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Biomass–Derived Porous Fe3C/WC/GC Nanocomposite for Efficient Electrocatalysis of Oxygen Reduction Ming Ma, Shijie You, Wei Wang, Guoshuai Liu, Dianpeng Qi, Xiaodong Chen, Jiuhui Qu, and Nanqi Ren ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10804 • Publication Date (Web): 08 Nov 2016 Downloaded from http://pubs.acs.org on November 10, 2016

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ACS Applied Materials & Interfaces

1

Biomass–Derived Porous Fe3C/WC/GC Nanocomposite for Efficient

2

Electrocatalysis of Oxygen Reduction

3 4

Ming Maa, Shijie Youa,*, Wei Wanga, Guoshuai Liua, Dianpeng Qib, Xiaodong Chenb, Jiuhui Quc,

5

and Nanqi Rena

6 7

a

8

73 Huanghe Road, Nangang District, Harbin, 150090, P. R. China

State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology,

9 10

b

11

Avenue, 639798, Singapore

School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang

12 13

c

14

Eco–Environmental Sciences, Chinese Academy of Sciences, Beijing, 200085, P. R. China

State

Key

Laboratory

of

Environmental

Aquatic

Chemistry,

Research

Center

for

15 16

*Corresponding author: E–mail: [email protected]; phone: (+86) 451 86282008; fax: (+86) 451

17

86282110

18 19 20 21 22 23 24 25 26 27 28 29 30 ACS Paragon Plus 1 Environment


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1

ABSTRACT

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Oxygen reduction reaction (ORR) draws an extensive attention in many applications and there is a

3

growing interest to develop effective ORR electrocatalysts. Iron carbide (Fe3C) is a promising

4

alternative to noble–metals (e. g. platinum), but its performances need further improvement and the

5

real role of Fe3C phase remains unclear. In this study, we synthesize Fe3C/WC/GC nanocomposite

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with waste biomass (i. e. pomelo peel) serving as carbon source using a facile, one–step

7

carbon–thermal reduction method. The nanocomposite is characterized by porous structure

8

consisting of uniform Fe3C nanoparticles encased by graphitic carbon (GC) layers with highly

9

dispersed nano–sized WC. The Fe3C provides the active sites for ORR, while the graphitic layers

10

and WC nanoparticles can stibilize the Fe3C surface, preventing it from dissociation in the

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electrolyte. The Fe3C/WC/GC nanocomposite is highly active, selective and stable toward

12

four–electron ORR in pH–neutral electrolyte, which results in 67.82% higher power density than

13

commercial Pt/C, and negligible voltage decay during a long-term phase of 33–cycle (2200 h)

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operation of microbial fuel cell (MFC). The density–functional–theory (DFT) calculations suggest

15

high activity for splitting O–O bond of molecular oxygen on the surface of Fe3C.

16 17

Keywords: biomass, Fe3C/WC/GC nanocomposite, porous structure, oxygen reduction reaction,

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microbial fuel cell, power density, durability

19 20 21 22 23 24 25 26 27 2

ACS Paragon Plus Environment

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INTRODUCTION

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The electrocatalysts for oxygen reduction reaction (ORR) are crucial to energy conversion efficiency

3

and overall cost of low–temperature fuel cell technologies.1 Despite great advancement made in

4

noble–metal electrocatalysts (e. g. Pt, Pd, Ru) during the past decades, the scale–up applications may

5

be still limited by high cost, scarcity and less stability.2 Within this context, much attention has been

6

attracted to noble–metal–free materials, including carbon–supported transition metal oxide,3 sulfide,4

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nitride,5 carbide,6 metal–free carbon materials,7 nitrogen–doped carbon and transition metal

8

materials.8,9

9 10

A close attention has been paid to Fe3C whose lattice contains carbon atoms located in trigonal

11

prismatic interstices among close–packed iron atoms with space group C (4c) (a = 5.032 Å, b =

12

6.708 Å, c = 4.477 Å). Such crystalline structure leads to high activation of iron atoms on carbide

13

surface,10 making it an efficient, economical and green catalyst toward adsorbing synthetic gas,11,12

14

and reducing hydrogen ions13 or oxygen molecules in many energy conversion applications.14

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Recently, Lee et al. reported the remarkable ORR activity of melamine foam functionalized by

16

Fe/Fe3C in alkaline medium, and Wen et al. found that the nitrogen–doped Fe/Fe3C nanorods were

17

active for ORR in pH–neutral electrolyte.15,16 These findings preliminarily suggest that the

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involvement of Fe3C may constitute an important factor that promotes oxygen reduction for these

19

catalysts. However, the ORR mechanism for Fe3C remains ambiguous thus far, due to difficulty of

20

decoupling the contribution of Fe3C, Fex–N and Nx–C in carbon–supported transition–metal

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nitrogen–doped composites. Besides, chemical instability of the Fe3C phase may cause the

22

transformation to α–Fe phase and/or Fe3O4 during preparation and dissolution in the electrolyte

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during application.17,18

24 25

In addition to catalyst itself, building up multi–scale porosity of support is also important to enable

26

efficient mass–transfer of three–phase interface where ORR takes place.19,20 Waste biomass like corn

27

stalks,21 pomelo peel,22 coir23 has gained a growing popularity in fabrication of carbon materials by 3



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virtue of its unique naturally–occurring porous structure, good accessibility and sustainability.

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However, the carbon materials obtained via direct thermal pyrolysis of biomass generally catalyze

3

the ORR through two–electron route to form peroxide species whose ongoing reduction (i. e.

4

four–electron ORR) needs further catalysis by metallic components.24

5 6

Based on the above considerations, we propose a facile, one–step carbon–thermal reduction method

7

for in–situ synthesis of Fe3C–based WC–assisted nanocomposites, i. e. Fe3C/WC/GC, by using

8

sustainable waste biomass (pomelo peel, PP) as carbon source. Incorporating iron into carbon matrix

9

can promote the graphitization of amorphous carbon to form Fe3C.25 The tungsten carbide (WC) is

10

selected specifically because the intimate interactions between iron and tungsten element can form

11

well dispersed and strongly embedded carbonized framework during the pyrolysis.17 The carbon

12

atoms seizing the interspaces of the close–packed iron atoms and tungsten atoms can mitigate

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nanoparticle accumulation and pore blockage. Besides, the carbonization–driven formed tungsten

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carbide (WC) has similar crystalline and electronic structure as platinum, making it possible assist

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the ORR via synergistic mechanism. Moreover, the use of porous PP carbon support can provide

16

high specific surface area and multi–scale porous structure for efficient mass transfer and reaction.

