ORIGINAL PAPER

Effect of surface modification of zinc oxideon the electrochemical performancesof [Ni4Al(OH)10]OH electrode

Xiaorui Gao & Lixu Lei & Le Chen & Yuqiao Wang &

Fei Ren & Yaobing Yin & Leqin He

Received: 29 March 2013 /Revised: 24 July 2013 /Accepted: 12 August 2013 /Published online: 30 August 2013# Springer-Verlag Berlin Heidelberg 2013

Abstract Surface modification of zinc oxide on the[Ni4Al(OH)10]OH has been performed by a chemical surfaceprecipitation method. Inductively coupled plasma measure-ments show that the amount of ZnO of prepared samplesincreases with the increase of initial concentration of Zn2+ inthe mother solution. Powder X-ray diffraction measurementsand scanning electron microscope images show that the mod-ification of ZnO has little effects on the lattice parameters andthe particle sizes of the [Ni4Al(OH)10]OH, but does change themorphology. The charge–discharge cycles results show that thedeterioration rate of discharge capacity for the electrode withZnO is only 4.0 % after 255 cycles, which is lower than that ofelectrode without ZnO (8.5 %); meanwhile, the maximal num-bers of exchanged electrons per nickel atom for the electrodeswith ZnO are basically over 1.83, which are higher than that ofthe electrode without ZnO (1.73), indicating that the modifica-tion of ZnO can improve the utilization of active material. Inaddition, the cyclic voltammogram tests results show that themodification of ZnO not only improves electrochemical cyclicreversibility but also elevates the oxygen evolution potential.Electrochemical impedance spectroscopy measurements showthat the modification of ZnO can lower the double layer capac-itance and the charge transfer resistance.

Keywords Layered double hydroxide . Nickel/metal hydridebattery . Electrochemical performance . Zinc oxide

Introduction

Nickel/metal hydride (Ni/MH) battery is considered as one ofthe most promising energy sources for electric vehicle andhybrid electric vehicle applications due to both its high ratecharge/discharge capability and high reliability [1]. The key ofa high performance secondary battery is the electrode materi-al. As the positive material for Ni/MH battery, nickel hydrox-ide has two polymorphic forms known as α-Ni(OH)2 and β-Ni(OH)2, which transform to γ-NiOOH and β-NiOOH, re-spectively, during charging [2]. But after extendedovercharging, β-Ni(OH)2 easily transforms to γ-NiOOH ac-companied by a large volumetric change, leading to poorelectric contact between the current collector and β-Ni(OH)2/β-NiOOH, increase of electrode resistance, and sub-sequent decrease of discharge capacity of the battery [3].However, for α-Ni(OH)2 electrode material, volume expan-sion is significantly reduced due to the similar lattices of α-Ni(OH)2 and γ-NiOOH [4]; moreover, it shows superiorelectrochemical properties in contrast to β-Ni(OH)2 with amuch higher discharge capacity, much more transferred elec-trons, and much longer cycle life.

However, α-Ni(OH)2 easily transforms to β-Ni(OH)2when stored in KOH solution (aging) and/or submitted tosuccessive charge/discharge processes [5]. To improve thestability of α-Ni(OH)2, many studies have been carried outthrough partial substitution of nickel ions in the nickel hy-droxide lattices by other metal ions, such as Al3+, to form Ni–Al layered double hydroxides (LDHs). They can be expressedby a typical formula [Ni1−xAlx(OH)2]

x+An−x/n·mH2O, where

A is a charge-balancing anion (e.g., NO3− or OH−). LDHs are

well known for their anionic exchange properties and their

X. Gao (*) :Y. Yin : L. HeCollege of Science, Hebei University of Engineering,Handan 056038, Chinae-mail: gxr_1320@sina.com

X. Gao : L. Lei :Y. Wang : F. RenSchool of Chemistry and Chemical Engineering,Southeast University, Nanjing 211189, China

L. ChenSchool of Chemistry and Chemical Engineering,Changzhou University, Changzhou 213164, China

J Solid State Electrochem (2014) 18:29–38DOI 10.1007/s10008-013-2226-9

stable structure [6], so they have been extensively applied inmany fields [7–12].

