YANG Yuanyuan, ZHU Pengli*, ZHANG Leicong, ZHOU Fengrui, LI Tingxi, BAI Ruiqin, SUN Rong*

(1. Shenzhen Institute of Advanced Electronic Materials, Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen 518055, Guangdong, China; 2. School of Materials Science and Engineering, Shandong University of Science and Technology, Qingdao 266590, Shandong, China)

Abstract: An elastic supercapacitor electrode material with Ni(OH)2 electrochemically active material modified melamine foam (MF) compressible framework was prepared by electroless plating and electrochemical plating. The elastic electrode of MF/Ni(OH)2 exhibits optimized capacitive behavior, such as cycling stability (the elastic electrode displays a capacitance retention of 90.63% even after 2 000 GCD cycles at 40 mA/cm-3) and compressibility (97.88% capacitance retention at 50% compression). The layered compressible supercapacitor consists of a MF/Ni(OH)2 elastic material as anode, a Ni/carbon (Ni/C)cathode, and a filter paper commonly used in laboratory acting as a diaphragm material. Such a supercapacitor device exhibits good electrochemical performance and superior compression stability under different compressions. Finally, the compressible supercapacitor was used to light a LED lamp under different compression conditions to demonstrate its application in flexible electronic device. These optimized electrochemical and mechanical properties indicate the availability of MF/Ni(OH)2 as candidate electrode in application of compressible supercapacitors.

Keywords: melamine foam; MF/Ni(OH)2 ; electroless plating; high compression stability; volumetric capacitance

Supercapacitor has received tremendous attentions in flexible[1], elastic[2]and wearable electronic devices for its great power density[3], high energy density[4-6], quick charge/discharge rate[7-8]and long service life[9], the rapid development of various portable and wearable electronic devices has stimulated the expeditious design and preparation of particular supercapacitor with optimized electrochemical and mechanical performance. Especially, the compressible supercapacitor has ability to maintain a stable electrochemical performance at different compressions and restore its initial shape after unloading, receiving widespread attentions in the field of flexible electronics. However, a successful compressible supercapacitor with stable electrochemical performance and enhanced mechanical performance mainly depends on suitable construction and design of innovational compressible electrode which exhibits excellent compression stability and enhanced capacitive behavior. Therefore, it is essential to design and prepare a rational compressible electrode which can avoid mechanical structural collapse and electrochemical performance degradation during different compressive conditions upon loading/unloading[10]. Intensively being investigated compressible electrode materials with excellent electrochemical performances[11-12]and mechanical behaviors[13-15]can be classified in several types according to the material used as their compressible skeletons: 1) Carbon nanotubes (CNTs) foam-basedcompressible electrode. CNTs foam is considered as an excellent compressible supercapacitor electrode material for its appropriate electrical conductivity[16], great surface area[11], exceptional mechanical properties and light weight[17], which can be used as a self-supporting compressible supercapacitor electrode material. The CNTs foam-based binary, ternary and laminate assembled composite compressible electrode materials were successively prepared[18-26]. 2) Graphene aerogel (GA)-based compressible electrode. GA is a light weight material with high porosity and excellent mechanical strength and flexibility[27-29]. GA can endure a compression strain as high as 90% without obvious irreversible physical deformation due to its three-dimensional interconnected structure[30]. However, the specific capacitance of GA is too low to satisfy the demands for practical applications. Thus, an efficient method for improving the specific capacitance of GA is to combine the GA with pseudocapacitive materials (such as polypyrrole (PPy)[31], polyaniline (PANI)[32]and other composites[21]) for preparing a hybrid aerogel electrode. 3) Melamine foam (MF) based compressible electrode. Commercial MF is a superior compressive substrate for elastic electrode material for its extremely low density[33], ultrahigh porosity and high elasticity[34]. Besides, the elastic property of commercial MF did not decrease significantly after high-temperature carbonization, and the obtained nitrogen-doped carbonaceous foams can be directly used as elastic electrode materials for supercapacitors[10]. Similar to GA, combining carbonized MF with pseudocapacitive materials is also used to composite elastic electrode for compressible supercapacitor[35]. In addition, researcher proposed a smart etching and catalytic method using KMnO4to prepare highly carbonized and etched nitrogen-doped carbon foam (ENCF)[14]. Moreover, MF was used as the carrier to integrate the electrode material due to its compressibility and porous structure[36].

