Synergy of nano-ZnO and 3D-graphene foam electrodes for asymmetric supercapacitor devices

Ultra-thin freestanding graphene films for efficient thermal insulation and electromagnetic interference shielding

The preparation of freestanding graphene films by convenient and environmentally friendly preparation methods is still the focus of attention in various industrial fields. Here, we first select electrical conductivity, yield and defectivity as evaluation indicators and systematically explore the factors affecting the preparation of high-performance graphene by electrochemical exfoliation, then further post-process it under volume-limited conditions by microwave reduction. Finally, we obtained a self-supporting graphene film with an irregular interlayer structure but excellent performance. It is found that the electrolyte is ammonium sulfate, the concentration is 0.2 M, the voltage is 8 V, and the pH is 11, which were the optimal conditions for preparing low-oxidation graphene. The square resistance of the EG was 1.6 Ω sq^-1, and the yield could be 65%. In addition, electrical conductivity and joule heat were significantly improved after microwave post-processing, especially its electromagnetic shielding performance with a shielding coefficient of 53 dB able to be achieved. At the same time, the thermal conductivity is as low as 0.05 W m^-1 K^-1. The mechanism for the improvement of electromagnetic shielding performance is that (1) microwave reduction effectively enhances the conductivity of the graphene sheet overlapping network; (2) the gas generated by the instantaneous high temperature causes a large number of void structures between the graphene layers, and the irregular interlayer stacking structure makes the reflective surface more disordered, thereby prolonging the reflection path of electromagnetic waves among layers. In summary, this simple and environmentally friendly preparation strategy has good practical application prospects for graphene film products in flexible wearables, intelligent electronic devices, and electromagnetic wave protection.

N-Doped Carbon Dots and ZnO Conglomerated Electrodes for Optically Responsive Supercapacitor Applications

The over-dependence of human society on fossil fuels for energy is exhausting the level of such non-renewable energy sources. Alternative energy storage systems have gained more popularity recently to counter this issue. In this context, we report the fabrication of N-doped carbon dot (N-CD)-decorated ZnO-based electrodes for supercapacitor applications. Due to the light-responsive nature of the N-CDs and ZnO, the electrode was also responsive under the influence of UV light. After the experimental tests, it was found that the areal capacitance value of the supercapacitor increased upto ∼58.9% when illuminated compared to that under the dark conditions. Moreover, the device showed a maximum areal capacitance of 2.6 mF/cm² after photocharging and galvanostatically discharging at a current density value of 1.6 μA/cm², which is quite comparable with the previously reported data. The doping of N-CDs with ZnO showed a significant improvement in the areal capacitance value under both illuminated (∼58.64%) and dark conditions (∼22.08%) compared to the case of pristine ZnO, which justifies the purpose of attaching N-CDs with ZnO. Therefore, in brief, we have fabricated a photoresponsive electrode material for supercapacitor application by combining N-CDs and ZnO. An explicit electrochemical characterization of the electrode was also done to identify the contribution from diffusion-controlled capacitance and double layer capacitance, and it was observed that the diffusion-controlled capacitance gets reduced from 59.1 to 33.6% when the scan rate is increased from 2 to 75 mV/s. Moreover, a detailed study has also been done to understand the reaction mechanism. It was confirmed that the defects in the electrode material played a vital role in the intercalation of K⁺ ions.

Reduced graphene oxide/ionic liquid composites with tunable interlayer spacing for improved charge/discharge kinetics in supercapacitors

The large specific surface area and high conductivity of reduced graphene oxide (RGO) make it a promising material for supercapacitors. However, aggregation of graphene sheets into graphitic domains upon drying hampers supercapacitor performance by drastically impeding ion transport inside electrodes. Here, we present a facile approach to optimize charge storage performance in RGO-based supercapacitors by systematically tuning their micropore structure. To this end, we combine RGOs with room temperature ionic liquids during electrode processing to impede stacking of sheets into graphitic structures with small interlayer distance. In this process, RGO sheets function as the active electrode material while ionic liquid serves both as a charge carrier and a spacer to control interlayer spacing inside electrodes and form ion transport channels. We show that composite RGO/ionic liquid electrodes with larger interlayer spacing and more ordered structure exhibit improved capacitance and charging kinetics.

