Calixarene intercalated NiCo Layered Double Hydroxide for

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Calixarene intercalated NiCo Layered Double Hydroxide for enhanced oxygen evolution catalysis Babasaheb Jayram Waghmode, Aarti P. Gaikwad, Chandrashekhar V. Rode, Shivaram D. Sathaye, Kashinath R. Patil, and Dipalee D. Malkhede ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.7b04788 • Publication Date (Web): 11 Jul 2018 Downloaded from http://pubs.acs.org on July 16, 2018

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ACS Sustainable Chemistry & Engineering

Calixarene intercalated NiCo Layered Double Hydroxide for enhanced oxygen evolution catalysis. Babasaheb J. Waghmode*†,‡, Aarti P. Gaikwad∥, Chandrashekhar V. Rode∥, Shivaram D. Sathaye£, Kashinath R. Patil‡ and Dipalee D. Malkhede† †

Centre for Advanced studies in Chemistry, Department of Chemistry, Savitribai Phule Pune University,

Ganeshkhind Road, Pune-411007, India. ‡

Centre for Materials Characterisation Divison, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road,

Pashan, Pune- 411008, India. ∥Chemical

Engineering and Process Development Division, CSIR-National Chemical Laboratory, Dr. Homi

Bhabha Road, Pashan, Pune 411008, India. £

759/83 Deccan Gymkhana, Pune 411004, India.

Corresponding Author: E-mail:[email protected] (BJW) Abstract Water splitting provides a promising sustainable way to resolve the problems due to depleting fossil fuels. The success needs development of low-cost and high-performance electrode materials. The oxygen evolution reaction (OER) is a crucial reaction in water splitting. Ni and Co oxides developed nanostructures having a small overpotential and fast kinetics of OER has drawn considerable attention, because of their theoretically high efficiency, high abundance, low cost, and environmental benignity in comparison with precious metal oxides, such as RuO2 and IrO2. However, the desired efficiency needs the developments of enhanced specific active area and conductivity. In the present communication, we address these issues. Specifically, exfoliation of layer double hydroxide (LDH) is applied to enhance the active surface area. The study reveals that intercalation by calixarene in NiCo LDH affords a multifunctional interlayer to deliver a large active surface area and fast electron transport toward the carbon nano onion (CNO) support. It favorably lowers the overpotentials in OER (290 mV) and small Tafel slope of 31 mV/decade. Enhanced conductivity is achieve by using CNO as supports for calixarene intercalated NiCo LDH. These developments offer synergistic effect in achieving superior electrocatalytic activity for OER. This work gives the insight to design binder-free electrodes in alkaline media with good stability for advanced OER activity. KEYWORDS: oxygen evolution reaction, electrocatalyst, calixarene, exfoliation, NiCo LDH

†Electronic supplementary information (ESI) available: Procedure for Conversion of Hg/HgO to RHE; IR spectra, TEM micrograph, TEM-EDS, SEM micrograph SEM-EDS and SEM-Elemental mapping of NiCo LDHs and its composites CNO NiCo, CNO NiSO42-Co and CNO NiSC4Co; TEM-EDS spectra of CNO NiSC8Co after OER study; Table of atomic percentage of elements of initial CNO NiSC8Co catalyst and after OER study obtained from TEM-EDS and SEM-EDS; Comparative XPS survey scan, C1s, O1s, S2p, Ni 2p and Co 2p spectra of NiCo LDHs and its composites respective composites; Table of atomic percentage of elements of initial CNO NiSC8Co catalyst and after OER study obtained from XPS; CV of NiCo and its respective composite; Table of OER performance of the reported LDH based catalyst.

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INTRODUCTION Increasing energy demands and environment awareness have promoted extensive research on the development of alternative energy conversion and storage technologies with high efficiency and environmental friendliness. Among them, water splitting is very appealing and is receiving more and more attention. It is considered as one of the most clean, environmentally friendly, and sustainable approach to generate hydrogen, a green fuel. Oxygen evolution reaction (OER) is the process in which molecular oxygen is generated through electrochemical oxidation of water. It plays a vital role in a number of important energy conversion and energy storage processes.1-3 During OER the formation of O=O bonds, after breaking of the O-H bonds is kinetically sluggish4,5 in both acidic and alkaline media, and usually needs a cell potential significantly greater than the thermodynamic value of 1.23 V.6,7 Mixed metal oxides are commonly employed as catalysts for the OER.8,9 In the past few years, much efforts have been devoted to the development of alternative OER electrocatalysts based on cost-effective 3d transition metal hydroxides/oxides/carbides as a low-cost, highly efficient, and stable catalyst which can lower the overpotential for efficient water splitting.10-13 The Ni and Co co-existing electrocatalyst system may offer synergistic effect on OER. 14,15 Also, the attractive electrocatalytic properties of the Co and Ni-based oxides reported recently are inspiring.15,16 Transition metal oxyhydroxides, layered hydroxides are proved to be active materials in the field of catalysis. The OER activity of several LDHs has been studied.17-24 It is obvious that the use of exfoliated LDHs would have increased surface area for catalytic reaction.25,26 After exfoliation, ion exchange methods can be used to introduce intercalating groups within the layers of LDHs,27,28 depending upon the charges on layers.29 The sizes of groups to be introduced are an important parameter, especially in case of anions.30 The stability of anions is another important parameter. Normally, inorganic anions are introduced within the layers. Organic compounds are rarely employed as intercalating agents.31,32 Calixarene is a macrocyclic molecule having a typical arrangement of benzene groups, which offer π-π interaction. Furthermore, the molecule can be suitably functionalized so that it can be processed as ionic moieties. The Sulfato functionalized calixarene becomes a stable, water soluble organic species, which could be a suitable consideration for intercalation of LDHs. The main efforts put in the research of OER or water splitting, in general, are focused on increasing the number of active centers via increasing the surface area of catalyst materials and lowering the impedance to charge transfer at the involved interfaces. Therefore, material foams were considered as a solution to the former problem,33 for the problem of lowering impedance, binders, carbon fibers were considered. Especially, in the use of Ni foam, although the surface area increases, the redox kinetics becomes unfavorable. The additives like binder may lower composition resistance at the cost of enhancing a number of interfaces and therefore prove to be controversial.34 Therefore, to eliminate the effects of metal foam, environmentally stable carbon-based materials were considered as support to improve the electrocatalytic performance of 2D LDH. It is vital to prepare nanostructures with the high surface area and well-defined morphology that integrated with conductive carbon-based substrates. Stacking of positively charged LDH nanosheets on negatively charged carbon material enables direct interfacial contact between 3d transition metals and carbon, expressively shortening the diffusion distance. The carbon materials act as a support for LDH catalysts and increase their dispersion, heat, and mass transfer during the reaction. Also, it can offer the mechanical strength for the whole composites. Thus, the catalytic activity of LDHs is enhance in their carbon based nanocomposites material. The application of suitable composites may


