Mechanistic Understanding of the Underlying Energy Storage Mechanism of α-MnO₂-based Pseudo-Supercapacitors

Manganese dioxide (α-MnO₂) has attracted significant research interest in supercapacitors recently. However, the reaction mechanism of α-MnO₂ in supercapacitors remains unclear. Therefore, a nano-supercapacitor using Environmental transmission electron microscopy (ETEM) is conducted and investigated the reaction mechanism of α-MnO₂ based on three ionic liquids (ILs). It found that in the aprotic ionic liquid (AIL) 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMIMOTF), α-MnO₂ nanowires (NWs) undergo an oxidation reaction due to the presence of an active proton at the second position (H2) of the imidazole ring. As a result, α-MnO₂ NWs undergo a phase transition and transform into Mn₃O₄, exhibiting pseudo-capacitive properties. Furthermore, characterization of the macroscopic α-MnO₂ electrodes after cycling reveals that after the initial charging cycles, the dominant energy storage mechanism of the supercapacitor transitions from pseudo-capacitance to a dual-layer capacitance formed by the combination of Mn₃O₄ and unreacted α-MnO₂. Simultaneously, due to the coexistence of these two energy storage mechanisms, the specific capacitance of the supercapacitor in EMIMOTF electrolyte reaches up to 80 F g^-1, and the cycle number reaches as high as 1000 cycles. The results are expected to provide insights into the selection of electrolytes in supercapacitors and offer a fundamental understanding of the internal reaction mechanisms in capacitors.