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Ceramics International ] (]]]]) ]]]–]]]www.elsevier.com/locate/ceramint

Cu-doped NiO for aqueous asymmetric electrochemical capacitors

Guohui Yuana,n, Yunfu Liua,b, Min Yuec, Hongju Lia, Encheng Liua, Youyuan Huangc,Dongliang Kongc

aSchool of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin 150001, ChinabCollege of Resources and Environmental Engineering, Heilongjiang Institute of Science and Technology, Harbin 150027, China

cShenzhen BTR New Energy Materials Inc., Shenzhen 528206, China

Received 3 January 2014; received in revised form 24 January 2014; accepted 25 January 2014

Abstract

In this work, the influence of Cu doping on the electrochemical properties of NiO was investigated regarding its performance as electrodes inaqueous supercapacitors. NiO and Ni1�xCuxO (x¼0.5–3%) cathode materials were synthesized via a citrate-gel process. Ni1�xCuxO sampleshave the same crystal structures as those of the un-doped NiO, indicating that copper atoms enter into the lattice of nickel mono-oxide. Comparedto NiO, Cu-doping materials significantly improve the specific capacitance and rate capability. Especially, among all of the investigated samples,Ni0.99Cu0.01O presents the best electrochemical performance as high as 559 and 389 F g�1, which are 90.14% and 86.12% higher than those ofNiO at 0.3 and 10 A/g, respectively. This work demonstrated that Cu-doping is an effective way to improve the pseudo-capacitance of nickelmonoxide.& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Citrate-gel process; Cu-doping; Electrochemical capacitor; Nickel oxide

1. Introduction

Electrochemical capacitors (ECs) were the typical represen-tative of high-power technologies with the characteristics ofhigh power capability, super-stable cyclibility, and low energydensity when compared to common batteries [1,2]. Theelectrode materials with quick Faradic reactions, such astransition metal oxides, have been widely employed toimprove energy density of ECs [3–5]. For example, theamorphous ruthenium oxides were one of the most famousmaterials in this purpose due to their quick kinetics and highcapacitance [6,7]. However, the extremely high cost ofruthenium seriously restricts its wide implementation in ECs.

In recent years, transition metal based oxides includingnickel oxides, cobalt oxides, and manganese oxides have beenextensively investigated as alternative electrode materials forsymmetric ECs. (The asymmetric design is an attractiveapproach to increasing the energy density of ECs as it canlead to an almost doubling of device capacitance [8].) Among

e front matter & 2014 Elsevier Ltd and Techna Group S.r.l. All ri10.1016/j.ceramint.2014.01.124

g author. Tel./fax: þ86 451 86413721.ss: ygh@hit.edu.cn (G. Yuan).

article as: G. Yuan, et al., Cu-doped NiO for aqueous asymmetricint.2014.01.124

them, NiO received the considerable attentions due to itsinexpensive characteristic, high capacitance and quick kineticssimilar to amorphous RuO2 [8–11]. Importantly, it can furtheraccelerate the commercial process and/or widen the workingfields of electrochemical capacitors [12–14].As the pseudo-capacitance is greatly dependent on the

interfacial structure of the electrode materials, the mosteffective way for high-capacitance materials is to preparehigh-surface-area electrode materials. Various synthetic meth-ods have been developed to prepare nickel oxides/hydroxide,including electrochemical deposition [15–18], chemical pre-cipitation [19,20], liquid-phase synthesis [21,22], hydrothermalmethod [23,24], sol–gel technique [25], and template methods[26]. Another efficient way is to prepare binary/multi-phasemetal compounds through doping [27–29]. Yu et al. [30]synthesized Ni(OH)2 doped with Cu by solid-state reaction atlow temperature, which presented the much improved capaci-tive behaviors. The possible reasons are attributable to thedoping of copper doping effect, rather than the direct con-tribution from CuO [31,32].In this study, for the first time, we investigated the influence

of Cu doping on capacitor performance of NiO. In doing so,

ghts reserved.

electrochemical capacitors, Ceramics International (2014), http://dx.doi.org/

0 150 300 450 6000

25

50

75

100

-100

-50

0

50

100

3rd stage

2nd stage

1st stage

TG /

%

Temprature / °C

TGDTA

DTA

/ uv

Fig. 1. TG–DTA curves of dried citrate-gel for nickel oxide precursors.

