Coupling Molecularly Ultrathin Sheets of NiFe-Layered Double

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Coupling Molecularly Ultrathin Sheets of NiFe-Layered Double Hydroxide on NiCoO Nanowire Arrays for Highly Efficient Overall Water Splitting Activity 2

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Zhiqiang Wang, Sha Zeng, Weihong Liu, Xing-Wang Wang, Qingwen Li, Zhigang Zhao, and Fengxia Geng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13075 • Publication Date (Web): 20 Dec 2016 Downloaded from http://pubs.acs.org on December 22, 2016

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ACS Applied Materials & Interfaces

Coupling Molecularly Ultrathin Sheets of NiFe-Layered Double Hydroxide on NiCo2O4 Nanowire Arrays for Highly Efficient Overall Water Splitting Activity Zhiqiang Wang,† Sha Zeng,‡ Weihong Liu,† Xingwang Wang,† Qingwen Li,‡ Zhigang Zhao,‡ Fengxia Geng†,* †

College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China

‡

Suzhou Institute of Nanotech and Nanobionics, Chinese Academy of Sciences, 398 Ruoshui Road, Suzhou Industry Park, Suzhou 215123, China *

E-mail: [email protected]

Abstract Developing efficient but non-precious bifunctional electrocatalysts for overall water splitting in basic media has been the subject of intensive research focus with the increasing demand for clean and regenerated energy. Herein, we report on the synthesis of a novel hierarchical hybrid electrode, NiFe-LDH molecularly ultrathin sheets grown on NiCo2O4 nanowire arrays assembled from thin platelets with nickel foam as the scaffold support, in which the catalytic metal sites are more accessible and active and most importantly strong chemical coupling exists at the interface, enabling superior catalytic power toward both oxygen evolution reaction (OER) and additionally hydrogen evolution reaction (HER) in the same alkaline KOH electrolyte. The behavior ranks top-class compared with documented non-noble HER and OER electrocatalysts and even comparable to state-of-the-art noble-metal electrocatalysts, Pt and RuO2. When fabricated as an integrated alkaline water electrolyzer, the designed electrode can deliver a current density 10 mA cm-2 at a fairly low cell-voltage of 1.60 V, promising the material as efficient bifunctional catalysts toward whole cell water splitting. Keywords: layered double hydroxide, molecular sheets, electrocatalyst, chemical coupling, water splitting 1

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Introduction Electrochemical water splitting, decomposition of water (H2O) into the constituent hydrogen (H2) and oxygen (O2), is attracting increasing attention as it is one of the most promising technology to produce clean and renewable energy in a chemical form.1 However, the process is so far rarely used in industrial applications because the two half splitting reactions, i.e. the anodic oxygen evolution reaction (OER) and cathodic hydrogen evolution reaction (HER), are strongly uphill and require much excess energy in the form of large overpotentials, for which electrocatalysts are essentially required to improve the electrolysis efficiency and minimize the dynamic overpotentials.2,3 Currently, Pt is acknowledged as the best electrocatalyst for HER with a near-zero overpotential, while IrO2 and RuO2 hold the benchmark for OER electrocatalysts.4,5 However, these noble-metal-based electrocatalysts suffer from severe element scarcity and high cost, which considerably impede their widespread technological use.6-8 It is thus highly desired to design alternative materials with low cost and high efficiency for catalyzing the water splitting reactions. Over the past few years, significant progress has been made in developing the catalysts based on the earth-abundant first-row transition metals, for example, phosphides, chalcogenides, carbides, and nitrides for HER,9-12 and oxides, hydroxides, nitrides, and phosphates for OER.13-16 Unfortunately, the best working conditions for these non-noble HER and OER catalysts often mismatch, HER preferably in acidic solution while OER in base environment, which would necessitate complicated procedures and accessories for overall water splitting.17 Therefore developing efficient bifunctional electrocatalysts that can work well for both HER and OER under the same condition may simplify the system, which is very attractive but highly challenging. Layered double hydroxide (LDH) has recently been demonstrated to be a new class of promising nonprecious OER electrocatalysts in alkaline electrolyte solutions.18-21 It is structurally characterized as sandwich stacking of positively charged layers, which is based on brucite (Mg(OH)2)-like layers with a proportion of divalent metals substituted by trivalent ones, and charge-balancing anions in the interspace. The identities of the di- and trivalent cations can be varied over a wide range, giving rise to a great number of isostructural members.22 In particular, NiFe-LDH hydrothermally synthesized on carbon nanotube or 2


