Hierarchical [email protected] layered double hydroxide heterostructures as a bifunctional electrocatalyst for overall water splitting

Hierarchical [email protected] layered double hydroxide heterostructures as a bifunctional electrocatalyst for overall water splitting

Journal of Alloys and Compounds 794 (2019) 261e267 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: http:...

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Journal of Alloys and Compounds 794 (2019) 261e267

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

Hierarchical Graphdiyne@NiFe layered double hydroxide heterostructures as a bifunctional electrocatalyst for overall water splitting Hua-Yan Si a, b, *, Qi-Xin Deng a, Li-Chuan Chen c, Liu Wang a, Xing-Yu Liu a, Wen-Shan Wu a, Yong-Hui Zhang e, Jin-Ming Zhou d, **, Hao-Li Zhang c a

School of Materials Science and Engineering, Hebei Provincial Key Laboratory of Traffic Engineering Materials, Shijiazhuang Tiedao University, Shijiazhuang, 050043, PR China Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA, 02139, USA c State Key Laboratory of Applied Organic Chemistry (SKLAOC), College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, 730000, PR China d Key Laboratory of Inorganic Nanomaterials of Hebei Province, College of Chemistry and Material Science, Hebei Normal University, Shijiazhuang, 050024, PR China e College of Materials and Chemical Engineering, Collaborative Innovation Center of Environmental, Pollution Control and Ecological Restoration, Zhengzhou University of Light Industry, Zhengzhou, 450002, PR China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 December 2018 Received in revised form 23 March 2019 Accepted 14 April 2019 Available online 26 April 2019

Hydrogen is an ideal energy with little impact on the environment to tackle the intense energy crisis. Development of highly efficient electrocatalysts for lowecost hydrogen production from water splitting is vital for the real-world practical applications. Herein, a three dimensional hierarchical graphdiyne (GDY)@NiFe layered double hydroxide (LDH) heterostructured catalyst toward overall water splitting is developed by coupling NiFe LDH and GDY growth on copper foam (CF). The catalyst exhibits high electrocatalytic activities in both oxygen evolution reaction (OER) and hydrogen evolution reaction with overpotentials of 220 mV and 163 mV at a current density of 10 mA cm2, respectively. The Tafel slope for OER is 39.33 mV dec1, which is much lower than that of the only NiFe LDH/CF with a value of 112 mV dec1. Thus, the GDY@NiFe LDH/CF heterostructure electrocatalyst leads to an improvement in the overall water splitting activity, with a current density of 20 mA cm2 at the voltage of 1.512 V. Density functional theory calculation indicates that the synergetic effects of 3d orbit of transition metal atoms and carbonecarbon triple bonds in GDY are responsible for the bifunctional excellent electrocatalytic activity for overall water splitting. © 2019 Published by Elsevier B.V.

Keywords: Graphdiyne@NiFe Layered double Hydroxide Bifunctional Electrocatalyst Overall water splitting Hierarchical

1. Introduction The electrochemical water splitting to achieve green and sustainable hydrogen energy has attracted enormous attentions due to its great potentials to replace the non-renewable and pollutive fossil energy [1,2]. The water splitting process includes two half reactions of hydrogen evolution reaction (HER) and oxygen

* Corresponding author. School of Materials Science and Engineering, Hebei Provincial Key Laboratory of Traffic Engineering Materials, Shijiazhuang Tiedao University, Shijiazhuang, 050043, PR China. ** Corresponding author. E-mail addresses: [email protected] (H.-Y. Si), [email protected] (J.-M. Zhou). https://doi.org/10.1016/j.jallcom.2019.04.150 0925-8388/© 2019 Published by Elsevier B.V.