In our previous studies [1, 13], it has been found that[Ni4Al(OH)10]OH·mH2O as a positive electrode material ofNi/MH battery has marvelous performances when it respec-tively is charged or discharged at high and low rates. But withthe increase of charge/discharge cycles in a strong alkali KOHsolution, the electrode capacity and charge/discharge perfor-mances decrease [14]. The reason is possibly that the materialloses some Al3+ and transforms to β-Ni(OH)2 becauseAl(OH)4

− is soluble in concentrated alkali solution. To solvethe problem, considerable efforts have been made in coatingvarious additives on the Ni–Al LDH by us such as Lu2O3 [15]and Ca(OH)2 [16]. Recently, β-Ni(OH)2 was also used asadditive to coat on the Al-substituted α-nickel hydroxide. Itis found that the surface modification can increase the con-ductivity, and the oxygen evolution potential and active ma-terial utilization of electrode, meanwhile, lower the rate of lossof Al ions. It is possible that surface modification of LDHsmay offer an efficient solution by providing a barrier againstthe intrusion of OH anions and the lost of Al ions.

Some studies on doping Zn2+ in LDHs as electrode mate-rials in Ni/MH battery have been carried out [17, 18]. It isfound that these materials have strong cycling stability andhigh activematerial utilization. However, no report about ZnOas an additive introduced in the LDH electrode is found.Hence, in this paper, we would like to see how the perfor-mance is if zinc oxide as modifier is coated on the surface of[Ni4Al(OH)10]OH.

Experimental

Reagents

All chemicals used in this research are analytical grade re-agents and used without further treatments. Distilled waterused in the experiments was boiled for 30 min to remove anydissolved gases.

Preparation of samples

Preparation of [Ni4Al(OH)10]OH was performed by a methodreported in our previous paper [1], and the product is used as thestarting material which was denoted as L. Surface modificationsamples of [Ni4Al(OH)10]OH coated by zinc oxide were pre-pared as follows: at room temperature, 3 mmol of the as-prepared starting material [Ni4Al(OH)10]OH was added into50 mL of aqueous solution of 1.5 mmol of Zn(NO3)2·6H2Ounder a N2 flow. Then, 0.5 mol L−1 of NaOH solution wasdropwise added into above suspension under vigorous stirringuntil the pH value reached 7. After 5 h, the resulted suspensionwas transferred into a Teflon-lined autoclave and had been kept

in an oven maintained at 180 °C for 24 h. It was then filteredand washed with distilled water for three times and dried invacuum at 60 °C. The dried product was denoted as B1 (themole ratio of Ni2+, Al3+, and Zn2+ is 4:1:0.5). Similarly, anothertwo surface modification samples were prepared and hereafter,respectively, denoted as B2 and B3 (the mole ratio of Ni2+,Al3+, and Zn2+ is 4:1:1.0 and 4:1:1.5, respectively).

Preparation of ZnO power: 10 mmol of Zn(NO3)2·6H2Owas dissolved in 50 mL of distilled water, and then1.0 mol L−1 of NaOH solution was slowly dropped into abovesolution under vigorous stirring until the pH value of suspen-sion reached 7. After 5 h, the resulted suspension was trans-ferred into a Teflon-lined autoclave and had been kept in anoven maintained at 180 °C for 24 h. It was then filtered andwashed with distilled water for three times and dried in vacuumat 60 °C. The preparation of ZnO is for determining whetherZnO is coated on the surface of [Ni4Al(OH)10]OH by thefollowing scanning electron microscopy (SEM) measurement.

Studies on the stability of sample coated by ZnO in an alkalisolution

Sample B2 was selected to test the structural stability in strongalkali solution. A 0.5000-g sample B2was added into a 40mLKOH solution (7 mol L−1) at 30 °C for 10, 20, and 32 days,respectively, and then, the suspensions were filtered. The threesolids were washed with water and acetone several times andthen dried in vacuum at 60 °C. They were sampled for metalcontent analyses andmorphologymeasurements and separate-ly called as aged sample B2′, B2″, and B2″′ hereafter todistinguish them from the original sample. Their structureand compositions were characterized by powder X-ray dif-fraction measurements (XRD) and inductively coupled plas-ma (ICP) spectrometer.

Characterizations of samples

The structures of samples were determined by XRD using aRigaku D/max 2000/PC diffractometer with Cu Kα radiation(λ =1.5406 Å, scan rate 0.02°/s). Peaks of the XRD spectrawere fitted with program Xfit (http://www.ccp14.ac.uk/tutorial/xfit-95/xfit.htm) to find the full width at half maximum(FWHM) and the diffraction angles (2θ) of the refractions.The Scherrer equation and the Chekcell software were respec-tively used to calculate the crystalline sizes and cell parametersof samples. Metal contents were determined by using a Jarrel-Ash J-A1100 ICP spectrometer. All the Fourier transform in-frared (FTIR) spectra were recorded on a Bio-Rad FTS 6000FTIR Spectrometer equipped with a DuraSamplIR II diamondaccessory in attenuated total reflectance mode in the range of400–4,000 cm−1; 100 scans at 4 cm−1 resolution were collected.The absorption in the range 2,500–1850 cm−1 is from theDuraSamplIR II diamond surface. The morphology and surface

30 J Solid State Electrochem (2014) 18:29–38

composition of the samples were tested by using a S4800 SEMand energy dispersive spectroscopy (EDS).

