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Journal of
Materials Chemistry A
Materials for energy and sustainability
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This article can be cited before page numbers have been issued, to do this please use: X. Liu, Y. ye and N.
Zhang, J. Mater. Chem. A, 2017, DOI: 10.1039/C7TA06906E.
Volume 4 Number 1 7 January 2016 Pages 1–330
Journal of
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Materials for energy and sustainability
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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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ARTICLE
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Amorphous NiFe(oxy)hydroxide Nanosheets Integrated Partially
Exfoliated Graphite Foil for High Efficiency Oxygen Evolution
Reaction
Yin-Jian Ye,a Ning Zhangb and Xiao-Xia Liu*a
The requirements for large-scale industrial electrolytic water splitting have motivated investigations of inexpensive and
stable oxygen evolution catalysts which would provide high current densities (>500 mA cm-2) at low overpotentials. Herein
we develop a scalable electrodeposition route to fabricate amorphous nickel-iron (oxy)hydroxide nanosheets directly onto
3D partially exfoliated graphite foil electrode. The integrated electrode combines the high OER catalytic activity of NiFebased materials and the excellent electric conductivity of the carbon substrate, while facile ion transport and gaseous
product (O2) diffusion is guaranteed by the hierarchical structure. The electronic structure of the Ni catalytic center, which
shows critical effects in determining the catalytic activity, can be controlled over Fe incorporation and/or
electrodeposition potential window. The optimal electrode catalyzes the OER process with a low overpotential of 214 mV
to reach 10 mA cm-2 current density in 1 M KOH. The Tafel slope is as small as 21 mV dec-1 so it is capable to deliver high
current densities of 500 mA cm-2 at an overpotential of only 251 mV. The OER reaction can be prolonged to 100 h at 10 mA
cm-2 and 48 h at 500 mA cm-2. The composite shows great potentials towards large-scale and long-term practical
applications.
1. Introduction
Electrolytic water splitting provides a clean pathway towards
1,2
energy conversion and hydrogen generation. However, the
oxygen evolution reaction (OER) at the anode (4OH → O2 +
2H2O + 4e in alkaline solution) is kinetically sluggish, resulting
in high overpotentials (η) above its thermodynamic potential
of 1.23 V vs. reversible hydrogen electrode (RHE) and
hindering the energy conversion efficiency of the overall water
3-6
splitting process. One strategy to overcome such barrier is to
apply an OER catalyst. RuO2 and IrO2, for instance, offers good
catalytic activity and is the current benchmark choice, but the
high cost and poor durability has limited its practical
7
applications.
Alternative catalysts have been investigated. Transition
metal based materials, not only cost much less due to their
earth abundance, but also show promising catalytic activity
8-12
towards OER process.
The NiFe(oxy)hydroxide based
catalysts, in particular, perform one of the best in alkaline
13,14
15,16
solutions.
Through fabricating nanostructure
and
hybridizing with carbon materials such as carbon quantum
a.
Department of Chemistry, Northeastern University, Shenyang, 110819, China.E*mail: xxliu@mail.neu.edu.cn
Institute of Advance Material Technology, Northeastern University, Shenyang,
110819, China.
†Electronic Supplementary Informa)on (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
b.
dots,17 CNTs18 and graphene19-21 to increase the conductivity,
the resulting composites greatly promote the OER process at
low current densities (eg. 10 mA cm-2). A strong synergistic
effect upon the incorporation of iron with nickel, even in trace
amount, is recognized to attribute to the high catalytic
activity.22 Higher oxidation state on Ni would further benefit
the catalytic performance,23-25 due to the increase of Ni-O
bond covalency which enhances the oxyl character and
promotes O-O formation according to Li et al.26 On the other
hand, amorphous materials have recently shown promising
OER catalystic activity.27-32 Yang’ group suggested that their
short-range order creates abundant catalytic active site on the
surface, greatly enhancing the electrocatalytic performance.
This opens a door to the applications of amorphous
nanomaterials as advanced OER electrocatalysts.27,28
Therefore, a Ni-based material combining those properties, i.e.
optimum Fe incorporation and Ni oxidation state in the
amorphous form, has great potential in efficiently catalyzing
OER process.
One important challenge remaining in the present OER
catalysts is the activity and stability at high current densities
(eg. 500 mA cm-2).4 Under such conditions, the large amount of
gas bubbles produced during OER would stay on the surface of
the electrodes. This will block the active sites of the catalysts
and impede ion transport, leading to high overpotentials. At
the same time, the vigorous gas evolution could cause catalyst
detachment from the electrode. Such circumstances are
especially serious for catalysts in powder form where electrical
insulating binders are used for electrode fabrication and poor
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-1
mechanical stability is resulted. Three dimensional
freestanding configurations, on the contrary, allow facile
oxygen diffusion and binder is not required, thus can
overcome these disadvantages. A couple studies involving Ni
foam as the catalyst substrate have shown promising OER
-2 33-35
activity under high current densities (hundreds mA cm ).
