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CoO-CeO/C as a highly active electrocatalyst for oxygen reduction reaction in Al-air batteries Kun Liu, Xiaobing Huang, Haiyan Wang, Fuzhi Li, Yougen Tang, Jingsha Li, and Minhua Shao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12294 • Publication Date (Web): 30 Nov 2016 Downloaded from http://pubs.acs.org on December 2, 2016

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ACS Applied Materials & Interfaces

Co3O4-CeO2/C as a Highly Active Electrocatalyst for Oxygen Reduction Reaction in Al-Air Batteries Kun Liua, Xiaobing Huangb, Haiyan Wanga,c*, Fuzhi Lia,d, Yougen Tanga*, Jingsha Lia, Minhua Shaoc* a

College of Chemistry and Chemical Engineering, Central South University, Changsha, 410083, P.R

China. b

College of Chemistry and Chemical Engineering, Hunan University of Arts and Science, Changde,

415000, P.R China c

Department of Chemical and Biomolecular Engineering, The Hong Kong University of Science and

Technology, Clear Water Bay, Kowloon, Hong Kong d

Institute of Packing & Material, Hunan University of Technology, Zhuzhou, 412008, P.R. China

*Corresponding Author

E-mail: [email protected] (H. Wang); [email protected] (Y. Tang); [email protected] (M. Shao). Tel: +86 0731 8830886; fax: +86 0731 8879616 ABSTRACT Developing high-performance and low-cost electrocatalysts for oxygen reduction reaction (ORR) is still a great challenge for Al-air batteries. Herein, CeO2, a unique ORR promoter was incorporated into ketjenblack (KB) supported Co3O4 catalyst. We developed a facile two-step hydrothermal approach to fabricate Co3O4-CeO2/KB as a high-performance ORR catalyst for Al-air batteries. The ORR activity of Co3O4/KB was significantly increased by mixing with CeO2 nanoparticles. In addition, the Co3O4-CeO2/KB showed a better electrocatalytic performance and stability than 20wt % Pt/C in alkaline electrolytes, making it good candidate for highly active ORR catalysts. Co3O4-

ACS Paragon Plus Environment


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CeO2/KB favoured a four-electron pathway in ORR due to the synergistic interactions between CeO2 and Co3O4. In full cell tests, the Co3O4-CeO2/KB exhibited a higher discharge voltage plateau than CeO2/KB and Co3O4/KB when used in cathode in Al-air batteries. KEYWORDS:

Al-air

batteries

·Co3O4-CeO2/C

·oxygen

reduction

reaction

·synergistic

effects· hydrothermal· adsorption of oxygen INTRODUCTION As a prospective energy conversion and storage device, aluminum (Al)-air batteries have attracted enormous attentions due to its high practical energy density, low cost and operating temperature.1-3 Although it is a primary battery, it can be mechanically recharged with adding new aluminium electrode to extend the discharge. An electric car featuring this technology co-developed by Phinergy and Alcoa showed a driving distance of about 1000 miles by toping up water.4 This technology has attracted considerable attention.5,6 Like that in fuel cells, the sluggish oxygen reduction reaction (ORR) in Al-air batteries is also a big issue and has hindered the commercial implementation of this promising technology.7,8 Pt-based materials with a four-electron process to produce hydroxide ions (OH-) directly in alkaline solutions, have been regarded as typical ORR electrocatalysts.9,10 However, the large-scale commercial applications are not feasible due to their high cost and inferior durability.11,12 Thus, developing low cost and highly efficient catalysts is extremely desirable but still challenging for Al-air batteries. Carbon-based materials (BP2000, ketjenblack, carbon nanotube, graphene, etc.) have been widely used in cathode for Al-air batteries. They not only serve as the supports, but also good conductive agents to ameliorate the catalytic active materials.13 Recently, transition metal (Fe,14 Co,15 and Mn16 ) oxides based catalysts with excellent ORR performance have been reported as promising alternatives to precious Pt-related electrocatalysts for metal-air batteries. Among these oxides, cobalt oxide (Co3O4) has been commonly studied as an electrocatalyst for metal-air batteries,17 including Li-O218,19 and Zn-air batteries20-23. Xu et al. found that the catalytic activity of Co3O4/C was greatly enhanced by


