Nitrogen-Doped Carbon Aerogels Prepared by Direct Pyrolysis of Cellulose Aerogels Derived from Coir Fibers Using an Ammonia-Urea System and Their Electrocatalytic Performance toward the Oxygen Reduction Reaction

In this paper, a new method using an ammonia-urea system was proposed to prepare cellulose aerogels from coir fibers as the starting material for non-ordered porous nitrogen-doped carbon aerogels. Direct pyrolysis of the as-prepared carbon aerogels has successfully produced nitrogen-doped carbon aerogels using the cellulose aerogel derived from the nitrogen-rich coir fibers. The as-prepared carbon aerogel inherits the three-dimensional nonordered porous network of the cellulose aerogel, maintaining its high specific surface area (SSA) and the large pore volume. In addition, the honeycomb-like structure of internal pores in individual fibers could also be maintained, and the pores are even larger than those of the corresponding cellulose aerogel. In the ammonia-urea system, ammonia not only served as an agent to assist the cellulose dissolution but also played an important role in exfoliating the carbon aerogel to form defects that cause a few layer disorders. The defects caused the SSA and the pore volume of the aerogel to increase significantly after carbonization. The surface area increased from approximately 70 to 3730 m2/g and the pore volume from 0.54 to 4.20 cm3/g. The porous nitrogen-doped carbon aerogel showed excellent electrocatalytic activity toward the oxygen reduction reaction (ORR) in alkaline media following a two-electron-transfer mechanism.