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Abstract Small molecule biomimetics inspired by the active site of the [FeFe]‐hydrogenase enzymes have shown promising electrocatalytic activity for hydrogen (H₂) generation. However, most of the active‐site mimics based on [2Fe‐2S] clusters are not water‐soluble which limits the use of these electrocatalysts to organic media. Polymer‐supported [2Fe‐2S] systems, in particular, single‐site metallopolymer catalysts, have shown drastic improvements for electrocatalytic H₂generation in aqueous milieu. [2Fe‐2S] complexes functionalized within well‐defined macromolecular supports via covalent bonding have demonstrated water solubility, enhanced site‐isolation, and improved chemical stability during catalysis. In this report, the synthesis of a new propanedithiolate (pdt)‐[2Fe‐2S] complex bearing a single α‐bromoester moiety for use in atom transfer radical polymerization (ATRP) is demonstrated as a novel metalloinitiator to prepare water‐soluble poly(2‐dimethylaminoethyl methacrylate) grafted (PDMAEMA‐g‐[2Fe‐2S]) metallopolymers. Using this approach, metallopolymers with controllable molecular weights (M_n= 5–40 kg mol⁻¹) and low dispersity (Đ,M_w/M_n= 1.09–1.36) are prepared, which allows for the first time observation of the effect of the metallopolymers' chain length on the electrocatalytic activity. The ability to control the composition and molecular weight of these metallopolymers enables macromolecular engineering via ATRP of these materials to determine optimal structural features of metallopolymer catalysts for H₂production. 

A molecular catalyst attached to an electrode sur-face can in principle offer the advantages of both homogeneous and heterogeneous catalysis. Unfortunately, some molecular catalysts constrained to a surface lose much or all of their solution performance. In contrast, we have found that when a small molecule [2Fe–2S] catalyst is incorporated into metallopolymers of the form PDMAEMA–g–[2Fe–2S] (PDMAEMA = poly(2-dimethylamino)ethyl methacrylate) and adsorbed to the sur-face, the observed rate of hydrogen production increases to kobs > 105 s-1 per active site with lower overpotential, increased life-time, and tolerance to oxygen. Herein, the electrocatalytic performances of these metallopolymers with different length polymer chains are compared to reveal the factors that lead to this high performance. It was anticipated that smaller metallopolymers would have faster rates due to faster electron and proton transfers to more accessible active sites, but the experiments show that the rates of catalysis per active site are largely independent of the polymer size. Molecular dynamics modelling reveals that the high performance is a consequence of adsorption of these metallopolymers on the surface with natural assembly that brings the [2Fe–2S] catalytic sites into close contact with the electrode surface while maintaining exposure of the sites to protons in solution. The assembly is conducive to fast electron transfer, fast proton transfer, and a high rate of catalysis regardless of polymer size. These results offer a guide to enhancing the performance of other electrocatalysts with incorporation into a polymer that provides optimal interaction of the catalyst with the electrode and with solution. 

Electrocatalytic generation of H₂is challenging in neutral pH water, where high catalytic currents for the hydrogen evolution reaction (HER) are particularly sensitive to the proton source and solution characteristics. A tris(hydroxymethyl)aminomethane (TRIS) solution at pH 7 with a [2Fe-2S]-metallopolymer electrocatalyst gave catalytic current densities around two orders of magnitude greater than either a more conventional sodium phosphate solution or a potassium chloride (KCl) electrolyte solution. For a planar polycrystalline Pt disk electrode, a TRIS solution at pH 7 increased the catalytic current densities for H₂generation by 50 mA/cm²at current densities over 100 mA/cm²compared to a sodium phosphate solution. As a special feature of this study, TRIS is acting not only as the primary source of protons and the buffer of the pH, but the protonated TRIS ([TRIS-H]⁺) is also the sole cation of the electrolyte. A species that is simultaneously the proton source, buffer, and sole electrolyte is termed a protic buffer electrolyte (PBE). The structure–activity relationships of the TRIS PBE that increase the HER rate of the metallopolymer and platinum catalysts are discussed. These results suggest that appropriately designed PBEs can improve HER rates of any homogeneous or heterogeneous electrocatalyst system. General guidelines for selecting a PBE to improve the catalytic current density of HER systems are offered.