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graphene support or electrodeposited on glassy carbon, nickel foam was reported to show high OER catalytic performances with low overpotentials, which is generally credited to several possible factors, including minor incorporation of Fe into Ni hydroxide, synergistic coupling with the substrate, and the enlarged surface area and increased number of accessible active sites endowed by the lamellar structure.23-25 However, report on its catalytic activity for HER and integration in overall water splitting is very few,26-28 because the transition metal hydroxides are typically unstable in acidic media and the HER activity in strong base is usually two or three orders lower than in acidic solution. Herein, for the first time, we demonstrate NiFe-LDH ultrathin sheets of several atomic layers grown on nickel cobalt oxide (NiCo2O4) nanowire arrays as an efficient bifunctional catalyst toward both HER and OER reaction, as schematically illustrated in Figure 1. Nickel (Ni) foam was used as the electrode scaffold support because of its earth abundance and porous three-dimensional structure. NiCo2O4, a typical OER electrocatalyst with high conductivity, was deposited on the Ni foam in the form of rhombus/hexagonal plates interconnected into perpendicular nanowire array morphology, efficiently facilitating electron transfer and electrolyte permeation. The electrical conductivity in NiCo2O4 has been believed to originate from the presence of Ni3+ and the electron transfer between Ni2+ and Ni3+. Importantly, the surface of NiCo2O4 was a Ni-rich layer, which served as the seed for the following hierarchical growth of NiFe-LDH, ensuring close contact and strong coupling at the interface. The NiFe-LDH sheets were ultrathin of only several atomic layers, combined with its strong coupling with NiCo2O4 and the unique hierarchical structure, enabled the hybrid electrode a remarkable overall water splitting performance of only 1.60 V to achieve 10 mA cm-2 current in single alkaline KOH eletrolyte.

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Figure 1. Schematic illustration for the construction of the hybrid hierarchical structure, i.e. ultrathin NiFe-LDH sheets grown on NiCo2O4 nanowire arrays with Ni foam as the support. The NiCo2O4 nanowires were actually rhombus/hexagonal plates interconnected into nanowire morphology, giving abundant exposed surfaces and meantime short pathway for electron transport. The NiFe-LDH ultrathin sheets were grown from the Ni-rich layer on the surface of NiCo2O4 nanowires, which enabled strong chemical coupling at the interface. The Ni foam electrodes grown with NiCo2O4 nanowire arrays and the NiFe-LDH/NiCo2O4 hybrid are denoted as NiCo2O4/NF and NiFe-LDH/NiCo2O4/NF, respectively.

Experimental section Materials and chemicals. The chemicals, including cobalt chloride hexahydrate (CoCl2·6H2O, ≥98%), nickel chloride hexahydrate (NiCl2·6H2O, ≥98%), iron nitrate hexahydrate (Fe(NO3)3·9H2O, ≥98%), nickel nitrate hexahydrate (Ni(NO3)2·6H2O, ≥98%), potassium hydroxide (KOH, ≥98%), trisodium citrate dihydrate (Na3C6H5O7·2H2O, ≥98%), and urea (CO(NH2)2), were purchased from Sinopharm Chemical Reagent Beijing Co., Ltd. and used as-received. RuO2 nanoparticles were obtained from Aladdin Reagent Co., Ltd. Distilled water was used in all experiments. Ni foam with a thickness of 1.6 mm was used as the substrate. Synthesis of NiCo2O4 nanowire arrays on Ni foam. NiCo2O4 nanowire array on Ni foam was prepared employing hydrothermal reaction followed by high-temperature calcination. In a typical synthesis, the Ni foam was first cleaned in 1.0 M HCl solution under sonication condition for 10 min to remove the nickel oxide passivation surface. The reactants of 4


