Effect of Stress and Composition on Electrochemical Degradation of Proton Exchange Membrane Fuel Cell (PEMFC) Cathode Catalysts

0931080 VirkarThis项目调查表面应力和组成对质子交换膜（PEM）燃料电池阴极催化剂降解的作用。阴极催化剂活性的损失通过四种主要机制发生：（a）颗粒从碳载体上脱离，（B）附聚和烧结，（c）奥斯特瓦尔德熟化，和（d）Pt在阴极处溶解并在膜中沉淀。膜中的Ostwald熟化和Pt沉淀取决于水性电解质和/或离聚物中的Pt离子浓度（Pt 2+和/或Pt 4+）。Ostwald熟化涉及Pt 2 +/Pt 4+离子通过水/离聚物介质和电子通过碳载体的耦合传输。团聚和烧结涉及Pt 2 +/Pt 4+通过水性电解质/离聚物介质的耦合传输和通过直接颗粒接触的电子传输。在机制（d）中，Pt沉淀通过Pt 2 +/Pt 4+和H2的反应发生。因此，三种机制-团聚/烧结，奥斯特瓦尔德熟化和沉淀的Pt在膜中取决于Pt 2 +/Pt 4+离子浓度。所有增加Pt 2 +/Pt 4+浓度的因素都会增加降解动力学。知识价值：许多因素决定了Pt 2 +/Pt 4+浓度，一些与材料有关，一些与操作条件有关。这项拟议的工作解决了决定Pt 2 +/Pt 4+浓度的基本材料相关性质，如合金系统的热力学和表面应力。感兴趣的基本热力学参数是Pt合金形成的偏摩尔焓。压力的作用也具有深远的意义。首先，已知较小颗粒生长的更大趋势是表面能效应，其本质上是压力对化学势的影响。表面压缩的幅度越大，化学势和降解动力学越大。在纯Pt催化剂中，只有颗粒尺寸决定这种应力。然而，在包含Pt壳和合金或非贵金属核的核-壳催化剂中，存在额外的内聚应力。通过适当选择晶格参数和界面结构，可以降低化学势，从而降低Pt 2+浓度，从而降低降解动力学。这提出研究a）Pt 2 + /Pt 4+浓度和温度对纯Pt、Pt合金和核-壳催化剂的动力学的影响;和B）应力和合金组成对使用电化学技术的Pt的化学势的作用。更广泛的影响：这项研究的结果应该为耐降解阴极的合成提供科学依据。所提出的方法是通用的，并适用于基本上所有的电化学装置，需要使用纳米材料的电极和水/离子介质的存在下。犹他州的大学有一个坚定的承诺，本科和研究生教育，并在提高社会代表性不足的群体的参与。一名本科生和一名研究生将参与这项研究。

0931080VirkarThis project investigates the role of surface stress and composition on cathode catalyst degradation in Proton Exchange Membrane (PEM) fuel cells. Loss of cathode catalyst activity occurs by four main mechanisms: (a) Particle detachment from carbon support, (b) Agglomeration and sintering, (c) Ostwald ripening, and (d) Pt dissolution at cathode and precipitation in the membrane. Ostwald ripening and Pt precipitation in the membrane depend on Pt ion concentration (Pt2+ and/or Pt4+) in aqueous electrolyte and/or ionomer. Ostwald ripening involves coupled transport of Pt2+/Pt4+ ions through aqueous/ionomer medium and electrons through carbon support. Agglomeration and sintering involve coupled transport of Pt2+/Pt4+ through aqueous electrolyte/ionomer medium and electron transport through direct particle contact. In mechanism (d), Pt precipitation occurs by a reaction of Pt2+/Pt4+ and H2. Thus, three mechanisms - agglomeration/sintering, Ostwald ripening and precipitation of Pt in the membrane depend upon Pt2+/Pt4+ ion concentration. All factors which increase Pt2+/Pt4+ concentration will increase degradation kinetics. Intellectual Merits: Many factors determine Pt2+/Pt4+ concentration, some materials related and some related to operating conditions. This proposed work addresses fundamental materials-related properties which determine Pt2+/Pt4+ concentration such as the thermodynamics of alloy systems and surface stress. Fundamental thermodynamic parameter of interest is the partial molar enthalpy of Pt alloy formation. The role of stress is also of profound significance. First, it is known that greater tendency for growth of smaller particles is the surface energy effect, which essentially is the effect of pressure on chemical potential. The greater the magnitude of surface compression, the greater is the chemical potential and degradation kinetics. In pure Pt catalysts, only particle size determines this stress. However, in core-shell catalysts comprising Pt shell and an alloy or non-noble metal core, additional coherency stresses exist. By suitable choices of lattice parameters and interfacial structure, the chemical potential can be reduced thereby reducing Pt2+ concentration and thus reducing degradation kinetics. This proposes to investigate a) the effect of Pt2+ /Pt4+ concentration and temperature on the kinetics of pure Pt, Pt alloy, and core-shell catalysts; and b) the role of stress and alloy composition on the chemical potential of Pt using electrochemical techniques. Broader Impacts: The results of this research should provide a scientific basis for the synthesis of degradation-resistant cathodes. The proposed methodology is general and applicable to essentially all electrochemical devices which require the use of nanosize materials in electrodes and the presence of aqueous/ionic medium. The University of Utah has a strong commitment to undergraduate and graduate education and in enhancing the involvement of socially under-represented groups. One undergraduate student and one graduate student will participate in this research.