简介概要

Co and N co-modified carbon nanotubes as efficient electrocatalyst for oxygen reduction reaction

来源期刊：Rare Metals2021年第1期

论文作者：Ying-Gang Zhu Chao-Qun Shang Zhen-Yu Wang Jian-Qiao Zhang Ming-Yang Yang Hua Cheng Zhou-Guang Lu

文章页码：90 - 95

摘要：It is a big challenge to prepare non-rare metal and high-activity electrocatalysts for oxygen reduction reaction（ORR）.In this paper,a cobalt/carbon nanotubes/chitosan composite gel was synthesized and then annealed under nitrogen atmosphere to yield the cobalt and nitrogen co-modified carbon nanotubes（Co-N-CNTs）nanocomposite electrocatalysts.In this strategy,the cobalt component considerably enhanced the ORR activity and improved the degree of graphitic structure to increase the electronic conductivity.The chitosan served as sustainable source for nitrogen doping.The Co-N-CNTs exhibit excellent oxygen reduction reaction（ORR） electrocatalytic activity due to the synergetic effect of Co species and N-doping.The Co-N-CNTs also deliver excellent methanol tolerance and superior long-term durability to that of commercial Pt/C,making it a promising ORR electrocatalyst.

详情信息展示

稀有金属(英文版) 2021,40(01),90-95

Co and N co-modified carbon nanotubes as efficient electrocatalyst for oxygen reduction reaction

Ying-Gang Zhu Chao-Qun Shang Zhen-Yu Wang Jian-Qiao Zhang Ming-Yang Yang Hua Cheng Zhou-Guang Lu

Department of Materials Science and Engineering,Southern University of Science and Technology

South China Academy of Advanced Optoelectronics,South China Normal University

作者简介：Chao-Qun Shang e-mail:chengh@sustc.edu.cn;

收稿日期：15 June 2018

基金：financially supported by the National Natural Science Foundation of China (No.21671096 and 21603094);the Natural Science Foundation of Guangdong Province (No.2016A030310376);the Guangdong Special Support for the Science and Technology Leading Young Scientist (No. 2016TQ03C919);the Guangdong Innovative and Entrepreneurial Research Team Program (No.2016ZT06G587);the Basic Research Project of the Science and Technology Innovation Commission of Shenzhen(No.JCYJ20170412153139454 and JCYJ20170817110251498);

Co and N co-modified carbon nanotubes as efficient electrocatalyst for oxygen reduction reaction

Ying-Gang Zhu Chao-Qun Shang Zhen-Yu Wang Jian-Qiao Zhang Ming-Yang Yang Hua Cheng Zhou-Guang Lu

Department of Materials Science and Engineering,Southern University of Science and Technology

South China Academy of Advanced Optoelectronics,South China Normal University

Abstract：

It is a big challenge to prepare non-rare metal and high-activity electrocatalysts for oxygen reduction reaction(ORR).In this paper,a cobalt/carbon nanotubes/chitosan composite gel was synthesized and then annealed under nitrogen atmosphere to yield the cobalt and nitrogen co-modified carbon nanotubes(Co-N-CNTs)nanocomposite electrocatalysts.In this strategy,the cobalt component considerably enhanced the ORR activity and improved the degree of graphitic structure to increase the electronic conductivity.The chitosan served as sustainable source for nitrogen doping.The Co-N-CNTs exhibit excellent oxygen reduction reaction(ORR) electrocatalytic activity due to the synergetic effect of Co species and N-doping.The Co-N-CNTs also deliver excellent methanol tolerance and superior long-term durability to that of commercial Pt/C,making it a promising ORR electrocatalyst.

Keyword：

Co-N-CNTs; N-doping; Long-term durability; Methanol tolerance; ORR;

Received： 15 June 2018

1 Introduction

The electrochemical oxygen reduction reaction (ORR) has received tremendous concerns due to its applications in energy conversion and storage,such as metal-air batteries and fuel cells [ 1, 2, 3, 4, 5, 6, 7, 8] .Serving as the most efficient and widely used electrocatalysts for ORR,Pt-based catalysts exhibit a full four-electron process to generate H₂O [ 9, 10, 11, 12, 13] .However,limited by its high cost,poor durability and limited supply,the large application of Pt is hindered [ 14, 15] .Among numerous alternatives to Pt,nonpreciousmetal/nitrogen-doped carbon hybrid materials,M-N-C,have been emerging as the most promising candidates due to their high electrocatalytic activity [ 16] .

Nowadays,numerous efforts have been devoted to the synthesis of M-N-C catalysts with one-dimensional (1D)carbon nanotubes (CNTs),which is attributed to their excellent electronic conductivity and large surface area.Usually,the precursors of nonprecious-metal and nitrogen co-modified carbon nanotubes (M-N-CNTs) catalysts are rich in N,forming chemical bonds with metal and incorporated into the carbon matrix [ 17, 18] .Besides,N-doping,on the one hand,contributes to the introduction of defects into the carbon structure,ensuring electrocatalytic activity,while on the other hand,it can also improve the possibility that the ORR happens through four-electron pathway in alkaline media [ 19, 20] .Chitosan with natural abundance possesses rich functional groups,such as-OH,and is a promising N source due to its high N content (6.89%) [ 15, 21] .Recently,several investigations about transforming chitosan into carbon-based ORR catalysts have been performed,though the ORR performance is still unsatisfied [ 15, 20, 22] .

