A mesoporous carbon-based catalyst derived from cobalt and boron co-doped melamine formaldehyde gel for oxygen reduction reaction

Journal Pre-proof A mesoporous carbon-based catalyst derived from cobalt and boron Co-doped melamine formaldehyde gel for oxygen reduction reaction Hehuan Cao, Jidong Cao, Fanghui Wang, Hong Zhu, Min Pu PII:

S0013-4686(19)32432-6

DOI:

https://doi.org/10.1016/j.electacta.2019.135560

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EA 135560

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Electrochimica Acta

Received Date: 22 October 2019 Revised Date:

10 December 2019

Accepted Date: 21 December 2019

Please cite this article as: H. Cao, J. Cao, F. Wang, H. Zhu, M. Pu, A mesoporous carbon-based catalyst derived from cobalt and boron Co-doped melamine formaldehyde gel for oxygen reduction reaction, Electrochimica Acta (2020), doi: https://doi.org/10.1016/j.electacta.2019.135560. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

A Mesoporous Carbon-based Catalyst Derived from Cobalt and Boron Co-doped Melamine Formaldehyde Gel for Oxygen Reduction Reaction Hehuan Cao, Jidong Cao, Fanghui Wang, Hong Zhu*, Min Pu* State Key Laboratory of Chemical Resource Engineering, Institute of Modern Catalysis, Department of Organic Chemistry, College of Chemistry, Beijing University of Chemical Technology, Beijing 100029, P. R. China.

Corresponding Author *

E-mail: [email protected] (Hong Zhu)

*

E-mail: [email protected] (Min Pu)

1

Abstract Non-precious metal catalysts are playing an increasingly important role in research on oxygen reduction reaction (ORR) catalysts for fuel cells due to their relatively low cost. In this work, a novel mesoporous catalyst derived from cobalt- and boron-co-doped melamine formaldehyde (MF) gel (Co, B-MF) was successfully synthesized by a facile self-assembly pyrolysis method. It was found that doping of Co and B could provide various active sites and an appropriate pyrolysis temperature was beneficial to increase the number of active sites. As a result, the obtained Co, B-MF-900 catalyst (pyrolyzed at 900 °C) exhibited an outstanding ORR performance in 0.1 M KOH solution with a high onset potential of 1.1 V versus reversible hydrogen electrode (vs RHE), which was 0.1 V higher than that of the commercial Johnson Matthey Pt/C. Additionally, Co, B-MF-900 displayed good stability and superior methanol tolerance. The superior ORR performance of Co, B-MF-900 can be attributed to its mesoporous structure, the synergistic effect of active sites, and the excellent conductivity of the carbon carrier.

Keywords: Non-precious metal catalysts; Oxygen reduction reaction; Fuel cell; Co-doping of cobalt and boron; Melamine-formaldehyde gel.

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1. Introduction Fuel cells are considered as the most promising energy conversion devices because of their high energy conversion efficiency, environmental friendliness, and abundant fuels. However, the commercial application of fuel cells remains challenging due to the sluggish kinetics of the oxygen reduction reaction (ORR) at the cathode, so it is essential to develop new catalysts to improve the activity.[1-3] Pure Pt is considered to be the most appropriate catalysts for ORR; however, it is scarce, expensive and has poor stability.[2, 4, 5] Therefore, researchers have sought new methods to reduce the use of Pt or replace it, including developing low-Pt and non-precious metal catalysts. So far, the preparation of low-Pt catalysts has been a more reliable approach, but in the long term, developing non-precious metal catalysts is a better way to solve the problems associated with cathodic catalysts. Among various non-precious metal catalysts, carbon-based materials doped with heteroatoms, such as nitrogen (N), boron (B), sulfur (S), and phosphorus (P), have been considered as promising candidates for replacing Pt-based catalysts due to their remarkable ORR activities, outstanding stabilities, and relatively low costs.[6-10] Introducing heteroatoms can improve the ORR activity of the catalyst by modifying its electronic structure. Especially, N-doped carbon materials have attracted much research attention and exhibit remarkable catalytic activity.[11-13] Further studies have shown that carbon materials doped with two or more heteroatoms generally have more active sites and thus higher activity than single N-doped carbon materials.[14-16] Kang et al. synthesized a carbon material doped with N, P, and B, and found that the 3

