Modification of carbon felt anodes using double-oxidant HNO3/H2O2 for application in microbial fuel cells

Carbon felt is widely used as an anode material in microbial fuel cells (MFCs) because of its high specific surface area, low cost, good electrical conductivity, and biocompatibility. In this paper, carbon felt samples were thermally treated with a mixed solution of concentrated HNO3 and 30% H2O2 with different volume ratios of 1 : 3 (MFC-1), 1 : 1 (MFC-2), and 3 : 1 (MFC-3). The electrochemical performance of the resulting MFCs were investigated by cyclic voltammetry, electrochemical impedance spectroscopy, chronoamperometry and polarization curve measurement. Fourier transform infrared spectroscopy and scanning electron microscopy were conducted to characterize the functional groups and the morphology of the carbon felts. After modification, the number of oxygen-containing functional groups in MFC-1, MFC-2, and MFC-3 increased compared with MFC-4 (bare anode MFC), the start-up time of the obtained MFCs was markedly shortened, and the charge transfer resistance of the bioanode was decreased. In MFC-2, the maximum power density was 758.2 mW m−2, which was 51.1% higher than MFC-4. Increases of oxygen-containing functional groups on the modified anodes favored the adsorption and growth of bacteria and acceleration of electron transport between the electrode and bacteria. Thus, the electrochemical characteristics of MFCs employing these anodes were improved.


Introduction
Microbial fuel cells (MFCs) are considered as a promising technology in the eld of renewable energy production and wastewater treatment. In these cells, exoelectrogenic bacteria directly converted chemical energy stored in organic waste into electrical energy. [1][2][3] Despite their obvious benets, however, MFCs suffered from low energy production and poor organic degradation. Thus, the performance of MFCs should be improved for practical application, which was inuenced by several factors, including microbial species, types and properties of the substrate, diaphragm material, electrode material, and system design. [4][5][6][7] The electrode material, especially anode material, played a crucial role in the power production of MFCs. The electrode material provided support for exoelectrogenic bacterial attachment and promoted the export of electrons generated in the redox reaction. Carbon materials, such as carbon paper, carbon cloth, carbon felt, carbon ber brush, graphite rods, and graphite plates, were the most widely used anode materials in MFC. High-performance anode materials should be inexpensive and presented good hydrophilicity, large specic surface area, good conductivity, excellent electrochemical properties, and good biocompatibility. [8][9][10][11] Several studies had shown that surface modication of anode materials mainly affected the electron transfer mechanism in two ways. First, changes in the structure of the material providing additional areas for bacterial adhesion. Second, the presence of functional groups favored electron transfer between the bacteria and electrodes. 12 According to Mohamed et al., 13 the power generation of MFC was signicantly affected by doping supercial nitrogen groups on the anode surfaces of carbon cloth and carbon paper. From Huang et al., 14 aer redox mediator modifying anodes prepared by electrodepositing riboavin and humic acid on the surface of graphite felt, MFC exhibited excellent electrocatalysis activity and showed decrease in internal resistance along with increase in maximum power density. Kang et al. 15 observed that electrochemical characteristics of MFC was enhanced through utilizing conductive polymer onto graphite felt base anodes. Li et al. 16 demonstrated that the performance of MFC was improved aer modifying carbon felt anode using two conductive polymer materials, polyaniline and poly(aniline-co-o-aminophenol).
This paper presented a new strategy to chemically treat carbon felts using mixed solutions of concentrated HNO 3 and 30% H 2 O 2 at different volume ratios. The morphology, hydrophilicity, and electrochemical properties of the treated carbon felts were characterized.

