Correlations between Properties of Pore-Filling Ion Exchange Membranes and Performance of a Reverse Electrodialysis Stack for High Power Density

Surface-Modified Pore-Filled Anion-Exchange Membranes for Efficient Energy Harvesting via Reverse Electrodialysis

In this study, novel pore-filled anion-exchange membranes (PFAEMs) modified with polypyrrole (PPy) and reduced graphene oxide (rGO) were developed to improve the energy harvesting performance of reverse electrodialysis (RED). The surface-modified PFAEMs were fabricated by varying the contents of PPy and rGO through simple spin coating and chemical/thermal treatments. It was confirmed that the PPy and PPy/rGO layers introduced on the membrane surface did not significantly increase the electrical resistance of the membrane and could effectively control surface characteristics, such as structural tightness, hydrophilicity, and electrostatic repulsion. The PPy/rGO-modified PFAEM showed excellent monovalent ion selectivity, more than four times higher than that of the commercial membrane (AMX, Astom Corp., Tokyo, Japan). This means that the PPy/rGO layer can effectively reduce the permeation of multivalent ions with a high charge intensity and a relatively large hydration radius compared to monovalent ions. The results of evaluating the performance of the surface-modified PFAEMs by applying them to a RED cell revealed that the decrease in potential difference occurring in the membrane was reduced by effectively suppressing the uphill transport of multivalent ions. Consequently, the PPy/rGO-modified membrane exhibited a 5.43% higher power density than the AMX membrane.

Heat to Hydrogen by RED-Reviewing Membranes and Salts for the RED Heat Engine Concept

The Reverse electrodialysis heat engine (REDHE) combines a reverse electrodialysis stack for power generation with a thermal regeneration unit to restore the concentration difference of the salt solutions. Current approaches for converting low-temperature waste heat to electricity with REDHE have not yielded conversion efficiencies and profits that would allow for the industrialization of the technology. This review explores the concept of Heat-to-Hydrogen with REDHEs and maps crucial developments toward industrialization. We discuss current advances in membrane development that are vital for the breakthrough of the RED Heat Engine. In addition, the choice of salt is a crucial factor that has not received enough attention in the field. Based on ion properties relevant for both the transport through IEMs and the feasibility for regeneration, we pinpoint the most promising salts for use in REDHE, which we find to be KNO3, LiNO3, LiBr and LiCl. To further validate these results and compare the system performance with different salts, there is a demand for a comprehensive thermodynamic model of the REDHE that considers all its units. Guided by such a model, experimental studies can be designed to utilize the most favorable process conditions (e.g., salt solutions).

Accurate Determination of Electrical Potential on Ion Exchange Membranes in Reverse Electrodialysis

Reverse electrodialysis is a promising membrane technology to generate energy from controlled mixing of water streams of different salinities. Electrical potentials generate on the ion exchange membranes (IEMs) when selective transport of cations and anions across the membranes driven by concentration difference. The accurate determination of the potentials developed on the IEMs is critical to fairly assess the feasibility of the technology. The Nernst–Planck–Poisson (NPP) equations for IEMs (the membranes with fixed charge) were solved numerically with the boundary updating scheme. The validity of this numerical method was verified by the identical values of Donnan potential obtained with the well-established analytical methods. The suitability and applicability of the classic Teorell–Meyer–Siever (TMS) model were assessed by comparison to the simulation results from the numerical method.