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CoCl2·6H2O (10 mmol), NiCl2·6H2O (5 mmol), and urea (15 mmol) were dissolved in 75 mL water and stirred thoroughly to form a homogeneous pink solution. After being stirred for 30 min, the solution was transferred to a stainless Teflon-lined autoclave of 100 mL inner volume, and a piece of the cleaned Ni foam was immersed into the mixture. Then the autoclave was sealed and placed still in an electric oven set at a temperature of 120 °C for 6 h. After completing the reaction, the system was allowed to cool down naturally to room temperature, and the Ni foam with brown precursor product evenly covering the surface was collected, which was washed repeatedly with water and ethanol for removing residual ingredients. To achieve a topochemical transformation to NiCo2O4, the precursor grown on Ni foam was subsequently annealed at 400 °C in air atmosphere for 3 h. Synthesis of NiFe-LDH thin sheets/NiCo2O4 nanowire array heterostructures. In a typical procedure, the annealed NiCo2O4/NF substrate was immersed into an aqueous solution of 100 mL containing 9 mmol of Ni (NO3)2·6H2O, 3 mmol of Fe (NO3)3·9H2O, 20 mmol urea, and 1 mmol of Na3C6H5O7·2H2O, which was hydrothermally treated at 150 °C for 48 h in an autoclave for growth of NiFe-LDH thin sheets with the NiCo2O4 nanowires on Ni foam as nucleating sites. After cooling down to room temperature, the product was washed with distilled water and ethanol followed by drying overnight at 60 °C to yield the desired heterostructure. The loading amount of the hybrid structure on Ni foam was approximately 4.9 mg cm-2. For comparison purposes, individual components, NiCo2O4 and NiFe-LDH, were also grown on Ni foam as control samples with the optical images provided in Figure S1. A detailed section including techniques of material characterization and eletrochemical measurements can be found in Supporting Information.

Results and discussion Experimentally, the hybrid nanostructure electrode on Ni foam was obtained via a three-step procedure, reacting NiCl2·6H2O and CoCl2·6H2O with urea (CO(NH2)2) to yield NiCo precursor in carbonate hydroxide form, annealing in air atmosphere at 400 °C, and topping a layer of vertically standing NiFe-LDH thin sheets by hydrothermal reaction of Ni, Fe nitrate salts in alkali environment of urea with the chelating assistance of trisodium citrate (Na3(C3H5O(COO)3), which is schematically elucidated in Figure 1. The morphology and 5



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structure of the products was examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) characterizations. The bare commercial Ni foam showed intricate open network structure with macropores of hundreds of micrometers, coupled with its improved mechanical strength and high conductivity, making it an excellent choice for 3D scaffold support for catalysts. After hydrothermal reaction and calcination treatment for growing NiCo2O4, the clean surface of Ni foam was evenly covered by nanowires having sharpened tips and diameter of approximately 200 nm (Figure 2a and Figure S2). A cross-sectional SEM view revealed that the nanowires were perpendicularly grown from the Ni substrate with height of ~15 µm (Supporting Information, Figure S3). As confirmed by the peaks in the X-ray diffraction (XRD) characterization, the nanowires were pure spinel NiCo2O4 in a cubic structure (Fd-3m, a = 0.811 (3) nm). It was noticed that the reflections exhibited obvious broadening, implying small dimensions for the NiCo2O4 crystallites (Supporting Information, Figure S4). The average sizes for the crystallites were estimated by Scherrer equation, which gave a value of approximately 26 nm. The sample was following peeled off from the Ni foam for TEM observations. Figure 2b presents a typical TEM image, which showed that the nanowire was constructed by a number of nanoparticles with mesopores distributed throughout the nanowire, and the nanoparticles were mostly in the shape of rhombus or hexagonal plates with lateral dimensions of tens of nanometers, in rough consistency with XRD results. From the high-magnification TEM (HR-TEM) image shown in Figure 2c, it was noticed that the NiCo2O4 rhombus/hexagonal plates were highly crystallized with well-resolved crossed lattice fringes having interplanar spacing of ca. 0.28 nm, which _

could be assigned to the (220) and (220) crystal planes of the NiCo2O4 phase, disclosing the preferential stacking growth of the rhombus sheets in the ab plane. Worthy of note, the rhombus/hexagonal nanoplates were interconnected with no observation of obvious cracks or lattice distortions at particle edges (Figure 2c and Figure S5). Such a structure ensured strong interaction with each other, helping to maintain good mechanical and electrical contacts, and meantime provide a vast number of exposing surface and active reaction sites. X-ray photoelectron spectroscopy (XPS) analysis of the surface composition of NiCo2O4 nanowires gave weight percentage of 41.3%, 31.4% and 17.8% for Ni, Co, and O, respectively, that is, 2.6 : 2 : 4.2 in molar ratio, implying that Ni was heavily enriched on the 6


