Preparation of multistage microporous and mesoporous nitrogen-doped carbon nanospheres and study on electrocatalytic oxygen reduction

. Non-metallic carbon-based catalysts are one of the most promising catalysts for electrocatalytic oxygen reduction (ORR). However, the controlled synthesis of the pore structures of carbon-based catalysts and the determination of catalytic sites still remains to be further explored. Based on this, two nitrogen-doped carbon-based catalysts with microporous and mesoporous structures were prepared in this paper. The two catalysts had different pore structures and nitrogen-doped species. The experimental results showed that the carbon-based catalyst (PC-NH 3 ) rich in microporous structure and pyridine nitrogen doping had better catalytic performance, the electrocatalytic oxygen reduction performance of the catalyst was comparable to that of the platinum catalyst, and it had excellent methanol resistance and stability. This study provided important guidance for the application of porous carbon materials in oxygen reduction.


Introduction
The technology of proton exchange membrane fuel cell could effectively alleviate the energy crisis and environmental pollution because of its high efficiency, green and clean advantages, which is an ideal alternative to fossil energy clean energy [1][2]. At present, the cathodic oxygen reduction catalysts for fuel cells are mainly precious metals such as Pt and Pd, and their alloys. Their high price greatly limits the large-scale industrial application of this technology [3][4][5]. Therefore, it is imperative to explore oxygen reduction catalysts with high activity, high stability, low cost and friendly environmental [6].
Non-metallic carbon-based catalysts have attracted extensive attention due to their advantages such as good conductivity, large specific surface area, abundant reserves and friendly environmental [7][8][9]. The controlled synthesis of pore structure and heteroatom doping are effective means to regulate carbon-based catalysts [10][11][12]. Usually, the hierarchical structure with pore size and pore volume composed of macropore, mesopore or micropore, which largely determines the kinetics of oxygen adsorption and diffusion [13]. In addition to the high surface area and controllable nanostructures, electrocatalysts also need good electron transfer conductivity, excellent activity and high chemical stability. Generally, doping nitrogen and other heteroatoms into the carbon matrix can regulate the local electronic structure of the carbon-based catalyst to improve the overall conductivity of the catalyst, and providing more active sites to enhance the interaction between carbon and oxygen molecules [14][15][16][17][18][19]. In carbon-based catalysts, the effects of macro-porous, mesoporous and microporous structures on oxygen adsorption and diffusion are different, and doping of different nitrogen species (pyridine nitrogen, graphite nitrogen, nitrogen oxide and etc.) has different effects on catalytic performance. However, up to now, the controlled synthesis of pore structure and precise doping of nitrogen species are still a major problem in the preparation of carbon-based catalysts [20][21][22]. Therefore, we developed a low concentration hydrothermal method to synthesize two nitrogen-doped spherical porous carbon-based catalysts. The two catalysts have different pore structures and nitrogen-doped species. The PC-NH 3 catalyst is rich in microporous structure and pyridine nitrogen doping, while the PC-N 2 catalyst is rich in mesoporous structure and graphite nitrogen doping. Performance tests show that, PC-NH 3 catalyst has better oxygen reduction performance, methanol resistance and stability. This indicates that compared with mesoporous structure and graphite nitrogen doping, the microporous structure and pyridine nitrogen doped carbon-based catalyst are more conducive to oxygen reduction reaction.

Sample preparation
Phenol (600 mg), aqueous formalin solution (3 mL, 37 wt.%) and NaOH aqueous solution (15 mL, 0.1 M) were mixed and heated at 70 °C for 1 h under intense stirring. Subsequently, melamine (100 mg) and Pluronic F127 (960 mg, Mw = 12600 g mol -1 ) dissolved in 15 mL of deionized water was added. The resulting mixture was then stirred at 80 °C for 7 h. Subsequently, the solution was diluted with 60 mL of deionized water and then maintained at 70 °C for 24h under constant stirring. Then, 50 mL of the resulting solution and 150 mL of deionized water were transferred into a Teflon-lined stainless steel autoclave and heated at 130 °C for 24 h. The porous polymer nanospheres formed were collected by centrifugation and washed repeatedly with deionized water. The porous polymer nanospheres were then transformed to defective N-doped carbon spheres by heating at 1000 °C for 2 h under an NH 3 or N 2 flow, marked as PC-NH 3 and PC-N 2 , respectively.

Performance and characterization of samples
Morphology and elemental analysis of the samples were performed using a JEOL-2100F transmission microscope with an acceleration voltage of 200 kV. The method of characterization was to add the catalyst sample solution on the ultra-thin carbon film, then dried it fully and characterized it. X-ray photoelectron spectroscopy (XPS) data were acquired on a VGESCALABMKII X-ray photo-electron spectrometer equipped with a non-monochromatized Al-Kα X-ray source (hν = 1486.7 eV). Raman spectra were obtained on a Renishaw in Via-Reflex spectrometer system. The spectra were excited using a 532 nm source.
The electrocatalytic oxygen reduction performance was tested in a three-electrode system on an electrochemical workstation (CHI 760E). Method for preparing the working electrode: the 8 mg sample was dispersed in a solution of 500 uL deionized water and 460 uL ethanol, 40 uL nafion solution was added in it, and sonicated for 2 h. And then, 3 uL of the sample dispersed droplets were added to the glassy carbon electrode and dried thoroughly. Saturated Ag/AgCl electrode and platinum wire electrode were used as reference electrode and counter electrode, respectively.

