Journal of Materials Science and Engineering B 2 (6) (2012) 376-380
Synthesis and Electrochemical Properties of Nickel Hydroxides
Danyang Xie1, Jianhua Liu2 and Ruijun Zhang2
1. Department of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, China
2. State Key Laboratory of Metastable Materials Science and Technology, Yan Shan University, Qinhuangdao 066004, China Received: April 09, 2012 / Accepted: April 28, 2012 / Published: June 25, 2012.
Abstract: In this endeavor to improve discharge capability of nickel hydroxide, the compounds with ethylenediamine and with Er3+ dope were synthesized, and structure and electrochemical properties were studied. The blank compound is of an amorphous structure, however, a theophrastite-Ni(OH)2 structure forms after u of complexing reagent during preparation. When Er3+ is doped upon preparation, Er2O3 appears. The u of complexing reagent increas discharge capacity and improves the cycle life remarkably although it delays the activation slightly. The high temperature disch
argeability increas from 52% to 88% for doped compound of Er3+ at 333 K. The cyclic voltammetry measurements show that proton diffusion coefficient is much higher in blank compound for numerous defects in amorphous structure.
Key words: Nickel hydroxide, electrochemical property, diffusion, Ni/MH battery.
1. Introduction
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Nickel/Metal hydride (Ni/MH) batteries have been widely ud in portable electronic devices, electric hand tools and even electric vehicles owing to their good performances and environmental friendliness [1]. The capacity of the Ni/MH batteries is usually designed according to positive electrode and the utilization of active materials. Generally, the positive electrode materials, nickel hydroxides which exist in two forms, namely α-Ni(OH)2 and β-Ni(OH)2 and β-type compounds are usually ud due to its excellent stability during charging/discharging [2, 3]. Up to now, a lot of rearch is devoted to improvement in electrochemical properties of nickel hydroxides. The common approach is to added Cobalt additives to get CoOOH which usually forms a conductive film on surface of the nickel hydroxides [4-6]. In order to improve electrochemical properties of nickel hydroxide electrodes, other additives, such as carbon materials (flake graphite, multi-walled carbon nanotubes), bimetallic additives (Cd-Zn, Cd-Mn,
Corresponding author:Ruijun Zhang, professor, rearch field:metallicmaterials.E-mail:***************.Cd-Co, Cd-Fe and Cd-Al), Calcium compounds (CaF2 and CaCO3) and some single metals are physically added to the electrodes [7-9]. However, physically mixing additives are not effective on improving conductivity of active materials and some work introduces additives by using co-precipitating method. Bardé et al. [10] prepared α-type precursors containing various amounts of Al, Cd, Co, Cr, Fe or Zn. And the reactions led to metal substituted γ-type nickel oxi-hydroxides except for cadmium and chromium containing samples. Li and Liu et al. [11, 12] prepared Al-Nd co-doped and Al doped nickel hydroxides by co-precipitating method. The compounds show better electrochemical reversibility and high discharge capacity. In the prent work, the Ni(OH)2 which is doped with Er3+ have been prepared by chemical coprecipitation method. Structure and electrochemical properties are tested.
2. Experiment
2.1 Synthesis and Characterization of Nickel Hydroxide
1.0 mol/L NaOH solution with extra amount was
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Synthesis and Electrochemical Properties of Nickel Hydroxides 377
added to mixed solution of nickel sulfate, erbium sulfate and ethylenediamine dropwi. Then the reaction system was kept at 55 ºC and pH was controlled between 12 and 13 using sodium hydroxide solution. The nickel hydroxide precipitate was filtered, rind with deionized water until pH was about 7, and dried at 60 ºC to constant weight, successively. Using this method, three kinds of nickel hydroxide powders A, B and C were made. A designates specimen without Er3+ dope and without ethylenediamine as complexing reagent, B designates specimen without Er3+ dope and with ethylenediamine as complexing reagent and C designates specimen with Er3+ dope and with ethylenediamine as complexing reagent (molar ratio of Ni2+ to Er3+ is 97:3) (for brevity hereafter).
The XRD patterns were obtained on a D/Max-2500/PC X-ray diffractometer (Cu Kαradiation) using powders of -300 meshes and analyzed using Jade-5 software.
2.2 Electrochemical Measurements
A simulated cell for electrochemical testing consisted of a nickel hydroxide electrode and a metal hydride (MH) electrode. The MH electrodes are commercially available with a discharge capacity much larger than nickel hydroxide one. The nickel hydroxide electrodes are prepared as follows: 0.34 g nickel hydroxide powder were well mixed with 0.06 g nickel powder, 3 wt.% PVA solution was added to get paste and then the mixture was pasted onto both sides of a foam nickel sheet. The pasted sheets were dried in vacuum at 60 ºC for 4 h. Finally dried sheet was cold presd at 10 MPa pressure to get testing electrode.
For cyclic voltammetric tests, the measurement was performed on a CHI660A potentiostat and in a tri-electrode system, consisting of nickel hydroxide electrode as working electrode, MH electrode as counter electrode and Hg/HgO electrode as reference electrode. The potential range was 0-0.8 V (vs. Hg/HgO electrode) and the scanning rate was 0.5 mV/s, 1 mV/s, 2 mV/s and 5 mV/s. 3 Results and Discussion
3.1 Structure of the As-prepared Compounds
Fig. 1 shows XRD patterns of the as-prepared Ni(OH)2 compounds. In compound A, veral broad peaks are found, which show amorphous structure of the compound. When comes to compound B,
a theophrastite-Ni(OH)2 structure (JCPDS No. 14-0017) is found. As for the compound C, peaks corresponding to Er2O3 appear, as well as tho corresponding to theophrastite-Ni(OH)2. It is en from Fig.1c that Er2O3 shows diffraction peaks with high intensity although small amount of Er ions are ud, which may be ascribed that Er2O3 microencapsulate at surface of the Ni(OH)2 particles.
