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Plasma-Assisted Synthesis of Self-Supporting Porous CoNPs@C Nanosheet as Efficient and Stable Bifunctional Electrocatalysts for Overall Water Splitting Qiuyan Jin, Bowen Ren, Dongqi Li, Hao Cui,* and Chengxin Wang* State Key Laboratory of Optoelectronic Materials and Technologies, School of Materials Science and Engineering, The Key Laboratory of Low-Carbon Chemistry & Energy Conservation of Guangdong Province, Sun Yat-sen University, Guangzhou 510275, China S Supporting Information *

ABSTRACT: The utilization of a highly active and robust bifunctional catalyst for simultaneously producing H2 and O2 is still a major challenging issue, which is vital for improving the efficiency of overall water splitting. Herein, we employ a novel plasma-assisted strategy to rapidly and conveniently synthesize the three-dimensional (3D) porous composite nanosheets assembled on monodispersed Co nanoparticles encapsulated in a carbon framework (CoNPs@C) on a carbon cloth. Such a novel 3D hierarchical porous nanosheet improves the exposure and accessibility of active sites as well as ensures high electroconductibility. Moreover, the coating of a few graphene layers on the surface of catalysts favors improvement of the catalytic activity. Benefited from these multiple merits, the CoNPs@C composite nanosheets enable a low overpotential of 153 mV at −10 mA cm−2 for hydrogen evolution reaction. Furthermore, they are also capable of catalyzing the oxygen evolution reaction with high efficiency to achieve current density of 10 mA cm−2 at the overpotential of 270 mV. Remarkably, when assembled as an alkaline water electrolyzer, the bifunctional CoNPs@C composite nanosheets can afford a water-splitting current density of 10 mA cm−2 at a cell voltage of 1.65 V. KEYWORDS: 3D porous structure, cobalt nanoparticles, binder-free, bifunctional catalyst, overall water splitting

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Co17 alloys, for HER and transition metal oxides/hydroxides,18−20 Fe−Co−W,21 and perovskite oxides22 for OER, respectively. Nevertheless, to accomplish a high energy conversion efficiency system, HER catalysts and OER catalysts should work in the same medium to minimize the overpotentials. Besides that, two-electrode configurations are similar to practical electrolytic cells. Unfortunately, the catalysts that are excellent in acidic electrolyte for HER always lose their performance in basic media; the same situation also happens to efficient catalysts in basic solution for OER. In addition, producing different catalysts for HER and OER needs different equipments and processes, which will increase the cost. Therefore, developing the efficient bifunctional catalysts in the same medium to accomplish overall water splitting is highly imperative. Recently, earth-abundant 3d transition metal cobalt (Co) catalyst has been considered as a promising candidate for water splitting,5,23,24 which attributes to metallic Co having a

INTRODUCTION Owing to the overuse of fossil fuel-related environmental issues, splitting water into hydrogen and oxygen has attracted great attention of the scientific community.1,2 The electrochemical water-splitting reaction, consisting of a cathodic hydrogen evolution reaction (HER) and an anodic oxygen evolution reaction (OER), is considered as one of the most promising ways to produce efficient renewable energy. However, it is a strongly uphill reaction that needs external energies to activate the reaction (Generally indicated at a cell voltage of 1.8−2.0 V for commercial electrolyzers, which is much higher than the theoretical value of 1.23 V.).3−5 The active electrocatalyst is necessary to reduce the value of the overpotential and advance the kinetic reaction of water splitting. Currently, Pt-group metals are the most active catalysts for HER, and RuO2/IrO2 are the benchmark catalysts for OER.6 Even though they have been recognized as state-of-the-art electrocatalysts, the scarcity and thus high cost of precious metal-containing compounds significantly prohibit their commercialization and widespread use. To date, a great deal of effort and progress has been made to develop the low-cost and effective alternative electrocatalysts for water splitting, such as 1-row transition metals,7−9 phosphides,10−12 sulfides,13,14 carbides,15 Ni−Mo,16 and Ni− © 2017 American Chemical Society

Received: July 1, 2017 Accepted: August 29, 2017 Published: August 29, 2017 31913

