- Email: [email protected]
Employment of fluorine doped zinc tin oxide (ZnSnOx:F) coating layer on stainless steel 316 for a bipolar plate for PEMFC
Materials Chemistry and Physics 128 (2011) 39–43
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Employment of fluorine doped zinc tin oxide (ZnSnOx:F) coating layer on stainless steel 316 for a bipolar plate for PEMFC Ji Hun Park a,b , Dongjin Byun b , Joong Kee Lee a,∗ a b
Advanced Energy Materials Processing Laboratory, Battery Research Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Republic of Korea Dept. of Materials Science & Engineering, Korea University, Seoul 136-701, Republic of Korea
a r t i c l e
i n f o
Article history: Received 11 February 2010 Received in revised form 30 November 2010 Accepted 10 January 2011 Keywords: Thin films Chemical vapor deposition (CVD) Electrochemical techniques Corrosion
a b s t r a c t The investigation of the electrochemical characteristics of the fluorine doped tin oxide (SnOx:F) and fluorine doped zinc tin oxide (ZnSnOx:F) was carried out in the simulated PEMFC environment and bare stainless steel 316 was used as a reference. The results showed that the ZnSnOx:F coating enhanced both the corrosion resistance and interfacial contact resistance (ICR). The corrosion current for ZnSnOx:F was 1.2 A cm−2 which was much lower than that of bare stainless steel of 50.16 A cm−2 . The ZnSnOx:F coated film had the smallest corrosion current due to the formation of a tight surface morphology with very few pin-holes. The ZnSnOx:F coated film exhibited the highest values of the cell voltage and power density due to its having the lowest ICR values. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The proton exchange membrane fuel cell (PEMFC) is one of the most promising next-generation electric vehicle applications owing to its low operating temperature (80 ◦ C); high current density and power density fast start time, simple design, and environmental-friendliness. Presently, there are still some challenges to the commercialization of PEMFCs, due to the high manufacturing cost of the fuel cell stack, as well as the characteristics of the component parts. One of the key components of the fuel cell stack is the bipolar plates that amount to more than 30% of the total cost [1–3]. The bipolar plates support the membrane electrode assembly (MEA) and provide conduits for the reactant gases namely hydrogen and oxygen. They also provide various functions such as the electrical connection between cells and facilitate water and heat management through the cell. Ideally, bipolar plates should have a high electrical conductivity, low gas permeability, high mechanical strength, high corrosion resistance, thermal conductivity, easy processing and low cost [4]. The compaction force of bipolar plates has several functions in a fuel cell stack. It required properties follow from its functions. It must be having electrical conductivity, impermeable to gases, adequate strength, lightweight and thermally conductive for compaction force of fuel cell. The most important
∗ Corresponding author. Tel.: +82 02 9585252; fax: +82 02 9585229. E-mail address: [email protected] (J.K. Lee). 0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.01.020
property of the fuel cell bipolar plates is the electrical contact resistance [14]. Currently graphite and graphite composites are considered as the standard materials for PEM bipolar plates because of their low contact resistance and high corrosion resistance. However graphite and graphite composites are classified as brittle and permeable to gases with poor cost effectiveness for high volume manufacturing process relative to metals such as aluminum, stainless steel, nickel, titanium etc. [5]. The main disadvantage of graphite is that it is not suitable for electric vehicle applications, due its low durability against shock and vibration. On the other hand, metals enjoy higher mechanical strength, better durability to shocks and vibration, impermeability and much superior manufacturability and cost effectiveness when compared to carbon based materials. However, the main handicap of metals is their lack of corrosion resistance in the harsh acidic and humid environment inside PEM fuel cells which leads to considerable power degradation [6]. Therefore, surface modification is needed in order to enhance the corrosion resistance. Aluminum, stainless steel, titanium and nickel are generally used as the base materials of the bipolar plates of PEMFCs. Moreover, the major disadvantage of these base metals is that they are easily corroded in the harsh acidic and humid environment inside the PEM fuel cell. Therefore, various kinds of metal alloys have been tried, but their performance was found to be insufficient to endure such an acidic environment [7]. Many recent reports revealed that metals with certain coatings have the potential to be used as bipolar plates which satisfy the demands of the commercial applications of PEMFCs. As a coating technology,
