We thoroughly investigate and quantify the chemical stability of an imidazolium-based alkaline anion exchange polymerized ionic liquid (PIL), poly(1-(2-methacryloyloxy)ethyl-3-butylimidazolium ...hydroxide) (poly(MEBIm-OH), over a broad range of humidities, temperatures, and alkaline concentrations using the combined techniques of electrochemical impedance spectroscopy and nuclear magnetic resonance spectroscopy. High chemical stability was observed under dry conditions (10% RH) at 30 °C, humid and saturated conditions up to 80 °C, and even in mild alkaline conditions (KOH < 1 M) at 25 °C. Degradation was only observed under more vigorous conditions: dry conditions (10% RH) at 80 °C or at higher alkaline concentrations (KOH > 1 M). Under these conditions, we suggest an imidazolium ring-opening mechanism as the primary degradation pathway, based on a detailed analysis of the 1H NMR spectra. Similar to poly(MEBIm-OH), other alkaline anion (carbonate (CO3 2–) and bicarbonate (HCO3 –)) exchange PILs were also synthesized in this study via salt metathesis of the PIL precursor, poly(1-(2-methacryloyloxy)ethyl-3-butylimidazolium bromide) (poly(MEBIm-Br)). The thermal and ion conductive properties of each PIL in this study were characterized. The ionic conductivity of the hydroxide conducting PIL, poly(MEBIm-OH), was the highest of these PILs investigated at 9.6 mS cm–1 at 90% RH and 30 °C with an Arrhenius activation energy of 17.1 kJ mol–1 at 90% RH.
In this study, we investigate the isolated effect of anion type on the chemical, thermal, and conductive properties of imidazolium-based polymerized ionic liquids (PILs). PILs with various anions at ...constant average chain length were prepared by ion exchange with a water-soluble PIL precursor, (poly(1-(2-methacryloyloxy)ethyl-3-butylimidazolium bromide) (poly(MEBIm–Br)). NMR, IR, and elemental analysis confirm that anion exchange of ploy(MEBIm–Br) with bis(trifluoromethanesulfonyl) imide (TFSI), tetrafluoroborate (BF
4), trifluoromethanesulfonate (Tf), and hexafluorophosphate (PF
6) in water resulted in nearly fully exchanged PILs. As a function of anion type, the glass transition temperature plays a dominant role, but not the sole role in determining ion conductivity. Other factors affecting ionic conductivity include the size and symmetry of the anion and dissociation energy of the ion pair. Both the Vogel-Fulcher-Tammann (VFT) and Williams-Landel-Ferry (WLF) equations were employed to investigate the temperature dependent ionic conductivities. The
C
1
g
(9.03) and
C
2
g
(168 K) values obtained from the WLF regression of these PILs greatly deviate from the classical WLF values originally obtained from the mechanical relaxation of uncharged polymers (
C
1
g
= 17.44,
C
2
g
= 51.6 K) and the WLF values obtained from the conductive properties of other polymer electrolytes. This suggests that the fractional free volume (
f (
T
g
) = B/(2.303
C
1
g
)) and Vogel temperature (
T
0 =
T
g
−
C
2
g
) are strong functions of ion concentration, where high free volume allows for ion mobility at temperatures farther below the glass transition temperature of the polymer.
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Electrochemical double layer capacitors (EDLCs), or supercapacitors, rely on electrosorption of ions by porous carbon electrodes and offer a higher power and a longer cyclic lifetime compared to ...batteries. Ionic liquid (IL) electrolytes can broaden the operating voltage window and increase the energy density of EDLCs. Herein, we present direct measurements of the ion dynamics of 1-ethyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide in an operating EDLC with electrodes composed of porous nanosized carbide-derived carbons (CDCs) and nonporous onion-like carbons (OLCs) with the use of in situ infrared spectroelectrochemistry. For CDC electrodes, IL ions (both cations and anions) were directly observed entering and exiting CDC nanopores during charging and discharging of the EDLC. Conversely, for OLC electrodes, IL ions were observed in close proximity to the OLC surface without any change in the bulk electrolyte concentration during charging and discharging of the EDLC. This provides experimental evidence that charge is stored on the surface of OLCs in OLC EDLCs without long-range ion transport through the bulk electrode. In addition, for CDC EDLCs with mixed electrolytes of IL and propylene carbonate (PC), the IL ions were observed entering and exiting CDC nanopores, while PC entrance into the nanopores was IL concentration dependent. This work provides direct experimental confirmation of EDLC charging mechanisms that previously were restricted to computational simulations and theories. The experimental measurements presented here also provide deep insights into the molecular level transport of IL ions in EDLC electrodes that will impact the design of the electrode materials’ structure for electrical energy storage.
