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NiFe2O4 Nanoparticles/NiFe Layered Double Hydroxide Nanosheets Heterostructure Array for Efficient Overall Water Splitting at Large Current Densities Zhengcui Wu, Zexian Zou, Jiansong Huang, and Feng Gao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07835 • Publication Date (Web): 16 Jul 2018 Downloaded from http://pubs.acs.org on July 16, 2018

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ACS Applied Materials & Interfaces

NiFe2O4 Nanoparticles/NiFe Layered Double Hydroxide Nanosheets Heterostructure Array for Efficient Overall Water Splitting at Large Current Densities Zhengcui Wu,* Zexian Zou, Jiansong Huang, and Feng Gao*

Anhui Laboratory of Molecule-Based Materials (State Key Laboratory Cultivation Base), The Key Laboratory of Functional Molecular Solids, Ministry of Education, College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241002, P. R. China.

ABSTRACT: Constructing catalysts with new and optimizational chemical components and structures, which can operate well for both anodic oxygen evolution reaction (OER) and cathodic hydrogen evolution reaction (HER) at large current densities, is of primary importance in practical water splitting technology. Herein, the NiFe2O4 nanoparticles/NiFe LDH nanosheets heterostructure array on Ni foam was prepared via a simple one-step solvothermal approach. The as-prepared heterostructure array displays high catalytic activity toward OER with a small overpotential of 213 mV at 100 mA cm−2, and can afford a current density of 500 mA cm−2 at an overpotential of 242 mV, and 1000 mA cm−2 at 265 mV. Moreover, it also presents outstanding HER activity, only needing a small overpotential of 101 mV at 10 mA cm−2, and can drive large current densities of 500 and 750 mA cm−2 at individual overpotentials of 297 and 314 mV. A two-electrode electrolyzer using NiFe2O4 nanoparticles/NiFe LDH nanosheets as both anode and cathode implements active overall water splitting, demanding a low voltage of 1.535 V to drive 10 mA cm−2, and can deliver 500 mA cm−2 at 1.932 V. The NiFe2O4 nanoparticles/NiFe LDH nanosheets array electrodes also 1

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show excellent stability against OER, HER and overall water splitting at large current densities. Significantly, the overall water splitting with NiFe2O4 nanoparticles/NiFe LDH nanosheets as both anode and cathode can be continuously driven by a battery of only 1.5 V. The intrinsic advantages and strong coupling effects of NiFe2O4 nanoparticles and NiFe LDH nanosheets make NiFe2O4 nanoparticles/NiFe LDH nanosheets heterostructure array abundant catalytically active sites, high electronic conductivity and high catalytic reactivity, which remarkably contributed to the catalytic activities for OER, HER and overall water splitting. Our work can inspire the optimal design of NiFe bimetallic heterostructure electrocatalyst for application in practical water electrolysis. KEYWORDS: NiFe LDH; NiFe2O4; heterostructure; electrocatalysis; water splitting

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INTRODUCTION Advancing active, stable and earth-abundant electrocatalysts that can work well in the same electrolyte for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), is greatly important for practical water splitting and many renewable energy conversion processes.1 Currently, the benchmark electrocatalysts for splitting water are IrO2 or RuO2 for OER2-4 and Pt-based materials for HER5-7 due to their low overpotentials and fast kinetics. But, the scarceness and high price of such noble-metal-based electrocatalysts restrict their extensive application. A large number of efforts and progresses have been made towards effective OER and HER electrocatalysts with earth-abundant resources and low cost materials. Among various prospective electrocatalysts obtainable for water splitting, the first-row transition metal compounds have been considered to be the fascinating substitutions since the electrocatalytic activity can be obtained by exact control of composition and structure.8-11 Recently, the first-row transition metal layered double hydroxides (LDH) have arisen as a kind of prospective OER and overall water splitting electrocatalysts in alkaline media. For LDHs, the metal cations in the brucite-like layers were flexibly tunable, the intercalated anions were easily exchangeable and the morphologies were controllable, which are favorable for electrochemical performance.12-15 However, their relatively inferior conductivity is often disadvantageous for electron transportation through the catalytic phase and full utilization of active sites, which eventually influences the electrocatalytic performance.16 Of these LDHs, NiFe LDH possessed the highest OER catalytic performance has been deeply researched.17-20 However, the intrinsic relatively low conductivity certainly impedes the full play of its OER catalytic performance. Researchers are motivated to couple it with other nanocomponents for constructing interface engineered hybrid nanomaterials or ternary LDHs to enhance the electrocatalytic activities 3



