稀有金属(英文版) 2020,39(12),1383-1394

In situ synthesis of Co₃O₄ nanoparticles confined in 3D nitrogendoped porous carbon as an efficient bifunctional oxygen electrocatalyst

Zhi-Yuan Wang Shun-Da Jiang Chan-Qin Duan Dan Wang Shao-Hua Luo Yan-Guo Liu

School of Materials Science and Engineering,Northeastern University

Key Laboratory of Dielectric and Electrolyte Functional Material-Hebei Province

作者简介：*Zhi-Yuan Wang,e-mail:zhiyuanwang@neuq.edu.cn;*Yan-Guo Liu,e-mail:lyg@neuq.edu.cn;

收稿日期：14 June 2020

基金：financially supported by the National Natural Science Foundation of China (Nos.51871046, 51902046,51874079,51571054,51771046 and 51674068);the Natural Science Foundation of Liaoning Province (No.201602257);the Natural Science Foundation of Hebei Province (Nos. E2019501097,E2018501091 and E2020501004);the Fundamental Research Funds for the Central Universities (Nos. N182304017,N182304015,N172302001 and N172304044);

In situ synthesis of Co₃O₄ nanoparticles confined in 3D nitrogendoped porous carbon as an efficient bifunctional oxygen electrocatalyst

Zhi-Yuan Wang Shun-Da Jiang Chan-Qin Duan Dan Wang Shao-Hua Luo Yan-Guo Liu

School of Materials Science and Engineering,Northeastern University

Key Laboratory of Dielectric and Electrolyte Functional Material-Hebei Province

Abstract：

The rational exploitation of non-precious metal catalyst with high activity,strong durability and low cost for the oxygen reduction reaction(ORR) and oxygen evolution reaction(OER) is of vital importance for metalair batteries.Herein,a composite of Co₃O₄ nanoparticles confined in three-dimensional(3 D) N-doped porous carbon(Co-NpCs) was prepared by a simple freeze-drying and in situ pyrolysis method.The effect of different dosages of Co(NO₃)₂ on the catalytic performance was discussed.The Co-NpC-12% exhibits the best catalytic performance(E_1/2=0.78 V,better stability than 20% Pt/C) in ORR and in OER among all the as-synthesized samples.Furthermore,it also exhibits the best bifunctional activity(ΔE=0.849 V).The excellent properties of Co-NpCs are mainly due to the synergy between Co₃O₄ and carbon.Firstly,a high Co₃O₄ loading amount can boost the defect level of the N-doped hierarchical porous carbon and expose more active sites.Secondly,the unique in situ pyrolysis guarantees a largearea contact between Co₃O₄ and carbon as well as a strong C-O-Co bonding,which promotes charge transfer,avoids the peeling of Co₃O₄ nanoparticles and effectively improves the stability of the material.This work is expected to offer a feasible strategy to produce metal oxide/carbon nanocomposite and push forward the development of bifunctional electrocatalyst with high activity and stability.

Keyword：

Bifunctional electrocatalyst; Co₃O₄; Nitrogendoped porous carbon; In situ synthesis;

Received： 14 June 2020

1 Introduction

Nowadays,the heavy use and dependence on fossil fuels have led to serious environmental pollution and energy shortage;therefore,the exploitation of clean,efficient and sustainable green energy is of great significance [ 1, 2, 3] .Metal-air batteries are widely considered to be promising renewable energy conversion technology because of their high theoretical energy densities [ 4, 5] .Unfortunately,as the critical step of the above technology,oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are commonly sluggish and have large overpotential,which limits the large-scale commercial application of metal-air batteries [ 6, 7, 8] .Therefore,catalysts,which can change the reaction pathway and enhance the kinetic of chemical reaction,are usually adopted.Up to now,precious metal(Pt/C) and precious metal oxide (RuO₂) are the most efficient electrocatalysts for ORR and OER,respectively [ 9, 10, 11] .Nevertheless,they always suffer from the shortcomings of reserve scarcity,high cost and poor stability and durability,limiting their commercialization [ 12, 13, 14] .Moreover,they cannot act as a desirable bifunctional ORR/OER electrocatalyst.Hence,it is very necessary to develop low-cost,earth-abundant and highly active bifunctional electrocatalysts for metal-air batteries.

