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Functional Nanostructured Materials (including low-D carbon)

Surface Engineering of Graphene Oxide Shells using Lamellar LDH Nanostructures Sarigamala Karthik Kiran, Shobha Shukla, Alexander Struck, and Sumit Saxena ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21265 • Publication Date (Web): 15 May 2019 Downloaded from http://pubs.acs.org on May 15, 2019

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ACS Applied Materials & Interfaces

Surface Engineering of Graphene Oxide Shells using Lamellar LDH Nanostructures Sarigamala Karthik Kiran1, Shobha Shukla2, Alexander Struck3 and Sumit Saxena2* 1

Centre for Research in Nanotechnology and Science, Indian Institute of Technology Bombay, Mumbai, MH, India - 400076. 2Nanostructures

Engineering and Modeling Laboratory, Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Mumbai, MH, India - 400076. 3Faculty

of Technology and Bionics, Rhein-Waal University of Applied Sciences, Kleve, Germany 47533. *corresponding

author: - [email protected]

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Abstract: The discovery of graphene oxide has made profound impact in varied areas of research due to its excellent physico-chemical properties. However, surface engineering of these nanostructures hold the key to enhanced surface properties. Here we introduce surface engineering of rGO shells by radially grafting Ni-Co LDH lamella on rGO shells to form Ni-Co LDH@rGO. The morphology of synthesized Ni-Co LDH@rGO mimics dendritic cell like 3D hierarchical morphologies. Silica nanospheres form self-sacrificial templates during the reduction of GO shells to form rGO shells during the template assisted synthesis. The radial growth of LDH lamellae during hydrothermal growth on GO shells provides access to significantly larger number of additional active redox sites and over-compensate the loss of pseudo-capacitive charge storage centers during the reduction of GO to form rGO shells. This enables in synthesis of novel surface engineered rGO nanoshells which provide large surface area, enhanced redox sites, high porosity and easy transport of ions. These synthesized 3D dendritic cell like morphologies of Ni-Co LDH@rGO shows a high capacitance of ~2640 Fg-1. A flexible hybrid device fabricated using this nanomaterial shows high energy density of ~35Wh kg-1 and power density of 750 Wkg-1 at 1Ag-1. No appreciable compromise in device performance is observed under bending conditions. This synthesis strategy may be used in development of functional materials useful for potential applications including sensors, catalyst, and energy storage.

Keywords: Graphene oxide; Hybrid material; Dendritic cell like morphologies; Energy storage; LDH


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INTRODUCTION Synthesis of low dimensional materials have created an avalanche effect in advancement of both fundamental research as well as their applications. Past few years have witnessed rapid growth in development of graphene based functional materials using several techniques including surface modification by chemical functionalization amongst others. Graphene oxide (GO) inherently contains oxygen functional groups1,2thatstrongly affect the electronic structure of graphene rendering useful electrical, mechanical and electrochemical properties along with ease of processability.3,4 These enhanced properties are desirable for several applications such as electronic devices,5chemical sensors,6 biosensors,7 biomedical applications8and supercapacitors. Supercapacitors are classified into two types EDLC (electrostatic accumulation of surface charges) and pseudocapacitors (Faradaic charge transfer process by thermodynamically and kinetically favored electrochemical redox reactions).Recently, GO or graphene based nanostructured materials are being explored for the fabrication of hybrid electrode materials forsupercapacitors.9,10Nanocomposites of GO with metal oxide based nanoparticles such as 3D delaminated flexible structures of SnO2/GO nonporous electrodes have been reported.11 Large aromatic donor as well as acceptor molecules are used to functionalize GO resulting in wide tunable electronic properties.12Conducting polymer based GO nanocomposite (PANI fibers absorbed on GO) through in situ polymerization technique in acidic environment exhibit high capacitive performance with good cyclic stability.13 Various kinds of enzymes have also been immobilized on GO sheets to form electrodes.14Several other methods of preparation and functionalization of GO using nanoparticles, organic compounds, polymers and biomaterials, exhibiting novel and interesting properties, are available in literature. The unique physicochemical properties along with possibilities of improvisation of these properties by utilizing their 3 ACS Paragon Plus Environment