40
J.H. Park et al. / Materials Chemistry and Physics 128 (2011) 39–43
chemical vapor deposition, physical vapor deposition and liquid phase chemical deposition techniques can be used [8]. The purpose of this study is to evaluate the corrosion resistance, as well as the electrical resistance, of stainless steels with various coatings. Through this work, the effects of metallic oxide composite film on the interfacial contact resistance and corrosion resistance of the bipolar plates in the strong hydro-acid environment of PEMFCs are investigated. In the present study, the characteristics of fluorine doped zinc tin oxide film (ZnSnOx:F) is investigated in comparison with fluorine doped tin oxide (SnOx:F) and bare stainless steel as a coating layer of PEMFCs. 2. Experimental An ECR-MOCVD (Electron Cyclotron Resonance-Metallic Organic Chemical Vapor Deposition) system was used for the deposition of the ZnSnOx:F and SnOx:F composite films. It consists of a microwave generator, two pairs of magnets, a CVD reactor, and a bubbler system for the organometallic precursor feeding system. The CVD reactor consists of cylindrical excitation and deposition chamber, respectively. A tunable microwave power of up to one 1000 W at a frequency of 2.46 GHz was used. The magnetic field of 875 Gauss is applied to the excitation chamber to form electron cyclotron resonance condition in the chamber. Electron cyclotron resonance is achieved when the frequency of the microwaves is equal to that of the circulating electrons under the magnetic field that is generated. The samples were prepared under the following conditions: a working pressure of 5 mTorr, a bubbler pressure of 70 Torr, a 4 sccm flow of TMT (tetramethyl-tin), a 0.8 sccm flow of DEZn (dethylzinc), a deposition time of 30 min, and a microwave power of 1000 W. A detailed description of ECR-MOCVD system is given in our previous report [9]. In order to measure the contact resistance, an experimental apparatus was devised. The setup shows that two pieces of conductive carbon paper are sandwiched between the prepared specimen and two gold-coated plates. The electrical current is provided via the two gold-coated plates. The potential difference between the gold-coated plates was measured while the compressive force was gradually increased to 300 N cm−2 . The interfacial contact resistance is obtained after calculating resistance based on Ohm’s law [10]. The corrosion characteristics of the coated stainless steel can be represented by the corrosion current which is determined by Tafel-extrapolation method from the polarization curves in the solution environment of the PEMFC. The corrosion test kit consists of various parts. A potentiostat was used to measure the change in the current between the SCE and the sample in the voltage range from −250 to 250 mV. Three electrodes are used for the corrosion measurements: a saturated calomel electrode as the reference electrode, a surface coated stainless steel plate as the working electrode and two electrodes as the counter electrode. The change in current between the SCE and working electrode is stored in the data acquisition system. The composition of the electrolyte mixture is one mole sulfuric acid and 2 ppm of HF and the temperature was maintained at 70 ◦ C during the corrosion test. The single cell is assembled with the catalyst loaded membrane, carbon papers, gasket and prepared bipolar plates. For the preparation of the MEA, a Nafion 212 membrane with platinum loading of 0.2 mg cm−2 (Johnson Matthey) and the CCMs (Corrosion Resistant Construction Materials) method was employed. The area of the active electrode was 5 cm2 . In order to operate the single cells, fully humidified hydrogen and oxygen gases were fed to the anode and cathode, respectively.
3. Results and discussion 3.1. Characteristics of the films Scanning electron microscopy was employed to investigate the difference in the surface morphology before and after the deposition of the metallic oxide composite coating. Fig. 1(a) shows the surface morphology of the bare stainless steel bipolar plate having smooth surface. Fig. 1(a), (b) and (c) is the morphology of bare stainless steel, SnOx:F coated steel and ZnSnOx:F coated stainless steel, respectively. The surface topography of the SnOx:F film consisted of a mixture of two types of grains, viz. elliptical shaped and sharp needle shaped grains. The surface topography of the ZnSnOx:F film has a tetrahedron shaped polycrystalline structure. It can be seen that both of the films are composed of grains with a wide distribution in size ranging from several tens to several hundreds of nanometers. The maximum grain sizes of the tin oxide and zinc tin oxide films are 120 nm and 250 nm, respectively. This variation may be caused by the rapid growth of the films resulting from
Fig. 1. Changes in surface morphology after the coating. (a) Bare stainless steel, (b) SnOx:F coating, and (c) ZnSnOx:F coating.