DOI: 10.1021/acsami.7b09487 ACS Appl. Mater. Interfaces 2017, 9, 31913−31921

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moderate H-bonding energy and thus being able to enhance the performance of HER.25,26 However, due to the low specific surface area and few catalytic reactive sites, the common Co materials have insignificant electrochemical catalytic activity with limited large-scale applications. Moreover, bare Co metal also becomes unstable under strong basic media and high overpotential. To solve aforementioned problems, a feasible method is to coat cobalt nanoparticles (NPs) with carbon materials (graphene, carbon nanotubes), which will avoid the agglomeration of neighboring metal NPs and protect metal NPs from strongly acidic or alkaline solution.27 Moreover, some previous studies also reveal that few-carbon-layersencapsulating metal NPs can enhance the HER performance.7,17 The high-temperature carbonization of C and Cocontained precursors is the common fabrication method for the carbon-encapsulated Co NP electrocatalyst. The method not only refers to the high carbonization temperatures and long treatment times but also could cause the sintering and agglomeration.28,29 Additionally, Co-based catalysts are usually prepared in powder form, and the polymer binder (Nafion or poly(vinylidene fluoride)) has to be used to adhere the powder particles to the current collector for water splitting. The polymer binder not only inevitably lowers the conductivity of materials but also buries active sites or blocks diffusion thus leading to decrease of the electrocatalytic activity of water splitting.24 In addition, the metal/carbon composites will drop from the electrode during the test due to the weak binder− carbon−metal connection. Thus, it is crucial for the development of a new strategy that can realize the rapid fabrication of the binder-free carbon-coated Co NP catalyst. Microwave plasma-enhanced chemical vapor deposition (MPECVD) is a frequently used technique for preparing different kinds of carbon materials, such as diamond and CNT.30 Plasma can offer nonequilibrium and highly reactive environment for the growth of nanostructure and have been recently utilized to conduct the rapid conversion of precursors into metal compound.15,31,32 Hence, we anticipate that PECVD can be employed to directly generate the carbon-coating layer on the surface of active materials without destroying the original structure treatment, which is a promising strategy to obtain the integrated carbon-coated composite electrode. Herein, we report a fast and convenient method based on plasma-assisted treatment for fabricating the self-standing, highly integrated three-dimensional (3D) porous CoNPs@C composite nanosheets on carbon cloth (CC). Few-grapheneencapsulated Co NPs can be rapidly formed within 10 min via plasma-assisted treatment, whereas the morphology of the precursor nanosheet can be well preserved. Furthermore, the obtained nanosheets hold a porous structure due to the etching effect of plasma.33 The CoNPs@C nanosheet is employed as an excellent bifunctional catalyst for the overall water splitting in strongly alkaline media. The CoNPs@C catalyst can afford current density of 10 mA cm−2 at the overpotential of 153 mV for HER and 270 mV for OER in 1 M KOH, respectively. Intriguingly, when fabricated as an alkaline water electrolyzer, the bifunctional CoNPs@C catalyst can operate at current density of 10 mA cm−2 with a small cell voltage of merely 1.65 V and performs robustly. Our results provide a universal route to designing and synthesizing a novel 3D self-standing porous nanostructure for efficient water splitting.

