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Research Article http://dx.doi.org/10.22180/na174

Volume 1, Issue 2, 2016

Highly Porous Nitrogen-Doped Carbon Nanofibers as Efficient Metal-Free Catalysts toward the Electrocatalytic Oxygen Reduction Reaction Yongfang Chen, ab Qian Liu a* and Jiacheng Wang a* a

State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China. b University of the Chinese Academy of Sciences, Beijing 100049, P. R. China. *Corresponding authors: [email protected] (Q. L.); [email protected] (J. W.) Received Feb. 25, 2016; Revised April 29, 2016

Citation: Y. Chen, Q. Liu and J. Wang, Nano Adv., 2016, 1, 79−89. Activated N-doped carbon nanofibers (ANCNFs) were successfully prepared by the combination of electrospinning and following chemical activation of polyacrylonitrile (PAN) as metal-free electrocatalysts for oxygen reduction reaction (ORR). The increase in the activation temperature leads to not only increased specific surface areas and pore volume, and larger percentage of microporosity, but also the gradual loss of nitrogen and continuous increase in the percentage of graphitic nitrogen species. The activation evidently improves the ORR activity of nanofibers in both alkaline and acidic medium. Among the activated samples, ANCNF-800 prepared by KOH activation at 800 °C showed the best electrocatalytic activity, which is comparable but has superior long-life stability and tolerance to methanol crossover to commercial Pt/C for the ORR with an inherit four-electron-transfer pathway in alkaline medium. It should be mainly ascribed to the largest percentage of graphitic nitrogen groups and increased exposure of catalytic sites resulting from significant increase in the specific surface area. KEYWORDS: Carbon nanofibers; Chemical activation; Electrospinning; Electrocatalysis; Oxygen reduction reaction

Considering their wide availability, lightweight, adjustable porosity, low-cost, fast adsorption kinetics, high chemical and thermal stability, and controllable chemical properties by heteroatom doping,4 porous carbon materials are widely studied in many fields including CO2 capture,5 electrochemical applications,6 H2 storage,4c, 6b, 7 catalytic supports,8 biomedical application,9 etc. Recently, lots of studies are focused on the application of various heteroatom (N, S, P, etc)-doped porous carbons materials as metal-free electrocatalysts for ORR because the introduction of heteroatoms could break the electroneutrality of carbon framework to produce the charged sites that are preferable to adsorb and activate oxygen molecules, thus increasing the ORR activity.10 Among these heteroatom-doped carbon materials, N-doped carbon materials (e. g. graphene,11 nanotubes,12 nanocubes,13 and hierarchical porous carbons 14) have shown to be the best for ORR compared to other heteroatom-doped carbon materials. For example, N-doped carbon nanotube arrays, synthesized by a chemical vapour deposition (CVD) process, demonstrated the high activity for ORR with long-term stability. The substitute of carbon by nitrogen in CNTs could form the positive charge density on the adjacent carbon atoms due to the larger electronegativity.

1. Introduction Fuel cells attract researchers’ interest for their high efficiency and environment friendlies. The oxygen reduction reaction (ORR) at the cathode of fuel cell plays a key role in controlling the performance of fuel cell.1 Prominent ORR electrocatalysts are essential in terms of desired current density, low overpotential and high chemical stability. Conventionally, noble metal-based materials, e. g. platinum-based materials, are extensively studied as the electrocatalysts for ORR.2 However, their large-scale production and application in commercial field suffer from some significant disadvantages of high cost, limited supply, activity deterioration with time, and crossover to methanol. Various transition metal-based compounds composited with various nano-carbon materials have been prepared as the electrocatalysts for ORR, but they also have unavoidable drawbacks, such as high-cost, low activity and durability in strong acid or basic electrolytes, detrimental environmental effects derived from catalyst wastes and unwanted side-products.3 Therefore, it is necessary to find efficient and economical substitute with good activity and long-term durability for ORR.

Nano Adv., 2016, 1, 79−89.

