A Solar Thermoelectric Nanofluidic Device for Solar Thermal Energy Harvesting

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE1 Jul 2021A Solar Thermoelectric Nanofluidic Device for Solar Thermal Energy Harvesting Zhong-Qiu Li†, Zeng-Qiang Wu†, Xin-Lei Ding, Ming-Yang Wu and Xing-Hua Xia Zhong-Qiu Li† State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023 , Zeng-Qiang Wu† State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023 , Xin-Lei Ding State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023 , Ming-Yang Wu State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023 and Xing-Hua Xia *Corresponding author: E-mail Address: [email protected] State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023 https://doi.org/10.31635/ccschem.020.202000366 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail Harvesting the low-grade (<100 °C) solar thermal energy with ionic heat-to-electricity conversion shows great promise but low efficiencies due to the challenges encountered in regulating ionic thermophoretic mobilities. Here, we used nanochannels to regulate thermal-driven ion transport properties and described a solar thermoelectric nanofluidic device (STEND). The localized heat generated by the broadband plasmonic absorption of the gold nanostructure is focused at the orifice of the nanochannel, which builds up a large temperature gradient inside the nanochannel. The following thermal-driven ionic charge separation was enhanced by the ion-selective nanochannel, resulting in large thermal membrane potential (TMP). The Seebeck coefficient and the TMP reached 0.76 mV/K and 23 mV, respectively, in an aqueous KCl solution. The performance of the device was improved further by the enhancement of electrostatic interaction between the ions and the nanochannnels, the increase of the membrane thermal resistance, and the decoupling of the ion concentration polarization (ICP) regions. This study supports the understanding of thermal-driven ion transport at nanoscale and provides a new strategy for harvesting solar thermal energy. Download figure Download PowerPoint Introduction Solar energy is one of the renewable energy sources1,2 considered to be the ultimate solution to the current energy crisis.3 The discovery of solar cells has achieved remarkable progress in solar technology over the past few decades, which has pushed the conversion efficiency to nearly 30%.4 However, a large portion of the solar energy is still being wasted in the form of heat, commonly termed solar thermal energy5 that mostly exists as low-grade heat source (<100 °C) and could be harvested with thermoelectric devices, as well as thermoelectrochemical cells. Thermoelectric devices are based on the thermal-driven electron–hole separation and show low conversion efficiencies in low-temperature regions due to the lack of high-performance thermoelectric materials.6,7 Thermoelectrochemical cells harvest low-grade heat with redox couples possessing temperature-dependent electrode potentials.8 Although they are environmentally friendly and cost effective, their conversion efficiency is limited by the electrode materials, which are typically <5% relative to the Carnot limit.9–11 Ionic heat-to-electricity conversion, which relies on the thermal-driven ionic charge separation, characterizes both high theoretical conversion efficiency and low cost.12,13 However, the development of this technology toward practical applications remains challenging due to the difficulty in regulating the thermophoretic mobilities of cations and anions. Tuning ion transport properties with nanochannels for energy harvesting has received increased attention nowadays.14–17 Various energy sources, such as pressure,18 salinity gradients,19–21 and capillary forces,22 have been converted successfully into electricity using nanochannels, which also hold great promise for ionic heat-to-electricity conversion. The interface properties of nanochannels could be regulated by constructing unique structures or chemical modification of the inner surface,23,24 which could enhance the thermal-driven ionic charge separation more effectively. A theoretical study has revealed that the thermovoltage in narrow and highly charged nanochannels might be >30 times larger than that in bulk liquids.25–27 However, for experimental study, bath temperature control is generally utilized to produce the temperature gradient inside the nanochannel,28,29 which not only wastes a lot of energy but also reduces the temperature gradient due to the temperature polarization effect (the temperature drop forms in the temperature-polarized layers at both sides of the nanochannels adjacent to the solutions).30,31 Moreover, the potential difference between two electrodes in different temperature solutions, which is usually the same order of magnitude as the thermal membrane potential (TMP), is seldom considered.32,33 In this study, we designed a solar thermoelectric nanofluidic device (STEND) to harvest solar thermal energy (Figure 1a). A broadband plasmonic absorption layer of a three-dimensional (3-D) gold nanostructure was deposited on one side of anodic aluminum