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Uniform MnCo2O4 Porous Dumbbells for Lithium Ion Batteries and Oxygen Evolution Reactions Xiangzhong Kong, Ting Zhu, Fangyi Cheng, Mengnan Zhu, Xinxin Cao, Shuquan Liang, Guozhong Cao, and Anqiang Pan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19719 • Publication Date (Web): 21 Feb 2018 Downloaded from http://pubs.acs.org on February 22, 2018

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ACS Applied Materials & Interfaces

Uniform MnCo2O4 Porous Dumbbells for Lithium Ion Batteries and Oxygen Evolution Reactions Xiangzhong Kong,† Ting Zhu,† Fangyi Cheng,‡ Mengnan Zhu,† Xinxin Cao,† Shuquan Liang,† Guozhong Cao§ and Anqiang Pan†,* †

School of Materials Science & Engineering, Central South University, Changsha,

Hunan, 410083, China ‡

Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education),

Nankai University, Tianjin 300071, China §

Department of Materials Science & Engineering, University of Washington, Seattle,

WA, 98195, USA

* Corresponding author: [email protected]

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Abstract Three-dimensional (3D) binary oxides with hierarchical porous nanostructures are attracting increasing attentions as electrode materials in energy storage and conversion systems due to their structural superiority which not only create desired electronic and ions transport channels, but also possess better structural mechanical stability. Herein, unusual 3D hierarchical MnCo2O4 porous dumbbells have been synthesized by a facile solvothermal method combined with a following heat treatment in air. The as-obtained MnCo2O4 dumbbells are composed of tightly stacked nanorods and show a large specific surface area of 41.30 m2 g-1 with a pore size distribution of 2-10 nm. As an anode material for lithium ion batteries (LIBs), the MnCo2O4 dumbbells electrode exhibits high reversible capacity and good rate capability, where a stable reversible capacity of 955 mAh g-1 can be maintained after 180 cycles at 200 mA g-1. Even at a high current density of 2000 mA g-1, the electrode can still deliver a specific capacity of 423.3 mAh g-1, demonstrating superior electrochemical properties for LIBs. In addition, the obtained 3D hierarchical MnCo2O4 porous dumbbells also display good oxygen evolution reaction (OER) activity with an overpotential of 426 mV at current density of 10 mA cm-2 and tafel slope of 93 mV dec-1.

Keywords: MnCo2O4, dumbbell-like, hierarchical porous structure, lithium ion batteries, oxygen evolution reaction

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1. Introduction Rechargeable LIBs with high energy density and long cycle life have attracted considerable attentions in various technological applications, such as portable electronic devices, hybrid and electric vehicles and other energy storage systems.1, 2, 3 As one of the most important components, the anode materials play a key role in the whole battery device. Graphite, the conventional anode material in LIBs, is becoming more difficult to meet the requirements for the next-generation LIBs due to its limited theoretical specific capacity of 372 mAh g-1. Therefore, it is highly desirable to develop alternative anode candidates with higher capacity and longer lifespan as well as high energy density and power density.4 Meanwhile, the electrocatalytic technology that produces hydrogen (H2) and oxygen (O2) from water splitting can efficiently regulate the balance of power demand-supply.5, 6, 7 As a significant process in water splitting, OER has intrinsically experienced sluggish kinetics, because of its complex proton coupling and transfer process, which greatly hampers the energy harvesting efficiency.8, 9

In recent years, micro/nanostructured binary transition metal oxides (AB2O4) with cubic spinel structure, such as FeMn2O4, CoMn2O4, NiMn2O4, NiCo2O4 and ZnCo2O4, have been studied as a group of promising candidates of anode materials for next-generation LIBs.10, 11, 12, 13 Moreover, they also exhibit desirable OER activity because of their structural flexibility of the spinel, variable valence states, and outstanding redox stability in aqueous alkaline solutions.14 They have shown