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tailoring the number of surface-exposed Co3+ ions, even better than that of Pd-based catalyst.24 Co3O4 grown on rmGO substrate also performed much higher activity towards ORR in alkaline solution than pure Co3O4. Additionally, surface-modified Co3O4 anchored on N-GO was developed as an advanced ORR catalyst for Zn-air battery with a comparable performance to Pt/C.21 CeO2 has been intensively studied as a promotor for many catalyst systems owing to the existence of Ce4+/Ce3+ redox couple.25-27 For example, CuO-CeO2 exhibited significantly enhanced activity toward CO oxidation.28 As a key catalyst candidate, this unique property makes it a good assisted catalyst applied in metal-air batteries. Compared with the commercial MnO2, MnOx/CeO2 nanorods showed a higher catalytic activity when used as catalyst for Li-O2 batteries.29 Our previous work disclosed a higher power density Al-air battery with Mn0.3Ce0.7O2 catalyst instead of pristine MnO2. It is believed that the activation of molecular oxygen on MnOx should be enhanced effectively because of the introducing of Ce-O.7 It was well reported that CeO2 would play an essential role in the adsorption of oxygen to regulate the oxygen density of catalyst surfaces.25,26, 29 Although some efforts have been devoted to develop MxOy-CeO2 catalyst for CO and NxOy oxidations,7, 30,31 only a few papers discussed their applications in ORR.32,33 In our previous work, we synthesized a MnOx-CeO2/ketjenblack (KB) catalyst by reducing potassium permanganate with the uncovered carbon in CeO2/KB. It exhibited significantly improved electrocatalytic activity compared to CeO2/KB and MnO2/KB alone owing to the synergistic effect between MnOx and CeO2. Note that the half-wave potential of this catalyst was 0.81 V, ~10 mV more negative than that of 20wt.% Pt/C.34 In order to understand the effect of CeO2 in MxOy-CeO2 towards ORR and develop more active catalysts, we synthesized Co3O4-CeO2/KB catalyst by a new two-step low-temperature hydrothermal method. The half-wave potential of Co3O4-CeO2/KB was 20 mV higher than that of MnOx-CeO2/KB.34 Meanwhile, the synthesized sample had better half-wave potential than the latest reported perovskitetype catalysts (La0.5Sr0.5Mn0.9Ni0.1O3-δ (δ = 0.06, LSMN)).35 For the first time, this Co3O4-CeO2/KB catalyst was considered as the air electrode material in Al-air batteries showing excellent performance.



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Interestingly, the limited current density and half-wave potential of Co3O4-CeO2/KB composite even outperformed those of 20wt. % Pt/C catalyst (Johnson Matthey). RESULTS AND DISCUSSION To clarify the phases of as-prepared KB, CeO2/KB, Co3O4/KB and Co3O4-CeO2/KB, XRD analysis was performed. The XRD pattern of KB in Fig. 1 exhibits a broad peak at approximately 24° (2 theta) with a low crystallinity, the characteristic peak of graphitic structure.36,37 This peak in CeO2/KB and Co3O4-CeO2/KB was weaker due to overlapping from CeO2. The main four sharp diffractions at approximately 28.6°, 32.3°, 48.2° and 56.4° are consistent with CeO2 (JCPDS#65-5923). For Co3O4/KB, the sharp diffractions except a broad peak for KB are indexed to Co3O4 phase (JCPDS#431003). All the diffraction peaks attributed to CeO2 and Co3O4 can be observed in Co3O4-CeO2/KB, which confirms the co-existence of Co3O4 and CeO2. Based on the Scherrer equation, the crystallize size of Co3O4 in Co3O4-CeO2/KB is calculated to be 9.7 nm which is slightly smaller than that (10.3 nm) in Co3O4/KB.

