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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