evolution reaction (OER). Generally, the noble metal based materials, such as Pt, Ir oxide, and Ru oxide are highly active electrocatalysts for HER or OER due to their advantages on overpotentials and fast kinetics for practical applications [3,4]. However, their shortages are the high cost, scarcity and poor stability [5,6]. To overcome the shortcoming of the precious metals, transition metal hydroxides/oxides [7e20], perovskites for OER [21,22] and metal compounds [23e30], heteroatom-doped carbons [31,32] for HER have been developed. However, using a single catalyst as both anode and cathode for overall water splitting in an alkaline solution remains a significant challenge. Very recently, the electrocatalyst with heterostructures and multi-functions were utilized to enhance the overall water splitting [33e40]. For example, hierarchical NiFe/NiCo2O4/Ni foam, or graphene foil/Co0.85Se/NiFe layered

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double hydroxide (LDH) bifunctional catalysts, active for both OER and HER, were reported for whole-cell water splitting [33]. A rational design of the bifunctional electrocatalyst is to stretch toward the full potentials of the multiple complementary components. A typical hetero-assembly is the graphene and other two dimensional (2D) nanolayers [41,42] in which the hybrid exhibits an improved electrocatalytic activity due to the synergistic effects of high conductive of graphene and highly exposed active centers on 2D nanolayers. In addition, there are also some works to construct the defective graphene and NiFe LDH nanosheets by electrostatic interaction to improve the activity of electrocatalysts [37]. In these works, stabilize and disperse transition metals on carbon matrix were considered to be two important factors for enhancing the electrocatalytic activity. However, the non-specific physical affinity between the functional transition metal and highly conductive graphene is unfavorable for the uniform distribution of the transition metal atom and stability. Thus, exploring a new carbon matrix, which has both superior conductive and abundant chemistry for anchoring metal atoms, is vital to construct the hierarchical hybrid structure with high electrocatalytic activity. Graphdiyne (GDY) has exhibited attractive properties, such as perfect semiconducting properties, superior electrical properties, and high stability [43e46]. Especially, GDY contains rich binding sites with transition metals through the sp and sp2 hybridized carbon-carbon bonds, which could form strong chemical bonds between transition metals and surrounding acetylenic bonds, enabling much more efficient catalytic performances and stability than the transition metals physically adsorbed on graphene. Herein, we design a micro/nano heterostructure GDY and NiFe LDH on copper foam for highly efficient overall water splitting. GDY/ copper foam (CF) has high conductivity and three-dimensional (3D) porous structure, which is beneficial for the flow of electrolytes and offers extraordinary specific surface area. Furthermore, such a hierarchical heterostructured composite (GDY@NiFe LDH) maximizes direct interfacial contact between 3d transition metal atoms and carbonecarbon triple bonds in GDY through the strong chemical binding, significantly enhancing electron transfer and shortening diffusion distance. A solar power assisted water-splitting device was designed based on the GDY@NiFe LDH/CF heterostructures. Enhanced performance and stability for both OER and HER with a high record of 20 mA cm2 at a voltage of only 1.512 V was demonstrated.

NiFe LDH/CF was the same except for using CF as the template. The mass of NiFe LDH catalyst on Cu foam or GDY/CF could be directly weighted after the growth of NiFe LDH [39]. 2.2. Characterization X-ray diffraction (XRD) patterns of the products were identified on a Bruker D8 ADVANCE by using Cu Ka radiation (l ¼ 0.1524 nm). Scanning electron microscopy (SEM) images were obtained on a Hitachi S-4800. The X-ray photoelectron spectroscopy (XPS) spectra were performed on Kratos Axis Ultra DLD-600W XPS system using Al Ka (1486.7 eV) radiation. 2.3. Electrochemical measurements Oxygen evolution reaction (OER) characterizations were measured on CHI660e electrochemical workstation, where the synthesized GDY@NiFe LDH/CF, the platinum plate, and Hg/HgO (1 M KOH) electrodes were utilized as the working electrode, counter electrode and reference electrode, respectively, with 1 M KOH as electrolyte. The Faradaic efficiencies of HER and OER are calculated as the ratio of the amount of experimentally collected gas to the theoretically generated gas estimated from the number of charge transfer (current density). Alkaline water electrolysis fullcell measurement was carried out in a standard two-electrode system by using GDY@NiFe LDH/CF as anode and cathode. 2.4. Theoretical simulation The DFT calculations are carried out by using the Vienna ab initio simulation package (VASP) [50]. The exchange correlation functional and General Gradient Approximate (GGA) of PerdewBurke Ernzerhof (PBE) were adopted [50]. The equilibrium lattice parameters and atoms positions for all structures are optimized to minimum total energy until the forces less than 0.01 eV/Å. The cut-off energy is set to 300 eV. The cell parameters are a ¼ b ¼ 90 , g ¼ 120 , a ¼ 18.89 Å, b ¼ 18.89 Å, c ¼ 15.00 Å, and the space was enough to avoid the interaction between periodical geometry. The K-point grid of Brillouin zone is sampled by a Monkhorst-Pack 3  3  1. The electron density difference is obtained by Dr ¼ rGDY@NiFe LDH-rGDY-rNiFe LDH. 3. Results and discussion

