【作者】 黄振宇；

【导师】 涂正凯；

【作者基本信息】 华中科技大学 ， 动力工程及工程热物理， 2021， 硕士

【摘要】 质子交换膜燃料电池是一种不经过燃烧直接将燃料的化学能转化为电能的能量转换装置。由于不受卡诺循环的限制,具有能量转换效率高的显著优点,并且生成物只有水,不污染环境,被认为是优秀的下一代机械动力源,受到了越来越广泛的关注。燃料电池作为机械动力源时,将不可避免地经历动态加载,这将导致响应电压发生“下冲”现象,即响应电压低于稳态电压。不合理的加载条件将增大“下冲”幅度,甚至使得运行电压下降至零或截至电压,造成加载失败的恶劣工况,不利于燃料电池的推广。因此,研究质子交换膜燃料电池的变载特性,提升其运行稳定性,具有重要的现实意义。研究了不同初始工况下质子交换膜燃料电池的动态响应特性。实验发现,未活化的燃料电池不能在1s内加载0.8A·cm^-2,而活化的燃料电池却能在1s内成功加载1.5A·cm^-2。研究结果表明,燃料电池高电流密度的加载过程中可能存在两次“下冲”。因为质子交换膜内阻不能随电流密度增加突然减小,对高阶跃电流密度的响应将产生第一次“下冲”。如果加载电流密度响应成功后发生阳极脱水工况,将发生第二次“下冲”,可能导致加载失败。最后优化了未活化燃料电池的动态加载策略,加载时间减少了32.65%。研究了氢空和氢氧燃料电池在不同运行温度、相对湿度、过量系数和工作压力下的变载特性,并采用电化学阻抗谱技术分析其内在机理。结果表明:合理提升工作温度能够显著降低氢空燃料电池的总极化阻抗和氢氧燃料电池的欧姆阻抗,提高电池性能。但过高的温度会导致电池失水,增加电池的欧姆阻抗和总极化阻抗,严重损害电池性能。氢空燃料电池在60℃下的响应电压比30℃降低了0.0638V。提升反应气体加湿度能够缓解电池失水导致的性能衰减。当相对湿度从20%提升至60%时,氢空燃料电池的总极化阻抗下降了5.561 m Ohm,响应电压提升了18.57%。提升过量系数将增大欧姆电阻,不利于氢氧燃料电池的性能,但能够大幅度降低氢空燃料电池的总极化阻抗,从而提升其性能。提升工作压力能够增大反应气体分压,减小氢空和氢氧燃料电池的反应阻力,电池性能变好。最后,在连续运行的工况下,由于更小的传质阻力,氢氧燃料电池比氢空燃料电池具有更好的运行稳定性。设计了一种内置障碍物与流道打断相结合的新型3D流道,并采用数值模拟进行研究分析和实验进行论证。模拟结果表明,新型3D流道将传统的扩散传质方式优化为扩散传质与对流传质相结合的方式,极大地增强了反应气体的传质能力。新型3D流道还增加了反应气体的流动速度,强化了反应气体对水的吹扫作用,明显降低了电池内部的水分压,提升了电池的水管理能力。实验结果表明,新型3D流道在0.3A·cm^-2的工作电流密度下稳态功率和瞬态输出功率分别提升了1.185W和2.382W,表明新型3D流道具有强化燃料电池变载特性的能力。更多还原

【Abstract】 The proton exchange membrane fuel cell（PEMFC）is an energy conservation device that directly converts the chemical energy of the fuel into electrical energy without burning.As is not limited by the Carnot cycle,it has the significant advantages of high energy conservation efficiency.And more importantly,its by-product is only water without polluting environment.The PEMFC qualified many advantages is regarded as the promising next-generation mechanical power source and the world is devoted to developing and popularizing it.When PEMFC is used as a mechanical power source,it inevitably undergoes dynamic loading,which can result in undershoot that means the response voltage becomes lower than steady-state voltage.Unreasonable loading conditions can increase the undershoot and may even lead to a failure,which can act as a disadvantage of PEMFC and limit its application.Therefore,it is of great practical significance to study the load changing characteristics of PEMFC and improve its operational stability.The transient responses of PEMFC under different initial working conditions were studied.The experimental results showed that the fuel cell cannot be successfully loaded at start-up when the current step is increased from 0 A·cm^-2to 0.8A·cm^-2.It could be loaded successfully with a step current density of 1.5A·cm^-2after activation.Two kinds of undershoot may occurred during the loading process of PEMFC with a large step current density.The first undershoot occurred immediately with the successful response to a step current density as the water content in the membrane couldn’t change quickly.The second undershoot may happened after the current step because of the anode dehydration and could result in a load failure.Finally,the load strategy of PEMFC without activation was optimized and the loading time was reduced by 32.65%.The load changing performance of hydrogen-air and hydrogen-oxygen PEMFC with activation were studied at varied operating conditions and the electrochemical impedance spectroscopy was used to analyze their intrinsic mechanisms.The experimental results showed that the total polarization resistance of the hydrogen-air fuel cell and the ohmic resistance of the hydrogen-oxygen fuel cell could be decreased tremendously with the increased properly operating temperature and thus the load changing performance improved.However,the ohmic and polarization resistance may be increased aroused by the dehydration of the membrane electrode assembly due to the over-elevated operating temperature.When the working temperature was increased to 60℃,the response voltage of the hydrogen-air fuel cell decreased by 0.0638V compared with that of 30℃.The adverse effects could be eliminated by the increased relative humidity.When the relative humidity was increased from20%to 60%,the total polarization resistance decreased by 5.561m Ohm and 18.75%of the increment of the response voltage was found.With the increased stoichiometry,the performance of the hydrogen-oxygen would be depressed as more water was brought away with the faster gas flow,whereas it could be improved as the total polarization resistance of the hydrogen-air fuel cell could be extremely reduced.Nevertheless,with the increased working pressure,the performance of hydrogen-air and hydrogen-oxygen fuel cells both got better due to the increased partial pressure of the gaseous reactant and the decreased reaction resistance.Finally,the hydrogen-oxygen fuel cell had more stable dynamic characteristics than the hydrogen-air fuel cell under continuous operation due to smaller mass transfer resistance.A novel 3D flow field patterned with built-in obstacles and flow channel interruption was designed and analyzed numerically and experimentally.The simulation results showed that the 3D flow field transformed the traditional diffusion mass transfer into the optimized combination of diffusion and convection mass transfer and thus the capacity of mass transfer of the novel flow field was considerably enhanced.The flow speed was also increased by the3D flow field.Its purging effects on water improved and water partial pressure decreased tremendously.The experimental results showed that the steady-state output power and transient output power of the new 3D flow field were increased by 1.185W and 2.382W at0.3A·cm^-2,respectively.Better load changing characteristics were observed in the fuel cell with the new designed flow field.更多还原