Structural Modulation of Co Catalyzed Carbon Nanotubes with CuâCo

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Energy, Environmental, and Catalysis Applications

Structural Modulation of Co Catalyzed Carbon Nanotubes with CuCo Bimetal Active Center to Inspire Oxygen Reduction Reaction Zhongtao Li, Tiantian Yang, Weinan Zhao, Tao Xu, Liangqin Wei, Jianze Feng, Xiujie Yang, Hao Ren, and Mingbo Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18496 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 6, 2019

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ACS Applied Materials & Interfaces

Structural Modulation of Co Catalyzed Carbon Nanotubes with Cu-Co Bimetal Active Center to Inspire Oxygen Reduction Reaction Zhongtao Li,* Tiantian Yang, Weinan Zhao, Tao Xu, Liangqin Wei, Jianze Feng, Xiujie Yang, Hao Ren and Mingbo Wu* State Key Laboratory of Heavy Oil Processing, School of Chemical Engineering, China University of Petroleum, Qingdao 266580, P. R. China *Corresponding author: [email protected]. KEYWORDS. Bimetal Catalyst, Nitrogen Doping, Oxygen Reduction Reaction, Synergistic Effect, Zinc-air Batteries

ABSTRACT. Rational design of highly efficient catalyst for ORR is critical for development of advanced air cathode in Zn-air cells and fuel cells. To optimize the ORR performance of Co based cathode, the structure of carbon nanotube from DCI-Co precursor could be controlled through modulate synthetic parameters. The optimized ORR catalyst Co@NCNT-700 exhibit larger BET area, higher content of Co-Nx and graphitic N, which performance could be improved through Cu doping. The experiment data approved that the activity of Co-Nx was enhanced by the synergistic effect with introduced Cu. Furthermore, the high-performance zinc-air batteries was fabricated with the bimetal catalyst CuCo@NCNT-700 as an air electrode. The high open-cycle

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potential (1.54V) and peak power density (0.275 W.cm-2 at 0.474 A.cm-2) were achieved, which would be potentially used to develop next generation energy conversion devices.

INTRODUCTION Exploit advanced cathode material for fuel cells and metal-air batteries is highly required due to the shortage of fossil fuels and environmental deterioration. At present, transition metal catalyst, especially cobalt-based catalyst, have sparked increasing attentions as electrocatalysts owing to their geographically ubiquitous, lower cost and less susceptible to be poisoned than commercialized Pt/C.1-4 In addition, introducing carbon support can promote electronic-conductivity of Co-based catalyst and then accelerated the electron transfer rate.5-9 In previous reports, incorporation of Co derivations with various carbon materials would result in higher electrocatalytic performance towards oxygen reduction reaction (ORR) through special synergistic effect. The preparation processes are generally through high temperature thermal treatments, and the morphology of the products are difficult to be controlled.10 However, the reaction kinetic of Co-based catalyst to ORR is still sluggish than Ptbased catalyst, which need to be improved to meet the request of practically applications. Recently, the widespread concerns on multi-transition metal oxides (MTMO) that with two or more transition metal oxides alloys, like FexMn3-xO4,11 CoxNi2-xO312 etc., were increased rapidly due to low cost, high conductivity and good electrochemical


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performance other than single metal oxides. Among these metals, Cu is one of intriguing series in spinel, which catalytic activity in ORR is limited by the strong absorption capability with oxygen.13 However, the earth abundance, low price, minimum environmental impact and multiple valences are beneficial for the application in electrochemical devices. Therefore, incorporation of Cu with Co-based catalyst would be able to provide additional synergistic properties to solve the shortcomings of monometallic catalysts during the electrocatalysis, which would active the catalytic center and varied redox potentials.14 In previous reports, various hollow structures were catalyzed by the Co-based catalyst through calcination the compositions of Co derivations and carbon resources.15-16 Generally, the uniform distribution of Co and optimized nanostructure to expose more active sites on surface are two of key effects to promoting the catalysts activity.17 Herein, 4,5-dicyanoimidazole (DCI), possessed strongly coordinative iminazole and polymerizable cyan, was adopted as ligands to develop a MOF precursor (DCI-Co) through solvothermal protocol. After calcination, some well-defined hollow carbon nanotubes (Co@NCNT) have been developed, which structures could be controlled through modulate calcination temperature. To further promote catalyst activity, Cu was introduced through dipping the Co@NCNT-700 into copper acetate solution. The experiment data and DFT calculation verified the synergistic effect between introduced Cu-Nx and Co-Nx, and the ORR activity of Co-Nx obviously improved. The synthetic protocol presented here would be practically used to developed alternative ORR catalyst for next generation of energy conversion device.



