Synthesis and characterisation of microcrystalline powder MOF-801

In a 500 mL screw-capped jar, 5.8 g (50 mmol) of fumaric acid (Fluka, 99%) and 16 g (50 mmol) of ZrOCl 2 ·8H 2 O (Alfa Aesar, 98%) were dissolved in a mixed solvent of DMF and formic acid (200 and 70 mL, respectively). The mixture was then heated in an isothermal oven at 130 °C for 6 h to give as-prepared MOF-801 as white precipitate. The precipitate from three reaction jars was collected by filtration apparatus using a membrane filter (45 µm pore size), washed three times with 100 mL DMF, three times 100 mL methanol, and dried in air. Air-dried MOF sample was transferred to a vacuum chamber. The chamber was first evacuated at room temperature for 5 h until the pressure dropped below 1 kPa. After that, the sample was heated in vacuum at 70 °C for 12 h, and then at 150 °C for another 48 h. This finally gave activated MOF-801 as a white powder (yield: 30 g).

Low-pressure gas (N 2 and Ar) adsorption isotherms (Supplementary Fig. 2) were measured using volumetric gas adsorption analyser (Autosorb-1, Quantachrome). Liquid nitrogen and argon baths were used for the measurements at 77 and 87 K, respectively. The powdered particle density (ρ P ) of activated MOF-801 was estimated to be 1400 ± 20 kg m−3 from the pycnometer (Ultrapyc 1200e, Quantachrome) (skeletal density ρ s = 2.6991 g cm−3) and BET pore volume measurements (V P = 0.3425 cm−3 g) using the following equation: \(\rho _{\mathrm{p}} = 1/(V_{\mathrm{p}} + 1/\rho _{\mathrm{s}})\). The particle size (see Supplementary Fig. 4) and intercrystalline diffusion characteristics of the powder MOF-801 were characterised and details are presented in Supplementary Note 2 and Supplementary Fig. 5. Additionally, the intra-crystalline diffusion characteristics were characterised as detailed in Supplementary Note 3 and Supplementary Fig. 6.

Synthesis and optical characterisation of OTTI aerogel

The OTTI silica aerogel was synthesised by sol-gel polymerisation of tetramethyl orthosilicate (TMOS, 131903, Sigma Aldrich), using an ammonia solution (NH 3 , 2.0 M in methanol, 341428, Sigma Aldrich) as a catalyst to promote both hydrolysis and condensation reactions9,10. TMOS was diluted by methanol (MeOH, 322415, Sigma Aldrich) followed by addition of NH 3 and water. The mixing molar ratio of chemicals was NH 3 :TMOS:water:methanol = 0.004:1:4:6. Then, the solution was gelled in a disposable polystyrene container. After 2 weeks, the container was dissolved away using acetone. The mother solvent was replaced with ethanol (EtOH, 89234-848, VWR) to be prepared for critical point drying (CPD, model 931, Tousimis) as EtOH is miscible with liquid CO 2 . To dry the wet gels in EtOH without cracks, it is important to dry them slowly to minimise capillary pressure during the CPD process. A bleed rate of 100 psi h−1 was used to decrease the CPD chamber pressure from ~1300 psi to ambient pressure. After drying, the monolithic aerogels were annealed at 400 °C for 24 h to maximise their transmittance. The aerogel was cut to the final size using a laser cutter (Epilog Zing). Experimentally measured solar transmittance and predicted thermal conductivity21,22 of the 8-mm-thick OTTI aerogel are shown in Supplementary Fig. 3a and b, respectively.

Device fabrication

The adsorber layer was fabricated by first brazing a porous copper foam (~100 pores per inch or ppi), 0.26 cm thick, onto a copper plate (5 × 5 × 0.17 cm). The activated MOF-801 was infiltrated into this foam-plate structure by immersion drying in a ~50 wt. % aqueous dispersion. The copper foam provided structural rigidity and helped enhance the effective thermal conductivity of the layer, given the intrinsically low thermal conductivity of the porous MOF. The layer was then dried under vacuum for 4 h at temperature of 70 °C and the total mass dehydrated MOF-801 was characterised to be 2.98 g. This corresponds to a packing density of 464 kg m−3 (dry) and a porosity of 0.67. In order to enhance solar absorption, the back side of the adsorber was coated with Pyromark paint. This coating was optically characterised using a UV-Vis-NIR spectrophotometer (Cary 5000, Agilent) and found to have a solar-weighted absorptivity of 0.95.

