Microstructure-tunable Torlon® membrane coatings for stable, high performance thermochemical heat storage

Porous millimeter-sized K 2 CO 3 granules are promising thermochemical energy storage (TCES) candidates; however, they are prone to dissolution, caking, and bed consolidation during repeated hydration/dehydration cycles. This study employs a nonsolvent-induced phase separation (NIPS) process to develop mechanically durable and water-vapor-permeable Torlon (polyamide-imide, PAI) shell layers for granule encapsulation. By modulating the polymer dope concentration (11–15 wt% Torlon in DMSO), the shell microstructure was systematically transitioned from a macrovoid-dominated, finger-like morphology to a dense, sponge-like network at the core–shell interface. Flat-sheet membrane evaluations (JIS L 1099 A-1) revealed that 11–12 wt% Torlon membranes provide high water-vapor transfer rates of approximately 780–900 g m −2 h −1 , whereas concentrations of 14–15 wt% cause significant transport inhibition (approximately 315–415 g m −2 h −1 ) due to suppression of finger-like microchannels. Cyclic assessment via simultaneous thermal analysis (STA) confirmed that 11–13 wt% samples maintain gravimetric energy densities of 530–580 kJ kg −1 , approaching the theoretical maximum, while 14 and 15 wt% samples exhibit incomplete conversion and substantial energy losses. Long-term durability testing in a confined packed bed over 50 cycles identified 13 wt% Torlon® as the optimal candidate. Micro-computed tomography (micro-CT) analysis demonstrated that the 13 wt% coating achieved a 72.8% survival rate and approximately 90% coating continuity, while effectively minimizing particle agglomeration under reactor-like confinement. These findings demonstrate that a mixed finger–sponge microstructure can accommodate repeated volumetric strain, providing a practical strategy for engineering durable, high-performance thermochemical heat storage materials. • Tunable Torlon® shell layers were developed for stable thermochemical heat storage granules. • Shell microstructure transitions from finger-like to sponge-like by varying polymer concentration. • A 13 wt% formulation offers the optimal balance of mechanical strength and vapor permeability. • The graded “finger-sponge” architecture effectively accommodates the salt core’s volumetric strain.