Iron Phthalocyanine Coated Nitrogen-doped Hollow Carbon Spheres for Efficient Catalysis of Oxygen Reduction Reaction

LI Ziruo，ZHANG Hongjuan，ZHU Guoxun，XIA Wei，TANG Jing，2*

（1.School of Chemistry and Molecular Engineering，Shanghai Key Laboratory of Green Chemistry and Chemical Processes，East China Normal University，Shanghai 200062，China；2.Institute of Eco-Chongming，Shanghai 202162，China）

Abstract It is imperative to develop low-cost and high-performance catalysts for oxygen reduction reaction（ORR）.Although iron phthalocyanine（FePc）has been proved as an efficient catalyst for ORR decades ago，it cannot replace commercial Pt/C due to its poor electronic conductivity and unsatisfied stability.Considering the chemical stability，high electronic conductivity，and intrinsic catalytic activity of nitrogen-doped carbons，herein，F(xiàn)ePc coated nitrogendoped hollow carbon（FePc-NHC）has been fabricated by carbonization of polystyrene@polydopamine spherical precursors，followed by the loading of FePc.A series of hollow spherical FePc-NHCS samples was prepared by adjusting the pyrolysis temperature and loading amount of FePc.Benefiting from the hierarchical porous structure，high specific surface area，proper nitrogen doping，and FePc loading，the optimized FePc-NHCS displayed an excellent catalytic activity for ORR in alkaline electrolyte，with a remarkable long-term stability and a high half-wave potential of 0.862 V（vs.RHE），outperforming Pt/C（0.848 V）.This work proved that hybridization is an effective pathway to strength the positive properties of each component.Also，structural and compositional modification is efficient to optimize the ORR performance of a catalyst.

Keywords Hard-template method；Non-noble metal catalyst；Hollow carbon sphere；Oxygen reduction reaction

1 Introduction

Thanks to the unique characteristics of porous carbons，including good electrical conductivity，excellent chemically stability，high specific surface area，and adjustable components［1］，porous carbon-based nanomaterials have been regarded as promising alternatives to high-cost Pt-based catalysts，for catalyzing the significant but sluggish oxygen reduction reaction（ORR）at the cathode in fuel cells or metal-air batteries［2，3］.Porous carbons without any modification might have poor catalytic performance for ORR［4］.To achieve high performance，it is necessary to adjust the active sites through morphology，structure，and composition of porous carbons.

Carbon materials have lots of dimensions and morphologies，including carbon quantum dot，carbon nanotube，graphene，spherical porous carbon，ordered porous carbon，and so on［5］.Among these forms，hollow carbon spheres（HCS）have regular geometry，low density，and controllable particle size from nanoscale to microscale，which are favorable to offer suitable pathway for mass diffusion and expose more active sites［6，7］.Moreover，the hollow spherical structure could significantly avoid the aggregation of active sites，promoting the usage efficiencies and stability.Therefore，hollow carbon spheres are often used as support for loading precious metal catalysts［8］，or unstable organic catalysts［9］.Louet al.［10］confined the Pt clusters in the mesoporous channels of hollow carbon spheres，which were used for boosting hydrogen evolution reaction.Zhenget al.［11］prepared a hollow spherical doped carbon catalyst impregnated/covered with iron phthalocyanines（FePcs），which combined the advantages of both carbon and FePc.FePc has been regarded as an efficient catalyst for ORR thanks to its precise Fe-N4active sites，which can accept and reserve multi electrons to optimize the binding energy of intermediates during the ORR reaction［12，13］，resulting in facilitating of ORR process.However，F(xiàn)ePc has a low electronic conductivity and is easy to aggregate and lose activity.Therefore，dispersing FePc onto stable and conductive porous carbon supports is a good strategy to improve the utilization of FePc.

