JPH0325903B2 - - Google Patents

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 into electric power and generates electricity.

[Prior art and its problems]

In general, Fig. 1 shows the change in cell voltage over time when a fuel cell is continuously operated under rated operating conditions. In other words, the output voltage is low immediately after the start of operation, and as time passes, the voltage gradually increases until it reaches the peak voltage, and after this peak voltage remains stable for a certain period of time, the output voltage tends to gradually start to decrease. show. This pattern of change in output voltage over time is greatly influenced by the affinity of the electrode for the electrolyte, in other words, the strength of the hydrophobicity of the electrode, especially the strength of the hydrophobicity of the oxidizer electrode, which influences the characteristics of fuel cells. known to give. In other words, if the oxidizer electrode has strong hydrophobicity, its affinity with the electrolyte will be weak, and this will lengthen the life of the electrode, but on the other hand, the electrode reaction at the beginning of operation will be insufficient, resulting in a low output voltage and a long time to reach the peak voltage. It takes time. On the other hand, if the hydrophobicity of the oxidizer electrode is weak, its affinity with the electrolyte will be strong, and the electrode reaction will be active from the beginning of operation, resulting in a shorter time for the output voltage to reach the peak voltage. They show contradictory trends in terms of output characteristics, such as shortening the life of the battery and accelerating the drop in output voltage after reaching the peak voltage. To this end, conventional efforts have been made to adjust the hydrophobicity of the electrodes that make up the fuel cell by changing the type and mixing ratio of electrode materials such as catalysts and water repellents, or by changing the method of constructing the electrodes from the electrode materials. I'm going. In this case, there are many examples in which the hydrophobicity is adjusted particularly by adjusting the amount of water repellent such as polytetrafluoroethylene and the method of adding it. However, when adjusting the hydrophobicity of the electrode itself using these methods,
Although it is desirable to have strong hydrophobicity from the viewpoint of electrode life, it is necessary to reach the specified peak output voltage soon after the start of operation, so conventional methods have been designed to reduce the electrode life to some extent. Although the hydrophobicity of the electrode is slightly lowered at the cost of production, this results in the problem of shortening the life of the fuel cell.

[Purpose of the invention]

This invention was made in consideration of the above points, and while maintaining the high hydrophobicity of the electrode necessary for extending the life of the battery, on the other hand, it can be used quickly after the start of operation without shortening the life of the battery. The object of the present invention is to provide a method of operating a fuel cell that allows a rated output voltage to be obtained.

[Key points of the invention]

According to the present invention, the above object is achieved by first performing a preliminary discharge operation for a predetermined period of time at a low pressure below the rated operating gas pressure (preferably below atmospheric pressure) when starting the initial operation of the fuel cell. Generally, it is well known that the life of fuel cell electrodes decreases if they are continuously operated at low operating gas pressure, but according to the knowledge obtained by the inventor through experiments, the lifespan of fuel cell electrodes decreases when the battery's output voltage reaches its peak value. It has been confirmed that the lower the operating gas pressure, the shorter the time. FIG. 2 shows the results of this experiment, in which the horizontal axis represents the discharge time, and the vertical axis represents the battery voltage when returning to the rated gas pressure after discharging under each gas pressure condition. As is clear from FIG. 2, the lower the initial operating gas pressure, the faster the initial voltage rises. The reason for this is presumed to be as follows. In other words, under normal operating conditions, increasing the reaction gas pressure slows down the progress of leakage due to the electrolyte at the electrode, but lowering the operating gas pressure slows down the wetting of the electrolyte necessary for the electrode reaction. It is thought that the progress speed will increase. Therefore, as mentioned above, the preliminary discharge operation is performed at a low gas pressure below the rated operating gas pressure only during the initial period of the start of operation of the fuel cell, and after the output voltage reaches the peak value, the operation is returned to the rated operating gas pressure. By doing so, it is possible to quickly shift to rated operation in which a predetermined output voltage can be obtained after the start of operation without impairing the hydrophobicity of the electrodes and, therefore, the life of the electrodes.

