Spinel CuCo2O4 Nanoparticles Supported on N-Doped Reduced

Oct 11, 2013 - Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia. ∥ Center of Excellence for Advanced ...
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Letter pubs.acs.org/Langmuir

Spinel CuCo2O4 Nanoparticles Supported on N‑Doped Reduced Graphene Oxide: A Highly Active and Stable Hybrid Electrocatalyst for the Oxygen Reduction Reaction Rui Ning,†,‡ Jingqi Tian,†,‡ Abdullah M. Asiri,§,∥ Abdullah H. Qusti,§,∥ Abdulrahman O. Al-Youbi,§,∥ and Xuping Sun*,†,‡,§,∥ †

State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, Jilin, China ‡ Graduate School of the Chinese Academy of Sciences, Beijing 100039, China § Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia ∥ Center of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah 21589, Saudi Arabia S Supporting Information *

ABSTRACT: In this Letter, for the first time, we demonstrated the preparation of a highly efficient electrocatalyst, spinel CuCo2O4 nanoparticles supported on N-doped reduced graphene oxide (CuCo2O4/N-rGO), for an oxygen reduction reaction (ORR) under alkaline media. The hybrid exhibits higher ORR catalytic activity than CuCo2O4 or N-rGO alone, the physical mixture of CuCo2O4 nanoparticles and N-rGO, and Co3O4/N-rGO. Moreover, such a hybrid affords superior durability to the commercial Pt/C catalyst.

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INTRODUCTION

activity, low cost, simple preparation, and high stability have been actively pursued.12 The use of a carbon support can increase the dispersion and utilization of the active catalyst, thereby improving the catalytic activity. Graphene, a 2D singlelayer sheet of sp2-hybridized carbon, has emerged as the most promising carbon support. The combination of its high surface area, enhanced mobility of charge carriers, and good stability makes graphene an ideal platform for growing or anchoring functional nanomaterials. It has been suggested recently that metal oxides should be strongly coupled to a graphene support to improve their electrocatalytic activity and stability.13,14 Although it is reported that CuCo2O4 is a better ORR catalyst than Co3O4,15 to the best of our knowledge no attention has been paid to the preparation of CuCo2O4/graphene hybrid. In this Letter, we demonstrate the first solvothermal preparation of spinel CuCo2O4 nanoparticles supported on N-doped

The development of electrocatalysts that can facilitate the sluggish kinetics of the oxygen reduction reaction (ORR) is vital to the development of electrochemical energy devices, including fuel cells1 and metal−air batteries.2 Although platinum (Pt) and Pt-based materials are the most efficient ORR catalysts, the high cost, poor durability, and scarcity of Pt have been shown as the major bottlenecks to large-scale commercial application. As such, considerable efforts have been devoted to finding alternative catalysts based on nonprecious materials and low-cost catalysts such as nonprecious metals3,4 or metal oxide,5,6 doped carbon materials,7−9 and carbonsupported transition-metal−nitrogen complexes10 have been successfully used as ORR electrocatalysts in alkaline solution. Nevertheless, effort is still required to explore an ORR catalyst that has a high activity comparable to that of Pt, higher durability, and lower cost. Mixed-valence transition-metal oxides with a spinel structure were identified as active catalysts for the ORR in alkaline media.11 Cobalt-based spinel oxides with the advantages of © XXXX American Chemical Society

