Evidence for a crystallite-rich skin on perfluorosulfonate ionomer membranes

Tang, Junkun; Yuan, Wang Zhang; Zhang, Jidong; Li, Hong; Zhang, Yongming

doi:10.1039/c3ra40430g

Cited by 23 publications

(24 citation statements)

References 49 publications

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“…Higher adhesion values typically indicates a higher PTFE-like content [46], which in this case, is associated with a backbone-rich phase; low adhesion indicates a water-rich phase, as discussed above. Close to the interface, more of the higher adhesive water-poor phase is present, which agrees with measurements at membranes [21]. The total thickness of this layer, which is oriented parallel to the interface, is approximately 2 mm.…”

Section: Interface Formation Of Ionomer Layer To Hydrophobic and Hydrsupporting

confidence: 77%

“…From GISAX measurements, a high crystallinity of an approximately 5-nm layer with an orientation parallel to the surface was observed [21]. From other scattering experiments, including synchrotron grazing incidence X-ray diffraction (GIXRD) and small angle X-ray scattering (GISAXS) analyses, a similar value was obtained [21], [22]. Recently, AFM measurements confirmed this thickness for the skin layer for membranes in air [23].…”

Section: Introductionsupporting

confidence: 60%

“…The total thickness of this layer, which is oriented parallel to the interface, is approximately 2 mm. This thickness is much larger than that reported in the literature [21], [19]; however, these layers were quite fresh and were not heated above the glass transition temperature. Under the common procedures used for fuel cells, the layers will most likely dry out further and shrink with time.…”

Section: Interface Formation Of Ionomer Layer To Hydrophobic and Hydrmentioning

confidence: 60%

See 2 more Smart Citations

Atomic Force Microscopy on Cross Sections of Fuel Cell Membranes, Electrodes, and Membrane Electrode Assemblies

Hiesgen

Morawietz

Handl

et al. 2015

Electrochimica Acta

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Dedicated to Prof. Jacek Lipkowski, in recognition of his achievements and on the occasion of his 70th birthday.Keywords: AFM PFSA conductive ionic network fuel cell catalytic layer ionomer content A B S T R A C T Using material-sensitive and conductive atomic force microscopy (AFM) on cross sections of perfluorinated and sulfonated membranes at low humidity, crystalline polymer lamellae were imaged and their thickness determined to approximately 6 nm. In the capacitive current, water-rich and waterpoor areas with different phase structures were investigated. The formation of a local electrochemical double layer within the water-rich ionically conductive areas at the contact of the AFM tip with the electrolyte enabled their visibility. The large water-filled ionically conductive areas include numerous ionic domains. Under equilibrium conditions, these areas are spherical (appearing circular in the images) and with distinct size distribution. Forcing a current through the membranes (current-induced activation) led to merging of the water-filled ionically conductive areas in the voltage direction and resulted in an anisotropic ionically conducting network with flat channels. The distribution of the current in the membrane and catalytic layers of a pristine membrane electrode assembly (MEA) was analyzed. From the adhesion force mappings, an inhomogeneous distribution of ionomer in the catalytic layer was detected. Cross currents between Pt/C particles through large ionomer particles within the catalytic layer were detected and the ionomer content across an electrode was evaluated.

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Section: Interface Formation Of Ionomer Layer To Hydrophobic and Hydrsupporting

confidence: 77%

Section: Introductionsupporting

confidence: 60%

Section: Interface Formation Of Ionomer Layer To Hydrophobic and Hydrmentioning

confidence: 60%

See 1 more Smart Citation

Atomic Force Microscopy on Cross Sections of Fuel Cell Membranes, Electrodes, and Membrane Electrode Assemblies

Hiesgen

Morawietz

Handl

et al. 2015

Electrochimica Acta

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“…35,19,36,37 The existence of a surface skin layer with a different orientation of phase structures, 21 different hydrophobicity, 37 and a different degree of crystallinity, 38 has been demonstrated. An investigation of the outer membrane surface may therefore not be representative of the bulk.…”

Section: Resultsmentioning

confidence: 99%

“…A 4.5 nm thick crystalliterich surface layer with a 40% higher degree of crystallinity based on synchrotron grazing incidence X-ray diffraction (GIXRD) and small angle X-ray scattering (GISAXS) measurements with crystallites oriented parallel to the surface was reported. 20 At the ionomer/water vapor interface, a hydrophobic surface is observed where the molecular backbone is present at the outer surface and the ionic side chains are switched to the interior. This interface is proposed to consist of polymer bundles predominantly oriented parallel to the surface, where, besides a few defects, no ionic channels have an exit for water/proton exchange with the environment.…”

mentioning

confidence: 99%

Insight into the Structure and Nanoscale Conductivity of Fluorinated Ionomer Membranes

