Integrating cobalt phosphide and cobalt nitride

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Integrating cobalt phosphide and cobalt nitrideembedded nitrogen-rich nanocarbons: highperformance bifunctional electrocatalysts for oxygen reduction and evolution† Xing Zhong, Yu Jiang, Xianlang Chen, Lei Wang, Guilin Zhuang, Xiaonian Li and Jian-guo Wang* The demand for cost-effective bifunctional oxygen electrocatalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) for application in rechargeable metal–air batteries and fuel cells operated in alkaline solutions has increased over the decades. We report for the first time an easy procedure for a unique nitrogen-rich sandwich-architectured catalyst (CoNP@NC/NG) as a highly efficient bifunctional electrocatalyst for ORR and OER. Physical characterizations confirmed the coexistence of Co2P and CoxN crystal phases in the nanostructure. The as-prepared CoNP@NC/NG exhibited potent bifunctional electrochemical performance with superior positive onset potential, large kinetic current density, and outstanding stability toward both ORR and OER, thereby showing excellent activities compared with Pt/C and state-of-the-art nonprecious catalysts. The excellent performance could have originated from the robust conjugation between the Co2P and CoxN crystal structures leading to a synergistic effect of the two interfaces, and the carbon shell also increased the number of nitrogen active sites. Moreover, the integrated structure of CoNP@NC/NG provided high electrical

Received 7th May 2016 Accepted 3rd June 2016

conductivity and facilitated electron transfer. Furthermore, the rechargeable zinc–air battery testing of CoNP@NC/NG-700 revealed good performance and long-term stability. The current work provided

DOI: 10.1039/c6ta03820d

a new pathway to design bifunctional catalysts with multiple crystal phases for energy conversion and

www.rsc.org/MaterialsA

storage.

Introduction Rechargeable metal–air batteries and regenerative fuel cells have stimulated extensive studies in the recent years to meet the requirements of future energy storage and conversion applications.1–6 Bifunctional electrocatalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are undoubtedly crucial for the practical use of such renewableenergy technologies.7–14 Precious metals, such as Pt, Ir, and Rubased materials, have been widely utilized in Zn–air batteries.15,16 However, such precious metals cannot provide dual attention to ORR and OER because of their prohibitive price and scarcity, leading to unsatisfactory bifunctional catalytic performance. Hence, immediate attention has been dedicated to seeking an effective nonprecious bifunctional electrocatalyst for both ORR and OER in Zn–air batteries.17–23 As expected, various low-cost alternative electrocatalysts have been College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, China. E-mail: [email protected] † Electronic supplementary information (ESI) characterizations, electrochemical measurements 10.1039/c6ta03820d

This journal is © The Royal Society of Chemistry 2016

available: results.

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developed, especially cobalt-based materials, such as Co3O4/ graphene hybrids,24 NG–NiCo,25 Co3O4 nanochains,26 and CoFe2O4.27 However, the inherent sluggish kinetics and high overpotential of cobalt-based materials are still less competitive than those of precious metal electrocatalysts. In addition, the undesired catalytic activity and stability degradation of cobaltbased materials during battery operation processes are mainly from corrosion under alkaline conditions. Therefore, the search for a highly efficient and robust cobalt-based bifunctional electrocatalyst for practical applications in Zn–air batteries is imperative. Despite the extensive research on single-cobalt active species for ORR and OER, reports on cobalt-based materials with different cobalt active species as bifunctional electrocatalysts have been rare. Cobalt phosphide (CoxP) and cobalt nitride (CoxN) take the unique advantages of intrinsic Co–P28–30 and Co– N bonding,31,32 which favor the exposure of an abundance of available active sites that may enhance the catalytic activity. From another point of view, both CoxP and CoxN have natural alkali-resistant properties, which may also help improve the stability. Encapsulating metal nanoparticles into a nitrogendoped carbon shell (NC) is a promising strategy to further

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Journal of Materials Chemistry A

prevent the corrosion and agglomeration of metal nanoparticles in an alkaline electrolyte, and the incorporation of NC with metal nanoparticles could improve the conductivity and charge transfer capability,33,34 and subsequently enhance the catalytic performance. However, as far as we know, producing a hybrid material composed of CoxP and CoxN embedded in a NC is a big challenge and has not been realized. Thus, exploring the integrated material as a bifunctional electrocatalyst for ORR and OER in Zn–air batteries is highly desirable. Based on the above concept, we present an easy and ingenious approach to fabricate a unique nitrogen-rich sandwicharchitectured catalyst composed of Co2P and CoxN nanoparticles embedded in a NC and nitrogen-doped graphene as a support (denoted as CoNP@NC/NG). The Co2P and CoxN nanoparticles are encapsulated in the NC and uniformly dispersed on the nitrogen-doped graphene sheets. The positive synergistic effect of the coexistence of Co–P and Co–N bonding, and the strong coupling between the NC and CoNP nanoparticles largely promote the interfacial electron transfer between the catalyst surface and reaction intermediates. The optimized CoNP@NC/NG-700 acts as an efficient bifunctional catalyst with high activity and excellent durability toward both ORR and OER in the alkaline solution. We also developed rechargeable Zn–air batteries using CoNP@NC/NG-700 as the air cathode, which demonstrates good performance and longterm stability.

Experimental section Synthesis procedures Preparation of CoNP@NC/NG-700. In a typical synthesis, pristine graphene (10 mg) was rst suspended in 10 mL of an ethanol/water (v/v ¼ 1 : 1) solution containing CoPc (10 mg) and aqueous cyanamide solution (0.4 mL, 50 wt%) and then ultrasonically treated for 0.5 h. Subsequently, the mixture was stirred at 80 C in an oil bath to remove the solvent. The obtained solid was vacuum dried at 80 C for 3 h, then ground to powder and placed into two separate positions in a quartz tube with a corundum boat loading of 0.2 g of (NH4)2HPO4 at the upstream side of the furnace. The system subsequently experienced a thermal treatment at 550 C for 2 h in an NH3 atmosphere with a ow rate of 45 sccm, then was further heated to 700 C with a ramp rate of 10 C min1 and maintained at 700 C for another 2 h. The composites were further heated to 600 C and 800 C for comparison, and the samples obtained are denoted as CoNP@NC/NG-600 and CoNP@NC/NG-800, respectively. Preparation of CoNP/NG-700. The CoNP@NG-700 catalyst was synthesized using the same method employed for CoNP@NC/NG, except that no cyanamide was added during the procedure. Electrode preparation. To prepare the working electrode for electrochemical tests, 1 mg of the as-synthesized catalyst was dispersed in ethanol (0.9 mL, 98 vol%) with Naon (0.1 mL, 5 wt%). Subsequently, the mixtures were subjected to an ultrasonic treatment for 5 min and a homogeneous catalyst ink was obtained. Before the tests, 10 mL of the dispersion was transferred on to a glassy carbon electrode (GCE) with a diameter of 4