G. Yuan et al. / Ceramics International ] (]]]]) ]]]–]]]2

NiO and Ni1�xCuxO samples were synthesized by the citrate-gel process. The prepared materials were characterized byXRD and SEM. In order to determine their supercapacitorsperformance, electrochemical characteristics of the materialswere investigated by cyclic voltammetry and galvanostaticdischarge–charge in 6.0 M KOH solution.

2. Experimental

2.1. Preparation of various nickel oxides

Given amounts of citric acid were dissolved in distilledwater, and then acetic nickel and copper were added. Thesolution was heated at 60–70 1C to form waterish citrate-gel.The prepared waterish citrate-gel was then dried at 80 1C for12 h in a vacuum oven to form dried citrate-gel as a nickeloxide precursor. After heating the dried citrate-gel powder atdifferent temperatures, the NiO or Ni1�xCuxO was obtained.

2.2. Electrodes preparation

The electrodes used for studies were fabricated as following.NiO or Ni1�xCuxO was used as the active material. Graphitepowders were mixed with the active material so as to achievegood electrical conductivity. The mixture containing the activematerial and graphite powders was then added to a solution ofcarboxymethylcellulose (CMC) and poly(tetrafluoroethylene).The suspension was subsequently spread onto a currentcollector. After drying it at a temperature of 120 1C, theelectrode was pressed to form a tablet of 20� 10� 0.8 mm3 insize. As a result, the electrodes consist of 80% active material,10% graphite powder, 3% carboxymethylcellulose and 7%poly(tetrafluoroethylene). The current collector was 1.6 mmnickel foam of 110 ppi and 450 g cm�2 surface density.

6.0 M KOH solution was used as the electrolyte in thisexperiment at a temperature of 29871 K. All potentials in thispaper are referred to HgO/Hg reference electrode immersed inKOH of the same concentration as the experimental electro-lyte. The high-surface-area activated carbon electrode wasused as the counter-electrode.

2.3. Characterization

Cyclic voltammetry was carried out in a three-electrode cell,using a computer-controlled potentiostat/galvanostat(EG&G273A). The charge/discharge tests at constant currentdensity were carried out on unit cell capacitors, which wereconstructed with an electrolyte impregnated microporouspolypropylene film sandwiched between the cathode andanode. Thermogravimetry–differential thermal analysis (TG–DTA) on dried citrate-gel for the nickel oxide precursor wascarried out using a ZRY-2P Synthesis thermal Analyzer madein Shanghai China. The crystal structure of NiO and Ni1�x-

CuxO was characterized by an XRD-6000 diffractometer madein Japan. Scanning electron microscopy (SEM) was performedusing a HITACHI S-4700 FE-SEM field emission highresolution scanning electron microscope.

Please cite this article as: G. Yuan, et al., Cu-doped NiO for aqueous asymmetric10.1016/j.ceramint.2014.01.124

3. Results and discussion

3.1. TG–DTA analysis on dried citrate-gel for the nickel oxideprecursor

In order to determine the optimum heating treatmenttemperature, dried citrate-gel for the nickel oxide precursorwas investigated by TG–DTA analysis as shown in Fig. 1. Thedecomposition of dried citrate-gel for the nickel oxide pre-cursor involved three stages. In the first stage, from roomtemperature to 180 1C, the dried citrate-gel lost its water andthere was about 8% weight loss. It was an endothermicprocess. During the second stage, from 180 1C to 310 1C,the weight loss was obviously increased to 44%. There was anexothermic peak at about 270 1C, which means a change fromcitrate to carbonate. During the third stage, from 310 1C to340 1C, the weight loss was about 30%. There was a bigexothermic peak at 330 1C, which was related to the decom-position of the carbonate [33]. Then, the TG–DTA curveremained stable at temperatures higher than 340 1C, suggestingcompletion of the decomposition of dried citrate-gel. The datadiscussed above indicated that the heating treatment tempera-ture for the dried citrate-gel should be higher than 340 1C.

