Cornstalk-Derived Nitrogen-Doped Partly Graphitized Carbon as

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Cornstalk-derived Nitrogen-doped Partly-Graphitized Carbon as Efficient Metal-free Catalyst for Oxygen Reduction Reaction in Microbial Fuel Cells Ye Sun, Yaqiang Duan, Liang Hao, Zipeng Xing, Ying Dai, Rui Li, and Jinlong Zou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06895 • Publication Date (Web): 13 Sep 2016 Downloaded from http://pubs.acs.org on September 14, 2016

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ACS Applied Materials & Interfaces

Cornstalk-derived Nitrogen-doped Partly-Graphitized Carbon as Efficient Metal-free Catalyst for Oxygen Reduction Reaction in Microbial Fuel Cells

Ye Suna,b, Yaqiang Duana,b, Liang Haoa,b, Zipeng Xinga,b, Ying Daia,c*, Rui Lia,b, Jinlong Zoua,b,*

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

Key Laboratory of Chemical Engineering Process and Technology for High-Efficiency Conversion,

College of Heilongjiang Province, School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, China.

c

School of Civil Engineering, Heilongjiang Institute of Technology, Harbin 150050, China.

Corresponding author (s): * Ying Dai, Jinlong Zou a

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

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

1

ACS Paragon Plus Environment


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ABSTRACT: The low electrocatalytic activity for oxygen reduction reaction (ORR) and high cost of cathode catalyst in microbial fuel cells (MFCs) are the important factors that limit the practical applications. The metal-free nitrogen (N)-doped partly-graphitized carbon (NPGC) as cathode catalyst is prepared at different temperatures (700–1050 oC) by using waste cornstalks as carbon source and melamine as N source. Scanning electron microscopy (SEM), X-ray diffraction (XRD), specific surface areas and transmission electron microscopy (TEM) have been used, in parallel with electrochemical activity tests including rotating disk electrode (RDE) and power output, to clarify how the active-constituents and structure of NPGC influence the MFCs performance. Carbonization temperature has a significant effect on the porous structure and N-doped defects (pyridinic, pyrrolic and graphitic N), which correspondingly influence the amount of active sites, ORR activity and long-time running durability in MFCs. The abundant functional oxygen-containing groups in the porous structure (1177.76 m2 g–1) of NPGC (1000 oC) contribute to the fast adsorption of molecular O2 onto the carbon skeleton. The N-induced charge delocalization facilitates the chemisorption of O2 and cleavage of O–O bonds to effectively enhance the four-electron O2 reduction on NPGC electrode. The maximum power density of NPGC-1000 is 1122 mW m–2 in MFCs, which is higher than that of Pt/C (988 mW m–2), and only has a decline of 10.2 % after 80 d. This work provides a metal-free, high-efficiency and cost-effective ORR electrocatalyst for MFCs.

KEYWORDS: metal-free; microbial fuel cells; nitrogen doping; oxygen reduction reaction; porous structure

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INTRODUCTION Microbial fuel cells (MFCs) are a bioelectrochemical system, which has attracted considerable attention as a green biotechnology for the efficient extraction of energy from organic matter in wastewater.1–3 Electrons generated from the organic substrates by the bacteria are transferred to the anode and flow to the cathode linked by a conductive material with a resistor.4 One of the bottlenecks for the large-scale practical application of MFCs is the sluggish oxygen reduction reaction (ORR) at the cathode side.5 ORR catalysts should be designed to consist of large surface area, tunnel structure, D-band center vacancies, high positive spin density or high positive atomic charge density, definite pore size, etc.6,7 The catalyst characteristics should afford the excellent opportunity for the selective adsorption and efficient reduction of oxygen (O2), which can also facilitate the mass transport and electrons transfer process.8,9

So far as we know, commercial Pt-based catalysts have been commonly used in MFCs because of their excellent catalytic performance for ORR.10,11 However, Pt is quite expensive and has poor long-time running stability due to the migration and coalescence of Pt nanoparticles poisoned by sulfur compounds.12 Consequently, extensive researches have been conducted to explore the Pt-free non-precious-metal and metal-free catalysts with high performance and long-time stability to replace the Pt-based catalysts.13–15 Up to now, many kinds of metal-free carbon materials served as the multi-function electrocatalysts for ORR have attracted much attention because of the high catalytic activity, low cost, long durability, and abundant active sites.16–18 Zhan et al.19 prepare the iodine/nitrogen co-doped graphene as a metal-free catalyst for ORR in both alkaline and acidic media. It is found that the modified graphene exhibits good catalytic activity and the ORR is a four-electron (4e–) process in both alkaline and acidic media. Zhang et al.20 synthesize the heteroatom-doped highly porous carbon from petroleum coke as cathode catalyst, which produces a maximum power density of 1029.77±99.53 mW m–2 in MFCs. Feng et al.21 use SBA-15 as the template to prepare the nitrogen (N)-doped carbon nanotubes (NCNTs), which can be considered as 3



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an efficient and durable metal-free catalyst for ORR in MFCs. MFCs with NCNTs as the cathotic catalysts can produce electricity more durably than that of Pt/C. Most of metal-free carbon catalysts are mainly derived from the heteroatom-doped (N, sulfur-S, boron-B, phosphorus-P, iodine-I, etc) carbon nanotubes or graphite, which aim to obtain the cathode catalyst with high ORR efficiency in MFCs.22–26 However, they have to face the challenge of high cost and complex synthesis.

