Polymer electrolyte fuel cell

Fuel cell is an electrochemical device that can convert a chemical energy of fuel directly to an electric current. Its practical application is very important to realize sustainable development of society. So far, polymer electrolyte fuel cells including direct methanol fuel cell have been commercialized. However, their performance is still poor. For improving fuel cell performance, we are developing new electrolyte membranes and catalysts based on structural designs in nano- and micro- scales.

1. Application of three-dimensionally ordered macroporous structure to electrolyte membranes

We are studying on new composite membranes composed of three-dimensionally ordered macroporous (3DOM) matrices and proton-conducting polymers for PEFCs. These types of composite membranes have high morphological stability due to high mechanical strength of 3DOM matrix, in which the expansion of polymer electrolyte is effectively suppressed by the 3DOM matrix. This matrix is also applicable to the template for preparing core-shell structures. As shown in Fig. 1, the formation of core-shell structure composed of proton-conducting polymer and methanol tolerant polymer in the 3DOM matrix can provide high transport selectivity to composite membranes.

Fig. 1 Selective transport model for the 3DOM composite membrane including core-shell polymer

2. Development of low-Pt loading catalyst by mechano-chemical bonding

The practical use of PEFC has been limited by the high cost of expensive Pt catalysts. We are trying to develop new low-Pt loading catalysts by using mechano-chemical bonding (MCB) technique, collaborated with Osaka University. This technique can provide novel solid-solid interface that is hardly obtained by conventional techniques such as heating and chemical reactions. So far, we prepared Pt-WC and Pt-SnO2 composite catalysts by using MCB technique, and succeeded to increase the catalytic activity of Pt. For further improvement of catalytic activity, the relationship between nano-structure of composite catalyst particle and its activity is under investigation.

Fig. 2 Transmission electron micrograph of Pt/WC composite catalyst

3. Ionic liquids for fuel cells

PEFC operation at high temperatures above 100 C is desirable to enhance the kinetics of electrode reactions and to improve CO tolerance of Pt catalyst. However, conventional electrolyte membranes can not be applied to such high temperature PEFC operation due to loss of water necessary for proton conduction. We are focusing on ionic liquids (ILs) that have unique properties as solvents such as high thermal and chemical stability, high ionic conductivity, and a wide electrochemical window, and are trying to develop new ILs optimized for high temperature PEFC. So far, we have investigated interfacial phenomena between Pt electrode and ILs in the course of oxygen reduction reaction (ORR) by in-situ FT-IR spectroscopy combining with electrochemical analyses, and revealed that the anion adsorption on Pt electrode is strongly concerned with the over-potential and kinetics of ORR. Based on this result, we are trying to synthesize a series of new ionic liquids with different anion structures as electrolytes for high temperature PEFC.

Fig. 3 Cyclic voltammograms of Pt in dema-FSI, dema-TFSI, and dema-BETI.