Power Generation Performance of a Pilot-Scale Reverse Electrodialysis Using Monovalent Selective Ion-Exchange Membranes

Composite Anion Exchange Membranes Based on Quaternary Ammonium-Functionalized Polystyrene and Cerium(IV) Phosphate with Improved Monovalent-Ion Selectivity and Antifouling Properties

The possibility of targeted change of the properties of ion exchange membranes by incorporation of various nanoparticles into the membranes is attracting the attention of many research groups. Here we studied for the first time the influence of cerium phosphate nanoparticles on the physicochemical and transport properties of commercial anion exchange membranes based on quaternary ammonium-functionalized polystyrenes, such as heterogeneous Ralex^® AM and pseudo-homogeneous Neosepta^® AMX. The incorporation of cerium phosphate on one side of the membrane was performed by precipitation from absorbed cerium ammonium nitrate (CAN) anionic complex with ammonium dihydrogen phosphate or phosphoric acid. The structures of the obtained hybrid membranes and separately synthesized cerium phosphate were investigated using FTIR, P31 MAS NMR, EDX mapping, and scanning electron microscopy. The modification increased the membrane selectivity to monovalent ions in the ED desalination of an equimolar mixture of NaCl and Na₂SO₄. The highest selectivities of Ralex^® AM and Neosepta^® AMX-based hybrid membranes were 4.9 and 7.7, respectively. In addition, the modification of Neosepta^® membranes also increased the resistance to a typical anionic surfactant, sodium dodecylbenzenesulfonate.

Fluidics for energy harvesting: from nano to milli scales

A large amount of untapped energy sources surrounds us. In this review, we summarize recent works of water-based energy harvesting systems with operation scales ranging from miniature systems to large scale attempts. We focus particularly on the triboelectric energy, which is produced when a liquid and a solid come into contact, and on the osmotic energy, which is released when salt water and fresh water are mixed. For both techniques we display the state of the art understanding (including electrical charge separation, electro-osmotic currents and induced currents) and the developed devices. A critical discussion of present works confirms the significant progress of these water-based energy harvesting systems in all scales. However, further efforts in efficiency and performance amelioration are expected for these technologies to accelerate the industrialization and commercialization procedure.

Power Generation Performance of Reverse Electrodialysis (RED) Using Various Ion Exchange Membranes and Power Output Prediction for a Large RED Stack

Reverse electrodialysis (RED) power generation using seawater (SW) and river water is expected to be a promising environmentally friendly power generation system. Experiments with large RED stacks are needed for the practical application of RED power generation, but only a few experimental results exist because of the need for large facilities and a large area of ion-exchange membranes (IEMs). In this study, to predict the power output of a large RED stack, the power generation performances of a lab-scale RED stack (40 membrane pairs and 7040 cm² total effective membrane area) with several IEMs were evaluated. The results were converted to the power output of a pilot-scale RED stack (299 membrane pairs and 179.4 m² total effective membrane area) via the reference IEMs. The use of low-area-resistance IEMs resulted in lower internal resistance and higher power density. The power density was 2.3 times higher than that of the reference IEMs when natural SW was used. The net power output was expected to be approximately 230 W with a pilot-scale RED stack using low-area-resistance IEMs and natural SW. This value is one of the indicators of the output of a large RED stack and is a target to be exceeded with further improvements in the RED system.

Renewable Power Generation by Reverse Electrodialysis Using an Ion Exchange Membrane

Reverse electrodialysis (RED) is a promising technology to extract sustainable salinity gradient energy. However, the RED technology has not reached its full potential due to membrane efficiency and fouling and the complex interplay between ionic flows and fluidic configurations. We investigate renewable power generation by harnessing salinity gradient energy during reverse electrodialysis using a lab-scaled fluidic cell, consisting of two reservoirs separated by a nanoporous ion exchange membrane, under various flow rates (qf) and salt-concentration difference (Δc). The current-voltage (I-V) characteristics of the single RED unit reveals a linear dependence, similar to an electrochemical cell. The experimental results show that the change of inflow velocity has an insignificant impact on the I-V data for a wide range of flow rates explored (0.01–1 mL/min), corresponding to a low-Peclet number regime. Both the maximum RED power density (Pc,m) and open-circuit voltage (ϕ0) increase with increasing Δc. On the one hand, the RED cell’s internal resistance (Rc) empirically reveals a power-law dependence of Rc∝Δc−α. On the other hand, the open-circuit voltage shows a logarithmic relationship of ϕ0=BlnΔc+β. These experimental results are consistent with those by a nonlinear numerical simulation considering a single charged nanochannel, suggesting that parallelization of charged nano-capillaries might be a good upscaling model for a nanoporous membrane for RED applications.

Ionic Transport Properties of Cation-Exchange Membranes Prepared from Poly(vinyl alcohol-b-sodium Styrene Sulfonate)

Membrane resistance and permselectivity for counter-ions have important roles in determining the performance of cation-exchange membranes (CEMs). In this study, PVA-based polyanions-poly(vinyl alcohol-b-sodium styrene sulfonate)-were synthesized, changing the molar percentages CCEG of the cation-exchange groups with respect to the vinyl alcohol groups. From the block copolymer, poly(vinyl alcohol) (PVA)-based CEMs, hereafter called "B-CEMs", were prepared by crosslinking the PVA chains with glutaraldehyde (GA) solution at various GA concentrations CGA. The ionic transport properties of the B-CEMs were compared with those previously reported for the CEMs prepared using a random copolymer-poly(vinyl alcohol-co-2-acrylamido-2-methylpropane sulfonic acid)-hereafter called "R-CEMs". The B-CEMs had lower water content than the R-CEMs at equal molar percentages of the cation-exchange groups. The charge density of the B-CEMs increased as CCEG increased, and reached a maximum value, which increased with increasing CGA. A maximum charge density of 1.47 mol/dm3 was obtained for a B-CEM with CCEG = 2.9 mol% and CGA = 0.10 vol.%, indicating that the B-CEM had almost two-thirds of the permselectivity of a commercial CEM (CMX: ASTOM Corp. Japan). The dynamic transport number and membrane resistance of a B-CEM with CCEG = 8.3 mol% and CGA = 0.10 vol.% were 0.99 and 1.6 Ωcm2, respectively. The B-CEM showed higher dynamic transport numbers than those of the R-CEMs with similar membrane resistances.

Comparison of Pressure-Retarded Osmosis Performance between Pilot-Scale Cellulose Triacetate Hollow-fiber and Polyamide Spiral-Wound Membrane Modules

Pressure-retarded osmosis (PRO) has recently received attention because of its ability to generate power via an osmotic pressure gradient between two solutions with different salinities: high- and low-salinity water sources. In this study, PRO performance, using the two pilot-scale PRO membrane modules with different configurations—five-inch cellulose triacetate hollow-fiber membrane module (CTA-HF) and eight-inch polyamide spiral-wound membrane modules (PA-SW)—was evaluated by changing the draw solution (DS) concentration, applied hydrostatic pressure difference, and the flow rates of DS and feed solution (FS), to obtain the optimum operating conditions in PRO configuration. The maximum power density per unit membrane area of PA-SW at 0.6 M NaCl was 1.40 W/m² and 2.03-fold higher than that of CTA-HF, due to the higher water permeability coefficient of PA-SW. In contrast, the maximum power density per unit volume of CTA-SW at 0.6 M NaCl was 4.67 kW/m³ and 6.87-fold higher than that of PA-SW. The value of CTA-HF increased to 13.61 kW/m³ at 1.2 M NaCl and was 12.0-fold higher than that of PA-SW because of the higher packing density of CTA-HF.