Facile synthesis of N-doped carbon layer encapsulated Fe2N as an efficient catalyst for oxygen reduction reaction

Accepted Manuscript Facile synthesis of N-doped carbon layer encapsulated Fe2N as an efficient catalyst for oxygen reduction reaction Zaojin Liu, Jing Yu, Xingyun Li, Lixue Zhang, Dong Luo, Xuehua Liu, Xiaowei Liu, Shuibo Liu, Hongbin Feng, Guanglei Wu, Peizhi Guo, Hongliang Li, Zonghua Wang, Xiu Song Zhao PII:

S0008-6223(17)31165-X

DOI:

10.1016/j.carbon.2017.11.051

Reference:

CARBON 12581

To appear in:

Carbon

Received Date: 8 October 2017 Revised Date:

15 November 2017

Accepted Date: 18 November 2017

Please cite this article as: Z. Liu, J. Yu, X. Li, L. Zhang, D. Luo, X. Liu, X. Liu, S. Liu, H. Feng, G. Wu, P. Guo, H. Li, Z. Wang, X.S. Zhao, Facile synthesis of N-doped carbon layer encapsulated Fe2N as an efficient catalyst for oxygen reduction reaction, Carbon (2017), doi: 10.1016/j.carbon.2017.11.051. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT Facile synthesis of N-doped carbon layer encapsulated Fe2N as an efficient catalyst for oxygen reduction reaction Zaojin Liu

a,b, 11

, Jing Yu

c,1

, Xingyun Li

a,b,*

, Lixue Zhang

c*

, Dong Luo

a,b

, Xuehua Liu

a,b

,

RI PT

Xiaowei Liu a,b, Shuibo Liu a,b, Hongbin Feng a,b, Guanglei Wu a,b, Peizhi Guo a,b, Hongliang Li a,b, Zonghua Wangc, Xiu Song Zhao a,b,d

Institute of Materials for Energy and Environment, Qingdao University, Qingdao, 266071, China

b

Laboratory of New Fiber Materials and Modern Textile, Breeding Basis for State Key Laboratory,

SC

a

c

M AN U

College of Materials Science and Engineering, Qingdao University, Qingdao, 266071, China Shandong Sino-Japanese Center for Collaborative Research of Carbon Nanomaterials, College of

Chemistry and Chemical Engineering, Qingdao University, Qingdao, 266071, China. School of Chemical Engineering, The University of Queensland, St Lucia Campus, Brisbane,

AC C

EP

QLD 4074, Australia

TE D

d

*

Corresponding author. E-mail address: [email protected] (X. Li).

1

These authors contribute equally to this work.

ACCEPTED MANUSCRIPT Abstract: Development of non-noble metal catalysts for oxygen reduction reaction (ORR) is of significant importance for the commercialization of fuel cells and metal-air batteries. Here we provide an

RI PT

efficient method to produce core-shell structured Fe-N-C catalyst via a facile in-situ chelating strategy by introducing ammonia iron citrate during the polymerization process of dopamine. The influence of calcination temperature and atmosphere on the physicochemical property and the

SC

activity of the catalyst are systematically evaluated. By calcination at 800 oC with NH3

M AN U

atmosphere, Fe2N encapsulated with N doped carbon layers shows excellent activity with close onset and half wave potential (E1/2) while better methanol crossover resistance than the Pt/C catalyst. The high activity could be due to the synergistic effect of Fe2N with the N-doped graphitic carbon layers and the mesoporous structure facilitating the mass transfer. Moreover, the

TE D

simple synthesis process could provide a versatile routine to construct core-shell structured metal-N-C composite for a wild catalytic application.

EP

1. Introduction

AC C

Oxygen reduction reaction (ORR) is a critical cathodic process in fuel cells and metal-air batteries which are regarded as two promising electrical devices with high energy efficiency and low environmental pollution. [1, 2] However the currently used Pt/C catalyst hinders its commercialization since the high price, limited reserve and low toxicity tolerance of Pt. [3-6] Hence the development of alternative non-noble metal catalyst has long been a hot topic in regards of both scientific research and industrial application. In the last several decades, tremendous efforts have been paid to nano-carbon based composite. [7] For instance, nitrogen doping is

ACCEPTED MANUSCRIPT adopted to endow pure carbon catalytic activity by creating positive charge on adjacent carbon atoms to facilitate the O2 activation. [8-10] Furthermore, introduction of transition metal into nitrogen doped carbon could significantly boost the ORR activity. [11] Vast of novel M-N-C

RI PT

(M=Fe, Co etc.) structures have been reported [12-17] , among which, Fe-N-C has been attracted particular research interest due to its promising catalytic performance. [18-25] However there is still no consensus on the reaction mechanism for the Fe-N-C system since various iron phases

SC

were identified to be catalytic active, e.g. Fe3O4 [26], Fe3C [27], Fe [28-30], FeN [31, 32], Fe2N

M AN U

[33, 34] etc. and even a dual active centers were proposed [35, 36]. Despite the above dispute, there is common view lies in that to construct iron species and nitrogen doped carbon composite is key to synergistically catalyze oxygen reduction reaction.[37] It is reported that iron could promote the graphitization and also tune the electronic states of the sp2 carbon which contribute to

TE D

the excellent ORR activity. [38-42] In turn, the graphitic carbon layer will benefit the electron transfer and bind with the catalyst to alleviate its agglomeration. Taking advantage of the electronic interactions between iron species and carbon, core-shell structured Fe-N-C could be

EP

prospective with a rational design strategy.

