Grazing Incidence X-ray Scattering Research Articles

First Sharp Diffraction Peak features of the intermediate phase glasses and amorphous thin films in the non-stoichiometric (GeS4)x(AsS3)1-x system

Grazing incidence X-ray scattering (GIXRS) patterns of thin solid films based on non-stoichiometric chalcogenide glasses (ChG) from the pseudo - binary system (GeS4)x(AsS3)1-x were studied with a focus on the First Sharp Diffraction Peak (FSDP), assigned to the middle range order (MRO) of the glassy material. The films were grown using explosive thermal evaporation in vacuum (10−4 Pa) of pulverized ChG, prepared from previously synthesized bulk glasses, onto mono-crystalline silicon substrates. Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray (EDX) spectroscopy were used to examine the morphology and elemental composition of the films, which were found to have similar composition to the bulk glasses. However, it was revealed that the molecular structure of the grown amorphous films differs from that of the initial ChG bulk material, as indicated by changes in the composition-dependent position and width of the FSDP. Additionally, the intensities of the FSDP in the films were higher compared to those of the bulk samples, suggesting that the molecular-like structure of ChGs is more pronounced in the form of thin films grown from the vapor phase.

Chemically Tuning Ionomer Thin Film Properties for Improved Electrode Function

Ion-conducting polymers (ionomers) are an important component of the heterogenous electrodes (catalyst layers, CL) in proton exchange membrane fuel cells (PEMFCs). In the CL, the nano-thin ionomer layer facilitates mass transport to and from the catalyst sites. Emerging chemistries have the potential to improve this mass transport by inserting additional acid groups to each sidechain or by modifying the backbone. This work aims to understand the impact of ionomer chemistry on the structure-property relationships relevant to fuel cells. Ionomer thin films (<100 nm) cast on planar supports model the CL ionomer. Film nanomorphology is characterized via in situ grazing-incidence x-ray scattering (GIXS); application-relevant properties, including hydration and proton conductivity, are probed via spectroscopic ellipsometry and electrochemical impedance spectroscopy, respectively. While both sidechain and backbone modifications lead to improvements in proton conductivity, the sidechain-modified ionomers show greater hydration and nanophase separation, and the backbone-modified ionomers show reduced hydration and nanophase separation, demonstrating the interplay between morphology, hydration, and film function. These results show that chemistry is a useful tool to tune thin film properties and provide insight to guide future ionomer design.

(Invited) Atomic-Scale Aspects of Nucleation and Growth at Liquid-Liquid Interfaces

