Preparation And Electrochemical Properties Of Two-dimensional Porous Transition Metal Base Carbonate

To solve the problem of the energy shortage,we must realize the efficient use of energy,which is the only way to realize the sustainable development of human society.As the most promising energy storage and conversion equipment in today’s society,supercapacitors have a considerably high power density and can rapidly charge and discharge.At the same time,supercapacitors also have a very stable cycling performance.The electrochemical performance of a supercapacitor depends on whether the electrode material has excellent conductivity,a larger specific surface area,a higher capacitance,and a stable structure during the chemical reaction.Therefore,to break the performance bottleneck of supercapacitors,researchers have devoted themselves to the research on the surface structure of electrode materials.Two-dimensional porous transition metal-based carbonate is a promising electrode material with large specific surface area and rich electrochemical reactivity sites.In addition,compared with other materials,carbonate has better structural stability,higher reversible capacity,and electrical conductivity.Besides,carbonate also has good hydrophilicity,which can enhance the moisture content on the electrode surface.This unique property allows the carbonate to exhibit unusual properties,such as effectively reducing polarization during charging and discharging.In this paper,the two-dimensional porous structure of carbonate flakes was successfully prepared by the hydrothermal method.XRD,XPS,SEM,TEM,BET,and other characterization methods were used to analyze the composition,morphology,specific surface area,and other information of the samples.The electrochemical properties were studied by some measurements including cyclic voltammetry.First,In this paper,using Ni（NO₃）₂·6H₂O as the nickel source,urea as the precipitant,and providing an alkaline environment,the two-dimensional porous nickel basic carbonate flakes were synthesized by hydrothermal method,and the effects of different conditions on the electrochemical properties of the materials were investigated.The results showed that the electrochemical properties of Ni₂（CO₃）（OH）₂·H₂O flakes were the best when reacted at 180°C for 12h.Through SEM and TEM tests,it can be seen that the product is a two-dimensional porous sheet structure with many smaller sheet structures interlaced and cross-linked to form a three-dimensional cross-linked structure.This structure has a larger specific surface area,so it has more active sites,which is conducive to the passage of ions,thus improving the electrochemical performance.By the BET test,the specific surface area of the two-dimensional slice was up to 102.4m²·g^-1.In terms of electrochemical test,when the current density in the three-electrode system is 1A·g^-1,the specific capacitance of the two-dimensional porous nickel carbonate flake is 2357.5 F·g^-1.When the current is 5A·g^-1,only 1.5% of the capacitance is lost after 2000 cycles.In the asymmetric supercapacitor system,Ni₂（CO₃）（OH）₂·H₂O two-dimensional porous sheet was used as the positive electrode and activated carbon（AC）as the negative electrode.When the current density is 1A·g^-1,the specific capacitance of the capacitor is 74.63F·g^-1,and when the power density is800W·kg^-1,the energy density can reach 26.53Wh·kg^-1,and when the power density is4800W·kg^-1,the energy density can still reach 12.8Wh·kg^-1.Second,In this paper,Co（NO₃）₂·6H₂O was used as the cobalt source.In the alkaline environment,Co₂（CO₃）（OH）₂·H₂O two-dimensional porous flakes with different crystallinity were synthesized by controlling the reactant concentration,temperature,and time in the hydrothermal method.The influence of the crystallinity of different morphologies on the electrochemical performance was also investigated.The results showed that the electrochemical performance of Co₂（CO₃）（OH）₂·H₂O flakes was the best when reacted at 180°C for 12h,and the specific capacitance was as high as1995.74F·g^-1.It was found by SEM and TEM that the product was a typical two-dimensional porous nanosheet structure.In the three-electrode system,the specific capacitance decreases from 1995.74F·g^-1 to 925.53F·g^-1 with the increase of current density(1A·g^-1～10A·g^-1),which is 49.1% of the initial specific capacitance.In addition,the specific capacitance remains at 98.5% after 2000 cycles under 5A·g^-1,showing excellent cycle stability.Finally,Co₂（CO₃）（OH）₂·H₂O two-dimensional porous flakes were assembled into Co₂（CO₃）（OH）₂·H₂O//AC asymmetric supercapacitor.The results showed that the specific capacitance of the device is 54.56F·g^-1 at the current density of 1A·g^-1,and the energy density can reach 19.4Wh·kg^-1 at the power density of 800W·kg^-1.Last,using the hydrothermal method,Ni（NO₃）₂·6H₂O as the nickel source,Co（NO₃）₂·6H₂O as the cobalt source and urea as the precipitant,the 2D porous nickel-cobalt basic carbonate flakes were successfully synthesized by combining Ni（NO₃）₂·6H₂O and Co（NO₃）₂·6H₂O with a molar ratio of 1:1.In the electrochemical performance study,when the current density is 1A·g^-1,the specific capacitance of the two-dimensional porous nickel-cobalt carbonate flake is 2605F·g^-1.In the asymmetric supercapacitor system,Ni Co（CO₃）（OH）₂·H₂O two-dimensional porous flake was used as the positive electrode and activated carbon（AC）as the negative electrode.When the current density is 1A·g^-1,the specific capacitance is 95.18F·g^-1;when the power density is 800W·kg^-1,the energy density can reach 33.8Wh·kg^-1;when the power density is 8000W·kg^-1,the energy density can still reach 12.44Wh·kg^-1.