Characterization and analysis of surface structure
As a precursor of nitrogenous non-metallic carbon spheres, the nitrogenous polymer spheres were first synthesized. Polymer molecules were synthesized from phenol and formaldehyde through the phenol-formaldehyde condensation reaction. The nitrogenous polymer spheres were synthesized in the environment of melamine with the action of F127, as shown in Fig. 1(a). Nitrogen-containing polymer spheres were calcined and carbonized at high temperature under NH 3 or N 2 atmosphere to form samples PC-NH 3 ( Fig. 1(b)) and PC-N 2 (Fig. 1(c)), respectively. It was obvious that the volume of a single sphere was significantly reduced after calcination, however, the spherical morphology was still maintained. The difference between PC-NH 3 and PC-N 2 could not be seen from the morphology alone. XPS was used to characterize the N content and N-doped species of the two catalysts, as shown in Fig. 2. The nitrogen contents of PC-NH 3 and PC-N 2 were 3.15 at% and 2.89 at%, respectively. The results were not that different, but the nitrogen content of the two species was very different, PC-NH 3 mainly contained pyridine nitrogen heteroatoms, while PC-N 2 mainly contained graphite nitrogen heteroatoms. This could be because NH 3 had an etching effect on carbon materials at high temperatures, the carbon material could be N-doped with its own N, and the N-doped form of the material itself was reconstructed [23][24][25].  Fig. 3. It was obvious that the specific surface area of PC-N 2 (931 m 2 ·g -1 ) was larger than that of PC-NH 3 (815 m 2 ·g -1 ). This meant that the etching of ammonia caused the pore structure of the catalyst to change at high temperatures, it reduced the specific surface area of the catalyst. In addition to the specific surface area of the catalyst, the pore size distribution of the catalyst directly affected the diffusion and adsorption process of oxygen, and then it affected the catalytic performance of the catalyst. Pore size distribution curves for PC-N 2 and PC-NH 3 were shown in Fig. 4. It proved that the pore size distribution of the two catalysts was quite different. The amount of micropore in PC-NH 3 sample was significantly higher than that in PC-N 2 sample. However, the amount of mesopore in the former was much lower than that in the latter. This might be due to the etching of ammonia led to the collapse of the mesoporous structure of the material itself at high temperature, at the same time, the microporous structure with relatively small pores was formed. BET specific surface areas calculated from N 2 physisorption data were 931 m 2 ·g -1 and 815 m 2 ·g -1 for PC-N 2 and PC-NH 3 , respectively. Based on the above experimental data, we successfully synthesized two kinds of porous carbon-based non-metallic catalysts with different pore structures and nitrogen-doped species. The obvious differences of micro-and mesoporous structures and N-doped species were beneficial to explore the relationship between the structure and catalytic performance of catalysts, which was the basis for the study of catalytic reaction mechanism.

Characterization of catalytic properties of oxygen reduction
The catalytic performance of PC-N 2 and PC-NH 3 is shown in Fig. 5(a) potentials of oxygen reduction of PC-N 2 and PC-NH 3 samples were -0.185 V and -0.138 V (vs Ag/AgCl), respectively, the catalytic performance of the latter was significantly better than that of the former, and it was closed to the half-wave potential of Pt witch was -0.141 V (vs Ag/AgCl). Comprehensive analysis of structural characterization data, the PC-NH 3 samples with microporous structure and pyridine nitrogen doping had excellent catalytic performance. This indicated that the microporous structure and pyridine nitrogen were more conducive to oxygen reduction among the types of carbon-based catalysts. Pt-based catalysts have an obvious disadvantage, during the catalytic process, it was easy to be poisoned by methanol used in the battery, and the catalytic activity was greatly reduced. Therefore, we compared the methanol tolerance of PC-NH 3 and Pt, as shown in Fig. 5(b), PC-NH 3 had excellent methanol tolerance.

Conclusions
In this paper, nitrogen-containing polymer carbon spheres were synthesized by a low concentration hydrothermal method. Two spherical porous carbon-based catalysts with different pore structures and nitrogen-doped species were synthesized by the calcination of NH 3 and N 2 on polymers at high temperature. The results of structural characterization showed that the catalyst PC-NH 3 was rich in microporous structure and pyridine nitrogen doping, while PC-N 2 was rich in mesoporous structure and graphite nitrogen doping. The performance test results showed that PC-NH 3 had better oxygen reduction performance and methanol resistance. This meant that compared with carbon-based catalysts contained mesoporous structures and graphite nitrogen doping, the carbon-based catalyst with microporous structure and pyridine nitrogen doping was more conducive to oxygen reduction reaction. This work laid an important research foundation for exploring more excellent non-metallic carbon-based catalysts and had a certain guiding role.