Block Copolymers for Fuel Cells Elabd, Yossef A; Hickner, Michael A
Macromolecules,
01/2011, Letnik:
44, Številka:
1
Journal Article
Recenzirano
Ion-containing block copolymers hold promise as next-generation proton exchange membranes in hydrogen and methanol fuel cells. These materials’ self-assembled ordered nanostructures facilitate proton ...transport over a wide range of conditions, a requirement for robust fuel cell performance. In this perspective, we will present an overview of the morphology and transport properties of ion-containing block copolymers that have been studied to gain insight into the fundamental behavior of these materials and, in some cases, are targeted toward applications in fuel cells and other electrochemical devices. We will discuss the challenges associated with predicting and obtaining well-ordered morphologies in block copolymers with high ion content, particularly those with chemistries that can withstand the chemical and mechanical stresses of the fuel cell, such as aromatic backbone block copolymers. New opportunities for ion-containing block copolymers in alkaline membrane fuel cells will also be reviewed.
A series of strongly microphase-separated polymerized ionic liquid (PIL) diblock copolymers, poly(styrene-b-1-((2-acryloyloxy)ethyl)-3-butylimidazolium bis(trifluoromethanesulfonyl)imide) ...(poly(S-b-AEBIm-TFSI)), were synthesized to explore relationships between morphology and ionic conductivity. Using small-angle X-ray scattering and transmission electron microscopy, a variety of self-assembled nanostructures including hexagonally packed cylinders, lamellae, and coexisting lamellae and network morphologies were observed by varying PIL composition (6.6–23.6 PIL mol %). At comparable PIL composition, this acrylate-based PIL block copolymer with strong microphase separation exhibited ∼1.5–2 orders of magnitude higher ionic conductivity than a methacrylate-based PIL block copolymer with weak microphase separation. Remarkably, we achieved high ionic conductivity (0.88 mS cm–1 at 150 °C) and a morphology factor (normalized ionic conductivity, f) of ∼1 through the morphological transition from lamellar to a coexistence of lamellar and three-dimensional network morphologies with increasing PIL composition in anhydrous single-ion conducting PIL block copolymers, which highlights a good agreement with the model predictions. In addition to strong microphase separation and the connectivity of conducting microdomains, the orientation of conducting microdomains and the compatibility between polymer backbone and IL moiety of PIL also significantly affect the ionic conductivity. This study provides avenues to controlling the extent of microphase separation, morphology, and ion transport properties in PIL block copolymers for energy conversion and storage applications.
Previously, nanofiber-nanoparticle electrodes produced via a simultaneous electrospinning and electrospraying (E/E) process (E/E electrodes) resulted in polymer electrolyte membrane fuel cells with ...high power densities at ultra-low platinum (Pt) loadings (<0.1 mgPt cm−2). In this study, E/E electrodes were fabricated at various Nafion contents to investigate the impact of ionomer content on catalyst layer transport resistances and fuel cell power density at ultra-low Pt loadings. Regardless of the Nafion content in the electrospray, the Nafion nanofiber diameters and catalyst aggregate particle sizes are constant in the E/E electrodes evidenced by electron microscopy. Therefore, this study allows for the exclusive investigation of the effect of transport resistances on fuel cell performances at different ionomer contents at a constant catalyst layer morphology, which differs from conventional electrodes. At higher magnifications, changes are evident in the micrographs around the catalyst aggregate particles, where an increase in ionomer thin film thickness is observed with increasing ionomer content. The maximum fuel cell performance and a minimum in catalyst layer resistance for E/E electrodes is observed at a total Nafion content of 62 wt%, which differs from conventional electrodes (ca. 30 wt%).
