C in High-Rate Hydrogen Evolution at

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NiTe2 Nanowire Outperforms Pt/C in High-Rate Hydrogen Evolution at Extreme pH Conditions Sengeni Anantharaj,†,‡ Kannimuthu Karthick,†,‡ and Subrata Kundu*,†,‡ †

Academy of Scientific and Innovative Research, Council of Scientific and Industrial Research, Central Electrochemical Research Institute (CSIR-CECRI), New Delhi, India ‡ Materials Electrochemistry Division (MED), CSIR-CECRI, Karaikudi, Tamil Nadu 630006, India S Supporting Information *

ABSTRACT: Better hydrogen generation with nonprecious electrocatalysts over Pt is highly anticipated in water splitting. Such an outperforming nonprecious electrocatalyst, nickel telluride (NiTe2), has been fabricated on Ni foam for electrocatalytic hydrogen evolution in extreme pH conditions, viz., 0 and 14. The morphological outcome of the fabricated NiTe2 was directed by the choice of the Te precursor. Nanoflakes (NFs) were obtained when NaHTe was used, and nanowires (NWs) were obtained when Te metal powder with hydrazine hydrate was used. Both NiTe2 NWs and NiTe2 NFs were comparatively screened for hydrogen evolution reaction (HER) in extreme pH conditions, viz., 0 and 14. NiTe2 NWs delivered current densities of 10, 100, and 500 mA cm−2 at the overpotentials of 125 ± 10, 195 ± 4, and 275 ± 7 mV in 0.5 M H2SO4. Similarly, in 1 M KOH, overpotentials of 113 ± 5, 247 ± 5, and 436 ± 8 mV were required for the same current densities, respectively. On the other hand, NiTe2 NFs showed relatively poorer HER activity than NiTe2 NWs, which required overpotentials of 193 ± 7, 289 ± 5, and 494 ± 8 mV in 0.5 M H2SO4 for the current densities of 10 and 100 mA cm−2 and 157 ± 5 and 335 ± 6 mV in 1 M KOH for the current densities of 10 and 100 mA cm−2, respectively. Notably, NiTe2 NWs outperformed the state-of-the-art Pt/C 20 wt % loaded Ni foam electrode of comparable mass loading. The Pt/C 20 wt % loaded Ni foam electrode reached 500 mA cm−2 at 332 ± 5 mV, whereas NiTe2 NWs drove the same current density with 57 mV less. These encouraging findings emphasize that a NiTe2 NW could be an alternative to noble and expensive Pt as a nonprecious and high-performance HER electrode for proton-exchange membrane and alkaline water electrolyzers.

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INTRODUCTION Storing intermittent energies as hydrogen through water electrolysis is on the top of the list for energy conversion and storage.1 Moreover, hydrogen is the fuel with the highest chemical energy density and a fuel with zero carbon emission upon combustion with oxygen. Because of these advantages, hydrogen has been accepted as the “fuel of the future”.1−3 Although conventional steam reforming of coal is used for hydrogen generation, water electrolysis is superior to coal reforming because it does not require high-temperature and -pressure conditions and the hydrogen produced by water electrolysis is the purest. The half-cell reactions of water electrolysis, namely, hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), are intensively studied with the objectives of improving the activity and replacing the noble and precious electrocatalysts.4−7 These half-cell reactions were catalyzed by Pt, IrO2/RuO2, and their alloys with other metals in earlier days.8−11 However, to make hydrogen production economically affordable, these half-cell reactions of water electrolysis must be catalyzed by nonprecious electrocatalysts without any loss in their activities.3 The recent evolution of nonprecious metals based on HER and OER electrocatalysts, such as metal chalcogenides,3 hydroxides, layered double hydroxides,2,4 and pnictogenides,1 shows the importance given to electrocatalysis of HER and © XXXX American Chemical Society

OER by researchers globally. OER was given greater attention over HER in earlier days because the sluggish kinetics associated with it was the major cause for increased cell voltage and energy loss.12 Recently, equal importance is being extended to HER also, with the sole aim of replacing Pt with nonprecious electrocatalysts.1,3 Sulfides, selenides, and phosphides of iron group metals have been reported as efficient HER electrocatalysts in the past few years.1,3 Among them, the phosphides of Ni, Co, and Fe are better HER catalysts than sulfides and selenides. However, phosphide13,14 is the most reported pnictogenide of 3d transition metals reported for HER electrocatalysis.1 In addition, nickel nitride is also reported to be an active catalyst for HER.15 In contrast, sulfides and selenides of 3d transition metals, mainly the iron group metals, are equally studied and reported in the literature.1,3 As far as Ni is concerned, there are reports for the HER activity of Ni foam itself in acidic as well as alkaline conditions.16,17 Among nickel chalcogenides, the sulfides and selenides are richer in the literature than the tellurides. Nickel sulfides have been reported in different stoichiometries such NiS,18−21 NiS2,22 and Ni3S2.23−26 Moreover, these sulfides have also been reported as composites with nanostructured carbon Received: November 20, 2017

A

DOI: 10.1021/acs.inorgchem.7b02947 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry materials.27 Similar to sulfides, selenides of Ni are also equally rich in the literature.28,29 Some of the important reports of nickel selenides with different stoichiometries are NiSe,30 NiSe2,31,32 Ni3Se2,33 and Ni3Se4.34 Like nickel sulfides, selenides of Ni are also reported as composites with nanostructured carbon materials with improved activities.3,35 In addition, both sulfides and selenides of Ni had been alloyed with other 3d iron group metals and reported with improved activities. Among them, NiFeSe2,36 NiFeS2,26,37,38 NiCoS2,39 and NiCoSe240 are the significant ones in the literature. As a new member of the metal chalcogenide HER electrocatalyst family, tellurides of 3d metals such as Co and Ni are comparatively new and poorly explored systems. Cobalt telluride has been documented for HER electrocatalysis.41 Similarly, nickel telluride has been reported in the forms of NiTe, NiTe2, and Ni3Te2 nanoparticles for HER electrocatalysis.42−44 In addition, there is also another report on NiTe grown on Ni foam by hydrothermal treatment for OER in alkaline conditions.45 Recently, Bhat et al. have reported the synthesis and water splitting characteristics of porous NiTe2.46 However, no effort has so far been made for promptly tellurizing Ni foam with optimized experimental conditions for its facile application in HER electrocatalysis for water electrolysis in extreme pH conditions. We have fabricated a nickel telluride catalyst of stoichiometry NiTe2 that has a higher percent content of heteroatoms on Ni foam. NiTe2 was obtained by shortening the hydrothermal treatment’s time span down to 2 h, unlike the earlier reports, which reportedly stated to have utilized 12−24 h of hydrothermal treatment. Moreover, the morphology was found to significantly differ when the precursors are different, such as NaHTe and Te metal powder with hydrazine hydrate. Such a difference in the morphology of NiTe2has also influenced their HER activity under identical conditions. The comparative HER studies indicated that NiTe2 nanowires (NWs) are better HER catalysts than NiTe2 nanoflakes (NFs) and state-of-the-art Pt.

