High Level Oxygen Reduction Catalysts Derived from the Compounds of Large Specific Surface Area Pine Peel Activated Carbon and Phthalocyanine Cobalt

equal contribution in this paper. *Corresponding authors. Department of physical science and technology, Lingnan Normal University, Zhanjiang 524048, PR China, E-mail: leizhaolingnan@163.com (L. Zhao) School of Electronics and Information Engineering, Guangdong Ocean University, Zhanjiang 524088, PR China, E-mail: yangwenhu@stu.gdou.edu.cn (W. Yang). Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 26 October 2021


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In recent years, in order to overcome the shortcomings of Pt-based catalysts, many nonprecious metal ORR catalysts with high electrocatalytic activity have been explored, including metal hydroxides [10], oxides [11], sulfides [12], phosphides [13], nitrides [14], selenides [15], and heteroatoms doped carbon materials [16][17][18]. Among them, heteroatoms doped carbon materials are very effective in improving the catalytic activity of ORR for the large SSA and lots of catalytic sites [19]. Doping carbon with heteroatoms (especially N) can improve the charge distribution of adjacent C atoms to provide high catalytic activity and high stability [20]. Because of the existence of pyridine and pyrrole-like N species, N-doped carbon materials exhibit excellent catalytic activity, and the higher the content of pyridine dinitrogen and pyrrole nitrogen, the more beneficial the improvement of ORR catalytic activity [21,22].
Besides, transition metals (for example, Co, Fe, and Ni) also have a pivotal role in ORR, but the scarcity of active sites and low electron transfer efficiency restrict their catalytic activity. And the M-N-C composite catalyst composed of N-doped carbon materials and transition metal has been widely studied for its ability to enrich abundant active sites and improve the conductivity and the strong synergy between the composition [23][24][25]. Recently, a predominant direction in the series of M-N-C is phthalocyanines metal catalyst, especially CoPc, which have shown prominent catalytic effect for reducing molecular oxygen and four-electron pathway [26].
However, in order to solve the problem of low conductivity of CoPc, various carbon materials have been compounded with CoPc to enhance the overall electrochemical performance of composite materials including carbon nanotubes [27] and graphene [28].
Compared with many carbon materials, biomass-derived carbon materials have been developed as a low-cost nonprecious metal catalyst thanks to their high availability, accessibility and recyclability. Biomass pine peels are widely distributed throughout China and are freely available, which will provide a good foundation for the development and application of nonprecious metal N-doped carbon catalysts.
Simultaneously, to promote the formation of catalytic sites to enhance catalytic performance, the most critical step is to design a catalyst with a large SSA and abundant nanopores to promote high mass transfer flux and high catalytic activity [29], Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 26 October 2021 4 which can be done by adjusting the concentration of the activator and the activation temperature [30]. Therefore, it is desirable to design and prepare Co-N-C catalysts with high electrocatalytic activity by converting N-doped pine peel into nanocarbon through pre-activation and heat treatment to regulate the chemical structure and morphology of carbon, and then conduct pyrolysis with an appropriate proportion of CoPc at high temperature. Compared with other precursors containing nitrogen and cobalt (such as aniline [31], melamine [32], pyrrole [33] and metal organic frameworks [34]), this method is simpler, more effective, lower cost, and more promising to prepare Co-N-C catalysts with synergistic effect and significantly enhanced ORR catalytic activity.
Herein, we synthesized a series of AC@CoPc composite catalysts through pre-activation treatment, high-temperature carbonization methods and precise control the composite proportion of AC derived from biomass pine peel and CoPc. And the characterization analysis and electrochemical study of AC@CoPc series composite catalysts under different composite proportions and temperature conditions were also carried out. Compared with other AC@CoPc series catalysts and Pt/C (20 wt%) catalysts, AC@CoPc-800-1-2 had excellent catalytic ability for ORR, which benefited from AC derived from biomass pine peel with a large SSA and many nanopores provided abundant particle attachment sites, N-doped activated carbon improved the charge distribution of adjacent C atoms and generated more charge sites as well as the formation of an enormous number of atomically dispersed Co nanoparticles encapsulated by graphitic carbon and synergistically with N promote the exposure of CoNx active sites.

Experimental
All reagents are analytically pure and can be used without further purification.

Preparation of AC
The pine peel was obtained locally in Harbin, China, washed by distilled water and crushed. First, the pine peel was mixed with KOH at a mass ratio of 1:4 and then heated at 900 ℃ for 1 h in tubular furnace with the heating rate of 5 ℃ 5 min -1 under N2 atmosphere with the flowing rate of 100 ml min -1 . After activated, the above samples were washed with 1 M HCl and distilled water several times and dried for 12 h at 60 °C. Finally, the obtained samples were denominated as AC.

