稀有金属(英文版) 2020,39(06),680-687

Nitrogen-,phosphorus-doped carbon-carbon nanotube CoP dodecahedra by controlling zinc content for high-performance electrocatalytic oxygen evolution

Xia-Xia Li Pei-Yao Zhu Qing Li Yu-Xia Xu Yan Zhao Huan Pang

School of Chemistry and Chemical Engineering,Yangzhou University

Guangling College,Yangzhou University

College of Chemistry and Materials Science,Sichuan Normal University

作者简介：*Yu-Xia Xu,e-mail:xuyuxia1105@163.com;*Huan Pang,e-mail:huanpangchem@hotmail.com;panghuan@yzu.edu.cn ;

收稿日期：22 January 2020

基金：financially supported by the National Natural Science Foundation of China(No.21671170);the Top-notch Academic Programs Project of Jiangsu Higher Education Institutions(TAPP);Program for New Century Excellent Talents of the University in China(No.NCET-13-0645);the Six Talent Plan (No.2015-XCL-030);the Program for Colleges Natural Science Research in Jiangsu Province(No.18KJB150036);

Nitrogen-,phosphorus-doped carbon-carbon nanotube CoP dodecahedra by controlling zinc content for high-performance electrocatalytic oxygen evolution

Xia-Xia Li Pei-Yao Zhu Qing Li Yu-Xia Xu Yan Zhao Huan Pang

School of Chemistry and Chemical Engineering,Yangzhou University

Guangling College,Yangzhou University

College of Chemistry and Materials Science,Sichuan Normal University

Abstract：

Here,N-and P-doped carbon-carbon nanotube CoP(NPC-CNTs-CoP) nanoparticles dodecahedra are achieved by multistep calcination of the Zn-doped zeolitic imidazolate framework ZIF-67 precursor(ZnCo-ZIF).In the structures,the presence of N and P atoms,abundant CNTs and the CoP nanoparticles can enhance electrochemical activity and promote the structural stability of materials.As the temperature increases,the Zn contents gradually reduce to zero,which provides more active sites for electrochemical testing.Furthermore,the high specific surface area and microporous behavior of NPC-CNTsCoP-9 make it excellent in electrocatalytic testing.NPCCNTs-CoP-9 shows a low overpotential of 224 mV at10 mA·cm^-2 in 1.0 mol·L^-1 KOH solution.The strategy of zeolitic imidazole framework-derived transition metal phosphides will provide a new sight for developing energy conversion materials.

Keyword：

Nitrogen; Phosphorus; Carbon; Carbon nanotube; CoP; Oxygen evolution;

Received： 22 January 2020

1 Introduction

As one of the most important reactions of electrochemical energy storage and conversion,oxygen evolution reaction(OER) is widely used in water splitting,fuel cells and metal-air batteries [ 1, 2, 3, 4, 5] .Some precious metal catalysts(such as RuO₂) [ 6, 7, 8, 9, 10] showed extraordinary performance in the oxygen evolution reaction (OER),but these precious metal catalysts have many obstacles in the large-scale application because of their low content and high cost [ 11, 12, 13] .The key to reducing costs and improving catalytic efficiency in the hydrolysis process is to find an excellent and common metal catalyst,such as carbides [ 14, 15, 16] ,nitride [ 17, 18] and phosphate [ 19, 20, 21] .

Transition metal phosphides (TMP) have good activity and durability over a wide pH range,and the M-P bond with bonding energy can facilitate the capture of catalyst intermediates to prevent the deactivation of catalysts,which has been widely studied in OER [ 22, 23, 24] .The catalytic efficiency of the catalyst has been maximized by optimizing the nanostructure and the composition of the catalyst,exposing more water cracking sites with catalytic activity [ 25, 26] .It can be known from the previous studies that both doping heteroatoms and compositing with porous carbon materials could improve the catalytic performance of transition metal phosphides [ 27, 28, 29, 30] .Zeolitic imidazolate frameworks (ZIFs) [ 31, 32, 33] can not only provide rich carbon and nitrogen sources,but also increase the number of coupled TM-N_x active sites and improve the reaction activity of active groups [ 34, 35] .By adding different metal cations in the process of ZIFs growth,not only the activity of OER reaction can be improved,but also the secondary metal atoms can be used as homogeneous dopants to promote the tuning of the coordination environment in the metal center [ 36, 37] .At the same time,the potential agglomeration in the pyrolysis process can provide the steric hindrance,thus improving the electrocatalytic activity [ 38] .As an excellent precursor,ZIFs have been designed as various nano-metal carbon composites and relevant phosphides.Furthermore,within all kinds of nanostructures,nanotube structures have good internal channels and large surface area,which is beneficial to the transfer of charge and mass in the process of electrocatalysis [ 39, 40] .Similarly,Li and co-workers reported the CoP nanoparticles embedded in a N-doped carbon nanotube hollow polyhedron catalyst with superior activity for overall water splitting [ 21] .Lou and co-workers [ 41] reported the use of Ni-Co mixed metal phosphides with amorphous carbon for OER.However,TMP with high electrocatalytic activity has yet to be developed.

