Investigation of (photo) electrocatalytic conversion of dinitrogen to ammonia using hybrid plasmonic nanostructures

Abstract

Meeting our future global energy needs in an environmentally responsible way is one of the most significant challenges in the twenty-first century. Currently, fossil fuels such as oil, gas, and coal satisfy more than 80% of the global energy demand. This results in more than 35 billion metric tons of carbon dioxide (CO2) emission annually. It is imperative to harvest renewable energy sources (e.g., solar, wind) to serve as a way to diversify from traditional fossil fuels for combatting the environmental challenges associated with greenhouse gases. Due to the intermittent nature of renewable energy sources, the development of a cost-effective and sustainable method of storing this vast amount of energy on an industrial scale when supply exceeds demand in the grid is an urgent need. As the cost of renewably derived electricity continues to decrease given the rapid progress in technology andeconomies of scale, there is a growing interest in fuels and chemicals electrosynthesis. The thrust of my Ph.D. research has focused on developing novel electrochemical technologies to use renewable electricity as an energetic driving force to convert low energy molecules (N2) to high value-added molecules (NH3) that can be utilized as either fuels, energy storage molecules, and/or chemicals. (photo) electrochemical nitrogen reduction reaction (NRR) for ammonia synthesis provides an attractive alternative to the long-lasting thermochemical process (Haber-Bosch) in a clean, sustainable, and decentralized way if the process is coupled to renewably derived electricity sources. In addition to ammonia’s widespread utility as a precursor for making fertilizer, ammonia also holds great promise as a carbon-neutral liquid fuel for storing intermittent renewable energy sources as well as for power generation due to the compound’s high energy density (5.6 MWh ton-1) and hydrogen content (17.6 wt.%). Electrification of ammonia synthesis on a large scale requires an effective electrocatalyst that converts N2 to NH3 with a high yield and efficiency. The selectivity of N2 molecules on the surface of the catalyst has been demonstrated to be one of the major challenges in enhancing the rate of photo electrochemical NRR in aqueous solution under ambient conditions. The rational design of electrode-electrolyte in the context of photo-electrochemical systems is required to overcome the selectivity and activity barrier in NRR. The scientific thrusts of this Ph.D. dissertation to address a critical obstacle to achieving the overarching goal of distributed ammonia synthesis are as follows: Thrust 1: Materials chemistry for the synthesis of a range of heterogeneous (photo) electrocatalysts including plasmonic and hybrid plasmonic-semiconductor nanostructures for selective and efficient conversion of N2 to NH3. Thrust 2: Novel reactor design to study the redox processes in the photo-electrochemical energy conversion system and to benchmark nanocatalysts’ selectivity and activity toward NRR. Thrust 3: Performing ex-situ and operando spectroscopy, including surface-enhanced Raman spectroscopy (SERS) to probe the reaction mechanism and to identify intermediate species relevant to NRR at the electrode-electrolyte interface. In addition, nuclear magnetic resonance (NMR) spectroscopy measurements is carried out to quantify the trace amount of products after the catalytic reaction and to distinguish 15NH4+ from 14NH4+ in isotopic labeling experiments using 15N2. The outcomes of this dissertation generate an integrated scientific framework, combining materials chemistry, photo-electrochemistry, and spectroscopy to overcome the challenges associated with renewable energy storage and transport. It also contributes to future electrification, decarbonization, and sustainability of modern chemical industry.