CuO Nanoplatelets with Highly Dispersed Ce-Doping Derived from

Sep 1, 2017 - Development of highly efficient, low-cost, and durable electrocatalysts for the oxygen reduction reaction (ORR) is still a major obstacl...
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Research Article pubs.acs.org/journal/ascecg

CuO Nanoplatelets with Highly Dispersed Ce-Doping Derived from Intercalated Layered Double Hydroxides for Synergistically Enhanced Oxygen Reduction Reaction in Al−Air Batteries Qingshui Hong, Huimin Lu,* and Junren Wang School of Materials Science and Engineering, Beihang University, XueYuan Road No. 37, HaiDian District, Beijing 100191, China S Supporting Information *

ABSTRACT: Development of highly efficient, low-cost, and durable electrocatalysts for the oxygen reduction reaction (ORR) is still a major obstacle that is preventing widespread commercialization of Al−air batteries. Herein, a cost-effective CuCe-layered double hydroxide (CuCe-LDH) was synthesized using a facile and scalable method of separate nucleation/growth steps, followed by an anion exchange process. The CuCe-LDH was successfully used as a precursor for the mass production of Ce-doped CuO nanoplatelets (Cu−Ce−O) as a Pt-free efficient electrocatalyst, which exhibited not only comparable electrocatalytic activity and durability in the ORR test but also a higher discharge voltage plateau when used in Al−air batteries compared to the benchmark Pt/C catalysts. A combination of structural characterizations and electrochemical analyses showed that the high ORR activity of the Cu−Ce−O catalyst was attributed to its high content of active species combined with their synergetic interactions. These results may provide an option for cost-effective production of ORR electrocatalysts at a large scale for practical applications of Al−air cathodes. KEYWORDS: Ternary layered double hydroxide, Cu−Ce−O electrocatalyst, Synergistic effects, Oxygen reduction reaction, Aluminum−air battery

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INTRODUCTION There is an urgent need for the development of alternative energy sources arising from the continuous increase of energy demands and fossil fuel depletion.1,2 Al−air batteries are one of the most attractive clean energy resources because of their abundant resources, low emissions, portability, high energy capacity (2980 Ah kg−1), low cost, and environmental compatibility.3,4 Nevertheless, its performance is still limited by the sluggish oxygen reduction reaction on the cathode, which has hindered the commercial implementation of this promising technology.5−7 Platinum-based materials with a fourelectron process to produce hydroxide ions (OH−) directly in alkaline solutions are well-known to be one of the most active catalysts for ORR; however, they suffer from the issues of high cost and inferior durability.8,9 Alternatively, nonprecious metal catalysts have been demonstrated to be effective for ORR in either reducing the usage of Pt or replacing it. For example, transition metal chalcogenides,10,11 oxides,12,13 carbides,14,15 and nitrides16 have shown reasonable ORR activity and practical durability. Among the transition metal oxide catalysts, copper oxide has been considered one of the most promising nonprecious metal catalysts in electrochemistry redox processes because of their large surface area, chemical stability and potential synergetic effects of surface-modified mixed valence metal oxides.17−20 © 2017 American Chemical Society

However, this material has not attracted much attention for applications in ORR because of its perceived limited catalytic abilities and relatively low electric conductivity. To further improve catalytic activity and conductivity, the combination of CuO with other functional components to form a hybrid structure arouses substantial interest.21−23 In this regard, ceria (CeO2) has been widely used as an oxygen storage medium and stabilizer in multielement oxide catalytic systems based on its ability to adsorb oxygen to regulate the oxygen density on catalyst surfaces. Recently, Wang et al. discovered that the ORR catalytic activity of the Co3O4−CeO2/KB composite depends on the amount of CeO2, from which they demonstrated that a certain amount of CeO2 on the composite could increase the oxygen transfer, enhancing the ORR activity. However, too much CeO2 may weaken the ORR performance due to the inferior intrinsic catalytic activity of CeO2.24 Although some efforts have been devoted to developing CuO/CeO2 catalysts for water25 and CO26,27 oxidation, few reports have discussed their applications in ORR. Layered double hydroxides (LDHs), with features of typical divalent (M2+) and trivalent (M3+) metal cations in a brucite Received: June 24, 2017 Revised: August 21, 2017 Published: September 1, 2017 9169