To date,non-precious metal materials,such as carbonbased materials and transition-metal-based materials (oxides [ 15, 16, 17, 18] ,sulfides [ 19, 20, 21] and phosphides [ 22] ),have been widely investigated as bifunctional oxygen electrocatalysts.Among these materials,heteroatom-doped carbon materials display outstanding ORR performance,but their OER performances remain to be lower than that of RuO₂.In addition,Co₃O₄ is regarded as another promising bifunctional catalyst for OER/ORR due to its low cost,high activity and stability.Nevertheless,the activity and stability of Co₃O₄ are still limited by low intrinsic conductivity and tendency of agglomeration.It has been proved that the combination of carbon material and Co₃O₄nanoparticles is an effective strategy to improve the performance of the catalyst through their synergetic effects [ 23, 24, 25] .However,although the electronic conductivity of Co₃O₄ could be boosted effectively by adding carbon materials,it usually undergoes a poor durability for the peeling off of Co₃O₄ from the carbon during durative reaction process;the main reason is that in most of the Co₃O₄/C composite materials,the Co₃O₄ nanoparticles are simply loaded on the surface of carbon,leading to a relatively weak interface bonding [ 26, 27] .To deal with this,encapsulating Co₃O₄ into porous carbon materials by in situ synthesis is a reasonable choice [ 12, 28] .The traditional in situ synthesis methods (such as silica template method) are harsh and time-consuming,and it also needs some toxic chemicals to remove template.Hence,exploiting a new-type metal oxide/carbon hybrid with high catalytic property and long durability though simple and efficient,environmentally friendly and low-cost technology route is urgent desirable.

Herein,we prepared a composite of Co₃O₄ nanoparticles spatially confined in three-dimensional (3D) nitrogen-doped porous carbon (Co-NpCs) by in situ watersoluble template-assisted in situ pyrolysis method.Because of the strong C-O-Co covalent bonds,the metal oxide particles are firmly pinned on the N-doped porous carbon wall,by which it not only effectively prevents the agglomeration of Co₃O₄ nanoparticles,but also enhances the interaction between metal oxide and carbon,and then it makes the maximum utilization of active sites and contributes to good catalytic performance as well as stability in ORR/OER process.Furthermore,the 3D porous carbon structure of the composite provides large specific surface area and abundant pore channels,leading to sufficient exposure of active sites and efficient electron and mass transfer.In addition,the atomic-level dispersed N produces plentiful defects and oxygen vacancies,which provides more active sites and increases electrical conductivity of the oxide.We also explored the effect of dosage of Co(NO₃)2 on catalytic properties to find an optimal content.As a result,Co-NpCs-12%hybrid catalyst displays remarkable bifunctional catalytic activity and excellent durability toward ORR and OER in an alkaline medium,which is promising to be next-generation high-efficiency bifunctional oxygen electrocatalysts.The synthesis method with good controllability can be popularized to synthesize other metal oxide/carbon hybrid material for energy storage and conversion device.

2 Experimental

2.1 Preparation of Co3O4/3D nitrogen-doped porous carbon material

In a typical preparation,1.25 g glucose,1.25 g urea and20.642 g NaCl were added to 70 ml deionized water to form a uniform transparent solution under stirring.Subsequently,different amounts of Co(NO₃)2·6H₂O were added to the previous solution and the solution was magnetically stirred for 12 h,and then,the mixed solution was frozen for at least 48 h.After the solution was frozen,it was quickly placed in a freeze-dryer under vacuum for 24 h to remove water.Finally,the powder was collected and heated to 650℃at a rate of20℃·min^-1 in a tube furnace under Ar/H₂ (200/100 ml·min^-1) atmosphere for 2 h,then oxidized in air atmosphere at 350℃for another 2 h and then naturally cooled to room temperature.The calcined sample was filtered with deionized water three to four times to remove NaCl and then dried in an oven at 80℃for 12 h to obtain a desired sample.Therein,three samples were prepared with different dosages of Co(NO₃)2·6H₂O and were named as Co-NpC-2%,Co-NpC-4%and Co-NpC-12%,respectively (2%,4%and 12%refer to the mass ratio of Co to carbon element in the precursor).

