Arousing the Reactive Fe Sites in Pyrite (FeS2) via Integration of Electronic Structure Reconfiguration and in Situ Electrochemical Topotactic Transformation for Highly Efficient Oxygen Evolution Reaction

Article abstract of DOI:10.1021/acs.inorgchem.9b01017

Despite significant advances in the development of highly efficient and robust oxygen evolution reaction (OER) electrocatalysts to replace noble-metal catalysts, commercializing OER catalysts with high catalytic activity for sustainable development still remains a great challenge. Especially, transition-metal Fe-based OER catalysts, despite their earth-abundant, cost-efficient, and environmentally benign superiorities over Co- and Ni-based materials, have received relatively insufficient attention because of their poor apparent OER activities. Herein, by rational design, we report Ni-modified pyrite (FeS₂) spheres with yolk-shell structure that could serve as pre-electrocatalyst precursors to induce a highly active nickel-iron oxyhydroxide via in situ electrochemical topological transformation under the OER process. Notably, as confirmed by the results of X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, and density functional theory (DFT) calculations, Ni doping could effectively regulate the intrinsic electronic structure of FeS₂ to realize a semiconductor-to-semimetal transition, which endows FeS₂ with dramatically improved conductivity and water adsorption ability, providing prequisites for subsequent topological transformation. Moreover, systematic post-characterizations further reveal that the optimal Ni-FeS₂-0.5 sample completely converts to amorphous Ni-doped FeOOH via an in situ electrochemical transformation with yolk-shell structure well-preserved under the OER conditions. The electronic structure modulation combined with electrochemical topotactic transformation strategies well stimulate the reactive Fe sites in Ni-FeS₂-0.5, which show impressively low overpotentials of 250 and 326 mV to drive the current densities (j) of 10 and 100 mA cm^-2, respectively, and a Tafel slope as small as 34 mV dec^-1 for the OER process. When assembled as a water electrolyzer for the overall water splitting, Ni-FeS₂-0.5 can display a low voltage of 1.55 V to drive a current density of 10 mA cm^-2, outperforming most of the transition-metal-based bifunctional electrocatalysts to date. This work may provide new insight into the rational design of other high-performance Fe-based OER electrocatalysts and inspire the exploration of cost-effective, ecofriendly electrocatalysts to meet the demand for future sustainable development.

Full text of DOI:10.1021/acs.inorgchem.9b01017

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Arousing the Reactive Fe Sites in Pyrite (FeS2) via Integration of
Electronic Structure Reconfiguration and in Situ Electrochemical
Topotactic Transformation for Highly Efficient Oxygen Evolution
Reaction
Zhi Tan,† Lekha Sharma,‡ Rita Kakkar,‡ Tao Meng,† Yan Jiang,† and Minhua Cao*,†
†Key Laboratory of Cluster Science, Ministry of Education of China, Beijing Key Laboratory of Photoelectronic/Electrophotonic
Conversion Materials, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, P. R. China
‡Computational Chemistry Laboratory, Department of Chemistry, University of Delhi, Delhi 110007, India
S
* Supporting Information
ABSTRACT: Despite significant advances in the development of
highly efficient and robust oxygen evolution reaction (OER)
electrocatalysts to replace noble-metal catalysts, commercializing
OER catalysts with high catalytic activity for sustainable development
still remains a great challenge. Especially, transition-metal Fe-based
OER catalysts, despite their earth-abundant, cost-efficient, and
environmentally benign superiorities over Co- and Ni-based
materials, have received relatively insufficient attention because of
their poor apparent OER activities. Herein, by rational design, we
report Ni-modified pyrite (FeS2) spheres with yolk−shell structure
that could serve as pre-electrocatalyst precursors to induce a highly
active nickel−iron oxyhydroxide via in situ electrochemical
topological transformation under the OER process. Notably, as
confirmed by the results of X-ray absorption spectroscopy, X-ray
photoelectron spectroscopy, and density functional theory (DFT) calculations, Ni doping could effectively regulate the intrinsic
electronic structure of FeS2 to realize a semiconductor-to-semimetal transition, which endows FeS2 with dramatically improved
conductivity and water adsorption ability, providing prequisites for subsequent topological transformation. Moreover,
systematic post-characterizations further reveal that the optimal Ni-FeS2-0.5 sample completely converts to amorphous Ni-
doped FeOOH via an in situ electrochemical transformation with yolk−shell structure well-preserved under the OER
conditions. The electronic structure modulation combined with electrochemical topotactic transformation strategies well
stimulate the reactive Fe sites in Ni-FeS2-0.5, which show impressively low overpotentials of 250 and 326 mV to drive the
current densities (j) of 10 and 100 mA cm−2, respectively, and a Tafel slope as small as 34 mV dec−1 for the OER process. When
assembled as a water electrolyzer for the overall water splitting, Ni-FeS2-0.5 can display a low voltage of 1.55 V to drive a current
density of 10 mA cm−2, outperforming most of the transition-metal-based bifunctional electrocatalysts to date. This work may
provide new insight into the rational design of other high-performance Fe-based OER electrocatalysts and inspire the
exploration of cost-effective, ecofriendly electrocatalysts to meet the demand for future sustainable development.
efficient OER electrocatalysts. Nevertheless, their natural
scarcity and high cost impede further applications.
1. INTRODUCTION
The energy crisis and environmental pollution have stimulated
the development of diverse sustainable energy conversion and
storage technologies. Among these technologies, electro-
chemical water splitting, which is a vital section of numerous
energy conversion techniques (such as rechargeable metal−air
batteries and regenerated fuel cells), has been recognized as
one of the most promising strategies to alleviate future energy
demands and environmental issues. However, the kinetically
sluggish oxygen evolution reaction (OER), owing to its
complex four-electron reaction pathway, severely hinders the
enhancement of efficiency for water splitting, which is the
bottleneck of water electrolysis. Noble-metal-based catalysts,
represented by IrO2 and RuO2, are well-recognized as the most
For these reasons, transition-metal-based (Ni, Co, Fe, etc.)
nonoxide materials involving their phosphides, selenides,
sulfides, and borides, etc., have been widely studied as
alternative OER electrocatalysts owing to their earth-abundant,
highly active, and low-cost features.1−4 However, most
researchers mainly focus on Co- and Ni-based materials, with
literature referring to Fe-based materials rarely reported,5,6
although they are more abundant and environmentally benign.
