Microwave-Initiated Facile Formation of Ni3Se4 Nanoassemblies for

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Microwave Initiated Facile Formation of Ni3Se4 Nanoassemblies for Enhanced and Stable Water Splitting in Neutral and Alkaline Media Sengeni Anantharaj, Jeevarathinam Kennedy, and Subrata Kundu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15980 • Publication Date (Web): 20 Feb 2017 Downloaded from http://pubs.acs.org on February 20, 2017

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ACS Applied Materials & Interfaces

Microwave Initiated Facile Formation of Ni3Se4 Nanoassemblies for Enhanced and Stable Water Splitting in Neutral and Alkaline Media Sengeni Anantharaj1,2, Jeevarathinam Kennedy4 and Subrata Kundu1,2,3* 1Academy

of Scientific and Innovative Research (AcSIR), CSIR-Central Electrochemical Research Institute (CSIR-CECRI) Campus, New Delhi, India 2Electrochemical

Materials Science (ECMS) Division, CSIR-Central Electrochemical Research Institute (CECRI), Karaikudi-630006, Tamil Nadu, India 3

Department of Materials Science and Mechanical Engineering, Texas A&M University, College Station, Texas, TX-77843, USA 4Central

Instrumentation Facility (CIF), CSIR-Central Electrochemical Research Institute (CECRI), Karaikudi-630006, Tamil Nadu, India

* To whom correspondence should be addressed, E-mail: [email protected]; [email protected], Phone: (+ 91) 4565-241486 and (+ 91) 4565-241487.

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ABSTRACT Molecular hydrogen (H2) generation through water splitting with minimum energy loss has become practically possible due to the recent evolution of high performance electrocatalysts. In this study, we had fabricated, evaluated and presented such a high performance catalyst which is the Ni3Se4 nanoassemblies that can efficiently catalyze water splitting in neutral and alkaline media. A hierarchical nanoassemblies of Ni3Se4 was fabricated by functionalizing the surface cleaned Ni foam using NaHSe solution as Se source with the assistance of microwave irradiation (300 W) for 3 min followed by 5 h of aging at room temperature (RT). The fabricated Ni3Se4 nanoassemblies were subjected to catalyze water electrolysis in neutral and alkaline media. For a defined current density of 50 mAcm-2, the Ni3Se4 nanoassemblies required very low overpotentials for oxygen evolution reaction (OER) viz., 232 mV, 244 mV and 321 mV at pH 14.5, 14.0 and 13.0 respectively. The associated lower Tafel slope values (33 mVdec-1, 30 mVdec-1 and 40 mVdec-1) indicate the faster OER kinetics on Ni3Se4 surfaces in alkaline media. Similarly, in hydrogen evolution reaction (HER), for a defined current density of 50 mAcm -2, the Ni3Se4 nanoassemblies required low overpotentials of 211 mV, 206 mV and 220 mV at pH 14.5, 14.0 and 13.0 respectively. The Tafel slopes for HER at pH 14.5, 14.0 and 13.0 are 165 mVdec-1, 156 mVdec-1 and 128 mVdec-1 respectively. A comparative study on both OER and HER was carried out with the state-of-the-art RuO2 and Pt under identical experimental conditions and the results of which revealed that our Ni3Se4 is a far better high performance catalyst for water splitting. Besides, the efficiency of Ni3Se4 nanoassemblies in catalyzing water splitting in neutral solution was carried out and the results are better than many previous reports. With these amazing advantages in fabrication method and in catalyzing water splitting at various pH, the Ni3Se4 nanoassemblies can be an efficient, cheaper, non-precious and high performance electrode for water electrolysis with low overpotentials.

Keywords: Electrolysis, Water splitting, Oxygen evolution, Hydrogen evolution, Ni3Se4 nanoassemblies, Overpotential, Tafel analysis.


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INTRODUCTION It has become mandatory to search for new and non-conventional energy sources due to the scarcity faced over carbonaceous fossil fuels and their associated harms to the environment.1 The fuel cell technology is one among the most trusted domain of science and technology that has the potential of meeting the energy requirements of the eco-caring future.2,3 In fuel cells, the purity of H2 supplied plays crucial role in enhancing life cycle as well as in increasing the cell efficiency. To harvest H2 in large scale industrially, the steam reforming of coal and water electrolysis are the most appropriate ones.4 The purity of H2 produced with the former method is quite low and cannot be employed in fuel cells. Hence, the water electrolysis has gained greater importance recently. In water electrolysis, the cathodic HER and anodic OER are the key electrochemical reaction responsible for the production of H2 and O2 respectively.5 Between, HER and OER, OER is the more complicated multistep electrochemical reaction thus requires huge overpotential in addition to the equilibrium potential of OER (1.23 V vs RHE).6 In earlier days these two processes were made catalytic utilizing the less abundant, noble and expensive metals and metal oxides such as Pt, Ru/RuO2 and Ir/IrO2.7–10 However, it is not good to have those precious materials for H2 production in large scale where the catalytic material is at high risk of corrosive dissolution into the electrolyte solution. This will retard the applications of these materials from implementing in large scale water electrolysis.11 To avoid the use of these three precious metals, the studies on 3d transition metal oxides/hydroxides,5 sulphides,12-17 selenides,18-20 phosphides21-24 layered double hydroxides (LDH)25,26 and borides27 are extensively being carried out to catalyze both OER and HER in acidic and alkaline media.6 Interestingly, Shi and coworkers have shown the ability of VOOH as a total water splitting catalysts recently. 28 Besides, the chalcogenides of some 4f transition metals (W and Re) and Mo are also being reported as efficient catalysts for HER.29,30 Among the non-noble transition metals and their compounds reported so far for the electrocatalysis of OER and HER, the iron group metals and their compounds are unique with the tremendous catalytic activity.6 Though the electrocatalytic OER activity of Fe, Co and Ni based oxide and hydroxide are well-documented in literature,5,31 the recent evolution on the electrocatalytic properties of their sulphides, selenides and phosphides have taken these metals based electrocatalysts to a newer height in the electrocatalysis of water splitting.6 The same was very recently highlighted by our group as a review covering the recent trends in the catalytic behavior of the above said electrocatalysts.6 3 ACS Paragon Plus Environment


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Among the sulphides, selenides and phosphides of iron group metals, metal phosphides are better HER catalysts than the other two.4,32 Similarly, the selenides of iron group metals are better OER catalysts than the other two in alkaline medium. However, it should be emphasized here that the HER activity of metal selenides are almost parallel to that of metal phosphides in alkaline medium.6 Among Fe, Co and Ni selenides, the Co based selenide catalysts are the more frequently reported ones than that of Fe and Ni.33–38 Between, Fe and Ni selenides very few reports are available on Fe selenides for HER alone.39 In case of Ni selenides also there are few reports on the electrocatalysis of HER and OER.39–42 The following are the significant reports among them. Tang et al. gave the first report on NiSe nanowire grown on Ni foam by hydrothermal method for total water splitting.40 Soon after, Swesi and coworkers deposited the nickel selenide over Ti foils and evaluated its performance for OER electrocatalysis in alkaline condition.41 Liu and coworkers modified the nickel selenide with cobalt doping and the fabricated nanoparticle (NP) thin film was screened for total water splitting in alkaline medium.43 Ming et al. have recently proposed an alternate way to construct such Co doped nickel selenide catalyst system by the decomposition of their respective metal organic frameworks (MOF).42 Very recently, the report of Xu and coworkers on the OER electrocatalysis of NiSe2 had revealed that the key roles of nickel hydroxides and oxides generated in situ under anodic conditions in the enhanced OER activity.44 However, it could be noticed from all the above reports that the employed method of preparation of nickel selenide is either the time consuming hydrothermal or the electrodeposition with high precautions and cares. Moreover, the most reported nickel selenides are the NiSe, NiSe2 and Ni3Se2. This survey implies that there are possibilities of obtaining better activity in both HER and OER with a different polymorph of nickel selenide with varying Ni to Se ratio. Most importantly, the preparation method needs to be simplified to make it applicable to large scale water electrolysis with minimum expenses. With this novel view, we have chosen the short term (3 min) microwave irradiation to initiate the nucleation of Ni3Se4 nanoassemblies formation on Ni foam with additional 5 h aging at RT that helped sufficient growth of the same. In this route, NaHSe solution derived by the dissolution of Se powder into NaBH4 was used as Se source. This is the first report on the Ni3Se4 as a catalysts matrix for both HER and OER in neutral and alkaline conditions. The excellent results observed with our catalyst are discussed in subsequent sections. Being a new nickel


