Cu2O Nanoparticles and CuNxCy Species to

Nov 3, 2017 - ... carbon derived from polyaniline. Zhi Zhou , Simin Gu , Yongli Zhang , Fangfang Wu , Nan Zhou. Journal of Alloys and Compounds 2018 7...
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Cu−MOF-Derived Cu/Cu2O Nanoparticles and CuNxCy Species to Boost Oxygen Reduction Activity of Ketjenblack Carbon in Al−Air Battery Jingsha Li,† Nan Zhou,‡ Jingya Song,† Liang Fu,§ Jun Yan,† Yougen Tang,*,†,∥ and Haiyan Wang*,†,∥

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†

Hunan Provincial Key Laboratory of Efficient and Clean Utilization of Manganese Resources, College of Chemistry and Chemical Engineering, Central South University, Lushan South Road, Changsha 410083, China ‡ College of Science, Hunan Agricultural University, Nongda Road, Changsha 410128, China § College of Chemistry and Chemical Engineering, Yangtze Normal University, Ju Xian Road, Fuling 408100, China ∥ Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai University, Weijin Road, Tianjin 300071, China S Supporting Information *

ABSTRACT: Developing a highly efficient and low-cost catalyst on a large scale is still a big challenge for fuel cells and metal−air batteries. Decorating carbon supports with some active sites (e.g., nitrogen doping, metal−nitrogen doping) seems to be a promising strategy. In this work, we first reported a Cu-centered metal organic framework (Cu−MOF) as a self-sacrificing precursor to modify the Ketjenblack (KB) carbon, where crystalline Cu/Cu2O nanoparticles and noncrystalline CuNxCy species were derived after calcination. The catalytic activity toward the oxygen reduction reaction (ORR) of this modified KB carbon was significantly boosted probably because of the synergistic effect between crystalline Cu/Cu2O nanoparticles and noncrystalline CuNxCy species. This hybrid catalyst exhibited a comparable half-wave potential (0.82 V versus reversible hydrogen electrode, RHE) and superior limiting-current density and durability to the commercial 20 wt % Pt/C. The excellent activity was also further confirmed by the application in home-made Al−air batteries, yielding a highly stable voltage of 1.53 V at a current density of 40 mA cm−2. KEYWORDS: Metallic Cu nanoparticles, Cu2O, Noncrystalline CuNxCy species, Oxygen reduction reaction, Al−air batteries

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Ar.9,11,12,17,19 However, both methods of introducing N hold the following problems. For example, the former makes the synthesis process complex, expensive, and dangerous. The heattreatment process of these nitrogen-rich precursors usually generates a lot of gas, which could take part of the product away under flowing Ar, thus leading to a low yield. Therefore, industry-scale application of these as-obtained M−N−C catalysts seems very difficult. Recently, metal−organic frameworks (MOFs) and their derivatives have been investigated as self-sacrificing templates for constructing carbon-based materials and/or metal oxides as efficient ORR catalysts or battery materials due to high surface area, long-ranging ordering frame with metal centers, inherent porosity, and ordered structure.20−29 Nitrogen-doped porous carbons derived from the ZIF-67/glucose composite exhibited comparable ORR catalytic activity but better stability and

INTRODUCTION

Facing problems of resource shortage, global warming, and environmental pollution, various efforts are devoted to developing advanced clean energy conversion and storage technologies including fuel cells, metal−air batteries, and so on.1−3 However, the sluggish kinetics of the oxygen reduction reaction (ORR) in these applications has restricted their further development. Conventionally, commercial Pt/C catalysts are widely used as the state-of-the-art catalyst to ameliorate the ORR.4,5 Nevertheless, their high price and sensitivity to CO poisoning have pushed forward a number of studies on low cost and highly efficient candidate catalysts to replace Pt/C. To date, metal-coordinated nitrogen-doped carbon materials (M−N−C, where M represents mainly Fe or Co) have been widely explored as highly efficient ORR catalysts, considering the merits of their good activity and durability.6−17 Generally, the nitrogen in these M−N−C catalysts was introduced by annealing the precursors under NH3 directly18 or a mixture of metal salts and nitrogen-rich precursors, such as melamine, cyanamide, dicyandiamide, and polyaniline under flowing © 2017 American Chemical Society

Received: August 3, 2017 Revised: October 23, 2017 Published: November 3, 2017 413

