Growth of NiCo2O4@MnMoO4 Nanocolumn Arrays with Superior

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Growth of NiCo2O4@MnMoO4 Nanocolumn Arrays with Superior Pseudocapacitors Properties Chunyu Cui, Jiantie Xu, Lei Wang, Di Guo, Minglei Mao, Jianmin Ma, and Taihong Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02962 • Publication Date (Web): 15 Mar 2016 Downloaded from http://pubs.acs.org on March 15, 2016

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ACS Applied Materials & Interfaces

Growth of NiCo2O4@MnMoO4 Nanocolumn Arrays with Superior Pseudocapacitors Properties Chunyu Cui,a Jiantie Xu,c Lei Wang,b Di Guo,a Minglei Mao,a Jianmin Mab,* and Taihong Wanga,b,* a State Key Laboratory of Chem-/Bio-Sensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, P. R. China b Key Laboratory for Micro-/Nano-Optoelectronic Devices of the Ministry of Education, School of Physics and Electronics, Hunan University, Changsha 410082, P. R. China c Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, OH 44106, United States

Corresponding authors: [email protected] (Jianmin Ma) [email protected] (Taihong Wang)

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ABSTRACT: Three-dimensional heterostructured NiCo2O4@MnMoO4 nanocolumn arrays (NCAs) on Ni foam were firstly fabricated through an improved two-step hydrothermal process associated with a successive annealing treatment. The hybrid NiCo2O4@MnMoO4 electrode exhibited remarkable pseudocapacitors property with high initial mass specific capacitance of 1705.3 F g-1 at 5 mA cm-2, and retained 92.6% after 5000 cycles, compared to the bare NiCo2O4 electrode with 839.1 F g-1 and 90.9%. The excellent capacitive property of the NiCo2O4@MnMoO4 hydrid was attributed to its high electron/ion transfer rate, large electrolyte infiltrate area and more electroactive reaction sites. KEYWORDS: NiCo2O4@MnMoO4, nanocolumn arrays, sacrificial template, supercapacitor, hydrothermal method, synergistic effect

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1 INTRODUCTION With the electric vehicles and portable electronic devices expanding, one of the most pressing challenges is to research conversion devices and electrochemical energy storages.1 Pseudocapacitors (PCs) have gained a substantial amount of attention due to its long cycling life, high power density, rapid charge and discharge properties, and environmental friendliness.2 During the development of high performance PCs, key to the success was the electrode materials. To date, the investigation of advanced PCs electrodes was primarily concentrated on hydroxides, transition metal oxides, or the combination of them, because their cost and toxicity were lower and they own great structures and morphology flexibility,3 including NiO,4-5 CoO,6 Ni(OH)x,7-8 Co(OH)x9-12 and MnO2.13-16 Because of limited kinetics during the redox reaction and unsteady structure,

17-21

however, these

materials generally suffer from poor cycling or rate performance, further limiting them in the practical application. Very recently, 3D hybrid nanostructures like Co3O4@MnO2 and Ni(OH)NO3@CoO have received more attention for PCs.22-23 These hybrids showed satisfying capacitance with enhanced rate capability benefitting from the synergistic effect between the materials with high electrical conductivity or good structural stability.24-25 Nevertheless, the capacitive properties of these hybrids were still unsatisfied to meet the high-demand PCs, such as Fe3O4@SnO2 core–shell nanorod film, when the current density was 0.2 mA cm-2, its area capacitance could attain 0.7 F cm-2, 26; Co3O4@NiCo2O4 nanoforests (1.6 mA cm-2, 0.89 F cm-2, 27), as shown in Table S1. Therefore, due to the great electrical conductivity and multifarious valence states, ternary oxides (eg., NiCo2O4,28-31 ZnCo2O4,32-33 NiMoO4,34-35 Zn2SnO4,36 CoMoO437 etc.) have been widely applied in PCs. Such as, Wang et al. synthesized the NiCo2O4@CoMoO4 nanowire/nanoplate arrays, when the current density was 10 mA cm-2, its capacitance could attain 14.67 F cm-2, with a capacitance fading of 25.9% after 1000 cycles.38 Mai et al. reported the design of MnMoO4/CoMoO4 hetero-structure nanowires, when the current density was 1 A g-1, 3