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enhanced cycling stability and rate capability compared to those of single transition metal oxides owing to their favourable complementarity and synergetic effects.15, 16 Among these candidates, MnCo2O4 is considered to be superior in LIBs because of its high theoretical capacity (906 mAh g-1), low cost and environmental friendliness.17 However, most of MnCo2O4-based electrode materials show unfortunately electrochemical stability and poor rate performance due to their intrinsically low electronic conductivity and large volume changes during the repeated cycling process.18 In order to alleviate this problem, tremendous efforts have been made in recent years to address these issues, including carbon coating, substitutional doping and porosity creating.19,

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For OER, The MnCo2O4 also suffers from undesirable

stability and electrochemical activity, which highly depends on the textural and morphological properties of the electrodes.8

More recently, preparation of MnCo2O4 materials with 3D hierarchical porous structures are great of interest.21 The large surface area endowed by porous structure can provide much more active sites, reduce the diffusion pathway of Li+ and improve the contact interface between electrode and electrolyte, thus resulting in enhanced electrochemical performances.22 For example, Li and co-workers have reported the synthesis of MnCo2O4 mesoporous spheres assembled with nanoparticles, which show a high reversible capacity of 600 mAh g-1 at a currrent density of 400 mA g-1 after 100 cycles.23 Besides, Yang and co-workers have synthesized MnCo2O4@ppy mixture, which displays an overpotential of 610 mV at a current density of 10 mA cm-2 in 0.1 M KOH for OER.24 However, the cycling stability performances reported therein are 4


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still not satisfactory yet. The fast capacity fading in LIBs may arise from the loose structure,

which

tends

to

be

pulverized

during

the

repeated

volume

expansion/contraction upon cycling. To date, common synthetic strategies for the synthesis of hierarchical porous structures usually employs sacrificial templates or surfactant, such as carbon, silica and Polyvinyl Pyrrolidone (PVP).25, 26, 27 For instance, hierarchical MnCo2O4 with mesoporous nanosheets have been fabricated by employing Polyvinyl Pyrrolidone.28 Porous MnCo2O4 hollow nanocages were also synthesized via a simple template method using carbon spheres as a template.25 However, the introduction of the template and surfactant to construct hierarchical porous structure is usually complicated and time-consuming.29

Herein, we report a facile solvothermal/annealing method to prepare 3D hierarchical MnCo2O4 porous dumbbells without using any surfactant or template. The as-prepared MnCo2O4 dumbbells are composed of numerous single crystalline nanorods, which are tightly attached together, leading to a good structural stability. The

final

3D

hierarchical

MnCo2O4

porous

dumbbells

exhibit

superior

electrochemical properties as anodes for LIBs and electrocatalysts for OER.

2. Experimental Section 2.1 Materials synthesis

The Mn0.33Co0.67CO3 dumbbells as the precursor for MnCo2O4 were prepared by a solvothermal process. Briefly, 1.2 mmol of Mn(CH3COO)2·4H2O, 2 mmol of Co(CH3COO)2·4H2O and 9 mmol urea were dissolved into a mixture solution which 5



contained 20 mL of distilled water and 10 mL of ethylene glycol under vigorous stirring for 30 min. The obtained pink solution was then transferred into a 50 mL Teflon-lined, stainless-steel autoclave, which was sealed and heated in an electronic oven at 180 oC for 24 h. After cooling down to room temperature, the precipitate was centrifuged and washed with water and ethanol for several times. The resulting solid was dried in air at 80 °C overnight before annealed in air at 600 oC for 4 h with a heating rate of 5 oC min-1 to obtain MnCo2O4 dumbbells. The MnCo2O4 spheres were prepared by a similar method, excepting that 2.4 mmol Mn(CH3COO)2·4H2O and 4 mmol Co(CH3COO)2·4H2O were added before solvothermal process. The concentration-dependent (0.05, 0.11, 0.13, 0.21 mol/L) and time-dependent (2, 6, 12, 24 h) experiments were also carried out to investigate the morphology evolutions of the precursor composites during solvothermal.

