Heteroatom self-doped graphitic carbon materials from Sargassum thunbergii with improved supercapacitance performance
image: Using Sargassum thunbergii as raw material, a kind of hierarchical porous carbon was prepared by direct carbonization and then K2FeO4 activation. The synergistic action of metal Fe and KOH obtained from the decomposition/reaction of K2FeO4 promotes the activation and graphitization of the material.
Credit: Beijing Zhongke Journal Publising Co. Ltd.
This research is led by Hui Xu (Huzhou Key Laboratory of Green Energy Materials and Battery Cascade Utilization, School of Intelligent Manufacturing, Huzhou College, Huzhou, China) and Hua Wang (Huzhou Key Laboratory of Medical and Environmental Applications Technologies, School of Life Sciences, Huzhou University, Huzhou, China).
Biomass-derived carbon materials stand out among many carbon based materials because of its low cost, good pore structure and other characteristics. Currently, the relatively low specific capacitance is an important problem limiting the application of biomass-carbon based materials in supercapacitors. Three methods can be applied to improve the capacitance performance, which is doping or self-doping heteroatoms based on biomass itself, a certain degree of graphitization after high temperature treatment and activating the biomass carbon by using activators to increase its porosity.
Traditional chemical activator can efficiently increase the porosity of biomass-based carbon. However, graphitized structure may be destroyed, reducing the graphitization degree. It is well-known that improvement of the graphitization degree of carbon materials can result in high conductivity while high conductivity can increase significantly the charge
storage capacity of carbon-based materials under high current density, thus achieving high rate capability. In order to improve the graphitization degree of carbon materials, the simplest method is high temperature (higher than 2000 ℃) treatment of carbon materials. However, a lot of energy is consumed at such a high temperature. At the same time, the specific surface area and porosity of carbon materials decrease dramatically. The transition metals such as Fe, Co, Ni were often employed as catalyst to reduce greatly the graphitization temperature (lower than 1000 ℃) of carbon-based materials. For example, using K3Fe(CN)6 and thiourea as graphitization catalysis and dopant, N, S co-doped carbon has been prepared by Ji et al. from bamboo fiber. N, S-codoped hierarchically porous carbon has been prepared by Wan et al. from rape pollen with mixed salts of ZnCl2 (chemical activation agent) and FeCl3 (catalyst for graphitization). Combining with Co2+-impregnation (catalyst for graphitization) and molten KOH activation, porous carbon with ultrahigh capacitance for supercapacitor was fabricated. By using Fe(NO3)3/KOH and Ni(NO3)2/KOH as the co-graphitization/activation catalysts, the carbon nanoflakes and porous carbon with high supercapacitance performance have been prepared respectively from the natural plane tree fluff. Currently, a common green disinfector and a drinking water treatment agent, K2FeO4 was selected as both the activation agent (KOH) and catalyst (Fe, a catalyst of graphitization) to obtain porous biomass carbon.
As aquatic organism, algae contains abundant cellulose and heteroatoms, which are ideal precursors for heteroatom self-doped carbons. Since the first report of algae carbon, the study on it increased exponentially. In previous study, N-doped porous carbon materials were prepared by one-step carbonization of Sargassum thunbergii and then used for energy storage, electroanalysis, and metal ion removal by our group. Although one-step carbonization is simple, the specific capacitance of 177 F g-1 in 6 M KOH at current density of 0.2 A g-1 is very low. In this work, by utilizing natural Sargassum thunbergii as a precursor, a simple and efficient two-step route to generate porous carbons with improved supercapacitance performance was demonstrated. The first step was the one-step carbonization of Sargassum thunbergii under nitrogen (N2) atmosphere at optimized temperature (700 ℃) to obtain a carbon species (bio-char). Then, the second step was the simultaneous chemical activation and graphitization of bio-char by using K2FeO4 in order to develop desired porous structure. The Sargassum thunbergii derived graphitized porous carbons (STGPCs) combine the advantages of well-defined porous texture, self-doping of heteroatoms and a certain degree of graphitization. Under optimal conditions, the obtained sample features a specific surface area of 1641.98 m2 g-1. N/O contents of 3.6 and 9.6 wt% demonstrate a capacitance of 325.5 F g-1 at 0.5 A g-1. The symmetric supercapacitor exhibits energy density as high as 21.3 Wh kg-1/14 Wh kg-1 (at a power density of 450W kg-1/129W kg-1) in1 M Na2SO4 electrolyte/6 M KOH. Excellent long-term cycling stability (97% capacitance retention after 3000 cycles at 5 A g-1) in 1 M Na2SO4 was also achieved.
See the article:
Heteroatom self-doped graphitic carbon materials from Sargassum thunbergii with improved supercapacitance performance
https://doi.org/10.1016/j.asems.2024.100102