Dc Kitty Nijmeijer

Anion-exchange membranes in electrochemical energy systems†

This article provides an up-to-date perspective on the use of anion-exchange membranes in fuel cells, electrolysers, redox flow batteries, reverse electrodialysis cells, and bioelectrochemical systems (e.g. microbial fuel cells). The aim is to highlight key concepts, misconceptions, the current state-of-the-art, technological and scientific limitations, and the future challenges (research priorities) related to the use of anion-exchange membranes in these energy technologies. All the references that the authors deemed relevant, and were available on the web by the manuscript submission date (30th April 2014), are included.

Doubled power density from salinity gradients at reduced intermembrane distance

The mixing of sea and river water can be used as a renewable energy source. The Gibbs free energy that is released when salt and fresh water mix can be captured in a process called reverse electrodialysis (RED). This research investigates the effect of the intermembrane distance and the feedwater flow rate in RED as a route to double the power density output. Intermembrane distances of 60, 100, 200, and 485 μm were experimentally investigated, using spacers to impose the intermembrane distance. The generated (gross) power densities (i.e., generated power per membrane area) are larger for smaller intermembrane distances. A maximum value of 2.2 W/m(2) is achieved, which is almost double the maximum power density reported in previous work. In addition, the energy efficiency is significantly higher for smaller intermembrane distances. New improvements need to focus on reducing the pressure drop required to pump the feedwater through the RED-device using a spacerless design. In that case power outputs of more than 4 W per m(2) of membrane area at small intermembrane distances are envisaged.

Thermodynamic, energy efficiency, and power density analysis of reverse electrodialysis power generation with natural salinity gradients

Reverse electrodialysis (RED) can harness the Gibbs free energy of mixing when fresh river water flows into the sea for sustainable power generation. In this study, we carry out a thermodynamic and energy efficiency analysis of RED power generation, and assess the membrane power density. First, we present a reversible thermodynamic model for RED and verify that the theoretical maximum extractable work in a reversible RED process is identical to the Gibbs free energy of mixing. Work extraction in an irreversible process with maximized power density using a constant-resistance load is then examined to assess the energy conversion efficiency and power density. With equal volumes of seawater and river water, energy conversion efficiency of ∼ 33-44% can be obtained in RED, while the rest is lost through dissipation in the internal resistance of the ion-exchange membrane stack. We show that imperfections in the selectivity of typical ion exchange membranes (namely, co-ion transport, osmosis, and electro-osmosis) can detrimentally lower efficiency by up to 26%, with co-ion leakage being the dominant effect. Further inspection of the power density profile during RED revealed inherent ineffectiveness toward the end of the process. By judicious early discontinuation of the controlled mixing process, the overall power density performance can be considerably enhanced by up to 7-fold, without significant compromise to the energy efficiency. Additionally, membrane resistance was found to be an important factor in determining the power densities attainable. Lastly, the performance of an RED stack was examined for different membrane conductivities and intermembrane distances simulating high performance membranes and stack design. By thoughtful selection of the operating parameters, an efficiency of ∼ 37% and an overall gross power density of 3.5 W/m(2) represent the maximum performance that can potentially be achieved in a seawater-river water RED system with low-resistance ion exchange membranes (0.5 Ω cm(2)) at very small spacing intervals (50 μm).

Salinity gradient energy

There exists a huge potential for the generation of energy from the mixing of saltwater and freshwater. The potential is 2.6 TW, which is more than the global electricity consumption (2.0 TW). Two membrane-based technologies exist to convert this potentially available energy into useful power: pressure-retarded osmosis (PRO) and reverse electrodialysis (RED). In PRO, water is transported through a semipermeable membrane from the less concentrated solution toward the concentrated salt solution to generate power. In RED, salt ions are transported from the concentrated salt solution through ion exchange membranes toward the less concentrated solution to extract the energy. Both technologies were developed in the 1970s and 1980s and both regained interest lately due to recent developments in membrane technology and the need for sustainable energy processes. This chapter describes the potential of salinity gradient energy of both processes in detail, and an overview of the relevant literature on both technologies is presented. Furthermore, it summarizes the recent developments, pilot testing, scale-up, and future expectations of both technologies.

