Zdravko Kochovski

Cryo-EM enables multimodal characterization of Li and post-Li metal anodes and interphases, offering mechanistic insights for next-generation battery design.

This study highlights the use of anomalous and small-angle X-ray scattering (ASAXS/SAXS) to monitor the evolution of micro- and mesoporosity during the synthesis of iron-nitrogen-carbon (Fe-N-C) electrocatalysts. A structural model-free SAXS approach enabled the determination of 13 structural parameters across five Fe-N-C electrocatalysts, compared to a commercial PMF-12704, Pajarito Powder. SAXS revealed hierarchical pore formation and structural changes spanning micro- to macroscales. Key features, such as pore curvature, porosity, and disorder, increased with synthesis modifications, correlating with enhanced oxygen reduction activity and reduced hydrogen peroxide yield. Notably, an average graphene layer curvature (l_R-Ruland length) above 3 nm was critical for forming curved pore walls, which improves selectivity and favors the 4-electron oxygen reduction pathway to water. These findings highlight the pore curvature and hierarchical pore architecture as crucial design parameters, guiding the development of next-generation electrocatalysts with enhanced efficiency and selectivity for sustainable energy applications. Specifically, optimizing the mesopore size distribution, with an increased proportion of mesopores within the range from 6 to 27 nm, and improving the transition from micropores to mesopores and macropores are essential strategies.

Covalent organic frameworks (COFs) have emerged as promising metal-free sulfur hosts to facilitate the conversion kinetics and suppress the shuttling effect of lithium polysulfides (LiPSs) in lithium-sulfur (Li-S) batteries. However, constructing COFs with stable and high electrocatalytic functionality for LiPS conversion remains unexplored. Herein, we develop a radical-cationic COF (R-TTF^•+-COF) with superior electrical conductivity of 3.9 S m^-1 at room temperature, which features both nucleophilic and electrophilic sites for effective LiPS chemisorption and conversion. With this novel radical-based catalyst, the Li-S battery achieves superior longevity of 1500 cycles with a capacity fading of 0.027% per cycle at a current density of 0.5 C. The capacity retention of the Li-S battery based on R-TTF^•+-COF at the current density of 2.0 C is nearly twice as high compared to a COF without radicals. The crucial role of radical cations in catalyzing LiPS conversion has been systematically elucidated through solid-state nuclear magnetic resonance spectroscopy, electron paramagnetic resonance spectroscopy, and theoretical simulations, which verify the reversible interactions between LiPSs and [TTF]₂^•+ moieties. This intriguing radical-assisted mechanism opens a new avenue for designing efficient catalytic sulfur hosts using organic molecules, offering a significant step toward the practical application of Li-S batteries.

The synthesis of homochiral two-dimensional covalent organic frameworks (2D COFs) from chiral π-conjugated building blocks is challenging, as chiral units often lead to misaligned stacking interactions. In this work, we introduce helical chirality into 2D COFs using configurationally stable enantiopure and racemic [5]helicenes as linkers in the backbone of 2D [5]HeliCOFs as powders and films. Through condensation with 1,3,5-triformylbenzene (TFB) or 1,3,5-triformylphloroglucinol (TFP), our approach enables the efficient formation of a set of homochiral and racemic 2D [5]HeliCOFs. The resulting carbon-based crystalline and porous frameworks exhibit distinct structural features and different properties between homochiral and racemic counterparts. Propagation of helical chirality into the backbone of the crystalline frameworks leads to the observation of advanced chiroptical properties in the far-red visible spectrum, along with a less compact structure compared with the racemic frameworks. Homogeneous thin films of [5]HeliCOFs disclosed photoluminescent properties arising from the controlled growth of highly ordered π-conjugated lattices. The present study offers insight into general chiral framework formation and extends the Liebisch-Wallach rule to 2D COFs.