DOI: 10.1021/acssuschemeng.7b02076 ACS Sustainable Chem. Eng. 2017, 5, 9169−9175

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Figure 1. (a) Schematic illustration for the synthesis of the CuZnAl-LDH-Ce(DTPA)2− and the resulting Cu−Ce−O oxide. (b) XRD patterns of CuZnAl-LDH-NO3−, CuZnAl-LDH-Ce(DTPA)2−, and their derived calcined products. STEM and elemental mapping analysis of (c) CuZnAl-LDHNO3− and (d) CuZnAl-LDH-Ce(DTPA)2−. (e) TEM and HRTEM images of Cu−Ce−O oxide. dissolving Cu(NO3)2·2.5H2O, Al(NO3)3·9H2O, and Zn(NO3)2·6H2O with a 1/1/1 molar ratio in freshly deionized water, resulting in a solution with a total cationic concentration of 1.5 M. The basic solution was 2 M NaOH. Equal volumes of salt and base solutions were then simultaneously added into a modified colloid mill at a rotor speed set at 3000 rpm. The resulting slurry was mixed for 5 min under different pH values by the addition of NaOH solution and then aged at 60 °C for 48 h. The resulting nitrate-intercalated CuZnAl-LDHs were obtained by centrifugal separation and washing thoroughly with deionized water several times. Anion Exchange (AE) and Preparation of Cu−Ce−O Oxides. A clear mixed solution (pH = 7) of diethylenetriaminepentaacetic acid (DTPA, 0.2 M, 50 mL) and NaOH was obtained by vigorous stirring. A solution of Ce(III) nitrate (0.2 M, 50 mL) was added slowly to the mixed solution. The pH value of the resulting solution was near 7, that is, Ce(DTPA)2− anion exchange solution. Then, a 2 g sample of the assynthesized CuZnAl-LDH was dispersed into the Ce(DTPA)2− anion exchange solution. The suspension was vigorously stirred for 24 h under flowing nitrogen to complete the exchange into the Ce(DTPA)2− form. The products were recovered by centrifugation, washed with deionized water, dried in air, and hereafter denoted as CuZnAl-LDH-Ce(DTPA)2−. The as-prepared CuZnAl-LDH-Ce(DTPA)2− precursors were placed in a ceramic boat and transferred into a temperature-programmed furnace. The annealing treatment was performed at 550 °C for 6 h with a heating rate of 5 °C min−1 in an air atmosphere. The residual Zn, Al component and other unstable impurities were removed by immersing the samples in a 6 M KOH solution for 24 h. The resulting black material was collected by centrifugation, washed with excess distilled water, and dried at 120 °C under vacuum for 12 h. The final products are denoted as Cu−Ce−O. For comparison, a DTPA5−-intercalated CuZnAl-LDH was prepared by a similar procedure followed by calcination and alkali leaching under the same conditions, which is denoted as Cu−O.

[Mg(OH)2]-like layer and an interlayer anion between the hydrated interlayer galleries, may be varied over a wide range because of the diversity of LDHs in metal cations, the M2+/M3+ molar ratio, and the interlayer anion.28 The LDHs are widely used in industry because of their versatility, low-cost and ease of mass production.29 After thermal decomposition, LDHs lose their layered structure and form mixed metal oxides with high stability, large specific surface area, and high metal dispersion, which lead to extensive applications of these materials.30,31 Moreover, compared with the materials prepared by other techniques (e.g., hydrothermal or nanocasting routes), the catalysts derived from LDH precursors are generally characterized by higher activities and longer lifetimes.28,32,33 In this work, we developed a facile and scalable method to produce Ce-containing ternary LDH composite precursors for mass production of Cu−Ce−O nanocomposites with highly dispersed metal ions. Cu is the active component for ORR, whereas Ce could promote the reducibility of Cu and increase the dispersion of Cu.34,35 As a result of the “Jahn−Teller effect” of the Cu ion, the Zn element was added to stabilize the host layer of LDH during preparation and increase the catalyst surface area after the alkali leaching treatment.36 The newly developed Cu−Ce−O-based catalysts demonstrate efficient ORR electrocatalytic performance with superior durability and show potential applications in Al-air batteries for the first time. Interestingly, the discharge voltage plateau of the Al-air battery at high current density with the Cu−Ce−O-based catalyst even outperforms that of 20 wt % Pt/C catalyst (Johnson Matthey).

