Hierarchical Hollow Nanoprisms Based on Ultrathin Ni-Fe Layered Double Hydroxide Nanosheets with Enhanced Electrocatalytic Activity towards Oxygen Evolution

Article abstract of DOI:10.1002/anie.201710877

The oxygen evolution reaction (OER) is involved in various renewable energy systems, such as water-splitting cells and metal–air batteries. Ni-Fe layered double hydroxides (LDHs) have been reported as promising OER electrocatalysts in alkaline electrolytes. The rational design of advanced nanostructures for Ni-Fe LDHs is highly desirable to optimize their electrocatalytic performance. Herein, we report a facile self-templated strategy for the synthesis of novel hierarchical hollow nanoprisms composed of ultrathin Ni-Fe LDH nanosheets. Tetragonal nanoprisms of nickel precursors were first synthesized as the self-sacrificing template. Afterwards, these Ni precursors were consumed during the hydrolysis of iron(II) sulfate for the simultaneous growth of a layer of Ni-Fe LDH nanosheets on the surface. The resultant Ni-Fe LDH hollow prisms with large surface areas manifest high electrocatalytic activity towards the OER with low overpotential, small Tafel slope, and remarkable stability.

Full text of DOI:10.1002/anie.201710877

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Accepted Article
Title: Hierarchical Hollow Nanoprisms Organized by Ultrathin Ni-
Fe Layered Double Hydroxide Nanosheets with Enhanced
Electrocatalytic Activity Toward Oxygen Evolution
Authors: Xiong-Wen (David) Lou
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201710877
Angew. Chem. 10.1002/ange.201710877
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http://dx.doi.org/10.1002/ange.201710877

10.1002/anie.201710877
Angewandte Chemie International Edition
Hierarchical Hollow Nanoprisms Organized by Ultrathin Ni-Fe Layered Double Hydroxide
Nanosheets with Enhanced Electrocatalytic Activity Toward Oxygen Evolution
Le Yu, Jing Fan Yang, Bu Yuan Guan, Yan Lu and Xiong Wen (David) Lou*
[*] Dr. L. Yu, J. F. Yang, Dr. B. Y. Guan, Dr. Y. Lu, Prof. X. W. Lou
School of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang
Drive, Singapore 637459, Singapore
Email: xwlou@ntu.edu.sg; davidlou88@gmail.com
Webpage: http://www.ntu.edu.sg/home/xwlou/
Abstract
Oxygen evolution reaction (OER) is involved in various renewable energy systems such as water
splitting cells and metal-air batteries. Ni-Fe layered double hydroxides (LDHs) have been reported
as promising OER electrocatalysts in alkaline electrolytes. The rational design of advanced
nanostructures for Ni-Fe LDHs is highly desirable to optimize their electrocatalytic performance.
Herein we report a facile self-templated strategy to synthesize novel hierarchical hollow nanoprisms
composed of ultrathin Ni-Fe LDH nanosheets. Tetragonal nanoprisms of nickel precursors are first
synthesized as the self-sacrificing template. Afterwards, these Ni precursors are consumed during the
hydrolysis of iron(II) sulfate to simultaneously grow a layer of Ni-Fe LDH nanosheets around the
surface. The resultant Ni-Fe LDH hollow prisms with large surface area manifest high
electrocatalytic activity toward OER with low overpotential, small Tafel slope and remarkable
stability.
Key words: Ni-Fe, layered double hydroxide, hollow, nanosheets, oxygen evolution reaction.
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10.1002/anie.201710877
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The fast depletion of fossil fuels and consequent cause of environmental problems have stimulated
the research and development of renewable energy systems.[1-3] Specially, electrochemical water
splitting is considered as a promising approach to obtain clean fuels from renewable but intermittent
energy sources.[4,5] However, the realization of a fully integrated water-splitting system is largely
hindered by the sluggish anodic oxygen evolution reaction (OER).[6] In this regard, an appropriate
catalyst is necessary to accelerate the OER under low overpotentials (η) for enhanced energy
conversion efficiency.[7,8] Many research efforts have been made to develop cost-effective
electrocatalysts to replace the state-of-the-art precious metal based catalysts.[9-19]
Among various promising alternatives, oxides/hydroxides containing first row transition metals
(Mn, Fe, Co and Ni etc.) have attracted tremendous interest owing to their earth-abundant nature and
remarkable OER performance.[12-19] In particular, Ni-Fe layered double hydroxides (LDHs) have been
reported as a promising class of the most effective OER catalysts under alkaline environment (pH =
13~14).[20-27] Although deep investigations for the identification of active sites are still needed, the
synergistic interactions between Ni and Fe can indeed bring dramatic enhancement of catalytic
activity compared with single Ni or Fe component.[28-33] As the OER process commonly takes place
on the surface of a catalyst, the geometric structure of the electrocatalysts can strongly influence their
activities.[12,34,35] Therefore, engineering nanostructures of Ni-Fe LDH based catalysts with targeted
functionalities is highly desirable to further boost their OER activity by achieving high participation
of electroactive species and facile transport of mass/electron in the catalytic process.