Preparation of electrodes

A paste was prepared by mixing 50 mg of as-prepared productwith 160mg ofNi powder, 40mg of cobalt powder, appropriateamount of polytetrafluoroethylene (PTFE), and carboxymethylcellulose suspensions. The paste was incorporated into a nickelfoam (ϕ 15 mm) using a spatula, dried at 80 °C for 24 h, andthen pressed at 20 MPa to assure good electrical contact be-tween the substrate and the active materials.

Electrochemical characterizations

Electrochemical characterizations were performed in a three-compartment electrolysis cell, in which a piece of Ni foamwas used as the counter electrode and an Hg/HgO electrode asthe reference electrode. Galvanostatic charge–discharge stud-ies were conducted on a LAND CT2001A cell performancetester. After the working electrode had been immersed in a7 mol L−1 KOH for 24 h, it was charged at a current density of100 mA g−1 for 4 h and then discharged to 0 V (vs. Hg/HgO in7 mol L–1 KOH aqueous solution) at the same current densityfor activation. After such 5 cycles, the electrode was chargedat a current density of 800 mA g−1 for 30 min and thendischarged to 0 V at a current density of 400 mA g−1 for250 cycles. The current densities and discharge capacities inthe context of this paper were all calculated according to theactual mass of ZnO coated [Ni4Al(OH)10]OH used in theelectrode preparations. The number of exchanged electronsper nickel atom (NEE) was determined according to theelemental analyses of nickel content in the samples. TheNEE for each powder was calculated using the formulaNEE=3600Cexp/nF [19], where Cexp is the discharge capac-ity in ampere-hours per gram of active material in the elec-trode, n is the number of moles of nickel per gram of activematerial, and F is Faraday’s constant (96,485 Cmol s−1).

Cyclic voltammetric studies were carried out using a fresh-ly prepared electrode at a scan rate of 0.10 mV s−1 from 0.0 to0.7 V vs. Hg/HgO on a CHI 660b electrochemical workstationat room temperature under an Ar flow. Electrochemical im-pedance spectra (EIS) were recorded under open circuit withfrequencies ranging from 10−3∼105 Hz, and the fitted exper-imental results were obtained by Zview2 software.

Results

XRD characterizations

XRD patterns of ZnO, [Ni4Al(OH)10]OH, ZnO coated[Ni4Al(OH)10]OH (B1, B2 and B3), and aged samples of

B2′, B2″, and B2″′ are shown in Fig. 1, and their metal contentanalysis data are listed in Table 1. The XRD pattern of ZnOpowder displays reflections attributed to ZnO (JCPDS 36-1451) and can be indexed on a hexagonal cell, space groupof P63mc. [Ni4Al(OH)10]OH has a stoichiometric formula of[Ni3.9Al(OH)9.8]OH·5.5H2O (shown in Table 1), and all thesediffraction peaks can be indexed on a hexagonal cell, spacegroup of R3m. It is obvious that other XRD patterns sharegreat similarities; these samples are composed of two phasesincluding LDH and ZnO.

The mole ratio of Ni to Al for [Ni4Al(OH)10]OH, B1, B2,and B3 samples has no change in Table 1, meanwhile a and cvalues of LDH are almost the same (Table 2). It seems thatcoating zinc oxide on the [Ni4Al(OH)10]OH surface appearsto have little effect on the crystal lattice of the LDH. However,the crystalline size decreases slightly as the amount of zincoxide in the sample increases, indicating that zinc oxide mayinfluence the morphology of the LDH particles during thecoating process. This result is in accordance with generalknowledge on this matter, as surface coating on a solid particleshould not change anything inherent of it [16].

For aged sample B2′ and B2″, their XRD patterns show thesimilarity with that of B2 sample which includes those diffrac-tion peaks from LDH phase and ZnO phase, but B2″′ sampleshows only diffraction peaks of LDH, indicating that the agingprocess did not destroy the structures of LDH and ZnO, and notransformation to β-Ni(OH)2 takes place in aged samples. Butwith the increase of aging time, ZnO gradually dissolved untildisappeared as B2 sample was soaked in 7 mol L−1 KOHsolution for 32 days. Another difference of original B2 sampleand aged samples is that the diffraction peaks of aged samplesin the LDH phase become narrower and stronger; however,those in ZnO phase of B2′ and B2″ become broader andweaker. The phenomenon in LDH phase is in good agreementwith the previous report [20], which evidences that the agingprocess is a recrystallization via a dissolution, nucleation, and

Fig. 1 XRD patterns of as-prepared samples and aged sample of B2′,B2″, and B2″′

J Solid State Electrochem (2014) 18:29–38 31

growth process. The calculated crystalline size of B2′, B2″, andB2″′ in LDH phase (shown in Table 2) confirms above result.