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offer simultaneous solutions to both the problems discussed above. 2D/1D/0D carbon and an active transition metal (TM) oxides stable in large pH range may form ideal composites for OER. The carbon part of the composite would favor charge transfer while TM oxide may show suitable redox properties. It is prevalent to use graphene as carbon component; but for 3D composition formation, it was thought, unsuitable for processing large volumes of it. Therefore, CNOs were consider for supporting component in the present studies. CNOs have almost all the advantages of graphene35,

36

while its density is much higher. CNOs exhibit exclusive

physicochemical properties due to their pronounced edge effects,37,

38

possesses small graphitic sp2 carbon

domains with highly localized π electrons and peripheral defects in the form of dangling bonds.39 The redox TM component, in layered form, was considered more suitable as it would value add to the properties of composition through suitable redox properties and also complement the requirement of the higher surface area. Furthermore, if the interlayer space is expanded through monitoring suitable intercalants within the layers, the increase in volume would be added advantage for charge storage, affecting redox process favorably. Thus, in present studies, we have chosen CNO, NiCo LDH and sulfo derivatives of calixarene as basic components of composites and studied their OER activity. In the current work, taking into account of the comparatively high surface area, high electron conductivity and low costs, organic anions/macromolecules intercalated NiCo LDH on CNO substrate, based nanocomposite material is prepared by co-precipitation and ion exchange method. Also, the effect of intercalation of SO42-, p-sulfonate calix[4]arene (SC4) and p-sulfonate calix[8]arene (SC8) in NiCo LDHs on the OER performance is studied. The association of this calixarene intercalated NiCo LDH with current collector CNOs, forms interconnected electrically conducting networks, which can efficiently promote the fast electron transport. It was revealed that the catalytic activity was enhanced with an increased the size of the intercalating agent in LDH nanosheets. The higher OER activity of intercalated LDHs is essentially accredited to an increase in electronic conductivity and the number of active edge sites.40 When a composition of SC8 intercalated NiCo LDH nanosheets and CNO was optimized, the overpotential could be reduced to 290 mV and the corresponding Tafel slope to 31 mV/decade in 1 M KOH aqueous solution.

EXPERIMENTAL SECTION Chemicals Nickel (II) Nitrate Hexahydrate [Ni(NO3)2·6H2O, 99.99%], Cobalt(II) Nitrate Hexahydrate [Co(NO3)2·6H2O, 99.99% ], Synthesized CNOs, Sodium Sulfate (Na2SO4, (>99 %), and Sodium Hydroxide (NaOH, (>97%) was purchased from Sigma-Aldrich. 4-Sulphocalix[4]arene Hydrate (>98 %) and 4-Sulphocalix[8]arene Hydrate (>98 %) was purchased from TCI. All the reagents were of analytical purity and used without further purification. Synthesis of CNO supported, SO42-/SC4/SC8 intercalated NiCo LDH The intercalated LDHs was prepared by the coprecipitation method under N2 atmosphere. Ni(NO3)2·6H2O (1.5 mmol) and Co(NO3)2·6H2O (0.5 mmol) were dissolved in distilled water (20 cm3) and the solution was added drop-wise to a solution of Na2SO4/SC4/SC8 (1 mmol) in water (100 cm3) over 1 h. Then, 0.1 mol/dm3 NaOH solution was added in above solution while maintaining pH 7. The mixture was aged at 40 ◦C for 1 h. The



precipitate was separated by centrifugation, washed with distilled water. This material denoted as Na2SO4/SC4/SC8 intercalated NiCo LDH. This synthesized intercalated NiCo LDH were loaded on CNOs to prepare their respective nanocomposite material. CNOs were prepared by the method described by Azagan et al.41For the synthesis of CNOs, the ghee is burned directly in the flame, the formed carbon black is collected on a glass plate. Then the collected carbon black was annealed at 800oC in an inert atmosphere to form graphitized CNOs. These synthesized CNOs (1mmol) was disperse in 5% MeOH solution (20 cm3) then, sonicate for a period of 1h to disperse CNOs. Then add this solution in above intercalated NiCo LDH solution. The solution is stirred at RT for 24 h. After filtering and washing with distilled water and ethanol several times, the resulting powder was dried first at room temperature and then dried at 40 ◦C for 24 h in a vacuum oven. The formed material is denoted as CNO NiSO42-Co, CNO NiSC4Co, and CNO NiSC8Co respectively. To study the effect of intercalation and effect of substrate, the control samples such as NiCo and CNO NiCo is prepared, and their OER performance is studied.

CHARACTERIZATION The synthesized nanocomposites were characterized by using various physiochemical techniques. X-ray photoelectron spectroscopy (XPS) was carried out on V. G. Scientific, UK. ESCA-3000 model operated at a pressure of 6×10-8 Pa with non-monochromatized Al Kα X-ray source (1,486.6 eV photons). (Pass energy of 50 eV, electrons take off angle of 55o and an overall resolution of 0.1 eV). All binding energy (BE) values were charge corrected to C 1s = 284.6 eV as an internal standard. X-ray Diffraction (XRD) was conducted using a Philips X'pert pro powder X-ray diffractometer (Cu-Kα radiation, Ni filter). The specific surface areas of nanomaterials were obtained from the Brunauer-Emmett-Teller (BET) surface area measurements (Quantachrome Quadrasorb automatic volumetric instrument). The morphological data was obtained by using scanning electron microscopy (FEI Quanta 200 3D Dual Beam E-SEM). Transmission electron microscopy (TEM) images were taken on a JEOL 1200-EX instrument with an accelerating voltage of 200 kV and highresolution transmission electron microscopy (HRTEM-JEOL 2010F) at an acceleration voltage of 300 kV. TEM samples were prepared by placing a drop of the catalyst sample in ethanol onto a carbon-coated Cu grid, dried in air, and loaded into the electron microscopic chamber.