Nowadays，various synthetic routes have been developed to prepare hollow carbon spheres，including hard-and soft-templating methods［14］.The general template synthetic procedures for HCS start with preparation of core-shell structured composites which consist of spherical template and coated carbon precursors，followed by carbonization，and removal of the template［15］.In order to apply HCS more efficiently for electrochemical catalysis，nitrogen-containing carbon precursors are usually adopted to prepare nitrogen-doped HCS.Because nitrogen-doped carbons have been demonstrated to be able to improve the performance for ORR［16］，by modifying charge and spin density on carbon atoms adjacent to electron-accepting nitrogen atoms［17，18］.Recently，dopamine（DA）was found to be an ideal nitrogen-doped carbon precursor because of its self-polymerization property under alkaline conditions at room temperature.And polydopamine（PDA）not only shows high yield after carbonization but also can be transferred to nitrogen-doped nanoporous carbon materials［19，20］.Tanget al.［21］prepared nitrogen-doped hollow carbon spheres（N-HCS）by dopamine as the precursor，SiO2and diblock polymer as the dual-template method.After that，Pt nanoparticles were loaded on the interior surface of the N-HCS to improve the usage efficiency and long-term stability of Pt nanoparticles［22］.However，extra etching was needed to the removal of SiO2template.

In this work，spherical polystyrene（PS），which can be simultaneously removed during carbonization without applying extra etching step，was chosen as the template.Nitrogen-doped hollow carbon spheres（NHCS）were prepared by using PS as the template and DA as the nitrogen and carbon precursors.Although DA-derived nitrogen-doped carbon has ORR performance，it is limited.In order to further improve the ORR performance，extra metal macrocycles of iron phthalocyanine（FePc）［23，24］，which has intrinsic high catalytic ability［25］but severe degradation during long-term operation［26］，was loaded onto the surface of the prepared NHCS in a highly dispersed state，and interacted strongly with NHCSvia π-πstacking［23，27］.Therefore，NHCS was post-coated with FePc to form FePc-NHCS，showing an improved catalytic activity compared with pure NHCS.

2 Experimental

2.1 Materials and Measurements

Potassium persulfate（K2S2O8，99%），polyvinyl pyrrolidone（PVP，K30），sodium hydroxide pellets（NaOH，99%），ammonia solution（NH3·H2O，99%），dichloromethane（CH2Cl2，99%），and ethyl alcohol（C2H5OH）were purchased from Sinopharm Chemical Reagent Co.，Ltd.Styrene（99.5%）was obtained from Shanghai Macklin Biochemical Co.，Ltd.Dopamine hydrochloride（C8H11NO2·HCl，98%）was purchased from J &K Scientific Ltd.Iron phthalocyanine（FePc，92%）was obtained from Shanghai Di Bai Biotechnology Co.，Ltd.All of the chemicals used in this work were of analytical grade and used without further purification.

The morphology of the as-synthesized materials was observed on a field emission scanning electron microscope（FESEM，Zeiss GeminiSEM450）.Transmission electron microscopy（TEM），scanning transmission electron microscopy（STEM），energy-dispersive X-ray spectroscopy（EDXS）and elemental mapping analyses were performed using a JEM-2100F microscope（JEM-2100F，Japan）.The crystal structure was characterized by a wide-angle X-ray diffractometer（XRD，Rigaku Ultima IV）at a scanning rate of 2°/min.The N2adsorption-desorption isotherms were characterized by a Micromeritics ASAP 2460 analyser at 77 K.The specific surface area was calculated by the Brunauer-Emmett-Teller（BET）equation based on the adsorption data.The surface element composition and valence were obtained by Shimadzu AXIS SUPRA X-ray photoelectronic spectrometer（XPS）with an AlKαradiation.

2.2 Synthesis of PS Spheres

The PS spheres here were prepared according to the reported literature［28］.Firstly，250 mg of PVP K30 which has been dissolved in 50 mL of water was added to a three-necked round-bottomed flask.Then，6.5 mL of styrene purified by 10%（mass fraction）NaOH solution was added.After bubbling nitrogen for 30 min，the mixture was fluxed at 75 ℃under magnetic stirring.After stirring for another 30 min，5 mL of aqueous solution containing 0.1 g of K2S2O8was quickly injected to initiate the polymerization reaction of styrene.With the help of reflux condenser，the mixture turned to milk-like liquid after stirring for 24 h at 75 ℃.Finally，the polystyrene aqueous solution was directly centrifuged and washed by water and ethyl alcohol several times to remove the impurities.The obtained precipitations were then dried at 60 ℃overnight.

