N-source for APEFC

Accepted Manuscript

Highly efficient Fe/N/C catalyst using adenosine as C/N-source for APEFC

http://www.journals.elsevier.com/ journal-of-energy-chemistry/

Huan Ren , Ying Wang , Xun Tang , Juntao Lu , Li Xiao , Lin Zhuang PII: DOI: Reference:

S2095-4956(17)30213-9 10.1016/j.jechem.2017.05.001 JECHEM 316

To appear in:

Journal of Energy Chemistry

Received date: Revised date: Accepted date:

17 March 2017 19 April 2017 2 May 2017

Please cite this article as: Huan Ren , Ying Wang , Xun Tang , Juntao Lu , Li Xiao , Lin Zhuang , Highly efficient Fe/N/C catalyst using adenosine as C/N-source for APEFC, Journal of Energy Chemistry (2017), doi: 10.1016/j.jechem.2017.05.001

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Highly efficient Fe/N/C catalyst using adenosine as C/N-source for APEFC Huan Ren, Ying Wang, Xun Tang, Juntao Lu, Li Xiao*, Lin Zhuang* College of Chemistry and Molecular Sciences, Hubei Key Lab of Electrochemical Power Sources, Wuhan University, Wuhan 430072, Hubei, China *

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Corresponding authors. E-mail: [email protected]; [email protected].

Abstract

An environmentally friendly precursor, adenosine, has been used as a dual source of C and N

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to synthesize nitrogen-doped carbon catalyst with/without Fe. A hydrothermal carbonization method has been used and water is the carbonization media. The morphology of samples with/without Fe component has been compared by HRTEM, and the result shows that Fe can promote the graphitization of carbon. Further electro-chemical test shows that the oxygen reduction reaction (ORR) catalytic activity of Fe-containing sample (C-FeN) is much higher

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than that of the Fe-free sample (C-N). Additionally, the intermediates of C-FeN formed during

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each synthetic procedure have been thoroughly characterized by multiple methods, and the function of each procedure has been discussed. The C-FeN sample exhibits high electro-catalytic stability and superior electro-catalytic activity toward ORR in alkaline media,

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with its half-wave potential 20 mV lower than that of commercial Pt/C (40 wt%). It is further

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incorporated into alkaline polymer electrolyte fuel cell (APEFC) as the cathode material and led to a power density of 100 mW/cm2.

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Keywords

N-doped carbon catalyst; ORR; Fuel cell; Alkaline polymer electrolyte; Fe/N/C 1. Introduction Alkaline polymer electrolyte fuel cells (in fuel cell test) have attracted widespread attentions due to their capability to use low-cost nonprecious metal catalysts [1–5]. As one of the potential nonprecious metal catalysts for application, N-doped carbon was considered 1

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promising for its low price, rich reserves, and good oxygen reduction reaction (ORR) catalytic activity [6–10]. Great efforts have been made to synthesis this kind of catalysts, including using various synthetic methods, such as thermal decomposition, pyrolysis and nitrogen-ion bombardment, and using various types of N-containing sources, such as compounds and polymers [11], gaseous N-precursor (NH3 or CH3CN) [12,13]. Among these methods, hydrothermal carbonization (HTC) method is considered to be mild, versatile, inexpensive,

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controllable and effective [14,15]. It is mild because the N-containing carbonaceous species can be produced below 300 oC. It is versatile and inexpensive because various precursors can be used as C/N source, from crude plants with complex chemical components [16–18] to isolated carbohydrates with well-defined formula [19–21]. For example, in Titirici’s work

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[19], several precursors, such as glucose, D-glucosamine, N-acetyl-D-glucosamine and phenolic compounds, were used to synthesize N-doped carbon aerogels with high BET surface areas and high nitrogen contents by HTC method. Moreover, this method is controllable and effective because the synthesized carbonaceous materials usually have

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porous structures with controllable morphology and functionality [14,15]. For example, Zhao and co-works use chitosan as a precursor to synthesize graphitic nitrogen-doped carbon

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nanoparticles (N-HCNPs) via a facile HTC method, and the final product possesses a high surface area and a highly desired four-electron ORR catalytic activity [20].