Electrocatalytic Study Electrochemical measurements were performed at room temperature using a rotating disk electrode made of glassy carbon (PINE, 5 mm diameter, 0.196 cm2) connected to a multichannel potentiostat (VMP-3 model BioLogic potentiostat) in a conventional three-electrode test cell with glassy carbon as the working electrode (The glassy carbon electrode was polished to a mirror finish and thoroughly cleaned before use.), Hg/HgO as the reference electrode and a graphite rod as the counter electrode. The potentials reported in work were referenced to the reversible hydrogen electrode (RHE) through RHE calibration in 1 M KOH. Procedure for calibration of electrode is provided in Electronic Supplimentary Information (ESI). The catalyst was synthesized for 5 times to check the reproducibility, and each time electrochemical activity is studied a couple of times. The preparation method of the working electrodes is as follows. In brief, 5 mg of catalyst powder was dispersed in 1 mL of 3:1 v/v water/isopropyl alcohol. The mixture was then ultrasonicated for about 0.5 h to generate a homogeneous ink.


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8 µL of the dispersion was transferred onto the glassy carbon disk, leading to a catalyst loading of ∼0.2 mg cm2

. Finally, the as-prepared catalyst film was dried at room temperature. For comparison, a bare glassy carbon

electrode that had been polished and cleaned was also dried for electrochemical measurement. Before the electrochemical measurement, the electrolyte (1 M KOH, 99.99% metal purity, pH ∼13) was purged by N2 (ultra-high grade purity, PRAXAIR) for at least 0.5 h to ensure the saturation of the electrolyte. The cyclic voltammetry (CV) curves were obtained by sweeping the potential from 0.96 to 1.96 V vs RHE at room temperature and 1600 rpm, with a sweep rate of 10 mV s-1. The impedance measurements were performed in the same configuration at open circuit potential over a frequency range from 20 kHz to 1 mHz at the amplitude of the sinusoidal voltage of 5 mV.

RESULT AND DISCUSSION The activity of multimetal LDH is improved by using exfoliation. The interlayer distance of LDH can be suitably expanded by changing the intercalating anion species; so that LDH can be easily exfoliated if the interlayer distance exceeds a certain threshold. Hunter et al. studied the effect of interlayer anions on the OER performance of NiFe LDH.30 Song and Hu obtained separated nanosheets by exfoliating LDHs with various metal compositions.42 The exfoliated nanosheets exhibited much better OER kinetics even with the similar surface areas of the pristine LDH. We have used wet synthesis methods for the preparations of nanocomposite material, where NiCo LDH get intercalate by using calixarene (SC4/SC8) molecule to increase the active surface area and conserve a homogeneous distribution of NiCo LDH sheets on the CNOs surface. LDHs have generally longer interlayer distances, it depending on the type of anion in the interlayer space. Using this dependence of the layer distance on the type of anions in the interlayer space, research to enhance the catalytic activity of LDHs has been extensively performed. Figure 1 depicts the synthesis of composite material. Detailed description of experimental method is given in the experimental section. The electrochemical measurements showed that the arrangements of intercalated NiCo-LDH on CNO support pays a synergistic effect, the nanocomposite material to the optimal OER activity and low voltage for water electrolysis with excellent long term stabilities. In this study, the intercalation of SO42-, SC4 and SC8 in the interlayer of the NiCo LDHs organized by co-precipitation method has been investigated and studied the effect of intercalation on OER performance of nanocomposite material. The aims of this effort are listed as following: (a) the anionic calixarene moieties is used as a intercalating agent between the layer of Ni-Co LDH (b) the uniformly distributed intercalated NiCo LDHs nanoplates on CNO substrate can exercise synergetic effects to efficiently convert water molecules to O2; (c) the cavity shaped porous component namely SC4/SC8 are, much beneficial to the charge transfer and mass transport (inward diffusion of electrolyte and outwards diffusion of gas bubbles). Electron microscopy was employed to investigate the microstructure of the synthesized nanocomposite materials. Morphology of as-obtained nanocomposites was characterized by TEM and SEM (Figure 2 and Figure 3). Figure 2A gives the TEM micrograph of CNONiSC8Co nanocomposite, it shows NiCo LDH nanoplates were uniformly dispersed over CNOs. Inset at bottom right observed a very thin exfoliated, hexagonal nanosheets of NiCo LDHs. The Selected Area Electron Diffraction (SAED) pattern (Insets at top right in Figure 2A) revealed the formation of polycrystalline nanosheets by the diffraction rings with hexagonally arranged spots over the diffused ring. The faint diffused halo of the SAED pattern is observed, because of relatively poor crystallinity, implying that the defects are present in the as-made nanocomposite. It