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EXPERIMENTAL SECTION

Synthesis of CuZnAl-LDHs. Nitrate-intercalated CuZnAl-LDH precursors were prepared by a scalable method of separate nucleation and aging steps (SNAS). Briefly, a mixed salt solution was prepared by 9170

DOI: 10.1021/acssuschemeng.7b02076 ACS Sustainable Chem. Eng. 2017, 5, 9169−9175

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Figure 2. XPS spectra of the Cu−Ce−O oxide. (a) Survey, (b) Cu 2p, and (c) Ce 3d.

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RESULTS AND DISCUSSION Figure 1a illustrates the preparation process, which involves the anion exchange of Ce(DTPA)2− with NO3− intercalating into the host CuZnAl-LDH, followed by a subsequent annealing treatment to achieve a mixed transition metal oxide-based ORR electrocatalyst. Through careful selection of pH during anion exchange (see Figure S1 and Table S2), we obtained CuZnAlLDH-Ce(DTPA)2− precursors with a well-defined LDH phase at pH = 7. The annealing treatment was performed at 550 °C for 6 h according to the results of the TG/DTA curves of the CuZnAl-LDH-Ce(DTPA)2−, in which almost no change in weight and heat were detected after 350 °C, indicating the relatively stable state of the sample (Figure S2). Figure 1b shows the X-ray diffraction (XRD) patterns of CuZnAl-LDHNO3−, CuZnAl-LDH-Ce(DTPA)2− and their derived calcined products after alkali leaching (denoted as Cu−O and Cu−Ce− O, respectively). The XRD pattern of CuZnAl-LDH-Ce(DTPA)2− displays the typical diffraction lines of an LDH phase. The basal (003) reflection is shifted to a lower angle compared to CuZnAl-LDH-NO3−, indicating an increase in basal spacing to 1.4 nm as a result of the replacement of NO3− by the larger Ce(DTPA)2− anions into the LDH interlayer galleries. After calcination and alkali leaching, the original peaks of CuZnAl-LDH-Ce(DTPA)2− are replaced by a series of new reflections that correspond to the peak positions of the crystal planes of face-centered cubic (fcc) CeO2 phase (JCPDS 655923), monoclinic CuO phase (JCPDS 45-0937) and cubic Cu2O phase (JCPDS 78-2076). Compared to Cu−O, cubic Cu2O phase is detected on the surface of the Cu−Ce−O sample, indicating that the stability of reducible Cu increased with the addition of Ce. No peaks attributed to the ZnO phase or CuAl2O4 spinel phase were observed in both samples. The mean crystalline size of Cu−Ce−O oxide was calculated to be ∼23.3 nm using the Scherrer formula. Figure 1c and d shows the STEM-EDS mapping analysis of the CuZnAl-LDH-NO3− and CuZnAl-LDH-Ce(DTPA)2−, which further verify the uniform distribution of the Cu, Zn, and Al elements throughout the LDH nanoplatelets. Additionally, Figure 1d clearly shows the presence of a uniform distribution of the Ce element throughout the LDH nanoplatelets, suggesting the successful preparation of Ce(DTPA)2− intercalated LDHs. Figure 1e shows the TEM and high-resolution TEM (HRTEM) images of the Cu−Ce−O oxide. The low-magnification TEM image displays a typical nanoplatelet structure of the as-prepared Cu− Ce−O. An enlarged view reveals that the Cu−Ce−O mixed grains are highly dispersed and anchored in the matrix with an average size of 10−30 nm. Furthermore, the HRTEM image shows the well-resolved (110) planes of CuO corresponding to

