去合金技术制备氧还原电催化剂

【作者】 张克；

【导师】 刘继锋；

【作者基本信息】 聊城大学 ， 分析化学， 2011， 硕士

【摘要】 氧还原反应（ORR）在聚合物电解质燃料电池和质子交换膜燃料电池等燃料电池中发挥着重要的作用。氧还原的电催化剂包括纳米颗粒、大环及裂解产物、碳、硫族化合物、酶和相应的复合物等,它们都已经有所报道。在这些催化剂中Pt及Pt合金纳米颗粒是使用最广泛的氧还原催化剂,但是由于它资源匮乏、价格昂贵,因此在实际应用中受到很大的限制。因此对于研究和开发燃料电池的最大的挑战就是开发具有Pt的高活性及低成本的电催化剂。本文具体工作如下:1.以石墨烯为载体的Pt-Co纳米颗粒在碱性溶液氧还原反应中产生羟基自由基的研究以石墨烯为载体的Pt-Co纳米催化剂的电催化氧还原活性,和纯Pt相比合金催化剂显示出较高的氧还原活性。合金的组分影响氧还原的活性及速率常数、过氧化氢百分率、电子转移数。Pt:Co原子比从1:0.25到1:1时氧还原活性、速率常数、过氧化氢百分率随着Co原子含量的增加而升高,当Co含量高于50%时氧还原活性、速率常数、过氧化氢百分率减小。因此我们得到了催化剂的组分与氧还原的活性呈“火山型”的关系。在碱性溶液中氧还原过程中产生OH自由基,同时我们也发现催化剂的组分与OH自由基的产量也呈“火山型”的关系,其中Pt1Co1产生的OH自由基最多。该工作将为燃料电池新型催化剂的开发和利用提供有力的理论及实验依据,并为催化剂的改性和可控性地产生活性氧提供新思路、新方法。2.去合金的以石墨烯为载体的PtNi合金在酸碱溶液中对氧还原的影响在燃料电池中溶解过渡金属是一个不可避免的过程,在这项工作中主要研究了Pt₁Ni_x （x=0、0.25、0.5、1和2）在酸性电解液及碱性电解液中的氧还原活性及在碱性电解质中氧还原产生OH自由基。合金通过浸渍-液相还原方法得到,合金的电子和结构特性通过X-射线光电子能谱（XPS）、X-射线衍射（XRD）、透射电镜（TEM）及高分辨透射电镜（HRTEM）来表征,催化剂的ORR的性能通过循环伏安法（CV）和旋转环盘电极技术（RRDE）来表征,实验中发现无论是在酸性介质还是在碱性介质合金的氧还原活性总是高于纯Pt催化剂,氧还原过程中我们还发现去合金的PtNi催化剂比去合金前氧还原活性增大。在不同原子比的合金催化剂中Pt₁Ni₁@graphene始终具有较高的氧还原活性。在碱性溶液中过氧化氢中间产物在合金表面分解生成OH自由基,其检测通过对苯二甲酸做为捕获剂,通过荧光技术在线检测,其中Pt₁Ni₁@graphene产生的OH自由基最多。这对ORR催化剂的设计或许有很大的帮助。更多还原

【Abstract】 Oxygen reduction reaction （ORR） plays an important role in fuel cells, such as polymer electrolyte fuel cells and proton exchange membrane fuel cells. Electrocatalysts including nanoparticles （NPs）, macrocycles and pyrolysis products, carbons, chalcogenides, enzymes, and coordination complexes have been reported for ORR use and reviewed recently. Comparatively, among the reported electrocatalysts, NPs of platinum and platinum based alloy are the most widely used and studied catalysts. However, because of the high cost and low abundance of Pt, practical use and introduction of fuel cells into markets are limited. So the most critical challenges for the study and application of fuel cells have been to develop alternative electrocatalysts that will retain the high activity of Pt while lowering the cost.The main works are as follows:1. Generation of OH Radicals in Oxygen Reduction Reaction at Pt-Co Nanoparticles Supported on Graphene in Alkanline SolutionsPt-Co alloy nanoparticles supported graphene were studied for electrocatalytic oxygen reduction. The alloy catalysts showed higher ORR activity than pure Pt catalysts. The compositon of the alloy affected had effects on ORR activities and rate constants, proportion of hydroperoxide, number of electron transfer. In the region of 1:0.25 to 1:1 of the Pt:Co atomic ratio, ORR activities and rate constants, proportion of hydroperoxide increased with increasing Co content, but number of electron transfer decreased. When Co content was higher than 50 at.%, ORR activities and rate constants, proportion of hydroperoxide decreased. Thus, obtaining a volcano dependence of ORR activity upon alloy composition. OH radicals was found during ORR at alloy catalyst in alkaline solutions and the generation also had a volcano dependence of ORR activity upon alloy composition with Pt1Co1 alloy giving highest quantity of OH radicals. This study may be helpful to designing ORR alloy catalysts with higher activity but low yield of OH radicals, because OH radicals are detrimental to the membrane electrode assembly of fuel cells.2. Effects of Acid Treatment of Pt-Ni Alloy Nanoparticles@Graphene on the Kinetics of the Oxygen Reduction Reaction in Acidic and Alkaline SolutionsAcidic dissolution of transition metals from Pt based alloy catalysts for oxygen reduction reaction （ORR） is an unavoidable process during fuel cell operation. In this work we studied effect of acid treatment of graphene-supported Pt₁Ni_x （x=0, 0.25, 0.5, 1 and 2） alloys on the kinetics of the ORR in both alkaline and acidic solutions together with the generation of OH radicals in alkaline solutions. The alloy nanoparticles were synthesized through co-impregnation and chemical reduction. The electronic and structural features of the alloy were characterized by X-ray photoelectron spectroscopy, X-ray diffraction, transmission electron microscopy and high-resolution transmission electron microscopy. The ORR performances were studied using cyclic voltammetry and rotating ring disk electrode techniques in 0.05 M H₂SO₄ and 0.1 M NaOH, respectively. The alloy catalysts were more active than pure Pt toward ORR, and after acid treatment the ORR activity of Pt-Ni alloy was enhanced in both acidic and alkaline media. The maximum activity of the Pt-based catalysts was found with ca. 50 at.% Ni content in the alloys （Pt₁Ni₁@graphene）. OH radicals were generated through dissociation of hydroperoxide at the catalysts’surface and detected by fluorescence technique using terephthalic acid as capture reagent, which readily reacts with OH radical to produce highly fluorescent product, 2-hydroxyterephthalic acid. More OH radicals were found to be generated at Pt1Ni1@graphene catalyst. This work may be valuable to design ORR electrocatalysts with higher ORR activity but lower efficiency of OH radical generation.更多还原