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Copper Cobalt Sulphide Nanosheets Realizing Promising Electrocatalytic Oxygen Evolution Reaction Meenakshi Chauhan, Kasala Prabhakar Reddy, Chinnakonda S. Gopinath, and Sasanka Deka ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b01831 • Publication Date (Web): 20 Jul 2017 Downloaded from http://pubs.acs.org on July 20, 2017

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Copper Cobalt Sulphide Nanosheets Realizing Promising Electrocatalytic Oxygen Evolution Reaction Meenakshi Chauhan,† Kasala Prabhakar Reddy,‡ Chinnakonda S. Gopinath,‡ Sasanka Deka*,† Department of Chemistry, University of Delhi, North campus, Delhi-110007 Catalysis Division and Center of Excellence on Surface Science, CSIR – National Chemical Laboratory, Pune-411 008, India Abstract: Nanostructured CuCo2S4, a mixed metal thiospinel is found be a benchmark electrocatalyst for oxygen evolution reaction (OER) in this study with low overpotential, low Tafel slope, high durability and high turnover frequency (TOF) at lower mass loading. Nanosheets of CuCo2S4 are realized from a hydrothermal synthesis method where the average thickness of the sheets is found in the range 8−15 nm. Aggregated nanosheets form a highly open hierarchical structure. When used as electrocatalyst, CuCo2S4 nanosheets offer an overpotential value of 310 mV at 10 mA cm-2 current density, which remains consistent for measured 10000 cycles in 1M KOH electrolyte. Chronoamperometric study reveals constant oxygen evolution throughout for 12 h at 10 mVs-1 scan rate without any degradation of the activity. Furthermore, calculated mass activity of CuCo2S4 electrocatalyst is found to be 14.29 A/g and it afford a TOF value of 0.1431 s-1 at 310 mV at a mass loading 0.07 mg cm−2. For comparison, nanostructures of Co3S4 and Cu0.5Co2.5S4 have been synthesized using the similar method followed for CuCo2S4. When compared the OER activities among these three thiospinels and standard IrO2, CuCo2S4 nanosheets offered efficiently highest OER activities at the same mass loading (0.7mg/cm2). Extensive XPS, EPR analyses for mechanism study reveal that, introduction of Cu in Co3S4 lattice enhances the oxygen evolution and kinetics by offering Cu2+ sites for utilitarian adsorption of OH, O and OOH reactive species and also by offering highly active high spin state of octahedral Co3+ OER catalysis. Keywords: Nanosheet, CuCo2S4, electrocatalyst, oxygen evolution, water splitting.

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INTRODUCTION The exploration of sustainable and highly efficient alternative energy to substitute exhaustible fossil fuels has been the major trend globally from last decade. In this scenario, the electrochemical and photocatalytic water splitting reactions have attracted great research attention in recent years because of their vital role in various energy conversion and storage technologies, such as hydrogen production, oxygen production, regenerative fuel cells, etc.1-5 In these processes, the kinetic bottleneck of water splitting is the oxygen evolution reaction (OER). In the electrochemical water splitting experiments, OER is one of the most important and technologically challenging reaction considering four electron transfer process and slow kinetics. The efficiency of OER is therefore limited by the requirement of large over potential and the cell voltage is governed by that of the anodic reaction.6,7 In OER the performance and efficiency of electrochemical water splitting can be drastically improved by trimming down cathodic overpotentials with the use of suitable and effective electro-catalysts. Although RuO2 and IrO2 are highly active OER catalyst exhibiting the lowest OER overpotentials, but cost limits the applicability of precious and expensive materials at large scale.8 Instead, metal oxides, such as Co3O4;9 MnO2, PtO2, NiCo2O4, ZnxCo3-xO4;10, and metal chalcogenides and phosphide such as CoP,11 CoS, NiCo2S4;12 Ni3S2;13 Cu2ZnSnS4,14 etc. have been greatly introduced in recent years as electrocatalyst for OER with excellent results. However, still improvement in the material part is required to replace those precious-metal based electrocatalyst to work in harsh conditions and for large scale production. Common pitfalls of these reported electrocatalysts are lesser number of active sites, poor electrical transport, inefficient electrical contact to the electrolyte and instability under operating conditions due to which either chemical exfoliation or functionalization by compounds having high surface area is needed.15 It is known that, in general the octahedral sites of spinels are catalytically active, however the tetrahedral sites are almost inactive.16,17 Knozinger et al. reported that {111} plane of spinel solely exposes octahedral coordinated cations, suggesting that these facets should have the maximum reactivity compared to other crystal facets.18 Therefore, constructing a spinel nanostructure with maximum exposed {111} octahedral planes (surface) is highly recommended for our catalytic activity, i.e. OER catalysis. Youwen et al. suggested that high-spin states of Co3+ in octahedral sites also significantly enhance the OER performance of spinel catalysts.17 2 ACS Paragon Plus Environment