2.2 Material characterization

The morphology and structure of the samples were characterized by a field emission scanning electron microscope(FESEM,Zeiss Supra 55,15 kV) and a high-resolution transmission electron microscope (HRTEM,JEM-21 00F,200 kV).The phase composition and crystallinity of the samples were examined by an X-ray diffractometer (XRD,DX-2500,Cu Kαradiation,40 kV,30 mA,4 (°)·min^-1)and a Raman spectrometer (Renishaw,inVia microscope,a532-nm laser).The surface composition,valence and bonding of the product were investigated by X-ray photoelectron spectroscopy (XPS,Thermo ESCALAB 250 XI).The specific surface area and pore size distribution of the material were characterized by N₂ adsorption and desorption experiments using a specific surface area analyzer(SSA-4300).

2.3 Electrochemical measurements

The working electrode was prepared as follows:15μl Nafion solution (1:1 in volume ratio;20 wt%Nafion/anhydrous ethanol) and 3 mg catalyst were dispersed into0.5 ml N,N-dimethylformamide (DMF) solvent,followed by ultrasound for at least 60 min to obtain a uniformly catalyst ink.Then,10μl ink was dropped on a glassy carbon electrode with a diameter of 5 mm (S=0.196 cm²)and dried at ambient temperature.The catalyst loading on the RDE was about 0.30 mg·cm^-2,the loading of the comparative Pt/C (20 wt%of Pt),and RuO₂ is consistent with the prepared catalyst.All electrochemical tests were performed on an electrochemical workstation CHI 760E by using a conventional three-electrode system.Hg/HgO electrode and platinum wire were used as a reference electrode and a counter electrode,respectively.

In the ORR experiments,O₂-or N₂-saturated0.1 mol·L^-1 KOH was used as electrolyte in all the electrochemical tests.Cyclic voltammograms (CV) was performed in the potential range from-0.135 to 1.065 V (vs.RHE) at a scan rate of 10 mV·s^-1,and linear sweep voltammetry (LSV) was tested in the potential range from0.2 to 1.0 V (vs.RHE) at a scan rate of 10 mV·s^-1.In addition,LSV was also tested at various rotating speeds from 400 to 2500 r·min^-1.Current-time (i-t) measurements were obtained at a constant voltage (0.57 V vs.RHE) with 1600 r·min^-1 to test the durability.The number of transferred electrons in the ORR process is calculated by Koutecky-Levich equation:

where j represents the measured current density,j1 is the diffusion limited current density,.jk represents the kinetic current density,B is the scale factor,ωis the angular velocity,n is the electron transfer number,F is the Faradic constant (96,485 C·mol^-1),A is the geometric area of glassy carbon electrode,C₀ is the bulk concentration of O₂(1.2×10^-6 mol·cm^-3),D₀ is the diffusion coefficient of O₂ in 0.1 mol·L^-1 KOH (1.9×10^-5 cm²·s^-1) and v is the kinematic viscosity of the electrolyte (0.01 cm²·s^-1).

In the OER experiments,O₂-saturated 1 mol·L^-1 KOH was used as electrolyte in all electrochemical tests.LSV experiments were performed in the potential range from 1to 1.8 V (vs.RHE) at a scan rate of 10 mV·s^-1.Electrochemical impedance spectroscopy (EIS) was measured in1 mol·L^-1 KOH at 1.62 V (vs.RHE) with the frequency ranging from 0.01 Hz to 100 kHz.The catalyst ink was dripped onto carbon paper (S=0.18 cm²,loading of0.65 mg·cm^-2),and the long-term durability of the material was evaluated by a chronopotentiometric method at a constant current density of 10 mA·cm^-2.Electrochemically active surface area (ECSA) of the material was determined by CV performed in non-Faradaic potential range.In this paper,the measured Hg/HgO potential(E) was converted to the potential of a reversible hydrogen electrode (RHE) using the Nernst equation.The conversion is as follows:E(V vs.RHE)=E(V vs.Hg/HgO)+0.0591pH+0.098.

