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Synthesis of nanosized nickel hydroxide by solid-state reaction at room temperature
Materials Letters 58 (2004) 1327 – 1330 www.elsevier.com/locate/matlet
Synthesis of nanosized nickel hydroxide by solid-state reaction at room temperature Xiaohong Liu, Lan Yu College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, PR China Received 10 July 2003; accepted 24 September 2003
Abstract The nanosized cathode material Ni(OH)2 powder for alkaline batteries was synthesized by solid-state reaction at room temperature through NiC2O42H2O as precursor, which was also prepared with solid-state reaction from nickel acetate and oxalic acid at ambient temperature. The precursor and the Ni(OH)2 samples were characterized by X-ray diffraction (XRD), infrared spectrometry (IR), transmission electron microscopy (TEM) and electrochemical testing. The results revealed that the as-synthesized Ni(OH)2 sample by this method is h(II)-type phase, and its shape is fibroid with the average particle size of 6 – 9 nm. Compared with microsized spherical h-Ni(OH)2, the nanosized h-Ni(OH)2 exhibits excellent electrochemical performance, such as lower polarization and better charge – discharge properties. D 2004 Elsevier B.V. All rights reserved. Keywords: Nickel hydroxide; Nanometer; Solid-state reaction; Synthesis; Alkaline batteries; Electrochemical performance
1. Introduction Alkaline rechargeable batteries such as Ni – metal hydride (Ni –MH), nickel – cadmium (Ni –Cd) and nickel – iron (Ni – Fe) are widely applied to today’s market covering domains ranging from power tools to portable electronics and electric vehicle. Nickel hydroxide is the active material for cathodes of these batteries. The development of nickel hydroxide has gone through from traditionally agglomerated nickel hydroxide to spherical h-Ni(OH)2 [1]. Traditionally, the theoretical capacity of nickel hydroxide electrode is believed to be 289 mA h/g if the electrode reaction during charge– discharge process involves one-electron transfer. However, in highpower applications such as electric vehicles, it is limited by the discharge – charge rate which determines the vehicle acceleration and recharge rate of the Ni –MH battery [2]. For further battery applications, single-phase nickel hydroxide products with good crystallinity, homogeneity, uniform morphology with submicrometer particle size distribution and high surface area are necessary. It has been reported that nickel hydroxide with a smaller crystalline size shows better charge– discharge cyclic characteristics and a higher proton diffusion coefficient [3]. US Nanocorp (USN) has developed an aqueous solution reaction (ASR) technique scalable for high-volume production of nanostructured nickel hydroxide for a wide range of applications; it is 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2003.09.054
expected to yield at least a 20% improvement in cathode energy content [4]. The method of solid-state reaction at low temperature is unique in synthetic chemistry, and it has been employed successfully in the synthesis of nanomaterials [5 –7]. Compared with synthetic methods of solution phase and solid phase at high temperature, there are many advantages in this method: no need for solvent, almost no pollution, high yields, low energy consumption and simple reaction at low temperature. In this paper, a new two-step solid-state reaction route at room temperature for the synthesis of nanosized nickel hydroxide powder is presented. Nanosized Ni(OH)2 powder synthesized by this route has not been reported to date. The precursor and the final product have been characterized by X-ray diffraction (XRD), infrared spectrometry (IR) and transmission electron microscopy (TEM). In addition, Compared with microsized spherical Ni(OH)2 powder, the electrochemical performance of nanosized Ni(OH)2 powder was also studied.
2. Experimental Ni(CH3COO)24H2O (nickel acetate), H2C2O42H2O (oxalic acid) and NaOH are analytical reagent grade. Nano-
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3. Results and discussion Fig. 1 shows the XRD pattern of the as-prepared precursor that was obtained by the solid-state reaction at room temperature between Ni(CH 3 COO) 2 4H 2 O and H2C2O42H2O. Its diffraction peak is quite consistent with the standard JCPDS card of NiC2O42H2O. The precursor was confirmed to be pure-phase NiC2O42H2O. The average particle size of the precursor NiC2O42H2O is about 11.5 nm, using the Scherrer formula [8] by calculating with line-width method. The reaction is as follows: NiðCH3 COOÞ2 4H2 O þ H2 C2 O4 2H2 O ! NiC2 O4 2H2 O þ 2CH3 COOH þ 4H2 O Fig. 1. XRD pattern of the as-synthesized precursor.