17

These unique properties will be expected to considerably enhance the ORR performances in

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electrochemical and bioelectrochemical systems. Moreover, the density–functional–theory (DFT)

19

calculations confirm the high activity of Fe3C for splitting O–O bond of molecular oxygen on its

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surface (001), i. e. the limiting step of ORR process, which provides a theoretical evidence for high

21

ORR efficiency of the Fe3C/WC/GC nanocomposite.

22 23

EXPERIMENTAL SECTION

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Materials preparation. The sponge–like part of pomelo peels were immersed into the mixed

25

solutions of K4Fe(CN)6 and Na2WO4 to form a PP–[Fe(CN)6]4-–WO42- precursor with a mass ratio of

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1:0.5:0.5. Then it was carbonized at 600−1100 °C for 1.5 h with a heating rate of 5 °C min−1 under a

27

highly pure N2 flow (50−60 mL min−1). Finally, the black samples were ground into powder and 4


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washed with deionized water three times. Similarly, the contrast samples (Fe3C/GC, WC/GC, and

2

GC) were prepared in the same way, and treated with diluted HCl solution to remove α–Fe or other

3

Fe species.

4 5

Materials characterization. X−ray diffraction (XRD) analysis was conducted on an X−ray

6

diffractometer (Bruke D8 Adv., Germany) using Cu K α radiation (λ = 0.15406 nm) at a power of 40

7

keV3 × 30 mA. Raman spectra were collected using a WITEC CRM200 Raman system equipped

8

with 532 nm laser source. X−ray photoelectron spectrometer (XPS) was carried out on a PH1−5700

9

ESCA system. Thermogravimetry (TG) and differential scanning calorimetry (DSC) experiments

10

were carried out with a NEJSCH STA 449 Instrument. The specific surface area of the materials was

11

calculated by the Brunauer−Emmett−Teller (BET) theory. The pore size distribution was computed

12

by using the Barrett−Joyner−Halenda (BJH) and Density−Functional−Theory (DFT) method from

13

the adsorption branch of the isotherm. Scanning electron microscopy (SEM) images were obtained

14

using a field emission scanning electron microscope (Guanta 200F, FEI, U.S.). The transmission

15

electron microscopy (TEM) and high−resolution TEM (HRTEM) were performed on F−30ST

16

(Tecnai, FEI, US) using high−resolution imaging.

17 18

Electrochemical characterization. The electrochemical properties were carried out using dual

19

working electrode PARSTAT (CHI750D, Chenhua Co. Ltd., China) electrochemical system at room

20

temperature (25 °C). All the experiments were conducted in 50 mM phosphate buffer solution (PBS,

21

pH = 7.3) saturated with gaseous O2. The conventional three−electrode electrochemical cell

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comprised a glassy−carbon (GC) working electrode (4 mm diameter, 0.126 cm2), saturated calomel

23

electrode (SCE, +0.242 V vs. SHE, 25 °C) and a platinum sheet counter electrode (1 cm2) as

24

reported.26 To prepare the catalyst−loaded working electrode, the catalyst was dispersed in a mixture

25

of 5 wt% Nafion solution and isopropyl alcolhol (volume ratio = 1:4) by sonication. 2 mg mL−1 of

26

the catalyst dispersion (10.00 μL) was transferred onto the GC electrode and the loading was

27

calculated to be 0.159 mg cm−2. The RDE, RRDE tests, the Koutecky−Levich equation and 5



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calculating parameters derived from the work reported before.27,28

2 3

MFCs experiment. The gas diffusion layers (GDLs) were prepared by rolling carbon black with

4

poly (tetrafluoroethylene) (PTFE) (60 wt%) with a mass ratio of 7:3 as repored.19 The Fe3C/WC/GC

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series electrocatalysts were dispersed with PTFE with a mass ratio of 6:1. The mixed catalyst ink

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was sonicated for at least 10 min, and the resulting material were rolled onto the stainless steel

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meshes (4 cm × 4 cm) used as the catalyst layers. The single−chamber MFCs reactor (28 mL) was

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made with an anode of carbon brush and an open−to−air cathode (7 cm2) placed on the opposite side

9

of plexiglas tube.

10 11

All the reactors were inoculated by using the effluent from MFCs well−operated in the laboratory,

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and operated at a fixed external circuit resistance (1000 Ω). The substrate solution contained glucose

13

(1 g L−1) and PBS (50 mM), which contained NH4Cl (0.31 g L−1), NaH2PO4·2H2O (3.321 g L−1),

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Na2HPO4·12H2O (10.3174 g L−1), KCl (0.13 g L−1), trace

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mL L−1).22 The MFCs operating temperature was maintained at 30 °C, and the medium was replaced

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with the fuel when the voltage decreased to less than 50 mV, which formed one complete cycle. To

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achieve statistical soundness, at least two replicates were carried out. The data acquisition system

18

(Model 2700, Keithley Instruments, Inc., Cleveland, OH, USA), polarization curves, and related

19

calculations derived from previously reported.29

minerals (12.5 mL L−1), and vitamins (5

20 21

Density functional theory (DFT) calculation. Our calculations were performed under the

22

framework of density functional theory as implemented in the VASP package.30–32 Exchange and

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correlation effects were treated self−consistently with a generalized gradient approximation (GGA)

24

using a Perdew−Burke−Ernzerhof (PBE) exchange−correlation functional.33 Electron−ion

25

interactions were described by the projector augmented plane−wave method (PAW),34,35 and the

26

wave functions were expanded in a plane−wave basis set with an energy cutoff of 400 eV.

