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Au Promoted Nickel-Iron Layered Double Hydroxide Nanoarrays: A Modular Catalyst Enabling High-Performance Oxygen Evolution Wenxin Zhu, Lizhi Liu, Zhihao Yue, Wentao Zhang, Xiaoyue Yue, Jing Wang, Shaoxuan Yu, Li Wang, and Jianlong Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 23 May 2017 Downloaded from http://pubs.acs.org on May 25, 2017

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ACS Applied Materials & Interfaces

Au Promoted Nickel-Iron Layered Double Hydroxide Nanoarrays: A Modular Catalyst Enabling High-Performance Oxygen Evolution

Wenxin Zhu, Lizhi Liu, Zhihao Yue, Wentao Zhang, Xiaoyue Yue, Jing Wang, Shaoxuan Yu, Li Wang, Jianlong Wang* College of Food Science and Engineering, Northwest A&F University, Yangling 712100, Shaanxi, China *Corresponding author. *E-mail: [email protected] (Jianlong Wang)

Abstract:

Oxygen evolution reaction (OER) plays a key role in various energy conversion

and storage technologies, such as water electrolysis, regenerative fuel cells, and rechargeable metal-air batteries. However, the slow kinetics of OER limit the performance and commercialization of such devices. Herein, we report on NiFe LDH@Au hybrid nanoarrays on Ni foam for much enhanced OER. By hybridization of electronegative Au and NiFe LDH with intrinsic remarkable OER catalytic activity, this modular electrode could drive an overall ultrahigh-performance and robust OER in base with the demand of overpotentials of only 221, 235 and 270 mV to afford 50, 100 and 500 mA cm-2, respectively. Also, it exhibits superior catalytic activity and durability toward OER in 30 wt.% KOH.

Keywords: oxygen evolution, NiFe LDH, Au, nanoarray, modular catalyst

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INTRODUCTION Continuous consumption of fossil fuels and ever-increasing environmental problems have triggered considerable research concern on searching for renewable clean alternatives.1,2 Among them, coupling renewable energy devices such as wind energy and solar energy with water electrolysis to produce pure hydrogen that has been recognized as the most ideal energy carrier, has been one of the most promising ways to cater for our need for future fuel applications.3 In this system, oxygen evolution reaction (OER), another vital component in water splitting cells with sluggish kinetics of proceeding through multistep proton-coupled electron transfer, is indeed the rate-limiting step and thus the bottleneck of commercial viability of industrial hydrogen production.4 Till now, rutile type oxides like RuO2 and IrO2 have been always considered as the benchmark OER catalysts both in acid and base owing to their lowest OER overpotentials and thereby high catalytic activity. However, the poor chemical stability in base, prohibitive cost and limited supply of Ru and Ir much hinder their practical application in industrial water electrolysis.5 Thus, considerable effort has been devoted recently in developing high-efficiency and durable OER catalysts composed of earth-abundant elements, like transition-metal (oxy)hydroxides,6-10 chalcogenides,11-15 and phosphides.16-20 Among aforementioned (oxy)hydroxides, transition-metal containing layered double hydroxides (LDHs), composed of positively-charged brucite-like host layers and charge-balancing interlayered anions,21 have been widely used in OER catalysis. The metal cations (M2+/3+) and incorporated anions among layers bring LDHs with unique redox characteristics and large interlayer spaces, respectively,22 offering them superior OER


