC, No. 11604088).
references
[1] Steele BCH. Fuel-cell technology e running on natural gas. Nature 1999;400:619e21. [2] Atkinson A, Barnett S, Gorte RJ, Irvine JTS, McEvoy AJ, Mogensen M, et al. Advanced anodes for high-temperature fuel cells. Nat Mater 2004;3:17e27. [3] Zhu B. Next generation fuel cell R&D. Int J Energ Res 2006;30:895e903. [4] Zhu B, Fan LD, Lund P. Breakthrough fuel cell technology using ceria-based multi-functional nanocomposites. Appl Energ 2013;106:163e75. [5] Zhu B, Raza R, Abbas G, Singh M. An electrolyte-free fuel cell constructed from one homogenous layer with mixed conductivity. Adv Funct Mater 2011;21:2465e9. [6] Fu CJ, Sun KN, Zhang NQ, Chen XB, Zhou DR. Evaluation of lanthanum ferrite coated interconnect for intermediate temperature solid oxide fuel cells. Thin Solid Films 2008;516:1857e63. [7] Zhao K, Xu Q, Huang DP, Chen M, Kim BH. Microstructure and electrode properties of La0.6Sr0.4Co0.2Fe0.8O3delta spincoated on Ce0.8Sm0.2O2delta electrolyte. Ionics 2011;17:247e54. [8] Simner SR, Anderson MD, Coleman JE, Stevenson JW. Performance of a novel La(Sr)Fe(Co)O-3-Ag SOFC cathode. J Power Sources 2006;161:115e22.
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[9] Zhang W, Cai Y, Wang B, Deng H, Feng C, Dong W, et al. The fuel cells studies from ionic electrolyte Ce0.8Sm0.05Ca0.15O2d to the mixture layers with semiconductor Ni0.8Co0.15Al0.05 LiO2d. Int J Hydrogen Energy 2016;41:18761e8. [10] Fan L, Su PC. Layer-structured LiNi0.8Co0.2O2: a new triple (Hþ/O2/e) conducting cathode for low temperature proton conducting solid oxide fuel cells. J Power Sources 2016;306:369e77. [11] Yoon JS, Lee J, Hwang HJ, Whang CM, Moon JW, Kim DH. Lanthanum oxide-coated stainless steel for bipolar plates in solid oxide fuel cells (SOFCs). J Power Sources 2008;181:281e6. [12] Li CX, Li CJ, Guo LJ. Effect of composition of NiO/YSZ anode on the polarization characteristics of SOFC fabricated by atmospheric plasma spraying. Int J Hydrogen Energy 2010;35:2964e9. [13] Vassen R, Hathiramani D, Mertens J, Haanappel VAC, Vinke IC. Manufacturing of high performance solid oxide fuel cells (SOFCs) with atmospheric plasma spraying (APS). Surf Coat Technol 2007;202:499e508. [14] Syed AA, Ilhan Z, Arnold J, Schiller G, Weckmann H. Improving plasma-sprayed yttria-stabilized zirconia coatings for solid oxide fuel cell electrolytes. J Therm Spray Technol 2006;15:617e22. [15] Lang M, Henne R, Schaper S, Schiller G. Development and characterization of vacuum plasma sprayed thin film solid oxide fuel cells. J Therm Spray Technol 2001;10:618e25. [16] Gindrat M, Damani R. LPPS hybrid technologies for emerging energy applications-recent developments. In: Int Therm Spray Conference; 2011. [17] Ahmed A, Raza R, Khalid MS, Saleem M, Alvi F, Javed MS, et al. Highly efficient composite electrolyte for natural gas fed fuel cell. Int J Hydrogen Energy 2016;41:6972e9.
