Titanium-based porous transport layers (PTLs) and iridium-based catalyst layers (CLs) are two main components of proton exchange membrane water electrolyzers (PEMWEs). PTLs are typically coated with platinum to minimize interfacial losses and to support long-term operation. Optimizing coatings and the PTL-CL interface requires comprehensive characterization. This study establishes time-of-flight secondary ion mass spectrometry (ToF-SIMS) as a valuable technique for PTL characterization, addressing capabilities and limitations related to PTL morphology. A methodology was developed that uses a Cs+ sputter beam for dynamic depth profiling, with data collected in both positive-ion (MCs+) and negative-ion modes to generate depth profiles, 2D ion maps, and 3D ion reconstructions. ToF-SIMS detected relative differences in platinum-layer thickness between samples; these trends were validated by cross-sectional scanning transmission electron microscope (STEM) measurements and flat-titanium substrate controls. Interfacial oxide layers are identified in both ion modes, with enhanced oxide sensitivity in negative mode. The technique's high sensitivity enables detection of nanometer-scale coatings and trace impurities within the bulk PTL structure. These results provide a methodological framework for analyzing Pt-coated PTLs, with the potential to extend to other components in PEMWEs and other electrolyzer systems.

Triple-conducting oxides (TCOs) are an emerging class of mixed ionic and electronically conducting materials that show great promise for oxygen reduction/evolution (ORR/OER) electrocatalysis-primarily in high-temperature ceramic electrochemical cells- but also in aqueous alkaline environments. Their high activity is attributed, at least in part, to their ability to incorporate and transport three mobile charge carriers: protons, oxygen vacancies, and electron-holes. Despite their promise, fundamental studies of TCOs are challenging, as transport dynamics from three charge carriers cannot be fully disentangled via traditional electrical measurement techniques. Characterizing proton dynamics in TCOs is particularly difficult as protons are generally the minority carrier, and their conduction response is typically obscured by the oxygen vacancies and electron holes. Here, we demonstrate successful isolation of the proton behavior in an archetypal TCO, BaCo0.4Fe0.4Zr0.1Y0.1O3-delta (BCFZY4411), using a combination of non-electrical techniques. We determine proton uptake and oxygen non-stoichiometry (delta) using thermogravimetric analysis (TGA). X-ray absorption near edge structure (XANES) and neutron diffraction (ND) are used to validate the oxidation state of Co and the delta values obtained through TGA. We apply H-1 solid-state magic-angle-spinning (MAS) nuclear magnetic resonance (NMR) to provide insights into local structure, dynamics, and proton kinetics. Finally, the proton transport properties are further quantified using tracer isotope exchange with time-of-flight secondary ion mass spectrometry (ToF-SIMS). Despite the very low proton concentrations in BCFZY4411 (<0.2 % under most conditions), our analysis suggests that the oxygen reduction and evolution reactions are nevertheless limited by the oxygen ion kinetics (e.g., oxygen surface exchange) rather than the proton kinetics at the reduced operating temperatures (<500 degrees C) that are targeted for electrochemical cell applications. These findings provide a comprehensive understanding of proton behavior in BCFZY4411 and pave the way for advancing the fundamental study of TCOs.

Type II silicon clathrates, with their unique cage-like structure, offer exciting potential for applications in thermoelectrics, photovoltaics, and quantum materials due to their tunable electronic and thermal properties. This study investigates the use of ion implantation to introduce targeted guest atoms, which act as dopants, into type II Si clathrate films. The focus is on Mg as a test case for implantation, a dopant previously unreported in type II Si clathrates. The effects of ion implantation on the metastable Si clathrate structure were examined through systematic investigation of implant parameters. Time-of-flight secondary ion mass spectrometry depth profiling confirmed the successful implantation of Mg, while X-ray diffraction and confocal Raman spectroscopy demonstrated minimal structural damage at lower fluences, with the clathrate framework retaining its integrity without converting to other phases. At the higher end of the fluence range implantation caused localized transitions from clathrate to amorphous silicon. Implant activation using rapid thermal annealing was examined with the clathrate structure stable up to 500 degrees C and being converted to diamond silicon above this temperature. Post-implantation and activation structural characterization showed evidence of damage reversal. Electron paramagnetic resonance studies provided indirect evidence of dopant incorporation. These findings establish a foundation for introducing alternative guests/dopants into the Si clathrate cages through ion implantation, advancing their tunability for next-generation quantum and optoelectronic devices.