27 6


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Spin−polarization calculation was included for iron systems to correctly account for the magnetic

2

properties. The k−points in the two dimensional Brillouin zone were sampled on a 2 × 2 mesh within

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the Monkhorst−Pack scheme for Fe3C(001) and WC(100) slabs modeling.36 The results of total

4

energy and Hellmann−Feynman forces are convergent within 0.1 meV Å-1 and 5 meV Å-1,

5

respectively. To evaluate the energy barrier of O2 dissociation progress, the climbing image nudged

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elastic band method (cNEB) was employed to investigate the saddle points and minimum energy

7

paths from the initial state to the final state.37,38

8 9

RESULTS AND DISCUSSION

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Formation of Fe3C/WC/GC Nanocomposite. The Fe3C/WC/GC nanocomposites were synthesized

11

by

12

highly–porous–structured pomelo peel (PP), serving as carbon source (Scheme S1). The [Fe(CN)6]4-

13

and WO42- were introduced into the skeleton of carbon scaffold through an ion–exchange process,

14

forming PP–[Fe(CN)6]4-–WO42- precursor. The [Fe(CN)6]4- acts as iron source and catalyst for

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graphitization, simultaneously. During the pyrolysis process at 1000 °C under N2 atmosphere, the PP

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scaffold was converted into conductive graphitic carbon. Meanwhile, the iron and tungsten species

17

were in–situ reduced to Fe3C and WC on the carbon skeletons. In order to investigate the evolution

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of crystalline phase, the samples pyrolyzed at 600–1100 °C were prepared. As is shown in Figure 1a,

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the FeWO4 crystalline formed after being carbonized at 600 °C, indicated by diffraction peaks at 2θ

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= 32.1°, 36.8°, and 44.7°.39 The W2C and WC phase initially emerged at 700 °C and 800 °C,

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respectively. The diffraction peaks at 2θ = 31.5°, 35.6°, and 48.5° are indexed as the (001), (100),

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(101) planes of hexagonal WC phase,40 and the slight peak at 2θ = 35.4° should be ascribed to the

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small amounts of W2C.41 Notably, the peak intensity of WC was observed to be enhanced as the

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temperature increases, suggesting the transformation of FeWO4 → W2C → WC along with the

25

progressive reduction of carbon element. Meanwhile, the Fe3C phase gradually transformed into

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α–Fe, indicated by the peak shift from 42.5° (Fe3C) to 44.5° (α–Fe),42,43 following the evolution of

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FeWO4 → Fe3C → α–Fe. These results were different from the literature reporting the final

using

in-situ

carbon–thermal

reduction

method

with

7


waste

biomass,

i.

e.


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formation of Fe3O4 and W2C driven by non–biomass carbon source.21,44 The characteristic peak at 2θ

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= 26.1° of GC(002) plane corresponds to the graphitization of carbon (Figure 1a).45

3 4

Raman measurements were performed for identifying the transformation of carbon phase. As shown

5

in Figure 1b, two peaks located at 1342.6 cm-1 and 1562.7 cm-1 represent the sp3 defect sites of

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graphite layers (D–band) and the E2g vibrational mode of sp2–bonded pairs (G–band), respectively.46

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As the pyrolysis temperature was increased from 600 °C to 1100 °C, the peak intensity of G–band

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becomes stronger while the D–band becomes weaker. The resultant ratio of IG/ID values are

9

increased from 1.06 to 3.76 (Figure S1). These results reveal the crystalline improvement of ordered

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carbon because higher temperature is favorable to form regular array of hexagonal sheets of carbon

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atoms, which is in good consistence with XRD data and prior studies reported.47 Besides, the D– and

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G–bands were shifted slightly to lower wave–number region, indicating a higher degree of carbon

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curling due to strong interaction between carbon and other particles. The 2D peaks can also be

14

observed at 2642 cm-1, and the intensity of the peak is also enhanced. This confirms the formation of

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multilayer graphitic carbon that protects the active nano–components from expropriating their

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activity during ORR process (Figure 1b).

17 18

The

thermogravimetry

and

differential

scanning

calorimetry

19

PP–[Fe(CN)6]4-–WO42- precursor were shown in Figure 1c. The initial weight loss of 8.8% at stage I

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(0–208 °C) was due to the removal of H2O and CO2 absorbed. The second–stage (208–318 °C)

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weight loss of 21.6% shows the formation of HCN and further removal of H2O and CO2. The abrupt

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weight decrease of 45.3% at about 318 °C of stage III ascribed to the decomposition of carbon

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skeleton precursor, which was accompanied by a wide endothermal peak around 526 °C together

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with the emissions of hydrocarbons gas. The gas formation and loss observed here were inferred

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from the literature where the same iron and tungsten species were used.48,49 Additionally, in

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683–941 °C, the oxygen reduction between iron, tungsten and carbon species could be seen. These

27

results, in combination with XRD data, confirmed the formation of W2C, WC, Fe3C, and α–Fe phase 8


(TG–DSC)

curves

of

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above 700 °C. No obvious weight–loss peak but a significant exothermic peak was found when the

2

temperature was further elevated to 1100 °C, indicating the stripping process of α–Fe from cementite

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at high temperature of 941–1100 °C (stage IV).

4 5

As illustrated in Figure 1d and 1e, the sponge–like part of pomelo peel has plenty of macroporous

6

tunnels made of naturally formed thick carbon skeleton (200 nm). Such porous structure can

7

accumulate abundant iron and tungsten species during the ion–exchange process, so as to facilitate

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the mass transfer and provide more active sites. With the evolution of crystalline phase following

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FeWO4 → Fe3C → α–Fe and W2C → WC, the morphology of nanocomposites changes from

10

short–needle shape to intertwined nanowire, and then to uniform nano–sphere (Figure 1f–1h).

