Membrane-electrolyte Interface Research Articles

The effect of electrostatically enhanced adsorption of polylysine on a heterogeneous cation-exchange membrane

Electrostatics influences the interactions between charged biopolymers and ion-exchange membranes. When carrying a charge opposite to that bound in the membrane, the biopolymers readily adsorb on the membrane surface, altering local conditions and, thus, affecting the desalination processes that proceed. To provide a coherent description of the consequences of such adsorption, we experimentally investigate the effects of polylysine as a model cationic polymer on the behavior of a heterogenous cation-exchange membrane. We show that even short exposure to polylysine changes the membrane behavior dramatically. Such a change reflects the influence of the adsorbed polylysine on natural convection, electroconvection, and water splitting. By directly observing the membrane-electrolyte interface, we find polylysine suppresses the beneficial electrical-field-induced convective transport at the membrane-electrolyte interface. Specifically, it diminishes electroconvection dominating at the membrane surface for polylysine-free solutions and attenuates natural convection, dictating the developed flow patterns. At the same time, it provides conditions enhancing a water-splitting reaction, significantly affecting the pH of the solutions adjacent to the membrane. All these effects are dependent on the polylysine concentration. Our study emphasizes the potentially harmful impact of biomolecular adsorption mediated by electrostatic interactions.

Evidence of transient potentials in ion-selective electrodes based on thin-layer ion-exchange membranes

Polymeric membranes with ion-exchange properties have found numerous applications in water treatment, dialysis, energy storage, chemical sensors, and bio-interfaces, among others. Notably, it is common to operate under non-equilibrium conditions while pursuing specific features (e.g., current generation) through an electron-to-ion mechanism. To maximize the final performance, it is crucial to understand the role of each interface within the system, which becomes complex when the device is tailored with several materials and films. This is the case for ion sensors based on thin membranes in backside contact with a redox active conducting polymer. Herein, we investigate such a system operating under a charge transfer mechanism, which features electroneutrality maintenance as the main driving force upon application of a linear sweep potential. This potential is modeled as being unequally distributed among the various system interfaces. Our results demonstrate and quantify the existence of a transient membrane potential at the membrane-electrolyte interface, owing to the implementation of a strategical measurement point on the buried membrane side and connected to a built-in electrometer for the exclusive acquisition of the potential difference at such an interface. The transient membrane potential was found to be <1 % of the total applied potential, meaning that the ion-transfer process at the electrolyte-membrane interface is less energetically costly that the electron transfer and doping processes occurring at the conducting polymer side. This small contribution can be potentiated by increasing the ion-exchange capacity of the membrane, which indirectly enlarges the system current and serves as a strategy for increasing the efficiency of the device.

Evaluating the Stability and Performance of Nasicon in Low-Cost High Charge Density Redox Flow Battery Electrolytes