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its capacitive attained 187.1 F g-1, with only 2% faded after 1000 cycles.39 Recently, Xiong

et

al.

synthesized

3D

hierarchical

NiCo2O4@NiMoO4

hybrid

nanowire/nanosheet arrays, when the current density was 10 A g-1, the electrodes faded 18.2% of the initial capacitance (5.80 F cm-2) after 5000 cycles.40 Despite the obtained progress, there are still necessity of exploring optimized structured materials to further improve their electrode performances for PCs Recently, NiCo2O4 and MnMoO4 have gained more and more attention as PCs due to their excellent electrochemical characteristics and environmental compatibility. 41-51

Herein, three-dimensional heterostructured NiCo2O4@MnMoO4 nanocolumn

arrays (NCAs) on Ni foam was firstly fabricated through an improved two-step hydrothermal process associated with a successive annealing treatment. With nanocolumn arrays interconnected on Ni foam, the hybrid NiCo2O4@MnMoO4 NCAs could offer plenty of structure and property superiorities, including i) hybrid nanowire arrays directly decorated on current collector significantly shorten the distance of electron transport and enhance the morphology flexibility of the electrode; ii) the distributed evenly NCAs with abundant free-space could facilitate the electrolyte penetration more quickly and accommodate volume expansion during the charge storage and release; iii) serve as sacrificial template, the NiCo2O4 nanoparticles uniformly confined on MnMoO4 nanosheets, which could enhance the electrons transfer of NiCo2O4@MnMoO4 NCAs. In addition, the structural stable MnMoO4 could ensure the cycling stability of NiCo2O4@MnMoO4 NCAs. Apart from the individual contributions of NiCo2O4 and MnMoO4 as electrode materials due to their internal structure and property features, the synergistic effect between them could further enhance their PCs’ performance. Benefiting from above advantages, the NiCo2O4@MnMoO4 NCAs on Ni foam exhibit remarkable pseudocapacitive performance compared to other reported electrodes, as shown in Supporting Information. 2 EXPERIMENTAL SECTION 4


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2.1 Material preparation. Analytically pure CoCl2·6H2O, NiCl2·6H2O, MnCl2, Na2MoO4·2H2O, urea (CO(NH2)2) and KOH were bought from Shanghai Aladdin Biological Technology Co., Ltd. Synthesis of NiCo2O4 NWAs: Firstly, tailor the Ni foam to 3 × 1.5 × 0.1 cm, and ultrasonic clean them for 15 minutes in deionized (DI) water, ethanol and acetone, respectively. The NiCo2O4 NWAs were synthesized as follow: 41 weigh urea (0.54 g), CoCl2·6H2O (1.19 g) and NiCl2·6H2O (0.59 g) into a 40 mL Teflon-lining, then add 35 mL DI water and keep magnetic stirring for 1h. Immerse a pre-cleaned Ni foam into the above-mentioned lining, Seal it into a stainless steel autoclave and keep it at 130 °C for 6 h in a constant temperature oven, followed by a natural cooling process. Collect and rinse the obtained Ni foam with ethyl alcohol for five times, and then anneal it in air for 2h at 350 °C, with the heating rate of 3°C min-1. Synthesis of NiCo2O4@MnMoO4 hybrid NCAs: Typically, weigh MnCl2 (0.39 g) and Na2MoO4·2H2O (0.48 g) into a 40 mL Teflon-lining, then add 35 mL DI water and keep magnetic stirring for 1h. Afterwards, immerse a piece of as-prepared NiCo2O4 NCAs on the Ni foam into the above-mentioned lining. Seal it into stainless steel autoclave and keep it at 130 °C for 6 h in a constant temperature oven, followed by a natural cooling process. Finally, collect and rinse the obtained Ni foam with ethyl alcohol for five times, then anneal it in at the same condition as the synthesis of NiCo2O4 NWAs. The mass loading calculation of electrode materials: Firstly, tailor the Ni foam to 3 × 1.5 × 0.1 cm, and ultrasonic clean them for 15 minutes in deionized (DI) water, ethanol and acetone, respectively, then anneal a pre-cleaned Ni foam in the same condition as the synthesis of NiCo2O4@MnMoO4 hybrid NCAs, and weigh the Ni foam as m1. Then, carry out the synthesis of NiCo2O4 NWAs and NiCo2O4@MnMoO4 hybrid NCAs, and weigh the NiCo2O4 NWAs on Ni foam as m2 and NiCo2O4@MnMoO4 hybrid NCAs on Ni foam as m3. Base on the following equation to calculate the mass loading of NiCo2O4 NWAs (m NiCo2O4): 5