synergistic effect of the heteroatoms strongly influenced the electrochemical performance of the carbon material.[17] Additionally, previous findings revealed that even trace amounts of transition metals can dramatically boost the activity of catalysts.[18-24] Various methods for preparing non-noble metal catalysts are currently available; the most common of these involves simply mixing precursors containing specified elements and then calcinating at a certain temperature.[17, 25] However, using this method, it is difficult to achieve a uniform distribution and high loading of active sites together with a high specific surface area. Herein, we successfully designed and synthesized a new mesoporous catalyst (Co, B-MF) with a large specific surface area, various active sites, and excellent conductivity using a facile self-assembly pyrolysis method. Melamine-formaldehyde (MF) gel was selected as the main framework of the catalyst due to its high porosity and high nitrogen content.[20, 26] These properties created many active sites and a large specific surface area in contact with the electrolyte. Vitamin B12 was chosen as a Co source over a conventional Co salt owing to its corrin structure. This structure, which contains a Co atom connected to four N atoms, is considered as a possible active site for the ORR.[27-29] Additionally, vitamin B12 is environmentally friendly, economical, and easy to prepare, so it can be used on a large scale. Co and B were introduced while preparing the MF gel so that active sites could be more uniformly distributed in the catalyst than by simple mixing. The effects of doping Co and B as well as that of the pyrolysis temperature on the ORR performances of the target catalysts have been systematically studied. The catalyst (Co, B-MF-900) doped with 4

Co and B and prepared at 900 °C had a high specific surface area and many active sites, and it exhibited the highest oxygen reduction activity among the catalysts studied. It also showed good durability and methanol tolerance. The results indicate that the as-prepared Co, B-MF catalyst is a feasible alternative to Pt-based catalysts for the ORR.

2. Experiment section 2.1. Chemicals and Materials Melamine, formaldehyde solution (37%), vitamin B12, boric acid (H3BO3), and sodium hydroxide (NaOH, 99%) were purchased from Beijing Chemical Works, China. Nafion solution (5 wt %) was provided by Dupont. 2.2. Synthesis of Co, B-MF-T catalysts 2.2.1. Preparation of Co, B-MF The catalyst precursor was derived from a modified MF gel (Figure 1). This was prepared according to an improved version of a method reported earlier.[26] Firstly, melamine (3.15 g) and a 37% formaldehyde solution (6.9 ml) were placed in a round bottom flask, which was then heated to 70 °C to dissolve the melamine. After adjusting the pH to 8–8.5 with a 0.1 M NaOH solution, the resulting mixture was prepolymerized at 80 °C for 30 minutes and then transferred to a beaker. Under vigorous stirring, 10 ml of a vitamin B12 aqueous solution (0.0315 g/ml) and 10 ml of boric acid solution (0.031 g/ml) were added to the solution. The mixed solution was stirred for 30 minutes until it was homogeneous. Subsequently, the pH was adjusted 5

to 2–2.5 with a 0.1 M HCl solution. Further polymerization of the prepolymer was performed in an oven at 60 °C for one day to form a wet MF gel, and then acetone was added to the wet gel once a day for 2 days to replace the solvent so that abundant holes were formed in the gel. After drying, the wet gel was ground into powder. Precursors without H3BO3 (Co-MF) or vitamin B12 (B-MF) were prepared under the same conditions as those used to prepare Co, B-MF. 2.2.2. Pyrolysis of Co, B-MF The prepared catalyst precursor was placed in a porcelain boat and heated to different target temperatures (700, 800, 900, and 1,000 °C) under N2 at a heating rate of 8 °C per minute in a tube furnace, held at the target temperature for 4 hours, and cooled to room temperature naturally to produce the target catalysts Co, B-MF-T (T=700, 800, 900, and 1000 °C). 2.3. Materials characterization The morphology, structure, and microstructure of the catalysts were observed by transmission electron microscopy (TEM) on a JEM-3010 microscope (JEOL). The energy dispersive X-ray spectroscopy (EDS) maps of the catalysts were measured by scanning electron microscopy (SEM) on a S-4700 microscope (HITACHI). The crystal structures of the catalysts were characterized by X-ray diffraction (XRD) using a D8 Advance X-ray diffractometer (Bruker) with Cu Kα radiation (λ = 1.54 Å). Elemental and quantitative analyses of the catalysts were performed by X-ray photoelectron spectroscopy (XPS) on an ESCALAB 250 X-ray photoelectron 6