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large surface area suggest that GO has vast potential for exploring new avenues and critical improvements in areas such as sensor development, catalysis and especially in field of energy storage15-19. Transition metal layered double hydroxides (TMLDHs) show excellent electro-activity, have flexible ion exchange property, lamellar structure and possess good redox activity. They form ideal candidates for surface functionalization of GO for applications such as energy storage, sensors and catalysis.20-22These are represented by M2+1-xM3+x(OH)2·An-x/n·zH2O, where M2+ and M3+may be divalent or trivalent metal ions, while An- is the charge-balancing anion residing in the inter-lamellar region.23,24Amongst all TMLDH's, nickel and cobalt based hydroxides have high theoretical specific capacitances, good cyclic stability, and are environmental friendly.25Both nickel and cobalt hydroxides have layered structures with large inter-planar distances, large porosity, and remarkable redox characteristics. The presence of Ni inNi1xCoxdouble

hydroxide reduces the resistance, mechanical stress and raises the oxygen over-

potential. Cobalt on the contrary participates in electrochemical redox reactions.26,27 The Ni-Co based layered double hydroxides (LDH) not only exhibits large charge transfer on the surface, but also through the bulk of the material by intercalation/de-intercalation of the ions in the electrolyte.28Several attempts have been made to synthesize engineered nanomaterials using several techniques such as in-situ layer by layer growth,29 solvothermal,30electrodeposition,31 coprecipitation,32 microwave assisted synthesis33 and hydrothermal techniques.34However, nanocomposites obtained from these processes have resulted in self-assembly of vertically stacked LDHs intercalated graphene sheets and are agglomerated. Rationally engineered nanostructures in form of dendritic cell like morphologies by surface functionalization of GO shells using LDH's are expected to significantly increase the surface area and enhance the charge


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storage properties of these materials. This will enable in access to large number of re-dox active centers in addition to those present on GO shell. This is also expected to provide easy access to electrolyte ions in an electrochemical setup. Crucial development of GO based materials for enhancing the surface properties thus lies in fine control over carefully tailored synthesis process. Here we report template assisted synthesis of3D engineered nanostructures of Ni-Co LDH sheets radially grafted on rGO conducting spherical shells (Ni-Co LDH@rGO)in dendritic cell like morphology. This involves the use of self-sacrificial silica nanospheres as template for making GO shell. The inherent advantage of the proposed synthesis method is that it prevents agglomeration of LDH nanosheets thereby forming dendritic cell like structures with radially grown LDH lamella, enabling easy dispersal in solution. Structural characterization and chemical analysis suggests that the silica template is removed in-situ during the synthesis and GO is reduced to rGO. The synthesized Ni– Co LDH@rGO displays a 3D architecture with a skeletal interior and membrane like Ni-Co LDH exterior structure extending radially out of a rGO shell grafted at the core. Such hierarchical structures in hybrid materials greatly enhance the electrical conductivity and reduce thelow diffusion resistance to ionic species. Large surface area, easy electrolyte penetration, easy access to increased number of redox sites facilitates efficient charge storage in these hybrid materials at high rates. The electrochemical characterization results suggest that a specific capacitance of as high as 2640 Fg-1is exhibited in these dendritic cell shaped Ni-Co LDH@rGO morphologies. A flexible hybrid device is also fabricated using the synthesized Ni-Co LDH@rGO hierarchical morphology. The device fabricated using this nanomaterial shows high energy density of ~35Wh kg-1 and power density of 750 Wkg-1 at 1Ag-1. It is observed that the device maintains its characteristics and performance even on rolling the electrode.Thus, this 5 ACS Paragon Plus Environment