Solid superionic conductor membranes are being considered as alternatives to polymer-based membranes for use in redox flow batteries (RFBs) due to their superior abilities to mitigate reactant crossover,1 and enable the deployment of aqueous electrolytes comprising small, inorganic earth-abundant reactants.2,3 Much however remains to be understood about the evolution of the electrochemical performance and microstructural stability of these membranes while they are immersed in aqueous electrolytes.In this work, we evaluate the suitability of von Alpen sodium superionic conductor (NaSICON) as a prospective RFB membrane material by examining its resistance, permeability and interfacial morphology over time as a function of electrolyte pH and composition, as well as temperature. NaSICON is found to have a stable resistance profile for several weeks while immersed in neutral to strongly alkaline ([OH-] = 3 M) aqueous electrolytes. Its permeability to polysulfide-based and permanganate-based small-molecule RFB reactants is negligible compared to that of Nafion. Its area-specific resistance falls dramatically with increasing temperature and decreasing membrane thickness; we project that a membrane with a thickness of 100 μm or lower, if operated slightly above ambient temperature (~ 40 °C), can enable power densities comparable to or better than those of conventional polymer membrane-containing RFBs. Nevertheless, the glassy phase of the NaSICON microstructure at the membrane-electrolyte interface was found to undergo small amounts of etching while in contact with aqueous electrolytes containing sodium ions; this etching became significantly more extensive when potassium ions were present in the electrolyte, leading in certain instances to complete disintegration of the membrane.The extraordinary high stability of NaSICON in strongly alkaline electrolyte permits the construction of flow cells containing a positive electrolyte based on permanganate, a high-potential, inexpensive reactant with high volumetric capacity (> 100 Ah/L). The flow cells had open-circuit voltages 1.2 V and greater, along with negligible reactant crossover and very low capacity fade (< 0.02 %/day). This work highlights the promise of ceramic membranes as effective separators in RFBs operating under neutral pH to strongly alkaline pH conditions. It also points to the need for further research on the long-term stability of the membrane and its interface with the electrolyte in solid-state membranes under investigation as separators in aqueous RFBs. (1) Yu, X.; Gross, M. M.; Wang, S.; Manthiram, A. Aqueous Electrochemical Energy Storage with a Mediator-Ion Solid Electrolyte. Advanced Energy Materials 2017, 7 (11), 1602454, https://doi.org/10.1002/aenm.201602454. DOI: https://doi.org/10.1002/aenm.201602454 (acccessed 2021/03/12).(2) Wei, X.; Xia, G.-G.; Kirby, B.; Thomsen, E.; Li, B.; Nie, Z.; Graff, G. G.; Liu, J.; Sprenkle, V.; Wang, W. An aqueous redox flow battery based on neutral alkali metal ferri/ferrocyanide and polysulfide electrolytes. J. Electrochem. Soc. 2016, 163 (1), A5150-A5153. DOI: 10.1149/2.0221601jes].(3) Colli, A. N.; Peljo, P.; Girault, H. H. High energy density MnO4-/MnO42- redox couple for alkaline redox flow batteries. Chem Commun (Camb) 2016. DOI: 10.1039/c6cc08070g.

(Digital Presentation) Evaluating Stability and Performance of Nasicon Membranes for Crossover Mitigation in Aqueous Redox-Flow Batteries

Most polymer-based membranes that are used in prototypical aqueous redox-flow batteries (RFBs) do not adequately prevent crossover of small-molecule reactants, causing high rates of capacity fade. Ceramic superionic conductor membranes are an attractive alternative due to their superior abilities to mitigate crossover;1 they can thus enable the deployment of electrolytes containing earth-abundant, small-molecule reactants.2 3 We test the performance and stability of a von Alpen sodium superionic conductor Na3.1Zr1.55Si2.3P0.7O11 (NaSICON) as an RFB membrane by examining its resistance, permeability and interfacial morphology as a function of electrolyte composition and temperature. The resistance of NaSICON is stable for several weeks while immersed in neutral to strongly alkaline ([OH-] = 3 M) aqueous electrolytes, and its permeability to polysulfide-based and permanganate-based small-molecule RFB reactants is negligible compared to that of Nafion. The glassy phase of the NaSICON microstructure at the membrane-electrolyte interface undergoes small amounts of etching while in contact with aqueous electrolytes containing sodium ions, which becomes more extensive when potassium ions are present in the electrolyte, leading in certain instances to complete disintegration of the membrane. We report a ferrocyanide-permanganate flow cell at a pH of 14.5 with a ~ 700 μm-thin NaSICON membrane supporting weeks of cycling with apparently negligible reactant crossover and very low capacity fade (< 0.04 %/day). Area-specific resistance of NaSICON falls dramatically with increasing temperature and decreasing membrane thickness, and a membrane that is 100 µm thick or thinner can enable power densities at above-ambient temperatures that are comparable to power densities of polymer membrane-containing flow cells.(1) Yu, X.; Gross, M. M.; Wang, S.; Manthiram, A. Aqueous Electrochemical Energy Storage with a Mediator-Ion Solid Electrolyte. Advanced Energy Materials 2017, 7 (11), 1602454, https://doi.org/10.1002/aenm.201602454. DOI: https://doi.org/10.1002/aenm.201602454 (acccessed 2021/03/12).(2) Wei, X.; Xia, G.-G.; Kirby, B.; Thomsen, E.; Li, B.; Nie, Z.; Graff, G. G.; Liu, J.; Sprenkle, V.; Wang, W. An aqueous redox flow battery based on neutral alkali metal ferri/ferrocyanide and polysulfide electrolytes. J. Electrochem. Soc. 2016, 163 (1), A5150-A5153. DOI: 10.1149/2.0221601jes].(3) Colli, A. N.; Peljo, P.; Girault, H. H. High energy density MnO4-/MnO42- redox couple for alkaline redox flow batteries. Chem Commun (Camb) 2016. DOI: 10.1039/c6cc08070g.

