Preparation And Electrochemical Performance Of Low-rank Coal Based Activated Carbon For Supercapacitor

Supercapacitor is a new energy storage device which between conventional capacitor and battery, it has wide application prospect and huge economic value. Activated carbon is the first choice electrode materials for supercapacitor, its structure and properties play a key role to supercapacitor performance. In this paper, activated carbons for supercapacitor were prepared from low-rank coal by KOH chemical activation. The effects of the preparation technological, low-rank coal types and organic maceral components on the pore structure and performance of activated carbon were systematically investigated by nitrogen adsorption at77K, XRD, SEM, FTIR and XPS. Preparation technology and method of activated carbon for supercapacitor prepared by microwave heating and moderate temperature (<600℃) activation condition were discussed. The influence of pore structure, microstructure and surface chemical properties of activated carbon to the electrochemical performance of electrode materials were also studied. Finally, activation mechanism of preparation low-rank coal based activated carbon by KOH chemical activation was investigated.The results showed that activated carbon with adjustable pore structure and surface chemical properties for supercapacitor can be prepared from low-rank coal. Abundant primitive pores and high volatile of raw coal were advantageous to develop mesopore, and the mineral not only weakened the activation reaction intensity, but also degraded the quality and performance of activated carbon. Activated carbons were prepared from lignite with KOH as activation agent, which specific surface area was665～3885m2/g, total pore volume was0.356～2.211cm3/g and the ratio of mesopore was18.8～74.9%. The pore structure and surface chemical properties of activated carbon can be tailored effectively by changing technological conditions, such as the weight ratio of KOH to coal, activation temperature and so on. Moreover, the pore structure, surface chemical properties and electrochemical performance of activated carbon were affected by maceral components. Activated carbon prepared from inertinite of low-rank coal had the largest specific surface area, highest ratio of mesopore and oxygen content, therefore its electrode materials had the most excellent electrochemical performance in KOH and (C2H5)4NBF4/PC electrolyte, secondly were activated carbons prepared from vitrinite and exinite. Based on investigating the electrochemical performance of activated carbon electrode materials, it was found that the contribution of micropore to the specific capacitance was slightly more important than mesopore in KOH electrolyte, when the specific surface area of activated carbon exceeded2500m2/g, surface chemical properties was more important than pore structure to improve the specific capacitance of electrode materials. In (C2H5)4NBF4/PC electrolyte, the contribution of mesopore to the specific capacitance was remarkable more important than micropore, the pore structure was crucial factor to affect the specific capacitance of activated carbon electrode materials, the surface oxygen-containing functional groups of activated carbon had no effect to improve the specific capacitance. In view of KOH and (C2Hs)4NBF4/PC electrolyte had different requirements to pore structure and surface chemical properties of activated carbon electrode materials, activated carbons were prepared from low-rank coal, which specific capacitance reached420F/g and197F/g, respectively. The opinion about designing and preparing coal based activated carbons for supercapacitor according to different electrolyte was also pointed outThe efficient activation of low-rank coal based activated carbon was realized by microwave heating. Activated carbon for supercapacitor was prepared from lignite by microwave heating with KOH as activation agent, which specific surface area was2097m2/g, total pore volume was1.193cm3/g and the ratio of mesopore was53.6%. The pore structure of activated carbon can be tailored effectively by changing the weight ratio of KOH to coal and microwave heating time.A new method of preparation activated carbon for supercapacitor under moderate temperature activation condition was explored. The activated carbon prepared under moderate temperature activation condition exhibited moderate specific surface area, abundant surface functional groups and high pressing density, and it had excellent electrochemical performance in KOH electrolyte. Furthermore, the gravimetric specific capacitance, surface specific capacitance and volumetric specific capacitance of the as-prepared activated carbon can reach369F/g,23.1μF/cm2and215F/cm3, respectively. The electrochemical performance in (C2Hs)4NBF4/PC electrolyte can be improved remarkably after reactivated by microwave heating.Segmented activation mechanism of preparation low-rank coal based activated carbon by KOH chemical activation was propounded, such as dehydration (<200℃) pre-activation (200～400℃), moderate temperature activation (400～600℃) and high temperature activation (>600℃). The main function of each stage are shown as following, dehydration was to removal moisture from activation materials, pre-activation was to induce active components on the raw coal surface, moderate temperature activation was to form and develop micropores, and high temperature activation was to widen micropores of activated carbon.