Nanoporous Sponges as Carbon-Based Sorbents for Atmospheric Water Generation

Abstract

Water scarcity threatens more and more people in the world. Moisture adsorption from the atmosphere represents a promising avenue to provide fresh water. Nanoporous sponges ("NPSs" ), new carbon-based sorbents synthesized from the pyrolysis of resorcinol-formaldehyde resin, can achieve comparable performance to metal organic framework-based systems, but at a significantly lower cost. Oxygen and nitrogen functionalities can be added to the NPS surface, through oxidation and addition of phenanthroline to the initial reagent mixture, respectively. The resulting NPS sorbents have high specific surface areas of 347 to 527 m 2 •g −1 and an average capillary-condensation-compatible pore size of 1.5 nm. When oxidized, the NPS can capture up to 0.28 g of water per gram of adsorbent at a relative pressure of 0.90 (0.14 g•g −1 at P/P sat = 0.40) and maintain this adsorption capacity over multiple adsorption/desorption cycles. Scaled-up synthesis of the NPS was performed and tested in an experimental water capture setup, showing good agreement between small-and larger-scale adsorption properties. Water adsorption isotherms fitted with the theoretical model proposed by Do and Do demonstrate that hydroxyl functionalities are of key importance to NPS behavior.

Experimental and Theoretical Assessment of Water Sorbent Kinetics

“…Despite these unfavorable properties, NPS was able to compete in terms of DWY because of a higher apparent density, enabling it to pack more material in the same layer thickness and open porosity. On top of that, the water uptake of 0.15 kg water /kg sorbent can be improved by post-pyrolysis functionalization with thermal oxidation, leading to a water uptake comprised between 0.20 and 0.30 kg water ·kg sorbent –1 . This water uptake could be potentially improved up to 0.50 kg water ·kg sorbent –1 , as demonstrated by Huber et al with a CO 2 activation that drastically improved the microporosity of initial resorcinol–melamine–formaldehyde resin …”

“…Sorbent Materials. The NPS are synthesized from the pyrolysis of resorcinol−formaldehyde resin, a procedure described thoroughly in Legrand et al 20 Type-A silica gel (Dry-Packs) was purchased from Amazon. Common silica gel (used as a desiccant) was selected over research grades since large-scale applications are more likely to employ a widely available and inexpensive product.…”

“…. 20 This water uptake could be potentially improved up to 0.50 kg water •kg sorbent −1 , as demonstrated by Huber et al with a CO 2 activation that drastically improved the microporosity of initial resorcinol−melamine−formaldehyde resin. 37 The design of a large-scale system would require accounting for energy transfer due to larger sorbent amounts and more confined spaces, promoting heat accumulation.…”

“…19 More recently, we reported the synthesis of nanoporous sponges (NPS) based on the pyrolysis of resorcinol− formaldehyde resins. 20 As synthesized, this material can capture 0.15 kg water •kg sorbent −1 , but when oxidized in postpyrolysis treatment, the water uptake can reach up to 0.20− 0.30 kg water •kg sorbent −1…”

An Asymmetric Hygroscopic Structure for Moisture‐Driven Hygro‐Ionic Electricity Generation and Storage

“…[16][17][18][19] The functional groups of these materials play a key role; not only do they provide affinity sites for water sorption, especially oxygen-containing species, but also produce free charge carriers (e.g., protons) on contact with water. [7,20] The directional movement of these moistureliberated carriers (for example, from one side of an ion-selective porous membrane to the other) drives the development of the electric field. [17] Thus, a difference in the concentration of mobile charge carriers is essential in enabling this directional diffusion for electric field generation.…”