Nitrogen-codoped hierarchical porous carbons as highly efficient electrocatalysts

Accepted Manuscript Hollow-structured conjugated porous polymer derived Iron/Nitrogen-codoped hierarchical porous carbons as highly efficient electrocatalysts Wenbei Zhang, Tianlu Cui, Lingyun Yang, Chao Zhang, Ming Cai, Sai Sun, Yefeng Yao, Xiaodong Zhuang, Fan Zhang PII: DOI: Reference:

S0021-9797(17)30232-1 http://dx.doi.org/10.1016/j.jcis.2017.02.061 YJCIS 22088

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

22 December 2016 30 January 2017 26 February 2017

Please cite this article as: W. Zhang, T. Cui, L. Yang, C. Zhang, M. Cai, S. Sun, Y. Yao, X. Zhuang, F. Zhang, Hollow-structured conjugated porous polymer derived Iron/Nitrogen-codoped hierarchical porous carbons as highly efficient electrocatalysts, Journal of Colloid and Interface Science (2017), doi: http://dx.doi.org/10.1016/j.jcis. 2017.02.061

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Hollow-structured conjugated porous polymer derived Iron/Nitrogen-codoped hierarchical porous carbons as highly efficient electrocatalysts Wenbei Zhang,a Tianlu Cui,a Lingyun Yang,b Chao Zhang,a Ming Cai,a Sai Sun,c Yefeng Yao,d Xiaodong Zhuang*a and Fan Zhang*a a

State Key Laboratory of Metal Matrix Composites, Shanghai Key Laboratory of

Electrical Insulation and Thermal Ageing, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Dongchuan Road 800, 200240 Shanghai, China. b

iHuman Institute, ShanghaiTech University, 100 Haike Road, Pudong, 201210 Shanghai,

China. c

Key Lab for Advanced Materials, Institute of Applied Chemistry, East China University

of Science and Technology, 130 Meilong Road, 200237 Shanghai, China. d

Physics Department & Shanghai Key Laboratory of Magnetic Resonance, East China

Normal University, North Zhongshan Road 3663, 200062 Shanghai, China. * E-mail: [email protected]; [email protected]

Graphical abstract：

A rational design towards Fe/N-codoped hierarchical porous carbon spheres with hollow structure was developed by using conjugated microporous polymer and silica sphere as precursor and template respectively.

Abstract: Iron and nitrogen (Fe/N) co-doped porous carbons have already shown great potential as electrocatalysts for oxygen reduction reaction in alkaline media. However, it still remains a great challenge to finely integrate a hierarchical porous structure and Fe/N co-doping effect into one material at the same time. In this work, a rational design towards Fe/N-codoped hierarchical porous carbon spheres was developed by the formation of an iron-porphyrin-containing conjugated microporous polymer sphere with hollow structure (HCMP) through a silica sphere template directed condensation of pyrrole and 1,4-phthalaldehyde, then etched with NaOH, and treated with FeCl2. The resulting HCMP-Fe polymer was readily converted to a series of Fe/N co-doped hierarchical porous carbons (HPC-Fe/N-X, X=700-900) upon pyrolysis at different temperatures and etching treatment. These porous carbons exhibit the high specific surface areas up to 518 m2 g-1 and the contents of N and Fe up to 3.28 at.% and 0.85 wt.%, respectively. Benefiting from the high surface area, Fe/N co-doping character, HPC-Fe/N-700 exhibited excellent electrochemical catalytic performance for oxygen reduction reaction under alkaline condition (0.1 M KOH) with a low half-wave potential (0.84 V), a dominant four-electron transfer mechanism (n = 3.89 at 0.65 V), as well as a high diffusion limiting current density (JL = 5.19 mA cm-2), comparable to those porous carbon-based ORR catalysts with excellent electrochemical performance. Keywords: conjugated microporous polymer, hollow sphere, Fe/N-doping, hierarchical porous carbon, oxygen reduction reaction Highlight: *Iron-porphyrin-containing conjugated microporous polymer spheres with hollow

structure (HCMP-Fe) were prepared. *A series of Fe/N-doped hierarchical porous carbons derived from HCMP-Fe were

prepared. *The as-prepared Fe/N co-doped porous carbons exhibited promising electrochemical

catalyzed ORR in both alkaline and acidic conditions.

