Template assisted hydrothermal synthesis of CoSnO3 hollow microspheres for electrocatalytic oxygen evolution reaction

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 1 6 2 3 e2 1 6 3 6

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Template assisted hydrothermal synthesis of CoSnO3 hollow microspheres for electrocatalytic oxygen evolution reaction Suryakanti Debata a,*, Sanchari Banerjee a, Sanchari Chakraborty b, Prashant K. Sharma a a

Functional Nanomaterials Research Laboratory, Department of Applied Physics, Indian Institute of Technology (Indian School of Mines), Dhanbad, JH 826004, India b Department of Applied Physics, Birla Institute of Technology, Mesra, Ranchi 835215, India

highlights
Systematic

method

graphical abstract to

develop

CoSnO3 HS by template assisted hydrothermal route.
CoSnO3 HS possesses higher BET surface area and greater Cdl than the pure CoSnO3.
CoSnO3 HS exhibits high OER catalytic activity and high stability in alkaline medium.
The TOF, specific as well as mass activities of CoSnO3 HS are also significant.

article info

abstract

Article history:

The practical complications suffered by the most recognized electrochemical energy sys-

Received 29 October 2018

tems, such as, water-electrolyzers and metal-air batteries reside in the half-cell oxygen

Received in revised form

evolution reaction. To resolve this problem, continuous colossal efforts are required to

30 May 2019

develop the active, affordable and sustainable electrocatalysts. Shape-tailoring of the

Accepted 16 June 2019

catalysts, constructed from non-noble metals is one of the emerging strategies to augment

Available online 11 July 2019

the activity of the material toward electrochemical reactions. In the present work, we demonstrate the template-assisted hydrothermal synthesis of hierarchical CoSnO3 hollow

Keywords:

microspheres, constructible from the wafer-thin sheets of CoSnO3. The hierarchical

Electrocatalytic

CoSnO3 hollow microspheres possess a high specific surface area of 153.59 m2/g, and

OER

mesoporous configuration, which are the essential pre-requisites of an electrochemical

Amorphous

system. In addition to this, the proposed CoSnO3 hollow microspheres possess adequate

CoSnO3 hollow microspheres

electroactive surface area (793.5 cm2) and happens to be a suitable candidate for driving the

Template assisted synthesis

oxygen evolution reaction with a low overpotential of 282 mV and Tafel slope of 96.5 mV/ dec in alkaline medium. The higher turnover frequency (0.0045 s1), high specific and mass

* Corresponding author. E-mail address: [email protected] (S. Debata). https://doi.org/10.1016/j.ijhydene.2019.06.094 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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activities (2.195 mA/cm2EASA and 28.752 mA/mg, respectively) were observed for CoSnO3 hollow spheres. Furthermore, the chronoamperometric measurement reveals a good stability of CoSnO3 hollow microspheres in alkaline condition, satisfying the fundamental demand of an energy system. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction The class of nanomaterials possessing unique morphology are hugely appraised for a wide range of technical applications, which include oil/dye sorption, catalysis, biomedical applications, biosensing and energy generation/storage [1e7]. The immense attraction of shape-tailored and porous nanomaterials in electrochemistry (exclusively in water electrolysis and metal-air energy-storage systems) is owed to the high surface-to-volume ratio and large exposure of active centres participating in the reaction. Also, the porous structures possess lower density in the equivalent volume of their bulk analogies, rendering better penetration of electrolyte inside the material, faster electron/ion transport process and adequate contact between the active sites and guest molecules. Since the last few years, a huge variety of shape-tailored porous nanomaterials have been passionately explored for electrochemical applications. Recently, Lin et al. have developed a highly ordered 3D-architecture of macroporous LaMnO3 with interconnected pores to improve the electroactive sites of the material [8]. Guan et al. have proposed a general method of preparing the multi-shell MneCo oxyphosphide spheres of micrometer dimensions, via the phosphidation of multilayered MneCo oxide particles, which were initially formed by the thermal treatment of MneCo coordination polymer precursor [9]. In addition to these, the porous nanowires [10e12], hollow cages [13,14], multishelled nanoboxes/nanotubes [15e17], hollow nanoellipsoids [18], hollow spheres [19] etc., made of nano-dimensional units have been brought into energy research for improving the performance of the electrochemical devices. The synthesis of hollow microstructures by template mediated approach is preferable because there always remains a chance of getting the desired shape of the material with uniform distribution of nano-dimensional units by this process. For example, Ma et al. have synthesized cobalt sulfide (CoSx) hollow nanospheres of three different molar compositions and phases by using CS2 templates. This methodology resulted in the construction of CoSx hollow spheres (HSs) of different diameters and high surface roughness, which were further applied for the electrochemical full-water-splitting [20]. Likewise, Zhao et al. have demonstrated the preparation of VN-HSs by utilizing carbon spheres as the hard template. The resulting material possesses a hierarchical macroporousemesoporous configuration, which helped in the efficient performance of VN-HSs in both lithium storage and electrocatalytic oxygen reduction reaction (ORR) [21]. Du et al. have described the overall water splitting ability of hollow micro-box-shaped N-doped carbon coated FeNiP (FeNiP/

