Real-Time Overpotential Evaluation and Control Method for PEMFC
Authors: Ohno Kohei , Akimoto Yutaro , Okajima Keiichi , Matsuki Takanori , Sakai Tomoharu , Nagaura Masao , Kasai Shin 
Published: December 2024

Fuel cells have an important role in promoting decarbonization because they can generate electric power from a chemical reaction between hydrogen and oxygen and do not emit carbon dioxide. Fuel cells can generate electric power through a chemical reaction between hydrogen and oxygen, and since they do not emit carbon dioxide, they play an important role in promoting decarbonization. PEMFCs are used in vehicles and household cogeneration systems because of their low operating temperatures, high power density, compact size, and light weight.

However, fuel cells decrease the output power depending on the internal conditions. Dry-out caused by drying electrolyte membranes and flooding due to the generated water remaining in the flow channels and catalysts. For the widespread use of fuel cells, it is necessary to detect these failures and immediately recover from the decreasing of power. Evaluation methods for diagnosing fuel cell failures are existed the individual calculation of overpotential and the Electrochemical Impedance Spectroscopy (EIS).

Individual calculation of overpotential is to diagnose internal failure of fuel cells by determining the overpotential, which is the amount of decrease from the theoretical electromotive force, for each factor calculated from cell voltage, cell resistance, and load current by calculation using the least-squares method. This method has the characteristics of identifying the cause of voltage decrease. However, the overpotential evaluation by calculating with cell voltage, stack current, and cell temperature allows real-time evaluation while making measurements, however it depends on the accuracy of the calculating may cause false evaluation. The EIS is a method of diagnosing internal conditions by sweeping frequency from high to low frequency and calculating resistance by factor from the Cole-Cole plot, which is the impedance response. Although this method can diagnose the internal conditions of the electrolyte membrane and catalyst in detail, it can only measure under constant current conditions, and the measurement time is long. Therefore, it is useful for detailed cell analysis, however, difficult to apply for real-time control. Real-time and highly accurate evaluation of internal conditions is necessary to control fuel cells to maintain their output, which varied as the output decrease due to drying of the electrolyte membrane and remaining of generated water.

The purpose of this study is to propose a method for highly accurate and real-time operational control. The study examined a diagnostic method that combines individual overpotential calculations and fixed-frequency impedance measurements. Fixed-frequency impedance measurement refers to measuring electrolyte membrane resistance by making impedance measurements at only a single fixed-frequency, unlike EIS. In other words, the method used in this study is to calculate the overpotential from real-time measurements by using the electrolyte membrane resistance obtained by fixed-frequency impedance measurements to calculate the individual calculation of overpotential. Electrolyte membrane resistance was calculated by applying only a constant high frequency (1 kHz) using the impedance measurement system ALDAS-F and each overpotential was evaluated by performing the individual calculation of overpotential using calculated electrolyte membrane resistance and the measured voltage and current. We found that with this method, the overpotential can be evaluated without calculation by the least-squares method while measuring in real time. To improves the stability and reliability of the fuel cell, this study controls the fuel cell operation using this real-time overpotential evaluation method and compare the results with the existing method.

In the experiment using a 500 W PEMFC at 20 A, the real-time overpotential evaluation was compared with the overvoltage evaluation by the EIS, which showed a similar trend in the decrease in overpotential before and after warm-up operation for each method. The results of each method also showed similar overpotential characteristics when compared with the load current. The results show that the proposed method can evaluate the fuel cell’s overpotential as well as the EIS. Moreover, we evaluated the overpotential by applying a frequency of 1 kHz, and each overpotential could be evaluated respectively. For diffusion overpotential, the increasing diffusion overpotential along the voltage as observed in the low current. In the high current, the overpotential decreased as the voltage recovered due to hydrogen purging. The cell resistance increased, and the resistive overpotential also increased.

These results imply that the overpotential evaluation is a referenceable metric in real-time control by operating the fuel cell with a constant frequency applied. Moreover, it was verified experimentally that the proposed method is not affected to calculate by least-squares method because it can measure the electric membrane resistance. This study made it possible to control the operation of fuel cells through real-time evaluation.