4. Subtask 4 : Development of hydrogen production technology

4.1 R&D Goals

The aim of this research is to establish hydrogen production technology for water electrolysis by the solid polymer electrolyte membrane (PEM) method, which has anticipated advantages in terms of high efficiency and low cost compared to the convention hydrogen production method, in the course of evolving the WE-NET under implementation since FY 1993.

In FY 1998, development efforts continuing from the previous year were directed at (i) elemental technology under the two hydrogen production techniques of electroless plating and hot pressing, and (ii) large cell stacking (electrode surface area: 2,500 cm2; 5 cells). Also, optimum criteria and conceptual design for a practical scale of hydrogen production as determined the previous year were re-examined, and the impact on hydrogen production cost studied. Under research on high-temperature resistant, solid polymer electrolytes, several types of new polymer electrolytes were synthesized and their properties evaluated.

Also, the present status of the ion exchange membrane, essential under the subject hydrogen production method, as well as literature pertaining to water electrolysis were studied.

4.2 Results in FY 1998

4.2.1 Present Status of Ion Exchange Membrane Technology

Ion exchange membrane technology has experienced remarkable progress since the shift from chlor-alkali electrolysis by the mercury method to the ion exchange membrane method. Recently, particularly close attention has been focused on the application of this technology to the polymer electrolyte fuel cell (PEFC), and efforts to improve the ion exchange membrane and upgrade its performance are being carried out with this new use in mind. Further to its application to chlor-alkali electrolysis and fuel cells, progress in various other directions as well is observed with regard to this technology.

During this fiscal year, the present status of membrane technology was studied based on information obtained from membrane manufacturers. Specifically, data provided by four membrane manufacturing firms were used to summarize the membrane characteristics of each firm. Abstracts were also prepared of technical data on ion exchange membranes from Du Pont, Tokuyama, Asahi Chemical and Asahi Glass. Here data pertaining to chlor-alkali electrolysis were ignored, and focus on the informations relevant to the present status of membrane technology application in other areas.

4.2.2 Development of Hydrogen Production Technology by Electroless Plating Method

1. Research objectives

  1. Improvement of fabrication technology for large-area cells
  2. Development of stacking technology for large-area cells
  3. Stack evaluation under high temperature and high pressure
  4. Evaluation of cell durability

2. Improvement of fabrication technology for large area cells

Various improvements and study were carried out for each step in the fabrication process for large-area cells, and the following results obtained.

1) Slurry membrane forming by screen printing

  1. Study of slurry composition confirmed that a glychol-based solvent and iron oxide are appropriate.
  2. Slurry control was studied, and a fabrication method including a drying process was evolved. After preparation, the slurry is then dried, pulverized and returned to a slurry state which improves viscosity and renders a good level of screen printing performance.
  3. A screen mesh study was carried out focusing on optimum parameters of aperture diameter 70 mesh and emulsion thickness over 30 mm.

2) Membrane bonding by improved roll-press method Bonding criteria under the roll-press method are pressing pressure, pressing speed and pressing temperature.

  1. Study of roll-pressing pressure confirmed that 0.6 MPa (Abs) is optimum in terms of bonding strength.
  2. Study of roll-pressing speed confirmed that 0.01 m/s is optimum.
  3. Study of roll-pressing temperature indicated 180°C as appropriate.

3) Membrane-electrode assembly fabrication by plating

  1. It was confirmed that a good membrane-electrode assembly can be fabricated by means of hot-dip galvanizing (without forced circulation of the plating fluid) (see Fig. 4-2-2-1)
  2. A good membrane-electrode assembly was obtained by plating a 2,500 cm2 porous surface membrane fabricated by screen printing and roll pressing.

3. Development of stacking technology for large-area cells

The parameters indicated in Fig. 4-2-2-2 were studied, and a cell stack and stack supporter with weighting device as shown in Fig. 4-2-2-3 were fabricated.

4. Development of technology for stack evaluation under high temperature and high pressure operation

Electrolytic testing was carried out for a 2,500 cm2 single-stage stack and four-stage stack under conditions of 120°C temperature and 0.5 MPa (Abs) pressure, and the following results were obtained (Fig. 4-2-2-4).