RESULTS AND DISCUSSION

Scheme 1. The synthesis process of Co@NCNT-700 (I) and CuCo@NCNT-700 (II). The precursor of MOF-Co (DCI-Co) was synthesized by hydrothermal 4,5dicyanoimidazole (DCI) with cobalt acetate in N,N-Dimethylformamide (DMF) at 120 oC

for 2 hours and then the depositions were collected to pyrolysis at elevated

temperature for a serial catalysts with different structures, which structures could be tunable through control the pyrolysis temperature (carbonized at 600 oC, 700 oC, 800 oC

namely Co@NCNT-600, Co@NCNT-700, Co@NCNT-800, respectively) in

scheme 1 process I. To further improve the catalyst activity, the bi-metal catalyst namely CuCo@NCNT-700 was obtained through dipping Co@NCNT-700 into copper acetate solution for 30 minutes and then thermal treated at 300 oC in scheme 1 process 2. As another control sample, Cu@NC-700 was synthesized through the same process as that of Co@NCNT-700 except addition of cobalt acetate instead of copper acetate. The structure of MOF precursors can be verified by the FT-IR spectroscopy. As shown in Figure 1a, the peaks of imidazole at 900-1350 cm-1 and 1350-1500 cm-1 could be found in DCI-Co and DCI, while the band below 800 cm-1 would also ascribe to the external vibration of imidazole ring in both samples. The strong peak both in DCI-Co and DCI at 638 cm-1 is attributed to the stretching mode of cyano groups. Additionally, it can be clearly observed that the tensile vibration peaks (2250 cm-1 and 3350 cm-1) of cyano groups in DCI disappeared after coordination of Co in DCI-Co and the new bands


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appeared at 1700 cm-1, which due to the formation of Co-Nx coordination bonds.20 The crystalline structure of Co-MOF was measured by XRD. As shown in Figure S1a, the clear diffraction peaks could be identified due to the perfect crystallinity of MOF-Co, which is very similar to the typical structure of the serials of zeolitic imidazole frameworks (ZIF).18-19

Figure 1. (a) FT-IR patterns of DCI and DCI-Co. (b) Raman images of Co@NCNT600, Co@NCNT-700 and Co@NCNT-800. (c) Pore-size distribution curves of Co@NCNT-600, Co@NCNT-700 and Co@NCNT-800. (d-f) The TEM image of Co@NCNT-600, Co@NCNT-700 and Co@NCNT-800 (The inset of Figure 1e is the HRTEM of Co@NCNT-700). In general, Co is considered as one classical catalyst for the growth carbon nanotubes (CNT) at high temperature.21-23 The TEM image of the Co@NCNTs that obtained through pyrolysis of DCI-Co at various temperature are shown in Figure 1. In Figure 1d, a small amount of short and irregular distributed carbon nanorods are formed after



annealing DCI-Co at 600 oC. While in Figure 1e, a large amount well-defined CNTs are easily identified after carbonized DCI-Co at 700 oC. When heat up the MOF-Co at 800 oC, the surface of CNT surface become roughness and diameter are increased (Figure 1f), which due to the violent side carbonization reaction at high temperature.2427