Received: August 11, 2013 Revised: October 3, 2013

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Figure 1. Schematic diagram to illustrate the two-step solvothermal preparation of the CuCo2O4/N-rGO hybrid catalyst. Powder X-ray diffraction (XRD) measurements were made on a RigakuD/MAX 2550 diffractometer with Cu Kα radiation (λ = 1.5418 Å). X-ray photoelectron spectroscopy (XPS) measurements were made on an ESCALABMK II X-ray photoelectron spectrometer using Mg as the exciting source. Electrochemical measurements were performed with a CHI 660D electrochemical analyzer (CH Instruments, Inc., Shanghai). A three-electrode cell is used, including a glassy carbon electrode (GCE, geometric area = 0.196 cm2) as the working electrode, a saturated calomel electrode as the reference electrode, and a platinum wire as the counter electrode. To prepare the working electrode, 4 mg of a prepared catalyst and 10 μL of a 5 wt % Nafion solution were ultrasonically dispersed in 1 mL of a water−isopropanol (3:1 v/v) mixed solvent to form a homogeneous ink, and 2 mg of Pt/ C was ultrasonically dispersed in 500 μL of EtOH with 17.5 μL of a 5 wt % Nafion solution. Then 15 μL of the catalyst ink was loaded onto a glassy carbon rotation disk electrode of 5 mm diameter (loading 0.3 mg/cm2). For the fabrication of the working electrode of the Pt/C catalyst, 10 μL of catalyst ink was loaded onto a glassy carbon rotating disk electrode of 5 mm diameter. The electrodes were air dried to allow solvent evaporation. Cyclic voltammetry measurements were carried out in an N2-saturated and O2-saturated aqueous solution of 1 M KOH at a scan rate of 5 mV/s. Rotating-disk electrode (RDE) measurements were conducted at different rotational speeds from 400 to 2025 rpm at a scan rate of 5 mV/s. Koutecky−Levich plots (J−1 vs ω−1/2) were analyzed at various electrode potentials. The slopes of their best linear fit lines were used to calculate the number of electrons transferred on the basis of the Koutecky−Levich equation17

reduced graphene oxide (CuCo2O4/N-rGO). The resulting hybrid exhibits a higher ORR catalytic activity than does CuCo2O4 or N-rGO alone, the physical mixture of CuCo2O4 nanoparticles and N-rGO, and Co3O4/N-rGO. Moreover, such a hybrid offers superior durability over the commercial Pt/C catalyst.

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EXPERIMENTAL SECTION

Materials. Graphite powder, H2SO4, NaNO3, and H2O2 (30%) were purchased from Aladin Ltd. (Shanghai, China). Pt/C (20 wt % Pt on carbon black) and Nafion (5 wt %) were purchased from SigmaAldrich. Concentrated hydrochloric acid and KMnO4 were purchased from Beijing Chemical Corp. Ammonia (25 wt %) and anhydrous ethanol (EtOH) were purchased from Tianjin Fuyu Fine Chemical Research Institute. Cobaltous acetate tetrahydrate was purchased from Tianjin Chemical Reagent Fu Chen Factory. Copper acetate monohydrate was purchased from Xilong Chemical Co., Ltd. All chemicals were used as received without further purification. The water used throughout all experiments was purified with a Millipore system. Preparation of GO. Natural graphite powder was used to synthesize GO according to a modified Hummers method.16 Briefly, a mixture of 1 g of graphite and 23 mL of 98% H2SO4 was stirred at room temperature over 24 h, and then 100 mg of NaNO3 was added to the mixture and stirred for another 0.5 h. Subsequently, the mixture was stirred below 5 °C in an ice bath, followed by the slow addition of 3 g of KMnO4 to the graphite powder solution. After vigorous stirring at 35−40 °C for another 0.5 h, 46 mL of deionized water was added to the above mixture. Finally, 140 mL of deionized water and 10 mL of 30% H2O2 were introduced into the mixture to complete the oxidation reaction. The GO solution was washed and filtered with HCl. Before complete drying, GO was dispersed in 800 mL of deionized water. The as-synthesized GO was ultrasonically dispersed in anhydrous ethanol for further use. Preparation of CuCo2O4 Nanoparticles Supported on an NDoped Reduced Graphene Oxide (CuCo2O4/N-rGO) Hybrid. The CuCo2O4/N-rGO hybrid was prepared by a simple solvothermal method: In a typical synthesis, 1.5 mL of a 0.2 M Co(OAc)2 aqueous solution and 0.75 mL of a 0.2 M Cu(OAc)2 aqueous solution were introduced into 20 mL of a 0.5 mg/mL GO EtOH suspension, followed by the addition of 1 mL of NH3·H2O at room temperature. The reaction was kept at 80 °C in a water bath with stirring for 24 h. After that, the reaction mixture was transferred to a 50 mL Teflonlined autoclave and placed in an oven at 160 °C for 3 h. The resulting product was collected by centrifugation and washed with ethanol and water. For control experimentation, N-rGO was made through the same procedure as CuCo2O4/N-rGO except for adding Co and Cu salts. Free CuCo2O4 nanoparticles were made through the same procedure except for adding GO. Co3O4/N-rGO was made through the same procedure except for adding Cu salt. Characterization. Transmission electron microscopy (TEM) measurements were made on a Hitachi H-8100 electron microscope (Hitachi, Tokyo, Japan) with an acceleration voltage of 200 kV.