5. Cell durability evaluation

Continuous electrolytic testing was carried out at a current density of 1A/cm2 and electrolytic temperature of 80°C using small cells with Nafion 115 and 1135 membranes. Although durability was better as a result of increased surface porosity and an improved nonelectrolytic plating method, the 115 membrane exhibited a drop in efficiency and an increase in oxygen concentration in the hydrogen after 5,000 hours. In the case of the 1135 membrane, continuous testing is at the 3,130 hour mark as of March 19. It is planned to continue the testing in the future for trend observation; nevertheless, it is necessary to further optimize the membrane-electrode assembly and improve the electron source material (Fig. 4-2-2-5).

6. Summary

  1. Fabrication technology for 2,500 cm2 large-area cells was improved. Specifically, screen printing, roll-press bonding and nonelectrolyte plating methods were optimized, and a stable membrane-electrode assembly produced.

  2. In order to achieve a 2,500 cm2, 4-stage cell stack, study was carried out on a header structure for circulated water and generated gas, and a design for uniform water supply to each cell. Also, a separator seal design for pressurization was studied, and a large-area stack separator was fabricated.

  3. The 2,500 cm2 4-stage stack incorporating the fabricated membrane-electrode assembly and separator was evaluated under conditions of high temperature and high pressure.

  4. Evaluation of cell durability indicated upgraded durability as a result of high surface porosity and improved nonelectrolyte plating. However, it is further necessary to pursue study on long-term durability.

4.2.3 Development of Technology for Hydrogen Production by Hot-pressing Method

Research and development of small cells to improve electrolytic properties and durability, and large cell development which were large cell fabrication technology, large stacking technology, high temperature and pressure base technology, etc. aimed at a greater hydrogen production scale were carried out and the following results achieved.

1. Research and development of small cells

  1. Study on anode catalyst particle size of the membrane-electrode assembly
    Five prototype anodes with varying catalyst particle size were fabricated, bonded to a 52 mm thick electrolyte membrane, and the electrolytic voltage of the membrane-electrode assembly measured.
    Results indicated that electrolysis was possible at the lowest voltage in the case of an electrode with particle size of 5~10 mm. Voltage in this case was 1.531 V under conditions of 80°C temperature and current density of 1A/cm2.

  2. Study on smoothing of the support collector
    The membrane-electrode assembly suffers damage as a result of gouging by the surface fiber of the titanium-fiber sintered plate used as the anode-side support collector. To prevent this, it is thus necessary to render this plate as smooth as possible. A prototype titanium-fiber sintered plate was accordingly fabricated by plugging the gaps between the titanium fibers with titanium powder, and re-sintering. Average surface coarseness value for this prototype plate was 10.5 mm compared to the conventional 22.9 mm. Maximum coarseness value was 83 mm compared to the conventional 168 mm, confirming a smoothening effect.

2. Durability testing

Continuous electrolytic testing was carried out for five cells (electrode surface area of 50 cm2) with differing anode-side support collector. In all cases, cells experienced a steady degradation in current efficiency.

Also, in the case of the cell subject to the longest testing, an energy efficiency of 91.1% at the outset under conditions of 80°C temperature and current density of 1A/cm2 steadily dropped to 84.1% after 21,850 hours of testing. When a low voltage below the theoretical electrolytic voltage was apllied to this cell, leak-current was observed. These results indicate that partial rupture of the membrane-electrode assembly occurred, causing direct contact between the anode-side and cathode-side support collector. In light of the fact that degradation of cell material was not evident, it is concluded that long-term durability would have been achieved if damage to the membrane-electrode assembly had not occurred.

3. Development of large cell fabrication technology

  1. Study on uniformity of electrolytic properties of the large membrane-electrode assembly
    An electrode with 2,500 cm2 surface area was fabricated applying an improved catalyst dispersion method to render a uniform catalyst substrate. Next, a membrane-electrode assembly was fabricated by bonding this electrode with a 52 mm electrolyte membrane using a hot press system with high pressure and temperature precision. In order to identify electrolytic property distribution, 13 samples were extracted which could be applied to a 50 cm2 cell, and electrolytic properties were observed. Results clearly indicated only a very small variation in electrolytic properties. Specifically, under conditions of 80°C temperature and current density of 1A/cm2, voltage was 1.533~1.553, current efficiency was 98.2~100%, and energy efficiency was 93.8~96.5%.

