Bridging of Ultrathin NiCo2O4 Nanosheets and Graphene with

Jul 26, 2016 - Ultrathin inorganic nanosheets that enable fast electrochemical reaction kinetics are highly required in many energy-related applicatio...
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Bridging of Ultrathin NiCo2O4 Nanosheets and Graphene with Polyaniline: A Theoretical and Experimental Study Juan Yang,† Chang Yu,† Suxia Liang,†,‡ Shaofeng Li,† Huawei Huang,† Xiaotong Han,† Changtai Zhao,† Xuedan Song,†,‡ Ce Hao,†,‡ Pulickel M. Ajayan,§ and Jieshan Qiu*,† †

State Key Lab of Fine Chemicals, Liaoning Key Lab for Energy Materials and Chemical Engineering, PSU-DUT Joint Center for Energy Research, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China ‡ School of Chemistry, Dalian University of Technology, Dalian 116024, China § Department of Materials Science and NanoEngineering, Rice University, Houston, Texas 77005, United States S Supporting Information *

ABSTRACT: Ultrathin inorganic nanosheets that enable fast electrochemical reaction kinetics are highly required in many energy-related applications. Herein, we report a simple strategy for in situ assembly of ultrathin NiCo2O4 nanosheets with enriched surface active sites on graphene surface in a vertical orientation way by employing polyaniline (PANI) as the structure coupling bridge between the two components (denoted by NiCo2O4−P-G). The as-made ultrathin NiCo2O4 nanosheets are rich in metal ions in high valence state and oxygen defective sites, and feature 3D open frameworks with hierarchical pore structure. It has been found that the nitrogen species derived from PANI building blocks as bridging sites tend to bond with metal ions, which effectively tune the electronic structural states and result in strong coupling effects with the NiCo2O4 nanosheets. Benefiting from these structural characteristics, the as-made NiCo2O4−P-G hybrids, when used as pseudocapacitive electrode materials, can deliver a high specific capacitance of 966 F g−1 (based on the mass of the active NiCo2O4 component) and an excellent rate capability of ca. 84% even the current density increased by 100 times and long-term stability. As the precious metal-free electrocatalyst for the oxygen evolution (OER) reaction, the NiCo2O4−P-G hybrids are also able to deliver a low overpotential of 0.32 V at a current density of 10 mA cm−2 in 0.1 M KOH aqueous electrolyte (only 70% iR compensation), holding promise for high performance yet cheap electrocatalysts for the OER reaction.

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INTRODUCTION The ever-increasing sustainable energy demands have become one of the key issues that lead to the development of electrochemical technologies for efficient energy storage and conversion.1−5 The surface-electrochemical reactions involved in these energy systems including pseudocapacitors (Faradaic redox reaction)6−8 and/or electrocatalytic water splitting (oxygen evolution reaction, OER)9−11 are one of the top concerns to be tackled. It is well-known that these electrochemical reactions usually involve a complicated multi-ion and multielectron transfer process which is limited greatly by the sluggish reaction kinetics and thus results in efficiency of the corresponding devices below expectation. To address these bottleneck problems, one needs to design and fabricate novel materials with tuned structure and high performance such as by incorporating and assembling two-dimensional (2D) inorganic nanosheets on high conductive nanocarbons, for example, by intercalation of 2D sp2-hybridized graphene with inorganic nanohybrids.12−16 These kinds of hybrids have a short distance for ion transport and a high conductivity, which helps to improve the electrochemical reaction kinetics involved in the electrode matrix, which has been demonstrated by some engineered nanohybrids made of inorganic nanosheets with © XXXX American Chemical Society

molecular-scale thickness bridged on the graphene, such as MoS 2 /graphene,17 MnO 2 /graphene,16,18 NiFe-LDH/graphene,19 and Ni(OH)2/graphene.20 Nevertheless, the coupling scheme and the interaction between inorganic nanosheets and graphene still need to be tuned precisely and conveniently, to ensure the electrolyte ions can get access to the surface and pores in the inorganic nanosheets with tuned layer number and/or thickness. The inorganic nanosheets grown on metal or graphene substrates in a vertical orientation way recently hold a significant promise in many energy-related applications.21−24 It is believed that there are a large number of exposed edges as contact points to easily bridge the current collector and further enhance the electron trapping capability. Moreover, such a configuration also exhibits high surface area and 3D open frameworks that enable efficient electrolyte ions transport. Recently, mixed transition metal oxide nanosheets with ultrathin features, typically binary metal oxides with two different metal cations, e.g., NiCo2O4, are a very promising Received: June 7, 2016 Revised: July 12, 2016