Correlating Stability and Performance of NaSICON Membranes for Aqueous Redox Flow Batteries.

Aqueous redox flow batteries (RFBs) are promising candidates for low-cost, grid-scale energy storage. However, the polymer-based membranes that are used in most prototypical systems fail to prevent crossover of small-molecule reactants, which results in high rates of capacity fade. In this work, we explore the feasibility of a von Alpen sodium superionic conductor Na3.1Zr1.55Si2.3P0.7O11 (NaSICON) as an RFB membrane by examining its resistance, permeability, and interfacial morphology as a function of electrolyte composition and temperature. The resistance of NaSICON is stable for several weeks while immersed in neutral to strongly alkaline ([OH-] = 3 M) aqueous electrolytes, and its permeability to polysulfide-based and permanganate-based small-molecule RFB reactants is negligible compared to that of Nafion. The glassy phase of the NaSICON microstructure at the membrane-electrolyte interface is susceptible to some etching while in contact with aqueous electrolytes containing sodium ions. This etching becomes more extensive when potassium ions are present in the electrolyte, leading in certain instances to complete disintegration of the membrane. A ∼0.7 mm-thin NaSICON membrane can nevertheless support over three weeks of cycling of a ferrocyanide|permanganate flow cell in a strongly alkaline electrolyte ([OH-] = 3 M), with apparently negligible reactant crossover and very low capacity fade (<0.04%/day). NaSICON's area-specific resistance also decreases dramatically with increasing temperature and decreasing membrane thickness; there is a 5.6× reduction from a 1.19 mm-thick membrane at 18 °C (101 Ωcm2) to a 0.61 mm-thick one at 70 °C (18 Ωcm2). Lowering the thickness of the membrane to 0.1 mm or lower will result in power densities at above ambient temperatures that are comparable to power densities of polymer membrane-containing flow cells. This work highlights the promise of ceramic membranes as effective separators in RFBs operating under neutral pH to strongly alkaline pH conditions.

Polarization-sensitive optoionic membranes from chiral plasmonic nanoparticles.

Optoelectronic effects differentiating absorption of right and left circularly polarized photons in thin films of chiral materials are typically prohibitively small for their direct photocurrent observation. Chiral metasurfaces increase the electronic sensitivity to circular polarization, but their out-of-plane architecture entails manufacturing and performance trade-offs. Here, we show that nanoporous thin films of chiral nanoparticles enable high sensitivity to circular polarization due to light-induced polarization-dependent ion accumulation at nanoparticle interfaces. Self-assembled multilayers of gold nanoparticles modified with L-phenylalanine generate a photocurrent under right-handed circularly polarized light as high as 2.41 times higher than under left-handed circularly polarized light. The strong plasmonic coupling between the multiple nanoparticles producing planar chiroplasmonic modes facilitates the ejection of electrons, whose entrapment at the membrane-electrolyte interface is promoted by a thick layer of enantiopure phenylalanine. Demonstrated detection of light ellipticity with equal sensitivity at all incident angles mimics phenomenological aspects of polarization vision in marine animals. The simplicity of self-assembly and sensitivity of polarization detection found in optoionic membranes opens the door to a family of miniaturized fluidic devices for chiral photonics.

Understanding Electrical Conduction and Nanopore Formation During Controlled Breakdown.

