Catalyzing the Hydrogen Evolution Reaction with [2Fe-2S] Metallopolymers in Neutral Water

Abstract

Oil, coal, and natural gas provided the energy that propelled humankind into modern prosperity. However, the world’s unsustainable dependence on fossil fuels causes geopolitical strife and environmental harm to our planet. Molecular hydrogen is a sustainable alternative fuel that is energy dense and carbon-free. However, the challenge remains to increase the economic viability of producing hydrogen commercially through non-carbon-emitting water electrolysis. To lower the energy requirement for hydrogen evolution, a catalyst is needed that is composed of inexpensive and Earth-abundant elements. To this end, we have developed synthetic metallopolymers featuring a [2Fe-2S] catalytic site with rates of hydrogen evolution of 105 s-1, which surpasses the [2Fe-2S] hydrogenase enzymes from which they were inspired by an order of magnitude. The [2Fe-2S] metallopolymer catalyst has the added benefit of operating efficiently in neutral aqueous conditions and, therefore, does not require the added cost of working in acidic or caustic media. This dissertation concentrates on establishing structure-activity relationships that lead to fast and efficient electrocatalysis for [2Fe-2S] metallopolymers. The components of the metallopolymer electrocatalytic system consist of the [2Fe-2S] active site, the polymer framework, and the media conditions. The modularity of the system design allows for these elements to be systematically tuned and optimized. With a goal toward decreasing the energy input and increasing the rate of hydrogen evolution for [2Fe-2S] metallopolymer systems, the influence of the solution conditions and the polymer support on electrocatalysis are examined using electrochemical methods. In neutral water, the appropriate choice and concentration of a protic buffer electrolyte is shown to significantly increase the rate of electrocatalytic generation of hydrogen for a standard platinum electrocatalyst and a 70-fold increase in the rate for a [2Fe-2S]-metallopolymer electrocatalyst. The term “protic buffer electrolyte” indicates a species that concomitantly buffers the pH of the solution near the electrode, increases the concentration of protons for electrocatalytic reduction, and serves as the electrolyte for water electrolysis. Changing the polymer composition further impacts the rate of catalysis at the same potential requirement and improves the metallopolymer’s stability under hydrogen evolving conditions. In optimized solution conditions, the electrocatalysis of two [2Fe-2S] metallopolymers are compared, one with protonated amines in the polymer chains and the other with functionality that is uncharged and unprotonated. Using Tafel analysis, mechanistic studies show the protonated amines are a crucial component of the polymer framework that promotes a low-energy pathway through the catalytic cycle. Additionally, for the [2Fe-2S] metallopolymer with protonated amine functionality on the polymer, the influence of the polymer size on the electrocatalytic behaviour is investigated for [2Fe-2S] metallopolymers that are nominally small, medium and large. The small metallopolymers are found to increase the amount of electroactive [2Fe-2S] catalytic sites adsorbed to the surface of the cathode, thereby, increasing the catalytic current density. The work in this dissertation on the [2Fe-2S] metallopolymer system illustrates the strategic design features for the primary, secondary, and tertiary structure about the active catalytic site as well as the solution conditions of the system for the optimal shuttling of protons and management of electrons. The strategies presented herein are broadly applicable to not only hydrogen evolving catalysts but to any cathodic electrocatalytic reaction involving proton and electron transfers.