Diffusion of native defects such as vacancies and their interactions with impurities are fundamental to semiconductor crystal growth, device processing, and design. However, the transient equilibration of native defects is difficult to directly measure. We used (AlxGa1−x)2O3/Ga2O3 superlattices (SLs) to detect and analyze transient diffusion of cation vacancies during annealing in O2 at 1000–1100 °C. Using a novel finite difference scheme for diffusion with time- and space-varying diffusion constants, we determined diffusion constants for Al, Fe, and cation vacancies, including the vacancy concentration dependence for Al. In the case of SLs grown on Sn-doped β-Ga2O3 (010) substrates, gradients observed in the extent of Al diffusion indicate a supersaturation of vacancies in the substrates that transiently diffuse through the SLs coupled strongly to Sn and thus slowed compared to undoped cases. In the case of SLs grown on (010) Fe-doped substrates, the Al diffusion is uniform through the SLs, indicating a depth-uniform concentration of vacancies. We find no evidence for the introduction of VGa from the free surface at rates sufficient to affect Al diffusion at at. % concentrations, establishing an upper bound on surface injection. In addition, we show that unintentional impurities in Sn-doped Ga2O3 such as Fe, Ni, Mn, Cu, and Li also diffuse toward the surface and accumulate. Many of these likely have fast interstitial diffusion modes capable of destabilizing devices, thus suggesting that impurities may require further reduction. This work provides a method to measure transients in diffusion-mediating native defects otherwise hidden in common processes such as ion implantation, etching, and film growth.

Advancements towards more efficient design and manufacturability of polymer electrolyte membrane electrolyzers include recent efforts to better integrate the porous transport layer (PTL) and adjacent anodic catalyst layer (CL), forming the porous transport electrode (PTE). It is well known that characterizations of the PTL alone, especially when combined with a protective coating, present numerous challenges. The PTE structure is even more complex, and it is difficult to comprehensively characterize all components and interfaces. In prior work, we established Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS) as a characterization technique for the PTLs coated with protective coatings. This work demonstrates ToF-SIMS as a powerful technique for the characterization of PTLs integrated with CLs. A series of sintered Ti PTLs were first coated with protective Pt coatings and then coated with various IrO x and IrRuO x catalysts via electrodeposition. Prepared PTE samples ranged in catalyst loadings and catalyst composition. Selected samples were also subjected to annealing post-treatments to further modify catalyst composition. X-ray photoelectron spectroscopy (XPS) was conducted to identify surface chemical speciation and states of the catalysts. ToF-SIMS analysis was conducted using Cs in both positive and negative modes to identify chemical species, including Ir, Ru, and their oxides as a function of catalyst layer depth. These two methods were complemented by Scanning Electron Microscopy and Energy Dispersive X-ray Spectroscopy (SEM- EDS) analysis to evaluate morphology and distribution of catalysts. This comprehensive physical and chemical characterization of the PTEs provides insights that are useful for the characterization of as produced, modified, and degraded PTE samples.

Postprocessing of additively manufactured (AM) metal parts to remove support structures or improve the surface condition can be a manually intensive process. One novel solution is a two-step, self-terminating etching process (STEP), which achieves both support removal and surface smoothing. While the STEP has been demonstrated for laser powder bed fusion (L-PBF) 316L stainless steel, this work evaluates the impact of pre-STEP heat treatments and resulting changes in dislocation density and microstructure on the resulting surface roughness and amount of material removed. Two pre-STEP heat treatments were evaluated: stress relief at 470°C for 5 h and recrystallization-solution annealing at 1060°C for 1 h. Additionally, one set of specimens was processed without the pre-STEP heat treatment (as-printed condition). Dislocation density and phase composition were quantified using X-ray diffraction along with standard, metallurgical stain-etching techniques. This work, for the first time, highlights the mechanisms of sensitization of AM L-PBF 316L stainless steel and provides fundamental insights into selective etching of these materials. Results showed that the sensitization depth decreased with increasing dislocation density. For samples etched at a STEP bias of 540 mVSHE, material removal terminated at grain boundaries; therefore, the fine-grained stress-relieved specimen had the lowest post-STEP surface roughness. For surface roughness optimization, parts should be stress relived pre-STEP. However, to achieve more material removal, pre-STEP solution annealing should be performed.

At low guest atom concentrations, Si clathrates can be viewed as semiconductors, with the guest atoms acting as dopants, potentially creating alternatives to diamond Si with exciting optoelectronic and spin properties. Studying Si clathrates with different guest atoms would not only provide insights into the electronic structure of the Si clathrates but also give insights into the unique properties that each guest can bring to the Si clathrate structure. However, the synthesis of Si clathrates with guests other than Na is challenging. In this study, we have developed an alternative approach, using thermal diffusion into type II Si clathrate with an extremely low Na concentration, to create Si clathrate with Li guests. Using time-of-flight secondary-ion mass spectroscopy, X-ray diffraction, and Raman scattering, thermal diffusion of Li into the nearly empty Si clathrate framework is detected and characterized as a function of the diffusion temperature and time. Interestingly, the Si clathrate exhibits reduced structural stability in the presence of Li, converting to polycrystalline or disordered phases for anneals at temperatures where the starting Na guest Si clathrate is quite stable. The Li atoms inserted into the Si clathrate lattice contribute free carriers, which can be detected in Raman scattering through their effect on the strength of Si-Si bonds in the framework. These carriers can also be observed in electron paramagnetic resonance (EPR). EPR shows, however, that Li guests are not simple analogues of Na guests. In particular, our results suggest that Li atoms, with their smaller size, tend to doubly occupy cages, forming "molecular-like" pairs with other Li or Na atoms. Results of this work provide a deeper insight into Li guest atoms in Si clathrate. These findings are also relevant to understanding how Li moves through and interacts with Si clathrate anodes in Li-ion batteries. Additionally, techniques presented in this work demonstrate a new method for filling the Si clathrate cages, enabling studies of a broad range of other guests in Si clathrates.

