Electrochemical Pseudocapacitance Of H(?)gg Compounds In Aqueous Solutions

As a kind of two-dimensional structural carbides or nitrides,MXenes are some of the most promising pseudocapacitive materials.They are obtained only after dealloying the A of MAX phases,and there are very little other preparation methods about MXenes.At present,the storage mechanism of the pseudocapacitance for MXenes is considered as the embedding mechanism,but it is based on the discussion in the two-dimensional structure.Whether there are more concise ways to prepare the MXenes,and very few reports about whether there is a quasi-two-dimensional pseudocapacitance storage mechanism beyond its two-dimensional structure.The H???gg compounds are a kind of interstitial phase intermetallic compound formed by carbon or nitrogen atoms filling the interstices that accumulating by transition metal atoms in tetrahedron or octahedron,and it is also a special kind of carbon or nitride.According to the crystallization principle,only the special H???gg compounds in nitride and carbide can be associated with the MXene materials mentioned above.The H???gg compound can be obtained in many simple ways.This bond of H???gg compounds makes the expansion of the MXene type and the simplification of macroquantization possible,and this realization no longer depends on the complex step of dealloy.The H???gg compounds can be considered as a two-dimensional or stacked multilayer MXene,it also makes the discussion of the origin of pseudocapacitance not only confined to the insertion of two-dimensional structures,that is,without weakening the blind spot of MXene structure in discussion.Due to the two-dimensional characteristics of MXene,the quasi-two-dimensional part of the pseudocapacitance mechanism in the charge and discharge process cannot be accurately understood by using it as a carrier material:whether the origin of the pseudocapacitance is two-dimensional or quasi-two-dimensional cannot be strictly answered.In this project,we take the MXenes as an entry point,and take a special kind of carbon or nitrogen interstitial phase H???gg compounds that can connectedly prepare the MXenes without attenuating and ignoring the structural blindness of the pseudocapacitive mechanism as a carrier,and then study systematically the origin and performance of pseudocapacitance,which can be expected to provide the material type and theoretical support for consequent design of MXene pseudocapacitive materials.The selection of H???gg compounds material carriers was selected reasonably in the principle of non-duplication and non-omission:metal element vanadium with multiple oxidation states,metal elements molybdenum and tungsten with regularity in the periodic table and rare earth represents lanthanum can form H???gg compounds with carbon or nitrogen that have simple geometric accumulation structures such as face centred cubic or body centered cubic.The specific materials are Mo₂N,W₂N,MnN,LaN,Mo₂C and W₂C.All these substances are prepared by dry method,that is,the corresponding oxides or ammonium salts are used to prepare nitrogen-H???gg compounds in the flowing ammonia atmosphere;the corresponding oxides or ammonium salts are added to the reactor to prepare carbon-H???gg compounds.The electrolyte system chooses the electrochemical classic aqueous solution system,which is beneficial to the result analysis,and the difference is that the choice of aqueous solution covers different acid-base properties.The research contents of this paper are the electrochemical pseudocapacitance of H???gg compounds:Mo₂N in 1 mol L^-1 H₂SO₄,W₂N in 1 mol L^-1 H₂SO₄,MnN in 1 mol L^-1 Na₂SO₄,LaN in 1 mol L^-1 Na₂SO₄,Mo₂C in 1 mol L^-1 H₂SO₄,W₂C in 1 mol L^-1 H₂SO₄.The Mo₂N electrode can possess a potential range from-1.0 to-0.2 V in 1 mol L^-1 H₂SO₄and a capacitance value of 171 F g^-1 at a current density of 1 A g^-1.The W₂N electrode can possess a potential range from-0.3 to 0.8 V in 1 mol L^-1 H₂SO₄ and a capacitance value of161 F g^-1 at a current density of 1 A g^-1.The MnN electrode can possess a potential range from-1.0 to 0.6 V in 1 mol L^-1 Na₂SO₄ and a volumetric capacitance value of 1044.3 F cm^-3 at a current density of 1 A g^-1.The LaN electrode possesses a potential range from-0.9 to 0.6 V in 1 mol L^-1 Na₂SO₄ and a volumetric capacitance value of 951.3 F cm^-3 at a current density of 1 A g^-1.The Mo₂C electrode can possess a potential range from-0.8 to-0.3 V in 1 mol L^-1H₂SO₄ and a capacitance value of 218 F g^-1 at a current density of 1 A g^-1.The W₂C electrode can possess a potential range from-0.3 to 0.8 V in 1 mol L^-1 H₂SO₄ and a capacitance value of158.2 F g^-1 at a current density of 1 A g^-1.All the electrodes have excellent high power charging-discharging performance and cycle stability in their respective systems.In addition,the origin of electrochemical capacitance is researched resting on the materials and the relevant electrolytes.Based on the above material systems,the Mo₂N//AC,Mo₂C//AC,W₂C//AC asymmetric supercapacitors and the W₂N//W₂N,MnN//MnN,LaN//LaN symmetric supercapacitors composed of various electrodes and activated carbon can show excellent electrochemical capacitance and electrochemical stability,and nice energy and power characteristics.All the results show the potential electrochemical pseudocapacitance energy storage applications of these materials.