2.2 Materials characterization

The crystallographic phases of the as-prepared precursor and its annealed product were investigated by powder X-ray diffraction (Rigaku D/max2500 X-ray diffractometer with non-monochromated Cu-Kα (λ=1.54178Å) radiation). The samples were scanned in a range of between 5° and 80°, with a step size of 0.02°. TG and DSC analysis were carried out on a combined differential scanning calorimetry and thermogravimetric analysis instrument (Netzsch STA 449C, Germany). The morphology of the products was examined by field-emission scanning electron microscopy (FESEM, FEI Nova NanoSEM 230) and transmission electron

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microscopy (TEM, JEOL-JEM-2100F). The molar ratio of Mn and Co in the precursor were carried out on a Thermal iCAP 6300 Inductive Coupled Plasma (ICP) spectrometer. Nitrogen adsorption-desorption measurements were performed at 77 K (NOVA 4200e,

Quantachrome

Instruments).

The

sample

weight

for

N2

adsorption-desorption test is about 200 mg.

2.3 Lithium-ion batteries characterizations

To prepare the working electrode, MnCo2O4 sample, acetylene black and poly (vinylidene fluoride) (PVDF) binder were dispersed in 1-methyl-2 pyrrolidone (NMP) solution at a weight ratio of 8: 1: 1 to form a slurry, which was then coated on a copper foil and dried in a vacuum oven at 100 oC overnight. The mass loading of the active material was around 1.0 mg cm-2. The electrodes were assembled into CR2016 coin cells in a glove-box (Mbraun, Garching, Germany) filled with pure argon gas. Lithium foil was used as the counter and reference electrode, while 1 M LiPF6 in ethylene carbonate/dimethyl carbonate/vinylene carbonate (EC/DMC, 1:1 v/v ratio, VC, 2% mass ratio) was used as the electrolyte. Cyclic Voltammetry (CV; 0.01-3 V, 0.01 mV s-1) measurements were recorded on an electrochemical workstation (CHI660E, China). Galvanostatic charge/discharge was carried out on a Land tester (Land CT 2001A, Wuhan, China) in the potential range of 0.01-3 V (vs. Li/Li+). The electrochemical

impedance

spectrometry

(EIS)

was

conducted

with

a

ZAHNER-IM6ex electrochemical workstation (ZAHNER Co, Germany) in the frequency range of 100 kHz to 10 mHz.

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2.4. Oxygen evolution reaction characterizations

Electrocatalytic activity evaluations were performed on the electrochemical workstation (CHI 660E) in a three-electrode system in 1.0 M KOH solution at room temperature. Samples were tested on the glassy carbon electrode (GCE, 0.071 cm2 in area) as the working electrode, Hg/HgO as the reference electrodes and a Pt foil as the counter electrode. 2 mg catalystswere dispersed in 1 mL of ethanol/Nafion (v/v = 9:1) by sonication for 30 min. Then, 4 uL well-dispersed catalysts were covered on the glassy carbon electrode with drying naturally for test. Linear sweep voltammetry (LSV) was conducted with a scan rate of 5 mV s-1. All the applied potentials were referenced to a reversible hydrogen electrode (RHE) scale. To investigate the stability of MnCo2O4 dumbbells and spheres, the technique of multi-current steps were operated at 10 and 20 mA cm-2 for 10 hours, respectively.

3. Results and Discussion Figure 1 illustrates the formation process of MnCo2O4 dumbbells. During the solvothermal process, composite nanoparticles composed of MnCO3 and CoCO3 are formed initially when both the Mn2+ and Co2+ reacted with hydrolyzed urea to assemble into smooth cylinders. Several symmetrical oblique faces can be observed to grow at both ends of the cylinders. With longer reaction time, selective dissolution takes place on the oblique faces because of their high surface energy, which makes these smooth faces rugged. After a secondary evolution, the faces further evolve into nanorods and tend to keep a certain angle with the Z axial direction, leading to a 8


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hierarchical dumbbell-like shape. The obtained MnCO3 and CoCO3 composites are then annealed at 600 oC in air for 4 h to be converted into the MnCo2O4 phase. Porous structures can be created and formed because of the decomposition of MnCO3 and CoCO3 during the annealing process.

Figure 1. Schematic illustration of the proposed growth mechanism for the 3D hierarchical MnCo2O4 porous dumbbells.

