From linkage chemistry to active-site engineering: strategic designs and progress in covalent organic frameworks for...

To meet the growing energy demand, sustainable energy sources are being explored, and covalent organic frameworks (COFs) have emerged as strong contenders for energy applications. COFs are a crystalline and innovative class of porous organic polymers consisting of covalently linked light heteroatoms. COFs have distinctive properties, including large surface area, tunable pore sizes, designable building blocks, and abundant active sites, making them a powerful platform in electrocatalysis. COF-based electrocatalysts have shown promising performance, but remains less explored than conventional catalyst systems for electrocatalytic water splitting, including the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). In this review, we focused on strategic design for COFs, particularly through linkage chemistry and heteroatom functionalization to improve redox activity, framework stability and charge transfer kinetics and metal coordination to introduce highly active catalytic centers and active site engineering. To date, Pt1@BCOF-600C has demonstrated the best HER performance, delivering an ultralow overpotential of 14.5 mV along with a small Tafel slope of 22 mV dec^-1. For the OER, IISER-COF1-RuO₂@370 shows superior activity, requiring an overpotential of only 215 mV and exhibiting a low Tafel slope of 65 mV dec^-1, indicating favorable reaction kinetics. This review also discusses the fundamentals of electrocatalytic water splitting along with tailored physicochemical properties of COFs and presents a comparative literature review for electrochemical HER and OER applications. We further discuss the key challenges and perspectives that currently limit their performance, including restricted pores that hinder mass transport, the presence of active sites, and intrinsic low conductivity that reduces charge carrier mobility. In the future, COF-based electrocatalysts are expected to gain greater attention by integrating highly dispersed single-atom active sites, donor-acceptor frameworks, and heteroatom engineering to further enhance intrinsic activity and conductivity.