Carbon Nanotube-Encapsulated Cobalt for High-Efficiency Zinc-Air Flow Battery: Integration of Single Atom Catalysis and Space Confinement

Carbon Nanotube-Encapsulated Cobalt for High-Efficiency Zinc-Air Flow Battery: Integration of Single Atom Catalysis and Space Confinement | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Carbon Nanotube-Encapsulated Cobalt for High-Efficiency Zinc-Air Flow Battery: Integration of Single Atom Catalysis and Space Confinement Jing Zhao, Mufei Liu, Jinqiao Dong, Hongxing Dong, Guiling Wang This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-3892690/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract To meet the practical demand of zinc-air battery cathode noble metal catalyst substitutes are required. Herein, we integrating non-precious single-atom catalysis and space confinement present an effective approach for the large-scale, in-situ growth of CoN 3 -doped carbon nanotubes (CNTs) coated with Co nanoparticles (Co@CoN 3 /CNTs), without adding additional additives. The in-situ grown CNTs serves a dual purpose by acting as a matrix for dispersed atomic CoN 3 sites and providing a space confinement effect on Co nanoparticles, resulting in lower energy barriers and superior mass transport capability. Furthermore, Co 3 C species derived from the Co-based zeolitic imidazolate frameworks (Co-ZIFs) act as catalysts for the direct arrangement of surrounding C-N groups. The resulting Co@CoN 3 /CNTs-800 displays remarkable oxygen reduction reaction (ORR) performance, with a half-wave potential of 0.84 V surpassing that of Pt/C counterparts. Moreover, the rechargeable zinc-air flow battery exhibits a peak power density of 169.5 mW cm -2 and superior recyclability. Physical sciences/Materials science/Materials for energy and catalysis/Electrocatalysis Physical sciences/Chemistry/Materials chemistry/Electronic materials Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction Rechargeable zinc-air energy storage devices have gained significant attention due to the increasing demand for energy resources, attributed to cost-effectiveness, and eco-friendliness. [ 1 – 3 ] The performance of zinc-air storage devices relies on the efficiency of ORR. However, their practical efficiency is often hindered by the sluggish ORR occurring at the air electrodes. Platinum-based materials have proven to be efficient catalysts, accelerating the typically slow four-electron ORR kinetics. [ 4 , 5 ] Regrettably, their high cost, limited availability, and suboptimal durability impede their widespread application. Consequently, there is an urgent need to develop non-noble metal ORR electrocatalysts for metal-air batteries that not only provide competitive pricing but also exhibit high ORR activity and exceptional electrochemical stability. [ 6 , 7 ] Alternatively, carbon materials encapsulated non-noble metal electrocatalysts (M@C) display promising potential due to their distinctive spatial structure and electronic configuration. Such heterostructures provide a competitive electronic configuration, allowing electrons to penetrate from the inner non-noble metal to the outer carbon layer to accelerate the electrocatalytic process. [ 8 ] Moreover, N-doping of the outer carbon layer can effectively increase the density of states near the Fermi energy level, further enabling the acceleration of electrocatalytic process. [ 9 ] Progress has been made in the application of M@C as cathodes for zinc-air batteries cathodes. Some groups tried to confine Co nanoparticles in porous carbon matrix. [ 10 , 11 ] While, traditional amorphous carbon has low conductivity. CNTs serve as effective carriers enabling faster electron conduction to accelerate the four-electron ORR process for metal-air batteries. [ 12 – 14 ] CNTs encapsulated non-noble metal (M@CNTs) catalysts are fabricated through grinding-calcination strategy. [ 12 ] However, the pre-synthesis procedure of CNTs is required, which are more prone to inhomogeneous growth, and involves material loss during transfer. The sacrificial template method is an effective way to grow M@CNTs. [ 15 ] However, the inclusion of the sacrificial template method complicated the process. The key challenge is achieving a low-cost and simple in-situ growth of M@CNTs to further enhance ORR activity. From a different perspective, in ORR processes, involving the breaking and generation of chemical bonds between reactants and products on the surface of electrochemical catalysts, superior ORR properties can be achieved by constructing heterojunctions for lower energy barrier, increasing the density/dispersion of active sites to accelerate kinetic processes, and improving the conductivity of the material for speeding up electron transfer. [ 16 ] Precise composition control of M@CNTs can be achieved through the design of appropriate Metal-organic frameworks (MOF) precursors, [ 17 – 20 ] in which heterojunctions are efficiently constructed. Moreover, the in-situ grown CNTs act as good carriers for loading atomically dispersed active sites and offering a fast pathway for electronic conduction [ 21 – 23 ] . Nevertheless, extreme environment such as high temperature/pressure conditions are usually required for initial MOF preparation. Furthermore, the auxiliary roles of the additional carbon or nitrogen additives are often emphasized, which are unfavorable for large-scale production due to the high cost and unstable reproductivity. Therefore, developing a simple method to achieve the autocatalytic in-situ growth of MOF derived M@CNTs without additional additives for assembling high-performance zinc-air batteries is highly desirable. Herein, we propose a facile method that combines functionalization and in-situ engineering for the large-scale preparation of seaweed-shaped carbon nanotubes. These CNTs simultaneously anchor Co single-atom catalysts (SACs) and encapsulate Co nanoparticles in CoN 3 -doped CNTs through the direct calcination of Co-ZIFs prepared by self-assembly strategy at room temperature and pressure. During the in-situ CNTs growth process the reliance on additional additives such as carbon/nitrogen sources is eliminated. The unique asymmetric CoN 3 moieties in the Co@CoN 3 /CNTs facilitate four-electron transfer in the ORR, outperforming the symmetric planar structure of CoN 4 sites. The resulting Co@CoN 3 /CNTs, with multiple active sites (atomically dispersed CoN 3 and N-doped CNTs structures), in-situ large-scale grown CNTs ensuring accelerated electron and mass transfer, a high degree of graphitization, strong metal-carrier interactions, and the absence of additional additives, contribute to lower energy barriers and superior ORR performance, surpassing that of Pt/C counterparts with a half-wave potential of 0.84 V. Furthermore, primary and reversible zinc-air flow batteries based on Co@CoN 3 /CNTs cathodes exhibit an excellent peak power density of 169.5 mW/cm² and exceptional cycling durability over 700 hours, with the voltage gap only 1.6% larger than the original. Results and discussion Material Design and Structural Characterization The construction of the Co@CoN 3 /CNTs were performed via a simple calcination process of Co-ZIFs precursor. Experimentally, the aqueous solution of Co 2+ ions and nitrogen-rich 2-MIM ligand were firstly prepared. Subsequently, to initiate the coordination of Co 2+ and 2-MIM, two aqueous solutions were directly mixed and Co atoms were encapsulated in the cage of Co-ZIFs. The resulting powder was then pyrolyzed to drive the in-situ growth of Co@CoN 3 /CNTs. 2-MIM ligand were selected as both a bridge to cobalt metal ions and carbon/nitrogen source, allowing the direct integration of the metal and the carbon sources required for CNTs growth in one framework without any additional additives, rather than introducing them in batches. [ 20 ] The in-situ grown CNTs embed Co nanoparticles in the tips of CNTs through strong metal-support interactions enhancing the kinetic process of four-electron ORR and extremely suited to capture metal atoms to load CoN 3 sites. [ 12 , 14 ] The mechanism diagram for the preparation of the Co@CoN 3 /CNT was shown in Fig. 1 . Briefly, during the pyrolytic process, Co-ZIFs could firstly decomposed into Co 3 C as the temperature gradually rises. [ 24 ] Subsequently, the cobalt species served as the catalyst for the direct arrangement of surrounding C-N groups from the ligand of Co-ZIFs precursor to initiate the growth of seaweed-shaped highly ordered gram-scale CoN 3 /CNTs. For the detailed formation mechanism of Co@CoN 3 /CNT, [ 24 , 25 ] to begin with, the surface carbide radicals were first fabricated via thermal decomposition of 2-MIM and dissolved on the surface of the small size Co nanoparticles. Then in the follow-up dissolution and diffusion phase stages, the carbide radicals diffused on the surface or in the bulk phase of Co nanoparticles to form metastable Co3C. After that, as C-N radicals derived from 2-MIM decomposition gradually dissolute and precipitate on the bottom of the metal nanoparticles, it would reach supersaturation and start to form a six-membered ring gradually forming N-doped graphite lamellar structure and then carbon nanotubes after oversaturation of carbon dissolution. Later, in the crystal growth stage, when the interaction between active component and support was relatively weak (the contact angle was acute), metal particles would leave the bottom during the growth process of CNTs and forming the tip growth model. Ultimately, as long as the metal particles have access to C-N source and the C-N radicals can diffuse, the CNTs will grow thus realizing the in-situ autocatalytic growth of CNTs from Co-ZIFs. [ 26 ] When the metal particles were completely encapsulated by the carbon, the CNTs stop growing. Therefore, the cobalt nanoparticles were simultaneously embedded in CoN 3 /CNTs to construct the Co@CoN 3 /CNTs hybrid nanostructures. Importantly, the N-doped graphite lamellar was an ideal matrix for the coordination of Co atoms to form Co-N 3 moieties. Thus, the CoN 3 sites can successfully simultaneously doped in N-doped CNTs walls during the growth of N-doped CNTs to fabricate CoN 3 /CNTs. Besides, this robust and open 3D architecture effectively boosted electrolytes accessibility, electrochemical stability, oxygen species capture and elevated reactant diffusion. The presence of Co-N 3 , pyridinic N, pyrrolic N, graphitic N, oxidized N, and Co nanoparticles provided numerous active sites for capturing and splitting oxygen molecules. Furthermore, the Co nanoparticles encapsulated by CoN 3 /CNTs with space confinement effect can effectively boost electrochemical stability, meanwhile the CoN 3 -doped CNTs layer can promote the electron conduction properties. [ 27 ] Due to the aforementioned well-defined electronic structure, the in-situ grown Co@CoN 3 /CNTs hybrids would present superior ORR properties and excellent stability when exposed to reactive environment. The specific crystal structure information of samples was measured by X-ray diffraction (XRD) measurements. The XRD results of the samples prepared by the precursor at a series of temperatures were shown in Fig. 2 a, and the results indicated that the as-prepared samples were Co/graphitization carbon hybrids and no impurity peaks were observed in the XRD results. Co@CoN 3 /CNTs-600, Co@CoN 3 /CNTs-700, Co@CoN 3 /CNTs-800 and Co@CoN 3 /CNTs-900 samples showed diffraction peaks located at 44.21, 51.52 and 75.85°, which were consistence with (111), (200) and (220) cubic planes of Co (PDF#15–0806). Besides, the diffraction peak situated at 26.51° was indexed to the C (002) plane. [ 8 , 28 ] As calcination temperature increased, the peaks of cobalt in the XRD characterization became sharper, it can be concluded that the crystallinity of cobalt in Co@CoN 3 /CNTs became higher with the increase of calcination temperature. To investigate the mechanism of CNTs growth, he Co-ZIFs precursor were calcined under relatively low temperature (400℃, 450℃ and 500℃) and characterized by XRD (Supplementary Fig. 1) and scanning electron microscopy (SEM) (Supplementary Fig. 2) measurements. XRD results showed that cobalt has not formed yet at around 400℃, and according to the SEM results CoN 3 /CNTs has not appeared at this time. When the calcination temperature was increased to 450℃, cobalt was detected, and at this time SEM results indicated that CoN 3 /CNTs was beginning to be generated, confirming the formation of Co species was a key factor in the preparation of CNTs. Upon pyrolysis at 450℃, the new peaks located at 38.64°, 44.14°,57.99° and 70.06° appeared and fitted well with the (100), (101), (102) and (110) diffraction peaks of Co 3 C crystals planes (PDF#43-1144). The formed Co 3 C could catalyze the adjacent C-N groups originating from the 2-methylimidazole (2-MIM) ligand for the gradually in-situ grown of CoN 3 /CNTs on the near-surface of cobalt nanoparticles. When the temperature risen to 500℃, CoN 3 /CNTs became more numerous and longer. To clarify the influence of H 2 SO 4 corrosion on Co in the catalysts, XRD characterization were also carried on the samples after acid washing. The diffraction peaks of washed Co@CoN 3 /CNTs-800A only presented a slight decrease in the peak intensity, indicating a high retention of cobalt after acid treatment (Fig. 2 a). The morphology of precursor were measured in Fig. 2 b, and more detailed characterization regarding the crystal properties and chemical bond structures of Co-ZIFs precursors were measured (Supplementary Figs. 3 and 4). The Co@CoN 3 /CNTs samples were in-situ directly prepared by calcining Co-ZIFs precursors. Cobalt nitrate (CoNO 3 ) as both the metal sources for Co-ZIFs frameworks and the catalyst precursor for in-situ growth of CoN 3 /CNTs. The in-situ decomposition of 2-methylimidazole (2-MIM) ligand upon heating provided a sufficient localized carbon feeding to promote growing CoN 3 /CNTs. Subsequently, the Co 3 C species served as catalysts for the construction of CoN 3 doped carbon nanotubes (CoN 3 /CNTs) from adjacent C-N groups originating from the 2-methylimidazole (2-MIM) ligand. The abundant Co nanoparticles were uniformly distributed within the nanotubes of the CoN 3 /CNTs, with a higher concentration at the tips, which effectively enhanced electrochemical stability. From the SEM images, a plenty of twisted CoN 3 /CNTs nanostructures were widely observed in the as-prepared Co@CoN 3 /CNTs-800, which has a diameter of approximately 15 nm and a length several micrometers (Fig. 2 c). To investigate the effect of calcination temperature on morphology, a series of Co@CoN 3 /CNTs-600, Co@CoN 3 /CNTs-700, Co@CoN 3 /CNTs-800 and Co@CoN 3 /CNTs-900 samples were prepared by calcining the Co-ZIFs precursors at different temperatures. The corresponding mass ratio of Co(NO 3 ) 2 ·6H 2 O and 2-MIM for the fabrication of Co-ZIFs precursor were chosen as 1:3. The results showed that Co@CoN 3 /CNTs were observed in the aforementioned all series of samples after calcination. As the increase of carbonization temperature in H 2 /Ar atmosphere, the SEM results demonstrated a gradually increase trend in the number of the prepared CoN 3 /CNT (Supplementary Fig. 5). However, the agglomeration was detected when the temperature increased to 900℃, which might