Light-Induced Heat Driving Active Ion Transport Based on 2D MXene Nanofluids for Enhancing Osmotic Energy Conversion

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE1 Apr 2021Light-Induced Heat Driving Active Ion Transport Based on 2D MXene Nanofluids for Enhancing Osmotic Energy Conversion Pei Liu, Teng Zhou, Yunfei Teng, Lin Fu, Yuhao Hu, Xiangbin Lin, Xiang-Yu Kong, Lei Jiang and Liping Wen Pei Liu CAS Key Laboratory of Bio-Inspired Materials and Interfacial Science, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Future Technology, University of Chinese Academy of Sciences, Beijing 100049 , Teng Zhou College of Mechanical and Electrical Engineering, Hainan University, Haikou 570228. , Yunfei Teng CAS Key Laboratory of Bio-Inspired Materials and Interfacial Science, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Future Technology, University of Chinese Academy of Sciences, Beijing 100049 , Lin Fu CAS Key Laboratory of Bio-Inspired Materials and Interfacial Science, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Future Technology, University of Chinese Academy of Sciences, Beijing 100049 , Yuhao Hu CAS Key Laboratory of Bio-Inspired Materials and Interfacial Science, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190 , Xiangbin Lin CAS Key Laboratory of Bio-Inspired Materials and Interfacial Science, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190 , Xiang-Yu Kong *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] CAS Key Laboratory of Bio-Inspired Materials and Interfacial Science, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190 , Lei Jiang CAS Key Laboratory of Bio-Inspired Materials and Interfacial Science, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Future Technology, University of Chinese Academy of Sciences, Beijing 100049 and Liping Wen *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] CAS Key Laboratory of Bio-Inspired Materials and Interfacial Science, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Future Technology, University of Chinese Academy of Sciences, Beijing 100049 https://doi.org/10.31635/ccschem.020.202000296 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Osmotic energy from the ocean, also called blue energy, serves as a clean, renewable, and vast energy source for the energy demands of the world. Reverse electrodialysis-based blue energy harvesting via ion-selective membranes, by the regulation and manipulation of directional ion transport, has been greatly developed recently. In particular, light has been employed to enhance directional ion transport for energy conversion through an increase in photo-induced surface charge. Here, the authors demonstrate a novel nanofluidic regulation strategy based on the phenomenon of light-induced heat-driven active ion transport through the lamellar MXene membrane. Due to the great light-induced heat effect, a temperature gradient appears as soon as illumination is applied to an off-center position, inducing an actively temperature gradient-driven ionic species transport. By employing this phenomenon, the authors conducted light-induced heat-enhanced osmotic energy conversion and doubled the osmotic energy conversion power density. This study has extended the scope of light-enhanced osmotic energy conversion and could further bring other photothermal materials into this field. Furthermore, the proposed system provides a new avenue of light-controlled ionic transport for ion gathering, desalination, and energy conversion applications. Download figure Download PowerPoint Introduction Osmotic energy from the salinity difference between ocean water and river water, also known as "blue energy," has attracted increasing attention in recent years.1,2 Reverse electrodialysis (RED) employing ion-selective membranes, especially nanofluidic technology, for osmotic energy harvesting shows great potential and significantly boosts the development of this field.3–6 To pursue better energy conversion performance of the RED system, researchers have focused on designing novel structures and materials for ion-selective membranes to finely manipulate ion transport behavior, by regulating the membrane thickness, pore size, micro/nanoscale morphology, charge distribution, and so on.7–9 In addition, in nanofluidic manipulation technology, the regulation methods of ion transport have been widely studied.10–13 In recent years, two-dimensional (2D) materials have also been applied in osmotic energy harvesting.14–17 The precisely controlled channel size, high surface charge density, and excellent mechanical properties of 2D materials lead to advances in their application in the osmotic energy harvesting field. Besides, several physical fields, especially light irradiation, have been applied to promote osmotic energy harvest by controlling ion transport with experimental and theoretical approaches.17–19 Compared with other types of physical stimuli, light stands out because of its flexibility in illumination positions, wavelengths, and intensities. With the advantage of the light-induced novel effects of 2D materials,20,21 light-regulated ion transport in 2D materials for osmotic energy harvest enhancement is particularly