In addition, variation tendency of lattice parameters of aand c values in LDH phase are different for aged samples. Theincrease of a and c values for B2′ and B2″ samples is possiblymainly resulted from the dissolution of part of ZnO in KOHsolution, meanwhile some Zn2+ ions enter the layer lattice ofLDH, which can be certified by the following SEM images ofB2′ and B2″. But how much are located in the layer and howmuch occur in the form of ZnO are difficult to be calculated.Only the total amounts of Zn can be tested by ICP for B2′ (Ni,37.9 wt.%; Al, 4.29 wt.%; and Zn, 7.29 wt.%) and B2″ (Ni,38.82 wt.%; Al, 4.33 wt.%; and Zn, 4.64 wt.%), and thecalculated molar ratio of Ni, Al, and Zn is 4.0:1:0.70 and4.1:1:0.45, respectively, which show that a small amount ofAl ions are lost as well. In addition, the crystalline size of B2′and B2″ in ZnO phase is smaller than that of B2, which canalso prove the dissolution of ZnO in strong alkaline solution[20, 21]. For B2″′ sample, both of a and c values decrease.The reason is from two aspects. On the one hand, ZnO and Znions in the sample are completely dissolved, which can also befound in SEM images; on the other hand, part of Al ions arealso dissolved which can be certified by ICP analysis data ofB2″′ (Ni, 44.7 wt.%; Al, 4.65 wt.%; and Zn, 0.00 wt.%), withcalculated molar ratio of Ni, Al, and Zn is 4.4:1:0. With the

dissolution of Zn ions, Al ions decreasemore quickly. It seemsthat the addition of Zn can delay the loss of Al ions, which canbe proved in the following EDS analysis data.

FTIR characterizations

Figure 2 gives FTIR absorption spectra for zinc oxide coated[Ni4Al(OH)10]OH samples. It shows the characteristic bandsfor hydroxide and adsorbedwater. A feature in the FTIR spectrais two absorption peaks between 3,200 and 3,800 cm−1 resultedfrom two different hydroxyl groups in the LDH phase. Thenarrow absorption about 3,635 cm−1 is attributed to stretchingvibration of hydroxyl groups (νOH) of the inorganic layers andthe board band centered at 3,538 cm−1 corresponds to a com-bination of the stretching vibration of the hydroxide groups andwater molecules between the layers [22, 23].

The absorption peaks at 1,625 and 970 cm−1 are assigned tothe bending vibrations of the interlayer water molecules andthe hydroxide groups in the brucite-like layers (δOH), respec-tively [23]. The peaks occurring at 1,358 and 701 cm–1 areassigned to the symmetric and antisymmetric bending vibra-tions of the interlayer hydroxide groups, respectively [24].The FTIR spectra further confirm the incorporation of intactOH anions into the LDH matrix. During the electrochemicalreaction, intercalated OH anions are inserted and deserted in

Table 1 Chemical compositionof ZnO coated [Ni4Al(OH)10]OHsamples

Sample, chemical formula Metal contents/wt.% obs. (calc.)

Ni Al Zn

L: [Ni3.9Al(OH)9.8]OH·5.5H2O 42.8 (42.5) 5.00 (5.01) 0.00 (0.00)

B1: [Ni3.9Al(OH)9.8]OH·5.5H2O·0.50ZnO 39.6 (39.7) 4.69 (4.70) 5.61 (5.60)

B2: [Ni3.9Al(OH)9.8]OH·5.5H2O·0.98ZnO 37.1 (37.2) 4.34 (4.36) 10.4 (10.3)

B3: [Ni3.9Al(OH)9.8]OH·5.5H2O·1.52ZnO 34.8 (34.7) 4.07 (4.07) 14.8 (14.9)

Table 2 Lattice parameters and particle sizes according to reflections of as-prepared samples and aged samples

Sample LDH (R3m) ZnO (P63mc)

Lattice parametersa (nm) Crystalline sizeb by reflection of (003) Lattice parameters (nm) Crystalline size by reflection of (100)

a (Å) c (Å) 2θ (°) FWHM (°) Particle size (nm) a (Å) c (Å) 2θ (°) FWHM (°) Particle size (nm)

