Electrospun Thin-Walled CuCo2O4@C Nanotubes as Bifunctional

Nov 22, 2017 - Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of Chemistry, Nankai University, Tianjin 300071,...
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Electrospun Thin-walled CuCo2O4@C Nanotubes as Bifunctional Oxygen Electrocatalysts for Rechargeable Zn-Air Batteries Xiaojun Wang, Yang Li, Ting Jin, Jing Meng, lifang jiao, Min Zhu, and Jun Chen Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b04502 • Publication Date (Web): 22 Nov 2017 Downloaded from http://pubs.acs.org on November 22, 2017

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Electrospun Thin-walled CuCo2O4@C Nanotubes as Bifunctional Oxygen Electrocatalysts for Rechargeable Zn-Air Batteries Xiaojun Wang,† Yang Li,† Ting Jin,† Jing Meng,† Lifang Jiao*,†,‡,Min Zhu# and Jun Chen†,‡ †

Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of Chemistry, Nankai University, Tianjin 300071, China

‡

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, China #

School of Materials Science and Engineering and Guangdong Provincial Key Laboratory of Advanced Energy Storage Materials, South China University of Technology, Guangzhou 510641, China

KEYWORDS: Thin-walled CuCo2O4@C nanotubes, coaxial electrospinning, bifunctional oxygen electrocatalyst, Zn-air batteries

ABSTRACT: Rational design of optimal bifunctional oxygen electrocatalyst with low cost and high activity is greatly desired for realization of rechargeable Zn-air batteries. Herein, we fabricate mesoporous thin-walled CuCo2O4@C with abundant nitrogen-doped nanotubes via coaxial electrospinning technique. Benefiting from high catalytic activity of ultrasmall CuCo2O4 particles, double active specific surface area of mesoporous nanotubes, and strong coupling with N-doped carbon matrix, the obtained CuCo2O4@C exhibits outstanding oxygen electrocatalytic activity and stability, which in terms of a positive onset potential (0.951 V) for oxygen reduction reaction (ORR) and a low overpotential (327 mV at 10 mA cm−2) for oxygen evolution reaction (OER). Significantly, when used as cathode catalyst for Zn-air batteries, CuCo2O4@C also displays a low charge−discharge voltage gap (0.79 V at 10 mA cm−2) and a long cycling life (up to 160 cycles for 80 h). With the desirable architecture and excellent electrocatalytic properties, the CuCo2O4@C is considered as one of the promising 1 ACS Paragon Plus Environment

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electro-catalyst for Zn-air batteries.

Increasing energy demand inspires developing alternative energy storage and conversion systems with high energy density.1, 2 Currently, rechargeable zinc-air (Zn-air) batteries have received world-wide attention due to their high theoretical energy density (1086 Wh kg-1), abundant resource and environmental benignity.3, 4 The state-of-the-art electro-catalysts for Zn-air batteries are remained noble metal-based materials (such as Pt, Ir and their oxides), which can efficiently accelerate oxygen reduction/evolution reactions (ORR/OER). However, such noble metal-based materials still face these issues such as high cost, limited cycling life and catalytic selectivity (Pt for ORR and Ir for OER).5-7 Therefore, exploring optimal bifunctional electro-catalysts for both ORR and OER with economic viability, efficient activity and superior durability to the realization of Zn-air batteries is still a challenge. Mixed-valence transition-metal oxides with spinel structure have sparked more attention as bifunctional electro-catalyst because of their low price and high catalytic efficiency.8-12 Among them, CuCo2O4 displays the advantage of high catalytic activity by the virtue of their tailored electron between Cu and Co ions. Importantly, the structural flexibility and variable valence states of the spinel oxides provide great potentials to fine-tune their catalytic performance.13, 14 However, the main issue along with bimetallic oxide materials is their low conductivity, resulting in inferior cycling stability. One effective approach is making the bimetallic oxides with coupling carbon substrate.15, 16 For example, CuCo2O4 nanoparticles supported on reduced graphene oxide exhibited a decrease of 14% in current density over 20000 s of continuous operation for ORR.13 Although carbon matrix can certainly improve the conductivity, the low availability of large specific surface area is still a weakness for further improving electro-catalytic properties owing to the poor contact areas between the active materials and electrolyte. Attempts have been made on exposing more active sites to increase the utilization rate of catalytic material.17-20 Recently, Cheng and colleagues 2 ACS Paragon Plus Environment

