Two Distinctive Hierarchical Products through the Hydrothermal Process for β-Co(OH)2 Reacting with NaH2PO2 and Their Morphological Effect on Electrochemical Hydrogen Storage

This paper reports a study of the reaction behavior of beta-Co(OH)(2) with NaH2PO2 under hydrothermal conditions, depending on the concentration of NaOH (09.0 M). Uniform sized beta-Co(OH)(2) microplatelets, as the precursor, were prepared by the method of homogeneous precipitation using hydrolysis reaction with hexamethylenetetramine as the base. After the hydrothermal reaction, two distinctive products were obtained: cobalt phosphite [Co-11(HPO3)(8)(OH)(6)] and hcp Co metal. The XRD analysis reveals that the Co-11(HPO3)(8)(OH)(6) appeared in the absence of NaOH. Then, Co-11(HPO3)(8)(OH)(6) and the hcp Co metal simultaneously appeared under 1.125 M NaOH. At 2.254.5 M NaOH, beta-Co(OH)(2) and hcp Co metal appeared concurrently, and only pure hcp Co metal appeared under 9.0 M NaOH. The FE-SEM observations indicated that the obtained particles were dendritic-like Co-11(HPO3)(8)(OH)(6) and flower-like Co metal. We found that the solubility of beta-Co(OH)(2) and the role of the NaH2PO2 were strongly influenced by the concentration of NaOH during this reaction. To investigate the morphological effect of the two obtained products on the electrochemical hydrogen storage performance, materials with the same crystal structures yet with different morphologies were used for comparison. The evaluations of electrochemical performance proved that the two products showed better reversibility, and higher storage capacity and rate dischargeability than the comparative materials. Their relatively good performances can be attributed to their morphology, which resulted in increased surface area, reduced diffusion pathway, and the accommodation of volume change during cycling.