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surface. The enriched Ni on the surface may serve as active nuclei sites for the subsequent hierarchical growth of NiFe-LDH, which would enable excellent contact and strong chemical coupling between the components. The SEM image for the hybrid sample in Figure 2d revealed that sheets featuring ultrathin thickness of several nanometers and lateral dimension of 100-200 nm uniformly covered on the nanowires in a vertically standing morphology. The ultrathin sheets appeared to be spatially interconnected, yielding an open and porous structure. Brunauer–Emmett–Teller (BET) analysis showed that the novel hybrid possessed a high specific area of approximately 107 m2 g-1 (Supporting Information, Figure S6) and the size distribution for mesopores was centered around 20 nm and 35 nm, which should be account by the regularly distributed pores between NiFe-LDH sheets and/or NiCo2O4 nanowires. The corresponding energy-dispersive X-ray spectrum (EDX) detected Co, Ni, Fe, O as the principle elements, evidencing the formation of NiFe-LDH/NiCo2O4 hybrid (Supporting Information, Figure S7). The elemental mapping showed homogeneous distribution of all elements, confirming the even growth of the hybrid electrode (Supporting Information, Figure S8). The Ni:Co:Fe ratio of sample peeled off from Ni foam substrate was determined to be 3.18:2:0.71, from which the LDH composition was deduced to be Ni3.1Fe-LDH, in rough consistency with the stoichiometry in the reaction system, and then hybrid formula to be NiCo2O4+0.7Ni3.1Fe-LDH.29 The hybrid peeled off from the Ni foam and sonicated in ethanol was further studied with TEM, as a typical image shown in Figure 2e, in which the striking contrast difference confirmed the ultrathin feature of the so-obtained NiFe-LDH sheets. The electron beam was almost completely transparent at some places as marked by arrows. Bending, crumpling, or overlapping of the sheets should account for the presence striped lines or dark areas. It should be mentioned that the thickness of the striped or dark lines ideally corresponds to at least twice of sheet thickness. The lines were measured to be approximately 5-8 nm, and accordingly thickness of the sheets would be below 4 nm. Considering the interlayer distance was 0.8 nm, as a result it could be deduced that the so-grown NiFe-LDH thin sheets were composed of 3-5 molecular layers. The HR-TEM view is given in Figure 2f, which manifested lattice fringes of different orientations, implying that the ultrathin sheets were polycrystalline. The crystal lattice spacings were measured to be 0.22 nm, corresponding to the distance of (100) and (010) planes of hexagonal NiFe-LDH structure, and the SAED 7



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pattern showed diffraction rings indexed to be (100), (200), (210), and (220) of hexagonal symmetry, which confirmed that the as-prepared sheets were NiFe-LDH. Of note, the Ni seed layer was also grown into NiFe-LDH with clear lattice fringes of hexagonal arrangement. The NiFe-LDH directly grown on surface-enriched Ni layer of NiCo2O4 support would enable rapid charge transfer and strong chemical coupling at the interface, the ultrathin sheet morphology of NiFe-LDH exposed almost all atoms as catalytically active centers, and the NiFe-LDH ultrathin sheets in open porous structure facilitated both ion and electrolyte diffusion, all of which provided strong structural support for the hybrid material as highly efficient electrocatalyst toward water splitting.

Figure 2. Structural characterization of NiCo2O4 and NiFe-LDH/NiCo2O4 hybrid. (a) SEM image of NiCo2O4 nanowire arrays, (b) TEM and (c) HRTEM images of an individual nanowire; (d-f) SEM, TEM, and HRTEM images of NiFe-LDH supported on the topmost of NiCo2O4 nanowires. Inset in (b) and (e): structural illustration for the preferential growth 8


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plane of NiCo2O4 plates and NiFe-LDH ultrathin sheets. Inset in (f): corresponding SAED pattern.

