Effects of anode and cathode areas on organic compounds removal and power generation in membraneless microbial fuel cell (MFC)

Electricity generation in microbial fuel cells (MFCs) is dependent on the biochemical reactions (electron donation) carried out by bacteria attached on anode surfaces and the electrochemical reactions (electron acceptance) on cathode surfaces [1]. This study focused on the effects of anode and cathode area on power generation and wastewater treatment in MFCs. It was hypothesized that increasing the anode area would result in an increased amount of bacteria and therefore more available electrons, while an increased cathode area would have a greater capacity to accept electrons and thus increase power generation in the MFC [2]. In this study, clean plain carbon cloth was used as the anode (geometric area of 10, 20 and 40 cm2) and clean platinum (Pt)-coated carbon cloth (Pt loading 0.09 mgPt/cm2) as the cathode (geometric area of 1.25, 2.5, 5 and 10 cm2). The single-chamber MFCs (volume: 0.13 L) were inoculated with raw wastewater. Sodium acetate was added weekly as a substrate at a concentration of 3 g/L. The operational period was 4 weeks. The configuration is shown in Figure 1 and previously used [2]. The results showed that when the cathode areas were kept the same, the power generation of the MFC did not vary with anode areas (10 to 40 cm2) (Figure 2). On the other hand, the power generation increased along with the increase in cathode areas, but only slightly. When the cathode areas was doubled from 1.25 to 2.5 cm2, the power generation increased from 230-236 to 241-251 μW) (Figure 2). When the cathode areas further increased to 5 and 10 cm2, the power generation reached 256-264 and 265-276 μW. Overall, the power generation only increased 10-16% when the cathode areas increased 9 times (from 1.25 to 10 cm2). The study indicated that the power generation of MFCs did not increase with anode areas, but slightly increased with cathode areas. The cathodic electrochemical reactions might be the limiting factor. The degradation of organic compounds (chemical oxygen demand: COD) was also investigated in MFCs and clearly showed the trend with anode and cathode areas. The COD degradation efficiency increased 1-7% when the anode areas increase from 10 to 20 cm2, and further increased 10-14% when the anode areas increased to 40 cm2 (Figure 3). This increase in COD removal was probably due to a greater amount of bacteria growing on larger anode surfaces. The anaerobic bacteria growing on anode surfaces degrade COD. The increase in cathode area substantially enhanced COD degradation (Figure 3). When the cathode area was 1.25 cm2, the COD degradation efficiency was 50-55% (anode area: 10-40 cm2); when the cathode area increased to 2.5 cm2, the COD degradation efficiency increased to 65-74%. The COD degradation continued increased with larger cathode areas, and reached the highest values (87-89%) when the cathode area increased to 10 cm2. The reason for the significant increase in COD removal but slight increase in power generation at larger cathode areas was that the increase in cathode areas enlarges the surface areas for the growth of aerobic bacteria on cathode, which enhanced the COD removal. Because aerobic COD removal is much faster than the anaerobic COD removal, the increase in cathode areas led to high COD removal efficiency. This study investigated the effects of the anode and cathode area on power generation and COD removal in MFCs. The results showed that an increase in anode area did not increase power generation in MFCs, and an increase in cathode area slightly increased power generation. On the other hand, enlarging the anode/cathode areas substantially increased the COD degradation efficiencies in MFCs, due to the higher amounts of anaerobic/aerobic bacteria growing on larger anode/cathode surfaces. The study demonstrated that the aerobic bacteria growing on cathodes effectively degrade COD in wastewater, which provides new understanding for the optimization of anode and cathode areas for power generation and wastewater treatment in MFCs.