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Electrochemical properties of La1−xSrxFeO3 (x = 0.2, 0.4) as negative electrode of Ni–MH batteries
Electrochimica Acta 54 (2009) 3910–3914
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrochemical properties of La1−x Srx FeO3 (x = 0.2, 0.4) as negative electrode of Ni–MH batteries Gang Deng, Yungui Chen ∗ , Mingda Tao, Chaoling Wu, Xiangqian Shen, Heng Yang School of Materials Science and Engineering, Sichuan University, Chengdu, 610065, PR China
a r t i c l e
i n f o
Article history: Received 4 December 2008 Received in revised form 22 January 2009 Accepted 4 February 2009 Available online 11 February 2009 Keywords: LaFeO3 Electrochemical properties Negative electrodes Ni–MH battery
a b s t r a c t La(1−x) Srx FeO3 (x = 0.2,0.4) powders were prepared by a stearic acid combustion method, and their phase structure and electrochemical properties were investigated systematically. X-ray diffraction (XRD) analysis shows that La(1−x) Srx FeO3 perovskite-type oxides consist of single-phase orthorhombic structure (x = 0.2) and rhombohedral one (x = 0.4), respectively. The electrochemical test shows that the reaction at La(1−x) Srx FeO3 oxide electrodes are reversible. The discharge capacities of La(1−x) Srx FeO3 oxide electrodes increase as the temperature rises. With the increase of the temperature from 298 K to 333 K, their initial discharge capacity mounts up from 324.4 mA h g−1 to 543.0 mA h g−1 (when x = 0.2) and from 147.0 mA h g−1 to 501.5 mA h g−1 (when x = 0.4) at the current density of 31.25 mA g−1 , respectively. After 20 charge–discharge cycles, they still remain perovskite-type structure. Being similar to the relationship between the discharge capacity and the temperature, the electrochemical kinetic analysis indicates that the exchange current density and proton diffusion coefficient of La(1−x) Srx FeO3 oxide electrodes increase with the increase of the temperature. Compared with La0.8 Sr0.2 FeO3 , La0.6 Sr0.4 FeO3 electrode is a more promising candidate for electrochemical hydrogen storage because of its higher cycle capacity at various temperatures. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction Ni–MH batteries have been widely studied owing to their high capacities, fast charge and discharge, environment-friendly and long cyclic stability [1–4]. All of the traditional negative electrodes of Ni–MH battery are hydrogen storage alloys, including AB5 [5,6], AB2 [7], AB [8], Mg-based alloys [9–11] and so on. Although AB5 type rare-earth metal alloys have been widely used in commercial batteries, the reversible capacities are only about 300 mAh g−1 . Mg-based materials are another promising negative electrodes because of its exceptional high capacities of more than 600 mAh g−1 . However, their poor cycling life leads to the limited practical applications. Therefore, on one hand, great efforts, such as the optimization of the alloy compositions and the surface modifications, have been undertaken to improve the electrochemical capacity and the high rate capability of these materials. On the other hand, many new hydrogen storage materials have been synthesized [12–15]. At present, few works were devoted to the study of protonconductive perovskite-type oxides, which promises to be used as negative electrode of Ni–MH secondary batteries. Esaka et al. [16] successfully prepared perovskite-type oxides ACe1−x Mx O3−ı (A = Sr
∗ Corresponding author. Tel.: +86 28 8540 7335; fax: +86 28 8546 6916. E-mail address: [email protected] (Y. Chen). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.02.007
or Ba and M = rare earth element) by a conventional solid-state reaction method and proposed it as innovative electrode material for Ni–MH batteries, where they denoted that the hydrogen storage mechanism is: BaCe(IV)0.95 Nd0.05 O3−ı +xH2 O + xe−
charge
BaCe(IV)0.95−x Ce(III)x Nd0.05 O3−ı H(I)x
discharge
+xOH−
(1)
Unfortunately, the maximum discharge capacity of the BaCeO3 based sample was only about 119 mA h g−1 .Up to now, further research results have rarely been reported. In the paper, both La0.8 Sr0.2 FeO3 and La0.6 Sr0.4 FeO3 were prepared by stearic acid combustion method, and their electrochemical properties were investigated as negative electrode. 2. Experimental 2.1. Preparation and structural characterization Both La0.8 Sr0.2 FeO3 and La0.6 Sr0.4 FeO3 were prepared by stearic acid combustion method. All the reagents were analytical grade chemicals (purity 98.5%). Stearic acid (C17 H35 COOH) was used as the solvent and dispersant. An appropriate amount of stearic
G. Deng et al. / Electrochimica Acta 54 (2009) 3910–3914
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acid was heated and melted and then stoichiometric amounts of La(NO3 )3 ·6H2 O, Fe(NO3 )3 ·9H2 O and Sr(NO3 )2 were added. The mixtures were stirred by the magnetic mixer until the homogeneous sol-like solution was formed. The solution was ignited in air and the obtained powders were calcined at 1123 K for 3 h in a muffle furnace. To analyze the phases of the calcined and discharged samples, X-ray diffraction (XRD, Dandongfangyuan) was carried out on a DX-2600 diffractometer with Cu K␣ radiation and a power of 25 kV × 30 mA. The XRD patterns were recorded between the range of 20 and 80◦ with a step of 0.05◦ /s and analyzed by Jade 5.0.
