Publications (2)4.95 Total impact
Article: Effects of reverse voltage and subzero startup on the membrane electrode assembly of a PEMFC[show abstract] [hide abstract]
ABSTRACT: Effects of reverse voltage and frozen fuel cell startup on the membrane electrode assembly (MEA) were investigated for a proton exchange membrane fuel cell (PEMFC). A single cell was started from a subzero temperature by applying reverse voltage. The voltages applied to the cell were 0.8 and 1.2 V. The fuel cell performance was measured with a polarization curve and by cyclic voltammetry (CV), electrochemical impedance spectra (EIS), linear scan voltammetry (LSV) after each experiment. From the results, it was concluded that the catalyst activity, electrochemical active surface area (ECA) and the membrane were not damaged by the reverse voltage if the voltage was below 0.85 V. In contrast, a reverse voltage improved cell performance slightly. If the reverse voltage was larger than 0.85 V, the cell performance degraded. Another single cell with an active area of 128 cm2 was started up at −15 °C by applying reverse voltage. The cell performance and MEA physical characteristic were tested before and after the freeze startup. From the results, the cell performance decayed MEA delamination was observed and the pore size distribution of the MEA changed.Journal of Power Sources 165(1):287-292. · 4.95 Impact Factor
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ABSTRACT: The effect of water removal and freeze/thaw cycles on proton exchange membrane PEM fuel cells was investigated by com-parative study of 20 on/off and freeze/thaw cycles after purging by reactant gas with relative humidity 58.0% at 25°C. Within 20 cycles, the cell performance, electrochemical active surface areas, and electrochemical impedance spectra responses were analyzed by the mean squared deviation method. No performance decay caused by water freezing was observed, and the water amount in the cell was reduced to an extent by which freeze degradation was avoided. From the mid 1980s, much attention has been focused on the research and development of proton exchange membrane PEM fuel cells for the terrestrial and space applications. Due to their merits such as high energy efficiency, high specific power, low op-eration temperature, and pollution-free quality, the PEM fuel cell is one of the most promising replacements for the traditional internal combustion engine. Now, the problem of PEM fuel cell storage and freeze start-up at subzero temperature has attracted increasing attention. 1-8 The U.S. Department of Energy DOE and NREL es-tablished a challenging technical target: rapid start-up of PEM fuel cell to 90% rated power from −20°C for 30 s to 2010. 9 Since water is generated at the cathode side due to the oxygen reduction reaction and supplied by humidified gas fed in the cell for both cathode and anode, it freezes once the cell shuts down and is kept at subzero temperature. When the cell temperature rises above the water freez-ing point after the cell starts up, the ice thaws. The volume change caused by water and ice phase transition destructs the fuel cell ma-terials and components, since the densities of water and ice are 0.9998 and 0.9168 g/cm 3 at 0°C, respectively. The effect of freeze/ thaw on PEM fuel cell performance has been studied. To simulate the situation of vehicle application, Cho and co-workers 1 operated a PEM fuel cell at an operating temperature of 80°C, stopped, cooled, and kept it at −10°C for 1 h, and heated it to 80°C again for the next operation. They reported that performance of the PEM fuel cell degraded with a rate of 2.8% at 0.6 V after four freeze/thaw cycles due to deformation of the catalytic layers accompanied by a de-crease in electrochemical active surface area and an increase in in-terfacial charge-transfer resistance and contact resistance between the membrane, electrodes, and separators. To prevent the damage on PEM fuel cell materials and performance degradation, water re-moval methods, i.e., dry N 2 gas purging, was applied. By dry N 2 gas purging the cell to below 3% relative humidity RH, the perfor-mance degradation rate was significantly reduced to 0.06% per ther-mal cycle. 2 Since pore size in the catalyst layer and diffusion layer ranges from nanometer to micrometer, water in these pores of the size range should have different freeze behavior. 10,11 It is possible to reduce the amount of water in the cell to an extent that the water freezing effect on the cell materials and performance is negligible. In addition, to eliminate the degradation caused by water freezing, using dry gas and purging the cell as dry as possible are reasonable, but the membrane dehydration during the purging progress is defi-nitely ineluctable, which may be a sincere impediment for success-ful start-up, especially for freeze start-up the next time. In this study, we used reactant gas with a 58.0% RH at 25°C to purge the cell to 16.6% RH at 50°C immediately after the cell op-eration. After the inlets and outlets of the cell were all sealed, the cell was frozen at −20°C for 1.5 h in a climate chamber. We applied 20 freeze/thaw cycles to investigate the effect of water freezing on the cell performance. Moreover, with the same type of membrane electrode assembly MEA, another cell following the same condi-tions but without freezing was tested through 20 normal on/off cycles. By comparative study, the evidence of the water freezing on the cell performance can be manifested.