Mechanisms of Liquid-Metal-Activated Aluminum-Water Reactions and Their Application
The work presented in this thesis contributes to the fundamental understanding of the liquid-metal-activated aluminum-water reaction system, as well as methods that leverage these insights to improve the practicality of aluminum-based fuels. Water-reactive aluminum is a promising energy storage mat...
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| Format: | Thesis |
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Massachusetts Institute of Technology
2022
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| Online Access: | https://hdl.handle.net/1721.1/139978 https://orcid.org/0000-0001-6613-6929 |
| Summary: | The work presented in this thesis contributes to the fundamental understanding of the liquid-metal-activated aluminum-water reaction system, as well as methods that leverage these insights to improve the practicality of aluminum-based fuels. Water-reactive aluminum is a promising energy storage material given its ability to generate hydrogen and heat at a high volumetric energy density. Accounting for only the hydrogen released in this aluminum-water reaction, energy densities up to 36.3 MJ/L can be achieved, compared to 7.2 MJ/L for liquid hydrogen. The ability for this reaction to generate hydrogen on-demand also eliminates safety concerns associated with gaseous or liquid hydrogen storage. In addition, the heat generated from the aluminum oxidation (15.8 MJ/kg) can be used to power thermal processes including seawater desalination, making aluminum a potentially attractive fuel source for disaster relief applications in which debris can be mined for energy to generate critical resources like electricity and potable water.
To make aluminum water-reactive, its natural oxide layer must first be disrupted. One promising activation approach is to introduce a liquid-phase gallium-indium eutectic (eGaIn) into the aluminum grain boundary network. While this method produces a highly reactive fuel with only roughly 5 wt.% added, viability for practical applications had hinged on several important but previously untested assumptions made in the literature. Specifically, the work presented in this thesis addresses the uncertainty around (1) whether the eGaIn can be recovered as a liquid post-reaction and recycled to activate more aluminum with minimal loss, (2) how the aluminum-water system performs at elevated pressures and with near-stoichiometric water inputs, and (3) whether this process can be applied to practical scrap aluminum with surface contamination and high alloying content.
In this research, SEM-EDS and XRD analysis showed that the activating eGaIn cannot be recovered under standard reaction conditions (i.e. deionized water, 1 bar, 100 degC) due to dealloying of the gallium and indium at the microscale. It was then discovered that in ionic aqueous solutions, liquid-phase eGaIn emerges from solution under specific ambient conditions. From this discovery, a method for recovering and recycling the eGaIn was developed, using NaCl in moderate concentrations as the only additive. In following experiments, >99% of the input eGaIn was recovered and recycled to produce aluminum fuel with no observed loss in performance. With support from additional experimental evidence, it was hypothesized that the recoverability in this method is due to the development of a passivating electronic double layer at the eGaIn-electrolyte interface, thereby inhibiting gallium oxidation and subsequent dealloying.
This proposed mechanochemical reaction theory was then extended to non-ionic solutions. A new experimental technique was developed in which aluminum and water can be reacted arbitrarily slowly in a non-aqueous environment via controlled exposure to room-temperature water vapor, enabling in-progress characterization via SEM-EDS and XRD techniques. This method was used to identify a two-part reaction mechanism in which the aluminum first disintegrates along its microstructure via a fractal-like exfoliation process, followed by its reaction with water at unoxidized sites along the freshly exposed grain surfaces. The indium in the activating alloy was shown to be crucial for the disintegration process specifically, and additional evidence suggests that the initial reaction driving the exfoliation is not a large-scale aluminum-water reaction, but possibly a gallium oxidation reaction instead. It was also shown that ambient oxygen in the reaction environment is capable of repassivating the exposed aluminum grains, severely limiting hydrogen yields.
This reaction mechanism was then studied for constant volume, elevated ambient pressure, and near-stoichiometric water input conditions. An idealized thermodynamics model was developed and validation experiments showed that actual hydrogen yields are suppressed under each of these conditions. Reduced reactivity was observed for <10x stoichiometric water inputs, and experimental evidence suggests excess water is both being taken up into the crystalline reaction products via intercalation and also serves as a physical barrier preventing repassivation by ambient oxygen. It was discovered that by increasing the pH of the reaction environment, a two-fold increase in hydrogen production under near-stoichiometric conditions can be achieved. To account for the reduced reactivity in isochoric reactions, it was observed that the disintegration phase of the reaction mechanism is inhibited at high ambient pressures. Using these insights and empirical parameterizations of reactivity under these various conditions, the accuracy of the thermodynamics model was improved.
Finally, a method for activating practical scrap aluminum using eGaIn was developed. Used aluminum beverage cans (UBCs) were selected as a challenging case study due to their thin geometry, polymer coatings, and high alloying content. It was demonstrated that shredding and compacting the UBCs into pellets under parameters optimized in this research for hydrogen production produces a fuel with consistent theoretical hydrogen yield fractions >0.97. The total energy input for this process was measured at 551 kJ/kg, only 1.8% of the embodied energy of the aluminum. A preliminary economics model that incorporates the results of this work predicts that the value of the electricity, potable water, and high-quality aluminum hydroxide produced by an aluminum oxidation power system is up to 9x that of the input scrap depending on location. In total, this work enables aluminum that would otherwise sit idle in a landfill to be mined locally for energy. |
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