XPS was employed to analyze the bonding state of elements and possible coupling between the components. Figure 3a shows the full-scan spectra for NiCo2O4 and NiFe-LDH/NiCo2O4 hybrid. All elements, including Ni, Co, Fe, and O, were detected. The O 1s region of both samples before and after hydrothermal growth of NiFe-LDH could be resolved into three oxygen species, from low to high energies attributing to oxygen bonding with metal, surface species including hydroxyls, chemisorbed oxygen, or undercoordinated lattice oxygen, and H2O bound to the surface of the sample.30 After hybriding NiFe-LDH, the fraction corresponding to surface hydroxyls became dominant (Figure 3b), in good agreement with the deposition of LDH phase on the topmost of NiCo2O4 nanowires. Meanwhile, both Ni 2p and Co 2p spectra in Figure 3c and d were well-fitted with two spin-orbit doublets as well as shake-up satellites, characteristic of mixed valence of Ni2+/Ni3+ and Co2+/Co3+.31,32 The higher oxidation states helped to facilitate fast charge transport across the electrode/electrolyte interface, for which, combining the 3D macroporous structure, the NiCo2O4 nanowire arrays on Ni foam in aqueous KOH did not show significant conductivity degradation compared with bare Ni foam in either charge transfer or electrolyte diffusion aspect (Supporting Information, Figure S9). After hybridization of the NiFe-LDH, the Co 2p3/2 peak shifted to lower numbers by 0.4 eV, unambiguously confirming the strong electronic interactions at the interface and guaranteeing fast electron transfer between NiCo2O4 and NiFe-LDH.

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Figure 3. XPS characterization for NiCo2O4 nanowires (bottom) and NiCo2O4/NiFe-LDH hybrid (top). (a) Full-scan XPS spectra, and high-resolution XPS spectra for (b) O 1s, (c) Ni 2p, and (d) Co 2p.

We first evaluated the OER activity of the as-prepared hybrid electrode in a conventional three-electrode electrochemical cell in 1.0 M aqueous KOH. All the potentials were referred to reversible hydrogen electrode (RHE) and no iR correction was applied (for iR-corrected polarization curves see Supporting Information, Figure S10). The linear sweep voltammetry (LSV) curves were collected after several continuous electrochemical conditioning cycles for achieving stability and reversibility (Supporting Information, Figure S11). The data shown in Figure 4a revealed that the hybrid presented a greater catalytic current density, reaching 50 mA cm-2 at a low overpotential (η) of 290 mV, lower than that for the respective components, NiCo2O4 nanowire (470 mV) or NiFe-LDH sheets (370 mV). Interestingly, while the oxidation peak corresponding to transformation from NiO to NiOOH was not observed in NiFe-LDH and barely detected in NiCo2O4 nanowires between 1.35 and 1.45 V, the signal was significantly increased for the hybrid electrode case, implying that the Ni sites in the hybrid possessed an improved activity. Consecutive CVs for the hybrid compared with the 10


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respective component, NiCo2O4 and NiFe-LDH on Ni foam, were collected, in which the designed hybrid presented the largest area among all the samples, verifying its excellent electrochemical behavior and enhanced activity of Ni sites (Supporting Information, Figure S12). The more active Ni species along with the above-mentioned increased number of active sites, and strong chemical coupling at the nanowire/ultrathin-sheet interface were considered to account for the improved catalytic activity. The Tafel slope in Figure 4b was derived from linear fitting of Tafel plots (η vs log(j)) and gave a low value of 53 mV dec-1, approaching the 50 mV dec-1 of RuO2,33 additionally confirming the remarkable OER reaction kinetics of NiFe-LDH/NiCo2O4 nanowires hybrid. Owing to the improved catalytic activity and kinetics, the hybrid electrode exhibited apparent higher current density than individual components and even commercial RuO2 catalyst over the entire measured potential range. Besides high OER activity, the hybrid electrode also featured excellent stability under the alkaline conditions. A electrochemical durability of the hybrid for electrolysis of water oxidation performed at a constant overpotential of 1.53 V (η = 300 mV) revealed that stable catalytic current densities were retained at approximately 50 mA cm-2 with negligible degradation in oxygen evolution for at least 10 h (Figure 4c). The cyclic voltammetric scanning for up to 1000 cycles shown in inset of Figure 4c exhibited negligible difference, further evidencing the excellent durability. The HER catalytic activities of the hybrid electrode were next assessed in the same electrolyte employing a typical three-electrode configuration. The LSV curve of hybrid compared with that for NiCo2O4 nanowires and NiFe-LDH on Ni foam at a scan rate of 5 mV s-1 is presented in Figure 4d. It was obvious that the hybrid electrode exhibited an obvious synergetic effect with a substantially low onset potential of 83 mV, significantly lower than the values required for NiCo2O4 (172 mV), NiFe-LDH (241 mV), and bare Ni foam (300 mV). Further scanning towards negative potential produced a rapid rise in HER current density along with vigorous evolution of H2 bubbles from the electrode surface, suggesting that the hybrid could serve as efficient cathode for water reduction and hydrogen release. The hybrid electrode achieved a 10 mA cm-2 current density at a negative potential of 192 mV, behaving favorably compared with bare Ni foam (395 mV), NiCo2O4 nanowires on Ni foam (294 mV), and directly grown NiFe-LDH (350 mV). Although the Pt-C electrode exhibited near-zero onset overpotential and gave the current of 10 mA cm-2 at 24 mV, the performance of our 11