However, metal forms still suffer from relatively weak
interaction with active material, poor flexibility and limited
durability. Carbon based substrates, on the other hand,
provide much better mechanical properties and stabilities due
to their structural flexibility, while the high electric
conductivity and open framework ensures the facile electric
36-38
and ionic transport.
Although most OER tests for catalysts
with carbon based substrates have only been carried out
39,40
under relatively low current densities,
they are
undoubtedly favorable candidates for high current density
applications.
Herein, we propose a facile one-step process to
electrodeposit amorphous NiFe(oxy)hydroxide nanosheets on
a three-dimensional partially exfoliated graphite foil substrate
(EG). The fabricated NiFe/EG composite is used as the catalyst
for high efficiency OER process over a wide current density
range. The amount of incorporated iron and nickel oxidation
state in NiFe/EG are controlled via Ni/Fe ratio in the electrolyte
and upper potential cutoff during electrodeposition,
respectively. Both effects on OER activity are investigated and
discussed. The EG substrate not only offers good electric
conductivity, but also provides large surface area for catalyst
deposition and pathways for O2/ion diffusion, thanks to its 3D
hierarchical structure. Furthermore, the EG interact strongly
with the NiFe(oxy)hydroxide species, so that the long-term
stability of the catalyst is guaranteed. The optimal catalyst
exhibits a remarkably low OER overpotential of 214 mV at a
-2
current density of 10 mA cm and a small Tafel slope of 21.2
-1
mV dec in 1 M KOH. More importantly, it shows superior
performance at high current densities, presenting a stable
voltage with low overpotential of 251 mV for 48 h at 500 mA
-2
cm . The outstanding OER activity of the NiFe/EG composite
offers the opportunity for large-scale practical applications in
water splitting.
dynamically scanned between 0.5 and 1.8 V at 20 mV s rate in
0.5 M K2CO3 aqueous solution for 6 cycles, with a platinum
plate and saturated calomel electrode (SCE) as the counter and
reference electrodes, respectively. Subsequently, the working
electrode was treated by advanced cyclic voltammetry from -1
0.9 to 1.9 V at 20 mV s in 1 M KNO3 solution for 10 cycles. At
the end of each cycle, the potential was held at 1.9 V for 5 s.
The obtained EG electrode was washed with ethanol and deionized water to remove inorganic residuals. Partial exfoliation
was similarly conducted on a piece of 2 cm x 2 cm graphite foil,
the obtained EG was investigated on a confocal laser scanning
microscope (Fig. S1A). The EG shows similar evenness on the
whole surface of the sample, implying that the whole surface
was similarly partial exfoliated. The sample also displays
similar roughness at different positions along the y direction
(Fig. S1Af), with Ra values from 43 to 47, which are essentially
higher than the non-treated graphite G (Fig. S1Bf with Ra
values from 10 to 12), further illustrating the evenness of the
partial exfoliation and the increased surface roughness. To
investigate the application of the partial exfoliation method on
other graphite-based materials, partial exfoliation was also
conducted on GR to get EGR.
2. Experimental section
2.4 Characterization
2.1 Materials
All the reagents were purchased from Sinopharm Chemical
Reagent Co., Ltd. The graphite foil (G) manufactured from
natural expanded graphite was obtained from SGL group
(Germany). Graphite rod (GR) was obtained from Beijing Jing
Long Te Tan technology co., LTD. (China).
2.2 Partially Exfoliated Graphite Foil
Partial exfoliation of graphite foil (G) was conducted by two
41
electrochemical steps as described in our previous report. In
2
the first step, the graphite foil (1×1 cm ) was potential
2.3 Electrochemical deposition of NiFe(oxy)hydroxide nanosheets
on partially exfoliated graphite
Electrochemical deposition of NiFe(oxy)hydroxide nanosheets
2
onto EG electrode (1×1 cm ) was performed by potential
dynamic scans between -1.2 V vs. SCE with different upper
-1
potential cutoffs at a scan rate of 100 mV s in electrolytes
containing 50 mM CH3COONH4 and 50 mM [Ni(NO3)2 ·
6H2O+Fe(NO3)3·9H2O]. The Ni:Fe precursor molar ratios of
1:0, 8:1, 4:1, 1:1 and 0:1 (0 indicating the absence of such
species) and upper potential cutoffs of 0 V, 0.8 V, 1.0 V, 1.2 V
and 1.4 V were under test. The nanosheets were also
electrochemically deposited on the non-treated graphite foil
and EGR substrates to study the influence of graphite
exfoliation and the type of graphite substrate. All electrodes
were washed with deionized water and subsequently freezedried to preserve the porous structure.
