Three-dimensional macroporous CNT–SnO₂ composite monolith for electricity generation and energy storage in microbial fuel cells

Abstract

A facile electrophoretic deposition method was used to prepare a three-dimensional macroporous CNT–SnO₂ monolith as a MFC anode. This 3D CNT–SnO₂ composite presents a clear micro-structure with CNTs inside and amorphous SnO₂ nanoparticles coating the CNT surface, and has both good electricity generation and energy storage in MFCs. Experimental results show that the CNT–SnO₂ composite possesses good biocompatibility and improved electrical conductivity. Compared with CNT, CNT–SnO₂ presents a much higher output current density (2.21 versus 0.47 mA cm⁻²) and power density (673.5 versus 443.1 mW m⁻²). The discharge–charge experiments show that the CNT–SnO₂ composite has a greater specific capacitance than the CNT electrode (382 versus 42.8 mF cm⁻²) with a discharge–charge current density of 1 mA cm⁻². These results reveal that the 3D CNT–SnO₂ composite has great promise as the anode material for MFCs.

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1. Introduction

Microbial fuel cells (MFCs) are bio-electrical devices that convert chemical energy into electrical energy by catalyzing the oxidation of organic substances from different sources (industrial, domestic and agricultural wastes) using bacteria, in order to produce electrical power.^1–3 With moderate operation conditions and widely available fuel sources such as wastewater, marine sediment and human excrement, MFCs present an innovative way of simultaneously performing wastewater treatment and electricity generation and thereby have very promising application perspectives.^4,5

The performance of MFCs is dominated by the electrochemically active microbes within the biofilm on the anodes, therefore, anode materials are a very important factor in determining the performance of an MFC.^6–8 Generally, desirable anode materials should fulfil several requirements such as good electrical conductivity, fine biocompatibility, a large specific surface area and so on.⁹ Among various kinds of MFC anode material, carbon materials, such as carbon cloth, carbon paper, carbon felt, carbon nanotubes and graphene, are the most widely used as anodes in MFCs due to their high conductivity, high specific surface area and good stability; however, they have poor catalytic activity and low porosity for microbial adhesion.^10–13 To increase the porosity and improve the microbial adhesion, three-dimensional macroporous structures have been discovered that present a distinct macroporous network structure with connected macropores and a high specific surface area: the connectivity of the macropores is convenient for the flow of liquid, and thus allows fast substrate transfer; a high specific surface area can increase the loading capacity per unit area, and provides more sites for the adhesion and growth of the microbes. Thus a 3D porous structure becomes helpful for the improvement of extracellular electron transfer and microbial energy production.^14,15 Additionally, to improve the catalytic activity of carbon material anodes, several modification methods are studied. Conductive polymers such as polyaniline, PPy and so on are used to modify the carbon materials and the MFCs appear to have enhanced performance.^16–19 Nanostructured metal oxides such as TiO₂, RuO₂ and so on are also applied in the modification of MFC anodes and show excellent performance.^20,21

Among various metal oxides, SnO₂ attracts great attention due to its unique properties and advantages such as high chemical stability, good catalytic activity and low cost.^22–24 Therefore, SnO₂ has been applied in many research fields including photocatalysis, sensors, solar cells, batteries, capacitors, environmental electrocatalysis and so on.^22–24 In addition, the biocompatibility of SnO₂ is confirmed in some research works.^25,26 It is also suggested that SnO₂ is more conductive than TiO₂ which is frequently used as an MFC anode material.²⁷ Because of these characteristics, tin oxide is beneficial as an MFC anode material.

In this work, a three-dimensional macroporous MFC anode combining CNTs and nanostructured SnO₂ was constructed via a facile electrochemical method. This electrode material appears to have good bio-capacitive performance, and exhibits a particular feature, that is, the integration of electricity generation and energy storage in an MFC to match the electricity generation and power demand. The 3D electrode material was characterized using scanning electron microscopy and an energy dispersive X-ray spectrometer. The electrode performance was measured using some electrochemical measurements including cyclic voltammetry, electrochemical impedance, polarization curves, charge–discharge experiments and so on. The results proved that the 3D CNT–SnO₂ composite electrode possesses excellent activity for electricity generation and energy storage.