11 12

The Morphology and Composition of Fe3C/WC/GC. As illustrated in Figure 2a, the diffraction

13

peaks at 2θ = 26.1° of GC(002) plane are strong for all the samples due to the graphitization effect

14

induced by iron species, giving high crystallinity and strong conductivity. The relatively weak

15

intensity of GC peak for WC/GC and Fe3C/WC/GC should result from the blockage of tungsten

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species to active sites where iron species catalyze the graphitization of carbon. Following

17

elimination of α–Fe by acid leaching, the peaks located at 2θ = 37.7°, 40.8°, 43.5°, 44.6°, 45.2°,

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46.1°, and 48.1° are indexed to (002), (201), (102), (220), (031), (112), and (131) planes of Fe3C

19

nanoparticles,50 and 2θ = 31.5°, 35.6°, and 48.5° are indexed as the (001), (100), (101) planes of

20

hexagonal WC phase,51 respectively. No additional definable diffraction peaks are detected,

21

indicating the high purity of Fe3C/WC/GC. For the other two samples, W and Fe can be detected

22

with the form of WC and Fe3C for WC/GC and Fe3C/GC. The only exception is the presence of

23

slight Fe3C peaks in GC sample. It further confirms that Fe3C cannot be completely removed by

24

hydrochloric acid, because the protective function of carbon layers and strong interaction between

25

carbon and iron phase.52

26 27

The small–sized and well–dispersed WC and Fe3C particles are spherical with diameters of 9



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approximately 12 nm and 35 nm on average, respectively (Figure 2b and Figure S2). This is also in

2

line with the results obtained from Scherrer equation based on XRD data. The similar darkness

3

nanoparticles were inlaid inside the nanocapsule structure of carbon layers with inner diameter of

4

50–70 nm and the thickness of 5–20 nm (Figure 2b). The WC and Fe3C particles are tightly

5

encircled by the walls of carbon nanocapsules composed of graphitic sheets with the spacing of

6

about 0.34 nm (Figure 2c and 2d) in accordance to the (002) plane of graphitic carbon lattice.45

7

Besides, the lattice spacing of 0.44 nm and 0.25 nm matches well with (001) plane of Fe3C, and (100)

8

plane of WC, respectively,53,54 denmonstrating the Fe3C/WC/GC nanocomposite could be

9

synthesized by waste biomass through facile one–step in–situ carbon–thermal reduction method.

10 11

The X−ray photoelectron spectrometer (XPS) technology is used to examine the chemical

12

composition of Fe3C/WC/GC. The characteristic peaks accounting for C, O, W, Fe can be clearly

13

seen from the overview scan spectra (Figure S3a). As shown in Figure 3a, the peak at 284.7 eV

14

represents the graphitic carbon with C–C, C=C, and C–H bonds.55 The peak of 285.7 eV is attributed

15

to the carbon involved in the formation of trace C–N during the reaction of NH3 and HCN gas with

16

the carbon in composite.56 Since the NH3 and HCN gas only generated as the reaction proceeds, so

17

the synthesized C–N is limited and the N1s peak cannot be observed in Figure S3a. The peaks at

18

higher binding energy of 286.6 eV, 287.5 eV, and 289.2 eV demonstrate the combination of carbon

19

with oxygen–containing groups in terms of C–O bond, O=C–O bond, and C=O groups,

20

respectively.57 The O1s can be seen as metal–O (529.9 eV), C=O (531.5 eV), C–O (533.1 eV,

21

including C–OH, C–O–C, C–O–OH), and the chemisorbed–O (533.1 eV, 534.5 eV, 535.9 eV),

22

respectively.58,59 However, the metal oxide phase was undetectable, suggesting the low content of

23

metal oxide in the nanocomposite (Figure 3b). The W4f spectrum in Figure 3c can be divided into

24

five sub–peaks. The peaks at 31.6 eV, 33.7 eV and 37.5 eV denote W2+ in W–C,60 the predominant

25

component in Fe3C/WC/GC. The peaks appearing at 32.3 eV and 35.3 eV are indexed to W4+ in

26

WO2 and W6+ in WO3, respectively.61 This result confirms further the analysis of O1s spectrum. To

27

elucidate the interaction between Fe3C and WC in Fe3C/WC/GC composite, we also give the W4f 10


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1

XPS spectrum of WC/GC. For WC/GC, the peaks of W from WC and WO3 located at 37.6 eV and

2

35.4 eV was not as strong as the corresponding W4f spectra for Fe3C/WC/GC located at 37.5 eV and

3

35.3 eV, indicating the introduction of iron species can benefit the interaction between tungsten and

4

carbon. Besides, this leads to a positive shift for Fe3C/WC/GC, which can be explained by the

5

electron transfer between Fe3C and WC as a consequence of intimate contact, incorporation, and

6

intense interaction.62 As shown in Figure 3d, the Fe2p spectrum has been deconvoluted into seven

7

peaks,25,63 in which Fe2p peaks of 708.3 eV, 711.3 eV, and 725.5 eV denote the combination of Fe

8

species with C and W,64 which provides further evidence for the interaction between Fe species and

9

WC in ternary Fe3C/WC/GC. Such interaction can also be revealed by comparing the Fe2p spectra

10

of Fe3C/GC and Fe3C/WC/GC. As shown in Figure 3d, The Fe2p spectrum for Fe3C/WC/GC shows

11

a positive shift of 0.1 eV compared with that for Fe3C/GC, and the positive shift in binding energy

12

corresponds to a decrease in electronic charge density on iron atoms present in Fe3C/WC/GC. The

13

most likely reason for such effect is the existence of metal–support interactions where the electrons

14

tend to be transferred from metal to WC/GC support.65

15 16

Surface Area and Porosity. The advantage of using pomelo peel as scaffold for preparing the

17

catalyst is the unique naturally occurring surface area and pore structure. Based on N2

18

adsorption–desorption isotherms, type IV curves for all the materials were characterized by sharp

19

upward extension branch (Figure S3b–S3e) according to IUPAC.66 Majority carbonized powders

20

from pomelo peel scaffold were porous carbon having random pore distribution and interconnected

21

pore structure (Figure S3f). The largest micropore area/volume was obtained for Fe3C/GC, a value

22

slightly higher than that for Fe3C/WC/GC, followed by that for WC/GC. This was the consequence

23

of small–sized WC nanoparticles blocking some microporous sites (Table 1). The hysteresis loops

24

for higher p/p0 values of all materials are characterized as type H3 that associated with capillary

25

condensation in mesopores.67 The steep shape in type H3 stands for absorption of mesopores and

26

probably macropores caused by stacking between nanoparticles and interconnected carbon skeletons

27

(Figure S3f). The total mesopore area for different composites follows the order of Fe3C/WC/GC > 11



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Page 12 of 38

1

WC/GC > Fe3C/GC > GC, which means the Fe3C/WC/GC can provide more channels and passages

2

for mass transfer to active species during ORR.