DOI: 10.1021/acssuschemeng.7b02661 ACS Sustainable Chem. Eng. 2018, 6, 413−421

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X-ray photoelectron spectroscopy (XPS) was measured on a KAlpha1063 spectrometer with a resolution of 0.3−0.5 eV from a monochromated aluminum anode X-ray source with Kα radiation (12 kV, 6 mA). The carbon content was analyzed on a CS744 carbon and sulfur analyzer in air. Nitrogen adsorption−desorption isotherms and the Barrett−Joyner−Halenda (BJH) pore size distribution were recorded at −196 °C using an Autosorb-1 apparatus. Before the measurement, the samples were degassed at 150 °C for 3 h under vacuum. The specific surface area was calculated by using the Brunauer−Emmett−Teller (BET) equation from the adsorption isotherm. The conductivities of the as-prepared CuNC/KB-X samples were conducted on an ST-2722 semiconductor powder conductivity tester (Suzhou Jingge Electronic Co., Ltd.) with the height of 2.0 mm at 4.0 MPa. Electrochemical Measurements. A 6 mg portion of catalyst powder was dispersed in a mixture of 950 μL of ethanol and 50 μL of Nafion with the aid of ultrasonic dispersion to form a homogeneous ink. Next, 10 μL of catalyst ink was dropped on the glassy carbon with a diameter of 5.61 mm (0.247 cm2 of geometric area) to obtain a catalyst layer, yielding a catalyst loading of 0.243 mg cm−2. Finally, the as-prepared catalyst film was dried under an infrared lamp for electrochemical measurements. The ORR activity of the as-prepared catalysts was evaluated by cyclic voltammetry (CV) and rotating ring-disk electrode (RRDE) measurements in O2-saturated 0.1 M KOH solutions. RRDE measurements were recorded on an electrochemical workstation (CHI760e) with a four-electrode cell at room temperature, in which the RRDE disk and ring were used as working electrodes, and a platinum wire and saturated calomel electrode (SCE) as counter and reference electrode, respectively. The disk electrode was scanned cathodically at a rate of 10 mV s−1 in the potential range from −1.0 to 0 V versus SCE, and the ring potential was constant at 0.1 V versus SCE. All the potentials measured against an SCE are converted to potential versus RHE.32,33 The number of electrons transferred (n) was calculated on the basis of the Koutecky−Levich equation:

tolerance to the methanol crossover effect than commercial Pt/ C.20 N-doped Fe/Fe3C@graphitic layer/carbon nanotube hybrids were fabricated by a one-step and in situ approach using MIL-101 (Fe-MOF) as precursor, and the resulting materials showed excellent bifunctional catalytic activity toward the ORR and OER.28 Co, N-doped porous carbons were successfully prepared by the pyrolysis of a bimetallic metal− organic framework based on ZIF-8 and ZIF-67 with a varied ratio of Zn/Co and exhibited excellent ORR catalytic activity approaching that of commercial Pt/C.30 In addition to iron and cobalt, copper has also been explored as a good ORR electrocatalyst owing to the second highest electrical conductivity (less than Ag), low price, and abundance.22 Highly exposed Cu(I)−N active sites within graphene31 were first reported as an advanced ORR catalyst via pyrolysis of copper phthalocyanine (CuPc) and dicyandiamide. However, CuPc is very expensive, and dicyandiamide decomposed to generate a lot of gas during the heat-treatment process, thus leading to a very low yield. Therefore, more deep and systematic studies of Cu−N−C should be implemented. Herein, we first reported a Cu−MOF sacrificing template method to modify the Ketjenblack (KB) carbon to form a novel hybrid catalyst, in which Cu/Cu2O nanoparticles and noncrystalline CuNxCy species could be confirmed. This method is facile, scalable, and of high yield. Probably because of their synergistic effects, this hybrid catalyst exhibits superior ORR catalytic activity in terms of half-wave potential of 0.82 V versus reversible hydrogen electrode (RHE) and limiting-current density (6.05 mA cm−2) in comparison to the commercial 20 wt % Pt/C. Furthermore, by cyclically scanning between 0.6 and 1.2 V vs RHE, the decreased half-wave potential of this hybrid (5 mV) is much less than that of the Pt/C catalysts (35 mV). When used in home-made Al−air batteries, it also outperforms the Pt/C catalyst.

