Highly Compressible Carbon Sponge Supercapacitor Electrode with

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Energy, Environmental, and Catalysis Applications

Highly Compressible Carbon Sponge Supercapacitor Electrode with Enhanced Performance by Growing Nickel-Cobalt Sulfide Nanosheets Xu Liang, Kaiwen Nie, Xian Ding, Liqin Dang, Jie Sun, Feng Shi, Hua Xu, Ruibin Jiang, Xuexia He, Zong-Huai Liu, and Zhibin Lei ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19043 • Publication Date (Web): 06 Mar 2018 Downloaded from http://pubs.acs.org on March 7, 2018

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ACS Applied Materials & Interfaces

Highly Compressible Carbon Sponge Supercapacitor Electrode with Enhanced Performance by Growing Nickel-Cobalt Sulfide Nanosheets

Xu Liang, Kaiwen Nie, Xian Ding, Liqin Dang, Jie Sun, Feng Shi, Hua Xu, Ruibin Jiang, Xuexia He, Zonghuai Liu, Zhibin Lei*

School of Materials Science and Engineering, Shaanxi Normal University, 620 West Chang’an Street, Xi'an, Shaanxi, 710119, China, Fax: 86-29-81530702; Tel: 86-29-81530810; Email: [email protected];

*Corresponding Author: Prof. Zhibin Lei, School of Materials Science and Engineering, Shaanxi Normal University, 620 West Chang’an Street, Xi'an, Shaanxi, 710119, China. Email: [email protected]; Tel: 86-29-81530810; Fax: 86-29-81530702

ORCID Zhibin Lei: 0000-0002-6537-9889

1

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ABSTRACT The development of compressible supercapacitor highly relies on the innovative design of electrode materials with both superior compression property and high capacitive performance. This work reports a highly compressible supercapacitor electrode which is prepared by growing electroactive NiCo2S4 (NCS) nanosheets on the compressible carbon sponge (CS). The strong adhesion of the metallic conductive NCS nanosheets to the highly porous carbon scaffolds enable the CS-NCS composite electrode to exhibit an enhanced conductivity and ideal structural integrity during repeated compression-release cycles. Accordingly, the CS-NCS composite electrode delivers a specific capacitance of 1093 F g−1 at 0.5 A g−1 and remarkable rate performance with 91% capacitance retention in the range of 0.5−20 A g−1. Capacitance performance under strain of 60% shows that incorporation of NCS nanosheets in CS scaffolds leads to over 5 times enhancement in gravimetric capacitance and 17 times enhancement in volumetric capacitance. These performances enable CS-NCS composite to be one of promising candidates for potential application in compressible electrochemical energy storage devices.

Keywords:

Carbon

sponge,

NiCo2S4

nanosheets,

Pseudocapacitive performance, Supercapacitor.

2


Compressible

electrodes,

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1. INTRODUCTION The demand for various portable and wearable electronic devices has stimulated the rapid development of light-weight and flexible energy storage systems with enhanced electrochemical performance and good mechanical flexibility.1 Currently, battery and supercapacitor represent two kinds of energy storage devices that have received tremendous attentions over the past decades.2-3 Supercapacitors, also known as electrochemical capacitors, store energy relying on electrical double layer capacitance (EDLC) on the electrode surface or pseudocapacitance arising from reversible faradaic redox reactions at the near surface of electrode.4-8 Because of their high power density, excellent electrochemical reversibility and extremely long cycling stability, supercapacitors hold great potentials as backup power sources for consumer electronic products, hybrid electric vehicles and un-interrupted power supply.9-10 In order to keep pace with the rapid development of thin, light-weight and wearable electronic devices, great efforts have been devoted to designing various unconventional supercapacitors that can be flexible, stretchable and compressible.11-12 In particular, the compressible supercapacitor has received great attentions as it can endure a large level of strain while maintaining a stable capacitive performance. However, successful fabrication of a compressible supercapacitor strongly relies on rational design of innovative electrode materials which not only possess superior compressible property but also exhibit high performance. In general, electrode materials applied in compressible supercapacitor are composed of the scaffolds and electroactive components. Presently, the scaffolds usually include conductive 3



carbon-based

materials

and

non-conductive

polymer-based

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materials.

Three-dimensional (3D) graphene foams or graphene aerogel have been applied as scaffold in compressible supercapacitor.13 For example, graphene aerogel decorated with electroactive polypyrrole (PPy) has successfully served as a compressible electrode with nearly unchanged capacitive performance even under 50% compression.14 However, the chemically derived graphene framework usually suffers from limited mechanical strength. Moreover, the strong π−π restacking and the poor inter-sheet connections brings a large interfacial resistance which in turn restrict graphene serving as a conductive scaffold for compressible supercapacitor. As compared with the 3D graphene architectures, CVD-derived carbon nanotubes (CNTs) sponge hold great promise as compressible supercapacitor electrode with respect to its good mechanical strength and high conductivity.15-16 But specific capacitance of CNTs sponge is too low to meet the ever-increasing requirement of high-energy device. In spite of decoration with PPy and MnO2 to enhance energy storage capacity, the capacitive performance of the CNTs sponge-based supercapacitor under various compression states has rarely been investigated.17 Apart from the carbon-based scaffolds, the non-conductive sponge composed of cellulose or polyester fibers applied as scaffold for compressible electrodes has also been reported. For example, by dip-coating single-walled CNTs aqueous suspension on commercially available sponge network, Niu and his coworkers have successfully fabricated a highly compressible supercapacitor which can make full utilization of the compression tolerance of sponges and the fault tolerance of SWCNTs.18 However, its volumetric 4