3 4

The unique pore structure in terms of high specific surface area (622.20 m2 g-1) and small–sized pore

5

diameter (1.13 nm) obtained for Fe3C/WC/GC originates from the interwoven sponge–like network

6

structure of pomelo peel substrate. Almost vertical climbing shape were observed for pore size

7

smaller than 2.0 nm in pore size distribution curves of Fe3C/WC/GC, Fe3C/GC, and WC/GC in the

8

micropore area/volume range (insert images in Figure S3b–S3e), and such discrimination further

9

confirms the micropores for biomass–based graphitic composites. The relative micropore area to

10

total area for different composites follows the order of Fe3C/GC (76.73%) > Fe3C/WC/GC (63.6%) >

11

WC/GC (50.55%). Despite lower content of micropores for Fe3C/WC/GC than that for Fe3C/GC,

12

this does not mean weaker catalytic activity as detailed below.

13 14

Electrochemical Measurements. To access the activity of electrocatalysts, the materials were

15

loaded onto glassy carbon electrode for measurements in O2−saturated phosphate buffer solution

16

(PBS, 50 mM, pH = 7.3). As shown in Figure 4a, the GC shows a minimal activity indicated by

17

extremely weak current response. The current was increased considerably when Fe3C or WC was

18

introduced, suggesting the activity of both Fe3C and WC towards ORR. Notably, the Fe3C/WC/GC

19

nanocomposite resulted in further increase of current much higher than Pt/C, and the improved ORR

20

performance by Fe3C/WC/GC could also be verified by electrochemistry impedance spectroscopy

21

(EIS), showing the lowest charge–transfer resistance (Figure 4b, Table S1).

22 23

To reveal the ORR kinetics of Fe3C/WC/GC nanocomposite, the rotating disk electrode (RDE)

24

measurements were conducted in 50 mM PBS. The limiting current density of Fe3C/GC was

25

improved by WC incorporation (1.70 mA cm-2) at the highest rotation rate (3600 rpm) (Figure 5a),

26

which was 9.7% higher than that of Fe3C/GC (1.55 mA cm-2). Meanwhile, the current density of

27

Fe3C/GC/GC was also much higher than that of WC/GC (0.75 mA cm-2) and GC (0.32 mA cm-2), 12


Page 13 of 38

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1

implying the enhancement by Fe (Fe–W and Fe–C moieties) species (Figure 5b–5e). The electron

2

transfer number (n) for different electrocatalysts were derived from Koutecky–Levich plots based on

3

RDE data (Figure 5f, Table S2). The electron transfer number was determined to be n = 3.95 for

4

Fe3C/WC/GC, n = 3.74 for Fe3C/GC and n = 3.59 for Pt/C, which were indicative of mainly

5

four–electron (4e-) ORR. This process may be accomplished through either direct 4e- pathway to

6

form H2O or two–electron (2e-) pathway to form H2O2 followed by further reduction to H2O. In

7

comparison, the WC/GC and GC exhibited much poor ORR activity with n = 2.86 and n = 1.87,

8

suggesting the predominance of 2e- oxygen reduction.

9 10

To verify the ORR catalytic pathway of Fe3C/WC/GC catalyst, the rotating ring–disk electrode

11

(RRDE) measurements were performed to monitor the H2O2 formation during ORR process (Figure

12

6). For the potential over the range of –0.8 V to –0.1 V, the H2O2 yields were lower than 5%,

13

amounting to n = 3.93–3.97 for Fe3C/WC/GC, n = 3.78–3.89 for Fe3C/GC, and n = 2.73–3.81 for

14

Pt/C, respectively (Figure 6d). The obvious H2O2 accumulation was observed for WC/GC and GC,

15

giving their electron transfer number of n = 2.97–3.72, and n = 1.10–2.75. This was consistent well

16

with the results obtained from the Koutecky–Levich plots of RDE measurements, indicating the

17

four–electron ORR catalyzed by Fe3C/WC/GC nanocomposite.

18 19

Performance Evaluation of MFCs. To further test the performances of Fe3C/WC/GC for ORR in

20

pH–neutral condition, we loaded the catalyst (loading rate of 30 mg cm-2) onto the Teflon–treated

21

gas diffusion electrode, and applied it to the air–cathode microbial fuel cells (MFCs) (Figure 7a). As

22

expected, the MFCs with Fe3C/WC/GC cathode produced the highest power density (1997±13 mW

23

m-2), a value 24.42% higher than Fe3C/GC (1605±24 mW m-2), 67.82% higher than Pt/C (1190±37

24

mW m-2). Much lower power density was observed for WC/GC (799±35 mW m–2) and GC (71±5

25

mW m–2) cathode (Figure 7b, Table S1). The highest power output yielded by Fe3C/WC/GC cathode

26

accompanies the largest cathode potential (Figure 7c), indicating the incorporation of WC phase can

27

largely decrease the ORR overpotential. The ternary Fe3C/WC/GC behaved better than binary 13



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Page 14 of 38

1

Fe3C/GC and other transition metal materials even precious–metal–modified catalysts with various

2

morphologies like nanorods, nanowires, nanospheres, nanoshells.16,68,69 The power density was

3

comparable to MNx/C catalyst reported in previous studies (Table S3).70–72

4 5

Transition–metal catalysts generally perform better under alkaline condition due to the lower

6

standard electrode potential than that under acidic condition. The current densities decreased with

7

time at both pH neutral and acidic conditions, especially exhibits a very slow attenuation and a high

8

relative current of 93.8% and 85.7% after 8000 s, respectively (Figure S4a). In particular, the cell

9

voltage for Fe3C/WC/GC (550±17 mV), Fe3C/GC (502±12 mV), and WC/GC (405±8 mV)

10

decreased slightly over 33 cycles (2200 h) (Figure 7d, Fig. S4b–S4f), demonstrating a remarkable

11

stability of Fe3C/WC/GC enabling a high kinetic activity than that of Pt/C in acidic and pH–neutral

12

condition.

