张 瑶 王春梅 孙鑫垚 高媛媛 高艳芳

(1内蒙古工业大学化工学院，呼和浩特 010050)

(2内蒙古工业大学大规模储能技术教育部工程研究中心，呼和浩特 010050)

Batteries and capacitors are considered the most efficient and practical storage methods in the field of accumulation of electric energy[1-5]. Supercapacitors have important research value because of their higher power density than various types of secondary batteries in energy storage technologies[6-8]. At present,the applications of supercapacitors are restricted by the narrow potential window and low specific capacitance during the energy storage process,which leads to the low energy density of supercapacitors. Therefore, the research focuses on increasing the energy density and maintaining the high power of supercapacitors. As an important part of asymmetric supercapacitors,the electrode determines the electrochemical performance of supercapacitors[9-10]. Therefore, it is an effective means to improve the electrochemical performance of supercapacitors to enhance the capacitance characteristics of electrodes.

As the cathode materials for supercapacitors,nickel- and cobalt-based oxides have gained wide attention because of their high theoretical capacitance and simple synthesis methods[11-13]. In the research of improving the specific capacitance of electrode materials, the researchers focus on changing the morphology of nickel-cobalt compounds and trying to improve the electrochemical properties of the materials by increasing the specific surface area of the materials[14-17]. However,in the energy storage process,the electrochemical performance of the electrode is not only related to its morphology and external reaction conditions but also depends on the structure and properties of the electrode materials. The electrochemical performances of nickel - cobalt compounds are determined by the amount of redox active sites, electronic conductivity,and wettability between electrode and electrolyte.Therefore, improving the structure of nickel - cobalt compounds can improve the electrochemical performance of nickel-cobalt-based electrodes.

The theoretical capacitance of NiO reaches up to 2 584 F·g-1[18]. However, the capacitance rate and cycle stability of NiO is poor due to its low electrical conductivity. Cobalt oxide has good electrical conductivity and exhibits excellent electrochemical properties in alkaline electrolytes. Therefore, it is extremely important to design and prepare new electrode materials with nickel-based high specific capacitance,cobaltbased high conductivity, and high exposure active site.According to the above analysis,Kong et al.successfully prepared Co0.56Ni0.44oxide nanosheets by a simple chemical coprecipitation. Electrochemical tests showed that bimetallic oxides exhibited advantages over monometallic oxides in energy storage[19]. Then, Lu and co-workers prepared NiCo2O4aerogel by sol-gel method, and NiCo2O4aerogel showed excellent electrochemical performance at a high loading of 0.4 mg·cm-2[20].The results show that the double transition metal electrode materials have more pseudocapacitance reactions than the single transition metal oxide electrode materials, which can effectively improve the electrochemical performance of the single transition metal oxide electrode materials.

The above studies indicate that double transition metal oxides prepared by the combination of Ni and Co transition metals can increase the electrode pseudocapacitance reaction, improve the retention rate of the electrode materials at high current density, and effectively improve the electrochemical performance of the electrode materials. However, the relationship between the rate of metal cations in bimetallic nickel-cobalt oxides and their electrochemical properties has rarely been reported[21].Herein,the nickel-cobalt oxide (NCO)nanosheets were synthesized by adjusted via arrangement of the molar ratios of nickel and cobalt ions in precursor solutions and further applied to asymmetric supercapacitors.

1 Experimental

1.1 Chemicals

Acetone was purchased from Tianjin Bohua Chemical Reagent Co., Ltd. Hydrochloric acid and potassium hydroxide were purchased from Sinopharm Chemical Reagent Co., Ltd. Cobaltous nitrate hexahydrate,nickel nitrate hexahydrateN-methylpyrrolidone were purchased from Shanghai Aladdin Reagent Co.,Ltd.

1.2 Material synthesis

All reagents used in the experiments were of analytical grade and used as received without further purification. Ni foam was cut into 1 cm×2 cm pieces. The cut Ni foam was washed using CH3COCH3and 1 mol·L-1HCl to remove the oxide and oil on the surface of it.

