Ruibin Liang‡
a,
Si Liu‡a,
Jianrong Lina,
Jingfei Daia,
Jingyi Penga,
Peiyuan Huanga,
Jianwen Chenb and
Peng Xiao*a
aGuangdong-Hong Kong-Macao Joint Laboratory for Intelligent Micro-Nano Optoelectronic Technology, School of Physics and Optoelectronic Engineering, Foshan University, Foshan 528225, China. E-mail: xiaopeng@fosu.edu.cn
bSchool of Electronic and Information Engineering, Foshan University, Foshan, 528000, China
First published on 15th November 2023
Mass loading is an important parameter to evaluate the application potential of active materials in high-capacity supercapacitors. Synthesizing active materials with high mass loading is a promising strategy to improve high performance energy storage devices. Preparing electrode materials with a porous structure is of significance to overcome the disadvantages brought by high mass loading. In this work, a Mn3O4/NiMoO4@NiCo layered double hydroxide (MO/NMO/NiCo LDH) positive electrode is fabricated on a carbon cloth with a high mass loading of 20.4 mg cm−2. The MO/NMO/NiCo LDH presents as a special three-dimensional porous nanostructure and exhibits a high specific capacitance of 815 F g−1 at 1 A g−1. Impressively, the flexible supercapacitor based on the MO/NMO/NiCo LDH positive electrode and an AC negative electrode delivers a maximum energy density of 22.5 W h kg−1 and a power density of 8730 W kg−1. It also retains 60.84% of the original specific capacitance after bending to 180° 600 times. Moreover, it exhibits 76.92% capacitance retention after 15000 charge/discharge cycles. These results make MO/NMO/NiCo LDH one of the most attractive candidates of positive electrode materials for high-performance flexible supercapacitors.
Nevertheless, the electrochemical performance of the above electrode materials was achieved with a low mass loading. In fact, the mass loading is the key parameter for some electrode materials in practical applications.6 It is usually necessary to achieve an active material mass loading of 8–10 mg cm−2 to build pseudocapacitors for commercial applications.7 However, due to the poor conductivity of metal oxides, the high mass loading affects the conductivity and ion diffusion of the electrode, thereby seriously reducing the specific capacitance, rate performance, and cycle life of supercapacitors. For example, manganese dioxide (MnO2) with a thickness of less than 1 μm can exhibit ultra-high specific capacitance. However, the high mass loading of MnO2 leads to a reduction in specific capacitance due to the close-packed structures causing limited active specific area.8 Therefore, preparing SCs with high energy density and high mass loading electrodes is full of challenges.9 Electrode materials with three-dimensional (3D) porous nanostructures can ensure the rapid transport of ions and electrons through the whole electrode and mechanical stability, which provide a guarantee for its good electrochemical performance on high mass loading.
In this work, we prepared high mass loading (20.4 mg cm−2) Mn3O4/NiMoO4@NiCo LDH (MO/NMO/NiCo LDH) on carbon cloth (CC) by electrodeposition followed by a two-step hydrothermal method. Sheet-like MO/NMO/NiCo LDH exhibited a 3D porous nanostructure. The MO/NMO/NiCo LDH electrode delivered 815 F g−1 at 1 A g−1. The MO/NMO/NiCo LDH//active carbon (AC) flexible supercapacitor (FSC) showed great potential in actual application. It exhibited an energy density of 22.5 W h kg−1 at 997.53 W kg−1. Even when the power density reached 8730 W kg−1, 1.944 W h kg−1 of energy density was retained. Importantly, the FSC keeps 60.84% of its original specific capacitance after bending to 180° 600 times. It also kept 184.62% of the original specific capacitance after 8000 charge/discharge cycles and eventually retained 76.92% after 15000 cycles. The MO/NMO/NiCo LDH electrode showed potential for use in a high-performance SC with a high mass loading electrode.