1 1 1 1 1 = + = + J JL JK JK Bω1/2 B = 0.62nFC0D0 2/3v−1/6 ; JK = nFkC0 where J is the measured current density, JK is the kinetic limiting current density, JL is the diffusion limiting current density, ω is the angular velocity of the rotating electrode, n is the number of electrons transferred in the reaction, F is Faraday’s constant, C0 is the bulk concentration of O2, D0 is the diffusion coefficient of oxygen, ν is the kinematic viscosity of the electrolyte, and k is the electron-transfer rate constant.

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RESULTS AND DISCUSSION Figure 1 presents a schematic diagram to illustrate the solvothermal preparation of the CuCo2O4/N-rGO hybrid catalyst. First, Co(OAc)2 and Cu(OAc)2 in a 2:1 molar ratio were reacted with GO in ethanol/water/ammonia at 80 °C, leading to the selective formation of a uniform coating on GO sheets with hydrolyzed precursors.18 This step is the nucleation step during which ammonia mediates the nucleation of nanoparticles onto the functional groups of GO and at the same time provides a nitrogen source for doping. Second, the solvothermal treatment of the solution at 160 °C caused the reduction of GO and the simultaneous incorporation of B

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Figure 2. (a) TEM image of the GO sheet, (b) low- and (c) high-magnification TEM images of the resulting CuCo2O4/N-rGO hybrid (insets show HRTEM images of one CuCo2O4 nanoparticle), and (d) XRD patterns of the CuCo2O4/N-rGO hybrid and the Co3O4/N-rGO hybrid.

was expected.19 The chemical composition of the resulting hybrid was determined by an energy-dispersive spectrum (EDS) (Figure S1). Several peaks corresponding to C, N, O, Cu, and Co elements (Au peak originating from sputtered Au for the conductive coating) are observed. The XPS survey spectrum of the CuCo2O4/N-rGO hybrid (Figure S2a) indicates the existence of C, N, Cu, Co, and O, and the total N content is 2.37%. The deconvoluted XPS N 1s spectrum (Figure S2b) contains two types of N chemical states, including pyridinic N and pyrrolic N. The XPS Cu 2p (Figure S2c) and Co 2p (Figure S2d) spectra indicate that copper exists in the Cu2+ state and cobalt has a spinel structure.20 Figure 3a shows the cyclic voltammograms (CVs) of the CuCo2O4/N-rGO hybrid and other ORR catalysts in an O2saturated or N2-saturated 1 M KOH solution. Featureless peak currents in the potential range of −0.8 to 0.1 V are observed for the CuCo2O4/N-rGO sample in the N2-saturated solution (dashed line). In contrast, CuCo2O4/N-rGO exhibits a welldefined cathodic peak in the O2-saturated solution (solid line). This cathodic peak is thereby logically attributed to the ORR process. The ORR onset potential of the CuCo2O4/N-rGO hybrid was −0.14 V versus the saturated calomel electrode (SCE), which is ∼30 mV more positive than that of the Co3O4/ N-rGO hybrid (onset potential of −0.17 V vs SCE). A welldefined cathodic peak of the CuCo2O4/N-rGO hybrid was obtained at around −0.20 V whereas it was −0.23 V for the Co3O4/N-rGO hybrid, thus CuCo2O4/N-rGO indicates enhanced ORR activity over that of Co3O4/N-rGO. This can be attributed to the fact that copper substitution increases the activity of the catalytic sites in the hybrid material and thus

