Quantitative in Situ Analysis of Ionomer Structure in Fuel Cell Catalytic Layers

Morawietz, Tobias; Handl, Michael; Oldani, Claudio; Friedrich, K. Andreas; Hiesgen, Renate

doi:10.1021/acsami.6b07188

Cited by 100 publications

(101 citation statements)

References 58 publications

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“…In Figure , AFM images taken at 50% RH of the cross‐section of a commercial MEA 0476 are represented, with the topography image (Figure a), the corresponding adhesion mapping (Figure b), and the same adhesion image as in Figure b with the ionomer marked in blue. The dark areas visible in Figure b indicate lower‐adhesive Pt/C agglomerates, the bright areas correspond to the ionomer present at the surface . Besides thin ionomer films that cover the agglomerates, also partly ionomer‐covered particles are present.…”

Section: Resultsmentioning

confidence: 99%

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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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Section: Resultsmentioning

confidence: 99%

“…The sample was heated in air with 50% RH. Upon heating of the electrode, the ionomer layer thickness distribution widened and the average thickness increased , . In Figure b, the dependence of average ionomer layer thickness on a temperature cycle is depicted.…”

Section: Resultsmentioning

confidence: 99%

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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“…19,35,36 In catalytic layers of fuel cells an operation-induced thinning of the ionomer films that partly encapsulate the platinum covered carbon (Pt/C) agglomerates has been reported. 13,37,38 The observed stronger thinning of the ionomer layers in the anode at samples cut close to the hydrogen inlet and with the use of thin membranes was explained by the higher concentration of highly active hydrogen radicals. 20 Since the thickness of the ionomer layers within electrodes ranges from roughly 4 nm to 20 nm, the conductive properties as well as the gas permeation are affected by the properties of such nanothin films.…”

Section: F3140mentioning

confidence: 99%

“…Depending on preparation and operation, a significant thinning of the ionomer films that cover the Pt/C agglomerates in the electrodes was observed. 13 In the SEM, the ionomer in the electrode is hardly visible due to significant shrinkage in the dry vacuum environment; also the contrast between ionomer and carbon is weak. AFM was therefore used as main method, because it works at elevated humidity and provides a large contrast between ionomer and Pt/C components.…”

Section: F3140mentioning

confidence: 99%

“…It has been reported that in adhesion mappings recorded with material-sensitive AFM, the ionomer exhibits a higher adhesion to the AFM tip than Pt/C agglomerates. 13 In Figure 8a, the area fraction with high adhesion according to ionomer is given. Similar to the decrease of non-conductive area given in Figure 7c the high-adhesion area decreased in a similar way with operation time from roughly 40% to 30%, slightly more at the anode.…”

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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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In Situ and Operando Characterization of Proton Exchange Membrane Fuel Cells

2019

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For proton-exchange-membrane fuel cells (PEMFCs) to become a mainstream energy source, significant improvements in their performance, durability, and efficiency are necessary. To improve their durability, there must be a solid understanding of how the structural and electrochemical processes are affected during operation to propose mitigation strategies. To this aim, in situ and operando characterization techniques locally identify structural and electrochemical processes, which cannot be captured using conventional techniques. Nevertheless, linking these properties in the same geometric area has been challenging due to its inherent limitations, such as sample size and imaging resolution. This has created a knowledge gap in structure-to-electrochemical performance relationships as operation and degradation unevenly affect different areas of the cell.

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Quantitative in Situ Analysis of Ionomer Structure in Fuel Cell Catalytic Layers

Cited by 100 publications

References 58 publications

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

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

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

In Situ and Operando Characterization of Proton Exchange Membrane Fuel Cells

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