Controlled breakdown has recently emerged as a highly appealing technique to fabricate solid-state nanopores for a wide range of biosensing applications. This technique relies on applying an electric field of approximately 0.4-1 V nm-1 across the membrane to induce a current, and eventually, breakdown of the dielectric. Although previous studies have performed controlled breakdown under a range of different conditions, the mechanism of conduction and breakdown has not been fully explored. Here, electrical conduction and nanopore formation in SiNx membranes during controlled breakdown is studied. It is demonstrated that for Si-rich SiNx , oxidation reactions that occur at the membrane-electrolyte interface limit conduction across the dielectric. However, for stoichiometric Si3 N4 the effect of oxidation reactions becomes relatively small and conduction is predominately limited by charge transport across the dielectric. Several important implications resulting from understanding this process are provided which will aid in further developing controlled breakdown in the coming years, particularly for extending this technique to integrate nanopores with on-chip nanostructures.

Ion Transport in Laser-Induced Graphene Cation-Exchange Membrane Hybrids.

Ion-exchange membranes hybridized with laser-induced graphene (LIG) might lead to membranes with functional surface effects such as antifouling, antibacterial, or joule heating effects; however, understanding the change in the electrical properties of the membrane is essential. Here we studied LIG-modified ion-exchange polymeric membranes using electrochemical impedance spectroscopy (EIS). The conductivity of the anionic sulfonated poly(ether sulfone) membranes and the effective capacitance of the membrane-electrolyte interface were obtained by fitting the EIS spectra to an electrochemical equivalent circuit and compared with LIG-modified nonionic poly(ether sulfone) films. The transport selectivity (as the relative permeability) of counterions (K+, Na+, Mg2+, Ca2+) across the membrane was quantified using the membrane's conductivities obtained from the EIS measurements. The total ohmic resistance of the membrane was directly correlated to the polymer thickness (with negligible contribution from the conductive LIG layer), thereby establishing EIS as a rapid, low-cost, and noninvasive method to accurately probe substrate usage in LIG modification.

Effect of Plasma Membrane Semipermeability in Making the Membrane Electric Double Layer Capacitances Significant.

Electric double layers (or EDLs) formed at the membrane-electrolyte interface (MEI) and membrane-cytosol interface (MCI) of a charged lipid bilayer plasma membrane develop finitely large capacitances. However, these EDL capacitances are often much larger than the intrinsic capacitance of the membrane, and all of these capacitances are in series. Consequently, the effect of these EDL capacitances in dictating the overall membrane-EDL effective capacitance Ceff becomes negligible. In this paper, we challenge this conventional notion pertaining to the membrane-EDL capacitances. We demonstrate that, on the basis of the system parameters, the EDL capacitance for both the permeable and semipermeable membranes can be small enough to influence Ceff. For the semipermeable membranes, however, this lowering of the EDL capacitance can be much larger, ensuring a reduction of Ceff by more than 20-25%. Furthermore, for the semipermeable membranes, the reduction in Ceff is witnessed over a much larger range of system parameters. We attribute such an occurrence to the highly nonintuitive electrostatic potential distribution associated with the recently discovered phenomena of charge-inversion-like electrostatics and the attainment of a positive zeta potential at the MCI for charged semipermeable membranes. We anticipate that our findings will impact the quantification and the identification of a large number of biophysical phenomena that are probed by measuring the plasma membrane capacitance.

Positive zeta potential of a negatively charged semi-permeable plasma membrane

The negative charge of the plasma membrane (PM) severely affects the nature of moieties that may enter or leave the cells and controls a large number of ion-interaction-mediated intracellular and extracellular events. In this letter, we report our discovery of a most fascinating scenario, where one interface (e.g., membrane-cytosol interface) of the negatively charged PM shows a positive surface (or ζ) potential, while the other interface (e.g., membrane-electrolyte interface) still shows a negative ζ potential. Therefore, we encounter a completely unexpected situation where an interface (e.g., membrane-cytosol interface) that has a negative surface charge density demonstrates a positive ζ potential. We establish that the attainment of such a property by the membrane can be ascribed to an interplay of the nature of the membrane semi-permeability and the electrostatics of the electric double layer established on either side of the charged membrane. We anticipate that such a membrane property can lead to such capabilities of the cell (in terms of accepting or releasing certain kinds of moieties as well regulating cellular signaling) that was hitherto inconceivable.

