Response surface optimization design and electrochemical performance of sword shell-based carbon

With the attention to the world ecology and economy, people are observing the abundant, low-cost, and clean renewable energy from sun and wind. However, most of the renewable energy sources are intermittent and cannot meet the needs for applications, except for converting to electricity. In order to satisfy the demand of people for new energy storage equipment, supercapacitors using biomass-based carbon materials as electrode materials have attracted much attention, because the multi-level structure of the natural biomass material is conducive to ion transmission. The fine structure of natural biomass cannot be synthesized artificially. The preservation of natural multi-scale structure can provide better electrochemical performance of the biomass-based carbon material. The biomass waste of sword beans shell with the characteristics of fast growth, a large amount of sword bean shells can be continuously produced as raw materials every year, but are often discarded or burned, which contributes to the environmental pollution. It is a promising precursor for obtaining hierarchically porous carbon-based material used as active component of high-storage capacity supercapacitors. The activated carbon with high surface area derived from sword shell by using KOH activation method and used as supercapacitor electrode materials. Taking the specific capacitance value of electrode material as the response value, the activation temperature and the activation ratio as the experimental factors, the Central Composite Design (CCD) method was employed to carry out the response surface optimization study, and the electrochemical performance of the activated carbon prepared under the optimal process conditions was explored. The research results show that the activation temperature and activation ratio have significant effects on the specific capacitance of activate carbon material. The coefficient R2 of the model is 97.54%, and the correction coefficient Adj R2 is 95.78%, indicating that the model can better predict the specific capacitance value of sword shell-based activated carbon with high reliability. The specific capacitance can reach a peak value under the condition of activation temperature 700 ℃ and the activation ratio 4∶1. The optimized process parameters determined by center composite design approach were the activation temperature of 694 ℃ and the activation ratio of 4.17∶1. The verification experiment shows that the average specific capacitance of the sword shell activated carbon material is 254 F/g, which is basically consistent with the predicted value. Furthermore, the Cyclic Voltammetry (CV) curves and Galvanostatic Charge-discharge (GCD) curves of different carbon materials were compared. A quasi-rectangular shape with wide hump peaks can be observed in CV curves, which can be attributed to the synergy between the Electric Double-layer Capacitance (EDLC) and the pseudocapacitance. The nitrogen fixation of legumes can provide carbon materials with nitrogen to produce redox reactions and provide pseudocapacitance. The GCD curves show nearly triangular shapes with a small deviation from linearity, which indicated excellent capacitive behavior of the electrode materials. The Nyquist plots from the Electrochemical Impedance Spectroscopy (EIS) analysis reveal that sword shell-based activated carbon has good electronic conductivity. Additionally, the physical properties of activated carbon was characterized. The apparent morphology of activated carbon was observed by Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM). The nitrogen adsorption-desorption was conducted to investigate the pore structure of the carbon material. The results showed that: sword shell-based activated carbons possess a large number of nanopores, which distributed on the surface of the material, and the maximum specific surface area, total pore volume and micropore volume can up to 3 129 m2/g, 1.68 cm3/g and 0.96 cm3/g, which is conducive to electrolyte circulation and electrolyte ion adsorption.