Freezing of water in a polymer electrolyte membrane fuel cell ͑PEMFC͒ may cause severe problems in driving a fuel cell vehicle during the winter time. Characteristics of PEMFC which suffered low temperatures below zero celsius were examined with thermal cycles during which the temperature of the environment chamber was cycled from 80 to Ϫ10°C. With the thermal cycles, the cell performance was degraded due to the phase transformation and volume changes of water. Effects of freezing of water in a PEMFC on the electrode structure and polarization resistance were examined by Brunauer, Emmett, Teller analysis, cyclic voltammetry, and ac impedance spectroscopy.

c Hyundai Motor Company, Fuel Cell Vehicle Team, Korea For the mobile application, performance of polymer electrolyte membrane fuel cells ͑PEMFCs͒ should be maintained with being exposed to subzero temperatures in the winter time. To simulate the situation, a PEMFC was operated at 80°C, stopped, cooled to and kept at Ϫ10°C for 1 h, and heated to 80°C for the next operation. With the thermal cycle, cell performance was measured and found to degrade at a degradation rate of 2.3% based on current density at a cell voltage of 0.6 V. The degradation was attributed to freezing of water that was produced during operation and remained in the PEMFC after the operation. To prevent the performance degradation, water was removed from the PEMFC by supplying dry gases or an antifreeze solution to the PEMFC before the cell temperature fell to below 0°C. By using the gas-purging and the solution-purging method designed in this work, the performance degradation rate was successfully reduced to 0.06 and Ϫ0.47%, respectively.The polymer electrolyte membrane fuel cell ͑PEMFC͒ is a promising alternative power source for electric vehicles in the near future. One of the challenges that the PEMFC faces is to maintain its integrity when outdoor temperatures are below 0°C in the winter. During operation of a PEMFC, water is produced at the cathode side by the electrochemical reaction and, in most cases, is supplied through the reactant gases to keep the membrane hydrated. 1,2 If operation of the PEMFC is stopped and outdoor temperature is below 0°C, the water remaining in the PEMFC may freeze with expanding its volume by about 9%, because the density of water and ice at 0°C is 0.9998 and 0.9168 g/cm 3 , respectively. 3 Then when the PEMFC is restarted, ice melts into water again, reducing its volume. Such a repetitive volume change of water occurring inside the PEMFC may degrade cell performance.Recently, effects of water freezing on the performance of PEMFCs have been studied. 4,5 Kagami et al. 4 started a PEMFC at temperatures below 0°C with feeding dry reactant gases and measured cell voltage at various constant current densities. Except at very low current densities, the cell voltage decreased quickly. The higher the current density was, the steeper the decrease in the cell voltage was due to increase in water and ice formation rate. Based on the results, they suggested that for self-starting of PEMFCs without external heating, the start-up temperature of a PEMFC should be above Ϫ5°C. Cho et al. 5 investigated effects of repetitive freezing and melting of water in PEMFCs on their characteristics. To simulate the situation of the mobile application, they operated a PEMFC at an operating temperature of 80°C, stopped, cooled, and kept it at Ϫ10°C for 1 h, and heated it to 80°C again for the next operation. They reported that with repetition of the thermal cycle, performance of the PEMFC degraded due to deformation of the catalytic layers accompanied by a decrease in electrochemical active surface area and an increase in charge-transfe...