Sun et al. NPG Asia Materials (2021) 13:50 https://doi.org/10.1038/s41427-021-00317-9 NPG Asia Materials
ARTICLE Open Access Thermoenhanced osmotic power generator via lithium bromide and asymmetric sulfonated poly (ether ether ketone)/poly(ether sulfone) nanofluidic membrane Yue Sun1,2,3,YadongWu1,3,YuhaoHu1,3, Congcong Zhu1,3,HaoGuo4, Xiang-Yu Kong 1,ErcangLuo4,LeiJiang1,3 and Liping Wen 1,3
Abstract Osmotic energy, existing between solutions with different concentrations, is a sustainable and ecofriendly resource for solving energy issues. However, current membrane-based osmotic energy conversion technologies focus on electricity generation from an “open” system by directly mixing salt (NaCl) solutions at room temperature. For the integrated utilization of thermal energy and higher power output performance, we demonstrate thermoenhanced osmotic energy conversion by employing highly soluble lithium bromide (LiBr) solutions, asymmetric sulfonated poly(ether ether ketone)/poly(ether sulfone) (SPEEK/PES) membranes, and LiMn2O4/carbon nanotube (LMO/CNT) electrodes. The thin top layer of this heat-resistant membrane contains hydrophilic groups (i.e., the sulfonated groups in SPEEK) that are beneficial for ion-selective transport. The thermal effect on each solution is investigated, and osmotic energy conversion can be improved by regulating the heat gradient. The power density is ~16.50 W/m2 by coupling with a
1234567890():,; 1234567890():,; 1234567890():,; 1234567890():,; temperature gradient (30 °C). This work is a step forward for promoting the performance of osmotic energy conversion with thermal energy assistance and provides the basis for a closed-loop system with regenerated osmotic energy from other energy forms. Moreover, the external field-osmotic hybrid energy conversion system shows powerful potential in the energy harvesting field.
Introduction developed to maximize the conversion of salinity gradient Salinity gradient energy is identified as a promising and energy into electricity. One of the most promising meth- abundant source of sustainable energy, which is obtained ods among these techniques is membrane-based reverse from the ionic gradient between sea water and fresh electrodialysis (RED), which utilizes an ion-selective water1,2. Since Pattle’s pioneering research on salinity membrane to directly generate electricity through ion – gradient energy in 19543, several techniques have been migration without mechanical components4 6. The ions selectively transit through the membrane to induce a dif- fusive voltage between the membrane7, which is largely 8 Correspondence: Xiang-Yu Kong ([email protected])or affected by the temperature distribution of the system .In Liping Wen ([email protected]) recent decades, synthetic polymeric membranes have 1 CAS Key Laboratory of Bio-inspired Materials and Interfacial Science, Technical become the core component in a wide variety of domains, Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, PR 9 10 China including liquid and gas separation ,ultrafiltration , and 2Key Laboratory of Green and Precise Synthetic Chemistry and Applications, energy generation/storage11,12. Recently, nanofluidic Ministry of Education, College of Chemistry and Materials Science, Huaibei membranes have been used in RED systems to achieve Normal University, Huaibei, Anhui, PR China 13,14 Full list of author information is available at the end of the article high power density . In addition, solutions with These authors contributed equally: Yue Sun, Yadong Wu
© The Author(s) 2021 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a linktotheCreativeCommons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/. Sun et al. NPG Asia Materials (2021) 13:50 Page 2 of 10 50
Fig. 1 Thermoenhanced osmotic power generator via an asymmetric SPEEK/PES blend membrane and LiBr solution. a Schematic of the energy conversion device (i), which combines an asymmetric SPEEK/PES blend membrane and a pair of LMO/CNT electrodes (ii). Cross-sectional SEM image of the SPEEK/PES blend membrane with a condensed ~1.1 μm surface layer (iii). b The device shows a very high power density of ~16.50 W/m2 under a temperature gradient of 30 °C, which is much higher than that under a temperature gradient of −30 °C. different salinities are also natural reservoirs, which can be different, which means that the thermal-field effect exists used as an intermediate for generating electricity from across the membrane. However, the current research other energy conversion methods. By employing a nano- using RED technology to generate electricity from artifi- fluidic RED membrane, the energy resources can be con- cial NaCl–water solutions or natural rivers and sea water nected together with proper system designs. is typically near room temperature, which largely reduces Osmotic energy conversion using RED technology is its application field. largely affected by several factors, including the (1) salt Herein, the feasibility of electricity generation via a LiBr solubility in water, (2) equivalent conductivity of the solution and heat gradient is validated experimentally in aqueous solution, (3) activity coefficient ratio, and (4) an RED system. An osmotic power generator for elec- – external field factors (e.g., thermal energy)15 17. In this tricity generation is