Effective Proton Mobility Research Articles

Evaluation of properties and performance of poly(ether sulfone ketone) membranes in proton exchange membrane fuel cells

AbstractTwo series of sulfonated poly(ether sulfone ketone)s have been synthesized. The effect of different ratios of sulfone: ketone functional groups on the properties and performance of membranes is studied. The density, ion‐exchange capacity, water absorption, swelling ratio, proton conductivity, thermal and oxidative stability of membranes are determined. The results showed that membranes with higher contents of sulfone functional groups show lower density and consequently higher water absorption and swelling ratio values. A precise evaluation of the effect of different sulfone: ketone functional group ratios on the analytical acid concentration and the effective proton mobility and their relationship with water absorption have been performed. The proton conductivity values are between 71–78 mS.cm−1 at 30°C and 117–208 mS.cm−1 at 80°C. The effective proton mobilities at 80°C are in the range of 10.27 × 10 −4 to 16.68 × 10 −4 cm2.V−1.s−1. The highest current density for prepared membranes in H2/O2 single cell at 100% relative humidity and 80°C is 1900 mA.cm−2.

Multi-component organic/inorganic blend proton exchange membranes based on sulfonated poly(arylene ether sulfone)s for fuel cells

To enhance the dimensional and oxidative stability of highly sulfonated aromatic hydrocarbon polymer membrane is currently focused to replace expensive perfluorosulfonic acid membrane. Polyethersulfone (PES) and/or sulfonated TiO2 (sTiO2) nanoparticles as additives were incorporated into sulfonated poly(aryl ether sulfone) (SPAES) blend membrane to balance the proton conductivity and water swelling. The blend polymer matrix contained three SPAES copolymers with different degrees of sulfonation in a weight ratio of 1:2:2 and modified blend membranes were casted and characterized using spectroscopy, thermal and mechanical stability and electrical characterization techniques. PES acted as a compatibilizer and reinforcer, which enhanced the dimensional and mechanical stability, while sTiO2 improved the effective proton mobility and proton conductivity of the modified blend membranes. Considerably, to improve physicochemical stability and fuel cell performance were achieved in the blend system, especially B-SPAES/sTiO2 and B-SPAES/PES/sTiO2, which exhibited good fuel cell performance during short-term operation. They displayed a maximum power output of 667 and 620 mW cm−2, respectively at 80 °C and 100% RH. These results show that SPAES blend membranes made from multi-component blend system have good application prospects in fuel cells.

Considerations of the Effects of Naphthalene Moieties on the Design of Proton-Conductive Poly(arylene ether ketone) Membranes for Direct Methanol Fuel Cells

Novel sulfonated poly(arylene ether ketones) (SDN-PAEK-x), consisting of dual naphthalene and flexible sulfoalkyl groups, were prepared via polycondensation, demethylation, and sulfobutylation grafting reaction. Among them, SDN-PAEK-1.94 membrane with the highest ion exchange capacity (IEC = 2.46 mequiv·g(-1)) exhibited the highest proton conductivity, which was 0.147 S· cm(-1) at 25 °C and 0.271 S·cm(-1) at 80 °C, respectively. The introduction of dual naphthalene moieties is expected to achieve much enhanced properties compared to those of sulfonated poly(arylene ether ketones) (SNPAEK-x), consisting of single naphthalene and flexible sulfoalkyl groups. Compared with SNPAEK-1.60 with a similar IEC, SDN-PAEK-1.74 membrane showed higher proton conductivity, higher IEC normalized conductivity, and higher effective proton mobility, although it had lower analytical acid concentration. The SDN-PAEK-x membranes with IECs higher than 1.96 mequiv·g(-1) also exhibited higher proton conductivity than that of recast Nafion membrane. Furthermore, SDN-PAEK-1.94 displayed a better single cell performance with a maximum power density of 60 mW·cm(-2) at 80 °C. Considering its high proton conductivity, excellent single cell performance, good mechanical stabilities, low membrane swelling, and methanol permeability, SDN-PAEK-x membranes are promising candidates as alternative polymer electrolyte membranes to Nafion for direct methanol fuel cell applications.