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study provides a favourable approach to design and synthesize structural assembly of high performance hybrid materials with enhanced surface area with potential applications in efficient charge storage, sensors and catalysis. RESULTS AND DISCUSSIONS Synthesis The synthesis of Ni-Co LDH@rGO nanostructures in form of dendritic cell like morphologies is schematically represented in figure 1.Graphene oxide(GO) nanosheets are synthesized using the modified Hummers method,35 while the self-sacrificial SiO2 nanoparticles used are synthesized using the modified Stober's method. The detailed synthesis protocols are provided in the section M1 and M2 respectively of the supporting information. Graphene-oxide coated silica (SG) is synthesized by wrapping graphene oxide over SiO2 nanoparticles (SiO2 /GO). The synthesis details of core-shell structures of (SiO2/GO) have been discussed in the section M3 of the supporting information.

Figure 1: - Schematic representation of synthesis process for Ni-Co LDH@rGO3D-dendritic cell like nanostructures obtained via template assisted growth process.


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Homogeneous mixing of GO and silica nano particles solution results in wrapping of GO sheets on the surface of the amino modified SiO2 spheres yielding SG core-shell structures (supplementary figure S1: TEM micrograph). During the hydrothermal reaction the surface functionalized anions of GO interact with the metallic cations to form nucleation sites for radial growth of Ni-Co LDH lamellae in presence of Hexamethylenetetramine (HMTA).During the hydrothermal process, HMTA hydrolyses into NH3 and HCHO, which act as a homogeneous precipitating agent and reducing agent respectively. The HCHO reduces GO so that Ni-Co LDH lamellae can deposit simultaneously on the surface of GO. Hydroxyl ions produced during the hydrolysis process as well as from the redox reactions between ethanol and NO3-, readily combines with Ni2+ and Co2+ to form Ni(OH)6 and Co(OH)6 octahedra with metal cations at the center of octahedra surrounded by hydroxyl groups. The water molecules and nitrate ions are retained in the inter-layers resulting in formation of Ni-CoLDH. Detailed synthesis procedure is described in the section M4 of the supporting information. The reactions involved during the synthesis are illustrated as follows: 4CH3CH2OH+NO3−→4CH3CHO + NH3 + 2H2O +OH− C6H12N4 + 6H2O → 6CH2O + 4NH3 NH3 + H2O → NH3. H2O NH3. H2O → NH4+ + OH(1-x) Ni2+ +xCo2+ +2OH- → (Ni1-xCox) (OH)2 (Ni1-xCox) (OH)2 + xNO3- + yH2O → [Ni1-x Cox (OH)2] (NO3-)x.yH2O + xe-



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During the hydrothermal process, GO is reduced to rGO forming cross-linked skeletal hydrogel. A large number of two dimensional LDH nanosheets self-assemble radially on the surface of the SG spheres to create a homogeneous three-dimensional dendritic cell like structures. As the amount of OH- increases, SiO2 template dissolves leading to formation of rGO skeletal structure inside and an array of LDH nanosheets on surface to form Ni-Co LDH@rGO dendritic cell like structures. Structural and Microstructural Characterizations. Structural characterization of synthesized SG, Ni-Co LDH and Ni-Co LDH@rGO samples is performed using powdered X-ray Diffraction (XRD). The XRD pattern of SG spheres in figure 2(a)shows all essential characteristic features of GO and SiO2. The broad amorphous peak corresponds to SiO2, while the Bragg's reflection around 11˚ suggest the presence of a few layers of GO36. The representative XRD pattern for synthesized SiO2, GO and rGO are shown as supplementary figure S2. The diffraction patterns of the pristine Ni-Co LDH in figure 2(a) show Bragg reflections corresponding to (003), (006), (009), (012), (015), (018), (110) and (113) planes suggesting the formation of typical hydrotalcite-like structure. Features corresponding to contaminant phases are not observed. The presence of (003), (006), (009) and (110) reflections in pristine Ni-Co LDH and Ni-CoLDH@rGOrepresentsR3msymmetry of LDH lamellae selfassembled radially on rGO spheres. The occurrence of d(003)=2d(006)=3d(009)suggests the formation of layered structure of LDH lamellae self-assembled radially on rGO. The presence of asymmetric reflection at 33.6 suggests the presence of stacking faults reported in the LDHs37. The sharp (003) peak and symmetric narrow bands of the Ni-CoLDH@rGO suggest that these synthesis conditions can be used to achieve high crystallinity in LDH lamellae on rGO shells. It is also to be noted that both Ni-Co LDH and Ni-Co LDH@rGO exhibit a large interlayer spacing 8 ACS Paragon Plus Environment

