Docheon Ahn

The durability of Pt/C electrocatalysts remains a central challenge for proton-exchange membrane fuel cells (PEMFCs), where Pt nanoparticle agglomeration and dissolution drive the loss of electrochemically active surface area (ECSA) during operation. Although ECSA loss is driven by coupled changes in Pt nanoparticle morphology and surface chemical state, these two contributions are difficult to separate. Here, we report a combined characterization using synchrotron-based operando small-angle X-ray scattering (SAXS) and ex situ photoemission spectroscopy to correlate nanoparticle growth with the evolution of surface chemical states in high-surface-area carbon-supported Pt catalysts. Operando SAXS reveals an overall increase in the apparent diameter of Pt nanoparticles during accelerated stress test (AST) cycling, consistent with irreversible particle growth. This increasing trend is periodically interrupted by decreases in the apparent diameter after each electrochemical cleaning step inserted between AST segments. Further ex situ photoemission analysis of the degraded Pt/C electrodes reveals Pt–O(H)-related surface signatures that depend on the preceding AST and cleaning conditions. These findings highlight the importance of combining hard X-ray operando scattering with ex situ soft X-ray surface-sensitive spectroscopy to relate the evolution of surface adsorption to that of nanoparticle morphology during electrocatalyst degradation.

Lithium carbonate (LC) is markedly cheaper than lithium hydroxide (LH) but remains underused for layered oxide cathodes because LC‐derived materials perform poorly. Here, using a mid‐Ni cathode, LiNi _0.65 Co _0.05 Mn _0.30 O ₂ (N65), we trace this performance gap to LC‐induced defects that span length scales, from atomic‐scale disorder to nanoscale and microscale porosity. These defects originate from three features of the carbonate pathway that act across the solid, liquid, and gas phases, namely decoupled dehydration and lithiation, late melting of the carbonate, and a locally oxygen‐deficient atmosphere from evolving CO ₂ . Guided by this picture, we heal the defects with a single secondary heat treatment (SH) that adds an optimized amount of LH, simultaneously replenishing lithium, restoring an oxidizing atmosphere, and densifying the structure. The resulting N65‐LC SH achieves discharge capacity, rate capability, and cycling stability comparable to those of the LH counterpart, while cutting the lithium precursor cost by 9% to 16% relative to conventional all‐LH synthesis. More broadly, the gap between LH and LC is not an intrinsic property of the carbonate but a consequence of a precursor‐specific pathway that, once traced, can be engineered around to unlock low‐cost, high‐performance Ni‐based layered oxide cathodes.

We have developed controlled H ₂ -flow-induced oxygen vacancies in NaCrO ₂ cathodes for Na-ion batteries. The presence of oxygen vacancies facilitated electron/ion transfer and robust structural stability.

Developing highly durable and efficient catalysts to overcome the sluggish oxygen reduction reaction (ORR) remains a critical bottleneck for the commercialization of heavy-duty proton exchange membrane fuel cells (PEMFCs). Conventional Pt-based catalysts supported on amorphous carbon are susceptible to significant performance loss due to nanoparticle instability and carbon support corrosion, particularly under high-potential and long-term PEMFC operating conditions. Herein, we report a highly durable ORR catalyst based on a fluorine-doped tin oxide (FTO)-modified highly crystalline carbon (HCC) hybrid support, designed to enhance catalyst activity and stability. The HCC framework provides intrinsic reliability under harsh electrochemical oxidation conditions, while the FTO promotes strong metal–support interactions with Pt catalysts, improving activity and durability. The interfacial coupling effectively regulates the electronic structure of Pt and strengthens catalyst–support bonding. The durability is enhanced by an order of magnitude compared with a commercial Pt/C catalyst, while maintaining stable ORR activity during 500,000 cycles of accelerated durability tests by effectively mitigating carbon oxidation and preserving the structural integrity of the catalyst layer. Furthermore, membrane electrode assembly-level durability tests confirmed stable single-cell performance, with only a 4.9% loss in power density after 5,000 cycles at 0.6–1.095 V. In particular, it showed no measurable degradation after 5,000 cycles at 1.0–1.5 V, which corresponds to the carbon corrosion potential range, approaching the durability performance of state-of-the-art ORR catalysts.

By quantitatively linking Ni charge heterogeneity to irreversible lithium loss, our study provides a mechanistic foundation for mitigating IRC loss and optimizing Ni-rich cathode performance.

The performance of anion exchange membrane water electrolysis, a key technology for achieving net-zero carbon emissions, can be improved by introducing an appropriate catalyst support. However, the ideal support material for water electrolysis remains debatable. Recent efforts focused on enhancing the corrosion resistance by using highly crystalline carbon as a carbon-based support. Despite this progress, the defects intrinsic to carbon emphasize the need for effective passivation strategies to ensure long-term stability and reliability. Addressing this challenge, here we introduce Ti to passivate these surface defects, resulting in oxide-hybridized supports with significantly improved corrosion resistance. The layered double hydroxide catalyst loaded on the Ti-hybridized carbon demonstrates notable performance (8.5 A cm ⁻² at 2 V) and durability (0.17 mV h ⁻¹ over 900 h at 1 A cm ⁻² ). The enhanced activity can be attributed to the efficient OH ⁻ supply, as confirmed by in -situ Fourier-transform infrared spectroscopy measurements and theoretical calculations. This study provides a foundation for the development of advanced catalyst supports for water electrolysis.

As phosphor-converted LEDs continue to displace traditional light technologies, developing new down-conversion materials remains essential for improving color quality and efficiency. This research introduces a cyan-emitting oxynitride phosphor, Na_13.57Ca_17.43Mg₅(PO_3.75N_0.25)₁₈:Eu²⁺, synthesized through ammonolysis of the oxide, Na₁₈Ca₁₃Mg₅(PO₄)₁₈. The presence of nitrogen was validated using a suite of advanced analytical techniques, including Rietveld co-refinement of synchrotron X-ray and neutron diffraction data, X-ray photoelectron spectroscopy, thermogravimetric analysis, and diffuse reflectance spectroscopy. This oxynitride phosphor was subsequently shown to exhibit a broad excitation spectrum covering the UV (>350 nm) to the violet (425 nm) portions of the electromagnetic spectrum, down-converting the absorbed light, generating an efficient cyan emission. A prototype light using the new Eu2+-substituted oxynitride produced a functional (daylight) white light when paired with a violet LED and commercial blue and red-emitting phosphor. This research not only introduces a promising phosphor for LED applications but also highlights a practical approach to obtain novel oxynitrides from disordered oxides with a cost-efficient synthesis method, potentially paving the way for advancements in phosphor-based lighting technologies.