High‐Performance 2.6 V Aqueous Asymmetric Supercapacitors based on In Situ Formed Na0.5MnO₂ Nanosheet Assembled Nanowall Arrays

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 MnO 2 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 MnO 2 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 MnO 2 electrode provides the opportunity to further increase the cell voltage of aqueous asymmetric supercapacitors beyond 2 V. To construct the asymmetric supercapacitor, carbon‐coated Fe 3 O 4 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 MnO 2 nanowall arrays as the cathode and carbon‐coated Fe 3 O 4 nanorod arrays as the anode. In particular, the 2.6 V Na 0.5 MnO 2 //Fe 3 O 4 @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 MnO 2 ‐based supercapacitors. This work provides new opportunities for developing high‐voltage aqueous asymmetric supercapacitors with further increased energy density.