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m NiCo2O4 (wt.%) = m2 – m1

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(1)

Calculate the mass loading of NiCo2O4@MnMoO4 hybrid NCAs on Ni foam m NiCo2O4@MnMoO4

as follow: m NiCo2O4@MnMoO4 (wt.%) = m3 – m1

(2)

Repeat the above calculation process for ten times to reduce the errors as much as possible. Due to the above way, the mass loading of NiCo2O4 NWAs and NiCo2O4@MnMoO4 hybrid NCAs is 0.87 and 0.95 mg cm-2, respectly. 2.2 Characterization. Scanning electron microscopy (SEM, Hitachi, S-4800) and high-resolution transmission electron microscopy (HRTEM, JEM-2100, 200KV) were used to observe the morphology and structure of the NiCo2O4 NWAs and NiCo2O4@MnMoO4 hybrid NCAs. X-ray diffraction (XRD, Rigaku Dmax-2500) patterns was used to investigate its crystal structure. 2.3 Electrochemical Measurements. Three-electrode test system on RST 4600 electrochemical workstation was used to conduct the pseudocapacitors measurements, and the electrolyte was 2 M potassium hydroxide aqueous. The standard calomel electrode (SCE) and Pt foil were used as the reference electrode and counter electrode, respectively, while the working electrode was the NiCo2O4 or NiCo2O4@MnMoO4 directly on Ni foam (1 cm2 in area). The following equation was used to calculate the area specific capacitance of NiCo2O4 and NiCo2O4@MnMoO4 electrodes:

C=

∫ i × d ∆u

(3)

v × ∆u

Herein, C represents the capacitance of the NiCo2O4 and NiCo2O4@MnMoO4 6


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electrodes, i and t represents the current density and time during the discharging process, ∆u represents the potential, and v represents its scan rate. Conduct the Electrochemical Impedance Spectroscopy (EIS) test from 10 mHz to 1 M Hz, with a 5 mV superimposed sinusoidal voltage. 3 RESULTS AND DISCUSSION

Scheme 1 Schematic fabrication process of NiCo2O4@MnMoO4 hybrid NCAs on Ni foam. The fabrication process of NiCo2O4@MnMoO4 hybrid NCAs is simply illustrated in Scheme 1. Though a hydrothermal process with NiCl2·6H2O and CoCl2·6H2O as raw materials and an annealing treatment, the NiCo2O4 nanowire arrays on Ni foam were initially synthesized. In the second hydrothermal process, the as-synthesized NiCo2O4 nanowires on Ni foam served as the templates. MnCl2 and Na2MoO4·2H2O reacted around the NiCo2O4 nanowires and formed MnMoO4 nanosheets, then plenty of nanosheets stacking and interweaving together to compose the NiCo2O4@MnMoO4 NCAs on Ni foam, while the NiCo2O4 nanowires broken into nanoparticles and dispersed uniformly on MnMoO4 nanosheets. It is noted that the 7



