Suitable Morphology Makes CoSn(OH)6 Nanostructure a Superior

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Suitable Morphology Makes CoSn(OH) Nanostructure a Superior Electrochemical Pseudocapacitor Ramkrishna Sahoo, Anup Kumar Sasmal, Chaiti Ray, Soumen Dutta, Anjali Pal, and Tarasankar Pal ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02568 • Publication Date (Web): 27 Jun 2016 Downloaded from http://pubs.acs.org on June 28, 2016

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ACS Applied Materials & Interfaces

Suitable Morphology Makes CoSn(OH)6 Nanostructure a Superior Electrochemical Pseudocapacitor Ramkrishna Sahoo†, Anup Kumar Sasmal†, Chaiti Ray†, Soumen Dutta†, Anjali Pal§ and Tarasankar Pal*,† †

Department of Chemistry, and §Department of Civil Engineering, Indian Institute of Technology, Kharagpur-721302, India E-mail: [email protected]

Abstract Morphology of a material with different facet, edge, kink, etc. generally influences the rate of a catalytic reaction.1,2 Herein, we account for the importance of altered morphology of a nanomaterial for a supercapacitor device and employed CoSn(OH)6 as an electrode material. Suitable fabrication of a stable aqueous asymmetric supercapacitor (AAS) using metal hydroxide as positive electrode can be beneficial if the high energy density is derived without sacrificing the power density. Here we have synthesized an uncommon hierarchical mesoporous nanostructured (HNS) CoSn(OH)6 to fabricate a pseudocapacitor. In this endeavour NH3 is found to be a well suited hydrolyzing agent for the synthesis.3 Serendipitously, HNS was transformed into favored cubic nanostructure (CNS) in NaOH solution. In solution, NaOH acts as a structure directing as well as an etching agent. Both the samples (HNS & CNS) were used as pseudocapacitor electrodes in KOH electrolyte independently, which is reported for the first time. The HNS exhibits very high specific capacitance value (2545 F/g at 2.5 A/g specific current) with better cyclic durability over CNS sample (851 F/g at 2.5 A/g specific current). To examine the real cell application, HNS sample was used as the positive electrode material with the activated carbon (AC) as the negative electrode material for the development of an aqueous asymmetric supercapacitor (AAS). The as-fabricated AAS exhibited very high specific capacitance value of 713 F/g at a specific current of 1.5 A/g and retained 92% specific capacitance value even after 10000 charge-discharge cycles. A maximum energy density of 63.5 Wh kg-1 and a maximum power density of 5277 W kg-1 were ascertained from the as-fabricated AAS, HNS CoSn(OH)6//AC. KEYWORDS: Hierarchical (HNS) and cubic (CNS) CoSn(OH)6 nanostructures, ammonia, etching, Sodium hydroxide, pseudocapacitor, aqueous asymmetric supercapacitor (AAS). 1 ACS Paragon Plus Environment


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INTRODUCTION Recently, the supercapacitor has been proved to be an affordable energy storage alternative mostly in the field of devices especially for the portable electronic devices and hybrid electric vehicles where high power energy resource is required in particular.4,5 Mostly, carbon based materials have been used as the supercapacitor electrode whose mechanism of action is based on the formation of electrical double layer (EDL).6 These carbon based materials exhibit excellent power density (> 10000 W kg-1) and long range life span, but their specific capacitance and energy density values are very low. On the other hand, rechargeable batteries exhibit high specific capacitance and high energy density, but their life span and power density are very low.

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Thus, fabrication of an advanced supercapacitor possessing high

specific capacitance and high energy density without sacrificing the power density has turned out to be extremely requisite. To address this problem, the asymmetric supercapacitor has been developed where a faradic type battery or pseudocapacitor material and an EDL type capacitor are combined. Here, individually both aqueous and non-aqueous solvents are employed as electrolyte.7 Between aqueous and non-aqueous asymmetric supercapacitors, aqueous asymmetric supercapacitors (AASs) have drawn much attention due to their low cost, non-flammability, high ionic conductivity and easy assembling in air.7,8,9 Due to the presence of two different types of materials in AASs, the working voltage becomes very high which significantly increases the energy density of the device. Thus by using AAS, the gap between capacitor and battery can be harmonized.7,8,9 In the AASs, mostly carbon based EDLC (e.g., AC, CNT, graphene, etc.) is employed as the negative electrode. On the other hand, redox active transition metal oxides (battery type pseudocapacitor) are found to be suitable positive electrode. Some oxide materials have also been used as the negative electrode, such as V2O5, MoO3 and Fe oxide/oxo hydroxides.10,11,12 Among the transition metal oxides, RuO2, MnO2, NiO/Ni(OH)2, CoO/Co3O4/Co(OH)2, etc. are most common.8,9,13-17 In recent days, some scientists are not like to use the term “pseudocapacitor” for those materials which exhibit peak shaped CV curve in aqueous electrolyte. They referred it as the battery material.18,19 But, according to definition by B. E. Conway, Pseudocapacitance is generated by the fast revesible redox reaction, electrosorption and intercalation on the surface of the material. Peak shaped curve is produced when the materials undergo redox charge transfer. B. E. Conway also mentioned in his book that Co, Fe based materials in aqueous electrolyte exhibit redox pseudocapacitnace behaviour. Thus we have used the term “pseudocapacitor” for our material in this work. Now due to the less 2 ACS Paragon Plus Environment