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hybrid electrode working in alkaline electrolyte ranked the top class among non-noble HER catalysts measured in not only alkaline but acidic solution. Furthermore, the NiFe-LDH after hybridization on NiCo2O4 nanowires exhibited a drastic improvement in reaction kinetics, manifesting a low Tafel slope of 59 mV dec-1 (Figure 4e), even comparable to the commercialized Pt-C of 57 mV dec-1. The Tafel slope for individual NiCo2O4 and NiFe-LDH was 107 and 139 mV dec-1, respectively, suggesting the possible optimization of reaction pathway and electron transport by the strong chemical coupling between NiFe-LDH and NiCo2O4. Figure 4f presents the excellent HER stability of the novel hybrid at a static potential of -0.32 V, showing -45 mA cm-2 cathodic current with no appreciable loss for more than 10 h of H2 evolution. Even after continuous CV scanning for 1000 cycles, the polarization curve was similar to the initial one (inset in Figure 4f), proving the excellent durability for HER reactions.

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Figure 4. Bifunctional catalytic performance of NiFe-LDH/NiCo2O4 hybrid electrode on Ni foam both in 1.0 M aqueous KOH electrolyte. OER: (a) anodic polarization curves, (b) corresponding Tafel plots and (c) chronoamperometry curve. HER: (d) cathodic polarization curves, (e) corresponding Tafel plots, and (f) chronoamperometry curves. Inset in (c, f): durability test for the hybrid electrode by CV scanning after 1000 cycles of repetition.

Based on the above studies on catalytic activities of the hybrid electrode toward OER and HER reaction, it is reasonably to anticipate that the hybrid electrode could serve as an efficient and stable bifunctional electrocatalyts for overall water splitting, integrated HER and OER, in alkaline media. Hence, a device of two-electrode configuration with the as-prepared 13



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hybrid electrode used as electrocatalysts for both anode and cathode was constructed, in which a catalytic current was observed when the applied potential was larger than 1.45 V. Larger potential brought a rapid rise in current, and a density of 10 mA cm-2 was quickly reached at a cell voltage of 1.60 V and 20 mA cm-2 at 1.67 V, and bubbles of hydrogen and oxygen were clearly seen to be evolved from the cathode and anode (Figure 5a). The required potential for giving a current density of 10 mA cm-2 was lower than that for individual NiCo2O4 (1.85 V) or NiFe-LDH (1.80 V) due to the synergy effect, and could even rival benchmark combination catalyst, Ir/C (anode)//Pt/C cathode (1.62 and 1.71 V at 10 and 20 mA cm-2).5 Contrastingly, the potential needed to drive a current density of 10 mA cm-2 was over 1.90 V for Ir/C//Ir/C electrode,5 over 1.75 V for Pt/C//Pt/C,5 1.70 V for Ni5P4/Ni foil//Ni5P4/Ni foil,34 1.78 V for Ni(OH)2/NiSe2//Ni(OH)2/NiSe2.35 The chronopotentiometry curve at 20 mA cm-2 revealed that the overall water splitting performance of NiCo2O4/ NiFe-LDH hybrid electrode was stable for at least 12 h (Figure 5b). All the results indicate that this hybrid material could be a very promising candidate catalyst for practical alkaline water electrolysis applications.

Figure 5. Overall splitting performance. (a) Polarization curves of overall water splitting for NiFe-LDH/NiCo2O4 hybrid electrode compared with respective component and bare Ni foam at a scan rate of 5 mV s-1 in 1.0 M aqueous KOH. (b) Chronopotentiometric curve of water electrolysis for the hybrid electrode serving as both cathode and anode under a constant current density of 20 mA cm-2. Inset: a photographic image of two-electrode water electrolysis device and the simultaneous evolution of H2 and O2 on cathode and anode.

