Graphdiyne-Supported NiFe Layered Double Hydroxide Nanosheets

May 16, 2018 - The precursor HEB was then obtained after evaporation. The HEB was directly dissolved in pyridine, then added slowly into a mixed solut...
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Graphdiyne-Supported NiFe Layered Double Hydroxide Nanosheets as Functional Electrocatalysts for Oxygen Evolution Guodong Shi,† Cong Yu,†,§ Zixiong Fan,† Junbo Li,§ and Mingjian Yuan*,†,‡ †

Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of Chemistry, Nankai University, Tianjin 300071, P. R. China ‡ Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300071, P. R. China § School of Chemistry and Environmental Engineering, Wuhan Institute of Technology, Xiongchu Avenue, Wuhan 430073, P. R. China S Supporting Information *

ABSTRACT: Graphdiyne (GDY), a novel two-dimensional fullcarbon material, has attracted lots of attention because of its high conjugated system comprising sp2 and sp-hybridized carbons. The distinctive structure characteristics endow it unique electronic structure, uniform distributed pores and excellent chemical stability. A novel GDY-supported NiFe layered double hydroxide (LDH) composite was successfully prepared for the first time. By taking advantage of the increased surface active areas and improved conductivity, the designed hierarchical GDY@NiFe composite exhibits outstanding catalytic activity that only required a small overpotential about 260 mV to achieve the current density of 10 mA cm−2. The nanocomposite shows excellent durability in alkaline medium implying a superior OER electrocatalytic activity. It is anticipated that the as-prepared GDY@NiFe composite electrocatalyst provide new insights in designing graphdiyne-supported electrocatalyst materials for oxygen evolution application. KEYWORDS: full-carbon material, graphdiyne, NiFe LDH, electrocatalyst, oxygen evolution

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INTRODUCTION Electrocatalytic water splitting is considered as a promising approach to generate clean energy hydrogen.1−3 In the electrolysis, oxygen evolution reaction (OER), also called water oxidation, is an important process. This process is widely considered as the major barrier of overall water splitting owing to the inactive four-proton-coupled electron transfer and kinetic energy uphill.4 Consequently, tremendous efforts have been devoted to developing efficient electrocatalysts to expedite kinetics and reduce the overpotential of the OER process. As far as we know, the state-of−the-art catalyst for OER, ruthenium dioxide (RuO2) or iridium dioxide (IrO2), still need overpotential of about 300 mV to obtain the current density of 10 mA cm−2. Furthermore, the price and scarceness of these noble metals bring many difficulties for their further scale-up application.5−9 Thus, it is necessary and important to find robust and stable OER electrocatalysts from earthabundant and low-cost resources.10 During the past decades, numerous efforts have been made to develop effective OER catalysts using earth-rich elements, such as phosphides,11−13 chalcogenides,14−16 and transition metal oxide.17−19 Among them, transition metal layered double hydroxides (LDHs), which are made up of positively charged brucite-like host layers, and hydrated exchangeable anions that © XXXX American Chemical Society

located in the interlayer gallery for charge balancing, have been regarded as promising electrocatalyst for OER. In general, LDHs possess two-dimensional (2D) layered structure featured with water molecules and anions species that situated in the interlayers of metal cations. Herein, the anions and water molecules locating in the layers lead to a wider interlayer space. The unique material structure offered LDHs extraordinary redox properties, which are suitable for OER catalytic process.20−23 Among of these LDHs, nickel−iron LDH has been intensively investigated due to its highest catalytic oxidation activity.24 For example, Grätzel et al. reported 3D Ni foam-supported NiFe LDH as ascendant OER electrocatalyst for the first time in 2014.25 Hu et al. demonstrated single-layer NiFe LDH exhibiting superior OER performance compared with NiCo and CoCo LDHs, that fabricated through liquid phase exfoliation.26 However, obstacles still exist for further improving OER catalytic activity of NiFe LDH due to its inherent relative-low conductivity. Keeping this in mind, Special Issue: Graphdiyne Materials: Preparation, Structure, and Function Received: February 26, 2018 Accepted: May 9, 2018

