Metal oxide modified (NH4)(Ni,Co)PO4·0.67H2O composite as high-performance electrode materials for supercapacitors

Metal oxide modified (NH4)(Ni,Co)PO4·0.67H2O composite as high-performance electrode materials for supercapacitors

Journal Pre-proofs Metal oxide modified (NH4)(Ni,Co)PO4·0.67H2O composite as high-performance electrode materials for supercapacitors Yong Zhang, Han-...

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Journal Pre-proofs Metal oxide modified (NH4)(Ni,Co)PO4·0.67H2O composite as high-performance electrode materials for supercapacitors Yong Zhang, Han-xin Mei, Hai-li Gao, Qing-yuan Huo, Xiao-dong Jia, Yang Cao, Shi-wen Wang, Ji Yan, He-wei Luo, Jing Yang, Ai-qin Zhang, Ke-zheng Gao PII: DOI: Reference:

S1387-7003(19)31136-0 https://doi.org/10.1016/j.inoche.2019.107696 INOCHE 107696

To appear in:

Inorganic Chemistry Communications

Received Date: Revised Date: Accepted Date:

5 November 2019 20 November 2019 21 November 2019

Please cite this article as: Y. Zhang, H-x. Mei, H-l. Gao, Q-y. Huo, X-d. Jia, Y. Cao, S-w. Wang, J. Yan, H-w. Luo, J. Yang, A-q. Zhang, K-z. Gao, Metal oxide modified (NH4)(Ni,Co)PO4·0.67H2O composite as highperformance electrode materials for supercapacitors, Inorganic Chemistry Communications (2019), doi: https:// doi.org/10.1016/j.inoche.2019.107696

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© 2019 Published by Elsevier B.V.

Metal oxide modified (NH4)(Ni,Co)PO4·0.67H2O composite as high-performance electrode materials for supercapacitors

Yong Zhanga,b,, Han-xin Meia, Hai-li Gaoa,*, Qing-yuan Huoc, Xiao-dong Jiaa, Yang Caoa, Shi-wen Wanga, Ji Yana, He-wei Luoa, Jing Yanga, Ai-qin Zhanga, Ke-zheng Gaoa

a

Department of Material and Chemical Engineering, Zhengzhou University of Light Industry, Zhengzhou 450002,

P.R. China b

Henan Provincial Key Laboratory of Surface & Interface Science, Zhengzhou University of Light Industry,

Zhengzhou 450002, China c

School of Arts and Design, Zhengzhou University of Light Industry, Zhengzhou 450002, China

ABSTRACT (NH4)(Ni,Co)PO4·0.67H2O (NNCP) was successfully synthesized by a simple one-step low-temperature hydrothermal method, and modified by metal oxides CuO, MnO2 and Co3O4, so that the composites with different length scales and different energy storage mechanisms could be synergistically integrated. The electrochemical behaviors of NNCP, NNCP/CuO, NNCP/MnO2 and NNCP/Co3O4 as supercapacitor electrode materials in 3 mol·L-1 KOH electrolyte were studied by cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical impedance spectroscopy (EIS), and the morphology and structure of the synthesized materials

* Corresponding authors. E-mail addresses: [email protected] (Y. Zhang), [email protected] (H. L Gao). 1 / 26

were studied by SEM, XRD and Raman spectrum. Compared with other samples, the optimized NNCP/MnO2 composites have higher specific capacitance and remarkable rate capability due to the micron flower-like rich interface structure, which provides a more active sites for electrochemical reaction, a shorter transport path for electrolyte ions, and the synergistic effect of NNCP and MnO2. The specific capacitances of NNCP/MnO2 at current densities of 1, 4, 7 and 10 A g-1 are 3626, 3336, 3140 and 3104 F g-1, respectively. These excellent properties confirm that the composite material has broad application prospects in the field of supercapacitor materials.

