Brief introduction of the results
Seawater flow and evaporation-induced power generation technologies hold great promise in driving the next generation of sustainable energy technologies. However, there are some limitations when operating in environments with high ion concentrations, such as natural seawater. To solve this problem,Prof. Hongfang Liu and Prof. Baoyu Xia (corresponding author) of Huazhong University of Science and TechnologyThe research group proposed a novel solution for seawater evaporation-induced power generation using the organic framework of calcium ** (MOFS, C12H6Ca2O19·2H2O).
The experimental results show that when calcium-based MOFs are immersed in seawater under natural conditions, the open-circuit voltage is 04 V with a short-circuit current of 14 A. Experimental and simulated studies have shown that sodium ions are selectively transported in the sub-nanochannels of these synthesized superhydrophilic MOFs. This selective ion transport creates a unipolar flow of solution, which drives the behavior of electricity generation in seawater.
This work not only demonstrates an efficient calcium-based MOFs for power generation through seawater flow evaporation, but also has important implications for our understanding of water-driven energy harvesting technologies and their potential applications in other environments. This research provides new ideas for the further development of water-driven energy harvesting technologies and contributes to the development of the field of sustainable energy.
Background:
Converting naturally occurring energy into electricity is one of the key strategies to address the growing global demand for electricity. Among them, the use of seawater to generate electricity through flow and evaporation is a promising way. Seawater evaporative power generation has two unique advantages: the generator's geographical location in the ocean is not limited, and the seawater evaporation process has a huge scale on Earth.
Despite the rapid development of pure water flow and evaporative power generation technologies in recent years, power generation through seawater evaporation still faces daunting challenges. This challenge is mainly due to the increase in ion concentrations in seawater. The basic principle of seawater evaporative power generation is the use of electrodynamic effects, in which the flow of water over a charged surface causes the accumulation of counterions in the diffusion layer, creating a flow potential. In this case, evaporation plays a key role in maintaining the continuity of the water flow.
However, in seawater, the presence of a large number of ions causes the thickness of the diffusion layer to decrease dramatically to the subangstrom level, which significantly reduces the number of counterions. As a result, the sharp reduction in the density of the counter ions leads to a significant weakening of the electrokinetic effects. As a result, power generation performance in highly concentrated salt solutions such as seawater has been neglected.
Despite the challenges, seawater evaporative power generation remains a promising area. Future research will focus on overcoming the limitations posed by ion concentrations in seawater to improve the efficiency and feasibility of seawater evaporative power generation. The success of this technology will provide an important solution for the field of sustainable energy and is expected to provide sustainable, environmentally friendly energy for global electricity needs**.
**Reading guide
Figure 1Strategies for seawater flow and evaporation-induced power generation
The wider capillary channels in the generator always result in a bulk flow with a large number of ions and therefore do not generate a flow potential (Figure 1a). On the other hand, in nanofluid channels with surface charge, a monopolar solution of counterions is always produced to maintain electrical neutrality. The authors' strategy to achieve seawater flow and evaporation-induced power generation is to construct MOFs single crystals with hydrophilic and charged sub-nanochannels for spontaneous seawater infiltration, resulting in a unipolar solution flow of counterions (Figure 1B). The flow of this unipolar solution in sub-nanochannels is expected to generate considerable flow potential.
Figure 2Physical properties of Cabha Mof
The authors synthesized CABHA (C12H6Ca2O19·2H2O) single crystals by slow evaporation crystallization using BHA and calcium chloride as raw materials (Fig. 2A). Through the analysis of single crystal X-ray diffraction, it is found that MOF Cabe crystallizes in the orthogonal space**212121 with a cell size of a = 68022(5) åb = 16.5632(11) åc = 18.8697(13) å
The authors succeeded in growing Cabaha crystals on a large scale on the surface of strong CaCO3 paper because it is essential for the manufacture of generators. Caco3 paper in 2After soaking in 95 mM BHA solution at room temperature for 48 h, the white solid was evenly deposited on the surface of CaCO3 paper. The X-ray diffraction (XRD) plot of CaBHA on CaCO3 paper is consistent with the simulated single crystal diffraction data (Fig. 2B), indicating that CaBHA can be formed directly on the surface of CaCO3 paper by slow coordination of BHA with Ca ions. Observed by field emission scanning electron microscopy (Fesem), Cabaha crystals form paper-like nanoribbon morphology on CaCO3 and these nanoribbons are disordered (Figure 2C). High-resolution transmission electron microscopy (HRTEM) images show that the Cabaha nanoribbons have a flat surface (Figure 2D).