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However, the synthesis procedures of the above carbon nanomaterials are relatively complicated, thus limiting their wide potential applications. In contrast, electrospinning is a simple and versatile technique to fabricate 1D carbon nanomaterials without need of complex set-up, which offers a promising route for large-scale production of nanofibers.15 The electrospun CNFs have been applied to electrochemistry as supercapacitors,16 electrodes for Li-ion batteries,17 biofuel cell,18 and catalyst supports,19 but few investigations were focused on their ORR properties in fuel cell.15b, 20 In spite of their high nitrogen contents, the as-electrospun nitrogen-doped CNFs showed low activity for ORR due to low surface areas because most nitrogen-containing groups are buried in carbon matrix,21 thus which are inaccessible to the reactants. Generally, the increase of the accessible surface area by creation of effective pores on the surface of CNFs could efficiently improve the performance of the CNFs in many fields. Recently, Qiu et al. synthesized the CNFs with unique structures by carbonizing electrospun PAN fibers in NH3 flow, and the observed steady-state diffusion current density for the ORR is over 4.5 mA cm−2 for the sample pyrolized at 1000 °C in NH3.20a The electrospun fibers could be modified by treatment with H2SO4/HNO3, heat treatment in NH3, and/or combined treatment of the above two processes, and the current density of ORR was observed 5 mA cm−2 and 3.8 mA cm−2 in 0.1 M KOH and 0.5 M H2SO4, respectively.22 Liu et al. prepared nitrogen-doped carbon nanofiber films based on PAN by combination of electrospinning and thermal treatment, the as-prepared nanofiber films showed the stable current density at 5 mA cm−2 in 0.1 M KOH.15b These values are evidently poorer than most of the previously-reported carbon-based electrocatalysts due to the un-activated surfaces of CNFs with very low surface areas. Herein, we report the preparation of activated N-doped carbon nanofibers (ANCNFs) with high catalytic activities for ORR in both alkaline and acid medium. The synthesis of ANCNFs was carried out by chemically activating electrospun PAN-based CNFs with KOH/carbon mass ratio of 4/1 at different temperatures. Chemical activation with alkali compounds such as KOH and NaOH is a well-known method to activate carbon materials with increased surface areas and pore volume especially in the level of micropore.6b, 21c, 23 The chemical activation reaction led to not only increased specific surface areas (up to 1362 m2 g−1), pore volume (0.604 cm3 g−1), larger percentage of microporosity (80.1%), and the thinning and disordering of graphitic layer on the fiber surface, but also the gradual loss of nitrogen and continuous increase in the percentage of graphitic nitrogen species with the increase in the activation temperature. Thus, the activation evidently improves the ORR activity of CNFs in terms of onset potential and limiting current density in both alkaline and acidic medium. Furthermore, ANCNFs promotes a close four-electron reaction pathway with very low selectivity for the production of unwanted peroxide and also demonstrate much better stability and increased tolerance to MeOH crossover effects than commercial Pt/C.

Nano Adv., 2016, 1, 79−89.

2.1 Preparation of PAN-based carbon nanofibers (CNFs) by electrospinning Typically, polyacrylonitrile (PAN) was dissolved in dimethylformamide (DMF) at room temperature to obtain an 8 wt% solution. Then, the solution was electrospun from a 20 mL syringe with a needle with an inner diameter of 0.34 mm. The flowing rate (3 mL h−1) was controlled by a programmable syringe pump. An electrostatic voltage of 16 kV was used for the electrospinning of nanofibers. The electrospun fibers were collected by a cylindrical electric drum collector (diameter: 6 cm, rotation speed: 50 rpm) which is 12 cm far from the needle. Then the as-spun nanofibers were stabilized at 280 °C for 2 h in air with a ramp rate of 1 °C min−1, followed by the carbonization at 800 °C for 1 h with a heating rate of 5 °C min−1 under a N2 flow (40 mL min−1). 2.2 Chemical activation of CNFs by KOH Activation of CNFs was conducted through a chemical procedure with KOH. Typically, KOH pellets and CNFs with a mass ratio of 4/1 were thoroughly mixed using a mortar with a pestle. After the mixture was pressed into several cylindrical monoliths with diameter of 10 mm and thickness of ca. 1 mm under a pressure of 20 MPa, they were put into an alumina ceramics crucible and heated to the desired temperature (600, 700 or 800 °C) for 1 h with a ramp rate of 5 °C min−1 under a N2 flow (40 mL min−1). In order to remove residual potassium compounds and carbonate, the activated samples were washed repeatedly with 1 M hydrochloric acid and distilled water for several times until neutral pH. Finally, the samples were dried at 90 °C overnight, and labeled as ANCNF-X in which ANCNF means activated nitrogen-doped carbon nanofibers and X stands for the activation temperature in °C. 2.3 Structural characterization The morphologies of the CNFs before and after the chemical activation were investigated using a JEOL S-4800 field emission scanning electron microscopy and a JEOL 2010F transmission electron microscope operating at 200 kV. X-ray photoelectron spectroscopy (XPS) measurements performed on an ESCALAB 250 X-ray photoelectron spectrometer with Al Kα (hυ = 1486.6 eV) radiation were used to ensure the chemical composition of ANCNFs. All spectra were calibrated using 285.0 eV as the line position of adventitious carbon. In order to determine the surface area and pore size distribution of the samples, nitrogen sorption-desorption isotherms were collected at −196 °C using an ASAP 2010 apparatus (Micromeritics Co., USA). Prior to the measurement, the samples were dried under vacuum at 200 °C overnight. The specific surface areas were calculated by the Brunauer–Emmett–Teller (BET) method. The pore size distribution curves were calculated based on the analysis of the adsorption branch of the isotherm using the Horvath–Kawazoe (HK) method. Raman spectra were recorded on a DXR Raman 80

doi: 10.22180/na174

Research Article

Nano Advances with different lengths of 0.5 to ca. 10 μm (Figure 1b), quite different from its precursor CNFs with long wires. It is reasonably speculated that the activation couldn’t result in the fracture of long CNFs because the activation reaction only happened on the surface of CNFs. The circumstance that long CNFs were fractured should take place when mechanically grinding CNFs with KOH. Indeed, the as-ground pure CNFs that haven’t been activated also show the same short-rod morphology (Figure S1 in the supporting information) as the activated sample ANCNFs, further proving the above speculation. Furthermore, the diameters of ANCNF-800 were about 220 ± 40 nm (the inset in Figure 1b), evidently smaller than those of pristine CNFs (the inset in Figure 1a). This decrease in the diameter of fibers is ascribed to the loss of carbon on the surface via KOH etching at high temperature. Despite this decrease in the diameter, the rod surface of ANCNF-800 is very smooth, similar to that of CNFs, implying that the chemical activation happened uniformly on the fiber surface and thus does not significantly increase surface toughness.

