Hierarchically Designed Ag@Ce6Mo10O39 Marigold Flower-like

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Hierarchically Designed Ag@Ce6Mo10O39 Marigold Flower-like Architectures: An Efficient Electrode Material for Hybrid Supercapacitors S. Chandra Sekhar, Goli Nagaraju, Bhimanaboina Ramulu, and Jae Su Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12527 • Publication Date (Web): 08 Oct 2018 Downloaded from http://pubs.acs.org on October 9, 2018

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ACS Applied Materials & Interfaces

Hierarchically Designed Ag@Ce6Mo10O39 Marigold Flower-like Architectures: An Efficient Electrode Material for Hybrid Supercapacitors S. Chandra Sekhar,a Goli Nagaraju,a,b Bhimanaboina Ramulu,a and Jae Su Yu a*

aDepartment

of Electronic Engineering, Institute for Wearable Convergence Electronics, Kyung

Hee University, 1732 Deogyeong-daero, Gihung-gu, Yongin-si, Gyeonggi-do 17104, Republic of Korea bDepartment

of Chemical Engineering, College of Engineering, Kyung Hee University, 1732

Deogyeong-daero, Gihung-gu, Yongin-si, Gyeonggi-do 17104, Republic of Korea.

*Address correspondence to [email protected]

KEYWORDS: cerium molybdates; EDTA; silver nanoparticles; hybrid supercapacitors; energy density.

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ABSTRACT We facilely prepared silver nanoparticles-decorated Ce6Mo10O39 marigold flower-like structures (Ag NPs@CM MFs) for use as an effective positive material in hybrid supercapacitors (HSCs). With an aid of ethylenediaminetetracetic acid (EDTA) as a chelating agent, the self-assembled CM MFs were synthesized by a single-step hydrothermal method. When the electrochemical properties were tested in aqueous alkaline electrolyte, the synthesized CM MFs with 0.15 g of EDTA exhibited a relatively high charge storage property (55.3 μAh/cm2 at 2 mA/cm2) with battery-type redox behavior. The high capacity performance is mainly due to the large surface area of CM MFs and the hierarchically connected nanoflakes provide wide-open wells for rapid accessibility of electrolyte ions and enable the fast transportation of electrons. Further improvement in electrochemical performance was achieved (62 μAh/cm2 at 2 mA/cm2) by decorating Ag NPs on the surface of CM MFs (i.e., Ag NPs@CM MFs), which is attributed to the high electric conductivity. Considering the synergistic effect and high electrochemical activity of Ag NPs@CM MFs, it was further employed as an effective positive electrode for the fabrication of pouch-type HSC with porous carbon (negative electrode) in alkaline electrolyte. The HSC exhibited high cell potential (1.5 V) with maximum energy and power densities of 0.0183 mWh/cm2 and 10.237 mW/cm2, respectively. The potency of HSC in practical applications was also demonstrated by energizing red and yellow light-emitting diodes as well as three-point pattern torch light.


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1. INTRODUCTION In view of rapid exhaustion of fossil fuels, global energy crisis and ever increasing energy need for an advancement of mankind in various areas, development of energy storages has been a promising research interest over the last decade.1-6 Particularly, supercapacitors (SCs) have attracted great attention as an eco-friendly energy storage device owing to their salient features of long-term durability, high power density and fast charging-discharging ability.7-10 However, the SCs are restricted to their vast usage in several electronic applications by the reason of having low energy density compared to the batteries. Therefore, one of the strategies to improve energy density of SCs without scarifying its power density is constructing the hybrid/asymmetric SCs which involve electric double layered capacitive (EDLC) materials as a power source and pseudocapacitive/battery-type materials as an energy source.11-14 The EDLC materials (activated carbons, reduced graphene oxide, etc.) accumulate the charges in non-faradaic manner, i.e., the electric charges are electrostatically adsorbed at the electrode/electrolyte interface.15-16 In contrast, the pseudocapacitive/battery-type materials (MnO2, Ni(OH)2, Co3O4, CoMoO4, etc.) with various morphologies store the charges in Faradaic manner by performing electrochemical redox reactions.17-21 Generally, the pseudocapacitive/battery-type materials are capable of storing nearly 10 times higher capacitance than the EDLC-type materials due to their multiple oxidation states and fast reversible redox behavior.22-23 Hence, synthesizing such materials with benefit-enriched morphologies could effectively improve the capacitance and hence the energy density of SCs. Recently, binary metal oxides have been extensively investigated as a SC active material, due to their synergistic multiple oxidation states of both metal oxides, natural abundance, environmental-friendly nature, low cost and high electrical conductivity compared to solitary metal oxides.8,

24-25

Especially, latest developed metal molybdates as a new class of advanced



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electrode materials have also demonstrated the remarkable energy storage performance because of their feasible oxidation states and better electrochemical conductivity resulted from the mixed metal oxides.26 Moreover, stable and corrosion resistive aspects of molybdenum element could be helpful for long life span of metal molybdate materials.27 So far, several metal molybdates with versatile morphologies have been broadly explored as electrode materials for SCs.12,

28-29

Particularly, cerium molybdate has attracted a significant research interest in various fields, like anti-corrosion, photocatalysis, ion exchanging, etc., due to its good catalytic-converting behavior, corrosion resistive nature and excellent photostability.30-33 However, to the best our knowledge, no literatures are found on the use of cerium molybdate (Ce6Mo10O39) as a positive electrode material in SC applications. Considering the advantageous features of cerium molybdate material with its multi-valent states, high redox reversibility and high electric conductivity of both cerium and molybdenum elements, the enhanced energy storage performance could be expected. To further elaborate the redox chemistry and electron transportation phenomena, decoration of conductive silver particles over the active materials is also meaningful to use as an efficient electrode material in HSC. Inspired by the above findings, we synthesized the Ce6Mo10O39 marigold flower-like structures