Please cite this article in press as: Yuan K, et al., Applying Low-Pressure Plasma Spray (LPPS) for coatings in low-temperature SOFC, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.215
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Applying Low-Pressure Plasma Spray (LPPS) for coatings in low-temperature SOFC Kang Yuan a,*,1, Jing Zhu b,1, Wenjing Dong b,**, Yueguang Yu a, Xiaoliang Lu a, Xiaojuan Ji a, Xunying Wang b a
Beijing General Research Institute of Mining and Metallurgy, Beijing 100160, China Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Key Laboratory of Ferro & Piezoelectric Materials and Devices of Hubei Province, Faculty of Physics and Electronic Science, Hubei University, Wuhan, Hubei 430062, China
b
article info
abstract
Article history:
Low-temperature solid oxide fuel cell (LTSOFC) has shown great potentials for commercial
Received 30 December 2016
applications in clean energy generation. Seeking for low cost and easy fabrication method
Received in revised form
is one of the most important issues for LTSOFC investigations. This paper introduces a new
1 April 2017
coating spray technology, namely Low-Pressure Plasma Spray (LPPS), for efficiently
Accepted 20 April 2017
manufacturing different functional coatings of LTSOFC. By applying the LPPS technique,
Available online xxx
uniform and dense Ni0.8Co0.15Al0.05LiO2d (NCAL) coatings were made on both solid bipolar plates and porous nickel foams to perform as protecting coatings and electrode catalyst
Keywords:
coatings respectively. Microstructure study showed that multi phases were formed and in-
LPPS
situ nano-micro crystallization occurred in the coatings during the LPPS process. Around
Coating
30 W output was achieved in a 4-cell stack indicating that the LPPS sprayed NCAL coatings
Low-temperature SOFC
on bipolar plates worked well. A fuel cell based on the NCAL-coated Ni foam reached an
In-situ nano-micro crystallization
open circuit voltage (OCV) at 1.08 V and a maximum power density of 717 mW cm2 at 550 C. This study reveals that LPPS is a promising technology for fabricating coatings of LTSOFC. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The solid oxide fuel cell (SOFC) has been developed extensively for several decades for the application of clean-energy power generation [1,2]. Some rare-earth oxides like yttriastabilized zircoina (YSZ) become good ionic conductor at high temperatures (800e1000 C) and have been widely studied and used as high temperature SOFCs (HTSOFCs). However, many
problems occurred for the HTSOFCs working at high temperatures like corrosion of the metallic interconnectors, sealing issues, and thermal stresses among different functional layers in fuel cells, which limit the commercialization of SOFCs. In the recent decade, the development of SOFC has moved to low-medium temperature range, and low temperature SOFCs (LTSOFCs) are widely investigated to achieve high power output at the temperature ranges of 300e600 C [3e5].
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (K. Yuan), [email protected] (W. Dong). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.ijhydene.2017.04.215 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Yuan K, et al., Applying Low-Pressure Plasma Spray (LPPS) for coatings in low-temperature SOFC, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.215
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The development of LTSOFC brings about reduced manufacturing costs and makes possible the application of inexpensive ferritic stainless steel bipolar plates as interconnectors. However, Stainless steel bipolar plate interconnectors suffer from oxidation and chromium poisoning, resulting in reduced cell performance and stability. Applying oxide coatings on interconnectors can avoid the corrosion of metallic bipolar plates and also provide high electronic conductivity. Oxidation protective coatings like lanthanum ferrite (LSF) and lanthanum cobalt ferrite (LSCF) can be coated on the interconnectors to provide anti-oxidation protection and decrease chromium poisoning content to the cathode [6e8]. Recent studies showed that some lithium battery electrode materials exhibited good performance as electrodes [9,10], therefore are potential to be coatings on fuel cells and interconnectors. To coat oxides on fuel cells and interconnectors, coating technologies can be used with low cost materials and high manufacture efficiency. Various technologies have been applied for making coatings in SOFCs, for example, technologies of thermal spray [6], spin coating [7], and dip coating [11]. As SOFC materials are typical oxide, thermal spray is the most time-saving technique to form coatings. Plasma spray technique has been used to make NiOeYSZ/YSZ/LaSrMnO sandwich layers functioning as anodeeelectrolyteecathode materials for HTSOFC [12,13]. For higher requirement of coating uniformity and large scale coating, plasma spray in vacuum is needed as larger heating zone can be generated so that powders can be more uniformly dispersed and heated in the plasma stream [14,15]. Recently a new technology, namely Low-Pressure Plasma Spray (LPPS), has been developed for coating spray by the company Oerlikon Metco. LPPS is capable to make uniform and dense coatings in short time, and has been applied to make coatings for the HTSOFC systems, like LaSrMnO (LSM) anti-corrosion electrode coating and yttriastabilized zirconia (YSZ) electrolyte coating [16]. However, LPPS technique has not been applied on LTSOFC so far. This paper will show some results of the cutting-edge research of LTSOFC manufacture using the LPPS technique by taking spraying NieCoeAleLi oxides (NCAL) coatings as the example. NCAL is a good charge collector with high electronic conductivity and shows good catalytic activity for both H2 and O2 [10]. In this paper, NCAL, as an oxide coating on electrodes and interconnectors, is investigated while the potential of the LPPS technology for making SOFCs is also discussed.