3.2 Proton Diffusion Coefficient
Typical cyclic voltammograms for as-prepared Ni(OH)2 compound electrodes at different scan rates are prented in Fig. 2. With the increa in scan rate, the peak current increas; the anodic peak potential and cathodic peak potential shifts towards a more positive and more negative direction, respectively. Generally, the electrochemical reaction process of Ni(OH)2 electrode is considered to be proton diffusion limited [13]. Therefore, it is of great importance to study the proton diffusion coefficient of the electrodes.服装跟单员
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Fig. 1 XRD patterns of Ni(OH)2 compounds, (a) compound A; (b) compound B and (c) compound C.
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378-600
-400
-2000200400C u r r e n t (m A )
-600
-400-200
200
400
600
800
C u r r e n t (m A )
C u r r e n t (m A )
Potential (mV, vs. Hg/HgO)
Fig. 2 Cyclic voltammograms of the as-prepared nickel hydroxides (a) compound A; (b) compound B; (c)
compound C.
For the controlled mi-infinite diffusion by voltammetry in liquid electrolytes, the oxidation peak current (i p ) of the voltammogram can be written for a mi-infinite diffusion and an irreversible transfer under the following form [14], as shown in Eq. (1): 53/21/21/21/2002.9810p i n AD V C α=⨯ (1)
where: n , A , D 0 and V are the number of transferred electron, the apparent surface area of the electrode, the proton diffusion coefficient, and scan rate, respectively. C 0 denotes the initial concentration of the Ni(OH)2 in solid, which can be expresd as Eq. (2) [12]:
a M C ⨯=/0ρ (2)
where: ρ, M and α are theoretical density of Ni(OH)2
(2.82 g/cm 3), molar mass of Ni(OH)2 (92.7 g/mol), and
molar fraction of Ni(OH)2 in codoped samples,
respectively. α is transfer coefficient of electro-active
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species, which can be calculated from Eq. (3) [14]: d E p /dlog v = 2.3RT /αnF (3) Combining Eq. (2) and Eq. (3), the proton diffusion
coefficient (D ) can be calculated by using Eq. (1).
Table 1 lists some calculated parameters from cyclic
voltammograms. The proton diffusion coefficient (D ) of compounds without complexing reagent is larger than that with complexing reagent. This is ascribed to that the compound without complexing reagent exhibits an amorphous structure with numerous defects, which facilitates the proton diffusion.
Fig. 3 shows discharge capacity of nickel hydroxide
electrodes. The maximum discharge capacity of the
报名职业学校as-prepared compound electrodes is 297 mAh/g, 320
mAh/g and 292 mAh/g for compounds A, B and C, respectively. Discharge capacity retention rate is ud to characterize cycle life of compounds, and defined as Eq. (4) max
100%n
n
C S C =⨯ (4)
where: S n is the capacity retention rate at Cycle n , C n is
the capacity at Cycle n and C max is the maximum discharge capacity. The discharge capacity retention rate at the 100th cycle is 86.5%, 92.8% and 89.7% for compounds A, B and C, respectively. This indicates that the u of complexing reagent upon preparation
improves cycle life of the nickel hydroxide electrode. Table 1 Parameters calculated from cyclic voltammograms. Samples
C 0 (mol/cm 3) α
D (× 10-8 cm 2/s) Compound A 0.0304 0.250 11.59 Compound B 0.0304 0.447 3.832 Compound C
0.0295
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Synthesis and Electrochemical Properties of Nickel Hydroxides
379
100
150200250
300350D i s c h a r g e c a p a c i t y (m A h /g )
Cycle number (n)
Fig. 3 Discharge capacity of nickel hydroxide electrodes vs. cycle number.
At high temperature positive electrodes of MH/Ni are usually subject to poor charge efficiency owing to oxygen evolution reaction, which leads directly to poor dischargeability of the batteries. In order to
show clearly the high temperature dischargeability, it is defined as shown in Eq. (5):
T 298
100%C
HTD C =⨯ (5)
where: HTD is high temperature dischargeability, C T is the discharge capacity at various temperature and C 298 is the discharge capacity at 298 K. The discharge capacity of undoped compound electrodes decreas sharply in spite of the u of complexing reagent. And the compound with Er 3+ dope shows excellent dischargeability at high temperature, with discharge capacity decreasing slightly at high temperature. When the temperature ris up to 333 K, HTD of the undoped electrode decreas to ～52%, while dischargeability of the Er 3+ doped electrode maintains ～88%. The improvement in high rate dischargeability should be ascribed to increa in oxygen evolution overpotential.
4. Conclusions
Three nickel hydroxide compounds were prepared, and their structure and electrochemical propertie
s were studied. After u of complexing reagent in preparation, the structure transforms from an amorphous structure
to a theophrastite-Ni(OH)2 structure reveals by XRD analysis. The Er 3+ dope introduces Er 2O 3 into the prepared compounds. The maximum discharge capacity increas from 297 mAh/g to 320 mAh/g, and the capacity retention rate at the 100th cycle increas from 86.5% to 92.8% when complexing reagent is ud in the preparation process. However, Er 3+ dope deteriorates discharge capacity and cycle life compared with compound B. The Er 3+ dope increas potential for oxygen evolution reactions and therefore improves the dischargeability at high temperature, the ratio of discharge capacity at 333 K to that at 298 K increas from 52% to 88%. The cyclic voltammetric measurements also show that proton diffusion coefficient is much higher in compound A for numerous defects in amorphous structure.
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