Ultrathin nanosheet-assembled hollow microplate CoMoO4 array derived from metal-organic framework for supercapacitor with ultrahigh areal capacitance

Journal of Power Sources 430 (2019) 51–59

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Ultrathin nanosheet-assembled hollow microplate CoMoO4 array derived from metal-organic framework for supercapacitor with ultrahigh areal capacitance Qin Li a, Yanli Li a, Jing Zhao b, Shihang Zhao a, Jiaojiao Zhou a, Chen Chen a, Kai Tao a, Rui Liu b, Lei Han a, * a

State Key Laboratory Base of Novel Functional Materials and Preparation Science, School of Materials Science & Chemical Engineering, Ningbo University, Ningbo, 315211, China b Ministry of Education Key Laboratory of Advanced Civil Engineering Material, School of Materials Science and Engineering, Institute for Advanced Study, Tongji University, Shanghai, 201804, China

A R T I C L E I N F O

A B S T R A C T

Keywords: Hollow microplates CoMoO4 Metal-organic frameworks Supercapacitors Areal capacitance

The rational control of metal-organic frameworks derived hollow nanomaterials with rapid mass transport and multiple active metal sites are desirable for emerging energy storage. Herein, CoMoO4 hollow microplate array on Ni foam is prepared via a facile self-sacrificing templated strategy from ion-exchange of cobalt-organic framework microplate array. Benefiting from hollow microplate structure constructed by ultrathin nanosheets, CoMoO4 as binder-free electrode for supercapacitor delivers ultrahigh areal capacitance/capacity (12.2 F cm 2/ 6120 C cm 2 at 2 mA cm 2), superior rate property (82.2% at high current density of 50 mA cm 2) and excellent cycling stability (90.5% retention after 5000 cycles). Moreover, an asymmetric supercapacitor device is assembled by using CoMoO4 as positive electrode and activated carbon as negative electrode, achieving high energy density of 0.321 mWh cm 2 at power density of 1.7 mW cm 2 and superior capacitance retention of 96.0% over 5000 cycles. Importantly, a blue light-emitting diode can be illuminated 2 min, indicating a great potential for practical applications. These excellent results demonstrate that this efficient strategy can extend to prepare various bimetallic oxides arrays with hollow and hierarchical microstructure for high-performance supercapacitors.

1. Introduction Supercapacitor (SC), as a renewable and environmentally friendly energy storage device, has been widely concerned and applied in portable electronic products and electromobile owing to their faster charging rate, higher power density, longer cycling life [1–4]. Therefore, the exploration of electrode materials for SCs with high-performance, inexpensive and simple process have attracted more attentions of many researchers. Among a large number of available electrode mate­ rials, Co-based materials such as Co3O4 [5], Co(OH)2 [6], Co9S8 [7] and NiCo2O4 [8], have been deeply studied in view of their high theoretical specific capacity and abundant redox reactions for capacitance genera­ tion. Particularly, many mixed transition metal oxides (eg. Co3O4@MnO2 [9], CuCo2O4@Co(OH)2 [10] and Co3O4/SnO2@MnO2 [11]) have attracted tremendous interest with excellent electrochemical

properties for SCs in consideration of their high electronic conductivity and multiple active metal sites. Notably, CoMoO4 has been regarded as one of the most desirable electrode materials for SCs due to several merits including low-cost, abundance and excellent electrochemical performances [12,13]. How­ ever, the practical capacitances of CoMoO4 are still lower than their theoretical values [14]. Hence, it is indispensable to make great efforts to boost the electrochemical properties of these materials. On the one hand, introducing the self-supported conductive substrates, such as carbon cloth [15], Ni foam [16], is an advanced strategy to optimize the electrochemical performances by increasing the electronic conductivity and cyclic stability. On the other hand, designing CoMoO4 with different microstructures (eg. microspheres [17], nanoplate [16], dandelion-shape [18]) has also been identified as an effective method to improve the electrochemical properties through increasing the exposure

* Corresponding author. E-mail address: [email protected] (L. Han). https://doi.org/10.1016/j.jpowsour.2019.05.011 Received 13 February 2019; Received in revised form 26 April 2019; Accepted 2 May 2019 Available online 10 May 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.

