New class of two-dimensional bimetallic nanoplatelets for high energy density and electrochemically stable hybrid supercapacitors

Currently, the application of supercapacitors (SCs) in portable electronic devices and vehicles is limited by their low energy density. Developing high-energy density SCs without sacrificing their advantages, such as their long-term stability and high power density, has thus become an increasing demand but a major challenge. This demand has motivated tremendous efforts, especially towards discovering and optimizing the architecture of novel electrode materials. To this end, we herein report the design, synthesis, and SC application of a new family of two-dimensional (2D) nanoplatelets, i.e., a transition-metal hydroxymethylate complex (Ni_xCo_1–x(OH)(OCH₃)). Bimetallic nanoplatelets were synthesized via a cost-effective approach involving a one-step solvothermal procedure. We for the first time tuned the metal composition of these 2D nanoplatelets over the entire molar-fraction range (0–1.0). Tuning the molar ratio of Ni/Co allowed us to optimize the structures and physicochemical properties of the nanoplatelets for SC applications. When tested in a half cell, SC electrodes using the nanoplatelets exhibited high electrochemical performance with a specific capacitance as high as 1, 415 F·g^–1 and a 96.1% retention of the initial capacitance over 5, 000 cycles. We exploited the novel 2D nanoplatelets as cathode materials to assemble a hybrid SC for full-cell tests. The resulting SCs operated in a wide potential window of 0–1.7 V, exhibited a high energy density over 50 Wh·kg^–1, and sustained their performance over 10, 000 charge–discharge cycles. The results suggest that the novel 2D nanoplatelets are promising alternative materials for the development of high-energy density SCs.