还原性气体诱导铂基合金高活性晶面暴露及其氧还原机理研究

Developing highly active and cost-effective electrocatalysts is one of the most important issues for the commercialization of fuel cells. In this project, the controllable preparation of high efficiency Pt-based oxygen reduction catalyst is achieved by reductive gas inducing the specific structure (nanowires, nanoframes, nanopolyhedrons and etc.) Pt alloys. Highly active oxygen reduction active site was exposed owing to the atomic structure on the surface of the alloy catalysts was rearranged by reductive gas inducing. Aberration-corrected scanning electron microscopy and X-ray absorption structure spectroscopy were used to characterize the crystal structure of the material surface in detail. The key factors that affect the arrangement of surface atoms were further studied and the structural change mechanism was explained. The interaction among the structure, compositions and activity, including ORR activity and the performance of Membrane electrode assembly, are carefully studied in order to reveal the structure-activity relationship, simultaneously with the assistance of the theoretical calculation that the mechanism of the catalytic performance improvement of the prepared materials can be demonstrated based on the comprehensive analysis. This project has an importantly theoretical and practical significance for re-understanding and developing high efficiency low platinum alloy catalyst from the interface perspective and promoting its practical application in proton exchange membrane fuel cell.

开发高效、低廉氧还原催化剂是实现燃料电池商业化的关键。本项目拟在特定结构（纳米线、纳米框架和纳米多面体等）铂基合金催化剂的基础上，利用还原性气体诱导，使合金催化剂表面原子结构发生重组，暴露出更高活性氧还原（ORR）催化位点，实现高效铂基催化剂的可控制备。结合球差校正扫描透射电子显微镜和X射线吸收谱等技术，对材料表面晶体结构进行精细表征，深入探究影响催化剂表面原子排布的关键因素，揭示其结构变化机理。同时，研究催化剂结构、组成与性能（包括ORR性能和膜电极性能）间的关系。结合理论计算，阐释性能改善机理，为更高层次低铂催化剂的发展指明方向。本项目对于从界面角度重新认识和发展高效低铂合金催化剂，推动其在质子交换膜燃料电池中的实际应用，具有重要的理论和实践意义。

燃料电池作为一种环保高效的能源转换技术受到了广泛的关注。然而，阴极缓慢的氧还原反应(ORR)需要大量的Pt催化剂，其储量稀缺和价格昂贵是燃料电池全面普及的巨大障碍。氧还原反应发生在催化剂表面，在原子尺度上对其表面组分的精准控制是有效提高催化剂性能、降低催化剂用量的重要手段。本项目实施过程中，首先我们利用溶剂热法合成了一系列特定结构铂基合金催化剂，经电化学性能测试筛选出性能最为优异的铂基合金纳米线；随后在特定氛围下对催化剂进行程序升温热处理，对其表层结构和组成进行细致调控；最后对得到的催化剂进行结构表征和氧还原性能测试，结合理论计算，阐释性能改善机理。研究结果表明：在不同的气氛中处理后催化剂表层的原子排列和组成发生了显著的变化，与PtNi/C NWs相比，PtNi/C-H2 NWs和PtNi/C-NH3的表层Pt/Ni原子比分别为4.1:1和3.7:1，镍的表层含量都有显著提升。我们推测这主要是由于催化剂在加热过程中，催化剂表层各元素的分布状况发生了重组。独特合金纳米线结构和表面组成使PtNi-NH3 NWs质量活性和比活性分别达到了1.02 A mgPt-1和3.86 mA cm-2，这一指标分别是商用JM Pt/C催化剂的5.7倍和11.4倍。值得注意的是，该催化剂还表现出极好的耐久性，即使在10,000次持续循环伏安扫描后也没有显著的活性衰减。此外，PtNi-NH3 NWs催化剂在室温和环境压力下的PEMFC中与JM Pt/C相比也显示出更好的全电池性能。0.8V时的电流密度达到了308 mA cm-2，这一指标高于美国能源部制定的2020行业标准。因此，我们通过构建以Pt为壳、表面原子排布可控的PtNi合金纳米线(NWs)得到了兼具高活性和稳定性的催化剂。根据实验结果和理论计算分析最终得出，PtNi/C-NH3 NWs突出的氧还原性能主要来源于其独特的结构、组成和恰当的ΔEO值。本项目的研究为提升合金纳米材料电催化活性提供了新的视角和方法，具有重要的理论和实践意义。