the high concentrations of metallic ions in and around the film substrate. Fig. 2 shows the changes in X-ray patterns after the deposition of the SnOx:F and ZnSnOx:F coatings. The diffraction patterns show that both SnOx:F and ZnSnOx:F films have a nano-polycrystalline tetragonal rutile structure [13]. The X-ray diffraction pattern of the SnOx:F film having a thickness of 1 m shows a (2 1 1) preferred orientation. The diffraction pattern of the fluorine doped zinc tin oxide film is shifted compared to that of the SnOx:F film, possibly due to zinc inclusion. The various diffraction pattern planes are shown in Table 1. It implies that the films are composed of polycrystalline in the nanometer scale. The incorporation of zinc atoms into the tin oxide composition leads to a decrease in the peak intensity in the diffraction patterns of the (1 1 0), (2 2 0) and (3 0 1) planes. These results are well matched with those in a previous work [11]. Fig. 3(a) and (b) shows the AES (Auger Electron Spectroscopy) depth profiles for the SnOx:F and ZnSnOx:F coating films, respectively deposited with a etching rate of 65 nm min−1 . As the etching time is increased, major profiles for Sn, Zn and O are observed. The amount of F is too small for it to be observed in the composition of either of the films. The Sn/O ratio through the film remained
J.H. Park et al. / Materials Chemistry and Physics 128 (2011) 39–43
41
Fig. 2. X-ray diffraction patterns of bare stainless steel (SUS316), SnOx:F (FTO) and ZnSnOx:F (ZTO:F) films coated on stainless steels.
almost constant at 0.8 for the fluorine doped tin oxide film. Our results showed employment of three components conductive oxide films for the application of bipolar plate, but Minami et al. [12] used them as the thin film transparent electrode of liquid crystal displays. 3.2. Electrochemical properties
Fig. 3. Auger electron spectra of (a) SnOx:F and (b) ZnSnOx:F films coated on stainless steel.
decrease in the interfacial resistance. Obviously, the interfacial conductivities of the SnOx:F and ZnSnOx:F coated stainless steels were higher than that of the bare stainless steel at all compaction forces. At a compaction pressure of 150 N cm−2 , the contact resistances of the bare stainless steel, fluorine doped tin oxide and fluorine
In order to simulate fuel cell stack conditions, the values of the interfacial contact resistance were evaluated when applying various compaction pressures. As shown in Fig. 4, the interfacial contact resistance of all of the specimens decreased rapidly at a low compaction pressure and then decreased gradually, probably due to the Table 1 Changes in X-ray patterns after the coating. ZnSnOx:F
SnO:F
FeCrNi (Austenite)
2
Pattern
2
Pattern
2
Pattern
26.9 34.28 38.26 44.78 47.16 52.02 54.96 62.24 66.26 74.91 78.86
(1 1 0) (1 0 1) (2 0 0) (1 1 1) (2 1 0) (2 1 1) (2 2 0) (3 1 0) (3 0 1) (2 1 2) (3 2 1)
26.84 52.04 55.14 66.2 78.92
(1 1 0) (2 1 1) (2 2 0) (3 0 1) (3 2 1)
44.82 47.2 51.23 65.3 75.1
(2 0 2) (1/3 3 1) (3 3 1) (1 0 1) (4 0 0)
Fig. 4. Comparison of ICR with compaction force for bare stainless steel, SnOx:F and ZnSnOx:F films coated on stainless steel.
42
J.H. Park et al. / Materials Chemistry and Physics 128 (2011) 39–43
Fig. 5. Corrosion current [A cm−2 ] in 1 M H2 SO4 with 2 ppm HF solution for bare stainless steel, SnOx:F and ZnSnOx:F films coated on stainless steel.
doped zinc tin oxide coated stainless steels were measured to be 279, 76 and 35 m cm2 , respectively. The interfacial resistance of the ZnSnOx:F coated stainless steel is almost the same as that of graphite, viz 30 m cm2 .
Fig. 7. Cell voltage [V] and power density [mW cm2 ] of bare stainless steel, SnOx:F and ZnSnOx:F films coated on stainless steel in current density range of 0–0.7 A cm−2 .