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catalytic performances to other transition-metal (oxy)hydroxides. Of these LDHs, nickel-iron LDHs (NiFe LDH) with the highest catalytic oxidation activity has been intensively investigated as OER catalysts.23-27 However, the inherent relative-low conductivity of NiFe LDH undoubtedly hinders the further enhancement of their OER catalytic performances,28 while their native high activity also limits the margin of improvement of OER. In view of this, several strategies have been well proposed: 1) hybriding NiFe LDH with conductive materials like carbon dots,29 carbon nanotubes,28 and graphene;30 2) Inserting another metal like Co and Mn into its structure;31,32 3) Direct growth or deposition of nanoarrayed NiFe LDH on commercial current collectors;23,24,26,27 4) Direct topotactic conversion of NiFe LDH into corresponding chalcogenides,33-35 phosphides,36 nitrides,37-39 and NiFe-based alloys.40 All these schemes could much improve the OER activity of NiFe LDH itself. In this Communication, we report on another novel strategy of integrating electronegative noble metals with arrayed LDHs and describe here our recent effort toward this direction in constructing NiFe LDH@Au hybrid nanoarrays on Ni foam (NiFe LDH@Au/Ni foam) through the combination of facile hydrothermal deposition and subsequent chemical deposition methods (Scheme 1). Delicate chemical valence change, improved material conductivity, enlarged active surface area and roughness, as well as this unique multi-stage topology make NiFe LDH@Au/Ni foam to be an ultrahigh-performance and robust OER catalyst with the demand of overpotentials of only 221, 235, and 270 mV to drive 50, 100 and 500 mA cm-2 in alkaline medium, respectively. Notably, this electrode also exhibits superior catalytic performance toward OER in 30 wt.% KOH.

Scheme 1



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EXPERIMENTAL METHODS Preparation of NiFe LDH/Ni foam: Nickel foam was soaked in diluted HCl solution (about 3.0 M) for 5 min to remove the surface NiO layer, and then washed with deionized water, absolute ethanol and water, successively. To prepare NiFe LDH/Ni foam, Ni(NO3)2•6H2O (0.5 mmol), Fe(NO3)3•9H2O (0.5 mmol) and CO(NH2)2 (5 mmol) were dissolved in 36 mL distilled water and stirred for at least 10 min to form a clear solution. A piece of Ni foam (2 cm × 3 cm, 0.5 mm in thickness) was inserted into a 50 mL Teflon-lined stainless steel autoclave with above solution, sealed and maintained at 120 °C for 12 h. The sample was then withdrawn from the solution, thoroughly rinsed with distilled water and dried at 80 °C for 3 h in a vacuum oven. Note that, because the Ni foam might have dissolved during hydrothermal treatment due to the decomposition of urea in aqueous environment, accurate measurement of the catalyst loading on Ni foam was challenging. Preparation of NiFe LDH@Au/Ni foam: In a typical synthesis, HAuCl4 solution (0.01 M) and sodium citrate solution (0.01 M) were added into a beaker. The NiFe LDH/Ni foam was then immersed in this solution, and fresh NaBH4 (0.01 M) was subsequently added. After reaction for 60 min, the samples were taken out, washed with deionized water to remove residual ions and then dried at 80 oC for 3 h. NiFe LDH@Au/Ni foam with different Au loading were then fabricated. For each sample, For each sample, the added amount of HAuCl4 solution was 100, 500 and 1000 µL, respectively, and the volume ratio of sodium citrate solution, NaBH4 solution and HAuCl4 solution were kept to 1:1:2. Electrochemical characterization: Electrochemical measurements were performed with a CHI 660E electrochemical potentiostat (CH Instruments, Inc., Shanghai) in a standard three-electrode system in an aqueous KOH electrolyte (1.0 M) using NiFe LDH@Au/Ni foam,


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NiFe LDH/Ni foam and bare Ni foam as the working electrodes (the surface area of each working electrode is 0.25 cm2), a graphite plate as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. In all measurements, SCE was calibrated against to the reversible hydrogen electrode (RHE). In 1.0 M KOH, E (RHE) = E (SCE) + 1.068 V. Polarization curves were conducted in base with a scan rate of 2 or 5 mV s-1 and then corrected by the iR loss according to the following equation: Ecorr = Emea – iR. Physical characterization. SEM measurements were performed on a Hitachi S-4800 field emission scanning electron microscope at an accelerating voltage of 20 kV. XRD measurements were performed using a Bruker D8 advanced diffractometer with Cu Kα radiation (40 kV, 40 mA). TEM measurements were made on a HITACHI H-8100 electron microscopy (Hitachi, Tokyo, Japan) with an accelerating voltage of 200 kV. XPS measurements were performed on an ESCALABMK II X-ray photoelectron spectrometer using Mg as the exciting source.