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Keeping these facts in mind, we chose CuCo2S4 as our study material which is paramagnetic and metallic in bulk state. CuCo2S4, a normal thiospinel having the ideal formula AB2S4, emerged as an interesting material in few new studies. In the spinel structure trivalent Co3+ ions occupies octahedral B sites and divalent Cu2+ ion occupy tetrahedral A site. In the case of CuCo2S4, there will be very less chance that catalytically active high-spin state Co3+ ions will populate the inert tetrahedral sites and consequently may reduce the overall activity. In the synthesis of CuCo2S4 nanostructures, Tang et al. have recently used hydrothermal method to synthesize CuCo2S4 nanoparticles and reported superior supercapacitance properties.19 Schaak’s group has followed solution synthesis method to prepare colloidal CuCo2S4 nanoparticles and used them as electrocatalyst.20 These are few recent reports on exploration of CuCo2S4 nanoparticles, however we do not find any other report in literature on the achievement of efficient OER activity using CuCo2S4 nanostructures. In this letter, we have modified an earlier synthesis method by introducing ethylenediamine as surfactant to prepare twodimensional (2D) CuCo2S4 nanosheets which are exposed mainly with (111) facets as evidenced from HRTEM studies (for experimental details, see Supporting Information, SI). These transparent nanosheets with 8-15 nm in thickness and are interconnected with each other, forming a highly open 3D hierarchical structure. This CuCo2S4 nanostructures showed benchmark OER activities over most of the reports related to spinels, thiospinels and are comparable to precious material (e.g. IrO2) catalysts activity, without any exfoliation or surface modification. At an electrocatalytic current density of 10 mA cm−2 the observed overpotential was as low as 310 mV which was consistent for 12 h. Moreover, in a 10000 cycles OER study, no deviation in current density as well as overpotential was seen in the current CuCo2S4 catalyst. For comparison we have also synthesized Co3S4 and Cu0.5Co2.5S4 nanosheets at the same conditions and carried out similar OER studies. In all studies it was found that CuCo2S4 is much better OER catalyst than Co3S4, Cu0.5Co2.5S4 and IrO2.

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Figure 1. (a) Powder XRD patterns of as-synthesized CuCo2S4, Co3S4 and Cu0.5Co2.5S4 compared with bulk XRD pattern of CuCo2S4. (b) Low magnification TEM image of a group of CuCo2S4 nanosheets showing the transparent nature of the as-synthesized nanosheets. (c) HRTEM image of a CuCo2S4 nanosheet laying flat on TEM grid (inset: calculated FFT pattern). (d) HRTEM image of a nanosheet laying perpendicular showing the thickness and few clear lattice fringes. (e) FESEM where the interconnected nanosheets formed a highly open 3D hierarchical structure (inset: an enlarged hierarchical particle). (f) TEM-EDS spectrum of CuCo2S4 nanosheets.

RESULTS AND DISCUSSION The powder XRD patterns of as-synthesized CuCo2S4, Co3S4 and Cu0.5Co2.5S4 are compared in Figure 1a to characterize the crystalline components and phase purity. The diffraction patterns are consistent with bulk Co3S4 and literature value,19,20 and the lattice parameter ‘a’ of CuCo2S4 (9.454 Å) and Cu0.5Co2.5S4 (9.451 Å) is calculated to be almost equal to Co3S4 (9.449 Å) since Cu2+ (IV coordination, tetrahedral site)21 and Co2+ (IV coordination, tetrahedral site)21 ions are almost equal in effective ionic radii. Presence of Cu in as-synthesized CuCo2S4 is determined from TEM-EDS (Figure 1f) where the elemental ratio is found to near to 1:2:4. No other crystalline phases are found in the XRD patterns revealing the pure phase of CuCo2S4, Cu0.5Co2.5S4 and Co3S4. Under the TEM imaging 2D nanosheets like morphology is seen, which are transparent (Figure 1b). However these nanosheets are not separate, are 4 ACS Paragon Plus Environment

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apparently agglomerated and few nanosheets are found laying flat on the TEM grid. When studied under HRTEM, we see the flat surface of the nanosheets are predominantly exposed with {111} planes (Figure 1c, d = 0.54 nm). All lattice fringes are corresponding to (111) facets, and this observation is actually against the powder XRD observation (Figure 1a). In XRD patterns the (111) peak is very weak, whereas (311) and (440) peaks are nearly 100% intense. Next we found few nanosheets laying perpendicular to TEM grid or parallel to the electron beam (Figure 1d). Under HRTEM studies the lattice fringes of these lateral side is found to be of solely {311} and {440} planes, where d = 0.285 nm and 0.167 nm, respectively. The average thickness of these nanosheets is found to be ~12 nm. The calculated FFT images are shown as insets in Figure 1c and 1d, which also reflect the {111}, {311} and {440} planes. The observed conflicting XRD and TEM results are now explained from FESEM analyses. As seen in Figure 1e, the transparent nanosheets are interconnected, first formed thick sheets and then formed a highly open 3D hierarchical structure (more images are given in Figure S1). Majority of the sheets are exposed with lateral sides, which are composed of {311} and {440} planes as corroborated from panel ‘d’. This is the reason for observation of very intense (311) and (440) peaks in XRD patterns than (111) peak. Low magnification TEM images and FESEM images of as-synthesized Cu0.5Co2.5S4 and Co3S4 are given in Figure S2 (see SI). In both the cases particles manifest identical morphology as CuCo2S4. The HAADF-STEM is a very powerful technique in the characterization of multicomponent materials with special topology. The HAADF-STEM and elemental mapping of the as-synthesized CuCo2S4 nanosheets were carried out (Figure S3). It is found that the distribution of Cu, Co and S over entire region is uniform corroborating a homogeneous phase of CuCo2S4.