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capacitance is only 3.4 F cm−3 even after polyaniline decoration. Most recently, a compressible supercapacitor electrode has been developed by a facile carbonization of commercial melamine sponge.19 The highly compressible property in combination with the good conductivity of carbon scaffold enables an excellent electrochemical performance to be achieved even under a large strain level. Further boosting its performance without scarifying the compression tolerance still remains a big challenge. Transition metal sulfides have been extensively explored as a new class of electrode materials for pseudocapacitors. In particular, nickel-cobalt sulfide (NiCo2S4, denoted as NCS thereafter), has drawn great research interests over the past several years because of its large theoretical capacity, abundant resource and rich redox reactions.20-26 Besides these merits, the conductivity of NCS was reported to be as high as 1.25 × 106 S m−1.27-28 The intrinsically metallic conductivity enables NCS to be one of promising electrode materials for high-rate supercapacitor. On the other hand, as NCS stores energy through reversible Faradaic redox reaction at the electrode/electrolyte interface, design of NCS architecture with more exposed electroactive sites and favorable ion pathways is highly desirable for improved performance.29-30 Inspired by these inherent properties, this work reports the sheet-like NCS which are vertically grown on carbon sponge (CS) for highly compressible supercapacitor electrode (CS-NCS). The compressible CS scaffold made up of interconnected carbon fibers not only provides large number of surface sites for uniform growth of electroactive NCS nanosheets, but also offers millimeter-scale 5



pores that can substantially promote electrolyte transport. Accordingly, the CS-NCS hybrid electrode delivers a specific capacitance up to 1093 F g−1 at current density of 0.5 A g−1, and retains 91% of its initial capacitance at 20 A g−1. More importantly, the structural integrity of the CS-NCS composite allows 90.4% geometric volume to be recovered and nearly 100% conductivity to be remained even after 100 repeated cycles of compression-release at a large strain of 60% (ε = 60%). When serving as a supercapacitor electrode, the CS-NCS electrode delivers a gravimetric capacitance of 369 F g−1 and volumetric capacitance of 20.9 F cm−3 at ε = 60%, which are over 5 times and 17 times higher than the CS electrode, respectively. 2. EXPERIMENTAL SECTION 2.1 Growth of NCS Nanosheets on CS. The CS was obtained by direct carbonization of the commercially available melamine sponge (MS) in flowing N2 atmosphere.19 Typically, the MS with geometric size of 2.5 cm × 1.5 cm × 1 cm was placed in an electronic tube furnace. The temperature of the furnace was increased to 900 °C with a rate ramp of 5 °C min−1, and then held at this temperature for 2 h. After naturally cooling down to room temperature, the black product with geometric size typical of 1.5 cm × 1.0 cm × 0.7 cm was obtained. Growth of NiCo2S4 nanosheets on CS skeleton was accomplished by a facile one-step hydrothermal method. Specifically, 1.25 mmol Ni(NO3)2⋅6H2O, 2.5 mmol Co(NO3)2⋅6H2O, 6.0 mmol thiourea, and 5.5 mmol hexamethylenetetramine (HMT) were dissolved in 15 mL deionized water and 10 ml ethanol to form a transparent pink solution. Afterward, CS of 2.5 mg was added and the mixture was continuously stirred 6


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for 30 min to allow sufficient penetration of the solution into the CS. Hydrothermal reaction at 160 °C for 10 h yielded a black product which was denoted as CS-NCS. This recipe produces CS-NCS composite with 60 wt% NCS, corresponding to a volumetric loading of 9.6 mg cm−3 for NCS. By adjusting the initial concentration of Ni(NO3)2⋅6H2O and Co(NO3)2⋅6H2O, the CS-NCS composite with tunable mass contents of NCS can be facilely obtained. In a control experiment, the NCS was also prepared by the same procedure except that CS was not added. In this case, the powder sample was denoted as p-NCS. 2.2 Material Characterization The morphologies and structure of samples were observed on Field-emission scanning electron microscope (SU8020) and transmission electron microscope (TEM, JEOL 2000) with an acceleration voltage of 200 kV. X-ray diffraction (XRD, Rigaku Miniflex600) measurements were performed on a DX-2700 X-ray diffractometer with Cu Kα (λ=0.154 nm, 40 kV, 30 mA). X-ray photoelectron spectroscopy (XPS) spectra were collected on an AXIS ULTRA spectrometer (Kratos Analytical) using a monochromatized Al Ka X-ray source (1486.71 eV). Raman spectra were collected on a Renishaw in Via Raman microscope with an excitation wavelength of 532 nm. Thermogravimetric analysis (TGA) curves were recorded on a TA Instruments Q600 TGA system. The stress-strain property of samples was carried out on a Song dun LDW-5 Universal Testing Machine connected with a program-controlled computer. The conductivities of samples under various compressive strains were measured by recording the I-V curves at applied voltage in the range of 0−9 mV. Prior to the 7