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of redox sites. Additionally, stimulated by the remarkable achievements of graphene, two-dimensional (2D) ultrathin nanosheets have received extensive research interests in the field of supercapacitors [19]. As electrode materials, the 2D ultrathin nanosheet structures can result in a higher specific surface area and further expose sufficient active sites for redox reaction, thus improving the electrochemical performances [20, 21]. Furthermore, the hollow mesoporous architectures have attracted extremely attentions as they can offer both more interfacial area and electrochemical active sites, as well as shorten the ions diffusion pathway and accelerate electrons transport which leads to both pseu­ docapacitance and electrochemical double layer capacitance [22,23]. By combining the design approaches mentioned above, we expect that hollow CoMoO4 architectures with numerous 2D ultrathin nanosheets grown on conductive substrates may effectively improve the electro­ chemical properties as electrode for SCs. Recently, metal-organic frameworks (MOFs) have been emerged as promising precursors or sacrificial templates for designing desirable nanostructured electrode materials (eg. porous carbon, metal oxides, carbides and sulfides) due to their high porosity, tunable morphology, pore size and chemical environment [24]. However, the direct conver­ sion and formation of CoMoO4 from MOF precursors has rarely been reported [25]. Therefore, herein we explore a facile self-sacrificing templated strategy to prepare CoMoO4 hollow microplate array on Ni foam (CoMoO4-HMPA/NF) from a cobalt-based MOF microplate array (Co-MOF-MPA/NF) through the ion exchange process. Remarkably, CoMoO4-HMPA/NF was constructed by numerous ultrathin nanosheets and kept its original framework of Co-MOF arrays. When served as binder-free electrode for supercapacitor, as-synthesized CoMoO4-HM­ PA/NF exhibited the enhanced electrochemical performances comparing to Co-MOF-MPA/NF, with ultrahigh areal capacitance/ca­ pacity (12.2 F cm 2/6120 C cm 2 at 2 mA cm 2), superior rate property (82.2% retention at 50 mA cm 2 with respect to the initial capacitance at 2 mA cm 2) and excellent cycling stability (90.5% retention over 5000 cycles). Furthermore, an asymmetric supercapacitor (ASC) device was assembled to evaluate its practical application, which could achieve a high energy density of 0.321 mWh cm 2 at a power density of 1.7 mW cm 2 and superior capacitance retention of 96.0% over 5000 cycles. Consequently, the as-obtained CoMoO4-HMPA/NF electrode is believed to have potential applications in high-performance supercapacitors.

autoclave was naturally cooled to room temperature, the Co-MOF-MPA/ NF was collected and washed by deionized water and ethanol for several times and dried at 60 � C for 12 h. 2.3. Preparation of CoMoO4-HMPA/NF CoMoO4-HMPA/NF was fabricated via hydrothermal ion exchange reaction. 40 mL Na2MoO4⋅2H2O solution (6 mg mL 1) was transferred into a Teflon-lined stainless-steel autoclave (50 mL) where the assynthesized Co-MOF-MPA/NF was immersed in the solution. The heat­ ing condition of the autoclave was set to 150 � C for 5 h. After the autoclave was naturally cooled to room temperature, the CoMoO4HMPA/NF was collected and washed by deionized water and ethanol for several times and dried at 60 � C overnight. The mass loading of CoMoO4HMPA is approximately 5.0 mg cm 2. In a control experiment, the CoMoO4 powder was also prepared by same condition without the presence of nickel foam. 2.4. Materials characterization FT-IR transmission spectrum was recorded on a Nicolet 6700 IR spectrophotometer. The phase structures and morphologies of the asfabricated samples were investigated with a X-ray Diffraction Spec­ trometry (XRD, BrukerD8 ADVANCE) with Cu Ka radiation (λ ¼ 0.154178 nm), field-emission scanning electronmicroscope (FESEM, Hitachi, S-4800) equipped with EDX detectors and transmission electron microscopy (TEM, JEOL, 2010F). The specific surface area and pore size distribution of products were studied by using a Quadrachrome adsorption instrument (Autosorb-iQ3; Quantachrome, America) at 77 K. X-ray photoelectron spectra (XPS) were detected on Axis Ultra DLD system witha monochromatic Al Kα radiation (1486.6 eV). 2.5. Electrochemical measurements All of the electrochemical measurements were tested with a CHI660E electrochemical working station (Chenhua, Shanghai) in a threeelectrode system by using platinum wire as auxiliary electrode and saturated calomel electrode (SCE) as reference electrode in 3 M KOH aqueous electrolyte. CoMoO4-HMPA/NF (or Co-MOF-MPA/NF, (1 � 1 cm) was directly used as working electrode. For CoMoO4 pow­ der (and Co-MOF powder), the working electrode was fabricated by forming a uniform slurry of active material (80%), PVDF (10%) and acetylene black (10%) in ethanol. Then, the slurry was uniformly covered on the surface of clean NF (1 � 1 cm) and dried at 60 � C over­ night. The cyclic voltammetry (CV), galvanostatic chargedischarge (GCD) and were recorded to evaluate the electrochemical properties of samples. When the electrochemical impedance spectroscopy (EIS) test was performed, the ac amplitude, frequency range, and the potential were set as 5 mV, 0.01 Hz 100 kHz, and open circuit potential, respectively. The open circuit potential for CoMoO4-HMPA/NF, CoMoO4 powder, Co-MOF-MPA/NF and Co-MOF powder was 1.20 V, 1.21 V, 1.25 V and 1.27 V vs. RHE, respectively. Generally, the areal specific capacitances of all electrodes in different current densities can be calculated based on the following equation (E1) and (E2):