Morphology and elemental distribution of the products were
examined by field emission scanning electron microscope
(SEM) equipped with an energy dispersive X-ray spectroscopy
(EDS) detector (LEO SUPRA 35, Carl Zeiss, Germany). The
atomic ratios of Ni:Fe in the samples were determined by
inductively coupled plasma-optical emission spectrometer
(ICP-OES, Perkin-Elmer, Optima 4300DV, USA). Confocal laser
scanning microscope investigation was conducted on a
Olympus OLS4100 (Japan), the surface roughness was
quantified by Ra value. Transmission electron microscopy
(TEM) was performed using a Philips CM 200 microscope
(Netherlands). X-ray diffraction (XRD) was performed on an X’
Pert Pro diffractometer (PANalytical B.V., Netherlands) with
Cu-Kα radiation. X-ray photoelectron spectroscopy (XPS)
2 | J. Name., 2012, 00, 1-3
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Scheme 1. Fabrication process of the NiFe/EG electrode.
analyses were conducted on an imaging photoelectron
spectrometer (Axis Ultra, Kratos Analytical Ltd., UK) with Al-Kα
radiation as the excitation source. Raman spectra were
obtained on a confocal Raman microscope (XploRA ONE,
Horiba Jobin Yvon, France). Fourier transform infrared (FT-IR)
spectroscopy was carried out on a spectrum one FT-IR
spectrophotometer (Perkin-Elmer, USA).
water, 245 μL ethanol and 5 μL 5 wt% Nafion solution and
drop casting the catalyst ink onto the GC electrode (with a
-2
catalyst loading of ∼0.6 mg cm ). The obtained electrode was
dried at room temperature and examined by LSV.
2.5 Electrochemical studies
3.1 Material characterization
All electrochemical measurements were performed in a
standard three electrode system on a multichannel
electrochemical
analyzer
(VMP3,
Bio-Logic-Science
Instruments,
France).
The
mass
loading
of
the
NiFe(oxy)hydroxide nanosheets on EG electrode were
controlled at ∼0.6 mg cm-2. The as-prepared composites were
used directly as the working electrode without further
treatment. A platinum plate and Hg/HgO electrode were
served as the counter and reference electrodes, respectively.
The electrocatalytic activity of the composites toward OER
process was examined by cyclic voltammetry (CV) and linear
sweep voltammetry (LSV) in 1 M KOH electrolyte with a low
scan rate of 5 mV s-1 and 1 mV s-1, respectively, in order to
minimize the capacitive current. The OER potentials
referenced to the Hg/HgO electrode were subsequently
converted to the reversible hydrogen electrode (RHE) using
the following equation: ERHE = EHg/HgO + 0.098 + 0.059pH. Tafel
slopes were derived from the OER polarization curves obtained
by LSV at 1 mV s-1 with 75% iR compensation, with i
representing the measured current and R representing the
compensated resistance between the working and reference
electrodes. The overpotential (η) was calculated using the
equation: η = ERHE –
1.23. Chronopotentiometric
measurements were carried out under the same experimental
setup
without
compensating
the
iR
drop.
The
electrochemically active surface areas (EASA) of the
composites were estimated from the electrochemical doublelayer capacitance of the electrodes.31 Electrochemical
impedance spectroscopy (EIS) measurements were conducted
in a frequency range of 10 mHz to 20 kHz at a potential of
1.504 V vs. RHE and an ac voltage magnitude of 10 mV. The
equivalent circuit for fitting of the EIS data was achieved with
ZView software. To compare the catalytic performance of our
FeNi/EG electrolyte with that of commercial noble-metal
based catalyst, RuO2 coated GC electrode was prepared by
sonicating 4 mg RuO2 (powder from Alfa Aesar) in 250 μL
The fabrication process of the NiFe(oxy)hydroxide on EG
(labeled as NiFe/EG) is illustrated in Scheme 1. Upon partial
exfoliation of the graphite foil, the obtained EG functions as a
freestanding substrate which offers a high surface area, good
electric conduction and strong mechanical robustness (Fig.