Figure 2. (a) AFM image of CuCo2S4 sample showing nanosheet tomography. (b) Line scanning of a very diluted sample to measure the thickness of the sheets. 5 ACS Paragon Plus Environment

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We have further tried to measure the thickness of our as-synthesized CuCo2S4 nanosheets with the help of atomic force microscopy. However, at first instance, we got bigger, dense clumps of sheets in aggregated form (Figure 2a). To avoid this interference we further diluted the sample solution to get particles in singular form and performed line scan mode of AFM. The result is shown in Figure 2b, where the average height of single-single particles is found to be in the range 8-15 nm (see Figure S4 for line scanning in a different region and additional AFM images). AFM result corroborated with HRTEM result to give average thickness of nanosheets in the similar range. Few controlled reactions were carried out to ascertain the synthesis of CuCo2S4 nanosheets and to get optimised synthesis conditions. We have varied reaction temperature and time, and found that reaction at 200 oC for 12 h is the best condition to get these nanostructures. If the reaction time was reduced to 8 h, the as-obtained agglomerated particles were found to be contaminated with Co9S8 and CoO impurity (see Figure S5, S6 in SI for XRD and TEM). If the reaction time is increased to 16 h, there is no change in physical appearance or properties of this sample as compared to 12 h sample. Further the reaction temperature was varied keeping the other parameters intact. When the reaction temperature was reduced to 180 oC, not only CuCo2S4 formed, but there are many other derivatives of Co-S formed. However, if the reaction temperature was increased to 220 oC, over oxidation takes place. We detected few peaks from Co3O4 and CoO along with CuCo2S4. Notwithstanding, it is important to mention that, CuCo2S4 sample prepared at 200 oC for 12 h showed the best OER activity than samples prepared at varied reactions conditions (discussed in later section).

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Figure 3. (a) Wide scan XPS survey spectra of as-synthesized CuCo2S4, Cu0.5Co2.5S4, Co3S4 nanosheets and CuCo2S4 sample after 10000 OER cycles. Comparison of high resolution XPS spectra of all fresh CuCo2S4, Cu0.5Co2.5S4, Co3S4 nanosheets samples: (b) Co 2p, (c) Cu 2p and (d) S 2p. (e) Temperature dependent EPR spectra of CuCo2S4 and Co3S4 nanosheets (only Co3S4 intensity is multiplied by 10 for clarity). (f) Raman spectrum of CuCo2S4 nanosheets measured at 25 oC.

The wide scan XPS survey spectra of CuCo2S4, Cu0.5Co2.5S4 and Co3S4 showed the presence of Co, Cu (except Co3S4), S, C, N and O (Figure 3a). In high resolution XPS of CuCo2S4, the core level of the Co 2p spectrum was deconvoluted in to two spin–orbit doublets and two shakeup satellites (Figure 3b). Those two prominent peaks at binding energies of 779.1 and 794.1 eV and the second at 781.7 and 796.7 eV corresponds to the Co 2p3/2 and Co 2p1/2 spin–orbit peaks, respectively, which confirmed the existence of two kinds of cobalt oxidation state: Co3+ and Co2+.19,22,23 Similar kinds of surface behavior have been observed in Cu0.5Co2.5S4 and Co3S4 samples too (Figure 3b). High intensity observed for Co3+ underscores the availability of larger amount of the same on the surface and suggests more amount of octahedral Co3+ on the surface. However, these results suggested a possibility of formation of minute amount of Co9S8 phase which is beyond the XRD detection limit. The strong feature of 781.7 and 796.7 eV doublets indicate a strong Co-S/Co-O interaction. Few interesting features are seen in the high 7 ACS Paragon Plus Environment