1. Introduction The impending global energy crisis has prompted intense research on energy conversion and storage systems.[1-4] Among these systems, fuel cells are one of the most promising clean energy sources as alternatives to conventional fossil fuel technology. Oxygen reduction reaction (ORR), which is the catholically reaction of fuel cells, plays a key role for efficiently consumption of fuel molecules, such as hydrogen, ethanol, etc. Therefore, development of high efficient catalysts for ORR is very

urgent.

Most

of

commercial

available

catalysts

for

ORR

are

precious-metal-based catalysts, such as Pt-based catalysts, due to the ultra-low onset potential and ultra-high diffusion current density. Although Pt-based materials have been proven to be the most efficient catalysts for ORR,[5] the high cost and scarcity of Pt metal severely limits the applications of such kinds of materials.[6] Thus, great efforts have been devoted to search for efficient, durable and inexpensive alternatives to Pt-based ORR catalysts.[7-16] Due to the abundant resources, unique electronic properties and structural characters, porous carbon materials,[17] in particular, heteroatom-doped carbon materials have been regarded as one of the most promising electrocatalysts for ORR, and thus arousing tremendous interest in the past decade. So far, N,[18-22] B,[23-26] S,[27-30] P[31-34] and halogen atom[35] have been widely used to prepare single, dual, or triple doped porous carbons for increasing ORR activity, especially, the transition metal (e.g., Fe, Co)-containing nitrogen-doped porous carbon materials have shown the promising results.[17, 36-39] Among them, great efforts have been devoted to catalysts based on transition metal nitrogen-coordinated porphyrins polymer,[17,

36, 37]

since Jasinski reported that transition metal porphyrins and

phthalocyanines show electrocatalytic activity toward ORR for the first time.[40] However, the electrocatalytic performances of most of these developed doped porous carbons are still far beyond the state-of-the-art precious metal based catalysts. In order to further improve the catalytic performance of heteroatom-doped porous carbons, high specific surface area and hierarchical porous structure, which favor the exposure of active sites and offer rapid mass transfer processes, and thus allowing for a facile access to active sites, have been taken into consideration. Such as Yu et al. report a strategy for controllable synthesis of 1D ZIF-8 nanofibers which can be subsequently transformed into doped carbon nanofibers by calcination. Such doped carbon materials with hierarchical pore structures and nanofibrous morphologies exhibit excellent electrocatalytic activity and long-term durability for ORR.[41] Lu et al. synthesized the 3D hierarchically porous N-doped carbon materials by using

p-MCEFs as both 3D macroporous templates and endogenous pore generating agents, which show super-high surface areas and hierarchical porosities, thus enabling sufficient exposure and improved accessibility of ORR-catalytic sites.[42] However, it still remains a great challenge for preparing porous carbon materials with hierarchical pore structures as well as controlled metal/N active sites. Herein, we report a simple synthetic approach to prepare a series of Fe/N-doped hierarchical porous carbons by using conjugated microporous polymer enwrapped silica spheres as precursors. As-produced porous carbons exhibit high specific surface areas up to 518 m2 g-1 and the high N and Fe contents of up to 3.28 at.% and 0.85 wt.%, respectively. Combination of the high surface areas and the Fe/N co-doping structures, the as-produced porous carbons can undergo excellent electrochemical catalytic performance for oxygen reduction reaction under alkaline condition. 2. Experimental section 2.1. Materials FeCl2·4H2O

was

purchased

from

Sigma.

