Nitrogen-doped carbon nanotubes as efficient and durable metal-free cathodic catalysts for oxygen reduction in microbial fuel cells

Recently, the application of microbial fuel cells (MFCs) with cost-effective and long durable cathodic catalysts to generate electricity sustainably, has drawn much attention. This study investigated the use of nitrogen-doped carbon nanotubes (NCNTs) as the cathodic catalyst for oxygen reduction in MFCs to produce electricity efficiently and durably. The obtained maximum power density was 1600 ± 50 mW m−2, which was higher than the commonly used platinum (Pt) catalyst (Pt/C) (1393 ± 35 mW m−2). Also, the drop percentage of power densities with NCNTs was lower than with Pt/C over 25 cycles, indicating that MFCs with NCNTs as the cathodic catalyst could generate electricity more durably than those with Pt/C. Further investigation of the mechanisms revealed that MFCs with the bamboo-shaped and vertically aligned NCNTs had lower internal resistance and higher cathode potentials. Rotating ring-disk electrode voltammogram, Raman microspectroscopy and X-ray photoelectron spectroscopic analyses suggested that NCNTs possessed a higher electrocatalytic activity for the oxygen reduction reaction (ORR) via a four-electron pathway in neutral pH phosphate buffer solution (PBS). Cyclic voltammograms on NCNTs and Pt/C electrodes before and after a continuous potentiodynamic swept in neutral PBS demonstrated that NCNTs had a better durability for cathodic ORR than Pt/C, which drove MFCs with NCNTs to generate electricity durably.