Unveiling Enhanced Oxygen Reduction in Multi-Walled Carbon Nanotube-Supported MnCo2O4: Experimental and Theoretical Insights into Tin Substitution for Octahedral Cobalt

In this work, Sn-doped manganese cobaltite (Sn_x-MnCo₂O₄) was synthesized by using a solvothermal method followed by an annealing process. To further increase its catalytic efficiency, it was integrated onto a carbon support, carboxylic acid-functionalized multiwalled carbon nanotubes (fMWCNTs). Among the synthesized materials, fMWCNT-supported Sn-doped MnCo₂O₄ exhibits the highest onset potential for oxygen reduction and shows a distinctly selective four-electron oxygen reduction, as demonstrated by the rotating disc electrode and rotating ring disc electrode experiments. X-ray photoelectron spectroscopy reveals a shift in the binding energy of Mn 2p owing to alterations in the electronic structure of the crystal upon incorporation of Sn into MnCo₂O₄. Computational studies proved the replacement of octahedral Co ions in the MnCo₂O₄ crystal structure by Sn⁴⁺ ions. The withdrawal of electron density by Sn⁴⁺ species from the active centers (Mn³⁺) leads to an increased electropositive character at the Mn³⁺ centers. Since Mn³⁺ centers are the effective active centers in this catalyst, oxygen is efficiently adsorbed at these active centers, resulting in enhanced electrocatalytic activity.