Impact of Hydration and Sulfonation on the Morphology and Ionic Conductivity of Sulfonated Poly(phenylene) Proton Exchange Membranes

Nonaqueous polyelectrolyte solutions have been recently proposed as high Li + transference number electrolytes for lithium ion batteries. However, the atomistic phenomena governing ion diffusion and migration in polyelectrolytes are poorly understood, particularly in nonaqueous solvents. Here, the structural and transport properties of a model polyelectrolyte solution, poly(allyl glycidyl ether-lithium sulfonate) in dimethyl sulfoxide, are studied using all-atom molecular dynamics simulations. We find that the static structural analysis of Li + ion pairing is insufficient to fully explain the overall conductivity trend, necessitating a dynamic analysis of the diffusion mechanism, in which we observe a shift from largely vehicular transport to more structural diffusion as the Li + concentration increases. Furthermore, we demonstrate that despite the significantly higher diffusion coefficient of the lithium ion, the negatively charged polyion is responsible for the majority of the solution conductivity at all concentrations, corresponding to Li + transference numbers much lower than previously estimated experimentally. We quantify the ion–ion correlations unique to polyelectrolyte systems that are responsible for this surprising behavior. These results highlight the need to reconsider the approximations typically made for transport in polyelectrolyte solutions.

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“…58 Identification of these mechanisms has been shown in previous works to be crucial to fully understanding trends in ionic conductivity. 59,60…”

Section: Resultsmentioning

confidence: 99%

Ion Transport and the True Transference Number in Nonaqueous Polyelectrolyte Solutions for Lithium Ion Batteries

et al. 2019

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“…Pulsed-Field-Gradient (PFG) NMR Analysis: 1 H pulsed field gradient (PFG) NMR experiments were performed on a Bruker AVANCE III 600 instrument using a DIFF50 (40 A, maximum strength of 1800 G cm −1 ) z-gradient 5 mm water-cooled diffusion NMR probe at 1 H observed frequency of 600.13 MHz. The bipolar stimulated echo pulse sequence was used, with the translational self-diffusion coefficients (DT) evaluated from the variation in signal intensity with gradient strength I g using the Stejkal and Tanner relation [38] as implemented in the TOPSPIN…”

Section: Methodsmentioning

confidence: 99%

“…b) Comparison of proton conductivity data for IPC‐COF membrane (solid stars), COF materials (open diamonds) [ 26–29,31 ] and state‐of‐the‐art PEMs (open triangles). [ 2,9,10,37–39 ] c) Proton conductivity of IPC‐COF membrane and Nafion 212 versus RH at 40 °C.…”

Section: Figurementioning

confidence: 99%

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Weakly Humidity‐Dependent Proton‐Conducting COF Membranes