a lattice spacing of 0.275 nm, whereas the (111) planes of CeO2 correspond to a lattice spacing of 0.312 nm. Additionally, the (111) facet of Cu2O with lattice fringes of 0.246 nm is also detected. These results are in good agreement with the XRD analysis (Figure 1b). The detailed information about the elemental character and oxidation state of the as-prepared Cu−Ce−O oxide derived from CuZnAl-LDH-Ce(DTPA)2− was conducted by XPS, as demonstrated in Figure 2. The full survey of the Cu−Ce−O oxide composite reveals the signals of Zn 2p, Cu 2p, Ce 3d, O 1s, C 1s, and Al 2p (Figure 2a) in the atomic ratios of 0.5%, 52.9%, 6.9%, 35.07%, 2.52%, and 2.2%, respectively (Table S1). The high-resolution peaks for Cu 2p and Ce 3d are shown in Figures 2b and c, respectively. Figure 2b shows the presence of a higher Cu 2p3/2 peak at 933.4 eV and a shakeup peak between 940 and 945 eV, which are characteristic peaks of CuO. The peak at 953.1 eV is assigned to Cu 2p1/2, which is confirmed by the shakeup peaks centered at 961.6 and 941.7 eV, which are typical of Cu2+ species. The appearance of a Cu 2p3/2 peak at slightly lower binding energies suggests the presence of Cu+ or metallic Cu0. Although the Cu+ and Cu0 phases are not stable after air exposure, a small fraction of Cu+ and Cu0 phases in the Cu−Ce−O sample after alkali leaching is detected both by XRD and XPS, confirming the Cu−Ce interactions to stabilize the reducible Cu. These Cu−Ce interactions could be explained briefly as follows: Ce3+ + Cu2+ → Ce4+ + Cu+.34,37 These interactions are expected to have a positive influence on the electrocatalytic ability of the Cu−Ce−O materials because they modify the chemical states of Cu sites and stabilize the more catalytically active Cu+ sites relative to Cu2+.38 The Ce 3d spectrum is shown in Figure 2b, which is characterized by two types of peaks: 3d3/2 and 3d5/2. The peaks denoted as V‴, V″, V, U‴, U″, and U are attributed to Ce4+, whereas the peaks marked as U′ and V′ are assigned to Ce3+. The Ce 3d spectrum suggests the coexistence of Ce3+ and Ce4+ species with Ce4+ being the main valence state in the Cu−Ce−O oxide. Oxygen vacancies and unsaturated chemical bonds are introduced in the crystal to create a charge imbalance resulting from the presence of Ce3+ in CeO2 nanocrystals.35 The surface area was further investigated by N2-adsorption/desorption measurements. As shown in Table S3, the specific surface area in the sample of Cu−Ce−O mixed oxide is 108.53 m2 g−1, significantly larger than that of Cu−O mixed oxide (9.87 m2 g−1) and other previously reported Cu−Ce−O analogs (∼46 m2 g−1, Table S3). Clearly, the larger intercalated Ce(DTPA)2− complex plays a critical role in producing channels, resulting from the decomposition of Ce(DTPA)2− and release of NO/NO2, CO2 and H2O volatile species, and preventing the aggregation 9171

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Figure 3. Electrochemical performance of the primary Al-air battery with Cu−Ce−O oxide catalyst as the cathode in 6 M KOH. (a) A schematic of the primary Al−air battery. (b) The discharge polarization curves of the battery using the bare CFP, CB-CFP, Cu−O/CB-CFP, Cu−Ce−O/CBCFP, and Pt/C/CB-CFP as the cathodes. (c) Dynamic potentiometric measurements of cited Cu−Ce−O/CB-CFP air cathode from 1 mA cm−2 to 150 mA cm−2 and inset shows the corresponding power density plot. (d) Typical discharge curves of primary Al−air batteries with Cu−Ce−O/CBCFP and Pt/C/CB-CFP as the cathodes (mass loading 0.05 mg cm−2) under continuous discharge until complete consumption of Al at the current density of 35 mA cm−2.

