اثر منجمد شدن /سیکلهای گرم شدن و روش پاکسازی گاز در سلول های سوخت غشاء پلیمر الکترولیت

At subzero temperature, the startup capability and performance of polymer electrolyte membrane fuel cell (PEMFC) deteriorates markedly. The object of this work is to study the degradation mechanism of key components of PEMFC—membrane-electrode assembly (MEA) and seek feasible measures to avoid degradation. The effect of freeze/thaw cycles on the structure of MEA is investigated based on porosity and SEM measurement The performance of a single cell was also tested before and after repetitious freeze/thaw cycles. The experimental results indicated that the performance of a PEMFC decreased along with the total operating time as well as the pore size distribution shifting and micro configuration changing. However, when the redundant water had been removed by gas purging, the performance of the PEMFC stack was almost resumed when it experienced again the same subzero temperature test. These results show that it is necessary to remove the water in PEMFCs to maintain stable performance under subzero temperature and gas purging is proved to be the effective operation.

Polymer electrolyte membrane fuel cell (PEMFC) is promising for its advantages of simple structure, relatively low operating temperature, high efficiency, convenient maintenance and rapid startup. PEMFCs can be used in many potential fields, especially as power sources for vehicles to replace normal combustion engine"'21. However, the availability of fuel cell vehicles is not quite close to us so far, although both concept design and demonstration have proved its feasibility. Actually, there are so many technical difficulties to be dealt with today, and the temperature impact is one of these issues encountered by the researchers and engineers. In the winter time or at the cold polar region, ambient temperature may drop below zero. So it is highly necessary for vehicle fuel cells to keep the ability of rapid startup and good self-maintenance from low temperature to normal operating temperature when they are used outdoor, for example in the range of -40°C to 80°C. Unfortunately, this demand is quite difficult for PEMFCs because of its high moisture content. Water is vital in PEMFCs to satisfy the hydration demand, which is the guarantee of favorable proton conductivity of the membrane and power generation capacity of PEMFCs. Water can be either brought into the fuel cell by external humidification or generated by electrochemical reaction^'^'. When the fuel cell is shut down at the ambient temperature below freezing point, water will freeze into ice and the ice crystal will hold the reactant back from getting to catalyst surface. When the fuel cell comes to normal conditions gradually, the ice will melt into water again. Thus reduplicate phase transition of water in PEMFCs will cause nonreversible damage to the materials and even components of fuel cells. Cho et aZ.[41f igured out that the performance exhibited degradation after four thermal cycles when the temperature decreased to - 10°C and kept for lh at this temperature. They also provided resistance difference between the electrode frozen and non-frozen through AC impedance measurement. Their conclusion was that the impedance between the electrode and membrane increased after freezehhaw cycles while the impedance of the membrane kept unchanged. Research on this topic is underway and to date the publication about operation at subzero temperature is few. However, some patents could be found in this field. Most of the patents focused on alteration and optimization of fuel cell system^'^-^'. The objective of this paper is to study the subzero temperature effects on PEMFCs. Firstly, changes of electrode structure and fuel cell power generation capacity before and after freezehaw cycles were examined via different methods. As a method of water elimination, gas purging was also investigated during the freezehaw cycles.

Water remaining in electrode of PEMFC is the immediate cause that results in structure change and performance degradation of PEMFCs in the thermal cycles from 50 to - 10°C. Gas purging is an effective method in preventing sharp performance degradation during freezehhaw cycles. The probably reason is that the structure breakage caused by phase transition of water can be avoided by effective water removing so that the components and performance of PEMFCs remain stable at subzero temperature conditions. Therefore, the method proposed in this paper is helpful to improve the durability of PEMFCs and its availability at subzero temperature.