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EXPERIMENTAL SECTION

Catalyst Synthesis. HNO3-functionalized KB was prepared as follows: Carbon support KB (Ketjenblack carbon, EC-300J) was first treated in concentrated HNO3 in an oil bath at 80 °C for 8 h to get rid of metal impurities and introduce oxygen-containing functional groups on the surface. After that, KB was washed with distilled water at least 10 times to remove the residual acid and then dried in vacuum oven at 110 °C for 12 h. Cu−MOF/KB was synthesized as follows: Different amounts of functionalized KB (300, 400, 500, 600 mg) were dispersed in 60 mL of N,N-dimethylformamide (DMF) with the aid of ultrasound. Then, 0.296 g of copper nitrate tetrahydrate [Cu(NO3)2·4H2O], 0.272 g of 1,4-benzenedicarboxylic acid, and 0.192 g of triethylene-diamine were added to the above mixture. After being stirred for 30 min, the final mixture was transferred into a Teflon vessel at 120 °C for 36 h followed by naturally cooling to room temperature. The blue-grayish crystals were collected by filtration, washed with ethanol several times, and dried under vacuum. The resulting Cu−MOF/KB powders in a quartz boat were placed in a furnace under Ar and heated to 800 °C for 1 h with a rate of 5 °C min−1. The obtained sample was denoted as CuNC/KB-X (X represents the KB mass, namely, 0, 300, 400, 500, or 600 mg). For emphasis of the role of Cu-based nanoparticles, CuNC/KB-400 was leached in 0.5 M H2SO4 at 80 °C for 24 h, collected through filtering, and washed several times with deionized water. Finally, the obtained product was dried at 60 °C under vacuum and denoted as CuNC/KB400-H2SO4. Physicochemical Characterization. The structure of as-prepared samples was characterized on the Dandong X-ray diffractometer (TD3500) with Cu Kα radiation (λ = 1.5406 Å). The morphology was investigated by transmission electron microscopy (Titan G2 60-300).

1 1 1 1 1 = + = + J Jl Jk Jk Bω1/2

(1)

B = 0.62nFCO(DO)2/3 v−1/6

(2)

where J is the measured current density, Jk and Jl are the kinetic and diffusion-limiting current densities, ω is the electrode rotating rate, n represents the transferred electron number, F is Faraday’s constant, CO is the bulk concentration of O2, DO is the diffusion coefficient of O2, and v is the kinematic viscosity of the electrolyte. The H2O2 yield and the electron-transfer number (n) were calculated based on the following equations:

%HO2− = 200 ×

n=4×

Ir / N Id + Ir /N

Id Id + Ir /N

(3)

(4)

where Id is disk current, Ir is ring current, and N is current collection efficiency of the Pt ring (0.37). Air Electrode Preparation. The air electrode consists of a gas diffusion layer, current collector, and the catalytic layer. Nickel foam was employed as the current collector in the air electrode because of its good conductivity and high intensity. The catalytic layer was fabricated as follows: Catalysts, active carbon, acetylene black, and polytetrafluoroethylene (PTFE) in a weight ratio of 3:3:1:3 were mixed well, and then, the paste was rolled until the thickness of this layer was about 0.2 mm. Finally, the film was pressed onto nickel foam (2 cm × 5 cm) at 15 MPa and then dried at 60 °C for 12 h. For comparison, a commercial Pt/C (20 wt % Pt, Johnson Matthey) catalyst was used. The air electrode using the Pt/C catalyst was obtained by the same process. 414

DOI: 10.1021/acssuschemeng.7b02661 ACS Sustainable Chem. Eng. 2018, 6, 413−421

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Figure 1. Synthetic process for the CuNC/KB-X catalysts.

Figure 2. (a) XRD patterns of the CuNC/KB-0 (blue), CuNC/KB-400 (red), and CuNC/KB-400-H2SO4 (black). (b) High-resolution spectra of the Cu 2p XPS peaks of CuNC/KB-400 and CuNC/KB-400-H2SO4. Al−Air Battery Tests. For the Al−air full cell test, an aluminum plate was used as the anode, and the electrolyte was composed of 6 M KOH with 0.01 M Na2SnO3, 0.0005 M In(OH)3 and 0.0075 M ZnO as corrosion inhibitors. A home-made apparatus as shown in the literature was used for electrochemical measurements.2,34