2. Experimental

2.1. Preparation of the 3D CNT–SnO₂ monolith

A 3D carbon nanotube (CNT) monolith was prepared via the facile electrophoretic deposition method with multiwalled carbon nanotubes (MWCNTs, 40–60 nm diameter, >5 μm length) as the source. To activate the MWCNTs and remove any remaining metals in the nanostructure, 1.00 g of MWCNTs was refluxed in a 40 mL mixed solution of concentrated HNO₃ and concentrated H₂SO₄ (1 : 3, v/v) at 85 °C for 30 min. Then the MWCNTs were washed with distilled water until the filtrate reached a neutral pH. They were dried in an oven. Ni foam as the 3D substrate framework was tailored to a size of 20 × 30 mm², and was pretreated by degreasing in acetone for 15 min and acid etching in a 5 M HCl solution for 15 min.

0.1 g of treated MWCNTs were ultrasonically dispersed in 100 mL of distilled water for 20 min at room temperature. A titanium sheet (20 × 70 mm²) and the treated Ni foam served as the cathode and anode, respectively, and the parallel distance between the two electrodes was 20 mm. A constant voltage of 30 V was used to deposit the MWCNTs onto the Ni foam for 10 min with low-speed magnetic stirring. Then the MWCNT electrode was taken out and dried at 60 °C.

A 3D monolith of the CNT–SnO₂ composite was prepared on a 3D CNT monolith via the electrodeposition method. The electrolyte solution contained 0.02 M SnCl₂·2H₂O, 0.1 M NaNO₃ and 0.1 M HNO₃. A 3D CNT electrode and a graphite electrode served as the cathode and anode, respectively. The electrodeposition process was conducted at a constant current density of 5 mA cm⁻² for 30 min. Then the CNT–SnO₂ electrode was taken out and dried at 100 °C for 10 h. Finally, the as-prepared 3D CNT–SnO₂ monolith was used as the anode in a microbial fuel cell, while the 3D CNT monolith was used as a control.

2.2. MFC construction

MFCs were constructed from two cylinder type chambers that were 10 cm in height and 4 cm in diameter, and the cathode and anode chambers were separated by a proton exchange membrane (Nafion 117); the as-prepared 3D CNT and CNT–SnO₂ monoliths served as anode materials placed in the anode chamber, and graphite rods served as the cathode in the other chamber. The anode chamber was inoculated with effluent from other active MFCs running on acetate. The MFC with the 3D CNT–SnO₂ composite monolith as the anode is abbreviated as CNT–SnO₂-MFC, and the MFC with the 3D CNT monolith as the anode is abbreviated as CNT-MFC. The components of the nutrient buffer solution were the following: 4.97 g L⁻¹ NaH₂PO₄·2H₂O, 2.75 g L⁻¹ Na₂HPO₄·12H₂O, 0.31 g L⁻¹ NH₄Cl, 0.015 g L⁻¹ CaCl₂, 0.2 g L⁻¹ MgSO₄, 0.56 g L⁻¹ (NH₄)₂SO₄, 0.13 g L⁻¹ KCl, 0.02 g L⁻¹ MnSO₄, 0.01 g L⁻¹ FeCl₃ and 2.5 g L⁻¹ NaAc. The nutrient buffer solution was refreshed when the voltage decreased below 50 mV, forming a fed-batch culture. The cathode chamber was fed with 10 g L⁻¹ K₃[Fe(CN)₆] as the electron acceptor.

2.3. Characterization and measurement

The morphologies and elemental compositions of the electrodes were examined using a scanning electron microscope (SEM, FEI QUANTA 200, America) equipped with an energy dispersive X-ray spectrometer (EDS).