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resolution deconvoluted XPS spectrum of Cu in CuCo2S4 and Cu0.5Co2.5S4 samples (Figure 3c). The first doublet at 932.9 and 935.4 eV and the second at 953.0 and 955.7 eV are attributed to Cu 2p3/2 and Cu 2p1/2 core levels, respectively. Very strong shake-up satellite peaks are observed at 944.4 and 964.0 eV, along with corresponding main line peaks at 935.4 and 955.7 eV, respectively; satellite feature attests the presence of Cu2+ oxidation state.24 These satellite peaks suggested a strong Cu-S interaction as well as significant ligand-metal charge transfer. Moreover, the second doublet, which is 935.4 and 955.7 eV is found to be more prominent and intense than the first one in the case of CuCo2S4. This again indicates strong interaction of copper ions with either S (from lattice) or O (adsorbed from solvent or from air atmosphere) when the amount of copper is more in CuCo2S4 than Cu0.5Co2.5S4. The binding energy of the S 2p peaks is 161.9 eV (2p3/2) and 163.5 eV (2p1/2), indicating that the S species exist as S2- (Figure 3d).19,25,26 Another peak at 168.1 eV suggested a strong interaction of adsorbed oxygen with S forming sulfate or sulfite-like species. From the low intensity of the above peak, it is clear that there is some oxidation on the surface, but limited exclusively to surface. Based on these XPS analyses we see that our nanosheet samples have some hydroxide and/or sulfate impurities at surface which were originated from adsorbed species, such as water. However, these impurities are beyond the detection limit of XRD. An additional XPS measurement has been carried out after in-situ surface cleaning of the CuCo2S4 sample by means of Ar+-sputtering and the resulting spectra were compared to that of fresh or unsputtered sample (Figure S7). The original peak at 935.4 eV which is due to strong Cu-O/Cu-S interaction is eliminated in the sputtered sample and also the peak at 955.7 eV is shifted to lower energy 953.7 eV (Figure S7a). Similar kinds of shifting of peaks to lower binding energy are observed in the case of cobalt too in the sputtered sample (Figure S7b). Thus the more prominent Cu-S and Cu-O peaks observed in the case of CuCo2S4 than Cu0.5Co2.5S4 might be due to more adsorption of atmospheric component in the former sample. Present in-situ sputter-cleaning method demonstrates the impurities are restricted to the surface, and probably it acts as a protective layer from further degradation. XPS studies revealed the presence of octahedral Co3+ at the surface of the nanosheets. Now to reveal the spin states of those cobalt ions, temperature dependent electron paramagnetic resonance (EPR) measurements have been carried out (Figure 3e). Because earlier it was revealed that spin states of active atoms is substantial factors in enhancing catalytic activity and EPR is a powerful technique to detect the 3d electron configuration (both eg and t2g 8 ACS Paragon Plus Environment

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electrons).17,27,28 Both the room (300 K) and low (100 K) temperature measurements exhibited an asymmetric broad resonance line in the EPR spectra for CuCo2S4 sample. There is a shifting of the spectral weight to lower magnetic fields at low temperatures (black curve). Moreover, the uncoupled Co2+ signal is absent at 3500 G in the case of CuCo2S4, suggests the Co2+ ferromagnetic coupled with surrounding other Co ions. All these observations infer the existence of ferromagnetic coupling in CuCo2S4 nanosheets as expected, which concludes that all of octahedral Co ions present with a high-spin state (t2g4eg2).17 Since the Co ions at octahedral sites are Co3+ and XPS revealed their presence at surfaces, hence all surface Co3+ are highly active high spin configuration, which should be beneficial for OER activity. However, in the case of Co3S4 nanosheets (both 300 and 100 K), we see weak signal which is not purely high spin, as we see low spin Co2+ signal (at ~3500 G) along with Co3+. Thus, introduction of Cu2+ in the Co3S4 lattice increases Co3+ high spin states which should make CuCo2S4 more beneficial as OER catalyst.17 In order to study the surface properties of the as-prepared CuCo2S4 sample, a detailed N2 adsorption–desorption measurement was carried out at a temperature of 77 K (Figure S8). It is clearly seen that the behavior of the isotherm is a combination of type II and type V, and the hysteresis is H3 type. By using the multipoint BET equation, the specific surface area of the assynthesized CuCo2S4 nanosheet sample was found to be 37.6 m2/g. This observed specific surface area is found to be relatively low owing to agglomerative nature of the nanosheets. We further performed Raman spectroscopic measurement on the thiospinel sample and the spectrum is shown in Figure 3f. Typically there used to be five allowed Raman transitions in normal cubic spinel, where one is Eg mode, another one is A1g mode and remaining three modes of vibrations are from F2g. In the present case, slight broadening of the observed peaks may be due to the creation of vacancies on catalyst surface, and some structural modification during experimental conditions. Figure 3f represented Raman spectrum of CuCo2S4, and the peaks observed at 458.5, 502.5, 599.1 and 653.8 cm-1 because of those different types of vibration modes. According to the literature the peaks at 458.5, 502.5 and 599.1 cm-1 are attributed to vibrational modes of Co-S bond and the one at 653.8 cm-1 is due S-S stretching vibration.29 There is no peak observed either at 422 and 467 cm-1 for Cu-S or at >680 cm-1 indicating absence of separate CoSx species.30 Hence, Raman studies give further evidences of pure phase of CuCo2S4 nanosheets.