2.3 Synthesis of PS@PDA and PDA Carbon Precursor

The PS@PDA precursor was synthesized through a method by spontaneous polymerization of dopamine on the hard-template.The dried polystyrene solid powder（100 mg）was dissolved in 72 mL of deionized water.Then the solution was sonicated for 1 h to obtain a uniform solution.After that，400 mg of dopamine was added under magnetic stirring.After stirring for 30 min，0.1 mL of ammonia solution was quickly injected to initiate the polymerization reaction of dopamine.Under weak alkaline conditions，it is beneficial to consume the H+generated during the polymerization of dopamine，which is conducive to the reaction moving towards the direction of polymerization.After stirring for 24 h at room temperature，the brown color mixture solution was centrifuged and washed by water three times.Finally，the obtained precipitations were then dried at 50 ℃overnight to obtain the PS@PDA carbon precursor.PDA carbon precursor was synthesized by the same method without adding the PS powder.

2.4 Synthesis of NHCS and NCS

The nitrogen-doped hollow carbon sphere（NHCS）was obtained by directly calcining the PS@PDA precursor in nitrogen atmosphere from 25 ℃to target temperature（900，1000，1100 ℃）with a heating rate of 5 ℃/min.After maintaining at target temperature for 2 h，the resulting product turned to be black.The obtained calcined carbon products were named as NHCS-x（x=900，1000，1100）.For comparison，nitrogendoped solid carbon sphere derived from only PDA was also synthesized by the same pyrolysis procedure，but the calcination temperature was 1000 ℃to get nitrogen-doped carbon sphere（NCS-1000）.

2.5 Synthesis of FePc-NHCS and FePc-NCS

In a typical synthesis，16 mg of the as-prepared NHCS-1000 was dissolved in 16 mL of dichloromethane and sonicated for 1 h to form a clear solution.Then 4 mL of dichloromethane solution containing 2 mg of iron phthalocyanine was added.To obtain a uniform mixture，the solution was treated under sonication for another 1 h.Afterward，the solution was transferred to a clean beaker and the reaction was continued overnight under magnetic stirring.After the mixture reacted completely，the dichloromethane was slowly evaporated under magnetic stirring to dry the samples.The collected products were further dried in a 60 ℃vacuum oven to get the final product named as FePc-NHCS-1000.FePc-NHCS-900，F(xiàn)ePc-NHCS-1100 and FePc-NCS-1000 were also prepared by the same method except using different carbons.Specifically，NHCS-1000 was treated with iron phthalocyanine of 1 and 0.5 mg，producing FePc-NHCS-1000-2 and FePc-NHCS-1000-3，respectively.In addition，F(xiàn)ePc-NHCS-1000 prepared using 2 mg of iron phthalocyanine was also labeled as FePc-NHCS-1000-1.

3 Results and Discussion

3.1 Description and Characterization of the Preparation Process

The preparation procedure and formation mechanism of iron phthalocyanine coated nitrogen-doped hollow carbon sphere（FePc-NHCS）is displayed in Scheme 1.In the first step，polystyrene sphere template and dopamine were dissolved in deionized water to form a homogeneous mixture.Dopamine started to selfpolymerize after adding ammonia solution，then polydopamine coated on the surface of PS to form PS@polydopamine（PS@PDA）composite.Then，nitrogen-doped hollow carbon sphere（NHCS）was obtained by pyrolysis of PS@PDA under a nitrogen atmosphere，realizing the simultaneous carbonization of PDA and removal of the PS template.Finally，iron phthalocyanine was loaded on NHCS to get the FePc-NHCS catalyst.