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successfully prepared spinel CuCo2O4 quantum dots anchoring on nitrogen doped carbon nanotubes, the enhanced conductivity and the enlarged specific surface areas (116.18 m2 g-1) of catalysts could contribute to the long lifetime (48 h in liquid state) for Zn air battery.21 Even though a lot of inspiring works make great progress in cycling stability for Zn-air batteries, the catalytic activity is still far away from meeting application of Zn-air batteries because the structure of narrow oxygen/hydroxyl transportation channels for catalysts result in sluggish diffusion kinetics. Thus considerable efforts in the rational design of advanced architecture with broadly diffused pathway are still desired. Thin-walled porous-nanotubes with open texture could not only provide large specific areas for catalyst reaction but offer more channels for oxygen/hydroxyl diffusion.16,

22

Compare to hydrothermal method,23 in-situ growth approach,24 and nucleation crystallization strategy,25 electrospinning is a facile technique to directly construct the architecture of thinwalled nanotubes with mesoporous surface. Peng’s group successfully prepared Fe-based nitrogen/oxygen codoped carbon nanotubes grown on carbon-nanofiber films (FeNO-CNTCNFFs) via a facile free-surface electrospinning technique, exhibiting the highest bifunctional oxygen catalytic activities in terms of a positive half-wave potential (0.87 V) for ORR and low overpotential (430 mV@10 mA cm−2) for OER.26 Mai’s group also synthesized the CoMn2O4 multilevel nanotubes via electrospinning method, showing high stability with about 92% current retention after 30000 s for ORR.27 Significantly, utilize the different pyrolysis rates of different molecular polymer to synthesis of thin-walled mesopores nanotubes with more gas channels that are covered CuCo2O4 nanoparticles has not yet been reported. It is of great interest to apply electrospinning to design such nanostructured CuCo2O4@C as the bifunctional electrocatalyst for Zn-air batteries. Herein, we report on the fabrication of N-doped thin-walled CuCo2O4@C nanotubes with mesoporous surface via coaxial electrospinning technique and their further application as bifunctional oxygen electrocatalyst for Zn-air batteries. The as-prepared CuCo2O4@C with a 3 ACS Paragon Plus Environment

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specific surface area of 514 m2 g−1 exhibits a positive half-wave potential of 0.850 V for ORR, and a low overpotential of 327 mV at 10 mA cm−2 for OER. Meaningfully, the as-built Zn-air batteries display a low discharge-charge voltage gap (0.79 V at 10 mA cm−2) and long cycle life (up to 160 cycles for 80 h). The excellent electrocatalytic properties are ascribed to the advanced structure of CuCo2O4@C, which provides a variety of superiorities: numerous mesoporous ensuring more channels for oxygen escape and hydroxyl diffusion, double active specific surface area offering more exposed active site for catalytic reaction, interconnected 1D open-ended nanotubes leading to superior electronic contact for external circuit, and doping N also contributing to oxygen redox catalysis.

Scheme 1 illustrates the synthesis procedure of thin-walled CuCo2O4@C (denoted as CCO@C) nanotubes via coaxial electrospinning technique. It mainly includes electrospinning fiber membrane, followed by stabilization, carbonization and oxidation treatment of the obtained film. It is worth mentioning that after the electrospinning process, the low- and highmolecular-weight PAN prone to be distributed into two layers under the coaxial action in the radial direction of nanowires. With the temperature gradually increasing, the inner low-weight PAN first pyrolysis and moves to the boundary between the low- and high- weight PAN, then creating the open-ended hollow nanotubes. Significantly, under high-temperature calcining in argon and then low-temperature sintering in air, the PAN and metal salts decompose to generate gases, so the uniform mesoporous on surface of tubes are obtained. Figure 1a presents the Rietveld refined X-ray diffraction (XRD) pattern of CCO@C composite. All diffraction peaks match well with cubic spinel-type CuCo2O4 (space group: Fd-3m, JCPDS card No. 78-2177), and refinement result shows that the unit cell structure consists of CuO4 tetrahedra and CoO6 octahedra (Figure 1b). The carbon exists in the CCO@C composites was demonstrated by Raman spectrum, just as shown in Figure S1 (supporting information, SI). The two prominent peaks at 1349 and 1591 cm-1 belong to 4 ACS Paragon Plus Environment

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defect-induced mode (D bands) and graphitic-induced mode (G bands) of carbon, respectively.28 The weak peaks around 453 cm-1 and 651 cm-1 are assigned to CuCo2O4. Element analysis (EA) applied to further determine the CuCo2O4 content of composite, which is approximately 19.64 wt%. This value is in consistent with the thermogravimetric analysis (TGA, Figure S9, SI) result.