Fig. 1 XRD patterns of KB, CeO2/KB, Co3O4/KB and Co3O4-CeO2/KB composite. The valence states and surface compositions of the Co3O4-CeO2/KB composite were conducted by XPS, as demonstrated in Fig. 2. The full survey of Co3O4-CeO2/KB composite displays the signals of Co 2p, Ce 3d, O 1s and C 1s (Fig. S1, ESI †). The high resolution peaks for Co 2p, Ce 3d and O 1s are shown in Fig. 2a, b, and c, respectively. In Fig. 2a, the peaks at 795.3 eV and 780.6 eV belong to Co2+,


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while the peaks at binding energies of 796.8 eV and 782.5 eV are ascribed to Co3+.38 Co3O4 possesses a spinel structure with a close-packed fcc configuration of O2- ions, where Co3+ ions fill one half of the

Fig. 2 XPS spectra of (a) Co 2p, (b) Ce 3d and (c) O 1s for Co3O4-CeO2/KB. octahedral B sites while Co2+ ions occupy the tetrahedral A sites.24, 39 Co3+ (t62ge0g) is substitutionally inert and low spin because of the higher oxidations state, whereas Co2+ (t52ge2g) with a high spin ion is substitutionally labile.40 Generally, Co3+ ions with the exposed active sites on Co3O4 play a critical role in the electrocatalytic performance for ORR.24 The Ce 3d spectrum (Fig. 2b) is characterized by two kinds of peaks: 3d3/2 and 3d5/2.41 The peaks denoted as U4, U3, U1, V4, V3 and V1 are attributed to Ce4+, whereas the peaks marked as U2 and V2 are assigned to Ce3+.42,43 The Ce 3d spectrum suggests the co-existence of Ce3+ and Ce4+ species with Ce4+ being the mainly valence state in the Co3O4-CeO2/KB.41 Oxygen vacancies and unsaturated chemical bond are introduced in the



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crystal to create a charge imbalance owing to the presence of Ce3+ in CeO2 nanocrystals.44 The O1s high resolution spectrum was deconvoluted into three typical peaks at 530.1, 531.7, and 533.2 eV (Fig. 2c). On the basis of previous reports, the energy at 530.1 eV corresponds to a typical metal-oxygen bond (denoted as Oα),45 the peak at 531.7 eV is a low defect oxygen coordination with small particle size (denoted as Oβ),38 and the binding energy at 533.2 eV is due to chemically bonded water and a multiplicity of physically on and within the surface (denoted as Oγ).46

Fig. 3 (a) SEM and (b) TEM images of Co3O4-CeO2/KB composite; HRTEM images (c) and (d) corresponding to the square region marked in TEM image (b). The microstructure of the as-prepared Co3O4-CeO2/KB was characterized by SEM, TEM and HRTEM images (Fig. 3). Fig. 3a demonstrates good dispersion of Co3O4-CeO2/KB composite. Fig. 3b shows a low-magnification TEM image, in which two square regions are marked and the corresponding HRTEM pictures are exhibited in Fig. 3c and 3d. HRTEM images confirm the homogeneous distribution of nanosized Co3O4 and CeO2 with intimate contact on KB matrix identified by lattice analysis. As seen in Fig. 3(c), (400) and (220) planes of Co3O4 correspond to 0.202 and


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0.286 nm of the lattice spacings, whereas that of 0.312 nm corresponds to (111) planes of CeO2. In Fig. 3d, (311) and (220) facets of Co3O4 crystals with the lattice spacings of 0.244 and 0.286 nm, respectively, are marked. At the same time, (220) and (111) facets of CeO2 with the lattice fringes of 0.191 and 0.312 nm, respectively, are also confirmed. Clearly, the intimate contact between Co3O4 and CeO2 particles would promise better synergistic effect towards catalytic reactions. STEM-EDS mapping analysis in Fig. 4 further verifies the uniform distribution of the Co, Ce and O elements throughout the carbon support. The mass loadings of carbon in CeO2/KB, Co3O4/KB and Co3O4CeO2/KB are about 80.8%, 76.7% and 62.4%, respectively, based on the DSC/TG testing results (Fig. 5c).