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EXPERIMENTAL SECTION

Materials. All reagents were directly used without further purification. The materials used were cobalt nitrate hexahydrate (Co(NO3)2·6H2O, Aladdin, Co., Ltd, ACS), Pt/C (Johnson Matthey Hispec 3600, Shanghai Hesen Electric Co., Ltd, 20 wt %), RuO2 (Aladdin, Co., Ltd, 99.9% metals basis), and potassium hydroxide (KOH, Sinopharm Chemical Reagent Co., Ltd, ≥85.0%). Pretreatment of Carbon Cloth. A carbon cloth (1 × 2 cm2) was soaked in concentrated sulfuric acid (H2SO4, 98%) over the night. Then, the carbon cloth was ultrasonicated for 15 min in ethanol solution. Finally, the carbon cloth was washed by deionized water several times. Synthesis of Co-Species Nanosheets Precursor on Carbon Cloth. Co-species precursor nanosheets were directly grown on carbon cloth by the electrodeposition method. Specifically, 5 mmol Co(NO3)2·6H2O was dissolved in 10 mL of ethanol and 40 mL of deionized water to form a clear solution. The pretreated carbon cloth inserted into the clear solution was used as a working electrode and platinum foil and Ag/AgCl (saturated KCl solution) were used as the counter electrode and reference electrode, respectively. The chronoamperometry procedure was conducted at −1.0 V for 20 min to obtain the green-surfaced Co-species nanosheets precursor. Synthesis of Porous CoNPs@C Nanosheets on Carbon Cloth. The as-prepared Co-species nanosheets precursor converted into CoNPs@C nanosheets by microwave plasma-enhanced chemical vapor deposition (MPECVD) system. The rectangular microwave waveguide coupled the microwave power in a quartz tube, which generated the plasma, and the as-prepared Co-species precursor was placed in the reaction chamber. The MPECVD system was evacuated to 2 mTorr, then the CH4 and H2 gas flowed into the reaction chamber at a flow rate of 15 and 20 sccm, respectively. CH4 and H2 plasma were generated at 700 W for 10 min with a base pressure of 25 Torr. The mass loading of the CoNPs@C nanosheet was estimated at about 3.43 mg cm−2 (see the caption of Figure S1). The control sample was prepared only under H2 plasma, named as Co−H2. Materials Characterizations. X-ray diffraction (XRD) patterns were obtained by X-ray diffraction (XRD) (Rigaku X-ray diffractometer D-MAX 2200 VPC) using Cu Kα radiation (λ = 0.15418 nm), with a scan step size of 10 (° s−1). The morphologies of samples were characterized by a scanning electron microscope (SEM, Carl Zeiss, Auriga-4525) at an acceleration voltage of 5 kV. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images and the high-angle annular dark-field/scanning transmission electron microscopy/energy-dispersive system (HAADF−STEM−EDS) data were taken on a FEI Tecnai G2 F30 microscope operated at 300 kV. X-ray photoelectron spectroscopy (XPS) measurements were recorded using an ESCA Lab250 spectrometer, using a twin-anode Al Kα (1486.6 eV) X-ray source. Nitrogen adsorption−desorption (Micromeritics ASAP 2460) was used to analyze the Brunauer−Emmett− Teller (BET) specific surface areas of the samples. Raman spectra were obtained on an inVia Renishaw Raman spectrometer system using 514.5 nm. The mass loading of the samples was measured by inductive coupled plasma (ICP) (ICP-AES PE2100DV). Electrochemical Measurements. The electrochemical measurements were performed in 1 M KOH at room temperature. Data were collected in a standard three-electrode system with a Ivium electrochemical workstation (IviumStat). A Hg/HgO electrode and a graphite rod served as the reference electrode and counter electrode (Pt as counter electrode will electroplate at the working electrode and improve the HER activity,34 so we chose a graphite rod as the counter electrode), respectively. The as-synthesized products on carbon cloth were directly used as a binder-free working electrode for the HER and OER test. The electrochemically inert silicon was used to determine the active coverage area (1 × 0.8 cm2). The reference electrode was calibrated with a reversible hydrogen electrode. Before the electrochemical measurement, the working electrode was preactivated by several cycle voltammetric scans at a scan rate of 50 mV s−1 until the CV curves became stable. Then, the linear sweep voltammetry (LSV) was measured from 0.1 to −0.4 V versus the reversible hydrogen 31914

DOI: 10.1021/acsami.7b09487 ACS Appl. Mater. Interfaces 2017, 9, 31913−31921

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ACS Applied Materials & Interfaces electrode (RHE) at a scan rate of 2 mV s−1 for HER and 1.2−1.8 V versus RHE at a scan rate of 2 mV s−1 for OER. Electrochemical impedance spectroscopy (EIS) was performed over a frequency range from 100 kHz to 0.01 Hz at the overpotential of −0.4 V versus RHE (HER) and 1.6 V versus RHE (OER), with an amplitude of 10 mV. All LSV data were reported with 90% iR compensation to fairly compare the performance of catalysts. CoNPs@C was used as both the anode and cathode in 1 M KOH for overall water splitting with the twoelectrode system. The potentials measured against the Hg/HgO electrode were converted to the reversible hydrogen electrode in 1 M KOH according to the Nernst equation: E (vs RHE) = E (vs Hg/ HgO) + E° (Hg/HgO) + 0.059 pH.

