部分氧化碳化硅提高钯催化氧还原反应性能

燃料电池具有能量转换效率高的优点,是能量转换与储存的高效器件之一.目前,燃料电池阴极氧还原反应(ORR)动力学缓慢,并且催化ORR大量使用铂碳(Pt/C)催化剂,由于Pt储量少,价格高,载体碳材料易发生碳蚀导致催化剂稳定性降低,限制了其进一步商业化应用.钯(Pd)与Pt为同族元素,具有相似的电子结构和化学性质,其储量是Pt的50倍,同时,Pd具有良好的抗甲醇毒性和抗一氧化碳毒性,因此,被视为燃料电池中阴极Pt催化剂的潜在替代品.但商用Pd/C催化剂的ORR活性较Pt/C差,因此,大量的研究工作集中在提高Pd基ORR催化剂的活性方面:将Pd与具有3d轨道的金属形成合金或将Pd负载到不同的载体上.通过选择合适的载体影响Pd的电子结构,从而提高催化剂活性和稳定性,是一种较简单的、有利于规模化生产Pd基ORR催化剂的方法.碳化硅(SiC)具有良好的电化学稳定性、热稳定性、机械强度和较强的供电子能力,可被用作ORR的金属催化剂载体.然而,由于金属与SiC作用较弱,需要制备特殊形貌的SiC或将SiC表面改性;通常,这些SiC基载体的制备过程复杂并且成本高.而在有氧条件下制备、保存或使用SiC时,其表面不可避免地被氧化,这种在温和条件下生成的表面具有含氧官能团的SiC,由于制备过程简便,可以大规模生产,且与金属有强的相互作用,是一种很有前景的ORR的Pd基催化剂载体.对于用于替代Pt基催化剂的负载型Pd基ORR催化剂的开发和大规模制造来说,对载体表面改性的深入了解是一个重要并且具有挑战性的课题.目前尚未发现关于SiC表面的含氧基团对ORR性能影响的报道.因此,详细考察SiC载体上含氧基团在ORR中的作用对于理解、设计和开发具有优异ORR性能的SiC负载催化剂至关重要.本文采用沉积沉淀法在表面部分氧化的碳化硅(O-SiC)均匀负载了平均直径为5.2 nm的Pd纳米颗粒.与20 wt%商业Pt/C相比,制备的2.5 wt%Pd/O-SiC催化剂显示出较好的ORR活性(半波电位正向移动10 mV),较好的稳定性(10 h后,电流密度损失3.5%vs.34.9%),和较高的抗甲醇毒性.结构表征及密度泛函理论(DFT)计算结果表明,与Pd/C相比,Pd/O-SiC具有优异的ORR性能主要是由于O-SiC载体对Pd纳米颗粒具有电子调控作用,使Pd带负电.富电子Pd增强了ORR关键中间体OOH的吸附,降低了反应的吉布斯自由能,从而提高了ORR活性.另外,O-SiC载体对Pd纳米颗粒具有大的结合能和较好的SiC稳定性,增强了Pd/O-SiC催化剂的抗甲醇毒性及稳定性.DFT计算结果表明,SiC表面部分氧化后,仍保持对Pd的较高结合能,同时大幅增强了催化剂对中间体的吸附,降低了ORR关键电化学步骤吉布斯自由能,从而提高了氧还原性能.因此,本工作明确了SiC表面氧化的作用,同时提供了一种简易大规模制备高效负载型铂基替代ORR催化剂的策略.

Enhanced oxygen reduction reaction performance over Pd catalysts by oxygen-surface-modified SiC

Obtaining a detailed understanding of the surface modification of supports is crucial; however, it is a challenging task for the development and large-scale fabrication of supported electrocatalysts that can be used as alternatives to Pt-based catalysts for the oxygen reduction reaction (ORR). In this study, commercial silicon carbide (SiC) was modified through surface oxidization (O-SiC) to support the use of Pd nanoparticles (Pd NPs) as electrocatalysts for ORR. The obtained Pd/O-SiC catalysts exhibited better ORR activity, stronger durability, and higher resistance to methanol poisoning than that exhibited by commercial Pt/C. The role of the support in enhancing the ORR performance, especially the oxidization of SiC surfaces, was discussed in detail based on the experimental charac-terizations and density functional theory calculations. The underlying mechanism of the superior ORR performance of Pd/O-SiC catalysts was attributed to the charge transfer from SiCxOy to Pd NPs on the surfaces of SiC and the strong metal–support interactions (SMSIs) between Pd and SiCxOy. The charge transfer enhanced the ORR activity by inducing electron-rich Pd, increased the adsorp-tion of the key intermediate OOH, and decreased the Gibbs free energy of the critical ORR step. Fur-thermore, SMSIs enhanced the ORR stability of the Pd/O-SiC catalyst. This study provided a facile route for designing and developing highly active Pd-based ORR electrocatalysts.