ZnO and reduced graphene oxide electrodes for all-in-one supercapacitor devices

Reduced graphene oxide/zinc oxide (rGO/ZnO) hybrid nanocomposites were prepared from synthesized GO and high energy ball milled (HEBM) ZnO for supercapacitor electrodes. Evolution of intrinsic point defects and defect-induced morphological, structural and size-dependent properties of rGO/ZnO hybrid nanocomposites were investigated using electron paramagnetic resonance (EPR) spectroscopy. CV, PEIS and GCPL techniques were employed to investigate the electrochemical behavior of the electrode materials and the effects of defects on the electrochemical performance of the electrodes by using the standard two-electrode cell in a 6 M KOH electrolyte. Analyses of the obtained CV and impedance profiles have shown the pseudocapacitive and EDLC-type contributions in the supercapacitors. Cycling stabilities were evaluated using galvanostatic charge-discharge curves at current densities between 0.10 and 2.40 A g^-1. The capacitance retention of all electrodes was found to be 100% after 30 cycles at 0.30 A g^-1. The electrochemical analyses revealed that the incorporation of ZnO that is rich in core defects improved the charge transfer performance and ion diffusion of the rGO electrode.

Flexible synthesis of high-performance electrode materials of N-doped carbon coating MnO nanowires for supercapacitors

The MnO/C composites were obtained by co-precipitation method, which used Mn₃O₄nanomaterials as precursors and dopamine solution after ultrasonic mixing and calcination under N₂atmosphere at different temperatures. By studying the difference of MnO/C nanomaterials formed at different temperatures, it was found that with the increase of calcination temperature, the materials appear obvious agglomeration. The optimal calcination temperature is 400 °C, and the resulting MnO/C is a uniformly dispersed slender nanowire structure. The specific capacitance of MnO/C nanowires can reach 356 F g^-1at 1 A g^-1. In the meantime, the initial capacitance of MnO/C nanowires remains 106% after 5000 cycles. Moreover, the asymmetric supercapacitor was installed, which displays a tremendous energy density of 30.944 Wh kg^-1along with a high power density of 10 kW kg^-1. The composite material reveals a promising prospect in the application of supercapacitors.

In situ hydrothermal synthesis of nickel cobalt sulfide nanoparticles embedded on nitrogen and sulfur dual doped graphene for a high performance supercapacitor electrode

Nickel cobalt sulfide nanoparticles (NCS) embedded onto a nitrogen and sulfur dual doped graphene (NS-G) surface are successfully synthesized via a two-step facile hydrothermal process. The electrical double-layer capacitor of NS-G acts as a supporting host for the growth of pseudocapacitance NCS nanoparticles, thus enhancing the synergistic electrochemical performance. The specific capacitance values of 1420.2 F g⁻¹ at 10 mV s⁻¹ and 630.6 F g⁻¹ at 1 A g⁻¹ are achieved with an impressive capability rate of 76.6% preservation at 10 A g⁻¹. Furthermore, the integrating NiCo₂S₄ nanoparticles embedding onto the NS-G surface also present a surprising improvement in the cycle performance, maintaining 110% retention after 10 000 cycles. Owing to the unique morphology an impressive energy density of 19.35 W h kg⁻¹ at a power density of 235.0 W kg⁻¹ suggests its potential application in high-performance supercapacitors.

Newly developed in situ hydrothermal synthesis governs morphology of Ni–Co–S embedded on N–S doped graphene thus providing exceptional capacitive behavior.

