N-Doped Graphitic Carbon

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Metallic state FeS anchored (Fe)/Fe3O4/N-doped graphitic carbon with porous sponge-like structure as durable catalysts for enhancing bioelectricity generation Xin Xu, Ying Dai, Jia Yu, Liang Hao, Yaqiang Duan, Ye Sun, Yanhong Zhang, Yuhui Lin, and Jinlong Zou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b01531 • Publication Date (Web): 14 Mar 2017 Downloaded from http://pubs.acs.org on March 14, 2017

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ACS Applied Materials & Interfaces

Metallic state FeS anchored (Fe)/Fe3O4/N-doped graphitic carbon with porous sponge-like structure as durable catalysts for enhancing bioelectricity generation

Xin Xua, Ying Daia,b, Jia Yuc, Liang Haoa, Yaqiang Duana, Ye Suna, Yanhong Zhanga,*, Yuhui Lina, Jinlong Zoua,*

a Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People's Republic of China, School of Chemistry and Materials Science, Heilongjiang University, Harbin, 150080, China;

b School of Civil Engineering, Heilongjiang Institute of Technology, Harbin, 150050, China;

c College of Aerospace and Civil Engineering, Harbin Engineering University, Harbin, China.

Corresponding author (s): *Yanhong Zhang, Jinlong Zou. a

Xuefu Road 74#, Nangang District, Harbin, 150080, China.

Tel.: +86-451-86608549; Fax: +86-451-86608549. E-mail: [email protected] (Y. H. Zhang); [email protected] (J. L. Zou).

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ABSTRACT: The critical issues in practical application of microbial fuel cells (MFCs) for wastewater treatment are the high cost and poor activity and durability of precious metal catalysts. To alleviate the activity loss and kinetic barriers for oxygen reduction reaction (ORR) on cathode, (Fe)/Fe3O4/FeS/N-doped graphitic carbon ((Fe)/Fe3O4/FeS/NGC) are prepared as ORR catalysts through a one step method using waste pomelo skins as carbon source. Various characterization techniques and electrochemical analyses are conducted to illustrate the correlation between structural characteristics and catalytic activity. MFCs with Fe/Fe3O4/FeS/NGC (900 oC) cathode produces the maximum power density of 930±10 mW m−2 (Pt/C of 489 mW m−2) and maintains a good long-term durability, which only declines 18 % after 90 d operation. Coulombic efficiency (22.2 %) obtained by Fe/Fe3O4/FeS/NGC (900 oC) cathode is significantly higher than that of Pt/C (17.3 %). Metallic state FeS anchored in porous NGC skeleton can boost electron transport through the interconnected channels in sponge-like structure to improve catalytic activity. Charge delocalization of C atoms can be strengthened by N atoms incorporation into carbon skeleton, which correspondingly contributes to the O2 chemisorptions and O–O bond weakening during ORR. Energetically-existed active components (Fe and N species) are more efficient than Pt to trap and consume electrons in catalyzing ORR in wastewater containing Pt-poisoning substances (bacterial metabolites). (Fe)/Fe3O4/FeS/NGC catalysts with the advantages of durable power outputs and environmental-friendly raw material can cover the shortages of Pt/C and provide an outlook for further applications of these catalysts.

KEYWORDS: biofilm overgrowth; durability; interconnected channels; power density; waste biomass.

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INTRODUCTION With the intensification of global energy consumption and the environmental impacts of traditional energy resources, energy shortage and environmental pollution have become serious issues for the sustainable developments of economics and society.1−4 Microbial fuel cells (MFCs) as a sustainable energy-generating technology has received increasing attention in both energy production and wastewater treatment areas.5 In MFCs, microorganisms attached on the anode surface can attract electrons from the oxidation of organic matters in wastewater to transfer them to the anode.6 Then the electrons flow to the air cathode to generate H2O through oxygen reduction reaction (ORR).3,5 During this process, it directly converts the chemical energy into electricity through electrochemical processes accompanied by the wastewater treatment, which undoubtedly meets the ever-increasing demand for both environmental protection and renewable clean energy.7,8 ORR efficiency on the air-cathode is considered as one of the key factors for governing the power output of MFCs.9 Owing to the high catalytic activity and efficient four-electron (4e−) pathway, Pt/C is commonly used as the cathode catalyst.10 However, Pt-based materials usually have the problems of high cost, serious aggregation, poor stability and easy poisoning, which prevent the practical application of MFCs.1,11,12 Thus, to overcome the shortages of Pt/C, current studies have focused on exploring the alternative ORR catalysts with low cost, which can still maintain the high performance in MFCs.

Carbon-based materials (such as graphene, carbon nanotubes, activated carbon, carbon black and graphitic carbon (GC)) with the advantages of high electrical conductivity, low cost and good mechanical properties are commonly considered as the alternative catalysts to replace the commercial Pt/C.13−17 Researches upon ORR catalysts have also indicated that partly-graphitized carbon or GC has exhibited a certain electro-catalytic activity, good electrical conductivity and long-term durability.18 However, via a two-electron pathway, the immanently low electrocatalytic activity of GC in ORR will fatally limit the power output of MFCs.19 Therefore, many attempts have been made to improve the catalytic activity of GC. Fe or Co species doped carbon catalysts 3



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have eagerly been investigated in recent studies. These composites can provide fast electron transport networks and sufficient active sites for ORR.20−22

Due to the desired physical and chemical characteristics, transition-metal sulfides have aroused great attentions of being applied in catalysis.23 Iron sulfide (FeS), possessing outstanding catalytic and electrochemical properties, has been intensively studied as a new type of anode material for lithium ion batteries.24,25 It is reported that the FeS electrode exhibits good cyclic stability and superior performance, but with a very low discharge capacity.24 To solve the problems of low discharge capacity, aggregation and poor durability, FeS and carbon materials are combined.26−28 Shangguan et al.28 synthesize FeS/C composite via a simple route and first evaluate it as host anode materials for nickel-iron batteries. It shows that the FeS/C composite exhibits considerably high charge/discharge capacity, excellent rate capability and superior cycling stability.28,29 The coated carbon layer on the particle surface can not only facilitate electron transport, but also prevent electrochemical aggregation to maintain high capacity. Therefore, the modifications of transition metal sulfides by carbon substrate aimed at enhancing catalytic activity and durability of catalysts are considered as significantly feasible.28,30,31 Shen et al.32 report the fabrication of nano-structured cobalt-iron double sulfides that are covalently entrapped in nitrogen-doped mesoporous graphite carbon (Co0.5Fe0.5S@N-MC). Moderate substitution and well-dispersion of iron in bimetallic sulfide composites are considered to have positive effect on the enhancement of ORR catalytic activity.32,33 Although such developments have been obtained, there are still persistently high expectations for deeply exploring the catalytic activity of FeS, such as ORR activity. The challenges still remain to optimize the structure of FeS or FeS complex to improve the ORR activity by reducing the energy gap between the oxygen 2p orbital and the highest occupied d-orbital of FeS.32

