Boron doping and structure control of carbon materials for supercapacitor application: the effect of freeze-drying and air-drying for porosity engineering

In the present work, we demonstrate a strategy of combining boron doping and porosity engineering for a highly modulated carbon component and pore structure, in which two contrasting drying methods (air-drying, freeze-drying) and their effects on supercapacitor performance are investigated in detail. Under freeze-drying and air-drying conditions, carbon nanoparticles and nanosheets are obtained, respectively. It is revealed that the carbon nanoparticles exhibit higher porosity (BET surface area of 1275 m(2) g(-1) and pore volume of 2.64 cm(3) g(-1)) than the nanosheets (BET surface area of 1109 m(2) g(-1) and pore volume of 1.72 cm(3) g(-1)); however, the boron content of the nanoparticles is lower (0.82 at.%) than that of the nanosheets (1.66 at.%). In the freeze-drying process, the direct sublimation of ice can prevent pore collapse, whereas stacking of nanosheets via Van der Waals interactions occurs during the air-drying process. With the air-drying method, the B-O-B structures produced by the evaporation process preferentially react with carbon, which promotes the production of more boron functional groups. As a result, the capacitive measurement indicates that the carbon nanoparticle electrode delivers larger capacitance of 129 F g(-1) at 1 A g(-1) and higher energy density of 41 Wh kg(-1) in the two-electrode system, in contrast to those of the nanosheets (capacitance of 98 F g(-1) and energy density 31 Wh kg(-1)) using EMIMBF4/AN as electrolyte. This kind of effect of freeze-drying and air-drying for porosity engineering is probably helpful for further supercapacitor applications.