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Figure 4. Comparison of electrochemical performances of CuCo2S4, Cu0.5Co2.5S4, Co3S4 nanosheets catalysts and standard IrO2 sample measured in 1.0 M KOH electrolyte at 25 oC. (a) OER polarization curves (iR-corrected). Scan rate was 10 mV s−1. (b) Tafel plots of all four samples (overpotential vs. log of the current density). (c) Nyquist plots of CuCo2S4, Cu0.5Co2.5S4 and Co3S4 electrodes (Z’ is the real and Z’’ is the imaginary impedance). Inset: Enlarged EIS activity of CuCo2S4 electrode. (d) Current density vs. scan rate plot of the nanosheets samples. To study OER activities electrochemical analyses were performed under a three electrode system constructed by depositing study materials on glassy carbon electrode (GC) to use as working electrode, platinum wire as counter electrode and Ag/AgCl (aq.) as reference electrode in 1M KOH electrolyte solution. To compare with standard and for referencing, similar measurements were conducted with IrO2 catalyst. Before initiating experiments O2 was bubbled in the electrolyte solution for 30 min and the internal resistance was corrected. OER measurements were performed by running polarization curves of nanosheets of Co3S4, CuCo2S4, Cu0.5Co2.5S4 and IrO2 with an applied potential window of 0.2−0.8 V vs. Ag/AgCl at a scan rate of 10 mVs-1 . Our as-synthesized mixed metal thiospinel (CuCo2S4) showed benchmark activity and stability to achieve the essential Faradaic efficiency for oxygen evolution reactions. 10 ACS Paragon Plus Environment

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In case of all samples except bare GC electrode a small hump at ~1.3 V is seen in LSV curves, which can be attributed to earlier onset potential (Figure 4a). However, in the case of CuCo2S4 major oxygen evolution starts at 1.43 V (RHE) and this potential increases for other samples. The polarization curves for CuCo2S4 exhibit immense activity with very less onset potential (0.2 V) and significantly low overpotential for oxygen evolution reaction. In stern comparison as-synthesized Co3S4 nanoparticles have very high onset potential for water oxidation reactions (0.43V). CuCo2S4 as a mix metal spinel species impart a current density of 10 mAcm-2 at an overpotential of 0.31V at the mass loading 0.7 mg cm−2 which is better than the IrO2 where the observed overpotential is 0.32 V at the same current density. Present CuCo2S4 electrocatalyst shows better activity than many previously reported Co containing oxides, sulphides and many other Co-Ni compounds.30-35 For the kinetics studies of the OER reaction, Tafel-slope was obtained by fitting the polarization curve into Tafel equation (ƞ = blogj + c, where j is the current density, b is the slope, ƞ is overpotential, c is constant) (Figure 4b). Tafelslope is an important kinetic parameter to reveal changes in the apparent OER mechanism. Kinetics of OER is inversely depends on the value of Tafel slope ‘b’, lower the value of b faster is the OER kinetics.36 As synthesized CuCo2S4 has a lower value of Tafel slope of 86 mV/decade in comparison to Cu0.5Co2.5S4 (98 mV/decade) and Co3S4 (144 mV/decade), however slightly higher than IrO2 (76 mV/decade), indicating that CuCo2S4 could be a better choice of low cost electrocatalyst for OER (faster kinetics). Further electrical impedance spectroscopy (EIS) measurements have also been performed at 0.1 Hz, which also suggested significant electrochemical activity of CuCo2S4. Nyquist plot are depicted in Figure 4c for Co3S4, CuCo2S4 and Cu0.5Co2.5S4 to find out resistivity and conductivity of these catalysts. The semicircular portion of Nyquist plot gave the value of charge transfer resistance (Rct). A lower Rct value is responsible for quick charge transfer process across the electrolyte solution and electrocatalyst and accelerates the kinetics of the reaction. The Rct values for CuCo2S4, Cu0.5Co2.5S4 and Co3S4 have been found to be 8.3 Ω, 13.02 Ω and 16.5 Ω respectively. CuCo2S4 had a lower Rct value which corresponds to faster rate of reaction than the rest. Linear part of Nyquist plot shows conductivity of catalyst come as a steep line which reveals that after crossing particular resistance point conductivity goes higher, which could be the reason for the observed early onset potential at ~1.3 V. Furthermore to estimate the reflection of intrinsic activity of catalyst and effect of charge transfer resistance during the reaction we 11 ACS Paragon Plus Environment

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have calculated exchange current density (iex) (see SI for calculations). The calculated iex value of CuCo2S4 11.04 mA cm-2 gave the evidence of lower resistance across the electrode interface and rapid electron transfer during the water splitting reaction. Thus the electrode kinetics of the CuCo2S4/GC in the OER from electrochemical impedance spectroscopy measurements revealed that the as-synthesized nanosheets exhibited more facile electrode kinetics, which is a very useful feature in enhancing the catalytic activity. Applied scan rate is very important factor in electrochemistry and the output current density varied accordingly with respect to the working materials properties. Interestingly, current density did not varied accountably with the applied scan rate in the present case as shown in Figure 4d, implying the stability. In the present case, figure 4d simply represent the variations in the values of limiting faradic current (catalytic current) with respect to the rate of charge transfer on electrode surface which accelerated with increase scan rate . The benefit of introduction of Cu into cobalt sulfide lattice could be seen from this plot, as the observed current density is the least for Co3S4 in all studied scan rates.