L 3.058 23.62 11.22 0.4073 19.38 – – – – –

B1 3.056 23.56 11.18 0.4980 15.85 3.249 5.190 31.77 0.2470 33.07

B2 3.056 23.56 11.32 0.5023 15.72 3.246 5.180 31.96 0.2163 37.78

B2′ 3.072 23.59 11.21 0.2923 27.00 3.258 5.194 31.71 0.3501 23.33

B2″ 3.064 23.51 11.28 0.2016 39.15 3.259 5.194 31.70 0.3732 21.88

B2″′ 3.049 23.35 11.36 0.1766 44.70 – – – – –

B3 3.056 23.62 11.32 0.5325 14.82 3.245 5.178 31.97 0.1819 44.92

a Lattice parameters were obtained by Xfit and Chekcell softwaresb Crystalline sizes were calculated by using the Scherrer equation,D =Kλ /βcosθ whereK =0.89,D is the particle size, λ=0.15406 nm, β is the FWHMin radians, and θ is the Bragg angle of the reflection

32 J Solid State Electrochem (2014) 18:29–38

order to balance the charge resulted from the reduction andoxidation of Ni ions.

SEM characterizations

The morphologies of as-prepared samples and aged samples aretested by SEM, and their images are shown in Fig. 3. The SEMimages of ZnO show that the particles are sheet-like aggregatesand quite small about 100–150 nm in diameter (Fig. 3, a1 anda2). [Ni4Al(OH)10]OH is composed of piles of irregular roundishdisks (Fig. 3, b1 and b2), which is similar to our previous report

[14]. The SEM images of sample B1, B2, and B3 show that theparticles keep the shape of the precursor [Ni4Al(OH)10]OH;meanwhile, it is found that some small sheet particles are dis-persed unevenly on the surface of [Ni4Al(OH)10]OH (Fig. 3, c1to e2). Furthermore, the amount of small sheet particles increaseswith the increase of ZnO added in raw material, which is inaccordancewith the ICP data. Comparedwith Fig. 3 a1 and a2, itcan be concluded that these sheet particles in samples B1, B2,and B3 are contributed to ZnO.

In Fig. 3 f1, f2, g1, and g2, these small sheet particles stillexist, but Fig. 3 h1 and h2 could not find them. It seems thatB2′ and B2″ still contain ZnO phase, but B2″′ has not. In orderto certificate it, EDS spectra for the surface of the B2′, B2″ andB2″′ particles are shown in Fig. 4 and the element contents areshown in Table 3. It is found that the amount of Zn decreasesgradually with the increase of aging time, part of Zn ions enterthe lattice of DH, others occur in form of ZnO for B2′ andB2″; as the aging time attains 32 days, ZnO and Zn ions weredissolved thoroughly, which are in good accordance withXRD patterns.

Galvanostatic charge–discharge performances

Gravimetric discharge capacities are given as functions ofcycle numbers in Fig. 5a, and the NEE analyses of the same

Fig. 2 FTIR spectra of zinc oxide coated [Ni4Al(OH)10]OH samples

Fig. 3 SEM images of ZnO (a1, a2), [Ni4Al(OH)10]OH (b1 , b2), B1 (c1 , c2), B2 (d1, d2), B3 (e1, e2), B2′ (f1 , f2), B2″ (g1, g2), and B2″′ (h1 , h2)

J Solid State Electrochem (2014) 18:29–38 33

data are shown in Fig. 5b. In Fig. 5a, the electrode[Ni4Al(OH)10]OH exhibits the highest discharge capacity of343 mAh g−1, which takes place in the initial stage of electro-chemical cycle. However, after the 25 cycles the dischargecapacity decreases quickly. At 255th cycle, discharge capacitydecreases to 314 mAh g−1 and its deterioration rate is 8.5 %.Contrastively, for the surfacemodification electrodes of B1, B2,and B3, the discharge capacities increase rapidly and attain themaximum of 330.2, 315.2, and 297.5 mAh g−1 at 13th, 16th,and 16th cycle, respectively. At 255th cycle, discharge capac-ities decrease to 308, 298.5, and 285.5 mAh g−1 and theirdeterioration rates are respectively 6.7, 5.3, and 4.0 %, whichare lower than that of [Ni4Al(OH)10]OH. It implies that thesurfacemodification of ZnO on [Ni4Al(OH)10]OH can improvethe cycling stability.

It is well known that Ni is the key factor to influence theelectrochemical performances because the electron transfer de-pends on it. So, in order to study the utilization of Ni, the NEEswere calculated and the curves of NEE to cycle number areshown in Fig. 5b. The maximal NEE of B1, B2, and B3 is 1.83,1.87, and 1.88, respectively, which are larger than that of[Ni4Al(OH)10]OH (1.73). It indicates that the doping of ZnOcan obviously improve the utilization of active material. Inaddition, the NEE increases with the increase of the amount ofZnO coating. It suggests that coating zinc oxide on the surface of[Ni4Al(OH)10]OH may increase the surface of contact, thusimproving the conductivity of electrodes and discharge capacity.