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mm, followed by the drying process at room temperature. The Pt/C catalyst (20 wt% Pt on carbon, Alfa Aesar) suspension was also prepared with the same method for comparison. Physicochemical characterizations. Transmission electron microscopy (TEM) images were recorded on a Tecnai G2F30STwin electron microscope at an operation voltage of 300 kV. Scanning electron microscopy (SEM) images were recorded on a SU8020 with an accelerating voltage of 2.0 kV. Thermogravimetric analysis (TGA) proles were recorded on a Perkin Elmer Thermal Analyzer from 10 to 900 C under an argon atmosphere at a ramp rate of 10 C min1. X-ray photoelectron spectroscopy (XPS) measurements were conducted on a Kratos AXIS Ultra DLD X-ray photoelectron spectrometer. X-ray diffraction spectra (XRD) measurements were carried out on an XPERT-PRO X-ray diffractometer utilizing CuKa radiation. Raman spectra measurements were carried out on an InVia Renishaw confocal spectrometer with a 532 nm excitation laser. Electrochemical measurements. The data of cyclic voltammetry (CV) and rotating disk electrode (RDE) measurements were recorded on a CHI760C electrochemical workstation from CHI Instrument Corp. Shanghai. The RDE tests were conducted on a RRDE-3A equipment from ALS Co., Ltd. Cyclic voltammetry tests. The CV tests were conducted in a three electrode system consisting of a Pt wire counter electrode, an Ag/AgCl/KCl (3 M) reference electrode, the as-prepared working electrode, and a solution (aq.) of KOH (45 mL, 0.1 M) in a 100 mL quartz beaker. The solution was bubbled with O2 for 30 min to reach an O2 saturated state before all the measurements. A ow of O2 was maintained over the electrolyte during the electrochemical tests. The scan rate for the CV tests was kept at 100 mV s1. Rotating disk electrode tests. The RDE tests were carried out with the same mass load as CV measurements on a rotating GCE (4 mm in diameter). Ag/AgCl/KCl (3 M) and a platinum wire were utilized as the reference and counter electrodes, respectively. For ORR, the linear sweep voltammogram (LSV) data of the as-prepared working electrode were recorded in O2 saturated 0.1 M KOH electrolyte with a scan rate of 20 mV s1 at various rotating speeds from 400 to 2025 rpm. Kinetics analysis was conducted by using the Koutecky–Levich (K–L) equation for obtaining the apparent number of electrons transferred per O2 involved during the ORR procedure: 1 1 1 ¼ þ j jk Bu1=2

B ¼ 0.2nFC0(D0)2/3v1/6 where j is the measured current density, jk is the kinetic current, u is the angular velocity (rpm) of the working electrode, and B is the Levich constant, n is the overall number of transferred electrons during the ORR process, F is the Faraday constant (96 485 C mol1), D0 is the diffusion coefficient under the test conditions (1.9 105 cm s1), C0 is the concentration of O2 (1.2 103 mol L1), and v is the kinetic viscosity (0.01 cm2 s1). For OER, the LSV data of the as-prepared working electrode


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were recorded in O2 saturated 0.1 M KOH electrolyte at the scan rate of 20 mV s1. The working electrode was kept rotating at 1600 rpm to remove the produced O2 bubbles. Due to the effect of ohmic resistance, iR compensation was applied for all the OER electrochemical measurements during the experiments. Electrochemical impedance spectra (EIS) were recorded in the frequency range from 1000 kHz to 1 Hz with an AC signal amplitude of 5 mV. RHE conversion. The potential scale was calibrated to a reversible hydrogen electrode (RHE) via the following Nernst equation: ERHE ¼ EAg/AgCl + 0.059pH + EqAg/AgCl in which ERHE is the calibrated potential of the RHE scale, EAg/ q AgCl is the measured potential during the experiment, and EAg/ AgCl is the normal potential of Ag/AgCl at 25 C, which is 0.1976 V under the experimental conditions. The electrochemical tests were performed in 0.1 M KOH at 25 C; thereby, ERHE ¼ EAg/AgCl + 0.9646 V. Zinc–air battery test. The measurements of the rechargeable zinc–air batteries were performed on home-built electrochemical cells. The as-prepared CoNP@NC/NG uniformly coating a carbon paper was used as the air cathode (catalyst mass loading of 1 mg cm2). A Zn foil (2.5 cm 1 cm 0.05 cm) was employed as the anode. The zinc–air battery was fabricated by pairing the CoNP@NC/NG-700 and Zn-foil electrodes in 100 mL of 6 M KOH with 0.2 M of zinc acetate solution. During the battery measurements, O2 (10 mL min1) was continuously purged into the electrolyte solution. The cycling performance of the rechargeable Zn–air batteries was investigated by charging at the current density of 5 mA cm2 and discharging at 0.5 mA cm2 using a battery test system (LANHE CT2001A). Computational details. All of these calculations were performed using the Vienna ab initio simulation package (VASP).35–37 In order to simulate the O, OH, OOH, key species of ORR and OER, adsorbed on Co2P (121) and Co2N (111), the ˚ between the adjacent layers for Co2P vacuum layer was 15 A (121) and Co2N (111), respectively. For the geometry optimizations, only the bottom layer was xed. The Brillouin zone integration was performed using the Monkhorst–Pack scheme with 4 4 1 periodic models. The convergence criteria of the force ˚ 1 and 0.01 meV, respectively. The and energy are 10 meV A adsorption energies (Eads) of O, OH, and OOH to Co2P (121) and Co2N (111) were calculated by the equation: Eads ¼ Etotal Esurf Emol where Etotal, Esurf and Emol represent the total energies of the combined, surface and molecular systems.

