【作者】 季孟波；

【导师】 魏子栋；

【作者基本信息】 重庆大学 ， 材料科学与工程， 2009， 博士

【摘要】 质子交换膜燃料电池（PEMFCs）的大规模商业化要求降低成本,增加电池的耐久性、可靠性以及实现其性能的最优化。合适的水管理对于实现PEMFCs的性能最优化和长耐久性发挥着至关重要的作用。然而一旦正极中氧还原反应（ORR）和电渗拖曳所产生的液态水的速率超过其通过反向扩散、蒸发、水蒸气扩散及液态水毛细传输等方法所排出水的速率时,就会发生水淹。而水淹一旦发生,就会阻止氧气扩散到催化剂活性位进行还原反应;取而代之的则是H+得到电子还原为氢气,由于后者的还原电位比前者低1.23伏,使得正极电位急剧下降,从而产生所谓的“负差效应”,进而对PEMFCs的输出性能带来重大的有时甚至是灾难性的损害。在过去的20年里,水淹问题已经得到了广泛的研究,包括数学模型的预测和实验诊断等。对于缓解水淹的研究,目前绝大部分的工作都放在了系统设计和工程方面,如设计电池的组件和优化操作条件等。然而这些策略都通常不可避免的会带来严重的寄生功率损耗。而对水淹的发源地——催化层自身的水淹研究却少有涉及。实际上既然水淹是发生在膜电极组件（MEA）里的,那么MEA组分的材料设计与工程对于水管理而言应该是一种比较简单的办法,况且它不会带来寄生负载,应该成为缓解水淹策略的首选。而本文就是通过修饰电极催化层的微观结构对MEA中组分（如正极和负极）的材料设计和工程做出了新的尝试。首先,本文在传统Pt/C电极的基础上添加MnO₂制成MnO₂-Pt/C复合电极,在因水淹导致的氧饥饿状态下,由与氧还原反应（ORR）有着相近Nernstian电位的MnO₂的还原反应维持正极反应以消除负差效应。研究结果表明在传统Pt/C电极中引入MnO₂不仅能够在一定程度上减轻因氧饥饿导致的负差效应,在富氧条件下它还能协同Pt/C催化剂共同促进催化氧还原反应。MnO₂-Pt/C复合电极和Pt/C电极的阻抗谱研究进一步证实了复合电极中的MnO₂在氧饥饿情况下扮演的是替代氧作为电子接受体的角色。此外无论是在富氧状态还是氧饥饿状态下,放电过的MnO₂完全能够恢复到它的初始状态。其次,通过将防水油——二甲基硅油渗入到传统Pt/C气体多孔电极中发明了一种新型抗溺水性气体多孔电极（AFE）,其中二甲基硅油主要分布在直径范围为20-70 nm的孔隙中。含有AFE正极的质子交换膜燃料电池不管在水淹状态下（大电流密度和氧气过增湿）还是在正常操作条件下都展示出了优于含有传统正极的燃料电池的输出性能。这种抗溺水性气体多孔电极在抗水淹方面的成功主要在于它解决的是发生在多孔电极自身中的水淹问题而非在双极板流场中水的积聚问题,该技术路线解决了发生在直径为20-70 nm孔隙中的水淹问题,而在这些孔隙中水淹经常发生而且常规措施对此却无能为力。再次,采用电化学阻抗谱（EIS）并结合催化层中的薄膜/水淹团块模型（thin film/flooded-agglomerate model）对抗溺水性Pt/C气体多孔电极（AFE）的抗溺水性能进行了分析,其结果表明:AFE在完全水淹状态下之所以体现出了优异的抗溺水性能是因为在水淹之前二甲基硅油预先占据了催化层中部分用于氧传输的团块（agglomerate）孔隙,进而在水淹来临之后阻止了这些具有团块扩散（agglomerate diffusion）性质的孔向薄膜扩散（thin film diffusion）性质孔的转化,从而为氧扩散提供了具有高溶解氧能力且不会为水占据的固定专用通道。第四部分,通过将二甲基硅油渗入到传统MnO₂/C氧电极中发明了一种有序化的抗溺水性氧电极,首次实现了氧传输通道和OH-离子扩散通道的分离。实验结果表明这种有序化的抗溺水性氧电极特别是在大电流密度下体现出了显著的抗水淹性能,它解决了碱性燃料电池和金属/空气电池氧电极中的PTFE降解和碱性电解质物理湿润现象所导致的催化层水淹问题。最后,在传统的PtRu/C电极（CPRE）中加入二甲基硅油（DMS）后制成了一种新型防止液封效应的DMFC用阳极（PLSE）。这种新型电极显示出了优异的防止液封效应的能力。当用PLSE替代CPRE后,单电池的输出功率从42.2 mW cm-2提高到了51.6 mW cm-2。PLSE这种阳极结构的成功之处在于CO₂在这种防水油DMS中的溶解度和扩散系数都要高于在甲醇水溶液中的溶解度和扩散系数。兼之DMS的憎水性,DMS为CO₂的扩散传质提供了专用快速通道,实现了CO₂传输通道和甲醇溶液扩散通道的分离,有效抑制了甲醇溶液对CO₂传输的液封效应。更多还原

