Ruthenium functionalized graphene aerogels with hierarchical and three-dimensional porosity as a free-standing cathode for rechargeable lithium-oxygen batteries

Although possessing extremely high energy density, lithium-oxygen (Li-O2) battery suffered from large charge overpotential, low round-trip efficiency and poor cycling life, which limited the practical application of this smart system. Ru particles functionalized graphene aerogels (Ru-GAs) were designed and directly used as a free-standing cathode for Li-O2 battery. The Ru-GAs showed hierarchical pores, which had the pore volumes of 2.8 and 14.1 cm3 g−1 below and above critical pore diameter of 100 nm. The Ru-GA cathode in Li-O2 battery delivered a high capacity of more than 12 000 mAh g−1 and excellent cycling retention, which was mainly attributed to the three-dimensional porosity, abundant active sites with Ru particles and chemical stability arising from the character of binder-free cathode. Based on the results of in situ gas chromatography–mass spectrometry analysis, the reaction mechanism during charge process in aprotic electrolyte was proposed by the theory of three oxidation stages. By overcoming common limitations of carbon cathodes for lithium–oxygen batteries, scientists in China have produced a high-capacity battery. Lithium–oxygen batteries are very promising for powering electric vehicles and domestic devices. Carbon would be an excellent cathode material for such batteries except that it both self-decomposes and catalyses decomposition of the electrolyte. A team led by Haoshen Zhou of Nanjing University has used graphene aerogels functionalized with ruthenium nanoparticles as a freestanding cathode for a lithium–oxygen battery. The battery displayed a high specific capacity and a superior cycling retention, which the researchers attribute to the three-dimensional hierarchical pores in the cathode acting as channels for oxygen and electrolyte diffusion. The high abundance of active sites on the ruthenium nanoparticles and good chemical stability also contributed to the favourable battery properties. The graphene aerogel (GA) cathode has unique properties with a hierarchically porous structure that facilitates electrolyte permeation and oxygen diffusion, three-dimensional network structures that can enable easy electron transfer through GA, high specific surface area that offers abundant active sites for electrochemical reaction and an ultra-large pore volume that can accommodate plenty of discharge products. Ru nanoparticles supported on graphene sheets also has superior catalytic activity toward oxygen evolution reaction and can efficiently catalyze the decomposition of the discharge product Li2O2.

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