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DOI: 10.1021/acsami.8b03345 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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several strategies have been proposed: (1) inlaid additional metal atoms into the structure of pristine catalysts;27,28 (2) hybrid with other conductive materials like graphene or nanocarbon;29,30 (3) growth of NiFe array on current transmitter.31,32 For example, Au nanoparticles modified NiFe LDH nanoarrays with ascended conductivity and enhanced oxygen evolution performance was successfully demonstrated by Wang group.33 Furthermore, Yang et al. combined NiFe LDH and graphene nanosheets through electrostatic attraction, obtained an excellent OER catalytic properties material with overpotential as low as 210 mV.29 Very recently, Yu et al. reported a 3D NiFe LDH covered Cu nanowire catalyst with a favorable electrocatalytic activity for both HER and OER, in where Cu nanowires acted as the effective current collector and NiFe LDH served as the reactive substance. The Cu@NiFe electrode required low overpotentials for OER, 199 and 281 mV, respectively, to achieve the current density of 10 and 100 mA cm−2.34 Obviously, the strategy aiming for improving the inherent electrical conductivity significantly truly boost the OER activity of catalysts. Carbon materials are promising for electrochemical application due to its intriguing properties, i.e., large specific surface areas, outstanding electrical conductivity, good thermal stability and excellent mechanical behavior. As a novel two-dimensional full-carbon materials, Graphdiyne (GDY) has attracted lots of attention since it was first synthesized via a facile coupling reaction method by Li group in 2010.35 GDY theoretically possesses a unique layered structure featuring with single-atom thick layer of sp- and sp2-hybridized carbon atoms. The benzene rings in GDY structure bonding together with diacetyllenic linkages, leading to a planar porous structure, which are favorable for the exposure of numerous accessible active sites and ensure smooth process of mass transport. Furthermore, GDY possesses excellent electrical conductivity benefiting from the characteristic electronic structure of sp and sp2 delocalized carbon hybrid π-conjugated network, lead to enhancement of electrochemical performance.36−38 Up to now, GDY has been widely explored in various fields, such as energy storage,39−42 photodetectors,43 water desalination44 and catalysis.45 Substantially, GDY was proved to be a brilliant cocatalyst in electrocatalysis. For instance, GDY supported NiCo2S4 nanowire as a useful 3D bifunctional catalyst for overall water splitting was reported by Li et al. The as-prepared bifunctional catalyst achieved a current density of 10 mA cm−2 at 1.53 V in alkaline solution.46 Wu et al. employed GDY as supporting electrocatalyst to demonstrate it can stabilize cobalt nanoparticles for oxygen evolution. By taking advantage of the enhanced active sites and electric conductivity, the as-prepared catalyst exhibited an excellent OER activity with a small overpotential about 300 mV.47 Lately, Huang and Long designed N-doped porous graphdiyne, which exhibited good performance for oxygen reduction reaction.48 Provoked by the aforementioned study, herein we integrate GDY and NiFe LDH to construct a novel electrocatalyst on copper foil for effective oxygen evolution reaction. The designed GDY@NiFe composite represent highly improved activity toward OER in alkaline medium. To the best of our knowledge, this is the first time to combine NiFe LDH with GDY via electrodeposition method for the improvement of electrocatalytic activity of oxygen evolution reaction. We believe that this kind of GDY@NiFe could bring the light on the devise and fabrication of novel GDY-based composite catalysts for electrocatalytic reaction.

