Advanced Cu3Sn and Selenized Cu3Sn@Cu Foam as

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Article Cite This: Inorg. Chem. 2019, 58, 9490−9499

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Advanced Cu3Sn and Selenized Cu3Sn@Cu Foam as Electrocatalysts for Water Oxidation under Alkaline and Near-Neutral Conditions Kannimuthu Karthick,†,‡ Sengeni Anantharaj,†,‡ Swathi Patchaiammal,§ Sathya Narayanan Jagadeesan,§ Piyush Kumar,§ Sivasankara Rao Ede,†,‡ Deepak Kumar Pattanayak,†,‡ and Subrata Kundu*,†,‡

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Academy of Scientific and Innovative Research (AcSIR), CSIR-Central Electrochemical Research Institute (CSIR-CECRI) Campus, New Delhi 630006, India ‡ CSIR-Central Electrochemical Research Institute (CECRI), Karaikudi 630003, Tamil Nadu India § Centre for Education (CFE), CSIR-Central Electrochemical Research Institute (CECRI), Karaikudi 630006, Tamil Nadu India S Supporting Information *

ABSTRACT: Water electrolysis is a field growing rapidly to replace the limited fossil fuels for harvesting energy in future. In searching of non-noble and advanced electrocatalysts for the oxygen evolution reaction (OER), here we highlight a new and advanced catalyst, selenized Cu3Sn@Cu foam, with overwhelming activity for OER under both alkaline (1 M KOH) and near-neutral (1 M NaHCO3) conditions. The catalysts were prepared by a double hydrothermal treatment where Cu3Sn is first formed which further underwent for second hydrothermal condition for selenization. For comparison, Cu7Se4@Cu foam was prepared by a hydrothermal treatment under the same protocol. The as-formed Cu3Sn@Cu foam, selenized Cu3Sn@Cu foam, and Cu7Se4@Cu foam were utilized as electrocatalysts and showed their potentiality in terms of activity and stability. In 1 M KOH, for attaining the benchmarking current density of 50 mA cm−2, selenized Cu3Sn@Cu foam required a low overpotential of 384 mV and increased charge transfer kinetics with a lower Tafel slope value of 177 mV/dec comparing Cu3Sn@Cu foam, Cu7Se4@Cu foam, and pristine Cu foam. Furthermore, potentiostatic analysis (PSTAT) was carried out for 40 h for selenized Cu3Sn@Cu foam and with minimum degradation in activity assured the long-term application for hydrogen generation. Similarly, under neutral condition selenized Cu3Sn@Cu foam also delivered better activity trend at higher overpotentials in comparison with others. Therefore, the assistance of both Sn and Se in Cu foam ensured better activity and stability in comparison with only selenized Cu foam. With these possible outcomes, it can also be combined with other active, non-noble elements for enriched hydrogen generation in future.

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INTRODUCTION Hydrogen is a promising fuel for the future and has the potential to replace finite fossil fuels in which hazardous CO2 emission is a setback that affects climate changes around the world.1,2 In the production of H2, water electrolysis offers a wide range of advantages compared to other methods, and the purity of H2 produced is greater. With the world’s current energy demand, water electrolysis offers approximately 4% in addition to other available reliable methods.2−4 Since the use of finite-supply fossil fuels is out of hand mostly at the end of this century, it is mandatory for us to reliably produce H2 without affecting environment much.3,5 If the H2 is produced by means of renewable energy sources like solar, wind, and tidal energy sources and if it is directly utilized in fuel cells, then this method will be a greener method with fast H2 production for energy needs in future. In water electrolysis, the oxygen evolution reaction (OER) with sluggish kinetics is a bottleneck compared to the hydrogen evolution reaction (HER) that reduces the overall efficiency of the system and drags its application at commercial scale.6,7 Since the noble metal catalysts, namely, Pt, IrO2, and RuO2, were only found to © 2019 American Chemical Society

be state-of-art for catalyzing the water-splitting reaction, researchers around the world have recently triggered a search toward finding cost-effective, less toxic, and earth abundant metals based electrocatalysts for efficient hydrogen production at large scale.8,9 Also, the corrosive dissolution of these IrO2 and RuO2 catalysts in acid over long-term usage restricts their use for continuous supply of hydrogen. Therefore, it is indeed urgently necessary to find alternates for H2 production with increased efficiency. For replacing noble metals based catalysts, 3d transition metals based catalysts were found to deliver comparable activity in OER in alkaline medium.10−12 Since the hydroxyl group adsorption and the cleavage of O2 molecules were important parameters in determining their efficiency, 3d transition metals based catalysts, particularly those with Fe, Co, Ni, and Cu, show extended activity in OER.13−16 For increasing the activity of these catalysts, there were more breakthrough works highlighted earlier by making them as Received: May 20, 2019 Published: June 25, 2019 9490