AC C

Dopamine is a mussel inspired biomimetic materials which can polymerize and coat on almost any substrate at high pH value. [43-45] The abundant hydroxyl and amino groups in dopamine offer itself great chelating ability with metal ions. [46-48] After calcination, PDA (polymerized dopamine) could be transformed into nitrogen doped porous carbon which will have the chances to enwrap metal species inside the final N-C materials. Taking the above features into account, here we demonstrate the feasibility of the complexing of Fe3+ with dopamine during polymerization process to construct the core-shell structured Fe-N-C composite. The precursor we

ACCEPTED MANUSCRIPT selected is ammonium ferric citrate (AFC) in that the interaction between Fe3+ with PDA could be enhanced by the possible hydrogen bonding since the existence of citrate and ammonium ions in AFC. To optimize the catalytic activity we concisely tune the final structure and iron crystalline

RI PT

phase by controlling the calcination temperature and atmosphere. By a fare comparison between different iron species of Fe3C, Fe2C, Fe3N, Fe4N, Fe2N, it is found that Fe2N encapsulated in nitrogen doped carbon showed outperformed performance with close activity and better stability

SC

to Pt/C catalyst. This work provides an attractive core-shell structured Fe2N-N-C as candidate to

construct other metal-N-C composite.

2. Experimental

TE D

2.1 Preparation of Fe-PDA composite

M AN U

replace Pt and moreover the simple synthesis process could be used as common method to

Firstly, 1.5 g dopamine hydrochloride (Shanghai Macklin Biochemical Co., Ltd) was dissolved in 300 mL deionized water to form a transparent solution. 0.5 g ammonium iron (Ⅲ)

EP

citrate (Sinopharm Chemical Reagent Co., Ltd) in 100 mL deionized water was added slowly to

AC C

the above solution, followed by stirring for 30 minutes. Then, the pH value of the mixture was adjusted to 8.5 using Tris-buffer (Tris(hydroxymethyl)aminomethane, Shanghai Macklin Biochemical Co., Ltd) and magnetically stirred at room temperature for 24 h. The black suspension was filtered and repeatedly washed with abundant deionized water. After drying at 60 o

C overnight, the precipitate was calcined at NH3 atmosphere with different calcination

temperature of 600 oC, 700 oC, or 800 oC for 3 h to obtain the final product which was named as NH3-Fe-N-C-600, NH3-Fe-N-C-700, NH3-Fe-N-C-800 respectively. As comparison, the sample

ACCEPTED MANUSCRIPT was calcined with Ar atmosphere at 800 oC to obtain Ar-Fe-N-C-800. The reference catalyst were also prepared by impregnating activated carbon (Shanghai Macklin Biochemical Co., Ltd) with ammonium iron citrate (Fe loading amount of 20 wt%) and calcined at 800 oC with Ar or NH3

RI PT

atmosphere respectively which was designated as Ar-Fe-AC and NH3-Fe-AC.

2.2 Physicochemial Characterization

SC

The morphology is characterized by JEOL JEM-2100 transmission electron microscope (TEM). Powder X-ray diffraction (XRD) was performed on a Rigaku Ultima IV X-ray

M AN U

diffractometer with Cu-Kα radiation (λ=0.15418 nm). N2 adsorption-desorption was carried out at Quantachrome Autosorb iQ3 and before the test the samples were degassed at 150 oC for 12 h. Raman spectra were collected using a Renishaw in Via Plus Micro-Raman spectroscopy system equipped with a 50 mW DPSS laser at 532 nm. X-ray photoelectron spectrometer (XPS) was

TE D

recorded in a Thermo Fisher ESCALAB 250Xi spectrometer with Al Kα X-ray source (1486.6 eV, operated at 15 kV and 10.8 mA). The iron loading was analyzed by inductively coupled

EP

plasma-atomic emission spectrometry (ICP-AES, SHIMADZU ICPS-8100).

