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【Research Background】

Supercapacitors are recognized as competitive energy storage components due to their fast charge and discharge speed, long life and good safety. According to electrochemical processes, supercapacitors can be divided into two categories: ion adsorption/desorption and Faraday redox reactions. The latter is a high-capacity/high-energy battery-like device that typically utilizes multiple redox states of transition metal oxides, known as Faraday tantalum capacitors. However, due to the inherent limitations of conventional bulk electrode materials, the volume and area energy densities of most of the reported asymmetric device ASCs still do not meet commercial needs. Among transition metal oxides, cobalt oxide has attracted extensive attention in energy conversion and storage devices in recent years, but its low conductivity and active site exposure are not ideal in practical applications. The electrochemical storage of metal oxides is primarily dependent on their specific surface area and oxidation state, while the conductivity of binary metal oxides is twice that of mono-oxides. Zn-based binary metal oxides have the advantages of high conductivity, low cost, and low toxicity, and are excellent candidates for high-capacitance ASCs. In addition, nano-engineering and surface/interface modification can improve the properties of the material. Symmetric supercapacitors have a finite voltage window (<1.2V) and their capacity is rapidly attenuated at high current densities. An asymmetric supercapacitor (ASC) expands the potential window by combining electrode materials of two different voltage windows, which enables the ASC to operate at a higher voltage range relative to the symmetric SC. Therefore, research on positive and negative materials with enhanced performance is currently a hot spot in international research. So far, a large amount of research has been invested in the research of positive electrode materials, and research on negative electrode materials as substitutes for carbon materials has received little attention. The most classic negative electrode is made of carbonaceous materials (carbon/graphene, carbon nanotubes). However, the energy density of the carbon-based anode material is limited due to the low specific capacity of the carbonaceous material. On the other hand, except for carbon, iron-based and vanadium-based materials, only a few materials have been studied as negative electrode materials. However, these materials are less stable in long-term cycling tests at high operating potentials. At the same time, in order to achieve the desired electrochemical performance, the anode material and the cathode material must be carefully matched. Therefore, the study of high capacitance, high cycle stability of the new anode material is ideal for assembling high energy density ASCs.

[Introduction]

Recently, the Institute of Physics of Jinan University and the Siyuan Laboratory of Guangdong Engineering Technology Research Institute collaborated with Professor Muhammad Sufyan Javed and Professor Wenjie Mai to report a carbon-based carrier based on battery-type bimetallic oxide (ZnCo2O4) nanopolyhedron (ZCO@CFT). As a positive electrode, and a capacitive structure layered Ti3C2Tx-MXene as a negative electrode material, a high performance all solid flexible a supercapacitor ASC. Before the assembly of ASC, the performance of the two electrodes in the aqueous electrolyte was systematically studied and optimized.

The results were published online in the Journal of Materials Chemisty A, a well-known academic journal in the field of international energy. The topics are: Achieving high rate and high energy density in an all-solid-state flexible asymmetric pseudocapacitor through the synergistic design of binder-free 3D ZnCo2O4 nanopolyhedra and 2D layered Ti3C2Tx MXenes

ZCO@CFT synthesis schematic

Figure 1. Microstructural characterization of CFT and ZCO@CFT (A) FESEM images of CFT (B and C) High-amplitude FESEM images of ZCO@CFT medium-low magnification FESEM images (D and E) nano-polyhedral assembled microspheres (F) Corresponding TEM image (illustration is HRTEM image, lattice fringe 0.3 nm, belongs to 220 plane of spinel phase ZnCo2O4)

Figure 2. (A) XRD pattern of ZCO@CFT (B) XPS full image (C - f) Co 2p, Zn 2p, C 1s and O 1s spectra

Figure 3. Electrochemical performance and kinetic analysis of ZCO@CFT electrode in electrolyte: (A) CV (B) GCD (C) as a function of capacitance and current density (D) Determination of linear currents of positive and negative peak currents The results are calculated as b values, which are 0.85 and 0.770 (inner (D)) (E), the contribution rate of Faraday diffusion and the charge storage process of capacitance and Faraday diffusion control when the capacitance charge storage (F) scan rate is 5 mVs-1. The typical separation (G) is the Nyquis curve of the EIS impedance spectrum, and (H) the cyclic stability at a current density of 25 A g-1.

Figure 4. Physical characterization of Ti3C2TxMXenes

Figure 5. Charge storage kinetics and electrochemical performance of MXene@CFT as a negative electrode in KOH solution electrolyte

Figure 6. Electrochemical performance of ZCO//MXene-ASC all-solid-state asymmetric supercapacitors: (A) Schematic diagram of ASC equipment in KOH electrolyte and charge storage mechanism (B) Voltage window of positive and negative electrodes (C) ZCO// MXene-ASC CV curve at different scanning speeds in the voltage window of 0.0 - -1.6 V (D) ZCO//MXene-ASC curve at different current densities GCD curve (E) Current density-dependent specific capacitance (F) EIS impedance spectrum The Nyquist plot of the spectrum (the inner graph is the region selected from the high frequency range) and (G) the capacitance with the number of cycles remains unchanged (the inner graph shows the first and last four GCD cycles)

Figure 7. (A) Energy comparison of ZCO//MXene-ASC with the recently reported ASC in the literature. (B) CV curve under different bending conditions, demonstrating that bending at a fixed scan rate of 50 mV/s does not affect it. Performance (C) Digital photo of all solid state ZCO//MXene-ASC, bent at different angles, display device flexibility (D) Three charged ZCO//MXene-ASCs are connected in series, continuously lighting 14 red Led (1.6 V, 15 mA) 2 minute photo

[Summary of this article]

In this paper, a high performance asymmetric supercapacitor is designed. The supercapacitor is based on the battery type bimetal oxide ZCO@CFT nanopolyhedron as the positive electrode, and the capacitive layer structure Ti3C2Tx-MXene as the negative electrode material. The ZCO@CFT positive electrode shows a higher tantalum capacitor charge storage capacity (63.22%), a specific capacitance of 2643.66 F g -1 at 2 A g-1, and a specific capacitance of MXene@CFT at 1.5 A g-1. It is 474.23 F g-1, > 95% excellent cycle stability. In addition, ASC was fabricated using ZCO@CFT and MXene@CFT as positive and negative electrodes (denoted as ZCO//MXene-ASC) to verify the feasibility of the proposed design. ZCO//MXene-ASC achieves a high specific capacitance of 281.25 F g-1 at 0.5 A g-1 and a high energy density of 99.94 W h kg-1 at a power density of 800 W kg-1, at 5000 cycles It has a good service life of >94% and has good flexibility. The energy density of ZCO//MXene-ASC is by far the highest among all binary metal oxides, carbonaceous materials and MXene-based SC and ASC. At the same time, the concept of using 2D metal carbides and metal oxides may open a new direction for the development of high-performance energy storage equipment.

Literature link:

DOI: 10.1039/c9ta08227a

Source: WeChat public account MXene Frontier