2.2. Electrochemical measurements
Fig. 1. The XRD patterns of the calcined and the discharged La(1−x) Srx FeO3 (x = 0.2 and 0.4) samples at the 20th cycle. (: La0.8 Sr0.2 FeO3 , : La0.6 Sr0.4 FeO3 , and : Ni. (a) The calcined powder La0.8 Sr0.2 FeO3 , orthorhombic, (b) the 20th discharged sample La0.8 Sr0.2 FeO3 , (c) the calcined powder La0.6 Sr0.4 FeO3 , rhombohedral, and (d) the 20th discharged sample La0.6 Sr0.4 FeO3 ).
Negative electrodes were prepared by cold-pressing the mixture of the calcined powder and carbonyl nickel powder with the weight ratio of 1:3 under 3.5 MPa to form a pellet of 10 mm in diameter. Electrochemical measurements were conducted by a DC-5 type computer-controlled battery test instrument with an open two-electrode cell placed in a thermostat water bath. NiOOH/Ni(OH)2 was used as the counter and reference electrode. In each charge–discharge cycle, the negative electrode was charged for 7 h at 125 mA g−1 and discharged at 32.5 mA g−1 and 125 mA g−1 to the cut-off voltage of −0.4 V (versus NiOOH/Ni(OH)2 ). The
Fig. 2. Charge–discharge curves of La(1−x) Srx FeO3 electrodes at various temperatures (a) at the discharge current density of 31.25 mA g−1 (x = 0.2), (b) at the discharge current density of 31.25 mA g−1 (x = 0.4), (c) at the discharge current density of 125 mA g−1 (x = 0.2), and (d) at the discharge current density of 125 mA g−1 (x = 0.4) (charge: () 298 K, (䊉)313 K, and () 333 K. Discharge: () 298 K, () 313 K, and ( ) 333 K).
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electrolyte is a 7 M KOH aqueous solution. The standing time between charge and discharge was 10 min and the temperature was 298 K, 313 K and 333 K, respectively. The hydrogen proton diffusion coefficient D in the bulk of the proton-conductive perovskite-type oxide and the exchange current density I0 were evaluated by an advanced electrochemical system PARSTAT2273 using the potential-step and linear polarization method, respectively. During the potential static step discharge process, the electrodes were in fully charged state, and then an overpotential of +600 mV was applied to them. The current–time transient curves were recorded after the potential step in this electrochemical analyzer. In addition, the linear polarization curves of the electrodes were plotted by scanning the electrode potential at the rate of 10 mV/s from −6 to 6 mV (versus open circuit potential) at 50% depth of discharge (DOD). 3. Results and discussion 3.1. Phase structure Fig. 1 shows the XRD patterns of the calcined La0.8 Sr0.2 FeO3 and La0.6 Sr0.4 FeO3 powders and their discharged samples at the 20th cycle. It can be seen that both La0.8 Sr0.2 FeO3 and La0.6 Sr0.4 FeO3 powders consist of nearly single phase and the diffraction data of them are in good agreement with those from JCPDS cards (JCPDS no: 35-1480 and JCPDS no: 82-1961), respectively. According to JCPDS cards, La0.8 Sr0.2 FeO3 holds an orthorhombic structure and La0.6 Sr0.4 FeO3 has a rhombohedral one. Compared with the diffraction peaks of La0.8 Sr0.2 FeO3 , those of La0.6 Sr0.4 FeO3 are broadened because of the coexistence of multi-peaks, and they shift from the left to the right, which was shown in the inset of Fig. 1. XRD patterns of the discharged samples show both La0.8 Sr0.2 FeO3 and La0.6 Sr0.4 FeO3 still remain perovskite-type structure as the peaks attributable to the Ni compounds presence are distinguishable in the patterns together with the peaks of the oxides. 3.2. Electrochemical properties The electrochemical properties of the oxide electrode were characterized by the charge, discharge and cycle curves. Initial charge and discharge curves of the La(1−x) Srx FeO3 oxide electrodes were presented in Fig. 2. It should be noted that the charge curves show a long and horizontal potential plateau, possibly due to the formation of stable chemical bonds between the perovskite-type oxides and protons. Nowick et al. [17] reported that vacancies could be replaced by protons, which resided on oxygen ions to form substitutional OH− ion defects in protonconductive perovskite-type oxides. The charge potential plateau decreases from 1.47 V to 1.4 V as the temperature increases from 298 K to 333 K. Even when the temperature rises up to 333 K, the discharge potential plateau of the electrode remains at about 1.1 V, which gain the advantage over the traditional hydrogen storage alloy, of which the voltage plateau rise does harm to the performances of the cell when the temperature increases [18–20]. Considering that the standard electrode potential of Fe3+ /Fe2+ (in La(1−x) Srx FeO3 electrode) and Ni2+ /Ni3+ (in NiOOH/Ni(OH)2 electrode) are −0.77 V and +0.52 V in alkaline solution, respectively, the obtained voltage was reasonable. It also can be seen that the initial discharge capacities at different current densities increase as the temperature rises from 298 K to 333 K. In Fig. 2(a and b), the capacities of La0.8 Sr0.2 FeO3 and La0.6 Sr0.4 FeO3 mount up from 324.4 mA h g−1 and 147.0 mA h g−1 to 543.0 mA h g−1 and 501.5 mA h g−1 at 31.25 mA g−1 , respectively. In Fig. 2(c and d), the corresponding values change from 54 mA h g−1 to 433.1 mA h g−1 and 51.6 mA h g−1 to 336.6 mA h g−1
Fig. 3. The discharge capacity versus cycle number of La(1−x) Srx FeO3 electrodes at various temperatures and under the charge–discharge current density of 125 mAg−1 (: 0.2 (298 K), 䊉: 0.4 (313 K), : 0.2 (333 K), : 0.4 (298 K), : 0.2 (313 K), and : 0.4 (333 K)).
