NiTe2 Nanowire Outperforms Pt/C in High-Rate Hydrogen Evolution at Extreme pH Conditions

Article abstract of DOI:10.1021/acs.inorgchem.7b02947

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.

Full text of DOI:10.1021/acs.inorgchem.7b02947

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
NiTe Nanowire Outperforms Pt/C in High-Rate Hydrogen Evolution
2
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 electro-
catalysts over Pt is highly anticipated in water splitting. Such an
outperforming nonprecious electrocatalyst, nickel telluride (NiTe ), has
2
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 NiTe NWs and NiTe NFs were comparatively screened for hydrogen evolution reaction (HER) in extreme pH
conditions, viz., 0 and 14. NiTe NWs delivered current densities of 10, 100, and 500 mA cm at the overpotentials of 125 ± 10,
2
2
−2
2
1
95 ± 4, and 275 ± 7 mV in 0.5 M H SO . Similarly, in 1 M KOH, overpotentials of 113 ± 5, 247 ± 5, and 436 ± 8 mV were
2 4
required for the same current densities, respectively. On the other hand, NiTe NFs showed relatively poorer HER activity than
2
NiTe NWs, which required overpotentials of 193 ± 7, 289 ± 5, and 494 ± 8 mV in 0.5 M H SO for the current densities of 10
2
2
4
−2
−2
and 100 mA cm and 157 ± 5 and 335 ± 6 mV in 1 M KOH for the current densities of 10 and 100 mA cm , respectively.
Notably, NiTe 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 at 332 ± 5 mV, whereas NiTe NWs drove the same current
density with 57 mV less. These encouraging findings emphasize that a NiTe NW could be an alternative to noble and expensive
2
−2
2
2
Pt as a nonprecious and high-performance HER electrode for proton-exchange membrane and alkaline water electrolyzers.
INTRODUCTION
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
■
Storing intermittent energies as hydrogen through water
electrolysis is on the top of the list for energy conversion and
12
1
voltage and energy loss. Recently, equal importance is being
extended to HER also, with the sole aim of replacing Pt with
storage. 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”.
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
1
,3
nonprecious electrocatalysts. Sulfides, selenides, and phos-
phides of iron group metals have been reported as efficient
1
−3
1
,3
HER electrocatalysts in the past few years. Among them, the
phosphides of Ni, Co, and Fe are better HER catalysts than
13,14
sulfides and selenides. However, phosphide
is the most
reported pnictogenide of 3d transition metals reported for HER
-
pressure conditions and the hydrogen produced by water
1
electrocatalysis. In addition, nickel nitride is also reported to
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
15
be an active catalyst for HER.
In contrast, sulfides and selenides of 3d transition metals,
mainly the iron group metals, are equally studied and reported
1
,3
4
−7
in the literature. As far as Ni is concerned, there are reports
and precious electrocatalysts.
These half-cell reactions were
for the HER activity of Ni foam itself in acidic as well as alkaline
catalyzed by Pt, IrO /RuO , and their alloys with other metals
2
2
16,17
8
−11
conditions.
Among nickel chalcogenides, the sulfides and
in earlier days.
However, to make hydrogen production
selenides are richer in the literature than the tellurides. Nickel
economically affordable, these half-cell reactions of water
electrolysis must be catalyzed by nonprecious electrocatalysts
without any loss in their activities.
The recent evolution of nonprecious metals based on HER
and OER electrocatalysts, such as metal chalcogenides,
hydroxides, layered double hydroxides, and pnictogenides,
shows the importance given to electrocatalysis of HER and
sulfides have been reported in different stoichiometries such
18−21
22
23−26
3
NiS,
NiS2, and Ni S .
Moreover, these sulfides have
3 2
also been reported as composites with nanostructured carbon
3
2
,4
1
Received: November 20, 2017
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b02947
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Inorganic Chemistry
Article
2
7
materials. Similar to sulfides, selenides of Ni are also equally
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 cm of the Ni foam
28,29
rich in the literature.
Some of the important reports of
2
30
nickel selenides with different stoichiometries are NiSe,
3
1,32
33
34
after tellurization, MNiTe2 is the molecular weight of NiTe , and M is
2
Te
NiSe2,
Ni Se , and Ni Se . Like nickel sulfides, selenides
3 2 3 4
the atomic weight of Te. Hence, the calculated loading of NiTe from
of Ni are also reported as composites with nanostructured
carbon materials with improved activities.
sulfides and selenides of Ni had been alloyed with other 3d iron
group metals and reported with improved activities. Among
2
3
,35
the area normalized weight increase x (note that x is calculated to be
In addition, both
−2
0
.804 mg cm 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
3
6
26,37,38
39
40
them, NiFeSe2, NiFeS2,
NiCoS2, and NiCoSe2 are
−2
Information (SI) for details]) using eq 3 was 1.98 mg cm . Thus,
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
fabricated NiTe NWs were ready for further studies and character-
ization.
