NiO hybrid coatings as supercapacitor electrode materials: Significant improvements in electrochemical performance

Journal of Electroanalytical Chemistry 742 (2015) 1–7

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One-step route synthesis of active carbon@La2NiO4/NiO hybrid coatings as supercapacitor electrode materials: Significant improvements in electrochemical performance Mei Zhou a, Yafeng Deng b, Kun Liang a,c, Xiaojiang Liu b, Bingqing Wei c, Wencheng Hu a,⇑ a b c

State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science & Technology of China, Chengdu 610054, PR China Institute of Electronic Engineering, China Academy of Engineering Physics, Mianyang 621900, PR China Department of Mechanical Engineering, University of Delaware, Newark, DE 19716, USA

a r t i c l e

i n f o

Article history: Received 13 September 2014 Received in revised form 16 January 2015 Accepted 27 January 2015 Available online 7 February 2015 Keywords: Active carbon@La2NiO4/NiO hybrid coatings Conductivity Specific capacitance Composite Porous structure

a b s t r a c t Active carbon@La2NiO4/NiO hybrid coatings are synthesized using different molar ratios of La/Ni to improve the electrochemical performance of active carbon materials. The morphology and microstructure of the composites are investigated via scanning electron microscopy (SEM), X-ray diffraction (XRD), and high-resolution transmission electron microscopy (HRTEM). Cyclic voltammetry (CV), galvanostatic charge/discharge test (GCD), and electrochemical impedance spectroscopy (EIS) are performed to characterize electrochemical performance. At a molar ratio of La/Ni 1:2, the electrochemical electrode shows that the resistance of the composite is 2.97 X, which is half of that in pure active carbon. The highest energy density of 70.37 W h kg1 is obtained at a current density of 1 A g1, whereas the highest power density of 32.4 kW kg1 is achieved at a current density of 10 A g1. The CV results present a high specific capacitance of 710.48 F g1 at a scan rate of 1 mV s1 in 7 M KOH aqueous solution. Ó 2015 Published by Elsevier B.V.

1. Introduction Given the increasing demand for power sources that deliver in high-power energy [1,2], studies on electrochemical capacitors (ECs) mostly focus on electrode materials and electrolytes [3–5], which affect the performance of ECs. Given their key function in ECs, electrode materials are crucial in determining the properties of ECs [6]. Many studies have investigated carbon-based materials [7–12]. Carbon has four nanostructures [13], namely zero-dimensional carbon nanoparticles (i.e., active carbon, carbon nanospheres, and mesoporous carbon) [14], one-dimensional nanostructures (i.e., carbon nanotubes and carbon nanofibers) [15], two-dimensional nanosheets (i.e., graphene and reduced graphene oxides) [16], and three-dimensional porous carbon nano-architectures [17]. Studies have investigated various carbon/metal oxide composite electrodes by focusing on different carbon nanostructures with metal oxide depositions [18–20]. Active carbon (AC) is used as the electrode material for ECs. This material provides several advantages, such as a high specific surface area, high porosity, and moderate electronic conductivity. AC also has an excellent cyclic stability ⇑ Corresponding author. Tel.: +86 28 83201171; fax: +86 28 83202550. E-mail address: [email protected] (W. Hu). http://dx.doi.org/10.1016/j.jelechem.2015.01.033 1572-6657/Ó 2015 Published by Elsevier B.V.

