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 the atomic layer deposition of Co3O4. The presence of P in (Ni,Co)-MoO4 caused the Mo to shift to a lower oxidation state, which improved the surface redox behavior of the material. Additionally, the formation of Ni2P and CoP3 nanoparticles on the surface of inflorescence architecture via nanoscale Kirkendall effect boosts the charge storage behavior 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 at a current density of 2 A/g with 79.8% capacity retention after 5000 cycles at 10 A/g 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 diffusion of P and Mo from the electrode material 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 owed to the inflorescence flower-like architecture, compositional arrangement after P-doping, and deposition of the Co3O4 atomic layer.