(Invited) Next Generation Ion Exchange Membranes for Electrochemical Polymer Membrane Processes

An electrochemical membrane process (EMP) with three compartments (anolyte, catholyte, and central compartment) based on in-house-prepared cation-exchange membrane (CEM) was developed to achieve in situ ion substitution and recovery of salicylic acid (SAH) from its sodium salt. The physicochemical and electrochemical properties of the ion-exchange membrane (cation- and anion-exchange membrane) under standard operating conditions reveal its suitability for the proposed reactor. Experiments using sodium salicylate (SANa) solutions of different concentrations were carried out under varied applied current density to study the feasibility of the process. Overall electrochemical reaction for the in situ ion substitution and separation of SAH from SANa under operating conditions is also proposed. Results showed that developed EMP with CEMs proved promising for the in situ ion substitution and separation of SAH with recovery of SAH with current efficiency close to 90% and energy consumption around 10 kW h/kg of the SAH produced. This process was completely optimized in terms of operating conditions such as initial concentration of SANa in the central compartment, the applied current density, Na+ flux, recovery percentage, energy consumption, and current efficiency. Furthermore, the process efficiency and energy consumption of EMP for the production of SAH were compared with electrodialysis (ED) used for the separation of Na2SO4 and SAH, formed due to acidification of SANa by H2SO4. It was observed that EMP showed high current efficiency, recovery, and low energy consumption, in comparison with ED under similar experimental concentrations. It was concluded that the proposed EMP is an efficient alternate for producing SAH from SANa by economical and environmental friendly manner. Also the production of NaOH in the cathode stream is a spin off of the EMP.

Ion exchange membrane (IEM) performance in electrochemical processes such as fuel cells, redox flow batteries, or reverse electrodialysis (RED) is typically quantified through membrane selectivity and conductivity, which together determine the energy efficiency. However, water and co-ion transport (i.e., osmosis and salt diffusion / fuel crossover) also impact energy efficiency by allowing uncontrolled mixing of the electrolyte solutions to occur. For example, in RED with hypersaline water sources, uncontrolled mixing consumes 20-50% of the available mixing energy. Thus, in addition to high selectivity and high conductivity, it is desirable for IEMs to have low permeability to water and salt in order to minimize energy losses. Unfortunately, there is very little quantitative water and salt permeability information available for commercial IEMs, making it difficult to select the best membrane for a particular application. Accordingly, we measured the water and salt transport properties of 20 commercial IEMs and analyzed the relationships between permeability, diffusion and partitioning according to the solution-diffusion model. We found that water and salt permeance vary over several orders of magnitude among commercial IEMs, making some membranes better-suited than others to electrochemical processes that involve high salt concentrations and/or concentration gradients. Water and salt diffusion coefficients were found to be the principal factors contributing to the differences in permeance among commercial IEMs. We also observed that water and salt permeability were highly correlated to one another for all IEMs studied, regardless of polymer type or reinforcement. This finding suggests that transport of mobile salt in IEMs is governed by the microstructure of the membrane, and provides clear evidence that mobile salt does not interact strongly with polymer chains in highly-swollen IEMs.