Indeed, Fe is used more as a dopant to enhance the
Received: April 8, 2019
© XXXX American Chemical Society
A
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performance of Ni/Co-based OER catalysts7−9 rather than a
host phase to accelerate the OER process, which is attributed
to the inferior activity of Fe-based OER catalysts caused by
their intrinsically low electrical conductivity and less exposed
active sites.6,10,11 Recently, many reports demonstrated by
combining experimental and theoretical studies that Fe is also
the active site for OER. For instance, Bell and co-workers
investigated the origin of dramatically enhanced activity of
mixed nickel−iron oxyhydroxides via operando X-ray absorp-
tion spectroscopy (XAS) and density functional theory (DFT)
calculation.12 The results show that Fe3+ in Ni1−xFexOOH
exhibits unusually short Fe−O bond distances and meanwhile
the OER intermediates have nearly optimal binding energy at
the Fe sites, confirming that Fe3+ species are the active sites for
OER. Boettcher and co-workers studied the roles of Fe and Co
in electrodeposited cobalt−iron (oxy)hydroxides through in
situ electrical measurements coupled with ex situ XRD and X-
ray photoelectron spectroscopy (XPS) characterizations and
drew the conclusion that Fe acts as a catalytic center, while Co
provides an electrically conductive, chemically stable host.10
Chen et al. synthesized an Fe-based OER electrocatalyst,
namely, ultrathin pyrrhotite (Fe7S8) nanosheets with a mixed-
valence state and intrinsic metallic character, which shows an
onset overpotential (η) of 270 mV at 10 mA cm−2 with a Tafel
slope of 43 mV decade−1, demonstrating highly active Fe sites
in the OER process.13 Song et al. also prepared a highly
efficient mono-Fe OER electrocatalyst, which displays an η of
283 mV at 10 mA cm−2 with a Tafel slope of 41.4 mV
decade−1, further confirming the activity of the Fe sites toward
OER.14
Inspired by the numerous contributions made above, we
attempted to design an Fe-based electrocatalyst with a high-
efficiency OER performance by addressing the factors that
cause its inferior apparent activity. We chose the earth-
abundant pyrite (FeS2) as the research subject for the OER
process although it is rarely studied as an OER catalyst because
of its poor performance. In view of the restraining factors in
the electrocatalytic process,15 namely, the intrinsic electrical
conductivity, the number of active sites, and the reaction
energy barrier, we sought to promote the OER property of
FeS2 from these aspects. Given that semiconductor character-
istics and less exposed active sites are the main factors that
cause the poor performance of FeS2,16,17 yolk−shell-structured
FeS2 spheres were first constructed to ensure more exposed
active sites, and then through a composition modulation
strategy, we introduce Ni into the lattice of FeS2 so as to
regulate its intrinsic properties like the electrical conductivity
and hydrophilicity.18−20 Meanwhile, considering the previous
reports that transition-metal sulfides/phosphides can be in situ
partially or entirely transformed into highly active metal
(oxy)hydroxides during the OER process,14,21,22 we anticipate
that the Ni doping strategy, together with sulfidation process,
will endow the catalyst with more rapid charge and mass
transfer, a more hydrophilic surface, and an optimal
composition so as to induce the entire conversion to the
most active nickel−iron (oxy)hydroxide species.23 Moreover, it
is believed that the sulfides are promising bifunctional catalysts,
and thus we also expect that sulfidation will benefit the water-
splitting process.6
oxyhydroxide via in situ electrochemical topological trans-
formation during the OER process. This electrochemical
reconstruction-derived catalyst could deliver current densities
of 10 and 100 mA cm−2 at low overpotentials of 250 and 326
mV, respectively, and a Tafel slope as low as 34 mV dec−1 for
the OER process, and this performance even surpasses those of
the benchmark IrO2 catalyst and most transition-metal-based
(Ni, Co, Fe, etc.) OER catalysts recently reported. XPS and
synchrotron-based XAS as well as DFT calculations were
conducted to manifest the effective reconfiguration of the
electronic structure of FeS2 by Ni doping, and the results
indicate that Ni doping induces a transition from semi-
conductor to semimetal and a promoted adsorption ability of
H2O for FeS2. To gain deep insight into the active form of the
catalyst during the catalytic process, Ni-FeS2-0.5 after long-
term OER testing was elaborately characterized by compre-
hensive ex situ measurements, which reveal that the Ni-doped
FeS2 spheres undergo a topotactic conversion during the
electrochemical oxidation process and transform into hier-
archical Ni-FeOOH spheres with the original frameworks and
Fe/Ni molar ratio retained. Finally, the water-splitting
performance of Ni-FeS2-0.5 was also evaluated, and, excitingly,
the Ni-FeS2-0.5 catalyst demonstrates an activity superior to
most of the transition-metal-based bifunctional electrocatalysts
to date. This work may inspire the exploration and design of
highly active Fe-based catalysts and provide a better under-
standing of the role that Fe plays in the OER process.
2. EXPERIMENTAL SECTION
Synthesis of Ni-FeS2-0.5. In a typical synthesis of a nickel−iron
alkoxide precursor, 8 mL of glycerol was dissolved in 42 mL of
isopropyl alcohol (IPA) to form a transparent colorless solution. Then
0.125 mmol of Ni(NO3)2·6H2O and 0.25 mmol of Fe(NO3)3·9H2O
were added to the above solution under vigorous stirring. After that,
the obtained solution was transferred into a Teflon-lined stainless
steel autoclave and kept at 140 °C for 12 h. Finally, the autoclave was
cooled naturally, and then the faint yellow precipitate was collected by
centrifugation, alternatively washed with absolute ethanol and water,
and dried. For the preparation of Ni-FeS2-0.5, the precursor and S
powder were placed in a quartz tube furnace, with the precursor in the
center of the tube, while the S powder was upstream. With a heating
rate of 5 °C min−1, the sample was then annealed at 370 °C and
maintained for 4 h under a N2 atmosphere.
Synthesis of Ni-FeS2-1. Ni-FeS2-1 was prepared with a procedure
similar to that of the Ni-FeS2-0.5 sample, except that the molar ratio
of Ni-to-Fe was changed to 1:1 [n(Ni) + n(Fe) = 0.375 mmol].
Synthesis of Ni-FeS2-0.25. Ni-FeS2-0.25 was prepared with a
procedure similar to that of the Ni-FeS2-0.5 sample, except that the
molar ratio of Ni-to-Fe was changed to 1:4 [n(Ni) + n(Fe) = 0.375
mmol].
Synthesis of FeS2. FeS2 was prepared with a procedure similar to
that of the Ni-FeS2-0.5 sample, except that Ni(NO3)2·6H2O was not
used and the molar amount of Fe(NO3)3·9H2O was increased to
0.375 mmol.
Synthesis of NiS2. NiS2 was prepared with a procedure similar to
that of the Ni-FeS2-0.5 sample, except that Fe(NO3)3·9H2O was not
used and the molar amount of Ni(NO3)2·6H2O was increased to
0.375 mmol and the holding time at 370 °C was reduced to 2.5 h.