Hollow nanostructures have been widely designed as a unique type of advanced architectures in
OER application due to their unique structural features.[34,36-39] Compared with bulk solid counterparts,
hollow configurations with higher surface area usually endow catalysts with a higher density of
surface exposed active sites. Moreover, their large void space could effectively reduce ionic transport
resistance and ion diffusion length for the surface reactions.[34] Very recently, a few of pioneering
works have demonstrated the successful generations of hollow nanostructured (oxy)hydroxides
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toward water oxidation reaction with encouraging results.[37,38,40-46] For example, Yan and co-workers
have reported the synthesis of α-Ni(OH)2 hollow nanospheres through a template-free method.[40] The
as-obtained hierarchical hollow nanospheres display enhanced OER activities compared with β-
Ni(OH)2 nanoplates/nanoparticles. In another case, Shi et al. have reported the preparation of VOOH
hollow nanospheres through a similar hydrothermal strategy.[41] As an OER catalyst, the hollow
structured VOOH exhibits smaller η compared with the solid control sample. Despite these advances,
hard templates (e.g., SiO2, Cu2O) are usually involved to obtain hollow nanostructured catalysts with
a uniform size distribution.[38,42] Facile self-engaged strategies without additional removal of inactive
inner cores have become more and more attractive for preparing hollow electrocatalysts.[47,48]
Encouraged by these findings, we herein develop a self-templated strategy to fabricate hierarchical
Ni-Fe LDH hollow nanoprisms assembled by ultrathin nanosheets. Solid prisms of nickel precursors
are selected as the starting materials to provide the Ni source. After a fast and well-controlled
hydrolysis of iron(II) sulfate, the coprecipitation process will generate an ultrathin Ni-Fe LDH shell
around the prism-like core. Of note, the inner template can be simultaneously dissolved and
transformed into active component without the removal step. Benefitting from the desirable
compositions and prominent structural advantages, the as-obtained Ni-Fe LDH hollow nanostructures
manifest enhanced electrocatalytic activity toward water oxidation reaction.
The straightforward synthesis of the novel Ni-Fe LDH hollow nanostructure is schematically
depicted in Figure 1. We first fabricate prism-like Ni precursors as templates through a modified
precipitation method. During the chemical transformation, the existence of water in the mixed solvent
and protons generated from the hydrolysis of Fe2+ could gradually consume the templates to release
the Ni2+ ions into the solution. At the same time, these released ions can participate in the precipitation
of Fe2+ ions to form a hierarchical shell constructed from Ni-Fe LDH nanosheets at the surface of the
Ni precursors. Then, the nanosheets continue to grow with the dissolution of inner core. Under a
steady control of the solvent composition and Fe2+ ion concentration, a well-defined hierarchical Ni-
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10.1002/anie.201710877
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Fe LDH hollow nanoprisms can finally be obtained.
The panoramic field-emission scanning electron microscopy (FESEM) image indicates the
template particles have a narrow size distribution with a prism-like configuration (Figure 2a). A
closer FESEM observation further reveals a smooth surface over these whole prisms (Figure 2b).
Besides, a uniform-contrast transmission electron microscopy (TEM) image of the Ni precursors
demonstrates their solid and dense nature (Figure 2c). As illustrated by X-ray diffraction (XRD), the
crystallographic structure of the Ni precursors is similar with the tetragonal cobalt acetate hydroxide
(JCPDS card No. 22-0582) (Figure 2d).
