Characterization and scalable synthesis
We prepared EMM-8 with hydrothermal synthesis according to the method reported by Afeworki et al.40 X-ray diffraction (XRD) measurements (Fig. 1a) confirms that the prepared aluminophosphate possesses the SFO topological structure. As shown in Figure 1b and Supplementary Fig. 2, the basic building units of EMM-8 are double four-rings (D4Rs), which form a two-dimensional framework with 12-ring and 8-ring channels. The 27Al and 31P magic-angle spinning (MAS) NMR spectrums (Fig. 1c) and X-ray photoelectron spectroscopy (XPS) measurements (Supplementary Fig. 3) of the calcined EMM-8 also confirm the tetrahedrally coordinated framework with Al and P atoms. Nitrogen adsorption results also show the EMM-8 has a high BET surface area of 879.63 m2·g-1 and a large pore volume of 0.59 cm3·g-1 (Fig. 1d and Supplementary Table 1). Moreover, we find a concentrated pore diameter distribution with an average micropore size of 0.7 nm, corresponding to the size of 12-ring windows. The nitrogen adsorption isotherm also confirms the presence of the mesopores, whose volume is as high as 0.288 cm3·g-1 that is close to that of micropores.
SEM and TEM images show two-edged sword-shaped and highly faceted EMM-8 particles (Fig. 2a-c), indicating the good crystallinity and high purity of synthesized samples. High-resolution TEM image (Fig. 2d) of the material shows clear lattice fringes, and selective area electron diffraction (SAED) pattern (Fig. 2e) clearly suggests its single crystal characteristics. Moreover, we find the zeolite-like crystals have uniform shapes in length (1–3 μm) and width (200–300 nm). SEM and AFM observations (Fig. 2b and f-h) reveal that the thicknesses of the EMM-8 nanoplates are distributed between 20 nm and 120 nm. The gap between the adjacent nanoplates contributes to the presence of mesopores. It is expected that the nano-sized plate-like structures with abundant intervals offer a high potential to achieve a faster transfer of water molecules within interparticle spaces and micropores.
For the water-sorption-based heating and cooling applications, it is crucial that the adsorbent has hydrothermally stable pores. Temperature-dependent powder XRD (Fig. 1a) and thermogravimetric analysis (TGA) (Supplementary Fig. 4) reveal the total removal of the SDA at ~400 °C and demonstrate the desirable thermal stability of the synthesized EMM-8 up to 700 °C, which is much better than that of MOFs. Also, we performed the hydrothermal stability tests by soaking the calcined EMM-8 in boiling water for 24 h, and the results point out that the obtained sorbent can maintain its crystallinity, framework structure, and microporosity, suggesting its stability under harsh working conditions. This conclusion was further evidenced by the NMR spectrum (Fig. 1c), XRD patterns, and N2 adsorption isotherms (Supplementary Fig. 5).
To build a real-life full-scale ADC device, it is imperative that the sample synthesis process of water adsorbents should be easily scaled up. Thus, the hundred-gram-scale production of this material is performed by a scaled-up reactor in this work. As shown in Supplementary Fig. 6a, we use a 2 L Teflon vessel to replace the 50 mL one in synthesis and introduce 40 times the number of raw materials in it. As a result, about 100 g EMM-8 with good crystallinity is obtained, suggesting the robust fabrication of this aluminophosphate (Supplementary Fig. 6b). Importantly, it is found that there is no significant difference between crystals of EMM-8 synthesized by 50-mL and 2-L Teflon vessels, evidenced by SEM images, XRD patterns, and N2 adsorption isotherms (Supplementary Fig. 7a-c). Moreover, it is worth mentioning that EMM-8 is easily shaped as pellets by pressing without any binders (Supplementary Fig. 7d).