Recently, varieties of natural biomass derived from plants have attracted a great deal of attention for preparation of N-doped carbon materials as ORR catalysts, because biomass as a renewable source is readily available in high quantity, low cost and environmental friendly. The direct conversion of natural biomass into metal-free carbon is an efficient way for obtaining highly active ORR electrocatalysts with low cost.27–30 Liu et al.27 report the biomass-derived nitrogen (pyridinic, pyrrolic and graphitic) self-doped porous carbon as effective metal-free catalysts for ORR. Water hyacinth is used as the raw material to obtain the metal-free catalysts by a simple activation method at temperatures of 600–800 °C.27 Gao et al.28 prepare the N and S co-doped porous carbon by using honeysuckles as the precursor at 800 °C, which exhibits excellent catalytic activity and good stability for ORR. Liu et al. 29 successfully use the straw as carbon source to prepare the N-doped carbon as ORR catalyst in MFCs through a combined process of hydrothermal treatment (180 °C) and carbonization (NH3, 900 °C). Generally, hydrothermal carbonization process is used for thermochemical conversion of biomass. Biomass can be liquefied in a closed oxygen-free reactor at a settled temperature and pressure using deionized water as the solvent.29 Zhou et al.30 use plant moss to prepare the self-constructed biomass carbon nanoparticles at 900 °C, which exhibit advanced ORR catalysts activity in MFC cathodes. As reported previously, N-species can in situ be doped in the skeleton of biomass-derived carbon to form the N-containing defects, which attract the protons through electrostatic attraction and act as the active centers for ORR.31 Furthermore, the N-species can induce the charge transfer to facilitate the O2 adsorption behavior, which effectively weaken the O–O bond and conduct the effective ORR process.21,32. 4


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In this work, natural biomass cornstalks (the main components are cellulose, hemicelluloses, and lignin) as carbon sources and melamine as nitrogen sources are used to synthesize the N-doped partly-graphitized carbon (NPGC) using hydrothermal carbonization method. Cornstalk as agricultural residue is usually disused or incinerated, which may cause serious environmental pollution problems, especially in northeast of China. The utilization of cornstalks (a fluffy sponge-like structure) can not only relieve the pollution problems, but also offer the derived carbon with a high surface area and a large pore volume for efficient mass transport. In the synthesis process, Fe-species originating from FeCl3 is used as a graphitization catalyst and then is removed from the structure of PGC by acid washing. Porous structure can be formed in NPGCs after carbonization, so that the surface area of the materials are highly increased, which provide abundant active sites for efficient cathodic oxygen reduction in MFCs. Moreover, N-species doped in the skeleton of PGC can be used as the highly active components for ORR. It is recognized that the synergistic effects between the porous structure with abundant functional groups and the defects introduced by N doping are significant to facilitate the ORR process. The performance of these materials as air-cathode catalysts for ORR is evaluated in electrochemical tests and MFCs systems.

EXPERIMENTAL SECTION Preparation of the NPGC Catalysts

The cornstalks (obtained from a farm in Harbin) were stripped the outside crust and cut into small pieces for further use. The stalk pieces were washed with deionized water to remove the impurities and dried at 60 oC in an oven. Typically, 2 g of washed cornstalk and 0.6 g of melamine were mixed in 50 mL of deionized water and then 2.5 mL of 0.5 M FeCl3 solution was added to form a homogeneous mixture. The mixture was put into a PTFE-lined stainless steel reaction kettle, which was sealed and heated at 180 oC for 12 h (at the working pressure of approximately 0.467 MPa) and then cooled to room temperature naturally, to obtain a black carbonaceous hydrogel. The black 5



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carbonaceous hydrogel was thoroughly dried at 60 oC for 12 h and carbonized at 700, 800, 900, 1000 and 1050 oC for 2 h with the heating rate of 3 oC min–1 under a highly pure N2 flow (50–60 mL min–1). The selected carbonization temperature range is referred from the results of our previous study.33 The resulting products were washed with 2 M HNO3 solution for 5 h at 80 oC (water bath) to thoroughly remove the Fe species in the carbon skeleton. Finally, the samples were filtered and washed with the mixture of deionized water and ethanol, followed by drying overnight. The final samples were marked as NPGC-x (x=700, 800, 900, 1000 and 1050).

Air-cathode Fabrication for MFCs.

