Pd-Co/CNF Composite Material as Electro-Catalysts for Lithium-Oxygen Batteries

Abstract

Lithium-Oxygen (Li-O2) batteries have attracted much attention owing to their high energy densities compared to conventional lithium ion batteries, and are one of the promising lithium secondary batteries for electric vehicles (EVs) and hybrid electric vehicles (HEVs). The theoretical specific energy density of the Li-O2 battery with the discharge reaction of 2Li++O2+2e-→Li2O2 is 3505 Wh kg-1. However, the low energy efficiency, poor rate capability, and limited cycle life of these batteries cause problems for their commercialization. The sluggish oxygen redox reaction results in low power and high overpotentials. Also, the pores of oxygen cathode can easily be blocked by insoluble discharge products, which cause incomplete discharge, inhibiting the flow of oxygen to the reaction sites. Therefore, the design of cathodes having oxygen diffusion pathway and uniform distribution of catalysts is the key role which can enhance the performances of Li-O2 batteries. This work focuses on the electrochemical effects of palladium-cobalt (Pd-Co)/carbon nanofiber (CNF) composite catalyst for a cathode. The nanosized Pd-Co catalysts are embedded evenly into CNFs. The high catalytic activity of Pd-Co catalysts reduce the overpotentials, which can improve the energy efficiency. Furthermore, the interconnected pore network structure creates effective gas diffusion channels, thus improving specific capacity and cycle life for Li-O2 cell. PdCo/CNF composite catalysts are synthesized by using polyol method and electrospinning method. Cells were tested at the current densities of 100 mA g-1 and 200 mA g-1with limited voltage of 2.0 to 4.5V. The morphology of the samples was observed on a field emission scanning electron microscope (FESEM) and transmission electron microscope (TEM). The structure of the samples was characterized by X-ray diffraction (XRD) spectroscopy and Inductively coupled plasma atomic emission spectrometry (ICP-AES). The result was analyzed by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV).[1] Y.C. Lu, B.M. Gallant, D.G. Kwabi, J.R. Harding, R.R. Mitchell, M.S. Whittingham, Y. Shao-Horn, Energy Environ. Sci., 6, 750 (2013).[2] L.J. Hardwick, P.G. Bruce, Curr. Opin. Solide. St. M., 16, 178 (2012).[3] M.A. Rahman, X. Wang, C. Wen, J. Appl Electrochem, 44, 5 (2014)[4] W. Wang, D. Zheng, C. Du, Z. Zou, X. Zhang, B. Xia, H. Yang, D.L. Akins, J. Power Sources, 167, 243 (2007)[5] B.W. Huang, X.Z. Liao, H. Wang, C.N. Wang, W.S. He, Z.F. Ma, J. Electrochem. Soc. 160, A1112 (2013)