Electrochemical hydrogen storage performance of hierarchical Co metal flower-like microspheres

Hierarchical cobalt metal flower-like microsphere (Co-FM) was synthesized by a facile hydrothermal process, and its electrochemical hydrogen storage performance was investigated. The Co-FM was generated by the reduction of beta cobalt hydroxide [β-Co(OH)2] platelets in the presence of a mild reducing agent without the use of any template or surfactant. The obtained sample was characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), and energy dispersive spectrometry (EDS). The XRD analysis reveals that this sample has a pure hexagonal close-packed (hcp) structure. The FE-SEM and HR-TEM observations indicate that the synthesized particles are flower-like microspheres with an average diameter of ∼1.5 μm, composed of individual Co metal platelets. The electrochemical hydrogen storage performance was investigated as the negative electrode for nickel-metal hydride (Ni-MH) battery in aqueous KOH solution. The electrochemical measurements demonstrated that this material showed better reversibility, higher hydrogen storage capacity, and higher rate dischargeability than the commercial Co metal powder with similar particle size (∼2 μm) under atmospheric temperature and pressure. Its maximum discharge capacity was ∼360 mA h g−1 and remained 300 mA h g−1 even after 100 cycles, and the capacity retention rate was ∼83%. This significant electrochemical hydrogen storage performance can be attributed to its hierarchical architecture, which leads to increasing the surface area, reducing the diffusion pathway, and buffering the volume change during cycling.