Interfaces between liquid metals and liquid electrolytes, in particular the liquid Hg - electrolyte interface, have played a key role in the development of the theory of the electrical double layer and electrochemical adsorption. More recently, liquid-liquid interfaces have regained interest in the field of material synthesis. Unlike solid interfaces, where strain and stress, heterogeneities, and defects strongly influence growth processes, fluid systems provide soft, defect- and stress-free interfaces. In addition, the growth process profits from the high mobility of atoms, molecules, and particles in both liquids, which allows growth from both sides of the phase boundary. A large variety of metallic and non-metallic nanomaterials has been prepared via electrochemical and electroless deposition at such liquid-liquid interfaces. As demonstrated by Maldonado and coworkers, electrodeposition at liquid metal electrodes even allows the growth of nanostructured crystalline semiconductors via a simple one-step, room-temperature electrochemical process [1].Understanding of the fundamental processes in nucleation and growth at liquid-liquid interfaces is hampered by difficulties in studying these interfaces experimentally on the atomic scale. Most surface-sensitive techniques, especially also scanning probe microscopy methods, cannot access these fluidic phase boundaries. For this reason, the majority of studies relies on electrochemical measurements, optical microscopy, and ex situ investigation of the deposit and thus provide little insight on the initial steps of the growth process. We have shown in the past that hard X-ray scattering methods, such as X-ray reflectivity (XRR) and grazing incidence X-ray scattering (GIXS), are unique tools for determining the atomic liquid-liquid interface structure. In this talk, we present case studies of electrochemically induced growth at liquid interfaces from the first monolayer up to several ten nanometer thick films.The first part discusses the growth of ionic compounds, using lead halides on Hg electrodes as an example. In PbBr2 containing NaF we observed previously growth of a PbBrF layer by operando X-ray scattering. This growth exhibited a complex nucleation and growth behavior, involving a crystalline precursor layer prior to 3D crystal growth [2]. The well-defined subnanometer thick precursor layer provided a template for the subsequent quasi-epitaxial growth of oriented 3D crystallites. Detailed studies on the potential-dependent nucleation and growth kinetics revealed with increasing overpotential a crossover from a low surface density film of large crystals to a compact PbBrF deposit with a saturation thickness of 25 nm [3,4]. In addition, growth on the liquid substrate was found to involve micromechanical effects, such as crystal reorientation and film breakup during dissolution. More recently, we extended these studies to growth in solutions containing only one type of halide anion (Br, Cl, or F). Also here, the formation of precursor layers was observed, indicating that this growth behavior is a general phenomenon.In the second part, joint X-ray scattering studies with Maldonado and coworkers on the electrochemical liquid-liquid-solid deposition of semiconductors from aqueous electrolyte are presented, focusing on Ge electrodeposition on Hg and HgxIn1-x alloy electrodes [5]. We provide evidence for the adsorption of GeO3 - anions on the liquid metal surface and the formation of a crystalline GeO2 adlayer at the positive end of the double layer region. Ge electrodeposition results in nanocrystals, which are separated from the Hg electrode by a water cushion. Furthermore, pronounced Hg surface segregation is found in HgxIn1-x, which protects the electrode surface from oxidation in the potential regime of Ge deposition.[1] Carim, A. I., Collins, S. M., Foley, J. M. & Maldonado, J. Am. Chem. Soc. 133, 13292 (2011)[2] A. Elsen, S. Festersen, B. Runge, C.T. Koops, B. M. Ocko, M. Deutsch, O. Seeck, B. M. Murphy, O. M. Magnussen, Proc. Nat. Acad. Sci., 110, 6663 (2013)[3] B.M. Murphy, S. Festersen, O.M. Magnussen, Nanoscale, 8, 13859 (2016)[4] S. Festersen, B. Runge, C. Koops, F. Bertram, B.M. Ocko, M. Deutsch, B.M. Murphy, O.M. Magnussen, Langmuir, 36, 10905 (2020)[5] D. Pattadar, Q. Cheek, A. Satori, Y. Zhao, P.R. Giri, B. Murphy, O.M. Magnussen, S. Maldonado, Cryst. Growth Des., 21, 1645 (2021)

Nucleation Kinetics and Structure Evolution of Quasi-Two-Dimensional ZnO at the Air-Water Interface: An In Situ Time-Resolved Grazing Incidence X-ray Scattering Study.

The design and synthesis of high-quality two-dimensional (2D) materials with desired morphology are essential for property control. One critical challenge that impedes the understanding and control of 2D crystal nucleation and growth is the inability of direct observation of the nanocrystal evolution process with high enough time resolution. Here, we demonstrated an in situ X-ray scattering approach that directly reveals 2D wurtzite ZnO nanosheet growth at the air-water interface. The time-resolved grazing incidence X-ray diffraction (GID) and grazing incidence X-ray off-specular scattering (GIXOS) results uncovered a lateral to vertical growth kinetics switch phenomenon in the ZnO nanosheet growth. This switch represents the 2D to three-dimensional (3D) crystal structure evolution, which governs the size and thickness of nanosheets, respectively. This phenomenon can guide 2D nanocrystal synthesis with rationally controlled size and thickness. Our work opens a new pathway toward the understanding of 2D nanomaterial growth kinetics based on time-resolved liquid surface grazing incidence X-ray techniques.

Characterizing Interfacial Structures of Dye-Sensitized Solar Cell Working Electrodes.

