**1. Introduction**

It is accepted fact that water is the most important substance to living organisms. Nonetheless, some areas lack access to water, which is concerning given how essential it is. Areas that lack natural clear water sources and/or access to water distribution have very few options are available to obtain clean water. One of these may be harvesting rainwater, though it comes with risks of chemical and microbiological contaminations [1]. Furthermore, it is not reliable to expect a consistent amount of rain throughout the year. Thus, more recently, efforts to generate clean water from atmospheric moisture are becoming more prominent [2–7].

Generally, generating clean water from atmospheric moisture consists of a twostage process. The first stage pertains to the harvesting of atmospheric moistures into a

hygroscopic absorbing media [1]. In the second stage, the absorbed moisture is separated from the hygroscopic media into a recovery vessel as clean water. This is achieved using well-established methods such as reverse osmosis [1] or distillation processes [1]. After moisture recovery, the absorbing media can reabsorb moisture and the cycle continues. It should be noted that the efficiency of the atmospheric clean water generation process depends on advancement in both the moisture recovery methods and the moistureabsorbing materials.

Several studies in the past have examined the moisture absorption capabilities of various media, including those that are complex and difficult to produce commercially. Some of the most distinguished media are CaCl2 in an alginate-derived matrix (Alg-CaCl2) [2], multiple versions of metal–organic framework (MOF) materials, including MIL-101(Cr) [3], Cr-soc-MOF-1 [4], Co2Cl2 (BTDD [5], activated carbon such as AC07 [6], and MOF-801 [7], as shown in Table 1.

**Table 1.** Comparative analysis of water uptake amongs<sup>t</sup> various media containing CaCl2.


The most prominent options are Cr-soc-MOF-1, which can absorb 2.0 g of water per gram of salt or 200% of its mass at 25 ◦C and 22.2 mbar of water vapor pressure, and Alg-CaCl2, which can absorb 2.88 g of water per gram of salt or 288% of its mass at 28 ◦C, 79% RH, and 30.0 mbar of water vapor pressure [2]. Despite these excellent innovations, scaling up moisture harvester technologies based on the materials may be challenging due to the intricate crystal making requirements—in the case of the MOFs—or the need to extract alginates from their origin; for example, the brown algae.

This work demonstrates the development of a simple and effective moisture harvester alternative material that is scalable and high performing. The moisture harvester is based on a common hygroscopic salt, CaCl2, which is praised as one of the most hygroscopic salts readily available. CaCl2 is also deliquescent, meaning that it absorbs moisture in the air until it dissolves to form a brine. Deliquescence is a property found to be maximized at low temperatures and high humidity [8], as it occurs when the vapor pressure of this brine solution is lower than the partial pressure of the vapor pressure of water in the air [9]. Although calcium chloride itself has excellent water absorbing capabilities, it must be complemented by another material due to its penchant to agglomerate. Agglomeration occurs when fine particles are chemically and physically bonded, "clumping up together in a floc" [10]. As shown in Figure 1, agglomeration occurs in calcium chloride when it liquifies and is dried again to be reused.

**Figure 1.** Illustration of agglomeration processes of calcium chloride salts upon moisture absorption and drying. The clumping reduces the amount of surface area in direct contact with the moist environment.

To solve this problem, we investigate the potential of utilizing a simple sponge salt system to overcome the agglomeration problem of deliquescent salts. First, four commercially available types of sponges were screened to determine their best performance. Then, a more thorough investigation was carried out for the best sponge. Moisture absorbance measurement was carried out over time at a constant temperature and humidity. The moisture absorption time dependence was analyzed using a basic exponential model, where the time constant and other absorption parameters were extracted and analyzed. The analysis revealed improvements in the sponge-salt system's water absorption rate and capacity than the salt system alone. A model is then proposed based on two mechanisms that highlight the existence of two distinct absorption mechanisms. Finally, a proof-of-concept water recovery Peltier-based system is proposed.