2. Experimental section 2.1. Catalyst synthesis 2.1.1. Synthesis of GDY on CF GDY on CF was synthesized according to the reported method [47,48]. Hexaethynylbenzene was added dropwisely into the mixture of N, N, N0 , N0 -tetramethyl ethylenediamine, pyridine, and acetone in the presence of copper foam. GDY films were then growth on the copper foam at 50  C for 12 h under an argon atmosphere. The synthesized GDY/CF was successively washed with heated acetone and N, N-dimethylformamide (DMF), respectively. 2.1.2. Synthesis of heterostructure GDY@NiFe LDH/CF GDY@NiFe LDH was synthesized as follows [49]. Firstly, 0.36 g Ni(NO3)2$6H2O, 0.16 g Fe(NO3)3$9H2O, 0.16 g urea, and 0.04 g trisodium citrate were put into a Teflon-lined stainless-steel autoclave filled with 80 mL of water solution. Then the as-obtained GDY on Cu foam was added into the above solution. Secondly, the mixture was heated at 150  C for 20 h. Thirdly, the product was washed by deionized water and ethanol, respectively. Finally, NiFe LDH@GDY/ CF was vacuum-dried at 80  C for 12 h. The process of preparing

The fabrication process of the hierarchical GDY@NiFe LDH/CF electrocatalysts by a CF templating approach is illustrated in Fig. 1a. The CF was used as structure guiding template to construct the micro-nano catalyst structures. Then, GDY vertical honeycomb-like nanolevel structures were well-distributed on the skeleton of CF, where a micro/nano two-tier hierarchical structure was produced. Subsequently, the NiFe LDH was in situ deposited through a simple hydrothermal method yielding a robust bifunctional electrocatalyst electrode (denoted as GDY@NiFe LDH/CF). The hierarchical structures of the GDY@NiFe LDH/CF electrocatalysts were investigated by SEM. The CF maintains its robust 3D structures with an average pore size of ~200 mm during the overall prepared process (Fig. 1 b-d). The CF skeletons is apparently smooth (Fig. 1e). After GDY growth, honeycomb-like GDY formed on the surface of CF skeletons (Fig. 1f), and many micro/nano hierarchical porous texture was successfully created through stable chemical bonding. As shown in Fig. 1g, the thickness of the GDY vertical walls gets thicker after hydrothermal treatment. Furthermore, the ordered nanostructures of NiFe LDH uniformly cover the whole 3D frame of CF in Fig. S1 (Supporting Information) with an average pore size of 150 nm. The cross section view shows that the

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Fig. 1. a) the schematic preparation process of GDY@NiFe LDH/CF catalysts. Typical SEM and HRTEM images of b,e,h) the copper foam, c,f,i) the GDY/CF, and d,g,j) the GDY@NiFe LDH/CF, with the inset showing the side-view images of GDY/CF and GDY@NiFe LDH/CF in f) and g), respectively. Corresponding SAED patterns are in inset of i).