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selenide catalyst (Ni3Se4) and with a very simple way of fabrication, this method could be economically more efficient than the others in large scale water electrolysis. EXPERIMENTAL Fabrication of Ni3Se4 nanoassemblies on Ni foam The most reported synthesis route for the formation of nickel selenide is either a time consuming hydrothermal method or the cumbersome electrodeposition which need high level care.39-44 Hence, here we have simplified the formation route of Ni3Se4 by initiating the reaction of NaHSe with the Ni metal surfaces of metal foam by a short term (3 min) microwave irradiation. Prior to the fabrication of Ni3Se4 nanoassemblies over Ni foam, a solution of NaHSe was prepared by stirring 0.393 g of Se metal powder in 10 mL of deionized water that contains 0.433 g of NaBH4 for several minutes until a clear solution appeared. Pieces of Ni foam with the dimensions of 3 cm length and 0.5 cm width were treated with 3 M hydrochloric acid for few minutes then washed several times with deionized water and ethanol to ensure the exposure of fresh metallic surface that gets into contact everywhere during the reaction between NaHSe and Ni foam. For the fabrication of Ni3Se4 on Ni foam, the surface cleaned Ni foam pieces were put into a beaker containing N2 purged 40 mL deionized water followed by which about 2 mL of the freshly prepared NaHSe solution was added and covered with paraffin foil. The whole solution was then irradiated with the microwave radiation of power 300 W continuously for 3 min with a regular pause of 5 sec after every 20 sec of irradiation to avoid spillage of reaction mixture out of container. Then the whole content was cooled naturally to RT and kept as such for 5 h of aging. The resultant Ni3Se4 grown Ni foam was black in color. The same was then gently washed a couple of times with deionized water and dried at 100 °C for 2 h in a vacuum oven. Then the resultant Ni3Se4 nanoassemblies fabricated Ni foam electrode was directly taken for material characterizations and electrochemical characterizations. The weight difference before and after formation of Ni3Se4 on Ni foam was used to calculate the loading following the report of Tang et al.22 and the calculated loading was found to be 2.45 mgcm-2. Besides, to ensure the role of each and every parameter in the above mentioned synthesis route, a set of controlled studies was carried that are explained below. We have found no nickel selenide formation either only with the microwave irradiation for 3 min or with the aging period for 5 h without initial microwave irradiation. Moreover, the silvery white color of Ni foam was not changed to a significant extent only by the microwave irradiation. This indicates no sufficient formation of nickel selenide on Ni 5 ACS Paragon Plus Environment


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foam surfaces was effected by the irradiation alone. In contrast, with 5 h of aging of Ni foam with NaHSe, the color of Ni foam surface was changed into reddish brown which was easily removed by a gentle water wash. This indicates that during aging for 5 h without any initial microwave irradiation, nucleation of nickel selenide was not promoted instead a simple deposition of Se occurred over the Ni surface which was the reason for the observed reddish brown color. Moreover, we have also optimized the period of aging to be 5 h to achieve complete Ni3Se4 nanoassemblies coverage over Ni surfaces. The overall controlled study implies that the initial microwave irradiation followed by 5 h of aging at RT are crucial in obtaining complete fabrication of Ni3Se4 nanoassemblies over Ni foam surfaces. The total synthesis and controlled studies are sketched as Scheme 1. The materials used for the fabrication and the details of the instruments used are provided in Supporting Information (SI). Electrochemical characterizations In electrochemical measurements, Hg/HgO reference electrodes filled with KOH of varying molarity viz., 0.1 M, 1 M and 3 M were used as reference electrodes for both OER and HER studies in alkaline conditions. An Ag/AgCl reference electrode filled with 1 M KCl with a salt bridge was used for OER and HER studies in phosphate buffer solution (PBS). In both alkaline and neutral medium a Pt foil counter electrode was used. For comparison purpose a paste of commercial RuO2 catalyst with comparable catalyst loading was prepared by homogenizing RuO2 with water, isopropyl alcohol and Nafion® binder in the volume ratio of 0.75:0.20:0.05 under ultra-sonication for 15 min. A piece of surface cleaned Ni foam of the same dimension was masked with a PTFE tap to insulate area other than the contact point and the area where the catalyst ink is to be coated. Then the same was dried at 60 °C for 2 h in a vacuum oven before use in comparative OER studies. For comparative HER studies the Pt foil electrode of geometrical area 1 cm-2 was directly used. Before acquiring electrochemical measurements for OER and HER studies, O2 and H2 were purged for 30 min continuously in the respective electrolyte. Linear sweep voltammograms (LSVs) and cyclic voltammograms (CV) were acquired at a scan rate of 5 mVs-1 with simultaneous iR compensation provided by electrochemical work station. Then the potential scale was converted into reversible hydrogen electrode (RHE) scale as per existing reports for ease of comparison of the results obtained in electrolytes varying pH.45–48 Accelerated degradation test was carried out for both HER and OER by subjecting the working electrode for 500 cycles CV at a scan rate of 200 mVs-1 at all pH 6 ACS Paragon Plus Environment

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studied here. Similarly, the stability of Ni3Se4 nanoassemblies upon potentiostatic electrolysis at high alkaline conditions for both OER and HER were tested by running chronoamperometry at iR uncompensated potentials of 1.46 V vs RHE and -0.21 V vs RHE respectively. The results are discussed in detail in subsequent sections. RESULTS AND DISCUSSION Material Characterizations The fabricated Ni3Se4 nanoassemblies on Ni foam was directly subjected X-ray diffraction (XRD) studies within the 2θ range of 10° to 90° with a scan rate of 5° min-1. The obtained XRD pattern is depicted as Figure 1a here from which three major high intense peaks characteristic to metallic Ni from the Ni foam substrate are clearly seen and distinguished by labeling with hash tag symbol. Besides, the peaks corresponding to the diffraction planes of (402), (-404), (-603) and (-606) from Ni3Se4 grown on Ni foam are also seen with comparatively very low intensities. This attributed to the smaller Ni3Se4 crystallites present in the Ni3Se4 nanoassemblies which were revealed by the high resolution transmission electron microscopic (HRTEM) results (discussed below). When the Y axis range is brought down to very low intensity (Figure S1 in SI), peaks originated from other diffraction planes of Ni3Se4 nanoassemblies are also seen with considerable intensities. The peak positions of Ni3Se4 are in good agreement with the ICDD reference card number of 01-089-2020. To further confirm the same we have gently sonicated the Ni3Se4/Ni in few mL of DI water to separate and disperse some of the grown Ni3Se4 in the same. Using this suspension, a thin film was fabricated on a glass substrate and acquired the XRD pattern as shown in Figure 1b. From this figure, it is evident the Ni3Se4 phase had been formed along with some SeO2 on the surfaces. The observed SeO2 phase was matched with the ICDD reference number of 04-0430. As a primary analytical tool to confirm the elemental composition of the fabricated Ni3Se4 nanoassemblies on Ni foam, the energy dispersive X-ray spectroscopic analysis (EDS) was employed. The EDS spectrum of the fabricated Ni3Se4 nanoassemblies on Ni foam (Figure S2 in SI) found to consist of many peaks corresponding to Se, Ni and O as expected. The microstructure information on the fabricated Ni3Se4 nanoassemblies on Ni foam was obtained through a combined field emission scanning electron microscopy (FESEM), HRTEM and EDS elemental color mapping studies. The FESEM micrographs (Figure 2, a-b) are showing the surface modification of Ni foam due to the growth of Ni3Se4 nanoassemblies with increasing 7 ACS Paragon Plus Environment


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magnification. Figure 2b shows that the smooth metallic surfaces of Ni foam (Figure S3, a-b in SI) had become rough and looks hairy all over due to the growth of Ni3Se4 nanoassemblies. Figure 2, c-d are the high magnification FESEM micrograph from which the hierarchical array formed out of Ni3Se4 can be clearly seen. The grown hierarchy of Ni3Se4 array looks to have a cactus-like morphology in nature. To ensure the uniform presence of both Ni and Se on the surface of the fabricated Ni3Se4 nanoassemblies on Ni foam, EDS color mapping in FESEM mode was employed. The color maps showing the surface distribution of Ni Ka1 and Se Ka1 are provided as Figure 2, e-f. From Figure 2e and Figure 2f, the uniform distribution of both Ni and Se is most certainly witnessed. This testifies the complete and uniform growth of Ni3Se4 over Ni surfaces in Ni foam. The EDS elemental color map showing the distribution O on the surfaces of the fabricated Ni3Se4 nanoassemblies on Ni foam is provided as Figure S4 in SI. From Figure S4, the distribution of O seems to be uniform everywhere which could be due to the surface oxides and hydroxides of Ni formed over the surfaces of the Ni3Se4 nanoassemblies. Similar surface oxidation was observed in the fabrication of selenides and sulphides of Ni and also that of Fe and Co where water was present during the reaction. HRTEM micrographs of the Ni3Se4 nanoassemblies dispersed in water by sonication of the fabricated Ni3Se4 nanoassemblies on Ni foam are shown as Figure 3, a-f. Figure 3, a-c are the large area low magnification micrographs of segregated Ni3Se4 nanoassemblies acquired at various regions of the prepared TEM specimen. These figures show that observed hierarchy of Ni3Se4 nanoassemblies in FESEM micrographs are actually composed of interwoven ultrathin sheets of Ni3Se4. The inset of Figure 3b is the selected area electron diffraction (SAED) pattern of the same which has two clearly visible diffuse rings. This certainly implies that the crystallites of Ni3Se4 making up the nanoassemblies on Ni foam must be very small due to which the electron diffraction is weak. This observation is in agreement with the XRD results where we have observed very small and broadened peaks for Ni3Se4. The observed rings of the SAED pattern were calibrated and assigned to the respective diffraction planes which are (-401) and (602). This observation is also matching with the same ICDD reference card number of 01-0892020 mentioned in XRD results. Further information on the fine structure of the material was revealed by the high magnification HRTEM micrographs acquired at various places of the same specimen as shown in Figure 3, d-f. The d-spacing of the lattice fringes seen in Figure 3, d-f are carefully measured and tagged with the same and the respective diffraction plane. The most 8 ACS Paragon Plus Environment