be caused by the excessive calcination temperature. In view of the above analysis, the SEM results indicated that the catalytic effect might have a limiting value through temperature regulation. The transmission electron microscopy (TEM) images of a typical product Co@CoN 3 /CNTs-800 using Co 2+ as the metal precursor were shown in Fig. 2 e, which further proved large number of CoN 3 /CNTs were in-situ grown, and numerous tiny metallic Co nanoparticles were homogeneously wrapped at the top of CoN 3 /CNTs matrix. The growth of CoN 3 /CNTs was catalyzed by the Co nanoparticles, and the mechanism has been unveiled. [ 15 ] According to (Fig. 2 f and 2 g), the high-resolution TEM (HR-TEM) image of Co@CoN 3 /CNTs-800 displayed seaweed-shaped CoN 3 /CNTs, this unique morphology can be attributed to the diffusion and continuous accumulation of carbon atoms at metal/graphene interface. [ 29 ] Hence an asymmetric distribution of carbon atoms occurred around cobalt species. Specifically, most of the carbon atoms were selected for nucleation at the bottom or edge of the deformed cobalt species catalytic centers for inducing graphitization, resulting in the formation of seaweed-like morphology in the outer layers. [ 30 , 31 ] The lattice fringes of 0.41 nm can be allocated to the (111) interplanar distance space of the (002) plane of the CoN 3 /doped graphene layer (Fig. 2 f). [ 8 , 28 ] The number of layers of multi-walled CoN 3 /CNTs coated with metallic Co was about 8, which can further effectively promoted the cyclic stability of cobalt nanoparticles in electrochemical tests. The HR-TEM results further proved Co nanoparticles about 5 nm was capsulated at the top of CoN 3 /CNTs. Importantly, the lattice spacing of the Co species was clearly observed at the wall of the CoN 3 /CNTs, and their lattice were measured to be approximately 0.20 nm assigned to the (111) cubic Co crystal phase (Fig. 2 g), this hinted the formation of CoN 3 sites at the wall of the CoN 3 /CNTs. It can be speculated that the as-synthesized composite was composed of graphitized carbon and cobalt, which was consistent with the XRD characterization results. As expected, the elemental mapping analysis (EDX) revealed the elements of C, N, Co, and O are detected (Fig. 2 h and Supplementary Fig. 6). The bright spots represented metallic Co species with high abundance. In addition, the EDX element mapping results further proved the abundant N element was uniformly distributed throughout the CNTs in Co@CoN 3 /CNTs-800 at such high magnification, and the Co element was mainly distributed in the tips of the tube parts of CoN 3 /CNTs. Importantly, The Co and N elements were highly overlapped, which indicated the formation of atomically dispersed CoN 3 moieties in Co@CoN 3 /CNTs-800. To further characterize the protective effect of graphene layers in CoN 3 /CNTs on the encapsulated Co nanoparticles, the as-prepared Co@CoN 3 /CNTs-800 were then leached in 2 M H 2 SO 4 for 1 day. Such protective wrapping of CoN 3 /CNTs would be profited to electro-catalytic stability. The final sample after acid leaching was named as Co@CoN 3 /CNTs-800A, which was further confirmed by the SEM (Supplementary Fig. 2d) and TEM measurement. It should be noted that large amount of metal nanoparticles completely protected by the seaweed-like N-CNTs still retained in Co@CoN 3 /CNTs-800A after acid treatment, in comparison with the product of Co@CoN 3 /CNTs-800 without acid treatment (Fig. 2 i). Therefore, Co nanoparticles with space confinement effect can be stable even with long-term acid dip. In addition, the thickness of the carbon nanotubes coated with metallic Co was about 2.5 nm, CoN 3 /CNTs were multi-wall CoN 3 /CNTs, and the Co nanoparticles were wrapped by multi-wall CoN 3 /CNTs, which can be clearly observed by HR-TEM (Fig. 2 j and 2 k). To further test the distribution of metal sites after acid washing, the corresponding EDX measurements were also carry out on Co@CoN 3 /CNTs-800A. High abundance of the C, N, Co, and O elements were detected (Fig. 2 l and Supplementary Fig. 7). Nitrogen atoms overlapped well with in-situ grown CNTs, indicating the uniform distribution of nitrogen in the seaweed-like Co@CoN 3 /CNTs-800A. Most of the cobalt nanoparticles were still retained after acid washing, which dispersed through the entire architecture of Co@CoN 3 /CNTs-800A, especially on the tips of the CoN 3 /CNTs. Moreover, the cobalt element overlapped well with nitrogen element, which reflected the stable presence of CoN 3 active sites on the walls of in-situ growth CoN 3 /CNTs after acid washing. To further investigate the impact of precursors with different morphologies on the calcined samples, the adding ratio of Co(NO 3 ) 2 ·6H 2 O and 2-MIM has been adjusted and 800℃ has been chosen as the calcination temperature. SEM characterization showed that the morphology of the precursor was indeed significantly changed by varying the mass ratio of Co 2+ and 2-MIM. Unlike the bamboo leaf-like morphology of Co-ZIFs, Co-ZIFs(1:2) exhibited a rod-like morphology while Co-ZIFs(1:4) exhibited a cross-like morphology (Supplementary Fig. 8). Furthermore, after calcining Co-ZIFs(1:2) and Co-ZIFs(1:4) under 800 ℃, Co@CoN 3 /CNTs(1:2)-800 and Co@CoN 3 /CNTs (1:4)-800 were successfully prepared. The results showed that both calcined samples successfully grew carbon nanotubes (Supplementary Fig. 9). However, a noticeable agglomeration of large cobalt balls (about 200 nm) were observed in Co@CoN 3 /CNTs(1:2)-800. The crystal structures of Co@CoN 3 /CNTs(1:2)-800 and Co@CoN 3 /CNTs(1:4)-800 were also measured (Supplementary Fig. 10). According to the Raman spectra (Supplementary Fig. 11), the intensity ratios of I D /I G value of Co@CoN 3 /CNTs-600, Co@CoN 3 /CNTs-700, Co@CoN 3 /CNTs-800 and Co@CoN 3 /CNTs-900 samples were calculated to be 1.078, 0.9482, 0.8724 and 0.7838. the I D /I G values of Co@CoN 3 /CNTs proved the increasing I D /I G values of the N-CNTs as the temperature rose from 600°C to 900°C, displaying a higher degree of graphitization. Moreover, the intensity ratios of I D /I G value of Co@CoN 3 /CNTs-800 and Co@CoN 3 /CNTs-800A samples were calculated to be 0.8724 and 0.8300, respectively. Such a close value further implied that the as-prepared carbon nanotube skeleton was stable and the long-time acid treatment has not changed carbon structure. X-ray photoelectron spectroscopy (XPS) measurement were performed to characterize the chemical states in the surface of the Co@CoN 3 -CNTs composites. The existence of Co, N, C and O elements can be verified, and the detailed atomic content of the Co, O, C and N elements were listed (Supplementary Table 1 ) . Figure 3 a confirmed the existence of CoN 3 species and Co metal. The two peaks of Co@CoN 3 /CNTs-800 appearing at 779.0 and 795.0 eV were assigned to Co 2p 1/2 and Co 2p 3/2 of Co metal, respectively. Furthermore, two peaks of Co-N 3 and two weak shake-up satellites corresponding to Co 2p 3/2 and Co 2p 1/2 were deconvoluted. [ 32 ] This confirmed partial Co atoms were coordinated with the surrounding nitrogen atoms to form the CoN 3 species in Co@CoN 3 /CNTs samples. After acid treatment, cobalt content of Co@CoN 3 /CNTs-800A sample was 1.05% respectively, indicating that there were still plenty of cobalt spices remained on the surface of the sample even after prolonged concentrated acid treatment. The fitted peaks of high-resolution N 1s ( Fig. 3 b ) , located at 398.7, 399.5 eV, 400.7, 401.7 eV, and 403.1 eV were assigned to the pyridinic N, Co-N 3 , pyrrolic N, graphitic N and oxidized N, respectively. [ 18 , 27 ] Furthermore, the content of different N species of Co@CoN 3 /CNTs materials were calculated (Supplementary Table 2 ) . According to Fig. 3 c, the content of graphitic N in Co@CoN 3 /CNTs samples significantly increased as the pyrolysis temperature rose, indicating an obvious change of N types in the carbon matrix via controlling pyrolysis temperature. Co coated by graphitic N doped carbon possessed more negative charge than pyridinic-N doping, leading to lower energy barriers for ORR catalysis. [ 8 ] It is reported that CoN 3 sites served as the active sites in ORR. Although the content of CoN 3 decreased with increasing temperature, the CoN 3 content still retained at high levels in these samples. The decrease in CoN 3 content may be attributed to the decreasing trend of pyridinic N with increasing temperature as the pyridinic N can act as an anchor to grab metal atoms to form CoN 3 sites. [ 33 – 36 ] Moreover, oxidized N species appeared when the calcination temperature reached 800℃, and the oxidized N content in Co@CoN 3 /CNTs increased with rising of calcination temperature. With the increase of calcination temperature, the N 1s peak of Co@CoN 3 /CNTs shifted toward the low binding energy direction, suggesting that the increase in temperature led to electron redistribution at the heterogeneous interface of Co@CoN 3 /CNTs. The electrons transferred from the encapsulated Co cobalt nanoparticles the N-doped CNTs promoted electron penetration from the encapsulated Co cobalt nanoparticles to the N-doped CNTs surface, resulting in better transfer paths and lower energy barrier for ORR catalytic performance. [ 8 , 37 ] The C 1s XPS spectrum of Co@CoN 3 /CNTs-800 sample (Supplementary Fig. 12a) exhibited three peaks at binding energies of 284.8 285.8 and 286.7 eV, assigning to the C − C, C − N and C = O bonds, respectively. [ 32 , 38 ] Particularly, the C − N peak in C 1s spectrum further verified the N doping into the carbon matrix. Co@CoN 3 /CNTs-800 showed high N atomic content of (4.16%), which contributed to the formation of Co-N 3 sites. Meanwhile, the O element can be deconvoluted in three existing states, corresponding to O-C = O (533.7 eV), C = O (532.0 eV), and metallic O (530.2 eV) oxidation states for the Co@CoN 3 /CNTs-800 sample (Supplementary Fig. 12b). It is noteworthy that the metallic O peak disappeared for the acid-treated Co@CoN 3 /CNTs-800A sample, indicating that the acid mainly etched away the oxidized cobalt, which might cause a slight decrease in ORR performance. XPS spectrum further confirmed the chemical formation and coordination environment of Co@CoN 3 /CNTs(1:2)-800 and Co@CoN 3 /CNTs(1:4)-800 (Supplementary Fig. 13). The Co, N, C and O elements were detected (Supplementary Table 1). According to the Co 2p spectra, Co@CoN 3 /CNTs(1:2)-800 obviously contained more cobalt monomers than Co@CoN 3 /CNTs(1:4)-800. Synchrotron X-ray absorption spectroscopy was employed to characterize the coordination environment of Co atoms in Co@CoN 3 /CNTs-800. Co K-edge X-ray absorption near-edge structure (XANES) spectra in Fig. 3 d illustrated that the absorption edge of Co@CoN 3 /CNTs-800 was located between that of Co foil and CoPc, suggesting an average oxidation state of + 1.70 (Supplementary Fig. 14). Fourier-transformed (FT) k2 -weighted extended X-ray absorption fine structure (EXAFS) curve (Supplementary Fig. 15) and EXAFS fitting spectrum in Fig. 3 e demonstrated the Co-N paths at 1.54 Å and the Co-Co path at 2.21 Å in R space, displaying the exist of Co SACs and Co-Co clusters in Co@CoN 3 /CNTs-800, which was consistent with the TEM and XPS results. [ 8 ] EXAFS fitting spectrum in Fig. 3 f further illustrated the Co-N pathway has a coordination number of about 2.7 and a bond length of about 1.90 Å, showing the formation of an Co-N 3 the Co single atoms coordinated with three N atoms (CoN 3 ) rather than CoN 3 C. [ 39 ] Interestingly, differ from the symmetric planar structure of CoN 4 , the unique CoN 3 coordination with an asymmetric active center were more conducive to the four-electron transfer ORR process. Besides, the Co-Co coordination number was approximately 4.9 with a bond length of 2.491 Å, which was much lower than the Co-Co coordination number of Co foil indicating the size of the sample Co particle was fairly small. Detailed Co-EXAFS fitting results of Co@CoN 3 /CNTs-800 were presented (Supplementary Table 3). Figure 3 g showed the wavelet transform (WT) of EXAFS for Co@CoN 3 /CNTs-800 sample, which exhibited two characteristic centers of Co-N path (~ 1.54 Å) and Co-Co path (~ 2.21 Å). [ 9 ] Based on the aforementioned analysis, the coordination environments of Co@CoN 3 /CNTs-800 were measured, of which the atomic structure model composed of single-atom dispersed CoN 3 and Co-Co clusters. Catalytic Properties for the oxygen reduction reaction The ORR performance was firstly investigated in O2-saturated 0.1 M KOH alkaline electrolyte and the commercial Pt/C was used as the ORR benchmarks. The polarization curves of Co@CoN 3 /CNTs-800 (Fig. 4 a) showed superior ORR performance with the half-potential (E 1/2 ) of 0.84 V and onset potential (E onset ) of 0.92 V, which were comparable or even surpass Pt/C catalyst (E 1/2 = 0.83 V, E onset = 0.97 V) and other reported catalysts (Supplementary Table 4). According to the Fig. 4 b, the kinetic current densities (J k ) of Co@CoN 3 /CNTs-800 at 0.80 and 0.84 V determined by the Koutecky-Levich (K-L) equation were calculated to be 16.35 and 5.24 mA/cm 2 , which were 1.31 and 1.02 times superior than Pt/C (12.49 and 5.15 mA cm 2 ) respectively. Furthemore, Co@CoN 3 /CNTs-800 displayed a much better ORR activity than Co@CoN 3 /CNTs-600 (E 1/2 = 0.79 V, E onset = 0.91 V, J k0.80V = 3.87 mA/cm 2 , J k0.84V = 1.47 mA/cm 2 ), Co@CoN 3 /CNTs-700 (E 1/2 = 0.82 V, E onset = 0.92 V, J k0.80V = 7.62 mA/cm 2 , J k0.84V = 2.87 mA/cm 2 ) and Co@CoN 3 /CNTs-900 (E 1/2 = 0.82 V, E onset = 0.90 V, J k0.80V = 7.11 mA/cm 2 , J k0.84V = 2.27 mA/cm 2 ) (Supplementary Table 5). Moreover, The Co@CoN 3 /CNTs-800 also performed lowest Tafel slope of 45.2 mV dec − 1 smaller than Pt/C (73.4 mV dec − 1 ), Co@CoN 3 /CNTs-600 (67.6 mV dec − 1 ), Co@CoN 3 /CNTs-700 (59.4 mV dec − 1 ) and Co@CoN 3 /CNTs-900 (47.5 mV dec − 1 ) in Fig. 4 c. The electrocatalytic kinetics of Co@CoN 3 /CNTs-800 was further evaluated by glassy carbon rotating disk electrode (RDE) measurements with the rotating speeds ranged from 400 to 2500 rpm (Fig. 4 d). As illustrated in Fig. 4 e, and 4 f, the fitting Koutecky-Levich (K-L) plots of Co@CoN 3 /CNTs-800 was mainly a four-electronic ORR pathway for the similar electron transfer number (n) at different potentials of 3.9. n value of Co@CoN 3 /CNTs-800 was higher than those of Co@CoN 3 /CNTs-600 (3.6), Co@CoN 3 /CNTs-700 (3.1), Co@CoN 3 /CNTs-900 (3.2) (Supplementary Fig. 16). ORR performance of Co@CoN 3 /CNTs produced a trend of increasing and then decreasing with increasing temperature, which can be attributed to higher temperature leading to the graphitization of CoN 3 /CNTs, while excessive temperature can lead to the agglomeration of cobalt nanoparticles. Furthermore, the chronoamperometric response (CA) measurement were conducted to test the durability of Co@CoN 3 /CNTs-800, Fig. 4 g showed the activity decay of Co@CoN 3 /CNTs-800 before and after a 6-hour CA response measurement during ORR. A negligible decay of E 1/2 shifted negatively by 6 mV, illustrating Co@CoN 3 /CNTs-800 possessed outstanding stability. Correspondingly, the structures of OOH*, O* and OH* intermediated on Co@CoN 3 /CNTs-800 during the ORR process were shown in Fig. 4 j. The asymmetric CoN 3 sites distributed on in-situ grown CNTs can efficiently reduce