3.2. Effects of heating temperature

Three nickel oxide samples were obtained by heat treatingdried citrate-gel at the temperatures of 350 1C, 400 1C and500 1C for 1 h. Fig. 2 shows the XRD patterns of the nickeloxides obtained at different temperatures. In the case of350 1C, the marked curve in the Fig. 2 belongs to NiOaccording to the JCPDS cards. The space group of the sampleis Fm-3m. The lattice constants a, b and c are equal to0.4177 nm. Peaks at 38.281, 43.261 and 62.921 of 2θ corre-spond to (111), (200) and (220) diffraction of NiO,respectively.The XRD patterns of samples obtained at 400 1C and

500 1C were the same as that of sample obtained at 350 1C.These results indicated that identical crystalline nickel oxideswere obtained by heat treatment at temperature higher than350 1C.

electrochemical capacitors, Ceramics International (2014), http://dx.doi.org/

20 40 60 80

(222)(311)

(220)

(200)(111)

NiO

Ni0.995Cu0.005O

Ni0.99Cu0.01O

Ni0.98Cu0.02O

inte

nsity

2 theta / degree

Ni0.97Cu0.03O

Fig. 2. XRD patterns of NiO and Ni1�xCuxO obtained at 350 1C.

G. Yuan et al. / Ceramics International ] (]]]]) ]]]–]]] 3

Fig. 3 shows the SEM images of the nickel oxides obtainedat 350 1C, 400 1C and 500 1C. It can be seen that the particlesizes of NiO are between 30 and 150 nm. Obvious particlesagglomerate is observed in Fig. 3(b) and (c), indicating that thehigher the heating temperature, the more severe the agglom-eration of the particles.

Fig. 4 shows the CV curves of various nickel oxidesobtained at 350 1C, 400 1C and 500 1C, at a potential rangingfrom �0.400 to 0.700 V. These CV curves indicate that all thesamples exhibit pseudcapacitive behavior. The lowest capaci-tive current appears on the sample heated at 500 1C, whichmay be due to the agglomeration. However, the currentmeasured for the sample heated at 350 1C is lower than thatat 400 1C. In addition, the electrochemical reversibility isbetter than that at 400 1C, which may be related to the lessagglomeration morphology. Hence, the highest capacitivecurrent response was obtained on the sample heat treated at350 1C.

Fig. 3. SEM images of nickel oxides obtained at (a) 350 1C, (b) 400 1C, and(c) 500 1C.

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8

-10

0

10

20

Cur

rent

den

sity

/ A

g-1

Potential / V

350°C400°C500°C

Fig. 4. Cyclic voltammograms of various nickel oxides obtained at differenttemperatures.

3.3. Effect of doping amount of Cu on capacitance of nickeloxide

Nickel oxides doped with four different Cu contents rangedfrom 0.5% to 3% were prepared using the heat treatment at350 1C. The XRD patterns of Cu-doped NiO are alsocompared in Fig. 2. According to the JCPDF cards, all thesamples are the face centered cubic crystal structures, whichare the same as NiO. No second phase was found on thesamples, indicating that no CuO formed during the heattreatment.

The lattice constants of NiO and NiO doped Cu werecalculated from XRD patterns. For NiO, a, b, or c is 4.17 Å,but for the Cu-doped NiO, a, b, or c is between 4.17 Å and4.18 Å. Some changes of crystal lattice related to the latticedistortion were found on Cu-doped NiO.

The SEM images show that the particle shape and size ofNi0.98Cu0.02O and Ni0.97Cu0.03O are similar as shown in Fig. 3(b) and (c), respectively. Their particle size was larger than100 nm. In contrast, Ni0.99Cu0.01O and Ni0.995Cu0.005O weresimilar to those shown in Fig. 3(a). The results indicate that the

Please cite this article as: G. Yuan, et al., Cu-doped NiO for aqueous asymmetric electrochemical capacitors, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.01.124

0.0 0.2 0.4 0.6 0.8

-20

0

20

40

60

50th cycle 1st cycle

Cur

rent

den

sity

/ A

g-1

Potential / V

Fig. 5. Cyclic voltammograms of Ni0.99Cu0.01O.

