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.