Keywords: hydrothermal method; metal oxides; (NH4)(Ni,Co)PO4·0.67H2O; properties; supercapacitors

1. Introduction As an important part of the competition in the world today, energy is of great importance, and the performance of energy storage devices is the most important part of the competition [1-3]. Supercapacitor is a new type of energy storage device in recent years. Compared with ordinary capacitors and secondary batteries, it has many advantages, such as good cycle stability, high power density, wide operating temperature range, etc [4-7]. It is one of the research focuses in the field of energy storage devices at home and abroad. At present, the energy density of supercapacitors is remain low, so it is still the focus of research to improve the performance of supercapacitors. Electrode materials play a decisive role in the performance of supercapacitors. Common electrode materials can be divided into three categories: carbon-based materials, conductive polymers and metal oxides. Supercapacitors can be divided into electric double-layer capacitors (EDLCs) and Faraday pseudocapacitors according to their energy storage mechanism 2 / 26

[8-10]. Carbon-based materials are mainly used in EDLCs, while the electrode materials in Faraday pseudocapacitors are mainly conductive polymers and metal oxides. Although the cycle stability of Faraday pseudocapacitors is poor, the cost is higher than that of carbon-based materials, the reaction process is complex, and some mechanisms are not clear, but the specific capacitance of Faraday pseudocapacitors is much larger than that of carbon-based materials of EDLCs. Therefore, it still has great potential for development [11-13]. As a binary transition metal compound, ammonium nickel-cobalt phosphate contains Ni and Co elements with good synergistic effect, which has been widely used in the field of inorganic catalysis. However, there are few studies in the field of electrochemistry, so it has great research potential [14, 15]. Liang et al. [16] synthesized NH4Co0.33Ni0.67PO4·H2O nanometer sheet by hydrothermal method. The material showed a specific discharge capacitance of 1037 F g-1 at a current density of 1 A g-1, and still had a capacity retention rate of 94% after 1000 cycles. Chen et al. [17] synthesized CoxNi1-xNH4PO4·H2O by chemical precipitation. The material exhibited an ultra-high specific capacitance of 1259 F g-1 at 1 A g-1 current density, with a capacity retention rate of 88.9% after 1000 cycles. Wang et al. [18] first synthesized the intermediate of Ni and Co hydroxides by a simple hydrothermal method, then using the principles of Kirkendall effect and Ostwald ripening, the core-shell structure (NH4)(Ni, Co)PO4·0.67H2O [email protected] crystal microchip was synthesized by another hydrothermal method. And the material provides an excellent energy density of 44.5 Wh kg-1 at a power density of 150 W kg-1. Even at a power of 7.4 KW, the energy density is still as high as 30 Wh kg-1 and has excellent cycling stability, with a capacity retention rate of 77.5% after 7000 cycles. In addition, composite modification is also an important way to improve the performance of 3 / 26

electrode materials. Modification of ammonium nickel-cobalt phosphate with metal oxides can increase the specific surface area of electrode materials, provide more binding active sites, enhance conductivity, accelerate redox reaction, and further improve electrochemical properties of electrode materials such as specific capacity [19, 20]. By means of hydrothermal method, Zhao et al. [21] combined the single-layer nanoparticle (Ni, Co)3(PO4)2·8H2O and single crystal microporous plate (NH4)(Ni, Co)PO4·0.67H2O to form a hollow composite materials with hollow hexagon and rectangle. Under the current density of 0.5 A g-1, the material showed a specific discharge capacity of 1128 F g-1, and the retention rate of discharge capacity was 95.6% after 5000 cycles. Existing literatures show that in the current research on the modification of ammonium nickel-cobalt phosphate by metal oxide, the modified materials are mostly homogeneous phosphates [22], and compared with the main materials of ammonium nickel-cobalt phosphate, most of them only have the difference in microstructure and crystal water content. Therefore, the modified material has a more complex microstructure, which also improves the electrochemical properties of the material to a certain extent, but the degree of improvement is limited. Based on the research idea of ternary lithium battery materials, this paper focuses on the synergistic effect of multiple metals, adopts simple metal oxides such as manganese dioxide, cobalt tetroxide and copper oxide as modified materials, and systematically studies the morphology, structure and capacitance performance of modified ammonium nickel-cobalt phosphate materials. The results show that ammonium nickel-cobalt phosphate modified by manganese dioxide has the best performanc, and its microstructure exhibits clear micron flower-like and rod-like structure. The characteristic peaks are obvious, the crystallinity is good, and the specific capacity in charge-discharge test is also better than that of the unmodified material. 4 / 26

