Application of plant-based porous carbon for supercapacitors

Abstract

Super capacitors have been widely used in various fields because of their high power density, long cycle life, and cost-effectiveness. Plant-based porous carbon continues to be the most suitable alternative for manufacturing the commercial electrode materials of super capacitors because of its good electrochemical performance, simple preparation process, high availability, and low cost. Although plant-based porous carbon prepared using physical activation has been widely used in commercial super capacitors, its performance is severely restricted because of its low value of specific surface area and highly micro porous structure. With a view to achieving high values of specific gravimetric/volumetric capacitances and outstanding rate performance in super capacitors, this review summarizes the recently developed methods for preparing plant-based ultrahigh specific surface area porous carbon materials, micro porous carbon materials, hierarchical porous carbon materials, and nitrogen-doped porous carbon materials. The factors affecting the electrochemical performance of plant-based porous carbon are also discussed. We also summarize some novel strategies to improve the volumetric electrochemical performance of plant-based porous carbon materials, such as preparing dense and porous carbon materials, performing heteroatom doping, and combining the carbon with pseudo capacitive materials (conductive polymers or metal oxides). Finally, the challenges and perspectives of using plant-based porous carbon in super capacitors are also proposed. In brief, when used as the electrode material for super capacitors, the ultrahigh surface area porous carbon prepared by KOH activation shows high value of specific capacitance at low current densities. However, the tortuous and deep micro pores in the plant-based porous carbon result in its sluggish ion-transport kinetics and high value of equivalent series resistance, which, in turn, result in poor rate performance. To improve the rate performance, tremendous efforts have been made to introduce micro pores in the carbon as ion-transport channels. However, this strategy usually involves the coalescence of a large number of micro pores, resulting in the reduced surface area as well as energy storage ability of the carbon. Hence, many researchers have utilized the inherent porous structure and inorganic templates of plants to prepare hierarchical porous carbon both with high specific surface area and high micro pore volume for use in devices with high capacitance and power. In addition to altering the surface area and pore structure of the carbon, doping with nitrogen is another promising approach to enhance the capacitance and electronic conductivity of the plant-based porous carbon. Surface nitrogen can be introduced by the direct carbonization/activation of nitrogen-rich plant precursors or by the reaction of the carbon with nitrogen-containing reagents. Porous carbon with large specific area and with developed micro porous structure may exhibit superior gravimetric capacitance but inferior volumetric capacitance because of the trade-off between its well-developed micro porous structure and packing density. To improve the volume performance, some methods, such as preparing dense and porous carbon with reasonably porous structure, using heteroatom-doped carbon, and incorporating the carbon with pseudo capacitive materials, have been developed. Although the electrochemical performance of plant-based porous carbon has been significantly improved using the aforementioned methods, yet issues such as the lack of green methods and low-cost activation methods to prepare large surface area porous carbon, the design and controlled modulation of carbon microstructures, the influence of heteroatom doping on pseudo capacitance, and weak interaction between pseudo capacitive components and plant-based porous carbon still need to be resolved. We hope that this review may provide the necessary background and ideas to develop more effective preparation methods for high-performance plant-based porous carbon.