- Email: [email protected]
Studying the substrate effects on energy storage abilities of flexible battery supercapacitor hybrids based on nickel cobalt oxide and nickel cobalt [email protected] molybdenum oxide
Electrochimica Acta 308 (2019) 83e90
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Studying the substrate effects on energy storage abilities of flexible battery supercapacitor hybrids based on nickel cobalt oxide and nickel cobalt oxide@nickel molybdenum oxide Wei-Lun Hong a, 1, Lu-Yin Lin a, b, *, 1 a b
Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei, Taiwan Research Center of Energy Conservation for New Generation of Residential, Commercial, and Industrial Sectors, Taipei, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 December 2018 Received in revised form 1 April 2019 Accepted 4 April 2019 Available online 5 April 2019
Assembly of the efficient flexible energy storage devices is indispensable for widening the application of the wearable electronics which require electricity for operating. The flexible substrate such as Ni foam and carbon cloth is one of the key components for establishing flexible energy storage devices. However, the effects of the substrate for growing the active materials on the physical and electrochemical properties of the energy storage devices are rarely investigated. In this work, the Ni foam and the carbon cloth are used as the flexible substrates for fabricating the positive electrodes composed of the nickel cobalt oxide and the nickel cobalt oxide@nickel molybdenum oxide active materials for battery supercapacitor hybrids. The larger surface area and the pore volume are obtained for the electrodes based on the carbon cloth with the 360 growing sites, whereas the higher electrical conductivity and extra nickel hydroxide and nickel oxide are obtained for the electrodes based on the Ni foam. The best electrochemical performance is obtained for the battery supercapacitor hybrid based on the Ni foam substrate with the nickel cobalt oxide@nickel molybdenum oxide as the active material. This device provides the highest energy density of 11.90 Wh kg1 at the maximum power density of 800 W kg1. This work proposes a novel investigation of the substrate effect on the energy storage ability for the battery supercapacitor hybrids. The unique physical properties of the electrodes composed of the Ni foam and carbon cloth flexible substrates are also understood. Another feasible ways are expected to apply for improving the energy storage ability of the battery supercapacitor hybrids with the understanding of the flexible substrate features. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Battery supercapacitor hybrid Carbon cloth Nickel cobalt oxide Nickel molybdenum oxide Nickel foam
1. Introduction To pursue high-quality life for human beings, the demands for wearable electronic devices are greatly increasing nowadays. The energy supply is significant for electronic devices, including the energy generation and storage installations. The high energy and power densities, excellent high-rate capacity and long cycle life are required for an efficient energy storage device. The batterysupercapacitor hybrid device (BSH) becomes more and more popular as one of the efficient energy storage devices, since the high energy density of the battery and the high power density as well as
* Corresponding author. Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei, Taiwan. E-mail address: [email protected] (L.-Y. Lin). 1 Equally contributed. https://doi.org/10.1016/j.electacta.2019.04.023 0013-4686/© 2019 Elsevier Ltd. All rights reserved.
the long cycle life of supercapacitor can be achieved by assembling the BSH [1e3]. To apply the BSH on the potential wearable electronic devices, the flexible substrates are required to assemble the BSH with high flexibility. The commonly used flexible substrate includes Ni foam [4e7] and carbon cloth [8e11]. The Ni foam can provide large surface area with the unique foam structure, and the carbon cloth is able to offering 360 growing sites with the threads for weaving cloth. On the other hand, the nickel cobalt oxide has attracted much attention as the battery-type material for BSH, due to the high electrical conductivity and high theoretical capacitance for energy storage [12,13]. To improve the energy storage ability of nickel cobalt oxide electrodes, the morphology design was widely applied to achieve the large surface area, high electrical conductivity, suitable pore size and small charge-transfer resistance for the battery-type electrodes. Zeng et al. prepared flower-like nickel cobalt oxide
84
W.-L. Hong, L.-Y. Lin / Electrochimica Acta 308 (2019) 83e90
electrode for energy storage devices via cathodic electrodeposition. A specific capacitance (CF) of 687.5 F g1 at 0.5 A g1 and the CF retention of 92% after 2500 cycles were achieved [14]. Ma et al. fabricated NiCo2O4 nanosheets assembled microtubes as the active material for energy storage devices, and obtained a CF value of 1387.9 F g1 at 2 A g1 and the CF retention of 89.4% after 12000 cycles [15]. Our group has paid much efforts on designing nickel cobalt oxide core-shell structures for achieving excellent electrocapacitive performance for the battery-type electrode [16]. We have also investigated the effects of the core morphology [4] and the core/shell growing sequence [17] on the configuration of the nickel cobalt oxide core-shell structures and the relating electrochemical performances. The reaction temperature [18], the reaction time [19], the precursor composition [13] and the post annealing process [20] were designed to produce the preferable morphology of the active materials for energy storage. However, the influence of the key component, the flexible substrate, on the morphology and pore structure of the battery-type electrode as well as the electrochemical properties for the flexible energy storage devices has been limitedly investigated. The material growth on different flexible substrates, i.e., Ni foam and carbon cloth, is necessary to understand for designing more efficient active materials for the flexible energy storage devices. Li et al. have reported a selfassembly strategy to fabricate the binder-free nickel cobalt layered hydroxide array on different substrates such as Ni foam, Cu foil, fabric, and carbon film using a hypothermal chemical coprecipitation strategy under low temperature and normal pressure reaction conditions. However, the author only aimed to promote that the synthesizing method proposed in their work is feasible to apply on several substrates. The detailed comparisons of the physical property for the nanomaterials and the electrochemical property of the energy storage devices are limited [21]. In this study, two flexible substrates of Ni foam and carbon cloth were applied for fabricating the battery-type electrodes. The active material of nickel cobalt oxide was fabricated on the flexible substrates by using a simple hydrothermal synthesis. The morphology, pore structure, and the electrochemical performance variation for the BSH based on the nickel cobalt oxide positive electrode with the flexible substrates of Ni foam and carbon cloth were carefully discussed. To have more reliable inference of the physical and electrochemical variation for the battery-type electrode based on different flexible substrates, the nickel cobalt oxide/nickel molybdenum oxide composite was also fabricated on the flexible substrates of Ni foam and carbon cloth, and the configuration and the electrocapacitive features for the flexible BSHs based on the nickel cobalt oxide/nickel molybdenum oxide positive electrodes were also compared. This work successfully demonstrates that the substrate plays important roles on the physical property and electrochemical performance for the BSH. The suitable experimental design is required for growing efficient active materials on different flexible substrates for energy storage devices. This work also provides reliable recipe for fabricating efficient flexible battery-type electrodes for application on promising flexible electronic devices. One of the feasible ways to study the effect of the substrate on the electrochemical performance of the BSH is to use the same electroactive material with possible different thicknesses. However, our work aims to study the effect of substrate on the growth of the active materials and further to study the effect of the active material morphology on the electrochemical performance of the BSH. Therefore, by using the same synthesizing method but different substrates, different morphologies and probably different thicknesses of the active materials could be obtained in this work. The variations on the electrochemical performance of the energy storage devices may be partially caused by the different morphologies and surface properties of the active materials. In this work, the
substrate effects were studied by growing the two materials of NiCo2O4 and NiCo2O4@NiMoO4 separately on the two substrates of NF and CC. The same synthesizing method of the hydrothermal synthesis was used to fabricate these electrodes. By using the same synthesizing method to fabricate different materials on NF and CC, the influences of the synthesizing method on the physical and electrochemical properties of the resulting electrodes could be excluded. By doing this, the substrate effect could be more emphasized. If different classes of materials such as conducting polymer and carbon were used to study the substrate effect, the synthesizing method could be different and thus the effect of the synthesizing method on the physical and electrochemical properties of the resulting electrodes could not be excluded. This situation may limit the substrate effect on the physical and electrochemical properties of the energy storage electrodes, and the main target we want to investigate in this work may be ignored in some extent. 2. Experimental 2.1. Synthesis of NiCo2O4 and NiCo2O4@NiMoO4 on flexible substrates The flexible substrates of Ni foam (110PPI, thickness ¼ 1.05 mm, Innovation Materials Co., Ltd, Taiwan) and carbon cloth (WOS 1002, CeTech) were applied for fabricating the electrodes for asymmetric supercapacitors (ASC). The flexible substrates with the size of 1 ⅹ 3 cm2 were cleaned as follows [5e7]. The Ni foam and carbon cloth were soaked in 6 M hydrochloric acid (HCl, 37%, SigmaAldrich) for 30 min under ultrasonic vibration and then washed using deionized water (DIW) and ethanol (C2H5OH, EtOH, 98.0%, Echo) sequentially. Finally, the cleaned Ni foam and carbon cloth were dried in a vacuum oven for 1 h before being applied as the substrate for growing active materials. The NiCo2O4 nanostructures were grown on the flexible substrates by using a hydrothermal synthesis coupled with a calcination process. The precursor solution was prepared by dissolving 0.5 mmol nickel nitrate hexahydrate (Ni(NO3)2$6H2O, 99.0%, Acros), 1 mmol cobalt nitrate hexahydrate (Co(NO3)2$6H2O, 99.0%, Showa), and 6 mmol urea (CO(NH2)2, BioReagent, SigmaAldrich) in 40 ml DIW under stirring for 30 min at room temperature. Following 15 ml of the as-prepared solution and the cleaned flexible substrates were transferred to a 100 ml Teflon-lined autoclave, which was heated at 120 C for 6 h. After the reaction the autoclave was cooled to room temperature, and samples were then rinsed by using DIW and EtOH for several times. The post calcination process for fabricating the NiCo2O4 coated flexible substrates was carried out at 350 C in air for 2 h. The NiCo2O4@NiMoO4 nanostructures were grown on the flexible substrates by using a hydrothermal synthesis coupled with a calcination process. The precursor solution was prepared by dissolving 1 mmol Ni(NO3)2$6H2O and 1 mmol Na2MoO4$2H2O (99%, Acros) in 40 ml DIW under stirring for 30 min at room temperature. The as-prepared solution and the cleaned NiCo2O4 coated flexible substrates were transferred to a 100 mL Teflon-lined autoclave which was heated at 130 C for 6 h. The autoclave was cooled to room temperature after the reaction, and the samples were then rinsed by using DIW and EtOH for several times. The post calcination process for fabricating the NiCo2O4@NiMoO4 coated flexible substrates was carried out at 350 C in air for 2 h. 2.2. Assembly of battery-supercapacitor hybrid devices Four kinds of the BSH were assembled with the activated carbon (AC) negative electrode. The positive electrodes include the NiCo2O4 electrode and the NiCo2O4@NiMoO4 electrode. Both of the
W.-L. Hong, L.-Y. Lin / Electrochimica Acta 308 (2019) 83e90
positive electrodes were fabricated on Ni foam and carbon cloth. The AC negative electrode was prepared using a doctor-blade method with the paste containing 10% acetylene carbon black (99.9900%, Echo Chemical CO., LTD), 80% charcoal activated carbon (99.99%, Showa) and 10% poly(vinylidene fluoride) (Scientific Polymer Products, INC.) binder in N-methyl-2-pyrrolidinone (99%, Echo Chemical CO, LTD). The substrate of the negative electrode is the same as that used for fabricating the positive electrode. The purpose for using the same substrate for both positive and negative electrodes is to emphasize the substrate effect. By using the same substrate for assembling the positive and negative electrodes for the BSH, the same reactions and the same influences from the substrate may be generated on both electrodes. For the case with only the substrate of the positive electrode being varied, the effect of the substrate on the positive electrode may be neglected once the influence of the negative electrode is more dominated. The mass of the positive and negative electrodes was balanced using Equation (1) as follows.
mþ I t ¼ m Iþ tþ
85
Fig. 1. The SEM images for Ni foam (a) in low magnification and (b) in high magnification; the SEM images for carbon cloth (c) in low magnification and (d) in high magnification.