built, which converts the osmotic respect, LiBr fits the requests as an RED working solution energy existing in LiBr solutions with different con- due to its excellent solubility (i.e., 13.13 M at 25 °C), which centrations and temperatures (Fig. 1a, i). This RED system is much higher than that of NaCl (i.e., 5.50 M at 25 °C). is composed of an asymmetric sulfonated poly(ether ether Thus, the use of LiBr leads to a higher concentration ketone)/poly(ether sulfone) (SPEEK/PES) blend mem- gradient in the RED system, which results in a higher brane and a pair of LiMn2O4/carbon nanotube (LMO/ output power density. Moreover, some factors, such as the CNT) electrodes. The high thermal stability and low cost thermal field, can significantly affect the osmotic con- of the membrane are important for practical applications. version performance. A temperature gradient is present The SPEEK/PES blend membrane shows a typical asym- between the high- and low-concentration solutions dur- metric finger-like structure with an average thickness of ing the membrane distillation (MD) process, which uti- ~37 μm (Fig. 1a, ii). The ion selective layer of the mem- lizes low-grade heat (e.g., solar energy, geothermal energy, brane is the thin top layer whose thickness is ~1.1 μm or waste heat) in the regeneration process of closed-loop (Fig. 1a, iii). The thin top layer contains hydrophilic RED. This is because MD utilizes a thermal field to pro- groups (i.e., the sulfonated groups in SPEEK) that are mote the transport of vapor in the hydrophobic mem- beneficial for ion selective transport in the membrane. brane. Thus, it is a sustainable method to regenerate high- Additionally, as a proof of concept, we combine the RED and low-concentration LiBr solutions, which is similar to with a heat gradient, and the system can reach a very high the regeneration of rivers from the evaporation of sea power density of ~16.50 W/m2 with a 30 °C temperature water. The temperatures of the two compartments are gradient at a 50-fold concentration gradient (Fig. 1b). Sun et al. NPG Asia Materials (2021) 13:50 Page 3 of 10 50
There is a heat gradient between the solutions in many membrane was immersed in IPA for 36 h and then lifted practical applications, while reported RED systems rarely and evaporated at room temperature for 24 h. consider this issue. The current work offers a promising method for enhancing electricity conversion from the Fabrication of LMO/CNT electrodes salinity gradient energy of LiBr solution with an applied Typically, 50 mg of CNTs was dispersed in 25 mL of heat gradient, indicating the prospect of the use of LiBr NMP with ultrasonic processing for 1 h. Then, 300 mg of solution in RED. LiMn2O4 was added into the mixed solution with further ultrasonic treatment for 0.5 h. The LMO/CNT composite Experimental section material was obtained by vacuum filtration. In a typical Materials and chemicals electrode preparation, the LMO/CNT composite material Poly(ether ether ketone) (PEEK, Sigma-Aldrich, and PVDF were mixed in a weight ratio of 9:1 in NMP to Shanghai), polyether sulfone (PES, BASF, Shanghai) and form a slurry, and then the slurry was uniformly pasted on N,N-dimethylacetamide (DMAc, Sigma-Aldrich, Shang- Al foil. Finally, the electrodes were vacuum dried at 45 °C hai) were utilized to fabricate membranes. Isopropanol to remove the solvent. (IPA, 99.7%) acquired from J&K Beijing was used for the posttreatment of the membranes. Lithium bromide (J&K Characterization Beijing) was used to determine the RED performance of Electrical characterization the membranes. LiMn2O4 (LMO, Sigma-Aldrich, Electrical measurements were obtained with a Keithley Shanghai), carbon nanotube (CNT, Aladdin, Shanghai), 6487 semiconductor picoammeter (Keithley Instruments, N-methyl pyrrolidone (NMP, Aladdin, Shanghai) and Cleveland, OH). The asymmetric SPEEK/PES blend poly(vinylidene fluoride) (PVDF, MTI, Shenzhen) were membranes were clamped between two compartments. used to prepare the electrode composite material. For the ionic transport measurement, identical KCl Degassed Milli-Q water was produced by a Milli-Q solutions from 0.1 μMto1Mwereusedtofill the two ultrapure water system (Millipore, USA). All chemicals chambers containing a pair of Ag/AgCl electrodes as were used as received. voltage application terminals. The CV measurements were carried out with an electrochemical workstation Synthesis of SPEEK (CHI-660E, Chenhua, Shanghai) utilizing a three- SPEEK was synthesized by sulfonating PEEK with electrode system. The membrane was placed between concentrated sulfuric acid. Briefly, 5 g of PEEK and two compartments of the two-cell system. Briefly, 20 mM 3- 3+ 75 mL of concentrated sulfuric acid (98 wt%) were anionic [Fe(CN)6] and cationic [Ru(NH3)6] probes added into a 250 mL three-necked flask and stirred for (the supporting electrolyte was 0.1 M KCl (pH≈4.3)) were 3 h until the PEEK dissolved completely. Then, the selected and added into one compartment, while the homogeneous solution was heated with stirring at 60 °C other compartment was added into Milli-Q water. After for another 2.5 h. Finally, the solution was slowly 1 h, exudation was used in the CV measurements. Plati- dropped into a large amount of ice-cold Milli-Q water num wires served as the working electrode and counter to precipitate