Synthesis and Electrochemical Analysis of Metallopolymers Containing Aryl-Bridged [2Fe-2S] Catalysts for Hydrogen Production

Abstract

The increased use of renewable energy sources faces a challenge due to their intermittent output not aligning with peak energy demands. Storing this energy by producing hydrogen (H2) from water through electrolysis is a possible solution. Proton exchange membrane (PEM) electrolyzers are currently the most effective for this, partly because they adapt well to fluctuating renewable energy output. However, the reliance of PEM electrolyzers on expensive metals like platinum and iridium for electrodes necessitates the search for more affordable alternatives. Small molecule catalysts inspired by the [FeFe]-hydrogenase active site show promise for the hydrogen evolution reaction (HER), but they struggle with oxygen instability, degradation, and poor water solubility.Previous research in this group led to the discovery of metallopolymer catalysts that overcome these limitations by covalently binding a polymer to the small molecule catalyst. These metallopolymers demonstrated superior HER catalysis in neutral aqueous solutions, greatly outperforming previous small molecule catalysts and even rivaling platinum at extremely low catalyst loadings (1-2 ppm) with less than 0.2V overpotential difference. Further research has explored how polymer composition and buffer choice influence catalytic output. The contributions of this dissertation aim to further our understanding of these metallopolymer catalysts by investigating how molecular weight limits catalysis, how the topology of the [2Fe−2S] catalyst site impacts output, and how azide-containing monomers can enhance functionality through click chemistry. When studying metallopolymer catalysts of different sizes, it was anticipated that smaller metallopolymers would have faster rates due to faster electron and proton transfers to more accessible active sites. However, the experiments show that the rates of catalysis per active site are independent of the polymer size, as it was discovered that the high performance is due to adsorption of the metallopolymer on the electrode surface, bringing 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 the polymer size. By designing a new metalloinitiator small molecule, the topology of metallopolymer catalysts was controlled, placing the [2Fe-2S] active site internal (in chain) or terminal (endgroup) in the polymer network. By comparing electrochemical performance of metallopolymer catalysts with the same composition, but with different topological active site placement, it was determined that the metallopolymer catalysts showed comparable current densities, Langmuir adsorption, and electronic limits. The adsorption of the metallopolymers to the electrode surface creates similar chemical environments for electrolysis, establishing that the active site can be placed anywhere that is most convenient in the metallopolymer topology without loss of performance. Through the incorporation of a methacrylic monomer with an azide pendant chain, a new synthetic methodology to functionalize [2Fe-2S] metallopolymers using atom transfer radical polymerization (ATRP) and post-polymerization functionalization using azide−alkyne “click” cycloaddition was developed. [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. These metallo-copolymers were found to retain Fe−CO bonds from the catalyst active site after the click chemistry reactions and, more importantly, retained electrocatalytic activity under pH-neutral aqueous conditions.