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Equation 3 was formed in the original report with the assumption that the reaction only yielded the desired product, and we also assumed the same here. In eq 3, x is the increase in weight for 1 cm2 of the Ni foam after tellurization, MNiTe2 is the molecular weight of NiTe2, and MTe is the atomic weight of Te. Hence, the calculated loading of NiTe2 from the area normalized weight increase x (note that x is calculated to be 0.804 mg cm−2 by subtracting the weight of whole Ni foam (90.3 mg) from the weight of the whole tellurized Ni foam (91.506 mg) followed by normalizing with the area of 1.5 cm2 [see the Supporting Information (SI) for details]) using eq 3 was 1.98 mg cm−2. Thus, fabricated NiTe2 NWs were ready for further studies and characterization. Fabrication of NiTe2 NFs. To fabricate NiTe2 NFs on Ni foam, the same protocol was followed except that the Te precursor used was NaHTe prepared by dissolving the tellurium metal powder (0.256 g) with 0.578 g of sodium borohydride at 90 °C. This is a modified synthetic protocol of an earlier report.46 In addition, the fabricated NiTe2 NFs on Ni substrates took an extra hour to dry at 80 °C than NiTe2 NWs. At the end of drying, NiTe2 NFs on Ni foam substrates were ready for further studies and characterization. The probable chemical reactions that occurred during the synthesis are as follows.

2Te + NaBH4(aq) → 2NaHTe(aq) + B(OH)4 − (aq) + H 2(g) (4) 2NaHTe(aq) + Ni(s) → NiTe2 + 2H+(aq) + 2Na +(aq)

The total mass loading of NiTe2 in NiTe2 NFs was calculated to be 2.02 mg cm−2 using eq 3, where the measured area normalized weight increase was 0.821 mg cm−2, which is calculated by subtracting the weight of the whole Ni foam (91.2 mg) from the weight of the whole tellurized Ni foam (92.431 mg), followed by normalizing with the area of 1.5 cm2 (see the SI for details).30,34 To ascertain that the chosen experimental conditions are essential for NiTe2 formation, a set of controlled studies were carried out, and the same is depicted in Scheme S1. When the acid-treated Ni foam was aged only with the Te metal powder, hydrazine hydrate, and NaHTe for 2 h, no reaction occurred to form any tellurides of Ni. This indicates that hydrothermal treatment is necessary for the formation of NiTe2 on Ni foam. Similarly, when the acid-treated Ni foams were hydrothermally treated under the same experimental conditions with the Te metal powder alone, no reaction occurred to form any tellurides of Ni. This implies that a reducing agent is necessary to convert the metallic Te to Te2−, which was done by hydrazine hydrate and borohydride here. Unlike selenium, which is capable of forming nickel selenides even without any assistance from a reducing agent, tellurium cannot do so because it is comparatively less reactive than selenium. The above controlled studies indicate that the chosen experimental conditions for the synthesis of NiTe2 NWs and NiTe2 NFs are essential. More information on materials used in the fabrication of NiTe2 NWs and NiTe2 NFs and technical details of instruments for characterization and HER studies along with the sample preparation methods for various characterization and HER studies are provided in the SI. Electrocatalytic HER Studies at Extreme pH Conditions. Both NiTe2 NWs and NiTe2 NFs on Ni foam substrates were directly used as electrocatalytic HER electrodes in 0.5 M H2SO4 and 1 M KOH corresponding to the pH values of 0 and 14, respectively. For comparison, bare Ni foam substrates and state-of-the-art Pt electrodes were also studied under the same experimental conditions of HER electrocatalysis. All polarization curves were recorded at a scan rate of 5 mV s−1 and corrected for iR drop by 100%. The primary polarization studies with NiTe2 NWs and NiTe2 NFs on Ni foam substrates were repeated five times to ascertain that the performances are reproducible and can be obtained with acceptable errors from the mean values. Electrochemical robustness of studies, viz., the accelerated degradation (AD) test for 1000 cycles at a high scan rate (300 mV s−1) by cyclic voltammetry and prolonged HER at fixed overpotentials of the ability to drive the 50 mA cm−2 current density for more than 1000 min, was carried out. The linear-sweep voltammograms (LSVs) acquired before and after the AD test were also corrected for iR drop by 100%. The

EXPERIMENTAL SECTION

Fabrication of NiTe2 NWs. The following synthetic protocol is totally different from all existing reports in that the reagent combination of Te metal powder and hydrazine hydrate has been used for the first time for tellurization of a metal substrate. For the fabrication of NiTe2 NWs on Ni foam, four pieces of Ni foam (3 cm × 0.5 cm) were treated with HCl (3 M) under sonication for 5 min and washed with deionized (DI) water before introduction into a Teflonlined autoclave of 50 mL volume containing 40 mL of DI water. Concomitantly, tellurium metal powder (0.256 g) and hydrazine hydrate (5 mL) were added to the same autoclave and sealed tightly. Then, a short-time (2 h) hydrothermal treatment was carried out on the reaction mixture at 180 °C. After the reaction mixture was naturally cooled to room temperature, the NiTe2-grown Ni foam (black in color) pieces were washed several times with DI water and dried at 80 °C for 2 h. In the above synthesis conditions, the following reactions were proposed to occur in the successful formation of NiTe2 NWs. 2Te + N2H4 ·H 2O → 2H 2Te(aq) + H+(aq) + N2(g)

(1)

2H 2Te(aq) + Ni(s) → NiTe2 + 4H+(aq)

(2)

The total mass loading of NiTe2 was calculated in the fabricated NiTe2 NWs following the procedure referred to in the literature.30,34 loading of NiTe2 = x mg × (M NiTe2 /M Te) = x mg × (313.89/127.6) = 2.46 × x mg

(5)

(3) B

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Figure 1. FESEM micrographs of NiTe2 NFs with gradually increasing magnification.

Figure 2. FESEM micrographs of NiTe2 NWs with gradually increasing magnification. chronoamperometric responses were presented without compensating for iR drop. All potential scales were converted to reversible hydrogen electrode (RHE) scale as per earlier reports.47−50 An electrochemical impedance spectroscopy (EIS) technique was used to examine the changes that occurred on the electrocatalytic interface before and after the AD test at the onset of overpotentials of HER in both 0.5 M H2SO4 and 1 M KOH. The results of the electrocatalytic studies are discussed in subsequent sections.