The High-resolution TEM image of Co@NCNT-700 indicated some nanoparticles

were sealed in the end of CNT. The inset in Figure 1e shows that the lattice spacings of these nanoparticles was 0.208 nm and the lattice spacings on the surface of particles was 0.338 nm, which are attributed to the (111) plane of metal Co nanoparticles and the (002) plane of graphene, respectively.28 The experiment data above indicated that the DCI-Co could be gradually reduced to metal Co during heating up with carbon, which could catalyze DCI to growth well-ordered CNTs at higher temperature such as 700 oC. To further explore the influence of temperature on chemical structure, the Raman spectra (Figure 1b) of Co@NCNT-600, Co@NCNT-700 and Co@NCNT-800 were tested, all of which exhibited two peaks at 1580 cm-1 and 1340 cm-1 (graphite sp2 carbon (G band) and disordered sp3 carbon (D band, respectively).29 The ratio of intensity between the D and G band (ID/IG) was 1.36 for Co@NCNT-600, which was much higher than 0.98 of Co@NCNT-700 and 0.97 of Co@NCNT-800. Therefore, the degree of graphitization seems obviously increased with the increasing of carbonization temperature.30 The results of BET analysis in Figure 1c and Figure S3 shown that the larger specific surface area (218.46 m2/g) of Co@NCNT-700 than those of Co@NCNT-800 (203.17 m2/g) and Co@NCNT-600 (156.61 m2/g). Besides, the ratio


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of micropore to mesopores is 11.1% (Co@NCNT-600), 9.2% (Co@NCNT-700) and 8.3% (Co@NCNT-800).

Figure 2. (a) The comparison of different types of metal nitrogen bond (left) and total cobalt content of Co@NCNT-600, Co@NCNT-700 and Co@NCNT-800. (b) The comparison of different types of nitrogen (left) and total nitrogen content of Co@NCNT-600, Co@NCNT-700 and Co@NCNT-800. (c) The CV curves of Co@NCNT-600, Co@NCNT-700 and Co@NCNT-800. (d) Linear sweep voltammetry (LSV) curves of Co@NCNT at different calcine temperatures and Pt/C in O2-saturated 0.1M KOH at 10 mV s-1 and the electrode speed rate is 1600 rpm. X-ray photoelectron spectroscopy (XPS) analysis of the Co@NCNTs was carried out to confirm the surface structure. The surface element concentration of Co@NCNT-600,



Co@NCNT-700 and Co@NCNT-800, was calibrated by XPS (Table S1). The content of cobalt decreases with the increase of temperature, which proves that the content of the uniformly dispersed cobalt MOFs are gradually convert to cobalt particles and embedded into the carbon nanotubes. When the temperature rises to 800 oC, the nitrogen content in Co@NCNT-700 is much lower than that of Co@NCNT-600 and Co@NCNT-700, which indicates that the deposition of N-containing groups at elevated temperature. This may also be one reason of the deterioration in performance of Co@NCNT-800. After Cu doping, the content of Co slightly decreased, which indicated the partly substitution of Co with Cu. In Figure S2a, three types of peaks at 778.39, 779.62 and 781.05 eV in high resolution spectra of Co could been obtained in all Co@NCNTs, which correspond to metallic Co, Co-Nx-Cy and Co-Nx, respectively.10, 29 As shown in Figure 2a, the content of Co on surface dramatically decreased from 600 oC (5.25%) to 700 oC (2.56%).31-33 When the temperature increased further from 700 oC to 800 oC, the content of Co kept stable (2.56% and 2.82%, respectively). Experiment data revealed that the thicker and longer nanotubes were grown on the surface of Co nanoparticles which lead to the decreasing of Co on surface. When the pyrolysis at 700 oC, all of the Co nanoparticles were embedded and the structure kept stable at even higher temperature. In Figure 2a, the content of Co-Nx are the highest after carbonized 700 oC. With the increasing of carbonized temperature, part of Co-Nx were decomposed to Co-Nx-Cy although the content of Co on nanotubes remained unchanged (Figure 2a). The nitrogen content decreased obviously with the increase of pyrolysis temperature (in Figure 2b), which would due to the conversion of


