简介概要

Self-etching Ni-Co hydroxides@Ni-Cu nanowire arrays with enhancing ultrahigh areal capacitance for flexible thin-film supercapacitors

来源期刊：Rare Metals2017年第9期

论文作者：Peng Guo Yang Shen Yu Song Jing Ma Yuan-Hua Lin Ce-Wen Nan

文章页码：691 - 697

摘要：Flexible thin-film supercapacitors with high specific capacitance are highly desirable for modern wearable or micro-sized electrical and electronic applications. In this contribution, Ni-Co hydroxides（NCH）nanosheets were deposited on top of Ni-Cu alloy（NCA）nanowire arrays forming a freestanding thin-film composite electrode with hierarchical structure for supercapacitors.During electrochemical cycling, the dissolution of Cu into Cu ions will create more active sites on NCA, and the redeposited copper oxide can be coated onto NCH, giving rise to substantial increase in specific capacitance with cycling. Meanwhile, NCA and NCH have excellent conductivity, thus leading to excellent rate performance. This flexible thin-film electrode delivers an ultrahigh initial specific capacitance of 0.63 F·cm^-2(or 781.3 F·cm^-3).During charge-discharge cycles, the specific capacitance can increase up to 1.18 F·cm^-2(or 1475 F·cm^-3) along with the"self-etching"process. The electrode presents a better specific capacitance and rate capability compared with previously reported flexible thin-film electrode, and this novel design of etching technique may expand to other binary or ternary materials.

详情信息展示

稀有金属(英文版) 2017,36(09),691-697

Self-etching Ni-Co hydroxides@Ni-Cu nanowire arrays with enhancing ultrahigh areal capacitance for flexible thin-film supercapacitors

Peng Guo Yang Shen Yu Song Jing Ma Yuan-Hua Lin Ce-Wen Nan

State Key Laboratory of New Ceramics and Fine Processing,School of Materials Science and Engineering, Tsinghua University

Laboratory of Advanced Energy Storage Materials and Devices,Research Institute of Tsinghua University in Shenzhen

收稿日期：10 May 2016

基金：financially supported by the National Basic Research Program of China (No. 2015CB654603);the National Natural Science Foundation of China (No.51572141, 51532003);Beijing Nova Program (No. XX2013037);the Research fund of Science and Technology in Shenzhen (No. JSGG20150331155519130);

Self-etching Ni-Co hydroxides@Ni-Cu nanowire arrays with enhancing ultrahigh areal capacitance for flexible thin-film supercapacitors

Peng Guo Yang Shen Yu Song Jing Ma Yuan-Hua Lin Ce-Wen Nan

State Key Laboratory of New Ceramics and Fine Processing,School of Materials Science and Engineering, Tsinghua University

Laboratory of Advanced Energy Storage Materials and Devices,Research Institute of Tsinghua University in Shenzhen

Abstract：

Flexible thin-film supercapacitors with high specific capacitance are highly desirable for modern wearable or micro-sized electrical and electronic applications. In this contribution, Ni-Co hydroxides(NCH)nanosheets were deposited on top of Ni-Cu alloy(NCA)nanowire arrays forming a freestanding thin-film composite electrode with hierarchical structure for supercapacitors.During electrochemical cycling, the dissolution of Cu into Cu ions will create more active sites on NCA, and the redeposited copper oxide can be coated onto NCH, giving rise to substantial increase in specific capacitance with cycling. Meanwhile, NCA and NCH have excellent conductivity, thus leading to excellent rate performance. This flexible thin-film electrode delivers an ultrahigh initial specific capacitance of 0.63 F·cm^-2(or 781.3 F·cm^-3).During charge-discharge cycles, the specific capacitance can increase up to 1.18 F·cm^-2(or 1475 F·cm^-3) along with the“self-etching”process. The electrode presents a better specific capacitance and rate capability compared with previously reported flexible thin-film electrode, and this novel design of etching technique may expand to other binary or ternary materials.

Keyword：

Nickel; Cobalt; Copper; Hydroxides; Supercapacitors;

Author： Yang Shen,e-mail:shyang_mse@tsinghua.edu.cn;

Received： 10 May 2016

1 Introduction

For the past decades,supercapacitors (SCs) have received great attention due to their large power density,short charging time and long cycle life [ 1] .Right now most commercial SCs are electric double-layered capacitors(EDLCs),made of carbon materials,but the low energy density limits its application.Pseudocapacitor,which delivers much higher energy density is therefore more promising for applications where high energy density is desirable [ 2] .The rapid development of wearable and micro-sized electronic devices also imposes increasing demands for flexible thin-film energy storage devices with high energy density [ 3, 4, 5] .