This experiment is not limited to the binary synergies that are widely studied at present, but innovatively introduces the ternary synergies of lithium batteries into the field of supercapacitors, and studies whether there is an analogy effect, which provides a new idea for the research of metal oxide modified supercapacitor electrode materials.

2. Experimental 2.1. Materials and chemicals The Ni foams matrix (Changsha Lyrun New Material Co., Ltd., Changsha, China) was degreased with acetone, corroded with hydrochloric acid for 5 min, then washed with deionized water, and dried before use. The reagents used in this experiment are all analytical grade reagents. They were purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd, and used directly without further purification. 2.2. Material preparation Metal oxide modified (NH4)(Ni,Co)PO4·0.67H2O (NNCP) was prepared on Ni foam substrate by hydrothermal method. Firstly, 1.8 mmol of nickel nitrate, 1.8 mmol of cobalt nitrate and 10 mmol of red phosphorus were mixed with 30 mg of CuO, 30 mg of MnO2 and 30 mg of Co3O4 water respectively, then 15 mmol of urea was added and stirred for 30 min. Finally, the homogeneous mixed water solution and treated Ni foams were put into 100 mL of hydrothermal reactor. The final samples of NNCP/CuO, NNCP/MnO2 and NNCP/Co3O4 were obtained by reaction at 160 °C for 12 h in an autoclave. The preparation conditions of pure NNCP were the same as those mentioned above. 2.3. Characterizations In order to characterize the electrochemical and physicochemical properties of NNCP 5 / 26

modified by different metal oxides, a CHI 660E electrochemical workstation (Chenhua Instruments, Shanghai, China) electrochemical workstation was used to analyze the cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and electrochemical impedance spectroscopy (EIS) of the prepared materials. The test systems are all three-electrode systems, in which the working electrode is the prepared sample, the counter electrode is a platinum plate of 20 mm×20 mm, the reference electrode is Hg/HgO electrode, and the electrolyte is KOH aqueous solution of 3 mol·L-1. The specific capacitance (Cm, F g-1) is calculated from the charge-discharge curve according to the following equation (1):

Cm 

C i  t  m m  u

(1)

Where, i (A), m (g), △t (s) and △u (V) represent the discharge current, the mass of active electrode material, the total discharge time and the potential window, respectively. In addition, scanning electron microscope (SEM, JEOL JSM-6490LV, Japan), X-ray diffraction (XRD, Bruker D8 Advance; Bruker Corp, Billerica) and Raman spectrometer—Lab RAM (LabRAM system, Dilor, Lille) were used to analyze the physicochemical properties of the samples.

3. Results and discussion 3.1. Electrochemical behavior The electrochemical behaviour of the pristine NNCP, NNCP/CuO, NNCP/MnO2 and NNCP/Co3O4 composite samples were characterized by CV, GCD, and EIS measurements in 3 mol·L-1 KOH aqueous solution within a potential range of 0-0.5 V vs. Hg/HgO. The representative CV curves of the as-synthesized samples are presented in Fig. 1 at scan rates of 2, 5, 10, 15 and 20 mV s-1 in a conventional three-electrode system. As is shown, due to the redox reactions of 6 / 26