specifies that exfoliated NiCo LDH nanosheets are in polycrystalline nature, and these nanosheets having the same phase as per their bulk LDHs. It confirmed that, without changing a basic structure of the layer, bulk LDHs was delaminated to a thin MO6 network of LDH. Figure 2B displays a high-angle annular dark-field scanning TEM and its corresponding elemental mapping images (Figure 2B-i to Figure 2B-v) of the CNONiSC8Co nanocomposite. The results clearly demonstrate uniform NiSC8Co LDHs dispersions on CNO. It reveals the presence of C, O, S, Co and Ni at their respective positions. Figure 2C gives the EDS peaks of CNO NiSC8Co nanocomposite material. In composites, the peak of C arises from CNO substrate. The presence of Co, Ni and O ascend from NiCo LDHs component. The peak of S came from sulphate anion of the intercalated SC8 molecule. The peak of O arises from both NiCo LDHs and intercalated SC8 moities. Figure 3 shows the SEM micrograph, EDS and Elemental mapping of CNOCoSC8Ni nanocomposite material. SEM Micrograph of Figure 3A displays uniformly distributed NiCo LDH nanosheets on CNO and micrograph B gives their EDS pattern. It discloses the presence of C, O, S, Co and Ni. The morphology and EDS pattern obtained by SEM is consistent with the results obtained by TEM. The SEM elemental colour mapping (Figure 3C) depicts a uniform distribution of Ni and Co on the CNO along with the S and O. It specifies the successful fabrication of the SC8 intercalated NiCo LDH nanocomposite material. This unique morphology gifted by SC8 advances the specific surface area and coarseness of the OER active materials. These structural changes carried out by intercalation of calixarene between the LDH material is expected to promote augmented ionic conductivity and having a favourable access of electrolyte to the active nanocomposite electrode materials.43 TEM micrograph of NiCo LDHs, CNO NiCo, CNO NiSO42-Co and CNONiSC4Co are shown in Figure S2(A, C, E and G) and their respective TEM EDS spectra are shown in Figure S2(B, D, F and H) of ESI. SEM micrograph of NiCo LDHs, CNO NiCo, CNO NiSO42-Co and CNOCoSC4Ni nanocomposite material are given in Figure S2 (A, D, G and J). It appears that the sheets of NiCo LDH are dispersed on the CNOs in the nanocomposite materials. SEM-EDS spectra of these materials are given in Figure S2 (B, E, H and K). The spectra of NiCo LDHs show the peaks of O, Co and Ni. Along with these peaks, the CNO NiCo observed one more peak of C. The peak of C arises from CNO support. In case of intercalated, CNO NiSO42-Co and CNOCoSC4Ni nanocomposite material, along with these 4 peaks of C, O, Co and Ni, one more peaks of S are observed. These peaks came from the sulphate ions present in the intercalated molecules. The SEM elemental mapping of NiCo LDHs, CNO NiCo, CNO NiSO42-Co and CNO CoSC4Ni are shown in Figure S2(C, F, I and L). It reveals the presence and position of their particular elements in respective nanocomposite materials. Figure S5 shows the TEM-EDS and SEM-EDS spectra of initial CNO NiSC8Co catalyst and after OER. Table S1 gives the details of atomic percentage of elements in CNO NiSC8Co catalyst, before and after after OER test. It observed that after OER study the atomic percentage of Ni get reduced as per their concentration in initial catalyst. FT-IR spectrum of CNO NiCo LDH and its intercalated composite material are shown in Figure S2. It advised the presence of sulfonate ions of calixarene functionality and water molecules in the layered structure of NiCo LDHs. A broad absorption peak in the region 3000-3600 cm-1 was assigned to OH group stretches of both hydroxides for the basal layer and the interlayer SC4/SC8 molecule. Other absorptions below 995 cm-1 are associated with Ni/Co-O stretching and Ni/Co-OH bending vibrations. The weak absorption peaks at ~2946 cm1

were observed due to the bridged methylene (-CH2-) group present in the intercalated SC4/SC8 molecules. The

strong absorption peaks at 1037-1040 cm-1 were observed due to S-O functionality present in the intercalated


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molecules. The FTIR spectra of the intercalating agents Na2SO4, SC4 and SC8 are shown that, there was no change in the structure, before and after the intercalation. XPS characterization was performed to confirm the chemical composition and surface electronic state of CNO, CNO NiCo and its intercalated nanocomposites CNO NiSO42-Co, CNO NiSC4Co and CNO NiSC8Co respectively. Comparative survey scan of CNO, NiCo LDHs and its respective nanocomposite in the region of 0-1000 eV shows S2p, C1s, O1s, Co 2p and Ni 2p elemental peaks signifying the presence of S, C, O, Co and Ni elements in the fabricated sample (Figure S6A). Also, the highly resolved comparative C1s, O1s, S2p, Ni2p, and Co2p spectra are shown in Figure S6 B-F. Figures 4A shows the XPS survey scan of, initial CNO NiSC8Co catalyst and catalyst after OER. We observed that the intensity of Ni2p and Co2p peaks get decreased after 30h of anodisation in OER. Figures 4B(a) and 4C(a) show the high resolution XPS spectra of Ni 2p and Co 2p of initial CNO NiSC8Co catalyst. Two spectra curves exhibited the spin-orbit splitting into 2p3/2 and 2p1/2 with shakeup satellites (indicated as S). For the Co 2p curves of Figure 4C(a), the binding energy values of 780.8 and 796.5 eV correspond to the Co 2p3/2 and Co 2p1/2 with spin-orbit characteristics of Co2+ and Co3+ in cobalt hydroxide respectively. The high resolution spectra of Ni 2p show the binding energy values of 855.3 and 779.3 eV for Ni 2p3/2 and Ni2p1/2 corresponds to Ni2+ [Figure 4B(a)]. Spectrum 4B(b) and 4C(b) shows the Ni2p and Co2p spectra of catalyst after OER study. XPS measurement indicate that the initial ratio of atomic percentage of Ni:Co get reduced as the amount of active Ni2+/3+ has decreased after OER reaction. Interestingly, after OER the atomic percentage of S2p get increased (Figure 4D). This increase in atomic percentage of S may arise due to the escalation of exposed sulfato functionality of intercalated SC8 moieties after long time OER reaction. The details of XPS analysis with species moieties of elements and its atomic percentage of initial CNO NiSC8Co and after OER are shown in Table S2 of ESI. The binding energy values observed at 536 eV for the O1s core level spectrum (Figure S6C) can be assigned for the hydroxyl ions in nanocomposite. The low spin-orbit splitting of Co 2p (15.7) and low intensity of satellite peak suggesting the co-existence of Co2+ and Co3+ in LDH nanocomposite.44,45 These results indicate the formation of hybrid NiCo LDH38 and SO42- ions /SC4/SC8 molecule have been intercalated in the composite. The XPS results confirm the composite consisting of Ni and Co hydroxides components can lead to superior electrochemical properties as a metal hydroxide based nanocomposite material for efficient OER perfrmance. The XPS experiments suggest that the oxidation states and electronic structures of NiCo LDH are quite complex and dynamic upon both exfoliation and OER measurements. The catalytic activity can be significantly enhanced by exfoliation of NiCo LDH nanosheets. Structural characterization showed that the exfoliation not only resulted in thinner layers with reduced size but also caused a change in the electronic structure. The intercalation of calixarene molecule opens up opportunities for rational design and improved OER activity of 2D layered materials with well-defined morphology. This exfoliated layer material on CNO support provides specific active sites to enhance the electrocatalytic OER performance of layered materials. Based on the values of atomic percentage of elements obtained from TEM EDS, SEM EDS and XPS analysis, ratio of Ni:Co in the initial CNO NiSC8Co nanocomposite was determined as 3:1, which is close to that in the reactant mixture of the precursor. After 30 h anodisation in OER, the ratio gets decreased as the atomic percentage of Ni gets reduced. The X-ray diffraction (XRD) patterns of the NiCo LDH and its nanocomposite materials with CNOs are shown in Figure 5. XRD is important techniques to the assessment of the exfoliation, and ion exchange of LDHs. As shown in Figure 5, XRD patterns of the NiCo LDH, CNO NiCo and their intercalated NiCo LDH