the SPEEK. SPEEK was filtered and electrode, while a Ag/AgCl electrode was used as the washed with Milli-Q water several times until the pH of reference electrode. The scan rate was set at 10 mV/s, and the percolate was ~7. the measurements were conducted at ~22 °C. For the energy conversion tests, a pair of LMO/CNT electrodes Fabrication of asymmetric SPEEK/PES blend membranes was utilized to apply a transmembrane potential. The Asymmetric SPEEK/PES blend membranes were fabri- feed and permeate chambers were filled with a high- cated via nonsolvent-induced phase inversion assisted by concentration LiBr solution and a low-concentration a nonsolvent additive. First, a certain mass ratio of SPEEK LiBr solution, respectively. The testing area of the and PES was mixed together and stirred in N,N-dime- membrane was 0.20 mm2. thylacetamide (DMAc) for 1.5 h at room temperature to form a homogeneous polymer solution. Then, 0.2 mL of Permeation experiments Milli-Q water was added and stirred for 0.5 h at 70 °C. Permeation experiments were performed using a two- After removing the air bubbles, the polymer solution was cell system separated by an asymmetric SPEEK/PES blend cast onto a clean glass plate at room temperature and membrane. The permeability rate was tested separately to below 50% relative humidity. Afterward, the glass plate avoid interference because the fluorescence emission of was immersed in Milli-Q water immediately to create a rhodamine 6 G is much stronger than that of sulforho- membrane with an asymmetric structure. These asym- damine. Either 0.1 mM rhodamine 6 G or 0.1 mM sul- metric membranes were peeled off from the glass plate forhodamine solution was added into the feed cell and soaked in Milli-Q water. Finally, the asymmetric facing the bottom side of the membrane. The exudations Sun et al. NPG Asia Materials (2021) 13:50 Page 4 of 10 50
in the receptor chamber were tested using a spectro- evaporation processes, forming a more hydrophilic chan- fluorophotometer every 5 min. nel surface with more sulfonated groups in the asymmetric SPEEK/PES blend membrane. Membrane structure characterization The microstructure of the membrane top surface dis- 1H NMR (nuclear magnetic resonance)measurements plays a dense structure (Supplementary Fig. S3a), while were obtained with a Bruker AVNANCE 400 spectro- the bottom surface morphology of the membrane shows meter. Field-emission SEM (scanning electron micro- large pores (Supplementary Fig. S3b, c, d). The pores in scope, Hitachi S-4800) at an accelerating voltage of 10 kV the dense layer of the SPEEK/PES membrane have a was used to observe the membranes. Cross-sectional diameter of ~11 nm. Membranes with different thick- membrane samples were cryogenically fractured through nesses were obtained by adjusting the height of the casting the use of liquid nitrogen. The membrane samples knife (Supplementary Fig. S4). The structure of the were sputter-coated with gold before the SEM test. membranes prepared by the phase inversion method is Contact angle measurements were obtained through the mainly determined by the solvent/nonsolvent exchange use of photographs, an OCA25 contact angle measuring rate, SPEEK/PES polymer interaction, and viscosity of the instrument (Dataphysics, Germany) and ImageJ software. casting solution22,23. With increasing casting solution Each membrane was measured five times, and the average concentration, the macrovoid size decreases and the was calculated. XPS (X-ray photoelectron spectroscopy) sponge-like region increases (Supplementary Figs. S5–S8). spectra were obtained on a PHI Quantera scanning X-ray These changes occur because the higher viscosity of the microprobe with a monochromated Al Kα radiation casting solution slows down the coagulation process and source at 1486.7 eV. Surface zeta potentials were deter- reduces the solvent/nonsolvent exchange rate24. The mined on a zeta potential analyzer (SurPASS 3, Anton major mass loss occurred in the temperature range of Paar, Austria). The permeability tests of fluorophores 500 − 650 °C, corresponding to polymer combustion were monitored by a spectrofluorophotometer (RF- (Supplementary Fig. S9). A high-temperature LiBr solu- 5301PC, SHIMADZU, Japan). tion can be utilized due to the heat resistance of the membrane. In addition, the cost of the SPEEK/PES Results and discussion membrane, approximately 8.7 USD($)/m2 (Table S1), is Membrane characterization and ionic transport properties much lower than the international cost target (11 USD Asymmetric SPEEK/PES blend membranes were fabri- ($)/m2) for ion-selective membrane-based RED technol- cated by utilizing nonsolvent-induced phase inversion with ogy. Collectively, this asymmetric structure is related to the assistance of trace water as the nonsolvent additive the solvent/nonsolvent exchange and polymer solidifica- (Supplementary Fig. S1). SPEEK was prepared by sulfo- tion processes during the phase inversion process. nating PEEK with concentrated sulfuric acid, and the The ionic transport properties of the asymmetric degree of sulfonation (DS) of SPEEK was 94.6% according SPEEK/PES blend membranes with and without IPA to the 1H NMR technique (Supplementary Fig. S2). To treatment were examined by obtaining current−voltage obtain an asymmetric finger-like membrane, trace Milli-Q (I − V) measurements. After IPA-induced membrane water was added to the SPEEK/PES casting solution as a reorganization, the ionic current increases from approxi- nonsolvent additive. Usually, some additives increase the mately 0.10 μA to 0.35 μA (Fig. 2a). The zeta potential viscosity of a casting solution, inhibiting the mutual dif- measurements confirm that the surface charge density of fusion process of the solvent and nonsolvent owing to the the membrane increases after reorganization, which 18 − reduced fluidity of a mixed solution . The nonsolvent means that the groups containing oxygen (–O–, –SO3 ,> water induced gelation of the casting solution by forming a C = O), especially the polar sulfonated groups, migrate to three-dimensional network due to hydrogen bonding the channel surface through the IPA treatment (Fig. 2a, interactions between the trace H2O and sulfonated groups inset). This migration can also be confirmed by the X-ray in SPEEK, which slowed down the movement of the photoelectron spectroscopy (XPS) results, which show polymer chains and the coalescence of the polymer-poor that the atomic content of sulfur and oxygen on the top phase19,20. The polymer solution was cast onto a glass layer of the membrane increases after IPA treatment plate, and the solution thickness was controlled by the (Tables S2,3). The transmembrane conductance deviates casting knife21. Then, the glass plate was immediately from bulk behavior at 0.01 M and gradually approaches transferred into a Milli-Q water bath at ~20 °C to form a plateaus at lower concentrations, which means that ionic membrane. Afterward, the membrane reorganization transport through the asymmetric SPEEK/PES blend process was conducted. The membrane was first membrane is governed by the surface charge (Fig. 2b). immersed in IPA for 36 h and then dried at room tem- Additionally, the reorganized membrane with more perature. The reorganization of polymer chains (hydro- charged groups in the channels demonstrate higher philic) occurs during both the IPA immersion and transmembrane conductance. Sun et al. NPG Asia Materials (2021) 13:50 Page 5 of 10 50
Fig. 2 Ionic transport properties of the asymmetric SPEEK/PES blend membrane. a I − V curves of the untreated and reorganized asymmetric SPEEK/PES blend membranes recorded in 10-5 M KCl solution. Inset is the surface zeta potential of these two SPEEK/PES blend membranes at different pH values. b Conductance measurement in KCl electrolyte. The ionic conductance of the asymmetric SPEEK/PES blend membrane changes as the electrolyte concentration decreases, and it clearly shows that at low concentrations, the ionic conductance of both membranes is governed by the surface charge. c Diffusion potential and diffusion current recorded with the series of concentration gradients of LiBr solution. Both the CV curves (d) and time-concentration curves in the permeation experiments (e) indicate the good cation selectivity of the reorganized asymmetric SPEEK/PES 3+ 3− blend membrane ([Ru(NH3)6] as a cationic electroactive probe (red trace); [Fe(CN)6] as an anionic electroactive probe (blue trace); rhodamine 6 G (Rh (+), red circle trace); sulforhodamine (Rh (−), blue square trace)). f Relationship between the ion selectivity of the membrane and concentration gradient. The low-concentration LiBr solution facing the top side of the membrane was fixed at 0.01 M.
To investigate the ion transport behavior of the mem- approximately 228 μAand55μA, respectively. Due to brane under a concentration gradient, Idiff and Vdiff were the negatively charged character of the membrane, the measured. Vdiff originates from the ion selectivity of the different diffusion abilities of the specific probe result in membrane, which can result in the difference in the dif- a distinct voltammetric response. To further confirm the fusive fluxes of anions and cations. The diffusion potential ion selectivity of the asymmetric SPEEK/PES blend can be calculated by Vdiff = Vapp−Eredox (Supplementary membrane, rhodamine (Rh) 6 G and sulforhodamine Note 1). Along with the increase in the concentration with opposite charges (Fig. 2e) were chosen as fluor- gradient, Idiff and Vdiff both increase (Fig. 2c). Vdiff origi- ophores due to their high quantum yields for trans- nates from the ion selectivity of the membrane, the main membrane transport measurements. Permeation + diffusive flux of cations (i.e., Li ). Due to the sulfonated experiments were conducted with a two-cell system, groups in SPEEK, the membrane exhibits excellent cation which was separated by an asymmetric membrane. The selectivity. The cation selectivity was first investigated by feed chamber was filled with fluorophore solutions 3+ −4 electroactive redox probes (i.e., [Ru(NH3)6] and [Fe (1.0 × 10 M), and the permeability of both fluor- 3− (CN)6] ) in cyclic voltammetry (CV) (Fig. 2d). The dif- ophores was monitored by taking the fluorescence 3+ 3− fusion coefficients of [Ru(NH3)6] and [Fe(CN)6] are spectra of the solution in the receptor chamber every − − 7.4 × 10 6 cm2/s and 7.6 × 10 6 cm2/s, respectively25. The 5 min. The permeability rate of Rh (+)ismuchlarger 2+/3+ reduction/oxidation peaks of [Ru(NH3)6] are than that of Rh (−), which corresponds to the CV test observed at −0.27 and −0.18 V, respectively. Additionally, results. The ion selectivity was quantified by calculating 3−/4− the reduction/oxidation peaks of [Fe(CN)6] are the transference number of cations. With the increase in observed at 0.094 and 0.17 V, respectively. These curves the concentration gradient, the transference number show strong electrochemical peaks for the transport of the reaches a peak value at a 50-fold salinity gradient of LiBr 3+ cationic probe [Ru(NH3)6] (red trace) and weak anionic solution (Fig. 2f). The decrease in transference number 3− peaks for the [Fe(CN)6] (blue trace) probe, and the peak under a higher salinity gradient could be due to the 2+/3+ 3−/4− 26,27 currents of [Ru(NH3)6] and [Fe(CN)6] are increase in concentration polarization . Sun et al. NPG Asia Materials (2021) 13:50 Page 6 of 10 50