The XRD pattern of the precursor synthesized by solvothermal treatment is shown in Figure 2a. All the peaks are assigned to Hexagonal MnCO3 (JCPDS Card No. 44-1472, space group R3c(167) and CoCO3 (JCPDS Card No. 11-0692, space group R3c(167)). ICP result shows that the molar ratio of Mn and Co in the composite is 1:2. Therefore, the carbonate precursor can be expressed as Mn0.33Co0.67CO3. According to the field-emission scanning electron microscope (FESEM) image (Figure 2b and c), the precursor dumbbells are highly uniform in rough surface, with a diameter of approximately 6 µm. A higher magnification image (Figure 2d) shows that the dumbbells are composed of superimposed nanorods, which are mainly distributed at the two ends of the particles against the Z axial direction with different 9



angles. This unique hierarchical MnCo2O4 dumbbells have rarely been reported before.

Figure 2. XRD pattern (a) and SEM images (b, c and d) of the 3D hierarchical precursor (Mn0.33Co0.67CO3) dumbbells prepared at 180 oC after solvothermal reaction for 24 h.

Time-dependent and concentration-dependent experiments are carried out to investigate the morphology evolutions of solvothermal precursors. As shown in Figure 3a, the smooth cylinders with inclined sections at both ends are formed after solvothermal reaction for only 2 h. Figure 3b shows SEM image of the product after a reaction for 6 h. It is found that the smooth inclined sections are broken into bulk particles. After a longer reaction time to 12 and 24 h, these particles are evolved into plates (Figure 3c) and nanorods (Figure 3d), respectively. The evolutionary process of inclined sections can be ascribed to the selective dissolution and secondary evolution to reduce the surface energy.30, 31 10


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Figure 3. SEM images of the precursor prepared with different solvothermal time: (a) 2 h, (b) 6 h, (c) 12 h, (d) 24 h. Moreover, the concentrations of Mn2+ and Co2+ also play a crucial role in the formation of hierarchical dumbbells. As shown in Figure 4a, the peanut-like crystals can be obtained at a low concentration (0.05 M) of Mn2+ and Co2+. When the concentration is increased to 0.11 M, nanorods appear at the two ends of the peanuts, thus leading to a dumbbell-like shape (Figure 4b). Further increasing the concentrations to 0.13 M and 0.21 M have moved the nanorods from ends to middle, and the morphologies of carbonate composites are transformed into notched spheres (Figure 4c) and complete spherules (Figure 4d), respectively. A similar evolution mechanism have been reported to prepare CaMoO4 microcrystallites by Chen and co-workers.32 They propose that the morphology change is governed by the oriented attachment process and reduction in surface energy is the primary driving force for this process. 11



Figure 4. FESEM images of the precursor solvothermally prepared by different concentration of Mn2+ and Co2+: (a) 0.05 mol L-1, (b)0.11 mol L-1, (c) 0.13 mol L-1, (d) 0.21 mol L-1.

The transformation of MnCo2O4 phase is further investigated by TG and DSC analysis. As shown in Figure 5a, a slight weight loss of 1.75% is observed around 324.5 oC, followed by a drastic mass loss (32.94%), which is along with an exothermic reaction in the DSC. The initial weight loss is mainly ascribed to the evaporation of physical water attached on the sample’s surface, while the abrupt loss between 324.5 oC and 560 oC refers to the decomposition of MnCO3 and CoCO3. Afterwards, negligible weight loss is found up to 700 oC, indicating that the decomposition reaction has completed at 560 oC. Therefore, we choose 600 oC as the optimal reaction temperature. The Mn0.33Co0.67CO3 precursor can be converted into MnCo2O4 phase by annealed in air at 600 oC for 4 h. The corresponding XRD pattern shown in Figure 5b demonstrates that all the diffraction peaks can be assigned to 12


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cubic MnCo2O4 phase (JCPDS Card: 23-1237, space group: Fd3m(227)). A weak peak located at 33o can be assigned to (Co, Mn)(Co, Mn)2O4 ((JCPDS Card: 18-0408). The formation of impure peak could possibly be ascribed to the incompletely reaction. A similar impure peak for MnCo2O4 has also been reported by other’s reports.33, 34

Figure 5. (a) TG and DSC results of the precursor from room temperature to 700 oC in air. (b) the XRD pattern of the MnCo2O4 obtained by annealing the precursor at 600 o

C for 4 h in air.

