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

ABSTRACT

1.INTRODUCTION

In WE-NET project, performance specification of final goal electrolyzer is as follows

To achive this goal, authors have been developing technologies of manufuctuaring membrane-electrode assembly by hot press method, that was a main component of solid polymer electrolyte electrolyzer. In the hot-press method, a catalyst film is superposed over an ion-exchange membrane, and the catalyst film is combined with the ion-exchange membrane by means of thermal fusion. This technique comes with the following features

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

In the development activities under way, the hot press method mentioned above is adopted and diverse types of membrane-electrode assemblies were test-produced by composing of various iridium dioxide powder as anode catalyst and various of ion-exchange membrane. 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 cellvoltage.

2.EXPERIMENT METHOD

The membrane-electrode assemblies used for the experiment were manufactured through the following prosses shown in Fig.1.
(1)Preparation of catalyst and PTFE mix solution.
(2)Film formation by Filtration 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 them hot-pressing.

Membrane-Electrode Assembles Composed of Various Iridium-dioxide Powder

Iridium dioxide was most superior as an electrocatalyst for anode. This material was fine powoder produced by pyrolisis of iridium by dioxide and the activety as anode catalyst differs dependent on temperature for pyrolisis, because the quantity of metal iridium content as by-product and the property of specific surface are changed by pyrolisis condition. Five spieces of iridium oxide powder were produced by changing temperature within 200-800 degree for pyrolisis. As shown in Table.1, diverse types of 50cm² membrane-electrode assemblies were test produced composed of iridium dioxide powder.

Membrane-Electrode Assemblies Composed of Various Ion-exchange Membrane

As shown in Table 2, diverse types of 50cm² membrane-electrode assemblies were test-produced composing of various ion-exchange membrane made by perfluorosulfonic acid in three chemicals companies that were different in thickness and ion-exchange resin weight(EW).

Table 2. Membrane-electrode assemblies composed of various ion-exchange membranes

Electrode area : 50cm²
Anode catalyst : Iridium dioxide powder prepared by pyrolysis of iridium hydroxide at 200degree
Anode catalyst Loading : 3mg/cm²
Cathode catalyst : Platinum black
Cathode catalyst Loading : 3mg/cm²

Sample No.	Specification of Membrane
	Thickness* mm	EW*	Tensile*** strength (Wet)kg/cm2	Manufacturer
6	89	920	1.25	A
7	122	920	1.71	A
8	220	920	3.08	A
9	52	1000	1.84	B
10	106	1000	2.53	B
11	114	-	-	B
12	110	-	3.28	B
13	54	1050	1.65	C
14	130	1050	3.47	C
15	182	1050	5.78	C

*Fully hydrated thickness at 25degree
**Equvalent weight
***Machine direction

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 sample membrane-electrode assemblies were placed in the laboratory cell and then cell voltages were measured at various current densities for 80degree using the operating system illustrated in Fig.3.

3.RESULTS AND DISCUSSION

As shown in Table 1, the higher the temperature at pyrolisis of iridium hydroxide, the larger the specific surface of iridium dioxide test-produced. Fig.4 shows the results of the experiment indicating the cell voltage vs the current density of diverse types membrane-electrode assemblies composed of various iridium dioxide powder as anode catalyst. The lower the temperature of iridium hydroxide pyrolizing, the cell voltage of sample membrane-electrode assemblies was decreased. Consequently, the sample membrane-electrode assemblies composed of iridium oxide powder produced by pyrolisis at 200degree registered as lower cell voltage 1.61V that the other. This suggests iridium dioxide powder used as anode catalyst that has larger specific surface, is effective to decrease cell voltage.

Table 1. Membrane-electrode assemblies composed of various iridium dioxide powders

Electrode area : 50cm²
Membrane : Perfluorosulfonic acid cation exchange membrne(thickness 120ミクロン)
Cathode catalyst : Platinum black
Cathode catalyst Loading : 3mg/cm²

Sample No.	Anode catalyst
	Varieties of Catalyst	Specific surface area m2/g	Loading mg/cm2
1	IrO2 pyrolyzed at 200degree	45	3
2	IrO2 pyrolyzed at 400degree	35	3
3	IrO2 pyrolyzed at 500degree	35	3
4	IrO2 pyrolyzed at 600degree	31	3
5	IrO2 pyrolyzed at 800degree	21	3

Effect Of Ion-Exchange Membrane Spieces

Fig.5 shows the results of the experiment indicating the cell voltage vs the current density of diverse types membrane-electrode assemblies composed of various ion exchange membrane. The thinner the thickness and the smaller the ion exchange resin weight of the ion-exchange membrane the lower the cell voltage. Consequently, No.9 sample membrane-electrode assembly composed of the ion-exchange membrane that is 52 mm thick and EW is 1000 was registered as lower cell voltage 1.59V, than the other.

Durability Of Membrane-Electrode Assemblies

Fig.6 shows the result of the experiment indicating the cell voltage vs time till 5000h of No.7 sample membrane-electrode assembly. The cell was registered 1.62〜1.63V of cellvoltage and 96.8% of current efficiency for a current density of 1A/cm² at 80degree. This suggested the durability of membrane-electrode assembly produced by the hot-press method is considerably high.

4.CONCLUSION

5.ACKNOWLEGEMENT

REFFERENCE