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NiCo2O4@MnMoO4 NCAs distribute sparser than NiCo2O4 NWAs, that is because during the formation process of MnMoO4, multiple NiCo2O4 nanowire adjacencies would be involved in and grown into single NiCo2O4@MnMoO4 nanocolumn. In addition, the relatively large MnMoO4 nanosheets should occupy larger space, therefore the diameter of NiCo2O4@MnMoO4 nanocolumn was larger than NiCo2O4 nanowire.

Figure 1 The high magnification (a-d) and low magnification (e, f) SEM images of NiCo2O4@MnMoO4 NCAs (b, d, f) and NiCo2O4 NWAs (a, c, e). The morphology and structures of the as-prepared NiCo2O4 NWAs and NiCo2O4@MnMoO4 NCAs were studied by SEM. As shown in Figure 1a and b, the 8


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Ni foam was uniformly covered by the NiCo2O4 NWAs and NiCo2O4@MnMoO4 NCAs nanoarrays, respectively. Figure 1c and e clearly displays the Ni foam was uniformly covered by NiCo2O4 nanowire arrays, with a wire length of ~ 2.2 um (Figure 1e) and a diameter of ~ 100 nm (Figure 1c). For NiCo2O4@MnMoO4 NCAs (Figure 1b and d), each single nanocolumn was composed of NiCo2O4@MnMoO4 nanosheets uniformly distributed with enough interval space between adjacent nanocolumns, facilitating the ions transporting and accommodating the volume expansion.

Figure 2 The TEM and HRTEM images of NiCo2O4 NWAs (a, c) and NiCo2O4@MnMoO4 NCAs (b, d). The detailed structure NiCo2O4 NWAs and NiCo2O4@MnMoO4 NCAs were studied by TEM and HRTEM. As shown in Figure 2a, the NiCo2O4 nanowire was composed by numerous interconnected nanoparticles, and its grain diameter was ~20 nm. The abundant free-space between particles could not only offer more electroactive surface sites but also facilitated the uniform formation of NiCo2O4@MnMoO4 nanostructure.29, 47 The HR-TEM of NiCo2O4 nanowire (Figure 9



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2c) confirmed the formation of the (220) face for cubic NiCo2O4 phase, with a lattice spacing of 0.29 nm.44 Figure 2b revealed that NiCo2O4@MnMoO4 NCAs were formed by MnMoO4 nanosheets and NiCo2O4 nanoparticles. The phase of MnMoO4 was confirmed by the formation of (022) plane, with an lattice spacing of 0.28 nm.39

Figure 3 The XRD patterns of NiCo2O4 NWAs and NiCo2O4@MnMoO4 NCAs on Ni foam. The XRD patterns of NiCo2O4@MnMoO4 NCAs and NiCo2O4 NWAs on Ni foam were shown on Figure 3. All the diffraction peaks of bare NiCo2O4 are well indexed to NiCo2O4 (PDF cards No. 20-0781).30-31 For the NiCo2O4@MnMoO4 NCAs, several strong peaks of 25.9°, 31.2° and 33.3° can be well indexed to MnMoO4 (PDF cards No. 50-1287),39 while the weak peaks of 19.06°, 36.8°, 58.72° and 64.98° are belong to NiCo2O4 nanoparticles. The peaks of Ni are really weak, further indicating that the Ni foam was completely covered by the NiCo2O4 NWAs and NiCo2O4@MnMoO4 NCAs.

