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Facile Synthesis of Unique Hexagonal Nanoplates of Zn/Co Hydroxy Sulfate for Efficient Electrocatalytic Oxygen Evolution Reaction Soumen Dutta, Chaiti Ray, Yuichi Negishi, and Tarasankar Pal ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00030 • Publication Date (Web): 17 Feb 2017 Downloaded from http://pubs.acs.org on February 19, 2017

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ACS Applied Materials & Interfaces

Facile Synthesis of Unique Hexagonal Nanoplates of Zn/Co Hydroxy Sulfate for Efficient Electrocatalytic Oxygen Evolution Reaction Soumen Dutta,a Chaiti Ray,a Yuichi Negishib and Tarasankar Pala* a

Department of Chemistry, Indian Institute of Technology, Kharagpur. Kharagpur 721302, West Bengal, India,

b

Department of Applied Chemistry, Tokyo University of Science, Tokyo-1628601, Japan E-mail: [email protected]

Abstract Cost-effective, highly active water oxidation catalysts are becoming increasingly demanding in the field of energy conversion and storage. Herein, simple modified hydrothermally (MHT) synthesized zinc and cobalt based hydroxyl double salt i.e., Zn4-xCoxSO4(OH)6· 0.5 H2O (ZCS) has been exfoliated for the first time as an efficient electrocatalyst for oxygen evolution reaction (OER) in alkaline medium. Morphology investigation suggests the evolution of unique hexagonal nanoplates of ZCS material. As OER catalyst, it requires only 370 and 450 mV overpotential to achieve 10 and 100 mA cm−2 current density respectively. More importantly, performance at the overpotential over 400 mV and durability of the designed material have been found to be superior to those of commercial RuO2 catalyst. In the designed ZCS material trace amount of cobalt species lead to attain higher mass activity of 146 A g-1 while compared to RuO2 catalyst (83 A g-1) at the same overpotential of 370 mV. The outstanding activity and stability of the cost-effective material emerges out from the promotional effect of Zn ions which are present as the principal constituent in the

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electrocatalyst and they also protect the cobalt ions in the matrix during its long-term electrochemical test. It is important to note that appropriate ratio of zinc and cobalt ions synergistically help to become an economically viable and environmentally suitable electrocatalyst in comparison to other related transition metal based materials. KEYWORDS: Water oxidation; energy conversion and storage; oxygen evolution reaction; overpotential; mass activity. INTRODUCTION Water electrocatalysis is widely considered to be a unique and promising technique for efficient renewable energy production, storage and usage including sustainable hydrogen generation, rechargeable metal–air batteries and fuel cells.1-3 Overall water splitting consists of two half reactions: the hydrogen evolution reaction (HER) at the cathode and the oxygen evolution reaction (OER) at the anode. However, the water oxidation process (i.e., OER) has the uphill thermodynamic barrier i.e., high overpotential and also the complexity of the reaction mechanism involving four electron transfer process in comparison to HER.4-6 The benchmark catalysts for high performance OER generally includes noble metal oxides e.g., RuO2 and IrO2.7,8 However, their high cost and low abundance seek urgency in developing cost-effective catalyst from earth abundant transition metal family.8 In this prospect Ni, Co, Fe, Mn based nanomaterials with interesting morphologies and also their various composites are reported to perform exceptionally well for excellent OER with long-term stability.9-14 Transition metal based layered double hydroxides (LDHs),11,13 spinels, metal-metal oxide/carbon composites10 are often found in the literatures for OER performance in alkaline electrolytes. Zinc based materials remain silent to exhibit OER performance, although few studies have been performed with the incorporation of Zn in mainly Co based matrices to increase their