nitrogen species into the graphene lattice to form N-rGO. In this step, CuCo2O4 nanoparticles also crystallized and grew on the graphene surface, leading to the CuCo2O4/N-rGO hybrid. Figure 2a shows a typical TEM image of the GO sheet, indicating that it is several micrometers in size. After the solvothermal reaction, a large number of nanoparticles were formed on the sheet surface, as is evidenced by the lowmagnification TEM image of the products shown in Figure 2b. The high-magnification TEM image (Figure 2c) further reveals that these nanoparticles have sizes in the range of 20−60 nm. The high-resolution TEM (HRTEM) images taken from one nanoparticle (see the insets in Figure 2c) reveal clear lattice fringes with interplanar spacings of 0.281 and 0.201 nm corresponding to the (220) and (400) planes of CuCo2O4, respectively. Figure 2d shows the XRD pattern of the hybrid thus prepared. For comparison, the XRD pattern of Co3O4/NrGO is also given. The hybrid shows a slight shift of its peaks to lower 2θ angles compared to those of Co3O4/N-rGO. The spinel lattice parameter, ao, estimated from the d spacings of the crystal planes, of the nanoparticles in the hybrid is 8.12 Å, which is larger than that of Co3O4 (8.08 Å). The lattice parameter of the nanoparticles is identical to the reported one.19 All of these observations indicate that these nanoparticles are spinel CuCo2O4. Because Cu2+ ions have excess octahedral stabilization energy values, they should preferentially occupy the octahedral sites of the spinel structure. Given that the ionic radius of the Cu2+ ion in the octahedral coordination (0.73 Å) is larger than that of Co3+ ions in the same coordination (0.55 Å for low spin and 0.61 Å for high spin), an increase in the lattice volume with the introduction of copper C

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Figure 3. (a) CVs of the CuCo2O4/N-rGO hybrid, CuCo2O4 + N-rGO mixture, Co3O4/N-rGO hybrid, CuCo2O4, and N-rGO on GCEs in an O2saturated (solid line) or N2-saturated (dashed line) 1 M KOH solution. (The dashed line indicates the peak position of Pt/C.) (b) LSVs of the CuCo2O4/N-rGO hybrid, CuCo2O4 + N-rGO mixture, Co3O4/N-rGO hybrid, CuCo2O4, N-rGO, and Pt/C in an O2-saturated 1 M KOH solution at 1600 rpm. (c) LSVs of the CuCo2O4/N-rGO hybrid in an O2-saturated 1 M KOH solution at various rotational rates at a scan rate of 5 mV/s. (d) Koutecky−Levich plots of the CuCo2O4/N-rGO hybrid in the potential range of −0.4 to −0.7 V.

Figure 4. (a) CVs of CuCo2O4/N-rGO and Pt/C in an O2-saturated (solid line) or N2-saturated (dashed line) 1 M KOH solution. (b) Current− time (i−t) chronoamperometric responses for ORR on CuCo2O4/N-rGO and Pt/C catalysts at −0.3 V in an O2-saturated 1 M KOH solution at a rotational rate of 200 rpm.

(Co3O4/N-rmGO) and MnCo2O4/N-rmGO catalysts.13 The high catalytic activity of CuCo2O4/N-rGO in our present study can also be attributed to the strong coupling between CuCo2O4 nanoparticles and N-rGO. Figure 3b compares the linear sweeping voltammograms (LSVs) of CuCo2O4/N-rGO, Co3O4/N-rGO, CuCo2O4 + N-rGO, CuCo2O4, and N-rGO in an O2-saturated 1 M KOH solution at a rotational rate of 1600 rpm, suggesting that CuCo2O4/N-rGO outperforms other catalysts examined in terms of disk current density and half-

enhances the catalytic activity. From the onset potential and the peak position for ORR, we conclude that free CuCo2O4 nanoparticles alone show very poor ORR catalytic activity, NrGO sheets alone show certain ORR catalytic activity, and the physical mixture of CuCo2O4 nanoparticles and N-rGO shows improved catalytic activity compared to that of each component alone but is still lower than that of the hybrid. Such observations are quite consistent with a previous report on Co3O4/N-doped reduced mildly oxidized graphene oxide D

