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Author?s Accepted Manuscript
Sub-3 nm Pores in Two-Dimensional Nanomesh
Promoting the Generation of Electroactive Phase
for Robust Water Oxidation
Junfeng Xie, Jianping Xin, Ruoxing Wang,
Xiaodong Zhang, Fengcai Lei, Haichao Qu, Pin
Hao, Guanwei Cui, Bo Tang, Yi Xie
www.elsevier.com/locate/nanoenergy
PII:
DOI:
Reference:
S2211-2855(18)30609-8
https://doi.org/10.1016/j.nanoen.2018.08.045
NANOEN2975
To appear in: Nano Energy
Received date: 21 July 2018
Revised date: 19 August 2018
Accepted date: 20 August 2018
Cite this article as: Junfeng Xie, Jianping Xin, Ruoxing Wang, Xiaodong Zhang,
Fengcai Lei, Haichao Qu, Pin Hao, Guanwei Cui, Bo Tang and Yi Xie, Sub-3
nm Pores in Two-Dimensional Nanomesh Promoting the Generation of
Electroactive Phase for Robust Water Oxidation, Nano Energy,
https://doi.org/10.1016/j.nanoen.2018.08.045
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Sub-3 nm Pores in Two-Dimensional Nanomesh Promoting the Generation of
Electroactive Phase for Robust Water Oxidation
Junfeng Xie,a,b,* Jianping Xin,a Ruoxing Wang,b Xiaodong Zhang,b Fengcai Lei,a,b Haichao Qu,a
Pin Hao,a Guanwei Cui,a Bo Tang,a,* and Yi Xieb,*
a
College of Chemistry, Chemical Engineering and Materials Science, Key Laboratory of Molecular
and Nano Probes (Ministry of Education), Collaborative Innovation Center of Functionalized
Probes for Chemical Imaging in Universities of Shandong, Institute of Molecular and Nano Science,
Shandong Normal University, Jinan, Shandong, 250014, P. R. China
b
Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and
Technology of China, Hefei, Anhui, 230026, P. R. China
xiejf@sdnu.edu.cn
tangb@sdnu.edu.cn
yxie@ustc.edu.cn
Abstract
Herein, abundant and uniform nanopores with sub-3-nm sizes are introduced to 1.5 nm-thick
nickel-iron layered double hydroxide (NiFe LDH) ultrathin nanosheets via an etching-aging process,
realizing remarkable enhancement in oxygen evolution reaction (OER) performance. Detailed
analyses revealed that the NiFe LDH phase around the nanopores can be preferentially oxidized into
electroactive Fe:NiOOH phase. In addition, the buffering space provided by the nanopores can
effectively avoid structural deformation during repeated redox cycling, leading to better
electrochemical stability, which synergistically make the catalyst a promising candidate for
commercial water splitting. This work highlights the positive role of porous structure in accelerating
the formation of active phases beyond just increment of surface area, which can provide insight in
designing advanced catalysts.
Graphical Abstract:
1
Well-distributed sub-3 nm pores are introduced to 2D NiFe LDH ultrathin nanomesh via a
controlled etching-aging process. The nanopores can effectively boost the generation of
catalytically active high-valence Ni species during the redox process and therefore lead to robust
water oxidation performance, making this earth-abundant catalyst promising for commercial water
splitting.
Keywords:
layered double hydroxide ? nanomesh ? porous material ? oxygen evolution reaction ? water splitting
1.
Introduction
Aiming at scalable hydrogen production, electrochemical water splitting has received substantial
attention owing to its high energy conversion efficiency and environmentally benign morality.[1, 2]
Unfortunately, as a half reaction, the sluggish OER severely hampers the overall efficiency of water
splitting for its complex four-electron redox processes.[3] Up to date, intensive research passion has
been devoted focused on exploring earth-abundant alternatives for the highly efficient but expensive
noble metal-based catalysts, such as the oxides of ruthenium and iridium, and enormous progresses
on this topic have been developed accordingly.[4-13]
During past decade, NiFe-based catalysts, including doped or mixed oxides,[14, 15]
oxyhydroxides[16-19] and layered double hydroxides (LDH),[20, 21] hold broad interests owing to
their low cost, easy availability and relatively high efficiency for OER. Both theoretical and
experimental results have proved that Fe plays a crucial role in improving the OER activity of
Ni-based host material,[14-21] but overloading of Fe(III) ions can induce the formation of FeOxHy
due to the presence of Fe-Fe neighboring atoms and cause dramatic degradation of activity.[22]
Therefore, realizing highly dispersed Fe(III) ions in Ni-based catalysts is urgently demanded. Bearing
this in mind, NiFe LDH with randomly dispersed Fe(III) ions and tunable Ni:Fe ratio offers a
2
promising platform to compensate this drawback. As a typical two-dimensional (2D) material, NiFe
LDH holds large surface area with high percentage of exposed atoms which are beneficial to
electrocatalysis.[23] Briefly, NiFe LDH can be regarded as Fe(III)-incorporated Ni(OH)2 layers in
which the extra positive charges brought by high-valence Fe(III) ions are compensated by anion
intercalation between layers, while the close-packed basal planes terminated by hydroxyl group
unfortunately hinder the fast electrochemical conversion to generate high-valence phases, thus
limiting the oxygen-evolving activity.[16, 24] As well accepted, the catalytically active phases in
NiFe-based catalysts are the electrochemically oxidized high-valence species, commonly in the
form of Fe:NiOOH, where the chemical state of Ni is +3~+4.[25-27] Therefore, enriching the
high-valence phases[27-29] or constructing microstructures that can boost the generation of active
species[30-33] have been regarded as effective routes to optimize OER performance, and for NiFe
LDH, constructing ion/gas permeable nanopores would be an effective but challenging strategy in
leaping over this obstacle.[34] Recently, catalysts with highly porous structure have attracted
intensive attention due to their large surface area.[32-37] For example, Zhang et al. developed
porous NiFe oxide as an efficient OER catalyst, which achieves high activity as well as superior
stability.[32] Up to date, most of currently explored catalysts with high porosity are polycrystalline,
for which the poor inter-grain conductivity is a main limiting factor that impedes the OER activity.