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as compared to other Ni-Co LDH morphologies reported elsewhere.38,39 The large tunable interlayer spacing results due to the intercalation of various anions can be adjusted by controlling the amounts of reagents. After the growth of LDH Lamellae on rGO shell, the diffraction peak corresponding to rGO becomes too weak to be observed due to extremely close proximity of LDH sheets on rGO shells. (a) (b)

(c)

Figure 2:- X-ray diffraction pattern of (a) SG (silica-GO) core-shell structure, Ni-Co LDH and Ni-Co LDH@rGO 3D-dendritic cell like structures. Representative SAED patterns for (b) Ni-Co LDH and (c) Ni-Co LDH@rGO 3D dendritic cell like nanostructures showing Bragg's planes observed in XRD patterns. The Bragg planes observed in X-Ray diffraction pattern for Ni-Co LDH and Ni-Co LDH@rGO 3D-dendritic cell like structures are identified as bright spots in the selected area electron diffraction (SAED) measurements shown in figure 2(b) and 2(c) respectively. The SAED is performed using high resolution transmission electron microscope (HRTEM) for both the samples. The cell parameters calculated from the XRD pattern are tabulated below in table 1. For hydrotalcites, the lattice parameter ‘c’ relies on numerous factors like anion size, hydratation and amount of interlayer anions. The lattice parameter ‘a’ of the LDHs can be associated with the cation–cation distance within the brucite-like layer. The unit cell values of ‘a’ and ‘c’ along with the basal spacing is calculated using the XRD pattern, 9 ACS Paragon Plus Environment


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a= 2d(110); c=(d(003) + 2d(006) + 3d(009)); Basal spacing= c/3 Table 1: - Calculated lattice parameters for the LDH lamellae in pristine Ni-Co LDH and Ni-Co LDH@rGO 3D-dendritic cell like nanostructure samples.

2θ of (hkl) Sample

(003) (006)

Ni-Co LDH Ni-Co LDH@rGO

Estimated Lattice parameters

(009)

(110)

a

c

Basal spacing

9.45

18.96 28.25

60.11

3.06

28.14

9.38

9.63

19.11 28.46

59.84

3.08

27.85

9.28

The elemental mapping of Ni-Co LDH and Ni-Co LDH@rGO using HRTEM shown in figure 3(a) and 3(b) respectively, is performed using energy dispersive X-ray analysis (EDAX). The homogenous distribution of Ni, Co and O in elementally mapped micrographs exhibit a compact lamellar structure of Ni-Co LDH. This is also observed in field electron scanning electron micrographs (FESEM) shown in supplementary figure S3a.

(a)

(b)


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Figure 3: -HRTEM micrographs of elementally mapped(a) Co, Ni, and O in Ni-Co LDH and (b) Co, Ni, O and C in Ni-Co LDH@rGO using EDAX. In order to inhibit self-aggregation resulting in loss of effective active area, it is necessary to construct a robust and rigid structure to support them. A representative elementally mapped TEM micrograph of 3D hybrid composite of Ni-Co LDH@rGO in spherical dendritic cell like morphology formed during the synthesis is shown in figure 3(b) as well as in SEM micrographs shown in supplementary figure S3(b). The Ni-Co LDH nanosheets grow radially outwards on the outer surface of SG. This prevents agglomeration of Ni-CoLDH nanosheets facilitating the electrolyte transport and also increases significantly the number of electrochemical active sites. The self-sacrificial nature of silica template verified using EDAX spectra is shown as supplementary image S4. This results in unique double skeletal structure with self-assembly of Ni-Co LDH lamellae radially outwards on rGO shell forming 3D-dendritic cell like hierarchical morphologies with spherical symmetry. Chemical Composition Analysis