AC C

2.3 Electrochemical measurements

The electrochemical properties of the catalysts were tested using a three-electrode system in

O2-saturated 0.1 M KOH and 0.1 M HClO4 solution by a Bio-logic VSP 300 electrochemical work station at room temperature. Catalyst-coated glassy carbon rotating disk electrode (RDE) or rotating ring-disk electrode (RRDE) were used as working electrodes, platinum mesh was used as the counter electrode and saturated calomel electrode (SCE) were used as the reference electrode. To prepare the working electrode, 10 mg catalysts or 3 mg 20 wt% Pt/C was dispersed in mixed

ACCEPTED MANUSCRIPT solution which consist of 20 µL Nafion solution (0.1 wt%) and 1 mL deionized water to form a homogeneous ink. 9 µL suspension were then casted onto the GC electrode and dried in air. The electrode was first activated by a few cycles of CV tests at 0.05 V/s and tested at 0.01

RI PT

V/s for electrochemical measurements. A rotating disk electrode (RDE) test was conducted at rotating speed from 400 rpm to 2025 rpm in O2-saturated 0.1 M KOH and 0.1 M HClO4. For the RRDE tests, the ring potential was set at 1.5 V versus RHE with a scan rate of 10 mV·s-1. The

SC

electron transfer numbers during the oxygen reduction reaction were determined from the

M AN U

Koutechy-Levich equation, which was analyzed at different potentials of RDE tests. Chronoamperometric measurements were performed at corresponding potential to deliver a current density of 10 mA·cm-2 for 6000 s.

The transferred electron numbers per O2 involved in the oxygen reduction can be calculated

TE D

by the Koutechy–Levichequation as given below: ଵ ଵ

ଵ

= + =

ଵ

+

ଵ

୎ ୎ై ୎ౡ ୆னభ/మ ୎ౡ

B= 0.2nF(DO)2/3 v-1/6 CO

EP

where n corresponds to the transferred number, F is the Faraday constant (F = 96485 C·mol-1), DO

AC C

is the diffusion coefficient of O2 in 0.1 M KOH (1.9×10-5 cm2·s-1), n is the kinetic viscosity (0.01 cm2·s-1), and CO is the bulk concentration of O2 (1.2×10-6 mol·cm-3). The

electron

transfer

number

from

RRDE

experiment

was

determined

by

the following equation:

n = 4ID/[ID+ (IR/N)] where ID is the disk current, IR is the ring current, and N is the ring correction coefficient. In RRDE experiment, N was determined to be 0.45 from the reduction of Fe(CN)64-/3- redox couple.

ACCEPTED MANUSCRIPT The ring potential was held at 1.5 V vs. RHE.

3. Results and discussion

RI PT

The Fe-N-C catalyst was prepared as illustrated in Fig. 1. Ammonium ferric citrate was firstly mixed with dopamine in aqueous solution under stirring, during which iron ions could be well chelated with the catechol and amino groups of dopamine [49]. Then, as the pH value was tuned

polymer. Upon calcination, Fe-N-C catalyst was obtained.

SC

to 8.5, dopamine slowly polymerized to form PDA with iron species being wrapped inside the

M AN U

Figs. 2(a) show the TEM image of NH3-Fe-N-C-800 catalyst. It can be seen that iron species are evenly loaded on porous carbon materials and from HRTEM in Fig. 2(b), we can clearly detect the core-shell structure with iron species thoroughly encapsulated by few layers graphene. The lattice spacing of the iron compound is calculated to be 0.34 nm corresponding to the (101) planes

TE D

Fe2N (JCPDS 73-2102) consistent with XRD patterns in Fig. 2(c). Meanwhile, from Fig. 2(b), we can also detect hollow vesicle, which may be created by the movement of iron species driven by

EP

the high temperature. The above results sufficiently prove the feasibility of in-situ iron ion

AC C

chelating method to construct the novel carbon layer encapsulated Fe2N composite.

Fig. 1 Illustration of the preparation of Fe-N-C catalyst.

By XRD characterization in Fig. 2(c), it is interesting to find that the crystalline phase is

ACCEPTED MANUSCRIPT sensitive to the calcination atmosphere as well as the calcination temperature. When the sample was calcined at 600 oC with NH3 atmosphere, pure Fe2C phase is obtained, nevertheless the catalyst phase is changed to both Fe4N and Fe3N as the calcination temperature increased to 700 C, and Fe2N finally occurred at 800 oC. This suggests that high temperature could facilitate the

RI PT

o

formation of Fe2N which may be more thermodynamically stable. As comparison, 800 oC calcined samples at Ar atmosphere showed pure Fe3C phase. To get deeper insight into in-situ metal

SC

complexation route and the role of PDA played for iron crystalline phase, we made reference

M AN U

catalyst by post loading ammonium ferric citrate onto activated carbon with the traditional impregnation method followed by calcination at 800 oC with Ar or NH3 atmosphere. XRD patterns in Fig. S1(a) showed that besides iron carbide there exist Fe2O3 and FeO phases for Ar-Fe-AC. Comparatively, the absence of iron oxide in Ar-Fe-N-C-800 highlights the unique reducing

