Abstract:

Increasing global energy consumption and the depletion of traditional energy sources pose severe challenges to environmental protection and energy supply security. Electrochemical decomposition of water is a green and promising technology and is also a key technology for efficient and sustainable energy production and storage by fuel cells and metal-air batteries. The electrocatalytic oxygen evolution reaction (OER), as the anode reaction for the electrolysis of water, requires large amounts of energy owing to multielectron participation, and to the breaking of O―H bonds and formation of O―O bonds. Many precious metal catalysts are expensive, and these are responsible for secondary environmental pollution, which is detrimental for the large-scale application of the OER. Therefore, it is necessary to develop a stable, clean, and efficient electrocatalyst to improve the efficiency of the OER. The application of covalent organic frameworks (COFs) to the electrocatalytic oxygen evolution reaction (OER) has received widespread attention. However, most metal-free covalent organic frameworks (COFs) have unsuitably poor conductivity for the OER. Herein, we report a 2D metal-free tetrathiafulvalene (TTF)-based COF, termed JUC-630. To improve the conductivity of COFs, we introduced TTF, which is a good electron donor, into the COF material. At the same time, compared with its analogue without TTF (Etta-Td COF), we found that JUC-630 has a large surface area, better crystallinity, and higher stability. Furthermore, we tested their OER performance in a 1 mol∙L⁻¹ KOH solution, and the results show that JUC-630 has a higher current density than Etta-Td COF and TTF at the same potential. For example, at a current density of 10 mA∙cm⁻², the overpotential of JUC-630 was 400 mV, which was significantly lower than that of Etta-Td COF (450 mV). This overpotential is comparable to or even better than those of the widely discussed carbon and graphene materials. The lower overpotential, Tafel slope values, and smaller electrochemical impedance illustrate that the introduction of TTF monomers into the COF material results in a significantly improved OER performance for JUC-630. This study proposes a strategy for the rational design of functional motifs that can greatly improve the OER catalytic activity of COF materials. Thus, the results should help to suggest new efficient approaches for the preparation of catalysts for energy conversion from water resources.

Key words: Covalent organic frameworks, Oxygen evolution reaction, Electrocatalysis, Metal-free catalyst