Sarah Blair PhD Thesis Defense

Sarah Blair
PhD Candidate
Chemical Engineering
Academic advisor: Professor Thomas F. Jaramillo

Abstract: Developing a Fundamental Understanding of Electrochemical Ammonia Formation

While industrial ammonia production for fertilizers has been critical to the support of the food supply for our dramatically increasing global population, this process is responsible for as much as 1.4% of global CO2 emissions, contributing to the rise in global temperatures, and is highly energy-intensive, using 1% of the world’s global energy supply. There is consequently a need for developing a more sustainable, decentralized means of fertilizer production to address such environmental consequences and provide fertilizer in heavily populated regions that lack transportation infrastructure. Electrochemical ammonia synthesis has attracted recent attention because of its potential for being coupled with renewable sources of electricity and operation at ambient temperatures and pressures. This dissertation describes work toward such a system in two areas: 1) wastewater recycling for NH3 production via aqueous nitrate reduction and 2) non-aqueous, Li-mediated direct electrochemical N2 reduction to NH3.

More than 50% of the fertilizer applied to crops is lost to groundwater as runoff, leading to eutrophication of water sources and human health concerns. Conversion of these nitrate pollutants to NH3 would be a means of closing the nitrogen cycle and “recycling” such pollutants. By mapping selectivity toward NH3 across a comprehensive range of electrochemical conditions on a Ti cathode, we identified a low pH and relatively high concentration of nitrate as providing high, stable NH3 selectivity. Thus, a higher availability of nitrate anions within an electrolyte with a high proton concentration allowed for the suppression of the HER while promoting reaction of these protons with adsorbed N-O intermediate species. We additionally found that the ability of this catalyst to suppress the HER in acidic conditions may be related to the formation of TiH2, with the Ti lattice preferentially absorbing protons during electrochemistry rather than facilitating the HER.

An alternative to wastewater recycling would be the direct electrochemical reduction of N2 to NH3. A non-aqueous, Li-mediated method (Li-N2R) that proceeds via the electrodeposition of Li and subsequent reaction with N2 and a proton source is the only such system that has thus far been demonstrated to reliably produce NH3, but there remains little experimental understanding of the electrode-electrolyte interface under reaction conditions. Here, an air-free electrochemical flow cell was designed for in situ synchrotron GI-XRD measurements and used to study the effect of the presence of N2 and a proton source on the formation of Li3N and additional Li-containing species. Li3N formation appears to be the slow step of this reaction, with the presence of EtOH limiting the accumulation of crystalline Li metal and Li3N during reaction, suggesting these species react quickly. To shed more light on these system dynamics, we used time-resolved in situ neutron reflectometry to probe the electrode-electrolyte interface under conditions relevant to Li-N2R. The interface was found to be highly dynamic, with Li-containing species having high mobility depending on the applied current density and porous SEI formation occurring on a short timescale.

This dissertation demonstrates the necessity for in situ characterization methods in developing mechanistic understandings of complicated non-aqueous electrochemical systems, as well as the general importance of electrochemical environment in tuning selectivity to desired products. Further work is required to conclusively identify the composition of SEI and Li-containing species in Li-N2R systems in order to engineer the SEI layer toward controlling electrolyte species diffusion rates to optimize NH3 selectivity toward more commercially relevant values.