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Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). N2 adsorption/ desorption isotherms of the samples were measured at −196 °C using an ASAP 2020 (Micromeritics). Thermogravimetric analysis (TGA) was carried out using a STA 449 F3 instrument in a temperature range of 30−750 °C with the heating rate of 10 °C min−1 in air. Electrochemical Characterization. For supercapacitors, the electrochemical properties of the samples were evaluated by a CHI 760D electrochemical workstation in a three-electrode cell with 6 M KOH aqueous electrolyte, in which platinum foil and Hg/HgO electrodes were used as the counter and reference electrodes, respectively. The working electrode was fabricated by mixing the sample powder, carbon black, and poly(vinylidene fluoride) in a mass ratio of 80:10:10 by adding a small amount of N-methyl-2-pyrrolidone, followed by ultrasonic treatment. Then, the mixture was coated on the current collectors and dried in a vacuum oven at 120 °C overnight. The mass loading of the samples, e.g., areal density of the electrode, is about 1 mg cm−2 for cyclic voltammetry (CV) and galvanostatic charge/discharge measurements. The specific capacitance of electrode was calculated by the following formula:

electrode material since it offers many intriguing advantages including rich surface active sites, relatively high electronic conductivity, and excellent electrochemical activity.15,25−28 Although these advance, it is still a main challenge to assemble the ultrathin NiCo2O4 nanosheets on high-stability graphene surface in a vertical orientation way because of the general incompatibility between graphene and NiCo2O4 species as well as the difficulties in controlling the surface properties of graphene. In this study, inspired by the polymer molecules functionalizing graphene and inorganic materials,29 we report a simple yet efficient strategy for in situ assembly of ultrathin NiCo2O4 nanosheets with the enriched surface active sites on graphene in a vertical orientation way by employing polyaniline (PANI) as the structural coupling bridge between the two components (denoted by NiCo2O4−P-G). The as-obtained ultrathin NiCo2O4 nanosheets are rich in metal ions in high valence state and oxygen defective sites and feature 3D open frameworks with hierarchical porous structure. Such a unique structure endows the NiCo2O4−P-G hybrids with numerous electroactive sites and channels for the access of electrolyte ions and transport of electron, thus further leading to deliver the fast reaction kinetics. The experimental and density-functional theory (DFT) investigations further reveal that the nitrogen species derived from PANI building blocks as bridging sites tend to bond with metal ions, which effectively tune the electronic structural states and result in strong coupling effects with the NiCo2O4 nanosheets. Benefiting from these structural characteristics, the as-made NiCo2O4−P-G nanohybrids, when used as the pseudocapacitive electrode, can deliver a high specific capacitance and excellent rate capability. Moreover, the NiCo2O4−P-G hybrids are also able to be a superior precious metal-free elctrocatalyst for the oxygen evolution reaction (OER).

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Cs = (I Δt )/(mΔV ) where Cs is specific capacitance of electrodes (F g−1), I is the discharge current (A), Δt is the discharge time (s), ΔV is the voltage range after ohmic drop (V), and m is the mass of the active material (g). It is noted that the calculated specific capacitances for all nanohybrids in this work are based on the total mass of the hybrids except for special instruction. For OER, the electrochemical measurements were carried out in standard three electrode systems with 0.1 M KOH aqueous electrolyte, where Pt wire and Ag/AgCl were used as counter and reference electrodes, respectively. The fabrication process of the working electrode is as follows: 4 mg of catalyst powder was dispersed in 985 μL of ethanol and 15 μL of 5 wt % Nafion solution by sonication for at least 10 min to form a homogeneous ink. Then, 10 μL of the catalyst ink was loaded onto a glassy carbon rotating disk electrode (GC-RDE, PINE, 5 mm in diameter, 0.196 cm2), with a constant catalyst loading of 0.20 mg cm−2. The glassy carbon electrode was polished to a mirror finish and thoroughly cleaned before use. The asprepared catalyst film was dried at room temperature. In this work, the potentials were displayed versus the RHE by the calibration: E(RHE) = E(Ag/AgCl) + 0.059 × pH + 0.197, where the value of pH is 13 in 0.1 M KOH solution. During the measurements, the working electrode was constantly rotated at 1600 rpm to remove generated O2. Linear sweep voltammetry (LSV) was carried out at 10 mV s−1 for the polarization curves and Tafel plots. All polarization curves were corrected with 70% iR-compensation, which is an optimal compensation level to achieve best curve shape. The stability measurements were recorded via the chronopotentiometric method at a current density of 2 mA cm−2. DFT Calculations. The spin-polarized DFT calculations were performed using projector augmented wave (PAW) potentials and the Perdew−Burke−Ernzarhof (PBE) functional implemented in the Vienna ab initio simulation package (VASP).30−32 The NiCo2O4 (311) surface with three atomic layers was modeled as a p(1 × 3) periodic slab. Two benzene rings coupled with two amino-groups and seven benzene rings were modeled as PANI and graphene, respectively. The atoms in the bottom layer were fixed, and all other atoms were fully relaxed. The neighboring layers were separated in the direction perpendicular to the surface by a vacuum distance of 15 Å. Relaxations were carried out using the conjugate-gradient algorithm. The occupancy of the one-electron states was calculated using Gaussian smearing (SIGMA = 0.1 eV). The energy convergence was selected as 1 × 10−4 eV atom−1. Brillouin-zone integrations were performed using Monkhorst−Pack grids of special points with 2 × 2 × 1 meshes. The kinetic-energy cutoff of the plane wave was set to 400 eV.

