Electrospinning derived hierarchically porous hollow CuCo2O4

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Electrospinning derived hierarchically porous hollow CuCo2O4 nanotubes as an effectively bifunctional catalyst for reversible Li-O2 batteries Haitao Wu, Wang Sun, Junrong Shen, Zhu Mao, Huaguo Wang, Huiqun Cai, Zhenhua Wang, and Kening Sun ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03646 • Publication Date (Web): 17 Sep 2018 Downloaded from http://pubs.acs.org on September 17, 2018

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ACS Sustainable Chemistry & Engineering

Electrospinning

derived

hierarchically

porous

hollow CuCo2O4 nanotubes as an effectively bifunctional catalyst for reversible Li-O2 batteries Haitao Wu,a Wang Sun,*,a,b Junrong Shen,a Zhu Mao,a Huaguo Wang,b Huiqun Cai,c Zhenhua Wang,a,d and Kening Sun*,a,d a

Beijing Key Laboratory for Chemical Power Source and Green Catalysis, School of Chemistry

and Chemical Engineering, Beijing Institute of Technology, No. 5 Zhongguancun South Avenue, Haidian District, Beijing 100081, China b

State Key Laboratory of Advanced Chemical Power Sources, Guizhou Meiling Power Sources

Co. Ltd., No. 705 Zhonghua Rd., Huichuan District, Zunyi, Guizhou 563003, China c

Yinlong Energy Co., Ltd, No. 16 Jinhu Rd., Sanzao Town, Jinwan District, Zhuhai City, China

d

Collaborative Innovation Center of Electric Vehicles in Beijing, No. 5 Zhongguancun South

Avenue, Haidian District, Beijing 100081, China

*Corresponding author: Email: [email protected] (Kening Sun); [email protected] (Wang Sun).

ACS Paragon Plus Environment

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ABSTRACT: The structural engineering and design of effectively bifunctional cathode catalysts perform a vital role in enhancing the oxygen-electrode kinetics for achieving highly reversible Li-O2 batteries. Hereon, one-dimensional CuCo2O4 nanotubes are fabricated, by a cost-efficient electrospinning technique, as a bifunctional cathodic catalyst for lithium-oxygen battery for the first

time.

The

as-fabricated

CuCo2O4

nanotubes

with

hollow

and

hierarchically

mesoporous/macroporous architecture accelerate the mass (O2 and Li+) transport and alleviate the clog of insoluble discharge products. Along with their highly intrinsic activity toward oxygen reduction and evolution reactions, the CuCo2O4 nanotubes based lithium-oxygen battery demonstrates remarkably improved electrochemical performance, such as low overpotential, high discharge capacities (8778 mAh g−1 at 100 mA g−1), superb rate capability, and superior reversibility up to 128 cycles under a controlled capacity of 1000 mAh g−1 at 200 mA g−1. The further ex-situ SEM and XRD analyses reveal that the disk-like toroidal-shaped Li2O2 product can be efficiently decomposed during recharging process, confirming the good reversibility of CuCo2O4 nanotubes based cathode. These outcomes demonstrate the good prospect of CuCo2O4 nanotubes as an effectively non-noble metal catalyst in the reversible Li-O2 battery.

KEYWORDS: CuCo2O4 nanotubes, electrospinning, mesoporous/macroporous structure, bifunctional catalysts, Li-O2 batteries

Introduction The ever-increasing energy requirements and adverse environmental effects, due to the fossil fuel consumption, have led to tremendous research efforts for exploitation of alternative energy conversion and storage systems. The sustainable energy, such as wind and solar energy, and


