Investigation on electrochemical behaviors of NiCo2O4 battery-type supercapacitor electrodes: the role of an aqueous electrolyte

Wei Jianga, Fang Hua, Qingyu Yanb and Xiang Wu*a
aSchool of Materials Science and Engineering, Shenyang University of Technology, Shenyang 110870, P. R. China. E-mail: wuxiang05@sut.edu.cn; wuxiang05@163.com
bSchool of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore

Received 8th July 2017 , Accepted 1st August 2017

First published on 3rd August 2017


Mesoporous NiCo2O4 flower-like structures are prepared successfully by a facile solvothermal route. The electrochemical performance of the as-synthesized products as electrode materials for battery-type supercapacitors is systematically investigated at various concentrations of potassium hydroxide (KOH) aqueous solution by cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy. NiCo2O4 supercapacitor electrode in the 6 M KOH electrolyte delivers high specific capacity (122.5 C g−1 at 1 A g−1), excellent rate performance (82.5 C g−1 at 10 A g−1) and cycling stability (21.7% capacity loss after 6000 cycles at 2 A g−1). The results show that KOH electrolyte at high concentrations plays an important role in improving electrochemical performance of NiCo2O4 battery-type supercapacitor electrodes.


Green, efficient and renewable energy storage systems have drawn great enthusiasm from researchers all over the world because of severe environmental problems and gradually exhausted non-renewable energy sources.1–5 Among various energy storage devices, lithium ion batteries and supercapacitors have been widely investigated as power sources for electric vehicles to integrate huge amount of electrical energy generated from a natural energy source (such as water or wind energy) into a power grid.6,7 A Li-ion battery stores energy with high energy density but limits its power density and cycling performance. However, supercapacitors can provide high power density and excellent cycle life because charge storage occurs at the interface between electrode materials and the electrolyte.8,9 To date, considerable efforts have been made to improve their energy density for extensive use of the supercapacitors.10–13

In general, based on energy storage mechanism, supercapacitor electrodes can be divided into three types: (1) electrochemical double layer electrodes, in which energy storage and release depend on nanoscale charge separation at the electrode/electrolyte interface. Typical electrode materials are carbonaceous materials with a large specific surface area.14–17 (2) Pseudocapacitive electrodes, in which energy storage arises from faradaic reactions on electrode surfaces and a relatively constant capacitance value is obtained within a potential window (RuO2, MnO2).18,19 (3) Battery-type electrodes, in which energy storage is also faradaic, but their specific capacitance value changes with voltage window and thus specific capacity in C g−1 or mA h g−1 is adopted as the metric (Co3O4, NiO, ZnCo2O4, Fe2O3, and NiCo2O4).20–24 Battery-type electrodes, exhibiting noncapacitive electrochemical behaviors, are commonly used in hybrid supercapacitors coupled with electrical double layer electrodes to balance the characteristic of batteries (high energy output) and supercapacitors (high power output).

NiCo2O4, one of important ternary transition metal oxides, has been investigated as a battery-type electrode material for supercapacitors due to its high theoretical capacity, excellent electrochemical activity and environmental benignity. Many reports about NiCo2O4 electrode focus on electrode design.25–27 A few studies investigate the effect of electrolyte concentration on electrochemical performance of supercapacitor electrode. In fact, electrolyte concentration has a great influence on final performance of electrode material.28,29

KOH aqueous electrolyte is widely used in NiCo2O4-based supercapacitors due to its high ionic conductivity and small hydrated ionic radius of K+; however, its concentration is generally selected randomly.30,31 The influence of KOH electrolyte concentration on electrochemical performance of NiCo2O4-based battery-type supercapacitors is ambiguous. Herein, we synthesize NiCo2O4 flower-like structures by a facile one-step solvothermal method. KOH solutions with the concentrations of 2 M, 4 M and 6 M are selected as the electrolytes, respectively. The electrolyte conductivity, hydrated ion radius, charge transfer resistance and ohmic resistance of KOH solution at various concentrations are investigated systematically. It reveals that 6 M KOH solutions possess the highest specific conductivity and smallest hydrated ionic radius of K+.