41
1a). The subsequent electrodeposition (Fig. S2) involves the
reduction of NO3 ions on the EG surface to generate OH
31,32,42
2+
during negative scans,
which then reacts with Ni and
3+
Fe ions to form NiFe(oxy)hydroxide in situ on the EG. Positive
scans facilitate the formation of species with higher oxidation
43,44
state Ni (see later for details).
The electrodeposition results in a brown thin film on the
EG substrate (Fig. S3). The SEM images of the representative
as-prepared NiFe/EG electrode NiFe2:1/EG-1.2V (Ni:Fe = 2:1 in
electrolyte, upper potential cutoff = 1.2 V, see experimental
section for details) are shown in Fig. 1b and 1c. The composite
maintains the well separated layers of the EG substrate (Fig.
1a), while rippled nanosheets of NiFe(oxy)hydroxide species
are grown uniformly on top. EDS elemental mapping shows
homogeneous distribution of Ni and Fe throughout the sample
(Fig. S4). The lateral dimensions of the nanosheets are around
hundred nanometers with the thickness generally below 10
nm, as shown in the SEM as well as the TEM of the
NiFe(oxy)hydroxide species removed from the substrate by
sonication (Fig. 1d). The corresponding selected area electron
diffraction (SAED) pattern demonstrates a broad and diffused
halo ring (Fig. 1e), suggesting that the NiFe(oxy)hydroxide is
45
amorphous. This is in agreement with the XRD pattern where
no additional peaks apart from those of the EG substrate are
observed (Fig. 1f). The amorphous nature of the NiFe-catalysts
can provide a higher activity in electrocatalytic reactions owing
to their structural flexibility as well as the larger density of
unsaturated coordination sites which gives rise to an easier
adsorption of the reactants compared to the counterpart
27,28
crystalline phases.
The as-prepared NiFe2:1/EG-1.2V
electrode was further characterized by XPS. The survey
3. Results
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Fig. 1. Characterizations of NiFe2:1/EG-1.2V. (a) SEM image of the EG substrate; (b)(c) SEM images; (d) TEM image; (e) SAED pattern; (f)
XRD comparison with EG substrate; high resolution (g) Ni 2p, (h) Fe 2p, (i) O 1s XPS; (j) Raman comparison with EG, non-treated
graphite foil G and NiFe2:1/G-1.2V fabricated by electrodeposition of NiFe(oxy)hydroxide on G; (k) FT-IR comparison with EG substrate.
spectrum (Fig. S5a) reveals the presence of C, O, Ni and Fe in
the composite. The high resolution Ni 2p spectrum shows the
characteristic spin-orbit peaks of Ni 2p3/2 and Ni 2p1/2 as well
as the corresponding shake-up satellites.46 The main peaks are
fitted by two Ni environments, with Ni2+ at 855.8 eV/873.6 eV
and Ni3+ at 857.2 eV/874.7 eV, respectively (Fig. 1g).16,47,48 The
Fe 2p spectrum shows similar peak splits and satellite
components, and the typical binding energies (712 eV and
725.6 eV for 2p3/2 and 2p1/2, respectively) correspond to an
Fe3+ state
(Fig. 1h).16 The O 1s spectrum (Fig. 1i) is
deconvoluted into three peaks: 530 eV for oxide, 531.9 eV for
hydroxide and ∼533 eV for structural/physisorbed water,24,25
suggesting the (oxy)hydroxide nature of the deposited
material. This is in accord with the Raman spectrum (Fig. 1j,
dark yellow), where the broad band from 460 to 550 cm-1 (i)
results from the overlapped peaks of Ni(OH)2 and NiOOH.28,49
The other two bands around 680 (ii) and 570 (iii) cm-1 are
ascribed to iron compounds; unfortunately, the identification
of the bands for iron compound is rather difficult owing to the
structural similarity among the many possible iron oxide and
oxyhydroxide phases.49 The presence of non-hydrogen-bonded
OH stretching vibration is also shown as the sharp peak at
3636 cm-1 in the FT-IR spectrum (Fig. 1k),50 on the shoulder of
the broad physisorbed water band at 3428 cm-1, again implying
the presence of hydroxide and oxyhydroxide species.
3.2 Ni:Fe ratio influence on the OER activity
The OER catalytic performance of the NiFe/EG-1.2V electrodes
with different cation ratios was tested by CV at the scan rate of
-1
-1
5 mV s (Fig. S6) and LSV at the scan rate of 1 mV s (Fig. 2a,b)
in 1 M KOH electrolyte. The Ni/Fe mixed materials show
improved OER catalytic activities than the single cation
composites – Ni/EG-1.2V or Fe/EG-1.2V – demonstrating the
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the Ni electronic structure and thus the catalytic activity (see
later for further discussion). With too high a Fe concentration
(Ni:Fe = 1:1), however, the potential for the formation of Ni
(III)/Ni (IV) activation site increases too much and delays the
onset of OER.