Redox Flow Batteries (RFBs) are inherently well suited for large-scale electrical-energy-storage (EES) applications [1]. RFBs are entering a period of renaissance, buoyed by both the increasing need for affordable long-duration EES solutions, as well as recent substantial advancements in cell performance that leverage state-of-the-art (SOA) flow-cell technologies, such as those originally developed for polymer-electrolyte fuel cells (PEFCs) [2, 3]. A good example of this approach has been the recent dramatic improvements in RFB power density, illustrated in Fig. 1. There are multiple opportunities for advanced RFB materials, especially cell-stack components and RFB active materials, since the remaining components of a RFB system are typically comprised of commercial off-the-shelf parts [2]. This talk will focus on the key requirements for advanced materials for SOA RFB cells, since high power density cells enable inherently lower cell-stack cost and size.First-generation RFB chemistries have been based on single-element active materials dissolved in aqueous electrolytes. Next-generation RFB chemistries are likely to be engineered molecules or complexes. Both aqueous and non-aqueous options are being pursued because non-aqueous electrolytes enable a broader window of electrochemical stability, which is obviously advantageous from both an energy-density and cell-voltage perspective. However, non-aqueous electrolytes also have significant disadvantages, such as higher solvent costs, higher viscosities, and lower ionic conductivities. Detailed techno-economic analysis have recently made a quantitative assessment of these trade-offs [5, 6], and a brief summary of the key requirements for RFB active materials will be briefly summarized. In addition, the key requirement for some less conventional RFB systems will be discussed (e.g., mediated RFB systems with solid-phase storage [7]).Most RFBs today utilize ion-exchange membranes (IEMs), similar to those used in PEFCs. IEMs provide high ionic conductivities, good selectivity for the transport of the desired charge carrier relative to the active materials, and good mechanical and chemical stability. However, IEMs are inherently expensive materials, especially fully-fluorinated IEMs, which are typically used in RFB cells since hydrocarbon-based IEMs are generally not sufficiently stable when exposed to the highly oxidative conditions present in the positive reactant solution (e.g., see [8]). SOA RFB cells employ relatively thin IEMs to reduce material cost and to enable higher ionic conductivities; however, ion selectivity is also key requirement, which is highly dependent on the type of RFB chemistry and what happens to active molecules at the counter electrode [9]. A fundamental understanding of the different causes of crossover in RFB cells (i.e., diffusion, migration, and electro-osmosis) under a various operating conditions [10], as well as how these are related to the physical properties of the separator and the active materials, is also required to intelligently develop alternative RFB separators. Transport-property requirements for RFB separators have been derived for both aqueous and non-aqueous RFB chemistries [9].Many first-generation RFB chemistries are able to utilize simple carbon electrodes because simple redox reactions are facile and involve outer-sphere electrocatalysis. However, the fundamental reaction kinetics of even the relatively mature all-vanadium RFB is not well understood [11]. A major contributing factor to the complexity of RFB reactions on carbon electrodes is the fact that carbon is an extremely complex material. There are many types of carbons, and pretreatments of even the same carbon material can yield very different surface species, which can dramatically impact catalytic activity. Additionally, carbon itself is electrochemically active in the potential window of interest for most RFBs. Therefore, a better understanding of fundamental carbon properties, and stability in an electrochemical environment, is required for RFB cells that rely on carbon as a catalyst. A major conclusion of a recent review article on carbon materials in RFBs was that additional studies on degradation mechanisms are needed [12]. Catalyst materials for redox reactions, beyond carbon, also deserve more attention. Acknowledgements Thanks to the organizers of this Symposia for the invitation to present. The author is also grateful to many collaborators, especially at Vionx Energy and UTRC. References M. Perry, et.al., IEEE Proceedings, 102, 976 (2014). M. Perry & A. Weber, JECS, 163, A5064 (2016). M. Perry, et.al., ECS Transactions, 53, 7 (2013). R. Darling & M. Perry, JECS, 161, A1381 (2014). R. Darling, et.al., Energy & Environmental Science, 7, 3459 (2014). R. Dmello, et.al., J. Power Sources, 330, 261 (2016). C. Jia, et.al., Science Advances, 1, 10 (2015). S. Kim, et.al., J. Appl. Electrochem. 41, 1201 (2011). R. Darling, et.al., JECS, 163, A5029 (2016). M. Perry, et.al., JECS, 163 , A5014 (2016). N. Pour, M. Perry, Y. Shao-Horn, et.al., J. Physical Chemistry C, V119, 5311 (2015). M. Chakrabarti, et.al., J. of Power Sources, 253, 150 (2014). Figure 1

Electrochemical membrane separation of chlorine from gaseous hydrogen chloride waste