Figure 5. Performance based electrocatalytic OER studies on CuCo2S4 electrode. (a) LSV curves of a single CuCo2S4 working electrode after 1st cycle, after 5000 and 10000 cycles. (b) Chronoamperometric curves obtained in constant current (j = 10 mA cm−2) bulk water 12 ACS Paragon Plus Environment

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electrolysis with CuCo2S4 coated GC electrode in 1 M KOH solution at η = 0.31V. (c) Upper panel: mass activity vs. overpotential and lower panel: TOF vs. overpotential. (d) Mass activity of CuCo2S4 vs. mass loading. Since the OER activity of CuCo2S4 was maximum among all four studied catalysts, further studies have been carried out on this material. CuCo2S4 electrode was found to be extremely stable over repeated catalytic runs. As seen in Figure 5a, polarization curves of initial cycle, after 5000 cycles and after 10000 cycles measured at the same condition are coincide and overlapped without any deviation, which is the novelty of this material. Furthermore, chronoamperometric measurement has been carried out at a scan rate 10 mV/s and at the constant overpotential voltage (η = 0.31V). In this case the observed anodic current density of CuCo2S4 working electrode is found to be consistent over 12 h of studied period (Figure 5b), representing the enduringness of the material for longer runs. Thus, notably CuCo2S4 has long lasting durability and reactivity as compare to Co3S4 as homogenous incorporation of Cu increases the activity of the catalyst. In addition to determine catalytic activities related to oxygen evolution, turn over frequency (TOF), mass activity and specific activity were calculated at fixed overpotential values (see SI for calculation details). The calculated mass activity of CuCo2S4 was found to be 14.29 A g-1, specific activity of catalyst was 0.038 mA cm-2, and it afforded a TOF value of 0.1431 s-1 at ƞ = 0.31V. All these values indicate the excellent reactivity of CuCo2S4 towards water oxidation reactions with durability of 10,000 LSV cycles without any considerable change in over potential. The mass activity of all samples (except Co3S4) increases in parabolic nature with the increasing overpotential value (Figure 5c, upper panel). However, mass activity of CuCo2S4 was found to be maximum among all thiospinel samples and slightly more than IrO2 catalyst. This indicated for same amount of mass loading CuCo2S4 is the best choice of low cost material for enhanced OER activity as compared to other reports too.31-35 On the other hand TOF for CuCo2S4 increases enormously when the overpotential crossed the threshold value 310 mV (Figure 5c, lower panel). The currently observed TOF value of 0.1431 s-1 at ƞ = 310 mV is one of the highest TOF value so far among the best results reported till now on similar materials.17,37 To find out the best mass loading for effective OER, few more experiments have been carried out considering a series of different mass loading (at constant potential) and the results of mass activity against mass 13 ACS Paragon Plus Environment

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loading are shown in Figure 5d. Interestingly, mass activity initially increases with mass loading and reaches maximum at 0.7 mg/cm2 mass loading. This suggests that, 0.7 mg/cm2 is the optimum loading for maximum OER activity, best conductivity and TOF, equivalent to formation of a layer of catalyst material and full coverage. Further increase in mass loading results in decreasing mass activity. Similar kinds of observation of increase in number of active sites, catalyst’s TOF, decrease in resistivity with decreasing mass loading (decreasing thickness of catalysts) and benefit of monolayer were reported in few previous studies.38,39 Increasing the mass loading simply thicken the catalyst layer and hinder electron movement and resulted in reduced mass activity or indirectly reduced OER activity. Notwithstanding to note, the experimental amount of nanosheets required for full coverage and to form a layer is found to be 0.049 mg (calculations in SI), whereas the theoretical value is found to be 0.026 mg. The higher amount in experimental value is due to the presence of ethylenediamine surfactant along with Nafion and isopropanol, which are not considered in the theoretical calculation.