and a long service lifetime considering the absence of any chemical reaction during its charge/discharge processes [21]. The accumulation of electrons on an electrode is a non-faradaic process [22]. However, the maximum specific capacitance and energy density of AC are lower than those of an electrochemical battery [23]. The electrochemical performance of AC is also limited by its electronic conductivity [24]. Surface modification of AC is necessary because it cannot fulfill the demand for power sources that deliver high-power energy. Multivalent metal oxides, graphene [25], and conductive polymers (i.e., polypyrrole [26,27] and polyaniline [28,29]), are usually blended with AC to minimize the internal resistance and to improve the electrochemical performance of the electrode. However, graphene oxide is a poor electrical conductor because of the disruption in the p-orbital structure during oxidation [30]. Polypyrrole is insoluble and infusible, which restrict its processing and applications [31]. The use of polyaniline as a conducting additive is hindered by structural defects [32–34]. However, the properties of metal oxides are relatively more stable than those of the abovementioned conducting materials [35,36]. Thus, active carbon/metal oxide composite materials are mainly studied in this paper. Various transition metal oxides, such as MnO2 [37,38], NiO [39– 41], Co3O4 [42,43], and RuO2 [44–46] are deposited on carbon materials to obtain large specific capacitance and high energy

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density. NiO is a promising electrode material that provides a higher specific capacitance than carbon-based active materials. However, the high resistivity of NiO causes a large IR loss at a high current density [47–49]. Moreover, the power density and the rate capability of NiO are not ideal, which hinder its practical application. As a result, the poor conductivity of NiO remains a great challenge. La2NiO4 has several advantages such as favorable chemical stability and high conductivity of ions and electrons [50,51]. La2NiO4 has a layered structure that is formed by the perovskite layer of LaNiO3 and the halite layer of LaO [52–54]. These two structures are alternately stacked [55]. Given the excellent transmission performance of electrons, La2NiO4 is a conductive oxide with a low electrical resistivity [56]. Therefore, La2NiO4 is expected to become a promising material for improving the conductivity of the carbonbased electrode materials. The co-existed La2NiO4 and NiO produce highly reduced IR loss at a high current density to provide a high power density. It is logical to produce a composite containing both La2NiO4/NiO and AC to combine the advantages and mitigate the shortcomings of all the components. In the composite, the AC structure not only serves as the physical support of La2NiO4/NiO but also provides the channels for charge transport [57]. La2NiO4 plays a beneficial conductive role. Capacitance is produced by the combination of the double-layer capacitance of AC and the pseudocapacitance of NiO. A synergistic effect of La2NiO4/NiO and AC could be expected. This study investigates the electrochemical performance of AC@La2NiO4/NiO hybrid coatings with different molar ratios of La/Ni to determine how these coatings can achieve a low resistance and a high specific capacitance. In the composite, La2NiO4 plays an excellent conductive role to improve the conductivity of composite and NiO is mainly to provide a high specific capacitance in the composite. The structure, morphology and composition of the composites are obtained via scanning electron microscopy (SEM), X-ray diffraction (XRD), and high-resolution transmission electron microscopy (HRTEM). Cyclic voltammetry (CV), galvanostatic

charge/discharge test (GCD), and electrochemical impedance spectroscopy (EIS) are used to characterize electrochemical performance.

2. Experiment 2.1. Preparation of AC@La2NiO4/NiO hybrid coatings All chemicals were of analytical grade and were used without further purification. La(NO3)36H2O and Ni(CH3COO)24H2O were dissolved in 25 mL of CH3COOH and 25 mL of 2,4-pentanedione by using a magnetic stir bar at a temperature of 80 °C, at molar ratios of La/Ni 1:1, 1:2, 1:3, and 1:4. Afterwards, 1 g of AC was evenly mixed into the obtained solution and was placed in a vacuum oven for 2 h. The resulting suspension was stirred at 60 °C for an additional 24 h. The solid suspension was filtered and dried at 80 °C. The as-prepared samples were pre-heated at 400 °C to remove residual materials, were heated up to 650 °C, and were then cooled down to room temperature by using a N2 oven. 2.2. Characterization The surface morphologies and nanostructures of the AC@La2 NiO4/NiO hybrid coatings were observed using an FE-SEM system (Inspect F, FEI Co., U.S.) at 20 kV and using an HRTEM (Libra 200FE, Germany). The XRD patterns of the products were determined by an X-ray diffractometer (D-Max-c type A, Rigaku Co., Japan) with a Ni-filtered Cu Ka radiation and a 0.15406 nm wavelength that was operated at 40 kV and 40 mA. Data were obtained from 2h = 20° to 90° at a scan rate of 4° per step. The textural characterization of the materials was based on the N2 adsorption/desorption isotherms from Brunauer–Emmett– Teller (BET) theory, and was determined using a high-speed surface area and pore size analyzers (NOVA 2000e, Quantachrome Instrument, US).