  2. Study on improving the thickness precision of the support collector
    Being in tight contact with the membrane-electrode assembly, the thicker areas of the support collector apply strong localized pressure to the assembly causing damage. The thickness of the ultra-fine titanium-fiber sintered plate used as the support collector is conventionally adjusted by means of roll pressing. For this study, however, a flat-pressing system was applied. In the case of a test specimen with dimensions 1 mm thick, 25 mm wide and 150 mm long, thickness precision was 10~20 mm. In the case of a test specimen with dimensions 1 mm thick, 90 mm wide and 400 mm long, this precision was 38 mm compared to 106 mm in the case of abrication by the conventional roll-press method. The flat-press method clearly results in improved thickness precision.

4. Development of large stack technology

In order to enlarge the electrolyzer, stacking technology for cells with large electrode surface area is crucial. To verify this technology, cell components including membrane-electrode assembly with 2,500 cm2 electrode area, support collectors, end plate and bipolar plates were fabricated and a prototype 5-cell stack electrolyzer with a bipolar plate contact type filter-press design as shown in Fig. 4-2-3-1. Testing equipment for evaluation purposes was also prepared. Results of property evaluation are given in Fig. 4-2-3-2 and indicate superior properties with values exceeding those for the small cell. With an electrolyte membrane thickness of 52 mm for example, an energy efficiency of 94.4% was achieved under conditions of 80°C temperature and current density of 1A/cm2.

5. Development of high temperature and high pressure base technology

A high temperature, high pressure operating system was designed and fabricated capable of controlling exhaust gas flow volume to high temperature and equipped with a heater to maintain temperature over 100°C by heating the purified water supply. This system was applied in testing of the electrolyzer with a five cell stack which electrode area were 200 cm2 per cell as shown in Fig. 4-2-3-3. This testing was carried out under conditions of temperature and pressure of 120°C / 3 ata and 140°C / 5 ata, respectively, and electrolysis was possible without damage occurring to the membrane-electrode assembly. It was confirmed that a higher energy efficiency can be achieved compared to conditions of normal pressure and temperature under 100°C. As shown in Fig. 4-2-3-4, energy efficiencies of 98.6% and 91.3%, respectively, were obtained for the two cases of current density of 1A/cm2 and 3A/cm2 under conditions of 140°C temperature and 5 ata pressure.

4.2.4 Economics of Hydrogen Production Plant - Sensitivity Analysis

4.2.4.1 Summary

The conceptual design and feasibility study of a 32,000 Nm3/h hydrogen production plant were carried out in FY 1997. Correspondingly, in FY 1998, the results were re-examined more accurately,and a sensitivity analysis was carried out to find the influence of factors on hydrogen production cost. Also, in order to assist a project pertaining to the verification of system performance of hydrogen fueling station, which is one of short-term programs of WE-NET Phase II, a 300 Nm3/h hydrogen production system (package type) was studied. In addition, the feasibility of the electrolyzer operation at high temperature (200°C) was examined in relation to the high temperature polymer electrolytes currently under development.

4.2.4.2 Sensitivity Analysis of Hydrogen Production Plant

(1) Parameters and its range for analysis

Sensitivity analysis was carried out using parameters within the range shown in Table 4-2-4-1. Figures in parentheses were applied to give standard conditions and to obtain the unit cost of hydrogen production. The unit cost is corresponding to the origin of Fig. 4-2-4-1.

(2) Sensitivity analysis

Fig. 4-2-4-1 shows the results of sensitivity analysis. It can be seen that (i) unit cost of electricity and cell voltage have a major impact, (ii) operation at higher temperature tends to reduce production unit cost, however, this is offset by a resultant rise in operating pressure, and (iii) life of membrane has more influence towards the shorter life. Influence of current density varies indicating the optimum point (optimum economic current density) and the most cost-effective current density is in the range of 2~2.5 A/cm2.

