Figure 3. (a) UV-Vis-NIR spectra of
MF@PPy10-OA,
MF@HPB-PPy10, MF@HPB-PPy10-OA. Inset
image is the air mass 1.5 global (AM 1.5 G) solar spectrum; (b) The
surface temperature rise of MF (reference), MF@PPy10-OA,
MF@HPB-PPy10, MF@HPB-PPy10-OA relative
to irradiation time under 1 sun. Inset: infrared image displaying the
temperature distribution after 180 s of irradiation time; (c) Plot
displaying the surface temperature of bulk water (reference) and
MF@PPy10-OA, MF@HPB-PPy10,
MF@HPB-PPy10-OA under 1 sun relative to irradiation
time. Inset: infrared image displaying the temperature distribution
after 600 s of irradiation time; (d) Mass change of
MF@PPy10-OA, MF@HPB-PPy10,
MF@HPB-PPy10-OA under 1 sun illumination, with pure
water as the control. And the corresponding (e) solar water evaporation
rate and energy efficiency; (f) Mass change over time with
MF@HPB-PPy10-OA under 0 to 2 sun (0 to 2 kW
m-2) radiation.
Figure 3d and Figure S8d, taking the bulk water as control group, the
overall mass change with MF@HPB-PPyn-OA is recorded
under 1 solar irradiation. As in some previous studies, a light cut-off
device with an aperture was placed between the lamp and the evaporator
to ensure the same size of light irradiation and evaporator
area.[51] The general agreement of the bulk water
evaporation rate value (0.43 kg m-2h-1) under this condition with the reported literature
values supports the reliability of this test
system.[9,22,52-57] Evaporation rates of 1.9 kg
m-2 h-1 and 2.9 kg
m-2 h-1 were obtained for the
evaporation systems driven by the hydrophobic
MF@PPy10-OA and hydrophilic
MF@HPB-PPy10, respectively (Figure 3d). Compared with
MF@PPy10-OA, the significant increase in evaporation
rate of MF@HPB-PPy10 proves the positive contribution of
hydrophilic HPB to the overall evaporation performance. When the
interfacial hydrophilic-hydrophobicity of the materials is further
modulated, MF@HPB-PPy10-OA presents optimal performance
with 3.4 kg m-2 h-1 evaporation rate
(Figure 3d and Figure S8e), which ranks among the highest levels
reported in recent years (Figure
S8f).[9,17,22,30,52-57] This is attributed to the
ability of the HPB-PPy10-OA self-assembled coating to
wrap the MF in a more stable and uniform manner, thus presenting
excellent light absorption and photothermal response. On the other hand,
the small and uniformly dispersed HPB clusters of the self-assembled
HPB-PPy10-OA allow more hydrophilic evaporation sites to
be exposed compared to HPB-PPy10 (Figure 2b and Figure
S2a). The excellent evaporative property of
MF@HPB-PPy10-OA exceeding the theoretical limit value
(1.47 kg m-2 h-1) should be
contributed to the active intermediate state of water molecules in the
hydrophilic region. The active intermediate water which is a state
intermediate between bound and free water is generally referred to as
activated water. [22,23,30] With its subtle
interactions between hydrophilic sites and adjacent water molecules, the
vaporization process of activated water consumes less energy than that
of bulk water. It is well known that POM has good affinity for water
molecules by virtue of its oxygen-rich surface structure and high
negative charge.[47,48] As shown by
thermogravimetric analysis (TG) (Figure S10), the water molecules in the
crystal structure of PMA evolve gradually in three stages. It is
noteworthy that in the first weight loss step, the free crystalline
water evolves with increasing temperature and endothermic peak 1 emerges
at a low temperature of 71.9 oC in the corresponding
DSC curve (while the endothermic peak of bulk water is around 100oC). The computational analysis indicates that the
evolution of crystalline water in pure POM (850.0 J
g-1) requires only low energy compared to bulk water
(2327.5 J g-1, see Figure S11 in Supporting
Information for details), confirming the active water molecule
intermediate state in the POM structure. In the DSC analysis of Figure
S11, water in MF@HPB-PPy10-OA similarly proceeds with
low energy consumption, indicating the activation process of water on
the hydrophilic region of HPB. The equivalent dark evaporation
measurements (Figure S13) results basically coincide with the DSC test
results, which supports this low-energy requirement evaporation process.
The energy efficiency (η) of the SVG in this system can be calculated
using the following equation:
Ƞ = ṁh/CoptP0
where, ṁ is the water net evaporation rate (ṁlight-
ṁdark kg m-2 h-1), h
is the evaporation enthalpy (J g-1) of the water in
MF@HPB-PPy10-OA, Copt is the optical
concentration on the evaporator surface, P0 is the solar
radiation power (1 kW m-2). Thus, the energy
conversion efficiency of MF@HPB-PPy10-OA reaches 94.9%
under 1 solar illumination (Figure 3e), obviously higher than that of
MF@PPy10-OA (76.0%) and MF@HPB-PPy10(88.2%). Meanwhile, the evaporation system is found to have only a low
heat loss in the energy loss assessment (see Section 4.4 in Supporting
Information for details). In addition, 3D
MF@HPB-PPy10-OA yields a satisfactory evaporation rate
of 5.6 kg m-2 h-1 with 2 solar
irradiations by virtue of its unique structure (Figure 3f).
Salt -water
separation in highly concentrated
brine(10 wt%)
It is worthwhile to mention that MF@HPB-PPy10-OA is
equally efficient in high salinity brine (10 wt%), and even
accomplishes complete salt-water separation.
As
shown in Figure 4a and Figure 4b, an inexpensive filter paper diffusion
layer (with a dimensional radius 0.2 cm larger than that of
MF@HPB-PPy10-OA) is introduced on the basis of the
conventional photothermal water evaporation system (including a light
absorber layer, insulation layer and delivery line) to reinforce the
transport of brine in MF@HPB-PPy10-OA. In the complete
salt-water separation, the average water evaporation rate of
MF@PPy10-OA is maintained at 1.8 kg
m-2 h-1 in the optimized evaporation
system, which is only slightly lower than that of bulk water (Figure
4c). Owing to the strong hydrophobicity of the
MF@PPy10-OA surface, salt starts to crystallize in the
diffusion layer instead of at the surface during up to 32 h of
continuous operation (Figure S14a). Eventually, a salt harvesting
efficiency of about 88.7% is obtained around the diffusion layer.
Unfortunately, after two cycles of salt-water separation, the
MF@PPy10-OA becomes significantly lighter in color and
has a significantly lower photothermal water evaporation efficiency,
which is assigned to the photobleaching effect of PPy. When
MF@HPB-PPy10 drives solar salt-water separation, the
average rate is improved. But the salt tolerance is visibly reduced
compared to that of MF@PPy10-OA (Figure 4c). Due to the
continuous evaporation of water, the brine concentration in the system
gradually increases and approaches saturation, inevitably leading to the
gradual precipitation of salt on