Results and discussion The fabrication procedure of the CoNP@NC/NG-700 hybrid is illustrated in Scheme 1. Pristine graphene, CoPc, and cyanamide solution were used to prepare the CoPc/pristine graphene


Scheme 1

Schematic illustration of the fabrication of CoNP@NC/NG

hybrids.

precursor (CoPc/PG) through a one-step mild physical process in the water/ethanol system, as described in the Experimental section. CoPc and graphene have good dispersibility in water/ ethanol solution, resulting in a uniform distribution of the CoPc on the graphene sheets aer the solvent evaporation. The water-soluble cyanamide can be adsorbed on the surface of the CoPc clusters because of the weak interaction between the cobalt center and the nitrogen atom of the cyanamide, leading to a higher concentration of cyanamide around the CoPc. During the annealing process in the NH3 atmosphere, the cyanamide experienced a thermal condensation process when the temperature was maintained at 550 C, which resulted in the formation of a polymeric carbon nitride (CN) that could be adsorbed on the PG surface. Meanwhile, the phosphate group carried by the owing gas is also easily deposited on the CN and PG surface.38 Subsequently, further increase of the annealing temperature led to the decomposition of CN, achieving the Ndoping into the graphene matrix.39 The acceleration of nitrogen doping of graphene could also be achieved with the assistance of the NH3 atmosphere.40 Moreover, the CoPc molecules began to aggregate into larger clusters aer the calcination at 550 C, and the cyano fragments (e.g., C2N2+, C3N2+, and C3N3+) produced by the pyrolyzation of carbon nitride started to deposit on the reduced Co2+, thus restricting its further growth and formed a NC shell outside the compounds. Meanwhile, the phosphate group previously adsorbed on the NC started to interact with the cobalt center to fabricate Co2P and CoxN with the assistance of the NH3 atmosphere. Finally, the nitrogen rich core/shell nanostructure with an inner core of crystalline Co2P and CoxN and external NC shell was fabricated. As a reference point, the CoPc/PG precursor was also prepared through the same method, except that no cyanamide solution was added, and named as CoNP/NG-700. The morphologies of the CoNP@NC/NG-700 sample were investigated via transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The TEM (Fig. 1a and S1a†) and SEM (Fig. 1b) images of CoNP@NC/NG-700 indicate the uniform dispersion of the CoNP nanoparticles over the NG

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sheets, where most of the nanoparticles exhibited spherical morphologies with average diameters of 35 nm (Fig. 1a, inset). The energy-dispersive X-ray spectroscopy (EDX) mapping analysis spectrum (Fig. 1c) conrmed the existence of C, N, Co, O, and P in CoNP@NC/NG-700. Fig. 1d reveals the well-recognized core shell nanostructure of the particles. High-resolution TEM (HRTEM) studies of CoNP@NC/NG700 (Fig. 1e) show that the thickness of the external nitrogendoped carbon sheet was about 2.8 nm. Moreover, two kinds of lattice fringes with interspaces of 0.22 and 0.21 nm, corresponding to the Co2P (112) and CoxN (211) planes, respectively, were observed.32 The nitrogen doped carbon shell was amorphous, which could have resulted from NH3 etching, leading to the gasication of disordered-phase carbon formed in the shells.32,41 In contrast, no lattice fringe and sign of NC shell were observed outside of the CoNP nanoparticles for the CoNP particles in CoNP/NG-700 (Fig. S2a and S2b†), indicating the bifunctional role of cyanamide in forming the NC shell and tailoring the crystal structure during the annealing procedure. In addition, Fig. S1b† shows that some of the CoNP nanoparticles in the CoNP/NG-700 sample exhibited an irregular morphology with a larger particle size (115 nm) than that of CoNP@NC/NG-700. This may be caused by the lack of cyano fragments to restrict the growth of the CoPc cluster particles during the annealing process. Moreover, no P element was detected in CoNP/NG-700 by the EDX measurement (Fig. S2c†), which could be attributed to the lack of carbon nitride intermediates to capture the phosphate groups. TEM measurements

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were also conducted for CoNP@NC/NG-600 and CoNP@NC/NG800. Fig. S3a and S3b† show that no regular particle can be observed, indicating that the CoPc failed to form a standard lattice structure at 600 C. Furthermore, a large amount of undecomposed sheet-like carbon nitrides and some large-sized irregular clusters were also observed, indicating that most of the carbon nitrides did not reach the pyrolysis step to release functional cyano fragments, which led to the random and unrestricted growth of CoPc clusters at 600 C. The CoPc clusters also failed to form well-dened crystal structures at 600 C. In contrast, Fig. S3c† illustrates that the sample annealed at 800 C possessed nanoparticles with well-dened spherical morphologies and a smaller average particle size (20.1 nm, Fig. S3d†), which could be attributed to the accelerated growth restriction effect towards CoPc clusters caused by the rapid pyrolysis of carbon nitrides as the temperature was raised from 550 C to 800 C. The PG sheets were not distinctively visible for CoNP@NC/NG-600 and CoNP@NC/NG-700 as illustrated in Fig. 1a and S3a,† which can be attributed to the undecomposed layer of CN adsorbed on the graphene surface, while the surface of the sample was relatively “clean” for the CoNP@NC/NG-800, indicating that the degree of the CN decomposition would increase with the temperature. Fig. 1g–j show the distributions of Co, P, N, and O elements in a certain CoNP@NC nanoparticle. The chemical constituents of cobalt phosphide and cobalt nitride can be suggested by the relatively strong signal of Co, P, and N. The presence of the O element can also be observed (Fig. 1j), which can be ascribed to the formation of

Fig. 1 TEM and SEM images of CoNP@NC/NG-700. (a) TEM image of CoNP@NC/NG with a scale bar of 200 nm. (b) SEM image of CoNP@NC/ NG with a scale bar of 250 nm. (c) EDX spectrum of a CoNP nanoparticle. (d and e) Enlarged image of spherical NC shell coated CoNP nanoparticles. (f) Index crystal plane of a CoNP nanoparticle. (g–j) EDS elemental mapping images of Co, P, N, and O.