measurement, CS or CS-NCS with nearly the same geometric size was sandwiched between two platinum foils to ensure a good electrical contact. The obtained sample resistance was converted to the corresponding conductivity by the equation: κ = l/(R × A),31 where A is the geometric area contacting the platinum foils, R is the measured resistance and l is the height of the sample. 2.3 Electrochemical Measurements The working electrode was prepared by directly attaching pristine CS or CS-NiCo2S4 hybrid between two pieces of nickel foam without need of any conductive additives and polymer binders. All electrochemical measurements were carried out in a three-electrode system with a Pt foil as the counter, Ag/AgCl electrode as reference electrodes, and 3.0 M KOH as aqueous electrolyte. The electrochemical performances of electrodes were evaluated by cyclic voltammetry (CV), galvanostatic charge-discharge and electrochemical impedance spectroscopy (EIS) on a Gamry Reference 3000 electrochemical workstation. For testing the electrochemical performance of electrodes at different strains, a home-made bracket that can be immersed into the KOH aqueous solution was applied (Figure S1), which allows the strains to be easily adjusted by tuning the electrode thickness. The gravimetric and volumetric capacitance of the electrode material were calculated from the discharge curves according to the following equations:32-33 Cm = I × ∆t/ (∆V × m) and Cv = I × ∆t/ (∆V × V), where I is the discharge current (A), ∆t is the discharge time (s), ∆V is the voltage change (V), m and V are mass (g) and geometric volume (cm−3) of the whole electrode. The EIS measurements were performed by employing an ac voltage 8


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of 5 mV in the frequency range of 0.01 Hz to 100 KHz. 3. RESULTS AND DISCUSSION Figure 1a shows the digital images of commercial MS before and after pyrolysis at 900 °C for 2 h. It is clearly that the pristine MS undergoes a substantial volume shrinkage accompanied with a significant weight loss as indicated by the TGA curve (Figure S2a). Nevertheless, the obtained CS is highly compressible and can fully restore its initial shape and size immediately after the release of the mechanical force (Figure 1a). SEM characterization reveals that the CS is actually composed of numerous branched carbon fibers with tens of micrometers in length (Figure 1b) and ~3.0 µm in diameter (Figure 1c). These fibers interconnect each other to form a highly porous network with a bulky density of 5.0 mg cm−3, which is slightly lighter than that of CNTs sponge (7.5 mg cm−3)15 and even comparable to that of graphene aerogel (2−3 mg cm−3).13 Figure S2b compares the Raman spectra of starting MS and CS. The disappearance of 976 and 675 cm−1 characteristic of MS, and occurrence of two peaks at 1348 and 1590 cm−1 clearly demonstrates the complete conversion of MS into carbon during pyrolysis process.19 The survey XPS spectrum of CS presents the C, N and O elements (Figure S2c). The N 1s XPS spectrum includes pyridine nitrogen (398.7 eV), pyrrole nitrogen (400.1 eV) and quaternary nitrogen (401.1 eV) (Figure S2d).34 Based on the XPS quantitative analysis, the doping level of N in CS is around 2 at%. These N atoms could not only enhance the bulky electric conductivity of CS, but also improve the wetting ability and offer an extra pseudocapacitance.19, 35 The CS with interconnected 3D network can serve as an ideal scaffold for 9



efficient growth of pseudocapacitive NCS. As shown in Figure 1d, even at 60 wt% mass contents of NCS, the CS-NCS composite can still restore to its initial shape upon releasing the manual force, suggesting its superior compression property. Figure 1d also includes a low-magnification SEM image of the composite. Clearly, foam-like network of CS is well preserved and growth of NCS does not destruct the porous scaffold. The structure of the NCS on the CS scaffold is magnified in Figure 1e. It is seen that the smooth surface of pristine carbon fibers has been fully and uniformly covered with the NCS sheets. These vertically grown NCS sheets form many voids which can dramatically shorten the ion diffusion length and enable easy access of electrolyte ions to the electrode surface. Figure 1f and 1g show the STEM image of CS-NCS and the corresponding elemental mapping, respectively. All the C, Ni, Co and S elements are homogenously distributed throughout the detected area, showing uniform growth of NCS sheets on the CS scaffold during hydrothermal synthesis. The structure of NCS sheets were further characterized by TEM (Figure 1h). It is seen that these NCS sheets are wrinkled and nearly transparent with sheet thickness varying from 17 to 23 nm. Figure 1i presents the XRD pattern of CS-NCS composite. Besides a broad diffraction peak characteristic of amorphous CS scaffold at 25.4°, the distinct diffraction peaks at 32.0°, 38.2° and 55.5° can be well indexed to (311), (400) and (440) planes of the cubic phase NiCo2S4 (JCPDS card No. 20-0782).29,36 Raman spectrum provides additional evidences that the NCS sheets have successfully grown on CS. As shown in Figure 2a, the distinct peaks at 461, 511 and 660 cm−1 correspond to the F2g, F2g and A1g models of NCS, respectively.37-38 The peak ratio of D band and 10


