Superior pore network retention of carbon derived from naturally dried ginkgo leaves and its enhanced oxygen reduction performance

Abstract

Obtaining a highly porous carbon has always been considered as an essential issue in many electrochemical applications. Ginkgo leaves have not only unique shape and color, but also interesting chemical and medical properties, which have inspired us to investigate them. In present approach, the naturally dried yellow ginkgo leaves, collected in autumn season, are directly used to prepare the porous carbon with simple two-step template-free procedure of pyrolysis at different temperatures followed by acid treatment for removal of inherent mineral salts. Interestingly, it is found that inherent salts present in the resulting carbon backbone can play as porogen to create high amount of pores in the carbon framework when the salts are removed by acid treatment. Effect of alternations in ginkgo leaves structure during the climate change, from spring to autumn, on ORR activity is examined for the first time on the carbons obtained by carbonizing different color, greenish and yellowish, ginkgo leaves at 1000°C. Yellow leaves can maintain their original tissue structure during the gradual drying in cold weather of late fall, which results in formation of more stable structure, leading to development of much more pores and larger surface area in the resulting carbon. The unforeseen results exhibit surprisingly higher ORR activity for carbon catalyst obtained from yellow leaves (LY-1000) compared with one prepared from green leaves collected in summer, (LG-1000). Higher surface area of LY-1000 is found to be the most important key factor for its enhanced ORR activity. Furthermore, electrocatalytic property of the carbon greatly depends on the carbonization temperature, which is a crucial factor to make a balance between electrical conductivity, heteroatom doping and surface area. As the temperature increases, the heteroatom doping decreases, which is not favorable for ORR, but at the same time, the conductivity and surface area increase, which is beneficial for ORR, indicating intriguing trade-off between them as a function of temperature, which needs to be optimized for best ORR performance. Moreover, present work enables a large-scale production of efficient heteroatom-doped porous carbon from ginkgo leaf waste without using any activation and templating agents.