The morphology of the obtained product is investigated by SEM and TEM. Figure 6a reveals that the MnCo2O4 are highly uniform and the hierarchical dumbbell-like structure is preserved well after annealing. No discernible aggregation or structural collapse is detected, indicating a good structural robustness. Moreover, Figure 6b gives a much clear picture of the sample. Porous structure is detected on the surface of dumbbells, which is constructed by the decomposition of the carbonate precursor. Figure S1a and S1b show the MnCo2O4 spheres with a mean diameter of 6 µm. However, no pores are detected on the spheres, which can be ascribed to their higher packing density than dumbbells. The elemental mapping results (Figure 6c-f) demonstrate the uniform distribution of Mn, Co and O elements throughout the 13



dumbbells. The TEM image (Figure 6g) further reveals the inner structure of the sample. The nanorods are composed of nanoparticles with a diameter of around 30 nm and abundant pores can be observed clearly at the nanoparticles boundaries. Figure 6h shows the representative HRTEM image. The Fourier transform (FFT) image of region (inset of Figure 6h) shows the diffraction spots of MnCo2O4, which indicates the single crystalline nature of the nanorods. By comparison with theoretical value, the lattice fringe with a planar distance of 4.7 Å corresponds to (111) face of MnCo2O4, confirming again the existence of MnCo2O4 phase.

Figure 6. SEM (a, b), elemental mapping (c-f), TEM (g) and HRTEM (h) images of the MnCo2O4 dumbbells. Figure 7a shows the nitrogen adsorption-desorption isotherms for MnCo2O4 dumbbells and spheres. Both adsorption-desorption isotherms can be described as 14


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type-Ⅳ isotherms with H3 hysteresis loops. According to the Brunauer-Emmett-Teller (BET) method, the surface area of MnCo2O4 dumbbells is calculated to be 41.30 m2 g-1, much higher than that of MnCo2O4 spheres (15.49 m2 g-1). As shown in Figure 7b, the Barrett-Joyner-Halenda (BJH) pore size distribution indicates that the MnCo2O4 dumbbells possess more abundant pores than MnCo2O4 spheres, which are mainly in the range of 2-10 nm. The relatively high surface area and porous structure can offer the materials 3D channels for the transport of electrolyte ions to facilitate the contact between electrode materials and electrolyte.35,

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XPS technique is employed to

investigate the elemental composition and valence states of elements. The survey spectrum in Figure 7c indicates the presence of Mn, Co and O. The Mn 2p spectrum (Figure 7d) can be fitted to two pairs of spin-orbit doublets (Mn 2p3/2 located at 642.0 eV and Mn 2p1/2 located at 653.5 eV). After refined fitting, the spectrum can be divided into four peaks. Among them, 643.2 and 652.7 eV can be ascribed to the existence of Mn2+, and other two peaks situated at 644.2 and 655.0 can be assigned to Mn3+. Similarly, the XPS spectrum of Co 2p (Figure 7e) can be best fitted by considering two spin-orbit doublet characteristics of Co2+ and Co3+ and two shake up satellites. The 780.9 eV (Co 2p3/2) and 795.7 eV (Co 2p1/2) can be attributed to the Co3+, while the 783 eV (Co 2p3/2) and 797 eV (Co 2p1/2) can be attributed to the Co3+. Therefore, it can be concluded that both Mn2+/Mn3+ and Co2+/Co3+ are existed in MnCo2O4 dumbbells. The O 1s spectrum (Figure 7f) can be divided into three peaks (located at 530.24, 530.79 and 532.14 eV), which can be assigned to oxygen in metal-oxygen bonds, hydroxyl, and surface adsorbed water, respectively. 15



