质子交换膜燃料电池反应物缺乏诱导的寄生反应机理的研究

发布时间：2018-07-30 07:27

【摘要】：质子交换膜燃料电池(PEMFC)因其低温、高效、无污染等优点，被认为是电动汽车、固定发电站和移动设备最有前途的电源之一，近年来受到了广泛的关注。然而，电池耐久性的不足是制约PEMFC大规模商业化发展的重要原因。在众多影响电池耐久性能的因素中，反应物的缺乏是最主要原因之一，其会引起电池内部发生多种寄生反应，如催化剂碳载体的氧化、Pt和Ru的溶解和脱落以及氢气的析出等，极大地影响电池的寿命。然而，先前该领域的研究绝大多数集中在电极材料的改进和运行工况的优化，很少有学者从传质的角度系统研究这类本质是由反应物传输能力不足引起的现象。鉴于此，本文利用数值模拟和实验研究相结合的方法，系统研究了PEMFC内多种由反应物缺乏而导致的寄生反应现象，着重分析了相应情况下反应物在电池内部的传输机理，以及电流和电势的空间分布状况。具体的研究内容和结论如下： 1. H2-PEMFC阳极氢气局部缺乏诱导的阴极碳腐蚀现象的研究。利用数值模拟的方法，对阳极流道水淹引起的氢气局部缺乏及其导致的阴极催化剂碳载体氧化现象进行了研究，着重分析了氢气在水淹区域扩散层和催化层中的传输机理，以及该情况下碳腐蚀速率的空间分布。研究发现，氢气在水淹区域内的传输分为边缘处的对流控制区和中心处扩散控制区，对流作用的产生源于氢气和水蒸气的同时消耗，而氮气的引入将极大地削弱对流作用进而引起缺氢区域面积的显著增加。同时，本文发现先前学者通常忽略的in-plane方向的质子传导作用对碳腐蚀速率的分布具有重要的影响，增大膜的质子传导率可以缩小发生碳腐蚀的区域面积。此外，本文发现在高的电池电压下，碳腐蚀速率的最大值完全决定于氧气在膜中的穿透量；而在低电压下，碳腐蚀速率的大小同时受到阳极的动力学特性和氧气在膜中穿透量的影响。 2. H2-PEMFC恒流放电时氢气整体缺乏诱导的电池电压反向和阳极碳腐蚀现象的研究。本文通过数值模拟研究了氢气的供应量不足以维持外电路电流需求条件下电池内部反应物浓度、电流密度和电极电势的空间分布。除了沿流动方向的分布外，本文着重强调了上述分布沿催化层厚度方向的不均匀性。研究发现，该情况下供给氢气的绝大部分均在距离阳极进口很短的区域内消耗，引起进口区域极高的电流密度，并引起进口区域的局部“氢泵”现象，，该现象为本文首次报道。此外，尽管阴极催化层的厚度仅有10μm，进口区域极高的电流密度将引起离子相电势沿阴极催化层厚度方向的显著变化，进而导致沿厚度方向依次发生析氢反应、氢气氧化反应和氧气还原反应，极大增加了该区域内传输现象的复杂度。 3.直接甲醇燃料电池(DMFC)内阴极氧气缺乏诱导的析氢现象的研究。本部分首先利用数值模拟研究了DMFC在开路状态下空气流量降低引起的阳极析氢现象。与先前学者的实验结论一致，模拟结果显示DMFC在开路状态且氧气供应不足情况下将分为上游的原电池区和下游的电解池(析氢)区。通过研究这两个区域交界面附近的传输现象，本文发现由于离子相电势在该界面处的急剧上升，引起质子沿in-plane方向的显著传导，从而导致在该界面附近存在一个局部的DMFC。同时，本文研究了不同阴极水淹程度对于该情况下电池内部电流密度分布的影响，发现即便在同样的空气流量下，不同的阴极水淹程度将引起电流密度分布的显著变化，这也解释了为什么此前Kulikovsky等的实验研究中在完全相同的测试条件下得到多种差别显著的电流分布的原因。此后，本文进一步研究了DMFC在恒流放电且供氧不足条件下的电池特性。通过实验监测单电池电压随空气流量下降而变化的规律，本文提出可以将空气流量分为三个区间：在区间1(高流量)，电压基本不随空气流量变化；而在区间2(中等流量)，电压随着空气流量的下降而显著降低，且降低速率逐渐加快；在区间3(极低流量)，电池电压变为负值。通过数值模拟，本文系统阐述了上述三个空气流量区间内电池的性能特性。在区间1，电池工作正常，无寄生反应发生；在区间2，氧气在电池下游发生局部缺乏，引起阳极的局部析氢现象。同时，随着空气流量在该区间内的下降，进口区域的电流密度显著上升，引起阳极侧较高的电极电势，进而导致阳极催化层中催化剂Ru的脱落；在流量区间3，析氢现象转而发生在阴极下游，且析出的氢气可以扩散至电池上游并发生氧化反应，导致进口区域阴极催化层中同时存在氧气还原、甲醇氧化和氢气氧化反应。
[Abstract]:Proton exchange membrane fuel cell (PEMFC) is considered to be one of the most promising power sources for electric vehicles, fixed power stations and mobile devices because of its advantages of low temperature, high efficiency and no pollution. However, the insufficient durability of the battery is an important reason for restricting the large-scale commercial development of PEMFC. Among the factors of long performance, the lack of reactants is one of the most important reasons, which can cause a variety of parasitic reactions within the battery, such as the oxidation of the catalyst carbon carrier, the dissolution and exfoliation of Pt and Ru, and the precipitation of hydrogen, which greatly affect the battery life. However, most of the previous studies in this field have been concentrated on the improvement of electrode materials. As well as the optimization of operating conditions, few scholars have studied the nature of the mass transfer from the point of view of mass transfer. In view of this, a variety of parasitic reactions caused by the lack of reactants in PEMFC are systematically studied by the combination of numerical simulation and experimental research. The transmission mechanism of the reactant in the battery and the spatial distribution of the current and potential are studied. The specific research contents and conclusions are as follows:
The partial lack of cathodic carbon corrosion induced by 1. H2-PEMFC anode hydrogen is studied. By numerical simulation, the partial lack of hydrogen and the carbon carrier oxidation of cathode catalyst caused by water flooding in the anode channel are studied by numerical simulation. The transmission mechanism of hydrogen in the diffusion layer and catalytic layer of water flooded area is analyzed. It is found that the transmission of hydrogen in the water flooded region is divided into the convection control area at the edge and the central diffusion control area at the edge. The convection is caused by the simultaneous consumption of hydrogen and water vapor, while the introduction of nitrogen will greatly weaken the convection and cause the area of the hydrogen deficient region. At the same time, it is found that the proton conduction in the in-plane direction, which the previous scholars usually neglect, has an important effect on the distribution of carbon corrosion rate. Increasing the proton conductivity of the membrane can reduce the area of the region of carbon corrosion. In addition, it is found that the maximum carbon corrosion rate under the high electric pressure of the battery is entirely determined by oxygen. The penetration rate of gas in the membrane, while at low voltage, the carbon corrosion rate is affected by the kinetic characteristics of the anode and the penetration of oxygen in the membrane.
