Carbon anode microbial fuel cell electrical characteristics of production
Because microbial fuel cell (microbial fuel cell, MFC) to purify the sewage in the same time, the organic matter in the chemical energy into electrical energy, low energy type for the new process of sewage treatment offers a new way of thinking, so in recent years has been widely attention [1 ~ 6]. However, the existing electricity generation capacity of less MFC, which makes the practical application of MFC is limited [7,8]. Therefore, improving the ability of MFC power production is one of the main objectives of the present study.
The composition from the MFC point of view, micro-organisms attached to the anode as the carrier of electricity production, electricity generation not only affect the amount of microbial adhesion, but also affect the electron transfer from microbes to the anode , to improve the electrical properties of MFC has a crucial production impact. Therefore, to improve electricity production capacity starting MFC, choose a potential anode material research, analytical properties of the anode material and surface electrical properties of the impact of microbial products, to improve the ability of MFC power production is of great significance. now been widely studied anode materials flat MFC can be divided into two categories, Category 1 is the plate material, such as carbon paper, graphite, flexible graphite, etc.; Category 2 is the powder electrode material through the binder pressed into plate, such as Zhang et al  using Teflon as a binder, pressed into the toner as the MFC anode plate, and expanded the range of options MFC anode.
In MFC, the anode should be easy to produce high-power micro-organisms attached to the growth and easy electronic transfer to the anode body from micro-organisms, and requested the internal resistance of the anode is small, highly conductive, anodic potential and stability. Nanotubes as a specific pore structure, high mechanical strength and toughness, a large specific surface area, high thermal stability and chemical inertness, electrical conductivity of strong and unique one-dimensional scale, could be a very good electrode material [11 ~ 13 ]. Qiao et al  will load in the polyaniline on carbon nanotubes, for producing electricity using E.coli microorganisms, their maximum capacity power density 42 mW/m2, that carbon nanotubes can improve the MFC producing electricity.
Based on the above analysis, this study, a room type MFC using carbon nanotubes as anode materials, study the electrical characteristics of its production. And has studied with two kinds of flexible graphite anode materials and activated carbon --- constructed flat MFC (FG-MFC and AC-MFC), and carbon anode MFC (CN-MFC) were compared to determine the surface characteristics of the anode 3, and 3 under the testing result of the electrical properties of MFC and coulomb efficiency of production differences, initially identified as carbon nanotubes the advantages of the anode materials. Since MFC anode 3 different resistance were determined by use of discharge stability at different times needed to finalize the 3 different external resistance of MFC when the minimum stable discharge time to accurately determine the internal resistance of MFC .
1 Materials and methods
1.1 MFC test device
Test using a room-type MFC, with the literature  shows, a total of 3 sets. Cathode air cathode are used, including the membrane electrode (GEFC-MEA, the catalyst platinum loading on the proton membrane PEM, 0.5 mg load / cm2, gold can, Beijing) and the diffusion layer (GDL-4, gold can, Beijing), the two pressed together through the porous plate.
3 sets of a room-type MFC carbon anode, respectively, flexible graphite, activated carbon powder of 3. In which carbon nanotubes were prepared by using vapor deposition, using 5% (mass fraction) of polytetrafluoroethylene (PTFE) as a sticky mixture of carbon nanotubes bonded together; activated carbon powder produced by Toray of Japan, with 5% (by mass) of activated carbon powder, PTFE as a binder to glue together; flexible graphite carbon was purchased from Beijing three industry companies, thickness of 1.5 cm. all the anode area were 3 cm 3 cm.
Anode chamber volume of 36 cm3. Anode chamber into the saturated calomel electrode (SCE, 212 type, Shanghai) as a reference electrode, the load with adjustable resistance box (ZX21 type, Tianshui). The output voltage from the signal acquisition system (DAQ2213, Dalink, Taiwan) automatically records storage, the minimum time interval between sampling rate of up to 1 ms.
1.2 Microbial inoculation and operation of MFC
Strains used in the experiment from the Beijing Qinghe sewage sludge, under anaerobic conditions by the acid from 3 d after the domestication of water distribution as seed sludge inoculation 10 mL. Anode chamber from the supply of substrate for the acid with water, mainly components: CH3COONa 1 640 mg / L; NH4Cl 500 mg / L; KH2PO4300 mg / L; MgCl2 6H2O 100 mg / L; CaCl2 2H2O 100 mg / L; KCl 100 mg / L; pH value of about 7 [16 ]. lid sealed on the anode chamber, test the solution before the exposure of the anode chamber of nitrogen 2 min, to remove one of the O2, the oxygen concentration was maintained at 0.2 mg / L below the temperature constant at 25 °C +/- 3 °C.
1.3 Test and calculation methods
Determination of steady-state discharge using MFC, the apparent resistance , measured using standard analytical methods out of the water COD concentration , anodic pore size distribution using mercury analyzer (Autopore 9510, the United States) determination. Multimeter measured using 3 conductive properties of the anode (test material length 5 cm, width 1 cm, thickness 0.1 cm). first with two plexiglass panels flatten the measured material, and then through the plexiglass panel meter resistance pen reserved hole contact with the anode materials were measured. anode to the protein content of biomass characterization. determination method: Remove the anode placed in a certain area of 5 mL deionized water, and then ultrasonic power 200 W 5 min, so attached to the electrode cells spread on the solution. centrifuged (2 000 r / min, 2 min) 0.5 mL supernatant was taken and 0.5 mL NaOH (0.1 mol / L) mixed, using modified Lowry method determination of protein concentration . coulomb efficiency E according to equation (1)  calculation:
Where, Ui MFC output voltage for the time ti (V), R for the foreign resistance (), Fi is the Faraday constant, 96 485 C / mol, bi corresponding to 1 mol COD number of electrons, equal to 4 e-mol / mol, S for the removal of COD concentration (mg / L), V for the use of matrix volume (L). M is the molecular weight of oxygen equal to 32 g / mol.
2 Results and Analysis
2.1 3 and the internal resistance of MFC distribution of electricity production
3 MFC inoculated with the same amount of sludge, foreign production to achieve stability in power, the use of MFC internal resistance and discharge method calculated the maximum electric power production as shown in Figure 1, where the results of the data for the determination of the average of 5 d (day Determination of 1), 5 times the standard deviation of the data as the error limit. It can be seen from Figure 1, the same inoculation conditions and operating conditions, carbon nanotubes, flexible graphite and activated carbon as the anode of the MFC internal resistance were 263,301 and 381, corresponding to the maximum capacity for the electric power density of 402,354 and 274 mW/m2. nanotubes as the anode the maximum density of the largest electric power production, its low internal resistance is the main reason.
Further determined using current interruption the internal resistance of MFC 3
Distribution, shown in Figure 2. Nanotube anode resistance and ohm resistance of flexible graphite and activated carbon were lower than the anode. And flexible than the graphite anode, carbon anode can accommodate more micro-organisms, so the CN-MFC anode resistance minimum. With the current interruption method measured the lowest ohmic resistance of FG-MFC, followed by CN-MFC, AC-MFC highest. and activated carbon powder, compared to carbon nanotube electrical conductivity, so the lowest ohmic resistance, which Huang Hui et al  using carbon nanotubes electrode impedance spectroscopy study found a significant reduction in the ohmic resistance is the same. And three kinds of resistance in the cathode MFC 150 ~ 160, or less, indicating that three kinds of MFC internal resistance of the main differences resistance and ohm resistance from the anode.
2.2 Coulomb efficiency of MFC 3
Coulomb used to measure the efficiency of electron use efficiency .3 kinds of MFC MFC resistance is 500, respectively, under the conditions of outer and stable operation of 48 h, determination of matrix degradation, and then according to equation (1) MFC calculated coulombic efficiency, the results shown in Figure 3. MFC in 3 different anode, CN-MFC Coulomb efficient. CN-MFC, AC-MFC, and FG-MFC5000 81, 0.77 and 0.69 mA, CN-MFC maximum current density, and the degradation of organic matter (see Figure 3, COD degradation rate) lower than the other 2 MFC, so the carbon nanotubes as the anode of the MFC of the Coulomb efficient.
2.3 3 Cumulative pore volume of the anode compared
Measured by mercury intrusion pore volume of total anode 3 with the pore size distribution shown in Figure 4. As is usually the case the size of individual bacteria in 0.45 m or more, while the anode surface only in the 0.45 m pore size than bacteria can only produce electricity in which the growth, therefore, Figure 4 lists the pore size> 0.45 m above the data. It can be seen from Figure 4, the cumulative pore volume of carbon nanotubes and activated carbon were higher than flexible graphite, which is due to carbon nanotubes and activated carbon powder particles smaller particle size, the use of PTFE as a binder pressed into the anode sheet after its larger faces. large pore volume available to accommodate more of the conditions for micro-organisms (protein content, see 2.4).