In this feature article, we discuss the fundamental use of materials-characterization methods that directly determine structural information on the dye···TiO2 interface in dye-sensitized solar cells (DSCs). This interface is usually buried within the DSC and submerged in solvent and electrolyte, which renders such metrological work nontrivial. We will show how ex-situ X-ray reflectometry (XRR), atomic-force microscopy (AFM), grazing-incidence X-ray scattering (GIXS), pair-distribution-function analysis of X-ray diffraction data (gaPDF), and in-situ neutron reflectometry (NR) can be used to deliver specific structural information on the dye···TiO2 interface regarding dye anchoring, dye aggregation, molecular dye orientation, intermolecular spacing between dye molecules, interactions between the dye molecules and the TiO2 surface, and interactions between the dye molecules and the electrolyte components and precursors. Some of these materials-characterization techniques have been developed specifically for this purpose. We will demonstrate how the direct acquisition of such information from materials-characterization experiments is crucial for assembling a holistic structural picture of this interface, which in turn can be used to develop DSC design guidelines. Moreover, we will show how these methodologies can be used in the experimental-validation process of "design-to-device" pipelines for big-data- and machine-learning-based materials discovery. We conclude with an outlook on further developments of this design-to-device approach as well as the materials characterization of more dye···TiO2 interfacial structures that involve known DSC dyes using the methods described herein. In addition, we propose to combine these formally disparate metrologies so that their complementary merits can be exploited simultaneously. New metrologies of this kind could serve as a "one-stop-shop" for the materials characterization of surfaces, interfaces, and bulk structures in DSCs and other devices with layered architectures.

Growth and Degradation Kinetics of Organic–Inorganic Hybrid Perovskite Films Determined by In Situ Grazing‐Incidence X‐Ray Scattering Techniques

Organic-inorganic halide perovskite (OIHP) solar cells hold a great promise for commercial breakthrough since their power conversion efficiency has been pushed beyond the mark of 25%, making them capable of competing with traditional crystalline silicon solar cells. The key to achieve efficient and stable perovskite solar cells is inherently related to the film morphology. The understanding of the kinetic processes of film formation and degradation opens up possibilities to tailor the film morphology via the regulation of precursor and processing parameters. In situ grazing-incidence X-ray scattering (GIXS) techniques allow for tracking the morphology evolution of thin films at different length scales and with high temporal resolution. In this review, the selected examples for application of in situ grazing-incidence wide-angle X-ray scattering and grazing-incidence small-angle X-ray scattering techniques to the growth and stability of OIHPs are summarized after a brief introduction to both techniques, highlighting particularly the morphological evolution of perovskite films over time. Then the correlated mathematical models are reviewed to give a toolbox for analyzing the mechanisms of film formation and degradation. Thus, an overview on the in situ GIXS methods is linked to the research of OIHP kinetics.

Structure-Property Relationships of PFSA-Based Ionomer Thin Films

In energy conversion devices (e.g. fuel cells), the ionomer layer within the heterogeneous catalyst layers (CLs) exhibits increased mass transport resistance, hindering device performance. This increased resistance is attributed to confinement effects of the ionomer to nanoscale thicknesses and greater influence of the ionomer-catalyst interactions under confinement. Chemical modifications to the ionomer side chains have the potential to improve transport and/or alter ionomer-catalyst interactions to overcome the performance limitations. To understand the impact of side chain and end group modifications, we investigate the structure-property relationships of perfluoro sulfonic acid (PFSA)-based ionomers thin films (20 to 100 nm) synthesized by 3M. Thin film water uptake and conductivity are determined via spectroscopic ellipsometry and electrochemical impedance spectroscopy (EIS), respectively, under controlled humidity and correlated with structural changes during hydration characterized via in situ grazing-incidence x-ray scattering (GIXS). The collected data are analyzed to draw structure-hydration-conductivity maps for various film chemistries and used to identify the chemical and structural factors controlling ionomer functionality. The results provide insight into the improvements and limitations on performance attainable by these PFSA-based chemistries. Furthermore, the impact of side chain chemistry on structure-property relationships can be used to guide ionomer design for improved device performance.