Fig. 1. SEM images of the composites at different molar ratios of La/Ni.

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To prepare the AC@La2NiO4/NiO hybrid coating electrode sheets, the as-prepared powders were mixed with carbon black and styrene-butadiene rubber (SBR, 20 mg mL1) at a mass fraction of 8:1:1. The viscous slurry was laminated on a 1 mm-thick nickel foam and was dried in a vacuum oven at 80 °C for 12 h. Electrochemical characterization was performed using a Metrohm Autolab B.V. electrochemical workstation (PGSTAT302N, Netherlands) with a two-electrode system using a standard symmetric 2032 coin cell. All tests were performed in a 7 M KOH aqueous solution at room temperature and normal pressure. The data were collected using the electrochemical workstation. GCD tests

Fig. 2. XRD pattern of the composite at a molar ratio of La/Ni 1:2.

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were performed in the two-electrode system by using a CT2001A rapid sampling battery testing system (LAND, China) at room temperature. The mass of the active material on each electrode in twoelectrode system is about 0.8 mg. 3. Result and discussion Fig. 1 shows the morphologies of the composites at different molar ratios of La/Ni. The actual surface appearance of the sample comprises AC and small nanoparticles. As shown in Fig. 1A, small nanoparticles are rarely deposited on the surface of AC at a molar ratio of La/Ni 1:1, because of the low concentration of the metal ions in the solution. At a molar ratio of La/Ni 1:2, small nanoparticles appear as uniform spheres, which can be seen at larger magnification as shown by the inset in Fig. 1B. These nanoparticles are arranged evenly before they are deposited on the surface of AC. The morphologies of the nanoparticles at molar ratios of 1:3 and 1:4 exhibit a cluster appearance, which are displayed by the insets in Fig. 1C and D, respectively. As shown in Fig. 1, the number and the grain size of the nanoparticles obviously increase with increasing proportion of Ni2+ in the solution. The grain size of the nanoparticle at a molar ratio of La/Ni 1:2 is more consistent than those of composites at molar ratios of 1:3 and 1:4. Therefore, a 1:2 M ratio of La/Ni is considered optimal. The insets images at larger magnification can be clearly obtained in Fig. S1. XRD is used to provide the structural information of the asprepared composites. Fig. 2 shows the XRD pattern of the samples at a molar ratio of La/Ni 1:2, in which the diffraction peaks of La2NiO4 and NiO can be observed. As shown in Fig. 2, the peaks at 37.09°, 43.1°, 62.57°, 75.04°, and 79.01° correspond to the (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) crystalline faces, respectively, which are related to NiO (PDF#65-2901). The lattice

Fig. 3. (A–D) HRTEM images of the 1:2 composite.