(1)
where mþ and m is the mass of active materials on the positive and negative electrodes, respectively; Iþ and I is the discharge current for the positive and negative electrodes, respectively; tþ and t is the discharge time for the positive and negative electrodes, respectively. The filter paper was used as the separator soaked in the electrolyte before using for assembling the BSH. The electrolyte for the BSH is 2 M KOH (analytical reagent grade, Fisher). 2.3. Measurements and characterizations The morphology of NiCo2O4 and the NiCo2O4@NiMoO4 grown on Ni foam and carbon cloth was examined using the fieldemission scanning electron microscopy (FE-SEM, Nova NanoSEM 230, FEI, Oregon, USA). The compositions of NiCo2O4 and the NiCo2O4@NiMoO4 grown on Ni foam and carbon cloth were examined using the energy-dispersive X-ray spectroscopy (EDX) equipped in the FE-SEM and the X-ray diffraction (XRD, X’Pert3 Powder, PANalytical) patterns. The surface area and pore properties of NiCo2O4 and the NiCo2O4@NiMoO4 grown on Ni foam and carbon cloth were investigated by using N2 adsorptionedesorption measurement on the BET apparatus (Micromeritics Gemini V) at liquid nitrogen. Pore size distributions were evaluated from the desorption branches of the nitrogen isotherms using the BarretteJoynereHalenda (BJH) model. The electrochemical performance of the BSH was evaluated using the Galvanostatic charge/discharge (GC/D), cyclic voltammetry (CV) techniques and the electrochemical impedance spectroscopy (EIS), which were conducted via the potentiostat/ galvanostatinstrument equipped with an FRA2 module (PGSTAT 204, Autolab, EcoeChemie, the Netherlands). The open-circuit potential was used as the applied bias voltage. The frequency ranges for carrying out EIS is from 0.01 Hz to 100 kHz. 3. Results and discussion 3.1. Physical properties of NiCo2O4 grown on flexible substrates and electrochemical analysis for flexible battery-supercapacitor hybrid devices based on NiCo2O4 The configurations of the flexible substrates, Ni foam and carbon cloth, were firstly observed prior to examining the morphology of the battery-type materials growing on different flexible substrates. Fig. 1(aed) shows the SEM images for the Ni foam and the carbon cloth, respectively. The Ni foam presents foam-like structure with
the width of around 50 mm. The carbon cloth is fabricated by weaving several carbon bundles in parallel and vertical directions on each other. A carbon bundle is composed of several carbon wires with the diameter of around 10 mm. The smaller size of the carbon wires is inferred to provide higher surface area for the nanomaterial growth. Furthermore, the morphology of the NiCo2O4 nanostructure on the flexible substrates of Ni foam and carbon cloth was examined by using the SEM images, as respectively shown in Fig. 2(a) and (b). The nanowire arrays with the similar size were observed for the NiCo2O4 grown on Ni foam and carbon cloth. The nanowires are not perfectly stand in the vertical direction on the surface of the substrate, but in a soft form to lie on each other. On the other hand, the EDX spectra for the NiCo2O4 nanostructure on Ni foam and carbon cloth were respectively shown in Fig. 2(c) and (d) for examining the composition. The signals of O, Co and Ni were obviously obtained in the spectra, suggesting the successful synthesis of nickel cobalt oxide. An extra signal for C was obtained in Fig. 2(d). This signal is attributed to the carbon cloth substrate. Fig. 2(e) shows the pore volume as a function of the pore width for the NiCo2O4 nanostructures on Ni foam and carbon cloth. The primary pore width of around 4 nm was found for the NiCo2O4 nanostructure on carbon cloth, whereas the NiCo2O4 nanostructure on Ni foam shows larger pore width for its primary pore volume. The surface area of 6.154 and 41.475 m2 g1 were respectively obtained for the NiCo2O4 nanostructure on Ni foam and carbon cloth. The seven-fold of the surface area for the NiCo2O4 nanostructure on carbon cloth comparing to that for the NiCo2O4 nanostructure on Ni foam is attributed to the 360 growing direction of the thread in carbon cloth. Also, the pore volumes of 0.00845 and 0.01176 cm3 g1 were obtained for the NiCo2O4 nanostructure on the Ni foam and the carbon cloth. The larger pore volume for the NiCo2O4 nanostructure on the carbon cloth is preferable for electrolyte diffusion. The surface area and the pore volume for the NiCo2O4 nanostructure on Ni foam and carbon cloth were shown in Table 1 for comparison. The electrochemical performance for the flexible BSH based on the NiCo2O4 positive electrode with the substrate of Ni foam and carbon cloth was further analyzed. Fig. 3(a) shows the CV curves for the BSH with the NiCo2O4/Ni foam positive electrode in the potential window of 0.6, 0.8, 1.0, 1.1 and 1.2 V. A serious increase on the current at the high potential region was observed in the CV curve with the potential window of 1.2 V. This phenomenon is caused by the oxidation of water. To avoid the serious generation of gas during the charge/discharge process, the largest potential
86
W.-L. Hong, L.-Y. Lin / Electrochimica Acta 308 (2019) 83e90
Fig. 2. The SEM images for (a) NiCo2O4/Ni foam, (b) NiCo2O4/carbon cloth; the EDX spectra for (c) NiCo2O4/Ni foam and (d) NiCo2O4/carbon cloth; (e) the pore distribution for NiCo2O4 on Ni foam and carbon cloth.
Table 1 The surface area and the pore volume for NiCo2O4 on flexible substrates and the specific capacitances for the BSH with the flexible NiCo2O4 positive electrodes. Substrate
Surface Area (m2 g1)
Pore Volume (cm3 g1)
CF (F cm2)
CF (F g1)
Q (mAh cm2)
Q (mAh g1)
Ni foam Carbon cloth
6.154 41.475
0.00845 0.01176
0.275 0.252
10.52 9.64
0.084 0.077
3.21 2.94
window of 1.1 V was used for further analysis. The CV curves with the potential window of 1.1 V measured at different scan rates were shown in Fig. 3(b). The peak separation in the CV curve is very small, even for the case measured at large scan rates. The result suggests the excellent high-rate charge/discharge capability for this BSH. Two couples of the redox peaks were observed in the CV curves, owing to the redox reactions of Ni2þ/Ni3þ as well as Co2þ/ Co3þ and Co3þ/Co4þ [22]. Furthermore, the GC/D curves measured at different current densities were shown in Fig. 3(c). A plateau at the potential of around 0.8 V was observed, which is attributed to
the redox reaction of Ni2þ/Ni3þ and Co3þ/Co4þ. The small plateau at the potential of around 0.6 V could be attributed to the redox reaction of Co2þ/Co3þ. The symmetry is high for the GC/D curves, especially for the curves measured using the larger current densities. This phenomenon again indicates the excellent high-rate charge/discharge capability for this BSH. On the other hand, the CV and GC/D curves were also measured for the BSH with the NiCo2O4/carbon cloth positive electrode. The potential windows of 0.7, 0.9, 1.1 and 1.3 V were used for measuring the CV curves, as shown in Fig. 3(d). The current increases largely at the high
Fig. 3. (a) The CV curves at different potential windows, (b) the CV curves at different scan rates and (c) the GC/D curves at different current densities for NiCo2O4/Ni foam; (d) the CV curves at different potential windows, (e) the CV curves at different scan rates and (f) the GC/D curves at different current densities for NiCo2O4/carbon cloth; (g) the CV curves at 50 mV s1 based on the weight of the active materials on positive and negative electrodes, (h) the Ragone plot based on the weight of the active materials on positive and negative electrodes (the inserted plot is based on the weight of the full cell including the current collectors) and (i) the Nyquist plot for NiCo2O4 on Ni foam and carbon cloth. These figures were obtained by measuring the full cell based on the AC negative electrode and the NiCo2O4 positive electrode.
W.-L. Hong, L.-Y. Lin / Electrochimica Acta 308 (2019) 83e90
potential region when the potential window of 1.3 V was applied for measuring the CV curve. Therefore, the potential window of 1.1 V was chosen for this BSH. The shape of the CV curve is similar to rectangular, which suggests the more important role for the carbon cloth on the charge/discharge behavior comparing to the active material of the nickel cobalt oxide. The CV curves were further measured at different scan rates, as shown in Fig. 3(e). The highly similar shapes with no distortion were observed for the CV curves measured using the scan rates of 10e50 mV s1. This phenomenon strongly indicates the excellent high-rate charge/discharge ability for this BSH based on carbon cloth. Fig. 3(f) shows the GC/D curves measured using the current densities of 5, 10, 15, 20 and 25 mA cm2. The larger IR drop was observed in the GC/D curves measured at higher current densities. This is a common phenomenon since the more irreversible reaction would occur when the higher charge/discharge rate is applied for the measurement. The shape of the GC/D curves is similar to triangle, which is the character for the carbon based energy storage device. The symmetry of the GC/D curves is very high, indicating the high reversibility for charging and discharging. The potential window of 1.1 V and the excellent high-rate charge/discharge ability were obtained for both of the BSH based on Ni foam and carbon cloth, but the electrochemical behavior is very different for the BSH prepared using the substrate of Ni foam and carbon cloth. The battery-type CV curves were observed for the BSH based on Ni foam, due to the metal nature of the substrate and the active material of nickel cobalt oxide. However, the capacitor-type CV curves were obtained for the BSH based on carbon cloth. The nearly invisible redox peaks resulting from the nickel cobalt oxide active material suggest that the electrochemical performance for the NiCo2O4 cannot be fully displayed when using the carbon cloth as the substrate. The electrocapacitive performance for the BSH based on Ni foam and carbon cloth was further compared using the CV curve, as shown in Fig. 3(g). Two couples of the extra redox peaks were clearly observed in the CV curve for the BSH based on Ni foam. The CF value was calculated using Equation (2) as follows.