About defect phenomena in ZnO nanocrystals

ZnO nanocrystals are receiving renewed attraction due to their multifunctional properties. Selective enhancement and tuning of their optical and electrical properties are essential for achieving novel devices with accurate sensing and conducting capabilities. The nature and type of intrinsic defects that occur in ZnO influence these properties. In this work, we investigate the intrinsic defect structure of ZnO via electron paramagnetic resonance (EPR) and photoluminescence (PL) spectroscopy and correlate the results with existing computational works. Mainly, the defects are analysed by taking the microscopic defect structure of the lattice into account. The results manifest the core-shell model of the defect structure in ZnO. By default, specifically for nanocrystals, oxygen vacancies localise on the surface, while zinc vacancies localise in the core. The investigations in this report demonstrate that the concentration of the intrinsic defects and their position can be tuned merely by changing the size of the nanocrystal. Additionally, the UV, green, orange and red emissions can be tuned by nanocrystal's size and post-annealing treatments.

Mass production of metal-doped graphene from the agriculture waste of Quercus ilex leaves for supercapacitors: inclusive DFT study

This work reports a facile, eco-friendly, and cost-effective mass-scale synthesis of metal-doped graphene sheets (MDGs) using agriculture waste of Quercus ilex leaves for supercapacitor applications. A single step-degradation catalyst-based pyrolysis route was used for the manufacture of MDGs. Obtained MDGs were further evaluated via advanced spectroscopy and microscopic techniques including Raman spectroscopy, FT-IR, XRD, SEM/EDX, and TEM imaging. The Raman spectrum showed D and G bands at 1300 cm^-1 and 1590 cm^-1, respectively, followed by a 2D band at 2770 cm^-1, which confirmed the synthesis of few-layered MDGs. The SEM/EDX data confirmed the presence of 6.15%, 3.17%, and 2.36% of potassium, calcium and magnesium in the obtained MDGs, respectively. Additionally, the FT-IR, XRD, TEM, and SEM data including the plot profile diagrams confirmed the synthesis of MDGs. Further, a computational study was performed for the structural validation of MDGs using Gaussian 09. The density functional theory (DFT) results showed a chemisorption/decoration pattern of doping for metal ions on the few-layered graphene nanosheets, rather than a substitutional pattern. Further, resulting MDGs were used as an active material for the fabrication of a supercapacitor electrode using the polymer gel of PVA-H₃PO₄ as the electrolyte. The fabricated device showed a decent specific capacitance of 18.2 F g^-1 at a scan rate of 5 mV s^-1 with a power density of 1000 W kg^-1 at 5 A g^-1.

ZnO and MXenes as electrode materials for supercapacitor devices

Supercapacitor devices are interesting owing to their broad range of applicability from wearable electronics to energy storage in electric vehicles. One of the key parameters that affect the efficiency of supercapacitor devices is selecting the ideal electrode material for a specific application. Regarding this, recently developed metal oxides, specifically nanostructured ZnO, and MXenes with their defect structures, size effects, as well as optical and electronic properties have been presented as electrode material in supercapacitor devices. The discussion of MXenes along with ZnO, although different in chemistry, also highlights the differences in dimensionality when it comes to defect-driven effects, especially in carrier transport. The volume under the influence of the defect centers is expected to be different in bulk and 2D structures, regardless of composition. Hence, analysis and discussion of both materials provide a fundamental understanding regarding the manner in which 2D structures are impacted by defects compared to bulk. Such an approach would therefore serve the scientific community with the material design tools needed to fabricate the next generation of supercapacitor devices.

The structure-directing role of graphene in composites with porous FeOOH nanorods for Li ion batteries

Graphene sheets that contain porous iron oxides including Fe2O3 and FeOOH nanorods were synthesized via a one-step hydrothermal route. A novel mechanism for controlling the structure of graphene-based composites was developed. Porous FeOOH nanorods with a high capacity for electron- and ion-transport were synthesized by controlling the composition of GO dispersion. The synthesized graphene/FeOOH composite anode exhibited an excellent electrochemical performance in which a reversible capacity of 304 mA h g-1 was reached with nearly 100% coulombic efficiency after 1000 cycles of charge and discharge under a high current rate of 5 A g-1.