at 125 mA g−1 , respectively. So the electrochemical test results suggest that the La(1−x) Srx FeO3 oxide electrodes hold a theoretical capacity which is much higher than that of traditional AB5 -type hydrogen storage electrode alloys. While Kleperis et al. [21] reported that the electrode made of LaNi5 alloy reaches a maximum capacity of 360 mA h g−1 and Raju et al. [18] denoted that of MmNi3.03 Si0.85 Co0.6 Mn0.31 Al0.08 hydrogen storage alloys falls from 283 mA h g−1 at 303 K down to 213 mA h g−1 at 328 K. Consequently, La(1−x) Srx FeO3 oxide electrode shows an advantage over the traditional hydrogen alloy upon both discharge capacity and voltage plateau at a higher temperature. The discharge cyclic properties of the oxide electrode at various temperatures and under the charge–discharge current density of 125 mA g−1 are shown in Fig. 3. Except for the charge–discharge cycle at 298 K, La0.8 Sr0.2 FeO3 oxide electrode can be discharged directly without activation, and the discharge capacity achieves its maximum in the first cycle. Generally, traditional hydrogen alloys including AB5, AB2 and La–Mg–Ni system alloy require more than three cycles for activation [5,7,22]. In the first three cycles, the discharge capacities decrease distinctly, down to 120 mA h g−1 and 220 mA h g−1 at 313 K and 333 K, respectively. Although the discharge capacity shows irregular fluctuation at 333 K after the 3rd cycle to the 20th cycle, the value always keeps 220 mA h g−1 above. The discharge capacities of La0.6 Sr0.4 FeO3 also keep steady at about 77 mA h g−1 and 170 mA h g−1 at 298 K and 313 K, respectively. When the temperature rises up to 333 K, its discharge capacity declines from 336.5 mA h g−1 to 278.3 mA h g−1 in the first three cycles and increases gradually after that. Finally, it reaches a value of about 360 mA h g−1 . Conclusively, La(1−x) Srx FeO3 electrodes show good capacity retention after three cycles, possibly due to be related to the stability of their structures after 20 charge–discharge cycles. Compared with the electrochemical properties of La0.8 Sr0.2 FeO3 at various temperatures, La0.6 Sr0.4 FeO3 electrode holds a higher electrochemical capacity, possibly due to more oxygen vacancies for hydrogen storage with increasing Sr content. 3.3. Kinetic characteristics The exchange current density (I0 ) is used to characterize the electro-catalytic activity of the charge-transfer reaction on the surface of the electrode. Fig. 4 shows the linear polarization curves of La(1−x) Srx FeO3 electrodes. Polarization resistance Rp can be
G. Deng et al. / Electrochimica Acta 54 (2009) 3910–3914
Fig. 4. Linear polarization curves of La(1−x) Srx FeO3 oxide electrodes (1: 298 K (x = 0.2), 2: 313 K (x = 0.2), 3: 333 K (x = 0.2), 4: 298 K (x = 0.4), 5: 313 K (x = 0.4), and 6: 333 K (x = 0.4)).