(CM

MFs)

in

a

single-step

hydrothermal

approach

with

an

aid

of

ethylenediaminetetracetic acid (EDTA). Primarily, the effect of EDTA on the morphological evolution was investigated in detail by introducing different amounts of EDTA. The CM MFs sample prepared with 0.15 g of EDTA was perceived as a better charge storage candidate than the remaining samples owing to the hierarchically linked nanoflakes (NFs) and wide-open wells. For further improvement in capacity storage performance, the silver nanoparticles (Ag NPs) were successfully decorated on CM MFs (Ag NPs@CM MFs) by metal-ion reduction method at room


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temperature (RT). Owing to the high conductive nature, multiple oxidation states and capability to perform redox reactions of Ag NPs, the Ag NPs@CM MFs electrode demonstrated an enhanced capacity performance compared to the solitary CM MFs. From the above worthwhile results, the pouch-type hybrid SC (HSC) was assembled with Ag NPs@CM MFs and porous activated carbon as positive and negative electrodes, respectively, in 1 M KOH aqueous electrolyte. The asassembled HSC demonstrated a superior electrochemical performance and long cycling stability with good capacitance retention. The practical applicability of our HSC was also demonstrated by energizing the light-emitting diodes (LEDs) and three-point pattern torch light with two HSCs in series connection. 2. EXPERIMENTAL 2.1 Chemicals: Cerium (III) nitrate hexahydrate (Ce(NO3)3·6H2O), tannic acid (C76H52O46), potassium carbonate (K2CO3) and potassium hydroxide (KOH) were purchased from SigmaAldrich Co., South Korea. Sodium molybdate (VI) dehydrate (Na2MoO4·2H2O), silver nitrate (AgNO3), EDTA (C10H16N2O8), N-mehtyl-2-pyrrolidone (NMP, C5H9NO), hydrochloric acid (HCl), polyvinylidene fluoride (PVDF, –(C2H2F2)n–) and other solvents used in this experiment were obtained from Daejung Chemicals Ltd., South Korea. Nickel (Ni) foam and super P carbon black (C65, TIMCAL) were received from MTI Korea, South Korea. All the chemical reagents used in our experiments were of analytical grade and used as obtained without any further purification. 2.2 Synthesis of Ce6Mo10O39 marigold flower-like structures (CM MFs): The hierarchically constructed Ce6Mo10O39 CM MFs were facilely prepared in a single-step hydrothermal method. In a typical procedure, 0.19 g of Na2MoO4·2H2O dissolved in 40 mL de-ionized (DI) water was added dropwise to the another beaker containing 0.34 g of Ce(NO3)3·6H2O and it was observed that the



solution color was gradually changed to pale grey. Then, 0.15 g of EDTA was introduced to above solution on the hot plate under vigorous magnetic stirring at RT for 30 min. Afterwards, the growth solution was transferred to a Teflon-lined stainless steel autoclave and heated at 180 ºC for 12 h. After cooling down the autoclave to RT, the dark yellow colored CM precipitate was obtained after centrifugation and washing with DI water several times. The precipitate was dried in a vacuum oven at 90 ºC for 12 h. Finally, the CM MFs were obtained after annealing the precursor powder in a muffle furnace at 350 ºC for 2 h. To investigate the effect of EDTA on the structural morphology of CM, different experiments were also conducted by varying the EDTA amount, i.e., 0, 0.05, 0.1 and 0.2 g under the similar growth conditions. 2.3 Preparation of silver nanoparticles on CM MFs (Ag NPs@CM MFs): Ag NPs were facilely prepared by a chemical reduction method at RT with tannic acid as a reducing agent. The asformed Ag NPs were uniformly decorated on the surface of CM MFs. Typically, 0.1 g of the prepared CM MFs (EDTA-0.15 g) were mixed in 100 mL of DI water and sonicated for about 5 min. After that, 0.51 g of tannic acid was added to the above solution and kept under vigorous constant magnetic stirring until it dissolved. Next, 0.05 g of potassium carbonate was dispersed in the above solution. Then, 0.02 g of silver nitrate was dissolved in the above growth solution and the resultant mixture was left under constant stirring for 1 h. Finally, Ag NPs decorated CM MFs (Ag NPs@CM MFs) powder was obtained by subsequent filtration, washing with DI water and finally dried in vacuum oven at 80 ºC for overnight. 2.4 Material characterization: The structural morphologies and elemental composition of the prepared samples were observed by using a field-emission scanning electron microscope (FE SEM) and a transmission electron microscope (TEM) equipped with an energy dispersive X-ray (EDX) spectroscopy. The crystallinity and phase purity of the samples were characterized by X-


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ray diffraction (XRD) patterns. The X-ray photoelectron spectroscopy (XPS) with an Al Kα radiation was used to determine the surface chemistry and oxidation states of elements existed in synthesized materials. The specific surface area and pore size distribution of the synthesized samples were calculated by BELSORP-max (MP) Brunauer-Emmett-Teller (BET) and BarrettJoyner-Halenda (BJH) methods, respectively. 2.5 Electrochemical measurements:

The electrochemical performance of prepared active

materials were analyzed by using an IviumStat workstation in a beaker-type three-electrode system at RT. Cyclic voltammetry (CV), galvanic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) analyses were carried out using as-prepared materials (CM MFs or Ag NPs@CM MFs) as a working electrode, Ag/AgCl as a reference electrode and Pt wire as a counter electrode in 1 M KOH aqueous electrolyte. Here, the working electrode was prepared by grinding 80 wt% of the synthesized samples as an active material, 10 wt% of super P carbon black as a conductive additive and 10 wt% of PVDF as a binder in an agate mortar using appropriate NMP solvent. Afterwards, the as-formed slurry was painted on the cleaned Ni foam (1 × 1 cm2 active area) using a painting brush and dried in an oven at 80 ºC for 4 h. After that, the electrode was pressed upto 10 MPa for strong attachment of material with the Ni foam. The mass loading of an active material was estimated to be 4 mg using an analytical balance with accuracy of 0.01 mg. The areal capacity of the as-prepared electrode materials in three-electrode system and the areal capacitance, areal energy and power densities of the HSC in two-electrode configuration are estimated using following formulae:34-36 CA =

𝐼 × ∆𝑡 𝑎 𝐼 × ∆𝑡

CS = 𝑚 × ∆𝑉


(1)

(2)


𝐼 × ∆𝑡

CAC = 𝑎 × ∆𝑉 1

Ed = 2 𝐶𝐴𝐶 × ∆𝑉2 𝐸𝑑

Pd = ∆𝑡

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(3)

(4)

(5)

where CA is the areal capacity (Ah/cm2), CS is the specific capacitance (F/g), I is the discharge current (A), Δt is the discharge time (s), a is an active area (cm2), ΔV is the potential window (V), CAC is the areal capacitance (F/cm2), Ed is the areal energy density (Wh/cm2) and Pd is the areal power density (W/cm2). 3. RESULTS AND DISCUSSION The schematic diagram shown in Figure 1 illustrates the successful formation of Ag NPs@CM MFs material. The novel CM MFs were synthesized in a single-step process with an aid of EDTA using hydrothermal method. As is well known, the EDTA plays a significant role as a chelating agent to produce stable complexes with metal atoms and also serves as a structure-directing agent to design the various benefit-enriched morphologies.37 The advantageous aspects including outstanding capping ability, strong chelating capability and efficient structure tunability of EDTA are mainly attributed to the existence of four carboxyl groups (–COOH) and a single electron pair located on each nitrogen atom.38 Accordingly, the adequate amount of EDTA was introduced into growth solution containing cerium nitrate and sodium molybdate (Figure 1(a)) to tune the surface morphology of product. The possible formation mechanism of CM MFs could be predicted as follows: initially, the EDTA molecules interact with released trivalent cerium ions in growth solution, thus forming an intermediate and stable Ce3+-EDTA complex. Meanwhile, the dissolved divalent molybdate ions also start to interact with EDTA by competing with the Ce3+ ions, as


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shown in Figure 1(b) which leads to the formation of Ce6Mo10O39 precursor nanoclusters (Figure 1(c)). The formed nanoclusters were grown into large and thin NFs as displayed in Figure 1(d) during the hydrothermal conditions. All these obtained NFs are tend to be self-assembled together to form a spherical marigold flower-like morphology through an Ostwald ripening process to lower the surface energy, as illustrated in Figure 1(e).39 Ultimately, the CM MFs with self-assembled hierarchical NFs are attained after annealing the CM MF precursor spherical flowers at 350 ºC for 2 h. Subsequently, Ag NPs are successfully deposited on the surface of CM MFs at RT via chemical reduction method by reducing the silver nitrate with tannic acid, which acts as reducing and stabilizing agent. During the synthesis, the phenolic hydroxyls groups presented in tannic acid can instantly reduce the dissolved Ag+ ions to metallic Ag due to direct redox reactions ensued between Ag+ ion and tannic acid.40 In the meantime, the as-formed Ag NPs were guided towards CM MFs by the continuous magnetic stirring and deposited over its entire architecture (Figure 1(f)). From Figure 1(g), it can be clearly seen that the Ag NPs are uniformly decorated on entire CM MFs. The formed CM MFs with wide-open wells constructed by hierarchically connected NFs empower the facile diffusion of electrolyte ions, leading to the complete wetting of entire MFs and generate the rapid reversible redox reactions. Additionally, the Ag NPs with high electronic conductivity decorated on CM MFs are more favorable for enhancing the rapid charge transfer ability. The structure and surface morphology of prepared samples were investigated by FE SEM and TEM images. Figure 2(a-i) displays the typical FE SEM image of CM MFs prepared via the hydrothermal method and followed by annealing process. From the SEM images, it is identified that the CM particles have the regular three-dimensional (3D) spherical shape with an average diameter of 4 μm. With increased magnification, it is obvious from Figure 2(a-ii) that the CM