NC), which was synthesized by the phosphorization of the cubical FeNi-metal organic framework, acting as both template as well as the source of carbon. In this way, the FeNiP nanoparticles of diameter 20e30 nm were aggregated to form the hollow boxes of convex edges, assuring less agglomeration and hence exposing an abundant electro-active site [22]. In addition to the morphology and porous structures, the emphasis on the chemical composition is equivalently important to achieve the enhanced activity of a nanomaterial for electrocatalytic applications [15,23]. As an example, Wei and co-workers have studied the electrochemical water splitting activity of the petal-like, self-standing FeCo/C (anode) and FeCoP/C (cathode) nanosheet arrays on Ni-foam, developed by a sacrificial template method. The self-standing electrode of FeCo/C nanosheets revealed an overpotential of 219 mV @10 mA/cm2 in the anodic OER [24]. In another report, Xiang et al. have emphasized on the study of the activity of Co@CoMoO4 core@shell type nanowires in hydrogen evolution reaction, which attained an overpotential as low as the commercial Pt/C electrode [25]. The oxides of noble metals especially, RuO2 and IrO2 are regarded as the high-standard catalysts to simplify the intricate oxygen evolution reaction (OER), which is a bottleneck reaction of one of the most captivating areas of renewable energy techniques, i.e. the electrochemical water splitting [26,27]. But the shortage of resources, unaffordability and low persistent activity of these noble-metal oxides constraint their wide-spread catalytic applications. Therefore, to find a permanent and feasible solution to this problem, the earth abundant, non-noble-metals are continuing to be extensively studied in the electrocatalytic applications [28e31]. The transition-metals in their oxide forms are appreciative in the economic OER catalytic studies due to their modulated electronic structures and tunable adsorption energies of the OER intermediates. It was also found that the binary/ternary metal-oxides have greater activity as compared to their singlecomponent forms. Recently, Hu et al. have proposed a coreshell type octahedral Mn3O4@CoxMn3-xO4 oxygen evolution catalyst, fabricated by facet evolution technique. As a result of the unique morphology and abundant oxygen vacancies, this material possessed a low overpotential (284 mV) and faster reaction kinetics (Tafel slope: 73.1 mV/dec) in OER [32]. Moreover, the amorphous materials are considered to be more active towards OER catalysis in comparison to the crystalline counterparts due to their structural flexibility, presence of a large number of defects and plenteous active sites of the material, which have been discussed in several recent reports [33,34]. For example, Sayed et al. have studied the activity of amorphous FeCoNiOxHy in OER, which was prepared by an

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This work 145.7 96.5 RHE: Reversible hydrogen electrode; NHE: Normal hydrogen electrode; SHE: Standard hydrogen electrode; SCE: Saturated calomel electrode.

358 282 1.53 1.43 CoSnO3 Pure CoSnO3 HS 10

RHE

[74] 46.2 318 @1 mA/cm2 Mesoporous-Sb-SnO2/IrO2 (50 wt%) 9

RHE

e

[69] [70] [71] [72] [73] 54 54 100 162 55 e e ~1.45 e e

Electro-spinning method Triblock polymers-assistant method Thermal decomposition Metallic electrodeposition Thermal treatment of mixed metal chlorides Soft template assisted colloidal synthesis Hydrothermal IrO2/SbeSnO2 nanowire IreSn oxide (S100eIr0.6Sn0.4O2) Ti/SnO2eSb(13x)eRu(x)ePt(3) (x ¼ 13 at.%) Ti/Pt/SnO2eSb Ti/Ir0.05Co0.05Sb0.1Sn0.8Ox 4 5 6 7 8

SCE SCE RHE Ag/AgCl, KCl sat. SHE

Sol-Gel Hydrolysis reaction Spin coating IrO2/SnO2 (2:1) Core-shell SnO2/Ag/Co(OH)2 Spheres (Sn,Ir)O2:F (10 wt% F)@Ti-foil 1 2 3

SCE RHE NHE

e 1.4 e

e 340 ~320 (calculated against 1.23 V vs. RHE) 8 e e ~600 438

79 80.04 62

[47] [67] [68]

Ref. Tafel slope (mV/dec) Overpotential (mV)

OER Parameters

Onset potential (V) Reference electrode

Synthesis process Material Sl. No.