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EXPERIMENTAL SECTION

Preparation of GDY. GDY was synthesized on the surface of copper according to the reported method.46 In a typical procedure, the solution of 200 mg of hexakis[(trimethylsilyl) ethynyl]benzene (HEBTMS) in 30 mL of tetrahydrofuran (THF) was added into 1.5 mL of tetrabutylammonium fluoride (TBAF) and stirred at 0 °C for 8 min. Then the mixture was diluted with ethyl acetate and washed with brine for 3 times, and dried with MgSO4. The precursor HEB was then obtained after evaporation. The HEB was directly dissolved in pyridine, then added slowly into a mixed solution of copper foils and 30 mL of pyridine in a nitrogen atmosphere. The mixture was maintained at 100 °C for another 72 h. After quenching the reaction, copper foils were washed with hot acetone and DMF several times. A black graphdiyne film was obtained on the copper foil. Preparation of GDY@NiFe. GDY supported NiFe LDH was synthesized via an electrochemical deposition method. In a threeelectrode system, Pt foil, saturated calomel electrode (SCE) and the prepared GDY on copper foil were used as counter electrode, reference electrode and working electrode, respectively. The electrolyte was achieved by dissolving 0.15 M of Ni(NO3)2·6H2O and 0.15 M of FeSO4·7H2O in 100 mL of deionized water under the argon atmosphere to avoid Fe2+ to Fe3+. Subsequently, electrodeposition was conducted at a constant potential of −1.0 V vs SCE, the electrodeposition time lasted for 90 s. Then the samples were washed with deionized water for three times, dried at room temperature for self-oxidation of Fe2+ to Fe3+. Pure NiFe LDH was fabricated through the same method, but using clean copper foil as working electrode. Preparation of Graphene Oxide (GO)@NiFe LDH. GO was synthesized by modified Hummers method. The prepared GO solution (5 mg mL−1) was added into a mixture solution which contained 0.4 mL of 1 M FeCl3, 1.2 mL of 1 M NiCl2, and 63 mL of distilled water. Subsequently, 5.9 mg of sodium citrate and 16.8 mg of urea added into the mixture. Then the solution was stirred for 30 min and heated up to 150 °C for 24 h. The product was then collected through centrifugation and washed with distilled water. Electrochemical Measurements. Electrochemical measurements were carried out on an electrochemical working station (CHI660E, Shanghai) in a standard three electrode system. The fabricated catalysts were employed as working electrodes, platinum foil and SCE electrode were used as counter electrode, reference electrode, respectively in 1.0 M KOH solution. The OER catalytic activity was investigated using linear sweep voltammetry (LSV) at a scan rate of 2 mV s−1 in O2-saturated KOH solution (1.0 M) . The Tafel slope was calculated from the Tafel equation: η = b log j + a, where j represents current density; a is the constant; and b stands for the Tafel slope. Electrochemical impedance spectroscopy (EIS) was measured using the frequency ranging from 0.1 Hz to 100 kHz. Cyclic voltammetry (CV) was carried out in a nonfaradaic region of the voltammogram with different scan rate in O2-saturated 1.0 M KOH to evaluate the double-layer capacitance. All of the above potentials were converted to RHE according to the equation (ENHE = ESCE + E0SCE + 0.0591pH), for 1.0 M KOH, ENHE = ESCE + 1.068 V. All the curves were recorded without iR-correction and the current densities were recorded with respect to the geometric area of an electrode. Materials Characterization. The morphology of the samples were observed using a field emission scanning electron microscopy (FESEM, JEOL JSM-7500F) and a high-resolution transmission electron microscopy (Tecnai G2 F20 S-TWIN(200 kV)). The crystalline phases were identified by using X-ray diffractometer (XRD, D/max-2500). X-ray photoelectron spectroscopy (XPS) was conducted on an Axis Ultra DLD (Kratos Analytical Ltd. UK). The banding energies were calibrated to the C 1s (284.6 eV). Raman spectra was used to provide more -structure information by employing a LabRAMHR Raman spectrometer with Ar+ (532 nm) laser excitation. Inductively coupled plasma-optical emission spectroscopy (ICP-OES) was carried out on X7 (Thermo Electron Corporation). B

DOI: 10.1021/acsami.8b03345 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. (a) Schematic illustration for the formation of GDY@NiFe architectures. SEM images of (b) pure Cu foil, (c) GDY, (d) NiFe LDH, and (e) the GDY@NiFe composite.