The Pt/C catalyst was commercially purchased from Johnson
Matthey Co. Ltd. (20 wt % Pt in C, crystallite size 3.5 nm, and carbon
black XC72R), and the IrO2/C catalyst was synthesized according to
the literature.24
Herein, we demonstrate a facile strategy to obtain yolk−
shell-structured Ni-doped FeS2 (Ni-FeS2) spheres as a high-
efficency OER precatalyst. The optimized Ni-FeS2-0.5 spheres
can induce an entire conversion to highly active nickel−iron
Materials Characterization. The morphology and structure of
the samples were characterized by scanning electron microscopy
(SEM; JSM-6490LV) and transmission electron microscopy (TEM;
JEOL JEM-2010) as well as high-resolution transmission electron
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microscopy (HRTEM). High-angle annular dark field (HAADF)
scanning transmission electron microscopy (STEM) images and
elemental mapping were conducted on a JEOL JEM-2100F
microscope (200 kV). Powder X-ray diffraction (XRD) was
conducted to analyze the crystal structure, which was in the range
of 10−80° (2θ) on a Bruker D8 Advance diffractometer. N2 sorption
measurements were measured at liquid-nitrogen temperature (77 K)
on a Belsorp-max sorption analyzer. Before the experiments, the
sample was degassed at 200 °C for 5 h. The surface area and pore-size
distribution were calculated by the Brunauer−Emmett−Teller (BET)
and Barrett−Joyner−Halenda (BJH) methods, respectively, from the
adsorption branch. X-ray photoelectron spectroscopy (XPS) was
exploited to identify the chemical states by using an ESCALAB 250
spectrometer (PerkinElmer), and the C 1s level at 284.8 eV was taken
as a reference to calibrate the binding energies. Fourier transform
infrared (FT-IR) spectra were collected on a Bruker VECTOR 22
spectrometer with the KBr disk method. The X-ray absorption near-
edge structure (XANES) was measured in transmission mode using a
Si(111) double-crystal monochromator at 1W1B station in Beijing
Synchrotron Radiation Facility (BSRF). The acquired extended X-ray
absorption fine structure (EXAFS) data collected in ambient
conditions were analyzed with the ATHENA module, implemented
in the IFEFFIT software packages. Raman spectra were obtained on
an Invia Raman spectrometer with a laser excitation of 532 nm.
Computational Methods. The electronic structure calculations
based on the generalized gradient approximation (GGA)-Perdew−
Burke−Ernzerhof (PBE) method with spin polarization were
performed in the Vienna ab initio simulation package (VASP) code.
The projected-augmented-wave potential together with the DFT+U
method was used with UFe= 2 eV for the Fe d orbitals and UNi = 3 eV
for the Ni d orbitals. The cutoff energy was 400 eV, and the K-point
mesh was 3 × 6 × 6. For the structure optimization, the residual
forces on all atoms were less than 0.01 eV Å−1. In this paper, we
explored the 2 × 1 × 1 supercells with and without Ni-doped FeS2.
The densities of states (DOSs) of all systems were calculated. First-
principles DFT calculations were performed using Dmol25,26 in the
Materials Studio 5.5 package from Accelrys Inc. Numerical basis sets of
double-ζ-quality plus polarization functions, which is the numerical
equivalent of the Gaussian basis, 6-31G**, but is much more accurate,
were used in calculations of the water adsorption energies. The cores
were treated using DFT semilocal pseudopotentials, specifically
designed for DFT calculations.26 The GGA-PBE functional27,28 was
employed in the calculations. Spin-unrestricted calculations were
performed using a Fermi broadening29 of 0.005 Ha to smear the
occupation of bands around the Fermi energy, and the results were
extrapolated to T = 0 K. The convergence criteria in energy, force,
and displacement were set to 2 × 10−5 Ha, 0.004 Ha Å−1, and 0.005
Å, respectively. The subsequent results are based on using a Brillouin
zone sampling by a Γ-centered Monkhorst−Pack scheme30 with a 4 ×
4 × 1 k-point grid. A geometry-optimized FeS2 cell was used to build
Fe-terminated slabs (with and without Ni doping) of thickness 4.4 Å
and a vacuum slab of 15 Å both above and below the layer. The water
adsorption energies of bare and Ni-doped (210) FeS2 slabs were
calculated based on the following equation:
CHI 760E electrochemical workstation (CH instrument, Chenhua,
China), wherein a graphite rod and a Hg/HgO (KOH-saturated)
electrode were used as the counter and reference electrodes,
respectively. The potentials measured were referenced to a reversible
hydrogen electrode (RHE) according to the Nernst equation E(RHE)
= E(Hg/HgO) + 0.098 + 0.059pH, and all current densities were
normalized to the geometrical surface area. Linear-sweep-voltammetry
(LSV) measurements were conducted, if not specified, at a scan rate
of 5 mV s−1 in a N2/O2-saturated 1 M KOH solution by using IR
compensation with a degree of 90%. The Tafel slope was obtained
from the corresponding LSV curves according to the Tafel equation η
= b log(j/j0). Electrochemical impedance spectroscopy (EIS) tests
were carried out from 500000 to 0.01 Hz with an amplitude of 5 mV
at an applied potential of 1.5 V versus RHE. An electrochemical
surface area (ECSA) was determined by the tested electrochemical
double-layer capacitance (Cdl) obtained from cyclic voltammetry
(CV) measurement. When Δj (ja − jc) was plotted against the scan
rate, a linear slope that is twice Cdl was acquired. The long-term
durability was conducted by a chronoamperometric curve in a 1 M
KOH solution.
With regard to the overall water-splitting measurements, Ni foam
(NF) was preprocessed by HCl, water, and ethanol, in turn, so as to
eliminate the oxides and impurities. Then, the catalyst was dropped
onto NF with a loading of 2 mg cm−2 and used as both the anode and
cathode. The polarization curve was recorded in N2-saturated 1 M
KOH at a scan rate of 5 mV s−1.
3. RESULTS AND DISCUSSION
In the current work, we developed a facile solvothermal
approach coupled with subsequent sulfidation to synthesize
highly uniform yolk−shell-structured Ni-FeS2 spheres. As
schematically illustrated in Figure 1, the synthesis strategy
Ewater ads = EFeS
− (Ewater + EFeS )
2
2/water complex
where Ewater ads is the water adsorption energy of the surface,
EFeS /water complex is the total energy obtained for the water-adsorbed
2
FeS2 surface, Ewater is the total energy of the water molecule, and EFeS
is the total energy of the FeS2 surface.
