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Nano Energy

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Sub-3 nm pores in two-dimensional nanomesh promoting the generation ofelectroactive phase for robust water oxidation

Junfeng Xiea,b,⁎,1, Jianping Xina,1, Ruoxing Wangb, Xiaodong Zhangb, Fengcai Leia,b,Haichao Qua, Pin Haoa, Guanwei Cuia, Bo Tanga,⁎, Yi Xieb,⁎

a College of Chemistry, Chemical Engineering and Materials Science, Key Laboratory of Molecular and Nano Probes (Ministry of Education), Collaborative InnovationCenter of Functionalized Probes for Chemical Imaging in Universities of Shandong, Institute of Molecular and Nano Science, Shandong Normal University, Jinan, Shandong250014, PR ChinabHefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, PR China

A R T I C L E I N F O

Keywords:Layered double hydroxideNanomeshPorous materialOxygen evolution reactionWater splitting

A B S T R A C T

Herein, abundant and uniform nanopores with sub-3-nm sizes are introduced to 1.5 nm-thick nickel-iron layereddouble hydroxide (NiFe LDH) ultrathin nanosheets via an etching-aging process, realizing remarkable en-hancement in oxygen evolution reaction (OER) performance. Detailed analyses revealed that the NiFe LDH phasearound the nanopores can be preferentially oxidized into electroactive Fe:NiOOH phase. In addition, the buf-fering space provided by the nanopores can effectively avoid structural deformation during repeated redoxcycling, leading to better electrochemical stability, which synergistically make the catalyst a promising candi-date for commercial water splitting. This work highlights the positive role of porous structure in accelerating theformation of active phases beyond just increment of surface area, which can provide insight in designing ad-vanced catalysts.

1. Introduction

Aiming at scalable hydrogen production, electrochemical watersplitting has received substantial attention owing to its high energyconversion efficiency and environmentally benign morality [1,2]. Un-fortunately, as a half reaction, the sluggish OER severely hampers theoverall efficiency of water splitting for its complex four-electron redoxprocesses [3]. Up to date, intensive research passion has been devotedfocused on exploring earth-abundant alternatives for the highly effi-cient but expensive noble metal-based catalysts, such as the oxides ofruthenium and iridium, and enormous progresses on this topic havebeen developed accordingly [4–13].

During past decade, NiFe-based catalysts, including doped or mixedoxides [14,15], oxyhydroxides [16–19] and layered double hydroxides(LDH) [20,21], hold broad interests owing to their low cost, easyavailability and relatively high efficiency for OER. Both theoretical andexperimental results have proved that Fe plays a crucial role in im-proving the OER activity of Ni-based host material [14–21], but over-loading of Fe(III) ions can induce the formation of FeOxHy due to thepresence of Fe-Fe neighboring atoms and cause dramatic degradation ofactivity [22]. Therefore, realizing highly dispersed Fe(III) ions in Ni-

based catalysts is urgently demanded. Bearing this in mind, NiFe LDHwith randomly dispersed Fe(III) ions and tunable Ni:Fe ratio offers apromising platform to compensate this drawback. As a typical two-di-mensional (2D) material, NiFe LDH holds large surface area with highpercentage of exposed atoms which are beneficial to electrocatalysis[23]. Briefly, NiFe LDH can be regarded as Fe(III)-incorporated Ni(OH)2layers in which the extra positive charges brought by high-valenceFe(III) ions are compensated by anion intercalation between layers,while the close-packed basal planes terminated by hydroxyl group un-fortunately hinder the fast electrochemical conversion to generate high-valence phases, thus limiting the oxygen-evolving activity [16,24]. Aswell accepted, the catalytically active phases in NiFe-based catalysts arethe electrochemically oxidized high-valence species, commonly in theform of Fe:NiOOH, where the chemical state of Ni is + 3~+4[25–27]. Therefore, enriching the high-valence phases [27–29] orconstructing microstructures that can boost the generation of activespecies [30–33] have been regarded as effective routes to optimize OERperformance, and for NiFe LDH, constructing ion/gas permeable na-nopores would be an effective but challenging strategy in leaping overthis obstacle [34]. Recently, catalysts with highly porous structure haveattracted intensive attention due to their large surface area [32–37]. For

https://doi.org/10.1016/j.nanoen.2018.08.045Received 21 July 2018; Received in revised form 19 August 2018; Accepted 20 August 2018

⁎ Corresponding authors.

1 J. Xie and J. Xin contributed equally to this work.E-mail addresses: xiejf@sdnu.edu.cn (J. Xie), tangb@sdnu.edu.cn (B. Tang), yxie@ustc.edu.cn (Y. Xie).

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Available online 21 August 20182211-2855/ © 2018 Published by Elsevier Ltd.