The MFC reactors with a cube shape and a cylindrical chamber (diameter of 3 cm, length of 4 cm and volume of 28 mL) were made of organic glass.10 Carbon fiber brush, which was washed with acetone and then heated at 450 oC in a muffle furnace before use, was used as the MFCs anode.21 The mixture of carbon black and 60 wt.% of PTFE with a mass ratio of 7: 3 was rolled onto one side of stainless steel mesh (SSM).33 Then, it was heated at 340 oC for 30 min in a muffle furnace to obtain the gas diffusion layer (GDL). The mixture of as-synthesized NPGC catalysts and PTFE (with a mass ratio of 2:1) was then rolled onto the other side of the SSM and dried overnight at room temperature. The SSM rolled with GDL and catalyst layer (CL, with layer thickness of 1 mm) was used as the air-cathode of MFCs.20

MFC Setup and Operation

The as-prepared NPGC-x (x=700, 800, 900, 1000 and 1050) were loaded on the MFC cathodes with the electrode area of 7 cm2. For comparison, commercial 10 wt.% Pt/C was used under the same operation condition.10 Bacteria originating from a stable running MFC of our research group was inoculated into the reactors. The used anodic bacteria initially originated from the efficient 6


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electrogenesis bacteria of Escherichia coli. Phosphate buffered solution (PBS) was used as neutral electrolyte, which consisted of NH4Cl, NaH2PO4·2H2O, Na2HPO4·12H2O, KCl, trace minerals and vitamins.33 The electrolyte consisted of glucose (1g L-1) and PBS was periodically added into MFC. The MFC was normally operated with a 1000 Ω external circuit resistance at a constant temperature around 30 oC. The feeding solutions were replaced once the voltage output down to 50 mV, which was considered as the ending of a cycle for power generation.21 The PBS with bacterial liquid and pure PBS were used as the replaced electrolytes for the initial (unstable) cycles and the stable cycles, respectively.26 To achieve the experimental data statistical soundness, at least three reactors were parallel operated.

Materials Characterization

The crystal structure was determined by an X-ray diffraction (XRD, D/max-IIIB) using Cu Kα radiation (λ=1.5406 Å) at step scan of 0.02o from 10o to 80o. The voltage was 40 kV and the applied current was 20 mA, respectively. XRD patterns were analyzed according to the Joint Committee on Powder Diffraction Standard (JCPDS) data. The Raman spectra were carried out using an HR 800 micro Raman Spectrometer (Jobin-Yvon, France) at 457.9 nm. FT-IR spectra of all samples were recorded using a Nicolet Magna 560 FT-IR spectrometer. The specific surface areas and pore size distributions

were

calculated

by

the

Brunauer-Emmett-Teller

(BET)

theory

and

the

Barrett-Joyner-Halenda (BJH) method, respectively. The analysis of the elements was performed by X-ray photoelectron spectroscopy (XPS, Kratos-AXIS UL TRA DLD, Al Ka X-ray source), XPS data for each atom were fitted with the ‘XPS peak’ software. Scanning electron microscopy (SEM) analyses were determined by an S-4800 scanning electron microscope (Japan) at an accelerating voltage of 5.0 kV. Transmission electron microscopy (TEM) images were taken on a JEM-2100 7



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electron microscope (JEOL) with an acceleration voltage of 200 kV. Dissolved oxygen (DO) was tested using a NeoFox fluorescence-sensing detector (NeoFox Sport, Ocean Optics Inc., America).

Electrochemical Analysis The electrochemical measurements were carried out on a CHI 760E electrochemical workstation (Shanghai Chenhua, China). A platinum sheet (1 cm2) and an Ag/AgCl electrode (+ 0.195V vs standard hydrogen electrode, saturated KCl) were used as counter electrode and reference electrode, respectively. All of the potentials reported in this work were versus the Ag/AgCl electrode. Linear sweep voltammetry (LSV) was scanned at 1 mV s–1 on the MFC cathode, and the reactor was filled with 50 mM PBS solution. The cyclic voltammetry (CV) test was conducted using a glassy carbon electrode (diameter of 3 mm) from – 0.8 V to + 0.3 V at a scan rate of 50 mV s–1.33 Rotating disk electrode (RDE) test was performed at rotation rates of 225–2025 rpm in 50 mM PBS solution, which was saturated with pure O2 flow. The average number of transferred electrons ( ) can be calculated through the Koutecky-Levich (K-L) equation 34:

1 1 1 = + ω −1/ 2 i ik 0.2nFCO2 DO22/ 3v −1/ 6 Where i is the measured current, F is the Faraday constant, DO2 is the diffusion coefficient of oxygen, v is the kinematic viscosity, CO2 is the concentration of oxygen in the solution, ω is the rotation rate of the electrode.25

Electrochemical impedance spectroscopy (EIS) was tested at the open circuit voltage (VOC) over a frequency range of 105 to 10–2 Hz with a sinusoidal perturbation of 10 mV amplitude.10 The charge transfer resistance (Rct) was calculated by fitting an equivalent circuit (EC). The output voltage was collected through a data acquisition system (PISO-813, ICPDAS, Taiwan). Polarization curves were 8