1.3 Synthesis of NCOs

Ni(NO3)2·6H2O and Co(NO3)2·6H2O were dissolved into 30 mLN-methylpyrrolidone (NMP). The molar ratios of Ni(NO3)2·6H2O and Co(NO3)2·6H2O were controlled at 1∶2(2 and 4 mmol,respectively),1∶1(3 and 3 mmol,respectively),and 2∶1(4 and 2 mmol,respectively), which were named as NCO-1, NCO-2,and NCO-3, respectively. The cleaned Ni foam was dropped into the above solution under vigorous stirring for 30 min. After stirring, the resulting solutions were transferred into 50 mL Teflon-lined stainless autoclaves and kept at 180 ℃for 12 h. Shortly afterward,the Ni foam substrates, which were covered with the precursor,were collected,washed,and followed by vacuum drying overnight at 80 ℃. After the reaction, the Ni foam substrates were annealed at 350 ℃in air for 3 h to obtain the NCOs.

1.4 Characterization of NCOs

X-ray diffraction (XRD) patterns were collected on a Bruker D8-Advance using a CuKαradiation (λ=0.154 06 nm), a current of 40 mA, a voltage of 40 mV,a scan speed of 3(°)·min-1,and a scan range of 5°-80°.The morphology and composition elements of the asobtained products could be observed by scanning electron microscope (SEM, S-4300, Hitachi) with an accelerating voltage of 20 kV. The surface chemical composition of NCOs was investigated by X-ray photoelectron spectroscopy (XPS, Kratos Amicus, Kratos Group PLC)through X-radiation MgKαwith an accelerating voltage of 12 kV. The inductively coupled plasma (ICP,Agilent, 725) with 1 150 W, and peristaltic pump speed 50 mL·min-1was used for quantitative analysis of metal composition in NCOs.

1.5 Preparation of electrode

To fabricate working electrodes, the as-prepared NCOs were pressed at 10 MPa for 60 s. The active material loadings of the NCO-1, NCO-2, and NCO-3 electrodes were 5.1, 4.7, and 5.3 mg, respectively. The activated carbon (AC) loadings of the electrodes were 4.5 mg.

1.6 Electrochemical measurements

The electrochemical measurements of the NCOs electrodes were carried out using a standard threeelectrode testing system with 6 mol·L-1KOH solutions.The NCOs were used as working electrodes. A platinum mesh (2.5 cm×2.5 cm) and Ag/AgCl electrode were used as the counter electrode and reference electrode, respectively. The above standard three-electrode testing system was immersed into 6.0 mol·L-1KOH solution for 30 min to enhance the wetness degree.Electrochemical impedance spectroscopy (EIS) was carried out on Parstat 2273 (Princeton Applied Research, America), and tested in a frequency range from 105-10-2Hz at modulation of 5 mV at the open circuit potential. Cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD)measurements were conducted on the CHI 760E (Chenhua, China) electrochemical workstation with the potential window 0.1-0.5 V. The cycling stability tests were performed on a LAND battery program-control test system. The specific capacitances (Csp) of NCOs were obtained from the formula as follows：

whereIis the discharge current,mis the mass of active materials, Δtis the discharge time, and ΔVis the potential window.

The energy density(E,Wh·kg-1)and power density(P, W·kg-1) of the NCOs asymmetrical supercapacitors were calculated as follows：

whereCspis the specific capacitance of the cell evaluated from the Eq.1, which is based on the total mass of active materials in a two-electrode system, andVis the cell voltage window.

2 Results and discussion

2.1 Structure and morphology characterizations of NCOs

The contents of Ni and Co of NCO-1, NCO-2, and NCO-3 were detected using ICP spectroscopy (Table 1). The contents of Ni and Co in the NCOs changed with the feeding molar ratios of Ni to Co. According to ICP, the atomic content percentages of Ni and Co in NCO-1, NCO-2, and NCO-3 were 1∶2 (Ni1Co2Ox), 1.5∶1.54 (Ni1Co1Ox) and 1.95∶1 (Ni1.95Co1Ox), respectively,which tallied with the theoretical values of Ni and Co in NCO-1,NCO-2,and NCO-3.