TE D

capability of PDA with catechol groups to facilitate the reduction of iron oxide as documented in other literatures [48, 50, 51], which may help the formation of iron carbide. The influence of calcination temperature on the carbon graphitization was characterized by Raman spectra as

EP

shown in Fig. S2, there are two typical peaks of G band at 1578 cm-1 derived from the vibration of

AC C

sp2 C and D band at around 1345 cm-1 corresponding to the amorphous carbon. The ratio of the intensity for D band to G band (ID/IG) decreased obviously with the increase of calcination temperature indicating that high temperature could facilely enhance the graphitization of N-C material. The specific surface area and the pore textures were obtained by N2 adsorption and desorption experiment. As shown in Fig. 2(d), the specific Brunauer-Emmett-Teller (BET) surface area of NH3-Fe-N-C-800 is calculated to be 124.5 m2/g with pore volume of 0.4 cc/g, and the material showed mesoporous structure with the DFT pore size distribution centered at around 3.8

ACCEPTED MANUSCRIPT nm, which will facilitate the exposure of active sites to the reactant and promote the mass

M AN U

SC

RI PT

diffusion through the solid catalyst.

TE D

Fig. 2 (a), (b) TEM and HRTEM of NH3-Fe-N-C-800, (c) XRD patterns of Fe-PDA calcined at different temperatures, (d) N2 adsorption-desorption isotherm and DFT pore size distribution of

EP

NH3-Fe-N-C-800.

XPS measurements were carried out to get the surface chemical composition information. As

AC C

shown in Fig. 3, there exist C, Fe, N and O peaks for NH3-Fe-N-C-800. The nitrogen content was calculated to be 3.5% with 43.3% pyridinic N (398.2 eV), 46.7% pyrrolic N (400.8 eV), 4.3% Fe-N (399.5 eV) and 5.7% quaternary N (402.6 eV)[52]. The excellent O2 activation activity of pyridinic and quaternary N [25, 53] make the N doped carbon surface could serve as the additional active sites to synergistically catalyze ORR. As comparison, we also investigated the surface composition of PDA calcined at 800 oC and NH3 atmosphere without adding iron. From Fig. S3 we can see that about 4% N is incorporated and the N peak is fitted into 29.6% pyridinic N, 21.0%

ACCEPTED MANUSCRIPT pyrrolic N and 49.4% quaternary N, which is quite different from that of NH3-Fe-N-C-800. This indicate that the existence of iron species during the polymerization and calcination process of dopamine could significantly influence the resulting surface N-C composition. The Fe spectrum

RI PT

could be deconvoluted into Fe3+ 2p2/3 (710.8 eV), Fe2+ 2p2/3 (713.0 eV), Fe3+ 2p1/2 (724.6 eV), satellite peak (719.0 eV) and Fe-N (707.5 eV) which are consistent with previously reported results [33, 54]. Based on the XPS peak area calculation, the amount of Fe content is estimated to

SC

be 2.3 % far lower than the ICP results (22%), suggesting that abundant of iron species were

TE D

M AN U

located underneath the carbon layers.

AC C

EP

Fig. 3 (a) XPS survey of NH3-Fe-N-C-800 and the deconvolution of (b) N and (c) Fe.

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

EP

Fig. 4 (a) Cyclic voltammetry (CV) curves of NH3-Fe-N-C-800 in O2 or N2-saturated 0.1 M KOH

AC C

solution, (b) hao, (c) LSVs of NH3-Fe-N-C-800 with various rotation rates in O2-saturated 0.1 M KOH solution, inset of (c) is the corresponding K-L plots (j-1 vs. w-1/2) at different potentials derived from RDE measurements, (d) RRDE voltammograms of NH3-Fe-N-C-800 in O2-saturated 0.1 M KOH solution, inset of (d) is the peroxide yield and the electron transfer number n, (e) Chronoamperometric (i–t ) responses at 0.6 V vs. RHE in O2-saturated 0.1 M KOH at 1600 rpm for NH3-Fe-N-C-800, (f ) Chronoamperometric responses at 0.6 V vs. RHE in O2-saturated 0.1 M KOH followed by addition of 3 M methanol.

ACCEPTED MANUSCRIPT Cyclic voltammetry (CV) measurements for NH3-Fe-N-C-800 in N2 or O2 saturated 0.1 M KOH solution were carried out. As the results shown in Fig. 4(a), there is a well-defined oxygen reduction peak located at 848 mV vs. RHE under the O2-saturated electrolyte for NH3-Fe-N-C-800,

RI PT

which indicates the obvious ORR catalytic activity of NH3-Fe-N-C-800. The influence of the proportion of the precursor on the catalytic performance was first evaluated. From Fig. S4 we can see that the ratio of dopamine to ammonium iron (Ⅲ) citrate at 3 is the preferred condition, and

SC

the following catalyst were all prepared with this optimized ratio. The electrocatalytic activity of