Scheme 1 Illustration for preparing iron phthalocyanine coated nitrogen-doped hollow carbon sphere(FePc-NHCS)

Spherical PS template with diameter of about 183 nm was shown in the scanning electron microscopy（SEM）and transmission electron microscopy（TEM）images in Fig.1（A）and Fig.S1（A）（see the supporting information of this paper）.Then，PDA was coated on the PS spheres，forming well-dispersed core-shell structured PS@PDA precursors［Fig.1（B）and Fig.S1（B）（see the supporting information of this paper）］.After carbonization of PS@PDA and loading FePc，NHCS and FePc-NHCS were obtained.Images in Fig.1（C）and Fig.S1（C）（see the supporting information of this paper）reveal that NHCS has hollow structure and the internal hollow size is about 128 nm.The average diameter of NHCS is smaller than that of the PS template，probably due to the obvious shrank of PS@PDA during carbonization at 1000 ℃.After loading FePc，F(xiàn)ePc-NHCS kept a good porous structure and maintained the original morphology without obvious changes［Fig.1（D）and Fig.S1（D）（see the supporting information of this paper）］.The coating of PDA on the surface of the PS spheres was confirmed by X-ray diffraction（XRD）.As shown in Fig.1（E），the diffraction peaks of PS@PDA locate closer to that of PS.The XRD patterns of NHCS and FePc-NHCS show two broad peaks at 23.4°and 44.0°，attributing to the（002）and（100）crystal planes of amorphous carbon（PDF#41-1487）.Also，F(xiàn)ePc-NHCS has diffraction peaks consistent with pure FePc（PDF#22-1771）.In order to understand the carbonization process of PS@PDA precursors，thermogravimetric（TG）analysis was carried out under nitrogen atmosphere.As shown in Fig.1（F），although PDA lost approximately 20% of weight due to dehydration，PS and PS@PDA did not lose much weight in the initial interval of 400 ℃.The weights of both PS and PS@PDA declined sharply from 400 ℃to 450 ℃，implying the fast decomposition of PS under this range.When the temperature continued rising from 450 ℃to 1000 ℃，PS@PDA and PDA show a similar trend in weight lose owing to the carbonization of PDA.Considering the behavior of PS@PDA before and after 450 ℃，it is inferred that PS did not affect the carbonization rate of PDA.To optimize the structural properties of FePc-NHCS and obtain a better performance for ORR，we calcined PS@PDA carbon precursor from 900 ℃to 1100 ℃and the obtained catalysts were named as NHCS-x（x=900，1000，1100），respectively.

Fig.1 SEM images of PS(A),PS@PDA(B),NHCS(C),and FePc-NHCS(D),XRD patterns of PS,PDA,PS@PDA,NHCS and FePc-NHCS(E) and TG curves of PS,PDA and PS@PDA measured under N2 atmosphere(F)