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3.3 Influence of upper potential cutoff during electrodeposition on
the OER activity
Fig. 2. (a)(b) iR-corrected LSV curves of the NiFe/EG-1.2V
electrodes prepared from the mixed nitrate precursors with
different Ni/Fe ratios (1:0, 8:1, 4:1, 2:1, 1:1, 0:1), together with
that of the RuO2/GC electrode. (c) Tafel plots of overpotential
versus log (i) derived from (a). (d) Overpotentials at 10 mA cm
2
-2
and 500 mA cm current densities of the six electrodes.
synergistic effect between Ni and Fe, which is in agreement
22,49,51
with previous observations.
The OER catalytic activity
shows a volcano shaped relation with Ni:Fe ratio, with the
optimum performance (lowest overpotential and smallest
Tafel slope) observed for NiFe2:1/EG-1.2V (Fig. 2d). Such
composite exhibits an onset potential of 1.42 V for water
-2
oxidation, reaching 10 mA cm current density at 1.444 V and
-2
500 mA cm at 1.481 V (vs. RHE with iR composition),
corresponding to the low overpotentials of 190 mV, 214 mV
and 251 mV, respectively. Such superior performance exceeds
most of the previously reported state-of-the-art OER catalysts
(Table S1) and the commercial noble metal based catalyst
including Ir/C, IrO2 and RuO2 (dark yellow line in Fig. 2a, 2b and
Table S2). The Tafel slope of the NiFe2:1/EG-1.2V electrode is
-1
only 21.2 mV dec (Fig. 2c), comparable to the lowest value
38
reported for the NiFe-based catalysts. The NiFe2:1/EG-1.2V
electrode also shows promising catalytic activity in lower
concentration electrolyte (Fig. S7). In 0.1 M KOH, a small
-2
overpotential of 248 mV is required to achieve 10 mA cm
-1
current density and Tafel slope is only 31.0 mV dec , superior
than most of the previously reported state-of-the-art OER
catalysts as well as the commercial noble-metal based catalyst
measured in the same low concentrated electrolyte (Table S1
and S2). Those results demonstrate the excellent catalytic
kinetics of the NiFe2:1/EG-1.2V composite.
It is interesting to note that the oxidation peak before 1.4
V, corresponding to the formation of Ni (III) or Ni (IV) species
18,32,40
that are the active sites to catalyze OER (Fig. 2b),
shows a
gradual shift to higher potential with the increase of Fe
concentration. It suggests a charge transfer between the Ni
and Fe centers in the material, resulting in a modification of
As mentioned earlier, the positive scans during the
electrodeposition of NiFe(oxy)hydroxide result in an oxidation
of Ni. The CV curve of the electrodeposition process shows
two redox peaks, attributed to the couples of Ni(OH)2/NiOOH
2+
and Ni (aq)/NiOOH at ∼0.9 V and 1.1 V (vs. SCE) for
43,44
oxidation, respectively (Fig. S2).
With the upper potential
terminated at a higher value, more of higher oxidation state Ni
species is expected in the composite, which would
consequently influence the OER performance. Such effect is
studied by varying the potential cutoffs with the Ni:Fe ratio in
electrolyte kept at constant 2:1. An improved OER activity –
decrease in overpotential and Tafel slope – is observed as the
potential cutoff increased from 0 V to 1.2 V (vs. SCE). However,
the OER activity becomes poorer when further increasing the
potential cutoff to 1.4 V (Fig. 3 and Fig. S8). The oxidation peak
before 1.4 V (vs. RHE, Fig. 3b) which corresponds to the
formation of catalytic active species Ni (III) or Ni (IV) shows a
decrease in current density with the catalyst obtained from
higher potential cutoff, but only minor potential shift is
observed. It demonstrates that the change in potential cutoff
Fig. 3. (a)(b) The iR-corrected LSV curves of the NiFe2:1/EG
electrodes prepared by potential dynamic scans between -1.2 V
vs. SCE and different upper potential cutoffs (0 V, 0.8 V, 1.0 V,
1.2 V, 1.4 V). (c) Tafel plots of overpotential versus log (i) derived
-2
-2
from (a). (d) Overpotentials at 10 mA cm and 500 mA cm current
densities of the five electrodes.
