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).