金属有机框架阵列材料光电水解微观机理的原位同步辐射研究

Splitting water into hydrogen and oxygen, driven by the solar energy, has been widely regarded as a promising approach to meet the globally energy demand and alleviate the environmental crisis in the twenty-first century. To achieve efficient solar energy conversion, it is urgently imperative to deeply understand the fundamental physical and chemical mechanisms of energy conversion process in photoelectrochemical water splitting cells; especially the reaction pathway of photo-generated carrier and the catalytic mechanism of water dissociation at the surface of photo-electrode. .In this proposal, we aim to develop an in-situ XAFS/EAS experimental method, based on synchrotron radiation facility, to provide real-time monitoring of photoelectric-driven water splitting process in the as-prepared transition-metal organic framework (MOF) based photoelectrodes under various catalytic operation conditions. In combination with other conventional characterizations and analysis methods, the in-situ XAFS/EAS technique is used to comprehensively and systematically study the surface capacitance of adsorbed water molecule, the transfer behavior of photo-generated carrier, and the structural evolution of the metal active sites in the transition metal organic framework array electrodes during photoelectrochemical water splitting process. By means of the first-principle theoretical calculations, the dissociation pathway of adsorbed water molecule and the catalytic mechanism of photo-generated carrier on the porous MOF-based electrode will be completely and deeply investigated. These obtained discoveries would offer fundamental and scientific insight into the advanced energy conversion mechanisms in the field of solar energy applications, and simultaneously provide experimental and theoretical basis for the optimized design and controllable preparation of high-performance low-cost transition-metal-based electrocatalysts towards efficient photoelectrochemical water splitting system in the future.

利用太阳能驱动水分解产生清洁的氢能是解决21世纪能源和环境问题的重要途径之一，为实现高效的太阳能转换，必须深入认识光解水的机理本质。本项目拟基于国内同步辐射大科学装置发展液相原位同步辐射X射线吸收谱(XAFS)和电化学导纳谱(EAS)联用实验方法，对高活性过渡金属有机框架纳米阵列电极的光电水解过程进行实时在线监测，获得电极表面水分子吸附/解离电容、光生载流子扩散长度以及金属活性中心局域原子电子结构在光电催化反应过程中动态变化的重要信息，并结合第一性原理理论计算重点揭示电极表面水分子吸附/解离构型变化以及表面反应活性中心结构演变的规律，阐明电极表面光生载流子反应以及表面水分子解离的新机制，从原子分子水平上深入认识光电催化水分解的微观机理，为高效、稳定和廉价的太阳能转换材料的优化设计提供指导原则。

依托北京同步辐射光源，设计和研制了适用于背照式荧光测量模式的XAFS/EAS联用原位测量实验装置，建立了光电化学原位XAFS/EAS实验方法。利用同步辐射XAFS/EAS技术，原位实时探索金属有机框架纳米阵列电极表界面光水解过程中外偏压、激发波长和光功率密度、电解液浓度和酸碱度等反应条件的改变对电极表界面水吸附形态、光生载流子迁移以及表面活性过渡金属节点中心原子和电子结构的影响。同时结合同步辐射红外测量和第一性原理计算分析电极表面光水解的反应路径，深入认识了过渡金属有机框架纳米阵列电极光解水的机理本质，建立了催化剂光解水的机理模型。利用建立的原位同步辐射技术，对NiZn-MOF催化剂的电催化氧反应过程进行深入研究，观察到在氧催化过程中，伴随着Ni活性中心价态的升高，氧反应关键中间产物*OOH出现并在Ni(2+δ)+ (0 < δ < 1)高价活性中心快速累积，这预示着双金属位点协同的NiZn-MOF催化剂实现了快速高效的两电子氧还原反应动力学机制。通过原位同步辐射等技术对材料进行了深入的结构表征分析，Br-Ni MOF催化剂中限域的卤素离子可以有效抑制吸附在金属活性位点上OOH中间体的断裂，从而进一步激发金属中心位的活性，促进了氧还原反应的两电子转移动力学特征。设计了一种Ni1-NC单原子催化剂模型，使参与反应的活性位点单一化来最大程度地增强表界面活性位产生的有效信号。然后利用建立的原位同步辐射技术，实时在线研究Ni1-NC催化剂在电催化氧还原过程中固液界面的结构动力学信息，直接观察到电压驱动下的关键中间体*O吸附在近自由的“之”字形活性位点Ni1(2-δ)+N2，促进了固液界面上高效的4e-电催化氧还原过程。依托本项目发展原位同步辐射测试平台一套，发表SCI论文11篇，授权发明和实用新型专利共2件。