sized Ni(OH)2 powder was obtained by two-step solid-state reaction synthesis at room temperature. In the first step, nickel acetate and oxalic acid were fully mixed by grinding 30 min at 1:1 mole ratio in an agate mortar, then the precursor was filtered, washed with distilled water and then dried in a vacuum desiccator at 70 jC for 4 h. In the second step, the precursor and NaOH was ground at 1:2 mole ratio in an agate mortar at room temperature for about an hour, then rinsed with distilled water and then oven-dried at 80 jC for several hours. The final product was green powder. The X-ray diffraction (XRD) powder patterns of the samples were carried out with a Shimadzu XRD 6000 ˚ ) operated at diffractometer. Cu Ka radiation (k = 1.5418 A 40 kV and 40 mA was used. The scan speed was 4j/min in 2h. The FT-IR of the precursor was measured on Thermo Nicolet Avatar 360 FT-IR instrument with KBr pellets from 4000 to 400 cm 1. The transmission electron microscope (TEM) image was performed by means of a JEOL JEM-100cx II microscope. Before observation, the sample was ultrasonically dispersed in ethanol. The specific surface areas of different Ni(OH)2 samples were obtained by the BET adsorption method using a surface area analyzer. Electrochemical properties tests were carried out as follows. Nanosized nickel hydroxide powder (or microsized spherical h-Ni(OH)2 powder), acetylene black and polytetrafluoroethylene (PTFE) binder were mixed completely in a weight ratio of 85:10:5, and pressed into a pellet. The pellet was mounted onto a 1 1-cm foam nickel for a cathode and dried at 120 jC for 24 h. Charge– discharge measurements were performed using one nickel hydroxide electrode coupling with a metal hydride (MH) electrode with an excessive capacity, and the electrolyte consisted of 6 mol l 1KOH and 15 g l 1 LiOH was used. A Hg/HgO (6 mol l 1 KOH + 15 g l 1 LiOH) electrode was employed as the reference electrode. Charge and discharge were performed at a rate of 0.2 C, and the terminal voltage of discharge was 0.1 V vs. Hg/HgO.
ð1Þ
Its structure is as follows [9]:
ð2Þ
The IR spectrum of the as-prepared NiC2O42H2O is illustrated in Fig. 2. The frequency values and the assignments of the peaks observed for the NiC2O42H2O precursor are listed in Table 1 [9]. Morphology of NiC2O42H2O precursor was investigated, and its TEM microscopy in Fig. 3 showed that its shape is fibroid. The particle sizes of the precursor are 5 – 15 nm, and it is in agreement with the data obtained from the XRD pattern. The powder XRD pattern of the as-synthesized Ni(OH)2 is shown in Fig. 4. All the reflectance peaks can be indexed as the hexagonal phase of Ni(OH)2 by comparison with the data of the JCPDS file no. 14-0117, which indicated that the crystalline state of the sample is typically
Fig. 2. IR spectrum of the as-synthesized precursor.
