Akaganeite (β-FeOOH) and hybrid active materials (akaganeite/maghemite (γ-Fe2O3)) containing carbon nanoparticles have been successfully developed through hydrothermal process using oxidation debris of graphene oxide and iron (II) chloride tetrahydrate. The obtained akaganeite sample and the hybrid material containing 29% akaganeite and 71% maghemite were confirmed using Mӧssbauer analysis. Two types of cathode made of akaganeite (β-FeOOH) and hybrid active materials supported on reduced graphene oxide (RGO) for RGO/AKA-100 and RGO/AKA-29 were taken as the main air electrode. The full-cell zinc–air battery prototypes (with 6 M KOH electrolyte) were tested for 500 cycles at room temperature. The result showed that the discharge capacity was achieved as high as 131.05 mAh/cm2 for RGO/AKA-100 and 137.26 mAh/ cm2 for RGO/AKA-29. These performances are better than that using zinc–air batteries with carbon black/MnO2 (CB/ MnO2) as air cathode, that give a discharge capacity of 115.7 mAh/cm2. The charge–discharge efficiency of RGO/AKA-100 and RGO/AKA-29 was examined in relation to their distinct catalytic activity for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) when incorporated into electrochemically rechargeable zinc–air batteries. In addition, the different morphology of zinc deposit and dendrite was characterized using SEM, TEM, and PXRD analysis. From this study, the high performance of active material was suggested to be due to the hybrid effect among akaganeite, maghemite, and reduced graphene oxide, which can produce a synergetic improvement.

We report a new route of akaganéite (β-FeOOH) formation and maghemite (γ-Fe2O3) formation. Akaganéite can be produced by stirring Fe2+ at room temperature for a day under mild conditions. We used FeCl2 ·4H2O as the precursor and mixed it with the Na-rich particle from the oxidation debris solution. The role of the concentration ratio between graphene oxide (GO) and NaOH was addressed to generate oxidation debris (OD) on the surface. In particular, the characterization of OD by transmission electron microscope (TEM) imaging provides clear evidence for the crystal formation of Na-rich particle under electron beam irradiation. For the base treatment process, increasing the concentration of a NaOH in Na-rich solution contributed primarily to the formation of γ-Fe2O3. The characterization by scanning electron microscope (SEM) and TEM showed that the morphology was changed from needle-like to small-oval form. In addition, β-FeOOH can be effectively produced directly using GO combined with FeCl2 ·4H2O at room temperature. More specifically, the role of parent material (Hummer's GO and Brodie's GO) was discussed, and the crystal transformation was identified. Our results concluded that β-FeOOH can be formed in basic and acidic conditions.