1,4-Phthalaldehyde,

pyrrole,

BF3-diethyl ether, silica nanosphere and p-chloranil were purchased from Aladdin (Shanghai, China). Ethanol, DMF and all other solvents were purchased from Adamas Reagent. All purchased chemicals were used without further purification. 2.2. Preparation Synthesis of hollow CMP spheres (HCMP): Core-shell type CMP enwrapped SiO2 (SiO2@CMP) was prepared by one-pot reaction by using 1,4-phthalaldehyde and pyrrole as starting materials in presence of SiO2 nanosphere.[43] First, silica nanosphere (4 g) and 1,4-phthalaldehyde (2 g, 15 mmol) were dissolved in dichloromethane (1 L). Then, the freshly distilled pyrrole (1 mL, 15 mmol) was added. Under nitrogen atmosphere, BF3-diethyl ether (3.0 mmol) was portion-wisely added to the off-white solution. The reaction color slowly turned to reddish purple. After stirring for 12 h, p-chloranil (2.8 g, 11.25 mmol) was added to the mixture. Then, the solution was stirred for further 12 h at 50 °C. The precipitate was collected by filtration, and washed with dichloromethane for several times before vacuum drying at 60 °C for 12 h. As-produced SiO2@CMP was etched by NaOH solution to produce hollow CMP spheres (denoted as HCMP). Preparation of Fe-containing hollow CMP spheres (HCMP-Fe): The as-prepared HCMP (1.5 g) was dissolved in DMF (250 mL) under nitrogen atmosphere. Then, FeCl2·4H2O (4.0 g) was added into the mixture and stirred for 12 h

at 150 °C. Afterwards, the mixture was cooled to room temperature, and the precipitate was collected by filtration, and washed by DMF, deionized water and ethanol for several times. After drying at 60 °C under vacuum for 12 h, Fe-containing hollow CMP (denoted as HCMP-Fe) was produced. Preparation of porous carbon materials: The Fe-coordinated composite (HCMP-Fe) was pyrolyzed at 700, 800 and 900 °C for 2 h at a heating rate of 5 °C min-1 in a N2 atmosphere, and then etched with HCl for removing off disassociated Fe to afford Fe/N co-doped porous carbons, denoted as HPC-Fe/N-700, HPC-Fe/N-800 and HPC-Fe/N-900, respectively. 2.3. Characterization Fourier transform infrared spectroscopy (FT-IR) was performed on a Spectrum 100 (Perkin Elmer, Inc., USA) spectrometer with a scan range of 4000-400 cm-1. The sample

powders

were

pulverized

with

KBr,

and

pressed

into

disks.

Thermogravimetric analysis (TGA) of the samples was performed using a Q5000IR (TA Instruments, USA) thermogravimetric analyzer at a heating rate of 20 °C min-1 under nitrogen flow. Scanning electron microscopy (SEM) was performed using an FEI Sirion-200 (FEI Co., USA) field emission scanning electron microscope. Transmission electron microscopy (TEM) observations were carried out with a JEOL-2100 (JEOL Ltd., Japan) electron microscope at an operating voltage of 200 kV. The specimens were dispersed in alcohol, and the suspensions were dropped onto a copper grid covered with lacey support films. X-ray diffraction (XRD) measurements were performed using a Rigaku D/Max 2500 X-ray di ractometer with Cu Kα radiation (k=1.54 Å) at a generator voltage of 40 kV and a generator current of 50 mA with a scanning speed of 5 ° min-1 from 5 ° to 60 °. Raman spectrums were recorded using an SEN TERRA spectrometer (Bruker Optics, Germany) at an excitation laser beam wavelength of 532 nm. X-ray photoelectron spectroscopy (XPS) was performed using an AXIS Ultra DLD system (Kratos Co., Japan) with Al Kα radiation as the X-ray source. The gas sorption isotherms were measured via an Autosorb-iQA3200-4 sorption analyzer (Quantatech Co., USA). Solid-state

13

C NMR analysis was

conducted on a Bruker AVANCE III 300 Spectrometer. Samples were spun at 5 kHz in a 4 mm zirconium rotor within a magic-angle spinning (MAS) probe. An acquisition time of 20 ms, a contact time of 1 ms, and a 6.5 μs pre-scan delay were used. The recycle time was 2 s in order to obtain fully relaxed spectra. Chemical shifts were externally referenced to adamantane at 38.48 ppm. 2.4. Cyclic voltammetry (CV) and rotating disk electrode (RDE) measurements