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G band is 0.988 for CS and 0.995 for the CS−NCS composite. This negligible difference suggests that introduction of NCS nanosheets does not affect the structure of CS. To analyze the surface chemical composition of the NCS, we performed the XPS analysis and supplied the survey XPS spectrum in Figure S3. The presence of C, N, O, Ni, Co and S signals coincide with the results of elemental mapping. By using a Gaussian fitting method, the Ni 2p and Co 2p spectra can be well-fitted with two spin-orbit doublets and two couples of shakeup satellites (Figure 2b, 2c). The peaks at 855.6 and 873.4 eV correspond to the Ni2+, while the fitting Ni 2p peaks at 861.1 and 880.1 eV are indexed to Ni3+.39-40 Meanwhile, the Co 2p spectrum presents two spin-orbit doublets at 781.1 and 796.8 eV, along with two shake-up satellites (Figure 2c), suggesting coexistence of Co2+ and Co3+.41 In the S 2p spectrum (Figure 2d), the component at 161.1 eV is typical of metal-sulphur bonds, and the peak at 162.4 eV can be ascribed to the sulphur ions in low coordination at the surface.23, 26 According to the above XPS analysis, the near surface of CS-NCS is comprised of Ni2+, Ni3+, Co2+, Co3+ and S2−, which agree well with the chemical composition of NiCo2S4 as reported in the literatures.35-36, 42 The highly porous network enables the CS scaffold to serve as a compressible electrode under a large strain level. Figure 3a compares the stress-strain curves of CS and CS-NCS composite under compressive strain of 40%, 60% and 70% (ε = 40%, 60% and 70%). Clearly, both CS scaffold and CS-NCS composite exhibit characteristic profile of compressible materials which are characterized by three regions, i.e. Hooke region at ε < 10%, plateau region from 10% to 45% and the 11



densification region at ε > 45%.18-19, 43 However, as compared with CS scaffold, the CS-NCS composite displays a relatively higher stress at the given strain, suggesting an enhanced mechanical strength due to the growth NCS nanosheets. Moreover, even at 60 wt% mass content of NCS, the stress curves of CS-NCS remain above zero in the releasing process, demonstrating its superior resilience even at ε = 70%. This compressible property is also validated by measuring the relative height of CS or CS-NCS during multiple compress-release cycles (Figure 3b). The CS recovers more than 96% of its original height in the first 60 repeated compression tests and keeps a stable height with a plastic deformation of only ~4% in the last 40 tests. Whereas, CS-NCS exhibits a slightly lower compressibility with 90.4% height preservation during the whole test. The nearly identical stress-strain curves with almost the same compression stress of ~19.31 kPa at different stages of compress-release cycles clearly demonstrate that the CS-NCS composite is highly durable and has reversible compression property (inset in Figure 3b), which is an important prerequisite for practical application of compressible electrode. The conductive behavior of CS-NCS composite under different compression states was systematically investigated and the results were compared with pristine CS scaffold (Figure 3c). Evidently, conductivity of both CS and CS-NCS strongly depends on the compressive strain applied. At normal state (ε = 0), the conductivity of CS and CS-NCS composite is measured to be 2.96 and 3.36 S m−1, and these values gradually increase with the strain and reach 10.33 and 25.87 S m−1 at ε = 80%, respectively, suggesting that closer contact of neighboring carbon skeletons at dense 12


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state benefits rapid electron motion. In addition, due to the uniform growth of NCS nanosheets with an extremely high conductivity (1.25 × 106 S m−1),27-28 the CS-NCS always exhibits a higher conductivity than CS scaffold at the given strain. To further evaluate the change of electrode conductivity during the repeated compress-release cycles, we measured their conductivities at a set strain of 60% and presented the results in Figure 3c. Notably, the conductivity of both electrodes remains highly stable and shows negligible change upon repeating the compress-release cycles for 100 times, demonstrating a good structural integrity during compression cycling. Figure 3d shows the resistance change of two electrodes in the first 5 compress-release cycles at ε = 60%. The ∆R/R0 ratios decreases by 86% for CS and 91% for CS-NCS composite, and the response of bulk electrical resistance is highly reproducible during the multiple cycles. The SEM images of both CS and CS-NCS after 100 cycles of compression-release experiment are shown in Figure S4. Interestingly, both CS and CS-NCS preserve their pristine interconnected network without noticeable structure damage even at ε = 60%. These results agree well with the stable conductivity (Figure 3c) and the nearly constant electric resistance (Figure 3d), thus further demonstrating the remarkable structural resilience of the highly compressible CS-NCS electrode under a large strain of 60%. In order to investigate the intrinsic electrochemical property of CS and CS-NCS electrodes, we pressed them between two nickel foams and tested their performance in three-electrode system with 3 M KOH as aqueous electrolyte. In this case, the strain of CS-NCS can be considered to be close to ~100%. It is seen that the CS 13



