Biomass-derived N-doped porous carbon nanosheets for energy technologies

Ultrathin porous carbon nanosheets (PCNs) with large aspect ratio and hierarchical porous layered structure provide numerous catalytic active sites along with chemical stability, large surface area and high electrical conductivity that is ideal for energy conversion and storage devices. The doping of PCNs with nitrogen and other heteroatoms improves the electrocatalytic properties needed for energy applications. However, routine synthesis methods based on molecular assembly or templates yield only small samples that restricts the use of PCNs in flexible electronics and energy devices and moreover, the use of toxic etching agents to remove templates is not environment-friendly. Therefore, the scalable and low-cost production of highly active and robust biomass-derived PCNs is of paramount importance and it can be achieved by the use of low-cost, abundantly available, sustainable biomass. In the present article, biomass-derived PCNs and their N-doping has been reviewed for energy applications, primarily as oxygen reduction reaction catalysts in fuel cells and as electrodes in supercapacitors. The synthesis of PCNs from twenty biomasses, ten for each application, and their N-doping by conventional (using ammonia or urea) and self-doped green (using biomass) approach has been discussed. In the green approach, same biomass is used as C as well as N (or heteroatom) precursor for N or heteroatom-doping of PCNs and it has recently emerged as a very promising green strategy for the scalable synthesis of N-doped PCNs from biomass. It can be further tuned to develop advanced carbon nanomaterials from diverse biomass materials for various sustainable energy related applications.