Lukas Zielke

Degradation of Li/S Battery Electrodes On 3D Current Collectors Studied Using X-ray Phase Contrast Tomography.

Lithium/sulphur batteries are promising candidates for future energy storage systems, mainly due to their high potential capacity. However low sulphur utilization and capacity fading hinder practical realizations. In order to improve understanding of the system, we investigate Li/S electrode morphology changes for different ageing steps, using X-ray phase contrast tomography. Thereby we find a strong decrease of sulphur loading after the first cycle, and a constant loading of about 15% of the initial loading afterwards. While cycling, the mean sulphur particle diameters decrease in a qualitatively similar fashion as the discharge capacity fades. The particles spread, migrate into the current collector and accumulate in the upper part again. Simultaneously sulphur particles lose contact area with the conducting network but regain it after ten cycles because their decreasing size results in higher surface areas. Since the capacity still decreases, this regain could be associated with effects such as surface area passivation and increasing charge transfer resistance.

In Situ Fabrication Of Quasi-Free-Standing Epitaxial Graphene Nanoflakes On Gold

Addressing the multitude of electronic phenomena theoretically predicted for confined graphene structures requires appropriate in situ fabrication procedures yielding graphene nanoflakes (GNFs) with well-defined geometries and accessible electronic properties. Here, we present a simple strategy to fabricate quasi-free-standing GNFs of variable sizes, performing temperature programmed growth of graphene flakes on the Ir(111) surface and subsequent intercalation of gold. Using scanning tunneling microscopy (STM), we show that epitaxial GNFs on a perfectly ordered Au(111) surface are formed while maintaining an unreconstructed, singly hydrogen-terminated edge structure, as confirmed by the accompanying density functional theory (DFT) calculations. Using tip-induced lateral displacement of GNFs, we demonstrate that GNFs on Au(111) are to a large extent decoupled from the Au(111) substrate. The direct accessibility of the electronic states of a single GNF is demonstrated upon analysis of the quasiparticle interference patterns obtained by low-temperature STM. These findings open up an interesting playground for diverse investigations of graphene nanostructures with possible implications for device fabrication.

Morphological Evolution of Electrochemically Plated/Stripped Lithium Microstructures Investigated by Synchrotron X-ray Phase Contrast Tomography

Due to its low redox potential and high theoretical specific capacity, Li metal has drawn worldwide research attention because of its potential use in next-generation battery technologies such as Li-S and Li-O2. Unfortunately, uncontrollable growth of Li microstructures (LmSs, e.g., dendrites, fibers) during electrochemical Li stripping/plating has prevented their practical commercialization. Despite various strategies proposed to mitigate LmS nucleation and/or block its growth, a fundamental understanding of the underlying evolution mechanisms remains elusive. Herein, synchrotron in-line phase contrast X-ray tomography was employed to investigate the morphological evolution of electrochemically deposited/dissolved LmSs nondestructively. We present a 3D characterization of electrochemically stripped Li electrodes with regard to electrochemically plated LmSs. We clarify fundamentally the origin of the porous lithium interface growing into Li electrodes. Moreover, cleavage of the separator caused by growing LmS was experimentally observed and visualized in 3D. Our systematic investigation provides fundamental insights into LmS evolution and enables us to understand the evolution mechanisms in Li electrodes more profoundly.

Tomography based screening of flow field / current collector combinations for PEM water electrolysis

Current collectors in PEM water electrolysis perform several functions that influence overall performance. These include conducting heat and electrons, as well as transporting water and gas. X-ray tomography, standardized reconstruction, and calculation methods are used to compare the morphological and transport parameters of eight different current collectors functioning as flow fields for PEM water electrolysis. We find simple exponential relations between through-plane thermal conductivity λ and porosity p (λ20°C(p) = 1749p−1.306 − 4.420), as well as between through-plane electrical conductivity σ and water permeability K (σ(K) = 10−6 × K−0.6376). In addition, we use both local and global concepts to investigate the pore space of current collectors. We hereby investigate homogeneity and characteristic sizes, like mean pore diameter or mean distance between solid parts at the catalytic interface. Moreover, we find that the local concept of mean chord lengths can be used to explain electrical and thermal conductivity anisotropies. These chords can be used to predict the direction of the largest conductivity for fibrous current collectors.

Multi-Scale Correlative Tomography of a Li-Ion Battery Composite Cathode.