ð IdV CF ¼
n,DV,A
(2)
R where I is the current density, IdV is the integrated area of the CV curve, n is the scan rate for measuring the CV curve, DV is the potential window, and A is the active area of the electroactive material in the electrode. The CF values based on unit working area and unit weight of the active material were shown in Table 1. The CF values of 0.275 and 0.252 F cm2 corresponding to the capacities of 0.084 and 0.077 mA h cm2 were obtained for the BSH based on Ni foam and carbon cloth, respectively. Due to the battery-type feature of the active material used in this work, the capacity is significant to provide for estimating the energy storage ability of the electrode [23,24]. Although the surface area and pore volume for the nickel cobalt oxide positive electrode based on carbon cloth are larger than those for the electrode prepared on Ni foam, the extra nickel hydroxide or nickel oxide formed on the Ni foam [25] and the high electrical conductivity for nickel may be responsible for the higher CF value for the latter case. In addition, the energy and power densities of the BSH based on Ni foam and carbon cloth were examined by using the Ragone plots, as shown in Fig. 3(h). The Ragone plot was calculated based on the mass of the active materials on the positive and negative electrodes. At all the power densities, the energy densities are higher for the BSH based on Ni foam than that for the BSH based on carbon cloth. At the maximum power density of 550 W kg1, the energy densities of 1.07 and 0.61 Wh kg1 were respectively obtained for the BSH based on Ni
87
foam and carbon cloth. Also, the energy density decay with increasing power densities is less for the BSH based on Ni foam, comparing to that for the BSH based on carbon cloth. These results suggest the better electrocapacitive performance for the BSH assembled using Ni foam as the substrate. To have more complete comparison, the Ragone plot calculated based on the whole electrode mass including the current collectors was shown as the insert in Fig. 3(h). Since the weight of carbon cloth is smaller than that for the Ni foam, the energy density is higher at all the power densities for the BSH based on the carbon cloth comparing to that for the BSH based on the Ni foam. The Nyquist plot for the BSH with the NiCo2O4 positive electrodes based on Ni foam and carbon cloth was shown in Fig. 3(i). The equivalent circuit for fitting the resistance data was inserted in this figure. The x-intercept can be fitted as the series resistance (Rs) which is in proportional to the electrical conductivity for the device. The semicircle at the high frequency region is an index to the charge-transfer resistance at the interface between the active material and the electrolyte (Rct). The Rs and Rct values are both smaller for the BSH based on the Ni foam, indicating the higher electrical conductivity and smaller interfacial resistance for the device using Ni foam as the substrate. The smaller resistances for this case is consistent with its faster energy storage ability comparing to that for the carbon cloth-based BSH. 3.2. Physical properties of NiCo2O4@NiMoO4 grown on flexible substrates and electrochemical analysis for flexible batterysupercapacitor hybrid devices based on NiCo2O4@NiMoO4 To more fairly investigate the substrate effect on the morphology and the electrochemical performance of the active material, other than the active material of NiCo2O4 discussed in the previous text, the NiCo2O4@NiMoO4 core/shell structure was also fabricated on the flexible substrates and the physical and electrochemical properties were further discussed in this part. The substrate effects on the growth of the core-shell structure should be less than that on the growth of the single layer, NiCo2O4, since the second layer of the NiMoO4 shell was grown on the first NiCo2O4 layer not directly on the substrate. However, there are still some influences of the substrate on the surface properties of the NiCo2O4@NiMoO4 electrodes. Hence the comparisons of the surface properties of the active materials and the electrochemical performances of the energy storage devices are still referable for this part. Fig. 4(a) and (b) respectively show the SEM images for the NiCo2O4@NiMoO4 grown on Ni foam and carbon cloth. The very similar sheet-on-wire core/shell structures were observed for the NiCo2O4@NiMoO4 growing on both of the flexible substrates. This phenomenon is the same as that observed for the NiCo2O4 grown on both of the flexible substrates. The morphology variation of the active materials grown on different substrates is inferred to be small. The composition of NiCo2O4@NiMoO4 grown on Ni foam and carbon cloth was further examined using the EDX spectra, as respectively shown in Fig. 4(c) and (d). The signals for Ni, Co and O were clearly observed in both of the spectra, indicating the formation of nickel cobalt oxide on the flexible substrate. The extra signal for Mo was found in both of the spectra, suggesting the participation of molybdenum in the active material and this is inferred to be the formation of the nickel molybdenum oxide as the shell. The signal of carbon was not obvious in the EDX spectrum for the NiCo2O4@NiMoO4 grown on carbon cloth. This phenomenon suggests that the active material is almost fully covered on the substrate, so the signal of carbon from the carbon cloth substrate is hard to detect. On the other hand, the pore distribution for the NiCo2O4@NiMoO4 grown on Ni foam and carbon cloth was examined, as shown in Fig. 4(e). The pore distribution profiles for the NiCo2O4@NiMoO4 growing on the two substrates are highly similar,
88
W.-L. Hong, L.-Y. Lin / Electrochimica Acta 308 (2019) 83e90
Fig. 4. The SEM images for (a) NiCo2O4/Ni foam, (b) NiCo2O4/carbon cloth; the EDX spectra for (c) NiCo2O4/Ni foam and (d) NiCo2O4/carbon cloth; (e) the pore distribution for NiCo2O4 on Ni foam and carbon cloth.
which is different from that observed for the NiCo2O4. This phenomenon indicates that the core/shell structure dominated more on the surface property than the substrate, and again suggests the full coverage of NiCo2O4@NiMoO4 on the flexible substrate. The surface area and the pore volume of NiCo2O4@NiMoO4 on the flexible substrate was shown in Table 2. The surface area of 70.062 and 74.337 m2 g1 were obtained for the NiCo2O4@NiMoO4 on Ni foam and carbon cloth, respectively. The electrode based on the carbon cloth again shows larger surface area than that of the electrode based on the Ni foam. The larger pore volume was also achieved for the electrode based on carbon cloth. This phenomenon may be owing to the 360 growing surface of the carbon cloth for providing more sites for active material deposition. Moreover, the electrochemical performance of the BSH with the NiCo2O4@NiMoO4 positive electrodes on Ni foam and carbon cloth was analyzed. The suitable potential window was firstly examined by measuring the CV curves with the potential windows of 0.8, 1.0, 1.2, 1.4 and 1.6 V for the BSH based on Ni foam, as shown in Fig. 5(a). The slight increases were observed at the high potential region when the potential window of 1.6 V was applied for the measurement. Hence the potential window of 1.4 V was used for further analysis. Fig. 5(b) shows the CV curves measured at different scan rates for the BSH based on Ni foam. The redox peaks are not obvious in the CV curves, and the shape distortion for the CV curves measured using different scan rates is small. This phenomenon indicates the excellent high-rate charge/discharge capability for this BSH. Furthermore, the GC/D curve measure at different current densities for the BSH based on Ni foam was shown in Fig. 5(c). The obvious plateau was observed in the curves, resulting from the redox reaction of Ni2þ/Ni3þ as well as Co2þ/Co3þ and Co3þ/Co4þ during in the charge/discharge process. The symmetry of the GC/D curve is very high at all the current densities, again suggesting the excellent high-rate charge/discharge capacity for this BSH based on Ni foam. On the other hand, the CV curves with different potential windows for the BSH based on carbon cloth were examined, as shown in Fig. 5(d). The redox peaks are also not obvious in the CV curves for the BSH based on carbon cloth, suggesting that the carbon cloth still plays visible roles on the electrochemical behavior for reducing the battery-like nature of the BSH. In addition, the
shape for these CV curves is less like the rectangular shape, but the NiCo2O4/carbon cloth-based BSH shows similar rectangular shape for its CV curve. It is inferred that the higher amount for the active material of NiCo2O4@NiMoO4 on the positive electrode may cause the BSH performing more similar to the battery, so the capacitor feature of the rectangular shape for the CV curve is hard to observe for this case. Due to the larger increases on the current at the high potential region in the CV curve measured using the potential window of 1.8 V, the suitable potential window of 1.6 V was used for further analysis. Fig. 5(e) shows the CV curves measured at different scan rates for the BSH based on carbon cloth. The carbon feature was also observed in the CV curves in the potential region of 0e0.7 V. Fig. 5(f) shows the GC/D curves for the BSH with the NiCo2O4@NiMoO4/carbon cloth positive electrode measured using different current densities. The plateau was also observed in these curves, and the high symmetry of the charge and discharge curves again indicates the excellent high-rate charge/discharge capacity. It is noted that the shapes of the CV curves are much similar for the NiCo2O4@NiMoO4-based BSH on Ni foam and carbon cloth, comparing to the NiCo2O4-based BSH on Ni foam and carbon cloth. This phenomenon indicates that the substrate plays more important roles on the electrochemical performance when the active material is less or the energy storage ability of the active material is worse. To more clearly compare the electrochemical performance of the BSH with the active material of NiCo2O4@NiMoO4 on different flexible substrates, the CV curves at the scan rate of 50 mV s1 were shown in Fig. 5(g). The CF value calculated using the CV curve was shown in Table 2 for comparison. The CF values of 1.294 and 0.443 F cm2 corresponding to the capacities of 0.503 and 0.172 mA h cm2 were obtained for the NiCo2O4@NiMoO4-based BSH with Ni foam and carbon cloth as the substrate, respectively. The much higher CF value was again obtained for the BSH based on Ni foam, as comparing to that for the BSH based on carbon cloth. On the other hand, the Ragone plots for the BSH composed of the NiCo2O4@NiMoO4 positive electrodes prepared using Ni foam and carbon cloth as the substrate were shown in Fig. 5(h). At the maximum power density of 800 W kg1, the energy densities of 11.90 and 5.06 Wh kg1 were respectively obtained for the BSH
Table 2 The surface area and the pore volume for NiCo2O4@NiMoO4 on flexible substrates and the specific capacitances for the BSH with the flexible NiCo2O4@NiMoO4 positive electrodes. Substrate
Surface Area (m2 g1)
Pore Volume (cm3 g1)
CF (F cm2)
CF (F g1)
Q (mAh cm2)
Q (mAh g1)
Ni foam Carbon cloth
70.06 74.34
0.02 0.06
1.29 0.44
42.99 14.38
0.50 0.17
16.72 5.59
W.-L. Hong, L.-Y. Lin / Electrochimica Acta 308 (2019) 83e90
89
Fig. 5. (a) The CV curves at different potential windows, (b) the CV curves at different scan rates and (c) the GC/D curves at different current densities for NiCo2O4@NiMoO4/Ni foam; (d) the CV curves at different potential windows, (e) the CV curves at different scan rates and (f) the GC/D curves at different current densities for NiCo2O4@NiMoO4/carbon cloth; (g) the CV curves based on the weight of the active materials on positive and negative electrodes for NiCo2O4@NiMoO4 on Ni foam and carbon cloth at 50 mV s1; (h) the Ragone plot based on the weight of the active materials on positive and negative electrodes (the inserted plot is based on the weight of the full cell including the current collectors) for NiCo2O4@NiMoO4 on Ni foam and carbon cloth. These figures were measured using the full cell based on the AC negative electrode and the NiCo2O4@NiMoO4 positive electrode.