In this work,N-and P-doped carbon-carbon nanotube CoP (NPC-CNTs-CoP) was fabricated from Zn-doped zeolitic imidazolate framework ZIF-67 precursor (ZnCoZIF) by multistep calcination methods.This work aims to provide a novel material specifically to detect electrocatalytic activity.The best samples show excellent OER performance,even superior to that of the commercial RuO₂catalysts.This work provides a method for the design and synthesis of other metal phosphides as electrocatalysts for energy conversion.

2 Experimental

2.1 Synthesis of ZnCo-ZIF

All purchased chemicals were used directly without further processing.In the precursor synthesis,1 mmol Zn(NO₃)2·6H₂O and 1 mmol Co(NO₃)2·6H₂O were dissolved in 4 ml anhydrous methanol under ultrasound conditions.Similarly,49.91 mmol of 2-methylimidazole was solubilized in 26 ml of anhydrous methanol.Then,two solutions were quickly mixed and allowed to rest at 25℃for 12 h.Finally,the substance precipitated at the bottom of the solution was centrifuged and washed three times with an hydrous methanol,and purple precursor of ZnCoZIFs was obtained after drying.

2.2 Synthesis of NC-CNTs-Co-x

The synthesized ZnCo-ZIFs with corundum boat were placed in tube furnace.The pipe furnace was heated to900℃(800 and 700℃) with 1℃·min^-1 in N₂ protection to prepare NC-CNTs-Co-x composites (x=7,8,9 representing the calcination temperature are 700,800,900℃).After the samples cooled to room temperature,the synthesized samples were collected.

2.3 Synthesis of NC-CNTs-Co3O4-x

The NC-CNTs-Co-x was placed in a tube furnace heated to350℃with a ramp rate of 5℃·min^-1,and kept for 2 h in N₂ to yield NC-CNTs-C0₃O_4-x.

2.4 Synthesis of NPC-CNTs-CoP-x

The NC-CNTs-Co₃O₄-x (50 mg) was placed in the middle of a porcelain boat,and 0.5 g NaH₂PO₂ was placed on the upstream side.The porcelain boat was put in a tube furnace,heated to 300℃with a ramp rate of 2℃·min^-1,and kept for 2 h in N₂ to yield NPC-CNTs-CoP-x.

2.5 Material characterizations

The products were tested on a Bruker D8 Advanced X-ray diffractometer (XRD) with Cu Ka radiation(λ=0.15406 nm) for the phase analysis.Scanning electron microscope (SEM,Zeiss＿Supra55) was used for observing the morphology of the samples at an acceleration voltage of5.0 kV.High-resolution transmission electron microscopy(HRTEM) images,selected area electron diffraction (S AED)images and energy dispersive X-ray spectroscopy (EDS)mapping were captured on a Tecnai G2 F30 transmission electron microscopy (TEM) at an acceleration voltage of300 kV.X-ray photoelectron spectroscopy (XPS) was carried out on a Thermo Scientific ESCALAB 250 apparatus.The N₂ adsorption-desorption isothermals and pore size distribution were obtained by Autosorb-iQ via BrunauerEmmett-Teller (BET) method.Thermal properties were determined using the PerkinElmer Pyris 1 thermogravimetric analyzer (TGA,USA),where the sample was heated to500℃at a heating rate of 5℃·min^-1 under N₂ flow.