M AN U

the Fe-N-C catalysts prepared with different conditions were compared with Pt/C catalyst by RDE techniques in O2-saturated 0.1 M KOH solution at a scan rate of 10 mV·s-1. From Fig. 4(b), we can see that there is slight catalytic activity for the PDA calcined at 800 oC which may be derived from the nitrogen doped carbon surface. Ar-Fe-N-C-800 shows a significantly improved ORR

TE D

catalytic activity compared with that of PDA, which highlights the importance of introducing iron species for the ORR catalytic process. Interestingly, the activity is further promoted for NH3-Fe-N-C-800 with an onset potential at 939 mV, that is very close to that of Pt/C (954 mV vs

EP

RHE) and the half wave potential (E1/2) is 0.869 V almost equal to that of the commercial Pt/C

AC C

catalyst (0.827 V vs. RHE). This could be due to the higher intrinsic catalytic ability for Fe2N obtained at NH3 atmosphere than that of Fe3C catalyst produced under Ar atmosphere even though Ar-Fe-N-C-800 have much higher specific surface area of 336.2 m2/g (in Fig. S5). By evaluation of the calcination temperature at NH3 atmosphere, we found that NH3-Fe-N-C-800 still achieved the highest activity. This indicates that Fe2N phase may be more preferable in ORR than Fe4N, Fe3N and Fe2C formed at lower temperature. And on the other hand the higher graphitization of the carbon layers in the core-shell structured catalyst could promote the electron conductivity

ACCEPTED MANUSCRIPT which may contribute to the activity improvement. We further investigate the ORR kinetics for NH3-Fe-N-C-800 by recording RDE curves. Fig. 4(c) displays that the limiting current density increases with the increase of rotation rate in O2 saturated electrolytes. The corresponding K-L

RI PT

(Koutecky-Levich) plots (j-1 vs. w-1/2) as inserted in Fig. 4(c) are nearly linear and parallel, suggesting an almost 4 electron transfer number for ORR. The RRDE voltammograms of the NH3-Fe-N-C-800 was shown in Fig. 4(d), and it can be found that the disk current density was

SC

rather small. The peroxide yield are 0.8%-6% and the average values of the electron transfer

M AN U

number (n) are calculated to be 3.94 (insert in Fig. 4d). As the durability test shown in Fig. 4(e), NH3-Fe-N-C-800 exhibits an excellent long-term stability with negligible degradation over 6000 s. Additionally we carried out toxicity resistant ability test, as the result shown in Fig. 4(f), there is typical methanol oxidation peaks appearing for the Pt/C catalyst. In contrast, NH3-Fe-N-C showed

TE D

no obvious change in the current density, suggesting that the NH3-Fe-N-C-800 catalyst had better tolerance to the methanol crossover effect. Furthermore, the CV curves and ORR performance of NH3-Fe-C-N-800 and Pt/C in 0.1 M HClO4 electrolyte was also evaluated as shown in Fig. S6. A

EP

well-developed ORR peaks was observed for the NH3-Fe-C-N-800 electrode and the onset

AC C

potential is 0.855 V and half-wave potential is 0.728 V quite close to that of Pt/C catalyst.

4. Conclusion

We have provided an efficient preparation method to construct core-shell structured Fe-N-C catalyst by complexing of ammonia iron citrate with dopamine during its polymerization process. The catalyst crystalline phase may derive into Fe3C, Fe2C, Fe3N, Fe4N, Fe2N at different preparation temperature and atmosphere, among which the obtained graphitic carbon wrapped

ACCEPTED MANUSCRIPT Fe2N under 800 oC and NH3 atmosphere showed the best performance with close activity and superior methanol crossover resistant ability than Pt/C catalyst in alkaline electrolyte. The high intrinsic activity of Fe2N, the higher graphitization of the adjacent carbon layers, the mesoporous

RI PT

structures are the contributions to the excellent catalytic behavior making itself potent candidate to replace the Pt/C catalyst. Moreover the simple preparation process could serve as an attractive

M AN U

composite for the application in catalysis and beyond.

SC

universal method to complex metal catalyst with polymer to produce core-shell metal-N-C

Acknowledgements

We acknowledge the financial support from Chinese Postdoctoral Science Foundation (2016M600519), Natural Science Foundation of Shandong Province (ZR2016BB03, and

TE D

ZR2016JL007), Qingdao Municipal Science and Technology Bureau (Grant No. 16-5-1-44-jch), the Thousand Talents Plan, the World-Class University and Discipline, the Taishan Scholar's Advantageous and Distinctive Discipline Program of Shandong Province and the world-Class

AC C

EP

Discipline Program of Shandong Province

References

[1] Su DS, Sun GQ. Nonprecious-Metal Catalysts for Low-Cost Fuel Cells. Angew. Chem. Int. Edit. 2011;50(49):11570-2.