Selected region electron diffraction (SAED) shows ordered lattices of (110) and (101), demonstrating the single-crystal structure of Cabaha nanoribbons (Figure 2D inset). CaBHA nanoribbons were characterized by solid-state nuclear magnetic resonance (SSSNMR), and two-dimensional 1H1H exchange spectroscopy showed 4The 1H chemical shifts at 8 ppm and 18 ppm correspond to H2O and COOH in CaBHA, respectively (Figure 2E). Due to the symmetry along the diagonal, the two peaks are correlated with each other, indicating that proton exchange occurs between the two structures in a strong magnetic field.
This result also confirms the internal hydrogen bonding network revealed by single-crystal X-ray diffraction. In 1The 1H chemical shift at 1 ppm is caused by the outermost -OH of the CaBHA crystal, indicating that CABHA and Ca-OH synergistically terminate. No associated peaks for this chemical shift were observed, suggesting that CaOH has a fixed structure on the surface. The water contact behavior of Cabe Nanoribbons on CaCO3 paper was observed by using an interfacial viscoelasticity measuring instrument and a high-speed camera. When a 3 L water droplet came into contact with the Cabe nanoribbon, the small water droplet rapidly diffused on the surface of the Cabaha nanoribbon within 40 ms and was completely invisible within 479 ms (Fig. 2F), indicating that the Cabaha MoF has a water hyper-diffusion behavior. The superspreading surface is determined by its intrinsic hydrophilicity and micro-nanostructure, while the water superspreading behavior of Cabe ha crystals is determined by the hydrophilic metal nodes and ligands and nanoband morphology.
Figure 3Research on the performance of seawater induction power generation based on Cabaha nanoribbons
As shown in Figure 3a, a Cabaha nanoribbon-based generator can produce 0An open-circuit voltage of 6 V and a short-circuit current of 1 A. Under the same environmental conditions, the short-circuit current in the simulated seawater increases dramatically to 14 A, which is one of the best values reported in both salt solutions and seawater. The good power generation performance of the Cabha nanoribbons in seawater shows that many counterions can migrate and accumulate at the top of the device as the water flows upward, driven by capillary forces. Through long-term performance evaluation, the authors found that the open-circuit voltage remained 0 after 3 days4 v。Considering that wind speed is an important factor of water evaporation, the effect of wind speed on the performance of Cabaha nanoribbon generators in simulated seawater was studied.
As shown in Figure 3c, the open-circuit voltage of the Cabaha nanoribbon generator in seawater remains almost 0 at different wind speeds4 v。When the wind speed increases to 2At 0 m s 1, the short-circuit current increases dramatically from 14 A to 50 A. The results of this evaporation-related experiment show that the electrical energy generated by the Cabaha nanoribbon generator comes from the internal energy of the air. To study the stability of the output performance, a variable resistor is connected in series as a load electrical component to a Cabaha nanoribbon-based generator to quickly adjust the resistance of the circuit. Figure 3d records the real-time currents of different load resistances in simulated seawater. When the load resistance is increased to 2 K, the output current gradually decreases, and when the load resistance is increased to 10 K, the output current decreases to 69 μa。As the load resistance decreases from 10 to 0 K, the output current of the generator returns to the initial value, indicating that the output performance of the Cabaha nanoribbon generator in simulated seawater is stable.
In addition, the authors further increased the total current of the Cabaha nanoribbon generator by parallel connection. Taking full advantage of the simple and modular method of synthesizing CaBHA nanoribbons on CaCO3 paper, the authors successfully fabricated 72 CABHA-based generators. A total of 72 CABHA nanoribbon generators were connected in parallel using flexible copper electrode attachment, as shown in Figure 3e. As expected, the short-circuit currents of the 72 Cabaha nanoribbon generators connected in parallel add up to a considerable current level of 1 mA in seawater (Figure 3f).
Figure 4Cabaha nanoribbon-based analysis of seawater-induced generators
Figure 4a compares the electric double layer of CABHA in water and seawater. The measured potential of CaCBA nanoribbons on CaCO3 paper in water is -7092 mV, indicating that its outer interface is negatively charged. This also means that H3O+ in the diffusion layer is the charge carrier of the Cabaha nanoribbons in pure water.