thickness of the NiFe LDH wall is about 200 nm. The composition of NiFe LDH can be verified by the appearance of Ni and Fe element throughout the GDY covered CF skeletons in energy dispersive spectroscopy (EDS) mapping analysis spectra (Fig. S1c d), suggesting a heterostructure coupling of NiFe LDH and GDY, where ordered honeycomb-like GDY is embedded in the NiFe LDH matrix. The energy-X-ray spectrum verifies the composition of GDY@NiFe LDH/ CF, in which the mass ratio of Ni and Fe is close to 3:1 as designed (Fig. S2, Supporting Information). However, the NiFe LDH growth on CF without GDY can not cover the entire interconnected 3D scaffolds of copper (Fig. S3, Supporting Information). The morphology is flower-shaped structure constituted by nanosheets, which are thicker than NiFe LDH growth on CF with GDY. This phenomenon reveals that the GDY/CF has strong chemisorption on the transition metal atoms (Ni, Fe), which could be ideal scaffolds for structuring electrocatalysts with superior durability and catalytic activity [51,52]. In addition, the high-resolution transmission electron microscopy (HRTEM) images further confirm the successful formation of hierarchical GDY@NiFe LDH/CF. As can be seen from Fig. 1h, the clear visible lattice spacing of about 0.208 nm are assigned to the (111) planes of copper. Fig. 1i reveals the lattice fringe is 0.365 nm,

consistent with interlayer spacing of GDY sheets [53]. The related selected area electron diffraction (SAED) patterns reveal the high crystallinity of GDY in certain areas. The lattice spacing of 0.25 nm is referred to the (012) planes of NiFe LDH (Fig. 1j). The results suggest that the NiFe LDH is successfully formed on the surface of GDY. The structure and surface information of GDY@NiFe LDH/CF electrocatalyst were studied systematically with XRD, Raman spectroscopy, XPS, and Nitrogen adsorption/desorption measurements. XRD patterns of pure GDY exhibit four characteristic peaks at 15.98 , 31.4 , 35.38 and 39.82 (Fig. S4, Supporting Information). The GDY@NiFe LDH/CF clearly shows the characteristic (003), (006), (012) and (110) peaks compared to the pure LDH, indicating the successful formation of GDY@NiFe LDH/CF composites. The successful preparation of GDY was further characterized by the Raman spectra. There are four clear peaks around 1389.8, 1583.1, 1932.6, and 2175.1 cm1 in Fig. 2a, which are typical Raman spectra of GDY [47,48,54]. GDY@NiFe LDH/CF composites show coincident Raman patterns to the NiFe LDH except for 1389.8 and 1576.1 cm1 peaks attributed to GDY, indicating that the NiFe LDH was successfully combined with GDY. Moreover, the peak at 1583 cm1 in the spectrum of the composites blue-shifts toward a high wavenumber (1595 cm1) compared with GDY, which may be attributed

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Fig. 2. a) Typical Raman spectra of NiFe LDH/CF, GDY/CF and GDY@NiFe LDH/CF, b) N2 adsorption/desorption measurement and the corresponding pore size distribution curves (inset) of GDY/CF, NiFe LDH/CF and GDY@NiFe LDH/CF, c-d) the high-resolution XPS spectra of the C1s of the GDY/CF and GDY@NiFe LDH/CF samples.