electrochemical method. The electroactivity of the tri-metallic oxide was considerably enhanced as compared to the mono/ bimetallic counterparts [35]. Indra et al. have verified the electrocatalytic and photo-electrocatalytic OER activity of both crystalline and amorphous cobalt iron oxides. The amorphous CoFe2O3.66 was found to possess better OER activity as compared to the crystalline CoFe2O4, exposing lower overpotential, low Tafel slope and better mass activity as compared to the crystalline one in both electrochemical as well as photo-electrocatalytic OER, owing to the high surface area and greater number of active centers in the amorphous CoFe2O3.66 [36]. The OER activity of amorphous Co-doped MoOx core-shell nanospheres have been studied by Guo and coworkers, revealing a significant catalytic activity with low overpotential (340 mV) and faster reaction kinetics (Tafel slope: 49 mV/dec) in anodic OER [37]. Among several other metal-oxide systems, nanodimensional cobalt-based oxides have been perceived as the efficient electrode materials in electrochemical energy systems including Li-/Na-ion batteries, supercapacitors, metal-air batteries and water electrolyzers [38e43]. The CoSnO3, as a bimetallic oxide, has grabbed a considerable interest in the research arena of Li-ion batteries. For example, Huang et al. have fabricated the amorphous CoSnO3 nanoboxes, tightly encapsulated in the two-dimensional rGO sheets, enabling high Li-storage capability [44]. In another report, Guan et al. have demonstrated the preparation of the hollow nanostructure of CoO-in-CoSnO3 (wire-in-tube) by atomic layer deposition strategy. This material, after being modified with carbon-coating was proclaimed as the highly efficient anode material, applicable in Li-ion battery [45]. The Na-storage behavior of rGO wrapped amorphous CoSnO3 nano-cubes has been studied by Dou et al., through which they propose a high rate capacity and good cycling permanence of amorphous CoSnO3@rGO as the anode material in Na-ion battery [46]. In some literatures, SnO2 has been employed as an electrocatalyst for water oxidation/reduction reaction due to its n-type behavior, affordability, and easy preparation. But most of the Sn-based compounds are integrated with noble metals (Pt/Ir/ Ru) to enhance their electrochemical activities, which can be better realized by looking into the list of materials in Table 1. At times, SnO2 has been used to dilute the content of the noble metal oxides (IrO2/RuO2) and hence to recompense the Ir/Ru content in OER without distressing the materials activity [47]. But the water-splitting capability of Co-incorporated Sn-based compounds, particularly CoSnO3 has been less explored. Kong et al. have studied the photocatalytic activity of nano-hybrid structure of graphene and cobalt-tin composite oxide (CoSnxOy/G), which was prepared by an in situ chemical accumulation process, followed by photo-reduction [48]. In the present study, we propose a systematic method of development of hierarchical CoSnO3 hollow microspheres (CoSnO3 HS) by a template-assisted hydrothermal synthesis approach. The spherical SiO2 has been used as the hard template for the growth of wafer-thin sheets of CoSnO3, which was subsequently etched by means of aqueous NaOH to get the hollow configuration. The high specific surface area, amorphous nature and sizable electrochemical active centers in CoSnO3 HS stimulates the material to be advantageous in electrocatalytic OER with a low over potential (282 mV), Tafel

Table 1 e Comparison of OER activity of CoSnO3 HS with the previously reported Sn-based oxide electrocatalysts. It can be noted that, the enhancement of catalytic activity of these electrocatalysts are arising from the involvement of noble-metals in the materials.

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slope of 96.5 mV/dec and prolonged stability over a period of 12 h. Additionally, the material possesses a double layer capacitance of 15.87 mF/cm2, which is higher than its counterpart (i.e. pure CoSnO3: 6.08 mF/cm2) by more than twofolds, which is an incentive property of CoSnO3 HS as an OER catalyst. The CoSnO3 HS exhibits higher turnover frequency (0.0045 s1), high specific activity (2.195 mA/cm2EASA) and mass activity (28.752 mA/mg) as compared to the pure form.

Synthesis of SiO2-microspheres

Experimental section

In order to prepare the colloidal silica (SiO2) microspheres, we have followed previously reported modified Stӧber's method [49]. At first, C2H6O (45 ml), DI water (5 ml), NH4OH solution (3 ml) and TEOS (1.5 ml) were mixed with stirring over a period of 12 h to obtain a homogeneous, colloidal dispersion. The arrival of faint-blue color in the reaction medium indicates the formation of SiO2-microspheres. Then the product was collected by centrifugation and washed several times using water and ethanol. The sample was collected and left to dry in a vacuum oven at 70  C.

Materials and reagents

Synthesis of SiO2eCoSnO3 core-shell structure

The reagents consumed in the synthesis process and electrochemical measurements include tetraethyl orthosilicate (TEOS), Cobalt (II) acetate (CH3(COO)2Co.4H2O) and dimethyl sulfoxide (DMSO), which were purchased from Thermo Fisher Scientific Pvt. Ltd., India. Stannic chloride pentahydrate (SnCl4.5H2O) was obtained from Central Drug House Pvt. Ltd., India. Ammonium hydroxide (NH4OH) was procured from Avantor Performance Materials India Ltd. Sodium hydroxide (NaOH) and potassium hydroxide (KOH) were obtained from Sisco Research Laboratories Pvt. Ltd., India. Polyvinylidene fluoride (PVDF), Potassium bromide (KBr) and urea (CH4N2O) were acquired from Merck Specialties Pvt. Ltd., India. Ethanol (C2H6O) was supplied by Changshu Hong Sheng Fine Chemical Co., Ltd. The reagents used throughout the experiment existed in the analytical grade and processed without any extra-purification. De-ionized water (DI water) was used in the experiment during the synthesis and for making the aqueous products.

The SiO2eCoSnO3 core-shell structure was prepared by a hydrothermal route. Initially, the as obtained SiO2 powder weighing 0.2 g was dispersed well in 50 ml distilled water with ultrasonic treatment for 1 h. After that, a 50 ml mixture of 1.0 mM CH3(COO)2Co.4H2O and 1.0 mM SnCl4.5H2O was dropwise added (over 2 h) to the above SiO2 suspension with continuous magnetic stirring. Then 0.18 g of urea was added to the above mixture and stirred continuously for 5 h so that the precursor gets well adsorbed over the SiO2 nanoparticles. The as-prepared mixture was sealed in a Teflon-lined stainless-steel autoclave bomb and heated at 120  C for 24 h. The precipitate was collected, washed with DI water and ethanol thoroughly and vacuum dried to get the powder sample. Finally, the as-prepared powder was annealed at 300  C for 4 h to obtain the SiO2eCoSnO3 core-shell structure.