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of OER and overall water splitting, such as carbon quantum dot/NiFe LDH,21 NiFe LDH/carbon nanotubes,22 NiFe LDH/graphene,23 NiFe LDH/Au nanoparticles,24 NiFe LDH/Ni2S3 nanosheets,25 and

ternary

NiFeMn

LDH26

for

OER;

NiCo2S4

nanotubes/NiFe

LDH

nanosheets,27

NiFe(oxy)hydroxide mesoporous nanosheets/NiCo2O4 nanoflakes,28 NiFe LDH nanosheets/Co0.85Se nanosheets/exfoliated graphene,29 exfoliated NiFe LDH nanosheets/defective graphene,30 NiCo2O4 nanowires/NiFe LDH molecular sheets,31 Cu nanowire@NiFe LDH nanosheets,32 and triple NiCoFe porous layered hydroxides33 for overall water splitting. Notably, NiFe2O4 is an inverse-spinel-structured oxide with earth abundance and environmental benignancy. It has presented good conductivity because of the electron hopping between different valence metals in O-sites and also supplies essential surface redox active metal centers for adsorption and activation of electroactive species.34, 35 Mesoporous NiFe2O4 nanorods,36 and the atomically thin NiFe2O4 quantum dots37 have been applied as electrocatalysts for OER. Sulfur-incorporated NiFe2O4 nanosheets built by ultra-small nanoparticles have been used for overall water splitting electrocatalysis.38 However, the obtained electrocatalytic performances of NiFe2O4 nanostructures toward OER or overall water splitting is still unable to meet the state-of-the-art requirements especially at large current densities due to the poor reactivity and limited surface active sites.35, 39 Some progresses have been made in constructing NiFe2O4-based nanocomposite electrocatalysts for enhanced OER applications, such as the mixed NiO/NiFe2O4 phase40 and the NiFe2O4 nanoparticles/α–Ni(OH)2 nanosheets.41 It is essential to elaborately design NiFe bimetallic nanocomposite with well-defined nanostructure that can combine the advantages of each component for overall water splitting, especially at large current densities. Herein, the NiFe2O4 nanoparticles/NiFe LDH nanosheets heterostructure array on Ni foam was synthesized by a simple 4


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one-step solvothermal approach in the mixed solvents of deionized water and methanol. It can be straight applied as an active electrode for electrocatalytic water splitting at large current densities. It shows outstanding OER performance with a small overpotential of 213 mV to drive 100 mA cm−2, and 265 mV to reach 1000 mA cm−2. Moreover, it also exhibits a good HER performance with a small overpotential of 101 mV at 10 mA cm−2, and 314 mV at 750 mA cm−2. A two-electrode electrolyzer with NiFe2O4 nanoparticles/NiFe LDH nanosheets as both anode and cathode only needs a voltage of 1.535 V to drive 10 mA cm−2 and 1.932 V to reach 500 mA cm−2 with outstanding stability. Significantly, the overall water splitting with this NiFe2O4 nanoparticles/NiFe LDH nanosheets array as electrodes can be continuously driven by a battery of only 1.5 V. The superior catalytic performances of delivering large current densities at low overpotentials and excellent durability make NiFe2O4 nanoparticles/NiFe LDH nanosheets heterostructure array a prospective electrocatalyst for practical overall water splitting.

EXPERIMENTAL SECTION Synthesis of NiFe2O4 nanoparticles/NiFe LDH nanosheets heterostructure array on Ni foam. A piece of Ni foam (NF 2×2 cm) was soaked in HCl solution (6 M) for 15 min and washed by deionized water and absolute ethanol for two times. 0.5 mmol of Ni(NO3)2·6H2O, 0.75 mmol of FeCl2·4H2O and 1.5 mmol of NaHCO3 were dissolved in the solvents of 20 mL of deionized water and 15 mL of methanol by agitation for 0.5 h. Afterwards, the solution was shifted to a 40 mL stainless steel autoclave with Teflon lined, and inserted cleaned Ni foam. Then, the autoclave was maintained at 120 °C for 2 h. After the reaction, the product covered on Ni foam was washed by deionized water and absolute ethanol for three times and vacuum drying at 50 °C for 6 h 5