Figure 7. (a) N2 adsorption/desorption isotherm. (b) Barrett-Joyner-Halenda (BJH) pore size distribution curves of MnCo2O4 dumbbells and spheres. (c) the XPS spectra, (d) Mn 2p, (e) Co (2p) and (f) O 1s of MnCo2O4 dumbbells. The electrochemical performances of the as-prepared materials are then evaluated. Figure 8a shows the first three consecutive CV curves of the MnCo2O4 dumbbells in the voltage of 0.01-3 V vs Li/Li+ at a scan rate of 0.1 mV s-1. For the first cycle, a cathodic peak located at 1.41 V indicates the reduction of Co3+ to Co2+. The reduction peak at 0.49 V which moves to 0.75 V in the second and third cycle represents the further reduction of Mn2+ and Co2+ to Mn and Co, respectively. In the anodic scan, the relatively broad anodic peak at 2.02 V can be attributed to the phase conversion from Mn and Co to MnO and CoO, respectively.37 The XRD patterns of MnCo2O4 dumbbells after discharging to 0.5 V and re-charging to 2.5 V (Figure S2a and b) further confirm the phase transformation of MnCo2O4 to Mn/Co and Mn/Co to MnO/CoO, respectively. Therefore, the entire electrochemical process can be

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expressed as follows: MnCo2 O4 + 8Li+ + 8e- →Mn+2Co+4Li2 O

(1)

Mn+2Co+ 3Li2 O ↔ MnO+2CoO+6Li+ +6e-

(2)

The discharge-charge curves of the MnCo2O4 dumbbells for the 1st, 20nd, 100th and 180th cycles at a current density of 200 mA g-1 are shown in Figure 8b. The initial discharge and charge capacities are 2073.0 and 1226.6 mAh g-1, respectively, corresponding to 59% of the first coulombic efficiency. The relatively large irreversible capacity loss can be attributed to the irreversible phase transition and the formation of the SEI layers occurred at the electrode interface.38, 39 To evaluate the cyclic stability of the MnCo2O4 dumbbells and spheres, the batteries are tested at a constant current density of 200 mA g-1 for 180 cycles and the results are shown in Figure 8c. The capacity of MnCo2O4 dumbbells decreases in the initial 30 cycles from its initial value of 2073.0 mAh g-1 to a minimum of 505.2 mAh g-1. Then, the capacity starts to increase and remains at 955 mAh g-1 after 180 cycles. The increasing capacity can be attributed to the activation processes during cycling. The gradual activation processes may be ascribed to two reasons: (1) The redox reaction of the Mn2+/Mn4+ couple at about 2.1 V can result in progressive increasing in capacity and lithium ion reactivity.40, 41, 42 (2) The lithiation-induced mechanical degradation caused by repeated cycling can effectively restructure the porous structure and optimize the stable solid electrolyte interfaces (SEI) layers, facilitating the ions and electrons transport.43 For MnCo2O4 spheres, the rapid capacity fading for the initial 30 cycles is also observed, which can be ascribed to the irreversible phase transition and repeated 17



volume changes upon cycling, continuously damaging the SEI layers and exposing active surfaces for the formation of new SEI layers. However, the capacity of MnCo2O4 spheres are stabilized at 110.8 mAh g-1 and cannot return to a higher value for the next cycles, indicating a poor cycle stability. Furthermore, the MnCo2O4 dumbbells can retain a stable capacity of 562 mAh g-1 after 800 cycles at 1000 mA g-1 (Figure S3), demonstrating its good long-term cyclic stability. The MnCo2O4 dumbbells also deliver a better cycle stability than some of the previous works, demonstrating that the tightly stacked porous structure owns a better structural stability than common porous or hollow structures.44, 45 Electrochemical impedance spectroscopy (EIS) of the MnCo2O4 dumbbells after different cycles at 200 mA g-1 and spheres are measured. As shown in Figure 8d, all the Nyquist plots are composed of two semicircles in the high and middle frequency regions and slanted lines in the low frequency region. The first intercepts on the Z’ axis in the high frequency region reveal the Ohmic resistance of the cell (Rs), which are contributed by the resistance of electrolyte, current collectors and electrode materials. While the semicircles are mainly ascribed to the charge transfer resistance (Rct) and CPE represents the double-layer capacitance. The slanted lines refer to the Warburg impedance of the lithium diffusion in the bulk of electrode materials (Zw). Cint indicates the capacitance caused by the accumulation or loss of Li+ in the crystal of electrode material. For MnCo2O4 dumbbells, the simulated Rct value for the fresh electrode is 83.45 Ω. After 20 cycles, the corresponding Rct are 206.3 Ω. The increasing charge transfer resistance can be ascribed to the formation of fractured SEI layers on the surface of MnCo2O4 18