As we move towards commercialization of the proton exchange membrane water electrolyzers (PEMWEs), fundamental understandings of individual components and their integration with focus on material composition and its correlation to performance and lifetime of electrolysis cell operation is of significant importance. Porous Transport Layers (PTLs) increase overpotential of the cell when the titanium passivates, i.e. develops a titanium oxide surface layer of a certain thickness. To prevent this passivation, platinum group metal coatings are typically applied to the PTL, but this adds significant costs. This presentation will cover recent developments in characterization of PTL coatings and PTL-coating interfaces using Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS) in correlation with Scanning Transmission Electron Microscopy and Energy Dispersive X-ray Spectroscopy (STEM-EDS) analysis. Although this technique is extremely beneficial, ToF-SIMS is typically applied to flat substrates and therefore has been mainly utilized in other scientific fields such as the thin film industry. This talk will discuss method validation and optimization that has been conducted for PTL analysis to allow its characterization by ToF-SIMS. Topics include understanding parameters and limitation of using SIMS on these morphologically challenging samples, as well as looking at elemental and species identification, and applying depth profiling to gauge information on coating and oxide thickness and their consistency. Correlations between real and flat substrates prepared under similar conditions are made, allowing standardization of depth profiling for future analysis of a wider variety of samples.

In Part 2 of this paper series, high-throughput tensile testing and characterization of porosity, microstructure, and oxygen content was carried out on Ti–6Al–4V samples that were fabricated via electron beam powder-bed fusion (PBF-EB) to test our hypothesis that current industrial powder mixing strategies lead to stochastic chemistry content and tensile property variation throughout a given build. Fifteen rectangular blocks were built at locations across the entire build plate and machined into high-throughput tensile racks that contained 25 mini tensile coupons each (375 total specimens). There was a spread of 138.9 MPa in yield strength (YS) across all specimens. Considering the YS spread was only 74.2 MPa for a virgin build with no mixing, as detailed in Part 1, the difference between these two YS ranges is expected to be attributed to mixing. To determine the cause for this observed spread in YS, systematic investigation of porosity, microstructure, and oxygen content was performed via scanning electron microscopy (SEM), micro X-ray computed tomography (μXCT), inert gas fusion, and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The large variation in YS for these specimens could not be attributed to internal porosity, fracture surface porosity, α-lath thickness, or crystallographic texture. Inert gas fusion measurements indicated large, stochastic variation in oxygen content, and measurements via ToF-SIMS near the fracture surfaces indicate localized chemistry variations that could be responsible for the large variations in tensile properties, specifically the yield strength. Specific alternative powder mixing and reuse methodologies are proposed to mitigate undesirable chemistry and tensile property variation within PBF-EB Ti–6Al–4V.

A BaCo0.7Fe0.1Zr0.1Y0.1O3−δ (BCFZY7111)-based button cell shows a peak power density of 695 mW cm−2 at 600 °C and a current density of 1976 mA cm−2 at 1.4 V in fuel cell and electrolysis modes.

A two-step process for the synthesis of the silicon clathrate film on a diamond silicon wafer is explored in detail. Key factors impacting the film quality are uncovered. We find that the optical properties of the films are strongly influenced by inhomogeneities and defect phases that dominate the top surface and grain boundaries of the material. For the first time, we systematically develop two approaches for minimizing the effects of defective structures and allow intrinsic properties of the clathrate material to be probed. One is separating the film surface from the Si substrate to expose the buried high-quality interface, and the other one is wet or dry etching of the clathrate film to remove the disordered material which is more heavily concentrated at the top surface. With high-quality clathrate surfaces and films produced, more reliable optical measurements are taken and interpreted. Techniques in this work provide a pathway for Si clathrate thin film toward an optically efficient alternative crystalline form of Si that can transform Si-based applications in optoelectronics.

Magnesium zinc oxide (MZO) is a promising front contact material for CdTe solar cells. Due to its higher band gap than traditional CdS, MZO can reduce parasitic absorption to significantly increase short-circuit current density while also providing a benefit of conduction band offset tuning through Mg:Zn ratio optimization. MZO has been successfully implemented into CdTe devices, however its stability has been of concern. The MZO stability issue has been attributed to the presence of oxygen in the CdTe device processing ambient, leading to double-diode behavior (S-kink) in the current density-voltage curves. Here we report on MZO thin films deposited by reactive co-sputtering. The reactively co-sputtered MZO thin films have encouraging stability, show no significant variation in work function of the surface over a period of 6 months, as measured by Kelvin probe. Energy conversion efficiencies of around 16% have been achieved both with and without presence of oxygen in device processing ambients across multiple research facilities. These efficiencies should be possible to increase further by tuning of the thin film deposition and device processing parameters, especially through optimization of the back contact.