Experimental Material preparation A commercial Ni0.8Co0.15Al0.05LiO2d (NCAL) powder from Tianjin Bamo company (China) was used to manufacture coatings on stainless steel bipolar plates and nickel foams. The as-received powders had good flowability in the powder feeders of LPPS equipment, and were therefore directly used for the coating manufacture including interconnector coatings on bipolar plates and electrode coatings on Ni foams. The NCAL coatings were manufactured in Sulzer Metco LPPS-TF system. The main components of the system and the
working process are schematically drawn in Fig. 1. In the upright combustion chamber (around 3 m, setting as 1.5 mbar atmospheric pressure), a long, wide and stable plasma stream can be generated (core heating zone > Ø20 cm). Via the sweeping of the plasma gun, the NCAL powders were heated and sprayed onto the workpieces to form uniform coatings. The workpieces can be exchanged via the transfer chamber without stopping the vacuum pump of the combustion chamber so that a successive manufacture process can be achieved with high working efficiency. The spray plasma power was about 70 kW and the spray distance was 900 mm. The surface temperature of the workpieces was in-situ monitored by an infrared pyrometer, and could reach 500e900 C during the spray. The coatings are usually denser than those made in air atmosphere because the powder were finer for easier melting and can be more sufficiently heated in the wider plasma stream with high energy. Large scale coating is allowed because of the widened plasma stream. In the LPPS spray process, 50 um thick coating can be obtained in less than 30 s, while at the same time high coating quality can be maintained as required. No other thin film technologies like CVD/PVD or sputtering can compete with it by producing efficiency. The coatings made by the LPPS technique also give high bonding strength with substrates not only because of the well-melting status of powders due to high plasma power (higher than that of APS which is usually less than 45 kW) and small size of the powders (easier for melting) but also due to that some powders are evaporated to promote atomic bonding with the substrate.
Electrochemical measurement The performance of NCAL protective coating (also as a charge collector) on bipolar plates was investigated through a stack test. The structure of the stack was illustrated in Fig. 2a. The stack used here was 6*6 cm2 with an active area of 25 cm2, and fuel cell was sandwiched in two NCAL-coated bipolar plates with the NCAL coating facing at the fuel cell. Following such structure, a 4-piece stack was made and measured at 550 C. To evaluate the electronic conductivity of NCAL, NCAL powders were pressed into a pellet using a dry-press method under a uniaxial pressure of 280 MPa. A thin layer of Ag was pasted on both sides of the pellet as electrode. Then, electrochemical impedance spectra (EIS) measurements were performed on the sample using an electrochemical workstation (Gamry Reference 3000, USA) in frequency range from 1 MHz to 0.1 Hz under an open circuit voltage (OCV) mode, with an amplitude of 10 mV at the temperature range from 350 to 600 C. In the test, hydrogen was used as fuel and ambient air as oxidant with flow rates in the range of 160e180 mL min1. The performance of NCAL coating on Ni foam was studied on device structured as Fig. 2b. The function layer of the fuel cell was made of samaria-doped ceria (Sm0.2Ce0.8O2, SDC) and NCAL (70 wt.% SDC mixing with 30 wt.% NCAL, named as SDCeNCAL in the following text) (Fig. 2b). The cell layer was fabricated by loading the SDCeNCAL composite powders between two LPPS spraying NCAL coated Ni foams under a uniaxial pressure of 280 MPa. The resultant fuel cell displayed a symmetric configuration, NCALeNi/SDCeNCAL/NCALeNi, with an active area of 0.64 cm2 and a thickness of 1 mm. The
Please cite this article in press as: Yuan K, et al., Applying Low-Pressure Plasma Spray (LPPS) for coatings in low-temperature SOFC, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.215
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Fig. 1 e Schematic drawing of the LPPS equipment for coating spray.
electrochemical performances (IeV and IeP characteristics) of the fuel cell (Fig. 2b) were tested at 550 C by a computer integrated instrument (IT8500), where pure (99.999%) and dry hydrogen was used as the fuel while air as oxidant. The flow rates for both gases were in the range of 160e180 mL min1.
Material characterization The X-ray diffraction (XRD) patterns of the powder and coatings were measured by a diffractometer (Bruker D8 Advance) with Cu-Ka radiation (l ¼ 1.54060 A). Phases in the materials were identified by analyzing the XRD spectrums with the species in standard database. The microstructures of the samples were detected in a scanning electron microscope (SEM, SUPRA55) equipped with an energy dispersive X-ray (EDX) spectrometer.