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OER performance.15-21 Previously, Zn-Co-LDHs,15,17 ZnxCo3−xO4,16 ZnCo2O4,18 Ni-Zn LDHs19 have been employed for OER in basic medium with required overpotential (η10, for 10 mA cm-2 current density) in the range of 320-500 mV. In all the cases, Zn(II) ions scarcely participate directly in OER, rather high catalytic activity has been explained in terms of Zn doping which leads to modified electronic structure or increased defect/active sites into the active cobalt on nickel ions.21 In the present report, we have shown for the first time the OER performance from a 2D material, Zn4(OH)6SO4 with minimum Co(II) ions incorporation to obtain economically as well as environmentally important OER catalyst. It is worthwhile that the amount of Co(II) in the 2D material, does matter to show efficient OER activity, but optimum Co(II) loading is advisable to get maximum performance from the designed material. The highly dispersible hexagonal nanoplates of Zn4-xCox(OH)6SO4 (designated as ZCS) with the optimized Zn/Co molar feeding ratio exhibit an overpotential of 370 V at 10 mA cm−2 and a Tafel slope of 60 mV dec−1, which are comparable to RuO2 and superior to various cobalt and nickel based reported materials.

EXPERIMENTAL SECTION Materials: All the reagents used in this report were of AR grade and used as received without further purification. All glasswares were cleaned using aqua-regia, subsequently rinsed with a copious amount of double distilled water, and dried well prior to use. Doubledistilled water was used throughout the course of the experiment. Synthesis of ZCS30: In a typical procedure, 0.2 mmol ZnSO4·7H2O and 0.4 mmol CoSO4·7H2O are dissolved in 10 mL distilled water. Then the aqueous solution is transferred into a screw capped test tube



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and then 30 µL ammonia (25%) has been added before the whole solution has been undergone heating at 160 ºC for 24 h though modified hydrothermal process (MHT).3 The resultant pinkish product has been washed thoroughly first by distilled water then by ethanol. Then the washed material is subjected to air drying before its further use. The experimental details have been shown in Table 1. Electrochemical study: All electrochemical experiments were carried out on a CHI 660E electrochemical workstation. A standard three-electrode system was used for all electrochemical experiments, which consisted of a platinum wire as a counter electrode, saturated calomel electrode (SCE) as reference electrode and a catalyst modified glassy carbon electrode (GCE) as the working electrode. The GCE was sequentially polished with alumina slurry (1.0, 0.3, and 0.05 µm) and rinsed carefully with double-distilled water. Then, the electrode was sonicated in water and finally in ethanol for 15 minutes. The GCE was then dried well at room temperature before use. All electrochemical tests are carried out at room temperature. Before the measurements, the electrolyte was removed oxygen by bubbling high-purity N2 for 20 min. During the measurements, magnetic stirring was used to eliminate bubble accumulation and limit the diffusion effect. OER study: Oxygen evolution reaction (OER) experiments were carried out in a three electrode cell taking 0.5 M KOH as electrolyte solution. Firstly, 7 µL of dispersed materials (2 mg mL-1 concentration in water) were loaded on GCE (with geometric area 0.07 cm-2). After complete drying, 7 µL of 0.1% Nafion solution (acts as binder) was further drop casted on it. The catalyst loading on GCE became 0.20 mg cm-2. Initially, 20 cyclic voltammetric (CV) cycles were performed in each case in order to achieve relatively steady state before linear sweep