2
Fabrication of NiTe NFs. To fabricate NiTe NFs on Ni foam,
2
2
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
41
HER electrocatalysis. Similarly, nickel telluride has been
reported in the forms of NiTe, NiTe , and Ni Te nanoparticles
2
3
2
46
42−44
synthetic protocol of an earlier report. In addition, the fabricated
for HER electrocatalysis.
In addition, there is also another
NiTe NFs on Ni substrates took an extra hour to dry at 80 °C than
2
report on NiTe grown on Ni foam by hydrothermal treatment
NiTe NWs. At the end of drying, NiTe NFs on Ni foam substrates
45
2
2
for OER in alkaline conditions. Recently, Bhat et al. have
reported the synthesis and water splitting characteristics of
were ready for further studies and characterization. The probable
chemical reactions that occurred during the synthesis are as follows.
4
6
porous NiTe2. 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.
−
2
Te + NaBH (aq) → 2NaHTe(aq) + B(OH) (aq) + H (g)
4
4
2
(4)
+
+
2
NaHTe(aq) + Ni(s) → NiTe + 2H (aq) + 2Na (aq)
(5)
We have fabricated a nickel telluride catalyst of stoichiometry
2
NiTe that has a higher percent content of heteroatoms on Ni
2
The total mass loading of NiTe in NiTe NFs was calculated to be
2
2
foam. NiTe was obtained by shortening the hydrothermal
−2
2
2
.02 mg cm using eq 3, where the measured area normalized weight
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.
−2
increase was 0.821 mg cm , 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
2
30,34
of 1.5 cm (see the SI for details).
To ascertain that the chosen
experimental conditions are essential for NiTe
formation, a set of
2
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 Te ,
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
Such a difference in the morphology of NiTe has also
influenced their HER activity under identical conditions. The
comparative HER studies indicated that NiTe2 nanowires
2
(
NWs) are better HER catalysts than NiTe nanoflakes (NFs)
2
and state-of-the-art Pt.
EXPERIMENTAL SECTION
■
2
−
Fabrication of NiTe NWs. The following synthetic protocol is
2
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 NiTe NWs on Ni foam, four pieces of Ni foam (3 cm ×
2
0
.5 cm) were treated with HCl (3 M) under sonication for 5 min and
washed with deionized (DI) water before introduction into a Teflon-
lined 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
information on materials used in the fabrication of NiTe NWs and
2
NiTe NFs and technical details of instruments for characterization
2
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
NiTe NWs and NiTe NFs on Ni foam substrates were directly used
naturally cooled to room temperature, the NiTe -grown Ni foam
2
2
2
as electrocatalytic HER electrodes in 0.5 M H SO and 1 M KOH
(
black in color) pieces were washed several times with DI water and
2
4
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
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.
+
−1
2
Te + N H ·H O → 2H Te(aq) + H (aq) + N (g)
(1)
(2)
5 mV s and corrected for iR drop by 100%. The primary polarization
2
4
2
2
2
studies with NiTe NWs and NiTe NFs on Ni foam substrates were
2
2
+
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
2
H Te(aq) + Ni(s) → NiTe + 4H (aq)
2
2
The total mass loading of NiTe was calculated in the fabricated NiTe
NWs following the procedure referred to in the literature.
2
2
3
0,34
−1
(AD) test for 1000 cycles at a high scan rate (300 mV s ) by cyclic
voltammetry and prolonged HER at fixed overpotentials of the ability
−2
loading of NiTe = x mg × (M /M )
to drive the 50 mA cm 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
2
NiTe2
Te
=
x mg × (313.89/127.6) = 2.46 × x mg
(3)
B
DOI: 10.1021/acs.inorgchem.7b02947
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Inorganic Chemistry
Article
Figure 1. FESEM micrographs of NiTe NFs with gradually increasing magnification.
2
Figure 2. FESEM micrographs of NiTe NWs with gradually increasing magnification.
2
chronoamperometric responses were presented without compensating
The FESEM micrographs of NiTe2 NFs with increasing
for iR drop. All potential scales were converted to reversible hydrogen
magnification are provided in Figure 1a−d.
47−50
electrode (RHE) scale as per earlier reports.
An electrochemical
Figure 1a is the electron micrograph of NiTe at ultralow
2
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
H SO and 1 M KOH. The results of the electrocatalytic studies are
magnification, which clearly reveals that NiTe NFs are self-
2
assembled 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
2
4
discussed in subsequent sections.
RESULTS AND DISCUSSION
■
Material Characterization. Fabricated NiTe nanostruc-
2
tures on Ni foam substrates were directly subjected for
microstructural characterization such as field-emission scanning
electron microscopy (FESEM), transmission electron micros-
copy (TEM), and elemental color mapping with energy-
51,52
preparation of the Te metal powder also.
Figure 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 NiTe2
nanostructures fabricated using the Te metal powder and
dispersive X-ray spectroscopy (EDS). NiTe nanostructures
2
fabricated on Ni foam using NaHTe as the Te precursor were
found to have a hierarchical coral-like assembly of NiTe NFs.