attractive. In the past decade, light-controlled directional ionic transport systems have elicited considerable interest in interdisciplinary fields19,22–27 and realized a series of new materials for potential application in nanofluidic devices and miniature osmotic energy conversion systems. Due to the excellent photoelectric effect of 2D materials, a series of light regulated ion transport systems have been fabricated. For a layered graphene oxide membrane (GOM)-based nanofluidic device and layered WS2 membrane24,25 and layered WS2 membrane,26 asymmetric simulated solar irradiation can induce an ionic current due to the generated potential from the light-induced electron–hole separation process. Single-layer MoS2 with a single nanopore exhibited an enormous single-pore power density.28 Later, in 2019, light was induced in the energy conversion system, and the photoelectrical effect of the single-layer MoS2 can greatly enhance the ionic diffusive process and double the power density under the same concentration gradient.17 In addition to the photoelectrical effect, light-induced heat technology, which can harvest and convert light (especially the most abundant and sustainable solar irradiation) into heat, also shows great application potential for beneficial usage. For example, MXenes with excellent photothermal effects have been applied for multiple imaging-guided tumor ablation.29 In addition, by employing the light-to-heat conversion effect of MXenes, a solar-driven interfacial evaporation device has been fabricated for water desalination.30,31 Compared with photoelectric conversion, photothermal conversion normally shows superior energy economy with a wide light absorption range.21,32 However, how the photothermal effect regulates ion transport for osmotic energy conversion is still unknown, but might open a new avenue for synergetic energy harvesting with solar energy and osmotic energy. Here, we present a series of laminar MXene nanofluidic devices for light-induced heat-regulated ion transport to evaluate the photothermal enhancement of osmotic energy conversion. MXenes, which constitute a class of well-developed photothermal conversion materials with strong light absorption and efficient photothermal conversion,20,29 were employed to efficiently convert light into heat. With asymmetric illumination, an uneven temperature gradient could be formed in the MXene membrane and used to drive ion transport for current generation. To determine the mechanism of this process, a model system was proposed for finite-element simulations. Furthermore, by using this device, the following three working modes were realized: (1) without a concentration gradient, showing the light-thermal-electric conversion; (2) with an anticoncentration gradient, presenting the light-induced antigradient ion transport under concentration gradient, also called ionic pumping; and (3) under a concentration gradient, exhibiting light-enhanced osmotic energy harvesting by capturing additional light energy in the concentration gradient conversion with the help of MXenes. Experimental Method Fabrication of the MXene membrane The MXene membrane was fabricated by the vacuum filtration method. First, a colloidal solution of few-layer 2D Ti3C2Tx nanosheets was prepared via sonication of multilayer Ti3C2Tx after selective etching of raw MAX-phase Ti3AlC2 powder using a LiF/HCl solution. Then, the dispersed Ti3C2Tx nanosheets self-assembled into a nanofluidic membrane upon vacuum filtration on polycarbonate (PC, an average pore size of 200 nm). After drying, the free-standing MXene membrane could be easily exfoliated from the substrate and could be easily cut into the desired shapes and sizes. Characterization of the MXene membrane X-ray photoelectron spectroscopy (XPS) of the MXene membrane identified the presence of oxygen and fluorine (see Supporting Information Figure S3), indicating the existence of hydroxide and fluoride surface groups. These functional groups create the negatively charged surface and cationic selectivity of Ti3C2Tx nanosheets in aqueous environments. The X-ray diffraction (XRD) pattern also proved the successful construction of the membrane. The (002) peak in the XRD pattern is a strong, single peak at 6.97° (see Supporting Information Figure S2), suggesting the highly uniform lamellar structure of the membrane. Device fabrication As schematically shown in Figure 1a, a rectangular MXene membrane with both sides embedded with polydimethylsiloxane (PDMS) precursor was cured in an oven for 4 h at 80 °C, in a two-compartment electrochemical cell (made of Teflon) to avoid leakage of the solution (see Supporting Information Figure S5). Then, the two sides of the sealed MXene membrane were trimmed off to expose to the electrolyte solution. A pair of Ag/AgCl electrodes was used to record the ion transport through the MXene membrane in the horizontal direction. Figure 1 | Photothermal-induced ion transport through the MXene membrane. (a) Schematic illustration of the current generation by partially irradiating the MXene membrane. (b) Cross-sectional SEM image of the MXene membrane, showing a layered structure. The inset shows an optical photograph of