Impedance characteristics of the direct methanol fuel cell under various operating conditions

In this paper we investigate the electrochemical impedance characteristics of the direct methanol fuel cell (DMFC) under various operating conditions such as cell temperature, methanol flow rate, cathode flow rate, methanol concentration, cathode backpressure, and type of oxidant gas. The impedance characteristics of the DMFC 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. 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 methanol flow rate and the methanol concentration. When oxygen was used as the oxidant gas, it was found that the impedance and the phase angle decreased more rapidly than when air was used because of the active electrochemical reaction caused by the higher oxygen concentration.