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Figure 4: - FTIR spectra of GO, Ni-Co LDH and Ni-Co LDH@rGO (from top to bottom) showing characteristic IR frequencies showing C=C backbone for both GO and Ni-Co LDH@rGO dendritic cell like nanostructures. Fourier transform infrared spectroscopy (FTIR) is used to understand the chemical structure of the synthesized samples. The absorption peaks at 3435cm-1 and 1054 cm-1in the FTIR spectra shown in figure 4 arise due to the stretching and deformation modes of OH bonds in all three samples. The two weak bands observed at 2856cm-1 and 2928cm-1 can be assigned to the stretching vibrations of C-H. The δ

(Ni-O-H)

and νCo-O stretching vibrations are observed as

absorption bands at 648cm-1 and 515cm-1 in the FTIR spectra for the Ni-Co LDH and Ni-Co LDH@rGO 3D hierarchical samples. In addition to observed bands, the transmittance dip at 1627cm-1 derives its origin from vibrations of the adsorbed water molecules, thereby confirming the existence of molecular water in the material structure. This also marks the contribution of the skeletal vibration of un-oxidized graphitic domains in the samples. The peak at 1385cm-1 reveals the presence of NO3- vibrations in the lattice.40 The band at 1734cm-1is attributed to the bending mode of carbonyl (C=O) stretching vibration in carbon materials. These investigations are furthered by studying the chemical composition and valence states of various elements in the Ni-Co LDH@rGO dendritic cell like morphologies using X-ray photon spectroscopy (XPS). The survey spectrum of the sample is shown as supplementary figure S5(a). It reveals the presence of Ni, Co, C and O elements in the composite material. The XPS survey spectrum of the composite shows the presence of C 1s and O 1s peaks along with the Ni 2p and Co 2p peaks. The Ni 2p spectrum in figure 5(a) shows two major peaks at 855.5eV and 873.2eV, corresponding to Ni 2p3/2 and Ni 2p1/2, besides two shakeup satellite peaks (indicated as ‘‘Sat’’). The satellite peaks at 861.3eV and 879.1eV, can be identified as the signals 12 ACS Paragon Plus Environment

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originating from Ni2+ ion. The Co 2p spectrum in figure 5(b)comprises of two spin–orbit doublets and two shake-up satellites (identified as ‘‘Sat.’’). One pair of binding energies centered at 780.2eV and 795.4eV corresponds to Co3+, while the other pair at the higher energies of 782.3eV and 797.3eV can be assigned to Co2+ besides two satellite peaks at 785.8eV and 802.9eV. These results indicate that there exist two oxidation states for Cobalt (i.e. Co2+ and Co3+) in the composite. The C1sspectrum in figure 5c can be de-convoluted into three different peaks centered at 284.5eV, 286.1eV and 288.5eV, corresponding to the C–C, C–O and O=C-O groups, respectively41. (a)

(b)

(c)

(d)

Figure 5:- XPS spectrum showing (a) Ni 2p, (b) Co 2p, (c) C 1s and (d) O1s spectrum for synthesized Ni-Co LDH@rGO 3D hierarchical nanostructures. The intensities of C–O and O=C-O peaks are considerably weaker than that of the C–C peak, further confirming the effective removal of oxygen-containing groups and reduction of GO to rGO during the hydrothermal process. The O 1s spectrum in figure 5d can be de-convoluted into three peaks at 531.1eV, 532.3eV and 533.5eV marked as O3, O2 and O1 respectively. These correspond to the metal bonded hydroxyl groups along with physisorbed/chemisorbed water.41 Electrochemical Analysis The electrochemical properties of the pristine rGO, Ni-Co LDH and Ni-Co LDH@rGO dendritic cell like 3D hierarchical structures were studied using cyclic voltammetry (CV) and