(Figures 2e and S3). About 3−10 graphene layers (Figure S4) can be observed on the surface of Co NPs, and the graphene interconnects throughout the whole nanosheet (Figure 2f), ensuring the conductivity of the whole nanosheets. Energy dispersive X-ray spectroscopy (EDX) verifies the presence of Co and C elements (Figure S5), and the intensity of oxygen is almost invisible, which indicates that the carbon layer well protects Co NPs from oxidation in the air. More details of the nanostructure are verified in high-resolution TEM (HRTEM). As shown in Figure 2g, the core has clearly observable lattice fringes. The adjacent planes of 2.03 and 1.48 Å are assigned to the (002) plane and (102) interplane of metal Co, respectively. The 3.45 Å matches well with the (002) plane of graphene. The selected area electron diffraction (SAED) pattern (Figure 2h) further confirms the presence of metal cobalt and graphene. X-ray photoelectron spectroscopy (XPS) is used to perform elemental analysis of the CoNPs@C nanosheets. Two sharp metallic Co 2p3/2 and Co 2p1/2 peaks appeared at 778.5 and 793.6 eV, respectively, implying the existence of abundant Co− Co bonds.36 The peak at around 780.1 eV is the typical Co3+, which is in good agreement with well-characterized Co3O4 (Figure 3a).37 The intensity of peak at Co3+ is very weak compared to that of metallic Co, which means very small amount of cobalt oxides exists in CoNPs@C samples. Because of the strong reducing property of H2 and CH4 plasma, the Cospecies precursor completely converts into pure metal Co NPs and the graphene layer rapidly forms on the surface of Co NPs simultaneously. The Raman spectrum further confirms this consequence. The Raman shifts between 400 and 1000 cm−1 belong to the characteristic mode of cobalt oxides that have not been detected (Figure 3b).4,38 The D band at about 1355 cm−1 is often related to the defect or disorder, and the G band at about 1585 cm−1 refers to tangential oscillations and vibrations of all sp2 carbon atoms. Therefore, the integrated intensity ratio ID/IG is considered as a measure of the graphitization degree and in our work the value of ID/IG is 0.84, implying that CoNPs@C holds good conductivity. Brunauer−Emmett− Teller (BET) measurement is carried out to investigate the porous structure of CoNPs@C. The BET surface area of nitrogen for the CoNPs@C with CC is 31.79 m2 g−1 (the CoNPs@C without CC is also calculated to be 213.92 m2 g−1, see the caption of Figure S6) and is far larger than that with pure CC (0.33 m2 g−1), further affirming the porous structure of CoNPs@C. The measurement of the electrocatalytic performance is carried out in a standard three-electrochemical system in 1 M KOH. A Hg/HgO electrode and a graphite rod served as the reference electrode and counter electrode, respectively. The assynthesized product on the carbon cloth is directly used as the working electrode. We first evaluate the HER activity of the CoNPs@C nanosheets. Co−H2 (prepared only with H2 plasma, Figure S7), bare CC, and 20% Pt/C are also tested for comparison. As shown in Figure 4a, the bare CC exhibits negligible activities, whereas 20% Pt/C possesses excellent HER activities (24 mV for 10 mA cm−2). It is worth noting that, as expected, the CoNPs@C electrocatalyst exhibits the most superior HER activity among these electrocatalysts, except Pt. To achieve current density of 1 and 10 mA cm−2, the CoNPs@ C requires small overpotentials at 36 and 153 mV, respectively. The HER performance of CoNPs@C is much higher than that of bare Co−H2, implying that the synergistic effect between functional carbon and inner metal can promote the HER activity. In fact, the catalytic activity of CoNPs@C nanosheets

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RESULTS AND DISCUSSION Figure 1 illustrates the preparation of CoNPs@C nanosheets through a two-step process: electrochemical deposition and

