Fabrication of Fe Nanoparticles into N-doped Mesoporous Carbon Nanotube Derived

Lu Decheng; Wang Wenyi; Chang Jiacheng; Wang Xueqin; Wang Yuanyuan; Song Hua

(Provincial Key Laboratory of Oil & Gas Chemical Technology, College of Chemistry & Chemical Engineering, Northeast Petroleum University, Daqing 163318)

Abstract: The development of non-noble metal oxidation reduction catalysts (ORR) to improve microbial fuel cell (MFC)performance remains extremely challenging. Herein, the nitrogen-doped iron-based porous carbon nanotube Fe/N@MC-T ORR catalysts were derived from Fe/N-MOF by pyrolyzation using acetonitrile as the nitrogen precursor in a low-cost organic solvent. The Fe/N@MC-T catalysts under different pyrolysis temperatures were characterized by SEM, TEM,BET, XRD, and XPS techniques. Fe/N-MOF showed a smooth rice-like structure with a particle size of about 400×50 nm2. The Fe species in Fe/N@MC-T mainly exists in the form of zero-valent iron with a small amount of Fe3C. The results of electrochemical tests revealed that the onset and half-wave potentials of Fe/N@MC-700 were 0.89 V and 0.80 V,respectively, which were only slightly lower than those of the commercial Pt/C (0.92 V and 0.82 V). The MFC with Fe/N@MC-700 showed a highest power density of 864.1 mW/m2, which was about 2.25 times that of MFC with carbon cloth, and was slightly lower than that of MFC with Pt/C (20%) (1002.0 mW/m2), which demonstrated that the Fe particles wrapped in carbon nanotubes possessed a relatively high ORR activity.

Key words: microbial fuel cells; metal organic framework; nitrogen doping; oxidation reduction reaction; Fe/N@MC-T catalyst.

1 Introduction

Recently, energy shortage and environmental pollution have become increasingly serious. Microbial fuel cells(MFCs) are an innovative method to generate electricity from organic matter using electrogenic microorganism.It can generate electricity and also treat wastewater simultaneously[1-4]. As a sustainable electrochemical device, MFC has received worldwide attention due to the huge and broad development potential of this emerging technology[5]. MFCs generate current mainly by releasing some electrons into the extracellular environment through consumption of soluble organic matter on anode by electro-producing bacteria. In the cathode chamber, an electron acceptor is reduced with the electrons transferred via an external circuit and the protons diffused through the proton exchange membrane[6]. In this process, the conversion efficiency of electrons is the main indicator of whether MFC can be industrialized. Therefore, the cathode reaction efficiency is an important factor to affect the electricity generation characteristics of MFC[7-10].The low cost and environmentally friendly oxygen is the most widely used electron-acceptor agent. However,the high overpotential of oxidation reduction reaction(ORR) would greatly reduce the electricity generation performance of MFC. Therefore, it is necessary to improve the cathode performance of MFC by loading oxidation reduction reaction (ORR) catalyst on the surface of cathode[7-11]. Some cost-effective transition metal compounds are used as ORR catalysts, by virtue of which the MFC power density was effectively improved[12-13]. Among these metals, the Fe, Co and Mn have been extensively studied, because they are cheap and plentiful[14]. Due to superior electrochemical properties, the nitrogen-doped carbon supported transition metal Fe, Co and Mn catalysts (M-N-C) have been widely studied. Among the above-mentioned three catalysts, the Fe-based catalyst possessed a highest ORR activity (Fe＞ Co ＞ Mn)[15]. It is well known that Fe is cheaper than Co and Mn, and is rich in terms of resources, which do not create pollution. Therefore, Fe-N-C is considered to be one of the alternatives to the precious metal Pt ORR catalyst used in commercial application. Carol Santoro,et al.[16]have developed new iron-based catalyst based on low cost organic precursors named niclosamide and ricobendazole for the ORR. The electrocatalytic activity of the iron-based catalyst was significantly higher than that of Pt in the cathodic polarization test. A maximum achievable power density of 195 ± 7 μW/m2was realized by MFC using the iron-based catalyst. It is well known that Fe-N-C can be made from a variety of nitrogen and carbon precursors through high temperature pyrolysis,which would lead to different activity values.

