Due to their unique 2D structure and outstanding intrinsic physical properties, such as extraordinarily high electrical conductivity and large surface area, graphene-based materials exhibit great potential for application in supercapacitors. In this review, the progress made so far for their applications in supercapacitors is reviewed, including electrochemical double-layer capacitors, pseudo-capacitors, and asymmetric supercapacitors. Compared with traditional electrode materials, graphene-based materials show some novel characteristics and mechanisms in the process of energy storage and release. Several key issues for improving the structure of graphene-based materials and for achieving better capacitor performance, along with the current outlook for the field, are also discussed.

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An overview of the applications of graphene-based materials in supercapacitors.

Electrochemical capacitors, so-called supercapacitors, play an important role in energy storage and conversion systems. In the last decade, with the increasing volume of scientific activity and publications in this field, researchers have developed better tools to improve electrode materials. Although carbonaceous materials seem the most suitable for supercapacitor applications, a large diversity of materials has been proposed and studied. Yet, in order to accomplish performance beyond the limitations of each material, mainly in terms of energy density and durability, composite materials have been implemented, most of them being the combinations of carbon-based materials and other components. In this review, we present the recent advances in the field of composite materials that include at least one carbon-based component for supercapacitor electrodes. We focus on cases in which a single material by itself suffers from a drawback that can be overcome by combining it with other components, enabling the fabrication of a composite material with enhanced performance. We present several important compositions as well as the major routes of synthesis, characterization and performance of composite materials in this field.

Carbon-based composite materials for supercapacitor electrodes: a review

Electrochemistry of graphene and related materials.

Nitrogen-doped graphene nanosheets (NGS) with the nitrogen level as high as 10.13 atom% were synthesized via a simple hydrothermal reaction of graphene oxide (GO) and urea. N-doping and reduction of GO were achieved simultaneously under the hydrothermal reaction. In the fabrication, the nitrogen-enriched urea plays a pivotal role in forming the NGS with a high nitrogen level. During the hydrothermal process, the N-dopant of urea could release NH3 in a sustained manner, accompanied by the released NH3 reacting with the oxygen functional groups of the GO and then the nitrogen atoms doped into graphene skeleton, leading to the formation of NGS. The nitrogen level and species could be conveniently controlled by easily tuning the experimental parameters, including the mass ratio between urea and GO and the hydrothermal temperature. Remarkably, in 6 M KOH electrolyte, the synthesized NGS with both high nitrogen (10.13 atom%) and large surface area (593 m2 g−1) exhibits excellent capacitive behaviors (326 F g−1, 0.2 A g−1), superior cycling stability (maintaining initial capacity even) and coulombic efficiency (99.58%) after 2000 cycles. The energy density of 25.02 Wh kg−1 could be achieved at power density of 7980 W kg−1 by a two-electrode symmetric capacitor test. A series of experiments results demonstrated that not only the N-content but also the N-type are very significant for the capacitive behaviors. In more detail, the pyridinic-N and pyrrolic-N play mainly roles for improving pseudo-capacitance by the redox reaction, while quaternary-N could enhance the conductivity of the materials which is favorable to the transport of electrons during the charge/discharge process. Hence, the approach in this work could provide a new way for preparing NGS materials which could be used as advanced electrodes in high performance supercapacitors.

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Nitrogen-doped graphene with high nitrogen level via a one-step hydrothermal reaction of graphene oxide with urea for superior capacitive energy storage

Being confronted with the energy crisis and environmental problems, the exploration of clean and renewable energy materials as well as their devices are urgently demanded. Two-dimensional (2D) atomically-thick materials, graphene and grpahene-like layered transition metal dichalcogenides (TMDs), have showed vast potential as novel energy materials due to their unique physicochemical properties. In this Review, we outline the typical application of graphene and grpahene-like TMDs in energy conversion and storage fields, and hope to promote the development of 2D TMDs in this field through the analysis and comparisons with the relatively natural graphene. First, a brief introduction of electronic structures and basic properties of graphene and TMDs are presented. Then, we summarize the exciting progress of these materials made in both energy conversion and storage field including solar cells, electrocatalysis, supercapacitors and lithium ions batteries. Finally, the prospects and further developments in these exciting fields of graphene and graphene-like TMDs materials are also suggested.

