Hierarchical NiFe Layered Double Hydroxide Hollow Microspheres

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Hierarchical NiFe Layered Double Hydroxide Hollow Microspheres with Highly-Efficient Behavior toward Oxygen Evolution Reaction Cong Zhang, Mingfei Shao, Lei Zhou, Zhenhua Li, Kaiming Xiao, and Min Wei ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12100 • Publication Date (Web): 24 Nov 2016 Downloaded from http://pubs.acs.org on November 26, 2016

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ACS Applied Materials & Interfaces

Hierarchical NiFe Layered Double Hydroxide Hollow Microspheres with Highly-Efficient Behavior toward Oxygen Evolution Reaction Cong Zhang, Mingfei Shao,* Lei Zhou, Zhenhua Li, Kaiming Xiao and Min Wei* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China.

ABSTRACT: The exploitation of highly-efficiency and low-cost electrocatalysts toward oxygen evolution reaction (OER) is a meaningful route in renewable energy technologies including solar fuel and water splitting. Herein, NiFe-layered double hydroxide (NiFe-LDH) hollow microsphere was designed and synthesized via a one-step in situ growth method by using SiO2 as a sacrificial template. Benefiting from the unique architecture, NiFe-LDH HMS shows a highly efficient OER electrocatalytic activity with a preferable current density (71.69 mA cm–2 at η = 300 mV) and a small onset overpotential (239 mV at 10 mA cm–2), which outperforms the 20 wt.% commercial Ir/C catalyst. Moreover, it exhibits a remarkably low Tafel slope (53 mV dec–1) as well as a satisfactory long-time stability. Electrochemical studies reveal that this hierarchical structure facilitates a full exposure of active sites and facile ion transport kinetics, accounting for the excellent performance. It is expected that the NiFe-LDH microsphere material can serve as a promising non-noble metal based electrocatalyst toward water oxidation reaction.

KEYWORDS: oxygen evolution reaction, layered double hydroxide, electrocatalysts, hollow microsphere, non-precious metal hydroxide

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INTRODUCTION The exploration of promising oxygen evolution reaction (OER) catalysts have boomed in the past few years owing to its essential role in various energy conversion and storage systems, including water splitting, fuel cells and metal–air batteries.1-4 To obtain an efficient OER performance, a promising OER electrocatalyst is necessary to accelerate the four-electron OER process for the development of energy technologies.5,6 At present, noble metal oxides, such as IrO2 and RuO2, are recognized as highly active electrocatalysts for OER but surfer from intrinsic limitations such as the high cost, scarcity and poor stability.7-9 To overcome these drawbacks, great efforts have been concentrated on the OER electrocatalysts based on non-noble metal materials, including transition metal oxides,10-12 hydroxides/oxyhydroxides13-16 and sulfides.17-19 Although some progresses have been made, the continuous development of durable earth-abundant alternatives for efficient water splitting process by virtue of material exploration and fabrication strategy is still urgent. Layered double hydroxides (LDHs) are a promising class of two-dimensional anionic clays, which can be used as a promising catalyst in the electrochemical water oxidation.20,21 So far, extensive efforts are focused on the synthesis and application of LDHs for OER by optimizing metal composition and ratio in the host layers,22-28 exfoliation into single-layer nanosheets29,30 or combining with specific functional materials.31-36 Although the developed LDHs-based catalysts have shown satisfactory activity in OER, an inefficient exposure of active sites and poor electron/ion transport ability of LDHs powdered samples tend to induce inferior OER performance. To solve these problems, an optimized architecture with maximized exposure of active sites, facile electron/ion transport and fast gas escape is highly desirable.37-42 Nanostructured materials with hierarchical architecture (e.g., core-shell or hollow microspheres) have been reported with interesting electrochemical properties compared with their low-dimensional counterparts.43,44 Fabricating well-designed nano-architecture with a large specific surface area and appropriate pore structure is a key challenge for LDHs in OER applications, in which a full participation