100
80
80
60
60
40 20 0 0
Areal capacitance Coulombic effciency
40 20
Coulombic effciency (% )
100
Capacitance retention (% )
based on Ni foam and carbon cloth. The nickel foam substrate has the metal nature with high electrical conductivity, and extra nickel hydroxide and nickel oxide would form on the surface of Ni foam. These features promote the better electrochemical performance for the energy storage device assembled using Ni foam, comparing to that for the device prepared using carbon cloth. The decay on the energy density with increasing power density is similar for both case, suggesting the similar high-rate charge/discharge capability for the NiCo2O4@NiMoO4 BSH composed of the Ni foam and carbon cloth substrates. The Ragone plot in Fig. 5(h) was calculated based on the mass of the active materials on the positive and negative electrodes. To have more complete comparison, the Ragone plot calculated based on the whole electrode mass including the current collectors was also shown as the insert in Fig. 5(h). Although the weight of carbon cloth is smaller than that for the Ni foam, the energy density is still higher for the BSH based on the Ni foam comparing to that for the BSH based on the carbon cloth at all the power densities. This result is different from that obtained for the BSH based on NiCo2O4, owing to the much higher energy densities for the BSH based on NiCo2O4@NiMoO4 with the Ni foam substrate comparing to that for the BSH based on NiCo2O4@NiMoO4 with the carbon cloth substrate. The best electrochemical performance was obtained for the BSH with the positive electrode composed of NiCo2O4@NiMoO4 on Ni foam. The stability test was further carried out for examining the cycling ability of this BSH. Fig. 6 shows the capacitance retention and the Coulombic efficiency as a function of the cycling number for the BSH with the NiCo2O4@NiMoO4/Ni foam positive electrode. After carrying out 3000 times of the charge/discharge cycles, the areal capacitance retention of 80% was obtained, and the Coulombic efficiency of higher than 90% was achieved during the whole cycling process. The excellent cycling stability was confirmed for
0 500 1000 1500 2000 2500 3000 Cycle number
Fig. 6. The relation between the areal capacitance retention and the Coulombic efficiency to the cycle number for the BSH with the NiCo2O4@NiMoO4/Ni foam positive electrode.
the BSH with the best-performed NiCo2O4@NiMoO4/Ni foam positive electrode. After separately discussing the substrate effects based on the NiCo2O4 and NiCo2O4@NiMoO4 electrodes, the electrochemical performances for the NiCo2O4 and NiCo2O4@NiMoO4 electrodes were further compared to realize the effect of the active material. The electrochemical performances for the similar electrodes reported in the previous literature were also compared to evaluate the energy storage ability of the electrodes proposed in this work. The capacities of 0.084, 0.077, 0.503 and 0.172 mA h cm2 were respectively obtained for the BSH with the NiCo2O4/Ni foam,
90
W.-L. Hong, L.-Y. Lin / Electrochimica Acta 308 (2019) 83e90
NiCo2O4/carbon cloth, NiCo2O4@NiMoO4/Ni foam and NiCo2O4@NiMoO4/carbon cloth positive electrode. The NiCo2O4@NiMoO4 core-shell electrodes showed much larger capacities than those for the NiCo2O4 electrodes based on the same substrate, owing to the higher surface area and pore volume as well as the higher electrical conductivity with the participation of molybdenum for the core-shell material. The optimized BSH with the NiCo2O4@NiMoO4/Ni foam positive electrode shows the energy density of 11.90 Wh kg1 at the maximum power density of 800 W kg1, and the maximum energy density of 15.40 Wh kg1 at the power density of 160 W kg1. In the previous literature, Zhang et al. assembled asymmetric supercapacitors of NiCo2O4 uninterrupted nanosheet arrays@NiMoO4 nanosheets//active carbon and obtain maximum energy density of 52.6 Wh kg1 at the power density of 332.4 W kg1 [26]. Huang et al. fabricated NiCo2O4@NiMoO4 on carbon cloth as the energy storage electrode and obtained a CF value of 2.917 F cm2 at 2 mA cm2 [27]. Chang et al. fabricated hierarchical NiCo2O4@NiMoO4 coreeshell nanowire/nanosheet arrays on Ni foam for pseudocapacitors. An energy density of 21.7 Wh kg1 was obtained at the power density of 157 W kg1 [28]. 4. Conclusions The substrate effect on the morphology, surface area, and pore structure for nickel cobalt oxide and the nickel cobalt oxide/nickel molybdenum oxide nanostructures on flexible substrates were investigated. The morphology is almost independent on the substrate, but the surface area and the pore volume are larger for the electrode fabricated using the carbon cloth as the substrate. The better electrochemical performance was obtained for the BSH assembled using the Ni foam, comparing to that for the BSH fabricated using the carbon cloth as the substrate, owing to the higher electrical conductivity and the extra formation of nickel hydroxide and nickel oxide on the surface of the Ni foam substrate. The highest energy and power densities were obtained for the BSH composed of the NiCo2O4@NiMoO4/Ni foam positive electrode. At the maximum power density of 800 W kg1 the energy density still remained 11.90 Wh kg1 for this case. It is inferred that the surface properties may be better when the carbon cloth was used as the substrate for assembling the energy storage device, whereas the nature for the nickel metal leads to the better electrochemical performance for the BSH assembled using the Ni foam substrate.
Acknowledgements This work was financially supported from the Young Scholar Fellowship Program by MOST in Taiwan, under Grant MOST 1072636-E-027-003. This work was financially supported in part by the “Research Center of Energy Conservation for New Generation of Residential, Commercial, and Industrial Sectors” from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
Y.Y. Huang, L.Y. Lin, ACS Appl. Energy Mater. (2018) 2979e2990. Y. Zhang, Z. Jiang, X. Yu, IEEE Energy 2030 Conference (2008), 2008, pp. 1e6. W. Zuo, R. Li, C. Zhou, Y. Li, J. Xia, J. Liu, Adv. Sci. (2017) 1600539. L.Y. Lin, H.Y. Lin, W.L. Hong, L.Y. Lin, Thin Solid Films, 2018, pp. 69e75. J.W. Cheng, L.Y. Lin, W.L. Hong, L.Y. Lin, H.Q. Chen, H.X. Lai, Electrochim. Acta (2018) 1245e1252. H.Y. Sun, L.Y. Lin, Y.Y. Huang, W.L. Hong, Electrochim. Acta (2018) 692e699. H.X. Lai, L.Y. Lin, J.Y. Lin, Y.K. Hsu, Electrochim. Acta (2018) 115e126. R. Wang, X. Yan, Sci. Rep. (2014) 3712. Y. Yang, Y. Zhou, Z. Hu, W. Wang, X. Zhang, L. Qiang, Q. Wang, J. Alloy. Comp. (2019) 683e692. P. He, Q. Huang, B. Huang, T. Chen, RSC Adv. (2017) 24353e24358. Y. He, S. Du, H. Li, Q. Cheng, V. Pavlinek, P. Saha, J. Solid State Electrochem. (2016) 1459e1467. H. Wan, L. Li, J. Zhang, X. Liu, H. Wang, H. Wang, Front. Energy Res. (2017) 00033. J. Zhang, F. Liu, J.P. Cheng, X.B. Zhang, ACS Appl. Mater. Interfaces (2015) 17630e17640. Z. Zeng, B. Xiao, X. Zhu, J. Zhu, D. Xiao, J. Zhu, Ceram. Int. (2017) S633eS638. F.X. Ma, L. Yu, C.Y. Xu, X.W. Lou, Energy Environ. Sci. (2016) 862e866. L.Y. Lin, L.Y. Lin, Electrochim. Acta (2017) 335e347. W.L. Hong, L.Y. Lin, L.Y. Lin, Electrochim. Acta (2017) 309e322. G. Xiong, P. He, L. Liu, T. Chen, T.S. Fisher, Front. Energy Res. (2015) 00039. X. Wang, C. Yan, A. Sumboja, P.S. Lee, Nano Energy (2014) 119e126. X. Shi, G. Zhou, Chem. Res. Chin. Univ. (2017) 939e945. T. Li, R. Li, H. Luo, J. Mater. Chem. A 4 (2016) 18922. C.F. Yu, L.Y. Lin, J. Colloid Interface Sci. (2016) 1e7. P. Simon, Y. Gogotsi, B. Dunn, Science (2014) 1210e1211. langer, W.L. Jeffrey, J. Electrochem. Soc. (2015) A5185eA5189. T. Brousse, D. Be J.M. Chiu, L.Y. Lin, P.H. Yeh, C.Y. Lai, K. Teng, C.C. Tu, S.S. Yang, J.F. Yu, RSC Adv. (2015) 83383. P. Zhang, J. Zhou, W. Chen, Y. Zhao, X. Mu, Z. Zhang, X. Pan, E. Xie, Chem. Eng. J. (2017) 687. L. Huang, W. Zhang, J. Xiang, H. Xu, G. Li, Y. Huang, Sci. Rep. (2016) 31465. D. Cheng, Y. Yang, J. Xie, C. Fang, G. Zhang, J. Xiong, J. Mater. Chem. A (2015) 14348.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Studying the substrate effects on energy storage abilities of flexible battery supercapacitor hybrids based on nickel cobalt oxide and nickel cobalt oxide@nickel molybdenum oxide Wei-Lun Hong a, 1, Lu-Yin Lin a, b, *, 1 a b
Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei, Taiwan Research Center of Energy Conservation for New Generation of Residential, Commercial, and Industrial Sectors, Taipei, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 December 2018 Received in revised form 1 April 2019 Accepted 4 April 2019 Available online 5 April 2019
Assembly of the efficient flexible energy storage devices is indispensable for widening the application of the wearable electronics which require electricity for operating. The flexible substrate such as Ni foam and carbon cloth is one of the key components for establishing flexible energy storage devices. However, the effects of the substrate for growing the active materials on the physical and electrochemical properties of the energy storage devices are rarely investigated. In this work, the Ni foam and the carbon cloth are used as the flexible substrates for fabricating the positive electrodes composed of the nickel cobalt oxide and the nickel cobalt oxide@nickel molybdenum oxide active materials for battery supercapacitor hybrids. The larger surface area and the pore volume are obtained for the electrodes based on the carbon cloth with the 360 growing sites, whereas the higher electrical conductivity and extra nickel hydroxide and nickel oxide are obtained for the electrodes based on the Ni foam. The best electrochemical performance is obtained for the battery supercapacitor hybrid based on the Ni foam substrate with the nickel cobalt oxide@nickel molybdenum oxide as the active material. This device provides the highest energy density of 11.90 Wh kg1 at the maximum power density of 800 W kg1. This work proposes a novel investigation of the substrate effect on the energy storage ability for the battery supercapacitor hybrids. The unique physical properties of the electrodes composed of the Ni foam and carbon cloth flexible substrates are also understood. Another feasible ways are expected to apply for improving the energy storage ability of the battery supercapacitor hybrids with the understanding of the flexible substrate features. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Battery supercapacitor hybrid Carbon cloth Nickel cobalt oxide Nickel molybdenum oxide Nickel foam
1. Introduction To pursue high-quality life for human beings, the demands for wearable electronic devices are greatly increasing nowadays. The energy supply is significant for electronic devices, including the energy generation and storage installations. The high energy and power densities, excellent high-rate capacity and long cycle life are required for an efficient energy storage device. The batterysupercapacitor hybrid device (BSH) becomes more and more popular as one of the efficient energy storage devices, since the high energy density of the battery and the high power density as well as
* Corresponding author. Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei, Taiwan. E-mail address: [email protected] (L.-Y. Lin). 1 Equally contributed. https://doi.org/10.1016/j.electacta.2019.04.023 0013-4686/© 2019 Elsevier Ltd. All rights reserved.