The working electrode was prepared by loading a catalyst sample film of 0.40 mg cm-2 onto a glass carbon electrode.[44] Firstly, 5 mg of catalyst was dispersed in 500 μL of 0.25 wt.% Nafion ethanol solution, and then sonicated for at least 30 min to form a homogeneous ink. Next, 6 μL of the catalyst ink (containing 0.06 mg of catalyst) was loaded onto a glassy carbon electrode, 5.61 mm in diameter (loading 0.24 mg cm-2). Pt/C ink was prepared by dispersing 5 mg of Pt/C (20 wt.% Pt) in 1 mL of 0.25 wt.% Nafion ethanol solution, and then 5 μL of Pt/C ink was loaded onto a glassy carbon electrode. The ink was air-dried before measurement. Electrochemical measurements (CV, RDE, and rotating ring-disk electrode (RRDE)) were conducted using an advanced electrochemical system (Pine Instrument Co., USA). A three-electrode cell system was employed incorporating a rotating glass carbon disk and a platinum ring electrode (Pine Instrument Co., USA) after loading the electrocatalyst as the working electrode, an Ag/AgCl (KCl, 3 M) electrode as the reference electrode, and a Pt wire as the counter electrode. The experiments were carried out in O2 -saturated 0.1 M KOH solution for the ORR. The potential range was cyclically scanned between -1 and 0 V at a scan rate of 100 mV s-1 at ambient temperature after purging O2 or N2 for 30 min. RDE measurements were conducted at a scan of 10 mV s-1 with a varying rotating speed of 225-1600 rpm. The slopes of the best linear fit lines were used to calculate the transferred electron number (n) per oxygen molecule in the ORR process on the basis of the Koutecky - Levich equations:[45]  



 ై



 ే





భ మ



 ే

(1)

  0.62   /  /

(2)

  

(3)

where J is the measured current density, JK and JL are the kinetic- and di usion-limiting current densities, ω is the angular velocity (ω=2πN, where N is the linear rotation speed), F is the Faraday constant (96486 C cm-1), C0 is the bulk concentration of O2 (C0=1.2×10-3 mol L-1), D0 is the di usion coefficient of O2 (1.9× 10-5 cm2 s-1), v is the kinematic viscosity of the electrolyte (0.01 cm2 s-1), and k is the electron transfer rate constant. 2.5. RRDE measurement For the RRDE measurements, catalyst inks and electrodes were prepared by the same method as mentioned above. The disk electrode was scanned cathodically at a rate of 10 mV s-1, and the ring potential was constant at 0.5 V versus Ag/AgCl. Based

on the RRDE result, the electron transfer number (n) and the HO2- concentration (%) were determined by the followed equations:[13] 

ీ

(4)

ీ ౎ /

% HO  

಺  ౎ ಿ

಺ ీ  ౎

(5)

ಿ

where ID is the disk current, IR is the ring current, and N is the current collection e ciency of the Pt ring (0.37, according to the manufacturer). The glassy carbon electrode with a diameter of 5.61 mm was used in this experiment. 3. Results and discussion

Scheme 1. The overall synthetic procedure of HPC-Fe/N-X. i) silica nanosphere, 1,4-phthalaldehyde and pyrrole in CH2Cl2 at room temperature for 12 h, then p-chloranil at 50 °C for 12 h; ii) etching of SiO2 templates in NaOH solution; iii) coordinated with FeCl2·4H2O in DMF at 150 °C for 12 h; iv) pyrolysis of HCMP-Fe under N2 at different temperatures and etching in HCl solution to produce HPC-Fe/N-X (X=700, 800 and 900 °C).

The strategy for synthesis of Fe/N-codoped hierarchical porous carbon was presented in Scheme 1. Firstly, 1,4-phthalaldehyde was reacted with pyrrole in the presence of silica nanosphere as hard template to form conjugated porous polymer-enwrapped SiO2 nanospheres (denoted as SiO2@CMP), featuring core-shell structure. Subsequently, SiO2 was etched in NaOH solution under vigorous stirring to generate hollow CMP spheres (denoted as HCMP). Typically, such condensation likely tend to form porphyrin structure in the networks as previously reported.[46] Thanks to the highly coordinated activity of porphyrin type moiety with transition

metal ions, HCMP was further treated by FeCl2 in solution to produced Fe (II) porphyrin based hollow CMP, which was denoted as HCMP-Fe. Upon pyrolysis of HCMP-Fe at different temperatures and etching with acid to remove Fe particles, Fe/N-doped hierarchical porous carbons (HPC-Fe/N-X, X=700, 800 and 900 °C) can be easily prepared.