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cobalt oxides on the surface of the nanoparticle caused by air contact.42 The X-ray diffraction (XRD) measurement of CoNP@NC/NG700 (Fig. 2a) shows a series of well-dened diffraction peaks, which are consistent with the simulated pattern for Co2P (JCPDS card no. 32-0306) and Co5.47N (CPDS card no. 41-0943). The diffraction peaks located at 40.7, 40.9, 43.3, and 52.1 are attributed to the (121), (201), (211), and (002) planes of Co2P, respectively, while the peaks at 43.5, 50.7, and 74.98 correspond to (111), (200), and (220) planes of Co5.47N, conrming the crystalline structure of Co2P and Co5.47N.30,32 In addition, the degree of crystallization for the nanoparticle increases with the annealing temperature, which is proven by the sharpest diffraction peaks of CoNP@NC/NG-800 among all the samples. Many crystalline metallic compounds have been proven to possess the ability to catalyze ORR or OER reaction. However, the increase of the crystallization degree also means the decrease of the defects in the crystal lattice, which may reduce the density of active sites and restrict the electrochemical activity of the catalyst. CoNP/NG-700 displays only weak diffraction peaks corresponding to Co5.47N (Fig. S4†), revealing the cyanamide function of tailoring the crystal structure of nanoparticles and forming carbon nitrides as intermediates to capture the phosphate group during the synthesis procedure. XPS measurement was carried out to gain further information on the valence states and elemental compositions of the

(a) XRD results of CoNP@NC/NG hybrids annealed at 600 C, 700 C, and 800 C. (b–e) High-resolution XPS spectra of the N1s, C1s, Co2p, and P2p of CoNP@NC/NG-700. (f) TGA measurement for PG, CoNP@NC/NG-700, and CoNP/NG-700. Fig. 2



CoNP@NC/NG-700 hybrids. Focusing on the high-resolution scan of the N1s electrons (Fig. 2b), the peaks at 398.4, 400.0, and 400.9 eV correspond to the pyridinic, pyrrolic, and quaternary N, respectively, indicating the successful introduction of N into the nitrogen-doped carbon shell and graphene matrix. The peak at 399 eV can be assigned to CoxN.32,43,44 Nitrogen doping could also be conrmed by the survey spectrum for the C1s region in Fig. 2c, where the peak at 285.3 eV is consistent with the C–N chemical bond.45 The Co2p and P2p regions of the XPS 2p spectrum of the CoNP@NC/NG-700 sample are shown in Fig. 2d and e, respectively. Fig. 2d shows the XPS spectrum of Co2p. The two main peaks at 778.3 and 780.9 eV and the two satellites at 783.8 and 785.8 eV are consistent with the Co2p3/2 of Co2P, while the main peak at 796.8 eV and the satellite at 802.8 eV match the Co2p1/2 of Co2P.42 The peaks simulated at 132.8 eV for P2p3/2 and 778.3 eV for Co2p3/2 are consistent with the binding energies for Co2p and P2p contributions in Co2P, respectively. Two main peaks appear at 782.1 and 800.2 eV, which correspond to the N-coordinated Co, conrming the presence of the Co–N bond.46 The tted peak of Co–O is overlapped by the Co–N peak because of the similar values of binding energy (BE). However, two weak and broad satellites are observed at 788.4 and 803.0 eV, which are usually retained by cobalt oxides.47 This result provided further evidence for the surface oxidation of nanoparticles caused by air exposure. The deconvolution of the P2p region shows two peaks at 133.3 and 132.8 eV, corresponding to the BEs of P2p1/2 and P2p3/2, respectively. The aforementioned binding energies are larger than those of published reports,29,30 which may be due to the effect of N on the dispersal of the negative charge on P and change of its electronic state. Another peak at 134.6 eV can be assigned to the oxidized phosphate species.42 Besides, no peak at 132.1 eV was observed, which corresponds to P–C bonding, indicating that the graphene is not transformed into P-doped graphene.48 We also detected the blank space of CoNP@NC/NG-700, there is also almost no P element observed (Fig. S5†). The high-resolution scan of the P2p region (Fig. S6†) for CoNP/NG-700 shows no obvious characteristic peak, indicating the failure of P doping without the assistance of the carbon nitride intermediates. The thermogravimetric analysis (TGA) of PG, CoNP@NC/NG700, and CoNP/NG-700 reveals that obvious weight loss occurs at about 660 C (Fig. 2f), which can be mainly assigned to the decomposition of CN and PG. Aer the temperature reaches 900 C, the total weight loss for PG, CoNP@NC/NG-700, and CoNP/ NG-700 is 13%, 6% and 10%, respectively, suggesting that CoNP@NC/NG-700 has the best thermostability among the catalysts. The same conclusion can also be obtained through analyzing the species concentrations of the various samples based on the XPS results. Table S1† shows that the concentration of Co decreased with rise of temperature, indicating the loss of active components because of thermal decomposition at higher temperatures. The Co concentration of CoNP/NG-700 (0.9%) is much lower than that of CoNP@NC/NG-700 (1.98%), suggesting that the formation of the NC shell can slow down the loss of Co thereby retaining more active sites for electrochemical catalysis. Raman spectra measurements were carried

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out for the comparison of the band intensity ratio aspects of D (1340 cm1) and G (1590 cm1). Fig. S7† shows that the ID/IG ratio of CoNP@NC/NG-700 (1.38) is much higher than that of PG (1.22), suggesting a higher density of defects in the carbon matrix. Almost equal values (1.26) of ID/IG for CoNP@NC/NG600 and CoNP@NC/NG-800 were obtained, indicating the similar defect densities of these two samples. These results can be attributed to three reasons. First, the introduction of CN means the introduction of more sp3 hybrid carbon atoms, thereby increasing the value of ID/IG for CoNP@NC/NG600 compared with PG. Second, aer 700 C, the temperature increase can speed up the thermal decomposition of CN before it can interact with the C atoms on the graphene surface and change their hybridization state by nitrogen doping. This can be proven by the TEM images of CoNP@NC/NG-600, CoNP@NC/ NG-700, and CoNP@NC/NG-800. Third, a higher annealing temperature can increase the graphitization degree of undecomposed CN, thus reducing the value of ID/IG.39 To investigate the ORR activity of the CoNP@NC/NG composites, cyclic voltammetry (CV) measurements were carried out in a 0.1 M KOH solution, saturated with O2 between 1.0 V and +0.2 V vs. Ag/AgCl. The ORR performance of the commercial Pt/C (20 wt% Pt on carbon black) was also measured under the same conditions for comparison. Fig. 3a shows that all the samples showed featured ORR peaks during the test. Compared with CoNP@NC/NG-600 and CoNP@NC/ NG-800, CoNP@NC/NG-700 displayed superior ORR activity with a more positive onset potential and higher cathode current density (3.36 mA cm2 at 0.78 V vs. RHE), indicating that the best annealing temperature was 700 C. In addition, Fig. 3a illustrates that CoNP@NC/NG-700 showed a much higher ORR performance than commercial Pt/C, which is indicated by its more positive onset potential (30 mV) than that of Pt/C. The catalytic performance of CoNP/NG-700 was also tested, which exhibited much worse ORR activity than CoNP@NC/NG-700 in terms of both the current density and onset potential. The catalytically active surface area of the various catalysts has been investigated by CV measurements at different scan rates (Fig. S8†). The calculated value of the electrochemical doublelayer capacitance (Cdl) can be used to evaluate the active surface area of the electrocatalysts. Fig. S9† illustrates that CoNP@NC/ NG-700 possesses the largest active surface area (5.9 mF cm2) among all the prepared samples, which can contribute to its electrochemical performance. To gain a deeper insight into the ORR performance of the catalyst, we carried out rotating disk electrode (RDEs) measurement in 0.1 M KOH saturated with O2 between 1.0 and +0.2 V vs. Ag/AgCl. A series of linear sweep voltammograms for various catalysts are displayed in Fig. 3b and S10.† Fig. 3c shows that the cathodic current of the CoNP@NC/NG700 sample started to sharply increase when the electrode potential shied negatively and reached 0.93 V vs. RHE, which is comparable with commercial Pt/C (0.93 V vs. RHE). However, the onset potential for CoNP/NG-700 is much more negative at 0.88 V vs. RHE. The CoNP@NC/NG samples annealed at different temperatures were also tested. Fig. 3c shows that both CoNP@NC/NG-600 and CoNP@NC/NG-800 exhibited a more