Arsenic removal from water by hybrid electro-regenerated anion exchange resin/electrodialysis process

Abstract One of the major public health concerns worldwide is arsenic presence in the water intended for human consumption in different countries. This paper proposes the use of the hybrid ion exchange/electrodialysis (IXED) process for arsenic removal from water. Its objective is to lower the concentration of dilute solutions of As(V) ions to values below the maximum limit (10 µg L −1 As) recommended by the World Health Organization. To prove this method, a 5-compartment cell was used: a central compartment filled with anion exchange resin and delimited by anion exchange membranes, two adjacent compartments with cationic membranes on the ends and two rinse compartments, one at each electrode. Ion exchange/electrodialysis process was performed as a batch wise operation by recirculating arsenic solutions through the central compartment and the two adjacent compartments. The As(V) ions in the solution are exchanged by OH − in the resin which is regenerated with OH − ions produced by water dissociation at the membrane-electrolyte interface, according to changes in pH, conductivity and As concentration in each compartment. The obtained results show that IXED is capable of removing arsenic from solutions at different concentrations ranging between 2.1 and 15 mg L −1 As, until reaching the final concentration ≤10 μg L −1 with a current efficiency between 8.2 and 47.7% and energy consumption between 7.5 and 63.9 kW h per kg of removed As(V).

Improved operation of SO2 depolarized electrolyser stack for H2 production at ambient conditions

In this paper, the performance of a robust SO2 depolarized electrolyser (SDE) stack that involves neither Pt catalyst nor carbon compounds at ambient conditions is demonstrated. In particular, the effect of different anolyte flow configurations to the overall H2 production rate and SO2 carry-over phenomenon is presented. These results indicate that serpentine anolyte flow (U-configuration) provides higher SO2 conversion, however, results in a more severe SO2 carry-over phenomenon due to increased membrane-electrolyte interface. In addition, individual unit cells in the stacks are monitored for further understanding about the different operational environment in each cell and to provide guidelines for future SDE stack design.

Dose-Dependent Interaction of trans-Resveratrol with Biomembranes: Effects on Antioxidant Property

The present study investigates dose-dependent effects of trans-resveratrol on the membrane fluidity using planar lipid bilayer and liposome models. The complex admittance plots obtained for the lipid bilayer show that resveratrol below 60 μM preferentially interacts with the polar headgroups at the membrane-electrolyte interface, leading to enhanced membrane admittance and vice versa at higher concentrations (>60 μM). This was confirmed using solid-state (13)C and (31)P NMR studies and membrane fluidization studies. The localization of resveratrol in the membrane bilayer was found to alter the membrane rigidity, which resulted in a dose-dependent blebbing and lysis of erythrocytes. The protective effect of trans-resveratrol against DPPH also confirms that its localization in the hydrophobic region prevents lipid peroxidation. The cytotoxic effect of resveratrol on a breast cancer cell line also displays a progressive pattern, indicating possible correlation with its membrane rigidifying properties and localization in the lipid bilayer.

Impedance characteristics and electrical double-layer capacitance of composite polystyrene–cobalt–arsenate membrane

In continuation of our previous work with composite polystyrene–cobalt–arsenate (PS–Co–As), we further extended impedance measurements. All calculations reported were extracted from experiments carried out in the frequency range of 1–5kHz and different concentrations (0.0001≤c(M)≤1) of KCl and NaCl at isothermal temperature (25±0.1°C). The membrane capacitance and resistance measurements were observed to depend on the concentration and the applied frequency of the electrolyte. The observed capacitances and resistances were used to calculate the membrane resistances (RM), capacitance (CM), reactance (XX), and also derive the impedance (Z). At higher frequencies, the capacitances became low and the impedance decreased with increasing frequency with a corresponding increase in the measured phase angle. On the other hand at the highest frequencies attainable, the phase angle became low. At low frequencies, the phase angle was become independent of the cation, while the impedance showed a clear dependence. The diffused double-layer polarization charge on the geometric capacitor played important role by affecting the overall membrane capacitance. The applied frequencies affected the double-layer capacitance due to the movement of ions across the membrane. At the membrane–electrolyte interface, the electrical double-layer was influenced in addition to being controlled by the transport of ions.