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-2
4. Discussion
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4.1 Effect of Fe incorporation
Fig. 4. (a) Multi-step chronopotentiometry curve for the OER
process with the optimal NiFe2:1/EG-1.2V catalyst in 1 M KOH
(without iR compensation). The first current density step is 50 mA
-2
-2
-2
cm and the last 500 mA cm , with an increment of 50 mA cm
between steps (every 300 s). (b)(c) Chronopotentiometry curves of
the NiFe2:1/EG-1.2V catalyst at constant current densities of 10 mA
-2
-2
cm and 500 mA cm (without iR compensation). (d) SEM image of
the NiFe2:1/EG-1.2V electrode after 48 h OER process at 500 mA
-2
cm .
mainly modifies the amount of high oxidation Ni species rather
than the Ni electronic structure as seen for the effect of Fe
incorporation. The two effects, in combination, result in the
superior catalytic activity of the optimal NiFe2:1/EG-1.2V
composite as shown in the previous section.
3.4 High current density performance of the optimal NiFe2:1/EG1.2V composite
Fig. 4a shows the multi-step chronopotentiometry curve of the
NiFe2:1/EG-1.2V electrode tested in 1 M KOH electrolyte with
-2
the current density increasing from 50 to 500 mA cm at an
-2
increment of 50 mA cm per 300 s. At the beginning of the 50
-2
mA cm step, the potential immediately levels off at 1.47 V
(vs. RHE without iR compensation) and remains constant for
the subsequent 300 s. Similar phenomena are observed for all
the other current densities as well, showing a remarkable
kinetics of the OER process with the NiFe/EG catalyst under
various conditions. Long-time durability of the OER activity was
further tested with the NiFe2:1/EG-1.2V electrode at current
-2
-2
densities of 10 mA cm and 500 mA cm . Excellent stabilities
are observed for both current densities, with the potential
maintaining constant at 1.48 V (vs. RHE without iR
-2
compensation) at 10 mA cm for 100 h and only minor
-2
increase of 20 mV after two days (48 h) at 500 mA cm (Fig.
4b,c). Such unique performance demonstrates the excellent
mass transport (inward diffusion of OH and outwards diffusion
of oxygen gas) and high electric conductivity of the composite.
SEM images of the NiFe2:1/EG-1.2V electrode after 48 h of
Based on ICP measurements, the Ni:Fe ratios in the
NiFe/EG-1.2V electrodes are 89.7: 10.3, 83.3: 16.7, 75.4: 24.6
and 44.3:55.7 for the samples fabricated in solutions with
Ni:Fe precursor ratios of 8:1, 4:1, 2:1 and 1:1 (Table S3). The
beneficial role that Fe plays in promoting the Ni-based OER
catalytic activity has been previously observed and generally
22,49,51
attributed to a “synergistic” effect between Ni and Fe.
In our experiments, a shift of the oxidation peak corresponding
to the formation of Ni (III) or Ni (IV) species towards a higher
potential is recognized upon Fe incorporation (see section 3.2,
Fig. 2b). Such result is in accord with XPS data which shows an
2+
3+
increase of binding energies of Ni and Ni peaks upon Fe
incorporation (855.6 eV and 856.7 eV in Ni/EG-1.2V compared
to 856.2 eV and 857.3 eV in NiFe2:1/EG-1.2V, Fig. 5). It
suggests a partial charge transfer between Ni and Fe centers,
3+
potentially due to the high electronegativity of Fe . The Ni
active site thus becomes more electropositive and shows a
stronger attraction towards the OH species, which is helpful to
promote the catalytic activity.
The Fe incorporation also helps to modify the
morphologies of the (oxy)hydroxide, as shown in their SEM
images (Fig. S9). In comparison with the dense nanosheets for
Ni/EG-1.2V, those for NiFe[8:1,4:1,2:1]/EG-1.2V show larger
and more separated rippled structures, with the lateral
extension of the sheets increased with Fe content. Such open
structure promotes a facile mass transport, ensuring the
excellent OER activity even under high current densities.
Further increase of Fe content, however, leads to the
aggregation of the nanosheets as shown for NiFe1:1/EG-1.2V.
This, in addition to the lower Ni component in the
(oxy)hydroxide,
explains
its
poorer
electrocatalytic
performance compared to the NiFe2:1/EG-1.2V electrode. The
Fig. 5. (a) Comparison of the high resolution Ni 2p XPS of Ni/EG-1.2
V (black) and NiFe2:1/EG-1.2V electrodes (red), showing a shift of
peak positions upon iron incorporation. (b) Detailed fits of the Ni 2p
spectrum of the Ni/EG-1.2 V electrode.