the rectangular MXene membrane. (c) Light-induced temperature change and current generation (light intensity of ∼200 mW·cm−2). Download figure Download PowerPoint Photothermal conversion property measurements A piece of rectangular MXene membrane was used for the photothermal conversion performance measurement. Light with different wavelengths was applied at a power density of ∼200 mW·cm−2. The temperature was monitored every 5 s by a Fluke (Ti450; Everett, Washington, USA) thermal imaging camera. Numerical simulation Theoretical calculations were carried out using the commercial finite-element software package COMSOL (version 5.4; COMSOL Inc., Stockholm, Sweden) Multiphysics based on the "electrostatics (Poisson equation)" and "Nernst–Planck without electroneutrality" modules. The coupled governing Poisson–Nernst–Planck (PNP) equations are shown below: J i = D i ( ▿ c i + z i F c i R T ▿ φ ) + c i u (1) ▿ 2 φ = − F ɛ Σ z i c i (2) ▿ · J i = 0 (3) Here, the physical quantities Ji, Di, ci, φ, u, R, F, T, and ɛ refer to the ionic flux, diffusion coefficient, ion concentration, electrical potential, fluid velocity, universal gas constant, Faraday constant, absolute temperature, and dielectric constant of the electrolyte solutions, respectively. Equation (1) is the Nernst–Planck equation, which describes the transport property of a charged nanochannel. Equation (2) is the Poisson equation, which describes the relationship between the electrical potential and ion concentrations. In addition, the flux should satisfy the time-independent continuity equation (3) when the system reaches a stationary regime. A simplified negatively charged (σ = −0.025 mC·m−2) channel with a length of 50 nm, a width of 8.3 nm, and a height of 0.27 nm was chosen as the simulated model (see Supporting Information Figure S15), and an equal concentration of electrolyte solution (KCl, 0.01 M) was added to both reservoirs. To carry out the calculations, the "electrostatics (AC/DC)" and "Nernst–Plank without electroneutrality" modules were used. In addition, to precisely set the temperature of the channel, the "heat transfer in fluids" module was coupled to this model. The model was divided into three equal parts, and with a temperature difference of 0, 4, 8, 12, 16, 20, 24, 28, 32, 36, and 40 K (room temperature is set to 298 K) was applied. The coupled equations (1–3) must be solved for a given geometry using appropriate boundary conditions. The boundary condition for the potential φ on the channel wall is → n · ▿ φ = − σ ɛ (4)The ion flux has zero normal components at the boundaries: → n · J = 0 (5)The parameter σ (σ is set to −0.025 mC·m−2) is the surface charge density of the channel walls. Then, the ionic current can be calculated by: I = ∫ F s ( z p j p + z n j n ) · → n d S (6) Results and Discussion Light-induced temperature change and directional ion transport Ti3C2Tx was chosen to fabricate the MXene membrane (see details in Supporting Information Notes 1 and 2). Briefly, the raw MAX-phase Ti3AlC2 was etched by a mild etching method33,34 to obtain few-layered micron-sized Ti3C2Tx nanosheets (see Supporting Information Figure S1a). The strong Tyndall scattering effect of the as-prepared Ti3C2Tx colloidal suspension (see Supporting Information Figure S1b) indicated good dispersibility. The vacuum-assisted filtered MXene membrane was embedded in PDMS, and a pair of Ag/AgCl electrodes was used to investigate the ion transport behavior through the MXene membrane in the horizontal direction (Figure 1a and Supporting Information Figure S3). The cross-sectional scanning electron microscopy (SEM) image (Figure 1b) of the membrane indicates a highly ordered lamellar structure with a layer spacing of ∼0.27 nm based on XRD data (see Supporting Information Figure S2 and Note 4).33,35 The negative surface, which was characterized by XPS and zeta potential (see Supporting Information Figures S3 and S4), is responsible for the cationic selectivity of the membrane. The cationic selectivity of nanochannels is crucial to achieve a high sensitivity of cation flow generation. With no applied voltage and an equal concentration of KCl solution (0.01 M), there is almost no ionic current in the system. Upon light irradiation (light intensity ∼200 mW·cm−2) of a partial MXene membrane, the temperature of the illuminated area quickly increases (∼19.91 °C, red line in Figure 1c) due to the good photothermal effect of MXene materials,20 and the corresponding ionic current increases sharply to approximately −37.5 nA (blue line in Figure 1c). When the illumination is interrupted, the current and temperature return to the original state, confirming that light irradiation induces the ion generation. In addition, the direction of the recorded ionic current, which is from the nonilluminated region (low temperature) to the illuminated region (high temperature), is in the same direction of the temperature gradient in the device.10 To investigate the heat transfer in the device, the temperature changes of the irradiation and non-irradiation regions, and also the photocurrent evolution with time have been recorded (see Supporting Information Figure S6). In addition, this study is also different