- Email: [email protected]
S battery
Solid State Ionics 192 (2011) 364–367
Contents lists available at ScienceDirect
Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s s i
Study of La0.8Sr0.2Co0.2Cr0.8O3 − δ as a candidate coating material for the positive current collector in Na/S battery Ying Huang, Zhaoyin Wen ⁎, Jianhua Yang, Yu Liu CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, PR China
a r t i c l e
i n f o
Article history: Received 30 August 2009 Received in revised form 20 August 2010 Accepted 6 September 2010 Available online 14 October 2010 Keywords: Perovskite Corrosion resistance Polarization Dip-immersion Molten salt
a b s t r a c t Perovskite La0.8Sr0.2Co0.2Cr0.8O3 − δ (LSCC) ceramic synthesized by the conventional ceramic processing technique was studied as a novel coating material for the cathode current collector in Na/S battery. Its structure, electrical conductivity, density and thermal expansion coefficient (TEC) were investigated. The corrosion performance of LSCC was in particular evaluated by electrochemical techniques in combination with long-term dip-immersion tests. The results indicated that LSCC exhibited excellent corrosion resistance in molten sodium tetrasulfide at 350 °C. The corrosion current density icorr (0.081 mA cm− 2) was much lower than that of 316 L stainless steel by approximately two orders of magnitude. The corrosion rate of LSCC deduced from immersion test was as low as about 12 μm year− 1. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Na/S battery has been in development as one of the most promising advanced energy storage technologies owing to its various advantages [1–3]. Generally, the major concern for Na/S battery is that few materials can simultaneously satisfy the main requirements of electrical conductivity and corrosion resistance for the sulfur electrode current collector which also acts as cell container. The corrosion of the positive current collector by the corrosive sodium polysulfide may lead to loss of capacity, increase of resistance and cycle-life limitations [1,4–10]. Material research efforts have demonstrated that the combinations of requirements mentioned above may be best fulfilled by a coated system [5,11,12]. Many candidate coating materials, e.g. molybdenum, super alloys, carbon, chromium carbide, or conducting ceramics have been examined [8,13–15], but most of them have certain drawbacks. Lanthanum chromite-based perovskite oxides are well known as conductive parts for electrochemical devices such as high-temperature solid oxide fuel cells, due to their good electrical conductivity and high stability in both oxidation and reduction environments [16,17]. Perovskite oxides may also seem to be promising candidates as the interior coating for the container of Na/S battery. In this work perovskite oxide La0.8Sr0.2Co0.2Cr0.8O3 − δ (LSCC) has been investigated for the first time as a possible candidate coating material for the current collector of Na/S battery. The
⁎ Corresponding author. Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, PR China. Tel.: + 86 21 52411704; fax: + 86 21 52413903. E-mail address: [email protected] (Z. Wen). 0167-2738/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2010.09.013
preparation of LSCC and its corrosion performance under the sodium polysulfide melt environment were presented in this paper.
2. Experimental La0.8Sr0.2Co0.2Cr0.8O3 − δ (LSCC) oxides were prepared by the conventional ceramic processing technique. Stoichiometric amounts of reagent-grade La2O3, SrCO3, Co2O3 and Cr2O3 were mixed by ball milling and then calcined at 1150 °C for 5 h in air. The resulting powders were ground and pressed into bars and discs, and finally sintered at 1510 °C for 1 h in air. The density of sintered specimens was measured according to Archimedes method using ethyl alcohol as the immersing media. The electrical conductivity measurements were carried out by the standard four-probe dc method. The thermal expansion coefficient (TEC) was evaluated on a NETZSCH DIL 402 C dilatometer (alumina holder) with a TSAC 414/4 controller. The corrosion resistance of LSCC against molten sodium tetrasulfide was evaluated by static dip-immersion tests as well as electrochemical measurements carried out under argon atmosphere using a three electrodes electrochemical cell. High density graphite rods which show sufficient stability in molten sodium polysulfide [5,8] were used as the reference electrode and counter electrode. Sintered specimen sized approximately 30 × 5 × 5 mm3, giving the exposure area of 6.5 cm2 was used as the working electrode. In the dip-immersion tests, the specimens to be tested and the desired sodium tetrasulfide used as corrosion medium were contained in hard glass tubes, which were filled in a glove box and then sealed under vacuum. At selected periods of exposure, the samples were taken out, ultrasonically cleaned in ethyl alcohol and
Y. Huang et al. / Solid State Ionics 192 (2011) 364–367
365
Table 1 Results of data analysis of polarization curve using POLFIT. Specimen
LSCC
316 L
χ2 R2 Icorr (mA) βa (mV/dec) βc (mV/dec) icorr (mA cm− 2)
0.00193 0.99911 0.523 ± 0.008 27.942 ± 0.297 25.002 ± 0.216 0.081 ± 0.001
0.00015 0.99996 5.303 ± 0.021 108.029 ± 0.362 102.565 ± 0.776 5.303 ± 0.021
that the electrical conductivity followed the relationship for the adiabatic small polaron hopping mechanism [19]: σ = ðA = kT Þ exp ð−Ea = kT Þ;
Fig. 1. Log σ T versus 1000/T in air for La0.8Sr0.2Co0.2Cr0.8O3 − δ.
