Design of an inflorescence-type phosphorus-doped (Ni,Co)–molybdate architecture with an atomically thin cobalt oxide layer for high-efficiency energy storage systems†

Abstract

High-performance asymmetric supercapacitors (ASCs) need to promote the diffusion of the electrolyte into the electrode, a process that is governed by the electron transfer kinetics and morphology of the electrode material. In line with this, the present study designed a phosphorus (P)-doped (Ni,Co)-MoO₄ electrode with an inflorescence-like architecture on a central stem and an atomically thin coating of Co₃O₄ to improve the electron conductivity of the resulting electrode. This unique structure was prepared using a three-step process that included hydrothermal processing, phosphorylation, and atomic layer deposition of Co₃O₄. The presence of P in (Ni,Co)-MoO₄ improves the surface redox behavior of the material. Additionally, the formation of Ni₂P and CoP₃ nanoparticles on the surface of the inflorescence architecture via the nanoscale Kirkendall effect boosts the charge storage behavior by increasing the active sites in the electrode system. As a result, the optimized P-doped (Ni,Co)-MoO₄ electrode with a 7 nm layer of Co₃O₄ (NCMP-7Co) achieved a high specific capacity of 2374 C g⁻¹ at a current density of 2 A g⁻¹ with 79.8% capacity retention after 5000 cycles at 10 A g⁻¹ current density. The mechanism for the optimized electrode based on electrochemical analysis indicates a dominant diffusion-governed behavior. On the other hand, postmortem analysis revealed dissolution of P and Mo from the electrode material to electrolyte with significant structural change. An ASC constructed device with reduced graphene oxide displays fairly high energy density compared to reported systems. Convincingly, a synergistic improvement with outstanding performance could be ascribed to the inflorescence flower-like architecture, compositional arrangement after P-doping, and deposition of the Co₃O₄ atomic layer.