Fig. 5 shows the potentiodynamic polarization curves for the bare stainless steel 316 and SnOx:F and ZnSnOx:F coated films in 1 M H2 SO4 + 2 ppm HF solution at 70 ◦ C [10]. The potential was varied from −250 to 250 mV at a scan rate of 1 mv s−1 . The current densities of the SnOx:F and ZnSnOx:F coated stainless steel and stainless steel 316, which determined by the Tafel-extrapolation method, are 1.2, 6.64 and 50.15 A cm−2 , respectively. As shown in the figure, the ZnSnOx:F coated stainless steel exhibited the smallest corrosion current in the simulated PEMFC environment. Considering the weak corrosion current of zinc, it can be construed that the films having a compact structure were more sustainable under acidic environment. This is probably due to the lesser formation of pin holes during the deposition of the ZnSnOx:F film which has a wider size distribution than that of the SnOx:F film. 3.3. Surface morphology Fig. 6(a)–(c) shows the SEM images of the specimens after potentiodynamic polarization test. In the case of the bare stainless steel in Fig. 6(a), exfoliated scale with a thickness of several hundreds of nanometers was observed over a broad region of the surface after the corrosion test. Fig. 6(b) and (c) shows effect of the SnOx:F and ZnSnOx:F coatings on the performance of the stainless steel substrate, with the bare stainless steel substrate still showing good stability in the acid solution. Pin hole defects and cracks are observed for the SnOx:F and ZnSnOx:F samples. In the case of the SnOx:F film, a broader region of the surface was dissolved than that in the case of the ZnSnOx:F film in the acidic environment. 3.4. Electrochemical performance of the single cell Fig. 7 shows the I–V curves of the single cells. The operating temperature is 70 ◦ C and the pressure is 1 atm. The stoichiometry of hydrogen and oxygen were 1.5 and 3, respectively. The cell voltages decreased with increasing current density. The fluorine doped zinc tin oxide coated bipolar plate exhibited the highest values of the cell voltage and power density, due to its having the lowest ICR values. The powder density of the fluorine doped zinc tin oxide coated bipolar plate of 350 mW cm2 at 0.7 A cm−2 is about 70% of that of graphite. 4. Conclusions
Fig. 6. Surface morphologies of (a) bare stainless steel, (b) SnOx:F and (c) ZnSnOx:F films coated on stainless steel after corrosion test obtained by SEM analysis. The arrow mark means the pin hole.
The deposition of the ZnSnOx:F layers on SUS 316 bipolar plates was carried out by ECR-MOCVD and the properties of the resulting plates were compared with those of the SnOx:F coated and the bare stainless steel 316 for the purpose of evaluating the
J.H. Park et al. / Materials Chemistry and Physics 128 (2011) 39–43
applicability of bipolar plates in PEMFCs. From the anode polarization tests in 1 M H2 SO4 + 2 ppm HF at 70 ◦ C, the ZnSnOx:F coating provided both the smallest corrosion current and the lowest interfacial contact resistance (ICR). The corrosion properties of the ZnSnOx:F coating need to be further evaluated. In the single cell test, the initial performance of the ZnSnOx:F coated bipolar plates was significantly improved compared with that of the bare bipolar plate, although it was still lower than that of the graphite bipolar plate. The ZnSnOx:F films provided a higher corrosion resistance, and lower contact resistance due to their low porosity and high density. Acknowledgment This work was supported by the following foundation: National Research Foundation [NRF-2010-C1AAA001-2010-0028958] of Korea Grant funded by the Korean Government (MEST).
43
References [1] P.R. Bueno, J.A. Varela, E. Longo, J. Eur. Ceram. Soc. 28 (2008) 505–529. [2] R.C. Makkus, A.H. Janssen, F.A. Bruijn, R.K. Mallant, J. Power Sources 86 (2000) 274–282. [3] D.P. Davies, P.L. Adcock, M. Turpin, S.J. Rowen, J. Power Sources 86 (2000) 237–242. [4] O. Diat, G. Gebel, Nat. Mater. 7 (2008) 13–14. [5] K. Kendall, Nat. Mater. 1 (2002) 211–212. [6] S.J. Lee, C.H. Huang, J.J. Lai, Y.P. Chen, J. Power Sources 131 (2004) 162–168. [7] A. Pozio, F. Zaza, A. Masci, R.F. Silva, J. Power Sources 179 (2008) 631–639. [8] H. Wang, J.A. Turner, X. Li, G. Teeter, J. Power Sources 178 (2008) 238–247. [9] J.H. Park, D. Byun, B.J. Jeon, J.K. Lee, J. Mater. Sci. 43 (2008) 3417–3423. [10] H. Wang, J.A. Turner, X. Li, R. Bhattacharya, J. Power Sources 171 (2007) 567–574. [11] J.H. Yu, G.M. Choi, Sen. Acr. B 52 (1998) 251–256. [12] T. Minami, Thin Solid Films 516 (2008) 5822–5828. [13] J.H. Park, D. Byun, J.K. Lee, J. Electroceram. 23 (2009) 506–511. [14] F. Barbir, J. Braun, J. Neutzler, Int. J. New Mater. Electochem. Sys. 2 (1999) 197–200.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Employment of fluorine doped zinc tin oxide (ZnSnOx:F) coating layer on stainless steel 316 for a bipolar plate for PEMFC Ji Hun Park a,b , Dongjin Byun b , Joong Kee Lee a,∗ a b
