Fe and N co-doped carbon derived from melamine resin capsuled biomass as efficient oxygen reduction catalyst for air-cathode microbial fuel cells

international journal of hydrogen energy xxx (xxxx) xxx

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Fe and N co-doped carbon derived from melamine resin capsuled biomass as efficient oxygen reduction catalyst for air-cathode microbial fuel cells Dongliang Wang a, Jingping Hu a,b,c, Jiakuan Yang a,b,c, Keke Xiao a,b, Sha Liang a,b, Jikun Xu a, Bingchuan Liu a,b,*, Huijie Hou a,b,** a School of Environmental Science and Engineering, Huazhong University of Science and Technology, Wuhan, 430074, PR China b Hubei Provincial Engineering Laboratory of Solid Waste Treatment, Disposal and Recycling, 1037 Luoyu Road, Wuhan, Hubei, 430074, China c State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology (HUST), 1037 Luoyu Road, Wuhan, Hubei, 430074, China

highlights

graphical abstract


Fe/N/C ORR catalyst was synthesized from melamine resin with biomass particle encapsulated.
Onset ORR potential for Fe/N/C was 207 mV more positively than AC&CB in neutral solution.
The Fe/N/C cathode MFC produced maximum

power

density

of

1166 mW m2, which was 140% higher than AC&CB cathode.
The moderate porosity and skeleton played by biomass particle contributed to excellent stability of Fe/N/C cathode.

article info

abstract

Article history:

Cathode oxygen reduction reaction (ORR) performance is crucial for power generation of

Received 12 July 2019

microbial fuel cells (MFCs). The current study provides a novel strategy to prepare Fe/N-

Received in revised form

doped carbon (Fe/N/C) catalyst for MFCs cathode through high temperature pyrolyzing of

24 September 2019

biomass capsuling melamine resin polymer. The obtained Fe/N/C can effectively enhance activity, selectivity and stability toward 4 ee ORR in pH neutral solution. Single chamber

* Corresponding author. School of Environmental Science and Engineering, Huazhong University of Science and Technology, Wuhan, 430074, PR China. ** Corresponding authors. School of Environmental Science and Engineering, Huazhong University of Science and Technology, Wuhan, 430074, PR China. E-mail addresses: [email protected] (B. Liu), [email protected] (H. Hou). https://doi.org/10.1016/j.ijhydene.2019.11.201 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Wang D et al., Fe and N co-doped carbon derived from melamine resin capsuled biomass as efficient oxygen reduction catalyst for air-cathode microbial fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.11.201

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Accepted 26 November 2019

MFC with Fe/N/C air cathode produces maximum power density of 1166 mW m2, which is

Available online xxx

140% higher than AC cathode. The improved performance of Fe/N/C can be attributed to the involvement of nitrogen and iron species. The excellent stability can be attributed to

Keywords:

the preferential structure of the catalyst. The moderate porosity of the catalyst facilitates

Fe/N doping

mass transfer of oxygen and protons and prevents water flooding of triple-phase boundary

Oxygen reduction reaction

where ORR occurs. The biomass particles encapsulated in the catalyst act as skeletons,

Microbial fuel cell

which prevents catalyst collapse and agglomeration.

Power density

© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Microbial fuel cells (MFCs) are green bioelectrochemical technologies that can directly convert chemical energy in wastewater into storable electricity [1]. Oxygen serves as the ideal electron acceptor on cathode for practical applications due to its high reduction potential and abundance in nature. However, the sluggish oxygen reduction reaction (ORR) kinetics in pH-neutral aqueous electrolyte is one of the main bottleneck for application of highly efficient MFCs [2,3]. To maximize power generation and energy conversion efficiency of MFC, the 4 ee ORR is preferential. Platinum-based materials is commonly utilized as a comprehensive excellent catalyst for cathode in lab scale MFCs, however, it’s not feasible for large scale applications due to its high cost and rapid performance degradation [4]. Intensive efforts have been devoted to explore non-noble-metal catalyst (NPMC) with ORR activity comparable or even superior to Pt/C catalyst. Among the extensive developed NPMCs, heteroatomdoped carbon materials have been proven as efficient catalyst toward 4 ee route ORR with high onset potentials and current densities. Doping carbonaceous materials with heteroatoms (e.g., N, S, B, P) could tune their electrochemical activities via electron modulation [5]. The improved activity of heteroatom-doped carbon catalysts can be attributed to doping-induced charge redistribution around the dopants, which changes the O2 adsorption and effectively weaken the OeO bonding and facilitate the ORR process [6e9]. In view of performance, stability and easy fabrication, nitrogen has become a most popular dopant element to improve ORR catalysis [10,11]. Introducing transition metal could further boost the activity of the catalyst [9,12], which can be attributed to the change of orbital energy level of the carbon atom induced by additional Fe or FeNx‒leading to easier electron transfer from catalyst to oxygen [13,14]. Although the mechanisms for the formation of active sites are not completely understood, the Fe/N co-doped carbon (Fe/N/C) materials have been considered to be one most promising ORR catalysts. Beside catalyst activity toward ORR, there still exist other challenges facing the catalyst in air cathode MFCs: the collapse and migration induced agglomeration of catalyst particles at triple-phase boundary can inevitably cause a loss of electrochemical activity [15]; and the electrode flooding when porosity of gas diffusion layer (GDL) beyond a certain point can lead to performance degradation [16]. Previous work has shown that a sufficiently low porosity of GDL will help to prevent air cathode flooding [17]. Therefore, increasing porosity beyond the flooding point will lead to fast decrease of