Table 1 ICP analyses of bimetal nickel and cobalt oxides

The crystalline structures of NCOs were characterized using an XRD as shown in Fig.1. As can be seen from Fig.1, the intensities of diffraction peaks of NCO-1, NCO-2, and NCO-3 were weak with increasing the Ni2+concentration, which is attributed to the increase in the disorder of atomic arrangement of NCOs with ionic radius difference between nickel (0.069 nm) and cobalt (0.745 nm). Furthermore, the full widths of the half-maximum (FWHM) of the NCOs were wide, which resulted in low crystallinity. The FWHM values of the NCOs, namely NCO-1 (80.8, 40.6, 52.6, 49.9, 59.6,45.5, and 43.7), NCO-2 (62.9, 39.7, 42.0, 52.6, 49.4,47.8, and 56.4), and NCO-3 (82.7, 37.1, 31.1, 59.8,54.8, 43.2, and 54.1), respectively, indicate variations in their crystal structures. The diffraction peaks, corresponding to NCO-1 (31.2°, 36.8°, 44.9°, 55.4°, 59.6°,64.9°, and 76.6°), NCO-2 (31.3°, 36.7°, 43.7°, 55.6°,59.1°, 64.6°, and 75.3°), and NCO-3 (31.1°, 37.1°,43.2°, 55.1°, 59.3°, 62.8°, and 75.5°), correspond to the (220), (311), (400), (422), (511), (440), and (533)planes of the rhombohedral structure of NiCo2O4(PDF No.02-1074), respectively. These observations highlight slight differences in the relative intensity and position of the crystal plane peaks among the NCO samples, resulting from minor adjustments in the feeding ratios of Ni and Co.Consequently,the crystal structures of the NCOs exhibit inherent distinctions from one another.

Fig.1 XRD patterns of NCO-1,NCO-2,and NCO-3

The morphologies and structures of NCO-1,NCO-2,and NCO-3 were observed by SEM. As can be seen from Fig.2a-2c, the SEM images of NCO-1, NCO-2,and NCO-3 exhibited nanosheet structures that were slightly wrinkled and interlaced. With the increase of Ni2+concentration in the initial solution, the size of NCOs nanosheets decreased from 200-300 nm(NCO-1) to 100 nm (NCO-2), and the porosity of the nanosheets increased. However, the nanosheets of NCO-3 overlapped with the feeding molar ratio of Ni2+and Co2+at 2∶1, which is shown in the black circle of Fig.2f.

Fig.2 SEM images of(a,c)NCO-1,(b,d)NCO-2,and(c,f)NCO-3

To effectively determine the surface composition and elemental valence of NCOs, NCO-1, NCO-2, and NCO-3 were characterized by XPS. As shown in Fig.3,the XPS spectra of NCO-1, NCO-2, and NCO-3 all showed the presence of Ni, Co, and O elements. In addition, the NCOs showed a C1sspectrum, which is attributed to that the NCOs were exposed to air to adsorb a small amount of CO2[22]. Fig.4 shows the XPS spectra of Ni and Co elements in NCO-1, NCO-2, and NCO-3.The peaks of Ni2pand Co2pof NCO-1,NCO-2,and NCO-3 were divided and fitted by XPSPEAK41 software. The Ni2pspectra of NCO-1, NCO-2, and NCO-3 in Fig.4a, 4c, and 4e showed two major peaks at 854.1 and 871.4 eV, 852.4 and 871.1 eV, and 852.4 and 871.0 eV accompanied with two shakeup satellites,respectively, which indicated the presence of Ni2+. Furthermore, the peaks of Ni were located at 855.9 and 872.8 eV, 854.2 and 871.7 eV, and 854.0 and 875.5 eV,respectively,which identified the existences of Ni3+in NCO-1, NCO-2, and NCO-3. Moreover, the highresolution Co2pspectrum of NCO-1 (Fig.4b) could be divided into four cobalt peaks which are located at 779.6 and 780.7 eV (corresponding to Co2+), 794.8 and 796.4 eV (corresponding to Co3+), respectively. The peaks of Co2pwere located at 778.2 and 793.4 eV(NCO-2： Co2+), 779.9 and 795.2 eV (NCO-2： Co3+),778.1 and 793.1 eV (NCO-3： Co2+), 779.6 and 794.9 eV (NCO-3： Co3+), respectively, which also indicated the existences of Co2+and Co3+in NCO-2 and NCO-3.The fitted data of NCO-1, NCO-2, and NCO-3 exhibited the peak positions of Ni2+, Ni3+, Co2+, and Co3+had minor changes with changing the feed ratios of Ni2+and Co2+,which were consistent with XRD results.