Advanced Energy Materials Processing Laboratory, Battery Research Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Republic of Korea Dept. of Materials Science & Engineering, Korea University, Seoul 136-701, Republic of Korea
a r t i c l e
i n f o
Article history: Received 11 February 2010 Received in revised form 30 November 2010 Accepted 10 January 2011 Keywords: Thin films Chemical vapor deposition (CVD) Electrochemical techniques Corrosion
a b s t r a c t The investigation of the electrochemical characteristics of the fluorine doped tin oxide (SnOx:F) and fluorine doped zinc tin oxide (ZnSnOx:F) was carried out in the simulated PEMFC environment and bare stainless steel 316 was used as a reference. The results showed that the ZnSnOx:F coating enhanced both the corrosion resistance and interfacial contact resistance (ICR). The corrosion current for ZnSnOx:F was 1.2 A cm−2 which was much lower than that of bare stainless steel of 50.16 A cm−2 . The ZnSnOx:F coated film had the smallest corrosion current due to the formation of a tight surface morphology with very few pin-holes. The ZnSnOx:F coated film exhibited the highest values of the cell voltage and power density due to its having the lowest ICR values. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The proton exchange membrane fuel cell (PEMFC) is one of the most promising next-generation electric vehicle applications owing to its low operating temperature (80 ◦ C); high current density and power density fast start time, simple design, and environmental-friendliness. Presently, there are still some challenges to the commercialization of PEMFCs, due to the high manufacturing cost of the fuel cell stack, as well as the characteristics of the component parts. One of the key components of the fuel cell stack is the bipolar plates that amount to more than 30% of the total cost [1–3]. The bipolar plates support the membrane electrode assembly (MEA) and provide conduits for the reactant gases namely hydrogen and oxygen. They also provide various functions such as the electrical connection between cells and facilitate water and heat management through the cell. Ideally, bipolar plates should have a high electrical conductivity, low gas permeability, high mechanical strength, high corrosion resistance, thermal conductivity, easy processing and low cost [4]. The compaction force of bipolar plates has several functions in a fuel cell stack. It required properties follow from its functions. It must be having electrical conductivity, impermeable to gases, adequate strength, lightweight and thermally conductive for compaction force of fuel cell. The most important
∗ Corresponding author. Tel.: +82 02 9585252; fax: +82 02 9585229. E-mail address: [email protected] (J.K. Lee). 0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.01.020
property of the fuel cell bipolar plates is the electrical contact resistance [14]. Currently graphite and graphite composites are considered as the standard materials for PEM bipolar plates because of their low contact resistance and high corrosion resistance. However graphite and graphite composites are classified as brittle and permeable to gases with poor cost effectiveness for high volume manufacturing process relative to metals such as aluminum, stainless steel, nickel, titanium etc. [5]. The main disadvantage of graphite is that it is not suitable for electric vehicle applications, due its low durability against shock and vibration. On the other hand, metals enjoy higher mechanical strength, better durability to shocks and vibration, impermeability and much superior manufacturability and cost effectiveness when compared to carbon based materials. However, the main handicap of metals is their lack of corrosion resistance in the harsh acidic and humid environment inside PEM fuel cells which leads to considerable power degradation [6]. Therefore, surface modification is needed in order to enhance the corrosion resistance. Aluminum, stainless steel, titanium and nickel are generally used as the base materials of the bipolar plates of PEMFCs. Moreover, the major disadvantage of these base metals is that they are easily corroded in the harsh acidic and humid environment inside the PEM fuel cell. Therefore, various kinds of metal alloys have been tried, but their performance was found to be insufficient to endure such an acidic environment [7]. Many recent reports revealed that metals with certain coatings have the potential to be used as bipolar plates which satisfy the demands of the commercial applications of PEMFCs. As a coating technology,
40
J.H. Park et al. / Materials Chemistry and Physics 128 (2011) 39–43