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Despite the achievements above, the function of metal component in nitrogen-doped carbon catalysts is still unclear. Although it has been announced that the ORR active site is

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created by pyridinic N [13,22], none of the “metal-free” catalysts really exhibits ORR catalytic activities comparable to that of Pt/C, especially under the same carbon loading.

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Hence, some researchers still believe the use of transition metals (Fe, Co) as the doping component is necessary to further activate N-doped carbon catalyst. These transition metal components are expected to either serve directly as the catalytic active sites (e.g. in a form of FeNx) [23–25], or act as a promoter to tune the electronic property of N-doped carbon [26– 28]. In this work, a highly efficient N-doped carbon catalyst has been synthesized by HTC method. An environmentally friendly precursor, adenosine, has been used as dual source of C 2

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and N. For comparison, the intermediates of sample with Fe component were carefully characterized to discuss the role that each synthetic procedure played. Moreover, samples with/without Fe component have been synthesized and tested, and the function of Fe has been discussed. The ORR tests show that the Fe-containing sample (C-FeN) has much better ORR catalytic activity than the intermediates and the Fe-free sample (C-N), and its performance is comparable with commercial Pt/C (40 wt%) in alkaline media with its half-wave potential

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shifted 20 mV negatively. The C-FeN sample also show superior performance in APEFC test, with the power density as 100 mW/cm2. 2. Experimental 2.1. Catalysts preparation

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FeCl3•6H2O (2.7 g) and adenosine (0.8 g) were dispersed in 8 mL ultrapure water. After fully mixed, the mixture was transferred into a stainless-steel autoclave (10 mL capacity) and then heated in an oven at 180 oC for 12 h. After cooled to room temperature, a black precipitate was obtained. This black precipitate was centrifuged and washed with ultrapure water, ethanol

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and acetone for several times, and then dried at 80 oC. The obtained black powder was further thermal decomposed at 800 oC under flowing argon for 3 h and cooled to room temperature. It

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was further ball-milled and stirred in 0.5 M H2SO4 at 80 oC for 10 h, and then centrifuged and washed with ultrapure water, ethanol and acetone for several times. After dried at 80o, the

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powder was treated at 800 oC under flowing argon for 3 h and cooled to room temperature to

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obtain the final product.

2.2. Physical characterization The crystalline structure was characterized by X-ray diffraction (XRD, Shimazu XRD-6000)

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at a scanning rate of 4 o/min. The morphology was characterized by scanning electron microscope (SEM, Supra 55, Zeiss) and high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2100). The Brunauer-Emmett-Teller (BET) surface area (SBET) was obtained by using N2 adsorption-desorption isotherms measured at 77 K using a constant-volume adsorption apparatus (Nova 4200e, Quantachrome). Prior to measurement, the samples were out-gassed at 100 oC under a N2 flow for 2 h in the pressure range, 10-6–10-5 3

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Torr. The SBET values ofthe samples were calculated by using adsorption isotherms for relative pressures (P/P0) at 0.987. The X-ray photoelectron spectroscopy (XPS) was used to analyze the chemical state of doped N element. The XPS measurements were recorded on a spectrometer (XSAM800, Kratos) using Mg K radiation (h = 1253.6 eV). The quantitative analyses of N were based on the peak intensities of N 1s. The intrepid XSP radial inductively coupled plasma optical emission spectrometry ICP-OES (Thermo) was used to analyze the Fe

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content in the samples. 2.3. Electrochemical measurement

To prepare the working electrode, 5 mg sample was dispersed ultrasonically in 1 mL Nafion alcohol solution (0.05 wt%). 10 L of the suspension was pipetted onto a glassy carbon (GC)

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electrode ( 4 mm). The coated electrode was dried under an infrared lamp, with the sample loading as 0.4 mg/cm2. For comparison, Pt/C (40%, Johnson Matthey Co.) was characterized by the same electrode preparation method, with the Pt/C loading as 51 g/cm2. ORR catalytic activity was evaluated in deaerated and O2 saturated 1 M KOH solution at

