고분자 전해질 연료전지의 성능특성에 관한 실험적 연구

Abstract

The purpose of this study is to investigate the performance characteristics of polymer electrolyte membrane fuel cell (PEMFC) according to the optimal operating conditions of fuel cell based on the analysis of performance, impedance and fuel crossover characteristics under various operating parameters for the reduction of performance loss and improvement of fuel cell performance. In this work, the experiment was performed by varying the operating parameters such as operating temperature, concentration, flow rate, humidified temperature, and backpressure. The performance was analyzed with a polarization curve, which was measured with the voltage-current density and power-current density. In order to analyze the performance loss in detail, the impedance characteristics were measured by using an alternating current impedance measurement system and simulated by an equivalent circuit at various frequencies. Nyquist diagram, impedance, and phase angle were expressed by real and imaginary components of impedance to investigate the ohmic and activation losses. Fuel crossover, which permeates from anode to the cathode through the membrane, experiment was performed by measuring crossover current density at an open circuit using humidified nitrogen instead of air at the cathode and applied voltage with a power supply. And, the effects of membrane thickness and catalyst loading quantity on the performance were investigated experimentally. The results revealed that the activation loss was decreased by increasing the cell temperature, cathode flow rate, and cathode backpressure. However, the impedance and phase angle were increased by increasing the anode flow rate and the alcohol concentration. When oxygen was used as the oxidant gas, it was found that the impedance and the phase angle decreased more significantly than when air was used because of the active electrochemical reaction caused by the higher oxygen concentration. Also, it was shown that fuel crossover increased by increasing cell temperature, alcohol concentration, anode flow rate and cathode flow rate, and by decreasing cathode backpressure. Under the same test conditions, the fuel cell performance was improved by the reduction of resistance for proton transport at the thinner membrane. The comparison of open circuit voltage shows that using of a thicker membrane results in a larger value than that of using a thinner membrane due to the decrease in fuel crossover. Moreover, there was a good agreement between empirical equation and experimental results under various operating conditions. The visualization experiment was conducted by transparent fuel cell because the flooding phenomenon, which occurs in the flow channel of fuel cell, was also considered as an important parameter on the performance characteristics. A transparent fuel cell was fabricated to observe the water flooding phenomenon in the cathode channel. Images of the flooding phenomena were obtained by digital and high-speed cameras. As the cell temperature was increased, the occurrence of flooding phenomena decreased due to evaporation, and performance was significantly improved. The water droplets in the cathode channel were also removed at high cathode flow rates, and the instances of flooding phenomenon were decreased by evaporation as the humidified temperature increased. When the cathode backpressure was increased, the flooding phenomenon increased due to high partial pressure. Also, the water distribution from using oxygen as an oxidant gas was increased compared to that using air. Also, the experiment was conducted for the application of dimethyl ether (DME) among various alternative fuels to fuel cell system. The DME aqueous solution is supplied into the fuel cell by DME vapor pressure. Experimental results showed that fuel cell with DME-methanol exhibited enhanced performance when compared to fuel cell with methanol only. Such performance enhancement was due to a decrease in activation losses by DME oxidation reactions. As the DME crossover through the membrane was reduced, the open circuit voltage (OCV) of the fuel cell increased.