microspheres are composed of numerous NFs on their outer surface and the entire structure reassembles like natural marigold flower-like shape. The high-magnification FE SEM image in Figure 2(a-iii) further reveals that the CM MF was assembled with two-dimensional (2D) crosslinked NFs with an average thickness of 15 nm. These NFs are analogous to the petals of marigold flower as shown in the inset of Figure 2(a-iii). Furthermore, the entire NFs created wide-open wells by cross-linking themselves, which can act as an electrolyte reservoirs to perform fast redox reactions. Figure 2(b-i to b-iii) illustrates the different resolutions of Ag NPs@CM MFs material prepared by a facile chemical reduction method. Even after deposition of Ag NPs, it is evident from low- and high-magnification FE SEM images presented in Figure 2(b-i and b-ii) that the CM 3D spherical shape of microstructures retained their MF morphology. An enlarged FE SEM image (Figure 2(b-iii)) was obviously displaying the well decoration of Ag NPs (diameter in the range of 40-45 nm) on the entire NFs. As-decorated Ag NPs effectively improve the conductivity of material, which leads to the better electrochemical performance. The TEM analysis was also performed to further examine the morphological and structural features of Ag NPs@CM MFs. Prior to the TEM analysis, the sample was dissolved in ethanol and sonicated for 15 min. Then, the well-dispersed solution was dropped on a copper grid holder and inserted into the TEM chamber. The TEM images presented in Figure 2(c-i and c-ii) further revealed the successful decoration of Ag NPs on CM MFs. The high-resolution TEM image in Figure 2(c-iii), which is taken from the circular marking area in Figure 2(c-ii), shows atomic planes with fringe spacing of ~0.45 nm. Figure 2(c-iv) represents the corresponding selected area electron diffraction pattern of the CM MFs and the obtained ring-like diffraction pattern with bright spots indicates good crystallinity of the prepared CM MFs. The EDX spectra in Figure 2(d-i) and Figure S2 demonstrate that the Ag NPs@CM MFs composite consists of only Ce, Mo, O and Ag elements without any


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impurity peaks, suggesting the formation of Ag NPs@CM MFs. Here, the originated Cu peak in Figure S2(a) is attributed to the copper grid holder. The elemental mapping images displayed in Figure 2(d-ii to d-v) confirmed the uniform distribution of Ce, Mo, O and Ag elements over the entire MFs (inset of Figure 2(d-i)). Therefore, these results further emphasized the successful formation of Ag NPs@CM MFs composite. Herein, the EDTA plays a vital role in potential morphology modification of CM MFs. Therefore, several experiments were performed by varying the EDTA amount with constant growth conditions. When EDTA was absent in the growth solution, i.e., 0 g, the formation of CM looks to be slab-like microstructures with an arbitrary shape as displayed in Figure 3(a), which may not be useful for enabling electroactive area and charge storage capacity. The formed microslabs had an estimated thickness of around 0.5 μm and several micrometers in length. Meanwhile, when the EDTA is introduced into the growth system (0.05 g), as shown in Figure 3(b), the construction of flower-like CM spherical architectures was resulted due to the chelating effect of EDTA with metal atoms (Ce and Mo). Also, the EDTA molecules in growth solution serve as a mediator for self-assembly of all nuclei to form NFs which are subsequently converted into flower-like shapes. This is the evidence that the EDTA regulates surface morphology of the CM. These flower-like CM structures were composed of a dense package of NFs with thickness of ~30 nm, as illustrated in Figure 3(b)(ii). The size of as-obtained flower-like particles is very large and measured to be in the range of 7-10 μm, which might be attributed to the less amount of EDTA in growth solution. When increasing the EDTA amount to 0.1 g, the size of microflowers was reduced to ~6 μm. Additionally, the length of NFs was slightly increased and the gap among them also became little broader (Figure 3(c)), which is favorable for facile penetration of electrolyte ions. Afterwards, the size of microflowers was further decreased to ~4 μm, when the



EDTA amount in the growth solution was reached to 0.15 g. The obtained microflowers appeared to be exclusive owing to the open well-like spaces generated by self-aligning the NFs. The asderived microflowers are more analogous to marigold flower. The experiment further proceeded by adding 0.2 g of EDTA to understand its impact on the morphology evolution. Surprisingly, the size of microflowers was abruptly increased to 20 μm (Figure 3(d)), which might be assigned to the supersaturation of EDTA amount in the growth solution. It can be noticeable from close observation of the resulted particles (Figure 3(d)(ii)) that all NFs were vanished from the surface, because of higher nucleation rate than the self-assembly of nuclei in supersaturation conditions. From the above investigations, it is apparent that the EDTA amount plays a significant role in tuning the morphology of particles, from which the sample prepared with 0.15 g of EDTA was considered as an optimized sample. The structural properties were further analyzed for the CM MFs sample prepared with 0.15 g of EDTA. Figure 4(a) shows the XRD patterns of CM MFs and Ag NPs@CM MFs samples. Almost all the sharp peaks in both diffraction patterns were well matched with the values of Joint Committee on Powder Diffraction Standards (JCPDS) card number #70-1085, indicating high crystallinity of CM MFs. The peaks originated at the 2θ values of 12, 12.4, 14.8, 15.6, 18.1, 20.4, 23.5, 27.3, 28.2, 30.5, 33.3 and 41.6º are corresponding to (hkl) values of (120), (021), (111), (1 30), (121), (210), (212), (032), (301), (033), (062) and (433), respectively. Moreover, a low intense peak originated at 38.5º in Ag NPs@CM MFs indicates the formation of Ag NPs.41 XPS analysis was performed to further reveal the successful formation of composite material by analyzing its surface elemental composition and oxidation states. From the surface scan spectrum (Figure 4(b)), it is obvious that the Ce, Mo, O and Ag peaks exist in the prepared composite material. The highresolution XPS spectra of Ce, Mo, O and Ag are well fitted using Gaussian method, as presented