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(a) ORR evaluation by the cyclic voltammograms of various catalysts in O2-saturated 0.1 M KOH. (b) LSV curves of CoNP@NC/NG700 under various rotating speeds and inset of corresponding K–L plots. (c) LSV curves of the various catalysts on a RDE (1600 rpm) in O2saturated 0.1 M KOH. (d) K–L plots of the various catalysts at 0.6 V vs. RHE. (e) Kinetic limiting current and the corresponding electrontransfer numbers at 0.6 V vs. RHE on the basis of the RDE data (1600 rpm) of various catalysts. (f) Tafel plots obtained from the RDE measurements on commercial Pt/C at 1600 rpm. Fig. 3

negative onset potential (about 0.86 V vs. RHE) than CoNP@NC/ NG-700. The difference between the limiting currents at 1600 rpm also exhibited apparent variations among the catalysts. For example, the limiting currents at 0.8 V vs. RHE (Fig. 3d) were 2.81 mA cm2 for CoNP@NC/NG-700, 1.38 mA cm2 for CoNP/NG-700, 1.19 mA cm2 for CoNP@NC/NG-600, 1.12 mA cm2 for CoNP@NC/NG-800, and 1.46 mA cm2 for commercial Pt/C, further demonstrating that CoNP@NC/NG700 is the best ORR catalyst among the samples both in onset potential and current density. The overall electron transfer numbers per oxygen molecule involved in a typical ORR procedure were calculated using the Levich equation. The calculation results for various catalysts at 0.6 V vs. RHE are listed in Fig. 3d. Following the excellent kinetics of Pt (electron transfer number of 4.0) in alkaline medium for ORR, CoNP@NC/NG-700 showed an electron transfer of 3.8, which is comparable to that of Pt/C, indicating a 4 electron pathway for ORR, whereas CoNP@NC/NG-600, CoNP@NC/NG-800, and CoNP/NG-700 showed 3.4, 3.11, and 3.2, respectively, which are lower than that of CoNP@NC/NG700. More calculation results for various samples at different potentials can be found in Fig. S11.† In addition, the K–L plots showed that CoNP@NC/NG-700 achieved a kinetic limiting current density (Jk) of 28.46 mA cm2 at 0.6 V vs. RHE (Fig. 3e),


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nearly 2.2 times higher than the value for commercial Pt/C (13.06 mA cm2). The superior ORR activity of CoNP@NC/NG700 can also be demonstrated by its considerably smaller Tafel slope (56 mV per decade) than that of CoNP@NC/NG600 (72 mV per decade), CoNP@NC/NG-800 (59 mV per decade), CoNP/NG-700 (75 mV per decade), and Pt/C (92 mV per decade) at low overpotentials (Fig. 3f). We calculated the mass activity to represent the intrinsic activity of the catalysts and compare the electrocatalytic activities of all evaluated catalysts for ORR. CoNP@NC/NG-700 exhibited a 2.04 times larger mass activity at 0.6 V vs. RHE than that with Pt/C, suggesting the highly efficient catalysis function of CoNP@NC/NG-700 toward ORR. A certain range of potential from 0 to +1.20 V vs. Ag/AgCl was applied on the working electrode with a scan rate of 20 mV s1 in O2 saturated 0.1 M KOH solution to estimate the OER catalytic performance of the prepared samples. The rotating speed for the electrode was maintained at 1600 rpm to remove the produced O2 bubbles. The good OER catalytic activities for CoNP@NC/NG-700 were reected by its more negative onset potentials and higher current than that of the CoNP@NC/NG600, CoNP@NC/NG-800, CoNP/NG-700, and Pt/C electrodes (Fig. 4a). The potential required for the current density of 10 mA cm2 is a metric related to the solar fuel, which has also been widely used as an essential parameter to evaluate the OER activity. As for CoNP@NC/NG-700 (Fig. 4b), the potential corresponding to the current density of 10 mA cm2 is 1.62 V vs. RHE, which is 0.23 V and 0.07 V lower than that of CoNP@NC/ NG-600 and CoNP@NC/NG-800, respectively, indicating the best catalytic performance of CoNP@NC/NG-700 among the samples annealed at various temperatures. When the CoNP@NC/NG-700 catalyst was loaded on a carbon cloth (1 1 cm), vigorous gas evolution was observed on the surface of the prepared working electrode at the potential of 2.0 V vs. RHE (see the video in the ESI Movie†), further demonstrating the excellent OER catalytic performance of CoNP@NC/NG-700.