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electrochemical energy storage, such as secondary ion batteries, have garnered significant research focus due to excellent efficiency and environmental sustainability.1-3 Among these systems, Li-O2 batteries (LOBs) are being heavily investigated because of the high energy density, cost-effectiveness, and eco-friendly nature.4,5 However, the successful realization of LOBs is hindered by large over-potential, low columbic efficiency, and poor cyclic stability. These shortages are basically caused by the poor conductivity and solubility of the discharge product (Li2O2) and the electrolyte decomposition due to the presence of radical intermediates such as O22− and O2−. Another important aspect is the complicated and sluggish kinetics of the air electrode because the oxygen reduction reaction (ORR), producing Li2O2 in the process of discharge, and oxygen evolution reaction (OER), breaking Li2O2 in the process of charge, both occurs at a three-phase interface (electrolyte/electrode/oxygen).6–8 To solve these issues, considerable endeavors have been devoted to utilize the diverse electro-catalysts,9 such as metal carbides and nitrides,10,11 precious metals,12,13 and transition metal oxides,14,15 to ameliorate the performance of LOBs. Among the various kinds of electro-catalysts, investigated for LOBs, the cobalt oxides are of great attraction for researchers by their low cost, excellent stability, controllable morphology, and catalytic efficiency towards ORR and OER.16–18 However, the limited cobalt (Co) reserves, in the earth crust, and high toxicity urge the partial replacement of Co with a cheaper and ecofriendly metallic element. More importantly, it has been confirmed that addition of metallic elements enhances the conductivity and electro-catalytic efficiency of Co-based ternary oxides. Therefore, ternary Co-based oxides are being developed by adding various transition metals such as Zn, Ni, Fe or Mn. For instance, Dai et al. have synthesized the MnCo2O4-graphene hybrid as an electro-catalyst for LOBs, which achieved good stability over 40 cycles.19 Moreover,


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NiCo2O4, CoFe2O4, and ZnCo2O4 have also been synthesized and investigated as efficient electro-catalysts for LOBs.20–22 In this regard, some research groups have proposed Cu addition into Co3O4, to accelerate the OER, and found much more excellent electrical conductivity and electrochemical performance of ternary CuCo2O4, compared to the mono-component copper or cobalt oxides.23–25 Sun et al. reported that the CuCo2O4/N-rGO composite shows improved ORR capability than N-rGO and Co3O4/N-rGO, and enhanced stability than Pt/C catalyst.26 Furthermore, copper cobalt complexes have been confirmed as effectively bifunctional materials for ORR and OER in both basic and neutral solutions.27–29 These outcomes imply that CuCo2O4 could also be a potential catalyst for LOBs, which are also closely involved in ORR and OER processes. In addition to its intrinsic catalytic activity, the morphology and pore architecture of a catalyst also perform a critical role in the LOBs’ performance.13,15,17,21 An ideal catalyst should have the high specific surface area, to offer numerous active sites, hierarchical pore structure, to facilitate mass transfer, and enough space to accommodate insoluble Li2O2 product. Recently, onedimensional (1D) nanostructures, such as nanowires, nanobelts, nanofibers, and especially nanotubes, have caught extraordinary researchers’ interest because of the larger aspect ratio, higher interfacial area, and shorter ion diffusion path.10,17,30,31 For instance, Li et al. reported that the electrospun NiCo2O4 nanotubes exhibit much better electrochemical performance of capacitors, compared with nanofibers and nanobelts counterparts.32 Kim et al. reported that the electrospun

hollow

manganese-cobalt

oxides

demonstrate

excellent

lithium

storage

performance.33 Zhang’s group used electrospun porous NiCo2O4 nanotubes as a cathodic catalyst for LOBs and achieved good reversibility over 110 cycles.34 These researches demonstrate that spinel Co-based metal oxides with porous nanotubular architecture are very attractive to high-


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performance LOBs. Therefore, synthesizing CuCo2O4 catalyst with desired morphology, grain size, and porous structure for LOBs can be a worthy and feasible way. Herein, CuCo2O4 catalysts with distinctively hollow and porous tubular architecture have been successfully fabricated, by a simple electrospinning method (Figure 1), and utilized as airelectrode catalysts in LOBs with high reversibility. The thin mesoporous walls and large hollow cavities effectively promote the mass transport and reduce the channel blockage caused by the insoluble Li2O2 product. The electrochemical measurements suggest that CuCo2O4 nanotubes exhibit superb catalytic activity in both aqueous and non-aqueous media. More importantly, the LOBs with CuCo2O4 nanotubes as air-electrode catalysts achieve high discharge capacities, low over-potential, and excellent cyclic performance over 120 cycles, showing obviously superior performances compared to other structural copper-cobalt oxides based LOBs.35-41 These results demonstrate the importance of a rationally designed catalyst and structural engineering to enhance the electrochemical performance of LOBs.