the ORR barrier and accelerate the electrocatalytic kinetics. The encapsulated abundant Co nanoparticles facilitated the kinetic process of four-electron oxygen reduction, and the CNTs components ensured outstanding mass transport capability. The cyclic voltammetry (CV) image profile after the stability test matched well with the pre-test, which indicated that the Co@CoN 3 /CNTs-800 material has high stability (Supplementary Fig. 17). To further prove the outstanding stability of Co@CoN 3 /CNTs-800, the ORR performance of acid treated sample Co@CoN 3 /CNTs-800A were tested (Supplementary Fig. 18 ) . We can conclude that after H 2 SO 4 treatment the Co@N-CNTs-800-A still retains the high E 1/2 value of 0.82 V, E onset value of 0.90 V, J k0.80V value of 7.54 mA/cm 2 , J k0.84V value of 1.93 mA/cm 2 and n value of 3.5. The reason for the decrease in catalyst ORR performance after acid treatment can be speculated to be although CoN 3 /CNT was protective of the Co embedded in it, the acid can wash out part of Co nanoparticles, leading to the reduction of space confinement effect on the ORR performance. To get a further insight into the intrinsic catalytic activity, the electrochemically active surface area (ECSA) of Co@CoN 3 /CNTs-800 was characterized via the CV tests with the non-Faradic voltage range of 1.07–1.16 V vs RHE with various scan rates (Supplementary Fig. 19). The as-calculated specific electrical double-layer capacitances (C dl ) of Co@CoN 3 /CNTs-800 was 13.38 mF/cm 2 which was higher than that of Co@CoN 3 /CNTs-600 (2.93 mF/cm 2 ), Co@CoN 3 /CNTs-700 (8.61 mF/cm 2 ) and Co@CoN 3 /CNTs-900 (4.01 mF/cm 2 ), respectively. Therefore, the highest ECSA of Co@CoN 3 /CNTs-800 revealed fastest reaction kinetics, largest electrochemical surface area and active sites. Furthermore, ORR performance was investigated under oxygen and nitrogen condition. The CV results of Co@CoN 3 /CNTs-800 presented an obvious cathodic peak appearing at about 0.83 V (vs. RHE) (Supplementary Fig. 20). ORR properties of precursors with different morphologies calcinated under 800℃ with different morphologies were explored (Supplementary Figs. 21 and 22 ) . The ORR activity of Co@CoN 3 /CNTs-800 was better than Co@CoN 3 /CNTs(1:2)-800 (E 0 = 0.967 V, E 1/2 =0.832 V, J, J k0.80V = 8.52 mA/cm 2 , J k0.84V = 2.51 mA/cm 2 ) and Co@CoN 3 /CNTs(1:4)-800 (E 0 = 0.967 V, E 1/2 =0.832 V, J k0.80V = 1.14 mA/cm 2 , J k0.84V = 0.33 mA/cm 2 ). The detailed information of ORR properties in this work were integrated (Supplementary Table 3). Therefore, the Co(NO 3 ) 2 : 2-MIM feeding ratio of 1:3 was an optimal solution. To further reveal the kinetic and the dynamic evolution properties of Co@CoN 3 /CNTs-800 electrode/electrolyte interfaces the in-situ electrochemical impedance spectroscopy (EIS) tests were performed (Fig. 4 h) in an O 2 -saturated 0.1 M KOH solution with a potential range of 1 to 0.4 V versus RHE recorded at constant potential with the frequency range of 0.01 Hz-100 KHz fitted based on equivalent circuit model (Supplementary Fig. 23 and Table 6). The ORR process of Co@CoN 3 /CNTs-800 can be divided into three sections, the kinetic-controlled section (at ≈ 1.05–0.9 V), the mixed-controlled section (at ≈ 0.8 − 0.6 V), and the diffusion-controlled section (at ≈ 0.5-0 V). Figure 4 i illustrated the effect of voltage on charge transfer resistance (R ct ), and solution resistance (R s ). Obviously, the R ct values were large when the potentials were higher than E onset , which elucidated that the charge transfer between the electrode and the reaction interface was weak. When potential started to fall below E onset , the R ct value undergone a sharp decrease, which indicated that a direct reduction reaction occurred at the electrode surface. [ 40 ] Whereas, when the potential continued to decrease to the diffusion control region, the R ct value increased for the formation and desorption at the electrode surface of H 2 O product. Zinc-Air Battery Performance The abundant asymmetric atomic CoN 3 sites, in-situ growth of CNTs matrix, a significant amount of confined Co nanoparticles and outstanding ORR catalytic activity of the Co@CoN 3 /CNTs-800 indicated that Co@CoN 3 /CNTs was ideal catalyst for the rechargeable zinc-air cell. Compared to static electrolyte in conventional zinc-air battery, rechargeable zinc-air flow battery has a flowing electrolyte for achieving higher cycle stability, and this flow electrolyte configuration can not only significantly inhibited the Zn electrode dendrite growth, but also effectively flushed away the unwanted byproducts generated during the charge/discharge process. [ 11 ] Therefore, the rechargeable zinc-air flow battery was constructed for evaluate practical application effect of the Co@CoN 3 /CNTs-800 as the air cathode catalyst and zinc foil as the anode (Fig. 5 a). The open circuit voltage was assessed in Fig. 5 b and 5 d, and the assembled Co@CoN 3 /CNTs-800 based zinc-air flow cell displayed a higher open circuit voltage of 1.44 V. Interestingly, it can continuous and steady powered a LED light (insert in Fig. 5 b), and powered a small fan (Supplementary Movie 1) showing its practical application. Co@CoN 3 /CNTs-800 based zinc-air flow battery demonstrated impressive discharge rate performance, the discharge plateau declined slowly with the current density from 1 to 50 mA cm − 2 (Fig. 5 c). When the current density reduced to 1 mA cm − 2 , discharge plateau resumed reversibly, indicating outstanding reversibility. Figure 5 e showed polarization and power density curves of Co@CoN 3 /CNTs-800. Co@CoN 3 /CNTs-800 catalyst showed an outstanding peak power density as high as 169.5 mW/cm² at the discharge density of 205.0 mA cm − 2 . This assembled zinc-air flow cell exhibited a much higher power density compared to that of Pt/C. Furthermore, according to Fig. 5 f, the Co@CoN 3 /CNTs-800 based zinc-air flow cell also displayed a much higher maximum power density compared to that of many previous works, manifesting its outstanding suitability for the application in electrocatalysts. Furthermore, the cycling performance of the zinc-air flow battery of Co@CoN 3 -CNTs-800 was also measured ( Fig. 5 g ) . Obviously, the rechargeable battery of Co@CoN 3 /CNTs-800 has displayed long lifetime (700 hours) than those of zinc-air flow battery assembled by Pt/C during discharging/charging cycling, illustrating its excellent long-term durability. Importantly, the assembled rechargeable battery delivered a stabled voltage gap, and after 700 hours the voltage gap only 1.6% larger than the original, indicating an outstanding charge-discharge performance. Moreover, the stability of Co@CoN 3 /CNTs-800 based zinc-air flow battery in this work were compared with other recently reported results (Supplementary Table 7). This excellent cycling stability was related to the unique structure of Co nanoparticles protected and confined by the in-situ grown CoN 3 /CNTs. Conclusion In summary, we have successfully implemented a comprehensive approach that integrates functionalization and in-situ engineering for the large-scale preparation of Co SACs. This method involves the simultaneous in-situ fabrication of seaweed-shaped carbon nanotubes and the encapsulation of cobalt nanoparticles, introducing a space confinement effect. The uniqueness of our approach is evident in two crucial aspects. Firstly, distinct from the symmetric planar structure observed in CoN 4 sites, the exclusive asymmetry of CoN 3 sites plays a pivotal role in enabling a four-electron transfer process during the oxygen reduction reaction. Secondly, the in-situ growth of N-doped carbon nanotubes is not only straightforward and free from any supplementary additives but also remarkably efficient in capturing metal atoms. The Co@CoN 3 /CNTs-800 sample prepared using this method demonstrates outstanding ORR activity in alkaline electrolyte, exhibiting remarkably ORR performance with a half-wave potential of 0.84 V, surpassing that of Pt/C counterparts. Furthermore, it shows distinguished stability, with a negligible decay of E 1/2 (6 mV) after a 6-hour chronoamperometric response measurement. Notably, the constructed zinc-air flow battery, utilizing the Co@CoN 3 /CNTs-800 cathode, achieves a peak power density as high as 169.5 mW/cm² and exceptional recyclability, with only a 1.6% larger voltage gap than the original after 700 hours of testing. This work opens a window for understanding the significance of CNTs matrix in metal-nitrogen-carbon catalysts, and may regulating the scale-up future tailor of heterostructure for metal-air batteries. Methods Materials Cobalt nitrate hexahydrate and 2-methylimidazole were purchased from Chengdu Organic Chemicals Co., China. Sulfuric acid (H 2 SO 4 , AR) was supplied by Chron Chemicals. Nafion solution (5 wt%) was bought from Sigma-Aldrich. Commercial Pt/C (20 wt%) catalyst was purchased from Aladdin. Deionized water was prepared by laboratory water purification system (HHitech Master-S30UVF). Synthesis of Co-ZIFs sample In a typical procedure, 0.6 g of Co(NO 3 ) 2 ⋅6H 2 O and 1.8 g of 2-methylimidazole (2-MIM) were dissolved in deionized water and stirred for 30 min, respectively. Subsequently, the two solutions were mixed quickly and aged for 20 h, and the as-synthesized samples were denoted as Co-ZIFs. Furthermore, using deionized water as solvent, Co(NO 3 ) 2 ⋅6H 2 O and 2-MIM were mixed with the mass ratio of 1:2 and 1:4, and the as-obtained samples were denoted as Co-ZIFs(1:2) and Co-ZIFs(1:4). Synthesis of Co@CoN 3 /CNTs electrocatalyst. The obtained Co-ZIFs sample was placed in an H 2 /Ar atmosphere and further pyrolyzed at different temperatures (600℃, 700℃, 800℃, and 900℃) to fabricate Co@CoN 3 /CNTs-600, Co@CoN 3 /CNTs-700, Co@CoN 3 /CNTs-800 and Co@CoN 3 /CNTs-900 samples, respectively. To verify the stability of CNTs-coated Co nanoparticles under strong acid conditions the Co@CoN 3 /CNTs-800A sample were prepared via further washing the Co@CoN 3 /CNTs-800 samples with 2 M H 2 SO 4 solution for 12 h. To investigate the effect of different precursors on the calcined samples, we chose 800℃ as the calcination temperature. The Co@CoN 3 /CNTs(1:2)-800 and Co@CoN 3 /CNTs(1:4)-800 sample were prepared by calcining Co-ZIFs(1:2) and Co-ZIFs(1:4) precursor at 800℃ under H 2 /Ar atmosphere, respectively. Electrochemical measurements ORR electrochemical measurements were carried on Autolab electrochemical workstation (PGSTAT 302N) using the glassy carbon rotating disk electrode (Ф = 5 mm) as working electrode, Ag/AgCl electrode as the reference electrode and the platinum as counter electrode. For preparation of the catalyst slurry, 5.0 mg of resultant electrocatalyst was dispersed in the 750 µl DI water 250 µl ethanol and 50 mL Nafion solution (5.0 wt%) and ultrasonicated. Subsequently, 12 µL of slurry was dropped onto the polished RDE. The linear sweep voltammetry (LSV) measurements were performed with 5 mV s − 1 . Moreover, the rotation speeds of RDE chose 400, 625, 900, 1225, 1600, 2050 and 2500 rpm. To avoid the affecting of the background capacitive current, the LSV measurement was conducted under the same conditions under an N2-saturated electrolyte condition. The electron transfer number of ORR process was calculated by the Koutecky-Levich (K-L) equation: $${\text{J}}^{-1}={{\text{J}}_{\text{K}}}^{-1}{+\left(\text{B}{{\omega }}^{\frac{1}{2}}\right)}^{-1}$$ $$\text{B}=0.62n\text{F}{\left({\text{D}}_{0}\right)}^{\frac{2}{3}}{\left({\nu }\right)}^{-\frac{1}{6}} {\text{C}}_{0}$$ $${\text{J}}_{\text{K}}=n\text{F}{\text{C}}_{0}$$ where J is the measured current density during ORR, J k and J L are the kinetic current density and diffusion limiting current density, respectively. ω represents the electrode rotating angular velocity, B is the slope of K-L plots, n is the electron transfer number per oxygen molecule, F represents the Faraday constant (F = 96485 C mol − 1 ), D 0 is the diffusion coefficient of O 2 in 0.1 M KOH (1.9 × 10 − 5 cm 2 s − 1 ), ν is the kinetic viscosity (0.01 cm 2 s − 1 ), C 0 is the bulk concentration of O 2 (1.2 × 10 − 3 mol L − 1 ). The in-situ EIS spectra were recorded under an AC amplitude of 10 mV at a frequency of 0.01 Hz-100 KHz under various electrochemical potentials in an O 2 -saturated 0.1 M KOH with potential range of 1 to 0.4 V versus RHE. EIS spectra were obtained using a VMP3 multichannel electrochemical station (VMP3). The EIS spectra were fitted to the equivalent circuit using commercial software (ZView, Scribner Associates Inc). The zinc-air flow battery tests were tested using the flow cell configuration with electrolyte flow rate of 6 mL min − 1 , and O 2 involved comes from air atmosphere. The gas diffusion electrode (GDE) coated with Co@CoN 3 /CNTs-800 catalyst was the air cathode, and Zn plate was anode. Furthermore, the electrolyte of liquid rechargeable zinc-air flow battery was composed of 6 M KOH and 0.20 M Zn(CH 3 COO) 2 . The catalyst layer was obtained by mixing the Co@CoN 3 /CNTs-800 catalyst (30 mg) and acetylene black (5 mg) in 5 mL of ethanol solution, followed by drop-casting the polytetrafluoroethylene (PTFE) emulsion (60 wt%, 22 µL). After mixing for 1 h and drying at 25°C for 12 h, the catalytic layer obtained was cut into circles with an effective surface area of 1 cm 2 . For comparison, the commercial Pt/C catalyst were assembled as air cathode of the zinc-air flow battery. The polarization curves were measured on the IVIUM electrochemical workstation (IVIUM TECHNOLOGIES BV). The charge/discharge tests were carried out on NEWARE battery testers (CT-3008 W) instrument at 5 mA cm − 2 , and one charging/discharging cycle time of was 10 min. Characterizations X-ray diffraction (XRD) was carried out by Rigaku TTR III with Cu-Kα radiation with λ = 1.5418 Å. Raman spectra were obtained by Jobin-Yvon HR800 with the 532 nm excitation line of an Ar ion laser. X-ray photoelectron spectroscopy (XPS) measurements were performed on ESCALAB XI+. Fourier Transform infrared spectroscopy (FT-IR) measurements were measured with a Thermo NICOLET iS50 FT-IR. Transmission electron microscopy (TEM) images and energy dispersive spectrum (EDS) mapping were acquired on TALOS G2 F200X. The X-ray absorption spectra (XAS) including X-ray absorption near-edge structure (XANES)and extended X-ray absorption fine stucture (EXAFS) of the sample at Ni K-edge was colleted at the Beamline of TLS07A1 in National Synchrotron Radiation Research Center (NSRRC), Taiwan. Declarations Data availability The data supporting the findings are available within the article and its Supplementary Information files. Supplementary Notes, Figs. 1 –23, Tables 1–7, and refs. 1–17. All the data reported in this work are available from the authors on request. Contributions Jing Zhao designed the study. Mufei Liu performed the electrochemical measurements. Mufei Liu and Guiling Wang. performed the measurements. Mufei Liu and Hongxing Dong discussed the Results. Mufei Liu, Jing Zhao, and Jinqiao Dong co-wrote the paper with contributions from all authors. Competing interests The authors declare no competing interests. Acknowledgements This work was supported by the Fundamental Research Funds for the Central Universities (No. 3072022QBZ1006) and the Natural Science Foundation of Heilongjiang Province jointly guided project (No. LH2021B007) References Li L et al (2021) Stretchable Energy Storage Devices Based on Carbon Material Small 17:2005015 Sumboja A et al (2017) All-Solid-State, Foldable, and Rechargeable Zn-Air Batteries Based on Manganese Oxide Grown on Graphene-Coated Carbon Cloth Air Cathode. Adv Energy Mater 7:1700927 Wu K et al (2020) An Iron-Decorated Carbon Aerogel for Rechargeable Flow and Flexible Zn-Air Batteries. 