2
Figure 1. Schematic illustration of the fabrication of yolk−shell-
structured Ni-FeS2 spheres.
Electrochemical Measurements. The working electrode was
prepared by dispersing a catalyst powder of 4 mg into 1 mL of
solution containing 900 μL of ethanol and 100 μL of a Nafion
solution (5 wt %). Afterward, the slurry was mixed by ultrasonication
to obtain a homogeneous ink. Then 30 μL of ink was extracted,
coated onto a glassy carbon (GC) electrode (D = 3 mm), and left to
dry naturally.
mainly consists of two steps. The first step is to prepare
uniform nickel−iron alkoxide solid spheres as the precursor by
using Fe(NO3)3·9H2O as the Fe source, Ni(NO3)2·6H2O as
the Ni source, and glycerol together with IPA as a mixed
solvent. Later on, by means of a nonequilibrium heat treatment
process, the nickel−iron alkoxide solid spheres could be easily
transformed into yolk−shell-structured nickel−iron sulfides
The hydrogen evolution reaction (HER) and OER performances
were evaluated in a standard three-electrode configuration using a
C
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Figure 2. SEM images of the Ni-Fe-0.5 precursor (a) and Ni-FeS2-0.5 (b and c). (d and e) TEM images of Ni-FeS2-0.5. (f) HRTEM image of Ni-
FeS2-0.5. The inset is the corresponding SAED pattern. (g and h) HAADF-STEM image of Ni-FeS2-0.5 and the corresponding line-scan profiles.
(i) Elemental mapping images.
(denoted as Ni−Fe−X for precursors and Ni-FeS2-X for
sulfurized products; X represents the Ni/Fe molar ratio of raw
materials). Previously, numerous synthetic strategies and
associated formation mechanisms had been proposed as
guidances for the design of hollow structures.31−33 Here, we
assume that the rational evolution might be that the large
temperature gradients along the radial direction will drive the
formation of the core−shell structure in the final product when
the heating rate was set at a relatively high degree (5 °C
min−1).34 Hence, on the interface between the shell and core,
there will exist two forces in opposing directions, namely, the
contraction force (Fc) induced by the oxidative degradation of
the organic species and the adhesion action (Fa) from the
relatively dense shell.35 Fc results in the inward shrinkage of the
nickel−iron alkoxide core, while Fa hinders the inward
contraction of the precursor core. When Fc is dominant, the
inner core will proceed to shrink as a consequence of the loss
of the organic species and thus a yolk−shell structure will be
generated. Synchronously, the S powder loaded in the
combustion boat will react with the precursor to form metal
sulfides.
The morphologies and microstuctures of the as-prepared
nickel−iron alkoxide precursor and corresponding sulfurized
products (Ni-FeS2) were investigated by an electron
microscopy technique (Figure 2a−e and Figures S1−S4).
The SEM (Figure 2a) and TEM (Figure S3) images clearly
show that the Ni-Fe-0.5 precursor was featured with a highly
uniform spherical morphology with an average diameter of 680
nm and that the spheres in nature are solid with a fairly smooth
surface. After sulfuration treatment, the resultant product still
maintains the spherical morphology of the precursor but with a
rougher surface. Meanwhile, the sulfurized product may
possess a yolk−shell structure from individual broken spheres
(marked by arrows; Figure 2b,c). Moreover, TEM images
further confirm that the yolk−shell structure is successfully
obtained via the sulfuration treatment (Figure 2d,e). The
HRTEM image (Figure 2f) recorded on the edge of a single
yolk−shell sphere shows clear lattice fringes with a spacing of
0.244 nm, which can be assigned to the (210) plane of FeS2.36
The selected-area electron diffraction (SAED) pattern (the
inset in Figure 2f) displays a polycrystalline nature of the yolk−
shell spheres. Energy-dispersive X-ray spectroscopy (EDS)
line-scan results demonstrate a well-defined yolk−shell
structure for Ni-FeS2-0.5 (Figure 2g), while the EDS mapping
images (Figure 2h,i) not only show a homogeneous
distribution of Fe, Ni, and S elements but also reveal that
the core and shell have the same components. Furthermore,
the specific surface area and porous nature of Ni-FeS2-0.5 were
also investigated by nitrogen sorption measurements. The BET
surface area was tested to be 45 m2 g−1, and the pore sizes are
mostly below 10 nm (Figure S5). All of the results above
indicate that well-defined yolk−shell-structured Ni-FeS2
spheres were achieved, and they might provide fast transfer
channels for electrons and more accessible active sites to the
electrolyte ions.
Powder XRD and FT-IR spectroscopy (Figures S6 and S7)
were conducted to investigate the crystal structure and
composition of the Ni-Fe-0.5 precursor spheres. As revealed
in Figure S6, the XRD pattern of the Ni-Fe-0.5 spheres shows a
weak diffraction peak at 10.5°, which is characteristic of metal
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Figure 3. (a) XRD patterns of the as-prepared Ni-FeS2-0.5, Ni-FeS2-0.25, and FeS2. The inset is magnified XRD patterns of the (200) peak. (b)
XPS survey spectrum of Ni-FeS2-0.5. (c) Comparison of high-resolution Fe 2p between bare FeS2 and Ni-FeS2-0.5. High-resolution XPS spectra of
Ni 2p (d) and S 2p (e) for Ni-FeS2-0.5.
Figure 4. (a) Fe K-edge XAFS k3[χ(k)] oscillation curves and (b) normalized Fe K-edge XANES spectra of Ni-doped FeS2 and FeS2 (the inset is
an enlarged view of peak B). FT-EXAFS spectra of (c) Fe and (d) Ni in Ni-doped and bare FeS2.
glycerate.37,38 Meanwhile, the FT-IR absorption peaks located
at 3335, 2920, and 2855 cm−1 are in good accordance with the
hydrogen-bonded hydroxyl groups and typical C−H stretching
vibrations,38 further confirming the existence of the organic
alkoxide in the precursor. Therefore, the as-formed Ni-Fe-0.5
precursor spheres are composed of amorphous metal alkoxide.
Figure 3a displays the XRD patterns of sulfurized samples with
different Ni/Fe molar ratios, all of which could be readily
42-1340). Particularly, no diffraction peaks related to Ni-based
phases could be detected. Also, the diffraction peaks of Ni-
FeS2-X (X = 0.25, 0.5) shift to lower angles compared with
their counterparts in FeS2 [magnified (200) peak in Figure 3a].