The flexibility of thin-film electrode relies on ultrathin substrates or freestanding structures.Though electrodes based on nickel foam or carbon paper exhibit high areal capacitance [ 6, 7] ,the substrates are not thin enough and the space inside pores (diameters～300μm) is not fully utilized,which limits the thickness and volume capacitance of the electrodes.Some efforts have been made in using ultrathin freestanding graphene and metal foil as thin-film electrodes [ 8, 9] ,yet the specific capacitance is not satisfying.Among freestanding structures explored so far,metal nanowire arrays are considered as one of the most promising substrates for thin-film supercapacitors for their large surface area,superior conductivity and excellent flexibility [ 10] .The final electrochemical performances of SCs will mainly depend on the active materials grown on them.

In recent years,Ni-Co hydroxides (NCH) have been widely investigated due to its low cost,abundance,none pollution,and high theoretical specific capacitance.Tremendous efforts have been made to improve their electrochemical performance,e.g.,building hierarchical structures [ 11, 12] ,changing morphologies into nanorods [ 13] ,nanospheres [ 14] ,nanosheets [ 15] ,nanobelts [ 16] ,or doping with Fe [ 17] ,Al [ 18] ,Cu [ 19] .According to the research by Lien et al. [ 19] ,introducing copper oxide can enhance the performance of NCH,though copper oxide is not electroactive in the working potential of NCH,which inspired us to design a special electrode to further improve the application of the enhancing mechanism in flexible thin-film electrode.Herein,a“self-etching technique”was employed to enhance the performance of SCs based on NCH and Ni-Cu alloy (NCA) nano wire arrays.This novel design of self-etching technique can be expected to enhance the performance of supercapacitors based on other binary or ternary materials.

2 Experimental

2.1 Synthesis of NCH@NCA nano wire arrays

For the deposition of nanowire arrays,anode aluminum oxide (AAO) membranes were used as templates.The AAO templates were prepared by a conventional two-step anodic oxidation process [ 20] .First,one side of the membrane was coated with Ni layer by vacuum evaporation,which acted as an electrode for electroplating NCA nanowires.Then,the electroplating was carried out in a two-electrode cell under constant current (10 mA·cm^-2)with a nickel plate anode at 55℃for 90 min.The plating solution consisted of 0.38 mol·L^-1 NiSO₄·6H₂O,0.04 mol·L^-1 CuSO₄·5H₂O,0.57 mol·L^-1 sodium citrate,0.50 mol·L^-1 H₃BO₃,0.10 mol·L^-1 NaCl in deionized(DI) water,and its pH was adjusted to 4.5 by adding sulfuric acid.Next,the sample was annealed in a vacuum oven at 160℃for 2 h and then was soaked in a 2 mol·L^-1NaOH to remove the AAO template.The NCA nano wire array was obtained followed by rinsing with DI water for several times.After that,the backside of the electrode was encapsulated with epoxy glue to eliminate the interference of the backside for electrochemical measurements.Finally,NCH were deposited on the array in a three-electrode system under constant voltage of-1.0 V versus a saturated calomel electrode (SCE) for 900 s.The plating solution consisted of 4 mmol·L^-1 Co(NO₃)₂·6H₂O,2 mmol·L^-1Ni(NO₃)₂·6H₂O in DI water.The composite electrode thus deposited was then thoroughly rinsed with DI water.

2.2 Microstructural characterization

The morphology of the samples was observed by scanning electron microscope (SEM,ZEISS MERLIN) and transmission electron microscope (TEM,JEOL2011).The phase composition and the surface chemical states were determined by X-ray diffractometer (XRD,Bruker D8) and X-ray photoelectron spectroscope (XPS,PerkinElmer).

2.3 Electrochemical measurements

All the electrochemical synthesis or measurements were conducted using an electrochemical workstation (CHI660E,Shanghai,China).The electrochemical measurements of single electrodes were performed in a three-electrode system in 6 mol·L^-1 KOH aqueous solution,with NCH@NCA as the working electrode,and a Pt flake and a saturated calomel electrode (SCE) as the counter and reference electrodes,respectively.