Poly(aryl ether ketone) containing flexible tetra-sulfonated side chains as proton exchange membranes

A new difluoride monomer containing electron-rich tetraphenylmethane was designed and synthesized. Based on this monomer along with 4,4′-(hexafluoroisopropylidene)diphenol (6FBPA) and 4,4′-difluorobenzophenone (DFB), a series of tetra-sulfonated poly(aryl ether ketone)s (TS-PAEK-x) were prepared by nucleophilic polycondensation, followed by a sulfonation reaction using chlorosulfonic acid. Tough, flexible, and transparent membranes were obtained by solvent casing. These membranes with ion exchange capacity (IEC) values ranging from 0.85 to 1.43 meq. g−1 exhibited good mechanical properties, excellent dimensional stability, and suitable proton conductivity. The largest swelling ratio (in-plane direction) with a value of 10.3% was observed from TS-PAEK-25 (IEC = 1.43 meq. g−1) membranes at 100 °C, which was much lower than that of Nafion 117 under the same conditions. The highest proton conductivity of 151 mS cm−1 was obtained from the TS-PAEK-25 membrane at 100 °C in the fully hydrated state. Compared to Nafion 117, TS-PAEK-25 with comparable water content exhibited a superior effective proton mobility (μ) value (up to 1.19 × 10−3 cm2 s−1 V−1). Under reduced humidity conditions, TS-PAEK-25 showed desired conductivity considering its lower IEC level. The results indicate that the TS-PAEK-x membranes are promising candidates for application as proton exchange membranes.

Theoretical calculation of proton mobility for collective surface proton transport

We present a theoretical study of surface proton mobility at a minimally hydrated array of protogenic surface groups. At dense packing, the array assembles into a 2D bicomponent lattice that is formed by sulfonate anions, which are only allowed to fluctuate about fixed equilibrium positions, and mobile hydronium ions. Proton transport on the lattice proceeds by collective translocations of hydronium ions. This type of motion is described within the framework of soliton theory. Our main objective in this article is to establish the relation between microscopic surface structure and effective proton mobility. To this end, we present an approach to calculate microscopic interaction parameters that determine hydronium ion motion. The developed formalism enables us to theoretically derive an expression for soliton mobility at a given surface structure and compare it with experimentally measured mobilities.

The importance of water transport on short-side chain perfluorosulfonic acid membrane fuel cells operating under low relative humidity

Polarization curves of fuel cells incorporating PFSA short-side chain (SSC) ionomer membranes having ion exchange capacity (IEC) 1.30, 1.37, 1.43 and 1.50 meq g−1 and NRE-211 are compared. Under low humidity conditions, fuel cells incorporating SSC membranes show higher performance than NRE-211. SSC PFSA membranes possessing an IEC of 1.37 meq g−1 exhibit the highest performance. Differences in fuel cell polarization curves are due to differences in the high frequency resistance, which in turn is found related to ex-situ measurements of both the effective proton mobility and the rate of water flux through the membrane, which also exhibit a maximum for membranes of IEC 1.37 meq g−1. Water permeation and proton mobility are shown to be inherently linked, but it is found that simply increasing the membrane's IEC does not necessarily translate to increased water transport and effective proton mobility, despite the increased water content.

Proton Mobility in Hydrated Acidic Polymers: Consequences for Optimization of Proton Conductivity

Proton conductivity and water uptake of hydrated acidic polymers can be combined to calculate the effective proton mobility u(H+) in the acidic solutions inside ionomers. The quantitative analysis shows common features for various ionomers, which can be related to percolation and tortuosity. The decrease of the proton mobility with increasing proton concentration c(H+) can be described by a power law above the percolation threshold of hydrated nanometric channels.

Sulfonated aromatic ionomers: Analysis of proton conductivity and proton mobility

Recent progress on proton-conducting Sulfonated Aromatic Polymers is reviewed, including composite and cross-linked ionomers. Proton conductivity and water uptake, studied as function of relative humidity, can be combined to calculate the effective proton mobility in ionomers. The plot of proton mobility versus proton concentration shows common features for various ionomers, which can be related to percolation and tortuosity of hydrated nanometric channels. Proton conductivity can even be enhanced in cross-linked ionomers, possibly due to a reduction of channel tortuosity.

Water, proton, and oxygen transport in high IEC, short side chain PFSA ionomer membranes: consequences of a frustrated network

The effect of ion exchange capacity (IEC) on the water sorption properties of high IEC, short side chain (SSC) PFSA ionomer membranes, and the relationships between water content, proton conductivity, proton mobility, water permeation, oxygen diffusion, and oxygen permeation are investigated. SSC PFSA ionomer membranes possessing 1.3, 1.4, and 1.5 mmol g(-1) IEC are compared to a series of long side chain (LSC) PFSA ionomer membranes ranging in IEC from 0.9 to 1.13 mmol g(-1). At 25 °C, fully-hydrated SSC ionomer membranes are characterized as possessing higher water contents (56-75 vol%), moderate λ values (15-18), high analytical acid concentrations (2-2.8 M), and moderate conductivity (88-115 mS/cm); but lower than anticipated effective proton mobility. Complementary measurements of water permeability, oxygen diffusion, and oxygen permeability also yield lower than expected values given their much higher water contents. Potential benefits afforded by reducing the side chain length of PFSA ionomer membranes, such as increased crystallinity, higher IEC, and high hydrated acid concentration are offset by a less-developed, frustrated hydrophilic percolation network, which provides a motivation for future improvements of transport properties for this class of material.