3 Results and discussion

3.1 Formation mechanism of materials

Co-NpCs (Co₃O₄ supported on nitrogen-doped porous carbon) were prepared by in situ pyrolysis process assisted by water-soluble template.The synthesis process is shown in Fig.1.In this process,the chelation between Co²⁺and C₆H₁₂O₆ in the homogeneously mixed precursor solution can improve the dispersion of Co²⁺on the carbon-based precursor,which is beneficial to the graphitization catalytic process of amorphous carbon.The precursor solution was freeze-dried to obtain a dry powder.The C₆H₁₂O₆-Co²⁺chelate in the powder evenly covered the outer surface of the crystalline sodium chloride cubic crystal to form an ultra-thin composite film.The precursor powder undergoes two key reactions during the high-temperature calcination process.Firstly,Co(NO₃)2·6H₂O in the C₆H₁₂O_6-Co²⁺ultra-thin composite film was reduced into Co nanoparticles under Ar/H₂ atmosphere at 650℃.In this process,the carbon layer produced by the carbonization of glucose,under the catalysis of the metallic Co nanoparticles,wraps the Co nanoparticles and limits their growth [ 29] .Secondly,when the temperature drops to 350℃,the metallic Co nanoparticles anchored on the carbon matrix were oxidized to Co₃O₄ nanoparticles in an air atmosphere.In addition,the uniformly distributed urea in the precursor powder decomposes during the high-temperature calcination,so that the nitrogen element is evenly distributed on the surface of the porous carbon.The sodium chloride cubic crystal undergoes a certain degree of deformation after high-temperature calcination,causing it to deviate from the cubic shape,and finally formed a 3D carbon network decorated with Co₃O₄ with a deformed sodium chloride crystal as the skeleton.After repeated washing with deionized water several times,the sodium chloride template was easily removed,and the N-doped 3D porous carbon material anchored with Co₃O₄ was successfully prepared.In addition,to determine the actual content of Co,an inductively coupled plasma (ICP) test was performed.Through ICP test,the contents of Co in different samples are 3.69%,7.86%and 17.66%,respectively.The results are higher than the design composition of 2%,4%and 12%,mainly due to the loss of carbon material during pyrolysis.

Fig.1 Schematic procedure for synthesizing Co-NpCs

3.2 Morphology and structure of materials

In order to investigate the morphology of the prepared materials,scanning electron microscopy (SEM) was performed,as shown in Fig.2.It can be clearly seen in Fig.2a,d,g that the materials with different dosages of Co(NO₃)2·6H₂O all present 3D porous structures composed of many uniformly distributed interconnected pores,and there is no big difference in morphology for the samples with different dosages of metal salt.Then,in the high-magnification SEM images (Fig.2b,e,h),uniform and ultra-thin carbon wall can be observed,which formed from the film of C₆H₁₂O₆-Co²⁺on the sodium chloride cubic crystals during the reduction process.The pore size(about 500 nm) and shape of porous carbon of the three samples are similar,which may be related to the same cubic crystals template and the same preparation temperature.However,with the increase in the amount of metal salt,it can be seen that the loading of Co₃O₄nanoparticles on porous carbon is increased obviously.To be more clear,higher-magnification SEM images are displayed in Fig.2c,f,i,when the amount of metal salt is2%,the Co₃O₄ nanoparticles on the porous carbon with a particle size of about 50-100 nm are hardly observed,while when the amount increases to 4%,the nanoparticles distributed on the carbon network skeleton turn to increase in amount,and as the amount is further increased to 12%,it can be clearly seen that a large number of nanoparticles are anchored uniformly on the entire carbon network,including the carbon network skeleton and ultrathin carbon walls.Based on the above results,it can be concluded that the amount of metal salt will not affect the3D porous structure of the material,but it affects the distribution (position and number) of Co₃O₄ nanoparticles on the carbon support.The 3D porous carbon framework with larger pore size not only facilitates sufficient contact between the material and the electrolyte,but also promotes the mass transfer process [ 30, 31] .Nano-sized Co₃O₄ produces a large specific surface area and is also conducive to full exposure of the active site.