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Co-Nx to Co-Nx-Cy at higher temperature. While, the content of graphitic N in Co@NCNT-700 was the highest. Moreover, it has been proved that the Co-Nx species own higher activities for ORR, and the higher content graphitic nitrogen is also beneficial to ORR performance.31, 34-36 The ORR electrocatalytic activity of Co@NCNT-600, Co@NCNT-700 and Co@NCNT-800 were examined in oxygen saturated 0.1 M KOH. In Figure 2c, an obvious higher cathodic peak of Co@NCNT-700 (0.854 V vs RHE) than that of Co@NCNT-600 (0.802 V vs RHE) and Co@NCNT-800 (0.844V vs RHE) indicates the higher ORR activity. As shown of linear sweep voltammetry (LSV) curves in Figure 2d, there was a significantly better ORR activity of Co@NCNT-700, which indicated much more positive onset potential 0.928V (vs RHE) and half-wave potential 0.826 V (vs RHE) compared with Co@NCNT-600 (0.787 V vs RHE and 0.898 V vs RHE) and Co@NCNT-800 (0.802 V vs RHE and 0.91 V vs RHE). The better ORR activity of Co@NCNT-700 would ascribe to the largest specific area, higher degree of graphitization and especially the highest content of Co-Nx with evenly distribution.33,36



Figure 3. (a) SEM and (b) TEM image of CuCo@NCNT-700. (c) EDS elemental mapping of CuCo@NCNT-700 showing the presence of C, Co, Cu and N. (d) Highresolution Co 2p of CuCo@NCNT-700 and Co@NCNT-700. (e) High-resolution Cu 2p of CuCo@NCNT-700 and Cu@NC-700. To further optimize the performance of the Co@NCNT-700, Cu is introduced to synthesis bi-metal/NCNT nanohybrids. To avoid the metal aggregation, the Co@NCNT-700 is dipped into copper salt solution for 30 minutes and then heat up to 300 oC to achieve a firmly binding of Cu with carbon substrate. In the TEM and SEM image (Figure 3a, b), the morphology of CNT in original CuCo@NCNT-700 are


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reserved after Cu doping without any obvious agglomerated nanoparticles and clusters, which indicated our protocol effectively avoid the aggregation of metal Cu. As compared, the TEM of Cu@NC-700 displayed in Figure S5, there were plenty of wrinkled graphene nanosheets. This result indicated that the CuCo@NCNT-700 has better dispersity and can expose more catalytic active sites. Similarly, the specific area of CuCo@NCNT-700 is 231.09 m2g-1 while the specific area of Co@NCNT-700 is 218.46 m2g-1 and Cu@NC-700 is 14.51 m2g-1 (Figure S6a). The Co catalyzed carbon nanotubes could effectively increase the surface area after thermocracking. After Cu doping, the specific area of CuCo@NCNT-700 is almost the same as Co@NCNT-700, which indicated the same nanostructure after Cu doping. From the Figure S7b, the pore size is slightly increased after doping with copper, which may be due to more defects produced during doping and the following thermal treatment. The EDS mapping image of TEM (Figure 3c) also approved that Cu atoms are uniformly distributed on the surface of CNT without obvious aggregation. While, the Co, Cu and N also uniformly distributed on CNT surface, which would due to the formation of Co-Nx and Cu-Nx on surface. While, the nanocrystals of metal Co that sealed inside CNT still reserved in sample CuCo@NCNT-700. To further explore chemical structure, the XPS measurements indicated the elemental composition and oxidation states of CuCo@NCNT-700 in Figure 3d and Table S1. The content of copper is almost the same in CuCo@NCNT-700 and Cu@NC-700. Highresolution Co 2p of CuCo@NCNT-700 spectrum was deconvoluted into three peaks centered at 777.9, 779.96 and 781.19 eV (compared with Co@NCNT of 778.39, 779.62



and 781.05 eV) which are assigned to metallic Co, Co-Nx-Cy and Co-Nx, respectively.10 In Figure 3e, the high-resolution Cu 2p peak of the CuCo@NCNT-700 composite show that two deconvoluted peaks at 932.04 and 934.59 eV (compared with Cu@NC of 932.24 eV and 934.26 eV), both of which correspond to Cu-N species. So, there is almost no metallic copper could be found on CNT surface. In comparison, Co-Nx and Cu-Nx in CuCo@NCNT-700 are all showed slight shifts than that in Co@NCNT-700 and Cu@NC-700 (0.14 eV and 0.33 eV) respectively, which would due to the synergistic effects to the changing of electronic clouds of Cu-Nx and Co-Nx.38-41 The synergistic effects between Cu-Nx and Co-Nx is further investigated through the following experiments and DFT calculations.