Scheme 1. Schematic illustration of preparation process for CuCo2O4@C nanotubes. 1) After coaxial electrospinning process, the low- and high-molecular-weight PAN tend to be distributed into two layers, 2) As the temperature is slowly increased, the inner low-weight PAN first pyrolysis and moves to the boundary of the low-/high-molecular-weight PAN, 3) With the temperature continues to rise, the PAN carbonized and metal salts decomposed, thus creating more porous on the tube surface, 4) After sintering in air, the CuCo2O4 nanoparticles supported on thin-walled nanotubes with uniform mesoporous surface are obtained.

The morphology and nanostructure of CCO@C were characterized via scanning electron microscopy (SEM) and transmission electron microscopy (TEM) technologies. As shown in Figure 1c and d, the CCO@C nanotubes are continuous and rough with abundant pores on 5 ACS Paragon Plus Environment

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fiber surface, which could improve the diffusion kinetics of hydroxide and favor the transportation of oxygen.29 TEM image (Figure 1e) displays all the CuCo2O4 nanoparticles are surrounded by amorphous carbon. It is worthwhile to emphasize that the nanotubes outer diameter is around 130 nm and a wall thickness is about 25 nm. The large interior channels (diameter of ~ 80 nm) effectively double the active surface area of 514 m2 g−1 (Figure S2, SI), thus improving the reaction activity between hydroxyl, oxygen and catalyst. Importantly, the nanoparticle size of CuCo2O4 in CCO@C is around 7 nm (Figure S3). As far as we know, the composite which simultaneously controls the smallest CuCo2O4 nanodots and ensures so large internal channels has never been reported. The clear lattice fringes with a d-spacing of 0.47 nm and 0.28 nm correspond to the (111) and (220) plane of CuCo2O4 with an angle of 35° between this two planes (Figure 1f), which is consistent with the standard value. TEM elemental mapping images (Figure 2g) revealing that Cu, Co, O, C and N are uniform distribution along the hollow carbon matrix. EA and XPS also illustrates that CuCo2O4@C contains 5.67 wt% N species (Figure S4 and Figure S9, SI). Significantly, different reaction conditions, mainly including diverse carbonization temperature and diverse oxidation temperature, were attempted to optimize the morphology and

catalytic

properties.

Figure

S5

(SI)

presents

the

SEM

images

of

Cu(NO3)2·3H2O/Co(CH3COO)2·4H2O@PAN (denoted as CuCo@PAN) nanofibers. When the composite carbonize at different temperature, the nanofibers display diverse architecture (Figure S6 (SI)). We can see that a lower temperature (700℃) leads to a smooth surface, while a higher temperature (900 ℃ ) results in rough surface with a certain degree of aggregation for nanoparticles. The possible mechanism that causes such difference mainly resulted from the pyrolysis degree of PAN, the decomposition of metal salts (Cu(NO3)2•3H2O/Co(CH3COO)2•4H2O@PAN). And more importantly, compared to the lower carbonization temperature (700 ℃), the external wall is more likely to collapse in the higher 6 ACS Paragon Plus Environment

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Figure 1. (a) Rietveld refined XRD pattern with experimental data (black line), calculated curves (red dots), position of allowed Bragg reflections (purple vertical bars), and difference profile (blue line); (b) Crystal structure with CuO4 tetrahedra (green) and CoO6 octahedra (blue); (c, d) SEM and (e, f) TEM images of CCO@C, (g) TEM elemental mapping images of CCO@C. carbonization temperature (900 ℃) because the inner hollow structure has been preliminarily formed. The morphology under 800℃ carbonized, by contrast, is much more suitable for next oxidation procedure owing to the ideal porousness. For comparison, we adjust the carbon content by altering the oxidation temperature (higher and lower oxidation temperature obtained production named H-CCO and L-CCO@C, respectively). The XRD patterns and Raman spectra of L-CCO@C and H-CCO are displayed in Figure S7 (SI). In combination with these samples SEM and TEM images (Figure S8, SI). L-CCO@C exists in a reticular morphology with indistinct hollow structure; it mainly results from the lower oxidation temperature not able to removal the carbon completely. However, the H-CCO tubes are stacked together with narrow vacant in the middle of nanotubes because the higher 7 ACS Paragon Plus Environment