DOI: 10.1021/acs.inorgchem.9b01467 Inorg. Chem. 2019, 58, 9490−9499

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activity at higher overpotentials than those of the other electrocatalysts. Therefore, in the future, these Cu foam based non-noble catalysts can find application in commercial H2 production.

composites, alloys, oxides, hydroxides, phosphides, chalcogenides, and others.13,17−22 Layered double hydroxides (LDH) systems were also found to show enriched activity for OER in alkaline medium but lacks in HER activity. For example, Feng et al. highlighted the oxyhydroxides of iron group metals for the enhanced OER.23−26 However, the oxide or hydroxide formed during anodization of these catalysts was only acted as a real catalyst. Hence, modulating the surface of the metal active sites like hydroxides and chalcogenides with various morphologies is highly desired for better performance, but the use of binder is necessary which indirectly inhibits the metal active sites from electrode−electrolyte interaction. Therefore, there are methods like direct growth of catalysts over external substrates such as carbon cloth (CC), carbon fiber paper (CFP), Cu foam, and Ni foam where the activity of the catalysts can be fine-tuned irrespective of the external binders like nafion.27−30 Recently, there are more reports that highlighted the direct growth of metals-based catalysts like NiSe, CuSe, and other oxides, hydroxides over Ni foam and Cu foam for the enhanced water oxidation.31−33 Therefore, it was found that growing highly active catalysts over Ni and Cu foam offers high activity and stability with low overpotential in OER.31−33 Recent literature findings revealed that selenide-based catalysts were found to deliver enriched activity in both OER and HER. There are more selenide-based catalysts reported, namely, NiSe, Ni0.89Co0.11Se2, and Ni0.76Fe0.24Se, which were grown over external substrates mostly on Ni foam.34−36 There are limited reports for the selenides-based catalysts grown over Cu foam for water splitting.32,37 A report by Masud et al. highlighted the synthesis of copper selenides in both chemical vapor deposition and hydrothermal conditions for the OER in alkaline medium.34 In another report by Tran et al., composites of MoSe2@Cu2Se were prepared, and Cu is selectively etched to give enhanced HER activity.38 Recently, from our group, Anantharaj et al. reported the Cu2Se@Cu foam for HER in acidic medium.32 Other than these, there are a limited number of reports that highlighted the use of Cu as selenides for OER. Therefore, it will be interesting to work on selenide-based catalysts that are directly grown on Cu foam and applying it for OER with increased cell efficiency with post-transition metals like Sn. Previously, Ni-based Sn materials were also explored for OER and HER studies in alkaline medium.39−42 The literature findings revealed that the influence of selenium can also be utilized under neutral conditions for the OER studies.43 The use of Sn as support in electrocatalysts in different media was also studied in detail.44 Therefore, Sn as support with selenides over the substrates like Ni and Cu foam will be a greater advantage in terms of activity and stability in different electrolytes.39−42,44 With these findings, in this work, we have successfully grown advanced Cu3Sn@Cu foam and selenized Cu3Sn@Cu foam via a double hydrothermal treatment for the first time. For comparison, Cu7Se4@Cu foam was prepared by a hydrothermal treatment by employing a similar protocol. The as prepared catalysts were directly utilized as electrodes for the eletrocatalytic water oxidation studies and showed fruitful results. For the OER in 1 M KOH, to attain 50 mA cm−2, the selenized Cu3Sn@Cu foam required just 384 mV as overpotential with a lesser Tafel slope value of 177 mV/dec. The long-term stability was analyzed by PSTAT analysis for 40 h in alkaline medium and showed less degradation in activity. In 1 M NaHCO3, selenized Cu3Sn@Cu foam showed better

