Study On The Startup-shutdown Process And The System Strategy Of Proton Exchange Membrane Fuel Cell

Some technological "bottlenecks" for current technology of Proton Exchange Membrane Fuel Cell (PEMFC) limited their further commercialization. For example, the relative short lifetime of PEMFC induced by degradation of materials is still unsatisfactory for stationary and mobile applications. For mobile applications, PEMFC must be operated under various conditions, such as startup-shutdown cycles. As catalyst support materials, the oxidation of carbon materials is considered as one of the major factors for the performance decay during startup and shutdown process, which must be mitigated in order to achieve acceptable durability. This work focuses on studying the degradation behaviors under different startup-shutdown processes, and investigating the factors which lead to the degradation, in order to provide the reference to establish the protection controlling strategy for PEMFC. Key research and conclusions are followed:(1) An experimental study is conducted on the attenuation performance of PEMFC stack with300cm2single cell area after frequent start-stop cycles. After500start-stop cycles, the voltage at100A is inspected. The polarization curves before and after500cycles are recorded to characterize the performance of PEMFC. In addition, the degradation behaviors and coherence of7single cells are compared in500cycles which is divided into the first200cycles and the latter300cycles. Because of the uniformity of gas distribution in the gas channel, the Ha/air boundary is also produced during the N2purge, resulting in the carbon corrosion. As the comparison of the polarization curves, the performance degradation which is caused by the uniformity of gas distribution is also very severe. As the current density was increasing, the degradation rate of the PEM fuel cell stack is rising. In addition, the degradation rate at100A during the former200cycles is3.76%which is about thrice as the rate during the latter300cycles. However, the uniformity of the single cell voltage don’t seem an obvious change after500startup-shutdown cycles.(2) The shutdown process of a single cell with the self-fabricated CCM is analyzed under different conditions (dummy loads, gas humidity and back pressures). The performance of the self-fabricated CCM is characterized with the polarization curves and the AC impedances. And it is compared with the performance of the commercial CCM (Gore5815). Under standard conditions, the self-fabricated CCM performs better than Gore CCM under low and medium current densities but slightly worse than Gore CCM under high current density; the discharge time of the fuel cell is decreasing as the increasing dummy load. The relationship between the discharge time and the dummy load can be fitted as the equation, y=-2E-06x3+0.0013x20.4197x+62.962(100%RH) and y=3E-07x3+0.0001x2-0.2158x+52.41(60%RH). From this equation, the value of the dummy load applied on the fuel cell system can be calculated according to the desired discharge time. According to the discharge time in UTC’s patents, the dummy load should be120mAcm-2～260mAcm-2.(3) The effect of cathode exhaust conditions on the degradation behaviors of fuel cell is investigated using two single cells, named open-ended cell and closed cell. The cathode inlet pressure during the introduction of dummy load is an important factor to analyze the performance decay of membrane electrode assemblies under different conditions. Electrochemical techniques including measurement of polarization curves, cyclic voltammetry and linear sweep voltammetry, and cross-sectional scanning electron microscopy of tested membrane electrode assemblies, are employed to evaluate the performance decay of fuel cells. The results show that closed cathode exhaust valve during the introduction of dummy load would significantly alleviate the performance decay and the decrease in the electrochemically active surface area, resulting in an improvement of fuel cell durability. No significant deterioration of membranes is observed for both open-ended cell and closed cell during frequent startup and shutdown processes.(4) The effect of the gas shutoff sequence with the dummy load, which is used to consume the residual gas in the flow field after shutting down PEMFCs on the degradation behaviors, is investigated under two different shutdown procedures. Theoretical analysis and experimental test indicated that different shutoff sequences have great effect on the oxygen permeation rate across the membrane which would produce the H2/O2interface during using the dummy load. Due to the greater oxygen permeation rate across the membrane after shutting off hydrogen and introducing the dummy load, there will be the H2/O2interface at the anode in the startup process, which lead to the carbon oxidation and the performance decay. Thus, shutting off air firstly and then introducing the dummy load to consume the residual oxygen after shutting down PEMFCs is an effective way to alleviate the degradation of the catalyst layer and improving the durability of PEMFCs, especially at high current density. The degradation rate of the cell voltage at1000mA/cm2is0.024mV/cycle for shutting off air firstly, which is half as the degradation rate for shutting off hydrogen firstly. The same result can be concluded by the active area test of the catalyst layer. Shutting off air firstly during the shutdown process can avoid the agglomeration of Pt particles with the cross-sectional SEM of MEAs and TEM of Pt/C catalyst.(5) A large-area single cell with a segmented cathode current collector is used to investigate the effects of dummy load, cathode inlet humidity, and cell temperature on current distribution during the gas starvation and shutdown processes under no back pressure. The obtained current distribution provides useful information for strategic operation during the gas starvation and the shutdown processes to mitigate performance degradation of PEMFCs. During the PEMFC shutdown process, the currents of all the segment cells decrease with time after application of a dummy load, but the rate of decline is different along the gas channel at different dummy loads, cathode humidities and cell temperatures. The currents of the segment cells decrease much more quickly at a lower cathode humidity and higher cell temperature, which will cause a non-uniform current distribution for a large area fuel cell.(6) Combination with the research in the above chapter, the detailed procedures of the system strategy for the startup and shutdown process of PEMFC are proposed as applying the dummy load and air purge during the shutdown process, applying the hydrogen purge and the dummy load to control the high potential during the startup process. Moreover, in order to reduce the possibility of the cell reversal during applying the dummy load, a modular fuel cell system is proposed for the startup and shutdown process of PEMFC system.