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The FESEM micrographs of NiTe2 NFs with increasing magnification are provided in Figure 1a−d. Figure 1a is the electron micrograph of NiTe2 at ultralow magnification, which clearly reveals that NiTe2 NFs are selfassembled and form a coral-like hierarchical array on Ni foam, which has a smooth surface, as can be seen in Figure S1a,b, where the corresponding EDS and the weight percent distribution of Ni and C, provided as Figure S1c,d, show that the as-cleaned Ni foam is free from surface oxides and contains a significant amount of C. This could have been associated with the fact that Ni foams are produced by carbothermal reduction from their molten salt precursor, and the same is true for the preparation of the Te metal powder also.51,52Figure 1 b is the closer view of the coral-like assemblies formed by NiTe2 nanostructures on Ni foam. Parts c and d of Figure 1 show that the observed coral-like assemblies are composed of NFs of varying sizes. Similarly, FESEM micrographs of NiTe 2 nanostructures fabricated using the Te metal powder and

RESULTS AND DISCUSSION

Material Characterization. Fabricated NiTe2 nanostructures on Ni foam substrates were directly subjected for microstructural characterization such as field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and elemental color mapping with energydispersive X-ray spectroscopy (EDS). NiTe2 nanostructures fabricated on Ni foam using NaHTe as the Te precursor were found to have a hierarchical coral-like assembly of NiTe2 NFs. C

DOI: 10.1021/acs.inorgchem.7b02947 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. (a) FESEM micrograph of NiTe2 NFs, where EDS/mapping was done. (b) EDS spectrum of NITe2 NFs. (c−h) EDS elemental electronic smart maps of Ni, Te, O, C, Na, and B, respectively.

Figure 4. (a) FESEM micrograph of NiTe2 NWs where EDS/mapping was done. (b) EDS spectrum of NiTe2 NWs. (c−e) EDS elemental electronic smart maps of Ni, Te, and O respectively.

is 200 ± 20 nm. To further confirm that the fabricated nanostructures are composed of Ni and Te, EDS/elemental color mapping was done for both NiTe2 NFs and NiTe2 NWs at low magnification. The electronic smart maps of Ni, Te, C, O, B, and Na in NiTe2 NFs on Ni foam are provided with the respective electron micrograph and EDS spectrum in Figure 3a−d. Figure

hydrazine hydrate are provided in Figure 2a−d. Parts a and b of Figure 2 show that the surface of Ni foam substrates are completely covered with an interwoven array of NWs. Parts c and d of Figure 2 are further magnified FESEM micrographs of the same NiTe2 NWs array on Ni foam, which impeccably show that the NWs are uniformly grown over all of the Ni foam substrate and the average diameter of a single NW D

DOI: 10.1021/acs.inorgchem.7b02947 Inorg. Chem. XXXX, XXX, XXX−XXX

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recovered from their ores by a carbothermal method.51,52 The overall EDS elemental color mapping results have given significant insight into the elemental composition and their distribution in coral-like NiTe2 NFs assemblies and in NiTe2 NWs assemblies fabricated on Ni foam substrates. It also confirmed that Ni and Te are the major components of the nanostructures fabricated over Ni foam with a percentage composition ratio of ∼1:2 in both and indicated that the stoichiometry is NiTe2. Having confirmed the presence of Ni and Te with the composition, the nature of crystallization of the same under hydrothermal treatment was analyzed through X-ray diffraction (XRD) studies. The XRD patterns of NiTe2 NWs and NiTe2 NFs on Ni foam were acquired at a scan rate of 3° min−1 within the 2θ range of 10−90° and are pictured in Figure 5. The

3a is the FESEM electron micrograph of NiTe2 NFs on Ni foam at low magnification, where the electronic smart maps of various elements in it were acquired. Figure 3b is the respective EDS spectrum that shows the presence of Ni, Te, O, C, Na, and B. To gain more information and relative composition of all of these elements, the weight and atomic percentages are provided in Figure S2 and Table S1, according to which the dominant presence of O is noted along with equal proportions of Ni, Te, and B. This indicates that, along with NiTe2, the oxides of B with some Na have also been incorporated. Parts c−h of Figure 3 are the electronic smart maps of the Ni K, Te K, O K, C K, Na K, and B K shells, respectively. From Figure 3c,d, it is clear that the observed coral-like self-assembly is composed mainly of Ni and Te. The presence of O can be seen with high intensity specifically at sites that look like buds and projected a little above the plane of other formed nanostructures on Ni foam. This could be due to the surface-oxidized Te and Ni. Similar surface oxidation of nickel sulfide and selenide catalysts fabricated on Ni foam by hydrothermal methods was reported earlier also. It is believed that the same surface oxidation must be the reason for the observed O in NiTe2 NFs. The presence of C must be from Te and Ni, which always contain some elemental C in them because they were produced by carbothermal reduction from their ores.51,52 The presence of Na and B from the reducing agent sodium borohydride used to prepare NaHTe. Interestingly, the percentage composition of B is 5.2%, which is significantly higher. This indicates that NiTe2 NFs also contain some oxides of B along with some Na. Such incorporation of Na and B into the NiTe2 self-assembled matrixes may significantly affect the HER electrocatalysis of the same. Similarly, EDS elemental color mapping was carried out on NiTe2 NWs on Ni foam to reveal the elemental composition. Figure 4a is the FESEM micrograph of a NiTe2 NW array on Ni foam at low magnification that shows the part of the specimen where the color mapping was carried out. Figure 4b shows the corresponding EDS spectrum acquired on the same part that revealed the presence of Ni, Te, C, and O. Interestingly, unlike NiTe2 NFs, NiTe2 NWs have shown no additional impurities. Parts c−e of Figure 4 are the electronic smart maps of the Ni K, Te K, and O K shells, respectively. The presence of O in NiTe2 NWs must be due to the surface oxidation of Te and Ni, as observed with NiTe2 NFs. The dominant presence of only Ni and Te with a 1:2 ratio of the percentage composition also allowed us to predict that the stoichiometry of the nickel telluride formed is NiTe2. The same is true for NiTe2 NFs also. Moreover, the percent composition of O in NiTe2 NWs is very less compared to that of NiTe2 NFs, which indicates that the surface oxidation is comparatively lesser on NiTe2 NW assemblies fabricated over Ni foam using the Te metal powder and hydrazine hydrate. In addition to the Ni, Te, and O peaks, the EDS spectrum also showed the presence of some C, as was observed for NiTe2 NFs also. Similarly, to gain relative information on the weights and atomic percentages of the elements present in NiTe2 NWs, plots of the atomic and weight percentage compositions are provided in Figure S3 and Table S2, according to which it was evidenced that the relative amounts of Ni and Te are almost equal to that of O. This indicates that, along with NiTe2, the oxides of Ni, Te, and C have also contributed to the overall percentage composition of O. The occurrence of C (Figure S4) in the fabricated material is again attributed to the same fact that the occurrence of residual C on Ni and Te surfaces is

Figure 5. XRD patterns of NiTe2 NFs (red) and NiTe2 NWs (blue) acquired in the diffraction angle range of 10−90° with a scan rate of 3 min−1. The asterisk symbols denote the peaks of metallic Ni from the substrate.