In this work, (Fe)/Fe3O4/FeS/NGC composites are prepared via a facile in situ reduction method by using pomelo skins as carbon source. To the best of our knowledge, no previous work has been 4


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reported about this type of (Fe)/Fe3O4/FeS/NGC catalyst. The pomelo skins consisted of a sponge-like porous structure can endow the carbon with a high specific surface area and a great number of active sites. Furthermore, the attractive assembly/incorporation of Fe-based nanoparticles ((Fe)/Fe3O4/FeS) in NGC is expected to exhibit higher activity and stability for ORR with hydroxyl ions (OH−) as the major intermediates. The synergetic effects among porous structure with a large surface area, (Fe)/Fe3O4/FeS with good stability, and the graphitized carbon with promising conductivity should contribute to the high ORR electrocatalytic activity, which in turn facilitate to the stable and robust performances in MFCs. This study highlights the important roles played by the majority of metallic state FeS on the electrocatalytic ORR activity of these (Fe)/Fe3O4/FeS/NGC composites and also discusses the possible ORR pathways. Effect of carbonization temperatures (600−1000 oC) on crystalline phase and structure evolutions of (Fe)/Fe3O4/FeS/NGC is investigated. Relationships between the (Fe)/Fe3O4/FeS/NGC structure and the long-time performance of MFCs are also explored. The prepared (Fe)/Fe3O4/FeS/NGC as the cathode catalysts are expected to have the promising ORR activity and stability, which may replace the use of Pt-based catalysts in MFCs.

EXPERIMENTAL SECTION Preparation of Fe3+-S2−-C precursors In this study, (Fe)/Fe3O4/FeS/NGC composites were prepared by using an in situ simultaneous synthesis method. Residual pomelo skins were used as carbon source for synthesis of the composites. The yellow parts outside the pomelo skins were completely peeled and the inside cores (the sponge-like parts) were kept in desiccators for further use. Ferric chloride (FeCl3) and thiourea (CN2H4S) were used as iron source and sulfur source, respectively. The mass ratio of FeCl3 to pomelo skins (dried) was fixed at 1: 2; meanwhile the molar ratio of FeCl3 to CN2H4S was fixed at 1: 2. The same volume of FeCl3 (0.5 mol L−1) and CN2H4S (1 mol L−1) solutions were poured into a beaker containing dried pomelo skins with lengths of approximately 0.5 cm. The mixture was vigorously stirred and then ultrasonically treated for 30 min. After 24 h immersion, the pomelo 5



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skins were almost completely dissolved in the mixed solution of FeCl3 and CN2H4S to form a homogeneous paste (Fig. S1, Supporting Information). Only a few crude fibers were not dissolved. The paste was dried at 50 oC for 24 h in the oven and then the Fe3+-S2−-C precursors were obtained. All of the used chemicals were of analytical grade and used without further purification.

Synthesis of (Fe)/Fe3O4/FeS/NGC composites The obtained Fe3+-S2−-C precursors were carbonized in a tubular furnace to prepare the (Fe)/Fe3O4/FeS/NGC. Before carbonization, highly pure N2 had been pumped into the tube for 10 min to get rid of the residual oxygen. Then, the precursors were heated at 600, 700, 800, 900 and 1000 oC for 2 h at a heating rate of 3 oC min−1 under N2 atmosphere at the flow rate of 60 mL min−1, followed by naturally cooling to room temperature (below 30 oC) under N2 atmosphere. The black samples were ground into powder and washed with deionized water several times to remove the impurities. The resulting powders were dried at 60 oC to obtain the catalyst samples, which were marked as (Fe)/Fe3O4/FeS/NGC-x (x=600, 700, 800, 900 and 1000).

Electrode preparation and MFCs configuration In the process of assembling the single-chamber air-cathode MFCs, a pretreated graphite fiber brush was used as the anode.15,34,35 The gas diffusion layer (GDL) and the catalyst layer were sequentially rolled onto the stainless steel mesh to form the air-cathode.36,37 Both the (Fe)/Fe3O4/FeS/NGC-x (x=600, 700, 800, 900 and 1000) and the commercial Pt/C (10 wt.%) were used as the cathode catalysts for MFCs. The single-chamber MFCs reactor was constructed with a cylindrical plexiglas chamber (3 cm in diameter and 4 cm in length). The inner volume of a reactor was 28 mL and the projected area of cathode was 7 cm2.15,38 The effluent with electricity-generating bacteria was used as inoculants to activate the reactors.38 The electrolyte contained glucose (1 g L−1) and phosphate buffered solution (PBS, pH=7.4).39 The external circuit resistance was fixed at 1000 Ω. The temperature of MFCs was kept at 30 oC in a thermostatically controlled incubator. Once the output 6


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voltage was below 50 mV, the fresh electrolyte was added to the reactors. To evaluate the accuracy and repeatability of MFCs, at least three reactors were parallel operated.

Materials characterization The crystal structure of catalyst was characterized by X-ray diffraction (XRD) analyses on a Rigaku D/max-IIIB diffractometer (λ=1.5406 Å). XRD patterns were analyzed according to the Joint Committee on Powder Diffraction Standard (JCPDS) data. Raman spectra were collected by an HR 800 micro-Raman spectrometer (Jobin-Yvon, France) at 457.9 nm. Thermogravimetry (TG) and differential scanning calorimetry (DSC) experiments were performed on a simultaneous DSC-TAG apparatus (NEJSCH STA 499C). The powder sample was heated from 30 to 1000 oC at 5 oC min−1 in air atmosphere with a flow rate of 60 mL min−1. The nitrogen adsorption/desorption isotherms were measured at 77 K using a Micromeritics Tristar II. The specific surface area was calculated by the Brunauer-Emmett-Teller (BET) method; meanwhile the pore size distribution was obtained using the Barrett-Joyner-Halenda (BJH) method. Scanning electron microscopy (SEM) images were taken on an S-4800 scanning electron microscope (Japan). Transmission electron microscopy (TEM) was obtained with a JEM-2100 electron microscope (JEOL) to determine the morphology of the catalyst. X-ray photoelectron spectroscopy (XPS; Kratos-AXISUL TRA DLD) was measured to analyze the elements on the surface of composite using a hemispherical analyzer and an aluminum anode (monochromatic Al Ka, 1486.6 eV) as the source (at 12−14 kV and 10−20 mA).

Electrochemical analysis Electrochemical measurements were carried out on an electrochemical workstation (BAS 100B electrochemical workstation, Germany) with a three-electrode cell. Linear sweep voltammetry (LSV) was measured at a scan rate of 1 mV s−1 on the cathodes of the tested MFCs with 28 mL of 50 mM PBS solution.18,39 The cyclic voltammetry (CV) scan was performed in the potential range from −0.8 to +0.3 V (50 mV s−1) on a glassy carbon electrode. The catalyst ink for CV tests was 7



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prepared according to the reported method.38 After 90 d operation, electrochemical impedance spectroscopy (EIS) was conducted at the open circuit voltage (VOC) over a frequency range from 105 to 10−2 Hz, with a sinusoidal perturbation of 10 mV amplitude.18 ZSimpWin 3.10 software (Echem, Lufkin, TX) was used to fit the EIS data to calculate the charge transfer resistance (Rct). The voltage output of MFCs was recorded by a data acquisition system (PISO-813, ICP-DAS, Taiwan) with an external circuit resistance of 1000 Ω at 30 oC. Polarization curves were measured at various external resistances from 5000 Ω to 50 Ω after 2 h operation with fresh substrate. Power density and Coulombic efficiency (CE) were also recorded. All of the electrochemical analyses were evaluated at 30 oC, consistent with the MFCs operation temperature. Concentration of chemical oxygen demand (COD) of effluents from the MFCs was measured according to the previous method.38

RESULTS AND DISCUSSION Characteristics analyses of (Fe)/Fe3O4/FeS/NGC composites XRD patterns of (Fe)/Fe3O4/FeS/NGC composites are shown in Fig. 1. Although the carbonization temperature is different (600, 700, 800, 900 and 1000 oC), the crystalline phases of the products are basically the same. The obvious characteristic peaks at 2θ of 29.83°, 33.70°, 43.50° and 52.95° are ascribed to the (100), (101), (102), and (110) lattice planes of FeS (JCPDS, No.65-1894), respectively. The diffraction peaks of Fe3O4 (JCPDS, No.89-6466) with a typical face-centered cubic structure can also be observed from the five samples at 35.10°.40 The weak diffraction peaks located at 23.28° correspond to the (222) planes of elementary S (JCPDS, No.42-1278). In addition, the diffraction peaks for graphitized carbon cannot obviously be noticed from the patterns. As reported previously, with the presence of Fe species, the high temperature carbonization should contribute to the high graphitization of carbon.41,42 With the increase of carbonization temperature, the emerged peaks at 44.66° can be attributed to the typical diffraction peak of α-Fe (FCPDS, No.65-4899), ascribing to the further reduction of Fe-species (Fe3O4/FeS) to metallic Fe in the 8


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Fe/Fe3O4/FeS/NGC-x (x=900 and 1000).

TG curves of the (Fe)/Fe3O4/FeS/NGC-x (x=600, 700, 800, 900, and 1000) obtained at different temperatures (heated to 1000 oC in an air stream) are shown in Fig. S2. As presented, the curves of the five (Fe)/Fe3O4/FeS/NGC samples demonstrate a similar variation tendency. The weight loss below 200 oC can be attributed to the evaporation of water. With the gradual increase of temperature, weight loss of each sample is mainly attributed to the combustion of carbon and/or decomposition of carbide. However, it can be seen that the weight loss for each sample is quite different. With the increase of x (x=600, 700, 800, 900, and 1000), the weight loss for (Fe)/Fe3O4/FeS/NGC exhibits a decreasing trend. The weight loss of Fe3O4/FeS/NGC-600 reaches the maximum value of 40.0 % among five samples. The weight losses of Fe3O4/FeS/NGC-700 and Fe3O4/FeS/NGC-800 correspond to 33.0 and 28.0 %, respectively, and the combustion/decomposition of carbon and carbide begins from 700−800 oC. The weight losses of Fe/Fe3O4/FeS/NGC-900 (20.0 %) and Fe/Fe3O4/FeS/NGC-1000 (16.0 %) are comparatively smaller. The major weight loss is related to the percentage content of carbon in Fe/Fe3O4/FeS/NGC. The weight loss of the samples during the period of carbon combustion is likely accompanied by the release of sulfur-containing gases (such as SO2, H2S, etc), which should originate from the oxidation and decomposition of sulfides. Furthermore, the oxidation reaction of α-Fe to Fe2O3 or Fe3O4 is inevitably existed in weight loss process for Fe/Fe3O4/FeS/NGC-x (x=900 and 1000).

N2 adsorption-desorption tests were carried out for (Fe)/Fe3O4/FeS/NGC-x (x=600, 700, 800, 900, and 1000) to evaluate the specific surface area (SBET) and porosity. As shown in Fig. S3, all of the samples exhibit reversible typical type-IV isotherms, which mainly consist of mesopores. H3 type hysteresis loops are related to the phenomenon of capillary condensation in mesopores or macropores. The wedge-shaped holes in (Fe)/Fe3O4/FeS/NGC are derived from the loose stack of flaky particles. The hysteresis loops of Fe/Fe3O4/FeS/NGC-x (x=900 and 1000) close at relative 9



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pressures (P/P0) of approximately 0.4, indicating the existence of mesopores with relatively small pore size (Table 1). It can be seen from Table 1 that with the increasing carbonization temperature, SBET and pore volume first increase and then decrease, while the average pore size exhibits the opposite change. The Fe/Fe3O4/FeS/NGC-900 has the highest SBET of 380.97 m2 g−1, which can provide more active sites for ORR in the porous structure.5 Porous structure consisted of a large number of interconnected holes and channels can serve plenty of pathways for the smooth transport of O2. In addition, the large surface area derived from the porous structure can provide more attachment points for active centers.5 Therefore, the synergistic effects between the sufficient active sites exposed on the surface of NGC skeleton and the smooth transport of O2 through the porous structure can significantly improve the ORR efficiency.

Surface morphologies (SEM) of (Fe)/Fe3O4/FeS/NGC-x (x=600, 700, 800, 900, and 1000) and carbonized pomelo skins are shown in Fig. 2a−f. It shows that with the increase of carbonization temperature, each sample exhibits different morphologies, originating from the decomposition of pomelo skins and the crystalline phase transition of Fe species in C skeleton. Hence, carbonization temperature has a great impact on morphology of (Fe)/Fe3O4/FeS/NGC-x (x=600, 700, 800, 900, and 1000). Pomelo skins-derived carbon (in the scale of 20 µm and 5 µm) exhibits an irregular particle-packing structure with flat and smooth surface. Fe3O4/FeS/NGC-600 reveals the densely stacked structure with sharp short-rods. There are many irregular spherical nanoparticles with diameters of 20−100 nm on the surface of Fe3O4/FeS/NGC-700. Structure of Fe3O4/FeS/NGC-800 consists of disordered stacks of dense short rods and sheets. Formation of the stacked pore channels with uneven pore size can provide paths for electrolyte and proton transfer so that they will enhance the catalytic activity of catalyst and reduce the phenomenon of electrode polarization.43 The images of Fe/Fe3O4/FeS/NGC-900 exhibit the regular sponge-like structure, which can provide a large number of interconnected holes and channels for mass transfer, and obtain the maximum SBET. These channels also provide more pathways for the permeability and transport of O2 so that the 10


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ORR

stability

and

efficiency

of

Fe/Fe3O4/FeS/NGC-900

can

be

improved.24,44

Fe/Fe3O4/FeS/NGC-1000 exhibits a denser packed structure with the smaller sized nanoparticles, which should originate from the clogging of sponge-like structure (Fe/Fe3O4/FeS/NGC-900).