power generation [16]. This raises difficulty in designing catalyst as a porous structure can render large specific surface areas and facilitate harnessing of active sites. One popular way to produce Fe/N/C catalysts was to heat a mixture of carbon precursor, nitrogen precursor and Fe source [18e20] or single precursor containing Fe, N and C [21]. In comparison, single precursor derived Fe/N/C catalysts exhibited superior ORR performance due to higher nitrogen contents and desirable Fe coordination [21,22]. Moreover, it is easy to design the textural structure and tailor preferential porosity for desired performance in MFC operation. Here in this paper, a facile synthesis strategy to prepare efficient Fe/N/C catalyst for MFCs cathode was proposed. Melamine resin encapsulating biomass derived from root tuber of giantarum, which has a relative high mass content of amino acid, was utilized as the precursor for nitrogen and carbon. Ammonia ferric citrate (AFC) was used as the precursor of Fe and Carbon. The Fe/N/C was synthesized through a simple polycondensation between methylolmelamines and AFC, and a subsequent pyrolysis process. The crosslinked polymer can result in moderate porosity after pyrolysis process, and the catalyst migration is unlikely to happen due to the retained cross-link structure. Fe and N species dopants offer highly active components for ORR. The encapsulated biomass particle acts as skeleton to prevent agglomeration of catalyst particles. These preferential properties are expected to significantly enhance the MFC performance.

Material and methods Catalyst synthesis Giantarum root was grinded and pyrolyzed at 800  C under N2 atmosphere for 2 h. The obtained Giantarum carbon (GC) was grinded to fine powder and sieved to sizes less than 30 mm. Then the GC powder of 0.25 g was added into 5 mL of formaldehyde solution (37e40%, AR) and ultrasonicated for 20 min. Melamine (99%, AR) of 2.5 g was then added into the above solution with constant stirring at 60  C for 20 min to form uniform mixture. Subsequently, formaldehyde solution of 3 mL containing ammonia ferric citrate (C6H11FeNO7, AFC, CP) with different concentrations (30 g/L, 50 g/L, 100 g/L) was mixed with the above solution respectively and stirred for 3 min. The complex solution was then added with 0.2 mL of acetic acid (99.5%, AR) and constantly stirred at 60  C for another 7 min to form Fe and N containing cross-linked carbon precursor (Fe-N-CP). The precursor was then dried in a

Please cite this article as: Wang D et al., Fe and N co-doped carbon derived from melamine resin capsuled biomass as efficient oxygen reduction catalyst for air-cathode microbial fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.11.201

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vacuum oven at 50  C overnight. The as-prepared precursors were labeled as Fe-N-CP-30, Fe-N-CP-50, Fe-N-CP-100 named after the concentration of the AFC added, respectively. Fe/N/C products were obtained by pyrolyzing the precursors at 900  C for 120 min with a heating rate of 1  C per minute under nitrogen atmosphere. The as prepared Fe/N/C products were grinded to fine powder. Fe/N/C products derived from Fe-NCP-30, Fe-N-CP-50, Fe-N-CP-100 were labeled as Fe/N/C-30, Fe/N/C-50, Fe/N/C-100 respectively. The pyrolyzed product from Fe-N-CP-50 at a temperature of 800  C was donated as Fe/ N/C-50-800. All chemicals were purchased from Sinopharm group Co. Ltd, China except stated otherwise.

Fabrication of cathode A paste-rolling process was utilized for the fabrication of cathode [23]. Typically, Fe/N/C material of 330 mg was mixed with 30 mg of carbon black and then agitated for 5 min. Polyvinylidene fluoride (PVDF, Arkema) of 2 g was mixed with N,NDimethylacetamide (DMAC, 99.5%, AR) of 20 mL and stirred for 8 h at temperature of 70  C. The powder complex was then blended with 1 mL of the above binder solution and stirred for 20 s. The blend was then paste onto one side of a round stainless steel mesh (50  50, type 304) of 11.3 cm2 to form a coating of ~1 mm thickness. The as-prepared cathode was immediately immersed in deionized water for 15 min to induce phase inversion and then dried in fume hood overnight.

MFC configuration and operation Cubic MFCs made from plexiglass with an inner volume of 28 mL was used for the characterization of the catalytic cathode [24,25]. Carbon cloth of 3.8 cm in diameter was sonicated with acetone and ethanol, and then equipped as the anode electrode. The anode and cathode were connected with a titanium wire as the electron collector. All MFCs were inoculated with the effluent of MFCs that operated for over six month. The medium was 1 g/L sodium acetate (AR) in 50 mM phosphate buffer solution (PBS) amended with 12.5 mL L1 minerals and 5 mL L1 vitamins [26]. All MFCs were operated in batch mode with a 1000 U external resistor (except as noted) at 25 ± 1  C. Voltages (U) were recorded across an external resistance (R) every 10 min using a data acquisition system (Keithley 2700, Keithley Inc., USA). Polarization curves were obtained using a single-cycle resistor change method by varying the external resistance from 1000 to 50 U. Each resistor was measured for 20 min to reach pseudo stability [27]. Current densities (J) and power densities (P) were normalized by cathode projected area (A ¼ 7 cm2), using equations J ¼ U/(RA) and P ¼ JU.