2.6 Electrochemical measurements

The electrochemical water oxidation experiments were carried out on an electrochemical working station (CHI760E,Shanghai Chenhua).A conventional three-electrode system was implemented for all the electrochemical measurements by utilizing a glassy carbon electrode (GCE)(diameter of 3.0 mm) as the working electrode,Pt wire as the counter electrode,and a Hg-HgO electrode as the reference electrode in 1.0 mol·L^-1 KOH electrolyte.According to the Nernst equation (E_RHE=E_Hg/HgO+0.059pH+0.098),the current density was normalized to the geometrical surface area and the measured potential vs.Hg/HgO was converted to a reversible hydrogen electrode(RHE).Prior to the electrochemical test,we first passed N₂gas for 0.5 h in 1.0 mol·L^-1 KOH to ensure the O₂/H₂O equilibrium at 1.23 V vs.RHE.Cyclic voltammograms(CVs) were obtained at a scan rate of 50 mV·s^-1,and the working electrodes were scanned several times until stabilization before CV data was collected.The linear sweep voltammograms (LSVs) were obtained with a scan rate of5 mV·s^-1.The working electrodes were scanned several times until the signals were stabilized,and then LSV data were collected,corrected for the internal resistance (iR)contribution within the battery.CV and LSV were applied for confirming the electrocatalytic properties of the working electrodes.

3 Results and discussion

The NPC-CNTs-CoP materials were synthesized via carbonization,oxidation and phosphorization treatment of the ZnCo-ZIF precursor (Scheme 1).The specific synthesis methods are shown in Supporting Information (SI).The Co/Zn mole ratio in bimetallic ZIFs can be controlled by adjusting the Co/Zo mole ratio in the reactants.As shown in Table S1,the molar ratio of Co/Zn in the products corresponds to that in the reactants.The precursors exhibit a regular dodecahedral with a size of about 200 nm and uniform distribution (Fig.S1 a).The precursor was consistent with the simulated ones,indicating the successful synthesis of ZnCo-ZIF (Fig.Slb).The progress of ZnCoZIF calcined to prepare NPC-CNTs-CoP was followed by thermogravimetric analysis (TGA) curve as shown in Fig.S2.The precursors were calcined at 700℃(800 and900℃) with 1℃·min^-1 in N₂ protection to prepare NC-CNTs-Co-x composites.SEM (Fig.S3a-c) and HRTEM(Fig.S3a1-c1) images demonstrate that NC-CNTs-Cox samples have a rough surface compared with the precursors,and the existence of Zn and Co nanoparticles promotes the formation of CNTs [ 42] .And the number of CNTs increases with the increase in calcination temperature.XRD result of NC-CNTs-Co-x (Fig.S4) shows that the peaks located at 26°(002),44°(111) and 51°(220) are corresponding to the graphitic carbon (PDF No.26-1077)and fee Co (PDF No.15-0806).

NC-CNTs-Co₃O₄-x and NPC-CNTs-CoP-x were synthesized by oxidation and phosphating heat treatment of NC-CNTs-Co-x.SEM and HRTEM images of the NC-CNTs-Co₃O₄-7,NC-CNTs-Co₃O₄-8,NC-CNTs-Co₃O₄-9,NPC-CNTs-CoP-7 and NPC-CNTs-CoP-8 are shown in Figs.S5,S6.And XRD results are consistent with those of NC-CNTs-Co₃O₄ in the previous literature (Fig.S7).After phosphorization,FESEM image of the NPC-CNTs-CoP-9shows high porosity polyhedron with carbon nanotubes(Fig.1a),maintained well polyhedral shape of the ZnCoZIF.Besides,parts of the Co-O bond become a Co-P bond.And HRTEM image indicates two fringe spacings in0.189 and 0.247 nm,which can represent the (211) and(111) lattice planes of CoP,respectively (Fig.1b-d).EDS elemental mapping analysis indicates the evidence of Co,P,C,N and O in NPC-CNTs-CoP-9 (Figs.1e,S8).From Fig.S9,the peaks at about 32°,36°,48°and 56°correspond to (011),(111),(211) and (301) facets of CoP (PDF No.29-0497),and the peak at about 45°corresponds to the Co₃O₄ (400) plane in NPC-CNTs-CoP-x materials.The results show that the three samples are the same kind of substances.