Focused ion beam/scanning electron microscopy tomography (FIB/SEMt) and synchrotron X-ray tomography (Xt) are used to investigate the same lithium manganese oxide composite cathode at the same specific spot. This correlative approach allows the investigation of three central issues in the tomographic analysis of composite battery electrodes: (i) Validation of state-of-the-art binary active material (AM) segmentation: Although threshold segmentation by standard algorithms leads to very good segmentation results, limited Xt resolution results in an AM underestimation of 6 vol% and severe overestimation of AM connectivity. (ii) Carbon binder domain (CBD) segmentation in Xt data: While threshold segmentation cannot be applied for this purpose, a suitable classification method is introduced. Based on correlative tomography, it allows for reliable ternary segmentation of Xt data into the pore space, CBD, and AM. (iii) Pore space analysis in the micrometer regime: This segmentation technique is applied to an Xt reconstruction with several hundred microns edge length, thus validating the segmentation of pores within the micrometer regime for the first time. The analyzed cathode volume exhibits a bimodal pore size distribution in the ranges between 0–1 μm and 1–12 μm. These ranges can be attributed to different pore formation mechanisms.

Comprehensive investigation of novel pore-graded gas diffusion layers for high-performance and cost-effective proton exchange membrane electrolyzers

Hydrogen produced by water electrolysis is a promising storage medium for renewable energy. Reducing the capital cost of proton exchange membrane (PEM) electrolyzers without losing efficiency is one of its most pressing challenges. Gas diffusion layers (GDL), such as felts, foams, meshes and sintered plates, are key stack components, but these are either inefficient or expensive. This study presents a new type of GDL produced via vacuum plasma spraying (VPS), which offers a large potential for cost reduction. With this technology, it is possible to introduce a gradient in the pore-size distribution along the thickness of the GDL by varying the plasma parameters and titanium powder particle sizes. This feature was confirmed by cross-section scanning electron microscopy (SEM). X-ray computed tomography (CT) and mercury intrusion porosimetry allowed determining the porosity, pore radii distribution, and pore entry distribution. Pore radii of ca. 10 μm could be achieved in the layers of the GDL close to the bipolar plate, while those in contact with the electrodes were in the range of 5 μm. The thermally sprayed Ti-GDLs allowed achieving PEM electrolyzer performances comparable to those of the state-of-the-art sintered plates and far superior than those of meshes. Moreover, a numerical model showed that the reduced capillary pressure and tortuosity eliminates mass transport limitations at 2 A cm−2. The results presented herein demonstrate a promising solution to reduce the cost of one of the most expensive components of the stack.

Three-dimensional morphology of the interface between micro porous layer and catalyst layer in a polymer electrolyte membrane fuel cell

Interfaces between the different layers in proton exchange membrane fuel cells are expected to influence transport properties and therefore cell performance. So far the interface between micro porous layer (MPL) and catalyst layer (CL) has been difficult to investigate due to its nanometer scale morphology. We apply focused ion beam scanning electron microscopy tomography with pore contrasting via atomic layer deposition to reconstruct a representative volume of 5.1 μm × 1.5 μm × 4.5 μm containing CL, MPL and their interface. We find that platinum in the CL results in brighter SEM image intensities, compared to the MPL. This allows (i) estimating the extension of the interfacial region (530 nm), (ii) evaluating Pt-content homogeneity in the CL and (iii) calculating the individual roughnesses for the CL (102 nm) and for the MPL (129 nm). We further calculate porosity, pore sizes, and oxygen diffusivities. Thus, we find that the values of the parameters of the interfacial region are between those of the CL and the MPL, meaning that on the investigated scale, the interface is a homogeneous transitional region. A representativeness analysis shows that our reconstructed volume is sufficiently large concerning all calculated parameters.

Three-Dimensional Analysis of the Porosity in MgB 2 Wires Using FIB Nanotomography

Porosity is one of several current limiting mechanisms in MgB2 wires. We have compared the microstructural homogeneity and the porosity distribution in different ex situ and in situ MgB2 powder-in-tube wires. The submicrometer structure was determined using focused ion beam nanotomography. The ex situ wires exhibit an isotropic microstructure, which has been quantified in terms of an identical tortuosity in transverse and longitudinal filament direction. The very homogenous microstructure in the new ex situ wire generation is probably one reason for its strongly improved critical current density. The in situ wire has an anisotropic microstructure with a lower tortuosity in the axial direction. The microstructural inhomogeneity of the in situ filaments makes microstructural characterization and the comparison between materials and superconducting properties particularly challenging.