T

example, Zhang et al. developed porous NiFe oxide as an efficient OERcatalyst, which achieves high activity as well as superior stability [32].Up to date, most of currently explored catalysts with high porosity arepolycrystalline, for which the poor inter-grain conductivity is a mainlimiting factor that impedes the OER activity. Recently, the authorsproposed an etching-intralayered Ostwald ripening process to constructnanopore structure in β-Ni(OH)2 nanosheets for electrocatalysis [30].However, the role that nanopores play in OER catalysis, whether im-proving the intrinsic activity or merely increasing the surface area, stilllacks experimental evidences, and unraveling this issue will be of greatsignificance for future design and optimization of porous catalysts.

Herein, by serving the ternary ZnNiFe LDH ultrathin nanosheets asthe precursor, single-crystalline NiFe LDH ultrathin nanomesh withabundant and well-distributed nanopores was fabricated for the firsttime (see Experimental section for details). In virtue of the existence ofnanopores, phase transformation from LDH to electroactive α-NiOOHphase can be significantly promoted, endowing it with excellent OERactivity. By means of structural characterizations combined with elec-trochemical normalization analyses, the role that the nanopores play inOER was confirmed. That is, the material around the nanopores is moreactive in undergoing electro-oxidation to generate catalytically activephase, therefore subsequently achieve higher OER performance thanthe nonporous counterpart where only edges are electroactive. Besides,for the porous catalyst, ample buffering space can be provided by thenanopores, which can effectively avoid structural deformation causedby the volume change during repeated redox reactions, thus guaran-teeing superior electrochemical stability, making the NiFe LDH nano-mesh catalyst a promising candidate for commercial water splitting.

2. Experimental section

2.1. Materials

All the reagents for synthesis were purchased from SinopharmChemical Reagent Co., Ltd. and used as received.

2.2. Preparation of ZnNiFe LDH bulk material

Typically, 0.15mmol Ni(NO3)2·6H2O, 0.05mmol Fe(NO3)3·9H2Oand 0.05mmol Zn(NO3)2·6H2O were dissolved in 80mL deionizedwater, then 0.66mmol urea was added to form a homogeneous solutionunder vigorous stirring. After stirring for 20min, the transparent solu-tion was transferred into a 100mL Teflon-lined stainless steel auto-clave, sealed and maintained at 120 °C for 24 h. Then the reactionsystem was allowed to cool down to room temperature naturally. Theobtained products were collected by centrifugation, washed withdeionized water and ethanol, and dried at 60 °C under vacuum.

2.3. Liquid exfoliation to fabricate ZnNiFe LDH nanosheets

To obtain the ternary ZnNiFe LDH nanosheets, liquid exfoliationprocess was conducted under ultrasonication [38]. Generally, 300mgZnNiFe LDH bulk material was dispersed in 100mL formamide andsubsequently sonicated for 6 h. After that, the dispersion was cen-trifugated at 4000 rpm to remove the unexfoliated bulk, and then theexfoliated nanosheets were collected from the supernatant by cen-trifugation at 12,000 rpm. The exfoliated nanosheets were washed withdeionized water and ethanol, and then dried at 60 °C under vacuum.

2.4. Synthesis of the NiFe LDH ultrathin nanomeshes

The single-crystalline NiFe LDH ultrathin nanomeshes were fabri-cated via a consequent hydrothermal alkaline-etching treatment of theexfoliated ternary ZnNiFe LDH nanosheets. Typically, 150mg ex-foliated ZnNiFe LDH nanosheets were dispersed in 10mL deionizedwater under vigorous sonication, then 30mL 1M NaOH solution was

added to form a homogeneous dispersion under subsequent ultra-sonication for 15min. The as-formed dispersion was then transferredinto a 50mL Teflon-lined stainless steel autoclave and maintained at140 °C for 12 h. After cooling down to room temperature, the as-ob-tained product was collected by centrifugation at 12,000 rpm, washedwith deionized water and ethanol, and dried at 60 °C under vacuum.

2.5. Fabrication of the nonporous NiFe LDH nanosheets

The nonporous NiFe LDH nanosheets were fabricated by exfoliatingthe corresponding binary LDH nanoplates synthesized according to amodified method reported in a previous literature [39]. Typically,0.9 mmol Ni(NO3)2·6H2O, 0.3mmol Fe(NO3)3·9H2O and 6mmol ureawere dissolved in 40mL of distilled water and stirred to form ahomogeneous solution. Then the aqueous solution was transferred intoa 50mL Teflon-lined stainless-steel autoclave, sealed, and maintainedat 120 °C for 12 h, after which the reaction system was allowed to coolto room temperature. The as-obtained product was washed by deio-nized water and ethanol for several times and dried at 60 °C undervacuum. Subsequently, 300mg as-synthesized NiFe LDH nanoplateswere dispersed in 100mL formamide and sonicated for 6 h. After that,the dispersion was centrifugated at 4000 rpm to remove the un-exfoliated bulk species, and then centrifugated at 12,000 rpm to obtainthe exfoliated nanosheets. The exfoliated NiFe LDH nanosheets werewashed with deionized water and ethanol, and then dried at 60 °Cunder vacuum.

