高活性和高稳定性的核壳结构PtPx@Pt/C催化氧还原

在质子交换膜燃料电池中,金属铂是最高效的阴极氧还原催化剂之一,但是铂昂贵的价格严重阻碍了其在燃料电池领域中的大规模商业化应用.通过铂与3d过渡金属(Fe、Co和Ni)合金化可以有效提高催化剂的氧还原活性,然而在实际的高腐蚀性、高电压和高温的燃料电池运行环境中,铂合金纳米粒子易发生溶解、迁移和团聚,从而导致催化剂耐久性差.同时过渡金属离子的溶出会影响质子交换膜的质子传导,并且一些过渡金属离子会催化芬顿反应,产生高腐蚀性?OH自由基,加快Nafion和催化剂的劣化.与过渡金属掺杂相比,非金属掺杂具有明显优势:一方面,非金属溶出产生的阴离子不会取代Nafion中的质子,也不会催化芬顿反应;另一方面,与3d过渡金属相比,非金属具有更高的电负性,其掺杂很容易调节Pt的电子结构.因此,本文通过非金属磷掺杂合成具有优异稳定性的核壳结构PtPx@Pt/C氧还原催化剂.通过热处理磷化商业碳载铂形成磷化铂(PtP2),经由酸洗处理产生富铂壳层,即PtPx@Pt/C.X射线粉末多晶衍射结果证明了PtP2相的存在,并且进一步通过电子能量损失谱对纳米粒子进行微区面扫描分析以及X射线光电子能谱分析证实了富铂壳层的存在,壳层厚度约1 nm.得益于核壳结构及磷掺杂引起的电子结构效应,PtP1.4@Pt/C催化剂在0.90 V(RHE)时的面积活性(0.62 mA cm–2)与质量活性(0.31 mAμgPt–1)分别是商业Pt/C的2.8倍和2.1倍.更重要的是,在加速耐久性测试中,PtP1.4@Pt/C催化剂在30000圈电位循环后质量活性仅衰减6％,在90000圈电位循环后仅衰减25％;而商业Pt/C催化剂在30000圈电位循环后就衰减46％.PtP1.4@Pt/C催化剂高活性与高稳定性主要归功于核壳结构、磷掺杂引起的电子结构效应以及磷掺杂增加了碳载体对催化剂粒子的锚定作用进而阻止了其迁移团聚.综上所述,本文为设计同时具有优异活性与稳定性非金属掺杂Pt基氧还原催化剂提供新的思路.

High activity and durability of carbon-supported core-shell PtPx@Pt/C catalyst for oxygen reduction reaction

Alloying Pt with transition metals can significantly improve the catalytic properties for the oxygen reduction reaction (ORR). However, the application of Pt-transition metal alloys in fuel cells is largely limited by poor long-term durability because transition metals can easily leach. In this study, we developed a nonmetallic doping approach and prepared a P-doped Pt catalyst with excellent durability for the ORR. Carbon-supported core-shell nanoparticles with a P-doped Pt core and Pt shell (denoted as PtPx@Pt/C) were synthesized via heat-treatment phosphorization of commercial Pt/C, followed by acid etching. Compositional analysis using electron energy loss spectroscopy and X-ray photoelectron spectroscopy clearly demonstrated that Pt was enriched in the near-surface region (approximately 1 nm) of the carbon-supported core-shell nanoparticles. Owning to P doping, the ORR specific activity and mass activity of the PtP1.4@Pt/C catalyst were as high as 0.62 mA cm–2 and 0.31 mAμgPt–1, respectively, at 0.90 V, and they were enhanced by 2.8 and 2.1 times, respec-tively, in comparison with the Pt/C catalyst. More importantly, PtP1.4@Pt/C exhibited superior sta-bility with negligible mass activity loss (6％after 30000 potential cycles and 25％after 90000 po-tential cycles), while Pt/C lost 46％mass activity after 30000 potential cycles. The high ORR activity and durability were mainly attributed to the core-shell nanostructure, the electronic structure ef-fect, and the resistance of Pt nanoparticles against aggregation, which originated from the enhanced ability of the PtP1.4@Pt to anchor to the carbon support. This study provides a new approach for constructing nonmetal-doped Pt-based catalysts with excellent activity and durability for the ORR.