Selection of oxygen reduction catalysts for secondary tri-electrode zinc–air batteries

Sample characterisationThe morphology of the ORR catalysts was characterised with SEM (Fig. 2) and X-ray powder diffraction measurements confirmed the crystal structures of the samples (Fig. 3). Structural parameters of the catalyst samples are reported in Table 1. Comparing the manganese oxides, the commercially procured MnO2 in Fig. 2a is composed of large granular particles and the diffraction pattern in Fig. 3a matches JCPDS 24-0735 for tetragonal β-MnO2. In comparison, the surface of MnOx in Fig. 2b displays smaller, finer, particles and this corresponds to the diffraction pattern in Fig. 3b which exhibits weak broad peaks characteristic of a non-crystalline nature; with peaks at 13.3°, 25°, 37° and 66° matching the (110), (220), (310), (211) and (541) peaks of tetragonal body-centred α-MnO2 hydrate, JCPDS 44-014028.Figure 2SEM images of (a) MnO2, (b) MnOx, (c) Co3O4,…

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