Bicontinuous Gyroid Phase of a Water-Swollen Wedge-Shaped Amphiphile: Studies with In-Situ Grazing-Incidence X-ray Scattering and Atomic Force Microscopy.

We report on formation of a bicontinuous double gyroid phase by a wedge-shaped amphiphilic mesogen, pyridinium 4′-[3″,4″,5″-tris-(octyloxy)benzoyloxy]azobenzene-4-sulfonate. It is found that this compound can self-organize in zeolite-like structures adaptive to environmental conditions (e.g., temperature, humidity, solvent vapors). Depending on the type of the phase, the structure contains 1D, 2D, or 3D networks of nanometer-sized ion channels. Of particular interest are bicontinuous phases, such as the double gyroid phase, as they hold promise for applications in separation and energy. Specially designed environmental cells compatible with grazing-incidence X-ray scattering and atomic force microscopy enable simultaneous measurements of structural parameters/morphology during vapor-annealing treatment at different temperatures. Such in-situ approach allows finding the environmental conditions at which the double gyroid phase can be formed and provide insights on the supramolecular structure of thin films at different spatial levels.

Development of an Evaluation Method for a Lithium/Electrolyte Interface Based on X-ray Reflectivity and Grazing Incidence X-ray Scattering Measurements

This study demonstrates the feasibility to investigate the electrolyte-side of the Li/electrolyte interface using an X-ray total reflection phenomenon. By means of a specially designed cell and synchrotron X-ray sources, the reflectivity curve is successfully obtained, where the critical angle is in reasonable agreement with the calculated value. Furthermore, we attempt to perform grazing incidence X-ray scattering measurement and find a characteristic signal, which should strongly reflect structural properties of the electrolyte in the interfacial region.

Influence of Substitutional Groups on the Ordering and Crystallization of Amphiphilic Silsesquioxanes at the Air-Water Interface.

We report on the surface ordering and crystallization sequences in differently organic-substituted amphiphilic polyhedral silsesquioxane (POSS) variants induced by regulated compression at the air-water interface. Such molecular systems floating at the interface serve as a model system to study dynamic crystallization mediated by weak interactions. In situ grazing incidence X-ray scattering (GIXS) measurements, performed at a synchrotron X-ray source using a liquid surface diffractometer and corroborated with Brewster angle microscopy, revealed transformations for the different POSS variants (viz. trisilanol isobutyl POSS (TBPOSS), trisilanol cyclohexyl POSS (TCHPOSS), disilanol octaisobutly POSS (DOBPOSS), and trisilanol isooctyl POSS (TOPOSS)) from a weakly correlated monolayer structure to appreciably different structural and crystalline phases in various packing schemes. GIXS measurements revealed a stable nature of the crystallization of DOBPOSS, varying degrees of metastable crystallization for TCHPOSS and TBPOSS, and complete absence of crystalline phase in TOPOSS molecules. Incidentally, for all POSS variants showing crystalline phases, the motifs always assembled in a triclinic lattice with P1̅ symmetry. For the metastable crystals, preferential surface ordering of the crystallites promotes selective crystalline planes to exhibit preferred tilt angles with respect to the interface. The structural transformations of the differently substituted POSS molecules and their variations therein are attributed to the changing balance of the hydrophobic vs hydrophilic interaction in the layers, which is determined by the anisotropic shape and distribution of substitutional groups over the nanosized core cage of the monomer, steric interaction between nearest dimeric neighbors, as well as the in-plane and out-of-plane assembly of the overlayers.

Study on Alignment Method for Grazing Incidence X-ray Scattering

Study on Alignment Method for Grazing Incidence X-ray Scattering

Revealing Structure and Crystallographic Orientation of Soft Epitaxial Assembly of Nanocrystals by Grazing Incidence X-ray Scattering.