RESULTS AND DISCUSSION

NiFe LDH@Au/Ni foam was prepared via the following three steps (detailed preparation process is described in Supplementary Information): 1) Ni foam with zig-zag skeleton and highly porous architecture was chosen here as the current collector (left in Figure S1, silver), which could increase the catalytic active surface area of loaded catalysts, promote electrolyte penetration and oxygen escape;41 2) NiFe LDH nanoarray was directly grown on Ni foam by a hydrothermal deposition process (middle in Figure S1, yellow); 3) Monodispersed Au nanoparticles were uniformly anchored on NiFe LDH nanosheets by a simple chemical



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deposition treatment (right in Figure S1, brown). Crystal phases of the NiFe LDH@Au/Ni foam were first investigated by X-ray powder diffraction (XRD) technique. Figure 1a shows the XRD pattern of NiFe LDH@Au/Ni foam with well-defined diffraction peaks. Of them, a series of Bragg reflections like (003), (006), (009) and (110) match well with the typical profile of LDH materials.42 Diffraction peaks at the Bragg angles of 38.1o, 44.4o, 64.5o, and 77.5o correspond well to the (111), (200), (220), and (311) facets of face-centered-cubic crystallite Au phase (JCPDS No. 04-0784),43 respectively. Three strong peaks at 44.5o, 51.8o, and 76.4o (marked with "#") are indexed to the (111), (200), and (220) planes of Ni originated from Ni foam substrate (JCPDS No. 87-0712). Figure S2 shows the typical scanning electron microscopy (SEM) images for open-cell Ni foam, suggesting its 3D macroporous skeleton morphology with a smooth surface. The low- and high-magnification SEM images of NiFe LDH/Ni foam (Figure 1b) show the complete coverage of Ni foam with vertical NiFe LDH nanosheets and the thickness of each nanosheet is approximately 25 nm (inset in Figure 1b). After chemical deposition with different Au content (0.1 mM, 0.5 mM, and 1.0 mM), Au nanoparticles were highly dispersed on the surfaces of NiFe LDH nanosheets in form of small grains (Figure 1c, Figure S3a, and Figure S3b). With the increase of Au content from 0.1–1.0 mM, the average size of Au particles varies from 9.1–31 and 31–42.7 nm while NiFe LDH nanosheets become thinner (change from 25–8.0 and 8.0–4.6 nm). Notably, when the Au content is less than 0.5 mM, both the nanoarray morphology and integrated nature could be well preserved (Figure S3a and Figure 1c). But when Au content is up to 1.0 mM, this nanoarray architecture would be etched by acid and collapse to some extent (Figure S3b). The energy-dispersive X-ray (EDX)


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spectrum of NiFe LDH@Au/Ni foam (Figure S4) confirms the co-presence of Ni, Fe, O and Au elements and corresponding elemental mapping images (Figure 1d) demonstrate the uniform distribution of these elements on the foam skeleton. Figure 1e shows the transmission electron microscopy (TEM) image for NiFe LDH@Au, further proving the uniform coverage of Au nanoparticles on NiFe LDH nanosheets. A closer view of such a hybrid structure (inset in Figure 1e) reveals the size of anchored Au particles (Au content is 0.5 mM) is about 10–30 nm, which coincides with the SEM result. The high-resolution TEM (HRTEM) image of the NiFe LDH/Au interface (Figure 1f) amplified from the chosen area shows that Au nanoparticles has clear lattice fringes with an interplanar spacing of 2.04 Å, which corresponds to the (200) plane of Au.44 In another side of the boundary, evident lattice fringes with uniaxial orientation could be assigned to NiFe LDH. Above all these results could well support the successful preparation of NiFe LDH@Au hybrid nanoarrays on Ni foam.