Fig.3 XPS survey spectra of NCO-1,NCO-2,and NCO-3

Fig.4 XPS spectra of NCO-1(a,b),NCO-2(c,d),and NCO-3(e,f)：Ni2p(a,c,e)and Co2p(b,d,f)

Table 2 Peak positions of Ni and Co elements of NCO-1,NCO-2,and NCO-3 eV

2.2 Electrochemical performance of NCOs

The CV and GCD measurements were conducted to evaluate the electrochemical performances of the as-prepared electrodes. Fig.5a shows the CV curves of the NCOs at 2 mV·s-1with a potential window of 0.0 to 0.5 V. The CV curves of NCO-1, NCO-2, and NCO-3 exhibited a pair of redox peaks, respectively, which presented that the pseudo capacitive characteristics were conducted through the faradaic process of Ni（Ⅱ）Ni（Ⅲ）and Co（Ⅱ）⇌Co（Ⅲ）⇌Co（Ⅳ）.As shown in Fig.5b,the GCD curves of NCOs presented at a current density of 0.5 A·g-1, demonstrated that a typical Faraday reaction occurred at the interfaces of electrodes with KOH solution. Furthermore, the GCD curves of NCOs all showed obvious plateaus at 0.2-0.3 V, which were consistent dollar-for-dollar with the results of the CV curves of NCOs.According to Eq.1,the specific capacitances of the NCO-1,NCO-2,and NCO-3 were 1 039.58,1 096.88, and 661.46 F·g-1at the current density of 0.5 A·g-1, respectively, which were suggested the specific capacitance of cobalt oxide was increased with an appropriate amount of nickel ion into the crystal lattice of cobalt oxide. Moreover, NCO-2 nanosheets with a suitable length and loose structure improved the electrochemical energy storage property owe to the high contact degree between the electrolyte solution and NCO-2 electrode, which were compared with the NCO-1 electrochemical property. Meanwhile, NCO-3 has reduced the specific capacitance because of the agglomerated nanosheets. The rate capabilities of NCO-1,NCO-2, and NCO-3 were discussed by GCD measurements at different current densities (0.5, 1, 2.5, 5, and 10 A·g-1) in Fig.5c. The specific capacitance values of NCO-1 were 1 039.58, 1 020.83, 1 000.00, 979.17,and 958.33 F·g-1, respectively, whereas for NCO-2, it was 1 096.88,1 037.50,973.96,937.50,and 916.67 F·g-1,respectively,and for NCO-3,it was 661.46,616.67,583.33, 572.92, and 541.67 F·g-1, respectively. The above specific capacitance values of the NCOs at different current densities indicated that the capacity retention ratios of NCO-1, NCO-2, and NCO-3 were 92.2%,83.57%, and 81.89%, respectively. EIS measurements were carried out to understand the molar ratios of Ni ions and Co ions in NCOs on the electronic conductivity behavior. Fig. 5d shows the Nyquist plots of the NCOs electrodes. All the plots showed a semicircle in the high-frequency region and a straight line in the lowfrequency region. It was obvious that the semicircle diameter of NCO-2 was the smallest, which indicated that NCO-2 had the lowest pseudocapacitive charge transfer resistance. Moreover, the slope of a straight line of NCO-2 at low frequency indicated the fastest electrolyte diffusion rate among the four electrodes.EIS data of the NCO-1,NCO-2,and NCO-3 were fitted by Zsimpwin software,whereRsrepresents the intrinsic resistance of the electrode material, the ionic resistance of the electrolyte, and the contact resistance between the electrode and the fluid collector,Rctis the charge transfer resistance at the electrode-electrolyte interface,CdlandCpsrepresent electric double layer capacitor and capacitance, respectively. The values ofRsandRctof NCO-2 were smaller (0.050 63 and 0.010 34 Ω, respectively) than those of NCO-1 (0.073 50 and 0.012 37 Ω, respectively) and NCO-3 (0.186 0 and 0.294 5 Ω, respectively), which indicated the capacitive performance of NCO- 2 was close to the ideal capacitor.