diffraction patterns of both NiTe2 NWs and NiTe2 NFs have three predominant peaks that correspond to the metallic Ni, which appeared because of diffraction from the Ni foam substrate and are labeled with asterisk symbols. These three peaks correspond to the diffraction planes of (111), (200), and (220), respectively, and were found perfect match with the ICDD reference card number 70-0989 and also with earlier report where Ni foam was used as the substrate for many watersplitting electrocatalysts.34 Other than these three Ni peaks, there are peaks with considerable counts in the pattern of NiTe2 NWs that are labeled with their corresponding hkl indices such as (011) at 31.7°, (002) at 34.1°, (012) at 43.9°, (110) at 47.2°, (200) at 55.0°, (201) at 58.0°, and (202) at 66.4° for the NiTe2 phase of ICDD reference card number 89-2021. This also matches well with the earlier report of NiTe2.42 Nevertheless, the diffraction pattern of NiTe2 NFs fabricated over Ni foam is relatively weaker in count but still recognizable, with similar features. This indicates that both NF and NW assemblies formed over Ni foam are NiTe 2 . This is in agreement with the stoichiometric prediction made from EDS elemental mapping analysis that revealed that the percent composition ratio of Ni and Te is ∼1:2 in both NiTe2 NW and NiTe2 NF assemblies fabricated on Ni foam substrates. E

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Figure 6. (a−c) TEM micrographs of NiTe2 NFs with increasing magnification. The insets of parts b and c are the SAED patterns and HRTEM micrographs of the same. (d−f) TEM micrographs of NiTe2 NFs with increasing magnification. The insets of parts e and f are the SAED pattern and HRTEM micrographs of the same.

pattern feature almost similar to that observed for NiTe2 NFs. The two primary rings observed in the SAED patterns are calibrated and found to belong to the diffraction planes of (002) and (112), as observed with NiTe2 NFs. This is also in full agreement with the ICDD reference card number 89-2021, according to which the corresponding d-spacing values of the (002) and (112) planes are 0.262 and 0.155 nm, respectively. Further, the HRTEM micrograph with fine lattice fringes of NiTe2 NWs at the edge was acquired and is provided in the inset of Figure 3f. The measured d spacing between two adjacent lines in the lattice fringes was 0.262 nm, which is in close agreement with the (002) plane as per the ICDD reference card number 89-2021. Both the SAED patterns and HRTEM micrographs with clear lattice fringes of NiTe2 NFs and NiTe2 NWs were confirmed once again to have anisotropic flakelike structures and hierarchical NW structures composed of NiTe2 crystalline sheets, respectively. This also revealed that the Na and B impurities that were found to be incorporated in NiTe2 NF assemblies did not alter the crystallinity or phase formation of NiTe2 and must be present as an amorphous impurity. The overall microstructural, spectroscopic, and diffraction studies confirmed the formation of NiTe2 in the case of both NF and NW assemblies. Although it has been confirmed that the material formed is NiTe2, it is mandatory to confirm the same with a more reliable tool such as X-ray photoelectron spectroscopy (XPS). The high-resolution XPS spectra observed with NiTe2 NFs are provided in Figure 7a−e for Ni 2p, Te 3d, O 1s, B 1s, and Na 1s, respectively. Figure 7a, which is the Ni 2p XPS spectrum, showed a distinct clear doublet with a splitting of 19.25 eV due to the phenomenon of spin−orbit coupling. The Ni 2p3/2 state upon deconvolution showed five different components, which are located at 850.1, 853.1, 854.9, 856.2, and 860.1 eV, corresponding to metallic Ni from the substrate, NiTe2, NiO,

To further ascertain the morphological and structural information gathered from FESEM, elemental mapping, and XRD analyses, both NiTe2 NFs and NiTe2 NWs were studied using high-resolution TEM (HRTEM) and selected-area electron diffraction analysis (see the SI for sample preparation). The HRTEM micrographs of NiTe2 NFs with gradually increasing magnification are provided in Figure 6a−c. These micrographs are consistent with the morphological information derived from the FESEM micrograph (Figure 1d) of the same. These micrographs also tell us that the flakes are anisotropic in size and shape and formed randomly during hydrothermal treatment on Ni foam with a NaHTe precursor. The inset of Figure 3b is the respective SAED pattern of NiTe2 NFs that shows dot and ring features. The two observed rings in the SAED pattern of NiTe2 NFs are calibrated and assigned to their corresponding diffraction planes of (002) and (112), respectively, as per the ICDD reference card number 892021. Similarly, the inset of Figure 3c is the HRTEM micrograph of NiTe2 NFs at a very high magnification, which shows clear lattice fringes. The measured distance between these two fringes was 0.264 nm, which is in very close agreement with the (002) diffraction plane of the NiTe2 phase, as per the ICDD reference card number 89-2021, according to which the standard d spacing for (002) is 0.262 nm. Similarly, HRTEM analysis of NiTe 2 NWs revealed the same morphological information as that observed with FESEM analysis (Figure 2a−d). The NWs of NiTe2 are clearly visible in the corresponding HRTEM micrographs provided in Figure 6d−f with increasing magnification. These HRTEM micrographs revealed that the NWs are composed of ultrathin sheets warped one over another. The average diameter of these wires is in the range of 190−220 nm, which is in close agreement with that of FESEM analysis. In addition, the SAED pattern of NiTe2 NWs is provided in the inset of Figure 3e, which has a F

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Figure 7. High-resolution XPS spectra of the Ni 2p, Te 3d, O 1s, B 1s, and Na 1s states in NiTe2 NFs.

a−c of Figure 8 are the high-resolution XPS spectra of the Ni 2p, Te 3p, Te 3d, and O 1s levels of NiTe2 NWs. Figure 8a shows a wide binding energy scale from 805 to 816 eV that covers the regions of both Ni 2p with a spin−orbit splitting of 19.1 eV and Te 3p with a splitting of 49.8 eV. Precise deconvolution of these peaks revealed significant information on the chemical nature of both Ni and Te. The Ni 2p3/2 peak resulted in four peaks upon deconvolution, characteristic of metallic Ni at 851.8 and 853.8 eV for NiTe2 and 857.8 eV for Ni(OH)2, in addition to their corresponding satellite peak at a considerably higher binding energy. Similar deconvoluted peak features were observed for Ni 2p1/2 also. This observation indicates that a maximum of the substrate surface is covered with NiTe2, which is accompanied with some hydroxide of Ni, which is possibly due to the hydration of surface-oxidized Ni. The elemental Ni peak is due to the Ni foam substrate used for fabrication. These results coincide well with earlier reports on NiTe2 and other related materials fabricated over Ni foam

Ni(OH)2, and the respective satellite peaks of all of them. The same pattern is observed with the Ni 2p1/2 state also, with the exception that it has also accompanied the peak of the Te 3p1/2 state located at 870.1 eV. Similarly, the Te 3d doublet with a splitting of 10.3 eV (Figure 7b) upon deconvolution showed mainly two peaks for the presence of Te− and TeO2. Both Te 3d5/2 and Te 3d3/2 had almost similar spectral features with an intensity ratio of 1:0.75 as expected. The Te 3d5/2 level had peaks at 572.1, 573.2, 576.3, 577.8, and 579.8 eV, corresponding to Te−, Te0, Te4+, and their satellite peaks, respectively. The same is true for the Te 3d3/2 state also. From Figure 7a,b, it is clear that NiTe2 NFs had the predominant formation of NiTe2, along with some surface oxides of Ni and Te. Other high-resolution spectra of O 1s, B 1s, and Na 1s, which are provided in Figure 7c−e, indicated that NiTe2 NFs have several oxygenated surface moieties that accompany both Na and B in their composition. The above observations are in good agreement with the earlier reports of NiTe2.42,44,46 Parts G

