Superhydrophilic nickel hydroxide ultrathin nanosheets enable high-performance asymmetric supercapacitors

Superhydrophilic surfaces have been applied for supercapacitor; however, during energy storage reaction, how the wettability affects the process of electrochemical reaction specifically is still unclear. Herein, we demonstrate superhydrophilic surface for promotion of electrochemical reactions by liquid affinity and further explain the mechanism, where the transition of the wettability state caused by the change in surface free energy is the main reason for the obvious increase in specific capacitance. Through citric acid assistance strategy, an intrinsically hydrophobic Ni(OH) 2 thick nanosheets (HNHTNs, 16 nm) can be transitioned into superhydrophilic Ni(OH) 2 ultrathin nanosheets (SNHUNs, 6.8 nm), where the water contact angle was 0° and the surface free energy increased from 8.6 to 65.8 mN·m −1 , implying superhydrophilicity. Compared with HNHTNs, the specific capacitance of SNHUNs is doubled: from 1230 F·g −1 (HNHTNs) to 2350 F·g −1 (2 A·g −1 ) and, even at 20 A·g −1 , from 833 F·g −1 (HNHTNs) to 1670 F·g −1 . The asymmetric capacitors assembled by SNHUNs and activated carbon show 52.44 Wh·kg −1 at 160 W·kg −1 and excellent stability with ~ 90% retention after 5000 cycles (~ 80% retention after 9500 cycles). The promotion of electrochemical performances is ascribed to the change of surface wettability caused by surface free energy, which greatly increase affinity of electrode to the surrounding liquid environment to reduce the interface resistance and optimize the electron transport path. Graphical Abstract