2.6. Electrocatalytic study

All the electrochemical measurements were performed in a three-electrode system on an electrochemical workstation (CHI660d) at roomtemperature, and all the potentials were calibrated to a reversible hy-drogen electrode (RHE). Typically, 4 mg of catalyst and 40 μL Nafionsolution (Sigma Aldrich, 5 wt%) were dispersed in 1mL water-iso-propanol mixed solution (volume ratio of 3:1) by sonicating for at least30min to form a homogeneous ink. Then 5 μL of the dispersion (con-taining 20 μg of catalyst) was loaded onto an L-shaped glassy carbonelectrode with 3mm diameter, leading to a catalyst loading of0.285mg cm-2. The as-prepared catalyst film was allowed to be dried atroom temperature. Linear sweep voltammetry with a scan rate of5mV s−1 was conducted in 1M KOH aqueous solution (pH=14, sa-turated with pure O2) using a Ag/AgCl (in saturated KCl solution)electrode as the reference electrode, a platinum gauze electrode(2 cm×2 cm, 60 mesh) as the counter electrode, and the glassy carbonelectrode loaded with various catalysts as the working electrode. Cyclicvoltammetry (CV) was conducted at 5mV s-1 to survey the electro-chemical reactions and operated at 50mV s-1 to investigate the cyclingstability. Chronoamperometry data were recorded for the NiFe LDHultrathin nanomesh catalyst at a static overpotential of 300mV. Themass activity curves for various catalysts were calculated from thecatalyst loading and the as-measured linear sweep voltammetry curves.In order to estimate the TOF value of the NiFe LDH nanomesh and itscounterparts, the total amount of Ni and Fe in mole are identified byICP analysis, and we assume that both Ni and Fe atoms are active sitesfor OER. Hence, the TOF values can be calculated as follows, whichrepresents the lowest limits of the model:

=TOF j·S /4F·ngeo

where j (mA cm-2) is the as-measured current density at variouspotentials, Sgeo (0.0707 cm-2) represents the surface area of the glassycarbon disk, the number 4 means a four-electron transfer during theformation of one mole of O2, F is the Faraday's constant(96,485.3 Cmol-1), and n is the moles of Ni and Fe atoms on the elec-trode which can be calculated by the loading weight of the metal atomsin the coated catalysts. The Faraday efficiency were calculated from thequantity of oxygen generated at various constant potentials by means of

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mass spectroscopy [40].

3. Characterization

X-ray diffraction (XRD) was performed on a Philips X’Pert Pro Superdiffractometer with Cu Kα radiation (λ=1.54178 Å). The transmissionelectron microscopy (TEM) was carried out on a JEM-2100F fieldemission electron microscope and a JEOL JEM-ARF200F TEM/STEM atan acceleration voltage of 200 kV, respectively. The high-resolutionTEM (HRTEM), high-angle annular dark-field scanning transmissionelectron microscopy (HAADF-STEM) and corresponding electron en-ergy loss spectroscopy (EELS) mapping analyses were performed on aJEOL 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 isothermswere carried out by using a Micromeritics ASAP 2000 system, and allthe gas adsorption experiments were performed at liquid-nitrogentemperature (77 K) after degassed at 200 °C for 6 h. X-ray photoelectronspectroscopy (XPS) analyses were performed on a VGESCALAB MKII X-ray photoelectron spectrometer with an excitation source of Mg Kα=1253.6 eV, and the resolution level was lower than 1 atom%. Theinductively coupled plasma (ICP) emission spectrum was conducted ona Perkin Elmer Optima 7300DV ICP emission spectroscope.

4. Results and discussion

Typically, ternary ZnNiFe LDH ultrathin nanosheets are firstly pre-pared as precursor via liquid exfoliation, and a following hydrothermalalkaline treatment is conducted to etch the amphoteric Zn ions, leadingto the formation of NiFe LDH primary porous nanosheets. Since Zn ionsare randomly distributed in the ternary LDH host layer, irregular pri-mary nanopores with uneven size are resulted in the primary porousnanosheets, in which the protruding edges around the primary nano-pores endow thermodynamically unfavorable feature according toKelvin equation (see Supporting information S1 for details) [30].Hence, the Ni or Fe atoms on protruding edges will tend to detach fromthe porous LDH matrix. Subsequently, the dissolved species will bondwith 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 uniformnanopores (Scheme S1).