obtained from Fig. 4, and consequently the exchange current density I0 can be calculated according to the following equation [23]: I0 =
RT FRp
(2)
where R is the gas constant (J/(mol K)), T is the absolute temperature (K) and F is the Faraday constant (C/mol). The results obtained are listed in Table 1. It can be seen that the value of Rp decreases and I0 increases when the temperature rises, indicating that the charge transfer rate increases. In addition, the hydrogen proton diffusion rate can be estimated by potential-step method. Fig. 5 illustrates the correspondence of anodic current versus discharge time of La(1−x) Srx FeO3 electrodes at fully charged state and at various temperatures. It can be seen that, after the application of the overpotential, the current–time responses in Fig. 5 can be divided into two time domains. In the first time region, the current rapidly declined due to a consumption of hydrogen proton on the surface. In the other time region, however, the current slowly declined in a linear fashion. In this region, the surface concentration of proton approaches zero. Thus, the electrode reaction will be controlled by proton diffusion in the oxide bulk. The proton diffusion coefficient D in the bulk of the oxide can be calculated by the following formula [24]: log i = log
6FD da2
(C0 − Cs ) −
2 D t 2.303 a2
(3)
where i is the anodic current (A), D the proton diffusion coefficient (cm2 /s), d the density of the oxide (g/cm3 ), a the radius of the oxide particle, C0 the initial proton concentration in the bulk of the oxide (mol/cm3 ), Cs the surface proton concentration of the oxide (mol/cm3 ) and t is the discharge time (s). Given that a is 9.3 nm, D can be calculated according to the equation, and is summarized Table 1 Electrochemical kinetics parameters of La(1−x) Srx FeO3 oxide electrode (where Rp is polarization resistance, T the absolute temperature, I0 exchange current density and D is hydrogen proton diffusion coefficient). Sample
T (K)
Rp (m)
I0 (mA g−1 )
D (×10−17 cm2 /s)
La0.8 Sr0.6 FeO3
298 313 333
678.9 581.4 344.8
37.8 46.4 83.2
1.62 2.02 4.04
La0.6 Sr0.4 FeO3
298 313 333
600.5 417.2 366.5
42.8 64.7 78.3
0.8 1.82 2.02
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Fig. 5. Correspondence of anodic current versus discharge time of La(1−x) Srx FeO3 oxide electrodes (1: 298 K (x = 0.2), 2: 313 K (x = 0.2), 3: 333 K (x = 0.2), 4: 298 K (x = 0.4), 5: 313 K (x = 0.4), and 6: 333 K (x = 0.4)).
in Table 1. It can be seen that the proton diffusion coefficient D of the oxide increases as the temperature increases, and is in good agreement with the responses of the discharge capacity versus temperature. 4. Conclusion The phase structure and electrochemical properties of La(1−x) Srx FeO3 oxide electrodes were investigated systematically. XRD shows that La(1−x) Srx FeO3 perovskite-type oxides consist of single-phase: orthorhombic structure (when x = 0.2) and rhombohedral one (when x = 0.4), respectively. After 20 cycles, they still remain perovskite-type structure. The electrochemical test shows that the reaction at La(1−x) Srx FeO3 electrodes are reversible. The discharge capacities of La(1−x) Srx FeO3 oxide electrodes increase as the temperature rises and decrease when the discharge current density grows. At discharge current density of 125 mA g−1 , their discharge capacities keep steady at about 54 mA h g−1 ,120 mA h g−1 and 220 mA h g−1 (when x = 0.2) and 77 mA h g−1 , 170 mA h g−1 and 360 mA h g−1 (when x = 0.4) at 298 K, 313 K and 333 K, respectively. Being similar to the relationship between the discharge capacity and the temperature, the exchange current density and the proton diffusion coefficient of La(1−x) Srx FeO3 electrodes also increase when the temperature rises. Compared with La0.8 Sr0.2 FeO3 , La0.6 Sr0.4 FeO3 electrode is a more promising candidate for electrochemical hydrogen storage because of its higher cycle capacity at various temperatures. References [1] S.Q. Shi, C.Y. Ouyang, M.S. Lei, J. Power Sources 164 (2007) 911. [2] Y.F. Liu, H.G. Pan, M.X. Gao, Y.F. Zhu, Y.Q. Lei, Q.D. Wang, Electrochim. Acta 49 (2004) 545. [3] M. Tliha, H. Mathlouthi, C. Khaldi, J. Lamloumi, A. Percheron-guegan, J. Power Sources 160 (2006) 1391. [4] M. Tliha, C. Khaldi, H. Mathlouthi, J. Lamloumi, A. Percheron-Gu’egan, J. Alloys Compd. 440 (2007) 323. [5] B. Liao, Y.Q. Lei, L.X. Chen, J. Power Source 129 (2004) 358. [6] T. Kohno, H. Yoshida, F. Kawashima, J. Alloys Compd. 311 (2000) 5. [7] M.Y. Song, D. Ahn, I.H. Kwon, S.H. Chough, J. Electrochem. Soc. 148 (9) (2001) A1041. [8] H. Yukawa, Y. Takahashi, M. Morinaga, Comput. Mater. Sci. 14 (1999) 291. [9] N.H. Goo, J.H. Woo, K.S. Lee, J. Alloy Compd. 288 (1999) 286. [10] T. Abe, T. Tachikawa, Y. Hatano, K. Watanabe, J. Alloys Compd. 332 (2002) 792. [11] J.W. Liu, H.T. Yuan, J.S. Cao, Y.J. Wang, J. Alloys Compd. 392 (2005) 300.
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[12] P. Chen, Z. Xiong, J. Luo, J. Lin, K.L. Tan, Nature 420 (2002) 302. [13] G. He, L.F. Jiao, H.T. Yuan, Y.Y. Zhang, Y.J. Wang, Electrochem. Commun. 8 (2006) 1633. [14] Y. Liu, Y.J. Wang, L.L. Xiao, D.W. Song, L.F. Jiao, H.T. Yuan, Electrochem. Commun. 9 (2007) 925. [15] M. Abdul, Seayad, M. David, Antonelli, Adv. Mater. 16 (2004) 765. [16] T. Esaka, H. Sakaguchi, S. Kobayashi, Solid State Ionics 166 (2004) 351. [17] A.S. Nowick, K.C. Yang Du, Liang, Solid State Ionics 125 (1999) 303. [18] M. Raju, M.V. Anantha, L. Vijayaraghavan, J. Power Sources 180 (2008) 830.
