Perfluorinated Sulfonic Acid Ionomers Research Articles

Effect of Hygrothermal Ageing on PFSA Ionomers' Structure/Property Relationship

Perfluorinated sulfonic-acid (PFSA) ionomers are widely used in polymer-electrolyte fuel cells (PEFCs) as electrolyte membranes for both separating reactant gases and transporting protons from anode to cathode. Ionomer properties like conductivity are largely affected by hydration (water uptake) and temperature. Thus, it is desirable for the membrane to operate at higher humidity and temperature. However, during practical operation, where there is high temperature and humidity, these ionomers frequently undergo hygrothermal ageing, which may, for example, result in the formation of anhydrides through a condensation reaction of the sulfonic-acid groups[1]. As a result, the properties and morphology change accordingly, which can significantly impact PEFC operation. Although there are some reports on the effects of hygrothermal ageing [2-4], much remains to be explored in terms of possible changed of the membrane’s underlying structure/property relationship. In this talk, ionomers subjected to hygrothermal ageing will be analyzed in terms of their properties to understand the role of hygrothermal ageing in membrane degradation. In this talk, the structure/property relationship of ionomers such as Nafion 212, 3M and Nafion XL will be examined both as received as well as after undergoing hygrothermal ageing at different conditions so as to correlate the separation-phased nanostructure with the membranes’ physical and transport properties. We will present the effect of pretreatment, temperature, humidity and purge flowrate on ageing and subsequent changes in mechanical, transport, uptake, and structural properties. It will be shown how the impact of ageing depends nonlinearly on humidity. We will also show how pretreatment like boiling and hot-pressing affect the impact of the ageing process. In addition, the possible effect of contamination will be discussed. Our findings provide new insight into how hygrothermal ageing affects the structure/property relationship of ionomers under different conditions.

A Small-Angle X-ray Scattering Study of the Development of Morphology in Films Formed from the 3M Perfluorinated Sulfonic Acid Ionomer

An extensive SAXS investigation of the 3M perfluorinated sulfonic acid ionomer was performed to investigate the morphological changes that occur during and after annealing at temperatures above the Tα. The effect of film thickness in the range studied, 11–45 μm, was found to be negligible. These properties were studied as a function of equivalent weight from 700 to 1100 and correlated with the water uptake as measured by dynamic vapor sorption. Isoscattering points were observed in dynamic annealing experiments of the unboiled annealed films at q = 0.023, 0.096 A–1. On initial water uptake these films also showed isoscattering points at q = 0.024, 0.220 A–1; q = 0.029, 0.223 A–1; and q = 0.030, 0.211 A–1 at 50, 80, or 95 °C, respectively, indicating a decrease in the symmetry of the scattering objects in these size regimes. Isoscattering points were absent in similar water uptake experiment for the films after boiling.

Role of Mechanical Factors in Controlling the Structure–Function Relationship of PFSA Ionomers

Ion-conducting polymers are ideal solid electrolytes for most energy storage and conversion devices where ion transport is a critical functionality. The system performance and stability are related to the transport and mechanical properties of the ionomers, which are correlated through physiochemical interactions and morphology. Thus, there exists a balance between the chemical and mechanical energies which controls the structure–function relationship of the ionomer. In this paper, it is reported how and why thermal treatments result in different water uptakes and nanostructures for a perfluorinated sulfonic acid (PFSA) membrane. The nanostructure of the PFSA membrane is characterized using small- and wide-angle X-ray scattering experiments. These changes are correlated with water content and mechanical properties and result in fundamental relationships to characterize the membrane with different thermal histories. Moreover, quasi-equilibrium water uptake and domain spacing both decrease with predrying or preconstraining the membrane, thereby suggesting that similar mechanical energies govern the structural changes via internal and external constraints, respectively. The findings suggest that heat treatments alter the balance between the chemical–mechanical energies where the interplay of the morphology and mechanical properties controls the structure–function relationship of the membrane. Finally, a model is developed using an energy-balance approach with inputs of the mechanical and structural properties; the dependence of water uptake on pretreatment is successfully predicted.

Preparation and electrocatalytic activity of palladium–platinum core–shell nanoalloys protected by a perfluorinated sulfonic acid ionomer

Palladium–platinum nanoalloys with a core–shell and nano-network structure were successfully synthesized by a hydrogen sacrificial protective method in an aqueous solution directly using a perfluorinated sulfonic acid (PFSA) ionomer as a protecting agent. The structure, local composition and electrocatalytic activity for the oxygen reduction reaction of the Pd/Pt/PFSA nanoalloys were investigated by transmission electron microscopy (TEM), aberration corrected scanning transmission electron microscopy (Cs-STEM), energy-dispersive X-ray spectrometry (EDS) and voltammetry. The core–shell structure was completed without contaminating reducing agents, organic solvents, useless protecting agents and a mediator. The Pd/Pt/PFSA core–shell nanoalloys realized a high electrochemical surface area and better electrocatalytic mass-activity for the oxygen reduction reaction than the Pt/PFSA nanoparticles.

