Clay (purchased from Art Publishing Educational Inc., Tokyo, Japan) was used in the ceramic separator for this experiment. D-glucose (Sigma-Aldrich Japan G.K., Tokyo, Japan) was also used as a fine powder in a mortar. Glucose was mixed with 30 g of clay to form sheets measuring 20 x 20 x 2 mm. The glucose was added in varying volume ratios of 0%, 10%, 20%, and 30% (as shown in Table 1). The sheets were then baked at 900°C for 3 h.

Figure 1 shows photos of the 0% and 30% separators after fabrication. It can be seen that the 0% separator has a smooth surface, whereas the 30% separator has an uneven surface. This indicates the effect of glucose on the separator structure.

Carbon felt (10 x 10 x 5 mm) was used as the anode electrode. The anode electrode was placed in paddy soil (Shiga, Japan) for 2 days prior to installation in the MFC to facilitate biofilm adhesion. Urethane (20 x 20 x 9 mm) (U.E.S. Inc., Osaka, Japan) and sumi ink (Kuretake Co., Ltd., Nara, Japan) were used as cathode electrodes. Bokuju is a traditional ink used in Japan and can be obtained at an affordable price. Bokuju was used in this case as a binder to connect the cathode electrode to the ceramic separator.

LB medium was used as an electrolyte in this experiment. To prepare it, 5 g of tryptone, 2.5 g of yeast extract, and 5 g of sodium chloride were dissolved in 500 ml of purified water. Then, 2.5 mL of a solution containing 0.5 g of sodium hydroxide, mixed with 50 mL of purified water, was added to adjust the pH of the LB medium. For the electrolyte used in MFC power generation, 100 mL of LB medium was prepared by mixing 500 mL of tap water with 1 mL of soil water collected from paddy fields.

Figure 2 shows a floating MFC with a separator attached. The ceramic separator was connected to the float using an adhesive. The cathode electrode is also created on this separator and fixed by drying the binder, Bokuju. MFCs were operated in a float type, as shown in Figure 3, with the ceramic separator placed in close contact with the cathode electrode. The distance between the electrodes was 35 mm, the external resistance was connected to 47 kΩ, and the external temperature was stabilized at 28°C ± 1°C. Tap water was added daily to the MFC electrolyte to maintain a stable water level.

After the ceramic separators were prepared, their surfaces were examined using a scanning electron microscope (SEM, S-4300, Hitachi, Ltd., Japan). Figure 4 shows those created containing (a) 0% and (b) 30% glucose. The 30% surface is more porous than the 0% surface. The use of separators with higher porosity improves the power generation in MFC as more protons are transported. Therefore, this ceramic separator has the potential to be an effective separator for MFCs.

Ag/AgCl reference electrodes were used to evaluate the performance of the anodes and cathodes in each MFC. These reference electrodes were made by soaking the surface of an Ag wire with AgCl ink and allowing it to dry. The voltage of each electrode relative to the Ag/AgCl reference electrode was measured using a digital multimeter. Figure 5 (a) shows the anode potential, with all anode electrodes stabilizing at approximately 500 mV. This indicates stable power generation by the microorganism at the anode electrode. Figure 5 (b) displays the cathode potential, where the cathode electrodes in MFCs with 0%, 10%, 20%, and 30% glucose stabilized at -10, 10, 15, and 25 mV, respectively. This indicates that the function of the cathode in MFCs improves with the addition of more glucose to the ceramic separator.

Figure 6 shows the power and current densities on day 5 for the 0% and 30% MFC. The external resistor connected to the MFC, 47 kΩ, was removed, and a variable resistor was connected in its place. The variable resistor varied from 47 kΩ to 12 kΩ, and the voltage was measured. The power density and current density were calculated from the resistance and voltage values. The maximum power density of the 0% MFC was 9.95 µW/㎠cm², whereas the 30% MFC was 13.27 µW/cm², representing an approximately 33% increase in power output. It can be said that ceramic separators made by mixing glucose are effective in increasing the power output of MFCs.

After running the 30% MFC, the anode electrode was removed from the float-type MFC and placed in alcohol for one day to kill any microorganisms on the electrode. The electrodes were then placed in ethanol for 30 minutes to dehydrate them and subsequently dried. A gold coating was applied to the dried electrode surface by sputtering to enhance its conductivity. The electrode surface was then observed using SEM. As shown in Figure 7, a significant amount of biofilm was formed on the electrode surface, confirming that power generation is due to microbial activity.

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sector/ individuals.

Not applicable.

The authors declare there is no conflict.