Characterization The morphology and microstructure of samples were investigated by scanning electron microscopy (SEM, JSM-7500F, JEOL CO.,Ltd, Japan) and transmission electron microscopy (TEM, Tecnai-G20, FEI, USA). Fourier transformed infrared radiation (FTIR) spectra were collected by a Bruker VERTEX 80

3

FTIR spectrophotometer (Bruker Corporation, Germany) in the range from 4000 to 400 cm1. Nitrogen adsorption/desorption isotherms were obtained on a JW-BK 122 W static nitrogen adsorption apparatus (Beijing JWGB Sci&Tech. Co., Ltd, China). The specific surface area and total pore volume were calculated according to Brunauer-Emmett-Teller method for nitrogen adsorption data in the relative pressure from 0 to 0.99 [28]. Pore size distribution was estimated by the Barret-JoynerHalenda method. X-ray diffraction (XRD) patterns were recorded from 10 to 80 using a Shimadzu diffractometer (Shimadzu, Japan) with Cu radiation sources. Raman spectra were recorded from 1900 to 900 cm1 using an NRS-3100 spectrometer (JASCO Electric CO.,LTD, USA) equipped with an Ar ion laser (wavelength 532.05 nm, 0.3 mW) as the excitation source. X-ray photoelectron spectroscopy (XPS) measurement was performed on an Escalab 250Xi spectrometer (Thermo Fisher Scientific, USA) with Al Ka X-ray radiation as the X-ray source for excitation.

Electrochemical measurements The catalytic activities of ORR catalysts were evaluated by liner sweep voltammetry (LSV) and cyclic voltammetry (CV) measurements in a three-electrode system. A CHI760E workstation (CH Co. Ltd., USA) and a rotating disk electrode (Pine, USA) were used for LSV measurement. A glassy carbon electrode (GCE) with an area of 0.196 cm2 loaded with catalyst was used as the working electrode, a platinum foil and a potassium chloride saturated Ag/AgCl electrode were employed as the counter and reference electrode. The catalyst was applied to the GCE as follows [24]: Fe/N/C catalyst (36 mg) and carbon black (CB, 3.6 mg) were mixed with 0.9 mL of isopropyl alcohol (99.7, AR) and then ultrasonicated for 10 min. Nafion of 0.1 mL (5 wt%, DuPont, USA) was added to the suspensions and mixed with ultrasonication for another 10 min. The catalyst ink (6 mL) was then drop coated onto the 0.5 cm diameter glassy carbon electrode and allowed to dry in the air overnight, leading to a sample loading of 1.2 mg cm2. The cyclic voltammetry (CV) measurements were conducted in O2 or N2-saturated PBS (50 mM) with a scan rate of 20 mV s1 with a VSP 300 electrochemical workstation (Biologic Inc., France). The working electrode was glass carbon electrode with a diameter of 3 mm with catalyst loading of 1.2 mg cm2. The reference and counter electrode were the same with LSV test. The chronoamperometry (CA) measurement was conducted with applied potential of 0 V vs. Ag/AgCl in O2-saturated PBS (50 mM) electrolyte. IR-correction was conducted for LSV and CV measurements to compensate potential drop resulted by resistance between the working and reference electrode. Here, 85% iR-compensation was applied for LSV tests and 90% compensation for CV tests. The overpotential can be calculated according to equation E ¼ E0 þ DE þ iR, where E0 represents the theoretical reduction potential of oxygen (1.23 V vs RHE), and DE represents the overpotential for ORR. The onset potential (E) is generally calculated as the potential at which the reduction current exceeds a threshold value (0.1 mA cm2). Potentials conversion between potassium chloride saturated Ag/AgCl and reversible hydrogen electrode (RHE) can be calculated as E vs. RHE ¼ E vs. Ag/AgCl þ 0.197 þ 0.0592 pH.

Please cite this article as: Wang D et al., Fe and N co-doped carbon derived from melamine resin capsuled biomass as efficient oxygen reduction catalyst for air-cathode microbial fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.11.201

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Electrochemical impedance spectroscopy (EIS) experiments of the cathode were conducted with initial open circuit voltage over frequency of 10, 000 Hze0.01 Hz with sinusoidal perturbation of 10 mV amplitude. EIS data were analyzed by fitting the spectra to an equivalent circuit model. The RRDE tests were adopted to measure the electron transfer number of catalysts for ORR. The n value can be calculated from ring current (Ir ) on Pt ring and disk current (Id ) on GCE disk electrodes using the following equation (1), and the collection efficiency (N) is 37%. n¼

4½Id  ½Id  þ Ir =N

(1)

