Identifying bottlenecks in the photocatalytic oxygen evolution reaction with covalent organic frameworks

Abstract

Covalent organic frameworks (COFs) have emerged as promising semiconducting materials for photocatalytic applications due to their large surface area, high crystallinity, and vast synthetic tunability. This is especially noticeable in the context of photocatalytic water splitting, where many COFs have been employed for the hydrogen evolution half-reaction. There, sacrificial reagents typically replace the kinetically demanding oxygen evolution half-reaction. On the contrary, only few reports focus on (sacrificial) water oxidation with COF photocatalysts. In most of these cases, cobalt species are employed as oxygen evolution cocatalyst, often with limited insight into their structure and detailed role in the catalysis. Herein, we use heterogenization of a molecularly defined iridium half-sandwich complex onto a bipyridine-based COF (Ir@TAPB-BPY COF) and provide detailed structural insights ensuring the integrity of the targeted cocatalyst. First, we demonstrate the retained catalytic activity of the anchored Cp*Ir­(III) motifs in chemical water oxidation experiments. In contrast, subsequent photocatalytic and electrocatalytic tests indicate that Ir@TAPB-BPY COF does not evolve oxygen and that careful control experiments have to be conducted in order to avoid false positive results, caused for example by the sacrificial electron acceptor. Using computational methods, we trace back the missing performance to thermodynamic and kinetic limitations of the employed systems. This work demonstrates the pitfalls associated with low-performing oxygen evolution photocatalysts as well as the indispensability of control experiments and their careful evaluation.