Graphene and graphene-like layered transition metal dichalcogenides in energy conversion and storage.

Transition metal oxides receive considerable attention in the area of electrochemistry not only due to their beneficial reported structural, mechanical or electronic properties, but because of their capacitive properties ascribed to their multiple oxide states they exhibit pseudo capacitances which carbon counterparts generally cannot. Typically transition metal oxides may be classified as noble transition metal oxides which exhibit excellent capacitive properties but have the drawback of generally being relatively expensive. Alternatively base metal oxides may also be utilised which are considerably cheaper and more environment friendly than noble transition metals as well as exhibiting good capacitive properties. In considering that nanostructured materials can help ameliorate the electrochemical performances of transition metal oxides, this review summarizes the recent investigations of fundamental advances in understanding the electrochemical reactivity of transition metal oxides, thus leading to an improved capacitive performance, which is essential for their continual use in a plethora of supercapacitor applications.

Electrochemical capacitors utilising transition metal oxides: an update of recent developments

Fundamental and solutions of microcrack in Ni-rich layered oxide cathode materials of lithium-ion batteries

Garnet Solid Electrolyte for Advanced All-Solid-State Li Batteries

A hexapod PtRuCu nanocrystalline alloy material is successfully fabricated by a facile approach based on Stranski–Krastanov growth and galvanic replacement and used as an efficient catalyst for direct methanol fuel cells (DFMCs). Because of the synergetic effect of metallic elements in the methanol oxidation, the activity and durability of the prepared PtRuCu/C catalyst is significantly enhanced. It has a mass activity of 1.35 A mgPt–1 and a specific activity of 3.92 mA cm–2, which are 3.8 and 8.2 times higher than those of Pt/C, respectively. Its mass activity only decreases 27% after 800 CV cycles compared with Pt/C (60% decline), indicating the admirable stability of the PtRuCu catalyst.

Hexapod PtRuCu Nanocrystalline Alloy for Highly Efficient and Stable Methanol Oxidation

The practical application of Li‐rich Mn‐based oxide cathode is predominately retarded by the capacity decline and voltage fading, associated with the structure distortion and anionic redox reactions. Here, a linkage‐functionalized modification approach to tackle these challenges via a synchronous lithium oxidation strategy is reported. The doping of Ce in the bulk phase activates the pseudo‐bonding effect, effectively stabilizing the lattice oxygen evolution and suppressing the structure distortion. Interestingly, it also induces the formation of spinel phase Li4Mn5O12 in the subsurface, which in turn constructs the phase boundaries, thereby arousing the interior self‐built‐in electric field to prevent the outward migration of bulk oxygen anions and boost the charge transfer. Moreover, the formed coating layer Li2CeO3 with oxygen vacancies accelerates Li+ diffusion and mitigates electrolyte cauterization. The corresponding cathode exhibits superior long‐cycle stability after 300 cycles with only a 0.013% capacity drop and 1.76 mV voltage decay per cycle. This work sheds new light on the development of Li‐rich Mn‐based oxide cathodes toward high energy density applications.

Wentao Deng

Papers

Electrochemical capacitors utilising transition metal oxides: an update of recent developments

Fundamental and solutions of microcrack in Ni-rich layered oxide cathode materials of lithium-ion batteries

Garnet Solid Electrolyte for Advanced All-Solid-State Li Batteries

Hexapod PtRuCu Nanocrystalline Alloy for Highly Efficient and Stable Methanol Oxidation

Pseudo-Bonding and Electric-Field Harmony for Li-Rich Mn-Based Oxide Cathode