The MFC polarization curve was obtained using a two-electrode system with a 3D CNT or CNT–SnO₂ anode serving as the working electrode and a graphite rod cathode as the counter electrode, and was recorded using an electrochemical workstation (CHI 760C) with a scan rate of 0.5 mV s⁻¹. The maximum power/current density was calculated by normalizing the anode surface area. All the other electrochemical measurements were conducted in the nutrient buffer solution with the three-electrode electrochemical system: the prepared anodes were used as the working electrodes, graphite rods were used as the counter electrodes, and the saturated Ag/AgCl (KCl) electrode was the reference electrode. The anode polarization curves were performed with a scanning rate of 1 mV s⁻¹. Electrochemical impedance spectroscopy (EIS) was conducted at the open circuit potential with a frequency range of 10 kHz to 1 Hz. In the electrochemical discharge experiments, −0.1 V was applied to the anode. Cyclic voltammetry (CV) was performed on the anode with a scanning rate of 2 mV s⁻¹ from −0.6 V to 0 V. Galvanostatic discharge–charge experiments were performed with a fixed current density of 1 mA cm⁻², and the charge–discharge range was from −0.6 V to −0.1 V.

3. Results and discussion

3.1. Material characterization

Fig. 1 shows the SEM images of the as-prepared 3D CNT and 3D CNT–SnO₂ composite monoliths. Fig. 1a shows the architecture of the three-dimensional macroporous monolith. CNT–SnO₂ composites were compactly deposited onto the Ni foam, and the connectivity of the macropore structure is still good. This kind of three-dimensionally structured electrode possesses a distinct macroporous network structure with connected macropores and a high specific surface area: the connectivity of the macropores is convenient for the flow of liquid, and thus allows fast mass transport; the high specific surface area can increase the loading capacity per unit area, provides more sites for the adhesion and growth of microbes, and thus becomes helpful for enhancing the bio-catalytic ability of the catalysts and the microbial energy production. Fig. 1b shows the uniform morphology of the CNT surface, in which the CNTs have a diameter of around 40–60 nm. As can be observed from Fig. 1c, very fine SnO₂ grains are electrodeposited onto the surface of the CNT layer. The aggregation of the SnO₂ grains is low and those fine SnO₂ grains are uniformly coated on the surface of the CNTs. Fig. 1d shows the EDS element composition of the CNT–SnO₂ composite. As seen from the table, the obtained atomic ratio of O to Sn is about 1.66, while the expected value is 2. This result may be ascribed to a high number of oxygen vacancies formed during the metal oxide electro-synthesis; it also can be attributed to the presence of a small amount of lower-valence Sn during the electrodeposition process. Fig. 1e and f are the SEM images of the CNT and CNT–SnO₂ anodes with a biofilm growth time of 48 h and show that the main bacteria species is the bacillus. Compared with the CNT anode, the CNT–SnO₂ anode displays a much greater amount of microbial growth on the surface, which implies that the CNT–SnO₂ anode is beneficial for the growth and adhesion of microbes.

Fig. 1 SEM images of (a) and (c) the CNT–SnO₂ monolith, and (b) the CNT monolith; (d) EDS result for the 3D CNT–SnO₂ monolith; SEM images of the (e) CNT and (f) CNT–SnO₂ anodes with biofilms.

3.2. Electrochemical measurements

The electrochemical behavior of K₃[Fe(CN)₆]₃ on the 3D CNT anode and the 3D CNT–SnO₂ anode reflects the electrochemical activity and stability of the anodes, and is shown in Fig. 2a. Cyclic voltammetry was tested with a scanning rate of 2 mV s⁻¹. On the CV plot of the 3D CNT–SnO₂ composite anode, there are a couple of well-defined redox peaks at potentials of 116 mV (reductive peak) and 274 mV (oxidative peak), which are attributed to the redox reaction of [Fe(CN)₆]³⁻/[Fe(CN)₆]⁴⁻; meanwhile, the well-defined redox peaks of the 3D CNT anode appear at 113 mV (reductive peak) and 278 mV (oxidative peak). As obtained from Fig. 2a, the potential separation for both anodes is greater than 59 mV, suggesting the divergence of reversible electrochemical behavior. The smaller the potential separation is, the more reversible the electrode is, and the better the electrochemical activity.²⁸ Moreover, the potential separation value for the 3D CNT–SnO₂ composite anode is 158 mV, and that for the 3D CNT anode is 165 mV, indicating that the 3D CNT–SnO₂ composite anode possesses better electrochemical activity. The CV plots reveal that the 3D CNT–SnO₂ anode has much larger current responses than the 3D CNT anode (6.9 mA cm⁻² vs. 1.9 mA cm⁻² for the anodic peak current density), indicating that the 3D CNT–SnO₂ anode has a higher electrochemical activity. The increase in the current response can be ascribed to the synergetic effects of the CNTs and SnO₂. As a consequence, the smaller potential separation and the greater electron transfer efficiency of the 3D CNT–SnO₂ composite anode reveals its suitability and good electrochemical activity for extracellular electron transfer from mediators or shuttles metabolized by microbes to the anode.

