Anjali Mulchandani1 Paul Westerhoff1

1, Arizona State University, Tempe, Arizona, United States

Instead of removing salts and pollutants from water, atmospheric water capture (AWC) removes clean water from air. Treating unconventional water supplies (e.g., oceans, brackish groundwater, wastewater, stormwater) to drinking water quality is more expensive than treating water supplies developed decades ago (e.g., pristine lakes or rivers). Therefore, in a paradigm shift, water can be obtained from an alternate freshwater reservoir – the atmosphere. An AWC system operates in a three step process: i) desiccant materials adsorb water vapor in the surrounding air; ii) heat drives desorption of vapor form the desiccant onto a cool surface, where iii) clean water is condensed and collected. Current desiccant-based AWC systems such as electrical dehumidifiers and outdoor solar heat driven systems are limited in their capabilities (1-2.5 L/m2/day). Assuming $0.10/kWh, electrical systems are energy intensive (0.29 L/kWh), and the cost of water production (~$0.35/L) is still 50x-100x more than ocean desalination.
We reduced energy demands of bulk heating of desiccants (step ii) through application of photothermal nanomaterials, thus almost eliminating energy operating costs for AWC systems. We have developed nanomaterial-enabled desiccants that are light-active, producing localized centers of heat directly on their surface in the presence of solar light. Heat transfer from the activated nanomaterial to the adjacent adsorbed water vapor molecule will increase kinetics of desorption. The regenerated desiccant can thus be cycled back as an adsorbent (step i) more quickly, allowing for a larger quantity of water to be harvested daily.
Commercially available silica gel desiccants (SiO2) were made light-active through silanization with 3-aminopropyltriethoxysilane followed by nanoparticle coating. Inexpensive Cabot Emperor 2000 carbon black (CBNP) was compared to gold (AuNP), a model plasmonic photothermal nanomaterial. UV-Vis measurements showed CBNP coated SiO2 to absorb light in the full visible spectrum, while AuNP coated SiO2 only absorbed light at resonant wavelengths correlated to nanoparticle size. Visible light spectra utilization efficiency correlates with differences in surface temperature and rate of heating under 1-sun of simulated solar irradiation (Xe/Hg lamp). In under 5 minutes, desiccant surface temperature increased 8x with CBNP monolayer vs 4x with AuNP monolayer, in comparison to bare SiO2. Thermodynamic and heat transfer models were developed to quantify heat generation by nanoparticles and calculate theoretical water vapor desorption potential. These results motivate development of an optimized, energy-efficient solar light and heat driven AWC system which can at least double quantity of water produced per day in comparison to commercial alternatives.