Development of High Performance Solid Polymer Electrolyte Water Electrolyzer in WE-NET

Mikimasa Yamaguchi, Kayoko Okisawa and Takahiro Nakanori
New Energy Laboratory
Fuji Electric Corporate Research and Development Ltd.
2-2-1, Nagasaka, Yokosuka City

Abstract

Commissioned to execute subtask 4, "Development of Hydrogen Production Technologies" as a part of the Ministry of International Trade and Industry's "Technologies for an International Clean Energy Network using Hydrogen Conversion Project", that is said WE-NET, Fuji Electric Corporate Research and Development, Ltd. has been developing technologies for high performance solid polymer electrolyte water electrolyzers. In term of technical features, Fuji Electric's technological approach call 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 spiecies and catalyst loading amount.
Based on the results of the performance evaluations of those sample we have obtained a test-produced high performance electrolyzer with platinum-plated titanium fiber plate, gold-plated stainless steel fiber plate and a membrane-electrode assembly composed of iridium dioxide, platinum black and perfluorocarbon sulfonic acid membrane, which registered 1.53V of cell voltage and 99.2% of current efficiency and 95.8% of energy efficiency for a current density of 1A/cm2 at a temperature of 80degree, under atmospheric pressure.


INTRODUCTION

In the WE-NET, a solid polymer electrolyte membrane water electrolyzer is expected to produce a large volume of hydrogen per unit its the efficiency and current density can be increased.
Performance specifications of final goal electrolyzer are as follows

Electrode area > 10,000cm2
Current density 1`3A/cm2
Energy efficiency >90%

To achieve those specifications, elemental technologies have been developed since 1994.
We have been investigating technologies of support collectors to minimize IR drops of cells and technologies of manufacturing membrane-electrode assembles by a hot-press method to decrease electrolysis voltage.
In the hot-press method, a catalyst film is superposed over an ion-exchange membrane, and the catalyst film is combined with an ion-exchange membrane by means of hot-pressing.
This technique comes with the following features.

(1) Different types of catalysts, including oxides, can be used.
(2) Three-dimensional electrode membrane interface can be achieved.

This paper presents the results of development of elemental technologies in regard to improvement of cell performances.

EXPERIMENTAL PROCEDURE

Structure of cell
Figure 1 shows the cross-sectional schematic of a developed cell which electrode area is 50cm2. This cell was built in a filter press types consisting of a membrane-electrode assembly with a support collector and a frame fitted on both sides.
The frame is a titanium plate with grooves of 2mm wide 2mm deep machined in parallel at a pitch 8mm.
The anode support collector is a sintered titanium fiber plate electroplated with platinum, because titanium fiber is highly resistant to electrochemical erosion in anode side.
The cathode support collector is a sintered stainless fiber plate electroplated with gold, because stainless fiber is more resistant to hydrogen embrittlement than titanium fiber.
Effect of coated materials and coating methods of support collectors on electrical contact resistance Figure 2 shows the micrographs of the support collectors.
The titanium fiber and stainless steel fiber, that are materials of anodic and cathodic support collectors respectively, have high electro-resistant oxide surface.
Therefore the electrical contact resistance of both support collectors to cell frames and membrane-electrode assemblies are not small for large electric current density electrolysis.
In order to make these resistance smaller, various types of current collectors were made by changing coated materials and coating methods.
Figure 3 shows the apparatus for measuring the resistance as IR drop.
The method of measuring IR drop was as follows

1. A test-produced support collector was sandwiched between an anode frame and a cathode frame.
2. Pressure was applied.
3. DC current was applied.
4. IR drop was measured.

Table 1 shows the results of IR drops between the anode frame and the cathode frame.
Regarding the titanium fiber plate, a platinum-plated plate was superior to a non-coated one or a platinum-iridium coated one by paint and pyrolysis.
Also, regarding the stainless fiber plate, a gold plated plate showed lower IR drop characteristics than a non-coated one or a platinum coated one by paint and pyrolysis.

Cell operating system
Figure 4 shows a schematic diagram of a cell operating system.
Cells were operated under constant electrolysis condition of temperature, pressure, and current density.
Consequently the performance of the cell was evaluated from voltage and the volume of generated hydrogen.
The cell operating system was operated according to the step described below.

1. Pure water was supplied from a reserve tank into a cell, which was kept at fixed temperature by a heater and a controller.
2. Constant current was applied to cell frames by a DC power supply.
3. Cell voltage was measured with a voltage meter, and the volume of generated hydrogen was measured with a fine membrane gas flow meter.

Fabrication method of membrane-electrode assemblies The membrane-electrode assemblies used for the experiments were manufactured through the following process shown in Figure 5.

(1) Preparation of a 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, then hot-pressing.

Performance Measurement of Membrane-Electorde Assembliess
The membrane-electrode assemblies were placed between anode and cathode support collectors of 50cm2 cells as shown in Figure 1. Under the electrolysis condition of 80degree and atmospheric pressure, the cell voltage and the volume of generated hydrogen were measured at various current densities using the operating system illustrated in Figure 4.

RESULTS

Effect of hot-pressing temperature on cell performance
Figure 6 shows the result of the measurement of cell voltage, energy efficiency, thickness and electrode area of the membrane-electode assemblies prepared by changing hot-pressing temperature. As the hot-pressing temperature become higher, the thickness of the assemblies and decreased. The thickness of the assemblies prepared at over 120degree was almost same to the thickness of original membrane. So the catalyst film was invested enough into the membrane in that condition. However, considering that the durability of a perfluoro-carbon sulfonic acid membrane is not enough at above 150degree, the suitable hot-pressing temperature is in the range from 140degree to 150degree. The cell voltage also reduced as the temperature increased. This is due to the decrease of the thickness of the membrane.

