Microscopic Analysis of Current and Mechanical Properties of Nafion® Studied by Atomic Force Microscopy

Hiesgen, Renate; Helmly, Stephan; Galm, Ines; Morawietz, Tobias; Handl, Michael; Friedrich, K. Andreas

doi:10.3390/membranes2040783

Cited by 51 publications

(73 citation statements)

References 35 publications

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“…In support of this assertion, we note the current-sensing atomic force microscopy (AFM) studies of Heisgen et al in which it is suggested that pores at the membrane's surface open, leading to better connectivity to the internal network of hydrophilic channels as a result of electrogenerated water being formed. 31 More specifically, when a bias potential is applied to the AFM electrode/membrane interface causing ORR, internal membrane pressures develop which is released by the opening of hydrophilic water channels at the surface, resulting in increased current flow. A similar effect is also postulated by Kreuer in conclusion of dynamic mechanical analyses (DMA) of membranes exposed to varying relative humidity, i.e., internal pressures in PFSA membranes resulting from increasing hydration levels are relieved by a reorganization of the morphology of the outer skin.…”

Section: Resultsmentioning

confidence: 99%

Time-Dependent Mass Transport for O2 Reduction at the Pt Perfluorosulfonic Acid Ionomer Interface

Novitski

Xie

Holdcroft

2014

ECS Electrochemistry Letters

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Chronoamperometric analysis of the oxygen reduction reaction (ORR) at Pt microelectrode | perfluorosulfonic acid (PFSA) ionomer interfaces in a solid state electrochemical cell reveals an increase in both the oxygen diffusion coefficient (D b Proton exchange membrane fuel cell (PEMFC) catalyst layers typically employ an ionomer component to reduce ionic resistance and improve electrochemical kinetics.)1,2 Recent studies reveal the ionomer in the catalyst layer may be present as an ultrathin film, and ex-situ studies have indicated that such films undergo a surface morphological reconstruction from hydrophobic to hydrophilic upon exposure to liquid water. Moreover, the reconstruction is observed to have a timedependency proportional to film thickness.3 Exposure to water vapor is found not to trigger this surface reconstruction. 3 The nature of these morphological reorganizations at the surface of the ionomer film is of considerable interest for understanding electrochemical mass transport at the ionomer | Pt interface. 4 However, there is little evidence concerning the role of these reorganizations on actual faradaic electrochemical activities. Overall, the complex morphological changes at ionomer surfaces are of increasing interest, where variables such as film thickness, substrate, and film preparation result in a markedly different outcome for the resulting ionomer properties. [5][6][7][8] Previous electrochemical studies have shown that mass transport through perfluorosulfonic acid (PFSA) ionomer membranes is dependent upon oxygen solubility in the hydrophobic, fluorinated domains and its transport into, and diffusion through, aqueous domains. 9-11The relatively high rate of oxygen permeation through PFSA ionomer is attributed to the relatively high oxygen solubility and large diffusion coefficient.9,12 The oxygen mass transport through the ionomer in a cathode catalyst layer (CCL), and its role in determining fuel cell performance, is relatively unexplored but is under increasing scrutiny. Studies based on limiting current techniques indicate that as Pt catalyst loading in the CCL is reduced, oxygen mass transport resistance in, and through, the PFSA ionomer increases, ultimately reducing the available power density of a fuel cell. [13][14][15] Other studies support this contention -that oxygen mass transport resistance through the catalyst layer ionomer plays a large role in determining the power that can be extracted from a fuel cell. 16 It has also been suggested that the total mass transport resistance is exacerbated by a mass transport resistance at the gas/ionomer interface, which becomes increasingly dominant as ionomer film thickness is reduced. [17][18][19] Understanding oxygen mass transport limitations at ionomer | Pt interfaces is therefore of growing interest.Ex-situ solid state electrochemical techniques provide a means to study ORR at Pt | ionomer interfaces under conditions of specific temperature, pressure, and RH. Pioneering work by Srinivasan and * Electrochemical Society Student Member....

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Section: Resultsmentioning

confidence: 99%

Time-Dependent Mass Transport for O2 Reduction at the Pt Perfluorosulfonic Acid Ionomer Interface

Novitski

Xie

Holdcroft

2014

ECS Electrochemistry Letters

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“…The fraction of non-conductive/conductive area was retrieved from area analysis of the images. More details on the AFM measurements can be found in Hiesgen et al 12,17,18,71 For AFM analysis of MEAs, freshly prepared cross-sections were imaged. The cross-sections were cut by microtome after embedding the MEA into 2-component polyurethane (Teromix, BASF) for stabilization.…”

Section: Methodsmentioning

confidence: 99%

“…With PeakForce QNM-mode, mapping of local height, adhesion, stiffness (DMT modulus, stiffness proposed by Derjagin, Muller, Toropov in 1975), deformation, and dissipation was performed. 12,17,18 Atomic force microscopy takes advantage of the higher contrast in adhesion (pulloff) force and stiffness between the ionomer and the carbon/platinum components. 13 The local ionic current at constant relative humidity in a gastight chamber was recorded using a platinum coated catalytically active tip.…”

Section: Methodsmentioning

confidence: 99%

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High-Resolution Analysis of Ionomer Loss in Catalytic Layers after Operation

Morawietz

Handl

Oldani

et al. 2018

J. Electrochem. Soc.

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The function of catalytic layers in fuel cells and electrolyzers depends on the properties of the ionically conductive phase, which are most commonly perfluorinated ionomers based on Nafion and Aquivion. An analysis by atomic force microscopy reveals that the ultrathin ionomer films around Pt/C agglomerates have a thickness distribution ranging from 3.5 nm to 20 nm. Their conductivity and gas permeation properties determine the fuel cell performance to a large extend. For electrodes in Aquivion-based membraneelectrode-assemblies operation-induced structure changes were investigated by means of material-and conductivity-sensitive atomic force microscopy, infrared spectroscopy and electron-dispersive X-ray analysis. The observed thinning of the ultrathin ionomer films was mainly caused by polymer degradation deduced from reduced swelling after long-time operation and a significant loss of ionomer with operation time detected by infrared spectroscopy. From the linear thickness increase of the ultrathin films with rising humidity, a mainly layered structure of the ionomer was deduced. An influence of thickness of such ultrathin ionomer films on fuel cell lifetime was found by analysis of differently prepared membrane-electrode-assemblies, where a linear increase of irreversible degradation rate with ionomer film thickness in the electrodes of unused membrane-electrode-assemblies was found.

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“…A skin layer thickness of 5 nm could be measured for Nafion NR212. 22,23 Upon contact with water, the ionic side groups can change their orientation within seconds, and the surface becomes hydrophilic. 18 At the interface with water, the structures tend to orient vertically to expose ionically conductive channels from the bulk to the exterior that supply an exit for water/proton exchange.…”

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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Microscopic Analysis of Current and Mechanical Properties of Nafion® Studied by Atomic Force Microscopy

Cited by 51 publications

References 35 publications

Time-Dependent Mass Transport for O2 Reduction at the Pt Perfluorosulfonic Acid Ionomer Interface

Time-Dependent Mass Transport for O2 Reduction at the Pt Perfluorosulfonic Acid Ionomer Interface

High-Resolution Analysis of Ionomer Loss in Catalytic Layers after Operation

Insight into the Structure and Nanoscale Conductivity of Fluorinated Ionomer Membranes

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