Metal organic frameworks (MOFs) and their modified materials are new porous materials with relatively high specific surface area, adjustable pore structure and high stability, which are widely used as carbon precursors. It has been found that the Fe-N-C catalysts derived from MOF materials possess a great application prospect as ORR catalysts for MFCs. Rossi, et al.[17]have prepared Fe-N-C catalyst by using powdered activated carbon(PAC), FeCl3, and 1,10-phenanthroline, and an optimal power density of 2780 ± 8 mW/m2was achieved in MFC,showing that the non-noble nitrogen-doped iron-based porous carbon materials are an efficient, low-cost ORR catatlysts for MFCs. Zhong, et al.[18]have synthesized Mn-Fe@g-C3N4catalyst by hydrothermal method and studied its ORR performance in MFC. They found that the Mn-Fe@g-C3N4catalyst exhibited a power density of 713 ± 2 mW/m2, which was even higher than that of Pt/C catalyst (333 ± 2 mW/m2). Research work had shown that the Fe-Nxsites and types of nitrogen in catalysts were directly related to the ORR performance. Non-noble nitrogen-doped iron-based porous carbon materials were recognized as an efficient, low-cost alternative to platinum for oxygen reduction in fuel cells. The aim of this study is to develop a simple method to obtain highly active low price ORR catalysts for MFCs cathode. In the future,the cathode catalyst will be studied in a bid to improve the performance and stability along with low production cost, and the metal-based MOF derived by new organic materials and metals will be widely used in MFC[19].

This study is aimed at developing a simple method for preparing highly active low price ORR catalysts for MFC cathode. For this purpose, the highly active nitrogendoped iron-based porous carbon nanotube Fe/N@MC-T ORR catalysts were derived from the rice-like Fe/N-MOF by pyrolyzation using acetonitrile as the nitrogen precursor in a low-cost organic solvent. The effect of pyrolysis temperature (T) on the structure and ORR performance of Fe/N@MC-T catalysts was studied. The electrochemical tests have revealed that the Fe/N@MC-T catalysts exhibit high electrocatalytic activity for ORR.

2 Experimental

2.1 Preparation of Fe/N-MOF and Fe/N@MC-T

In a typical procedure, 30 mg of Fe(NO3)3·9H2O were dissolved in DMF (N,N-dimethylformamide) (4 mL).Terephthalic acid (PTA) (64 mg) was then added into the above solution under stirring for 30 min at room temperature. After thorough mixing, acetonitrile (4 mL)was slowly added to the above solution. The resulting mixture was placed into a 30-ml Teflon-lined autoclave and heated at 120oC for 15 h prior to being air-cooled to room temperature. The light red powder was obtained by centrifugation and was washed with methyl alcohol and DMF for three times and was then dried at 80oC in vacuum overnight. The as-prepared Fe/N-MOF was heated to target temperature (T=500oC, 600oC, 700oC, 800oC) at a temperature increase rate of 3°C/min and was kept at the target temperature for 2 h under the protection of N2flow.The obtained macroporous black powder was named as Fe/N@MC-T(withTrepresenting the pyrolysis temperature inoC). The Fe/N@MC-700 was treated in 0.1 M HCl for 24 h and the obtained catalyst was denoted as Fe/N@MCacid.