spectrometer (Thermo Fisher Scientific). The specific surface area and pore distribution were measured using an AS-1C-VP instrument (Quantachrome). The degree of carbon structural disorder in the catalysts was determined by Raman spectroscopy on a Raman Spectrometer with an argon ion laser. 2.4. Electrochemical test To prepare the working electrode, 4 mg of the catalyst was added to a mixture of 390 µl of isopropanol, 600 µl of water, and 10 µl of Nafion solution (5 wt %) and sonicated for 30 min to prepare a catalyst ink, of which 20 µl was dropped onto a polished glassy carbon electrode at a catalyst loading of 0.4 mg cm-2 and dried naturally at room temperature. Electrochemical tests were performed at room temperature with a three-electrode system. Rotating disk electrode (RDE) tests were performed on a Zahner Zennium electrochemical workstation (Germany). Rotating ring disk electrode (RRDE) tests were performed on a PARSTAT MC electrochemical workstation (USA). Hg/Hg2SO4 (saturated K2SO4) and Ag/AgCl (saturated KCl) electrodes were selected as reference electrodes under alkaline and acidic conditions, respectively. A Pt wire was used as the counter electrode. A glassy carbon RDE (diameter 5 mm, Pine) and a RRDE (glassy carbon disk diameter 5.5 mm, surrounded by a Pt ring with an outer diameter of 8.5 mm, Pine) with a collection efficiency of 0.38 served as working electrodes. The electrodes were polished with alumina, rinsed with ethanol and deionized water and dried before use. All the electrode potentials were converted to the standard reversible hydrogen 7

electrode (RHE) potentials according to the Nernst equation. Onset potential is the potential corresponding to the intersection of the tangent to the polarization curve in low overpotential region and the baseline at which the current density is 0.[21] These tests were performed under oxygen- or nitrogen-saturated conditions, with the gases injected into the electrolyte for 30 minutes before the tests. The electron transfer number (n) of a catalyst can be calculated using the Koutecky-Levich (K-L) equation:[30-32] 1 1 1 1 1 = + = +      



 = 0.62 ( )   where J, JL and JK are the measured current density, diffusion-limited current density, and kinetic density, respectively. ω represents the electrode rotation rate, Faraday constant named as F is 96485 × 103 C/mol. The oxygen saturation concentration, diffusion coefficient of oxygen, and kinematic viscosity of the electrolyte are named as C0, D0, and ν, respectively.[33]

3. Results and discussion 3.1. Structure and characterization To study the effect of the pyrolysis temperature on the morphology of Co, B-MF, we performed TEM on the catalysts prepared at different temperatures. The TEM images are shown in Figure 2 and Figure S1. Notably, all the catalysts have open pores formed by connections between carbon particles, which can facilitate mass transfer during the ORR, increasing the catalytic activity. The magnified TEM shows that some spherical particles are uniformly distributed on the carbon carriers of the 8

catalysts prepared at 900 °C and above. The lattice parameter of these particles measured from the High-resolution TEM (HRTEM) image (Figure 2(c)) is about 0.223 nm, corresponding well to the (111) planes of the Co2P phase.[27] The HRTEM image of Co-MF-900, shown in Figure S2(b), also exhibits the same lattice fringes. Therefore, it can be concluded that the observed spherical particles were a Co2P complex, which was obtained from the high-temperature decomposition of vitamin B12. Vitamin B12 decomposed during the pyrolysis to form Co-NX chelates, which offered highly efficient active sites. However, these Co-NX chelates were unstable at temperatures exceeding 900 °C and partially decomposed to form a Co2P complex.[27] When the temperature reached 1000 °C, the number of Co2P complex increased sharply, indicating that more Co-NX chelates in Co,B-MF-1000 is converted into Co2P complex. Additionally, the TEM images shown in Figure 2(b) and Figure S2(a) shows that Co, B-MF-900 has fewer metal particles, a smaller particle size, and a more uniform particle distribution than Co-MF-900. As shown in figure S3, the SEM and corresponding EDS maps shows that the Co, B-MF consisted of C, N, O, Co, and B, and that these elements were distributed evenly. Figure 3(a) shows XRD patterns of Co, B-MF-T prepared at different temperatures. All the samples shows a broad peak at 2θ = 25° and a small peak at 44°, corresponding to the (002) and (101) planes, respectively, of graphitic carbon in the carbon carrier [20, 34, 35]. These peaks demonstrates that these catalysts have a turbostratic carbon structure that consisting of a two-dimensional long-range ordered graphitic carbon structure along with an amorphous carbon structure.[36] Particularly, 9