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RESULTS AND DISCUSSION Figure 1a showed the fabrication process for GDY@NiFe composite. The first step was synthesizing GDY via a crosscoupling reaction method. Subsequently, NiFe LDH nanosheets were electrodeposited onto the GDY substrate. Highmagnification scanning electron microscopy (SEM, Figure 1b− e) was employed to investigate the morphology of the all prepared samples. Compared with pure copper foil (Figure 1b), the GDY grown on the surface of copper foil exhibited a porous, fluffy structure (Figure 1c). The porous structure offered enhanced active surface area and suitable condition for subsequent growth of NiFe LDH nanosheets. Furthermore, as a full carbon material, GDY also act as a good conductive bridge to bind copper substrate and NiFe LDH in the later electrodeposition process. This phenomenon was in good agreement with the previous report.46 The low magnification SEM image for GDY was showed in Figure S1b. The NiFe LDH array showed a randomly stacked nanowall-like structure (Figure 1d) providing abundant of catalytic sites for oxygen evolution. The interlaced nanosheets formed a porous structure, that facilitating of the electrolyte diffusion and gaseous product release during the process of electrocatalytic oxygen evolution. Low-magnification SEM image of NiFe LDH (Figure S1c) also presented a uniform coverage. As showed in Figure 1e, the NiFe LDH nanosheets vertically and uniformly grown on the surface of GDY with reserved morphology demonstrated a hierarchical structure. In the lower right corner of the image, the GDY substrate was clearly observed. The lowmagnification SEM image of NiFe LDH@GDY (Figure S1d) also showed the unbroken nanosheets structure, indicating uniform and effective interconnected behavior between GDY substrate and NiFe LDH. Similar structure that NiCo LDH anchored on the surface of graphene was reported by Hu et al.49 Thus, It is also reasonable to believe that the direct interconnect between GDY and NiFe LDH, which could effectively reduce the collector resistance, improve the electron transport properties and lead to a good performance for oxygen evolution. The structure information on pristine GDY powder, NiFe LDH and GDY@NiFe composite was identified by X-ray diffraction (XRD Figure 2a). It could be observed that in pristine GDY, a broad peak positioned at 23° could be assigned to plane (002) of graphite-type carbon, corresponding to the previous report.50 For both NiFe LDH and GDY@NiFe composite, three strong peaks positioned at 43°, 50° and 74° were assigned to plane (111) (200) and (220) of cooper, respectively. Only the characteristic peak of (006) plane of

Figure 2. (a) XRD patterns for GDY powder, NiFe LDH and GDY@ NiFe; (b, c) TEM results of GDY@NiFe in low magnification, the insets are the enlarged image of labeled zone.

NiFe LDH was represented and no well-defined diffraction peak of GDY for GDY@NiFe sample was observed25 because of the strong diffraction influence of Cu substrate and the low crystallization of GDY on the surface of Cu foil. In order to further investigate the structural characteristic of the prepared sample, GDY@NiFe composite with ultrasonic-treated was also characterized through transmission electron microscopy (TEM). For the low magnification images (Figure 2b and c), it can be obviously seen that distinct petal-like NiFe LDH nanosheets with randomly interconnected structure were anchored on the substrate, in agreement with the SEM results. In addition, NiFe LDH exhibited a very thin morphology, and thus providing numerous active sites for hydroxyl radicals adsorption. Furthermore, the formed interconnecting nanosheets result in a channels structure that will be helpful for the electrolyte diffusion and gaseous products release. In highresolution TEM images (insets in Figure 2c), characteristic lattice fringes were observed in both NiFe LDH and GDY substrate. The lattice fringe with interplanar spacing of 0.25 nm was assigned to the (012) plane of NiFe LDH and the interlayer spacing of GDY layer was about 0.365 nm.51,52 It was worth to note that NiFe LDH remained integrity and anchored on the surface of GDY closely after a long time ultrasonic process. Those results indicate the intimate contact between NiFe LDH and GDY substrate, which would form an C

DOI: 10.1021/acsami.8b03345 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 3. (a) Full X-ray photoelectron spectroscopy (XPS) spectra of GDY@NiFe composite; high-resolution spectra of (b) C 1s, (c) Ni 2p, and (d) Fe 2p for GDY@NiFe electrode.