Fig. 2 (a) CV and (b) EIS plots of abiotic CNT and CNT–SnO₂ anodes in K₃[Fe(CN)₆]₃ solution.

Electrochemical impedance analysis was performed to evaluate the charge transfer resistance of the anode materials. As observed from Fig. 2b, the electrochemical impedance spectra include two parts, the charge-transfer dependent semicircle in the high frequency region and the Warburg diffusion straight line in the low frequency region. The ohmic resistance (R_s) results from the electrolyte, the outer connection, the intrinsic resistance of the electrode and so on, and is shown as the high-frequency intercept; and the charge transfer resistance (R_ct) is shown as the diameter of the semicircle. The equivalent circuit model was obtained by fitting the Nyquist plot data and shows that the anode reaction is controlled by the diffusion process and the electrochemical kinetics process. The constant phase element (CPE_ct) rather than a capacitor was used to model the double-layer capacitor due to the frequency dispersion effects possibly caused by the surface roughness and the inhomogeneous distribution of the reactions. The charge transfer resistance represents the electrochemical reaction activity occurring at the electrode–electrolyte interface. The smaller the charge transfer resistance is, the faster the charge transfer rate is. Based on Fig. 2b, the charge transfer resistances of the 3D CNT anode and the 3D CNT–SnO₂ anode are 3.62 ohm and 2.42 ohm, respectively. The charge transfer resistance of the 3D CNT–SnO₂ anode is lower than that of the 3D CNT anode. The reduced charge transfer resistance of the 3D CNT–SnO₂ anode indicates that the CNT–SnO₂ composite has a greater electrochemical performance and a faster charge transfer rate between the electrode surface and the electrolyte.

3.3. Bio-electrocatalytic performance of the MFC anodes

Anode polarization curves of the CNT and CNT–SnO₂ anodes are shown in Fig. 3a. Bacteria-free controls for the polarization curves are given and the results show that the anodes with biofilms have significantly negatively shifted open circuit potentials, indicating the successful growth of the biofilm on the anode materials. There is a difference in the open circuit potential for the two anodes with biofilms, that is, a more negative open circuit potential is found for the CNT–SnO₂ anode with the biofilm. It is observed that the current density of the CNT–SnO₂ anode with the biofilm is significantly higher than that of the CNT anode at the same potential, for example, at −0.35 V, the current density of the CNT–SnO₂ anode reaches 0.61 mA cm⁻², while that of the CNT anode is only 0.3 mA cm⁻². In addition, there is a great decrease in the slope of the anode polarization curve using the CNT–SnO₂ anode versus the CNT anode. When the current density ranges from zero to 0.35 mA cm⁻², for example, the CNT anode potential rises from −0.428 V (open circuit potential) to −0.328 V, while the CNT–SnO₂ anode potential increases only from −0.448 V (open circuit potential) to −0.413 V, showing a reduction of the polarization state for the CNT–SnO₂ anode. Based on these results, the CNT–SnO₂ anode exhibits enhanced bioelectrocatalytic activity compared with the CNT anode. In addition, compared with a carbon belt, the CNT–SnO₂ anode has excellent activity (the anode polarization curve of a carbon belt with a biofilm is shown in Fig. S1 of the ESI†).

Fig. 3 (a) Anode polarization curves and (b) CV plots of the CNT and CNT–SnO₂ anodes without and with biofilms. The inset shows the anode polarization curves of the CNT and CNT–SnO₂ anodes without biofilms.