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recorded by measuring the stable voltage generated at various external resistances (50–5000 Ω) after the reactors run with fresh substrate for about 2 h.23 The concentration of chemical oxygen demand (COD) of effluents discharging from the MFCs was measured by using the potassium dichromate oxidation method.33

RESULTS AND DISCUSSION Characteristics Analysis XRD patterns can reflect the crystalline structure and structural purity of the prepared samples. Figure 1a shows the typical XRD patterns of the samples carbonized at different temperatures. The strong peak located at around 22o is assigned to the (002) crystal plane of graphite, while the peak at around 44o is assigned to the (100) crystal plane of graphite structure, indicating the co-presence of crystalline and non-crystalline structures.26 The broad and scatter peaks at around 22o and 44o indicate the highly amorphous structure of NPGCs. It is observed that the peak intensities of the (100) crystal face of NPGC-1050 and NPGC-1000 are higher than those of others, indicating that the degree of graphitization of NPGC-1050 and NPGC-1000 is much higher than the others. This is a proof of the successful doping of external atom N into the carbon structure.16

Figure 1b shows the FT-IR spectra of NPGCs. The FT-IR spectra reflect the presence of functional groups in the structure of NPGCs. As shown in Figure 1b, the band at around 1558 cm–1 can be attributed to the C=C, while the active hydrogen components including N–H and O–H are located at approximately 3200–3650 cm–1. The peak at around 1198 cm–1 can be attributed to C–N or C–O groups, illustrating that N species is successfully doped in the skeleton of NPGCs. The C–N groups with negative charges can attract the protons with opposite electric charge, providing more 9



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primitive reaction centers for ORR. Additionally, the C–O groups can enhance the hydrophilicity of the active centers and further improve the reaction efficiency. The presence of these two types of surface groups (C–N and C–O groups) may contribute to the catalytic activity and long-time stability of NPGCs.34

Figure S1 (Supporting Information, SI) shows the morphology (SEM) of NPGCs. All of the samples exhibit a porous structure, which is mainly derived from the particle stacks and acid treatment. The formation of porous structure may favor the in situ incorporation of N-species, thereby facilitating the electro-conductivity (contributed by pyridinic N and graphitic N) and catalytic activity of the NPGCs according to the literatures.21,27,30

The large surface area and porous structure of the catalysts can afford more active sites for ORR. As shown in Figure S2a (SI), the N2 adsorption/desorption isotherms exhibit a combined type of type I and IV, indicating the co-existence of micropores and mesopores. As shown in Figure S2b (SI), macropores are also be existed in the NPGCs structure. The porous structure, which consists of micropores, mesopores and macropore, can make great contribution to the BET surface area (SBET) and pore volume.35 For metal-free catalyst, the porous structure with abundant active centers can greatly contribute to the ORR activity. Table 1 shows the surface area, total pore volume, and average pore width of NPGCs. The SBET of NPGC-1000 is 1177.76 m2 g–1, which is the largest. The SBET of NPGC-1050 decreases to 1014.76 m2 g–1 due to the pore collapse caused by high pyrolysis temperature, consist with the result of SEM. The average pore widths of all of NPGCs are approximately 2–3 nm, demonstrating that mesopores make a major contribution to the volume. It is reported that mesopores are beneficial for ORR and the migration of electrolyte ions (proton) into 10


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inside layers.36 It is proposed that, on the one hand, the complete elution of Fe species can probably promote the formation of pore channels in NPGCs, leading to a higher pore volume.37 On the other hand, during the pyrolysis process, melamine decomposition and heteroatom (N) doping are favorable for pore generation and SBET augment.37 The introduction of Fe species can improve the graphitization degree of carbon, while the removal of Fe species by nitric acid can promote the formation of active centers (sites) in NPGCs. Irregular porous structures with abundant functional O-containing groups should be formed during the Fe-removal process. Hence, NPGC-1000 with the highest surface area and pore volume may exhibit a more excellent electrocatalytic performance than other NPGCs.

The XPS spectra (Figure 2a) of NPGCs clearly show the presence of C, O, and N at around 284.6, 531.5 and 399.8 eV, respectively, confirming that N species has been successfully doped into the carbon skeleton. As shown in Table S1 (SI), residual Fe cannot be observed in the NPGCs, suggesting that the Fe species is completely removed by nitric acid treatment. Meanwhile, abundant vacancies and defects are in situ formed in the metal-free catalysts through the Fe removal. With the increase of the pyrolysis temperature, the contents of N and O decrease while the content of C increases, attributing to that parts of the N and O species are volatilized from the carbon phase at high temperature. It implies that more defects may be formed in the structure of NPGCs at high temperature.

The C 1s spectra (Figure S3, SI) of NPGC-x (x=700, 800, 900, 1000 and 1050) can be deconvoluted into three components, which can be identified as graphitic C (at around 284.5 eV), C–N (at around 285.6 eV) and C–O (at around 286.7 eV). It should be noted that the presence of graphitic C 11



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contributes to the high electrical conductivity, which is crucial in ORR due to the acceleration of electron transfer.21,27,30 As shown in Figure S4 (SI), by decomposing the O 1s peaks, two components can be observed at around 530.6 and 532.2 eV, originating from physically absorbed O and C=O/O–C=O, respectively. The existence of the hydrophilic C=O/O–C=O means that abundant O-groups are formed on the surface of C skeleton after acid treatment, which should facilitate to the adsorption and mass transfer of the dissolved oxygen (DO) on cathode.38 It may further enhance the ORR activity of NPGCs.