Nanoporous and proton conductive hydrophobic-hydrophilic copolymer thermoset membranes

Copolymers of hydrophobic diglycidyl ether of bisphenol A (DGEBA) vinyl ester (VE) and hydrophilic 2-acrylamido 2-methyl 1-propane sulfonic acid (AMPS) were evaluated as proton conducting membranes for fuel cell applications. Membranes were synthesized using free radical copolymerization in the presence of a common solvent for both monomers, dimethyl formamide (DMF), followed by solvent removal by supercritical CO2 to induce porosity. Micrographs revealed pore sizes below 60 nm with porosity proportional to the initial solvent fraction used. Studies on the states of water showed that the presence of this pore volume significantly altered the freezable water fraction at equivalent AMPS concentrations. Comparison of the moles of water per mole of sulfonic acid (λ) between copolymer membranes and AMPS monomer solutions showed that the nonfreezable water (λ|nonfr) was depressed at high AMPS concentrations, suggesting that differences in interatomic distances between sulfonic acid groups might alter λ|nonfr. The highest average through plane conductivity of membranes was determined to be 30 mS/cm and was comparable to that of Nafion®117 (27 mS/cm). The effective proton mobility, μeff, was calculated and suggested to be a parameter used to capture the effects of membrane structure and swelling while acting as a comparison between different membrane types. Fuel cell tests on membranes at low ion exchange capacities were compared to Nafion®117, with suggestions on improvements of copolymer structures for improved performance. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1245–1255, 2010

Transport properties of composite membranes containing silicon dioxide and Nafion ®

Silicon dioxide (SiO 2) nanoparticles were incorporated into Nafion 115 membranes using the sol–gel method in order to investigate their effect on water retention/transport, proton concentration, effective proton mobility, and proton conductivity. By adjusting the sol–gel reaction time, Nafion/SiO 2 membranes were fabricated with SiO 2 content ranging from 5.9 to 33.3 wt%. Because the density of the membranes decreased with increasing SiO 2 content and because dimensional changes with swelling in water of the composite membranes were less than that of unmodified Nafion 115 despite having increased water content, the theory that rigid scaffolding is formed inside the membrane is supported. Water content increases with increasing SiO 2 content due to void space formed inside the membrane. This increase in water content dilutes the protons in the membrane leading to lower proton concentration and therefore lower proton conductivity. A decreasing effective proton mobility with increasing SiO 2 content, likely due to an increase in the tortuosity of the proton-conducting pathway, also contributes to the decreasing conductivity. However, as evidenced by the similar water vapour permeance values, the SiO 2 nanoparticles do not increase the effective tortuosity of the water vapour transmission pathways.

Relationships of Acid and Water Content to Proton Transport in Statistically Sulfonated Proton Exchange Membranes: Variation of Water Content Via Control of Relative Humidity

An in-depth analysis for proton exchange membranes to examine the effects of acid concentration and effective proton mobility upon proton conductivity as well as their relationship to water content was carried out on two main-chain, statistically sulfonated polymers at 25 degrees C. These polymer systems consisted of poly(ethylenetetrafluoroethylene-graft-polystyrenesulfonic acid) (1) and sulfonated trifluorostyrene (BAM) membrane (2). Nafion (3) was used for comparison. Water content (as represented by Xv, the water volume fraction, where Xv = volume of water in hydrated PEM / volume of hydrated PEM), for each sample was varied by adjusting the relative humidity (RH) of the membrane environment from 50% to 98%. It was found that, at low RH (RH < 70%), the major factor determining proton conductivity is proton mobility. In order to remove the differences in acid strength for the membranes, proton mobility values at infinite dilution (Xv = 1.0) and 25 degrees C were calculated and found to be 2.6 +/- 0.2 x 10-3 (average of 1a-c), 1.6 +/- 0.3 x 10-3 (average of 2a-e), and 2.32 +/- 0.01 x 10-3 cm2 s-1 V-1 (3). These were then compared to the theoretical value for the mobility of a free proton at infinite dilution and to previously reported data. Possible differences in tortuosity and the juxtaposition of acid groups are proposed in order to account for the significant deviations of all samples from the theoretical value.