2.2 Preparation of cathode catalyst and electrodes

Scheme 1 Schematic diagram for the synthesis of FeN-MC-T

Preparation of the cathode catalyst: Firstly, 50 mg of carbon black (Vulcan xc-72, USA) and 600 μL of PTFE emulsion (60%, Hesen Electrical Co., Ltd., Shanghai,China) were thoroughly mixed and evenly coated on the side of a 4 cm×4 cm carbon cloth (30% waterproof treatment, E-TEK, USA), and was then subjected to heat treatment at 370oC for 20 min. After natural cooling at room temperature, the surface of carbon cloth was coated with a 60% PTFE emulsion and dried by air, the electrode was again subjected to heat treatment at 370oC for 20 min. After cooling at room temperature, the brushing, airdrying, heating, and cooling operations were repeated for three times to complete the preparation of the diffusion layer. The carbon cloth with diffusion layer was tailored to form wafers with a diameter of 4 cm. Finally, the 60-mg as-prepared catalyst was thoroughly mixed with 50 μL of deionized water, 400 μL of Nafion emulsion (5%, Sigma-Aldrich Co., Ltd., China) and 200 μL of propanol (Alladdin Biochemical Technology Co., Ltd., Shanghai, China) and was then uniformly coated on the other side of the carbon cloth diffusion layer (5 mg/cm2) prior to being dried at room temperature for 24 hours. For comparison, the Pt/C (10%,E-TEK, USA) was also coated with the same method.

Preparation of the anode electrode: The anode was made from carbon fiber brush with a diameter of 3 mm and a length of 2.5 cm. Before use, the brush was soaked in acetone solution for 24 hours to remove contaminants and was then washed with distilled water several times, prior to being treated at 470oC for 30 min, and was then cooled down to room temperature[20].

2.3 MFC setup and operation

The cube-shaped, single chamber MFCs were constructed by placing anode and cathode on opposite sides in a plastic cylindrical chamber, 4 cm in length and 3 cm in diameter (with a liquid volume of 28 mL, and a projected cathode surface area of 7 cm2), as previously reported[21]. The electrodes with Fe/N@MC-T were used as MFC cathodes. All MFCs were inoculated with 20% of domestic wastewater collected from a sewage treatment plant (Daqing, China) and 80% of medium,which contained glucose (1 g/L), 50 mM phosphate buffer solution, vitamins (5 mL/L) and minerals (12.5 mL/L)prepared as described in literature[22]. The medium was replaced when the voltage decreased below 50 mV. All reactors were operated in duplicate in the fed-batch mode at 30oC with an external resistance of 1000 Ω.

2.4 Analysis and calculation

The voltage of each reactor was recorded every 10 min with a data acquisition system (Model 2700, Keithly Instruments, USA). The polarization curves and power density were tested by varying an external resistor in the range of 5000 Ω-50 Ω, and the steady-state voltage and electrode voltage were recorded.

The output voltage was recorded every 30 min after the loading of external resistance, with the plot of current density (calculated as follows) versus time in the first cycle including a fast start up phase. The fast increased startup current during the start-up is due to the nature of Shewanella spp. mentioned in the previously reported works[23]. At the steady-state of the MFC, the polarization and power density curves were obtained by measuring the stable voltage (V) generated at various external loading resistance values (200—50000 Ω).

2.5 Structural characterization

The sample was characterized by X-ray diffraction (XRD),recorded on an (Philips X’Pert Pro Super) X-ray powder diffractometer with Cu Kα radiation, operating at a tube voltage of 40 kV, a tube current of 30 mA, and a scanning angle (2θ) range of 0.5° to 10° with a scanning speed of 1/6oC/min. The specific surface area was measured using the Brunauer-Emmett-Teller (BET) method and the pore size distribution was calculated using the classical Barrett-Joyner-Halenda (BJH) model. Scanning electron microscope (SEM) images were obtained on a SIGMA scanning electron microscopy instrument (Carl Zeiss AG, Germany) under vacuum conditions with an accelerating voltage of 100 V to 30 kV to magnify the sample to 2000—10000 times. The X-ray photoelectron spectroscopy (XPS) spectra were acquired using an ESCALAB MKII spectrometer operating under vacuum.The XPS measurements have been performed for Mg radiation (E=1253.6 eV) equipped with a hemi-spherical analyzer operating at a fixed pass energy of 40 eV. The recorded photoelectron binding energy was referenced against the C 1s contamination line at 284.8 eV. The transmission electron microscope (TEM) examinations were performed using a JEM-1010 instrument made by JEOL. The samples were dispersed in ethanol and placed on a carbon grid before TEM examinations.