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voltammogram (LSV) has been recorded. All LSV polarization curves were iR-corrected considering the solution resistance (Rs) of the electrolyte. In 0.5 M KOH solution we have converted the potential window following ERHE = ESCE + 1.069. The AC impedance measurements were taken at frequencies ranging from 0.1 Hz to 10 000 Hz. Overpotential has been calculated as (η) = ERHE -1.23 V. The linear portion of the Tafel plot is fitted to the Tafel equation, η = b log j + a, where η is the overpotential, j is the current density, b is the Tafel slope, and a is the intercept relative to the exchange current density (J0). Electrochemical double layer capacitance (Cdl) is considered to be directly proportional to the ECSA values of the materials. Here the potential range of the CV profiles has been restricted without the faradic reactions and the difference between charging and discharging current density is demonstrated to be a linear function of the applied scan rate. Now Cdl value is calculated by considering slope = 2×Cdl. RESULTS AND DISCUSSION In the present study, we have synthesized double hydroxide salts22 Zn4-xCoxSO4(OH)6 · 0.5H2O (ZCS) from a aqueous solution of cobalt and zinc salts in presence of ammonia under hydrothermal condition at 160 °C. FESEM images of the synthesized ZCS30 (material prepared with 30 µL of ammonia) materials suggest evolution of 100% hexagonal plate like morphology with 400-500 nm edge length (Figure 1a, S1). Elemental mapping analysis suggests the well distributed Zn, Co, O and S in the as-prepared sample (Figure S2). Thin hexagonal shaped plates are observable from the corresponding TEM (Figure 1b) and STEM images (Figure S3a, b). Tapping mode AFM image of ZCS30 reveals the synthesized nanoplates have the thickness of 5-8 nm (Figure S4). High resolution TEM (HRTEM) study reveals interlayer spacing of 0.31 nm for (012) plane whereas its bright spots in SAED pattern indicate single crystalline nature of the material (Figure 1c and S3c).



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XRD pattern of the ZCS30 material in Figure 2a matches with the standard XRD of hexagonal Zn4SO4(OH)6·0.5 H2O (JCPDS 44-0674).23 It is important to note that heating the as-synthesized product in air at 400 °C for 4 h leads to the formation of ZnO-CoO (Figure S5).23 XPS spectra of ZCS30 have been shown in Figure 2b which suggest the characteristic peaks for the elements Zn, Co, S and O.24-28 High resolution Zn 2p XPS spectra contains one doublets at 1043.7 and 1020.7 eV for Zn 2p1/2 and Zn 2p/2 respectively (Figure 2c). These peak positions are shifted a little towards lower binding energy than expected, which may be due to the incorporation of cobalt ions.16 Two spin energy levels are separated by 23 eV which is typical for Zn(II) species.24 Two core level signals at 779.9 and 795.9 eV can be assigned for Co 2p3/2 and Co 2p1/2, respectively as shown in Figure 2d. Spin-orbit energy level spacing of 16.0 eV suggest the presence of Co(II) ions in the synthesized material as indicated in the previous reports.25,26 In addition, signals at almost 5 eV higher binding energy correspond to satellite peaks of Co(II) centres.27 It is worthwhile that S2p XPS spectra (Figure 2e) can be deconvulated in doublets at 168.1 and 169.3 eV for S2p3/2 and S2p1/2 respectively, which are corresponding to sulfate ion present in the matix.28 Recently, we have found ammonia to be a potential and specific growth directing reagent for the fabrication of two dimensional (2D) nanosheets of metal oxides or hydroxides29,30 due to its mild nature, volatility, hydrogen bond formation ability and facile metal coordination propensity. In the present case, 2D morphology could not be obtained with commonly used hydrolyzing agents like urea, NaOH etc. These reagents exhibit anomalous activities may be due to easy Zn2CO3(OH)2 formation in presence of urea24 (Figure S6) or etching nature of NaOH30,31 (Figure S7). In the present study, ammonia and that too with its optimized amount is one of the key parameters reported by us to prepare morphologically important 2D ZCS plate like materials. Here we have varied the amount of ammonia from 15 to 200 µL. It has been observed that ZCS15, ZCS30 and ZCS60 with 15, 30 and 60 µL ammonia respectively