XPS spectra of (Fe)/Fe3O4/FeS/NGC-x (x=600, 700, 800, 900, and 1000) are shown in Fig. S4. Five elements (S, C, N, O, and Fe) are detected on the surface of the (Fe)/Fe3O4/FeS/NGC catalysts. The predominant peaks centered at around 284.8 eV refer to the C 1s. The peaks located at around 389 eV and 530 eV can be assigned to N 1s and O 1s, respectively. In addition, the peaks of Fe 2p and S 2p are observed at around 710 eV and 161 eV, respectively, confirming that the Fe and S species are successfully incorporated into the NGC skeleton, consistent with above results.

Fig. 3a−e presents the high resolution XPS of N1s spectra for (Fe)/Fe3O4/FeS/NGC-x (x=600, 700, 800, 900, and 1000). As shown in Fig. 3a−e, the peaks of N species in Fe3O4/FeS/NGC-x (x=600, 700 and 800) can be decomposed to three components corresponding to Fe-bonded N, pyridinic N and pyrrolic N, whereas the peaks of N1s in Fe/Fe3O4/FeS/NGC-x (x=900 and 1000) can be deconvoluted into two peaks including pyridinic N and pyrrolic N. It can be observed from Table S1 that pyrrolic N occupies the maximum proportion among all of the N-species, followed by pyridinic N. Previous studies have reported that the existence of Fe-bonded N does favor to the enhancement of ORR catalytic activity.12,20,45 Pyridinic N and pyrrolic N also have great impact on ORR catalytic activity of catalyst.20 Pyridinic N and pyrrolic N with planar structure are bonded with sp2 C in sp2 hybrid orbital, which can energetically be served as active sites to catalyze the reduction of O2.46 In the N-doping process, partly graphitized structure is gradually formed, which is also accompanied by both the incorporation of N species into graphite structure and the volatilization of N species from the structure. The loss of N species is inevitable in the heating process, which can build the porous structure to provide supplementary active sites for ORR. The incorporation of N-species into carbon structure can lead to the charge delocalization of C atoms, which facilitates the 11



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chemisorption of molecular oxygen and the O–O bond weakening during the ORR.40,47,48

As shown in Fig. S5, C 1s peaks of (Fe)/Fe3O4/FeS/NGC-x (x=600, 700, 800, 900, and 1000) can be divided into three components, which indicate the existence of abundant defects originating from the embedded particles such as FeS and (Fe)/Fe3O4 in NGC structure. NGC structure can influence the binding status of C1s, whose binding energy is related to the bonded atoms or groups. The peaks at around 284.5 eV can be assigned to C in graphite (C–H or C–C). The symmetry center of the predominant sp2 C peaks is at approximately 284.5 eV. The C 1s spectra also show the presence of C–N (285.1 eV) and C–O (287.1 eV).38 In Fig. S6, the O 1s peaks can be deconvoluted into three peaks corresponding to physically-absorbed O (around 530.6 eV), Fe–O (around 531 eV) and C=O, O–C=O (around 532.2 eV). The presence of O–C=O groups implies the high hydrophilicity of (Fe)/Fe3O4/FeS/NGC-x (x=600, 700, 800, 900, and 1000), which is favorable to the absorption and mass transfer of aqueous electrolyte.39,44 The presence of abundant oxygen-containing functional groups implies the sufficient exposure of catalytic active sites, which can be served as the preferred locations for the absorption, activation, and reduction of oxygen on the (Fe)/Fe3O4/FeS/NGCs surface.11,12,26 In the S 2p spectra (Fig. S7), the peaks at around 164.0 and 168.0 eV are referred to S 2p3/2 and S 2p1/2, respectively, suggesting the presence of a predominant amount of S2– (FeS) and a small amount of SO32– (such as FeSO3). The generation of ferrous sulfite or ferrous sulfate can be attributed to the acidic synthesis system (the mixture of thiourea, ferric chloride and pomelo skins).

The Fe 2p spectra (Fig. 3f−j) can be fitted to two asymmetric resolved doublets resulting from the spin-orbit splitting, which are assigned to Fe 2p3/2 at lower binding energy and Fe 2p1/2 at higher binding energy. Fe 2p peaks can be decomposed into five components, including Fe 2p3/2 Fe–S bond (around 709.7 eV), Fe 2p3/2 ferric state (around 711.2 eV), Fe 2p1/2 Fe–S bond (around 720.2 eV), Fe 2p1/2 ferric state (around 724.1 eV) and metallic iron (α-Fe, around 706.7 eV). The peaks of Fe 2p3/2 α-Fe observed in the spectra of Fe/Fe3O4/FeS/NGC-x (x=900 and 1000) are in agreement 12


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with the XRD results. As carbonization temperature increases, the crystalline phase transitions from Fe-species to α-Fe are accompanied by the weight loss of S-species, resulting in a gradual decrease of Fe–S bond (FeS), which can be considered as one of the main active components for ORR.39 For Fe–S bond, the valence band edge is dominated by nonbonding Fe2+ 3d states, while the conduction band above the Fermi level corresponds to a mixture of antibonding orbitals originated from the S2– and 3d states from Fe2+.49 It can be deduced from S 2p and Fe 2p XPS spectra (Fig. S7 and Fig. 3f−j) that the FeS particles (Fe–S bond) are successfully embedded in the carbon skeleton. The intrinsically metallic state of FeS can be excited by encapsulating in NGC skeleton to directly improve the charge transfer capacity and ORR activity of FeS.

As presented in Raman spectra (Fig. S8), three small peaks at around 220, 280, and 390 cm–1 can be observed, corresponding to the characteristic peaks of Fe–S species, which are most likely to be FeS. The D band (1350 cm–1) can be ascribed to the asymmetric nano-structure of carbon, while the G band (1580 cm–1) corresponds to the vibration mode for sp2-hybridized graphitic carbon. Generally, the intensity ratio of D band to G band (ID/IG) is calculated to evaluate the graphitization degree of NGC.8,47 No peaks at D band and G band can be observed from spectra of Fe3O4/FeS/NGC-600. Except for Fe3O4/FeS/NGC-600, with the increase of carbonization temperature, the ID/IG ratios decrease gradually, which means the increase of graphitization degree. The introduction of Fe species can not only promote the carbon graphitization (i.e. the electrical conductivity), but also act as the main active components in (Fe)/Fe3O4/FeS/NGC.50