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Figure 4. (a) CVs of CuCo@NCNT in Ar and O2 saturated 0.1 M KOH solution at a scan rate of 50 mV/s (b) LSV curves of CuCo@NCNT-700, Co@NCNT-700, Cu@NC700 and Pt/C. (c) The electron transfer number of CuCo@NCNT-700, Co@NCNT-700 and Cu@NC-700 and Pt/C. (d) LSV curves of CuCo@NCNT-700 with and without SCN- in 0.5M H2SO4. (e) Chronoamperometric responses of CuCo@NCNT-700 and Pt/C catalysts in O2-saturated 0.1 M KOH. (f) Chronoamperometric response of CuCo@NCNT-700 and Pt/C.



The

ORR

electrocatalytic

activity

of

Cu@NC-700,

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Co@NCNT-700

and

CuCo@NCNT-700 were examined in 0.1 M KOH saturated with oxygen bubbling (Figure 4a and Figure S7). An obvious higher cathodic peak of CuCo@NCNT-700 (0.883 V vs RHE) than that of Co@NCNT-700 (0.854 V vs RHE) indicates higher ORR activity. While, Cu@NC-700 exhibit the poorest ORR activity (0.834V vs RHE) without element Co. The ORR performances of the as-prepared catalysts were studied further by Linear Sweep Voltammetry (LSV). In Figure 4b, a clear advantage of CuCo@NCNT-700 was observed both ORR onset potential and half-wave potential. The onset potential and half-wave potential CuCo@NCNT-700 are 0.964 V (vs RHE) and 0.844 V (vs RHE), while Co@NCNT-700 and Cu@NC-700 shown deteriorated performances (0.928 V vs RHE onset, 0.826 V vs RHE half-wave and 0.864 V vs RHE onset, 0.678 V vs RHE half-wave potential, respectively). In order to gain further insight into electron transfer process, the n value of CuCo@NCNT-700 was calculated as 3.78, which close to Pt/C (3.94) and higher than Co@NCNT-700 (3.5) and Cu@NCNT-700 (3.2) (Figure 4c). Experiment data further implies the most efficient four electron transfer process during CuCo@NCNT-700 catalyzed ORR reaction among as-prepared samples. As shown in Figure S8, the Tafel slope of CuCo@NCNT700 (92 mV) is close to commercial Pt/C (90 mV) which indicated that CuCo@NCNT700 has the similar Kinetic reaction path as Pt/C.42 The durability and methanol tolerance are another two key parameters for ORR catalysts.10 The chronoamperometric method was adopted to investigated the ORR stability at constant voltage (0.5 V vs RHE). After stability test, the current retention of


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CuCo@NCNT-700 was 96%, while the commercial Pt/C suffered about 50% loss of initial current density (Figure 4e). The above results suggested a better durability of CuCo@NCNT-700 during a continuous ORR process. Moreover, we also compared the

anti-methanol

properties

of

CuCo@NCNT-700

with

Pt/C

(20%)

by

chronoamperometric measurements in Figure 4f. Almost no fluctuation of CuCo@NCNT-700 could be observed after the addition of methanol, while an obvious decline was observed of commercial Pt/C (20%) after the injection of methanol, which suggested that the CuCo@NCNT-700 possessed excellent durability and robust antimethanol performance. Based on the experiment data above, the Co nanoparticles embedded in the top of the nanotube and Co-Nx active center on the catalytic surface are existing simultaneously in CuCo@NCNT-700. Through dip-doping, the Cu have been introduced into the surface of catalyst, which lead to the increasing of ORR activity. Even through the Co nanoparticles in the catalyst have high activity in some reports, the activity of Co nanoparticles herein seems hardly be influenced by the introduced Cu on the catalytic surface due to the hinder of the carbonic coating on Co nanoparticles. Therefore, the improving of ORR activity after dip-doping of Cu are supposed through the synergistic effect between active centers of introduced Cu-Nx and Co-Nx, both of which are dispersed on the surface of catalyst. To verify this phenomenon, the leaching experiments was performed. As we know, SCN- ion can poison Co-Nx in catalyzing ORR.43-44 Due to the reaction of SCN- ion with KOH, the “poison” experiment to detect active sites of ORR in CuCo@NCNT-700 was measured in 0.5M H2SO4, which LSV



