Abstract
Reverse electrodialysis (RED) technology represents a promising electro-membrane process for renewable energy harvesting from aqueous streams with different salinity. However, the performance of the key components of the system, that is, the ion exchange membranes, is limited by both the presence of multivalent ions and fouling phenomena, thus leading to a reduced generated net power density. In this context, the behavior of anion exchange membranes (AEMs) in RED systems is more severely affected, due to the undesirable interactions between their positively charged fixed groups and, mostly negatively charged, foulant materials present in natural streams. Therefore, controlling both the monovalent anion permselectivity and the membrane surface hydrophilicity is crucial. In this respect, different surface modification procedures were considered in the literature, to enhance the above-mentioned properties. This review reports and discusses the currently available approaches for surface modifications of AEMs, such as graft polymerization, dip coating, and layer-by-layer, among others, mainly focusing on preparing monovalent permselective AEMs with antifouling characteristics, but also considering hydrophilicity aspects and identifying the most promising modifying agents to be utilized. Thus, the present study aimed at providing new insights for the further design and development of selective, durable, and cost-effective modified AEMs for an enhanced RED process performance, which is indispensable for a practical implementation of this electro-membrane technology at an industrial scale.
Highlights
The continuous increase of the global energy demand, which is expected to be significantly increased by 80% in 2050 [1], as well as global warming concerns, owing to the combustion of fossil fuels, is leading to the development of environment-friendly strategies and technologies, to ensure sustainable and alternative energy resources
The development of highly selective anion exchange membranes (AEMs) for Reverse electrodialysis (RED) applications is clearly beneficial for other electro-membrane devices, such as fuel cells, especially for alkaline anion exchange membrane fuel cells (AAEMFC). These were reported to present remarkable advantages over proton exchange membrane fuel cells (PEMFC), since AAEMFC might progress the implementation of low-platinum or platinum-free fuel cell technologies, which is favorable in terms of process costs [23]
PEMFC, especially those based on Nafion-type membranes, are far studied much more, because in AAEMFC, improving the membrane chemical stability under alkaline conditions still represents the key challenge hindering the practical application of this novel fuel cell technology
Summary
The continuous increase of the global energy demand, which is expected to be significantly increased by 80% in 2050 [1], as well as global warming concerns, owing to the combustion of fossil fuels, is leading to the development of environment-friendly strategies and technologies, to ensure sustainable and alternative energy resources. The development of highly selective AEMs for RED applications is clearly beneficial for other electro-membrane devices, such as fuel cells, especially for alkaline anion exchange membrane fuel cells (AAEMFC) These were reported to present remarkable advantages over proton exchange membrane fuel cells (PEMFC), since AAEMFC might progress the implementation of low-platinum or platinum-free fuel cell technologies, which is favorable in terms of process costs [23]. PEMFC, especially those based on Nafion-type membranes, are far studied much more, because in AAEMFC, improving the membrane chemical stability under alkaline conditions still represents the key challenge hindering the practical application of this novel fuel cell technology In this respect, strategies followed so far for developing monovalent permselective and hydrophilic AEMs for RED applications could be incorporated into the AAEMFC technology, in order to move forward in seeking improvements of the AEM structure, surface properties, etc. The objective of the authors was to provide novel insights into the continuous design and development of innovative cost-effective, sustainable, durable, stable, and selectively modified AEMs, for an improved overall RED process efficiency
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