Commercial MF has been used in many fields due to its light flame retardant, high temperature resistance, aging resistance[37], low cost, good chemical stability, hygienic non-toxicity, and good processing and molding[38]. Due to its outstanding 3D interconnected architecture, commercial MF can be used as an elastic skeleton for compressible electrode in supercapacitor. However, the non-conductivity of MF is the biggest obstacle to its application in compressible energy storage devices. Transition metal oxides and hydroxides (such as NiO, Ni(OH)2and Co(OH)2) have been widely used in electrode materials for batteries and supercapacitors due to their excellent electrochemical performances and low cost[39-40].The compressible supercapacitor based on Ni(OH)2possesses high theoretical specific capacitance, low-cost, good environmental compatibility and high rate performance[41]. The electrode materials based on transition metal oxides and hydroxides can be fabricated by many different methods, such as chemical bath deposition (CBD)[42], electrochemical deposition[41,43], self-template protocol[44]and dip-coating method[45]. Among them, electrochemical deposition method is appealing due to its high efficiency and low-cost. In this work, a MF/Ni(OH)2elastic material prepared by electrochemical deposition and electroless plating was investigated. The MF/Ni(OH)2elastic electrode was prepared via two steps: 1) Coating a uniform layer of Ni metal particles on the commercial MF fiber surface by chemical deposition to enable its conductivity, and 2) the Ni(OH)2electrochemically active material was deposited on MF/Ni conductive material surface. In addition, a layered compressible supercapacitor consists of acquired elastic anode and Ni/C cathode material was assembled, which are separated by filter paper, and the entire device is filled with PVA-KOH gel electrolyte, assess the reliability of MF/Ni(OH)2elastic material as an electrode in application of compressible supercapacitor.

1 Experimental

1.1 Materials

The commercial MF is provided by Outlook Company (Chengdu). NiSO4·6H2O and polyvinyl alcohol (PVA) were received from Sinopharm Chemical Reagent Co., Ltd. Tin dichloride (SnCl2), palladium dichloride (PdCl2), citric acid monohydrate (C6H8O7·H2O) and N-methyl pyrrolidone(NMP) solvent were received from Aladdin Industrial Co., Ltd.

1.2 Preparation of MF/Ni conductive material

Commercial MF was cut into appropriate size and put into the 0.1 mol/L HCl solution for half an hour to improve its hydrophilicity, and then acquired MF was carefully cleaned by deionized (DI) water until neutral. Afterwards, the treated MF was soaked into 50 mL sensitizing solution consists of 0.05 mol/L SnCl2and 0.12 mol/L HCl solution for half an hour and cleaned by DI water. Next, the received MF was soaked into the activation bath (containing 0.03 mol/L HCl and 100 mg/L PdCl2) for half an hour and cleaned carefully by DI water. Finally, we soaked the treated MF into an electroless plating bath(including 0.10 mol/L C6H8O7·H2O and 0.05 mol/L NiSO4·6H2O), and keep the solution pH=10 by adding ammonia water simultaneously, and then the reducing agent of 0.10 mol/L NaH2PO2·H2O solution was slowly dropped into above mentioned electroless plating bath. Finally, the obtained mixed solution containing treated MF was put into the 70 ℃ water bath for 1 h. After electroless plating, the acquired foam was completely cleaned using DI water and then placed into a 60 ℃ oven for 12 h[46-48], acquiring the sample of MF/Ni.

1.3 Manufacture of MF/Ni(OH)2 elastic material

For preparing MF/Ni(OH)2elastic material by electrochemical plating, we set a three-electrode system in electrochemical workstation (Zennium, Zahner, Germany), containing a working electrode of acquired MF/Ni elastic material, a counter electrode of platinum sheet and a reference electrode of saturated Hg/HgCl. The electrochemical deposition was conducted by potential dynamic method in 1 mol/L KOH electrolyte, via potential cycling of MF/Ni conductive material with a potential window ranging from -0.1 to 0.6 V at 25 mV/s for 70 cycles. After electrochemical deposition, the sample was carefully washed using DI water and put into a 60 ℃ oven for 12 h to acquire MF/Ni(OH)2elastic material.

1.4 Assembly of layered asymmetrical compressible supercapacitor

Carbon powder and polyvinylidenedifluoride (PVDF) with a mass ratio of 9∶1 were dispersed in the NMP to form the slurry, and then the received slurry was attached on commercial Ni foam surface (area: 1.5 cm×1 cm) to prepare the anode material for compressible device, and the obtained anode material was put into a 60 ℃ oven for 3 h to evaporate excess solvent. Then, 1.200 g PVA and 1.122 g KOH were added into a beaker containing 20 mL DI water and then the obtained solution was heated to 90 ℃ under magnetic stirring for 6 h to get the gel electrolyte. After that, the above received MF/Ni(OH)2, Ni/C and diaphragm are put into PVA-KOH gel electrolyte for 12 h. At last, the MF/Ni(OH)2as well as other obtained components are assembled to a compressible supercapacitor with layered structure and packaged by PI film for electrochemical performance testing.