Recently, the authors proposed an etching-intralayered Ostwald ripening process to construct
nanopore structure in ?-Ni(OH)2 nanosheets for electrocatalysis.[30] However, the role that
nanopores play in OER catalysis, whether improving the intrinsic activity or merely increasing the
surface area, still lacks experimental evidences, and unraveling this issue will be of great
significance for future design and optimization of porous catalysts.
Herein, by serving the ternary ZnNiFe LDH ultrathin nanosheets as the precursor,
single-crystalline NiFe LDH ultrathin nanomesh with abundant and well-distributed nanopores was
fabricated for the first time (see Experimental section for details). In virtue of the existence of
nanopores, phase transformation from LDH to electroactive ?-NiOOH phase can be significantly
promoted, endowing it with excellent OER activity. By means of structural characterizations
combined with electrochemical normalization analyses, the role that the nanopores play in OER was
confirmed. That is, the material around the nanopores is more active in undergoing
3
electro-oxidation to generate catalytically active phase, therefore subsequently achieve higher OER
performance than the nonporous counterpart where only edges are electroactive. Besides, for the
porous catalyst, ample buffering space can be provided by the nanopores, which can effectively
avoid structural deformation caused by the volume change during repeated redox reactions, thus
guaranteeing superior electrochemical stability, making the NiFe LDH nanomesh catalyst a
promising candidate for commercial water splitting.
2.
Experimental Section
2.1 Materials
All the reagents for synthesis were purchased from Sinopharm Chemical Reagent Co., Ltd. and
used as received.
2.2 Preparation of ZnNiFe LDH bulk material
Typically, 0.15 mmol Ni(NO3)2�2O, 0.05 mmol Fe(NO3)3�2O and 0.05 mmol
Zn(NO3)2�2O were dissolved in 80 mL deionized water, then 0.66 mmol urea was added to form
a homogeneous solution under vigorous stirring. After stirring for 20 min, the transparent solution
was transferred into a 100 mL Teflon-lined stainless steel autoclave, sealed and maintained at
120 癈 for 24 h. Then the reaction system was allowed to cool down to room temperature naturally.
The obtained products were collected by centrifugation, washed with deionized water and ethanol,
and dried at 60 癈 under vacuum.
2.3 Liquid exfoliation to fabricate ZnNiFe LDH nanosheets
To obtain the ternary ZnNiFe LDH nanosheets, liquid exfoliation process was conducted under
ultrasonication.[38] Generally, 300 mg ZnNiFe LDH bulk material was dispersed in 100 mL
formamide and subsequently sonicated for 6 hours. After that, the dispersion was centrifugated at
4000 rpm to remove the unexfoliated bulk, and then the exfoliated nanosheets were collected from
the supernatant by centrifugation at 12000 rpm. The exfoliated nanosheets were washed with
deionized water and ethanol, and then dried at 60 癈 under vacuum.
2.4 Synthesis of the NiFe LDH ultrathin nanomeshes
The single-crystalline NiFe LDH ultrathin nanomeshes were fabricated via a consequent
hydrothermal alkaline-etching treatment of the exfoliated ternary ZnNiFe LDH nanosheets.
Typically, 150 mg exfoliated ZnNiFe LDH nanosheets were dispersed in 10 mL deionized water
4
under vigorous sonication, then 30 mL 1M NaOH solution was added to form a homogeneous
dispersion under subsequent ultrasonication for 15 min. The as-formed dispersion was then
transferred into a 50 mL Teflon-lined stainless steel autoclave and maintained at 140 癈 for 12 h.
After cooling down to room temperature, the as-obtained product was collected by centrifugation at
12000 rpm, washed with deionized water and ethanol, and dried at 60 癈 under vacuum.
2.5 Fabrication of the nonporous NiFe LDH nanosheets
The nonporous NiFe LDH nanosheets were fabricated by exfoliating the corresponding binary
LDH nanoplates synthesized according to a modified method reported in a previous literature.[39]
Typically, 0.9 mmol Ni(NO3)2�2O, 0.3 mmol Fe(NO3)3�2O and 6 mmol urea were dissolved in
40 mL of distilled water and stirred to form a homogeneous solution. Then the aqueous solution was
transferred into a 50 mL Teflon-lined stainless-steel autoclave, sealed, and maintained at 120 癈 for
12 h, after which the reaction system was allowed to cool to room temperature. The as-obtained
product was washed by deionized water and ethanol for several times and dried at 60 癈 under
vacuum. Subsequently, 300 mg as-synthesized NiFe LDH nanoplates were dispersed in 100 mL
formamide and sonicated for 6 hours. After that, the dispersion was centrifugated at 4000 rpm to
remove the unexfoliated bulk species, and then centrifugated at 12000 rpm to obtain the exfoliated
nanosheets. The exfoliated NiFe LDH nanosheets were washed with deionized water and ethanol,
and then dried at 60 癈 under vacuum.