oxide (AAO) membrane that could convert light into heat efficiently via surface plasmon resonance (SPR). The generated localized heat established a large temperature gradient inside the nanochannels, which drove the ionic charge separation to create a TMP. The designed STEND was able to convert light into electricity, demonstrating a great promise in harvesting solar thermal energy. Figure 1 | Design and characterization of the STEND. (a) Schematic design of the STEND. (b) SEM image of the cross-section of the asy-AAO. Insets: detailed morphology of the large-pore and small-pore portions. (c) SEM image of a 3-D gold nanostructure in asy-AAO-Au. (d) Absorption spectra of the asy-AAO (orange curve), asy-AAO-Au-back (irradiating from the small-pore side, green curve), and asy-AAO-Au-front (irradiating from the large-pore side, red curve) in the visible to NIR region. STEND, solar thermoelectric nanofluidic device; SEM, scanning electron microscope; AAO, anodic aluminum oxide; NIR, near-infrared. Download figure Download PowerPoint Experimental Methods Fabrication of asymmetric-AAO-Au membrane The AAO membrane was fabricated by a two-step anodization technique ( Supporting Information Figure S1). The aluminum substrate (Al, thickness 0.1 mm, purity 99.99%, Xinjiang Zhonghe Limited Corp., Xinjiang, China) was placed in acetone, 1 M KOH and water successively for ultrasonic cleaning and then electrochemically polished using a 1∶9 mixture solution of HClO4 and CH3CH2OH (2 °C) at a voltage of 10 V for 5 min. The mirror-smooth Al substrate was first anodized in a 0.2-M H3PO4 solution (8 °C) at a constant voltage of 165 V for 2 h. Then the anodization voltage was decreased gradually from 165 to 60 V at 1.5 V/min to thin the barrier layer. The resulting Al-AAO composite was etched further in a 5% H3PO4 solution (30 °C) for 1 h to remove the barrier layer completely. Subsequently, a second anodization was performed in a 0.3 M H2SO4 solution (−0.3 °C) at a constant voltage of 25 V for 20 h. After anodization, a free-standing AAO was formed by removing the aluminum substrate with a 0.11 M CuCl2 solution (in 6 M HCl) and opening the bottom with a 5% H3PO4 solution. The as-synthesized asymmetric (asy)-AAO membrane (asy-AAO) was annealed at 750 °C in the Argon atmosphere for 3 h to reduce the lattice defects and enhance the transparency. Finally, a ∼60-nm-thick layer of gold was deposited onto the large-pore side of the membrane by a magnetron sputtering method (SCD500 Spatter Coater, BAL-TEC), forming asy-AAO-Au. Characterization of asy-AAO-Au membrane The absorption spectra of the asy-AAO and asy-AAO-Au membranes were measured in the range of 400–2500 nm on a Shimadzu UV-3600 (UV–vis—near infrared [NIR]) spectrophotometer attached with an integrating sphere (ISR-3100). The surface morphology of the membrane was acquired on a field-emission scanning electron microscopy (FESEM) (S-4800, Hitachi, Tokyo, Japan) at an accelerating voltage of 5 kV. Measurement of TMP TMP was measured by a homemade four-cell device. The membrane was mounted between two vertically assembled cells, and light (Xenon lamp, LSPX150; Zolix, Beijing, China) was irradiated down to the membrane. The irradiation intensity was monitored by an optical power meter (LP-3A; Physcience Opto-Electronics, Beijing, China), and the optical power density was adjusted by inserting attenuation plates (25%, 50%, and 75%). Two other cells were adopted as extensions of the upper and lower cells, and a pair of Ag/AgCl electrodes were placed in these two cells to measure the TMP. A shutter was used to separate the photosensitive Ag/AgCl electrodes from the lamp, and each of these cells contained a 50 mL solution, which ensured a constant temperature of the bulk solution during the experiment. The TMP was recorded by a CHI 650 electrochemical workstation (Chenhua, Shanghai, China), and all measurements were performed under ambient conditions. Results and Discussion The two-layer AAO membrane fabricated by a two-step anodization technique showed an asymmetric structure.34,35 As revealed by SEM images, the asy-AAO is composed of a 60-μm-long portion with a pore diameter of 25 nm and a 6-μm-long portion with a pore diameter ranging from 200 to300 nm (Figure 1b and Supporting Information Figure S2). The small-pore portion had a pore diameter comparable to the thickness of the electric double layer (EDL), which endowed the membrane with ion selectivity. While, the large-pore portion showed a 3-D porous structure,36,37 which facilitated the deposition of the 3-D gold nanostructure, forming asy-AAO-Au (Figure 1c and Supporting Information Figure S2d). The plasmonic absorption of the gold nanostructure could convert light efficiently into heat and raise the localized temperature. The original transparent asy-AAO became black after the deposition of gold ( Supporting Information Figure S3). As displayed in Figure 1d, the asy-AAO was transparent for the whole waveband (absorption <5%, orange curve), while the absorption of the asy-AAO-Au was extensively enhanced (80–90% in the visible region and ∼90% in the NIR region) when irradiating from the small-pore