The effect of spatial confinement of Nafion ® in porous membranes on macroscopic properties of the membrane

Polyelectrolytes were incorporated into porous reinforcing materials to study the properties of ionomers in confined spaces and to determine the effect of the porous material on the behaviour of the membranes. Nafion ® was imbibed into porous polypropylene (Celgard ®), ultra-high-molecular weight polyethylene (Daramic ®), and polytetrafluoroethylene (PTFE) films. Through the use of reinforcing materials, it is possible to prepare membranes that are thinner, but stronger than pure ionomer membranes. Thin reinforced membranes have advantages such as lower areal resistance (as low as 0.14 Ω cm 2 for 57 μm CG3501 + Nafion ® compared to 0.34 Ω cm 2 for 89 μm cast Nafion ®) and lower dimensional changes due to swelling (as low as a 4% change in length and width for WDM + Nafion ® compared to 13% for cast Nafion ®). Using reinforcing materials results in a reduction in important membrane properties compared to bulk Nafion ®, such as proton conductivity (as low as 0.016 S cm −1 for CG3401 + Nafion ® compared to 0.076 S cm −1 for cast Nafion ®), effective proton mobility (as low as 3.2 × 10 −4 cm 2 V −1 s −1 CG3401 + Nafion ® compared to 7.6 × 10 −4 cm 2 V −1 s −1 for cast Nafion ®), and water vapour permeance (as low as 0.036 g h −1 Pa −1 m −2 for WDM + Nafion ® compared to 0.056 g h −1 Pa −1 m −2 for cast Nafion ®). By normalizing the membrane properties with respect to ionomer content, it was possible to examine the properties of the Nafion ® inside the pores of the membranes. The proton conductivity (as low as 0.032 S cm −1 for CG3401 + Nafion ®), effective proton mobility (as low as 3.6 × 10 −4 cm 2 V −1 s −1 for CG3401 + Nafion ®), and water vapour permeability (as low as 2.7 × 10 −6 g h −1 Pa −1 m −1 for PTFE MP 0.1 + Nafion ®) of the ionomer in the membrane are also diminished compared to bulk Nafion ® due to decreased connectivity of the ionomer and a restriction in macromolecular motions caused by the pore walls. A series of porous materials with increasing pore were also examined. As the pore size of the PTFE MP materials increased from 0.1 μm to 10 μm, the proton conductivity (0.022 S cm −1 to 0.041 S cm −1), effective proton mobility ((4.1 to 5.6) × 10 −4 cm 2 V −1 s −1), and water vapour permeability ((2.4 to 4.3) × 10 −6 g h −1 Pa −1 m −1) of the reinforced membranes improved with increasing pore size and the properties of the ionomer inside the membranes approached the value of bulk Nafion ®.

Main-chain, statistically sulfonated proton exchange membranes: the relationships of acid concentration and proton mobility to water content and their effect upon proton conductivity

An in-depth analysis has been developed for proton exchange membranes to examine the effect of acid concentration and effective proton mobility upon proton conductivity as well as their relationship to water content. The analysis was carried out on a series of main-chain, statistically sulfonated polymers with varying ion-exchange capacities. These polymer systems consisted of: sulfonated poly(ether ether ketone) (1), poly(ethylenetetrafluoroethylene-graft-polystyrenesulfonic acid) (2), sulfonated polyimide (3) and BAM® membrane (4) with Nafion® (5) as baseline. They represent membranes comprising polyaromatic polymers (1 and 3), one of which is also a rigid-rod polymer (3), vinylic polymers (4) and a vinylic polymer polymerized inside a polymer matrix (2). In order to remove the differences in acid strength for the membranes, proton mobility values at infinite dilution (Xv = 1.0) and 25 °C were calculated and found to be 3.2 (±0.4) × 10−3 cm2 s−1 V−1 (1), 2.9 (±0.4) × 10−3 cm2 s−1 V−1 (2), 1.6 (±0.7) × 10−3 cm2 s−1 V−1 (3) and 2.1 (±0.2) × 10−3 cm2 s−1 V−1 (4). These were then compared with the theoretical value for the mobility of a free proton at infinite dilution. Significant deviations from this value were theorized to be due to possible differences in tortuosity and proximity of acid groups.