2.6 Electrocatalytic study
All the electrochemical measurements were performed in a three-electrode system on an
electrochemical workstation (CHI660d) at room temperature, and all the potentials were calibrated
to a reversible hydrogen electrode (RHE). Typically, 4 mg of catalyst and 40 ?L Nafion solution
(Sigma Aldrich, 5 wt%) were dispersed in 1 mL water-isopropanol mixed solution (volume ratio of
3:1) by sonicating for at least 30 min to form a homogeneous ink. Then 5 ?L of the dispersion
(containing 20 ?g of catalyst) was loaded onto an L-shaped glassy carbon electrode with 3 mm
diameter, leading to a catalyst loading of 0.285 mg cm-2. The as-prepared catalyst film was allowed
to be dried at room temperature. Linear sweep voltammetry with a scan rate of 5 mV s ?1 was
conducted in 1M KOH aqueous solution (pH=14, saturated with pure O2) using a Ag/AgCl (in
saturated KCl solution) electrode as the reference electrode, a platinum gauze electrode (2 cm � 2
5
cm, 60 mesh) as the counter electrode, and the glassy carbon electrode loaded with various catalysts
as the working electrode. Cyclic voltammetry (CV) was conducted at 5 mV s -1 to survey the
electrochemical reactions and operated at 50 mV s-1 to investigate the cycling stability.
Chronoamperometry data were recorded for the NiFe LDH ultrathin nanomesh catalyst at a static
overpotential of 300 mV. The mass activity curves for various catalysts were calculated from the
catalyst loading and the as-measured linear sweep voltammetry curves. In order to estimate the TOF
value of the NiFe LDH nanomesh and its counterparts, the total amount of Ni and Fe in mole are
identified by ICP analysis, and we assume that both Ni and Fe atoms are active sites for OER.
Hence, the TOF values can be calculated as follows, which represents the lowest limits of the
model:
TOF = j稴geo/4F穘
where j (mA cm-2) is the as-measured current density at various potentials, Sgeo (0.0707 cm-2)
represents the surface area of the glassy carbon disk, the number 4 means a four-electron transfer
during the formation of one mole of O2, F is the Faraday's constant (96485.3 C mol-1), and n is the
moles of Ni and Fe atoms on the electrode which can be calculated by the loading weight of the
metal atoms in the coated catalysts. The Faraday efficiency were calculated from the quantity of
oxygen generated at various constant potentials by means of mass spectroscopy.[40]
3. Characterization
X-ray diffraction (XRD) was performed on a Philips X?Pert Pro Super diffractometer with Cu K?
radiation (? = 1.54178 �). The transmission electron microscopy (TEM) was carried out on a
JEM-2100F field emission electron microscope and a JEOL JEM-ARF200F TEM/STEM at an
acceleration voltage of 200 kV, respectively. The high-resolution TEM (HRTEM), high-angle
annular dark-field scanning transmission electron microscopy (HAADF-STEM) and corresponding
electron energy loss spectroscopy (EELS) mapping analyses were performed on a JEOL
JEM-ARF200F TEM/STEM with a spherical aberration corrector. Atomic force microscopy (AFM)
was performed using a Veeco DI Nano-scope MultiMode V system. Nitrogen adsorption-desorption
isotherms were carried out by using a Micromeritics ASAP 2000 system, and all the gas adsorption
experiments were performed at liquid-nitrogen temperature (77 K) after degassed at 200 癈 for 6 h.
X-ray photoelectron spectroscopy (XPS) analyses were performed on a VGESCALAB MKII X-ray
6
photoelectron spectrometer with an excitation source of Mg K? = 1253.6 eV, and the resolution
level was lower than 1 atom%. The inductively coupled plasma (ICP) emission spectrum was
conducted on a Perkin Elmer Optima 7300DV ICP emission spectroscope.
4. Results and Discussion
Typically, ternary ZnNiFe LDH ultrathin nanosheets are firstly prepared as precursor via liquid
exfoliation, and a following hydrothermal alkaline treatment is conducted to etch the amphoteric Zn
ions, leading to the formation of NiFe LDH primary porous nanosheets. Since Zn ions are randomly
distributed in the ternary LDH host layer, irregular primary nanopores with uneven size are resulted
in the primary porous nanosheets, in which the protruding edges around the primary nanopores
endow thermodynamically unfavorable feature according to Kelvin equation (see Supporting
Information S1 for details).[30] Hence, the Ni or Fe atoms on protruding edges will tend to detach
from the porous LDH matrix. Subsequently, the dissolved species will bond with the
low-coordinated sites, i.e., the concaved edges or pore fringes, thereby reducing the pore size,
smoothing edges and the pore fringes, and finally resulting in NiFe LDH ultrathin nanomesh with
uniform nanopores (Scheme S1).