X-ray diffraction (XRD) analysis was conducted to investigate thestructural information of the samples at different stages in the synthesisprocedure (Fig. 1A). As can be seen, the NiFe LDH ultrathin nanomeshexhibits only one weak and broadened diffraction peak, which can beattributed to the reduced layer thickness and subsequent structure re-laxation along c-axis. Detailed analysis reveals that the exfoliation ofternary LDH can lead to a slight enlargement of interlayer spacing from6.6 Å to 6.7 Å (Fig. 1B); while after removing Zn ions from the hydro-xide layer, a dramatically increased interlayer spacing of 7.0 Å can beidentified, which may arise from the weakened interaction betweenadjacent layers induced by the presence of abundant nanopores. Ofnote, the temperature for the etching-aging process is a crucial factorfor preparing NiFe LDH ultrathin nanomesh, since lower temperature isnot enough to remove Zn ions completely (Table S2), whereas tem-perature higher than 140 °C will trigger phase separation to generate β-Ni(OH)2 and FeOx (Fig. S2). Besides, the quantity of metal salt pre-cursors is also crucial for the generation of NiFe LDH nanomesh. Whenthe proportion of Fe reaches 40%, the product exhibits obvious phaseseparation (Fig. S3), indicating the severe damage of the LDH structureinduced by the high component of M(III) ions [24]. When more Zn atomswere introduced into the ternary precursor, the 2D structure of theproduct after etching-aging process will be unstable rather than gen-erating more nanopores. That is, the removal of high-ratio Zn compo-nent in the exfoliated ternary LDH nanosheets may lead to poor struc-tural stability of the as-formed primary porous NiFe LDH nanosheets,

which further cause the collapse of the 2D nanostructures rather thanundergoing 2D-confined aging process to form ultrathin nanomeshes(Fig. S4).

Low-resolution transmission electron microscopy (TEM) images in-dicate that the product maintains sheet-like morphology with lateralsize of several hundreds of nanometers, and the homogeneous contrastsuggests the uniform thickness (Fig. S5). As shown in Fig. 1C, abundantand uniformly distributed nanopores with size of approximately 2–3 nmcan be clearly observed and the layer thickness is identified to be1.5 nm from a curled edge, corresponding to two Fe-incorporated Ni(OH)2 layers in NiFe LDH. In addition, a piece of monolayered nano-mesh overlapped on the bilayered nanomesh can also be revealed, in-dicating the ultrathin thickness and the strong tendency in forminglayer-by-layer assemblies to minimize the surface energy [41]. In sharpcontrast with the highly porous NiFe LDH nanomesh, the ZnNiFe LDHbulk precursor, the ZnNiFe LDH ultrathin nanosheets as well as the NiFeLDH ultrathin nanosheets exhibit nonporous nanosheet morphologywith high crystallinity (Fig. S6–9). Atomic force microscopy (AFM)image further confirms the uniform thickness of 1.5 nm for the NiFeLDH nanomesh (Fig. S10). Of note, the nanopores cannot be detectedsince the size of AFM probe is larger than the nanopores [42]. Nitrogenadsorption-desorption isotherms were conducted to investigate thespecific surface area as well as the pore size distribution. As can be seenfrom Fig. S11 and Table S1, the NiFe LDH nanomesh exhibits a highBrunauer-Emmett-Teller (BET) specific surface area of 57.7m2 g-1,which is approximately 3.1, 4.0, and 52.5 times larger than that of thenonporous NiFe LDH nanosheets, ZnNiFe LDH nanosheets and the bulkZnNiFe LDH. The corresponding Barrett-Joyner-Halenda (BJH) poresize distribution curve (Fig. S11B) derived from the N2 desorptionbranch further reveals the presence of nanopores with diameter of ~2.4 nm, which is consistent with the result from HRTEM analysis. Be-sides, a broad peak in the range of 3–5 nm can be observed for all thesamples, which can be attributed to the stacking spacing of nanosheetsor nanomeshes. This phenomenon matches well with the results fromprevious SEM, TEM and AFM analyses. High-resolution TEM (HRTEM)was applied to survey the crystal structure of the NiFe LDH ultrathinnanomesh, from which the hexagonal symmetry with interplaner spa-cing of 2.70 Å can be indexed (Fig. 1D), matching well with previousreport on NiFe LDH [21]. Notably, although abundant nanopores existin the basal plane, the nanodomains are interconnected and the crystalfringes are grown along the same orientation, indicating that the single-crystalline nature is retained, which may favor the electrocatalyticprocess owing to better intralayered conductivity when compared withthe polycrystalline catalysts [43,44]. The corresponding selected areaelectron diffraction (SAED) pattern (Fig. 2D, inset) further confirms thesingle-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 (Fig. 1E), fromwhich the abundant nanopores can be further corroborated. Corre-sponding elemental mapping analyses were recorded under electronenergy loss spectroscopy (EELS) mode (Fig. 1F), where nickel, iron andoxygen are homogenously distributed in the whole nanomesh. Theatomic ratio of elements was analyzed by inductively coupled plasma(ICP) emission spectrum, and a Ni:Fe molar ratio of 3.20:1 can beidentified without Zn residual, which is close to that of ZnNiFe LDHnanosheet precursor (Table S2). It is worth noting that, the Fe con-centration in the NiFe LDH ultrathin nanomesh is in the range of broadmaximum in activity, suggesting its high potential for OER [14].