Microscope (Thermal Scientific Co., USA) with 532 nm excitation length. 2.4 Preparation of the working electrodes The catalyst ink was prepared by dispersing the nanofibers (5 mg) into the mixture of water (0.5 mL) and ethanol (0.5 mL) containing 25 μL Nafion solution (5 wt%) under ultrasonic irradiation for 3 h until a homogeneous mixture was formed. Then, a 20 μL aliquot of ink containing 62.5 μg catalyst was dropped onto glassy carbon disk electrode with a diameter of 5 mm to obtain a catalyst level of 0.32 mg cm−2. The electrode with the catalyst ink was dried at 50 °C in air, which was used as the working electrode for further electrochemical measurements. For 20 wt% Pt/C commercial catalyst (Johnson Matthey), the electrode was prepared following the same procedure. 2.5 Electrochemical tests Electrochemical properties of the working electrodes were evaluated using a standard three-electrode cell with a Pt plate as the counter electrode and Ag/AgCl (3 M KCl) as the reference electrode. The electrochemical cell was controlled by a bipotentiostat (Pine Instrument Co.). An aqueous solution of 0.1 M KOH or 0.5 M H2SO4 was used as the electrolyte for the electrochemical studies. The ORR activity of the working electrodes was studied by cyclic voltammetry (CV), rotating disk electrode (RDE), and rotating ring disk electrode (RRDE), respectively. Prior to measurement, the electrolyte was saturated by bubbling O2 (or N2) for at least 20 min. Before recording the CV date, the working electrode was cycled fifty times at a scan rate of 100 mV s−1. The RDE measurements were performed at different rotating rates varying from 400 to 2025 rpm with a scan rate of 10 mV s−1. In the case of RRDE measurement, ring current (iR) and disk current (iD) were collected using a Pt ring-disk electrode in O2-saturated 0.1 M KOH (or 0.5 M H2SO4) solution at a rotating speed of 1600 rpm with a sweep rate of 10 mV s−1. The Pt ring electrode was polarized at 0.2 V in 0.1 KOH electrolyte or 1.0 V in 0.5 M H2SO4 electrolyte. The peroxide (H2O2) yield and the electron transfer number (n) were determined by the following equations: 𝑛 =4×

𝑖𝐷 𝑖𝐷 + 𝑖𝑅 /N

%(𝐻2 𝑂2 ) = 200 ×

𝑖𝑅 /𝑁 𝑖𝐷 + 𝑖𝑅 /N

where N is the collection efficiency of the Pt ring electrode with a value of 0.37.15b

Figure 1. SEM images of (a) CNFs and (b) ANCNF-800, TEM images of (c) CNFs and (d) ANCNF-800, and high-resolution TEM images of (e) CNFs

3. Results and discussion

and (f) ANCNF-800. The insets in panel a and b show the corresponding fiber diameter distribution, and the insets in panel e and f indicate the

The as-spun CNFs show uniform wire-like morphologies, which are continuous and long without any particles on the surface, as seen in the scanning electron microscopy (SEM) image (Figure 1a). The diameters of CNFs were measured to be 240 ± 20 nm shown in the inset of Figure 1a. After chemical activation of CNFs with KOH by physically grinding and then pyrolyzing at 800 °C, the resulting ANCNF-800 is composed of many rods Nano Adv., 2016, 1, 79−89.

corresponding selected area electron diffraction (SAED) image.

The low resolution TEM images also confirm that ANCNFs have a smooth surface as CNFs (Figure 1c-d). However, the high resolution TEM images clearly show the difference in surface microstructure of pristine and activated samples (Figure 1e-f). There is a 10 nm thick graphitic layer, in which the graphene 81

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layers are generally aligned along the fiber axis observed on the surface of CNFs (Figure 1e). The lattice fringes on the surface of CNFs are corresponding to the (002) plane of graphite as confirmed by SAED image (the inset in Figure 1e). After chemical activation with KOH, the HRTEM image shows that the graphitic layer on the surface of ANCNFs significantly decreases to 5 nm (Figure 1f), thus leading to the decrease in the fiber diameter. Moreover, the graphitic layers on the surface of ANCNFs become more disordered compared to the pristine one. This increase in the disorder of graphene layers is necessary for improving porosity in the level of micropore in the KOH-activated carbon samples. During the activation process, several chemical reactions between KOH and CNFs consisting of the solid–solid/solid–liquid processes stoichiometric proceed as the following equations:24 4 KOH + −CH2− → K2CO3 + K2O + 3 H2↑

