NiO-bridged MnCo-hydroxides for flexible high-performance fiber-shaped energy storage device

NiO-bridged MnCo-hydroxides for flexible high-performance fiber-shaped energy storage device

Accepted Manuscript Full Length Article NiO-bridged MnCo-hydroxides for flexible high-performance fiber-shaped energy storage device Libo Gao, Rong Fa...

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Accepted Manuscript Full Length Article NiO-bridged MnCo-hydroxides for flexible high-performance fiber-shaped energy storage device Libo Gao, Rong Fan, Ran Xiao, Ke Cao, Peifeng Li, Weidong Wang, Yang Lu PII: DOI: Reference:

S0169-4332(19)30018-2 APSUSC 41411

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

22 November 2018 22 December 2018 2 January 2019

Please cite this article as: L. Gao, R. Fan, R. Xiao, K. Cao, P. Li, W. Wang, Y. Lu, NiO-bridged MnCo-hydroxides for flexible high-performance fiber-shaped energy storage device, Applied Surface Science (2019), doi: https://

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NiO-bridged MnCo-hydroxides for flexible high-performance fiber-shaped energy storage device Libo Gao1,2*, Rong Fan2,3, Ran Xiao2, Ke Cao2, Peifeng Li4, Weidong Wang1*, and Yang Lu2,5* 1

School of Mechano-Electronic Engineering, Xidian University, Xian 710071, China


Department of Mechanical Engineering, City University of Hong Kong, Hong Kong

SAR, Kowloon 999077, Hong Kong; 3

School of Automotive Engineering, Faculty of Vehicle Engineering and Mechanics,

Dalian university of technology, Dalian 116024, Liaoning 4

College of Materials Science and Engineering, Shenzhen University, Shenzhen

518060, China 5

Shenzhen Research Institute, City University of Hong Kong, Shenzhen 518057, China

* Author








[email protected]; [email protected]; [email protected] ;

Abstract Flexible fiber-shaped supercapacitors (FFSSs) hold promising prospect to meet the increasingly high requirements of the wearable electronics. However, today it remains a great challenge to construct advanced supercapacitor with high areal capacity and favorable rate capability to achieve superior energy density in facile route is a great challenge. Herein, we directly used the low-cost nickel wire as the fiber substrate to in-situ grow compacted NiO buffer layer capable of strongly grafting the outer MnCo-Layered double hydroxide (MnCo-LDH) with high electrochemical reversibility. Compared to MnCo-LDH directly growing on nickel fiber in the absence of NiO, the [email protected] exhibited 210% enhancement in areal capacity (165.6 mC cm-2/368.1 mF cm-2 at 0.5 mA cm-2) and ultrahigh rate capability (85% retention at 20 mA cm-2), as synthesized NiO buffer not only served as “nano glue” to strongly immobilize the active materials on the metal substrate but also positively supplied extra capacitance. Thusly, the assembled hybrid/asymmetric fiber device presented a high energy density of 0.0198 mWh cm-2 at a power density of 0.38 mW cm-2 to drive a digital watch, demonstrating its promising potential application in electronic devices. This rational design sheds light on the synthesis of nickel fiber-based supercapacitor with high energy delivery.

Keywords: flexible, fiber supercapacitor, nickel wire, NiO, MnCo-LDH

Introduction Flexible fiber-shaped supercapacitors (FFSSs) recently have attracted people’s attention due to the rapid progress of the high demand of the commercially available deformable organic light emitting diode (OLED), flexible displays and sensors.[1–5] However, how to rationally design and build the FFSSs with high energy density in a tiny body to meet the strict requirement regarding high energy storage and wearable ability is still a challenge. A limiting factor in achieving a high capacitive performance of the FFSSs is the fiber scaffold. These fiber scaffolds are highly related to the efficiency of electron, and the accommodation of foreign active materials. Typically, the base scaffold for fabricating FFSSs includes carbonaceous fibers (such as carbon nanotube fibers[6–8], graphene fibers[9–11], carbon fiber[12,13]) and polymer fiber[14,15] as well as natural fibers[16,17]. Although these fiber scaffolds already have been extensively employed, some knotty issues have yet to be solved. To our knowledge, for instance, the ultralow surface area hinders the large-mass active materials loading on carbon fiber to achieve high capacitance, though it has superior mechanical performance.[18] Poor conductivity and weak adhesion for foreign materials is the bottleneck for polymer and natural fibers though they show good deformability.[15,19] In this regard, the metal fibers recently were extensively used as alternative candidates owing to their superior conductivity, high flexibility and in particular, the low-cost manufacturing than the above-mentioned fibers.[4,20–24] Nevertheless, directly growing foreign active materials on the plain metal substrate would lead to the interface resistance of electrons transportation, causing declined rate capability and shorter cycle life.[25,26] Also, the thick active materials would be easily peeled off from the substrate due to the limited surface area, smooth surface, and its relatively increased gravity. How to effectively boost the surface area without sacrifice of the excellent conductivity of the metal fiber, therefore, is an advanced research direction. In this context, hierarchical CuO nanowire was fabricated on Cu wire with further deposition of FeCo-Layered double hydroxide (FeCo-LDH) to significantly enhance the specific capacitance, rate capability and long