Electrochemical measurements were carried out to verify thestructural benefits of the NiFe LDH ultrathin nanomesh catalyst. Asshown in Fig. 2A, the polarization curve of NiFe LDH ultrathin nano-mesh shows an early oxidation peak for Ni(II)-to-Ni(III) conversion lo-cated at 1.44 V vs. RHE, which is ~ 120mV lower than that of thenonporous nanosheet. This phenomenon can be attributed to the highly

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porous structure which can offer more reactive edge sites for oxidationreactions [34,45]. The early phase conversion can bring in more cata-lytically active high-valence species at a certain overpotential, realizinghigher OER activity. For instance, a high current density of 234.5 mAcm-2 can be achieved at η=500mV for the NiFe LDH ultrathin nano-mesh catalyst, which is 6.5-, 93.8- and 156.3-folds larger than those ofthe nonporous NiFe LDH nanosheet, ZnNiFe LDH nanosheet and thebulk ZnNiFe LDH. Moreover, the overpotential required to drive anobvious water splitting, i.e., η for jgeo = 50mA cm-2 with elimination ofthe interference of the Ni(II)-Ni(III) oxidation reaction, is as low as268mV for the nanomesh catalyst, which is among the lowest valuesfor earth-abundant OER catalysts (Table S3) [16,45–47]. In addition,the commonly used parameter, η for jgeo = 10mA cm-2, is also roughlyestimated to be 184mV from the reverse scan of CV curves as a re-ference (Fig. S13), which inevitably contains non-negligible errors dueto the existence of double-layer capacitance as well as pseudocapaci-tance brought by high surface area and the reversible Ni(II)-Ni(III) redoxreactions.

Tafel plot of the NiFe LDH ultrathin nanomesh also shows pre-dominance in kinetic aspect when compared with the other three re-ferences (Fig. 2B). A small Tafel slope of 30mV decade-1 can be iden-tified, suggesting the facile OER process for the nanomesh catalyst. Thesmall Tafel slope will lead to a strongly enhanced OER rate at a mod-erate increase of overpotential [48]. The facile catalytic process couldbe attributed to the highly porous morphology which provides moreion-accessible and gas-permeable channels that are perpendicular to theultrathin layers, thereby exposing more reactive edges, facilitating the

generation of catalytically active species, and guaranteeing easy releaseof oxygen bubbles to offer empty sites for subsequent catalysis [34,45].

In order to further understand the influence of the KOH con-centration to the OER performance, the LSV curves measured in O2-saturated 0.1 M KOH solution were investigated. As shown in Fig. S14A,the NiFe LDH nanomesh catalyst exhibits excellent OER activity with ahigh current density of 96.1mA cm-2 at η=500mV and a low over-potential of 403mV to drive a rapid water splitting rate with currentdensity of 50mA cm-2, demonstrating the high OER activity of the NiFeLDH nanomesh catalyst even in lower pH condition.

Turnover frequency (TOF) is a key parameter to evaluate an ad-vanced OER catalyst. Unfortunately, the determination of the TOF peractive sites for NiFe-based catalysts is still controversial, mainly owingto the ambiguous nature of the active sites (i.e., Ni or Fe, bridge μ2-OHor on top μ1-OH) and lack of reliable methods to identify them [16]. Inorder to estimate the TOF value of the NiFe LDH nanomesh and itscounterparts, the total amount of Ni and Fe in mole are measured byICP analysis, and we assume that both Ni and Fe atoms are active sitesfor OER, which can derive the lowest limits of TOF for various catalysts.As shown in Fig. 2C, the TOF values of the NiFe LDH nanomesh catalystare calculated to be 1.49 s-1 and 4.74 s-1 at η=300mV and 500mV,respectively, which show significant superiority to those of the NiFeLDH nanosheets, ZnNiFe LDH nanosheets and bulk ZnNiFe LDH mate-rial. The value of 1.49 s-1 at η=300mV has surpasses that of the Ni-FeOx film (TOF = 0.21 s-1; on Au/Ti, in 1M KOH) [26], bulk and ex-foliated NiFe LDH (TOF = 0.01 s-1 and 0.05 s-1, respectively; loaded onglassy carbon (GC) electrode, in 1M KOH) [45], the NiFe LDH/CNT

Fig. 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 LDHultrathin nanomesh. (E) HAADF-STEM image and (F) elemental maps of Ni, Fe and O detected under EELS mode.

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hybrid catalyst (TOF = 0.56 s-1; loaded on carbon fiber paper, in 1MKOH) [21], and even the state-of-the-art gelled FeCoW oxyhydroxide atthe same conditions (0.46 s-1 in 1M KOH, loaded on GC electrode)(Table S3) [47], confirming the excellent OER activity of the NiFe LDHnanomesh catalyst. When measured in 0.1 M KOH electrolyte, the TOFof the NiFe LDH nanomesh catalyst is also sound (Fig. S14B). The TOFvalue in 0.1M KOH solution reaches 0.99 s-1 at η=400mV, showing9.3 times enhancement than the nonporous NiFe LDH nanosheets. Thisvalue is also larger than some previously developed earth-abundantOER catalysts [33,49,50].