(1)

8 KOH + 2 –CH= → 2 K2CO3 + 2 K2O + 5 H2↑

(2)

K2O + C → 2 K + CO↑

(3)

K2CO3 + 2 C → 2 K + 3 CO↑

(4)

At low temperature, the activation reaction goes as eqn. 1 and 2. When the activation temperature is over 700 °C, K2O and K2CO3 also can be reduced by carbon to produce metallic K with the emission of CO, as shown in eqn. 3 and 4. The etching reaction happened from outside to inside accompanying the loss of large amount of carbon, so the graphitic layer on the surface of fibers becomes thinner. Meanwhile, the remaining tiny graphene stacks on the surface of fibers become incompactly after activation,20a, 25 thus resulting in not only the increased disorder of graphene layers, but also the improved porosity in the micropore level as discussed below. The pore textures of CNFs and ANCNFs were studied by nitrogen adsorption-desorption isotherm measurements performed at −196 °C. The nitrogen sorption isotherms and pore size distributions are shown in Figure 2a and 2b, respectively. The textural properties are listed in Table 1. As shown in Figure 2a, the nitrogen uptake evidently increases especially when the activation temperature goes up to 700 °C or higher, indicating the significant impact of the activation temperature on the pore

Figure 2. (a) N2 adsorption-desorption isotherms of CNFs, ANCNF-600, ANCNF-700 and ANCNF-800 and (b) pore size distributions of ANCNF-600, ANCNF-700 and ANCNF-800.

microstructure. The adsorption isotherms for ANCNFs show type I behaviour, because they have significant nitrogen uptakes at a relative lower pressure than 0.1. At relative pressures higher than 0.1, the adsorption-desorption isotherms become approximately flat with no any hysteresis loop, typical characters for microporous materials. These findings match well with the previous results that KOH activation of carbon materials evidently increases the amount of micropores. As shown in Table 1, the specific surface areas for CNFs and ANCNFs have the following order: CNF (12 m2 g−1) < ANCNF-600 (40 m2 g−1)

Table 1. Physical properties of the pristine CNFs and ANCNFs prepared by KOH activation of CNFs at different temperatures.

SBET Samples

a

2

−1 a

Vtotal 3

−1 a

Vmicro 3

−1 c

Vmeso 3

−1 d

Percent of Vmicro

(m g )

(cm g )

(cm g )

(cm g )

(%)

CNFs

12

0.028

0.002

0.026

5.4

ANCNF-600

60

0.039

0.006

0.033

15.4

ANCNF-700

516

0.313

0.185

0.128

59.1

ANCNF-800

1361

0.604

0.484

0.120

80.1

Specific surface area determined by the BET equation; b Total pore volume; c Micropore volume; d Mesopore volume.

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< ANCNF-700 (516 m2 g−1) < ANCNF-800 (1361 m2 g−1). It is obvious that the surface area values increase rapidly with the raising of the activation temperature. However, ANCNF-600 also has a very low surface area, implying that the 600 °C activation couldn’t efficiently promote the KOH etching action. The increase of the activation temperature to 700 °C dramatically resulted in the enhancement of the surface area by over 40 times, because carbon could be efficiently etched by KOH at this temperature as shown in eqn. 1 and 2. The surface area continuously increases to 1361 m2 g−1 for ANCNF-800 due to more carbon burning as suggested in eqn. 3 and 4. This surface area is evidently larger than those previously reported CNFs,15b, 20a, 21−22 and such a high surface area leads to the increased exposure and achievability of the catalytic sites. The total pore volumes for ANCNFs also have the same trends with increasing the activation temperature as the specific surface areas (Table 1). The total pore volume for ANCNF-600 is 0.039 cm3 g−1, a bit higher than 0.028 cm3 g−1 for CNFs. However, it sharply increases to 0.313 cm3 g−1 for ANCNF-700 and 0.604 cm3 g−1 for ANCNF-800, respectively. It is notably pointed that the larger percent of micropore volume is obtained with increasing the activation temperature. As shown in Table 1, the percent of micropore volume for ANCNF-600 is as low as 15.4%, but it significantly increases to 59.1% for ANCNF-700 and 80.1% for ANCNF-800, respectively. It implies that chemical activation by KOH is very beneficial for the development of porous carbon with high microporosity. The increased surface area and high microporosity are suitable for fast transportation of both aqueous electrolyte and gaseous oxygen,15c thus resulting in the improved electrocatalytic activity for ORR. The existence of large amount of micropores in the ANCNFs could be further investigated by the HK pore size distribution curves shown in Figure 2b. A one single peak in the region of micropore was clearly observed at 0.99 nm for ANCNF-600, 0.96 nm for ANCNF-700, and 0.95 nm for ANCNF-800, respectively. And there is no peak found in the region of mesopore or macropore, showing that ANCNFs are really microporous resulting from the etching effect of KOH.26 Furthermore, the formation of another micropore system (1.1 nm)

Intensity(a.u.)