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clearly seen planes are the (-401) and (-311) which are in good agreement with the reference ICDD card number of 01-089-2020 and with the XRD results too. The formation of Ni3Se4 nanoassemblies had almost been confirmed by the above discussed diffraction and microstructure analyses. However, it is essential to confirm the oxidation states of both Ni and Se to prove the formation Ni3Se4 which would be having Ni2+ and Ni3+ ions with the Se2-. Hence, the X-ray photoelectron spectroscopic (XPS) studies were carried out directly on fabricated Ni3Se4 nanoassemblies on Ni foam. The high resolution XPS spectrum of Ni 2p3/2 and Se 3d states are shown as Figure 4, a-b. The deconvolution of Ni 2p3/2 state had revealed that there are four different peaks which correspond to four different chemical environments around Ni. The low intense broad peak located at the lower binding energy 852.6 eV is due to the metallic Ni from the Ni foam substrate over which the Ni3Se4 was grown.49,50 Next two the metallic Ni peak, two other high intense peaks at 853.6 eV and 854.9 eV are observed which are attributed to the Ni2+ and Ni3+ ions from the Ni3Se4 nanoassemblies which is directly exposed to the X-ray unlike the substrate Ni foam. The position of nickel selenide around 852 eV to 853.5 eV is exactly matching with the earlier reports.39–42,44 This is the reason for the high intense peaks observed for Ni3Se4. Another small intense peak located at a binding energy value of 856.3 eV must be due to some nickel hydroxide formed on the surfaces of the fabricated Ni3Se4 nanoassemblies on Ni foam upon exposure to the environment. Other three peaks seen with considerable intensities beyond nickel hydroxide peak in Ni 2p3/2 state are their respective satellite peaks. All these observation are matching with earlier reports of nickel,49,50 nickel selenide39–42,44 and nickel hydroxide respectively.51–54 The high resolution XPS spectrum of Se 3d state (Figure 4b) is found to have two different sets of peaks upon deconvolution due to the presence of both Se2- and Se4+ ions as Ni3Se4 and SeO2 on the as prepared Ni3Se4/Ni surfaces. These peaks correspond to the Se2- ions are observed at 52 eV and 54.5 eV. Similarly, the peaks of SeO2 are observed at 55 eV and 56.8 eV respectively. These result have also found good agreement with the earlier report.17,19,44 The observation made from both Figure 4, a-b have certainly confirmed the formation of Ni3Se4 on Ni foam with some surface nickel hydroxide which could play key role in enhancing the OER activity of Ni3Se4 under alkaline conditions as highlighted by Xu et al. recently.44 Other than these spectra, the high resolution spectra of Se 3p and O 1s states were also acquired, deconvoluted and provided as Figure S5, a-b in SI. Similar to Se 3d spectrum, the Se 3p spectrum had also revealed the presence of both Se2- as Ni3Se4 and 9 ACS Paragon Plus Environment


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Se4+ as SeO2 as indicated in Figure S5a. The peak positions are nicely matching with the earlier reports as well. 17,19,38-43 The deconvolution of O 1s spectrum (Figure S5b) had also revealed the presence of two different O in the fabricated Ni3Se4 nanoassemblies on Ni foam which are attributed to the surface oxides and hydroxides of Ni formed upon exposure to the environment. These results are also nicely resonating with the earlier reports.44,51-54 The overall XPS analysis has proven the successful formation of Ni3Se4 on Ni foam. Thus fabricated and characterized Ni3Se4/Ni was subjected for various electrochemical characterizations in order to evaluate its electrocatalytic performance towards HER and OER in neutral and alkaline medium. The results are discussed below. Comparative Electrocatalytic Studies on Ni3Se4 Nanoassemblies on Ni Foam for Water Splitting in Neutral and Alkaline Media The electrocatalytic water splitting studies on Ni3Se4 nanoassemblies were carried out in KOH electrolyte of varying molar concentrations viz., 0.1 M, 1 M and 3 M that correspond to the pH of 13, 14 and 14.5 respectively for both HER and OER. Similarly, the Ni3Se4 nanoassemblies was also screened for HER and OER in neutral electrolyte (PBS) of pH 7 and discussed concomitantly. During OER measurements in all the above mentioned medium the-state-of-theart OER electrocatalyst RuO2 with comparable loading was studied simultaneously for comparison purposes. Similarly, for HER, a platinum foil electrode of geometrical surface area 1 cm-2 was used for comparison. The HER polarization curves obtained by acquiring LSVs at a scan rate of 5 mVs-1 in alkaline and neutral electrolytes on Ni3Se4 nanoassemblies are provided along with the HER polarization curves of bare Ni foam and Pt foil electrode as Figure 5, a-b respectively and the corresponding iR uncompensated curves for the same is provided as Figure S6a in SI. In alkaline conditions, for HER, the overpotential required by Ni3Se4 nanoassemblies at a current density of 50 mAcm-2 at pH 14.5, 14.0 and 13 are -211 mV, -208 mV and -220 mV vs RHE respectively. At the same current density (50 mAcm-2) the state-of-the-art HER electrocatalyst Pt required an overpotential of -220 mV vs RHE at a pH of 14.5. However, it should not be forgotten that at lower current densities the overpotential required by Pt is relatively smaller than that of Ni3Se4. On the other hand, the bare Ni foam has no significant contribution to the catalytic current. These observations indicate that though Ni3Se4 nanoassemblies required little higher overpotential than the noble Pt at lower current densities, it had overcome the catalytic activity of Pt at higher overpotential region which indirectly implies 10 ACS Paragon Plus Environment

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the facile HER kinetics on Ni3Se4 nanoassemblies at higher overpotentials in alkaline solutions. Similarly, the activity in neutral electrolyte (PBS) was following the same trend in activity (Figure 5b) and the corresponding iR uncompensated curves for the same is provided as Figure S6b in SI. Though the Ni3Se4 nanoassemblies had little higher onset overpotential (-50 mV vs RHE) from that of Pt, it had overcome the HER activity of Pt at high overpotential region in neutral medium. For a defined current density of 50 mAcm-2, the overpotential required by both Ni3Se4 nanoassemblies and Pt are -282 mV and -343 mV vs RHE. The observed overpotentials with Ni3Se4 nanoassemblies in alkaline electrolytes are better than the other existing reports on similar materials. However, such a comparison for the observed HER performance in neutral solution could not be made due to the unavailability any relevant earlier report. The Tafel analysis on the HER electrocatalytic activity on Ni3Se4 nanoassemblies, Ni foam and Pt in alkaline and neutral solutions are provided as Figure 5, c-d. The HER Tafel plots of Ni3Se4 nanoassemblies, Ni foam and Pt acquired in alkaline electrolyte (Figure 5c) is showing the respective Tafel slopes. The Tafel slopes of Ni3Se4 nanoassemblies at pH 14.5, 14.0 and 13.0 are 128 mVdec-1, 156 mVdec-1 and 165 mVdec-1 respectively. This trend in the Tafel slope indicates that the HER kinetics on Ni3Se4 nanoassemblies is comparatively more facile in high alkaline condition and when the alkalinity of the electrolyte brought down, the HER kinetics also decreased regularly. Besides, the HER Tafel slopes of Pt and Ni foam in high alkaline condition (pH = 14.5) are 297mVdec-1 and 178 mVdec-1 respectively. This information implies the poor HER kinetics on Pt surfaces in high alkaline conditions and reveals the superior HER activity of Ni3Se4 nanoassemblies in alkaline electrolytes. The HER Tafel slopes of Ni3Se4 nanoassemblies and Pt acquired in PBS electrolyte are 101 mVdec-1 and 178 mVdec-1 respectively. This is in sharp contrast to the overpotential trend observed with the respective LSVs and implies that though HER is thermodynamically favored (reflected by lower overpotentials) on Pt surfaces, the kinetics seems to be more facile on the surfaces of Ni3Se4 nanoassemblies. The HER polarization analysis and Tafel analysis have revealed the better HER activity on Ni3Se4 nanoassemblies surfaces in alkaline electrolytes and better HER kinetics in both neutral and alkaline electrolytes. The OER polarization curves (LSVs) were obtained at a scan rate of 5 mVs-1 in alkaline and neutral electrolytes of the same pH used for the measurements of HER polarization curves on Ni3Se4 nanoassemblies, Ni foam and RuO2 loaded Ni foam are provided as Figure 6, a-b and the corresponding iR uncompensated curves for the same are provided as Figure S7, a-b in SI. 11 ACS Paragon Plus Environment