To investigate the effect of SnO₂ on the extracellular electron transfer in the electrochemically active biofilm, cyclic voltammetry was performed. Fig. 3b shows the cyclic voltammograms obtained for the CNT and CNT–SnO₂ anodes with and without biofilms. It can be seen from Fig. 3b that the current produced on the CNT–SnO₂ anode is far larger than that in relation to the CNT anode. And after the biofilm grows, the current density on the CNT–SnO₂ anode increases obviously in comparison to that without biofilm growth. This results from the catalytic acceleration effect of SnO₂ on microbial electro-genesis as well as the significant synergistic effect of the CNTs and SnO₂. A 3D-structured pseudocapacitive interface is created by in situ deposition of SnO₂ on the 3D CNT electrode and larger amounts of exoelectrogenic bacteria can attach onto the interface, thus accounting for the improvement in the anode electron transfer efficiency and the power performance. The charge capacities of the CNT anode and the CNT–SnO₂ anode were calculated from the results of the CV and the values are 52.3 mC cm⁻² and 628 mC cm⁻², respectively. This result further confirms that the 3D CNT–SnO₂ anode possesses a greater value for the active reaction area proportional to the anode surface area, and this can be expected to provide the MFC with excellent power output performance.

The interaction between the biofilm and the anode surface is very important for the overall MFC performance. To further verify the interfacial interactions between the microbial biofilm and the prepared anode materials, electrochemical impedance experiments were performed in MFCs for both anodes at the open circuit potential. In order to interpret the EIS spectra, the equivalent circuit was used to fit and to model the EIS behavior, and the fitted results are listed in Table 1. Fig. 4a shows the Nyquist plots of both anodes without biofilms. The resistance at the high frequency intercept of the real axis can be attributed to ohmic resistance, and it results from the wire connection to the external circuit, the electrolyte and the intrinsic resistance of the active materials.²⁹ The ohmic resistance (R_s) values of the CNT and CNT–SnO₂ anodes are approximately 1.57 ohm and 3.19 ohm, respectively; this shows the higher ohmic resistance of the CNT–SnO₂ anode, which can be attributed to the intrinsic low conductivity of SnO₂. In the high frequency region, the EIS spectra of the anodes without biofilms show only one depressed semicircle, with the diameter of the semicircle representing the electron transfer resistance. The electron transfer resistance (R_ct) of the CNT–SnO₂ anode is 0.92 ohm, while the value for the CNT anode is 2.56 ohm. Therefore, we can conclude that the CNT–SnO₂ composite features a lower electron transfer resistance, and a remarkable enhancement of the catalytic reaction and electron transfer in the CNT–SnO₂ anode.

Table 1 EIS fitting results of the CNT and CNT–SnO₂ composite in MFCs

Anode	Rs/ohm	Rct/ohm	CPEct/F	n1	Rbiofilm/ohm	CPEbiofilm/F	n3	Zw/ohm	n2
CNT	1.57	2.56	9.39 × 10^−3	0.68	—	—	—	0.54	0.48
CNT–SnO2	3.19	0.92	1.62 × 10^−3	0.79	—	—	—	0.12	0.36
Bio-CNT	2.60	0.72	6.27 × 10^−4	0.77	14.5	0.037	0.46	16.1	0.42
Bio-CNT–SnO2	2.29	0.67	3.32 × 10^−3	0.77	1.74	0.14	0.64	0.60	0.48

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Fig. 4 Nyquist plots of CNT and CNT–SnO₂ (a) without and (b) with biofilms in MFCs; (c) and (d) are the corresponding equivalent circuit models.

As observed from Fig. 4b, compared to the one semicircle that is present before the electro-active biofilm is formed, two unequal semicircles can be observed for both anodes, showing a distinct two-step electrochemical reaction process. This phenomenon indirectly proves the formation of the biofilm and the biocompatibility of the anode materials. The two-step electrochemical process on the anode can be reasonably considered as the following:³⁰ the first step (R_ct–CPE_ct) is similar to the electrochemical process without the biofilm, and can be considered as the process occurring on the active material/biofilm interface; the second step (R_biofilm–CPE_biofilm) can be interpreted as the bio-electrochemical process, that is, the mass transfer process occurring on the biofilm/electrolyte interface; this result is in accord with the irreversible redox reaction observed in CV. In the MFC system, the microbial oxidation of organic substrates and the extracellular electron transfer are complicated, and the electron transfer from the microbes to the electrode goes through three possible pathways: soluble electron-shuttling molecules, reactive proteins on the cell membrane, and conductive pili; therefore, desirable anode materials can enhance the ability of the microbes and facilitate the heterogeneous electron transfer.³¹ The fitted results show that the electron transfer resistances of both anodes decrease; this can be ascribed to the electrochemical enhancement effects of microbial electro-genesis. Moreover, the CNT–SnO₂ anode has a lower R_ct value than the CNT anode (0.67 ohm vs. 0.72 ohm), showing an improved extracellular electron transfer. Compared with the CNT anode, the CNT–SnO₂ anode has an evidently reduced R_biofilm value (1.74 ohm vs. 14.5 ohm) in the bio-electrochemical process; this reveals the enhanced catalytic effect of SnO₂ on the microbial activity. These results demonstrate that the introduction of SnO₂ promotes the interactions among microbes, the organic substrate and the anodes.