Faraday efficiency is another important parameter to investigate theconversion efficiency from electric energy to chemical energy. TheFaraday efficiency of the NiFe LDH nanomesh catalyst for OER in 1Mand 0.1M KOH electrolyte at various constant potentials was identified

by using mass spectroscopy, respectively [40]. As can be seen fromFig. 2D, the Faraday efficiency for oxygen evolution is measured to be~ 95% at an applied potential of 1.50 V vs. RHE in 1M KOH electrolyte,and further reaches ~ 98% at higher potentials. The lower Faradayefficiency at 1.50 V may arise from the incomplete oxidation reactionthat consumes charges for Ni(II)-Ni(III) phase conversion, while at higherpotentials, OER becomes the predominant reaction, which displays highFaraday efficiency for the NiFe LDH nanomesh catalyst. Of note, theFaraday efficiency of ~ 98% is comparable to many earth-abundantOER catalysts, demonstrating the high activity of the NiFe LDH nano-mesh catalyst [40,51–53]. Furthermore, the Faraday efficiency of theNiFe LDH nanomesh catalyst in 0.1 M KOH solution was also in-vestigated, which reaches ~ 96% at potentials higher than 1.6 V vs.RHE where OER process is predominant, revealing the high conversion

Fig. 2. Electrochemical characterizations on OER activity. (A) Polarization curves and (B) corresponding Tafel plots of NiFe LDH ultrathin nanomesh and coun-terparts. Scan rate: 5mV s-1. (C) TOF plots with respect to applied potentials of various LDH-based catalysts in 1M KOH electrolyte. (D) Faraday efficiency of the NiFeLDH nanomesh catalyst in 1M and 0.1M KOH electrolyte at various constant potentials. (E) Mass activity of diverse LDH-based OER catalysts. (F) Estimation of Cdl

for various catalysts.

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efficiency for OER even at lower pH conditions. The high Faraday ef-ficiency and the high TOF value further confirm the excellent OERactivity of the NiFe LDH nanomesh catalyst.

Mass activity is also crucial for commercial water splitting. As de-picted in Fig. 2E, the mass activity of NiFe LDH ultrathin nanomeshreaches 257.8 A g-1 and 818.6 A g-1 at η=300mV and 500mV, re-spectively, indicating the high OER activity (Table S3). In sharp con-trast, nonporous NiFe LDH nanosheet only gains 32.0 A g-1 and125.3 A g-1 at the same overpotentials, further confirming the structure-performance relationship of the nanomesh catalyst. Electrochemicaldouble layer capacitance (Cdl) was measured to better understand themerits of the nanomesh morphology, which is linearly proportional tothe electrochemically active surface area (ECSA) [34,44]. As shown inFig. 2F, the NiFe LDH ultrathin nanomesh shows a Cdl of 8.3 μF, whichis roughly 4.9, 3.2 and 27.7 times larger than the values of NiFe LDHnanosheet, ZnNiFe LDH nanosheet and the bulk ZnNiFe LDH, respec-tively, revealing the high exposure of electrochemically active and ion-accessible sites in the nanomesh catalyst, which is responsible for theenhanced OER activity. Interestingly, the increment of ECSA by formingnanopores (4.9 times enlargement) is more obvious than that for thecomparison in BET specific surface area (3.1 times enlargement). Thatis, both nanosheet and nanomesh catalysts have strong tendency inaccumulating into layer-by-layer assemblies during the solution-pro-cessed preparation of catalyst-coated working electrode to lower theirsurface energy [41]. Under this circumstance, the basal surfaces of thenonporous nanosheets undergo severe overlapping, making the as-measured Cdl values underestimated. While for the NiFe LDH nanomeshcatalyst, although the overlapping of individual nanomeshes is in-evitable, the abundant nanopores can act as effective channels forvertical ion penetration, thus achieving a higher Cdl value and furtherresulting in significant enhancement in OER activity [30]. By means ofnormalization of the LSV curves by Cdl values (Fig. S16), the nanomeshcatalyst exhibits the highest normalized current among all the testedsamples, which confirms the increased intrinsic activity in OER cata-lysis. The high intrinsic activity of the nanomesh catalyst may arisefrom the highly porous structure, which can provide more reactive sitesfor electro-oxidation to generate active phases for OER [32,34,36,37].