Such a negative shift indicates a lattice expansion, further
proving the substitution of Ni atoms at the Fe sites within the
materials.18 When the Ni/Fe molar ratio is increased to X = 1,
additional peaks belonging to ternary Ni0.5Fe0.5S2 are detected
at 32.1°, 39.6°, and 54.6°, indicating the occurrence of phase
̅
indexed to cubic-phase FeS2 with the Pa3 space group (JCPDS
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Figure 5. (a) Structure representations and the corresponding DOSs of pristine FeS2, Ni-FeS2-0.25, and Ni-FeS2-0.5. (b) Geometry-optimized
structures and relevant water adsorption energies of water-adsorbed pristine FeS2, Ni-FeS2-0.25, and Ni-FeS2-0.5, wherein the yellow spheres
denote S, the bluish-violet spheres Fe, the rose spheres Ni, the red spheres O, and the white spheres H.
separation in the Ni-FeS2-1 sample (Figure S8).17 Further-
more, the inductively coupled plasma atomic emission
spectroscopy (ICP-AES) and EDS analysis for Ni-FeS2-X (X
= 0.25, 0.5) samples demonstrate that the actual Fe/Ni atom
ratios in Ni-FeS2-0.25 and Ni-FeS2-0.5 are 6.0 and 2.7,
respectively (Table S1 and Figure S9). The results above
suggest that the Ni atoms, in the form of the doping state, are
probably stabilized in the FeS2 lattice.
XPS was carried out to investigate the effect of Ni doping on
the chemical state of Ni-FeS2-0.5. The survey XPS spectrum
reveals the presence of Fe, Ni, and S elements (Figure 3b),
consistent with the result from the elemental mapping images
(Figure 2i). Furthermore, as depicted in Figure 3c, the high-
resolution Fe 2p XPS spectrum displays two peaks located at
about 707.2 and 720.0 eV, which could be assigned to the
spin−orbit doublet of the 2p orbital of Fe2+ in FeS2.16
Especially, these two peaks both show an obvious downshift in
contrast to those of pure FeS2, suggesting that the electrical
potential environment of the Fe centers has been modified
through Ni doping.39−41 The broad shoulder peak at 711.1 eV
possibly belongs to the Fe3+ species from the surface oxidation
under ambient air.42 In high-resolution Ni 2p XPS spectrum
(Figure 3d), the doublets located at 853.6 and 870.8 eV have
binding energies between nickel metal and nickel oxides,
indicative of the presence of nickel sulfides,43−46 while the
other two peaks situated at 855.5 and 873.3 eV, respectively,
are attributable to oxidized Ni species.41,47 The remaining two
peaks are satellite peaks. With regard to the S 2p spectrum, the
two sharp peaks appearing at about 162.4 and 163.6 eV
(Figure 3e) are assigned to S 2p3/2 and S 2p1/2
,
respectively.48−50 The minor peak occurring at about 164.5
eV is due to polysulfides (Sn2−),51 while the weak peak at 168.7
eV could be recognized as a S−O bond derived from surface
oxidization.52−54
Synchrotron XAS was performed to further investigate the
local structure and electronic nature of Ni-FeS2-X (X = 0.25,
0.5). As displayed in Figure 4a, the Fe K-edge k3[χ(k)]
oscillation curve of Ni-FeS2-X is very similar in profile to that
of FeS2, confirming the good maintenance of the FeS2
structure after Ni incorporation.18 XANES was exploited to
evaluate the effect of Ni doping on the electronic structure of
Ni-FeS2-X. As indicated in the Fe K-edge XANES spectra
(Figure 4b), the profiles of Ni-FeS2-X are consistent with that
of pristine FeS2, further suggesting that the Ni-FeS2-X samples
have structures analogous to that of FeS2. In addition, the Fe
K-edge XANES spectra of Ni-FeS2-X involve three main
excitations, which are labeled as A−C, respectively. Here, peak
A is a pre-edge peak generally arising from a Fe 1s → 3d
dipole-forbidden transition, which becomes allowed in the
condition of distorted FeO6 octahedral or deviation from
centrosymmetric coordination.55,56 The shoulder peak marked
as B represents a ligand-to-metal charge transfer, and peak C is
due to a Fe 1s → 4p electric dipole-allowed transition.55−57
For all of the samples, peak A shows a weak signal intensity
ascribed to the transition from the Fe 1s core level to the 3d
unoccuped state, which emerged as a result of a distorted FeO6
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Figure 6. (a) OER polarization curves and (b) the corresponding Tafel plots of Ni-FeS2-0.5, Ni-FeS2-0.25, FeS2, Pt/C, and IrO2/C at 5 mV s−1 in 1
M KOH. (c) Summary and comparison of the overpotentials at 10 mA cm−2 and Tafel slopes for Ni-FeS2-0.5, Ni-FeS2-0.25, FeS2, Pt/C, and IrO2/
C, respectively. (d) Cdl of Ni-FeS2-0.5, Ni-FeS2-0.25, and FeS2. (e) Nyquist plots of post-OER Ni-FeS2-0.5, Ni-FeS2-0.25, and FeS2 at an
overpotential of 270 mV. The inset shows a partial zoomed-in image. (f) Durability test of Ni-FeS2-0.5 at an overpotential of 250 mV. The inset
displays the SEM image for Ni-FeS2-0.5 after a 15 h stability test.
octahedron and the Fe 3d−4p orbital mixing enabled by
deviation from centrosymmetric coordination.17,56 However, a
relatively obvious distinction is detected at peak B among these
three materials (the inset in Figure 4b). Notably, Ni-FeS2-0.5
exhibits the lowest intensity, followed by Ni-FeS2-0.25 and
FeS2, revealing an increased occupancy of the Fe valence state
after Ni doping. This result further demonstrates that a
secondary charge transfer has taken place from Ni to Fe in the
Ni-FeS2-X samples,58,59 which well matches with the result
from previous XPS measurements. Moreover, the bonding
environment of Fe atoms was further investigated by Fourier-
transformed (FT-)EXAFS at the Fe edge for Ni-FeS2-0.5,
which shows a dominant peak at approximately 1.85 Å (Figure
4c), corresponding to the Fe−S coordination,17,60 whereas as
in the case with Ni-FeS2-0.25, the peak intensity of Ni-FeS2-0.5
is slightly decreased compared with FeS2, implying surface
disorder caused by Ni incorporation (Figure S10).61 Figure 4d
shows Ni K-edge EXAFS spectra of Ni-FeS2-X in the R space,
which reveal a main peak assigned to Ni−S bonding at about
1.93 Å.62 Moreover, no Ni−Ni bonding is found in the
coordination shells. Besides, the Ni K-edge XANES spectra
and k3[χ(k)] oscillation curve of Ni-FeS2-X are also dissimilar
to that of Ni foil (Figure S11). These results above confirm not
only the successful doping of Ni into the crystal structure of
FeS2 but also the electronic structure changes of Ni-doped
FeS2, which might offer more active sites to promote the
catalytic performance of FeS2.
