Use of impedance spectroscopy to investigate factors that influence the performance and durability of proton exchange membrane (PEM) fuel cells

Impedance spectroscopy provides the opportunity for in-situ identification and quantification of physical processes and has been used extensively to study the behavior of the fuel cell. However, a key question to be answered is whether the features seen in the impedance response are caused by an artifact or represent a physical process taking place in the system. The measurement model developed by our group can be used to identify the frequency ranges unaffected by bias errors associated with instrument artifacts and non-stationary behavior.;Impedance measurements were performed with the 850C fuel-cell test station supplied by Scribner Associates and with a Gamry Instruments FC350 impedance analyzer coupled with a Dynaload electronic load. All electrochemical measurements were performed with a two-electrode cell in which the anode served as a pseudo-reference electrode. The experiments were conducted in galavanostatic mode for a frequency range of 0.001-3000 Hz with 10 mA peak-to-peak sinusoidal perturbation, and ten points were collected per frequency decade. Ultra pure hydrogen was used as the anode fuel, and compressed air was used as oxidant.;The measurement model was used to show that low-frequency inductive loops were, in some cases, fully self consistent, and, therefore, the inductive loops could be attributed to processes occurring in the fuel cell. Then we developed first-principle models that incorporate processes that may be responsible for the inductive response seen at low frequencies. We found that side reactions producing hydrogen peroxide intermediates and reactions causing Pt deactivation could yield inductive loops. These side reactions and the intermediates can degrade fuel cell components such as membranes and electrodes, thereby reducing the lifetime the fuel cells. The hypothesized reaction involving of peroxide and PtO formation were supported by microstructural characterization.;A more sensitive manner of using impedance spectroscopy to gain an insight into the problem of flooding which adversely affects the performance of the fuel cell was established. A comprehensive model for base-level noise in impedance measurements for normal (non-flooded) conditions was developed and actual noise in flooded conditions was calculated by transient fixed-frequency measurements. A comparison of the actual noise to the base-level noise was used to detect onset of flooding.;Also, graphical methods were used to interpret impedance spectra in terms of interfacial capacitance. The effective interfacial capacitance decreased with increase in current and decreased slowly with time. The decreases in interfacial capacitance with higher current density can be attributed to an excess amount of water i.e., flooding; whereas, the decrease in interfacial capacitance with time may be related to catalyst dissolution and deactivation.