In addition, CoNP/NG-700 exhibited a much higher potential (1.72 mV) at 10 mA cm2, suggesting that a superior lattice structure and carbon shell outside the nanoparticles are also vital factors in enhancing the catalytic performance for OER. Furthermore, the semicircular diameter in the electrochemical impedance spectrum (Fig. S12†) of CoNP@NC/NG-700 indicates a smaller charge-transfer impedance than that of CoNP/NG-700, certifying that the NC in our system can reduce charge transfer resistance and provide high electrical conductivity. Meanwhile, the OER current on the Pt/C reached 10 mA cm2 at 1.95 V vs. RHE, suggesting much worse electrochemical activity for OER than CoNP@NC/NG-700. The Tafel slope for the CoNP@NC/NG700 catalyst was about 78 mV per decade (Fig. 4c), which is smaller than that of CoNP@NC/NG-600 (138 mV per decade), CoNP@NC/NG-800 (89 mV per decade), CoNP/NG-700 (98 mV per decade) (Fig. 4c), and commercial Pt/C (164 mV per decade), which further conrmed the best OER performance of CoNP@NC/NG-700. The overall activity of the oxygen electrode can be displayed by DE obtained using the following equation:49 DE ¼ EOER EORR where EOER and EORR represent the potentials corresponding to the current density of 10 and 3 mA cm2, respectively. The calculated results are listed in Fig. 5d and Table S2 (ESI†), which show that the value of DE for CoNP@NC/NG-700 is 0.84 V, much lower than that of commercial Pt/C. Furthermore, the overall electrode potential of the as-prepared sample is much smaller than those of previously published studies (Fig. 5d and Table S2†), indicating that CoNP@NC/NG-700 is one of the most active bifunctional catalysts. Cyclic potential sweeps between 0 V and 1.16 V at a sweep rate of 100 mV s1 were utilized in an O2-saturated 0.1 M KOH solution to evaluate the durability of the catalysts for ORR.

(a) ORR endurance test of CoNP@NC/NG-700, CoNP/NG-700, and Pt/C for 2000 cycles in O2-saturated 0.1 M KOH at 0.78 V vs. RHE. (b) OER endurance test of CoNP@NC/NG-700 and CoNP/NG-700 for 5000 s in O2-saturated 0.1 M KOH at an overpotential of 335 mV. (c) Methanol crossover tolerance test by studying the chronoamperometric response in 0.1 M KOH aqueous electrolyte at 0.78 V. (d) Overall activity of the oxygen electrode for various catalysts. Fig. 5

Fig. 4 (a) OER currents of various catalysts and commercial Pt/C

electrodes in O2-saturated 0.1 M KOH solution at a scan rate of 20 mV s1. (b) Potential required for 10 mA cm2 of various catalysts. (c) Tafel plots of various catalysts and commercial Pt/C calculated from polarization curves. (d) Oxygen electrode activities within the ORR and OER potential windows of various catalysts in O2-saturated 0.1 M KOH.


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Fig. 5a and S13† show that CoNP@NC/NG-700 shows a slight loss of 8.4% aer 2000 cycles. In contrast, the commercial Pt/C showed a considerable loss of 30.0%, suggesting that the durability of Pt/C is worse than that of CoNP@NC/NG-700. CoNP/NG-700 showed a signicant loss of 14.4%, which can be attributed to the lack of NC protection. Chronoamperometric response and successive cyclic voltammetric (CV) scanning were used to investigate the durability of catalysts for OER. Aer maintaining a static overpotential of 335 mV for 5000 s, the CoNP@NC/NG-700 catalyst retained 94.2% of the original current density, while CoNP/NG-700 preserved only 80.7%, suggesting the protective effect of the NC shell toward the catalyst for both OER and ORR. Furthermore, aer scanning for 1000 cycles at 100 mV s1, the CoNP@NC/NG-700 catalyst suffered a slight current drop (4.4% at 2.1 V vs. RHE, Fig. S14†). Furthermore, the well-retained core/shell morphology was revealed by the TEM image of a certain nanoparticle in CoNP@NC/NG-700 (Fig. S15†), further conrming the good stability of CoNP@NC/NG-700. A methanol crossover test on the CoNP@NC/NG-700 sample was also performed through chronoamperometric responses. Fig. 5c illustrates that the cathodic ORR currents of CoNP@NC/NG-700 and CoNP/NG700 were nearly unchanged aer the addition of methanol (1.5 mL, 3 mol L1), while a dramatic decrease in the cathodic ORR current was observed for Pt/C, indicating that the resistance ability of CoNP@NC/NG-700 toward methanol is better. The outstanding electrocatalytic activities of the CoNP nanocomposites can be mainly assigned to the synergistic effect of the two structures. Adsorption energies of O, OH, and OOH, the key species of ORR and OER reactions, on Co2P (121) and Co2N (111) surfaces were investigated using density functional theory (DFT) calculations (Fig. 6a). On both surfaces, it was found that the adsorption energy of OOH is much weaker than that of O and OH. And the adsorption energies are nearly the same on the two surfaces except that the adsorption energy of OOH is slightly stronger on Co2P (121) than on Co2N (111). Meanwhile, the total densities of states (DOS) for the two surfaces are also shown in Fig. 6b. It indicates that the total DOS of Co2N (111) is more localized on the Fermi level than that of Co2P (121). The robust conjugation between Co2P and CoxN crystal structures provides the as-prepared sample a unique structure, which is expected to maximize the number of exposed active sites for electrochemical reactions. Both the geometric and electronic factors are mainly responsible for the improvement of catalytic activity. In addition, the nitrogen-doped carbon shell and the graphene substrate contribute to increasing the dispersion of CoNP nanoparticles during the carbonization process. Moreover, graphene also improves the electroconductivity and facilitates the charge transfer in the catalyst, thereby enhancing the electrocatalytic performance.50 The carbon shells hamper the aggregation of CoNP and help boost the endurance during the reaction. The CoNP@NC/NG catalyst exhibits a potent bifunctional electrochemical performance benet from the synergistic catalytic effects of the aforementioned aspects. A rechargeable zinc–air battery prototype was fabricated (as shown in Fig. 7a) according to the literature to explore future

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Fig. 6 (a) Adsorption energy of O, OH and OOH on Co2P (121) and Co2N (111). (b) TDOS of Co2P and Co2N.