3.2 Characterization and Electrochemical Measurements of NHCS and NCS

The SEM images and XRD patterns of NHCS-x（x=900，1000，1100）［Fig.2（A1）—（A3）and（B）］show that there is no significant difference in the morphology and structure among these carbons，indicating the good morphological stability of NHCS.The X-ray photoelectron spectra（XPS）of NHCS-xwere shown in Fig.2（C）.The high-resolution N1sspectra can be deconvoluted into four peaks at binding energies of 398.4，399.3，401.1，and 403.0 eV，corresponding to pyridinic N（N1），pyrrolic N（N2），graphitic N（N3），and oxidized N（N4）species，respectively［28，29］.Table S1（see the supporting information of this paper）shows the percentages of different nitrogen types in NHCS-x.For NHCS-x，the ratios of N1 and N2 declined while the ratio of N3 increased after increasing the calcination temperature.That is because graphitic N is more stable than pyridinic N and pyrrolic N，the higher thermal treatment temperature results in the reduction of unstable substances［30］.Moreover，the Raman spectra were displayed in Fig.2（D）.The intensity ratio ofID/IGcan be used to characterize the degree of graphitization of carbon materials.The higher theID/IGvalue is，the lower the degree of graphitization is［20］.Among NHCS-x，NHCS-900 shows the highestID/IGvalue of 2.50，followed by 2.37 of NHCS-1000 and 2.27 of NHCS-1100，revealing that the degree of graphitization increased with the increasing calcination temperature.Specific surface area（SSA）and pore size distribution（PSD）were investigated by N2adsorption-desorption analysis.According to Fig.2（E），the N2adsorption-desorption isotherms of NHCS-xexhibit type-IV isotherms with well-defined hysteresis loops in the latter half part（p/p0=0.6—1.0），indicating the co-existence of micropores，mesopores，and macropores.NHCS-1000 and NHCS-1100 show higher nitrogen uptake at low relative pressure（p/p0＜0.1）than NHCS-900，implying the existence of more micropores after increasing the carbonization temperature.The calculated SSA and pore volume of NHCS-xwere shown in Table S2（see the supporting information of this paper）.When the temperature increased from 900 ℃to 1100 ℃，the SSA of NHCS-xobviously increased from 369 m2/g to 566 m2/g.The pore volume of NHCS-1100 is 1.837 cm3/g，larger than that of NHCS-900 and NHCS-1000.The pore size distribution curves of NHCS-xwere shown in Fig.2（E），demonstrating the presence of hierarchical nanopores，which provide low-resistant channels for ions transfer during electrochemical reaction.What’s more，NCS-1000 was also prepared by directly carbonization of polydopamine at 1000 ℃without using PS template.And its detail information was displayed in Figs.S2 and S3（see the supporting information of this paper）.Compared with NCS-1000，NHCS-xpossess hollow spherical structure and hierarchical porous structure，which can facilitate the exposure of active sites and the rapid mass transportation for ORR.

Fig.2 SEM images(A1—A3),XRD patterns(B),high resolution N1s XPS spectra(C),Raman spectra(D),N2 adsorption-desorption isotherms(E)and pore size distributions(inset)of NHCS-x(x=900,1000,1100)

The electrocatalytic performances of NHCS-xand Pt/C were evaluated with a rotating ring-disk electrode（RRDE）in alkaline electrolyte.As depicted in Fig.S4（see the supporting information of this paper），all NHCS-xexhibit similar cyclic voltammetry（CV）curves in O2-saturated electrolyte.Among them，NHCS-1000 displays the ORR peak at 0.684 Vvs.RHE，which is more positive than that of NHCS-900（0.657 V）and NHCS-1100（0.678 V）.The linear sweep voltammetry（LSV）curves plotted in Fig.3（E）were also used to evaluate the catalytic performance for the ORR.The onset potential（Eonset）and half-wave potential（E1/2）are the most important performance indicators in LSV.Eonsetis defined as the corresponding potentials when the current density is 0.1 mA/cm2.NHCS-1000 exhibits the most promising ORR activity among the samples，withEonsetof 0.863 V andE1/2of 0.721 V.Besides，NHCS-1000 has a larger diffusion-limited current density（Jd）of 3.06 mA/cm2，compared to that of NHCS-900（2.48 mA/cm2）and NHCS-1100（2.64 mA/cm2）.Above all，NHCS-1000 shows the best ORR catalytic performance.To investigate the influence of morphology on ORR catalytic activity，NCS-1000 was also tested and it showed anEonsetof 0.710 V，anE1/2of 0.548 V，and aJdof 2.28 mA/cm2.By comparing NCS-1000 and NHCS-1000，theEonsetandE1/2of NHCS-1000 increase by almost 153 mV and 173 mV，and theJdalso has a remarkable enhancement，indicating that hollow porous morphology is beneficial to improve the performance for ORR.Nevertheless，the ORR catalytic activity of NHCS-xis still far behind that of Pt/C（Eonset=0.978 V，E1/2=0.848 V，Jd=5.67 mA/cm2）.