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participation of the electrode materials and low conductivity, they parade short life span and low rate capability. Thus, the fabrication of a highly rate capable and highly stable, superior pseudocapacitor using those transition metal oxides becomes very much challenging for the researchers. To make the pseudocapacitor material stable, conductive and durable for long term use, researchers started to employ mixed oxide and hydroxide materials instead of single metal oxide or hydroxide. In this respect, spinel type compounds, such as MCo2O4, MFe2O4 (M = metal ion) accomplish much success, where one Fe or Co ion from the spinel structure is replaced with another suitable metal ion.20-22 Among the binary metal oxides and hydroxides, Ni-Co mixed oxides or hydroxides nanomaterials are mostly used by the researchers.20,23 Their interconnected arrangement and high specific surface area make them superior pseudocapacitor electrode materials.23 Another interesting binary oxide/hydroxide charge storage material is Co-Sn oxide/hydroxide (e.g., Co2SnO4, CoSnO3, CoSn(OH)6).24-26 Yet, these have not been used as pseudocapacitor till today except as anode material for Liion battery. In most of the cases, these composite materials inherit cubic structure with increased dimensionality. Probably, due to the thick wall of the cubic structures, these Co-Sn based materials have not been exploited as the pseudocapacitor material.24-26 Some researchers have used redox active material to increase the efficiency of the pseudocapacitor.27,28 Occasionally, it has been observed that even mixed transition metal oxides/hydroxides are not able to satisfy the reasearchers with their enhanced electrochemical activities over the single metal oxides/hydroxides. Sometimes, they exhibit low specific capacitance value, low energy density value, etc. presumably due to the lack of participation of their whole body. In most of the cases, structures of transition metal oxides are larger in size (micron range). On the other hand, it is reported that ion can diffuse through ~20 nm range of the electrode material.3 Thus, a major area of these electrode materials remain silent in terms of electrochemical activity. Again, due to lower transportation rate of ion after few charge-discharge cycles, the electrode materials become aggregated and lose their inherited stability and structure. Thus, the morphology of the metal oxides/hydroxides plays a vital role when they are used as the charge storage electrode material. In our previous paper, we developed 2D nanosheet of Co based oxides to resolve this perennial problem and some other researchers are also working with two-dimensional materials.3,29,30 Introduction of porosity in the 2D materials or fabrication of porous hierarchical structure can resolve all the issues related to the existing limitations of the metal oxides or binary metal oxides at a time. Porous structure and ultrathin wall of the nano or microstructures significantly improve the specific 3 ACS Paragon Plus Environment


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capacitance value and electrochemical stability, which is regarded due to the ultrathin porous wall of the material that can minimize the diffusion path of the ion. Eventually, ion transportation or diffusion of ions becomes facile. Herein, we used CoSn(OH)6 as a pseudocapacitor electrode material. Due to the common cubic nature of CoSn(OH)6, it was very challenging to use it as the pseudocapacitor electrode material. First, we synthesized hierarchical porous nanostructure (HNS) which involved 2D ultrathin porous nanosheets as the building block. Thereafter, we logically applied concentrated NaOH treatment upon HNS and eventually obtained CoSn(OH)6 cubic nanostructure (CNS). The as-prepared HNS inherited very high specific surface area and pore volume compared to the CNS. We compared both the as-synthesized materials, i.e.,HNS and CNS samples as pseudocapacitor electrode material in KOH electrolyte, individually. It was observed that HNS performed better as superior electrode material over CNS in all respects, i.e. specific capacitance value, rate capability, electrochemical stability, etc. As the HNS exhibited superior electrochemical activity, we used it as the positive electrode by combining it with AC to fabricate a real asymmetric cell, HNS CoSn(OH)6 //AC. The AASs exhibit very high specific capacitance value and energy density value without sacrificing the power density. EXPERIMENTAL SECTION: Materials and Instruments: The related information is briefly discussed in the supporting information (S1). Synthesis of CoSn(OH)6 nanomaterial: Synthesis of hierarchical nanostructure (HNS), C1: We have prepared HNS CoSn(OH)6 by co-precipitation method using our laboratory developed modified hydrothermal technique (MHT).31 First, 2 mL of 0.05M CoSO4.7H2O and 2 mL of 0.05 M SnCl2. 2H2O (soluble in conc. HCl) aqueous solutions were taken in screw capped test tube. Then it was sonicated for 5 min to make the reaction mixture homogeneous. After that, 300 µL conc. ammonia solution was added to the mixture and was shaken well. Then the screw capped test tube was kept in the modified hydrothermal condition at 180 ºC. After 24 h, the product was isolated and pink colour product was obtained. The product was washed thoroughly by centrifugation using distilled water until the product was neutral. Finally, it was dried and collected. 4 ACS Paragon Plus Environment

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Synthesis of cubic nanostructure (CNS), C2: The as-prepared 100 mg of the pink product (HNS sample) was taken in a 10 mL beaker with 5 mL of 0.1 M NaOH solution. It was kept at room temperature condition. After 45 h the pink product (no significant change in colour) was thoroughly washed with distilled water by centrifugation until the product becomes neutral. Then it was dried and collected. Electrochemical measurements: The electrochemical measurements for both the samples, HNS and CNS have been carried out using both three-electrode system and two-electrode system at room temperature. Aqueous 3 M KOH solution was used as the electrolyte. Electrochemical studies were carried out by cyclic voltammetry (CV), chronopotentiometry (CP) or galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopic (EIS) technique. For three-electrode system, first we dispersed HNS/CNS sample, acetylene black and polyvinylidene fluoride (PVDF) in NMP solvent in the weight ratio of 85:10:5. Here Ni foam was used as the current collector. First the Ni foam was treated with 6 M HCl, followed by washing with ethanol and water. Then it was dried. The as obtained slurry was pasted on 1×1 cm2 activated Ni foam using a spatula. Then the electrodes were dried at 120 °C for 12 h. This was used as the working electrode. Pt wire and saturated calomel electrode (SCE) were used as the counter and reference electrode, respectively. Before starting the experiments, electrodes were dipped in 3 M KOH electrolyte for 15 min. All the electrochemical measurements were carried out using CHI 660E electrochemical workstation. The weight of the working electrode was 1.2 mg (excluding the weight of the acetylene black and the binder, PVDF). A detailed characterization and electrochemical studies of the bare and active Ni foam (acid treated) have been discussed in the supporting information (S2). Calculation details of the electrochemical analysis: ௩ଶ