EXPERIMENTAL SECTION

Fabrication of PANI Functionalized Graphene Oxide (GO) Nanosheets (P-G). For a typical run, aniline monomer solution (300 μL) and a certain amount of sodium dodecyl sulfate (SDS) were added into aqueous GO dispersion (0.30 mg mL−1, 100 mL), followed by vigorous stirring and ultrasonication for 30 min to form a homogeneous suspension. Then, ammonium persulfate (APS, 0.7 g) was dissolved into diluted HCl solution (1 M, 50 mL) and slowly added into the mixture above under vigorous stirring at 0 °C for 12 h. The as-synthesized P-G samples were collected and washed several times with deionized (DI) water and ethanol and then dried at 80 °C for 12 h. PANI-Mediated Vertically Oriented Assembly of Ultrathin NiCo2O4 Nanosheets on Graphene (NiCo2O4−P-G). In a typical procedure, the as-synthesized P-G (100 mg) in the first step was dispersed into water to form a composite dispersion (250 mL) under vigorous stirring and ultrasonication for 30 min, and then NiCl2·6H2O, CoCl2·6H2O (molar ratio of 1:2), and hexamethylenetetramine (HMT) were dissolved into composite solution with the total metal ion and HMT concentrations of 11.25 and 35 mM, respectively. Subsequently, the composite solution was transferred into a threenecked round-bottomed flask and refluxed at 100 °C under continuous magnetic stirring for 5 h. After reaction, the powder was collected by centrifugation and washed several times by water and annealed at 300 °C for 2 h in air to obtain NiCo2O4−P-G hybrids. For comparison, the NiCo2O4-G and pristine NiCo2O4 were also prepared under the same conditions in the absence of PANI and P-G, respectively. Material Characterization. The as-obtained samples were examined by field-emission scanning electron microscopy (FE-SEM, FEI NOVA NanoSEM 450), transmission electron microscopy (TEM, FEI Tecnai G2 F30), X-ray diffraction (XRD) (Cu Kα, λ = 1.5406 Å), B

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RESULTS AND DISCUSSION The overall strategy for in situ assembly of the ultrathin NiCo2O4 nanosheets with enriched surface active sites on graphene in a vertical orientation is illustrated in Scheme 1. The Scheme 1. Fabrication Process for In-Situ Growth and Assembly of Ultrathin NiCo2O4 Nanosheets with Enriched Surface Active Sites on Graphene in a Vertical Orientation Way via Employing PANI as the Structural Coupling Bridge, Followed by Annealing Treatments

GO was first coated with PANI to form 2D P-G hybrid sheets by in situ polymerization of aniline in a GO suspension.29 The surface charges of P-G sheets further were monitored by zeta potential measurements, of which the typical P-G sheets are negatively charged in alkaline solution (Figure S1). The reason for this is attributed to the nitrogen (N)- and oxygen (O)containing species derived from PANI (Table S1). Benefiting from these characteristics, the metal ions (Co2+ and Ni2+) could easily adsorb onto the P-G sheet by complexation and electrostatic interactions under alkaline environments. Remarkably, the PANI on GO surface further functions as the structural coupling bridge to mediate the heterogeneous nucleation, growth, thus tuning the assembly of the NiCohydroxide precursor nanosheets on the P-G surface. Afterward, the vertically oriented ultrathin NiCo2O4 nanosheets with enriched surface active sites and well-defined porous structures grown on the P-G surface can be obtained via a facile thermal annealing treatment in air. The surface chemical properties of GO and P-G were analyzed by FT-IR, the detailed results are shown in Figure S2. Compared with GO, the peaks at 1583 and 784 cm−1 are consistent with the profiles of PANI, corresponding to the vibration of −C-N, −N-H, respectively, which may act as bonding sites for the in situ assembly of 2D ultrathin NiCo2O4 nanosheets, and it is also noted that these N-containing species are still present on the P-G surface even though suffering from annealing treatment of 300 °C in air, evidenced by FT-IR, thermogravimetric analysis (TGA, Figure S3), and some previous literature.33,34 The FE-SEM and TEM images of the P-G (Figure S4a-b) reveal uniform 2D sheet structure with a smooth surface. The high magnification TEM images further reveal the homogeneous distribution of PANI on the GO surface (Figure S4c-d), indicative of strong coupling between PANI and GO via intermolecular interaction. When metal salts are added to the refluxing process followed by growth and annealing treatment, the crystallographic structure and phase purity of as-obtained NiCo2O4−P-G hybrids are examined by typical powder XRD as shown in Figure 1a. All of the