2. Experimental section 2.1. Materials All reagents were used without further purification. Cobalt nitrate hexahydrate (Co(NO3)2⋅6H2O), polyvinylpyrrolidone (PVP) and acti­ vated carbon (AC) were purchased from Aladdin chemical Co. Ltd., Sodium molybdate dihydrate (Na2MoO4⋅2H2O) was purchased from Shanghai Titan Scientific Co., Ltd. Nickel foam, terephthalic acid (H2BDC) was purchased from Alfa aesar (Tianjin) chemical Co. Ltd. Dimethylformamide (DMF), absolute ethyl alcohol (C2H5OH), poly­ vinylidene fluoride (PVDF), acetylene black, poly(vinyl alcohol) (PVA) and potassium hydroxide (KOH) were purchased from Sinopharm. Platinum wire and saturated calomel electrode (SCE) were purchased from Chenhua (Shanghai).

(1)

Cs ¼ IΔt=SΔV

(2)

Qs ¼ IΔt=S

2.2. Preparation of Co-MOF-MPA/NF

2

2

Where Cs (F cm ) and Qs (C cm ) represent specific capacitance and capacity, respectively. I (A) represents the discharging current, Δt (s) is the discharge time, ΔV (V) represents the potential window; and S (cm2) is the geometrical area of the working electrode. The asymmetric supercapacitor (ASC) was constructed by using CoMoO4-HMPA/NF electrode as the positive electrode and activated carbon (AC) as the negative electrode in 3 M KOH solution. The negative electrode was fabricated by mixing together AC, PVDF and acetylene

Typically, 250 mg Co(NO3)2⋅6H2O was dissolved in 20 mL deionized water, and 71.4 mg H2BDC was dispersed in 40 mL C2H5OH/DMF (1 : 1 v/v). The above two solutions were mixed homogeneously before add­ ing 250 mg PVP. Then, the mixture solution was transferred to a Teflonlined stainless-steel high-pressure autoclave (100 mL) and a piece of clean nickel foam (NF, 2 � 4 cm) was immersed in the solution. The heating condition of the autoclave was set to 80 � C for 60 h. After the 52

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black with mass ratio of 8:1:1, which was dissolved in ethanol and grounded adequately to form uniformly slurry. Then, the slurry was uniformly coated on the surface of clean NF (1 � 1 cm) and dried at 60 � C overnight. For achieving optimized performance, the charges of both positive and negative should be balanced (qþ ¼ q ). The optimized mass ratio of two electrodes depends on the following equation (E3): mþ Cs ΔV ¼ þ þ m Cs ΔV

in Fig. 1a, the observed characteristic peaks can be indexed to those simulated peaks from the single crystal data (CCDC 153067), demon­ strating the formation of pure Co-MOF [Co2(OH)2BDC] phase [26]. The SEM and TEM images were recorded to characterize the morphology and microstructures of Co-MOF arrays. As depicted in Fig. 1b, the Co-MOF microplate arrays (Co-MOF-MPA) uniformly covered on the whole Ni foam surface. From the enlarged SEM image (Fig. 1c), Co-MOF-MPA with a smooth surface and a thickness of 1–2 μm could be clearly seen. The corresponding TEM image (Fig. 1d) confirmed the solid nature of Co-MOF-MPA. The FTIR spectra (Fig. S2) of Co-MOF scraped from NF displayed a sharp peak located at 3595 cm 1 corresponding to the stretching vibration of the OH groups. Obvious C–H stretching vibra­ tions in aromatic rings were observed at peaks of 1141, 1094, 1010, and 809 cm 1, respectively. Other two intense absorption peaks appeared at 1580 and 1368 cm 1 were ascribed to the symmetric and anti-symmetric stretching vibrations of –COO– functional groups coming from BDC2 ligand, which are in accordance with the previously reported results [27]. Based on the above XRD, SEM and FTIR analysis, these results fully proved the successful preparation of Co-MOF microplate array on Ni foam. Additionally, the Co-MOF powder in the absence of NF substrate was fabricated by using the same experimental conditions, and the corresponding SEM image (Figs. S3a–b) showed the microplate structure as like as the Co-MOF supported on Ni foam, but severely aggregation. It could be observed that CoMoO4 inherited the microplate frame­ work of Co-MOF precursor from the panoramic SEM image in Fig. 2a. Also as depicted in the red circle, the hollow interior could be clearly seen from a broken CoMoO4 microplate. Intriguingly, the enlarged SEM image (Fig. 2b) showed the shell of CoMoO4 hollow microplate was composed of numerous interlaced ultrathin nanosheets, forming a hi­ erarchical structure. The corresponding TEM image (Fig. 2c) indicated the whole CoMoO4 microplate consisting of self-assembled nanosheets and the core was hollow, further verifying the above SEM results. The HRTEM images (Fig. 2d–f) displayed a crystal lattice distance of 0.336 nm, corresponding to the (002) planes of CoMoO4 (JCPDS no. 21–0868) [18]. And the homogeneously distribution of Co, Mo and O throughout the entire hollow CoMoO4 microplates was confirmed by the element mapping analysis (Fig. 2g–j). The very weak C signal (Fig. 2k) indicates the complete conversion of Co-MOF to CoMoO4. The Co/Mo atomic ratio (1: 0.89) was provided by EDS spectrum (Fig. S4), indi­ cating the formation of CoMoO4. Furthermore, the crystalline structure of the synthesized CoMoO4 was investigated by XRD patterns (Fig. S5), and the appeared diffraction peaks at 26.1, 32.9, 36.8, and 59.5� could be ascribed to the (002), (022), (400) and ( 352) reflections of the CoMoO4 phase (JCPDS no. 21–0868) [18,27,28], which matched well with HRTEM result. The peaks at 26.1 and 59.5� were offset to the left by about 0.4� , which was likely to be a reflection of the lattice distortion caused by macroscopic residual stress. FT-IR spectrum (Fig. S2) of CoMoO4 scraped from NF displayed the main absorptions at 887 cm 1 (the vibrational modes of distorted MoO4), 690 cm 1 (Mo–O vibration), and 481 cm 1 (vibrations due to the Co and Mo building blocks of CoMoO4) [29]. The peaks at 3000–3600 cm 1 regions along with 1656 cm 1 corresponded to the adsorbed water on the surface of prod­ uct [30,31]. The results further demonstrated the complete conversion

