Pd/Pt二元合金电极的制备及氧还原性能

本文利用欠电位沉积亚单层的Cu及Pt置换取代Cu的方法, 制备了具有不同表面元素组成的Pd/Pt二元合金电极(用Pd/Ptx表示, x指欠电位沉积Cu-Pt置换取代Cu过程的次数)，并对其表面元素组成、氧还原性能进行了表征. 在控制欠电位沉积Cu的下限电位恒定(0.34 V)的前提下, 表面Pt/Pd的元素组成比通过重复欠电位沉积Cu及Pt置换取代Cu的次数(1~5次)来可控地调变. 光电子能谱(XPS) 以及红外光谱实验表明，Pd/Ptx电极表层区的Pt：Pd元素组成比随着Pt沉积次数增加而增加, 对Pd/Pt4电极, 在电极表层区约2~3 nm内的Pt/Pd的原子比大约是1:4，而最表层裸露Pd原子的比例仍在20%以上。循环伏安结果显示, 随着Pt沉积次数的增加(1-5次), Pd/Ptx电极表面越不易被氧化。氧还原测试结果显示随着Pt沉积次数的增加(1~4次), Pd/Ptx二元金属电极的氧还原活性依次增加, 经过第3次沉积后其氧还原活性已优于纯Pt，而经4次以上沉积，其氧还原活性基本不变。在其它反应条件相同条件的前提下, Pd/Pt4电极上氧还原的半波电位与纯Pt相比右移约25 mV。结合本文与文献的实验结果，我们初步认为Pd/Ptx二元金属体系氧还原性能改善主要源自表层Pd原子导致其邻近的Pt原子上含氧物种吸附能的降低.

Preparation of Pd/Pt Bimetallic Electrodes and Its Activity Toward Oxygen Reduction Reaction

Pd/Pt bimetallic electrodes are prepared using under-potential deposition (UPD) of Cu following with galvanic displacement of Cu by Pt(denoted as Pd/Ptx, here x denoted the cycle No. of UPD Cu and the subsequent Pd-Cu displacement processes). The atomic ratios of Pt:Pd in the surface region can be tuned by holding the low potential limit for Cu UPD at 0.34 V, and tuning the cycle No. of UPD Cu and Pd-Cu displacement processes. The surface composition of Pd/Pt bimetallic electrodes were examined by x-ray photon-electron spectroscopy (XPS) and electrochemical infrared spectroscopy (EC-IRS) using CO as a probe. And the activity of Pd/Ptx bimetallic electrodes toward oxygen reduction reaction (ORR) have been studied using cyclic voltammetry (CV) in a flow cell. XPS and EC-IRS demonstrate that the atomic ratios of Pt:Pd increase monotonically with the cycle No. of Cu-upd and Pt-Cu displacement. For Pd/Pt4 electrode, the atomic ratios of Pt:Pd in the region of ca. 2~3 nm from the surface is 1:4, while Pt:Pd ratio at the surface is ca. 5:1. With the increase in Pt ratio at the surface, Pd/Ptx electrodes become more and more oxidation resistant. The ORR activity increase with x up to 4 and it remains unchanged at x?4. For Pd/Ptx electrode (with x>3), its ORR activity is higher than that of polycrystalline(pc) Pt. Under otherwise identical condition, the half wave potential for ORR at Pd/Pt4 electrode is about 25 mV more positive than that at pc-Pt electrode. The enhancement in ORR activity of Pd/Ptx bimetallic electrodes is attributed to the reduction of adsorption energy of O containing species on Pt atoms induced by adjacent Pd atoms at the surface or by Pd atoms under the surface Pt layer.