As presented in TEM images of Fe3O4/FeS/NGC-800, some nanoparticles with relatively good dispersion are embedded in the NGC skeleton. The NGC structure with the incorporated FeS nanoparticles may enhance the electrical conductivity of the catalyst.24 The crystalline lattice spacing of nanoparticles in Fe3O4/FeS/NGC-800 is 0.208 nm, corresponding to the (102) plane of FeS (Fig. 2h). As shown in Fig. 2i, the morphology of Fe/Fe3O4/FeS/NGC-900 is quite different 13



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from that of Fe3O4/FeS/NGC-800. The skeleton of Fe/Fe3O4/FeS/NGC-900 is mainly consisted of parallel interconnected channels, which can provide sufficient pathways for O2 transport and sufficient active sites for ORR.31 The NGC sheets with a two-dimensional robust structure can provide a large surface area to facilitate the dispersion of active components and prevent the aggregation of Fe/FeS/Fe3O4 nanoparticles (Fig. 2i).10 This special structure can dramatically enhance the electron transport and adhesion between active components and oxygen, and improve the structural stability for long-term usage, which consequently increase the volumetric efficiency and desirable interface in the cathode.1,8,24 As shown in Fig. 2j, the lattice spacing of 0.253 nm marked in the pattern is attributed to the (311) planes of Fe3O4 (HRTEM image).

ORR activity comparison of (Fe)/Fe3O4/FeS/NGC composites LSV tests are conducted to investigate the electrocatalytic activity of the (Fe)/Fe3O4/FeS/NGC and Pt/C for ORR. As shown in Fig. 4a, Fe3O4/FeS/NGC-600 exhibits barely any ORR activity. Pt/C has the maximum current density of –3.4 mA cm–2, which implies the highest ORR catalytic activity. Fe/Fe3O4/FeS/NGC-900 reveals the highest current density among the (Fe)/Fe3O4/FeS/NGC composites, which should possess the relatively high catalytic activity for ORR. As discussed, the presence of Fe-bonded N, pyridinic N and pyrrolic N on the porous NGC surface can efficiently enhance the ORR catalytic activity by providing more active sites. The charge delocalization caused by the inducement of nitrogen can alter the adsorption behavior of graphitic carbon toward oxygen, which should effectively weaken the O–O bonding to facilitate the ORR process.1,47 Furthermore, the embedded Fe species can not only be served as the active components, but also can stabilize the incorporation of N-species within the carbon matrix, promoting the electron transfer for ORR.8

The electrochemical performances of (Fe)/Fe3O4/FeS/NGC and Pt/C are also evaluated by CV tests. As shown in Fig. 4b, no obvious redox peaks can be observed for (Fe)/Fe3O4/FeS/NGC-x (x=600, 700, 800, 900, and 1000) and Pt/C, which can be attributed to the limitation of the CV testing 14


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methods for MFCs as previously reported.50 It shows that Pt/C has the maximum current density of –6.08 mA cm–2, implying that Pt/C may have the highest activity for ORR. Among the (Fe)/Fe3O4/FeS/NGCs, Fe/Fe3O4/FeS/NGC-900 attains the maximum current density of –5.81 mA cm–2, attributing to the synergistic effects between the special structure and active components, which can promote the mass transfer efficiency and O2 reduction efficiency on the Fe/Fe3O4/FeS/NGC-900 surface. The good electrical conductivity and interconnected-channels structure are also conductive to the proton transfer from electrolyte and the electron transfer from external circuit, which correspondingly improve the rate and efficiency of ORR.1,8 Except for Fe3O4/FeS/NGC-x (x=600 and 700), (Fe)/Fe3O4/FeS/NGC-x (x=800 and 1000) also exhibits a certain catalytic activity for ORR, consistent with the LSV results.

Performance of MFCs with different cathodes (Fe)/Fe3O4/FeS/NGC and Pt/C are tested as the cathode catalysts in single-chamber MFCs. Fig. S9 shows the voltage outputs of all of the MFCs during the whole operational period for more than 90 d. All of the reactors equipped with the same anodes are only distinguished from the cathode catalysts. As presented, all of reactors with (Fe)/Fe3O4/FeS/NGC catalysts can normally be started after 7 d. As a contrast, reactor with Pt/C cathode has the shortest start-up time (approximately 3–4 d) and shows a relatively high voltage output in initial start-up stage. However, the high voltage output of reactor with Pt/C cathode only lasts for approximately 30 d and then gradually decreases below 0.4 V, which demonstrates the poor durability of Pt/C catalyst in the electrolyte (wastewater). The voltage output (140 mV) of Fe3O4/FeS/NGC-600 cathode is far lower than those of others. (Fe)/Fe3O4/FeS/NGC-x (x=700, 800 and 900) catalysts exhibit relatively good durability during the operation process. Among the cathodes, Fe/Fe3O4/FeS/NGC-900 has the highest voltage output (0.59±0.05 mV) and durability. The abundant channels in the sponge-like structure of Fe/Fe3O4/FeS/NGC-900 with large surface area can promote the transport of O2, proton and electron.8 Sufficient active components (including Fe-N species and Fe/Fe3O4/FeS with enhanced 15



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metallic state) are energetically existed on the surface of Fe/Fe3O4/FeS/NGC-900, which should be more efficient than metallic Pt to trap and consume electrons in catalyzing ORR in wastewater containing Pt-poisoned substances and bacterial metabolites.39

All of the MFCs are conducted with intermittent replacement of electrolyte at the end of each cycle. Fig. 4c shows the maximum cell voltage of each cycle in all MFCs. It can be observed that the highest voltage output is obtained by Fe/Fe3O4/FeS/NGC-900, followed by Fe3O4/FeS/NGC-800. The (Fe)/Fe3O4/FeS/NGC-x (x=800 and 900) composites with porous structure can facilitate the transfer of electrons and protons on the cathode surface.8 By using (Fe)/Fe3O4/FeS/NGC-x (x=800 and 900) cathodes, less energy is consumed to overcome the activation energy barrier for oxygen reduction and then high ORR efficiency and voltage output can be obtained.

The COD removal rate and CE of MFCs with the six cathodes are measured at each cycle (Fig. 4d). It reveals that MFCs with Fe/Fe3O4/FeS/NGC-900 cathode has the highest COD removal rate (above 90 %) and stability even at the end stage of the operation. The average COD removal rate of Fe3O4/FeS/NGC-800 cathode can reach 85 %. Fe3O4/FeS/NGC-700, Fe/Fe3O4/FeS/NGC-1000 and Pt/C cathodes show the COD removal rates of approximately 80 %. The high removal rates within one cycle can be assigned to the good degradation effects of the electricigens and other heterotrophic microorganisms with capability of organic matter degradation.39 As presented in Fig. 4d, CE of MFCs shows differences in every cycle, which can be attributed to the different durability and activity of electricigens. Generally, the durability and activity of electricigens on anode are closely related to the performance of cathode. The highest CE (average value of 22.2 %) is obtained by Fe/Fe3O4/FeS/NGC-900 cathode. Fe3O4/FeS/NGC-800 and Fe/Fe3O4/FeS/NGC-1000 cathodes also exhibit higher CEs than that of Pt/C (17.3 %), suggesting that the introduction of Fe/Fe3O4/FeS can enhance the catalytic activity of composites and consequently facilitate the reaction kinetics of cathode, which reduce the loss of the generated electrons to thereby obtain a high CE.40 The high 16


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COD removal rate and CE for Fe/Fe3O4/FeS/NGC-900 cathode directly reflect the high activity of electricigens on anode and the high ORR efficiency on cathode, which ensure the stable and continuous power output, and the fast and smooth reactions in MFCs.