Structural transformation during the charge–discharge cycles

In order to observe the transformation of the active materialduring the charge–discharge cycles, after the electrodes of[Ni4Al(OH)10]OH and B2 had been charged and dischargedfor 100, 200, and 255 cycles, respectively, they were washedwith water for several times, dried at 60 °C under vacuumcondition, and then detected by XRD. The results were shownin Fig. 6. The peak at about 18° is from PTFE, which is usedas binder in the electrode. Peaks indicated with Ni(111),Ni(200), and Co(111) are from two different conductingagents, i.e., metallic nickel powder and cobalt powder, respec-tively. After the two electrode were charged and discharged in7 mol L−1 KOH solution for 100 cycles, which takes about5 days, there is no obvious change in structure of LDH. Afterthe 200 cycles, which takes about 10 days, the small broad-ened peaks of β-Ni(OH)2 are observed in [Ni4Al(OH)10]OHelectrode, but do not appear in B2 electrode in which the LDHmaterial keeps its layered structure, but the intensity of dif-fraction peaks decreases greatly. After 255 cycles, which takesabout 12 days, the peaks ofβ-Ni(OH)2 are observed in both of[Ni4Al(OH)10]OH and B2 electrodes. Comparing the intensityof diffraction peak of β-Ni(OH)2 (001), it seems that the β-Ni(OH)2 formed is only in very small amount in B2 electrode.Moreover, obviously, under the condition of electrochemicalcycles, the transformation of LDH into β-Ni(OH)2 proceedsmore quickly; however, B2 sample still shows only LDH

Fig. 4 EDS spectra of regions Aand B of B2′, C of B2″, and D ofB2″′

Table 3 Results of EDS analysisfor the surface compositions ofB2′ at regions A and B, B2″ atregion C, and B2″′ at region D

Region Element

Weight (%) Atomic (%)

Ni Al Zn O C Ni Al Zn O C

A 63.07 20.14 16.79 26.64 34.77 38.60

B 51.88 8.66 13.89 25.58 29.30 10.64 7.04 53.01

C 71.11 6.75 9.68 12.46 50.71 10.48 6.20 32.61

D 84.47 5.99 9.54 63.76 9.84 26.41

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structure when soaked in 7 mol L−1 KOH solution for 32 days.Probably due to the structural transformation, the dischargecapacity of the active material drops. Such a transformationcould play a key role in the electrochemical behaviors of theelectrode, thus requiring a detailed inspection on the electro-chemical behaviors of the materials.

Cyclic voltammetric behaviors

Figure 7 gives the cyclic voltammograms of [Ni4Al(OH)10]OHdoped with different amount of ZnO by surface modificationat the scanning rate of 0.1 mV s−1 after 20 charge–discharge cycles. The results describing the features ofthe voltammograms are given in Table 4 in detail. Asseen in Fig. 7, two peaks appear when the electrodesare scanned catholically, which can be assigned to theoxidation potential EO and oxygen evolution potentialEOE, respectively. During the following anodic process,only reduction peak is observed, which can be assignedto the reduction potential ER.

From Table 4, all electrodes with ZnO show significantlylower EO and higher ER than the electrode without ZnO. Thedifference between EO and ER usually is taken as an estimateof the reversibility of the electrode reaction; accordingly, theabove results suggest that these electrodes with ZnO havebetter cycling reversibility than the electrode without ZnO.This may lead to more efficient charging, hence higher energytransformation efficiency. Especially for B3, the differencebetween EO and ER is the smallest, which indicates that ithas more excellent electrochemical reversibility.

The difference between EOE and EO for the electrodewithout ZnO is 38 mV, lower than those of electrodes withZnO. The large value allows the electrode to be charged fullybefore the oxygen evolution. Therefore, the electrodes withZnO can efficiently restrain the oxygen evolution reaction andimprove the charge efficiency. Moreover, with the increase ofthe amount of ZnO, the differences between EOE and EO

gradually increase.