Figure 4. OER performance conducted in KOH solution (1.0 M). (a) Polarization curves for all electrode with a scan of 2 mV s−1. (b) Overpotentials at the current density of 10 (pink) and 100 mA cm−2 (blue). (c) Corresponding Tafel curves. (d) Chronopotentiometry curves of GDY@NiFe at a constant overpotential of 1.52 versus RHE for 6 h, the inset is the polarization curve of GDY@NiFe before and after 6 h test.

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DOI: 10.1021/acsami.8b03345 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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LDH and pristine GDY to reach the same current density, respectively. For a straightforward comparison, Figure 4b showed the overpotentials at 10 and 100 mA cm−2 for the three electrodes. For those two current densities, GDY@NiFe clearly showed the minimum overpotential among the various electrodes. The quantity of NiFe deposited on the GDY@ NiFe was measured by ICP-OES to be 1.03 mg cm−2, which was similar to that of NiFe LDH (1.13 mg cm−2). We further evaluated the mass activity of NiFe and GDY@NiFe sample at 1.55 V. As shown in Figure S4, the mass activity of GDY@NiFe was calculated to be ∼45 mA mg−1, higher than that of NiFe sample (∼21 mA mg−1). Additionally, the activity of NiFe LDH with different electrodeposition time was also studied and presented in Figure S5. The NiFe LDH with the electrodepostion time of 90 s, exhibited a better activity than the two others. Furthermore, the corresponding Tafel curve were shown in Figure 4c. GDY@NiFe revealed the smallest Tafel slope of 95 mV dec−1 among NiFe LDH (149 mV dec−1) and GDY (282 mV dec−1). There explicitly represents the inherent excellent activity toward OER for GDY@NiFe. In order to further illustrate the advantage of GDY, the graphene oxide (GO) supported NiFe LDH was prepared and employed as a working electrode in OER process. The SEM image and OER polarization of GO@NiFe LDH were shown in Figure S6. To achieve a current density of 10 mA cm−2, GO@NiFe LDH needed an overpotential of about 320 mV, which was 60 mV more than that of GDY@NiFe. Additionally, stability is an important factor to evaluate a catalyst. A long time stability test of GDY@NiFe was performed at a constant potential of 1.52 V vs RHE. As displayed in Figure 4d, it can be clearly seen that during a 6 h test, GDY@NiFe maintained a stable current density, indicating the remarkable stability. The inset in Figure 4d demonstrated the comparison of polarization curves for GDY@NiFe before and after 6 h electrolysis. Likewise, no obvious variation was observed in the Polarization curves for GDY@NiFe. The slight difference existed in figure also revealed the remarkable stability of GDY@NiFe sample in a long time electrochemical process. Moreover, the SEM image of GDY@NiFe after 6 h electrochemical test was also examined. As presented in Figure S7, compared to the fresh GDY@NiFe, the GDY substrate became chapped and the supernatant NiFe LDH nanosheets were aggregated partially. But it is worth noted that the supernatant NiFe LDH was still in close contact with the GDY substrate even though the existence of cracks. The results indicate that the smooth connectivity between GDY and NiFe LDH, which provides a favorable condition for the rapid electron transport in the process of electrocatalytic reaction. For comparison, the stability test of pristine NiFe LDH conducted at a constant potential was displayed in Figure S8a. As the reaction goes on, the current density declines appreciably. Furthermore, from the SEM image (Figure S8b), an aggregation of fragments were appeared after the stability test, indicating the inferior stability of pristine NiFe LDH. In addition, the oxygen products from electrolysis process by GDY@NiFe electrode was also detected by using gas chromatography. As shown in Figure S9, The total of detected oxygen fitted well with the calculated result, demonstrating an approximately 100% Faradaic efficiency. Electrochemical impedance spectra (EIS) and the cyclic voltammetry (CV) were carried out. EIS measurement is considered as an effective mean to assess the kinetics of charge transfer in the hybrid composites.46 For comparison, the EIS of other samples were measured and the equivalent circuit model