Figure 1. TEM images of SiO2@CMP (a) and HCMP (c); SEM images of SiO2@CMP (b) and HCMP (d); (e) Solid-state 13C CP/MAS NMR spectrum of HCMP (signals with * are sidebands); (f) TGA curve of HCMP-Fe.

The morphologies of as-prepared materials were investigated by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The TEM image of SiO2@CMP (Fig. 1a) shows the nanosphere structure with a diameter of around 45 nm, which is similar to the value evaluated in SEM image (Fig. 1b). Large-area SEM image (Fig. S1) revealed the formation of uniform size SiO2@CMP nanosphere. After etching, the TEM image shows that the resulting HCMP consists of hollow particles with shell thicknesses of ca. 10 ± 2 nm (Fig. 1c), and the silica cores of SiO2@CMP have been completely removed off. There is no obvious size change between SiO2@CMP and HCMP according to the SEM images (Figs. 1b and 1d). Fourier-transform infrared (FTIR) spectroscopy was further used to study the chemical structure of the as-prepared material (Fig. S2). The peak at 1692 cm-1 (C=O stretching) was absent or strongly attenuated after polymerization, suggesting that the monomers were almost completely consumpted to produce HCMP. The peaks at 1604 cm-1 (C=C stretching of pyrrole ring), 1484 cm-1 (C=C vibration modes of the phenyl rings), 803 cm-1 (C-H out-of-plane bending of the phenyl rings) confirm the direct

incorporation of the pyrrole moiety and the formation of microporous porphyrin in the network. New vibration band in the series of HCMP-Fe at 1003 cm-1 should be attributed to the strong coordination of Fe in porphyrin units (N-Fe moiety).[46] Solid-state

13

C NMR spectrum of HCMP shows four broad peaks at 143.9, 140.5,

132.8, and 120.9 ppm, respectively, assignable to phenylene moiety and porphyrin macrocycle (Fig. 1e).[47]

X-ray photoelectron spectroscopy (XPS) was further

applied to analyze the elemental compositions and nitrogen-bonding configurations in the samples (Fig. S3). The high-resolution N 1s spectra of HCMP shows one type of nitrogen, pyrrole nitrogen (399.8 eV), indicative of the presence of porphyrin units. The thermogravimetric analysis (TGA) curve of HCMP-Fe under nitrogen flow was shown in Fig. 1f. HCMP starts to decompose at around 200 °C and shows weight loss of only 42 wt.% at 800 °C, indicating HCMP can be feasibly transformed into carbon material in a high carbon yield.

Figure 2. (a) TEM image of HPC-Fe/N-700; (b) SEM image of HPC-Fe/N-700; XRD patterns (c) and Raman spectra (d) of HPC-Fe/N-700, HPC-Fe/N-800 and HPC-Fe/N-900; (e) Nitrogen adsorption/desorption isotherms and (f) corresponding pore size distribution curves of HPC-Fe/N-700, HPC-Fe/N-800, HPC-Fe/N-900 and HCMP-Fe.