Figure 1

The different surface valence states of each element in NiFe LDH@Au/Ni foam were investigated by X-ray photoelectron spectroscopy (XPS) to characterize the binding energies (BEs) of Ni, Fe and Au. The survey spectrum of NiFe LDH@Au/Ni foam (Au content is 0.5 mM) (Figure 2a) confirms the existence of Ni, Fe, O and Au elements, which is consistent with the EDX result. Figure 2b-2d present the high-resolution XPS spectra in Ni 2p, Fe 2p and Au 4f regions, respectively. In the Ni 2p region, BEs of Ni 2p1/2 and Ni 2p3/2 located at 856.3 and 874.1 eV could be assigned to Ni2+ in NiFe LDH, accompanied by two prominent



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shakeup satellite peaks (862.1 and 879.7 eV).45 The Fe 2p region shows two peaks at 713.0 and 726.2 eV, which are usually the typical characteristics of Fe3+ species.23,36 The XPS spectrum of Au 4f exhibits two strong peaks at 83.4 and 87.2 eV, which could be attributed to the Au 4f5/2 and Au 4f7/2 for typical metallic Au0 species.43,46 Moreover, as mentioned previously, Au, as a highly electronegative metal, could work as an electron adsorbate to generate and stabilize nickel cation to higher oxidation states and thereby improve the overall OER efficiency.40,41 Compared with those of our synthesized NiFe LDH/Ni foam (Figure S5) and other reported NiFe LDH-based catalytic materials,21,44 the binding energy values of Ni 2p in NiFe LDH@Au/Ni foam have a positive shift of about 0.4 eV, which indicates that the incorporated Au could promote the formation of Ni3+ cation to some extent and these formed NiOOH may act as active centre for OER.41

Figure 2

To verify the enhancement of electrocatalytic performance of NiFe LDH@Au/Ni foam compared with that for NiFe LDH/Ni foam, OER test was performed in 1.0 M KOH in a typical three-electrode setup (as shown in Figure S6). Ohmic potential drop losses from the solution resistance were applied to all initial data,47 and calculated current densities were based on the projected geometric area of an electrode. Figure 3a and 3b show linear sweep voltammetry curves of NiFe LDH@Au/Ni foam, NiFe LDH/Ni foam and bare Ni foam on the reversible hydrogen electrode (RHE) scale. Obviously, bare Ni foam has very inferior catalytic activity toward OER, while NiFe LDH/Ni foam exhibits very high current response with the need of overpotentials of 243 and 310 mV to deliver 100 and 500 mA cm-2,


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respectively. In sharp contrast, NiFe LDH@Au/Ni foam (Au content is 0.5 mM) is much superior in catalytic activity to NiFe LDH/Ni foam and capable of affording 50, 100, and 500 mA cm-2 at overpotentials of just 221, 235, and 270 mV, respectively. It should be noted that, from the aforementioned comparison of XPS data of NiFe LDH@Au/Ni foam and NiFe LDH/Ni foam, the incorporated Au species indeed induces the peak shift of Ni 2p and promotes the generation of Ni3+ species. But both the small positive shift value of Ni 2p of about 0.4 eV (From 855.89 to 856.31 eV and from 873.65 to 874.1 eV) and the pre-oxidative peak of Ni(OH)2-NiOOH redox couple in NiFe LDH@Au/Ni foam still at about 1.43 V indicate that compared with the inhibition by the highly-charged Fe(III) ions occupying the surrounding positions, the promotion of Ni2+ to Ni3+ species by Au is still relatively limited. Moreover, when the scan rate of LSV curve of NiFe LDH@Au/Ni foam was set to be 5 mV s-1, the comparison of small current density of 50 mA cm-2 between these two materials cannot be well performed because the interference of the large oxidative peak of Ni in NiFe LDH@Au/Ni foam. Here, to decrease the current density of pre-oxidation peak of NiFe LDH@Au/Ni foam and compare more clearly of 50 mA cm-2, the scan rate of LSV curve of NiFe LDH/Ni foam was set to be 5 mV s-1 while that of NiFe LDH@Au/Ni foam was adjusted to be 2 mV s-1 in the OER test in 1.0 KOH, which could explain the phenomenon that the pre-oxidation peak obtained with NiFe LDH@Au/Ni foam is even smaller than that for NiFe LDH/Ni foam on account of the difference between scan rates of these two materials (Figure 3a and 3b). Besides, the OER performances of NiFe LDH loaded on glassy carbon electrode (NiFe LDH/GCE) and Au deposited on Ni foam (Au/Ni foam) were also measured here (Figure S7). Apparently, compared with NiFe LDH@Au/Ni foam and NiFe LDH/Ni



