基于5d过渡金属的多元合金电催化体系的构筑及其超高活性起源的研究

Proton exchange membrane fuel cell (PEMFC), as one of the most promising clean energies alternative to traditional fossil fuels, can be widely used in the fields of new energy vehicles, electric propulsion ships, portable devices and so on. However, the commercial application of PEMFC is seriously hindered by the low activity and poor durability of the current catalysts like Pt for the oxygen reduction reaction (ORR). In this work, we introduce cheap refractory transition metals (M=W，Ta，Hf) into Pt-Co solid alloy supported on commercial or home-made carbons, aiming to fabricate carbon-supported novel Pt-Co-M ternary alloys with ultrahigh ORR activity and chemical stability, by using a facile one-pot ethylene glycol reduction method. After achieving the co-deposition of the above three metals on the carbon, the effect of the ternary alloy’s types, compositions, and structures on the electrocatalytic properties will be investigated systematically. Note that these refractory 5d transition metals have some unique characteristics, such as strong electronegativity, highly unsaturated 5d orbitals, and high-valency states coordinated with oxygen-containing groups. Based on their unique physical and chemical properties, we will explore the generated orbital hybridization and electron transfer between the 5d metal and platinum or cobalt components in the ternary alloys, which highly correlates with the enhanced ORR performance. The physical origin of the synergetic electro-catalytic activity toward ORR will be discussed in detail. This study provides a new understanding of the structure-property relationship for the as-synthesized Pt-Co-M/C catalysts. Moreover, DFT calculation is also used to study the unique adsorption energy levels and state density of oxygen reduction-produced intermediates on the surface of the alloy at the atomic level. Through studying both active oxygen chemical adsorption and product desorption processes catalyzed by the alloy catalysts, we maybe give some insight into the detail information about the accelerated ORR kinetics, caused by the added 5d metals in the alloy systems. In addition, the practical application of the synthesized ternary alloys in fuel cell vehicles is explored based on the evaluation of single cell operation.

针对当前质子膜燃料电池中氧还原电催化材料活性低、耐久性差等问题，采用乙二醇合成体系，以商业或自制的炭材料为载体，将高熔点的5d过渡族金属（M=W，Ta，Hf等）作为关键第三组元，引入到铂钴二元合金系统中，制备出具有超高活性的Pt-Co-M/C复合电催化材料。在实现多元合金在炭载体表面可控共沉积的基础上，探明合金的种类、成分以及结构与性能之间的对应关系。从5d金属具有的较高电负性、大量的5d空轨道以及高氧化价态等特性为基础和出发点，重点研究此类金属与铂或钴组元之间的轨道杂化和电子配位效应对合金性能的调控作用和机制。采用DFT计算的方法，在原子水平上，研究材料表面各种氧还原反应产物的吸附能及态密度等信息，重点从活性氧分子的化学吸附及反应产物脱附两个方面入手，分析5d金属的嵌入对于大幅促进氧还原反应动力学过程的机制。此外，在单电池评价的基础上，探讨材料在燃料电池汽车等领域中应用的可行性。

针对质子膜燃料电池关键电极材料活性低、耐久性差等问题，以商业或自制炭材料为载体，引入5d过渡族金属等作为关键第三组元，制备出一系列高性能多元合金催化剂，如PtCoW、PtCuCoW等，提出多元合金化协同策略是大幅提高氧还原电催化性能的可行方法。采用同步辐射（XAS）等测试手段，结合密度泛函理论（DFT）计算，在原子水平上，探讨了多元成分、相结构、表面化学状态与性能之间对应关系。首先探索了多元合金不同晶面、表面化学重构对性能的影响规律，发现合金化诱导产生的（111）晶面择优取向、掺杂金属稳定的表面低配位亚结构以及晶格内应力等对活性和稳定性能强化起到关键作用，首次提出痕量金属钨掺杂引起的微合金化效应是大幅提高催化剂活性和稳定性能的可行方法。其次，从单个金属原子、团簇到超晶体等金属聚集态结构角度出发，定量研究其对电催化性能影响规律，建立了合金聚集态结构与性能关系图谱。此外，在分子水平上解析氧还原动力学基础上，建立了合金催化微观动力学模型。首次理论计算得到中熵合金电催化氧还原反应的动力学电流,揭示了外来金属原子配位结构和表面化学重构能够引起合金内部强烈的轨道杂化和电子耦合效应,大幅提高氧还原反应速率。PtCoW等催化剂已初步实现中试生产，在氢氧燃料电池测器件试中，电池峰值功率密度达到2.1W cm-2，比质量功率密度高达到21W mgPt-1，远高于商业Pt/C的。在3万次电化学循环加速破坏测试中，比质量活性仅降低3%。这从根本上解决了二元铂合金电催化活性受到“降低氧分子吸附能力”与“提高还原产物脱附能力”两个相互制约作用造成性能不高的共性难题。相关结果为制备高效催化剂提供了新颖的思路，也为开发下一代适用于质子膜燃料电池的超高活性和耐久性能电极材料的应用提供了理论支持。本项目在执行期内（2019-2022）发表SCI学术论文19篇，申请或授权中国发明专利9件，国际会议报告3次，培养毕业博士/硕士研究生10人以上。