Fig.5 Electrochemical performances of three electrodes：(a)CV curves at a scan rate of 2 mV·s-1;(b)GCD curves at a current density of 0.5 A·g-1;(c)gravimetric capacitances as a function of current density;(d)Nyquist plots(Inset：the magnification plots of the high-frequency region and the equivalent circuit)

As shown in Fig.6a, the CV at different scan rates was used to further identify the electrochemical performance of NCO-2. The CV curves had a pair of oxidation and reduction peaks, which exhibited pseudocapacitive characteristics of the NCO-2 electrode. Fig.6b presents the GCD curves of NCO-2 at different current densities. All the GCD curves with voltage plateaus indicated that the NCO-2 electrode was an ideal pseudocapacitive material[23]. Furthermore, the cycling stability of the NCO-2 electrode was investigated through GCD measurements at 10 A·g-1for 5 000 cycles.Fig.6c shows the 78.26% specific capacitance retention of the NCO-2 electrode, which demonstrated good electrochemical stability of the NCO-2.

As a measure for evaluating the electrochemical performance of the electrode,the NCO-2//AC asymmetric supercapacitor was assembled,in which NCO-2 and AC were the anode and cathode, respectively. Fig.7a shows the CV curves of NCO-2 and AC at a scan rate of 5 mV·s-1, which presents that the potential window of NCO-2 and AC were 0-0.5 V and -1.0-0 V, respectively. As shown in Fig.7b, the CV curves of the NCO-2//AC asymmetric supercapacitor with different scan rates were presented in the potential window of 0-1.5 V, which indicated that the supercapacitor stored and released electrical energy. The GCD curves of the NCO-2//AC asymmetric supercapacitor are shown in Fig.7c, from which the specific capacitance values of 34.8,31.0,25.0,and 20.0 F·g-1at various current densities 0.5, 0.6, 5.0, and 40 A·g-1could be deduced. At low current density, it is easy to generate a charging and discharging platform,while at high current density,the charging and discharging platform is easy to disappear[24-25]. At the current density of 0.5 A·g-1, there was a charging and discharging platform,which droped rapidly before reaching the platform. When the current density increased slightly, the platform disappeared. In addition, the duration of this stage is related to voltage value, ambient temperature, discharge rate, and so on.In the Ragone plots (Fig.7d),the maximum energy density of the NCO-2//AC was found over 57.70 Wh·kg-1at a power density of 576 W·kg-1.Even at a high-power density of 14.4 kW·kg-1,the energy density of NCO-2//AC could still reach a value of 32 Wh·kg-1, which was higher than the asymmetric supercapacitors under similar systems[26-30]. The electrochemical performance of NCO-2//AC was relatively better than that of various nickel-cobalt oxide//AC supercapacitors.

3 Conclusions

In summary, the nickel-cobalt oxide nanosheets were synthesized by controlled hydrothermal and calcined methods. The NCO nanosheets with a length of 100-200 nm exposed a large number of active sites which improved the electrochemical reaction between Ni2+and Co2+with OH-in electrolyte solution to produce pseudocapacitance. It was found that the pseudocapacitance reactions of the nickel and cobalt oxides were increased by appropriately adjusting the proportion of nickel and cobalt atoms.The NCO-2 nanosheets showed a superior specific capacitance of 1 096.88 F·g-1at 0.5 A·g-1with a cycling stability of 78.26% after 5 000 cycles. For asymmetric supercapacitors, the NCO-2-based device exhibited the maximum energy density of 57.70 Wh·kg-1and the power density of 14.4 kW·kg-1, which had significantly exceeded those of previously reported nickel- and cobalt-based asymmetric supercapacitors. Therefore, this work inspired related research on bimetallic nickel cobalt oxides for potential applications in energy storage and conversion.