D

G

R=1.04

ANCNF-800

R=1.03

ANCNF-700

R=1.03

ANCNF-600

R=1.02

750

1000

is observed for the sample prepared at the highest activation temperature (800 °C). Also the gradual widening of pore sizes in the region of 1.0~1.8 nm suggests the broadening of the micropore sizes with increasing temperature. The data from nitrogen adsorption isotherms and pore size distributions evidently reveal an increase of the specific surface area and percent of micropore volume as well as a little enlargement of micropore size as the activation temperature increases, which closely match those previously reported for other carbonaceous materials.6b The differences in structural and electronic properties of pristine CNFs and ANCNFs were assessed using Raman spectroscopy. Figure 3 displays representative first-order Raman spectra of the samples, in which the D band and G band are located at around 1301 and 1554 cm−1, respectively. The D band is defect-induced, associated with disordered or graphene edges, while the G band is the result of the first-order scattering of the E2g mode of sp2 carbon domains, corresponds to phonons propagating along the graphene sheets.27 The area ratio (R) of D band and G band (R = ID/IG) shows the graphitization degree of carbon. The R ratio of the fibers increases with the activation temperature; R is 1.02 for non-activated CNF, and it increases to 1.03, 1.03 and 1.04 for ANCNF-600, ANCNF-700 and ANCNF-800, respectively, revealing a lower graphitization degree for the activated samples. This result is well consistent with that from TEM observation. During the activation process, KOH prefers to react with less graphitic carbon, so the un-activated graphitic layers become tiny during the activation process. Therefore, the residual graphitic planes rearrange disorderly, leading to a lower graphitization degree.25 Besides the differences in R ratios, the Raman spectra exhibit a marked broadening of the full width at half-maximum (FWHM) of G band for the activated samples. The FWHM of G band increased gradually from 210 cm−1 for pristine CNFs to 212 cm−1 for ANCNF-600, 220 cm−1 for ANCNF-700 and 232 cm−1 for ANCNF-800, respectively. This result is well in agreement with the studies of Maldonado et al, indicating that the broadening of the first order band correlates strongly with the degree of graphitic disorder.28 Furthermore, both D and G bands shift to the higher frequency region with increasing the activation temperature. The D band evidently shifts from 1283 cm−1 for CNFs up to 1324 cm−1 for ANCNF-800, and the G band moves from 1535 cm−1 for CNFs up to 1580 cm−1 for ANCNF-800, which are consistent with the previous theoretical simulations and experimental observations on N-doped CNTs. This up-shift of G and D bands is an important characteristics of p-type doping of graphene and CNTs by the substitution.29 The activation influences not only the microstructures of the fibers, but also the elemental composition. The change in the composition of CNFs and ANCNFs could be confirmed by the X−ray photoelectron spectroscopy. As shown in Figure 4a, the XPS spectra display the presence of three peaks ascribed to carbon, nitrogen, and oxygen, respectively, for CNFs and ANCNFs. No peak due to potassium was found, implying that the complete removal of potassium compounds in ANCNFs by washing using HCl solution. The elemental compositions for the

CNF 1250

1500

1750

2000

2250

-1

Raman Shift (cm ) Figure 3. Raman spectra of the CNFs, ANCNF-600, ANCNF-700 and ANCNF-800. R = ID/IG.

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Table 2. Elemental composition of CNFs and ANCNFs determined by XPS analysis.

Samples

C1s (at%)

N1s (at%)

O1s (at%)

