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CNFs synthesized from biogenic material is being considered as an alternative catalyst for the oxygen reduction reaction. Zhou et al. (2016) [47] have fabricated heteroatom-doped (N and S) porous CNFs via pyrolysis of natural spider silk as a precursor. The CNFs have exhibited very good oxygen reduction activity (half-wave potential of 0.85 V and on-set potential of 0.98 V vs. RHE), superior to that of the Pt/C and many metal-free carbon catalysts under alkaline conditions. The catalytic proficiency of the SS-derived CNFs is attributed to the highly positive charges on the carbon atoms due to the presence of the electronegative nitrogen and sulfur atoms within the carbon lattice; and their high surface area and large number of active sites due to their nano-fibrillar structure and abundant pores. They suggested that since these CNFs exhibited excellent oxygen reduction activity in neutral solution (pH 7.0), they used it as cathode catalysts in microbial fuel cells (MFCs), which gave a maximum power density of 1800 mW/m² that is 1.56 times more than with a Pt/C cathode.