3 Results and discussion

Figure 1 illustrates the procedure for the synthesis of composite electrode by a mild electrochemical deposition method and the process of self-etching during cycling.The detailed morphologies of prepared specimens were investigated by SEM,as shown in Fig.2.Figure 2a shows SEMimage of the prepared AAO templates through a traditional two-step anodic oxidization process,exhibiting extremely ordered nano-channels with diameters of 55-75 nm.A nickel layer was deposited on one side of the AAO template by vacuum evaporation to serve as the seed layer for the subsequent electroplating,and then NCA nano wires were deposited along the channel.After removing AAO template,an array of NCA nano wires is obtained,as shown in Fig.2b.Figure 2c,d shows SEM images of the ultrathin nanosheets of NCH covered on top of the NCA array.Within the NCA array,the space left unfilled by NCH is favorable for the contact with electrolyte.The electrode has a thickness of only 8μm as shown in Fig.2e,yet obviously providing a large surface area for the growth of active materials.The inset in Fig.2e shows photograph of flexible electrode.Owing to the ultrathin thickness,the freestanding NCA has a good flexibility and could work as flexible substrate as well as current collector.Figure 2f shows SEM image of NCH@NCA after the test of cycling life.The morphology of the active materials is preserved except that the twisted nanosheets of NCH become denser than before,indicating the deposition of copper oxides onto NCH@NCA.

Fig.1 Schematic illustration of fabrication of NCH@NCA and self-etching process

Fig.2 SEM images of samples:a AAO template,b Ni-Cu nano wire arrays,c top view of NCH@NCA,d cross section of NCH@NCA,e side view of NCH@NCA,and f NCH@NCA after cycling

More detailed structure of NCH@NCA after self-etching was investigated by TEM.Figure 3 a shows TEMimage of NCH@NCA after cycling.The trace of etching can be seen on NCA,and it is still entwined with NCH.The high-resolution image is shown in Fig.3b.A layer of Cu₂O and CuO has been coated on the surface of NCH.The lattice fringe spacings of 0.246 and 0.232 nm are in agreement with Cu₂O (JCPDS 74-1230) and CuO (JCPDS48-1548),respectively [ 21, 22] .The analysis of composition is also consistent with the result of selected area electron diffraction (SAED) pattern shown in Fig.3c.A series of diffraction rings corresponding to Cu₂O and CuO are found in SAED pattern without any other diffraction spots or rings,suggesting an amorphous phase for Ni-Co hydroxide.

The phase composition of NCH@NCA was further determined by XRD.As shown in Fig.4,all of the diffraction peaks could be indexed to NCA (JCPDS47-1406) in the patterns before self-etching process,which also suggests that NCH deposited is amorphous.After selfetching process,the diffraction peaks of CuO (JCPDS48-1548) and Cu₂O (JCPDS 74-1230) are observed,indicating that a small amount of Cu in NCA are turned into copper oxide and coated on the surface of NCH@NCA again during self-etching process.

The valence bond information of NCH@NC A after selfetching process is presented in XPS spectra shown in Fig.5.The typical O Is spectrum in Fig.5a can be pided into four peaks,which correspond to metal-oxygen bonds(529.6 eV),metal-OH bonds (531.2,531.7 eV) and absorbed water (533.1 eV) [ 23, 24, 25, 26] ,respectively.Figure 5b shows Ni 2p spectrum,and two main peaks with two obvious satellites (shown as“sat.”) can be assigned to Ni²⁺ [ 24, 25, 27] .Similarly,the Co 2p spectrum confirms the existence of Co²⁺ [ 28, 29] ,and the Cu 2p spectrum reveals the coexistence of Cu¹⁺and Cu²⁺ [ 30, 31] .The small peak area of Cu 2p indicates that a very small amount of copper oxide is generated during self-etching process.

Fig.3 TEM images of NCH@NCA after cycling a,high-resolution image of NCH b and corresponding SAED pattern c

Fig.4 XRD patterns of NCH@NCA before and after self-etching

The electrochemical performance was tested in an alkaline electrolyte with a three-electrode system.Figure 6a shows a series of cyclic voltammetry (CV)curves at different scan rates in the potential of-0.1 to0.4 V.Figure 6b shows the charge-discharge curves at different current densities (2-128 mA·cm^-2) in the potential of-0.1 to 0.4 V,using the chronopotentiometry (CP)technique.The symmetric shape with typical plateau suggests the good electrochemical reversibility and pseudocapacitive behaviors,which also corresponds with the CV curves.

