Polymer electrolyte fuel cells (PEFC) systems have attracted considerable attention in that they can be powered by the hydrogen that is produced as a by-product of various industrial processes, such as caustic soda production and petrochemical processes. Most of the by-product hydrogen is discarded as a waste stream or used for heat production. Therefore, such plants could realize significant energy savings by installing PEFC systems, allowing them to generate power with a high level of efficiency by using the by-product hydrogen to fuel PEFC systems. However, the by-product hydrogen produced by these plants typically contains impurities. Although most of these impurities can be removed, a small amount inevitably remains in the hydrogen and can cause the performance of a PEFC system to degrade. Therefore, the influence of these impurities must be clarified before installing a PEFC system.

Given the above, we have been developing a means of quickly evaluating the influence of impurities on a PEFC. The measurement of the current distribution is an important analysis approach that provides information on the local electrochemical reaction conditions in each of the cells of a PEFC. These cells are composed of an electrolytic membrane, a catalyst layer, a diffusion layer, and a flow channel. We have devised a means of measuring the current distribution using current distribution measurement sensors (S++ Simulation Services, Germany) which are inserted between the gas flow channel and current collector of either electrode. This method of measuring the current distribution allows us to quickly analyze the influence of impurities on the PEFC performance, relative to conventional voltage measurements, and can clarify which areas in a cell have degraded because the voltage change is the average of the values for every area in a cell, whereas the current change appears as an absolute value in a local area. Electrochemical impedance spectroscopy (EIS) is a valuable electrochemical analysis tool that can be applied to many kinds of electrochemical devices, including fuel cells. EIS can provide much useful information on the electrolyte membrane, the catalyst layer, and the reactant transfer in a PEFC cell. Although the measurement of the electrochemical impedance distribution in a cell provides a useful means of analyzing the influence of impurities on the PEFC performance, to the best of our knowledge, no previous studies have attempted a high-resolution analysis of the distribution. Therefore, we have developed a new measurement system that enables us to measure both the electrochemical impedance distribution and current distribution. The measurement system is an improved current distribution measurement sensor, intended for application to a JARI-standard cell, with an active area of 25 cm² and which has 81 segmented internal current shunt resistances for measuring DC current and 81 segmented external connectors for measuring AC current signals.

In the present study, to simulate impurities in the hydrogen, we added NH₃ (25 ppm) and analyzed its influence on the PEFC performance. We used the measurement system to measure the change in the electrochemical impedance distribution and current distribution over time. The operating temperature, humidification temperatures of the anode and cathode, and current density of the cell were set to 65 ºC, 65 ºC, and 0.3 A/cm² respectively. The AC current for measuring the electrochemical impedance was 1 A_p-p and the frequency range was 100 mHz–100 kHz.

The current distribution measurement (Fig. 1) revealed that the influence of the NH₃ was greatest at the inlet to the cell. Furthermore, from the results of the electrochemical impedance distribution measurement, it was found that the current distribution and some aspects of the resistance distribution, as determined from the electrochemical impedance, were correlated. Meanwhile, the most significant factor influencing the effect of NH₃ on the PEFC performance is the increase in the activation overvoltage at the inlet of the cell. In addition, it was also thought that the cathode reaction resistance of the cell increases, which is caused by the catalyst-specific surface area decreasing at the inlet of the cell.

In conclusion, our new measurement method provides an effective means of determining the largest degraded area and the largest degradation factor affecting PEFC performance, as a result of the presence of impurities in hydrogen. Therefore, relative to conventional measurements, we can quickly evaluate the influence of impurities on the PEFC performance by focusing on and analyzing these areas and factors. In the future, we will apply our new measurement method to the evaluation of the influence of impurities on the PEFC performance by acquiring information on the influence of such impurities on PEFC performance.