Surfactant-assisted polymer electrolyte nanocomposite membranes for fuel cells

Polymer electrolyte nanocomposite membranes were prepared by using a commercial perfluorinated sulfonic acid ionomer (Nafion ®), nonporous hydrophilic fumed silica (Aerosil 380 ®), and anionic fluorosurfactant (Zonyl ® TBS). The fluorosurfactant-containing an acid group, which includes sulfonic acid, contributes to well-dispersion of the silica nanoparticles and simultaneously enhances the proton conductivity of the nanocomposite membrane. Hence, the addition of inorganic fillers such as silica to the fluorosurfactant can significantly improve the properties of the Nafion ®-based polymer electrolyte nanocomposite membrane. The membranes were characterized in terms of their mechanical properties ( e.g., dimensional change and tensile strength) and fundamental electrochemical properties ( e.g., proton conductivity and current–voltage ( I– V) curve). Zonyl ® TBS, a commercial anionic fluorosurfactant with sulfonic acid groups at the end of the surfactant chain, added to the Nafion ® matrix showed improved mechanical and electrochemical properties. The sulfonic acid groups in Zonyl ® TBS allowed for the additional transport of protons. As a result, the nanocomposite membrane with surfactant exhibited an improved performance. In a single cell test on a proton exchange membrane fuel cell (PEMFC), a membrane electrode assembly (MEA) prepared from the Nafion ®–silica nanocomposite membrane with TBS exhibited the highest PEMFC performance. The cell was fed with H 2 and O 2 humidified at 75 °C for 80 °C, respectively at ambient pressure. The current density of this membrane was 1600 mA cm −2 at 0.6 V and 80 °C, superior to 999 mA cm −2 for Nafion ® 112.

Chemically Tuned Anode with Tailored Aqueous Hydrocarbon Binder for Direct Methanol Fuel Cells

An anode for direct methanol fuel cells was chemically tuned by tailoring an aqueous hydrocarbon catalyst (SPI-BT) binder instead of using a conventional perfluorinated sulfonic acid ionomer (PFSI). SPI-BT designed in triethylamine salt form showed lower proton conductivity than PFSI, but it was stable in the catalyst ink forming the aqueous colloids. The aqueous colloidal particle size of SPI-BT was much smaller than that of PFSI. The small SPI-BT colloidal particles contributed to forming small catalyst agglomerates and simultaneously reducing their pore volume. Consequently, the high filling level of binders in the pores, where Pt-Ru catalysts are mainly located on the wall and physically interconnected, resulted in increased electrochemical active surface area of the anode, leading to high catalyst utilization. In addition, the chemical affinity between the SPI-BT binder and the membrane material derived from their similar chemical structure induced a stable interface on the membrane-electrode assembly (MEA) and showed low electric resistance. Upon adding SPI-BT, the synergistic effect of high catalyst utilization, improved mass transfer behavior to Pt-Ru catalyst, and low interfacial resistance of MEA became greater than the influence of reduced proton conductivity in the electrochemical performance of single cells. The electrochemical performance of MEAs with SPI-BT anode was enhanced to almost the same degree or somewhat higher than that with PFSI at 90 degrees C.

Toward Large Arrays of Multiplex Functionalized Carbon Nanotube Sensors for Highly Sensitive and Selective Molecular Detection.

Arrays of electrical devices with each comprising multiple single-walled carbon nanotubes (SWNT) bridging metal electrodes are obtained by chemical vapor deposition (CVD) of nanotubes across prefabricated electrode arrays. The ensemble of nanotubes in such a device collectively exhibits large electrical conductance changes under electrostatic gating, owing to the high percentage of semiconducting nanotubes. This leads to the fabrication of large arrays of low-noise electrical nanotube sensors with 100% yield for detecting gas molecules. Polymer functionalization is used to impart high sensitivity and selectivity to the sensors. Polyethyleneimine coating affords n-type nanotube devices capable of detecting NO2 at less than 1 ppb (parts-per-billion) concentrations while being insensitive to NH3. Coating Nafion (a polymeric perfluorinated sulfonic acid ionomer) on nanotubes blocks NO2 and allows for selective sensing of NH3. Multiplex functionalization of a nanotube sensor array is carried out by microspotting. Detection of molecules in a gas mixture is demonstrated with the multiplexed nanotube sensors.