3 Results and Discussion

3.1 Characterization of samples

Figure 1 illustrates the morphology of Fe/N-MOF (a-b) and Fe/N@MC-T (c-e) obtained by SEM. It can be seen from Figure 1 (a—b) that Fe/N-MOF shows a smooth rice-like structure with a particle size of about 400×50 nm2. After calcination at different temperatures (500oC, 600oC, 700oC),Fe/N@MC-T presents a loose small spherical structure.

Figure 1 (a-b) SEM of Fe/N-MOF, (c) SEM of Fe/N@MC-500, (d) SEM of Fe/N@MC-600, (e) SEM of Fe/N@MC-700

TEM images of Fe/N@MC-700 are shown in Figure 2. It can be seen from Figure 2 (a—b) that the Fe nanoparticles with different sizes owing to aggregation of nanoparticles,which have been embedded in the graphite carbon and amorphous carbon, are identified. In addition, the carbon nanotube structure can be seen. Furthermore, it can be clearly seen from high-magnification TEM image (Figure 2(b)) that the Fe nanoparticles with a diameter of about 15 nm are wrapped in carbon nanotubes with a lattice spacing of 0.337 nm, which matches with the (002) plane of carbon.

3.1.2 Specific surface area and porosity

The N2adsorption-desorption isotherms of Fe/N-MOF and Fe/N@MC-T are shown in Figure 3, with the related textural data presented in Table 1. The isotherm of Fe/N-MOF (Figure 3 (a)) displays a type-II adsorption isotherm with very weak hysteresis loops, indicating the abundant existence of micropores[24-25]. While the isotherms of Fe/N@MC-T belong to type-IV with type-H 4 hysteresis loops, indicating the coexistence of mesopores and macropores in the Fe/N@MC-T, which is in accordance with the pore size (Dp) distribution in Figure 3 (b). TheDpdistribution of Fe/N-MOF is centered at 2.6 nm, while theDpof Fe/N@MC-T is centered between 17.1 nm and 17.9 nm and increases with an increasing pyrolysis temperature. This is true, because the high pyrolysis temperature may burn the organic ligands away, while leaving more pores[25]. It is verified that the surface area (SBET) and pore volume (Vp) of Fe/N@MC-T catalysts also increase with an increasing pyrolysis temperature. The Fe/N@MC-700 exhibits the highestSBET(195.3 m2/g) andVp(0.47 cm3/g) (Table 1), as compared to samples derived at other pyrolysis temperature.

Table 1 Surface properties of Fe/N-MOF and Fe/N@MC-T catalysts obtained from BET and XPS analyses

Figure 2 TEM images of Fe/N@MC-700

3.1.3 XRD results

Figure 4 shows the XRD patterns of the Fe/N@MC-T obtained from Fe/N-MOF. It can be observed that a strong diffraction peak corresponding to the plane (002)of graphite carbon (JCPDS NO.13-0148) appears near 2θ=26.6°. Moreover, its intensity increases with an increase of pyrolysis temperature, indicating that the graphitization degree of Fe/N@MC-T increases with an increasing pyrolysis temperature[26]. For all Fe/N@MC-T samples, the strong diffraction peaks at 2θ=44.8oand 65obelong to zerovalent iron (JCPDSN zero-valent iron 0.06-0696), showing that the zero-valent iron is the predominant phase[27]. In addition, with an increasing pyrolysis temperature the intensity of these two peaks also increases obviously,demonstrating that more zero-valent iron phase has formed.The weak diffraction peaks of Fe/N@MC-T appearing at 2θ= 37.6°, 39.6°, 40.8°, 44.8°, 45.9°, 48.6°, 49.2°, 51.9°,and 54.3° can be attributed to the normal phase of Fe3C(JCPDS No. 35-0772)[28]. The above results indicate that Fe element in Fe/N@MC-T mainly exists in the form of zerovalent iron with a small amount of Fe3C.