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(NiFe2O4/NiFe LDH with 1:1.5 Ni and Fe molar ratio). For varied composition, NiFe2O4/NiFe LDH with 1:1 or 1:2 Ni and Fe molar ratio was gained via adding Ni(NO3)2·6H2O and FeCl2·4H2O with 0.625 and 0.625 mmol or 0.42 and 0.84 mmol, respectively, while keeping other parameters constant. Unless otherwise noted, the NiFe2O4/NiFe LDH product with Ni and Fe molar ratio of 1:1.5 was simplified as NiFe2O4/NiFe LDH. Synthesis of NiFe LDH nanosheets array on Ni foam. The parameters are similar with NiFe2O4/NiFe LDH heterostructure array with 1:1.5 Ni and Fe molar ratio except the solvents were 5 mL of deionized water and 30 mL of methanol. The product was simplified as NiFe LDH. Synthesis of NiFe2O4 nanosheets array on Ni foam. The precursor nanosheets array was firstly prepared with 0.5 mmol of Ni(NO3)2·6H2O and 1 mmol of FeCl2·4H2O while other parameters kept constant as that of NiFe LDH nanosheets array. The precursor array on Ni foam was cleaned by deionized water and absolute ethanol for three times and vacuum drying at 50 °C for 6 h. Next, the precursor array was calcined under air at 300 °C for 2 h to acquire NiFe2O4 nanosheets array on Ni foam. The product was simplified as NiFe2O4. Characterizations. X-ray diffraction (XRD) patterns were carried out with a D8 advance diffraction with Cu Kα radiation source. Scanning electron microscope (SEM) was performed with a Hitachi S-4800 instrument with an energy dispersive X-ray analysis spectroscopy (EDX). Transmission electron microscopy (TEM), and scanning transmission electron microscopy (STEM) were conducted with a JEOL JEM-ARF200F. Raman spectra were obtained with a M-9836-3991-04-A Raman spectrometer. X-ray photoelectron spectroscopy (XPS) was carried out with an ESCALAB MK II X-ray photoelectron spectrometer. Electrochemical measurements. The electrochemical measurements were performed on CHI 760e 6


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electrochemical workstation (Chenhua Corp., China) with a three electrode experimental setup at 25 °C in 1.0 M KOH electrolyte. The working electrode was the NiFe2O4/NiFe LDH/NF, NiFe LDH/NF and NiFe2O4/NF with a size of 0.5×0.5 cm by cut the Ni foam with product deposited (The loading mass was 2.8, 2.5 and 3.1 mg cm−2, respectively.). Platinum wire was used for counter electrode, and Ag/AgCl electrode for reference electrode. Linear sweep voltammetry (LSV) was performed at 5 mV s−1 with 90% iR-correction. The iR-correction of the potential was conducted according to the equation: ERHE(iR-correction) = EAg/AgCl + 1.023 – iR×90%, in which i is the current, R is the uncorrected ohmic electrolyte resistance tested under open-circuit potential. The multi-current process was measured without iR-correction. The stability test of current density curve was carried out at a constant potential. The electrochemically active surface areas (ECSAs) were evaluated through electrochemical double-layer capacitance (Cdl) via CV measurements in no apparent Faradaic region from 0.95 to 1.05 V (vs. RHE). The electrochemical impedance spectroscopy (EIS) was conducted on the open-circuit potential with a frequency scope from 100 kHz to 0.01 Hz. Commercial RuO2 and Pt/C was loaded on Ni foam for individually comparing the OER and HER activity of the catalyst. RuO2 or Pt/C (20 mg) was scattered in Nafion solution (0.5 wt.%, 0.2 mL) and ethanol (0.8 mL) by ultrasound, which was controllably dropped onto Ni foam to acquire a loading mass about 3 mg cm−2. Overall water splitting tests using NiFe2O4/NiFe LDH/NF as both anode and cathode were performed in a two-electrode configuration through linear sweep voltammetry (LSV) polarization curve at 5 mV s−1 (with 90% iR-correction) and current density curve at a constant potential. The experimentally evolved O2 were measured by gas chromatography (GC-2010 plus, Shimadzu, Japan).

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RESULTS AND DISCUSSION The products were first examined by X-ray diffraction (XRD) to investigate the crystalline phases and compositions. Figure 1 shows the product is the mixed phases of NiFe2O4 and NiFe LDH when the raw material ratio of Ni/Fe was 1:1.5 in 20 mL of deionized water and 15 mL of methanol. The diffraction peaks at 18.4o, 30.3o, 35.7o, 43.4o, 53.8o, 57.4o, 63.0o and 74.5o can be attributed to the (111), (220), (311), (400), (422), (511), (440) and (533) planes of cubic NiFe2O4 (JCPDS No. 74-2081). The diffraction peaks at 11.3o, 22.7o, 33.5o, 34.4o, 38.8o, 46.0o, 60.0o and 61.2o can be indexed to the (003), (006), (101), (012), (015), (018), (110) and (113) planes of NiFe LDH, consistent with the α-phase Ni(OH)2 (JCPDS No. 38-0715).22, 42 When the raw material ratio of Ni/Fe was kept 1:1.5 in 5 mL of deionized water and 30 mL of methanol, the product primarily showed the reflections of NiFe LDH. As the raw material ratio of Ni/Fe was raised to 1:2 in 5 mL of deionized water and 30 mL of methanol, only the NiFe2O4 phase was presented after calcination. It should be noted the precursor before calcination was the phase of NiFe LDH (Figure S1).