The adsorber layer was then integrated into an enclosure constructed with acrylic sheets (0.318 cm thick). The top face was designed with a cut-out, equal in size to the adsorber layer (5 cm × 5 cm) and pilot holes to suspend the adsorber layer with nylon strings. Any gaps found between the side walls of the adsorber layer and the cut out were sealed with high temperature vacuum grease (Dow Corning). In addition, a layer of transparent polyethylene wrap was stretched over the entire top face and sealed against the side walls. Both these measures prevented leakage of any desorbed vapour. Thermal insulation (white in colour) was attached on all side walls except the view port. The adsorber side was completed by placing a piece of OTTI aerogel measuring 5 × 5 × 1 cm. The bottom face of the enclosure was made with a 4 × 4 cm cut-out to enable integration with a condenser assembly. The condenser assembly comprised a 4 × 4 × 0.6 cm polished copper piece that was bonded with high conductivity thermal epoxy (Omega Therm, Omega Engineering) to a heat pipe heat sink (NH-L9x65, Noctua). The air-cooled heat sink consisted of a finned heat pipe array with a fan that consumed ~0.9 W of electrical power to dissipate the condensation heat. The finished device measured 7 × 7 × 3.2 cm (excluding the heat sink, fan, insulation and aerogel) and was mounted on a stage with adjustable tilt to enable experiments under both GHI (no optical concentration, no tilt) and GNI (with optical concentration of 1.8× and tilt at elevation angles of 55−75° and azimuth angles of 100−180°).

Experimental procedure

The water harvesting experiment comprises two phases: night-time vapour adsorption and day-time water harvesting and condensation. During vapour adsorption, typically started at 20:00 hours local time (UTC/GMT—7 h), the adsorber layer with its acrylic frame was mounted into the cover of an air-tight food storage container with the pyromark coated side up for night-time radiative cooling (Supplementary Fig. 1). The sides of the air-tight container were modified to fit a fan (0.9 W; 12 VDC) and enable cross flow of ambient air (vapour source). Two T-type thermocouple (5TC series, Omega Engineering) were used to measure the temperature of the adsorber layer during adsorption. In order to estimate the extent of radiative cooling and ambient temperature, another T-type thermocouple was placed in the air stream of another fan. Relative humidity measurements were made with a capacitive RH sensor (RH820U, Omega Engineering). Transparent polyethylene wrap was used to suppress convective heat loss on the black absorber side of the layer. Prior to the exposure to the clear sky, the container cover was wrapped in aluminium foil so that the MOF layer could equilibrate with the ambient air. Once the foil was removed, the MOF layer was exposed to the sky and an instant temperature drop of ~3 K below ambient was observed, as shown in Fig. 3a, b, due to radiative cooling. Adsorption was allowed to occur overnight and the sample was sealed into the device (between 06:00 and 07:00 hours local time) to prevent undesired loss of water due to the RH swing.

The procedure for water release and condensation typically started between 10:00 and 11:00 hours local time. In addition to the two T-type thermocouples embedded into the adsorption layer, three additional T-type thermocouples were used to measure temperatures: two for the copper condenser plate and one for the vapour space between the adsorber layer and the condenser plate. Ambient humidity and temperature conditions were recorded as described during the adsorption phase. The heat of condensation was dissipated to the ambient through the heat sink and fan operating at 0.9 W. The incoming solar irradiation (both global horizontal (GHI) and global normal (GNI) irradiations) was measured with a pyranometer (LP02-C, Hukseflux). The measured GNI was used to evaluate the DNI as: DNI = GNI – DI (diffuse irradiance) according to the weather data available from a weather station in Tucson, AZ, USA (available at NREL, SOLRMAP University of Arizona (OASIS)) for clear days in May 2017. The ratio between the GNI and DNI was found to be 0.93 and it matched well to an available correlation23. The area ratio between the lens and solar absorber surface was ~2.5; however, the achieved optical concentration of 1.8× was due to transmittance loss of the lens and the square solar absorber area being circumscribed by the circular concentrated solar irradiance (Fig. 2a). The actual optical concentration achieved with the concentrating lens was characterised to be 1.8× for the focal distance during the outdoor experiments with a thermopile detector (919P-040-50, Newport) and a solar simulator (92192, Newport Oriel). Solar transmittance of the transparent polyethylene wrap, ~0.93, was characterised with the pyranometer under direct solar irradiance. Images were acquired with a digital camera (EOS DS126211, Canon) to visualise the condensation process (see Fig. 3 and Supplementary movies 1 and 2). At the end of desorption, the MOF layer and its acrylic frame were extracted from the device to prevent re-adsorption of condensed water and isolated in an air-tight box. The adsorber assembly was only removed in the evening time to restart the adsorption phase for the next cycle. A total of five water harvesting cycles are reported in this study: cycle 1 (Supplementary Fig. 8a and b), cycle 2 (Fig. 3a, c), cycle 3 (Supplementary Fig. 8c and d and Supplementary movie 2), cycle 4 (Supplementary Fig. 8e and f), and cycle 5 (Fig. 3b, d).