400-H2SO4 [C (96.47 atom %), O (1.81 atom %), N (1.63 atom %), and Cu (0.09 atom %)] is much lower, revealed by the weakened characteristic peak of Cu (Figure 2b, and Figure S2 in the SI). The high-resolution Cu 2p spectrum of CuNC/ KB-400 could be fitted with four types. The peaks at 932.6 and 952.4 eV correspond to the Cu 2p1/2 and Cu 2p3/2 spin−orbit peaks of Cu(0), respectively.22,35 The Cu 2p peaks at 935.0 and 954.5 eV belong to Cu(II),22 which should be attributed to the oxidation of Cu2O on the surface. The Cu 2p XPS spectrum of CuNC/KB-400-H2SO4 is negligible (Figure 2b), further verifying that the Cu-based nanoparticles were almost removed. The N 1s spectra of CuNC/KB-400 (Figure S3, in the SI) could be deconvoluted into four peaks, namely, pyridinic N (∼398.6 eV), pyrrolic N (∼400 eV), graphitic N (∼401.2 eV), and oxidized N (∼402.5 eV).36−39 It is worth noting that the pyridinic N and graphitic N are generally regarded as ORR active sites.32,40 The morphologies of Cu−MOF/KB-400, CuNC/KB-400, and CuNC/KB-400-H2SO4 were characterized by transmission electron microscopy (TEM) in Figure 3. As shown in Figure 3a, the sizes of Cu−MOF nanoparticles are larger than those of CuNC/KB-400 after carbonization at 800 °C under Ar (Figure 3b), indicating that Cu−MOF is well dispersed on the surface of KB carbon. Figure 3c shows that the diameter of the metallic Cu sphere, which is partially attached to carbon, is about 150 nm. The high-resolution TEM (HRTEM) image (Figure 3d) indicates that part of Cu nanoparticles are encapsulated to the carbon layer, and the lattice distance is determined to be 0.25 nm, corresponding to the (110) plane of cubic metallic Cu. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and elemental mapping images (Figure 3g) display that C, N, and O elements are uniformly dispersed throughout the entire CuNC/KB-400, while the

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RESULTS AND DISCUSSION The synthesis process of CuNC/KB-X catalysts is shown in Figure 1. First, the Cu−MOF/KB precursors were prepared by solvent-thermal method at 120 °C for 36 h with HNO3functionalized KB, Cu(NO3)2·4H2O, 1,4-benzenedicarboxylic acid, and triethylenediamine as raw materials, and were then subjected to carbonization at 800 °C for 1 h under Ar flow. The crystal phases of as-prepared samples were investigated by powder X-ray diffraction (XRD) in Figure 2a. For CuNC/KB0, the sharp peaks at 43.4°, 50.6°, and 74.3° are assigned to the cubic phase of Cu with lattice parameters, a = b = c = 3.608 Å, and space group, Fm3̅m (225; JCPDS 65-9743), suggesting that partial Cu(II) in Cu−MOF (Figure S1, in the SI) was reduced to metallic Cu. After 400 mg of KB was added into the precursor of Cu−MOF, the hybrid phases of metallic Cu, graphitic carbon (2θ = 26.3°), and Cu2O (JCPDS 05-0667) could be seen. It is noted that the patterns of Cu2O are very weak, indicating the slight content of an impurity phase. The leach process makes the diffraction peaks of Cu2O and Cu almost disappear, revealing that Cu2O and Cu nanoparticles in the sample were almost removed during the acid treatment process. We also performed X-ray photoelectron spectroscopy (XPS) for CuNC/KB-400 before and after acid treatment to further investigate the elemental composition and valence states. The results show that CuNC/KB-400 is mainly composed of C (96.74 atom %), O (1.38 atom %), N (1.59 atom %), and Cu (0.28 atom %). However, the Cu content in the CuNC/KB415

DOI: 10.1021/acssuschemeng.7b02661 ACS Sustainable Chem. Eng. 2018, 6, 413−421

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Figure 3. TEM images of (a) Cu−MOF/KB-400, (b, c) CuNC/KB-400, and (e) CuNC/KB-400-H2SO4. HRTEM images of (d) CuNC/KB-400 and (f) CuNC/KB-400-H2SO4. HAADF-STEM images and copper, carbon, nitrogen, and oxygen elemental mapping images of (g) CuNC/KB-400 and (h) CuNC/KB-400-H2SO4.