conductor to a semimetal is observed for both Ni-FeS2-0.25
and Ni-FeS2-0.5, indicating an enhanced charge-transfer
kinetics. Such a transition behavior might be caused by the
electron transfer from Ni to Fe, as elaborated by the XPS and
XANES results above (Figures 3c and 4b), where new energy
levels of Ni are induced within the forbidden zone of FeS2 and
are close to overlapping with the conduction band.19
Moreover, considering that the adsorption energy of water
plays a vital role in estimating the HER and OER activity of a
catalyst in an alkaline solution, DFT calculations were further
conducted to study the effect of Ni doping on the water
adsorption ability of FeS2. As displayed in Figure 5b, geometry-
optimized structures for FeS2, Ni-FeS2-0.25, and Ni-FeS2-0.5
were established. The derived water adsorption energies on Ni-
FeS2-0.5 and Ni-FeS2-0.25 are −1.43 and −1.2 eV, respectively,
which are much lower than that of FeS2 (−0.93 eV), indicating
a favorable water activation process on the surface of Ni-
modified FeS2. The enhanced conductivity combined with
good hydrophilicity of Ni-FeS2-0.5 might accelerate in situ
electrochemical transformation to obtain highly active nickel−
iron (oxy)hydroxide species.
To evaluate the effects of electronic structure modification
of FeS2 by Ni on its electrocatalytic performance, the OER
activities of Ni-FeS2-X, FeS2, Pt/C, and IrO2/C were measured
using a standard test system in alkaline media (pH = 14). The
LSV curves (Figure 6a) suggest that both the FeS2 and Pt/C
electrodes exhibit low OER activities. However, after the
introduction of Ni in FeS2, the electrocatalytic activity toward
OER is enhanced dramatically, even surpassing the state-of-
the-art IrO2/C and most transition-metal-based (Ni, Co, Fe,
etc.) OER catalysts reported to date (Table S2). More
specifically, Ni-FeS2-0.5 that outperforms the rest of the
catalysts can readily achieve an onset η of 206 mV and requires
η values of 250 and 326 mV to drive the current densities (j) of
10 and 100 mA cm−2, respectively. Subsequently, only 300 mV
To further get through the role of Ni in the electronic
structure reconfiguration of FeS2, the optimized crystal
structures of FeS2, Ni-FeS2-0.25, and Ni-FeS2-0.5 were
constructed and the corresponding electronic DOS of these
three samples were calculated (Figure 5a). The calculation
results reveal that the pristine FeS2 is a semiconductor with a
band gap close to the experimental value of 0.95 eV.63 Upon
the substitution of Ni in FeS2, a transition from a semi-
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Figure 7. High-resolution XPS spectra of (a) S 2p, (b) Fe 2p, and (c) Ni 2p before and after long-term OER tests. (d) Normalized Fe K-edge
XANES spectra and (e) Fe K-edge FT-EXAFS spectra of Ni-FeS2-0.5 before and after long-term OER tests. (f) Fe K-edge XAFS k3[χ(k)]
oscillation curves of Ni-FeS2-0.5 after long-term OER tests together with Fe foil, Fe2O3, and FeOOH. (g) Normalized Ni K-edge XANES spectra
and (h) Ni K-edge FT-EXAFS spectra of Ni-FeS2-0.5 before and after long-term OER tests. (i) Ni K-edge XAFS k3[χ(k)] oscillation curves of Ni-
FeS2-0.5 after long-term OER tests together with Ni foil and Ni(OH)2.
is required for Ni-FeS2-0.25 to reach a current density (j) of 10
mA cm−2, which also precedes the benchmark IrO2/C, further
proving the superiority of Ni-doped FeS2 toward OER.
Meanwhile, the reaction kinetics of the catalysts, which are
reflected by their Tafel slopes, also follow the same trend. As
depicted in Figure 6b, the Ni-FeS2-0.5 catalyst exhibits the
lowest Tafel slope of about 34 mV dec−1, rather close to that of
NiFe LDH (32 mV dec−1), by which Ni-FeS2-0.25 (68 mV
dec−1), IrO2/C (73 mV dec−1), Pt/C (94 mV dec−1), and FeS2
(151 mV dec−1) are followed. Distinct comparisons with
regard to the overpotentials at 10 mA cm−2 and the
corresponding Tafel slopes for all of the samples are illustrated
in Figure 6c, which further indicates the overwhelming
advantage for Ni-doped FeS2. To further improve the OER
performance of Ni-FeS2-0.5, we utilized a NF as the substrate
to support Ni-FeS2-0.5 and measured its activity toward the
OER performance under different scan rates (Figure S12). As
expected, the NF-supported Ni-FeS2-0.5 shows an enhanced
OER catalytic activity, yielding a much lower overpotential of
230 mV and a negligible change in the performance at different
scan rates, which might benefit from the metallic nature and
3D nanostructure of NF.64,65 In addition to these, the OER
performance was also assessed by comparing the ECSAs
through the tested electrochemical Cdl (Figure S13). As shown
in Figure 6d, Cdl of Ni-FeS2-0.5 (27.3 mF cm−2) is 5.3 times
larger than that of pure FeS2 (5.2 mF cm−2), indicating more
accessible catalytic sites and higher current densities. However,
given the fact that Ni-FeS2-0.5 has the smallest specific surface
area of 45 m2 g−1 compared with those of Ni-FeS2-0.25 (49 m2
g−1) and FeS2 (76 m2 g−1) (Figure S14), it might make more
sense to attribute the enhanced active sites to the role of Ni in
the electronic structure modification of Fe insead of the
contribution of the specific surface area. EIS (Figure 6e)
unravels a remarkably favorable conductivity for post-OER Ni-
FeS2-X (X = 0.25, 0.5) in contrast to the post-OER FeS2,
indicating enhanced charge-transfer kinetics during the OER
process. This further supports an effective optimization of the
catalyst and also proves an inheritance of the conductivity from
the optimal sulfide precursor. The durability and long-term
stability of Ni-FeS2-0.5 toward OER were further tested to
manifest its practical applications. As displayed in Figure S15,
the polarization curve of Ni-FeS2-0.5 suffers a negligible
difference after 2000 CV cycles in comparison with the original
data, proving its good durability. The chronoamperometry
curve (Figure 6f) at a constant j of 10 mA cm−2 was observed
with a slight decline at first and then a continuous increase,
which probably results from the electrochemical self-
reconstruction of the Ni-FeS2-0.5 catalyst during the OER
conditions (more details will be discussed later). The SEM
image (the inset in Figure 6f) for Ni-FeS2-0.5 after the stability
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Figure 8. (a) HER polarization curves of Ni-FeS2-0.5, Ni-FeS2-0.25, FeS2, and Pt/C at 5 mV s−1 in 1 M KOH. (b) Durability test of Ni-FeS2-0.5 at