13 14

ORR Mechanisms. According to previous works, Fe3C was assumed to be active toward ORR, but

15

there was very limited evidence for real role of pure Fe3C phase thus far. It is known that the

16

sluggishness of ORR originates inherently to the difficulty of splitting oxygen molecule (O2 → O +

17

O, 494 KJ mol-1).73,74 Theoretical studies were performed to identify thermodynamic progress of O2

18

molecule dissociation on Fe3C by using DFT calculations. Figure S5 shows six stable active sites on

19

Fe3C(001) surface for adsorption of atomic oxygen after full relaxation (Figure S6 and Table S4). As

20

such, the Eads is calculated to be 1.244 eV for the most stable site where O2 molecules are placed

21

vertically, whereas the Eads is increased to –4.63 eV following the complete dissociation of O2

22

molecule to O atom on Fe3C(001). Based on normalization of the adsorption energy for each step,

23

we obtain the energy level diagrams for stepwise dissociation of molecular oxygen on Fe3C(001)

24

surface. As can be seen from Figure 8, the O–atom dissociation from O2 molecule to stable sites

25

occurs in a thermodynamically favourable manner with a very small energy barrier (0.06 eV) along

26

with the reaction coordinate, which corresponds to a nearly thermoneutral process. When Fe3C is

27

employed as ORR electrocatalyst, such barrier can be overcome easily, driven by a negative 14


Page 15 of 38

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1

potential applied to the cathode in an electrochemical system. This clearly suggests a remarkable

2

activity of Fe3C for cleaving the O–O bond of oxygen molecule on its (001) surface. In such case,

3

the oxygen atoms tend to be adsorbed on two individual sites with partially filled d orbitals for

4

bonding with π* orbitals of oxygen molecule, giving rise to what is called “bridge model”, thereby

5

the four–electron ORR generally predominates.75 These results show that the strong affinity of O2 for

6

Fe3C surface makes it easy break the bottleneck of O–O cleavage. This is in good consistence with

7

the prior study reported by Hu et al., who developed ORR catalyst with the form of hollow sphere

8

containing Fe3C nanoparticles encased by graphitic layers, and the catalyst showed good activity and

9

stability in acidic and alkaline media.6 Overall, Fe3C possesses high activity for splitting O–O bond

10

of molecular oxygen on its surface, i. e. the well–acknowledged limiting step of ORR process,76,77

11

which may provide the most likely explanation to the reason why those Fe3C–involved catalysts can

12

catalyze ORR efficiently.

13 14

The Role of WC on ORRs. Fe3C is highly active for splitting O–O bond of molecular oxygen on its

15

surface according to DFT calculations (Figure 8), whereas WC surface is stable and resistant to

16

bulk-like clustering as is commonly encountered for platinium–based catalyst. Many previous

17

studies proved that the WC could stabilize the Fe3C(001) surface, as the alloyed cementite doped

18

with transition metals (e. g. Co, Ni, Cr, Mn, V, Mo, W) exhibited more stable surface in (001) planes,

19

with all of the Fe2MC(001) surface atoms relax towards the bulk–like layer, and the

20

relaxation–caused normal–direction displacement of the (001) surface C atom is maximal.78,79 Thus,

21

the main role of WC is assumed to enhance the strength of the Fe–C bond of the cementite, which

22

resulted in more stable surface of the composite than that of pure Fe3C. On the other hand, there are

23

also literatures showing that the adsorption of ORR active metal catalyst (Pt, Ag) on WC(100)

24

system can reduce the surface energies, and the O2 dissociation at high oxygen coverage can

25

generate new catalytic active sites with low activation energy barrier and favorable

26

thermodynamics.80 The enhanced stability and durability as a result of the incorporation of WC can

27

also be indicated from the stable voltage ouput of 550±17 mV within the tested period as long as 15



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1

Page 16 of 38

2200 h (Figure S4b).

2 3

Besides, from perspective of structure properties and chemical interactions, the Fe3C and WC

4

nanoparticles with remarkable cementite properties are embedded in the thick graphitic layers,

5

making it of high stability and activity during the acid leaching process. Specifically, these iron and

6

tungsten species incorporation is more conducive to chemisorption of oxygen or further convert into

7

lattice oxygen for ORR. Besides, the strong interaction between Fe3C and WC should play a

8

significant role in improving the performance of Fe3C/WC/GC electrocatalyst in the electrochemical

9

system. Furthermore, since the three phase interface (TPI) where ORR takes place can mainly be

10

established on the surface of micropores rather than mesopores or macropores,19 the high content of

11

micropores from biomass–derived porous carbon precursor for all materials implies the capability of

12

providing more active sites for improved electrocatalytic performances.

13 14

CONCLUSIONS

15

In summary, the Fe3C/WC/GC nanocomposite was fabricated by using a facile ion exchange and

16

biomass–thermal reduction process. As indicated by CVs, EIS, RDE and RRDE measurements, the

17

Fe3C/WC/GC nanocomposite was shown highly active and stable for electrocatalysis of oxygen

18

reduction in pH–neutral electrolyte. According to the DFT calculations, the barriers of ORR could be

19

overcome by Fe3C whose (001) surface was active for dissociating O–O bond of oxygen molecules.

20

The Fe3C provides the active sites for ORR, while the graphitic layers and WC nanoparticles can

21

stibilize the Fe3C surface, preventing it from dissociation in the electrolyte. The Fe3C/WC/GC

22

nanocomposite is highly active, selective and stable toward four–electron ORR in pH–neutral

23

electrolyte, which results in 67.82% higher power density than commercial Pt/C, and negligible

24

voltage decay during a long-term phase of 33–cycle (2200 h) operation of microbial fuel cell (MFC).