Ni2+/Ni3+ and Co2+/Co3+ in KOH solution of the samples [23], a well-defined redox peaks on the cathodic as well as anodic side under various scan rates from 2 to 20 mV s-1 were observed in all these metal phosphate electrodes and their shapes are different from the conventional EDLCs, indicating the capacity behavior of the materials mainly resulted from the faradaic pseudocapacitive capacitance. Interestingly, the peak current of all the CV profiles upsurges as scan rates increase, suggesting the good rate capability in the given scan rates [24]. Furthermore, with the increase of scanning rate, the anode current peak gradually shifts to the positive direction, while the cathode current peak gradually shifts to negative direction, and the peak potential difference between the two peaks gradually increases, indicating that the reversibility of the material is worse. The main reason is the voltage loss caused by uncompensated resistance in solution and the polarization of electrode under fast scanning rates [25]. It indicates that the peak potential difference of NNCP/MnO2 is the smallest at all scanning rates, which indicates its reversibility is the best. Besides, the NNCP/MnO2 has the largest CV area, indicating the highest specific capacity. In addition to the minimum resistance (Fig. 3b), the maximum specific capacitance of NNCP/MnO2 electrode should be attributed to the synergistic effect of nickel, cobalt and MnO2 in phosphate, as well as the suitable structural morphology. -80

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The GCD profiles of these metal phosphate electrodes confirm the high capacitance of nickel-cobalt phosphate electrodes. As shown in Fig. 2, the platform observed in every charge/discharge curve under different current densities indicates the pseudocapacitive characteristics of the metal phosphate electrode [26, 27], and the results are consistent with the CV test. It can be seen that the time required for charging and discharging decreases with the increase of current densities, which is related to the limitation of ion diffusion rate [28, 29]. Compared with the other three samples, the NNCP/MnO2 electrode has higher charge and longer discharge time, showing the best capacitance characteristics.

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The specific capacities at different current densities are calculated via Eq. (1) and plotted in Fig. 3a. As shown in Fig. 3a, the specific capacitances of NNCP, NNCP/CuO, NNCP/MnO2 and NNCP/Co3O4 at current densities of 1, 4, 7 and 10 A g-1 are 2960, 1048, 3626 and 2172 F g-1; 2874, 871, 3336 and 1905 F g-1; 2780, 791, 3140 and 1336 F g-1; 2634, 736, 3104 and 935 F g-1, respectively, which are larger than those of Ni2Co(CO3)2(OH)2 hollow microspheres [30], ZnCo2O4 nanoparticles [31], NiCo2O4 electrode [32], and CdS microspheres [33]. The high electrochemical performance of NNCP/MnO2 electrode might be attributed to the suitable micron flower-like rich interface structure, which provides abundant active sites for fast and easy electrochemical reaction. The addition of cobalt improves the electronic conductivity of the 9 / 26

material, which provides fast electron donor. In addition, MnO2 also can undertake reversible electrochemical reaction, which is another factor contributing the performance of metal phosphate electrode. To obtain more information about the conductivity and electrochemical activity of the as-prepared NNCP, NNCP/CuO, NNCP/MnO2 and NNCP/Co3O4 electrodes, the EIS analyses were performed with the frequency increasing from 0.01 Hz to 100 kHz at amplitude of 5 mV, and the corresponding Nyquist plots are presented in Fig. 3b. As shown in the figure, the Nyquist plots of all the samples is composed of a semicircle at high- and medium-frequency regions and a sloped line at low frequency region, the semicircle corresponding to the constant phase component (CPE) and charge transfer resistance (Rct) generated at the working electrode/electrolyte interface, similar to the frequency requirement of ion diffusion/transmission in electrolyte, while the oblique line represents the Warburg resistance (ZW) and related with the diffusion of the redox substance in the electrolyte [34-36]. The intercept between Nyquist diagram and real axis in high frequency region represents the equivalent series resistance (Rs) of electrodes, including electrolyte resistance and contact resistance of electrode/electrolyte interface [37-39]. Obviously, as can be seen from Fig. 3b, the NNCP/MnO2 electrode shows the smallest Rs and Rct, corresponding to its maximum electronic conductivity and the fastest charge transfer process, and the linear slope in the low frequency region is the largest, indicating that the ion diffusion resistance is low, which also means that its capacitance behavior is more than that of the battery [40, 41]. The above results show that MnO2 doping can significantly improve the conductivity, charge transfer and capacitance behavior of the electrode, which should be attributed to the synergistic effect of Co, NNCP and MnO2. In this structure, Co can reduce internal resistance and provide effective 10 / 26