By fitting, the value of Cabacha nanoribbons on CaCO3 paper in seawater is -073 mv。Ion-rich seawater has a small Debye length (043 nm), the electrical bilayer formed on the surface of the Cabe nanoribbon is significantly compressed, and the sliding layer is not obvious. As a result, the electrodynamic effects on the outer surface of the CaBA nanoribbons are weak. Attenuated total reflection infrared spectroscopy (ATR-IR) was used to study the interaction between NaCl and CABHA (Fig. 4B). The ATR-IR spectra of the O H group in CaBHA can be obtained by Gaussian fitting to obtain O c o h, ca oh and h o h. For CABHA with water, the wavenumbers of the O C o H, Ca Oh, and H o H tensile vibration peaks increase from and 3168 to and 3139 cm1, respectively.
This redshift results from hydrogen bonding interactions between water molecules and these groups. In the presence of NaCl, the wavenumbers of the O C O H, Ca Oh, and H O H peaks are converted to and 3176 cm1, respectively. The blue shift of OH is due to the destruction of the internal hydrogen bond structure formed by H2O and hydroxyl groups. This is because Na and Cl ions displace some of the H2O molecules, occupying positions around the outer surface of the CaBABHA and around the oxygen groups on the inner wall.
To further determine the dominance of NA ion transport in seawater for power generation, at 04 M NaCl solution, when the generator voltage reaches a steady state (about 04 V), the CABAHA fraction (2 cm 2 cm) exposed to air was quickly cut into two small sections (1 and 2 cm) of equal area (1 cm 1 cm) and the Na ion concentration in these two sections was measured (Figure 4C, right). If H2O+ is the charge carrier generated by the dominant current potential, the Na ion concentration in Part 1 and Part 2 is the same due to the solution homogeneity.
Figure 4c shows that the NA ion peak area for Parts 1 and 2 is 22., respectively604 s min and 9490 μs min。After linear fitting, the Na ion mass of Part 1 and Part 2 was 0., respectively815 and 0342 mg。Significant salinity differences in parts 1 and 2 (238 times) fully illustrates that the CaBHA surface will absorb more Cl ions, while the free Na ions will move towards the top of the generator with the flow of seawater, creating a voltage difference between the top and bottom of the generator. Therefore, the power generation of Cabha nanoribbon generators in seawater is mainly the movement of Na ions rather than the movement of H+, and its electrodynamic effect is completely different from that in water.
On the other hand, ion chromatography results also suggest that some Na ions in CaBA are transported with water flow in sub-nano channels, because the wide spacing between CaBHA nanobands cannot achieve the separation of charged ions at high concentration ions, and thus the potential difference cannot be established.
In order to study the influencing factors of power generation in seawater, a two-dimensional geometric model (2cm 2cm) of a CABACHA component generator exposed to air was established, and the three equations were coupled for finite element simulation. The finite element simulation results are shown in Figure 4d-f, which show the potential, Na ion concentration, and seawater velocity distributions on the generator at steady state, respectively. When moving with the flow, the concentration of Na ions is 36 10 8 mol m3, the potential at the top of the generator can reach the experimental value (0.).4 v)。Due to the effects of concentration diffusion and electromigration flux, the two-dimensional potential distribution does not grow linearly along the vertical direction (Figure 4d). As a result, the concentration of Na ions that can flow upward with seawater at steady state is close to zero across the entire plane, as shown in Figure 4E. For seawater laminar flow, the generator bottom velocity is set to 1 10 7 m s 1 under znormal conditions.