Fig.3 SEM images(A1—A3),XRD patterns(B),high resolution N1s(C) and Fe2p XPS spectra(D) of FePc-NHCS-x(x=900,1000,1100),and LSV of NCS-1000,FePc-NCS-1000,NHCS-x,FePc-NHCS-x(x=900,1000,1100),and Pt/C(20%)at 1600 r/min with a scan rate of 10 mV/s(E)

3.3 Characterization and Electrochemical Measurements of FePc-NHCS-x Catalysts

FePc-NHCS-x（x=900，1000，1100）were obtained after loading FePc onto NHCS-x.Based on previous reports and preliminary experiments［25］，the initial ratio of NHCS to FePc is selected to be 8 in this work.For comparison，NCS-1000 was also coated with FePc to obtain the FePc-NCS-1000.From the SEM images in Fig.3（A1）—（A3），F(xiàn)ePc-NHCS-xretain its original morphology inherited from the corresponding NHCS-x，which was a hollow shape with a rough surface.From the XRD patterns in Fig.3（B），there are obvious diffraction peaks at 7.0°，15.6°and 24°，indicating the loading of FePc after comparing with the XRD pattern of pure FePc.XPS data of FePc-NHCS-xare displayed in Table 1 and Table S1，F(xiàn)ig.3（C）and（D）.The highresolution N1sspectra［Fig.3（C）］can be divided into five bands including the four types in the NHCS mentioned above，and additional Fe-Nxat 398.8 eV（N5）caused by the FePc［31，32］.The nitrogen content of FePc-NHCS decreased from 6.63%，4.33%，to 3.50% with the increase of temperature（Table 1）.What’s more，the atom ratio of pyridinic N and pyrrolic N in nitrogen specie decrease with the rising of temperature，while the ratio of graphitic N was increasing.The trends above are understandable just as the trends changed in NHCS.The high-resolution Fe2pspectrum［Fig.3（D）］can be deconvoluted into six peaks［33，34］.The peaks at 709.8 and 712.0 eV are attributed to Fe2+（Fe1）and（Fe2）peaks，respectively.While the peaks at 722.5 and 724.5 eV are correspond to the Fe2+（Fe3）and（Fe4）peaks，respectively［31，35］.The last two are satellite peaks at 717.9（Fe5）and 732.0（Fe6）eV.

Table 1 Elemental content(%,atom fraction)in FePc-NHCS-x(x=900,1000,1100)

The ORR performance of FePc-NHCS-xand FePc-NCS-1000 were investigated by CV and LSV tests，using the same conditions and parameters as the test of NHCS-xand NCS-1000.As shown in Fig.S5（see the supporting information of this paper），all of them have well-defined ORR peaks at potentials between 0.81 V and 0.86 V in the O2-saturated alkaline solution，but not in the N2-saturated alkaline solution.FePc-NHCS-1000 and FePc-NHCS-1100 display the same ORR peak at 0.855 V，which is slightly greater than that of FePc-NHCS-900（0.850 V），but all the three are apparently more positive than FePc-NCS-1000.Moreover，the LSV curves of FePc-NHCS-xand FePc-NCS-1000 were also recorded，as shown in Fig.3（E）.FePc-NHCS-1000 has the most positiveEonsetof 0.941 V andE1/2of 0.862 V among the four catalysts.FePc-NHCS-900 hasEonsetof 0.938 V andE1/2of 0.847 V，and FePc-NHCS-1100 hasEonsetof 0.939 V andE1/2of 0.853 V.Besides，theJdof FePc-NHCS-1000（Jd=5.36 mA/cm2）is close to that of FePc-NHCS-1100（Jd=5.49 mA/cm2），but is a bit better than that of FePc-NHCS-900（Jd=5.09 mA/cm2）.Also，F(xiàn)ePc-NCS-1000 was tested as a comparison sample，which displayed a worse performance（Eonsetof 0.904 V，E1/2of 0.776 V，Jdof 4.64 mA/cm2）compared with FePc-NHCS-1000，proving that the hollow morphology is beneficial to catalyze ORR.Above all，the ORR activities of FePc-NHCS-xsamples all surpass that of the original NHCS-xsamples［Fig.3（E）］.Considering FePc-NHCS-xand NHCS-xcatalysts have almost the same morphology and degree of graphitization，the increased ORR performance can be attribute to the loading of FePc.Due to the comprehensive structural and componential properties，F(xiàn)ePc-NHCS-1000 shows the best ORR catalytic performance，and a more positive half-wave potential（E1/2=0.862 V）than that of Pt/C（E1/2=0.848 V）.