1.5 Material characterization and electrochemistry measurement

The morphologies of commercial MF, MF/Ni conductive material and MF/Ni(OH)2elastic material were investigated by Field Emission Scanning Electron Microscopy (FESEM, Nova NanoSEM 450). X-ray photoelectron spectroscopy (XPS)(PHI 5800) was tested to research electronic structure and chemistry composition of MF/Ni(OH)2elastic material. The electrochemical performance of MF/Ni(OH)2elastic material was investigated by three-electrode system in 6 mol/L KOH solutionin electrochemical station (Zennium, Zahner, Germany). The CV curves of elastic electrode material were measured with potential window from -0.2 to 0.7 V at various scan rate of 10-100 mV/s, and its GCD curves were performed with potential window from -0.2 to 0.7 V at various current density of 10-60 mA/cm3. Besides, the electrochemical performances of MF/Ni(OH)2elastic electrode material at different compressions were also performed to evaluate its compression stability. Finally, we investigated electrochemical impedance spectroscopy(EIS) of MF/Ni(OH)2elastic electrode with ac perturbation of 5 mV in frequency range from 100 mHz to 100 kHz.

2 Results and discussion

Fig.1 shows the schematic illustration for preparing MF/Ni(OH)2elastic material, the preparation process mainly includes the following steps: 1) The commercial MF with 3D interconnected structure was utilized as compressible skeleton for attaching metal Ni particles via electroless plating. The formation of MF/Ni conductive material can be described as follows: the surface hydrophilicity of MF was firstly improved by HCl solution for better absorption of Sn2+on MF fiber surface in sensitization process, the Pd particles with catalytically active were attached on MF surface via reaction between Sn2+and Pd2+in activation process, the Ni2+ions could be reduced into metal Ni particles in electroless plating bath containing reducing agent, and these Ni metal particles act as seed crystals to accelerate formation of a continuous and uniform metal Ni particles layer, and electroless nickel process could be finished until Ni2+ions and reducing agent are completely consumed.2)Afterwards, a thin Ni(OH)2layer act as electrochemical active material was deposited on MF/Ni conductive material surface through potential dynamic to obtain a MF/Ni(OH)2elastic electrode. A layered compressible device was assembled to assess the applicability and dependability of MF/Ni(OH)2elastic material as compressible supercapacitor electrode, which utilizing Ni/C and MF/Ni(OH)2elastic material as cathode and anode, respectively, and the cathode and anode were separated by filter paper and the entire supercapacitor was filled with PVA-KOH gel electrolyte. In addition, the photos of MF/Ni(OH)2material revealing its elastic performance are shown in Fig.1, the MF/Ni(OH)2elastic material can keep its original shape and size of commercial MF under and remove the load.

Fig.1 Schematic illustration for fabrication of MF/Ni(OH)2 elastic electrode and assembly of compressible supercapacitor, photos of MF/Ni(OH)2 electrode revealing its compressible performance (inside)

Fig.2a, b and c depict the SEM images of commercial MF, MF/Ni conductive material and MF/Ni(OH)2the tightly stacked rod-shape attachments, which are supposed to be Ni(OH)2electrochemically active material as shown in Fig.2f. The energy-dispersive X-ray (EDX) spectrum and elemental mapping of MF/Ni(OH)2in a specific area were performed to explore the spatial distribution of Ni(OH)2in the 3D MF/Ni(OH)2elastic material. The elements such as C, N, O and Ni are presence in MF/Ni(OH)2elastic material as shown in Fig.2g. Fig.2i-l display the element Ni(OH)2elastic material, respectively. It can be clearly seen that the 3D interconnected architecture composed of numerous dendritic fibers with several micrometers diameters of commercial MF can be completely maintained after electroless plating and electrochemical deposition. Fig.2d, e and f display the higher magnification SEM images of individual fiber of MF, MF/Ni and MF/Ni(OH)2. For MF/Ni, apparently, the original 3D interconnected network architecture of MF is completely maintained (Fig.2b), whereas the surface becomes rougher compared to pure MF as shown in Fig.2e. Fig.2c depicts the morphology of MF/Ni(OH)2elastic material fibers with increased diameters compared with MF and MF/Ni. The surface roughness of MF/Ni(OH)2is further increased due to mapping corresponding to the region in Fig.2h, which further indicate the distribution of elements C, N, O and Ni in MF/Ni(OH)2elastic material.