2
C
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Figure 3. (a) FESEM micrograph of NiTe NFs, where EDS/mapping was done. (b) EDS spectrum of NITe NFs. (c−h) EDS elemental electronic
2
2
smart maps of Ni, Te, O, C, Na, and B, respectively.
Figure 4. (a) FESEM micrograph of NiTe NWs where EDS/mapping was done. (b) EDS spectrum of NiTe NWs. (c−e) EDS elemental electronic
2
2
smart maps of Ni, Te, and O respectively.
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
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 NiTe NFs and NiTe NWs
2
2
at low magnification.
micrographs of the same NiTe NWs array on Ni foam, which
The electronic smart maps of Ni, Te, C, O, B, and Na in
2
impeccably show that the NWs are uniformly grown over all of
the Ni foam substrate and the average diameter of a single NW
NiTe NFs on Ni foam are provided with the respective
electron micrograph and EDS spectrum in Figure 3a−d. Figure
2
D
DOI: 10.1021/acs.inorgchem.7b02947
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Inorganic Chemistry
a is the FESEM electron micrograph of NiTe 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,
Article
5
1,52
3
recovered from their ores by a carbothermal method.
The
2
overall EDS elemental color mapping results have given
significant insight into the elemental composition and their
distribution in coral-like NiTe NFs assemblies and in NiTe
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
2
2
stoichiometry is NiTe .
2
along with NiTe , the oxides of B with some Na have also been
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
2
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
(XRD) studies. The XRD patterns of NiTe NWs and NiTe
2 2
−1
NFs on Ni foam were acquired at a scan rate of 3° min within
the 2θ range of 10−90° and are pictured in Figure 5. The
observed O in NiTe NFs. The presence of C must be from Te
2
and Ni, which always contain some elemental C in them
because they were produced by carbothermal reduction from
5
1,52
their ores.
The presence of Na and B from the reducing
agent sodium borohydride used to prepare NaHTe. Interest-
ingly, the percentage composition of B is 5.2%, which is
significantly higher. This indicates that NiTe NFs also contain
2
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
NiTe NWs on Ni foam to reveal the elemental composition.
2
Figure 4a is the FESEM micrograph of a NiTe NW array on
Figure 5. XRD patterns of NiTe NFs (red) and NiTe NWs (blue)
2
2
2
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 NiTe NFs, NiTe NWs have shown no
acquired in the diffraction angle range of 10−90° with a scan rate of 3
−1
min . The asterisk symbols denote the peaks of metallic Ni from the
substrate.
2
2
additional impurities. Parts c−e of Figure 4 are the electronic
diffraction patterns of both NiTe NWs and NiTe NFs have
2
2
smart maps of the Ni K, Te K, and O K shells, respectively. The
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 water-
presence of O in NiTe NWs must be due to the surface
2
oxidation of Te and Ni, as observed with NiTe NFs. The
2
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 NiTe . The same
2
is true for NiTe NFs also. Moreover, the percent composition
2
34
of O in NiTe NWs is very less compared to that of NiTe NFs,
splitting electrocatalysts.
2
2
which indicates that the surface oxidation is comparatively
Other than these three Ni peaks, there are peaks with
lesser on NiTe NW assemblies fabricated over Ni foam using
considerable counts in the pattern of NiTe NWs that are
2
2
the Te metal powder and hydrazine hydrate. In addition to the
Ni, Te, and O peaks, the EDS spectrum also showed the
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
presence of some C, as was observed for NiTe NFs also.
55.0°, (201) at 58.0°, and (202) at 66.4° for the NiTe phase of
2
2
Similarly, to gain relative information on the weights and
ICDD reference card number 89-2021. This also matches well
42
atomic percentages of the elements present in NiTe NWs,
with the earlier report of NiTe2. Nevertheless, the diffraction
2
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
pattern of NiTe 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
2
equal to that of O. This indicates that, along with NiTe , the
Ni foam are NiTe . This is in agreement with the
2
2
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
stoichiometric prediction made from EDS elemental mapping
analysis that revealed that the percent composition ratio of Ni
and Te is ∼1:2 in both NiTe NW and NiTe NF assemblies
2
2
fabricated on Ni foam substrates.
E
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Figure 6. (a−c) TEM micrographs of NiTe NFs with increasing magnification. The insets of parts b and c are the SAED patterns and HRTEM
2
micrographs of the same. (d−f) TEM micrographs of NiTe NFs with increasing magnification. The insets of parts e and f are the SAED pattern and
2
HRTEM micrographs of the same.
To further ascertain the morphological and structural
information gathered from FESEM, elemental mapping, and
XRD analyses, both NiTe NFs and NiTe NWs were studied
pattern feature almost similar to that observed for NiTe NFs.