cycling lifespan of the FFSSs.[27] Also in Li’s work, TiO2 buffer layer was introduced to modify the Ti wire, which notably enhanced the mechanical adhesion of the MoS2 on the Ti wire.[1] Nevertheless, there is still a big room for the improvement of the capacitive performance, especially in terms of the rate capability of the fiber supercapacitor by a more simple way. To address this challenge, in our previous research, we have rationally designed and fabricated











Co3O4/PANI,[32] showing admirable energy storage performance. This design concept gives solid guidance for us on how to fabricate the FEES with superior performance. Herein, we directly used the commercially available nickel wire as the substrate with an acid-assisted process to grow a rough NiO layer. The compact, light scattering layers not only offer large electrode surface area to grow more MnCo-LDH active materials, but also is regarded as the active materials itself. Also, thanks to the superior conductivity and unique porous structure of the MnCo-LDH, the [email protected] exhibited exceptional areal capacitance and ultrahigh rate capability. Finally, the assembled hybrid supercapacitor (ASC) showed a superior energy density, outperforming most reported fiber supercapacitor.

Materials and methods Fabrication of the [email protected] The fabrication procedure of the Ni/NiO is similar to our previous article.[33] Briefly, a pre-treated nickel wire (400 μm in diameter, 5 cm in length) which has been treated using acetone, ethanol and deionized (DI) water for 15 min respectively was directly immersed into 3 M HCl solution for 20 min at 90 °C. After pulling out from the solution with the following washing with DI water, the nickel wire was dried at 60 °C for one day until a light green layer was observed. A simple electrodeposition method was employed to synthesize of [email protected] In a typical procedure, the Ni/NiO wire, graphite and Hg/HgO was regarded as the working, counter and reference electrodes, respectively.[34] Then the wire was kept at a constant voltage of -1 V in a

mixed solution of 0.045 M Co(NO3)2 and 0.005 M MnSO4 for 2 min. After finally washed using DI water and dried at room temperature, the [email protected] is obtained. For a controlled comparison, MnCo-LDH directly grown on the nickel wire with same conditions was fabricated. Fabrication of the negative electrode To prepare the negative electrode, the activated carbon(AC) powder, acetylene black and poly-vinylidene fluoride (PVDF) were mixed together by a weight ratio of 8:1:1 in N-methylpyrrolidone (NMP) solution at 70 °C. The slurry then is painted on the nickel wire homogeneously. After drying at 80 °C to evaporate the NMP, the final sample is obtained. Then the slurry can be uniformly coated on the nickel wire through dip-coating method. Assembly of solid fiber supercapacitor The fabrication procedure is shown in our previous procedures.[28] Briefly, 3g of polyvinyl alcohol (PVA) and 20 mL of DI water were mixed at 100 °C with stirring for 1 h. And then 10 mL of 0.3 g mL-1 of KOH solution was added into above clear solution drop by drop. After continuously stirred at 100 °C for 2 h, the positive and negative electrode were immersed into the PVA/KOH solution for 5 min and dried at room temperature. Finally, the two electrodes were assembled carefully and coated with a PVA/KOH gel electrolyte again to guarantee there is no any short-circuit. Characterization and measurements The element analysis and morphology of the sample were performed by field emission scanning electron microscope (FESEM, Quanta 450), transmission electron microscope (JEOL JEM 2100) equipped with energy dispersive spectroscopy (EDS), and select area electron diffraction (SAED). X-ray powder diffractometer (XRD, RigakuSmartLab) with monochromatic Cu Ka radiation was employed to characterize the crystal structure. All of the electrochemical experiments were performed on the electrochemical workstation (CHI 7660E, Chenhua). For the three-electrode measurements, the cyclic

voltammetry (CV) and galvanostatic charge/discharge (GCD) curves, as well as the electrochemical impedance spectroscopy (EIS) were investigated in 2 M KOH electrolyte, while Pt foil (1×1 cm2) was used as the counter electrode and saturated calomel electrode (SCE) was used as the reference electrode. For the calculation of the specific capacitance (C) of the positive electrode, below formula was employed.[35] (1) However, for the negative electrode, the specific capacitance (C) can be calculated based on the following equation: (2) The areal capacity (Q, C cm-2) was based on the formula: Q