from the recent report of photothermal electricity generation based on a nanofluidic water pump from Lao et al.36 In their system, asymmetric light irradiation could cause gradient water evaporation and directional water flow, thus drive the ion flow to generate current. A comparison between the mechanism in our study and others has been summarized (see Supporting Information Table S1). To examine the relationship between the ionic current direction and light irradiation, the irradiated regions of the membrane are shifted accordingly. As shown in Figure 2a, when illumination was applied to the left part of the MXene membrane, the horizontal ionic current increased to approximately −37.5 nA (Figures 2a-i and 2a-iv). When the simulated solar illumination was applied to the middle section of the MXene membrane, no significant ionic current could be observed (Figures 2a-ii and v). Furthermore, the direction of the current was reversed when the illumination shifted to the right part of the MXene membrane, without altering its magnitude significantly (approximately +37.4 nA; Figures 2a-iii and vi). To better illustrate the detailed relationship between the position of the light irradiation and the magnitude of the current, more points have been set from the left to the right side ( Supporting Information Figure S7). Considering the light intensity issue (350 nm light), the photothermal-induced current increased with increasing light intensity (Figure 2b). In addition, the wavelength dependence of the photothermal ionic current was measured (Figure 2c and Supporting Information Figure S8). The ionic current recorded at different light wavelengths generally agreed with the absorption spectrum of the Ti3C2Tx dispersion (Figure 2c), further confirming the light-induced heat-driven ionic current generation. Figure 2 | Light-induced ion transport performance. (a) Time series of the current (iv—vi) obtained by light irradiation (light intensity of ∼200 mW·cm−2) separately on three different regions of the MXene membrane [(i) left, (ii) middle, and (iii) right]. (b) Ionic current under different light intensities at 350 nm (65, 200, 280, 394, and 690 mW·cm−2). (c) UV–vis absorption spectrum of Ti3C2Tx dispersion (black solid curve), and the generated currents of the MXene membrane at a series of wavelengths (orange open circles). Download figure Download PowerPoint Photothermal conversion performance As the irradiation-induced ionic current is strongly related to the light-induced heat generation, the real-time temperature of MXene was recorded with an IR thermal camera to investigate the photothermal performance of the fabricated device. Before irradiation (350 nm), the MXene membrane showed the same temperature as the testing cell (Figure 3a-i). Upon irradiation with 350 nm light (light intensity 200 mW·cm−2), the MXene membranes underwent a significant temperature increase (Figure 3a-ii). The corresponding temperature distribution indicates that the temperature of the MXene membrane could increase from ∼28 to ∼60 °C (Figures 3a-iii and iv). The temperature increased as soon as the simulated solar irradiation began, owing to the quick photothermal responsive performance of the MXene membrane (Figure 3b). The directly irradiated surface reached ∼60 °C in less than 5 s. It was found that the temperature increases significantly after irradiating the MXene membrane with different wavelengths of light (see Supporting Information Figure S9). The current and the maximum temperature difference decreased gradually with increasing wavelength. The temperature reached 68.34, 60.92, 54.27, 55.23, 55.72, 55.20, 53.58, 45.57, and 33.54 °C at the wavelength of 350, 380, 420, 475, 500, 550, 600, 650, and 700 nm, respectively (Figure 3c and Supporting Information Figure S10). As the nanosheets, which hold a large number of transition metals, remain metallic upon any type of functionalization,37–39 Ti3C2Tx was unlikely to undergo the light-induced electron-hole separation process due to the lack of a band gap in Ti3C2Tx, which is not the situation in other photoresponsive materials.19,24–26 Considering the potential practical application of the membrane, the thermal stability of our material was further estimated by its weight loss at an increasing temperature under a nitrogen protective atmosphere. Thermogravimetric analysis measurements show <5% weight loss even when the MXene membrane was heated to 600 °C (see Supporting Information Figure S11), indicating excellent thermal stability. Figure 3 | Photothermal conversion performance of the MXene membrane. (a) IR camera images (350 nm, ∼200 mW·cm−2) and the corresponding temperature distribution along with the MXene membrane. (b) Temperature response curve of the membrane under illumination (350 nm, ∼200 mW·cm−2). (c) The changes in the generated currents (blue open circles) and the maximum temperature difference (ΔTmax, red open square) at each wavelength (∼200 mW·cm−2). Download figure Download PowerPoint To further prove that the light-induced temperature change generated the ion current, parallel experiments were conducted. Here, a heated solution was used to simulate the photothermal effect. Since