side (asy-AAO-Au-back, green curve). This was due mainly to the distinct gold nanostructure, which consisted of a perforated 60-nm-thick gold film on the AAO surface and gold nanoparticles (AuNPs) with a continuous size distribution on the inner wall of the nanochannels (Figure 1c). The SPR hybridization of the closely packed mixture of the small and large AuNPs endowed the membrane with broadband absorption, and the highly reflective gold film, despite weak absorption, could reflect light into the AuNPs region, which increased the optical path length and enhanced the absorption.38,39 However, when irradiating from the large-pore side (asy-AAO-Au-front, red curve), asy-AAO-Au showed a much weaker absorption in contrast to asy-AAO-back, because the compacted gold film reflected a large portion of the light. These results confirmed highly efficient broadband plasmonic absorption of the asy-AAO-Au. The surface property of the nanochannels in asy-AAO-Au was evaluated through measuring the transmembrane ionic conductance. As the salt concentration decreased, the ionic conductance first reduced and then reached saturation (<10−3 M) ( Supporting Information Figure S4). This provides evidence for the existence of the surface charge on the nanochannels.40,41 The surface charge density was calculated to be −5.21 mC/m2 in the neutral solutions, indicating cation selectivity. A homemade four-cell device was adopted to measure the TMP ( Supporting Information Figure S5). Notably, the temperature-sensitive and photosensitive Ag/AgCl electrodes were separated from the light using a shutter to keep their potentials constant during the experiment. As a result, the TMP could be obtained directly by measuring the change of the open circuit potential (OCP). The light-induced temperature difference across the asy-AAO-Au membrane, which acted as the driving force for the charge separation, was first studied indirectly by measuring the temperature change of the gold nanostructure (ΔTAu). Considering that the resistance change of the gold nanostructure (ΔRAu) is proportional to the ΔTAu,42 the gold nanostructure was made into a plasmonic thermistor. The ΔRAu was monitored and then converted into the ΔTAu with the established relation (ΔTAu = 132.8 K/Ω·ΔRAu) ( Supporting Information Figure S6). It is reasonable to take ΔTAu as the temperature difference across the membrane (ΔTm), in light of the relatively large thermal resistance of the solution and the alumina. The ΔTm-time curve for the asy-AAO-Au upon illumination in the KCl solution was then obtained (Figure 2a, blue curve). The temperature difference across the membrane responded rapidly to the switching of the illumination, and it reached saturated values within 30 s. It was observed that the temperature difference increased as high as 35 K, which could be ascribed to the efficient broadband plasmonic absorption and the photothermal conversion of the 3-D gold nanostructure. Figure 2 | Demonstration of the STEND. (a) ΔTm (blue curve) and OCP (orange curve) response of asy-AAO-Au to light. The yellow and cyan areas indicate whether the light source was on or off, respectively. (b) TMP of the asy-AAO, sym-AAO-Au, and asy-AAO-Au. (c) OCP response of the asy-AAO-Au when irradiating from the large-pore (asy-AAO-Au-front, orange curve) and small-pore (asy-AAO-Au-back, blue curve) sides. (d) TMP and ΔTm of asy-AAO-Au-front and asy-AAO-Au-back. (e) TMP of asy-AAO-Au under different ΔTm. If not mentioned, all the experiments were performed in 0.1 mM KCl, the power density of the light was 8 kW/m2, and the light was irradiated from the small-pore side. STEND, solar thermoelectric nanofluidic device; OCP, open circuit potential; AAO, anodic aluminum oxide; TMP, thermal membrane potential. Download figure Download PowerPoint The OCP-time curve (Figure 2a, orange curve) was highly consistent with the trend of the temperature, indicating a heat-induced potential. The TMP can be obtained by creating a baseline and measuring the change of OCP. The influence of the SPR-induced water splitting could be ruled out, as revealed by the constant pH values of the solution monitored before and after illumination ( Supporting Information Figure S7). We found that the TMP of the asy-AAO-Au reached 23.6 ± 0.3 mV, while it was not observed in the case of the control systems of the bare asy-AAO and the symmetric 200-nm-diameter AAO deposited with 60-nm-thick gold (sym-AAO-Au) (Figure 2b). This demonstrated that both the SPR-enabled localized heating (large-pore portion) and the ion-selective nanochannel (small-pore portion) played key roles in generating the TMP. Also, we found that with prolonged illumination, the TMP decreased gradually after illuminating for 500 s ( Supporting Information Figure S8). This is because with the increase of the illumination time, the heat accumulated at the membrane region, which reduced the temperature gradient inside the membrane. The maximum output power density (Pmax) was calculated as Pmax = ITMC × VTMP, where ITMC is the thermal membrane current (e.g., short-circuit current), and VTMP is the TMP. For an optimized condition (0.1 mM KCl; power