高性能Pd-Pt纳米晶电催化剂的氨络合法制备及构-效关系研究

The cost, activity, durability and poison tolerance of electrocatalyst are involved in the key issues for the commercialization of proton exchange membrane fuel cell. To obtain high performance Pd-Pt nanocrystal electrocatalyst with low preparation cost, this project intends to develop a template-free ammonia complexation method for the synthesis of polyhedral or dendrite-like Pd-Pt nanocrystal electrocatalysts, to bypass the difficult template removal process and to avoid the activity loss from residual-template-adsorption during the template synthesis process. Various high performance polyhedral or dendrite-like Pd-Pt nanocrystals with different fine structures (including the morphology, nanosize, exposed facet, et al.) will be template-free obtained by controlling the nucleation and growth rates of Pd-Pt nanocrystals, which can be achieved by the synergic effect of the complexation between the ammonia and the Pd or Pt ion, the supporting and anchoring of carbon support for nanocrystal, as well as the direction-adsorption of capping agent. The effects of synthesis conditions on the fine strucutre of polyhedral or dendrite-like Pd-Pt nanocrystals will be studied by tuning the surface structure of carbon supports, the Pt/Pd ratios in precursors, the ammonia complexation, the reduction agent and the capping agent. Meanwhile, the influence of fine structure (morphology, nanosize, surface composition) on catalytic properties (activity, stability and poison tolerance) will also be studied by evaluating the catalytic activities, stabilities and poison tolerance of various as-prepared Pd-Pt electrocatalysts for oxygen reduction reaction, to clarify the “structure-performance” correlations, which is beneficial for the further design of the advanced Pd-Pt electrocatalysts on nano-scale.

电催化剂的成本、活性、稳定性及抗毒性均为质子交换膜燃料电池商业化应用的关键影响因素。为提高Pd-Pt纳米晶电催化剂性能并降低制备成本，本项目针对多面体、树枝状等Pd-Pt纳米晶合成过程中面临的模板剂去除困难及残余模板剂吸附造成活性损失等问题，拟采用氨络合法，通过NH3分子与Pd、Pt前驱体的络合作用、碳载体的支撑与“锚定”作用及小分子、阴离子等的定向吸附作用的协同效应，调控金属颗粒成核与生长速度，实现在无模板剂条件下调控Pd-Pt纳米晶的形貌、尺寸、暴露晶面等精细结构，获得高性能Pd-Pt纳米晶催化剂。研究并揭示合成体系中载体表面状态、前躯体Pt/Pd比、氨络合强度、还原剂及定向吸附剂等条件对Pd-Pt纳米晶精细结构的影响机制；同时，研究Pd-Pt纳米晶形貌、尺寸、表面状态等对其催化活性、稳定性及抗毒性的影响, 阐明构-效关系，为从纳米结构层面设计高性能Pd-Pt电催化剂提供理论参考。

为解决低温燃料电池电催化剂成本、活性、抗毒性等问题，本项目采用简单的氨络合法制备碳载多面体Pd-Pt纳米晶电催化剂，通过调变合成体系中金属前躯体的Pt/Pd比、氨络合强度以及碳载体表面状态，控制纳米晶的形貌、尺寸及表面状态，获得了不同精细结构的多面体Pd-Pt纳米晶电催化剂；研究结果表明，利用表面光滑且具有大π键的石墨烯作为载体以及含N小分子氨作为络合剂和高能晶面的保护剂可在无表面活性的条件下获得具有规整多面体结构的Pd-Pt纳米晶电催化剂，避免了表面活性残留对电催化剂活性的毒化作用，得到的Pd6Pt/C电催化剂在Pt含量远低于商业化Pt/C的条件下表现出相近的ORR电催化活性及优异的抗甲醇毒化性能，具有良好的应用前景；项目探究了合成条件对Pd-Pt纳米晶结构的影响机制，结果表明，石墨烯大π键对多面体边、角等高能位置的电子缓冲稳定作用、氨分子络合对Pd、Pt前驱体还原速度的调控作用以及氨分子在Pd-Pt纳米晶高能位置的吸附保护作用是多面体结构形成的关键。项目采用CV、LSV等电化学测试评价技术考察不同结构Pd-Pt纳米晶的氧还原反应（ORR）电催化活性、抗甲醇毒化性能，探究了多面体Pd-Pt纳米晶催化剂结构与性能之间的构-效关系，结果表明，多面体Pd-Pt纳米晶暴露的高活性（111）晶面、丰富的边、角缺陷位点是多面体Pd-Pt纳米晶电催化剂获得高活性的原因，这些研究结果为进一步从纳米结构层面设计高性能电催化剂提供理论依据。