et al. 2020

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phase-separated copolymer membranes, often suffer from dramatically declined proton conductivity at low relative humidity (RH), [6-10] because the severe water loss under low RH destructs watermediated hydrogen-bonding networks. [8,11] For decades, great effort has been devoted to reduce the humidity-dependence of conductivity through preserving membrane hydration under low RH, such as incorporating hydrophilic groups [12,13] or hygroscopic porous nanofillers into membranes [7,14] to improve water retention properties, and installing nanovalves to retard water desorption of membranes. [2] However, the water retention remains a daunting challenge due to the "flexible polymer networks" nature of these PEMs. The amorphous, flexible ion nanochannels are prone to shrinking and even breaking upon dehydration, leading to poorly connected water channels and consequently exponential decline in conductivity and fuel cell performance. [6,15] Therefore, the design of electrolyte materials with weakly humidity-dependent proton-conducting feature remains a challenge. Materials with crystalline and rigid nanochannels are able to firmly retain water by capillary forces [16-20] and may promote membrane hydration, thereby facilitating proton transport efficiently over a wide range of humidity. Covalent organic frameworks (COFs), a class of crystalline porous polymers with pre-designable pore structures, hold grand promise. [21-23] Unlike amorphous, flexible crosslinked networks, the crystalline, rigid organic frameworks of COFs are expected to afford well-defined and stable proton-conducting nanochannels, [24-30] as well as good water retention capability. [18] However, leveraging COF materials in the field of COF-based PEMs for practical application remains a great challenge arising from the poor processing ability of COF materials and the poor structural integrity of COF membranes. Moreover, the proton transport mechanism in crystalline and rigid nanochannels of COF membranes remains elusive. Herein, we report a diffusion and solvent co-mediated modulation strategy for the direct synthesis of highly crystalline IPC-COF nanosheets (NUS-9) that can be readily processed into defect-free, robust IPC-COF membranes by subsequent vacuum-assisted self-assembly. These IPC-COF membranes exhibit weakly humidity-dependent conductivity and fuel cell performance over a wide range of humidity (30-98%). We further demonstrate that the crystalline and rigid ion nanochannels render the IPC-COF membranes exceptional State-of-the-art proton exchange membranes (PEMs) often suffer from significantly reduced conductivity under low relative humidity, hampering their efficient application in fuel cells. Covalent organic frameworks (COFs) with pre-designable and well-defined structures hold promise to cope with the above challenge. However, fabricating defect-free, robust COF membranes proves an extremely difficult task due to the poor processability of COF materials. Herein, a bottom-up approach is developed to synthesize intrinsic proton-conducting COF...

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“…53,59 Previous research on similar materials has found that proton conductivity at high degrees of acid functionalization and high hydration number, such as the materials discussed in this work, stems from a combination of the Grotthuss (structural diffusion) and vehicular mechanisms of proton transport. 60 The relationship between proton conductivity and proton mobility is highlighted in Eqn (4), 53,59 where ⊞ H+ is the proton conductivity, F is Faraday's constant, [SO 3 H] is the analytical acid concentration www.soci.org N Peressin, M Adamski, S Holdcroft wileyonlinelibrary.com/journal/pi within a swollen membrane (as per Eqn (5)), and ⊘ 0 H+ is the effective proton mobility. The consequence of this relationship is somewhat contradictory: an increase in water sorption (water content) within a membrane may result in increased proton conductivity due to the presence of more bulk-like water, and yet decreased analytical acid concentration, which in turn lowers effective proton mobility and proton conductivity.…”

Section: Mechanical Propertiesmentioning

confidence: 99%

Effect of steric constraints on the physico‐electrochemical properties of sulfonated polyaromatic copolymers

Peressin

Adamski

Holdcroft

2020

Polymer International

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The impact of incorporating additional steric restrictions into highly sterically encumbered sulfonated polyaromatic polymers was investigated. Copolymers possessing between 0 and 10% nonlinear ortho or meta biphenyl units in an otherwise linear para biphenyl-containing sulfo-phenylated poly(phenylene) were synthesized in yields >80% and evaluated on the basis of their physical and electrochemical properties. When incorporated into sulfonated copolymers in ≤5 mol%, ortho and meta linked biphenyl moieties reduced membrane swelling in water by up to 23 and 19 vol%, respectively, compared to strictly para biphenyl-linked copolymers. Despite this, copolymers possessing nonlinear, biphenyl-linked monomers displayed a decrease in proton conductivity and mechanical strength. This study reinforces the importance of considering restricted rotation, backbone flexibility, and chain entanglement in the design of polymers aimed at improving their physical and electrochemical properties.

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Impact of Hydration and Sulfonation on the Morphology and Ionic Conductivity of Sulfonated Poly(phenylene) Proton Exchange Membranes

Cited by 65 publications

References 102 publications

Ion Transport and the True Transference Number in Nonaqueous Polyelectrolyte Solutions for Lithium Ion Batteries

Ion Transport and the True Transference Number in Nonaqueous Polyelectrolyte Solutions for Lithium Ion Batteries

Weakly Humidity‐Dependent Proton‐Conducting COF Membranes

Effect of steric constraints on the physico‐electrochemical properties of sulfonated polyaromatic copolymers

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