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room temperature. A sheet of carbon paper was used as the counter electrode to avoid contamination of Pt containing species of Pt counter electrode dissolved during

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electrochemical measurement [29]. Hg/HgO in the same solution was used as the reference electrode. All potentials in this paper were converted to refer to RHE. The potentiostat was a

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CHI-600A electrochemical station. The rotation rate was 900 rpm, and the ORR curves were recorded by scanning at 5 mV/s. The background capacitive current measured in Ar-saturated

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1 M KOH solution has been subtracted to plots the ORR polarization curves. To evaluate the H2O2 yield and the electron transfer number of the catalysts, the Pt ring

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potential was set to 1.3 V (vs. RHE) in the course of RRDE measurements. The H2O2 yield was calculated by the following equation:

H 2O2 (%)  200 

I R / N0 ( I R / N0 )  I D

Where IR and ID are the ring and disk currents, respectively, and N0=0.22 is the collection efficiency of Pt ring [30]. 2.4. Fuel cell test 4

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A H2-O2 Fuel Cell was tested (850e Multi range, Scribner Associate Co.) under a galvanic mode using fully humidified H2 and O2 gases flowing at a rate of 200 mL/min and at temperature of 50 oC, 0 MPa. C6 xaQAPS was used as the alkaline membrane to produce the catalyst-coated membrane (CCM) [31–33]. The anode and cathode were 0.4 mg/cm2 Pt (60 wt%, Johnson Matthey Co.) and 2 mg/cm2 C-FeN, with the area of the electrode as 4 cm2. The weight percentage of C12 ionomer in both the anode and the cathode was calculated to be 20

GDS3250) to make the membrane electrode assembly (MEA). 3. Results and discussion

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3.1. Physical-chemical characterization

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wt%. The resulting CCM were pressed between two pieces of carbon paper (AvCard

To fully understand the function of each synthetic procedure, the intermediates and the final sample were carefully characterized. Samples named as C-hydro, C-pyrolysis, C-acid and C-FeN, refer to samples that obtained after hydrothermal carbonization, first pyrolysis, acid-leaching and second pyrolysis process, respectively. As shown in Figure 1, the XRD

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pattern of each sample was compared with the standard XRD pattern of Fe (PDF#06-0696), Fe15.1C (PDF#52-0512) and Graphite C (PDF#65-6212). Only a broad peak was found for

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C-hydro, indicating that the sample was amorphous after hydrothermal process. After the first pyrolysis, the feature of graphite, Fe and Fe15.1C appeared in sample C-pyrolysis, indicating

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the formation of these compositions at 800 oC. Further acid-leaching step maintained the

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composition of graphite, Fe and Fe15.1C in sample C-acid, but the peak intensity of Fe was greatly decreased, indicating that the majority of Fe was dissolved in 0.5 M H2SO4 during

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stirring. After the second pyrolysis, only features of graphite carbon and Fe15.1C can be found in sample C-FeN, while features of Fe was almost disappeared, indicating that Fe left in sample C-acid reacted with carbon to form Fe15.1C during the second pyrolysis process. The peak intensity of graphite at 26.5o was greatly intensified comparing with that of C-acid, indicating that the graphitization degree was increased by pyrolysis treatment.

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Figure 1. (a) XRD patterns of samples C-hydro, C-pyrolysis, C-acid and C-FeN. (b) N2 adsorption/desorption isotherms for C-hydro, C-pyrolysis, C-acid and C-FeN. Inset: pore size

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distribution of the samples calculated by BJH method.