nanocomposite materials are alike with the standard XRD pattern of NiCo LDH, but with peaks being shifted.46 Spectra a of Fig 5 shows the XRD pattern of NiCo LDH. Where we get a peaks at 2θ= 09.70 (003), 19.80 (006), 34.20 (101) and 60.10 (110). XRD pattern of CNO NiCo shows all the peaks at same 2θ values to that of XRD pattern of NiCo LDH, only intensity of each peak is lower. Figure 5 c, d and e gives the XRD pattern of expanded LDH based nanocomposite material, CNO NiSO42-Co, CNO NiSC4Co and CNO NiSC8Co respectively. The intercalated LDH nanocomposite material shows the diffraction peaks of NiCo LDH and each peak get shifted to lower 2θ values with lowering intensity. The asymmetric peaks may arise due to stacked layer structure. The diffraction patterns approves the presence of α-Ni(OH)2 and α-Co(OH)2 LDHs.47 Along with 4 intense peaks, the NiCo LDH and CNO NiCo LDH observe the low intensity peak at 2θ=38.25 (101), these peak arises due to the presence of β-Ni(OH)2 impurity in the composite material. The shifting of peaks implies that the intercalation of SO42-, SC4 and SC8 is superior to that of the H2O molecules in their respective nanocomposite materials. This expanded LDH structure is observed due to the intercalation of anions between the Co-Ni LDH. The arrangement of intercalating group in the interlayer space of the LDHs was supposed to be different by the number of dissociated OH group and the strong point of the electrostatic force of attraction between the negative intercalating group and the positive LDH basal layer. Also, the formation of `Co/Ni-anion´ complexes would block the interface between the dissociated OH groups and the LDH basal layer. Therefore, SC4/SC8 cavity axis was alleged to orient horizontally to the LDH basal layer and form a porous network structure in their respective nanocomposite material. TEM and SEM images revealed that, calaxirane can coordinate to specific organization of NiCo LDH and govern the porosity of a catalyst, thus, influence the mass-transport phenomena at the surface. This is an important factor in the promotion of early bubble detachment during OER. In catalysis, surface area plays an important role, suitable modifications in nanostructures of nanocomposite material which increase the active surface area is often predominantly responsible for improved catalytic activity. The cavity shaped sulfato calixarene, due to its water solubility, has been employed as suitable intercalating moieties. The delocalised π electrons in calixarene moieties may improve the conductivity of nanocomposite material. The OER on high-surface-area catalytic NiCo LDH layers proceeds with an increased active site density of catalyst after intercalation of calixarene moieties. This means that reaction sites in the “inner” surface also become active with an increasing size of the intercalated species so that the reaction “spreads” into the inner catalytic layers. We observe the enhanced OER activity of exfoliated LDH comparing to the catalytic activity of bulk LDH. The Surface area of the nanocomposite material is measured by BrunauerEmmett-Teller (BET) method. The difference in electrochemically active surface areas is mainly influence on the catalytic activity of the particular nanocomposite material. The order of electrochemical activity can be speculated based on the differences in the BET surface area of nanocomposite material after interaction. The calixarene intercalated, CNO NiSC8Co (134.360 m2/gm) and CNO NiSC4Co (87.683 m2/gm) nanocomposite exhibited a relatively larger surface area than those of a SO42- intercalated, CNO Ni SO42-Co (67.776 m2/gm) and their corresponding precursors of CNO NiCo, (44.053 m2/gm), NiCo (31.297 m2/gm) respectively. Here, we observe that as the size of intercalating component increase from SO42- to SC4 to SC8, the surface area of their respective nanocomposite material is increases. This leads to increase the accessible electrochemically active inner surface of LDH. It is clear that the specific surface area of CNONiSC8Co (134.360 m2/gm) is much larger than other nanocomposite materials. Therefore, the high electro catalytic performance of CNONiSC8Co


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could be partly ascribed to (a) the enormous surface area arise due to the exclusive layered structure of NiCo LDH, and (b) the intercalated SC8 exposes the active catalytic sites of the NiCo LDHs. The rigid multilayers of LDH transform into bi layer/mono layers of LDH through exfoliation. These exfoliated LDH assembled on a conducting substrate has better electron transport property than the bulk LDHs. Thus, the improvement in conductivity might be a contributor for the enhancement in activity of catalyst. Exfoliation of LDH improves the chemical and electronic properties of its nanocomposite material, leads to increase in OER activity.48,49

Electrochemical performance of the OER catalyst The water oxidation catalytic activities of the as-synthesized exfoliated NiCo LDHs nanocomposite with CNO were tested for electrochemical OER in 1M KOH solutions in a standard three-electrode setup. The nanocomposite catalysts were electrochemically pre-conditioned by 50 cyclic voltammetric scans and galvanostatic oxidation to reach a stable state (Figure S7B). During the electrochemical test, produced oxygen bubbles were removed by continuously rotating the working electrode at 1600 rpm. Individual Co/Ni hydroxides have been known to be proficient OER catalys.50 However, nanocomposite material with exfoliated nanosheets of NiCo LDH exhibit analogous and often imposing activity to the best known metal oxide catalysts.51 LDHs are fundamentally different from their hydroxide through a topotactic transformation, the structure of the LDH has galleries containing Co and Ni cations that have been partly oxidized to higher valence states. To examine whether the active surface area of catalyst would affect its OER performance, composite electrode materials with different intercalating groups between the layers of LDHs were fabricated. Linear Sweep Voltammetry (LSV) curves of the composite (Figure 6A) became more pronounced as the size of intercalating group between NiCo LDH increased. LSV curves of exfoliated NiCo LDH nanocomposite materials of CNO NiSO42-Co, CNO NiSC4Co and CNONiSC8Co catalysts are shown in Figure 6A (d, e and f) and their components such CNO, NiCo and CNO NiCo was also tested for comparison purpose [Figure 6A (a, b and c)]. Based on the results of LSV obtained at Figure 6A, it shows that the catalytic currents were shifted to expressively lower overpotentials upon exfoliation of bulk NiCo LDH materials. Accordingly, the exfoliated NiCo LDH is better catalysts than their bulk counterparts. Different parameters were used to quantify the improvement of activity, these are: (a) the overpotential at a current density (J), 10 mA/cm2, and (b) the Tafel slope. The current density 10 mA/cm2 was chosen because it represents the current density from a device with 10% efficiency of solar-to-fuel conversion, which is at the upper end of a realistic device. CNONiSC8Co catalyst exhibits significantly higher anodic current and lower onset potential than that of the other catalysts [Figure 6A (f)]. It achieves the current density of j=10 mA/cm2 at the over potential of 290 mV, which is much better than that of the CNONiSC4Co catalysts (305 mV), CNONiSO42-Co (315 mV), CNONiCo (324 mV), NiCo (337mV), and CNO (381 mV) in the same experimental condition. Table S3 of ESI shows the electrocatalytic performance of some reported LDH based nanocomposite electrodes for overall water splitting. The obtained results demonstrate that the samples consisting of exfoliated NiCo LDHs and CNOs have decent OER activity. Remarkably, we observe that the catalytic activity of nanocomposite material is increased as the size of intercalating group increases. However, SC4 and SC8 show maximum exfoliation of LDHs than that of SO42- intercalation. Regardless of the comparable OER overpotentials of the exfoliated NiCo LDH catalysts, at the same applied potential, CNO NiSC8Co can achieve the highest current density. This result give emphasis to the significance of assembling of SC8 intercalated NiCo LDH materials on CNOs substrate for efficient



