Simultaneous atmospheric water production and 24-hour power generation enabled by moisture-induced energy harvesting

Water and electricity scarcity are two global challenges, especially in arid and remote areas. Harnessing ubiquitous moisture and sunlight for water and power generation is a sustainable route to address these challenges. Herein, we report a moisture-induced energy harvesting strategy to realize efficient sorption-based atmospheric water harvesting (SAWH) and 24-hour thermoelectric power generation (TEPG) by synergistically utilizing moisture-induced sorption/desorption heats of SAWH, solar energy in the daytime and radiative cooling in the nighttime. Notably, the synergistic effects significantly improve all-day thermoelectric power density (~346%) and accelerate atmospheric water harvesting compared with conventional designs. We further demonstrate moisture-induced energy harvesting for a hybrid SAWH-TEPG device, exhibiting high water production of 750 g m−2, together with impressive thermoelectric power density up to 685 mW m−2 in the daytime and 21 mW m−2 in the nighttime. Our work provides a promising approach to realizing sustainable water production and power generation at anytime and anywhere.


Theoretical analysis of the energy balance of hybrid SAWH-TEPG for water and electricity co-generation
The structure of the SAWH-TEPG device is schematically shown in Figure 1a, 1b.
During daytime, the device is closed and the SAWH-TEPG device is exposed to solar irradiance for water releasing (desorption).The solar energy is first converted into thermal heat and then transferred from the solar absorber to the sorbent through the TEPG module.The desorbed water vapor diffuses from the sorbent layer to the condenser due to a concentration gradient.Accumulation of vapor in the enclosure leads to saturation conditions and consequently, the condensation process occurs at ambient temperature.The heat of condensation is dissipated to the ambient by a heat sink.The energy balance model of this device during solar-driving water desorption and harvesting process can be expressed as: where  is the absorbed solar energy,  is the converted electrical power,  , and  , are the radiative and convective heat loss of the dual-functional coating layer,  , and  , are the radiative and convective heat loss of the sorbent,  desorption heat of the sorbent, which can be calculated as the product of the desorption rate ( , g s -1 ) and the desorption enthalpy (ℎ , J gwater -1 ):   ℎ .As a result, the thermal heat passing through the hot-side and cold-side of TEPG module ( ) are determined by: where  is the area of sorbent,  is the Stefan-Boltzmann constant,  is the emissivity of the sorbent,  and  are the temperatures of the sorbent and the ambient, ℎ is the conductive heat transfer coefficient.The thermal resistance networks.
According to heat transfer characteristics, the Q can also be calculated as: where  is the area of TEPG module,  is the thermal resistance of the TEPGmodule,  and  are the hot-side and cold-side temperatures of TEPG module.
Additionally, the maximum electrical power of a TEPG module ( ) is given by the following equation: where n is the number of thermoelements,  is the loading resistance,  is the Seebeck coefficient of TEPG materials.Notably, in addition to the intrinsic merit of the TEPG material and the  , the theoretical maximum output power ( ) depends only on the temperature difference of the TEPG module (Δ ).
Compared with the conventional solar-driven thermoelectric device where the residual thermal heat is mainly released through natural convection and radiative heat loss, the introduction of sorbent leads to a huge thermal consumption ( ) owing to the water desorption process.Thus, the cold-side temperature of TEPG module decreases from  , to  and the resultant temperature difference of TEPG module enlarges ∆ , to ∆ .As a result, the combination of sorbent leads to an increment in the maximum output power of TEPG module.
During nighttime, the proposed SAWH-TEPG device is opened and the sorbent is exposed to the ambient to saturate with vapor from the natural flow of air and passively cooled with radiation to the sky.Radiative cooling is a passive cooling technique that cools object by radiating a fraction of the object's thermal radiation to the cold of outer space.Different from the conventional AWH device, in which a considerable amount of sorption heat is wasted through nature convection, the radiative cooling surface of our device is applied as the cold-side of the TEPG module, while the hot-side is attached with a sorbent and heated through the sorption heat.Thus, a temperature difference is passively created and electricity can be generated by the TEPG.The energy balance model during radiative cooling and water sorption process can be described as: where  is the sorption heat, which can be calculated as the product of the adsorption rate ( , g s -1 ) and the adsorption enthalpy (ℎ , J gwater Where  the area of dual-functional coating layer,  is the emissivity of the dualfunctional coating layer,  and  are the temperatures of the dual-functional coating layer and the ambient, ℎ is the conductive heat transfer coefficient.According to heat transfer characteristics, the  can also be calculated as: where  and  are the hot-side and cold-side temperatures of TEPG module. The electric power is generated through the combination of adsorption heating and radiative cooling.Compared with conventional radiative cooling-based thermoelectric electricity generation, the synergistic effect of sorption heating and passive cooling is able to enlarge the hot-side temperature of TEPG module ( , to  ) and promote the temperature difference between hot-side and cold-side of TEPG module (∆ , to ∆ ), thus improves the electricity generation performance.Additionally, compared with the naturally cooled AWH devices, the passive radiative cooling facilitates the dissipation of sorption heat, which lowers the sorbent temperature and increases water uptake, leading to a significant improvement in water harvesting output of the SAWH device.