Periodic Feedwater Reversal and Air Sparging As Antifouling Strategies in Reverse Electrodialysis

Renewable energy can be generated using natural streams of seawater and river water in reverse electrodialysis (RED). The potential for electricity production of this technology is huge, but fouling of the membranes and the membrane stack reduces the potential for large scale applications. This research shows that, without any specific antifouling strategies, the power density decreases in the first 4 h of operation to 40% of the originally obtained power density. It slowly decreases further in the remaining 67 days of operation. Using antifouling strategies, a significantly higher power density can be maintained. Periodically switching the feedwaters (i.e., changing seawater for river water and vice versa) generates the highest power density in the first hours of operation, probably due to a removal of multivalent ions and organic foulants from the membrane when the electrical current reverses. In the long term, colloidal fouling is observed in the stack without treatment and the stack with periodic feedwater switching, and preferential channeling is observed in the latter. This decreases the power density further. This decrease in power density is partly reversible. Only a stack with periodic air sparging has a minimum of colloidal fouling, resulting in a higher power density in the long term. A combination of the discussed antifouling strategies, together with the use of monovalent selective membranes, is recommended to maintain a high power density in RED in short-term and long-term operations.

Multifunctional polyelectrolyte multilayers as nanofiltration membranes and as sacrificial layers for easy membrane cleaning

This manuscript investigates the modification of an ultra-filtration (UF) membrane support with polyelectrolyte multilayers (PEMs) consisting of the weak polyelectrolytes poly(allyl amine) hydrochloride (PAH) and poly(acrylic acid) (PAA). These prepared polyelectrolyte multilayer membranes have a dual function: They act as nanofiltration (NF) membranes and as sacrificial layers to allow easy cleaning of the membranes. In order to optimize the conditions for PEM coating and removal, adsorption and desorption of these layers on a model surface (silica) was first studied via optical reflectometry. Subsequently, a charged UF membrane support was coated with a PEM and after each deposited layer, a clear increase in membrane resistance against pure water permeation and a switch of the zeta potential were observed. Moreover these polyelectrolyte multilayer membranes, exhibited rejection of solutes in a range typical for NF membranes. Monovalent ions (NaCl) were hardly rejected (<24%), while rejections of >60% were observed for a neutral organic molecule sulfamethoxazole (SMX) and for the divalent ion SO3(2-). The rejection mechanism of these membranes seems to be dominated by size-exclusion. To investigate the role of these PEMs as sacrificial layers for the cleaning of fouled membranes, the prepared polyelectrolyte multilayers were fouled with silica nano particles. Subsequent removal of the coating using a rinse and a low pressure backwash with pH 3, 3M NaNO3 allowed for a drop in membrane resistance from 1.7⋅10(14)m(-1) (fouled membrane) to 9.9⋅10(12)m(-1) (clean membrane), which is nearly equal to that of the pristine membrane (9.7⋅10(12)m(-1)). Recoating of the support membrane with the same PEMs resulted in a resistance equal to the resistance of the original polyelectrolyte multilayer membrane. Interestingly, less layers were needed to obtain complete foulant removal from the membrane surface, than was the case for the model surface. The possibility for backwashing allows for an even more successful use of the sacrificial layer approach in membrane technology than on model surfaces. Moreover, these PEMs can be used to provide a dual function, as NF membranes and as a Sacrificial coating to allow easy membrane cleaning.

Highly Selective Amino Acid Salt Solutions as Absorption Liquid for CO2 Capture in Gas–Liquid Membrane Contactors

The strong anthropogenic increase in the emission of CO(2) and the related environmental impact force the developments towards sustainability and carbon capture and storage (CCS). In the present work, we combine the high product yields and selectivities of CO(2) absorption processes with the advantages of membrane technology in a membrane contactor for the separation of CO(2) from CH(4) using amino acid salt solutions as competitive absorption liquid to alkanol amine solutions. Amino acids, such as sarcosine, have the same functionality as alkanol amines (e.g., monoethanolamine=MEA), but in contrast, they exhibit a better oxidative stability and resistance to degradation. In addition, they can be made nonvolatile by adding a salt functionality, which significantly reduces the liquid loss due to evaporation at elevated temperatures in the desorber. Membrane contactor experiments using CO(2)/CH(4) feed mixtures to evaluate the overall process performance, including a full absorption/desorption cycle show that even without a temperature difference between absorber and desorber, a CO(2)/CH(4) selectivity of over 70 can be easily achieved with the sarcosine salt solution as absorption liquid. This selectivity reaches values of 120 at a temperature difference between absorber and desorber of 35 degrees C, compared to a value of only 60 for MEA under the same conditions. Although CO(2) permeance values are somewhat lower than the values obtained for MEA, the results clearly show the potential of amino acid salt solutions as competitive absorption liquids for the energy efficient removal of CO(2). In addition, due to the low absorption of CH(4) in sarcosine compared to MEA, the loss of CH(4) is reduced and significantly higher CH(4) product yields can be obtained.