Figure 3 Nitrogen adsorption-desorption isotherms and BJH pore size distribution curves of Fe/N-MOF and Fe/N@MC-T

Figure 4 The XRD analysis results of Fe/N@MC-T catalysts

3.1.4 XPS results

The chemical components of Fe/N@MC-T were characterized by XPS, with the results shown in Figure 5. The peaks located at a binding energy of approximate 708 eV, 286.3 eV, 531 eV, and 397 eV in survey spectra of Fe/N-MOF and Fe/N@MC-T (Figure 5 (a)) can be attributed to Fe, C, O, N elements, respectively, showing that nitrogen was embedded and the Fe/N@MC-T catalysts were successfully obtained. As shown in Figure 5 (b),the N 1s spectra of Fe/N@MC-T can be divided into four peaks, namely pyridinic N (397.9 eV), Fe-Nx(399.1 eV), pyrrolic N (400.2 eV), and graphite N (401.2 eV)[29].The Fe-Nxsites in Fe/N@MC-T are proved to be the active sites for ORR, therefore, the atomically dispersed Fe in the N-doped carbon materials should have higher activity[30].Meanwhile the pyridinic N, pyrrolic N, and graphite N also play the important role in ORR. The Fe content on the surface of the sample is about 1.55%-1.58%, which is in line with that of the reported Fe-N-C catalyst[31]. The Fe 2p spectra of Fe/N@MC-T (Figure 5 (c)) can be divided into five peaks. The peaks appearing at a binding energy of 710.3 eV and 711.8 eV correspond to the Fe 2p3/2 orbitals of Fe2+and Fe3+, respectively. The peaks appearing at a binding energy of 723.6 eV and 726.4 eV correspond to the Fe 2p1/2 orbitals of Fe2+and Fe3+, respectively. The peak appearing at a binding energy of 718.4 eV can be regarded as the satellite peak. In addition, the Fe 2p3/2 XPS spectra show a peak at a binding energy of 711.8 eV, which can be attributed to Fe-Nxbonds[32]. The C 1s spectra of Fe/N@MC-T (Figure 5 (d)) can be divided into three peaks at C=C (284.6 eV), C-N (285.2 eV), and C=N (288.7 eV).

3.1.5 Electrocatalytic performance towards ORR

The ORR performance of Fe/N@MC-T catalysts was tested by the cycle voltammetry (CV) in an O2saturated 0.1 M KOH solution at a scanning rate of 100 mV/s,using a three-electrode configuration (Figure 6 (a) and Table 2). All Fe/N@MC-T catalysts showed a couple of well-defined oxidation reduction peaks with a half-wave potential at 0.75―0.80 V, indicating that Fe/N@MC-T possessed efficient catalytic activity towards ORR[19,33].

