Water electrolysis produces hydrogen and oxygen using electricity. Conventional electrolysers couple the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) that occur simultaneously in cells divided by separators into cathodic and anodic compartments. This division prevents hazardous hydrogen and oxygen mixing, but increases the cost of electrolysers and limits their efficiency and dynamic operation under variable load conditions that characterize solar and wind energies. In this Review, we discuss strategies and technologies that decouple the HER and OER in time and/or place. Decoupled water electrolysis utilizes redox mediators such as the polyoxometalate phosphomolybdic acid (H₃PMo₁₂O₄₀) or nickel (oxy)hydroxide (Ni(OH)₂/NiOOH) to mediate ion (H⁺ or OH^–) exchange between the cathode and the anode that generate hydrogen and oxygen in different stages or separate cells. Consequently, the risk of hydrogen and oxygen mixing is reduced, in comparison to conventional electrolysis, especially at low currents, enabling safe operation under partial load conditions, which is important for operation with solar and wind energies. Alternatively, redox couples such as silicotungstic acid (H₄SiW₁₂O₄₀) or bromide/bromate (Br^–/BrO₃^–) can store and release hydrogen or oxygen by turn, dividing the HER or OER into electrochemical and chemical subreactions that can yield high efficiency decoupled water electrolysis.

Borate buffer was found to have both structural and functional roles within a low‐cost tri‐copper electrocatalyst for homogeneous water oxidation that exhibits a high turnover frequency of 310 s⁻¹. The borate buffer was shown to facilitate the catalytic activity by both bridging the three Cu ions and participating in O−O bond formation. Phosphate and acetate buffers did not show such roles, making borate a unique player in this catalytic system.

A dinuclear Cu-peptoid, Cu₂(BDiE)₂, having a diol side chain was developed as a homogeneous electrocatalyst for oxygen evolution reaction (OER) at neutral pH. The molecular structure of Cu₂(BDiE)₂ was characterized and concluded by ESI-MS, UV-vis, and single-crystal X-ray diffraction. Electrochemical, spectroscopic, and mechanistic studies revealed that borate buffer (the solution medium) has a minor effect during electrocatalysis; however, the diol side chain promotes a second coordination sphere effect via multiple hydrogen bonds which highly stabilize the complex, leading to an OER. Based on these observations and the collected data, we also suggest two different and unique mechanism pathways: a major one, which involves interactions of radical intermediates, which is buffer independent, and a minor one that resembles water nucleophilic attack (WNA) and is assisted by the borate buffer.

Studies on a series of molecular dicopper peptoid complexes showed that the CuCu distances measured in X-ray single-crystal diffraction are typically in the range of 4.2-6.9 & Aring;. Herein, we designed a new peptoid, L1, having 2,2 '-bipyridine, propyl, and pyridyl side chains and discovered that although it forms a typical dicopper self-assembled structure (complex 1), the CuCu distance is exceedingly long -8.043 & Aring;. By analyzing its structure and surface properties in comparison to a control Cu-peptoid complex (2), in which the pyridyl side chain is modified by an ethanolic side chain, we suggest that the long CuCu distance is contributed by the hydrophilic-hydrophobic interaction influenced by the pyridyl side chain and the steric hindrance of the propyl side chain. This result may motivate the use of dinuclear Cu peptoid complexes for wider applications, such as cooperative catalysis and luminescence.

Water oxidation (WO) is the first step in the water-splitting process aiming at the production of hydrogen as a green renewable fuel. To successfully perform WO, potent strategies for overcoming the high energetic barrier and slow kinetics of this reaction are urgently required. One such strategy is the use of molecular catalysis. Specifically, Cu-based catalysts have been highlighted over the last decade due to their stability and fast kinetics. Among them, Cu-peptoids, where peptoids are peptidomimetics akin to peptides and are N-substituted glycine oligomers, can act as stable and active catalysts for oxidation transformations including electrocatalytic WO. Previously, we suggested that a benzyl group incorporated as a side chain near the catalytic site within a Cu-peptoid electrocatalyst for WO has a structural role in the activity of the electrocatalyst in phosphate buffer (PBS). Herein, we aimed to test this hypothesis and understand how an incorporated structural element side chain affects WO. To this aim, we prepared a set of peptoid trimers each with a different structural element replacing the benzyl group by either naphthyl, cyclohexyl, benzyl, propyl chloride, or propyl side chains as well as a peptoid lacking a structural element. We studied the structure of their Cu complexes and tested these complexes as electrocatalysts for WO. We discovered that while all the peptoids self-assemble to form dinuclear Cu-peptoid complexes, the duplex that has no structural side chain, Cu₂(BE)₂, is structurally different from the others in the solid state. Moreover, Cu₂(BE)₂ remains dinuclear in a PBS at pH 11, while all the other duplexes are mononuclear in the PBS. Finally, though most of the complexes showed low electrocatalytic activity for WO, the dinuclear complex Cu₂(BE)₂ performed with the highest turnover frequency of 484 s⁻¹. Nevertheless, this dinuclear complex slowly decomposes to the corresponding mononuclear complex as a more stable species during WO, while the other mononuclear complexes retain their structure in solution but display much slower kinetics (ca. 5 to 8 s⁻¹) under the same conditions. Overall, our results demonstrate that bulkier side chains hamper the stability of dinuclear Cu-peptoids in a PBS, and hence, their efficiency as WO electrocatalysts is also hampered.