Fig. 1 (a) The XRD pattern of MO/NMO/NiCo LDH. (b and c) SEM images of MO/NMO/NiCo LDH. (d–h) Element mapping images corresponding to the SEM. |
The X-ray photoelectron spectroscopy (XPS) spectrum of MO/NMO/NiCo LDH is shown in Fig. S1† and includes Ni, Co, Mn, Mo, O and C. As shown in Fig. 2a, there are two major peaks and two satellite peaks in the Ni 2p XPS spectrum.10 The peaks at 855.46 eV and 857.26 eV accord with Ni 2p3/2. The peaks at 872.95 eV and 874.77 eV originate from Ni 2p1/2. The peaks at 861.39 eV (Ni 2p3/2 sat.) and 879.58 eV (Ni 2p1/2 sat.) are satellite peaks. This proves the existence of Ni2+.11 Fig. 2b shows that the Co 2p3/2 peaks (780.61 eV and 783.37 eV) and Co 2p1/2 peaks (796.59 eV and 798.79 eV) contribute to the major peaks of the Co 2p spectrum and the peaks at 786.94 eV and 802.66 eV correspond to the satellite peaks. The existence of Co2+ is further confirmed by the binding energy difference between the peaks of Co 2p3/2 (781.03 eV) and Co 2p1/2 (797.01 eV), which is near 16.00 eV. As shown in the Mo 3d spectrum (Fig. 2c), the existence of Mo6+ is revealed by the peaks at 232.10 eV (Mo 3d5/2) and 235.17 eV (Mo 3d3/2).12 Fig. 2d shows the XPS spectrum of Mn 2p. The peaks at 638.05 eV (Mn2+ 2p3/2), 643.89 eV (Mn2+ 2p3/2) and 650.89 eV (Mn2+ 2p1/2) are from Mn2+. The peak at 646.83 eV corresponds to Mn 2p3/2. The binding energy difference between 641.26 eV and 653.05 eV is 11.79 eV, which proves the existence of Mn3O4.
Fig. 3a shows the cyclic voltammetry (CV) curves of each sample at the scan rate of 30 mV s−1. All the CV curves show redox peaks, which means that energy is stored by redox reaction.13 In addition, the CV area of MO/NMO/NiCo LDH is obviously larger than those of MO/NMO and MO/NiCo LDH, indicating that MO/NMO/NiCo LDH has better capacitance performance than the other samples. Fig. 3b shows the galvanostatic charge/discharge (GCD) curves at 1 A g−1 of the synthesized samples. Due to the energy storage advantages of composite materials and porous structure, the MO/NMO/NiCo LDH positive electrode displays a high specific capacitance of 815 F g−1, which is much larger than those of MO/NMO (525 F g−1) and MO/NiCo LDH (135 F g−1). As shown in Fig. 3c, all electrochemical impedance spectroscopy (EIS) curves consist of a quasi-semicircle (representing the charge transfer resistance (Rct) caused by redox reaction) and an oblique line (revealing the diffusion behavior of ions).14,15 The EIS curve of MO/NMO/NiCo LDH displays a smaller diameter semicircle and a larger slope in the linear part, which means that MO/NMO/NiCo LDH possesses a smaller Rct and faster ion-diffusion rate than the other samples.
Fig. 3 The curves of MO/NMO/NiCo LDH, MO/NMO/NiCo LDH, and MO/NMO/NiCo LDH: (a) the CV curve at 30 mV s−1, (b) GCD curve at 1 A g−1, and (c) EIS curve. |
Recently, mass loading has been an important parameter to evaluate the application potential of an active material in a high capacity supercapacitor. Synthesizing an active material with high mass loading is still a promising strategy to improve high performance energy storage devices, though high mass loading leads to a sacrifice in electrochemical performance. Therefore, we investigated the electrochemical properties of MO/NMO/NiCo LDH with different mass loadings. It can be easily seen in Fig. S2† that the CV curve area and discharge time increase with the mass loading. In other words, the electrochemical performance of MO/NMO/NiCo LDH is enhanced instead of sacrificed with a high mass loading of 20.4 mg cm−2. This reveals that MO/NMO/NiCo LDH has great application potential in high-capacity supercapacitors. Table 1 shows the specific capacitances and mass loadings of reported electrode materials. Compared to the reported materials,16–20 the MO/NMO/NiCo LDH electrode in our work exhibited higher specific capacitance and higher mass loading. However, MO/NMO/NiCo LDH and MO/NMO/NiCo LDH show worse specific capacitance, which may be due to the poor conductivity and low ion diffusion brought by high mass loading.