[Embodiments of the invention]

Next, embodiments of this invention will be described. First, the configuration of the test fuel cell is shown below. Fuel electrode: Platinum-added furnace black catalyst with polytetrafluoroethylene as a hydrophobic agent.
A fuel electrode was constructed by forming a thin film on an electrode base material by adding and mixing the mixture in a weight percent. Oxidizer electrode: An oxidizer electrode was constructed by forming a thin film of a mixture of platinum-added acetylene black catalyst and polyetrafluoroethylene in an amount of 30% by weight on an electrode base material. Electrolyte: 7N potassium hydroxide aqueous solution was used. Next, the fuel cell was operated under the following conditions. First, perform a preliminary discharge operation for 250 hours with the operating gas pressure at -500 mmH ₂ O relative to atmospheric pressure. After confirming that the battery output voltage has reached its peak value, the operating gas pressure is then rated. The gas pressure was returned to (500mmH ₂ O・G) and rated operation was performed. Figure 3 shows the change in output voltage over time (solid line A) when the above pre-discharge operation is performed, and the rated operating gas pressure from the beginning of operation using the same fuel cell without performing the above-mentioned pre-discharge operation. In addition, the change in voltage over time during operation (dotted line B) is calculated from the results of the operation test and is shown. As can be seen from this operating characteristic diagram, by performing the pre-discharge operation at a low gas pressure, the time for the output voltage to reach its peak value is significantly shortened compared to the case where no pre-discharge is performed. On the other hand, in order to confirm the influence on the lifespan of the fuel cell, the inventor conducted a test to compare the lifespan of the fuel cell by operating the fuel cell according to the above embodiment and the following fuel cell continuously for a long time. The results are shown in Figure 4. In other words, the characteristic line C in Fig. 4 corresponds to the characteristic line A in Fig. 3, and when a pre-discharge operation using a low gas pressure is performed on a fuel cell constructed with highly hydrophobic electrodes as described above. Figure 2 shows the voltage change characteristics of a fuel cell over time. On the other hand, characteristic line 2 shows that after the fuel cell is configured with a low hydrophobic electrode in which the amount of water repellent added to the oxidizer electrode has been adjusted to a reduced amount, and operation is started under the conventional operating conditions of operating at the rated gas pressure from the beginning. This figure shows the voltage change characteristics over time of a fuel cell whose output voltage reaches its peak value in about 250 hours. As is clear from this figure, the operating method of this invention has no effect on shortening battery life, and is superior to the method that uses electrodes that have been adjusted to have low hydrophobicity in advance to accelerate the initial voltage rise. However, a method in which a highly hydrophobic electrode was used and a pre-discharge operation was performed at a low gas pressure yielded better results with a longer life.

【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 least at a pressure lower than the rated operating gas pressure before shifting to rated operation. While maintaining the high hydrophobicity imparted to the electrode, the time from the start of operation until the output voltage reaches its peak value can be significantly shortened compared to conventional operation methods without impairing the electrode life, and its practical effects is extremely large.

[Brief explanation of the drawing]

Figure 1 is a characteristic diagram showing the general change in output voltage over time due to continuous operation of a fuel cell, and Figure 2 is a characteristic diagram showing the relationship between the time it takes for the cell voltage to reach its peak value and the operating gas pressure. , FIG. 3 is a characteristic diagram of battery voltage change over time comparing the battery voltage change over time according to the embodiment of the present invention and the case where no pre-discharge operation is performed, and FIG. 4 is a graph showing the operation according to the embodiment of the present invention. FIG. 3 is a characteristic diagram of voltage change over time due to continuous operation, comparing the battery life according to the method and the conventional operation method of a fuel cell configured with a low hydrophobic electrode. In the figure, A and C: Output characteristics of the battery obtained by the embodiment of the present invention, B and D: Output characteristics of the battery obtained by the conventional operating method.

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 power by supplying fuel and oxidizer reaction gas to each electrode,
1. A method of operating a fuel cell, which comprises performing a preliminary discharge operation for a predetermined period of time at least at a pressure lower than the rated operating gas pressure before transitioning to rated operation when starting the operation of the fuel cell.