In this critical review, metal oxides-based materials for electrochemical supercapacitor (ES) electrodes are reviewed in detail together with a brief review of carbon materials and conducting polymers. Their advantages, disadvantages, and performance in ES electrodes are discussed through extensive analysis of the literature, and new trends in material development are also reviewed. Two important future research directions are indicated and summarized, based on results published in the literature: the development of composite and nanostructured ES materials to overcome the major challenge posed by the low energy density of ES (476 references).

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A review of electrode materials for electrochemical supercapacitors

The is a comprehensive review on the needs and potential storage technologies for electrical grid that is expected to integrate significant levels of renewables. This review offers details of the technologies, in terms of needs, status, challenges and future R&d directions.

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Electrochemical Energy Storage for Green Grid

Hierarchical flowerlike nickel hydroxide decorated on graphene sheets has been prepared by a facile and cost-effective microwave-assisted method. In order to achieve high energy and power densities, a high-voltage asymmetric supercapacitor is successfully fabricated using Ni(OH)2/graphene and porous graphene as the positive and negative electrodes, respectively. Because of their unique structure, both of these materials exhibit excellent electrochemical performances. The optimized asymmetric supercapacitor could be cycled reversibly in the high-voltage region of 0–1.6 V and displays intriguing performances with a maximum specific capacitance of 218.4 F g−1 and high energy density of 77.8 Wh kg−1. Furthermore, the Ni(OH)2/graphene//porous graphene supercapacitor device exhibits an excellent long cycle life along with 94.3% specific capacitance retained after 3000 cycles. These fascinating performances can be attributed to the high capacitance and the positive synergistic effects of the two electrodes. The impressive results presented here may pave the way for promising applications in high energy density storage systems.

Advanced Asymmetric Supercapacitors Based on Ni(OH)2/Graphene and Porous Graphene Electrodes with High Energy Density

Asymmetric supercapacitor with high energy density has been developed successfully using graphene/MnO2 composite as positive electrode and activated carbon nanofibers (ACN) as negative electrode in a neutral aqueous Na2SO4 electrolyte. Due to the high capacitances and excellent rate performances of graphene/MnO2 and ACN, as well as the synergistic effects of the two electrodes, such asymmetric cell exhibits superior electrochemical performances. An optimized asymmetric supercapacitor can be cycled reversibly in the voltage range of 0–1.8 V, and exhibits maximum energy density of 51.1 Wh kg−1, which is much higher than that of MnO2//DWNT cell (29.1 Wh kg−1). Additionally, graphene/MnO2//ACN asymmetric supercapacitor exhibits excellent cycling durability, with 97% specific capacitance retained even after 1000 cycles. These encouraging results show great potential in developing energy storage devices with high energy and power densities for practical applications.

Asymmetric Supercapacitors Based on Graphene/MnO2 and Activated Carbon Nanofiber Electrodes with High Power and Energy Density

In order to achieve high energy and power densities, we developed a high-voltage asymmetric electrochemical capacitor (EC) based on graphene as negative electrode and a MnO2 nanowire/graphene composite (MGC) as positive electrode in a neutral aqueous Na2SO4 solution as electrolyte. MGC was prepared by solution-phase assembly of graphene sheets and α-MnO2 nanowires. Such aqueous electrolyte-based asymmetric ECs can be cycled reversibly in the high-voltage region of 0−2.0 V and exhibit a superior energy density of 30.4 Wh kg−1, which is much higher than those of symmetric ECs based on graphene//graphene (2.8 Wh kg−1) and MGC//MGC (5.2 Wh kg−1). Moreover, they present a high power density (5000 W kg−1 at 7.0 Wh kg−1) and acceptable cycling performance of ∼79% retention after 1000 cycles. These findings open up the possibility of graphene-based composites for applications in safe aqueous electrolyte-based high-voltage asymmetric ECs with high energy and power densities.

High-energy MnO2 nanowire/graphene and graphene asymmetric electrochemical capacitors.

A new cheap asymmetric aqueous supercapacitor: Activated carbon//NaMnO2

γ-Graphyne is a new nanostructured carbon material with large theoretical Li+ storage due to its unique large conjugate rings, which makes it a potential anode for high-capacity lithium-ion batteries (LIBs). In this work, γ-graphyne-based high-capacity LIBs are demonstrated experimentally. γ-Graphyne is synthesized through mechanochemical and calcination processes by using CaC2 and C6 Br6 . Brunauer-Emmett-Teller, atomic force microscopy, X-ray photoelectron spectroscopy, solid-state 13 C NMR and Raman spectra are conducted to confirm its morphology and chemical structure. The sample presents 2D mesoporous structure and is exactly composed of sp and sp2 -hybridized carbon atoms as the γ-graphyne structure. The electrode shows high Li+ storage (1104.5 mAh g-1 at 100 mA g-1 ) and rate capability (435.1 mAh g-1 at 5 A g-1 ). The capacity retention can be up to 948.6 (200 mA g-1 for 350 cycles) and 730.4 mAh g-1 (1 A g-1 for 600 cycles), respectively. These excellent electrochemical performances are ascribed to the mesoporous architecture, large conjugate rings, enlarged interplanar distance, and high structural integrity for fast Li+ diffusion and improved cycling stability in γ-graphyne. This work provides an environmentally benign and cost-effective mechanochemical method to synthesize γ-graphyne and demonstrates its superior Li+ storage experimentally.

Mechanochemical Synthesis of γ‐Graphyne with Enhanced Lithium Storage Performance

Poly(ethylene oxide) (PEO)-based solid polymer electrolytes (SPEs) with nonflammability, shape flexibility, and high SPE/Li interfacial stability are poised to be an enabler for solid-state lithium batteries, but their application is restricted by low room-temperature ionic conductivity and poor mechanical strength at elevated temperatures. Herein, hyperstar polymers are synthesized with hyperbranched PEO serving as the star core and linear polystyrene (PS) serving as the arms. The hyperbranched topological structure suppresses crystallization and facilitates PEO segmental motion, and the steric hindrance for PEO segmental motion is tuned by controlling the average chain length of the branched PEO segments. The rigid PS arms entangle during phase separation and form mechanically strong physical cross-links. An all-solid-state Li/LiFePO4 battery based on our hyperstar SPE delivers a stable capacity of 142 mAh g–1 at 0.2C owing to fast ion transport and stable electrolyte/electrode interfaces.

Yang Chen

Papers

A new cheap asymmetric aqueous supercapacitor: Activated carbon//NaMnO2

Mechanochemical Synthesis of γ‐Graphyne with Enhanced Lithium Storage Performance

Hyperbranched PEO-Based Hyperstar Solid Polymer Electrolytes with Simultaneous Improvement of Ion Transport and Mechanical Strength

In situ synthesis of carbon incorporated TiO2 with long-term performance as anode for lithium-ion batteries

Annealing-free synthesis of carbonaceous Nb2O5 microspheres by flame thermal method and enhanced photocatalytic activity for hydrogen evolution