Experimental Section Synthesis of CuCo2O4 Nanotubes. The porous CuCo2O4 nanotubes were synthesized by an easy and low-cost electrospinning technique. In a typical procedure, 1.35 g of polyvinylpyrrolidone (PVP, Mw =1 300 000 g mol−1) was dissolved in the mixture of 0.1965 g of Cu(NO3)2•3H2O, 0.4035 g of Co(Ac)2•4H2O, and 5 mL of N, N-dimethylformamide. Then stir for 24 h to get a uniform precursor and subsequently transferred into a plastic syringe. The applied voltage and the distance from the needle tip of syringe to the roller collector were 20 kV and 18 cm, respectively. The feed rate was 0.3 mL h−1 and the electrospun precursor was


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collected by a roller collector. The precursor fibers were dried at 70 °C for 24 h and finally sintered in air at 400 °C for 2 h with a speed of 1 °C min−1 to form CuCo2O4 nanotubes. Synthesis of CuCo2O4 Bulks. CuCo2O4 bulks were prepared via a sol-gel combustion way. In a typical reaction, 5 mmol of cupric nitrate trihydrate, 10 mmol of cobaltous nitrate hexahydrate, 12 mmol of glycine and 12 of mmol citric acid were mixed in 200 mL ultrapure water, and then continue stirring at 80 °C until transparent gel was formed. Then, the gel was presintered at 250 °C for 2.5 h and finally calcined in air at 400 °C for 4 h to get the CuCo2O4 bulks product for comparison studies. Material Characterization. The thermal property of the electrospinning precursor was analyzed by a thermogravimetric analyzer (TGA2050, TA Instruments) with a speed of 5 °C min−1, under air atmosphere. The crystal natures of as-synthesized materials were probed by Xray diffraction (XRD). The microstructure and morphology were viewed via scanning electron microscopy (SEM, QUANTA FEG 250) and transmission electron microscopy (TEM, JEOL JEM-2100F). The energy dispersive X-ray spectroscopy (EDS) mapping was performed to analyse the elemental distributions, while the X-ray photoelectron spectroscopy (XPS) was performed via Physical Electronics 5400 ESCA. The specific surface area and pore characteristics were obtained based on nitrogen sorption isotherms through an Autosorb-IQ2MP-C instrument. Electrode Fabrication and Electrochemical Measurements. The ORR and OER performance of CuCo2O4 catalysts in alkaline aqueous solution was measured by a rotating disk electrode (RDE) system. To fabricate the working electrode, 1 mg of CuCo2O4 catalyst and 1 mg of conductive carbon (Super P) were dispersed into the mix of 15 µL of Nafion solution (5 wt.%, Dupont) and 985 µL of isopropanol. The mixture was ultrasonically dispersed for 1 h to obtain a


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uniform ink. Then 10 µL inks were spread on the surface of the working electrode (0.247 cm2) and dried naturally. Meanwhile, the purchased Pt/C (20 wt.%, Alfa Aesar) and RuO2 (99.9%, Sigma) were also investigated for contrast owing to the recognized excellent ORR activity of Pt/C and OER activity of RuO2. The Pt wire and Hg/HgO were employed as a counter and a reference electrode, respectively. The electrolyte was 0.1 M KOH aqueous solution. The cyclic voltammetry (CV) curves were tested with a sweep speed of 10 mV s−1, within −0.8–0.3 V, under the O2-saturated condition. The linear sweep voltammetry (LSV) of ORR (−0.8–0.3 V) and OER (0.3–1.0 V) were measured at 5 mV s−1. The ORR electron number was determined according to the Koutecky-Levich (K-L) formula.42 The lithium-oxygen battery performance was tested by a Swagelok-type cell, in a pure oxygen atmosphere, at ambient temperature. The oxygen electrodes were fabricated by spreading slurry, consisted of a mixture of 40 wt.% of catalysts (CuCo2O4 nanotubes or bulks), 50 wt.% of Super P and 10 wt.% of polyvinylidene fluoride (PVDF) binder (or 90 wt.% Super P and 10 wt.% PVDF), onto a carbon fiber paper, and then dried at 100 °C. The total load was about 0.6 mg cm−2. The performances of LOBs were calculated on the weight of the Super P and the catalyst. The Li-O2 battery was assembled by an air electrode, a separator (Whatman, GF/D), 1 M LiTFSI/tetraethylene glycol dimethyl ether (TEGDME) electrolyte, and lithium metal in Arfilled glove box. The galvanostatic testing was performed on a LAND CT2001A apparatus, in the voltage range of 2.2 to 4.4 V. The CV was recorded on a CHI 660D apparatus in the potential range of 2.0 to 4.5 V with a sweep rate of 0.1 mV s−1.

