## 4 Conclusions  
A detailed study of the structural morphology and electrochemical characteristics of composites based on MWCNTs and manganese oxide with the addition of rhenium was carried out using a combination of SEM, EDX, XPS and CV methods. It has been established that, as a result of the thermal treatment of the MnO\({}_{\mathrm{x}}\)/MWCNTs composite in vacuum condition at 580 \({}^{\circ}\)C, crystalline manganese oxide nanoparticles are formed, well distributed over the CNTs surface. Phase analysis performed on this sample indicates the presence of Mn\({}_{3}\)O\({}_{4}\) (dimanganese(III)). The addition of rhenium to a composite annealed at 580 \({}^{\circ}\)C leads to a decrease in the average size of nanoparticles to 15 nm, as well as additional oxidation of manganese oxide and an increase in the proportion of Mn(IV). It is shown that MWCNTs preserve their crystalline structure after heat treatment and the deposition of manganese and rhenium. In this case, the phase composition of metal oxides is represented mainly by Mn\({}_{3}\)O\({}_{4}\) (dimanganese(III)) and KReO\({}_{4}\). The formation of the KReO\({}_{4}\) phase is the result of the interaction of the KMnO\({}_{4}\) decomposition products and rhenium (IV) oxide, formed due to the decomposition of the NH\({}_{4}\)ReO\({}_{4}\) compound. It has been shown that the addition of rhenium to annealed composites leads to the additional oxidation of manganese oxide and an increase in the proportion of Mn(IV). The electrochemical tests of the composites showed that the best specific capacitance result (in both variants at high and low potential scan rates) is observed for the MnO\({}_{\mathrm{x}}\)/MWCNTs sample annealed at 580 \({}^{\circ}\)C with the addition of rhenium oxide. Such a high result is due to the synergistic effect achieved by combining the properties of manganese and rhenium oxides, which, on the one hand, provides an increase in the proportion of Mn(IV) oxides in the composite and leads to an increase in the proportion of pseudocapacitance (reversible RedOx processes), as well as an increase in the electrical conductivity of nanoparticles, and, on the other hand, leads to a noticeable increase in the contribution of the EDL during charge accumulation. Thus, this composite can be further used as an electrode material in the development of new electrochemical devices for storing and converting energy with good technical characteristics. The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app122412827/s1](https://www.mdpi.com/article/10.3390/app122412827/s1)]([https://www.mdpi.com/article/10.3390/app122412827/s1](https://www.mdpi.com/article/10.3390/app122412827/s1)), Figure S1: SEM image of Mn-0 composite (a) and the EDX spectrum recorded in the selected area (b); Figure S2: SEM image of Mn-580 + Re composite (a) and the EDX spectrum recorded in the selected area (b); Figure S3: Overview spectra of MWCNTs and composites; Figure S4: K2p photoemission spectra for different composites; Figure S5: The dependence of the reciprocal of the total capacitance (C\({}^{-1}\), (F/g)\({}^{-1}\)) on the square root of the scan rate (v\({}^{0.5}\), mV\({}^{0.5}\),s\({}^{-0.5}\)) and the total capacitance (C, F/g) on the reciprocal of the square root of the scan rate (v\({}^{-0.5}\), mV\({}^{-0.5}\),s\({}^{0.5}\)), respectively for the electrodes: MWCNTs (a) and (b), Mn-0 (c) and (d), Mn-580 (e) and (f), Mn-700 + Re (g) and (h); Figure S6: Change in specific capacitance depending on the number of cycles for electrodes based on MWCNTs (a) Mn-0 (b), Mn-580 (c) and Mn-580 + Re (d). Conceptualization, P.M.K. and S.N.N. ; methodology, P.M.K. and S.N.N.N. ; formal analysis, P.M.K. and S.N.N.N. ; writing--original draft preparation, P.M.K. and S.N.N. ; writing--review and editing, P.M.K. and S.N.N.N; visualization, P.M.K. and S.N.N.N. ; supervision, P.M.K. and S.N.N. All authors have read and agreed to the published version of the manuscript.