Characterization techniques The prepared materials were characterized by the following techniques. The powder X-ray diffraction (PXRD) technique was necessary to identify the phase of the materials, which were conducted on a Panalytical Xpert PRO MPD X-ray diffractometer (source: Cu Ka radiation (l ¼ 1.54
A)). The morphology and the atomic compositions were studied by means of field-emission scanning electron microscopy (FESEM, ZEISS Supra 55, Germany) and energy dispersive X-ray (EDX) spectroscopy, equipped with the FE-SEM respectively. Further details of the microstructure were studied using transmission electron microscopy (TEM, Tecnai G2 20 (200 kV TEM)). The Fourier transform infrared (FTIR) spectroscopic analysis was conducted on PerkinElmer infrared photospectrometer, using KBr palette method. To further confirm the shallow elemental composition of the material and chemical states of the constituent atoms, X-ray photoelectron spectroscopy (XPS) (ESCAþ (Omicron Nanotechnology, Oxford Instrument Germany) was employed. These characterizations were conducted at room temperature (25±3 ). BrunauereEmmetteTeller (BET) specific surface area and BarrettJoyner-Halenda (BJH) pore size distribution analyses were carried out with the help of Micromeritics 3 Flex Surface Characterization Analyzer. Further instrumental details are enlisted in Table S1 of the supplementary information.

Preparation of CoSnO3 HS In order to obtain the desired CoSnO3 hollow spheres, the SiO2 template was removed from the SiO2eCoSnO3 core-shell structure by chemical etching with 5.0 M NaOH solution at room temperature. Typically, 0.2 g of the as retrieved SiO2eCoSnO3 powder was submerged in 10 ml of the prepared NaOH solution in a 15 ml polypropylene centrifuge tube. The etching process was continued up to two days with the fresh NaOH solution exchanged each day. The etched product was again washed with DI water and ethanol for a number of times and dried in a vacuum oven to get the final material.

Preparation of pure CoSnO3 nanoplates The pure CoSnO3 nanoplates were prepared separately without using the SiO2 template, by using the same procedure as of the preparation of SiO2eCoSnO3 core-shell structure.

Electrochemical measurements The electrochemical measurements were executed by an electrochemical cell containing three electrodes; Ag/AgCl (Sat. KCl), Pt-wire and the catalyst modified pencil graphite lead (PGL) as the reference, auxiliary and working electrodes respectively. This three-electrode set-up containing aqueous KOH electrolyte was controlled by a CH Instrument (440C). The working electrode was prepared by adopting a cost-effective approach, using PGL (2.0 mm diameter, Kokuyo Camlin Ltd.),

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 1 6 2 3 e2 1 6 3 6

which are easily available in the market. The purchased PGLs were washed with 10% HNO3 and gently polished with cotton in order to obtain a smooth surface. The prepared catalyst (95 wt%) and the PVDF binder (5 wt%) were mixed well by mortar-pestle with addition of DMSO to make the ink. The as obtained catalyst inks were loaded on the PGLs by dropcasting method, such that the geometrical area of 0.33 cm2 of each PGL was modified with the catalyst. These modified PGLs were mounted on a micro pipette tip and the copper wires were wound over the PGLs for external connections. The linear sweep voltammetry (LSV), cyclic voltammetry (CV) and chronoamperometric (CA) scans were recorded to estimate the catalytic properties and stability of the materials in OER. The potential values, which are obtained with reference to Ag/ AgCl (Sat. KCl) electrode, can be converted in the scale of reversible hydrogen electrode (RHE) based on the Nernst equation; ERHE ¼ E Ag þ 0:059 pH þ E qAg AgCl

AgCl

[1]

where, the pH value corresponding to 1.0 M KOH is taken to be 14 and the value for E qAg is taken as 0.1976 at room temperaAgCl ture (~25  C) [50].

Electroactive surface area (EASA) calculation The EASA of the prepared materials were estimated from the CV measurements. At first, the CV scans were recorded between 1.15 and 1.25 V vs. RHE at different scan rates (5e200 mV/s) (Fig. 4c and d) to estimate the non-Faradaic capacitive currents and the electrochemical double layer capacitance (Cdl ) was measured form the slope of the straight line, Dj ¼ 2vCdl

[2]

where, j is the measured current density at a particular potential (we have taken the current density at the potential

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value of 1.201 V vs. RHE) and v represents the scan rate. Thereafter, the EASA was measured by the ratio of the calculated Cdl to the specific capacitance of an atomically smooth material (C0dl ¼ 20 mF/cm2).  Cdl EASA ¼

C0dl

mF cm2



 

[3]

mF cm2

Calculation of different activity metrics in OER (a) Overpotential (h): The overpotential associated with the OER activity of the electrocatalysts can be calculated by, h ¼ ERHE  1:23 V:

[4]

(b) Tafel slope (b): OER kinetics of the materials can be estimated from Tafel slope measurement, guided by the Tafel equation, h ¼ blogjjj þ a

[5]

where, the symbols b and j represent the Tafel slope and the obtained current density respectively. (c) Turnover frequency (TOF): TOF is regarded as the excellent measure for the intrinsic OER catalytic activity of a material, describing the number of O2 molecules evolved by an electrocatalytic active center per unit time. The TOF can be calculated from the following formula. TOF ¼

j   4F m M

[6]

Here, the symbols F, m and M are representing the Faraday's constant (F ¼ 96485.3 C mol1), loading mass of the catalyst over the electrode per unit cm2 (m ¼ 4.54 mg/cm2) and molecular weight of the catalyst (M ¼ 225.64 g/mol) respectively. The factor 4 in the denominator is the representative of

Fig. 1 e FE-SEM images of (a) SiO2-microspheres, (b, c) SiO2eCoSnO3 core-shell structure, (d, e, f) CoSnO3 HSs (Inset of (e): FESEM image of CoSnO3 HS at different portion).