Calculation of radiative cooling power, sorption heating power and desorption cooling power
A thermal model is developed to analyze the performance of the SAWH-TEPG device (Supplementary Figure 1).The energy balance model of the SAWH-TEPG device during daytime and nighttime can be expressed as: Daytime: Nighttime: where  is the desorption cooling power,  is the incident solar irradiation In the energy balance models, we approximate that the total heat flows are dominated by sorption heat and neglect heat generation and absorption due to the Seebeck effect and Joule heating in the thermoelectric legs.This simplifying approximation is made due to the power conversion efficiency of the thermoelectric module is expected to be well below 0.5%.Therefore, the above parameters can be calculated by the following equations:  HCl solution until the surface color turned red, and then immersed in ethanol and deionized water alternatively for ultra-sonic cleaning 5 min.Afterwards, the clean copper foam was dried at 50 °C.1 g prepared MIL-101(Cr) powders were mixed with 8mL ethanol and 2 mL deionized water.The suspension was homogenized in an ultrasonic bath for 20 min, and 0.37 g silicate sol ( ~30 wt%) was added as binder [5].The prepared copper foam was immersed in the as-synthesized MOF suspension slurry and then dried at 80 °C for 24 h.A robust MOF coating forms on the skeleton of porous CF with a high mass fraction of MIL-101(Cr) up to 90wt %.

Characterizations of MIL-101(Cr)@CF composite
The morphology of MIL-101(Cr) and the MIL-101(Cr)@CF composite were observed to reduce the heat loss during both daytime and night.In the hybrid SAWH-TEPG device, the TEPG module is embedded in SAWH module, placed between a dual-functional coating layer and MIL-101(Cr)@CF through a small aluminum block as thermal transport framework.The dual-functional coating layer works as light-to-heat absorber for radiative heating in daytime or thermal emitter for radiative cooling in nighttime.
Moreover, the working modes of dual-functional coating layer can be automatically switched between absorber and emitter owing to its blackbody radiation characteristics.
The employed thermoelectric generator not only converts the excessive solar-thermal energy to be electricity during daytime but also enables electricity generation by simultaneously harvesting coldness from the sky and obtaining sorption heat during water vapor capture of MOF at night.

Performance of hybrid SAWH-TEPG device (i) Indoor experiments
For nighttime proof-of-concept experiments, in prior to the experiments, the hybrid SAWH-TEPG device undergoes a thorough desorption stage at 85 o C for 5h.Then, we placed the device in an environment-controlled chamber with 65% RH and 25 °C to perform water harvesting and radiative cooling involved electricity generation.In order to simulate the radiative cooling effect in the night sky, a water-cooling head with an inlet water temperature of 20 o C was adopted and attached to the surface of the dualfunctional coating layer plate [6].suppress convective heat loss on the dual-functional coating layer of the device.Prior to the exposure to the clear sky, the container cover was wrapped in aluminum 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 performed vapor adsorption.The output voltage and the temperature evolution of the dual-functional coating layer, TEPG module, sorbent, environment temperature and RH were recorded through voltmeter, thermocouples and thermo/hygrometer (HF335, Rotronic).After the sorbent was saturated overnight, the device was sealed to prevent undesired water loss due to the RH swing, and the solar-driven water desorption and harvest was performed on March 26, from 9:30 am to 2:30 pm (UTC+8).Ambient humidity and temperature conditions were recorded as described during the adsorption phase.In addition to the output voltage of the TEPG module and the temperatures of the TEPG module, sorbent and the dualfunctional coating layer, the solar flux was measured by a pyranometer (TBQ-DL, Jinzhou Sunshine Technology), and the mass variation of the device during water capture and release processes were measured by using a precision electronic balance (ME503TE, METLER TOLEDO).Notably, the water collection performance of this device was also studied by measuring the mass gain for several nights between March and April.

𝜀Supplementary Note 3 .
can be defined by the correlation of  0.741 0.0062 . is the dew point temperature.ℎ and ℎ are the convective heat transfer coefficients of the absorber and the sorbent, and they are assumed to be 5 W m -2 K -1 according to the literatures[1][2][3].Preparation of MIL-101(Cr)@CF compositeChemicals including chromium nitrate (Cr(NO3)3 9H2O), 1,4-benzenedicarboxylic acid (H2BDC), N,N-Dimethyl formamide (DMF), and ethanol were purchased from Sigma-Aldrich.MIL-101(Cr) nanoparticles were synthesized based on a reported procedure[4].At first, 1 mmol chromic chloride hexahydrate and 1 mmol terephthalic acid were dissolved into 7.2 mL deionized water.Subsequently, the mixed solution was vigorously stirred for complete dissolution before hydrothermal treatment at 190 o C for 24 h.After the reaction finished, the resultant solution was naturally cooling down to room temperature, the recrystallized terephthalic acid was removed by low-speed centrifugation (350 g, 3 min).Next, the MIL-101(Cr) product in the green color supernatant was collected by centrifugation (7700 g, 15 min), washing with DMF and ethanol respectively, and drying at 80 °C for 4 h.Copper foams (50 × 50 × 5 mm and 100 × 100 × 5 mm) were immersed in 0.5 mol L -1