25

In particular, the recyclable waste biomass and facile synthesis strategy make the ORR catalyzed by

26

Fe3C/WC/GC more economical and more sustainable toward practical applications of MFCs

27

technology. 16


Page 17 of 38

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1 2

ASSOCIATED CONTENT

3

Supporting Information

4

Additional details are available including schematic illustration for synthesis procedure of

5

Fe3C/WC/GC composite (Scheme S1), IG/ID ratio (Figure S1), STEM and corresponding MAPPING

6

images of Fe3C/WC/GC (Figure S2), XPS, N2 adsorption/desorption isotherms and pore size

7

distributions curves of different materials (Figure S3), the durability of Fe3C/WC/GC (Figure S4),

8

DFT calculations (Figure S5 and S6), and corresponding fitting results (Table S1 to S4). This

9

material is available free of charge via the Internet at http://pubs.acs.org/.

10 11

AUTHOR INFORMATION

12

Corresponding Author

13

*Tel: (+86) 451 86282008; Fax: (+86) 451 86282110; Email: sjyou@hit. edu. cn.

14

Notes

15

The authors declare no competing financial interest.

16 17

ACKNOWLEDGEMENTS

18

Project supported by the National Natural Science Foundation of China (No. 51378143), Singapore

19

National Research Foundation (CREATE Programme of Nanomaterials for Energy and Water

20

Management), State Key Laboratory of Urban Water Resource and Environment (Grant No.

21

2015TS01), the Fundamental Research Funds for the Central Universities (Grant No.

22

HIT.BRETIII.201419).

23 24

REFERENCES 17



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7

and O2 Dissociation on WxC Low–Index Surfaces. J. Phys. Chem. C 2014, 118 (25),

8

13525–13538.

9 10 11 12 13 14 15 16 17 18 19 20 21

27







 



G 2D

D

 

     

10 20 30 40 50 60 70 80 90

2 (degree)

I

1100 C 1000 C 900 C 800 C 700 C 600 C

1100 C 1000 C 900 C 800 C 700 C 600 C



80

500 μm

II

DTG

1

0

-1

-1

III

20 0

-1

1

2

0

40

1000 1500 2000 2500 3000 3500

200

400

600

IV

800

TG

-2

-2

-3 1000 1200

-3

Temperature (C)

e

f

30 μm

500 μm

DSC

60

Raman shift (cm )

d

3

c100

Deriv. weight (% min-1)



b

Heat flow (W g-1)



Graphite  WC  W2C Fe3C a-Fe FeWO4   

Intensity (a.u.)



Weight (%)

a

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

Page 28 of 38

g

30 μm

h

10 μm

Figure 1. (a) XRD patterns and (b) Raman spectra of composites carbonized at 600–1100 °C, (c) TG–DSC curves for thermal pyrolysis of PP–[Fe(CN)6]4-–WO42- precursor, (d–h) SEM images of pure pomelo peel, PP–[Fe(CN)6]4-–WO42- precursor, precursors carbonized at 600 °C, 800 °C, and 1000 °C, respectively.

28


Page 29 of 38

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




a 

 

b

Graphite

WC



Fe3C





         







Fe3C/WC/GC Fe3C/GC



WC/GC GC

10 20 30 40 50 60 70 80 90

100 nm

2 (degree)

c

d=0.34 nm

d

d=0.34 nm dFe3C(001)=0.44 nm d=0.34 nm dWC(100)=0.25 nm

dGraphite(002)=0.34 nm 5 nm

5 nm

Figure 2. (a) XRD patterns of different composites carbonized at 1000 °C, (b) TEM and (c–d) HRTEM images of Fe3C/WC/GC composite.

29



Raw CN

Sum CO

b

CC, CH, CC OCO CO

Intensity (a.u.)

Intensity (a.u.)

a

C 1s 284.7 eV

289.2 eV

285.7 eV 286.6 eV 287.5 eV

Raw Sum OMetal CO COH, COC, COOH, chemisorbedO Displacement of chemisorbed oxygen

O 1s

531.5 eV 533.1 eV

W4f7/2: W2+ in WC W4f5/2: W2+ in WC

W4f7/2: W6+ in WO3

W5p3/2: W2+ in WC

d Intensity (a.u.)

Raw Sum W4f7/2: W4+ in WO2

35.3 eV

W 4f WC/GC 37.5 eV

33.7 eV

44

42

40

31.5 eV

32.3 eV

Fe3C/WC/GC 38

36

34

32

537

534

531

528

525

Binding energy (eV)

Binding energy (eV)

c

529.9 eV

534.5 eV 535.9 eV

540

294 292 290 288 286 284 282 280 278

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

Page 30 of 38

30

28

Raw Sum Fe2p3/2: Fe (II) Satellite Fe (II) Fe2p1/2: Fe (II)

Fe 2p

Carbide Fe2p3/2: Fe (III) Satellite Fe (III) Fe2p1/2: Fe (III)

725.5 eV

711.3 eV

/GC Fe 3C /GC /WC Fe 3C 740 735 730 725 720 715 710 705

Binding energy (eV)

Binding energy (eV)

Figure 3. XPS spectra of (a) C1s, (b) O1s of Fe3C/WC/GC, (c) W4f of WC/GC and Fe3C/WC/GC, and (d) Fe2p of Fe3C/GC and Fe3C/WC/GC, respectively.

30


Page 31 of 38

0.3

b

0.2 0.1 0.0 -0.1

Fe3C/WC/GC

-0.2

Pt/C Fe3C/GC

-0.3 -0.4

-0.4

-0.2

0.0

0.2

100

Fe3C/WC/GC

Pt/C

WC/GC

GC

Fe3C/GC

80 60 40 20

WC/GC GC

-0.6

120

 Zim ()

a Current (mA)

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

0.4

50

100

150

200

250

Zre ()

Potential (V vs. SCE)

Figure 4. (a) CV curves and (b) Nyquist plots of different catalysts for ORR in O2–saturated PBS at scan rate of 10 mV s-1.