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Fig. 2 e (aed) TEM images of CoSnO3 HS at different magnifications (the dotted portion in (c) representing hollow cavity of CoSnO3 HS), (e) HR-TEM and (f) SAED pattern of CoSnO3 HS. (g) STEM image and (hek) EDX elemental mapping of CoSnO3 HS.

Fig. 3 e (a) XRD and (b) FTIR patterns of SiO2, SiO2eCoSnO3 core-shell structure and CoSnO3 HS. (c) XPS survey scan and highresolution spectra: (d) Co 2p, (e) Sn 3d and (f) O 1s of CoSnO3 HS.

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Fig. 4 e (a, b) N2 adsorption-desorption isotherms (inset: BJH pore size distribution) and (c and d) CV curves in the nonFaradic potential range of 1.15e1.25 V vs. RHE at different scan rates (5e200 mV/s) for pure CoSnO3 and CoSnO3 HS, respectively. (e) Capacitive current densities (anodic and cathodic) and (f) linear plots showing the variation of double layer current density with respect to scan rates for the measurements of Cdl of pure CoSnO3 and CoSnO3 HS. the involvement of four electrons in OER. The current density, j is obtained at the overpotential h ¼ 0.35 V in the OER polarization curve. The value of TOF is represented in terms of s1, which is calculated by considering all the metal atoms as the catalytically active centers.

(d) Specific activity (SA): In the SA measurement, the unapproachable bulk sites are neglected, which supports as a complementary metric to the TOF. The SA values can be calculated by normalizing the j value at h ¼ 0.35 V to the EASA by the formula,

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j SA ¼ EASA

[7]

where, SA stands in the unit of mA/cm2. (e) Mass activity (MA): The mass normalized activity of the catalyst is a significant metric from the perspective of industrial applications. The MA value can also be evaluated from the formula, MA ¼

j m

[8]

where, j is the current density at h ¼ 0.35 V and m is the loading mass of the catalysts (i.e. 4.54 mg/cm2, for our present study). All these calculations are performed by following the previous reports [51e55].

Result and discussion The method of preparation of CoSnO3 hollow spheres is schematically represented in Scheme 1. Highly dispersed spherical SiO2 particles were used as the template to fabricate CoSnO3 hollow spheres. An aqueous solution containing Co2þ and Sn4þ ions was added drop-wise to the SiO2 dispersion and continuously stirred to ensure the complete adsorption of metal ions on the SiO2 templates. In the course of hydrothermal treatment, the metal ions undergo nucleation and form the sheet-like structures over the SiO2 particles. After the heat treatment (300  C) and NaOH etching of SiO2 particles, the CoSnO3 hollow spheres were obtained. The morphological information of the prepared samples is acquired from the FE-SEM analysis (Fig. 1). The FE-SEM image

of the as obtained SiO2 microspheres is depicted in Fig. 1a. The size of the SiO2 particles varies from ~250 to 300 nm in diameter. Fig. 1b and c represents the spherical SiO2eCoSnO3 core-shell structure of diameter ~530 nm, which was collected after the hydrothermal treatment, followed by annealing. In this compound, the nano-dimensional CoSnO3 sheets of thickness ~30 nm are well arranged over the spherical SiO2 particles. The thickness of the shell is thus expected to be ~200 nm. The EDX elemental mapping of SiO2eCoSnO3 coreshell structure is represented in Fig. S1. The uniform distribution of the elements such as Co, Sn and O over the entire space indicates that the nanodimensional CoSnO3 plates are uniformly grown on the SiO2 template in the hydrothermal synthesis process. The successful etching of the SiO2 template from the SiO2eCoSnO3 core-shell structure can be realized from Fig. 1def. The cavities in the CoSnO3 hollow microspheres, generated after removing the SiO2 templates can be recognized from Fig. 1e and f (indicated by arrows). The FESEM images of rectangular CoSnO3 nanoplates, which were prepared in the absence of SiO2 templates are represented in Figs. S2a and b in the supplementary information. The thickness of each CoSnO3 nanoplate was found to be ~50 nm and a single nanoplate measures an area of ~2 mm2 along the plane. The hollow spherical structure of the material can be clearly perceived from the TEM images, depicted in Fig. 2aec. The dotted transparent section of the TEM image specifies the high electron transparency, indicating the presence of the cavity in the CoSnO3 HS. Upon higher magnification, the wafer-thin, wrinkled sheets of CoSnO3 can be visualized (Fig. 2d). Furthermore, the high resolution-TEM (HR-TEM: Fig. 2e) analysis has been performed to investigate the nature of the material and the inter-planar spacing of CoSnO3 HS was thereby calculated. It was found that CoSnO3 HS exhibits

Scheme 1 e Schematic depiction of the growth of CoSnO3 hollow spheres.