as the pyrolysis temperature increases, the intensity of the (101) peak of graphitic carbon first decreases and then increases, with a minimum at 900 °C. This clearly reveals that the catalyst is most amorphous when the pyrolysis temperature is 900 °C. This unusual tendency may be related to the heteroatom doping, since the catalyst prepared at 900 °C has the most heteroatoms doped into the carbon matrix according to the XPS elemental analysis results (Table S1). This doping can create more defects in the carbon matrix and thus provide more active sites for the catalyst. As shown in Figure S4(a), the XRD pattern shows a decrease in the intensity of the (101) peak of graphitic carbon after Co and B co-doping into the catalyst, which further demonstrates that the change in the carbon structure is caused by the heteroatom doping. At temperatures exceeding 900 °C, a small sharp peak appears at 2θ = 42 ° in the XRD pattern of Co, B-MF-900 and Co-MF-900, but this does not appear in the XRD pattern of B-MF-900. Based on these XRD patterns, combined with the HRTEM analysis, we attributes the peak to the (111) planes of the Co2P phase.[29] Figure 3(b) shows the Raman spectra of Co, B-MF-T prepared at different temperatures. It is clear from the figure that the peak positions of each sample are almost the same at about 1340 and 1600 cm-1, corresponding to the D band (amorphous carbon) and G band (graphitic carbon), respectively.[37, 38] The D band is generated by the disordered carbon structure with atomic lattice defects, and the G band originates from the stretching vibrations of the sp2-hybridized carbon atoms. The ratio of the intensity of the D band to that of the G band (ID/IG) is usually used to denote the degree of disorder and defects in the carbon structure.[39, 40] With higher 10

ID/IG, the carbon structure has the more defects and it is more disordered. ID/IG increased and then decreased as the pyrolysis temperature increased, and the catalyst prepared at 900 °C had the highest ID/IG. Therefore, Co-MF-900 had the highest degree of disorder in the carbon structure, which was consistent with the XRD results. Compared with Co-MF-900 and B-MF-900, Co, B-MF-900 had a higher degree of structural disorder (Figure S4(b)), indicating that doping of Co and B facilitated the formation of the disordered carbon structure. The above analysis shows that the pyrolysis temperature and doping of Co and B can affect the structure of carbon, which is consistent with the XRD results. Figure 4(a) and Figure S5(a) shows the results of the full XPS elemental analysis of all the catalysts. The peaks corresponding to C, N, O, and Co can be clearly seen. Figure 4(b) shows that the C 1s peak of Co, B-MF-900 located at about 285 eV has an obvious asymmetric tail, which indicates disorder in the graphitic carbon structure.[41] The C 1s peak is deconvoluted into five peaks at 284.7, 285.7, 286.5, 287.2, and 289.1 eV, corresponding to sp2-hybridized graphite-like carbon (C–C sp2), sp3-hybridized diamond-like carbon (C–C sp3), C–O/C–N, C=O/C=N, and O=C–O, respectively.[33, 41] In the Co 2p spectra of Co, B-MF-900 (Figure 4(c)), the peaks at 778.2, 793.5, 780.8, and 797.0 eV are assigned to Co 2p3/2, Co 2p1/2, Co2+ 2p3/2, and Co2+ 2p1/2, and the peaks at 786.0 and 803.4 eV are fitted to shake-up satellite peaks.[25, 42] The main peak corresponds to Co2+ in the form of Co-NX, which provides an active site for the ORR.[43] According to the B 1s spectra of Co, B-MF-900 in Figure 4(d), the peaks at 189.6, 191.1, and 192.5 eV are attributed to B–C, B–N, and B–O bonds, 11