31



-1.5 -1.8

-2.1 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1

d 0.0 -0.3

Potential (V vs. SCE) WC/GC

-0.6 -0.9 -1.2 -1.5 -1.8

-2.1 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0


Current density (mA cm-2)

400 900 1600 2500 3600

-0.9 -1.2 -1.5 -1.8 -2.1 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1

e 0.0 -0.3

0.0


-0.3

Fe3C/GC

-0.6 -0.9 -1.2 -1.5 -1.8 -2.1

-0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0

f 3.0

GC

-0.6 -0.9 -1.2 400 900 2500 3600

-1.5 -1.8 -2.1 -0.7

0.0


2

-1.2

-0.6

-1

-0.9

-0.3

Pt/C

2.5 2.0

-1

-0.6

c

0.0

Current density (mA cm )

-2


-0.3

b

Fe3C/WC/GC

Current density (mA cm-2)

-2


a 0.0

Current density (mA cm -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 32 of 38

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0.0


1.5

Fe3C/WC/GC

Pt/C

WC/GC

GC

Fe3C/GC

1.0 0.5 0.04 0.06 0.08 0.10 0.12 0.14 0.16 -1/2 -1/2 -1/2 (rad s ) 

Figure 5. (a–e) RDE voltammograms and (f) corresponding Koutechy–Levich plots for different electrocatalysts in O2–saturated 50mM PBS electrolyte.

32


-1.0

Fe3C/WC/GC

-1.5

Pt/C Fe3C/GC

-2.0 -0.8

-0.4

25

Fe3C/WC/GC

20

Pt/C Fe3C/GC

70

WC/GC

60

15 10 5

50 40

GC

30 -0.75 -0.60 -0.45 -0.30 -0.15 Potential (V vs. SCE)

0

8 6 4 2

WC/GC GC -0.2 0.0 0.2


H2O2 (%)

c

-0.6

10

0 -0.8

d 4.0

-0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1


-0.6

-0.4

-0.2

0.0

0.2


3.8

Number of electrotransfer

-0.5

b

IR (A)

0.0

Number of electrotransfer

a

H2O2 (%)

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


ID (mA cm-2)

Page 33 of 38

3.6 3.4 3.2

2.8 2.4 2.0 1.6 1.2 -0.75 -0.60 -0.45 -0.30 -0.15 Potential (V vs. SCE)

3.0 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1


Figure 6. RRDE measurements of different electrocatalysts in O2–saturated PBS. (a) Disk current, (b) ring current, (c) H2O2 productivity and (d) electron transfer number as function of potential applied.

33



b 2.5

e-

-2

e-

Power density (W m )

a

O2 + 4H+ + 4e-→2H2O

e

-

CO2

H2O O2

eH+

Anode

Glucose

Cathode

Fe3C/WC/GC

Pt/C

WC/GC

GC

Fe3C/GC

1.5 1.0 0.5 0

2

4

6

8

10

-2

Current density (A m )

d 0.6 Maximum voltage (V)

0.0 Cathode

-0.2 -0.4 Anode

-0.6

2.0

0.0

c 0.2 Electrode potential (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 38

0

2

4

6

8

-2

10

0.5 0.4 0.3 Fe3C/WC/GC

0.2

Pt/C Fe3C/GC

0.1 0.0

WC/GC GC

0

5

Current density (A m )

10

15

20

25

30

Recycle number (n)

35

Figure 7. (a) Schematic single–chamber MFCs reactor for testing the performances of electrocatalysts, (b) power density and (c) electrode potential as function of current density, and (d) the maximum cell voltage recorded during 33–cycle operation.

34


Page 35 of 38

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


Energetics of molecular O2 dissociation on Fe3C (001)

0.00 Adsorption of molecular O2

0.06 Transition state

-2.28 Dissociation of O-O bond

-3.77

Fe atom

Transition state

C atom O atom

-4.63 Adsorption of atomic O

Figure 8. Calculated energetic of dissociation of molecular oxygen on the Fe3C(001) surface. The details of theoretical calculations are provided in the Supporting Information.

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

Page 36 of 38

Table 1. Pore parameters of different electrocatalysts. Specific surface area (BET), m2 g-1 Micropore area, m2 g-1 Mesopore area, m2 g-1 Amicro/Atotal, % Micropore volume, cm3 g-1 Total volume, cm3 g-1 Pore width (DFT), nm

Fe3C/WC/GC 622.20

Fe3C/GC 650.17

WC/GC 392.75

GC 224.87

395.69 226.51 63.6 0.17 0.58 1.13

498.88 151.29 76.73 0.21 0.59 1.18

198.54 194.21 50.55 0.086 0.38 3.97

− 122.19 − − 0.28 3.83 (BJH)

36


Page 37 of 38

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


Biomass–Derived Porous orous Fe3C/WC/GC Nanocomposite anocomposite for Efficient Electrocatalysis of Oxygen xygen Reduction

Ming Maa, Shijie Youa,*, Wei ei Wanga, Guoshuai Liua, Dianpeng Qib, Xiaodong Chenb, Jiuhui Quc, and Nanqi Rena

Keywords: biomass, Fe3C/WC/GC nanocomposite nanocomposite, porous structure, oxygen reduction reaction, microbial fuel cell, power densityy, durability

Biomass derived Fe3C/WC/GC nanocomposite for highly active, selective and stable oxygen reduction. The biomass–based synthesized Fe3C/WC/GC nanocomposite is highly active, active selective and stable toward four–electron oxygen reduction reduction, as result of inherent high reactivity of Fe3C for dissociating O–O O bonds on its surface, the pporous structure for providing active sites and mass transfer, and the assistant catalysis effect of WC.

37



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Biomass–Derived Porous Fe3C/WC/GC Nanocomposite for Efficient Electrocatalysis of Oxygen Reduction

Ming Maa, Shijie Youa,*, Wei Wanga, Guoshuai Liua, Dianpeng Qib, Xiaodong Chenb, Jiuhui Quc, and Nanqi Rena

Keywords: biomass, Fe3C/WC/GC nanocomposite, oxygen reduction reaction, electrochemical and bioelectrochemical system

Biomass derived Fe3C/WC/GC nanocomposite for highly active, selective and stable oxygen reduction. The biomass–based synthesized Fe3C/WC/GC nanocomposite is highly active, selective and stable toward four–electron oxygen reduction, as result of inherent reactivity of Fe3C for dissociating O–O bonds on its surface, the porous structure for providing active sites and mass transfer, and the assisted catalysis of WC.


Graphitic Carbon

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