All the raw materials were of analytical grade and used without further purification. In a typical synthesis process, 0.5816 g nickel nitrate, 0.5820 g cobalt nitrate, and 2.0 g urea were added to 80 ml of absolute ethyl alcohol and stirred until complete dissolution. Then, transparent solution was transferred into the 100 ml of a Teflon-lined stainless autoclave and maintained at 120 °C for 4 h. After cooling down to room temperature, a pink precipitate was collected by centrifugation and washed with deionized water and ethanol several times. The as-synthesized sample was dried at 80 °C for 12 h. Finally, the pink precursor was calcined at 350 °C for 2 h in a muffle furnace with a temperature ramping rate of 2 °C min−1.

The crystal phase of the as-synthesized sample was studied by power X-ray diffraction (XRD, 7000, Shimadzu) using Cu Kα radiation (λ = 1.5406 Å) under 40 kV.

Nitrogen absorption–desorption isotherm (Gold APP V-Sorb 4800) measurements were conducted at 77 K to evaluate specific surface area and the porosity of the products. X-ray photoelectron spectroscopy (XPS, Thermo Scientific Escalab 250Xi) was performed to determine elemental compositions and valence state of the samples. Surface morphology and microstructure of the as-synthesized samples were characterized by using a field emission scanning electron microscope (FESEM, SU8010, Hitachi), a high resolution transmission electron microscope and selected area electron diffraction (SEAD) (HRTEM, JEOL-2010).

A working electrode was prepared by mixing 70 wt% of active materials, 20 wt% of the conductive agent (Super P) and 10 wt% of the binder (PTFE 60 wt%) with a little NMP as the solvent. The as-prepared slurry was coated on a nickel foam substrate (1.0 × 1.0 cm2) and dried in a vacuum oven at 110 °C for 6 h. The electrochemical performance of the as-fabricated sample was investigated on a CHI 660E electrochemical workstation in a three-electrode system with the KOH solution as the electrolyte, where mercuric oxide electrode (Hg/HgO) and Pt foil were used as a reference electrode and a counter electrode, respectively. Cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) and electrochemical impedance spectroscopy (EIS) techniques were performed to evaluate electrochemical characteristics of the products. EIS measurements were carried out in a frequency range of 105 Hz to 0.01 Hz with an AC amplitude of 5 mV.

Fig. 1a shows a typical XRD pattern of the as-synthesized sample. The diffraction peaks located at 2θ values of 18.9 (111), 31.2 (220), 36.8 (311), 38.4 (222), 44.7 (400), 55.5 (422), 59.1 (511), 65.0 (440) and 68.4° (531) can be assigned to cubic spinel NiCo2O4 structure (JCPDs card no. 20-0781) and no diffraction peaks from any other phases can be detected, revealing a high purity of the as-synthesized samples. N2 adsorption–desorption isotherm and the pore diameter distribution curve of the samples are depicted in Fig. 1b. A typical type IV adsorption–desorption isotherm with a H4 hysteresis loop shows mesoporous structure of the as-synthesized sample. BJH analysis indicates that the as-synthesized NiCo2O4 products possess a narrow pore size distribution centered in the range of 2–10 nm (the inset in Fig. 1b). According to multi-point BET method, a high specific surface area of 66.3 m2 g−1 is achieved with a large pore volume of 0.308 cm3 g−1.


image file: c7qi00391a-f1.tif
Fig. 1 (a) X-ray diffraction pattern of NiCo2O4 product. (b) Nitrogen adsorption/desorption isotherms and the representative pore size distribution (inset) of NiCo2O4 product. (c) XPS full spectra of NiCo2O4 product. (d) Ni 2p X-ray photoelectron spectra. (e) Co 2p X-ray photoelectron spectra. (f) O 1s X-ray photoelectron spectra.