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Appl Catal B-Environ 316:121674 Sabhapathy P (2023) Axial Chlorine Induced Electron Delocalization in Atomically Dispersed FeN 4 Electrocatalyst for Oxygen Reduction Reaction with Improved Hydrogen Peroxide Tolerance. Small 19:2303598 Additional Declarations There is NO Competing Interest. Supplementary Files SI.docx MovieS1.mov video of Co@CoN3-CNTs-800 based zinc-air flow battery continuous and steady powered a small fan. Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-3892690","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":270098586,"identity":"93a3cae5-cae1-4d13-95c6-330978fb7fe0","order_by":0,"name":"Jing Zhao","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAs0lEQVRIiWNgGAWjYDACCSBibGCQY2NvP0CsFmawFmM+njMJpGlJnCfhYECcDvnZ/Qdv/txxJ71NgiGB4UfFNsJaGOccZraQPPMst0268QBjz5nbhLUwSySzSRi2Hc5tkzmQwMzYRoQWNpCWxLbD6WwSCQbEaeEBaTnYdjiBeC0SEsnGlo1thw3bgIF8kCi/yM9IfHjzZ9thefn29oMPflQQoQUFHCBR/SgYBaNgFIwCXAAAYWw5t/x29l0AAAAASUVORK5CYII=","orcid":"https://orcid.org/0000-0002-8951-6462","institution":"Harbin Engineering University","correspondingAuthor":true,"prefix":"","firstName":"Jing","middleName":"","lastName":"Zhao","suffix":""},{"id":270098587,"identity":"df4478a0-e681-401b-a5d3-c7de0b4fb305","order_by":1,"name":"Mufei Liu","email":"","orcid":"","institution":"Harbin Engineering University","correspondingAuthor":false,"prefix":"","firstName":"Mufei","middleName":"","lastName":"Liu","suffix":""},{"id":270098588,"identity":"78da9453-b3ce-4f3e-b87a-b3ab91456b7f","order_by":2,"name":"Jinqiao Dong","email":"","orcid":"","institution":"Shanghai Jiao Tong University","correspondingAuthor":false,"prefix":"","firstName":"Jinqiao","middleName":"","lastName":"Dong","suffix":""},{"id":270098589,"identity":"4e458388-c3e1-43e8-bc85-d8aee70ff8f3","order_by":3,"name":"Hongxing Dong","email":"","orcid":"","institution":"Harbin Engineering University","correspondingAuthor":false,"prefix":"","firstName":"Hongxing","middleName":"","lastName":"Dong","suffix":""},{"id":270098590,"identity":"814b7953-075e-4251-bc8d-8aee4f74120b","order_by":4,"name":"Guiling Wang","email":"","orcid":"","institution":"Harbin Engineering University","correspondingAuthor":false,"prefix":"","firstName":"Guiling","middleName":"","lastName":"Wang","suffix":""}],"badges":[],"createdAt":"2024-01-24 03:05:40","currentVersionCode":1,"declarations":{"humanSubjects":false,"vertebrateSubjects":false,"conflictsOfInterestStatement":false,"humanSubjectEthicalGuidelines":false,"humanSubjectConsent":false,"humanSubjectClinicalTrial":false,"humanSubjectCaseReport":false,"vertebrateSubjectEthicalGuidelines":false},"doi":"10.21203/rs.3.rs-3892690/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-3892690/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":50405343,"identity":"f882f7dc-1bea-48d9-b968-fef5fa904234","added_by":"auto","created_at":"2024-01-31 04:52:27","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":184679,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic representation of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs.\u003c/p\u003e","description":"","filename":"floatimage1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3892690/v1/edd525ff01649f55dcc98e37.jpg"},{"id":50405347,"identity":"a183c661-c7d5-4ce2-aa04-2ec435042c09","added_by":"auto","created_at":"2024-01-31 04:52:27","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":518608,"visible":true,"origin":"","legend":"\u003cp\u003e(a) XRD patterns of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-600, Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-700, Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800, Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-900, and Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800A. SEM image of (b) Co-ZIFs precursor, (c) Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 and (d) Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800A. (e)TEM images and (f, g) HRTEM image, (h) HAADF-STEM image and corresponding elemental mapping of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800. (i) TEM images and (j, k) HRTEM image (l) HAADF-STEM image and corresponding elemental mapping of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800A.\u003c/p\u003e","description":"","filename":"floatimage2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3892690/v1/dd0a1a10151611748c04c70e.jpg"},{"id":50405346,"identity":"449acb54-3d6b-43ee-ae27-51477da46ae8","added_by":"auto","created_at":"2024-01-31 04:52:27","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":325788,"visible":true,"origin":"","legend":"\u003cp\u003eHigh-resolution XPS spectra of (a) Co 2p, and (b) N 1s of the Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-600, Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-700, Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800, Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-900, and Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800A. (c) The corresponding content of different N types of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs.\u003cstrong\u003e \u003c/strong\u003e(d) The Co K-edge XANES spectra, and (e) k\u003csup\u003e2\u003c/sup\u003e-weighted χ(k)-function of Co-EXAFS spectra and Co-EXAFS fitting of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800, Co foil, Co\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and CoPc. (f) Co-EXAFS fitting curves of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 at R space. (g) Co-EXAFS fitting curves of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800, Co foil, Co\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and CoPc.\u003c/p\u003e","description":"","filename":"floatimage3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3892690/v1/ba13e95e4c468756b13d5647.jpg"},{"id":50405344,"identity":"bbd2d7de-8952-4355-84b7-05ac494a3482","added_by":"auto","created_at":"2024-01-31 04:52:27","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":334977,"visible":true,"origin":"","legend":"\u003cp\u003e(a) LSV curves and (b) differences between the E\u003csub\u003e1/2\u003c/sub\u003e, E\u003csub\u003eonset\u003c/sub\u003e and J\u003csub\u003ek\u003c/sub\u003e, (c) corresponding Tafel plots for Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-600, Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-700, Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800, Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-900 and Pt/C catalysts tested in O\u003csub\u003e2\u003c/sub\u003e-saturated 0.1 M KOH electrolyte at 1600 rpm. (d) LSV curves of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 at different electrode rotating speeds from 400 to 2500 rpm. (e) Corresponding fitted K-L plots and (f) n at different potentials for Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800. (g) LSV curves of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 before and after CA test. (h) In-situ EIS tests, and (i) the effect of voltage on R\u003csub\u003ect\u003c/sub\u003e and R\u003csub\u003es\u003c/sub\u003e values of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800. (j) Schematic representation of 4e\u003csup\u003e-\u003c/sup\u003e transfer of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800.\u003c/p\u003e","description":"","filename":"floatimage4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3892690/v1/d0a610dc9a74b858c0894d55.jpg"},{"id":50405768,"identity":"455766dc-16bb-4682-a9d8-9eef41783d03","added_by":"auto","created_at":"2024-01-31 05:00:27","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":238574,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Diagram of the zinc-air flow battery, (b) open circuit voltages, (c) chronopotentiometric response at different current densities, (d) open circuit voltage measured by digital voltmeter of Co@CoN\u003csub\u003e3\u003c/sub\u003e-CNTs-800 based zinc-air flow battery. (e) Polarization and power density curves of Co@CoN\u003csub\u003e3\u003c/sub\u003e-CNTs-800 and Pt/C-based based zinc-air flow batteries. (f) Comparison of maximum power density and corresponding discharge current density with previous works. (g) Long-term charge/discharge curve at 5 mA cm\u003csup\u003e-2\u003c/sup\u003e of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 and Pt/C-based zinc-air flow batteries. Insert in (b): digital photograph of LED lighted by Co@CoN\u003csub\u003e3\u003c/sub\u003e-CNTs-800 based zinc-air flow battery.\u003c/p\u003e","description":"","filename":"floatimage5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3892690/v1/8fc91a5af7dfbd23962c59d8.jpg"},{"id":52143947,"identity":"981b7f32-2f88-4aa7-8121-4f861062483d","added_by":"auto","created_at":"2024-03-07 11:53:21","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1140727,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-3892690/v1/4fc023ef-60da-45c8-89c9-b820994d50b9.pdf"},{"id":50405348,"identity":"b553738b-ffaf-497b-80d5-d14df452aeb9","added_by":"auto","created_at":"2024-01-31 04:52:28","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":5019336,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"SI.docx","url":"https://assets-eu.researchsquare.com/files/rs-3892690/v1/f7edcf27616dc7a8bc32a9d8.docx"},{"id":50405351,"identity":"28056a76-50d2-4efe-a53c-1e21236ab506","added_by":"auto","created_at":"2024-01-31 04:52:28","extension":"mov","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":27998965,"visible":true,"origin":"","legend":"\u003cp\u003evideo of Co@CoN3-CNTs-800 based zinc-air flow battery continuous and steady powered a small fan.\u003c/p\u003e","description":"","filename":"MovieS1.mov","url":"https://assets-eu.researchsquare.com/files/rs-3892690/v1/163f2d19d172ad26c5761952.mov"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Carbon Nanotube-Encapsulated Cobalt for High-Efficiency Zinc-Air Flow Battery: Integration of Single Atom Catalysis and Space Confinement","fulltext":[{"header":"Introduction","content":"\u003cp\u003eRechargeable zinc-air energy storage devices have gained significant attention due to the increasing demand for energy resources, attributed to cost-effectiveness, and eco-friendliness.\u003csup\u003e[\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]\u003c/sup\u003e The performance of zinc-air storage devices relies on the efficiency of ORR. However, their practical efficiency is often hindered by the sluggish ORR occurring at the air electrodes. Platinum-based materials have proven to be efficient catalysts, accelerating the typically slow four-electron ORR kinetics.\u003csup\u003e[\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]\u003c/sup\u003e Regrettably, their high cost, limited availability, and suboptimal durability impede their widespread application. Consequently, there is an urgent need to develop non-noble metal ORR electrocatalysts for metal-air batteries that not only provide competitive pricing but also exhibit high ORR activity and exceptional electrochemical stability.\u003csup\u003e[\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]\u003c/sup\u003e Alternatively, carbon materials encapsulated non-noble metal electrocatalysts (M@C) display promising potential due to their distinctive spatial structure and electronic configuration. Such heterostructures provide a competitive electronic configuration, allowing electrons to penetrate from the inner non-noble metal to the outer carbon layer to accelerate the electrocatalytic process.\u003csup\u003e[\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]\u003c/sup\u003e Moreover, N-doping of the outer carbon layer can effectively increase the density of states near the Fermi energy level, further enabling the acceleration of electrocatalytic process.\u003csup\u003e[\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]\u003c/sup\u003e Progress has been made in the application of M@C as cathodes for zinc-air batteries cathodes. Some groups tried to confine Co nanoparticles in porous carbon matrix.\u003csup\u003e[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]\u003c/sup\u003e While, traditional amorphous carbon has low conductivity. CNTs serve as effective carriers enabling faster electron conduction to accelerate the four-electron ORR process for metal-air batteries.\u003csup\u003e[\u003cspan additionalcitationids=\"CR13\" citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]\u003c/sup\u003e CNTs encapsulated non-noble metal (M@CNTs) catalysts are fabricated through grinding-calcination strategy.\u003csup\u003e[\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]\u003c/sup\u003e However, the pre-synthesis procedure of CNTs is required, which are more prone to inhomogeneous growth, and involves material loss during transfer. The sacrificial template method is an effective way to grow M@CNTs.\u003csup\u003e[\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]\u003c/sup\u003e However, the inclusion of the sacrificial template method complicated the process. The key challenge is achieving a low-cost and simple in-situ growth of M@CNTs to further enhance ORR activity.\u003c/p\u003e \u003cp\u003eFrom a different perspective, in ORR processes, involving the breaking and generation of chemical bonds between reactants and products on the surface of electrochemical catalysts, superior ORR properties can be achieved by constructing heterojunctions for lower energy barrier, increasing the density/dispersion of active sites to accelerate kinetic processes, and improving the conductivity of the material for speeding up electron transfer.\u003csup\u003e[\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]\u003c/sup\u003e Precise composition control of M@CNTs can be achieved through the design of appropriate Metal-organic frameworks (MOF) precursors,\u003csup\u003e[\u003cspan additionalcitationids=\"CR18 CR19\" citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]\u003c/sup\u003e in which heterojunctions are efficiently constructed. Moreover, the in-situ grown CNTs act as good carriers for loading atomically dispersed active sites and offering a fast pathway for electronic conduction \u003csup\u003e[\u003cspan additionalcitationids=\"CR22\" citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]\u003c/sup\u003e. Nevertheless, extreme environment such as high temperature/pressure conditions are usually required for initial MOF preparation. Furthermore, the auxiliary roles of the additional carbon or nitrogen additives are often emphasized, which are unfavorable for large-scale production due to the high cost and unstable reproductivity. Therefore, developing a simple method to achieve the autocatalytic in-situ growth of MOF derived M@CNTs without additional additives for assembling high-performance zinc-air batteries is highly desirable.