Other than the structure of the material, surface composition, especially the chemical environment of nitrogen plays an important role on the ORR catalytic activity of the nitrogen doped carbon catalysts [34,35]. Figure 2 shows the XPS spectra of each sample at N 1s region,

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and Table 1 listed the corresponding nitrogen species and their percentage on the surface. Only one peak was found for sample C-hydro at 400.06 eV, which was attributed to amine N

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[36], indicating that no N-doped carbon was formed during hydrothermal process. After the first pyrolysis process, the N 1s peak was split into five peaks, which were ascribed to the

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formation of oxidized N (403.0 eV), graphiticN (401.2 eV), pyrrolic N (400.1 eV), Fe-Nx (399.2 eV), and pyridinic N (398.3 eV), respectively. The XPS spectra of sample C-acid and

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C-FeN share a similar profile as that of C-pyrolysis, with the proportion of each N-containing species slightly changed. As shown in Table 1, the percentage of total N decreased by half

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after acid-leaching, from 2.47% to 1.15%, indicating that one or more N species are not stable in acidic environment. The percentage of Fe-Nx significantly decreased from 0.66% to 0.05%, indicating that Fe-Nx is very unstable in acidic media. At the same time, the percentage of pyrrolic N and graphitic N was reduced to ~1/5 and 1/2, respectively, but the percentage of pyridinic N remain unchanged, indicating that pyridinic N is the most stable N specie in the acidic media. After second pyrolysis, the percentage of the total N and each N-containing species in sample C-FeN was similar as that of sample C-acid, indicating that the 6

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N-containing species after acid-leaching were relatively stable during second pyrolysis

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process.

Figure 2. N 1s XPS spectra of (a) C-hydro, (b) C-pyrolysis, (c) C-acid, and (d) C-FeN.

Sample

Ntotal

Pyridinic N

Fe-Nx

Pyrrolic N

Graphitic N

Oxidized N

Amine N

SBET

Pore size

2

(%)

(%)

(%)

(%)

(%)

(%)

(m /g)

(nm)

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(%)

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Table 1. XPS data and BET results for samples C-hydro, C-pyrolysis, C-acid, and C-FeN.

7.53

0

0

0

0

0

7.53

312.2

3.69

C-pyrolysis

2.47

0.31

0.66

0.37

0.98

0.15

0

320.7

3.70

C-acid

1.15

0.34

0.05

0.07

0.48

0.21

0

232.5

3.67

C-FeN

1.18

0.24

0.09

0.05

0.58

0.22

0

364.7

3.69

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C-hydro

Calculated as the atomic percent of N in the sample when N, C and Fe were considered.

The morphology of the catalyst can have strong influence on its catalytic activity. Figure 3 shows the SEM images of the samples in two magnifications. Sample C-hydro gives a 7

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smooth surface and spherical shape, with the diameter from 2 to 5 m (Figure 3a), which is a typical shape for carbon synthesized by HTC method [14,37,38]. After pyrolysis, the shape and size of the particles remain unchanged, but the surface of the particles was no longer smooth but with cracks all over the surface (Figure 3a). Further ball mill and acid-leaching step split the carbon spheres into submicron particles (Figure 3c), and the second pyrolysis did not further change the morphology of these particles (Figure 3d). Although it is apparent

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to find that the morphology of the samples changed greatly from sample C-hydro to sample C-FeN, the SBET surface area was found to be almost identical for these two samples, which is 312.2 m2/g and 320.7 m2/g, respectively. The pore size of these two samples was also the same as 3.69 nm according to BET characterization, indicating that the pores were formed

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during the hydrothermal process, and the following treatments did not create new pore

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structure (Figure 1b).

Figure 3. SEM images of samples obtained in each synthesis step. (a) C-hydro, (b) C-pyrolysis, (c)

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C-acid, (d) C-FeN. The scale bars are 400 nm in image (a-1) to (d-1), and 10 m in image (a-2) to (d-2).

The role that Fe played in the catalyst is pretty important, but the details remain unclear. Some researchers believe that Fe improve the ORR activity by forming the FeNx active site [39,40]. Others, such as Xing’s and Li’s groups, believe that FexCy can improve the ORR catalytic activity by modulating the electronic structure of carbon [27,41]. In this work, in order to elucidate the influence of Fe to the sample, we used the same synthesis procedure 8

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without using FeCl3 precursor as a comparison, and this sample was named as C-N. Figure 4(a) and 4(b) compared the HRTEM images of final sample with/without using FeCl3 precursor. As shown in Figure 4(b1), it was clear that there were uniformly dispersed nanoparticles with size around 10 nm carbon when FeCl3 precursor was added, while no such dark spots were found without using FeCl3 (Figure 4a1). By combining this observation with the XRD feature of C-FeN, we thus infer these nanoparticles observed in C-FeN to be Fe15.1C.

