Iron/carbon-black composite nanoparticles as an iron electrode material in a paste type rechargeable alkaline battery

Iron/carbon-black composite nanoparticles were synthesized by chemically reducing the iron salt mixing with carbon black by adding NaBH₄ in the aqueous solution. Carbon-black particles, with a mean particle size of approximately 40 nm, function as the nucleation cores for iron deposition. Additionally, core-shell iron composite particles are observed to be 30-100 nm with spherical sharp. At the first time discharge, the iron/carbon-black composite nanoparticle discharged 1200 mAh g⁻¹(Fe) at plateau one and 400 mAh g⁻¹at plateau two at a high current density of 200 mA g⁻¹(Fe). The capacity is larger than the theoretical value, which is attributed to the formation of iron hydride (FeH_x) while the iron was reduced by NaBH₄, followed the hydrogen reaction as an active material while the battery discharge occurs. In further cycles, the iron/carbon-black composite iron electrode shows a good reversibility of about 600 mAh g⁻¹(Fe) when the nickel-iron battery operated between 1.65 and 0.8 V. XRD analysis results indicate that the carbon black in the core of the iron/carbon-black composite enhances the reduction/oxidation reactions of iron, as achieved by the carbon black forming an enhanced electronic conductive network while iron is discharged as the insulator species such as Fe(OH)₂ and Fe₃O₄. SEM images reveal that the iron/carbon-black composite keeps particle sizes smaller than 300 nm during the electrode cycling, indicating that carbon black also acts as the nucleation cores for the dissolution-deposition of iron.