Except for the activity, electrochemical stability is another key cri-terion to evaluate an electrocatalyst. Long-term cyclic voltammetry(CV) was first applied to the NiFe LDH ultrathin nanomesh. As shown inFig. 3A, the anodic current shows slight increment after 5 CV cycles,and reaches a maximum after 100 cycles, showing a 17.9% incrementin catalytic current at η=300mV. This phenomenon is common forearth-abundant OER catalysts, which can be ascribed to the activationprocess with substantial accumulation of active high-valence species[54,55]. When applying further cycling, the current undergoes slightdegradation, but is still higher than that of the initial cycle even after3000 CV cycles, indicating the superior stability of the nanomesh cat-alyst. Of note, the onset overpotential exhibits negligible degradationduring long-term CV cycling, revealing the high OER activity againstperformance fading. The high stability of the nanomesh catalyst mayarise from the highly porous structure which offers ample space tobuffer the volume change during repeated redox reactions [44]. Inaddition, the electrochemical stability was further evaluated bychronoamperometry under fixed overpotential. As shown in Fig. 3B, atη=300mV, the anodic current of the nanomesh catalyst exhibitsconsiderable increment within first 36 h and shows only slight de-gradation until 48 h, confirming the superior electrochemical stabilityin long-term OER operation. In contrast, the nonporous NiFe LDH na-nosheet catalyst undergoes a similar activation process but subse-quently a severe performance fading, indicating its poor operationalstability (Fig. S18). X-ray photoelectron spectroscopy (XPS) was appliedto investigate the valence of nickel for the catalyst after OER operation.As can be seen from Fig. 3C, the Ni 2p spectrum of the catalyst afterundergoing 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 twopeaks 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 canbe 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 sa-tellite peaks of 2p3/2 and 2p1/2 spin orbits. Therefore, the existence ofcatalytically active Ni(III) species can be confirmed for the post-OERnanomesh catalyst, which may be responsible for the enhanced activityduring the initial cycling (from 1st cycle to 100th cycle, Fig. 3A). Inorder to verify the morphological benefits in fast generation of activehigh-valence species for the NiFe LDH ultrathin nanomesh catalyst,HRTEM was conducted to the nanomesh and nanosheet catalysts afterrepeated CV cycles. As shown in Fig. 3D, the hexagonal LDH structureof the nanomesh catalyst is generally maintained, and the original po-sition of nanopores can be observed from the lower diffraction contrast,suggesting the high structural stability of the nanomesh. Of note, a neworthorhombic phase can be observed around the original position ofnanopores, which can be indexed as α-NiOOH, indicating that the poreregion are more active to undergo electro-oxidation from Ni(II) to Ni(III)

[29]. In sharp contrast, the HRTEM image of the nonporous NiFe LDHnanosheet catalyst after continuous CV cycling can be seen from Fig.S20, where the hexagonal LDH structure is basically maintained in theinternal area of the nanosheet, and the α-NiOOH phase can only beidentified in the edge area. This phenomenon indicates that the edges ofthe nonporous nanosheets have a higher tendency in undergoing redoxreactions than the close-packed internal basal planes of the LDHstructure. For the NiFe LDH nanomesh catalyst, the fringes around thenanopores possess similar structure with the edges, which are rich inlow-coordinated metal atoms and more active for the Ni(II)-Ni(III) redoxreactions [37]. Therefore, the abundant sub-3 nm pores in the NiFe LDHnanomesh catalyst can effectively promote the generation of electro-active high-valence phase during the OER operation rather than justincrease surface area. Therefore, the intrinsic electrochemical activitycan be improved via introducing pore structure, in line with the resultfrom Cdl normalization study of the OER activity (Fig. S16). Besides, theelectroactive α-NiOOH phase accumulated in repeated CV operationmay also contribute to the improved OER activity, which has beenproved in Fe-incorporated Ni-based catalysts [27]. As indicated inHRTEM analysis (Fig. 3D), considerable α-NiOOH phase can only beobserved after approximately 100 CV cycles. Interestingly, LSV curvesindicate that the OER activity reaches a maximum after 100 CV cycles(Fig. 3A), which is coincident with the content trend of Ni(III) species.Therefore, the accumulation of Ni(III) species in the nanomesh catalystmay serve as the activation effect for OER catalysis, leading to the in-creased OER activity during repeated CV scanning.

A schematic illustration is depicted in Fig. 4 to intuitively highlightthe structural benefits of the NiFe LDH ultrathin nanomesh catalyst. Asillustrated, the active high-valence phases are mainly formed in thefringes of nanopores and the edges, which is benefited from the pre-sence of abundant nanopores that ensures facile ion interaction andsubsequently leads to fast phase conversion from LDH to the high-va-lence electroactive phase. This fast phase conversion is deemed to beresponsible for the enhanced OER activity. Moreover, owing to the highporosity of the nanomesh catalyst, sufficient space could be offered toavoid the structural deformation induced by frequent redox phaseconversion, which guarantees superb electrochemical stability in long-term OER operation.