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in Figure 4(c-f). Figure 4(c) represents the core-level spectrum of Ce 3d deconvoluted into Ce 3d5/2 and Ce 3d3/2 and all the detected eight peaks in spectrum are associated to various oxidation states of Ce3+ and Ce4+. The peak labelled as ‘a’ obtained at binding energy value of 879.7 eV is assigned to Ce4+ ionization in Ce 3d5/2 along with its satellite peaks originated at 883 eV (a1) and 894.8 eV (a2). On the other hand, the peak labelled with ‘b’ (898.8 eV) and its satellite peak provoked at b1 (903.8 eV) are attributed to Ce4+ oxidation state in Ce 3d3/2. The remaining peaks labelled with ‘c’ (880.8 eV), ‘d’ (901.8 eV) and ‘c1’ (885.1 eV) are corresponding to Ce3+ state of Ce 3d5/2 and Ce 3d3/2 energy levels. The deconvoluted peaks in Ce 3d core-level spectrum clearly demonstrate that the Ce element in the prepared material had Ce3+ and Ce4+ oxidation states.42 The high-resolution Mo 3d spectrum displayed in Figure 4(d) consists of two peaks positioned at 231.2 and 234.3 eV, representing the Mo 3d5/2 and Mo 3d3/2 energy levels, respectively. Accordingly, it is evident from this spectrum that the Mo in the synthesized compound possesses +6 oxidation state.27 The O corelevel spectrum in Figure 4(e) shows the characteristic peaks of metal to oxygen bonds in the prepared sample. The high-resolution Ag 3d spectrum in Figure 4(f) shows two distinct peaks at 366.7 and 372.6 eV, which are attributed to Ag 3d5/2 and Ag 3d3/2 energy levels, respectively.43 These results collectively verify the successful formation of CM MFs and Ag NPs@CM MFs with high purity. Moreover, the surface area and pore size distribution of the prepared samples were investigated by BET analysis. Figure S3(a) and (b) shows the typical N2 adsorption/desorption isotherms and their respective BJH pore-size distribution plots (insets) of CM MFs and Ag NPs@CM MFs samples, respectively. Both samples exhibit the type-IV isotherm behavior with hysteresis loops, specifying the presence of many mesopores in both CM MFs and Ag NPs@CM MFs materials. The specific surface area and pore size of both the samples were calculated using BET and BJH methods. The corresponding values for Ag NPs@CM MFs samples are 33.7 m2 g-1



and 4.25 nm, respectively. While, the CM MFs sample exhibited relatively lower specific surface area (16 m2 g-1) and pore size (3.75 nm). Thus, an improved electrochemical performance could be expected for Ag NPs@CM MFs, because the high surface area can provide abundant electroactive sites to perform numerous redox reactions and the large pore size enables for rapid penetration of ample electrolyte ions into the material. Electrochemical properties of all the prepared samples were investigated by using a common three-electrode system in 1 M KOH aqueous electrolyte. Figure 5(a) shows the comparative CV curves of the CM samples prepared with different amounts of EDTA, i.e., 0, 0.05, 0.1, 0.15 and 0.2 g at a constant scan rate of 20 mV/s. It is clear from CV curves that the prepared materials demonstrated strong redox peaks in the given potential frame of 0-0.55 V, indicating the battery-type behavior of CM samples. In particular, it is noticeable that the high current response and large enclosed CV area were observed for the sample prepared with 0.15 g of EDTA compared to the other samples, which could be due to the smaller size of MF, higher electroactive area, and wider-open wells of NFs with in it (Figure 3(a)). These advantages of CM MFs facilitate the deep penetration of electrolyte ions into the interiors of MFs, which ensures the plenty of redox reactions and rapid electron transportation. The presence of Mo in prepared material is helpful to increase an electrochemical conductivity instead of performing Faradaic reactions during the measurement. This is in well agreement with the previous reports.44-45 Figure 5(b) shows the discharge curves of the samples prepared with EDTA-0 g, 0.05 g, EDTA-0.1 g, EDTA-0.15 g and EDTA-0.2 g measured at a constant current density of 2 mA/cm2. Evidently, the CM MFs with EDTA-0.15 g sample delivered higher discharge time than all the remaining samples, which is well-consistent with the CV curves. All the CM samples prepared with different amounts of EDTA exhibited a pair of strong redox peaks in CV curves and the non-linear GCD curves, which are mainly ascribed


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to the transitions between Ce3+-Ce4+ and vice versa.46 The improved performance of CM MFs can be ascribed to fast surface electro-kinetics and large number of electroactive sites generated on the surface of MFs during the measurement. The calculated areal capacity values based on discharge times of the prepared samples are plotted in Figure 5(c). At 2 mA/cm2, the estimated values are 29.6, 35, 41.6, 56 and 27 μAh/cm2 for EDTA-0 g, EDTA-0.05 g, EDTA-0.1, EDTA-0.15 g and EDTA-0.2 g, respectively. Noticeably, the sample prepared with 0.15 g of EDTA delivered superior areal capacity to the other samples owing to the high electrochemical activity. Accordingly, the CM MFs prepared with EDTA-0.15 g was considered as an optimum sample and further electrochemical characterizations were carried out using the same sample. Figure 5(d) depicts the CV curves of corresponding electrode performed at various scan rates of 10-100 mV/s in the voltage range of 0-0.55V. The two strong redox peaks observed in all CV curves indicate the Faradaic process involved in the charge storage process. With increasing the scan rate, the anodic and cathodic peak currents were also increased without affecting the CV shapes, implying good rate capability of the material. Moreover, the shift of oxidation and reduction peaks to more right and left with increasing the scan rate is attributed to the electric polarization when increasing the scan rate. The GCD tests were also performed to the same sample to further investigate its electrochemical performance. Figure 5(e) displays the GCD curves of CM MFs (EDTA-0.15 g) sample measured in the voltage range of 0-0.5 V with various current densities in the range of 215 mA/cm2. Obviously, the non-linear and nearly symmetric GCD curves further revealed the battery-type behavior and good electrochemical reversibility of material. Based on discharge times, the areal capacity values of an optimized sample were estimated and plotted against current density values, as shown in Figure 5(f). The CM MFs prepared with 0.15 g of EDTA delivered the maximum areal capacity values of 55.3, 46.8, 41, 36.5, 31.6 and 27.5 μAh/cm2 at current densities



