Fig. 2

The current density of the two systems and the time that polarization curve was measured (a). The meso-MFC immediately reaches 780 mA m^− 2 (based on the cathode area), and gradually increases to plateau at 1900 mA m^− 2; the current density of the thermo-MFC gradually increases to 875 mA m^− 2. The polarization and power density of the two systems (b). The highest voltage is 0.38 V for the meso-MFC and 0.28 V for the thermo-MFC. The highest power densities for the meso-MFC and thermo-MFC are 330 and 105 mA m^− 2