Figure 1. Structure characterization of the samples: (a) typical XRD patterns of the pristine NiCo2O4, NiCo2O4-G, and NiCo2O4−P-G nanohybrids; (b) Raman spectra of the P-G and NiCo2O4−P-G nanohybrids; (c) high-resolution XPS spectra of N 1s for the P-G and NiCo2O4−P-G nanohybrids; (d,e,f) high-resolution XPS spectra of Ni 2p, Co 2p, and O 1s of the pristine NiCo2O4, NiCo2O4-G, and NiCo2O4−P-G nanohybrids.

diffraction peaks in the XRD pattern can be well indexed to (111), (220), (311), (400), (511), and (440) plane reflections of the spinel NiCo2O4 phase (JCPDS Card no. 20-0781), which is consistent with that of the as-compared pristine NiCo2O4 and NiCo2O4-G sample. No diffraction peaks of the P-G can be observed in the NiCo2O4−P-G hybrids due to its relatively low diffraction intensity. Nevertheless, the presence of P-G can be evidenced by the Raman spectra (Figure 1b). It is also noted that compared with that of the pristine P-G, the N-containing characteristic bands of NiCo2O4−P-G hybrids for C−N and C−N+/CN slightly shift toward low (a shift of ca. 4 cm−1) and high Raman positions (a shift of 9−10 cm−1), respectively. This implies that there are strongly electronic interactions between P-G and NiCo2O4 species.35 In order to obtain more information about the surface electronic states of the asobtained samples, the NiCo2O4−P-G nanohybrids and the correspondingly compared samples (P-G, NiCo2O4, and NiCo2O4-G nanohybirds) were further examined by XPS spectroscopy. It can be clearly seen that the Ni, Co, C, N, and O elements are the main components of the NiCo2O4−P-G nanohybrids (Figure S5). The three peaks in the N 1s regions of XPS are observed (Figure 1c): the quinonoid imine (−N) at 398.9 eV, the benzenoid amine (−NH−) at 400.1 eV, and the positively charged nitrogen (−N+−) at 401.0 eV. Moreover, the ratio of −NH− components on the surface of NiCo2O4−PG nanohybrids is higher than that of P-G (Table S2), indicating that more reduction states are present in PANI induced by gaining electron from ambient NiCo2O4 species.36,37 The Ni 2p3/2 and Ni 2p1/2 emission spectra (Figure 1d) were fitted with two spin−orbit doublets characteristic of Ni2+ and Ni3+. In C

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ultrathin features is uniformly bridged on the graphene surface in the presence of PANI (Figure 2d). The size of the NiCo2O4 nanosheets changes in a range of 500−700 nm is also observed in comparison to the pristine NiCo2O4 sheets (Figure 2e). High-resolution FE-SEM images of NiCo2O4−P-G hybrids further reveal that these vertically oriented nanosheets are interconnected with each other to form a well-defined hierarchically porous structure on the graphene substrate (Figure 2f). These results suggest that PANI plays a critical role in mediating the nucleation and assembly of the ultrathin NiCo2O4 nanosheets on graphene by the bonding effects between metal ions and nitrogen species from PANI. The elemental mapping analysis of the NiCo2O4−P-G hybrids (Figure 2g) also illustrates the uniform distribution of the carbon, nitrogen, nickel, cobalt, and oxygen components, and the corresponding percentage of various elements are presented in Table S5. This further shows the intimate interaction between NiCo2O4 nanosheets and P-G in the nanohybrids. In addition, the specific surface area of the NiCo2O4−P-G hybrids determined by Brunauer−Emmert−Teller (BET) analysis reaches 140 m2 g−1 (Figure S6 and Table S6), being a slight increase in comparison to that of the pristine NiCo2O4 nanosheets (115 m2 g−1) and NiCo2O4-G nanohybrids (119 m2 g−1). The further pore-size distribution analysis reveals that NiCo2O4−P-G hybrids have a multilevel pore structure with an average pore size of 10.7 nm and pore volume of 0.38 cm3 g−1 calculated by the Barrett−Jouner−Halenda (BJH) model. Accordingly, the porous features with large specific surface area and multilevel pore structure can provide numerous channels for the access of electrolyte ions, thus leading to the fast reaction kinetics.42,43 The representative TEM and high-resolution TEM (HRTEM) images of the NiCo2O4−P-G hybrids are shown in Figure 3. The low-magnification TEM images (Figure 3a,b) clearly show that dozens of vertically oriented NiCo2O4