Fig. 4e and f shows the Nyquist plots of (Fe)/Fe3O4/FeS/NGC and Pt/C cathodes, both of which display a depressed semicircle along with a straight line. The semicircle arc suggests the charge transfer resistance (RCT), which reflects the electro-catalytic activity of catalyst for ORR in the cathode. To interpret the EIS results, an equivalent circuit is employed as described in the inset. MFCs with Fe3O4/FeS/NGC-600 cathode exhibit an RCT of 466.3 Ω, which is far higher than those of other cathodes. MFCs with Fe3O4/FeS/NGC-800 and Fe/Fe3O4/FeS/NGC-900 cathodes have the RCT of 26.5 and 27.0 Ω, respectively, both of which are lower than that of Pt/C cathode (48.0 Ω). The RCT of MFCs with Fe3O4/FeS/NGC-700 and Fe/Fe3O4/FeS/NGC-1000 cathodes are 86 and 78 Ω, respectively. The low RCT of (Fe)/Fe3O4/FeS/NGC-x (x=800 and 900) cathodes can contribute to an improvement of catalytic activity for ORR and a higher voltage output, implying that the synergism among Fe3O4, FeS and NGC can enhance the charge transfer capacity.50

Power density and electrode potential of MFCs with the six cathodes are measured at the initial and final cycles (Fig. 5). Fig. 5a shows that the maximum power density (930±10 mW m−2) and VOC (0.80 V) are obtained by Fe/Fe3O4/FeS/NGC-900 cathode, followed by Fe3O4/FeS/NGC-800 cathode (756±5 mW m−2, 0.76 V). MFCs with Fe3O4/FeS/NGC-700 and Fe/Fe3O4/FeS/NGC-1000 cathodes obtain the PMAX of approximately 400−500 mW m−2 (VOC of approximately 0.65−0.7 V), which are comparable to the performance of Pt/C (489 mW m−2 and 0.64V). Although Pt/C exhibits the promising catalytic activity for ORR (LSV and CV data), (Fe)/Fe3O4/FeS/NGCs have better electricity production capacity than that of Pt/C in MFCs, demonstrating that these prepared (Fe)/Fe3O4/FeS/NGCs catalysts possess superior catalytic effects and durability, especially when faces to the electricity-generating bacteria and wastewater (PBS). Summary of the performances of 17



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MFCs with Fe/Fe3O4/FeS/NGC-900 and other recently-reported cathodes are shown in Table 2. Fig. 5b and 5d show the distinctive changes of the cathode potentials and the unobvious changes of anode potentials. It fully proves that the different catalytic activity of cathode catalyst is responsible for the different outputs of MFCs.3,5,9,12,51 The maximum electrode potential is obtained by Fe/Fe3O4/FeS/NGC-900 cathode in both the initial and final cycles (Fig. 5b and 5d). The cathode potentials of these MFCs exhibit the same trend as that of the power density. Power density of MFCs in the last cycle is presented in Fig. 5c. Fe/Fe3O4/FeS/NGC-900 still exhibits the best performance among the six MFCs, whose power density only declines 18 % (760±10 mW m−2 and 0.71 V) after 90 d operation. It should be noted that the power density of Pt/C declines 33 % in the final stage, attributing to that too much biofilm attached on the surface of cathode (Fig. S10) can dramatically lower the efficiency of proton transfer and ORR catalytic activity.5

As shown in Fig. S10, a thick layer of biofilm attached on the surface of cathode is formed after 90 d running. Note that the biofilm coated on the Pt/C cathode is thicker than those of other cathodes, which may completely cover the active sites on the Pt/C surface to inhibit its ORR activity. Heterotrophic bacteria gathered in cathodic biofilm will compete for oxygen gas (O2) with ORR catalyst, which may have an adverse effect on ORR efficiency.39 Moreover, the oxidation reaction of organic matter conducted inside and outside the biofilm will lead to a higher overpotential, which may directly influence the total power output of MFCs by reducing the CEs.11,12 The active components on the active surface of (Fe)/Fe3O4/FeS/NGCs and Pt/C are inevitably coated by the biofilm, which changes the Gibbs free energy of ORR and inhibits the catalytic activity of catalyst.39 The active components in the (Fe)/Fe3O4/FeS/NGC catalysts, such as FeS, can boost the chemisorptions of O2 and weaken the bond energy of O−O bond to facilitate the consumption of O2, which correspondingly restrains the overgrowth of aerobic bacteria on cathode surface.39

The coated biomass formed on the surface of (Fe)/Fe3O4/FeS/NGC-x (x=600, 700 and 1000) 18


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cathodes is lower than those of other cathodes (Fig. S10). It can be attributed to the low oxygen permeability and the insufficient amount of heterotrophic bacteria and electricigens in MFCs with (Fe)/Fe3O4/FeS/NGC-x (x=600, 700 and 1000) cathodes that make it difficult to form a thick biofilm on anode or cathode surface.5 Although the biofilm on Fe/Fe3O4/FeS/NGC-900 cathode is relatively thick, the best performance is still obtained by it due to the inherent catalytic activity and high oxygen permeability, which can accelerate the splitting of O−O bond, relieve the activity loss and gain more oxygen from the competing-consumption process during ORR (Fig. 6). Generally, oxygen is continuously diffused to the cathode surface under the driving force of concentration difference, meanwhile the oxygen from cathode reacts with the proton (H+) and electrons from anode to generate H2O.3 When the balance of this ORR consumption is achieved, the aerobic bacteria can utilize the residual oxygen to grow on the surface of (Fe)/Fe3O4/FeS/NGC-900 cathode, which will slightly influence the rate of ORR. It can be deduced from Fig. S10 that the more efficient the ORR is, the less the cathode-coated biofilm is.18,40 Furthermore, the active components (FeS or Fe/N species) embedded in the NGC skeleton can maintain the synergistic effects between the components and structure to obtain the promising electrical conductivity, mass transfer ability and electro-catalytic activity, which also contribute to the balance between the oxygen supply and consumption for (Fe)/Fe3O4/FeS/NGC-900.1,8,24,52

CONCLUSION In summary, the (Fe)/Fe3O4/FeS/NGC composites are in situ synthesized via a facile reduction method by using the residual biomass (pomelo skins) as carbon source. The promising catalytic ORR activity of (Fe)/Fe3O4/FeS/NGC cathodes in single-chamber MFCs can be attributed to its porous structure and highly active components. Defects originated from active components (anchored FeS or Fe/N species) in NGC structure with large SBET can accelerate the chemisorptions of O2 and weaken the energy of O−O bond to facilitate the consumption of O2. The electrical conductivity, mass transfer ability and electrocatalytic activity can be greatly enhanced by the 19



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synergistic effects among the components ((Fe)/Fe3O4/FeS/NGC). In addition, the interconnected holes and channels can provide sufficient active sites for ORR and facilitate the transport of O2 and electron, which can improve the power output and CEs of MFCs. The maximum power density is obtained by (Fe)/Fe3O4/FeS/NGC-900 cathode, demonstrating that its electricity production capacity is better than that of Pt/C. The high voltage outputs of (Fe)/Fe3O4/FeS/NGC cathodes with good durability in long-time operation suggests that the (Fe)/Fe3O4/FeS/NGC catalysts should be promising alternatives to the costly Pt/C catalyst in practical application of MFCs.