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of electroactive species and an easy transport pathway of mass/electron in the OER catalytic process can be guaranteed. In this work, an efficient OER electrocatalyst based on NiFe-LDH hollow microsphere (HMS) has been prepared via a one-step in situ growth technique by using SiO2 as a sacrificial template. The as-synthesized NiFe-LDH HMS electrocatalyst gives a current density of 71.69 mA cm−2 at η = 300 mV, which is 3.75 and 5.36 folds than that of NiFe-LDH nanoparticles (NPs) and 20 wt% Ir/C commercial catalyst, respectively. Furthermore, the current density (η = 300 mV) is rather stable and even increases by 4.5% after 40000 s of testing, indicating a prominent durability. The superior OER performances can be ascribed to the highly-dispersed nanoplatelets with fully exposed active species and facile electron/ion transport kinetics, confirmed by various electrochemical investigations. In addition, this preparation strategy is also demonstrated in other three LDHs materials (CoFe-, CoNi- and NiAl-LDH) with satisfactory OER performance. EXPERIMENTAL SECTION Reagents and Materials All the chemical reagents including nitrate, tetraethyl orthosilicate (TEOS) and 20 wt% Ir/C catalyst are analytical grade, which is obtained from Aladdin reagent Co., Ltd. Hexamethylenetetramine (HMT) and NaOH were purchased from Beijing Chemical Co., Ltd. Preparation of LDHs electrocatalysts Synthesis of NiFe-LDH HMS: The monodispersed silica spheres were first synthesized according to a reported method with some modification.45 0.1 g of synthesized SiO2 spheres were dispersed in 100 mL of HMT solution (9 mM), heated to 120 °C. Then Ni(NO3)2·6H2O (1.5 mM) and Fe(NO3)3·9H2O (0.5 mM) was fully mixed, which was added dropwise (over 2 h) into the above suspension under stirring, followed by aging in an autoclave at 120 °C for 24 h. After thoroughly wash and dry at room temperature, the NiFe-LDH HMS product was finally synthesized. For the preparation of LDHs HMSs containing different Ni/Fe ratios, Ni(NO3)2·6H2O was changed to 1.67, 1.60, 1.50, 1.33 and 1.00 mM, and the amount ACS Paragon Plus Environment

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of Fe(NO3)3·9H2O was 0.33, 0.40, 0.50, 0.67 and 1.00 mM accordingly, giving rise to a molar ratio of Ni2+/Fe3+ to be 5:1, 4:1, 3:1, 2:1 and 1:1, respectively. Synthesis of NiFe-LDH NPs: The NiFe LDH NPs were obtained through a modified coprecipitation method.46 Briefly, a mixed metal salt solution (150 mL) consist of Ni(NO3)2·6H2O (0.16 M) and Fe(NO3)3·9H2O (0.055 M) was adjusted to pH= 9.0 by using NaOH solution (1.5 M), followed by aging in an autoclave at 120 °C for 24 h. The colloidal LDH suspension was centrifuged and washed thoroughly for three times, followed drying at 60 °C for 6 h. Structural Characterizations The morphology of NiFe-LDH HMS and NiFe-LDH NP was conducted using a scanning electron microscope (SEM; Zeiss SUPRA 55) and a high-resolution transmission electron microscopy (HRTEM) system (JEM 2100), combined with energy dispersive X-ray (EDX) spectroscopy for determining the metal composition of Ni, Fe, C and O. For the X-ray diffraction patterns (XRD) measurement, the NiFeLDH HMS and NiFe-LDH NP were carried out on a Shimadzu XRD-6000 diffractometer with a scan step of 10° min−1. A Thermo VG ESCALAB 250 X-ray photoelectron spectrometer using Al Kα radiation at a pressure of 2×10−9 Pa was used for the measurement of X-ray photoelectron spectra (XPS). Brunauer– Emmett–Teller (BET) method was chosen for the specific surface areas calculation of the LDH materials. Electrochemical Measurements We use a standard three electrode system controlled by a CHI 660E electrochemistry workstation (Shanghai Chenhua Instrument Co., China) for the electrochemical studies (counter electrode: platinum wire; reference electrode: Ag/AgCl). The working electrode was fabricated as follows: the catalyst (3.5 mg) was firstly dispersed in a mixture solution (0.77 mL of water, 0.20 mL of ethanol), and then 0.03 mL of Nafion solution (5.0 wt%) was added, followed by sonication (1.0 h) to form a homogeneous suspension. The above suspension (5.0 μL) was dripped onto a clean glass carbon electrode (GCE). After the solvent evaporation for 10 min in air, the working electrode was used for electrochemical