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EXPERIMENTAL SECTION

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RESULTS AND DISCUSSION

Synthesis of Cu3Sn, Cu7Se4@Cu Foam and Selenized Cu3Sn@Cu Foam. For synthesizing both Cu3Sn and selenized Cu3Sn over Cu foam catalysts, we have followed a double hydrothermal treatment. At first, to prepare Cu3Sn over Cu foam, acid pretreated Cu foams (4 × 0.5) were cut down and to which 0.5 M of SnCl2 is added along with 3 M KOH in 30 mL of H2O and kept in a 50 mL autoclave at 180 °C for 12 h. After this, the as formed Cu3Sn@Cu foam which is gray white color was again subjected to another hydrothermal condition. In second, Cu3Sn@Cu foam is taken along with 0.1 M NaBH4 and 0.02 M Se and was added in 30 mL of H2O in a 50 mL autoclave and again subjected to hydrothermal condition at 180 °C for 12 h. Similarly, Cu7Se4@Cu foam was prepared by following similar protocols except the addition of SnCl2 into the reaction mixture. After which, the as-formed black selenized Cu3Sn@Cu foam was washed with DI water and ethanol several times and dried for further studies. Details of the materials used and sample preparation techniques for characterization have been given in the Supporting Information. Electrochemical Characterizations. The potentiality of the asprepared Cu3Sn, Cu7Se4@Cu foam, and selenized Cu3Sn over Cu foam were analyzed in the OER in alkaline and near-neutral media. Electrocatalytic studies for Cu3Sn@Cu foam and selenized Cu3Sn@ Cu foam were carried out in a conventional three-electrode cell where Hg/HgO reference electrode and platinum counter electrode were used along with Cu3Sn@Cu foam and selenized Cu3Sn@Cu foam as working electrodes. In 1 M NaHCO3 electrolyte, Ag/AgCl reference electrode was used. Prior to use, Cu foam was acid-treated to remove surface oxides. The average loading in 0.5 × 0.5 cm2 was 1.8 mg cm−2 (varied from 1.75 to 1.85 mg cm−2 in each catalyst) in Cu3Sn@Cu foam, and the average loading was 0.255 mg cm−2(varied from 0.24 to 0.26 mg cm−2 in each catalyst) for selenized Cu3Sn@Cu foam and used for electrochemical studies. The loading was 0.383 mg cm−2 (varied from 0.37 to 0.39 mg cm−2 in each catalyst) in the case of Cu7Se4@Cu foam. Polarization curves were acquired at a sweeping rate of 5 mV s−1, and backward CV was taken for evaluating the activity. iR compensation (100%) was done manually from the Rs as observed from EIS taken at 10 mA cm−2. Long-term stability was analyzed by potentiostatic analysis (PSTAT) at an overpotential of 374 mV for 40 h with selenized a Cu3Sn@Cu foam working electrode in alkaline medium. An accelerated degradation (AD) study was also carried out at a very high sweep rate of 200 mV s−1 for 500 cycles to ensure the stable nature of the catalysts prepared in alkaline medium. Impedance analysis was performed in the frequency range of 0.01 Hz to 10 kHz with an amplitude potential of 0.005 V. Under near-neutral conditions, PSTAT was done at an overpotential of 716 mV. The electrode potentials applied have been converted into RHE scale as per the literature survey.45

Characterization of Cu3Sn@Cu Foam, Cu7Se4@Cu Foam, and Selenized Cu3Sn@Cu Foam. The formations of Cu3Sn@Cu foam, Cu7Se4@Cu foam, and selenized Cu3Sn@ Cu foam were confirmed through advanced materials characterization techniques such as XRD, FE-SEM, TEM, HR-TEM and XPS analyses, respectively. A brief elaboration of confirmation of Cu3Sn@Cu foam, Cu7Se4@Cu foam and selenized Cu3Sn@Cu foam is discussed in detail below. At first, an X-ray diffraction study was performed to know the successful formation of Cu3Sn@Cu foam and Cu7Se4@Cu foam which has been given as Figures S1 and S2, respectively. From Figure S1, we can see the formation of Cu3Sn@Cu 9491