\u003c/p\u003e \u003cp\u003eHerein, we propose a facile method that combines functionalization and in-situ engineering for the large-scale preparation of seaweed-shaped carbon nanotubes. These CNTs simultaneously anchor Co single-atom catalysts (SACs) and encapsulate Co nanoparticles in CoN\u003csub\u003e3\u003c/sub\u003e-doped CNTs through the direct calcination of Co-ZIFs prepared by self-assembly strategy at room temperature and pressure. During the in-situ CNTs growth process the reliance on additional additives such as carbon/nitrogen sources is eliminated. The unique asymmetric CoN\u003csub\u003e3\u003c/sub\u003e moieties in the Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs facilitate four-electron transfer in the ORR, outperforming the symmetric planar structure of CoN\u003csub\u003e4\u003c/sub\u003e sites. The resulting Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs, with multiple active sites (atomically dispersed CoN\u003csub\u003e3\u003c/sub\u003e and N-doped CNTs structures), in-situ large-scale grown CNTs ensuring accelerated electron and mass transfer, a high degree of graphitization, strong metal-carrier interactions, and the absence of additional additives, contribute to lower energy barriers and superior ORR performance, surpassing that of Pt/C counterparts with a half-wave potential of 0.84 V. Furthermore, primary and reversible zinc-air flow batteries based on Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs cathodes exhibit an excellent peak power density of 169.5 mW/cm\u0026sup2; and exceptional cycling durability over 700 hours, with the voltage gap only 1.6% larger than the original.\u003c/p\u003e"},{"header":"Results and discussion","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eMaterial Design and Structural Characterization\u003c/h2\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe construction of the Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs were performed via a simple calcination process of Co-ZIFs precursor. Experimentally, the aqueous solution of Co\u003csup\u003e2+\u003c/sup\u003e ions and nitrogen-rich 2-MIM ligand were firstly prepared. Subsequently, to initiate the coordination of Co\u003csup\u003e2+\u003c/sup\u003e and 2-MIM, two aqueous solutions were directly mixed and Co atoms were encapsulated in the cage of Co-ZIFs. The resulting powder was then pyrolyzed to drive the in-situ growth of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs. 2-MIM ligand were selected as both a bridge to cobalt metal ions and carbon/nitrogen source, allowing the direct integration of the metal and the carbon sources required for CNTs growth in one framework without any additional additives, rather than introducing them in batches.\u003csup\u003e[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]\u003c/sup\u003e The in-situ grown CNTs embed Co nanoparticles in the tips of CNTs through strong metal-support interactions enhancing the kinetic process of four-electron ORR and extremely suited to capture metal atoms to load CoN\u003csub\u003e3\u003c/sub\u003e sites.\u003csup\u003e[\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]\u003c/sup\u003e The mechanism diagram for the preparation of the Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNT was shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. Briefly, during the pyrolytic process, Co-ZIFs could firstly decomposed into Co\u003csub\u003e3\u003c/sub\u003eC as the temperature gradually rises. \u003csup\u003e[\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]\u003c/sup\u003e Subsequently, the cobalt species served as the catalyst for the direct arrangement of surrounding C-N groups from the ligand of Co-ZIFs precursor to initiate the growth of seaweed-shaped highly ordered gram-scale CoN\u003csub\u003e3\u003c/sub\u003e/CNTs.\u003c/p\u003e \u003cp\u003eFor the detailed formation mechanism of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNT, \u003csup\u003e[\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]\u003c/sup\u003e to begin with, the surface carbide radicals were first fabricated via thermal decomposition of 2-MIM and dissolved on the surface of the small size Co nanoparticles. Then in the follow-up dissolution and diffusion phase stages, the carbide radicals diffused on the surface or in the bulk phase of Co nanoparticles to form metastable Co3C. After that, as C-N radicals derived from 2-MIM decomposition gradually dissolute and precipitate on the bottom of the metal nanoparticles, it would reach supersaturation and start to form a six-membered ring gradually forming N-doped graphite lamellar structure and then carbon nanotubes after oversaturation of carbon dissolution. Later, in the crystal growth stage, when the interaction between active component and support was relatively weak (the contact angle was acute), metal particles would leave the bottom during the growth process of CNTs and forming the tip growth model. Ultimately, as long as the metal particles have access to C-N source and the C-N radicals can diffuse, the CNTs will grow thus realizing the in-situ autocatalytic growth of CNTs from Co-ZIFs. \u003csup\u003e[\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]\u003c/sup\u003e When the metal particles were completely encapsulated by the carbon, the CNTs stop growing. Therefore, the cobalt nanoparticles were simultaneously embedded in CoN\u003csub\u003e3\u003c/sub\u003e/CNTs to construct the Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs hybrid nanostructures. Importantly, the N-doped graphite lamellar was an ideal matrix for the coordination of Co atoms to form Co-N\u003csub\u003e3\u003c/sub\u003e moieties. Thus, the CoN\u003csub\u003e3\u003c/sub\u003e sites can successfully simultaneously doped in N-doped CNTs walls during the growth of N-doped CNTs to fabricate CoN\u003csub\u003e3\u003c/sub\u003e/CNTs. Besides, this robust and open 3D architecture effectively boosted electrolytes accessibility, electrochemical stability, oxygen species capture and elevated reactant diffusion. The presence of Co-N\u003csub\u003e3\u003c/sub\u003e, pyridinic N, pyrrolic N, graphitic N, oxidized N, and Co nanoparticles provided numerous active sites for capturing and splitting oxygen molecules. Furthermore, the Co nanoparticles encapsulated by CoN\u003csub\u003e3\u003c/sub\u003e/CNTs with space confinement effect can effectively boost electrochemical stability, meanwhile the CoN\u003csub\u003e3\u003c/sub\u003e-doped CNTs layer can promote the electron conduction properties.\u003csup\u003e[\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]\u003c/sup\u003e Due to the aforementioned well-defined electronic structure, the in-situ grown Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs hybrids would present superior ORR properties and excellent stability when exposed to reactive environment.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe specific crystal structure information of samples was measured by X-ray diffraction (XRD) measurements. The XRD results of the samples prepared by the precursor at a series of temperatures were shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea, and the results indicated that the as-prepared samples were Co/graphitization carbon hybrids and no impurity peaks were observed in the XRD results. Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-600, Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-700, Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 and Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-900 samples showed diffraction peaks located at 44.21, 51.52 and 75.85\u0026deg;, which were consistence with (111), (200) and (220) cubic planes of Co (PDF#15\u0026ndash;0806). Besides, the diffraction peak situated at 26.51\u0026deg; was indexed to the C (002) plane.\u003csup\u003e[\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]\u003c/sup\u003e As calcination temperature increased, the peaks of cobalt in the XRD characterization became sharper, it can be concluded that the crystallinity of cobalt in Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs became higher with the increase of calcination temperature.\u003c/p\u003e \u003cp\u003eTo investigate the mechanism of CNTs growth, he Co-ZIFs precursor were calcined under relatively low temperature (400℃, 450℃ and 500℃) and characterized by XRD (Supplementary Fig.\u0026nbsp;1) and scanning electron microscopy (SEM) (Supplementary Fig.\u0026nbsp;2) measurements. XRD results showed that cobalt has not formed yet at around 400℃, and according to the SEM results CoN\u003csub\u003e3\u003c/sub\u003e/CNTs has not appeared at this time. When the calcination temperature was increased to 450℃, cobalt was detected, and at this time SEM results indicated that CoN\u003csub\u003e3\u003c/sub\u003e/CNTs was beginning to be generated, confirming the formation of Co species was a key factor in the preparation of CNTs. Upon pyrolysis at 450℃, the new peaks located at 38.64\u0026deg;, 44.14\u0026deg;,57.99\u0026deg; and 70.06\u0026deg; appeared and fitted well with the (100), (101), (102) and (110) diffraction peaks of Co\u003csub\u003e3\u003c/sub\u003eC crystals planes (PDF#43-1144). The formed Co\u003csub\u003e3\u003c/sub\u003eC could catalyze the adjacent C-N groups originating from the 2-methylimidazole (2-MIM) ligand for the gradually in-situ grown of CoN\u003csub\u003e3\u003c/sub\u003e/CNTs on the near-surface of cobalt nanoparticles. When the temperature risen to 500℃, CoN\u003csub\u003e3\u003c/sub\u003e/CNTs became more numerous and longer. To clarify the influence of H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e corrosion on Co in the catalysts, XRD characterization were also carried on the samples after acid washing. The diffraction peaks of washed Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800A only presented a slight decrease in the peak intensity, indicating a high retention of cobalt after acid treatment (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea).\u003c/p\u003e \u003cp\u003eThe morphology of precursor were measured in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb, and more detailed characterization regarding the crystal properties and chemical bond structures of Co-ZIFs precursors were measured (Supplementary Figs.\u0026nbsp;3 and 4). The Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs samples were in-situ directly prepared by calcining Co-ZIFs precursors. Cobalt nitrate (CoNO\u003csub\u003e3\u003c/sub\u003e) as both the metal sources for Co-ZIFs frameworks and the catalyst precursor for in-situ growth of CoN\u003csub\u003e3\u003c/sub\u003e/CNTs. The in-situ decomposition of 2-methylimidazole (2-MIM) ligand upon heating provided a sufficient localized carbon feeding to promote growing CoN\u003csub\u003e3\u003c/sub\u003e/CNTs. Subsequently, the Co\u003csub\u003e3\u003c/sub\u003eC species served as catalysts for the construction of CoN\u003csub\u003e3\u003c/sub\u003e doped carbon nanotubes (CoN\u003csub\u003e3\u003c/sub\u003e/CNTs) from adjacent C-N groups originating from the 2-methylimidazole (2-MIM) ligand. The abundant Co nanoparticles were uniformly distributed within the nanotubes of the CoN\u003csub\u003e3\u003c/sub\u003e/CNTs, with a higher concentration at the tips, which effectively enhanced electrochemical stability. From the SEM images, a plenty of twisted CoN\u003csub\u003e3\u003c/sub\u003e/CNTs nanostructures were widely observed in the as-prepared Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800, which has a diameter of approximately 15 nm and a length several micrometers (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec). To investigate the effect of calcination temperature on morphology, a series of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-600, Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-700, Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 and Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-900 samples were prepared by calcining the Co-ZIFs precursors at different temperatures. The corresponding mass ratio of Co(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e\u0026middot;6H\u003csub\u003e2\u003c/sub\u003eO and 2-MIM for the fabrication of Co-ZIFs precursor were chosen as 1:3. The results showed that Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs were observed in the aforementioned all series of samples after calcination. As the increase of carbonization temperature in H\u003csub\u003e2\u003c/sub\u003e/Ar atmosphere, the SEM results demonstrated a gradually increase trend in the number of the prepared CoN\u003csub\u003e3\u003c/sub\u003e/CNT (Supplementary Fig.\u0026nbsp;5). However, the agglomeration was detected when the temperature increased to 900℃, which might be caused by the excessive calcination temperature. In view of the above analysis, the SEM results indicated that the catalytic effect might have a limiting value through temperature regulation.\u003c/p\u003e \u003cp\u003eThe transmission electron microscopy (TEM) images of a typical product Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 using Co\u003csup\u003e2+\u003c/sup\u003e as the metal precursor were shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee, which further proved large number of CoN\u003csub\u003e3\u003c/sub\u003e/CNTs were in-situ grown, and numerous tiny metallic Co nanoparticles were homogeneously wrapped at the top of CoN\u003csub\u003e3\u003c/sub\u003e/CNTs matrix. The growth of CoN\u003csub\u003e3\u003c/sub\u003e/CNTs was catalyzed by the Co nanoparticles, and the mechanism has been unveiled.\u003csup\u003e[\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]\u003c/sup\u003e According to (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ef and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eg), the high-resolution TEM (HR-TEM) image of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 displayed seaweed-shaped CoN\u003csub\u003e3\u003c/sub\u003e/CNTs, this unique morphology can be attributed to the diffusion and continuous accumulation of carbon atoms at metal/graphene interface.\u003csup\u003e[\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]\u003c/sup\u003e Hence an asymmetric distribution of carbon atoms occurred around cobalt species. Specifically, most of the carbon atoms were selected for nucleation at the bottom or edge of the deformed cobalt species catalytic centers for inducing graphitization, resulting in the formation of seaweed-like morphology in the outer layers.\u003csup\u003e[\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]\u003c/sup\u003e The lattice fringes of 0.41 nm can be allocated to the (111) interplanar distance space of the (002) plane of the CoN\u003csub\u003e3\u003c/sub\u003e/doped graphene layer (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ef).