Fig.2 SEM images of (a) MF, (b) MF/Ni and (c) MF/Ni(OH)2 , Enlarged SEM images of fiber in (d) MF, (e) MF/Ni and (f) MF/Ni(OH)2 , (g) EDX, (h)-(l) elemental mapping of MF/Ni(OH)2 electrode material

Fig.3a depicts the XPS results of MF/Ni(OH)2elastic material. The several characteristic peaks of carbon (C 1s), oxygen (O 1s) and nickel (Ni 2p) can be observed, the characteristic peak at 284.86 eV is corresponding to C 1s. Fig.3b depicts the detailed analysis of Ni 2p, which can be divided into four obviously peaks, and two comfortable singles at 855.74 and 873.44 eV can attribute to Ni 2p3/2and Ni 2p1/2, respectively, with a spin-energy separation of 17.70 eV, corresponding to the characteristic peak of Ni(OH)2phase, which is in agreement with previous literature reports[49-50]. Two peaks at 861.55 and 879.45 eV are attributed to the satellite signals of Ni 2p3/2and Ni 2p1/2, respectively[51]. The higher-solution O 1s spectrum is presents in Fig.3c, a single peak located at 531.30 eV is corresponding to the feature of -OH[52]. These XPS results demonstrate an identity that the Ni(OH)2electrochemically active material was generated on MF/Ni material surface.

Fig.3 (a) XPS spectrum of MF/Ni(OH)2 , (b) high resolution XPS survey of Ni 2p in MF/Ni(OH)2 , (c) the high-resolution XPS peaks of O 1s of the MF/Ni(OH)2

Fig.4a presents the CV curves of MF/Ni(OH)2elastic material with a potential ranging from -0.2 to 0.7 V at various scan rates. The redox peaks in CV curve demonstrate that the capacitance behaviors of MF/Ni(OH)2elastic material are governed by faradaic reactions, and the anode peak at 0.48 V might owe to the redox reaction that Ni(OH)2is oxidized to NiOOH in KOH electrolyte, and the cathode peak at 0.35 V could be ascribed to the inverse reaction of the above oxidation process that NiOOH is reduced to Ni(OH)2[53].The redox peaks in MF/Ni(OH)2elastic material reveal the Faradaic behavior between Ni2+and Ni3+, which can be described as following redox reaction[54]:

Ni(OH)2+OH-?NiOOH+H2O+e-

(1)

The peak current and integral area of MF/Ni(OH)2gradually increase as the scan rate increases. Whereas the shape of CV curve had no obviously change with the scan rate increases from 5 to 80 mV/s, which indicates the highly reversible electrochemical reaction and excellent rate performance of MF/Ni(OH)2elastic material. Fig.4b shows volume specific capacitances of MF/Ni(OH)2elastic material calculated from its CV curvesusing following equation:

(2)

Whereiis the current in CV curve,Vis the operating potential,vrepresents scan rate,Veindicates geometry volume of elastic material, and ΔV is potential window in CV curve of MF/Ni(OH)2elastic material. It indicates that the volume capacitances of MF/Ni(OH)2elastic material gradually decrease from 1.08 to 0.48 F/cm3as the scan rate increases. Fig.4c displays GCD curves of MF/Ni(OH)2elastic material at various current densities. The symmetrical GCD curve shape of MF/Ni(OH)2elastic material reveals the highly reversible electrochemical reaction during charge and discharge. The discharge time of MF/Ni(OH)2elastic material gradually decreases as current density increases, and which exhibits a discharge time retention of 13 s even at 60 mA/cm3. Based on the GCD curve of MF/Ni(OH)2elastic material, the volume specific capacitances at various current densities were calculated according to the following equation:

(3)

WhereIis the peak current in discharge curve, ΔVrepresents the potential range in discharge curve, Δt express the discharge time in GCD curve andVis geometry volume of MF/Ni(OH)2elastic material. The MF/Ni(OH)2elastic material exhibits a maximum volumetric capacitance of 5.422 F/cm3at 10 mA/cm3, and the volumetric capacitances gradually decrease with current density increases from 10 to 60 mA/cm3according to the calculated results shown in Fig.4d.