Results and discussion


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Figure 1. Schematic of the electrospinning route carried out to synthesize porous CuCo2O4 nanotubes. As depicted in Figure S1a, the XRD result of the as-spun precursor demonstrated typical characteristics of polymer. The thermogravimetric analysis (TGA, Figure S1b) was performed to probe the thermal behavior and determine the calcination temperature of the electrospun precursor. The initial weight drop of ~8.8% before 200 °C is due to the evaporation of the residual solvent, the loss of absorbed water, and the deprivation of crystal water from the metal salt.32 The two sharp peaks, at 229 and 245 °C, can be ascribed to the decomposition of cupric nitrate and cobalt acetate, respectively, whereas the notable drop, at about 285 °C, can be assigned to the elimination of PVP. Afterwards, no obvious weight loss was found in the range of 300 to 600 °C, implying the full decomposition of the metal-salt precursor and PVP organic polymer. Although, the higher calcination temperature results in improved crystallinity and conductivity, the resulting grains get easily sintered together and form large particles with damaged structure and lower specific surface area. Therefore, the calcination temperature of 400 °C was adopted for the as-spun precursor to obtain a pure phase of CuCo2O4.38,39,41


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Figure 2. SEM of (a, b) the electrospining precursor and (c, d) the porous CuCo2O4 nanotubes after calcination. The SEM was utilized to get the morphology of the as-electrospun nanofibers and calcined CuCo2O4 nanotubes. The electrospun precursor exhibited typically interconnected web-like morphology, consisting of numerous smooth nanofibers (Figure 2a and b). The nanofibers had a mean diameter of 360 nm and ranged from 270 to 430 nm (Figure S2a). Interestingly, a hollow nanostructure was achieved after sintering at 400 °C in the air (Figure 2c and d) and the diameter of the obtained CuCo2O4 nanotubes decreased to approximately 163 nm, with a relatively concentrated distribution (Figure S2b). The significant decrease in diameter can be ascribed to


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the decomposition of metal salts and PVP polymer.32 Moreover, it can be readily observed that the nanotubes wall is consisted of massive CuCo2O4 nanoparticles and the wall thickness is only about 30 nm. The sintered nanotubes also possess rough surface with small pores, which may be caused by the outward extravasation of gases produced during the sintering.34

Figure 3. (a) The XRD spectra, (b) N2 isotherms and the pore-size distribution plots, and (c–f) wide-range and high-resolution Cu 2p, Co 2p and O 1s XPS spectra of CuCo2O4 nanotubes.


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The XRD was employed to elucidate the crystallinity and phase structure of the products. As depicted in Figure 3a, all diffraction peaks could be well indexed to cubic spinel CuCo2O4, with a space group of Fd-3m (227) (JCPDS No. 76-1887), confirming the pure phase structure of the CuCo2O4 nanotubes. In addition, we did not observe any impurity peaks in the XRD pattern of bulk CuCo2O4 (Figure S3), suggesting that pure spinel phase of CuCo2O4 bulks (Figure S4) were also obtained by the sol-gel process. The porous characteristic and surface area of CuCo2O4 materials were investigated by the N2 adsorption isotherms tests, as given in Figure 3b and Figure S5. The adsorption/desorption isotherm shows a clear type IV isotherm, with a H3 hysteresis loop, confirming the mesoporous feature of the CuCo2O4 nanotubes. Moreover, the mesoporous nature was further confirmed by the pore-size curve (insert of Figure 3b). The broad pore size distribution, with two prominent peaks at around 3.4 and 90 nm, indicates the hierarchical mesoporous/macroporous nature of the CuCo2O4 nanotubes. The smaller pore size can be attributed to the gaps among primary nanograins and holes on the surface of nanotubes, whereas the larger one is assigned to the internal diameter of hollow nanotubes. Moreover, the BET area and pore volumes of CuCo2O4 nanotubes are determined to be about 22.1 m2 g−1 and 0.11 cm3 g−1, respectively, much larger than that of CuCo2O4 bulks (Table S1) synthesized by the sol-gel method. This particularly hierarchical porous structure with relatively high surface area is desirable for electron movement, electrolyte impregnation, and O2 flow, resulting in improved electrochemical performance.33,34 Figure 3c shows the XPS survey spectrum of CuCo2O4 nanotubes, confirming the existence of Cu, Co, and O in the calcined product. The Cu 2p3/2 spectrum (Figure 3d) can be separated with three spin-orbit doublets at 932.4, 934.2, and 935.2 eV, corresponding to Cu+, Cu2+, at the octahedral site, and Cu2+, at the tetrahedral site, respectively.28,31 The X-ray stimulated reduction