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amorphous nature along with the presence of some nanocrystals with inter-planar spacing of 0.31 nm and 0.43 nm, corresponding to different set of planes of CoSnO3. The SAED pattern (Fig. 2f) further confirms the amorphous nature of CoSnO3 HS. The EDX elemental mapping was also carried out to visualize the spatial distribution of the constituent elements. The scanning transmission electron microscopy (STEM) image and elemental mapping of the constituent elements (Co, Sn and O) are represented in Fig. 2gek respectively. It was found that, Co, Sn and O are uniformly distributed over the space to form the CoSnO3 HS. The crystal structures of the prepared materials were analyzed by PXRD technique. The resulted diffraction patterns of SiO2, SiO2eCoSnO3 core-shell structure and CoSnO3 HS are represented in Fig. 3a. The PXRD pattern of pure CoSnO3 is represented in the Fig. S2c, given in the supplementary information. In the XRD pattern of SiO2, the broad peak near 22 represents the successful formation of the SiO2 in the Stӧber's method. After the hydrothermal treatment, the amorphous CoSnO3 was successfully grown on the spherical SiO2 particles, which is confirmed from the emergence of small peaks at 34.2 , 42.5 and 59.24 in the PXRD pattern of SiO2eCoSnO3 core-shell structure. The removal of SiO2 from the intermediate core-shell structure by the NaOH etching can be successfully verified from the diffraction pattern of CoSnO3 HS. It is worthy to mention that the intensity of the diffraction planes are enhanced after the NaOH etching. The peaks at 19.26 , 25.36 , 34.1 , 46 , 50.28 , 53.8 , 59.46 , 64.3 , 71.43 and 83.09 in the PXRD pattern of CoSnO3 HS are clearly visible and matched with the standard JCPDS card# 28e1236. The diffraction pattern of the material (pure CoSnO3), as represented in Fig. S2c confirms the formation of CoSnO3 in the hydrothermal-annealing synthesis process. FTIR spectral analysis helps in the identification of bonding configuration of materials. The FTIR spectral patterns of SiO2, SiO2eCoSnO3 core-shell structure and CoSnO3 HS are depicted in Fig. 3b. In all three spectra, the broad peaks in the wavenumber range of 3430e3460 cm1 are observed, which correspond to OeH stretching vibrational modes of absorbed water molecules. The bands near 2923 cm1 and 1634 cm1 correspond to aromatic CeH stretching vibrational mode and bending mode of HeOeH respectively [56]. The low-intensity peaks are detected at 1400 cm1, which is assigned to OeH deformation vibration. The bands near 1450 cm1 in all the spectra are ascribed to in-plane scissoring vibration of alkyl CeH [57,58]. An intense peak near 1110 cm1 in the spectral pattern of SiO2 is observed, which is the characteristic peak of SiO2 nanoparticles, arising from the stretching vibration of SieO. In addition to this, two other peaks of low intensity are observed at 474 and 801 cm1 in the spectrum of SiO2, which correspond to SieO rocking and bending vibrations respectively. The peak at 958 cm1 is appearing from the vibration of SieOH in the material [59]. The stretching vibration of SieO at 1110 cm1 is also observed in the FTIR spectral pattern of SiO2eCoSnO3 core-shell structure. In addition to this, a new band appears at 1027 cm1, which is ascribed to the CeO stretching vibration of acetate groups present in the material. The additional peaks appearing in the low-frequency region (i.e. below 670 cm1) are ascribed to the vibrations of metal-oxygen bonds (SieO,

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CoeO, SneO and SneOeCo) in the lattice. The removal of SiO2 from the SiO2eCoSnO3 core-shell structure by the NaOH etching process disappears the stretching vibration of SieO (i.e. the band appearing at 1105 cm1 for the SiO2-based compounds). The peaks appeared in all four spectra, in the wavelength range of 670 to 450 cm1 are assigned to the metal-oxygen (M  O) vibrational modes (SieO, SneO, CoeO and SneOeCo) in the materials [60]. The FTIR pattern of pure CoSnO3 is depicted in Fig. S2d, which is also resembling the presence of metal-oxygen characteristic bands along with the additional vibrational modes, similar to the other synthesized compounds. Further, the XPS characterization of CoSnO3 HS was helpful in obtaining the valence states of the constituent elements (Sn, Co and O), and the as obtained XPS spectral patterns are represented in Fig. 3cef. The survey spectrum (Fig. 3c) approves the existence of Sn, Co and O in the material. Apart from these characteristic peaks, a small peak appearing at 283.50 eV can be assigned to the carbon impurity present in the material. In the high-resolution Co 2p spectrum (Fig. 3d), the deconvoluted peaks at 780.99 eV (Co2þ 2p3/2) and 796.99 eV (Co2þ 2p1/2) affirms the presence of Co2þ in the material. In addition to these, two satellite peaks are appearing at 785.6 and 802.6 eV (denoted by Sat. in the spectrum) [61,62]. The XPS spectral pattern in the binding energy range of 481e497 eV corresponds to Sn 3d spin-orbit level of Sn4þ oxidation state. The Lorentzian peaks at 486.21 and 494.70 eV in the Sn 3d spectrum (Fig. 3e) resemble the Sn 3d5/2 and Sn 3d3/2 electronic states of Sn4þ, which is in accordance with the expected result [63]. The high-resolution XPS O 1s spectrum is further deconvoluted into two distinct bands, as represented in Fig. 3f. The multi-fitted peaks in the O 1s spectrum are located at 530.99 and 532.09 eV, which are assigned to metal-oxygen (SneO and CoeO) lattice vibrations and the oxygen species (H2O/OH) adsorbed on the surface respectively [64]. In summary, we can explain that, the co-existence of the chemical states Co2þ, Sn4þ and O2 in the as-synthesized CoSnO3 HS were revealed from the XPS analysis, which is in agreement with the previous literature. The porosity and high specific surface area of the electrocatalysts greatly enhance the performance of the electrode material in an electrochemical reaction. Therefore, in order to analyze the pore structure of the as-prepared materials, we have performed the N2 adsorption-desorption isotherm measurements (Fig. 4a and b). The specific surface area of the pure CoSnO3 and CoSnO3 HS were estimated from the BET method, which revealed the BET surface area of 153.59 m2/g and 104.06 m2/g for pure CoSnO3 and CoSnO3 HS respectively. The higher BET surface area was obvious for CoSnO3 HS, which may be useful to afford a large electroactive site for the heterogeneous reactions. In both the cases, the appearance of the hysteresis beyond P/P0~0.5 indicates that the materials exhibit mesoporous structure. In addition to this, the respective BJH pore size distribution curves of the prepared catalysts are represented in the inset of Fig. 4a and b. The pore diameters of pure CoSnO3 and CoSnO3 HS were found to be lying between 4 and 8 nm (7.0 nm and 4.7 nm respectively), which indicates that the materials are certainly mesoporous in nature, thereby providing a high surface area for the electrochemical reaction [65].