detail, the Ni 2p3/2 spectra show two peaks at 854.7 and 856.2 eV that correspond to the Ni2+ and Ni3+ ions, respectively.27,38 The overall ratios of Ni3+ ions on the surfaces of NiCo2O4−P-G nanohybrids are relatively higher than that of NiCo2O4 and NiCo2O4-G hybrids (Table S3). This is also the case for Co 2p XPS spectra of the NiCo2O4−P-G nanohybrids, as fitted from the relative intensities of Co3+ and Co2+ peaks in the Co 2p core level spectra (Figure 1e), and the relative content of Co3+ ions (779.6 eV) in the NiCo2O4-G hybrids is higher in comparison to that in other samples (Table S3).38,39 These results further confirm that strongly electronic interactions are present between PANI and NiCo2O4, which is consistent with that of Raman spectra. The further high-resolution spectrum for the O 1s region (Figure 1f) shows three oxygen contributions (denoted as O1, O2, and O3), which can be attributed to metal−oxygen bonds (529.3 eV), defect sites (531.0 eV), and physi- and chemisorbed water at or near the surface (532.7 eV), respectively.15 The area of the O2 peak at 531.0 eV is the largest for the NiCo2O4−P-G nanohybrids among the three samples, which indicates that the NiCo2O4−P-G nanohybrids possess more oxygen vacancies than the other counterparts (Table S4). On the basis of the results discussed above, it is not difficult to conclude that the complexly coupled effects between the nitrogen species from PANI building blocks and transition metal atoms (Ni and Co) can effectively tune the surface electronic states of as-made NiCo2O4−P-G hybrids. The metal ions in high valence state and oxygen defective sites are present in the NiCo2O4−P-G hybrids, which will play a favorable role for the high electrochemical performance of electrode materials for surface-dependent electrochemical reactions.10,40,41 The morphology and microstructures of the as-made series of samples were investigated by FE-SEM. A typical FE-SEM image (Figure 2a) indicates that the pristine NiCo2O4 obtained

Figure 2. Typical FE-SEM images of (a) pristine NiCo2O4; (b,c) NiCo2O4-G nanohybrids; (d−f) NiCo2O4−P-G nanohybrids; and (g) corresponding elemental mapping of C, N, Ni, Co, and O from the square region marked in the FE-SEM image of the NiCo2O4−P-G hybrids.

by free nucleation and growth process tends to form a flowerlike aggregated structure composed of nanosheets with a large size of 2−3 μm. For the NiCo2O4-G nanohybrids, some irregular NiCo2O4 nanosheets with paralleled structure are anchored on the graphene surface (Figure 2b,c). Surprisingly, in sharp contrast with the NiCo2O4-G hybrids, the large amount of vertically oriented NiCo2O4 nanosheets with

Figure 3. TEM (a−d) and HR-TEM (e,f) images of the vertically oriented NiCo2O4−P-G hybrids. D

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implies that more metal ions in high valence state will be produced in the NiCo2O4−P-G hybrids, which is consistent with the XPS results above. The correspondingly calculated average tranferred charge number (nM‑C/N) between C/N atoms drived from PANI and neighboring metal atoms (Ni and Co) is 1.37, being more than that between graphene and NiCo2O4 (1.08, Table 1), indicative of strong electronic

nanosheets with the size of 500−700 nm are uniformly assembled on the P-G surface, featuring hierarchical porous properties. An enlarged view further reveals that the vertically oriented NiCo2O4 nanosheets with an ultrathin wall are also well decorated and bridged on the P-G surface (Figure 3c,d), which is consistent with the FE-SEM images above. In order to observe the presence of P-G, the NiCo2O4−P-G hybrid samples suffer from long-time sonication treatment, and the typical TEM images are shown in Figure S7, in which PANI functionalized graphene nanosheets can be clearly observed. The ca. 24.0 wt % of the P-G is included in the NiCo2O4−P-G hybrids on the basis of TGA result (Figure S8). Furthermore, a typical NiCo2O4 nanosheet with visible lattice fringes can be distinctly observed in a HR-TEM image (Figure 3e, inset), with a lattice spacing of ca. 0.20 nm that can be indexed to the (400) plane of the spinel NiCo2O4 phase. The fast Fourier transformation (FFT) electron diffraction pattern reveals that the NiCo2O4 nanosheets are polycrystalline (Figure S9, inset) and that the crystallite size is about 5−10 nm (Figure S9). Furthermore, the nanosheets with thickness of about 4 nm were also demonstrated in Figure 4e. Meanwhile, a lattice

Table 1. Transferred Charge Number (nM‑C/N) and Distance (dM‑C/N) of C/N Atoms Derived from Graphene and PANI on the NiCo2O4 (311) Surfaces graphene on NiCo2O4 surface bonding

nM‑C/e

Co1−C1 Co2−C2 Co2−C3 Ni1−C4 Ni1−C5 Average

1.26 1.22 1.24 0.88 0.82 1.08

−

PANI on NiCo2O4 surface

dM‑C (Å)

bonding

nM‑C/N/e−

dM‑C/N (Å)