Water adsorption evaluation
The water adsorption isotherms of EMM-8 were measured by a gravimetric adsorption analyzer at three different temperatures (25, 40, and 50 °C) (Fig. 3a). The results represent a perfect S-shaped water adsorption isotherm with a step-wise water uptake in the extremely narrow relative pressure range of P/P0 = 0.15–0.17, indicative of the presence of uniform micropores. EMM-8 exhibits a relatively high water adsorption capacity of 0.283 gH2O·gsorbent‒1 at 30 °C and P/P0 = 0.3, comparable to that of the selected state-of-the-art water adsorbents (Supplementary Fig. 8), including SAPO-34, MOF-801, CAU-10, COF-TpPa41, and Co-CUK-142.
A calculated average isosteric enthalpy of water adsorption was determined to be 46.76 kJ·mol−1 (Fig. 3b) by using the Clausius–Clapeyron equation based on the data of water adsorption isotherms at three different temperatures. We found this value of EMM-8 is lower than that of many other benchmark sorbents (Fig. 3b and Supplementary Table 3) and merely higher than the evaporation enthalpy of water (44 kJ·mol−1), indicating a relatively lower energy consumption for water desorption and thus higher COP for sorption-based heating or cooling system. Unlike the strong electrostatic interaction between the framework and water molecules for conventional zeolite, the zeotypic EMM-8 with electrical neutrality endows a weak hydrogen-bonded network of water molecules within micropores, leading to lower adsorption enthalpy, which will be detailed in the following section. The characteristic curves defining the relations of adsorption potential and water uptake16 are determined using adsorption isotherms of EMM-8 and other reference materials, as shown in Fig. 3c. In the context of meeting the typical operating conditions for both refrigeration and heat pump applications, EMM-8 exhibits a significantly lower adsorption potential than conventionally-used zeolite, SAPO-34, and Co-CUK-1, indicating the lower required driving temperature.
In order to further confirm the suitability of the material for applications on ADC and AHP, cycling stability tests were carried out under the condition of regeneration temperature of 110 °C and adsorption temperature of 30 °C. The results reveal no significant loss of the water uptake for EMM-8 over 30 adsorption/desorption cycles (Fig. 3d).
To further explore the adsorption mechanism at the microscopic level, the water adsorption isotherm of EMM-8 was calculated by using Grand Canonical Monte Carlo (GCMC) simulations at 30 °C. The obtained result matches the experimental adsorption isotherm well, as shown in Supplementary Fig. 9. At a low relative pressure of P/P0=0.01, only a small amount of water molecules enter into the 12-ring channels, associated with a high adsorption enthalpy (71 kJ·mol−1, Supplementary Fig. 10). The water molecules are preferentially coordinated to aluminum atoms on the pore wall (Fig. 4a), resulting in the change of the aluminum atom from tetrahedral coordination to octahedral coordination. This phenomenon is very common for water adsorption on AlPOs39,43-45 and consistent with the 27Al NMR experimental findings (Supplementary Fig. 11a). When the relative water vapor pressure is above 0.25, there is a sudden increase in the water adsorption capacity and the water molecules tend to fill the entire 12-ring 1D pore openings of EMM-8, as shown in Fig. 4b and c. It also can be seen that a hydrogen bond network of water molecules is formed with a total number of hydrogen bonds per water molecule of 2.9 (Supplementary Fig. 12), which is close to that of bulk water. This observation is supported by the 1H NMR spectra (Supplementary Fig. 11b), which shows the low-field shift of the water signal, indicating the presence of the hydrogen-bonded water molecules. This low-energy water–water molecular interaction suggests a low enthalpy of adsorption (40–50 kJ·mol−1, Supplementary Fig. 10) and thus results in the facile regeneration of EMM-8.
