Atomic Layer Deposition of Catalytic Manganese Oxide for High Surface Area Zinc-Air Battery Electrodes

Abstract

Driven by the need to move away from the use of fossil fuels, clean energy technologies such as wind and solar have seen tremendous technological and economic development in the last decade. The rapidly falling cost of wind and solar energy has made these technologies economically competitive with fossil fuels, promoting large growth. Renewable energy grew by 14.1% in 2016 alone.[1,2] With the continuing growth of renewable technologies, comes the need for cheap, safe and reliable energy storage solutions. Currently Li-ion batteries are seeing widespread use for renewable energy storage. However, the use of metallic Li in these batteries makes them expensive and unsafe. Zinc-air batteries are an attractive alternative to Li-ion batteries because they are inexpensive, safe, environmentally benign, and have excellent energy density.[3] The cathode for Zn-air batteries has a number of design requirements in order to facilitate the reduction of oxygen from the air. The cathode must be hydrophobic and porous to allow for diffusion of oxygen into the cell while maintaining intimate contact with the electrolyte within the cell. It also must be conductive and have sufficient loading of an effective catalyst to improve the poor kinetics of the oxygen reduction reaction. Catalyst distribution on/in the cathode is very important as well, as effective surface area of the catalyst is critical to battery performance. Since the oxygen reduction reaction utilizes oxygen from the air, three phase boundaries between air, electrolyte, and catalyst are of key importance.[3] Atomic layer deposition (ALD) is a gas phase deposition technique capable of producing high purity thin films of a wide variety of materials. The development of ALD techniques has largely been driven by the strict material and design challenges of the semiconductor industry. ALD utilizes alternating pulses of reactants that each undergo self-limiting reactions on the sample surface. These self-limiting reactions give rise to a number of useful properties of ALD films such as uniformity, conformality, composition control and thickness control on the order of Ångstroms.[4] ALD can be used to deposit catalytic material directly onto high surface area electrodes, such as porous carbon paper, so that the internal surfaces of the electrode can all be coated. By coating the entire porous structure of the electrode, the effective catalyst surface area and three phase boundary area can be greatly increased. In this work, an ALD process is developed to deposit Mn oxide (MnOx) catalytic films directly onto porous carbon for application as the air electrode in Zn-air batteries. Initial depositions are conducted on Si wafers so that in-situ spectroscopic ellipsometry can be used to monitor deposition behavior. Once optimal ALD parameters are finalized, MnOx films of varying thicknesses are prepared on carbon electrodes and tested for their reactivity towards the oxygen reduction reaction. MnOx has a variety of oxidation states and crystal structures, each with varying degrees of catalytic activity. In order to maximize performance, various deposition and annealing conditions will be used to generate different MnOx phases. Deposits are characterized using a variety of electrochemical and materials techniques including linear sweep voltammetry, electrochemical impedance spectroscopy, galvanostatic cycling, scanning and transmission electron microscopy, x-ray diffraction, x-ray photoelectron spectroscopy and Raman spectroscopy. [1] BP Statistical Review of World Energy 66th Edition, June 2017 [2] G. Jifan, “The next energy revolution is already here”, World Economic Forum, September 20 2017, [online] https://www.weforum.org/agenda/2017/09/next-energy-revolution-already-here/ [3] J. Lee, S. T. Kim, R. Cao, N. Choi, M. Liu, K. T. Lee, “Metal–Air Batteries with High Energy Density: Li–Air versus Zn–Air”, Adv. Energy Mater., 1 (2011) 34-50 [4] S. M. George, “Atomic Layer Deposition: An Overview”, Chem. Rev., 110 (2010) 111-131