Results and discussion NCAL powder and LPPS spray
Fig. 2 e Fuel cell device illustration: (a) coating on bipolar from the device to stack, (b) structures of a single fuel cell.
The NCAL powder had typical agglomeration morphology, as shown in Fig. 3, with a diameter ranging from several micrometers to dozens of micrometers. The XRD pattern of the powder (Fig. 4) was identified as Li(Ni0.9Co0.1)O2 (PDF No. 704311). The LPPS spray was processed in the vacuum chamber where the atmospheric pressure was set as ~1.5 mbar. A wide plasma stream was generated in the chamber and the sprays can be uniformly coated on the substrates. Fig. 5 shows the typical working status in the LPPS chamber. When the stream was swept onto the workpieces, the molten powders struck onto the surface of the workpieces with high speed (hundred meters per sec) to form dense coatings. The turbulence at the back side of the fixture was a typical low pressure-induced fluid phenomenon. A coating of 50 mm thickness was achieved with 30 s spray process.
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Fig. 3 e SEM image of the morphology of the NCAL powder.
Fig. 4 e XRD spectrum of NCAL powder. The phase is identified as Li(Ni0.9Co0.1)O2 (PDF No. 70-4311).
Fig. 5 e The working status of NCAL coating spray in the LPPS vacuum chamber.
NCAL coating on bipolar electrode plate The XRD result of the coated NCAL on bipolar electrode plate was presented in Fig. 6. The coating contains three phases, i.e. 49.6 vol.% Li0.3Ni0.7O2 (PDF No. 75-0543), 41.4 vol.% Li2CO3 (PDF No. 87-0729), and 9.0 vol.% Li(Ni0.9Co0.1)O2 (PDF No. 70-4311). As the NCAL powder only had single Li(Ni0.9Co0.1)O2 phase, the phases of Li0.3Ni0.7O and Li2CO3 were evidently formed during the coating spray process. Li0.3Ni0.7O was probably formed due
Fig. 6 e XRD spectrum of NCAL coating made by LPPS. Three main phases were identified: blue lines for Li(Ni0.9Co0.1)O2 (PDF No. 70-4311), green lines for Li0.3Ni0.7O (PDF No. 75-0543), and red lines for Li2CO3 (PDF No. 870729). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
to high-temperature phase transformation from Li(Ni0.9Co0.1) O2; while the formation of Li2CO3 could be due to a reaction of the powder with CO2 [2]. The carbon source should be further ascertained, but can be possibly from the spray environment or the metal substrate or impurities in the powder. The multiphase formed could bring abundant grain and phase boundaries for the coating. As shown in Fig. 7a, the NCAL coating was quite dense comparing with a lab-made membrane via powder metallurgy. Furthermore, the coating showed the occurrence of in-situ nano-micro crystallization as presented by Fig. 7b (see the “ridge” morphology). The “ridges” were homogeneously distributed through the whole coating. The dense coating morphology was also identically confirmed by the cross section image in Fig. 8. Pores can be found locally in the coating but the coating's porosity was quite low. A quick image analysis by using ImageJ software showed less than 5% porosity. Furthermore, in the coating matrix, as presented in Fig. 8b, melting status between the powder particles has been observed. Such melting-induced strong bonding among the powders would result in high gas tightness of the coating, which is benefit for avoiding the oxidation of bipolar plates. EDX mapping and composition analyses showed no heterogeneous distribution of the elements such as carbon, nickel and cobalt. That indicated the new phases, i.e. Li0.3Ni0.7O and Li2(CO3), could be in-situ formed at Li(Ni0.9Co0.1)O2 phase. If so, the nano-micro crystals may be the mixtures of all the phases. A 4 cell stack was made using NCAL coated bipolar plates as interconnectors. An electrical power output of around 30 W was obtained with an active area of 25 cm2 for each cell. It demonstrates that NCAL coating on bipolar plates by LPPS technique works well for collecting and transport electrons. After the stack performance test, NCAL coating spallation from the steel bipolar plate occurred due to thermal stress induced by temperature change. Further work is needed to increase the bonding strength by, for example, introducing a metallic bond layer under the oxide coating to improve thermal stress resistance. Fig. 9 displays the temperature-
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Fig. 8 e SEM images showing (a) cross section of NCAL coating on bipolar plate, (b) melting-bonding morphology in the coating. Fig. 7 e Surface morphology of NCAL coating in SEM: (a) an overview, (b) “ridge”-like structures.
dependent electrical properties of powder-pressed NCAL. The conductivity of the NCAL was 1e3 S cm1 at the temperature range between 350 and 600 C.
presented in Fig. 11. As can be seen, a maximum power density (Pmax) of 717 mW cm2 and OCV of 1.08 V was achieved, indicating the adaptation of the LPPS-made NCAL coatings with good electrochemical property for the LTSOFC devices.