(3)

2 þ Where mþ (g), Cþ s (F cm ) and ΔV (V) represent the mass loading, the area specific capacitance and the potential window of positive electrode, respectively, and m–, Cs and ΔV– are the mass loading, area specific capacitance and the potential window of negative electrode, respectively. The energy density (E) and power density (P) of ASC devices were calculated based on the following equation (E4) and equation (E5), respectively. R I V dt E¼ (4) S

P¼

E Δt

(5)

Where I, V, t, and S represent the discharge current, potential window, discharging time and the geometrical areas of the working electrodes for the assembled ASC devices, respectively. In order to fabricate the all solid-state asymmetric supercapacitor device, the PVA/KOH gel electrolyte was prepared firstly. 10 mL of KOH (0.3 g mL 1) aqueous solution was added to 20 mL of poly(vinyl alcohol) (PVA, 0.15 g mL 1) aqueous solution. Then the mixture was stirred for 2 h under 80 � C to form PVA/KOH gel electrolyte. To assemble all solidstate asymmetric supercapacitor device, the positive electrode (CoMoO4-HMPA/NF) and negative electrode (AC coated on nickel foam) were assembled together face-to-face, where the PVA/KOH gel electro­ lyte served as ion-porous separator. All the potentials in this paper were discussed relative to the reversible hydrogen electrode (RHE, E (RHE) ¼ E(SCE) þ0.0591 pH þ 0.24). 3. Results and discussion The two-step preparation process of ultrathin nanosheet-constructed CoMoO4 hollow microplate arrays on Ni foam was shown in Scheme 1. Firstly, Co-MOF template arrays was grown on Ni foam surface through one-pot solvothermal self-assembling of Co2þ and terephthalic acid ligand, and the color of Ni foam changed from silver to dark red (Fig. S1). Subsequently, the molybdate anion (MoO24 ) released from sodium molybdate was considered to replace terephthalate anion (BDC2 ) of Co-MOF via ion exchange reaction to form CoMoO4, and the color of the product correspondingly turned from dark red to lavender (Fig. S1). This facile synthetic strategy is simple and efficient to prepare metal molybdate arrays from MOFs precursor as the self-sacrificial template. The XRD pattern of as-prepared Co-MOF scraped from NF was shown

Scheme 1. Schematic illustration of the synthesis of CoMoO4-HMPA/NF. 53

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Journal of Power Sources 430 (2019) 51–59

Fig. 1. (a) XRD patterns of Co-MOF-MPA scraped from Ni foam; (b–c) SEM images of Co-MOF-MPA/NF; (d) TEM image of Co-MOF-MP scraped from Ni foam.