Fig. 3 High-performance osmotic energy conversion of asymmetric SPEEK/PES blend membranes. a I−V curves of the membrane with two 2 opposite concentration gradient configurations. The JSC and VOC change from 49 A/m and 190 mV (concentrated KCl solution on the bottom side of the membrane, red circle) to 37 A/m2 and 196 mV (concentrated KCl solution on the top side of the membrane, blue square), respectively; thus, the
corresponding inner resistance (Rchannel) increases by ~36.6%. b JSC and VOC as functions of the concentration gradient. The low-concentration (LiBr) solution was placed on the top side of the membrane and fixed at 0.01 M. The output power density as a function of the (c) thickness of the casting solution, (d) polymer concentration in the casting solution, and (e) SPEEK/(SPEEK+PES) weight ratio. The high-salinity (LiBr) solution was placed on the bottom side of the membrane and fixed at 0.5 M, while the low-concentration (LiBr) solution was placed on the top side and fixed at 0.01 M. f Output power densities of the membrane under different concentration gradients from 5 to 1000. The top side of the membrane faced the low- concentration (LiBr) solution (0.01 M).
Asymmetric SPEEK/PES blend membranes for osmotic In the reversed concentration gradient configuration, the 2 power generation at room temperature absolute value JSC decreases to ~37 A/m , and the VOC The osmotic energy conversion performance of the increases to ~196 mV. The calculated inner resistance asymmetric SPEEK/PES blend membrane was examined (Rchannel) increases by ~36.6%, impeding the energy con- by applying a series of LiBr concentration gradient con- version. Ion transport is dominated by the thin top layer ditions. Additionally, the LiBr-water solution presents of the asymmetric membrane, which contains a large application prospects due to its higher theoretical power number of nanoscale channels. The thickness of the density and open-circuit voltage under the same salinity electrical double layer (EDL) in the nanoscale channels is gradients15. Two opposite salinity gradient test directions determined by the ionic concentration, which largely were adopted to optimize the concentration gradient affects the energy conversion7. Therefore, the former (Fig. 3a). When the diluted KCl solution (i.e., 10 μM) faces configuration was chosen in our following tests. For the the top thin layer, the absolute values of the short-circuit energy conversion experiments, a pair of LMO/CNT current density (JSC) and the open-circuit voltage (VOC) electrodes was used to offer a transmembrane potential. are approximately 49 A/m2 and 190 mV, respectively. This is because the commonly used Ag/AgCl electrode Sun et al. NPG Asia Materials (2021) 13:50 Page 7 of 10 50
reacts with Br- to form AgBr that decomposes easily solution (i.e., 8%), the membrane showing large pores in when irradiated with light. Normally, LMO is employed the top layer (Supplementary Fig. S5) induces relatively + as a cathode in aqueous Li-ion batteries because Li -ion poor ion selectivity along with low output current den- (de)intercalation occurs near the upper limit of the sta- sity (Supplementary Fig. S15a) and power density. bility window of the aqueous electrolyte28. The electrode Additionally, the continual increase in casting solution reactions in the high-concentration solution are concentration generally increases the compactness of 3+ 4+ 3+ 4+ + − + LiMn Mn O4 = Li1-xMn 1-xMn 1+xO4 + xLi +xe membranes, which reduces the Li -ion flux through the (0 < x < 1). The electrode reactions in the low- membrane. In addition, with increasing power density 3+ 4+ + 2 2 concentration solution are LiMn Mn O4 + xLi + from ~1.52 W/m to ~9.26 W/m , the SPEEK/(SPEEK + − 3+ 4+ 29 xe = Li1+xMn 1+xMn 1-xO4 (0
Fig. 4 Osmotic energy conversion performance under heat gradients. a Schematic illustration of the energy harvesting process by using the
asymmetric SPEEK/PES blend membrane with a series of temperature gradients. The ΔT was calculated by ΔT = Thc − Tlc. The temperature on the low-concentration (Tlc) side remained at 25 °C; the temperatures on the high-concentration (Thc) side gradually changed from -5 °C to 10, 25, 40, and 55 °C. Thus, the resulting ΔT values were -30, -15, 0, 15, and 30 °C. b Changes in the measured JSC and VOC with increasing ΔT. c Output power densities of the asymmetric SPEEK/PES blend membrane with a 50-fold concentration difference with a series of ΔT from −30 to 30 °C. d Current density and load resistance at the peak power density change along with ΔT.
