Formation of Nanocrystalline Cobalt Oxide-Decorated Graphene for Secondary Lithium-Air Battery and Its Catalytic Performance in Concentrated Alkaline Solutions.

A potent cathode catalyst of octahedral cobalt oxide (CoO) was synthesized onto graphene (GR) nanosheets via a two-step preparation method. The precursor cobalt solution reacted with GR during the initial hydrolysis step to form intermediates. A subsequent hydrothermal reaction promoted CoO crystallinity with a crystalline size of 73 nm, resulting in octahedral particles of 100-300 nm in size. Scanning electron microscopy, Raman spectroscopy, and X-ray diffraction analysis confirmed the successful formation of the CoO/GR composite. This catalyst composite was sprayed onto a carbon cloth to form a cathode for the hybrid electrolyte lithium-air battery (HELAB). This catalyst demonstrated improved oxygen reduction and oxygen evolution capabilities. The HELAB containing this catalyst showed a higher discharge voltage and stable charge voltage, resulting in a 34% reduction in overall over-potential compared to that without the CoO/GR composite. The use of saturated LiOH in 11.6 M LiCl aqueous electrolyte at the cathode further reduced the over-potential by 0.5 V. It is proposed that the suppressed dissociation of LiOH expedites the charging reaction from un-dissociated LiOH. This CoO/GR composite is a promising bi-functional catalyst, suitable as a cathode material for a HELAB operating in high relative humidity and highly alkaline environment.