(3) In these formulas, i (A) represents the areal (mass) current density, Vi (V) and Vf (V) is the initial and final voltage in the discharging procedure, t (or Δt, s) is the discharge time. For the constructed flexible solid fiber supercapacitor, the specific capacitance was calculated as expressed in equation (1). The energy density (E, Wh cm-2) and power density (P, W cm-2) can be calculated respectively by the following relations, (4) (5) Where C (F cm-2) is the areal capacitance of the two-electrode system, V (V) is voltage range of the ASC, and t (s) is corresponding discharge time. Also, charging balance is important for the ASC to achieve superior performance without obvious fading according to the following equation[36]: (6) Where m+ (m-) designates the mass loading of the positive (negative) electrode, v (mV

s-1) represents the scan rate, V (V) is the voltage and

is the integral area of the

cyclic voltammogram for the positive or negative electrode at 5 mV s-1. Here, the mass ratio of the positive to negative of the ASC is thus determined to be 0.28 as shown in Figure S7 (the specific mass of the positive electrode is 0.4 mg cm-2).

Results and discussions Structure characterization The limited surface area is one of the obstructions for the metal wire current collector to be used as an effective backbone to strongly support more active materials.[33] Figure 1a schematically illustrates our synthetic approach to settle this challenge. After simple corrosion and re-oxidation process, the grain boundaries on the nickel wired was selectively corroded, and finally, a rough surface-modified nickel wire was obtained (Figure 1b). Such unique morphology not only enhances the surface area but also act as the branch to tightly immobilize foreign active materials. To identify the phase structure of the rough micro-heaves with the size of 1-5 μm (Figure 1c) on the nickel wire, XRD was employed. The weak yet observable characteristic patterns belong to NiO patterns. This weak observation is mainly due to the thin thickness of NiO film and partially covered by the strong Ni patterns as shown in Figure S1, which is similar to our previous observation.[33] And the NiO film can be further confirmed by the SEM-EDX, in which the O element increased greatly as shown in Figure S2. Importantly, numerous voids (Figure 1c) generated at the root interface between the heaves and nickel wire, which is beneficial to tightly growing more MnCo-LDH materials and ions accessible permeation. As expected, the MnCo-LDH was conformally grafted on the treated nickel wire, and numerous orderly interconnected MnCo-LDH layers with 200 nm sized opening porous structure were observed (Figure 1d).

Figure 1. Illustration of the fabrication procedure and FESEM characterization of the [email protected] (a) Simple two-step method to prepare the [email protected] (b) FESEM images of the nickel wire, Ni/NiO, and [email protected] (c) Large view of NiO anchored on nickel wire. (d) FESEM images of MnCo-LDH and the inset is corresponding high magnification SEM images.

TEM was employed to observe more information of the MnCo-LDH coating. As shown in Figure 2a, stacking wrinkled LDH layer is in good accordance with the observation in Figure 1d. Importantly, 1.4 nm sized pores are distributed on the ultrathin individual LDH layer (Inset in Figure 2a and Figure S3), which facilitate the electrolyte ions to penetrate quickly into the inner part of the LDH and thus improve the rate capability. The pores were supposed to be primarily derived from two items. Firstly, as cobalt nitrate typically show low pH value, thus it would lead to high hydrolysis rate to produce more nucleation positions, resulting in LDH layers with lots of nanopores inside.[37,38] Secondly, the water deintercalation maybe is responsible for forming the pores during the process of drying the LDH product.[39] Also, apparently the LDH features a polycrystal structure and the interplanar spacing of 2.4 Å corresponds well to the (101) plan, which belongs to the MnCo-LDH (Inset in Figure 2b and Figure

2c).[40] TEM-EDS mapping analysis is shown in Figure 2d. Only Co, O and Mn elements were uniformly detected. No Ni element has been found, further indicating that the NiO particles have a firm bonding on the nickel wire.

Figure 2. TEM and EDS element maps of the [email protected] (a-b) Low magnification of TEM images of MnCo-LDH. Inset is its high magnification on the pore size. The corresponding SAED pattern of (b) region indicates the polycrystal structure. (c) HRTEM images of the MnCo-LDH, further confirming the successful fabrication of MnCo-LDH. (d) STEM images and corresponding EDS mapping of the structure.