of Co-MOF precursor to CoMoO4. In the control experiment, the CoMoO4 powder without NF substrate was collected via using the same experi­ mental conditions, and the corresponding SEM image (Fig. S3c) dis­ played the formation of ultrathin nanosheets shell structure, but severely aggregation and collapse. Notably, the formation of well-defined CoMoO4-HMPA depended on the balance between the etching of template and the formation of nanosheets shell [23], which was controlled by the concentration of sodium molybdate. In the addition of Na2MoO4 with low concentration (1 mg mL 1), the limited molybdate ions only reacted on the surface of Co-MOFs, which resulted in core-shell structure (Fig. S6a). As increased the concentration of Na2MoO4, the core part gradually decreased and disappeared (3 mg mL 1 for Figs. S6b and 6 mg mL 1 for Fig. S6c). When further using a high concentration Na2MoO4 (10 mg mL 1), the disso­ lution rate of precursor exceeded the formation rate of CoMoO4, resulting in frame collapse (Fig. S6d). The corresponding XRD patterns of the samples obtained at different concentrations were shown in Fig. S7. The XRD patterns at concentration of 1 and 3 mg mL 1 mainly displayed the characteristic peaks of unreacted Co-MOFs. However, in the case of 6 and 10 mg mL 1, the diffraction peaks of CoMoO4 were observed clearly and no other impurity peaks appeared. Based on these analyses, the formation mechanism of CoMoO4 could be explained to ion exchange as follows: the generated MoO24 could combine with Co2þ released from Co-MOF to form ultrathin CoMoO4 nanosheet on the outer surface, while the continuous consumption of Co-MOF core resulted in hollow microstructure assembled with ultrathin CoMoO4 nanosheets. The surface areas and pore characteristics of samples were recorded by N2 adsorption/desorption measurements. As shown in Fig. 3a, compared to the precursor, an obviously well-defined hysteresis loop was appeared for CoMoO4-HMPA/NF, indicating the existence of mes­ oporous structure. The pore size distribution (Fig. 3b) of CoMoO4HMPA/NF concentrated at 3.9 nm, further implying the improved

uniformly mesoporous feature when compared to the Co-MOF-MPA/NF with negligible pore. The BET surface areas were calculated to be 160.45 and 9.08 m2 g 1 for CoMoO4-HMPA/NF and Co-MOF-MPA/NF, respec­ tively. For the reference samples, the surface areas (Fig. S8) presented to be proportional to the concentration of Na2MoO4, which might be ascribed to the formation of larger proportion of CoMoO4 ultrathin nanosheets. Consequently, the complete transformation from Co-MOF to CoMoO4 could lead to higher surface area and more mesopores, which might offer sufficient active sites for redox reaction and accelerate the ions diffusion kinetics for supercapacitors. X-ray photoelectron spectroscope (XPS) measurements were used to confirm the composition and chemical state of CoMoO4-HMPA/NF. The survey XPS spectrum (Fig. 3c) demonstrated that the microplate con­ sisted of Co, Mo, and O elements. The presence of low intensity C peak supported the complete conversion of Co-MOF to CoMoO4. The atomic ratio of Co/Mo could be calculated as 1: 0.92, which was in agreement with the EDS result. The Co 2p spectra (Fig. 3d) exhibited two major peaks at the binding energies (BEs) of 780.9 eV (Co 2p3/2) and 797.1 eV (Co 2p1/2) accompanying two weak shakeup satellites (named as “Sat.”). And the fitting peaks at 780.8 eV and 782.2 eV could demonstrate the existence of Co2þ species, which was in consistent with the reported results [32–35]. The high-resolution spectrum of Mo 3d (Fig. 3e) could be divided into two peaks at 235.3 and 232.2 eV with a separated width of 3.1 eV, which was assigned to Mo 3d3/2 and Mo 3d5/2, respectively, testifying the existence of Mo6þ in CoMoO4 [36]. The peak at BE of 530.8 eV corresponded to O 1s level (Fig. 3f) in CoMoO4, and a very weak peak at 532.5 eV might attribute to the OH of H2O. The above results indicated that Co-MOF completely transformed into CoMoO4, which reconfirmed the EDS, XRD and FTIR analyses. The capacitive behaviors of CoMoO4-HMPA/NF, CoMoO4 powder, Co-MOF-MPA/NF and Co-MOF powder were studied in a three-electrode system with 3 M KOH electrolyte. Fig. S9 showed the galvanostatic 54

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Fig. 2. (a–b) SEM images of CoMoO4-HMPA/NF; (c) TEM and (d–f) HRTEM images of CoMoO4-HMP; (g–k) EDS mapping of CoMoO4-HMP.

Fig. 3. (a) N2 adsorption-desorption isotherms and (b) corresponding pore-size distribution curves of CoMoO4-HMPA and Co-MOF-MPA scraped from Ni foam; XPS spectra of synthesized CoMoO4-HMPA: (c) survey spectrum, (d) Co 2p, (e) Mo 3d, and (f) O 1 s spectra.

charge–discharge (GCD) curves of as-obtained CoMoO4-HMPA/NF treated with different Na2MoO4 concentrations. Notably, when the concentration of Na2MoO4 solution was 6 mg mL 1, the fully converted

CoMoO4-HMPA/NF possessed the longest discharge time at the same current density. The result revealed that CoMoO4-HMPA/NF was the optimized electrode compared with the partially transformed product, 55