X. Liu, L. Yu / Materials Letters 58 (2004) 1327–1330
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Table 1 Frequencies and band assignments in NiC2O42H2O precursor Frequencies (cm 1)
Assignment
3396.0 3132.5 1633.8 1400.4 1317.2 828.3 778.9 558.7 488.1
vas(H2O) vs(H2O) vas(C = O) vs(CUO) + v(CUC) vs(CUO) + d(OUC = O) vs(CUO) + d(OUC = O) d(OUC = O) + v(NiUO) v(NiUO) + v(CUC) v(NiUO)
h(II)-type Ni(OH)2 phase. The qualitatively large peak widths indicate that the crystalline size is very small. According to Scherrer’s equation [8], the mean size of the h-Ni(OH)2 crystals is calculated to be about 7.3 nm. The spherical Ni(OH)2 powder used in the experiments is also h(II) phase, and its average particle size is about 8 –12 Am. The BET surface areas of microsized spherical Ni(OH)2 powder and nanosized Ni(OH)2 powder are 10.8 and 39.6 m2 g 1, respectively. TEM image of the final product is shown in Fig. 5. The resultant product is a nanosized fiber cluster. The fibers are approximately 3 –9 nm in diameter by 10 –50 nm long, while the average particle size of is approximately 6 –9 nm, which is consistent with the result of XRD analysis. Coincident with its strong peaks’ intensities in XRD pattern, its TEM investigation showed that it is in good crystallinity. On the basis of the experiment facts, the mechanism helping to understand the nanosized Ni(OH)2 particles formation in the solid-state reaction synthesis at room temperature is as follows: The existence of crystalline water in the reactants and sufficient grinding are the two key factors for the occurrence of a solid-state reaction at room temperature. On the one hand, when the reactant Ni(CH3COO)24H2O was reacted with H2C2O42H2O, and subsequently, the NiC2O42H2O precursor was reacted with NaOH, the crystalline water was considered to be released from the structure of its molecules forming a layer of liquid film over particles, which actually
formed a micro-aqueous environment for the reactant molecules. Perhaps these reactions offered the possibility to synthesize nanosized products. On the other hand, sufficient grinding is also required to provide the reactant molecules with more contact opportunities [10]. In the synthetic process, the nanosized NiC2O42H2O precursor was very important, which helped to the mixing of the reactant molecules at a molecule level and finally resulted in the formation of nanosized Ni(OH)2 particles. Moreover, in the conversion process of OH substituting C2O42 , the C2O42 controlled the solid-state reaction speed. Compared with the method of direct precipitation by the reaction of nickel ions and sodium hydroxide in water solution, the conversion process was much slower. The slow substitution of the C2O42 ions maintained the morphology of NiC2O42H2O precursor. The results shown by the XRD and TEM were coincident with the above supposition. Fig. 6 shows the charge– discharge curves of the microsized spherical h-Ni(OH)2 electrode and the nanosized hNi(OH)2 electrode at a rate of 0.2 C. As seen from Fig. 6, the charging potentials of the nanosized Ni(OH)2 electrode are lower than those of spherical Ni(OH)2 electrode, but the
Fig. 3. TEM image of NiC2O42H2O precursor.
Fig. 5. TEM image of Ni(OH)2 powder.
Fig. 4. XRD pattern of Ni(OH)2 powder.
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with microsized spherical Ni(OH)2 [3,11]. As we know that the step of proton diffusion is the rate-determining step in nickel hydroxide electrodes [12,13], the increase of the rate of proton diffusion means that the reaction activity of nickel hydroxide is enhanced; moreover, contact between the active material and the electrolyte is enhanced and results in the decrease of concentration polarization of electrode during charge–discharge process. Accordingly, the utilization of nanosized Ni(OH)2 is improved, and the nanosized Ni(OH) 2 electrode exhibits excellent electrochemical performance.
4. Conclusion Fig. 6. Charge – discharge curves of different samples of Ni(OH)2 powder.
discharging potentials of the nanosized Ni(OH)2 electrode are higher in comparison. There are two charging potential plateaux in both charge curves. It is suggested that the lower potential plateau corresponds to the reaction: Ni(OH)2 + OH ! NiOOH + H2O + e, while the higher potential plateau corresponds to oxygen evolution. The plateau of nanosized Ni(OH)2 oxidation is longer than that of spherical Ni(OH)2 because the former material can be oxidized more completely during the charging process. The discharging potential plateau of the nanosized Ni(OH)2 electrode is higher and more level than that of spherical Ni(OH)2 electrode. Moreover, the discharging capacity of the nanosized Ni(OH)2 electrode is markedly greater than that of spherical Ni(OH)2 electrode, namely, 234 vs. 214 mA h/g, which revealed that the utilization of nanosized Ni(OH)2 is greater than that of spherical Ni(OH)2. Based on the above analysis, it can be concluded that the nanosized Ni(OH)2 electrode has exceptional electrochemical properties in comparison with spherical Ni(OH)2 electrode. By definition, nanosized materials have at least one physical dimension less than 100 nm in length, an attribute which imparts exceptional electrochemical properties to nanosized Ni(OH)2 because the nanosized Ni(OH)2 particle dimensions are close to atomic dimensions and there are a very high fraction of atoms residing at nanocrystalline grain boundaries on the one hand. On the other, nanosized Ni(OH)2 powder synthesized by this method has a very small crystalline size and a large specific surface area, Thus, it shows a higher proton diffusion coefficient in comparison
Nanosized Ni(OH)2 was synthesized by the solid-state reaction method at room temperature. Characterization by XRD and TEM showed that it has a typical h(II)-phase structure and its shape is fibroid with the average particle size of 6 – 9 nm. Electrochemical measurements revealed that the performance of nanosized Ni(OH)2 is superior to that of microsized spherical Ni(OH)2, e.g., lower polarization and better charge–discharge properties.