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The surface characteristic properties of the electrocatalysts have also been studied by measuring their electrochemical active surface area (EASA), by a method demonstrated by Gao et al. [51] and the calculations are detailed previously, in Section Electroactive surface area (EASA) calculation. Fig. 4e represents the anodic and cathodic capacitive currents of pure CoSnO3 and CoSnO3 HS, measured at the potential value of 1.2 V vs. RHE, from their respective CV curves (Fig. 4c, d). The double layer currents obtained from the CV scans are achieved due to the capacitive charging [66]. From Fig. 4f, it can be explained that CoSnO3 HS possesses a higher Cdl value (15.87 mF/cm2) as compared to the pure CoSnO3 (6.08 mF/cm2). The EASA values for different electrodes were calculated by using Equation (3), which were obtained to be 793.5 cm2 and 304 cm2 for CoSnO3 HS and pure CoSnO3 respectively. The two-fold increase in the EASA can be accredited to the higher porosity, larger surface area and large extent of electroactive centers in the CoSnO3 HS catalyst as compared to pure CoSnO3. The electrochemical activities of the prepared catalysts were studied in aqueous KOH solution. At the very beginning of the OER catalytic activity study of CoSnO3 HS, we have optimized the analytical parameters, such as, scan rate, the concentration of the alkaline electrolyte and the loading mass of the catalysts by LSV technique. The obtained results are depicted in Fig. S3 in the electronic supplementary information. These findings support the fact that, the optimum catalytic activity of CoSnO3 HS was achieved in 3.0 M KOH solution, when the scan rate was maintained at 1 mV/s and the loading mass of the catalyst was 0.4 mg/0.33 cm2 over the PGL. Such a slow scan rate helps in the minimization of capacitive current during the OER process. Retaining these optimized conditions, we have further analyzed the OER catalytic activity of both CoSnO3 HS and pure CoSnO3 from a comparison point of view. The LSV profiles of both of the catalysts are represented in panel-(a) of Fig. 5. It was found that, the pure CoSnO3 possesses an onset potential of 1.53 V vs. RHE and demands an overpotential of 358 mV to accomplish a current density value of 10 mA/cm2. On the other hand, CoSnO3 HS modified PGL exhibits a lesser onset potential (1.43 V vs. RHE) and a lower overpotential (h10 ¼ 282 mV) to attend a current density of 10 mA/cm2, which is following our expectation. Again, the Tafel plots (Fig. 5b), obtained from the linear portion of the LSV curves provide the information of OER kinetics of the catalysts. From the Tafel slope calculations it was found that, the pure CoSnO3 exhibits a Tafel slope of 145.7 mV/dec, while the slope value is 96.5 mV/dec for CoSnO3 HS. This result indicates that the CoSnO3 HS favors OER more actively, in comparison to pure CoSnO3, allowing a better electron transport at the electrode and electrolyte interface. As we have been discussing in the introduction, the high OER catalytic activity of CoSnO3 HS might be triggered from its amorphous nature, high surface area, existence of abundant defects, high porosity and thereby a huge electroactive centers available in the material. The OER activity of CoSnO3 HS has also been compared with the previously reported Sn-based materials, which is enlisted in Table 1. It is worth mentioning that, most of the reported Snbased catalysts have been integrated with the noble metals,

