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Photo-thermo-electric
hydrogel with interlocking photothermal layer and hydrogel for
enhancement of thermopower generation
Jingjie Shen, Chenhui Yang*, Yanli Ma, Mengnan Cao, Zifa Gao, Shuo Wang,
Jian Li, Shouxin Liu, Zhijun Chen*, Shujun Li*.
Key Laboratory of Bio-based Material Science and Technology of Ministry
of Education, Northeast Forestry University, Hexing Road 26, Harbin
150040, P.R. China.
E-mail: yangch@nefu.edu.cn; chenzhijun@nefu.edu.cn; lishujun@nefu.edu.cn
Keywords: thermoelectrochemical cells, solar-thermo-electric energy
conversion, solar energy, interlocking structure, heat conduction
Abstract:
Photothermal devices and thermoelectric cells hold great promise for
energy generation but integration of the two remains a considerable
challenge in real-life power supply for sensors. Here, a novel
photo-thermo-electric hydrogel (PTEH-Interlocking) was constructed by
synthesis of a photothermal layer on a thermoelectric hydrogel with the
redox pair
Fe(CN)63−/Fe(CN)64−.
The smart design of using oxidation of pyrogallic acid by
Fe(CN)63− to construct the
photothermal layer for photo-to-heat conversion protected the redox
couple of the thermogalvanic ion pair from ultraviolet damage, as well
as triggered the formation of an interlocking structure at the interface
of the photothermal layer and the thermoelectric hydrogel. The
as-prepared PTEH-Interlocking have shown a high Seebeck coefficient and
rapid heat transfer, boosting the photo-thermo-electric conversion. As a
demonstration of a practical application, the PTEH-Interlocking cells is
successfully used as the energy supply for a mechanical sensor.
1. Introduction
Solar energy, the most abundant resource on earth, is the ultimate clean
and renewable source of energy to ease the global energy crisis.1, 2 Various solar thermal technologies have been
developed to generate heat to meet the needs of daily life, including
water evaporation 3-7and other approaches.8-11 Thermoelectric cells can convert ambient heat
originating from solar energy into electricity, without using cables or
batteries that need to be recharged periodically.12-17Therefore, a solar-thermal-electric integration with photothermal
materials and thermoeelctric cells is highly desired.
Flexible thermoelectric cells are mechanically adaptable to dynamic
interfaces, enabling their used as wearable power supplies for
sensors.9, 18-25 For example, Zhou and coworkers have
prepared wearable and flexible thermoelectric devices using polyvinyl
alcohol as the gel solution, with the addition of ferric/ferrous
chloride or potassium ferricyanide/ferrocyanide
couples.26, 27 Hydrogel-based flexible thermoelectric
cells are particularly interesting because of their eco-friendly nature,
shapable liquid electrolytes and relatively high Seebeck coefficients.21, 28-32 In such cells, thermogalvanic ions are used
as redox couples, such as
Fe(CN)63−/Fe(CN)64−,
Fe2+/Fe3+ and
Co2+/Co3+, of which are trapped in a
molecular hydrogel network and can generate or accept electrons at two
electrodes at two different temperatures. 15, 33-36The solar-thermal-electric integration to convert ubiquitous solar
energy into heat and then generate electricity has, however, rarely been
reported because of the relatively weak resistance of the redox couples
to solar light, expecially in the ultraviolet (UV) light.
Herein, we describe the construction of a solar-driven
photo-thermo-electric hydrogel with
interlocking structure (PTEH-Interlocking), which enables to generate
stable electricity from solar light as the energy supply for a
mechanical sensor (Figure 1 ). The thermo-electric hydrogels
(TEH) were constructed via the crosslinking of polyacrylamide and
carboxymethylcellulose with
Fe(CN)63−/Fe(CN)64−)
as the thermogalvanic redox couple. The
PA-PEI-Fe photothermal film were
prepared in situ via the crosslinking with pyrogallic acid (PA)
and polyethyleneimine (PEI) after the oxidation of
Fe(CN)63−(PA-PEI-Fe). Interestingly, the
PA-PEI-Fe photothermal film permeated into the TEH, forming a well
interlocking structure at the interface. The dark and dense PA-PEI-Fe
photothermal film adsorbs and converts sunlight into heat, and protects
the redox couple of the thermogalvanic ion pair from UV damage. More
importantly, the interlocking structure can rapidly convert
solar-generated heat into thermoelectric ions for enhanced electricity
generation. As a result, PTEH-Interlocking shows an ultra-stable
electricity generation, and could be successfully used as the power
supply for a mechanical sensor.
