## 3 Results  
In this study, the uniform manganese-cobalt spinel nanoparticles and their composites with carbon nanotubes have been prepared through a straightforward two-step methodology. The structural formation and crystallization characteristics of the samples were initially verified through X-ray diffraction (XRD) analysis. Illustrations in Figure 1a demonstrate that the specific diffraction peaks' positions and intensities correspond precisely with those of the cubic MnCo\({}_{2}\)O\({}_{4}\) spinel structure (JCPDS no. 23-1237), devoid of any extraneous diffraction peaks indicating impurities. It is indicated that cubic spinel MnCo\({}_{2}\)O\({}_{4}\) has been successfully synthesized via the two-step process with precursor Mn/Co molar ratio of 1:2 (Mn/Co). Meanwhile, the XRD patterns of the products prepared with a precursor Mn/Co molar ratio of 2:1 (Mn/Co) are presented in Figure 1b. The characteristic diffraction peaks can be indexed as the tetragonal CoMn\({}_{2}\)O\({}_{4}\) phase (JCPDS no. 77-0471). This suggests the formation of CoMn\({}_{2}\)O\({}_{4}\) characterized by a slightly altered spinel framework. Notably,the XRD patterns illustrate no marked distinction between the two manganese-cobalt spinel nanoparticles and MnCo\({}_{2}\)O\({}_{4}\) (CoMn\({}_{2}\)O\({}_{4}\))-CNTs composites, due to the limited amount of CNTs and presence of MnCo\({}_{2}\)O\({}_{4}\) and CoMn\({}_{2}\)O\({}_{4}\) in the composites [41; 42]. Subsequent examinations of the microstructures and morphological characteristics of the synthesized samples were conducted using transmission electron microscopy (TEM). Figure 2a-c show the image of pure MnCo\({}_{2}\)O\({}_{4}\), and uniform nanoparticles with an average diameter of approximately 7 nm can be observed. The high-resolution TEM (HRTEM) image, depicted in Figure 2c, distinctly displays lattice fringes, with the measured d-spacing of 0.25 and 0.48 nm aligning perfectly with the (311) and (111) planes of the cubic spinel MnCo\({}_{2}\)O\({}_{4}\) structure, respectively [43]. Furthermore, in the MnCo\({}_{2}\)O\({}_{4}\)-CNT nanocomposites illustrated in Figure 2d-f, the MnCo\({}_{2}\)O\({}_{4}\) nanoparticles, denoted as MCO, exhibit a reduced average size of about 4 nm and are evenly distributed on the carbon nanotubes' surfaces. The TEM imagery of unadulterated CoMn\({}_{2}\)O\({}_{4}\), presented in Figure S1a-c, shows CoMn\({}_{2}\)O\({}_{4}\) nanoparticles mirroring the morphology of MnCo\({}_{2}\)O\({}_{4}\) but with a larger average size of 9 nm. Additionally, the observed lattice fringes of the CoMn\({}_{2}\)O\({}_{4}\) nanoparticles, exhibiting an inter-fringe spacing of 0.49 nm, correspond to the (101) plane of the tetragonal spinel CoMn\({}_{2}\)O\({}_{4}\) phase, which is shown in Figure S1c [44]. Figure S1d-f depict the CoMn\({}_{2}\)O\({}_{4}\)-CNT nanocomposites, where carbon nanotubes are uniformly encapsulated by CoMn\({}_{2}\)O\({}_{4}\) nanoparticles averaging 5 nm in size. Notably, MnCo\({}_{2}\)O\({}_{4}\) and CoMn\({}_{2}\)O\({}_{4}\) nanoparticles were not detected outside the CNTs following ultrasonication in ethanol for TEM analysis, suggesting a robust interaction between the CNTs and the manganese-cobalt spinel nanoparticles [45]. Additionally, it is noteworthy that the integration of CNTs curtails the size expansion of manganese-cobalt nanoparticles, attributable to the abundant hydroxyl functionalities on the CNTs-OH surfaces serving as profile nucleation sites for the spinel nanoparticles [46]. To delve deeper into the composition and valence states of the surface elements, X-ray photoelectron spectroscopy (XPS) analyses were conducted. The outcomes are presented in Figure 3. Figure 3a displays the survey spectra for both MnCo\({}_{2}\)O\({}_{4}\)-CNTs and CoMn\({}_{2}\)O\({}_{4}\)-CNTs, definitively confirming the presence of Mn, Co, O, and C elements. Furthermore, the Mn/Co atomic ratios in each sample are conspicuously inverse, aligning with the ratios used in the initial precursors.