Development of Efficient Electrocatalysts for Metal-Air Batteries

Abstract

Metal-air batteries have a lot of merits whereas there are still facing various fundamental issues to reach commercial applications such as poor cycleability, low round-trip efficiency and poor rate capability. For instance, the round-trip efficiency of metal-air batteries with bare carbon electrode is below 70% in contrast with that of the conventional LIBs-in case LiCoO2 is around 95%. These challenges are stem from instability of not only Li metal but also binder and electrolyte. Above all, the major challenge is the sluggish kinetics of oxygen reduction and evolution reactions at the oxygen electrode during battery discharge and charge. Therefore, the development of electrochemically active, stable and non-precious bifunctional catalyst is highly required for the commercialization of this technology for practical metal-air batteries. We first focus on designing an inexpensive and highly active OER electrocatalyst, Co3V2O8 with 1D morphology consisting of nanotubes and nanorods which can be used in Zn-air battery. In addition to looking for new cost-effective materials with stable structure and also tuning the morphology of the existing material to improve their catalytic activity which is directly related with battery performance has been considered to improve oxygen evolution reaction. From this point of view, one dimensional (1D) nanostructure materials possess better triple phase boundary to facilitate efficient transport pathways for electrons and ions. Moreover, high surface area of 1D nanostructure expected to provide high performance to suppress sluggish oxygen electrode kinetics. To understand the effect of Co3V2O8 morphology on OER activity, we have synthesized 1D & 0D nanostructures and discussed their performance. The result demonstrates that the 1D-Co3V2O8 cathode exhibits superior OER activity and long term stability to those of 0D-Co3V2O8 and even for commercial precious metal catalysts. The excellent OER performance and long-term durability is attributed to the well-designed one dimensional nanorods and nanotubes like structure, the synergistic effect of different metal ions, and the presence of amorphous nitrogen-doped carbon. In the second part of research, we developed Co-CoO/CNR catalysts as a bifunctional air cathode for the OER and ORR for application of Li-O2 batteries. High power density could be achieved with this system since the Li-O2 batteries possess higher open-circuit voltage of 2.96 V than that of Zn-air batteries (1.65 V). The Co-CoO/CNR cathode achieved a discharge capacity of 10569 mAh gcatalyst-1 at a current density of 100 mA g-1, which is higher than that of CNR electrode (7087 mAh gcatalyst-1). This result demonstrates that Co-CoO/CNR catalyst exhibits good oxygen reduction activity. Moreover, the Co-CoO/CNR cathode shows almost 6 times better cycling performance than CNR electrode with a cutoff capacity of 1000 mAh gcatalyst-1. The poor cycleability of Li-O2 batteries with CNR electrode should be caused by the accumulation of Li2CO3, which is the one of the major products in this oxygen electrode. The enhancement of discharge capacity and voltage observed for Co-CoO/CNR electrode may due to the presence of uniform mesoporous nanostructure with high surface area so that it could diffuse Li+ easily and provide space to accommodate discharge solid products. Furthermore, Co-CoO nanoparticles on CNR electrode might help to minimize the oxidation of carbon structure and form nanosized Li products during the discharge process. ⓒ 2016 DGIST