Microbial fuel cells (MFCs) generate low-pollution power by feeding organic matter to bacteria; MFC applications have become crucial for energy recovery and environmental protection. The electrode materials of any MFC affect its power generation capacity. In this research, nine single-chamber MFCs with various electrode configurations were investigated and compared with each other. A fabrication process for carbon-based electrode coatings was proposed, and
As technology advances, energy consumption and shortfalls of energy supply are inevitable. Moreover, the rise of environmental awareness has motivated the development of minimally polluting renewable energy sources, such as microbial fuel cells (MFCs). Thus, MFCs have been extensively studied in the last decade. MFCs utilize microorganisms as catalysts to decompose organic or inorganic matter and harvest electrical energy [
The major obstacles towards commercialization of MFC are its high cost of fabrication and low power output. The cost of MFC mainly depends on the design of reactors, membrane separator, and electrode catalysts [
The power output of an MFC is affected by several factors, including the microbial inoculation, electrode materials, ionic concentration, catalyst, internal resistance, and electrode spacing [
In recently years, carbon nanotubes (CNTs) and graphene have been intensively studied and explored in various applications for advanced technologies due to their fascinating properties, such as high electrical conductivity, surface area, and stability [
The process of electrode modification was described as follows. For anodic modification, carbon ink was prepared by dispersing 20 mg of multiwall CNTs (MW-CNTs; average diameter larger than 50 nm, length between 10 and 20
For cathodic modification, polytetrafluoroethylene (PTFE) solution, with or without carbon-based materials, was used for waterproofing. The treated PTFE solution was prepared by dispersing 20 mg of MW-CNTs or MG in 19.98 g of PTFE solution (preparation 60 wt% dispersion in H2O). The dispersion was performed using ultrasonic vibration to obtain a homogeneous solution. A piece of SSM was soaked in the pure or treated PTFE solution for 1 hour, removed, and then baked at 235°C for 1 hour. The soaking-baking process was repeated four times to obtain a cathode with excellent waterproofing. In some study cases, platinum (0.5 mg/cm−2, 20 wt% Pt/C) was used to catalyze the oxygen reaction. For those case studies, the inner side of the cathode, which served as the contact surface with the reaction environment, was covered with a Pt catalyst after waterproofing and baked for 30 minutes at 350°C.
A single bacterium,
The structure of the air-cathode MFC used in this study, which resembles previously reported MFCs [
Schematic of the air-cathode MFC used in this study. Different types of SSMs can be used as electrodes and connected to external resistance loads with copper wires.
Nine types of single-chamber MFCs were constructed, as shown in Figure
Electrode conditions of the MFC systems.
Experimental cases | Electrode | |
---|---|---|
Anode | Cathode | |
MFC-NE | Normal | PTFE |
MFC-GE | MG | PTFE |
MFC-CE | MW-CNTs | PTFE |
MFC-NPE | Normal | Pt + PTFE |
MFC-GPE | MG | Pt + PTFE |
MFC-CPE | MW-CNTs | Pt + PTFE |
MFC-NGE | Normal | MG + PTFE |
MFC-NCE | Normal | MW-CNTs + PTFE |
MFC-NPGE | Normal | Pt + MG + PTFE |
Schematic of the electrode conditions in the studied case.
Stable values of power density and internal resistance were selected to compare the effects of MW-CNTs and MG on MFC system performance. The methods of measurement and calculation are described as follows. A digital electronic multimeter was employed to record the cell voltage. Before determination, a resistance of 1 kΩ was used as the external load. When the voltage output was stable, the cell voltage was recorded to evaluate the power generation. External resistance was varied from 1 to 20 kΩ to obtain the cell voltage of the MFC under testing, and then the performance of the MFCs was evaluated through the polarization and power density curves. Power density
The results of the anodic modification were photographed through a scanning electron microscope (SEM) and are shown in Figures
SEM images of MG on an SSM anode.
SEM images of MW-CNTs on an SSM anode.
The results of cathode modification are shown in Figure
SEM images of cathodes covered with treated PTFE. (a) PTFE mixed with MG. (b) PTFE mixed with MW-CNTs.
To realize the diverse effects of differently coated anodic surfaces on MFC performance, three types of anodes with PTFE-coated cathodes (MFC-NE, MFC-GN, and MFC-CN) were selected for analysis. The results are shown in Figure
Performance of systems with PTFE-coated cathodes and with anodes coated with different materials.