Figure 1. Schematic illustration of the preparation of CoNPs@C nanosheets and corresponding sample photos.

plasma processing. The green-surfaced Co oxide/hydroxide (named Co-species) precursors are directly grown on the graysurfaced CC via the electrodeposition process. Then, the Cospecies precursor nanosheets get converted into black-surfaced CoNPs@C nanosheets by a plasma-assisted process. The X-ray diffraction (XRD) are carried out to analyze the crystal structure. The Co-species precursor can be mainly indexed as Co(OH)2 (Figure S2). Some diffraction peaks for Co3O4 and Co-OOH are also present, indicating that the electroplated Co has been oxidized to various oxidation forms. As shown in Figure 2a, the Co-species precursor converts into the metal Co after plasma processing. Their morphologies are observed with a scanning electron microscope (SEM). Figure 2b shows that the Co-species precursor nanosheets with smooth surfaces (inset) uniformly cover the carbon cloth; they are interconnected with each other. After plasma processing, the CoNPs@C still remain as the original nanosheet morphology, as shown in Figure 2c. In fact, the rugae-like morphology could improve hydrogen or oxygen production and the gap between the nanosheets may be beneficial to the absorption of water molecules. Further, high-magnification SEM shows that the surface of CoNPs@C nanosheets became rough (the inset image of Figure 2c), which is attributed to the etching effect of plasma. The transmission electron microscopy (TEM) images reveal that the CoNPs@C nanosheets consist of high-density Co nanoparticles and the nanoparticles are separated from each other by the carbon framework (Figure 2d,e). Plenty of mesopores can be observed in the CoNPs@C nanosheets (Figure S3). The pores expose more active sites and allow them direct contact with the electrolyte, which is beneficial to the diffusion and charge transfer. Such highly open 3D network is believed to improve the performance of catalysts.35 These nanoparticles are of narrow size distribution of about 5−16 nm 31915

DOI: 10.1021/acsami.7b09487 ACS Appl. Mater. Interfaces 2017, 9, 31913−31921

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Figure 2. (a) XRD patterns of the CoNPs@C sample, (b) High- (inset) and low-magnification SEM images of the Co-species precursor (c) High(inset) and low-magnification SEM images of porous CoNPs@C, (d, e) TEM images of CoNPs@C with particle size statistics (inset), (f, g) HRTEM image of CoNPs@C, and (h) SAED image.

Tafel slope can be used to reveal the inherent reaction processes of the HER39 and the linear part at the low overpotential fitted to the Tafel equation (η = b log j + a, where

reported here is also better than that of most cobalt carbon composites, cobalt N-doped carbon composites, and even comparable to some Co-based compounds (Table S1). The 31916