Figure 1. The XRD patterns of NiFe2O4/NiFe LDH, NiFe LDH and NiFe2O4, which were obtained with the powders separated from Ni foam by ultrasound.

Raman spectroscopy was further applied to explore the products. Figure 2 showed the peaks at 211, 275 and 683 cm−1 were presented in pure NiFe2O4 product. These modes correspond to the 8


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asymmetric and symmetric bending of oxygen atom in Fe–O bond at octahedral void and symmetric stretching of oxygen atom in Fe–O bond at tetrahedral void, respectively.36, 43 The bands at 455, 525 and 703 cm−1 in pure NiFe LDH product can be individually assigned to the Ni–O stretching vibration, defective or disordered Ni(OH)2,44-46 and the Fe-O vibrations in disordered FeOOH clusters.47 As for NiFe2O4/NiFe LDH product, the bands at 211, 275, 458, 533 and 683 cm−1 indicate the NiFe2O4 and NiFe LDH phases are coexisted.

Figure 2. Raman spectra of NiFe2O4/NiFe LDH, NiFe LDH and NiFe2O4, which were obtained with the powders separated from Ni foam by ultrasound.

The FESEM images of the NiFe2O4/NiFe LDH in Figure 3a and 3b showed the product was nanosheets array structure, with uniform nanoparticles attached on the nanosheets, of which the nanosheets were ca. 1.5 µm in lateral size and the nanoparticles were about 100 nm in diameter. While the NiFe LDH product consisted of smaller nanosheets with ~500 nm in lateral size (Figure S2), and the NiFe2O4 product was composed of nanosheets with about 1 µm in lateral size (Figure S3). The EDX results of three products in Figure S4 revealed the atomic ratio of Ni and Fe is basically in accordance with the raw material ratio of Ni and Fe source. The further quantitative analyses of the ratio of NiFe LDH and NiFe2O4 are estimated from the XRD patterns by the strongest peak of NiFe LDH at (003) and that of NiFe2O4 at (311).48 It revealed the NiFe2O4/NiFe 9



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LDH product with Ni/Fe ratio of 1:1 was about 84.1 wt % of NiFe LDH and 15.9 wt % of NiFe2O4, while the NiFe2O4/NiFe LDH product with Ni/Fe ratio of 1:1.5 was about 77.7 wt % of NiFe LDH and 22.3 wt % of NiFe2O4, and the NiFe2O4/NiFe LDH product with Ni/Fe ratio of 1:2 was about 54.8 wt % of NiFe LDH and 45.2 wt % of NiFe2O4. Figure 3c presented the STEM image on NiFe2O4/NiFe LDH, from which the nanoparticles with a size about 100 nm attached on the nanosheets can be clearly identified. The corresponding elemental mapping images reveal that Ni, Fe and O elements were distributed across the nanosheet, where the domain of NiFe2O4 nanoparticles can be discerned, especially from Fe and O elements. The HRTEM image of the NiFe2O4/NiFe LDH in Figure 3d revealed the adjacent fringes were 0.30 and 0.27 nm, respectively, which were individually in accordance with the d-spacing of (220) planes of NiFe2O4 and that of (101) planes of NiFe LDH.

Figure 3. (a, b) Low- and high- magnification FESEM images of NiFe2O4/NiFe LDH. (c) The STEM image of NiFe2O4/NiFe LDH and corresponding elemental mapping images. (d) The HRTEM image of NiFe2O4/NiFe LDH. 10