Specific capacitance, Csp = ቀ‫׬‬௩ଵ ܸ݅݀‫ ݒ‬ቁ /݉‫ܸ(ݒ‬2 − ܸ1) ----------------- (1) Where, the numerator stands for the total charge, m for electrode mass, ν for the scan rate, and (V2 −V1) is for the potential window. Specific capacitance in three-electrode system, CSP = it/m∆V ----------------- (2) Specific capacitance in two-electrode system, CSP (2 electrode cell) = 4×CSP ----------------(3) Where, I denotes the constant cathodic current, t signifies the discharge time, m is the weight of the active mass and ∆V is the potential window. 5 ACS Paragon Plus Environment


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Coulombic efficiency, η = tD/tC× 100 ---------------- (4) tD is the discharging time and tC is the charging time. Energy density, E=1/(2×3.6)CSP∆V2 Or ௧ଶ

E =ቀ‫׬‬௧ଵ ܸ݅݀‫ ݐ‬ቁ = ½ C (Vmax + Vmin) (Vmax + Vmin)--------------- (5) Power density, P= (3600×E)/T ------------------ (6) Where, C denotes the specific capacitance at specific current, I is the discharge current and ∆V is the potential window, and T is the discharge time. For two electrode system we used HNS as the positive electrode and activated carbon (AC) as the negative electrode. First, we dispersed HNS sample, acetylene black and polyvinylidene fluoride (PVDF) in NMP solvent in the weight ratio of 85:10:5. For the negative electrode we dispersed AC, acetylene black and PVDF in NMP solvent in the weight percentage of 85:10:5. Here also the activated Ni foam was used as the current collector. The slurry was pasted on 1×1 cm2 activated Ni foam using a spatula. Then the electrodes were dried at 120 °C for 12 h. Full cell measurement was performed using HNS as the positive electrode and AC as the negative electrode , separated by a distance of 2 cm. Pt wire was used to connect the electrodes and the instrument. Aqueous 3 M KOH was used as the electrolyte. We performed the asymmetric cell study in the voltage window of 1.6 V. To achieve the maximum specific capacitance value of the asymmetric cell, the storage capacity of negative and positive electrodes has to be balanced by the following equation, 1/Ctotal = 1/Cpositive + 1/Cnegative-------------7 Charge storage capacity of the asymmetric cell was balanced by adjusting the mass ratio of HNS CoSn(OH)6 and AC. Mass ratio was calculated by achieving the charge balance using the following equation, m+/m- = (C- × ∆E-)/(C+ × ∆E+ ) -------------8 where q+, q_, m+, m_, C+, C_, E+, and E- are the charge, mass, specific capacitance and potential windows obtained in the three-electrode measurement for the positive and negative electrode. For the asymmetric cell weight of the active material was 2.8 mg (excluding the weight of acetylene black and PVDF) (for HNS CoSn(OH)6//AC, HNS CoSn(OH)6: AC was 1 : 6).

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RESULTS AND DISCUSSION Scheme 1 represents the schematic overview of the synthesis of CoSn(OH)6 hierarchical nanostructure (HNS) and its conversion to cubic nanostructure (CNS). First, HNS was synthesized via the co-precipitation of Co(II), Sn(II) using liquor ammonia in aqueous solution. To switch the HNS sample to CNS, the former was treated with 0.1 M NaOH solution.

Scheme 1: Schematic presentation of the synthesis of hierarchical nanostructure (HNS) and cubic nanostructure (CNS) of CoSn(OH)6. The details of the construction of two structure of CoSn(OH)6 have been discussed in the later part of the manuscript. To comprehend the morphology of the as-synthesized CoSn(OH)6, we performed the FESEM, TEM, STEM and AFM image analysis for both the samples. It is evident from the results that C1 is hierarchical in nature and the other hand, C2 bears cubic

Figure 1: TEM images of HNS sample (a, b) and CNS (c) sample. nanostructure morphology. Figure S1 demonstrates the FESEM (Figure S1a), STEM (dark field) (Figure S1b) and AFM images (Figure S1c,d) of the as-synthesised C1 images. Figure 1a,b demonstrate TEM images of the same sample. From these images, it is evident that the C1 sample is of hierarchical morphology. Highly magnified images (Figure 1b and S1d) of 7 ACS Paragon Plus Environment


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the sample reveals that the hierarchical structure has been constructed by the self-assembly of 2D thin nanosheets. Height profile of the AFM images notify that the hierarchical structures are 250-300 nm in height and ~ 2.5 µm in length (Figure S2a) and each 2D sheet is 4-5 nm in height and 250 nm in length (Figure S2b). This above discussion confirms that the assembly of 2D ultrathin nanosheets is the reason behind the construction of hierarchical nanostructure (HNS) of CoSn(OH)6 sample. Figure 1c and S3a display the TEM and FESEM images of the C2 sample respectively. It has been detected from the images that the hierarchical structure of the C1 sample has been transformed into nanocube structure (C2) by the NaOH (0.1M) treatment in 45 hours. To verify the elements present in the as-synthesized materials we performed the EDX analysis from the FESEM images for both the samples, HNS and CNS (Figure 2a and b). EDX spectra for both of the samples exhibit the signature peak for Co, Sn and O. Peak for H was not detected due to the limitation of the instrument. Elemental area mapping of the HNS sample (Figure 2c to f) illustrates the homogeneous distribution of Co, Sn and O over the entire HNS samples. To study the growth mechanism of the morphological transformation of hierarchical nanostructure (HNS) to cubic nanostructure (CNS), we present the FESEM images of the NaOH treated sample at different time intervals (Figure 3). From Figure 3 it is observed that after 5 h of NaOH (0.1 M) treatment, HNS started to transform into cubes and after 28 h of aging time the formation of microcubes was complete. After that, with the increase in ageing time, the microcube structure disintegrated and was dissolved in the NaOH medium.

Figure 2: EDX spectra of (a) HNS and (b) CNS sample. (c-f) Elemental area mapping of the HNS sample.