DOI: 10.1021/acsami.7b09487 ACS Appl. Mater. Interfaces 2017, 9, 31913−31921

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and interconnected graphene. To gain insights into the activity of the catalysts, the electrochemical double-layer capacitance (Cdl) was measured to calculate the electrochemical active surface area (ECSA). Circle voltammetry (CV) curves are measured in the potential range from 0.08 to 0.18 V (vs RHE) without redox processes(Figure S10a). The Cdl of CoNPs@C, Co−H2, and CC was 144, 80, and 0.04 mF cm−2, respectively (Figure S10b). CoNPs@C holds the biggest Cdl, which can be mainly attributed to the novel 3D porous structure with a large specific surface area. Durability is another important criterion to evaluate the performance of HER electrodes. The polarization curves of CoNPs@C after 5000 potential cycles just slightly decrease (Figure S11a); then, the 20 h constant-current test (23 mA cm−2) is carried out to further verify the viability of CoNPs@C nanosheets. The catalytic potential of CoNPs@C nanosheets almost remains unchanged for 20 h (Figure 4c), whereas that of Co−H2 decreases to 120 mV for comparison, suggesting that the carbon layer can protect the inner Co NPs from corrosion of the electrolyte and thus ensure the stability of CoNPs@C. The catalytic ability of CoNPs@C toward OER is also investigated in 1 M KOH solution. As shown in Figure 4d, CoNPs@C also exhibits a small overpotential of only 270 mV at 10 mA cm−2, which is lower than that of RuO2 with 365 mV at 10 mA cm−2 and that of Co−H2 with 317 mV at 10 mA cm−2. CoNPs@C could reach current densities of 1, 10, and 100 mA cm−2 at overpotentials of 140, 270, and 330 mV, respectively, lower than those of RuO2 and most other recently reported excellent OER catalysts (Table S2). The Tafel slope of CoNPs@C is 59 mV dec−1, which is much lower than that of bare Co−H2 with 80 mV dec−1 and similar to the Tafel slope of RuO2 with 56 mV dec−1 (Figure 4e). The close values of the Tafel slopes of CoNPs@C and RuO2 demonstrate similar OER kinetics between the two catalysts. Then, the semicircle of CoNPs@C from the Nyquist plot’s pattern is the smallest during the OER process, indicating a more efficient electron transport at the CoNPs@C−electrolyte interface (Figure S9b). The electrochemical active surface area (ECSA) for OER is also estimated by double-layer capacitance (Cdl) (Figure S10c,d). The Cdl of CoNPs@C (48 mF cm−2) is double that of Co−H2 (24 mF cm−2), indicating that CoNPs@C expose more accessible active site. It is worth noting that the ECSAs of the CoNPs@C during the HER and OER processes are significantly different, implying that the active sites for HER and OER are not necessarily the same,32 and the difference of ECSAs will be discussed later. The durability of CoNPs@C for OER is examined after 5000 CVs, and the polarization curves are almost unchanged (Figure S11b). As shown in Figure 4f, the constant-current test (23 mA cm−2) further confirms the excellent long-term stability of CoNPs@C nanosheets and the potential of CoNPs@C nanosheets only decreases by 0.8% (By contrast, the potential of Co−H2 decreases by 3.6%, which is 4 times than that of CoNPs@C nanosheets.). To investigate the optimized structure of porous CoNPs@C nanosheets, we test the catalytic performance of CoNPs@C samples under different powers by MPECVD. The samples are treated at 600 W (CoNPs@C-600), 700 W (CoNPs@C-700), 800 W (CoNPs@C-800), and 900 W (CoNPs@C-900). All CoNPs@C samples exhibit much higher HER activity compared to that of Co−H2 (Figure S12a), implying that the extraordinary catalytic activity is derived from structural properties of the porous 3D cobalt/graphene core/shell nanosheets. In addition, the Tafel slopes of CoNPs@C

Figure 3. (a) XPS spectra of CoNPs@C (b) Raman spectrum of CoNPs@C.

η stands for overpotential, j for current density, and b for Tafel slope) determined the Tafel slope. Typically, there are two mechanisms during the HER process in an alkaline solution, the Volmer process (electrochemical hydrogen adsorption: H2O + e− → H ads + OH− with 120 mV dec−1) followed by either the Heyrovsky process (electrochemical desorption: Had + H2O + e− → H2 + OH− with 40 mV dec−1) or Tafel process (chemical desorption: Hads + Hads → H2 with 30 mV dec−1).40 Meanwhile, the rate-determining step can be reflected by the value of the Tafel slope. Thus, the 20% Pt/C with a Tafel slope of 28 mV dec−1 follows the Volmer−Tafel mechanism and the ratedetermining step is the Tafel reaction (Figure 4b).41 CoNPs@ C showed a Tafel slope of 106 mV dec−1 in alkaline solution, indicating that the HER process may be controlled by a Volmer−Heyrovsky mechanism because the Tafel slope is almost identical to that for smooth Ni (120 mV dec−1)42 and the rate-determining step for CoNPs@C follows the Volmer step. To evaluate the catalytic efficiency, the exchange current density (J0) serves as the key descriptor. By extrapolation of Tafel plots to the J axis, calculating j0 of CoNPs@C gives 0.337 mA cm−2, which is much better than that of bare Co−H2 with j0 of 0.017 mA cm−2 (Figure S8). The high J0 value suggests the favorable HER kinetics of CoNPs@C. Electrical impedance spectroscopy (EIS) data is collected to further examine the HER kinetics of CoNPs@C (Figure S9a). The charge-transfer resistance (Rct) obtained from diameter of the semicircles in the high-frequency zone is related to the interfacial charge transfer, and the lower Rct value indicates a faster reaction rate.43 CoNPs@C exhibits the smallest semicircular diameter in the high frequencies among all samples, which means fast electron transfer kinetics. The superior electron mobility can be achieved due to the high conductivity of metallic Co NPs 31917