After the hydrolysis reaction of iron(II) sulfate, the as-obtained discrete sample still preserves the
prism-like morphology (Figure 3a). A magnified FESEM image clearly shows that the smooth
surface of the prism-like precursor finally turns into a hierarchical shell constructed by randomly
oriented nanosheets (Figure 3b). XRD pattern of the resultant hierarchical prisms can be assigned to
a pure LDH phase, indicating the complete consumption of the Ni precursor template during the
reaction (Figure S1, see Supporting Information). In addition, all the diffraction peaks are
significantly broadened, which might be due to small crystallite size and stacking faults. Energy-
dispersive spectroscopy (EDX) analysis validates the existence of Ni, Fe, C and S elements in these
LDH prisms (Figure S2, see Supporting Information). Meanwhile, Fourier transform infrared (FTIR)
spectrum indicates the co-existence of sulfate and acetate anions as the intercalating anions (Figure
S3, see Supporting Information). As shown in Figure 3c, the interior cavity of these LDH prisms is
clearly displayed by the sharp contrast between the hierarchical shell and the well-defined central
void space. An enlarged TEM image verifies that the interconnected LDH nanosheets are loosely
packed to form a highly porous texture (Figure 3d). Moreover, the visible dark strips are the folded
edges or wrinkles of the nanosheets, indicating their ultrathin nature. Owing to these unique structural
features, the Ni-Fe LDH hollow nanoprims exhibit a high Brunauer-Emmett-Teller (BET) specific
surface area of 245.3 m2 g-1 (Figure S4, see Supporting Information). Besides, a set of distinct lattice
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10.1002/anie.201710877
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fringes with a spacing of 0.26 nm can be recognized in the shell, which is consistent with the (012)
plane of LDH structure (Figure 3e). To investigate the spatial distribution of Ni and Fe elements,
element mapping analysis under scanning transmission electron microscope (STEM) mode is carried
out on an individual Ni-Fe LDH hollow particle. High-angle annular dark-field (HAADF)-STEM
image and EDX mapping results confirm the uniform distribution of Ni and Fe elements throughout
the hollow nanoframe (Figure 3f-i).
It is noteworthy that the solvent composition plays an imperative role in controlling the growth of
the Ni-Fe LDH and the removal of the internal template during the hydrolysis reactions. More ethanol
in the mixed solvent can slow down the transformation process. As a result, the Ni precursors are not
dissolved completely and the released Ni2+ ions are not sufficient to support the growth of the newly
formed Ni-Fe LDH nanosheets around the template surface (Figure S5a, see Supporting Information).
Whereas, the reaction rate is accelerated with the increase of water content to form Fe-rich Ni-Fe
LDH nanoclusters without prism-like appearance (Figure S5b and Figure S6, see Supporting
Information). Besides, the morphology of the as-obtained sample could also be affected by the
amount of Fe source. Specifically, a thin Ni-Fe LDH layer produced by insufficient Fe salt is not
strong enough to withstand the outward mass transport during the dissolution of the inner template,
leading to the collapse of the prism (Figure S7a, see Supporting Information). While, an excessive Fe
dosage will generate some cluster-like by-products around the Ni-Fe LDH prisms (Figure S7b, see
Supporting Information). When the usage of iron(II) sulfate is controlled in an appropriate range, the
converted nanostructures exhibit similar hierarchical configurations composed by nanosheet subunits.
Compared with the Ni-Fe LDH hollow prisms, the Ni-Fe LDH-10 (amount of iron(II) sulfate: 10 mg)
sample shows lower Fe content and the other Ni-Fe LDH-50 (amount of iron(II) sulfate: 50 mg)
sample has higher Fe/Ni atomic ratio (Figure S8, see Supporting Information).
Next, the water oxidation activities of the hierarchical Ni-Fe LDH hollow nanoprism
electrocatalyst are investigated. Figure 4a presents the iR-compensated polarization curve.
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Apparently, a redox peak around 1.4 V relative to the reversible hydrogen electrode (RHE) is
observed, which is assigned to the oxidation process of Ni2+ to Ni3+.[23,49] For OER performance
evaluation, the operating potentials required at a current density (j) of 10 mA cm-2 are usually
compared. In the present case, the anodic electrode based on the hierarchical hollow nanoprisms
requires a small potential of 1.51 V (vs. RHE) to obtain the j of 10 mA cm-2, corresponding to a low
η of 280 mV. For comparison, the polarization curves of the control samples with iR correction are
also investigated (Figure S9, see Supporting Information). As shown, the hierarchical Ni-Fe LDH
hollow nanoprisms display apparent structural and compositional superiority with much higher
catalytic activity over the Fe-rich Ni-Fe LDH nanoclusters, Ni-Fe LDH-10 and Ni-Fe LDH-50. Some
previous studies have demonstrated that Ni and Fe elements have played vital roles in the control of
active catalytic sites for Ni-Fe LDH, although further comprehensive study is still needed for the
specific mechanism.[24,28,29] As indicated in Figure 4b, the linear portion of the Tafel plot is fitted into
the Tafel equation (η = b log j + a, where η is the overpotential, j is the current density, and b is the
Tafel slope), yielding a small Tafel slope of 49.4 mV dec-1 for the hierarchical Ni-Fe LDH hollow
nanoprisms.[20] Such high electrocatalytic activities of the Ni-Fe LDH hollow nanoprism product
compare favorably to those of many high-performance Ni-Fe LDH based electrocatalysts (Table S1,
see Supporting Information).[20,21,23,25,27,49,50] The enhanced performance might be ascribed to their
unique hollow features and optimized chemical composition. Specifically, besides the synergistic
effect between Ni and Fe elements, the hierarchical hollow nanoprisms with large surface area could
provide sufficient electroactive sites for electrocatalytic reactions. As indicated by the cyclic
voltammograms (CVs) in a proper potential range without redox processes and the corresponding
capacitive current plots against the scan rate, the Ni-Fe LDH hollow nanoprism sample has much
larger double-layer capacitance (Cdl) compared with the Ni-Fe LDH nanocluster catalyst,
demonstrating the higher electrochemically active surface area (ECSA) of the hollow structure for
the enhanced OER performance (Figure S10, see Supporting Information).