the long cycle life of supercapacitor can be achieved by assembling the BSH [1e3]. To apply the BSH on the potential wearable electronic devices, the flexible substrates are required to assemble the BSH with high flexibility. The commonly used flexible substrate includes Ni foam [4e7] and carbon cloth [8e11]. The Ni foam can provide large surface area with the unique foam structure, and the carbon cloth is able to offering 360 growing sites with the threads for weaving cloth. On the other hand, the nickel cobalt oxide has attracted much attention as the battery-type material for BSH, due to the high electrical conductivity and high theoretical capacitance for energy storage [12,13]. To improve the energy storage ability of nickel cobalt oxide electrodes, the morphology design was widely applied to achieve the large surface area, high electrical conductivity, suitable pore size and small charge-transfer resistance for the battery-type electrodes. Zeng et al. prepared flower-like nickel cobalt oxide
84
W.-L. Hong, L.-Y. Lin / Electrochimica Acta 308 (2019) 83e90
electrode for energy storage devices via cathodic electrodeposition. A specific capacitance (CF) of 687.5 F g1 at 0.5 A g1 and the CF retention of 92% after 2500 cycles were achieved [14]. Ma et al. fabricated NiCo2O4 nanosheets assembled microtubes as the active material for energy storage devices, and obtained a CF value of 1387.9 F g1 at 2 A g1 and the CF retention of 89.4% after 12000 cycles [15]. Our group has paid much efforts on designing nickel cobalt oxide core-shell structures for achieving excellent electrocapacitive performance for the battery-type electrode [16]. We have also investigated the effects of the core morphology [4] and the core/shell growing sequence [17] on the configuration of the nickel cobalt oxide core-shell structures and the relating electrochemical performances. The reaction temperature [18], the reaction time [19], the precursor composition [13] and the post annealing process [20] were designed to produce the preferable morphology of the active materials for energy storage. However, the influence of the key component, the flexible substrate, on the morphology and pore structure of the battery-type electrode as well as the electrochemical properties for the flexible energy storage devices has been limitedly investigated. The material growth on different flexible substrates, i.e., Ni foam and carbon cloth, is necessary to understand for designing more efficient active materials for the flexible energy storage devices. Li et al. have reported a selfassembly strategy to fabricate the binder-free nickel cobalt layered hydroxide array on different substrates such as Ni foam, Cu foil, fabric, and carbon film using a hypothermal chemical coprecipitation strategy under low temperature and normal pressure reaction conditions. However, the author only aimed to promote that the synthesizing method proposed in their work is feasible to apply on several substrates. The detailed comparisons of the physical property for the nanomaterials and the electrochemical property of the energy storage devices are limited [21]. In this study, two flexible substrates of Ni foam and carbon cloth were applied for fabricating the battery-type electrodes. The active material of nickel cobalt oxide was fabricated on the flexible substrates by using a simple hydrothermal synthesis. The morphology, pore structure, and the electrochemical performance variation for the BSH based on the nickel cobalt oxide positive electrode with the flexible substrates of Ni foam and carbon cloth were carefully discussed. To have more reliable inference of the physical and electrochemical variation for the battery-type electrode based on different flexible substrates, the nickel cobalt oxide/nickel molybdenum oxide composite was also fabricated on the flexible substrates of Ni foam and carbon cloth, and the configuration and the electrocapacitive features for the flexible BSHs based on the nickel cobalt oxide/nickel molybdenum oxide positive electrodes were also compared. This work successfully demonstrates that the substrate plays important roles on the physical property and electrochemical performance for the BSH. The suitable experimental design is required for growing efficient active materials on different flexible substrates for energy storage devices. This work also provides reliable recipe for fabricating efficient flexible battery-type electrodes for application on promising flexible electronic devices. One of the feasible ways to study the effect of the substrate on the electrochemical performance of the BSH is to use the same electroactive material with possible different thicknesses. However, our work aims to study the effect of substrate on the growth of the active materials and further to study the effect of the active material morphology on the electrochemical performance of the BSH. Therefore, by using the same synthesizing method but different substrates, different morphologies and probably different thicknesses of the active materials could be obtained in this work. The variations on the electrochemical performance of the energy storage devices may be partially caused by the different morphologies and surface properties of the active materials. In this work, the
substrate effects were studied by growing the two materials of NiCo2O4 and NiCo2O4@NiMoO4 separately on the two substrates of NF and CC. The same synthesizing method of the hydrothermal synthesis was used to fabricate these electrodes. By using the same synthesizing method to fabricate different materials on NF and CC, the influences of the synthesizing method on the physical and electrochemical properties of the resulting electrodes could be excluded. By doing this, the substrate effect could be more emphasized. If different classes of materials such as conducting polymer and carbon were used to study the substrate effect, the synthesizing method could be different and thus the effect of the synthesizing method on the physical and electrochemical properties of the resulting electrodes could not be excluded. This situation may limit the substrate effect on the physical and electrochemical properties of the energy storage electrodes, and the main target we want to investigate in this work may be ignored in some extent. 2. Experimental 2.1. Synthesis of NiCo2O4 and NiCo2O4@NiMoO4 on flexible substrates The flexible substrates of Ni foam (110PPI, thickness ¼ 1.05 mm, Innovation Materials Co., Ltd, Taiwan) and carbon cloth (WOS 1002, CeTech) were applied for fabricating the electrodes for asymmetric supercapacitors (ASC). The flexible substrates with the size of 1 ⅹ 3 cm2 were cleaned as follows [5e7]. The Ni foam and carbon cloth were soaked in 6 M hydrochloric acid (HCl, 37%, SigmaAldrich) for 30 min under ultrasonic vibration and then washed using deionized water (DIW) and ethanol (C2H5OH, EtOH, 98.0%, Echo) sequentially. Finally, the cleaned Ni foam and carbon cloth were dried in a vacuum oven for 1 h before being applied as the substrate for growing active materials. The NiCo2O4 nanostructures were grown on the flexible substrates by using a hydrothermal synthesis coupled with a calcination process. The precursor solution was prepared by dissolving 0.5 mmol nickel nitrate hexahydrate (Ni(NO3)2$6H2O, 99.0%, Acros), 1 mmol cobalt nitrate hexahydrate (Co(NO3)2$6H2O, 99.0%, Showa), and 6 mmol urea (CO(NH2)2, BioReagent, SigmaAldrich) in 40 ml DIW under stirring for 30 min at room temperature. Following 15 ml of the as-prepared solution and the cleaned flexible substrates were transferred to a 100 ml Teflon-lined autoclave, which was heated at 120 C for 6 h. After the reaction the autoclave was cooled to room temperature, and samples were then rinsed by using DIW and EtOH for several times. The post calcination process for fabricating the NiCo2O4 coated flexible substrates was carried out at 350 C in air for 2 h. The NiCo2O4@NiMoO4 nanostructures were grown on the flexible substrates by using a hydrothermal synthesis coupled with a calcination process. The precursor solution was prepared by dissolving 1 mmol Ni(NO3)2$6H2O and 1 mmol Na2MoO4$2H2O (99%, Acros) in 40 ml DIW under stirring for 30 min at room temperature. The as-prepared solution and the cleaned NiCo2O4 coated flexible substrates were transferred to a 100 mL Teflon-lined autoclave which was heated at 130 C for 6 h. The autoclave was cooled to room temperature after the reaction, and the samples were then rinsed by using DIW and EtOH for several times. The post calcination process for fabricating the NiCo2O4@NiMoO4 coated flexible substrates was carried out at 350 C in air for 2 h. 2.2. Assembly of battery-supercapacitor hybrid devices Four kinds of the BSH were assembled with the activated carbon (AC) negative electrode. The positive electrodes include the NiCo2O4 electrode and the NiCo2O4@NiMoO4 electrode. Both of the
W.-L. Hong, L.-Y. Lin / Electrochimica Acta 308 (2019) 83e90
positive electrodes were fabricated on Ni foam and carbon cloth. The AC negative electrode was prepared using a doctor-blade method with the paste containing 10% acetylene carbon black (99.9900%, Echo Chemical CO., LTD), 80% charcoal activated carbon (99.99%, Showa) and 10% poly(vinylidene fluoride) (Scientific Polymer Products, INC.) binder in N-methyl-2-pyrrolidinone (99%, Echo Chemical CO, LTD). The substrate of the negative electrode is the same as that used for fabricating the positive electrode. The purpose for using the same substrate for both positive and negative electrodes is to emphasize the substrate effect. By using the same substrate for assembling the positive and negative electrodes for the BSH, the same reactions and the same influences from the substrate may be generated on both electrodes. For the case with only the substrate of the positive electrode being varied, the effect of the substrate on the positive electrode may be neglected once the influence of the negative electrode is more dominated. The mass of the positive and negative electrodes was balanced using Equation (1) as follows.
mþ I t ¼ m Iþ tþ
85
Fig. 1. The SEM images for Ni foam (a) in low magnification and (b) in high magnification; the SEM images for carbon cloth (c) in low magnification and (d) in high magnification.