density of light: 8 kW/m2), VTMP was 23.6 ± 0.3 mV and ITMC was 0.188 ± 0.011 μA/cm2; therefore, Pmax was ∼4.5 nW/cm2. Although the light-to-electricity conversion efficiency is very low, it might be enhanced substantially using nanochannels with high thermal resistance and a high surface charge density. The OCP-response of the asy-AAO-Au showed good stability and reproducibility when switching the light between "on" and "off" (Figure 2c). The ΔTm and TMP values were much higher when irradiating from the small-pore side than they were for the large-pore side (Figure 2d), which is attributed to the difference in the plasmonic absorption as shown in Figure 1d. Besides, when the membrane was flipped upside down without changing any other part of the system, the direction of the TMP was reversed because of the reversed temperature gradient. Both the ΔTm and the TMP increased with an increase in the power density of the light ( Supporting Information Figure S9), and the extracted ΔTm and TMP showed a linear relationship (Figure 2e). The Seebeck coefficient in KCl solutions, which could be obtained from the slope of the fitting line, reached 0.76 ± 0.17 mV/K, an order of magnitude larger than the Seebeck coefficient of the simple monovalent electrolytes in bulk.23 However, for the bath temperature control thermoelectric system using the same asy-AAO-Au membrane ( Supporting Information Figure S10), the Seebeck coefficient was only 0.13 ± 0.01 mV/K, and thus, a small TMP value of 3.6 ± 0.2 mV was obtained at a temperature difference of 30 K. In the bath temperature control system, there was a deviation between the membrane-surface temperature and the bath temperature because layers with temperature gradients existed on both sides of the membranes adjacent to the solution. This effect, known as temperature polarization, reduced the temperature gradient inside the membrane considerably. The salt concentration has a significant impact on the EDL inside the nanochannels, which could directly change the TMP. For the KCl electrolyte (Figure 3a, blue rhombuses), the TMP was not observed at high concentrations (>10−2 M). Instead, it increased with a decrease in the salt concentration in a moderate range (10−4–10−2 M). After reaching a maximum at 0.1 mM, it decreased again, which was inconsistent with the theoretical prediction.23 This phenomenon could be ascribed to changes in the EDL thickness with salt concentration. At a high concentration, the surface charge on the wall of the nanochannels was shielded, and the membrane lost its ion selectivity. This shielding effect disappeared gradually with decreasing salt concentration, which recovered the ion selectivity of the membrane and enhanced the TMP. The abnormal TMP at extremely low concentrations (<10−4 M) would be discussed further below. Then we investigated the influence of the electrolyte type by comparing the TMP in electrolytes with various alkali metal ions (LiCl, NaCl, KCl, RbCl, and CsCl) and organic carboxylate ions (HCOONa, CH3COONa, C2H5COONa, C6H5COONa, and C7H15COONa) ( Supporting Information Figure S11). The TMP was almost constant in these electrolytes because the nanochannel size (25 nm) was much larger than the size of the hydrated ions (<1 nm). However, for cations with different valences (KCl, CaCl2, and LaCl3), the higher cationic valence state resulted in a smaller TMP (Figure 3a), which was due to the much stronger shielding effect of multivalent cations, compared with the monovalent ones.43 These results showed that the surface charge had a great influence on the charge separation inside the nanochannels. Figure 3 | Influence of electrostatic interaction between ions and nanochannels on TMP. (a) TMP of the asy-AAO-Au in electrolytes of different types (KCl, CaCl2, and LaCl3) and concentrations. (b) TMP of the asy-AAO-Au in 1 mM KCl with different pH values. If not mentioned, the power density of the light was 8 kW/m2, and the light was irradiated from the small-pore side. (c) electric potential of the EDL inside the nanochannels with and without temperature gradients in different (KCl, CaCl2, and (d) electric potential the direction of the nanochannels with and without temperature The potential was by the surface potential of the nanochannels. (e) TMP in different salt concentrations. TMP in different length with different surface charge The electrolyte concentration, pore surface charge and temperature difference across the nanochannels, not mentioned, are 0.1 mM, 25 and K, respectively. TMP, thermal membrane potential; AAO, anodic aluminum oxide; electric double layer. Download figure Download PowerPoint The influence of properties of the nanochannels (e.g., diameter and surface charge on the TMP was also For the nanochannels with a larger diameter (e.g., a much smaller TMP was to the low EDL low ion selectivity ( Supporting Information Figure We further the surface charge density of the nanochannels by changing the solution pH (Figure We found that when the pH was higher than the TMP increased with the pH due to the increased surface charge by the enhanced of of the However, for a pH lower than the TMP was reversed