N-Doping of Graphene Aerogel as a Multifunctional Air Cathode for Microbial Fuel Cells

One of the main challenges faced by microbial fuel cells (MFCs) generating voltage is how to facilitate the oxygen reduction reaction (ORR) process using a specifically designed air cathode, especially by optimizing a three-phase catalytic interface and enhanced O₂ diffusion on it. Herein, a three-dimensional porous N-doped graphene aerogel (NGA) is polymerized onto a steel mesh (SM) to construct a simple structure of an air cathode (NGA-x/SM) via hydrothermal synthesis and subsequent freeze-drying treatment; more specifically, NGA was simultaneously used as an efficient ORR catalyst layer and breathable gas diffusion layer to improve the performance of MFCs. In this system, the NGA-5/SM (with a precursor concentration of x = 5.0 mg mL^-1) makes itself a perfect candidate to be used as an air cathode. Characterization parameters reveal that sub-micrometer micropores, defective multilayer structures, and the highest proportion of pyridinic-N (48.1%) exist in NGA-5/SM. Furthermore, electrochemical measurements demonstrate that it has an oxygen reduction peak potential of 0.63 V, a Tafel slope of 187 mV dec^-1, and closest 4e^- transfer pathway (n = 3.2-3.5). These data prove that a three-phase boundary can naturally form in NGA-5/SM, where the ORR occurs. More importantly, this work provides a proof of concept that a Pt-free air cathode could be prepared with high-efficiency NGA by a two-step preparation method to achieve a MFC maximum power density of 1593 mW m^-2.