\u003csup\u003e[\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]\u003c/sup\u003e The number of layers of multi-walled CoN\u003csub\u003e3\u003c/sub\u003e/CNTs coated with metallic Co was about 8, which can further effectively promoted the cyclic stability of cobalt nanoparticles in electrochemical tests. The HR-TEM results further proved Co nanoparticles about 5 nm was capsulated at the top of CoN\u003csub\u003e3\u003c/sub\u003e/CNTs. Importantly, the lattice spacing of the Co species was clearly observed at the wall of the CoN\u003csub\u003e3\u003c/sub\u003e/CNTs, and their lattice were measured to be approximately 0.20 nm assigned to the (111) cubic Co crystal phase (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eg), this hinted the formation of CoN\u003csub\u003e3\u003c/sub\u003e sites at the wall of the CoN\u003csub\u003e3\u003c/sub\u003e/CNTs. It can be speculated that the as-synthesized composite was composed of graphitized carbon and cobalt, which was consistent with the XRD characterization results. As expected, the elemental mapping analysis (EDX) revealed the elements of C, N, Co, and O are detected (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eh and Supplementary Fig.\u0026nbsp;6). The bright spots represented metallic Co species with high abundance. In addition, the EDX element mapping results further proved the abundant N element was uniformly distributed throughout the CNTs in Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 at such high magnification, and the Co element was mainly distributed in the tips of the tube parts of CoN\u003csub\u003e3\u003c/sub\u003e/CNTs. Importantly, The Co and N elements were highly overlapped, which indicated the formation of atomically dispersed CoN\u003csub\u003e3\u003c/sub\u003e moieties in Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800.\u003c/p\u003e \u003cp\u003eTo further characterize the protective effect of graphene layers in CoN\u003csub\u003e3\u003c/sub\u003e/CNTs on the encapsulated Co nanoparticles, the as-prepared Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 were then leached in 2 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e for 1 day. Such protective wrapping of CoN\u003csub\u003e3\u003c/sub\u003e/CNTs would be profited to electro-catalytic stability. The final sample after acid leaching was named as Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800A, which was further confirmed by the SEM (Supplementary Fig.\u0026nbsp;2d) and TEM measurement. It should be noted that large amount of metal nanoparticles completely protected by the seaweed-like N-CNTs still retained in Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800A after acid treatment, in comparison with the product of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 without acid treatment (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ei). Therefore, Co nanoparticles with space confinement effect can be stable even with long-term acid dip. In addition, the thickness of the carbon nanotubes coated with metallic Co was about 2.5 nm, CoN\u003csub\u003e3\u003c/sub\u003e/CNTs were multi-wall CoN\u003csub\u003e3\u003c/sub\u003e/CNTs, and the Co nanoparticles were wrapped by multi-wall CoN\u003csub\u003e3\u003c/sub\u003e/CNTs, which can be clearly observed by HR-TEM (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ej and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ek). To further test the distribution of metal sites after acid washing, the corresponding EDX measurements were also carry out on Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800A. High abundance of the C, N, Co, and O elements were detected (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003el and Supplementary Fig.\u0026nbsp;7). Nitrogen atoms overlapped well with in-situ grown CNTs, indicating the uniform distribution of nitrogen in the seaweed-like Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800A. Most of the cobalt nanoparticles were still retained after acid washing, which dispersed through the entire architecture of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800A, especially on the tips of the CoN\u003csub\u003e3\u003c/sub\u003e/CNTs. Moreover, the cobalt element overlapped well with nitrogen element, which reflected the stable presence of CoN\u003csub\u003e3\u003c/sub\u003e active sites on the walls of in-situ growth CoN\u003csub\u003e3\u003c/sub\u003e/CNTs after acid washing.\u003c/p\u003e \u003cp\u003eTo further investigate the impact of precursors with different morphologies on the calcined samples, the adding ratio of Co(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e\u0026middot;6H\u003csub\u003e2\u003c/sub\u003eO and 2-MIM has been adjusted and 800℃ has been chosen as the calcination temperature. SEM characterization showed that the morphology of the precursor was indeed significantly changed by varying the mass ratio of Co\u003csup\u003e2+\u003c/sup\u003e and 2-MIM. Unlike the bamboo leaf-like morphology of Co-ZIFs, Co-ZIFs(1:2) exhibited a rod-like morphology while Co-ZIFs(1:4) exhibited a cross-like morphology (Supplementary Fig.\u0026nbsp;8). Furthermore, after calcining Co-ZIFs(1:2) and Co-ZIFs(1:4) under 800 ℃, Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs(1:2)-800 and Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs (1:4)-800 were successfully prepared. The results showed that both calcined samples successfully grew carbon nanotubes (Supplementary Fig.\u0026nbsp;9). However, a noticeable agglomeration of large cobalt balls (about 200 nm) were observed in Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs(1:2)-800. The crystal structures of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs(1:2)-800 and Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs(1:4)-800 were also measured (Supplementary Fig.\u0026nbsp;10).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAccording to the Raman spectra (Supplementary Fig.\u0026nbsp;11), the intensity ratios of I\u003csub\u003eD\u003c/sub\u003e/I\u003csub\u003eG\u003c/sub\u003e value of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-600, Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-700, Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 and Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-900 samples were calculated to be 1.078, 0.9482, 0.8724 and 0.7838. the I\u003csub\u003eD\u003c/sub\u003e/I\u003csub\u003eG\u003c/sub\u003e values of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs proved the increasing I\u003csub\u003eD\u003c/sub\u003e/I\u003csub\u003eG\u003c/sub\u003e values of the N-CNTs as the temperature rose from 600\u0026deg;C to 900\u0026deg;C, displaying a higher degree of graphitization. Moreover, the intensity ratios of I\u003csub\u003eD\u003c/sub\u003e/I\u003csub\u003eG\u003c/sub\u003e value of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 and Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800A samples were calculated to be 0.8724 and 0.8300, respectively. Such a close value further implied that the as-prepared carbon nanotube skeleton was stable and the long-time acid treatment has not changed carbon structure.\u003c/p\u003e \u003cp\u003eX-ray photoelectron spectroscopy (XPS) measurement were performed to characterize the chemical states in the surface of the Co@CoN\u003csub\u003e3\u003c/sub\u003e-CNTs composites. The existence of Co, N, C and O elements can be verified, and the detailed atomic content of the Co, O, C and N elements were listed (Supplementary Table\u0026nbsp;1\u003cb\u003e)\u003c/b\u003e. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea confirmed the existence of CoN\u003csub\u003e3\u003c/sub\u003e species and Co metal. The two peaks of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 appearing at 779.0 and 795.0 eV were assigned to Co 2p\u003csub\u003e1/2\u003c/sub\u003e and Co 2p\u003csub\u003e3/2\u003c/sub\u003e of Co metal, respectively. Furthermore, two peaks of Co-N\u003csub\u003e3\u003c/sub\u003e and two weak shake-up satellites corresponding to Co 2p\u003csub\u003e3/2\u003c/sub\u003e and Co 2p\u003csub\u003e1/2\u003c/sub\u003e were deconvoluted.\u003csup\u003e[\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]\u003c/sup\u003e This confirmed partial Co atoms were coordinated with the surrounding nitrogen atoms to form the CoN\u003csub\u003e3\u003c/sub\u003e species in Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs samples. After acid treatment, cobalt content of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800A sample was 1.05% respectively, indicating that there were still plenty of cobalt spices remained on the surface of the sample even after prolonged concentrated acid treatment. The fitted peaks of high-resolution N 1s \u003cb\u003e(\u003c/b\u003eFig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb\u003cb\u003e)\u003c/b\u003e, located at 398.7, 399.5 eV, 400.7, 401.7 eV, and 403.1 eV were assigned to the pyridinic N, Co-N\u003csub\u003e3\u003c/sub\u003e, pyrrolic N, graphitic N and oxidized N, respectively.\u003csup\u003e[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]\u003c/sup\u003e Furthermore, the content of different N species of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs materials were calculated (Supplementary Table\u0026nbsp;2\u003cb\u003e)\u003c/b\u003e. According to Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec, the content of graphitic N in Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs samples significantly increased as the pyrolysis temperature rose, indicating an obvious change of N types in the carbon matrix via controlling pyrolysis temperature. Co coated by graphitic N doped carbon possessed more negative charge than pyridinic-N doping, leading to lower energy barriers for ORR catalysis. \u003csup\u003e[\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]\u003c/sup\u003e It is reported that CoN\u003csub\u003e3\u003c/sub\u003e sites served as the active sites in ORR. Although the content of CoN\u003csub\u003e3\u003c/sub\u003e decreased with increasing temperature, the CoN\u003csub\u003e3\u003c/sub\u003e content still retained at high levels in these samples. The decrease in CoN\u003csub\u003e3\u003c/sub\u003e content may be attributed to the decreasing trend of pyridinic N with increasing temperature as the pyridinic N can act as an anchor to grab metal atoms to form CoN\u003csub\u003e3\u003c/sub\u003e sites.\u003csup\u003e[\u003cspan additionalcitationids=\"CR34 CR35\" citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]\u003c/sup\u003e Moreover, oxidized N species appeared when the calcination temperature reached 800℃, and the oxidized N content in Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs increased with rising of calcination temperature. With the increase of calcination temperature, the N 1s peak of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs shifted toward the low binding energy direction, suggesting that the increase in temperature led to electron redistribution at the heterogeneous interface of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs. The electrons transferred from the encapsulated Co cobalt nanoparticles the N-doped CNTs promoted electron penetration from the encapsulated Co cobalt nanoparticles to the N-doped CNTs surface, resulting in better transfer paths and lower energy barrier for ORR catalytic performance.\u003csup\u003e[\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]\u003c/sup\u003e The C 1s XPS spectrum of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 sample (Supplementary Fig.\u0026nbsp;12a) exhibited three peaks at binding energies of 284.8 285.8 and 286.7 eV, assigning to the C\u0026thinsp;\u0026minus;\u0026thinsp;C, C\u0026thinsp;\u0026minus;\u0026thinsp;N and C\u0026thinsp;=\u0026thinsp;O bonds, respectively.\u003csup\u003e[\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e, \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]\u003c/sup\u003e Particularly, the C\u0026thinsp;\u0026minus;\u0026thinsp;N peak in C 1s spectrum further verified the N doping into the carbon matrix. Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 showed high N atomic content of (4.16%), which contributed to the formation of Co-N\u003csub\u003e3\u003c/sub\u003e sites. Meanwhile, the O element can be deconvoluted in three existing states, corresponding to O-C\u0026thinsp;=\u0026thinsp;O (533.7 eV), C\u0026thinsp;=\u0026thinsp;O (532.0 eV), and metallic O (530.2 eV) oxidation states for the Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 sample (Supplementary Fig.\u0026nbsp;12b). It is noteworthy that the metallic O peak disappeared for the acid-treated Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800A sample, indicating that the acid mainly etched away the oxidized cobalt, which might cause a slight decrease in ORR performance. XPS spectrum further confirmed the chemical formation and coordination environment of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs(1:2)-800 and Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs(1:4)-800 (Supplementary Fig.\u0026nbsp;13). The Co, N, C and O elements were detected (Supplementary Table\u0026nbsp;1). According to the Co 2p spectra, Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs(1:2)-800 obviously contained more cobalt monomers than Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs(1:4)-800.\u003c/p\u003e \u003cp\u003eSynchrotron X-ray absorption spectroscopy was employed to characterize the coordination environment of Co atoms in Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800. Co K-edge X-ray absorption near-edge structure (XANES) spectra in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed illustrated that the absorption edge of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 was located between that of Co foil and CoPc, suggesting an average oxidation state of +\u0026thinsp;1.70 (Supplementary Fig.\u0026nbsp;14). Fourier-transformed (FT)\u003csub\u003ek2\u003c/sub\u003e-weighted extended X-ray absorption fine structure (EXAFS) curve (Supplementary Fig.\u0026nbsp;15) and EXAFS fitting spectrum in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ee demonstrated the Co-N paths at 1.54 \u0026Aring; and the Co-Co path at 2.21 \u0026Aring; in R space, displaying the exist of Co SACs and Co-Co clusters in Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800, which was consistent with the TEM and XPS results.\u003csup\u003e[\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]\u003c/sup\u003e EXAFS fitting spectrum in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ef further illustrated the Co-N pathway has a coordination number of about 2.7 and a bond length of about 1.90 \u0026Aring;, showing the formation of an Co-N\u003csub\u003e3\u003c/sub\u003e the Co single atoms coordinated with three N atoms (CoN\u003csub\u003e3\u003c/sub\u003e) rather than CoN\u003csub\u003e3\u003c/sub\u003eC.\u003csup\u003e[\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]\u003c/sup\u003e Interestingly, differ from the symmetric planar structure of CoN\u003csub\u003e4\u003c/sub\u003e, the unique CoN\u003csub\u003e3\u003c/sub\u003e coordination with an asymmetric active center were more conducive to the four-electron transfer ORR process. Besides, the Co-Co coordination number was approximately 4.9 with a bond length of 2.491 \u0026Aring;, which was much lower than the Co-Co coordination number of Co foil indicating the size of the sample Co particle was fairly small. Detailed Co-EXAFS fitting results of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 were presented (Supplementary Table\u0026nbsp;3). Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eg showed the wavelet transform (WT) of EXAFS for Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 sample, which exhibited two characteristic centers of Co-N path (~\u0026thinsp;1.54 \u0026Aring;) and Co-Co path (~\u0026thinsp;2.21 \u0026Aring;).