Cation substitution is a promising strategy for modulating the structural properties and optimizing the electrochemical performance of spinel cobalt oxide (Co₃O₄); however, the underlying mechanism of this action induced by different cation substitutions has not yet been clearly addressed.

Cobaltite systems with spinel structures are promising cathode materials for next-generation high-performance electrochemical capacitors because of their high electrochemical stability. However, increasing the mass loading of active materials without sacrificing the geometry of the nanostructures remains a challenge. In this study, we propose vertically stacked bilayer spinel heterostructures constructed from hierarchical CuCo₂O₄/MnCo₂O₄ on graphite paper as highly capable supercapacitor electrodes. A two-step hydrothermal method with post annealing treatment is used in the preparation of the heterostructures. The CuCo₂O₄/MnCo₂O₄ electrode delivers a remarkable specific capacitance of 1434 F g⁻¹ at 0.5 A g⁻¹, considerable high-rate capability (810 F g⁻¹ at 15 A g⁻¹), and an excellent cycling stability, maintaining 81.4% at 10 A g⁻¹ after 5000 cycles. An electrochemical capacitor device operating at 1.6 V is also constructed using CuCo₂O₄/MnCo₂O₄ and graphene as positive and negative electrodes, respectively. The device shows a high energy density of 42.1 W h kg⁻¹ at a power density of 400 W kg⁻¹, as well as good cycling stability (88.4% retention after 10 000 cycles). The concept of stacking heteronanostructures can potentially enrich the electrochemical performance of metal oxides for next-generation electrochemical capacitors.

Black phosphorus (BP) is rediscovered as a 2D layered material. Since its first isolation in 2014, 2D BP has triggered tremendous interest in the fields of condensed matter physics, chemistry, and materials science. Given its unique puckered monolayer geometry, 2D BP displays many unprecedented properties and is being explored for use in numerous applications. The flexibility, large surface area, and good electric conductivity of 2D BP make it a promising electrode material for electrochemical energy storage devices (EESDs). Here, the experimental and theoretical progress of 2D BP is presented on the basis of its preparation methods. The structural and physiochemical properties, air instability, passivation, and EESD applications of 2D BP are discussed systemically. Specifically, the latest research findings on utilizing 2D BP in EESDs, such as lithium‐ion batteries, supercapacitors, and emerging technologies (lithium–sulfur batteries, magnesium‐ion batteries, and sodium‐ion batteries), are summarized. On the basis of the current progress, a few personal perspectives on the existing challenges and future research directions in this developing field are provided.