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Figure 4 CV curves of NiCo2O4 NWAs (a, b) and NiCo2O4@MnMoO4 NCAs (a, c) at various scan rates. Three-electrode test system was used to investigate the pseudocapacitive performance of NiCo2O4 NWAs and NiCo2O4@MnMoO4 NCAs electrodes, and the electrolyte was 2 M potassium hydroxide aqueous. Set the scan rates at 5, 10, 20, 40, 60, 80 and 100 mV s-1 and the voltage range from 0.1 V to 0.7 V to measure the cyclic voltammogram (CV) curves of NiCo2O4 NWAs and NiCo2O4@MnMoO4 NCAs, as shown in Figure 4. Every CV curve owns visible redox peaks, which indicated that the Faradaic redox reactions govern the main pseudocapacitive reaction process. In the CV curves of 5 mV s-1 (Figure 4a), the 0.22 V of anodic peaks for NiCo2O4 and 0.24 V for NiCo2O4@MnMoO4 electrode were clearly observed, and the cathodic peaks at 0.53 V and 0.54 V were also revealed correspond to the their reverse process.39, 44 Remarkably, the NiCo2O4@MnMoO4 NCAs own much larger enclosed area within current-potential curve than NiCo2O4, which means that the hybrid electrodes owns larger areal specific capacitance （ASC）. There were peak potential shifting phenomenon when the scan rate increased (Figure 4b-c), which was caused 11



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by the electrode polarization.49 However, the NiCo2O4@MnMoO4 hybrid electrodes show smaller shift, and its peak potential could still be observed even at 80 and 100 mV s-1 compared to NiCo2O4, indicating improved electron conduction and mass transportation for NiCo2O4@MnMoO4 hybrid electrode.

Figure 5 charge–discharge plot (a, b), ASC (c) and cycling performance (d) of NiCo2O4 and NiCo2O4@MnMoO4 electrodes at various current densities. For estimating the capacitance of the NiCo2O4 and NiCo2O4@MnMoO4 hybrid electrodes, the constant-current charge–discharge measurements at different current densities of 1, 2, 5, 10 and 20 mA cm-2 was conducted and shown in Figure 5a-b, the voltage range is 0-0.5 V. Obviously there were distinct voltage plateau regions in every curve. The charge-discharge curves could be used to calculate the ASC of the NiCo2O4 and NiCo2O4@MnMoO4 electrodes, as shown in Figure 5c. As can be seen, at different current densities of 1, 2, 5, 10 and 20 mA cm-2, the NiCo2O4@MnMoO4 hybrid electrode delivered the ASCs of 1.91, 1.78, 1.62, 1.39 and 1.19 F cm-2, and the corresponding mass specific capacitance were 2010.5, 1873.7, 1705.3, 1463.2 and 1252.6 F g-1, much higher than NiCo2O4 electrode of 1069.0, 965.5, 839.1, 712.6 and 12


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597.7 F g-1, whose ASCs were 0.93, 0.84, 0.73, 0.62 and 0.52 F cm-2, respectively. Therein, the NiCo2O4@MnMoO4 electrodes could still exhibit a high capacitance of 1252.6 F g-1 even at 20 mA cm-2, much higher than NiCo2O4 electrodes of 597.7 F g-1. To investigate the cycling stability of the NiCo2O4 and NiCo2O4@MnMoO4 hybrid electrodes, the repeated charge-discharge measurements of 5000 cycles was carried out at 5 mA cm-2 and shown in Figure 5d, the voltage range is 0-0.5 V. Remarkably, the initial mass specific capacitance of NiCo2O4@MnMoO4 hybrid electrodes was 1705.3 F g-1, with an capacity retention ratio of 92.6% after 5000 cycles, perform better than NiCo2O4 (839.1 F g-1, 90.9% after 5000 cycles). Compared to previous reports, the mass specific capacitance of NiCo2O4@MnMoO4 hybrid electrodes could attain 1705.3 F g-1, and 1578.9 F g-1 still be maintain after 5000 cycles at 5 mA cm-2 (Table S1).