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measurements. For the Ni foam electrode, 70.6 μL of the above-mentioned catalyst ink was dropped onto a 1 cm  1 cm  0.1 cm Ni foam (loading density: 0.25 mg cm−2). The electrochemical performances were assessed by using cyclic voltammetry (CV) measurements (at 50 mV s−1) and linear sweep voltammetry (LSV; at 5 mV s−1). For both NiFe-LDH HMS and NiFe-LDH NP samples, the CVs were recycled for 50 times before the data were collected for comparison. The electrochemically active surface area (EASA) was obtained by CVs at various scan rates. The plotting of  j = (ja  jc) at 0.25 V vs. Ag/AgCl against scan rate was estimated, whose linear slope is twice of the double layer capacitance (Cdl). The apparent turnover frequency (TOF) value was obtained according to the reported method.29 The Faradaic efficiency which was tested by using rotating ring disk electrode (RRDE) is calculated as follows: Faradaic efficiency = napp × iR/iD × N; where napp is the apparent number of electron (which is 2 at a rotating rate of 1600 rpm); iR and iD are the measured ring and disk current, respectively; while N is the collection efficiency (0.29 ± 0.01). An airtight H shape cell was designed for the electrolysis experiment of Faradaic efficiency.41 RESULTS AND DISCUSSION

Scheme 1. Scheme of the synthesis of NiFe-LDH HMS. ACS Paragon Plus Environment

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The fabrication process of NiFe-LDH HMS is displayed in Scheme 1. The monodisperse SiO2 microspheres with a narrow size distribution (diameter ~250 nm; Figure 1A,D and Figure S1A,D) were firstly prepared by a modified Stöber method,45 followed by dispersion in HMT solution at 120 °C. Afterwards, a mixed metal salts solution containing Ni2+ and Fe3+ ions (2.0 mM in total; molar ratio Ni:Fe= 3:1 ) was added dropwise into the above suspension under stirring, which results in a uniform coating of NiFe-LDH microcrystals on the surface of SiO2 nanospheres (Figure S1B,E). During the subsequent aging process (120 °C, 12 h), well-defined LDH microspheres (~350 nm) were obtained with highly-distribute LDH nanoplatelets (lateral size: ~50 nm; thickness: ~5 nm) (Figure 1B,C and Figure S1C). The corresponding TEM images (Figure 1E,F and Figure S1F) reveal the hollow spherical structure with a LDH shell thickness of ~50 nm. The energy dispersive X-ray analysis (EDX) shows the presence of Ni and Fe (Figure S2) in the outer shell of microspheres. Moreover, the mapping analysis of TEMEDX spectrum exhibits both Ni and Fe elements are homogeneously distributed throughout the hierarchical structure of LDH shell (Figure 1G). In addition, the XPS spectrum of NiFe-LDH HMS reveals signals of Ni, Fe, C and O element with a content of 11.75%, 3.68%, 20.52% and 64.05%, respectively, approximately consistent with the results of EDX spectra (Figure S2 and Table S1). The elemental analysis confirms a Ni/Fe ratio of 3.19, in line with the nominal ratio (Ni:Fe = 3:1). Typical SEM images of NiFe-LDH HMSs with different Ni/Fe ratios (denoted as NixFe-LDH; x= 1, 2, 3, 4 and 5) are given in Figure S3 (see ESI for details). Various LDHs HMSs (CoFe-LDH, CoNi-LDH and NiAl-LDH) were successfully synthesized via this in situ growth route (Figure S4, see ESI for detailed synthesis), indicating the generality of this method.