electrode exhibits nearly rectangular CV profiles (Figure S5a) and displays almost linear charge-discharge curves (Figure S5b), which are characteristic of electrical double-layer (EDL) charge storage mechanism. The CS delivers a specific capacitance of 180 F g−1 at 0.5 A g−1 and 142 F g−1 at 20 A g−1. The electrochemical performance of CS is dramatically enhanced by growing NCS nanosheets. Figure S5c compares the charge-discharge profiles of CS-NCS electrodes with various mass contents of NCS. The significantly increased discharging time means the enhanced charge storage capacity due to the NCS growth. The dependence of specific capacitance and the maximum strain on the mass contents of CS-NCS electrodes are presented in Figure 4a. Clearly, a higher NCS content gives rise to a larger specific capacitance but leads to a slightly decreased compression property. Therefore, in the following text, we select CS-NCS with 60 wt% mass content as the representative electrode to systematically investigated its pseodocapacitiev performance and compression property. Figure 4b shows the CV curves of CS-NCS with 60 wt% mass content at scan rate ranging from 5 to 20 mV s−1. Clearly, one broad oxidation peak and two well-defined reduction peaks are observed in all the CV curves. These redox peaks can be associated with the following electrochemical redox reactions between NCS and OH−:29, 44 NiCo2S4 + OH− + H2O → NiSOH + 2CoSOH + 2e− CoSOH + OH− → CoSO + H2O + e− Because the similar redox potentials of the two reactions are seriously overlapped, only one broad oxidation peak is observed in all the CV curves. Figure 4c plots the 14


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galvanostatic charge-discharge profiles of CS-NCS at different current densities. The occurrence of two voltage plateaus at around 0.26 and 0.14 V in the discharge curve agrees well with the CV results. Meanwhile, the nearly symmetric charge and discharge profiles at different current densities indicate the highly reversible redox reactions between NCS sheets and OH− ions. In addition to the efficient electrochemical reactions, the internal resistance of CS-NCS electrode is also extremely small, as evidenced from the low voltage drop (0.032 V) at a high current density of 5 A g−1. Clearly, the small internal resistance of CS-NCS electrode is not only arisen from its enhanced conductivity, but also is closely related to the sheet-like NCS that enable easy access of electrolyte ions to electrode surface and reduce the ion diffusion resistance. These combined effects afford the CS-NCS a specific capacitance of 1093 F g−1 at 0.5 A g−1 (Figure 4d), corresponding to 1701 F g−1 of NCS by considering the capacitance of CS-NCS arising from EDL of CS and pseudocapacitance of NCS. This specific capacitance is 6 times higher than CS electrode (180 F g−1 at 0.5 A g−1), and also higher than most of previous NCS-based electrodes (Table S1). Notably, the capacitance retention of CS-NCS is as high as 91% over a wide range of current density (0.5−20 A g−1). To our knowledge, this rate capability is among the highest value reported for NCS-based electrode (Table S1), including NCS nanoneedles on carbon fibers (1−20 A g−1, 62.3%),45 NCS nanosheets/rGO (3−20 A g−1, 52.4%),46 tube-like NCS (1−20 A g−1, 50.1%),29 caterpillar-like NCS on nickel foam (1−20 A g−1, 40.5%),36 and porous NCS (1−20 A g−1, 85%).40 15



To further elucidate the role of CS for improving the capacitance performance of NCS, we prepared the p-NCS sample by the identical hydrothermal method except the CS was not added. Shown in Figure S6 are the SEM images of p-NCS. The p-NCS tends to assemble into flower-like aggregates by the NCS nnaosheets. Specific capacitance of p-NCS at various current densities is included in Figure 4d for comparison. The p-NCS only shows a capacitance of 971 F g−1 at 0.5 A g−1, which is much lower than 1701 F g−1 of NCS and 1093 F g−1 of CS-NCS composite. Moreover, capacitance of p-NCS rapidly decreases to 438 F g−1 as the current increases to 20 A g−1. This dramatically enhanced performance of CS-NCS highlights the improved conductivity of composite electrode as well as the vital role of the CS scaffold, which not only effectively inhibits the severe aggregation of NCS sheets by offering large number of growth sites, but also provides sufficient space favorable for fast ion transport. This conclusion is partially supported by comparing EIS spectra of CS, p-NCS and CS-NCS (Figure 4e). It is seen that growth of NCS nanosheets on CS significantly reduces the charge transfer resistance of CS electrode. The long-term stability of CS-NCS electrode was also evaluated and the results are supplied in Figure 4f. Clearly, even after 8000 continuous charge-discharge cycles at current density of 8 A g−1, CS-NCS electrode still retains 83% capacitance and 98% Coulombic efficiency, suggesting an efficient and reversible Faradaic reaction between NCS sheets and electrolyte. To understand the capacitance decay, the CS-NCS electrode after cycling test was analyzed by SEM and representative SEM images are shown in Figure S7. Like the initial CS-NCS, the NCS still remain 16