advantageous electron transport channel, ensure a rapid transport of electrons between NiFe LDH catalyst and GDY current collectors in the process of oxygen evolution. To further study the chemical state and composition of the samples, X-ray photoelectron spectroscopy (XPS) was employed. As showed in Figure 3a, the signals presented in the full XPS spectra of GDY@NiFe confirmed the existence of C, O, Ni and Fe elements in the composite. Figure 3b−d showed the XPS spectrum of C, Ni and Fe, respectively. As shown in Figure 3b, the C 1s orbits of the GDY@NiFe consist of four sub peaks located at 284.4, 285.1, 286.5, and 288.7 eV, assigning to sp2, sp, C−O, and CO, respectively. The partial oxidation of some terminal alkyne is the reason why the presence of oxygen-containing carbon species in the composite, in accordance with previous reports.47,53 As displayed in Figure 3c, the two peaks positioned at 873.4 and 855.6 eV belong to Ni 2p1/2 and Ni 2p3/2, respectively, which indicate the Ni2+ in NiFe LDH. The two satellite peaks of Ni were also found. Similarly, in Fe 2p region (Figure 3d), binding energies of Fe 2p1/2 and Fe 2p3/2 located at 725.8 and 712.6 eV correspond to the Fe3+ species in NiFe LDH.34 Those above results demonstrate the successful synthesis of NiFe LDH on the surface of GDY. Raman spectroscopy was further employed to study the prepared samples. Figure S2 displayed the Raman spectra of pristine GDY and GDY@NiFe composite. For pristine GDY, two prominent peaks around 1382 and 1569 cm−1 were observed, and were ascribed to the D band and G band, respectively. Moreover, two characterization peaks located at 1926 and 2189 cm−1 correspond to the vibration of conjugated diyne links (CCCC), which were in accordance with previous reported work.54 After electrodeposition of NiFe LDH, as shown in Figure S2b, the main peaks of the GDY were clearly presented at 1377, 1557, 1959, and 2139 cm−1, corresponding to the D band, G band, and characteristic peak of the conjugated diyne links, respectively, whereas the peaks at 476 and 547 cm−1 corresponded to the Ni2+ONi2+ in Ni(OH)2 and Fe3+OFe3+ due to the introduction of Fe.30 The XPS and Raman results further confirmed the successful fabrication of expected GDY@NiFe composite. To investigate the property of GDY@NiFe for oxygen evolution reduction, the linear sweep voltammetry (LSV) polarization curves were first conducted at a sweeping rate of 2 mV s−1 in 1.0 M KOH solutions. In Figure 4a, the pristine GDY displayed a negligible increase of anodic current as the increase of applied potential, revealing the intrinsic inertia of GDY itself in terms of the oxygen evolution. This result was in conformity with the previous report that used graphdiyne-supported NiCo2S4 nanowires or cobalt nanoparticles as the catalysts for oxygen evolution.46,47 For comparison, the OER activity of copper foil was also measured and no apparent anodic current was observed (Figure S3). In comparison with the current curve of pristine GDY, the NiFe LDH showed a fast-growing current density with the increased potential, which was attributed to the brilliant inherent electrocatalytic activity for oxygen evolution. As expected, the GDY@NiFe exhibited the optimal current response with the minimum onset potential, which meant a bright performance than NiFe LDH and pristine GDY. The remarkable activity of GDY@NiFe was further identified by its lower overpotential values recorded at the same current densities. It needs only 260 mV overpotential for GDY@NiFe to reach the current density of 10 mA cm−2. In contrast, 280 and 460 mV overpotantial are required for NiFe E

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Figure 5. (a) Nyquist plots in KOH solution for the as-prepared samples. (b) Capacitive currents at 0.975 V versus RHE as a function of scan rates for GDY@NiFe, NiFe LDH and GDY. The inset in a is the detailed EIS curve of the samples and that in b is CV curve of GDY@NiFe electrode.