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profiles were compared with before and after the addition of 0.01 M KSCN. Figure 4d shown that both the half-wave potential and the onset potential decreases significantly after addition of SCN-. The greatly depression of catalytic activity of CuCo@NCNT700 can be attributed to SCN- on blocked the Co-Nx activity site during catalyzing ORR, which suggested that Co-Nx was the activity site of [email protected] Table 1. DFT calculation results of the bond length of O2 adsorbed on CuCo@NCNT700, Co@NCNT-700 and Cu@NC-700 and the absorption energy of O2 on CuCo@NCNT-700, Co@NCNT-700 and Cu@NC-700 (the Cu or Co using the Italics in table 1 represent the adsorption sites of oxygen). Catalysts O2 Co@NCNT-700 Cu@NC-700 CuCo@NCNT-700 CuCo@NCNT-700

Adsorption energy (eV) --0.797 -0.232 -0.928 -0.279

Bond length (O2)( Å) 1.211 1.283 1.289 1.291 1.252

Figure 5. The adsorption energy of Co@NCNT-700, Cu@NC-700, CuCo@NCNT-700 (The red atom is the adsorption site).


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In order to further understand the synergistic effect of Co-Nx and Cu-Nx on the CuCo@NCNT-700, the adsorption energy and the length of the O-O with the connection at different metal active center are explored in Table 1, Figure 5 and Figure S9.44-46 The adsorption energy of the vital species on Co-N4 in CuCo@NCNT-700 is strongest (-0.928 eV) than other three species (Co-N4 of Co@NCNT-700 is -0.797 eV, Cu-N4 of Cu@NCNT-700 is -0.232 eV and Cu-N4 of CuCo@NCNT-700 is -0.279 eV, respectively) in Figure 5 and Table 1. Many studies have proved that the adsorption energy is higher, the oxygen reduction is more likely to occur.47-48 Similarly, the length of O-O on Co-N4 of CuCo@NCNT-700 is longer (1.291 Å) than other three species ((Co-N4 of Co@NCNT-700 is 1.283 Å, Cu-N4 of Cu@NCNT-700 is 1.289 Å and CuN4 of CuCo@NCNT-700 is 1.252 Å, respectively) which suggested that absorbed oxygen molecules are more active for reaction.49 The theoretical results are in good agreement with the experimental results. The synergistic effect between Co-N4 and CuN4 can affect the electronic structure and change the intermediate adsorption energy, thus significantly improving the catalytic activity of the catalyst.



Figure 6. (a) A schematic diagram of primary Zn-air battery. (b) Photograph of the corresponding open-circle potential (OCP). (c) Polarization curves and the corresponding power density plots of on the CuCo@NCNT-700 and Pt/C, and (d) Galvanostatic discharge plots of the Zn–air battery with the CuCo@NCNT-700 and Pt/C air-cathode at various current densities In order to further explore the practically application of CuCo@NCNT-700 in practical energy equipment, a homemade Zn-air battery was fabricated by 6 m KOH electrolyte, a zinc plate anode and CuCo@NCNT-700 loaded carbon cloth as air cathode (Figure 6a).33-34 For comparison, Pt/C was utilized as the rechargeable noble metal air cathode in the control battery. The structure of Zn-air battery schematically shown in Figure 6a. In Figure 6b, the CuCo@NCNT-700-based battery exhibit a high open-circuit potential (OCP) at 1.54 V. As shown in Figure 6c, the polarization and power density curves


