## 4 Conclusions  
In summary, manganese-cobalt spinel nanoparticles-carbon nanotubes composites of cubic MnCo\({}_{2}\)O\({}_{4}\)-CNTs and tetragonal CoMn\({}_{2}\)O\({}_{4}\)-CNTs have been effectively synthesized via a two-step method by involving the meticulous calibration of precursor ratios. Compared with a simple physical mixture of MnCo\({}_{2}\)O\({}_{4}\) and CNTs (MnCo\({}_{2}\)O\({}_{4}\) + CNTs), MnCo\({}_{2}\)O\({}_{4}\)-CNTs showed more positive starting potential, superior current density, and greater durability. These spinel oxides, characterized by their mixed valence properties, showcase a uniform dispersion of nanoparticles across the carbon nanotubes, anchored securely through robust hybrid coupling. Remarkably, these nanocomposites have demonstrated ORR activities on par with, and OER activities surpassing, those observed in the commercially available Pt/C catalysts. This work provides an attractive approach for designing efficient ORR/OER electrocatalysts and will open exciting avenues for exploring new strategies for the design of high-performance electrocatalysts for renewable energy applications.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app132312702/s1](https://www.mdpi.com/article/10.3390/app132312702/s1)]([https://www.mdpi.com/article/10.3390/app132312702/s1](https://www.mdpi.com/article/10.3390/app132312702/s1)), Figure S1: TEM images of (a-c) CoMn\({}_{2}\)O\({}_{4}\) nanoparticles, and (d-f) CoMn\({}_{2}\)O\({}_{4}\)-CNTs nanocomposites; Figure S2: ORR polarization curves of MCO, MCO-CNTs, and MCO + CNTs samples; Figure S3: The current density at the electrode potential of 0.8 V vs. RHE of MnCo\({}_{2}\)O\({}_{4}\), MnCo\({}_{2}\)O\({}_{4}\)-CNTs, and CoMn\({}_{2}\)O\({}_{4}\) + CNTs with the rotation of 1600 rpm; Figure S4: Chronoamperometric curves of MnCo\({}_{2}\)O\({}_{4}\)-CNTs, MnCo\({}_{2}\)O\({}_{4}\) + CNTs, and Pt/C with the rotation of 1600 rpm; Figure S5: OER polarization curves of MnCo\({}_{2}\)O\({}_{4}\)-CNTs, CoMn\({}_{2}\)O\({}_{4}\)-CNTs, and commercial Pt/C catalyst at the rotation of 1600 rpm.  
**Author Contributions:** The conceptualization and design of the experiments were the collective effort of [PERSON], [PERSON] and [PERSON]. The experimental work was carried out by [PERSON], [PERSON] and [PERSON]. [PERSON] and [PERSON] were responsible for data analysis. Reagents, materials, and analytical tools were provided by [PERSON], [PERSON] and [PERSON]. The manuscript was principally authored by [PERSON] All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research received funding from the National Natural Science Foundations of China (Grant Nos. 51902027, 61874014, 51976143, 61874013, 61672108, 61976025, 61674019, and 61974011), National Basic Research of China (Grant No. 2015 CB932500), Fundamental Research Funds for the Central Universities (Grant No. 2019 RC20, 2023 ZCJH03), Teaching Reform Projects at BUPT (2022 CXCY-B03), Fund of State Key Laboratory of Information Photonics and Optical Communications (Beijing University of Posts and Telecommunications, P.R. China) and BUPT Excellent Ph.D. Students Foundation (CX2022240).  
**Data Availability Statement:** The data from this research are available upon request from the corresponding author. The data are not publicly available because they are part of the ongoing research of the authors.  
**Conflicts of Interest:** The authors report no conflict of interest regarding this publication.