enhancing conductivity between KB and Cu-based nanoparticles. It is well-known that the good conductivity could facilitate the ORR process.22,41,42 The BET surface area and pore size distribution of the asprepared samples were obtained by the N2 adsorption− desorption isotherm. As shown in Figure 4 and Table S2, the BET surface area of CuNC/KB-0 is 19.3 m2 g−1, and it increases significantly with increasing KB content. The CuNC/ KB-400 has a high surface area of 394.3 m2 g−1. The N2 adsorption−desorption isotherm of CuNC/KB-0 presents type-II curves, indicating the presence of both mesopores and macropores concentrated at 3.7 and 78.2 nm (Figure 4a). After KB is added into the precursors, an obvious hysteresis of N2 adsorption−desorption isotherms at high relative pressure appears, and the pore distribution peak moves to ∼39 nm. The carbon and sulfur analysis results show that the carbon contents of CuNC/KB-0 and CuNC/KB-400 are 20.9% and 76.8%, respectively. The ORR electrocatalytic activity of CuNC/KB-X samples was evaluated by rotating disk electrode (RDE) measurements in O2-satured 0.1 M KOH, and the commercial Pt/C catalyst was used as reference (Figure 5a). Among these CuNC/KB-X catalysts, CuNC/KB-400 has the best ORR activity with a halfwave potential of 0.82 V versus RHE and a limiting-current

copper element is mainly focused on the copper particle. After acid leaching, the metallic Cu particles are removed, which could be confirmed by the TEM (Figure 3e), HRTEM (Figure 3f), and HAADF-STEM and elemental mapping images (Figure 3h) of CuNC/KB-400-H2SO4. However, the energydispersive spectroscopy (EDS; Figure S4, in the SI), HAADFSTEM, and elemental mapping images (Figure 3h) of CuNC/ KB-400-H2SO4 also suggest that part of the copper is maintained in the form of noncrystalline CuNxCy species. That is, there are three types of copper in CuNC/KB-400: Cu2O, metallic Cu nanoparticles, and noncrystalline CuNxCy species. This result is also verified by the difference of the TEM and HRTEM images of CuNC/KB-400-H2SO4 and HNO3functionalized KB (Figure S5, in the SI). The conductivity test results of the as-prepared samples (Table S1, in the SI) show that the bare CuNC/KB-0 exhibits the poorest conductivity (0.96 S cm−1). After KB is added, the conductivity of these composites is significantly improved. CuNC/KB-400 has the highest conductivity of 5.62 S cm−1, much better than bare KB, CuNC/KB-0, and the sum of both KB and CuNC/KB-0. Moreover, the conductivity of CuNC/KB-400 is slightly enhanced after the acid treatment process. These results also prove that KB has higher conductivity than Cu-based nanoparticles, and there is a good synergistic effect in 416

DOI: 10.1021/acssuschemeng.7b02661 ACS Sustainable Chem. Eng. 2018, 6, 413−421

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Figure 4. N2 adsorption−desorption isotherms and the (inset) corresponding pore size distribution curves of (a) CuNC/KB-0, (b) CuNC/KB-300, (c) CuNC/KB-400, and (d) CuNC/KB-500.

Figure 5. (a) LSV curves for various as-prepared CuNC/KB-X samples, KB, and the commercial Pt/C catalyst with catalyst loading of about 0.243 mg cm−2 in O2-saturated 0.1 M KOH at a sweep rate of 10 mV s−1. (b) CV curves of the CuNC/KB-400 in O2-saturated (solid line) and N2saturated (dashed line) 0.1 M KOH at 10 mV s−1. (c) LSV curves of the CuNC/KB-400 in O2-saturated 0.1 M KOH at 10 mV s−1 at different rotating rates. The inset is the Koutecky−Levich (K−L) plots at different potentials (0.50, 0.55, 0.60, 0.65, and 0.70 V vs RHE). (d) Tafel plots for CuNC/KB-400 (black line) and commercial Pt/C (red line) catalysts calculated from part a.

density of 6.05 mA cm−2, even outperforming the commercial 20 wt % Pt/C catalyst. Bare CuNC/KB-0 and KB exhibit

relatively poor activity. After KB support is added in the Cu− MOF precursor, the ORR performance is significantly 417