Limits to the minimum concentrations of Ca and minimum number of Ca atoms that can be detected are determined for EELS in TEM using the Ca L ₂₃ signal, and EDX in TEM using the Ca Kα line.

Understanding the oxygen evolution reaction (OER) and Ir dissolution mechanisms in amorphous, hydrous iridium oxides (am-hydr-IrO_x) is hindered by the reliance on crystalline iridium oxide theoretical models to interpret its behaviour. This study presents a comprehensive investigation of hydrous iridium oxide thin films (HIROFs) as a model for am hydr-IrO_x to elucidate electronic and structural transformations under OER conditions of Proton Exchange Membrane Water Electrolyzers (PEM-WE). Employing in situ and operando Ir L₃-edge X ray absorption spectroscopy supported by density functional theory calculations, we introduce a novel surface H terminated nanosheet model that better characterizes the short-range structure of am-hydr-IrO_x compared to previous crystalline models, which exhibits elongated Ir-O bond lengths compared to rutile-IrO₂. This atomic model unveils the electronic and structural transformations of am-hydr-IrO_x, progressing from H-terminated nanosheets to structures with multiple Ir vacancies and shorter bond-lengths at OER potentials. Notably, Ir dissolution emerges as a spontaneous, thermodynamically driven process, initiated at potentials lower than OER activation, which requires a parallel mechanistic framework describing Ir dissolution by Ir defect formation. Moreover, our results provide mechanistic insights into the activity-stability relationship of am-hydr-IrO_x by systematically screening the DFT-calculated OER activity of diverse Ir and O chemical environments. This work challenges conventional perceptions of iridium dissolution and OER mechanisms in am-hydr-IrO_x, providing an alternative perspective within a dual-mechanistic framework.

The influence of different synthesis parameters on CeO₂ and Pt nanoparticle (NP) deposition on Ketjenblack carbon (C(KB)) was examined. The Pt NP diameter (3.1–4.1 nm) was not influenced by CeO₂ synthesis parameters. The CeO₂ NPs synthesized using ultrasound sonication contribute to a better durability of the Pt–CeO₂/C against CO poisoning. In contrast, CeO₂ synthesized using the microwave heating method contributes to better methanol oxidation reaction (MOR) activity at low electrode potential. Synthesis parameters of CeO₂ and Pt NPs developed for the C(KB)-based catalysts were applied for C(Cr₃C₂)-based catalysts. The Pt NP diameter of C(Cr₃C₂)-based catalysts was slightly higher (7.2 nm) as some Pt NPs were agglomerated. The C(Cr₃C₂) support facilitates the MOR and CO stripping, especially in the case of the Pt/C on C(Cr₃C₂) support. The MOR activity at 0.85 V of Pt NPs on the C(Cr₃C₂) support is as good as the MOR activity for the best Pt–CeO₂ on the C(KB) support. The C(Cr₃C₂) support also improves the CO removal from the Pt surface. All the synthesized catalysts had better MOR activity than the commercial Pt/C(Vulcan) catalyst. The oxygen reduction reaction activity of Pt–CeO₂/C catalysts with higher CeO₂ content synthesized with the microwave heating method was very good.

A small series of copoly(α,l-glutamic acid/dl-allylglycine)s with the same chain length and allylglycine content (∼10 mol %) but different spatial distribution of allylglycine units was synthesized and subsequently glycosylated via thiol-ene chemistry. Dilute aqueous copolypeptide solutions (0.1 wt %, physiological saline) were analyzed by circular dichroism spectroscopy, dynamic light scattering, and cryogenic transmission electron microscopy. The copolypeptides adopted a random coil or α-helix conformation, depending on solution pH, and the glycosylated residues either distorted or enhanced the folding into an α-helix depending on their location and spatial distribution along the chain. However, regardless of their secondary structure and degree of charging, all partially glycosylated copolypeptides self-assembled into 3D spherical structures, supposedly driven by a hydrophilic effect promoting microphase separation into glucose-rich and glutamate-rich domains.