of 2, 3, 5, 7, 10 and 15 mA/cm2, respectively. Long-term cycling stability is one of the critical requirements for practical applicability of SC electrode materials. Therefore, the cycling performance of CM MFs (EDTA-0.15 g) was evaluated by repeating the charge-discharge cycles upto 2000 at a constant current density of 3 mA/cm2. As depicted in Figure 5(g), the CM MFs electrode demonstrated a good electrochemical stability and 65.8% of its initial capacity was maintained even after 2000 cycles. To better understand the electrochemical conductivity, the EIS test was also performed for the prepared electrode. Figure 5(h) shows the Nyquist plots of CM MFs (EDTA-0.15 g) measured before and after cycling test in the frequency range of 0.01-100000 Hz by applying the bias potential with 5 mV amplitude. It is evident that both EIS spectra exhibited a depressed semi-circle in a high frequency region and nearly straight line in the low frequency region, which represents the charge-transfer resistance (Rct) and diffusion resistance, respectively. The Rct value can be measured from the diameter of semi-circle shape of curves, which can be clearly seen from the inset of Figure 5(h), and the estimated value is 1.87 Ω before cycling test. Even, after completion of cycling, the CM MFs material still exhibited low Rct (1.92 Ω), suggesting the fast charge transfer at an electrode/electrolyte interface. Moreover, the internal resistance (Ri) of electrode can be calculated from the first intercept of EIS curve with X-axis in the highfrequency region and the values are estimated to be 0.53 and 0.72 Ω before and after cycling test. The lower Ri and Rct values were further revealing the good electrochemical conductivity and fast electro-kinetics of CM MFs active material. High conductivity Ag NPs were successfully deposited on the surface of CM MFs by chemical reduction method at RT to further enhance the electrochemical performance. Figure 6(a) shows the CV curves of CM MFs and Ag NPs@CM MFs electrodes evaluated at a scan rate of 20 mV/s in 1 M KOH electrolyte. Clearly, the Ag NPs@CM MFs electrode exhibited an enhanced


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redox peak currents and large enclosed CV area compared to the CM MFs. Consequently, an improvement in capacity storage could be expected from the Ag NPs@CM MFs electrode. The discharge curves examined at a constant current density of 2 mA/cm2 again corroborated that the Ag NPs@CM MFs delivered longer discharge time than the sample without Ag NPs, as presented in Figure 6(b). The enhanced redox property and improved discharge time is predominantly attributed to the excellent electric conductivity and good redox property of decorated Ag NPs on the CM MFs. The EIS analyses of both Ag NPs@CM MFs and CM MFs electrodes were compared as shown in Figure 6(c) (The magnified part of EIS curves in the high-frequency region is shown in the inset). Compared to the CM MFs material, the Ag NPs@CM MFs material exhibited relatively lower Ri (0.28 Ω) and Rct (1.6 Ω) values, due to good electrical conductivity and redox property of Ag NPs. Accordingly, we further investigated the electrochemical characteristics of Ag NPs@CM MFs electrode material. Figure 6(d) displays the CV curves of the corresponding electrode studied at different scan rates from 10 to 100 mV/s in a three electrode-system. All the CV curves exhibited non-rectangular shapes with strong redox peaks, indicating the battery-type behavior of material. Even at high scan rate (i.e., 100 mV/s), the Ag NPs@CM MFs electrode still showed the redox behavior without much degradation of its shape, implying the good rate capability. The GCD curves of Ag NPs@CM MFs electrode tested at various current densities of 2-15 mA/cm2 are presented in Figure 6(e). Utilizing high electrical conductivity of Ag NPs and several structural benefits of CM MFs, the Ag NPs@CM MFs composite delivered high areal capacity values of 62, 57.6, 52, 47.2, 41.7 and 35.3 μAh/cm2 at current densities of 2, 3, 5, 7, 10 and 15 mA/cm2, respectively. Moreover, the specific capacitance values were also calculated using Eq. (2) and the obtained values were 109.1, 103.73, 96.26, 91.6, 85.5 and 78.7 F/g at the above current densities, respectively. Figure 6(f) shows the comparative capacity storage performance of



two electrodes with and without Ag NPs at the same current densities, respectively. Evidently, the Ag NPs@CM MFs electrode demonstrated better charge storage performance than the electrode without Ag NPs. This is attributed to the deposition of high conductive Ag NPs. Furthermore, the capacity storage performance of Ag NPs@CM MFs electrode is competitive or higher than the previously reported metal oxide/molybdate based materials, such as graphene-NiMoO4·nH2O (13.75 μAh/cm2 at 1.5 mA/cm2),47 NiO nanoflakes (40.1 μAh/cm2 at 0.5 mA/cm2),48 CoMoO4/C (54.16 μAh/cm2 at 1 mA/cm2),49 Graphene/cerium oxide nanoparticles (11.09 F/g at 5 mV/s),50 CoMoO4 nanostructures (98.3 F/g at 5 mV/s),51 CoMoO4/MWCNTs (96 F/g at 1 A/g)52 and CoMoO4 nanorods (70 μAh/cm2 at 1.2 mA/cm2).53 The detailed comparative electrochemical performance of these materials were summarized in the Table S1 of Supporting Information. Figure 6(g) demonstrates the cycling test of Ag NPs@CM MFs electrode investigated at 3 mA/cm2. After completion of 2000 cycles, the electrode material maintained 74.1% of initial capacity, indicating the good electrochemical stability. After cycling test, the EDX and elemental mapping analyses were carried out for the Ag NPs@CM MFs sample to further demonstrate the structural stability of material. The EDX spectrum in Figure S4(i) demonstrated the Ce, Mo, O and Ag peaks and the elemental mapping images in Figure S4(ii-v) displayed the uniform distribution of Ce, Mo, O and Ag elements on Ag NPs@CM MF particle, suggesting the good electrochemical stability of material. The schematic diagram in Figure 6(h) demonstrates the morphology features of the prepared Ag NPs@CM MFs composite: (i) the deposited Ag NPs unambiguously increase an electrochemical performance of the composite material owing to its higher electric conductivity than other several metals. Moreover, the good redox property of Ag NPs can effectively enhance the surface electro kinetics. (ii) The CM MFs consisting of wide open-wells are more favorable for easy diffusion of electrolyte ions into interior parts of MFs and to completely wet an entire