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Figure 8. (a) High-resolution XPS spectra of the Ni 2p and Te 3p levels in NiTe2 NWs. (b and c) High-resolution XPS spectra of the Te 3d and O 1s levels in NiTe2 NWs.

substrates through hydrothermal treatment.34,42,45 From the same Figure 8a, peaks corresponding to Te 3p3/2 and Te 3p1/2 can be seen at 820.1 and 870.1 eV, respectively. The locations of these peaks at these binding energy values clearly suggest that Te is mainly in the negative monovalent oxidation state (Te−). This observation once again suggests that the material formed is NiTe2 and also in agreement with an earlier report.42 Figure 8b is the high-resolution XPS spectrum of the Te 3d level with a splitting of 10.3 eV, which shows a doublet due to spin−orbit coupling that splits into Te 3d5/2 and Te 3d3/2, respectively. When Te 3d5/2 was deconvoluted, four peaks at 572.5, 574.1, 575.1, and 576.1 eV were observed, which correspond to Te−, Te0, TeO2, and its satellite peak. However, the peak that corresponds to Te− is the dominant one, indicating that the formed material is mainly NiTe2 with some Te0 and TeO2 due to surface oxidation. The same deconvoluted peak features were also observed for Te 3d3/2. These results are in agreement with the earlier reports on NiTe and NiTe2.42,45 To further gain information on the oxidized species, the highresolution XPS spectrum of the O 1s level was acquired and is provided in Figure 8c after deconvolution. Four different components within the O 1s spectrum can be seen in Figure 8c at 530, 531.8, 532.4, and 534.8 eV, which indicate that there are M−O−M, M−O−, M−O−H, and moisture on the surface, respectively. This observation is consistent with the spectral results of Ni 2p and Te 3d, which suggested the surface oxidation of both Ni and Te.34,42,45,53−55 Moreover, a recent report on NiTe2 for OER also presented similar XPS

characteristics.56 Interestingly, the presence of O as the M− OH functionality is in resonance with the Ni(OH)2 peak observed in the Ni 2p spectrum. This observation also matches the EDS elemental color mapping results and confirms that there was significant surface oxidation of both Ni and Te, which is commonly encountered in the solution-based synthesis of nanomaterials in water. The features of the O 1s spectrum are in good agreement with earlier reports as well.29,36,37,40 The overall XPS analysis confirmed once again that the formed material is NiTe2 with some surface oxidation, which is on par with the results of EDS mapping, XRD, SAED, and HRTEM analyses. Moreover, the selective formation of NiTe2 over that of the other tellurides of Ni is attributed to the short reaction time under hydrothermal conditions, which restricted the formation of Ni-rich telluride by not providing extended access to the Ni atoms in the Ni foam substrate. This is true because all of the three existing reports on the nickel telluride synthesis utilized a long-term hydrothermal process that resulted in the formation of either NiTe or Ni3Te2; both of them are comparatively rich in Ni content compared to NiTe2. The overall specific binding energy values of each deconvoluted peak are tabulated in Table 1 along with their corresponding species. This could be a fruitful advantage in HER electrocatalysis because it has already been proven that an increase in the content of heteroatoms like P, S, and Se increases their HER activity, which was investigated in detail earlier by many. Having confirmed the formation of Te-rich NiTe2 as NF and NW assemblies over Ni foam substrates, H

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Electrocatalytic Hydrogen Evolution Studies at Extreme pH Conditions. As a new member of the nickel chalcogenide family of HER electrocatalysts, nickel telluride had been applied to HER electrocatalysis in recent days. However, there are no data on comparatively Te-rich NiTe2 NWs and NFs grown on Ni foam for HER electrocatalysis in both highly acidic and highly alkaline media. To fill this gap, we studied our materials (NiTe2 NFs and NiTe2 NWs grown on Ni foam) in detail for HER electrocatalysis in 0.5 M H2SO4 as well as in 1 M KOH. As a primary evaluation study, polarization curves of NiTe2 NWs, NiTe2 NFs, Ni foam, and Pt/C were acquired at a slower scan rate of 5 mV s−1 in both 0.5 M H2SO4 and 1 M KOH, which correspond to pH 0 and 14, respectively. LSVs obtained with 100% iR compensation are pictured in parts a and b of Figure 9 respectively for pH 0 and 14. Figure 9a shows the LSVs of Ni foam, NiTe2 NWs, NiTe2 NFs, and Pt/C within the potential range of −0.6 to +0.2 V versus RHE, from which we can see that the onset of Pt/C is exactly at 0 V versus RHE, which showed better activity than other studied catalysts in lower overpotential regions. However, the onset potentials for HER on NiTe2 NWs and NiTe2 NFs in pH 0 are 115 ± 5 and 150 ± 8 mV, respectively. Similarly for the defined current densities of 10, 100, and 500 mA cm−2, NiTe2 NWs showed 125 ± 10, 195 ± 4, and 275 ± 7 mV as overpotentials in pH 0, wherein NiTe2 NFs required overpotentials of 193 ± 7, 289 ± 5, and 494 ± 8 mV for the same current densities in pH 0. This implies that the HER activity of NiTe2 NWs is better than that of NiTe2 NFs. Interestingly, when the electrocatalytic interfaces were subjected to a higher current density like 500 mA cm−2, the activity

Table 1. Specific Binding Energy Values of Each Deconvoluted Peak with Its Respective Species catalyst NiTe2 NFs

state

binding energy (eV)

corresponding species

Ni 2p3/2

850.1, 853.1, 854.9, 856.2, and 860.1 866.9, 870.1, 871.6, 873.1, 875.5, and 878.1 572.1, 573.2, 576.3, 577.8, and 579.8 583.1, 584.2, 586.7, and 589.9 530, 531.8, 532.4, and 534.8 192.2 and 193.9 1072.9 and 1074.2 851.8, 853.8, 857.8, and 861 869.9, 873.1, 876.7, and 879.2 820.7 871.5 572.9, 574.1, 575.1, and 576.1 582.8, 583.5, 584.6, and 586.1 530, 531.8, 532.4, and 534.8

Ni, Ni2+ of NiTe2, Ni2+ of NiO, Ni2+ of Ni(OH)2, and satellite Ni, Te2− of NiTe2 as Te 3p1/2, Ni2+ of NiO, Ni2+ of Ni(OH)2, and satellite Te−, Te0, Te4+, satellite 1, and satellite 2 Te−, Te0, Te4+, satellite 1, and satellite 2 M−O−M, M−O−, M−O−H, and satellite B−O and B−OH/B−OB O−Na and B−O−Na Ni, Ni2+ of NiTe2, Ni2+ of Ni(OH)2, and satellite Ni, Ni2+ of NiTe2, Ni2+ of Ni(OH)2, and satellite Te− Te− Te−, Te0, TeO2, and satellite

Ni 2p1/2 and Te 3p1/2 Te 3d5/2 Te 3d3/2 O 1s

NiTe2 NWs

B 1s Na 1s Ni 2p3/2 Ni 2p1/2 Te 3p3/2 Te 3p1/2 Te 3d5/2 Te 3d3/2 O 1s

Te−, Te0, TeO2, and satellite M−O−M, M−O−, M−O−H, and moisture

electrocatalytic HER studies in extreme pH conditions were conducted, the results of which are discussed below.