Thermal-field effect on energy conversion using LiBr solution generates a higher output power The thermal-field effect on energy conversion under a density than that of NaCl solution. The counterion flux certain concentration gradient was evaluated. Figure 4ais density is almost zero under open-circuit conditions, and a schematic illustration of the energy conversion process the concentration gradient inside the membrane is also under a series of temperature gradients. The high and low almost negligible. Therefore, the physical explanation of concentrations of LiBr solutions were set at 0.5 M and the generation of a potential drop in the membrane caused 0.01 M, respectively; the temperature of the low salinity by the thermal-field effect can be explained by the equa- 32 chamber (Tlc) was set at 25 °C, and the temperature of the tion below : high salinity chamber (Thc) was gradually increased from m m −5 °C to 10 °C, 25 °C, 40 °C, and 55 °C. The ΔT of the two Qi d ln T þ ziFdϕ ¼ 0 chambers is defined as ΔT = Thc − Tlc. The corresponding m JSC and VOC values under different temperature gradients where Qi is the heat of transport of ionic species i (i = 1, are shown in Fig. 4b. Both the JSC and VOC values increase 2), T is the thermodynamic temperature, zi is the charge gradually as ΔT increases and reach maximum values of number of ionic species i, F is the Faraday constant, and m approximately 107 A/m2 and 203 mV, respectively. Under ϕ is the electrostatic potential inside the membrane. m the 50-fold salinity gradient, the corresponding power If Qi > 0, the ions tend to move toward chambers of densities increase from approximately 2.29 W/m2 to lower temperature. Thus, an electric field is generated 16.50 W/m2 when ΔT increases from −30 to 30 °C (Fig. 4c inside the membrane, and current occurs under closed- and Supplementary Figs. S18, 19). When both cells are set circuit conditions. Moreover, the corresponding conver- at 55 °C, the output power density reaches ~18.8 W/m2 sion efficiencies increase from approximately 15.2% to (Supplementary Fig. S20). According to the previous the- 46.8% (Table S6), indicating that the elevated temperature oretical calculation, the LiBr-water solution offers a higher in the high salinity cell can largely promote energy con- power density than the NaCl-water solution15, and the version. Additionally, the current density and corre- comparison among existing technologies and this work sponding peak power density increase as ΔT increases are shown in Table S5. The osmotic energy conversion (Fig. 4d and Supplementary Fig. S21). The increase in Sun et al. NPG Asia Materials (2021) 13:50 Page 9 of 10 50
current density along with the temperature elevation applying a LiBr solution-based RED system for high- can be described by the equation J = 75.1+1.88ΔT. performance osmotic energy conversion and further Interestingly, the resistance changes with temperature, demonstrates the great potential of RED-based hybrid which can be described as RL = 3.80−0.00267ΔT systems for harvesting other energy sources. In this +0.000361ΔT2−0.0000108ΔT3, and shows similar beha- regard, with a properly designed RED system, different vior with the reported resistance changing of a lithium- energy sources, such as low-grade heat, light, sound, wind ion battery33. The resistance decreases rapidly when T energy, and pressure, could be effectively extracted and increases from −5 to 10 °C, while it decreases slowly when converted into electricity, providing new viewpoints for T increases from 10 to 55 °C. The slow decrease in combined energy harvesting. resistance might be because the low salinity solution Acknowledgements resistance is the main part of the total resistance. A higher This work was supported by the National Natural Science Foundation of China peak power density value is obtained when the tempera- (21625303, 21905287, 21988102), the National Key R&D Program of China ture in the high salinity chamber increases, which may be (2017YFA0206904, 2017YFA0206900, 2020YFA0710401), and the Key Research due to the increasing transmembrane migration of Program of the Chinese Academy of Sciences (QYZDY-SSW-SLH014). + fl 34–36 Li -ion ux driven by the temperature gradient . The Author details traditional processes for converting low-grade energy to 1CAS Key Laboratory of Bio-inspired Materials and Interfacial Science, Technical power are normally based on the production of Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, PR China. 