The specific capacitance (C_s) could be calculated as follows:

where I is the constant discharge current (A),Δt is the discharge time (s),ΔV is the width of potential window during discharge (V),and S is the area of the electrode(cm^-2).Figure 6c shows series of specific capacitance under different current densities.The initial C_s is calculated to be 625 mF·cm^-2 (781.3 F·cm^-3 or 2790 F·g^-1)at 2 mA·cm^-2 and still 519 mF·cm^-2 at 128 mA·cm^-2(83%retention),showing an excellent rate capability.During cycling,the self-etching takes place and C_s rises by 70%from the initial value after 5000 cycles and shows no signs of decay,as shown in Fig.6d.The RGO reduced graphene oxide,NWA nanowire arrays,LDH layered double hydroxidedischarge curves of NCH@NCA after self-etching are shown in Fig.6e;thus,the specific capacitance of NCH@NCA after cycling can be calculated,which certifies a retentive good rate capability as shown in Fig.6c.The specific capacitance at 2 mA·cm^-2 increases to1.18 F·cm^-2 after cycling.The rising capacitance during cycling is induced by designed self-etching process.The Cu in the NCA is oxidized into Cu ions thus generating more active sites for NCH,and the re-deposited copper oxides coated on NCH enhance the performance of NCH in return [ 19] ,which occurs repeatedly during cycling process.

Fig.5 XPS results of NCH@NCA after self-etching process:a O 1s,b Ni 2p,c Co 2p,and d Cu 2p

Fig.6 Electrochemical performance of NCH@NCA before and after cycling:a CV curves of NCH@NCA at different scan rates before cycling,b galvanostatic charge-discharge curves at different current densities before cycling,c areal-specific capacitance calculated from galvanostatic charge-discharge GCD curves as a function of current densities,d cycle performance of NCH@NCA and NCH@NA at 32 mA·cm^-2,e CP discharge curves of NCH@NCA after cycling,and f CV curves of NCH@NCA at a scan rate of 5 mV·s^-1

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Table 1 Electrochemical performance for reported flexible thin-film supercapacitors electrode

CV curve of NCH@NCA after cycling is shown in Fig.6f.The peak shift is caused by the introduced copper oxide,which is also accordant with Ref. [ 19] .To further confirm this mechanism,a Ni nanowire array without copper element (NA) was also prepared for contrast.The cycling performance of NCH@NA is shown in Fig.6d and little rise could be seen during cycling,indicating that the enhancement comes mainly from the special etching design of the electrode.In addition,the as-prepared NCH@NCA electrode presents a competitive specific capacitance and rate capability compared with that of previously reported flexible thin-film electrode,as shown in Table 1.Therefore,the special NCH@NCA electrode is a promising candidate for flexible thin-film supercapacitors application,and the etching technique may expand to other binary or ternary materials.

4 Conclusion

In this study,NCH@NCA as a flexible thin-film electrode with hierarchical structure for supercapacitors was fabricated and a novel self-etching design was used to enhance the performance of the thin-film electrode.This flexible thin-film electrode delivers an ultrahigh initial specific capacitance of 0.63 F·cm^-2 (or 781.3 F·cm^-3) and excellent rate capability.Experimental results show that,during charge-discharge cycles,Cu will dissolve from NCA and copper oxide will be re-deposited on NCH,which is called a“self-etching”process.This dissolution of Cu will create more active sites,and copper oxide will also enhance the capacitance of NCH.Therefore,the specific capacitance can increase up to 1.18 F·cm^-2 (or 1475 F·cm^-3) along with the“self-etching”process.This novel design of selfetching technique can greatly enhance the areal capacitance of the NCH@NCA electrode during charge-discharge cycles,and this technique may also expand to other binary or ternary materials for supercapacitor applications.

Acknowledgements This study was financially supported by the National Basic Research Program of China (No.2015CB654603),the National Natural Science Foundation of China (No.51572141,51532003),Beijing Nova Program (No.XX2013037) and the Research fund of Science and Technology in Shenzhen (No.JSGG20150331155519130).