NCAL coating on Ni foams In our devices, the NCAL coating on the Ni foam functions as electrode catalysts (both anode and cathode) and current collectors. The morphology of the as-brought foam is presented in Fig. 10a. Large pores and high porosity (99%) can be observed. The macro structure of the foams was not changed after LPPS spray (Fig. 10b), indicating that the Ni networks contains a good creep resistance during the high-temperature spray process (~800 C). Fig. 10c shows that the NCAL coatings on the Ni networks had similar microstructure as the coatings made on the bipolar electrodes (Fig. 7a). The cross sections of the SDC-NCAL fuel cells with the LPPS spraying NCAL coated Ni foams were observed in SEM before and after the IeV/IeP measurement, showing no obvious pores in the fuel cell layer. Furthermore, the fuel cell worked well during the electrochemical test, demonstrating that the cell was robust. The typical electrochemical performances of LTSOFC using the plasma sprayed NCAL on porous Ni foam (not suppressed) as electrode is
Fig. 9 e Arrhenius curve showing the conductivity of NCAL at different temperatures.
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Fig. 12 e Schematic structure of a proposed EFFC made by LPPS technology.
demonstrated the essential function of boundaries and interfaces inside the functional layer in fuel cells [4,17]. Therefore, the LPPS technology could be not only used for making electrode catalyst layer but also very potential for making ionic conductive layer for LTSOFC. As a result, in our incoming research we aim to fabricate electrolyte (layer)-free fuel cells (EFFCs) by LPPS. The schematic structure of such EFFC is presented in Fig. 12. The substrate can be made via a powder-metallurgy approach for the standing of the coatings [13,16], while all the layers (porous catalyst layers and dense function layer) will be made via LPPS coating technology. The key difference of this EFFC from conventional SOFC is that the functional layer is the mixture of semiconductor NCAL and ionic conductor SDC.
Conclusions Fig. 10 e SEM images showing (a) nickel foam before coating, (b) nickel foam after coating, (c) the morphology of the NCAL coating on the nickel foam.
As LPPS is powerful especially for rapid preparation of dense and uniform coatings, it is expected to be applied for the fuel cell layer manufacture as well. This study has demonstrated that LPPS can make coatings with multiphase and insitu nano-micro crystalline structures which may improve fuel cell properties. For LTSOFC, many investigations have
In this study, LPPS technology was applied to manufacture NCAL coatings as interconnector coatings on bipolar plates and also as electrode coatings on Ni foam for low-temperature SOFC (LTSOFC). The LPPS made NCAL coating on bipolar plates presented a dense structure which helps to protect the bipolar plates from oxidation or corrosion. The LPPS sprayed NCAL coatings on bipolar plates (6*6 cm2) demonstrated potential use in stack since a power of about 30 W was generated by a 4cell stack at 550 C. The same LPPS coated NCAL has been proved to be active on Ni foam when used as the electrode catalyst layer of LTSOFC, which has achieved the fuel cell performances at Pmax of 717 mW cm2 and OCV of 1.08 V. The results demonstrated that the LPPS technique provides a new fast and effective way to manufacture continuous, uniform and dense coatings for LTSOFCs. Phase transformation occurred during the coating spray process due to the partial melting of the powders. Nano-micro crystallization also took place in the coatings which can be beneficial for improving fuel cell properties since interfaces like grain/phase boundaries in the functional layers of LTSOFC are important.
Acknowledgments
Fig. 11 e IeV/IeP characteristics of cell based on spray coated NCAL on Ni foam at 550 C.
This work was supported by the funding from the project in Beijing General Research Institute of Mining and Metallurgy (Grant Nos. QC-201702 and YJ-2016-04), the International Cooperation project (Grant No. 2015DFA51530), the Natural
Please cite this article in press as: Yuan K, et al., Applying Low-Pressure Plasma Spray (LPPS) for coatings in low-temperature SOFC, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.215
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Science Foundation of Hubei Province (Grant No. 2015CFA120) and the National Natural Science Foundation of China (NSFC, No. 11604088).
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Please cite this article in press as: Yuan K, et al., Applying Low-Pressure Plasma Spray (LPPS) for coatings in low-temperature SOFC, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.215