Herein,the fabrication of cobalt and nitrogen co-modified carbon nanotubes (Co-N-CNTs) via the modification of chitosan was demonstrated due to its rich functional groups (e.g.,-OH and-NH₂).The as-prepared Co-N-CNTs exhibit a promising ORR performance in the aspect of durability,activity and tolerance to methanol,which can be ascribed to its 1D nanostructure with hierarchical pores,providing fast electron transfer pathway,sufficient ORR active sites and large contact area between the catalyst and the electrolyte.

2 Experimental

Analytically pure chemicals were used directly without further purification.Typically,0.2 g chitosan (Alfa Aesar)was added into 40 ml acetic acid solution (1 wt%) under vigorous stirring.After stirring for 1 h,0.2 g carboxylic acid-functionalized multi-walled CNTs (Aladdin,multiwalled carbon nanotubes,MWCNTs) were added and stirred for another 30 min,followed by the dissolution of0.2 g Co(NO₃)2·6H₂O (Aladdin) into the above suspension,stirring for 48 h.After it was freeze-dried,the final suspension was then pyrolyzed for 2 h at 800℃with a heating rate of 5℃·min^-1 under Ar atmosphere (denoted as Co-N-CNTs).For the sake of comparison,the N-CNTs were prepared in the same routine without using Co salt.The suspension without the addition of MWCNTs was freeze-dried and pyrolyzed in the same condition (denoted as Co-CS).The annealing CNTs were pyrolyzed directly at800℃for 2 h under Ar.

Electrochemical measurements were conducted with a typical three-electrode system (Biologic VMP3 workstation) with a rotating disk electrode (RDE) or rotating ring disk electrode (RRDE,Pine,USA) as the working electrode,an Hg/HgO electrode as the reference electrode and a platinum plate as the counter electrode [ 23] .0.408 mg·cm^-2 electrocatalysts were deposited on the working electrode,and 0.1 mo1·L^-1 KOH solution was used as electrolyte.Either N₂ or O₂ was bubbled into the solution for 30 min prior the testing.The morphology and microstructure of the prepared Co-N-CNTs catalysts were investigated by transmission electron microscope (TEM,FEI-Tecnai F30),scanning transmission electron microscope (STEM,Titan Themis) and field emission scanning electron microscope (FESEM;Tescan-MIRA3) measurements.The phase identification was measured by X-ray diffractometer (XRD;Bruker-D8) and Raman spectroscopy(Jobin-Yavon T6400).X-ray photoelectron spectroscopy(XPS;VG scientific ESCALAB 220iXL) was carried out to evaluate the elemental components near surface region,especially to further explore the chemical states of elemental compositions in Co-N-CNTs.

3 Results and discussion

Figure la shows a typical FESEM image of the Co-N-CNTs after pyrolysis.The nanotubes are interlaced with one another to ensure excellent electron transport properties.As shown in Fig.1b,the nanocarbons still keep tubular morphology after pyrolysis.The high-resolution TEM (HRTEM) image in the inset of Fig.1b exhibits that the carbon layers are crystalline,with the separation of the distinct lattice fringes with an interplanar distance of about0.34 nm,corresponding to the (002) plane of graphite.Elemental mapping results indicate the homogeneous distribution of nitrogen and cobalt throughout the Co-N-CNTs catalyst as depicted in Fig.lc.

Figure 2a exhibits XRD patterns of the annealed CNTs,N-CNTs and Co-N-CNTs,three of which display a common characteristic reflection peak at 26°,corresponding to the characteristic (002) lattice plane of ordered graphitic carbon.As indicated in Fig.2b,the G band associated with graphitic sp²-carbon and D band assigned to disordered carbon structure are located at 1581 and 1324 cm^-1,respectively.The intensity ratio of the D band to the G band (denoted as ID/IG) value of N-CNTs is about 0.55.However,a decreasing value of ID/IG (0.31) is shown in Co-N-CNTs,demonstrating that the introduction of Co during pyrolysis increases the graphitic fraction in the product.The high degree of graphitization of Co-N-CNTs improves the electrical conductivity,which is favorable to enhance the electron transfer during ORR.

The pore distribution and specific surface area of the Co-N-CNTs were investigated by N₂ adsorption/desorption measurements (Fig.2c).The pore size of Co-N-CNTs,as illustrated in the inset of Fig.2c,indicates a wide distribution including mesopores and micropores.The hierarchical pores providing efficient accesses for electrolyte are beneficial for fast transport of species related to ORR,such as 0₂,OH^-and H₂0 in the process of electrocatalysis.