The morphologies of HPC-Fe/N-X were also investigated and shown in Fig. 2a-b (HPC-Fe/N-700) and Fig. S4 (HPC-Fe/N-800 and HPC-Fe/N-900). All these samples showed the nanometer-sized hollow structures and similar pore sizes in comparison with HCMP (Fig. 1c, 1d), demonstrating that the well-defined mesoporous hollow structure of HCMP was maintained after high temperature treatment. Furthermore, the corresponding elemental mapping images of the SEM unit revealed that carbon, nitrogen and iron elements were distributed homogeneously in the as synthesized porous carbon materials (Fig. S5). X-ray diffraction (XRD) was carried out to investigate the graphitic crystallinities of various samples in Fig. 2c. The pronounced graphitic peaks at 26 ° (002) and 43 ° (101) suggested the formation of graphitic structure for all samples during pyrolysis.[48] Raman spectroscopy widely used to characterize the structure of carbon materials, was carried out to further survey the defects and the degree of the ordering of carbon.[49] As shown in Fig. 2d, all the samples showed two bands which should be attributed to D band and G band of carbon. As known, the D band is associated with structural defects and partially disordered structures of sp2 domains, while the G band is related to the E2g vibration mode of sp2 carbon domains, which can be used to evaluate the degree of graphitization.[50] The intensity ratio of D to G band (ID/IG) decreases from 1.01 (HPC-Fe/N-700) to 0.97 (HPC-Fe/N-900), suggesting that the graphitic degrees of the carbonized samples have been improved with the increasing of temperature.[49] Meanwhile, the defects and disordering, with respect to the partial sp2 domains were also restored at different levels. The porous structures of Fe/N co-doped porous carbons were confirmed by nitrogen physisorption measurements (Fig. 2e-f and Table S1). It was found that the isotherms of Fe/N co-doped porous carbons are typical type IV (Fig. 2e). The steep and high capillary condensation steps revealed well-developed mesopores, which can be confirmed by the pore size distribution (Fig. 2f). The Brunauer-Emmett-Teller (BET) surface areas of HCMP-Fe, HPC-Fe/N-700, HPC-Fe/N-800 and HPC-Fe/N-900 are 486, 518, 281 and 277 m2 g-1, respectively. The difference in BET surface areas could be caused by the degradation of polymers and recombination of fragments under the different carbonization temperatures.[28] Notably, micropores (<2 nm, Fig. 2f) could also be observed in all these samples, which is beneficial to fully exposing the catalytic active sites of catalytic materials.[51] All these analyses clearly confirmed the hierarchical porous structures of these as-made Fe/N co-doped porous carbons.

Figure 3. (a) High-resolution N 1s spectra of HCMP-Fe, HPC-Fe/N-700, HPC-Fe/N-800 and HPC-Fe/N-900; N content (b) and Fe content (c) of HPC-Fe/N-700, HPC-Fe/N-800 and HPC-Fe/N-900, respectively.

X-ray photoelectron spectroscopy (XPS) was further applied to analyze the elemental compositions and nitrogen-bonding configurations of the as-prepared porous carbon materials (Fig. 3a). The N 1s core-level spectra of HPC-Fe/N-700, HPC-Fe/N-800 and HPC-Fe/N-900 can be fitted into two peaks: pyridine nitrogen (398.5 eV) and graphitic nitrogen (401.0 eV). The peak at the binding energy of 398.5 eV should also include a contribution from nitrogen binding to metal (metal-nitrogen), due to the small difference between the binding energies of nitrogen-metal and pyridine-type nitrogen.[51] Different from N 1s spectra of HCMP-Fe and HCMP (Fig. 3a, S3), the Fe/N co-doped porous carbon samples show more than one kind of nitrogen excepting pyrrole-type nitrogen, indicating that pyrrole-type nitrogen transforms into other kinds of nitrogen during pyrolysis. The nitrogen contents of HPC-Fe/N-700, HPC-Fe/N-800 and HPC-Fe/N-900 are 3.28 at.%, 2.71 at.% and 2.01 at.%, respectively, including pyridinic nitrogen contents of 1.54 at.%, 1.16 at.% and 0.56 at.%, respectively (Fig. 3b). Along with the pyrolysis temperature increasing, the amount of graphitic nitrogen remains constant, whereas pyridinic-type nitrogen largely decrease, hence implying that this species is less stable at high temperatures. Thus, such different amounts of N-bonding configurations in HPC-Fe/N-X samples must exert a large influence on their electrocatalytic performances for oxygen reduction.[52] The Fe contents are 0.85 wt.%, 0.88 wt.% and 0.90 wt.% for HPC-Fe/N-700, HPC-Fe/N-800 and HPC-Fe/N-900, respectively (Fig. 3c) according

to ICP analysis.

Figure 4. (a) RRDE voltammogram for HPC-Fe/N-700 in 0.1 M KOH solution saturated with O2. The electrode rotation rate was 1600 rpm and the Pt ring electrode was held at 0.5 V (inset: electron transfer number and percentage of H2O2 as a function of potential); (b) RDE voltammograms for HPC-Fe/N-700 in 0.1 M KOH solution saturated with O2; (c) Koutecky-Levich plots obtained from the RDE curves; (d) Oxygen reduction polarization curves for HPC-Fe/N-700, HPC-Fe/N-800, HPC-Fe/N-900 and Pt/C at 1600 rpm in 0.1 M KOH.