electron transfer. At the same time, there are a large number of interfaces between different phases of NNCP/MnO2, which makes the fast capacitive interface charge transport have better charge transport kinetics, and the extra charges (such as holes, electrons or voids) on the interface would contribute more capacity [42, 43]. And the results of EIS are consistent with those of CV and

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NNCP/MnO2 and NNCP/Co3O4.

3.2. Morphology and structure The morphologies of the as-synthesized pure NNCP and NNCP/MnO2 composites (the best electrochemical performance) were examined via SEM, and the results are presented in Fig. 4 at low and high magnification respectively. Fig. 4a and b shows the SEM images of NNCP at two different magnifications. As can be clearly observed from the figure, the pure NNCP presents a rod structure of varying lengths and thicknesses, as well as a cocoon-like spherical mixed structure. The diameter of rod crystal is about 0.5 µm, the length is about 5-10 µm, and the diameter of spherical materials is about 3 µm. In addition, the NNCP surface is smooth and regular, but the arrangement is messy. The surface morphology of NNCP/MnO2 composites after modification by 11 / 26

MnO2 is shown in Fig. 4c and d. As shown in the SEM results, the obtained NNCP/MnO2 samples exhibit a loose micrometer flower-like structure. The micron flower-like morphology is composed of microrods with a diameter of about 1 µm and a length of about 20-30 µm. The single microrod has smooth layered structure. The larger layered structure is conducive to the infiltration of electrolyte and the implantation of ions [44, 45] can provide sufficient electroactive sites for surface redox reaction [7], and accelerate the redox reaction rate. The disappearance of cocoon-like spherical structure of NNCP may be due to the destruction of material structure at the microscopic level by the presence of MnO2.

(a)

(b)

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Fig. 4. SEM images of (a, b) NNCP, and (c, d) NNCP/MnO2.

The structure and phase purity of the as-synthesized NNCP/MnO2 composites (the best 12 / 26

electrochemical performance) were further carried out by XRD, and the obtained results are displayed in Fig. 5a. As shown, all the diffraction peaks can be mainly assigned to NNCP (JCPDS 38-0315) [46]. In the XRD pattern, the strong and narrow diffraction peaks at 10.1° and 11.3° corresponding well to the (010) and (110) plane confirms the crystalstate of the sample, which indicates the good crystallinity of the material. The good crystallinity of the material is helpful for its stability in electrochemical application and the experimental results with less deviation [47]. Apart from the strong peaks, some small diffraction peaks are observed at 20.3°, 26.7°, 32.1°, 39.7°, and 44.4° in the XRD spectra can be indexed to the (020), (111), (121), (131), and (140) crystal surface of NNCP/MnO2 respectively, which is consistent with previous findings [24, 42]. Meanwhile, no other peaks of phosphite or phosphate were observed from these patterns, indicating that the synthesized phosphate phase was pure. Moreover, the XRD spectra does not exhibit any MnO2 diffraction peaks, which could be due to its amorphous nature. However, the diffraction peaks of NNCP/MnO2 are slightly shifted to lower values compared with NNCP, which may be caused by the addition of MnO2 that destroys the original crystal structure of the material and causes lattice distortion [48]. The phenomenon is due to the fact that the atomic radius of Mn atom is larger than that of Ni and Co atoms. With the addition of MnO2, the plane spacing of the microscopic layer is decreasing, so the diffraction peak is shifted to a lower angle [49]. In order to further uncover in detail the degree of hybridisation, crystal texture, formation any surface defects, and extent of chemical modification in the composite, Raman spectrometry of the as-synthesized NNCP/MnO2 composites were carried out and shown in Fig. 5b. In Raman spectra of the sample, two major peaks were observed near 937 cm-1 and 991 cm-1, corresponding to the symmetric stretching and vibrational mode of PO43- [50]. The peak at 2434 cm-1 is the blue shift of 13 / 26