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Figure 6. Post-characterization results of CuCo2S4 nanosheets sample after 10000 cycles. (a) Powder XRD pattern compared with as-synthesized and bulk patterns as indicated. (b-d) High resolution XPS spectra of Co 2p, Cu 2p and S 2p, respectively. (e) TEM image and (f) SEM image. Post-characterization of the OER working electrode material is very important to know the phase decomposition, morphology, electronic states and surface oxidation states after rigorous OER cycles in alkaline medium. In the present study, CuCo2S4 nanosheets sample was characterized after 10000 OER cycles and the results are depicted in Figure 6. Few peaks of Co9S8 and CoS are seen in the pXRD pattern after catalyst, implying minor phase decomposition of original CuCo2S4, however major phase remained as CuCo2S4 (Figure 6a). In order to understand why the present CuCo2S4 nanosheets showed extensive OER activity, a comparative study has been done by comparing the XPS results of the as-synthesized samples and results of post-characterization. There is no deviation in the XPS peak positions after 10000 LSV cycles (Figure 6b-d) when compared with XPS spectra of as-synthesized sample in Figure 3b-d. In fact, large intensity observed for Co3+ and Cu2+ reiterates their dominant presence on the surface of CuCo2S4. Very likely, this might be the reason for OER activity observed for 10000 cycles. Although some surface changes and degradation occurs, it hardly affects the OER activity over a large number of cycles. The observed more prominent Cu-S and Cu-O peaks are due to surface impurities in the fresh sample (Figure 3b-d) are still present in the sample even after 10000 cycles (Figure 6b-d), fully indicating the impurities hardly impact the OER activity, and they may be considered as mere spectators. The effect of introduction of Cu in Co3S4 lattice on the OER output is studied using FTIR spectroscopy too. Few pseudo in-situ FTIR measurements on Co3S4 and CuCo2S4 working electrodes were performed (Figure S9) after 500 LSV cycles. Many stretching and bending frequencies related to –OH are seen in the CuCo2S4 electrode (for instance, at 1423, 1361 cm-1 for -OH medium bending, at 3001 and 2914 cm-1 for strong OH stretching), which are either absent or weak in the case of Co3S4 electrode. This data depicts that the introduction of Cu in Co3S4 lattice can enhance adsorption of at least -OH species. Recent studies have revealed that if we can intensify the conductivity of catalysts we can get better OER activity. For this purpose, people grew their catalysts on conductive supports.13,40 Very recently, Zou and co-workers predicted from DFT calculations that, introduction of a metal into a metal sulphide lattice can modify it’s Gibb’s fee energy to enhance conductivity of catalyst.41 Hence, 15 ACS Paragon Plus Environment

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this hypothesis is found to be correct in the present study, as conductivity as well as OER activities increased with increasing Cu doping as evidenced. Furthermore, TEM and SEM imaging presented that the nanosheets morphology of CuCo2S4 particles retained after 10000 LSV cycles, demonstrating the overall stability of the material. CONCLUSIONS In summary, we have demonstrated an efficient hydrothermal approach for the creation of nanosheets of CuCo2S4. In the developed approach ethylenediamine catalyzed the formation of 2D CuCo2S4 nanosheets which are exposed mainly with (111) facets. These transparent nanosheets with 8-15 nm in thickness and are interconnected with each other, formed a highly open 3D hierarchical structure. When applied as the electrocatalyst of OER in alkaline medium, 2D CuCo2S4 nanosheets exhibit very high catalytic activity with the current density of 10 mA cm−2 at overpotential of 310 mV and Tafel slope of 86 mV/decade. These values were consistent for 12 h of OER run. Moreover, in a 10000 cycles OER study, no deviation in current density as well as overpotential was seen in our CuCo2S4 catalyst. The observed mass activity, specific activity and TOF values are very impressive at very low mass loading. The overall OER activities of 2D CuCo2S4 nanosheets are found to be excellent than other related thiospinels and oxides as noted earlier. This is found to be due to the presence of high spin Co3+ at particle surface as well as the replacement of Co2+ with Cu2+ in the crystal lattice as evidenced from XPS and EPR. EXPERIMENTAL SECTION: Materials: Cobalt(II) nitrate hexahydrate (Co(NO3)2.6H2O, 99.5%) was acquired from Thomas Baker India, copper(II) nitrate trihydrate (Cu(NO3)2.3H2O, 99.0%) and thiourea (99%) were purchased from Merk, India. Ethylenediamine (99%) from SRL, India and absolute ethanol and isopropanol (99%) were purchased from Ficsher scientific, India. KOH from Rankem-India and Nafion (5 wt%) and IrO2 (99.9%) were obtained from Sigma Aldrich, USA. All the chemicals were used as received without further purification. Synthesis of CuCo2S4 nanosheets: A hydrothermal procedure19 has been modified by increasing the amount of ethylenediamine and reaction temperature at reduced time to synthesize CuCo2S4 nanosheets. In a typical synthesis procedure 2 mmol of Co(NO3)2.6H2O and 1 mmol of 16 ACS Paragon Plus Environment