Electrochemical performance Figure 3 displays the electrochemical performance of the positive electrode in 2 M KOH electrolyte. As shown in Figure 3a, CV curves of NiO, MnCo-LDH, and [email protected] were investigated at a scan rate of 5 mV·s-1. These curves show

typical redox peaks which belonged to the battery-type characteristics of the active materials. The [email protected] combines both the redox features of NiO and MnCo-LDH, together with an enhanced area of the CV curves, indicating the strong synergistic effect between NiO and MnCo-LDH (note that the capacity produced by pure nickel wire can be ignored as shown in Figure S4). This is further confirmed by the GCD curves as shown in Figure 3b. The areal capacity of the [email protected] is 165.6 mC cm-2/368.1 mF cm-2 while the individual MnCo-LDH and NiO are respectively 80 mC cm-2/178 mF cm-2 and 60.2 mC cm-2/133.8 mF cm-2 at a current density of 0.5 mA cm-2, respectively, demonstrating the as-made composite structure is not just the simple mixture hybrid. Further, for the [email protected], with continue increasing the scan rates from 5 to 50 mV s-1 as shown in Figure 3c, the potential disparity between reduction and oxidation peaks increased with scan rates, due to the charge diffusion polarization in term of the sacrifice of redox reaction reversibility.[41,42]







[email protected] are observed at various current densities, demonstrating the reversibility of the faradaic redox reactions in Figure 3d. Here we should note that there is a slight difference of potential range between CV and GCD curves, avoiding of the overpotential existed in battery-type electrode materials during CV process.[2][43] In addition, as shown in Figure 3e and Figure S5, the [email protected] achieves 85% capacity retention when the current density increases 40 folds, which is slightly lower than that of MnCo-LDH (86%) yet much higher than that of the NiO (21%). To our knowledge, this value is superior higher than that of other reports based on battery-type electrode materials such as, NiCoAl-LDH/rGO



NiCo2S4/PANI (72%,


CuCo2O4/NiO (63.5%, 20-fold),[45] [email protected](OH)2 (56%, 10-fold),[46] NiCo2O4/NiO (83.2%, 10-fold)[47]. Also, the areal capacity of [email protected] is 2.1 and 2.8 times higher than the pure MnCo-LDH and NiO at the current density of 0.5 mA cm-2, further demonstrating the jointly enhancing effect. The EIS measurement was employed to analyze the kinetic and charge-transfer process. As shown in Figure 3f, the charge-transfer resistance of [email protected] slightly

increased in comparison to the MnCo-LDH, which is possibly due to the low conductivity of the NiO electrode.

Figure 3. Electrochemical performance of the [email protected] (a) CV and (b) GCD curves of NiO, MnCo-LDH, and [email protected] (c) CV curves and (d) GCD curves of [email protected] (e) The areal capacity of three different electrode materials as a function of the current density, indicating the [email protected] is not the simple composite mixture. (f) EIS spectrum of three different kinds of electrode materials. Inset is corresponding large view at high frequency.

However, one question then naturally arose: why the MnCo-LDH coating would increase the charge-transfer ability of NiO? We deduced that two factors could explain this doubt as shown in Figure 4. Firstly, as shown in Figure 1c-d, more conductive MnCo-LDH layers were able to sufficiently pad the gap and the crack between NiO nanoparticles (poor conductivity) produced during the oxidation procedure, thus shortening the charge transport path through merging the NiO nanoparticles together. Secondly, the unique open porous architecture of the MnCo-LDH with high surface area (Figure S3) would greatly enhance the active sites and improve the reaction kinetics. Finally, the coarse surface produced by NiO shall enhance the mechanical adhesion of the MnCo-LDH on the nickel wire and importantly, numerous voids (Figure 1c) generated at the root interface between the heaves and nickel wire, which is beneficial to tightly growing more MnCo-LDH materials and ions accessible permeation. Then one may question what is the role of NiO as a low-conductivity core in the electrochemical enhancement? Actually, as mentioned above in Figure 1c, more cracks and gaps produced at the root between the NiO particles and nickel core, MnCo-LDH would grow tightly to keep close adhesion with the nickel current collector through the enhanced mechanical adhesion. Note that the electrodeposition time is also an important factor in determining the whole electrochemical performance as illustrated in Figure S6 and Figure S7. Excess time would lead to the depressed electrochemical behavior, which is similar with other reports and can be explained that the densely packed LDH would cause less exposure of active sites in reducing the reactions and charge transportation.[27] Also the too-long deposition time would lead to the peeling off of the active materials from the wire substrate in decreasing the areal capacitance.