to the partly CeC (sp) of GDY could convert into CeC (sp2) for the instability of the diacetylenic linkages in GDY during the solvothermal process [44,55]. The intensity ratio of the D and G bands (ID/IG) for GDY@NiFe LDH/CF (0.92 vs 0.74 of GDY) further implies augmenting the structural defects, which would increase active sites to enhance the activities of both OER and HER [52]. In addition, the nearly disappeared of 1932.6 and 2175.1 cm1 peaks in GDY@NiFe LDH/CF may be due to the reduction in the number of the eC^CeC^C functional groups in the composite [44,52]. Fig. 2b displays the characterization of surface area and porosity properties of the bulk GDY, NiFe LDH and GDY@NiFe LDH powder which is peeled off the 3D scaffolds of copper foam. All the isotherms of GDY, NiFe LDH and GDY@NiFe LDH exhibit a type-H3 hysteresis loop and the shape is more like type IV, indicating the non-uniform mesopores [56]. The BrunauereEmmetteTeller surface area and the mesopore volume of GDY, NiFe LDH and GDY@NiFe LDH are 131.94, 95.031 and 205.0186 m2 g1, respectively. The distribution pores size at 3.24, 7.69 and 32.66 nm can be observed, demonstrating the mesoporous structure of GDY (inset of Fig. 2b) [57]. The pore size distribution of NiFe LDH is 2.8, 8.7, and 13.9 nm. The new pore size of 5.1 nm and larger surface area in heterostructures GDY@NiFe LDH indicate further increase the micro/nano two-tier hierarchical structure which are expected to be beneficial to transfer ion and increase the area of interfacial electrochemical sites. The composition and element valence of the heterostructure of GDY@NiFe LDH/CF were investigated by XPS (Fig. 2ced, Fig. S5, Supporting Information). The XPS survey scans of Fig. 2ced presents high resolution asymmetric C 1s XPS spectra of GDY/CF and GDY@NiFe LDH/CF. Compared with bare GDY/CF, the C 1s peak in GDY@NiFe LDH/CF can be fitted into three subpeaks at 284.66, 286.45, and 288.60 eV, corresponding to CeC (sp2), CeO, and OeC]O, respectively [51,52,54]. The nearly disappeared of CeC

(sp) in GDY@NiFe LDH composites relative to bare GDY, shows shrinking in the number of the CeC (sp) [52], which is in agreement with the Raman result. The additional CeO peak (286.45 eV) is attributed to the restoration of the delocalized p conjugation, which indicates the strong interactions between GDY and NiFe LDH species. Fig. S5 reveals that the valence of Fe and Ni is þ3 and þ 2 in the as synthesized composite, respectively [7,37]. However, there is no apparent shift in the binding energies of Ni and Fe between GDY@NiFe LDH/CF and NiFe LDH/CF or bare NiFe LDH samples (Fig. S5f, Supporting Information). The catalytic performances of as-prepared GDY@NiFe LDH/CF used as an integrated 3D OER anode were investigated in a typical three-electrode configuration in 1 M KOH aqueous solution. GDY/ CF, NiFe LDH/CF and the commercial GDY@RuO2/CF were also examined for comparison. Fig. 3a shows representative LSV curves of GDY@NiFe LDH/CF as well as GDY@NiFe LDH/CF, GDY/CF, NiFe LDH/CF and GDY@RuO2/CF. GDY@NiFe LDH/CF exhibits the smallest onset potential of 1.45 V and overpotential of 0.22 V at the current density of 10 mA cm2, equivalent to the best result of NiFe LDHNS@NG10 which outperforms most of the 3d transition metal hydroxide/oxide OER catalysts reported to date [37]. The peak around 1.44 V of GDY@NiFe LDH/CF is due to the Ni(II)/Ni(III or IV) redox process [7,52]. As the Ni: Fe ratio is 3:1, the composites exhibit the best catalytic performance toward OER (Fig. S6, Supporting Information) [58]. The overpotentials of GDY@NiFe LDH/CF at the current densities of 10, 20, and 30 mA cm2 are lower than those of GDY/CF, NiFe LDH/CF and GDY@RuO2/CF (Fig. S7, Supporting Information), demonstrating that GDY can significantly increase their OER catalytic activities. The GDY@NiFe LDH/CF heterostructures show a Tafel slope of 39.33 mV dec1, which is much smaller than the value of 112 mV dec1 for NiFe LDH/CF (Fig. 3b). The OER performance on GDY@NiFe LDH/CF is comparable to or even better

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Fig. 3. a) OER linear sweeping voltammetry (LSV) curves of different electrocatalysts in 1 M KOH solution. b) Tafel plots for OER. c) time-dependent current density curves of the GDY@NiFe LDH/CF and GDY@RuO2/CF. d) HER linear sweeping voltammetry curves of different electrocatalysts in 1 M KOH solution. e) Tafel plots for HER. f) time-dependent current density curves of the GDY@NiFe LDH/CF and 20% Pt/C/CF.