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of Cu2+ ions is supposed to be responsible for the emergence of Cu+ ions.43 Furthermore, the two apparent satellites (940.4 and 942.7 eV) correspond to the Cu2+ ions and the prominent peak at 934.2 eV indicates that copper ions mainly existed in the form of Cu2+ ions and located at octahedral sites for the energetically stable structure.31,43 Moreover, the divalent ions, located at the octahedral sites, can more effectively improve the catalytic efficiency, as compared to the ions located at tetrahedral sites.43–45 The Co2p spectrum (Figure 3e) exhibits two peaks at 779.6 and 794.8 eV, which belong to Co 2p3/2 and Co 2p1/2, respectively. The energy gap of 15.2 eV, between Co 2p3/2 and Co 2p1/2, confirms the co-existence of Co2+ and Co3+.31 The first doublet, at 779.4 and 794.5 eV, corresponds to the Co3+ and the second doublet, at 780.8 and 796.4 eV, refers to the Co2+.30 Additionally, the double weak shakeup satellites (Sat.) are the typical feature of spinel structures, where trivalent cations are located at octahedral lattice sites and divalent cations occupy tetrahedral sites.31 The O 1s curve (Figure 3f) can be resolved into three peaks located at 529.4, 531.1, and 532.4 eV. The most prominent peak, at 529.4 eV, represents the metal-oxygen bonding in the form of O2- ions.37 The peak observed at 531.1 eV can be assigned to considerable oxygen coordination defect sites in nanomaterials with tiny grains.30 The peak at 532.4 eV can be ascribed to the surface-bound water of CuCo2O4 nanotubes.31


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Figure 4. (a) TEM and (b, d) HRTEM images of CuCo2O4 nanotubes; (c) the SAED pattern of CuCo2O4 nanotubes; (e) the STEM image and (f-h) the corresponding EDS images of CuCo2O4 nanotubes. The microstructure and crystallinity of the hierarchical porous CuCo2O4 nanotubes were further explored by TEM. As given in Figure 4a, the porous structure can be clearly observed. Moreover, the sharp contrast between the dark edge and light centre, in the high-resolution image (Figure 4b), reveals the hollow porous tube-like structure and indicates a wall thickness of about


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31 nm. The wall-thickness distribution of CuCo2O4 nanotubes mainly centred at ~30 nm (Figure S6), further confirming the good consistency of the nanotube’s morphology. Figure 4b also indicates that the walls of the nanotubes consist of numerous CuCo2O4 nanograins, with a grain size of about 20 nm, and bountiful mesopores of around 2–8 nm in diameter. The SAED pattern (Figure 4c) shows well-defined diffraction rings corresponding to the (311), (331), (220), (222), and (422) planes, which confirm the polycrystalline nature of the CuCo2O4 nanotubes. The HRTEM image (Figure 4d) exhibits the d-spacing of 0.267 and 0.441 nm, corresponding to the (331) and (551) facets of the spinel CuCo2O4 (JCPDS No. 76-1887). In addition, the STEM image and corresponding EDS maps of a single nanotube, presented in Figure 4e–h, demonstrate the existence and uniform dispersion of Cu, Co and O in the hollow porous nanotube. Based on the above characterizations and analyses, it can be deduced that the hierarchical mesoporous/macroporous spinel CuCo2O4 nanotubes have been successfully fabricated.


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Figure 5. (a) CV profiles of CuCo2O4 nanotubes, CuCo2O4 bulks, and Pt/C catalysts. (b) LSV plots for ORR at 1600 rpm. (c) Rotating-disk voltammograms of the CuCo2O4 nanotubes and corresponding K-L profiles (insert). (d) Chronoamperometric responses of catalytic materials at −0.6 V (1600 rpm). (e) OER polarization profiles at 1600 rpm. (f) The bifunctional ORR and OER activity at 1600 rpm.