References [1] M. Oshitani, H. Yufu, K. Takashima, S. Tsuju, J. Electrochem. Soc. 136 (1989) 1590. [2] S.R. Ovshinsky, M.A. Fetcenko, J. Ross, Science 260 (1993) 176. [3] K. Watannabe, T. Kikuoka, J. Appl. Electrochem. 25 (1995) 219. [4] D.E. Reisner, A.J. Salkind, P.R. Strutt, T. Danny Xiao, J. Power Sources 65 (1997) 231. [5] X.R. Ye, D.J. Jia, J.Q. Yu, X.Q. Xin, Z. Xue, Adv. Mater. 11 (1999) 941. [6] F. Li, X.H. Yu, H. Pan, M.C. Wang, X.Q. Xin, Solid State Sci. 2 (2000) 767. [7] F. Li, L.Y. Chen, Z.Q. Chen, J.Q. Xu, J.M. Zhu, X.Q. Xin, Mater. Chem. Phys. 73 (2002) 335. [8] H.P. Klug, E.A. Leroy, X-ray Diffraction Procedure, Wiley, New York, 1974, p. 656. [9] K. Nakamoto, Infrared Spectra of Inorganic and Coordination Compounds, Wiley, New York, 1963. [10] Q. Li, G. Luo, J. Li, X. Xia, J. Mater. Process. Technol. 137 (2003) 25. [11] Z. Zhou, J. Yan, J. Lin, Y.S. Zhang, Chin. J. Power Sources 23 (1999) 319 (in Chinese). [12] R. Barnard, C.L. Randell, F.L. Tye, J. Appl. Electrochem. 10 (1980) 109. [13] S. Motupally, C.C. Streiz, W.J. Weidner, J. Electrochem. Soc. 145 (1998) 29.
Synthesis of nanosized nickel hydroxide by solid-state reaction at room temperature Xiaohong Liu, Lan Yu College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, PR China Received 10 July 2003; accepted 24 September 2003
Abstract The nanosized cathode material Ni(OH)2 powder for alkaline batteries was synthesized by solid-state reaction at room temperature through NiC2O42H2O as precursor, which was also prepared with solid-state reaction from nickel acetate and oxalic acid at ambient temperature. The precursor and the Ni(OH)2 samples were characterized by X-ray diffraction (XRD), infrared spectrometry (IR), transmission electron microscopy (TEM) and electrochemical testing. The results revealed that the as-synthesized Ni(OH)2 sample by this method is h(II)-type phase, and its shape is fibroid with the average particle size of 6 – 9 nm. Compared with microsized spherical h-Ni(OH)2, the nanosized h-Ni(OH)2 exhibits excellent electrochemical performance, such as lower polarization and better charge – discharge properties. D 2004 Elsevier B.V. All rights reserved. Keywords: Nickel hydroxide; Nanometer; Solid-state reaction; Synthesis; Alkaline batteries; Electrochemical performance
1. Introduction Alkaline rechargeable batteries such as Ni – metal hydride (Ni –MH), nickel – cadmium (Ni –Cd) and nickel – iron (Ni – Fe) are widely applied to today’s market covering domains ranging from power tools to portable electronics and electric vehicle. Nickel hydroxide is the active material for cathodes of these batteries. The development of nickel hydroxide has gone through from traditionally agglomerated nickel hydroxide to spherical h-Ni(OH)2 [1]. Traditionally, the theoretical capacity of nickel hydroxide electrode is believed to be 289 mA h/g if the electrode reaction during charge– discharge process involves one-electron transfer. However, in highpower applications such as electric vehicles, it is limited by the discharge – charge rate which determines the vehicle acceleration and recharge rate of the Ni –MH battery [2]. For further battery applications, single-phase nickel hydroxide products with good crystallinity, homogeneity, uniform morphology with submicrometer particle size distribution and high surface area are necessary. It has been reported that nickel hydroxide with a smaller crystalline size shows better charge– discharge cyclic characteristics and a higher proton diffusion coefficient [3]. US Nanocorp (USN) has developed an aqueous solution reaction (ASR) technique scalable for high-volume production of nanostructured nickel hydroxide for a wide range of applications; it is 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2003.09.054
expected to yield at least a 20% improvement in cathode energy content [4]. The method of solid-state reaction at low temperature is unique in synthetic chemistry, and it has been employed successfully in the synthesis of nanomaterials [5 –7]. Compared with synthetic methods of solution phase and solid phase at high temperature, there are many advantages in this method: no need for solvent, almost no pollution, high yields, low energy consumption and simple reaction at low temperature. In this paper, a new two-step solid-state reaction route at room temperature for the synthesis of nanosized nickel hydroxide powder is presented. Nanosized Ni(OH)2 powder synthesized by this route has not been reported to date. The precursor and the final product have been characterized by X-ray diffraction (XRD), infrared spectrometry (IR) and transmission electron microscopy (TEM). In addition, Compared with microsized spherical Ni(OH)2 powder, the electrochemical performance of nanosized Ni(OH)2 powder was also studied.