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•Nanofiber-nanoparticle electrodes via simultaneous electrospinning/electrospraying.•High fuel cell power densities at ultra-low platinum loadings.•Ability to maintain similar catalyst layer morphology at different Nafion contents.•Exclusive investigation of transport resistances at different Nafion contents.•Optimum Nafion content at 62 wt percent.
A series of polymerized ionic liquid (PIL) block and random copolymers were synthesized from an ionic liquid monomer, 1-(2-methacryloyloxy)ethyl-3-butylimidazolium bis(trifluoromethanesulfonyl)imide ...(MEBIm-TFSI), and a nonionic monomer, methyl methacrylate (MMA), at various PIL compositions with the goal of understanding the influence of morphology on ion transport. For the diblock copolymers, the partial affinity between the PIL and PMMA blocks resulted in a weakly microphase-separated morphology with no evident long-range periodic structure across the PIL composition range studied, while the random copolymers revealed no microphase separation. These morphologies were identified with a combination of techniques, including differential scanning calorimetry, small-angle X-ray scattering, and transmission electron microscopy. Surprisingly, at similar PIL compositions, the ionic conductivity of the block copolymers were ca. 2 orders of magnitude higher than the random copolymers despite the weak microphase-separated morphology evidenced in the block copolymers. We attribute the higher conductivity in the block copolymers to its microphase-separated morphology, since significant differences in conductivity are still observed even when differences in glass transition temperature are considered. This work demonstrates that local confinement and connectivity of conducting ions in nanoscale ionic domains in PIL block copolymers can accelerate ion transport significantly.
Herein, we present the synthesis of five styrene-based poly(ionic liquids) (PILs) containing (covalently linked) saturated N-heterocyclic cations with various ring sizes (i.e., methylpyrrolidinium, ...methylpiperidinium, methylazepanium, methylazocanium, and methylazonanium). High alkaline chemical stability was confirmed by 1H NMR spectroscopy after 4 weeks in 40 mol equiv of KOH (1.0 M KOH in D2O) at 80 °C for PILs with 5-, 6-, 7-, and 8-membered ring cations; a requirement for polymer electrolyte separators in long-lasting alkaline fuel cells. Additionally, ion conductivity of PILs increased by 4 orders of magnitude with increasing water content, where a master percolation power law curve was observed, that is, similar conductivity versus water volume fraction for all PILs, regardless of cation size.
Polymerized ionic liquid (POIL) block copolymers represent a unique class of materials for fundamental studies of single ion conduction as a function of morphology in microphase-separated polymer ...electrolytes for energy storage and conversion applications. We describe the synthesis of a series of poly(styrene-b-4-vinylbenzylalkylimidazolium bis(trifluoromethanesulfonyl)imide) (PS-b-PVBn(alkyl)ImTFSI; alkyl = CH3 (Me), n-C4H9 (Bu), n-C6H13 (Hex)) diblock copolymers (2.7–17.0 mol % POIL) via exhaustive functionalization and ion exchange of relatively narrow molecular weight dispersity poly(styrene-b-4-vinylbenzyl chloride) precursors derived from nitroxide-mediated block copolymerizations. The solid-state morphology of these PS-b-PVBn(alkyl)ImTFSI copolymers were studied using a combination of temperature-dependent synchrotron small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). From electrochemical impedance spectroscopy measurements, we observe that lamellar samples having similar compositions exhibit comparable values of conductivity (0.1 mS cm–1 at 150 °C) regardless of imidazolium alkyl substituent. The ionic conductivity of a compositionally varied series of PS-b-PVBnHexImTFSI diblocks depends nonlinearly on POIL composition (0.01 mS cm–1 for 8.6 mol % POIL and 0.1 mS cm–1 for 17.0 mol % POIL at 150 °C), thus highlighting the influence of morphology on the observed ionic conductivity of POIL block copolymers for the first time. By using different polymer processing strategies, we further demonstrate that the ionic conductivity of a single sample (8.6 mol % POIL) may vary by more than one order of magnitude depending on the long-range ordering of the microphase separated morphology. These studies indicate that macroscopic connectivity and morphological defects strongly affect the observed conductivity in these materials.