Preparation of AC@CoPc
The cobalt phthalocyanine compound was purchased from Shanghai, China. To obtain AC@CoPc series composite catalysts, AC and CoPc were mixed with the mass ratio of 2:1, 1:1, and 1:2 by carbonizing at 700, 800, and 900 ℃ for 1 h in a N2 atmosphere, respectively. Then the obtained samples were grinded for 1 h with ethanol solution in the glass dish, and dried for 12 h at 60 °C. Scheme 1 is a schematic illustration of AC@CoPc series composite catalysts. The AC@CoPc series composite catalysts were named as self-defined pattern, such as the composed materials of AC and CoPc mixed with the mass ratio of 2:1 was dominated as AC@CoPc-700-2-1.

Scheme 1
The schematic illustration of the synthesis of AC@CoPc series catalysts.

Structure characterizations
Scanning electron microscopy (SEM, Hitachi S-4800) was used to investigate the surface morphology and structure of the catalyst samples. X-ray diffraction (XRD) patterns of the samples were obtained on a Shimadzu XRD-6000 X-ray diffractometer using Cu Ka radiation with 4°min -1 . Transmission electron microscopy (TEM) and selected area mapping were collected were operated on a JEM-2100F instrument with acceleration voltage of 100 kV. X-ray photoelectron spectroscopy (XPS) analysis was investigated using a VG Scientific ESCALAB 250 iXL spectrometer with an Al Ka X-ray source. 6

Electrochemical characterizations
Electrochemical experiments were conducted at room temperature on an electrochemical workstation (RST5200F, Zhengzhou shiruisi Instrument Co., Ltd. Under the same experimental conditions, Pt/C (20 wt%) purchased from Shanghai He Sen Electric Co., Ltd. was used for the above experimental comparison.

Calculation of electron transfer number (n)
The electron transfer number (n) is determined by the Koutecky-Levich equation at a series of potentials: Where J is the measured current density (mA cm -2 ), JL and JK are the diffusion-limiting and kinetic current densities (mA cm -2 ), ω is the angular velocity of the disk (ω = 2πN, N is the linear rotation speed), n is the overall number of electrons transferred per oxygen molecule during ORR, D0 is the diffusion coefficient (cm s -1 ),

Results and discussion
The SEM images of AC and different proportion AC@CoPc series composite catalysts are shown in Figure 1 and These results suggest that the rich distribution and size of nanopores can be regulated and controlled by pyrolysis temperature and activation, thereby forming more nanopores to expose more active areas and promote the ability of electron transfer [35,36], but the appropriate temperature and proportion are more conducive to the recombination of AC and CoPc as well as can provide more adhesion sites for CoPc.  The XRD patterns of AC@CoPc series composite catalysts (Figure 3a) all correspond to Co (JCPDS 15-0806) [37] and graphite (JCPDS 01-0646) [38].
In Figure 4a, many vague small black spots are exposed as active sites on the carbon skeleton, and they are uniformly distributed. HRTEM images of Figure 4c confirmed that these active sites (circled in yellow circles in Figure 4b) are Co nanoparticles active sites with a lattice spacing of 0.20 nm corresponding to (111) crystal plane of Co, which have been embedded into the carbon skeleton, while 0.33 nm corresponds to the (002) crystal plane of graphite [40]. This value is larger than the spacing of (002) in graphite, showing a disordered effect in the catalyst, and the graphite carbon tightly wraps the active sites of Co nanoparticles in the carbon skeleton, which also enhances the mechanical stability of nanostructured composites [41,42]. The element mappings  pyridinic-N (N1) [43]. While the major part of nitrogen moieties in AC@CoPc-800-1-2 exhibits the higher contribution of pyridine-N and a high amount of Co and N association in the CoNx structure. Except the two catalysts mentioned above, AC@CoPc-900-1-2 shows lower CoNx and pyrrolic-N content in Table 1. Therefore, we have reason to infer that pyridine-N sites and CoNx have a substantial role in ORR.
For Co 2p, the XPS spectra of these three composite catalysts (Figure 5b) show that three main peaks at 780.3 eV, 781.8 eV, and around 783 eV are assigned to Co, CoxOy or CoCxNy, and CoNx respectively [44][45][46]. The cobalt content percentages of these three composite catalysts are 0.77%, 0.52%, and 0.51%, respectively. As the temperature increased, the cobalt content gradually decreased, while the nitrogen content is 6.63%, 3.35% and 2.91%, also showing a downward trend (Figure 5d). This indicates that high temperature (800 °C) can increase the reaction rate between Co and N, but too high temperature (900 °C) will cause a large loss of N. And compared with AC@CoPc-700-1-2 and AC@CoPc-900-1-2, cobalt content in AC@CoPc-800-1-2 shows higher CoNx content and the result is consistent with the analysis of N2 moiety.  In order to identify the properties of catalysts in Figure 6a, Table 2 Table 3.

Conflicts of Interest:
The authors declare no conflicts of interest.