Ion-exchange membrane (IEM) performance in electrochemical processes such as fuel cells, redox flow batteries, or reverse electrodialysis (RED) is typically quantified through membrane selectivity and conductivity, which together determine the energy efficiency. However, water and co-ion transport (i.e., osmosis and salt diffusion/fuel crossover) also impact energy efficiency by allowing uncontrolled mixing of the electrolyte solutions to occur. For example, in RED with hypersaline water sources, uncontrolled mixing consumes 20-50% of the available mixing energy. Thus, in addition to high selectivity and high conductivity, it is desirable for IEMs to have low permeability to water and salt to minimize energy losses. Unfortunately, there is very little quantitative water and salt permeability information available for commercial IEMs, making it difficult to select the best membrane for a particular application. Accordingly, we measured the water and salt transport properties of 20 commercial IEMs and analyzed the relationships between permeability, diffusion, and partitioning according to the solution-diffusion model. We found that water and salt permeance vary over several orders of magnitude among commercial IEMs, making some membranes better suited than others to electrochemical processes that involve high salt concentrations and/or concentration gradients. Water and salt diffusion coefficients were found to be the principal factors contributing to the differences in permeance among commercial IEMs. We also observed that water and salt permeability were highly correlated to one another for all IEMs studied, regardless of polymer type or reinforcement. This finding suggests that transport of mobile salt in IEMs is governed by the microstructure of the membrane and provides clear evidence that mobile salt does not interact strongly with polymer chains in highly swollen IEMs.

Treatment of complexed Copper(II) solutions with electrochemical membrane processes

The independent control of power capability, which is affected by reactor design, and energy capacity, which is affected by stored electrolyte volume, enables redox flow batteries (RFBs) to be potential candidates for large-scale energy storage systems. Typical RFBs consist of two large tanks with positive and negative electrolyte pumped to an electrochemical reactor through a network of pipes, while the reactor consists of high surface area porous electrodes (such as carbon felt) separated by an ion exchange membrane (IEM) or a non-selective separator (NSS). Previous research has established inorganic (such as vanadium) and organic/semi-organic (such as TEMPO) molecules for use in RFBs with aqueous or non-aqueous electrolytes. IEMs are used to electronically insulate the two electrodes and reduce crossover of the redox species by charge selectivity. IEMs are expensive and can contribute as much as 20% to the total battery cost depending on the size of the reactor [1]. Reduction of RFB capital costs could incentivize the adoption of RFBs for grid-energy arbitrage [2], and, hence, the development of RFBs with inexpensive NSSs with minimal crossover could enhance RFB use. In this regard, redox active polymers (RAPs) have been developed in both aqueous and non-aqueous systems to replace IEMs with NSSs. Here, NSSs mitigate crossover by size exclusion of RAPs [3,4]. However, the viscosity of such polymer based redox molecules increases with concentration leading to higher pumping pressures required to maintain the flow rate, thus adding to the pumping costs. In addition, higher pumping pressures increase electrolyte crossover which induces RFB capacity fade from cycle-to-cycle. The objectives of the present work are (1) to model multi-component transport during simultaneous electrochemical reactions within RFBs using NSSs, (2) to assess the tradeoffs in rate capability and cycle life incurred when replacing IEMs with NSSs, and (3) to predict performance as a function NSS design (including pore size and thickness). In this context, we are developing a two-dimensional model using porous electrode theory that explicitly captures porous-media flow, electronic current conservation, and conservation of molecular species (typically four redox-active species and two supporting ionic species that are inert) with simultaneous electrochemical reactions. These processes produce coordinated molecular fluxes can result in crossover and shuttling of redox species. In this model, molecular fluxes arise due to (1) pressure differences across the membrane (advection) (2) concentration gradient between the two electrolytes in the reactor (diffusion) and (3) migration due to the gradient in the electrolyte potential. In addition, we observe electrolyte volume losses which arises due to bulk electrolyte flow through the porous membrane because of viscosity (and therefore pressure) difference between the two electrolytes. Each electrode experiences primary reactions due to the major redox couple in the electrolyte and secondary reactions arising from the species that crossover from the counter electrode. We find that the secondary reactions experience a much higher overpotential than the primary reactions. The shuttling process occurs when the products of the secondary reactions diffuse back into the parent electrode. This shuttling process is used in bio sensing devices to amplify and detect the amount of catechol in human nervous system [5] and for overcharge protection in Li-ion batteries [6]. In the context of RFBs, this must be controlled to minimize capacity fade. The present porous-electrode model and associated reduced-order models will enable mechanistic interpretation of experimental data from which performance will be correlated as a function of several non-dimensional parameters that will aid in the design of NSSs. Further, we will validate our porous electrode model with experiments using aqueous RFBs having interdigitated flow fields using various NSSs and IEMs. We gratefully acknowledge the financial support of the Joint Center for Energy Storage Research (JCESR). References Arora, P., Zhang, Z. M., Chem. Rev., 2004, 104, 4419−4462.Darling et al., Energy Environ. Sci., 7 ,2014.Gavvalapalli et al., Am. Chem. Soc., 136 (46),2014Janoschka et al., Nature 527, 78–81, 2015Bernhard Wolfrum et al., Analytical Chemistry, Vol. 80, No. 4, 2008Jun Chen et al., Solid-State Lett., volume 8, issue 1, A59-A62, 2005