the temperature of the left part of the membrane reached ∼50.38 °C under light irradiation (light intensity ∼200 mW·cm−2, see Supporting Information Figure S12a-iii), an electrolyte solution with a similar temperature (∼51.32 °C) in the left reservoir (see Supporting Information Figure S12b-iii) and a room-temperature solution in the right reservoir were established to simulate the light irradiation experiment. Under such test conditions, ionic currents without externally applied voltage were recorded (see Supporting Information Figure S13). As expected, the ionic current flowed from the low-temperature region to the high-temperature region, consistent with the direction of the current generated by asymmetric illumination. Therefore, the variation of the Gibbs free energy of the electrolyte (dG) caused by the temperature difference is as follows10: d = − S d T + d p + Σ i = 1 i d n i Here, is the Gibbs free energy, S is the T is the temperature, is the and p is the and are the potential and the number of species respectively. When light irradiation was partially applied upon the layer MXene membrane, the Gibbs free energy of illumination regions decreased with increasing temperature, and the Gibbs free energy of the electrolyte in the region by the Gibbs free energy diffusion and a ionic current is (see details in Supporting Information Note 5 and Figure transport from the low-temperature region to the high-temperature region, which could for the Therefore, transport in the same direction of the temperature light energy could be to electricity via thermal gradient directional ion transport. Numerical simulation To further the proposed photothermal-induced current finite-element based on coupled equations were to the experimental For the negatively charged MXene membrane, a simplified negatively charged (σ −0.025 mC·m−2) channel with a length of 50 nm, a width of 8.3 nm, and a height of 0.27 nm was established for the simulation (see section and Supporting Information Figure and electrolyte of equal concentration (KCl, 0.01 M) were added to both reservoirs. In this negative channel, were the of the generated ionic current under a light-induced temperature Based on three different ionic could be obtained by a thermal at three different of the channel (Figure When a heat source was applied the left side of the channel, the ionic from right to left could be observed (Figure and Supporting Information Figure When the heat source was to the middle of the channel, the ionic was almost zero (Figure and Supporting Information Figure the ionic from left to right from the heat source applied to the right side (Figure and Supporting Information Figure In addition, the direction of cation transport was in the same direction of the temperature were the same as our experimental In addition, the ionic was to the increase in temperature Figure 4 and Supporting Information Figures and the increasing diffusion currents were observed along with the temperature gradient in the model (Figure and showed the with the current system. It is that the high ion are used to the of the ionic current along with the temperature, and the absolute are from the simplified which is from the The ion might the and theoretical simulations. Figure 4 | Numerical (a) The ionic obtained by a thermal at three different of the left, (ii) middle, and (iii) (b) The ionic at different temperature when the thermal is applied to the left side of the (c) of current as a of temperature difference in Download figure Download PowerPoint osmotic energy conversion For working modes in solutions, from to high concentration from concentration solutions, the light-induced ion transport shows great potential for applications. Considering the anticoncentration several ion have been in recent years, and our system used light-induced thermal energy as the (see Supporting Information Note 5 and Figure attracted the most was the light-enhanced ion transport under concentration which could a new for osmotic energy conversion. recent that the photoelectrical effect could enhance the ionic diffusive process on single-layer MoS2 under the same concentration gradient.17 Here, our light-induced heat-driven ion transport could also help to increase the osmotic energy conversion performance but with different light-enhanced In the proposed the direction of the ionic current induced by the concentration gradient is in the same direction as the light-induced ion transport (Figure Under a concentration difference = = M), the diffusion current was approximately Upon irradiation mW·cm−2) the left side of the MXene membrane, the ionic current increased to approximately nA (Figure In addition, the measured response time for the increase increasing from to of the peak and from to of the peak in the photocurrent are and indicating a response (see Supporting Information Figure As a of we fabricated an osmotic energy conversion device with river water (0.01 and ocean water by employing the light-induced heat-driven ion transport phenomenon, and the generated osmotic energy could be to the to an (Figure Before irradiation, the current density on the decreased as the increased under salinity and the power density,