2.14 2.13 2.08 2.18 2.24 2.15

Ni1−N1 Co1−C1 Co2−C2 Ni2−C3 Ni2−N2 Average

1.88 0.99 1.12 0.97 1.90 1.37

2.07 2.12 2.04 2.05 1.94 2.04

interactions between PANI and NiCo 2 O 4 . Meanwhile, compared with the optimized graphene and NiCo2O4 model, the obvious migration of metal atoms in NiCo2O4 surface derived from unique interaction of N atoms is also observed (Figure S10). It was found that the average distance (dM‑C/N) of about 2.04 Å between C/N atoms drived from PANI and near metal atoms is demonstrated, which is smaller than that between C atoms from graphene and metal atoms (2.15 Å, Table 1 and Figures S11, S12). These results clearly indicate that the strongly chemical coupling interaction is present between the PANI and NiCo2O4, thus further implying that PANI plays the structural switching agent and coupled bridging role between the NiCo2O4 and graphene components. The electrochemical performance of the as-fabricated samples as pseudocapacitive electrodes for supercapacitors was evaluated in a three-electrode cell in 6 M KOH aqueous electrolyte. It should be noted that the bare P-G substrate deilivers a small current in comparison to that of the NiCo2O4− P-G hybrid electrodes, indicating that the contribution of the PG substrate toward the capacitance is negligible under the present adopted conditions (Figure S13). A pair of redox peaks derived from the faradaic reaction that is related to MO/ MOOH (M represents Ni or Co) can be observed in the CV curves (at a scan rate of 20 mV s−1) of the pristine NiCo2O4, NiCo2O4-G and NiCo2O4−P-G hybrid electrodes, as shown in Figure 5a. The peak intensities of the CV curves for the NiCo2O4−P-G hybrid electrodes are higher than that of the other electrodes; moreover, all of the CV curves for the NiCo2O4−P-G hybrid electrode at scan rates ranging from 5 to 100 mV s−1 have similar shapes (Figure 5b), implying that the NiCo2O4−P-G hybrid electrodes feature much faster redox reaction kinetics. Such excellent transport characteristics of the charge and ion carriers within the NiCo2O4−P-G hybrid electrodes were also further confirmed by electrochemical impedance spectroscopy (EIS) (Figure S14). Figure 5c shows charge/discharge curves of the pristine NiCo2O4, NiCo2O4-G, and NiCo2O4−P-G hybrid electrodes at a current density of 1 A g−1, in which the NiCo2O4−P-G hybrids demonstrate the longest charge/discharge time, indicating the highest specific capacitance among all samples fabricated. The corresponding specific capacitance of the samples varies with the current densities calculated and is shown in Figure 5d. The as-made

Figure 4. Schematic illustration for the adsorption of graphene and PANI unit on the spinel NiCo 2 O 4 (311) surface and the corresponding partial charge density of atoms: (a) side-view and (b) top-view of the as-optimized graphene and NiCo2O4 calculation model; (c) side-view and (d) top-view of the as-optimized PANI and NiCo2O4 calculation model.

spacing of ca. 0.25 nm in nanoscaled dimension corresponds to the (311) crystal plane (Figure 3f), indicative of the {311} plane preferential orientation, which is further confirmed by the XRD analysis above.15 To have further insight into the electronic structural states and unique coupling effects of PANI and NiCo2O4 components in the NiCo2O4−P-G nanohybrids, the DFT calculations were performed, and the detailed results are shown in Figure 4. Therein, seven benzene rings and two benzene rings coupled with two amino-groups were optimized as graphene and PANI unit, respectively, and the corresponding spinel NiCo2O4 (311) surface with three atomic layers was modeled as the p(1 × 3) periodic slab. It can be clearly seen from Figure 4a and c (sideview) that the electrons of some carbon (C) and nitrogen (N) atoms derived from graphene and PANI, and neighboring metal atoms (Ni and Co) become delocalized. It is interesting that these kinds of delocalized C atoms from graphene just occur on its unit edge in comparison to that of PANI (Figure 4b and d, top-view). The further Bader charge analysis shows that the electrons are transferred from the Ni and Co atoms to surrounding N and C atoms (Tables S7 and S8). This also E