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X-ray photoelectron spectroscopy (XPS) was applied to analyze the valence states of Ni and Fe in NiFe2O4/NiFe LDH. Figure 4a showed the fitting peaks of Ni 2p lied at 855.5 and 873.3 eV are characteristic of Ni2+,30, 49, 50 and those at 861.4 and 879.4 eV were Ni2+ satellite peaks.8 The peaks at 856.5 and 874.3 eV correspond to Ni3+.37 A peak at 706.4 eV in Fe 2p is the pre-peak of Fe 2p3/2 (Figure 4b).51 The peaks at 711.8 and 724.7 eV can be assigned to Fe3+,30, 50 accompanied by a satellite peak of Fe3+ at 719.4 eV.51 The binding energies at 709.4 and 723.1 eV correspond to Fe2+.37 The O 1s spectrum in Figure 4c presented three binding energies at 529.9, 531.5 and 533.3 eV, which can be attributed to the lattice of oxides, hydroxides and absorbed water, respectively.51 It is generally deemed the oxidation states of metal elements in NiFe LDH were mostly in +2 for Ni and +3 for Fe,22, 30, 50, 52 while those in NiFe2O4 were the coexistence of Ni2+/Ni3+ and Fe2+/Fe3+ cations.37, 38 The XPS results of the product demonstrated coexistence of NiFe LDH and NiFe2O4 from another point of view. To clarify the effect of FeCl2·4H2O on the formation of NiFe2O4/NiFe LDH heterostructure, an experiment with FeCl3·6H2O instead was carried out. After reaction, only NiFe LDH nanosheets array were acquired (Figure S5). The results indicated that Fe2+ ions in FeCl2·4H2O have a particular effect on the formation of the NiFe2O4/NiFe LDH heterostructure. The morphology evolution of the intermediates showed the product is nanosheets array at 30 min (Figure S6a), and with time prolonged to 60 min, some tiny nanoparticles were attached on the nanosheets (Figure S6b). At 90 min, more and larger nanoparticles were covered on the nanosheets (Figure S6c). And the number of nanoparticles attached on the nanosheets was further increased at 2 h (Figure 3a and 3b). Time resolved experiments revealed that the nanosheets array was first constructed, then, the

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Figure 4. XPS spectra of Ni 2p (a), Fe 2p (b) and O 1s (c) of NiFe2O4/NiFe LDH.

nanoparticles were generated and covered on the nanosheets. As the reaction time further prolonged, more intensive and larger nanoparticles were attached on the nanosheets, forming nanoparticles/nanosheets array. According to the above results, the formation mechanism of the NiFe2O4 nanoparticles/NiFe LDH nanosheets array on Ni foam was proposed. Firstly, Fe2+ ions were partly oxidized into Fe3+ by oxygen dissolved in the solution.53 Then, Ni2+ and Fe3+ ions hydrolyzed and produced NiFe LDH nanosheets, which directly deposited on Ni foam and generated nanosheets array. Next, the NiFe2O4 nanoparticles produced by the hydrolysis of Ni2+, Fe2+ and Fe3+ ions deposited on NiFe LDH nanosheets. Then, the NiFe2O4 nanoparticles on NiFe LDH nanosheets gradually increased and grew up. Thus, NiFe2O4 nanoparticles/NiFe LDH nanosheets heterostructure array was formed. The formation mechanism of NiFe2O4/NiFe LDH is schematically shown in Scheme 1.

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Scheme 1. Formation process of NiFe2O4/NiFe LDH heterostructure array. (a) NiFe LDH nanosheets array grew on Ni foam. (b) NiFe2O4 nanoparticles deposited on NiFe LDH nanosheets.

The NiFe2O4/NiFe LDH array was straight utilized as an electrode for OER catalysis. The OER catalytic performance of the catalyst was first examined via linear sweep voltammetry (LSV). The polarization curve in Figure 5a showed NiFe2O4/NiFe LDH array possessed eminent OER activity, only demanded a low overpotential of 213 mV to drive 100 mA cm−2. For NiFe LDH, NiFe2O4 and commercial RuO2 catalysts, the requirement overpotentials to reach 100 mA cm−2 were individually 247, 274 and 322 mV. Remarkably, NiFe2O4/NiFe LDH shows much better OER catalytic activity compared to NiFe LDH, NiFe2O4 and RuO2. Moreover, NiFe2O4/NiFe LDH array can reach a large current density of 500 mA cm−2 at a quite small overpotential of 242 mV, and 1000 mA cm−2 at 265 mV (inset in Figure 5a). The results truly fulfilled the needs for commercial water splitting electrolysis (e. g., j ≥ 500 mA cm−2 at η ≤ 300 mV).20,

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The outstanding OER activity of