ð1Þ
where A is a material constant containing the carrier concentration term, Ea the activation energy for hopping conduction, k the Boltzmann's constant and T the absolute temperature. 3.2. Electrochemical corrosion behaviors
then analyzed by X-ray diffraction analysis (XRD) and electron microprobe analysis.
3. Results and discussion 3.1. Characterization of the sintered LSCC As confirmed in Fig. 4, all the major XRD peaks of the sintered ceramic belonged to the perovskite structure, indicating the formation of a single perovskite phase. The relative density of LSCC measured by Archimedes method was 96.4%. The thermal expansion coefficient of the perovskite ceramic tested under N2 environment was 17.2 × 10− 6 K− 1 between room temperature and 800 °C. This value was very close to that of 316 L stainless steel (18 × 10− 6 K− 1) [18] which was often chosen as a substrate material for container of Na/S battery. Electrical conductivity measurements were performed with the standard four-probe dc method from room temperature to 600 °C in air. The results showed that electrical conductivity changed little within the temperature range of 250 to 600 °C, and it reached 10 S cm− 1, an acceptable level for current collector in the Na/S battery [15]. The electrical conductivity was plotted as log σT versus 1/T in Fig. 1. The slope for the linear portion of the data curve yielded the activation energy about 0.089 eV, suggesting
Fig. 2. Potentiodynamic polarization curve measured in the potential range of − 30 mV to 30 mV and result of data fitting for LSCC and 316 L stainless steel exposed to molten sodium tetrasulfide at 350 °C respectively.
The electrochemical corrosion behavior of LSCC was studied using potentiondynamic polarization tests conducted at a scanning rate of 0.166 mV s− 1 in the vicinity of the corrosion potential (from − 30 mV to 30 mV versus graphite reference electrode). The data fitting analysis was carried out with the software program POLFIT [20,21] based on the Butler–Volmer equation [22]: 2:3ðE−Ecorr Þ = βa −2:3ðE−Ecorr Þ = βc ; iðEÞ = icorr e −e
ð2Þ
where icorr is the corrosion current density, Ecorr the corrosion potential, βa and βc the anodic and cathodic Tafel slope respectively. The potentiodynamic polarization curves and the results of the data fitting were shown in Fig. 2. For the purpose of comparison, the electrochemical behaviors of the 316 L stainless steel were also studied under the same conditions. The fit parameters βa, βc and icorr as well as their errors were listed in Table 1. The goodness of fit was represented by the Chisquare (χ2) parameter. As given in the table, the icorr of LSCC was approximately two orders of magnitude lower than that of 316 L stainless steel. It is known that icorr is proportional to corrosion rate [23]. Therefore, it is reasonable to consider that LSCC exhibits a
Fig. 3. Potentiodynamic polarization curves performed in the potential range of − 1.0 V to 1.5 V for LSCC and 316 L stainless steel exposed to molten sodium tetrasulfide at 350 °C respectively.