2. Results and disscussion
2.1. Synthesis and Structure Characterization of the PTEH-Interlocking
Mix acrylic amide, ammonium persulfate, N, N-methylene bisacrylamide,
and sodium carboxymethyl cellulose to form a solution. Pour the solution
into a mold to make a hydrogel, then immerse it in a redox electrolyte
solution of FeCN
([Fe(CN)6]3−/[Fe(CN)6]4−/LiBr),
hydrogel becomes TEH (thermo-electric hydrogel). The as-prepared TEH
were soaked into mixture solution of PA and PEI to constructed the
PTEH-Interlocking cells. After the in situ oxidation of PA and
PEI by [Fe(CN)6]3−, the dark and
dense PA-PEI-Fe photothermal film (Figure 2a ) prevents the
continued penetration of
[Fe(CN)6]3−/[Fe(CN)6]4−into photothermal film, which retains the redox activity of
thermoelectric cells. 37, 38 Both the digital picture
and the scanning transmission electron microscopy (SEM) images of the
photothermal film shows a relatively smooth surface of PA-PEI-Fe
photothermal film (Figure 2b ). The SEM elemental mapping of
photothermal layer presents a uniform distribution of C, N, O and Fe in
the film matrix. Fourier transform infrared (FTIR) spectroscopy and
X-ray photoelectron spectroscopy (XPS) were used to confirm the
coordination of
[Fe(CN)6]3−/[Fe(CN)6]4−with the phenolic hydroxyl groups in the PA-PEI-Fe photothermal film.
The FTIR spectra of the PA-PEI film (where the pure PA-PEI film was
constructed using PA (0.1 mol L−1) and PEI (0.2 g
L−1) under atmospheric conditions) and PA-PEI-Fe
photothermal film (Figure 2d ) showed obvious characteristic
peak at 3400 cm-1 corresponding to the stretching
vibration of C-OH, and the peak at 1650 cm-1corresponding to the stretching vibration of C=O. While, that FTIR
spectrum of PA-PEI-Fe photothermal film shows an additional
characteristic peak at obvious pepeks at 2054 cm−1corresponding to C≡N stretching vibrations and obvious peak at 1129
cm−1 corresponding to the stretching vibration of
-C-O- bonds. The XPS survey shows that photothermal film is composed of
C, N, O and Fe species, with an atomic ratio of 36.32: 2: 32.21: 0.46
(Figure S1 and Table S1 ), indicating very little usage of
[Fe(CN)6]3−/[Fe(CN)6]4−.
The C 1s and N 1s XPS
spectra of PA-PEI-Fe film confirm the C, N, and O species related
bonding. The O 1s XPS spectra in Figure 2e of the
PA-PEI-Fe photothermal film consistently shows an obvious binding energy
peak of “-O-Fe-” bonds at 531.3 eV. 39 These results
suggest that the coordination of
[Fe(CN)6]3−/[Fe(CN)6]4−with the phenolic hydroxyl groups in the PA-PEI-Fe photothermal film.
The phenolic content was estimated using the Lowry method. The total
phenolic content of PA-PEI was 1.1 μmol g−1, whilst
that total phenolic content of the PA-PEI-Fe photothermal film is only
0.1 μmol g−1 (Figure S2 ). The reduction of
phenolic content of the PA-PEI-Fe photothermal film is hence due to the
oxidation of PA-PEI by
[Fe(CN)6]3−. The photothermal film
is more hydrophobic than PA film and has a smaller contact angle with
water (107° vs. 123°) (Figure S3 ), which is possibly due
to the reduced phenolic content. The UV-Vis absorption intensity of the
mixture solution of PA-PEI and
[Fe(CN)6]3−/[Fe(CN)6]4−is obviously higher than that of PA-PEI solution (Figure 2f )
and the absorption peak is red-shifted by 12 nm (~228
nm) in comparison to that of PA-PEI solution (~216 nm).
The absorption of the mixture solution of
[Fe(CN)6]3− and PA-PEI is
red-shifted and shows an obvious enhancement, whereas the absorption of
the mixture solution of
[Fe(CN)6]4− and PA-PEI only shows
a marked enhancement but is not red-shifted (Figure S4, video
S1 ). The red shift is therefore, mainly due to the coordination between
[Fe(CN)6]3− and PA-PEI.40, 41