铁在催化活性位中的作用 - FeN/C与CNx的活性位结构对氧还原活性和稳定性的影响研究

Non-precious metal catalysts (NPMCs) have attracted great attention due to their low cost and high activity toward oxygen reduction reaction (ORR). Recent studies showed FeN/C catalysts have high initial ORR activity close to the level of platinum based catalysts, However, it demonstrated poor durability. In parallel, CNx showed good durability but poor initial activity. These two types of catalysts have different active site structure and the role of Fe in active sites is the key parameter to find the differences between FeN/C and CNx, which the key factor to prepare ORR catalysts with high activity and stability. In this project, we will design and prepare NPMCs with high activity and durability for ORR in proton exchange membrane fuel cells (PEMFCs). The proposed work will undertake an investigation of the nature of active sites, and the degradation mechanism for ORR in NPMCs using fundamental approaches based on catalysis and electrochemistry. The main objective is arriving at a molecular level understanding of the electrocatalytic phenomena involved in the ORR over NPMCs. The nature of the active sites and the ways to create these active sites for ORR will be investigated in an effort to control electrocatalytic properties by molecular tailoring of the catalyst nanostructure and its surface moieties. Catalysts will be characterized using an array of techniques to acquire information about their physical characteristics (surface area, pore size distribution, particle diameter, crystal structure, crystal plane exposure, defects, etc.), and structural and molecular characteristics. Activity and durability measurements will be performed using kinds of electrochemical techniques. Experimental work will be closely coupled with modeling studies to provide a molecular-level understanding of the factors that contribute to the activity and durability in NPMCs for ORR. The practical relevance of the works in this project lies in the need for developing NPMCs for ORR in PEMFCs and the long-term objectives include building a knowledge base that would facilitate the production of active, stable and inexpensive cathode electrocatalysts that would realize the wide-spread application of fuel cells for energy generation.

低价、高效的氧还原（ORR）非贵金属催化剂（NPMCs）是质子交换膜燃料电池研究的热点。FeN/C已在活性上取得突破，但稳定性仍是难点；CNx有较好的稳定性但活性较差。这两类催化剂的催化活性位有不同结构，铁在其中的作用是解开催化剂ORR活性与稳定性差异的关键。项目将在制备FeN/C与CNx催化剂的基础上，对催化剂各组分前躯体及制备方法进行考察，优化制备方法；利用电化学方法测试催化剂的ORR活性和稳定性，分析其ORR动力学特性；利用探针分子或毒物分子（CO，氰化物，硫化物等）研究活性位中铁的价态和存在形态；利用物性和光谱表征方法（如TGA, BET, XRD, XPS, TEM, Mossbauer谱等）对催化剂的结构进行表征；结合电化学与物性和光谱表征结果，探索研究催化剂中催化活性位的组成、结构与活性、稳定性之间的关系，为高稳定、高活性的NPMCs的设计制备提供理论指导。

非贵金属催化剂（NPMCs）是目前质子交换膜燃料电池研究的热点之一，研究催化剂前驱体的组成及制备过程对获得高活性、高稳定性的氧还原（ORR）催化剂至关重要。项目针对NPMCs前驱体中氮源、铁源、碳载体、氮源含量、铁含量以及热处理条件等对催化剂ORR的性能影响进行了系统研究。使用比表面积和孔径分布测试（BET），热重分析（TGA），透射电子显微镜(TEM)，X射线衍射（XRD），X射线光电子能谱（XPS）等方法对催化剂及催化剂前驱体的结构及物性等进行表征，探索催化剂结构与氧还原性能之间的关系。主要结果如下：.1. 催化剂前驱体中氮的种类、含量以及铁源中铁的价态对在热解过程中形成的催化剂形貌、粒径、比表面积及表面氮元素含量等有着较明显的影响，当前驱体中1,10-菲啰啉（phen）与碳载体（BP）的质量比为20/80时，所得催化剂有着较高的ORR活性。催化剂的比表面积与孔结构、催化剂表面氮元素的含量等是影响催化剂ORR活性的重要因素。前驱体中氮源的含量及铁的不同的价电子结构及配体单元等对热解过程中催化剂氧还原活性位的形成有显著影响。.2. 载体的比表面积、孔结构等显著影响FeN/C催化剂的ORR活性。氨气气氛、900℃条件下对催化剂进行二次热处理可提高催化剂的ORR活性。.3.利用电化学快速扫描方法初步探索了二次热处理前后催化剂的稳定性，催化剂粒径的增大、表面氮元素含量的降低等因素是催化剂稳定性下降的可能原因。.4.以自制沸石咪唑酯骨架材料（ZIFs），1,10菲啰啉，硫酸亚铁为催化剂前驱体组分制备得到的FeN/ZIF催化剂在酸性及碱性环境中表现出明显优于商品Pt/C催化剂的ORR活性。可归因于ZIFs特有的超高的比表面积、不同孔径分布和有序的多孔结构，易于实现非金属或金属元素的掺杂。进一步的研究正在进行中。.综合上述结果，本项目研究为研制高活性、高稳定性的NPMCs奠定了较为坚实的基础，同时对NPMCs催化活性位的研究提供了重要的线索和依据。