3D Photothermal Cryogels for Solar-Driven Desalination

Abstract

This paper reports the fabrication of photothermal cryogels for freshwater production via the solar-driven evaporation of seawater. Photothermal cryogels were prepared via in situ oxidative polymerization of pyrrole with ammonium persulfate on preformed poly(sodium acrylate) (PSA) cryogels. We found that the pyrrole concentration used in the fabrication process has a significant effect on the final PSA/PPy cryogels (PPCs), causing the as-formed polypyrrole (PPy) layer on the PPC to evolve from nanoparticles to lamellar sheets and to consolidated thin films. PPC fabricated using the lowest pyrrole concentration (i.e., PPC10) displays the best solar-evaporation efficiency compared to the other samples, which is further improved by switching the operative mode from floating to standing. Specifically, in the latter case, the apparent solar evaporation rate and solar-to-vapor conversion efficiency reach 1.41 kg m^-2 h^-1 and 96.9%, respectively, due to the contribution of evaporation from the exposed lateral surfaces. The distillate obtained from the condensed vapor, generated via solar evaporation of a synthetic seawater through PPC10, shows an at least 99.99% reduction of Na while all the other elements are reduced to a subppm level. We attribute the superior solar evaporation and desalination performance of PPC10 to its (i) higher photoabsorption efficiency, (ii) higher heat localization effect, (iii) open porous structure that facilitates vapor removal, (iv) rough pore surface that increases the surface area for light absorption and water evaporation, and (v) higher water-absorption capacity to ensure efficient water replenishment to the evaporative sites. It is anticipated that the gained know-how from this study would offer insightful guidelines to better designs of polymer-based 3D photothermal materials for solar evaporation as well as for other emerging solar-related applications.

Keywords: interfacial solar evaporation; poly(sodium acrylate); polypyrrole; porous hydrogels; solar−thermal conversion.

Figure 1

SEM images showing (a) the top surface and (b) the cross-sectional morphology of the samples. (c) High-magnification SEM images elucidating the detailed morphology of the samples.

Figure 2

(a) Narrow-range FTIR spectra of the samples with annotated peaks. (b) Derivative thermogram of the samples. Note that PPC is represented by PPC10.

Figure 3

(a) Static water contact angles of the samples. Dynamic water-absorption profiles for the samples in (b) Milli-Q water and (c) synthetic seawater (3.5 wt %). (d) Maximum water-absorption capacities of the samples in Milli-Q water and synthetic seawater. (e) Water vapor sorption capacities of the samples.

Figure 4

IR thermal imaging of the dry PPC samples showing their photothermal conversion ability when exposed to 1 Sun irradiation.

Figure 5

(a) Cumulative mass loss of water and (b) solar evaporation rates of water only and of water through the PPC samples due to evaporation under 1 Sun irradiation. (c) Temperature profiles of the sample surface versus bulk water as determined from IR thermal imaging; the insets show the digital photograph of the sample and the corresponding thermal images before and after 1 Sun irradiation. (d) Temperature gradient along the z-axis of the PPC samples, i.e., from the sample midpoint to the bulk water, showing the difference in heat localization efficiency.

Figure 6

(a) Thermal images of PPC10 comparing their temperature distribution when irradiated with 1 Sun under floating or standing modes. (b) Temperature gradient between PPC10 and the environment (i.e., air or water) across the lateral axis, x. (c) Solar evaporation rates of PPC10 in different testing modes.

Figure 7

(a) Long-term solar evaporation of PPC10 in synthetic seawater under 1 Sun irradiation tested in the floating mode; the insets are the photographs of PPC10 after the 48-h test showing the absence of salt accumulation on both sides of the sample (top surface is exposed to solar irradiation). (b) Concentration of the major elements, as determined using inductively coupled plasma-optical emission spectrometry (ICP-OES), in synthetic seawater (3.5 wt %) and in the distillate collected using fresh PPC10 or PPC10 after being used for solar evaporation under 1 Sun in synthetic seawater for 24 h.