In the cases of MFC-NPE, MFC-GPE, and MFC-CPE, all of the cathodes were covered with pure PTFE and then coated with Pt, whereas each of the three anodes received a different treatment (MG, MW-CNTs, or nothing). The results of these three cases are shown in Figure
Performance of systems with anodes coated with MG, MW-CNTs, or nothing, and with Pt/PTFE-coated cathodes.
The effects of modified cathodes on MFC performance were evaluated, as shown in Figures
Performance of systems with cathodes coated with different materials and with uncoated SSM anodes.
Performance of systems with cathodes coated with graphene-treated PTFE and platinum.
To compare the maximum power densities among various MFC systems as shown in Figure
Figure
The objective of this study was to realize the modification effects of SSM electrodes by using carbon-based materials (MW-CNTs and MG). Therefore, experiments were conducted using a single species of microorganism. No electron transfer mediators were used because mediators might have distorted the accurate assessment of the modification effects of electrodes; this is also why the MFC power output levels in this study were lower than those reported in most related research. Besides, among all the experimental trials, the MW-CNTs-modified electrodes showed higher power generation than the MG-modified electrodes, regardless of whether the MW-CNTs were on the anode or on the cathode. A possible explanation is the differences in the surface area of MW-CNTs- and MG-coated SSMs. MW-CNTs and MG are adsorbed on SSM via carbon-metal bonding [
According to the measurement data, the internal resistance was evaluated using the polarization slop method; Table
Internal resistances of the MFC system for different electrode conditions.
Experimental cases |
|
---|---|
MFC-NE | 101 |
MFC-GE | 30 |
MFC-CE | 19 |
MFC-NPE | 20 |
MFC-GPE | 7 |
MFC-CPE | 4 |
MFC-NGE | 83 |
MFC-NCE | 36 |
MFC-NPGE | 14 |
Regarding cathodic modification, the internal resistance was reduced from 101 kΩ (in the case of MFC-NE) to 83 kΩ for the case of the MG-modified cathode (in the case of MFC-NGE) and markedly decreased to 36 kΩ for the case of the MW-CNTs-modified cathode (the case of MFC-NCP). However, when the cathode was coated with pure PTFE and Pt (in the case of MFC-NPE), the internal resistance was 20 kΩ. Thus, a cathode coated with Pt has a more favorable internal resistance than does a device coated with PTFE mixed with MW-CNTs or MG. This may be explained by the amount of contact area between the materials used for modification and the reaction liquid. Additionally, internal resistance can be further improved by using Pt and treated PTFE. The internal resistance was reduced from 20 kΩ for a normal electrode with Pt catalyst (in the case of MFC-NPE) to 14 kΩ when the PTFE waterproof layer was mixed with MG (in the case of MFC-NPGE).
Comparing the performance levels of modified anodes with those of cathodes, it was noted that the anodic modification showed more obvious effects on the performance than did the cathodic modification. Moreover, the MFC systems with MW-CNTs-modified SSM electrodes showed lower internal resistance levels than those with MG-modified electrodes for the same electrode conditions. In summary, MW-CNTs- or MG-modified electrodes have lower internal resistance levels. The excellent electrical conductivity values and high surface areas of MW-CNTs and MG improve the efficiency of electron transmission.
Air-cathode MFCs were constructed using different types of composite electrodes, with or without MW-CNTs- or MG-modified SSMs. The experiments confirmed that the addition of MW-CNTs and MG enhanced the power density and reduced the internal resistance. Using MW-CNTs- or MG-modified anodes increased the maximum power density by approximately 7.1 and 3.1 times, respectively, compared with that of an untreated anode. Cathodes coated with PTFE solutions mixed with MW-CNTs and MG have maximum power densities, approximately 4.5 and 1.7 times, respectively, those of a cathode coated with pure PTFE solution. Additionally, internal resistances were substantially reduced from 101 kΩ for the normal case (MFC-NE) to 19 kΩ and 30 kΩ for the cases of anodic electrode modified by MW-CNTs (MFC-CE) and MG (MFC-GE), respectively. The electrodes modified with MW-CNTs showed superior power density and lower internal resistance than did those modified with MG. The surface area of the stacked MG may be lower than that of the MW-CNTs. The MW-CNTs coatings appeared to form interconnected networks rather than loosely overlaid MG, leading to the superior conductivity levels of MW-CNTs-coated SSMs.
The authors declare no conflicts of interest.
The authors are grateful to Professor Hwan-You Chang (Department of Medical Science, National Tsing Hua University) for discussion and support regarding microbes. In addition, the authors acknowledge National Nano Device Laboratories for helping SEM imaging.