Hierarchical porous carbons with honeycomb-like macrostructure derived from steamed-rice for high performance supercapacitors

To promote the advancement of electrochemical energy storage systems and meet the demand for rationalization of resource utilization, sustainable biomass porous carbon is being widely explored as a promising material for supercapacitor applications. Here rice grain was used as biomass precursor and steamed. After the steaming process, the rice grains formed a fluffy material with honeycomb-like structure. The structure not only contributes to the full infiltration of the activator to upgrade the activation efficiency, but helps to impede the tendency of crossing-linking between adjacent nanosheets during the subsequent activation and thus thinner and high-quality 2D carbon flakes could be obtained. Additionally, In the mild-activation route (the mass ratio of KOH and steamed-rice is only 1:1), numerous and tunable micro/mesoporous structures emerged on the basis of honeycomb-like macrostructure, creating a high specific surface area and unobstructed pore channel for electrolyte ions. Experimentally, the electrode material from steamed-rice at an activation temperature of 850°C has the optimal micropore/mesopore proportion and achieves the highest specific capacitance (350.2 F·g-1) at 0.2 A·g-1 in a 6 M KOH electrolyte. Meanwhile, it displays outstanding cycling stability (91.34% retention after 10000 cycles), and excellent energy density (9.775 Wh·kg-1 at 180 W·kg-1) when assembled into supercapacitors. This work shows that the "steaming effect" combined with chemical activation is beneficial for pore structure distribution in biomass materials, which contributes to rapid fabrication of carbon electrodes for electrochemical energy storage equipment.