[2] Winter M, Brodd RJ. What are batteries, fuel cells, and supercapacitors? Chem. Rev. 2004;104(10):4245-69. [3] Shao MH, Chang QW, Dodelet JP, Chenitz R. Recent Advances in Electrocatalysts for Oxygen Reduction Reaction. Chem. Rev. 2016;116(6):3594-657. [4] Wang B. Recent development of non-platinum catalysts for oxygen reduction reaction. J. Power Sources. 2005;152(1):1-15. [5] Lv M, She XL, Li Q, Sun J, Li HL, Zhao XS, et al. Synthesis of Magnetic MnFe2O4/Polyaniline Composite Microspheres and Their Electrocatalytic Activity for Oxygen Reduction Reaction. Sci. Adv.

ACCEPTED MANUSCRIPT Mater. 2015;7(9):1686-93. [6] Li Z, Gao K, Han GT, Wang RY, Li HL, Zhao XS, et al. Solvothermal synthesis of MnFe2O4 colloidal nanocrystal assemblies and their magnetic and electrocatalytic properties. New J. Chem. 2015;39(1):361-8. [7] Deng DH, Pan XL, Yu LA, Cui Y, Jiang YP, Qi J, et al. Toward N-Doped Graphene via Solvothermal Synthesis. Chem. Mater. 2011;23(5):1188-93. [8] Liang J, Jiao Y, Jaroniec M, Qiao SZ. Sulfur and Nitrogen Dual-Doped Mesoporous Graphene

RI PT

Electrocatalyst for Oxygen Reduction with Synergistically Enhanced Performance. Angew. Chem. Int. Edit. 2012;51(46):11496-500.

[9] Qu LT, Liu Y, Baek JB, Dai LM. Nitrogen-Doped Graphene as Efficient Metal-Free Electrocatalyst for Oxygen Reduction in Fuel Cells. Acs Nano. 2010;4(3):1321-6.

[10] Sheelam A, Ramanujam K. Nitrogen Functionalized Few Layer Graphene Derived from

SC

Metal-Organic Compound: A Catalyst for Oxygen Reduction Reaction. Electrochim. Acta. 2016;216:457-66.

[11] Bashyam R, Zelenay P. A class of non-precious metal composite catalysts for fuel cells. Nature. 2006;443(7107):63-6.

M AN U

[12] Perez-Alonso FJ, Dominguez C, Al-Thabaiti SA, Al-Youbi AO, Salam MA, Alshehri AA, et al. Evidences of the presence of different types of active sites for the oxygen reduction reaction with Fe/N/C based catalysts. J. Power Sources. 2016;327:204-11.

[13] Li QR, Wan G, Zhao H, Pan LY, Wang N, Zhao WP, et al. Nitrogen-Doped Carbon Vesicles with Dual Iron-Based Sites for Efficient Oxygen Reduction. Chemsuschem. 2017;10(3):499-505. [14] Dominguez C, Perez-Alonso FJ, Salam MA, Al-Thabaiti SA, Pena MA, Barrio L, et al. Effect of the N content of Fe/N/graphene catalysts for the oxygen reduction reaction in alkaline media. J. Mater.

TE D

Chem. A. 2015;3(48):24487-94.

[15] Liu C, Wang J, Li JS, Luo R, Sun XY, Shen JY, et al. Fe/N decorated mulberry-like hollow mesoporous carbon fibers as efficient electrocatalysts for oxygen reduction reaction. Carbon. 2017;114:706-16.

[16] Ma XX, Lei ZC, Feng WM, Ye YL, Feng CH. Living Fe mineral@bacteria encrustation-derived

EP

and self-templated preparation of a mesoporous Fe-N-C electrocatalyst with high activity for oxygen reduction. Carbon. 2017;123:481-91.

[17] Videla AHAM, Ban S, Specchia S, Zhang L, Zhang JJ. Non-noble Fe-N-x electrocatalysts

AC C

supported on the reduced graphene oxide for oxygen reduction reaction. Carbon. 2014;76:386-400. [18] Bezerra CWB, Zhang L, Lee KC, Liu HS, Marques ALB, Marques EP, et al. A review of Fe-N/C and Co-N/C catalysts for the oxygen reduction reaction. Electrochim. Acta. 2008;53(15):4937-51. [19] Hu Y, Jensen JO, Zhang W, Martin S, Chenitz R, Pan C, et al. Fe3C-based oxygen reduction catalysts: synthesis, hollow spherical structures and applications in fuel cells. J. Mater. Chem. A. 2015;3(4):1752-60.

