Excessive emission of greenhouse gases has serious adverse effects on global climate. How to reduce carbon emissions has become a global problem. Supercapacitors have advantages of long cycle life, high power density and relatively low carbon emissions. Developing supercapacitor energy storage is an effective measure to build the reliable future energy system. In recent years, MXene materials have achievedgreat popularity in the field of supercapacitor energy storage applications due to their excellent hydrophilicity, electrical conductivity, high electrochemical stability, and surface chemical tunability. However, the serious self-stacking problem of MXene limits its performance in energy storage. Developing advanced MXene materials is critical for next generation high-performance electrochemical energy storage devices. This paper reviews the research progress of MXene material in the field of supercapacitor energy storage. Firstly, the structure and energy storage properties of MXene are introduced, followed by analysis of the energy storage mechanism of MXene. Secondly, nanoengineering of structure design to improve the performance of MXene electrode is depicted. Thirdly, structure-performance relationship of MXene composite materials and its latest research progress in application of supercapacitor are summarized. Finally, research and development trends of MXene as an electrode for supercapacitor are broadly prospected.

The microreactor was constructed by using the block copolymer (P123)/sodium dodecyl sulfate (SDS) hybrid emulsion. Horseshoe-shaped hollow porous carbon was prepared by hydrothermal carbonization of xylose. The results showed that hydrothermal reaction of xylose occurred at interface between microreactor and solution. Hydrophilicity of PEO (hydrophilic block in P123) decreased at hydrothermal temperature of 160 ℃. Hybrid emulsion was swelled and destroyed gradually because PEO ran into the interior of emulsion. Furthermore, the morphology of microreactor could be regulated by the mass ratio of P123/SDS and the opening angle, and cavity diameter could be controlled by the hydrothermal time. Owing to the open cavity, the capacity of charges and ions was magnified and transport distance was reduced. In addition, specific capacitance and energy density of porous carbons were improved and showed positive correlation with cavity diameter. The horseshoe-shaped hollow porous carbon with largest opening angle (63°), cavity diameter (80 nm) and optimal supercapacitor performance was obtained at a P123/SDS mass ratio of 1.25 : 1 by hydrothermal method for 12 h. In a three-electrode system, the product showed a high specific capacitance of 292 F·g^-1 at a current density of 1 A·g^-1. In a two-electrode system, the product showed an excellent energy density (6.44 Wh·kg^-1), specific capacitance of 185 F·g^-1 at a current density of 0.2 A·g^-1 and outstanding cycling stability (94.83%) after 5000 cycles at a current density of 5 A·g^-1.

In the long river of human history, every technological revolution is accompanied by transition of cognition, development and utilization of energy. At present, China has become the No. 1 in the world in both production and consumption of energy, which continue rising in the excepted future. Developing energy technology is still a key way to solve the problems of excessive dependence on traditional fossil energy and environmental pollution, construct a reasonable social structure, promote the sustainable development of human society, and achieve the goals of carbon emission peaking and carbon neutrality. In 2020, renewable energy in China such as photovoltaics and wind power evolved marvelously which occupied 1/3 of global total volume. In this regard, energy materials are indispensable components, which play the core role in realizing conversion and utilization of clean energy, developing new energy technologies, and supporting the entire energy system.
In recent years, energy materials have achieved extensive and sustainable development in many fields, including secondary batteries, fuel cells, solar cells, supercapacitors, photoelectric catalysis, and energy-containing materials. For example, high nickel ternary materials as cathode material in the lithium-ion battery are leading the future of a new generation of automotive power battery technology towards faster charging speeds, longer service life and longer mileage^[1-4]. The ever increasing demand for energy storage has also spawned simultaneously a series of new battery technologies, such as lithium-sulfur^[5], lithium-air^[6] and solid-state batteries^[7]. They have advantages in energy density, economy and safety, but technical defects (e.g., shuttle effect in Li-S battery attributed to polysulfides, blockage of matrix pores in Li-air battery attributed to discharging product, unsatisfactory electrical conductivity of electrolyte in solid-state battery) are frustrating. Technological improvement and industrialization are strongly dependent on the innovative design and structural optimization of electrode and electrolyte materials. To promote the share of renewable energy in primary source, photovoltaics, the representative of new energy, received great expectation. In addition, halogen perovskite-based third-generation solar cell technology has achieved a solar energy conversion efficiency comparable to that of silicon single crystal, showing a prosperous photovoltaic industry in the future^[8]. However, its sensitivity to temperature, humidity, light, and oxygen^[9], and inevitable Pb-containing raw material in preparation still need to find a solution in the underlying materials design. Moreover, as continuously optimizing the traditional catalyst materials, like Pt and Pd, as well as the non-precious and non-metallic catalysts, the energy conversion efficiency of fuel cells has been gradually improved with reduction of their technical costs, meeting a certain degree of commercial application^[10-11]. Besides, photocatalytic and electrocatalytic technologies for CO₂ reduction and nitrogen fixation also provide a new way for the storage and utilization of renewable energy, technically support the carbon emission peak in 2030 and carbon neutrality in 2060^[12-13].
In the context of the era of sustainable development and the fiercely competitive international scientific and technological frontier research environment, in the energy materials research, including the exploration of physical and chemical properties, functional discovery, precise design and preparation of nanomaterials, and advanced device assembly, China has made many important breakthroughs. In order to focus on displaying the research results of Chinese scholars in this field, to promote academic exchanges among peers, and to stimulate interest in energy materials from all walks of life, Nanjing University of Science and Technology, Shanghai Institute of Ceramics, Huazhong University of Science and Technology, etc. hereby organize the publication of “Energy Materials Special Issue”, containing the latest research articles and reviews related to energy materials involved with perovskite photovoltaics, semitransparent solar cell, Li-ion battery, Mg battery, Li-S battery, thermoelectrics, CO₂ splitting, etc. It is hoped that this Special Issue can offer useful references for the scientific research and disciplinary development of energy materials in China.