(1)
where mþ and m is the mass of active materials on the positive and negative electrodes, respectively; Iþ and I is the discharge current for the positive and negative electrodes, respectively; tþ and t is the discharge time for the positive and negative electrodes, respectively. The filter paper was used as the separator soaked in the electrolyte before using for assembling the BSH. The electrolyte for the BSH is 2 M KOH (analytical reagent grade, Fisher). 2.3. Measurements and characterizations The morphology of NiCo2O4 and the NiCo2O4@NiMoO4 grown on Ni foam and carbon cloth was examined using the fieldemission scanning electron microscopy (FE-SEM, Nova NanoSEM 230, FEI, Oregon, USA). The compositions of NiCo2O4 and the NiCo2O4@NiMoO4 grown on Ni foam and carbon cloth were examined using the energy-dispersive X-ray spectroscopy (EDX) equipped in the FE-SEM and the X-ray diffraction (XRD, X’Pert3 Powder, PANalytical) patterns. The surface area and pore properties of NiCo2O4 and the NiCo2O4@NiMoO4 grown on Ni foam and carbon cloth were investigated by using N2 adsorptionedesorption measurement on the BET apparatus (Micromeritics Gemini V) at liquid nitrogen. Pore size distributions were evaluated from the desorption branches of the nitrogen isotherms using the BarretteJoynereHalenda (BJH) model. The electrochemical performance of the BSH was evaluated using the Galvanostatic charge/discharge (GC/D), cyclic voltammetry (CV) techniques and the electrochemical impedance spectroscopy (EIS), which were conducted via the potentiostat/ galvanostatinstrument equipped with an FRA2 module (PGSTAT 204, Autolab, EcoeChemie, the Netherlands). The open-circuit potential was used as the applied bias voltage. The frequency ranges for carrying out EIS is from 0.01 Hz to 100 kHz. 3. Results and discussion 3.1. Physical properties of NiCo2O4 grown on flexible substrates and electrochemical analysis for flexible battery-supercapacitor hybrid devices based on NiCo2O4 The configurations of the flexible substrates, Ni foam and carbon cloth, were firstly observed prior to examining the morphology of the battery-type materials growing on different flexible substrates. Fig. 1(aed) shows the SEM images for the Ni foam and the carbon cloth, respectively. The Ni foam presents foam-like structure with
the width of around 50 mm. The carbon cloth is fabricated by weaving several carbon bundles in parallel and vertical directions on each other. A carbon bundle is composed of several carbon wires with the diameter of around 10 mm. The smaller size of the carbon wires is inferred to provide higher surface area for the nanomaterial growth. Furthermore, the morphology of the NiCo2O4 nanostructure on the flexible substrates of Ni foam and carbon cloth was examined by using the SEM images, as respectively shown in Fig. 2(a) and (b). The nanowire arrays with the similar size were observed for the NiCo2O4 grown on Ni foam and carbon cloth. The nanowires are not perfectly stand in the vertical direction on the surface of the substrate, but in a soft form to lie on each other. On the other hand, the EDX spectra for the NiCo2O4 nanostructure on Ni foam and carbon cloth were respectively shown in Fig. 2(c) and (d) for examining the composition. The signals of O, Co and Ni were obviously obtained in the spectra, suggesting the successful synthesis of nickel cobalt oxide. An extra signal for C was obtained in Fig. 2(d). This signal is attributed to the carbon cloth substrate. Fig. 2(e) shows the pore volume as a function of the pore width for the NiCo2O4 nanostructures on Ni foam and carbon cloth. The primary pore width of around 4 nm was found for the NiCo2O4 nanostructure on carbon cloth, whereas the NiCo2O4 nanostructure on Ni foam shows larger pore width for its primary pore volume. The surface area of 6.154 and 41.475 m2 g1 were respectively obtained for the NiCo2O4 nanostructure on Ni foam and carbon cloth. The seven-fold of the surface area for the NiCo2O4 nanostructure on carbon cloth comparing to that for the NiCo2O4 nanostructure on Ni foam is attributed to the 360 growing direction of the thread in carbon cloth. Also, the pore volumes of 0.00845 and 0.01176 cm3 g1 were obtained for the NiCo2O4 nanostructure on the Ni foam and the carbon cloth. The larger pore volume for the NiCo2O4 nanostructure on the carbon cloth is preferable for electrolyte diffusion. The surface area and the pore volume for the NiCo2O4 nanostructure on Ni foam and carbon cloth were shown in Table 1 for comparison. The electrochemical performance for the flexible BSH based on the NiCo2O4 positive electrode with the substrate of Ni foam and carbon cloth was further analyzed. Fig. 3(a) shows the CV curves for the BSH with the NiCo2O4/Ni foam positive electrode in the potential window of 0.6, 0.8, 1.0, 1.1 and 1.2 V. A serious increase on the current at the high potential region was observed in the CV curve with the potential window of 1.2 V. This phenomenon is caused by the oxidation of water. To avoid the serious generation of gas during the charge/discharge process, the largest potential
86
W.-L. Hong, L.-Y. Lin / Electrochimica Acta 308 (2019) 83e90
Fig. 2. The SEM images for (a) NiCo2O4/Ni foam, (b) NiCo2O4/carbon cloth; the EDX spectra for (c) NiCo2O4/Ni foam and (d) NiCo2O4/carbon cloth; (e) the pore distribution for NiCo2O4 on Ni foam and carbon cloth.
Table 1 The surface area and the pore volume for NiCo2O4 on flexible substrates and the specific capacitances for the BSH with the flexible NiCo2O4 positive electrodes. Substrate
Surface Area (m2 g1)
Pore Volume (cm3 g1)
CF (F cm2)
CF (F g1)
Q (mAh cm2)
Q (mAh g1)
Ni foam Carbon cloth
6.154 41.475
0.00845 0.01176
0.275 0.252
10.52 9.64
0.084 0.077
3.21 2.94
window of 1.1 V was used for further analysis. The CV curves with the potential window of 1.1 V measured at different scan rates were shown in Fig. 3(b). The peak separation in the CV curve is very small, even for the case measured at large scan rates. The result suggests the excellent high-rate charge/discharge capability for this BSH. Two couples of the redox peaks were observed in the CV curves, owing to the redox reactions of Ni2þ/Ni3þ as well as Co2þ/ Co3þ and Co3þ/Co4þ [22]. Furthermore, the GC/D curves measured at different current densities were shown in Fig. 3(c). A plateau at the potential of around 0.8 V was observed, which is attributed to
the redox reaction of Ni2þ/Ni3þ and Co3þ/Co4þ. The small plateau at the potential of around 0.6 V could be attributed to the redox reaction of Co2þ/Co3þ. The symmetry is high for the GC/D curves, especially for the curves measured using the larger current densities. This phenomenon again indicates the excellent high-rate charge/discharge capability for this BSH. On the other hand, the CV and GC/D curves were also measured for the BSH with the NiCo2O4/carbon cloth positive electrode. The potential windows of 0.7, 0.9, 1.1 and 1.3 V were used for measuring the CV curves, as shown in Fig. 3(d). The current increases largely at the high
Fig. 3. (a) The CV curves at different potential windows, (b) the CV curves at different scan rates and (c) the GC/D curves at different current densities for NiCo2O4/Ni foam; (d) the CV curves at different potential windows, (e) the CV curves at different scan rates and (f) the GC/D curves at different current densities for NiCo2O4/carbon cloth; (g) the CV curves at 50 mV s1 based on the weight of the active materials on positive and negative electrodes, (h) the Ragone plot based on the weight of the active materials on positive and negative electrodes (the inserted plot is based on the weight of the full cell including the current collectors) and (i) the Nyquist plot for NiCo2O4 on Ni foam and carbon cloth. These figures were obtained by measuring the full cell based on the AC negative electrode and the NiCo2O4 positive electrode.
W.-L. Hong, L.-Y. Lin / Electrochimica Acta 308 (2019) 83e90
potential region when the potential window of 1.3 V was applied for measuring the CV curve. Therefore, the potential window of 1.1 V was chosen for this BSH. The shape of the CV curve is similar to rectangular, which suggests the more important role for the carbon cloth on the charge/discharge behavior comparing to the active material of the nickel cobalt oxide. The CV curves were further measured at different scan rates, as shown in Fig. 3(e). The highly similar shapes with no distortion were observed for the CV curves measured using the scan rates of 10e50 mV s1. This phenomenon strongly indicates the excellent high-rate charge/discharge ability for this BSH based on carbon cloth. Fig. 3(f) shows the GC/D curves measured using the current densities of 5, 10, 15, 20 and 25 mA cm2. The larger IR drop was observed in the GC/D curves measured at higher current densities. This is a common phenomenon since the more irreversible reaction would occur when the higher charge/discharge rate is applied for the measurement. The shape of the GC/D curves is similar to triangle, which is the character for the carbon based energy storage device. The symmetry of the GC/D curves is very high, indicating the high reversibility for charging and discharging. The potential window of 1.1 V and the excellent high-rate charge/discharge ability were obtained for both of the BSH based on Ni foam and carbon cloth, but the electrochemical behavior is very different for the BSH prepared using the substrate of Ni foam and carbon cloth. The battery-type CV curves were observed for the BSH based on Ni foam, due to the metal nature of the substrate and the active material of nickel cobalt oxide. However, the capacitor-type CV curves were obtained for the BSH based on carbon cloth. The nearly invisible redox peaks resulting from the nickel cobalt oxide active material suggest that the electrochemical performance for the NiCo2O4 cannot be fully displayed when using the carbon cloth as the substrate. The electrocapacitive performance for the BSH based on Ni foam and carbon cloth was further compared using the CV curve, as shown in Fig. 3(g). Two couples of the extra redox peaks were clearly observed in the CV curve for the BSH based on Ni foam. The CF value was calculated using Equation (2) as follows.