NiFe2O4/NiFe LDH array was confirmed by its very small Tafel slope of 28.2 mV dec−1 (Figure 5b), revealing inherent excellent OER activity. The Tafel slopes of NiFe LDH nanosheets, NiFe2O4 nanosheets and RuO2 catalysts were individually 47.3, 65.9 and 87.9 mV dec−1. The low overpotential and small Tafel slope of NiFe2O4/NiFe LDH array indicated its excellent OER catalytic activity, which are comparable to or better than those of other NiFe bimetallic high-performance OER catalysts, as listed in Table S1. Additionally, the NiFe2O4/NiFe LDH

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products with other Ni/Fe atomic ratio of 1:1 and 1:2 was also investigated (Figure S7). The OER activities of the catalysts with Ni/Fe atomic ratio of 1:1 and 1:2 are inferior to that of Ni/Fe atomic ratio of 1:1.5. Figure 5c presents a multistep chronopotentiometric curve of NiFe2O4/NiFe LDH electrode, in which the current density was stepwisely raised from 50 to 500 mA cm−2. The potential promptly stabilized at 1.48 V at 50 mA cm−2 for 500 s; similar results are also showed for

Figure 5. (a) The OER polarization curves of NiFe2O4/NiFe LDH, NiFe LDH, NiFe2O4, commercial RuO2 and naked Ni foam. Inset: The whole LSV curve of NiFe2O4/NiFe LDH. (b) The corresponding Tafel plots of NiFe2O4/NiFe LDH, NiFe LDH, NiFe2O4 and RuO2. (c) The multi-step chronopotentiometric curve of NiFe2O4/NiFe LDH without iR-compensation. (d) Chronoamperometric curves of NiFe2O4/NiFe LDH at overpotentials of 202, 230 and 242 mV.

subsequent other steps till 500 mA cm−2, demonstrating NiFe2O4/NiFe LDH array electrode provided the superior conductivity, mass transportation and mechanical robustness for OER. The 14


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potentials in multi-step chronopotentiometric curve were consistent with the values of uncorrected LSV curve shown in Figure S8. Encouragingly, the NiFe2O4/NiFe LDH array electrode also presents significant stability during the OER test. Figure 5d shows the current densities at overpotentials of 202, 230 and 242 mV all retain over 97% during the 20 hours session tested, which manifests the exceptional stability of NiFe2O4/NiFe LDH array electrode in OER electrocatalysis, even at large current densities. After 20 h OER chronopotentiometry test at overpotential of 202 mV, the morphology and microstructure of NiFe2O4/NiFe LDH were barely changed, as demonstrated by the SEM images (Figure S9a), XRD pattern (Figure S9b) and the Raman spectrum (Figure S9c). The XPS spectra of post Ni 2p and Fe 2p showed Ni3+ and Fe3+ were slightly increased (Figure S9d and Figure S9e, respectively), which are due to the slight oxidation on electrode surface. The results suggest the high tolerance of NiFe2O4/NiFe LDH array against corrosion under long-term strong oxidizing conditions. The OER Faraday efficiency at 100 mA cm-2 was measured by the actual amount of experimentally evolved O2 divided theoretical value. The experimentally evolved O2 was determined during electrolysis at 100 mA cm−2 by gas chromatography. As shown in Figure S10, the generated O2 were basically in accordance with expected calculated values, giving the Faradaic efficiency of over 92.6%. The result demonstrates that the current density totally derives from the OER process, not from other side reaction. The HER activity of NiFe2O4/NiFe LDH was further evaluated. Encouragingly, the NiFe2O4/NiFe LDH also exhibited promising HER activity (Figure 6a). It brought about an overpotential of 101 mV to drive 10 mA cm−2, and 229 mV for 100 mA cm−2, better than those of NiFe LDH (126 mV for 10 mA cm−2 and 270 mV for 100 mA cm−2) and NiFe2O4 (132 mV for 10 15



