Self-supporting nitrogen-doped reduced graphene oxide@carbon nanofiber hybrid membranes as high-performance integrated air cathodes in microbial fuel cells

The traditional air cathode in microbial fuel cell (MFC) usually consists of catalyst layer (CL), supporting layer (SL) and conductive gas diffusion layer (GDL), the overall MFC performance is inevitably affected by the additional and expensive adhesives and conductive agents. Here, we developed an integrated air cathode in MFC without any additional SL, GDL or adhesives. The integrated air cathode was selfsupporting nitrogen-doped reduced graphene oxide@carbon nanofiber (N-rGO@CNF) hybrid membranes fabricated by electrospinning with subsequent heat-treatment under ammonia atmosphere. The as-fabricated N-rGO@CNFs possessed far superior MFC performance and oxygen reduction reaction (ORR) activity to the pristine nitrogen-doped carbon nanofibers (NCNF) and commercial activated carbon (CAC). The amount of rGO embedded into CNF had prominent influence on their ORR activities and MFC performances. N-5-rGO@CNF had the lowest resistance and the maximal exchange current density, exhibiting desirable oxygen reduction performance via a four-electron pathway. The maximum power density of N-5-rGO@CNF can reach 826 mW m-2 in MFC, which is approximately 9, 2.53 and 1.82 times of pristine NCNF, CAC and Pt/C with values of 91, 327 and 454 mW m-2. The outstanding performance of the integrated air-cathodes originates from the integrality, brevity and hybrid composition of the electrospun nanofiber membrane. The appropriate embedded rGO not only improves the bulk conductivity of the rGO@CNF to promote ion adsorption, but also provides vacancies to accommodate ions, the doped nitrogen atoms facilitate O2 adsorption and/or subsequent O-O bond breaking, thus improving the electrochemical performance of N-rGO@CNF in MFC.