4.2.4.3 Study on a 300 Nm3/h hydrogen production system (package type)

(1) Setting of basic criteria

Considering a hydrogen fueling station, a conceptual design of 300 Nm3/h hydrogen production system was conducted. The basic criteria adopted for the conceptual design is shown in Table 4-2-4-2.

(2) Assumption in fluctuation of equipment cost

  • The cost of a cell is in proportion to the electrode area (base:1.2 million yen/m2)
  • The cost of a rectifier and power receiving equipment are in proportion to the power for electrolysis required
  • The cost of heat exchanger, etc, is in proportion to the heat-transfer area necessary for cooling oxygen and hydrogen gases (both including steam) and for cooling circulating water under each condition
  • The cost of cooling tower is in proportion to the power of 0.7 of the total cooling quantity of heat

(3) System description and cost estimation

With consideration to the fact that the system is a package type, the facility is to be broken down into units, to be fabricated and assembled at the factory and installed in site. Table 4-2-4-3 shows the approximate dimensions for each piece of major equipment and the size of the support-base for each unit. Dimensions of the fully assembled system are 7.5 meter in length, 4.3 meter in width and 5.25 meter in height. The fabrication cost of the facility was estimated at about 200 million yen.

4.2.4.4 Study on electrolysis at high temperature

(1) Operating conditions at 200°C

Under the condition of heat balance at the electrolyzer, the operating conditions at 200°C are calculated as shown in Table 4-2-4-4 assuming extrapolation of cell voltage is valid based on the present performance of the cell. It is noted that the operating pressure exceeds 30 kgf/cm2 G for any case considered.

(2) Operating conditions at atmospheric pressure (<10 kgf/cm2G)

At this moment, the appropriate range for optimum operating conditions is considered as shown in Table 4-2-4-5. These conditions are reasonable with consideration of the heat resistance of the present polymer electrolyte, the most cost-effective current density for electrolysis and the advantage of operation at moderate pressure.

4.2.5 Development of High Temperature Resistant Polymeric Electrolyte

SRI International is developing novel high-temperature high-strength polymer electrolytes, as alternative to Nafion or other perfluorinated hydrocarbon sulfonate ionomers, for use in high-temperature electrolyzers. The goal of this project is to develop a solid-state, high-temperature solid polymer electrolyte electrolyzer that produces hydrogen more efficiently than existing solid polymer electrolyte electrolyzers. The basis of this cell is a solid polymer electrolyte that will operate at high temperatures (200°-300°C). Requirements of these new polymer electrolytes include high proton conductivity, high thermal stability, electrochemical and chemical stability under reducing and oxidizing conditions, and good mechanical properties.

We have developed high temperature polymer electrolytes which are chemically designed to form channel-like domains with high concentration of sulfonic acid groups for high proton conductivity. These new polymers are fully fluorinated for optimum thermal and chemical stability, especially under oxidizing conditions. We have developed processing methods for the fabrication of polymer films with good handling properties. The polymer films, typically 75 mm to 125 mm thick, are currently prepared by solution casting. We have shown that these polymers have a proton conductivity of 0.066 S/cm at 200°C and 0.062 S/cm at 150°C, at 100% relative humidity, as tested by 4-electrode AC impedance analysis (see Fig. 4-2-5).

4.2.6 Study of Literature Pertaining to Water Electrolysis

Water electrolysis is an extremely important industrial process which has been carried out over the years to produce the hydrogen necessary in the chemi-industrial sector for ammonia synthesis, etc. Although it has been somewhat eclipsed in recent years by cheap hydrogen production through steam-reforming of petroleum, natural gas, etc., it still garners focus as the only industrially established method for extraction of hydrogen from water as a clean, secondary energy source. It is thus concluded that a thorough grasp of the present status of research in this regard is extremely important.

In this light, a study was carried out of water electrolysis related literature published in recent academic journals, etc. This study focused on publications rendered during the period July 1997 ~ June 1998, and sets out a summary of the research papers presented as well as important diagrams and figures contained therein. Here, attention is directed at the main orientation of this research. For more detailed understanding of the content of each treatise it is recommended that reference be made to the original publication.

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