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galvanostatic charge-discharge measurements (GCD) to evaluate the suitability of this material for charge storage applications. A three-electrode system in 3M KOH solution as electrolyte was used. The CV curves in figure 6 (a) shows that both the samples exhibit a pair of redox peaks in the CV plots which corresponds to Ni(OH)2 peaks arising from the pseudocapacitive reaction of Ni2+/Ni3+, and two pairs of peaks in the unitary Co(OH)2arising from the redox reactions of Co2+/Co3+ and Co3+/Co4+. The CV of rGO in figure 6(b) showsa typical rectangular shape which infers that the capacitive behavior of rGO is mostly dueto EDLC type characteristics.

Figure 6: -(a) CV for synthesized Ni-Co LDH@rGO with dendritic cell like morphologies and Ni-Co LDH in 3M KOH solution using three electrode configuration at scanning speed of 5mV/s. (b) CV for rGO at 5mV/s. (c) Galvanostatic charge-discharge curves for the Ni-Co 14 ACS Paragon Plus Environment

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LDH@rGO (□) and Ni-Co LDH(+) and rGO (○) at 1A/g. The potential window for rGO is 0.9V - 0.1 V (left y-axis) while for the Ni-Co LDH@rGO and Ni-Co LDH is 0V - 0.4 V (right yaxis).(d) Comparison of specific capacitance obtained from CD measurements along with comparison with reported data (represented as lines) for similar material compositions in other morphologies. It is also evident that the charge-discharge curve of rGO evidently displays a linear and symmetrical nature with no internal resistance drop which depicts that the electrode material has good electrical conductivity. The high electrical conductivity of rGO will enhance the electron charge transfer at the interface for fast surface adsorption and desorption reaction. Evidently, the 3D-dendritic cell like Ni-Co LDH@rGO nanocomposite electrode exhibits larger area under the CV curve and high peak current density compared to pristine LDH nanosheets which implies that these 3D hierarchical structures are capable of large charge storage. The corresponding reversible Faradaic reactions occur during the electrochemical measurements are given below Ni(OH)2 + OH−↔ NiOOH + H2O + e− Co(OH)2 + OH−↔ CoOOH + H2O + e− CoOOH + OH− ↔ CoO2 + H2O + e− To explore charge storage capacity of the Ni-Co LDH@rGO, the samples were fabricated as a supercapacitor electrode and characterized using GCD measurements. Figure 6(c) presents a comparison of charge–discharge curves of rGO, Ni-Co LDH and Ni-Co LDH@rGO 3D hierarchichal nanostructures in 3M KOH solution in the potential range of 0–0.40 V at current



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density of 1Ag-1. The CD cycles for Ni-Co LDH and Ni-Co LDH@rGO is shown in supporting information as figure S8. It is evident that there is an apparent deviation in the discharge profile from a straight line, which validates that the obtained capacitance is predominantly from the redox reactions of both nickel and cobalt species in the sample. The Ni-Co LDH exhibits less charge discharge times than the synthesized Ni-Co LDH@rGO 3D hierarchical nanohybrid material suggesting the crucial role of rGO shells in enhancing the electrochemical activity of the synthesized morphology. It is apparent from the GCD profiles that the discharge curves have two characteristic regions one region with linear discharge profile which is dependent on the surfacelimited charge storage process due to the discharge of charges from the surface of the active material and a wide stretched plateau region with a large discharge end due to the redox species. The specific capacitance(C) is calculated from the equation C= It/ΔVm; where I is the constant current (A), t is the discharge time(s), ΔV is the potential window (V), and m is the mass of active materials (g). The results indicate that specific capacitance of the Ni-Co LDH@rGO 3D hierarchical hybrid nanostructure electrode exhibits a specific capacitance of about 2640 Fg-1 at current density of 1 Ag-1 as compared to 902 Fg-1 for the pristine Ni-Co LDH and 140 Fg-1 for rGO at same current density. A comparison of various graphene based hybrid materials and their electrochemical activity is tabulated in table 2. Table 2: - Comparison of various LDH graphene composites Material