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bear hexagonal plate like structures whereas further increase in NH3 concentration causes disruption of the 2D morphology (Figure S8). ZCS30 material prepared under room temperature or 100 °C heating has somewhat two dimensional architectures, but certainly they are way behind unique hexagonal plate like morphology (Figure S9). It is worth noting that Co or Zn free cases but with the prescribed amount of ammonia, as mentioned earlier, also evolved plate like structures (Figure S10 and S11). XRD pattern of Zn free material i.e., CS30 matches with the cobalt oxide as shown in Figure S12. Calcination of ZCS30 yields stacked nanosheets like morphology (Figure S13). In addition, we have varied the ratio of zinc to cobalt precursor concentration and studied their morphology evolution also in great detail (Figure S14-S17). Perfect hexagonal plates for ZCS30 are evolved when the ratio is maintained to Zn : Co = 2 : 4. ICP-MS results of various ZCS30 materials prepared from varying ratio of zinc and cobalt have been shown in Table S1. Critical experimental analysis reveals that in Zn4-xCoxSO4(OH)6 · 0.5H2O material with the x value varied from 1.88 to 0.44. Oxygen Evolution Reaction (OER): Water oxidation capability of ZCS30 has been investigated through a series of electrochemical tests using standard three electrode cell in 0.5 M KOH solution under stirring at 1000 rpm with a fixed mass loading of 0.20 mg cm-2 on glassy carbon electrode (GCE). Figure S18 suggests about the unchanged morphology and composition of ZCS30 after initial 20 CV cycles for activation of the catalyst. Figure S19 represents the anodic polarization graphs (not iR corrected) at a slow scan rate of 2 mV s-1 with the electrocatalyst modified GCE, which clearly indicates the standout performance from our designed ZCS30 in terms of lower overpotential and higher current density in comparison to other related materials such as ZS30, CS30, and ‘ZCS30 calcined’ employed under identical condition. Bare GCE doesn’t have any OER activity which is also quite true for only zinc based similar material i.e., ZS30 (η10= 530 mV) in comparison to ZCS30. Figure 3a represents the iR-corrected polarization profiles for ZCS30, CS30, calcined ZCS30



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and commercial RuO2 catalyst. It is worth mentioning that ZCS30 exhibits onset potential at 1.55 V (vs. reversible hydrogen electrode, RHE) and showing just 370 mV overpotential (η10) to achieve 10 mA cm-2 current density (required for 12% efficient solar to fuel conversion devices).32,33 The cobalt based material synthesized through similar procedure (i.e., CS30) has shown η10= 420 mV, while calcined product of ZCS30 is able to produce 10 mA cm-2 current density at 400 mV overpotential. It is important to note that the OER performance of ZCS30 is comparable to the state of art catalyst RuO2 (commercial), with barely 20 mV higher overpotential (η10). However, ZCS30 achieves 50 and 100 mAcm-2 current density at 420 and 450 mV overpotential whereas RuO2 requires 450 mV overpotential to attain only 50 mAcm2

current density i.e., ZCS30 exhibits 2 times higher OER activity in terms of obtained current

density than commercial material once the overpotential reaches to 450 mV, which suggests its superior performance. ZCS30 materials prepared under various reaction temperatures e.g., room temperature or under hydrothermal treatment at 100 °C clearly fall short in terms of electrocatalytic activity than the sample prepared under MHT at 160 °C. The observed overpotential values (η10) are found to be 30-40 mV higher and obtained current densities at 450 mV overpotential are ~2.3 times lower in these cases in contrast to ZCS30 (160 °C) as shown in Figure S20. ZCS prepared with varying amount of ammonia leads to morphological and compositional differences in the prepared material as described in Figure S8. OER performances with these ZCS electrocatalysts are shown in Figure S21. It has been shown that increasing ammonia volume from 15 µL to 30 µL to prepare ZCS nanomaterial leads to improved OER activity (∆η10 = 30 mV). However, further increment of added NH3 (30-200 µL) leads to lowering of OER performance (∆η10 = 45 mV) suggesting zinc cobalt based material prepared with 30 µL ammonia i.e., ZCS30 is the best suited OER catalyst among the tested materials in terms of