【Abstract】 At present, despite the great advances in proton exchange membrane fuel cells （PEMFCs） technology over the past two decades through intensive research and development activities, their large-scale commercialization is still hampered by the higher materials cost and lower reliability and durability. In this context, proper water management is of vital importance to achieve maximum performance and durability from PEMFCs. However, when the liquid water generation rate at the cathode, by electro-osmotic drag and the ORR, exceeds the liquid water removal rate from the cathode by back diffusion to the anode, evaporation, water vapor diffusion and liquid water capillary transport through the GDL, the water flooding appears. With water accumulation and the cathode flooded, oxygen starvation,and even oxygen depletion would occur at the cathode. In this case, protons H+ reduction reaction （PRR） carries on at the cathode of PEMFC rather than oxygen reduction reaction （ORR）. The potential of PRR is 1.23V less than that of ORR; it would cause a remarkable decline of cathode potential as ORR was replaced by PRR. The output voltage of a single cell with oxygen starvation would be likely reversed. This phenomenon was defined as“voltage reversal effect”（VRE） in this paper, which will lead to a significant, sometimes catastrophic, decrease in cell performance. Over the last two decades, extensive research work has been carried out on water flooding, including prediction through numerical modeling, detection by experimental measurements. The flooding mitigation strategies are mainly focused on system design and engineering such as the design of cell components and the manipulation of operating conditions, which is often accompanied by significant parasitic power loss. However, there are few reports that aimed to overcome water flooding happening in the pores of a porous electrode so far, which is just the headstream of water flooding.In fact, since water flooding happens within the membrane electrode assemblies （MEA）, a simpler approach for water management through material design and engineering of the components of the MEA is preferred because it does not usually have an associated parasitic load. Thus, as a new attempt of material design and engineering of the cathode and/or anode to address water management in the PEMFCs, modification of the microstructures of the catalyst layer has been carried out in this paper.Firstly, a MnO₂–Pt/C composite electrode was designed to solve the VRE caused by oxygen starvation, which was based upon the fact that the electrochemical reduction of MnO₂ has almost the same Nernstian potential as the ORR. It has been found that the introduction of MnO₂ into the Pt/C catalyst not only can alleviate, to a certain extent, the problem VRE in the case of oxygen starvation, but also play a synergistic role with Pt/C in catalysis of the ORR in the case of oxygen rich conditions. The impedance spectra of the MnO₂–Pt/C and Pt/C electrodes further confirm that MnO₂ in the composite electrode does substitute for oxygen as an electron-acceptor in the case of oxygen starvation. The discharged MnO₂ can recover to its initial state regardless of oxygen rich or oxygen starvation conditions. Thus, it may be possible to apply the proposed composite MnO₂–Pt/C electrode for practical use.Secondly, an anti-flooding electrode （AFE） was prepared by introduction of water-proof oil, dimethyl-silicon-oil （DMS） into the conventional Pt/C electrode. The experiments results indicate that the DMS mainly distributes in the pores with diameter ranging from 20 to 70 nm. The single PEMFC cell with the AFE cathode displays much better power output not only in the case of water flooding but also in the case of a well-designed operational condition than the cell with the conventional cathode. The novel anti-flooding electrode displays outstanding anti-flooding capability, especially in the case of a large current density and over-humidification. The success of the AFE in anti-flooding lies in that （1） it solves the water flooding to the porous electrode itself rather than the water accumulation in the gas channels of the bipolar plate, and （2） it solves the water flooding of the pores with a diameter of 20 to 70 nm, in which water flooding frequently happens and is not easy to remove by the routine ways.In the part three, electrochemical impedance spectroscopy （EIS） studies have been carried out in order to evaluate the anti-flooding capacity of the AFE. From the dependences of the EIS on the overpotentials, the impedance properties were analyzed in terms of the thin film/flooded-agglomerate dynamics in the catalyst layer. The EIS study demonstrated that the excellent anti-flooding capability of the AFE in the case of completely flooding lies in that AFE alleviates the thin film diffusion effect in the case of flooding due to the DMS has occupied partial pores in the agglomerate before they are flooded by product water, thus prevents the conversion of oxygen diffusion type from faster agglomerate diffusion to slower thin film diffusion to some extent, and therefore provides the unoccupied channels with high solubility of oxygen in such DMS-filled channels for oxygen transportation.In the part four, an ordered anti-flooding oxygen electrode was prepared by adding water-proof oil DMS into the conventional MnO₂/C electrode, thus the channels for oxygen transportation and OH- ions are orderly allotted between the channels/pores occupied by DMS and electrolyte, which makes the oxygen and OH- hold their own fixed and stabile transport channels, respectively. The success of the ordered anti-flooding oxygen electrode in anti-flooding lies in that it solves the water flooding to the catalyst layer of oxygen electrode used in AFE and metal/air batteries due to the PTFE degradation and physical wetting phenomenon in alkaline electrolyte.Finally, A novel anode for preventing liquid sealing effect in the DMFC was invented by adding water-proof oil DMS into the conventional PtRu/C electrode. The novel electrode displays outstanding capability in preventing liquid sealing effect. The performance of DMFC was increased from 42.2 to 51.6 mW cm-2 with substitution of the PLSE for the CPRE. The success of the novel anode structure lies in that the solubility and diffusion coefficient of CO₂ in the water-proof oil DMS are higher than in methanol-water solution. Then, the hydrophobic DMS supplies the unoccupied channels for CO₂ transportation, which are separated from the chnnels for methanol solution diffusion. Therefore, the introduction of MDS effectively prevents the Liquid Sealing Effect （LSE） to CO₂.更多还原