366
Y. Huang et al. / Solid State Ionics 192 (2011) 364–367
Fig. 4. XRD patterns of LSCC before and after immersed in molten sodium tetrasulfide melt.
much better corrosion resistance than 316 L stainless steel in the described environments. The Tafel slopes are considered to be indicators of the influence of the diffusion process on the rate
controlling step. Tafel slopes lower than 100 mV/dec are typical for activation controlled systems, whereas Tafel slopes with larger values are typical for systems which are not purely activation or diffusion controlled [24]. It can be clearly seen from Table 1 that the Tafel slopes for LSCC were significantly lower than 100 mV/dec, implying that the corrosion rate controlling step was activation control. On the other hand, the Tafel slopes for 316 L stainless steel were slightly higher than 100 mV/dec, implying a complex corrosion mechanism including diffusion process as well as electron transport process. Fig. 3 illustrated the potentiodynamic polarization curves of LSCC and 316 L stainless steel measured in the wider potential range from −1.00 V to 1.50 V. No active–passive transition region was visible in the two curves, indicating that both of the materials were able to reach a passive condition spontaneously in the melt. Moreover, no current spike which indicated breakdown processes of the passive layer was recorded for both the materials studied. However, it was clearly seen that the corrosion potential of LSCC (− 77 mV) was much higher than that of 316 L stainless steel (−187 mV). Moreover, the passive anodic current density of LSCC was far lower than that of the 316 L stainless steel. Therefore, the anticorrosion property of LSCC was superior to 316 L stainless steel in the molten sodium tetrasulfide. This conclusion was in well accordance with the results of the data fitting in the pre-Tafel region.
Fig. 5. Morphology of the corrosion surface of LSCC immersed in molten sodium tetrasulfide melt for: (a) 0 days; (b) 10 days; (c) 130 days; (d) cross-section micrograph of LSCC immersed for 130 days.
Y. Huang et al. / Solid State Ionics 192 (2011) 364–367
3.3. Dip-immersion tests It's difficult to identify any sign of corrosion by visual examination on the surface of LSCC after exposing to the sodium tetrasulfide melt even for 130 days. Qualitative analysis was conducted by a combination of XRD analysis and electron microprobe technique. Figs. 4 and 5 presented the XRD patterns and micro morphology of LSCC before and after the exposure respectively. Compared to the specimen without exposure, small amounts of phases corresponding to the corrosion products were detected on the spectrum for the sample immersed for 10 days. Furthermore, much more complicated corrosion products formed on the surface of the specimens after immersed for 50 days, as illustrated in the XRD patterns which comprised of Cr3S4 and other uncertain phases. Cr3S4 possesses a very low resistivity of 6 × 10− 3 Ω cm even at room temperature [5], thus the resistance of the battery will be almost unchanged by the corrosion layer. As for the sample immersed for 130 days, the intensity of the peaks of the XRD pattern became lower, which might suggest the formation of less crystalline layer, as would be expected for a highly corroded sample. It can be observed from Fig. 5 that the surface of LSCC exposed to sodium tetrasulfide for 10 days (Fig. 5(b)) showed well faceted grains with morphology very similar to that of the unexposed specimen (Fig. 5(a)). It was therefore indicated that little corrosion product formed on the surface and LSCC could well withstand the attack of the sodium tetrasulfide melt. This might be due to the negligibly small lattice diffusion coefficient of sulfur in perovskite-type ceramic oxides [25]. Fig. 5(c) demonstrated that when the exposure time increased to 130 days, LSCC was covered by a corrosion layer with some visible grains on the surface. This result was in well agreement with XRD analysis. The cross-section micrograph of LSCC immersed for 130 days was shown in Fig. 5(d). As seen, spallation occurred on the