N/C molar ratio

CNFs

83.2

11.0

5.8

0.132

ANCNF-600

82.7

8.4

8.9

0.101

ANCNF-700

85.8

8.4

5.8

0.098

ANCNF-800

92.3

1.8

5.9

0.020

sites, efficiently adsorbing and activating oxygen molecules.31 The detailed chemical surroundings of the N atoms doped into CNFs and ANCNFs could be analysed by high-resolution N 1s XPS spectroscopy. Figure 4b-e show the N 1s spectra for CNFs, ANCNF-600, ANCNF-700 and ANCNF-800, respectively. Typically, they can be deconvoluted into four components: pyridinic N (398.4 eV), pyrrolic N (399.8 eV), graphitic N (401.2 eV), and oxidized N (403 eV).20b It is clearly observed that the activation by KOH has a great impact on the nitrogen species. Figure 4f summarizes the quantitative analysis based on the deconvolution, presenting the evolution of the percentage of N functionality. The percentages of pyrrolic-N and oxidized-N have no regular trends. However, a clear trend could be found for the percentages of pyridinic-N and graphitic-N with increasing temperature. Upon activation, the percentage of pyridinic-N drops from 43.1% for CNFs to 28.9% for D G ANCNF-600, 25.7% for ANCNF-700, and 8.4% for D G ANCNF-800, respectively. In contrast, the percentage of graphitic-N increases with the activation temperature (Figure 4f) Graphitic-N ANCNF-800from 32.9% for CNFs to 34.8% for ANCNF-600, 43.8% for R=1.04 ANCNF-800 R=1.04 ANCNF-700, and 48.4% for ANCNF-800, respectively. These R=1.03 R=1.03 ANCNF-700 ANCNF-700 findings are consistent with the fact that graphitic-N is more R=1.03 ANCNF-600 R=1.03 ANCNF-600 stable than pyridinic-N at high temperature.32 Both pyridinic-N CNF R=1.02 and graphitic-N doped into porous carbons play important roles CNF R=1.02 in the electrocatalysis of ORR.33 Accordingly, ANCNFs with 750 1000 1250 D 1500G 1750 200075022501000 1250 D 1500G 1750 2000 2250 -1 -1 Raman Shift (cm ) adjustable nitrogen contents and increased surface areas will be Raman Shift (cm ) Graphitic-N used as the electrocatalysts for ORR, as will be discussed later. Graphitic-N ANCNF-800 ANCNF-800 The electrocatalytic activity of pristine CNFs and ANCNFs R=1.04 R=1.04 was evaluated by rotating disk electrode (RDE) and rotating ring R=1.03 R=1.03 ANCNF-700 ANCNF-700 disk electrode (RRDE) measurements using a standard three R=1.03 R=1.03 ANCNF-600 ANCNF-600electrode electrochemical station. The ORR activities for samples were tested in O2-saturated alkaline media (0.1 M KOH) R=1.02 CNF R=1.02 CNF and acidic media (0.5 M H2SO4), respectively. Figure 5 shows 750 1000 1250 1500 1750 200075022501000 1250D 1500 G 1750 2000 2250 D -1 G -1 the ORR electrocatalytic activities of various samples tested in Raman Shift (cm ) Raman Shift (cm ) alkaline media (0.1 M KOH). As shown in Figure 5a, there is a Graphitic-N Graphitic-N cathodic current peak in all cyclic voltammograms (CV) curves ANCNF-800 R=1.04 ANCNF-800 R=1.04 of ANCNFs, indicating the occurrence of ORR on the surface of R=1.03 ANCNF-700 activated samples. However, there is no such a peak for pristine R=1.03 ANCNF-700 R=1.03 ANCNF-600 CNFs, implying that the KOH activation causes an evident R=1.03 ANCNF-600 improvement in the ORR activity of CNFs. R=1.02 CNF R=1.02 CNF The RDE measurements were also carried out to study the 750 1000 1250 1500 1750 2000 2250 catalytic performance of all samples in O2-saturated 0.1 M KOH -1 750 1000 1250 1500 1750 2000 2250 Raman Shift (cm ) (b-e) Figure 4. (a) XPS survey spectra of CNFs and activated samples, -1 Raman Shift (cm ) at a rotating speed of 1600 rpm with a sweep rate of 10 mV s−1. high-resolution N1s XPS spectra with deconvoluted peaks for CNFs (b), As shown in Figure 5b, the pristine CNFs have an ORR onset ANCNF-600 (c), ANCNF-700 (d), and ANCNF-800 (e), and (f) the potential at −0.12 V (vs. Ag/AgCl). It is evident that the dependence of the percentage of various nitrogen-containing groups on the activated samples have the more positive onset potentials, e. g. activation temperature.

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Intensity(a.u.) Intensity(a.u.)

Intensity(a.u.)

Intensity(a.u.)

Intensity(a.u.)

Intensity(a.u.)

samples before and after activation are listed in Table 2. It reveals that CNFs contain 83.2 at% carbon, 11.0 at% nitrogen, and 5.0 at% oxygen. The KOH activation evidently results in the decrease of nitrogen contents to 8.4 at% for both ANCNF-600 and ANCNF-700, and 1.8% for ANCNF-800 (Table 2). The loss of nitrogen atoms was especially remarkable as the activation temperature increases to 800 °C, which accompanies the dramatic increase in the specific surface area (Table 1). This trend is in good agreement with the previous literature.30 As expected, the calculated N/C molar ratio goes down with increasing the activation temperature: 0.132 (CNFs) > 0.101 (ANCNF-600) > 0.098 (ANCNF-700) > 0.020 (ANCNF-800), because of the higher temperature leading to the formation and emission of more gaseous nitrogen-containing compounds.5b The residual nitrogen-containing groups in ANCNFs are key active

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Nano Advances the increased ORR activity;1a, 38 (2) ANCNF-800 has the largest percentage of graphitic N groups, which have been proved to efficiently improve the electrocatalytic performance of carbon-based materials.1b Moreover, it is notable that ANCNF-800 has the smallest amount of N-doping, implying the N content is not directly related to the electrochemical performance.31 Figure 5c and S2 show LSV curves of ANCNF-800 and other samples at a rotation speed varying from 400 rpm to 2025 rpm. The voltammetric profiles show that the current density is increased with an increase in rotating speeds due to the improved mass transport on the electrode surface. In order to further confirm the high catalytic activity of ANCNF-800, we used RRDE technique to test the selectivity for four-electron reaction pathway of ORR by the catalyst in O2-saturated 0.1 M KOH solution at a rotation rate of 1600 rpm and the Pt ring electrode was polarized at 0.2 V (vs. Ag/AgCl). Figure 5d shows the RRDE curves of CNF, ANCNF-800 and commercial 20 wt% platinum on carbon black (Pt/C) (Johnson