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foam, both Au/Ni foam and NiFe LDH/GCE have inferior catalytic activities toward OER (a slight higher than that of bare Ni foam), demonstrating the superiority of this binder-free system and the relatively-poor OER catalytic activity of Au itself. Meanwhile, with the increase of Au content from 0.1–1.0 mM, the catalytic performance of NiFe LDH@Au/Ni foam goes up first and then down (Figure S8). That could be due to that too much Au particles coating on NiFe LDH layer will block the contact of active phases with electrolyte. Also note that in Figure S8, when the dosage of HAuCl4 is up to 1.0 mM, the nanoarray architechture of NiFe LDH would be etched by acid and collapse to some extent (as shown in Figure S3b), which might lead to a non-persistent OER catalysis when the current density is above some value and thus a more noisy LSV curve of NiFe LDH@Au/Ni foam (1.0 mM) than those for NiFe LDH@Au/Ni foam (0.5 mM and 0.1 mM). And the curve of NiFe LDH@Au/Ni foam with 0.1 mM HAuCl4 is also a slight more noisy compared with the one with 0.5 mM HAuCl4, which might be due to that HAuCl4 with relatively low concentration (0.1 mM) could not well etch the NiFe LDH nanosheets and form a stable NiFe LDH/Au interface for OER catalysis. A more clear comparison of catalytic performance for different electrodes in the form of histogram could be seen in Figure 3c, showing that NiFe LDH@Au/Ni foam has a more rapid rising OER current density at high potential in contrast to NiFe LDH/Ni foam. This phenomenon could also be reflected in Tafel plots, as shown in Figure 3d. Tafel slope for NiFe LDH@Au/Ni foam (48.4 mV dec-1) is much lower than that for NiFe LDH/Ni foam (71.1 mV dec-1), implying NiFe LDH@Au/Ni foam has a more rapid OER kinetics. It is also worth mentioning that, the catalytic performance (100 mA cm-2@235 mV) of this electrode


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compares favorably to those for Fe-Ni3S2/Ni foam (253 mV),49 Fe-NiSe/FeNi foam (264 mV),11 Fe-Ni2P/Ni foam (235 mV),36 NiFe/Ni foam (about 360 mV),26 NiFe LDH@FeOOH/Ni foam (about 250 mV),48 and NiCoFe LDH/Ni foam (about 270 mV),31 and much surpasses those of all recently-reported high-performance OER catalysts listed in Table S1.