\u003csup\u003e[\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]\u003c/sup\u003e Based on the aforementioned analysis, the coordination environments of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 were measured, of which the atomic structure model composed of single-atom dispersed CoN\u003csub\u003e3\u003c/sub\u003e and Co-Co clusters.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003eCatalytic Properties for the oxygen reduction reaction\u003c/h2\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe ORR performance was firstly investigated in O2-saturated 0.1 M KOH alkaline electrolyte and the commercial Pt/C was used as the ORR benchmarks. The polarization curves of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea) showed superior ORR performance with the half-potential (E\u003csub\u003e1/2\u003c/sub\u003e) of 0.84 V and onset potential (E\u003csub\u003eonset\u003c/sub\u003e) of 0.92 V, which were comparable or even surpass Pt/C catalyst (E\u003csub\u003e1/2\u003c/sub\u003e = 0.83 V, E\u003csub\u003eonset\u003c/sub\u003e = 0.97 V) and other reported catalysts (Supplementary Table\u0026nbsp;4). According to the Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb, the kinetic current densities (J\u003csub\u003ek\u003c/sub\u003e) of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 at 0.80 and 0.84 V determined by the Koutecky-Levich (K-L) equation were calculated to be 16.35 and 5.24 mA/cm\u003csup\u003e2\u003c/sup\u003e, which were 1.31 and 1.02 times superior than Pt/C (12.49 and 5.15 mA cm\u003csup\u003e2\u003c/sup\u003e) respectively. Furthemore, Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 displayed a much better ORR activity than Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-600 (E\u003csub\u003e1/2\u003c/sub\u003e = 0.79 V, E\u003csub\u003eonset\u003c/sub\u003e = 0.91 V, J\u003csub\u003ek0.80V\u003c/sub\u003e = 3.87 mA/cm\u003csup\u003e2\u003c/sup\u003e, J\u003csub\u003ek0.84V\u003c/sub\u003e = 1.47 mA/cm\u003csup\u003e2\u003c/sup\u003e), Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-700 (E\u003csub\u003e1/2\u003c/sub\u003e = 0.82 V, E\u003csub\u003eonset\u003c/sub\u003e = 0.92 V, J\u003csub\u003ek0.80V\u003c/sub\u003e = 7.62 mA/cm\u003csup\u003e2\u003c/sup\u003e, J\u003csub\u003ek0.84V\u003c/sub\u003e = 2.87 mA/cm\u003csup\u003e2\u003c/sup\u003e) and Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-900 (E\u003csub\u003e1/2\u003c/sub\u003e = 0.82 V, E\u003csub\u003eonset\u003c/sub\u003e = 0.90 V, J\u003csub\u003ek0.80V\u003c/sub\u003e = 7.11 mA/cm\u003csup\u003e2\u003c/sup\u003e, J\u003csub\u003ek0.84V\u003c/sub\u003e = 2.27 mA/cm\u003csup\u003e2\u003c/sup\u003e) (Supplementary Table\u0026nbsp;5). Moreover, The Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 also performed lowest Tafel slope of 45.2 mV dec\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e smaller than Pt/C (73.4 mV dec\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-600 (67.6 mV dec\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-700 (59.4 mV dec\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-900 (47.5 mV dec\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec. The electrocatalytic kinetics of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 was further evaluated by glassy carbon rotating disk electrode (RDE) measurements with the rotating speeds ranged from 400 to 2500 rpm (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed). As illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee, and \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ef, the fitting Koutecky-Levich (K-L) plots of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 was mainly a four-electronic ORR pathway for the similar electron transfer number (n) at different potentials of 3.9. n value of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 was higher than those of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-600 (3.6), Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-700 (3.1), Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-900 (3.2) (Supplementary Fig.\u0026nbsp;16). ORR performance of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs produced a trend of increasing and then decreasing with increasing temperature, which can be attributed to higher temperature leading to the graphitization of CoN\u003csub\u003e3\u003c/sub\u003e/CNTs, while excessive temperature can lead to the agglomeration of cobalt nanoparticles.\u003c/p\u003e \u003cp\u003eFurthermore, the chronoamperometric response (CA) measurement were conducted to test the durability of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800, Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eg showed the activity decay of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 before and after a 6-hour CA response measurement during ORR. A negligible decay of E\u003csub\u003e1/2\u003c/sub\u003e shifted negatively by 6 mV, illustrating Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 possessed outstanding stability. Correspondingly, the structures of OOH*, O* and OH* intermediated on Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 during the ORR process were shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ej. The asymmetric CoN\u003csub\u003e3\u003c/sub\u003e sites distributed on in-situ grown CNTs can efficiently reduce the ORR barrier and accelerate the electrocatalytic kinetics. The encapsulated abundant Co nanoparticles facilitated the kinetic process of four-electron oxygen reduction, and the CNTs components ensured outstanding mass transport capability.\u003c/p\u003e \u003cp\u003eThe cyclic voltammetry (CV) image profile after the stability test matched well with the pre-test, which indicated that the Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 material has high stability (Supplementary Fig.\u0026nbsp;17). To further prove the outstanding stability of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800, the ORR performance of acid treated sample Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800A were tested (Supplementary Fig.\u0026nbsp;18\u003cb\u003e)\u003c/b\u003e. We can conclude that after H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e treatment the Co@N-CNTs-800-A still retains the high E\u003csub\u003e1/2\u003c/sub\u003e value of 0.82 V, E\u003csub\u003eonset\u003c/sub\u003e value of 0.90 V, J\u003csub\u003ek0.80V\u003c/sub\u003e value of 7.54 mA/cm\u003csup\u003e2\u003c/sup\u003e, J\u003csub\u003ek0.84V\u003c/sub\u003e value of 1.93 mA/cm\u003csup\u003e2\u003c/sup\u003e and n value of 3.5. The reason for the decrease in catalyst ORR performance after acid treatment can be speculated to be although CoN\u003csub\u003e3\u003c/sub\u003e/CNT was protective of the Co embedded in it, the acid can wash out part of Co nanoparticles, leading to the reduction of space confinement effect on the ORR performance. To get a further insight into the intrinsic catalytic activity, the electrochemically active surface area (ECSA) of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 was characterized via the CV tests with the non-Faradic voltage range of 1.07\u0026ndash;1.16 V vs RHE with various scan rates (Supplementary Fig.\u0026nbsp;19). The as-calculated specific electrical double-layer capacitances (C\u003csub\u003edl\u003c/sub\u003e) of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 was 13.38 mF/cm\u003csup\u003e2\u003c/sup\u003e which was higher than that of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-600 (2.93 mF/cm\u003csup\u003e2\u003c/sup\u003e), Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-700 (8.61 mF/cm\u003csup\u003e2\u003c/sup\u003e) and Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-900 (4.01 mF/cm\u003csup\u003e2\u003c/sup\u003e), respectively. Therefore, the highest ECSA of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 revealed fastest reaction kinetics, largest electrochemical surface area and active sites. Furthermore, ORR performance was investigated under oxygen and nitrogen condition. The CV results of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 presented an obvious cathodic peak appearing at about 0.83 V (vs. RHE) (Supplementary Fig.\u0026nbsp;20). ORR properties of precursors with different morphologies calcinated under 800℃ with different morphologies were explored (Supplementary Figs.\u0026nbsp;21 and 22\u003cb\u003e)\u003c/b\u003e. The ORR activity of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 was better than Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs(1:2)-800 (E\u003csub\u003e0\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;0.967 V, E\u003csub\u003e1/2\u003c/sub\u003e=0.832 V, J, J\u003csub\u003ek0.80V\u003c/sub\u003e = 8.52 mA/cm\u003csup\u003e2\u003c/sup\u003e, J\u003csub\u003ek0.84V\u003c/sub\u003e = 2.51 mA/cm\u003csup\u003e2\u003c/sup\u003e) and Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs(1:4)-800 (E\u003csub\u003e0\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;0.967 V, E\u003csub\u003e1/2\u003c/sub\u003e=0.832 V, J\u003csub\u003ek0.80V\u003c/sub\u003e = 1.14 mA/cm\u003csup\u003e2\u003c/sup\u003e, J\u003csub\u003ek0.84V\u003c/sub\u003e = 0.33 mA/cm\u003csup\u003e2\u003c/sup\u003e). The detailed information of ORR properties in this work were integrated (Supplementary Table\u0026nbsp;3). Therefore, the Co(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e : 2-MIM feeding ratio of 1:3 was an optimal solution.\u003c/p\u003e \u003cp\u003eTo further reveal the kinetic and the dynamic evolution properties of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 electrode/electrolyte interfaces the in-situ electrochemical impedance spectroscopy (EIS) tests were performed (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eh) in an O\u003csub\u003e2\u003c/sub\u003e-saturated 0.1 M KOH solution with a potential range of 1 to 0.4 V versus RHE recorded at constant potential with the frequency range of 0.01 Hz-100 KHz fitted based on equivalent circuit model (Supplementary Fig.\u0026nbsp;23 and Table\u0026nbsp;6). The ORR process of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 can be divided into three sections, the kinetic-controlled section (at \u0026asymp;\u0026thinsp;1.05\u0026ndash;0.9 V), the mixed-controlled section (at \u0026asymp;\u0026thinsp;0.8\u0026thinsp;\u0026minus;\u0026thinsp;0.6 V), and the diffusion-controlled section (at \u0026asymp;\u0026thinsp;0.5-0 V). Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ei illustrated the effect of voltage on charge transfer resistance (R\u003csub\u003ect\u003c/sub\u003e), and solution resistance (R\u003csub\u003es\u003c/sub\u003e). Obviously, the R\u003csub\u003ect\u003c/sub\u003e values were large when the potentials were higher than E\u003csub\u003eonset\u003c/sub\u003e, which elucidated that the charge transfer between the electrode and the reaction interface was weak. When potential started to fall below E\u003csub\u003eonset\u003c/sub\u003e, the R\u003csub\u003ect\u003c/sub\u003e value undergone a sharp decrease, which indicated that a direct reduction reaction occurred at the electrode surface.\u003csup\u003e[\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e]\u003c/sup\u003e Whereas, when the potential continued to decrease to the diffusion control region, the R\u003csub\u003ect\u003c/sub\u003e value increased for the formation and desorption at the electrode surface of H\u003csub\u003e2\u003c/sub\u003eO product.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003eZinc-Air Battery Performance\u003c/h2\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe abundant asymmetric atomic CoN\u003csub\u003e3\u003c/sub\u003e sites, in-situ growth of CNTs matrix, a significant amount of confined Co nanoparticles and outstanding ORR catalytic activity of the Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 indicated that Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs was ideal catalyst for the rechargeable zinc-air cell. Compared to static electrolyte in conventional zinc-air battery, rechargeable zinc-air flow battery has a flowing electrolyte for achieving higher cycle stability, and this flow electrolyte configuration can not only significantly inhibited the Zn electrode dendrite growth, but also effectively flushed away the unwanted byproducts generated during the charge/discharge process.\u003csup\u003e[\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]\u003c/sup\u003e Therefore, the rechargeable zinc-air flow battery was constructed for evaluate practical application effect of the Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 as the air cathode catalyst and zinc foil as the anode (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea). The open circuit voltage was assessed in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed, and the assembled Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 based zinc-air flow cell displayed a higher open circuit voltage of 1.44 V. Interestingly, it can continuous and steady powered a LED light (insert in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb), and powered a small fan (Supplementary Movie 1) showing its practical application. Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 based zinc-air flow battery demonstrated impressive discharge rate performance, the discharge plateau declined slowly with the current density from 1 to 50 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec). When the current density reduced to 1 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e, discharge plateau resumed reversibly, indicating outstanding reversibility. Figure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ee showed polarization and power density curves of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800. Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 catalyst showed an outstanding peak power density as high as 169.5 mW/cm\u0026sup2; at the discharge density of 205.0 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e. This assembled zinc-air flow cell exhibited a much higher power density compared to that of Pt/C. Furthermore, according to Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ef, the Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 based zinc-air flow cell also displayed a much higher maximum power density compared to that of many previous works, manifesting its outstanding suitability for the application in electrocatalysts. Furthermore, the cycling performance of the zinc-air flow battery of Co@CoN\u003csub\u003e3\u003c/sub\u003e-CNTs-800 was also measured \u003cb\u003e(\u003c/b\u003eFig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eg\u003cb\u003e)\u003c/b\u003e. Obviously, the rechargeable battery of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 has displayed long lifetime (700 hours) than those of zinc-air flow battery assembled by Pt/C during discharging/charging cycling, illustrating its excellent long-term durability. Importantly, the assembled rechargeable battery delivered a stabled voltage gap, and after 700 hours the voltage gap only 1.6% larger than the original, indicating an outstanding charge-discharge performance. Moreover, the stability of Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 based zinc-air flow battery in this work were compared with other recently reported results (Supplementary Table\u0026nbsp;7). This excellent cycling stability was related to the unique structure of Co nanoparticles protected and confined by the in-situ grown CoN\u003csub\u003e3\u003c/sub\u003e/CNTs.