We study the structural coherence of a self-assembled overlayer of PbS nanocrystal (NC) superlattice onto an underlying PbS NC monolayer, which acts as a template. We explore the effect of the templating layer on the structure of the overlayer asemblies by varying interfacial strain and determine the impact of new ligands on their superlattice structure. The overlayers and templates are analyzed by grazing-incidence X-ray scattering and microscopy. We find that differences in the lattice parameters of 7.7% between the two layers are tolerated in terms of a "soft epitaxial" assembly into the body-centered tetragonal superstucture and lead to structural registry within the overlayer. Conversely, at the interface, a lattice mismatch of 24.4% is too large for soft epitaxy and invokes a change in the superlattice. Upon ligand treatment, the overlayer superlattices transform their orientation axis and the NCs orient preferentially. These results provide new insights into mitigating defects in layered, heterostructured NC assemblies.

Morphological Study of Blend Thin Films of Poly(3-hexylthiophene)-&lt;i&gt;block&lt;/i&gt;-polyisobutylene-&lt;i&gt;block&lt;/i&gt;-poly(3-hexylthiophene):Poly(3-hexylthiophene) and Their Application to Photovoltaics

The detailed morphological studies of poly(3-hexylthiophene)(P3HT)-b-polyisobutylene(PIB)-b-P3HT:P3HT blend thin films by grazing incidence X-ray scattering (GIXS) were reported. The results of GIXS experiments indicated the formation of phase separation between P3HT and PIB domains even after blending the P3HT homopolymer below 30 wt%. The change in d-spacing values of the phase-separated under strains was suppressed by increasing the weight ratio of the P3HT homopolymer, probably due to the disruption of the microphase separation. The blending the P3HT homopolymer induced the edge-on orientation during the strain process below 75%, probably by improving the interaction between P3HT domains in the blend thin films. The OPV characteristics were obtained with the device structure of ITO/PEDOT:PSS/P3HT-b-PIB-b-P3HT:P3HT:PC61BM/Ca/Al, significantly improving the JSC and FF values by increasing the P3HT homopolymer weight ratio. The excellent elongation behavior of P3HT-b-PIB-b-P3HT:P3HT blend bulk films (P3HT < 30 wt%) could be achieved, probably due to the formation of microphase separation between semi-crystalline P3HT and rubbery PIB domains.

Fractionation of clay colloids and their synthetic utility in vanadium hydroxide-clay thin film formation

Stable mineral colloids play a role in the mobilization of actinide contaminants in the environment. In this paper we present a method to isolate and identify colloids released from high surface area sorbents associated with a proposed clay mineral barrier in a nuclear waste repository. The mineral colloids, which have a strong tendency to remain dispersed in solution, were separated from Al13- or (CeAl)-pillared clay sediment, adsorbed on glass together with hydrolyzed vanadium(IV) species and studied by a variety of techniques including grazing-incidence X-ray scattering (GIXS), small-angle X-ray scattering (SAXS), Fourier transform-infrared spectroscopy (FT-IR) and atomic force microscopy (AFM). We show that the clay platelets are dimensionally orientated in the continuous, transparent 100 nm thick film until the onset of vanadium oxides above 350 °C. Organic matter and the secondary mineral quartz limit the swelling capacity of the platelets, an indication that the stable colloids originate from a part of the native clay that could not be intercalated by aluminum cations. X-ray scattering from oriented clay films intercalated by aluminum- and mixed cerium/aluminum oxo-hydroxy cations are also investigated by the GIXS technique.

Effect of Film Thickness on Induction Time of Isothermal Crystallization from the Melt of Poly(3-hydroxybutyrate) Investigated by Time-resolved Grazing-incidence X-ray Scattering Measurements Using Synchrotron Radiation

Effect of Film Thickness on Induction Time of Isothermal Crystallization from the Melt of Poly(3-hydroxybutyrate) Investigated by Time-resolved Grazing-incidence X-ray Scattering Measurements Using Synchrotron Radiation

Nanometer-Sized Crystalline Clusters of IGZO Films Determined from the Grazing Incidence X-ray Scattering and Anomalous X-ray Scattering Data Combined with Reverse Monte Carlo Simulations