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constants are calculated as a = b = c = 4.195 Å and a = b = c = 90°. The diffraction peaks are located at 28.20°, 33.11° ,42.93°, 43.56°, 57.90°, and 58.61°, which can be indexed to La2NiO4 (PDF#340984) and correspond to the (0 0 4), (1 1 0), (1 0 5), (1 1 4), (2 1 3), and (0 0 8) crystalline faces, respectively. The lattice constants are calculated as a = b = 3.85 Å, c = 12.6 Å and a = b = c = 90°. The nanostructural and crystalline characteristics of the composites are also investigated. The HRTEM images in Fig. 3 show the nanoporous morphology of the 1:2 composite. The crystalline faces (2 1 3), (1 0 5), (1 1 0), and (0 0 4) of La2NiO4 based on the crystalline face spacings are consistent with the XRD result and are shown in Fig. 3B and C. The crystalline faces (2 0 0) and (2 2 0) of NiO can also be seen in Fig. 3B and D, which show that La2NiO4 and NiO are arranged in a parallel manner. The nano structure can take advantage of both components of the composite to produce a low resistance and a high specific capacitance. The selected area electron diffraction pattern of 1:2 composite is shown in Fig. S2 (in the Supporting information). The diffraction pattern shows the ring pattern substantiating polycrystallinity. The welldefined rings corresponding to the diffracted planes of La2NiO4 and NiO indicate the good crystallinity. The specific surface area is measured through N2 adsorption– desorption by using the BET technique. Fig. 4 shows the typical N2 adsorption–desorption isotherms and the corresponding pore size distribution plots of composites at molar ratios of La/Ni1:1, 1:2, 1:3 and 1:4, respectively, while those plots of pure AC are displayed in Fig. S3 (in the Supporting information). The pore size

distribution is investigated by the N2 sorption isotherm through the Barrett Joyner Halenda (BJH) approach. As shown in Fig. S3 and Fig. 4B, both the pure AC and the 1:2 composite possess a large number of pores in the mesopore range, which help the ECs achieve a specific capacitance. The specific surface areas of the pure AC and the 1:2 composite are 1814.86 and 1139.62 m2 g1, respectively. As shown in Fig. 4 and Fig. S3, metal oxides are deposited into the mesopore of the AC to decrease the specific surface area of the AC. Given the little amount of the mesopore with deposition, the pore size slightly decreases less. It can be indicative that 1:2 M ratio of La/Ni is sufficiently optimal to obtain a better porous structure than other composites, which contributes to an efficient transmission performance of ions and electrons among electrode materials and electrolyte. To investigate the electrochemical performance of AC@La2NiO4/ NiO composites, the effect of NiO and La2NiO4 in the composites is obtained independently. In the Supporting information, Fig. S4(A) shows the Nyquist plots of the EIS spectra of AC@La2NiO4 and pure AC. With the good conductivity of La2NiO4, it is beneficial for efficient electrolyte ion transport, not only at the active material surface but also throughout the bulk. Therefore, it can clearly exhibit that the Faradaic interfacial charge-transfer resistance of AC@La2NiO4 is much smaller than that of pure AC, indicating that La2NiO4 plays an excellent conductive role to improve the conductivity of composite. The CV curves of AC@La2NiO4 and AC@NiO at scan rate of 100 mV s1 are displayed in Fig. S4(B). As shown in Fig. S4(B), all CV curves maintain a quasi-rectangular or a

Fig. 4. N2 adsorption–desorption isotherms of composites at molar ratios of La/Ni1:1, 1:2, 1:3 and 1:4, respectively. The insets display the corresponding pore size distribution investigated by BJH approach.

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rectangular shape. With doping of La2NiO4, the specific capacitance of AC composite is not significantly improved, while the capacitance of AC@NiO is higher than that of pure AC. It is confirmed that NiO is mainly to provide a high specific capacitance in the composite. Cyclic voltammetry (CV) is employed for the determination of the electrochemical properties of the composites at different molar ratios of La/Ni in 7 M KOH aqueous solution. Only one electron exchange is performed during the process because of the NiO conversion to NiOOH. The redox mechanism describes the faradic reaction as follows [58]:

NiO þ OH $ NiOOH þ e The CV results in Fig. 5(A–C) are obtained using a standard symmetric 2032 coin cell. Fig. 5A shows the CV curves of the composites within a stable potential window of 0.9 V to 0.9 V at different molar ratios of La/Ni at a scan rate of 100 mV s1. As shown in Fig. 5A, the CV result of the 1:2 composite has a better quasi-rectangular curve compared with those of other composites. The specific capacitance of the 1:2 composite is higher than those of other composites, which indicates that such a molar ratio of La/ Ni is optimal and can take advantage of both components of the composite to obtain a high specific capacitance. Fig. 5B shows the CV curves of the 1:2 composite at scan rates of 100, 50, 10, 5, and 1 mV s1. The CV curves maintain a quasi-rectangular shape at scan rates ranging from 1 mV s1 to 100 mV s1, which indicates an ideal electrochemical behavior. Fig. 5C shows the calculated specific capacitances of the 1:2 composite and of the AC without deposition. The specific capacitance of the 1:2 composite increases quickly as the scan rate decreases. The specific capacitance of the

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1:2 composite is calculated as 710.48 F g1 at scan rates of 1 mV s1. Such a rapid increase in specific capacitance may be attributed to the activation of active components at a high scan rate and the sufficient amount of time for the exchange of electrons between the electrode and electrolyte at a low scan rate. Furthermore, the specific capacitance of pure AC is 182.51 F g1 at a scan rate of 1 mV s1. As shown in Fig. 5C, the highest specific capacitance of the 1:2 composite is nearly four times higher than that of the AC without deposition. Additionally, the specific capacitance of 1:2 composite (334.32 F g1) is much higher than the reported value of AC@NiO materials as the electrode material for symmetric supercapacitor (90 F g1[59]) at 10 mV s1. As shown in Fig. 5A, the specific capacitance of 1:4 is smaller than that of 1:3 composite, but higher than that of AC, indicating an improvement for the specific capacitance of composite. In 1:4 composite, with much more NiO, La2NiO4/NiO nanoparticles arrange to absolutely encircle the AC nanoparticle hindering the contact between the AC nanoparticles and the electrolyte. Being barely in contact with the electrolyte, the electrochemical performance of AC is limited. And the lower conductivity also limits the electrochemical performance of 1:4 composite. Electrical resistivity is an important property that should be considered when using La2NiO4 as an electrode. The excellent conductivity of La2NiO4 enhances the electrochemical performance of the AC material electrode. Fig. 5D shows the Nyquist plots of the EIS spectra of composites at different molar ratios of La/Ni. Insets (a) and (b) display the expanded plots at the high-frequency region and the equivalent circuit that is delivered from the Nyquist plots, respectively. In inset (b), R1 is the bulk solution resistance, R2 is the Faradaic interfacial charge-transfer resistance, C1 is the constant

Fig. 5. Electrochemical performances of composites in a two-electrode system: (A) CV curves of composites by different molar ratios of La/Ni at the scan rate of 100 mV s1; (B) CV curves of 1:2 composite at different scan rates; (C) specific capacitances of 1:2 composite and pure AC at different scan rate based on CV curves; (D) Nyquist plots for composites, inset (a) the expanded plots at high frequency region, inset (b) the equivalent circuit derived from the Nyquist plots.

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phase element that delivers a double-layer capacitance, and C2 is the Faradaic pseudocapacitor. R1 and R2 can be obtained from Fig. 5D. The R1 and R2 values of the pure AC and the obtained composites are displayed in Table 1. La2NiO4 is a conductive oxide with a low electrical resistivity. With a little amount of NiO, it is logical that the resistance of AC@La2NiO4/NiO at the 1:2 ratio is higher than that of AC@La2NiO4, but still lower than that of pure AC. The R2 values of the 1:3 and 1:4 composites become higher than those of the 1:1 and 1:2 composites as well as that of the AC when the amount of NiO increases. The possible reason for such an increase is that the nanoparticles of NiO are coated by La2NiO4 to generate a low resistance at small molar ratios of Ni2+ in the solution, whereas La2NiO4 is gradually coated by NiO to produce a poor conductivity as the molar ratio of Ni2+ in the solution increases. The composite achieves the best conductivity at a molar ratio of La/Ni 1:2, which significantly decreases the electrical resistivity of composite as the electrode for ECs. As shown in Fig. 6A, a galvanostatic charge/discharge experiment is performed on the 1:2 composite at different current densities of 10, 5, 2.5, and 1 A g1 within a stable window of 0–0.9 V. The GCD curves have a quasi-triangular shape, which

Table 1 The R1 and R2 values of the pure AC and the obtained composites.