ASSOCIATED CONTENT Supporting Information Additional details are available, including nitrogen atomic weight percentage (wt.%) of various chemical states in (Fe)/Fe3O4/FeS/NGC-x (x=600, 700, 800, 900 and 1000) (Table S1), photos of pomelo skins before and after immersion (Fig. S1), TG curves of the (Fe)/Fe3O4/FeS/NGC-x (x=600, 700, 800, 900 and 1000) (Fig. S2), N2 adsorption/desorption isotherms and pore size distributions (inset) for the (Fe)/Fe3O4/FeS/NGC-x (x=600, 700, 800, 900 and 1000) (Fig. S3), XPS spectra of (Fe)/Fe3O4/FeS/NGC-x (x=600, 700, 800, 900 and 1000) (Fig. S4), high resolution XPS of C1s spectra for (Fe)/Fe3O4/FeS/NGC-x (x=600, 700, 800, 900 and 1000) (Fig. S5), high resolution XPS of O1s spectra in (Fe)/Fe3O4/FeS/NGC-x (x=600, 700, 800, 900 and 1000) (Fig. S6), high resolution XPS of S2p spectra in (Fe)/Fe3O4/FeS/NGC-x (x=600, 700, 800, 900 and 1000) (Fig. S7), Raman spectra of the (Fe)/Fe3O4/FeS/NGC-x (x=600, 700, 800, 900 and 1000) (Fig. S8), voltage outputs of MFCs with (Fe)/Fe3O4/FeS/NGC-x (x=600, 700, 800, 900 and 1000) and Pt/C cathodes during the whole operation (Fig. S9), photos of six cathodes after 90 d running (Fig. S10). This material is available free of charge via the Internet at http://pubs.acs.org/. 20


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AUTHOR INFORMATION Corresponding Author *Phone/Fax: (+86) 451 8660 8549; E-mail: [email protected]; [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS We acknowledge the support by National Natural Science Foundation of China (51578218, 51108162, 21473051), Natural Science Foundation of Heilongjiang Province (B201411, QC2015009), Postdoctoral Science Foundation of Heilongjiang Province (LBH-Q14137), Scientific and technological innovation talents of Harbin (2016RQQXJ119), and Excellent Young Teachers Fund of Heilongjiang University and Hundred Young Talents in Heilongjiang University.

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Iannaci,

A.;

Mecheri,

B.;

D'Epifanio,

A.;

Elorri,

M.

J.

L.;

Licoccia,

S.

Iron–Nitrogen-Functionalized Carbon as Efficient Oxygen Reduction Reaction Electrocatalyst in Microbial Fuel Cells. Int. J. Hydrogen Energy 2016, 41, 19637-19644. (54) Nguyen, M. T.; Mecheri, B.; Iannaci, A.; D’Epifanio, A.; Licoccia, S. Iron/Polyindole-Based Electrocatalysts to Enhance Oxygen Reduction in Microbial Fuel Cells. Electrochim. Acta 2016, 190, 388-395. (55) Liu, Z. Q.; Ge, B. C.; Li, K. X.; Zhang, X.; Huang, K. The Excellent Performance and Mechanism of Activated Carbon Air Cathode Doped with Different Type of Cobalt for Microbial Fuel Cells. Fuel 2016, 176, 173-180.

27



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Page 28 of 36

Table 1 Textural properties of (Fe)/Fe3O4/FeS/NGC-x (x=600, 700, 800, 900 and 1000) Surface area

Pore volume

Average pore width

m2 g−1

cm3 g−1

nm

Fe3O4/FeS/NGC-600

3.88

0.01

18.28

Fe3O4/FeS/NGC-700

16.41

0.02

17.27

Fe3O4/FeS/NGC-800

140.42

0.07

11.82


380.97

0.18

6.94


345.92

0.16

7.31

Samples

28


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2 Summary of the performances of MFCs with (Fe)/Fe3O4/FeS/NGC and other recently-reported catalysts.

Catalysts

PMAX

VOC

Rct

(W m–2)

(V)

(Ω)

Conditions

References


0.927

0.800 27.0

PBS+glucose

This work

Commercial Pt/C

0.484

0.642 48.0

PBS+glucose

This work

Iron-nitrogen-functionalized carbon

0.742

0.617

PBS+sodium acetate

53

Nitrogen/iron co-doped carbon

0.745

0.653 37.9

PBS+sucrose

45

Nitrogen-doped MoS2/carbon

0.815

0.752

8.2

PBS+glucose

35

Iron/Polyindole/carbon nanotubes

0.799

0.650

--

PBS+sodium acetate

54

Activated carbon

0.847

0.632

6.0

PBS+sodium acetate

55

--

29



& #

#

Fe/Fe3O4/FeS/NGC-1000 #

@ &

#

#*

#

Fe/Fe3O4/FeS/NGC-900 #

@ &

#--FeS *--Fe &--S @--Fe3O 4

* #

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

#

# @

Fe3O4/FeS/NGC-800 # &

&

#

#@

#

#

20

Fe3O4/FeS/NGC-700 #

#

#

Fe3O4/FeS/NGC-600 #

@

10

Page 30 of 36

30

40

2 Theta (deg)

50

60

70

Fig. 1. XRD patterns of (Fe)/Fe3O4/FeS/NGC-x (x=600, 700, 800, 900 and 1000)

30


Page 31 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 2. SEM images of (Fe)/Fe3O4/FeS/NGC-x (x=600 (a), 700 (b), 800 (c), 900 (d) and 1000 (e)) and pomelo skins-derived carbon (f); TEM images of (Fe)/Fe3O4/FeS/NGC-x (x=800 (g and h) and 900 (i and j)).