respectively.[44] The presence of B–C clearly indicates that B is successfully doped into the carbon matrix. As shown in Table S1, the total N content first increases and then decreases as the temperature increases from 700 °C to 1,000 °C and it is highest (about 11.1%) in Co, B-MF-900. The rapid decline in the N content above 900 °C may be a consequence of destruction of C–N bonds, since these bonds can decompose rapidly above 900 °C.[41] In the N 1s spectra of all the catalysts shown in Figure 4(e-h) and Figure S5(a), four types of N can be discerned: pyridinic N (398.5 eV), pyrrolic N (399.8 eV), graphitic N (401.3 eV), and oxidized N (403.6 eV).[45, 46] As the pyrolysis temperature increases, the change in the relative content of pyridinic N shows the same trend as that for the total N content, which is consistent with the results of other studies indicating that the formation of pyridinic N is preferred at higher total N contents.[41, 47] Co, B-MF-900 has the highest relative content of pyridinic N. Many studies reported that pyridinic N was an active site for ORR. The high content of pyridinic N may contribute to improving the catalytic activity of Co, B-MF-900. The relative contents of pyrrolic N and oxidized N gradually decrease as the temperature increases from 700 °C to 1,000 °C, indicating that pyrrolic N and oxidized N are unstable at high temperatures. The detailed calculated contents of all the catalysts are listed in Table S2. The total N contents of Co-MF-900 and B-MF-900 are both lower than that of Co, B-MF-900 (Table S1), indicating that the presence of B and Co is beneficial for doping of N in the carbon structure. The high contents of total N and pyridinic N can contribute to the improved electrical conductivity and catalytic activity of Co, B-MF-900. The pyrolysis temperature and 12

doping of Co and B affect the doping of N in the carbon matrix and also the type of N. The Co content of Co, B-MF-900 were calculated to be about 1.4% based on the TGA curves (Figure S6), which was a little higher than the surface Co content measured by XPS (Table S1). This may be due to that some Co atoms were wrapped in the carbon substrate. The specific surface areas of the catalysts prepared at different temperatures were measured using the BET test. The specific surface areas of Co, B-MF-700, Co, B-MF-800, Co, B-MF-900 and Co, B-MF-1000 were 527, 667, 671, and 510 m2/g, respectively. To further characterize the pore structure, we obtained the nitrogen adsorption and desorption isotherms and pore size distribution of the most representative catalyst, Co, B-MF-900. Figure 5(a) shows a type IV isotherm with a H3 hysteresis loop. The hysteresis loop begins to form at a relative pressure (P/P0) of about 0.4, indicating the mesoporous characteristics of Co, B-MF-900.[48, 49] The pore size distribution graph of Co, B-MF-900 shown in Figure 5(b) clearly shows a narrow distribution with pore sizes mainly concentrated at about 3.9 nm, further indicating that the porosity of Co, B-MF-900 is mainly composed of mesopores. The mesopores in Co, B-MF-900 play an important role in exposing active sites and promoting mass transfer due to their suitable size.[50] The high surface area of Co, B-MF-900 also allows the attachment of many active sites. 3.2. Electrochemical tests Linear sweep voltammetry tests were operated at a scan rate of 10 mV/s in each of O2-saturated 0.1 M KOH and HClO4 solutions at 1,600 rpm to evaluate the effect of 13

pyrolysis temperature on the catalytic activity, and the results are shown in Figure 6(a) and Figure S7. All the catalysts studied have a higher onset potential and half-wave potential than the commercial Johnson Matthey (JM) Pt/C in an alkaline solution, and a good onset potential in an acidic solution. Especially, Co, B-MF-900 has the highest activity and the maximum limiting current density under both alkaline and acidic conditions, so we speculate that the optimum pyrolysis temperature of Co, B-MF was about 900 °C. The change in the limiting current density of Co, B-MF-T is the same as the change in N and B content, but different from the change in Co content (measured by XPS). We infer that N and B play a major role in improving activity. B (2.04) and N (3.04) with different electronegative from C (2.55) doped into carbon structure would destroy the electroneutrality of carbon, thereby creating more active sites for oxygen adsorption and improving the conductivity of the catalyst.[17] Moreover, the interaction between B and N atoms can accelerate the electronic transfer and tune oxygen adsorption energy, thus improving the catalytic efficiency.[51] When the temperature reaches 1000 °C, the amount of Co2P increases while the amount of Co-NX decreases, and the catalytic activity decreases severely. Therefore, we infer that the Co2P has almost no effect on improving catalytic activity. Co, B-MF-900 shows excellent activity in an alkaline solution, with an onset potential of 1.1 V, which is 0.1 V higher than that of the JM Pt/C, a half-wave potential of 1.02 V, and a limiting current density (Ji) of 5.75 mA cm-2 at 0.2 V, which is slightly higher than that of the JM Pt/C (5.25 mA cm-2 at 0.2 V). Combining the electrochemical and XPS results, we can conclude that among four type of N, 14

