More Like this

1. Suppressing vanadium crossover using sulfonated aromatic ion exchange membranes for high performance flow batteries

   https://doi.org/10.1039/D0MA00508H  

   Wang, Tongshuai; Han, Junyoung; Kim, Kihyun; Münchinger, Andreas; Gao, Yuechen; Farchi, Alain; Choe, Yoong-Kee; Kreuer, Klaus-Dieter; Bae, Chulsung; Kim, Sangil (October 2020, Materials Advances)

   null (Ed.)

   Liquid flow batteries have potential to achieve high energy efficiency as a large-scale energy storage system. However, the ion exchange membranes (IEMs) currently used in flow batteries do not have high ion selectivity and conductance at the same time, owing to the trade-off between ionic membrane resistance and ion selectivity. Here, we report a rationally designed sulfonated aromatic polymer membrane which can greatly mitigate the trade-off limitation and achieve high performance vanadium RFB. Small-angle X-ray scattering studies and density functional theory calculations indicated that the narrowly distributed aqueous ionic domain of just the right width (<7 Å) and the strong hydrogen bond interaction of vanadium species with a unique polymer side chain structure play a key role in improving the ion selectivity. Our systematic studies of the polymer structures, morphologies, and transport properties provide valuable insight that can aid in elucidating the structure–property relationship of IEMs and in establishing design criteria for the development of high-performance membranes. 

   more » « less
2. The effect of explicit counterion binding on the transference number of polyelectrolyte solutions

   https://doi.org/10.1063/5.0083414  

   Lytle, T. K.; Yethiraj, A. (March 2022, The Journal of Chemical Physics)

   Polyelectrolyte solutions have been proposed as a method to improve the efficiency of lithium-ion batteries by increasing the cation transference number because the polymer self-diffusion coefficient is much lower than that of the counterion. However, this is not necessarily true for the polymer mobility. In some cases, negative transference numbers have been reported, which implies that the lithium ions are transporting to the same electrode as the anion, behavior that is often attributed to a binding of counterions to the polyion. We use a simple model where we bind some counterions to the polymer via harmonic springs to investigate this phenomenon. We find that both the number of bound counterions and the strength of their binding alter the transference number, and, in some cases, the transference number is negative. We also investigate how the transference number depends on the Manning parameter, the ratio of the Bjerrum length to charge separation along the chain. By altering the Manning parameter, the transference number can almost be doubled, which suggests that charge spacing could be a way to increase the transference number of polyelectrolyte solutions. 

   more » « less
3. Forbidden ion transport through cation exchange membranes

   https://doi.org/10.1016/j.talanta.2024.126581  

   Chaudhary, Chandan K; Dasgupta, Purnendu K (November 2024, Talanta)

   Cation exchange membranes (CEMs) are widely used in many applications. The fixed anionic groups e.g., COO􀀀 , –SO3 - , etc. in the polymer matrix ideally allows the passage only of oppositely charged cations, driven by a potential or a concentration gradient. Anions, charged negative, the same as the membrane matrix, cannot pass through the membrane due to electrostatic repulsion. Such “Donnan-forbidden” passage can, however, occur to some degree, if the electrical or concentration gradient is high enough to overcome the “Donnan barrier”. Except for salt uptake/transport in concentrated salt solutions, the factors that govern such Forbidden Ion Transport (FIT) have rarely been studied. In most applications of transmembrane ion transport, whether electrically driven as in electrodialysis, or concentration-driven, it is the transport of the counterion to the fixed charged groups, such as that of the proton through a CEM, that is usually of interest. Nevertheless, CEMs are also of interest in analytical chemistry, specifically in suppressed ion chromatography. As used in membrane suppressors, both transport of permitted ions and rejection of forbidden ions are important. If the latter is indeed governed by electrostatic factors, other things being equal, the primary governing factor should be the charge density of the membrane, tantamount to its ion exchange capacity (IEC). In fabricating microscale suppressors, we found useful to synthesize a new ion exchange polymer that can be easily molded to make tubular microconduits. Despite a high IEC of this material, FIT was also found to be surprisingly high. We measured several relevant properties for thirteen commercial and four custom-made membranes to discover that while FIT is indeed linearly related to 1/ IEC for a significant number of these membranes, for very high water-content membranes, FIT may be overwhelmingly governed by the water content of the membrane. In addition, FIT through all CEMs differ greatly among strong acids, they may still be transported as the molecular acids and the extent is in the same order as the expected activity of the molecular acid in the CEM. These results are discussed with the perspective that even for strong acids, the transport does take place as un-ionized molecular acids. 

