Two Methods And Related Mechanisms To Improve The Specific Energy Of Gallium Nitride-based Supercapacitors

Supercapacitor as a high specific power energy storage device is a hot research topic in the field of energy storage.As a wide band gap semiconductor material,GaN has attractive application prospects in supercapacitor field with its good chemical stability and electrical conductivity;however,the formation of Ga-N bond reduces the ability of GaN surface atoms to provide d electrons to the adsorbent,resulting in the unsatisfactory energy storage specific capacity of GaN interface.In addition,the limitation of the electrochemical stability potential window（ESPW）of acidic aqueous electrolyte makes the output voltage of GaN-based supercapacitor limited to about 1 V,leading to its low specific energy.Therefore,how to improve the specific capacity of GaN-based electrode materials and broaden the ESPW of acidic aqueous electrolytes are the key issues to improve the specific energy of GaN-based supercapacitors in acidic aqueous electrolytes.Based on the above considerations,the first part of the thesis utilizes the thermal instability of GaN to prepare a new electrode material of graded structure porous GaN/nitrogen-doped carbon by the thermal reduction of carbon in a confined system.Using the high stacking density of GaN and the high specific surface of graded structured porous GaN,the dilemma that the supercapacitor electrode material cannot perform high area specific capacity and high volume specific capacity at the same time is solved.The research results show that:（1）By controlling the heat treatment time,it is possible to regulate the morphology,specific surface,pore structure,relative content of GaN/nitrogen-doped carbon and defect concentration of GaN,and thus the electrochemical properties of the sample electrodes.（2）The honeycomb structured GaN/nitrogen-doped carbon（S-C-8）samples were obtained when the heat treatment time was 8 h.When the active substance loading was3.4 mg,S-C-8 had 448 m F cm^-2 at a 1 m A cm^-2 S-C-8 has a discharge specific capacitance of 448 m F cm^-2 at a current density of 1 m A cm^-2;even if the current density is increased to 200 m A cm^-2,S-C-8 still has a discharge specific capacitance of 310 m F cm^-2 with a capacity The retention rate is 69.2%.（3）Based on the fitting results of Nicholson and Nyquist curves,the excellent electrochemical performance of S-C-8 is explained from the kinetic point of view,attributed to the surface-confined electrochemical-dominated energy storage process and the unique honeycomb structure that provides excellent mass and charge transfer rates.（4）In view of the superior multiplicative performance of S-C-8,the possibility of practical application of the electrode with high active substance loading was further explored against the commercial supercapacitor standard.When the active substance loading was 7.6 mg cm^-2,the discharge specific capacity of the sample electrode was1076 m F cm^-2 at 1 m A cm^-2;at 200 m A cm^-2,the discharge specific capacity was 730 m F cm^-2,with a capacity retention rate of 67.8%.（5）Symmetrical acidic aqueous(1 mol L^-1 H₂SO₄)supercapacitor based on high loading(7.6 mg cm^-2)S-C-8 at 10-50 and then back to 10 m A cm^-2 current density for20,500 cycles with a discharge specific capacity of 157 m F cm^-2 and a capacity retention of 98.1%.The device was able to exhibit area/volume specific energy and area/volume specific power of 27.2μW h cm^-2/3.9 m W h cm^-3 and 50.0 m W cm^-2/7142.9 m W cm^-3.The limitation of the water decomposition voltage（1.23 V）greatly limits the specific energy of water-based energy storage devices.To broaden the ESPW of the electrolyte,the electrochemical effects of phosphoric acid electrolytes with mass concentrations of33.3-66.7%on activated carbon and honeycomb structured GaN-based supercapacitors were investigated in the second part of the thesis,taking into account the binding effect of strong hydrogen bonding on water molecules,and the results showed that:（1）the conductivity of P-W-1 electrolyte with mass concentration of 50%is 166m S cm^-1,which is higher than that of P-W-0.5 and P-W-2 electrolytes with mass concentrations of 33.3%and 66.7%.which is attributed to the moderate hydrogen bonding of P-W-1.（2）The broadening of ESPW for P-W-x（x=0.5-2）was explained using Raman and infrared spectroscopy tests:with increasing phosphoric acid concentration,the free water content in the electrolyte decreased,leading to a decrease in the reactivity of water and broadening the ESPW.（3）The P-W-1 based carbon-based symmetrical supercapacitor(single electrode active substance loading of 3.7 mg cm^-2)has a discharge specific capacity of 158 m F cm^-2 at 2 m A cm^-2;at 100 m A cm^-2 is,the discharge specific capacity is 50 m F cm^-2 with a capacity retention rate of 31.6%and no capacity loss after 11,000 charge/discharge cycles.（4）The GaN-based symmetrical supercapacitor based on P-W-1(single electrode active substance loading of 3 mg cm^-2)has a discharge specific capacity of 110 m F cm^-2at 2 m A cm^-2;at The discharge specific capacitance is 50 m F cm^-2 at 100 m A cm^-2 with a capacity retention of 45.4%;at 2 m A cm^-2,the discharge specific capacitance of the S-C-8/P-W-1 device has a 175%higher discharge specific capacitance compared to the S-C-8/1 mol L^-1 H₂SO₄ device with the same loading.（5）S-C-8/P-W-1 shows excellent cycling stability and specific energy/specific power.It shows a discharge specific capacity of 88 m F cm^-2 after 20,000 cycles at 10-50and back to 10 m A cm^-2 current density,with a capacity retention rate of 93.6%.Meanwhile,S-C-8/P-W-1 was able to exhibit a maximum of 34.4μW h cm^-2/5.7 m W h cm^-3 and 112 m W cm^-2/18.8 W cm^-3 in terms of surface/volume specific energy and surface/volume specific power.