outer surface of the corrosion scale. However, it didn't influence the evenness and coherence of the corrosion layer. The thickness of the continuous and compact corrosion layer formed on the ceramic surface was approximately 4 μm. The corrosion rate of LSCC could be simply calculated to be about 12 μm year− 1, which was significantly lower than that of aluminum (150 μm year− 1) and the steel (AISI 446) (90 μm year− 1) reported in Ref. [10]. By combining the results of the electrochemical measurements and the dip-immersion tests, it was shown that the perovskite ceramic LSCC had excellent corrosion resistance against sodium tetrasulfide compared to conventional materials such as aluminum and steel. 4. Conclusions Perovskite La0.8Sr0.2Co0.2Cr0.8O3 − δ (LSCC) ceramic was investigated as a novel coating material for the sulfur electrode current collector in Na/S battery. The electrical conductivity of LSCC was sufficient as an inner coating layer for container in Na/S battery, and its thermal expansion coefficient was very close to that of the suggested substrate
367
material 316 L stainless steel. The results of corrosion behavior measurements demonstrated that LSCC exhibited favorable corrosion resistance against molten sodium tetrasulfide. The corrosion rate deduced from immersion test was about 12 μm year− 1. The corrosion current density of LSCC was about 0.08 mA cm− 2, approximately two orders of magnitude lower than that of 316 L stainless steel. The perovskite oxide LSCC seems to be a promising candidate for the interior coating for the Na/S battery container. Further works will concentrate on studying the mechanism of the anticorrosion performance of LSCC and deposition techniques for the LSCC coating onto the 316 L stainless steel surface. Acknowledgements This work was financially supported by NSFC Project No. 50730001, research projects of Chinese Science and Technology Ministry No. 2007BAA07B01 and No. 2007CB209700, and research projects from the Science and Technology Commission of Shanghai Municipality Nos. 06DE12213, 07DE12004 and 08DZ2210900. References [1] R. Okuyama, H. Nakashima, T. Sano, E. Nomura, Journal of Power Sources 93 (2001) 50. [2] Zhaoyin Wen, Jiadi Cao, Gu. Zhonghua, Xu. Xiaohe, Fuli Zhang, Zuxiang Lin, Solid State Ionics 179 (2008) 1697. [3] American Electric Power website, http://www.aep.com/newsroom/newsreleases/ ?id=872. [4] Roger J. Bones, Trevor L. Markin, Journal of The Electrochemical Society 125 (1978) 1587. [5] B. Hartmann, Journal of Power Sources 3 (1978) 227. [6] B. Dunn, M.W. Breiter, D.S. Park, Journal of Applied Electrochemistry 11 (1981) 103. [7] K.R. Kinsman, W.L. Winterbottom, Thin Solid Films 83 (1981) 417. [8] R. Knodler, Journal of Applied Electrochemistry 18 (1988) 653. [9] Jiulin Wang, Jun Yang, Yanna Nuli, Rudolf Holze, Electrochemistry Communications 9 (2007) 31. [10] K.E. Heusler, A. Grzegorzewski, R. Knodler, Journal of The Electrochemical Society 140 (1993) 426. [11] Robert R. Dubin, Materials Performance 20 (1981) 13. [12] J.L. Sudworth, A.R. Tilley, The Sodium Sulfur Battery, Chapman & Hall, New York, 1985, p. 207. [13] D.S. Park, D. Chatterji, Thin Solid Films 83 (1981) 429. [14] A.P. Brown, Journal of The Electrochemical Society 134 (1987) 1921. [15] H.S. Wroblowa, R.P. Tischer, G.M. Crosbie, G.J. Tennenhouse, Corrosion Science 26 (1986) 193. [16] S.P.S. Badwal, Solid State Ionics 143 (2001) 39–46. [17] Jeffrey W. Fergus, Solid State Ionics 171 (2004) 1–15. [18] S. Frangini, A. Masic, Surface and Coatings Technology 184 (2004) 31–39. [19] L.-W. Tai, M.M. Nasrallah, H.U. Anderson, D.M. Sparlin, S.R. Sehlin, Solid State Ionics 76 (1995) 259–271. [20] H. Shih, F. Mansfeld, ASTM STP 1154 (1992) 174. [21] F. Mansfeld, Corrosion Science 47 (2005) 3178. [22] P. Marcus, J. Oudar (Eds.), Corrosion Mechanisms in Theory and Practice, Marcel Dekker, Inc., New York, 1995. [23] Chunan Cao, Principles of Corrosion Electrochemistry, Chemical Industry Press, Beijing, 2004, p. 71. [24] W. Skinner, British Corrosion Journal 22 (3) (1987) 172–175. [25] S.P.S. Badwal, Solid State Ionics 143 (2001) 39.