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The overpotential required by Ni3Se4 nanoassemblies in alkaline electrolytes of pH 14.5, 14.0 and 13.0 at a current density of 50 mAcm-2 are 232 mV, 244 mV and 321 mV vs RHE respectively. The bare Ni foam did not reach the benchmarking current density of 50 mAcm-2 within the selected experimental window. On the other hand, the RuO2 with comparable loading supported on Ni foam required a very high overpotential of 407 mV at the current density of 50 mAcm-2. These observations certainly indicate that the OER is thermodynamically highly favored on Ni3Se4 nanoassemblies surfaced than either on Ni foam or RuO2 loaded Ni foam alone. In PBS electrolyte, the Ni3Se4 nanoassemblies had only shown considerable activity compared to Ni foam and RuO2 loaded Ni foam. The overpotential required by Ni3Se4 nanoassemblies at a current density of 10 mAcm-2 is 480 mV which is significant in neutral conditions. This observation certainly indicates that Ni3Se4 nanoassemblies are far better OER catalysts in neutral medium than RuO2. The corresponding OER Tafel slopes of Ni3Se4 nanoassemblies in alkaline electrolytes are provided in comparison with the RuO2 loaded Ni foam and Ni foam alone as Figure 6c. The observed Tafel slopes for Ni3Se4 nanoassemblies at pH 14.5, 14.0 and 13.0 are 33 mVdec-1, 30 mVdec-1 and 40 mVdec-1 respectively. This shows the facile OER kinetics on Ni3Se4 nanoassemblies surfaces. Besides, the Tafel slopes of RuO2 loaded Ni foam and Ni foam are 132 mVdec-1 and 228 mVdec-1 respectively which again implies that the OER kinetics is far more facile on Ni3Se4 nanoassemblies surfaces than the state-of-the-art OER electrocatalysts RuO2 and bare Ni foam under such high alkaline conditions. The Tafel slopes of Ni3Se4 nanoassemblies, RuO2 loaded Ni foam and bare Ni foam acquired in PBS electrolyte are 116 mVdec-1, 157 mVdec-1 and 581 mVdec-1 respectively. This is in agreement with the LSV results of the same. The OER kinetics on Ni3Se4 nanoassemblies is better than that of RuO2 loaded Ni foam and bare Ni foam in neutral medium. The overall comparative polarization and Tafel analyses have certainly proven the superior OER and HER activities in alkaline and neutral electrolytes with facile kinetics than the commercially used state-of-the-art HER (Pt) and OER (RuO2) catalysts. The observed trends in both HER and OER were again ascertained from the corresponding Nyquist plots (Figure S8 in SI). The Nyquist plots have clearly given the picture on the nature of the interfaces towards electron transfer. The bare Pt foil electrode had shown the lower charge transfer resistance (RCT) of 40 ohm among other studied interfaces. Similarly, the bare Ni foam had shown a lower RCT of 50 ohm. The RCT values of Ni3Se4 at pH 13, 14, 14.5 and that of RuO2/Ni at a pH of 14.5 are 70 ohm, 65 ohm, 60 ohm and 12 ACS Paragon Plus Environment

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53 ohm respectively. Though the RuO2/Ni had lower RCT than our catalyst even with the same mass loading, it had failed in competing with our catalyst under identical conditions. The reason behind this was revealed by the evaluation of the corresponding double layer capacitance (Cdl) which is directly proportional to the electrochemically active surface area (ECSA) of the interface. Similar, study was included as an evaluation parameter of understanding the activity trends of various catalysts earlier.23-26 The difference in the anodic and cathodic double layer charging currents in the regions closer to HER (0.12 V vs. RHE) and OER (1.12 V vs. RHE) were plotted against the scan rates from which the Cdl was determined from the slop of the data plot for each catalyst as shown in Figure S9, a-b (SI). The increased Cdl in case of Ni3Se4 assemblies on Ni foam in the regions nearer to both HER and OER (Table S1, in SI) have clearly shown the reason for enhanced catalytic performance of the same. Moreover, the ECSA normalized overpotentials from the respective polarization curves (Figure S10, a-d in SI) were also calculated and included in the Table 1 for comparison. The stability of electrocatalytic nanoassemblies must be evaluated to emphasize its advantages and possibilities to applying for large scale and prolonged water electrolysis. The most accepted two methods of assessing the stability of a nanoassemblies electrocatalytic electrode in water splitting are the accelerated degradation (AD) test by CV experiment and the galvanostatic or potentiostatic water electrolysis for hundreds of cycles and several hundred minutes respectively.6 Here, we have assessed the stability of Ni3Se4 nanoassemblies for both OER and HER by AD test of 1000 cycles at a scan rate of 200 mVs-1 in alkaline and neutral electrolytes at the end of which the increase in overpotential at a fixed current density was taken as the parameter to asses. The LSVs acquired on Ni3Se4 nanoassemblies, Ni foam and Pt for HER before and after AD test in alkaline and neutral electrolytes are shown as Figure 7, a-b. The cathodic shifts in HER overpotential at a current density of 50 mAcm-2 at pH 14.5, 14.0, 13.0 and 7.0 on Ni3Se4 nanoassemblies are 25 mV, 24 mV, 26 mV and 160 mV after 1000 cycles respectively. This indicates the superior cycling stability of Ni3Se4 nanoassemblies toward HER in alkaline electrolytes and a moderate stability in PBS. Similarly, the LSVs acquired on Ni 3Se4 nanoassemblies, Ni foam and RuO2 loaded Ni foam for OER before and after AD test in alkaline and neutral electrolytes are shown as Figure 7, c-d. The anodic shifts in OER overpotential at a current density of 50 mAcm-2 at pH 14.5, 14.0 and 13.0 on Ni3Se4 nanoassemblies are 20 mV, 24 mV and 28 mV after 1000 cycles respectively. Similarly, the measured anodic shift in OER 13 ACS Paragon Plus Environment


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overpotential at a current density of 10 mAcm-2 in PBS electrolyte is 44 mV. These results indicate the high stability upon cycling in both alkaline and neutral electrolytes. The stability test upon prolonged potentiostatic electrolysis for both HER and HER was carried out in high alkaline condition (pH = 14.5) where the Ni3Se4 nanoassemblies had shown better activity compared to other electrolytic medium. The j vs. t plots for both HER and OER on Ni3Se4 nanoassemblies surface for 1000 min in alkaline and neutral conditions are shown as Figure 8, ab. Both HER and OER chronoamperometric curves showed the excellent stability of Ni3Se4 nanoassemblies upon potentiostatic electrolysis without any further reduction in the current density. Interestingly, the HER chronoamperometric curves have shown a steady increase in both alkaline and neutral conditions. This could be due to the increase in the ECSA by the reduction of SeO2 to Se2- which could in turn combine with the Ni2+ to form the NiSe active sites. The same was evidenced with post-HER XPS analysis which is discussed in the subsequent sections. The overall stability tests carried out on Ni3Se4 nanoassemblies in both alkaline and neutral electrolytes have testified the suitability of the same for large scale, prolonged water electrolysis with minimum energy loss. Total Water Splitting with Ni3Se4 Assemblies on Ni Foam The Ni3Se4 nanoassemblies had been found to be showing excellent bi-functional catalytic activity toward water splitting in alkaline electrolyte. The same was witnessed once again by constructing a simple two electrode water electrolyser filled with 1 M KOH where the Ni3Se4 nanoassemblies were employed as both anode (OER electrode) and cathode (HER electrode). The two electrode water electrolyser was powered by a AAA alkaline battery which has the ability of delivering a constant optimum potential of 1.5 V. The snapshot captured during the operation of the constructed two electrode water electrolyser is provided as Figure S11, in SI. The gas bubbles evolved out of Ni3Se4 nanoassemblies/Ni foam electrode are clearly visible from Figure S6. This simple test had proven the ability of our Ni3Se4 nanoassemblies for sustained total water splitting with a very low potential window of 1.5 V. Further, the polarization curves of Ni3Se4/Ni || Ni3Se4/Ni in 3M KOH was obtained where the cathode and the anode were the same Ni3Se4 assemblies on Ni Foam and provided as Figure 9a. This assembly required the combined oxygen evolution overpotential of 382 mV at a current density of 50 mAcm-2. This is a highly significant merit for a non-noble metal catalyst based electrolyser. Similarly, the endurance of this two electrode assembly was assessed by subjecting 14 ACS Paragon Plus Environment

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the same for continuous galvanostatic electrolysis that extended into several hours at a current density of 50 mAcm-2. The same is pictured here as Figure 9b. From Figure 9b, it is clear that the two electrode assembly made up of our catalyst have shown consistent performance by requiring 1.66 V vs. RHE to deliver the fixed current density of 50 mAcm-2 and also found good agreement with the polarization curve (iR uncompensated) as seen in Figure 9a. These experiments have once again proved the ability of Ni3Se4 assemblies as an efficient bi-functional water splitting catalyst. The results of the overall comparative electrocatalytic studies are provided comprehensively in Table 1. Moreover, we have also made a detail survey on the existing nickel selenide reports in comparison with our Ni3Se4 nanoassemblies and found our catalysts is superior in showing better electrocatalytic performance in alkaline medium and the same is shown in Table 2.12-28,39-44 From Table 1 and Table 2, it is perceivable that our Ni3Se4 nanoassemblies is a better catalyst than many of the reported ones. The overall comparative electrocatalytic studies have implied the superiority of our Ni3Se4 nanoassemblies for efficient and durable bulk electrolysis of water.