X-ray photoelectron spectroscopy (XPS) experiments are conducted to analyze oxidation states and elemental composition of NiCo2O4 products. Fig. 1c shows full spectrum of XPS. Two spin–orbit doublets of Ni 2p emission spectra (Fig. 1d) confirm the presence of Ni3+ and Ni2+, and two shakeup satellites are located in the spectra.32 Co 2p emission spectra (Fig. 1e) match with two spin–orbit doublets characteristic of Co3+ and Co2+ and one shakeup satellite.33 The high-resolution spectrum for O 1s (Fig. 1f) contains three oxygen distributions marked as O1, O2, and O3, respectively. The component O1 located at 529.4 eV is characteristic of a metal–oxygen bond.34 The component O2 at 531.0 eV corresponds to oxygen ions in low coordination on the surface.35 Well-resolved O3 at 532.5 eV is associated with hydroxide species.36

The morphology of the as-prepared NiCo2O4 sample is observed by SEM. Fig. 2a shows a low magnification SEM image of the sample, revealing that flower-like structures are composed of many thin nanosheets, and these nanosheets are interconnected with each other. The nanosheets possess a thickness of less than 10 nm and porous characteristic, as shown in Fig. 2b. The microstructure of NiCo2O4 microflower is characterized by TEM. Fig. 2c shows that a single nanosheet possesses a thickness of several nanometers, and numerous pores can be found in the nanosheet (inset). HRTEM measurements are conducted to further study the structure of NiCo2O4 sample, as shown in Fig. 2d. The calculated interplanar spacings are 0.203 nm, 0.248 nm and 0.286 nm, respectively, which are assigned to the d-spacing (400), (311), and (220) lattice planes of NiCo2O4. SEAD pattern (the inset in Fig. 2d) indicates polycrystallinity of NiCo2O4 microflower.


image file: c7qi00391a-f2.tif
Fig. 2 (a) Low magnification SEM images of NiCo2O4 product. (b) High magnification SEM images of NiCo2O4 product. (c) TEM images of NiCo2O4 product, and the inset is partial enlarged view of the nanosheet. (d) HRTEM image and the SEAD pattern (inset) of NiCo2O4 product.

Cyclic voltammetry experiments are carried out to investigate electrochemical behaviors of battery-type supercapacitor electrodes with NiCo2O4 microflowers as active materials at various scan rates with potential windows of 0 to 0.6 V in 2 M, 4 M, and 6 M KOH aqueous electrolytes, respectively. It can be found in Fig. 3c that supercapacitor electrode in 6 M KOH electrolyte shows representative curves of battery-type supercapacitors with a rapid current response at the position of voltage reversal even when sweep rates increase from 2 mV s−1 to 50 mV s−1.37 However, for cyclic voltammograms in 2 M (Fig. 3a) and 4 M (Fig. 3b) KOH electrolytes, large deformation appears and current response presents obvious hysteresis with scan rates increasing from 2 mV s−1 to 50 mV s−1, revealing a poor rate performance. In order to further compare electrochemical behaviors of NiCo2O4 product at different concentrations of KOH electrolyte, CV curves at a scan rate of 20 mV s−1 are presented in Fig. 3d. All CV profiles at various concentrations of KOH electrolyte show a similar shape. However, the calculated area of CV curve in 6 M KOH electrolyte is much larger than those in 2 M and 4 M KOH electrolytes, which means that the specific capacity in 6 M KOH electrolyte is the highest. The results show that electrolyte concentrations exert a great influence on electrochemical performance of supercapacitor electrode, which can be ascribed to the variation of the sizes of hydrated K+ in various concentration electrolytes. With the decreased concentration of KOH electrolyte, a larger size of hydrated K+ ions will be attained. When entering and diffusing to the pores, large hydrated ions not only need overcome more obstacles but also lead to a lower electrolyte ion transport rate, which reduces the capacity of the electrode.38,39


image file: c7qi00391a-f3.tif
Fig. 3 (a) CV curves of NiCo2O4 electrode in 2 M KOH electrolyte. (b) CV curves of NiCo2O4 electrode in 4 M KOH electrolyte. (c) CV curves of NiCo2O4 electrode in 6 M KOH electrolyte. (d) CV profiles of NiCo2O4 electrode at a scan rate of 20 mV s−1 at various concentrations of KOH electrolyte. (e) Specific capacity of supercapacitor electrode in various electrolytes at different scan rates. (f) Normalized specific capacity of supercapacitor electrode in various electrolytes at different scan rates.