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The intrinsic electrocatalytic activity of CuCo2O4 nanotubes toward ORR and OER was explored in a basic medium, by RDE method, and results are given in Figure 5 and Table S2. The CV profiles (Figure 5a) displayed a more positive cathodic peak for CuCo2O4 nanotubes (−0.244 V) than CuCo2O4 bulks (−0.302 V), indicating excellent electron transfer in the CuCo2O4 nanotubes. The LSV polarization measurements were conducted to assess the kinetic performance of several catalysts toward ORR. As depicted in Figure 5b, the onset ORR potential for CuCo2O4 nanotubes clearly shifted towards positive voltage, as compared to the bulk CuCo2O4, although slightly lower than that of Pt/C. Figure 5c presents the ORR polarization behaviour of the CuCo2O4 nanotubes under different rotating speeds. Moreover, the corresponding K-L plots can be well linearly fitted and the resulting electron transfer numbers, according to K-L plots, were 3.92–3.94, implying a four-electron ORR route on the CuCo2O4 nanotubes catalyst. The commercial Pt/C also exhibits the similar four-electron ORR process.42,46 Importantly, the CuCo2O4 nanotubes exhibited excellent durability in basic solutions, with slight drop in ORR activity, for over 20000 s of consecutive operation (Figure 5d). Figure 5e presents the OER polarization curves of CuCo2O4 nanotubes, CuCo2O4 bulks, and RuO2. It is obvious that the OER half-wave potential, at 10 mA cm−2, for the CuCo2O4 nanotubes (0.828 V) is much lower than that of the CuCo2O4 bulks (>1.0 V) and close to that of RuO2 (0.795 V), suggesting the improved OER activity of CuCo2O4 nanotubes. Remarkably, the CuCo2O4 nanotubes reveal the most desirable performance when considering both ORR and OER, as given in Figure 5f. The Pt/C catalyst possesses good ORR activity, but its OER activity is weak, while it is just the opposite for RuO2. Briefly, these results illustrate that the rationally designed nanostructured CuCo2O4, like other spinel Co-based ternary metal oxides, is an effective and promising bifunctional catalyst in alkaline aqueous.


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Figure 6. CV profiles of Li-O2 cells with CuCo2O4 nanotubes, CuCo2O4 bulks, and pure Super P based electrodes. The electrochemical performance of the CuCo2O4 nanotubes toward ORR and OER in organic system was studied by CV. As given in Figure 6, although the pure Super P based Li-O2 cell shows a clearly broad ORR peak, there is no obvious oxidation peak in the subsequent charging process. This shows that the Super P has weak catalytic activity toward the discharge product. In contrast, the CuCo2O4 nanotubes based Li-O2 battery not only exhibits much more positive ORR onset and peak potentials, but also shows much clearer oxidation peak, lower anodic onset potential, and larger oxidation-reduction current density as compared to pure Super P and CuCo2O4 bulks, suggesting that CuCo2O4 nanotubes can effectively promote the generation and degradation of the product. To further prove the ORR and OER peaks are related to Li-O2 reactions, the same CuCo2O4 nanotube based battery was tested in pure Ar gas. The resulted CV


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curve (Figure S7) suggested no clearly redox reactions occurred in an inert gas-saturated electrolyte. The superior catalytic activity of CuCo2O4 nanotubes, compared with Super P and CuCo2O4 bulks, can be attributed to their intrinsic activity and hollow porous architecture with large pore volume, high surface area, and small grain size, which facilitate the electron and mass transfer and also provide numerous active sites.17,23,29 The above results confirm that CuCo2O4 nanotubes can be used as a high-efficiency bifunctional catalyst in both aqueous and nonaqueous media.6,37

Figure 7. (a) The discharge/charge curves of LOBs with CuCo2O4 nanotubes, CuCo2O4 bulks, and pure Super P based electrodes at 100 mA g−1 in the potential range of 2.2–4.4 V and (b) the rate performance. (c) The cyclic test of the LOBs at 200 mA g−1 and (d) the change of the corresponding terminating discharge voltage with the number of cycles for CuCo2O4 nanotubebased electrodes.


