Design of Monovalent Ion Selective Membranes for Reducing the Impacts of Multivalent Ions in Reverse Electrodialysis

Abstract

Reverse electrodialysis (RED) represents one of the most promising membrane-based technologies for clean and renewable energy production from mixing water solutions. However, the presence of multivalent ions in natural water drastically reduces system performance, in particular, the open-circuit voltage (OCV) and the output power. This effect is largely described by the "uphill transport" phenomenon, in which multivalent ions are transported against the concentration gradient. In this work, recent advances in the investigation of the impact of multivalent ions on power generation by RED are systematically reviewed along with possible strategies to overcome this challenge. In particular, the use of monovalent ion-selective membranes represents a promising alternative to reduce the negative impact of multivalent ions given the availability of low-cost materials and an easy route of membrane synthesis. A thorough assessment of the materials and methodologies used to prepare monovalent selective ion exchange membranes (both cation and anion exchange membranes) for applications in (reverse) electrodialysis is performed. Moreover, transport mechanisms under conditions of extreme salinity gradient are analyzed and compared for a better understanding of the design criteria. The ultimate goal of the present work is to propose a prospective research direction on the development of new membrane materials for effective implementation of RED under natural feed conditions.

Keywords: monovalent selective membranes; multivalent ions; reverse electrodialysis; salinity gradient power; uphill transport.

Figure 1

Schematic illustration of reverse electrodialysis (RED) for salinity gradient power generation. The high concentration compartment (HCC) and low concentration compartment (LCC) are created by a series of alternative cation exchange membranes (CEMs) and anion exchange membranes (AEMs). The electrical energy is generated by the redox reactions occurring over the two electrodes placed at the ends of the membrane pile.

Figure 2

Transport in an AEM contacted with a NaCl feed solution.

Figure 3

Illustration of uphill transport.

Figure 4

The impact of multivalent ions for RED systems tested using different commercial membranes (Ralex, Neosepta, or Fujifilm membranes). (a) Ppen-circuit voltage (OCV), (b) ohmic resistance, and (c) gross power density as a function of the molar fraction of MgSO₄ of the total dissolved salts in the feed solutions. Experimental results are reported as an average of a stationary data series measured over 1 h. Reproduced with permission from [8]. Copyright 2015 Royal Society of Chemistry.

Figure 5

Variation in internal area resistance per cell with the composition of the feed solutions (single membrane area: 100 cm²; number of cell pairs: 25). Reproduced with permission from [9]. Copyright 2014 Royal Society of Chemistry.

Figure 6

(a) Polarization curves (Voltage (V) vs. current (I)) and (b) gross power density (P_d) as a function of current density for RED tests using multivalent ions (NaCl/MgCl₂) of different molal compositions. In pure MgCl₂ solution, the power density and the OCV decreased by 94% and 57%, respectively, with respect to pure NaCl solution. Reproduced with permission from [5]. Copyright 2016 Elsevier.

Figure 7

(a) Mitigation of uphill transport using monovalent selective cation exchange membrane. LCC: low concentration solution; HCC high concentration solution; CEM: cation exchange membrane. (b) Enhancement in OCV and power density with the monovalent selective membranes based on polypyrrole/chitosan composites. Reproduced with permission from [39]. Copyright 2016 Elsevier.

Figure 8

Surface modification of an anion exchange membrane (AEM) by infiltration and photo-cross-linking using 4,4-diazostilbene-2,2-disulfonic acid disodium salt (DAS). The azido group in DAS reacted to a nitrene group under UV irradiation which immobilized it on the membrane surface, thereby creating covalent cross-linking: The sulfonate group facilitated the water solubility and infiltration into the surface layer structure of the membrane, providing the negative charge groups and also improving the monovalent anion selectivity. Reproduced with permission from [57]. Copyrights 2018 American Institute of Chemical Engineers.

Figure 9

(a) The transport number ratio of SO₄²⁻ and Cl⁻ ions in ED as a function of the number of layers for the AMX membrane modified with PSS end layers [51]. (b) Variations in the permselectivity of the Neosepta CMX with the number of PEI/PSS bilayers [54]. Reproduced with permission from [51,54]. Copyright 2013, Elsevier, and copyright 2014, American Chemical Society.