an overpotential of 258 mV. (c) Polarization curves of a two-electrode alkaline electrolyzer using Ni-FeS2-0.5 loaded onto NF as both the cathode
and anode at a scan rate of 5 mV s−1 in N2-saturated 1.0 M KOH. Optical photograph presenting the generation of H2 and O2 bubbles during the
overall water-splitting process. (d) Durability test of Ni-FeS2-0.5∥Ni-FeS2-0.5 electrodes for water splitting at 1.57 V. (e) Comparison of the water-
splitting performance of Ni-FeS2-0.5 with other transition-metal-based bifunctional electrocatalysts. Data in the graph: 10% VNS/NF;18
Ni1.5Fe0.5P/CF;76 Fe-CoP/Ti;77 NiSe/NF;78 NiCo2O4/NF;79 CoP/GO-400;80 Co0.85Se@NC/NF.81
test shows the typical (oxy)hydroxide-like nanosheets, and this
phenomenon preliminarily reveals self-reconstruction, which
might be responsible for the activated process.39,66,67
ratio of Fe-to-Ni remains nearly the same as the case before the
OER process (Table S1), indicating a well-controlled
compositional modulation. Considering that the Ni content
cannot be neglected for the post-OER catalysts, we further
prepared NiS2 by a similar method and tested its OER
performance (Figure S18,S19). As displayed in Figure S19, Ni-
FeS2-0.5 only requires an η of 250 mV to drive the j of 10 mA
cm−2, while NiS2 needs an η of 283 mV. This further proves
the effective modification of FeS2 by Ni, which dramatically
stimulates the activity of Fe toward the OER process. XPS
techniques were further utilized to investigate the chemical
composition and surface valence state in pristine and
galvanostatic activation (GA)-treated Ni-FeS2-0.5. Notably,
the survey XPS survey spectrum shows that the S content on
the surface of GA-treated Ni-FeS2-0.5 decreases distinctly
while the O content increases to a dominant role, suggesting
the generation of excessive O-containing species (Figure S20).
High-resolution S 2p lines in Figure 7a also indicate that the
signal intensity of the S element on the surface significantly
decreases after the OER test but with the relatively increased
peak intensity of S−O in the S 2p region.68 In the high-
To gain deep insight into the origin for the remarkable OER
activity of Ni-FeS2-0.5, a series of ex situ measurements were
conducted for analysis from aspects in the structure,
component, and chemical state for Ni-FeS2-0.5 after the
stability test. The XRD pattern of post-OER Ni-FeS2-0.5
indicates an obvious phase change after electrocatalysis (Figure
S16), and the as-formed amorphous phase resulting from the
in situ electrochemical transformation may be explained by
disorder of the crystal structure. TEM images of Ni-FeS2-0.5
after the stability test (Figure S17) further reveal that an in situ
electrochemical topological transformation has happened,
during which the yolk−shell structure is well-preserved.
More interestingly, the outer single shell has transformed
into a double-layer shell, and many in situ generated small
nanosheets could also be distinguished from the shell, which
may benefit the contact with electrolyte ions. ICP-AES tests
were exploited to detect the change in the composition after a
long-term OER test (Table S3). As listed in the chart, the atom
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resolution Fe 2p spectra (Figure 7b), the signal from Fe−S
almost disappears for Ni-FeS2-0.5 after a long-term OER test,14
while the two peaks at 711.3 and 724.4 eV, which occupy the
dominant content after the OER process, may be indexed to Fe
2p3/2 and 2p1/2 of FeOOH.22,69 Likewise, the Ni−S peaks after
the OER test also could not be detected, and only peaks
assigned to oxidized Ni species are recognized (Figure 7c).
The results above demonstrate that Ni-FeS2-0.5 on the surface
is reconstructed and Fe, Ni, and S species have been oxidized
to high-valence states, which might be responsible for the high
OER activity.
possibly attributed to the following: (1) the precatalyst of FeS2
is a metastable material,75 which may easily experience self-
reconstruction under the OER conditions; (2) the optimized
Ni-doped FeS2 yolk−shell spheres are endowed with superior
conductivity, hydrophilicity (proven by the DFT calculations
mentioned above), and many accessible active sites, which may
accelerate complete conversion to amorphous Ni-FeOOH
species;23 (3) the loss of S may also facilitate the progress of
reconstruction.
Another critical factor for water splitting, the HER activities
of Ni-FeS2-X, FeS2, and Pt/C were also studied in N2-saturated
alkaline media (pH = 14). The results obtained from the LSV
curves indicate that the incorporation of Ni in FeS2 not only
dramatically promotes the OER performance but also induces
enhanced HER activities. As shown in Figure 8a, Ni-FeS2-0.5
delivers a lower overpotential with 258 mV at 10 mA cm−2 in
contrast to bare FeS2 (332 mV) but still larger than that of Pt/
C (30 mV). To further improve the HER performance, we
utilized NF as a substrate to support Ni-FeS2-0.5 and measured
its activity toward HER. As shown in the LSV curves, Ni-FeS2-
0.5 on NF exhibits a much smaller overpotential of 164 mV
compared with the one on the GC electrode (258 mV), which
might benefit from the good conducitivity and 3D porous
nanostructure of NF (Figure S22). The electrochemical
stability of Ni-FeS2-0.5 for HER was also evaluated. Figure
8b demonstrates that Ni-FeS2-0.5 could be retained at a j of 10
mA cm−2 within 14 h, proving its excellent long-term durability
for practical applications. In view of the excellent OER and
HER performances of Ni-FeS2-0.5, a water electrolyzer with a
two-electrode configuration was assembled, in which the
catalyst was loaded onto NF as both the anode and cathode.
As depicted in Figure 8c, the Ni-FeS2-0.5 couple is capable of
delivering a water-splitting j of 10 mA cm−2 at an applied
potential of 1.55 V, which even surpasses the conventional
noble-metal-based electrocatalysts (1.60 V for Pt/C-IrO2/C).
Meanwhile, the ever-bubbling H2 and O2 could be observed on
both electrodes (the inset in Figure 8c). Furthermore, the
stable i−t curve indicates that the electrolyzer exhibits
negligible degradation in a period of 22 h, demonstrating its
superior durability (Figure 8d). In addition, compared with the
other transition-metal-based bifunctional electrocatalysts re-
ported so far, Ni-FeS2/NF also shows a better or comparable
performance for water splitting (Figure 8e). These results
suggest that the Ni-modified Fe-based catalyst with a
remarkable performance toward the overall water splitting is
promising for further applications.