Figure 9. (a and b) LSVs of Ni foam, NiTe2 NWs, NiTe2 NFs, and Pt/c acquired at a scan rate of 5 mV s−1 in 0.5 M H2SO4 and 1 M KOH corresponding to pH 0 and 14, respectively. (c and d) Steady-state polarization curves of the same in 0.5 M H2SO4 and 1 M KOH corresponding to pH 0 and 14, respectively. I

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Figure 10. Plot of current density versus overpotential with standard deviations from the mean value as measured from the first five consecutive experiments.

To further investigate the activity trend observed in polarization studies, steady-state polarization analysis was also done for all four catalytic interfaces studied here including the bare Ni foam substrate in both pH 0 and 14. The resultant Tafel plots are pictured in Figure 9c,d. The Tafel plots of Ni foam, NiTe2 NWs, NiTe2 NFs, and Pt/C in pH 0 (Figure 9c) show their corresponding slope values (V dec−1). In acidic conditions, Pt/C showed the minimum Tafel slope of 0.031 V dec−1 to which the Tafel slope of NiTe2NWs was closest (0.036 V dec−1). This indicates that NiTe2 NWs show almost parallel kinetics to that of Pt/C in acidic conditions. Similarly, the Tafel slope of NiTe2 NFs was lower (0.048 V dec−1) but not as low as those of Pt/C and NiTe2 NWs. This once again indicates that, in acidic conditions, NiTe2 NWs are kinetically better than NiTe2 NFs. However, the Tafel slope of Ni foam is the largest (0.091 V dec−1) among all of them, and this indicates that metallic Ni surfaces are poor in HER kinetics under acidic conditions. The kinetic trend observed in pH 14 just reflects that observed in pH 0 for all four catalytic interfaces, with the exception that NiTe2 NWs showed a lower Tafel slope than Pt/ C (0.120 V dec−1). This indicates that, in high alkaline conditions, NiTe2 NWs are better kinetic HER interfaces than Pt/C. Steady-state polarization analysis revealed that NiTe2 NWs are better than NiTe2 NFs in both pH 0 and 14, while they are on par with Pt/C in pH 0 and better than Pt/C in pH 14. The repeatability of the results of polarization studies on both the NiTe2 NWs and NiTe2 NFs interfaces was tested by running the experiments consecutively five times under identical conditions. From this study, a plot of fixed current densities versus corresponding overpotentials is given in Figure 10. The result of such a repeatability study in pH 0 and 14 is indicated as a bar diagram in Figure 10 with the corresponding mean deviations as error bars. For both NiTe2 NWs and NiTe2 NFs, it can be seen that the overpotentials required for all current densities are much lower in pH 0 than the ones required in pH 14 for the same. This indicates that NiTe2 NWs and NiTe2 NFs are better at catalyzing HER in acidic conditions than in alkaline conditions. Moreover, it is also once again evident from Figure 10 that NiTe2 NWs are better than NiTe2 NFs in catalyzing HER in both pH 0 and 14. Like the plot of current density versus overpotential, an equivalent

trend was changed. For a high-rate current density of 500 mA cm−2, Pt/C and NiTe2 NWs required 335 ± 5 and 275 ± 7 mV, respectively. This indicates that, at high-rate electrolysis, our NiTe2 NWs outperformed the state-of-the-art Pt/C and delivered 500 mA cm−2 with 57 ± 5 mV lesser overpotential. In addition, NiTe2 NFs reached a current density of 500 mA cm−2 with 494 ± 8 mV as the overpotential. This indicates that NiTe2 NWs are high-performance catalysts for HER in pH 0 and perform better than Pt, which is highly significant and will surely be beneficial for large-scale proton-exchange membrane (PEM) water electrolysis. The polarization curves obtained in pH 14 (in 1 M KOH), pictured in Figure 9b, also show an activity trend similar to that observed in pH 0 (in 0.5 M H2SO4). The onset overpotentials of Pt/C, NiTe2 NWs, and NiTe2 NFs are almost the same as those observed in pH 0, with negligible difference. However, the overpotentials of 113 ± 5, 247 ± 5, and 436 ± 8 mV were required to reach 10, 100, and 500 mA cm−2. Pt/C is still the better HER electrocatalyst even in alkaline conditions when operations at lower overpotentials are considered. For 10 mA cm−2, Pt/C, NiTe2 NWs, and NiTe2 NFs required 66 ± 3, 113 ± 5, and 157 ± 5 mV as overpotentials. However, when highrate electrolysis was considered, NiTe2 NWs were the only catalysts to drive 500 mA cm−2 by requiring just 436 ± 8 mV. This indicates that, whether the solution pH is 0 or 14, the activity of NiTe2 NWs is always better than that of Pt/C as far as high-rate electrolysis for 500 mA cm−2 is concerned. This once again indicates that, for large-scale alkaline water electrolyzers, NiTe2 NWs grown on Ni foam substrates could be an efficient and inexpensive alternate to noble Pt/C for HER. It is also important to notice here that the activity of a bare Ni foam substrate in both highly acidic and alkaline conditions is negligible within the studied window of potential, which indicates that the activity observed with both NiTe2 NWs and NiTe2 NFs is only of catalytic NiTe2. The above polarization studies revealed that, between NiTe2 NWs and NiTe2 NFs, NiTe2 NWs are the best HER catalysts in both pH 0 and 14. Similarly, although Pt/C behaves better in loweroverpotential HER operations, NiTe2 NWs outperformed at high-current-density operations like 500 mA cm−2, which indicates that, for large-scale bulk electrolysis, NiTe2 NWs are better HER electrocatalysts than the state-of-the-art Pt/C. J

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ing scan rates acquired at both 0.5 M H2SO4 and 1 M KOH, and the same is provided in Figure 12a,b.

plot of overpotential versus mass activity is also given by calculating the mass activity for both NiTe2 NWs and NiTe2 NFs at fixed overpotentials in Figure 11. Figure 11 also implies

Figure 11. Plot of overpotential versus mass activity with standard deviations from the mean value as measured from the first five consecutive experiments.