practical applications.19,51 CoNP@NC/NG-700 loading on carbon cloth with a mass density of 1 mg cm2 was employed as the air cathode, Zn foil as the anode, and 6 M KOH with 0.2 M zinc acetate as the electrolyte. A stable open circuit voltage of 1.43 V was observed for CoNP@NC/NG-700, (Fig. 7b) which is slightly higher than that of 20% Pt/C (1.40 V), indicating that the Zn–air battery using the CoNP@NC/NG-700 catalyst can work smoothly. Fig. 7c shows that the maximum power density of the Zn–air battery was determined to be as high as 87.3 mW cm2, demonstrating that the CoNP@NC/NG-700 catalyst could be used as a bifunctional cathode catalyst for Zn–air batteries. Fig. 7d shows the discharge and charge polarization curves, where the charge performance of CoNP@NC/NG-700 was observed to be signicantly better, while the commercial Pt/C catalyst exhibited a comparable discharge performance. The sum of the charging and discharging overpotentials of CoNP@NC/NG-700 is 1.25 V at a current density of 100 mA cm2, which is lower than that of Pt/C (1.53 V). This phenomenon is consistent with the performance comparison in halfcells. Furthermore, the cycle stability of CoNP@NC/NG-700 as a bifunctional air electrode catalyst for rechargeable zinc–air batteries was investigated with the battery charged at a current density of 5 mA cm2 and discharged at 0.5 mA cm2 (Fig. 7e). The CoNP@NC/NG-700 air-cathode produced an initial charge potential of 2.13 V and discharge potential of 1.28 V, with a small voltage gap of 0.85 V. Aer 100 cycles (about 30 h), the overall voltage gap for the charge and discharge processes for


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(a) Schematic representation of the rechargeable Zn–air battery with the CoNP@NC/NG-700 catalyst loaded on carbon cloth (1 mg cm2); (b) open circuit plot; (c) discharge polarization and power density curves; (d) charge and discharge polarization curves; (e) galvanostatic discharge–charge cycling curves of rechargeable Zn–air batteries with CoNP@NC/NG-700; (f) digital photograph of the lighting of LEDs connected with two Zn–air batteries with the CoNP@NC/NG-700 air-cathode connected in series. Fig. 7

CoNP@NC/NG-700 was 0.89 V, showing a slight performance loss compared with the initial voltage gap, which demonstrates virtually negligible voltage fading aer prolonged cycling. The practicality of the CoNP@NC/NG-700 catalyst is further elucidated in Fig. 7f to emphasize its electrocatalytic performance in a realistic environment. We lit up 40 LEDs using two seriesconnected Zn–air batteries based on the CoNP@NC/NG-700 air cathode. All the aforementioned results indicate that CoNP@NC/NG-700 can be an efficient air electrode in rechargeable metal–air batteries, which also suggests its great potential as a replacement for precious-metal-based catalysts for real applications.

Conclusions A unique nitrogen-rich sandwich-architectured CoNP@NC/NG catalyst was successfully fabricated. Physical characterizations revealed the presence of both Co2P and CoxN lattice structures in one nanoparticle. Most of the nanoparticles were encapsulated in the nitrogen-doped carbon shell and uniformly dispersed on the graphene sheets. CoNP@NC/NG-700 can behave as a highly efficient bifunctional electrocatalyst for both ORR and OER with superior positive onset potentials, large kinetic current density, and outstanding stability. The electrocatalysis performance of CoNP@NC/NG-700 is among the most active bifunctional catalysts. CoNP@NC/NG-700 can also be an efficient air electrode in rechargeable zinc–air battery testing, and revealed good performance and long-term stability. The impressive performance originates from the positive synergistic


effect of the two interfaces between the Co2P and CoxN crystal structures, and the nitrogen-rich nanostructure also helped to increase nitrogen active sites, thus improving the catalytic activity. Our present strategy, which uses multiple metal crystal phases encapsulated in nitrogen-rich carbon, introduces a new pathway to explore the design of high-efficiency multifunctional electrocatalysts for practical applications.

Acknowledgements The authors acknowledge the nancial support from the National Basic Research Program (973 program, No. 2013CB733501) and the National Natural Science Foundation of China (No. 21306169, 21101137, 21136001, 21176221 and 91334013).

Notes and references 1 F. Cheng and J. Chen, Chem. Soc. Rev., 2012, 41, 2172–2192. 2 Y. Li and H. Dai, Chem. Soc. Rev., 2014, 43, 5257–5275. 3 M. Liu, R. Zhang and W. Chen, Chem. Rev., 2014, 114, 5117– 5160. 4 Y. J. Wang, N. Zhao, B. Fang, H. Li, X. T. Bi and H. Wang, Chem. Rev., 2015, 115, 3433–3467. 5 R. Bashyam and P. Zelenay, Nature, 2006, 443, 63–66. 6 M. K. Debe, Nature, 2012, 486, 43–51. 7 X. Ge, Y. Liu, F. W. Goh, T. S. Hor, Y. Zong, P. Xiao, Z. Zhang, S. H. Lim, B. Li, X. Wang and Z. Liu, ACS Appl. Mater. Interfaces, 2014, 6, 12684–12691.