by
Scanning Electron Microscopy (SEM, Sirion 200 instrument, FEI) equipped with an energy-dispersive X-ray spectrometer (EDS, INCA X-Act attachment, Oxford).The nitrogen adsorption of synthesized MIL-101(Cr) was measured by the volumetric method using the physisorption apparatus (Autosorb-IQ3, Quantachrome) at 77K followed by the evacuated degas at 393K for 12h.The specific surface area is determined to be 2850 g cm-3 by the Brunauer-Emmett-Teller (BET) method.The thermal conductivities of the MIL-101(Cr) powder, MIL-101(Cr) tablet and MIL-101(Cr)@CF composite were measured by the laser flash method (LFA 447, Netzsch) and Hot Disk thermal constants analyzer (TPS3500, Hot Disk AB Company, Sweden).The PXRD patterns were measured by an X-ray diffractometer (Ultima IV, Rigaku).FT-IR spectra were measured by an FT-IR spectrometer (Nicolet 6700, Thermo Fisher Scientific) The water sorption isotherms were measured using an accelerated surface area and porosimetry analyzer (ASAP2020, Micromeritics) under a water vapor atmosphere with controllable vapor pressure.The sample temperature was set as a constant (15, 25, and 35 °C), and the relative pressure of the water vapor was increased from 0 to 1 according to a set of pressure intervals.The TGA tests were carried out by a commercial assembling MOF sorbent with a TEPG module.Specifically, the hybrid SAWH-TEPG device consists of four parts: a square wall made from transparent acrylic tube (length × width × thinness 20 cm × 20 cm × 1 cm), a copper foam (CF) block adhered with MIL-101(Cr) powder (length × width × thinness 100 × 100 × 5 mm, MOF sorbent weight: 8.1 g), a commercial TEPG module (length × width × thinness 40 × 40 × 4 mm) and a copper plate painted with a commercial black paint (length × width × thinness 100 × 100 × 0.8 mm).The top of the hybrid SAWH-TEPG device is enclosed with an infraredtransparent wind cover made from 12.5 mm-thick low-density polyethylene previously The weight change of the SAWH-TEPG device was measured by a precision electronic balance (ME503TE, METLER TOLEDO) with a resolution of 1 mg which was in real-time communication with a computer.A voltmeter is used to measure the output voltage of the SAWH-TEPG device, and several temperature sensors are placed on the dual-functional coating layer, hot and cold sides of the TEPG module, and composite sorbent to measure the temperature changes during the water collecting and electricity generation process.As a contrast, another SAWH-TEPG device without MOF sorbent was also fabricated and tested to investigate the effect of the sorbent on the performance of the electricity generation.For daytime proof-of-concept experiments, in prior to the experiments, the hybrid SAWH-TEPG device undergoes a thorough saturation stage in an environment of 90% RH and 20 •C overnight.The water harvesting experiments were firstly carried out in our laboratory under artificial lighting generated by a solar simulator with an irradiation intensity of 500, 750, and 1000 W m -2 (UHE-NS-100, SCIENCETECH, spot diameter of 100 mm × 1 cm), with uniformity of 10 %, temporal instability of 10 %, and spectral match classification of Class AAA.As a contrast, another SAWH-TEPG device without MOF sorbent was also fabricated and tested.The weight changes of the SAWH-TEPG device under different simulated sunlight intensity were measured by a precision electronic balance (ME503TE, METLER TOLEDO).The output voltage and the temperature evolution of the solar dual-functional coating layer, TEPG module, and sorbent were also recorded.Notably, during the experiments, the enclosure sidewalls of the aluminum fins plate are opened to allow the free migration of the disported water vapor.All experiments of the SAWH-TEPG devices were carried out in an environmentcontrolled chamber and the swings of temperature and RH were kept below 0.5°C and 2%, measured by the thermo/hygrometer (HF335, Rotronic).(ii) Outdoor experiments The outdoor water harvesting experiments were implemented on the roof of our laboratory.The water and electricity co-generation experiment comprise two phases: night-time vapor adsorption and radiative cooling involved electricity generation; daytime solar-driving water harvesting, condensation and electricity co-generation.During night-time vapor adsorption, the experiment was first conducted on March 25, 2021, from 9:30 pm to 2:30 am (UTC+8) for radiative cooling enhanced water sorption and electricity generation at night.Prior to the first cycle, the MOF layer was heated and dehydrated through the desorption stage at 85 o C for 5h.The dual-functional coating layer was positioned to face the clear sky to enable passive radiative cooling, thus reducing the MOF layer temperature.Transparent polyethylene film was used to ∆ H 2 O is the water sorption enthalpy (44 kJ mol -1 water),  H 2 O is the relative molecular mass of water.