the dynamic galvanostatic discharge curves of the battery with the Cu−Ce−O/CB cathode. Obviously, with increasing current density, the discharge voltage plateau decreases accordingly. Notably, no obvious voltage drop is observed for each discharge voltage plateau after a long-term galvanostatic discharge, indicating good catalytic stability of the Cu−Ce−O/CB cathode for ORR. Its peak power density is 78.1 mW cm−2, which was achieved at 0.78 V (inset in Figure 3c). Figure 3d shows that the Cu−Ce−O/CB cathode yields comparable specific capacity to the Pt/C cathode, which is 2912.6 mA h g−1 when normalized to the mass of consumed Al, corresponding to a high energy density of 4077 Wh kg−1 (Table S4). These results confirm that the Cu−Ce−O/CB cathode shows comparable electrocatalytic activity and cycling stability compared to commercial Pt/C. Cyclic voltammetry (CV) in 0.1 M KOH solution saturated with N2 or O2 at 50 mV·s−1 was evaluated to further study the ORR catalytic activities of Cu−Ce−O oxide. For comparison, commercial 20 wt % Pt/C and Cu−O (see Figure S4) were measured under the same conditions. As shown in Figure 4a, both the samples of Cu−Ce−O and Pt/C recorded in N2saturated electrolyte show no obvious reduction peak. In contrast, when O2 was saturated in the electrolyte, the profile curve appears as a clear reduction peak, suggesting remarkable ORR performance. The CV curves of Cu−Ce−O oxide in O2 show an obvious peak potential of ∼−0.64 V (vs Hg/HgO), which is approximately 0.12 V lower than that of Pt/C. Furthermore, during the ORR studies (Figure 4b), the Cu−

and fusion of the mixed oxide nanoparticles during the hightemperature annealing treatment. A newly designed Al-air cell with an Al anode in 6 M KOH (Figure 3a) was used to determine the cathode catalyst performance of Cu−Ce−O mixed oxides under real battery operation conditions. The air cathodes were prepared by coating a mixture of the CB and PTFE on hydrophobic CFPs, followed by the loading of Cu−O, Cu−Ce−O, or Pt/C on the surface of the electrode. The discharge polarization curves shown in Figure 3b clearly reveal that the performance of the battery with a Cu−Ce−O/CB cathode is superior to those with CB and Cu−O/CB cathodes, and slightly outperforms the Pt/ C benchmark cathode at high current ranges. This phenomenon shows that adding Cu−Ce−O catalyst can effectively improve the electrochemical performance compared to that of Cu−O catalyst. Conversely, the bare CFP and pristine CB show insignificant activity toward the ORR during the battery discharge. Many reports have inferred that the CeO2 often acts as an oxygen buffer that can store O2 in an oxygen-rich condition and remove it in an oxygen-insufficient condition because of the Ce4+/Ce3+ redox couple.35,39−41 As a result, the presence of CeO2 on the catalyst surface would easily transfer O2 to the contiguous Cu−O active sites.42 During the redox process, these factors are helpful to the catalytic activity in accordance with the Mars−Krevelen redox mechanism.24 Therefore, the existence of Ce3+ can remarkably enhance the catalytic activity of the Cu−Ce−O/CB composite and realize the effective activation of molecular oxygen. Figure 3c displays 9172

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Figure 4. (a) CV curves of Cu−Ce−O oxide and Pt/C catalysts in N2 and O2 saturated 0.1 M KOH solution. (b) ORR polarization curves of Cu− Ce−O oxide and Pt/C catalysts in O2 saturated 0.1 M KOH solution without rotation. (c) Tafel plots and (d) K−L plots at −0.8 V for the Cu−Ce− O oxide and Pt/C catalysts (catalyst loading 33 μg cm−2).

Figure 5. (a) Chronopotentiometry (i−t) measurements of Cu−Ce−O in O2-saturated 0.1 M KOH on catalyst-modified GC electrode. Insets: CVs for ORR before cycling and after 1000 and 2000 cycles. (b) Long-time discharge curves of a primary Al-air battery using Cu−Ce−O/CB-CFP air cathode at three different current densities.