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mA cm−2 and 337 mV for 100 mA cm−2). Although they are not as good as commercial Pt/C on Ni foam (37 mV for 10 mA cm−2 and 135 mV for 100 mA cm−2), they are comparable with or better than other NiFe bimetal catalysts (Table S2). Moreover, it can drive a large current density of 500 mA cm−2 at a low overpotential of 297 mV, and 750 mA cm−2 at 314 mV. The resulting Tafel plot of NiFe2O4/NiFe LDH is 67.1 mV dec−1 (Figure 6b), smaller than that of NiFe LDH (86.6 mV dec−1) and NiFe2O4 (105.4 mV dec−1). The HER performance of the NiFe2O4/NiFe LDH with different ratio of Ni and Fe presented the catalytic activity achieves optimum at Ni/Fe ratio of 1:1.5 (Figure S11). The low Tafel slope of NiFe2O4/NiFe LDH indicates a comparable or better HER activity than some recently reported NiFe bimetallic HER catalysts (Table S2). The multi-step chronopotentiometric curve of NiFe2O4/NiFe LDH from 50 to 500 mA cm−2 presents prompt voltage response with the increase of current density (Figure 6c), indicating it also possesses outstanding mechanical robustness and mass transport properties for HER. The potentials in multi-step chronopotentiometric curve were in accordance with the values of uncorrected LSV curve presented in Figure S12. The long-term stability of NiFe2O4/NiFe LDH was evaluated by 20 h constant overpotential electrolysis at 150, 259 and 298 mV, respectively. Figure 6d shows the current densities were all only slightly dropped, suggesting the steady currents during the whole electrolytic course for HER in low and high current densities. The post catalysis characterizations of the catalyst after 20 h HER controlled overpotential electrolysis at 150 mV showed the SEM image (Figure S13a), the XRD pattern (Figure S13b) and the Raman spectrum (Figure S13c) of the catalyst were almost unchanged. The deconvolved Ni 2p and Fe 2p peaks of XPS spectra showed Ni3+ and Fe3+ were slightly increased, respectively (Figure S13d and Figure S13e), revealing the product underwent slight surface oxidation at highly alkaline environment. The results 16


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demonstrated the NiFe2O4/NiFe LDH is also cathodically stable in the strongly alkaline media.

Figure 6. (a) The HER polarization curves of NiFe2O4/NiFe LDH, NiFe LDH, NiFe2O4, commercial Pt/C and naked Ni foam. (b) The corresponding Tafel plots of NiFe2O4/NiFe LDH, NiFe LDH, NiFe2O4 and Pt/C. (c) The multi-step chronopotentiometric curve of NiFe2O4/NiFe LDH without iR-compensation. (d) chronoamperometric curves of NiFe2O4/NiFe LDH at overpotentials of 150, 259 and 298 mV.

The electrochemical active surface areas (ECSAs) of NiFe2O4/NiFe LDH, NiFe LDH and NiFe2O4 are compared (Figure S14). The Cdl of NiFe2O4/NiFe LDH was 11.9 mF cm−2, while that of NiFe LDH and NiFe2O4 was 7.7 and 6.6 mF cm−2, respectively. The result demonstrates that integration of NiFe2O4 nanoparticles on NiFe LDH nanosheets can expose a bigger catalytically active surface area and a higher interfacial contact area with the electrolyte, which guarantees rapid interfacial charge transfer and is favorable for electrochemical reactions. Generally, the specific activity of electrocatalyst with the current normalized to ECSA can efficiently demonstrate its intrinsic

17



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electrocatalytic activity.55 Therefore, the electrocatalytic activities of NiFe2O4/NiFe LDH, NiFe-LDH and NiFe2O4 for OER and HER were normalized to real ECSA (Figure S15). The results indicate that NiFe2O4/NiFe LDH still possesses the highest OER and HER activity, revealing its high intrinsic electrocatalytic activity. The NiFe2O4/NiFe-LDH heterostructure has a large number of interfaces, where the strong electronic reciprocity cause charge separation and modify the binding strength between OER intermediates and catalysts, which extremely benefit the OER kinetics.56, 57 The electrochemical impedance spectroscopy (EIS), a powerful technique to investigate the behaviors of electrode-electrolyte interfaces, was used to evaluate the differences between the electrocatalysts. The charge transfer resistance (Rct) of the electrochemical reaction corresponds to the semicircle diameter of the Nyquist plot of -Z" against Z' at the high frequency region, revealing the charge transfer kinetics of the electrocatalysts including NiFe2O4/NiFe LDH, NiFe-LDH and NiFe2O4 at the electrode interface. As shown in Figure 7, NiFe2O4/NiFe LDH displays the smallest impedance, indicating that NiFe2O4/NiFe LDH has the fastest electron transport ability. The fitting results of the semicircles in Figure S16 reveal the Rct of NiFe2O4/NiFe LDH was 0.34 Ω, while that of NiFe-LDH and NiFe2O4 was 1.90 and 1.00 Ω, respectively, further demonstrating the difference of conductivity among three catalysts. The mass transfer of the electrochemical reaction relates to the slopes of lines at low frequency region in Nyquist plot. The largest slope of NiFe2O4/NiFe LDH in Figure 7 suggests its best mass transfer performance. The Ni foam substrate with three-dimensional macropores supplies a stable frame structure for deposition of NiFe2O4/NiFe-LDH array, offering good electrical contact between the catalyst and the substrate,58, 59 and supplying more chances for interreactions between the active sites and the 18


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substrate,60 which is favorable for strain relaxation, assuaging catalyst exfliation due to electrolysis. Moreover, the large interspace between the nanosheets can not only supply abundant reachable electroactive sites, but also cushion great volume variation under reversible oxidation-reduction reactions, therefore, preventing the mechanical deformation during long-term work.61

Figure 7. The Nyquist diagrams of NiFe2O4/NiFe LDH, NiFe LDH and NiFe2O4.