Fig.4 (a) CV curves of MF/Ni(OH)2 , (b) volume capacitances versus scan rates, (c) GCD curves of MF/Ni(OH)2 , (d) volume capacitances versus current densities

Fig.5a presents the electrochemical impedance spectroscopy (EIS) of MF/Ni(OH)2elastic material in the frequency from 100 mHz to 100 kHz with an ac perturbation of 5 mV. It is usually accepted by the public is that the intercept in real axis at high frequency indicates the contact resistance between electrochemically active material and current collector in MF/Ni(OH)2elastic material and the electrolyte resistance in testing. The semicircle (depressed arcs) at high frequency is corresponding to the charge transfer resistances of MF/Ni(OH)2elastic material, which is corresponding to the rate performance of MF/Ni(OH)2elastic electrode material. In addition, the slope of straight line at low frequency is attributed to ion diffusion resistance of MF/Ni(OH)2elastic material[55-56].The equivalent series resistance (ESR) of MF/Ni(OH)2elastic material is calculated to be 2.934 Ω according to thex-axis intercept at high frequency. The equivalent charge transfer resistance of MF/Ni(OH)2elastic material is 0.550 Ω calculated from the semicircle at high frequency in curve fitting of EIS[57-58]. MF/Ni(OH)2elastic material displays diffusion-controlled process for charge storage in the straight line at low frequency. Furthermore, the Nyquist plots of MF/Ni(OH)2elastic material at different compressions were tested to investigate its compressibility. The EIS curves of MF/Ni(OH)2electrode materials almost coincide in the high frequency region at different compressions.

Fig.5 (a)Nyquist plot of MF/Ni(OH)2 material, (b) Nyquist plot of MF/Ni(OH)2 at different compressions

We evaluated the cycling stability of MF/Ni(OH)2elastic material through 2 000 GCD cycles at 40 mA/cm3. Fig.6a shows GCD curves of MF/Ni(OH)2elastic electrode, the discharge time reduced from 33 to 30 s after 2 000 cycles, and the volume capacitance only reduced from 1.733 to 1.570 F/cm3, which exhibits a high retention (90.63%) of volume capacitance after 2 000 cycles. Fig.6b and 6c display the SEM images of MF/Ni(OH)2elastic material after 2 000 GCD cycles, which maintains the 3D interconnected structure of MF/Ni(OH)2electrode material even after 2 000 GCD cycles. The surface roughness of MF/Ni(OH)2electrode after 2 000 GCD cycles increased compared with the original MF/Ni(OH)2material, and some nanoparticles with a diameter of about 291 nm are attached on fiber surface, which may be the NiOOH formed during the electrochemical testing.

Fig.6 (a) The cycling stability of the MF/Ni(OH)2 electrode using the galvanostatic charge/discharge at a current density of 40 mA/cm3, (b) and (c) SEM images of MF/Ni(OH)2 after 2 000 GCD cycles

In order to assess the compressive performance of MF/Ni(OH)2elastic material, its electrochemical properties at different compressions were measured. Fig.7a presents the CV curves of MF/Ni(OH)2elastic material under various compression sat 50 mV/s. The shape and integral area of CV curve did not obviously change under different compressions as shown depicted in Fig.7a. Fig.7b depicts the GCD curves of MF/Ni(OH)2elastic material under different compressions at 50 mA/cm3. The discharge time of MF/Ni(OH)2elastic material did not obviously decline as compression increases, illustrating the compressive stability of MF/Ni(OH)2. Based on the CV and GCD curves of MF/Ni(OH)2elastic material at various compressions, which shows a volume capacitance retention of 97.88% (Fig.6c) at 50% compression according to the volume capacitances calculated from Eq.(2) (Fig.7c) and (3) (Fig.7d). MF/Ni(OH)2electrode material illustrates a superior compression stability, again indicating that the MF/Ni(OH)2is an acceptable electrode material in application of compressible supercapacitors.

Since the superior electrochemical and mechanical properties of MF/Ni(OH)2elastic material, a simple sandwich structured compressible supercapacitor was assembled using MF/Ni(OH)2and Ni/C as anode and cathode, respectively, the filter paper used as a diaphragm for separating anode and cathode, and the entire device was filled with PVA-KOH gel electrolyte. The assemble device is used to evaluate the reliability and applicability of MF/Ni(OH)2material as compressible electrode in supercapacitor after packaged by polyimide (PI) film. The CV curves of Ni/C electrode material at different scan rate were tested to investigate its capacitance (Fig.8). The CV curves of device with a potential window of 1.5 V at various scan rates are shown in Fig.9a, the redox peaks observed in picture are related to the oxidation and reduction reactions described above. There is no significant change in shapes of CV curves in device, and its integral area is slightly increased with increases of scan rate. Fig.9b depicts the GCD curves of assembled device at various current densities, the discharge time of the device slightly decreases as current density increases, indicating the superior capacitive behavior of device containing MF/Ni(OH)2anode.

