DEVELOPMENT ON SOLID POLYMER ELECTROLYTE WATER ELECTROLYSIS TECHNOLOGY FOR HIGH CURRENT DENSITY AND ENERGY EFFICIENCY

M. Nagai, H. Tazima and A. Sakanishi
Chemical Laboratory, Nagasaki Research & Development Center,
Technical Headquarters, Mitsubishi Heavy Industries, Ltd.,
5-717-1, Fukahori-machi, Nagasaki, 851-03, Japan

N. Hisatome Product Development Section, Power Plant Engineering Dept.,
Nagasaki Shipyard & Machinery Works, MHI,
1-1, Akunoura-machi, Nagasaki, 850-91, Japan

S. Ohkura Product Development Dept., Power Systems Engineering Center,
Power Systems Headquarters, MHI,
3-3-1, Minatomirai, Nishi-ku, Yokohama, 220-84, Japan

Abstract

Mitsubishi Heavy Industries, Ltd. (MHI) has been participating in the sub-task of WE-NET Project since 1993 to develop the solid polymer electrolyte water electrolysis technology. The ultimate goal of the WE-NET Project is to build a hydrogen energy system on a worldwide scale. In this project, water electrolysis plays an important role as the conversion technology of electrical energy to hydrogen energy. Compared with other technologies, the water electrolysis technology using solid polymer electrolyte can be expected highly promising because this technology can ensure a high energy efficiency and a high purity of hydrogen gas.
MHI has been conducting the fundamental research on this technology using membrane electrode assembly made through the nonelectrolytic plating. 50cm2 and 200cm2 cells were used in the experiments for the elevation of cell performance. The improvement of cell components, especially the material and structure of current collector, decreased the cell electrolytic voltage. As a result, the energy efficiency data at 80℃ were increased to 84% at 1A/cm2 and 73% at 3A/cm2.

1.INTRODUCTION

Conversion of electrical energy to hydrogen energy can play an important role in the hydrogen energy system. There are some hydrogen production methods using electrolysis, and among them, the method through water electrolysis using solid polymer electrolyte, in particular, can be expected highly promising because this method can ensure a high efficiency and a high purity of hydrogen gas. For these reasons, MHI has been developing this technology as a hydrogen production process since 1987.
The electrolytic cell is constructed of the membrane electrode assembly with its electrocatalyst formed over Nafion membrane surface by means of the nonelectrolytic plating process, which is sandwiched between bipolar metal plates. Government Industrial Research Institute of Osaka provided technical advice on the nonelectrolytic plating process.1),2)
Currently, MHI has participated in WE-NET Project to conduct researches aimed at its improvement scale up and cost reduction.
The development targets in the WE-NET Project to 2020 are as follows;

electrode are a :>10,000cm2
current density : 1-3A/cm2
energy efficiency : 80-90%

This paper presents the test results of the improvement of cell performances.

2.RESULTS

2.1 Test Apparatus

Fig.1 shows the flow sheet of test apparatus. The operational parameters are temperature and current density. Cell voltage, current efficiency and gas purity are measured to calculate the energy efficiency of electrolysis.

2.2 Membrane Electrode Assembly

Fig.2 shows the outline of the process to fabricate a catalytic electrode. The adsorption reduction growth method that was a variation of non-electrolytic plating method is used to fabricate membrane electrode assemblies in this test. Fig.3 shows the structure of laboratory cell.

2.3 Effect of Electrode Material3),4)

In this test, iridium and platinum were used as the electrocatalyst materials. The case that iridium existed in both the anode and the cathode showed the lowest cell electrolytic voltage in all the test cases. Because iridium showed a marked effect especially as the anodic catalyst, iridium was used as the electrocatalyst in all the subsequent tests.

2.4 Effect of Current Density

In the WE-NET Project, operation with the highest possible current density has been planned for cost reduction of hydrogen production. Because there are no published reports on experiments at 3A/cm2, the operational feasibility on the availability of current density elevation was first studied in this research. As shown in Fig.4, it was clarified that operation at 3A/cm2 was possible.

2.5 Effect of Mass of Electrode

For the cost reduction, decrease of mass of electrocatalyst is desirable. So, we conducted tests by varying the iridium quantity.

Anode Support CollectorPlatinum-coated expanded titanium mesh
Cathode Support CollectorPorous carbon plate
Cell Temperature80C

The results are shown in Fig.4. The cell electrolytic voltage increased linearly with increasing current density in all samples of any iridium quantity. Voltage in the sample of 0.5mg/cm2 showed the highest value. The cell electrolytic voltage at 1mg/cm2 was lowest in the samples tested in the current density range up to 3A/cm2. The use of 1mg/cm2 appears to pose no particular problems from the aspect of the initial performance. However, a further evaluation will be needed because the cell durability can be a major issue in the practical system.

2.6 Effect of Material of Support Collector

The support collector requires various features including its electrical contact performance, passage functions for water, hydrogen and oxygen, and cushioning property for the membrane electrode assembly, and thus, its material and structure are factors which can greatly affect the water electrolysis performance. The effects of the materials on the water electrolysis performance were studied by using support collectors of different materials and structures respectively for the cathode and the anode.