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Considering their rational structure and favorable catalytic activities, as illustrated above, the CuCo2O4 nanotubes are supposed to be a high-efficiency electrocatalyst for rechargeable LOBs. Figure 7a exhibits the initial discharged/charged profiles of the LOBs with different cathodes. Obviously, the CuCo2O4 nanotubes based battery achieved a higher discharge capacity of 8778 mAh g−1 and a higher discharge voltage platform of 2.74 V (vs. Li/Li+) than that of CuCo2O4 bulks (6163 mAh g−1 and 2.69 V) and pure Super P (4900 mAh g−1 and 2.64 V). Moreover, the charge potential plateau and overpotential in case of CuCo2O4 nanotubes electrodes are much lower than CuCo2O4 bulks electrodes. Under a limited charge potential of 4.4 V, the first charge capacities of pure Super P based battery only reached 590 mAh g−1, with a low discharge-charge efficiency of ~12%, indicating the poor reversibility of this electrode, which is accordant with the CV result. These outcomes confirm that the CuCo2O4 nanotubes catalyst can effectively promote the oxygen kinetics, with high OER and ORR catalytic activity, in non-aqueous LOBs. As depicted in Figure 7b, the rate performance of LOBs, with three different electrodes, was also investigated by discharging and charging at various current densities. Although the discharge capacity decreased with an increase in current density, due to increased polarization, the nanotubes based cell still delivered much higher discharge capacities, compared to that of CuCo2O4 bulks and Super P. In addition, a comparative chart of average initial and final specific capacities with error-bar was also provided to avoid the measurement errors when using coin cell batteries. As shown in Figure S8, it is obvious that the synthesized catalyst does increase the specific cyclic capacities. The cyclic performance of the CuCo2O4 nanotubes based LOBs was measured under a restricted capacity of 1000 mAh g−1 within 2.2–4.4 V (Figure 7c). During the initial cycling process, the battery showed a high discharge platform and a low charge plateau potential ~3.74


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V, suggesting the excellent reversibility of CuCo2O4 nanotube-based electrodes. The charge potential during the initial few cycles (~10 cycles) rapidly increased owing to the production of solid electrolyte interface (SEI) film on lithium anode and accumulation of the partially undecomposed Li2O2 product on the O2 electrode, which increase the ohmic resistance and result in a rapid increase of charge voltage.15 Figure 7d demonstrates that the batteries with the CuCo2O4 nanotubes based cathodes exhibit excellent cyclic stability up to 128 cycles. In contrast, the Li-O2 cell using CuCo2O4 bulks as a catalyst only achieved 24 cycles and the terminated charge voltage had reached 4.5 V after 10 cycles (Figure S9). These further confirm the superior catalytic capability of the hierarchical porous CuCo2O4 nanotubes in LOBs.


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Figure 8. (a) The XRD patterns of CuCo2O4 nanotubes based cathodes. The SEM images of (b) pristine, (c) discharged and (d) recharged CuCo2O4 nanotubes based cathodes. To further understand the reversibility and discharge/charge behaviours of the CuCo2O4 nanotubes based LOBs, the discharge products and electrode morphologies were explored by XRD and SEM. As depicted in Figure 8a, the new diffraction peaks in XRD pattern for the discharged electrode confirm that the dominant product was crystalline Li2O2, despite that the peak intensity is weak due to the effect of the carbon paper substrate. After recharged, all diffraction peaks of the product disappeared, revealing the excellent reversibility of the electrode. The SEM image of the CuCo2O4 nanotubes based electrode after discharge, presented in Figure 8c, shows that most of the discharge products are the thin disk-like and toroidal-shaped particles, accordant with the previous researches on the morphology of Li2O2.6,8,47 Furthermore, as given in Figure 8d, the vast majority of products on the electrode (especially nanotubes) surface can be thoroughly decomposed after recharge, preserving the pristine electrode (Figure 8b) morphology. This further confirms that the CuCo2O4 nanotubes can effectively catalyze the degradation of the Li2O2 product. Raman spectroscopy was also performed to further assure the composition and reversibility of products, as depicted in Figure. S10.48,49 The Raman results of discharged electrodes show the presence of two clear Li2O2 peak at ~250 and 780 cm−1.50 Besides Li2O2, the another two weak peaks at about 1120 and 1480 cm−1 reveal the existence of a small proportion of LiO2, which is usually considered to be the intermediate product of Li2O2. Compared with Li2O2, the poor crystalline LiO2 product is easier to be decomposed but it has higher activity, which can induce side reactions with electrolyte and carbon material.50,51 After recharge, all corresponding peaks of Li2O2 and LiO2 disappeared, suggesting the degradation of Li2O2 and LiO2 during charging


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process. In addition, Raman spectrum (Figure S10e) show that a small quantity of Li2CO3 (~1080 cm−1) also generated after long-term operation. The formation of Li2CO3 may attribute to the degradation of electrolyte and conductive carbon induced by the radical O22−/O2−.15,52 The formed Li2CO3 could not be fully decomposed after recharging (Figure S10f), which could passivate the air electrode due to the gradual accumulation of side products.6,15,52

Figure 9. Schematic illustration of the structure of a rechargeable Li-O2 battery and the working mechanism of the oxygen electrode with CuCo2O4 nanotubes.


