【作者】 余意；

【导师】 潘牧；

【作者基本信息】 武汉理工大学 ， 新能源材料， 2013， 博士

【摘要】 质子交换膜燃料电池(PEMFC,Proton Exchange Membrane Fuel Cell)的商业化发展一直被一些技术“瓶颈”所制约,比如应用于固定电源和移动电源领域的PEMFC,由于材料性能的衰减导致其寿命相对较短。对于车用燃料电池,PEMFC电堆不可避免的要经历一些工况,如启停工况。在启停工况下,催化剂载体碳材料的氧化被认为是造成电池性能衰减的重要因素,其根本原因是启动和停机过程中阳极氢气／氧气界面的存在导致阴极形成高电位。因此,本文旨在研究PEMFC在频繁启停工况下的性能衰减,比较不同启停程序下电池性能的衰减速率,为提出保护性的PEMFC启停控制策略提供参考性意见。主要研究内容和结论如下：(1)考察了PEMFC电堆在启停工况下的性能衰减,以活性面积为330cm2的电堆为研究对象,考察燃料电池经历非保护性频繁启停操作后的性能衰减,重点记录PEMFC在启停前后的极化曲线。同时,将500次连续的启停循环分为前200个循环和后300个循环,比较两个不同阶段电堆在工作电流下的单片电压衰减速率,并考察其单片均一性在启停前后的变化。实验结果表明,PEMFC电堆停机后由于反应气体在各单片内的分布浓度不一致,会造成单片电压在N2吹扫过程中下降速率不一致,甚至会出现某一片或者几片出现反极的现象；PEMFC电堆随着频繁启停循环的增多,电池性能的下降会变慢。以100A下的电压为例,经历500次循环后,前200次启停工况平均电压衰减速率为后300次衰减速率的3.32倍；而电堆的单片均一性并未发生明显的恶化。(2)设计启停程序,对单电池(自喷涂膜电极)在不同辅助负载、气体加湿度和背压下的停机过程进行研究和分析。自喷涂膜电极的性能通过极化曲线和交流阻抗谱的方式来表征。通过与商业的Gore5815膜电极的性能进行比较,在标准条件下,自喷涂膜电极的性能在中低电流密度时的性能要优于商业的Gore5815膜电极,而在高电流密度下性能要略差于商业的Gore5815膜电极。通过记录不同辅助负载下的放电时间,得出放电时间随着辅助负载大小的增加而降低,通过拟合得出放电时间与负载大小的关系式,y=-2E-06x3+0.0013x2-0.4197x+62.962(100%RH)和y=3E.07x3+0.0001x2-0.2158x+52.41(60%RH)。通过这两个公式,我们可以根据放电时间值来确定燃料电池系统所需要的辅助负载的最佳值。根据UTC专利里对放电时间的规定,PEMFC停机时放电电流的大小范围应为120mAcm-2～260mAcm-2.(3)为了提出一种新颖的有效的减少PEMFC在启停过程中性能衰减的方法,并模拟车用PEMFC发动机真实的启停过程,以单电池为研究对象,采用一个特殊的启停程序,研究阴极尾气阀的开启和关闭状态对PEMFC启停过程中性能衰减的重要影响。在使用辅助负载的过程中,阴极气体压力的变化是研究不同停机条件下膜电极性能衰减的一个重要参数。PEMFC性能的衰减主要通过极化曲线、循环伏安扫描和线性扫描等电化学方法以及膜电极的断面扫描来表征。结果发现,PEMFC在启停过程中,关闭阴极尾气阀能够有效的减少催化剂的衰减,这一点可以通过催化剂活性面积以及催化层的厚度变化看出。开口系PEMFC在启停过程中的衰减很严重,特别是在高电流密度下,催化剂载体的氧化对PEMFC的性能及其耐久性造成很严重的负面影响。在频繁启停过程中,PEMFC性能的衰减主要是由于催化剂的腐蚀造成的,而质子交换膜在这个过程中并没有出现明显的失效。(4)通过理论计算,考察气体的不同关闭顺序对氢气／空气界面的影响,并设计了两种启停控制程序,在使用放电负载降电压的基础上考察氢气和空气的关闭顺序对燃料电池耐久性的影响。结果表明,反应气体的关闭顺序对氧气通过质子交换膜的渗透速率有很大影响,而氧气通过膜的渗透会导致H2/02的形成。由于先关闭氢气并使用辅助负载时,氧气渗透速率更大,使得催化剂载体碳材料的腐蚀和电池性能衰减速率更大。因此,先关闭空气后使用放电负载消耗掉阴极的氧气,能够更有效的防止燃料电池性能的衰减并提高燃料电池的耐久性,特别是高电流密度区域的效果更为明显。在1000mA/cm2下,电池的电压衰减速率为0.024mV/cycle;而如果是先关闭氢气后放电,其衰减速率为0.045mV/cycle,大约为前者的两倍。通过对两者的催化层活性面积的测试,会得到相同的结果；通过膜电极断面SEM测试和Pt/C催化剂的TEM测试,先关闭空气后放电比先关氢气后放电更能够防止催化剂Pt颗粒的团聚。(5)利用集流板分块的方法,对大面积燃料电池在不同放电负载、阴极湿度和电池温度下反应气体饥饿和启停过程的电流密度分布进行研究。本章所获得的电流密度分布的数据和结果,能够为减小燃料电池在气体饥饿和启停过程中性能衰减提供参考意见。研究结果指出,在空气和氢气缺气条件下电流分布很不均匀,尤其是当电流密度高于400mA cm-2时,而氢气缺气比空气缺气对电流分布的影响更大；燃料电池停机过程,电流随着辅助负载的连接逐渐下降,但是每一个子电池下降的速率不均匀。在高负载、低湿度和高温度下子电池电流的下降速率更大。(6)结合以上研究,本文最后提出了行之有效的燃料电池启动和停机过程的系统控制策略,主要思路是停机利用辅助负载和空气吹扫相结合,开机利用氢气吹扫并采用辅助负载控制开机高电位的方法。另外,为了减小辅助负载连接过程中电池反极的可能性,本文提出了一种模块化放电的燃料电池系统。更多还原

【Abstract】 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.更多还原