Dual‐Gradient Structure of Component and Channel Size in Co–Ni Hydroxides Boosts Conductivity and Suppresses Self‐Discharge for High‐Performance Supercapacitors

ABSTRACT Nickel‐based hydroxides are promising pseudocapacitive materials, yet their poor conductivity, limited cycling stability, and severe self‐discharge hinder applications. Here, a facile one‐step electrochemical co‐deposition strategy, combining continuously varied metal‐ion ratios with multi‐current steps (ISTEP), was developed to directly construct NiCo hydroxide nanosheet arrays with dual gradients in composition and channel size (DG). The DG electrode delivers 2200 F g −1 at 1 A g −1 , 45.1% retention at 20 A g −1 , and 88% after 10 000 cycles, outperforming the traditional non‐gradient NiCo hydroxide (NG) by 74%, 9%, and 16%, respectively. Under open‐circuit conditions, the voltage drop within 2 h is only 140 mV, compared with 190 mV for NG. The assembled DG||AC asymmetric supercapacitor achieves 525 F g −1 at 1 A g −1 , retains 60% at 50 A g −1 , and delivers 146 Wh kg −1 at 750 W kg −1 , outperforming NG||AC and most reported hydroxide‐based devices. More importantly, theoretical calculations and finite‐element simulations reveal that the compositional gradient enhances intrinsic conductivity, while the channel‐size gradient suppresses ion migration during self‐discharge. This facile and scalable one‐step strategy, together with the excellent pseudocapacitive performance, highlights the considerable application potential of DG‐based ASC and provides new insights for advanced green energy storage.