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Figure 5. Electrochemical performance of the samples for supercapacitors: (a) CV curves of the pristine NiCo2O4, NiCo2O4-G, and NiCo2O4−P-G hybrid electrodes at a scan rate of 20 mV s−1; (b) CV curves of the NiCo2O4−P-G hybrid electrodes at various scan rates of 5−100 mV s−1; (c) galvanostatic charge/discharge curves of the pristine NiCo2O4, NiCo2O4-G, and NiCo2O4−P-G hybrid electrodes at a current density of 1 A g−1; (d) the specific capacitances of the pristine NiCo2O4, NiCo2O4-G, and NiCo2O4−P-G hybrid electrodes at different current densities; (e) the specific capacitances based on active NiCo2O4 component of the NiCo2O4−P-G hybrid electrodes at different current densities; (f) cycling performance of the NiCo2O4−P-G hybrid electrodes at a current density of 6 A g−1; (g) galvanostatic charge/discharge curves of the NiCo2O4−P-G hybrid electrodes after cycling test for first and 5000th cycles at a current density of 6 A g−1; (h) Nyquist plots (expanded view of the high-frequency region in the inset) and (i) frequency responses of EIS for the NiCo2O4−P-G hybrid electrodes after the cycling test for first and 5000th cycles.

current density of 6 A g−1. It can be noted that the specific capacitance of NiCo2O4−P-G hybrid electrodes can be increased to 815 F g−1 from 699 F g−1 at a current density of 6 A g−1 (based on the total mass of the NiCo2O4−P-G hybrids) after 5000 cycles. The possible reason for this is that the electrode materials are activated electrochemically, which helps to create rich electroactive sites for fast redox reaction.45,46 The electrochemical performance of the NiCo2O4−P-G hybrids after cycling test for 1 and 5000 cycles was further investigated by using EIS (Figure 5h). It is found that the arc increment from the first to the 5000th cycles is not obvious, indicating that the nanostructures are well kept with little structural deformation after 5000 cycles. This is also evidenced by the typical FE-SEM images of the NiCo2O4−P-G hybrids after 5000 cycles (Figure S16). The semicircle in the high frequency corresponds to the charge transfer resistance (Rct) at the electrode/electrolyte interface. The slightly increased Rct can be attributed to the loss of adhesion of some active materials that block the transport pathways of ions during the charge/ discharge process. Furthermore, the solution resistance (Rs) of NiCo2O4−P-G hybrids after cycling test for 5000 cycles, which is obtained from the X-intercept of the Nyquist plot, is nearly in keeping with that of the first cycle (inset in Figure 5h), suggesting a good ion response, which is related to improved wettability derived from long-term charge/discharge or redox

NiCo2O4−P-G hybrid electrodes exhibit an excellent specific capacitance of 734 F g−1 (based on the total mass of the NiCo2O4−P-G hybrids) at a current density of 1 A g−1, which is higher than that of NiCo2O4-G (520 F g−1 at 1 A g−1) and pristine NiCo2O4 (301 F g−1 at 1 A g−1), and a capacitance of around 84% (618 F g−1) could be retained even at a high current density of 100 A g−1. Accordingly, the NiCo2O4−P-G hybrid electrodes also deliver a areal capacitance of 503 mF cm−2 at a current density of 1.4 mA cm−2 (Figure S15). Furthermore, the high specific capacitance of 966 F g−1 for NiCo2O4−P-G hybrids can be obtained at the current density of 1.3 A g−1 based on the mass of the active NiCo2O4 component (Figure 5e), and the NiCo2O4−P-G hybrid electrodes also show the ultrahigh capacitance of 813 F g−1 even at the current density of 130 A g−1. To the best of our knowledge, such superior electrochemical performance has rarely been observed for transition metal oxide-based electrode materials (Table S9).23,25,29,44 In addition, the NiCo2O4−P-G hybrid electrodes also keep a good electrochemical stability at a current density of 6 A g−1 for 5000 cycles (Figure 5f). It is found that the capacitive retention rate is not stumbling over the long-term cycling, even when an increase by 11% of its initial capacitance happened in the first 1000 cycles. Figure 5g shows charge/discharge curves of the NiCo2O4−P-G hybrid electrodes after the cycling test for 1 and 5000 cycles at a F

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Chemistry of Materials reaction process. The NiCo2O4−P-G hybrids after cycling for 5000 cycles also demonstrate similar operating frequency to that of the first cycle, at which the capacitance equals 50% of its maximum value (Figure 5i). The corresponding operating frequencies are about 0.83 Hz, indicative of the fast and stable frequency response for NiCo2O4−P-G hybrids. All in all, these superior electrochemical properties of the as-obtained NiCo2O4−P-G hybrid electrodes for supercapacitors are attributed to the uniquely structural features and the synergistic effects between NiCo2O4 nanosheet and P-G, although the conductivity of P-G may slightly be reduced in alkaline media. The interconnected hierarchically porous structure formed by the vertically oriented NiCo2O4 nanosheets can serve as continuous channels for the access of electrolyte ions. Moreover, metal ions in high valence state and oxygen defective sites are present, leading to numerous electroactive sites and delivering the fast reaction kinetics. Besides, the strongly coupled interaction between the NiCo2O4 nanosheets and P-G in the nanohybrids provides a sufficient guarantee for structural stability, and the ultrathin NiCo2O4 sheets assembled on graphene in a vertical orientation also ensure high charge storage and fast transport of electrolyte ions by utilizing both sides of the NiCo2O4 sheets. In order to show the multifunctional applications of the NiCo2O4−P-G hybrids, the electrocatalytic OER activity of the NiCo2O4−P-G hybrids and other contrast samples was investigated in 0.1 M KOH alkaline solution in a standard three-electrode system, where the ohmic potential drop (iR) that arises from the solution resistance was corrected (only 70% iR compensation, Figure S17). As shown in Figure 6a, the polarization curve for the NiCo2O4−P-G catalyst in LSV exhibits slightly low onset potential (about 1.50 V versus the RHE) and high oxygen-evolving current in comparison to those of NiCo2O4-G and NiCo2O4 catalysts, even commercial Pt/C catalyst (20 wt %) and the state-of-the-art RuO2 catalyst (Sigma-Aldrich). In sharp contrast, no obvious voltammetric