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is well-selenized and formed as selenized Cu3Sn@Cu foam. The possible mechanism of formation of selenized Cu3Sn@Cu foam has been given as Scheme S1. After this, the corresponding morphological outcomes in the cases of both Cu3Sn@Cu foam and selenized Cu3Sn@Cu foam were merely analyzed by HR-TEM analysis which were given here as Figure 2. Figure 2a shows the HR-TEM micrograph of the prepared Cu3Sn@Cu foam with a diameter of ∼1.53 μm with the morphology of a starlike structure. When the same Cu3Sn@Cu foam is selenized to form selenized Cu3Sn@Cu foam, the morphology became like a square star with a diameter of ∼1.21 μm as we can see from Figure 2b,c. Figure 2d showed the highmagnification HR-TEM micrograph with the fine lattice fringes of Cu3Sn@Cu foam with the interplanar distances of 0.204 and 0.221 nm showing different phases of both Cu and Sn from Cu3Sn which is in accordance with PDF no. 00−001−1240. In Figure 2e, for selenized Cu3Sn@Cu foam, the lattice fringes with the interplanar distances of 0.205 and 0.231 nm were assigned with respect to PDF no. 00−023−0602 which clearly showed that even after selenization the interplanar spacing was almost similar as observed from Cu3Sn@Cu foam. The selected area diffraction pattern of selenized Cu3Sn@Cu foam showed perfect crystalline nature and is in accordance with the results of the XRD studies. In addition to this, transmission electron microscopy (TEM) was also performed for selenized Cu3Sn@Cu foam, which was found to have square-star-like morphology, provided as Figure S3. Figure S3a−c are the low- and high-magnification micrographs of selenized Cu3Sn@Cu foam with fine morphological outcomes. Microscopic analyses together revealed that the formed selenized Cu3Sn@Cu foam has a square-star-like morphology and showed fine lattice fringes with perfect crystalline nature as observed from the SAED pattern. Furthermore, for ensuring the presence of the elements, FESEM color-mapping was carried out for both Cu3Sn@Cu foam and selenized Cu3Sn@ Cu foam and is provided as Figures S4 and 3 here, respectively. The color-mapping results of Cu3Sn@Cu foam from Figure S4 clearly revealed that there was a homogeneous distribution of both Cu and Sn over the Cu foam with some oxides is formed along them. Similarly, the selenized Cu3Sn@Cu foam as showed in Figure 3a−f, have shown the presence of Cu, Sn,

foam which was confirmed in reference to the Powder Diffraction File (PDF) no. 00−001−1240 (International Centre for Diffraction Data (ICDD), [1927]). The corresponding crystal system and space group were orthorhombic and Cmcm, respectively. The formation of Cu7Se4@Cu foam was confirmed from Figure S2 where high-intensity peaks at 27.081 and 44.903° exactly matched with those of PDF no. 00−026−0557 (ICDD, [1971]). Here, the crystal system was cubic, and the space group was P and in accordance with X-ray studies. After the selenization process of Cu3Sn@Cu foam, it was subjected for XRD analysis again which showed successful formation of selenized Cu3Sn@Cu foam as we can see from Figure 1.

Figure 1. XRD pattern of selenized Cu3Sn@Cu foam prepared by a double hydrothermal treatment compared to PDF nos. 01−085− 1326, 00−001−1240, and 00−023−0602 (ICDD, [1953, 1927, 1970]).

A closer look at the figure revealed that the pattern matches with Cu from Cu foam, Cu3Sn, and SnSe2 corresponding to the PDF nos. 01−085−1326, 00−001−1240, and 00−023− 0602 (ICDD, [1953, 1927, 1970]), respectively. The SnSe2 sample had a hexagonal crystal system, and the space group was P3̅m1, in accordance with PDF no. 00−023−0602 (ICDD, [year]). These results together revealed that the formed Cu3Sn

Figure 2. (a) HR-TEM micrograph of Cu3Sn@Cu foam, (b and c) the HRTEM micrographs of selenized Cu3Sn@Cu foam, (d and e) fine lattice fringes of Cu3Sn@Cu foam and selenized Cu3Sn@Cu foam, and (f) corresponding SAED pattern of selenized Cu3Sn@Cu foam. 9492

DOI: 10.1021/acs.inorgchem.9b01467 Inorg. Chem. 2019, 58, 9490−9499

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Figure 3. (a−f) FE-SEM color mapping images of the prepared selenized Cu3Sn@Cu foam: area taken, mix, Cu K1, Sn L1, Se K1, and O K1, respectively.

Figure 4. X-ray photoelectron spectroscopy (XPS) analysis of selenized Cu3Sn@Cu foam. (a−d) Cu 2p, Sn 3d, Se 3d, and O 1s high-resolution spectra, respectively.

and Se from mix, Cu K1, Sn L1, Se K1, and O K1, respectively,

formed for both Cu3Sn@Cu foam and selenized Cu3Sn@Cu

which were grown all around and confirmed the fruitful incorporation of selenium over the Cu3Sn@Cu foam to form

foam and has been given as Figures S5 and S6, respectively.46 The EDS has again confirmed the presence of Cu and Sn

selenized Cu3Sn@Cu foam. To validate the presence of

with some oxides and in case of Cu3Sn@Cu foam according to

elements, energy-dispersive spectroscopy (EDS) was per-

earlier report. The presence of Cu, Sn, Se and O in selenized 9493

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Figure 5. (a) Backward sweep from CV taken at 5 mV s−1. (b) Corresponding Tafel slopes. (c) Backward sweep acquired after AD. (d) PSTAT analysis at 374 mV.

Figure S9c showed O 1s with two distinct peaks at binding energy values of 530.3 and 531.7 eV that correspond to the Sn−O and Sn−OH of Cu3Sn@Cu foam. These results clearly portray the fruitful formation of Cu3Sn@Cu foam. Selenized Cu3Sn@Cu foam was then subjected to XPS analysis and is given here as Figure 4. Figure 4a shows the high-resolution spectrum of Cu 2p, and for Cu 2p1/2 and Cu 2p3/2, the binding energy values observed at 932.5 and 952.5 eV correspond to the oxides of Cu. At high binding energies, 934.4 and 954 eV are the respective satellite peaks of the same. In the case of Sn 3d, high-resolution spectrum in Figure 4b shows that at Sn 3d5/2 a peak appeared near 486.1 eV for SnSe with surface oxides that confirming the formation of selenized Cu3Sn@Cu foam. Similarly, the peak at 495 eV for Sn 3d3/2 is attributed to the oxides of Sn from the selenized Cu3Sn@Cu foam.48,49 From Figure 4c, the Se 3d high-resolution spectrum showed two peaks at 53.8 and 54.7 eV which were related to the SnSe2 and Se4+ states from the selenized Cu3Sn@Cu foam with oxides.48,49 In the case of Figure 4d, two peaks appeared at the binding energy values of 530.2 and 531.3 eV for O 1s which are correlated to the Sn−O and Sn−OH bonds, respectively. From the XPS analysis, it is concluded that both Cu3Sn@Cu foam and selenized Cu3Sn@Cu foam were formed.47−49 Similarly, the XPS analysis was performed for Cu7Se4@Cu foam that has been given as Figure S10. Figure S10a showed Cu 2p high-resolution spectra which split into Cu 2p3/2 and 2p1/2, respectively. The binding energy values observed at 931.7 and 951.6 eV correspond to Cu 2p3/2 and 2p1/2,

Cu3Sn@Cu foam that assured the presence of all the expected elements.46 For the identification of the presence of all the expected elements, Cu7Se4@Cu foam was also subjected to FE-SEM color-mapping and EDS analysis, and the results are given as Figures S7 and S8, respectively. EDS showed the presence of Cu and Se with some oxides over the surface as seen in Figure S7. From Figure S8, the presence of Cu and Se is confirmed in Cu7Se4@Cu foam in the order of area taken, Cu K1, Se K1, and O K1, which are observed with some oxides formed over the surface. After confirming the formation and morphological outcomes through XRD and microscopic analyses, the chemical nature of both Cu3Sn@Cu foam and selenized Cu3Sn@Cu foam were confirmed by X-ray photoelectron spectroscopy (XPS) studies. First, the formation of Cu3Sn@Cu foam was carried out which was shown as Figure S9. Figure S9a shows a doublet because of spin−orbit coupling, which has been observed as Cu 2p1/2 and Cu 2p3/2, respectively. For Cu 2p3/2 and Cu 2p1/2, two peaks observed at 933.2 and 952.8 eV that correspond to Cu3Sn and oxides of Cu, respectively, as per the literature survey.47 Other peaks observed near 935.6 and 954.8 eV are the corresponding satellite peaks of the same. In Figure S9b, the Sn 3d has been split into 3d3/2 and 3d5/2, respectively. In Sn 3d3/2, two peaks observed near 484.8 and 486.5 eV corresponding to metallic Sn and SnO2, respectively, confirm the formation of alloys of Cu3Sn with oxides formed over the surface. In addition, at Sn 3d5/2, binding energy values of 495 and 493.3 eV are related to the surface oxides of Sn. 9494

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Figure 6. (a) Backward sweep from CV taken at 5 mV s−1. (b) Corresponding Tafel slopes. (c) EIS taken at 2 V. (d) PSTAT analysis at 716 mV.

