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.
Highlights
The ever-increasing demand for energy due to population growth, industrialization, and urban area expansion along with the fossil fuel runout drives the need for a vigorous energy supply toward sustainable growth and improved living standards
We critically review the impacts of multivalent ions on power generation by reverse electrodialysis (RED) along with the effect of the feed salinity conditions
Uphill transport is one of the key challenges for the implementation of RED under natural feed salinity conditions with a mixture of monovalent and multivalent ions. The presence of both divalent cations and anions (Mg2+, Ca2+, SO4 2− ) in feed solutions imposed a significant negative effect on open-circuit voltage (OCV) and the power output of RED system. This effect is not well-understood, as the transport phenomenon in RED under conditions of mixture with monovalent and multivalent ion solutions is complex and requires advanced study through modeling approaches combined with experimental outputs
Summary
The ever-increasing demand for energy due to population growth, industrialization, and urban area expansion along with the fossil fuel runout drives the need for a vigorous energy supply toward sustainable growth and improved living standards. Membranes 2020, 10, 7 energy, referred to as “blue energy” is the energy obtained by the conversion of the chemical potential difference to electrical/mechanical energy by mixing two different salt solutions [2,3]. 1 m3 of freshwater into the sea produces around 0.8 kWh theoretical energy In this sense, the total freshwater flow of the major rivers into the sea generates nearly 2 TW of salinity gradient power (SGP) [3,4]. The compartments are separated by spacer materials which provide a space between the membranes, thereby allowing mixing of the salt solutions When these compartments are supplied with respective high concentration solutions and low concentration solutions, a potential gradient is created that drives the selective transport of ions through the membranes.
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