ð IdV CF ¼
n,DV,A
(2)
R where I is the current density, IdV is the integrated area of the CV curve, n is the scan rate for measuring the CV curve, DV is the potential window, and A is the active area of the electroactive material in the electrode. The CF values based on unit working area and unit weight of the active material were shown in Table 1. The CF values of 0.275 and 0.252 F cm2 corresponding to the capacities of 0.084 and 0.077 mA h cm2 were obtained for the BSH based on Ni foam and carbon cloth, respectively. Due to the battery-type feature of the active material used in this work, the capacity is significant to provide for estimating the energy storage ability of the electrode [23,24]. Although the surface area and pore volume for the nickel cobalt oxide positive electrode based on carbon cloth are larger than those for the electrode prepared on Ni foam, the extra nickel hydroxide or nickel oxide formed on the Ni foam [25] and the high electrical conductivity for nickel may be responsible for the higher CF value for the latter case. In addition, the energy and power densities of the BSH based on Ni foam and carbon cloth were examined by using the Ragone plots, as shown in Fig. 3(h). The Ragone plot was calculated based on the mass of the active materials on the positive and negative electrodes. At all the power densities, the energy densities are higher for the BSH based on Ni foam than that for the BSH based on carbon cloth. At the maximum power density of 550 W kg1, the energy densities of 1.07 and 0.61 Wh kg1 were respectively obtained for the BSH based on Ni
87
foam and carbon cloth. Also, the energy density decay with increasing power densities is less for the BSH based on Ni foam, comparing to that for the BSH based on carbon cloth. These results suggest the better electrocapacitive performance for the BSH assembled using Ni foam as the substrate. To have more complete comparison, the Ragone plot calculated based on the whole electrode mass including the current collectors was shown as the insert in Fig. 3(h). Since the weight of carbon cloth is smaller than that for the Ni foam, the energy density is higher at all the power densities for the BSH based on the carbon cloth comparing to that for the BSH based on the Ni foam. The Nyquist plot for the BSH with the NiCo2O4 positive electrodes based on Ni foam and carbon cloth was shown in Fig. 3(i). The equivalent circuit for fitting the resistance data was inserted in this figure. The x-intercept can be fitted as the series resistance (Rs) which is in proportional to the electrical conductivity for the device. The semicircle at the high frequency region is an index to the charge-transfer resistance at the interface between the active material and the electrolyte (Rct). The Rs and Rct values are both smaller for the BSH based on the Ni foam, indicating the higher electrical conductivity and smaller interfacial resistance for the device using Ni foam as the substrate. The smaller resistances for this case is consistent with its faster energy storage ability comparing to that for the carbon cloth-based BSH. 3.2. Physical properties of NiCo2O4@NiMoO4 grown on flexible substrates and electrochemical analysis for flexible batterysupercapacitor hybrid devices based on NiCo2O4@NiMoO4 To more fairly investigate the substrate effect on the morphology and the electrochemical performance of the active material, other than the active material of NiCo2O4 discussed in the previous text, the NiCo2O4@NiMoO4 core/shell structure was also fabricated on the flexible substrates and the physical and electrochemical properties were further discussed in this part. The substrate effects on the growth of the core-shell structure should be less than that on the growth of the single layer, NiCo2O4, since the second layer of the NiMoO4 shell was grown on the first NiCo2O4 layer not directly on the substrate. However, there are still some influences of the substrate on the surface properties of the NiCo2O4@NiMoO4 electrodes. Hence the comparisons of the surface properties of the active materials and the electrochemical performances of the energy storage devices are still referable for this part. Fig. 4(a) and (b) respectively show the SEM images for the NiCo2O4@NiMoO4 grown on Ni foam and carbon cloth. The very similar sheet-on-wire core/shell structures were observed for the NiCo2O4@NiMoO4 growing on both of the flexible substrates. This phenomenon is the same as that observed for the NiCo2O4 grown on both of the flexible substrates. The morphology variation of the active materials grown on different substrates is inferred to be small. The composition of NiCo2O4@NiMoO4 grown on Ni foam and carbon cloth was further examined using the EDX spectra, as respectively shown in Fig. 4(c) and (d). The signals for Ni, Co and O were clearly observed in both of the spectra, indicating the formation of nickel cobalt oxide on the flexible substrate. The extra signal for Mo was found in both of the spectra, suggesting the participation of molybdenum in the active material and this is inferred to be the formation of the nickel molybdenum oxide as the shell. The signal of carbon was not obvious in the EDX spectrum for the NiCo2O4@NiMoO4 grown on carbon cloth. This phenomenon suggests that the active material is almost fully covered on the substrate, so the signal of carbon from the carbon cloth substrate is hard to detect. On the other hand, the pore distribution for the NiCo2O4@NiMoO4 grown on Ni foam and carbon cloth was examined, as shown in Fig. 4(e). The pore distribution profiles for the NiCo2O4@NiMoO4 growing on the two substrates are highly similar,
88
W.-L. Hong, L.-Y. Lin / Electrochimica Acta 308 (2019) 83e90
Fig. 4. The SEM images for (a) NiCo2O4/Ni foam, (b) NiCo2O4/carbon cloth; the EDX spectra for (c) NiCo2O4/Ni foam and (d) NiCo2O4/carbon cloth; (e) the pore distribution for NiCo2O4 on Ni foam and carbon cloth.
which is different from that observed for the NiCo2O4. This phenomenon indicates that the core/shell structure dominated more on the surface property than the substrate, and again suggests the full coverage of NiCo2O4@NiMoO4 on the flexible substrate. The surface area and the pore volume of NiCo2O4@NiMoO4 on the flexible substrate was shown in Table 2. The surface area of 70.062 and 74.337 m2 g1 were obtained for the NiCo2O4@NiMoO4 on Ni foam and carbon cloth, respectively. The electrode based on the carbon cloth again shows larger surface area than that of the electrode based on the Ni foam. The larger pore volume was also achieved for the electrode based on carbon cloth. This phenomenon may be owing to the 360 growing surface of the carbon cloth for providing more sites for active material deposition. Moreover, the electrochemical performance of the BSH with the NiCo2O4@NiMoO4 positive electrodes on Ni foam and carbon cloth was analyzed. The suitable potential window was firstly examined by measuring the CV curves with the potential windows of 0.8, 1.0, 1.2, 1.4 and 1.6 V for the BSH based on Ni foam, as shown in Fig. 5(a). The slight increases were observed at the high potential region when the potential window of 1.6 V was applied for the measurement. Hence the potential window of 1.4 V was used for further analysis. Fig. 5(b) shows the CV curves measured at different scan rates for the BSH based on Ni foam. The redox peaks are not obvious in the CV curves, and the shape distortion for the CV curves measured using different scan rates is small. This phenomenon indicates the excellent high-rate charge/discharge capability for this BSH. Furthermore, the GC/D curve measure at different current densities for the BSH based on Ni foam was shown in Fig. 5(c). The obvious plateau was observed in the curves, resulting from the redox reaction of Ni2þ/Ni3þ as well as Co2þ/Co3þ and Co3þ/Co4þ during in the charge/discharge process. The symmetry of the GC/D curve is very high at all the current densities, again suggesting the excellent high-rate charge/discharge capacity for this BSH based on Ni foam. On the other hand, the CV curves with different potential windows for the BSH based on carbon cloth were examined, as shown in Fig. 5(d). The redox peaks are also not obvious in the CV curves for the BSH based on carbon cloth, suggesting that the carbon cloth still plays visible roles on the electrochemical behavior for reducing the battery-like nature of the BSH. In addition, the
shape for these CV curves is less like the rectangular shape, but the NiCo2O4/carbon cloth-based BSH shows similar rectangular shape for its CV curve. It is inferred that the higher amount for the active material of NiCo2O4@NiMoO4 on the positive electrode may cause the BSH performing more similar to the battery, so the capacitor feature of the rectangular shape for the CV curve is hard to observe for this case. Due to the larger increases on the current at the high potential region in the CV curve measured using the potential window of 1.8 V, the suitable potential window of 1.6 V was used for further analysis. Fig. 5(e) shows the CV curves measured at different scan rates for the BSH based on carbon cloth. The carbon feature was also observed in the CV curves in the potential region of 0e0.7 V. Fig. 5(f) shows the GC/D curves for the BSH with the NiCo2O4@NiMoO4/carbon cloth positive electrode measured using different current densities. The plateau was also observed in these curves, and the high symmetry of the charge and discharge curves again indicates the excellent high-rate charge/discharge capacity. It is noted that the shapes of the CV curves are much similar for the NiCo2O4@NiMoO4-based BSH on Ni foam and carbon cloth, comparing to the NiCo2O4-based BSH on Ni foam and carbon cloth. This phenomenon indicates that the substrate plays more important roles on the electrochemical performance when the active material is less or the energy storage ability of the active material is worse. To more clearly compare the electrochemical performance of the BSH with the active material of NiCo2O4@NiMoO4 on different flexible substrates, the CV curves at the scan rate of 50 mV s1 were shown in Fig. 5(g). The CF value calculated using the CV curve was shown in Table 2 for comparison. The CF values of 1.294 and 0.443 F cm2 corresponding to the capacities of 0.503 and 0.172 mA h cm2 were obtained for the NiCo2O4@NiMoO4-based BSH with Ni foam and carbon cloth as the substrate, respectively. The much higher CF value was again obtained for the BSH based on Ni foam, as comparing to that for the BSH based on carbon cloth. On the other hand, the Ragone plots for the BSH composed of the NiCo2O4@NiMoO4 positive electrodes prepared using Ni foam and carbon cloth as the substrate were shown in Fig. 5(h). At the maximum power density of 800 W kg1, the energy densities of 11.90 and 5.06 Wh kg1 were respectively obtained for the BSH
Table 2 The surface area and the pore volume for NiCo2O4@NiMoO4 on flexible substrates and the specific capacitances for the BSH with the flexible NiCo2O4@NiMoO4 positive electrodes. Substrate
Surface Area (m2 g1)
Pore Volume (cm3 g1)
CF (F cm2)
CF (F g1)
Q (mAh cm2)
Q (mAh g1)
Ni foam Carbon cloth
70.06 74.34
0.02 0.06
1.29 0.44
42.99 14.38
0.50 0.17
16.72 5.59
W.-L. Hong, L.-Y. Lin / Electrochimica Acta 308 (2019) 83e90
89
Fig. 5. (a) The CV curves at different potential windows, (b) the CV curves at different scan rates and (c) the GC/D curves at different current densities for NiCo2O4@NiMoO4/Ni foam; (d) the CV curves at different potential windows, (e) the CV curves at different scan rates and (f) the GC/D curves at different current densities for NiCo2O4@NiMoO4/carbon cloth; (g) the CV curves based on the weight of the active materials on positive and negative electrodes for NiCo2O4@NiMoO4 on Ni foam and carbon cloth at 50 mV s1; (h) the Ragone plot based on the weight of the active materials on positive and negative electrodes (the inserted plot is based on the weight of the full cell including the current collectors) for NiCo2O4@NiMoO4 on Ni foam and carbon cloth. These figures were measured using the full cell based on the AC negative electrode and the NiCo2O4@NiMoO4 positive electrode.