Post-electrochemical Characterizations on Ni3Se4 Assemblies on Ni Foam It is necessary to assess the catalysts robustness after subjecting the material for a prolonged electrochemical treatment which could either be the cycling or the galvanostatic or potentiostatic electrolysis test. To get useful information on the changes occurred on the catalytically active sites after OER and HER at high alkaline conditions (where our catalyst had shown better performance than in neutral medium), we have subjected the Ni3Se4 assemblies on Ni foam for a complete reverse characterizations set that includes XRD, LASER-Raman, FESEM, HRTEM and XPS techniques. As a primary tool to detect the changes occurred on the catalyst surface the XRD pattern and Raman spectrum were obtained on Ni3Se4/Ni subjected to both HER and OER. The XRD pattern acquired after HER and OER characterizations are shown as Figure S12, a-b in SI from which as an initial indication formation of NiO after both HER and OER and NiSeO3 after OER can be observed. The signal to noise ratio is very low due to the predominant metallic peaks of Ni from the Ni foam substrate which is removed in order to show other peaks at least in the observable intensities. The formation of NiO after cathodic polarization is due to the displacement of Se by OH- at high alkaline medium and its subsequent conversion to NiO. Similarly, after anodic polarization, NiO was observed along with the 15 ACS Paragon Plus Environment


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NiSeO3. The formation of NiO and NiSeO3 here is mainly attributed to the applied anodic oxidizing potential which is so obvious at such high overpotential. However, it is hard to conclude such formation of NiO and NiSeO3 only with the XRD patterns that had shown very weak signals. Hence, we moved further to analyze the Raman spectra of Ni3Se4/Ni as such and after subjecting for OER and HER characterizations. The acquired spectra are given here as Figure 10. Figure 10 has three Raman spectral features in which (a) is of the Ni 3Se4/Ni before electrochemical characterizations, (b) is of Ni3Se4/Ni after anodic polarization and (c) is of Ni3Se4/Ni after cathodic polarization. The Raman spectrum of Ni3Se4/Ni before electrochemical treatment has only three notable peaks at 443 cm-1, 238 cm-1 and 139 cm-1 in the regions where the metal selenides and sulphides are reported to show peaks and indicating the dominant presence of Ni3Se4 on the surface.55,56 After anodic polarization the Ni3Se4/Ni have shown peaks for the presence of NiO at 813 cm-1 and 1575 cm-1 along with two broadened peaks for the presence of NiSeO3 at 1360 cm-1 and 1222 cm-1. The observed peak positions of NiO are well matching with the earlier reports.57,58 Unfortunately, there is no data available on the Raman spectrum of NiSeO3. Hence, we have compared it with other metal selenates and found good agreement.59 The formation of NiO and NiSeO3 after anodic polarization as predicted from the corresponding XRD pattern is further supported here by the above result. Similarly, the Raman spectrum of Ni3Se4/Ni after cathodic polarization have shown an addition peak at 813 cm-1 characteristic to NiO in addition with the peaks of Ni3Se4 and is in agreement with the corresponding XRD pattern. Both XRD and Raman analyses have revealed significant surface functionality changes. However, it is essential to note also that the basic Ni3Se4 phase also coexist along with the new phases formed and not completely degraded. With this view, the morphological robustness of Ni3Se4/Ni was analyzed by FESEM, HRTEM and EDX color mapping subsequently. For FESEM, the Ni3Se4/Ni was directly taken. The electronic micrographs of FESEM and HRTEM are provided here as Figure 11, a-i. Figure 11, a-c are the FESEM micrographs that show the complete morphological retention even after some chemical modification on the surfaces. Figure 11a is the low magnification large area FESEM micrograph that show similar morphological look observed before electrochemical characterizations. Figure 11b and Figure 11b are the high magnification FESEM micrographs of Ni3Se4/Ni after anodic and cathodic polarizations respectively. From both of these figures, almost a complete retention in morphology can be witnessed with highly negligible degradations. 16 ACS Paragon Plus Environment

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This indicates the catalysts robustness even after such harsh electrochemical treatment. Similarly, the fine structures and morphologies of Ni3Se4 sheets that make up the Ni3Se4 assembly on Ni foam after cathodic and anodic polarizations are witnessed from their respective HRTEM micrographs and the SAED patterns as shown in Figure 11, d-i. From both Figure 11d and Figure 11g, it can be seen that after cathodic and anodic polarizations, the scrambled sheet like morphology was not affected to any notable extent and the same is testifying the morphological robustness of Ni3Se4/Ni once again here. The fine structures (Figure 11e and Figure 11h) of these Ni3Se4 sheets after cathodic and anodic polarization are found to have the fringes that are mostly of Ni3Se4 phase and fringes of NiO and NiSeO3 can hardly be detected. However, the presences of these two phases along with Ni3Se4 after anodic polarization and NiO alone with Ni3Se4 after cathodic polarization were witnessed from their respective SAED patterns (Figure 11i and Figure 11f). The presence of dots that are calibrated and assigned to NiO in Figure 11f along with the two rings characteristic to Ni3Se4 phase indicates the significant formation NiO after cathodic polarization. Similarly, the presence of dot patterns related to NiO and NiSeO3 along with the ring patterns of Ni3Se4 phase were observed in Figure 11i. The results of SAED patterns are in agreement with the XRD and Raman results as well. Further, the EDAX elemental color mapping with the respective EDAX spectrum and HAADF micrograph of Ni3Se4 nanosheets after anodic and cathodic polarizations are given as Figure S13, a-e and Figure S14, a-e respectively in SI. As expected, the presence of Ni, Se, O are predominantly observed. This observation also resonates well with the other characterization results. More importantly, we have not seen any trace level Fe in both the EDAX spectra. This primarily indicates that there was no Fe impurity assisted enhancement in the observed OER activity of Ni3Se4/Ni. The above combined microscopic and electronic diffraction studies have once again proven the morphological robustness of Ni3Se4/Ni and indicated the surface chemical changes and found good agreement with the XRD and Raman analyses. Though the surface changes have been observed to occur after anodic and cathodic polarizations on Ni3Se4/Ni by the above techniques, it is essential to look into the chemical nature of all the elements mainly Ni, Se and O by XPS analysis to get more useful information. The XPS high resolution spectra of Ni 2p3/2, Se 3d, Se 3p and O 1s after cathodic and anodic polarizations are provided here as Figure 12, a-h in which Figure 12, a-d are the Ni 2p3/2, Se 3d, Se 3p and O 1s spectra after cathodic polarization and Figure 12, e-h are the Ni 2p3/2, Se 3d, Se 17 ACS Paragon Plus Environment


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3p and O 1s spectra after anodic polarization. The Ni 2p3/2 spectrum (Figure 12a) taken after cathodic polarization have three peaks that are corresponding to the Ni3Se4 and NiO along with their satellite peak and in accordance with the earlier results of XRD, Raman, FESEM, HRTEM and SAED results. The absence of Ni peak may be due to the high thickness of the catalyst materials in the area of the material chosen for the analysis. The Ni 2p3/2 spectrum (Figure 12e) taken after anodic polarization have four peaks that are corresponding to the Ni3Se4, NiO and the metallic Ni along with their satellite peak and this also found good agreement with the earlier results of XRD, Raman, FESEM, HRTEM and SAED results and earlier reports.38–41,43 The Se 3d spectrum acquired after cathodic polarization (Figure 12b) differs significantly from the one that was acquired after anodic polarization (Figure 12f). The presence of only Se2- as seen in Figure 12b indicates the reduction of Se4+ ions that were present before the cathodic polarization to Se2-. This could be the key reason for the observed increasing trend in the current densities while subjecting the Ni3Se4/Ni for chronoamperometric studies in both alkaline and neutral media. On the other hand, the Se 3d spectrum acquired after anodic polarization retained its features as observed before electrochemical characterization and indicates the significant presence of Se4+ ions which is another direct proof for the formation of NiSeO3 on Ni3Se4/Ni surface after anodic polarization. These results are also in agreement with the other prior characterizations done after electrochemical characterizations and earlier reports.38–43 Similar trends were obsevred with the Se 3p and O 1s spectra acquired after cathodic and anodic polarizations respectively and the result are in agreement with the earlier reports as well. 38-43,50-53 The overall reverse characterizations on Ni3Se4/Ni after electrochemical characterization have given a clear insight on the chemical changes occurred on the catalyst’s surface along with the morphological robustness of Ni3Se4/Ni. Having such an exceptional electrochemical performance in addition with extraordinary stability, our catalyst had assured its place instead of noble metal and metal oxides (Pt, IrO2 and RuO2) as an alternate, abundant and efficient electrode material for large scale, durable water electrolysis with minimum energy loss.