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lower η10 value and more than 2 times obtained current density at 450 mV overpotential (I450) during OER as depicted in Figure 3b. Co based materials especially Co3O4 nanocatalysts are investigated quite elaborately towards OER in basic medium by various groups.34-36 It is interesting to note that Co3O4 nanosheets,29 prepared from same procedure and precursors (Figure S22) exhibits lower electrocatalytic performance as the observed overpotential (η10= 433 mV) and current density at 450 mV overpotential (3.7 times lower than ZCS30) speaks very positive in favour our developed ZCS30 material (Figure S23). Mixture of ZS30 and CS30 also fails to exhibit similar activity of ZCS30 as the mixture electrocatalyst needs 30 mV higher overpotential (as η10) and provides 1.9 times lesser current density at 450 mV overpotential in comparison to ZCS30 suggesting the intrinsic property of the designed material rather than its collective activity (Figure S23). In order to scrutinize the genuine catalytically active sites of the ZCS30 material for OER performance, we have studied a series of ZCS30 based materials catalyzed OER, where the ratio of the two precursors i.e. ZnSO4 and CoSO4 has been deliberately varied (Figure S24). ZCS30 with 0.2 mmol ZnSO4 and 0.4 mmol CoSO4, designated as ZCS30 (2:4) represents the highest catalytic activity among all the tested materials and the rest of the materials follow the trend like ZCS30 (2:4) > ZCS30 (2:6) > ZCS30 (2:2) > ZCS30 (6:2) ≈ ZCS30 (4:2). This study reveals that when Co/Zn ratio is very small e.g., 0.33 and 0.5 in ZCS30 (6:2) and ZCS30 (4:2) respectively, the OER activity goes down. With increasing the ratio of Co/Zn to 1 and then to 2, the activity is found to be improved slightly in ZCS30 (2:2) and significantly for ZCS30 (2:4) materials, respectively. However, further increase in ratio leads to decrease in electrocatalytic activity in ZCS30 (2:6) in contrast to ZCS30 (2:4). From ICP-MS experiment we have checked the number of atomic cobalt loading in ZCS30 materials and the trend exactly follows with the added precursor’s ratio into the reaction medium as shown in



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Table S1. Figure 3b represents the comparative account of OER performance from these ratio dependent various ZCS30 materials which clearly claims the superiority of ZCS30 (2:4). So, the presence of Co is proved to be crucial to show OER activity but its relative amount with respect to Zn is also very much the subject of interest to design a high performance OER catalyst. It is worthwhile that the catalytic performance of non-supported ZCS30 (2:4) i.e., Zn4-xCoxSO4(OH)6· 0.5 H2O (x = 1.78) catalyst exhibiting η10 = 370 mV is superior to various highly active Co and Ni based recent reports such as Zn–Co-LDH (η10 ≈ 470 mV),17 ZnCo2O4 (η10 = 390 mV),18 ZnCo LDH/graphene (η10 = 430 mV),20 Co3O4 (η10 = 401 mV),36 Co3O4/NRGO (η10 = 420 mV),37 Co@Co3O4/NC (η10 = 410 mV),38 rGO–Co3O4 YSNCs (η10 = 410 mV),39 Co2P2O7@C (η10 = 397 mV),40 NiSx (η10 = 408 mV),41 C-Co NPs (η10 = 390 mV),42 Fe-N-C/NiO (η10 = 390 mV),43 MWCNTs@NiCo silicate hydroxide nanosheets (η10 = 440 mV),44 NixCo3-xO4 (η10 = 490-530 mV),45 CoSe2 (η10 = 484 mV),46 Co/Co2P (η10 = 390 mV),47 CoP polyhedron (η10 = 400 mV)48. A tabulated form on comparative account has been shown in Table S2. The OER kinetics by various catalysts was further evaluated from Tafel plots as demonstrated in Figure 4a.17,36-48 In our study, ZCS30 shows Tafel slope of 60 mV dec-1 which is smaller than CS30 (73 mV dec-1) and ZCS30 calcined (71 mV dec-1) materials, describing the favourable OER kinetics with ZCS30. This Tafel slope value is also comparable to that of commercial RuO2 (56 mV dec-1). The electrochemical impedance spectra (EIS) for all the above applied catalysts were recorded at 1.66 V vs. RHE for better insight into the kinetics of the OER performance (Figure 4b). The semicircle at lower frequency indicates the charge transfer resistance (Rct) in the electrode-electrolyte interface. As shown in Figure 4b, ZCS30 has the smallest Rct value of 14.5 Ω whereas CS30, RuO2, ZCS30 calcined and Co3O4 have 55, 44.2, 34.1 and 40.8 Ω respectively (Figure S25).15-21,36 It is important to note that smaller Rct value means faster interfacial electron transfer dynamics,


