Abstract
Covalent organic frameworks (COFs) have shown promise as electrocatalysts for water splitting. However, there are significant challenges, including limited active sites and poor electrical conductivity. These issues stem from various factors within the structures of the COFs. Firstly, the limited number of active sites can be attributed to active sites being deeply embedded in the complex pore network of COFs, making them less accessible for catalytic reactions. Secondly, non-metallic active sites within the COFs may suffer from insufficient catalytic activity, which impacts their overall catalytic performance. Additionally, uncontrolled polymerization during COF synthesis can introduce structural defects and irregularities that disrupt electron transfer pathways; this hinders effective charge transport and lowers the catalytic activity in water-splitting reactions
This doctoral research study addresses these challenges using two distinct strategies. Firstly, we explore band structure modulation to improve the electrical conductivity and catalytic activity of metal-modified salen COFs. A stable Zn-salen COFEDA complex was synthesized, which allows for the incorporation of various metal ions (Cu, Ni, Co, Fe, and Mn). This fine-tuning of the band structure optimizes the free energy intermediate species during the HER process. The resulting metal-salen COFEDA complexes maintain their inherent crystal structure and high crystallinity. Conductive macromolecular poly(3,4-ethylenedioxythiophene) (PEDOT) is then integrated into the metal-salen COFEDA framework, creating PEDOT@metal-salen COFEDA heterostructures via an in situ solid-state polymerization process. Among these complexes, PEDOT@Mn-salen COFEDA exhibits remarkable electrochemical activity, with an overpotential of 150 mV and a Tafel slope of 43 mV dec‒1. The theoretical and experimental results reveal that the continuous energy band structure modulation enhances electron transport during the HER process. At the same time, charge density difference calculations demonstrate that the heterostructures facilitate intramolecular
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charge transfer and create highly active interfacial sites.
Secondly, we drew inspiration from nature by designing well-defined conjugated reticular oligomers (CROs) using an aqueous micellar strategy. These CROs possess precise chemical structures and can be regarded as conjugated oligomers or defect-free COF segments. When combined with conducting polymers, they form ‘muscle’-biomimetic electrocatalysts used for water splitting. These self-assembled ‘muscle’-like structures ensure the exposure of active sites, efficient electron conduction, and effective mass transfer. The PEDOT/CROs-Ru catalysts outperform COFs-Ru, with PEDOT/CROOH-8-Ru exhibiting mass activity and TOF values approximately 95 and 38 times higher than the bulk Py-COFOH, respectively. Theoretical calculations and experimental results demonstrate electron enrichment of electrocatalyst surfaces by CROs, which leads to enhanced carrier mobility. The improved water electrolysis activity of CROs-Ru can be attributed to the Schottky heterojunction, which mitigates electron backflow and facilitates the adsorption of hydrogen protons and hydroxides.
This research introduces innovative strategies to address the challenges associated with COFs and enhance their performance as electrocatalysts for water electrolysis and advancing the field of sustainable energy conversion.