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Figure 4. (a) HER polarization curve of CoNPs@C in comparison with that of 20% Pt/C, Co−H2, and CC in 1 M KOH, with a scan rate of 2 mV s−1; (b) Tafel plots of CoNPs@C, 20% Pt/C, and Co−H2 for HER; (c) long-term HER stability test (without iR corrected) of CoNPs@C and Co− H2; (d) OER polarization curve of CoNPs@C in comparison with that of RuO2, Co−H2, and CC in 1 M KOH, with a scan rate of 2 mV s−1 (e) Tafel plots of CoNPs@C, RuO2, and Co−H2 for OER; and (f) long-term OER stability tests (without iR corrected) of CoNPs@C and Co−H2.

nanosheets with different powers are close to each other, which suggests the similar HER kinetics (Figure S12b). Furthermore, the morphology of nanosheets is influenced by different powers. As shown in Figure S12c, the carbon tubes are generated at the surface of CoNPs@C-600 nanosheets, which leads to a slight degradation of the catalytic performance. CoNPs@C-700 and CoNPs@C-800 possess integrated nanosheet morphologies (Figure S12d,e); therefore, the catalytic performance of CoNPs@C-700 is similar to that of CoNPs@C800. The edge of nanosheets is degraded, and the nanosheets become smaller under the power of 900 W (Figure S12f). Once the power is too high, the morphology of nanosheets is damaged and thus the catalytic performance declines. Hence, CoNPs@C-700 has the optimal configuration to take full advantage of these structural properties.

To understand the catalytic performance of CoNPs@C, further analysis is carried out after electrochemical measurement. First, the samples are observed by SEM once again after the stability test. One can clearly see that the samples still retain the morphology of nanosheets for both HER and OER (Figure 5a,b). Then, the crystal structure of the sample is also analyzed by XRD. The metal Co peaks after the HER test are the same as those of the original sample (Figure 5c). After the OER stability test, the metal Co peaks become weak in Figure 5d, which may be derived from the partial oxidation of cobalt NPs after the OER stability test. Thus, we also perform the XPS measurements to analyze the change of compositions after the stability test. The peak after HER catalysis is the same as that of the initial one, with a dominated metallic Co peak at 778.5 eV (Figure S13a). After OER catalysis, the X-ray photoelectron 31918

DOI: 10.1021/acsami.7b09487 ACS Appl. Mater. Interfaces 2017, 9, 31913−31921

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Figure 5. SEM images of CoNPs@C (a) after HER and (b) after OER. XRD patterns of CoNPs@C (c) after HER and (d) after OER.

degradation (10 mV) than that of the benchmarking combination (230 mV for Pt/C-RuO2), as shown in Figure 6b. The excellent catalytic activity and long-time stability of the CoNPs@C electrode has potential application in low-cost and energy-efficient overall water electrolysis. The outstanding electrocatalytic activity and stability of the CoNPs@C electrode could be attributed to the following aspects: (1) Highly integrated, hierarchical 3D porous nanostructure improves exposure and accessibility of active sites. In addition, porosity of CoNPs@C makes the diffusion of electrolytes into the carbon layer easier, facilitating the transport of products and reactants. (2) Few-graphene-layerscoating on the surface of Co NPs can not only avoid the agglomeration of neighboring NPs and sustain the stability in the strong basic media but can also improve the catalytic activity of the nanostructure through the synergistic effect. (3) The uniformly distributed small Co NPs encapsulated in interconnected graphene ensure the good conductivity of the whole CoNPs@C nanosheet and thus accelerate the electron transfer during the catalytic process. (4) The self-supporting electrode avoids the use of the polymer binder, which is favorable for the electrolysis process.