Grazing incidence X-ray scattering measurements have been carried out on c-axis aligned crystalline-indium gallium zinc oxide (CAAC-IGZO) film and nanocrystalline category-indium gallium zinc oxide (NC-IGZO) film and the following results were obtained: (1) the characteristic layered structure of the IGZO crystal did not hold its shape and the X-ray scattering profile showed only a relatively sharp first peak at the wave vector (Q) = 21.8 for CAAC film and 23.1 nm−1 for NC film, respectively, and additional weak broad peaks were observed at a higher angle. (2) In the case of the CAAC film, tiny peaks were observed at Q = 7 and 14 nm−1, corresponding to the positions of the 003 and 006 reflections, respectively, of the IGZO crystal. Such tiny peaks were not detected in the case of NC film but the asymmetry of the first peak at the low angle side was clearly observed. (3) These structural features implied that more than three polyhedral units, such as InOx (x = 4–6), GaOy (y = 4–6), and ZnOz (z = 4–6), were likely to coexist. It is appropriate to call this structural feature as cluster-1. (4) A composite-type structure formed by combining these polyhedral units is also likely to exist and leads to middle-range ordering. This structure is called cluster-2. The size of such cluster-2 has been estimated to be 2.2 nm for CAAC film and 1.8 nm for NC film using the measured pair distribution function. To gain insights into the structural features of IGZO films, realistic atomic-scale models were obtained to fit not only the ordinary interference function of grazing incidence X-ray scattering but also the environmental interference function of the anomalous X-ray scattering (AXS) with Zn-absorption edge using reverse Monte Carlo (RMC) simulation. (5) The resultant models indicated the complex and irregular atomic arrangements of two types of IGZO films, which are well characterized by nanometer-sized crystalline clusters. This characteristic feature may be referred to as crystalline–cluster–composite (triple C) structure.

(Invited) Interactions in Ionomers: From Cations to Confinement

Ion-conductive polymers (ionomers) have been widely used as the solid-electrolyte in energy conversion devices, such as polymer-electrolyte fuel cells (PEFCs) fuel cells, redox flow batteries and water-splitting devices. Ionomers, such as perfluorosulfonic-acid (PFSA), function as the separator-electrolyte in these devices due their high proton conductivity and chemical-mechanical stability.1 In PEFCs, however, ionomers play another key role in Catalyst Layer (CL) structure, where they exist as nanometer-thick electrolyte “thin film" to facilitate transport of ionic and gaseous species to the catalytic particles. As a cathode catalyst film, an ionomer is expected to transport the oxygen molecules to the catalytic sites, which differs dramatically from its gas-separator functionality.2 Once confined to these nanometer thicknesses (< 100 nm), ionomer properties change drastically from the bulk (solid-electrolyte PEM), and are significantly more influenced by its interfaces with the free surface (air) and the catalytic surfaces comprised of carbon and platinum(Pt)-alloyed particles.1,2 In particular, mass-transport limitations in CLs are believed to be related to the transport resistances at the ionomer thin-film, mitigation of which requires a fundamental understanding of ionomer interfaces.3-5 In addition, ionomers, whether as PEMs or in CLs, are exposed to various cations arising from contamination, migration of radical scavengers (such as Cerium), dissolution of catalyst particles (such as Co-alloys), all of which alter the ionomer’s interactions and impact the cell performance.3,6 Thus, throughout this thickness range, it is not only the functionality of ionomer that differs, but also its morphology and transport properties, which are controlled by various interactions. This talk will present an overview of various interactions, from cationic and catalytic particles to dynamic interfaces, and how they influence an ionomer’s intrinsic morphology and transport properties across lengthscales, with a focus on thin films. The latter is in particularly important as the interactions and confinement-driven changes in thin films create an intriguing interplay governing ionomer functionality. We will explore structure-property relationship PFSA ionomers of several chemistries and thicknesses (from micrometer to 10’s of nanometers) in the presence of various types and doping levels of cations to elucidate the underlying origins of governing interactions. Water uptake and swelling behavior as well as nano-morphology of cation-exchanged PFSA membranes and thin films are studied to identify the key factors and interactions impacting their behavior. We will also discuss the role of side-chain chemistry in these interactions, especially in nano-confined thin films. Lastly, I will summarize recent progress in ionomer thin-film research, including improved model systems for thin films that can mimic fuel-cell catalyst structure and environment, and advanced characterization techniques, such as Grazing-Incidence X-ray scattering (GIXS), and how they can be used to probe various interactions. Acknowledgements We acknowledge Adam Z. Weber and Douglas Kushner of Berkeley Lab for helpful discussions, and Michael Yandrasits and Andrew Haug of 3M for helpful discussions and supplying ionomer materials. This work made use of facilities at the Advanced Light Source (ALS), supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy, and was funded under the Fuel Cell Performance and Durability Consortium (FC-PAD), by the Fuel Cell Technologies Office, of the office of the Energy Efficiency and Renewable Energy (EERE), of the U. S. Department of Energy (under contract number DE-AC02-05CH11231). References A. Kusoglu and A. Z. Weber, Chem. Rev., 117, 987 (2017).A. Kusoglu, in Encyclopedia of Sustainability Science and Technology, R. A. Meyers Editor, p. 1, Springer New York, New York, NY (2018).A. Kongkanand and M. F. Mathias, J Phys Chem Lett, 7, 1127 (2016).A. Z. Weber and A. Kusoglu, J Mater Chem A, 2, 17207 (2014).M. Tesfaye, A. N. MacDonald, P. J. Dudenas, A. Kusoglu and A. Z. Weber, Electrochem Commun, 87, 86 (2018).A. M. Baker, R. Mukundan, D. Spernjak, E. J. Judge, S. G. Advani, A. K. Prasad and R. L. Borup, J. Electrochem. Soc., 163, F1023 (2016).