Fig. 4 STEM and elemental mapping analysis of Co3O4-CeO2/KB composite; (a) Typical STEM image; (b) STEM image taken from the middle selected region marked in (a); (c-h) the corresponding elemental mapping images of (c) C-K, (d) O-K, (e) Co-K, (f) Co-L, (g) Ce-L and (h) Ce-M. The LSV curves for different samples at a rotating speed of 1600 rpm are shown in Fig. 6a. It can be seen that KB exhibits a very low ORR electrocatalytic activity. After introducing the transitional metal oxides, more positive half-wave potentials of CeO2/KB (~0.71 V) and Co3O4/KB (~0.73 V) are observed, indicating enhanced ORR performance. Notably, the half-wave potential of Co3O4-CeO2/KB composite is ~0.83 V, which is much higher than those of CeO2/KB and Co3O4/KB. It is also about 10 mV more positive than that of Pt/C (~0.82 V). Interestingly, the incorporation of CeO2 effectively increases the ORR activity of Co3O4-CeO2/KB. In addition, the limiting current density of Co3O4-



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CeO2/KB is ~ -5.35 mA cm-2, higher than those of CeO2/KB and Co3O4/KB. It even exceeds that (5.25 mA cm-2) of the commercial Pt/C. Noted that the specific surface area of Co3O4-CeO2/KB (325.8 m2 g-1) is much lower than those of Co3O4/KB (588.1 m2 g-1) and CeO2/KB (369.1 m2 g-1), as seen in Fig. 5d, because the load partially blocks the pores of nanoparticles. The more the metal oxides support, the smaller the specific surface areas become. However, Co3O4-CeO2/KB with lower specific surface area exhibits a better catalytic property than those of CeO2/KB and Co3O4/KB, revealing the formation of more active sites on Co3O4 and CeO2 nanoparticles.34 Therefore, a substantial number of active sites residing on Co3O4-CeO2/KB composite may be more easily accessible to O2, making a great contribution to catalytic activities.47,48 For comparison, the ORR catalytic performance of Co3O4CeO2/KB is superior to that of Co3O4/N-doped reduced graphene oxide nanocomposites reported previously.49 Moreover, CeO2 could serve not only as an effective catalyst for ORR, but also as an “oxygen buffer” for metal-air batteries to relieve oxygen insufficiency.29 The significantly improved performance should be contributed to the synergistic effects between Co3O4 and CeO2.50 Cyclic voltammetry (CV) in 0.1M KOH solution saturated with N2 or O2 at 10 mV·s-1 was evaluated to further study their ORR catalytic activities (Fig. 6b). All the samples recorded in Ar-saturated electrolyte show no obvious reduction peak between 0 and 1.2 V. In contrast, when O2 was saturated in electrolyte, the profile curve appears a clear reduction peak, suggesting the remarkable ORR performance. The CV curves of KB, CeO2/KB and Co3O4/KB electrode in O2 show a obvious peak potential of ~0.73 V, ~0.74 V and ~0.76 V (vs. RHE), respectively, which are more negative in comparison with that of commercial Pt/C (20wt%). Note that Co3O4-CeO2/KB exhibits a reduction peak potential of ~0.82 V (vs. RHE), which is about 20 mV higher than that of Pt/C. The significantly improved ORR performance of Co3O4-CeO2/KB compared with CeO2/KB and Co3O4/KB should be mainly ascribed to the synergistic interaction between CeO2 and Co3O4.