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electrode.46 However, the Rct decreases to 98.4 Ω after 100 cycles, which can be attributed to the structure refinement and the formation of stable SEI layers. The decline of Rct for MnCo2O4 electrode after long cycles is also reported in other’s work.37 However, the Rct of MnCo2O4 spheres (409.5 Ω) is much higher than that of the MnCo2O4 dumbbells, indicating weakened electron diffusion rate. The little charge transfer resistance for MnCo2O4 dumbbells can be ascribed to its high surface area and porous structure, which provide easy path way for the electron transportation. Figure 8e shows the rate capability of the MnCo2O4 dumbbells and spheres. The batteries are tested for 100 cycles at 1000 mA g-1 for initial stabilization procedure. At the current densities of 100, 200, 500, 800, 1000, 1500, 2000 mA g-1, the MnCo2O4 dumbbells deliver specific capacities of 901.8, 772.5, 705.9, 635.6, 594.4, 481.6, 423.3 mAh g-1, respectively. When the current density is reset to 100 mA g-1, the capacity can be recovered to 901.5 mAh g-1, indicating the excellent rate capability. For the MnCo2O4 spheres, the specific capacity is only 70.1 mAh g-1 at the current density of 2000 mA g-1 and is not recovered to the initial capacity when the current density is reset to 100 mA g-1, implying the worse rate capability. The relatively high specific surface area and abundant of porous structure of MnCo2O4 dumbbells are believed to supply more Li+ insertion/extraction sites for the reactions and accelerate the electrolyte penetration and activation process upon repeated cycling, which is beneficial to the improvement of electrochemical performances.

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Figure 8. (a) The first three consecutive CV curves of the MnCo2O4 dumbbells in the voltage range of 0.01-3.0 V vs Li/Li+ at a scan rate of 0.1 mV s-1. (b) The discharge-charge curves of MnCo2O4 dumbbells at a current density of 200 mA g-1. (c) Cycle performances of the MnCo2O4 dumbbells and spheres at a current density of 200 mA g-1. (d) Nyquist plots of the MnCo2O4 dumbbells and spheres and the dumbbells after different cycles (inset: the corresponding equivalent circuit and calculated Kinetic parameters). (e) Rate capacities of MnCo2O4 dumbbells and spheres at different current densities.

To investigate the structural stability of the electrode, the MnCo2O4 dumbbells

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after 180 cycles at 200 mA g-1 were disclosed and characterized by SEM technique and the result is shown in Figure S4. The morphology of the MnCo2O4 dumbbells is still in good reservation, demonstrating the good structural stability of the MnCo2O4 dumbbells as anode materials for lithium ion batteries.

To verify the electrochemical activity of OER, the slurry of MnCo2O4 dumbbells were deposited onto a glassy carbon electrode and then investigated as a working electrode using a typical three-electrode cell setup in a 1.0 M KOH solution. For comparison, the MnCo2O4 spheres were also prepared and evaluated. Figure 9a shows the OER linear sweep voltammetry (LSV) curves of the two samples. The MnCo2O4 dumbbells exhibit an overpotential of 426 mV achieve a current density of 10 mA cm-2, which is much better than that of the MnCo2O4 spheres (485 mV). Meanwhile, the MnCo2O4 dumbbells shows a electrocatalytic current of 49 mA cm-2 at an overpotential of 0.55 V, approximately 2 times of the MnCo2O4 spheres, demonstrating a more efficient OER activity of the dumbbell-like MnCo2O4. In addition, the corresponding Tafel curves were plotted to further investigate the OER kinetics (Figure 9b). The Tafel slope for the MnCo2O4 dumbbells is 93 mV dec-1, which is much smaller than that of MnCo2O4 spheres (128 mV dec-1) revealing the favorable kinetics towards electrochemical OER.47, 48 The comparison of OER activity between our work and previous reports and IrO2 is shown in Table S1 and the result shows that the overpotential of MnCo2O4 porous dumbbells is closer to IrO2 (320 mV) than MnCo2O4/CNT (513 mV) and MnCo2O4/ppy (583 mV). The enhanced OER activity could be attributed to the unique 3D hierarchical porous structure, which is 21