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which may be related to the unique hollow structure and composition. Fig. 4a presented the cyclic voltammetry (CV) curves of CoMoO4HMPA/NF, CoMoO4 powder and Co-MOF precursors in a potential window of 1–1.6 V (vs. RHE) at a scan rate of 5 mV s 1. Clearly, a pair of distinguishing and separated redox peaks could be seen for the four samples, indicating a battery-like behavior that comes from Faradic redox reaction of Co2þ/Co3þ [37,38]. For CoMoO4 sample, the Mo atoms did not undergo redox reactions and its function was to enhance electrical conductivity of electrode material, thus improving the elec­ trochemical properties [39,40]. The corresponding Faradic redox re­ actions were as following equations [41,42]: 3Co(OH)2 þ 2OH ↔ Co3O4 þ 4H2O þ 2e

(6)

Co3O4 þ H2O þ OH ↔ 3CoOOH þ e

(7)

CoOOH þ OH ↔ CoO2 þ H2O þ e

(8)

powder (74.1%), Co-MOF-MPA/NF (72.7%) and Co-MOF powder (71.5%) when the current density increased 25 times, implying the significantly improved electrochemical properties of CoMoO4-HM­ PA/NF. In addition, such a CoMoO4-HMPA/NF electrode also had excellent capability compared with previously reported electrode ma­ terials based on transition metal oxides and hydroxides (Table 1) [12, 43–54]. To the best of our knowledge, as-synthesized CoMoO4-HM­ PA/NF has the highest areal capacitance in all reported CoMoO4 related electrode materials. This ultrahigh areal capacitance and superior rate property endow that CoMoO4-HMPA/NF have potential applications in high-performance supercapacitors. The electrochemical impedance spectroscopy (EIS) tests were per­ formed to evaluate the charge transfer kinetics of as-fabricated elec­ trodes. The EIS curves (Fig. 4e) were composed of a semicircle at high frequency corresponding to the interface charge-transfer resistance (Rct) and a linear part at low frequency relevant to Warburg resistance (Wd) [55–57]. Compared to the CoMoO4 powder, Co-MOF-MPA/NF and Co-MOF powder, CoMoO4-HMPA/NF had a lower value of Rct which could be derived from the radius in the high frequency region and a higher slope based on the linear component in the low frequency, indicating the higher charge transfer kinetics through the interface of electrode/electrolyte. Furthermore, the CoMoO4-HMPA/NF possessed a smaller intersection at the real X-axis corresponding to the equivalent series resistance (Rs) that included the contact resistance, ionic resis­ tance and the intrinsic resistance of electrode materials. Cycling per­ formances of electrode materials were assessed by 5000 continuous GCD tests at 50 mA cm 2. As shown in Fig. 4f, the high capacitance retention of 90.5% for CoMoO4-HMPA/NF which was superior to that of CoMoO4 powder (81.1%), Co-MOF-MPA/NF (74.1%) and Co-MOF powder (62.8%), illustrating its greatly promoted cycling stability. During the cycling stability test, the comparison of the first 8 and the last 8 GCD cycles of CoMoO4-HMPA/NF were shown in Fig. S11, which further proved its excellent superior cycling stability. In addition, the coulombic efficiency of four electrodes was all nearly 100%, implying a favourable reversible charge storage and transfer process (Fig. 4f). The stability could be investigated by recording the SEM images of CoMoO4-HM­ PA/NF after the long-term cycling tests. As shown in Fig. S12, the negligible morphological changes could be observed, demonstrating the

Specifically, the CoMoO4-HMPA/NF exhibited higher redox peak current and larger surrounded area of CV curve, indicating its stronger charge storage capability compared to CoMoO4 powder, Co-MOF-MPA/ NF and Co-MOF powder. The CV curves of CoMoO4-HMPA/NF at various scan rates were depicted in Fig. 4b, where the redox peak current was incremental by faster scan rate. Fig. 4c presents GCD curves of CoMoO4-HMPA/NF that were recorded in a current density range of 2–50 mA cm 2 and a potential range of 1–1.5 V vs. RHE. The apparent nonlinear GCD curves with obvious plateaus further indicated a batterylike feature. Such feature was consistent with the above CV results [37, 38]. Additionally, the CV curves (Figs. S10a–c) of the reference samples (CoMoO4 powder, Co-MOF-MPA/NF and Co-MOF powder) also dis­ played obviously separated redox peaks on account of the behavior of battery-like, and the relevant GCD profiles were recorded in Figs. S10d–f. The area specific capacitances of the prepared electrodes could be obtained based on the GCD curves by using equation (E1). The CoMoO4-HMPA/NF delivered the highest areal capacitance/capacity of 12.2 F cm 2/6120 C cm 2 in comparison with CoMoO4 powder Co-MOF-MPA/NF (4.98 F cm 2/ (2.24 F cm 2/1124 C cm 2), 2490 C cm 2) and Co-MOF powder (0.93 F cm 2/465 C cm 2) at a cur­ rent density of 2 mA cm 2. Remarkably, as depicted in Fig. 4d, the rate performance of CoMoO4-HMPA/NF (82.2%) outperformed the CoMoO4