Conclusion A hierarchical 3D assembly of Ni3Se4 was fabricated on Ni foam in a comparatively easier methodology which utilized a simple microwave initiation for first 3 min and 5 h of aging at RT where the NaHSe solution derived by dissolving NaBH4 and Se metal powder was used as 18 ACS Paragon Plus Environment

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Se source. The above proposed method is faster, handy to conduct and does not require sophisticated instruments and avoids high temperature and pressure reactions. Thus fabricated Ni3Se4 nanoassemblies have shown excellent HER and OER activities in neutral and alkaline electrolytes and overcome the state-of-the-art Pt (for HER) and RuO2 (for OER) under identical experimental conditions. The observed very low overpotentials at a defined current density of 50 mAcm-2 for HER and OER in neutral and alkaline medium along with the lower Tafel slopes have testified the superior electrocatalytic bi-functional water splitting behavior of Ni3Se4 nanoassemblies. These results mainly imply the better HER and OER kinetics and the less thermodynamic barrier. With these prolific and beneficial performance, the Ni3Se4 nanoassemblies have proven itself to be an efficient, cheaper and durable electrocatalyst for total water splitting in neutral and alkaline medium. Moreover, the proposed synthesis method can also be opted for the facile formation of other metal chalcogenides on suitable substrates. ASSOCIATED CONTENT Supporting Information (SI) Available Information on the materials, technical details on the instruments used, sample preparation methods and figures related XRD, EDS, EDS mapping of O, high resolution XPS spectra for Se 3p and O 1s states, iR uncompensated polarization curves, Nyquist plots, plot of double layer charging currents vs. scan rates, ECSA normalized polarization curves, Table S1, XRD patterns and the EDAX color mapping results after HER and OER characterizations along with the optical image that shows the bi-functional water splitting activity of Ni3Se4 nanoassemblies in the two electrode water electrolyser are provided. This materials is available at free of cost at http://www.pubs.acs.org/ ACKNOWLEDGEMENTS We are grateful to the support and encouragement extended by The Director, Dr. Vijayamohanan K Pillai. S.A acknowledges CSIR-New Delhi for the Senior Research Fellowship (SRF). Authors thankfully acknowledge the support from Dr. B. Subramanian, Sr. Scientist, ECMS-Division, V. Prabhu (FESEM), A. Rathishkumar (TEM), P. Nagesh Reddy (HRTEM) and other faculties of CIF-CECRI, Authors also acknowledge the help during synthesis and characterization from M. Venkatesh, T. V. S. V. Simha and Ashish S. Salunke of CFE-CECRI, Karaikudi, India. 19 ACS Paragon Plus Environment


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Rhenium Sulfide on Free-Standing Three-Dimensional Electrodes Is Highly Catalytic for the Hydrogen Evolution Reaction: Experimental and Theoretical Study. Electrochem. commun. 2016, 63, 39–43. (31)

Subbaraman, R.; Tripkovic, D.; Chang, K.-C.; Strmcnik, D.; Paulikas, A. P.; Hirunsit, P.; Chan, M.; Greeley, J.; Stamenkovic, V.; Markovic, N. M. Trends in Activity for the Water Electrolyser Reactions on 3d M(Ni,Co,Fe,Mn) Hydr(oxy)oxide Catalysts. Nat. Mater. 2012, 11, 550–557.

(32)

Xiao, P.; Chen, W.; Wang, X. A Review of Phosphide-Based Materials for Electrocatalytic Hydrogen Evolution. Adv. Energy Mater. 2015, 5, 1–13.

(33)

Carim, A. I.; Saadi, F. H.; Soriaga, M. P.; Lewis, N. S. Electrocatalysis of the HydrogenEvolution Reaction by Electrodeposited Amorphous Cobalt Selenide Films. J. Mater. Chem. A 2014, 2, 13835.

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Xu, X.; Du, P.; Chen, Z.; Huang, M. An Electrodeposited Cobalt–selenide-Based Film as an Efficient Bifunctional Electrocatalyst for Full Water Splitting. J. Mater. Chem. A 2016, 4, 10933–10939.

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Liao, M.; Zeng, G.; Luo, T.; Jin, Z.; Wang, Y.; Kou, X.; Xiao, D. Three-Dimensional Coral-like Cobalt Selenide as an Advanced Electrocatalyst for Highly Efficient Oxygen Evolution Reaction. Electrochim. Acta 2016, 194, 59-66.

(36)

Hou, Y.; Lohe, M. R.; Zhang, J.; Liu, S.; Zhuang, X.; Feng, X. Vertically Oriented Cobalt selenide/NiFe Layered-Double-Hydroxide Nanosheets Supported on Exfoliated Graphene Foil: An Efficient 3D Electrode for Overall Water Splitting. Energy Environ. Sci. 2016, 9, 478–483.

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Xiao, M.; Miao, Y.; Tian, Y.; Yan, Y. Synthesizing Nanoparticles of Co-P-Se Compounds


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as Electrocatalysts for the Hydrogen Evolution Reaction. Electrochim. Acta 2015, 165, 206–210. (38)

Zhang, H.; Yang, B.; Wu, X.; Li, Z.; Lei, L.; Zhang, X. Polymorphic CoSe2 with Mixed Orthorhombic and Cubic Phases for Highly Efficient Hydrogen Evolution Reaction. ACS Appl. Mater. Interfaces 2015, 7, 1772–1779.

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Wang, Z.; Li, J.; Tian, X.; Wang, X.; Yu, Y.; Owusu, K. A.; He, L.; Mai, L. Porous Nickel–Iron Selenide Nanosheets as Highly Efficient Electrocatalysts for Oxygen Evolution Reaction. ACS Appl. Mater. Interfaces 2016, 8, 19386–19392.

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Tang, C.; Cheng, N.; Pu, Z.; Xing, W.; Sun, X. NiSe Nanowire Film Supported on Nickel Foam: An Efficient and Stable 3D Bifunctional Electrode for Full Water Splitting. Angew. Chem., Int. Ed. 2015, 54, 9351–9355.

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Swesi, A. T.; Masud, J.; Nath, M. Nickel Selenide as a High-Efficiency Catalyst for Oxygen Evolution Reaction. Energy Environ. Sci. 2016, 9. 1771–1782.

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Ming, F.; Liang, H.; Shi, H.; Xu, X.; Mei, G.; Wang, Z. MOF-Derived Co-Doped Nickel selenide/C Electrocatalysts Supported on Ni Foam for Overall Water Splitting. J. Mater. Chem. A 2016, 4, 15148–15155.

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Liu, T.; Asiri, A. M.; Sun, X. Electrodeposited Co-Doped NiSe2 Nanoparticles Film: A Good Electrocatalyst for Efficient Water Splitting. Nanoscale 2016, 8, 3911–3915.

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Xu, X.; Song, F.; Hu, X. A Nickel Iron Diselenide-Derived Efficient Oxygen-Evolution Catalyst. Nat. Commun. 2016, 7, 12324–12330.

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Cheng, Y.; Jiang, S. P. Advances in Electrocatalysts for Oxygen Evolution Reaction of Water Electrolysis-from Metal Oxides to Carbon Nanotubes. Prog. Nat. Sci. Mater. Int. 2015, 25, 545–553.



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Louie, M. W.; Bell, A. T. An Investigation of Thin-Film Ni−Fe Oxide Catalysts for the Electrochemical Evolution of Oxygen. J. Am. Chem. Soc. 2013, 135, 12329–12337.

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Chemelewski, W. D.; Lee, H.-C.; Lin, J. F.; Bard, A. J.; Mullins, C. B. Amorphous FeOOH Oxygen Evolution Reaction Catalyst for Photoelectrochemical Water Splitting. J. Am. Chem. Soc. 2014, 136, 2843–2850.

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Kamnev, A. A.; Ezhov, B. B. Electrocatalysis of Anodic Oxygen Evolution at the Nickel Hydroxide Electrode by Ferric Hydroxo Species in Alkaline Electrolytes. Electrochim. Acta 1992, 37, 607–613.

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Catalytic System for Methanol Oxidation in Alkaline Solution. J. New Mater.


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Su, C.; Suarez, D. L. Selenate and Selenite Sorption on Iron Oxides: An Infrared and Electrophoretic Study. Soil Sci. Soc. Am. J., 2000, 64, 101-111.



a

#

3000

# - Ni foam $ - Ni3Se4

Intensity (a.u)

2500 2000 #

1500 1000

#

500 $

$

0 10

20

30

40

50

60

70

80

90

2(degree)

b

140

SeO2 PDF #04-0430 Ni3Se4 PDF #89-2020

120

Obsevred pattern 100

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 28 of 44

80 60 40 20 0 10

20

30

40

50

60

70

80

90

2(degree)

Figure 1: (a) XRD pattern of as prepared Ni3Se4/Ni. (b) XRD pattern of Ni3Se4 thin fil fabricated on a glass substrate.


Page 29 of 44

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


(b)

(a)

20 µm

100 µm (c)

(d)

1 µm

500 nm (f)

(e)

Ni Ka1

Se Ka1

20 µm

20 µm

Figure 2: (a-b) Low magnification FESEM micrographs of Ni3Se4 nanoassemblies on Ni foam. (c-d) High magnification FESEM micrographs of Ni3Se4 nanoassemblies on Ni foam. (e-f) EDS colour mapping of Ni Ka1 and Se Ka1 acquired exactly at the same region shown in Figure 2b.



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

(a)

Page 30 of 44

(c)

(b)

(-401) (-602)

200 nm (d)

(f) 2d = 0.404 nm

(e) 3d = 0.687 nm (-311) 2d = 0.404 nm (-401)

(-401)

d = 0.202 nm (-401) d = 0.202 nm (-401)

2d = 0.502 nm (-401)

d = 0.23 nm (-311)

Figure 3: (a-c) Low magnification HRTEM micrographs of Ni3Se4 nanoassemblies on Ni foam. Inset of (b) is the SAED pattern of Ni3Se4 nanoassemblies on Ni foam. (d-f) High magnification HRTEM micrographs of Ni3Se4 nanoassemblies on Ni foam.


