A comparative study of composite electrodes consisting of nanoparticles of iron compounds with fibrous nanocarbon in alkaline electrolyte was conducted to ascertain the morphological or composition factors that can increase the iron capacity. The preparation method and the iron content of a composite electrode affect the capacity per unit of iron mass. Particularly by increasing the iron content using a solution process, the capacity can be increased considerably up to ca. 460 mAh (g Fe) −1 for an Fe(OH) x /carbon composite. Such improved iron utilization in these composites suggests a catalytic effect of some intermediate species. Addition of a model candidate of an intermediate species, FeOOH, even of only 3 wt%, can improve the iron utilization in a Fe(OH) x /carbon composite.