ADC performance evaluation
Materials-based COPs for cooling, which are usually used to assess the energy efficiency of the ADC devices, are evaluated according to the methodology by de Lange et al.18 (Supplementary Note 2) at different boundary temperatures for evaporation (Tev), condensation (Tcon), and regeneration (Tdes). Figure 5a shows the calculated maximum COPC for EMM-8 and its corresponding driving temperature for a specific refrigeration condition, i.e., Tev = 5 °C and Tcon = 30 °C. The results indicate that EMM-8 exhibits an exceptionally high COPC of 0.85 at the ultralow driving temperature of 63 °C, remarkably exceeding the existing state-of-the-art materials, including the recently reported best-in-class adsorbents, such as AlPO-LTA (0.75), MIP-200 (0.78), KFM-1 (0.75), and Co-CUK-1 (0.83). Additionally, the very high thermal efficiencies and working capacities on cooling retain for a wide range of evaporation temperatures, as illustrated in Supplementary Fig. 13a and 14. More importantly, the required driving temperatures, achieving the highest COP and working capacity of EMM-8, are lower than those of reference materials by 5-15 °C, as shown in Fig. 5a and b, which is significantly meaningful for the efficient exploitation of ultra-low-grade thermal energy. The complementary metric, specific energy capacity, i.e., the provided cooling capacity from the evaporator in one cooling cycle (Qev), is evaluated and compared with that of other benchmark materials (Supplementary Fig. 15). The excellent volumetric specific energy capacity for EMM-8 is also found even at a low regeneration temperature of 65 °C (Fig.5c and Supplementary Table 3), which is another indication of the extraordinary cooling performance of EMM-8 at ultralow driving temperatures.
Moreover, under a standard heat pump condition, i.e., Tev = 15 °C and Tcon = 45 °C, EMM-8 also gives a very high COPH of 1.75 at a driven temperature of 82 °C (Fig. 5d and Supplementary Fig. 13b). The comparison of this value between EMM-8 and other excellent materials confirms that EMM-8 outperforms both other AlPOs and most MOFs for heat pump applications. This value is also as high as that of the best water adsorbents reported so far, i.e., Co-CUK-1 (1.77) and KMF-1 (1.74). However, in comparison with Co-CUK-1, EMM-8 shows higher volumetric working capacities at lower desorption temperatures (Fig. 5e). To the best of our knowledge, therefore, the obtained unprecedentedly high cooling and heating efficiencies suggest that EMM-8 is one of the best water adsorbents to date for realizing ultra-low-temperature-driven ADC and AHP applications.
Alongside COP, the specific cooling power (SCP) of the water adsorbent is also a key performance indicator that dictates the power density of ADC devices. EMM-8 was shaped via pressing, crushing, sieving, and then deposited on a flat-plate adhesive plate (Supplementary Fig. 17b) to assess its dynamic sorption performances under varying ADC operating conditions (Supplementary Table 5). The water desorption profiles of EMM-8 show the fast desorption rates under the operating conditions of Tcon=30 °C, and Tdes=65 °C (Fig. 5f), demonstrating its superiority in the deep utilization of ultra-low-temperature heat. According to the evaluations of SCPs based on kinetic measurements (Supplementary Note 3), EMM-8 has significantly high power density in ADCs operated at low evaporator temperature (5-10 °C) and low driven temperatures (65-80 °C), as seen in Supplementary Table 6 and 7. Typically, even under the rather harsh working conditions (Tev=5 °C, Tcon=30 °C, and Tdes=65 °C), a superior high SCPmax of 2.22 kW·kgsorbents-1 and SCP80% of 1.1 kW·kgsorbents-1 for this material with the size of 0.45-0.6 mm is achieved. The SCP80% value of EMM-8 surpasses those of both commercial water sorbents and promising MOFs (Supplementary Table 8), suggesting a unique opportunity to construct a compact and lightweight sorption system using this adsorbent.
The production cost of water sorbents is also a key concern for the application of the ADC systems. The cost of reagent-grade raw materials for EMM-8 is one percent to one third of the outstanding MOFs considered here (Supplementary Table 9), indicating its extremely attractive advantage in commercialization potentials. Accordingly, together with its outperforming energy efficiency, power density, and level of production maturity, the proposed EMM-8 clearly shows a successful combination of superior performance and economic potential.