Figure 3

The superior OER catalytic performance of NiFe LDH@Au/Ni foam could be rationally explained as follows. 1) The unique multi-stage topology comprising 3D macroporous Ni skeleton covered with high-density upright hybrid nanoarrays will largely expose the number of active sites and facilitate the diffusion of electrolyte and the generated gas bubbles.41,43 2) As a highly electronegative metal, Au could work as an electron adsorbate to generate and stabilize nickel cation to higher oxidation states (Ni3+) and thereby improve the overall OER efficiency.44 3) The strong electrophilic Ni3+ cation will accelerate to form the hydroperoxy species (OOH) that are key intermediates in the evolution of O2 via nucleophilic reaction with O.48 4) The Electrochemical impedance spectroscopy (EIS) measurements (Figure S9) show that the NiFe LDH@Au/Ni foam has a smaller polarization resistance in contrast to that for NiFe LDH/Ni foam, demonstrating a faster electron transfer rate and enhanced OER catalytic performance of this modular electrode. 5) The capacitances of the double layers at solid/liquid interfaces of NiFe LDH@Au/Ni foam and NiFe LDH/Ni foam were measured to estimate their electrochemically active surface areas. Figure S10a and S10b show the typical cyclic voltammograms of NiFe LDH@Au/Ni foam and NiFe LDH/Ni foam with various scan rates



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(5–100 mV s−1) in the region of 0.1–0.2 V (vs. SCE), respectively, where the current responses should be only due to the charging of double layers. Figure S10c shows the fitting curves of current density differences at 0.15 V (vs. SCE) against the scan rate. The capacitances for NiFe LDH@Au/Ni foam and NiFe LDH/Ni foam are calculated to be 5.05 and 2.88 mF cm-2, respectively, indicating a higher surface area and thus more active sites of NiFe LDH@Au/Ni foam. 6) The increased surface roughness caused by acidic etching during preparation could further enlarge the active surface area. Besides the catalytic activity, durability is another key factor to evaluate the performance of electrocatalysts. Figure 3e shows a multistep chronopotentiometric curve for NiFe LDH@Au/Ni foam with the anodic current density increasing from 10–110 mA cm-2 (10 mA cm-2 per 500 s). The potential immediately levels off at 1.43 V (vs. RHE) at the initial current value and remains constant for over 500 s, and subsequent other steps also show similar results, proving the superior mechanical robustness, mass transportation and conductivity of NiFe LDH@Au/Ni foam.26 Also note that in the inset of Figure 3e, when current density changes from one step to another, the response time is less than 3 s, which also demonstrates the rapid mass and electron transfer. Long-term durability of this electrode was performed by continuous electrolysis at a static current density (Figure 3f). A constant potential of about 1.5 V could be maintained for over 22 h without degradation. After this prolonged electrolysis, we tested again the OER performance of this electrode in 1.0 M KOH and found that the polarization curve of it only exhibits a negligible positive shift of about 10 mV (inset in Figure 3f), demonstrating excellent stability of this electrode under alkaline conditions.

Figure 4


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To further examine the viability of NiFe LDH@Au/Ni foam in practical water electrolysis, we also tested its OER activity in 30 wt.% KOH. Figure 4a shows the polarization curves of different electrodes. Of them, NiFe LDH@Au/Ni foam displays the highest activity toward OER with a rapidly rising OER response with the increase of overpotentials and a very small overpotential of 226 mV to drive 500 mA cm-2. Tafel curves for these electrodes (Figure 4b) also shows that NiFe LDH@Au/Ni foam has the lowest Tafel slope of 60.8 mV dec-1 and thus the most rapid OER kinetics. Moreover, after continuous 5000 CV cycles, this electrode exhibits negligible loss in current density and a small positive shift of 15 mV in overpotentials (Figure 4c). Notably, this NiFe LDH@Au/Ni foam electrode exhibits a much improvement of OER performance at a higher temperature of 60 °C and only requires a much smaller overpotential of 162 mV to gain 500 mA cm-2 (Figure 4d).

CONCLUSIONS In summary, Au embedded NiFe LDH hybrid nanoarrays are successfully developed on Ni foam via the combination of facile hydrothermal deposition and subsequent chemical deposition methods. As an ultrahigh-performance catalyst for OER, this electrode requires ultra-low overpotentials of 221, 235 and 270 mV for delivering 50, 100 and 500 mA cm-2 with a very small Tafel slope of 48.4 mV dec-1. Besides, this electrode also works well in 30 wt.% KOH with requiring an overpotential of only 226 mV to deliver a high current density of 500 mA cm-2. The ultrahigh OER catalytic performance and durability, accompanied with this facile and controllable preparation process, promising NiFe LDH@Au/Ni foam as a low-cost, ultrahigh-performance and robust OER catalyst in industrial water-electrolysis devices for the mass production of hydrogen.