Hiesgen

Morawietz

Handl

et al. 2014

J. Electrochem. Soc.

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A material-sensitive atomic force microscopic (AFM) tapping mode was combined with current measurements to investigate structure, phase separation, and conductive structure of surfaces and cross sections of long side chain Nafion and short side chain AQUIVION PFSA ionomer membranes. We found unexpected large-scale ordered structures consistent with a dominant lamellar polymer structure at the cross sections. The highly terraced areas of both ionomers have a wide distribution of layer thicknesses from sub-nanometer to a few nanometers. In both broad size distributions, preferential sizes were identified that reflect the different lengths of the molecular side chains, indicating a stacking in layers. The nanoscale phase separation of the ionomer was analyzed by using the capacitive current distribution. In AQUIVION PFSA, larger connected water-rich ionic areas were found than in Nafion with same total ionic area. A steady-state current at the cross sections evolved only after an activation period by enforcing current flow though the membrane. A comprehensive and heterogeneous current distribution was observed with highly conductive areas. In contrast, on outer membrane surfaces, only non-continuous spot-like currents were observed. In general, our measurements are consistent with conduction in water layers in-between polymer chains and a bi-continuous structure under faradaic current flow. Perfluorinated sulfonated ionomer membranes (PFSA) have been established as proton-conducting electrolytes in many technical applications, most notably in brine electrolysis but also with increasing importance as electrolytes for fuel cell applications. For highperformance operation of fuel cells, the water management of the membrane is of great importance. Therefore, the nature of water transport and the conductive structure of the ionomer membrane are still under investigation. In particular, the nanoscopic ionomer properties at non-equilibrium conditions under steady-state current flow close to operation conditions are expected to differ from equilibrium state and have not been characterized in detail.During solidification of perfluorinated polymers (PFSA) casted from dispersions, a phase separation occurs that is the key factor in providing ionic conductivity. The hydrophilic sulfonate-terminated end groups of the side chains cluster together to form an ionic phase. The perfluorinated molecular backbone is assumed to form bundles due to the hydrophobic interaction and to undergo partial crystallization that is considered to enhance mechanical strength.1 The size of these crystalline areas, as determined from scattering data, is reported to be within 0.5 to 10 nm.2,3 The spacing of the average hydrophobic/hydrophilic separations was obtained from the position of the ionomer peaks in scattering data and determined to approximately 3-7 nm depending on the water content.4 While the hydrophobic phase provides mechanical stability, the ionic phase provides a waterbased proton-conducting network. It is generally assumed that this i...

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Influence of Water and Temperature on Ionomer in Catalytic Layers and Membranes of Fuel Cells and Electrolyzers Evaluated by AFM

et al. 2018

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The major difference for the durability of the materials in PEM fuel cells and PEM electrolyzers is the exposure to humidified gases and to liquid water, respectively. Aquivion and Nafion ionomer was investigated at surfaces of membranes and cross‐sections of electrodes by material‐sensitive atomic force microscopy. The dimensional behavior of electrodes and membranes in a membrane‐electrode assembly cross section was studied as a function of combined humidity and temperature changes. The observed opposite effects of ionomer dimensional changes in membrane and catalytic layers are important for understanding mechanical stress factors and degradation. For the lamellar polymer phase at moderate humidity, a linear swelling behavior and the enclosure of multiples of 0.8 nm‐thick water layers was observed. Submersed in water, 3 nm‐thick enclosed water layers were found. At the water‐immersed Aquivion membrane derived from alcoholic dispersion larger 30–50 nm sized water‐rich areas were found. In contrast, for membranes derived from water‐based dispersion the water‐rich phase was smaller with 11 nm. The thickness of the ultrathin ionomer layers in the electrode was investigated as a function of humidity and temperature. A linear swelling was found for humidity changes whereas nonlinear behavior with 100% expansion and hysteresis was seen for temperature cycles.

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Evidence for a crystallite-rich skin on perfluorosulfonate ionomer membranes

Cited by 23 publications

References 49 publications

Atomic Force Microscopy on Cross Sections of Fuel Cell Membranes, Electrodes, and Membrane Electrode Assemblies

Atomic Force Microscopy on Cross Sections of Fuel Cell Membranes, Electrodes, and Membrane Electrode Assemblies

Insight into the Structure and Nanoscale Conductivity of Fluorinated Ionomer Membranes

Influence of Water and Temperature on Ionomer in Catalytic Layers and Membranes of Fuel Cells and Electrolyzers Evaluated by AFM

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