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To avoid the potential physical loss of active materials on glassy carbon (GC) during high-speed
rotating, these hollow structures are transferred onto Ni foam for the stability test. The contribution
from the conductive substrate toward OER can be ignored due to its rather inferior activity (Figure
S11, see Supporting Information). The long-term durability of these prism-like hollow nanostructures
is tested by a chronopotentiometry (CP) measurement on Ni foam under the η of 295 mV. The
obtained result demonstrates the j generally remains stable over 6 h with small degradation,
corroborating the good stability in basic condition (Figure 4c). To further assess the stability of the
Ni-Fe LDH hollow nanoprisms, consecutive CV scans are carried out. As seen from Figure 4d, CV
curves show a small shift of about 10 mV at the j of 10 mA cm-2 after 1000 repeated cycles. Although
the cycling causes considerable damage to the nanosheets, the overall structure of prisms is generally
retained after the long-term stability test (Figure S12, see Supporting Information). The above
experimental results suggest good stability of the Ni-Fe LDH hollow nanoprisms toward OER.
In summary, we have reported a facile self-engaged strategy to prepare hollow nanoprisms
constructed by interconnected ultrathin Ni-Fe layered double hydroxide (LDH) nanosheets. With the
steady control of hydrolysis process of iron(II) sulfate, prism-like Ni precursors can be dissolved and
simultaneously converted to the final Ni-Fe LDH shell with tailorable chemical composition. Owing
to the open and porous structural features and synergistic effect between Ni and Fe elements, the as-
prepared hierarchical Ni-Fe LDH hollow nanoprisms manifest enhanced electrochemical
performance toward the oxygen evolution reaction. Specifically, the electrode based on these hollow
nanoprisms could reach a current density of 10 mA cm-2 at a small overpotential of 280 mV with a
small Tafel slope of 49.4 mV per decade and excellent stability in alkaline electrolyte.
Acknowledgements
X. W. L. acknowledges the funding support from the National Research Foundation (NRF) of
Singapore via the NRF investigatorship (NRF-NRFI2016-04).
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Figures and Captions
Figure 1. Schematic illustration on the formation of hierarchical Ni-Fe LDH hollow nanoprisms via
a self-templated strategy.
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Figure 2. (a,b) FESEM images, (c) TEM image and (d) XRD pattern of prism-like Ni precursors.
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Figure 3. (a,b) FESEM and (c,d) TEM images of the hierarchical Ni-Fe LDH hollow prisms obtained
after the hydrolysis reaction. (e) Lattice fringes from the Ni-Fe LDH shell. (f-i) HAADF-STEM
image and elemental mapping images of an individual Ni-Fe LDH hollow prism.
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Figure 4. (a) Polarization curve and (b) Tafel plot of the Ni-Fe LDH hollow prisms on GC. (c) Time-
dependent current density curve of the Ni-Fe LDH hollow prisms on Ni foam. (d) CV curves of the
Ni-Fe LDH hollow prisms on Ni foam before and after 1000 repeated cycles.
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for Table of Content Entry
Hierarchical hollow nanoprisms composed of ultrathin Ni-Fe layered double hydroxide (LDH)
nanosheets have been prepared via a facile self-templated strategy. During the hydrolysis process of
iron(II) sulfate, prism-like Ni precursors can be dissolved and converted to a shell of Ni-Fe LDH
around the surface. Due to the structural and compositional advantages, these well-defined
hierarchical hollow nanoprisms manifest enhanced electrochemical activity as an electrocatalyst for
oxygen evolution.
15
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