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ASSOCIATED CONTENT Supporting Information Optical photographs of bare Ni foam, NiFe LDH/Ni foam, and NiFe LDH@Au/Ni foam. SEM images of bare Ni foam, NiFe LDH@Au/Ni foam with 0.1 mM Au content and 1.0 mM Au content. EDX spectrum of NiFe LDH@Au/Ni foam. XPS spectra of NiFe LDH/Ni foam. Digital photo of NiFe LDH@Au/Ni foam in long-term OER catalysis. OER performances of Au/Ni foam, NiFe LDH/Ni foam, NiFe LDH/GCE, and NiFe LDH@Au/Ni foam with different Au content (0.1 mM, 0.5 mM, and 1.0 mM HAuCl4). Nyquist plots of NiFe LDH@Au/Ni foam and NiFe LDH/Ni foam in 1.0 M KOH. Typical cyclic voltammograms and correlation curves of capacitive current density as a function of scan rate for NiFe LDH@Au/Ni foam and NiFe LDH/Ni foam in 1.0 M KOH.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by Grants from National Natural Science Foundation of China (Nos. 21675127 and 31371813) and Fundamental Research Funds for the Northwest A&F University of China (2014YB093, 2452015257).

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Mater. Chem. A 2015, 3, 23207–23212.



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List of Figure Captions Scheme 1. A schematic diagram to illustrate the fabrication process of NiFe LDH@Au/Ni foam via the combination of facile hydrothermal deposition and subsequent chemical deposition methods. Figure 1. (a) XRD pattern for NiFe LDH@Au/Ni foam. SEM images for (b) NiFe LDH/Ni foam and (c) NiFe LDH@Au/Ni foam. (d) SEM image and corresponding EDX elemental mapping analysis for Ni, Fe and Au for NiFe LDH@Au/Ni foam. (e) TEM analysis for NiFe LDH@Au/Ni foam. (f) HRTEM image of the NiFe LDH/Au boundary amplified from the chosen area. Figure 2. (a) XPS survey spectrum of NiFe LDH@Au/Ni foam. XPS spectra of the (b) Ni 2p, (c) Fe 2p, and (d) Au 4f regions. Figure 3. (a) Polarization curves of NiFe LDH@Au/Ni foam (at a scan rate of 2 mV s-1), NiFe LDH/Ni foam and bare Ni foam (at scan rates of 5 mV s-1). (b) Enlarged polarization curves derived from (a). (c) The bar chart of comparison of overpotentials for different electrodes. (d) Corresponding Tafel plots. (e) Multi-current process of NiFe LDH@Au/Ni foam. The current density started at 10 mA cm-2 and ended at 110 mA cm-2, with an increment of 10 mA cm-2 per 500 s without iR correction (Inset: The response time of current density changes from one step to another). (f) Chronopotentiometric curve of this electrode under a constant current density of 60 mA cm-2 without iR correction (Inset: Polarization curves for NiFe LDH@Au/Ni foam before and after over 22 h electrolysis). All experiments were performed in 1.0 M KOH. Figure 4. (a) Polarization curves of NiFe LDH@Au/Ni foam, NiFe LDH/Ni foam and bare


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Ni foam at a scan rate of 5 mV s-1. (b) Corresponding Tafel plots. (c) Polarization curves recorded for NiFe LDH@Au/Ni foam before and after 5000 CV cycles. (d) Polarization curves of NiFe LDH@Au/Ni foam at different temperatures. All experiments were carried out in 30 wt.% KOH.



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Scheme 1


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Figure 1



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Figure 2


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Figure 3



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Figure 4


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