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particles and N-rGO, and Co3O4/N-rGO in catalytic activity. It also possesses better activity retention than the commercial Pt/ C catalyst as a result of the strong coupling between these two components. Our present study is important for the following two reasons: (1) it is the first demonstration of the integration of CuCo2O4 onto a graphene sheet as a novel robust ORR hybrid catalyst with outstanding catalytic activity; (2) this hybrid may hold great promise as a highly efficient catalyst in alkaline fuel cells1 and metal−air batteries2 and as an anode material for lithium ion batteries.23

wave potential. To gain further insight into the electrocatalytic ORR process, the reaction kinetics was studied by voltammetry at a rotating-disk electrode (RDE). Figure 3c shows the LSVs of CuCo2O4/N-rGO at different electrode rotational rates. Two separate potential regions can be identified, namely, a kineticdiffusion-controlled region (−0.15 to −0.3 V) followed by a diffusion-controlled region with the appearance of a diffusionlimiting current. The diffusion-limiting current density increases from 1.93 to 3.63 mA/cm2 with increasing rotational speed from 400 to 2025 rpm. High rotational rates quicken oxygen flux to the electrode surface and result in large currents. The number of electrons transfer is calculated to be 3.80 between −0.4 and −0.7 V from the slope of the Koutecky−Levich plots (Figure 3d). Figure S3a,b shows the LSVs and Koutecky− Levich plots of Co3O4/N-rGO, respectively. The number of electrons transferred is calculated to be 3.86 between −0.4 and −0.7 V. These observations suggest that both CuCo2O4/NrGO and Co3O4/N-rGO undergo a four-electron oxygen reduction process. We compared the electrocatalytic properties of CuCo2O4/NrGO to those of commercial Pt/C. Figure 4a shows CVs of CuCo2O4/N-rGO (red curve) and Pt/C (black curve) in an O2-saturated (solid line) or N2-saturated (dashed line) 1 M KOH solution at a scan rate of 5 mV/s. The CV profile for the Pt/C catalyst is consistent with the reported data.18,21 CuCo2O4/N-rGO gives an ORR peak at −0.20 V in the O2saturated solution, which is 50 mV more negative than the ORR peak at −0.15 V observed on the Pt/C electrode. RDE measurements also show that the onset potential and half-wave potential of CuCo2O4/N-rGO are a little more negative than those of the commercial Pt/C catalyst (Figure 3b). Because durability is one of the major concerns in current fuel-cell technology, the stability of CuCo2O4/N-rGO was further tested at a constant voltage of −0.3 V in a 1 M KOH solution saturated with O2 at a rotational rate of 200 rpm (Figure 4b). Remarkably, the corresponding current−time (i−t) chronoamperometric response of CuCo2O4/N-rGO shows a decrease of 14% in current density over 20 000 s of continuous operation, whereas a decrease of 30% in current density is observed for the Pt/C catalyst. Figure S4 shows the CVs of CuCo2O4/N-rGO before and after the durability test. It can be seen that only the current density changed a little after cycling. This result suggests that CuCo2O4/N-rGO shows superior durability over the Pt/C catalyst, which could be due to the fact that the strong coupling of CuCo2O4 to N-rGO can effectively prevent the nanoparticles from agglomeration during electrochemical tests. We also tested the stability of the Co3O4/N-rGO catalyst, as shown in Figure S5. It is seen that Co3O4/N-rGO shows a decrease of 17% in current density over 20 000 s of continuous operation. It can be inferred that CuCo2O4/N-rGO exhibited better stability than Co3O4/N-rGO. The inferior stability of Co3O4/N-rGO to Co3O4/N-rmGO reported by Liang et al.22 could be due to the use of different GO samples and testing conditions.

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ASSOCIATED CONTENT

S Supporting Information *

EDS and XPS spectra of CuCo2O4/N-rGO. LSVs and Koutecky−Levich plots of Co3O4/N-rGO. CVs of CuCo2O4/ N-rGO before and after the durability test. Stability test of Co3O4/N-rGO. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel/Fax: 0086-431-85262065. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (no. 21175129), the National Basic Research Program of China (no. 2011CB935800), and the Scientific and Technological Development Plan Project of Jilin Province (nos. 20100534 and 20110448).

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REFERENCES

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CONCLUSIONS Spinel CuCo2O4 nanoparticles have been integrated onto NrGO through a two-step solvothermal method with the use of Co(OAc)2, Cu(OAc)2 and GO as precusors, ethanol/water as solvent, and ammonia as a nucleation and doping agent for metal oxide and graphene, respectively. As a novel ORR catalyst, the CuCo2O4/N-rGO hybrid outperforms CuCo2O4 or N-rGO alone, the physical mixture of CuCo2O4 nanoE

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