active sites for oxygen evolution. (4) The intrinsically superior activity toward OER of NiFe LDH originates from effective diffusion of water and fast gaseous product release.34 (5) The increased electrochemical surface area is measured by the capacitances of the double layers, indicating a larger active area and more catalytic sites after combining NiFe LDH with GDY.56 Therefore, it is reasonable to say that GDY@NiFe exhibits a preeminent performance for oxygen evolution. Therefore, from those discussion, a schematic illustration of oxygen evolution over GDY@NiFe composite was displayed in Scheme 1. In brief, prepared GDY acted as an electrical bridge

was presented in Figure S10. As shown in Figure 5a, it could be clearly observed that GDY@NiFe had the smallest semicircle radii among all samples. A more clear comparison of impedance could be seen in the inset. It is well-known that the radius at high frequency area is regarded as relating to charge transfer and a smaller radius means a lower resistance. Because the diameter of the semicircle of GDY@NiFe was much smaller than others, this composite possesses a desirable efficiency of charge transfer, which could own a significant contribution for oxygen evolution reduction. The EIS also confirmed the favorable electrical contacts between NiFe LDH and GDY substrate in accordance with the SEM results. In addition, CV measurements were performed to verify the double layer capacitance. The capacitance was thought to be proportional to electrochemical active surface area (ECSA).55 The inset in Figure 5b showed the CV curves of GDY@NiFe composite measured at the scanning rate from 10 to 100 mV s−1 and CVs of GDY and NiFe LDH were also recorded in Figure S11. The GDY@NiFe exhibited a Cdl of 61.2 mF cm−2, which was much higher than that of GDY (2.71 mF cm−2) and NiFe LDH (9.4 mF cm−2). The significantly increased Cdl value indicates the GDY@NiFe composite possessing the largest catalytic active surface area along with the maximum exposure of active sites among three materials. Therefore, the significantly enhanced OER catalytic activities of the GDY@NiFe could be ascribed to the increased ECSA, which could be beneficial to the water molecule adsorption, intimate contacting with the electrolyte and exposure of electrocatalytic activity sites. Some logical speculations may account for the extraordinary catalytic performance of GDY@NiFe toward oxygen evolution. (1) Li et al. reported that the GDY served as a highly conductive supporting matrix in the composite electrocatalyst.46 As a carbon material, GDY possesses the unique electronic structure and electric conductivity, leading to a rapid electron transfer during the process of OER. (2) The NiFe LDH rooted into the GDY directly without any polymer binder, results in a good mechanical adhesion and electrical contacts. This close contact is favorable for decreasing contact resistance and charge-transfer resistance, further accelerating the reaction kinetics, which are in accordance with the EIS results. (3) The NiFe LDH nanosheets grown on the GDY substrate directly, lead to a multistage decentralized architecture, will largely expose abundant edges and provide abundant

Scheme 1. Schematic Diagram of the Proposed Process of OER on the GDY@NiFe Sample

to anchor NiFe LDH, established a high speed electronic transfer path. The protons then migrated to the active sites of NiFe LDH for the oxygen evolution reduction.

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CONCLUSIONS In summary, the first GDY-supported NiFe LDH electrocatalyst was successfully synthesized via a coupling reaction and subsequent electrodeposition methods. Such an integrated and synergetic electrode exhibited brilliant electrocatalytic activity for oxygen evolution in alkaline electrolytes. The designed hierarchical GDY@NiFe composite not only enhances surface active areas, but also improves electronic conductivity, resulting in a significantly enhancement of OER activity with a small overpotential of 260 mV at the current density of 10 mA cm−2 and a good stability over 6 h after continuous electrolysis F

DOI: 10.1021/acsami.8b03345 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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operation at 1.52 V. The prepared composites with admirable activity for oxygen evolution is accomplished by the synergetic effect of GDY and NiFe LDH. It is predicted that the GDY@ NiFe may give us more ideas for GDY-supported electrocatalysts applied in oxygen evolution reaction and further overall water splitting.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b03345. Low-magnification SEM images of prepared samples, Raman spectrum of GDY and GDY@NiFe, OER performance of GO@NiFe, SEM results of GDY@ NiFe after stability test, CV curves of prepared catalysts, equivalent circuit and faradaic efficiency of GDY@NiFe (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Mingjian Yuan: 0000-0002-2790-9172 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The author acknowledged financial support from the National Natural Science Foundation of China (21771114) and Natural Science Foundation of Tianjin (17JCYBJC40900). M.Y. thanks to the financial support from “Thousand Youth Talents Plan of China”.

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REFERENCES

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DOI: 10.1021/acsami.8b03345 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX