1.2 Volt manganese oxide symmetric supercapacitor

Capacitance fading of MnO2 supercapacitor electrode under negative polarization below 0.0 V (versus Ag/AgCl/sat. KCl(aq)) arises from extensive reduction of Mn(IV) to form inactive Mn(II) species, and this has typically limited the operating voltage window of an aqueous symmetric MnO2 supercapacitor to be no greater than 0.8 V. As this lower potential limit is close to the onset potential of MnO2-catalyzed oxygen reduction reaction (ORR), the fading problem can be alleviated by effectively passing the accumulated electrons in the oxide electrode to the dissolved oxygen molecules in electrolyte in order to avoid the formation of the surface Mn(II) species. This has been demonstrated by either increasing the dissolved oxygen content or using the Ti(IV)/Ti(III) redox couple in the electrolyte as a charge-transfer mediator to enhance the electrocatalytic activity of MnO2 for ORR. Therefore, a MnO2 symmetric supercapacitor showing remarkable cycling stability over an operating voltage window of 1.2 V has been achieved by using Ti(IV)-containing neutral electrolyte (1 M KCl(aq)).

► Formation of Mn(II) on MnO2 electrode at low potentials causes capacitance fading.
► Working voltage window of a typical MnO2 symmetric supercapacitor is limited to 0.8 V.
► A new approach is introduced to alleviate the capacitance fading.
► Electrons are passed to dissolved oxygen in electrolyte to avoid Mn(II) formation.
► MnO2 supercapacitor with remarkable cycling stability over 1.2 V is achieved.