The two primary rings observed in the SAED patterns are
calibrated and found to belong to the diffraction planes of
2
2
2
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
(002) and (112), as observed with NiTe NFs. This is also in
2
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
NiTe NWs at the edge was acquired and is provided in the
2
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
Figure 3b is the respective SAED pattern of NiTe NFs that
2
shows dot and ring features. The two observed rings in the
HRTEM micrographs with clear lattice fringes of NiTe NFs
2
SAED pattern of NiTe NFs are calibrated and assigned to their
and NiTe NWs were confirmed once again to have anisotropic
2
2
corresponding diffraction planes of (002) and (112),
flakelike structures and hierarchical NW structures composed of
respectively, as per the ICDD reference card number 89-
NiTe crystalline sheets, respectively. This also revealed that the
2
2021. Similarly, the inset of Figure 3c is the HRTEM
Na and B impurities that were found to be incorporated in
micrograph of NiTe NFs at a very high magnification, which
NiTe NF assemblies did not alter the crystallinity or phase
2
2
shows clear lattice fringes. The measured distance between
these two fringes was 0.264 nm, which is in very close
formation of NiTe and must be present as an amorphous
impurity. The overall microstructural, spectroscopic, and
2
agreement with the (002) diffraction plane of the NiTe phase,
diffraction studies confirmed the formation of NiTe in the
2
2
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 NiTe2 NWs revealed the same
morphological information as that observed with FESEM
case of both NF and NW assemblies.
Although it has been confirmed that the material formed is
NiTe , it is mandatory to confirm the same with a more reliable
2
tool such as X-ray photoelectron spectroscopy (XPS). The
analysis (Figure 2a−d). The NWs of NiTe are clearly visible in
high-resolution XPS spectra observed with NiTe NFs are
2
2
the corresponding HRTEM micrographs provided in Figure
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 2p state
6d−f with increasing magnification. These HRTEM micro-
graphs 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
3
/2
upon deconvolution showed five different components, which
are located at 850.1, 853.1, 854.9, 856.2, and 860.1 eV,
NiTe NWs is provided in the inset of Figure 3e, which has a
corresponding to metallic Ni from the substrate, NiTe , NiO,
2
2
F
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Inorganic Chemistry
Article
Figure 7. High-resolution XPS spectra of the Ni 2p, Te 3d, O 1s, B 1s, and Na 1s states in NiTe NFs.
2
Ni(OH) , and the respective satellite peaks of all of them. The
a−c of Figure 8 are the high-resolution XPS spectra of the Ni
2
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
2p, Te 3p, Te 3d, and O 1s levels of NiTe NWs. Figure 8a
2
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
splitting of 10.3 eV (Figure 7b) upon deconvolution showed
−
mainly two peaks for the presence of Te and TeO . Both Te
2
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,
metallic Ni at 851.8 and 853.8 eV for NiTe and 857.8 eV for
2
−
0
4+
corresponding to Te , Te , Te , and their satellite peaks,
respectively. The same is true for the Te 3d3/2 state also. From
Ni(OH) , in addition to their corresponding satellite peak at a
2
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
Figure 7a,b, it is clear that NiTe NFs had the predominant
2
formation of NiTe , along with some surface oxides of Ni and
2
Te. Other high-resolution spectra of O 1s, B 1s, and Na 1s,
with NiTe , which is accompanied with some hydroxide of Ni,
2
which are provided in Figure 7c−e, indicated that NiTe NFs
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
2
have several oxygenated surface moieties that accompany both
Na and B in their composition. The above observations are in
4
2,44,46
good agreement with the earlier reports of NiTe2.
Parts
NiTe and other related materials fabricated over Ni foam
2
G
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Figure 8. (a) High-resolution XPS spectra of the Ni 2p and Te 3p levels in NiTe NWs. (b and c) High-resolution XPS spectra of the Te 3d and O
2
1
s levels in NiTe NWs.
2
3
4,42,45
56
substrates through hydrothermal treatment.
From the
characteristics. Interestingly, the presence of O as the M−
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 NiTe and also in agreement with an earlier report.
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
OH functionality is in resonance with the Ni(OH) peak
2
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
−
(
42
2
29,36,37,40
in good agreement with earlier reports as well.
overall XPS analysis confirmed once again that the formed
The
spin−orbit coupling that splits into Te 3d and Te 3d3/2
,
material is NiTe with some surface oxidation, which is on par
with the results of EDS mapping, XRD, SAED, and HRTEM
5
/2
2
respectively. When Te 3d was deconvoluted, four peaks at
5
/2
5
72.5, 574.1, 575.1, and 576.1 eV were observed, which
analyses. Moreover, the selective formation of NiTe over that
2
−
0
correspond to Te , Te , TeO , and its satellite peak. However,
the peak that corresponds to Te is the dominant one,
indicating that the formed material is mainly NiTe with some
Te and TeO due to surface oxidation. The same deconvoluted
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.
2
−
2
0
2
peak features were also observed for Te 3d3/2. These results are
in agreement with the earlier reports on NiTe and NiTe2.
