DEVELOPMENT OF 2500cm2 SOLID POLYMER ELECTROLYTE
Mikimasa Yamaguchi, Taizo Shinohara, Harutaka Taniguchi,
WATER ELECTROLYZER IN WE-NET
Takahiro Nakanori and Kayoko Okisawa
New Energy Laboratory
Fuji Electric Corporate Research and Development, Ltd.
2-2-1, Nagasaka, Yokosuka City, 240-0194, Japan
Fuji Electric Corporate Research and Development, Ltd. has been developing technologies for high performance solid polymer electrolyte water electrolyzers. In term of technical features, our companie's technological approach calls for membrane-electrode assemblies to be formed by a hot-press method. In the development activities, diverse types of current collectors were test-produced by changing coating materials and methods, and various types of 50cm2 membrane-electrode assemblies were fabricated by changing anode catalyst species, membrane species and catalyst loading amount.
Based on the results of the performance evaluations of those samples, we have obtained a test-produced electrolyzer of 2500cm2 electrode area with a platinum-plated titanium fiber sintered plate, a gold-plated stainless steel fiber sintered plate, a titanium plate frame and a membrane-electrode assembly. The assembly was composed of iridium dioxide, platinum black and perfluorocarbon sulfonic acid membrane. The electrolyzer registered 1.54V of cell voltage and 99.4% of current efficiency and 95.5% of energy efficiency for a current density of 1A/cm2 at a temperature of 80°C under atmospheric pressure.
In the WE-NET, a solid polymer electrolyte membrane water electrolyzer is expected to produce a large volume of hydrogen per unit and to increase its energy efficiency and current density. The final performance targets of the electrolyzer is as follows.
To achieve this target, elemental technologies have been developed since 1994.
We have been investigating technologies of manufacturing membrane-electrode assemblies by a hot-press method to decrease electrolysis voltage and technologies of support collectors and frames to minimize IR drop of the electrolyzer. In 1996, we obtained a 50cm2 test-produced high performance electrolyzer, which registered 1.53V of electrolysis voltage, 99.2% of current efficiency and 95.8% of energy efficiency for a current density of 1A/cm2 at a temperature of 80°C under atmospheric pressure. Based on these technologies, we developed developing a 2500cm2 electrolyzer. This paper presents the results of the test-manufactured 2500cm2 electrolyzer and the discussion for this results.
2. TEST- MANUFACTURED ELECTROLYZER
Structure of cell
Figure 1 shows the horizontal cross-sectional schematic of the test- manufactured cell which electrode area is 2500cm2. This cell was built in a filter press types consisting of a membrane electrode assembly with support collectors and frames fitted on both sides.
Figure 2 shows a test-manufactured plate frame made of titanium with 31 channels of 3mm wide, 3mm deep, 1006mm long machined at a pitch 8mm on a support collector side. The frame was polished to come the roughness R max to 0.8 and the acurracy of parallel to 50mm, because its surface has to fit the surface of the support collector precisely together extending over all area. .
Table 1 and Figure 3 shows the specification and the appearance of anode and cathode support collectors respectively. An anode support collector was a titanium fiber sintered plate electroplated with platinum to reform electric conductivity of the oxide surface. A cathode support collector was a stainless fiber sintered plate elector plated with gold to reform electric-conductivity of the oxide surface The collectors were made the thickness uniform over all area with a press roller. On the result of the measurement of thickness, the differences between minimum value and maximum value of the thickness were 184mm for the anode support collector and 159mm for the cathode support collector. These thickness differences were not enough for both support collectors from the viewpoint of uniformity.
Figure 4 shows a 2500cm2 membrane-electrode assembly that we manufactured. The membrane-electrode assemblies were manufactured through the process shown in Figure 5. In order to obtain high electrolysis performance on the 2500cm2 cell, it is important to apply enough catalyst loading of IrO2 over all electrode area. Therefore, IrO2 catalyst powder was purified enough by washing and filtering repeatedly on the purification process A and B shown in Figure 6, and was dispersed to microparticles as small as possible Also, in order make the thickness of catalyst film uniform and to make membrane-electrode bonding uniform in hot pressing, it was needed to make the temperature and pressure uniform over all electrode area. Therefore, a highly precise hot press machine was used. In addition, it is necessary that ion exchange membrane have uniform thickness over all electrode area .Table 2 shows species of test-used ion exchange membrane. The differences of thickness between maximum and minimum values were 14mm in B2 membrane, 23mm in B4 membrane and 22mm in A12 membrane. Table 3 shows data of thickness and electrolysis voltage of at 30 samples that were cut out from three species of test manufactured 2500cm2 membrane-electrode assemblies. The differences of the thickness between maximum and minimum values were 25mm for No.1 assembly, 26mm for No.2 assembly and 47mm for No.3 assembly. The difference of electrolysis voltage between maximum and minimum values was 35mV for No.2 assembly. There result shows that the uniforms of membranes are not necessarily satisfactory although we had to use them.