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Figure 6 SEM images (a, b) of the NiCo2O4 and NiCo2O4@MnMoO4 electrodes after 5000 cycles; (c): Electrochemical impedance spectra before and 5000th cycles of NiCo2O4 and NiCo2O4@MnMoO4 electrodes, (Top: the equivalent circuit diagram). The morphologies of the NiCo2O4 and NiCo2O4@MnMoO4 hybrid electrodes after 5000 cycles are also further investigated in Figure 6a and b. As can be seen, the morphology and structures of NiCo2O4 and NiCo2O4@MnMoO4 are well preserved even after 5000 cycles. For investigating the charge transfer resistance of NiCo2O4 and NiCo2O4@MnMoO4 electrodes before and after 5000 cycles, EIS measurements were conducted and shown in Figure 6c. The equivalent circuit diagram fitting the EIS plots is also presented. The equivalent model includes the Warburg impedance of diffusive resistance (W), the charge-transfer resistance (Rct), and the bulk solution resistance (Rs).52 Therein, Rct is the key indicator of kinetics. As expected, Rct of the electrodes increased from 1.47 Ω (before cycling) to 2.87 Ω (after 5000 cycles) for NiCo2O4@MnMoO4 and from 1.33 Ω (before cycling) to 3.21 Ω for NiCo2O4 (after 5000 cycles), indicating there is no distinct change of Rct for both electrodes after 5000 cycles. These analyzes are consistent to the electrochemical performance for both electrodes. 4 CONCLUSION In summary, 3D hybrid NiCo2O4@MnMoO4 NCAs on Ni foam was firstly fabricated through an improved two-step hydrothermal process associated with a successive annealing treatment. The as-synthesized hybrid NiCo2O4@MnMoO4 electrodes exhibited remarkable pseudocapacitive properties of 1705.3 F g-1 at 5 mA cm-2, with an capacity retention ratio of 92.6% after 5000 cycles. The excellent performance was attributed to its unique structures that providing rich active sites for ions storage, and facilitating electrolyte quick penetration and ions fast diffusion. We believe that the hybrid NiCo2O4@MnMoO4 NCAs hold great application potential for the high-performance PCs in future due to its excellent electrochemical performance. 5 ASSOCIATED CONTENT 14


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Supporting Information Electrochemical performance: NiCo2O4@MnMoO4 NCAs vs other reported electrodes.

Acknowledgements

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Scheme 1 Schematic fabrication process of NiCo2O4@MnMoO4 hybrid NCAs on Ni foam. 218x160mm (150 x 150 DPI)


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Figure 1 The high magnification (a-d) and low magnification (e, f) SEM images of NiCo2O4 NWAs (a, c, e) and NiCo2O4@MnMoO4 NCAs (b, d, f). 169x191mm (150 x 150 DPI)



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Figure 2 The TEM and HRTEM images of NiCo2O4 NWAs (a, c) and NiCo2O4@MnMoO4 NCAs (b, d). 220x165mm (150 x 150 DPI)


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Figure 3 The XRD patterns of NiCo2O4 NWAs and NiCo2O4@MnMoO4 NCAs on Ni foam. 176x126mm (150 x 150 DPI)



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Figure 4 CV curves of NiCo2O4 NWAs (a, b) and NiCo2O4@MnMoO4 NCAs (a, c) at various scan rates. 224x162mm (150 x 150 DPI)


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Figure 5 charge–discharge plot (a, b), ASC (c) and cycling performance (d) of NiCo2O4 and NiCo2O4@MnMoO4 electrodes at various current densities.



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Figure 6 SEM images of the NiCo2O4 (a) and NiCo2O4@MnMoO4 (b) electrodes after 5000 cycles; (c): Electrochemical impedance spectra before and 5000th cycles of NiCo2O4 and NiCo2O4@MnMoO4 electrodes, (Top: the equivalent circuit diagram). 134x125mm (150 x 150 DPI)


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Growth of NiCo2O4@MnMoO4 Nanocolumn Arrays with Superior

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