Page 31 of 44

a

94000 92000

Ni 2p3/2

Ni

90000 Sat

cps

88000

3+

Ni

Ni(OH)2

2+

86000 84000 Ni

82000

0

80000 78000 864 862 860 858 856 854 852 850 848

Binding energy (eV) b 6000

Se 3d 4+

Se 5500

2-

Se

5000

cps

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


4500

4000

3500

3000 58

57

56

55

54

53

52

51

50

Binding energy (eV) Figure 4: (a) High resolution Ni 2p3/2 state XPS spectrum of Ni3Se4 nanoassemblies on Ni foam. (b) High resolution Se 3d state XPS spectrum of Ni3Se4 nanoassemblies on Ni foam.



a

b 0

0

Ni-foam Ni3Se4/Ni-foam

-15

-20

Pt foil

-220 mV

-206 mV

-60

-2

-2

-40

j (mAcm )

-220 mV

j (mAcm )

-211 mV Ni-foam @ pH 14.5 Ni3Se4/Ni-foam @ pH 13

-80

Ni3Se4/Ni-foam @ pH 14

-30 -343 mV -45 -282 mV

-60

Ni3Se4/Ni-foam @ pH 14.5

-100

-75

Pt foil @ pH 14.5

-0.4

-0.3

-0.2

-0.1

0.0

-0.3

-0.2

E / V vs RHE

d -0.5

Ni-foam @ pH 14.5 Ni3Se4/Ni-foam @ pH 13

Overpotnetial () / V

-0.3

Ni3Se4/Ni-foam @ pH 14.5 Pt wire @ pH 14.5

156 mV/dec

178 mV/dec

-0.2

128 mV/dec 165 mV/dec

-0.1

-0.4 Ni3Se4/Ni-foam


-0.4

-0.1

Pt foil -0.3 101 mVdec

-1

-0.2

-0.1 178 mVdec

2.5

2.0

1.5

1.0

-1

pH = 7

297 mV/dec

3.0

pH = 7 0.0

E / V vs RHE

c Overpotential () / V

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 32 of 44

0.5

0.0 0.50

0.75

1.00

1.25 -2

log j (mAcm )

-2

log j (mAcm )

Figure 5: (a) HER LSVs acquired at scan rate of 5 mVs-1 on Ni3Se4 nanoassemblies on Ni foam, Ni foam and Pt foil surfaces in alkaline electrolytes of mentioned pH. (b) HER LSVs acquired at scan rate of 5 mVs-1 on Ni3Se4 nanoassemblies on Ni foam, Ni foam and Pt foil surfaces in neutral PBS electrolyte. (c) The corresponding Tafel plots for Ni3Se4 nanoassemblies on Ni foam, Ni foam and Pt foil surfaces in alkaline electrolytes of mentioned pH. (d) The corresponding Tafel plots for Ni3Se4 nanoassemblies on Ni foam and Pt foil surfaces in neutral PBS electrolyte. 32 ACS Paragon Plus Environment

Page 33 of 44

a

-2

j (mAcm )

125 100

14

Ni-foam @ pH 14.5 Ni3Se4/Ni-foam @ pH 13

RuO2/Ni-foam @ pH 14.5 321 mV

244 mV

50 232 mV

25 0 1.1

RuO2@Ni foam

10

Ni3Se4/Ni-foam @ pH 14.5

75

Ni foam Ni3Se4@Ni foam

12

Ni3Se4/Ni-foam @ pH 14 -2

150

b

j (mAcm )

175

407 mV

8

-2

@10 mAcm = 480 mV 6 4 2

pH = 7 1.2

1.3

1.4

1.5

1.6

0 1.2

1.7

E / V vs RHE 0.5

d

Ni-foam @ pH 14.5 Ni3Se4/Ni-foam @ pH 13 Ni3Se4/Ni-foam @ pH 14.5 RuO2/Ni-foam @ pH 14.5 40 mV/dec

228 mV/dec

0.3 30 mV/dec

132 mV/dec

1.4

1.5

1.6

1.7

1.8

0.2

RuO2@Ni foam

1.0

2.0

157 mV/dec

0.4 116 mV/dec 581 mV/dec 0.2

33 mV/dec

0.5

1.9

0.6 Ni foam Ni3Se4@Ni foam


0.4

1.3

E / V vs RHE

Overpotential () / V

c Overpotential () / V

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


1.5

0.0

2.0

pH = 7 -2

-2

-1

0

1

-2

log j (mAcm )

log j (mAcm )

Figure 6: (a) OER LSVs acquired at scan rate of 5 mVs-1 on Ni3Se4 nanoassemblies on Ni foam, Ni foam and RuO2 loaded Ni foam surfaces in alkaline electrolytes of mentioned pH. (b) OER LSVs acquired at scan rate of 5 mVs-1 on Ni3Se4 nanoassemblies on Ni foam, Ni foam and RuO2 loaded Ni foam surfaces in neutral PBS electrolyte. (c) The corresponding Tafel plots for Ni3Se4 nanoassemblies on Ni foam, Ni foam and RuO2 loaded Ni foam surfaces in alkaline electrolytes of mentioned pH. (d) The corresponding Tafel plots for Ni3Se4 nanoassemblies on Ni foam, Ni foam and RuO2 loaded Ni foam surfaces in neutral PBS electrolyte. 33 ACS Paragon Plus Environment


a

b 0

0

-2

-20

-50

j / mAcm

j / mAcm

-2

-25

Ni3Se4/Ni foam @ pH=13

-75

Ni3Se4/Ni foam @ pH=13 after AD Ni3Se4/Ni foam @ pH=14

-40

-60

Ni3Se4/Ni foam @ pH=14 after AD

-100

Ni3Se4/Ni foam @ pH = 7

Ni3Se4/Ni foam @ pH=14.5

Ni3Se4/Ni foam @ pH = 7 after AD

Ni3Se4/Ni foam @ pH=14.5 after AD

-0.4

-0.3

-0.2

-0.1

-80 -0.4

0.0

-0.3

E / V vs RHE

-0.1

0.0

d

100 75

Ni3Se4/Ni foam @ pH=13 after AD

12

Ni3Se4/Ni foam @ pH=14 Ni3Se4/Ni foam @ pH=14 after AD Ni3Se4/Ni foam @ pH=14.5 after AD

50 25 0 1.1

Ni3Se4/Ni foam @ pH = 7 Ni3Se4/Ni foam @ pH = 7 after AD

10

Ni3Se4/Ni foam @ pH=14.5

-2

125

14

Ni3Se4/Ni foam @ pH=13

j / mAcm

150

-2

-0.2

E / V vs RHE

c

j / mAcm

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 34 of 44

8 6 4 2

1.2

1.3

1.4

1.5

0 1.2

1.6

1.3

1.4

1.5

1.6

1.7

1.8

E / V vs RHE

E / V vs RHE

Figure 7: (a-b) HER LSVs acquired before and after AD test of CV cycle on Ni3Se4 nanoassemblies on Ni foam surfaces in alkaline and neutral PBS electrolytes respectively. (c-d) OER LSVs acquired before and after AD test of CV cycle on Ni3Se4 nanoassemblies on Ni foam surfaces in alkaline and neutral PBS electrolytes respectively.


Page 35 of 44

0

200

400

600

800

45

1000

(d)

Ni3Se4/Ni at 1.73 V vs. RHE

30 15

pH = 7

0 60

(c)

Ni3Se4/Ni at 1.46 V vs. RHE

-2

50

j / mAcm

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


pH = 14.5

40 -160

(b)

Ni3Se4/Ni at -0.300 V vs. RHE

-120 -80

pH = 7

-40 -100

(a)

Ni3Se4/Ni at -0.210 V vs. RHE

-75 -50

pH = 14.5

-25 0

200

400

600

800

1000

Time / min Figure 8: (a) 1000 min of potentiostatic electrolysis taking Ni3Se4 nanoassemblies on Ni foam as HER electrode at -0.210 V vs RHE at pH 14.5. (b) 1000 min of potentiostatic electrolysis taking Ni3Se4 nanoassemblies on Ni foam as HER electrode at -0.300 V vs RHE at pH 7. (c) 1000 min of potentiostatic electrolysis taking Ni3Se4 nanoassemblies on Ni foam as OER electrode at 1.46 V vs RHE at pH 14.5. (d) 1000 min of potentiostatic electrolysis taking Ni3Se4 nanoassemblies on Ni foam as OER electrode at 1.46 V vs RHE at pH 7.



a 200

LSV of Ni3Se4/Ni II Ni3Se4/Ni

175

LSV of Ni3Se4/Ni II Ni3Se4/Ni (iR free)

150

-2

125

j / mAcm

100



75

50

= 382 mV

50 25 0 1.0

1.1

1.2

1.4

1.3

1.5

1.6

1.7

1.8

1.9

Voltage / V

b

1.80 -2

1.75

E / V vs. RHE

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 36 of 44

GSTAT electrolysis at 50 mAcm on Ni3Se4/Ni II Ni3Se4/Ni system

1.70

at pH 14.5

1.65 1.60 1.55 1.50 0

1

2

3

4

5

6

7

8

9

Time / h

Figure 9: (a) The polarization curves with and without iR compensation acquired at a scan rate of 5 mVs-1 for the two electrode assembly Ni3Se4/Ni||Ni3Se4/Ni in 3 M KOH. (b) The chronopotentiometric response acquired at a current density of 50 mAcm-2 for the two electrode assembly Ni3Se4/Ni||Ni3Se4/Ni in 3 M KOH. 36 ACS Paragon Plus Environment

Page 37 of 44

500

400

1000

1500

200

2000

2500

c - Ni3Se4/Ni after HER

300

NiO

Ni-Se

100

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


0 900

b - Ni3Se4/Ni after OER

600

Ni-Se

NiSeO3

NiO

300

NiO(100)

0 900 600

a - Ni3Se4/Ni as such Ni-Se

300 0 500

1000

1500

2000

2500

-1

Raman Shift / cm

Figure 10: (a-c) Raman spectra of Ni3Se4/Ni before electrochemical characterizations, after anodic polarization studies and after cathodic polarization studies respectively.