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Fig. 5 TG/DSC curves of (a) CeO2/KB, (b) Co3O4/KB and (c) Co3O4-CeO2/KB; (d) N2 adsorptiondesorption isotherms of CeO2/KB, Co3O4/KB and Co3O4-CeO2/KB. The Koutecky-Levich (K–L) plots on the basis of their corresponding RDE curves (Fig. S2, ESI†) are given in Fig. 6c. The corresponding K–L plots (J-1 vs. ω-1/2) 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 at different potentials (Fig. S2, ESI†).50-52 The electron transfer numbers (n) of KB, CeO2/KB, Co3O4/KB, Co3O4-CeO2/KB and 20wt% Pt/C at 0.5 V are calculated to be 2.01, 2.63, 3.61, 3.91 and 3.97, respectively. To further confirm the reaction mechanism of the above catalysts, we obtained the percentage of peroxide (H2O2%) and electron transfer number (n) at various potentials by RRDE (Fig. 7). The assessed H2O2 yields of both Co3O4-CeO2/KB and Pt/C between 0.3 and 0.8 V (vs. RHE) are close to 1%, which are much less than those for KB (close to 50%), CeO2/KB (close to 30%) and Co3O4/KB (close to 5%). The 2.3, 2.6, 3.8 for KB, CeO2/KB and Co3O4/KB, respectively. For Co3O4CeO2/KB and Pt/C, their electron transfer numbers approach to 4, indicating the as-prepared Co3O4CeO2/KB favours a four-electron transport throughout the reaction process. To comprehensively



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understand the difference of ORR kinetics performance between Co3O4-CeO2/KB and Pt/C, the Tafel slopes were obtained from the linear plots of LSV at 1600 rpm (Fig. S2 (g, i), ESI†). As seen from Fig. 6d, the Tafel slope (83.9 mV/decade) of Co3O4-CeO2/KB is close to that of 20wt% Pt/C (79.2 mV/decade), suggesting its good kinetic behavior for ORR.

Fig. 6 (a) ORR polarization curves for different catalysts in O2-saturated 0.1 M KOH solution at 1600 rpm; (b) CVs of KB, CeO2/KB, Co3O4/KB, Co3O4-CeO2/KB composite and Pt/C in Ar and O2 saturated 0.1M KOH solution; (c) K-L plots of different samples at 0.5 V; (d) Tafel plots of kinetic current for Co3O4-CeO2/KB and Pt/C; the ORR activity of (e) Co3O4-CeO2/KB and (f) Pt/C, measured before and after 2000 cycles. Insets: CVs for ORR at the Co3O4-CeO2/KB (e) and Pt-C (f) before cycling and after 2000 cycles.


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Fig. 7 (a) The percentage of peroxide (H2O2%) and (b) electron transfer number (n) of different catalysts at different potentials. We assessed the durability of Co3O4-CeO2/KB and Pt/C catalyst under potential cycling conditions between 0.6 and 1.2 V at 50 mV·s-1 (Fig. 6e and 6f). After 2000 continuous cycles, the CV curve of Co3O4-CeO2/KB scarcely changes, while larger difference is observed for 20wt% Pt/C. The half-wave potentials (E1/2) of Co3O4-CeO2/KB and 20wt% Pt/C exhibit a negative shift of ~16 mV and ~35 mV, respectively. These results confirm that the Co3O4-CeO2/KB shows better cycling stability than the commercial Pt/C. Many reports have inferred that the ORR takes place at the cobalt oxide surface with active sites of Co3+. The Co3+ ions can serve as donor-acceptor reduction sites.20 To understand the active sites of Co3O4, these results are described to comprehend Co3+/Co4+ redox by the following reactions53,54: H2O + OH- + Co3O4 → 3CoOOH+ e- (1) OH- + CoOOH → CoO2 + H2O + e-

(2)

H2O + CoO2 + e- →CoOOH + OH-

(3)

On the other hand, CeO2 has the excellent ability of reversibly exchanging oxygen. The process can be represented with the following equation55: CeO2

୶

⇔ CeO2-x + O2 ଶ

(0≤x≤0.5)

Hence, CeO2 often acts as an oxygen buffer owing to the Ce4+/Ce3+ redox couple. It can store O2 in an oxygen-rich condition and remove it in oxygen-insufficient condition. When O2 escapes from the ACS Paragon Plus Environment