In order to further reveal the microstructure of the material and the distribution of elements,transmission electron microscopy (TEM) was carried out on Co-NpCs-12%.As shown in Fig.3a,it can be clearly observed that the circular holes with a diameter of about 500 nm and Co₃O₄ nanoparticles with a particle size of 50-100 nm distributed on the carbon support,which are consistent with the results shown in SEM images.As shown in Fig.3b,it can be clearly seen that most of the Co₃O₄ nanoparticles are confined in porous carbon,which is different from the conventional point-to-plane contact.This large-area contact not only avoids the peeling of nanoparticles,but also can inhibit migration and agglomeration of nanoparticles.Figure 3c shows a high-resolution image of Co₃O₄nanoparticles.It can be seen the lattice spacings of 0.283and 0.462 nm,which correspond to the (220) and (111)planes of Co₃O₄.It can be seen from Fig.3d-h that there are four elements C,O,N and Co in the material,where N is evenly distributed on the surface of the carbon matrix,indicating that N is successfully incorporated into the material.In addition,Co is mainly distributed in the location where Co₃O₄ nanoparticles exist,and O is not only distributed in the position of Co₃O₄,but also uniformly distributed on the surface of the carbon matrix.The uniform distribution of N and O on the surface of the carbon matrix can adjust the surface electronic structure [ 32] ,which is beneficial to improve the catalytic performance of the oxygen electrocatalyst.

Fig.2 SEM images of Co-NpCs with different dosages of Co(NO₃)₂·6H₂O:a-c Co-NpC-2%,d-f Co-NpC-4%and g-i Co-NpC-12%

Fig.3 a,b TEM,c HRTEM and d STEM images of Co-NpC-12%;corresponding elemental mapping images of e C,f Co,g N and h O for Co-NpC-12%

The crystal structure of the prepared materials was investigated by XRD.Figure 4a shows that there is a broad peak corresponding to porous carbon at 20≈14.5°,and the sharp peaks at 19°,31.3°,36.9°,44.8°,59.4°and 65.2°correspond to (111),(220),(311),(400),(511) and (440)planes of spinel Co₃O₄ (JCPDS No.42-1467);no other impurities are found,which is consistent with TEM results.In addition,it can be clearly seen that as the dosage of metal salt increases,the diffraction peak of the prepared sample becomes stronger and stronger,which also confirms that the metal salt successfully decomposed and converted to Co₃O₄.Figure 4b shows Raman spectra of the three samples.The two strong peaks at 1350 and 1590 cm^-1 are distinctive characteristics of carbon-based materials,which represent D-band and G-band,respectively.The G-band coincides with the sp² hybrid carbon atoms in an ideal graphite lattice,and the D-band describes the edge,disordered carbon and other defects.The intensity ratio of D-band and G-band (I_D/I_G) can be applied to evaluate the disordered degree of carbon structure,and the larger the value is,the higher the degree of disorder is [ 33] .It is worth mentioning that theI_D/I_G values of Co-NpC-2%,CoNpC-4%and Co-NpC-12%are 0.83,0.90 and 0.92,respectively,suggesting that with the increase in dosage of metal salt,more Co₃O₄ nanoparticles are embedded in the carbon matrix,resulting in more fracture edges and defects,which could boost the number of active sites and improve the electron conductivity.

Fig.4 a XRD patterns and b Raman spectra of Co-NpC-2%,Co-NpC-4%,Co-NpC-12%

The specific surface area and pore size distribution of Co-NpCs were analyzed by N₂ adsorption and desorption experiments.As depicted in Fig.S1,the specific surface areas of Co-NpC-2%,Co-NpC-4%and Co-NpC-12%are183.1,173.4 and 178.3 m²·g^-1,respectively.As shown in Fig.S1a,c,e,all the three materials have the similar Type IV isotherms and H3 hysteresis loops [ 34] ,suggesting the mesopore nature of the material.Moreover,the rapid increase in volume at the relative pressure (p/p₀) between0.9 and 1.0 indicates the presence of macropores,which is consistent with the results observed by SEM and TEM.In addition,the pore size distribution curves of the three materials indicate that the pore size distribution of the mesopores is wide,with the main pore size around 4.0 nm.The above results confirm the hierarchical porous structure of the material.Unlike the macropores formed by the sodium chloride template,these mesopores may be due to the volatilization of urea during pyrolysis.Moreover,Fig.S1 shows that the loading of Co₃O₄ has a slight effect on the specific surface area of the prepared materials,while it has no significant effect on the pore size distribution of porous carbon.A large proportion of mesopores and macropores are conducive to the rapid transfer of electrolytes and reactants and effectively enhance the reaction kinetics of the oxygen electrode reaction [ 35] .