Contents lists available at ScienceDirect
Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s s i
Study of La0.8Sr0.2Co0.2Cr0.8O3 − δ as a candidate coating material for the positive current collector in Na/S battery Ying Huang, Zhaoyin Wen ⁎, Jianhua Yang, Yu Liu CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, PR China
a r t i c l e
i n f o
Article history: Received 30 August 2009 Received in revised form 20 August 2010 Accepted 6 September 2010 Available online 14 October 2010 Keywords: Perovskite Corrosion resistance Polarization Dip-immersion Molten salt
a b s t r a c t Perovskite La0.8Sr0.2Co0.2Cr0.8O3 − δ (LSCC) ceramic synthesized by the conventional ceramic processing technique was studied as a novel coating material for the cathode current collector in Na/S battery. Its structure, electrical conductivity, density and thermal expansion coefficient (TEC) were investigated. The corrosion performance of LSCC was in particular evaluated by electrochemical techniques in combination with long-term dip-immersion tests. The results indicated that LSCC exhibited excellent corrosion resistance in molten sodium tetrasulfide at 350 °C. The corrosion current density icorr (0.081 mA cm− 2) was much lower than that of 316 L stainless steel by approximately two orders of magnitude. The corrosion rate of LSCC deduced from immersion test was as low as about 12 μm year− 1. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Na/S battery has been in development as one of the most promising advanced energy storage technologies owing to its various advantages [1–3]. Generally, the major concern for Na/S battery is that few materials can simultaneously satisfy the main requirements of electrical conductivity and corrosion resistance for the sulfur electrode current collector which also acts as cell container. The corrosion of the positive current collector by the corrosive sodium polysulfide may lead to loss of capacity, increase of resistance and cycle-life limitations [1,4–10]. Material research efforts have demonstrated that the combinations of requirements mentioned above may be best fulfilled by a coated system [5,11,12]. Many candidate coating materials, e.g. molybdenum, super alloys, carbon, chromium carbide, or conducting ceramics have been examined [8,13–15], but most of them have certain drawbacks. Lanthanum chromite-based perovskite oxides are well known as conductive parts for electrochemical devices such as high-temperature solid oxide fuel cells, due to their good electrical conductivity and high stability in both oxidation and reduction environments [16,17]. Perovskite oxides may also seem to be promising candidates as the interior coating for the container of Na/S battery. In this work perovskite oxide La0.8Sr0.2Co0.2Cr0.8O3 − δ (LSCC) has been investigated for the first time as a possible candidate coating material for the current collector of Na/S battery. The
⁎ Corresponding author. Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, PR China. Tel.: + 86 21 52411704; fax: + 86 21 52413903. E-mail address: [email protected] (Z. Wen). 0167-2738/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2010.09.013
preparation of LSCC and its corrosion performance under the sodium polysulfide melt environment were presented in this paper.
2. Experimental La0.8Sr0.2Co0.2Cr0.8O3 − δ (LSCC) oxides were prepared by the conventional ceramic processing technique. Stoichiometric amounts of reagent-grade La2O3, SrCO3, Co2O3 and Cr2O3 were mixed by ball milling and then calcined at 1150 °C for 5 h in air. The resulting powders were ground and pressed into bars and discs, and finally sintered at 1510 °C for 1 h in air. The density of sintered specimens was measured according to Archimedes method using ethyl alcohol as the immersing media. The electrical conductivity measurements were carried out by the standard four-probe dc method. The thermal expansion coefficient (TEC) was evaluated on a NETZSCH DIL 402 C dilatometer (alumina holder) with a TSAC 414/4 controller. The corrosion resistance of LSCC against molten sodium tetrasulfide was evaluated by static dip-immersion tests as well as electrochemical measurements carried out under argon atmosphere using a three electrodes electrochemical cell. High density graphite rods which show sufficient stability in molten sodium polysulfide [5,8] were used as the reference electrode and counter electrode. Sintered specimen sized approximately 30 × 5 × 5 mm3, giving the exposure area of 6.5 cm2 was used as the working electrode. In the dip-immersion tests, the specimens to be tested and the desired sodium tetrasulfide used as corrosion medium were contained in hard glass tubes, which were filled in a glove box and then sealed under vacuum. At selected periods of exposure, the samples were taken out, ultrasonically cleaned in ethyl alcohol and
Y. Huang et al. / Solid State Ionics 192 (2011) 364–367
365
Table 1 Results of data analysis of polarization curve using POLFIT. Specimen
LSCC
316 L
χ2 R2 Icorr (mA) βa (mV/dec) βc (mV/dec) icorr (mA cm− 2)
0.00193 0.99911 0.523 ± 0.008 27.942 ± 0.297 25.002 ± 0.216 0.081 ± 0.001
0.00015 0.99996 5.303 ± 0.021 108.029 ± 0.362 102.565 ± 0.776 5.303 ± 0.021
that the electrical conductivity followed the relationship for the adiabatic small polaron hopping mechanism [19]: σ = ðA = kT Þ exp ð−Ea = kT Þ;
Fig. 1. Log σ T versus 1000/T in air for La0.8Sr0.2Co0.2Cr0.8O3 − δ.