The valence state and electronic structure of Zn,Co,Pand N were tested and shown in Fig.2a.With the increase in temperature,Zn species have been evaporated completely,providing more active sites for electrochemical testing (Fig.S10).The N 1s spectra in Fig.2b exhibit thatthere are three types of N (pyridinic,pyrrolic and graphitic).The gradual decrease in pyridinic N and the significant increase in pyrrolic N from NPC-CNTs-CoP-7 to NPC-CNTs-CoP-9 show that the two types of nitrogen species can promote the OER electrocatalytic activity.The P 2p region of the three samples exhibits two energy bands of～129.4 eV (P 2p_3/2),～130.8 eV (P 2p_1/2) and～134.6 eV (oxidized P)(Fig.2c).In addition,the peak appeared at～134.5 eV corresponds to the P-C bonds,which shows that P is also successfully doped into the carbon lattice.Figure 2d shows the major Co 2p_3/2 peak at778.1 eV (777.9,778.0 eV) and the Co 2p_1/2 peak at793.2 eV (793.3,793.4 eV),which are deconvoluted into Co-P in composites.The characteristic fitting peaks at778.1 eV (777.9,778.0 eV) and 793.2 eV (793.3,793.4 eV) belong to Co³⁺,while those at 781.2 eV (781.0,781.2 eV) and 797.5 eV (797.2,797.6 eV) are ascribed to Co²⁺ [ 43, 44] .NPC-CNTs-CoP-9 contains more Co-Pbonds than the other two samples,suggesting that Co-Pbonds can have a significant impact on OER performance.

Scheme 1 Illustration of synthesis process of NPC-CNTs-CoP

Fig.1 a SEM image of NPC-CNTs-CoP-9;b,c HRTEM images of NPC-CNTs-CoP-9;d SAED pattern;e EDX elemental mapping images of Co,P,C,N and O in NPC-CNTs-CoP-9

Fig.2 a Survey spectrum,b N 1s spectrum,c P 2p spectrum and d Co 2p spectrum of NPC-CNTs-CoP-7,NPC-CNTs-CoP-8 and NPC-CNTs-CoP-9

In order to evaluate the catalytic performance of the NPC-CNTs-CoP-x for the OER,we measured the OER catalytic activities of the different samples in 1.0 mol·L^-1 KOH electrolyte using a standard three-electrode system(Fig.S11).The working potential at 10 mA·cm^-2 is a significant parameter for estimating the OER performance.Figure 3a shows LSV graph of different electrodes measured at a scanning speed of 5 mV·s^-1.The operating potential of NPC-CNTs-CoP-9 denoted at 1.464 V vs.RHE to drive a current density of 10 mA·cm^-2,corresponds to an overpotential of 224 mV.In comparison with NPC-CNTs-CoP-7(363 mV),NPC-CNTs-CoP-8 (325 mV) and RuO₂(349 mV),NPC-CNTs-CoP-9 indicates the most outstanding OER activity (Figs.3a,4b).Overpotential is the potential difference between the thermodynamically determined reduction potential of the half-reaction and the experimentally observed potential of the redox reaction.The OER activity of NC-CNTs-Co-x and NC-CNTs-Co₃O_4-x is shown in Figs.S12,S13.The electrocatalytic activity of NPC-CNTs-CoP-9 is also better than that of NC-CNTs-Co-7,NC-CNTs-Co-8,NC-CNTs-Co-9,NC-CNTs-Co₃O_4-7,NC-CNTs-Co₃O₄-8 and NC-CNTs-Co₃O₄-9,which are 518,377,349,448,358 and 329 mV,respectively.The smaller Tafel slope means the smaller resistance in the polarization,which indicates outstanding OER performance.NPC-CNTsCoP-9 has the lowest Tafel slope (89 mV·dec^-1) contrast to NPC-CNTs-CoP-7,NPC-CNTs-CoP-8 and RuO₂ (99,116,and 184 mV·dec^-1)(Fig.3b).

The electrochemical surface area of the samples can be calculated through electrochemical double-layer capacitance (C_d1).The cyclic voltammetry curves with different sweep speeds were tested in the non-faradaic region to calculate the corresponding C_d1.Figure S14 shows that NPC-CNTs-CoP-9 shows a high C_d1 value (4.64 mF.cm^-2),about2.32 and 2.09 times those of NPC-CNTs-CoP-7(2.00 mF·cm^-2) and NPC-CNTs-CoP-8 (2.22 mF·cm^-2),respectively.The large C_d1 value means greater roughness and more active sites for electrode,and further indicates that the NPC-CNTs-CoP-9 composites are more favorable for catalytic reaction.In addition,the durability of the sample is a crucial factor to investigate the catalytic reaction.Compared with the polarization curve after 1 cycle,the current density of the polarization curve after 1000 cycles is attenuated to some extent,which is caused by sample shedding during the test (Fig.3c).As shown in Fig.3d,the current density of NPC-CNTs-CoP-9 slightly decreases after12,000 s,which is due to the partial drop of catalyst attached to the working electrode during long-term test.After 12,000-s stability,negligible changes in SEM are observed(Fig.S15).