To make a further investigation of the activity of Fe/N@MC-T towards ORR, the linear sweep voltammetry (LSV)of Fe/N@MC-T catalysts was performed in an O2-saturated 0.1 M KOH solution at a scan rate of 5 mV/s, with the results shown in Figure 6 (b) and Table 2. The LSV results exhibit that compared with the commercial Pt/C catalyst,the onset potential and half-wave potential (E1/2) of Fe/N@MC-T catalysts were slightly negatively shifted. The E1/2of Fe/N@MC-700 and Fe/N@MC-800 is both 0.80 V, which is slightly lower than that of the commercial Pt/C (0.82 V). The onset potential of Fe/N@MC-700 and Fe/N@MC-800 is 0.950 V and 0.947 V, respectively, which is significantly lower than that of the commercial Pt/C (0.96 V), indicating that Fe/N@MC-700 has high electrocatalytic activity for ORR[19]. This outcome can be attributed to the Fe nanoparticles wrapped in carbon nanotubes, which can possess high activity towards ORR. The CV, LSV and Nyquist curve of Fe/N@MC-700acidcatalyst were tested,with the results shown in Figure 6 (a, b and f) and Table 2. As compared to Fe/N@MC-700 (Figure 6(a, b)), there was a significant left shift in the oxidation peak position for Fe/N@MC-700acid. The open circuit voltage and half wave potential of Fe/N@MC-700acidwere 0.939 V and 0.78 V, respectively, which were lower than those of the Fe/N@MC-700 without acid leaching. This phenomenon can occur, possibly because the acid leaching has removed most of the metal iron ions in the catalyst, showing that the Fe-Nxcomprises the active sites of the catalyst.

Figure 5 (a) XPS survey spectra of Fe/N-MOF and Fe/N@MC-T; (b) XPS spectra of N 1s in Fe/N@MC-700; (c) XPS spectraof Fe 2p in Fe/N@MC-700; (d) XPS spectra of C 1s in Fe/N@MC-700

Table 2 Electrochemical performance and resistance of Fe/N@MC-X catalysts

The RDE tests and the Koutecky-Levich (K-L) plots of Pt/C (c, d) and Fe/N@MC-700 (e, f) in a 0.1 M KOH solution saturated with 50 mM O2with a sweeping speed of 5 mV/s at various rotation rates are shown in Figure 6 (c―f). As the rotation rate increases from 400 rpm to 2000 rpm, the corresponding limiting current density increases. The K-L curves represent the first-order ORR kinetics. The slope of the K-L curve obtained by linear fitting can be used to calculate the electron transfer number (n). The average n of Pt/C is 3.86, indicating that Pt/C follows an efficient fourelectron transfer path in the oxidation reduction reaction.

The averagenof Fe/N@MC-700 is 3.61, indicating that the ORR catalyzed by Fe/N@MC-700 is a reaction process in which two electrons and four electrons coexist, but the process of four-electron transfer is dominant.

Figure 6 (g) and Table 2 show the Nyquist curves of cathode manufactured with the Fe/N@MC-T catalysts. All the Fe/N@MC-T samples revealed a smaller semicircle as compared with that of bare carbon cloth, indicating the lower charge transfer resistance (Rct) of Fe/N@MC-T. This result is possible, because the large mesopores (17.1―17.9 nm) of Fe/N@MC-T can facilitate the electron transfer between electrolyte and electrodes[34]. In addition, the large mesopores can act as an ion buffer reservoir, shortening the distance to the active sites on the inner surface, which can promote the mass transfer of reactants and products[35-36].This phenomenon could accelerate the dynamic process of diffusion. The total resistance (Rct) of Fe@MC-500 (11.7 Ω),Fe@MC-600 (11.6 Ω), Fe@MC-700 (8.3 Ω), and Fe@MC-800 (10.1 Ω) was decreased by 32.6%, 33.3%, 52.1%, and 41.9%, respectively, as compared to that of carbon cloth (17.4Ω). Fe/N@MC-700 possessed a lowestRctof 20 Ω at the catalyst/electrolyte interface, suggesting a favorable catalytic activity toward ORR. This could occur, because Fe/N@MC-700 had the highest SBET (Table 1), which could promote the dispersion of Fe species on the surface of catalysts. It also possessed the largest pore size/volume, which could facilitate the mass and electron transfer. It is well known that the electrochemical reaction rate is negatively related to theRctand therefore the lower value ofRctshows a faster reaction rate[37-38]. Thus, the Fe/N@MC-700 could efficiently decrease the resistance of the cathodes, and would finally influence the performance of the MFC.