[20] Yu DS, Wei L, Jiang WC, Wang H, Sun B, Zhang Q, et al. Nitrogen doped holey graphene as an efficient metal-free multifunctional electrochemical catalyst for hydrazine oxidation and oxygen reduction. Nanoscale. 2013;5(8):3457-64. [21] Wang ZJ, Jia RR, Zheng JF, Zhao JG, Li L, Song JL, et al. Nitrogen-Promoted Self-Assembly of N-Doped Carbon Nanotubes and Their Intrinsic Catalysis for Oxygen Reduction in Fuel Cells. Acs Nano. 2011;5(3):1677-84. [22] Yang WX, Liu XJ, Yue XY, Jia JB, Guo SJ. Bamboo-like Carbon Nanotube/Fe3C Nanoparticle

ACCEPTED MANUSCRIPT Hybrids and Their Highly Efficient Catalysis for Oxygen Reduction. J. Am. Chem. Soc. 2015;137(4):1436-9. [23] Zhou TS, Zhou Y, Ma RG, Liu Q, Zhu YF, Wang JC. Achieving excellent activity and stability for oxygen reduction electrocatalysis by hollow mesoporous iron-nitrogen-doped graphitic carbon spheres. J. Mater. Chem. A. 2017;5(24):12243-51. [24] Lee JS, Park GS, Kim ST, Liu ML, Cho J. A Highly Efficient Electrocatalyst for the Oxygen Reduction Reaction: N-Doped Ketjenblack Incorporated into Fe/Fe3C-Functionalized Melamine Foam.

RI PT

Angew. Chem. Int. Edit. 2013;52(3):1026-30.

[25] Wu R, Wang J, Chen K, Chen SG, Li J, Wang QM, et al. Space-Confined Pyrolysis for the Fabrication of Fe/N/C Nanoparticles as a High Performance Oxygen Reduction Reaction Electrocatalyst. Electrochim. Acta. 2017;244:47-53.

[26] Wang HT, Wang W, Gui MX, Asif M, Wang ZY, Yu Y, et al. Uniform Fe3O4/Nitrogen-Doped

SC

Mesoporous Carbon Spheres Derived from Ferric Citrate-Bonded Melamine Resin as an Efficient Synergistic Catalyst for Oxygen Reduction. Acs Appl. Mater. Inter. 2017;9(1):335-44. [27] Hou Y, Huang TZ, Wen ZH, Mao S, Cui SM, Chen JH. Metal-Organic Framework-Derived Nitrogen-Doped Core-Shell-Structured Porous Fe/Fe3C@C Nanoboxes Supported on Graphene Sheets

M AN U

for Efficient Oxygen Reduction Reactions. Adv. Energy. Mater. 2014;4(11).

[28] Dominguez C, Perez-Alonso FJ, Salam MA, Al-Thabaiti SA, Pena MA, Garcia-Garcia FJ, et al. Repercussion of the carbon matrix for the activity and stability of Fe/N/C electrocatalysts for the oxygen reduction reaction. Appl. Catal. B-Environ. 2016;183:185-96.

[29] Chen XQ, Yu L, Wang SH, Deng DH, Bao XH. Highly active and stable single iron site confined in graphene nanosheets for oxygen reduction reaction. Nano Energy. 2017;32:353-8. [30] Wang J, Wu HH, Gao DF, Miao S, Wang GX, Bao XH. High-density iron nanoparticles

TE D

encapsulated within nitrogen-doped carbon nanoshell as efficient oxygen electrocatalyst for zinc-air battery. Nano Energy. 2015;13:387-96.

[31] Liu BC, Huang BB, Lin C, Ye JS, Ouyang LZ. Porous carbon supported Fe-N-C composite as an efficient electrocatalyst for oxygen reduction reaction in alkaline and acidic media. Appl. Surf. Sci. 2017;411:487-93.

EP

[32] Yin H, Zhang CZ, Liu F, Hou YL. Hybrid of Iron Nitride and Nitrogen-Doped Graphene Aerogel as Synergistic Catalyst for Oxygen Reduction Reaction. Adv. Funct. Mater. 2014;24(20):2930-7. [33] Huang XX, Yang ZY, Dong B, Wang YZ, Tang TY, Hou YL. In situ Fe2N@N-doped porous

AC C

carbon hybrids as superior catalysts for oxygen reduction reaction. Nanoscale. 2017;9(24):8102-6. [34] Lai QX, Zheng LR, Liang YY, He JP, Zhao JX, Chen JH. Meta-Organic-Framework-Derived Fe-N/C Electrocatalyst with Five-Coordinated Fe-N-x), Sites for Advanced Oxygen Reduction in Acid Media. Acs Catal. 2017;7(3):1655-63. [35] Tylus U, Jia QY, Strickland K, Ramaswamy N, Serov A, Atanassov P, et al. Elucidating Oxygen Reduction Active Sites in Pyrolyzed Metal-Nitrogen Coordinated Non-Precious-Metal Electrocatalyst Systems. J. Phys. Chem. C. 2014;118(17):8999-9008. [36] Jiang WJ, Gu L, Li L, Zhang Y, Zhang X, Zhang LJ, et al. Understanding the High Activity of Fe-N-C Electrocatalysts in Oxygen Reduction: Fe/Fe3C Nanoparticles Boost the Activity of Fe-N-x. J. Am. Chem. Soc. 2016;138(10):3570-8. [37] Wang M, Yang YS, Liu XB, Pu ZH, Kou ZK, Zhu PP, et al. The role of iron nitrides in the Fe-N-C catalysis system towards the oxygen reduction reaction. Nanoscale. 2017;9(22):7641-9. [38] Deng DH, Yu L, Chen XQ, Wang GX, Jin L, Pan XL, et al. Iron Encapsulated within Pod-like