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temperature induces some shrinkage of nanofibers.30, 31 Considering the uniform nanotubes morphology, appropriate carbon content (Figure S9, SI) and optimal specific surface areas (Figure S10, SI), 350 °C should be the suitable oxidation temperature. The ORR activities of CuCo2O4 samples were measured and compared using rotating ring-disk electrode (RRDE) in O2-saturated 0.1 M KOH solution (Figure 2a). Remarkably, the CCO@C shows a much more positive onset potential (0.951 V) than those of the LCCO@C (0.927 V) and H-CCO (0.888 V). Significantly, the ORR properties of CCO@C nanotubes also show close to that of Pt/C (onset potential: 0.961 V, half-wave potential: 0.862 V). The electron transfer number per oxygen molecule (n) of CCO@C composite was determined from the linear sweep voltammetry (LSV) curves (Figure 2b), n is calculated to be about 3.9, implying our CCO@C composite is able to straightly reducing the H2O to OHby an efficient four electron reaction. This results mainly because the advanced morphology of thin-walled CCO@C nanotubes with mesoporous surface could offer more active sites and more transport channels for gas transportation. Although the HO2- yield above (6.3 %) than that of Pt/C (3.55 %) electrode (Figure S11, SI), this value is better than many of the nonnoble metal-based catalysts that have been reported.32 The electrochemical durability in terms of ORR was presents in Figure 2c, where Pt/C electrode is observed to degrade more severely after 3000 repeated cycles with larger positive potential shift, while the CCO@C electrode is relatively stable even after 3000 CV cycles. During the methanol resistance test (Figure S12, SI), interestingly, the Pt/C catalyst shows obviously degraded current density after addition of methanol; however, CCO@C still exhibits a stable current density without distinct recession, indicating the strong methanol resistance and stability of CCO@C. Figure 2d exhibits the OER polarization curves for L-CCO@C, CCO@C, H-CCO and commercial IrO2 catalysts. The CCO@C nanotubes shows a lower overpotential (327 mV) compared with L-CCO@C (461 mV) and H-CCO (492 mV) at a current density of 10 mA cm-2. The catalytic kinetics for OER are also evaluated by Tafel slope. As revealed in Figure 8 ACS Paragon Plus Environment

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2e, the Tafel slope of CCO@C nanotubes is about 74.0 mV dec-1, which is smaller than those

Figure 2. (a) ORR polarization curves of L-CCO@C, CCO@C, H-CCO, and Pt/C samples. (b) ORR polarization curves of the CCO@C nanotubes at different rotation speeds and the corresponding K−L plots at different potentials (inset), (c) ORR polarization curves of CCO@C and Pt@C samples before and after 3000 potential cycles, (d) OER polarization curves of L-CCO@C, CCO@C, H-CCO, IrO2 and the overpotential schematic at 10 mA cm-2 (inset), (e) Tafel plots derived from (d), (f) OER polarization curves of CCO@C and IrO2 during cycling durability test.

of L-CCO@C (91.3 mV dec-1) and H-CCO (105.9 mV dec-1), indicating the outstanding OER kinetics of CCO@C samples. Additionally, the overpotential and Tafel slope of CCO@C nanotubes were comparable to that of commercial IrO2 (316 mV, 58.1 mV dec-1). To further explore the reasons of outstanding OER performance of CCO@C, we have measured the electrochemically active surface area (ECSA) from the electrochemical double-layer capacitance (Cdl). As shown in Figure S13 (SI), the ECSA of CCO@C, L-CCO@C, H-CCO were estimated to be 313.8 cm2, 80.0 cm2, 61.3 cm2, respectively, which means that CCO@C has the largest electrochemically active surface among the three catalysts. The increased ECSA for CCO@C can render a large specific surface area for it’s catalytically active sites, excellent gas bubble diffusion ability, and thus leading to the superior catalytic performance. The excellent OER kinetics of CCO@C composite was also demonstrated by the EIS test. As shown in Figure S14 (SI), the semicircle at high frequency is corresponding to the formation 9 ACS Paragon Plus Environment

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of electrical double layer, and the semicircle at low frequency can be ascribed to the Faradaic reaction of hydrogen evolution. The CCO@C presents superior performance, which mainly because the large external and internal hollow tubular structure and abundant pores would facilitate the charge transfer kinetics. The stability of CCO@C and noble-metal IrO2 catalyst was assessed through 3000 continuous potential cycles test at 1.45-1.65 V. The polarization curves of CCO@C nanotubes shows slightly decline. In contrast, the stability of IrO2 is much worse than CCO@C (Figure 2f).