31



a

b

Fe3O4/FeS/NGC-600

c

Fe 3O 4/FeS/NGC-700

d

Fe 3O 4 /FeS/NGC-800


e


392

g

Fe3O4/FeS/NGC-600

700

(3)

720

402 404

390 392 394 396 398 400 402 404 406 408

406

h

Fe 2p 3/2

730

Binding Energy (eV)

740

700

(2) (1)

710

i

Fe 2p 3/2

720

(4)

730

Binding Energy (eV)

390

392

394

740

700

(2)

710

720

730

400

402

404

406

Intensity(cps) 390

392

394 396

(2)

j

740

(3)

Fe 2p 3/2

(5)

710

720

730

Binding Energy (eV)

400

402

404

406


(4)

(1)

700

398

Binding Energy (eV)

Fe 2p 1/2

Fe 2p 3/2

(3)

Binding Energy (eV)

398


Fe 2p 1/2 (4)

(1)

396

pyridinic N 398.9eV

Binding Energy (eV)

Fe3O4/FeS/NGC-800

Fe 2p 1/2 (3)

pyridinic N 398.6 eV Fe-N 398.1eV

Binding Energy (eV)

(4)

(1)

710

400

Fe3O4/FeS/NGC-700

Fe 2p 1/2

Intensity (cps)

(2)

398

Intensity (cps)

Fe 2p 3/2

396

Binding Energy (eV)

Binding Energy (eV)

f

394

pyridinic N 398.8eV

pyrrolic N 400.6eV

Intensity (cps)

390

390 392 394 396 398 400 402 404 406 408

pyrrolic N 400.6eV

Fe-N 398.1eV

Intensity(cps)

Fe-N 398.1eV

pyrrolic N 400.7eV

Intensity (cps)

Intensity(cps)

Intensity(cps)

pyridinic N 398.6 eV

Intensity(cps)

pyrrolic N 400.9eV pyrrolic N 400.5eV

pyridinic N 398.5eV

Intensity (cps)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 36

740

700

Fe 2p 1/2 (4)

(2) (1)

(3)

(5)

710

720

730

740

Binding Energy (eV)

Fig. 3. High resolution XPS of N1s and Fe 2p spectra for (Fe)/Fe3O4/FeS/NGC-x (x=600 (a and f), 700 (b and g), 800 (c and h), 900 (d and i) and 1000 (e and j)); (1) Fe 2p3/2→Fe–S bond, (2) Fe 2p3/2→Fe3+, (3) Fe 2p1/2→Fe–S bond, (4) Fe 2p1/2→Fe3+, and (5) Fe 2p3/2→Fe0 (α-Fe).

32


Page 33 of 36

1.0

b

4 3 2

0.0

-2

-2

Current Density(mA cm )

0.5

Current Density(mA cm )

a

-0.5 -1.0 -1.5 -2.0 Fe 3O4/FeS/NGC-600 Fe 3O4/FeS/NGC-700 Fe 3O4/FeS/NGC-800 Fe/Fe 3O 4/FeS/NGC-900 Fe/Fe 3O 4/FeS/NGC-1000 Pt/C

-2.5 -3.0 -3.5 -4.0 -0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

1 0 -1 -2 -3 Fe3O4/FeS/NGC-600 Fe3O4/FeS/NGC-700 Fe3O4/FeS/NGC-800 Fe/Fe3O 4/FeS/NGC-900 Fe/Fe3O 4/FeS/NGC-1000 Pt/C

-4 -5 -6 -7 -0.8

0.4

-0.6

-0.4

Potential(V)

c

600 500 450 400 350 300 Fe3O4/FeS/NGC-600 Fe3O4/FeS/NGC-700 Fe3O4/FeS/NGC-800 Fe/Fe3O4/FeS/NGC-900 Fe/Fe3O4/FeS/NGC-1000 Pt/C

250 200

COD removal rate (%)

Maximum cell voltage (mV)

0.2

0.4

100

90

550

80

80 70

60

60 50 Fe3O4/FeS/NGC- 600 Fe3O4/FeS/NGC-800 Fe/Fe3O4/FeS/NGC-1000

40

Fe3O4/FeS/NGC- 700 Fe/Fe3O4/FeS/NGC-900 Pt/C

30

150

40

20

20

100

10

50 0

5

10

15

20

25

0 0

30

2

4

6

80

f

Fe3O4/FeS/NGC-700 Fe3O4/FeS/NGC-800 Fe/Fe3O4/FeS/NGC-900 Fe/Fe3O4/FeS/NGC-1000 Pt/C

70 60

8 10 12 14 16 18 20 22 24

Cycle number (n)

Cycle number(n) 1000

Fe3O4/FeS/NGC-600

800

600

50

-Zim(Ω)

-Zim(Ω)

0.0

d 100

650

e

-0.2

Potential(V)

Coulumbic efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40

400

30 200

20 10

0

0 20

40

60

80

100

120

0

140

200

400

600

800

1000 1200 1400 1600

Zre(Ω)

Zre(Ω)

Fig. 4. LSV (a) and CV (b) curves of (Fe)/Fe3O4/FeS/NGC-x (x=600, 700, 800, 900 and 1000) and Pt/C in PBS medium; the maximum cell voltage of MFCs with the six cathodes in each cycle (c); COD removal rate and CEs of MFCs with the six cathodes in each cycle (d); Nyquist curves (e and f) of the six cathodes in MFCs; inset is the corresponding equivalent circuit.

33



1100 -2

Fe3O4/FeS/NGC-600 Fe3O4/FeS/NGC-800 Fe/Fe3O4/FeS/NGC-1000

1000

Power Diensity (mW m )

0.7

0.9

a

Fe3O4/FeS/NGC-700 Fe/Fe3O4/FeS/NGC-900 Pt/C

0.6

0.8

800

0.6

700

0.5

600 0.4

500 400

0.3

300

0.2

200 0.1

100


b

0.4 0.3

cathode

0.2 0.1 0.0 -0.1 -0.2

anode

-0.3

0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

2

3.5

0.0 4.0

0.0

0.5


1.0

0.6

0.9

0.5

800

0.8

700

0.7

600

0.6

500

0.5

400

0.4

300

0.3

200

0.2

100

0.1

Electrode Potential (V)

-2


Voltage (V)

Power Diensity (mW m )

900

c

1.0

1.5

2.0

2.5

3.0

2

3.5

Current Density (A/m )

Current Density (A/m ) 1000


0.5

0.7

900

Electrode Potential (V)

1200

Voltage (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 36

d



0.4 0.3

cathode

0.2 0.1 0.0 -0.1 -0.2

0 0.0

0.5

1.0

1.5

2.0

2.5

anode

-0.3

0.0 3.0

0.0

0.5

2

1.0

1.5

2.0

2

2.5

3.0



Fig. 5. Power density and the corresponding electrode potentials of MFCs (vs Ag/AgCl) with different cathodes as a function of current density at the initial (a and b) and last (c and d) cycles.

34


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Fig. 6. Oxygen permeation and reduction routes on the cathodic reaction interfaces for (Fe)/Fe3O4/FeS/NGC composites.

35



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Page 36 of 36

Graphical Abstract


N-Doped Graphitic Carbon

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