Effects of Water Removal on the Performance Degradation of PEMFCs Repetitively Brought to <0°C

Abstract

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...

Cold start characteristics and freezing mechanism dependence on start-up temperature in a polymer electrolyte membrane fuel cell

“…The plots clearly show that the cell voltage after the shutdown at the -10 °C cold start -8 -does not recover to the value before the cold start, while there is little difference between the voltages before and after the cold start shutdown at -20 °C. This deterioration after the -10 °C shutdown could be recovered by the dry nitrogen purge with flow rate of 3000 SCCM for 5 minutes, and very similar cell resistances in the range from 0.08 to 0.09  cm 2 were recorded during all of the operations at 30 °C (not shown in Fig. 10).…”

“…Cyclic voltammetry studies on the degradation of the electrochemically active Pt area of the cathode catalyst layer due to the ice formation were conducted for a cell containing a membrane with high water content after freezing/thawing cycling [1,2] and during and post-subzero start-up of a PEFC [3]. Here, together with the permanent degradation caused by structural alterations of the cathode catalyst layer [1][2][3], a temporary deterioration of the cell performance was reported after start-up from subzero temperatures and the warm-up to 25 °C with all ice in the catalyst layer melted [3]. Tajiri et al proposed a strict gas purge process before cold start operation, and estimated the ice distribution in the cathode catalyst layer at the end of cold starting at -30 °C at low and high current densities [5].…”

A review on performance degradation of proton exchange membrane fuel cells during startup and shutdown processes: Causes, consequences, and mitigation strategies

“…Wu et al [6] published a comprehensive review on PEMFC degradation mechanisms and mitigation strategies, in which durability tests under steady state [7][8][9][10][11][12][13][14][15][16][17] and dynamic state [18][19][20][21][22][23][24][25][26][27][28][29][30] conditions were also briefly summarized. In addition, Wu et al [6] also discussed the major failure modes and mitigation strategies of different components in PEMFCs.…”

Degradation aspects of water formation and transport in Proton Exchange Membrane Fuel Cell: A review

“…Alternatively, Cho et al [325] tried purging the anode with dry N 2 and the cathode with O 2 . The performance degradation rate of the cell was significantly reduced from 2.3% to 0.06%, and the reduction in Pt utilisation of the catalyst was also very small (~ 3% during 7 testing cycles).…”

“…Purging the cell with antifreeze solutions is another alternative to the dry gas approach. Cho et al [325] used 30% methanol and 35% ethylene glycol solutions for purging and by doing so the performance degradation rate of the cell was reduced to -0.16% and 0.47%, respectively.…”