As mentioned in the last section，F(xiàn)ePc-NHCS-xcatalysts were synthesized with a FePc/NHCS ratio of 1/8 based on previous reports and preliminary experiments［25］.In order to decrease the use of expensive FePc while keep the good performance of FePc-NHCS，another two samples of FePc-NHCS-1000-2（the mass ratio of FePc/NHCS is 1/16）and FePc-NHCS-1000-3（the mass ratio of FePc/NHCS is 1/32）were prepared as the control samples to determine the optimized dosage of FePc.The initial FePc-NHCS-1000（the mass ratio of FePc/NHCS is 1/8）was also named as FePc-NHCS-1000-1 in the following parts.The above three samples are collectively referred to as FePc-NHCS-1000-y（y=1，2，3）.SEM images in Fig.4（A1）—（A3）show that loading FePc have little influence on the spherical morphology of NHCS.As observed from the TEM image［Fig.4（B）］，the hollow carbon has a central hollow core of about 128 nm in diameter，which is consistent with the SEM results.To further explore the elemental distribution of FePc-NHCS-1000-y，elemental mapping of the representative FePc-NHCS-1000-2 was obtained.Fig.4（C1）—（C4）exhibit that carbon，nitrogen and iron are homogeneously dispersed in the sample.XRD patterns of different FePc-NHCS-1000-y［Fig.4（D）］show that only FePc-NHCS-1000-1 has obvious diffraction peaks consistent with pure FePc，indicating the good disperse of FePc in FePc-NHCS-1000-2 and FePc-NHCS-1000-3 after decreasing the loading amount of FePc.XPS data of FePc-NHCS-1000-ywere displayed in Fig.4（E）and Fig.S6，Table S1 and Table S3（see the supporting information of this paper）.From Table S1，the percentages of Fe-Nxspecies in Fe-NHCS-1000-ydecline from 22.8%（y=1），21.3%（y=2），to 17.3%（y=3），which is consistent with the trend of loading dosage of FePc.

Fig.4 SEM images of FePc-NHCS-1000-y(y=1,2,3 from left to right)(A),TEM(B),STEM(C1)and elemental mapping(C2—C4) images of FePc-NHCS-1000-2,and XRD patterns(D)and high resolution N1s XPS spectra(E)of FePc-NHCS-1000-y(y=1,2,3)

Similarly，the electrocatalytic performance of FePc-NHCS-1000-ywas evaluated with a rotating ring-disk electrode（RRDE）in alkaline electrolyte.As seen in Fig.S5（see the supporting information of this paper），F(xiàn)ePc-NHCS-1000-2 and FePc-NHCS-1000-3 display almost the same ORR peaks at 0.856 and 0.854 V，and exhibit similar CV curves to FePc-NHCS-1000-1.The LSV curves of FePc-NHCS-1000-2 and FePc-NHCS-1000-3 were also tested on the RRDE in O2-saturated alkaline solution，as shown in the Fig.5（A）.FePc-NHCS-1000-2 has the comparable ORR performance（Eonset=0.944 V，E1/2=0.862 V）to FePc-NHCS-1000-1（Eonset=0.941 V，E1/2=0.862 V），but FePc-NHCS-1000-3（Eonset=0.939 V，E1/2=0.827 V）exhibits worse catalytic performance.What’s more，theJdof FePc-NHCS-1000-2 and FePc-NHCS-1000-3 is 5.67 and 5.65 mA/cm2，respectively，both better than that of FePc-NHCS-1000-1（Jd=5.36 mA/cm2）.Taking all these factors into consideration，F(xiàn)ePc-NHCS-1000-2 with a moderate dosage of FePc is the most cost-effective sample.The catalytic activity of FePc-NHCS-1000-ywas mostly originated from FePc.Therefore，increasing the loading of FePc could provide more active sites for ORR.However，F(xiàn)ePc started to agglomerate when FePc was over-loaded on the NHCS，resulting in a worse electrocatalytic performance.

