| Summary: | ZnMn<sub>2</sub>O<sub>4</sub> spinels are prepared by a simple hydrothermal route with control of the reaction time, ranging from 6 h to 18 h. The evolution of the structural and morphological parameters under the effect of time was analyzed by XRD, ATR-FTIR, XPS, and SEM-EDS. The XRD results show that for longer reaction times (18 h), the ZnMn<sub>2</sub>O<sub>4</sub> spinel samples present a tetragonal structure with high crystallinity and an average crystallite size of 32.3 ± 1.7 nm, larger than those obtained for 6 h and 12 h. The ATR-FTIR spectra confirm the structural results, with well-defined peaks related to stretching vibrations of M-O (M = Zn, Mn) functional groups. XPS reveals the co-existence of several metal oxides and hydroxides at the outermost surface. SEM analysis shows that the samples present a pyramid morphology, better defined at 18 h, with an average particle size of 6.2 ± 1.5 µm. EDS analysis of ZnMn<sub>2</sub>O<sub>4</sub> (18 h) reveals atomic ratios of 0.45, 0.22, and 0.50 for Zn/Mn, Zn/O, and Mn/O, respectively, in good agreement with the expected values. Based on the CVs, the synthesized ZnMn<sub>2</sub>O<sub>4</sub> samples formed at 18 h showed the most promising electrochemical properties, with a specific capacity of 102 F g<sup>−1</sup>, offering great potential in supercapacitor applications.
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