DOI: 10.1021/acssuschemeng.7b02661 ACS Sustainable Chem. Eng. 2018, 6, 413−421

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Figure 6. (a) LSV curves for the CuNC/KB-400 catalyst before and after acid treatment. (b) CV curves for the CuNC/KB-400 catalyst before and after acid treatment in O2-saturated 0.1 M KOH at a sweep rate of 10 mV s−1. (c) H2O2 yield and (d) electron transfer numbers of the CuNC/KB400 catalyst before and after acid treatment at a potential range 0.1−0.8 V calculated from RRDE voltammograms.

indicating first-order kinetics over the potential range (0.50− 0.70 V). The electron transfer number of CuNC/KB-400 calculated from the K−L equation is about 4, confirming a fourelectron pathway for the ORR process. The Tafel slope (Figure 5d) of CuNC/KB-400 is 69.7 mV per decade, less than that of the Pt/C (82.8 mV per decade), further confirming that the CuNC/KB-400 has the better kinetic process for the ORR. For a better comprehension of the role of metallic Cu and Cu2O nanoparticles in the resultant CuNC/KB-400 hybrid, the sample was leached in hot acid to remove Cu-based nanoparticles, which was verified by the XRD and XPS results. The ORR performance of CuNC/KB-400-H2SO4 was measured by LSV and CV curves. As shown in Figure 6a, CuNC/ KB-400-H2SO4 exhibits a much more inferior ORR activity (more negative onset and half-wave potential, lower limitingcurrent density) than CuNC/KB-400. It is a remarkable fact that CuNC/KB-400-H2SO4 has slightly better electronic conductivity than CuNC/KB-400. Generally, good conductivity will facilitate the ORR process. In other words, Cu and Cu2O nanoparticles in the CuNC/KB-400 play a significant role in promoting the ORR process.22 In fact, metallic Cu and Cu2O nanoparticles have been identified as ORR active sites in many references.22,53,54 The obvious enhanced ORR performance of CuNC/KB-400-H2SO4 compared with that of bare KB indicates that CuNxCy species make a great contribution to the catalytic activity of CuNC/KB-400-H2SO4.17 In summary, there is a good synergistic effect between Cu/Cu2O nanoparticles and CuNxCy species in catalyzing the ORR process. As a matter of fact, both the noncrystalline MeNxCy moieties17,22,31 and the crystalline metal-based nanoparticles45,47 encased in surrounding graphitic carbon layers have been proposed as the main active sites for the ORR. Our recent work discloses that crystalline Fe/Fe3C and noncrystalline FeNxCy

enhanced, which should be attributed to the enhanced BET surface area and electronic conductivity.32,40,43,44 More importantly, Cu−MOF-derived Cu/Cu2O nanoparticles and noncrystalline CuNxCy species on the surface of the KB support could serve as active sites to accelerate the ORR.45−47 Nevertheless, when the added carbon is more than 400 mg, the performance is instead reduced, especially the limitingcurrent density. This reveals that more active sites should be exposed in the CuNC/KB-400 hybrid since CuNC/KB-500 has an approaching conductivity and much larger BET surface area. It is noted that this CuNC/KB-400 hybrid exhibits much better ORR performance than Cu−N−C composites or KB-based materials reported in recent references (see Table S3, in the SI).24,27,43,46−49 Cyclic voltammetry (CV) curves of CuNC/KB-400 in O2saturated and N2-saturated 0.1 M KOH are compared in Figure 5b. As observed, there are two characteristic peaks at 0.62 and 0.88 V under both O 2- and N 2 -saturated conditions, corresponding to the reduction peaks of Cu(I) and Cu(II).50 Obviously, the CV curve in O2-saturated KOH has a strong peak at 0.80 V for the ORR, while no apparent peak is observed in the N2-saturated medium. A small hydrogen adsorption between 0.05 and 0.4 V is also observed.51 It is noted that the sample in N2-saturated solution presents a weak peak at 0.60 V, which may be attributed to the oxidation of Cu(0) in CuNC/ KB-400. LSV curves of CuNC/KB-400 at various rotating rates and the corresponding Koutechy−Levich (K−L) plots at different potentials are given in Figure 5c. It is worth noting that LSV curves show a sudden change at lower rotation speed. The decrease of the current density at relatively low rotation speed can be attributed to the full integration between catalyst and oxygen causing the rapid depletion of the latter.40,52 The K−L plots in Figure 5c (inset) display a good linearity, 418

