High‐Performance 2.6 V Aqueous Asymmetric Supercapacitors based on In Situ Formed Na_0.5MnO₂ Nanosheet Assembled Nanowall Arrays

Abstract

The voltage limit for aqueous asymmetric supercapacitors is usually 2 V, which impedes further improvement in energy density. Here, high Na content Birnessite Na_0.5 MnO₂ nanosheet assembled nanowall arrays are in situ formed on carbon cloth via electrochemical oxidation. It is interesting to find that the electrode potential window for Na_0.5 MnO₂ nanowall arrays can be extended to 0-1.3 V (vs Ag/AgCl) with significantly increased specific capacitance up to 366 F g^-1 . The extended potential window for the Na_0.5 MnO₂ electrode provides the opportunity to further increase the cell voltage of aqueous asymmetric supercapacitors beyond 2 V. To construct the asymmetric supercapacitor, carbon-coated Fe₃ O₄ nanorod arrays are synthesized as the anode and can stably work in a negative potential window of -1.3 to 0 V (vs Ag/AgCl). For the first time, a 2.6 V aqueous asymmetric supercapacitor is demonstrated by using Na_0.5 MnO₂ nanowall arrays as the cathode and carbon-coated Fe₃ O₄ nanorod arrays as the anode. In particular, the 2.6 V Na_0.5 MnO₂ //Fe₃ O₄ @C asymmetric supercapacitor exhibits a large energy density of up to 81 Wh kg^-1 as well as excellent rate capability and cycle performance, outperforming previously reported MnO₂ -based supercapacitors. This work provides new opportunities for developing high-voltage aqueous asymmetric supercapacitors with further increased energy density.