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material, resulting in the number of redox reactions. (iii) The thin NFs offer numerous electroactive sites to generate the plenty of electrons and also they can readily transfer the produced electrons from Ag NPs and MFs to the current collector. To further demonstrate the practical applicability of the as-prepared composite, HSC was fabricated with Ag NPs@CM MFs as a positive electrode and porous activated carbon (PAC) as a negative electrode in aqueous KOH electrolyte. As illustrated in the schematic diagram of Figure 7(a), the HSC was assembled in a facile manner by sandwiching the two electrodes along with a filter paper separator in between them to prevent short-circuit. After that, the entire configuration was introduced into a pouch-type polyethylene bag with a few mL of aqueous KOH electrolyte and sealed properly by using a commercial grade sealer. Before assembly of the device, the mass values on both electrodes were optimized using the following equation to balance the charge storage:54 𝑄+ × 𝑚+

𝑚 ― = 𝐶 ― × ∆𝑉 ―

(5)

where, m+ and Q+ are the mass loading and areal capacity values of Ag NPs@CM MFs, respectively and m-, C- and ΔV- are the mass loading, areal capacity and operating voltage of PAC, respectively. The mass loading of PAC on Ni foam substrate was optimized to be ~1.3 mg. Moreover, the electrochemical characteristic of PAC/Ni foam was also studied in the potential window of -1.0 to 0 V by using a three-electrode system in 1 M KOH electrolyte and the corresponding results were displayed in Figure S1. Therefore, the operating voltage window of the as-assembled HSC was optimized to be 1.5 V by considering the working potentials of both positive and negative electrode materials and further electrochemical characterizations were carried out within this voltage window. It is apparent from Figure 7(b) that the CV curves measured



at different scan rates from 10 to 100 mV/s demonstrate the capacitive-type behavior as well as battery-type behavior of both materials, signifying the successful assembling of HSC. As the scan rate increased from 10 to 100 mV/s, the current response and area under the CV curve also increased, and the HSC still maintained its CV shape even at a high scan rate, suggesting the good rate capability of the device. As presented in Figure 7(c), the GCD curves of HSC were also explored at various current densities of 1-15 mA/cm2 in the optimized potential window (1.5 V). The nearly symmetric and non-linear behavior of GCD curves was well consistent with the CV curves. The areal capacitance values of assembled HSC were calculated using Eq. (3) based on the discharge curves and plotted against current densities as shown in Figure 7(d). The calculated areal capacitance values of the device were 60.4, 59, 54.6, 53.3, 50.4, 47.8, 47.1 and 45.9 mF/cm2 at current density values of 1, 2, 3, 4, 5, 7, 10 and 15 mA/cm2, respectively, with the superior rate capability of 76%. The areal energy and power densities of the Ag NPs@CM MFs//PAC HSC were estimated using Eqs. (4) and (5), respectively, as displayed in Figure 7(e). From the benefits of both electrode materials, the as-constructed Ag NPs@CM MFs//PAC HSC delivered a maximum areal energy density of 0.0183 mWh/cm2 (areal power density of 0.739 mW/cm2) at 1 mA/cm2 and the device still maintained 0.0119 mWh/cm2 at high power density of 10.237 mW/cm2. The energy density performance of the Ag NPs@CM MFs//PAC HSC is comparable or higher than the previously reported works.55-59 The stability test for HSC was also conducted at 4 mA/cm2 to evaluate its durability. Noticeably, the HSC still remained 78.1% of its capacitance retention after 2000 continuous charge-discharge cycles (Figure 7(f)), indicating the good cycling stability. The last five charge-discharge cycles of the fabricated hybrid device are also shown in the inset of Figure 7(f), which indicates the stable charge-discharge time of the fabricated device. Moreover, the EDX and elemental mapping were performed for the Ag NPs@CM MFs sample