A comparative summary of the performance of state-of-the-art catalysts for OER, HER, 14


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and integrated reactions in recent literatures is provided in Table S1-3. The hybrid electrode behaved favourably compared with respective component and most documented non-noble electrocatalysts, for which the unique structure and the chemical coupling at the interface of NiCo2O4 and NiFe-LDH should be responsible. The 3D Ni foam with macroporous structure as the support enabled good mechanical strength, improved surface area, and good electrical conductivity, facilitating both ion and electrolyte diffusion; the NiCo2O4 nanowires composed of interconnected nanoparticles ensured good mechanical, electrical contacts and increased accessible active surface area, and the vertically aligned morphology provided open channels both for electrolyte diffusion into inner active sites and for rapid release of evolved H2 or O2 bubbles; the ultrathin thickness of NiFe-LDH and the polycrystalline nature increased the number of catalytic sites and lowered reaction activation energy, additionally improving the reaction efficiencies. In such a case, it is expected that the catalytic power is mostly optimized when the Ni foam is completely grown with perpendicular NiCo2O4 nanowires and the nanowires are fully covered with ultrathin NiFe-LDH sheets. We endeavored to obtain such a structure by firstly depositing overdose NiCo2O4 and NiFe-LDH followed by a short ultra-sonication in deionized water and ethanol to remove the debris, by which the samples those were weakly attached would be off. If any component was too much some catalytic sites would be blocked; similarly if any component was less the number of catalytic sites would be reduced, for both of which the catalytic performance would be degraded (Supporting Information, Figure S13). To estimate the electrochemically active surface area and verify the activity of electrodes, the electrochemical double layer capacitances (Cdl), being proportional to effective active surface area of electrode, were measured using a simple cyclic voltammetry (CV) method in the region of 1.1-1.2 V versus RHE. The current response in the potential window used for CV should be due to the charging of double layer. The value for the hybrid electrode was 233 mF cm-2 for NiFe LDH/NiCo2O4, much higher than 197 mF cm-2 for NiCo2O4/NF and 53 mF cm-2 for bare Ni foam (details in Supporting Information, Figure S14). Last and most importantly, the NiFe-LDH was grown on the enriched surface Ni layer of NiCo2O4 nanowires, which ensured intimate contact and strong chemical coupling between the two components at the interfaces and efficient charge transfer across the hybrid electrode, realizing simultaneous HER and OER in the same electrolyte. To demonstrate the importance 15



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of interface coupling, the catalytic activity of physically mixed NiCo2O4 and NiFe-LDH with the same chemical composition was examined for comparison, for which an overpotential of 350 and 257 mV was required to afford OER current density of 50 mA cm-2 and HER of 10 mA cm-2, higher than the value of 290 and 192 mV for the hybrid electrode (Supporting Information, Figure S15). Further, no shift in XPS spectra related to chemical coupling was observed in the physical mixture (Supporting Information, Figure S16). The results explicitly substantiated that the chemical coupling at the component interface played a critical role in enhancing the catalytic activity of the electrode toward HER and OER reactions.

Conclusion In summary, we have demonstrated that NiFe-LDH molecularly ultrathin sheets on NiCo2O4 nanowires assembled from thin platelets with Ni foam as the support could work as efficient bifunctional electrocatalyst toward overall water splitting in base KOH. Owing to the unique hieratical architecture and strong chemical coupling between NiCo2O4 and NiFe-LDH at the interface, the hybrid electrode showed superior catalytic activity for not only OER but also HER, achieving a current density of 10 mA cm-2 at a low cell-voltage of 1.60 V in a two-electrode system. The novel hybrid outperforms the respective component and most documented bifunctional non-noble electrocatalysts. This work may shed new light on rational design of novel nanostructure systems for efficient non-noble electrocatalysts.

Acknowledgement. The authors acknowledge financial support from the National Natural Science Foundation of China (51402204). Thousand Young Talents Program, Jiangsu Specially-Appointed Professor Program, and a project funded by the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions.

Supporting Information. Characterization methods, XRD patterns for the samples, more SEM and TEM images, EDS spectrum and mapping for the samples, catalytic performance comparison with the physical mixture, detail for estimation of electrochemically active surface area, table comparing OER, HER and integrated overall splitting behavior of the 16


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novel hybrid electrode with recently documented materials,

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Table of Contents Graphic

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Coupling Molecularly Ultrathin Sheets of NiFe-Layered Double

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