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After 45 h of aging time, a uniformly distributed 3D nanocube was formed. So NaOH stands to be an etching and growth directing agent owing to its dissolution capability. The details of the mechanism have been discussed below. Most commonly evolved Co-Sn based oxide and hydroxide materials replicate cubic morphology. Thus, a plausible mechanism is utmost important to explain the formation of exceptional hierarchical structure of CoSn(OH)6 and its transformation to nanocube after NaOH treatment. Here for the synthesis of CoSn(OH)6 HNS, we used NH3 as the hydrolyzing agent which plays the crucial role to develop the HNS. Here, ammonia simultaneously acts as a hydrolysing as well as a bridging agent.3 In the reaction medium NH3.H2O becomes NH4+ and OH- (equation 9). Hydroxyl ion (OH-) acts as the hydrolyzing agent (equation 10). NH3 binds to the surface of the particle through hydrogen bonding. Oxygen presents in the air act as the oxidising agent and transform Sn(II) to Sn(IV). NH4+ + OH-------------- (9)

NH3.H2O Sn2+ (aq.) + Co2+ (aq.) + O2(g) + 6OH-(aq.)

CoSn(OH)6 (s)------------- (10)

The whole CoSn(OH)6 evolution process may be divided into four steps: dissolution, crystallization, oriented attachments and self-assembly.3,32,33 At first, during hydrothermal treatment, at high temperature and high autogenic pressure the dissolution and crystallization of the amorphous particles occurred very fast. After that NH3 presumably passivated the surface of CoSn(OH)6 particle through hydrogen bonding (-N…H-O or N-H….O-H). During this process, formation of hierarchical structure started through oriented attachment and selfassembly process.

Figure 3: FESEM images of the HNS sample after 0.1 M NaOH treatment at different time intervals (a-f) at room temperature. 9 ACS Paragon Plus Environment


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In oriented attachment, to optimize the crystallographic orientation at low surface energy, particles rotate by themselves and irreversible attachment occurs through atom by atom addition. Here, due to the surface passivation by NH3, the surface energy of the particles became low. Again, due to the two energetically different types of hydrogen bonding with two different metal centres, the surface energy of the particles varied.3 Thus irreversible selfassembly process made the hierarchical structure reproducible. To justify the effect of hydrogen bonding for the formation of hierarchical structure, we introduced additional NH4Cl into the reaction mixture. Due to the presence of excess NH4+ ion, self-assembly process was much more favourable and thus we obtained more number of fine hierarchical structures (Figure S3b). Hierachical nanostructure (HNS) of CoSn(OH)6 evolved due to the hydrogen bonding in the presence of NH3. HNS is not a favorable structure for the evolved CoSn(OH)6. Upon 0.1 M NaOH treatment HNS transformed into CNS. Thus, the growth mechanism for the transformation of HNS to CNS is very much demanding here. When 0.1 M NaOH added to the HNS sample, hydrogen bonding got disrupted and NH3 was released. As a consequence to that, the hierarchical structure started to disrupt. We have monitored the effect of NaOH on HNS sample by time dependent FESEM imaging. From Figure 3a, it is observed that after 5 h of aging, few microcubes started to grow leaving most of the portion of the hierarchical structure intact. Figure 3b reveals that after 15 h, number of microcubes have increased though the formation of the cube was not fully developed. After 28 h of aging time (Figure 3c), formation of the micocubes was complete. This transforamation is facile, as the cube is the most stable and favourable structure of the compound. Here NaOH behaves as the structure directing agent. Again considering the amphoteric nature of CoSn(OH)6, dissolution of microcubes in NaOH started via the formation of Co(OH)42- and Sn(OH)62- (equation 11). CoSn(OH)6 + 4OH-

Co(OH)42- + Sn(OH)62-------------- (11)

Thus, in Figure 3c, d and e broken surface of the microcubes is observed. From the Figure 3d, it is observed that few of the microcubes got etched by OH- ions and etching continuously increased with time (Figure 3c,d and e). Subsequently the microcubes started transforming into nanocubes (Figure 3d). After 45 h, as observed from Figure 3e, all the microcubes were transformed into the nanocubes. Here, NaOH acted as the etching agent.26 This discussion supports our assumption over the dual role of NaOH during the formation of CNS from HNS.


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Figure 4: (a, b) High resolution TEM images (with fringe spacing) of HNS sample. (c, d) SAED pattern of HNS and CNS samples, respectively. Figure S4 demonstrates the FESEM images of C1 sample at different concentration ratio of the precursor materials (Co(II) and Sn(II)). When the ratio of Co(II) and Sn(II) was 2:1, the composite did not change its actual morphology, i.e. hierarchical structure was retained. But when the ratio was 1:2, then cubic structures predominated with minimum number of hierarchical structures. These experimental results demonstrate that when concentration of Sn(II) was more, then cubic structure predominated. Though morphology varies with the ratio of the precursor salt, yet in all cases the final compound was CoSn(OH)6 (discussed in the XRD part). It has already been discussed that Sn based or Co-Sn based oxides or hydroxides mostly exhibit cubic structure. Thus, when the concentration of Sn(II) increased, the materials try to achieve their most favoured cubic structure. The high resolution TEM images pointing up the lattice fringe spacing of the as-synthesized samples. Figure 4a,b demonstrate the interplanar distances of ~0.27 nm and ~0.38 nm for HNS sample which are very much in agreement with the lattice spacing of (200) and (220) planes, respectively.26 For CNS sample (Figure S5), the interplanar distances are around 0.382 nm, 0.27 nm and 0.24 nm indicating the lattice fringe spacing for (200), (220) and (310) planes, respectively. Figure 4c,d represents the SAED patterns for HNS and CNS samples. In Figure 4c, small dots forming the well-defined rings indicate the poly crystalline nature of the HNS sample and also the rings can be well manifested to the (420) and (220) planes. Bright spots in Figure 4d confirm the single crystalline nature of the CNS sample and dotted lines indicate the (420) and (200) planes.