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after cycling test, as shown in Figure S4. Even after cycling test, the EDX and elemental mapping images further demonstrate the structural stability of Ag NPs@CM MFs sample. After cycling test, the EDX spectrum in Figure S4(i) demonstrated the existence of Ce, Mo, O and Ag elements. In addition, the elemental mapping images in Figure S4(ii-v) displayed the uniform distribution of Ce, Mo, O and Ag elements in the entire structure of Ag NPs@CM MF particle, suggesting the good electrochemical stability of material. The absence of semicircle in EIS spectrum (inset of Figure 7(e) suggests the superior charge transfer process in fabricated device. To demonstrate the practical applicability, the fabricated HSCs were tested to glow various LEDs and torch lights. Figure 7(g) illustrates the schematic diagram of two HSCs in series connection and the photographic image elucidating the as-fabricated devices before connecting to LEDs. With the maximum energy density and power density, the HSCs efficiently lit-up three-point pattern torch light, yellow LED and red LED upto for about 20, 50 and 150 sec, respectively, as shown in Figure 7(h) and Figure S5. From the remarkable features of novel CM MFs and highly conductive Ag NPs, the synergistic Ag NPs@CM MFs electrode material could be expected as a promising positive electrode material for construction of HSCs. 4. CONCLUSIONS In summary, we have successfully prepared the CM MFs with chelating agent (EDTA) assisted hydrothermal method. The amount of EDTA plays a crucial role for the self-assembly of MFs and hence, the effect of EDTA has been elucidated effectively by introducing different amounts in the growth solution. Subsequently, the Ag NPs were uniformly decorated on entire surface of CM MFs using a facile metal reduction method. Owing to the beneficial morphological features including hierarchically connected NFs and wide-open wells, the CM MFs material demonstrated a superior electrochemical performance. With an inclusion of Ag NPs, the capacity performance



was significantly enhanced due to their high electric conductivity and charge storage properties. The resultant composite material (Ag NPs@CM MFs) exhibited a maximum areal capacity of 62 μAh/cm2 at 2 mA/cm2 and also showed good cycling stability (74.1 % of retention). Furthermore, the pouch-type HSC fabricated with Ag NPs@CM MFs as positive electrode and PAC as negative electrode and the as-assembled HSC demonstrated a maximum areal energy density of 0.0183 mWh/cm2 and areal power density of 10.237 mW/cm2 with the optimum potential window of 1.5 V. Considering the facile preparation methods and remarkable results, the Ag NPs@CM MFs composite material could pave a new-path in the fabrication of novel electroactive materials for energy storage systems. ASSOCIATED CONTENT Supporting Information Supporting Information contains the electrochemical properties of negative electrode, EDX and elemental mapping images of Ag NPs@CM MFs before and after cycling test, BET and BJH plots of CM MFs and Ag NPs@CM MFs samples and real-time applications of fabricated HSC. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. NOTES The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2017R1A2B4011998 and No. 2018R1A6A1A03025708).


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Figure 1. Schematic diagram illustrating the preparation process of CM MFs and Ag NPs@CM samples using hydrothermal and metal reduction methods: (a) Preparation of growth solution, (b) metal-EDTA complexes, (c) intermediate CM nanoclusters, (d) growth of CM NFs, (e) marigold flower-like CM particle, (f) Ag reduction at the surface of CM MF and (g) uniform decoration of Ag NPs over CM MFs.


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Figure 2. Low to high magnification FE-SEM images of (a)(i-iii) CM MFs, (b)(i-iii) Ag NPs@CM MFs, respectively. (c)(i and ii) Low and high magnification TEM images of Ag NPs@CM MFs sample. (c)(iii and iv) HR TEM image and SAED pattern of Ag NPs@CM MFs, respectively. (d)(i) EDX spectrum of Ag NPs@CM MFs sample. Elemental mapping images of (d-ii) Ce, (diii) Mo, (d-iv) O and (d-v) Ag elements in the prepared sample.



Figure 3. Low and high magnification FE SEM images of CM material prepared with (a)(i and ii) 0 g, (b)(i and ii) 0.05 g, (c)(i and ii) 0.1 g and (d)(i and ii) 0.2 g of EDTA, respectively.


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Figure 4. (a) XRD patterns of CM MFs and Ag NPs@CM MFs samples. (b) XPS survey scan spectrum of Ag NPs@CM MFs composite. High resolution XPS analysis of (c) Ce, (d) Mo, (e) O and (f) Ag elements of Ag NPs@CM MFs sample.



Figure 5. Comparative (a) CV curves, (b) GCD curves and (c) areal capacities of the samples prepared with EDTA- (i) 0 g, (ii) 0.05 g, (iii) 0.1 g, (iv) 0.15 g and (v) 0.2 g. (d-e) CV and GCD curves of the optimized sample, i.e., CM MFs prepared with 0.15 g of EDTA. (f) Areal capacity values and (g) cycling stability of CM MFs (EDTA-0.15 g) electrode. (h) Comparison of EIS spectra before and after cycling test of the optimized sample. Inset in (h) displays the magnified view of EIS spectrum.


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Figure 6. Comparison of (a) CV, (b) GCD curves and (c) EIS spectra of Ag NPs@CM MFs electrode with CM MFs electrode. (d) CV and (e) GCD curves of Ag NPs@CM MFs electrode. (f) Relationship between obtained areal capacity values and current density values of CM MFs electrode with and without Ag NPs. (g) Cycling stability of Ag NPs@CM MFs electrode conducted at 3 mA/cm2. (h) Schematic diagram illustrating the Ag NPs@CM MFs morphological features. Inset in (c) shows the magnified EIS spectra.



Figure 7. (a) Schematic representation of assembled HSC with Ag NPs@CM MFs and PAC as positive and negative electrodes, respectively in 1 M KOH electrolyte. (b) CV and (c) GCD curves of HSC device measured at different scan rates and current densities, respectively. (d) Calculated areal capacitance values of HSC at different current densities. (e) Ragone plot and (f) cycling stability of HSC device. (g) Schematic and photographic images of HSCs and (h) demonstrating the practical applicability of HSC by powering commercial LEDs and torch light. Insets in (e) and


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(h) show EIS of the device before and after cycling test and the last five cycles of HSC during cycling test, respectively.



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Hierarchically Designed Ag@Ce6Mo10O39 Marigold Flower-like

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