The oxygen reduction reaction (ORR) is of great importance in fuel cells and other electrochemical devices.[53] The electrocatalytic properties of the as-prepared Fe/N co-doped porous carbons catalyst toward ORR were evaluated using rotating ring-disk electrode (RRDE) technique in 0.1 M KOH solution at room temperature. To gain insight into the performance of HPC-Fe/N-700 in the ORR, RRDE and RDE were measured. The onset potential for HPC-Fe/N-700 was approximately 0.92 V (Fig. 4a). The calculated electron transfer number and H2O2 concentration were 3.89 and 5.4% at 0.65 V vs. RHE (Fig. 4a inset), respectively, indicating the direct oxygen to water transformation in a four-electron transfer mechanism by using HPC-Fe/N-700 as catalyst. The RDE voltammetric profiles in O2-saturated 0.1 M KOH showed that the current density was enhanced by an increase in the rotation rate (from 225 to 1600 rpm, Fig. 4b). The corresponding Koutecky-Levich plots (K-L plots, Fig. 4c) which were calculated from linear sweep voltammetry (LSV) curves (Fig. 4b) at various rotation rates exhibit good linear relationship for the HPC-Fe/N-700. Linearity of the plots is usually taken as an indication of the first-order reaction kinetics with respect to the concentration of dissolved O2.[54] The

slopes remain approximately constant over the potential range from 0.3 V to 0.8 V, manifesting that the electron transfer numbers for oxygen reduction at different electrode potentials are similar. The electron transfer number (n) was calculated to be 3.89 at 0.65 V. Again, this result suggests that HPC-Fe/N-700 lead to a four-electron transfer mechanism in oxygen reduction. The electrocatalytic properties of HPC-Fe/N-800 and HPC-Fe/N-900 were show in Fig. S6. The LSV curves for the as-prepared samples and Pt/C in O2-saturated 0.1 M KOH solution at a rotation rate of 1600 rpm were compared in Fig. 4d. HPC-Fe/N-700 shows the lowest onset potential at 0.92 V and the highest current density of up to 5.19 mA cm-2, which is slightly lower than that of Pt/C. The half-wave potential of HPC-Fe/N-700 occurs at 0.84 V, which is only 10 mV higher than that of Pt/C (0.85 V). Given that HPC-Fe/N-700 possesses a higher pyridinic nitrogen content (1.54 at.%) and a larger specific surface area (518 m2 g-1), it can be concluded that the pyridinic nitrogen and the specific surface area are possibly more favorable for ORR catalysis.

Figure 5. (a) RRDE voltammogram for HPC-Fe/N-700 in 0.5 M H2SO4 solution saturated with O2. The electrode rotation rate was 1600 rpm; (b) Electron transfer number and percentage of H2O2 as a function of potential; (c) RDE voltammograms for HPC-Fe/N-700 in 0.5 M H2SO4 solution saturated with O2; (d) Koutecky-Levich plots obtained from the RDE curves.

Furthermore, the catalytic performance of HPC-Fe/N-700 for ORR in acidic condition (0.5 M H2SO4) was also studied. The half-wave potential (E1/2) of the

sample of HPC-Fe/N-700 in RDE voltammogram was approximately 0.65V vs. RHE (Fig. 5a). The n and H2O2 concentration were calculated to be 3.9 and 1.2% at 0.65 V vs. RHE respectively (Fig. 5b), which are better than those of its performance in alkaline media. The RDE voltammetric profiles in O2-saturated 0.5 M H2SO4 solution showed that the current density increased by increasing rotation rate (Fig. 5c). K-L plots (Fig. 5d) with a well-fitted linear relationship for HPC-Fe/N-700 were calculated from linear sweep voltammetry (LSV) curves (Fig. 5c) at various rotation rates. Oxygen reduction polarization curves for the HPC-Fe/N-700 and Pt/C in an O2-saturated 0.5 M H2SO4 solution at a rotation rate of 1600 rpm are compared in Fig. S7, the JDL values were 5.05, and 5.45 mA·cm-2 for the HPC-Fe/N-700 and Pt/C, respectively. The JDL value of HPC-Fe/N-700 was only 0.4 mA·cm-2 lower than the commercial precious metal Pt/C catalyst. All in all, HPC-Fe/N-700 delivered superior ORR performances under both alkaline and acidic conditions, potentially applicable for some practical energy devices, such as fuel cells and Zn-air batteries.