Cu 3 Sn@Cu foam showed around 177 mV/dec, and Cu7Se4@Cu foam showed 188 mV/dec From these observations, it is clear that even though the activity was almost similar the charge transfer kinetics was facile with Sn and selenized Cu foam compared only to selenized Cu foam. In the cases of Cu3Sn@Cu foam and bare Cu foam the values are 230 and 344 mV/dec, respectively, that indirectly mean the charge transfer kinetics are facile in selenized Cu3Sn@Cu foam.37 An accelerated degradation (AD) study was performed in order to know the material stability under severe scan rate of 200 mV/s for 500 cycles, and the resultant polarization curves after AD have been portrayed here as Figure 5c. The changes in overpotential before and after AD at 50 mA cm−2 were 10 and 18 mV for selenized Cu3Sn@Cu foam and Cu7Se4@Cu foam, respectively. The highly stable nature of selenized Cu3Sn@Cu foam is attributed to the use of Sn and Se in Cu foam compared to only Se in Cu foam. Therefore, from the above observations made, both the Sn and Se were found to be the most advantageous for stable oxygen evolution for prolonged time. In the Cu3Sn@Cu foam, the activity is actually enhanced (Figure 5c), and the overpotential is decreased which implied that Sn(OH)42− is leached from the Cu3Sn@Cu foam and enhanced the active sites of Cu. After the AD test, the active sites of Cu enriched with the leaching of Sn(OH)42− under alkaline conditions. Long-term stability was assured by PSTAT analysis at an overpotential of 374 mV for selenized Cu3Sn@ Cu foam for 40 h and is shown here as Figure 5d. The degradation in activity is negligibly small even after prolonged exposure showing the real scale stability of the prepared

respectively, and peaks observed at high binding energies are the respective satellite peaks of the same. This Cu 2p highresolution spectrum confirms the presence of Cu2+ oxidation states.50 Figure S10b, the Se 3d high-resolution spectrum, showed two peaks at 53.7 and 54.6 eV from the Se 3d5/2 and Se 3d3/2 levels that confirm the presence of Se2− oxidation states.50 From Figure S10c, the peak observed in a highresolution spectrum for O 1s corresponded to the surface oxides and oxides of selenium from Cu from Cu7Se4@Cu foam. The excess oxides formed or are related to the formation of oxides of Cu along with the surface oxides and from the oxides of selenium. From XPS analysis of all three catalysts, it has been found that there was concurrent formation of Cu3Sn@Cu foam, Cu7Se4@Cu foam, and selenized Cu3Sn@ Cu foam. Electrocatalytic Water Oxidation of Cu3Sn@Cu Foam, Cu7Se4@Cu Foam, and Selenized Cu3Sn@Cu Foam under Alkaline Condition (1 M KOH). The prepared Cu3Sn@Cu foam and selenized Cu3Sn@Cu foam were subjected to the OER in alkaline medium, and the resultant polarization curves have been shown here as Figure 5a. From this figure, we can see that to reach a current density of 50 mA cm−2 selenized Cu3Sn@Cu foam required less overpotential around 384 mV, while Cu7Se4@Cu foam required 387 mV as overpotential. Both Cu foam and Cu3Sn@Cu foam delivered poor activity under the same potential window. To gain deeper insight in the activity, Tafel slopes were obtained with 100% iR compensated backward sweep of CV curve and have been shown here as Figure 5b. The Tafel slope values are in exact agreement with the polarization curves where selenized 9495

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Figure 7. (a−c) Low- and high-magnification micrographs of selenized Cu3Sn@Cu foam after OER in alkaline medium as observed from TEM. (d) Corresponding SAED pattern.

