Volume 2, Issue 2, Pages 337-348 (February 2018) Durable and Efficient Hollow Porous Oxide Spinel Microspheres for Oxygen Reduction  Hao Wang, Ruiping Liu, Yutao Li, Xujie Lü, Qi Wang, Shiqiang Zhao, Kunjie Yuan, Zhiming Cui, Xiang Li, Sen Xin, Ru Zhang, Ming Lei, Zhiqun Lin  Joule  Volume 2, Issue 2, Pages 337-348 (February 2018) DOI: 10.1016/j.joule.2017.11.016 Copyright © 2017 Elsevier Inc. Terms and Conditions

Joule 2018 2, 337-348DOI: (10.1016/j.joule.2017.11.016) Copyright © 2017 Elsevier Inc. Terms and Conditions

Figure 1 Morphologies and Element Distribution of As-Obtained Microspheres (A) TEM images of hollow porous ZnCo2O4 (x = 0). Scale bar, 500 nm. (B) ZnMnCoO4 (x = 1) microspheres. Scale bar, 600 nm. (C–F) Elemental mapping of hollow porous ZnMnCoO4 microspheres: (C) Co, (D) Mn, (E) Zn, and (F) O. Scale bar, 1 μm. Joule 2018 2, 337-348DOI: (10.1016/j.joule.2017.11.016) Copyright © 2017 Elsevier Inc. Terms and Conditions

Figure 2 XRD Pattern and XPS of the As-Obtained Microspheres (A) XRD patterns of as-prepared hollow porous ZnMnxCo2−xO4 microspheres and commercially available Co3O4 powder. The standard XRD profiles of ZnMn2O4 (JCPDS no. 77-0470) and Co3O4 (JCPDS no. 74-2120) are shown in pink and green, respectively. (B) Rietveld analysis of the synchrotron XRD of ZnMnCoO4 (open circles, experimental measurement; red line, fitted profile). The crystal structure of spinel ZnMnCoO4 is shown as an inset. (C and D) XPS spectra of (C) Co and (D) Mn in hollow porous ZnMnCoO4 microspheres. Joule 2018 2, 337-348DOI: (10.1016/j.joule.2017.11.016) Copyright © 2017 Elsevier Inc. Terms and Conditions

Figure 3 The ORR Performances of the As-Obtained Samples (A) ORR polarization curves of hollow porous ZnMnxCo2−xO4 microspheres with different value of x, solid Co3O4 powder, and the Pt/C powder as catalysts in an O2-saturated 0.1 M KOH solution stirred at 1,600 rpm. (B) Comparison of the ORR polarization curves among the ZnCo2O4, ZnMnCoO4, and Pt/C catalysts in an O2-saturated 0.1 M KOH solution stirred at 1,600 rpm. (C) The Tafel slope of ZnMnxCo2−xO4, Co3O4 powder and Pt/C catalysts. (D) The chronoamperometric curves of ZnCo2O4 and ZnMnCoO4 catalysts and Pt/C in an O2-saturated 0.1 M KOH electrolyte at the fixed voltage of 0.6 V stirred at 1,600 rpm. Joule 2018 2, 337-348DOI: (10.1016/j.joule.2017.11.016) Copyright © 2017 Elsevier Inc. Terms and Conditions

Figure 4 The Exploration of ORR Mechanism for ZnMnxCo2-xO4 (A and B) Magnetic susceptibility of hollow porous (A) ZnCo2O4 and (B) ZnMnCoO4 microspheres. (C) The proposed ORR process occurring on the surface of Co3+ ions. (D) The relationship between the ORR activity (i.e., onset potential) of a series of catalysts and the B3+-O bond length within the spinel oxide catalysts (B = Co/Mn). Joule 2018 2, 337-348DOI: (10.1016/j.joule.2017.11.016) Copyright © 2017 Elsevier Inc. Terms and Conditions

Figure 5 The DFT Calculations for Correlating the ORR Activity with the Spinel Structure of Crafted ZnMnxCo2−xO4 Catalysts (i.e., ZnCo2O4 at x = 0 and ZnMnCoO4 at x = 1) (A and B) Optimized atomic structures for the main process of an ORR: (A) ZnCo2O4 and (B) ZnMnCoO4. The red circles and gray circles represent O atom and Zn atom, respectively. The blue circle refers to Co atom in (A) and represents B atom (i.e., 50% Mn and 50% Co atoms) in (B). Joule 2018 2, 337-348DOI: (10.1016/j.joule.2017.11.016) Copyright © 2017 Elsevier Inc. Terms and Conditions