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Figure 5: (a) XRD spectra and (b) FTIR spectra of HNS (black line) and CNS (red line) samples. To comprehend the phase purity and crystallinity, we have opted the X-ray powder diffraction study. Figure 5a demonstrates the XRD of the samples (black line for HNS sample and red line for CNS sample). The sharp diffraction peak confirms the crystalline nature of the sample. The diffraction peak positions for both the sample are similar and the diffraction patterns are very much equivalent to the CoSn(OH)6 [JCPDS No-74-0365, cubic, primitive lattice, space group-Pn3m(224), cell parameter a= 7.780 Å]. Very small peaks around 33°, 53°, 63° and 72° in both the curves are due to the presence of the little amount of Co3O4 (JCPDS No-42-1467).3 Amount of Co3O4 with respect to CoSn(OH)6 is very small for both the sample. In the XPS part, it has categorically been discussed. We have also performed the XRD analysis for the samples which were synthesized using different Co(II) and Sn (II) salt ratios (Figure S6a). Patterns for all the three samples (ratio 1:1, 2:1, 1:2) are similar (Figure S6a). This illustrates that for all types of ratios, composition of the as-synthesized materials is same, i.e. CoSn(OH)6. These results establish that the CoSn(OH)6 can be prepared by different molar ratio of the precursor salts adopting our synthetic protocol. To confirm the composition of the as-synthesized materials, we have performed the FTIR analysis. Both the samples (HNS & CNS) showed the same type of FTIR spectra (Figure 5b, black line for HNS and red line for CNS). A large band at 3235 cm-1 and a small pinnacle at 1632 cm-1 designate the O-H stretching vibration and bending vibration, respectively.34 The main characteristic peaks at around 540 cm-1 and at 1177 cm-1 signify the stretching vibration of Sn-O bond and bending vibration of the Sn-OH bond, respectively.34 A large band at 778 cm-1 is for the H2O-H2O hydrogen bonding.34


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Figure 6: (a) Wide range XPS spectra of HNS (black line) and CNS sample (red line). (b-d) narrow range XPS spectra for HNS sample (b: Co 2p, c: Sn3d, d: O 1s). The composition of the as-synthesized material has been further confirmed from the XPS analysis. Wide range XPS spectra confirm that both the samples are composed of Co, Sn and O (Figure 6a). Figure 6b and Figure S6b demonstrate the high resolution XPS for Co 2p for HNS and CNS samples, respectively. By deconvoluting the spectra, two major peaks have been found, positioned at ~ 780.5 eV for 2p3/2 (for HNS, 780.63 eV and for CNS, 780.49 eV) and at ~ 797.2 eV for2p1/2 (for HNS, 797.16 eV and for CNS, 797.1 eV) which signify the presence of pure Co(II) in the samples.35 Here, the peaks are generated due to the spin-orbit coupling at 2p3/2 component (780.63, 782.81, 785.46 and 789.96 eV) and 2p1/2 (797.16 and 802.67 eV) component of HNS sample. Both the components include the same qualitative chemical information. Here the main characteristic peak is the shakeup satellite peak, which is situated at ~785.4 eV in both the sample. This added spectral line generally occurs either due to the excitation of multielectron (shakeup) or due to the coupling between the unpaired electrons (multiplet splitting).35 This additional satellite peak is totally absent in case of Co(III) and intensity is very low in case of Co3O4 [i.e., Co(II) and Co(III)]. Here, in both the samples, the intensities of the satellite peaks are very high which confirms that in both the assynthesized nanomaterials, Co is present in its Co(II) form only. Figure 6c and Figure S6c exhibit the high resolution Sn 3d spectra of HNS and CNS samples, respectively. Both the spectra demonstrate two high intense peaks positioned at ~ 486.5 eV, for 3d5/2 (for HNS, 13 ACS Paragon Plus Environment


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486.46 eV and for CNS, 486.51 eV) and 494.9 eV for 3d3/2 (for HNS, 494.91 eV and for CNS, 494.99 eV). In both cases, the peak splitting of 8.4 eV is observed. These spectral data confirm that in HNS and CNS samples, Sn is present in Sn(IV) state.36 Figure 6d and Figure S6d display the high resolution XPS of O 1s for HNS and CNS samples, respectively. Deconvolution of the spectra offers three peaks in each case. The large peak at around 531.1 eV indicates the binding energy of the hydroxyl group with metal ion. Another two small peaks around ~ 529.2 eV and ~ 532.6 eV stand for the presence of oxide and structural water with the CoSn(OH)6 compound.3,36,37 The peak for oxide compound originates due to the presence of trace amount of Co3O4 (The ratio of CoSn(OH)6: Co3O4 = 25:1). Specific surface area and pore size distribution of the materials are absolutely crucial for a material to behave as a charge storage electrode material. Figure S7a demonstrate the N2 adsorption-desorption curve for HNS and CNS samples at 77K within the relative pressure of 0 to1 (P/P0) region. These two isotherm curves follow type IV category of isotherm, which suggests the samples to be mesoporous in nature. Pore size distribution curves (using BJH method) (Figure 7Sb) also depict the mesoporous nature of the as-synthesized nanostructures (pore diameter of HNS sample is 3.96 nm and that of CNS sample is 3.8 nm). The calculated BET surface area for HNS sample and CNS samples are 298.1 m2/g and 66.6 m2/g, respectively. Pore volumes of the samples are 1.225 cm3/g and 0.318 cm3/g for HNS and CNS samples, respectively. Here, the hierarchical structure constructed by the self-assembly of 2D ultrathin nanosheets makes HNS sample more surface active. The high surface area and pore volume of the HNS sample increase the wettability of the electrodes and thus facilitate the transportation of electrolyte.3 Electrochemical behaviour: Here the as-synthesized CoSn(OH)6 samples have been used as the electrode materials for pseudocapacitors. Evaluation of the electrochemical behaviour has been done by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), chronopotentiometry (CP) or galvanostatic charge-discharge (GCD) in both three-electrode and two-electrode systems. In all cases, 3 M KOH was used as the electrolyte. Three-electrode system:


