Fuel assisted crystal structure tailoring of manganese oxides and their surface reactivity towards oxygen evolution reaction

A simple and robust method has been proposed to synthesize manganese oxide polymorphs wherein the phase transition among the manganese oxides crystal structures was evidenced under the assistance of fuels via solution combustion route. The cubic-Mn₂O₃ dominated with glycine and urea, while spinel tetragonal-Mn₃O₄ was favored with sucrose, citric acid and maleic acid fuels. The average particle size of 17 nm and 21 nm is observed for Mn₂O₃ derived from urea and glycine respectively. The Mn₂O₃ derived from urea (Mn₂O₃–U) exhibits high fraction of Mn³⁺ content and oxygen defects on the surface compared to the Mn₂O₃ derived from glycine (Mn₂O₃–G). In addition, Mn₂O₃–U exhibits ~ 5 times higher electrochemical active surface area (237 cm⁻²) compared to Mn₂O₃–G (47.5 cm⁻²). Owing to the enhanced surface properties, Mn₂O₃–U demonstrates superior OER performance where it exhibits a low overpotential of 270 mV at the current density of 10 mA cm⁻² compared to Mn₂O₃–G (590 mV 10 mA cm⁻²). The Mn₃O₄ derived from sucrose (Mn₃O₄–S), citric acid (Mn₃O₄–C) and maleic acid (Mn₃O₄–M) exhibits inferior OER performance compared to Mn₂O₃ and followed the order: Mn₂O₃–U > Mn₂O₃–G > Mn₃O₄–S > Mn₃O₄–C > Mn₃O₄–M. The findings of the results collectively suggest that the fuel alters the surface structure and crystallization process, with further impacts the electrocatalytic performance.

Fuels used in the synthesis step altered the crystallization kinetic and surface structural features.