Tuning Carbon Materials for Supercapacitors by Direct Pyrolysis of Seaweeds

The sea provides a large variety of seaweeds that, because of their chemical composition, are fantastic precursors of nanotextured carbons. The carbons are obtained by the simple pyrolysis of the seaweeds under a nitrogen atmosphere between 600 and 900 °C, followed by rinsing the product in slightly acidic water. Depending on the origin of the seaweed and on the pyrolysis conditions, the synthesis may be oriented to give an oxygen‐enriched carbon or to give a tuned micro/mesoporous carbon. The samples with a rich oxygenated surface functionality are excellent as supercapacitor electrodes in an aqueous medium whereas the perfectly tuned porous carbons are directly applicable for organic media. In both cases, the specific surface area of the attained carbons does not exceed 1300 m² g⁻¹, which results in high‐density materials. As a consequence, the volumetric capacitance is very high, making these materials more interesting than activated carbons from the point of view of developing small and compact electric power sources. Such versatile carbons, obtained by a simple, ecological, and cheap process, could be well used for environment remediation such as water and air treatment.     

Flexible Wire‐Shaped Supercapacitors in Parallel Double Helix Configuration with Stable Electrochemical Properties under Static/Dynamic Bending

Wire‐shaped flexible supercapacitors (SCs) have aroused much attention due to their small size, light weight, high flexibility, and deformability. However, the previously reported wire‐shaped SCs usually involve complex assembly processes, encounter potential structural instabilities, and the influence of dynamic bending on the electrochemical stability of wire‐shaped SCs is also not clear. Here, a parallel double helix wire‐shaped supercapacitor (PDWS) protocol has been developed with two symmetric titanium@MnO₂ fiber electrodes winded on a flexible nylon fiber by a simple and reliable process. The PDWSs show an operate voltage of 0.8 V, a high capacitance of 15.6 mF cm^–2 and an energy density of 1.4 µWh cm^–2. Due to rational structure design, the PDWSs demonstrate excellent mechanical and electrochemical stability under both static and dynamic deformations. Over 3500 bending cycles, 88.0% of the initial capacitance can still be retained. In terms of dynamic bending, it is found that the cyclic voltammetry curves show periodically fluctuations simultaneously with the bending frequency and the intensity of fluctuation increases with higher bending frequency, while the dynamic capacitance is almost not affected. With extraordinary mechanical flexibility and excellent electrochemical stability, the high performance PDWS is considered to be a promising power source for wearable electronics.     

Fabrication of Hollow Microporous Carbon Spheres from Hyper‐Crosslinked Microporous Polymers

Porous carbon materials prepared from the porous organic polymers are currently the subject of extensive investigation. On the basis of their interesting applications, it is highly desirable to develop new synthetic methodologies to obtain carbon materials with controllable pore size and morphology. Herein, a facile synthesis of hollow microporous carbon spheres (HCSs) from hollow microporous organic capsules (HMOCs) with a good control over the pore morphology, hollow cavity, and the shell thickness is reported. The highly porous hollow carbon spheres are prepared by the pyrolysis of HMOCs‐based microporous polymers. The synthetic parameters, such as hypercrosslinking and pyrolysis conditions, are optimized to modify the porous structures and the properties. The morphology and porosity as well as energy storage applications of the microporous structures HCSs, derived through a combination of divinylbenzene‐crosslinking and micropore‐generating hypercrosslinking, are discussed. These findings provide a new benchmark for fabricating well‐defined HCSs with great promise for various applications.     

High Volumetric Energy Density Asymmetric Supercapacitors Based on Well‐Balanced Graphene and Graphene‐MnO2 Electrodes with Densely Stacked Architectures

The well‐matched electrochemical parameters of positive and negative electrodes, such as specific capacitance, rate performance, and cycling stability, are important for obtaining high‐performance asymmetric supercapacitors. Herein, a facile and cost‐effective strategy is demonstrated for the fabrication of 3D densely stacked graphene (DSG) and graphene‐MnO₂ (G‐MnO₂) architectures as the electrode materials for asymmetric supercapacitors (ASCs) by using MnO₂‐intercalated graphite oxide (GO‐MnO₂) as the precursor. DSG has a stacked graphene structure with continuous ion transport network in‐between the sheets, resulting in a high volumetric capacitance of 366 F cm^–3, almost 2.5 times than that of reduced graphene oxide, as well as long cycle life (93% capacitance retention after 10 000 cycles). More importantly, almost similar electrochemical properties, such as specific capacitance, rate performance, and cycling stability, are obtained for DSG as the negative electrode and G‐MnO₂ as the positive electrode. As a result, the assembled ASC delivers both ultrahigh gravimetric and volumetric energy densities of 62.4 Wh kg^–1 and 54.4 Wh L^–1 (based on total volume of two electrodes) in 1 m Na₂SO₄ aqueous electrolyte, respectively, much higher than most of previously reported ASCs in aqueous electrolytes.     

Electrochemical cyclability mechanism for MnO2 electrodes utilized as electrochemical supercapacitors

The electrochemical cyclability mechanism of nanocrystalline MnO₂ electrodes with rock salt-type and hexagonal ?-type structures was investigated to determine the relationship between physicochemical feature evolution and the corresponding electrochemical behaviour of MnO₂ electrodes. Rock salt MnO₂ and hexagonal ?-MnO₂ electrodes, with fibrous and porous morphologies, evolve into the antifluorite-type MnO₂ with a petal-shaped nanosheet structure after electrochemical cycling, similar to that observed in nanocrystalline antifluorite-type MnO₂ electrodes after electrochemical cycling. However, a different impedance response was observed for the rock salt MnO₂ and hexagonal ?-MnO₂ electrodes during the charge–discharge cycles, compared with the improved impedance response observed for the cycled antifluorite-type MnO₂. A dissolution–redeposition mechanism is proposed to account for the impedance response of the MnO₂ electrodes with different morphologies and crystal structures.