Fig.5 LSV of FePc-NHCS-1000-y(y=1,2,3)and Pt/C(20%)at 1600 r/min with a scan rate of 10 mV/s(A),different rotation speeds(B),and RRDE curves(C) of FePc-NHCS-1000-2 in O2-saturated 1 mol/L KOH with a scan rate of 10 mV/s,electron transfer number and peroxide yield of FePc-NHCS-1000-2 calculated from(C)(D)

FePc-NHCS-1000-2 was further investigated by conducting the LSV curves at various rotation speeds［Fig.5（B）］.As illustrated in the inset images of Fig.5（B），the K-L plots all show an excellent linearity.The calculated average value of the electron transfer number calculated from K-L plots is 3.98，indicating that the FePc-NHCS-1000-2 mainly performs the four-electron dominant ORR pathway.To evaluate the ORR catalytic selectivity of FePc-NHCS-1000-2，the yield of H2O2and electron transfer number were calculated from the corresponding RRDE curves［Fig.5（C）and（D）］.FePc-NHCS-1000-2 exhibits low yields of H2O2from 2% to 0 and high values of electron transfer number more than 3.95，implying the four-electron dominated ORR pathway，which is consistent with the finding from the K-L plots.In addition to the abovementioned，the stability test is another significant index to consider for evaluating the ORR catalytic activity.CV cycling test was conducted on Pt/C catalyst，and the most active catalyst，F(xiàn)ePc-NHCS-1000-2，between 0.6 V to 1.1 V for 3000 cycles.From Fig.S7（see the supporting information of this paper），the LSV curves of FePc-NHCS-1000-2 and Pt/C catalysts before and after the stability test can be observed.After the stability test，the diffusion limiting current density of FePc-NHCS-1000-2 displays 88%original value，which is higher than that of Pt/C（76%）.FePc-NHCS-1000-2 shows a negative shiftEonsetof 16 mV，andE1/2of 14 mV.In contrast，the Pt/C catalyst has a more negative shift ofEonset=67 mV，andE1/2=15 mV.Totally，F(xiàn)ePc-NHCS-1000-2 reveals a better long-term durability than Pt/C.

4 Conclusions

PS@PDA composite was prepared by using PS as the template and PDA as the carbon precursor.And after calcination，nitrogen-doped hollow carbon sphere（NHCS）was obtained.The NHCS was then loaded with iron phthalocyanine（FePc）to afford FePc-coated nitrogen-doped hollow carbon sphere（FePc-NHCS）.FePc-NHCS-xwere hollow carbon spheres with high content of uniformly dispersed nitrogen and iron，and possessed a high specific surface area，and hierarchical porous structure.All of this endowed FePc-NHCS with unusual catalytic performance towards ORR in alkaline conditions.Through the exploration of pyrolysis temperature，it was found that this factor affected the ORR performance in a small range.But hollow sphere exhibited a more excellent ORR activity than solid sphere，verifying the advantages of the hollow sphere shape.Besides，F(xiàn)ePc-NHCS-xsamples showed a much better ORR performance than the samples without FePc loading.What’s more，the most suitable loading percentage of FePc was found by changing the loading dosage of FePc.Through the above comparative experiments，F(xiàn)ePc-NHCS-1000-2（mass ratio of FePc/NHCS is 1/16）exhibited an excellent ORR catalytic activity，with more positiveE1/2and better long-term durability compared to that of Pt/C in alkaline solution.The excellent ORR performance of FePc-NHCS is ascribed to the hollow shape，and the introduction of FePc.This work provides a simple method to synthesize non-precious metal carbon-based ORR catalyst and exhibits a great potential in the future development of renewable energy devices.

The supporting information of this paper see http://www.cjcu.jlu.edu.cn/CN/10.7503/cjcu20200677.