Figure 3. (a) The scheme of the as-assembled two-electrode rechargeable Zn–air batteries, (b) Charge and discharge polarization curves of L-CCO@C, CCO@C, H-CCO and Pt/C+IrO2 samples, (c) Galvanostatic discharge-charge cycling curves at 2 mA cm-2 using CCO@C and Pt/C+IrO2 as air electrodes and the Optical images of an LED illuminated using CCO@C as air electrodes (inset), (d) schematic illustration of the advantages of the CCO@C nanotubes as electrocatalysts. As a proof-of-concept application, a primary Zn-air battery is assembled to further evaluate its performance under real battery conditions (Figure 3a). Figure 3b shows typical discharge-charge polarization curves. The open circuit voltage (OCV) of Pt/C-IrO2 is 1.45 V, which slightly higher than that CCO@C (1.41 V). Whereas the L-CCO@C presents lower 10 ACS Paragon Plus Environment

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OCV of 1.37 V owing to the narrow channels for oxygen/hydroxyl diffusion. The H-CCO also exhibits inferior OCV (1.35 V) mainly because the barely carbon contained in the composite lead to poor conductivity. The CCO@C electrode also exhibits a lower discharge−charge voltage gap of 0.79 V at 10 mA cm-2 (Figure 3b). Significantly, when CCO@C repeatedly charge-discharge at 2 mA cm-2 for a total of 160 cycles with a 30 min per cycle period, only a little charge/discharge voltage change is observed, which is superior to L-CCO@C, H-CCO, and Pt/C-IrO2 as cathode for Zn-air batteries (Figure 3c, Figure S15 (SI)). The cycling stability of CCO@C nanotubes at higher current density was also remarkable (10 mA cm-2, Figure S16), the discharge voltage still maintain stable up to 60 cycles. One important proofof-concept illustration is to power two light-emitting diode (LED) by series-connected Zn–air batteries based on a CCO@C air cathode, and the LED could still emit constantly and steadily (Figure 3c (inset) and Video S1 (Supporting Information)). On the basis of the advanced structural and outstanding electro-catalytic properties of CCO@C nanotubes (Figure 3d, Table S1, S2， ，S3 (SI)), we think that four critical aspects are responsible for their excellent ORR and OER activity and stability for Zn-air batteries: 1) The large interior hollows (diameter of ~ 80 nm) and a mass of mesoporous among carbon nanotubes could double the active surface area, which would effectively offer more channels for oxygen/hydroxyl diffusion and more active sites for catalytic reaction. 2) The ultrasmall CuCo2O4 nanoparticles (~7 nm) could improve the utilization rate, and thus increase the catalytic activity. 3) The structure of uniform 1D nanotubes interconnected into 3D conductive network would provide the superior electronic contact to external circuit. 4) The N-doping, (especially for pyridinic-N and graphitic-N) may also contribute to oxygen redox catalysis.

Combining advantages of bimetallic oxides and carbon matrix, we synthesis the mesoporous thin-walled CuCo2O4@C nanotubes with abundant nitrogen element contained by facile coaxial electrospinning method. Rational design of surface structure through multi11 ACS Paragon Plus Environment

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porous auxiliary and strong coupling with N-doped carbon substrate could leads to a high activity and a strong durability for bifunctional catalysis toward ORR (positive onset potential: 0.951 V) and OER (low overpotential: 327 mV at 10 mA cm−2). When used as cathode for Znair batteries, CCO@C electrode exhibits a low discharge−charge voltage gap of 0.79 V at 10 mA cm-2 and a long cycle life up to 160 cycles for 80 h at a current density of 2 mA cm-2. Such promising prepared method as well as remarkable electro-catalytic properties would provide a certain reference to the relevant materials.

ASSOCIATED CONTENT Supporting Information. Experimental Section, additional Figures (Figure S1-S16) and Tables (Table S1-S3) are provided in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].（L.J.）

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (51622102, 51231003, 51571124), MOST(2016YFB0901502), and the 111 Project (B12015).

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