Materials and interfaces for electrocatalytic hydrogen production and utilization.

Mass industrialization over the last few centuries has created a global economy which is dependent upon fossil fuels to satisfy an exponentially increasing demand for energy. Aside from the possible depletion of this finite resource, the combustion of fossil fuels releases greenhouse gases into the atmosphere which cause the global temperature to rise – a phenomenon which has already begun to create geologic and geopolitical instability and shows no signs of abatement. One proposed method to rid humanity of its dependence on fossil fuels is to use green hydrogen as an energy carrier. In this scheme, excess electricity from a robust renewable energy generation infrastructure is diverted to grid-scale electrolyzers which split water into hydrogen and oxygen. The hydrogen is in turn distributed via supply chain to end users who employ fuel cells to locally convert the energy stored in the chemical bond of hydrogen into electricity on-demand. This vision for an alternative global energy economy has been inhibited by several factors including the low utilization of renewable energy generating technologies, the considerable cost of precious metals required for electrolyzer electrodes, and the relatively low efficiency of the cathodic fuel cell reaction. This vi dissertation is a compilation of experimental work related to the development of materials and interfaces for enhanced electrocatalytic hydrogen evolution using non-precious materials and hydrogen utilization in highly-efficient proton exchange membrane fuel cells. Chapter 1 begins with an overview of the global energy diet and the problem of climate change followed by a discussion of renewable energy technologies, ending with a proposal of how the large-scale implementation of green hydrogen technologies may fit into the futuristic energy landscape. Chapter 2 presents a brief review of the mechanism of hydrogen evolution electrocatalysis including its thermodynamic, kinetic, and mass transport limitations. In Chapter 3, we demonstrate a simple and scalable fabrication process for highly-active non-precious molybdenum sulfide electrolyzer cathodes. Molybdenum sulfide and other transition metal dichalcogenides have received considerable attention in recent years as electrolyzer catalysts due to their environmental benignity, high stability, good catalytic activity, and low cost. Aside from the issue of low efficiency, industrial implementation of most reported molybdenum sulfide fabrication procedures is complicated by extreme and/or lengthy processing conditions. Our process is roll-to-roll amenable and produces a catalyst film in milliseconds without the use of harsh processing conditions or excessive chemicals. Ex-situ characterization of the resulting electrode confirms transformation of the precursor to molybdenum sulfide and reveals a hydrogen evolution reaction overpotential of 200 mV at 10 mA cm-2 which is comparable to that of other reported highly-active molybdenum sulfide catalysts. Chapter 4 deals with the development and ex-situ characterization of a novel non-precious electrocatalyst platform for hydrogen evolution based upon a nickel-centered thiosemicarbazone molecular framework. The enzyme hydrogenase, found in most forms of life, has served as a source of inspiration for researchers due to its ability to reversibly catalyze hydrogen evolution efficiently. However, efforts to translate the activity of the hydrogenase active site to an electrode have been limited in their success. We report a class of molecular catalysts inspired by previous work on hydrogenase active site analogues. Successful translation and retention of the catalyst to the electrode surface is confirmed, and ex-situ testing reveals a hydrogen evolution reaction overpotential of 450 mV at 10 mA cm-2 with remarkable stability – a promising milestone which lays the foundation for further development of this class of materials. Beginning in Chapter 5, the focus of the dissertation is shifted away from materials for hydrogen evolution and toward the development of highly-efficient proton exchange membrane fuel cell devices. Chapter 5 details the interplay between chemical transport, catalytic kinetics, thermodynamic considerations, and Ohmic losses which influence fuel cell efficiency. In Chapter 6, we introduce a structured proton exchange membrane fuel cell cathode which utilizes an array of bulk-like ionomer channels to improve the interface between the catalyst layer and membrane. The channels serve as conduits for rapid proton transport, dramatically decreasing the cathode sheet resistance and enabling a reduction in electrode ionomer content which attenuates active site inhibition from sulfonate adsorption. Using this approach, the mass activity of Vulcan carbon-based Pt viii catalyst is improved by up to 80 %, and the power density is improved by up to hundreds of mW cm-2 at benchmark cell potentials – particularly under arid conditions. Further performance gains may be realized by tuning the electrode geometry and/or applying this technology to other catalyst types including those based upon high-surface-area carbon. The results described in this dissertation 1) advance the understanding of materials and fabrication processes for hydrogen evolution electrodes and 2) provide a simple tool for mitigating chemical transport and catalyst poisoning limitations in the proton exchange membrane fuel cell cathode.