Fig. 4. (a) CV curves of CoMoO4-HMPA/NF, CoMoO4 powder, Co-MOF-MPA/NF and Co-MOF powder at a scan rate of 5 mV s 1 in 3 M KOH; (b) CV curves of CoMoO4-HMPA/NF at different scan rates; (c) GCD curves of CoMoO4-HMPA/NF at different current densities; (d) Areal capacitances versus current densities, (e) EIS spectra, and (f) Cycling performances and coulombic efficiency of CoMoO4-HMPA/NF, CoMoO4 powder, Co-MOF-MPA/NF and Co-MOF powder. 56

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Table 1 Comparison of CoMoO4-HMPA/NF with other reported electrode materials based on transition metal oxides and hydroxides in capacitive performance. Electrodes CoMoO4 microplates@CoMoO4 microprisms carbon cloth@CoMoO4@NiCo-LDH CoMoO4⋅0.9H2O MnMoO4/CoMoO4 CoMoO4 nanoplate arrays CoMoO4@NiMoO4 ZnO nanorod@CoMoO4 Ni(OH)2@CoMoO4 CuCo2O4 nanobelts Co3O4@NiMoO4 Ni0.33Co0.67(OH)2–CNFP graphene@NiO/MoO3 CoMoO4@MnO2 NiO@CoMoO4 CoMoO4�0.75H2O/PANI CoMoO4/C CoMoO4/rGO CoMoO4 HMSA/NF

Cs (F cm 2) Cm (F g 1)

Current density

Potential range

Electrolyte

Ref.

4.33 F cm 2 2024 F g 1 326 F g 1 187.1 F g 1 1.26 F cm 2 3.30 F cm 2 1.52 F cm 2 5.23 F cm 2 2.42 F cm 2 5.69 F cm 2 2.03 F cm 2 2.14 F cm 2 2.27 F cm 2 848 F g 1 380 F g 1 451.6 F g 1 336.1 F g 1 12.2 F cm 2

50 mA cm 2 1Ag 1 5 mA cm 2 1Ag 1 4 mA cm 2 8 mA cm 2 2 mA cm 2 8 mA cm 2 2 mA cm 2 30 mA cm 2 2.1 mA cm 2 6 mA cm 2 3 mA cm 2 0.5 A g 1 1Ag 1 1Ag 1

0–0.45 V vs. SCE 0–0.5 V vs. Hg/HgO 0.2–0.45 V vs. SCE 0.6-0.4 V vs. Ag/AgCl 0–0.5 V vs. SCE 0–0.5 V vs. SCE 0–0.5 V vs. SCE 0–0.4 V vs. SCE 0–0.45 V vs. SCE 0–0.53 V vs. SCE 0–0.35 V vs. SCE 0–0.6 V vs. SCE 0–0.5 V vs. SCE 0–0.4 V vs. SCE 0.2–0.7 V vs. SCE 0.1–0.32 V vs. SCE 0.3-0.25 V vs. SCE 1.0–1.5 V vs. RHE (0–0.5 V vs. SCE)

3 M KOH 2 M KOH 2 M KOH 2 M NaOH 2 M KOH 2 M KOH 1 M KOH 2 M NaOH 2 M KOH 2 M KOH 1 M KOH 2 M KOH 1 M KOH 2 M KOH 1 M Na2SO4 3 M KOH

12 15 29 41 43 44 45 46 47 48 49 50 51 52 53 54

3 M KOH

This work

2 mA cm

2

structure stability of CoMoO4-HMPA/NF. The excellent stability might be attributed to the strong interaction between sample and NF, as well as tightly interlaced hierarchically nanosheet structure, which could pre­ vent collapse and aggregation of sample during the electrochemical reaction. To further assess the porous CoMoO4-HMPA/NF electrode for the practical application, an asymmetric supercapacitor (ASC), CoMoO4HMPA/NF//AC, was constructed and tested in a two-electrode system, where CoMoO4-HMPA/NF as the positive electrode and AC (mass loading depended on equation (E3)) as the negative electrode. Accord­ ing to the CV and GCD curves (Figs. S13a–b) in three-electrode system, the AC electrode showed the double-layer capacitance feature and it delivered a high areal capacitance of 700 mF cm 2 (700 C cm 2) at a current density of 2 mA cm 2 (Fig. S13c). When the current density increased 25 times, the AC electrode retained 57% of the initial specific capacitance. Based on the CV measurements at a scan rate of 20 mV s 1