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surface of CeO2, a reduction reaction from Ce4+ to Ce3+ occurs; while O2 is captured on the surface of the CeO2, Ce3+ is oxidized to Ce4+.56 When oxygen concentration is lower in the high current density region, the oxygen adsorbed on CeO2 sites receive electrons to form HO2−, which would be easily transferred to the contiguous Co3O4 active sites. Therefore, the existence of Ce3+ could also remarkably enhance the catalytic activity of Co3O4-CeO2/KB composite and realize the effective activation of molecular oxygen.34,57-59 In other words, the existence of CeO2 increases the oxygen storage capacity on the surface of the catalyst. During the redox process, these factors are helpful to the catalytic activity in accordance with the Mars-Krevelen redox mechanism.60 Thus, a certain amount of CeO2 on Co3O4-CeO2/KB composite can increase the oxygen transfer, finally enhance the ORR activity. However, too many CeO2 may weaken the ORR performance due to the inferior intrinsic catalytic activity of CeO2 in comparison with Co3O4.7 The ORR activities of various Co3O4-CeO2/KB synthesized by changing mass ratio of Ce(NO3)3·6H2O:KB are compared in Fig. S3, ESI†. The results demonstrate that the sample with a Ce(NO3)3·6H2O:KB mass ratio of 1:15 displays the best ORR activity. The elemental analysis was further conducted by ICP analysis, and the molar ratio of Co:Ce in the optimized sample (Ce(NO3)3:KB=1:15) was found to be close to 1:5, while those of Co:Ce in the other samples (Ce(NO3)3:KB=1:10 and Ce(NO3)3:KB=1:20) were be close to 1:10 and 2:5, respectively.

Moreover, the intimate contact between Co3O4 and CeO2 particles seen from the

HRTEM images would be conducive to ORR enhancement. The CeO2 particles not only improve the dispersion of Co3O4, but also activate oxygen directly involved in the redox processes. It has been confirmed that CeO2 in MxOy-CeO2 composite could generate the local electrons of Ce 4f orbit, in which electrons could be stored and transported to the adsorbed small molecules (CO, NO, O2, etc.).61,62 By accepting the local electrons, the small molecules adsorbed on CeO2 carrier could be activated, thereby increasing the catalytic performance of the catalyst.63 In addition, we believe that the transfer of the adsorbed oxygen reaction intermediates to Co3O4 could further enhance the ORR performance. Accordingly, we hypothesize that a synergistic coupling of the redox properties between Co3O4 and CeO2 on KB substrate greatly contribute to the superior catalytic properties. It was reported


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that oxygen-storage CeO2 in MnOx-CeO2/KB was a high-performance synergistic component for ORR due to containing the function of adsorbing oxygen molecules in oxygen reduction.34 In Co3O4CeO2/KB system, it is inferred that some oxygen should be firstly reduced to HO2- at CeO2 and further reduced to OH- at Co3O4 in alkaline media. Furthermore, the high specific surface area, small charge transfer resistance, and good synergistic effect of KB, Co3O4 and CeO2 could be the main reasons for the better ORR activity of Co3O4-CeO2/KB. To evaluate the feasibility of Co3O4-CeO2/KB in practical application, we fabricated a primary Alair battery for constant current discharge test. The testing results of the Al-air batteries with different catalysts at 50 mA cm-2 are shown in Fig. 8. We recorded the data every 30 min (discharge for 10h). As can be seen, all cells show the increased discharge voltage in the beginning due to the activation process of Al anode, on which the passive film was dissolved gradually. Co3O4-CeO2/KB catalyst exhibits a working voltage plateau of about 1.27 V, which is about 0.04 V and 0.15 V higher than that of Co3O4/KB (1.23 V) and CeO2/KB (1.12 V), respectively. The pristine KB declines quickly in several hours, revealing the poor electrochemical activity towards ORR. Note that the Co3O4-CeO2/KB catalyst demonstrates a very stable voltage during the discharge process. In the light of these results, it is palpability that the air cathode with Co3O4-CeO2/KB catalyst shows much better discharge performance than CeO2/KB or Co3O4/KB alone.