X-ray diffraction (XRD) analysis was conducted to investigate the structural information of the
samples at different stages in the synthesis procedure (Figure 1A). As can be seen, the NiFe LDH
ultrathin nanomesh exhibits only one weak and broadened diffraction peak, which can be attributed
to the reduced layer thickness and subsequent structure relaxation along c-axis. Detailed analysis
reveals that the exfoliation of ternary LDH can lead to a slight enlargement of interlayer spacing
from 6.6 � to 6.7 � (Figure 1B); while after removing Zn ions from the hydroxide layer, a
dramatically increased interlayer spacing of 7.0 � can be identified, which may arise from the
weakened interaction between adjacent layers induced by the presence of abundant nanopores. Of
note, the temperature for the etching-aging process is a crucial factor for preparing NiFe LDH
ultrathin nanomesh, since lower temperature is not enough to remove Zn ions completely (Table S2),
whereas temperature higher than 140 癈 will trigger phase separation to generate ?-Ni(OH)2 and
FeOx (Figure S2). Besides, the quantity of metal salt precursors is also crucial for the generation of
NiFe LDH nanomesh. When the proportion of Fe reaches 40%, the product exhibits obvious phase
separation (Figure S3), indicating the severe damage of the LDH structure induced by the high
7
component of M(III) ions.[24] When more Zn atoms were introduced into the ternary precursor, the
2D structure of the product after etching-aging process will be unstable rather than generating more
nanopores. That is, the removal of high-ratio Zn component in the exfoliated ternary LDH
nanosheets may lead to poor structural stability of the as-formed primary porous NiFe LDH
nanosheets, which further cause the collapse of the 2D nanostructures rather than undergoing
2D-confined aging process to form ultrathin nanomeshes (Figure S4).
Low-resolution transmission electron microscopy (TEM) images indicate that the product
maintains sheet-like morphology with lateral size of several hundreds of nanometers, and the
homogeneous contrast suggests the uniform thickness (Figure S5). As shown in Figure 1C,
abundant and uniformly distributed nanopores with size of approximately 2~3 nm can be clearly
observed and the layer thickness is identified to be 1.5 nm from a curled edge, corresponding to two
Fe-incorporated Ni(OH)2 layers in NiFe LDH. In addition, a piece of monolayered nanomesh
overlapped on the bilayered nanomesh can also be revealed, indicating the ultrathin thickness and
the strong tendency in forming layer-by-layer assemblies to minimize the surface energy.[41] In
sharp contrast with the highly porous NiFe LDH nanomesh, the ZnNiFe LDH bulk precursor, the
ZnNiFe LDH ultrathin nanosheets as well as the NiFe LDH ultrathin nanosheets exhibit nonporous
nanosheet morphology with high crystallinity (Figure S6-9). Atomic force microscopy (AFM)
image further confirms the uniform thickness of 1.5 nm for the NiFe LDH nanomesh (Figure S10).
Of note, the nanopores cannot be detected since the size of AFM probe is larger than the
nanopores.[42] Nitrogen adsorption-desorption isotherms were conducted to investigate the specific
surface area as well as the pore size distribution. As can be seen from Figure S11 and Table S1, the
NiFe LDH nanomesh exhibits a high Brunauer-Emmett-Teller (BET) specific surface area of 57.7
m2 g-1, which is approximately 3.1, 4.0, and 52.5 times larger than that of the nonporous NiFe LDH
nanosheets, ZnNiFe LDH nanosheets and the bulk ZnNiFe LDH. The corresponding
Barrett-Joyner-Halenda (BJH) pore size distribution curve (Figure S11B) derived from the N2
desorption branch further reveals the presence of nanopores with diameter of ~2.4 nm, which is
consistent with the result from HRTEM analysis. Besides, a broad peak in the range of 3~5 nm can
be observed for all the samples, which can be attributed to the stacking spacing of nanosheets or
nanomeshes. This phenomenon matches well with the results from previous SEM, TEM and AFM
8
analyses. High-resolution TEM (HRTEM) was applied to survey the crystal structure of the NiFe
LDH ultrathin nanomesh, from which the hexagonal symmetry with interplaner spacing of 2.70 �
can be indexed (Figure 1D), matching well with previous report on NiFe LDH.[21] Notably,
although abundant nanopores exist in the basal plane, the nanodomains are interconnected and the
crystal fringes are grown along the same orientation, indicating that the single-crystalline nature is
retained, which may favor the electrocatalytic process owing to better intralayered conductivity
when compared with the polycrystalline catalysts.[43, 44] The corresponding selected area electron
diffraction (SAED) pattern (Figure 2D, inset) further confirms the single-crystalline nature of the
nanomesh, from which the typical six-fold symmetry with six independent diffraction spots can be
revealed, agreeing well with the hexagonal structure of NiFe LDH in the basal xy-plane. Scanning
transmission electron microscopy (STEM) under high-angle annular dark-field (HAADF) mode was
conducted (Figure 1E), from which the abundant nanopores can be further corroborated.
Corresponding elemental mapping analyses were recorded under electron energy loss spectroscopy
(EELS) mode (Figure 1F), where nickel, iron and oxygen are homogenously distributed in the
whole nanomesh. The atomic ratio of elements was analyzed by inductively coupled plasma (ICP)
emission spectrum, and a Ni:Fe molar ratio of 3.20:1 can be identified without Zn residual, which is
close to that of ZnNiFe LDH nanosheet precursor (Table S2). It is worth noting that, the Fe
concentration in the NiFe LDH ultrathin nanomesh is in the range of broad maximum in activity,
suggesting its high potential for OER.[14]
9
Figure 1. Characterizations of the NiFe LDH ultrathin nanomesh. (A) XRD patterns of various
LDH-based materials. (B) Enlarged peak region. (C) TEM image and (D) HRTEM image of the
NiFe LDH ultrathin nanomesh. The inset displays the SAED pattern, which clearly indicates the
single-crystalline nature of the NiFe LDH ultrathin nanomesh. (E) HAADF-STEM image and (F)
elemental maps of Ni, Fe and O detected under EELS mode.