The specific capacity of NiCo2O4 microflowers at various scan rates of 2 mV s−1, 5 mV s−1, 10 mV s−1, 20 mV s−1 and 50 mV s−1 is depicted in Fig. 3e, respectively. Specific capacity (SC) can be calculated from eqn (1):

 
image file: c7qi00391a-t1.tif(1)
SC, s, m, i, and V are specific capacity (C g−1), scan rates (mV s−1), the mass (g) of active materials, response current (A) and cell voltage (V), respectively. A high specific capacity of about 135 C g−1 can be obtained in 6 M KOH at a scan rate of 2 mV s−1. When a scan rate is increased to 50 mV s−1, specific capacity in 6 M KOH electrolyte is 84.6 C g−1, which is much larger than those in 2 M (66.3 C g−1) and 4 M (72.2 C g−1) KOH. The corresponding normalized specific capacity is depicted in Fig. 3f. It can be seen that specific capacity in 6 M KOH decreases to 37.5% with scan rate increasing from 2 mV s−1 to 50 mV s−1, which is less than those in 4 M (43.6%) and 2 M (47.6%) KOH solutions. The above results show that NiCo2O4 electrode in 6 M KOH electrolyte shows a higher specific capacity and better rate performance than those in 2 M and 4 M KOH electrolytes.

Galvanostatic charge/discharge (GCD) measurements at different current densities are further studied to evaluate electrochemical performance of NiCo2O4 product in 2 M, 4 M, and 6 M KOH electrolytes, respectively. The typical GCD profiles of battery-type supercapacitors with a discharge plateau at about 0.35–0.5 V (2 M), 0.32–0.5 V (4 M), and 0.28–0.5 V (6 M) at a current density of 1 A g−1 are observed in Fig. 4a–c. The mentioned plateaus correspond to discharge capacity from faradaic reactions of NiCo2O4, which is in good agreement with above CV curves. Symmetric GCD curves at different current densities show good reversibility and high coulombic efficiency of NiCo2O4 sample in above electrolytes. Fig. 4d shows GCD curves of NiCo2O4 microflowers in various KOH electrolytes at a current density of 1 A g−1, respectively. Obviously, GCD curves in 6 M KOH show a longer discharge time than those of other KOH concentrations, revealing that the supercapacitors in 6 M KOH can obtain a large specific capacity. Specific capacity (SC) can be determined by using the following equation:

 
image file: c7qi00391a-t2.tif(2)
where SC, i, Δt, and m are specific capacity (C g−1), current (A), discharge time (s), and the mass (g) of active materials, respectively.


image file: c7qi00391a-f4.tif
Fig. 4 (a) GCD curves of NiCo2O4 electrode in 2 M KOH electrolyte. (b) GCD curves of NiCo2O4 electrode in 4 M KOH electrolyte. (c) GCD curves of NiCo2O4 electrode in 6 M KOH electrolyte. (d) GCD curves of NiCo2O4 electrode in various electrolytes at a current density of 1 A g−1. (e) Specific capacity of supercapacitor electrode in various electrolytes at different current densities. (f) Normalized capacity of supercapacitor electrode in various electrolytes.

Specific capacity versus various current densities from 1 A g−1 to 10 A g−1 in different electrolytes is depicted in Fig. 4e. A large specific capacity of up to 122.5 C g−1 can be obtained in 6 M KOH electrolyte at 1 A g−1, which is higher than those in 4 M (110 C g−1), 2 M (104.4 C g−1) and 1 M (86.87 C g−1 NiCo2O4 electrode).40 Even at current density of 10 A g−1, the supercapacitor in 6 M KOH electrolyte still delivers a high specific capacity value of 82.2 C g−1. In Fig. 4f, the corresponding normalized capacity shows that a specific capacity loss in 6 M KOH electrolyte is 32.2% when current density increases from 1 A g−1 to 10 A g−1, which is much less than the loss 41.8% and 51.7% in 4 M and 2 M, respectively. The above results reveal that NiCo2O4-based supercapacitors in a high concentration KOH electrolyte possess a better capacity performance and rate capability than that in a low concentration KOH electrolyte.