polarization curves were obtained at a scan rate of 5 mV/s and are shown as Figure 6a. From the LSV curves, it is clear that both Cu and Cu3Sn@Cu foam showed poor activity, while the selenized ones showed better activity. Among Cu7Se4@Cu foam and selenized Cu3Sn@Cu foam, former showed better activity at lower overpotentials, whereas at higher overpotentials, the latter showed increased activity. At a current density of 10 mA cm−2, Cu7Se4@Cu foam showed a lower overpotential of 624 mV, and selenized Cu3Sn@Cu foam showed an overpotential of 680 mV. At higher anodic potentials, selenized Cu3Sn@Cu foam delivered better activity. Therefore, as observed under alkaline condition and under neutral condition, Sn and Se addition to the Cu foam showed fruitful results considering activity and stability. The Tafel slopes (Figure 6b) clearly confirmed that the charge transfer kinetics were too facile in the case of selenized Cu3Sn@Cu foam with a value of 263 mV/dec which was lower than others. The EIS analysis (Figure 6c) was fitted with the corresponding equivalent circuit and showed that at higher potential of 2 V the charge transfer resistance (Rct) was too low for selenized Cu3Sn@Cu foam followed by Cu7Se4@Cu foam. Furthermore, the long-term stability study (PSTAT) was performed (Figure 6d) in order to know the real scale stability of the catalyst prepared. Here, the selenized Cu3Sn@Cu foam delivered reasonably better activity for a prolonged time scale at a constant anodic overpotential of 716 mV. These findings clearly portray the influence of Sn and Se in Cu foam that directed better activity and stability. As observed before under alkaline condition, here also under near neutral condition, the activity and stability were assured due to the presence of both Sn and Se over the Cu foam compared to only Se over Cu foam. Post-OER Analysis of Selenized Cu3Sn@Cu Foam. After the electrochemical studies of the prepared selenized

selenized Cu3Sn@Cu foam. In addition, an electrochemical impedance spectrum (EIS) was carried out to study the nature of interface and have been given in Figure S11. From Figure S11a, the charge transfer resistance (Rct) was the lowest in selenized Cu3Sn@Cu foam with the Rct value around 5 Ω, and Cu7Se4@Cu foam showed an Rct value of 7 Ω, implying superior kinetics in the former one. Even after AD test (Figure S11b), the Rct value being almost the same in both implied the superior activity of the catalysts in comparison with that of bare Cu foam. The decrease in Rct for Cu3Sn@Cu foam is wellreflected with the observed LSV curves, and as expected, Cu3Sn@Cu foam showed better activity. The inset of the figures showed the corresponding equivalent circuits of the observed EIS spectra. Furthermore, electrochemical active surface area (ECSA) was calculated based on the Cdl (doublelayer capacitance) method and was in good agreement with the observed trends of electrochemical studies which was shown as Figure S12. The calculated Cdl values were 23.2, 28.9, 20.9, and 9.5 μF for selenized Cu3Sn@Cu foam, Cu7Se4@Cu foam, Cu foam, and Cu3Sn@Cu foam, respectively. Combined electrocatalytic water oxidation results highlighted that selenized Cu3Sn@Cu foam delivered enriched activity compared to Cu7Se4@Cu foam, Cu3Sn@Cu foam, and bare Cu foam in terms of activity and stability and can be used for large-scale studies. To evaluate the activities, a comparison table is given as Table S1. Electrocatalytic Water Oxidation of Cu3Sn@Cu Foam, Cu7Se4@Cu Foam, and Selenized Cu3Sn@Cu Foam under Near-Neutral Condition (1 M NaHCO3). After the initial observations that the presence of Sn and Se enhanced the electrocatalytic water oxidation under alkaline conditions, the potentiality was again monitored by employing them as catalysts in 1 M NaHCO3 (pH 8.5). The observed electrochemical results have been shown here as Figure 6. The 9496

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Figure 8. X-ray photoelectron spectroscopy (XPS) analysis of selenized Cu3Sn@Cu foam after OER. (a−d) Cu 2p, Sn 3d, Se 3p, and O 1s highresolution spectra, respectively.

are for the oxides and hydroxides that are formed during the anodization in OER with the corresponding satellite peaks. Figure 8c shows the O 1s spectrum where 533.2 and 533.4 eV are for the M−O and M−OH bonds, respectively.47−49 In Figure 8d, the high-resolution spectrum of Se 3p was shown where peaks observed at 163.2 and 170.8 eV are for the surface oxidized selenium (SeO2) formed during the anodization.47−49 These findings clearly state that the formed selenized Cu3Sn@ Cu foam is found to have robustness even after harsh anodic conditions when undergone for the OER studies at longer times. Therefore, the material robustness was achieved with respect to the utilization of Sn and Se over the Cu foam for prolonged OER studies. The fruitful incorporation of Sn and Se as selenized Cu3Sn@Cu foam could deliver activity and stability under both alkaline and near-neutral conditions compared to others that highlight the importance of it for large scale production of hydrogen.