Figure 6 (a) CV of Fe/N@MC-T in O2 saturated 0.1 M KOH solution with a scanning rate of 5 mV/s; (b) LSV of Fe/N@MCT; (c-f) RDE tests and Koutecky-Levich plots of Pt/C (c, d) and Fe/N@MC-700 (e, f) at various rotation rates; (g) Nyquist curves of the Fe/N@MC-T(b)

3.2 Performance of MFCs with Fe/N@MC-T

The output voltage of MFCs with Fe/N@MC-T and the power density of the MFC were recorded after 40 days of stabilization. It can be seen from Figure 7 (a) that the cell voltage of Fe/N@MC-T was sharply increased at the 16thday and then gradually increased. The optimal power density of MFC with the bare carbon cloth cathode is 384.5 mW/m2, which is quite low (Figure 7 (b) and Table 2). The optimal power density of MFCs with Fe/N@MC-T catalysts has been much higher than that of MFC with the bare carbon cathode. The optimal power density of MFC with Fe/N@MC-T catalysts increased with an increasing pyrolysis temperature, which was possibly owing to the increase in the graphitization degree and formation of more Fe nanoparticles with the increase of the pyrolysis temperature. Among the tested Fe/N@MC-T catalysts, the MFC with Fe/N@MC-700 showed a highest power density of 864.1 mW/m2, which was about 2.25 times that of MFC with carbon cloth, and was slightly lower than that of MFC with Pt/C (20 wt%)(1002.0 mW/m2). The excellent performance of MFC with Fe/N@MC-700 can be explained in two ways. (i)The Fe nanoparticles wrapped in carbon nanotubes are in possession of excellent electrocatalytic performance towards ORR. (ii) The large mesopores (17.9 nm) of Fe/N@MC-700 can facilitate the electron transfer and mass transfer, leading to a low internal resistance, thus greatly increasing the output power of the MFC through the external circuit.

Figure 7 Performance of MFCs with Fe/N@MC-T (a) Output voltage plots; (b) Polarization curves

4 Conclusions

This study is aimed at developing an alternative way to obtain highly active low price ORR catalysts for MFCs cathode. For this purpose a series of Fe/N@MC-T catalysts with high ORR activity was derived from rice-like MOF Fe/N-MOF by pyrolyzation using acetonitrile as the nitrogen precursor in a low-cost organic solvent. Test results showed that Fe/N-MOF possessed a smooth rice-like structure with a particle size of about 400×50 nm2. However, the Fe/N@MC-T showed a loose small spherical structure with a mesopore size centered diameter of around 17.1―17.8 nm. TEM image of Fe/N@MC-700 showed that the Fe nanoparticles with a grain size of about 15 nm were wrapped in carbon nanotubes,which mainly exist in the form of zero-valent iron with a small amount of Fe3C (verified by XRD analysis).The electrochemical tests reveal that the onset and halfwave potentials of Fe/N@MC-T were 0.89 V and 0.80 V,respectively, which were slightly lower than those of the commercial Pt/C (0.92 V and 0.82 V), indicating that Fe/N@MC-700 had a high electrocatalytic activity for ORR.The MFC with Fe/N@MC-700 showed a highest power density of 864.1 mW/m2, which was about 2.25 times that of MFC with carbon cloth, and was slightly lower than that of MFC with Pt/C (20%) (1002.0 mW/m2).The excellent performance of MFC with Fe/N@MC-700 can be explained in two ways. (i) The Fe nanoparticles wrapped in carbon nanotubes possessed excellent electrocatalytic performance towards ORR. (ii) The large mesopores (17.8 nm) of Fe/N@MC-700 can facilitate the electron transfer and mass transfer, leading to a low internal resistance, thereby greatly increasing the output power of the MFC through the external circuit. This work provides a different way to design non-noble metal ORR catalysts, instead of expensive metallic materials used in energy conversion technologies.