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 Ni Te ;
42,45
To further gain information on the oxidized species, the high-
resolution 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
3
2
both of them are comparatively rich in Ni content compared to
NiTe . The overall specific binding energy values of each
2
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
−
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
34,42,45,53−55
oxidation of both Ni and Te.
Moreover, a recent
report on NiTe2 for OER also presented similar XPS
NiTe as NF and NW assemblies over Ni foam substrates,
2
H
DOI: 10.1021/acs.inorgchem.7b02947
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Inorganic Chemistry
Article
Table 1. Specific Binding Energy Values of Each
Deconvoluted Peak with Its Respective Species
Electrocatalytic Hydrogen Evolution Studies at Ex-
treme 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
catalyst
NiTe
state
binding energy (eV)
corresponding species
2
+
, Ni2+ of NiO,
Ni of Ni(OH) , and satellite
866.9, 870.1, 871.6, Ni, Te2− of NiTe
2
Ni 2p3/2
850.1, 853.1, 854.9, Ni, Ni of NiTe
2
2
+
NFs
856.2, and 860.1
2
as Te 3p1/2, Ni2+
of NiO, Ni of Ni(OH)
satellite
Ni 2p1/2
and Te
2
2
+
873.1, 875.5, and
878.1
2
, and
both highly acidic and highly alkaline media. To fill this gap, we
3
p1/2
studied our materials (NiTe NFs and NiTe NWs grown on
2
2
−
0
4+
Te 3d5/2
572.1, 573.2, 576.3, Te , Te , Te , satellite 1, and
77.8, and 579.8 satellite 2
583.1, 584.2, 586.7, Te , Te , Te , satellite 1, and
Ni foam) in detail for HER electrocatalysis in 0.5 M H SO as
2
4
5
well as in 1 M KOH. As a primary evaluation study, polarization
curves of NiTe NWs, NiTe NFs, Ni foam, and Pt/C were
−
0
4+
Te 3d3/2
and 589.9
satellite 2
2
2
−1
−
acquired at a slower scan rate of 5 mV s in both 0.5 M H SO
O 1s
530, 531.8, 532.4,
and 534.8
M−O−M, M−O , M−O−H, and
satellite
2 4
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, NiTe NWs, NiTe NFs, and Pt/C
B 1s
192.2 and 193.9
B−O and B−OH/B−OB
O−Na and B−O−Na
Na 1s
1072.9 and 1074.2
2
+
2+
NiTe2
Ni 2p3/2
851.8, 853.8, 857.8, Ni, Ni of NiTe , Ni of
2
2
2
NWs
and 861
Ni(OH)
2
, and satellite
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 NiTe NWs and NiTe NFs in pH 0 are 115 ± 5
869.9, 873.1, 876.7, Ni, Ni2+ of NiTe , Ni of
2+
Ni 2p1/2
2
and 879.2
820.7
871.5
2
Ni(OH) , and satellite
Te−
Te 3p3/2
Te 3p1/2
Te 3d5/2
Te−
−
0
572.9, 574.1, 575.1, Te , Te , TeO
and 576.1
2
, and satellite
2
2
and 150 ± 8 mV, respectively.
−
0
Te 3d3/2
O 1s
582.8, 583.5, 584.6, Te , Te , TeO
and 586.1
2
, and satellite
Similarly for the defined current densities of 10, 100, and 500
−
2
mA cm , NiTe NWs showed 125 ± 10, 195 ± 4, and 275 ± 7
−
2
530, 531.8, 532.4,
and 534.8
M−O−M, M−O , M−O−H, and
mV as overpotentials in pH 0, wherein NiTe NFs required
moisture
2
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 NiTe NWs is better than that of NiTe NFs.
2
2
electrocatalytic HER studies in extreme pH conditions were
conducted, the results of which are discussed below.
Interestingly, when the electrocatalytic interfaces were sub-
jected to a higher current density like 500 mA cm , the activity
−2
−1
Figure 9. (a and b) LSVs of Ni foam, NiTe NWs, NiTe NFs, and Pt/c acquired at a scan rate of 5 mV s in 0.5 M H SO and 1 M KOH
2
2
2
4
corresponding to pH 0 and 14, respectively. (c and d) Steady-state polarization curves of the same in 0.5 M H SO and 1 M KOH corresponding to
2
4
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.
trend was changed. For a high-rate current density of 500 mA
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
−2
cm , Pt/C and NiTe NWs required 335 ± 5 and 275 ± 7 mV,
2
respectively. This indicates that, at high-rate electrolysis, our
NiTe2 NWs outperformed the state-of-the-art Pt/C and
−2
delivered 500 mA cm with 57 ± 5 mV lesser overpotential.