As shown in Figure 2b–2f, the N 1s spectra can be deconvoluted into four characteristic peaks at around 398.1, 399.5, 400.5, and 405.4 eV, which correspond to pyridinic, pyrrolic, graphitic, and oxidized N, respectively. The various chemical states of N including the oxidized N are fitted with the 'XPS peak' software. The various chemical states of N in the NPGCs should play a critical role in the ORR process. It should be mentioned that the pyridinic N and pyrrolic N on the carbon surface can affect the reduction of oxygen over the carbon materials.27 Note that the pyridinic N has a lone electron pair in the plane of carbon matrix, which can improve the electron-donor property of NPGCs. Moreover, the bonds between oxygen and nitrogen and/or the adjacent carbon atoms can contribute to the chemisorption of molecular oxygen, and facilitate the O–O bond weakening.21 The percentage contents of graphitic and oxidized N increase as the pyrolysis temperature increases (Table 2). It implies that pyridinic N and pyrrolic N may be converted to graphitic N and oxidized N, which improve the electronic density of the N-neighboring carbon and the ORR activity of NPGC. It is reported that graphitic N is the dominant species for obtaining high catalytic activity of N-doped metal-free catalyst.27,30 Generally, graphitic N atoms are firmly incorporated into the carbon structure by substituting the C atoms and the presence of graphitic N definitely facilitates a 12


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relatively high and stable ORR activity.27,30 The percentage content (50.2 %) of the graphitic N of NPGC-1000 is higher than those of other samples, which is comparable to those of the studies reported by Yuan et al. (53.9 %) and Liu et al. (47.6 %).39,40 The centered peaks of graphitic N of NPGC-1000 and NPGC-1050 are both at around 401.2 eV, while the others are at around 400.8 eV. The slight skewing should be attributed to the increase of binding ability between the atomic nucleus and extranuclear electron of N atoms, which may contribute to the combination of N and O atoms. Therefore, the doping of N into C skeletons is believed to induce charge delocalization of carbon atoms, which changes the chemisorption model of oxygen during the ORR.16

Electrochemical Characterization As shown in Figure 3a, the obtained current densities of NPGCs are quite different in CV curves. NPGC-1000 has the maximum current density (-10.70 mA cm–2) among the NPGCs, which is slightly higher than that of Pt/C (-10.17 mA cm–2). As shown in LSV curves (Figure 3b), NPGC-1000 has the maximum current density (-1.73 mA cm–2), which is higher than that (1.60 mA cm–2) of Pt/C. These results are consistent with the description in CV analyses. As previously reported,14, 33 the catalyst with higher current density (both CV and LSV) generally exhibits higher ORR activity. The ORR activity of NPGCs should be attributed to the formation of the N-containing defects in the skeleton of biomass-derived carbon after N doping. The heteroatom (N) doping is favorable for the formation of asymmetric spins and better charged structures in carbon, contributing to the better electrocatalytic property.20 Furthermore, the abundant O-containing groups can contribute to the fast electrons transfer and also act as the active centers for ORR. Note that the synergistic effects between N and O species in the NPGCs are different from each other because of their different doping contents at different temperatures, rendering them different ORR 13



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activities.34 As carbonization temperature increases to 1050 oC, the percentage contents of the N and O groups decrease in the NPGC-1050 (Table S1), which cannot afford enough active sites (N and O) for obtaining high ORR performance (lower than NPGC-1000).

Performance of MFC Equipped with NPGC Cathode. To investigate the stability of power output, the MFCs are operated for approximately 80 d. Figure 4a shows the maximum cell voltage of each cycle in MFCs with NPGCs and Pt/C cathodes. The average maximum cell voltage of NPGC-1000 is 0.57 V, which is higher than that of Pt/C (0.54 V) and is comparable to that of the metal-free NCNTs-MFCs reported by Feng et al.21 (0.59 V). As a comparison, the average maximum cell voltage of NPGC-1050, NPGC-900, NPGC-800, NPGC-700 is 0.56, 0.56, 0.54 and 0.44 V, respectively. In the final cycle, the maximum cell voltage of Pt/C cathode is declined obviously (26.8 %), which may result from the poison of sulfides and the biofilm attached on the MFC cathodes.41 However, there is no obvious voltage decline for NPGC-1000 (1.7 %), which exhibits the excellent long-time stability. The average COD removal rate and coulumbic efficiency for MFC with NPGC-1000 cathode (Figure 4b) are 92.1 and 23.5 %, respectively. They are comparable to those of Pt/C cathode (92.3 and 21.9 %), indicating that NPGC-1000 has a similar electronic recycling efficiency to that of Pt/C.