2. Experimental Ni(CH3COO)24H2O (nickel acetate), H2C2O42H2O (oxalic acid) and NaOH are analytical reagent grade. Nano-
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3. Results and discussion Fig. 1 shows the XRD pattern of the as-prepared precursor that was obtained by the solid-state reaction at room temperature between Ni(CH 3 COO) 2 4H 2 O and H2C2O42H2O. Its diffraction peak is quite consistent with the standard JCPDS card of NiC2O42H2O. The precursor was confirmed to be pure-phase NiC2O42H2O. The average particle size of the precursor NiC2O42H2O is about 11.5 nm, using the Scherrer formula [8] by calculating with line-width method. The reaction is as follows: NiðCH3 COOÞ2 4H2 O þ H2 C2 O4 2H2 O ! NiC2 O4 2H2 O þ 2CH3 COOH þ 4H2 O Fig. 1. XRD pattern of the as-synthesized precursor.
sized Ni(OH)2 powder was obtained by two-step solid-state reaction synthesis at room temperature. In the first step, nickel acetate and oxalic acid were fully mixed by grinding 30 min at 1:1 mole ratio in an agate mortar, then the precursor was filtered, washed with distilled water and then dried in a vacuum desiccator at 70 jC for 4 h. In the second step, the precursor and NaOH was ground at 1:2 mole ratio in an agate mortar at room temperature for about an hour, then rinsed with distilled water and then oven-dried at 80 jC for several hours. The final product was green powder. The X-ray diffraction (XRD) powder patterns of the samples were carried out with a Shimadzu XRD 6000 ˚ ) operated at diffractometer. Cu Ka radiation (k = 1.5418 A 40 kV and 40 mA was used. The scan speed was 4j/min in 2h. The FT-IR of the precursor was measured on Thermo Nicolet Avatar 360 FT-IR instrument with KBr pellets from 4000 to 400 cm 1. The transmission electron microscope (TEM) image was performed by means of a JEOL JEM-100cx II microscope. Before observation, the sample was ultrasonically dispersed in ethanol. The specific surface areas of different Ni(OH)2 samples were obtained by the BET adsorption method using a surface area analyzer. Electrochemical properties tests were carried out as follows. Nanosized nickel hydroxide powder (or microsized spherical h-Ni(OH)2 powder), acetylene black and polytetrafluoroethylene (PTFE) binder were mixed completely in a weight ratio of 85:10:5, and pressed into a pellet. The pellet was mounted onto a 1 1-cm foam nickel for a cathode and dried at 120 jC for 24 h. Charge– discharge measurements were performed using one nickel hydroxide electrode coupling with a metal hydride (MH) electrode with an excessive capacity, and the electrolyte consisted of 6 mol l 1KOH and 15 g l 1 LiOH was used. A Hg/HgO (6 mol l 1 KOH + 15 g l 1 LiOH) electrode was employed as the reference electrode. Charge and discharge were performed at a rate of 0.2 C, and the terminal voltage of discharge was 0.1 V vs. Hg/HgO.
