Recently,transition metal carbides and/or nitrides called MXenes as a novel series of two-dimensional nanolaminates have achieved extensive attention by virtue of unique properties like good electrical conductivity,hydrophilic and tunable compositions.Both theoretical and experimental research confirm that MXenes can serve as promising candidates for many applications like energy storage,catalysis and biosensing.Among these attractive applications,MXene shows a bright prospect for supercapacitors.However,the decrease of specific surface area caused by collapse and re-stacking and weakening of electrical conductivity caused by functionalization restrain the development of MXene-based supercapacitors.Chemical doping with heteroatoms is an effective and common strategy to improve the electrochemical performance of 2D materials by manipulating their structural properties and electron transportation process.It has been proved that nitrogen doping can be used as a facile approach to tailor the electrochemical properties of MXenes.However,the underlying doping mechanisms for nitrogen element is still in controversy,especially over the specific existing forms.Moreover,the relationship between the existing forms of nitrogen atoms and electrochemical performance of the MXene is still open to discussion.In this work,a combination of theoretical calculation and experimental research is applied to investigate the doping mechanism for nitrogen element and electrochemical performance of Ti3C2Tx.it is found that there are three types of existing forms for nitrogen:lattice substitution,functional substitution(FS)and surface adsorption(SA)from first-principles calculations.Moreover,the electrical conductivity can be enhanced for SA-Ti3C2Tx.Correspondingly,three nitrogen doped Ti3C2Tx(N-Ti3C2Tx)with nitrogen dopants locating at expected sites(Ti3CNTx,hydrothermal N-Ti3C2Tx,plasma N-Ti3C2Tx)are designed and synthesized using different nitrogen doping strategies.Results indicate that the spacing of d-Ti3C2Tx layer increases and electrochemical performance is enhanced with the existence of nitrogen.For all the three types of N-Ti3C2Tx,the specific capacitance is higher than that of Ti3C2Tx at the same scanning rate.The specific capacitance of Ti3C2Tx,Ti3CNTx,hydrothermal and plasma N-Ti3C2Tx is 101.6,186.5,131.3 and 115.4 F g-1,respectively(at the scanning rate of 5 m V s-1).In hydrothermal N-Ti3C2Tx,functional groups are substituted by nitrogen and electrochemical behaviors are mainly influenced by electrical double-layer capacitance(EDLC)and pseudocapacitance(PC)mechanisms collectively.For plasma N-Ti3C2Tx,nitrogen elements mainly exist with the form of SA and electrochemical behaviors are influenced by PC mechanism.For Ti3CNTx,lattice carbon atoms are substituted by nitrogen and electrochemical behaviors are strongly influenced by EDLC mechanism.In summary,the doping mechanism for nitrogen element and electrochemical performance of N-Ti3C2Tx are investigated in this work.The results offer an improved understanding of surface chemical modification method to improve the electrochemical performance of MXene-based supercapacitors. |