Results and discussion Composition and chemical properties of Fe/N/C catalyst The fabrication process of Fe/N/C was schematically illustrated in Fig. 1. The polymerization process consisted of three steps: prepolymerization of hydroxymethylation followed by polymerization of methylolmelamines and finally acid catalyzed polycondensation. The GC powder was uniformly dispersed in methylolmelamines resulted from hydroxymethylation of the melamine and formaldehyde molecules. Then the introduction of AFC allowed further polymerization of methylolmelamines and AFC (esterification between carboxyl groups eCOO and eCH2OH), which was confirmed by the FTIR spectra of the Fe-N-CP precursor (Fig. S1). The peaks at 1552 cm1 and 1200 cm1 can be ascribed to stretch vibration of C]O and OeCeO in the ester groups, respectively, confirming the polycondensation reaction between methylomelamines and AFC, which extended the polymeric chain and created tight coordination of Fe. The final addition of acetic acid accelerated the condensation reaction finished in few minutes to form bulk precursor with GC particles uniformly dispersed (Fig. 2a). The 3D monolith of Fe-N-CP precursor (Fig. 2a) shrunk and converted to fluffy and porous structure after heat treatment at 900  C (Fig. 2b), as a great amount of N, C, O elements in the polymerized precursor was evaporated and left pores during

evaporation the pyrolysis process, while the Fe element was retained [29]. SEM image shows the Fe/N/C-50 comprised of irregular flakes grinded from the monolith framework (Fig. 2). TEM images shows the flakes were loosely linked graphene structure decorated with quadrangular particles ranging from 10s to 100 nm (Fig. 2def). The exposed lattice fringe of these particles was 0.46 nm (Fig. 2f). The loose and porous graphene structure, which converted from the condensed polymer during high temperature pyrolysis, constituted the main structure of Fe/N/C-50 catalyst. The morphology of carbon in the Fe/N/Cs was directly related to the amount of AFC addition and the pyrolysis temperature (Fig. 3a and b). XRD patterns showed that the peak at 26 for all Fe/N/C catalysts represents the [002] lattice face of graphite (Fig. 3a), revealing the formation of graphitic structure in all prepared catalysts with different synthetic parameters, which was confirmed with Raman spectroscopy that All Fe/N/Cs exhibited smaller ID/IG value than GC powder (Fig. 3b). The peak at 26 on Fig. 3a became weakened gradually from Fe/N/C-30 to Fe/N/C-100 as the amount of AFC increased in precursors, indicating that higher mass ratio of AFC led to lower graphitization degree. The pyrolysis temperature also affected the degree of graphitization. The degree of graphitization achieved at 800  C higher than that at 900  C (Fig. 3a) is inconsistent with many reports for carbon pyrolysis [30e32]. This may be ascribed to the escape of sulphide contained in the GC powder at higher temperature, leaving holes and cracks that deteriorated the degree of graphitization [33]. This can be proved by XPS spectra which showed sulfur residue in Fe/N/C-50-800 sample but no sulfur on Fe/N/C-50 (Fig. S2). The surface mass percentages of Fe in Fe/N/C-30, Fe/N/C50 and Fe/N/C-100 measured from XPS survey spectra were 0.09%, 0.08% and 0.11% respectively (Table 1). XRD patterns exhibited diverse combination states of Fe in the Fe/N/C samples (Fig. 3a). Fe/N/C-100 was mainly comprised of Fe3O4 according to diffraction peaks at 30.5 , 35.9 , 43.6 , 57.6 , and 63.2 . Fe/N/C-50 exhibited two peaks located at about 40.9 and 48.1 , relating respectively to [001] and [131] plane of Fe3C, which matches well with the exposed lattice space of 0.46 nm for [001] plane of Fe3C as presented on HRTEM

Fig. 1 e Illustration diagram of reaction process for FeeNeC precursor. Please cite this article as: Wang D et al., Fe and N co-doped carbon derived from melamine resin capsuled biomass as efficient oxygen reduction catalyst for air-cathode microbial fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.11.201

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Fig. 2 e Optics images of (a) Fe-N-CP-50 precursor and (b) Fe/N/C-50; SEM image of Fe/N/C-50. (d) (e) (f) TEM images of Fe/N/C50 with different magnifications.

Fig. 3 e (a) XRD patterns and (b) Raman spectra for Fe/N/Cs.

(Fig. 2f). Fe/N/C-30 exhibited a weak peak at 43.6 related to (111) lattice face of zero valent Fe. It was obvious the characteristic peak for Fe increased with more AFC in the precursor. No diffraction peak related to Fe was observed for Fe/ N/C-50-800, implying the temperature of 800  C was not sufficient for Fe crystallization.