6 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
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Journal of Materials Chemistry A Accepted Manuscript
bulk water electrolysis at 500 mA cm show that the
NiFe(oxy)hydroxide nanosheets are still well attached to the
EG substrate (Fig. 4d), suggesting an outstanding physical
31
stability and mechanical robustness of the catalyst.
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DOI: 10.1039/C7TA06906E
ARTICLE
Fig. 6. Detailed fits of the high resolution Ni 2p XPS of (a) NiFe2:1/EG-0V, (b) NiFe2:1/EG-1.2V and (c) NiFe2:1/EG-1.4V electrodes,
3+
2+
demonstrating an increase of Ni /Ni ratio upon increasing potential cutoff during electrodeposition.
electrochemically active surface areas (EASA) of the NiFe/EG
catalysts are similar in each case (Fig. S10), excluding such
effect on the catalytic activity.
3+
4.2 Effect of increasing Ni species
2+
3+
XPS suggests the co-existence of Ni and Ni species in the
3+
2+
NiFe2:1/EG electrodes. A detailed fitting reveals a Ni /Ni
ratio of 0.92 in NiFe2:1/EG-0V, which increases to 1.24 for
NiFe-EG 1.2V and 1.35 for NiFe-EG 1.4V (Fig. 6). This agrees
with the expectation that increasing the upper potential cutoff
2+
3+
during electrodeposition allows more Ni oxidized to Ni and
3+
results in a higher Ni content in the composite. Such preoxidation process results in more stabilized and larger amount
of catalyst centers, as shown in the decrease current density of
the peak at around 1.4 V during LSV (Fig. 3b), thus facilitating
the OER process. Some structural modification would also take
place during the pre-oxidation to further promote the OER.
Similar effect has been observed with the nickel borate OER
52
catalyst.
The upper potential cutoff during electrodeposition also
affects the morphology of the (oxy)hydroxide (Fig. S11). The
lateral dimensions of the nanosheets decrease as the upper
potential cutoff increases from 0 V to 1.2 V (Fig. S11a∼c).
Further increasing the cutoff to 1.4 V, however, causes
apparent
oxygen
evolution
reaction
during
the
electrodeposition process and leads to the aggregation of the
nanosheets (Fig. S11d), resulting in a poorer electrocatalytic
performance. The EASA of each catalyst is similar as well (Fig.
S12), again excluding such effect on the catalytic activity.
4.3 Effect of the EG substrate
The nanostructure of the EG substrate also plays essential
roles in the excellent OER catalytic performance of the
composite. EG provides a large surface area, which allows the
deposition of more catalyst nanosheets as well as efficiently
prevents them from agglomerating. This is in contrast to the
case with non-treated graphite foil as the substrate
(NiFe2:1/G-1.2V, Fig. S13). The EASA test results in a high
-2
double layer capacitance (Cdl) of 23.02 mF cm for the
-2
NiFe2:1/EG-1.2V electrode, while only 2.53 mF cm Cdl is
obtained for NiFe2:1/G-1.2V (Fig. S14a), suggesting a nine
times higher surface area by using EG substrate. The open and
hierarchical structure offered by the EG substrate allows facile
mass transport of hydroxide ions towards the catalyst and
gaseous product (O2) away from the electrode, which is
especially essential for the OER performance at high current
densities.
To study the interaction of the (oxy)hydroxide with EG,
Raman spectra were collected for NiFe2:1/EG-1.2V and EG, as
well as for NiFe2:1/G-1.2V and G (Fig. 1j). The prominent peaks
-1
at around 1350 and 1580 cm correspond to the D and G
-1
bands of the graphite substrate. A 5 cm blue shift is observed
for the G band of NiFe2:1/EG-1.2V compared to that of EG,
whereas no band shift was detected between NiFe2:1/G-1.2V
and G. It has been reported that the G band shift in the
carbon-based composites relates to the charge transfer
between the carbon and other compounds present. Therefore,
the band shift observed in NiFe2:1/EG-1.2V indicates a charge
transfer between EG and the (oxy)hydroxide, implying the C-OM (Ni and/or Fe) formation as happened in the composite of
53
NiO and graphene. At the same time, the EG surface is
functionalized with oxygen-containing groups (C-O, C=O, O54,55
C=O, etc.)
as revealed by its C 1s XPS (Fig. S5b). This allows
additional chemical interactions with the (oxy)hydroxide
grown on top. As a result, a more seamless connection
between the (oxy)hydroxide and EG substrate than the
untreated graphite foil G can be expected, ensuring the
improved mechanical stability and electric conductivity for the
NiFe/EG composite.
