“…So far, electrode materials can be classified as metal oxides, polymers, and carbon-based composites [15][16][17]. Metal oxides and composites such as NiO, CoO, Co 3 O 4 /Ni(OH) 2 , MnO 2 , CuO, and CoS 2 exhibit higher specific capacitance than carbon and polymer materials [18][19][20][21][22][23][24] Experimental section Material preparation Figure 1 presents the hydrothermal fabrication process of the NiMn 2 O 4 nanosheets. In a typical synthesis, 0.582 g Ni(NO 3 ) 2 · 6H 2 O (purity 99.9 %, Sigma-Aldrich, USA), 0.965 g MnCl 2 · 4H 2 O (purity 99.9 %, Sigma-Aldrich, USA), and 0.4 g urea (AR, China) were stirred in a mixture of 5 mL ethanol and 35 mL deionized water for 30 min to obtain a clear solution.…”