100
80
80
60
60
40 20 0 0
Areal capacitance Coulombic effciency
40 20
Coulombic effciency (% )
100
Capacitance retention (% )
based on Ni foam and carbon cloth. The nickel foam substrate has the metal nature with high electrical conductivity, and extra nickel hydroxide and nickel oxide would form on the surface of Ni foam. These features promote the better electrochemical performance for the energy storage device assembled using Ni foam, comparing to that for the device prepared using carbon cloth. The decay on the energy density with increasing power density is similar for both case, suggesting the similar high-rate charge/discharge capability for the NiCo2O4@NiMoO4 BSH composed of the Ni foam and carbon cloth substrates. The Ragone plot in Fig. 5(h) was calculated based on the mass of the active materials on the positive and negative electrodes. To have more complete comparison, the Ragone plot calculated based on the whole electrode mass including the current collectors was also shown as the insert in Fig. 5(h). Although the weight of carbon cloth is smaller than that for the Ni foam, the energy density is still higher for the BSH based on the Ni foam comparing to that for the BSH based on the carbon cloth at all the power densities. This result is different from that obtained for the BSH based on NiCo2O4, owing to the much higher energy densities for the BSH based on NiCo2O4@NiMoO4 with the Ni foam substrate comparing to that for the BSH based on NiCo2O4@NiMoO4 with the carbon cloth substrate. The best electrochemical performance was obtained for the BSH with the positive electrode composed of NiCo2O4@NiMoO4 on Ni foam. The stability test was further carried out for examining the cycling ability of this BSH. Fig. 6 shows the capacitance retention and the Coulombic efficiency as a function of the cycling number for the BSH with the NiCo2O4@NiMoO4/Ni foam positive electrode. After carrying out 3000 times of the charge/discharge cycles, the areal capacitance retention of 80% was obtained, and the Coulombic efficiency of higher than 90% was achieved during the whole cycling process. The excellent cycling stability was confirmed for
0 500 1000 1500 2000 2500 3000 Cycle number
Fig. 6. The relation between the areal capacitance retention and the Coulombic efficiency to the cycle number for the BSH with the NiCo2O4@NiMoO4/Ni foam positive electrode.
the BSH with the best-performed NiCo2O4@NiMoO4/Ni foam positive electrode. After separately discussing the substrate effects based on the NiCo2O4 and NiCo2O4@NiMoO4 electrodes, the electrochemical performances for the NiCo2O4 and NiCo2O4@NiMoO4 electrodes were further compared to realize the effect of the active material. The electrochemical performances for the similar electrodes reported in the previous literature were also compared to evaluate the energy storage ability of the electrodes proposed in this work. The capacities of 0.084, 0.077, 0.503 and 0.172 mA h cm2 were respectively obtained for the BSH with the NiCo2O4/Ni foam,
90
W.-L. Hong, L.-Y. Lin / Electrochimica Acta 308 (2019) 83e90
NiCo2O4/carbon cloth, NiCo2O4@NiMoO4/Ni foam and NiCo2O4@NiMoO4/carbon cloth positive electrode. The NiCo2O4@NiMoO4 core-shell electrodes showed much larger capacities than those for the NiCo2O4 electrodes based on the same substrate, owing to the higher surface area and pore volume as well as the higher electrical conductivity with the participation of molybdenum for the core-shell material. The optimized BSH with the NiCo2O4@NiMoO4/Ni foam positive electrode shows the energy density of 11.90 Wh kg1 at the maximum power density of 800 W kg1, and the maximum energy density of 15.40 Wh kg1 at the power density of 160 W kg1. In the previous literature, Zhang et al. assembled asymmetric supercapacitors of NiCo2O4 uninterrupted nanosheet arrays@NiMoO4 nanosheets//active carbon and obtain maximum energy density of 52.6 Wh kg1 at the power density of 332.4 W kg1 [26]. Huang et al. fabricated NiCo2O4@NiMoO4 on carbon cloth as the energy storage electrode and obtained a CF value of 2.917 F cm2 at 2 mA cm2 [27]. Chang et al. fabricated hierarchical NiCo2O4@NiMoO4 coreeshell nanowire/nanosheet arrays on Ni foam for pseudocapacitors. An energy density of 21.7 Wh kg1 was obtained at the power density of 157 W kg1 [28]. 4. Conclusions The substrate effect on the morphology, surface area, and pore structure for nickel cobalt oxide and the nickel cobalt oxide/nickel molybdenum oxide nanostructures on flexible substrates were investigated. The morphology is almost independent on the substrate, but the surface area and the pore volume are larger for the electrode fabricated using the carbon cloth as the substrate. The better electrochemical performance was obtained for the BSH assembled using the Ni foam, comparing to that for the BSH fabricated using the carbon cloth as the substrate, owing to the higher electrical conductivity and the extra formation of nickel hydroxide and nickel oxide on the surface of the Ni foam substrate. The highest energy and power densities were obtained for the BSH composed of the NiCo2O4@NiMoO4/Ni foam positive electrode. At the maximum power density of 800 W kg1 the energy density still remained 11.90 Wh kg1 for this case. It is inferred that the surface properties may be better when the carbon cloth was used as the substrate for assembling the energy storage device, whereas the nature for the nickel metal leads to the better electrochemical performance for the BSH assembled using the Ni foam substrate.
Acknowledgements This work was financially supported from the Young Scholar Fellowship Program by MOST in Taiwan, under Grant MOST 1072636-E-027-003. This work was financially supported in part by the “Research Center of Energy Conservation for New Generation of Residential, Commercial, and Industrial Sectors” from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
Y.Y. Huang, L.Y. Lin, ACS Appl. Energy Mater. (2018) 2979e2990. Y. Zhang, Z. Jiang, X. Yu, IEEE Energy 2030 Conference (2008), 2008, pp. 1e6. W. Zuo, R. Li, C. Zhou, Y. Li, J. Xia, J. Liu, Adv. Sci. (2017) 1600539. L.Y. Lin, H.Y. Lin, W.L. Hong, L.Y. Lin, Thin Solid Films, 2018, pp. 69e75. J.W. Cheng, L.Y. Lin, W.L. Hong, L.Y. Lin, H.Q. Chen, H.X. Lai, Electrochim. Acta (2018) 1245e1252. H.Y. Sun, L.Y. Lin, Y.Y. Huang, W.L. Hong, Electrochim. Acta (2018) 692e699. H.X. Lai, L.Y. Lin, J.Y. Lin, Y.K. Hsu, Electrochim. Acta (2018) 115e126. R. Wang, X. Yan, Sci. Rep. (2014) 3712. Y. Yang, Y. Zhou, Z. Hu, W. Wang, X. Zhang, L. Qiang, Q. Wang, J. Alloy. Comp. (2019) 683e692. P. He, Q. Huang, B. Huang, T. Chen, RSC Adv. (2017) 24353e24358. Y. He, S. Du, H. Li, Q. Cheng, V. Pavlinek, P. Saha, J. Solid State Electrochem. (2016) 1459e1467. H. Wan, L. Li, J. Zhang, X. Liu, H. Wang, H. Wang, Front. Energy Res. (2017) 00033. J. Zhang, F. Liu, J.P. Cheng, X.B. Zhang, ACS Appl. Mater. Interfaces (2015) 17630e17640. Z. Zeng, B. Xiao, X. Zhu, J. Zhu, D. Xiao, J. Zhu, Ceram. Int. (2017) S633eS638. F.X. Ma, L. Yu, C.Y. Xu, X.W. Lou, Energy Environ. Sci. (2016) 862e866. L.Y. Lin, L.Y. Lin, Electrochim. Acta (2017) 335e347. W.L. Hong, L.Y. Lin, L.Y. Lin, Electrochim. Acta (2017) 309e322. G. Xiong, P. He, L. Liu, T. Chen, T.S. Fisher, Front. Energy Res. (2015) 00039. X. Wang, C. Yan, A. Sumboja, P.S. Lee, Nano Energy (2014) 119e126. X. Shi, G. Zhou, Chem. Res. Chin. Univ. (2017) 939e945. T. Li, R. Li, H. Luo, J. Mater. Chem. A 4 (2016) 18922. C.F. Yu, L.Y. Lin, J. Colloid Interface Sci. (2016) 1e7. P. Simon, Y. Gogotsi, B. Dunn, Science (2014) 1210e1211. langer, W.L. Jeffrey, J. Electrochem. Soc. (2015) A5185eA5189. T. Brousse, D. Be J.M. Chiu, L.Y. Lin, P.H. Yeh, C.Y. Lai, K. Teng, C.C. Tu, S.S. Yang, J.F. Yu, RSC Adv. (2015) 83383. P. Zhang, J. Zhou, W. Chen, Y. Zhao, X. Mu, Z. Zhang, X. Pan, E. Xie, Chem. Eng. J. (2017) 687. L. Huang, W. Zhang, J. Xiang, H. Xu, G. Li, Y. Huang, Sci. Rep. (2016) 31465. D. Cheng, Y. Yang, J. Xie, C. Fang, G. Zhang, J. Xiong, J. Mater. Chem. A (2015) 14348.