Electrochemical measurements were carried out to verify the structural benefits of the NiFe LDH
ultrathin nanomesh catalyst. As shown in Figure 2A, the polarization curve of NiFe LDH ultrathin
nanomesh shows an early oxidation peak for Ni(II)-to-Ni(III) conversion located at 1.44 V vs. RHE,
which is ~120 mV lower than that of the nonporous nanosheet. This phenomenon can be attributed
to the highly porous structure which can offer more reactive edge sites for oxidation reactions.[34,
45] The early phase conversion can bring in more catalytically active high-valence species at a
certain overpotential, realizing higher OER activity. For instance, a high current density of 234.5
10
mA cm-2 can be achieved at ? = 500 mV for the NiFe LDH ultrathin nanomesh catalyst, which is
6.5-, 93.8- and 156.3-folds larger than those of the nonporous NiFe LDH nanosheet, ZnNiFe LDH
nanosheet and the bulk ZnNiFe LDH. Moreover, the overpotential required to drive an obvious
water splitting, i.e., ? for jgeo = 50 mA cm-2 with elimination of the interference of the Ni(II)-Ni(III)
oxidation reaction, is as low as 268 mV for the nanomesh catalyst, which is among the lowest
values for earth-abundant OER catalysts (Table S3).[16, 45-47] In addition, the commonly used
parameter, ? for jgeo = 10 mA cm-2, is also roughly estimated to be 184 mV from the reverse scan of
CV curves as a reference (Figure S13), which inevitably contains non-negligible errors due to the
existence of double-layer capacitance as well as pseudocapacitance brought by high surface area
and the reversible Ni(II)-Ni(III) redox reactions.
Tafel plot of the NiFe LDH ultrathin nanomesh also shows predominance in kinetic aspect when
compared with the other three references (Figure 2B). A small Tafel slope of 30 mV decade-1 can be
identified, suggesting the facile OER process for the nanomesh catalyst. The small Tafel slope will
lead to a strongly enhanced OER rate at a moderate increase of overpotential.[48] The facile
catalytic process could be attributed to the highly porous morphology which provides more
ion-accessible and gas-permeable channels that are perpendicular to the ultrathin layers, thereby
exposing more reactive edges, facilitating the generation of catalytically active species, and
guaranteeing easy release of oxygen bubbles to offer empty sites for subsequent catalysis.[34, 45]
In order to further understand the influence of the KOH concentration to the OER performance,
the LSV curves measured in O2-saturated 0.1 M KOH solution were investigated. As shown in
Figure S14A, the NiFe LDH nanomesh catalyst exhibits excellent OER activity with a high current
density of 96.1 mA cm-2 at ? = 500 mV and a low overpotential of 403 mV to drive a rapid water
splitting rate with current density of 50 mA cm-2, demonstrating the high OER activity of the NiFe
LDH nanomesh catalyst even in lower pH condition.
11
Figure 2. Electrochemical characterizations on OER activity. (A) Polarization curves and (B)
corresponding Tafel plots of NiFe LDH ultrathin nanomesh and counterparts. Scan rate: 5 mV s -1.
(C) TOF plots with respect to applied potentials of various LDH-based catalysts in 1 M KOH
electrolyte. (D) Faraday efficiency of the NiFe LDH nanomesh catalyst in 1 M and 0.1 M KOH
electrolyte at various constant potentials. (E) Mass activity of diverse LDH-based OER catalysts. (F)
Estimation of Cdl for various catalysts.
12
Turnover frequency (TOF) is a key parameter to evaluate an advanced OER catalyst.