Figure 5 shows the polarization curves and the corresponding electrode potentials. It is observed from Figure 5b and 5d that the cathode potentials decline rapidly with the increase of current density. In the initial cycle, the MFC with NPGC-700 has the lowest power density of 502±13 mW m–2. The maximum power density of NPGC-800 and NPGC-900 are 830±17 and 996±21 mW m–2, respectively. Remarkably, NPGC-1000 generates the highest power density of 1122±32 mW m–2, 14


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which is higher than that of Pt/C (988±11 mW m–2) and other metal-free catalysts as reported previously23,25. The high electrocatalytic activity of NPGCs is attributed to the N-induced charge delocalization, which facilitates the chemisorption of O2 and the cleavage of O–O bonds to effectively enhance the ORR on NPGCs electrode.27 Moreover, the higher ORR activity of NPGC-1000 is also attributed to its larger surface area, more crinkled surfaces, and higher amount of O-groups.34,35,37

After 80 d of operation (Figure 5c and 5d), the maximum power density of MFCs declines in different degrees. The MFC with NPGC-1000 still produce the highest maximum power density of 1008±27 mW m–2, which has only 10.2 % of decline (Table S2, SI). The durability of NPGC-1000 is slightly better than that of Fe/Fe3C/NPGC (decline of 11.9 %) in our previous study.42 The C–N groups are formed in NPGC-1000 to generate the active centers to attract the protons by their negative electric charges. Moreover, the synergistic effect between C–N and C–O may also contribute to the ORR stability of NPGC-1000.34 The MFC with Pt/C cathode has a decline of 37.8 %, which should mainly be caused by the biofouling of cathode surface. The results illustrate that the MFCs with NPGCs cathodes have a better long-time stability than that of Pt/C cathode.

The results of EIS for different cathodes are shown in Figure 6 and Table S3 (SI). As previously described, an equivalent circuit is used to analyze the EIS results.33 The charge transfer resistance (Rct) of NPGC-1000 is the lowest (27.6±0.2 Ω) of NPGCs, which is slightly higher than that of Pt/C (23.8±0.3 Ω). However, NPGC-1000 as cathode catalyst exhibits a better electrochemical performance in MFC among the catalysts. The structural defects caused by N-doping facilitate the electron transfer across the carbon skeleton.43 This phenomenon confirms that the doped N species 15



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(pyridinic N and graphitic N) can improve the conductivity of NPGCs.21,27,30,43 Moreover, the porous structure is also beneficial to the transport (penetration) of the electrolyte (proton) and the oxygen28, which should correspondingly enhance the ORR activity and lower the Rct.

Detailed Structures and ORR Pathway of NPGC-1000 To investigate the relationship between performance and structure, TEM images of NPGC-900 and NPGC-1000 are shown in Figure S5 (SI) and Figure 7, respectively. The sample exhibits a highly interconnected 3D framework and porous structure, which may be favorable for the transport of protons and electrons. NPGC-1000 with a stereoscopic spongy structure has crinkled surface and more irregular pores, which has a larger BET surface area than that of NPGC-900. It is reported that the graphitic with many irregular nanopores is multilayer and the crinkled structure is favorable for the formation of defects originating from the N doping.16,28,44 The highly amorphous structure of NPGC-1000 can be observed in Figure 7b, which is consistent with the results of XRD and Raman spectroscopy (Figure S6, SI).

RDE measurements is used to investigate the kinetics of ORR. From the LSV curves (Figure 8a and 8c), it can be observed that the limiting current density increases with the rotation rates increase from 225 to 2025 rpm. The K-L equation is used to calculate the electron transfer number (n) of ORR.45 The average n values under different potentials are calculated (Figure 8b and 8d). The n value of Pt/C is 3.98, indicating that the ORR pathway of Pt/C proceeds with a high-quality 4e– reaction process. The calculated n value of NPGC-1000 for ORR is approximately 3.75, which is similar to those of the metal-free catalysts reported by Meng et al. 25 (3.80) and Liu et al.34 (3.87), indicating that the main pathway for ORR is the 4e– reaction on the active sites.46 The presence of N species in NPGC-1000 plays a key role in promoting the adsorption, activation and reduction of

16


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oxygen. Correspondingly, it makes the materials to easily donate electrons to either the oxygen or the peroxide intermediates adsorbed on the NPGC-1000 surface, leading to the high reaction efficiency in ORR.16 Although it is reported that carbon materials with more O-groups are known to favor the 2e– process in ORR,36,47 4e– reaction process is still conducted on the NPGC-1000 because of the high amount of O-groups on its crinkled surface.