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In three-electrode system, HNS/CNS CoSn(OH)6 was used as the working electrode, while Pt wire and saturated calomel electrode (SCE) were used as the counter and reference electrode,

Figure 7: Comparative study of (a) CV curves at 50 mV/sec and (b) charge-discharge curves at 2.5 A/g for HNS (red line) and CNS (black line) sample. (c) Graphical presentation of percentage capacitance retention (red line) and coulombic efficiency retention (blue line) of HNS sample up to 10000 cycles and (d) Nyquist plot for HNS and CNS sample before and after the 10000 charge-discharge cycles at three-electrode system. respectively. Figure S8a,b represents the CV curves for the HNS and CNS samples at different scan rate in the potential window -0.1 to 0.4 V. Both the CV curves are constructed by two regions. In the first region (from -0.1 V to 0V), there in no peak and in the second region (0 to 0.4 V), a peak remains present. The initial small part signifies the formation of double layer and the other part which is the large one stands for the faradic charge transfer. This result suggests that the capacitive nature of the as-synthesized material originates mostly from the faradic redox reaction, i.e. pseudocapacitive and a small contribution of electrical double layer capacitor.6 The peaks are due to the redox reaction of Co(II)/Co(III) couple.3,38 With increasing scan rate from 10 mV/s to 100 mV/s, specific current increases. This indicates the high transportation of ions through the electrode at high scan rate. Figure S8c,d displays the galvanostatic charge-discharge (GCD) curves of both the materials at different specific currents. Figure 7a and 7b demonstrate the comparative CV curves at 50 mV/sec and charge-discharge curves at 2.5 A/g for both the CoSn(OH)6 materials (HNS and CNS) where we can see that the curves are very much similar i.e., mechanism of action for both the



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materials are same. In both the curves, it is observed that charging curve and discharging curve are almost symmetrical. This observation suggests the excellent reversibility of the assynthesized materials.5 The calculated specific capacitances for the materials at different specific currents have been shown in Table S1. According to the Table S1, HNS CoSn(OH)6 exhibits high specific capacitance, 2545 F/g at 2.5 A/g whereas the CNS sample shows 851 F/g at the same specific current. Table S2 demonstrates the reported specific capacitance values of Cobalt based different electrode materials in three-electrode system. Compared to these values, our HNS sample exhibits higher specific capacitance value. Table S1 indicates that the rate capability of the HNS sample is comparatively very high than that of the CNS sample. The specific capacitance retention for CNS sample is only 30 % at 25 A/g specific current where the value for HNS sample is 61% at the same specific current. Even at 60 A/g specific current the HNS CoSn(OH)6 exhibits 42 % capacitance retention. Interestingly, both the materials are of same composition, yet they exhibit electrochemical property which are poles apart, in terms of acitivity as the pseudocapacitor electrode. Here, actually morphology plays a pivotal role. HNS sample is composed of 2D building block whose individual thickness is ~5 nm (from AFM measurement), whereas the dimension of cubic sample is ~100 nm (from FESEM and TEM image). It has already been discussed in our previous paper that electrolyte can diffuse up to ~ 20 nm depth of the active pseudocapacitor material.3 High specific capacitance value of a pseudocapacitor electrode depends on the utilization of active material in the sample. Here, HNS sample offers a whole body participation of the material (i.e., amount of electro-active material is very high). On the other hand, for CNS sample, participation of the electro-active material is comparatively quite less. Thus, HNS sample shows very high specific capacitance value compared to CNS sample. The ion transportation through the material is the fundamental reason behind a material to be rate capable. Accordingly, higher the transport of ion in the material, higher will be the rate capability. From the BET analysis, it is confirmed that both the samples are of mesoporous in nature but due to flat sheet like structure of HNS sample active surface area is much larger than that of the CNS sample. Again, pore volume of HNS sample is higher compared to CNS sample. Thus, high surface area and pore volume of HNS sample augment the electrode wettability and thus ion transportation in HNS sample becomes high compared to the CNS sample. As a matter of fact, rate capability of HNS sample is higher than that of the CNS sample. To examine the cyclic durability, we have performed the cyclic charge-discharge experiment of the HNS CoSn(OH)6 sample up to 10000 cycles at 40 A/g. It has been observed that for first few cycles, specific capacitance increases and after 1000 cycles it returns to its initial value. 16 ACS Paragon Plus Environment

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This is due to the activation of surface in KOH electrolyte at high specific current. From Figure 7c it is observed that specific capacitance decreases with the increase in chargedischarge cycles but the decrease is very minimal. After 10000 cycles, specific capacitance retention is 92%. This large electrochemical cyclic stability of the HNS CoSn(OH)6 sample may be due to the synergistic effect of Co and Sn in the mixed hydroxide. We have also calculated the coulombic efficiency (η) of the sample to determine the kinetic irreversibility of the material. Coulombic efficiency was calculated from the charging time (tc) and discharging time (td) using the equation, (η = td/tc×100). Lower ∆t (∆t = td-tc) value signifies the lower kinetic irreversibility and higher coulombic efficiency. In case of the HNS sample, we have seen that from 2.5 A/g to 15 A/g specific current, coulombic efficiency increases from 91% to ~100% and remain unchanged up to 60 A/g. The high coulombic efficiency of the materials supports the high OH- ion transport through the HNS sample. The unit of the HNS CoSn(OH)6 sample is 2D sheet and these 2D sheets have been assembled together to build up a highly porous hierarchical structure. Thus, very low thickness (~5 nm) and high surface of the electrode material facilitate the fast ion transport. At low specific current structural activation of the electrode material is not so high, thus coulombic efficiency is also comparatively low. We have also calculated the coulombic efficiency of the material at different charge-discharge cycles (Figue 7c). In general, it has been observed that after few charge-discharge cycles coulombic efficiency decreases. In our case, we have checked it up to 10000 cycles at 40 A/g specific current and it has been observed after 10000 cycles change is minimum (only 3%). This superior electrochemical activity of HNS CoSn(OH)6 is also due to the mixed metal hydroxide nature of the compound. Presence of Sn inside the Co matrix makes it better conductive, robust compared to the single metal hydroxide compound.3 Thus, it exhibited better structural stability (Figure S9). On the other hand, we have checked the charge-discharge cyclic stability up to 10000 cycles and also the coulombic efficiency of the CNS sample up to 10000 cycles at 25A/g (Figure S10a). For the latter sample (CNS CoSn(OH)6), it has been observed that only after 500 cycles capacitance decreases to 93.5 % and continues to decrease further. After 10000 cycles retention of capacitance is only 73%. Coulombic efficiency of the cubic sample at 2.5 A/g was 81% and at 25 A/g it was 96%. We have also checked the coulombic efficiency up to 10000 cycles and we observed that it gradually decreases with the increase in charge-discharge cycle. Coulombic efficiency was found to be 85 % after 10000 cycles. Though the CNS sample is also robust due to the mixed hydroxide material, yet thickness of the material is very high. Again from the BET measurement, it has been observed that its surface area and pore volume are not so large. 17 ACS Paragon Plus Environment