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electrochemical catalysts. The catalysts has the OER activity order as CNO NiSC8Co> CNO NiSC4Co> CNO NiSO42-Co> CNO NiCo > NiCo> CNO for both exfoliated LDH nanocomposite and their bulk counterpart. Figure S7A shows the cyclic voltammograms (CV) of of CNO, NiCo, CNO NiCo, and its exfoliated nanocomposites namely, CNO NiSO42-Co, CNO NiSC4Co, CNO NiSC8Co respectively. The CV of nanoomposite exhibits oxidation peaks at potentials (E) 1.47 V vs NHE before the catalytic wave. These peaks are due to cationic redox steps. It can be dispensed toward the Co(III)/Co(IV) transition.52 Then, a large catalytic current arises with an onset potential of 1.52 V. The CV results indicate that, to perform the water oxidation reaction efficiently. The back scans shows a broad cathodic peak, signifying the de oxidation of the oxidized species. The absence of the Ni redox peak is possibly due to its overlap with the OER wave. In general mechanism of OER, once the water molecule coordinates to a surface active site of the metal oxide, the surface MOH species is formed via electron transfer and simultaneous proton release.53 Intercalation of SC8 between layers of NiCo LDH is believed to promote the active site density. The exfoliated NiCo LDHs based nanocomposite material needs lower energy than bulk NiCo LDH. In aqueous electrolyte, faradic reaction sites on the intercalated NiCo LDH increase, because, it gets easy access of the ions to the electrode/electrolyte interface and thus increases the faradic reaction sites in nanocomposite material. Figure 6A shows that at J=10 mA/cm2, the over potentials were decreased upon exfoliation of LDHs. Also, it shows that the current densities were enhanced by intercalation of SO42-/SC4/SC8. According to Henrys law, when the rate of the OER becomes high, the oxygen may overcome the saturation concentration and gas bubbles start to form and behave like a “sink” for further dissolved gas.54 It can block portions of the electroactive surface area, and inhibit the kinetics of the electrode, unfavourably, leading to increase in overpotential.55 We have used a key approach to performance optimization, where, calixarene can nucleate gas bubbles and restricted their growth. The CNO NiSC8Co catalyst can stimulate the detachment of small bubbles with a high frequency and leads the acceleration of gas evolution through intercalated SC8 moieties resulting in the decrease in overpotential. To obtain further insights into the oxygen evolution activity, Tafel plots of various catalysts were investigated Figure 6B displays the Tafel plots derived from the polarization curves in Figure 6A. Tafel plot is used to analyse mechanism of the catalytic reaction, the Tafel equation

where η is the over potential, b

is the Tafel slope, j is the current density, and j0 is the exchange current density. The Tafel slopes of CNO NiSO42-Co, CNO NiSC4Co, and CNO NiSC8Co are 53, 45, and 31 mV/decade, respectively [Figure 6B (d, e and f)] , while those of their bulk counterparts CNO NiCo is 59 mV/decade and for unsupported NiCo, it is 63 mV/decade, and for CNO is 166 mV/decade [Figure 6B (a, b and c)]. The electron transport is more facile on few layer nanosheets than in bulk LDHs, therefore after intercalation of organic anion, the formed nanocomposite material become electronically dynamic. The closely parallels LSV result (Figure 6B) direct that the exfoliated NiCo LDH exhibit higher current densities than the others at the same applied potential, and manifests again the decreased charge-transfer resistance owing to the intercalation of anions in LDHs. As expected, the CNO NiSC8Co nanocomposite show favourable kinetics of water oxidation. The Tafel result advances further support to the conclusion that the unconventional catalytic performances of these nanocomposite on OER is derived from the strong interactions between the NiSC8Co and the CNOs at the molecular level. To reveal the kinetics on the surface of the electrodes, Nyquist plots were obtained by electrochemical impedance measurement at a constant potential of -1.2 V versus RHE. Figure 6C shows the electrochemical


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impedance response of electrode materials recorded over a range of potentials accompanying with active oxygen evolution. The Nyquist plots (-Zim vs. Zreal) of the above catalysts consist of a depressed semicircle in the highfrequency region (corresponding to charge transfer resistance, Rct) and a quasi-sloping line in the low-frequency region (corresponding to mass transfer resistance). One semicircle suggested a one-time-constant behaviour for both NiCo LDHs and its CNO supported nanocomposite material.