[19] S.A. Gamboa, P.J. Sebastian, M. Geng, D.O. Northwood, Int. J. Hydrogen Energy 26 (2001) 1315. [20] J.O. Jensen, T.S. Møller, N.J. Bjerrum, J. Alloys Compd. 330–332 (2002) 215. [21] J. Kleperis, G. Wojcik, A. Czerwinski, J. Skowronski, M. Kopczyk, M. BeltowskaBrzezinska, J. Solid State Chem. 5 (2001) 229. [22] T. Kohno, H. Yoshida, F. Kawashima, T. Inaba, I. Sakai, M. Yamamoto, M. Kanda, J. Alloys Compd. 311 (2000) L5. [23] P.H.L. Notten, P. Hokkeling, J. Electrochem. Soc. 138 (1991) 1877. [24] G. Zhang, B.N. Popov, R.E. White, J. Electrochem. Soc. 142 (1995) 2695.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrochemical properties of La1−x Srx FeO3 (x = 0.2, 0.4) as negative electrode of Ni–MH batteries Gang Deng, Yungui Chen ∗ , Mingda Tao, Chaoling Wu, Xiangqian Shen, Heng Yang School of Materials Science and Engineering, Sichuan University, Chengdu, 610065, PR China
a r t i c l e
i n f o
Article history: Received 4 December 2008 Received in revised form 22 January 2009 Accepted 4 February 2009 Available online 11 February 2009 Keywords: LaFeO3 Electrochemical properties Negative electrodes Ni–MH battery
a b s t r a c t La(1−x) Srx FeO3 (x = 0.2,0.4) powders were prepared by a stearic acid combustion method, and their phase structure and electrochemical properties were investigated systematically. X-ray diffraction (XRD) analysis shows that La(1−x) Srx FeO3 perovskite-type oxides consist of single-phase orthorhombic structure (x = 0.2) and rhombohedral one (x = 0.4), respectively. The electrochemical test shows that the reaction at La(1−x) Srx FeO3 oxide electrodes are reversible. The discharge capacities of La(1−x) Srx FeO3 oxide electrodes increase as the temperature rises. With the increase of the temperature from 298 K to 333 K, their initial discharge capacity mounts up from 324.4 mA h g−1 to 543.0 mA h g−1 (when x = 0.2) and from 147.0 mA h g−1 to 501.5 mA h g−1 (when x = 0.4) at the current density of 31.25 mA g−1 , respectively. After 20 charge–discharge cycles, they still remain perovskite-type structure. Being similar to the relationship between the discharge capacity and the temperature, the electrochemical kinetic analysis indicates that the exchange current density and proton diffusion coefficient of La(1−x) Srx FeO3 oxide electrodes increase with the increase of the temperature. Compared with La0.8 Sr0.2 FeO3 , La0.6 Sr0.4 FeO3 electrode is a more promising candidate for electrochemical hydrogen storage because of its higher cycle capacity at various temperatures. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction Ni–MH batteries have been widely studied owing to their high capacities, fast charge and discharge, environment-friendly and long cyclic stability [1–4]. All of the traditional negative electrodes of Ni–MH battery are hydrogen storage alloys, including AB5 [5,6], AB2 [7], AB [8], Mg-based alloys [9–11] and so on. Although AB5 type rare-earth metal alloys have been widely used in commercial batteries, the reversible capacities are only about 300 mAh g−1 . Mg-based materials are another promising negative electrodes because of its exceptional high capacities of more than 600 mAh g−1 . However, their poor cycling life leads to the limited practical applications. Therefore, on one hand, great efforts, such as the optimization of the alloy compositions and the surface modifications, have been undertaken to improve the electrochemical capacity and the high rate capability of these materials. On the other hand, many new hydrogen storage materials have been synthesized [12–15]. At present, few works were devoted to the study of protonconductive perovskite-type oxides, which promises to be used as negative electrode of Ni–MH secondary batteries. Esaka et al. [16] successfully prepared perovskite-type oxides ACe1−x Mx O3−ı (A = Sr
∗ Corresponding author. Tel.: +86 28 8540 7335; fax: +86 28 8546 6916. E-mail address: [email protected] (Y. Chen). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.02.007
or Ba and M = rare earth element) by a conventional solid-state reaction method and proposed it as innovative electrode material for Ni–MH batteries, where they denoted that the hydrogen storage mechanism is: BaCe(IV)0.95 Nd0.05 O3−ı +xH2 O + xe−
charge
BaCe(IV)0.95−x Ce(III)x Nd0.05 O3−ı H(I)x
discharge
+xOH−
(1)
Unfortunately, the maximum discharge capacity of the BaCeO3 based sample was only about 119 mA h g−1 .Up to now, further research results have rarely been reported. In the paper, both La0.8 Sr0.2 FeO3 and La0.6 Sr0.4 FeO3 were prepared by stearic acid combustion method, and their electrochemical properties were investigated as negative electrode. 2. Experimental 2.1. Preparation and structural characterization Both La0.8 Sr0.2 FeO3 and La0.6 Sr0.4 FeO3 were prepared by stearic acid combustion method. All the reagents were analytical grade chemicals (purity 98.5%). Stearic acid (C17 H35 COOH) was used as the solvent and dispersant. An appropriate amount of stearic
G. Deng et al. / Electrochimica Acta 54 (2009) 3910–3914
3911
acid was heated and melted and then stoichiometric amounts of La(NO3 )3 ·6H2 O, Fe(NO3 )3 ·9H2 O and Sr(NO3 )2 were added. The mixtures were stirred by the magnetic mixer until the homogeneous sol-like solution was formed. The solution was ignited in air and the obtained powders were calcined at 1123 K for 3 h in a muffle furnace. To analyze the phases of the calcined and discharged samples, X-ray diffraction (XRD, Dandongfangyuan) was carried out on a DX-2600 diffractometer with Cu K␣ radiation and a power of 25 kV × 30 mA. The XRD patterns were recorded between the range of 20 and 80◦ with a step of 0.05◦ /s and analyzed by Jade 5.0.