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which is consistent with the polarization curves. Furthermore noted that all the catalysts have shown similar solution resistance value (Rs) at higher frequency region, which has been applied for iR-correction of the polarization curves of all the catalysts during OER. In addition, we have checked the electrochemically active surface area (ECSA) of ZCS30, CS30 and the calcined product of ZCS30 (Figure S26, S27). Electrochemical double layer capacitance (Cdl) is considered to be directly proportional to the ECSA values of the materials.49 ZCS30 has 21.2 mF cm-2 double layer capacitance which is higher than that of CS30 (7.8 mF cm-2) and the calcined product (2.4 mF cm-2). This result suggests that there are much more electrochemically active sites in ZCS30 in contrast to other catalysts and that is also been reflected in the superior OER performance of ZCS30. Stability and long-term durability of the electrocatalysts are very important parameters for practical application of the designed material. We have checked the long term stability of ZCS30 material by conducting chronoamperometric (CA) test at 380 mV overpotential and it shows ~96% retention of OER activity on prolong 11 h of experiment under continuous stirring (Figure 5a). In addition, ZCS30 material shows almost unchanged linear sweep voltammogram (LSV) after 3000 CV cycles (operated in between 1.5 to 1.8 V vs. RHE), whereas RuO2 losses its activity very fast even after 1000 CV cycles as the overpotential (η10) is increased by 100 mV. It is important to note that CS30 and ZCS30 calcined product have also experienced 60 and 30 mV increase in overpotential after 1000 CV cycles (Figure S28). Thus, our designed Zn4-xCoxSO4(OH)6 · 0.5H2O material stands out to be an outstanding OER electrocatalyst with exceptionally high activity and stability in comparison to other related materials and commercial RuO2.



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Effect of Zn during OER in ZCS30: In ZCS30 cobalt centre is the most active centre for its OER activity. However, ZCS30 has almost 4 times superior OER activity than CS30 (i.e., Co3O4 as confirmed from XRD analysis) although it has lower Co amount than CS30. The obtained current densities at 450 mV overpotential are 100 and 27.2 mA cm-2 for ZCS30 and CS30 respectively. In addition η10 is favoured by 50 mV for ZCS30 over its Zn free counterpart. It is also important to note that both have similar morphology i.e., two dimensional plates like structures and also possess similar BET surface areas as shown in Figure S29. ZCS material has BET surface area of 49.6 m2 g-1 with average pore size of 2.1 nm, while CS30 material possesses 47.1 m2 g-1 surface area and 2.6 nm average pore diameter. So, the integration of active Co species in the cost-effective Zn based matrix is crucial to show such impressive OER performance by ZCS30. In general, at higher potential Co(II) transforms into Co(III) and then Co(IV) valance states which help proton coupled electron transfer (PCET) process in OER.50,51 The adsorbed O on the active CoIV surface experiences nucleophilic attack to form -OOH species which under deprotonation route produces O2.50 Zinc ions make easier the charge transfer process as confirmed from EIS results (14.5 Ω vs. 55.0 Ω) and also help to generate higher oxidation states of cobalt ions, which are responsible for water oxidation reaction.37 In short, Co is the active centre for OER and Zn acts as a promoter in ZCS30 (Scheme 1). Zn ions also accountable for high retention of OER activity over prolong electrochemical test (Figure 5b and S28). FESEM image of ZCS30 material after ~11 h of CA test reveals the aggregation of hexagonal nanoplates and in some cases they even form nanosheets (Figure S30a, b). TEM analysis suggests that in association of the nanosheets there is few smaller size particles generated on its surface (Figure S30c). HRTEM study exposes that the nanosheets are amorphous in nature whereas newly generated particles are crystalline (Figure S30d, e). Elemental mapping and EDX results in Figure S30f, g demonstrate that the


