JPS61225775A - Operation of fuel cell - Google Patents

Abstract

PURPOSE:To obtain high output voltage in the early stage of operation as high hydrophobic nature is maintained by discharging a cell at high current density exceeding a rated load for a specified period before transferring to rated operation after starting. CONSTITUTION:Before transferring to rated operation after starting, a fuel cell is preminarily dischaeged at high current density exceeding a rated load, preferably at current density two times high than the rated current density, for a specified period. By increasing current density, temperature rise within electrode is quickened and electrolyte concentration is varied, and electrode reaction becomes active. High initial voltage is rapidily obtained as high hydrophobic nature is maintained without sacrifice of the life of electrode.

Description

[Detailed description of the invention] [Technical field to which the invention pertains] This invention has a fuel electrode and an oxidizer electrode facing each other with an electrolyte layer in between, and by continuously supplying fuel and oxidizer to each electrode from the outside, the chemical energy of the fuel can be directly converted into electricity. This invention relates to a method of operating a fuel cell that converts energy to generate electricity. [Prior art and its problems]

In general, Figure 1 shows the change in battery voltage over time when a fuel cell is operated continuously under rated operating conditions.The output voltage is low immediately after the start of operation, and the voltage gradually increases as time passes. This pattern of output voltage changes over time shows a tendency for the output voltage to reach a peak voltage, remain stable for a certain period of time, and then gradually start to decrease. The strength of the hydrophobicity of the oxidizer electrode, especially the strength of the hydrophobicity of the oxidizer electrode, which influences the characteristics of the fuel cell, has a great influence. In other words, if the oxidizer electrode is highly hydrophobic, it has a weak affinity with the electrolyte, and this results in a longer electrode life, but on the other hand, the electrode reaction at the beginning of operation is insufficient, resulting in a low output voltage, and the output voltage is low until the peak voltage is reached. It takes a long time. On the other hand, if the hydrophobicity of the oxidizer electrode is weak, the electrode reaction will be active from the beginning of operation, which will shorten the time it takes for the output voltage to reach the peak voltage, but on the other hand, the life of the battery will be shortened, and the peak voltage They show contradictory trends in terms of output characteristics, with the output voltage decreasing more quickly after reaching . To this end, conventional efforts have been made to adjust the hydrophobicity of the electrodes that make up the fuel cell by changing the types and mixing ratios of electrode materials such as catalysts and FA hydrocarbons, and by changing the manufacturing method used to make the electrodes. I was going. In particular, in the past, hydrophobicity was often adjusted by adjusting the amount of water repellent such as polytetrafluoroethylene and the method of adding it. Despite the fact that it is desirable to have strong hydrophobicity, in order to obtain the specified output voltage early after the start of operation, it is necessary to sacrifice some electrode life to increase the hydrophobicity of the electrode. The fuel cell was manufactured to be slightly lowered, which resulted in a problem that shortened the life of the fuel cell.

[Purpose of the invention]

This invention was made in consideration of the above points, and while adopting an electrode with high hydrophobicity, it also allows for a high output voltage within an early period after the start of operation without shortening the life of the battery. It is an object of the present invention to provide a method of operating a fuel cell in which characteristics can be obtained.

[Key points of the invention]

The above object is achieved by the present invention, when starting the operation of a fuel cell, a preliminary discharge is carried out for a predetermined period at a high current density that exceeds at least the rated load, preferably at least twice the rated current density, before transition to rated operation. This is accomplished by performing an operation. In other words, it is generally well known that the life of fuel cell electrodes tends to be shortened when continuous operation is performed with a high discharge current density; According to the results of experiments conducted by the inventor, the time required for this to occur can be shortened by increasing the discharge current density!
! It has been certified. Figure 2 shows this situation, and the vertical axis shows the change over time in the battery's output voltage when the load is returned to the rated load from each discharge condition shown.As is clear from the characteristics shown, the discharge conditions The higher the current density, the faster the voltage rises. The reason for this is that as the current density increases, the temperature inside the electrode increases faster, and this causes changes in the electrolyte concentration (for example, in alkaline fuel cells, 5. The electrolyte concentration inside the oxidizer electrode becomes high. It is presumed that this results in a phenomenon in which the capillary pressure of the electrolytic solution increases, promoting wetting of the electrolytic solution to the electrodes, thereby invigorating the electrode reaction. Therefore, as mentioned above, only at the beginning of fuel cell operation, a preliminary discharge operation is performed at a current density that exceeds the rated load, and after the output voltage reaches the peak voltage, the load is returned to the rated load and continuous operation is performed. This makes it possible to maintain high hydrophobicity of the electrode and quickly obtain a good initial voltage without impairing the electrode life.

