Long-term stability of moisture-capturing hydrogels by preventing metal-mediated degradation

Sorbent-based atmospheric water harvesting promises passive, geography-independent, and economical freshwater production. Achieving low water cost through moisture harvesting demands inexpensive, high-performance, and durable sorbents. Among all sorbents, hydrogel salt-composites have exceptional performance at a low cost. However, hydrogel durability has so far been overlooked, ultimately preventing moisture capture from realizing reliable, inexpensive water production. In this work, we systematically study hydrogel-salt composite degradation under different conditions. We demonstrate that commonly used polyacrylamide-lithium chloride (PAM-LiCl) are intrinsically durable, with a limited decrease (~50%) in their elastic moduli, even at elevated temperatures (75 °C) and for prolonged durations (>8 months). In contrast, degradation quickly occurs (<3 weeks) when these hydrogels interface with copper and copper oxides, as is common practice in moisture-harvesting devices or for polyvinyl alcohol-lithium chloride hydrogels (in <50 days). We rationalize these results through a proposed metal-mediated degradation mechanism involving hydroxyl radical generation, which is consistent with our PAM-LiCl observations, including ion concentration measurements and experiments with other metals. With the insights from our experiments and proposed mechanism, we implement coatings which prevent hydrogel degradation. This enables stable cyclic moisture absorption-desorption (>190 cycles) and provides a path towards <0.01 $/L water from moisture.