XPS was used to explore the surface composition and bonding configurations in the near-surface region of CoNpC-12%.It can be found that there are four elements in the sample,namely C,N,O and Co (Fig.5a),which is in accordance with the results of elemental mappings in Fig.3e-h,and the nitrogen-doping content is calculated as5.04 at%.The fitted C 1s XPS spectrum (Fig.5b)demonstrates the presence of three types of carbon species,namely C=C,C-N/C-O-C and O-C=O.The presence of C-O bonds strengthens the bond between Co₃O₄nanoparticles and 3D porous carbon,which effectively avoids the flaking of Co₃O₄ from carbon and inhibits the migration and agglomeration of nanoparticles.As shown in Fig.5c,pyridine-N (398.17 eV),graphite-N (400.45 eV),pyrrole-N (399.33 eV) and oxidized-N (403.31 eV) are present in Co-NpC-12%,of which pyridine-N and graphite-N species account for the vast majority,and it is generally believed that pyridine-N and graphite-N play a significant role in the reversible reaction of oxygen [ 36, 37] .This also further confirms that the nitrogen atom is successfully doped into the carbon matrix.Figure 5d suggests that the peaks centered at 780.2 and 795.4 eV in the Co 2p high-resolution spectrum should be derived from the Co 2p3/2 and Co 2p_1/2 species,indicating the two valence states of Co²⁺/Co³⁺in the oxide.

3.3 Electrocatalytic performances

To evaluate the catalytic activity of as-prepared catalysts for ORR,the cyclic voltammetry (CV) tests were first performed in O_2-and N₂-saturated 0.1 mol·L^-1 KOH electrolyte,respectively.As shown in Fig.S2a,b,there is no obvious cathodic reduction peak under N₂,but welldefined reduction peaks are observed under O₂,which are attributed to the oxygen reduction reaction.It is worth noting that cathodic peak shifts the positive direction as the Co₃O₄ loading increases,which indicates that the increase of Co₃O₄ as the main active site significantly improves the catalytic performance of the material.To further confirm the main active site of Co-NpC-12%for ORR,NpC was used as the blank for comparison (Fig.S2).As displayed in Fig.S2,the peak current of the cathode peak increases significantly after the carbon matrix was loaded with Co₃O₄ nanoparticles,and the peak potential has an obvious positive shift,which demonstrates that Co₃O₄ is the main active site of the material.In order to clarify the activity of the Co-NpCs more accurately,the LSV of the catalyst was implemented using a rotating disk electrode (RDE) in0.1 mol·L^-1 KOH at rotation rate of 1600 r·min^-1.As displayed in Fig.6a,the onset potentials of Co-NpC-2%,Co-NpC-4%and Co-NpC-12%are all about 0.88 V.In addition,the higher the Co₃O₄ loading is,the higher the limit current density is.On the one hand,the Co₃O₄ itself has a certain catalytic performance;on the other hand,the porous carbon with Co₃O₄ loading displays more defects,and it is reported that more defects will cause the porous carbon to expose more active sites [ 38] ,thereby improving the catalytic performance.The synergy effect of the two aspects makes the limiting current density of the material increase with the increase in dosage of metal salt.Among the three as-prepared samples,although Co-NpC-12%shows the highest limiting current density,the half-wave potentials of the three samples are similar,which manifests that the increase of Co₃O₄ does not continue to effectively enhance the performance of catalyst.To investigate the reaction kinetics of Co-NpC-12%in ORR process,LSV measurements were executed at different rotating speeds(Fig.6b).It can be seen that as the speed increases,the current density also increases,which is a manifestation of diffusion-controlled process.Furthermore,according to Koutecky-Levich (K-L) equation,the corresponding electron transfer number (n) was calculated as 3.94 and approximately equal to 4,which shows that the ORR process is a four-electron pathway without H₂O₂ generation.As shown in Fig.S4,when potential is fixed at 0.5 V,the corresponding n values of Co-NpC-2%and Co-NpC-4%are 3.33 and 3.70.In addition,the K-L plots of CoNpC-12%(Fig.6c) under different voltages maintain a good linear relationship,and the different curves are near parallel,which means that this catalytic reaction of dissolved oxygen is a first-order reaction kinetic process [ 39] .In addition to catalytic activity,long-term durability is another key factor for ORR catalysts.The rotating speed of the rotating disk electrode was maintained at 1600 r·min^-1in 0.1 mol·L^-1 KOH electrolyte,and the current versus time curve was recorded at an E=0.57 V (vs.RHE).As shown in Fig.6d,the current retention rate of Co-NpC-12%after 15,000 s is 92.0%,which is much better than that of Pt/C (68%).There is only a slight loss of current density in as-prepared Co-NpC-12%catalyst after long-term testing,indicating a perfect stability of the composite.The superior stability is mainly due to the strong binding force(C-O-Co) between Co₃O₄ nanoparticles and porous carbon,which can effectively avoid the flaking,migration and aggregation of the nanoparticles during the electrochemical testing,so that the material can continue to fully expose the active sites.