According to the above results, the CNT–SnO₂ composite is proved to have improved electrochemical performance as an anode material for MFCs. Such an improvement can be attributed to the synergetic effect between the CNTs and SnO₂. The 3D structure with a macroporous network can facilitate the organic substrate transfer, thus being favorable to microbial colonization inside the anode material; and the 3D macroporous structure ensures good interaction between the biocatalyst and the substrate.²⁸ Meanwhile, the CNT–SnO₂ composite also exhibits good biocompatibility and facilitates biofilm formation. Hence, the microbe-generated electrons can be transferred more quickly from the microbes to the anode.

3.4. MFC performance for electricity generation and energy storage

To fairly evaluate the anode effect in MFCs, an H-type MFC is set up to investigate the electricity generation by putting different anodes into the same anode chamber without direct contact. Through these means, the two kinds of anodes (the CNT anode and the CNT–SnO₂ anode) can share the same MFC. The MFC performance was evaluated by plotting the polarization and power density curves. Fig. 5 compares the polarization curves obtained from the CNT-MFC and the CNT–SnO₂-MFC after inoculation. The two MFCs show nearly the same V_oc of 0.73 V, suggesting similar thermodynamics in both MFCs. However, the CNT–SnO₂-MFC attains a maximum current density of 0.41 mA cm⁻², which is about 1.41 times the maximum current density from the CNT-MFC (0.29 mA cm⁻²). This result indicates a higher kinetic activity in the CNT–SnO₂-MFC. Meanwhile, the CNT–SnO₂-MFC produces a maximum power density of 673.5 mW m⁻², which is 1.52 times that from the CNT-MFC (443.1 mW m⁻²). Compared with a carbon felt-MFC (171.8 mW m⁻², shown in Fig. S1b of the ESI†), the CNT–SnO₂-MFC demonstrates excellent performance for electricity generation. In addition, to evaluate the effectiveness of the data, the polarization and power density curves were repeatedly tested. The results are shown in Fig. S2.† For the 1^st, 3^rd and 7^th tests, the maximum power densities of the CNT–SnO₂-MFC are 673.5, 730.3 and 681.0 mW m⁻², respectively. These results indicate the effectiveness of the experimental data. The increased power output may be attributed to the binding of cytochrome and SnO₂ nanoparticles via electrostatic and coulombic bindings.³² These data strongly show that the 3D CNT–SnO₂ anode enables extracellular electron transfer and possesses superior MFC performance, which can be attributed to the 3D open structure.

Fig. 5 Polarization curves and power density output of the CNT-MFC and the CNT–SnO₂-MFC.

The large surface area and open macroporous structure of the 3D CNT–SnO₂ anode should facilitate internal microbial colonization and attachment, substrate transport to the attached microorganisms, efficient transfer of electrons from the microorganisms to the electrode surface, and efficient collection of current from all regions of the electrode. A 3D-structured pseudocapacitive interface is created by in situ deposition of SnO₂ onto the 3D CNT electrode. The pseudocapacitive SnO₂ can improve the activity of the exoelectrogenic biofilm and facilitate the extracellular electron transfer.