beneficial to the transport of ions and electrons.49, 50 The electrochemical impedance spectroscopy (EIS) was performed to value the electrical conductivity of the samples and the Nyquist impedance spectra of MnCo2O4 dumbbells and spheres at 426 mV is shown in Figure S5. The semicircle in the Nyquist plots is attributed to the charge transfer within MnCo2O4. The MnCo2O4 dumbbells exhibit smaller semicircles than MnCo2O4 spheres, suggesting that the MnCo2O4 dumbbells have a higher charge transfer efficiency. Cycling stability is another key parameter to estimate the performance of an electrocatalyst. Figure 9c shows the long-term stability test of MnCo2O4 dumbbells and spheres, which are continuously conducted at 10, 20 and 10 mA cm-2 for 10 h, respectively. The MnCo2O4 dumbbells exhibit high activity and stable required overpotentials over prolonged testing. Even at 20 mA cm-2, the overpotential is still retained at 500 mV, which is much better than that of the MnCo2O4 spheres (583 mV). When the current density returns to 10 mA cm-2, the corresponding overpotential is 450 mV, indicating an excellent durability of the MnCo2O4 dumbbells. After 30 h testing, the LSV plot were collected again. As shown in Figure 9d, the overpotentials before and after stability testing is 426 and 459 mV at 10 mA cm-2, respectively, further demonstrating its noticeable electrocatalytic stability.

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Figure 9. (a) LSV curves. (b) Corresponding tafel plots. (c) Electrochemical stability of the MnCo2O4 dumbbells and spheres collected at different current densities for 10 h. (d) LSV comparison of the MnCo2O4 dumbbells between initial state and cycling after 30 h (Inset: Optical image of the working OER device).

4. Conclusions In summary, we have successfully fabricated 3D hierarchical Mn0.33Co0.66CO3 dumbbells by a facile solvothermal method. The dumbbells are uniform and composed of nanorods, which are tightly stacked together. After annealing in air at 600 oC for 4 h, the carbonate precursor can be transformed to pure MnCo2O4 phase with highly porous structure. When evaluated as an anode material for LIBs, the 3D hierarchical MnCo2O4 porous dumbbells display a high reversible capacity of 955 mA h g-1 after 180 cycles at a current density of 200 mA g-1. The electrode also displays excellent rate capability for LIBs. Moreover, these 3D hierarchical MnCo2O4 porous dumbbells demonstrate superior OER activity with a low overpotential of 426 mV at 23



current density of 10 mA cm-2 and a small Tafel slope of 93 mV dec-1 as well as good stability. The enhanced electrochemical performances and OER activity can be ascribed to the tightly stacked hierarchical porous structure.

Supporting information SEM images of the MnCo2O4 spheres; The cycling performance of MnCo2O4 dumbbells at 1000 mA g-1; XRD patterns of the MnCo2O4 dumbbells after discharging and re-charging; SEM images of MnCo2O4 dumbbells after 180 cycles; The comparison of OER activity between our work and previous reports and IrO2; Electrochemical impedance spectroscopy of MnCo2O4 dumbbells and spheres.

Acknowledgments This work was supported by the National Natural Science Foundation of China (No 51302323), the Program for New Century Excellent Talents in University (No. NCET-13-0594), and the Innovation-Driven Program of Central South University (No. 2017CX001).

Notes The authors declare no competing financial interest.

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