Identifying the active species in a cobalt-based covalent organic framework for the electrochemical oxygen evolution reaction

While considerable efforts have been devoted to developing functionalized covalent organic frameworks (COFs) as oxygen evolution electrocatalysts in recent years, studies related to the identification of the true catalytically active species for the oxygen evolution reaction (OER) remain lacking in the field. In this work, we investigated the active species of a cobalt-functionalized COF (TpBpy-Co) as electrochemical OER catalyst through a series of electrochemical measurements and post-electrolysis characterizations. Our results demonstrate that Co(II) ions, coordinated to the COF backbone, are transformed to cobalt-based nanoparticles when exposing TpBpy-Co to alkaline media. These nanoparticles act as the true active species for oxygen evolution. It remains unclear whether intact TpBpy-Co acts as a secondary catalytic species, due to its structural instability in alkaline electrolyte and its inferred lower catalytic activity compared to cobalt-based nanoparticles. Our results highlight that caution is warranted when identifying the active species for COF electrocatalysts formed under catalyst working conditions. Specifically, strong coordination between COFs and metal centers under electrochemical operation conditions is crucial to avoid unintended transformation of COF electrocatalysts. Our study thus contributes to the rational development of earth-abundant COF OER catalysts for the production of green hydrogen from renewable resources.