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exhibited a close peak power density at higher current density of CuCo@NCNT-700based battery (0.275 W cm-2 at 0.474 A cm-2) to the Pt/C-based one (0.26 W cm-2 at 0.372 A cm-2). To evaluate the stability of the batteries, galvanostatic discharge measurements were employed. At 10, 20, 50, and 100 mA cm-2 discharge current densities, the CuCo@NCNT-700-based battery exhibits stable potential plains over 1.0 V, which suggest that the excellent discharge performance and durability. The CuCo@NCNT-700 generates a specific capacity of 800 mAh g-1Zn at 10 mA cm-1 (corresponding to an energy density of 1041.2 Wh kgZn-1). In Figure S7b, the battery could also exhibit remarkable specific capacity of 760 mAh g−1 (corresponding to an energy density of 820.8Wh kgZn-1) even at a higher discharge rate (100 mA cm-1). CONCLUSIONS In summary, a serial of Co@NCNTs catalysts were synthesized through pyrolysis a DCI-Co precursor at various temperature. The experiment data above indicated that the DCI-Co could be gradually reduced to metal Co during heating up, which could catalyze DCI to growth well-ordered CNTs at higher temperature. When the pyrolysis at 700 oC, the well-defined nanotubes were grown on the surface of Co nanoparticles in Co@NCNT-700, which lead to optimized ORR activities due to lager BET area, higher content of Co-Nx and graphitic N. To further improve the ORR activities, CuCo bimetal ORR catalyst was synthesis through dip doping of Cu into Co@NCNT-700. The synergistic effect between Cu-Nx and Co-Nx obviously improved the ORR activities, which could be used as cathode of Zin-air battery. The battery could also



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exhibit remarkable specific capacity of the specific capacity of the battery can reach 760 mAh g-1 (equivalent to the energy density of 820.8Wh kgZn-1) even at a higher discharge rate (100 mA cm-1). The superiority of the bimetal catalyst would provide an additional option to develop novel energy conversion devices. EXPERIMENTAL SECTION Syntheses and Characterization. All chemicals and reagents were not further purified. For details on the synthesis and characterization of materials, please refer to Supporting Information. ACKNOWLEGEMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 21572269, 21502227 and 51873231), and the Fundamental Research Funds for the Central Universities (17CX05015, 15CX08005A), the Financial Support from Taishan Scholar Project, the Key Research and Development Program of Shandong Province, China (2017GGX40118). ASSOCIATD CONTENT Support information is available free of charge from the ACS Publications Website. Figure from S1 to S10 and Table S1 (PDF). The details of experimental section, structure

characterization,

electrochemical

characterization,

Zinc-Air

evaluation and density functional theory (DFT) calculation of catalyst.


battery

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AUTHOR INFORMATION Corresponding Authors *Email: [email protected]. ORCID Zhongtao Li: 0000-0003-0157-6098 Mingbo Wu: 0000-0003-0048-778X Notes The authors declare no competing financial interest REFERENCES (1) He, D.; Zhang, L.; He, D.; Zhou, G.; Lin, Y.; Deng, Z.; Hong, X.; Wu, Y.; Chen, C.; Li, Y., Amorphous Nickel Boride Membrane on a Platinum–Nickel Alloy Surface for Enhanced Oxygen Reduction Reaction. Nat. Commun. 2016, 7, 12362. (2) Dai, L.; Mo, S.; Qin, Q.; Zhao, X.; Zheng, N., Carbon Monoxide-Assisted Synthesis of Ultrathin PtCu3 Alloy Wavy Nanowires and Their Enhanced Electrocatalysis. Small 2016, 12, 1572-1577. (3) Bao, M.; Amiinu, I. S.; Peng, T.; Li, W.; Liu, S.; Wang, Z.; Pu, Z.; He, D.; Xiong, Y.; Mu, S., Surface Evolution of Pt Cu Alloy Shell over Pd Nanocrystals Leads to Superior Hydrogen Evolution and Oxygen Reduction Reactions. ACS Energy Letter. 2018, 3, 940-945.



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Structural Modulation of Co Catalyzed Carbon Nanotubes with CuâCo

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