Method

NiAl LDH/graphene43

Hydrothermal

Graphene/NiAl LDH44

Hydrothermal followed by

Morphology corrugated and scrolled sheets

Specific capacitance(Fg-1) 781 Fg-1 at 5mVs-1

915 Fg-1 at 2 Ag-1 Nanosheets 16

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LDH/rGO45

chemical reduction Layer by layer assembly

CoAl LDH/rGO46

electrostatic heteroassembly

GNS/CoAl-LDH47

Reflux method

LDH48

Electrostatic self-assembly

Hexagonal morphology

CoAl LDH/rGO/Nickel foam49

Hydrothermal Method

3D Nanosheets

1671 Fg-1 at 1 Ag-1

CoNi LDH/rGO46

Electrostatic heteroassembly

650 Fg-1 at 5Ag-1

NiCo LDH/rGO50

Solvothermal method One-pot hydrothermal method Chemical precipitation

Heterostacked LDH and rGO nanosheets Nanosheet architecture

1911 Fg-1 at 2Ag-1

Ultrathin Nanonosheets

1691 Fg-1 at 0.5 Ag-1

Sandwich like structure

1866 Fg-1 at 1 Ag-1

CoAl

rGO/CoAl

NiCo hydroxide@rGO51 RGO(25)@CoNiAl‐LDH52

Nanosheets

1204 Fg-1 at 5mVs-1

Heterostacked LDH and rGO nanosheets Laminated structure

450 Fg-1 at 5Ag-1 711 Fg-1 at 1 Ag-1 825 Fg-1 at 1 Ag-1

rGO/Ni1−xCoxAl-LDH53

In situ growth Technique

sandwich architecture

1902 Fg-1 at 1Ag-1

GNS/NiCoAl-LDH54

Hydrothermal method

3D Nanosheets

1962 Fg-1 at 1 Ag-1

Ni-Co LDH@rGO (This work)

Template assisted growth on graphene core shells

3D-Dendritic cell structure

2640 Fg-1 at 1 Ag-1

Performance analysis of Ni-Co LDH@rGO dendritic cell like morphologies In order to, demonstrate the potential of this novel materiala flexible hybrid supercapacitor device (FHSD) was assembled using Ni-Co LDH@rGO 3D hierarchical hybrid material as



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positive electrode while rGO was used as the negative electrode. Both the electrodes are assembled with PVA/KOH gel electrolyte sandwiched in between them. The entire device is represented as Ni-CoLDH@rGO//rGO. The specific capacitances of the hybrid device were calculated taking into account of the total active mass of both the electrodes. An optimum ratio is maintained to fabricate the electrodes of Ni-Co LDH@rGO and the rGO electrodes with a mass ratio calculated from the equation55, 56 m+/m− = C− × ΔV−/ (C+ × ΔV+)

… (1)

Figure 7(a) shows the Ragone plot for the hybrid device which gives a relation between specific energy density and specific power density which were calculated from the charge discharge profiles (supplementary figure S9) using the following standard equations.57 E = [1/ (7.2)] *CV2

Whkg-1

… (2)

P = (3600*E)/t

Wkg-1

… (3)

Where, E is the specific energy density (Wh kg−1), P is the specific power density (W kg−1) and Δt is the discharge time (s). C is device specific capacitance (=IΔt/VM; where, I is the discharge current (mA), V is the potential window and M is the total mass of both the electrodes).The device exhibits the high energy density of 35 Wh kg−1 at a power density of 750 W kg−1, and retains energy density of 19 Wh kg−1 at high power density of 3760 W kg−1 which is superior than the previous works reported elsewhere. A comparison of device performance of other relevant similar material composition/device architecture is tabulated as supplementary table S1. The comparison suggests that the present work exhibits superior rate capability in terms of energy storage during charge and discharge and a high cyclic performance.Notably, the obtained


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energy densities are higher than that of asymmetrical supercapacitors. The fabricated flexible supercapacitor described herein can deliver higher energy density under high power conditions.