Fig.5 a Wide XPS survey spectrum of Co-NpC-12%;b high-resolution C 1s,c N 1s and d Co 2p spectra of Co-NpC-12%

Fig.6 a LSV curves of Co-NpC-2%,Co-NpC-4%,Co-NpC-12%and Pt/C for ORR catalytic activity in O₂-saturated 0.1 mol·L^-1 KOH solution with a RDE (1600 r·min^-1);b LSV curves of Co-NpC-12%tested at different rotation rates;c fitting lines according to K-L equations at various potentials from 0.2 to 0.6 V (vs.RHE);d i-t curve of Co-NpC-12%in O₂-saturated 0.1 mol·L^-1 KOH solution with a RDE (1600 r·min^-1)

Moreover,the OER activities of the Co-NpC-2%,CoNpC-4%,Co-NpC-12%and RuO₂ samples were also investigated by LSV.As shown in Fig.7a,although the onset potentials of the three self-made catalysts are larger than that of RuO₂,their onset potentials decrease with the increase in the metal salt dosage.In addition,compared with Co-NpC-2%and Co-NpC-4%,the Co-NpC-12%displays a more obvious oxidation peak at a low potential,indicating that the amount of Co₃O₄ nanoparticles has significant effect on the oxidation current.In addition to the onset potential,it is well known that the overpotential at10 mA·cm^-2 is an important parameter for characterizing catalyst activity [ 40] .Figure 7a shows that as the amount of metal salt increases,the overpotentials of the as-prepared catalysts at 10 mA·cm^-2 become smaller and smaller.When the amount of metal salt reaches 12%,the material exhibits the smallest overpotential (400 mV),which is close to that of RuO₂ (340 mV).Although the over-potential of Co-NpC-12%at this current density still has a certain gap with RuO₂,it is interesting that the current density of Co-NpC-12%shows a rapid increase after the catalytic reaction started.At the same current density,the overpotential difference between Co-NpC-12%and RuO₂ is getting smaller and smaller,showing the characteristics of rapid kinetics.Although the other two self-made catalysts also show similar trends,their changes are not obvious relative to Co-NpC-12%.This interesting phenomenon may be due to that as the applied voltage increases,the surface of Co₃O₄ nanoparticles undergoes oxidative reconstruction to form new active sites [ 41] ,thereby accelerating the reaction kinetics.In addition,there is a large area of contact between porous carbon with superior conductivity and Co₃O₄ nanoparticles with surface reconstruction,which promotes the charge transfer and mass transfer,ultimately resulting in a rapid increase in the current with the potential increasing.As we all know,the Tafel slope is an important indicator reflecting the reaction kinetics of the material,that is,a smaller Tafel slope indicates faster reaction kinetics [ 42] .As shown in Fig.7b,as the dosage of metal salt increases,the Tafel slope decreases significantly.The Co-NpC-12%displays the lowest Tafel slope among the prepared materials,which is consistent with the catalytic performance shown in the LS V curves,further illustrating the fast reaction kinetics of Co-NpC-12%.The interface charge transfer impedance can be used to characterize the electron transfer ability of the catalyst and then explain the reaction kinetics of the catalyst.As shown in Fig.7c,it can be clearly seen that as the dosage of metal salt increases,the interface charge transfer impedance of the catalyst decreases sharply,and more Co₃O₄ nanoparticles anchored on the porous carbon significantly improve the charge transfer ability of the material,which is consistent with the trend of the reaction kinetics previously analyzed.