ACCEPTED MANUSCRIPT Carbon Nanotubes for Oxygen Reduction Reaction. Angew. Chem. Int. Edit. 2013;52(1):371-5. [39] Strickland K, Elise MW, Jia QY, Tylus U, Ramaswamy N, Liang WT, et al. Highly active oxygen reduction non-platinum group metal electrocatalyst without direct metal-nitrogen coordination. Nat. Commun. 2015;6. [40] Yang F, Deng DH, Pan XL, Fu Q, Bao XH. Understanding nano effects in catalysis. Natl. Sci. Rev. 2015;2(2):183-201. [41] Li YR, Guo CZ, Li JQ, Liao WL, Li ZB, Zhang J, et al. Pyrolysis-induced synthesis of iron and

RI PT

nitrogen-containing carbon nanolayers modified graphdiyne nanostructure as a promising core-shell electrocatalyst for oxygen reduction reaction. Carbon. 2017;119:201-10.

[42] Deng J, Deng DH, Bao XH. Robust Catalysis on 2D Materials Encapsulating Metals: Concept, Application, and Perspective. Adv. Mater. 2017;DOI: 10.1002/adma.201606967.

[43] Liu YL, Ai KL, Lu LH. Polydopamine and Its Derivative Materials: Synthesis and Promising

SC

Applications in Energy, Environmental, and Biomedical Fields. Chem. Rev. 2014;114(9):5057-115.

[44] Liu MY, Zeng GJ, Wang K, Wan Q, Tao L, Zhang XY, et al. Recent developments in polydopamine: an emerging soft matter for surface modification and biomedical applications. Nanoscale. 2016;8(38):16819-40.

M AN U

[45] Lee H, Dellatore SM, Miller WM, Messersmith PB. Mussel-inspired surface chemistry for multifunctional coatings. Science. 2007;318(5849):426-30.

[46] Ang JM, Du YH, Tay BY, Zhao CY, Kong JH, Stubbs LP, et al. One-Pot Synthesis of Fe(III)-Polydopamine Complex Nanospheres: Morphological Evolution, Mechanism, and Application of

the

Carbonized

Hybrid

Nanospheres

2016;32(36):9265-75.

in

Catalysis

and

Zn-Air

Battery.

Langmuir.

[47] Zeng T, Yu MD, Zhang HY, He ZG, Zhang XL, Chen JM, et al. In situ synthesis of cobalt

TE D

ferrites-embedded hollow N-doped carbon as an outstanding catalyst for elimination of organic pollutants. Sci. Total. Environ. 2017;593:286-96. [48] Kong

JH,

Shahabadi

SIS,

Lu

XH.

Integration

of

inorganic

nanostructures

with

polydopamine-derived carbon: tunable morphologies and versatile applications. Nanoscale. 2016;8(4):1770-88.

EP

[49] Sever MJ, Weisser JT, Monahan J, Srinivasan S, Wilker JJ. Metal-mediated cross-linking in the generation of a marine-mussel adhesive. Angew. Chem. Int. Edit. 2004;43(4):448-50. [50] Ma ZY, Jia X, Hu JM, Zhou F, Dai B. Mussel-inspired chemistry for one-step synthesis of

AC C

N-doped carbon-gold composites with morphology tailoring and their catalytic properties. Rsc. Adv. 2014;4(4):1853-6.

[51] Wang F, Song SY, Li K, Li JQ, Pan J, Yao S, et al. A "Solid Dual-Ions-Transformation" Route to S,N Co-Doped Carbon Nanotubes as Highly Efficient "Metal-Free" Catalysts for Organic Reactions. Adv. Mater. 2016;28(48):10679-+. [52] Chen JL, Li WB, Xu BQ. Nitrogen-rich Fe-N-C materials derived from polyacrylonitrile as highly active and durable catalysts for the oxygen reduction reaction in both acidic and alkaline electrolytes. J. Colloid. Interf. Sci. 2017;502:44-51. [53] Yu L, Pan XL, Cao XM, Hu P, Bao XH. Oxygen reduction reaction mechanism on nitrogen-doped graphene: A density functional theory study. J. Catal. 2011;282(1):183-90. [54] Kamiya K, Hashimoto K, Nakanishi S. Instantaneous one-pot synthesis of Fe-N-modified graphene as an efficient electrocatalyst for the oxygen reduction reaction in acidic solutions. Chem. Commun. 2012;48(82):10213-5.