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

Abstract

AbstractManganese 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.