Effect of kinds of anode catalyst on cell performance
Table 2 gives the membrane-electode assemblies composed of various anode catalyst powders. Figure 7 shows the results of the experiment indicating the cell voltage and the energy efficiency vs the current density of the cell using these samples . Figure 8 shows the cell voltage change with time of the membrane-electorde assemblies composed of RuO2 pyrolyzed at 400degree, RuO2 pyrolyzed at 600degree, Ir-Ru mixture Ir-Pt mixture and IrO2 pyrolyzed at 200degree as anode catalysts. Based on the results shown in Figure 7 and 8, the following things were elucidated in regard to the characteristics of the electrolysis performance and durability of the catalysts.

(1) Electrolysis performance
The superior sequence of various anode catalysts on electrolysis performance was as follows.
RuO2 pyrolyzed at 400degree>RuO2 pyrolyzed at 600degree>Ir-Ru mixture IrO2 pyrolyzed at 200degreeIr-Pt mixture >Ir black >Ir2O3 > Rh2 O3 >Pt black

(2) Durability
1. RuO2 pyrolyzed at 400degree :The cell voltage was in the range of 1.47`1. 55V till 400hours, but thereafter rapidly increased over 2.1V.
2. RuO2 pyrolyzed at 600degree : The cell voltage was in the range of 1.55` 1. 57V till 400hours, but thereafter rapidly increased oven 1.7V.
3. Ir-Ru mixture and Ir-Pt mixture :Both cell voltages incresed slightly with time .
4. IrO2 :The cell voltage was constant in the range of 1.54`1.55V.

Effect of kinds of ion-exchange membranes on cell performance
Table 3 gives the membrane-electrode assemblies composed of various kinds of ion-exchange membranes. Figure 9 shows the cell voltage and energy efficiency vs. the current density of these samples. Thinner and smaller EW ion-exchange membrane gave lower cell voltage and higher energy efficiency . The superior sequence of various ion-exchange membranes in electrolysis performance was as follows.

B2 > D > C2 > A120 > B4 > C5

The membrane B2 having, 51mm thickness and 1000 EW registered the lowest cell voltage of 1.533V and the highest energy efficiency of 95.8% at 1A/cm2.

Effect of anode catalyst loading on cell performance
Figure 10 shows the results of the experiment indicating the cell voltage and the energy efficiency vs. the current density in the membrane-electrode assemblies with different anode catalyst loading of 1.7, 2.5, 3.3, 4.3 mg/cm2. The cell voltage of the assembly with catalyst loading of 1.7mg/cm2 was higher than that of the other assemblies which catalyst loading were over 2.5mg/cm2 , and the cell voltage of the assemblies with catalyst loadings of 2.5, 3.3.,4.3 mg/cm2 were almost same. From this result, the catalyst loading enough for anodes was found to be about 3mg/cm2.

Effect of cathode catalyst loading on cell performance
Figure 11 shows the results of the experiment indicating the cell voltage and the energy efficiency vs. the current density of the membrane-electrode assemblies fabricated by changing cathode catalyst loading of 0.2,0.5,1.5,3.0 mg/cm2. The cell voltage of the assemblies with catalyst loading of 0.2mg/cm2 was higher than that of the other assemblies which catalyst loading were over 0.5mg/cm2. And the cell voltage of the assemblies with catalyst loading 0.5, 1.5, 3.0 mg/cm2 were almost same. Base on this result, the catalyst loading enough for cathode was determined to be 0.5mg/cm2.

CONCLUSION

A 50cm2 cell having high electrolysis performance, and high durability was made using following components:

(1)Anode and cathode frames Platinum-electroplated titanium plate with grooves of 2mm wide and 2mm deep machined in parallel at 8mm pitch.
(2)Anode support collector Platinum-electroplated titanium fiber sintered plate of 1.0mm thickness.
(3)Cathode support collector Gold-electroplated stainless steel fiber sintered plate of 1.0mm thickness.
(4)Membrane-electrode assembly Membrane Type B2, 51mm thickness and 1000EW Anode electrode :

Iridium pyrolyzed at 200degree
Loading about 3mg/cm2 Cathode electrode :
Platinum black
Loading about 0.5mg/cm2 Hot-pressing temperature 140degree

This cell registered 1.533V, 1.665V of electrolysis voltage, 99.2%, 98.7% of current efficiency and 95.8%, 87.8% of energy efficiency at 1A/cm2,3A/cm2 respectively at 80degree under atmospheric pressure.

ACKNOWLEDGEMENT

This work was performed as an R&D program of the New Energy Development Organization (NEDO) under the WE-NET project of the Agency of Industrial Science and Technology, MITI.

REFFERENCE

1)M.Yamaguchi, T.Shinohara, K.Okisawa,International Hydrogen and Clean Energy Symposium `95.P.205-208(1995)
2)M.Yamaguchi, K. Yagiuchi, K. Okisawa,Proceedings of the 11th world Hydrogen Energy Conference, P.781-786(1996)
3)T.Nakanori, M.Yamaguchi, K. Okisawa,Proceeding of the 64th meeting of the Electrochemical Society of Japan, P.91.