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Figure 1. SEM and TEM images of (A, D) SiO2 nanosphere, (B, E) NiFe-LDH HMS, (C, F) the enlarged images of NiFe-LDH HMS and (G) EDS mapping results for a single NiFe-LDH HMS. The structural information on the NiFe-LDH HMS sample was further studied by X-ray diffraction (XRD) (Figure 2A). The broad peak in the XRD pattern of SiO2 indicates its amorphous phase. After the in situ growth process, the resulting SiO2@NiFe-LDH microspheres exhibit a series of (003), (012), and (110) reflection indexed to an LDH phase. The subsequent aging process leads to the disappearance of SiO2 core caused by the hydrolysis of HMT, and a further crystallization of LDHs. The XPS spectrum further verified the element of Ni, Fe, C, N and O in NiFe-LDH HMS (Figure 2B), in accordance with the EDS results. The binding energies at 856.5 eV and 874.1 eV are attributed to Ni 2p3/2 and Ni 2p1/2, respectively, indicating Ni2+ species in NiFe-LDH microsphere. The signals at 713.3 eV and 726.6 eV correspond to Fe 2p3/2 and Fe 2p1/2, implying a Fe3+ oxidation state (Figure S5). The surface properties (such as specific surface area and pore-size distribution) are important factors of OER electrocatalysts, and N2-adsorption/desorption measurements (Figure 2C) are performed over the NiFe-LDH HMS and ACS Paragon Plus Environment

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NiFe-LDH NP. The sample of NiFe-LDH HMS shows a largely enhanced specific surface area of 155.4 m2 g−1, compared with NiFe-LDH NP (22.9 m2 g−1). Moreover, a typical IV isotherm with H3-type hysteresisloops (P/P0 > 0.4) is observed for NiFe-LDH HMS, indicating the presence of mesopores. This is further verified by the isotherms: NiFe-LDH HMS consists of a mesopore distribution in the range 3– 5 nm; whilst NiFe-LDH NP shows no clear porous structure (Figure 2C, inset). It has been reported that electrocatalysts with superhydrophilicity usually show enhanced wettability and rapid electrolytic ion transport in OER.43 The contact angle tests over NiFe-LDH NP and NiFe-LDH HMS which uses deionized water as the wetting liquid was displayed in Figure 2D, from which the contact angle decreases from 22.5 (NiFe-LDH NP) to 0 (NiFe-LDH HMS), manifesting a remarkably enhanced surface wettability for the microsphere sample.

Figure 2. (A) XRD patterns of SiO2, SiO2@NiFe-LDH HMS, and NiFe-LDH HMS; (B) XPS survey spectrum of NiFe-LDH HMS; (C) N2-sorption isotherms and pore size distribution (inset) of NiFe-LDH HMS and NiFe-LDH NP; (D) contact angle tests over NiFe-LDH HMS and NiFe-LDH NP.