2D peak, which may be caused by the carbon elements contained in the reactant urea resides on the NNCP/MnO2 material, and the double resonance scattering process in carbon causes phonon dispersion, which is usually referred to as G* or D+D" [51]. The materials also show Raman mode at 95-440 cm-1 comparing with the literature these modes could be assigned to Ni-Co-O phases [52]. The broad bands located in the range 450-550 cm-1 can be attributed to MnO2, corresponding to the deformation modes of the metal‑ oxygen the Mn-O-Mn chains in the MnO2 octahedral lattice and the stretching modes of the Mn-O bonds in MnO6 octahedra occur, respectively [53]. The width of the broad bands indicates the poor crystallinity of MnO2, which is consistent with XRD results.

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4. Conclusions In conclusion, NNCP were prepared by a simple one-step low-temperature hydrothermal method, and modified by metal oxides CuO, MnO2 and Co3O4 for supercapacitor applications. The electrochemical and physicochemical properties of the as-fabricated NNCP, NNCP/CuO, NNCP/MnO2 and NNCP/Co3O4 were evidenced by various techniques, such as CV, GCD, EIS, 14 / 26

SEM, XRD and Raman spectrum. The specific capacitances of NNCP, NNCP/CuO, NNCP/MnO2 and NNCP/Co3O4 at current densities of 1, 4, 7 and 10 A g-1 are 2960, 1048, 3626 and 2172 F g-1; 2874, 871, 3336 and 1905 F g-1; 2780, 791, 3140 and 1336 F g-1; 2634, 736, 3104 and 935 F g-1, respectively. Compared with other samples, the optimized NNCP/MnO2 composites exhibits the higher specific capacitance and remarkable rate capability essentially due to the micron flower-like rich interface structure, which provides a more active sites for electrochemical reaction, a shorter transport path for electrolyte ions, and MnO2 also can undertake reversible electrochemical reaction, which is another factor contributing the performance of NNCP/MnO2. All these results evidently suggest the NNCP/MnO2 composite is a great potential candidate material for high-energy storage devices with aqueous/solid electrolyte.

Acknowledgements

This work is supported by the Key Scientific Research Project of the Higher Education Institutions of Henan Province of China (Grant No. 20A530001), and the National Natural Science Foundation of China (Grant No. 21503193, U1404201).

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Graphical abstract

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HIGHLIGHTS ► (NH4)(Ni,Co)PO4·0.67H2O (NNCP) was successfully synthesized by a simple one-step low-temperature hydrothermal method. ► We give a novel method to adjust the electrochemical performance of NNCP. ► Experiments on capacitors prepared with NNCP, NNCP/CuO, NNCP/MnO2 and NNCP/Co3O4 composites. ►The synergistic effect of NNCP and MnO2, and the micron flower-like rich interface structure was the main reason for its superior capacitive performance.

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Declaration of Interest Statement None.

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Author Contribution Statement Yong Zhang: Conceptualization, Methodology, Software, Writing- Original draft preparation. Han-xin Mei: Data curation, Writing- Original draft preparation. Hai-li Gao: Conceptualization, Methodology, Software, Supervision. Qing-yuan Huo: Software, Investigation. Xiao-dong Jia: Visualization, Investigation. Yang Cao: Supervision, Software. Shi-wen Wang: Software, Validation. Ji Yan:Writing- Reviewing and Editing. He-wei Luo: Data Curation. Jing Yang: Writing - Original Draft. Ai-qin Zhang: Funding acquisition. Ke-zheng Gao: Data Curation, Software.

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