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elemental content for Zn and S decrease significantly which is also supported by XPS analysis. XPS analysis indicates the presence of all the elements as before catalysis. However, a considerable amount of Co(III) has been generated after prolong electrochemical operation and these in-situ formed oxidised Co surfaces lead to high OER performance with time (Figure S31). Moreover, probable leaching of Zn and sulfate ions leads to the creation of amorphous layers on ZCS surface to enhance the OER performance of the catalyst.13, 26, 52 It is important to note that TEM images of the tested electrocatalyst after 2 h of CA operation suggests the formation of amorphous layer however the crystalline particles (seen after 11 h of CA) are not found within this period (Figure S32a, b). So, the leaching of zinc and sulfate ions introduces certain defects in the ZCS30 matrix close to active cobalt centres which enhances OER performance as supported by lowering Tafel slope = 56.2 mV dec-1 after 2 h of CA test (Figure S32c). The crystalline particles may be coming from in-situ generated CoOOH or CoOx (d-spacing = 0.247 nm) with prolong electrochemical test. Considering the electrochemical activity of cobalt atoms we have calculated mass activity of ZCS30 material at 370 mV overpotential. ZCS30 shows remarkably higher mass activity of 146 A g-1 in contrast to commercial RuO2 catalyst (83 A g-1) at the same overpotential. Thus, cobalt and zinc in appropriate ratio synergistically help to show remarkable OER activity and stability of the ZCS30 material. CONCLUSION In summery one step synthesis of hexagonal nanoplates of Zn4-xCoxSO4(OH)6 · 0.5H2O material in gram quantity has been reported. The designed material shows 370 mV overpotential to obtain 10 mA cm-2 anodic current density as OER electrocatalyst in basic medium. The obtained OER performance stands to be superior in comparison to expensive RuO2 considering the 2 times higher obtained current density at 450 mV overpotential and long term stability. Optimum ratio of the active Co(II) and supporting Zn(II) ions content



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happens to be the leading factor to exhibit such a high OER activity and stability. The highly recyclable material could be a good alternative to the expensive electrocatalysts for the application in fuel cell and metal-air batteries.

Acknowledgements: The authors are thankful to DST, India, and CSIR, New Delhi, for financial assistance, IIT Kharagpur for instrumental support. The authors are also thankful to Mr. Takumi Terui of Tokyo University of Science, Tokyo, Japan, for ICP-MS analysis. Supporting Information: Detailed instrumentation; FESEM, EDX, elemental mapping, XRD, AFM, STEM, ICP-MS result of various synthesize material; LSV, EIS of various controlled material; comparative table on OER performance; BET results of ZCS30 and CS30; XPS, FESEM, TEM analysis of spent ZCS30 material. This material is available free of charge via the Internet at http://pubs.acs.org.


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Tables and Figures

Table 1: ZCS materials prepared with varying ratio of precursors’ amount.

Material

a, b

25% NH3 amount (µL)

ZnSO4·7H2O

CoSO4·7H2O

(mmol)

(mmol)

ZCS30a

30

0.2

0.4

ZCS15

15

0.2

0.4

ZCS60

60

0.2

0.4

ZCS100

100

0.2

0.4

ZCS200

200

0.2

0.4

CS30

30

0

0.4

ZS30

30

0.2

0

ZCS30 (2:4)b

30

0.2

0.4

ZCS30 (2:2)

30

0.2

0.2

ZCS30 (2:6)

30

0.2

0.6

ZCS30 (4:2)

30

0.4

0.2

ZCS30 (6:2)

30

0.6

0.2

Both are same.



a

c

b

Figure 1. (a) FESEM (inset, at higher magnification), (b) TEM and (c) HRTEM image of ZCS30 material.

