UC Davis

Hydrogen is a clean, zero-emission technology with significant potential to replace fossil fuels in a variety of applications and industries, especially those that are hard to decarbonize like metals and chemical manufacturing. Hydrogen has high gravimetric energy density but low volumetric energy density due to its small size and must be compacted into a liquid or compressed gas for practical uses like long term storage and transportation. Moreover, safely containing this volatile and flammable species when compressed still presents challenges and requires widespread rigorous testing as well as standardized safety codes. It has been shown that through a reaction between aluminum and water, hydrogen gas and aluminum hydroxide can be quickly and spontaneously produced with no carbon emissions. Aluminum and water are the energy and hydrogen sources, respectively. Aluminum on its own, however, is passivated with an aluminum oxide layer that prevents any reaction. The aluminum must be activated to overcome this barrier. One activation method is by combining with other metals to form low melting temperature alloys. In this activation, gallium is used to alloy with aluminum, which has a eutectic temperature of 26.6C at a composition of 0.8 wt% Al-99.2 wt% Ga. Therefore, aluminum-gallium alloys can react with water at near room temperature conditions which is advantageous for practical applications. Solid aluminum-gallium alloys are in the activated state when liquid alloy is present at the interior grain boundaries. These alloys are high in aluminum and low in gallium weight percent. However, they suffer from low reaction rates and result in less than 100% yields. Liquid aluminum-gallium alloys react at significantly higher rates and do achieve 100% yields. This thesis focuses on low aluminum content liquid alloys in the water-splitting reaction. This technology provides a pathway for safe hydrogen production on-demand and at the point of use by storing and transporting aluminum, gallium and water instead of hydrogen, alleviating the need for this gas to be compressed. Being able to produce hydrogen safely on-demand at the point of use is an advantage that creates new opportunities for this fuel including re-fueling stations, back-up power generation via fuel cells, and power for remote off-grid locations. However, large challenges exist in bringing down the costs of producing hydrogen in this way to Department of Energy (DOE) cost targets of $2 to $3 per kilogram of H2. By leveraging the other reaction product, aluminum hydroxide, the Woodall process has the potential to achieve this cost target. The aluminum hydroxide formed in the reaction is known as gibbsite (-Al(OH)3), and its calcined form of alumina (-Al2O3) carries significant economic value at purities of at least 99.99 wt% (4Ns) due to its use in a variety of applications including sapphire manufacturing for semiconductors and LED lighting in displays. However, the purity of the aluminum hydroxide formed in the reaction is less than this threshold due to the presence of gallium which adheres to it. Reducing the impurity levels of gallium in gibbsite is the motivation for the work in this thesis. First, experimental methods used in reactant preparation, reaction with water, and precipitate collection stages of the process were refined to reduce the large variations observed in previous repeatability experiments. This improved experimental process was used to investigate specific reaction conditions. Prior research demonstrated that slowing down the rate of reaction resulted in lower gallium contamination of gibbsite. Empirical evidence from this work also determined that the formation of oxides of gallium and aluminum appear to increase the gallium contamination. Therefore, reaction conditions that manipulated the reaction rate and amount of oxidation of alloy were investigated. The gallium contamination is found to be in two phases: elemental gallium and an oxidized gallium. It was found that pre-reaction oxidation in air could increase gallium concentrations to as high as 29.0 wt% Ga, driven by the presence of more elemental gallium. Lowering the reaction rate by reducing aluminum concentration in alloys to below 1 wt% Al had the most significant effect observed, with gallium concentrations dropping to 0.4 wt% Ga. Raising the temperature of the water before reaction with alloy lowered gallium levels down to 0.4 wt% Ga, which was contrary to expectation that higher water temperature would raise the reaction rate. The gallium in these low Ga wt% samples was in the oxidized gallium phase, as almost no elemental gallium was observed. Lastly, gallium removal after the reaction was investigated, and it was shown that an effective method is by high temperature exposure to hydrogen gas which lowered gallium levels to below the detection limit of Energy Dispersive X-ray Spectroscopy detector of the Scanning Electron Microscope (instrument reported value = 0.0 wt% Ga). Future research recommendations are proposed to further understand the effect of these key process factors. New process methods are also proposed to study their impact on lowering the amount of gallium in the aluminum hydroxide formed during the reaction. The findings in this thesis and future work will help drive this technology towards cost-effectively producing hydrogen and help realize this fuel’s potential for deep decarbonization across a variety of industries.