\u003c/p\u003e \u003c/div\u003e"},{"header":"Conclusion","content":"\u003cp\u003eIn summary, we have successfully implemented a comprehensive approach that integrates functionalization and in-situ engineering for the large-scale preparation of Co SACs. This method involves the simultaneous in-situ fabrication of seaweed-shaped carbon nanotubes and the encapsulation of cobalt nanoparticles, introducing a space confinement effect. The uniqueness of our approach is evident in two crucial aspects. Firstly, distinct from the symmetric planar structure observed in CoN\u003csub\u003e4\u003c/sub\u003e sites, the exclusive asymmetry of CoN\u003csub\u003e3\u003c/sub\u003e sites plays a pivotal role in enabling a four-electron transfer process during the oxygen reduction reaction. Secondly, the in-situ growth of N-doped carbon nanotubes is not only straightforward and free from any supplementary additives but also remarkably efficient in capturing metal atoms. The Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 sample prepared using this method demonstrates outstanding ORR activity in alkaline electrolyte, exhibiting remarkably ORR performance with a half-wave potential of 0.84 V, surpassing that of Pt/C counterparts. Furthermore, it shows distinguished stability, with a negligible decay of E\u003csub\u003e1/2\u003c/sub\u003e (6 mV) after a 6-hour chronoamperometric response measurement. Notably, the constructed zinc-air flow battery, utilizing the Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 cathode, achieves a peak power density as high as 169.5 mW/cm\u0026sup2; and exceptional recyclability, with only a 1.6% larger voltage gap than the original after 700 hours of testing. This work opens a window for understanding the significance of CNTs matrix in metal-nitrogen-carbon catalysts, and may regulating the scale-up future tailor of heterostructure for metal-air batteries.\u003c/p\u003e"},{"header":"Methods","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eMaterials\u003c/h2\u003e \u003cp\u003eCobalt nitrate hexahydrate and 2-methylimidazole were purchased from Chengdu Organic Chemicals Co., China. Sulfuric acid (H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, AR) was supplied by Chron Chemicals. Nafion solution (5 wt%) was bought from Sigma-Aldrich. Commercial Pt/C (20 wt%) catalyst was purchased from Aladdin. Deionized water was prepared by laboratory water purification system (HHitech Master-S30UVF).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003eSynthesis of Co-ZIFs sample\u003c/h2\u003e \u003cp\u003eIn a typical procedure, 0.6 g of Co(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e\u0026sdot;6H\u003csub\u003e2\u003c/sub\u003eO and 1.8 g of 2-methylimidazole (2-MIM) were dissolved in deionized water and stirred for 30 min, respectively. Subsequently, the two solutions were mixed quickly and aged for 20 h, and the as-synthesized samples were denoted as Co-ZIFs. Furthermore, using deionized water as solvent, Co(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e\u0026sdot;6H\u003csub\u003e2\u003c/sub\u003eO and 2-MIM were mixed with the mass ratio of 1:2 and 1:4, and the as-obtained samples were denoted as Co-ZIFs(1:2) and Co-ZIFs(1:4).\u003c/p\u003e \u003cp\u003e \u003cb\u003eSynthesis of Co@CoN\u003c/b\u003e \u003csub\u003e \u003cb\u003e3\u003c/b\u003e \u003c/sub\u003e \u003cb\u003e/CNTs electrocatalyst.\u003c/b\u003e \u003c/p\u003e \u003cp\u003eThe obtained Co-ZIFs sample was placed in an H\u003csub\u003e2\u003c/sub\u003e/Ar atmosphere and further pyrolyzed at different temperatures (600℃, 700℃, 800℃, and 900℃) to fabricate Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-600, Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-700, Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 and Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-900 samples, respectively. To verify the stability of CNTs-coated Co nanoparticles under strong acid conditions the Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800A sample were prepared via further washing the Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 samples with 2 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e solution for 12 h. To investigate the effect of different precursors on the calcined samples, we chose 800℃ as the calcination temperature. The Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs(1:2)-800 and Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs(1:4)-800 sample were prepared by calcining Co-ZIFs(1:2) and Co-ZIFs(1:4) precursor at 800℃ under H\u003csub\u003e2\u003c/sub\u003e/Ar atmosphere, respectively.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003eElectrochemical measurements\u003c/h2\u003e \u003cp\u003eORR electrochemical measurements were carried on Autolab electrochemical workstation (PGSTAT 302N) using the glassy carbon rotating disk electrode (Ф = 5 mm) as working electrode, Ag/AgCl electrode as the reference electrode and the platinum as counter electrode. For preparation of the catalyst slurry, 5.0 mg of resultant electrocatalyst was dispersed in the 750 \u0026micro;l DI water 250 \u0026micro;l ethanol and 50 mL Nafion solution (5.0 wt%) and ultrasonicated. Subsequently, 12 \u0026micro;L of slurry was dropped onto the polished RDE. The linear sweep voltammetry (LSV) measurements were performed with 5 mV s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Moreover, the rotation speeds of RDE chose 400, 625, 900, 1225, 1600, 2050 and 2500 rpm. To avoid the affecting of the background capacitive current, the LSV measurement was conducted under the same conditions under an N2-saturated electrolyte condition. The electron transfer number of ORR process was calculated by the Koutecky-Levich (K-L) equation:\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$${\\text{J}}^{-1}={{\\text{J}}_{\\text{K}}}^{-1}{+\\left(\\text{B}{{\\omega }}^{\\frac{1}{2}}\\right)}^{-1}$$\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equb\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equb\" name=\"EquationSource\"\u003e\n$$\\text{B}=0.62n\\text{F}{\\left({\\text{D}}_{0}\\right)}^{\\frac{2}{3}}{\\left({\\nu }\\right)}^{-\\frac{1}{6}} {\\text{C}}_{0}$$\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equc\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equc\" name=\"EquationSource\"\u003e\n$${\\text{J}}_{\\text{K}}=n\\text{F}{\\text{C}}_{0}$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere J is the measured current density during ORR, J\u003csub\u003ek\u003c/sub\u003e and J\u003csub\u003eL\u003c/sub\u003e are the kinetic current density and diffusion limiting current density, respectively. ω represents the electrode rotating angular velocity, B is the slope of K-L plots, n is the electron transfer number per oxygen molecule, F represents the Faraday constant (F\u0026thinsp;=\u0026thinsp;96485 C mol\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), D\u003csub\u003e0\u003c/sub\u003e is the diffusion coefficient of O\u003csub\u003e2\u003c/sub\u003e in 0.1 M KOH (1.9 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e cm\u003csup\u003e2\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), ν is the kinetic viscosity (0.01 cm\u003csup\u003e2\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), C\u003csub\u003e0\u003c/sub\u003e is the bulk concentration of O\u003csub\u003e2\u003c/sub\u003e (1.2 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e mol L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e).\u003c/p\u003e \u003cp\u003eThe in-situ EIS spectra were recorded under an AC amplitude of 10 mV at a frequency of 0.01 Hz-100 KHz under various electrochemical potentials in an O\u003csub\u003e2\u003c/sub\u003e-saturated 0.1 M KOH with potential range of 1 to 0.4 V versus RHE. EIS spectra were obtained using a VMP3 multichannel electrochemical station (VMP3). The EIS spectra were fitted to the equivalent circuit using commercial software (ZView, Scribner Associates Inc).\u003c/p\u003e \u003cp\u003eThe zinc-air flow battery tests were tested using the flow cell configuration with electrolyte flow rate of 6 mL min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, and O\u003csub\u003e2\u003c/sub\u003e involved comes from air atmosphere. The gas diffusion electrode (GDE) coated with Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 catalyst was the air cathode, and Zn plate was anode. Furthermore, the electrolyte of liquid rechargeable zinc-air flow battery was composed of 6 M KOH and 0.20 M Zn(CH\u003csub\u003e3\u003c/sub\u003eCOO)\u003csub\u003e2\u003c/sub\u003e. The catalyst layer was obtained by mixing the Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 catalyst (30 mg) and acetylene black (5 mg) in 5 mL of ethanol solution, followed by drop-casting the polytetrafluoroethylene (PTFE) emulsion (60 wt%, 22 \u0026micro;L). After mixing for 1 h and drying at 25\u0026deg;C for 12 h, the catalytic layer obtained was cut into circles with an effective surface area of 1 cm\u003csup\u003e2\u003c/sup\u003e. For comparison, the commercial Pt/C catalyst were assembled as air cathode of the zinc-air flow battery. The polarization curves were measured on the IVIUM electrochemical workstation (IVIUM TECHNOLOGIES BV). The charge/discharge tests were carried out on NEWARE battery testers (CT-3008 W) instrument at 5 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e, and one charging/discharging cycle time of was 10 min.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003eCharacterizations\u003c/h2\u003e \u003cp\u003eX-ray diffraction (XRD) was carried out by Rigaku TTR III with Cu-Kα radiation with λ\u0026thinsp;=\u0026thinsp;1.5418 \u0026Aring;. Raman spectra were obtained by Jobin-Yvon HR800 with the 532 nm excitation line of an Ar ion laser. X-ray photoelectron spectroscopy (XPS) measurements were performed on ESCALAB XI+. Fourier Transform infrared spectroscopy (FT-IR) measurements were measured with a Thermo NICOLET iS50 FT-IR. Transmission electron microscopy (TEM) images and energy dispersive spectrum (EDS) mapping were acquired on TALOS G2 F200X. The X-ray absorption spectra (XAS) including X-ray absorption near-edge structure (XANES)and extended X-ray absorption fine stucture (EXAFS) of the sample at Ni K-edge was colleted at the Beamline of TLS07A1 in National Synchrotron Radiation Research Center (NSRRC), Taiwan.\u003c/p\u003e \u003c/div\u003e "},{"header":"Declarations","content":"\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003eData availability\u003c/h2\u003e \u003cp\u003eThe data supporting the findings are available within the article and its Supplementary Information files. Supplementary Notes, Figs.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e\u0026ndash;23, Tables\u0026nbsp;1\u0026ndash;7, and refs. 1\u0026ndash;17. All the data reported in this work are available from the authors on request.\u003c/p\u003e \u003c/div\u003e\n\u003ch2\u003eContributions\u003c/h2\u003e \u003cp\u003eJing Zhao designed the study. Mufei Liu performed the electrochemical measurements. Mufei Liu and Guiling Wang. performed the measurements. Mufei Liu and Hongxing Dong discussed the Results. Mufei Liu, Jing Zhao, and Jinqiao Dong co-wrote the paper with contributions from all authors.\u003c/p\u003e \u003ch2\u003eCompeting interests\u003c/h2\u003e \u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e\u003ch2\u003eAcknowledgements\u003c/h2\u003e \u003cp\u003eThis work was supported by the Fundamental Research Funds for the Central Universities (No. 3072022QBZ1006) and the Natural Science Foundation of Heilongjiang Province jointly guided project (No. LH2021B007)\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eLi L et al (2021) Stretchable Energy Storage Devices Based on Carbon Material Small 17:2005015\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSumboja A et al (2017) All-Solid-State, Foldable, and Rechargeable Zn-Air Batteries Based on Manganese Oxide Grown on Graphene-Coated Carbon Cloth Air Cathode. 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Chem Eng J 426:131801\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLi P (2022) Bifunctional electrocatalyst with CoN\u003csub\u003e3\u003c/sub\u003e active sties dispersed on N-doped graphitic carbon nanosheets for ultrastable Zn-air batteries. Appl Catal B-Environ 316:121674\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSabhapathy P (2023) Axial Chlorine Induced Electron Delocalization in Atomically Dispersed FeN\u003csub\u003e4\u003c/sub\u003e Electrocatalyst for Oxygen Reduction Reaction with Improved Hydrogen Peroxide Tolerance. Small 19:2303598\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-3892690/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-3892690/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eTo meet the practical demand of zinc-air battery cathode noble metal catalyst substitutes are required. Herein, we integrating non-precious single-atom catalysis and space confinement present an effective approach for the large-scale, in-situ growth of CoN\u003csub\u003e3\u003c/sub\u003e-doped carbon nanotubes (CNTs) coated with Co nanoparticles (Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs), without adding additional additives. The in-situ grown CNTs serves a dual purpose by acting as a matrix for dispersed atomic CoN\u003csub\u003e3\u003c/sub\u003e sites and providing a space confinement effect on Co nanoparticles, resulting in lower energy barriers and superior mass transport capability. Furthermore, Co\u003csub\u003e3\u003c/sub\u003eC species derived from the Co-based zeolitic imidazolate frameworks (Co-ZIFs) act as catalysts for the direct arrangement of surrounding C-N groups. The resulting Co@CoN\u003csub\u003e3\u003c/sub\u003e/CNTs-800 displays remarkable oxygen reduction reaction (ORR) performance, with a half-wave potential of 0.84 V surpassing that of Pt/C counterparts. Moreover, the rechargeable zinc-air flow battery exhibits a peak power density of 169.5 mW cm\u003csup\u003e-2\u003c/sup\u003e and superior recyclability.\u003c/p\u003e","manuscriptTitle":"Carbon Nanotube-Encapsulated Cobalt for High-Efficiency Zinc-Air Flow Battery: Integration of Single Atom Catalysis and Space Confinement","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-01-31 04:52:22","doi":"10.21203/rs.3.rs-3892690/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"ac5b4b25-e8e9-4baf-87a3-37545801a646","owner":[],"postedDate":"January 31st, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":28467081,"name":"Physical sciences/Materials science/Materials for energy and catalysis/Electrocatalysis"},{"id":28467082,"name":"Physical sciences/Chemistry/Materials chemistry/Electronic materials"}],"tags":[],"updatedAt":"2024-03-07T11:45:13+00:00","versionOfRecord":[],"versionCreatedAt":"2024-01-31 04:52:22","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-3892690","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-3892690","identity":"rs-3892690","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}