A Copper-Peptoid as a Highly Stable, Efficient, and Reusable Homogeneous Water Oxidation Electrocatalyst

Water electrolysis is among the simplest methods to generate hydrogen, which can be used as a clean and renewable energy source. Within this process, the oxidation of water into molecular oxygen is considered as the bottleneck reaction because it involves the transfer of four electrons toward the oxidation of a highly stable small molecule. Challenges in this area include the development of stable and effective electro- and photocatalysts that utilize readily available metal ions. Herein we report a copper-peptidomimetic complex as an electrocatalyst for water oxidation, which is both highly stable and efficient. Inspired by enzymatic catalysis, which is largely based on intramolecular cooperativity between a metal center and functional organic molecules located on one scaffold, we have designed and synthesized a peptoid trimer bearing a 2,2'-bipyridine (bipy) ligand, an -OH group, and a benzyl group. Both experimental and computational data reveal that binding of Cull to this peptoid in aqueous medium occurs via the bipy group and two hydroxyl moieties from the solution. Based on a systematic electrochemical study, we show that this complex is an active electrocatalyst for water oxidation in aqueous phosphate buffer solution enabling oxygen evolution at pH 11.5 with a turnover frequency of 5.8 s(-1) and a Faradaic efficiency of up to 91%. Importantly, this catalyst is highly stable over at least 15 h of electrolysis. Thus, we could reuse it for at least 9 times in 40 min electrolysis experiments, demonstrate that it retains its activity in every experiment, and obtain oxygen evolution with an overall turnover number record (based on moles oxygen to moles catalyst) of >56 in 6 h. Moreover, based on electrochemical experiments, spectroscopic data, and density functional theory-D3 calculations, we identified a key peroxo intermediate and propose an intramolecular cooperative catalytic path for this reaction, which suggests that the -OH group has a major role in the high stability of the complex.