Cu3Sn@Cu foam in 1 M KOH, it was subjected to TEM and XPS analysis in order to know the material robustness and chemistry behind them. First, the morphological robustness of the catalyst was investigated with TEM, and the results of TEM are given as Figure 7. Figure 7a−c are the low- and high-magnification micrographs that showed the similar morphological outcomes as observed before the OER with some slighter changes inferring the robustness of selenized Cu3Sn@Cu foam even after the OER. Also, the SAED pattern from Figure 7d showed similar crystalline structure. These findings clearly state that the prepared selenized Cu3Sn@Cu foam was stable even after harsh anodic conditions and prolonged exposure under alkaline conditions. Therefore, from TEM analysis it has been observed that even after the OER conditions, the basic morphological outcomes was not varied, showing the robustness of the selenized Cu3Sn@Cu foam. To have further insight into the stability of catalysts and to confirm the presence of Se after electrochemical studies, EDS spectrum was carrried out after cycling study for selenized Cu3Sn@Cu foam (Figure S13). The presence of Cu, Sn, Se, and O was verified, and the presence of K is from the electrolyte KOH. Similarly, XPS analysis was performed after the OER and has been shown here as Figure 8. In Figure 8a, for Cu 2p3/2 high-resolution spectrum, a peak observed at 936.5 eV is for oxides of Cu and that at 945.2 eV is the corresponding satellite peak.47−49 From Figure 8b, binding energy values observed near 485.6, 488.8, 494, and 497.5 eV

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CONCLUSION The findings of non-noble metals based catalysts for OER are always preferred, and in this work, the non-noble Cu based Cu3Sn@Cu foam and selenized Cu3Sn@Cu foam were successfully formed for the first time by a double hydrothermal treatment. Similarly, for comparison, Cu7Se4@Cu foam was prepared under hydrothermal condition. The selenization of Cu3Sn@Cu foam could give enriched performance in terms of activity and stability for the OER in both alkaline and nearneutral media. In OER (1 M KOH), for reaching a current 9497

DOI: 10.1021/acs.inorgchem.9b01467 Inorg. Chem. 2019, 58, 9490−9499

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Inorganic Chemistry density of 50 mA cm−2, the selenized Cu3Sn@Cu foam required overpotential of just 384 mV with a lesser Tafel slope value of 177 mV/dec. Similarly, for OER in 1 M NaHCO3, it showed a better activity trend compared to that of Cu7Se4@Cu foam at higher anodic overpotentials. Furthermore, long-term stability was observed with less degradation in alkaline media for selenized Cu3Sn@Cu foam. Post-OER characterizations revealed that the morphological outcome was retained even after harsh anodic conditions that indirectly proved the robustness of the selenized Cu3Sn@Cu foam. Therefore, the incorporation of post-transition metals like Sn can also be combined with other metals-based catalysts for achieving better activity and stability in near future.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b01467. Reagents used in the synthesis of materials and characterization techniques and figures related to electrochemical studies (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. ORCID

Kannimuthu Karthick: 0000-0003-2689-0657 Sengeni Anantharaj: 0000-0002-3265-2455 Sivasankara Rao Ede: 0000-0002-1122-4179 Subrata Kundu: 0000-0002-1992-9659 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge Dr. Vijayamohan K. Pillai (Former Director) and Dr. N. Kalaiselvi (Present Director), CSIR-CECRI, for their continuous support and encouragement. K.K. acknowledges UGC for a SRF award. S.A. and S.R.E. acknowledge CSIR for SRF awards. S.K. acknowledges the Department of Science and Technology (DST) for EMR research funding (no. EMR/2017/000860, 11-05-2018 with institute OM no. 18-29-03/(27/2018)-TTBD-CSIR-CECRI on 29-10-2018).

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