This paper reports a study of the reaction behavior of β-Co(OH)2 with NaH2PO2 under hydrothermal conditions, depending on the concentration of NaOH (0-9.0 M). Uniform sized β-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 [Co11(HPO3)8(OH)6] and hcp Co metal. The XRD analysis reveals that the Co11(HPO3)8(OH)6 appeared in the absence of NaOH. Then, Co11(HPO3)8(OH)6 and the hcp Co metal simultaneously appeared under 1.125 M NaOH. At 2.25-4.5 M NaOH, β-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 Co11(HPO3)8(OH)6 and flower-like Co metal. We found that the solubility of β-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.