than those of previously reported LDH-related OER catalysts (Supplementary Table 1). The best OER performance may be attributed to several reasons: (1) GDY is more powerful for the NiFe LDH anchoring due to the carbonecarbon triple bonds and carboxyl bonds to attach metal atoms enhanced chemical and electronic coupling between NiFe LDH and GDY; (2) the extraordinary micro/ nano two-tier hierarchical 3D porous structure increases mass transport [59,60]; and (3) self-supporting electrode can also enhance the conductivity due to decrease series resistance, deactive sites and the length of diffusion [7,26,61]. Electrochemical impedance spectroscopy measurements were executed to investigate the reaction kinetics (Fig. S8, Supporting Information). The smallest solution resistance and the charge transfer resistance of GDY@NiFe LDH/CF at the potential of 1.40 V (close to the OER onset potential), indicates a highest conductivity and fastest chargetransfer ability in GDY@NiFe LDH/CF [40]. Besides, as GDY is more powerful for the LDH anchoring and possesses two-tier hierarchical porous structure, leading to larger relative electrochemically active surface area as compared to NiFe LDH/CF by using only CF as a support. Importantly, the durability of GDY@NiFe LDH/CF catalyst was evaluated at the current densities of 10 mA cm2. As shown in Fig. 3c, GDY@NiFe LDH/CF exhibits the superior durability, which may be due to the intense interaction between GDY and LDH. Moreover, the Faradiac efficiency was calculated to be 98.2% after 20 h chronoamperometric test at an anode current density of 10 mA cm2. The morphology, oxidation state and the crystal structure of GDY@NiFe LDH/CF after the long-term test is shown in Fig. S9. The SEM image (Fig. S9a, supporting information) shows almost no change, indicating the GDY/CF can effectively afford robust stability during the OER process in the strong alkaline solutions. The XPS spectra (Figs. S9c and S9d, supporting information) after 20 h electrolysis reveal the formation of Ni(III), noting that Fe specie inside NiFe LDH is still at its high valence (þ3), which was not changed during OER process [62,63]. Raman spectroscopy analysis further concludes the formation of NiOOH (Fig. S9e, supporting information), suggesting that NiOOH was the active phase for OER [62]. To further examine the structure of GDY@NiFe LDH/CF, the

XRD characterization was measured in Fig. S9f. There are no peaks of the NiFe LDH except for the Cu substrate, which may be ascribed to the NiFe LDH had changed into amorphous FeeNiOOH after OER test [38,64]. The HER electrocatalytic performances of the GDY@NiFe LDH/CF are also assessed in 1 M KOH. GDY@NiFe LDH/CF shows excellent HER activities as compared to pristine NiFe LDH/CF (Fig. 3d). The overpotential at the current density of 10 mA cm2 for GDY@NiFe LDH/CF, 20% Pt/C, GDY/CF and NiFe LDH/CF is 163, 49, 243 and 401 mV, respectively. The GDY@NiFe LDH/CF presents a relative small Tafel slope of 105.99 mV dec1 in the linear region (Fig. 3e). It is noteworthy that these values are better than those of the pristine NiFe LDH/CF and most previously reported nonprecious HER electrocatalysts in basic media (Supplementary Table 2). The steady property for GDY@NiFe LDH/CF was further measured at overpotential of 163 mV for almost 20 h in 1 M KOH (Fig. 3f). Remarkably, the current density is almost constant. The Faradaic efficiency is 96.2% after 20 h chronoamperometric test at a cathode current density of 10 mA cm2. The SEM image of GDY@NiFe LDH/CF collected after the long-term test (Fig. S9b, Supporting Information) shows the robust stability of GDY@NiFe LDH/CF in HER. The excellent catalytic HER activity and durability of GDY@NiFe LDH/CF may result from coupling interaction between 3d transition metal atoms and the carbonecarbon triple bonds and carboxyl bonds in GDY. There are some reports about composites based on GDY possessing favorable HER activity [59,65,66]. Based on experimental results aforementioned, both the HER and OER activities are further enhanced in the GDY@NiFe LDH heterostructure, indicating the synergistic effect may play a vital role in HER and OER. Naturally, we supposed that the coupling interaction between GDY and NiFe LDH may efficiently enhance the HER and OER. To investigate the charge distributions on this heterostructure, we undertook a density functional theory (DFT) calculations. Fig. 4a and Fig. 4b show the geometries of the fully relaxed GDY@NiFe LDH in top view and in side view. Fig. 4c displays the electron density difference between GDY and NiFe LDH. It can be seen apparently that the charge density accumulation around the interface between 3d transition metal atoms and