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A closer look at these two samples will indicate that the carbon structure in C-N is amorphous, without lattice fringe (Figure 4a2). While clear lattice fringe, pointed by yellow arrows, can be found in carbon structure for sample C-FeN (Figure 4b2), indicating that Fe can promote the graphitization of carbon [42]. The bulk content of Fe was further characterized by

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ICP-OES, and the result shows that it was 3.29 wt% for C-hydro, 4.94 wt% for C-pyrolysis, 0.70 wt% for C-acid and 1.03 wt% for C-FeN, respectively, indicating acid-leaching dissolved ~80% of Fe in the sample C-pyrolysis, and there was still ~20% of Fe left in the final sample. Because the XPS result shows no Fe 2p signal on the surface of C-FeN, Fe was

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considered to form Fe15.1C in C-FeN and covered by carbon shell. According to Xing and Li’s study, this kind of encased carbide nanoparticles may activate the graphitic layers on their

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surface, and further promote the ORR catalytic activity [27].

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Figure 4. HRTEM images of (a-1, a-2) C-N, the scale bar in (a-1) and (a-2) is 100 nm and 5 nm,

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respectively. (b-1, b-2) C-FeN, the scale bar in (b-1) and (b-2) is 50 nm and 5 nm, respectively.

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3.2. ORR studies

The ORR catalytic activity of all samples was tested in 1 M KOH by a rotation disk

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electrode (RDE) method. As shown in Figure 5(a), the ORR catalytic activity of C-hydro is the lowest with no significant ORR current generated until 0.75 V vs. RHE. After the first

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pyrolysis, the ORR catalytic activity of C-pyrolysis is much improved comparing with C-hydro, with the onset potential positively moved to 0.95 V vs. RHE, indicating that the

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pyrolysis after hydrothermal process is necessary. The catalytic activity improvement may due to the graphite formation at high temperature according to XRD characterization (Figure 1) and the formation of pyridinic N and Fe-Nx active sites according to XPS characterization. (Figure 2). Nevertheless, the diffusion limit current (Id) of C-hydro is much lower than Id with four-electron transferred pathway, which may due to the covering of active sites by unstable phase (eg: Fe) [43]. After acid leaching, the ORR activity of C-acid was clearly improved with Id larger than that of C-pyrolysis, and this improvement is possibly due to the removal of 10

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the majority of Fe during acid-leaching step according to XRD and XPS characterization. The second pyrolysis process further increased the ORR activity of C-FeN, which may due to the increase of graphitization degree according to XRD characterization (Figure 1) and the increase of the sample surface area according to BET analysis (Table 1). The ORR catalytic activity of C-FeN is compared with that of a commercial Pt/C catalyst (40 wt%, Johnson Matthey Co., with a Pt loading of 51 g/cm2). The ORR polarization curve of C-FeN has an

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onset potential close to that of Pt/C, and a half-wave potential (E1/2, ca. 0.875 V) negatively shifted by 20 mV comparing with that of Pt/C(Figure 5b). The Id of C-FeN is the same as Pt/C, indicating the C-FeN catalyst has a four-electron transferred pathway for ORR. In contrast, without the use of FeCl3 precursor, the ORR activity of C-N is greatly decreased, with a much

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negative E1/2 (0.84 V vs. RHE) and an Id smaller than that of four-electron reduction reaction, indicating that the Fe-containing species play an important role in the ORR catalytic activity. The Tafel plots for C-FeN and Pt/C were shown in the insert of Figure 5(b). At low over-potential region, the Tafel slope for C-FeN is 69.3 mV per decade, which is almost the

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same as that of Pt/C (69.4 mV/decade), indicating that the ORR kinetics of C-FeN is similar to that of Pt/C at low over-potential region. The Tafel slope of for C-N is slightly bigger than

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that of Pt/C, indicating a mixed ORR pathway.