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intersected nanosheets which are still vertically yet strongly adhered onto the CS skeleton (Figure S7a, S7c). However, comparison of the magnified images reveals that NCS nanosheets become thicker and turn to be slightly irregular (Figure S7b, S7d), suggesting gradually decreased electrode/electrolyte contact area which might responsible for the decayed performance upon cycling. Nevertheless, the excellent performance of CS-NCS electrode can be ascribed to the following structure merits: i) 3D porous CS with mechanical compressibility and electrical conductivity offers continuous network which accelerates electron motion and promotes fast ions transport within the interior electrode; ii) hierarchical NCS thin sheets not only allows more electroactive sites to be accessed by the electrolyte, but also dramatically reduces the ion diffusion resistance. Both help to enhance the utilization efficiency of electrode and contributes to a high-rate capability; iii) Strong adhesion of conductive NCS sheets on CS helps to maintain the structure integrity during the repeated cycling, leading to a satisfying cycling performance and excellent Coulombic efficiency. To demonstrate the potential application of CS-NCS as compressible electrode, we evaluate its performance under different strains in aqueous KOH electrolyte using a home-made bracket (Figure S1). By tuning the electrode thickness, the strains of CS-NCS can be facilely controlled. Figure 5a shows the CV profiles of CS-NCS electrode measured in 3.0 M KOH under different strains. Like the CV curves measured in nickel foam (Figure 4b), the CS-NCS electrode also displays roughly similar redox peaks except that the oxidation peaks move to more positive region and reduction peaks to negative region as compared with the curves in Figure 4b, 17



suggesting larger electrode resistance at low strains.47-48 Nevertheless, all the charge-discharge curves exhibit nearly symmetric profiles with the IR drop decrease from 0.030 V to 0.014 V upon increasing the strains from 0 to 60%, further showing the reduced internal resistance at higher compression. According to the discharge curves, the specific capacitances of CS-NCS under various strains are plotted in Figure 5c. The CS-NCS deliver a specific capacitance of 302 F g−1 at ε = 0 and 369 F g−1 at ε = 60%. In contrast, the CS electrode only shows a low and nearly invariable specific capacitance of 73 F g−1 despite the change of strain from 0 to 60% (Figure 5c). This result clearly demonstrates the significant contribution of pseudocapacitance from the electroactive NCS nanosheets. By taking their corresponding geometric volume into account, the volumetric capacitance of CS-NCS electrode is calculated to be 20.9 F cm−3 at ε = 60% (Figure 5c), which is over 17 times higher than the CS electrode (1.2 F cm−3). Figure 5d presents the long-term stability of CS-NCS measured at 2.0 A g−1 with strain varying from 0 to 60%. During each 500 cycles of charge-discharge tests at a given strain, the capacitance retention is over 90%. Even after charge-discharge cycles at ε = 0, 20%, 40% and 60% for every 500 cycles, CS-NCS electrode can still recover to its initial charge storage capacity and deliver nearly the same specific capacitance (234 vs 232 A g−1), suggesting the electroactive NCS nanosheets keep in good contact with the CS scaffolds during the whole cycling process. This structural integrity is responsible for the constant conductivity as described in Figure 3c and is highly desirable for compressible supercapacitor. While CS-NCS composite electrode exhibits substantially enhanced performance 18


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than the CS electrode, its specific capacitance at 0 ≤ ε ≤ 60% is much smaller than that (1093 F g−1) at dense state (ε ≈ 100%). To understand the mechanism of the performance difference, we performed the EIS of CS-NCS composite electrode under various strains and presented the Nyquist plots in Figure 5e. It is seen that all the Nyquist plots display similar shape consisting of an arc in the higher frequency region followed by a spike at low frequency. The points intersecting the real axis are referred to the solution resistance (Rs), which includes electrolyte resistance, the internal resistance of electrode and the contact resistance at the interfaces of electrode/current collector.49 The semicircle at high frequency represents the charge transfer resistance (Rct) that is mainly determined by the electrode conductivity, whereas the 45° slope line known as the Warburg resistance (W0) reflects the ion diffusion resistance within the interior of electrode.50 By fitting the EIS using the equivalent circuit given in Figure 5f, the Rs, Rct and W0 at 0 ≤ ε ≤ 100% were derived and they were listed in Figure 5f. As expected, the variation of Rs in the range of 0 ≤ ε ≤ 80% is negligible because all the EIS was performed using the identical bracket cell with same electrolyte. However, Rct is found to decrease with the increase of the strains, showing gradually enhanced charge transfer capability at the electrode/electrolyte interfaces due to the improved conductivity at higher strains (Figure 3c). Besides the variation of Rct, another important change is the W0. As shown in Figure 5e and 5f, the continuous decrease of W0 from 11.1 ohm at normal state to 3.12 ohm at ε = 80% suggests the reduced ion diffusion resistance due to the drastically shortened ion diffusion pathway under large strain state. In particular, compressing CS-NCS in nickel foam with strain 19