responses were observed for the pure P-G sheets. It is very meaningful to achieve the low overpotential requirements at the current density of 10 mA cm−2, which is a metric relevant to solar fuel synthesis. Remarkably, the vertically oriented NiCo2O4−P-G hybrid can afford such current density at a low overpotential of about 0.32 V, much smaller than those of the other catalysts (Figure 6b). Such an overpotential is also smaller than that of the well-investigated Co and/or Ni-based oxides OER catalysts in the literature (Table S10).10,38,40,47 The OER kinetics of the catalysts above is probed by corresponding Tafel plots at a scan rate of 10 mV s−1, as shown in Figure 6c. The resulting Tafel slopes are found to be about 76, 80, 83, 91, and 139 mV dec−1 for NiCo2O4−P-G, NiCo2O4-G, NiCo2O4, RuO2, and commercial Pt/C, respectively. The as-made NiCo2O4−P-G hybrid exhibits the smallest Tafel slope and is therefore the most efficient electrocatalyst among the adopted materials. It is interesting to note that the Tafel slope of about 83 mV dec−1 for the pristine NiCo2O4 is also smaller than that of RuO2, suggesting the outstanding intrinsic OER kinetics of this kind of NiCo-based oxides material even comparable of RuO2 catalyst. The further reduced Tafel slope for NiCo2O4− P-G hybrid demonstrates that the synergistic enhancement of OER activity takes effect in the hybrids. The long-term stability is another critical parameter that determines the practicability of electrocatalysts. To assess this, the catalytic stability of NiCo2O4−P-G is investigated by chronopotentiometric measurements at a current density of 2 mA cm−2 for 13 h (Figure 6d). The NiCo2O4−P-G shows a potential of about 1.52 V versus the RHE at the first time, and a slight potential decrease can be observed, which should be attributed to the wettabilityinduced enhancement of OER activity. Moreover, the polarization curve of NiCo2O4−P-G hybrids after the long-term test for 13 h also shows a slightly increased current density to initial time (Figure S18).These results demonstrate that the NiCo2O4−P-G hybrids are capable of retaining their catalytic activity over long-term testing.

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CONCLUSIONS In summary, we have developed a simple yet efficient strategy for in situ assembly of the ultrathin NiCo2O4 nanosheets on graphene in a vertical orientation to construct the hierarchical NiCo2O4−P-G nanohybrids via employing PANI as the structural coupling bridge between the two components. The as-obtained NiCo2O4−P-G nanohybrids with enriched surface active sites, together with numerous channels for the access of electrolyte ions and transport of electron, deliver the fast reaction kinetics and the enhanced electrochemical activity. As an evidence of its multifunctional applications, the NiCo2O4− P-G nanohybrids deliver an excellent rate capability and longterm cycling performance for supercapacitors, as well as good electrocatalytic performance as a precious metal-free catalyst for OER.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b02303. Zeta potential data, FT-IR spectra, SEM and TEM images, TG data, nitrogen adsorption/desorption isotherms, XPS data, DFT calculations, and additional electrochemical data (PDF)

Figure 6. Electrochemical performance of the as-made samples for OER: (a) the polarization curves of the P-G, pristine NiCo2O4, NiCo2O4-G, NiCo2O4−P-G hybrid electrodes, and commercial Pt/C catalyst (20 wt %) as well as the state-of-the-art RuO2 catalyst at a scan rate of 10 mV s−1; (b) corresponding onset potentials and potentials required to reach the current density of 10 mA cm−2; (c) Tafel plots of the samples at the scan rate of 10 mV s−1; (d) the catalytic stability of NiCo2O4−P-G at current density of 2 mA cm−2. G

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

J.Y. and C.Y. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by the Natural Science Foundation of China (Nos. 21336001, 21522601), the Fundamental Research Funds for the Central Universities (DUT16ZD217), and the Education Department of the Liaoning Province of China (No. T2013001).

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