The study on the voltage reversal and anode carbon corrosion of the battery induced by the total lack of hydrogen in 2. H2-PEMFC constant current discharge is studied in this paper. The spatial distribution of the reactant concentration, current density and electrode potential in the battery is not sufficient to maintain the external current demand. In addition, this paper emphasizes the inhomogeneity of the distribution along the thickness direction of the catalytic layer. It is found that in this case, most of the hydrogen supply is consumed in a short area of the inlet of the anode, causing high current density in the imported region and causing local "hydrogen pump" in the import region. This phenomenon is the first report in this paper. In addition, although the thickness of the cathode catalytic layer is only 10 mu m, the high current density in the inlet region will cause a significant change in the direction of the ion phase potential along the thickness of the cathode catalytic layer, which leads to the hydrogen evolution reaction in the direction of the thickness, hydrogen oxidation and oxygen reduction, which greatly increases the complexity of the transmission phenomenon in the region.
The study of hydrogen evolution induced by the lack of oxygen induced by oxygen in 3. direct methanol fuel cell (DMFC). This part first uses numerical simulation to study the phenomenon of the anodic hydrogen evolution caused by the decrease of air flow in the open circuit of DMFC. The results agree with the experimental conclusions of previous scholars. The simulation results show that the DMFC is in the open state and the oxygen supply is insufficient. It is divided into the upstream battery area and the downstream electrolytic cell (hydrogen evolution) area. By studying the transfer phenomena near the interface of the two regions, it is found that the sharp rise of the ion potential at the interface causes the significant conduction of the proton along the in-plane direction, which leads to the existence of a local DMFC. near the interface. The effect of different degree of water flooding on the distribution of current density in the battery has been studied. It is found that even under the same air flow, different levels of water flooding will cause significant changes in the distribution of current density. This also explains why the previous experimental studies of Kulikovsky and so on were obtained under the same test conditions. To a variety of significant differences in current distribution, this paper further studies the characteristics of DMFC under constant current discharge and insufficient oxygen supply. Through the experimental monitoring of the variation of single cell voltage with the decrease of air flow, this paper proposes that the air flow can be divided into three intervals: in the interval 1 (Gao Liuliang), the voltage is basic. In the interval 2 (medium flow), the voltage decreases significantly with the decrease of air flow, and the reduction rate increases gradually; the battery voltage becomes negative in the interval 3 (extremely low flow). By numerical simulation, the performance characteristics of the battery in the above three air flow intervals are described in this paper. At interval 1, battery workers As normal, no parasitic reaction occurs; in the interval 2, the partial lack of oxygen in the lower reaches of the battery causes the local hydrogen evolution of the anode. At the same time, with the decrease of the air flow in this area, the current density in the inlet region rises significantly, causing a higher electrode potential on the anode side, which leads to the loss of the catalyst Ru in the anode catalytic layer. In the flow interval of 3, the phenomenon of hydrogen evolution occurs at the downstream of the cathode, and the precipitated hydrogen can spread to the upper reaches of the battery and oxidize, which leads to the simultaneous reduction of oxygen, methanol oxidation and hydrogen oxidation in the cathode catalytic layer of the imported regions.
【学位授予单位】：上海交通大学
【学位级别】：博士
【学位授予年份】：2014
【分类号】：TM911.4

【共引文献】