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Thus, ion transport through the CNS sample is not so facile. That is why coulombic efficiency and electrochemical stability of the material are less for the cubic sample. Figure S9 demonstrates the FESEM images of HNS and CNS sample after 10000 charge-discharge cycles. From this figure, it is clear that after 10000 cycles, HNS sample almost maintained its hierarchical structure, but few cubes have been deformed. This result also justifies the excellent cyclic durability of HNS over CNS. These above results and discussions illustrate that only morphology can play a significant role to behave as a superior electrode material for charge storage. From the aforesaid results and discussion, it is obvious that HNS CoSn(OH)6 sample is a superior pseudocapacitor electrode material over the CNS sample. To explain the results, we have performed the electrochemical impedance spectroscopy (EIS) using both samples. EIS has been performed over the frequency range from 100 KHz to 10 mHz at 0.3 V for both the samples (AC amplitude was 5 mV) in three-electrode system. Figure 7d demonstrates the Nyquist plot for HNS and CNS samples before and after 10000 charge-discharge cycles. It has been observed from the figure that for HNS sample both the curves are almost symmetrical. This observation clarifies the high electrochemical stability of the HNS sample. But for CNS sample, the curves before and after the 10000 charge-discharge cycles are completely different which suggests the instability of the cubic sample before and after the charge-discharge cycles. Both the curves were fitted with the Randles circuit (Figure S10b) and the data obtained from this (Table S3) also supports the high stability of HNS sample and low stability of CNS sample. We also have plotted the Bode plot (Figure S10c) for both the samples. For an ideal super capacitor, i.e. double layer capacitor, at low frequency range the phase angle shifts almost to -90º and for a pseudocapacitor, the phase angle is well below 90º. From S10c, it is observed that phase angle for HNS sample is -80° and that of CNS is 72°. These results suggest that both the materials exhibit capacitive property which originates mostly from the pseuducapacitor (0.05 V to 0.4 V) but participation of electrical double layer is also there (from the CV curve -01 V to 0.05 V).3,39 Two-electrode system:


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Scheme 2: Schematic presentation of the mechanism of action of the HNS CoSn(OH)6//AC when it was used as the two-electrode system. To verify the excellent electrochemical activity of the HNS sample, we have used it as the positive electrode material in the two-electrode system assembling with AC as the negative electrode material (Scheme 2). The details of the electrochemical behaviour of AC have been discussed in the supporting information (S2). The real cell has ensured the superior electrochemical activity due to the hybridization of the redox active HNS CoSn(OH)6 pseudocapacitor having a high energy density and EDLC based high power active AC. Figure 8a demonstrates the CV curve of HNS CoSn(OH)6 and AC at 50 mV/sec at 3 M KOH in three-electrode system. Rectangular shaped CV curve, ranging from -1.2 to 0.0 V, confirms the EDLC nature of the AC electrode. The pseudocapacitive nature of the HNS sample has already been discussed (CV curve ranges from -0.1 to 0.4 V). Thus, theoretically the total voltage window of the real cell is 1.6 V. (summing the working potential window of AC and HNS CoSn(OH)6 sample).



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Figure 8: (a) CV curve of AC and (red line) and HNS CoSn(OH)6 (black line) at 50 mV/s in three-electrode system, (b) CV curve at different voltage window, (c) CV curves at different scan rate and (d) charge-discharge curve at different specific currents of HNS CoSn(OH)6//AC asymmetric cell. Figure 8b demonstrates the CV curve of the asymmetric cell at different voltage window and it has been observed that the cell is stable up to 1.6 V. Thus, we have performed the whole electrochemical experiment of the HNS CoSn(OH)6//AC asymmetric cell in the working voltage of 1.6 V. Figure 8c illustrates the CV curve of the full asymmetric cell at different scan rates (10-100 mV/sec). This figure depicts that the cyclic voltamogram of the asymmetric cell which is contributed by both polarization of charge (0.0 to 0.9 V) and faradic redox reaction (0.9 V to 1.6 V). The unchanged CV curve at different scan rate actually reveals the reversible nature of the electrode and excellent rate capability as well. Figure 8d displays the charge-discharge curve of the asymmetric cell at different specific currents. From the graph, it is clear that the charging and the discharging curves are almost symmetrical. This result also verifies the reversibility and superior rate capability of the real cell having very low ohmic resistance value. By excluding the ohmic resistance from the total working cell voltage, we have calculated the specific capacitance of the asymmetric cell at different specific currents from the equation 3. The asymmetric cell shows very high specific capacitance value, 713 F/g at 1.5 A/g. Table S4 illustrates the specific capacitance value of the as-fabricated two-electrode system at different specific currents. Table S5 illustrates the specific capacitance values of different reported AASs which suggest that our cell exhibits better specific capacitance value compared to the reported asymmetric system. According to this table S4, specific capacitance of the asymmetric cell is 301 F/g at 7 A/g i.e., capacitance retention at such high specific current is ~42% which supports the high rate capability of the system. Highly porous nature of the AC and HNS CoSn(OH)6 makes the asymmetric cell better rate capable. To investigate the electrochemical stability of the real cell, we have performed the cyclic charge-discharge cycles of the system up to 10,000 cycles at 7 A/g. Again, to verify the kinetic reversibility of the cell we have calculated the coulombic efficiency of the cell at different charge-discharge cycles. Figure S11a demonstrates the electrochemical stability and coulombic efficiency of the two-electrode system up to 10000 charge-discharge cycles. For first few cycles (up to ~900), specific capacitance increases and becomes fixed at 104% and after 980 cycles it starts decreasing and maintains its initial capacitance value. The rate of decrease of specific capacitance value is minimum and after 20 ACS Paragon Plus Environment