DOI: 10.1021/acssuschemeng.7b02661 ACS Sustainable Chem. Eng. 2018, 6, 413−421

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Figure 7. LSV curve comparison of (a) Pt/C and (b) CuNC/KB-400 for the ORR in O2-saturated 0.1 M KOH at a sweep rate of 200 mV s−1 before (red) and after (black) 2000 cycles. The constant current discharge curves of Al−air batteries with Pt/C (red ●) and CuNC/KB-400 (black ■) as cathode catalysts at (c) 20 and (d) 40 mA cm−2.

cathode catalyst exhibits a trend similar to that with Pt/C at 20 mA cm−2. However, when the discharge current density increases to 40 mA cm−2, it shows a much higher and more stable cell voltage (1.53 V) than the commercial Pt/C (1.45 V), revealing better power density in a practical application.

species contributed greatly to the superior ORR catalytic activity of the Fe−N−C sample with relatively high Fe content.17 The CV curve shows only a well-defined cathodic peak at 0.768 V for the ORR in O2-saturated 0.1 M KOH (Figure 6b), more negative than that without acid treatment. It is noteworthy that the reduction peaks of Cu(I) and Cu(II) disappear, further confirming the removal of these Cu-based nanoparticles after the acid leaching. The ORR performance difference of the samples before and after acid treatment was further manifested by the rotating ring-disk electrode (RRDE) voltammograms. In the potential range 0−0.8 V, the H2O2 percentage (Figure 6c) of CuNC/KB-400-H2SO4 reaches 16.9%, corresponding to an electron transfer number of 3.66−3.81 (Figure 6d). However, that of CuNC/KB-400 remains below 3.7%, and the electron transfer number varies from 3.92 to 3.94, indicating that the ORR process in CuNC/ KB-400 mainly goes through a 4e mechanism. This conclusion is in good agreement with the above K−L equation. All results further highlight the good synergistic effect of Cu/Cu2O nanoparticles and noncrystalline CuNxCy species derived from Cu−MOF toward the ORR. The durability of CuNC/KB-400 was evaluated by cycling the catalyst in the potential range 0.6−1.2 V in O2-saturated 0.1 M KOH at 200 mV s−1. After 2000 cycles, the half-wave potential shows a very small negative shift of 5 mV and a limiting-current density decrease of 0.3 mA cm−2 (Figure 7b). However, the commercial Pt/C exhibits a 35 mV negative shift of half-wave potential (Figure 7a). There is no doubt that CuNC/KB-400 possesses excellent catalytic activity and durability for the ORR in alkaline solution. The ORR performance of the CuNC/KB-400 catalyst was also evaluated by the practical Al−air batteries in 6 M KOH electrolyte.2 As shown in Figure 7c, the air electrode with CuNC/KB-400 as a

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CONCLUSIONS

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ASSOCIATED CONTENT

In summary, the Cu−MOF was first used as a self-sacrificing precursor to modify the KB carbon by a combination of a facile solvothermal strategy and subsequent heat treatment. It was found that Cu−MOF-derived Cu/Cu2O nanoparticles and noncrystalline CuNxCy significantly boosted the ORR activity of KB carbon. This hybrid catalyst exhibited a comparable catalytic activity and superior durability as compared to the commercial Pt/C. It also showed much higher working voltage in homemade Al−air batteries at a relatively high current density, indicating much better power density. This work can provide an important guidance for constructing high-performance and scalable KB-based catalysts for the ORR.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02661. XPS survey spectra, EDS, TEM image, conductivities, BET surface area, corresponding pore diameter calculated from N2 adsorption−desorption isotherms, and summary for Cu-based and KB-based ORR catalysts in the literature (PDF) 419

DOI: 10.1021/acssuschemeng.7b02661 ACS Sustainable Chem. Eng. 2018, 6, 413−421

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jun Yan: 0000-0002-6158-0614 Haiyan Wang: 0000-0002-2242-6534 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by the National Nature Science Foundation of China (21671200 and 21571189), Fujian Provincial Natural Science Foundation of China (2015J01072), the Hunan Provincial Science and Technology Plan Project of China (2016TP1007), the Science and Technology Plan Project of Changsha (hk1601153), and Innovation-Driven Project of Central South University (2016CXS009 and 2016CX037).

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DOI: 10.1021/acssuschemeng.7b02661 ACS Sustainable Chem. Eng. 2018, 6, 413−421

Research Article

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