exactly the same trend in the mass activity as that observed with the plot of current density versus overpotential. This has once again proven that NiTe2 NWs are better than NiTe2 NFs in both acidic and alkaline conditions. The results of these electrochemical characterizations clearly showed that NiTe2 NWs are better than NiTe2 NFs. Hence, it is now essential to interrogate these two systems for the observed difference in their activity. To do so, EIS spectra for both NiTe2 NWs and NiTe2 NFs were obtained at their onset potential of HER with a sinusoidal alternating-current amplitude of 20 mV. Figure S5 gives the Cole−Cole plots of NiTe2 NWs and NiTe2 NFs in pH 0 and 14. It can be seen from these plots that NiTe2 NWs showed lesser charge-transfer resistance (Rct) than NiTe2 NFs in both pH 0 and 14. The measured Rct values for NiTe2 NWs in pH 0 and 14 are 34 and 8 Ω, respectively. This indicates that the charge-transfer ability of NiTe2 NWs is better in alkaline conditions than in acidic conditions. Similarly, NiTe2 NFs have Rct values of 44 and 51 Ω in pH 0 and 14, respectively. Unlike, NiTe2 NWs, NiTe2 NFs showed higher Rct values in alkaline conditions. This trend tells us the reason behind the difference in the observed activity between NiTe2 NWs and NiTe2 NFs. The lowered chargetransfer ability of NiTe2 NFs is attributed to the amorphous impurities of Na and B incorporated in it, which may have a serious effect in decreasing the conductivity of NiTe2 NFs. In addition, to obtain a correlation between the relative electrochemically active surface area (ECSA) determined from the double-layer capacitance (Cdl), Ni foam, NiTe2 NFs, and NiTe2 NWs were taken. It is important to emphasize here that the Pt/ C interface cannot be subjected to such a study under identical conditions where the H2 adsorption and desorption peaks would strongly interfere with the double-layer charging current. The cyclic voltammograms (CVs) recorded for Ni foam, NiTe2 NFs, and NiTe2 NWs in 0.5 M H2SO4 in a double-layer charging region are provided in Figure S6a−c. Similarly, the CVs recorded in 1 M KOH for the same interfaces in their respective double-layer charging region closer to HER onset are provided in Figure S7. The difference in the double-layer charging current densities was plotted against the correspond-

Figure 12. Plot of double-layer charging current density versus scan rate for Ni foam, NiTe2 NFs, and NiTe2 NWs in acidic (a) and alkali (b) conditions.

The measured slope values of Ni foam, NiTe2 NFs, and NiTe2 NWs were 2.74, 11.05, and 13.5 μF cm−2, respectively, in 0.5 M H2SO4. Similarly, in 1 M KOH, the same interfaces were 2.02, 5.24, and 7.72 μF cm−2, respectively. These 2Cdl values were related to the ECSA of the studied catalytic interfaces by definition. Hence, from the observed trend, it is once again clear that NiTe2 NWs were the only catalytic interfaces that had a better relative ECSA value than both Ni foam and NiTe2 NFs. This observation further backs up the activity trend observed in both the voltammetric study and Tafel analysis. Having found that NiTe2 NWs are far better electrocatalytic HER electrodes than NTe2 NFs, they were subjected to stability studies under harsh electrochemical conditions to assess their suitability for long-term use in PEM and alkaline water electrolyzers in place of Pt as a nonprecious HER catalyst. AD tests were carried out on NiTe2 NWs for 1000 cycles of cyclic voltammetry at a very high scan rate of 300 mV s−1. During this, LSVs were acquired before and after the same and corrected for iR drop by 100%, and these are pictured in Figure 13a. From Figure 13, it is clearly evidenced that degradation in the activity is very low in acidic conditions and considerably high in alkaline conditions at a current density of 500 mA cm−2. This shows that the NiTe2 NWs’ stability under such harsh electrochemical cycling is comparatively higher in pH 0 than in K

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pH 0 and 14. The resultant chronoamperometric curves are pictured in Figure 13b. It is significant here to emphasize that the current density of 50 mA cm−2 was not decreased at all throughout this prolonged potentiostatic electrolysis. This testifies its suitability to both PEM and alkaline water electrolyzers as cathodes for HER in place of noble Pt/C. The change in the nature of charge transfer was studied after the AD test once again by EIS analysis. The Nyquist plots of NiTe2 NWs in pH 0 and 14 acquired before and after AD tests are provided in Figure S8. The percent increases in Rct after the AD test of the NiTe2 NWs’ interface in pH 0 and 14 are 37.14% and 174%, respectively. The higher percent increase in Rct in alkaline conditions after the AD test reveals the reasons behind the observed large degradation in the activity of NiTe2 NWs. The results of the comparative HER studies carried out are provided in Table 2. The results of all electrochemical characterizations testified clearly that NiTe2 NWs are better than all catalytic systems studied including the state-of-the-art Pt/C in both highly acidic (0.5 M H2SO4) and alkaline (1 M KOH) conditions as far as high-rate H2 production is concerned. Moreover, the observed activity is far better than those of metal tellurides reported so far, which are also compared in Table S3.43−45 These encouraging results will surely help to improve the performance of the electrolyzers by substituting our NiTe2 NWs for a noble and expensive Pt cathode. Reverse Material Characterization after a HER Study in Highly Corrosive Acidic Conditions. Because the 3d nonprecious metal-based electrocatalysts are highly prone to dissolution in acidic conditions, we opted to study the robustness of both NiTe2 NFs and NiTe2 NWs after their HER studies in acidic conditions. Figure S9a shows the XRD patterns of NiTe2 NFs (red) and NiTe2 NWs (blue) acquired after HER studies. Both of them have shown prominent retention of the NiTe2 phase even after such harsh HER studies in acidic conditions. This was the first evidence that the prepared NiTe2 materials are stable enough in acidic conditions. Parts b and c of Figure S9 show the surface morphologies of NiTe2 NWs and NiTe2 NFs with the exact features evidenced before electrochemical studies and indicate that both of them were morphologically robust under HER conditions. Parts a and b of Figure S10 are the respective EDS

Figure 13. (a) LSVs acquired at 5 mV s−1 on NiTe2 NWs in both pH 0 and 14 before and after AD tests of 1000 rapid cyclic voltammetry cycles. (b) Prolonged potentiostatic electrolysis with NiTe2 NWs in both pH 0 and 14 for a current density of 50 mA cm−2.

pH 14. The increases in the overpotential observed at 500 mA cm−2 after AD tests for NiTe2 NWs in pH 0 and 14 are 25 and 51 mV, respectively. However, compared to earlier studies on the high-rate HER catalysts, the observed degradations in both pH 0 and 14 are much smaller, which indicates the superiority of the same over the others. Second, to assess the stability of the NiTe2 NWs’ catalytic interfaces, prolonged potentiostatic electrolysis was carried out for more than 1000 min at the overpotentials of 175 and 210 mV respectively in solutions of

Table 2. Results of the HER Studies Carried Out in 0.5 M H2SO4 and 1 M KOH medium 0.5 M H2SO4 (pH 0)