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 38 of 44

b c

a

100 µm

1 µm

d

e

200 nm

2d = 0.458 nm (-311)

f (-602)

(-401)

(111) NiO

2d = 0.455 nm (-311)

200 nm g

5 nm

10 nm-1

h

i

2d = 0.452 nm (-401)

2d = 0.502 nm (-401) (-602) (-313)

(111) NiO

2d = 0.455 nm (-401)

5 nm

(213) NiSeO3

10 nm-1

(212) NiSeO3

Figure 11: (a) FESEM low magnification micrograph of Ni3Se4 after OER. (b-c) FESEM high magnification micrographs of Ni3Se4 after OER and HER characterizations respectively. (d-e) HRTEM micrographs of Ni3Se4 after OER with the corresponding SAED pattern (f). (g-h) HRTEM micrographs of Ni3Se4 after HER with the corresponding SAED pattern (i).


Page 39 of 44

(a)15000

(b)

1400

Ni 2p3/2

Se 3d

14500 3+

Ni

NiO

Se

2+

13500

1000

cps

cps

2-

1200

Ni

Sat

14000

13000

800

12500 12000

600

11500 866

(c)

864

862

860

858

856

854

852

850

57

56

55

54

53

52

51

Binding energy (eV)

Binding energy (eV)

(d) 12000

2000

3p3/2

Se 3p

O 1s

1900

11000

3p1/2

1800

10000

Sat 1700

2-

Se

Sat

cps

cps

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


1600 2-

Se

9000 8000

1500 7000 1400 166

164

162

160

158

156

154

535

Binding energy (eV)

534

533

532

531

530

Binding energy (eV)

Figure 12: (a-d) high resolution XPS spectra of Ni 2p3/2, Se 3d, Se 3p and O 1s states of Ni3Se4 after HER studies.


529

528


(e) 20000

(f) Ni 2p3/2 Ni

Sat

4+

Se

3+ 1400

NiO

Ni

2+

18000 0

Ni 17000

1000

16000

800

862

860

858

856

854

852

850

848

60

Binding energy (eV) 2600

3p3/2,Se

Se 3p

(h)

58

56

54

52

50

Binding energy (eV) 13000

4+

O 1s 12000

2-

2400

2-

Se

cps

cps

1200

864

3p1/2,Se

Sat

Sat 2200

11000 2-

3p3/2,Se

cps

(g)

1600

Se 3d

19000

cps

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 40 of 44

10000

2000 9000 1800 8000 168

166

164

162

160

158

156

154

534

Binding energy (eV)

532

530

528

526

Binding energy (eV)

Figure 12: (e-h) high resolution XPS spectra of Ni 2p3/2, Se 3d, Se 3p and O 1s states of Ni3Se4 after OER studies.


524

Page 41 of 44

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


Table 1: Results of the comparative electrocatalytic total water splitting studies. Electrocatalytic process

Catalyst

Medium

Overpotential Overpotential (ηgeo) (ηECSA)

Ni3Se4/Ni

3 M KOH

Ni3Se4/Ni

1 M KOH

Ni3Se4/Ni

0.1 M KOH

Ni3Se4/Ni

PBS

Pt foil

3 M KOH

Pt foil

PBS

Ni3Se4/Ni

3 M KOH

Ni3Se4/Ni

1 M KOH

Ni3Se4/Ni

0.1 M KOH

Ni3Se4/Ni

PBS

RuO2/ Ni

3 M KOH

RuO2/ Ni

PBS

HER

OER

-211 @ 50 mAcm-2 -206 @ 50 mAcm-2 -220 @ 50 mAcm-2 -282 @ 50 mAcm-2 -220 @ 50 mAcm-2 -343 @ 50 mAcm-2 232 @ 50 mAcm-2 244 @ 50 mAcm-2 321 @ 50 mAcm-2 480 @ 10 mAcm-2 407 @ 50 mAcm-2 -(a)

-195 @ 10 mAcm-2 -203 @ 10 mAcm-2 -208 @ 10 mAcm-2 -269 @ 10 mAcm-2 -82 @ 10 mAcm-2 -132 @ 10 mAcm-2 231 @ 10 mAcm-2 243 @ 10 mAcm-2 309 @ 10 mAcm-2 457 @ 2 mAcm-2 313 @ 2 mAcm-2 -(a)

Mass activity @ ηgeo (Ageog-1) 40.92 @ -250 mV 39.29 @ -250 mV 28.14 @ -250 mV 13.97 @ -250 mV -(b)

Tafel slope (mVdec-1)

297

Shift in η after 1000 cycles of CV 25 @ 50 mAcm-2 25 @ 50 mAcm-2 26 @ 50 mAcm-2 160 @ 50 mAcm-2 -(c)

-(b)

108

-(c)

83.3 @ 232 mV 83.3 @ 244 mV 83.3 @ 321 mV 6.5 @ 530 mV 33.3 @ 470 mV -(b)

33

132

30 @ 50 mAcm-2 25 @ 50 mAcm-2 20 @ 50 mAcm-2 44 @ 10 mAcm-2 -(c)

157

-(c)

128 156 165 101

30 40 116

Note: (a)RuO2 does not show any significant activity to measure overpotentials and mass activity. (b)

The mass activity of Pt foil was not calculated as it was a planar electrode. (c)The ADS tests

were not carried out on commercial catalysts and for bare Ni foam. (–) signs given before the HER overpotentials indicate that it is cathodic in nature.



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Page 42 of 44

Table 2: Comparison with the existing reports that are closely related to our catalyst Electrocatalytic process

HER

OER

Catalyst

Loading (mgcm-2)

Medium

Ni3Se4/Ni Ni3Se4/Ni Ni3Se4/Ni Ni3Se4/Ni Co-Ni-Se/C/NF Amorphous Co-Se film Ni3Se2 film Co-Se film CoSe/NiFe LDH/EG Polymorphic CoSe2 NiSe nanowire film Co doped NiSe/C Co doped NiSe2 Ni3Se4/Ni Ni3Se4/Ni Ni3Se4/Ni Ni3Se4/Ni Co-Ni-Se/C/NF Amorphous Co-Se film Ni3Se2 film Co-Se film Coral-like CoSe CoSe/NiFe LDH/EG Ni-Fe-Se nanosheets NiSe nanowire film Electrodeposited NiSe Co doped NiSe/C

2.4 2.4 2.4 2.4 -(a) 3.8 3.0 3.5 -(a) -(a) 2.8 -(a) 1.67 2.4 2.4 2.4 2.4 -(a) 3.8 3.0 1.7 0.28 -(a) 1.5 2.8 21.7 -(a)

3 M KOH 1 M KOH 0.1 M KOH PBS 1 M KOH 1 M KOH 1 M KOH 1 M KOH 1 M KOH 1 M KOH 1 M KOH 1 M KOH 1 M KOH 3 M KOH 1 M KOH 0.1 M KOH PBS 1 M KOH 1 M KOH 1 M KOH 1 M KOH 1 M KOH 1 M KOH 1 M KOH 1 M KOH 1 M KOH 1 M KOH

Overpotential (ηgeo) @ 50 mAcm-2 (mV) -211 -206 -220 -282 -151 -198 ~ -200 ~ -190 -390@30 mAcm-2 ~ -170 ~ -390 ~ -190 ~ -125 232 244 321 480 ~ 300 ~ 340 ~ 310 ~ 320 ~ 325 ~ 280 ~ 280 ~ 295 ~ 420 ~ 290

Tafel slope (mVdec-1) 128 156 165 101 81 84 98 39 160 31.2 120 81 63 33 30 40 116 63 69 80 68 40 57 47.2 64 97.1 63

Note: (a)The corresponding loading information was not available with the cited reports. (–) signs given before the HER overpotentials indicate that it is cathodic in nature. (~) signs given before the overpotentials indicate that it is approximately calculated from the available polarization curves of the cited reports. TW* means ‘This Work’.


Ref.

TW* TW* TW* TW* 41 18 20 33 35 37 39 41 42 TW* TW* TW* TW* 41 18 20 33 34 35 38 39 40 41

Page 43 of 44

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NaHSe

Ni3Se4

Microwave irradiation 300 W, 3 min

Not sufficient for efficient electrocatalysis of HER and OER

nucleated Ni foam

NaHSe Ni foam

Microwave irradiation 300 W, 3 min

Ni3Se4

5 h of aging at RT

Ni3Se4/Ni

nucleated Ni foam

foam

NaHSe 5 h of aging at RT

No Reaction

Ni foam Scheme 1: The graphical sketch of overall synthesis and the controlled experiments.



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Table of Contents


Page 44 of 44

Microwave-Initiated Facile Formation of Ni3Se4 Nanoassemblies for

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