~100% yields a dramatically reduced Rs, Rct and W0. These results clearly demonstrate that full contact of the CS-NCS composite electrode with current collector (nickel foam) can dramatically improve the charge transfer capacity, while compressing the CS-NCS at dense state would substantially reduce ion diffusion length. Both contribute to the minimized internal resistance and enable full utilization of NCS for redox reaction, leading to the significantly enhanced capacitive performance. 4. CONLUSIONS In summary, NCS nanosheets have been successfully grown on the carbon sponge to prepare a highly compressible supercapacitor electrode. The composite electrode exhibits ideal compression property under the strain as high as 70% and remains a constant conductivity during the 100 repeated cycles of compression-release tests at the set strain of 60%. Moreover, the sheet-like NCS and the millimeter-scale pores dramatically enhance the utilization efficiency of electroactive NCS for redox reaction. As a result, CS-NCS composite delivers a specific capacitance of 1093 F g−1 at 0.5 A g−1 and presents an extremely high capacitance retention of 91% at 20 A g−1 with a satisfactory electrochemical stability. Under strain of 60%, the CS-NCS electrode delivers gravimetric capacitance of 369 F g−1 and volumetric capacitance of 20.9 F cm−3, which are over 5 times and 17 times higher than the CS electrode, respectively. These results show great potential of CS-NCS composite as highly compressible supercapacitor electrodes for practical application. ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (Nos. 20


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51772181, 21373134) and 111 project (B14041). The Supporting Information is available free of charge on the ACS Publication website at DOI: 10.1021/XXXXX Digital photo of the home-made bracket electrochemical cell, TGA and Raman curve of MS, Raman and survey and N 1s XPS spectra of CS, SEM image of CS and CS-NCS before and after compression cycles at ε = 60% for 100 times, CV and galvanostatic charge-discharge profiles of CS and CS-NCS with various mass contents of NCS, Performance comparison of our CS-NCS with previously published references, SEM images of p-NCS, and SEM images of CS-NCS composite electrode before and after charging/discharging for 8000 cycles.

21



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Ultrahigh-Rate Energy Storage. Science 2017, 356, 599-604.

27



Captions for Figures

Figure 1. (a) Digital photo of MS, CS and compression property of CS. (b, c) SEM image of CS with different magnifications. (d) Low-magnification SEM image of CS-NCS composite and digital photo showing its compressibility. (e) SEM, (f) STEM and (g) elemental mapping of CS-NCS composite with mass content of 60 wt%. (h) TEM images of NCS and (i) XRD patterns of CS and CS-NCS composite. Figure 2. (a) Raman spectra, XPS spectra of (b) Ni 2p, (c) Co 2p and (d) S 2p of the CS-NCS composite. Figure 3. Physical properties of CS and CS-NCS with 60 wt% NCS. (a) The stress-strain curves of CS and CS-NCS at different strains. (b) The height variation of CS-NCS composite during 100 repeated compress-release cycles at ε = 60% with inset giving the stress-strain curves of CS-NCS recorded at different cycling stages. (c) The conductivity of CS and CS-NCS at different strains and their corresponding variations with the compression times at ε = 60%. (d) The resistance changes of CS and CS-NCS during the repeated cycles of compress-release with ε = 60%. Figure 4. (a) Dependence of specific capacitance and maximum strain on the CS-NCS with various mass contents. (b) CV curves and (c) galvanostatic charge-discharge profiles of CS-NCS electrode containing 60 wt% NCS. (d) Capacitance retention and (e) EIS spectra of CS, p-NCS and CS-NCS electrodes. (e) Cycling performances of CS and CS-NCS electrodes at 8 A g−1. Figure 5. Electrochemical performance of CS and CS-NCS (60 wt% NCS) under different compression states. (a) CV curves and (b) galvanostatic charge-discharge curves of CS-NCS at 0.2 A g−1 under different strains. (c) Comparison of the gravimetric and volumetric capacitance of CS and CS-NCS electrodes. (d) Cycling performance and corresponding compression state of CS-NCS electrode at different strains. (e) Nyquist plots of CS-NCS under different strains. (f) The equivalent circuit used for fitting the Nyquist plots and the derived parameters including Rs, Rct and W0 under different strains.

28


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Figure 1. (a) Digital photo of MS, CS and compression property of CS. (b, c) SEM image of CS with different magnifications. (d) Low-magnification SEM image of CS-NCS composite and digital photo showing its compressibility. (e) SEM, (f) STEM and (g) elemental mapping of CS-NCS composite with mass content of 60 wt%. (h) TEM images of NCS and (i) XRD patterns of CS and CS-NCS composite.

29



(a)

1348 cm

(b)

-1

-1

1590 cm

Ni 2p

2p3/2

660 cm

Intensity (a.u.)

Intensity (a.u.)

+2

-1

-1

461 cm

2p1/2

Sat.

Sat.

CS-NCS 511 cm

300

600

-1

CS

900

1200

1500 Raman shift (cm-1)

(c)

885 880 875 870 865 860 855 850 845

1800

Binding energy (eV)

2p3/2

(d)

Co 2p

2p1/2

2p1/2

S 2p 2p3/2

Intensity (a.u.)

Sat.

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 35

Sat.

810

805

800

795

790

785

780

Sat.

176

775

172

168

164

160

Binding energy (eV)

Binding energy (eV)

Figure 2. (a) Raman spectra, XPS spectra of (b) Ni 2p, (c) Co 2p and (d) S 2p of the CS-NCS composite.