The pyrolysis temperature had significant influence on nitrogen content and speciation in Fe/N/C samples. The high resolution N1s spectra in Fig. 4 showed four types of nitrogen groups on Fe/N/Cs: pyridinic-N (398.3 ± 0.1 eV), pyrrolic-N (400.3 ± 0.1 eV), graphitic-N (401.0 ± 0.1 eV) and oxidized-N (404 ± 0.2 eV) [34,35]. It can be found that with the

Table 1 e Elemental contents and proportions of N species in synthesized samples determined by XPS measurement. Sample

Fe/N/C-30 Fe/N/C-50 Fe/N/C-100 Fe/N/C-50-800

Element content (at. %) O

C

Fe

N-Total

Pyridinic-N

Pyrrolic-N

Graphitic-N

Oxidized-N

4.62 5.14 6.08 6.07

91.03 92.52 90.22 79.46

0.09 0.08 0.11 0.20

4.26 2.26 3.59 14.27

0.98 0.52 1.13 5.71

0.14 0.11 0.14 7.71

2.77 1.42 1.92 0.18

0.40 0.21 0.39 0.55

Please cite this article as: Wang D et al., Fe and N co-doped carbon derived from melamine resin capsuled biomass as efficient oxygen reduction catalyst for air-cathode microbial fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.11.201

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Fig. 4 e High resolution N 1s XPS spectra of (a) Fe/N/C-30 (b) Fe/N/C-50 (c) Fe/N/C-100 and (d) Fe/N/C-50-800.

temperature increasing from 800 to 900  C, the total nitrogen decreased from 14.27% to 2.26% (Fe/N/C-50-800 vs Fe/N/C-50). Pyridinic and pyrrolic N were predominant at 800  C, whereas graphitic and pyridinic N were predominant at higher temperature, indicating the conversion of pyrrolic N to graphitic N as the temperature increased. The proportions of nitrogen species in the Fe/N/Cs are listed in Table 1. The graphitic and pyridinic N could be critical to enhance ORR performance. The graphitic N promotes electron transport and enhances the conductivity of carbon materials [36]. It is also reported that the graphitic N is stable in some extreme conditions and would help electron transfer from adjacent C electronic bands to oxygen anti-bonding orbitals, leading to higher ORR activity [37,38]. Pyridinic N could greatly enhance ORR performance. According to Lewis acid-base theory, the reduction of oxygen on the catalyst could be hypothesized as a series of Lewis acidbase reaction, which initiates with adsorption of Lewis acid of oxygen on the Lewis base of carbon atom. The carbon atom could act as Lewis base by dopant of pyridinic nitrogen, which alters localized density of states in the occupied region near the Fermi level of adjacent carbon atom [39]. It could be assumed that the coordination of moderate content of graphitic and pyridinic nitrogen lead to excellent ORR performance.

Textural properties The BET surface area, pore volume and average pore size of Fe/ N/Cs and GC were analyzed and listed in Table 2. Fe/N/C-50 possessed the highest BET surface area and pore volume. The specific surface areas were 71.6, 385.3, 34.5 m2 g1 for Fe/

N/C-30, Fe/N/C-50 and Fe/N/C-100 respectively, indicating too much AFC addition decreased the surface area of the final catalyst. The Fe/N/Cs synthesized with pyrolysis temperature of 900  C exhibited typical type IV isotherm (Fig. 5a). The hysteresis loop at relative pressure of around 0.5 suggested the existence of meso-pores in the materials. However, Fe/N/ C-50-800 exhibited type III isotherm, similar to that of GC, which represented weak adsorption. The pore volume were 0.12, 0.298 and 0.092 cm3 g1 for Fe/N/C-30, Fe/N/C-50 and Fe/ N/C-100 respectively, suggesting the addition of 50 mg AFC resulted bigger pore volume than that of 30 mg, and further increase of AFC addition decreased the pore volume. The poor pore volume of Fe/N/C-100 may be caused by the pore occupation of massive Fe3O4 particle, which accumulated in the small and medium sized pore and consequently reduced the BET surface area. The most probable pore diameters of Fe/N/ C-50 and Fe/N/C-100 were 1.9 and 3.5 nm (Table 2), respectively, which were the smallest and largest among all Fe/N/Cs

Table 2 e Textural properties of synthesized Fe/N/C samples.

BET Surface Area (m2 g1) Average pore diameter (nm) BJH cumulative volume of pores (cm3 g1) Most probable pore diameter (nm)

Fe/N/ C-30

Fe/N/ C-50

Fe/N/ C-100

Fe/N/C50-800

71.6 6.63

385.3 3.09

34.5 10.61

224.40 2.53

0.12

0.298

0.092

0.142

2.15

1.9

3.5

2.05

Please cite this article as: Wang D et al., Fe and N co-doped carbon derived from melamine resin capsuled biomass as efficient oxygen reduction catalyst for air-cathode microbial fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.11.201

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Fig. 5 e (a) Nitrogen adsorption-desorption isotherms of Fe/N/Cs. (b) Pore distribution of Fe/N/Cs.

(Fig. 5b). The average pore size of Fe/N/C-50 is 3.09 nm, also smaller than both Fe/N/C-100 and Fe/N/C-30 (Table 2), suggesting that Fe/N/C-50 has more pores for mass transport and migration. The BET surface area and total pore volume of Fe/ N/C-50 were not so large compared to some reported porous ORR catalysts (e.g ~1100 m2 g1 for BET surface area, ~0.6 cm3 g1 for pore volume) [40,41]. Undoubtedly the welldeveloped porous structure is conducive to catalytic performance. However, it may not be beneficial to performance stability if the porosity beyond the flooding point. Indeed there has report on fast degradation of power generation for MFC adopting porous cathode [20].

