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ABSTRACT: We review in this Viewpoint recent progress on the development of a new class of sustainable electrocatalytic systems for water-splitting and molecular hydrogen generation using diiron disulfide ([2Fe-2S]) active site methacrylate metallopolymers. To date, noble metal catalysts (e.g, platinum) remain the best electrocatalysts for molecular hydrogen generation using water as the chemical feedstock and proton donor. However, there remains a need for the synthesis of efficient electrocatalytic systems using low cost, earth abundant materials for sustainable H2 generation. We focus on our recent work in this area using well-defined single site [2Fe-2S]-metallopolymer catalysts. Thus far, these systems have demonstrated rates of hydrogen production >25 times faster than [FeFe]-hydrogenase enzymes and match the current densities of platinum with only 0.2 V higher potential when operating in water at neutral pH with tris(hydroxymethyl)aminoethane (TRIS) buffer (Faradaic yields 100±3%). The molecular design and synthesis of [2Fe-2S]-metallopolymers are reviewed along with mechanistic studies unraveling the causality of efficient H2 production from this catalytic system. The overall current output and overpotential are improved by (a) the reversible electrostatic adsorption of the metallopolymer on the carbon electrode surface that enhances the proton and electron transfer rates and (b) the use of protic buffer electrolytes (PBEs) that enhance the availability of protons. The schemes summarized here to improve the performance of [2Fe-2S] catalysts by incorporation into metallopolymers may be used to enhance the performance of other molecular electrocatalysts at the electrode surface. 

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. 

ABSTRACT: The diverse functionalization of the polymeric support phase of diiron disulfide[2Fe–2S] metallopolymer elec-trocatalysts offers a route to enhanced generation of molecular hydrogen production via water-splitting. Click chemistry has been shown to be a useful tool in post-polymerization functionalization for a wide range of polymeric materials under mild conditions which is a requirement for [2Fe-2S] metallopolymers due to the presence of iron carbonyl (Fe-CO) bonds in the active site. In this study, we developed a new synthetic methodology to functionalize [2Fe–2S] metallopolymers using atom transfer radical polymerization (ATRP) and post-polymerization functionalization using azide-alkyne “click” cycloaddition. Azide functional [2Fe–2S] metallopolymers were prepared by the ATRP of 3-azidopropyl methacrylate (AzPMA) with either methyl methacrylate (MMA), or 2-(dimethylamino)ethyl methacrylate (DMAEMA), followed by copper-catalyzed “click” cycloaddition with functional terminal alkynes. Both families of PMMA and PDMAEMA functional [2Fe–2S] metallo-co-polymers were found to be retain Fe-CO bonds from the catalyst active site after the click chemistry reactions, and more importantly, exhibited high electrocatalytic activity for electrochemical water-splitting under pH neutral aqueous conditions.Not Available 

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.