UC Santa Barbara

With the growing threat of climate change, it is important that we rapidly transition from fossil fuels to renewable energy use. Renewable energy technologies have developed considerably making them cost competitive with fossil fuels; however, the key issue inhibiting our transition is storage. The United States has a 261 GW generation capacity from renewables, but only an energy storage capacity of 24.2 GW. Renewables, such as solar and wind, are intermittent, therefore, we cannot fully rely on renewables until we develop new technologies for storage.Ammonia (NH3) is a promising storage medium for excess electricity generated from renewables. Instead of curtailing renewable energy plants during peak hours to prevent grid overload, the excess electricity can be used to electrolyze water to produce H2 which can then be used to in the Haber Bosch process to produce NH3. NH3 meets the standards set by the United States Department of Energy for hydrogen storage and can be utilized in a direct ammonia fuel cell (DAFC) to generate electricity. However, the oxidation of NH3 to N2, nitrogen evolution reaction (NER), is kinetically challenging and incurs large overpotentials under ambient conditions. Developing robust catalysts to mediate the NER are therefore needed. Herein, we report a series of high-valent tetranuclear nickel clusters isolated from the chemical oxidation of an all Ni(II) ([Ni4]) neutral cluster. Electrochemical analysis of [Ni4] reveals three reversible sequential oxidations at 0.248 V (1e-), 0.678 V (1e-), and 0.991 V (2e-) vs. Fc+/Fc corresponding to mono-, di-, and tetra-oxidized species, [Ni4]+,[Ni4]2+, [Ni4]4+, respectively. Using spectroscopic, crystallographic, magnetometric, and computational techniques, we assign the primary loci of oxidations to the Ni centers in each case, thus resulting in the isolation of the first tetranuclear all-Ni(III) cluster,[Ni4]4+. The [Ni4]4+ contains the most high-valent nickel atoms to be reported in a molecule to be crystallographically characterized. Preliminary studies indicate the [Ni4] and its derivatives are promising candidates for investigating electrocatalytic NER and is described in this thesis. Additionally, we describe a transition from studying NER molecular catalysts in organic solvents to water. For smaller devices, aqueous DAFC show promise for industrial viability with volumetric energy densities comparable to or larger than compressed H2 and ~2000x higher than in organic solvents. Ruthenium Bipyridinedicarboxylate (RuBda) complexes are renowned for catalyzing water oxidation at rapid rates (~300 s-1) and were recently reported to catalyze NER in acetonitrile at sluggish rates (~0.5 s-1). We report a RuBda derivative capable of catalyzing the NER electrochemically in water achieving high faradaic efficiencies (>84%) and several turnovers. Under aqueous conditions, the complex exhibits the highest TOF (~4130 s-1) to be reported for a NER electrocatalyst and displays impressive stability in realistic commercial concentrations of NH3 (14,300 equivalents excess) without any indication of decomposition. Our kinetic analyses suggest that the catalyst operates via a unimolecular mechanism which is highly applicable for commercially viable fuel cells.