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

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.