ð1Þ
where A is a material constant containing the carrier concentration term, Ea the activation energy for hopping conduction, k the Boltzmann's constant and T the absolute temperature. 3.2. Electrochemical corrosion behaviors
then analyzed by X-ray diffraction analysis (XRD) and electron microprobe analysis.
3. Results and discussion 3.1. Characterization of the sintered LSCC As confirmed in Fig. 4, all the major XRD peaks of the sintered ceramic belonged to the perovskite structure, indicating the formation of a single perovskite phase. The relative density of LSCC measured by Archimedes method was 96.4%. The thermal expansion coefficient of the perovskite ceramic tested under N2 environment was 17.2 × 10− 6 K− 1 between room temperature and 800 °C. This value was very close to that of 316 L stainless steel (18 × 10− 6 K− 1) [18] which was often chosen as a substrate material for container of Na/S battery. Electrical conductivity measurements were performed with the standard four-probe dc method from room temperature to 600 °C in air. The results showed that electrical conductivity changed little within the temperature range of 250 to 600 °C, and it reached 10 S cm− 1, an acceptable level for current collector in the Na/S battery [15]. The electrical conductivity was plotted as log σT versus 1/T in Fig. 1. The slope for the linear portion of the data curve yielded the activation energy about 0.089 eV, suggesting
Fig. 2. Potentiodynamic polarization curve measured in the potential range of − 30 mV to 30 mV and result of data fitting for LSCC and 316 L stainless steel exposed to molten sodium tetrasulfide at 350 °C respectively.
The electrochemical corrosion behavior of LSCC was studied using potentiondynamic polarization tests conducted at a scanning rate of 0.166 mV s− 1 in the vicinity of the corrosion potential (from − 30 mV to 30 mV versus graphite reference electrode). The data fitting analysis was carried out with the software program POLFIT [20,21] based on the Butler–Volmer equation [22]: 2:3ðE−Ecorr Þ = βa −2:3ðE−Ecorr Þ = βc ; iðEÞ = icorr e −e
ð2Þ
where icorr is the corrosion current density, Ecorr the corrosion potential, βa and βc the anodic and cathodic Tafel slope respectively. The potentiodynamic polarization curves and the results of the data fitting were shown in Fig. 2. For the purpose of comparison, the electrochemical behaviors of the 316 L stainless steel were also studied under the same conditions. The fit parameters βa, βc and icorr as well as their errors were listed in Table 1. The goodness of fit was represented by the Chisquare (χ2) parameter. As given in the table, the icorr of LSCC was approximately two orders of magnitude lower than that of 316 L stainless steel. It is known that icorr is proportional to corrosion rate [23]. Therefore, it is reasonable to consider that LSCC exhibits a
Fig. 3. Potentiodynamic polarization curves performed in the potential range of − 1.0 V to 1.5 V for LSCC and 316 L stainless steel exposed to molten sodium tetrasulfide at 350 °C respectively.