Electrochemical impedance spectroscopy (EIS) is investigated at equilibrium potential (0 V vs. Hg/HgO) to further analyze electrochemical performance in various electrolytes. As shown in Fig. 5a, Nyquist plots in different KOH electrolytes show analogous shapes. In a high frequency zone, the diameter of a quasi-semicircle represents charge transfer resistance (Rct), which corresponds to charge transfer at the electrode/electrolyte interface. It can be clearly seen that quasi-semicircle area decreases with the increase in electrolyte concentration, which means a lower Rct value and a faster charge transfer process in a high concentration electrolyte. The first intersection point at real axis at a high frequency corresponds to ohmic resistance (Rs), which represents the total resistance of the electrolyte, the separator, electrode materials and electrical contacts.41 Moreover, Rs values in various electrolytes are 0.544 Ω (2 M), 0.314 Ω (4 M) and 0.265 Ω (6 M). The low Rs value in 6 M KOH electrolyte is mainly attributed to large specific conductivity and small size of hydrated K+ ions.42


image file: c7qi00391a-f5.tif
Fig. 5 (a) EIS curves of supercapacitor electrode in various electrolytes. (b) Specific conductivity vs. molarity at 25 °C. (c) Cycling performance of supercapacitor electrode in various electrolytes at a current density of 2 A g−1. (d) Normalized capacity retention of supercapacitor electrode in various electrolytes during 6000 cycles at 2 A g−1.

As is well known, specific conductivity of the electrolyte plays a critical role in determining electrochemical performance of supercapacitors.42,43 Therefore, in order to improve electrochemical performance of the supercapacitors, the electrolyte with a high ionic conductivity is commonly used. Here, an empirical relationship between specific conductivity and concentration of KOH solution and temperature is proposed. The specific conductivity of KOH electrolyte can be calculated from the equation:44

 
k = A(M) + B(M2) + C(M·T) + D(M/T) + E(M3) + F(M2·T2) (3)
where k is specific conductivity (S cm−1), M is the molarity (mol L−1), T is the temperature (K), and letters from A to F are constants. The values of specific conductivity can be calculated using eqn (3) at various concentrations with a constant temperature (298 K), as shown in Fig. 5b. Moreover, KOH electrolyte with a concentration higher than 6 M is avoided due to peeling off of active materials from a conductive substrate.45 The specific conductivity of 6 M KOH solution is 0.6266 S cm−1, which is higher than 0.3778 S cm−1 (2 M) and 0.6164 S cm−1 (4 M). The excellent electrochemical performance of the supercapacitors in 6 M KOH electrolyte can be attributed to high specific conductivity of the electrolyte.

The long-term cycling stability of the supercapacitors at different KOH concentrations is performed by GCD tests at a current density of 2 A g−1 within a potential window of 0–0.5 V, as shown in Fig. 5c. After 6000 cycles, the supercapacitors in 6 M, 4 M, and 2 M KOH deliver specific capacities of 88.05 C g−1, 56.7 C g−1 and 48.65 C g−1, respectively. In addition, an uptrend of specific capacity can be observed in an initial cycling process in 6 M KOH electrolyte, revealing an activation process of NiCo2O4 electrode.46 As shown in Fig. 5d, the corresponding normalized capacity retentions are calculated in different electrolytes. After 6000 cycles, a higher retention of 78.3% is achieved in 6 M KOH than those of 58.3% in 4 M and 54.6% in 2 M KOH. Long-term cycle tests further prove superior electrochemical performance of the supercapacitors in high concentration KOH electrolyte.

In summary, the effects of electrolyte concentrations on electrochemical performances of NiCo2O4 supercapacitor electrodes are investigated in detail. The results show that high electrolyte conductivity and small hydrated ionic radius can obtain a high specific capacity (135 C g−1 at 2 mV s−1) and rate capability (84.6 C g−1 at 50 mV s−1) because of fast ion/charge diffusion and exchange. The long-term cycling stability is strongly dependent on the concentration of the electrolyte. After 6000 cycles at 2 A g−1, specific capacity retentions of 54.6%, 58.3%, and 78.3% are achieved in 2 M, 4 M, and 6 M KOH electrolytes, respectively. This work provides new insight into rational selection of electrolyte concentration for high performance battery-type supercapacitors.

Acknowledgements

This work was supported by initial funding for top level talents of Shenyang University of Technology (004593) and Nature Science Fund of Liaoning province (no. 20170540671).

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