Integrating Electronic-Storage Piperazine into Covalent Organic Frameworks for Promoting Oxygen Reduction Reaction

Metal-free covalent organic frameworks (COFs) have emerged as potential electrocatalysts for oxygen reduction reaction (ORR) in new environmental-friendly electrochemical energy conversion technologies. However, their catalytic activity is hindered by inefficient electron transfer from electrodes to catalytic sites along extended frameworks. To overcome this bottleneck, herein, we first incorporated redox-active piperazine units into the COFs to catalyze ORR. The redox-active piperazine units enable to storage electrons, thus accelerate the electron transfer to the catalytic sites. Furthermore, the introduction of ─OH group-containing building blocks induces keto-enol tautomerism (enabling reversible ─OH / ─C═O interconversion), improving framework polarity with a dipole moment of 6.87 Debye (5.8 times increase compared to non-hydroxylated COFs). This polarity enhancement strengthens the intermediates binding ability, thereby improving the catalytic activity. As a result, the optimized PD-COF-OH exhibits a high half-wave potential of 0.76 V, turnover frequency (TOF) of 0.045 s^-1, and electrochemically active surface area of 9.4 mF cm^-2, surpassing most reported metal-free COFs. Theoretical calculations further reveal synergistic roles of ─OH and ─C═O groups in stabilizing OOH* and OH* intermediates, contributing to the improved catalytic activity. This work establishes a novel design paradigm for catalytic COFs through a rational integration of electron reservoir units and tautomerism-enabled polarity modulation.