J. Mater. Chem. A, 2016, 4, 10575–10584 | 10583

View Article Online



8 J. I. Jung, H. Y. Jeong, J. S. Lee, M. G. Kim and J. Cho, Angew. Chem., Int. Ed., 2014, 53, 4582–4586. 9 K. Sakaushi, T. P. Fellinger and M. Antonietti, ChemSusChem, 2015, 8, 1156–1160. 10 T. Takeguchi, T. Yamanaka, H. Takahashi, H. Watanabe, T. Kuroki, H. Nakanishi, Y. Orikasa, Y. Uchimoto, H. Takano, N. Ohguri, M. Matsuda, T. Murota, K. Uosaki and W. Ueda, J. Am. Chem. Soc., 2013, 135, 11125–11130. 11 F. Cheng, J. Shen, B. Peng, Y. Pan, Z. Tao and J. Chen, Nat. Chem., 2011, 3, 79–84. 12 J. Wang, K. Wang, F. B. Wang and X. H. Xia, Nat. Commun., 2014, 5, 5285. 13 J. Zhang, Z. Zhao, Z. Xia and L. Dai, Nat. Nanotechnol., 2015, 10, 444–452. 14 G. L. Tian, M. Q. Zhao, D. Yu, X. Y. Kong, J. Q. Huang, Q. Zhang and F. Wei, Small, 2014, 10, 2251–2259. 15 Z. L. Wang, D. Xu, J. J. Xu and X. B. Zhang, Chem. Soc. Rev., 2014, 43, 7746–7786. 16 J. Suntivich, H. A. Gasteiger, N. Yabuuchi, H. Nakanishi, J. B. Goodenough and Y. Shao-Horn, Nat. Chem., 2011, 3, 546–550. 17 J. Fu, D. U. Lee, F. M. Hassan, L. Yang, Z. Bai, M. G. Park and Z. Chen, Adv. Mater., 2015, 27, 5617–5622. 18 X. Liu, M. Park, M. G. Kim, S. Gupta, G. Wu and J. Cho, Angew. Chem., Int. Ed., 2015, 54, 9654–9658. 19 Y. Li, M. Gong, Y. Liang, J. Feng, J. E. Kim, H. Wang, G. Hong, B. Zhang and H. Dai, Nat. Commun., 2013, 4, 1805. 20 M. G. Park, D. U. Lee, M. H. Seo, Z. P. Cano and Z. Chen, Small, 2016, 12, 2707–2714. 21 D. U. Lee, M. G. Park, H. W. Park, M. H. Seo, X. Wang and Z. Chen, ChemSusChem, 2015, 8, 3129–3138. 22 D. U. Lee, J. Y. Choi, K. Feng, H. W. Park and Z. W. Chen, Adv. Energy Mater., 2014, 4, 1301389. 23 B. Y. Xia, Y. Yan, N. Li, H. B. Wu, X. W. Lou and X. Wang, Nature Energy, 2016, 1, 15006. 24 Y. Liang, Y. Li, H. Wang, J. Zhou, J. Wang, T. Regier and H. Dai, Nat. Mater., 2011, 10, 780–786. 25 S. Chen, J. Duan, M. Jaroniec and S. Z. Qiao, Angew. Chem., Int. Ed., 2013, 52, 13567–13570. 26 P. W. Menezes, A. Indra, D. Gonz´ alez-Flores, N. R. Sahraie, I. Zaharieva, M. Schwarze, P. Strasser, H. Dau and M. Driess, ACS Catal., 2015, 5, 2017–2027. 27 P. Li, R. Ma, Y. Zhou, Y. Chen, Z. Zhou, G. Liu, Q. Liu, G. Peng, Z. Liang and J. Wang, J. Mater. Chem. A, 2015, 3, 15598–15606. 28 V. V. T. Doan-Nguyen, S. Zhang, E. B. Trigg, R. Agarwal, J. Li, D. Su, K. I. Winey and C. B. Murray, ACS Nano, 2015, 9, 8108– 8115.

10584 | J. Mater. Chem. A, 2016, 4, 10575–10584

Paper

29 K. Y. Chen, X. B. Huang, C. Y. Wan and H. Liu, Chem. Commun., 2015, 51, 7891–7894. 30 C. Han, X. J. Bo, Y. F. Zhang, M. Li, A. X. Wang and L. P. Guo, Chem. Commun., 2015, 51, 15015–15018. 31 P. Chen, K. Xu, Z. Fang, Y. Tong, J. Wu, X. Lu, X. Peng, H. Ding, C. Wu and Y. Xie, Angew. Chem., Int. Ed., 2015, 54, 14710–14714. 32 X. Zhong, L. Liu, Y. Jiang, X. Wang, L. Wang, G. Zhuang, X. Li, D. Mei, J.-g. Wang and D. S. Su, ChemCatChem, 2015, 7, 1826–1832. 33 H. T. Chung, J. H. Won and P. Zelenay, Nat. Commun., 2013, 4, 1922. 34 Y. Liu, G. Yu, G.-D. Li, Y. Sun, T. Asefa, W. Chen and X. Zou, Angew. Chem., Int. Ed., 2015, 54, 10752–10757. 35 G. Kresse and J. Hafner, Phys. Rev. B: Condens. Matter Mater. Phys., 1993, 48, 13115–13118. 36 G. Kresse and J. Furthm¨ uller, Phys. Rev. B: Condens. Matter Mater. Phys., 1996, 54, 11169–11186. 37 G. Kresse and D. Joubert, Phys. Rev. B: Condens. Matter Mater. Phys., 1999, 59, 1758–1775. 38 S. Guo, Z. Deng, M. Li, B. Jiang, C. Tian, Q. Pan and H. Fu, Angew. Chem., Int. Ed., 2016, 55, 1830–1834. 39 K. Parvez, S. B. Yang, Y. Hernandez, A. Winter, A. Turchanin, X. L. Feng and K. Mullen, ACS Nano, 2012, 6, 9541–9550. 40 H. F. Huang, G. S. Luo, L. Q. Xu, C. L. Lei, Y. M. Tang, S. L. Tang and Y. W. Du, Nanoscale, 2015, 7, 2060–2068. 41 L. Liu, X. Yang, N. Ma, H. Liu, Y. Xia, C. Chen, D. Yang and X. Yao, Small, 2016, 12, 1295–1301. 42 Y. P. Zhu, Y. P. Liu, T. Z. Ren and Z. Y. Yuan, Adv. Funct. Mater., 2015, 25, 7337–7347. 43 B. F. Cao, G. M. Veith, J. C. Neuefeind, R. R. Adzic and P. G. Khalifah, J. Am. Chem. Soc., 2013, 135, 19186–19192. 44 K. Niu, B. Yang, J. Cui, J. Jin, X. Fu, Q. Zhao and J. Zhang, J. Power Sources, 2013, 243, 65–71. 45 Y. L. Zhai, C. Z. Zhu, E. K. Wang and S. J. Dong, Nanoscale, 2014, 6, 2964–2970. 46 Y. M. Yang, J. Liu, Y. Z. Han, H. Huang, N. Y. Liu, Y. Liu and Z. H. Kang, Phys. Chem. Chem. Phys., 2014, 16, 25350–25357. 47 T. Warang, N. Patel, A. Santini, N. Bazzanella, A. Kale and A. Miotello, Appl. Catal., A, 2012, 423, 21–27. 48 J. Wu, C. Jin, Z. Yang, J. Tian and R. Yang, Carbon, 2015, 82, 562–571. 49 Y. Gorlin and T. F. Jaramillo, J. Am. Chem. Soc., 2010, 132, 13612–13614. 50 B. Y. Xia, Y. Yan, X. Wang and X. W. Lou, Mater. Horiz., 2014, 1, 379–399. 51 S. Chen, J. Duan, P. Bian, Y. Tang, R. Zheng and S.-Z. Qiao, Adv. Energy Mater., 2015, 5, 1500936.