because the was at a solution pH lower than its which the ion selectivity from cations to These results showed that the thermal-driven transport properties of cations and could be regulated by the surface charge density of the nanochannels, which confirmed that the electrostatic interaction between the surface charge and the ions was a key for charge separation inside the nanochannels. the experimental the and of the TMP in the STEND were using the method ( Supporting Information Figure The existence of the temperature difference in the STEND endowed the ions with different resulting in different of the ion The between the electrostatic interaction of the ions with the nanochannels wall and the ion the of the EDL inside the nanochannels. to the the electric potential in an EDL decreased with increased temperature (Figure which demonstrated that ion energy to the electrostatic The electric potential the direction of the nanochannels showed a potential difference between two sides of the nanochannels under a temperature gradient (Figure In the electric potentials of the EDL were much lower for multivalent ions than monovalent ions because of the electrostatic interaction of the multivalent ions by the charged thus, low were observed (Figure The theoretical of the TMP was in good with the experimental results at high salt concentrations and However, at low the theoretical TMP a which not the trend observed the electrostatic interaction between the ions and nanochannel on the surface charge density than the salt concentrations at a low concentration TMP constant with a further decrease in salt concentration. The observed TMP in the extremely low concentration range is by the of the surface charge density of the AAO or the change in ion thermophoretic mobilities. Moreover, it was that the TMP could be increased by the surface charge density (Figure with a surface charge density of a TMP value as high as mV was consistent with the experimental (Figure in which surface charge density was by changing the solution These results confirmed that the surface charge density to enhance the electrostatic interaction between the nanochannels and ions is a for the TMP of the STEND. The heat inside the STEND also had a significant impact on the energy conversion which was mainly by the thermal resistance coefficient (e.g., thermal resistance of the asy-AAO-Au membrane. The thermal resistance coefficient of the membrane was proportional to the length and to the heat (e.g., (Figure ΔTm increased with the thermal resistance coefficient and a at (Figure as from the results shown in Supporting Information Figure A thermal resistance TMP was observed in the nanochannels with a length (Figure This is because the of the temperature gradient inside the nanochannels, which acted as the driving force for the charge separation, has on the thermal resistance coefficient of the membrane. Figure | Influence of heat and effect on TMP. and ΔTm and TMP across nanochannels with different thermal resistance in the influence of the on the thermal resistance coefficient of the nanochannels with different and concentration across the and and 10 (e) for the across the membranes with a and 10 The electrolyte concentration of KCl, pore surface charge and temperature difference across the nanochannels in not mentioned, were 0.1 mM, 25 and K, respectively. ion concentration TMP, thermal membrane potential. Download figure Download PowerPoint for membranes with the same thermal resistance such as a membrane with one and a membrane with 10 they the same the TMP of the membrane was much larger than that of the membrane (Figure We this phenomenon to the of the ion concentration polarization (ICP) regions in the membrane. The of the concentration in these two membranes showed compared with the membrane, the concentration of and ions decreased due to the of the region in the membrane, which a reduced of charge separation, to a smaller TMP. the performance of the STEND could be improved further by decoupling the regions by the demonstrated a technique for harvesting solar thermal energy by the heating and the thermal-driven charge This strategy utilized localized heating of bath temperature control to the temperature which not only the temperature polarization effect but also the influence of temperature on electrode We showed that the nanochannels could regulate the thermal-driven ionic charge separation through of the electrostatic interaction between surface charge and A Seebeck coefficient as high as 0.76 mV/K was achieved in order of magnitude larger than that of the simple monovalent electrolytes in the bulk We also revealed the of heat and the effect in the of the TMP. designed STEND might the understanding of nanoscale transport and shows potential for solar thermal energy Supporting Information Supporting Information is of is of to Information This study is by the of and and for a Energy of the and for Solar Thermal in Energy the with Solar with for a and of Solar for Thermoelectric Energy K. the of for Energy in the of and to to and as for Energy and Thermal Energy K. 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