碳载金属-金属氧化物共价复合材料纳米相结构的设计及其电催化协同机制的研究

Oxygen reduction reaction (ORR) has long been considered as one of the key electrochemical reactions in electrochemical energy storage and conversion devices such as fuel cells and metal-air batteries. However, the developing of efficient ORR electrocatalysts is still urgent because of the low activity, poor selectivity and insufficient durability of the current ORR catalysts such as precious metal catalysts and transition metal oxides alone. In this work, we first synthesize carbon-supported metal oxide nanoparticles (such as La2O3, Co3O4, Mn3O4, etc.) as structure-directed templates for the synthesis of novel metal-metal oxide-carbon nanocomposites as advanced catalysts for ORR. Subsequently, some precious metal nanoparticles (such as Ag, Pd, Pt) with high catalytic activity for ORR are controlled deposited on the surface of carbon-supported metal oxide templates by the method of a controlled electrochemical oxidation-reduction reaction, whereby forming the desirable metal-metal oxide-carbon multiphase nanocomposites with completely and partially phase-segregated structures or core-shell structure, respectively. The effects of tailoring of surface-functionalization of carbon supports, sizes, morphologies and crystalline structures of the loaded metal oxide nanoparticles on carbon in these templates on the nanoscale phase structures and catalytic properties for the as-synthesized nanocomposites are investigated. The main objective of this study is to describe new findings of a detailed investigation of such nanoscale phase structures and their structure-catalytic activity correlation for as-synthesized nanocomposites prepared with controllable nanophase structures. We will systematically study the unique chemical and electronic structures of these nanocomposites that could be tuned by the substantial electron transfer between metals and metal oxide nanoparticles through their common carbon support, which highly correlates with the enhanced ORR performance. Importantly, the unique electronic coordination effects, particle-to-particle ligand, ensemble effects and spatial confinement effects in these nanocomposites on the ORR kinetics will be investigated. Achieving synergy between precious metals and inexpensive metal oxides is a key requirement for the development of highly active, economical composites as next-generation catalysts for ORR. Therefore, this work may also provide an opportunity to tune the chemical and electronic structure of the materials and offer desirable electrochemical performance of the ORR in practical applications.

针对当前燃料电池、金属-空气电池中氧还原电催化材料活性低、选择性差和耐久性不足等关键问题，提出以碳载金属氧化物（如La2O3、Co3O4、Mn3O4等）作为结构导向模板，采用可控电化学氧化-还原反应的方法，将高催化活性的贵金属（如Ag、Pd、Pt系）选择性地担载在模板材料表面上，制备出具有完全相分离、部分相分离以及核壳结构的金属-金属氧化物/碳三相共价复合电催化材料。在阐明碳材料表面功能化设计、金属氧化物晶型及形貌对复合材料纳米相结构的调控作用和机制的基础上，重点研究材料中金属、氧化物与碳载体界面之间的共价电子定向转移特性对材料界面/表面电子结构的调控作用，建立多相杂化材料的结构与性能之间的构效关系理论。重点探明金属与氧化物纳米粒子之间的电子配位效应、结构耦合效应以及纳米限域催化效应对于氧还原反应动力学过程的促进作用和机制。这一研究结果为开发下一代氧还原电催化材料及应用提供了基础理论支持。

针对当前氢燃料电池等新能源系统中催化剂活性低、选择性差和耐久性不足等问题，首先采用浸渍-煅烧、水热等方法制备出炭担载的尖晶石型（Co3O4）、钙钛矿型（LaMnO3）以及稀土氧化物（La系）等共价杂化导向模板材料，发现上述氧化物与碳载体之间均存在强烈的共价键（O-C=O-Metal），该化学键是调控氧化物/炭复合材料性能的关键活性位点之一。其次以上述模板材料为载体，采用电化学、油浴、微波水热以及固相光化学等多种方法，将多种金属担载在上述模板表面上，进而制备出一系列具有完全相分离、部分相分离以及核壳结构的金属/氧化物/碳三相复合电催化材料。从材料的化学成分设计入手，重点开展了多相催化剂的纳米相结构的精确调控，提出了低铂和非铂复合催化剂的制备新方法。对于低铂催化剂，具有相分离结构的铂-氧化物-炭表现出高的氧还原电催化活性。对于非铂催化剂，具有典型多相异质结构的Fe3C/Fe2O3/Fe/C也表现出明显高于商业Pt/C的氧还原电催化活性。这种高的活性主要归结于金属、氧化物与碳三者之间在相界面上形成了特殊的O-Fe-C-Fe-C化学键。为深入理解多相电催化的机制，将金属或氧化物纳米粒子的尺寸减小、甚至达到原子的水平，提出一种从原子水平精确分析金属、氧化物或碳之间化学作用的新策略和可行方法。为此，首次合成出具有原子厚度的钴氧化物包覆金属钴多相催化剂，当氧化物壳层厚度控制在1.1 nm时，其电催化性能最高。金属/金属氧化物/炭复合催化剂的活性起源可以归因于以下原因：（1）炭载体担载的氧化物和金属纳米粒子之间的电子配体效应；（2）氧还原反应产物从金属表面向金属氧化物表面溢出迁移效应等；（3）活性组分功函数的调控等。上述研究结果对于推动低铂或非铂基多相复合催化剂的高效制备及其在氢氧燃料电池中的商业化应用具有重要的意义。