Exploring the Surface Segregation of Rh Dopants in PtNi Nanoparticles through Atom Probe Tomography Analysis

Proton-exchange membrane fuel cells hold promise as energy conversion devices for hydrogen-based power generation and storage. However, the slow kinetics of oxygen reduction at the cathode imposes the need for highly active catalysts, typically Pt or Pt based, with a large available area. The scarcity of Pt increases the deployment and operational cost, driving the development of novel highly active material systems. As an alternative, a Rh-doped PtNi nanoparticle has been suggested as a promising oxygen reduction catalyst, but the three-dimensional distributions of constituent elements in the nanoparticles have remained unclear, making it difficult to guide property optimization. Here, a combination of advanced microscopy and microanalysis techniques is used to study the Rh distribution in the PtNi nanoparticles, and Rh surface segregation is revealed, even with an overall Rh content below 2 at. %. Our findings suggest that doping and surface chemistry must be carefully investigated to establish a clear link with catalytic activity that can truly be established

Effect of Heat Treatment Temperature on the Crystallization Behavior and Microstructural Evolution of Amorphous NbCo_1.1Sn

Heat treatment-induced nanocrystallization of amorphous precursors is a promising method for nanostructuring half-Heusler compounds as it holds significant potential in the fabrication of intricate and customizable nanostructured materials. To fully exploit these advantages, a comprehensive understanding of the crystallization behavior of amorphous precursors under different crystallization conditions is crucial. In this study, we investigated the crystallization behavior of the amorphous NbCo1.1Sn alloy at elevated temperatures (783 and 893 K) using transmission electron microscopy and atom probe tomography. As a result, heat treatment at 893 K resulted in a significantly finer grain structure than heat treatment at 783 K owing to the higher nucleation rate at 893 K. At both temperatures, the predominant phase was a half-Heusler phase, whereas the Heusler phase, associated with Co diffusion, was exclusively observed at the specimen annealed at 893 K. The Debye–Callaway model supports that the lower lattice thermal conductivity of NbCo1.1Sn annealed at 893 K is primarily attributed to the formation of Heusler nanoprecipitates rather than a finer grain size. The experimental findings of this study provide valuable insights into the nanocrystallization of amorphous alloys for enhancing thermoelectric properties

Trace-Level Cobalt Dopants Enhance CO₂ Electroreduction and Ethylene Formation on Copper

The development of Cu-based catalysts for electrochemical CO2 reduction reaction (CO2RR) with stronger CO-binding elements had been unsuccessful in improving multicarbon production from the CO2RR due to CO-poisoning. Here, we discover that trace doping levels of Co atoms in Cu, termed CoCu single-atom alloy (SAA), achieve up to twice the formation rate of CO as compared to bare Cu and further demonstrate a high jC2H4 of 282 mA cm–2 at −1.01 VRHE in a neutral electrolyte. From DFT calculations, Cu sites neighboring CO-poisoned Co atomic sites accelerate CO2-to-CO conversion and enhance the coverage of *CO intermediates required for the formation of multicarbon products. Furthermore, CoCu SAA also exhibits active sites that favor the deoxygenation of *HOCCH, which increases the selectivity toward ethylene over ethanol. Ultimately, CoCu SAA can simultaneously boost the formation of *CO intermediates and modulate the selectivity toward ethylene, resulting in one of the highest ethylene yields of 15.6%

Atom-Scale Chemistry in Chalcopyrite-Based Photovoltaic Materials Visualized by Atom Probe Tomography

Chalcopyrite-based materials for photovoltaic devices tend to exhibit complex structural imperfections originating from their polycrystalline nature; nevertheless, properly controlled devices are surprisingly irrelevant to them in terms of resulting device performances. The present work uses atom probe tomography to characterize co-evaporated high-quality Cu­(In,Ga)­Se2 (CIGS) films on flexible polyimide substrates either with or without doping with Na or doping with Na followed by K via a post-deposition treatment. The intent is to elucidate the unique characteristics of the grain boundaries (GBs) in CIGS, in particular the correlations/anti-correlations between matrix elements and the alkali dopants. Various compositional fluctuations are identified at GBs irrespective of the presence of alkali elements. However, [Cu-poor and Se/In,Ga-rich] GBs are significantly more common than [Cu-rich and Se/In,Ga-poor] ones. In addition, the anti-correlations between Cu and the other matrix elements are found to show not only regular trends among themselves but also the association with the degree of alkali segregation at GBs. The Na and K concentrations exhibited a correlation at the GBs but not in the intragrain regions. Density functional theory calculations are used to explain the compositional fluctuations and alkali segregation at the GBs. Our experimental and theoretical findings not only reveal the benign or beneficial characteristics of the GBs of CIGS but also provide a fundamental understanding of the GB chemistry in CIGS-based materials