Electrical conductivity is also the key factor affecting OER catalytic activity.The Schottky barrier becomes smaller,while the conductivity becomes higher between catalyst-electrode and catalyst-electrolyte interfaces.In order to study the electron transfer ability of the sample,electrochemical impedance spectroscopy (EIS) tests were performed.Compared with NPC-CNTs-CoP-7 and NPC-CNTs-CoP-8,NPC-CNTs-CoP-9 has lower charge transfer resistance,which possesses faster charge transfer and higher electrical conductivity.Moreover,the smaller the impedance is,the higher the conductivity is,which is beneficial to the OER and the improvement of the electrocatalytic reaction efficiency (Fig.S16).Additionally,the NC-CNTs-Co-x and NC-CNTs-C0₃O_4-xsamples show a similar regular in Figs.S17,S18.

The N₂ adsorption-desorption isotherm test of NPC-CNTs-CoP-9 shows a typical type III curve,and the nitrogen absorption increases exponentially under all pressures (Fig.4a).The large Brunauer-Emmett-Teller(BET) surface area of NPC-CNTs-CoP-9 is 418 m²·g-1and the pore volume is 1.11 cm³·g-1.Barrett-JoynerHalenda (BJH) size distribution (inset in Fig.4a) points out that the majority pores in NPC-CNTs-CoP-9 are about1.88 nm in size.The high specific surface area and abundant porosity of the NPC-CNTs-CoP-9 catalyst contribute to charge and mass transport electrocatalysis.

The special composition and structure of NPC-CNTsCoP-9 contribute to its excellent catalytic performance.Controlling calcination temperature is the key to control Zn content.The intensity ratio of D band to G band (I_D/I_G)value of NPC-CNTs-CoP-9 is significantly higher than those of NPC-CNTs-CoP-7 and NPC-CNTs-CoP-8,indicating that NPC-CNTs-CoP-9 has higher defect density and disorder (Fig.S19).In this catalytic reaction,Co atoms are oxidized to CoP/CoOOH as the active center,and CoOOH is activated via CoP providing an effective electron pathway for accelerating the OER process [ 22, 45] .The introduction of nitrogen can promote electron interaction with C/CoP and introduce more defect active sites into the carbon frames [ 46] .In addition,partial carbon nanotubecoated CoP inhibits polymerization,improves material stability and further improves electrocatalytic activity.Furthermore,the high surface area,the abundant pore structure and the rough framework structure with interconnected CNTs also play a vital role for the strengthened electrocatalytic activity and stability,due to their enhanced fast charge,mass transport and excellent conductivity between the materials and electrolyte [ 47, 48, 49] .

Fig.3 a LSV curves for OER in N₂-saturated 1.0 mol·L^-1 KOH electrolyte at 5 mV·s^-1 and b Tafel slopes of NPC-CNTs-CoP-7,NPC-CNTs-CoP-8,NPC-CNTs-CoP-9 and RuO₂;c durability test of NPC-CNTs-CoP-9 for 1000 cycles and d chronoamperometric testing of NPC-CNTs-CoP-9 for 12,000 s at a static overpotential of 234 mV in N₂-saturated 1.0 mol·L^-1 KOH electrolyte

Fig.4 a N₂ adsorption-desorption isotherms at 77 K and corresponding pore size distribution plot (inset) of NPC-CNTs-CoP-9 (STP:standard temperature and pressure;V:volume;RP:real pore);b overpotential of NPC-CNTs-CoP-7,NPC-CNTs-CoP-8,NPC-CNTs-CoP-9 and RuO₂

4 Conclusion

In summary,we successfully prepared nanocomposite NPC-CNTs-CoP-x by using the ZnCo-ZIF precursor through a multi-step calcination method and used it as an excellent OER electrocatalyst.Compared with the other two samples,the NPC-CNTs-CoP-900 showed smaller charge transfer resistance and better durability.Beneficial from the unique features of this structure,such as the synergistic effect between CoP and the N-doped carbon nanotube,large specific surface area and porous channels,the obtained NPC-CNTs-CoP-9 exhibits promising performance and excellent cycling stability in OER.NPC-CNTs-CoP-9 has an overpotential of 224 mV in1.0 mol·L^-1 KOH to achieve a current density of10 mA·cm^-2.The results will provide a deep understanding of the design and synthesis of other metal phosphating electrocatalysts in the future,and will pave the way for the development of energy materials in fuel cells,metal air cells,water cracking units and other renewable energy systems.

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