Phosphorus Covalent Triazine Framework-Based Nanomaterials for Electrocatalytic Hydrogen Evolution Reaction

The production of hydrogen via electrocatalytic reduction of water using metal-free nanomaterials as the catalyst is a promising and ultimate green approach. Graphitic carbon nitride, covalent organic frameworks, and covalent triazine frameworks (CTFs) are some of the nanostructured materials that are investigated for this purpose. Currently, these materials still lack the efficiency to compete with other techniques (electrolysis). This is because the reaction mechanism and active sites are, in many cases, still poorly understood. In this work, we report a set of metal-free nanostructure-based electrocatalysts, phosphorus covalent triazine frameworks (PCTFs), for electrocatalytic hydrogen production. The hydrogen evolution reaction (HER) performance of PCTF-based nanomaterials is ascribed to the synergistic effect of isolated single nitrogen and phosphorus sites on the large surface area. By combining both experimental and theoretical studies, we found that especially the pyridinic-nitrogen species are the most active sites for the HER. The presence of phosphorus next to the pyridinic-N enhances the HERs. The present results provide a better understanding of the importance of different heteroatoms in nanomaterials as active sites in HERs. Theoretical studies confirmed that phosphorus, being electron rich, creates high electron densities on the nearby N atoms of the CTF materials and intensifies the HER process.

以无金属纳米材料为催化剂，通过电催化水还原制备氢气，是一种极具潜力的近乎完美的绿色制氢路径。石墨相氮化碳（graphitic carbon nitride）、共价有机框架（covalent organic frameworks）以及共价三嗪框架（covalent triazine frameworks, CTFs）均是当前被广泛研究用于该方向的纳米结构材料。但目前这类材料的催化效率仍无法与其他制氢技术（如电解水制氢）相抗衡，究其原因，该类催化反应的机制与活性位点在多数情况下仍未被充分阐明。本研究开发了一类基于无金属纳米结构的电催化剂——磷共价三嗪框架（phosphorus covalent triazine frameworks, PCTFs），用于电催化制氢。该类PCTFs基纳米材料的析氢反应（hydrogen evolution reaction, HER）性能，归因于大比表面积上孤立分散的单个氮与磷活性位点的协同效应。结合实验与理论研究，本研究发现，吡啶氮物种是该析氢反应中最主要的活性位点；吡啶氮旁的磷掺杂可显著强化析氢反应过程。本研究结果有助于更深入地理解纳米材料中不同杂原子作为析氢反应活性位点的重要性。理论研究证实，具有富电子特性的磷可在共价三嗪框架材料的邻近氮原子上营造高电子云密度，进而强化析氢反应进程。