[Embodiments of the invention]

Next, this invention will be explained in detail. First, the fuel electrode that makes up the fuel cell is made of a palladium-doped activated carbon catalyst mixed with polytetrafluoroethylene, which is formed into a thin film on a bridge base, and the oxidizer electrode is made of a highly hydrophobic platinum-doped carbon black catalyst. A sufficient amount of polytetrafluoroethylene was added and mixed so as to give properties, and a yI film was formed.The fuel cell was heated at a pre-discharge current density of 300 s in an electrolytic solution of 7N potassium hydroxide aqueous solution. /cd
(The rated discharge current density was 100 mA/a, and a pre-discharge operation was performed for 96 hours, followed by rated load operation. Figure 3 shows the change in output voltage over time (solid line) when the above pre-discharge operation was performed. A) and the voltage change over time (dotted line) obtained from the experimental results when the same highly hydrophobic electrode was used and rated operation was performed from the beginning of operation without performing a pre-discharge operation, As can be seen from the illustrated characteristic diagram, by performing the pre-discharge operation, the time until the voltage reaches the peak value Vo is significantly shortened.Furthermore, Fig. 4 shows the case of operation with the above-mentioned pre-discharge operation. Then, using an electrode configured to have low hydrophobicity by adjusting the amount of water repellent added to the oxidizer electrode in advance, and without performing the pre-discharge operation described above, operation was started at the rated load from the beginning.1: It is an output characteristic diagram of the voltage change over time due to the solid line, −
1° C shows the characteristic line according to the embodiment of the present invention, and dotted line 2 shows the characteristic line according to the conventional operating method. As is clear from this characteristic diagram, no reduction in battery life was observed due to the operating method according to this invention, and the battery life was longer when the pre-discharge operation was performed using a highly hydrophobic electrode. Good results were obtained with regard to longevity.

【Effect of the invention】

As described above, according to the present invention, when starting the operation of the fuel cell, a preliminary discharge operation is performed in which discharge is performed for a predetermined period at a high current density that exceeds at least the rated load before shifting to rated operation. A practical effect that can significantly shorten the time from the start of operation until the output voltage reaches its peak value compared to conventional operation methods, while maintaining the high hydrophobicity imparted to the electrode in advance and without impairing the electrode life. It is possible to provide a highly efficient driving method.

[Brief explanation of drawings]

Figure 1 is an output characteristic diagram showing the general change in cell voltage over time during continuous operation of a fuel cell, and Figure 2 shows the relationship between the discharge current density and the change over time until the cell voltage reaches its peak value. A characteristic diagram, FIG. 3 is a characteristic diagram of output voltage change over time comparing the case where a pre-discharge operation is performed according to the operating method of the present invention and the case where a pre-discharge operation is not performed, and FIG. FIG. 3 is a diagram showing voltage change characteristics over time due to continuous operation, comparing the operating method and the conventional operating method. In the figure, A and C: Output characteristics of the battery obtained by the example of this invention. Tatsu Todoroki, Director of the Institute of Industrial Science and Technology

Claims (1)

[Claims] 1) A method of operating a fuel cell, which has a fuel electrode and an oxidizer electrode facing each other with an electrolyte layer in between, and generates electricity by supplying fuel and oxidizer reaction gas to each electrode, the method of operating a fuel cell. 1. A method of operating a fuel cell, which comprises performing a pre-discharge operation for a predetermined period of time at a high current density that exceeds at least a rated load before transitioning to rated operation.