   more » « less
4. Combination of Counterion Size and Doping Concentration Determines the Electronic and Thermoelectric Properties of Semiconducting Polymers

   https://doi.org/10.1002/adma.202313863  

   Baustert, Kyle_N; Bombile, Joel_H; Rahman, Md_Tawabur; Yusuf, Augustine_O; Li, Ruipeng; Huckaba, Aron_J; Risko, Chad; Graham, Kenneth_R (May 2024, Advanced Materials)

   Abstract In both chemical and electrochemical doping of organic semiconductors (OSCs), a counterion, either from the electrolyte or ionized dopant, balances the charge introduced to the OSC. Despite the large influence of this counterion on OSC optical and electronic response, there remains substantial debate on how a fundamental parameter, ion size, impacts these properties. This work resolves much of this debate by accounting for two doping regimes. In the low‐doping regime, the Coulomb binding energies between charge carriers on the OSC and the counterions are significant, and larger counterions lead to decreased Coulomb interactions, more delocalized charge carriers, and higher electrical conductivities. In the high‐doping regime, the Coulomb binding energies become negligible due to the increased dielectric constant of the films and a smoothing of the energy landscape; thereby, the electrical conductivities depend primarily on the extent of morphological disorder in the OSC. Moreover, in regioregular poly(3‐hexylthiophene), rr‐P3HT, smaller counterions lead to greater bipolaron concentrations in the low‐doping regime due to the increased Coulomb interactions. Emphasizing the impact of the counterion size, it is shown that larger counterions can lead to increased thermoelectric power factors for rr‐P3HT. 

   more » « less
5. Molecular Dynamics Study of the Effect of Grafting Density on Ion Diffusivity in a MARTINI Coarse-Grained Strong Polyelectrolyte Brush

   https://doi.org/10.1021/acs.macromol.4c01018  

   Boyle, Michael J; Radhakrishnan, Ravi; Composto, Russell J (July 2024, Macromolecules)

   Because surface-grafted polyelectrolyte brushes (PEBs) are responsive to external stimuli, such as electric fields and ionic strength, PEBs are attractive for applications ranging from drug delivery to separations technologies. Essential to PEB utilization is understanding how critical parameters like grafting density (σ) impact PEB structure and the dynamics of the PEB and counterions. To study the effect of σ on PEB and counterion structure and dynamics, we fine-tune a coarse-grained model that retains the chemical specificity of a strong polyelectrolyte, poly[(2-(methacryloyloxy)ethyl) trimethylammonium chloride] (PMETAC), using the MARTINI forcefield. Using “salt-free” conditions where the counterion concentration balances the charge on the brush, we build coarse-grained (CG) molecular dynamics simulations for MARTINI PMETAC brushes (N=150 monomers; MW = 31.2 kg/mol) at experimentally relevant values of σ = 0.05, 0.10, 0.20, and 0.40 chains/nm2. Using 5 µs simulations, we investigate the effects of grafting density on PEB structure, ion dissociation dynamics, polymer mobility, and counterion diffusivity. Results show that competition between electrostatic interactions, steric hindrance, and polymer mobility controls counterion diffusivity. The interplay of these factors leads to diffusivity that depends non-monotonically on σ, with counterion diffusivity peaking at an intermediate σ = 0.10 chains/nm2. 

   more » « less