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To provide a comprehensive overview of the current performance level of our CuCo2O4 nanotubes based LOBs, the battery performance with some typical catalysts and CuCo2O4 based catalysts with various morphologies, such as nanoparticles, nanowires, nanosheets, and microflowers, are collated in Table S3. It visually reflects that our CuCo2O4 nanotubes based battery presented a very competitive performance in discharge capacity and cyclic-ability, compared to some previous reports. The excellent electrochemical performance of the CuCo2O4 nanotubes based LOBs is benefited from their following desirable advantages: (a) the high specific surface area offers numerous three-phase interface and active sites for the generation/degradation of the products; (b) the high aspect ratio, thin wall thickness, and excellent crystallinity of CuCo2O4 nanotubes improve the conductivity and accelerate electron transport;47 (c) the hierarchical mesoporous/macroporous hollow structure not only promotes the oxygen diffusion, Li+ transport, and electrolyte infiltration, but also provides sufficient space for deposition of discharge products. As illustrated in Figure 9, the mass transport (O2, Li+, O2−, etc.) does not happen only through the surface mesoporous but also takes place in the longitudinal direction from the larger hollow holes. This indicates that the formation and decomposition of products can occur at both inner and outer surface of the nanotubes, which accelerates the reaction rate. And the large hollow interiors can also effectively accommodate more discharge products and alleviate the product congestion. Moreover, the ternary spinel CuCo2O4 has the high intrinsic catalytic capability as compared to the binary copper or cobalt oxides.23–29 These favorable factors, combined together, contribute to the high performance of the CuCo2O4 nanotubes based LOBs.

Conclusions


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In summary, novel CuCo2O4 nanotubes with big hollow cavities and substantially mesopores in the tube wall were designed and fabricated using a simple electrospinning technique. The onedimensional porous nanotube structure effectively accelerates the electrolyte infiltration, electron migration, and oxygen diffusion, and meanwhile, provides numerous surface active sites for the electrochemical reactions and sufficient room for the accommodation of products. Combined with their highly intrinsic catalytic capability, CuCo2O4 nanotubes exhibited high ORR and OER catalytic activity in both basic-aqueous system and organic system. Consequently, the LOBs with CuCo2O4 nanotubes as cathodic catalysts achieved significantly enhanced electrochemical performance, including high discharge capacities of 8778 mAh g−1 at 100 mA g−1, superb rate capability, and superior cyclic performance up to 128 cycles. Moreover, the ex-situ XRD, SEM and Raman results confirm that the main discharge product was disk-like toroidal-shaped Li2O2 particles and can be almost completely decomposed during recharging. This contributes to the superior reversibility of CuCo2O4 nanotubes based electrodes in LOBs. This report shows the important role of intrinsic activity and morphology engineering in developing a practical catalyst, and also provides an effective approach to design effectively non-noble metal catalysts for reversible Li-O2 batteries. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: . XRD pattern and TGA curve of the CuCo2O4 precursor nanofibers, diameter distribution histograms of CuCo2O4 nanofibers and nanotubes, XRD patterns and SEM images of CuCo2O4


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bulks, pore parameters of CuCo2O4 samples, rotating disk electrode results of catalysts, CV profile of the CuCo2O4 nanotubes based electrodes in pure argon gas, cyclic performance and Raman spectra of the Li-O2 battery with CuCo2O4 bulks based cathode, and the battery performance comparison of Li−O2 cells between our work and some others (PDF) AUTHOR INFORMATION Corresponding Author *Email: [email protected] (Kening Sun); [email protected] (Wang Sun). NOTES The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (Grants No. 21576028, 21376001 and 21506012), the Opening Project of State Key Laboratory of Advanced Chemical Power Sources (SKL-ACPS-C-19) and the Program for Innovative and Entrepreneurial team in Zhuhai (ZH01110405160007PWC). REFERENCES (1) Sum, T. C.; Mathews, N. Advancements in perovskite solar cells: photophysics behind the photovoltaics. Energy Environ. Sci. 2014, 7, 2518-2534, DOI 10.1039/c4ee00673a. (2) Chou, S. L.; Dou, S. X. Next-Generation Batteries. Adv. Mater. 2017, 29, 1705871, DOI 10.1002/adma.201705871.


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TOC

Hollow porous CuCo2O4 nanotubes are fabricated by an electrospinning technique and served as a bifunctional catalyst for rechargeable Li-O2 batteries, achieving remarkably improved electrochemical performance.


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Electrospinning derived hierarchically porous hollow CuCo2O4

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