366
Y. Huang et al. / Solid State Ionics 192 (2011) 364–367
Fig. 4. XRD patterns of LSCC before and after immersed in molten sodium tetrasulfide melt.
much better corrosion resistance than 316 L stainless steel in the described environments. The Tafel slopes are considered to be indicators of the influence of the diffusion process on the rate
controlling step. Tafel slopes lower than 100 mV/dec are typical for activation controlled systems, whereas Tafel slopes with larger values are typical for systems which are not purely activation or diffusion controlled [24]. It can be clearly seen from Table 1 that the Tafel slopes for LSCC were significantly lower than 100 mV/dec, implying that the corrosion rate controlling step was activation control. On the other hand, the Tafel slopes for 316 L stainless steel were slightly higher than 100 mV/dec, implying a complex corrosion mechanism including diffusion process as well as electron transport process. Fig. 3 illustrated the potentiodynamic polarization curves of LSCC and 316 L stainless steel measured in the wider potential range from −1.00 V to 1.50 V. No active–passive transition region was visible in the two curves, indicating that both of the materials were able to reach a passive condition spontaneously in the melt. Moreover, no current spike which indicated breakdown processes of the passive layer was recorded for both the materials studied. However, it was clearly seen that the corrosion potential of LSCC (− 77 mV) was much higher than that of 316 L stainless steel (−187 mV). Moreover, the passive anodic current density of LSCC was far lower than that of the 316 L stainless steel. Therefore, the anticorrosion property of LSCC was superior to 316 L stainless steel in the molten sodium tetrasulfide. This conclusion was in well accordance with the results of the data fitting in the pre-Tafel region.
Fig. 5. Morphology of the corrosion surface of LSCC immersed in molten sodium tetrasulfide melt for: (a) 0 days; (b) 10 days; (c) 130 days; (d) cross-section micrograph of LSCC immersed for 130 days.
Y. Huang et al. / Solid State Ionics 192 (2011) 364–367
3.3. Dip-immersion tests It's difficult to identify any sign of corrosion by visual examination on the surface of LSCC after exposing to the sodium tetrasulfide melt even for 130 days. Qualitative analysis was conducted by a combination of XRD analysis and electron microprobe technique. Figs. 4 and 5 presented the XRD patterns and micro morphology of LSCC before and after the exposure respectively. Compared to the specimen without exposure, small amounts of phases corresponding to the corrosion products were detected on the spectrum for the sample immersed for 10 days. Furthermore, much more complicated corrosion products formed on the surface of the specimens after immersed for 50 days, as illustrated in the XRD patterns which comprised of Cr3S4 and other uncertain phases. Cr3S4 possesses a very low resistivity of 6 × 10− 3 Ω cm even at room temperature [5], thus the resistance of the battery will be almost unchanged by the corrosion layer. As for the sample immersed for 130 days, the intensity of the peaks of the XRD pattern became lower, which might suggest the formation of less crystalline layer, as would be expected for a highly corroded sample. It can be observed from Fig. 5 that the surface of LSCC exposed to sodium tetrasulfide for 10 days (Fig. 5(b)) showed well faceted grains with morphology very similar to that of the unexposed specimen (Fig. 5(a)). It was therefore indicated that little corrosion product formed on the surface and LSCC could well withstand the attack of the sodium tetrasulfide melt. This might be due to the negligibly small lattice diffusion coefficient of sulfur in perovskite-type ceramic oxides [25]. Fig. 5(c) demonstrated that when the exposure time increased to 130 days, LSCC was covered by a corrosion layer with some visible grains on the surface. This result was in well agreement with XRD analysis. The cross-section micrograph of LSCC immersed for 130 days was shown in Fig. 5(d). As seen, spallation occurred on the