−0.10, −0.06 and −0.03 V (vs. Ag/AgCl) for ANCNF-600, ANCNF-700 and ANCNF-800, respectively. It is mainly ascribed to the increased ratios of graphitic-N to pyridinic-N with increasing activation temperature, which shift the onset potential positively for ORR.1b, 34 Graphitic N atoms have the same configuration as graphitic carbon atoms, but they could introduce extra electrons in the delocalized π-system, showing a nucleophilic attack on oxygen.1a, 35 ANCNF-800 has the largest limiting current density of −5.49 mA cm−2 among these activated samples, higher than those reported values of electrospun carbon fibers 22, 36 and CNFs treated in NH3 (Table S1),20a and comparable to those of N-doped graphene materials prepared by annealing polypyrrole/GO, polyaniline/GO, NH3/GO, etc.37 ANCNF-800 has the optimum ORR activity mainly ascribed to the following two points: (1) ANCNF-800 shows the high surface area (1361 m2 g−1) and large percentage of micropore. Such a high specific area and porous structure lead to the exposure and achievability of more active centres, thus giving

Figure 5. Rotating disc electrode (RDE) and rotating ring disc electrode (RRDE) experiments of samples. (a) Cyclic voltammograms (CVs) of CNFs and ANCNFs in O2-saturated 0.1 M solution of KOH at a scan rate of 50 mV s−1, (b) LSV curves of CNFs and ANCNFs in O2-saturated 0.1 M solution of KOH at a scan rate of 10 mV s−1 at 1600 rpm, (c) LSV curves of ANCNF-800 at different rotation speeds, (d) RRDE voltammograms in O2-saturated 0.1 M KOH solution at a sweep rate of 10 mV s −1 with the electrode rotation speed of 1600 rpm (the Pt ring electrode was polarized at 0.2V), (e) yield of H2O2 calculated from (d), and (f) electron transfer number (n) of CNFs, ANCNF-800 and Pt/C calculated from (d).

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Matthey). The low ring current suggests that a low amount of peroxides reached the ring electrode. ANCNF-800 demonstrates a high disk current density, even a little superior to commercial Pt/C. The electron transfer number (n) and peroxide yields were calculated from the RRDE data. For commercial Pt/C, the n value is 3.90~3.95, and the peroxide yield is ca. 5.0% in the potential ranging from −0.8 to −0.2 V (vs. Ag/AgCl). The pristine CNFs showed a very high H2O2 yield of 32.5 to 39.5% and the calculated n value varied from 3.34 to 3.46, implying a combination of two-electron and four-electron reduction processes in the ORR on the surface of CNFs. In contrast, only 6.3 % of H2O2 was produced on ANCNF-800, showing a large n value of 3.86~3.88. These values are close to those for commercial Pt/C, implying that an intrinsic four-electron pathway for ANCNF-800 to obtain maximum efficiency.1b, 39 The ideal catalyst for ORR should exhibit satisfactory long-term durability and tolerance to crossover effect. We measured the current–time (i–t) chronoamperometric responses at 0.2 V (vs. Ag/AgCl) in O2-saturated 0.1 M KOH solution at a rotation rate of 1600 rpm (Figure 6). After 5.6 h (20000 s), ANCNF-800 showed a 1.2% loss of current density, while the commercial Pt/C suffered from 29.9% decrease (Figure 6a). When 3 M methanol is introduced into the O2-saturated alkaline electrolyte at 5 min, 91.2% of kinetic current density was

remained at 20 min for ANCNF-800, much higher than 68.2% for Pt/C. These results signify that ANCNF-800 shows better durability and resistance to crossover effect than commercial Pt/C, possibly resulting from the metal-free textures and high chemical stability of actives sites effectively preventing the loss of catalytic sites. The as-prepared ANCNFs also demonstrate improved electrocatalytic activity in acidic media (0.5 M H2SO4), where is a more harsh environment than in alkaline condition for ORR. As shown in Figure 7a, CNFs and ANCNF-600 have no evident ORR peak. However, the samples activated at higher temperatures show an obvious one at 0.3 V (vs. Ag/AgCl), indicating the significantly increased ORR activity for ANCNF-700 and ANCNF-800. Figure 7b shows the LSV curves of the samples in 0.5 M H2SO4 at a rotation speed of 1600 rpm. It is clearly seen that both ANCNF-700 and ANCNF-800 have the onset potential of 0.6 V (vs. Ag/AgCl), which are evidently superior to 0.42 V for ANCNF-600 and 0.35 V for pristine CNFs. Moreover, the limiting current density of ANCNF-800 (−6.1 mA cm−2) is larger than that of ANCNF-700 (5.5 mA cm−2), both of which exceed those of ANCNF-600 and CNFs due to large surface areas and increased exposure of catalytic sites for ANCNF-700 and ANCNF-800. The limiting current density of ANCNF-800 evidently outperforms those of other previously reported non-metal electrocatalysts in acidic medium (Table S1).11a, 38a Figure 7c and S3 show LSV curves of the samples at a rotation speed from 400 to 2025 rpm. We further used RRDE technique to test the electron transfer number (n) and H2O2 yields for the samples. And the Pt ring electrode was polarized at 1.0 V (vs. Ag/AgCl). Figure 7d show the RRDE curves of CNFs, ANCNF-800 and commercial Pt/C, the calculated H2O2 yields and n values are shown in Figure 7e-f . For commercial Pt/C, n is calculated to 3.87 at potentials of -0.1 to 0.2 V, and the corresponding H2O2 yield is about 6.6%. Both CNFs and ANCNF-800 demonstrate higher selectivity of H2O2 and lower n values than Pt/C. However, the H2O2 selectivities for both CNFs and ANCNF-800 are 10.0%~13.8% and 12.0%~22.0%, respectively, giving n values of 3.72~3.80 and 3.56~3.76, respectively. Thus, CNFs and ANCNF-800 could result in the ORR process toward a close four-electron transfer process even in harsh acidic electrolyte. It is notably seen that the activation evidently leads to the positive shift of the onset potential and thus decrease the over potential of the ORR in the acidic medium, possibly due to the largest proportion of graphitic-N for ANCNF-800 among these samples. The activity for ANCNF-800 in acidic solution is significantly poorer than in alkaline medium because the protons prefer coordinating with the extra electrons of N atoms in the nitrogen-containing functional groups.1a, 40