chemical vapor deposition, physical vapor deposition and liquid phase chemical deposition techniques can be used [8]. The purpose of this study is to evaluate the corrosion resistance, as well as the electrical resistance, of stainless steels with various coatings. Through this work, the effects of metallic oxide composite film on the interfacial contact resistance and corrosion resistance of the bipolar plates in the strong hydro-acid environment of PEMFCs are investigated. In the present study, the characteristics of fluorine doped zinc tin oxide film (ZnSnOx:F) is investigated in comparison with fluorine doped tin oxide (SnOx:F) and bare stainless steel as a coating layer of PEMFCs. 2. Experimental An ECR-MOCVD (Electron Cyclotron Resonance-Metallic Organic Chemical Vapor Deposition) system was used for the deposition of the ZnSnOx:F and SnOx:F composite films. It consists of a microwave generator, two pairs of magnets, a CVD reactor, and a bubbler system for the organometallic precursor feeding system. The CVD reactor consists of cylindrical excitation and deposition chamber, respectively. A tunable microwave power of up to one 1000 W at a frequency of 2.46 GHz was used. The magnetic field of 875 Gauss is applied to the excitation chamber to form electron cyclotron resonance condition in the chamber. Electron cyclotron resonance is achieved when the frequency of the microwaves is equal to that of the circulating electrons under the magnetic field that is generated. The samples were prepared under the following conditions: a working pressure of 5 mTorr, a bubbler pressure of 70 Torr, a 4 sccm flow of TMT (tetramethyl-tin), a 0.8 sccm flow of DEZn (dethylzinc), a deposition time of 30 min, and a microwave power of 1000 W. A detailed description of ECR-MOCVD system is given in our previous report [9]. In order to measure the contact resistance, an experimental apparatus was devised. The setup shows that two pieces of conductive carbon paper are sandwiched between the prepared specimen and two gold-coated plates. The electrical current is provided via the two gold-coated plates. The potential difference between the gold-coated plates was measured while the compressive force was gradually increased to 300 N cm−2 . The interfacial contact resistance is obtained after calculating resistance based on Ohm’s law [10]. The corrosion characteristics of the coated stainless steel can be represented by the corrosion current which is determined by Tafel-extrapolation method from the polarization curves in the solution environment of the PEMFC. The corrosion test kit consists of various parts. A potentiostat was used to measure the change in the current between the SCE and the sample in the voltage range from −250 to 250 mV. Three electrodes are used for the corrosion measurements: a saturated calomel electrode as the reference electrode, a surface coated stainless steel plate as the working electrode and two electrodes as the counter electrode. The change in current between the SCE and working electrode is stored in the data acquisition system. The composition of the electrolyte mixture is one mole sulfuric acid and 2 ppm of HF and the temperature was maintained at 70 ◦ C during the corrosion test. The single cell is assembled with the catalyst loaded membrane, carbon papers, gasket and prepared bipolar plates. For the preparation of the MEA, a Nafion 212 membrane with platinum loading of 0.2 mg cm−2 (Johnson Matthey) and the CCMs (Corrosion Resistant Construction Materials) method was employed. The area of the active electrode was 5 cm2 . In order to operate the single cells, fully humidified hydrogen and oxygen gases were fed to the anode and cathode, respectively.
3. Results and discussion 3.1. Characteristics of the films Scanning electron microscopy was employed to investigate the difference in the surface morphology before and after the deposition of the metallic oxide composite coating. Fig. 1(a) shows the surface morphology of the bare stainless steel bipolar plate having smooth surface. Fig. 1(a), (b) and (c) is the morphology of bare stainless steel, SnOx:F coated steel and ZnSnOx:F coated stainless steel, respectively. The surface topography of the SnOx:F film consisted of a mixture of two types of grains, viz. elliptical shaped and sharp needle shaped grains. The surface topography of the ZnSnOx:F film has a tetrahedron shaped polycrystalline structure. It can be seen that both of the films are composed of grains with a wide distribution in size ranging from several tens to several hundreds of nanometers. The maximum grain sizes of the tin oxide and zinc tin oxide films are 120 nm and 250 nm, respectively. This variation may be caused by the rapid growth of the films resulting from
Fig. 1. Changes in surface morphology after the coating. (a) Bare stainless steel, (b) SnOx:F coating, and (c) ZnSnOx:F coating.
