Recent Progress on Zinc-Air Batteries.

Abstract

Zinc-air batteries are promising as inexpensive, safe and environmentally benign power sources with high energy density. The active material in the positive electrode is oxygen in air, and thus the negative electrode (zinc) can occupy most of the space in the battery. Primary zinc-air batteries have been used for many applications such as power sources for hearing aid. Rechargeable types have been intensively studied, however, further development is required for its commercialization. Gas diffusion electrodes consisting of carbon substrate and fluorocarbon binders are widely used in the pos- itive side in order to offer gas (oxygen)-liquid (electrolyte solution)-solid (current collector) interfaces. The poor energy efficiency of the battery arises from low activity of carbon materials for oxygen reduction. High dispersion of active catalysts on the substrate is effective for improvement. Another issue of the air electrode is corrosion of the carbon substrate during oxygen evolution on charging, which limits the lifetime of the battery. Possible solu- tions are use of graphite or foamed metal substrate. Self-catalytic oxide substrate materials are also attractive, despite of thier low surface area (hence limited high-power capability). Zinc electrodes exhibit low polarization, however, the shape changes is a major problem for long cycle life of the battery. As soluble zinc species are chiefly responsible, some efforts are made to limit the solubility. It is also effective to decrease the apparent current density in the negative electrode. A new approach for vehicle appli- cation is “mechanical charging”, where zinc species formed during discharging are taken from the car battery and are regenerated in a different place (e.g. a factory) while new zinc particles are refilled instantly in the fuel station. Thus electrodes suitable for discharging and charging can be used in the car and factory, respectively. This system requires large infrastructure for zinc delivery and regeneration, hence also social-consensus.