4. Conclusions In summary, we have prepared activated nitrogen-doped carbon nanofibers (ANCNFs) by KOH activation of electrospun PAN-based CNFs with KOH/ carbon ratio of 4/1. Various characterization results show the activation reaction led to

Figure 6. Chronoamperometric responses of ANCNF-800 and Pt/C electrodes at −0.6 V (vs. Ag/AgCl) in O2-saturated (a) 0.1 M KOH solution (b) 0.1 M KOH solution with 3 M methanol solution added at about 5 min.

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increased specific surface areas, larger percentage of microporosity, and the thinning and disordering of graphitic layer on the fiber surface. Especially, ANCNF-800 prepared by activation of CNFs at 800 °C has a surface area of up to 1362 m2 g−1, pore volume of 0.604 cm3 g−1, and uniform micropore size of 0.99 nm. Such a unique fiber structure with high specific surface area and pore volume could have better electron and mass transport properties, which render it a better backbone support for catalytically active sites. The activation resulted in not only the gradual decrease of N/C molar ratio from 0.132 for pristine CNFs to 0.02 for ANCNF-800 with increasing the activation temperature, but also the continual increase in the percentage of graphitic N groups and decrease in the percentage of pyridinic N groups in the fibers. Although it resulted in the decrease of total N content, the activation evidently improves the ORR activity of CNFs in both

alkaline and acidic medium, especially in alkaline environment. Among the activated fibers, ANCNF-800 demonstrates the optimum ORR activity in both acidic and alkaline electrolyte in terms of the onset potential and limiting kinetic current density in spite of its lowest nitrogen contents. It also demonstrates much better stability and increased tolerance to MeOH crossover effects than commercial Pt/C in alkaline solution. Moreover, ANCNF-800 promotes a four-electron reaction pathway revealed by the RRDE measurements, implying low yield of side peroxide in alkaline solution. In contrast, ANCNF-800 has similar electron transfer numbers in acidic solution as the pristine CNFs and the peroxide yield varies between 12.0% and 22.0%, higher than those in alkaline solution. The activity of ANCNF-800 in harsh acidic solution is evidently poorer than that in alkaline one, ascribed to easy protonation of basic N-containing groups, thus decreasing the ORR activity. The

Figure 7. Rotating disc electrode (RDE) and rotating ring disc electrode (RRDE) experiments of the samples. (a) Cyclic voltammograms (CVs) of CNFs and ANCNFs in O2-saturated 0.5 M solution of H2SO4 at a scan rate of 50 mV s−1, (b) LSV curves of CNFs and ANCNFs in O2-saturated 0.5 M solution of H2SO4 at a scan rate of 10 mV s−1 at 1600 rpm, (c) LSV curves of ANCNF-800 at different rotation speeds, (d) RRDE voltammograms in O2-saturated 0.5 M H2SO4 solution at 1600 rpm with a sweep rate was 10 mV s −1 (the Pt ring electrode was polarized at 1.0 V), (e) yield of H2O2 calculated from (d), and (f) electron transfer number ( n) of CNFs, ANCNF-800 and Pt/C calculated from (d).

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improved ORR activity of ANCNF-800 is mainly due to its large specific surface area and pore volume, and increased percentage of graphitic N groups. It is believed that ANCNFs with high surface areas, large microporosity, and adjustable nitrogen contents also have potential applications in the field of supercapacitor, gas storage, catalytic supports, etc.

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Acknowledgements

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The authors thank the financial support from Shanghai Institute of Ceramics, Key Project for Young Researcher of State Key Laboratory of High Performance Ceramics and Superfine Microstructure, One Hundred Talent Plan of Chinese Academy of Sciences, National Natural Science Foundation of China (No. 21307145), the International Cooperation Program of Shanghai Municipal Science and Technology Commission (No. 15520720400), and the Research Grant (No.14DZ2261203) from Shanghai Government.

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