Donor-acceptor engineering of NiPc-based covalent organic frameworks for boosting oxygen reduction and hydrogen evolution reactions

Abstract

Covalent organic frameworks (COFs), particularly imine-linked COFs, have emerged as viable electrocatalytic materials to promote oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER). However, a slow intramolecular charge transfer significantly limits their electrocatalytic activity. One effective strategy to overcome this drawback involves the introduction of donor-acceptor (D-A) structural units to the imine-linked COFs. Herein, with a donor-acceptor engineering strategy, three novel imine-linked COFs with D-A structural units that serve as efficient ORR/HER bifunctional electrocatalysts have been synthesized using 2,5-thiophenedicarboxaldehyde (TDA), thieno[3,2-b]thiophene-2,5-dicarboxaldehyde (TT) and benzo[1,2-b:3,4-b':5,6-b'']trithiophene-2,5,8-tricarbaldehyde (BTT) as electron donors, and 2,9,16,23-tetraaminophthalocyanine nickel (II) (NiPc) as an electron acceptor (denoted as TDA-NiPc-COF, TT-NiPc-COF and BTT-NiPc-COF). When compared with TDA-NiPc-COF and BTT-NiPc-COF, TT-NiPc-COF exhibits the higher ORR/HER activity thanks to its larger specific surface area, more abundant catalytic sites and stronger intrinsic charge transfer ability. Theoretical calculations have revealed that TT-NiPc-COF has the smallest HOMO-LUMO energy gap and lowest Gibbs free energy values for intermediates on the Ni–N and thiophene-S sites, indicating the fastest charge transfer ability and the most positive catalytic ability for ORR/HER. In addition, a negatively shifted d-band center energy provided by TT-NiPc-COF indicates an optimized adsorption strength of ORR-correlated intermediates, resulting in substantial improvement in ORR activity. This study provides a new insight into the rational design of novel imine-linked COFs with D-A structural units for application in the electrocatalytic field.