Ce−O oxide catalyst exhibits an onset potential of −0.23 V and a half-wave potential of −0.37 V (vs Hg/HgO). Although these values are still 20 and 70 mV more negative than the corresponding values for Pt/C of −0.21 V and −0.30 V, respectively, it is still among the highest reported ORR activity of nonprecious metal-based oxide catalysts (see Table S5). To analyze the kinetic properties of ORR using the Cu−Ce−O oxide, the Tafel slope was obtained from the linear plots of LSVs (Figure 4c). The plot shows typical two-stage linear regions at low overpotential (ηL, where the overall ORR speed is determined by the surface reaction rate on the catalyst) and high overpotential (ηH, where the overall ORR rate is

dependent on the oxygen diffusion). Both the Tafel slopes at ηL (146 mV dec−1) and ηH (191 mV dec−1) for the Cu−Ce−O oxide are slightly higher than those of the commercial Pt/C catalyst (115 mV dec−1 at ηL and 89 mV dec−1 at ηH), suggesting that the Cu−Ce−O oxide possesses a relatively fast electron transfer rate and efficient reactant diffusion. The K-L plots on the basis of their corresponding RDE curves (Figure S3) are given in Figure 4d. The corresponding K-L plots have good linearity at different electrode potentials. The linearity and parallelism of the K-L plots indicate the first order reaction kinetics toward the amount of dissolved oxygen in electrolyte and the electron transfer numbers for ORR for different 9173

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ACS Sustainable Chemistry & Engineering catalysts. The electron transfer numbers (n) of Cu−Ce−O oxide and Pt/C at −0.8 V are calculated to be 3.45 and 3.59, respectively, which demonstrate an apparent quasi-four electron process in the Cu−Ce−O oxide catalyst. Furthermore, chronopotentiometry (i−t) measurements were conducted at two constant cathodic current densities of 0.4 and 0.8 mA cm−2 (Figure 5a) for long-term catalytic durability tests. Cu−Ce−O exhibits high ORR stability with no significant activity loss over 20 h. The inset shows that the CV curve of Cu−Ce−O hardly changes after 2000 continuous cycles. Furthermore, the primary Al-air batteries made with Cu−Ce−O catalyst were very robust. When galvanostatically discharged at 10, 50, or 100 mA cm−2 for 24 h, no obvious voltage drop was observed (Figure 5b), indicating the stability of Cu−Ce−O for ORR, which may be ascribed to the strong coupling effect between Cu and Ce.

ACKNOWLEDGMENTS

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REFERENCES

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CONCLUSIONS In summary, we have explored a facile and effective synthetic method for the mass production of low-cost and Pt-free Cu− Ce−O oxide electrocatalysts for Al-air batteries by using Ce(DTPA)2− intercalated CuZnAl-LDH as a precursor. The presence of cerium in the Cu−Ce−O oxide is found to influence the specific surface area and chemical state of the resulting oxide materials. The catalyst exhibits comparable electrocatalytic activity and durability compared with commercial Pt/C catalysts because of the synergistic effect between Cu and Ce. The Cu−Ce−O oxide favors a four-electron pathway in ORR. During the battery tests, the Cu−Ce−O/CB exhibited a higher discharge voltage plateau than bare CFP, CB/CFP, and Cu−O/CB, and even outperformed the benchmark Pt/C at a high discharge current density when used as the cathode in Al−air batteries. The reported route could be extended to design and produce various nanostructured catalysts in a costeffective and scalable way in light of the versatility of LDH precursors for Al−air batteries to replace costly Pt/C. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02076. Instrumentation and methods, electrochemical measurements, XRD patterns of the CuZnAl-LDHs, TG/DTA curves of the CuZnAl-LDH-Ce(DTPA)2−, CVs and ORR polarization curves of Cu−Ce−O oxide and Cu− O, elemental contents of the Cu−Ce−O oxide determined by XPS spectra, elemental contents of CuZnAl-LDH-NO3−-pH, CuZnAl-LDH-Ce(DTPA)2−, and their derived calcined products, mixed oxides BET surface area, Al−air batteries using the Cu−Ce−O oxide electrode and commercial 20% Pt/C electrode, and ORR catalytic activity of Cu−Ce−O and relevant leading transition metal oxides ORR catalysts (PDF)

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This work was supported by grant from the Science and Technology Ministry of China (“863” project 2012AA062302).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Huimin Lu: 0000-0002-6363-0902 Notes

The authors declare no competing financial interest. 9174

DOI: 10.1021/acssuschemeng.7b02076 ACS Sustainable Chem. Eng. 2017, 5, 9169−9175

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DOI: 10.1021/acssuschemeng.7b02076 ACS Sustainable Chem. Eng. 2017, 5, 9169−9175