R1 (X) R2 (X)

1:1

1:2

1:3

1:4

AC

2.11 3.09

1.52 2.97

2.08 7.40

2.10 9.12

1.89 5.94

indicates a favorable capacitive behavior. It clearly displays that the discharge curve is made up with two lines with different slope at the current density of 1 A g1. During the first short section of discharge curve, an efficient electrolyte ion adsorption–desorption on the surface of AC is performed to produce the double-layer capacitance. And the redox-type reactions are mainly performed during the second long section of discharge curve. It is indicating that the specific capacitance of the composite is produced by the combination of the double-layer capacitance of AC and the pseudocapacitance. Fig. 6B shows the calculated specific capacitances. The specific capacitance of the composite reaches 625.49 F g1 at a current density of 1 A g1, which covers 88.04% of the CV-calculated value obtained in two-electrode system and is much higher than the reported value of AC@NiO materials in the GCD test (194.02 F g1[60]). Electrochemical performance is also evaluated in terms of cycling stability and capacitance retention. Fig. 6C shows the specific capacitance retention of the 1:2 composite over 5000 cycles. The cycling test is performed at a current density of 1 A g1 within the stable potential window of 0–0.9 V. A 6.21% capacitance loss up to 5000 cycles shows an excellent cycling performance. The favorable cycling stability reflects a stable energystorage process during long-cycle charging/discharging. Power density and energy density are important parameters for electrochemical performance and are also used to characterize the 1:2 composite. These parameters are both calculated from charge– discharge curves with current densities of 10, 5, 2.5, and 1 A g1. Fig. 6D shows the Ragone plot of the 1:2 composite. The highest energy density of 70.37 W h kg1 is obtained at a current density of 1 A g1, whereas the highest power density of 32.40 kW kg1

Fig. 6. (A) GCD curves of 1:2 composite with different current densities. (B) Specific capacitances at different current densities based on charge–discharge curves. (C) The cycling performance of the 1:2 composite exhibits capacitance retention after 5000 cycles at a charge–discharge current density of 1 A g1. (D) Energy and power densities with various charge–discharge current densities.

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is achieved at a current density of 10 A g1, which suggests that the 1:2 composite is a promising electrode material for power sources that deliver high-power energy. The highest energy density is much higher than that of normal active carbon-based material (The energy density of the currently commercial carbon-based ECs is typically 3–5 W h kg1 [61,23]), due to the good conductivity of La2NiO4. 4. Conclusions The preparation of AC@La2NiO4/NiO hybrid coating materials with favorable capacitance properties for ECs is a simple, low-cost process. The 1:2 composite shows an excellent electrochemical performance that is primarily attributed to its low resistance, high specific capacitance, as well as acceptable power and energy density. The specific capacitance of the AC materials is highly improved after they are blended with La2NiO4/NiO. Capacitance is produced by the combination of the double-layer capacitance of AC and the pseudocapacitance that is associated with surface redox-type reactions. A 1:2 M ratio of La/Ni is sufficiently optimal to take advantage of both components of the composite by combining the mesopore of AC. La2NiO4 is a favorably conductive inorganic metal oxide. The excellent electrochemical performance of the 1:2 composite suggests that such composite will become a promising electrode material for ECs. Conflict of interest The authors declare no competing financial interest. Appendix A. Supplementary material SEM, SAED, BET, CV results obtained in a three-electrode system are displayed in the Supporting Information. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jelechem.2015.01.033. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]

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