Catalytic activity The catalytic activity toward ORR was firstly examined by CV experiments in O2 or N2 saturated PBS solution at room temperature. Fe/N/C-50 showed a reduction peak at 0.557 V in oxygen saturated solution, which is more positive than AC with a pseudo reduction peak at 0.412 V (Fig. 6a and b), implying superior ORR activity of Fe/N/C-50. Fe/N/C-100 exhibited a similar reduction curve than Fe/N/C-50 with a reduction peak at 0.584 V, while the Fe/N/C-30 and Fe/N/C-50800 showed no reduction peak (Fig. 6a). The LSV tests were executed to eliminate diffusion effect for ORR reaction. Fe/N/ C-50 exhibited higher reduction current than AC at all potentials. The onset potentials were 0.878 and 0.671 mV for Fe/ N/C-50 and AC respectively (Fig. 6c and d), indicating large positive shift of onset potential compared to AC. The Fe/N/C100 exhibited lower reduction current than Fe/N/C-50, but showed larger reduction current than AC at potential range of 0.8e0.3 V. The LSV curve for ORR activity of Fe/N/C-50 and AC were also conducted in 0.1 M KOH solution (Fig. S5). The onset potential for Fe/N/C-50 was 0.896 V, very similar to that of Pt (0.9 V vs RHE) [42] The average electron transfer number (n) of Fe/N/C-30, Fe/N/C-50, Fe/N/C-100, Fe/N/C-50-800 and AC in pH neutral PBS were 2.87, 3.96, 2.95, 3.96 and 3.87 respectively according to ring-disk polarization curves (Fig. 6e and f). It is speculated that the physical property of catalyst influences the electron transfer number from RRDE experiment. The mechanism behind this influence is the probability variation of H2O2 diffusing away from catalyst layer. The ring current depends on how much H2O2 produced at the disk electrode can diffuse to the ring electrode. For the case of

catalyst with well-developed porous structure, H2O2 produced on catalyst layer may diffuse throughout the disk electrode prior to its release in the bulk electrolyte and subsequent detected by outer ring electrode. Along the internal diffusion path, H2O2 may further react on catalytic sites, either to produce water (H2O2 þ 2Hþ þ 2ee / 2H2O) or produce oxygen (H2O2 / 2H2O þ O2). Oxygen molecule released by the disproportionation reaction may then again be reduced into H2O2. So all H2O2 produced in the bulk of disk electrode can be converted to water, resulting in 4 ee path for ORR. However, for the case of catalyst with less porous structure, the diffusion of H2O2 molecules along the catalyst layer is more difficult, so they have higher probability of diffusing into the bulk electrolyte than being reduced to water at internal active sites. We believe the scenario is similar to that of different loading on disk electrode [43]. The CV curve after 1000 scanning circles was close to that of the initial CV for Fe/N/C-50, implying good durability of the catalyst in PBS (Fig. S4). The existence of Fe element has been claimed have positive influence on the catalytic activity of some Fe/N doped carbon materials [13,44e47]. Theoretical calculations (e.g, density function theory, frontier molecular orbital theory) have been used to speculate how the Fe dopant influences the ORR. Based on theoretical calculations, the active site of Lewis basic Fe2þ-N and Fe/Fe3C nanoparticles can promote ORR. These effects have been proved in some experiments [13,48e50]. Here, it is worth studying how the Fe3C crystal influenced the ORR performance of Fe/N/C-50. The ORR catalytic performance was significantly reduced upon the removal of Fe with a leaching process, showing the onset and half wave potential shifted negatively about 54 and 87 mV (Fig. 7a and b). This was further proved by deactivating the Fe activate sites with SCN and F [13,51], where the addition of KSCN blocked the Fe3þ active site and induced a negative shift of the onset potential by 20 mV (Fig. 7c) and a decrease of current density from 0.28 mA cm2 to 0.26 mA cm2 (Fig. 7d). Further addition of NaF decreased the current density to 0.24 mA cm2 by the chelating reaction of F with Fe3þ. Experiment detail for this section can be seen in Supporting Information.

Performance of MFCs with Fe/N/C cathodes The maximum power density (MPD) of Fe/N/C-50 cathode MFC (1166 mW m2) was superior to Fe/N/C-30 (575 mW m2)， Fe/

Please cite this article as: Wang D et al., Fe and N co-doped carbon derived from melamine resin capsuled biomass as efficient oxygen reduction catalyst for air-cathode microbial fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.11.201

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Fig. 6 e (a) CV curves of Fe/N/Cs in O2 saturated 50 mM PBS. (b) CVs of Fe/N/C-50 and AC modified GCE in 50 mM PBS saturated with O2 or N2. (c) ORR LSV curves of AC, Fe/N/C-50 and Fe/N/C-100 in 50 mM PBS with a disk rotating rate of 1600 rpm. (d) ORR Tafel plots derived from (c). (e) Ring-disk test of Fe/N/Cs and AC catalysts. (f) Electron transfer numbers for Fe/N/Cs and AC calculated from (e).