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Fig. 4. DFT calculation studies of GDY@NiFe LDH. a) The top views of and b) the 2D side views of GDY@NiFe LDH based composite interfaces. c) The 3D side views of interfacial electron density difference (the iso-surface value is 0.001e/Å3) between a GDY sheet and a NiFe LDH layer. Blue isosurfaces conveys charge accumulation between GDY and NiFe LDH. Grey, purple, blue, red and white balls represent C, Fe, Ni, O and H atoms, respectively. d) The schematic of the probable reaction mechanism of GDY@NiFe LDH for HER and OER. the red and blue spheres stand for electrons and holes, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

carbonecarbon triple bonds in GDY. Such modulation mainly traces to the electron transfer between transition metal atoms and the GDY induced by the strong coupling between transition metal atoms and GDY. Obvious charge transfer arises from the transition metal atoms to GDY sheet as shown in Fig. 4d. Here, the localized electrons accumulation at the GDY is supposed to enhance the HER. Meanwhile, the electron transfer leads to the holes accumulation on NiFe LDH, which is favor to OER. Inspired by the active and stable performance in OER and HER, the GDY@NiFe LDH/CF is served as both anode and cathode to evaluate its electrocatalytic function in a prototype electrolytic cell in 1 M KOH at room temperature. The overall water splitting performance of GDY@NiFe LDH/CF bifunctional catalyst exhibits more superior electrolytic performance than GR@NiFe LDH/CF, GDY/CF, GDY@RuO2/CF and NiFe LDH/CF catalysts with a current density of 20 mA cm2 at 1.512 V (Fig. 5a). The long-term stability of this system was also tested for 10 h in 1.0 M KOH. A potential of 1.5 V is observed at the beginning. Subsequently, the potential around 1.46 V is steady after 10 h electrolysis test (Fig. 5b). The GDY@NiFe LDH/CF heterostructures still maintain its morphology and integration after long-term water electrolysis (Fig. S10, Supporting

Information). It is considerable that our catalyst is equivalent with other bifunctional electrocatalysts previously reported [37] (Supplementary Table 3). Furthermore, a solar power assisted water-splitting device was build for the real application. We could clearly observe the evolution of both oxygen and hydrogen bubbles by powering with a 1.5 V solar panel under the sun which could be potentially applied in distributed energy storage technologies (Fig. 5b, inset and Movie S1, Supporting Information). Supplementary video related to this article can be found at https://doi.org/10.1016/j.jallcom.2019.04.150. In summary, an engineer design to construct a 3D hierarchical GDY@NiFe LDH/CF heterostructure through a facile two-step method by using GDY/CF as both robust 3D porous skeleton and catalyst is presented for overall water splitting. The as-prepared GDY@NiFe LDH/CF combines the high OER catalytic activity of the FeNi LDH as well as the remarkable HER enhancement together with the charge conducting of GDY synergistically. The experimental results are consistent with the DFT calculation interface data. A solar panel of only 1.5 V assisted water-splitting device was constructed to reveal the as-prepared GDY@NiFe LDH/CF hybrid materials as bifunctional catalysts for both OER and HER. The

Fig. 5. a) Linear sweeping voltammetry curve of water electrolysis for different electrocatalysts in a two-electrode configuration. b) Chronopotentiometric curve water electrolysis for GDY@NiFe LDH/CF in a two electrode system at a current density of 20 mA cm2. c) Designing a water-splitting device assisted by a solar power with the outputted voltage of 1.5 V.

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