30


Page 31 of 35

(b) 40% 60% 70%

CS

96.0%

100

CS-NCS

25 20 15 10

90.4%

80

20 Compressive stress (kPa)

40% 60% 70%

30

Relative height (%)

Compressive stress (kPa)

(a) 35

60 40 20

5

10 times 50 times 100 times

15

5

0 10

0

20

18 CS-NCS

20

12

16 CS

9

12 6

8 4 0 0

20 40 60 Strain (%)

80

0

0

60

0

50

60

40 60 80 Compression cycles 60

0

60

0

60

0

100

60

0

Strain (%)

CS-NCS

15 Conductivity (S m-1)

CS CS-NCS

(d) 20

30 40 Strain (%)

CS

0 -20

∆R/R0 (%)

(c) 28 24

20

Strain (%)

Strain (%)

CS

ε = 60%

CS-NCS

0 0 10 20 30 40 50 60 70 0 10 20 30 40 50 60 70

CS-NCS

10

0

0

Conductivity (S m-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-40 -60

3

-80

0 20 40 60 80 100 Compression times

-100 0

1

2

3

4

5

Cycle number

Figure 3. Physical properties of CS and CS-NCS with 60 wt% NCS. (a) The stress-strain curves of CS and CS-NCS at different strains. (b) The height variation of CS-NCS composite during 100 repeated compress-release cycles at ε = 60% with inset giving the stress-strain curves of CS-NCS recorded at different cycling stages. (c) The conductivity of CS and CS-NCS at different strains and their corresponding variations with the compression times at ε = 60%. (d) The resistance changes of CS and CS-NCS during the repeated cycles of compress-release with ε = 60%.

31



100

(b) 40

(c)

600 40 400 20

200 0 68 50 36 60 Mass contents of NCS in CS-NCS (wt%)

(d)1200

20 10 0 -10

-1

5 mV s -1 10 mV s -1 15 mV s -1 20 mV s

-20 -30 -0.2 -0.1

0 26

Potential (V vs Ag/AgCl)

60

-1

800

Current density (A g )

80

Maximum strain (%)

1000

0.1

0.2

0.3

0.4

0.5

-Z'' (ohm)

CS

p-NCS

-1

1.0

0.5 80%

0 0

5

10

15

20

0

0.6

Specific capacitance (F g )

-1

p-NCS

0.1

400

800 1200 Time (s)

1600

(f) 1200 CS-NCS CS

1.5

CS-NCS

0.2

98%

900

300

0.3

0.0 0.0

(e) 2.0

45%

-1

0.5 A g -1 1 Ag -1 2 Ag -1 5 Ag

Potential (V vs. Ag/AgCl)

91%

600

0.032 V

0.4

30

-1


1200

0.0 0.0

83%

800

1.0 Z' (ohm)


1.5

2.0

80

CS-NCS

60

600

40

400 CS 98%

200 0

0.5

-1

100

1000

0

2000

4000

6000

20

Coulombic efficiency (%)

(a)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 32 of 35

0 8000

Cycle number

Figure 4. (a) Dependence of specific capacitance and maximum strain on the CS-NCS with various mass contents. (b) CV curves and (c) galvanostatic charge-discharge profiles of CS-NCS electrode containing 60 wt% NCS. (d) Capacitance retention and (e) EIS spectra of CS, p-NCS and CS-NCS electrodes. (e) Cycling performances of CS and CS-NCS electrodes at 8 A g−1.

32


0% 20% 40% 60%

2 0

0% 20% 40% 60%

-2 -4

0.3

0.2

Specific

Potential (V vs Ag/AgCl)

10 mV s-1

-1


4

0.1 -1

0.2 A g

0.0

0.1

0.2

0.3

0.4

0.5

300

600

900

(e) 25

(d)

15

200 10

100

-1

20

-1

232 F g

200

-1

250 F g

96%

-1

302 F g

-1

273 F g

-1

234 F g

90%

94%

92%

-Z'' (ohm)

600

400

85%

15

5

Rs

0

0

1500

2000

2500

20

40

60

Rct

W0

0

0 1000

1500

10

Current density: 2 A g 500

1200

Strains (%)

-1

0

5

0% 20% 40% 60% 80% 100%

strain:0%

strain:60%

strain:40%

strain:20%

strain:0%

CS-NCS

20

Time (s)

Potential (V vs. Ag/AgCl)

800

CS

25

0

0

0.6

CS-NCS

300

0.0

-0.2 -0.1

CS

-3

(c) 400

(b) 0.4

Volumetric capacitance (F cm )

(a)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


capacitance (F g-1)

Page 33 of 35

0

5

10

15

20

25

Z' (ohm)

Cycle number

Figure 5. Electrochemical performance of CS and CS-NCS (60 wt% NCS) under different compression states. (a) CV curves and (b) galvanostatic charge-discharge curves of CS-NCS at 0.2 A g−1 under different strains. (c) Comparison of the gravimetric and volumetric capacitance of CS and CS-NCS electrodes. (d) Cycling performance and corresponding compression state of CS-NCS electrode at different strains. (e) Nyquist plots of CS-NCS under different strains. (f) The equivalent circuit used for fitting the Nyquist plots and the derived

33


ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

parameters including Rs, Rct and W0 under different strains.

34


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TOC Graphic

35


Highly Compressible Carbon Sponge Supercapacitor Electrode with

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