1. Introduction
Solar
vapor generation (SVG) technique holds great promise for applications in
solving two of the most serious global challenges: the water crisis and
energy shortages.[1-6] The main bottleneck of this
technique is the contradiction between high-efficiency water evaporation
and salt crystallization, which dramatically restricts the practical
application of SVG, especially in the treatment of highly concentrated
brine.[7-11] Owing to the ultra-low diffusivity of
salt in water (10-9 m2s-1, and the diffusivity of water vapor in air is
about 10-5 m2s-1),[12] the salt inevitably
crystallizes and precipitates on the surface of the photothermal
material as the water vapor continues to escape
rapidly.[13, 14] This problem severely hinders the
light absorption, water transport and water vapor escape in the
continuous water evaporation process, significantly degrading the
performance of SVG and terminating the operation of the device. Thus,
the development of photothermal water evaporation materials with both
efficient water evaporation performance and self-cleaning/anti-salt
fouling properties is a key issue to drive SVG from the laboratory to
large-scale practical applications.
To achieve the above aim, the SVG materials should possess three key
properties. One is displaying the excellent solar light capture and
photothermal conversion capability. The other is possessing efficient
water evaporation pathway. The third one is having suitable
hydrophilic-hydrophobic dual interfaces that can improve the anti-salt
fouling ability. Recently, a series of reports by Yu et al. have
demonstrated that hydrogen bonding or electrostatic interactions between
hydrophilic functional groups and water molecules facilitate the
evaporation of bulk water as activated
water,[15-23] providing a fast and efficient water
evaporation pathway (exceeding the theoretical evaporation rate value of
1.47 kg m-2h-1).[24] Such evaporation
process requires less energy than the conventional single-molecule water
evaporation that requires breaking the hydrogen bonds between all water
molecules. On the other hand, several studies including
hydrophilic-hydrophobic janus and biomimetic structures aim to enhance
the anti-salt fouling ability of evaporators through hydrophobic
modifications.[7,25-32] For example, the
hydrophobic surface structure designed by Zhu et al. based on natural
water lily can effectively inhibit the crystallization of salt on its
surface and achieve the complete separation of salt and water. However,
the strongly hydrophobic surface weakened the effective contact of water
with the photothermal active site and the water activation capacity,
which just yielded an evaporation rate of 1.39 kg m-2h-1. Therefore, the combination of three properties
into one material is obviously a challenging task for exploring
efficient, anti-salt fouling and durable SVG evaporators.
Heteropoly blues (HPBs) that possess
a classic blue-black color are reduced polyoxometalates
(POMs).[33-39] They exhibit excellent potential
applications in the fields of photothermal catalysis and photodynamic
therapy based on their broad and high light absorption properties,
unique photothermal activity and stability in the solar spectral
range.[40-46] Moreover, the oxygen-rich surface
structure of HPBs strongly supports their affinity for
water,[47,48] exhibiting the potential in the
construction of hydrophilic-photothermal evaporation sites. Importantly,
the HPBs that hold high-negative charge can be electrostatically
self-assembled with cationic polymers to avoid water leaching problems
and achieve efficient and stable loading. Further regulation of the
surface microenvironment of the functional self-assembled materials is
expected to achieve system management and optimization of the surface
local hydrophilic-hydrophobic region, offering an effective strategy for
constructing hydrophilic-hydrophobic interfacial SVG evaporators.
Given the above considerations, a new
type of photothermal water evaporation materials (abbr.MF@HPB-PPyn-OA) were designed and constructed based on
the melamine foam (MF) skeleton, which is covered by a
hydrophilic-hydrophobic composite interfacial material via a one-step
self-assembly of HPB, oleic acid (OA) and polypyrro (PPy). On the
surface of MF@HPB-PPyn-OA, the hydrophilic regions of
HPB and the hydrophobic regions of OA show spatial separation features
due to the electrostatic mutual repulsion of HPB and OA molecules.
Benefiting from the advantages of the self-assembly strategy and the
unique surface structure, the local hydrophilic-hydrophobic interface of
this SVG material can be regulated and optimized so as to realize the
ultrahigh water evaporation and salt crystallization simultaneously. In
a series of MF@HPB-PPyn-OA,
MF@HPB-PPy10-OA exhibits the best SVG performance,
yielding 3.3 kg m-2 h-1 of
evaporation rate and 96.5% of salt harvesting efficiency in the
complete salt-water separation process of high-salinity (10 wt%) brine
at 1 solar irradiation, and realizes zero liquid discharge, which
belongs to the record-high of high salinity systems reported so far.
Moreover, MF@HPB-PPy10-OA can operate continuously and
stably for over 100 h at an ultrahigh evaporation rate of 3.3 kg
m-2 h-1 (corresponding to an energy
efficiency of 92.1%) in continuous solar desalination of high-salinity
brine (10 wt%), and enables real-time anti-fouling and maintains
surface cleanliness. The low cost, excellent mechanical properties and
processability of MF@HPB-PPy10-OA in terms of
manufacturing exhibit promising prospects for commercial applications.
Results and discussion