pyridinic N significantly affects the catalytic activity. Figure 6(b) shows that the activity of Co, B-MF-900 is higher than that of B-MF-900, demonstrating that the doping of Co positively affects the catalytic activity due to the introduction of Co-NX. The improvement of catalytic activity by doping B further proves the promoting effect of B. Co and B co-doped catalyst has higher activity than Co or B doped catalyst, indicating that the synergistic effect between B, Co and MF is important in boosting this activity. To further verify the oxygen reduction performance of the catalyst, cyclic voltammetry (CV) tests on Co, B-MF-900 were performed in each of a N2- and O2-saturated 0.1 M KOH solutions at a scan rate of 10 mV/s. The results are shown in Figure 7(a). No cathode reduction peak appears with the N2-saturated solution, indicating that this catalyst is inactive in N2. However, a noticeable reduction peak appears at about 0.95 V in the O2-saturated solution, confirming aerobic reduction and excellent ORR catalytic activity.[37] In order to further prove the superior performance of the catalyst, Tafel slopes of catalysts shown in Figure S8 were obtained by analyzing the corresponding LSV plots at low overpotentials. From the Figure S8, we can see that Co, B-MF-900 has a smaller Tafel slope than JM Pt/C, indicating that Co, B-MF-900 has a faster electron transfer rate. And, because B and Co atoms can synergistically adjust the oxygen affinity, Co, B-MF-900 behaves smaller Tafel slope compared to Co-MF-900 and B-MF-900. In addition to its activity, a catalyst’s durability and methanol tolerance are also important properties. Therefore, we performed accelerated durability test and 15

methanol tolerance tests on the catalyst with the best catalytic activity. CV (5,000 cycles) was performed on Co, B-MF-900 in each of O2-saturated 0.1 M KOH and HClO4 solutions at a scan rate of 500 mV/s. By comparing the ORR polarization curves of Co, B-MF-900 before and after 5000 cycles of CV, as shown in Figure 7(b) and Figure S9, we can see that the onset potential is almost unchanged, the half potential decreases by only about 0.05 V in the alkaline solution, and the ORR polarization curves are almost unchanged by the acidic solution. That is, the catalyst has a high durability that allows it to last for a long time in both alkaline and acidic solutions with minimal loss of activity. The methanol tolerances of Co, B-MF-900 and the JM Pt/C were determined from chronoamperometric measurements performed in O2-saturated 0.1 M KOH solution. The i–t curves are shown in the inset of Figure 7(b). The i–t curve for Co, B-MF-900 shows no obvious change after adding methanol (3 M) at 600 s, showing an outstanding tolerance to methanol. Conversely, the i–t curve for the JM Pt/C shows a significant jump after adding methanol (3 M), indicating that the JM Pt/C is susceptible to methanol poisoning. Therefore, Co, B-MF-900 has better methanol tolerance than the JM Pt/C. To determine the catalytic pathway and kinetics, we obtained the polarization curves of Co, B-MF-900 in O2-saturated 0.1 M KOH solution at rotation speeds ranging from 400 to 1,600 rpm (Figure 8(a)). Figure 8(b) shows the K–L plots at potentials ranging from 0.4 to 0.8 V. The fitted lines at different potentials are almost parallel with high fitting degrees, indicating that the electron transfer numbers at different potentials are almost identical. The electron transfer number calculated using 16

K–L equation is close to 4, indicating that the catalytic oxygen reduction process mainly through a four-electron pathway. To further calculate the electron transfer number and H2O2 yield, a RRDE measurement was performed in O2-saturated 0.1 M KOH solution at a rotation speed of 1,600 rpm (Figure 9(a)). The electron transfer number (n) and H2O2 yield were calculated using the following equations.[52] !" ⁄# !% + !" ⁄# !% n=4× !% + !" ⁄#

%H O = 200 ×

where IR is the ring current, ID is the disk current, and N is the collection efficiency of the RRDE.As shown in Figure 9(b), the electron transfer number of Co, B-MF-900 between 0.2 and 0.8 V was about 3.88, and the H2O2 yields were below 9.7%. These results further demonstrate that the catalyst promotes the four-electron reduction of O2 to H2O.