3 2.4 The growth of microorganisms in MFC
3 MFC anode running stable protein content determination, while the corresponding results are listed in Table 1.3 kinds of resistance MFC anode size and protein content was negatively correlated with Huang Xia et al  compared to other anode materials similar result. the highest density of carbon nanotubes on the protein, indicating that the use of carbon nanotubes in the MFC anode is not strong inhibitory effect on microorganisms, Morozan  New research also shows that carbon nanotubes as the MFC anode showed good biocompatibility sex.
2.5 Comparison of 3 Conductivity
3 using the multimeter test the conductivity of anode materials, test table pen spacing 4 cm, each material measuring 3 times the average value as the measurement results, shown in Figure 5 .3 kinds of anode materials, the conductivity of the most flexible graphite well, followed by carbon nanotubes, carbon minimum.
Determination of three kinds of MFC 2.6, the minimum resistance stability time were determined by steady-state discharge CN-MFC, AC-MFC, and FG-MFC internal resistance, from resistance changes with stable time relationship shown in Figure 6 . we can see, with the increase of settling time, 3 the determination of MFC internal resistance increases. For the FG-MFC, stable time> 300 s the measured results when the resistance is essentially the same, for the AC-MFC, stable time> 1 200 s Determination of resistance when the results are basically unchanged, while the CN-MFC, and stable at least 1 800 s time to get more accurate within the resistance. MFC internal resistance measurement to three kinds of line in front of the text when the settling time are involved in resistance 1 800 s.
3 different anode materials, MFC, the MFC anode carbon nanotubes as the largest electric power producing the highest density, lowest resistance, while the highest coulombic efficiency. Nanotubes as the anode of the MFC internal resistance of minimum performance and the lowest in the anode ohmic resistance resistance minimum two aspects, which are associated with the anode material characteristics. anode material properties, including the table face size, conductivity and capacitance.
Anode resistance anode will affect the speed of electrochemical reaction on the anode power production on the micro-organisms (anode catalysts) the more the electrons from the microbial body faster transfer to the anode, the anode resistance is smaller. Nanotubes volume table developed faces, and flexible graphite, compared to accommodate more microorganisms. experimental test showed that carbon nanotubes attached to the high protein content of microorganisms, producing electricity as a microbial anode catalysts the higher the density the lower the anode resistance. Compared with activated carbon, although carbon anode face smaller table, but the carbon nanotubes are rich in carboxyl functional groups, also has features such as wall defects, can promote the electron transfer . For the above reasons are may lead to carbon nanotubes as the anode of the MFC anode resistance decreased.
Ohmic resistance characterized by MFC when the internal charge transfer resistance, MFC-ohm resistance, including charged ions (eg protons) to pass the time in the electrolyte solution resistance and electrons are passed by the resistance of the electrode. In this study, referred to in 3 MFC exactly the same electrolyte solution, and the resistance of the electrode on the electron transfer is different from carbon nanotubes with good electrical conductivity, so the current interruption method using measured minimum ohmic resistance, which Huang Hui et al  using impedance spectroscopy was found that carbon nano- ohmic electrodes tube significantly reduced the same. add multi-walled carbon nanotube electrode material may be more conducive to electronic transmission, to improve the conductivity of electrode materials [23,24].
Meanwhile, the carbon nanotubes with good electrical conductivity, and the carbon nanotubes may be one-dimensional scale for electricity production on the bacterial cell membrane nanowires (cilia and flagella) of electron transfer , resistance to further decline. Morozan  recently found that carbon nanotubes and micro-organisms producing electricity with biocompatibility, the use of this nature, may increase the ability of MFC to produce electricity.
(1) carbon nanotubes, activated carbon, and flexible graphite as the anode of the MFC power density of the maximum electricity production were 402,354 and 274 mW/m2, which produce electric power MFC carbon anode density. Nanotubes, activated carbon, and flexible graphite as MFC internal resistance of the anode, respectively 263,301 and 381, carbon nanotubes can reduce ohmic resistance and MFC internal resistance of the anode.
(2) external resistance in the same conditions, the carbon anode Coulombic efficiency of MFC than the other two kinds of materials for the anode of the MFC.
(3) carbon nanotubes and activated carbon anode surface is higher than the cumulative pore volume of flexible graphite anode, the same mixed strain inoculation conditions, the protein content of carbon anode 149g/cm2, the protein content of carbon anode 132g/cm2, Flexible graphite anode of protein content 92g/cm2, protein content on the anode was negatively correlated with the anode resistance.
(4) measurement CN-MFC, AC-MFC, and FG-MFC internal resistance of the stability of the time required were 1 800,1 200 and 300 s.