Well-Defined Conjugated Reticular Oligomer “Blood Cells” and Conducting Polymer “Neurons” Constructing “Muscle”-Biomimetic Electrocatalysts for Water Electrolysis

Covalent organic frameworks (COFs) provide a tunable platform for water electrolysis. However, it is difficult to perform explicitly structural characterization for COFs due to the uncontrollable polymerization via the solvothermal method, which hinders the clear-cut exploration of the COFs’ structure-performance relationship in further applications. Here, the well-defined conjugated reticular oligomers (CROs) are designed for the first time using an aqueous micellar strategy. The CROs have definite chemical structure and can be regarded as conjugated oligomers or defect-free COFs segment. Using CROs and conducting polymer, three “muscle”-biomimetic electrocatalysts are engineered for splitting water to H₂ and O₂. The self-assembled “muscle”-like structures guarantee fully exposed active sites, fast electron conduction and mass transfer (H₂O/H₂/O₂). The “muscle”-biomimetic poly(3,4-ethylenedioxythiophene) (PEDOT)/CROs-Ru exhibit superior electrochemical performance than the COFs-Ru. In particular, the mass activity and turnover frequency (TOF) value of PEDOT/CRO_OH-8-Ru are ≈95 and 38 times that of the counterpart bulk Py-COF_OH. The theoretical calculation and the experimental results demonstrate that the CROs endow the electrocatalyst with an electron-rich surface and enhance carrier mobility. The enhanced water electrolysis activity of CROs-Ru can be attributed to the Schottky heterojunction suppressing the electron backflow, which facilitates the adsorption of hydrogen protons and hydroxides.     

Bifunctional Covalent Organic Framework-Derived Electrocatalysts with Modulated p-Band Centers for Rechargeable Zn–Air Batteries

Fine control over the physicochemical structures of carbon electrocatalysts is important for improving the sluggish oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in rechargeable Zn–air batteries. Covalent organic frameworks (COFs) are considered good candidate carbon materials because their structures can be precisely controlled. However, it remains a challenge to impart bifunctional electrocatalytic activities for both the ORR and OER to COFs. Herein, a pyridine-linked triazine covalent organic framework (PTCOF) with well-defined active sites and pores is readily prepared under mild conditions, and its electronic structure is modulated by incorporating Co nanoparticles (CoNP-PTCOF) to induce bifunctional electrocatalytic activities for the ORR and OER. The CoNP-PTCOF exhibits lower overpotentials for both ORR and OER with outstanding stability. Computational simulations find that the p-band center of CoNP-PTCOF down-shifted by charge transfer, compared to pristine PTCOF, facilitate the adsorption and desorption of oxygen intermediates on the pyridinic carbon active sites during the reactions. The Zn–air battery assembled with bifunctional CoNP-PTCOF exhibits a small voltage gap of 0.83 V and superior durability for 720 cycles as compared with a battery containing commercial Pt/C and RuO₂. This strategy for modulating COF electrocatalytic activities can be extended for designing diverse carbon electrocatalysts.     

The Role of Defects in Metal–Organic Frameworks for Nitrogen Reduction Reaction: When Defects Switch to Features

The electrochemical nitrogen reduction reaction (NRR), a contributor for producing ammonia under mild conditions sustainably, has recently attracted global research attention. Thus far, the design of highly efficient electrocatalysts to enhance NRR efficiency is a specific focus of the research. Among them, defect engineering of electrocatalysts is considered a significant way to improve electrocatalytic efficiency by regulating the electronic state and providing more active sites that can give electrocatalysts better physicochemical properties. Recently, metal–organic frameworks (MOFs), along with their derivatives, have captured immense interest in electrocatalytic reactions owing to not only their large surface area and high porosity but also the ability to create rich defects in their structures. Hence, they can provide plenty of exposed active sites for electron transfer, NN cleavage, and N₂ adsorption to enhance NRR performance. Herein, the concept, the in situ characterizations techniques for defects, and the most common ways to create defects into MOFs have been summarized. Furthermore, the recent advances of MOF-based electrocatalysts towards NRR have been recapitulated. Ultimately, the major challenges and outlook of defects in MOFs for NRR are proposed. This paper is anticipated to provide critical guidelines for optimizing NRR electrocatalysts.