| Direct methanol fuel cells (DMFCs) are considered as a promising alternative power source due to its great advantages, such as simple system, high efficiency, low working temperature, low pollution, high power density and so on. It is mainly used as mobile and portable power sources for microelectronic devices,microelectronic engines,micromedical instruments and individual communications.The diversitical usages need DMFC have a ability to adapte at various temperatures, of course, the operation temperature which is lower than room temperature even 0℃will be faced. Once the temperature is lower than the freezing point of water, the water inside the cell will become frozen which may cause damage to the electrode and decrease in cell performance. Thus, the subfreezing operation store ,cold start and subfreezing durability are key issues in DMFC commercialization, which are also challenging research areas of fuel cells.In this study, the characteristics of cell performance, subfreezing store and cold start of a DMFC were investigated experimentally with a home-made membrane electrode assembly (MEA) operating at temperatures below 25℃. The effect of operation parameters including methanol concentration, the flow rate of methanol solution and oxygen flow rate on the performance in a temperature range of 25～5℃was discussed. Additionally, the effect of freeze-thaw (F/T) cycles on the properties and performance of the MEA was characterized by scanning electron microscope (SEM) , hydrophobicity measurement, cyclic voltammetry (CV),current interrupt and anode polarization method. Moreover, the method for effective subfreezing store of the MEA of a DMFC was obtained. Finally, the cell performance during cold start in variational and steady subfreezing temperature environment was studied. The main results are summarized below:①The MEA was made by the thin-film method. It was found that the cell performance was greatly improved after being activated by pumping 70℃methanol solution into the anode for 17 h. SEM results reveale that the contact between the layers inside the home-made MEA was good. In addition, contact angle testing by sessile drop method results showed that the hydrophobicity of electrode was improved with the increasing of the PTFE content.②The effect of operation temperature on the cell performance at a range of 25℃～5℃were studied. It was found that cell performance decreased significantly as the temperature decreased. This can be attributed to the reason that the electrochemical kinetics and the mass transfer of reactants as well as the proton conductivity decrease with decreasing operation temperature.③The effect of operation parameters on the performance at 5℃was also studied. The results exibited that the methanol crossover rate was closely related to the cell operation temperature and the methanol concentration. The best cell performance was obtained at 4M methanol. The performance was improved with the decline of the methanol flow rate, which results from the less heat removed by the methanol solution. Similarly, the optimal oxygen flow rate was found to be 100mL·min-1 due to the bargain between the removal of heat and oxygen transfer from the flow field to the catalyst layer.④10-time freeze-thaw cycle tests were conducted from 25℃to -5℃and the cell performances were measured at 25℃. With the repeatation F/T cycles, the cell performance decreases and became steady finally because the water content of the MEA component remained unchanged.⑤By investigating the MEA structure after the freeze-thaw cycle tests, some cracks and grooves were observed on the surface of the gas diffusion layer (GDL) and damages were found to both microporous layer (MPL) and catalyst layer (CL). Contact angle results showed that the surface hydrophobicity of the MEA components decreased after the freeze-thaw cycles. Electrochemical measurements revealed that the electrochemical active surface area (ECSA) of Pt decreased and the contact resistance increased after the freeze-thaw tests. The above-mentioned reasons resulted in a decrease in the cell performance after the F/T cycles.⑥The low temperature operation characteristics of a DMFC under various temperatures was also studied. It was revealed that the cell can operate successfully with the decrease in temperature from 25℃to -5℃. Increasing the temperature reduction rate appropriately showed a benefit to the steady operation of the cell at low temperature. With progrossing operation time, the cell performance descended finally, though it may improve halfway.⑦Studies on the characteristics of cold start of a DMFC at -5℃were also performed in this study. The results showed that the cell start successfully at -5℃and the maximal current is 205 mA, which was 25% of the performance at room temperature. It was found that the optimal methanol concentration and oxygen flow rate were 4M and 250 mL min-1, respectively. The performance of cell cold-start was found to be influenced by the current density. A lower current density tends to a fast start and a long steady operation at the subfreezing temperature.
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