2.2. Electrochemical measurements
Fig. 1. The XRD patterns of the calcined and the discharged La(1−x) Srx FeO3 (x = 0.2 and 0.4) samples at the 20th cycle. (: La0.8 Sr0.2 FeO3 , : La0.6 Sr0.4 FeO3 , and : Ni. (a) The calcined powder La0.8 Sr0.2 FeO3 , orthorhombic, (b) the 20th discharged sample La0.8 Sr0.2 FeO3 , (c) the calcined powder La0.6 Sr0.4 FeO3 , rhombohedral, and (d) the 20th discharged sample La0.6 Sr0.4 FeO3 ).
Negative electrodes were prepared by cold-pressing the mixture of the calcined powder and carbonyl nickel powder with the weight ratio of 1:3 under 3.5 MPa to form a pellet of 10 mm in diameter. Electrochemical measurements were conducted by a DC-5 type computer-controlled battery test instrument with an open two-electrode cell placed in a thermostat water bath. NiOOH/Ni(OH)2 was used as the counter and reference electrode. In each charge–discharge cycle, the negative electrode was charged for 7 h at 125 mA g−1 and discharged at 32.5 mA g−1 and 125 mA g−1 to the cut-off voltage of −0.4 V (versus NiOOH/Ni(OH)2 ). The
Fig. 2. Charge–discharge curves of La(1−x) Srx FeO3 electrodes at various temperatures (a) at the discharge current density of 31.25 mA g−1 (x = 0.2), (b) at the discharge current density of 31.25 mA g−1 (x = 0.4), (c) at the discharge current density of 125 mA g−1 (x = 0.2), and (d) at the discharge current density of 125 mA g−1 (x = 0.4) (charge: () 298 K, (䊉)313 K, and () 333 K. Discharge: () 298 K, () 313 K, and ( ) 333 K).
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electrolyte is a 7 M KOH aqueous solution. The standing time between charge and discharge was 10 min and the temperature was 298 K, 313 K and 333 K, respectively. The hydrogen proton diffusion coefficient D in the bulk of the proton-conductive perovskite-type oxide and the exchange current density I0 were evaluated by an advanced electrochemical system PARSTAT2273 using the potential-step and linear polarization method, respectively. During the potential static step discharge process, the electrodes were in fully charged state, and then an overpotential of +600 mV was applied to them. The current–time transient curves were recorded after the potential step in this electrochemical analyzer. In addition, the linear polarization curves of the electrodes were plotted by scanning the electrode potential at the rate of 10 mV/s from −6 to 6 mV (versus open circuit potential) at 50% depth of discharge (DOD). 3. Results and discussion 3.1. Phase structure Fig. 1 shows the XRD patterns of the calcined La0.8 Sr0.2 FeO3 and La0.6 Sr0.4 FeO3 powders and their discharged samples at the 20th cycle. It can be seen that both La0.8 Sr0.2 FeO3 and La0.6 Sr0.4 FeO3 powders consist of nearly single phase and the diffraction data of them are in good agreement with those from JCPDS cards (JCPDS no: 35-1480 and JCPDS no: 82-1961), respectively. According to JCPDS cards, La0.8 Sr0.2 FeO3 holds an orthorhombic structure and La0.6 Sr0.4 FeO3 has a rhombohedral one. Compared with the diffraction peaks of La0.8 Sr0.2 FeO3 , those of La0.6 Sr0.4 FeO3 are broadened because of the coexistence of multi-peaks, and they shift from the left to the right, which was shown in the inset of Fig. 1. XRD patterns of the discharged samples show both La0.8 Sr0.2 FeO3 and La0.6 Sr0.4 FeO3 still remain perovskite-type structure as the peaks attributable to the Ni compounds presence are distinguishable in the patterns together with the peaks of the oxides. 3.2. Electrochemical properties The electrochemical properties of the oxide electrode were characterized by the charge, discharge and cycle curves. Initial charge and discharge curves of the La(1−x) Srx FeO3 oxide electrodes were presented in Fig. 2. It should be noted that the charge curves show a long and horizontal potential plateau, possibly due to the formation of stable chemical bonds between the perovskite-type oxides and protons. Nowick et al. [17] reported that vacancies could be replaced by protons, which resided on oxygen ions to form substitutional OH− ion defects in protonconductive perovskite-type oxides. The charge potential plateau decreases from 