There has been a surge of research interest in the lithium polysulfide (Li-PS) redox flow batteries (RFBs) for large scale energy storage applications because of their high theoretical energy density (~2600 Wh/kg and 2199 Wh/L for elemental Li and S) and low material cost. They also can bring additional advantages, such as design flexibility, easy scalability, and safe operation condition. However, the Li-PS battery systems suffer from polysulfide crossover (polysulfide shuttling) between their two working electrodes which decreases the batteries’ columbic efficiency and shortens their cycle lives. This shuttling effect is more detrimental in Li-PS RFBs which doesn’t have carbon matrix to immobilize the solid-state sulfur. In conventional redox flow batteries (e.g. vanadium redox flow battery), the membrane separators has an important role by suppressing the active species crossover between catholyte and anolyte. However, commercial porous battery separators (e.g. Celgard) cannot be adopted due to their low rejection for polysulfide (PSn-) active species and fast capacity decay. Recent few research works revealed the feasibility of using ion exchange membranes (IEMs) as a barrier layer to selectively transport Li+ ions and block PSn- ions. Unfortunately, most commercial IEMs are unstable in the organic polysulfide electrolyte because of its high swelling ratio which can cause the electrolytes crossover. Hence, there is a critical need to develop high performance membrane materials for the Li-PS RFB applications. In this work, we will present our recent progress on developing novel ion exchange membrane materials for the Li-PS RFBs which can greatly suppress the polysulfide shuttling while maintaining high Li+ conductivity and mechanical stability. Our biphenyl polymer membrane (BP-SA) presents excellent stability the DOL/DME organic electrolyte solution, exhibiting potential for Li-PS RFBs application. We studied its selective ion transport properties through diffusion and electrochemical measurement. Diffusion test shows that there is close to 100 % rejection of Li-PS ions by the BP-SA membrane, while considerable amount of Li-PS crossover was found for Celgard and Nafion. Moreover, the BP-SA membrane possess comparable Li conductivity (0.13 mS/cm2) to Nafion (0.29 mS/cm2) in organic solvent. As a result, the Li-PS single cell with the IEM/Celgard composite membrane shows better efficiencies in comparison to the cell with Celgard 2325. Our results unambiguously demonstrate that the BP-SA membrane has greater Li+/PS anion selectivity than Nafion and can be a promising base material to make separators for Li-PS RFBs.

Characterization of protein, peptide and amino acid fouling on ion-exchange and filtration membranes: Review of current and recently developed methods

Acid resistant sulphonated poly(vinylidene fluoride-co-hexafluoropropylene)/graphene oxide composite cation exchange for water splitting by iodine-sulfur bunsen process for hydrogen production