ð1Þ
Its structure is as follows [9]:
ð2Þ
The IR spectrum of the as-prepared NiC2O42H2O is illustrated in Fig. 2. The frequency values and the assignments of the peaks observed for the NiC2O42H2O precursor are listed in Table 1 [9]. Morphology of NiC2O42H2O precursor was investigated, and its TEM microscopy in Fig. 3 showed that its shape is fibroid. The particle sizes of the precursor are 5 – 15 nm, and it is in agreement with the data obtained from the XRD pattern. The powder XRD pattern of the as-synthesized Ni(OH)2 is shown in Fig. 4. All the reflectance peaks can be indexed as the hexagonal phase of Ni(OH)2 by comparison with the data of the JCPDS file no. 14-0117, which indicated that the crystalline state of the sample is typically
Fig. 2. IR spectrum of the as-synthesized precursor.
X. Liu, L. Yu / Materials Letters 58 (2004) 1327–1330
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Table 1 Frequencies and band assignments in NiC2O42H2O precursor Frequencies (cm 1)
Assignment
3396.0 3132.5 1633.8 1400.4 1317.2 828.3 778.9 558.7 488.1
vas(H2O) vs(H2O) vas(C = O) vs(CUO) + v(CUC) vs(CUO) + d(OUC = O) vs(CUO) + d(OUC = O) d(OUC = O) + v(NiUO) v(NiUO) + v(CUC) v(NiUO)
h(II)-type Ni(OH)2 phase. The qualitatively large peak widths indicate that the crystalline size is very small. According to Scherrer’s equation [8], the mean size of the h-Ni(OH)2 crystals is calculated to be about 7.3 nm. The spherical Ni(OH)2 powder used in the experiments is also h(II) phase, and its average particle size is about 8 –12 Am. The BET surface areas of microsized spherical Ni(OH)2 powder and nanosized Ni(OH)2 powder are 10.8 and 39.6 m2 g 1, respectively. TEM image of the final product is shown in Fig. 5. The resultant product is a nanosized fiber cluster. The fibers are approximately 3 –9 nm in diameter by 10 –50 nm long, while the average particle size of is approximately 6 –9 nm, which is consistent with the result of XRD analysis. Coincident with its strong peaks’ intensities in XRD pattern, its TEM investigation showed that it is in good crystallinity. On the basis of the experiment facts, the mechanism helping to understand the nanosized Ni(OH)2 particles formation in the solid-state reaction synthesis at room temperature is as follows: The existence of crystalline water in the reactants and sufficient grinding are the two key factors for the occurrence of a solid-state reaction at room temperature. On the one hand, when the reactant Ni(CH3COO)24H2O was reacted with H2C2O42H2O, and subsequently, the NiC2O42H2O precursor was reacted with NaOH, the crystalline water was considered to be released from the structure of its molecules forming a layer of liquid film over particles, which actually
formed a micro-aqueous environment for the reactant molecules. Perhaps these reactions offered the possibility to synthesize nanosized products. On the other hand, sufficient grinding is also required to provide the reactant molecules with more contact opportunities [10]. In the synthetic process, the nanosized NiC2O42H2O precursor was very important, which helped to the mixing of the reactant molecules at a molecule level and finally resulted in the formation of nanosized Ni(OH)2 particles. Moreover, in the conversion process of OH substituting C2O42 , the C2O42 controlled the solid-state reaction speed. Compared with the method of direct precipitation by the reaction of nickel ions and sodium hydroxide in water solution, the conversion process was much slower. The slow substitution of the C2O42 ions maintained the morphology of NiC2O42H2O precursor. The results shown by the XRD and TEM were coincident with the above supposition. Fig. 6 shows the charge– discharge curves of the microsized spherical h-Ni(OH)2 electrode and the nanosized hNi(OH)2 electrode at a rate of 0.2 C. As seen from Fig. 6, the charging potentials of the nanosized Ni(OH)2 electrode are lower than those of spherical Ni(OH)2 electrode, but the
Fig. 3. TEM image of NiC2O42H2O precursor.
Fig. 5. TEM image of Ni(OH)2 powder.
Fig. 4. XRD pattern of Ni(OH)2 powder.
