Cu-Pt纳米颗粒去合金化过程中特征结构形成与演化的原子模拟

Electrocatalyst is one of the key materials in low-temperature fuel cells, which determines the performance, lifetime and cost of the fuel cells. Very recently, Pt based core-shell structured electrocatalysts have attracted increasing attention due to their unique advantages in reducing Pt loading and improving the oxygen reduction activity. Electrochemically dealloying is the most promising method for synthesis of Pt based core-shell nanocatalysts. While the durability of dealloyed Pt alloys is tested under voltage-cycling conditions. The oxygen reduction activity and durability of Pt based nanocatalyst are strongly influenced by the formation and evolution of the characteristic structures during dealloying process, thus the dealloying mechanism is the key to optimizing electrocatalyst. Dealloying behavior refers to the selective dissolution of the non-noble metal and the surface diffusion of Pt. The dissolution or diffusion barriers are closely related to potentials and surface binding energy, which is local coordination-dependent and related to the morphology, composition, and chemical ordering of nanoparticle. Most of these factors can hardly be measured by experimental means, so that there is still a lack of deep understanding of dealloying mechanism. In this project, the embedded atom method will be use to calculate the surface binding energy, the kinetic Monte Carlo method to simulate the formation and evolution of characteristic structures of Cu-Pt nanoparticles during dealloying process. Our purposes here are threefold: to reveal the dealloying mechanism of Cu-Pt nanocatalysts, to bridge the gap between the microstructure evolution and experimental polarization curves, to provide a theoretical basis for the design of Pt-based electrocatalyst.

电催化剂是决定燃料电池性能、寿命和成本的关键材料之一。去合金化制备的铂基纳米催化剂因其低铂载量和高氧还原活性受到重点关注。铂基纳米催化剂的高氧还原活性与去合金化过程中特征结构的形成密不可分，但长时间去合金化处理中非贵金属的流失会导致特征结构退化。去合金化过程中特征结构形成与演化的微观过程、物理机制、影响因素等问题的解决是调控铂基纳米催化剂结构与性能的关键。去合金化行为本质上是非贵金属的选择性溶解和铂的表面扩散。由于电位、颗粒形貌、成分、化学有序度等因素或独立或协同地影响表面原子溶解或扩散能垒，仅采用实验方法难以揭示去合金化动态过程和微观机理。本项目拟采用嵌入原子多体势计算原子表面结合能，利用动力学蒙特卡洛方法模拟铜铂颗粒去合金化过程中特征结构的形成与演化及其影响因素以揭示纳米尺度去合金化机理，建立微观结构演化与极化曲线、氧还原活性面积之间的联系，为设计铂基纳米电催化剂提供理论基础。

构筑“低铂核—铂壳”结构阴极催化剂可极大的减少燃料电池铂载量，提高其ORR比活性。深入理解去合金化过程中“低铂核—铂壳”结构的形成与演化机制及其影响因素，是设计低铂、耐久、高活性ORR催化剂的关键。本项目基于原子模拟程序库：构建了包括Cu-Pt在内的多个合金体系MAEAM势并计算了体相的基本物性；获得了Cu-Pt粒子在不同合金组分、形貌、尺寸、温度和电位条件下的成分、有序相、应力、应变与势能分布；模拟了恒电位下不同尺寸、成分和初始结构的Cu-Pt粒子的演化过程，包括铜快速溶解与缓慢再刻蚀，表面铂富集、表面迁移、骨架形成及平整化。.原子模拟结果表明：单晶纳米粒子表面Cu富集和核内有序均为一般规律且相互竞争，而多壳层结构的形成体现了二者的精妙协同；多重孪晶结构本征的不均匀和各向异性的应变/应力分布可诱导成分沿径向或等价位重排，为实现纳米合金的成分与结构调控提供了新思路。对去合金化过程的模拟表明：电位、形貌、组分、初始成分分布等因素直接改变原子结合能，从而间接影响临界溶解电位、去合金化组分阈值、选择性溶解速率、表面扩散速率，最终决定微观结构演变过程。对于固溶式初始结构，其临界溶解电位与成分存在反火山关系，粒子最终形成八面体（小尺寸）或凹多面体（大尺寸）的铂壳多壳层结构。以铜壳多壳层结构为前驱体，则得到铂利用率约80%、高电化学稳定性、表面铂处于约2%-4%压应变的多层铂壳—富铜核结构。通过DFT计算应变条件下Pt@Cu@Pt(111) 模型合金表面（简化的多层铂壳—富铜核结构）的ORR反应特征能量与可能路径。结果表明，1.5%-4%压应变的模型合金表面可避开高活化能的O2解离路径，以OOH 或H2O2解离路径进行ORR反应并获得较高的活性。此外，将二元合金共近邻分析方法拓展至第二近邻并程序化，可用于定量识别块体和纳米结构中8种典型有序相，对于研究相变等微结构演化过程意义重大。