1 M KOH (pH 14)

catalytic interface

loading (mg cm−2)

overpotential at 10 mA cm−2 (mV)

overpotential at 100 mA cm−2 (mV)

overpotential at 500 mA cm−2 (mV)

Tafel slope (mV dec−1)

overpotential at a mass activity of 50 A g−1 (mV)

NiTe2 NWs NiTe2 NFs Pt/C/Ni foam Ni foam NiTe2 NWs NiTe2 NFs Pt/C/Ni foam Ni foam

1.98

125 ± 10

195 ± 4

275 ± 7

36

194 ± 5

2.02

193 ± 7

289 ± 5

494 ± 8

48

290 ± 6

1.8

28 ± 5

89 ± 5

332 ± 5

31

84 ± 5

564 ± 6 247 ± 5

b

91 69

c

1.98

418 ± 5 113 ± 5

249 ± 6

2.02

157 ± 5

335 ± 6

d

91

332 ± 8

1.8

66 ± 3

245 ± 5

d

120

232 ± 5

403 ± 8

595 ± 5

d

110

c

a

a

436 ± 8

Loading cannot be calculated for Ni foam substrate electrodes. bNi foam did not deliver 500 mA cm−2 within the experimental window of potential. Mass activities cannot be calculated for Ni foam substrate electrodes. dThese catalysts did not reach 500 mA cm−2.

a c

L

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500 mA cm−2 and lower Tafel slopes than NiTe2 NFs. Similarly, NiTe2 NWs were found to be better in catalyzing HER in high-current reaction rates in both pH 0 and 14. NiTe2 NWs required 275 ± 7 and 436 ± 8 mV as overpotentials to deliver 500 mA cm−2 in the solutions of pH 0 and 14, respectively. For the same current density of 500 mA cm−2 under identical conditions, the state-of-the-art Pt required 90 ± 5 and 165 ± 7 mV higher than our NiTe2 NWs, which certainly testifies that our catalyst had outperformed the noble Pt. In addition, NiTe2 NWs also showed better stability upon rapid cycling and 100% retention in activity during potentiostatic electrolysis for more than 1000 min. These encouraging findings certainly testify to the advantages of NiTe2 NWs over Pt for HER electrocatalysis in both high acidic and alkaline conditions. With these advantages, NiTe2 NWs can be utilized for large-scale water electrolysis in place of Pt as nonprecious and abundant catalysts.

spectra of NiTe2 NFs and NiTe2 NWs, which show the presence of all expected elements in addition to the incorporation of S because the HER study was carried out in sulfuric acid. Interestingly, the EDS spectrum of NiTe2 NFs showed the complete absence of B, which indicates that, upon HER study, the B moieties present along with the catalyst were extensively leached out. The weight percentage compositions of all elements in both NiTe2 NFs and NiTe2 NWs are provided as bar diagrams in Figure S10c,d for a better understanding. To gain further information on the chemical nature of the expected elements in both NiTe2 NFs and NiTe2 NWs, XPS analysis was done. Parts a−c of Figure S11 are high-resolution XPS spectra of the Ni 2p, Te 3d, and O 1s states of NiTe2 NWs after HER studies. The Ni 2p3/2 state of Figure S11a showed exactly the same features as those observed before HER studies, in which the peaks corresponded to metallic Ni, NiTe2, NiO, Ni(OH)2, and satellite. The same is true for the Ni 2p1/2 state. The Te 3d (Figure S11b) spectrum showed three distinct peaks for the presence of Te2−, Te−, and TeO2, which is slightly contradictory to the observation made before HER studies. The overreduction of Te− to Te2− is attributed to the exposure of NiTe2 NWs to the cathodically polarized environment during HER studies. Figure S11c also showed the presence of different O moieties, like those observed before HER studies. On the other hand, NiTe2 NFs showed interesting changes in the chemical nature of the expected elements after HER studies (Figure S12a−e). However, the Ni 2p spectrum (Figure S12a) of NiTe2 NFs showed features upon deconvolution, which showed the presence of metallic Ni, NiTe2, NiO, Ni(OH)2, and satellite peaks, similar to those observed before HER studies. Figure S12b, the Te 3d spectrum of NiTe2 NFs after HER studies, showed the presence of only Te− and TeO2. No overreduction of Te− to Te2− was seen with NiTe2 NFs, unlike that observed with NiTe2 NWs. The O 1s spectrum of NiTe2 NFs after HER studies (Figure S12c) also showed the presence of different O moieties, just like that of NiTe2 NWs. Interestingly, the B 1s spectrum of NiTe2 NFs after HER studies (Figure S12d) does not show any peak for the presence of B, which is in exact agreement with the EDS study. This information also implies that B has completely been leached out of NiTe2 NFs during HER studies in acid. In contrast, the Na 1s peak (Figure S12e) still showed the notable presence of Na in NiTe2 NFs even after such harsh HER studies in acid. All of the inferences made in the XPS studies are in good agreement with the earlier reports.41,42,45,46,53 The overall reverse material characterization mainly implied that both NiTe2 NFs and NiTe2 NWs are stable enough under harsh acidic HER environments and indicated that these materials can be taken as alternates to the expensive Pt/C catalysts.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02947. Details on the materials used and technical details on the instruments used for characterization along with the sample preparation methods for various characterizations and electrochemical studies, scheme related to controlled studies of the synthesis, and figures related to the C smart map of NiTe2 NWs and Nyquist plots (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] or [email protected]. Phone: (+91) 4565-241486 or (+91) 4565-241487. ORCID

Sengeni Anantharaj: 0000-0002-3265-2455 Kannimuthu Karthick: 0000-0003-2689-0657 Subrata Kundu: 0000-0002-1992-9659 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Vijayamohanan K. Pillai, Director, CSIRCECRI, for his support and encouragement. S.A. and K.K. thank the CSIR, New Delhi, and UGC, New Delhi, for the awards of SRF and JRF, respectively. The authors also extend their gratitude to V. Prabu, J. Kennedy, A. Rathishkumar, Mr. Ranjith, and other staff members of the Central Instrumentation Facility in CSIR-CECRI for their continuous support throughout this study.

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CONCLUSION A nickel telluride electrocatalyst of stoichiometry 1:2 (NiTe2) has been fabricated on a Ni foam substrate with two distinct morphologies such as NWs and NFs with short-time hydrothermal treatment of Ni foam and two different Te precursors, viz., NaHTe and Te powder with hydrazine hydrate. Consequently, fabricated NiTe2 NW and NiTe2 NF assemblies on Ni foam were studied for the first time for HER electrocatalysis at extreme pH conditions of pH 0 and 14 in comparison with the bare Ni and state-of-the-art Pt/C 20 wt % loaded Ni foam electrode. Between NiTe2 NWs and NiTe2 NFs, the former one has been found to be better in catalyzing HER in pH 0 and also in pH 14 with lower overpotentials at

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REFERENCES

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Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.7b02947 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.7b02947 Inorg. Chem. XXXX, XXX, XXX−XXX