Figure 6. (a) Current-time (i-t) chronoamperometric response of HPC-Fe/N-700 and Pt/C at -0.4 V in O2-saturated 0.1 M KOH at a rotation rate of 1600 rpm; (b) Chronoamperometric responses in O2-saturated 0.1 M KOH after addition (at 300 seconds) of 2% (v/v) methanol for the HPC-Fe/N-700 and Pt/C at -0.4V.

Durability is one of the major concerns in current fuel cell technology, the stability of HPC-Fe/N-700 was further tested at a constant voltage of -0.4 V for 20000 s in 0.1 M KOH solution saturated with O2 at a rotation rate of 1600 rpm (Fig. 6a). Remarkably, the corresponding current-time chronoamperometric response of HPC-Fe/N-700 exhibited a very slow attenuation after a fast decrease of 7% within the first 1000 s, and a high relative current of 93% still persisted after 20000 s. In contrast, Pt/C showed a gradual decrease, with a current loss of approximately 18% measured after 20000 s. This result suggests that the durability of HPC-Fe/N-700 is superior to that of the commercially available Pt/C catalyst. We further measured the

electrocatalytic selectivity of HPC-Fe/N-700 and Pt/C against the electro-oxidation of methanol for the ORR (Fig. 6b). After the adding of 2% (v/v) methanol, the ORR current for HPC-Fe/N-700 was almost unchanged, while Pt/C showed a sharp decrease and even exhibited a negative current. Therefore, HPC-Fe/N-700 has a much better selectivity for methanol and a remarkable ability to avoid crossover effect, which is superior to the commercially available Pt/C catalyst. 4. Conclusions In this work, we developed a facile and low cost synthetic method to prepare a series of Fe/N-doped hierarchical porous carbons with hollow structure by using conjugated microporous polymer and silica spheres as precursor and template respectively. The resulting carbon materials possess high surface areas and high-content Fe/N co-doping characters, rendering them suitable for being as non-precious metal electrochemical catalysts for ORR reaction. The sample achieved at 700 °C (HPC-Fe/N-700) exhibited excellent electrochemical performance in both alkaline and acidic conditions in comparison with the commercially available Pt/C catalysts. This work paves a new avenue to design hierarchical porous carbons with controlled multiple dopants for high performance energy storage and conversion. Acknowledgements We are grateful for the financial supported from the National Basic Research Program of China (973 Program: 2013CBA01602), National Natural Science Foundation of China (21574080, 51403126, 61306018), the Shanghai Committee of Science and Technology (15JC1490500, 16JC1400703) and the Open Project Program of the State Key Laboratory of Photocatalysis on Energy and Environment (SKLPEE-KF201702). References [1] Chu, S., Majumdar, A., Opportunities and challenges for a sustainable energy future. Nature 2012, 488(7411): 294-303. [2] Wang, Y., Liu, X., Li, Z., Cao, Y., Li, Y., Zhao, Y., Constructing Synergetic Trilayered TiO2 Photoanodes Based on a Flexible Nanotube Array/Ti Substrate for Efficient Solar Cells. ChemNanoMat, 2017, 3(1): 58-64. [3] Liu, X., Yang, J., Zhao, W., Wang, Y., Li, Z., Lin, Z., A Simple Route to Reduced Graphene Oxide-Draped Nanocomposites with Markedly Enhanced Visible-Light Photocatalytic Performance. Small, 2016, 12(30): 4077-4085. [4] Liu, X., Iocozzia, J., Wang, Y., Cui, X., Chen, Y., Zhao, S., Li, Z., Lin, Z., Noble metal-metal oxide nanohybrids with tailored nanostructures for efficient solar energy conversion, photocatalysis and

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