Fig.7 a LSV plots and b Tafel plots of Co-NpC-2%,Co-NpC-4%,Co-NpC-12%and RuO₂;c electrochemical impedance spectroscopy (Z':real part of impedance,-Z":imaginary part of impedance) measured at 1.62 V (vs.RHE);d chronopotentiometric measurement atj=10 mA·cm^-2

In order to investigate the effect of the electrochemically active surface area of the material on the performance by assessing double-layer capacitance (Cd₁),a CV test was performed in the non-Faraday interval at different scan rates,and the results are shown in Fig.S3.It can be seen from Fig.S3d that the electrochemically active surface area of the material significantly increases with the metal salt increasing.And the increased active surface area provides more catalytic sites,thereby improving the catalytic activity of the material.To evaluate long-term durability of as-synthesized catalysts for OER,the chronopotentiometric measurements at.j=10 mA·cm^-2 were recorded at constant current density 10 mA·cm^-2 by testing on carbon paper in 1 mol·L^-1 KOH electrolyte.As shown in Fig.7d,the Co-NpC-12%(decay of 3.5%) shows a smaller potential decay than RuO₂ (decay of 4.2%) after 15,000 s,indicating an outstanding stability of the composite.The superior stability is mainly due to the large-area contact and the strong bonding (C-O-Co) between porous carbon and Co₃O₄,which effectively avoids the exfoliation,migration and agglomeration of nanoparticles during the electrocatalysis process.In order to study the changes in morphology,impedance and valence state of the as-prepared materials after the stability test,the SEM of Co-NpC-12%after ORR stability test and the EIS and XPS of CoNpC-12%after OER stability test were performed.As shown in Fig.S5,after ORR stability test,the 3D porous structure of the material is stable and not damaged,and the Co₃O₄ nanoparticles are evenly distributed on the porous carbon matrix.The stable combination of Co₃O₄nanoparticles and the carbon matrix makes the material show good stability.In addition,by comparing the impedance before and after OER stability test (Fig.S6),it can be found that the impedance of the sample has been significantly reduced after the stability test.This may be due to the reconstruction of the Co₃O₄ surface to generate more active oxyhydroxide,which accelerates the electron transfer process [ 43, 44] .By analyzing XPS spectrum (Fig.S7)of Co-NpC-12%after OER stability test,it is found that the amount of Co³⁺increases,which will improve the OER catalytic performance of the material.

Fig.8 a Polarization curve measured for Co-NpCs in whole region of OER (1 mol·L^-1 KOH) and ORR (0.1 mol·L^-1 KOH) and b potential difference between OERs

and ORR (E_1/2) for Co-NpCs

The potential difference (ΔE) between ORR (E_1/2) andOER ( ) is usually an important parameter for evaluating the performance of bifunctional oxygen electrocatalysts [ 45] .The smaller the potential difference (ΔE)a catalyst displays,the higher bifunctional activity the catalyst has.The polarization curve corresponding to the whole voltage window including ORR and OER is shown in Fig.8a.In all the prepared samples,the Co-NpC-12%(Fig.8b) exhibits the best bifunctional activity (ΔE=0.849 V),which is more excellent than that of those materials reported in Refs. [ 46, 47, 48, 49] .In addition,compared with the recently reported metal oxide catalysts (Table S1),the materials in this article also exhibit excellent dualfunctional catalytic performance.

4 Conclusion

In summary,we designed a composite of Co₃O₄ nanoparticles confined in three-dimensional N-doped porous carbon material by simple in situ template-as sis ted hightemperature pyrolysis method and employed it as ORR and OER bifunctional electrocatalysts.The optimal dosage ratio (m_Co/m_C=12%) can be obtained by changing the dosage of metal salt.The as-synthesized Co-NpC-12%exhibits the best catalytic performance in ORR and in OER,and it also exhibits the best bifunctional activity(△E=0.849 V).The excellent properties of the prepared materials are mainly due to the synergy between Co₃O₄and carbon.Firstly,the high Co₃O₄ loading boosts the defect level of the N-doped hierarchical porous carbon and the exposure of more active sites,and the porous carbon matrix promotes mass and electron transfer processes,leading to enhanced catalytic performance.Secondly,in situ pyrolysis causes Co₃O₄ and carbon to form a largearea contact with a strong C-O-Co bonding to promote charge transfer and avoid the peeling of Co₃O₄ nanoparticles and aggregation,and effectively improves the stability of catalyst.The high activity,excellent stability,as well as the facile and low-cost synthesis method make it a promising candidate to apply to fuel cells and metal-air batteries.

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