Elimination of nitrate from drinking water by electrochemical membrane processes

Ion exchange membranes are used in practically every industry; however, most of them have defects such as low permeability and poor oxidation resistance. In this paper, cation-exchange membranes were prepared with poly (vinylidene fluoride) (PVDF) blended with nano-SiO2, nano-Al2O3 and nano-ZnO. Sulfonic acid groups were injected into the membrane prepared by styrene grafting and sulfonation. The methods used for characterizing the prepared membranes were Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and electrochemical measurements. Membrane performance, such as the ion exchange capacity (IEC), water uptake (WU), transport number, membrane permselectivity, membrane resistance, functional groups, and morphology were also evaluated. The hydrophilia, IEC, and permselectivity of cation-exchange membranes depended on the nanoparticle content of the membrane matrix. High transport property values were obtained, which increased with increasing nano-SiO2/Al2O3/ZnO weight fractions. Finally, the cation-exchange membranes prepared with 1.5% nano-SiO2, 2.0% nano-Al2O3 or 2.0% nano-ZnO all exhibited excellent membrane properties, including membrane permselectivity (PVDF/2% ZnO-g-PSSA membranes, 94.9%), IEC (PVDF/2% Al2O3-g-PSSA membranes, 2.735 mmol·g−1), and oxidation resistance (PVDF/1.5% SiO2-g-PSSA membranes, 2.33%). They can be used to separate applications in a variety of different areas, such as water treatment, electro-driven separation, heavy metal smelting, or other electrochemical processes.

The rational design of ion exchange membranes (IEMs) is becoming more pertinent as their usage becomes broader and as their staple applications (i.e., electrodialysis, flow batteries, and fuel cells) improve in commercial viability. Such efforts would be catalyzed by an improved fundamental understanding of ion transport in IEMs. This review discusses recent progress in modeling ion partitioning and diffusion in IEMs in an effort to relate IEM performance metrics to fundamental membrane properties over which researchers and membrane manufacturers possess direct and sometimes precise control. Central focus is given to the Donnan‐Manning model for ion partitioning and the Manning‐Meares model for ion diffusion in IEMs. These two frameworks, which are derived from Manning's counter‐ion condensation theory for polyelectrolyte solutions, have been widely used within the IEM literature since their recent introduction. To explore this topic, the mathematical derivation of both models is revisited, followed by a survey of experimental and computational discussions of counter‐ion condensation in IEMs. Alternative models which fulfill similar roles in predicting IEM transport properties are compared. This review concludes by highlighting the uniquely favorable positions of the Donnan‐Manning and Manning‐Meares models and discussing their prospects as leading predictors of IEM partitioning and diffusive properties.

Impact of the Donnan electrolytes on selectivity of cation exchange membranes evaluated via the ionic membrane conductivity

This review focuses on charged polymer membranes motivated by their growing importance in membrane-based separation technologies. Charged polymers have a long history in ion exchange chromatography, and thus charged polymer membranes are commonly termed ion-exchange membranes (IEMs). IEMs can be used in energy-efficient reverse osmosis desalination and are being studied for recovering valuable minerals from aqueous waste streams. Types of IEMs are first introduced, categorized by charge type, charge distribution and porosity. Synthesis of charged polymers is briefly discussed. Considerable attention is given to important membrane properties and methods for characterizing them. These properties include ion-exchange capacity (IEC), water content, structure, ionic conductivity, permeability, selectivity, and thermal and mechanical properties. A key challenge in membrane design is achieving high IEC, which is desired for high IEM selectivity. This is a challenge due to the high water uptake that accompanies high IEC. Relevant aspects of membrane structure include percolated ion channels, porous morphology and inert mechanical reinforcement phases. Membrane structure is essential in addressing the challenge of achieving high IEC and optimizing membrane performance. Structure is predominantly dictated by membrane processing. Thus, membrane processing methods, their benefits and drawbacks and their impact on structure are described in detail. These methods include solution casting, the paste method, extrusion, electrospinning, phase inversion, and an emerging method to form a composite IEM. Finally, specific IEM applications are discussed that hold great promise for circular economies. These applications include lithium extraction from battery waste, mining of desalination brine, and mineral recovery from semiconductor waste. A major driver for the growing interest in these applications is the demonstrated cost-effectiveness of membranes in commercial desalination. With on-going research advances, such success is probable in these extraction and recovery applications.