2Key Laboratory of Green and Precise Synthetic Chemistry and mechanical energy. The energy is converted to electricity Applications, Ministry of Education, College of Chemistry and Materials Science, through a turbine, such as steam Rankine cycles and Huaibei Normal University, Huaibei, Anhui, PR China. 3University of Chinese 4 organic Rankine cycles. Several other technologies, Academy of Sciences, Beijing, PR China. Key Laboratory of Cryogenics, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, including closed-loop RED technology, can also realize Beijing, PR China thermal-electric conversion, and we list all these tech- nologies in Table S7 for comparison. Traditional techni- Competing interests ques have demonstrated that the conversion of heat at low The authors declare no competing interests. temperatures (<100 °C) to electricity have low efficiencies, ’ high costs, and short lifetimes. The closed-loop reverse Publisher s note Springer Nature remains neutral with regard to jurisdictional claims in electrodialysis (RED) system generates electricity from published maps and institutional affiliations. low-grade heat at low temperature (<100 °C)37. In this regard, closed-loop RED is proposed as a suitable alter- Supplementary information The online version contains supplementary material available at https://doi.org/10.1038/s41427-021-00317-9. native to convert low-grade heat (<100 °C) to electricity. Received: 4 February 2021 Accepted: 1 June 2021 Conclusions In this study, we demonstrate the feasibility of designing a high-performance osmotic power generator using LiBr solutions. The osmotic power generator is composed of References an asymmetric SPEEK/PES blend membrane and a pair of 1. Logan, B. E. & Elimelech, M. Membrane-based processes for sustainable power generation using water. Nature 488, 313–319 (2012). LMO/CNT electrodes. An asymmetric membrane with a 2. Yip, N. Y. & Elimelech, M. Thermodynamic and energy efficiency analysis of finger-like structure is prepared via nonsolvent-induced power generation from natural salinity gradients by pressure retarded phase inversion assisted by trace nonsolvent additives and osmosis. Environ. Sci. Technol. 46, 5230–5239 (2012). 3. Pattle,R.E.Productionofelectricpower by mixing fresh and salt water in the followed by IPA reorganization treatment. More nega- hydroelectric pile. Nature 174, 660 (1954). tively charged groups are brought to the channel surface 4. Mei, Y. & Tang, C. Y. Y. Recent developments and future perspectives of during the reorganization process, inducing excellent reverse electrodialysis technology: a review. Desalination 425,156–174 (2018). cation selectivity. The robust membrane can be used to 5. Xin, W. et al. Biomimetic nacre-like silk-crosslinked membranes for osmotic harvest the Gibbs free energy of LiBr solutions with dif- energy harvesting. ACS Nano 14, 9701–9710 (2020). ferent concentrations and temperatures. The generator 6. Chen, J. et al. Ultrathin and robust silk fibroin membrane for high-performance osmotic energy conversion. ACS Energy Lett. 5,742–748 (2020). presents high-performance osmotic energy conversion 7. Huang, X. et al. Engineered PES/SPES nanochannel membrane for salinity 2 using LiBr solutions with a power density of ~9.26 W/m gradient power generation. Nano Energy 59,354–362 (2019). under a 50-fold concentration gradient, and it can even 8. Rui, L., Zhengfei, K., Zhichun, L. & Wei, L. Temperature regulated reverse 2 electrodialysis in charged nanopores. J. Membr. Sci. 561,1–9(2018). achieve ~33.00 W/m under a 1000-fold concentration 9. Hou, L. et al. Separation of organic liquid mixture by flexible nanofi- gradient. In addition, the energy conversion of the system brous membranes with precisely tunable wettability. NPG Asia Mater. 8, can be further promoted by thermal field regulation and e334 (2016). 2 10. An, X., Hu, Y., Wang, N., Wang, T. & Liu, Z. Breaking the permeability–selectivity reach a power density up to ~16.50 W/m under a tem- trade-off in thin-film composite polyamide membranes with a PEG-b-PSF-b- perature gradient (ΔT = 30 °C) at a 50-fold concentration PEG block copolymer ultrafiltration membrane support through post- gradient. The current work shows the great potential of annealing treatment. NPG Asia Mater. 11, 13 (2019). Sun et al. NPG Asia Materials (2021) 13:50 Page 10 of 10 50