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with microsized spherical Ni(OH)2 [3,11]. As we know that the step of proton diffusion is the rate-determining step in nickel hydroxide electrodes [12,13], the increase of the rate of proton diffusion means that the reaction activity of nickel hydroxide is enhanced; moreover, contact between the active material and the electrolyte is enhanced and results in the decrease of concentration polarization of electrode during charge–discharge process. Accordingly, the utilization of nanosized Ni(OH)2 is improved, and the nanosized Ni(OH) 2 electrode exhibits excellent electrochemical performance.
4. Conclusion Fig. 6. Charge – discharge curves of different samples of Ni(OH)2 powder.
discharging potentials of the nanosized Ni(OH)2 electrode are higher in comparison. There are two charging potential plateaux in both charge curves. It is suggested that the lower potential plateau corresponds to the reaction: Ni(OH)2 + OH ! NiOOH + H2O + e, while the higher potential plateau corresponds to oxygen evolution. The plateau of nanosized Ni(OH)2 oxidation is longer than that of spherical Ni(OH)2 because the former material can be oxidized more completely during the charging process. The discharging potential plateau of the nanosized Ni(OH)2 electrode is higher and more level than that of spherical Ni(OH)2 electrode. Moreover, the discharging capacity of the nanosized Ni(OH)2 electrode is markedly greater than that of spherical Ni(OH)2 electrode, namely, 234 vs. 214 mA h/g, which revealed that the utilization of nanosized Ni(OH)2 is greater than that of spherical Ni(OH)2. Based on the above analysis, it can be concluded that the nanosized Ni(OH)2 electrode has exceptional electrochemical properties in comparison with spherical Ni(OH)2 electrode. By definition, nanosized materials have at least one physical dimension less than 100 nm in length, an attribute which imparts exceptional electrochemical properties to nanosized Ni(OH)2 because the nanosized Ni(OH)2 particle dimensions are close to atomic dimensions and there are a very high fraction of atoms residing at nanocrystalline grain boundaries on the one hand. On the other, nanosized Ni(OH)2 powder synthesized by this method has a very small crystalline size and a large specific surface area, Thus, it shows a higher proton diffusion coefficient in comparison
Nanosized Ni(OH)2 was synthesized by the solid-state reaction method at room temperature. Characterization by XRD and TEM showed that it has a typical h(II)-phase structure and its shape is fibroid with the average particle size of 6 – 9 nm. Electrochemical measurements revealed that the performance of nanosized Ni(OH)2 is superior to that of microsized spherical Ni(OH)2, e.g., lower polarization and better charge–discharge properties.
References [1] M. Oshitani, H. Yufu, K. Takashima, S. Tsuju, J. Electrochem. Soc. 136 (1989) 1590. [2] S.R. Ovshinsky, M.A. Fetcenko, J. Ross, Science 260 (1993) 176. [3] K. Watannabe, T. Kikuoka, J. Appl. Electrochem. 25 (1995) 219. [4] D.E. Reisner, A.J. Salkind, P.R. Strutt, T. Danny Xiao, J. Power Sources 65 (1997) 231. [5] X.R. Ye, D.J. Jia, J.Q. Yu, X.Q. Xin, Z. Xue, Adv. Mater. 11 (1999) 941. [6] F. Li, X.H. Yu, H. Pan, M.C. Wang, X.Q. Xin, Solid State Sci. 2 (2000) 767. [7] F. Li, L.Y. Chen, Z.Q. Chen, J.Q. Xu, J.M. Zhu, X.Q. Xin, Mater. Chem. Phys. 73 (2002) 335. [8] H.P. Klug, E.A. Leroy, X-ray Diffraction Procedure, Wiley, New York, 1974, p. 656. [9] K. Nakamoto, Infrared Spectra of Inorganic and Coordination Compounds, Wiley, New York, 1963. [10] Q. Li, G. Luo, J. Li, X. Xia, J. Mater. Process. Technol. 137 (2003) 25. [11] Z. Zhou, J. Yan, J. Lin, Y.S. Zhang, Chin. J. Power Sources 23 (1999) 319 (in Chinese). [12] R. Barnard, C.L. Randell, F.L. Tye, J. Appl. Electrochem. 10 (1980) 109. [13] S. Motupally, C.C. Streiz, W.J. Weidner, J. Electrochem. Soc. 145 (1998) 29.