Preparation of electrodes
Carbon felts (Shanghai Lishuo Composite Material Technology Co. Ltd., Shanghai, China) with a 2 Â 5 cm 2 geometric area were treated with different proportions of mixed HNO 3 and H 2 O 2 .
The concentration of concentrated HNO 3 and H 2 O 2 was 16 mol L À1 and 8.8 mol L À1 , respectively. The carbon felts were submerged in 200 mL of mixed solution of double oxidant, ultrasonically dispersed at room temperature for 0.5 h, then heated in air at 450 C for 0.5 h in a muffle furnace. The resultant samples were washed repeatedly with deionized water until a constant pH was achieved. The samples were dried at 60 C overnight to obtain the modied anode material. MFC-4 referred to the bare anode MFC, in which carbon felt anode was not modied using double-oxidant HNO 3 /H 2 O 2 . Table 1 showed the treatment conditions of each sample.

MFC construction and operation
The cathode and anode chambers of the MFCs were of similar geometries, and their effective volume was also 80 mL. The two chambers were separated by a proton exchange membrane (Naon 115, DuPont) with an area of 4 cm 2 . The carbon felt (2 Â 4 cm 2 ) served as anode, and a stainless steel mesh (2 Â 2 cm 2 , 99.99%) served as cathode. The external circuit was connected to a constant external resistance of 1000 U, and the anode chamber was sealed to cut off air. Thus, an anaerobic environment was maintained in MFC. The sludge came from a local domestic sewage treatment plant in Taiyuan, China.
The MFCs were operated in batch mode. When the output current was less than 0.02 mA, the substrate was replaced. When the output maximum current reached a stable value, a mature biolm was formed and the battery was started up stably.

Analyses and calculations
All electrochemical tests were performed using a multichannel potentiostat (Princeton VMP III, US) with a three-electrode system consisting of a working electrode, a saturated calomel electrode (SCE) as reference electrode, and a stainless steel mesh electrode as cathode. Cyclic voltammetry (CV) was performed by applying a potential ramp at a scan rate of 5 mV s À1 over the potential range from À0.5 V to 0.5 V to the working electrode. The electrochemical impedance of anode was measured at frequencies ranging from 100.000 kHz to 5.000 mHz with a potential amplitude of 10 mV. The EIS tests were conducted at open circuit condition. Scanning data were tted and simulated using ZSimpWin 3.10 soware (Echem). Chronoamperometry was performed at a constant potential of À0.3 V (vs. SCE). Polarization curve measurements were obtained at 20 mV s À1 within a certain potential range. The performance of the fuel cells was critically evaluated based on power output. The power density curves were obtained by varying the external resistances. Current production during steadily operating of fuel cell was monitored by connecting to various external resistances (100 U to 100 kU) using a multimeter. Power output (mW) was calculated using the equation P ¼ IU. Power density (mW m À2 ) and current density (mA m À2 ) were calculated as a function of the anodic surface area (m 2 ). Fourier transform infrared spectroscopy (FTIR) was performed to analyze the functional groups formed on the electrochemically oxidized carbon felts. Treated felts (2 mg) were cut and mixed with KBr (200 mg), and the samples obtained were analyzed by a FTIR spectrometer (Nicolet is 5, US). Scanning electron microscopy (JSM-7001F, JEOL, Japan) was performed to analyze the bacterial morphology. Water contact angle measurement (Phoenix-300, Korea) was used to analyze the hydrophilic nature and the hydrophobic nature.

Acclimation of MFC
Aer inoculation, the MFCs took batch operation mode, each cell had different operating period. As shown from Fig. 1, the time to form the mature biolm were as follows: MFC-1, 400 h; MFC-2, 160 h; MFC-3, 220 h; and MFC-4, 450 h. The time required by the MFCs to reach the peak power output was shortened by anode modication. Modifying with a mixed solution of HNO 3 and H 2 O 2 could evidently help reduce the start-up time of the MFCs, thereby improving their electrochemical performance. Compared with mixed solutions of other ratios, the mixed solution with a volume ratio of 1 : 1 (MFC-2) was the most effective in improving the biochemical properties of carbon felt. These results were due to enhancements in the specic surface area of anode caused by acidinduced surface modication. 17 Acid treating increased the roughness of anode surface and provided a more conductive environment for microbial reproduction. The modifying hastened biolm formation on carbon felt surfaces, effectively enhanced the power outputs. 18 Increases of -OH and -COOH (Fig. 7) on the modied carbon felt beneted the adhesion and reproduction of bacteria, thereby enabling rapid formation of mature biolms.
Electrochemical measurement Cyclic voltammetry. The electrochemical behavior of the modied carbon felts was characterized by CV (Fig. 2). The cyclic voltammograms of MFC-1 and MFC-3 showed two pairs of reversible redox peaks, while MFC-2 presented a single pair of reversible redox peaks. The redox peaks of MFC-4 were especially weak. These results suggested that multistep electrochemical reaction took place on the biolm. Different functional groups on the modied carbon felt anode surfaces directly affected the positions and sizes of the redox peaks. The weak redox peaks of MFC-4 indicated that the electrochemical activity of biolm was low. Evident redox peaks were observed in MFC-1, MFC-2, and MFC-3. The two pairs of redox peaks of MFC-1 were observed at (0.21 mV, 8.85 mA; 0.17 mV, À1.67 mA) and at (0.005 mV, 5.47 mA; À0.058 mV, À2.54 mA); there was another oxidation peak (À0.31 mV, 2.81 mA). The two pairs of redox peaks of MFC-3 were observed at (0.21 mV, 6.33 mA; 0.18 mV, À1.35 mA) and at (À0.14 mV, 3.00 mA; À0.017 mV, À1.53 mA); there was also another oxidation peak (À0.33 mV, 1.69 mA). These characteristics indicated that the mechanism of electrochemical reaction occurring on the biolm was extremely similar in MFC-1 and MFC-3, while strong and reversible electrochemical oxidation and reduction reaction took place on the biolms. The bioelectrochemical reaction occurring was multi-step reaction. MFC-2 showed a pair of redox peaks at (0.21 mV, 13.00 mA; 0.17 mV, À9.54 mA), this might be attributed that two large redox peaks were superimposed into a single peak, in particular the peak current reached 13.00 mA. This meant that electrochemical activity of the bacteria on the modied felts was the highest among all MFCs. Differences between the observed redox peaks among the samples could be attributed to differences of the volume ratio of HNO 3 and H 2 O 2 . The much higher electrochemical activity might have resulted from enhancements of electron transmit between the bacteria and the modied carbon felts, the increased attachment of bacteria on the treated electrodes might also explained the results. 19 AC impedance. The Nyquist plots were shown in Fig. 3. The experimental spectra was t into an equivalent circuit according to Wagner to estimate the impedance data quantitatively. 20 The inset gure in Fig. 3 showed an equivalent circuit consisted of a solution resistance, followed by Randlestype charge transfer resistance, a Warburg diffusion resistance, and a constant phase element (CPE). A CPE suggests a rough electrode surface, which is used to simulate the nonideal behavior of a distributed capacitor. The experimental spectra were tted and simulated by ZSimpWin 3.10 soware (Echem). The charge-transfer resistance dominated the internal resistance of bioanodes. Table 2 showed that chargetransfer resistances of MFC-1, MFC-2, MFC-3, and MFC-4 were 130.11, 115.72, 116.26 and 148.23 U, respectively, likely because the double-oxidant modifying increased the C]C groups of the carbon felt, which could hasten the electron transfer rate on carbon felts. Meanwhile oxygen-containing functional groups on the treated carbon felts also increased such as biocompatibility, specic surface area, and the number of exoelectrogenic bacteria.   Chronoamperometry. Changes of current as a function of time under a constant anode potential of À0.3 V (vs. SCE) were shown in Fig. 4. The bioanodes exhibited electrochemical activity under a particular anode potential. The initial current values of MFC-1, MFC-2, MFC-3, and MFC-4 were fairly large, decreased sharply with time, and then reached steady currents of 0.77, 2.03, 0.69, and 0.13 mA, respectively. This result indicated that MFC-2 presented the highest bioelectrochemical activity among all MFCs.