Fig. 8 Electrochemical test of Al-air batteries with various as-prepared catalysts.



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CONCLUSIONS We developed a facile two-step hydrothermal approach to fabricate Co3O4-CeO2/KB as a highperformance ORR electrocatalytic catalyst for Al-air batteries. Interestingly, the ORR activity of Co3O4/KB was significantly increased by mixing with CeO2 particles, which even outperformed the benchmark Pt/C in alkaline electrolyte, making the Co3O4-CeO2/KB one of the best-performing transition metal oxide based catalysts. The Co3O4-CeO2/KB favoured a four-electron pathway in ORR due to the synergistic effect between CeO2 and Co3O4. In full cell tests, the Co3O4-CeO2/KB exhibited a higher discharge voltage plateau than CeO2/KB and Co3O4/KB when used in cathode in Al-air batteries. The strategy of using CeO2 as catalyst promoter can be used to other transitional metal oxides. By virtues of their high performance and low cost, Co3O4-CeO2/KB catalysts here can be applied as a good candidate for Al-air batteries to replace the costly Pt/C.

METHODS Chemicals. The main chemicals, HNO3, Ce(NO3)3·6H2O, Co(CH3COO)2·4H2O), C6H8O7·H2O and CO(NH2)2 were purchased from Sinopharm Chemical Reagent Co., Ltd. and used directly. Ketjenbalck EC-300J (KB) was purchased from Shanghai Teng Min Industrial Co., Ltd. (Shanghai, China). Before using, the KB was pretreated as follows. Firstly, 2.0 g of KB was added into 100 mL HNO3 and then refluxed at 80 °C for 8 h. The mixture was rinsed using deionized water to remove the acid residual and then dried at 80 °C overnight in an oven. The gas diffusion layers used in this work were provided by Midea bus factory (Kunming, China). Synthesis of Co3O4-CeO2/KB, CeO2/KB and Co3O4/KB. Co3O4-CeO2/KB was synthesized via a simple two-step hydrothermal method and the detailed procedures are illustrated in Scheme 1. Briefly, 0.2 g Ce(NO3)3·6H2O, 1.2 g of urea and 2.1 g of C6H8O7·H2O were first dissolved into 80 ml deionized water, then 1.5 g of pre-treated KB was added. The suspension was quickly transferred into a 100 mL Teflon-lined autoclave after ~30 min of ultrasonic processing and kept at 120 °C for 24 h. The precipitate was filtered and dried in air at 80 °C for 12h to obtain CeO2/KB.


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The above-mentioned CeO2/KB was added to the mixed solution containing 0.35 g of Co(CH3COO)2·4H2O and 3.2g of urea. After continuous and vigorous stirring for at least 10 min, the suspension was rapidly poured into a Teflon-lined autoclave (100 mL) and reacted at 140 °C for 12 h. The product was filtered and dried at 80 °C for 12 h in air, followed by an annealing process in air at 300 °C for 2 h at a heating rate of 5 °C min-1 in a muffle furnace. The product denoted as Co3O4CeO2/KB was collected after cooling down. Controlling different contents of CeO2 in Co3O4-CeO2/KB were performed to obtain the optimal catalytic activity. For comparison, Co3O4/KB was synthesized by the similar method but KB instead of CeO2/KB was used.