To further identify the active form of the catalyst during the
OER process, XAS measurements were performed. As
presented in the Fe K-edge XANES spectra (Figure 7d), Ni-
FeS2-0.5 is oxidized with increased Fe valence after the OER
test. Furthermore, in the FT-EXAFS spectra, the local structure
around the Fe sites that previously feature Fe−S coordination
has transformed into Fe−O coordination, indicating a distinct
structural evolution (Figure 7e). Besides, the Fe k-space
oscillation of post-OER Ni-FeS2-0.5 shows obvious differences
from that of Fe2O3 but is more similar to that of FeOOH
(Figure 7f), confirming that the Fe component in the Ni-FeS2-
0.5 catalyst has been in situ oxidized to FeOOH. This
deduction is in accordance with the XPS results above (Figure
7a), and it has been further verified by Raman spectra. As
shown in Figure S21, there are five peaks, located at around
243, 300, 398, 498, and 691 cm−1, respectively, all of which
show a slight shift compared with those of the literature-
reported FeOOH.20,22,70,71 In addition, the characteristic
bands ascribed to Ni-based vibration modes are not detected.
This result suggests that the Ni atoms also exist in a doping
form in the in situ formed FeOOH. In order to confirm the
hypothesis above, ex situ Ni K-edge XAFS analyses were
carried out to identify the oxidation states and local atomic
structure of Ni in post-OER Ni-FeS2-0.5 (Figure 7g−i). As
indicated in Figure 7g, the Ni K-edge of Ni-FeS2-0.5
undergoing the long-term OER test shifts to a higher energy
close to the oxidation state 2+. Furthermore, the XANES
profile of Ni-FeS2-0.5 after the OER test well resembles to that
of Ni(OH)2, suggesting that the post-OER Ni-FeS2-0.5
exhibits a structure similar to that of Ni(OH)2.72 This result
is further supported by the Ni k-space oscillations (Figure 7i).
Notably, Ni-FeS2-0.5 after the OER test and Ni(OH)2 exhibit
well-matched EXAFS oscillations, demonstrating that both of
them share an analogous structure. FT-EXAFS analysis at the
Ni K-edge also reveals that Ni−S bonds in GA-treated Ni-
FeS2-0.5 are destroyed and oxidized to Ni−O bonds (Figure
7h). Considering that the oxidation state of Ni after OER
catalysis still remains in 2+ without a transition from Ni(OH)2
to NiOOH, a rational explanation for this discrepancy might
be that Ni is incorporated into the as-formed FeOOH host and
stabilized in a lower oxidation state as previously reported.73,74
Thus, the actual active state during OER catalysis could be
described as Ni2+Fe3+OOH. From the systematic postchar-
acterizations implemented above, we may draw the conclusion
that the optimized Ni-doped FeS2 spheres undergo a topotactic
conversion during the electrochemical oxidation process and
transform into hierarchical Ni-doped FeOOH spheres with the
original frameworks and Fe/Ni molar ratio retained. Also, the
hierarchical structure, yolk−shell-structured framework, and
optimal metal composition of Ni-doped FeOOH are
responsible for its remarkable OER performance. As for the
occurrence of such a transformation, we consider that it is
4. CONCLUSIONS
In summary, we have demonstrated a novel approach to
stimulating reactive Fe sites by integrating the electronic
structure reconfiguration and in situ electrochemical topotactic
transformation strategies so as to accelerate the sluggish OER
kinetics at low overpotential. The obtained Ni-FeS2-0.5 spheres
can transform into nickel−iron oxyhydroxide via an in situ
electrochemical reconstruction that could exhibit a remarkable
OER performance with overpotentials of 250 and 326 mV to
drive j values of 10 and 100 mA cm−2, respectively, and a Tafel
slope as small as 34 mV dec−1. In addition, when used as a
bifunctional catalyst for the overall water splitting, the Ni-FeS2-
0.5 sample also shows a better or comparable performance
(1.55 V at j10) in contrast to other transition-metal-based
bifunctional electrocatalysts reported so far. The experimental
results combined with the DFT calculations demonstrate that
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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.inorg-
chem.9b01017.
Field-emission SEM images of the Ni-Fe-0.25 precursor,
Fe precursor, Ni-FeS2-0.25, and FeS2, TEM images of
the Ni-Fe-0.5 precursor, Ni-FeS2-0.25, and FeS2, N2
adsorption−desorption isotherms of Ni-FeS2-0.5, Ni-
FeS2-0.25, and FeS2, XRD patterns of the Ni-Fe-0.5
precursor and Ni-FeS2-1, FT-IR spectra of the as-
prepared Ni-Fe-0.5 precursor, EDS spectrum of Ni-FeS2-
0.5, Fe K-edge k3-weighted EXAFS spectra, Ni K-edge
XANES spectra and XAFS k3-weighted oscillation
curves, OER polarization curves of NF and Ni-FeS2-
0.5 on NF and of Ni-FeS2-0.5 on NF at different scan
rates, CV curves of Ni-FeS2-0.5, Ni-FeS2-0.25, and FeS2
at the different scan rates, XPS survey spectra of Ni-
FeS2-0.5 before and after OER tests, Raman spectrum
and XRD pattern of Ni-FeS2-0.5 after OER tests, and
HER polarization curves of NF and Ni-FeS2-0.5 on NF
in 1 M KOH (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: caomh@bit.edu.cn.
■
ORCID
Lekha Sharma: 0000-0001-8890-4378
Rita Kakkar: 0000-0002-8666-2288
Minhua Cao: 0000-0002-2232-6859
Author Contributions
(13) Chen, S.; Kang, Z.; Zhang, X.; Xie, J.; Wang, H.; Shao, W.;
Zheng, X.; Yan, W.; Pan, B.; Xie, Y. Highly Active Fe Sites in
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The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (Grants 21872008, 21471016, and
21271023) and the 111 Project (B07012). The authors
thank the Analysis & Testing Center, Beijing Institute of
Technology, for performing field-emission SEM and TEM
measurements.
(17) Wang, D. Y.; Gong, M.; Chou, H. L.; Pan, C. J.; Chen, H. A.;
Wu, Y.; Lin, M. C.; Guan, M.; Yang, J.; Chen, C. W.; Wang, Y. L.;
Hwang, B. J.; Chen, C. C.; Dai, H. Highly Active and Stable Hybrid
Catalyst of Cobalt-Doped FeS2 Nanosheets-Carbon Nanotubes for
Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2015, 137, 1587−
1592.
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