Characterization of home-made silver sulphide based iodide selective electrode

Polycrystalline silver sulphide/silver iodide ion selective electrodes (ISEs) with four different compositions, 9:1, 2:1, 1:1, 1:9 Ag 2S–AgI mole ratios, have been fabricated in the laboratory and characterized by X-ray diffractometry (XRD), scanning electron microscopy (SEM), and electrochemical impedance spectroscopy (EIS). X-ray diffraction studies show the presence of Ag 3SI, Ag 2S and AgI crystalline phases in the electrode material. The electrode surfaces have been found to become smoother and lustrous with increasing percentage of silver sulphide in silver iodide. ISE 1:1, ISE 2:1 and ISE 9:1 all responded in Nernstian manner with slopes of about 60 mV/decade change in iodide ion concentration in the linear range of 1 × 10 −1 to 1 × 10 −6 M while ISE 1:9 showed sub-Nernstian behavior with slope of about 45 mV up to the concentration 1 × 10 −5 M. Two capacitive loops, one corresponding to the charge transfer process at metal electrode and the back contact and a second loop corresponding to the charge transfer process at membrane–electrolyte interface have been observed at high and low frequency ranges, respectively. Mott–Schottky analysis shows that the materials are n-type semiconductors with donor defect concentrations in the range of 5.1 × 10 14 to 2.4 × 10 19/cm 3.

The electrical state of NAFION coated electrodes emersed from liquid electrolytes

Electrodes were coated with a perfluorosulfonate polymer (NAFION). After equilibration in liquid electrolytes they were withdrawn from the electrolytes under potentiostatic control (“emersed”). The apparent work functions of the emersed coated electrodes were determined from measurements of Volta potential differences. They were conducted with Kelvin's method of the vibrating capacitor plate. The apparent work functions at constant emersion potential show the linear correlation with the logarithm of the solution concentration expected for the equilibrium potential drop across the membrane-electrolyte interface (the Donnan potential). From comparison with the apparent work function of the electrolyte, at the same potential, the surface dipole potential of the solvated polymer after the emersion is derived.

Interactions of divalent cations with negatively charged membrane surfaces. I. Discrete charge potential

The present paper is concerned with the mathematical description of the local potential produced by an arbitrary arrangement of polar groups in contact with an electrolyte solution containing divalent cations. The membrane-electrolyte interface is modeled, for that purpose, as a three regions—three dielectric constants system namely: a non-polar region of dielectric constant ϵ m , an adsorption or polar region of dielectric constant ϵ ⩾ ϵ m in which the polar head groups are presumed to be located, and an aqueous phase of dielectric constant ϵ 0 ⩾ ϵ. Using a mathematical procedure involving Fourier-Bessel integrals, a general expression for the local value of the surface potential is derived. For membrane systems of low surface charge densities, the proposed general solution is found to be mathematically equivalent to the expressions obtained from a linearized treatment of the discrete charge potential problem. It is shown moreover, that the space average value of the local surface potential at a given distance from the interface, corresponds formally to the surface potential produced by a uniform surface charge distribution. In addition, the present discrete charge theory is found to reproduce satisfactory Schaufs experimental observations on the effect of divalent cations on the Na + conductance of Myxicola giant axon (Schauf, 1975). A numerical analysis of several surface charge arrangements leads us finally to conclude, that for highly charged membranes, the local value of the surface potential at one point depends more upon the point macro than micro-environment. In fact, the effect on the local surface charge potential of a change in micro-environment is found to be extremely sensitive to the average value of the membrane surface charge density.

Contribution to the electrochemistry of bimolecular lipid membranes.

The electrical properties and the mechanism of the electrical conductivity of artificial bimolecular lipid membranes (BLM) have been investigated in the presence of iodine and iodide. It is shown that a model different from simple ion diffusion must be considered to account for the observations. On the basis of experimental results the following can be concluded: a) each membrane-electrolyte interface behaves as a semiconductor electrochemical electrode, b) the bulk membrane phase behaves as an electronic conductor, transferring the charge carriers generated by the electrode reactions, c) the membrane can also serve as a matrix for certain components of the redox system present in the bathing solutions.