N/C-100 (442 mW m2) and Fe/N/C-50-800 (574 mW m2) (Fig. 8a), which was mainly attributed to the variations of cathode performance as exhibited on polarization curves (Fig. 8b). It should be noted although Fe/N/C-100 showed little inferiority to Fe/N/C-50 toward ORR catalysis (Fig. 6a,c), the MPD of the latter was more than 2-folds higher. This can probably be ascribed to the difference of textural property, as poor BET surface area and pore volume hindered mass diffusion and transport to the active sites. Moreover, the charge transfer resistance (Rct) of Fe/N/C-100 cathode was much higher than that of Fe/N/C-50 (15.45 vs 2.97 U) (Fig. 8c), which indicated slower charge transfer for oxygen reduction on pore wall surface of gas diffusion cathode. The power generation of Fe/N/Cs were compared with activated carbon and carbon black (AC) cathode, which exhibited similar performance to Pt/C air cathode [23]. All Fe/

N/Cs produced higher MPD than AC cathode (478 mW m2) except Fe/N/C-100 (442 mW m2). The MPD of Fe/N/C-50 was 140% higher than AC cathode, implying the proposed catalyst as an excellent ORR cathode. Here some recently reported MFC applications adopting Fe/N co-doping carbon based cathodes and similar reactor configuration were listed (Table 3). The power generation for Fe/N/C-50 cathode in this study was lower than Fe/N-doped hollow carbon nanospheres cathode that derived from aniline-co-pyrrole copolymer [52] and Fe/NeC gained from aminobenzimidazole/benzimidazole precursor with relative tedious process [20], but higher than those in other studies. In addition, Fe/N/C-50 cathode showed superior stability in multi-cycle operation. The plateau cell voltage of Fe/N/C-50 cathode MFC kept at 570 ± 5 mV for 30 consecutive cycles (Fig. 8d), and the MPD only decreased by 7% after 60- day operation‒a degree lower than some porous

Please cite this article as: Wang D et al., Fe and N co-doped carbon derived from melamine resin capsuled biomass as efficient oxygen reduction catalyst for air-cathode microbial fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.11.201

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Fig. 7 e (a) XRD patterns of Fe/N/C-50 before and after acid leaching. (b) CV curves in oxygen saturated PBS of Fe/N/C-50 before and after acid leaching (c) CV curves of Fe/N/C-50 in O2 saturated PBS before and after addition of 1 mM KSCN. (d) Chronoamperometry test of Fe/N/C-50 in O2 saturated PBS with successive addition of 1 mM KSCN and 1 mM NaF.

Fig. 8 e (a) Power densities and (b) anode and cathode polarization curves of MFCs. (c) EIS Nyquist plots for MFC cathodes measured at initial open circuit potential with a sinusoidal perturbation of 10 mV amplitude. (d) Plateau voltages and MPD of Fe/N/C-50 cathode MFC during 60 day operation. Please cite this article as: Wang D et al., Fe and N co-doped carbon derived from melamine resin capsuled biomass as efficient oxygen reduction catalyst for air-cathode microbial fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.11.201

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Table 3 e Comparison of this work with other recently reported Fe and N co-doped carbon as cathode catalyst in MFCs. Catalyst FePc/PID/CNTs NFe/CNSb FeeNeC FeeN/AC Fe/NeC FeeNeCc Fe/NeC Fe/N/C-50 a b c

d

a

Precursor/Source

Anode

FePc, PID Carbon black, melamine Iron aminoantipyrine (FePc)-coated AC Xerogel BZIM/ABZIMd Anilineecoepyrrole Melamine resin/biomass

Graphite fiber brush Carbon brush carbon veil fiber Carbon brush Graphite rods Graphite fiber brush Graphite fiber brush Carbon cloth

MFC configuration Air cathode Air cathode Air cathode Air cathode DCMFC Air cathode Air cathode Air cathode

SCMFC SCMFC flow DCMFC DCMFC SCMFC SCMFC SCMFC

MPD (mW m2)

Ref

799 ± 41 866.5 ± 7 1060 1092 207 1620 ± 30 1300 ± 64 1166

[57] [19] [58] [59] [60] [20] [61] This work

Polyindole (PID) and iron phthalocyanine (FePc) on carbon nanotubes (CNTs)/carbon Vulcan. N and Fe co-doped porous carbon nanospheres. Maximum power density (MPD) of was 62% higher than AC, this number is lower than 140% (Fe/N/C-50 vs AC) gained in this paper. MPD of FeeNeC decreased by ~40% after 20 days operation. Benzimidazole and Aminobenzimidazole.

catalysts [20,53]and much lower than Pt/C [23]. The performance degradation of cathode can be mainly ascribed to catalyst agglomeration, collapse and water-flooding. It has been reported that nano and near nanoscale particles are prone to agglomeration (eg., Pt/C) [54]. The Fe/N/C-50 catalyst is unlikely to aggregate owning to its well-developed crosslink structure and relative larger particle size. Catalyst collapse results in deteriorated connectivity of the solid phase, which is inevitable in fuel cell operation [15]. Here it can be alleviated by encapsulating biomass particles in the cross-link network of Fe/N/C catalyst. The particle encapsulated network is also beneficial to prevent catalyst agglomeration [55]. The well-developed pore distribution provides adequate active sites for electrochemical reaction but can lead to waterflooding [16]. However it is probably unlikely to occur on the moderate porous Fe/N/C-50 catalyst. Contact angle measurement for pristine and used Fe/N/C50 as well as the counterpart AC cathodes (Fig. 9) showed that Fe/N/C-50 cathode suffered less water-flooding than AC.