outer surface of the corrosion scale. However, it didn't influence the evenness and coherence of the corrosion layer. The thickness of the continuous and compact corrosion layer formed on the ceramic surface was approximately 4 μm. The corrosion rate of LSCC could be simply calculated to be about 12 μm year− 1, which was significantly lower than that of aluminum (150 μm year− 1) and the steel (AISI 446) (90 μm year− 1) reported in Ref. [10]. By combining the results of the electrochemical measurements and the dip-immersion tests, it was shown that the perovskite ceramic LSCC had excellent corrosion resistance against sodium tetrasulfide compared to conventional materials such as aluminum and steel. 4. Conclusions Perovskite La0.8Sr0.2Co0.2Cr0.8O3 − δ (LSCC) ceramic was investigated as a novel coating material for the sulfur electrode current collector in Na/S battery. The electrical conductivity of LSCC was sufficient as an inner coating layer for container in Na/S battery, and its thermal expansion coefficient was very close to that of the suggested substrate
367
material 316 L stainless steel. The results of corrosion behavior measurements demonstrated that LSCC exhibited favorable corrosion resistance against molten sodium tetrasulfide. The corrosion rate deduced from immersion test was about 12 μm year− 1. The corrosion current density of LSCC was about 0.08 mA cm− 2, approximately two orders of magnitude lower than that of 316 L stainless steel. The perovskite oxide LSCC seems to be a promising candidate for the interior coating for the Na/S battery container. Further works will concentrate on studying the mechanism of the anticorrosion performance of LSCC and deposition techniques for the LSCC coating onto the 316 L stainless steel surface. Acknowledgements This work was financially supported by NSFC Project No. 50730001, research projects of Chinese Science and Technology Ministry No. 2007BAA07B01 and No. 2007CB209700, and research projects from the Science and Technology Commission of Shanghai Municipality Nos. 06DE12213, 07DE12004 and 08DZ2210900. References [1] R. Okuyama, H. Nakashima, T. Sano, E. Nomura, Journal of Power Sources 93 (2001) 50. [2] Zhaoyin Wen, Jiadi Cao, Gu. Zhonghua, Xu. Xiaohe, Fuli Zhang, Zuxiang Lin, Solid State Ionics 179 (2008) 1697. [3] American Electric Power website, http://www.aep.com/newsroom/newsreleases/ ?id=872. [4] Roger J. Bones, Trevor L. Markin, Journal of The Electrochemical Society 125 (1978) 1587. [5] B. Hartmann, Journal of Power Sources 3 (1978) 227. [6] B. Dunn, M.W. Breiter, D.S. Park, Journal of Applied Electrochemistry 11 (1981) 103. [7] K.R. Kinsman, W.L. Winterbottom, Thin Solid Films 83 (1981) 417. [8] R. Knodler, Journal of Applied Electrochemistry 18 (1988) 653. [9] Jiulin Wang, Jun Yang, Yanna Nuli, Rudolf Holze, Electrochemistry Communications 9 (2007) 31. [10] K.E. Heusler, A. Grzegorzewski, R. Knodler, Journal of The Electrochemical Society 140 (1993) 426. [11] Robert R. Dubin, Materials Performance 20 (1981) 13. [12] J.L. Sudworth, A.R. Tilley, The Sodium Sulfur Battery, Chapman & Hall, New York, 1985, p. 207. [13] D.S. Park, D. Chatterji, Thin Solid Films 83 (1981) 429. [14] A.P. Brown, Journal of The Electrochemical Society 134 (1987) 1921. [15] H.S. Wroblowa, R.P. Tischer, G.M. Crosbie, G.J. Tennenhouse, Corrosion Science 26 (1986) 193. [16] S.P.S. Badwal, Solid State Ionics 143 (2001) 39–46. [17] Jeffrey W. Fergus, Solid State Ionics 171 (2004) 1–15. [18] S. Frangini, A. Masic, Surface and Coatings Technology 184 (2004) 31–39. [19] L.-W. Tai, M.M. Nasrallah, H.U. Anderson, D.M. Sparlin, S.R. Sehlin, Solid State Ionics 76 (1995) 259–271. [20] H. Shih, F. Mansfeld, ASTM STP 1154 (1992) 174. [21] F. Mansfeld, Corrosion Science 47 (2005) 3178. [22] P. Marcus, J. Oudar (Eds.), Corrosion Mechanisms in Theory and Practice, Marcel Dekker, Inc., New York, 1995. [23] Chunan Cao, Principles of Corrosion Electrochemistry, Chemical Industry Press, Beijing, 2004, p. 71. [24] W. Skinner, British Corrosion Journal 22 (3) (1987) 172–175. [25] S.P.S. Badwal, Solid State Ionics 143 (2001) 39.