The NPGCs-MFCs exhibit an excellent performance, including high efficiency and long-time stability. This promising activity toward ORR can be attributed to the synergetic effects between N and O species that are firmly implanted in the skeleton of metal-free NPGC. The reaction mechanism of NPGCs cathode in MFCs is shown in Figure 9. These metal-free catalysts are obtained by removing the residual Fe species through the nitric acid treatment, meanwhile the defects as the ORR active sites are in situ formed in the surface of pore walls.48 The highly porous structure can afford the large SBET, which is favorable for the exposure of the key active sites of pyridinic N and graphitized N groups.49,50 Meanwhile, many oxygen-containing groups are formed and exposed in the NPGCs structure, which should facilitate the adsorption and activation of molecular oxygen. The O–O bond can efficiently be weakened at these active sites, leading to the smooth occurrence of ORR.27 The oxygen atoms can easily combine with the protons and electrons to conduct the 4e– ORR on the NPGCs electrode. According to the RDE tests, the main pathway of NPGC-1000 for ORR is the 4e– reaction, implying that the direct O2 reduction with little intermediates is happened.28,36 Through the efficient 4e– reaction, NPGC-1000 can be considered as a promising ORR catalyst with lower energy loss that can be further used in MFC cathode.

CONCLUSION In summary, the metal-free NPGC catalysts obtained from residual biomass exhibit porous structure, large SBET and excellent ORR performance in MFCs. The N species originating from melamine are 17



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successfully introduced into the PGC skeleton through the carbonization of cornstalks at 700–1050 °C. The doped N species can induce charge delocalization of carbon atoms and change the chemisorption model of oxygen at the active sites. During the ORR, O2 can be fast adsorbed onto the NPGCs surface with abundant O-groups, and then the O–O bond is easily fractured at the active sites (defects). The NPGC-1000 catalyst exhibits the 4e– (main) ORR pathway and the highest power density (1122 mW m–2). Furthermore, the MFCs with NPGC-1000 cathode also exhibits the highest voltage output and the best long-time stability, which are intrinsically connected with the abundant defects and specific structure of NPGC. Therefore, this work presents an attractive idea for preparation of low-cost metal-free catalysts, which can be considered as a good reference for improving MFCs performance.

ASSOCIATED CONTENT Supporting Information Additional details are available, including the XPS results of element content analysis of NPGCs (Table S1), VOC and Pmax for MFCs with different cathodes (Table S2), the electrochemical impedance fitting results of different cathodes (Table S3), SEM of NPGCs (Figure S1), N2 adsorption-desorption isotherms and pore-size distributions of NPGCs (Figure S2), high resolution XPS of C 1s spectra for NPGCs (Figure S3), high resolution XPS of O 1s spectra for NPGCs (Figure S4), TEM images of NPGC-900 (Figure S5), and Raman spectrum of NPGC-1000 (Figure S6). This material is available free of charge via the Internet at http://pubs.acs.org/.

AUTHOR INFORMATION Corresponding Author *Tel: +86 451 8660 8549 (Y.Dai); (+86)451 8660 9115 (J.L.Zou). Email: [email protected] (Y. Dai); [email protected] (J.L.Zou). 18


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Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS We acknowledge the support by National Natural Science Foundation of China (51578218, 51108162, 51210105014, 21473051), Natural Science Foundation of Heilongjiang Province (B201411,

QC2015009),

Postdoctoral

Science

Foundation

of

Heilongjiang

Province

(LBH-Q14137), and Excellent Young Teachers Fund of Heilongjiang University and Hundred Young Talents in Heilongjiang University.

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25



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Table 1 Pore characteristics of NPGCs. BET surface area

Total pore volume

Average pore

(m2 g–1)

(cm3 g–1)

width (nm)

NPGC-700

520.33

0.27

2.11

NPGC-800

565.99

0.31

2.19

NPGC-900

830.68

0.45

2.21

NPGC-1000

1177.76

0.74

2.51

NPGC-1050

1014.76

0.59

2.31

Samples

26


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Table 2 Percentage contents (at. %) of various chemical states of N in NPGC-x (x=700, 800, 900, 1000, and 1050) Pyridinic N

Pyrrolic N

Graphitic N

Oxidized N

(at. %)

(at. %)

(at. %)

(at. %)

NPGC-700

30.01

22.89

26.10

21.00

NPGC-800

26.69

9.39

37.56

26.19

NPGC-900

27.08

10.16

40.36

22.14

NPGC-1000

6.67

9.21

50.16

34.29

NPGC-1050

5.06

2.25

44.38

48.31

Samples

27



a

NPGC-700 NPGC-900 NPGC-1050

b

NPGC-800 NPGC-1000


Intensity (a.u.)

NPGC-1050

Intensity (a.u.)