Investigation of Drying Process of Catalyst Ink for Polymer Electrolyte Fuel Cells By Grazing-Incidence X-Ray Scattering

Polymer electrolyte fuel cells (PEFCs) are attractive power converters for various applications including electric vehicles and stationary powers without emitting CO2. The electrochemical reaction to generate electric power occurs at the interface of catalyst nanoparticle and ionomer in a catalyst layer on electrodes. The catalyst layers consist of Pt catalyst nanoparticles supported on carbon (Pt/C) and ionomer as the binder. The fine structure of the catalyst layer such as the connected structure of carbon support, and the interconnected pore structure, and the state of ionomer coverage on carbon support, governs the performance of PEFCs. The catalyst layer is fabricated by coating of the catalyst ink consisting of the Pt/C, ionomer, and solvents. Precise structural analyses of the catalyst ink and the catalyst layer have been reported. However, the determining factor for the catalyst layer formation is still unclear. In this study, we performed in situ grazing-incidence small-angle x-ray scattering (GISAXS) measurements to investigate the aggregation process of Pt/C during the drying process of the catalyst ink. The catalyst ink was prepared by blending Pt/C (TEC10E50E, Tanaka Kikinzoku Kogyo), ionomer (Nafion® dispersion, DE2020, Chemours) and mixture of water/1-propanol (NPA) (1/1 v/v) at 6.5/3.5/90 w/w/w. The weight ratio of ionomer to carbon was 1:1. The catalyst ink was coated on a Si wafer using an automatic film applicator at 25 °C. The stainless-steel applicator with a gap of 100 μm height and 40 mm width was used as the blade. The drawing speed was fixed at 20 mm s-1 for all coatings. In situ GISAXS measurements were carried out at the BL45XU beam line of SPring-8. The X-ray with 0.100 nm in wavelength impinged onto the coating film under an incident angle of 0.12° and the scattering data were collected with 2D detector (PILATUS3X 2M Dectris Ltd.) located at a distance of 3.5 m from the sample. Figure 1a and 1b show 2D GISAXS patterns collected at several elapsed times (t), and the scattering intensities obtained from vertical line cuts of 2D patterns at qy = 7.37×10−2 nm−1 in a width of Δqy = 6.13×10−2 and plotted against qz , respectively. As seen in Figure 1b, the intensity profile at t = 70 s includes a maximum at qz = 0.2 nm−1, which shifts toward smaller qz with increasing t from 70 s to 250 s. It suggests that 30-nm-sized Pt/C primary particles aggregate to increase the size and spacing with the solvents evaporate. With further increasing t, the intensity profile did not change, indicating that the nanoscale structure was fixed at t = 250 s. In contrast, the gravimetry for the same process indicates that the decrease of the catalyst ink weight due to solvents evaporation completes at t = 1600 s. These results show that the aggregation of Pt/C particles mostly completes in the first half of drying process This work was supported by the PEMFC Research and Development Program for “Highly-Coupled Analysis of Phenomena in MEA and its Constituents and Evaluation of Cell Performance” from the New Energy and Industrial Technology Development Organization, NEDO, Japan. The synchrotron radiation experiments were performed at BL45XU beam line in SPring-8 with the approval of JASRI (Proposal No. 2017A1002 and 2017B1006). Figure 1