The investigation on overall water splitting performance was encouraged by the exceptional OER activity as well as decent HER activity of NiFe2O4/NiFe LDH, which was carried out in a two-electrode layout using NiFe2O4/NiFe LDH as both the anode and cathode. For comparison, Pt/CǀǀRuO2 couple was also examined. As shown in Figure 8a, Pt/CǀǀRuO2 couple exhibited an outstanding catalytic activity at low current density. But, the glued Pt/CǀǀRuO2 pair easily detached from Ni foam substrate at large current density, hence, which was only tested to less than 125 mA cm−2. The NiFe2O4/NiFe LDHǀǀNiFe2O4/NiFe LDH couple afforded the current density of 10 mA cm−2 at 1.535 V. This potential is comparable to or outperformed some representative NiFe bimetallic electrocatalysts, as listed in Table S3. Impressively, to achieve a large current density of 500 mA cm−2, the NiFe2O4/NiFe LDH couple only needs a voltage of 1.932 V. At the same time, the long-term testing of the NiFe2O4/NiFe LDH electrodes for 20 hours chronoamperometric tests at 1.60, 1.82 and 1.94 V all showed only a slight degradation of the current density (Figure 8b), 19



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revealing very good stability of the electrodes. The long-term stability of NiFe2O4/NiFe LDH electrodes can be steadily acquired for OER, HER and overall water splitting in low and high current densities by chronoamperometric tests (Figure S17), in which the current densities were all remained over 94.4% for exceeding 40 h, indicating extremely excellent durability of the electrodes. The high catalytic activity and stability at large current densities make NiFe2O4/NiFe LDH electrodes a prospective electrocatalyst for industrial overall water splitting. Moreover, a battery of only 1.5 V can drive overall water splitting of this NiFe2O4/NiFe LDHǀǀNiFe2O4/NiFe LDH couple with continuous gas bubble release (Movie S1 in supporting information), verifying high efficiency of the electrodes.

Figure 8. (a) LSV curves for overall water splitting with NiFe2O4/NiFe LDHǀǀNiFe2O4/NiFe LDH and RuO2ǀǀPt/C in a two-electrode configuation. (b) Chronoamperometric measurements of NiFe2O4/NiFe LDHǀǀNiFe2O4/NiFe LDH couple at 1.60, 1.82 and 1.94 V for 20 h, respectively.

CONCLUSIONS The NiFe2O4 nanoparticles/NiFe LDH nanosheets heterostructure array has been synthesized on Ni foam via a facile one-step solvothermal approach. It can be applied as highly efficient, freestanding water splitting electrocatalyst at large current densities. The direct integration of NiFe2O4

20


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nanoparticles on NiFe LDH nanosheets significantly enhances active surface, efficient mass transport and charge transfer within the electrode. The NiFe2O4 nanoparticles/NiFe LDH nanosheets array not only exhibited prominent durability with small overpotentials at large current densities for OER, but also decent performance for HER as well. Consequently, a two-electrode overall water electrolyzer with the NiFe2O4 nanoparticles/NiFe LDH nanosheets array electrodes achieves a current density of 10 mA cm−2 at 1.535 V, and 500 mA cm−2 at 1.932 V with outstanding stability. Moreover, the overall water splitting can be continuously driven by a battery of only 1.5 V. This work demonstrates the importance of integrating different nanocomponents to optimize the electronic and surface structures of the electrocatalyst for superior catalytic performances, advancing fundamental research for practical electrocatalytic overall water splitting.

ASSOCIATED CONTENT Supporting information The Supporting Information is available free of charge on the ACS Publications website. XRD patterns, SEM images, EDX results, Tables comparing the OER and HER performances, LSV curves, Raman and XPS spectra, and CV curves of the catalysts.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]; [email protected].

ORCID Zhengcui Wu: 0000-0001-8368-8418 21



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Feng Gao: 0000-0003-3173-1650 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The Natural Science Foundation of China (21575004 and 21671006) was acknowledged.

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