Session: K6-07: Thermal Energy Storage Systems - II

Paper Number: 184142

184142 - Tuning Energy and Power Density in Hydrogel Thermal Batteries via Isotherm-Informed Heat and Mass Transfer Modeling 

Abstract: 

Sorption-based thermochemical energy storage (TCES) is gaining momentum as an attractive pathway for compact, high-energy density, and sustainable thermal management. While zeolites and metal–organic frameworks have long dominated the TCES landscape because of their sharp isotherms and rapid vapor transport, their relatively modest uptake capacity and higher production cost limit scalability. Hygroscopic hydrogels—polymer networks infused with inorganic salts—offer a fundamentally different materials platform. Their low cost, manufacturability, structural tunability, and ability to host high salt loadings position them as promising candidates for thermal batteries that operate at low to medium temperatures. Yet the same property that enables their high energy density—the incorporation of hygroscopic salts—introduces significant complexity, as salts transition between crystalline and deliquesced phases inside the polymer network during sorption. This phase behavior alters both equilibrium uptake and transport pathways, making it difficult to reliably predict performance using conventional models.

The performance of TCES materials hinges on two intertwined metrics: energy density, set by water uptake and the enthalpy of sorption, and power density, set by how fast water molecules can move through the material during charging and discharging. Hydrogels introduce a structural design variable absent in rigid sorbents: their network can be engineered to be dense and non-porous, or highly porous through freeze-drying, salt templating, or other processing routes. Porous architectures provide short diffusion distances and large accessible surface area, while dense hydrogels offer mechanical robustness and reduced sensitivity to salt leakage. The effect of these structural choices on sorption pathways is nontrivial because the effective diffusivity in a hydrogel is not a constant material property. Instead, it depends on both the microstructure and the thermodynamics encoded in the water sorption isotherm.

A key challenge arises from the steep isotherm transitions that occur when salt domains begin to deliquesce. In sorbents with sharp steps, the thermodynamic factor, defined as the sensitivity of vapor pressure to changes in water concentration, expressed as ∂ln(p)/∂ln(c), can change by more than an order of magnitude across a narrow concentration window. This phenomenon directly alters the effective driving force for vapor transport, meaning that ignoring its concentration dependence can lead to substantial misprediction of kinetic behavior. For hydrogels, where salt dissolution and pore-scale restructuring both influence the isotherm, this effect becomes even more pronounced. To address this gap, the present work develops an isotherm-informed, concentration-dependent transport model for predicting sorption kinetics in both porous and non-porous hygroscopic hydrogels. Rather than imposing a constant diffusivity, the model captures how the internal polymer network, porosity, and shifting isotherm slope collectively shape mass transport. The model extends to address the kinetics during even when operating in during crystallization. We parameterize the model using experimentally measured water vapor isotherms for polyacrylamide-based hydrogels and their porous analogues prepared by freeze-drying. These isotherms reveal significant variations in slope during deliquescence, providing clear evidence that transport behavior is tightly coupled to salt phase transitions.

We use this framework to evaluate adsorption and desorption dynamics under TCES-relevant conditions, with charging temperatures between 40-70°C and operating humidities spanning the isotherm step. Preliminary findings show that porous hydrogels can accelerate sorption kinetics by a factor of 3-5× relative to MOFs and inorganic salts. However, the strong concentration dependence of the thermodynamic factor results in nonlinear kinetic behavior that cannot be captured by traditional Fickian models. As a result, constant-diffusivity approaches can misestimate achievable power density by 30–70%, depending on porosity and operating humidity. These results highlight the importance of linking isotherm thermodynamics with transport physics to accurately describe hydrogel-based TCES behavior. By resolving how deliquescence, network morphology, and thermodynamic slope interact, this work establishes a predictive framework for designing next-generation hydrogel sorbents. The insights enable rational tuning of porosity, salt loading, and polymer architecture to optimize both energy density and power density, ultimately guiding the engineering of thermal batteries that are scalable, low-cost, and capable of fast cycling for distributed and grid-integrated thermal energy storage.

Presenting Author: Ibrahim Halil Sahin Georgia Institute of Technology

Presenting Author Biography: Ibrahim Halil Sahin is a PhD candidate in the department of Mechanical Engineering at the Georgia Institute of Technology, working under the supervision of Dr. Bachir El Fil. His research focuses on transport phenomena in solid sorbents and hydrogels, with applications in thermal energy storage and atmospheric water harvesting.

Authors:

Ibrahim Halil Sahin Georgia Institute of Technology
Bachir El Fil Georgia Institute of Technology