(b)

(a)

Flat position Flexible position of device (c)

(d)

Bending angle

Figure 7: - (a) Ragone plot for the hybrid energy-storage device consisting of theNi-Co LDH@rGO//rGO in comparison to various energy storage devices (b) Photographs showing flexible hybrid supercapacitor device powering an electronic gadget in flat position (c) CV curves of flexible supercapacitor at various bending angles (d) Wearable supercapacitor device in working during bending position.



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Inorder to verify the flexibility of the fabricated solid state capacitor CV profiles were obtained under different bending positions with a constant scan rate of about 60 mVs-1 as shown in figure 7(c). The CV profiles are nearly same and no notable deterioration under normal and different angles of bending conditions is observed. These results prove good mechanical stability in real time application. CONCLUSION In a nutshell, we report the synthesis of surface enhanced GO shells ofNi-Co LDH@rGO in form of 3D dendritic cell like nanostructures. The method involves synthesis of spherical rGO shells coated with Ni-Co LDH lamellae during hydrothermal reaction. The SiO2 core within the GO shell gets annihilated with simultaneous reduction of GO shell. The formation of rGO is accompanied with growth of Ni-CO LDH nanosheets during this hydrothermal deposition process. The formation of hierarchical dendritic cell like morphology is validated by elemental mapping in HRTEM micrographs. Structural and spectroscopic analysis suggest the formation of Ni-Co LDH as well as other constitutive morphologies, which are used during the synthesis of Ni-Co LDH@rGO showing dendritic cell like morphologies. The presence of C=C signature in FTIR and characteristic peaks in C 1s spectrum of Ni-Co LDH@rGO confirms the hypothesis of presence of reduced graphene oxide shell in these dendritic cell like hierarchical structures. Electrochemical studies using CV show significant increase in charge storage capacity of these surface engineered rGO shells. GCD measurement suggest a high specific capacitance of ~2640 Fg-1 at 1Ag-1. A flexible device fabricated using the Ni-Co LDH@rGO show promising results as no noticeable compromise in device performance was observed with bending of the flexible device.


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Overall, this study reports a novel synthesis protocol and establishes that surface modification of rGO shells by lamellar LDHs with simultaneous reduction of GO results in synthesis of dendritic cell like nanostructures. These are superior nanostructured functional materials as compared to traditional GO, rGO, LDH and LDH pristine and nanocomposites. On a wider perspective, the proposed synthesis technique may be potentially used to synthesize other optimal nanocomposite formulations along with the possibility for providing enhanced surface area for high performance materials for energy storage and catalysis. Acknowledgements: The authors would like to thank Prof. Tim Albrecht at the department of chemistry, University of Birmingham for technical discussions. Supporting information: The Supporting Information is available free of charge on the ACS Publications website at DOI: xxx Sections M1 - M4 describes in details different parts of synthesis techniques used. The Materials characterization sections contains additional data such as TEM micrographs, XRD, FESEM micrographs, EDX, XPS survey spectrum, Raman measurements. The electrochemical Characterization section contains data on galvanostatic charge-discharge curves using 3 electrode system and impedance spectra for material characterization and measurements using 2 electrode system for device performance. The table S1 shows comparison of device performance of devices fabricates using similar reported material systems at 1 Ag-1.



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Surface Engineering of Graphene Oxide Shells ... - ACS Publications

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