Rct can be directly measured as the

semicircular arc diameter. The values of Rct for CNO, NiCo, CNO NiCo, CNO NiSO42-Co, CNO NiSC4Co, and CNO NiSC8Co composite electrodes are 16.63, 11.54, 8.83, 6.96, 4.26 and 2.94 Ω respectively. Clearly, the CNO NiSC8Co nanocomposite exhibit Rct much lower than that of other component materials; which demonstrates that the anions intercalated LDH on CNOs enhances conductivity and improves charge transfer performance of composite electrode material. However, the supported catalyst had a lower Tafel slope as compared to unsupported catalyst. Spectroscopic analyses indicated that the CNOs support induced a cathodic shift and enhancement of the catalyst redox wave in the precatalytic potential range. The support also enhanced OER activity by particle dispersion, allowing a larger population of active sites to become accessible. This was solid evidence of the synergistic effect of the exfoliated NiCo LDH and the CNO support. Stability is another important factor in energy conversion systems. Tailoring the morphology of OER catalysts is a key factor in controlling the accessible active sites and utilization of the catalyst. The available active area determines mass-transport phenomena, such as bubble detachment, which plays an important role in the stability and activity of an OER catalyst.56 Fundamental studies focusing on the catalytic performance of inaccessible crystal faces of a catalyst have helped in improving the design of catalysts for OER. Exfoliation of LDH by calixarene adapts the exposed facets of nanostructured catalysts for better performance. The CNONiSC8Co catalyst exhibits good stability in the alkaline solutions. Figure 6D gives the durability of catalyst in 1 M KOH solution. After 10 h, the CNONiSC4Co, CNO NiSO42-Co, CNO NiCo and Bulk NiCo LDHs catalyst electrode degrades by 14.80%, 17.91%, 25.90% and 29.83% of the initial value [Figure 6D(a-d)]. However, the CNONiSC8Co electrode shows outstanding durability up to 30h. The catalyst shows substantial decrease of normalised current for first 2 hours and then lowers the activity decay. The catalyst shows 6.46 % loss of current for first 10h, we have continued the test for 30h and only 8.38% of activity decay was observed compared with the initial value [Figure 6D(e)]. The anodic electrode current observed is often referred to as corrosion current or it is assumed to correspond with the loss of supported carbon material. In electrochemical studies carbon supported metals catalyst may cause corrosion, however, the analysis of the electrode current is further complicated by the electrochemical reactions of the supported metal particles. The corrosion studies may also focus on the dissolution of electrodes composite material. In TEM study, no obvious morphology change and decay is observed in the TEM image of CNONiSC8Co after long-term stability testing (see Inset of Figure 6D), suggesting that the catalyst can tolerate long-term corrosion with robust mechanical properties. Concerning the role of NiCo LDHs, and in light of the significant partial carbon oxidation current observed, it should be emphasised that there is a crucial difference between NiCo LDHs catalysing the corrosion of CNOs, and it may catalysing the oxidation of corrosion products. Alkaline media can be electrochemically oxidized to β- phases of Ni(OH)2, it acts as a corrosion resistance toward the OER. During OER, the electrode surfaces blocked by gas bubbles will not participate in the reaction, leading to an increase in the current passing through the remaining catalytically active sites and causing a higher stress on the active parts of the electrode. Good conductivity of CNO support and strong interaction between



active LDH layer and CNO support which plays a very important role to reduce the stress. Also, it is proposed that the cavity shape of intercalated calixarene would facilitate faster bubble detachment. It demonstrates that a high surface area structure of porous calixarene intercalated LDH will enhance the number of preferential surface active sites; it can simultaneously improve activity and stability of catalyst. 57 It demonstrates that a high surface area structure with preferential surface sites can simultaneously improve the activity and stability of catalyst.

49

Fig. 6A of LSV shows that exfoliated NiCo LDHs on the CNO substrate shows more reductive

potentials; the OER activity increases, eventually reaching a current density greater than 10 mA cm-2 with a low overpotential value. In practice, the factors affecting the Tafel slope are complicated. The big Tafel slopes mainly occur from the poor electrical conductivity of NiCo LDHs. To verify it, the anion intercalated NiCo LDH hybrids, such as of NiSO42-Co, NiSC4Co, NiSC8Co with CNOs was synthesized and its morphology is confirmed by SEM and TEM, where we observed that, the exfoliated NiCo nanosheets uniformly adhere to the surface of CNOs. As expected, the value of Tafel slope decreases with increasing size of intercalant molecule. It may be attributed to intercalant calixarene which exposes the inner part of LDHs. In turn, it leads to increase in number of active sites of catalyst. Nyquist plots can be understood by considering CNOs offers conducting paths and expedite electron transport. The influence of size of intercalant in nanocomposite materials is also evaluated. As shown in Figure 6A the OER activity increases with increasing size of intercalant; while the highest activity is obtained with an SC8 intercalated nanocomposite material. Mechanism contains four-electron transfer is a commonly acceptable OER mechanism.58 LDH based catalyst possibly shows following four simple steps to generate O2. OH- + *

→ OH* + e-

*

OH + OH *

-

→ O + H2O(l) + e

-

*

O + OH *

(I)

*

→ OOH + e -

-

(II)

-

OOH + OH → * + O2(g) + H2O(l) + e

(III) -

(IV)

The initial OER step involves OH* formation/adsorption; followed by subsequent oxidation of OH* into O* [step (II)]. Then the O* react with OH- to form OOH* is step (III); The final oxidation step releases O2 gas by the combination of OOH* with OH- species and leaves a free metal site available for repeated catalytic cycles. Relation of onset potentials and Tafel slopes in various anions intercalated LDH with CNO support such as CNO NiSO42-Co and CNO NiSC4Co indicates that the intercalation of anions do influence the inherent OER activity of the NiCoLDH based catalyst. In the present case, CNO NiSC8Co produced an average Tafel slope of 31 mV per decade. As per the mechanism, a higher surface area would favour the OER activity. Thus, the intercalation of calixarene favours NiCo LDHs structures, facilitate the OER. The presence of intercalated LDHs would be useful for availability of OH- which is necessary for lower overpotential. The other important part for lower overpotential is fast charge transfer which is favoured by the presence of CNOs. These factors are beneficial to the enhancement of the OER performance of the nanocomposite. However, lower overpotential for OER is an important factor which decides the efficiency of the catalyst. The presence of intercalated LDHs would be useful for the availability of OH- which is necessary but not a sufficient condition for lower overpotential. The other part important for lower overpotential is fast charge transfer which is favoured by the presence of CNOs. The coupling of LDH nanosheets with intercalated calixarene is to surge the conductivity. These factors are beneficial to the enhancement of the OER performance of the nanocomposite with the least


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overpotential. XPS observations support the above explanation, after the OER, Ni is in the oxidised state which will adversely affect the charge transfer. CNO NiSC8Co electrode maximizes direct interfacial contact between 3d transition metal Ni/Co active matrix and conductive CNOs, significantly enhancing electron transfer through shortening diffusion distance. Also, the strong interaction between active NiCo LDHs phase and CNO induces local electronic structural change around metal centres and thus the redox behaviour leading to more active catalysts. The uniformly dispersed SC8 intercalated NiCo LDH on CNOs enable fast charge transfer pathways by avoiding the use of binders and conducting additives. This nanostructure ensures improved electronic conductivity owing to the contribution of intercalated calixarene moieties. The delocalised π electrons within the calixarene may ensure a good electrical connection between the electroactive NiCo LDH nanosheets. Furthermore, these CNOs serve as the current collector, and more importantly, prevents the use of polymer binder and conductive additives and substantially reduces the electrode “dead volume.” After taking into account our morphological study, spectroscopic characterization as well as electrochemical studies, we propose that the observed enhancement in the OER activity of NiCo LDHs after the SC8 intercalation of CNO NiSC8Co caused by the local participation of LDHs in the OER catalysis. This interpretation is consistent with all of the results of our study and is also supported by the published literature in the area of OER catalysis for LDH based materials. Table S3 of the ESI† convincingly demonstrates that the samples consisting of both calixarenes exfoliated NiCo LDHs and CNOs had anomalously high OER activity when compared to other active LDH based OER catalysts.