Polyacrylonitrile (PAN)/crown ether composite nanofibers for the selective adsorption of cations

In this study, we prepared electrospun polyacrylonitrile (PAN) nanofibers functionalized with dibenzo-18-crown-6 (DB18C6) crown ether and showed the potential of these fibers for the selective recovery of K+ from other both mono- and divalent ions in aqueous solutions. Nanofibers were characterized by SEM, FTIR and TGA. SEM results showed that the crown ether addition resulted in thicker nanofibers and higher mean fiber diameters, in a range of 138 to 270 nm. Batch adsorption experiments were conducted in order to evaluate the potential of the crown ether modified nanofibers as an adsorbent for ion removal. The maximum adsorption capacity of the crown ether modified nanofibers for K+ was 0.37 mmol g−1 and the nanofibers followed the selectivity sequence of K+ > Ba2+ > Na+ ∼ Li+ for single ion experiments. Adsorption of Ba2+ ions onto crown ether-modified nanofiber was examined by XPS and the results confirmed the adsorption of the ion. Mixed ion adsorption experiments revealed competitive adsorption between K+ and Ba2+ ions for the available binding sites. This effect was not observed for the other monovalent ions present in the solution and exceptionally high selectivities for K+ over Li+ and Na+ were obtained. Also the crown ether modified nanofibers exhibited good regeneration properties and a good reusability over multiple consecutive adsorption–desorption cycles. Electrospinning is thus shown to be a very versatile tool to prepare crown ether functional polymer adsorbents for the selective recovery of ions.

Monovalent cation selective crown ether containing poly(arylene ether ketone)/SPEEK blend membranes

Blend membranes of sulfonated poly(ether ether ketone) (SPEEK) and poly(arylene ether ketone) (PAEK) derivatives containing crown ether units in the main chain (CPAEK) were prepared and characterized in terms of water swelling and ion exchange capacity (IEC). The miscibility of the polymers was verified by DSC and HR-SEM. Ion transport characteristics of the membranes were established for the monovalent ions Li+ and K+ and the separation of these ions by the cation exchange membranes was investigated. Diffusion experiments for aqueous KCl, LiCl and their mixtures were carried out with pure SPEEK membranes as well as with the CPAEK/SPEEK membranes. Blending significantly decreased the ion permeability due to cation-crown ether complexation and increased the hydrophobicity of the matrix. The K+ over Li+ selectivity of the SPEEK membrane was enhanced by blending SPEEK with CPAEK by a factor of nearly 4, indicating that the presence of a crown ether polymer changes the relative transport of the ions in the membrane.

Dominant factors controlling the efficiency of two-phase flow cleaning in spiral-wound membrane elements

Two-phase flow cleaning has been successfully applied to control fouling in spiral wound membrane elements. This study focuses on its experimental optimization using a Taguchi Design of Experiment method (L-25 orthogonal arrays) to elucidate the influence of different factors and to reveal the important one(s) affecting the cleaning efficiency of two-phase flow cleaning. All possible combinations of the factors, i.e. feed type, spacer geometry, gas/liquid ratio, and liquid velocity, each at five levels were evaluated. The main effect of each factor on the efficiency of two-phase flow cleaning was measured by determining the performance response (mean of cleaning efficiency) and by calculating the mean signal-to-noise ratio. An analysis of variance was applied to calculate the relative contribution of each factor on the efficiency of two-phase flow cleaning. The results showed that the feed type is by far the most essential factor contributing to the cleaning efficiency. The spacer geometry is ranked second, followed by the gas/liquid ratio and the liquid velocity, which both have an only very minor effect on the cleaning performance. In terms of practical application, the operator should consider first the type of foulant prior to taking a decision on whether or not two-phase flow cleaning will be effective. Once the foulant type is defined, the use of the highest gas/liquid ratio, the highest liquid velocity, and the thickest feed spacer (diamond type) are recommended to achieve maximum two-phase flow cleaning efficiency.