Scheme 1. The schematic diagram for the preparation of Co3O4-CeO2/KB. Sample characterizations. The structure of as-prepared samples were measured by an X-ray diffractometer (Dandong Haoyuan, DX-2700) using a CuKα1 source. Scanning electron microscope (SEM) pictures of Co3O4-CeO2/KB were taken utilizing a Nova NanoSEM 230 SEM. High resolution transmission electron microscope (HRTEM) images, scanning TEM (STEM) and elemental mapping were detected using a FEI Tecnai G2 F20 S-TWIX TEM. X-ray photoelectron spectroscopy (XPS, KAlpha1063) was used to determine the chemical compositions of Co3O4-CeO2/KB. Differential scanning calorimetry and thermogravimetric analysis (DSC/TGA) was recorded by a NETZSCH STA 449C with a heating rate of 10 °C min-1 from room temperature to 800 °C in air. A Builder SSA-4200 apparatus Surface Area Analyzer was employed to determine the BET surface areas of CeO2/KB, Co3O4/KB and Co3O4-CeO2/KB samples. The cerium content and its concentration in the Co3O4-



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CeO2/KB were conducted on a Perkin Elmer Optima 5300 inductively coupled plasma-optical emission spectroscopy (ICP-OES). Rotating disk electrode tests. A CHI760E electrochemical station was used for electrochemical measurements in a standard three-compartment electrochemical cell. The counter electrode was a platinum wire and the reference electrode was a saturated calomel electrode (SCE). The working electrode was sample coated glassy carbon electrode. All potentials were converted to reversible hydrogen electrode (RHE). The testing samples were prepared by ultrasonically mixing 4 mg of asprepared samples and 100µL of Nafion (5wt %) in 1900µL ethanol for more than 40 min. Then 10µL of the testing samples was loaded onto the work electrode. Cyclic voltammetry (CV) was conducted in N2- and O2-saturated 0.1 M KOH electrolyte at room temperature to test the catalytic properties of the as-prepared materials. The scan rate was 10 mV s-1 and the voltage range was from 0 to 1.2 V. Linear sweep voltammetry (LSV) curves were obtained in oxygen-saturated 0.1 M KOH solution with a sweep rate of 10 mV s-1 at various rotating speeds ranging from 400 to 1600 rpm. In order to make the electrolyte saturated with oxygen, oxygen (99.999%) was introduced into the reactor for 20 min before each measurement. O2 flow was promised during the test. Electron transfer number (n) was calculated by the Equations (S1, S2, S3†) in the electronic supplementary information (ESI). In case of the long-term durability test, the electrodes were first cycled 2000 cycles from 0.6 to 1.2 V at 50 mV s-1. Al-air batteries test. To assess the electrochemical activities of the as-prepared samples in Al-air batteries, we fabricated air electrodes with a three-layer structure using the hot press method which was composed of a gas diffusion layer, a current collector, and a catalytic layer. The current collector in the air electrode utilized Ni foams due to its superior conductivity and intensity. To fabricate the catalyst layer, catalysts, ketjenblack, polytetrafluoroethylene and acetylene black (mass ratio of 3:3:3:1) were mixed and dispersed in ethanol to form a slurry. After it turned into a paste, it was rolled to the thin sheet with the thickness of ~0.2 mm on glass plate.7 The total thickness of the air electrode was


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0.4-0.6 mm (3 cm×5 cm) after pressing at 15 MPa. Finally, the air electrode was dried at 60 °C overnight in a vacuum oven. For full cell tests, a home-made Al-air battery was utilized and tested on a Neware battery testing system (CT-3008W). The detailed information could be seen in Reference.8

Supporting Information Available: Koutecky-Levich equation, XPS spectra of survey spectrum for Co3O4-CeO2/KB composite, The RDE at various rotation rates, and the K-L plots, the linear polarization curves of the various mass ratio of Ce(NO3)3·6H2O/KB in Co3O4-CeO2/KB composite. These materials are available free of charge via the Internet at http:// pubs.acs.org. Acknowledgements. The authors thank for the financial support from National Nature Science Foundation of China (No. 21571189, No.21671200 and No. 51304077), Hunan Provincial Natural Science Foundation of China (No.14JJ3022). The work at the Hong Kong University of Science and Technology was supported by a start up fund and the “Hong Kong Scholar” fund.

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Graphic Abstract


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Graphical Abstract 186x78mm (300 x 300 DPI)


C as a Highly Active Electrocatalyst ... - ACS Publications

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