1.47 V to 1.4 V as the temperature increases from 298 K to 333 K. Even when the temperature rises up to 333 K, the discharge potential plateau of the electrode remains at about 1.1 V, which gain the advantage over the traditional hydrogen storage alloy, of which the voltage plateau rise does harm to the performances of the cell when the temperature increases [18–20]. Considering that the standard electrode potential of Fe3+ /Fe2+ (in La(1−x) Srx FeO3 electrode) and Ni2+ /Ni3+ (in NiOOH/Ni(OH)2 electrode) are −0.77 V and +0.52 V in alkaline solution, respectively, the obtained voltage was reasonable. It also can be seen that the initial discharge capacities at different current densities increase as the temperature rises from 298 K to 333 K. In Fig. 2(a and b), the capacities of La0.8 Sr0.2 FeO3 and La0.6 Sr0.4 FeO3 mount up from 324.4 mA h g−1 and 147.0 mA h g−1 to 543.0 mA h g−1 and 501.5 mA h g−1 at 31.25 mA g−1 , respectively. In Fig. 2(c and d), the corresponding values change from 54 mA h g−1 to 433.1 mA h g−1 and 51.6 mA h g−1 to 336.6 mA h g−1
Fig. 3. The discharge capacity versus cycle number of La(1−x) Srx FeO3 electrodes at various temperatures and under the charge–discharge current density of 125 mAg−1 (: 0.2 (298 K), 䊉: 0.4 (313 K), : 0.2 (333 K), : 0.4 (298 K), : 0.2 (313 K), and : 0.4 (333 K)).
at 125 mA g−1 , respectively. So the electrochemical test results suggest that the La(1−x) Srx FeO3 oxide electrodes hold a theoretical capacity which is much higher than that of traditional AB5 -type hydrogen storage electrode alloys. While Kleperis et al. [21] reported that the electrode made of LaNi5 alloy reaches a maximum capacity of 360 mA h g−1 and Raju et al. [18] denoted that of MmNi3.03 Si0.85 Co0.6 Mn0.31 Al0.08 hydrogen storage alloys falls from 283 mA h g−1 at 303 K down to 213 mA h g−1 at 328 K. Consequently, La(1−x) Srx FeO3 oxide electrode shows an advantage over the traditional hydrogen alloy upon both discharge capacity and voltage plateau at a higher temperature. The discharge cyclic properties of the oxide electrode at various temperatures and under the charge–discharge current density of 125 mA g−1 are shown in Fig. 3. Except for the charge–discharge cycle at 298 K, La0.8 Sr0.2 FeO3 oxide electrode can be discharged directly without activation, and the discharge capacity achieves its maximum in the first cycle. Generally, traditional hydrogen alloys including AB5, AB2 and La–Mg–Ni system alloy require more than three cycles for activation [5,7,22]. In the first three cycles, the discharge capacities decrease distinctly, down to 120 mA h g−1 and 220 mA h g−1 at 313 K and 333 K, respectively. Although the discharge capacity shows irregular fluctuation at 333 K after the 3rd cycle to the 20th cycle, the value always keeps 220 mA h g−1 above. The discharge capacities of La0.6 Sr0.4 FeO3 also keep steady at about 77 mA h g−1 and 170 mA h g−1 at 298 K and 313 K, respectively. When the temperature rises up to 333 K, its discharge capacity declines from 336.5 mA h g−1 to 278.3 mA h g−1 in the first three cycles and increases gradually after that. Finally, it reaches a value of about 360 mA h g−1 . Conclusively, La(1−x) Srx FeO3 electrodes show good capacity retention after three cycles, possibly due to be related to the stability of their structures after 20 charge–discharge cycles. Compared with the electrochemical properties of La0.8 Sr0.2 FeO3 at various temperatures, La0.6 Sr0.4 FeO3 electrode holds a higher electrochemical capacity, possibly due to more oxygen vacancies for hydrogen storage with increasing Sr content. 3.3. Kinetic characteristics The exchange current density (I0 ) is used to characterize the electro-catalytic activity of the charge-transfer reaction on the surface of the electrode. Fig. 4 shows the linear polarization curves of La(1−x) Srx FeO3 electrodes. Polarization resistance Rp can be
G. Deng et al. / Electrochimica Acta 54 (2009) 3910–3914
Fig. 4. Linear polarization curves of La(1−x) Srx FeO3 oxide electrodes (1: 298 K (x = 0.2), 2: 313 K (x = 0.2), 3: 333 K (x = 0.2), 4: 298 K (x = 0.4), 5: 313 K (x = 0.4), and 6: 333 K (x = 0.4)).