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The OER activity of the as-obtained HMS and the control samples (NiFe-LDH NP and 20 wt% Ir/C catalyst; see Figure S6 for detailed characterization) was evaluated in 1 M KOH. All the samples were firstly deposited uniformly onto a GCE (catalyst loading: 0.25 mg cm−2). As shown in Figure 3A, the NiFe-LDH HMS sample presents the lowest onset potential (~1.45 V vs. RHE) and the highest current density at the certain overpotential, indicating its superior intrinsic electrocatalytic activity for OER relative to the compared catalysts. A small overpotential (239 mV) was required to reach a current density (j) of 10 mA cm−2 for the as-prepared NiFe-LDH HMS, which is decreased by 41 mV and 50 mV compared with that of NiFe-LDH NP and Ir/C catalyst, respectively (Figure 3B). This is also among the best performance in comparison with previously reported electrocatalysts (Figure S7). Moreover, NiFeLDH HMS affords a high current density of 71.69 mA cm−2 at η = 300 mV, which is 3.75 and 5.36 folds than that of NiFe-LDH NP and Ir/C catalyst modified electrode, respectively. This current density (η = 300 mV) can be further increased to 90.30 mA cm−2 by using Ni foam as substrate electrode (loading amount: 0.25 mg cm−2; Figure S8). For comparison, the grinded NiFe-LDH HMS sample was obtained via a route of mechanical trituration, whose SEM image showed a cracked and irregular morphology (Figure S9). A lower current density (30.64 mA cm–2) at η=300 mV is obtained for the grinded sample compared with NiFe-LDH HMS, demonstrating the advantage of this unique hierarchical morphology. The Tafel slope (η vs. log(j)) of NiFe-LDH HMS is measured to be 53 mV dec−1 (Figure 3C), clearly lower than that of LDH NP (62 mV dec−1) and Ir/C catalyst (75 mV dec−1), further indicating a superior OER performance of NiFe-LDH HMS. EIS was carried out to study the electrode kinetics under OER condition. Moreover, the NiFe-LDH HMS modified electrode shows a much smaller Rct value (1.94 Ω cm–2) than NiFe-LDH NP (3.23 Ω cm–2) (Figure S10) in Nyquist plots, indicating an accelerated charge transfer kinetics for water electrooxidation over NiFe-LDH HMS. The EASA of NiFe-LDH HMS and NP were studied using their double-layer capacitance (Cdl), which can be used as a reasonable parameter to represent their active surface areas.22,27 It is found that NiFeLDH HMS gives 4.6 times higher Cdl than that of NiFe-LDH NP (Figure 3D), which is attributed to the highly dispersed active sites in NiFe-LDH HMS. The intrinsic activity was investigated by using apparent ACS Paragon Plus Environment

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turnover frequencies (TOFs).24,29 The NiFe-LDH HMS gives TOF values of 0.206, 0.402, 0.610, 0.820 and 1.029 s−1 at overpotential of 300 mV, 350 mV, 400 mV, 450 mV, and 500 mV (Figure 3E), respectively, which is obviously larger than those of corresponding NiFe-LDH NP (0.055, 0.130, 0.216, 0.306 and 0.390 s−1, respectively). High energy efficiency is one of the major issues for electrocatalysts, which is also a great challenge for water oxidation.47 The Faradaic efficiency was first measured by the RRDE technique (Figure S11 and S12). As a result, the faradic efficiency of NiFe-LDH HMS (97.8 %) is significantly higher than that of NiFe-LDH NP (91.7 %) at the initial potential for oxygen formation (1.450 and 1.472 V vs. RHE for NiFe-LDH HMS and NiFe-LDH NP, respectively; Figure 3E). In addition, the total Faradic efficiency normally declines as the potential is positively shifted, which implies that the existence of O2 bubble hinders the electron transfer between electrocatalyst and electrolyte.48 However, NiFe-LDH HMS shows a very slow decline of Faradaic efficiency, suggesting an easy escape of O2 bubble on the surface of NiFe-LDH HMS, probably due to the hydrophilicity of this hierarchical nanostructure.48 Moreover, by using water displacement method, the faradaic yield of O2 formation for the HMS was collected at a current density of 100 mA cm−2 (Figure S13). Remarkably, the Faradaic efficiency of LDH HMS is calculated to be 99.6% during 10000 s, indicating a satisfactory availability of electric energy. To demonstrate the effect of NiFe-LDH crystallinity on OER performances, the NiFeLDH NP with a similar low-crystallinity to NiFe-LDH HMS was synthesized as a control sample (see ESI for detailed characterizations including SEM and XRD: Figure S14). The results show that the current density at j= 300 mV for NiFe-LDH HMS is 17.3 times higher compared with the low-crystallinity NiFeLDH NP (Figure S15), indicating the neglectable impact of crystallinity on the electrocatalytic property. In conclusion, both a high surface area (155.4 m2 g−1) and suitable mesopore (3–5 nm) of the hierarchical NiFe-LDH microspheres, which largely facilitate the charge/mass transportation, play a key role in OER activity. Moreover, the superhydrophilicity of NiFe-LDH HMS also gives contribution by improving the interface contact between catalyst surface and electrolyte.48 It is reported that metal ratio could be the dominant factor affecting electrocatalytic performance of NiFe (oxy) hydroxide.49-51 To give a clear insight into this effect, the influence of Ni/Fe molar ratio of ACS Paragon Plus Environment