The Nyquist plot of compressible supercapacitor containing MF/Ni(OH)2anode at the frequency ranging from 100 mHz to 100 kHz with an ac perturbation of 5 mV was tested for further illustrating its electrochemical performance. The equivalent series resistance (ESR) of compressible supercapacitor containing MF/Ni(OH)2anode is 1.471 Ω calculated fromx-axis intercept at high frequency of Nyquist plot showing in Fig.10a. Furthermore, the cycling stability of compressible supercapacitor containing MF/Ni(OH)2anode was investigated by 2 000 GCD cycles at 7 mA/cm3. Fig.10b presents the GCD curve of compressible supercapacitor before and after 2 000 GCD cycles, the discharge time reduced from 20 to 18 s after 2 000 GCD cycles, illustrating a volume capacitance retention of 90% even after 2 000 GCD cycles.

Fig.7 (a) CV and (b) GCD curves of MF/Ni(OH)2 material at different compressions, volumetric capacitances of MF/Ni(OH)2 material calculated from (c) Eq. (2) and (d) Eq. (3) based on its CV and GCD curves as a function of strain

Fig.8 (a) CV curves of Ni/C electrode material at different scan rate, (b) capacitance of Ni/C electrode

The electrochemical properties of supercapacitor containing MF/Ni(OH)2anode under various compressions were tested for investigating its compression stability. Fig.11a presents the CV curves of supercapacitor under various compressions at 50 mV/s, the shapes of CV curves have no obvious change under various compressions, and the volumetric capacitances of device can maintain 104.85% even under 50% strain (Fig.11c). The GCD curves of device under various compressions were tested to further investigate its compression stability. Fig.11b expresses the GCD curves of device under various compressions at 6 mA/cm3, the discharge time and shape of device did not show significant changes even under different compressions, and exhibited the volumetric capacitances retention of 105.00% at 50% strain (Fig.11d). These optimized electrochemical properties of device indicate that the MF/Ni(OH)2is a suitable electrode material in application of compressible supercapacitors. In addition, the volumetric capacitances of supercapacitor containing MF/Ni(OH)2anode after different compression times was calculated using Eq. (1) and CV curves to assessits compressive performance. Fig.11e presents the volumetric capacitances of the device with the increase of compression times, the volumetric capacitances of the device remains at 88.47% even after 1 000 compression cycles, indicating the excellent compression stability of supercapacitor containing MF/Ni(OH)2anode. Fig.11f and g show the photos of the supercapacitors and the LED light powered by the device. The LED lamp can be lighted by two supercapacitors in series (Fig.11f), and the brightness of LED lamp did not change significantly even the 100 g weight was loaded on each device (Fig.11g). These appropriate results illustrate that the MF/Ni(OH)2is a suitable candidate in application of compressible supercapacitor electrode material.

Fig.11 (a) CV and (b) GCD curves of supercapacitor at different compressions, the volume capacitance calculated from (c) Eq. (1) and (d) Eq. (2) versus strain, (e) the volume capacitance versus compression times, (f) the photo of LED lamp lighted by supercapacitor

3 Conclusions

In summary, we successfully prepared an elastic material for layered compressible supercapacitor by electroless plating and electrochemical plating. The electrochemical property of MF/Ni(OH)2elastic material was investigated, its volume capacitance can achieve 5.422 F/cm3at 10 mA/cm3, and the compressibility of MF/Ni(OH)2elastic material was performed in the same testing system, the CV and GCD curves have no significant change at various compressions. These beneficial results illustrate that MF/Ni(OH)2elastic material is an acceptable electrode in application of compressible supercapacitor. Furthermore, we assembled a layered compressible supercapacitor containing MF/Ni(OH)2elastic material and Ni/C as anode and cathode, respectively, and tested the electrochemical properties of device under different scan rates, current densities and compression conditions. These results indicate that the excellent electrochemical performance, cyclic stability, compression stability and rate property of supercapacitor containing MF/Ni(OH)2anode. Therefore, we may offer an interesting strategy to devise and prepare elastic electrode materials for compressible supercapacitorsin application of new electronic devices.