直接甲醇燃料电池阳极动态响应的原位测试及衰减机理分析

Direct methanol fuel cell (DMFC) anode is one of the key materials that control fuel cell's characteristic, service life and costs. Its characteristic is often evaluated in simulated environment. These results gotten from off-line environment can't reflect the real state of anode at operating mode and provide micro-area information of anode. Thus, it is difficult to reveal the process of anode change under operating environment. In order to explain the degradation mechanism of anode, In situ test of DMFC anode is conducted under operating mode. That is, online current data of anode are recorded by Scanning Electrochemical Microscopy (SECM) and Localized Electrochemical Impedance Spectroscopy (LEIS) during loading. The anode changes are actualized in 3D imaging. Also, anode characteristic parameters such as surface electronic structure, size, morphology of catalyst, are also analyzed during this process. Based on these data, the main factors influencing its performance are analyzed. Also, orbital change of catalyst is calculated by density functional theory. Based on these results, the degradation mechanism is revealed at molecular level. These conclusions have an important theoretical and practical significance on selecting catalyst, preparing anode and improving fuel cell performance and service life.

直接甲醇燃料电池（DMFC）阳极是决定电池性能、寿命和成本的关键材料之一。阳极性能常采用模拟环境进行评价，而离线环境下的评价结果既不能真实反映在线工况下的状态，又不能提供电极微小区域的信息，因而很难从本质上揭示实际加载状态下阳极的变化过程。为了解实际工况下电极的性能、分析其衰减机理，本课题创新性的实现DMFC阳极在线状态下的原位测试，即采用扫描电化学显微镜（SECM）和局部交流阻抗（LEIS）原位测试阳极在线状态下的动态响应，并以三维图像展现电极性能的变化。检测电极特征参数（如催化层表面电子结构、尺寸、形貌等）的变化，分析影响其性能的敏感参数。进一步运用量子化学密度泛函理论，计算催化金属原子轨道的变化，从分子水平上揭示电极性能衰减的机理。课题的实施对实现燃料电池关键零部件的在线测试、指导催化剂选择和电极制备、提高电池性能和寿命具有重要的理论和实际意义。

鉴于DMFC阳极的性能评价多采用常规电化学手段如循环伏安、动电位极化等，评价结果均是单位面积上电极性能的平均值，不能反映电极微观性能的变化。本项目采用扫描电化学显微镜和局部交流阻抗技术，研究了DMFC阳极在恒压和变压加载情况下微区形貌和阻抗的变化，再结合常规电化学测试和物理测试，分析了电极性能的衰减。结果表明，DMFC阳极表面催化剂的催化活性呈不均匀峰分布状态。在相同的加载电位下，超过4h后，阳极表面面扫描锯齿状电流峰出现急剧变化，锯齿状电流峰数量和峰电流随加载时间的延长逐渐减少和降低。微区阻抗测试尽管规律并不如扫描电化学表现明显，但变化趋势基本一致。物理分析表明，随加载时间延长及电位增加，催化剂粒径呈增大的趋势，Pt/Ru比增加。如0.6V下加载16h和72h后，催化剂粒径从3.4nm分别增加到3.6nm和4.4nm，增加5.88%和29.41%。0.8V下加载72h后，催化剂中Pt/Ru比由2:1提高到3.9:1。.利用密度泛函理论(DFT)理论，构建了CH3OH和CO在Pt、Ru上的吸附模型，并计算了吸附能和部分态密度（PDOS）。结果表明，尽管CH3OH优先吸附在Pt上，但Pt、Ru催化剂都可离散CH3OH分子，且Ru离散CH3OH分子后，其电子轨道能量变化较小，表现出更佳的稳定性。中间产物CO在Pt（100）面上吸附时以优先吸附C原子，而在Ru（100）面上则优先吸附其中的O原子,由于Pt吸附CO后其Pt5d轨道能量被削弱，导致其易发生腐蚀反应。Ru吸附CO后，其轨道能量变化较小，表现出更好的抗CO稳定性。利用DFT理论进行了非金属催化剂掺杂及性能的模拟计算，实际测试结果印证了模拟计算结果，研究结果对指导DMFC阳极的制备和开发新型的催化剂具有一定的指导意义。