Accelerated durability test was performed to assess the stability of catalysts in alkaline

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media by cycling the electrode potential between 0.6 V and 1.0 V for 10000 circles. The change of C-FeN experienced a decay by ca. 8 mV negative shift in half-wave potential

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(Figure 5c).

The H2O2 yield and electron transfer number of C-FeN, Pt/C and C-N were measured by

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RRDE method in 1 M KOH in the range of 0.3~0.8 V (vs. RHE) as shown in Figure 5(d). The H2O2 yield of C-FeN is below 13% that corresponds to the electron transfer number of 3.87±0.1, indicating a four-electron transferred reaction pathway of C-FeN, which is the same as Pt/C. In contrast, the H2O2 yield of C-N is much higher than that of C-FeN and Pt/C, with the electron transfer number as 3.66±0.15, which can be regarded as a mixed pathway of two-electron and four-electron transfer reaction.

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Figure 5. (a) ORR curves of C-hydro, C-acid, C-N and C-FeN. (b) ORR curves of C-N, C-FeN and Pt/C; insert: Tafel plots of C-N, C-FeN and Pt/C. (c) Stability test of C-FeN. (d) H2O2 yield

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test by RRDE, the potential of the ring electrode was holding at 1.3 V vs. RHE. All the RDE tests

mV/s.

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are performed at a rotation rate of 900 rpm in O2-saturated 1 M KOH, and the scanning rate is 5

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3.3. Fuel cell performance

The performance of the H2-O2 single APEM fuel cell with the C-FeN cathode catalyst is

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shown in Figure 6. Pt/C catalysts are used in the anode with a Pt loading of 0.4 mg/cm2. The maximum power output was 100 mW/cm2 at 50 oC. This result shows that the C-FeN catalyst is not only highly active in electrochemical performance, but also applicable in fuel cells.

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Figure 6. APEFC single-cell performance test. Anode: 60 wt% Pt/C, Pt loading 0.4 mg/cm2. Cathode: C-FeN, loading 2 mg/cm2. Membrane: C6xa-QAPS. APE ionomer in electrodes:

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C12xa-QAPS (20 wt% in the catalyst layer). Fuel cell operating conditions including: temperature, 50 °C, backpressure of gas, 0 MPa at each side of the cell. Fully humidified H2 and O2 were fed in

4. Conclusions

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a flow rate of 200 mL/min.

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In the present work, we report a new precursor to synthesize Fe/N/C catalyst as a dual

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source for N and C. Adenosine was first polymerized by HTC method, and further treated by pyrolysis, acid-leaching and a second pyrolysis process. The morphology, structure and ORR

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catalytic activities of the intermediates were carefully characterized to identify the function of each synthetic procedure. The obtained C-FeN catalyst has a superior ORR catalytic activity in comparison with Pt/C(40 wt%) and sample without using of Fe precursor (C-N) in alkaline media. Its high ORR catalytic activity is attributed to Fe-containing species and pyridinic N formed on its surface. The H2-O2 single APEM fuel cell performance of C-FeN was also tested, with a maximum power output as 100 mW/cm2 at 50 oC, indicating that the C-FeN catalyst is applicable in fuel cells. We believe that the performance of C-FeN can be further improved by increasing the density of the pyridinic N and Fe-Nx active sites, and by enlarging 13

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the pore size to improve the mass transfer.

Acknowledgments This work was financially supported by the National Natural Science Foundation of China (21573167, 21633008, 91545205, 21125312), National Key Research and Development Program (2016YFB0101203), the National Basic Research Program (2012CB932800,

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2012CB215500), the Doctoral Fund of Ministry of Education of China (20110141130002) andthe Fundamental Research Funds for the Central Universities (2014203020207). We are grateful to Dr. Luxi Shen at Cornell University for editing the manuscript.

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Graphical Abstract

Description

Adenosine has been used to synthesize highly active Fe/N/C catalyst. The ORR catalytic

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activity of the catalyst is superior not only on RDE test but also on alkaline polymer

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electrolyte membrane fuel cell application.

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