CONCLUSIONS The synthetic expediency of LDHs and its ease of exfoliation make them decent applicants to substitute bulk noble metals in renewable energy applications. There has been great interest in multimetal systems as an effective strategy to enhance the catalytic efficiency of metal-oxide-based OER catalysts. This work is marked by a binder-free hybrid system of intercalated NiCo LDHs on CNOs support. By charismatic improvement of the comparatively high surface area, enhanced conductivity, and synergistic effects; the well-designed CNOCoSC8Ni nanocomposite electrode exhibits outstanding OER performance with small over potential of 0.29 V at 10 mA/cm2 and decent stability in alkaline medium. The electrocatalytic performances were primarily due to the synergistic effect between the active bimetallic NiCo LDH structure and the enhancing electron transport of the CNOs. These properties, as well as the durability and cost-effectiveness of the NiCo LDH, make them promising materials for replacing noble metal-based catalysts for water splitting. The OER activity of the CNO NiSC8Co was significantly enhanced mainly because of the reinforced active surface area, electron transport, and the synergetic effect in addition to rich redox properties of the LDH structure. Therefore, this study successfully demonstrates an accessible strategy through designing the inexpensive calixarene intercalated NiCoLDH-based structural design of the catalyst toward global scale clean energy production.

ACKNOWLEDGEMENTS The Authors thank Mr. Suhas Deo, Mr. Ramkrishna Gholap and Mrs. Pooja Kumbhar for technical assistance in XPS, TEM and SEM characterisation and insightful discussions regarding this work.



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Figures Table of content: Figure 1. Schematic illustration of the synthesis of SO42-/SC4/SC8 intercalated NiCo LDH and its nanocomposite electrocatalyst. Figure 2. (A) TEM micrograph; Inset at top right shows the corresponding SAED pattern, Inset at bottom right shows the highly resolved exfoliated LDHs, (B) dark-field scanning TEM and TEM Element mapping image for (B-i) carbon, (B-ii) oxygen (B-iii) sulphur (B-iv) nickel, and (B-v) Cobalt, and (C) TEM-EDS of CNO NiSC8Co electrocatalyst. Figure 3. (A) SEM micrograph (B) SEM-EDS and (C) SEM Element mapping image for (C-i) carbon, (C-ii) oxygen (C-iii) sulphur (C-iv) nickel, and (C-v) Cobalt of CNO NiSC8Co electrocatalyst. Figure 4. (A) XPS survey and XPS spectra of the (B) Ni2p, (C) Co2p and (D) S2p peaks of (a)initial CNO NiSC8Co and (b) after OER CNO NiSC8Co electrocatalyst. Figure 5. Comparative X-ray diffraction patterns of (a) NiCo, (b) CNO NiCo, (c) CNO NiSO42-Co, (d) CNO Co SC4Ni, and (e) CNO NiSC8Co electrocatalysts. Figure 6. Comparative (A) LSV plots, (B) Tafel plots, (C) EIS Nyquist plots of (a) CNO, (b) NiCo, (c) CNO NiCo, (d) CNO NiSO42-Co, (e) CNO NiSC4Co, and (f) CNO NiSC8Co electrocatalysts, in 1 M KOH electrolytes, and (D) Chronoamperometric responses of (a) NiCo, (b) CNO NiCo, (c) CNO NiSO42-Co, (d) CNO NiSC4Co, and (e) CNO NiSC8Co electrocatalysts, in 1 M KOH electrolytes.

Figure 1. Schematic illustration of the synthesis of SO42-/SC4/SC8 intercalated NiCo LDH and its nanocomposite electrocatalyst.


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Figure 2. (A) TEM micrograph; Inset at top right shows the corresponding SAED pattern, Inset at bottom right shows the highly resolved exfoliated LDHs, (B) dark-field scanning TEM and TEM Element mapping image for (B-i) carbon, (B-ii) oxygen (B-iii) sulphur (B-iv) nickel, and (B-v) Cobal, and (C) TEM-EDS of CNO NiSC8Co electrocatalyst.

Figure 3. (A) SEM micrograph (B) SEM-EDS and (C) SEM Element mapping image for (C-i) carbon, (C-ii) oxygen (C-iii) sulphur (C-iv) nickel, and (C-v) Cobalt of CNO NiSC8Co electrocatalyst.



Figure 4. (A) XPS survey and XPS spectra of the (B) Ni2p, (C) Co2p and (D) S2p peaks of (a)initial CNO NiSC8Co and (b) after OER CNO NiSC8Co electrocatalyst.

Figure 5. Comparative X-ray diffraction patterns of (a) NiCo, (b) CNO NiCo, (c) CNO NiSO42-Co, (d) CNO Co SC4Ni, and (e) CNO NiSC8Co electrocatalysts.


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Figure 6. Comparative (A) LSV plots, (B) Tafel plots, (C) EIS Nyquist plots of (a) CNO, (b) NiCo, (c) CNO NiCo, (d) CNO NiSO42-Co, (e) CNO NiSC4Co, and (f) CNO NiSC8Co electrocatalysts, in 1 M KOH electrolytes, and (D) Chronoamperometric responses of (a) NiCo, (b) CNO NiCo, (c) CNO NiSO42-Co, (d) CNO NiSC4Co, and (e) CNO NiSC8Co electrocatalysts, in 1 M KOH electrolytes.



For Table of Content use only

! The developed CNO NiSC8Co is a Low cost, environmental friendly and efficient OER catalyst to attain the increasing energy demands.


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Calixarene intercalated NiCo Layered Double Hydroxide for

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