Unfortunately, the determination of the TOF per active sites for NiFe-based catalysts is still
controversial, mainly owing to the ambiguous nature of the active sites (i.e., Ni or Fe, bridge ?2-OH
or on top ?1-OH) and lack of reliable methods to identify them.[16] In order to estimate the TOF
value of the NiFe LDH nanomesh and its counterparts, the total amount of Ni and Fe in mole are
measured by ICP analysis, and we assume that both Ni and Fe atoms are active sites for OER,
which can derive the lowest limits of TOF for various catalysts. As shown in Figure 2C, the TOF
values of the NiFe LDH nanomesh catalyst are calculated to be 1.49 s-1 and 4.74 s-1 at ? = 300 mV
and 500 mV, respectively, which show significant superiority to those of the NiFe LDH nanosheets,
ZnNiFe LDH nanosheets and bulk ZnNiFe LDH material. The value of 1.49 s-1 at ? = 300 mV has
surpasses that of the NiFeOx film (TOF = 0.21 s-1; on Au/Ti, in 1 M KOH),[26] bulk and exfoliated
NiFe LDH (TOF = 0.01 s-1 and 0.05 s-1, respectively; loaded on glassy carbon (GC) electrode, in 1
M KOH),[45] the NiFe LDH/CNT hybrid catalyst (TOF = 0.56 s-1; loaded on carbon fiber paper, in
1 M KOH),[21] and even the state-of-the-art gelled FeCoW oxyhydroxide at the same conditions
(0.46 s-1 in 1 M KOH, loaded on GC electrode) (Table S3),[47] confirming the excellent OER
activity of the NiFe LDH nanomesh catalyst. When measured in 0.1 M KOH electrolyte, the TOF of
the NiFe LDH nanomesh catalyst is also sound (Figure S14B). The TOF value in 0.1 M KOH
solution reaches 0.99 s-1 at ? = 400 mV, showing 9.3 times enhancement than the nonporous NiFe
LDH nanosheets. This value is also larger than some previously developed earth-abundant OER
catalysts.[33, 49, 50]
Faraday efficiency is another important parameter to investigate the conversion efficiency from
electric energy to chemical energy. The Faraday efficiency of the NiFe LDH nanomesh catalyst for
OER in 1 M and 0.1 M KOH electrolyte at various constant potentials was identified by using mass
spectroscopy, respectively.[40] As can be seen from Figure 2D, the Faraday efficiency for oxygen
evolution is measured to be ~95% at an applied potential of 1.50 V vs. RHE in 1 M KOH
electrolyte, and further reaches ~98% at higher potentials. The lower Faraday efficiency at 1.50 V
may arise from the incomplete oxidation reaction that consumes charges for Ni (II)-Ni(III) phase
conversion, while at higher potentials, OER becomes the predominant reaction, which displays high
Faraday efficiency for the NiFe LDH nanomesh catalyst. Of note, the Faraday efficiency of ~98% is
13
comparable to many earth-abundant OER catalysts, demonstrating the high activity of the NiFe
LDH nanomesh catalyst.[40, 51-53] Furthermore, the Faraday efficiency of the NiFe LDH
nanomesh catalyst in 0.1 M KOH solution was also investigated, which reaches ~96% at potentials
higher than 1.6 V vs. RHE where OER process is predominant, revealing the high conversion
efficiency for OER even at lower pH conditions. The high Faraday efficiency and the high TOF
value further confirm the excellent OER activity of the NiFe LDH nanomesh catalyst.
Mass activity is also crucial for commercial water splitting. As depicted in Figure 2E, the mass
activity of NiFe LDH ultrathin nanomesh reaches 257.8 A g-1 and 818.6 A g-1 at ? = 300 mV and
500 mV, respectively, indicating the high OER activity (Table S3). In sharp contrast, nonporous
NiFe LDH nanosheet only gains 32.0 A g-1 and 125.3 A g-1 at the same overpotentials, further
confirming the structure-performance relationship of the nanomesh catalyst. Electrochemical double
layer capacitance (Cdl) was measured to better understand the merits of the nanomesh morphology,
which is linearly proportional to the electrochemically active surface area (ECSA).[34, 44] As
shown in Figure 2F, the NiFe LDH ultrathin nanomesh shows a Cdl of 8.3 ?F, which is roughly 4.9,
3.2 and 27.7 times larger than the values of NiFe LDH nanosheet, ZnNiFe LDH nanosheet and the
bulk ZnNiFe LDH, respectively, revealing the high exposure of electrochemically active and
ion-accessible sites in the nanomesh catalyst, which is responsible for the enhanced OER activity.
Interestingly, the increment of ECSA by forming nanopores (4.9 times enlargement) is more
obvious than that for the comparison in BET specific surface area (3.1 times enlargement). That is,
both nanosheet and nanomesh catalysts have strong tendency in accumulating into layer-by-layer
assemblies during the solution-processed preparation of catalyst-coated working electrode to lower
their surface energy.[41] Under this circumstance, the basal surfaces of the nonporous nanosheets
undergo severe overlapping, making the as-measured Cdl values underestimated. While for the NiFe
LDH nanomesh catalyst, although the overlapping of individual nanomeshes is inevitable, the
abundant nanopores can act as effective channels for vertical ion penetration, thus achieving a
higher Cdl value and further resulting in significant enhancement in OER activity.[30] By means of
normalization of the LSV curves by Cdl values (Figure S16), the nanomesh catalyst exhibits the
highest normalized current among all the tested samples, which confirms the increased intrinsic
activity in OER catalysis. The high intrinsic activity of the nanomesh catalyst may arise from the
14
highly porous structure, which can provide more reactive sites for electro-oxidation to generate
active phases for OER.[32, 34, 36, 37]
Figure 3. Stability tests and analyses on structure-performance relationship. (A) Stability test
of the NiFe LDH ultrathin nanomesh by long-term CV cycling. (B) Chronoamperometry data
detected at ? = 300 mV. (C) XPS Ni 2p spectrum after 100 CV cycles indicates the generation of
Ni(III) species. (D) HRTEM image of the nanomesh catalyst after 100 CV cycles. The original
positions of nanopores are highlighted by green arrows, and the orthorhombic ?-NiOOH
nanodomains are indexed by dashed circles.
Except for the activity, electrochemical stability is another key criterion to evaluate an
electrocatalyst. Long-term cyclic voltammetry (CV) was first applied to the NiFe LDH ultrathin
nanomesh. As shown in Figure 3A, the anodic current shows slight increment after 5 CV cycles,
15
and reaches a maximum after 100 cycles, showing a 17.9% increment in catalytic current at ? = 300
mV. This phenomenon is common for earth-abundant OER catalysts, which can be ascribed to the
activation process with substantial accumulation of active high-valence species.[54, 55] When
applying further cycling, the current undergoes slight degradation, but is still higher than that of the
initial cycle even after 3000 CV cycles, indicating the superior stability of the nanomesh catalyst.