Cathode support collector
Because the cathodic side is in hydrogen atmosphere, titanium and stainless steel materials prone to hydrogen embrittlement are not usually used for the cathode support collector. The porous carbon free from hydrogen embrittlement is often used as this material. However, the carbon material is generally brittle, showing concerns about its collapse during the water electrolysis cell assembly or about reduced reliability during the long-term operation.
In this test therefore, attention was positively given to the stainless steel material which can solve the above potential problems in the use of porous carbon for the support collector, and its initial performance was evaluated. In the test, porous carbon was used as the conventional support collector and sintered stainless steel fibers as the stainless steel support collector to measure their current-voltage characteristics and electrolytic performances under given test conditions.
The test results showed that prevention of deformation of sintered stainless steel fibers was needed though the stainless steel support collector would be available for operation at 1A/cm2. Similarly, it was found that voltage rise in the current density range above 1A/cm2 would require some preventive measures. Furthermore, it was deemed necessary to design improvement of the material properties in the high current zone and perform the long-term operation assessment taking hydrogen embrittlement, etc., into account.

Anode support collector
Because the anodic side is in oxidation atmosphere, the expanded titanium mesh highly resistant to oxidation is often used. However, the use of the expanded titanium mesh can pose concerns on reliability decline over the long-term operation because this material limits its surface contact with the membrane electrode assembly, prone to its bite into the membrane electrode assembly. In this test therefore, attention was directed to porous titanium material similar to the cathode support collector, aimed at the improvement of contact between the anode support collector and the membrane electrode assembly. For the test, three different types of materials were used; the conventional expanded titanium mesh, the sintered titanium fiber, and the sintered titanium fiber post-treated with titanium powder flame-spraying, and surfaces of all the support collectors were coated with platinum. The respective current-voltage characteristics and cell performances at 80℃ under given test conditions were measured by using the above anode support collector materials and porous carbon plate for the cathode.
The current-voltage characteristics in the use of the above three different anode support collectors are shown in Fig.5. The expanded titanium mesh and the sintered titanium fiber showed almost similar current-voltage characteristics whereas the sintered titanium sprayed with titanium powder showed current-voltage characteristics superior to those of the expanded titanium mesh. The titanium-sprayed sintered compact has a structure of a fine titanium particle layer laminated over one side (membrane electrode assembly side) of a coarse titanium fiber layer, and it is estimated that its contact with the membrane electrode assembly improved, compared with the expanded titanium mesh and the sintered titanium.
The cell performance showed a tendency of improvement in an order of the expanded titanium mesh < the sintered titanium fiber < the titanium-sprayed sintered titanium fiber. The cell voltage drop suggests an improved contact between the anode support collector and the membrane electrode assembly and the improved current efficiency and the decreased oxygen concentration also an improvement of gas passability through the contact interface. Thus, it was found that the water electrolysis cell performance could be improved by chaning the structure of the support collector. Based on the foregoing results, the best electrolytic cell performance was obtained from the water electrolysis cell using the titanium-sprayed sintered titanium fiber for the anode support collector and showed the following values at 80℃ and at 1atm.

Current
density		 Cell
Voltage		 Current
Efficiency		 Energy
Efficiency		 Hydrogen
Purity
1 A/cm2 1.70 V 96.6% 84.1% 99.998%
3 A/cm2 2.01 V 98.9% 72.8% 99.999%

2.7 200cm2 Cell

Fig.6 shows the I-V characteristics of 200cm2 cell. The cell voltage is slightly higher than 50cm2 cell.

4.CONCLUSION

It was confirmed that water electrolysis operation at the current density of 3A/cm2 was possible in solid polymer electrolyte water electrolysis using the membrane electrode assembly made by nonelectrolytic plating process and using 50cm2 cell and the electrocatalyst of iridium. 1mg/cm2 of the catalyst mass rate was sufficient. Also, porous carbon showed a good performance as the cathode support collector and platinum-coated sintered titanium fibers treated with titanium powder flame-spraying a good performance as the anode support collector.
Energy efficiency obtained was approximately 84% at 1A/cm2 and 73% at 3A/cm2. Similar performances were also obtained from 200cm2 cell. We are now confirming the effects of properties of the membrane for a further improvement of the cell performance.

5.ACKNOWLEDGEMENT

This research was conducted as a part of WE-NET Project, sponsored by New Energy and Industrial Technology Development Organization.

6.LITERATURE REFERENCES

1)H. Takenaka, E. Torikai, Y. Kawami, N. Wakabayashi, T. Sakai, Denki Kagaku, 53, 261 (1985)
2)H. Takenaka, Soda & Enso, 37, 327 (1989).
3)H. Takenaka, Y. Kawami, I. Uehara, T. Sakai, E. Torikai, Denki Kagaku, 57, 229 (1989)
4)T. Sakai, H. Takenaka, N. Wakabayashi, K. Kawami, E. Torikai, J. Electrochem. Soc., 132, 1328 (1985)