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 40nm, function as the nucleation cores for iron deposition. Additionally, core–shell iron composite particles are observed to be 30–100nm with spherical sharp. At the first time discharge, the iron/carbon-black composite nanoparticle discharged 1200mAhg−1(Fe) at plateau one and 400mAhg−1at plateau two at a high current density of 200mAg−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 600mAhg−1(Fe) when the nickel–iron battery operated between 1.65 and 0.8V. 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 300nm during the electrode cycling, indicating that carbon black also acts as the nucleation cores for the dissolution–deposition of iron.