| Energy is an important material foundation for the sustainable development of human society and thus has become a topic of common concern to countries around the world.The rapid development of economy,rapid growth of population,rapid development of hybrid electric vehicles,and increasing demand for portable electronic devices inevitably accelerate the consumption rate of global energy,sounding the alarm of energy shortage for human.Looking for clean,sustainable and renewable new energy sources and developing advanced,low-cost and environmentally friendly energy storage technologies are urgent.As a new kind of energy storage device bridging between traditional capacitors and batteries,supercapacitors have aroused intense research interest due to their advantages of high power density,long cycle life,fast charge/discharge rate,wide range of working temperature,low cost and high safety.Nevertheless,the relatively low energy density severely hampers further development of supercapacitors towards large-scale practical applications.As is known to all,the performance of supercapacitors essentially depends on the properties of electrode materials,so the key issues in developing supercapacitors with both high energy density and high power density are the design and research of high-performance electrode materials.In recent years,transition metal nitrides have emerged as alternative high performance electrode materials due to their advantages of excellent electrical conductivity,high melting point,superior chemical stability and unique characteristics possesing excellent cycle stability of electric double layer capacitor electrode materials and high energy density of Faradic pseudo-capacitor electrode materials.Among them,CrN and TiN with a higher theoretical capacitance has shown great promise for the use in the field of supercapacitors.In view of the main advantages(low deposition temperature,fast deposition rate,good repeatability,good adhesion between film and substrate,and avoidance of using binder and toxic gas)of magnetron sputtering,this work mainly focuses on the reactive magnetron sputtered CrN and TiN based nanostructured thin films and their applications in the supercapacitor fields.The effects of deposition pressure and nitrogen gas flow on the morphology,structure,roughness,porosity and electrochemical performance of thin film electrodes were systematically investigated and the structure-activity relationship between process parameters,structure,and electrochemical performance was explored.In order to solve the problems involved in conventional reactive magnetron sputtering,such as high density,simple morphology,small specific surface area,and low specific capacitance,a combined reactive magnetron sputtering and chemical modification process(selective chemical etching or laser processing)was developed to achieve the construction of porous CrN and three-dimensional(3D)array nanostructured TiN thin films.Moreover,the effects of porous and 3D array nanostructures on the supercapacitor performance were analyzed.A new universal multi-target reactive magnetron co-sputtering strategy is proposed to realize the controllable preparation of ternary transition metal nitride CrVN and TiNbN thin films with solid solution nanostructure,and the effects of active metal doping and synergistic effects between metals on the supercapacitor performance were explored.Based on the above research,symmetric or asymmetric supercapacitor devices were assembled to investigate the practical application value.The specific works were outlined as follows:(1)By simply varying the deposition conditions,the morphology,porosity,and roughness of thin films and thus the electrochemical performance can be readily tuned.The CrN thin films deposited under 3.5 Pa achieve the optimal performance with a high areal specific capacitance of 12.8 mF cm-2 at 1.0 mA cm-2 and excellent stability with a capacitance retention of 92.1%after 20 000 GCD cycles.Moreover,a symmetric supercapacitor device using a pair of CrN thin film electrodes delivers a high energy density of 8.2 mW h cm-3 and a power density of 0.7 W cm-3,suggesting the promising applications of CrN thin films as electrode materials for high performance supercapacitors.(2)The effect of nitrogen gas flow on morphology,structure,porosity,and roughness was investigated to optimize the electrochemical performance of TiN thin film electrodes.The TiN thin film electrodes with 9%N2 content show long-term cycling performance(98.2%capacitance retention after 20000 cycles at 2.0 mA cm-2)with an optimized specific capacitance of 27.3 mF cm-2 at 1.0 mA cm-2,which excels most of the previous transition metal nitrides.Furthermore,a symmetric supercapacitor device based on TiN thin films can deliver a maximum energy density of 17.6 mWh cm-3 and a maximum power density of 10.8 W cm-3 with remarkable cycling stability.These findings suggest that TiN thin films possess good application prospects in the field of supercapacitor.(3)The porous CrN thin films for binder-free supercapacitor electrodes by reactive magnetron co-sputtering and selective chemical etching were successfully prepared.Benefiting from excellent electrical conductivity,porous structure,and high surface area,the porous CrN thin films exhibit much higher electrochemical performance than directly synthesized non-porous CrN when used as supercapacitor electrodes.Specifically,a high specific capacitance of 31.3 mF cm-2 at 1.0 mA cm-2 along with remarkable cycling stability with 94%capacitance retention over 20000 cycles were achieved on porous CrN.In addition,the symmetric supercapacitor devices based on porous CrN demonstrate a maximum energy density of 14.4 mWh cm-3 and a maximum power density of 6.6 W cm-3,which is superior to other reported nitride-based supercapacitor devices.(4)The synthesis strategy of 3D nanoarray structured TiN thin films involves a simple three-step procedure,including photoetching,deep silicon etching and magnetron sputtering processes.Owing to a unique 3D array nanostructure,the resulting 3D array nano structured TiN thin film electrodes deliver a high specific capacitance of 43.8 mF cm-2 at 1.0 mA cm-2 as well as outstanding long-term cycling stability(no obvious deterioration after 20000 cycles at 2.0 mA cm-2).Moreover,the symmetric supercapacitor devices based on the 3D nanoarray structured TiN thin films possess a high energy density of 20.5 mW h cm-3 at a power density of 0.86 W cm-3,along with excellent cycling performance.(5)CrVN films were prepared by reactive magnetron co-sputtering,and the effect of V doping on the electrochemical performance of CrN thin films was investigated.Compared with directly deposited CrN thin film electrodes,the introduction of V endows CrN thin film electrodes with more active sites and higher specific surface area,achieving higher specific capacitance at the same current density(a specific capacitance of 22.8 mF cm-2 at a current density of 1.0 mA cm-2).In addition,the symmetric supercapacitor devices assembled on the basis of CrVN thin films can achieve a maximum energy density of 11.2 mWh cm-3 and a maximum power density of 7.5 W cm-3,suggesting that metal doping or replacement is an effective method to improve electrochemical performance.(6)TiNbN thin film electrodes were prepared by magnetron co-sputtering under mild conditions,and the influence of Nb doping on the supercapacitor performance of TiN thin films was investigated.The incorporation of Nb into TiN greatly boosts the performance because of the synergy of Ti and Nb.As a result,a high specific capacitance of 74.1 mF cm-2 at 1.0 mA cm-2 was achieved,which is much higher than that of TiN and NbN(33.6 and 49.5 mF cm-2,respectively)as well as many other reported high performance transition metal nitrides.More importantly,we report for the first time an all metal nitride TiNbN//VN asymmetric supercapacitors.The device can deliver a high energy density of 74.9 mWh cm-3 at a power density of 8.8 W cm-3 along with outstanding stability,outperforming other reported nitride-based devices,which opens up new possibility for the rational construction of all nitride based high performance asymmetric supercapacitors. |
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