INVESTIGATING NEW CLASSES OF NON-DELIQUESCENT MATERIALS FOR THERMOCHEMICAL ENERGY STORAGE

Kavin Chakravarthy Thangaraj

Thermochemical energy storage using salt hydrates offers high energy densities and near-zero standing losses, making it attractive for industrial waste heat recovery in the 100–300 °C range. However, most inorganic salt hydrates suffer from deliquescence, corrosion, and capacity fade under cycling, which have prevented their practical deployment. This dissertation explores a design strategy based on selecting materials that are sparingly soluble or insoluble in water, so that liquid-phase formation and the associated failure modes are avoided by the material's intrinsic chemistry rather than by system-level workarounds.Two material tracks are investigated. The first is copper hydroxychloride (CHC; Cu2(OH)3Cl), a sparingly soluble inorganic compound that stores heat through reversible dehydroxylation in the 200–300 °C range. A multimodal characterization approach combining TGA/DSC, ex-situ and in-situ XRD, XPS, and Calvet calorimetry establishes the full dehydroxylation pathway and identifies the conditions under which the Cu2OCl2 intermediate can be stabilized against chloride loss. The key finding is that continuous gas flow during dehydroxylation drives Cl2 loss and progressive capacity fade by removing chlorine from the local environment and shifting equilibrium toward irreversible dechlorination, causing enthalpy to decline from ~402 J/g in cycle 1 to ~79 J/g by cycle 10. Suppressing gas flow stabilizes Cu2OCl2 through 325 °C, eliminates dechlorination entirely across 10 full cycles, and results in 95 wt% CHC recovery with hydration enthalpies reaching 599 J/g. These results establish CHC as a proof-of-concept member of the hydroxychloride subclass of double-anion hydroxide salts, an underexplored structural family for TCES. The second track introduces organic pharmaceutical salt hydrates as a new class of TCES materials. Ibuprofen sodium dihydrate (ISD; C13H17NaO2·2H2O) is identified through systematic screening of over one million crystalline compounds and characterized for its potential as a low-temperature TCES material. ISD dehydrates reversibly between 60 and 110 °C with a dehydration enthalpy of 440 ± 10 J/g, achieves 99.9% cycling efficiency over 100 cycles, and does not deliquesce under any tested humidity condition. In-situ characterization reveals that ISD stores energy through a coupled mechanism involving both chemical dehydration and a concurrent solid-state phase transition from a racemic compound to a racemic conglomerate, which accounts for roughly 50% additional enthalpy beyond dehydration alone. Long-term cycling to 150 cycles confirms 99.99% efficiency with stable thermal and structural performance, with surface porosity developing gradually and improving hydration kinetics rather than degrading them. No self-discharge is detected after 15 days of storage at 10% RH. Two additional pharmaceutical hydrates are tested to assess how broadly this approach extends across different hydrate structural classes. A techno-economic analysis of non-API ibuprofen sodium synthesis at production scale estimates a material cost of approximately $60–63/kWhth under 2024 industrial commodity pricing, which exceeds the DOE materials screening benchmark. However, on a per-cycle basis, this cost falls to approximately $0.060–0.063/kWhth at 1,000 cycles, reaching levels tractable within system-level cost targets for daily load shifting applications. More broadly, the analysis identifies a design principle: the structural tunability of organic frameworks allows backbone length, functional group density, and coordination geometry to be modified simultaneously to improve energy density, shift hydration temperature, and reduce synthesis cost, a degree of freedom that inorganic salt hydrates do not offer. Together, these results demonstrate that the non-deliquescent design strategy is viable across both inorganic and organic material classes and across two temperature windows of practical relevance to industrial waste heat recovery.

INVESTIGATING NEW CLASSES OF NON-DELIQUESCENT MATERIALS FOR THERMOCHEMICAL ENERGY STORAGE

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