Figure 4. Electron transportation and ion diffusion mechanism of the [email protected]

To test its practical application in electronic devices, a solid ASC was assembled through using the [email protected] as a positive electrode, AC as the negative electrode and PVA/KOH as the gel electrolyte (Figure 5a and Figure S8-9). Figure S10 shows that the ASC device remained an operating potential as high as 1.5 V at a scan rate of 100 mV s-1. Figure 5b shows the CV curves of the ASC at various scan rates (10-100 mV s-1), apparent redox peaks belonged to the [email protected] are observed. The GCD curves of the device with various current densities (0.5-10 mA cm-2) are exhibited in Figure 5c, in which the areal capacitance and volume capacitance can be obtained accordingly in Figure 5d. A superior high areal capacitance of 63.3 mF cm-2 (5.9 F cm-3) and rate capability of 81% were achieved. Simultaneously, the energy and power density are key factors to evaluate the performance of the ASC. As shown in Figure 5e, the ASC gave a maximum energy density of 19.8 µWh cm-2 at a power density of 380 µW cm-2. Even further increasing the power density to 9030 µW cm-2, the energy density is surprisingly kept at 16 µWh cm-2. This result is superior to the most reported fiber devices, for instance, the [email protected]@MnO2//CNT








NiO/Ni(OH)2/PEDOT//AC (11µWh cm-2 and 7800 µW cm-2)[49], NiCo2O4//AC (9.46 µWh cm-2 and 2041 µW cm-2)[12], MnO2//CNT (5.4 µWh cm-2 and 2531 µW

cm-2)[50], and Ni(OH)2//AC (10 µWh cm-2 and 7300 µW cm-2)[51]. Ultrahigh recyclability of 95.1% of the ASC was realized as shown in Figure S11, demonstrating its long-term stability.

Figure 5. Electrochemical performance of the solid assembled ASC. (a) Schematic illustration of the assembled ASC. (b) CV and (c) GCD curves of the ASC. (d) Areal capacitance and volume capacitance of the ASC as a function of various current density. (e) Ragone plots of the constructed device and other reported related materials. (f) CV curves of the ASC when subjected various bending deformation. (g) CV curves of two ASCs connected in parallel. (h) GCD curves of two ASCs connected in series. (i) The assembled device can successfully power a digital watch.

Additionally, for the fiber supercapacitor, flexibility is another imperative factor when subjected to external mechanical deformation without any fading behavior. As shown in Figure 5f, the ASC device remains its original CV shape without any obvious loss during the bending operation. Figure 5g and h show the output currents and working

potential are doubled without obvious change when two ASC was connected in parallel and series, respectively. Finally, two ASCs were connected in series to test its practical application ability. They can successfully power a digital watch, demonstrating its great potential in practical application (Figure 5i).

Conclusions In summary, typical nickel fiber with oxidation operation was introduced as the scaffold to deposit the active materials (MnCo-LDH). The NiO buffer layer not only bridged the MnCo-LDH with nickel substrate but also supplied extra capacitance. Therefore, the composite [email protected] exhibited high areal capacity of 165.6 mC cm-2/368.1 mF cm-2 with superior rate capability (85% capacitance retention from 0.5 to 20 mA cm-2). The assembled device showed the high energy of 19.8 µWh cm-2 (at a power density of 380 µW cm-2) enough to successfully power the electronic devices. This strategy guides advanced research on constructing fiber supercapacitor with high energy delivery for practical application.

Supplementary data XRD, SEM-EDX and other electrochemical performance can be found in the supporting information.

Conflict of interest The authors report no conflict of interest.

Author Contributions Libo Gao, Rong Fan and Ran Xiao contributed equally to this work. Yang Lu supervised the project. Libo Gao designed and fabricated samples, conducted experiments, analyzed data, and wrote the manuscript. Ke Cao performed the TEM experiments. All authors have approved the better final version of the paper.

Acknowledgements The authors wish to thank the Research Grants Council of the Hong Kong Special

Administrative Region of China (GRF No. CityU 11216515), City University of Hong Kong (Project Nos. 6000604 and 9667153), and the Shenzhen Basic Research Grant (No. JCYJ20160401100358589) and the Natural Science Basic Research Plan in Shanxi Province of China (grant no. 2017JM5003).

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Highlights 

NiO buffer layer was introduced via a simple route.

MnCo-LDH with high reversibility in electrochemical performance was employed.

The assembled fiber supercapacitor achieved high energy density of 0.0198 mWh cm-2.

The flexible device holds great promising in application of wearable electronic device.

Graphical abstract