During the MFC charge–discharge experiment, the MFCs were alternately held at open circuit and at an anode potential of −0.1 V. The electrodes are charged when the electric circuit is open, and they are discharged when the anode potential is controlled. The anodic current density behavior with a discharge potential of −0.1 V is given in Fig. 6. Both anodes have a peak in the current density after the potential of −0.1 V is applied. This process represents the capacitive behavior of the electrodes. The presence of a peak in the current density indicates that more charge is produced and stored during the open circuit period. After this peak, the current density of each anode decreases towards a stable current density. When the current becomes stable, the MFC electrodes show non-capacitive behavior, which is related to the bio-electrochemical activity.

Fig. 6 The current density behavior of anodes with biofilms at a constant discharge potential of −0.1 V (vs. Ag/AgCl).

A transient peak current that is much larger than the steady-state current is observed for both the CNT and CNT–SnO₂ anodes. The higher the transient current produced, the more charge is stored during the open circuit period. The CNT–SnO₂ anode has a greater peak of 11.1 mA cm⁻² compared to the CNT anode (4.8 mA cm⁻²). The transient stage is integrated to obtain the capacitive charge value, and the calculated results for the CNT and CNT–SnO₂ anodes are 111 mC cm⁻² and 228 mC cm⁻², respectively. This indicates that the amount of stored charge is larger for the CNT–SnO₂ anode compared with the CNT anode. Upon adding tin dioxides, the response current is significantly increased: the steady-state current density increases by 4.7 times (from 0.47 mA cm⁻² to 2.21 mA cm⁻²). This can be attributed to the biocatalytic acceleration effect of tin oxide for extracellular electron transfer. This better performance may be explained by the charge storage: electrons are stored inside the anode and the produced protons accumulate in the biofilm during the charging period, and the electrons are released from the anode to the cathode and the accumulated protons are transferred from the biofilm to the bulk solution during the discharging period; since the protons are most likely transported out of the biofilm through the protonation of the conjugate base of the buffer, the immediate release of all stored protons would lead to an increase in buffer capacity at the biofilm, and this would enable a high current density output.^33,34

Constant charge–discharge tests were performed to obtain the specific capacitance of the CNT and the CNT–SnO₂ composite with biofilms, and the results are shown in Fig. 7. The specific capacitances (C) of the anodes are calculated according to the following equation:

C = (j × t)/V (1)

where j (mA cm⁻²) is the charge/discharge current density, t is the discharge time and V is the discharge potential range. The results are displayed in Fig. 7. The specific capacitance of the CNT–SnO₂ composite is 382 mF cm⁻² at a current density of 1 mA cm⁻², and is 8.9 times as much as that of the CNT (42.8 mF cm⁻²). The enhanced capacitance performance can be ascribed to the synergistic effect between the CNTs and tin oxides. Table 2 provides some electrochemical measurement results after the biofilm growth, and the average loading capacities of the CNT and CNT–SnO₂ composite are 8.88 and 3.68 mg cm⁻², respectively. The corresponding quality specific capacitances of the CNT and CNT–SnO₂ composite are 4.82 F g⁻¹ and 103.8 F g⁻¹, respectively.

Fig. 7 The charge–discharge curves of CNT and CNT–SnO₂ with biofilms at a constant current density of 1 mA cm⁻².

Table 2 Summary of some electrochemical measurement results after the biofilm growth

Anode material	CNT	CNT–SnO2
Power density/mW m^−2	443.1	673.5
Anode open circuit potential/mV	−0.428	−0.448
Steady-state current density/mA cm^−2	0.47	2.21
Specific capacitance/mF cm^−2	42.8	382
Average loading capacity/mg cm^−2	8.88	3.68

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4. Conclusions

In summary, we have demonstrated a facile method to prepare a three-dimensional macroporous CNT–SnO₂ composite monolith as the bioanode for microbial fuel cells. The CNT–SnO₂ composite has evidently improved performance as a bioanode material for MFCs. The good electrochemical energy storage performance can be attributed to the synergetic effect between the CNTs and SnO₂ with good biocompatibility and improved electrical conductivity. In addition, this 3D macroporous CNT–SnO₂ monolith improves the mass transport and facilitates the growth of microbes, promoting the interactions among microbes, organic substrates and bioanodes.

Acknowledgements

The project was supported by the National Natural Science Foundation of China (No. 21476053 and No. 51179033), and the Doctoral Program of the Ministry of Education (No. 20132304110027).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra11869k

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