Tafel analysis. The bioelectrocatalytic ability of biolm on the anodes was evaluated through Tafel slope analysis (Fig. 5). This analysis facilitated interpretation of the electrochemical activity of biolm on carbon felt. The semi-empirical Tafel equations of oxidative and reductive reaction can be expressed as follows: where i (mA) represents current; i 0 (mA) represents exchange current; E (V) is applied voltage; a a , a c represent the electron transfer coefficient of oxidative and reductive reaction, respectively; n is the number of electrons transmitted at the rate-limiting step; F is Faraday's constant (96 485 C mol À1 ); R is the gas constant (8.314 J mol À1 K À1 ); T is the temperature in kelvin (298 K). These equations simplify the description of kinetics of electron transfer-controlled processes to two parameters, namely, the exchange current density (i 0 ) and Tafel slope. The Tafel slope is inversely proportional to the electrocatalytic activity and electron transfer efficiency of biocatalyst. Tafel analysis showed marked variations in electron transfer efficiency and exchange current density among bioanodes. 21 The evaluation of biolm activity through Tafel analysis showed a gradually decreasing oxidative slope from MFC-4 (1.134 V dec À1 ), MFC-1 (1.116 V dec À1 ), MFC-2 (0.915 V dec À1 ) to MFC-3 (0.434 V dec À1 ), meanwhile a gradually decreasing reductive slope from MFC-4 (0.559 V dec À1 ), MFC-3 (0.441 V dec À1 ), MFC-2 (0.331 V dec À1 ) to MFC-1 (0.273 V dec À1 ). Higher Tafel slope indicates the lower bio-electro catalytic activity along with electron transfer efficiencies, so MFC-4 had the worst bioelectrocatalytic activity towards oxidation and reduction comparing with the other three. According to Tafel equation, the y-axis intercept was logarithm of the exchange current density (ln i 0 ). The i 0 calculated at maximum performance depicted clear variations among the anodes, 22 the values of MFC-1, MFC-2, MFC-3, and MFC-4 were determined to be 8.11, 11.97, 10.82, and 7.76 mA cm À2 , respectively. These results clearly indicated that i 0 of MFC-2 was the highest among MFCs studied.
Power output of MFC. The output power densities of MFCs were shown in Fig. 6. With increasing of current density, the power density gradually increased till attaining the maximum, then decreased. Carbon felt modication signicantly affected the anodic activity and power generation of MFCs. The output peak power density of MFC-1, MFC-2, MFC-3, and MFC-4 were 453.0, 758.2, 438.0, and 387.8 mW m À2 , respectively. Among the samples, MFC-2 showed the maximum enhancement of 51.1% relative to MFC-4. No signicant differences among MFC-1, MFC-3, and MFC-4 were found. The carbon felt treated with a suitable volume ratio of the mixed acids solution showed enhanced activity. Thus, we speculated that HNO 3 /H 2 O 2 modifying could altered the physical and chemical properties of carbon felts. The modifying increased the surface roughness  and specic surface area of carbon felt, which enhanced the hydrophilicity and provided a more suitable environment for bacterial growth. The carbon felt treated with volume ratio of 1 : 1 (V HNO 3 : V H 2 O 2 ) exhibited the best results. FTIR analysis. FTIR was employed to investigate the functional groups on carbon felts (Fig. 7). The untreated anode possessed a high O : C ratio, which probably resulted from the presence of surface contaminants (e.g., alkaloids, resins, etc.) formed during fabrication. Aer modifying, remarkable changes in O : C ratio were observed. The relative intensity of the broad peak at approximately 3417 cm À1 , which indicated the stretching vibrations of -OH within -COOH; and the peak at approximately 1400 cm À1 , which indicated the bending vibrations of -OH, increased considerably. This result indicated that the quantity of -OH and -COOH increased remarkably. The peak at approximately 1710 cm À1 , indicated the C]O stretching vibrations in -COOH, signicantly increased, thus con-rming that a large number of carboxyl-containing functional groups were generated. The relative intensity of the peak at 1225 cm À1 , representing the stretching vibrations of C-O, also increased. The peak at 1597 cm À1 might be associated with the stretching vibrations of aromatics (C]C) and/or the bending vibration of physisorbed H 2 O.
The FTIR spectra also showed that the functional group contents varied with modifying condition. HNO 3 was a strong oxidant that favored the introduction of carboxyl and carbonyl groups to a carbon felt, whereas H 2 O 2 was a weak oxidant that favored the introduction of hydroxyl groups. The volume ratio of HNO 3 : H 2 O 2 (1 : 1) resulted in the optimal proportions of carboxyl, carbonyl groups and hydroxyl groups. Oxygencontaining functional groups on felt surfaces enhanced exoelectrogenic bacterial attachment, increased hydrophilicity, and consequently, improved electrical-chemical performance of MFCs.