obtained from Fig. 4, and consequently the exchange current density I0 can be calculated according to the following equation [23]: I0 =
RT FRp
(2)
where R is the gas constant (J/(mol K)), T is the absolute temperature (K) and F is the Faraday constant (C/mol). The results obtained are listed in Table 1. It can be seen that the value of Rp decreases and I0 increases when the temperature rises, indicating that the charge transfer rate increases. In addition, the hydrogen proton diffusion rate can be estimated by potential-step method. Fig. 5 illustrates the correspondence of anodic current versus discharge time of La(1−x) Srx FeO3 electrodes at fully charged state and at various temperatures. It can be seen that, after the application of the overpotential, the current–time responses in Fig. 5 can be divided into two time domains. In the first time region, the current rapidly declined due to a consumption of hydrogen proton on the surface. In the other time region, however, the current slowly declined in a linear fashion. In this region, the surface concentration of proton approaches zero. Thus, the electrode reaction will be controlled by proton diffusion in the oxide bulk. The proton diffusion coefficient D in the bulk of the oxide can be calculated by the following formula [24]: log i = log
6FD da2
(C0 − Cs ) −
2 D t 2.303 a2
(3)
where i is the anodic current (A), D the proton diffusion coefficient (cm2 /s), d the density of the oxide (g/cm3 ), a the radius of the oxide particle, C0 the initial proton concentration in the bulk of the oxide (mol/cm3 ), Cs the surface proton concentration of the oxide (mol/cm3 ) and t is the discharge time (s). Given that a is 9.3 nm, D can be calculated according to the equation, and is summarized Table 1 Electrochemical kinetics parameters of La(1−x) Srx FeO3 oxide electrode (where Rp is polarization resistance, T the absolute temperature, I0 exchange current density and D is hydrogen proton diffusion coefficient). Sample
T (K)
Rp (m)
I0 (mA g−1 )
D (×10−17 cm2 /s)
La0.8 Sr0.6 FeO3
298 313 333
678.9 581.4 344.8
37.8 46.4 83.2
1.62 2.02 4.04
La0.6 Sr0.4 FeO3
298 313 333
600.5 417.2 366.5
42.8 64.7 78.3
0.8 1.82 2.02
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Fig. 5. Correspondence of anodic current versus discharge time of La(1−x) Srx FeO3 oxide electrodes (1: 298 K (x = 0.2), 2: 313 K (x = 0.2), 3: 333 K (x = 0.2), 4: 298 K (x = 0.4), 5: 313 K (x = 0.4), and 6: 333 K (x = 0.4)).
in Table 1. It can be seen that the proton diffusion coefficient D of the oxide increases as the temperature increases, and is in good agreement with the responses of the discharge capacity versus temperature. 4. Conclusion The phase structure and electrochemical properties of La(1−x) Srx FeO3 oxide electrodes were investigated systematically. XRD shows that La(1−x) Srx FeO3 perovskite-type oxides consist of single-phase: orthorhombic structure (when x = 0.2) and rhombohedral one (when x = 0.4), respectively. After 20 cycles, they still remain perovskite-type structure. The electrochemical test shows that the reaction at La(1−x) Srx FeO3 electrodes are reversible. The discharge capacities of La(1−x) Srx FeO3 oxide electrodes increase as the temperature rises and decrease when the discharge current density grows. At discharge current density of 125 mA g−1 , their discharge capacities keep steady at about 54 mA h g−1 ,120 mA h g−1 and 220 mA h g−1 (when x = 0.2) and 77 mA h g−1 , 170 mA h g−1 and 360 mA h g−1 (when x = 0.4) at 298 K, 313 K and 333 K, respectively. Being similar to the relationship between the discharge capacity and the temperature, the exchange current density and the proton diffusion coefficient of La(1−x) Srx FeO3 electrodes also increase when the temperature rises. Compared with La0.8 Sr0.2 FeO3 , La0.6 Sr0.4 FeO3 electrode is a more promising candidate for electrochemical hydrogen storage because of its higher cycle capacity at various temperatures. References [1] S.Q. Shi, C.Y. Ouyang, M.S. Lei, J. Power Sources 164 (2007) 911. [2] Y.F. Liu, H.G. Pan, M.X. Gao, Y.F. Zhu, Y.Q. Lei, Q.D. Wang, Electrochim. Acta 49 (2004) 545. [3] M. Tliha, H. Mathlouthi, C. Khaldi, J. Lamloumi, A. Percheron-guegan, J. Power Sources 160 (2006) 1391. [4] M. Tliha, C. Khaldi, H. Mathlouthi, J. Lamloumi, A. Percheron-Gu’egan, J. Alloys Compd. 440 (2007) 323. [5] B. Liao, Y.Q. Lei, L.X. Chen, J. Power Source 129 (2004) 358. [6] T. Kohno, H. Yoshida, F. Kawashima, J. Alloys Compd. 311 (2000) 5. [7] M.Y. Song, D. Ahn, I.H. Kwon, S.H. Chough, J. Electrochem. Soc. 148 (9) (2001) A1041. [8] H. Yukawa, Y. Takahashi, M. Morinaga, Comput. Mater. Sci. 14 (1999) 291. [9] N.H. Goo, J.H. Woo, K.S. Lee, J. Alloy Compd. 288 (1999) 286. [10] T. Abe, T. Tachikawa, Y. Hatano, K. Watanabe, J. Alloys Compd. 332 (2002) 792. [11] J.W. Liu, H.T. Yuan, J.S. Cao, Y.J. Wang, J. Alloys Compd. 392 (2005) 300.
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