In addition, NiTe NFs reached a current density of 500 mA
foam, NiTe NWs, NiTe NFs, and Pt/C in pH 0 (Figure 9c)
2 2
2
−1
−2
show their corresponding slope values (V dec ). In acidic
conditions, Pt/C showed the minimum Tafel slope of 0.031 V
dec to which the Tafel slope of NiTe NWs was closest (0.036
cm with 494 ± 8 mV as the overpotential. This indicates that
NiTe NWs are high-performance catalysts for HER in pH 0
2
−1
and perform better than Pt, which is highly significant and will
surely be beneficial for large-scale proton-exchange membrane
2
−1
V dec ). This indicates that NiTe NWs show almost parallel
2
kinetics to that of Pt/C in acidic conditions. Similarly, the Tafel
(
PEM) water electrolysis.
−1
slope of NiTe NFs was lower (0.048 V dec ) but not as low
The polarization curves obtained in pH 14 (in 1 M KOH),
2
as those of Pt/C and NiTe NWs. This once again indicates
pictured in Figure 9b, also show an activity trend similar to that
observed in pH 0 (in 0.5 M H SO ). The onset overpotentials
2
that, in acidic conditions, NiTe NWs are kinetically better than
2
2
4
NiTe NFs. However, the Tafel slope of Ni foam is the largest
2
of Pt/C, NiTe NWs, and NiTe NFs are almost the same as
2
2
−1
(
0.091 V dec ) among all of them, and this indicates that
those observed in pH 0, with negligible difference. However,
the overpotentials of 113 ± 5, 247 ± 5, and 436 ± 8 mV were
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
−2
required to reach 10, 100, and 500 mA cm . Pt/C is still the
better HER electrocatalyst even in alkaline conditions when
operations at lower overpotentials are considered. For 10 mA
exception that NiTe NWs showed a lower Tafel slope than Pt/
2
−
1
−2
C (0.120 V dec ). This indicates that, in high alkaline
cm , Pt/C, NiTe NWs, and NiTe NFs required 66 ± 3, 113
2
2
conditions, NiTe NWs are better kinetic HER interfaces than
2
±
5, and 157 ± 5 mV as overpotentials. However, when high-
Pt/C. Steady-state polarization analysis revealed that NiTe2
rate electrolysis was considered, NiTe NWs were the only
2
NWs are better than NiTe NFs in both pH 0 and 14, while
−2
2
catalysts to drive 500 mA cm by requiring just 436 ± 8 mV.
they are on par with Pt/C in pH 0 and better than Pt/C in pH
This indicates that, whether the solution pH is 0 or 14, the
1
4.
The repeatability of the results of polarization studies on
both the NiTe NWs and NiTe NFs interfaces was tested by
activity of NiTe NWs is always better than that of Pt/C as far
2
−2
as high-rate electrolysis for 500 mA cm is concerned. This
once again indicates that, for large-scale alkaline water
2
2
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
electrolyzers, NiTe NWs grown on Ni foam substrates could
2
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,
1
0.
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 NiTe NWs and NiTe
which indicates that the activity observed with both NiTe2
2
2
NWs and NiTe NFs is only of catalytic NiTe . The above
2
2
NFs, it can be seen that the overpotentials required for all
current densities are much lower in pH 0 than the ones
polarization studies revealed that, between NiTe NWs and
2
NiTe NFs, NiTe NWs are the best HER catalysts in both pH
required in pH 14 for the same. This indicates that NiTe NWs
2
2
2
0
and 14. Similarly, although Pt/C behaves better in lower-
and NiTe2 NFs are better at catalyzing HER in acidic
conditions than in alkaline conditions. Moreover, it is also
overpotential HER operations, NiTe NWs outperformed at
2
−2
high-current-density operations like 500 mA cm , which
once again evident from Figure 10 that NiTe NWs are better
2
indicates that, for large-scale bulk electrolysis, NiTe NWs are
than NiTe NFs in catalyzing HER in both pH 0 and 14. Like
2
2
better HER electrocatalysts than the state-of-the-art Pt/C.
the plot of current density versus overpotential, an equivalent
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plot of overpotential versus mass activity is also given by
ing scan rates acquired at both 0.5 M H SO and 1 M KOH,
2
4
2
calculating the mass activity for both NiTe NWs and NiTe2
and the same is provided in Figure 12a,b.
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 NiTe NWs are better than NiTe NFs in
2
2
both acidic and alkaline conditions.
The results of these electrochemical characterizations clearly
showed that NiTe NWs are better than NiTe NFs. Hence, it
2
2
is now essential to interrogate these two systems for the
observed difference in their activity. To do so, EIS spectra for
both NiTe NWs and NiTe NFs were obtained at their onset
2
2
potential of HER with a sinusoidal alternating-current
amplitude of 20 mV. Figure S5 gives the Cole−Cole plots of
NiTe NWs and NiTe NFs in pH 0 and 14. It can be seen
Figure 12. Plot of double-layer charging current density versus scan
rate for Ni foam, NiTe NFs, and NiTe NWs in acidic (a) and alkali
b) conditions.