5. Conclusions

In summary, highly porous NiFe LDH ultrathin nanomesh withuniformly distributed nanopores in size of sub-3 nm was synthesized viaan etching-aging process by serving ZnNiFe LDH nanosheets as theprecursor. The amphoteric Zn ions in the ternary precursor can be se-lectively etched by alkaline treatment, and a subsequent aging process

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can result in the homogenization of nanopores. Benefited from the 2Dhighly porous morphology, the generation of catalytically active high-valence phase can be effectively promoted, realizing remarkably en-hanced OER activity. Detailed structural analyses revealed that the poreregion is more electroactive in undergoing the redox reaction to gen-erate active α-NiOOH phase, which is responsible for the enhanced OERactivity. In addition, the abundant nanopores in 2D layers can provideample space for buffering the volume change in repeated redox

reactions, which effectively avoids structural deformation and guaran-tees superior stability for OER. This work provides an efficient strategyon optimizing the OER performance, and may enlighten future de-signing of catalysts for energy-related applications.

Acknowledgements

This work was financially supported by the National Natural Science

Fig. 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 η=300mV. (C) XPS Ni 2p spectrum after 100 CV cycles indicates the generation of Ni(III) species. (D) HRTEM image of thenanomesh catalyst after 100 CV cycles. The original positions of nanopores are highlighted by green arrows, and the orthorhombic α-NiOOH nanodomains areindexed by dashed circles.

Fig. 4. Schematic illustration depicts the structural benefits of the nanomesh catalyst.

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Foundation of China (grant number 21501112, 21401181, 21535004,U1532265, 21331005, 11621063, 21390411 and U1632149), theNatural Science Foundation of Shandong Province (grant numberZR2018JL009), and the Key Research Program of Frontier Sciences(grant number QYZDY-SSW-SLH011).

Appendix A. Supporting information

Supplementary data associated with this article can be found in theonline version at doi:10.1016/j.nanoen.2018.08.045.

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Prof. Junfeng Xie received his Bachelor's degree inChemistry from Qufu Normal University in 2009 and Ph.D.in Inorganic Chemistry at University of Science andTechnology of China (USTC) in 2014. Currently, he is anassociate professor of Inorganic Chemistry in College ofChemistry, Chemical Engineering and Materials Science,Shandong Normal University (SDNU). His research interestsmainly focus on function-oriented design and synthesis oflow-dimensional electrocatalysts for energy conversion.

Jianping Xin received his BS degree in Chemistry fromShihezi University (2014) and MS degree in AppliedChemistry from Shandong Normal University (SDNU,2017). Currently, he is a doctoral student in MaterialsPhysics and Chemistry at Shandong University. His researchinterests include the designs and synthesis of nanos-tructured materials for highly efficient electrocatalyticwater splitting.

Ruoxing Wang received his BS degree from University ofScience and Technology of China (USTC) in 2015 and cur-rently she is pursuing her Ph.D. at Purdue University. Herresearch interests mainly focus on structural optimizationof low-dimensional electrocatalysts for energy conversion.

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Prof. Xiaodong Zhang obtained his BS degree in theDepartment of Chemistry at Anhui University (2008) andPh.D. degree from the University of Science andTechnology of China (USTC) in 2013. After that he joinedthe Collaborative Innovation Center of Chemistry as apostdoctoral associate. Currently, Prof. Zhang is a researchprofessor in Department of Chemistry, USTC. His currentresearch areas are centered in controllable synthesis andfunctionality of two-dimensional semiconductors withatomic thickness.

Dr. Fengcai Lei received her BS degree in Physics fromShandong Normal University (SDNU) in 2011 and Ph.D. inCondensed Matter Physics at University of Science andTechnology of China (USTC) in 2016. Now she is an assis-tant research fellow in Shandong Normal University. Hercurrent interests include the theoretical computation andthe underlying physics during studying the atomically thininorganic graphene analogues for energy conversion.

Haichao Qu received her BS degree in Chemistry fromHarbin University of Science and Technology in 2016 andcurrently pursuing her MS degree at Shandong NormalUniversity (SDNU). Her research interests include materialdesign 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, ShandongNormal University (SDNU). Her research interests mainlyfocus on design and synthesis of functional materials forelectrochemical energy conversion and storage.

Prof. Guanwei Cui received his Ph.D. from ShandongNormal University (SDNU) in 2013 and currently he is aprofessor of the College of Chemistry, ChemicalEngineering and Materials Science, Shandong NormalUniversity (SDNU). His research interests mainly focus ondesign and synthesis of photocatalysts for water splitting.

Prof. Bo Tang received his Ph.D. degree from the NankaiUniversity in 1994. He is currently a full professor of theCollege of Chemistry, Chemical Engineering and MaterialsScience, Shandong Normal University (SDNU). His currentcutting-edge research interests are mainly focused on thedesign and synthesis of molecular and nano probes forbioimaging and functional materials for (photo)electro-chemical energy conversion.

Prof. Yi Xie obtained her BS degree from XiamenUniversity (1988) and a Ph.D. from the University ofScience and Technology of China (USTC, 1996). She iscurrently a Principal Investigator of Hefei NationalLaboratory for Physical Sciences at the Microscale and a fullprofessor of the Department of Chemistry, USTC. Her cur-rent cutting-edge research interests are mainly focused atfour major frontiers: solid state materials chemistry, nano-technology, energy science and theoretical physics.

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