(Fig. S14a) in a three-electrode system, the CoMoO4-HMPA/NF and AC electrodes had a potential window ranging of 1–1.6 V and 0–1 V vs. RHE, respectively. Accordingly, the CV voltage window of the fabricated CoMoO4-HMPA/NF//AC ASC device could be set to 0–1.6 V without polarization (Fig. S14b). Fig. 5a displayed the CV curves of CoMoO4HMPA/NF//AC at various scan rates. Obviously, the shape of curve was no obvious deformation with increased scan rates, demonstrating the ASC device had a good capacitive property [57,58]. The GCD curves of CoMoO4-HMPA/NF//AC ASC at various current densities were shown in Fig. 5b. According to the GCD results and equation (E1), the obtained areal capacitances of ASC device reached to 902 mF cm 2 (1353 C cm 2) at a current density of 2 mA cm 2. The areal capacitance still remained at 324 mF cm 2 (486 C cm 2) even at a high current density of 50 mA cm 2, indicating a good rate performance (36%) (Fig. S14c) of ASC device. Furthermore, the CoMoO4-HMPA/NF//AC ASC device could remain almost 96.0% of the initial capacitance after 5000

Fig. 5. (a) CV curves of CoMoO4-HMPA/NF//AC ASC at different scan rates; (b) GCD curves of CoMoO4-HMPA/NF//AC ASC at different current densities; (c) Cycling performance of CoMoO4-HMPA/NF//AC ASC device; (d) Energy density and power density of CoMoO4-HMPA/NF//AC ASC,in comparison with reported data; (e) Schematic illustration of ASC; (f) LED indicator lighted up by two ASC in-series. 57

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Journal of Power Sources 430 (2019) 51–59

continuous charge/discharge cycles at a current density of 50 mA cm 2, which exhibiting superior cycling stability (Fig. 5c). The Ragone plot (Fig. 5d) displayed the relationship between energy density and power density of CoMoO4-HMPA/NF//AC ASC device at different current densities. Notably, this ASC exhibited a maximum energy density of 0.321 mWh cm 2 at a power density of 1.7 mW cm 2, and it still kept an energy density of 0.114 mWh cm 2 even at an high power density of 42.2 mW cm 2, indicating excellent properties of CoMoO4-HMPA/N­ F//AC ASC device in comparision with those previous reported elec­ trodes [59–63]. Simultaneously, to further confirm the practical application of CoMoO4-HMPA/NF, an all solid-state asymmetric super­ capacitor device (Fig. 5e) was constructed by using CoMoO4-HMPA/NF, AC and PVA/KOH as positive electrode, negative electrode and gel electrolyte, respectively. A blue light-emitting diode (LED) could be powered successfully for 2 min by suing two-in-series all solid-state ASC devices (Fig. 5f). These results further certified that CoMoO4-HMPA/NF might be a promising electrode material for supercapacitor applications. The remarkable electrochemical performance of CoMoO4-HMPA/NF could be reasonably interpreted as follows: (1) the active materials grown on the surface of conductive NF provided good mechanical strength and conductivity, accelerating the charge transport process between the CoMoO4-HMPA and the current collector; (2) the freestanding CoMoO4-HMPA/NF could be directly employed as electrode material for SCs without adding any conductive agents and binder, increasing the utilization rate of the electrode; (3) the interconnected nanosheet-constructed hollow microplate structure in CoMoO4 possessed larger specific areas and uniform mesopores, offering enough active sites and efficient ion transport during electrochemical reaction process; and (4) the orderly CoMoO4 arrays could prevent collapse and aggregation of sample during the electrochemical reaction.

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4. Conclusions In summary, CoMoO4 ultrathin nanosheet-constructed hollow microplate arrays supported on Ni foam (CoMoO4-HMPA/NF) has been successfully prepared by a simple self-sacrificial template route from the ion exchange of cobalt-based MOF microplate array (Co-MOF-MPA/NF). When applied as free-standing electrode for supercapacitors, such CoMoO4-HMPA/NF performs ultrahigh areal specific capacitance (12.2 F cm 2/6120 C cm 2 at 2 mA cm 2), rate capability (82.2% retention at a high current density of 50 mA cm 2) and excellent cycling stability (90.5% capacitance retention over 5000 cycles). The superior electrochemical behaviors of CoMoO4-HMPA/NF may be ascribed to the ultrathin nanosheet-constructed microstructures with abundant elec­ trochemical active sites and fast charge transfer kinetics, as well as improved conductivity provided by NF. Furthermore, an assembled CoMoO4-HMPA/NF//AC ASC exhibits outstanding cycling stability of 96.0% over 5000 cycles and a high energy density of 0.321 mWh cm 2 at a power density of 1.7 mW cm 2. Consequently, such efficient MOFs-derived synthetic strategy opens new paths for designing various hollow and hierarchical mixed metal oxide arrays, which can be applied to diverse electrochemical fields. Acknowledgements We are grateful for the financial support from the National Natural Science Foundation of China (No. 21471086, 51572272), the Natural Science Foundation of Ningbo (No. 2017A610062, 2017A610065), and the K.C. Wong Magna Fund in Ningbo University. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.05.011.

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