such as, Pt, Ru or Ir to enhance the catalytic properties. However, the CoSnO3 HS is found to be better than or comparable with the other similar oxide-based materials in terms of onset potential, overpotential and Tafel slope values. The CoSnO3 HS also competes well with the commercial catalysts such as RuO2 and IrO2, which have been reported earlier. For better presentation, we have provided a comparison table (Table S3: Please see the electronic supplementary information), which displays a good OER catalytic activity of CoSnO3 HS in terms of the overpotential value. Therefore, we can assume that the highly active CoSnO3 HS can reasonably undertake the commercial applications in water-electrolyzers and metal-air batteries as the anodic/cathodic materials respectively. Further insights of OER catalytic performance of the prepared materials have been studied from the calculations of their corresponding TOF, specific and mass activities with reference to the overpotential, h ¼ 0.35 V (The detailed calculations are described previously). These are the basic but significant metrics to inspect the OER activity of the materials. Fig. 5c represents the bar-chart of the TOF, SA, MA and Cdl for CoSnO3 HS and pure CoSnO3 and the values are also detailed in Table S2 (Please see the electronic supplementary information). The SA and MA values were found to be 2.195 mA/ cm2EASA and 28.752 mA/mg, respectively for CoSnO3 HS. While, for pure CoSnO3, the SA and MA were obtained to be 1.445 mA/cm2EASA and 7.250 mA/mg respectively. Assuming all the metal atoms to be electrochemically active for the OER process, the TOFs of the prepared materials have been calculated. The TOF of CoSnO3 HS (0.0045 s1) is found to be higher than that of pure CoSnO3 (0.0011 s1). Thus, from the above results it can be inferred that the CoSnO3 HS is more electrochemically active in comparison to the pure form, owing to the enhanced surface area and excellent linkage of the CoSnO3 sheets in the hollow spherical structure. The energy devices predominantly anticipate the high stability of the catalyst to enhance the longevity of the electrochemical system. Therefore, we have investigated the stability of the CoSnO3 HS by chronoamperometry (CA) technique. The multistep CA study was performed in the potential range of 0.6 Ve0.95 V, with continuously rising the potential value in 1800 s by 0.05 V in each step. The as obtained result is depicted in Fig. 5d, where we can observe the enhancement of current density value at each step of the potential increment, along with the sustained current density over the entire step (throughout 1800 s). Furthermore, the continuous CA measurement for a long-period of 12 h (43200 s), with h10 ¼ 281 mV is represented in Fig. 5e. The current density-time plot reveals a feeble change in the current density after the long-duration stability test, suggesting a high persistence of the material in OER. Again to cross-check the OER activity of the material, we have recorded the LSV curve of CoSnO3 HS modified PGL after the CA measurement and compared with the initial result (represented in the inset of Fig. 5e). The OER catalytic activity of CoSnO3 HS remains almost unaffected, except a small change in current density in the LSV measurement. Furthermore, we have verified the morphology of the material after the stability test by FE-SEM analysis (Fig. 5f). The shape of the material is found to be unchanged after the 12 h of continuous CA analysis, suggesting high stability of CoSnO3 HS in the alkaline medium. The FE-SEM image of CoSnO3 HS, which was

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Fig. 5 e OER catalytic activity: (a) polarization curves, (b) Tafel plots of CoSnO3 HS and pure CoSnO3 and (c) bar chart showing the comparison between the OER activities of the two materials. Stability test: (d) multistep CA measurement of CoSnO3 HS with increasing potential and (e) CA measurement of CoSnO3 HS for a duration of 12 h and (Inset: comparison of electrocatalytic activity (LSV study) of CoSnO3 HS after 12 h of CA measurement with the initial activity), (f) FE-SEM image of CoSnO3 HS modified PGL after the 12 h CA measurement (Inset: enlarged view of the same showing no change in the shape of the material).

recorded after eight months from the date of synthesis displays no change in the morphology (Fig. S4, please see the supplementary information). This indicates the high storage stability of CoSnO3 HS, which may be advantageous from the commercial point of view.

Conclusion The hierarchical CoSnO3 HS was designed by a template assisted hydrothermal strategy, in which the thin sheets of CoSnO3 were grown over the spherical SiO2 templates, which were further etched out to obtain the hollow structure. The as obtained CoSnO3 HS possesses a high specific surface area, mesoporous feature and higher EASA as compared to the pure CoSnO3, which is advantageous to carry out the electrocatalytic OER more effectively. A lower overpotential of 282 mV was afforded to achieve a current density of 10 mA/ cm2 and a low Tafel slope (96.5 mV/dec) was also calculated for CoSnO3 HS, which supports in the high catalytic activity of the material toward OER in alkaline medium. Additionally, the higher turnover frequency (0.0045 s1), high specific and mass activities (2.195 mA/cm2EASA and 28.752 mA/mg, respectively) was observed in case of CoSnO3 HS, which support in the high catalytic property of the material in OER. The CoSnO3 HS also possesses a good stability over a period of 12 h, which is advantageous for the longevity of the energy systems. The porous configuration, unique morphology and high electrocatalytic activity of CoSnO3 HS paves the way toward different energy generation/storage applications. Moreover, this template assisted hydrothermal synthesis approach can be

adopted for the fabrication of different transition metal oxides, useful for the energy applications.

Acknowledgement The experimental works, starting from synthesis to the electrochemical study of the materials have been performed by Suryakanti. She has taken the help of S. Chakraborty during material preparation. She has compiled and written the manuscript with the help of S. Banerjee and Dr. P. K. Sharma. Suryakanti also acknowledges Dr. R. Madhuri, Dept. of Applied Chemistry, IIT (ISM) Dhanbad for assisting in some of the laboratory facilities. The authors sincerely acknowledge TIFR Mumbai, IIT (BHU) Varanasi for XRD and TEM characterizations respectively. XPS characterization has been done at Sprint Testing Solutions, India on payment basis. Authors acknowledge the research infrastructure provided by Central Research Facility, IIT (ISM) Dhanbad, India.

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.06.094.

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