Hydrothermal Aging of SAPO-34/Nexar Composite Coatings for Thermal Energy Storage

Efficient Thermal Energy Storage (TES) systems are vital for advancing energy efficiency and integrating renewable sources. Within this context, innovative composite coatings offer a promising avenue to enhance TES system effectiveness and longevity. This study focuses on developing and characterizing novel composite coatings, specifically tailored for TES applications, consisting of a sulfonated pentablock terpolymer (Nexar) matrix, known for its excellent water vapor permeability, filled with SAPO-34 zeolite filler, a microporous substance with high heat exchange capabilities at lower temperatures. A significant challenge for adsorbent coatings and composite materials lies in their susceptibility to aging, particularly when the polymeric matrix is crucial for maintaining structural and functional integrity. Prolonged exposure to adsorbates like water vapor can severely degrade the coating's performance. Therefore, assessing the composite's aging response is crucial for confirming its practical viability.

In this work, we subjected SAPO-34/Nexar composite coatings, with varying zeolite concentrations, to hydrothermal aging to verify their stability. The aging protocol involved hundreds of wet (30°C, 80% RH) and dry (40°C, 20% RH) cycles within a controlled climatic chamber. We evaluated the coatings' mechanical properties (including pull-off strength and scratch resistance) and adsorption/desorption capacity (by means of DVS analysis), both before aging and after specific aging durations. The mechanical properties of the coatings decrease with an increasing number of cycles: a decline that is most pronounced at lower zeolite concentrations. The 90 wt% zeolite composition offers superior mechanical stability over cycles, while exceeding this percentage leads to a degradation of properties due to the reduced amount of polymer binder. DVS analysis indicated only a minimal reduction (between 2.9% and 3.5%) in adsorption capacity. These findings highlight that a balanced composition is crucial for the practical viability of high-filler content SAPO-34/Nexar coatings in long-term TES applications.