Charge and discharge curves

The 28th charge–discharge curves for all electrodes are shownin Fig. 8. The charge curves of all electrodes have two plateaus

Fig. 5 (a) Gravimetric discharge capacity and (b) number of exchangeelectrons (NEE ) as functions of cycle number for electrode with[Ni4Al(OH)10]OH (L) and zinc oxide-coated [Ni4Al(OH)10]OH (B1,B2, and B3)

Fig. 6 XRDpatterns of a β-Ni(OH)2 (JCPDS 3-177); [Ni4Al(OH)10]OHelectrode: been charged and discharged forb100,d 200, and f 255 cycles,in comparison with B2 electrode: been charged and discharged for c 100,e 200, and g 255 cycles. The star shows that the (001) diffraction of β-Ni(OH)2

Fig. 7 Cyclic voltammograms of electrodes with [Ni4Al(OH)10]OH andzinc oxide-coated [Ni4Al(OH)10]OH

J Solid State Electrochem (2014) 18:29–38 35

apparently, corresponding to the oxidation reaction of Ni andthe oxygen evolution reaction. The charging potentials and theoxygen evolution potentials of electrodes with ZnO are higherthan that of the electrode without ZnO, which is in accordancewith above cyclic voltammetric results. The discharge poten-tials are little higher than that of electrode without ZnO.

The majority of the researchers regarded the reason for thesimilar improvement of electrochemical performances as thecontribution of the over-potential increase for oxygen evolu-tion reaction [25–27]. The oxygen over-potential is the keyfactor to affect electrochemical cycling stability of nickelelectrodes [28, 29]. The larger oxygen over-potential can leadto the higher charge efficiency and charge acceptance [30]. Inour study, both cyclic voltammograms (CV) and charge/discharge test results have shown that coating ZnO can effec-tively enhance the oxygen over-potential. Their CV andcharge–discharge curves show the same result which is thatthe oxygen over-potentials increase with the amount of ZnOcoated. It indicates that the improvement of electrochemicalperformance of nickel electrode is not only related to theeffects of additive itself but also depends on the amount anddistribution of ZnO on the surface [Ni4Al(OH)10]OH. Thecoating surface with dense and porous morphology, largerrelative surface content, and higher utilization ratio of zinc

are more effective on suppressing the oxygen evolution reac-tions and improving the electrochemical performances.

Electrochemical impedance spectroscopy behaviors

Figure 9a gives the EIS of all electrodes which were measuredat the 100 % depth of discharge at the open circuit potential. Amode of circuit is used to fit these experimental data as shownin Fig. 9b, in which R s is the ohmic resistance of the alkalielectrolyte, R t is the charge transfer resistance of the elec-trodes which is related to the ionic transportation, Q c is adouble layer capacitance, and Zw is the Generalized FiniteWarburg impedance of the solid phase diffusion. Figure 9ashows that each EIS curve comprises a depressed semicircle athigh frequency and a straight line at lower frequency. Thesemicircles at high frequencies (HF) are the characteristic ofthe charge-transfer resistance (R t) acting in parallel with thedouble layer capacitance (Qc) [31]. At low frequencies (LF),the straight lines having an angle with the real axis indicate alinear Warburg portion, being the characteristic of the semi-infinite diffusion.

In our studies, much smaller semicircles are observed forall electrodes with ZnO. But for L electrode, the semicircle isbigger, which means that the electron transfer impedance, R t

is bigger. This implies that the reaction occurring at the elec-trode without ZnO is under a charge-transfer control; howev-er, the electrode reaction occurring at the electrodes with ZnOis under joint control by the charge-transfer and proton diffu-sion processes [32]. At lower frequencies, a linear Warburgportion can be seen. The Warburg slope is interpreted here asan empirical parameter related qualitatively rather than quan-titatively to the diffusion resistance where a higher slope

Table 4 Summary of the characteristic potentials vs. Hg/HgO of all theelectrodes

Sample EO (mV) EOE (mV) ER (mV) EOE−EO

(mV)EO−ER

(mV)

L 621 659 268 38 353

B1 548 596 282 48 266

B2 553 600 292 47 261

B3 543 594 289 51 254

Fig. 8 The 28th charge–discharge curves for electrodes of B1, B2, andB3 compared to that of L electrode

Fig. 9 a Electrochemical impedance spectroscopy of electrodes of B1,B2, and B3 and compared to the electrode of L. b The equivalent circuitused for experimental result analyses

36 J Solid State Electrochem (2014) 18:29–38

signifies a slower rate of diffusion and a lower slope a morerapid rate of diffusion. In Figure 9a, it can be found that theslopes of the electrodes with ZnO are basically lower than thatof the electrode without ZnO, suggesting that the doping ofZnO enhances the ions diffusion [33, 34].