Water quality analysis

In order to quantitatively characterise the harvested water, a bench-top adsorption cycling system was constructed. A schematic of the water collection apparatus for ICP-MS analysis is shown in Supplementary Fig. 11a. The system consists of five main components, namely, adsorption and condenser chambers, a glass flask that serves as a reservoir for HPLC water (OmniSolv HPLC grade water, VWR), two temperature-controlled thermoelectric stages (CP-200HT-TT, TE Tech), and a vacuum pump. The adsorption and condenser chamber were custom-designed copper vacuum chambers (2 × 2 × 1 cm) with a removable lid. The adsorption chamber additionally had a layer of copper foam (2 × 2 × 0.8 cm) brazed to the bottom, which was infiltrated with activated MOF-801 (~1.5 g). These chambers were individually placed in thermal contact with a temperature-controlled thermoelectric stage that allowed for continuous cycling. Thermocouples (5TC series, Omega Engineering) were inserted into pilot holes made in the side walls of the copper chambers. For cycling, a pair of electronically controlled vacuum valves were used to link the adsorbent chamber to either the water reservoir during adsorption or the condenser chamber during desorption.

The adsorption−desorption cycles were performed under evacuated conditions to enable efficient transport of vapour across distances of ~0.5 m through the hoses and valves as shown in Supplementary Fig. 11a. The water in the glass flask was first degassed to remove non-condensable gases by connecting it to the vacuum pump and freezing the water. The flask was then heated to melt the ice under evacuation and reduce the solubility of non-condensable gasses. This cycle was repeated three times. The adsorption and condenser chambers were heated to 60 °C for 2 h under evacuated conditions to ensure there was no residual water in the system. The cycling experiments started with the adsorption phase, where the water reservoir was exposed to the adsorbent chamber. The dry adsorbent triggered evaporation and the generated vapour was adsorbed. During adsorption, the chamber was held at a constant temperature of 30 °C to extract the adsorption heat as well as prevent any condensation of vapour from the reservoir kept at ~20 °C. After complete adsorption (~40 min), the adsorption chamber was isolated from the water reservoir and exposed to the condenser chamber. The thermoelectric stage of the adsorption chamber was programmed to ramp up to 60 °C at this stage while the condenser stage was always maintained at 0.5 °C. The desorption was allowed to continue for 40 min at the end of which the adsorption chamber was opened to the reservoir and simultaneously cooled to 30 °C for the next cycle. Representative temperature and pressure profiles for a desorption−adsorption cycle are shown in Supplementary Fig. 11b. This cycle was repeated 18 times and about 8 g of condensed water was collected (i.e., ~0.3 L of water per kg of MOF per cycle).

The HPLC grade water from the reservoir was used as a control sample. The concentration of potentially contaminant elements was analysed using an inductively coupled plasma–mass spectroscopy system (ICP-MS, Agilent 7900, 68403A). Both the harvested water and control sample were analysed for the following elements: iron (from tubes/hoses), copper (from foam, chambers and braze alloy), silver, indium (both from braze alloy) and zirconium (from MOF compound). In addition, FT-IR spectra of control water (HPLC grade) and collected water from MOF-801 were collected in-house using a Bruker ALPHA Platinum ATR-FT-IR Spectrometer equipped with a single reflection diamond ATR module.

Data availability

The authors declare that all data supporting the findings of this study are available within the article and its Supplementary Information or from the corresponding author upon reasonable request.