2
2
from these plots that NiTe NWs showed lesser charge-transfer
2
2
2
(
resistance (R ) than NiTe NFs in both pH 0 and 14. The
ct
2
measured R values for NiTe NWs in pH 0 and 14 are 34 and
ct
2
8
Ω, respectively. This indicates that the charge-transfer ability
The measured slope values of Ni foam, NiTe NFs, and
2
−2
of NiTe NWs is better in alkaline conditions than in acidic
NiTe NWs were 2.74, 11.05, and 13.5 μF cm , respectively, in
2
2
conditions. Similarly, NiTe NFs have R values of 44 and 51 Ω
0.5 M H SO . Similarly, in 1 M KOH, the same interfaces were
2
ct
2
4
−2
in pH 0 and 14, respectively. Unlike, NiTe NWs, NiTe NFs
2.02, 5.24, and 7.72 μF cm , respectively. These 2C values
2
2
dl
showed higher R values in alkaline conditions. This trend tells
us the reason behind the difference in the observed activity
were related to the ECSA of the studied catalytic interfaces by
definition. Hence, from the observed trend, it is once again
ct
between NiTe NWs and NiTe NFs. The lowered charge-
clear that NiTe NWs were the only catalytic interfaces that had
2
2
2
transfer ability of NiTe NFs is attributed to the amorphous
a better relative ECSA value than both Ni foam and NiTe NFs.
2
2
impurities of Na and B incorporated in it, which may have a
This observation further backs up the activity trend observed in
both the voltammetric study and Tafel analysis.
serious effect in decreasing the conductivity of NiTe NFs. In
2
addition, to obtain a correlation between the relative electro-
chemically active surface area (ECSA) determined from the
double-layer capacitance (C ), Ni foam, NiTe NFs, and NiTe
2
NWs were taken. It is important to emphasize here that the Pt/
C interface cannot be subjected to such a study under identical
Having found that NiTe NWs are far better electrocatalytic
2
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.
dl
2
conditions where the H adsorption and desorption peaks
AD tests were carried out on NiTe NWs for 1000 cycles of
2
2
−1
would strongly interfere with the double-layer charging current.
The cyclic voltammograms (CVs) recorded for Ni foam, NiTe2
NFs, and NiTe NWs in 0.5 M H SO in a double-layer
cyclic voltammetry at a very high scan rate of 300 mV s .
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
2
2
4
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-
−2
high in alkaline conditions at a current density of 500 mA cm .
This shows that the NiTe NWs’ stability under such harsh
2
electrochemical cycling is comparatively higher in pH 0 than in
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Inorganic Chemistry
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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
0 mA cm− was not decreased at all throughout this prolonged
2
5
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 NiTe NWs in pH 0 and 14
2
acquired before and after AD tests are provided in Figure S8.
The percent increases in R after the AD test of the NiTe
ct
2
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 NiTe NWs. The
2
results of the comparative HER studies carried out are provided
in Table 2.
The results of all electrochemical characterizations testified
clearly that NiTe NWs are better than all catalytic systems
2
studied including the state-of-the-art Pt/C in both highly acidic
(
0.5 M H SO ) and alkaline (1 M KOH) conditions as far as
2 4
high-rate H production is concerned. Moreover, the observed
2
activity is far better than those of metal tellurides reported so
4
3−45
far, which are also compared in Table S3.
These
encouraging results will surely help to improve the performance
of the electrolyzers by substituting our NiTe NWs for a noble
2
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
Figure 13. (a) LSVs acquired at 5 mV s−1 on NiTe NWs in both pH
2
0
and 14 before and after AD tests of 1000 rapid cyclic voltammetry
robustness of both NiTe NFs and NiTe NWs after their
cycles. (b) Prolonged potentiostatic electrolysis with NiTe NWs in
2
2
2
−2
both pH 0 and 14 for a current density of 50 mA cm .
HER studies in acidic conditions. Figure S9a shows the XRD
patterns of NiTe NFs (red) and NiTe NWs (blue) acquired
2
2
pH 14. The increases in the overpotential observed at 500 mA
after HER studies. Both of them have shown prominent
retention of the NiTe phase even after such harsh HER studies
in acidic conditions. This was the first evidence that the
prepared NiTe materials are stable enough in acidic
conditions. Parts b and c of Figure S9 show the surface
morphologies of NiTe NWs and NiTe NFs with the exact
−2
cm after AD tests for NiTe NWs in pH 0 and 14 are 25 and
2
2
5
1 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
2
2
2
the NiTe 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
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
2
Table 2. Results of the HER Studies Carried Out in 0.5 M H SO and 1 M KOH
2
4
catalytic
loading
overpotential at 10
overpotential at 100
overpotential at 500
Tafel slope
overpotential at a mass
interface (mg cm−2)
mA cm (mV)
−2
mA cm (mV)
−2
mA cm (mV)
−2
(mV dec
−1
)
activity of 50 A g (mV)
−1
medium
0
1
.5 M H SO4 NiTe2
1.98
2.02
125 ± 10
193 ± 7
28 ± 5
195 ± 4
289 ± 5
89 ± 5
275 ± 7
36
194 ± 5
2
(
pH 0)
NWs
NiTe2
NFs
494 ± 8
48
31
290 ± 6
Pt/C/Ni
foam
1.8
a
332 ± 5
84 ± 5
c
b
Ni foam
418 ± 5
113 ± 5
564 ± 6
247 ± 5
91
69
M KOH
pH 14)
NiTe2
NWs
1.98
436 ± 8
249 ± 6
(
d
NiTe2
NFs
2.02
157 ± 5
66 ± 3
335 ± 6
245 ± 5
91
120
110
332 ± 8
d
d
Pt/C/Ni
foam
1.8
232 ± 5
a
c
Ni foam
403 ± 8
595 ± 5
a
b
−2
Loading cannot be calculated for Ni foam substrate electrodes. Ni foam did not deliver 500 mA cm within the experimental window of potential.
