Pomelo peel-derived, N-doped biochar microspheres as an efficient and durable metal-free ORR catalyst in microbial fuel cells

Microbial fuel cells (MFCs) are widely explored for promising green and renewable energy generation; however, their strong reliance on noble metal-based catalysts causes high fabrication costs and thus limits their widespread applications. Herein, metal-free alternatives derived from naturally abundant, renewable pomelo peels are explored. Two kinds of pomelo peel-derived novel N-doped carbocatalysts are presented, i.e., biochar microspheres (BCMs) and their activated porous counterpart (a-BCMs). In comparison with BCMs, the thermal activation processing endowed a-BCMs with significantly enhanced porosity, an elevated N doping content (2.41 vs. 0.51 at%), a 5-fold increased specific surface area (314.3 vs. 62.8 m(2) g(-1)), and a much improved electrical conductivity (0.13 vs. 0.006 S cm(-1)). This metal-free a-BCM catalyst is subsequently employed to construct an MFC cathode for the oxygen reduction reaction (ORR), and an impressive electrocatalytic activity is achieved in 0.1 mol L-1 phosphate-buffered saline (PBS) buffer. The maximum power density reaches as high as 907.2 mW m(-2), comparable to that of the costly Pt/C counterpart (1022.9 mW m(-2)). The long-term durability of the a-BCM electrocatalyst is also demonstrated by continuous running for 90 days, even superior to that of Pt/C. The origin of this excellent electrocatalytic performance of a-BCMs is deeply analyzed and discussed. Furthermore, the 4e(-) reduction pathway is also unraveled for the efficient and durable carbocatalysis of the ORR over a-BCMs in MFCs.