To further explore OER kinetics on NiFe2:1/EG-1.2V and
NiFe2:1/G-1.2V, electrochemical impedance spectroscopy (EIS)
investigations were conducted at the same applied potential of
1.504 V vs. RHE (Fig. S14b). The EIS data are fitted with
equivalent circuit (inset of Fig. S14b) which is comprised of a
resistor (Rs) in series with two parallel combinations of a
resistor (R1, Rct) and a constant phase element (CPE1, CPE2).
The Rs represents the ohmic resistance arising from the
electrolyte and all contacts. The R1 may be related to the
interfacial resistance resulting from the electron transport
between the NiFe nanosheets and the graphene skeleton
underneath, while the Rct reflects the charge transfer
resistance at the interface between the NiFe nanosheets and
J. Name., 2013, 00, 1-3 | 7
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Journal Name
the National Natural Science Foundation of China (Grant No.
21673035).
Notes and references
1
2
3
4
5
6
7
8
5. Conclusion
9
We develop a high efficiency OER catalyst through facile
electrodeposition
of
amorphous
NiFe(oxy)hydroxide
nanosheets onto 3D partially exfoliated graphite substrate.
The hierarchical composite not only allows facile electric, ionic
and mass transport in the catalyst, but also offers superior
stability even upon vigorous gas evolution under high current
OER process. XPS reveals the critical role that the electronic
structure of the active Ni center plays in the catalytic activity of
NiFe(oxy)hydroxide, which is modified either through Fe
incorporation or tuning the electrodeposition potential
window. The optimal composite requires only 214 mV
-2
overpotential to afford 10 mA cm current density and the
-1
Tafel slope is as low as 21.2 mV dec , achieving one of the best
OER catalytic performance among the present reports. The
catalyst also shows a remarkable long-term stability over 48 h
of OER process with only 251 mV overpotential under a high
-2
current density of 500 mA cm . The results satisfactorily meet
the requirements for commercial water electrolysers (for
−2
example, > 500 mA cm at < 300 mV overpotential). The high
activity, good stability and low price of the NiFe/EG composite
demonstrates a step towards building an OER catalyst for
large-scale and long-term application. At the same time, the
catalyst fabrication method can be extended to other graphitebased substrates.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Acknowledgements
25
We thank Dr. Xiaoqi Sun in University of Waterloo, Canada
very much for her constructive suggestions and the valuable
discussions. We gratefully acknowledge financial support from
26
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Journal of Materials Chemistry A Accepted Manuscript
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56
the electrolyte. Although NiFe2:1/EG-1.2V displays a slightly
larger Rs (0.30 Ω) than EG (0.28Ω) (Fig. S14c), its Rs is smaller
than that of NiFe2:1/G-1.2V (0.37 Ω), showing the effect of
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demonstrating the rapid charge transfer kinetics for the OER
9,56
on NiFe2:1/EG-1.2V.
The catalyst fabrication method can be extended to other
graphite-based substrates as well. Using a graphite rod as the
starting material (Fig. S15a), the partially exfoliation was
conducted and NiFe(oxy)hydroxide was electrodeposited on
the obtained EGR by the similar procedure of NiFe2:1/EG-1.2V
(Fig. S15b-d). The resulting NiFe2:1/EGR-1.2V presents similar
-2
double layer capacitance Cdl of 22.38 mF cm (Fig. S14a) with
the NiFe2:1/EG-1.2V electrode. Although the former shows
slightly larger overpotential and Tafel slope (Fig. S16) –
possibly due to the higher Rs (1.47 Ω) and Rct (2.80 Ω, Fig.
S14b), it is much improved compared to the catalyst with G
substrate. This demonstrates the beneficial role of partial
exfoliation of the graphite substrates and the wide application
of the exfoliation procedure.
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J. Name., 2013, 00, 1-3 | 9
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Journal of Materials Chemistry A Accepted Manuscript
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Amorphous NiFe (oxy)hydroxide Nanosheets Integrated Partially Exfoliated
Graphite Foil for High Efficiency Oxygen Evolution Reaction
Yin-Jian Ye,a Ning Zhangb and Xiao-Xia Liu*a
a
b
Department of Chemistry, Northeastern University, Shenyang, 110819, China
Institute of Advance Material Technology, Northeastern University, Shenyang, 110819, China.
*Corresponding author: Xiao-Xia Liu, xxliu@mail.neu.edu.cn
Superior OER performance on NiFe(oxy)hydroxide integrated in EG with 21 mV/dec Tafel slope and
251 mV overpotential at 500 mA/cm2.
Journal of Materials Chemistry A Accepted Manuscript
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