Imaging and Microanalysis of Thin Ionomer Layers by Scanning Transmission Electron Microscopy

Abstract

Optimized conditions for imaging and spectroscopic/elemental mapping of thin perfluorosulfonic acid (PFSA) ionomer layers in fuel cell electrodes by scanning transmission electron microscopy (STEM) have been investigated. The proper conditions were first identified using model systems of either Nafion ionomer-coated nanostructured thin film catalysts or thin films on nanoporous Si. These analysis conditions were then applied in a quantitative study of the ionomer through-layer loading for two differently-prepared electrode catalyst layers using electron energy loss (EELS) and energy dispersive X-ray spectroscopy (EDS) in the STEM. The electron-beam induced damage to the PFSA ionomer was quantified by following the fluorine mass loss with electron dose/exposure and was mitigated by several orders of magnitude using cryogenic specimen cooling and a higher incident electron voltage. Multivariate statistical analysis was applied to the analysis of both EELS and EDS spectrum images for data de-noising and unbiased separation of the independent components related to the catalyst, ionomer, and support distributions within the catalyst layers.Perfluorosulfonic acid (PFSA) ionomer is a key component within the electrode layers of polymer electrolyte fuel cells (PEFCs). The PEFC electrode layer is typically constructed at a ∼10 μm thickness and is comprised of a dispersed Pt nanoparticle catalyst supported on a highly structured carbon black support with a distributed PFSA ionomer film. This percolating solid polyelectrolyte in the electrode provides an efficient proton transport path to the active catalyst sites. The carbon and polymer occupy ∼20% volume fraction each in the electrode, which leaves ∼50-60% pore volume for transport of the reactant hydrogen/air and product water to/from the active Pt sites.Both the uniformity of the PFSA ionomer loading on a 100-nm length scale and the uniformity of the actual film thickness distribution surrounding the carbon support and catalyst nanoparticles on a 1-nm length scale are critical to electrode performance, and quantitative measurements of both these properties are highly desired. Scanning transmission electron microscopy (STEM) is an attractive tool for characterizing the distribution of ionomer within PEFC electrodes, especially when coupled with spectroscopic techniques such as electron energy loss spectroscopy (EELS) or energy dispersive X-ray spectroscopy (EDS). 1 While electron microscopy is more than capable of fulfilling the spatial resolution and chemical sensitivity requirements necessary for analysis of the PFSA ionomer, further method optimization of the STEM imaging and analysis parameters is required due to the beam-sensitive nature of the ionomer films.Fluorinated compounds, such as the PFSA ionomer, can be highly sensitive to electron beam radiation damage. 2 The high electron doses needed to acquire spectroscopic maps by either EDS or EELS can induce severe structural and chemical changes to the ionomer within the electrode, as previously demonstrated on PEF...

Correlating Cathode Microstructure with PEFC Performance Using FIB-SEM and TEM

“…In order to observe Nafion ionomer more effectively, K. More et al utilized electron microscopes with higher resolution than SEM, such as TEM and STEM. 16,17 The idea of "staining" ionomers using Cs has also been explored, 17 and has become an important technique for observing ionomer materials by TEM.…”

Hybrid approach combining multiple characterization techniques and simulations for microstructural analysis of proton exchange membrane fuel cell electrodes

“…Besides the catalyst and support characterization, using atomic force microscopy (AFM) and TEM, Xie et al [15] showed that the ionomer content changes the sizes of C aggregates and the thickness of the ionomer film covering the C surface. In addition, recently advanced TEM techniques such as aberration-corrected scanning transmission electron microscopy (STEM) with energy-dispersive spectroscopy (EDS) [16], and electron tomography (ET) [17], coupled with cesium ion exchange, were able to provide information about the microstructure of the ionomer phase in the electrode.…”

“…Direct imaging of the three-dimensional ionomer network inside the electrode still remains a challenge. Advanced TEM techniques have been used to image the ionomer via either microtoming and two-dimensional imaging of a series of slices or via three-dimensional imaging (ET) for a few ionomer-coated carbon particles[16,17].…”

Experimental Methodologies to Understand Degradation of Nanostructured Electrocatalysts for PEM Fuel Cells: Advances and Opportunities

“…The high sensitivity of PFSA ionomer to electron beam radiation damage via radiolysis represents the main limitation of the former strategy. Through the investigation of different model systems, Cullen et al [68] identified the optimal conditions (electron dose, temperature, acceleration voltage) to minimize fluorine losses during the acquisition of EELS or EDS spectra. High acceleration voltages and cryo (liquid N 2 ) conditions were employed to obtain high spatial resolution EELS maps of Nafion-Pt/C catalysts minimizing the beam damage.…”