N/S doped carbon nanotubes were prepared with natural mineral fibrous brucite as template, sucrose as carbon source, and thiourea as nitrogen and sulfur source. Experimental results indicate that the doped carbon nanotubes inherit the one-dimensional columnar structure of the fibrous brucite template. In addition, it presents a hollow tubular structure, which increases the specific surface area and pore volume of the template carbon. In 6 mol·L^-1 KOH electrolyte, the electrochemical performance significantly improves after doping. CNT-N/S presents a high specific capacity of 172.0 F·g^-1 at current density of 1 A·g^-1, higher than those of CNT (62.2 F·g^-1) and CNT-N (97.0 F·g^-1). The capacitance of the N/S doped carbon nanotubes remains 89% after 1000 charge-discharge cycles. Furthermore, the assembled symmetrical supercapacitor also shows good capacitance performance.

Carbon materials are favorable for supercapacitors but suffer from insufficient capacitance. Heteroatom doping, especially nitrogen (N) doping, is an effective method to significantly improve the electrochemical performance, but it is still a big challenge to achieve high active nitrogen content in carbon materials. This work successfully tuned nitrogen species and content by interaction between Si-O-Si network and aluminum oxide. Besides, the structure of carbon materials varies from a coral-like network to three-dimensional structure by adjusting the precursor composition. Oxygen (O) in oxides bonds with N in carbon materials during the reaction, which makes it difficult to escape, achieving high nitrogen content of 5.29at% at 1000 ℃. On the other hand, the interaction empowers the carbon material with large pore volume of ~1.78 cm³·g^-1 and broad pore size distribution of 0.5-60 nm. Thus, the N-rich carbon material harvests high capacitance of 302 F·g^-1 at 1 A·g^-1 and excellent rate capability of 177 F·g^-1@120 A·g^-1. This unique nitrogen fixation method is a promising strategy for preparing high performance electrode materials of supercapacitors.

High-entropy oxides have attracted more and more attention due to their unique structures and potential applications. In this work, M₃O₄(M=FeCoCrMnMg) high entropy oxide powders were synthesized by a facile solid-state reaction method. The powders were characterized by different methods. Furthermore, M₃O₄/Ni foam (M₃O₄/NF) electrode was prepared by a coating method, followed by investigation of its supercapacitor performance. The results showed that, with the increase of calcining temperature, Fe₂O₃(H)/Co₃O₄(S)/Cr₂O₃(E) and Mn₂O₃(B) dissolved successively in the crystal lattice of spinel structure. After M₃O₄ powders being calcined at 900 ℃ for 2 h, single spinel structure (FCC, Fd-3m, a=0.8376 nm) was obtained with uniform distribution of Fe, Co, Cr, Mn, and Mg elements, the typical characteristic of high entropy oxide. In addition, the mass specific capacitance of M₃O₄/NF composite electrode is 193.7 F·g^-1, with 1 mol/L KOH as electrolyte and 1 A·g^-1 as current density, which indicated that the M₃O₄ high entropy oxide can be considered as a promising candidate for the electrode material in the field of supercapacitor applications.