Hierarchical urchin-shaped [alpha]-MnO2 on graphene-coated carbon microfibers: a binder-free electrode for rechargeable aqueous Na-air battery

With the increasing demand of cost-effective and high-energy devices, sodium-air (Na-air) batteries have attracted immense interest due to the natural abundance of sodium in contrast to lithium. In particular, an aqueous Na-air battery has fundamental advantage over non-aqueous batteries due to the formation of highly water-soluble discharge product, which improve the overall performance of the system in terms of energy density, cyclic stability and round-trip efficiency. Despite these advantages, the rechargeability of aqueous Na-air batteries has not yet been demonstrated when using non-precious metal catalysts. In this work, we rationally synthesized a binder-free and robust electrode by directly growing urchin-shaped MnO2 nanowires on porous reduced graphene oxide-coated carbon microfiber (MGC) mats and fabricated an aqueous Na-air cell using the MGC as an air electrode to demonstrate the rechargeability of an aqueous Na-air battery. The fabricated aqueous Na-air cell exhibited excellent rechargeability and rate capability with a low overpotential gap (0.7 V) and high round-trip efficiency (81%). We believe that our approach opens a new avenue for synthesizing robust and binder-free electrodes that can be utilized to build not only metal-air batteries but also other energy systems such as supercapacitors, metal-ion batteries and fuel cells.