4. Conclusions In summary, we have developed a simple method for preparing a mesoporous high-efficiency ORR catalyst from the MF gel doped with Co and B. We focused on the effects of the pyrolysis temperature and doping of Co and B on the catalytic activity. The optimal catalyst Co, B-MF-900 showed the highest catalytic activity, with an onset potential of about 1.1 V in 0.1 M KOH solution. This catalyst also showed good durability, excellent methanol tolerance and a catalytic process mainly based on a four-electron pathway. The mesoporous structure of Co, B-MF-900 17

facilitates mass transfer and increases the area in contact with the electrolyte during the catalytic process. The synergistic effect between Co, B, and MF also plays an important role in increasing the oxygen reduction activity. Furthermore, this work provides a novel method for doping heteroatoms into an organic polymer to prepare ORR catalysts.

Acknowledgements We gratefully appreciate the financial supports from the National Key Research and Development Program of China (No. 2016YFB0101203), the Joint Funds of the National Natural Science Foundation of China (No. U1705253), the National Natural Science Foundation of China (No. 21776014 and 21776012), Beijing Municipal Science and Technology Program (No. Z171100000917019), the International S&T Cooperation Program of China (No. 2013DFA51860), and the Fundamental Research Funds for the Central Universities (No. 12060093063).

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27

Figure 1. The synthesis route of the catalyst Co, B-MF-T.

Figure 2. (a–b) TEM and (c) HRTEM images of Co, B-MF-900

Figure 3. (a) XRD patterns and (b) Raman spectra of Co, B-MF-T prepared at different temperatures.

Figure 4. (a) Survey scan XPS spectras of Co, B-MF-T prepared at different temperatures, (b) C 1s XPS spectra, (c) Co 2p XPS spectra, and (d) B 1s XPS spectra of Co, B-MF-900, and N 1s XPS spectra of (e) Co, B-MF-700, (f) Co, B-MF-800, (g) Co, B-MF-900, and (h) Co, B-MF-1000.

Figure 5. (a) N2 adsorption-desorption plots and (b) pore size distribution of Co, B-MF-900.

Figure 6. (a) ORR polarization curves of Co, B-MF-T prepared at different temperatures in O2-saturated 0.1 M KOH and (b) ORR polarization curves of Co, B-MF-900, Co-MF-900, and B-MF -900 in O2-saturated 0.1 M KOH. (rotation rate: 1,600rpm; scan rate: 10 mV/s)

Figure 7. (a) CV curves of Co, B-MF-900 in N2- and O2-saturated 0.1 M KOH solutions at a scan rate of 10 mV/s, (b) ORR polarization curves of Co, B-MF-900 before and after 5000 cycles of CV in O2-saturated 0.1 M KOH solution (rotation rate: 1,600rpm; scan rate: 10 mV/s), and inset of (b) current–time (i–t) chronoamperometric responses of Co, B-MF-900 and the JM Pt/C over 1,200 s in O2-saturated 0.1 M KOH solutions (rotation rate: 1,600rpm; scan rate: 10 mV/s).

Figure 8. (a) ORR polarization curves of Co, B-MF-900 in O2-saturated 0.1 M KOH solution at different rotation rates at a scan rate of 10 mV/s and (b) K–L plots of Co, B-MF-900 at various potentials.

Figure 9. (a) RRDE voltammograms of Co, B-MF-900 in O2-saturated 0.1 M KOH solution at a rotation rate of 1,600 rpm with a scan rate of 10 mV/s and (b) Electron transfer numbers and H2O2 yields of Co, B-MF-900 in O2-saturated 0.1 M KOH solution at various potentials based on the RRDE rate.

Highlights:


High activity catalysts were prepared by a facile self-assembly pyrolysis method.




The doping of Co and B into melamine-formaldehyde gel made much active sites.




Co, B-MF/C-900 showed high activity, great durability, and good methanol tolerance.




The ORR catalytic process of Co, B-MF/C-900 was based on a four-electron pathway.

Hehuan Cao: Investigation, Writing-Original Draft. Jidong Cao: Validation, Data Curation. Fanghui Wang: Writing-Review and Editing and supervision. Hong Zhu: Conceptualization, Writing-Review and Editing, Project Administration and Funding Acquisition.

Min Pu: Conceptualization, Writing-Review and Editing.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: