Thermodynamic analysis on synthesis of fibrous Ni-Co alloys precursor and Ni/Co ratio control

According to the principles of simultaneous equilibrium and mass equilibrium, a series of thermodynamic equilibrium equations in Ni(II)-Co(II)-C2O42−-NH3-NH4+-H2O system at ambient temperature were deduced. The diagrams of logarithm ion concentrations versus pH values at different solution compositions were drawn. The results show that Ni2+ and Co2+ can completely precipitate at pH less than 5.0 and the predefined Ni/Co ratios can be well kept in the precursor. The precursor morphology is granular aggregation. However, rod aggregation precursor is obtained in the pH range of 5.0–8.0, and fibre-shape precursor is got at pH value higher than 8.0. The Ni/Co ratios in the above two precursors are not reproduced as that in the feed due to the formation of multi-coordinated Ni(NH3)n2+ and Co(NH3)n2+ (n=1−6). Modification of precipitation medium is favorable for the precursors to keep the predefined Ni/Co ratios of the feed in the pH range of 2.0–8.6. Meanwhile, the precursors with fibrous morphology can be obtained.