R&D OF HIGH PERFORMANCE SOLID POLYMER ELECTROLYTE WATER ELECTROLYZER IN WE-NET

ABSTRACT

1. INTRODUCTION

In WE-NET project, the performance target of electrolyzers is as follows:
Energy efficiency 80 �`90%
Current density 1 �`3A/cm²
To achieve this target, we have been developing technologies of manufucturing membrane-electrode assemblies' which are main components of solid polymer electrolyte electrolyzers, by a hot-press method. In this method, a catalyst film is supurposed over an ion-exchange membrane, and is combined with the ion-exchange membrane by means of hot-pressing. This technique comes with the following features:

(1)Different types of catalysts, including oxides,can be applied.
(2)Because carrier catalysts can be used, use of noble metal catalysts can be reduced.
(3)Three-dimensional electrode/membrane interface can be achieved.

In the development activities under way, the hot-press method mentioned above was adopted and diverse types of membrane-electrode assemblies were fabricated by composing of various iridium dioxide powders as anode catalysts and various ion-exchange membranes. Based on the results of performance evaluations of those samples, studies were being conducted to elucidate the composition of materials in a membrane-electrode assembly with low cell voltage.

2.EXPERIMENT METHOD

(1)Preparation of catalyst and PTFE mix solution.
(2)Catalyst film formation from the mix solution.
(3)Natural drying
(4)Heat treatment of the catalyst film
(5)Laminating the catalyst film and an ion-exchange membrane, and then hot-pressing.

Membrane-Electrode Assemblies Composed of Various Iridium Dioxide Powder
Iridium dioxide is the best electrocatalyst for anodes. This material is fine powder prepared by pyrolysis of iridium hydroxide, and the activity as anode catalysts depends on the temperature of the pyrolysis, because the content of metal iridium as by-product and the property of specific surface area are changed by pyrolysis condition. Five iridium oxide powder samples were prepared by changing pyrolysis temperature in the range of 200�`800 degree. As shown in Table 1, diverse types of 50cm² membrane-electrode assemblies composed of different iridium dioxide powders were fabricated.

Membrane-Electrode Assemblies Composed of Various Ion-Exchange Membranes
As shown in 2, diverse types of 50cm² membrane-electrode assemblies were fabricated. Those were composed of various ion-exchange membranes with different thickness and equivalent weight (EW) made of perfluorosulfonic acid by three chemicals companies.

Performance Measurement of Membrane-Electrode Assemblies
A laboratory cell was built in filter-press type consisting of a membrane electrode assembly with a support collector and a current collector fitted on both sides, as can be seen in Fig.2. The support collector was a titanium fiber-sintered plate, which was platinum-coated. The current collector was a titanium plate with grooves of 2mm wide 3mm deep mounted in parallel at a pitch of 4mm, which was also platinum-coated. The membrane-electrode assemblies were placed in the laboratory cell and their cell voltages were measured at various current densities at 80 degree using the operating system illustrated in Fig.3.

3.RESULTS AND DISCUSSION

Effect of Varieties of Ion-Exchange Membranes on Cell Performance
Figure 5 shows the curves of the cell voltage vs the current density of diverse types of membrane-electrode assemblies composed of various ion exchange membranes. Thinner and smaller EW ion-exchange membranes gave lower cell voltage. No.9 membrane-electrode assembly composed of the ion-exchange membrane of 52 mm thickness and 1000 EW registered the lowest cell voltage of 1.59V at 1A/cm². Durability of Membrane-Electrode Assemblies Figure 6 shows the cell voltage change with time for 5400hours in No.7 membrane-electrode assembly. The cell registered 1.62 �`1.63V of cell voltage and 9 6.8% of current efficiency at 1A/cm² and 80 degree during the operation. This suggests the durability of membrane-electrode assemblies fabricated by the hot-press method is very high.

4.CONCLUSION

Acknowledgement

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