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Cu(NO3)2.3H2O were dissolved in 30 ml of distilled water. Solution was stir for 10 min then 4 mmol of thiourea was added to this pink colour solution followed by vigorously stirring for 15 min. 2 ml of ethylenediamine was added into this solution and the colour of solution become brown and the whole solution was transferred to the 50 ml teflon lined autoclave. The autoclave was sealed and kept at 200 oC for 12 h. Subsequently the reaction mixture was allowed to cool down upto room temperature. The as-obtained black coloured product was washed with distilled water and ethanol for 2 times followed by drying at 50 oC for 6 h and stored for further studies. Co3S4 and Cu0.5Co2.5S4 were synthesized in the similar way as mentioned for CuCo2S4 nanosheets, where no Cu salt was added during the synthesis of Co3S4. In the case of Cu0.5Co2.5S4, 2.5 mmol of Co(NO3)2.6H2O and 0.5 mmol of Cu(NO3)2.3H2O were used during the reaction. Instrumentation: Transmission electron microscopy (TEM), phase-contrast high resolution TEM (HRTEM), electron diffraction for X-ray analysis (EDAX) measurements, the high angle annular dark field imaging and scanning transmission electron microscopy (HAADF-STEM) were executed with a FEI Technai G2-20 transmission electron microscope operating at an escalating voltage of 200 kV. X-Ray Diffraction (XRD) patterns of as-synthesized and reused catalyst were obtained at 298 K using a Bruker D8 Advance diffractometer system employing monochromatized Cu Kα radiation (λ = 1.54056 Å) source. Scanning electron microscopy (SEM) and EDAX measurements were performed with a JEOL JSM 6610 at 20 kV, width distance 10 mm and spot size 30. EDAX was accomplished at a resolution of 135.2 eV. AFM imaging was done by using a Bruker multimode 8 scanning probe microscope and Bruker silicon cantilever through tapping mode analysis. The sample for AFM were prepared by drop casting 5 µL solution dispersed in ethanol on silicon wafer sheet. Prior to this silicon sheets were cleaned through sonication in acetone/ethanol for 20 min. XPS measurements were made using a custombuilt ambient pressure XPS system from Prevac equipped with a Scienta monochromator (MX650) using an Al Ka anode (1486.6 eV).42 The energy of the photoelectrons was determined using a Scienta R3000HP differentially pumped analyser. The spectra were recorded at a pass energy of 50 eV. Raman spectra of the as-synthesized samples were recorded at 25 oC in a Renishaw in Via Raman spectrometer equipped with a laser having a wavelength of 514 nm. The textural properties (BET surface area, BJH pore volume) of the as-synthesized sample were derived from N2 sorption analysis, which was carried out at 77 K using an automatic micropore 17 ACS Paragon Plus Environment

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physisorption analyzer (Micromeritics ASAP 2020, USA) after the sample was degassed at 110 °C for 8 h under 10−3 Torr pressure. The FT-IR spectra (KBr disk, 4000−400 cm−1) were recorded on a PerkinElmer FT-IR 2000 spectrophotometer. Electrochemical measurements: Electrochemical studies were accomplished by using a CHI 660E potentiostat and a suitable three electrode system. The system was made up of a glassy carbon electrode (GC, 3 mm diameter, geometric area 0.07 cm2), a platinum wire as counter/auxiliary electrode and Ag/AgCl as reference electrode. For the preparation of working electrode finely powdered catalyst (5 mg) and 20 µL Nafion (5 wt%) solution were dispersed in isopropanol (1mL). The solution was ultrasonicated for 30 min to acquire an ink type suspension. 10 µL of this ink was drop-casted on the surface of glassy carbon electrode and dried in oven at 40 oC for overnight and used as working electrode. Prior to each experiment, GC electrode was polished with 0.03, 0.1 micron alumina powder for mirror shine surface and the experimental electrolyte KOH (1M) was degassed by bubbling O2 for 30 min. For OER activity cyclic voltammetry (CV), linear sweep voltammetry (LSV), impedance spectroscopy (EIS) experiments were performed at a scan rate of 10 mVs−1 along a potential window of 0.2−0.8 V vs. Ag/AgCl. For the activation of catalyst we run 20 CV sweep a higher scan rate of 100 mV s−1 at the same potential window before staring the actual experiment. The electrochemical impedance spectroscopy was performed from 0.1 Hz to 1 MHz. For the stability and durability of catalyst we run 10000 LSV cycles at a scan rate of 10 mV s−1. All potentials were converted to reversible hydrogen electrode (RHE) using standard equation (see SI). Author Information. Corresponding Author *E-mail: [email protected] Notes: The authors declare no competing financial interest. Associated content Supporting Information Details of method and characterization, calculations on exchange current density, mass activity, specific activity, turn over frequency, additional TEM and SEM images of Cu0.5Co2.5S4 and Co3S4 nanosheets, STEM-HAADF image, AFM image, XRD patterns and TEM images of control synthesis of CuCo2S4 particles at different temperature and time, sputtering XPS, BET, 18 ACS Paragon Plus Environment

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CV sweeps at different scan rates, FTIR spectra. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgement MC thanks CSIR-India for providing research fellowship. SD thank CSIR-New Delhi (01(2773)/14/EMR-II) and SERB-New Delhi (EMR/2016/004833), for financial assistance. We thank USIC-DU, SAIF-AIIMS, AIRF-JNU and MNIT-Jaipur for instrumentation facility. Authors thank Mr. Ashmeet Singh, INST-Mohali for the help with AFM measurement.

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