Hyung-Juhn Kim

Effects of Purging on the Degradation of PEMFCs Operating with Repetitive On/Off Cycles

Degradation of polymer electrolyte membrane fuel cells (PEMFCs) that is facilitated by on/off cycles is one of the most important issues for commercialization of fuel cell vehicles. When a PEMFC stack is shut down, residual hydrogen could induce high voltage equivalent to open-circuit voltage to the cathode side that might cause sintering of the Pt catalyst and facilitate the formation of hydrogen peroxide radicals at the anode side that might decompose the Nafion membrane. In this study, the degradation of PEMFCs exposed to repetitive on/off cycles was investigated by measuring current density-voltage characteristics, ac impedance, cyclic voltammograms, gas leak, cross-sectional scanning electron microscopy images, and transmission electron microscopy images. To prevent the degradation of PEMFCs caused by the residual gases, hydrogen was removed from the anode gas channel by air-purging, which was found to be a very effective method.

Complex Capacitance Analysis of Ionic Resistance and Interfacial Capacitance in PEMFC and DMFC Catalyst Layers

For polymer electrolyte membrane fuel cell (PEMFC) and direct methanol fuel cell (DMFC) catalyst layers (CLs), a complex capacitance analysis of impedance data was developed to evaluate the catalyst/ionomer interfacial capacitance and ionic resistance of ionomer networks without nonlinear data fitting. First, assuming no faradaic reactions, equivalent circuits for the CLs were suggested, which are similar to electric double-layer capacitor systems with porous carbon electrodes. Then, with the simulated complex capacitances, it was confirmed that the plots of the real and imaginary parts as a function of ac frequency are determined by the catalyst/ionomer interfacial capacitances and the ionic resistances time constants, which are important characteristics for high fuel cell performances. Experimentally, the condition of no faradaic reactions was realized by supplying nitrogen or water to the cathodes instead of air and by fixing the dc potential at 0.4 V during the impedance measurements. By performing a complex capacitance analysis, the interfacial capacitances and ionic resistances of PEMFC membrane electrode assemblies (MEAs) can be obtained at various relative humidities, proving that the catalytic activity in fuel cell operation depends on ionic resistances as well as on the catalyst/ionomer interfacial area. The effects of various MEA preparation methods on the ionomer distributions and DMFC performances were analyzed by a complex capacitance analysis.

Gradient Catalyst Coating for a Proton Exchange Membrane Fuel Cell Operation under Nonhumidified Conditions

Operation under nonhumidified conditions was demonstrated with a membrane electrode assembly (MEA) for a proton exchange membrane fuel cell. The Pt catalyst was coated with a gradient on the active area of a MEA; the catalyst amount was reduced gradually from cathode inlet to outlet. The MEA with the gradient coating method produced more water near the inlet site than that with the uniform coating method. The water was used to mitigate dryness of the MEA. The cell performance was improved by 17% at 800 mA/cm 2 under nonhumidified conditions by effective water management of the gradient coating method.

Novel Membrane Electrode Assembly (MEA) Fabrication for PEMFC Operations under Non-humidified Conditions

The operation under non-humidified condition was demonstrated with novel MEA (membrane electrode assembly) for a PEMFC (proton exchange membrane fuel cell). For a conventional PEMFC operation, Pt is loaded uniformly on the whole active area of MEA. In the report, Pt catalyst was coated with a gradient on the active area of MEA. The catalyst amount was reduced gradually from cathode inlet to outlet. When the cell was fully humidified, the performance of gradient catalyst coated MEA was lower than that of the uniform catalyst coated one. However, when the cell was operated under non-humidified condition, the cell performance of the gradient catalyst coated MEA was higher than that of the uniform catalyst coated MEA. In the case of the gradient catalyst coated electrode, the amount of water which was generated at the cathode inlet site was relatively large compared to uniform catalyst coated one and the water was used to hydrate the MEA properly, resulting in higher cell performance under non-humidified condition.