the high concentrations of metallic ions in and around the film substrate. Fig. 2 shows the changes in X-ray patterns after the deposition of the SnOx:F and ZnSnOx:F coatings. The diffraction patterns show that both SnOx:F and ZnSnOx:F films have a nano-polycrystalline tetragonal rutile structure [13]. The X-ray diffraction pattern of the SnOx:F film having a thickness of 1 m shows a (2 1 1) preferred orientation. The diffraction pattern of the fluorine doped zinc tin oxide film is shifted compared to that of the SnOx:F film, possibly due to zinc inclusion. The various diffraction pattern planes are shown in Table 1. It implies that the films are composed of polycrystalline in the nanometer scale. The incorporation of zinc atoms into the tin oxide composition leads to a decrease in the peak intensity in the diffraction patterns of the (1 1 0), (2 2 0) and (3 0 1) planes. These results are well matched with those in a previous work [11]. Fig. 3(a) and (b) shows the AES (Auger Electron Spectroscopy) depth profiles for the SnOx:F and ZnSnOx:F coating films, respectively deposited with a etching rate of 65 nm min−1 . As the etching time is increased, major profiles for Sn, Zn and O are observed. The amount of F is too small for it to be observed in the composition of either of the films. The Sn/O ratio through the film remained
J.H. Park et al. / Materials Chemistry and Physics 128 (2011) 39–43
41
Fig. 2. X-ray diffraction patterns of bare stainless steel (SUS316), SnOx:F (FTO) and ZnSnOx:F (ZTO:F) films coated on stainless steels.
almost constant at 0.8 for the fluorine doped tin oxide film. Our results showed employment of three components conductive oxide films for the application of bipolar plate, but Minami et al. [12] used them as the thin film transparent electrode of liquid crystal displays. 3.2. Electrochemical properties
Fig. 3. Auger electron spectra of (a) SnOx:F and (b) ZnSnOx:F films coated on stainless steel.
decrease in the interfacial resistance. Obviously, the interfacial conductivities of the SnOx:F and ZnSnOx:F coated stainless steels were higher than that of the bare stainless steel at all compaction forces. At a compaction pressure of 150 N cm−2 , the contact resistances of the bare stainless steel, fluorine doped tin oxide and fluorine
In order to simulate fuel cell stack conditions, the values of the interfacial contact resistance were evaluated when applying various compaction pressures. As shown in Fig. 4, the interfacial contact resistance of all of the specimens decreased rapidly at a low compaction pressure and then decreased gradually, probably due to the Table 1 Changes in X-ray patterns after the coating. ZnSnOx:F
SnO:F
FeCrNi (Austenite)
2
Pattern
2
Pattern
2
Pattern
26.9 34.28 38.26 44.78 47.16 52.02 54.96 62.24 66.26 74.91 78.86
(1 1 0) (1 0 1) (2 0 0) (1 1 1) (2 1 0) (2 1 1) (2 2 0) (3 1 0) (3 0 1) (2 1 2) (3 2 1)
26.84 52.04 55.14 66.2 78.92
(1 1 0) (2 1 1) (2 2 0) (3 0 1) (3 2 1)
44.82 47.2 51.23 65.3 75.1
(2 0 2) (1/3 3 1) (3 3 1) (1 0 1) (4 0 0)
Fig. 4. Comparison of ICR with compaction force for bare stainless steel, SnOx:F and ZnSnOx:F films coated on stainless steel.
42
J.H. Park et al. / Materials Chemistry and Physics 128 (2011) 39–43
Fig. 5. Corrosion current [A cm−2 ] in 1 M H2 SO4 with 2 ppm HF solution for bare stainless steel, SnOx:F and ZnSnOx:F films coated on stainless steel.
doped zinc tin oxide coated stainless steels were measured to be 279, 76 and 35 m cm2 , respectively. The interfacial resistance of the ZnSnOx:F coated stainless steel is almost the same as that of graphite, viz 30 m cm2 .
Fig. 7. Cell voltage [V] and power density [mW cm2 ] of bare stainless steel, SnOx:F and ZnSnOx:F films coated on stainless steel in current density range of 0–0.7 A cm−2 .