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NiFe-LDH HMS on its OER performances was further studied. Figure S16 shows representative CVs of NixFe-LDH HMSs (x= 1, 2, 3, 4 and 5). All these samples exhibit typical characteristics: a redox couple at 1.42 V vs. RHE is attributed to the valence transformation between Ni(OH)2 and NiOOH (Ni(OH)2 + OH− ↔ NiOOH + H2O + e−); the other oxidation current at potentials larger than 1.45 V is due to the OER (4OH− → 2H2O + O2 + 4e−). With the increase of Fe content (lower than 25%), the Ni(OH)2/NiOOH redox peaks shift positively; however, as the Fe content is larger than 25%, this redox couple become less pronounced. Meanwhile, the OER activity is dependent on Fe content significantly: the potential decreases with the rise of Fe content (below 25%), but increases with a further enhancement of Fe content. Moreover, the lowest onset-potential (220 mV) for Ni3Fe-LDH HMS indicates the most favorable catalytic performance toward OER with a Ni:Fe ratio of 3:1, in good accordance with other reports.52,53


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Figure 3. (A) LSV curves; (B) current densities at η= 300 mV and overpotential at j= 10 mA cm–2; (C) Tafel slopes of NiFe-LDH HMS, Grinded NiFe-LDH HMS, NiFe-LDH NP and Ir/C catalyst; (D) Cdl, (E) TOFs and (F) Faradaic efficiency of NiFe-LDH HMS and NiFe-LDH NP. The water oxidation activity of CoFe-LDH, CoNi-LDH, NiAl-LDH HMSs and NPs was also measured by LSV curves to highlight the influence of LDH type. The CoFe-LDH, CoNi-LDH and NiAl-LDH NPs show moderate OER performance, in agreement with previous reports.27,29,37 In contrast, the hierarchical LDHs mirospheres display siginifantly improved performance toward OER compared with their NP counterparts (Figure 4A). For instance, CoFe-, CoNi- and NiAl-LDH HMSs show an overpotential of 271, 329 and 377 mV at j = 10 mA cm−2, which is 41, 30 and 58 mV less than the corresponding NP sample, respectively (Figure 4B). The current density of CoFe-, CoNi- and NiAl-LDH HMS (at η = 300 mV) is 2.79, 1.87, and 4.75 times larger than that of NP counterparts (Figure 4C). The TOFs value of CoFe-, CoNi- and NiAl-LDH HMSs is 0.076, 0.016 and 0.011 s−1 at η = 300 mV respectively (Figure 4D), larger than the correspoinding NP samples (0.027, 0.009 and 0.003 −1, respectively). The results above confirm the OER activity follows the sequence order of CoFe-LDH HMS>CoNi-LDH HMS>NiAl-LDH HMS, demonstrating the superior behavior of Fe-containing LDH material. Moreover, the intrinsic OER performance of LDHs can be promoted by constructing hierarchical microsphere architecture.