The contact angle for pristine Fe/N/C-50 cathode was higher than pristine AC (113.41 vs 94.7 ), indicating a more hydrophobic characteristic of pristine Fe/N/C-50 cathode. The increase of water contact angle can prevent flooding in gas diffusion layer as it controls wetting and further water transport [56]. The Fe/N/C-50 cathode exhibited minor decrease of contact angle from 113.41 to 90.76 after 60-day operation, indicating excellent stability. Whereas the AC cathode showed sharp decrease of contact angle from 94.7 to 30.64 after 60-day operation, which means it became hydrophilic and may have lost the ability to prevent flooding. It was noticed that Fe/N/C-30 and Fe/N/C-100 cathode MFCs suffered from power overshoot at relative low current densities (Fig. 8a), which negatively affected the power generations. Similar power overshoot have been reported for polarization tests with different MFC systems [27,62e64]. Previous work showed that it was low redox activity of exoelectrogens and insufficient anode electron capacitance that led to power overshoot [63]. In this work, the anodes were

Fig. 9 e Contact angle for Fe/N/C-50 and AC cathodes before and after 60-day operation. Please cite this article as: Wang D et al., Fe and N co-doped carbon derived from melamine resin capsuled biomass as efficient oxygen reduction catalyst for air-cathode microbial fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.11.201

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identical carbon clothes with very low capacitance, which may lead to power overshoot in polarization test. The occurrence and elimination of power overshoot on Fe/N/Cs may be ascribed to difference in cathode capacitances. The improved performance of Fe/N/C-50 catalyst can be attributed to several unique properties. Firstly, the cross-link FeeNeC-50 precursor converted into loose structure during pyrolysis, which provided moderate porosity. This facilitated the interfacial contact of reactants and mass transfer and benefited for long term stability as the porosity was probably far from the flooding point. Secondly, the Fe source improved graphitization during pyrolysis, and thus increased the conductivity of the catalyst. Moreover, The Fe3C nanoparticles formed at high temperature can catalyze ORR efficiently due to strong affinity of O2 and easy OeO cleavage due to strong FeeC coordination as elaborated by Ma et al. [65]. Thirdly, the moderate content of pyridinic N (0.52%) and graphite N (1.42%) converted at high temperature played critical role for ORR. The pyridinic-N could greatly improve ORR catalysis by forming exposed edge plane, where the adjacent carbon is imparted Lewis basicity due to lone pair of electrons and conjugated p bond of pyridinic nitrogen [66]. The graphite N not only promoted electron transport and enhanced the conductivity, but also induced higher electrochemical characteristics including ORR activity. The Fe/N/C-50-800 had much higher pyridinic N content than Fe/N/C-50 (5.71 vs 0.52%). However, its graphitic N content is very low (0.18%), leading to relatively higher electron transfer resistance and inferior ORR activity. What’s more, the low Rct for Fe/N/C-50 facilitated fast electron transfer for oxygen reduction. Lastly, the GC particle not only prevented the catalyst particles from agglomerating but also provided active sites, which enhanced the ORR performance.

Conclusions In summary, Fe/N/C was prepared by pyrolyzing biomass capsuling polymer, where the melamine resin was used as carbon and nitrogen source, the ACF was used as iron source, and the biomass particle derived from giantarum acted as skeleton in the catalyst framework. The obtained Fe/N/C sample exhibited enhanced activity and excellent stability toward 4 ee ORR. The pyridinic and graphitic nitrogen dopant and iron-carbon coordination (Fe3C) improved the ORR activity. The moderate porosity enabled fast mass transport to the triple-phase boundary, and prevented flooding which may occur on catalyst with porous structure. The biomass particles capsuled in the catalyst framework can alleviate structure collapse and prevent agglomeration of the catalyst. The Fe/N/ C-50 cathode MFC produced 140% higher power density than commercial AC. This study for preparing biomass particle encapsulated Fe/N/C catalyst provides a cost-effective and green route to produce noble-metal-free ORR catalyst for MFCs.

Acknowledgement This work is supported by General program of Natural Science Foundation of Hubei Province (2016CFB538 and 2016CFB539),

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Natural Science Foundation of China (21607046, 51508213, 51608217 and 31700511), and Independent Innovation Foundation of HUSTeExploration Fund (2016YXMS288, 2016YXMS291 and 2017KFYXJJ217). The authors would like to thank the Analytical and Testing Center of Huazhong University of Science and Technology for providing the facilities to conduct the characterization work.

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.11.201.

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Please cite this article as: Wang D et al., Fe and N co-doped carbon derived from melamine resin capsuled biomass as efficient oxygen reduction catalyst for air-cathode microbial fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.11.201