11. Kim,D.W.,Lee,J.H.,Kim,J.K.&Jeong,U.Materialaspectsoftriboelectric 24. Wang, W. Y. et al. Preparation and characterization of SLS-CNT/PES ultra- energy generation and sensors. NPG Asia Mater. 12, 6 (2020). filtration membrane with antifouling and antibacterial properties. J. Membr. Sci. 12. Park,S.,Lee,H.,Kim,Y.-J.&Lee,P.S.Fully laser-patterned stretchable micro- 548,459–469 (2018). supercapacitors integrated with soft electronic circuit components. NPG Asia 25. Kim,J.&Bard,A.J.Electrodepositionofsinglenanometer-sizePtnanoparticles Mater. 10, 959–969 (2018). at a tunneling ultramicroelectrode and determination of fast heterogeneous 3+ 13. Zhang, Z. et al. Ultrathin and ion-selective janus membranes for high- kinetics for Ru(NH3)6 reduction. J. Am. Chem. Soc. 138,975–979 (2016). performance osmotic energy conversion. J. Am. Chem. Soc. 139, 8905–8914 26. Kim,S.J.,Ko,S.H.,Kang,K.H.&Han,J.Directseawaterdesalinationbyion (2017). concentration polarization. Nat. Nanotechnol. 5,297–301 (2010). 14. Zhang, Z. et al. Improved osmotic energy conversion in heterogeneous 27. Dlugolecki, P., Gambier, A., Nijmeijer, K. & Wessling, M. Practical potential of membrane boosted by three-dimensional hydrogel interface. Nat. Commun. reverse electrodialysis as process for sustainable energy generation. Environ. 11, 875 (2020). Sci. Technol. 43,6888–6894 (2009). 15. Tamburini, A. et al. Reverse electrodialysis heat engine for sustainable power 28. Okubo, M. et al. Fast Li-ion insertion into nanosized LiMn2O4 without domain production. Appl. Energy 206, 1334–1353 (2017). boundaries. ACS Nano 4, 741–752 (2010). 16. Veerman, J., Saakes, M., Metz, S. J. & Harmsen, G. J. Reverse electrodialysis: 29. Zampardi,G.,Batchelor-McAuley,C.,Katelhon,E.&Compton,R.G.Lithium-ion- performance of a stack with 50 cells on the mixing of sea and river water. J. transfer kinetics of single LiMn2O4 particles. Angew. Chem. Int. Ed. 56, 641–644 Membr. Sci. 327, 136–144 (2009). (2017). 17. Long, R., Kuang, Z., Liu, Z. & Liu, W. Temperature regulated reverse electro- 30. Lu, W. et al. Solvent-induced rearrangement of ion-transport channels: a way dialysis in charged nanopores. J. Membr. Sci. 561,1–9 (2018). to create advanced porous membranes for vanadium flow batteries. Adv. 18. Wang, Z.-G., Xu, Z.-K. & Wan, L.-S. Modulation the morphologies and perfor- Funct. Mater. 27, 1604587 (2017). mance of polyacrylonitrile-based asymmetric membranes containing reactive 31. Wang, T., Wang, Y.-Q., Su, Y.-L. & Jiang, Z.-Y. Antifouling ultrafiltration mem- groups: effect of non-solvents in the dope solution. J. Membr. Sci. 278, brane composed of polyethersulfone and sulfobetaine copolymer. J. Membr. 447–456 (2006). Sci. 280,343–350 (2006). 19. Zhang, L. et al. Preparation of PES/SPSf blend ultrafiltration membranes with 32. Jokinen,M.,Manzanares,J.A.,Kontturi,K.&Murtomäki,L.Thermalpotentialof high performance via H2O-induced gelation phase separation. J. Membr. Sci. ion-exchange membranes and its applicationtothermoelectricpowergen- 540,136–145 (2017). eration. J. Membr. Sci. 499,234–244 (2016). 20. Zhao, Y. et al. Robust sulfonated poly (ether ether ketone) nanochannels for 33. Ahmed,S.H.,Kang,X.S.&Shrestha,S.O.B.Effectsoftemperatureoninternal high-performance osmotic energy conversion. Natl.Sci.Rev.7, 1349–1359 resistances of lithium-ion batteries. J. Energy Resour. Technol. 137, 031901 (2020). (2015). 21. Sun, Y. et al. Tailoring a poly(ether sulfone) bipolar membrane: osmotic-energy 34. Benneker,A.M.,Klomp,J.,Lammertink,R.G.H.&Wood,J.A.Influence of generator with high power density. Angew.Chem.Int.Ed.59, 17423–17428 temperature gradients on mono- and divalent ion transport in electrodialysis (2020). at limiting currents. Desalination 443,62–69 (2018). 22. Rahimpour, A. & Madaeni, S. S. Polyethersulfone (PES)/cellulose acetate 35. Suo, L. et al. Advanced high-voltage aqueous lithium-ion battery enabled by phthalate (CAP) blend ultrafiltration membranes: preparation, morphology, “water-in-bisalt” electrolyte. Angew. Chem. Int. Ed. 55,7136–7141 (2016). performance and antifouling properties. J. Membr. Sci. 305, 299–312 (2007). 36. Zhang, C. et al. Anion-sorbent composite separators for high-rate lithium-ion 23. Liu, Y., Koops, G. H. & Strathmann, H. Characterization of morphology con- batteries. Adv. Mater. 31, 1808338 (2019). trolled polyethersulfone hollow fiber membranes by the addition of poly- 37. Ortega-Delgado, B. et al. Boosting the performance of a Reverse ethylene glycol to the dope and bore liquid solution. J. Membr. Sci. 223, Electrodialysis–Multi-Effect Distillation Heat Engine by novel solutions and 187–199 (2003). operating conditions. Appl. Energy 253, 113489 (2019).