Under these boundary conditions, the velocity distribution along the horizontal direction is symmetrical (Figure 4f). The flow velocity in the middle is greater than that on both sides. Figure 4g shows the effect of bottom movable Na ion concentration and bottom flow rate on the generator top potential. The top potential increases linearly as the bottom concentration of Na ions, which can move upwards with seawater, increases. As the velocity of the bottom water increases, the top potential remains constant. The result that the top potential is not affected by the bottom water flow velocity is due to the high diffusion coefficient of the water (16 × 10−4 m2 s−1)。
This simulation also reasonably explains the experimental phenomenon that the effect of wind on voltage is not obvious, which is due to the rapid diffusion of water in the Cabha nanoribbons. The authors further investigated the short-circuit current density distribution on the generator at steady state, and the results are shown in Figure 4h. Essentially, sodium ions flow in a Cabaha nanoribbon-based generator to form an electric current. Thus, the net current density can be expressed as the product of the Na ion concentration, the seawater flow velocity, and the Faraday constant. Figure 4h clearly shows that the pattern of water flow density distribution is similar to that of sea velocity. When the bottom Na ion concentration is 0024 mol m3, when the bottom seawater velocity is 1 10 7 m s 1, the central current at the top of the generator is close to the experimental value (35 × 10−4 a cm−2)。
In contrast, the boundary concentration of Na ions is much higher under the short-circuit condition than under the open condition. In short-circuit conditions, more Na ions can flow upward with seawater without hindering the electric field. In addition, the effect of the bottom movable NA ion concentration and the bottom flow velocity on the central current at the top of the generator is shown in Figure 4i. According to the product relationship, the current density increases completely linearly with the concentration of Na ions and the flow velocity. This also fully explains the experimental phenomenon that wind energy significantly increases the current due to the increase in the bottom flow velocity due to rapid evaporation. From these finite element simulation results, it can be confirmed that the power generation performance of Cabaha nanostrip generators in seawater is highly dependent on the concentration of Na ions flowing with seawater and the flow velocity at steady state at the bottom of the device.
Figure 5Evaluate the stability and utility of Zn Di-PH-SE batteries
It is important to understand the electrodynamic effect, i.e., the flow of a unipolar solution in the CaBHA nanoribbons. The authors found that Cabaha nanoribbons have different flow potentials in different neutral cationic solutions (Figure 5A). Cabaha nanoribbon-based generator at 00A voltage of 42 V, which is almost the same in simulated seawater. On the other hand, at 00Current potential of 28 V. In 0Low flow potentials (008 v), but at 0No significant flow potential was observed in the 4 M CSCL solution.
The results of this comparative experiment reflect the existence of the flow of the unipolar solution, and the significant flow potential difference is generated due to the different flow of the unipolar solution in the sub-nanometer channel of the cabha. This remarkable cation selectivity is related to the difference in ion hydration structure caused by the arrangement of oxygen in a confined environment, and Na and Li ions are beneficial for water transport in the CaBA sub-nano channel due to the appropriate size, but due to the large size or strong electrostatic interaction, K+, NH4+, Ca2+, Mg2+, and CS+ cannot be transported in the CaBBAHA sub-nano channel. Since the Cabha nanoribbon grows in one dimension (1D) along the C-axis, the benzene ring is associated with coordination water (0.).36 nm) is a remote channel for water and ions (Figure 5B).
The authors further investigated the effect of pH on the flow potential of the aqueous solution of NaCl (Figure 5C). In 0In 4 M NaCl solution, the negative current potential of the Cabaha nanoribbon generator is -0 when the pH is 5 and 655 V, when the pH is 8, its positive current potential is 025 v。In 0No significant flow potential was observed under the conditions of 4 M NaCl and pH = 9. The authors also observed that the cabha nanoribbon was effective in glycerol and 0There is no significant flow potential in glycerol of 4 NaCl (Figure 5D). The reversal of the flow potential is attributed to a sign change in the surface charge. Therefore, this current potential transition from positive to negative with pH should be the result of a change in surface charge in different aqueous media, which is caused by protonation and deprotonation of the Cabha inner group.
The authors used density electron theory (DFT) calculations to show the electronic localization function (ELF) of CABHA (Figure 5E). Cabha cells contain conjugated acid-base pairs for Coo-Cooh, in which hydrogen is shared. The H in the CoOH molecule is naked, while the H in the H2O molecule is not bare and has an isopar value of 086。This reflects that the H atom in CoOh has lower electron locality and is more easily ionized than the H atom in H2O. On the other hand, lone pairs appear around oxygen (blue region), indicating that COO COOH is able to accept external protons. In acidic media, the Cabha sub-nanochannel promotes the migration of Cl ions due to the positive charge of the inner wall caused by the protonation of COO COOH. In neutral media, COO COOH deprotonation predominates, resulting in negative charge of the inner wall and migration of Na ions.
Bibliographic information
zhengyun wang, yuchen huang, tiansui zhang, kunqi xu, xiaoling liu, airong zhang, you xu, xue zhou, jiawei dai, zhineng jiang, guoan zhang, hongfang liu*, and bao yu xia*. unipolar solution flow in calcium-organic frameworks for seawater-evaporation-induced electricity generation.jacs