0 100 200 300 400 500

0.2

0.3

0.4

0.5 Ni0.99Cu0.01O Ni0.995Cu0.005O Ni0.98Cu0.02O Ni0.97Cu0.03O NiO

Pot

entia

l / V

(vs.

HgO

/Hg)

Times / s

Fig. 6. Galvanostatic discharge curves of NiO and Ni1�xCuxO.

0 15 30 45

150

300

450

600

NiO

Ni0.99Cu0.01O

Ni0.995Cu0.005O

Ni0.98Cu0.02O

Ni0.97Cu0.03O

Spe

cific

cap

acita

nce

/ F g

-1

Cycle number

Fig. 7. Relationship between specific capacitances of cathodes and cyclenumbers.

Table 1Comparison of specific capacitance between Ni0.99Cu0.01O and NiO.

Samples 0.3 A g�1 1 A g�1 5 A g�1 10 A g�1

Ni0.99Cu0.01O 559 526 476 389NiO 294 275 251 209

G. Yuan et al. / Ceramics International ] (]]]]) ]]]–]]]4

agglomeration of the Cu-doped NiO results from excess Cucontent.

Fig. 5 shows the representative CV curves of Ni0.99Cu0.01O,where the scanning potential range was from �0.000 to0.700 V. The capacitive current response of the Cu-doped

Please cite this article as: G. Yuan, et al., Cu-doped NiO for aqueous asymmetric10.1016/j.ceramint.2014.01.124

NiO at 50th cycle was obviously larger than that at 1st cycle.This indicates that the specific capacitance of the nickel oxideis increased with cycle numbers. Fig. 6 shows the representa-tive galvanostatic discharge curves of NiO and Ni1�xCuxO,where the samples were used as a cathode in supercapacitorand the constant current was 0.3 A g�1. No obvious linearitywas found in these curves and all the samples were pseudca-pacitive. The capacitances of all Ni1�xCuxO were higher thanthose of NiO. It was not found that the capacitance ofNi1�xCuxO was enhanced with the doping amount of Cu.This indicates that small quantities of Cu in NiO did notchange the specific capacitance, which is in good agreementwith Yu's research [30]. The enhancement on the specificcapacitance of an Ni1�xCuxO electrode may be related tolattice distortion of NiO due to the Cu doping. The capacitanceof Ni0.98Cu0.02O and Ni0.97Cu0.03O was less than that ofNi0.99Cu0.01O, which may be related to the agglomeration ofthe sample.Fig. 7 shows the relationship between specific capacitances

of cathodes made of NiO or Ni1�xCuxO (x¼0.5–3%) andcycle numbers. It was found that the specific capacitances ofall electrodes were increased with cycle numbers at the initialstage, which is the same as the results of CV measurements.This result means that the enhanced capacitances can beobtained on cathodes made from NiO and Ni1�xCuxO afteractivation by charge–discharge cycles. In general, the capaci-tances were stable when the cycle number was about 10. Thespecific capacitance of Ni0.99Cu0.01O and NiO at differentconstant currents is compared in Table 1. The specificcapacitance of the Ni0.99Cu0.01O electrode was 559 F g�1,90.14% higher than that of NiO electrode. At the highercurrent density of 10 A/g the Ni0.99Cu0.01O electrode stillexhibits a capacitance of 389 F g�1, 86.12% higher than thatof NiO.

4. Conclusions

The influence of copper doping on supercapacitors perfor-mance of NiO was investigated in this work. Cu doping plays apromotional role in enhancing NiO behavior in electrochemi-cal capacitors. Ni0.99Cu0.01O presented the best electrochemi-cal performance among the materials studied in this work. Itdelivered 559 F g�1 at 0.3 A/g, holding a promise to beapplied in aqueous asymmetric electrochemical capacitors.

Acknowledgments

The authors gratefully acknowledge financial support fromthe Natural Science Foundation of China (21076050) andNational Science and Technology Support Program of China(2013BAE04B04).

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