Nanotechnology for Material Development on Future Energy Storage

Abstract

Various kinds of energy devices have been developed as power sources for portable electronic devices and electric vehicles. Fuel cell, rechargeable lithium ion battery, and super capacitor are the most interesting devices, and they have been extensively studied to improve their electrochemical performance around the world [1, 2]. In these electrochemical devices, chemical energy is directly converted to electric energy through charge transfer process occurring at an interface between electrode and electrolyte. The electrochemical reactions take place at the interface and their reaction rates strongly depend on the nature of interface consisting of electrode and electrolyte materials. In some case, the electrode reaction is so slow that the electrode reaction kinetics should be carefully investigated in order to improve charge transfer reaction rate. On the other hand, the slow electrode reaction can be technically overcome by a large interface area for the electrode reaction, leading to an improvement of apparent reaction rate. For example, the true surface area of the porous electrode used in practical battery and fuel cell is much larger than that of flat electrode. When the surface area is 100 times larger than that of flat electrode, the apparent electrode reaction rate is also 100 times. However, this is too simple to estimate the advantage of the porous electrode. The porous electrode has so many problems that the reaction rate may not become 100 times [3]. Figure 4.1 shows the electrode reaction occurring on flat electrode and porous electrode. In the case of the flat electrode, the electrode reaction takes place uniformly on an entire electrode surface. On the other hand, the electrode reaction taking place on the porous electrode surface has a distribution of electrode reaction rate depending on its porous nature and a kind of electrode material. For example, both electronic and ionic conductivities of porous electrode are very important properties to establish an electrochemical interface and to realize apparently high charge transfer rate. One of the key technologies for porous electrodes used in electrochemical energy conversion system is a fabrication process of porous electrode with three-dimensionally ordered porous structures. Recently, three-dimensionally ordered macroporous materials have been extensively studied on various application fields, such as catalyst, photonic material, sensor, and so on [4–11]. At first silica porous materials have been prepared by using colloidal crystal templating method. This study has inspired a lot of scientists working in the field of material science. So far, many kinds of macroporous materials, such as zirconia, titania, carbon, and so on, have been successfully prepared and applied to various applications. In this section, three applications of three-dimensionally ordered materials to electrochemical energy conversion systems are introduced.