c
d
−2
Mass activities cannot be calculated for Ni foam substrate electrodes. These catalysts did not reach 500 mA cm .
L
DOI: 10.1021/acs.inorgchem.7b02947
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
spectra of NiTe NFs and NiTe NWs, which show the
500 mA cm− and lower Tafel slopes than NiTe2 NFs.
2
2
2
presence of all expected elements in addition to the
incorporation of S because the HER study was carried out in
Similarly, NiTe NWs were found to be better in catalyzing
2
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− in the solutions of pH 0 and 14,
respectively. For the same current density of 500 mA cm−
under identical conditions, the state-of-the-art Pt required 90 ±
sulfuric acid. Interestingly, the EDS spectrum of NiTe NFs
2
2
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 NiTe NFs and NiTe NWs are provided
2
5 and 165 ± 7 mV higher than our NiTe NWs, which certainly
2
2
2
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 NiTe NFs and NiTe NWs, XPS
testifies that our catalyst had outperformed the noble Pt. In
addition, NiTe NWs also showed better stability upon rapid
2
cycling and 100% retention in activity during potentiostatic
electrolysis for more than 1000 min. These encouraging
2
2
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 NiTe NWs
findings certainly testify to the advantages of NiTe NWs
2
2
after HER studies. The Ni 2p3/2 state of Figure S11a showed
exactly the same features as those observed before HER studies,
over Pt for HER electrocatalysis in both high acidic and alkaline
conditions. With these advantages, NiTe NWs can be utilized
2
in which the peaks corresponded to metallic Ni, NiTe , NiO,
for large-scale water electrolysis in place of Pt as nonprecious
and abundant catalysts.
2
Ni(OH) , and satellite. The same is true for the Ni 2p state.
2
1/2
The Te 3d (Figure S11b) spectrum showed three distinct peaks
2
−
−
for the presence of Te , Te , and TeO , which is slightly
ASSOCIATED CONTENT
2
■
contradictory to the observation made before HER studies. The
overreduction of Te to Te is attributed to the exposure of
NiTe 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, NiTe 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 NiTe NFs showed features upon deconvolution, which
showed the presence of metallic Ni, NiTe , NiO, Ni(OH) , and
satellite peaks, similar to those observed before HER studies.
*
S
Supporting Information
−
2−
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b02947.
2
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
2
(
2
2
2
smart map of NiTe NWs and Nyquist plots (PDF)
2
Figure S12b, the Te 3d spectrum of NiTe NFs after HER
2
−
studies, showed the presence of only Te and TeO . No
AUTHOR INFORMATION
Corresponding Author
E-mail: skundu@cecri.res.in or kundu.subrata@gmail.com.
2
■
*
−
2−
overreduction of Te to Te was seen with NiTe NFs, unlike
that observed with NiTe 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.
2
2
Phone: (+91) 4565-241486 or (+91) 4565-241487.
ORCID
Interestingly, the B 1s spectrum of NiTe NFs after HER
2
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
Sengeni Anantharaj: 0000-0002-3265-2455
Kannimuthu Karthick: 0000-0003-2689-0657
Subrata Kundu: 0000-0002-1992-9659
out of NiTe NFs during HER studies in acid. In contrast, the
2
Notes
Na 1s peak (Figure S12e) still showed the notable presence of
The authors declare no competing financial interest.
Na in NiTe NFs even after such harsh HER studies in acid. All
2
of the inferences made in the XPS studies are in good
ACKNOWLEDGMENTS
41,42,45,46,53
■
agreement with the earlier reports.
The overall
The authors thank Dr. Vijayamohanan K. Pillai, Director, CSIR-
CECRI, 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 Instrumenta-
tion Facility in CSIR-CECRI for their continuous support
throughout this study.
reverse material characterization mainly implied that both
NiTe NFs and NiTe NWs are stable enough under harsh
2
2
acidic HER environments and indicated that these materials can
be taken as alternates to the expensive Pt/C catalysts.
CONCLUSION
A nickel telluride electrocatalyst of stoichiometry 1:2 (NiTe )
■
2
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 NiTe NW and NiTe NF assemblies
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