Zn 2p

a

b

Intensity (a.u.)

75000 O 1s Co 2p

50000

25000 C 1s

0

1000

750

500

S 2p

250

Binding Energy (eV) 4000

35000 Zn 2p

25000

Zn 2p1/2

20000 15000

Co 2p3/2

d

16000

Intensity (a.u.)

c

30000

3/2

Intensity (a.u.)

Intensity (a.u.)

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

Page 24 of 27

Co 2p1/2 12000

8000 1050

1035

Binding Energy (eV)

1020

810

800

790

780

770

e

3500 S 2p3/2 3000

S 2p1/2

2500

2000 164

Binding Energy (eV)

166

168

170

172

174

Binding Energy (eV)

Figure 2. (a) XRD pattern and (b) XPS survey of ZCS30, and narrow range XPS spectra for (c) Zn 2p, (d) Co 2p and (e) S 2p in ZCS30.


Overpotential (η10)/mV

RuO

10

ZCS30

75

b

430

a

CS30 100

2

Calcined ZCS

50 25

100

420

80 410

60

400 390

40 380

Potential/v vs RHE

(6 S3 :2) 0 (4 ZC S3 :2) 0 (2 ZC :2 S3 ) 0 (2 ZC S3 :4) 0 (2 :6 )

1.7

S3 0

1.6

ZC

1.5

ZC

1.4

ZC S1 5 ZC S3 0 ZC S6 0 ZC S1 00 ZC S2 00

1.3

Various catalysts

Figure 3. (a) Polarization curves after iR-correction of various samples for OER in 0.5 M KOH solution at 2 mV s-1 sweep rate under 1000 rpm stirring condition and (b) activity comparison among various ammonia and metal ratio dependent ZCS products (η10 stands for overpotential to achieve 10 mA cm-2 current density and I450 represents the obtained current density at 450 mV overpotential).

30

0.45 ZCS30

0.35

RuO2 ZCS30 calcined

73

e /d mV

0.30 56

-0.4

71

c

ec /d mV 60

/d mV

0.0

b

RuO2

25

ZCS30 calcined

-Z'' (ohm)

0.40

ZCS30

a

CS30

ec /d mV

ec

CS30

20 15 10 5

0.4

0.8 -2

log (j/mAcm )

1.2

0

0

10

20

30

40

50

60

Z' (ohm)

Figure 4. (a) Tafel plots and (b) EIS Nyquist plots at 430 mV overpotential for various electrocatalysts applied for OER.


70

-2

20

370

0

Overpotential (V)

-2

125

Current Density (I450)/mA cm

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


Current Density (mA cm )

Page 25 of 27

a

50 12.8 mA cm

-2

-2

12.3 mA cm

0

-50

-100

0

2

4

6

Time (h)

8

10

-2

120

b 80 1

st

Cycle

3000

th

Cycle

40

0 1.3

Page 26 of 27


-2

100


-2

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



1.4

1.5

1.6

1.7

1.8

Potential/V vs RHE

70 60 50 40

c 1

st

cycle

1000

th

cycle

30 20 10 0 1.3

1.4

1.5

1.6

1.7

Potential/V vs RHE

Figure 5. (a) Time dependence of anodic current density for ZCS30 at a fixed overpotential of 380 mV (b) iR-corrected polarization curves of ZCS30 before and after 3000 CV cycles, (c) iR-corrected polarization curves of RuO2 before and after 1000 CV cycles.

Scheme 1. Schematic representation of OER in ZCS30 surface; Zn and Co species are indicated as grey and blue spheres, respectively.


1.8

Page 27 of 27

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Table of Content (TOC)


Co Hydroxy

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