Electrode | Specific capacitance | Mass loading | Reference |
---|---|---|---|
Co–Ni LDH | 811 F g−1 | 3.59 mg cm−2 | 16 |
Graphene films | 340 F g−1 | 4.8 mg cm−2 | 17 |
Polyaniline-graphite | 607 F g−1 | 5.89 mg cm−2 | 18 |
N-Doped porous carbon | 457 F g−1 | 1 mg cm−2 | 19 |
Ni–Co–S | 640 F g−1 | 8.84 mg cm−2 | 20 |
MO/NMO | 525 F g−1 | 15 mg cm−2 | This work |
MO/NiCo LDH | 135 F g−1 | 13.8 mg cm−2 | This work |
MO/NMO/NiCo LDH | 815 F g−1 | 20.4 mg cm−2 | This work |
Fig. 4a shows the CV curves of MO/NMO/NiCo LDH at different scan rates with the mass loading of 20.4 mg cm−2. When the scan rate reaches 100 mV s−1, both the oxidation peak and reduction peak are still visible, revealing the good energy storage behavior of MO/NMO/NiCo LDH. To confirm the pseudocapacitance behavior of MO/NMO/NiCo LDH, kinetic calculations were performed according to the CV curves. The relationship between the scan rate (v) and peak current (i) is described by the following formulas,21
i = avb |
logi = blogv + loga |
i(v) = k1ν + k2ν1/2 |
To reveal the application potential of high mass loading MO/NMO/NiCo LDH in a flexible supercapacitor, a FSC was assembled using MO/NMO/NiCo LDH as the anode, AC/CC as the cathode and polyvinyl alcohol (PVA)/KOH as the gel electrolyte in a sandwich structure, as shown in Fig. 5a. Fig. S4a† shows that the CV curve at 2.0 V displays no obvious polarization, which means the FSC can work stably at 2.0 V.24 Fig. 5b shows the CV curves of the FSC at different scan rates from 10 mV s−1 to 200 mV s−1. The redox peaks can be easily seen at the high scan rate of 200 mV s−1, revealing the outstanding rate performance of the FSC. Fig. 5c shows the GCD curves of the FSC at different current densities and the corresponding specific capacitances of each GCD curve are 40.5 (1 A g−1), 22.2 (2 A g−1), 15.3 (3 A g−1), 8 (5 A g−1) and 3.5 F g−1 (7 A g−1). Fig. 5d displays the relationship between the energy density and the power density of the FSC. The maximum energy density and power density of the FSC are 22.5 W h kg−1 and 8730 W kg−1. It can be clearly seen that the FSC in this work possesses better performance than those of other reported supercapacitors, including the HPC-4//HPC-4 all-solid-state symmetric supercapacitor,25 PANI@CNFs//PANI@CNFs flexible supercapacitor,26 BIC-Co3O4//BIC-Co3O4 symmetric cell,27 APC4/1//APC4/1,28 PANI/GF//PANI/GF device,29 Ni33/ZIF-67/rGO20//Ni33/ZIF-67/rGO20 all-solid-state supercapacitor30 and L-Ti3C2Tx/E-ANF based flexible supercapacitor.31
Whether the FSC can work normally after deformation is an important indicator to measure its application potential in flexible devices. Fig. 5e shows the CV curves under different bending conditions. The CV curves under bending conditions show little difference and are obviously smaller in curve area than that of the original status. The corresponding area data of each curve are shown in Table S1.† The decrease in the CV curve area means that the performance of the FSC weakens during folding. This can be further proved by the GCD test. Fig. S4b† shows that the GCD curves under bending addition show little difference in shape to the original, which means that the FSC can work stably under bending conditions. In addition, the discharging time of each status is 81.2 s (original status, 40.5 F g−1), 56.6 s (folded 200 times, 28.3 F g−1), 54 s (folded 300 times, 27 F g−1), 52.4 s (folded 400 times, 26.2 F g−1), 50.2 s (folded 500 times, 25.1 F g−1) and 49.4 s (folded 600 times, 24.7 F g−1). In another words, the FSC kept 60.84% of the original specific capacitance after bending 600 times. The reduction of the CV curve area and discharging time may be attributed to the exfoliation of active material from the substrate. It is clearly shown in Fig. S4c† that the EIS curves under bending conditions possess smaller slopes than the original. To simulate the scenario of practical application, the long-term cycle performances of the FSC are evaluated at 5 A g−1, and Fig. 5f shows the relationship between capacitance retention and cycles. The FSC possessed a high capacitance retention of 184.62% after 8000 GCD cycles and also retained 76.92% of the original specific capacitance after 15000 cycles. The superior capacitance retention is due to the special structure of MO/NMO/NiCo LDH. During the high-current charge/discharge process, the 3D porous nanostructures of MO/NMO/NiCo LDH provide a large number of transport channels for electrons/ions as well as reactive active sites, which ensure the smooth progress of the redox reaction. Meanwhile, there is ample space for material expansion/contraction, greatly improving the cycle stability during long charge/discharge processes.
The electrochemical growth of MO was processed by the cyclic voltammetry (CV) method, modified from Raut's work.10 5 mM MnCl2·4H2O and 5 mM NaCl were dissolved in 50 mL deionized water to prepare the precursor solution. The electrodeposition was performed in a three-electrode system, with the CC as the working electrode with a soak area of 1 cm × 1 cm, a platinum plate as the counter electrode, and a saturated calomel electrode as the reference electrode. The CV process was performed in the potential window of 0–1 V at 10 mV s−1 for 4 cycles. After the deposition, the electrodeposited MO was cleaned and dried for 6 h. The mass of MO deposited on the CC was 7.4 mg.
The total mass of MO/NMO/NiCo LDH was 20.4 mg cm−2. In addition, for comparison, MO/NMO and MO/NiCo LDH were synthesized by the same process.
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ra06937k |
‡ These authors contributed equally to this work. |
This journal is © The Royal Society of Chemistry 2023 |