Water contact angle measurement
The results of water contact angle and hydrophilicity tests of carbon felt were shown in Fig. 8(a) and (b), respectively. When the liquid (water or oil) is dropped onto a solid surface, the droplet either completely spreads or disperses at a certain angle. The included angle of the tangent from the interface of gas, liquid and solid interface is dened as contact angle. The contact angles of various solid surfaces are as follows: q < 5 , super-hydrophilic/oil; 5 < q < 90 , hydrophilic/oil; 90 < q < 150 , hydrophobic/oil; and q > 150 , super hydrophobic/oil. Most microorganisms are negatively charged by nature. Therefore, the hydrophobic/hydrophilic surfaces of the electrode affected microbial attachment and biolm formation. From Fig. 8(a), the untreated carbon felt in MFC-4 presented a water contact angle q > 120 , the distilled water drop was absorbed by the treated carbon felt fastly, however, the water contact angle was close to zero in other MFCs. 23 The surface wettability of solid material is associated with its surface chemical composition and morphology. Therefore, we speculated that the mixed solution of HNO 3 and H 2 O 2 changed the surface characteristics of the carbon felts. Chemically-oxidizing remarkably reduced the water contact angles, probably owing to the increased number of the oxygen-containing functional groups on carbon felt surfaces. From Fig. 8(b), the carbon felts were placed in water for 12 h at room temperature, MFC-4 was aoat, whereas MFC-1, MFC-2, and MFC-3 sank to bottom of the vessel. 24 This phenomenon suggested that HNO 3 /H 2 O 2 treatment could increase the availability of hydrophilic functional groups on the surface of carbon felts. The best results was shown by MFC-2. This volume ratio of V HNO 3 : V H 2 O 2 (1 : 1) was most benecial to hydrophilic nature of carbon felt. Modifying with increasing concentrations of HNO 3 resulted in more rougher surface on carbon felts owing to more carbon loss, therefore provided more space for bacterial attachment. A marked change of morphology on carbon felt was caused by microbe adhering to them and colony formation, likely because oxygen-containing functional groups on the modied carbon felt surfaces signicantly increased their biocompatibility. Differences kinds of functional groups presented selectivity toward different bacteria, which could lead to markedly different electrical properties of biolms. Meanwhile more functional groups might introduce more producing-electricity

Conclusions
Carbon felts were oxidized using HNO 3 /H 2 O 2 to increase hydrophilicity, improve biocompatibility, enhance electron transfer rates, promote the electrocatalytic properties of bio-lm, especially increase the power output of the resultant MFCs substantially. Different HNO 3 /H 2 O 2 volume ratios resulted in varying numbers and kinds of oxygen-containing functional groups. The optimal volume ratio was 1 : 1. Excessively high or low mixing proportions could result in longer start-up time, larger charge transfer resistance, lower electrochemical activity, consequently lower power output.

Conflicts of interest
There are no conicts to declare.