spectra show the obvious change compared to that of the initial one (Figure S13b). The dominated peak at 781.2 eV and the peak at 783 eV could be assigned to Co3+ and Co2+,37 respectively, which indicated the oxidation of the sample during the OER process. Moreover, their corresponding shakeup satellites (denoted as “sat”) can also be observed in Figure S13b. Previous reports have demonstrated the cobalt hydr(oxy)oxides formed during the OER test.44,45 In fact, the really active species of Co-based catalysts for OER are in situgenerated cobalt oxides and hydroxides, which have been illustrated in the previous literature.46,47 Thus, the difference of ECSAs for HER and OER are as shown in Figure S10a,c. It’s worth noting that although the intensity of the peak of metallic Co (778.5 eV) obviously decreases, the peak still remains during long-term OER catalysis (20 h), with a high oxidation current (current density: 23 mA cm−2). Inspired by the aforementioned performance test, we further use CoNPs@C as anode and cathode for overall water splitting in alkaline media. Thus, a two-electrode configuration is constructed. Because Pt is the most effective catalyst for HER and RuO2 for OER, Pt−C (20 wt %) and RuO2 are also loaded to the carbon cloth as control samples. Figure 6a shows the polarization curves for overall water splitting, and the Pt/CRuO2 two-electrode configuration exhibit the best catalytic performance with current density of 10 mA cm−2 at 1.62 V. Impressively, CoNPs@C-CoNPs@C couple can afford current density of 10 mA cm−2 at the cell voltage of 1.65 V, which is only 30 mV larger than the Pt/C-RuO2 couple and comparable to the catalytic materials reported recently for overall water splitting (Table S3). Meanwhile, the CoNPs@C-CoNPs@C couple can withstand continuous electrolysis under the constant current density of 23 mA cm−2 for 20 h with less

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CONCLUSIONS In conclusion, we utilize a plasma-assisted process to form a fast and convenient strategy for synthesizing the self-supporting 3D porous CoNPs@C nanosheets. The obtained CoNPs@C nanosheets serve as bifunctional electrocatalysts with high electrocatalytic performance in alkaline solutions for overall water splitting. This novel hierarchical porous nanostructure affords sufficient active sites and outstanding conductivity. Meanwhile, the synergistic effect of the few graphene coating 31919

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Research Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Fax: +86-20-8411-3901 (H.C.). *E-mail: [email protected] (C.W.). ORCID

Chengxin Wang: 0000-0001-8355-6431 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grants Nos. 51472276, 51772338, and U1401241), Pearl River S&T Nova Program of Guangzhou (Grant No. 201610010085), and Guangdong special support program (Grant No. 2014TQ01C483).

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Figure 6. (a) Polarization curves of CoNPs@C-CoNPs@C (black) and RuO2−20% Pt/C (red) for overall water splitting in 1 M KOH at a scan rate of 2 mV s−1 (b) long-term stability test of CoNPs@CCoNPs@C (black), RuO2−20% Pt/C (red).

layers and active materials can promote the catalytic activity. As expected, the CoNPs@C electrode demonstrates the high HER and OER activity with long-term stability. Finally, CoNPs@C serves as a bifunctional catalyst to conduct the overall water splitting and a cell voltage as low as 1.65 V can afford the current density of 10 mA cm−2. This work may be applicative for improving the electrocatalytic performance and provide us a new vision for the design of binder-free diverse metal or multimetal carbon composites, which should be more suitable to the practical application of overall water splitting.

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REFERENCES

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b09487. EDS patterns of CoNPs@C without carbon cloth, XRD patterns of the Co-species precursor, TEM images of CoNPs@C, EDX patterns of CoNPs@C, N2 adsorption−desorption isotherm of CoNPs@C, SEM images of Co−H2 and CoNPs@C with different powers, exchange current densities of CoNPs@C for HER, electrochemical impedance spectrum of CoNPs@C, circle voltammetry curves of CoNPs@C, polarization curves of CoNPs@C with different powers, XPS patterns of CoNPs@C after the stability test (PDF) 31920

DOI: 10.1021/acsami.7b09487 ACS Appl. Mater. Interfaces 2017, 9, 31913−31921

Research Article

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