XPS survey spectrum (Fig.2d) confirms the presence of Co,C,N and O,with the elemental components of Co-N-CNTs revealed in Table 1.As for the Co 2p spectrum in Fig.2e,the two distinct Co 2p1/2 and Co 2p3/2 peaks located at 796.5 and 780.3 eV are accompanied by the two satellite peaks (802.3 and 786.2 eV),serving as the typical features of Co²⁺ [ 24] .The high-resolution N 1s spectra(Fig.2f) of Co-N-CNTs are deconvoluted into five different peaks corresponding to oxidized N (403.2 eV),graphitic N (401.5 eV),pyrrolic N (400.9 eV),Co-N,(399.3 eV) and pyridinic N (398.6 eV),respectively [ 25, 26, 27, 28, 29, 30] .Pyridinic N,among these nitrogen species,plays a vital role in forming Co coordinate sites to serve as ORR active centers [ 21, 31] .

Fig.1 a Typical FESEM image of Co-N-CNTs;b representative TEM and corresponding HRTEM image (inset);c STEM image and corresponding elemental mapping images

Fig.2 a XRD patterns of annealed CNTs,N-CNTs and Co-N-CNTs;b Raman patterns of N-CNTs and Co-N-CNTs;c nitrogen adsorption and desorption isotherms of Co-N-CNTs,along with corresponding pore size distribution (inset),where V_abs is absorption volume of N₂ and D is diameter of pores;d XPS spectrum of Co-N-CNTs;high-resolution XPS spectra of e Co 2p and f N 1s in Co-N-CNTs

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Table 1 Elemental contents of Co-N-CNTs from XPS (at%)

Figure 3a shows the linear sweep voltammetry (LSV)tests.It is clear that the Co-N-CNTs,compared to the rest of the samples,possess a higher diffusion limited current density (-3.75 mA·cm^-2) and a more positive onset potential (0.87 V vs.RHE),revealing the improved electrocatalytic performance of Co-N-CNTs for the coexistence of N and Co elements.As depicted in Fig.3b,Co-N-CNTs show an evident cathodic peak at 0.762 V vs.RHE in the O2-saturated solution.The peak current density of Co-N-CNTs (0.72 mA·cm^-2) is much higher than that of Pt/C catalyst (0.45 mA·cm^-2).It is seen from Fig.3c that the limiting currents of Co-N-CNTs increase with the increasing speed (from 400 to 2500 r·min^-1).KouteckyLevich (K-L) plots are calculated to further investigate the electrocatalytic activity of Co-N-CNTs (inset of Fig.3c).The calculated transfer number of Co-N-CNTs,slightly lower than that of Pt/C (3.96),is 3.84,indicating the quasifour-electron process of Co-N-CNTs.The RRDE results of Co-N-CNTs (Fig.3d) show that the ORR of Co-N-CNTs proceeds a 3.73-electron reaction with a hydrogen peroxide yield of 12.4%,further confirming oxygen of Co-N-CNTs catalyzes via a four-electron mechanism.

Fig.3 a LSV curves of all catalysts in 0.1 mo1·L^-1 KOH saturated with O2 at 1600 r·min^-1;b CV profiles of Co-N-CNTs as well as commercial Pt/C in N_2-and O_2-saturated 0.1 mol·L^-1 KOH;c LSV curves of Co-N-CNTs at rotation rates of 400-2500 r·min^-1 and K-L curves of Co-N-CNTs at various potentials (inset),where J is the current density and w is angular rotation speed;d disk and ring current density of Co-N-CNTs in RRDE measurements (rotation speed:1600 r min^-1)

It has been regarded that the ORR performance greatly depends on strong methanol tolerance and durability of the electrocatalyst.Co-N-CNTs,regardless of the presence of methanol,exhibit no obvious change in its cyclic voltammetry (CV) traces,as shown in Fig.4a.However,the Pt/C shows typical large oxidation peaks in a metfhanol-containing KOH solution (Fig.4b),demonstrating the high tolerance to methanol of Co-N-CNTs.Good stability of the Co-N-CNTs can be confirmed by chronoamperometric tests.Co-N-CNTs exhibit good stability with a current retention of 91.5%after 10,000 s,prevailing over that of Pt/C (86.2%),as shown in Fig.4c.At the rotating speed of1600 r·min^-1,displaying neglectable performance loss after 1000 CV cycles,LSV curves of Co-N-CNTs exhibit a little negative shift of only 21 mV in half-wave potential(E_1/2),indicating a good cycling stability (Fig.4d).

Fig.4 CV results of a Co-N-CNTs and b Pt/C in presence of methanol;c I-t responses of Co-N-CNTs and commercial Pt/C in 0.1 mol·L^-KOH saturated with 0₂;d LSV plots of Co-N-CNTs before and after 1000 cycles,as well as 10th,500th and 1000th CV curves (inset)

4 Conclusion

Co-N-CNTs,due to its specific structure and composition,high specific surface area and high graphitization degree during pyrolysis,show excellent catalytic activity.The CoN-CNTs,compared to commercial Pt/C,also display superior methanol tolerance and better long-term durability,making it a potential ORR electrocatalyst.