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10000 cycles the specific capacitance value is 92% of its initial value. Indeed, this result is extraordinary. On the other hand, after 7000 cycles coulombic efficiency has been faded to 94% and then it was stable up to 10000 cycles. This high coulombic efficiency reveals the high reversibility of the asymmetric cell.

Figure 9: (a) Ragone plot of HNS CoSn(OH)6//AC cell and (b) Nyquist plot of the same at before and after 10000 charge-discharge cycles. To evaluate the overall performance of the cell, we have calculated the energy density and power density of the whole asymmetric cell. Figure 9a demonstrates the Ragone plot of the real cell which compares the energy density of the asymmetric cell with its power density at different specific currents. As per the plot the asymmetric real cell shows very high energy density of 63.5 Wh kg-1 at power density of 1196 W kg-1 and it decreases to ~27 Wh kg-1 when its power density increases to 5277 W kg-1. Comparing with the reported values (plotted in the Figure 9a), the hierarchical mesoporous CoSn(OH)6 nanostructures is very much promising in the field of supercapacitor.40-48 To explain the stability and superiority of the real cell, the EIS analysis of the system was performed. The experiment has been performed in the frequency range of 100 KHz to 10 mHz at 0.8 V (AC amplitude was 5 mV). Figure 9b illustrates the Nyquist plot of the asymmetric cell and the Bode plot of the system before and after 10000 cycles. After fitting with their equivalent circuit, we have found the corresponding parameters for both the plots which have been enlisted in Table S6. From the table S6, it is clear that before and after 10000 charge-discharge cycles, the difference in RS, i.e. internal resistance value is 0.09 ohm and difference in RCT i.e., charge transfer resistance value is 2.11 ohm. These values also indicate the electrochemical stability of the material. From the Bode plot (Figure S11b), it is observed that at low frequency region (below 10 Hz) the phase angle starts increasing and 21 ACS Paragon Plus Environment


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shifts towards more negative value. In our case, at 0.01 Hz the value is -82º which suggests that the capacitive nature of the hybridized asymmetric cell originates from both electrical double layer capacitor type (0 V to 0.9 V) and pseudocapacitor type (0.9 V to 1.6 V) (Figure S10b).39,49,50 Another important factor to explain the activity of a supercapacitor system is the relaxation time constant τ0 (τ0= 1/f0, f0 = frequency at −45°). In our case, τ0 is only 1.58 s. This low relaxation time constant reveals the deep penetration of OH- ion into the electrode and the supercapacitor reached at its maximum capacitance value in low charging time.51 Actually shorter relaxation time involves the fast charge-discharge reversibility and high efficiency.52 This also justifies the high energy density and high power density of the asymmetric cell. Thus, almost symmetric Nyquist plot and Bode plot before and after the 10000 charge-discharge cycles, and shorter relaxation time constants justify the cyclic durability, high energy storage and high power delivery of HNS CoSn(OH)6//AC asymmetric supercapacitor. Conclusion: This work demonstrates a simple NH3 mediated way to synthesize ultrathin 2D sheet based hierarchical CoSn(OH)6 nanostructured (HNS) material. Aqueous NaOH treatment over the HNS sample directs morphological transformation towards the most favoured cubic CoSn(OH)6 nanostructure (CNS). Synthesis of Co–Sn based oxides of hydroxides other than cubic structure is highly challenging. Therefore, our ammonia mediated co-precipitation route to synthesize HNS CoSn(OH)6 would draw the attention of the researchers. In addition to this ultrathin wall and high porosity of HNS sample make it a better pseudocapacitor electrode over CNS sample. Specific capacitance value of HNS sample is 2545 F/g at 2.5 A/g specific current, whereas that of CNS sample is 851 F/g at the same specific current. Thus, the result proves that only morphology can drastically alter the electrochemical property of a material. Aqueous asymmetric supercapacitor (AAS) was fabricated using HNS as the positive electrode material and AC as the negative electrode material. Specific capacitance value of the AAS system is 713 F/g and the cyclic durability of the system is also very high. High capacitance value and large potential window provide very high energy density (maximum energy density value is 63.5 Wh kg-1) of the AAS without losing the power density (maximum power density is 5277 W kg-1).


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Acknowledgements The authors are thankful to Prof. D. Pradhan, Materials Science Centre, IIT Kharagpur for BET measurements, DST and CSIR, New Delhi, India for financial assistance, and the IIT Kharagpur for instrumental support.

Supporting Information descriptions Description of instruments used during the experiments. Comaparative study of bare Ni foam and activated Ni fom. Electrochemistry of AC. Tables for experimental and reported specific capacitance values for both three-electrode and two-electrode systems, Electrochemical parameters obtained from Nyquist plots for both systems. TEM image of CNS sample. AFM image and height profile of HNS sample. FESEM images of HNS sample at different precursors ratios. HRTEM image of CNS sample. XRD pattern of HNS sample at different molar ratios of Co(II) and Sn(II) salts. Narrow range XPS for CNS sample. BET analysis. CV curves, ch-dch curves, Nyquist plot and randles circuit for both thee-electrode and twoelectrode system. FESEM images of HNS and CNS sample after 10000 cycles. Stability and Bode plot for CNS sample.



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Suitable Morphology Makes CoSn(OH)6 Nanostructure a Superior

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