Construction of 2500cm2 cell
Figure 7 shows a 2500cm2 cell assembled with frames, support collectors and a membrane-electrode assembly. In order to make the contact pressure of each components uniform over all area, the cell was set on the cell stand, and was compressed with two pressure plates operated by oil pressure cylinder. The uniformity of the compression was checked using pressure indication films.
Figure 8 shows color generating of the film sandwiched between the anode and cathode support collectors. The color density of the film depends on the pressure applied. As shown in the picture, there were some parts where the color did not generate enough in the film. This means that the cell did not contact uniformly over all area.
3.2500cm2 CELL OPERATING SYSTEM
Figure 9 shows a schematic diagram of a cell operating system. Figure 10 shows the appearance this system. This system consisted of purified water feed pumps, gas-water separators, a gas chiller, a DC power supply, a control board, a monitor board, various sensors and data recorders. Cells were operated under constant electrolysis condition of temperature, pressure, and current density. The performance of the cell was evaluated form voltage and the volume of hydrogen. This system was operated according to the step described below.
- purified water was supplied from a reserve tank into a cell. The water was kept at constant high by level controller and at constant temperature by a heater and a controller.
- Constant current was applied to frames through twelve electric cables connected to DC power supply.
- Cell voltage was measured with a voltage meter, and the volume of generated hydrogen was measured with accurate gas volume accumulator that had rotary basket dipped in water bath.
4.ELECTROLYSIS PERFORMANCE MEASUREMENT
Method of measurement
No.1, No.2, and No.3 cells using B2, B4 and A12 membranes respected were prepared to measure their performances. Under the temperature of 40°C, 60°C, 80°C and atmospheric pressure, the cell voltage and the volume of generated hydrogen were measured at various current density. Current efficiency and energy efficiency were calculated according to next equation.
Current efficiency % =
Volume of generated Hydrogen(Nl / h)x96,500x2x100
Energy efficiency % =
Electrolysis voltage (V)
xCurrent efficiency %
Results of electrolysis performance measurement
Table 4 shows the results of the electrolysis performance measurement of No.1, No.2 and No.3 cells. Based on these results, the following things were elucidated
- All of these cells registered almost the same electrolysis performance whith 50cm2 cells. The superior sequence of cells on electrolysis performance was as follows.
No.1 > No.2 >= No.3
- No.1 cell with the thinnest B2 membrane (thickness 50mm, EW1,000) of the three cells registered the lowest cell voltage of 1.540V and the highest energy efficiency of 95.5% at 1A/cm2 and at 80°C.
- No.2 cell with B4 membrane (thickness 100mm, EW 1,000) registered higher energy efficiency of 91.8% and 93.2% than target value of 90% under the condition of 60°C, 1A/cm2 and 80°C, 1A/cm2 respectively. This cell also registered high energy efficiency of 81.4% under the condition of 3A/cm2 and 80°C.
- No.3 cell unit with the thickest A12 membrane (thickness 120mm, EW 920) registered similar electrolysis voltage and energy efficiency to those of No.2 cell, because this membrane had smaller equivalent weight than that of B4 membrane.
Three types of 2500cm2 cells were constructed using different species of membranes. All of the cells registered higher efficiency than target value. Especially, No.1 cell that had the thinnest membrane B2 (thickness 50mm, EW 1,000) registered the highest electrolysis performance of electrolysis voltage 1.540V and energy efficiency 95.5% under the condition of 80°C. 1A/cm2 . Though the thicknesses of the titanium fiber sintered plate, the stainless fiber sintered plate and the ion exchange membrane were not necessarily uniform. We will make effort to in prove the uniformity on thickness of those components of 2500cm2 in order to achieve higher performance then present one.
The studies are administrated through the New Energy and Industrial Technology Development Organization (NEDO) as a part of the International Clean Energy Network Using Hydrogen Conversion (So-called WE-NET) program with funding from the Agency of Industrial Science and Technology (AIST) in Ministry of International Trade and Industry(MITI) of Japan.
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