Investigation of Drying Process of Catalyst Ink for Polymer Electrolyte Fuel Cells by Grazing-Incidence X-Ray Scattering

The performance of polymer electrolyte fuel cells (PEFCs) depends on the structure of catalyst layer which is formed by coating catalyst ink. This study investigated the nanostructural formation of catalyst layer during ink drying process using time-resolved grazing-incidence small-angle X-ray scattering (GISAXS). GISAXS patterns measured for the catalyst ink coated with a 100 μm thickness on a silicon wafer suggest that Pt/C aggregation completed at drying time (t) of 250 sec although the ink was completely dried at t = 1600 sec. Thus, the Pt/C aggregate structure was mostly pinned in the early stage of the drying.

Ionomer Thin Films for PEM Fuel Cells

Ion-conductive polymers (ionomers) such as perfluorosulfonic-acid (PFSA) have been widely adopted in polymer electrolyte fuel cells (PEFCs) as a polymer-electrolyte membrane (PEM) due to their ability to conduct ions in hydrated conditions without losing stability. The role of ionomers in PEFCs, however, is not limited to PEM as it also extends to the porous catalyst layers (CL) where ionomers exist as nanometer-thick electrolyte thin films binding the catalytic agglomerates and facilitating proton transport. Moreover, the ionomer’s properties differ from the bulk (PEM) when it is confined to these nanometer thicknesses where its behavior is influenced by its interfaces with the air and the catalytic sites, such as carbon/platinum. In particular, this phenomenon of confinement onto Pt sites could have important implications for the mass-transport limitations observed in fuel-cell catalyst layers, which have been related to the transport resistances at the ionomer thin-film. Thus, despite an ever-growing number of studies on ionomer membranes for PEM, for desired next-generation PEFCs, it is important to characterize catalyst ionomers and elucidate factors controlling their transport properties. In particular, if is of great interest to develop model systems for thin films that can mimic catalyst ionomer behavior and establish their structure/functionality to optimize ionomer interfaces and improve CL performance. This tutorial talk will present an overview of PFSA ionomer thin films and their current state of understanding with a focus on recent developments in advanced diagnostics and the information they can provide to establish their structure/function relationship. In addition, applicability of the various diagnostics techniques in the thin-film regime (5 to 500 nm) will be discussed, including the use of state-of-the-art techniques, such as Grazing-Incidence X-ray scattering (GIXS). First, thin films of dispersion-cast ionomers on model substrates will be explained in relation to catalyst ionomers. Swelling and water uptake behavior as well as various transport properties of these thin films will be examined. Then, it will be discussed how an ionomer’s phase-separated nanostructure and properties are correlated and impacted by film thickness (i.e., confinement effect) and substrate interactions, especially below 50 nm. Lastly, it will be explored how EW and side-chain chemistry influence the nature of ionomer-substrate interactions and thus the structure and properties of PFSA thin films. The collected data set will be analyzed to illustrate the bulk-to-film transition of ionomers of various equivalent weights used in fuel-cells. The results will be elucidated to highlight the key aspects of ionomer thin films, provide insight into optimization of their structure and functionalities for fuel-cell catalysts, and discuss implications for PEFC performance.