Nanostructured composites as electrochemical catalysts for Li-air batteries
Park, Hey Woong
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Advanced energy conversion and storage systems are drawing tremendous research attention as the energy demand for portable electronics and electric vehicle (EV) applications continues to increase. Although lithium (Li) ion batteries are currently used for various applications, including EVs, these batteries are still unable to fulfill the ultimate driving range requirements. Li-air batteries are an attractive alternative due to their extremely high theoretical energy density; however some technical hurdles remain, including insufficient cycle stability, low energy efficiency and poor rate capability. These challenges are directly related to the sluggish oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), which are the two main reactions that govern the discharge and charge processes, respectively. Therefore, the development of a highly efficient bi-functional catalyst active towards both ORR and OER is a significantly important research area to commercialize Li-air batteries. In the first part of this study, a facile method which utilizes one-step rapid heating is introduced to synthesize reduced graphene oxide with heterogeneously doped nitrogen atoms (NRGO), with the morphology and nitrogen doping investigated by electron microscopy and x-ray photoelectron spectroscopy, respectively. Electrochemical half-cell experiments show that NRGO has comparable ORR activity to that of state-of-the-art commercial Pt/C catalysts. Next, by combining NRGO synthesized via the facile method with La0.5Sr0.5Co0.8Fe0.2O3 perovskite oxide with a porous rod morphology prepared by electrospinning (LSCF-PR), a novel composite catalyst (LSCF-PR/NRGO) is created. Electrochemical testing of the LSCF-PR/NRGO composite in alkaline medium shows that it has excellent ORR and OER activities, verifying the effectiveness of the composite for bi-functional activity. These results highlight the importance of morphology on the catalytic activity for metal-air battery applications. Next in the study, one-dimensional carbon nanotube-based composite catalysts have been investigated such as ethylene diamine (EDA)-based nitrogen-doped carbon nanotubes (NCNT) synthesized on RGO, prepared by chemical vapor deposition (CVD). The NCNT/RGO composites exhibit ORR performance similar to that of commercial Pt/C catalysts, with much superior OER activity. Last in the study, a one-pot facile synthetic is developed to prepare advanced bi-functional composite catalysts (op-LN), which combine NCNT and perovskite oxide La0.5Sr0.5Co0.8Fe0.2O3 nanoparticles (LSCF-NP). Electron microscopy confirms that NCNTs are densely grown on the metal oxide. Half-cell experiments show that the synergy of combining LSCF-NP with NCNT yields good bi-functionality towards both ORR and OER, attributed to the synergistic effects arising from the two component species. op-LN has been evaluated in a practical rechargeable Li-air battery and demonstrated superior charge and discharge performance and cycle stability compared to those of LSCF-NP, NCNT and Pt/C. Based on the excellent results, the simple route to prepare op-LN is very attractive for obtaining a highly effective and durable bi-functional catalyst for potential commercialization of rechargeable metal air batteries.