Of note, the onset overpotential exhibits negligible degradation during long-term CV cycling,
revealing the high OER activity against performance fading. The high stability of the nanomesh
catalyst may arise from the highly porous structure which offers ample space to buffer the volume
change during repeated redox reactions.[44] In addition, the electrochemical stability was further
evaluated by chronoamperometry under fixed overpotential. As shown in Figure 3B, at ? = 300 mV,
the anodic current of the nanomesh catalyst exhibits considerable increment within first 36 hours
and shows only slight degradation until 48 hours, confirming the superior electrochemical stability
in long-term OER operation. In contrast, the nonporous NiFe LDH nanosheet catalyst undergoes a
similar activation process but subsequently a severe performance fading, indicating its poor
operational stability (Figure S18). X-ray photoelectron spectroscopy (XPS) was applied to
investigate the valence of nickel for the catalyst after OER operation. As can be seen from Figure
3C, the Ni 2p spectrum of the catalyst after undergoing 100 CV cycles can be fitted as two
spin-orbit doublets, characteristic of Ni(II) and Ni(III), and two shakeup satellites.[56-58] The binding
energy of Ni 2p3/2 region can be deconvoluted into two peaks at 857.5 eV and 859.3 eV that match
the oxidation states of Ni(II) and Ni(III), respectively; while the binding energy of Ni 2p1/2 region can
be deconvoluted into two peaks centered at 875.0 eV and 876.5 eV, corresponding to Ni(II) and Ni(III),
respectively.[56-58] Furthermore, two broad peaks at 863.0 eV and 881.2 eV can be indexed to the
satellite peaks of 2p3/2 and 2p1/2 spin orbits. Therefore, the existence of catalytically active Ni(III)
species can be confirmed for the post-OER nanomesh catalyst, which may be responsible for the
enhanced activity during the initial cycling (from 1st cycle to 100th cycle, Figure 3A). In order to
verify the morphological benefits in fast generation of active high-valence species for the NiFe
LDH ultrathin nanomesh catalyst, HRTEM was conducted to the nanomesh and nanosheet catalysts
after repeated CV cycles. As shown in Figure 3D, the hexagonal LDH structure of the nanomesh
catalyst is generally maintained, and the original position of nanopores can be observed from the
16
lower diffraction contrast, suggesting the high structural stability of the nanomesh. Of note, a new
orthorhombic phase can be observed around the original position of nanopores, which can be
indexed as ?-NiOOH, indicating that the pore region are more active to undergo electro-oxidation
from Ni(II) to Ni(III).[29] In sharp contrast, the HRTEM image of the nonporous NiFe LDH
nanosheet catalyst after continuous CV cycling can be seen from Figure S20, where the hexagonal
LDH structure is basically maintained in the internal area of the nanosheet, and the ?-NiOOH phase
can only be identified in the edge area. This phenomenon indicates that the edges of the nonporous
nanosheets have a higher tendency in undergoing redox reactions than the close-packed internal
basal planes of the LDH structure. For the NiFe LDH nanomesh catalyst, the fringes around the
nanopores possess similar structure with the edges, which are rich in low-coordinated metal atoms
and more active for the Ni(II)-Ni(III) redox reactions.[37] Therefore, the abundant sub-3 nm pores in
the NiFe LDH nanomesh catalyst can effectively promote the generation of electroactive
high-valence phase during the OER operation rather than just increase surface area. Therefore, the
intrinsic electrochemical activity can be improved via introducing pore structure, in line with the
result from Cdl normalization study of the OER activity (Figure S16). Besides, the electroactive
?-NiOOH phase accumulated in repeated CV operation may also contribute to the improved OER
activity, which has been proved in Fe-incorporated Ni-based catalysts.[27] As indicated in HRTEM
analysis (Figure 3D), considerable ?-NiOOH phase can only be observed after approximately 100
CV cycles. Interestingly, LSV curves indicate that the OER activity reaches a maximum after 100
CV cycles (Figure 3A), which is coincident with the content trend of Ni(III) species. Therefore, the
accumulation of Ni(III) species in the nanomesh catalyst may serve as the activation effect for OER
catalysis, leading to the increased OER activity during repeated CV scanning.
Figure 4. Schematic illustration depicts the structural benefits of the nanomesh catalyst.
17
A schematic illustration is depicted in Figure 4 to intuitively highlight the structural benefits of
the NiFe LDH ultrathin nanomesh catalyst. As illustrated, the active high-valence phases are mainly
formed in the fringes of nanopores and the edges, which is benefited from the presence of abundant
nanopores that ensures facile ion interaction and subsequently leads to fast phase conversion from
LDH to the high-valence electroactive phase. This fast phase conversion is deemed to be
responsible for the enhanced OER activity. Moreover, owing to the high porosity of the nanomesh
catalyst, sufficient space could be offered to avoid the structural deformation induced by frequent
redox phase conversion, which guarantees superb electrochemical stability in long-term OER
operation.
5.