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N-H,O-H

NPGC-700

C=C

C-N,C-O 10

20

30

40

50

60

70

80

500

2 Theta degree

1000

1500

2000

2500

3000

3500

4000

-1

Wavenumber (cm )

Figure 1. XRD patterns (a) and FT-IR spectra (b) of NPGC-x (x=700, 800, 900, 1000, and 1050)

28


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NPGC-700 NPGC-800 NPGC-900 NPGC-1000 NPGC-1050

C 1s O 1s

Intensity (cps)

N 1s

0

200

400

600

800

1000

b pyridinic N

Intensity (cps)

a

1200

graphitic N oxidized N

pyrrolic N

392

394

396

Binding Energy (eV)

398

400

402

404

406

408

410

Binding Energy (eV)

c

d graphitic N oxidized N

graphitic N

Intensity (cps)

Intensity (cps)

pyridinic N pyrrolic N

392

394

396

398

400

402

404

406

408

410

pyrridic N

oxidized N

pyridinic N

390

392

394

396

Binding Energy (eV)

398

400

402

404

406

408

410

Binding Energy (eV)

e

f graphitic N

oxidized N graphitic N

Intensity (cps)

oxidized N

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


pyrridic N pyridinic N

390

392

394

396

398

400

402

404

406

408

410

Binding Energy (eV)

pyrridic N pyridinic N

390

392

394

396

398

400

402

404

406

408

410

Binding Energy (eV)

Figure 2. XPS survey spectra (a) and High resolution XPS of N 1s spectra (b–f) of the NPGC-x (x=700, 800, 900, 1000, and 1050)

29



15

-2

Current Density ( mA cm )

a

10 5

NPGC-700 NPGC-800 NPGC-900 NPGC-1000 NPGC-1050 Pt/C

0 -5 -10 -0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

Potential(V)

b

0.0

-2


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 30 of 37

-0.5

-1.0


-1.5

-2.0 -0.4

-0.2

0.0

0.2

0.4

0.6

Potential (V)

Figure 3. (a) Cyclic voltammograms (CV) of NPGCs and Pt/C in PBS medium at a scan rate of 50 mV s–1, (b) Linear-sweeping voltammograms (LSV) of NPGCs and Pt/C in PBS medium at a scan rate of 1 mV s–1.

30


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a

0.65

Maximum cell (V)

0.60 0.55 0.50 0.45 0.40 0.35


0.30 0.25 0

10

NPGC-800 NPGC-1000 Pt/C

20

30

40

Cycle number (n)

b

100

90 50 80


70


40

30 60 20

Coulumbic efficiency (%)

60

COD removal rate (%)

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


50 0

5

10

15

20

25

30

Cycle number (n)

Figure 4. (a) The maximum cell voltage of each cycle for NPGC-x (x= 700, 800, 900, 1000 and 1050) and Pt/C cathodes, (b) COD removal rates and coulumbic efficiencies of MFCs using different cathodes.

31



1600

1.0


-2

Power Density(mW m )

1400 1200


b

0.8

1000

0.6

800 0.4

600 400

0.2 200

0.4 0.3 0.2

cathode

0.1 0.0 -0.1

anode

-0.2

0

0.0 0

1

2

3

4

-0.3

5

0

1

2

-2

1.0

d

0.6

5


0.5 0.8

1000 0.6

800 600

0.4

400 0.2 200

Electrode Potential(V)

-2


Voltage(V)

Power Density(mW m )

1200

4

Current Density(A m )

1400


3 -2


c


0.6 0.5

Electrode Potential(V)

a

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 32 of 37

0.4 0.3 0.2

cathode

0.1 0.0

anode

-0.1 -0.2

0

0.0 0

1

2

3

4

-0.3 0

-2

1

2

3

4 2



Figure 5. Power densities and the corresponding electrode potentials of MFCs with different cathodes as a function of current density at the initial cycle (a and b) and the final cycle (c and d).

32


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90


80 70 60

-Zim(Ω)

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


50

C

Rs

40 30

Rct

Zw

20 10 0 0

20

40

60

80

100

120

140

Zre(Ω)

Figure 6. Nyquist plot of the NPGCs and Pt/C cathodes in MFCs.

33



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

Figure 7. TEM images of NPGC-1000 (a) and high resolution TEM images of NPGC-1000 (b).

34


Page 34 of 37

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0

b

0.6

0.5

-2


a

-2

-6

-2 -1

225 rpm 400 rpm 625 rpm 900 rpm 1225 rpm 1600 rpm 2025 rpm

-4

j (mAcm )

0.4

-8 -0.6

0.3

0.2

0.2V 0.3V 0.4V

0.1

0.0

-0.4

-0.2

0.0

0.2

0.4

0.02

c

0.04 -1/2

Potential (V)

ω

0

d

(rpm

0.06 -1/2

0.08

)

0.8 0.7

-2


-1

0.6

-2

j (mAcm )

225 rpm 400 rpm 625 rpm 900 rpm 1225 rpm 1600 rpm 2025 rpm

-2

-3

-4 -0.6

-1

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


0.5 0.4 0.3

0.2V 0.3V 0.4V

0.2 0.1 0.0

-0.4

-0.2

0.0

0.2

0.4

0.02

Potential (V)

0.04

ω

-1/2

(rpm

0.06 -1/2

0.08

)

Figure 8. RDE polarization curves of Pt/C (a, b) and NPGC-1000 (c, d) at various rotation rates.

35



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

Figure 9. The reaction mechanism of NPGCs cathode in MFCs.

36


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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


Graphic Abstract


Cornstalk-Derived Nitrogen-Doped Partly Graphitized Carbon as

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