| Supercapacitors have emerged as a crucial option for effective energy storage devices due to their exceptional features like rapid charge-discharge and extended cycling stability.The performances of supercapacitors are significantly impacted by the selection of electrode materials.Transition metal nitrides（TMNs）have been proven to be electrode materials with great potential in the field of energy storage due to their excellent corrosion resistance,wide potential window,outstanding electronic conductivity,and high theoretical specific capacitance.As a kind of transition metal nitride with high conductivity and high theoretical specific capacitance,vanadium nitride（VN）is expected to be a high-performance electrode material for supercapacitors.However,when applied as a supercapacitor electrode,VN faces the issues of limited electrochemical active surface area,decreased actual specific capacitance under high loads,and poor cycling performance.In this paper,the synthesis and modification of VN is taken as a case study to address the above-mentioned issues.Multiple strategies,including the use of environmentally friendly nitrogen sources,design of porous microstructure,composite electrochemically stable materials,and construction of self-standing electrodes,were adopted to achieve the creative development of the synthesis process of TMNs,improvement of electrochemical stability,and expansion of energy storage applications.These strategies effectively solved several key common problems of TMNs in the application of supercapacitors.The research content and progress of this paper are summarized as follows:（1）Hydrogel-derived nitrogen-doped porous carbon framework and vanadium nitride particle composites were parepared and used as high-performance vanadium nitride-based supercapacitors with excellent cycling stability.Polyacrylamide（PAM）hydrogel,a commonly used solid electrolyte,was used as both the nitrogen and carbon sources.After absorbing vanadium ions in solution,a composite powder material consisting of nitrogen-doped porous carbon with vanadium nitride nanoparticles（VN/NPC）was obtained by combining freeze-drying with a hard template method and annealing in an inert atmosphere.Compared to bare VN nanoparticles,the long cycling stability of the composite powder material was greatly improved when tested in an aqueous alkaline electrolyte.The advantages of the nanomaterials were highlighted by the stable three-dimensional interconnected hierarchical porous structure,which alleviated the aggregation of VN nanoparticles and provided efficient channels for ion/electron transport.When assembled into a solid-state symmetric supercapacitor,the energy density reached 21.97μWh cm-2 at a power density of 0.5 mW cm-2,and the specific capacitance remained at 90.9%of its initial value after 18000 charge/discharge cycles at a current density of 20 mA cm-2.（2）Low-cost fabrication of self-supported VN films on large silicon substrates were achieved using a chemical solution method.The self-standing electrode avoids the need of neither binders nor conductive additives during the process for the preparation of electrodes.Due to the excellent conductivity and multi-level interconnected pore structure of VN,the film electrode exhibited an excellent areal capacitance of approximately 60 mF cm-2 in 1 M KOH electrolyte at a scan rate of 5 mV s-1.The solidstate symmetric device,assembled with polyvinyl alcohol（PVA）/KOH electrolyte and a separator,showed highly reversible electrochemical reactions dominated by capacitive behavior according to electrochemical kinetic analysis.The energy densities were 21.2 and 12.8 mWh cm-3 at power densities of 2.0 and 16.8 W cm-3,respectively.Moreover,the solid-state electrolyte played a significant role in mitigating structural collapse,with only an 8.8%decrease in specific capacitance observed after 15000 long cycles.（3）On the basis of the template-free solution method for preparing porous electrode materials which mentioned above,a simple immersion and pullback annealing nitridation method was developed to achieve mass loading and performance indicators for commercialized electrodes.A high-performance self-standing vanadium nitride/carbon cloth（VN/CC）electrode with high mass loading(28.3 mg cm-2)was obtained,which has a continuous conductive network for electron transfer and a fully interconnected layered pore structure for ion transport.The areal capacitance of VN/CC can reach 3.34 F cm-2 at a current density of 5 mA cm-2,and it exhibits excellent rate performance.A sandwich-shaped flexible solid-state symmetric device was assembled using VN/CC as the negative electrode and nickel foam-supported zinc-nickel-cobalt oxide（ZNCO/NF）as the positive electrode,demonstrating high power density(22.4 mW cm-2)and energy density(0.185 mWh cm-2),and the capacitance could still maintain 87%after 8000 cycles.（4）To increase the proportion of active material in the self-supported electrode and move towards integration,a low-cost melamine foam was used to absorb vanadium-containing water solution.By using freeze-drying and one-step argon annealing,self-standing vanadium nitride supported on nitrogen-doped carbon foam（VN@NCF）integrated electrode with retained 3D skeleton was obtained.The porous material has a highly continuous electron conductive network and sufficient space for electrode-electrolyte contact.A solid-state device assembled without a membrane using PAM/KOH gel electrolyte has an areal capacitance of 224.2 mF cm-2.The energy density can reach 31.14 and 24.38μWh cm-2(at power densities of 0.5 and 5 mW cm-2,respectively),and the rate performance is outstanding.The capacitance of the device composed of integrated electrodes only decreased by 6.7%after long-cycle tests（12000 cycles）. |
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