Salinity Gradient Energy Conversion Using Permselective Organic Liquid Membranes
Authors: Yamada Yusuke , Kitazumi Yuki , Kano Kenji , Shirai Osamu 
Published: December 2020

The increases of worldwide energy consumption and the concern about the environment have made us pay attention to eco-friendly and renewable energy, such as solar power, wind power, water power, geothermal power, biofuel energy, etc. [P. A. Owusu and S. Asumadu-Sarkodie, Cogent Eng., 3, 1167990 (2016)]. The salinity gradient between seawater and river water must be one of such energy sources. Reverse electrodialysis (RED) is a power generation system which translates salinity gradient energy to electricity by the use of ion-exchange membranes [Y. Mei and C. Y. Tang, Desalination, 425, 156 (2018)]. Since the waters are sufficient on Earth and RED using them does not generate harmful waste, RED can supply electricity in a sustainable and eco-friendly manner. However, the drop in the power capacity owing to the large resistance of the river water, the high cost of ion-exchange membranes, deterioration and clogging of ion-exchange membranes, etc. are serious problems to be solved.
We used organic liquid membranes instead of ion-exchange membranes as the permselective membranes for a RED system. Liquid membranes are often employed in the field of ion sensors in order to give electrodes the selectivity on the objective ions. The permselectivity of the liquid membranes is attributed to both the difference in the transfer energies of coexisting ions and the concentration ratio of them. The characteristics of the liquid-membrane type electric cell were studied.
A Na+-permselective liquid membrane (Na+-LM) was prepared in the following manner. First, a 0.2-mm-thick polytetrafluoroethylene (PTFE) sheet opening a hole of 1.0 cm diameter was put on a flat glass plate and was fixed with clips. 74 mg of Sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (NaTFPB), 100 mg of polyvinyl chloride (PVC), and 0.8 mL (L = dm3) of 2-nitrophenyl octyl ether (NPOE) were dissolved in 4 mL of tetrahydrofuran (THF). Then, 0.1 mL of the THF solution was poured into the hole, and the solvent was dried for a day. Finally, a Na+-LM was formed within the hole on the glass plate. The membrane was peeled off from the glass plate and was used along with the PTFE sheet. Similarly, a Cl−-permselective liquid membrane (Cl−-LM) was prepared by the use of a THF solution containing 42 mg of tetraoctylammonium chloride (TOcACl), 100 mg of PVC, and 0.8 mL of NPOE in 4 mL of THF. The thickness of the membranes (Na+-LM and Cl−-LM) was 13 μm.
The cell is mainly composed of three aqueous phases (W1, W2, and W3), two permselective liquid membranes (Na+-LM and Cl−-LM), and two Ag|AgCl electrodes (RE1 and RE2). The composition of the cell is as follows: RE1|W1|Na+-LM|W2|Cl−-LM|W3|RE2. W1 and W3 contained 1.0 mol L−1 NaCl aqueous solutions, which corresponded to seawater. A 50 mmol L−1 NaCl aqueous solution simulating river water was used as W2. The thickness of W1 and W3 and that of W2 are 2.3 cm and 2.8 cm, respectively.
The power density of the cell was evaluated by normal pulse voltammetry. The potential of the cell, E, was taken as the potential of RE2 against RE1. In each step, E was maintained at the electromotive force E
emf (or the open circuit potential) for 370 s, and then the rectangular pulse voltage was applied for 50 s. The current due to the Na+ flow from W1 to W3 was taken to be positive. The measurement was conducted at 298 ± 1 K.
The result is shown in the attached figure. The time-course of the current density j and that of E were expressed as a solid line and a broken line, respectively. The measured value of E
emf was 138 mV under the conditions. The calculated value of E
emf by the Nernst equation using the activities of Na+ and Cl− is 140 mV, and the agreement between the two E
emf values demonstrates that Na+-LM and Cl−-LM played as ideal permselective membranes. In order to evaluate the power density of the cell, the values of j at the last 2 s of each pulse were averaged and were related to the E values. The maximum power density was 14 μW cm−2 at 69 mV.
In the present study, the liquid membranes were used instead of ion-exchange membranes for the RED system. The NPOE solution of NaTFPB and that of TOcACl showed the good permselectivity in the NaCl aqueous solution. The liquid membranes are able to replace the ion-exchange membranes used in RED systems.

Figure 1