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Figure 4. (A) LSV curves, (B) overpotential at j =10 mA cm−2, (C) current densities at η =300 mV and (D) TOF values at η =300 mV for CoFe-LDH, CoNi-LDH, NiAl-LDH HMSs and NPs. A durability test over NiFe-LDH HMS was conducted by a chronoamperometric measurement at η = 300 mV (Figure 5A). The density of oxidation current is rather stable and even increases by 4.5% after 40000 s of testing, indicating the inherent stability of the NiFe-LDH HMS in the tested condition. However, the current densities of the NiFe-LDH NP and Ir/C decreases from 19.3 to 15.7 mA cm−2 and from 17.4 to 13.4 mA cm−2 within 20000 s, respectively. The LSV curve of NiFe-LDH HMS after 40000 s usage is overlapped with that of the fresh sample (Figure 5A, inset), confirming its strong durability. As shown in Figure 5B, the electrode maintains similar activity with the initial value during the successive biasing at anodic current density of j = 10, 20, 30, 50 and then 100 mA cm−2 each for 2000 s, revealing a relatively stable potentiometric response for NiFe-LDH HMS. Moreover, the nanostructure of the used NiFe-LDH HMS was examined (Figure 5B, inset), from which the hierarchical structure is maintained after the OER test. A prototype electrolyzer cell is fabricated to test the superiority of NiFe-LDH HMS in practical water splitting (Figure 5C), where NiFe-LDH HMS modified Ni foam serves as anode, Pt wire as cathode and powered with a 1.5 V AA battery. From the aforementioned water electrolysis cell, the ACS Paragon Plus Environment

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bubbles are clearly observed when the electrolysis test occurs (Figure 5D). Furthermore, a continuous bubble escapes from both anode and cathode within 2 h test, indicating the favourable activity and durability of the NiFe-LDH HMS electrode in water-splitting.

Figure 5. (A) Chronoamperometric response of NiFe-LDH HMS and Ir/C electrocatalyst at η = 300 mV in 1 M KOH solution (inset: LSV curves before and after 40000 s OER test); (B) chronopotentiometry response of NiFe-LDH HMS at j =10, 20, 30, 50 and then 100 mA cm−2 (inset: SEM images of the fresh and used NiFe-LDH HMS); (C) water-splitting system powered with a battery and (D) the corresponding enlarged image.

CONCLUSIONS In conclusion, NiFe-LDH HMS was prepared by in situ synthesis of NiFe-LDH nanoplatelets on the SiO2 spheres followed by subsequent dissolution of SiO2 core. The as-obtained NiFe-LDH HMS gives a very high surface area, mesoporous structure and excellent hydrophilicity, resulting in an efficient accessibility of active sites toward water oxidation and an easy release of oxygen. Consequently, NiFeACS Paragon Plus Environment

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LDH HMS exhibits superior electrochemical features in OER with a low overpotential (239 mV at 10 mA cm–2), large anodic current density (71.69 mA cm–2 at η= 300 mV) and excellent stability, which is comparable to the best performed LDH-based OER catalysts as well as commercial Ir/C catalyst. It is expected that the hierarchical LDH microspheres synthesized in this work is promising in the electrochemical energy storage and conversion systems.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: XXX. The synthesis processes of control samples; the details for the water contact angle test; figures including SEM, EDS, XPS, Raman spectrum for NiFe-LDH based materials; comparison results with other reported OER catalysts. Corresponding Author E-mail: [email protected]; [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (NSFC), the 973 Program (Grant No. 2014CB932102) and the Fundamental Research Funds for the Central Universities (buctrc201506; YS 1406). M. Wei particularly appreciates the financial aid from the China National Funds for Distinguished Young Scientists of the NSFC. ACS Paragon Plus Environment

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Hierarchical NiFe Layered Double Hydroxide Hollow Microspheres

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