High-performance asymmetric supercapacitors fabricated by amorphous MnO₂ on 3D-Ni foam as positive electrodes in a mixed electrolyte

A high-performance supercapacitor based on an amorphous MnO₂ electrode on Ni foam (MnO₂/NF) in a Na₂SO₄/KOH mixed electrolyte was fabricated and studied. XRD, Raman, TEM, and SEM characterizations highlighted the nanostructure of these amorphous MnO₂ materials. The electrochemical behaviors of amorphous MnO₂/NF were studied using cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) measurements, and the cycling performance. The results reveal that the amorphous MnO₂/NF in the Na₂SO₄/KOH electrolyte exhibited a high capacitance. The Na₂SO₄/KOH mixed electrolyte with ionic interactions in the mesopores increased the capacitance. However, weaker ion-interacting cations can diffuse into microspores without losing their capacitive ability. An asymmetric supercapacitor has been developed successfully with amorphous MnO₂ and graphene nanosheets (GNs)/multiwalled carbon nanotubes (MWCNTs) as the positive and negative electrodes, respectively. The MnO₂//GNS–MWCNT (GM) asymmetric supercapacitor exhibits a maximum energy density of 43.3 Wh kg⁻¹ at a power density of 500 W kg⁻¹.