Fig. 5 shows the potentiodynamic polarization curves for the bare stainless steel 316 and SnOx:F and ZnSnOx:F coated films in 1 M H2 SO4 + 2 ppm HF solution at 70 ◦ C [10]. The potential was varied from −250 to 250 mV at a scan rate of 1 mv s−1 . The current densities of the SnOx:F and ZnSnOx:F coated stainless steel and stainless steel 316, which determined by the Tafel-extrapolation method, are 1.2, 6.64 and 50.15 A cm−2 , respectively. As shown in the figure, the ZnSnOx:F coated stainless steel exhibited the smallest corrosion current in the simulated PEMFC environment. Considering the weak corrosion current of zinc, it can be construed that the films having a compact structure were more sustainable under acidic environment. This is probably due to the lesser formation of pin holes during the deposition of the ZnSnOx:F film which has a wider size distribution than that of the SnOx:F film. 3.3. Surface morphology Fig. 6(a)–(c) shows the SEM images of the specimens after potentiodynamic polarization test. In the case of the bare stainless steel in Fig. 6(a), exfoliated scale with a thickness of several hundreds of nanometers was observed over a broad region of the surface after the corrosion test. Fig. 6(b) and (c) shows effect of the SnOx:F and ZnSnOx:F coatings on the performance of the stainless steel substrate, with the bare stainless steel substrate still showing good stability in the acid solution. Pin hole defects and cracks are observed for the SnOx:F and ZnSnOx:F samples. In the case of the SnOx:F film, a broader region of the surface was dissolved than that in the case of the ZnSnOx:F film in the acidic environment. 3.4. Electrochemical performance of the single cell Fig. 7 shows the I–V curves of the single cells. The operating temperature is 70 ◦ C and the pressure is 1 atm. The stoichiometry of hydrogen and oxygen were 1.5 and 3, respectively. The cell voltages decreased with increasing current density. The fluorine doped zinc tin oxide coated bipolar plate exhibited the highest values of the cell voltage and power density, due to its having the lowest ICR values. The powder density of the fluorine doped zinc tin oxide coated bipolar plate of 350 mW cm2 at 0.7 A cm−2 is about 70% of that of graphite. 4. Conclusions
Fig. 6. Surface morphologies of (a) bare stainless steel, (b) SnOx:F and (c) ZnSnOx:F films coated on stainless steel after corrosion test obtained by SEM analysis. The arrow mark means the pin hole.
The deposition of the ZnSnOx:F layers on SUS 316 bipolar plates was carried out by ECR-MOCVD and the properties of the resulting plates were compared with those of the SnOx:F coated and the bare stainless steel 316 for the purpose of evaluating the
J.H. Park et al. / Materials Chemistry and Physics 128 (2011) 39–43
applicability of bipolar plates in PEMFCs. From the anode polarization tests in 1 M H2 SO4 + 2 ppm HF at 70 ◦ C, the ZnSnOx:F coating provided both the smallest corrosion current and the lowest interfacial contact resistance (ICR). The corrosion properties of the ZnSnOx:F coating need to be further evaluated. In the single cell test, the initial performance of the ZnSnOx:F coated bipolar plates was significantly improved compared with that of the bare bipolar plate, although it was still lower than that of the graphite bipolar plate. The ZnSnOx:F films provided a higher corrosion resistance, and lower contact resistance due to their low porosity and high density. Acknowledgment This work was supported by the following foundation: National Research Foundation [NRF-2010-C1AAA001-2010-0028958] of Korea Grant funded by the Korean Government (MEST).
43
References [1] P.R. Bueno, J.A. Varela, E. Longo, J. Eur. Ceram. Soc. 28 (2008) 505–529. [2] R.C. Makkus, A.H. Janssen, F.A. Bruijn, R.K. Mallant, J. Power Sources 86 (2000) 274–282. [3] D.P. Davies, P.L. Adcock, M. Turpin, S.J. Rowen, J. Power Sources 86 (2000) 237–242. [4] O. Diat, G. Gebel, Nat. Mater. 7 (2008) 13–14. [5] K. Kendall, Nat. Mater. 1 (2002) 211–212. [6] S.J. Lee, C.H. Huang, J.J. Lai, Y.P. Chen, J. Power Sources 131 (2004) 162–168. [7] A. Pozio, F. Zaza, A. Masci, R.F. Silva, J. Power Sources 179 (2008) 631–639. [8] H. Wang, J.A. Turner, X. Li, G. Teeter, J. Power Sources 178 (2008) 238–247. [9] J.H. Park, D. Byun, B.J. Jeon, J.K. Lee, J. Mater. Sci. 43 (2008) 3417–3423. [10] H. Wang, J.A. Turner, X. Li, R. Bhattacharya, J. Power Sources 171 (2007) 567–574. [11] J.H. Yu, G.M. Choi, Sen. Acr. B 52 (1998) 251–256. [12] T. Minami, Thin Solid Films 516 (2008) 5822–5828. [13] J.H. Park, D. Byun, J.K. Lee, J. Electroceram. 23 (2009) 506–511. [14] F. Barbir, J. Braun, J. Neutzler, Int. J. New Mater. Electochem. Sys. 2 (1999) 197–200.