Conclusions
In summary, highly porous NiFe LDH ultrathin nanomesh with uniformly distributed
nanopores in size of sub-3 nm was synthesized via an etching-aging process by serving ZnNiFe
LDH nanosheets as the precursor. The amphoteric Zn ions in the ternary precursor can be
selectively etched by alkaline treatment, and a subsequent aging process can result in the
homogenization of nanopores. Benefited from the 2D highly porous morphology, the generation of
catalytically active high-valence phase can be effectively promoted, realizing remarkably enhanced
OER activity. Detailed structural analyses revealed that the pore region is more electroactive in
undergoing the redox reaction to generate active ?-NiOOH phase, which is responsible for the
enhanced OER activity. In addition, the abundant nanopores in 2D layers can provide ample space
for buffering the volume change in repeated redox reactions, which effectively avoids structural
deformation and guarantees superior stability for OER. This work provides an efficient strategy on
optimizing the OER performance, and may enlighten future designing of catalysts for
energy-related applications.
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (grant
number 21501112, 21401181, 21535004, U1532265, 21331005, 11621063, 21390411 and
U1632149), the Natural Science Foundation of Shandong Province (grant number ZR2018JL009),
18
and the Key Research Program of Frontier Sciences (grant number QYZDY-SSW-SLH011). J. Xie
and J. Xin contributed equally to this work.
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Vitae
Prof. Junfeng Xie received his Bachelor's degree in Chemistry from Qufu Normal University in
2009 and Ph.D. in Inorganic Chemistry at University of Science and Technology of China
(USTC) in 2014. Currently, he is an associate professor of Inorganic Chemistry in College of
22
Chemistry, Chemical Engineering and Materials Science, Shandong Normal University (SDNU).
His research interests mainly focus on function-oriented design and synthesis of
low-dimensional electrocatalysts for energy conversion.
Jianping Xin received his BS degree in Chemistry from Shihezi University (2014) and MS
degree in Applied Chemistry from Shandong Normal University (SDNU, 2017). Currently, he is
a doctoral student in Materials Physics and Chemistry at Shandong University. His research
interests include the designs and synthesis of nanostructured materials for highly efficient
electrocatalytic water splitting.
Ruoxing Wang received his BS degree from University of Science and Technology of China
(USTC) in 2015 and currently she is pursuing her Ph.D. at Purdue University. Her research
interests mainly focus on structural optimization of low-dimensional electrocatalysts for energy
conversion.
23
Prof. Xiaodong Zhang obtained his BS degree in the Department of Chemistry at Anhui
University (2008) and Ph.D. degree from the University of Science and Technology of China
(USTC) in 2013. After that he joined the Collaborative Innovation Center of Chemistry as a
postdoctoral associate. Currently, Prof. Zhang is a research professor in Department of Chemistry,
USTC. His current research areas are centered in controllable synthesis and functionality of
two-dimensional semiconductors with atomic thickness.
Dr. Fengcai Lei received her BS degree in Physics from Shandong Normal University (SDNU)
in 2011 and Ph.D. in Condensed Matter Physics at University of Science and Technology of
China (USTC) in 2016. Now she is an assistant research fellow in Shandong Normal University.
Her current interests include the theoretical computation and the underlying physics during
studying the atomically thin inorganic graphene analogues for energy conversion.
24
Haichao Qu received her BS degree in Chemistry from Harbin University of Science and
Technology in 2016 and currently pursuing her MS degree at Shandong Normal University
(SDNU). Her research interests include material design and fabrication for energy-related
electrocatalysis.
Dr. Pin Hao received her Ph.D. from Shandong University (SDU) in 2015 and currently works
in College of Chemistry, Chemical Engineering and Materials Science, Shandong Normal
University (SDNU). Her research interests mainly focus on design and synthesis of functional
materials for electrochemical energy conversion and storage.
25
Prof. Guanwei Cui received his Ph.D. from Shandong Normal University (SDNU) in 2013 and
currently he is a professor of the College of Chemistry, Chemical Engineering and Materials
Science, Shandong Normal University (SDNU). His research interests mainly focus on design
and synthesis of photocatalysts for water splitting.
Prof. Bo Tang received his Ph.D. degree from the Nankai University in 1994. He is currently a
full professor of the College of Chemistry, Chemical Engineering and Materials Science,
Shandong Normal University (SDNU). His current cutting-edge research interests are mainly
focused on the design and synthesis of molecular and nano probes for bioimaging and functional
materials for (photo)electrochemical energy conversion.
26
Prof. Yi Xie obtained her BS degree from Xiamen University (1988) and a Ph.D. from the
University of Science and Technology of China (USTC, 1996). She is currently a Principal
Investigator of Hefei National Laboratory for Physical Sciences at the Microscale and a full
professor of the Department of Chemistry, USTC. Her current cutting-edge research interests are
mainly focused at four major frontiers: solid state materials chemistry, nanotechnology, energy
science and theoretical physics.
Highlights
? Uniform sub-3-nm nanopores are firstly introduced into the NiFe LDH nanosheets which
significantly optimize the OER activity and stability.
? We firstly show that the region around the nanopore structure in NiFe LDH nanosheets can
accelerate the generation of high-valence nickel species which are catalytically active for OER
catalysis.
? The highly porous NiFe LDH nanosheet catalyst with abundant sub-3-nm nanopores and optimal
Ni:Fe ratio exhibits the best OER activity among all samples with 234.5 mA cm-2 at an
overpotential of 500 mV vs. RHE and a small overpotential of 184 mV at 10?mA?cm?2.
27
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