In order to compare clearly these experimental data fromEIS, the fitted experimental parameters obtained by ZView.2software are shown in Table 5. According to the equivalentcircuit, it is obvious that the values of R s, R t, Qc, and Zw forelectrodes with ZnO are basically lower than that of theelectrode without ZnO. The Q c is related to double electriclayer and may be affected by surface roughness of solidelectrodes [35, 36]. Based on this view of point, it seems thatthe electrodes becomemore porous as the doping of ZnO. Ourprevious report [15] mentioned that the loss of aluminumcould become more serious with the increase of charge–dis-charge cycles, which causes cracks and instability of[Ni4Al(OH)10]OH. We thought that similar thing happenshere. The decrease of R t implies that electrochemical polari-zation decreases with the doping of ZnO. The electrochemicalpolarization is correlated with the discharge potential (thesmaller polarization, the larger discharge potential) [37]. Thus,the results of EIS suggest that the discharge potentials ofelectrodes with ZnO are higher than the electrode withoutZnO, which is basically in accord with the results obtainedfrom Fig. 8. In a word, these experimental parameters of EISimply that the doping of ZnO can enhance charge-transfer andproton diffusion, thus improving the electrochemical perfor-mances of electrodes.

Discussion

The entire data reported above show that ZnO coated[Ni4Al(OH)10]OH samples have better electrochemical per-formances. We believe that it is resulted from the structuralstability, fast anion exchange capability, and fast anion trans-portation of OH−.

Firstly, during the charge–discharge process, part of ZnO isdissolved in KOH solution, and then, some Zn2+ ions in theelectrolyte may insert into the lattice of LDH [38]. The incor-poration of Zn2+ ions can improve the structural stability [17,31, 39]. Due to the incorporation of Zn2+ ions, more anions(OH−) have to be intercalated to the interlayer of LDH in order

to keep charge neutrality. In the oxidation process, some ofprotons drop off from the hydroxyl and react with the originaland entered interlayer OH− anions to produce water mole-cules. That process prevents excessive accumulation of OH−

anions. As a result, more water molecules are accumulated inthe interlayer space of LDH. The water molecules either moveout of the interlayer space or form hydrogen bonds with thelayer oxygen atoms to compensate their loss of hydrogenatoms and thus stabilize the lamellar structure. When thematerial is reduced, the layers absorb protons directly fromthe interlayer water and generate more OH− anionswhich will move away from the interlayer space to keepits electric neutrality.

Secondly, it is known that H+ and OH− run the fastest inaqueous solution because there is a relay system [31], in whichneighbor water molecules exchange protons very rapidly. Theincrease of additives makes more space for water molecules,which strengthens the relay system of the transportation of H+

[40]. Therefore, the proton diffusion is enhanced and thecharge-transfer resistance is decreased with increase of ZnOadditives. The relay system is always taking place while thematerial is oxidized or reduced and makes the OH− anions runvery fast in or out as required by the reaction, which improvesthe ionic conductivity of materials.

Thirdly, the defined interlayer space could help the migra-tion of OH− anions in and out of the interlayer space of LDH.The materials must have channels for fast movements of ionsand electrons, since the speed of the electrode reactions iscontrolled by both their inherent properties and the flux of theion and electron currents [41]. The incorporation of Zn2+ ionsmakes more OH− anions and water molecules to be interca-lated in the interlayer space, thus probably enlarging theinterlayer space which can be proved by the increase of cvalue of B2′ and B2″ samples in LDH phase.

Conclusion

In this paper, samples of [Ni4Al(OH)10]OH coated with ZnOwere prepared by a chemical precipitation method, and theirstructure and electrochemical performances were compared.XRD and SEM images of the surface modification samplesshow that themodification of ZnO has little effects on the latticeparameters and the particle sizes of the [Ni4Al(OH)10]OH, butdoes change the morphology. Based on the electrochemicalperformances, it is found that ZnO can improve the electro-chemical reversibility of [Ni4Al(OH)10]OH and can increasethe oxygen evolution potential and the utilization of activematerial. The addition of ZnO promotes NEEs. The maximalNEEs for the electrodes with ZnO are basically over 1.83, whilethat for the electrode without ZnO is 1.73. EIS studiesshow that the charge transfer resistance decreases as theamount of ZnO increases.

Table 5 The fitted EIS experimental parameters of all electrodes

Sample Rs (Ω cm2) R t (Ω cm2) Zw (Ω cm2) Qc (F cm2)

L 2.552 1.021 0.259 2.758

B1 1.117 0.115 0.161 2.427

B2 1.263 0.108 0.138 1.168

B3 1.269 0.097 0.181 1.057

J Solid State Electrochem (2014) 18:29–38 37

Acknowledgments Wewould like to thank the National Science Foun-dation of China (no. 51202054, 21206026, and 81271665), NaturalScience Foundation of Hebei Province (no. B2012402006 andB2012402011), and Handan City Science and Technology Researchand Development Project of China (no. 1221120095-4) for financialsupports.

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