Woo-Jae Lee

Uniform nickel nanoparticles (Ni NPs) on carbon black were synthesized by fluidized-bed reactor atomic layer deposition (FBR-ALD). ALD behavior was first established on SiO2 using Ni(dmamb)2 and NH3, yielding pure Ni films with a saturated growth rate as precursor and reactant pulse times increased and exhibiting linear thickness depending on ALD cycle. Based on this, a Ni catalyst was synthesized and its NP size was controlled by varying ALD cycles from 5 to 30. Thermogravimetric analysis demonstrated that Ni loading on carbon increased linearly with the ALD cycle. Transmission electron microscopy showed uniformly dispersed Ni NPs with size ranging from 2.08 to 3.95 nm, and a decrease in areal density from about 2000 to about 1400 counts/μm2 as cycles increased, without severe agglomeration. X-ray photoelectron spectroscopy indicated a size-dependent balance between metallic Ni and surface oxide. Small NPs were readily oxidized, and larger NPs displayed a stronger metallic binding state due to the lower surface-to-volume ratio. In 1 M KOH, hydrogen evolution reaction tests were conducted, indicating that the 20 cycle Ni/C catalyst showed the best performance, with an overpotential of 51 mV at 10 mA/cm2 and a Tafel slope of 114 mV/dec. This ALD route enables uniform Ni NPs with tunable size on carbon black supports, demonstrates the scalability of FBR-ALD, and provides a versatile platform for Ni-based alloy catalysts and atom-level control toward efficient hydrogen catalysis.

To address the intermittent nature of renewable energy, highly efficient high-performance energy storage devices are essential. Among various options, supercapacitors have attracted significant attention because they provide high power density, rapid charge–discharge capability, and long cycle life. In this study, a composite of amorphous Cu(OH)2 grown on a metal–organic framework ZIF-67 (Cu(OH)2@ZIF-67) was successfully synthesized via a facile solvothermal method. Detailed structural and surface analyses, including scanning and transmission electron microscopy as well as X-ray diffraction measurements, revealed that ultrathin Cu(OH)2 nanosheets were uniformly grown on the ZIF-67 surface. The combination of the amorphous phase with the MOF provided abundant electrochemically active sites and facilitated rapid ion transport, significantly contributing to greatly improved charge storage behavior. Electrochemical characterization demonstrated that Cu(OH)2@ZIF-67 exhibited a specific capacitance of 168 F/g at 3 A/g, approximately four times higher than that of pristine ZIF-67, while maintaining superior energy density across a wide power range. These findings demonstrate that introducing an amorphous structure onto MOF-derived metal supports not only effectively enhances the overall performance of supercapacitors but also provides valuable insights for the rational design, optimization, and potential practical and technological applications of hybrid electrode materials in next-generation high-performance energy storage systems.

Lithium-ion batteries serve as a key technology, establishing the advancement of energy storage devices and playing a vital role in the global shift toward sustainable and green energy. However, the growing demand for high capacity nickel (Ni)-rich lithium-ion batteries accelerates the optimization of their energy density. A high-mass loading electrode design is a promising strategy for enhancing the energy density of LIBs, enabling the improved performance for commercial applications. The factors that limit the rate capability of the high-mass loading electrode are associated with the underutilization of active materials and increased polarization, which can be further attributed to slow electronic/ionic transport within the electrode. In this work, the conductive networks and porous characteristics of the Ni-rich LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) cathode electrode is preciously tailored through the incorporation of multidimensional carbon conductive additives, facilitating enhanced electron transport and optimized porosity enhancing lithium-ion diffusion within the high-mass loading electrode. As a result, the NCM811 cathode electrode with dual-multidimensional conductive additives, such as carbon black and carbon nanofiber (CB + CNF), exhibits an outstanding performance, achieving a capacity retention of 94.8% over 100 cycles at 1 C. It is also observed that long-structured CNFs contributes significantly to the formation of efficient conductive networks in a high-mass loading thick electrode (∼23 mg cm^-2) exhibiting excellent performance at 0.2 C. The simple yet fundamental principle uncovered through this work demonstrates that the integration of the dual-carbon system synergistically enhances the conductive networks and optimizes electrode porosity. This microstructural optimization effectively reduces Ohmic contact resistance, contributing to a significantly enhanced electrochemical performance.

Mn‐based energy storage systems are gaining attention as promising candidates for next‐generation aqueous batteries, owing to their higher theoretical energy density and capacity compared to conventional Zn‐based systems. This advantage is primarily attributed to the lower standard redox potential of the Mn anode (−1.19 V vs SHE) relative to that of Zn (−0.76 V vs SHE). In this study, an Mn ² ⁺/H⁺ hybrid aqueous battery system utilizing LiV₃O₈ is presented as the cathode material, which delivers a high specific capacity of 204.58 mAh g ⁻¹ and excellent capacity retention of 76.2% after 7,000 cycles. The charge storage mechanism of LiV₃O₈ is thoroughly investigated through structural characterization, as well as diffusion pathway and energy barrier analyses. Proton insertion is identified as the dominant charge carrier and is found to induce the formation of Mn(OH)₂ on the electrode surface, as confirmed by spectroscopic techniques. Notably, the Mn//LiV₃O₈ cell achieved an operating voltage of 1.1–0.2 V higher than that of the conventional Zn//LiV₃O₈ cell. This study underscores the potential of Mn ² ⁺/H⁺ hybrid systems as next‐generation aqueous batteries and offers a comprehensive understanding of the associated reaction mechanisms, providing valuable guidance for the future design of Mn‐based aqueous energy storage technologies.

AlN-MgO composites with different compositions were prepared by spark plasma sintering, and the effects of their composition on their microstructure, thermal properties, and mechanical properties were systemically investigated. MgO compositions in the AlN-MgO composites were controlled from 20 to 80 wt%. The results indicated that a phase transition did not occur during the sintering process, and different solid solutions were formed within the MgO and AlN lattices. The AlN-MgO composites exhibited finer-grain microstructures than those of the sintered pure AlN and MgO samples. Transmission electron microscopy analysis showed that both oxygen-rich, low-density grain boundaries and clean boundaries with spinel phases were present in the composites. The sintered pure AlN sample exhibited the highest thermal conductivity (53.2 W/mK) and lowest coefficient of thermal expansion (4.47 × 10-6 /K) at 100 °C. And, the sintered pure MgO sample exhibited moderate thermal conductivity (39.7 W/mK) and a high coefficient of thermal expansion (13.05 × 10-6 /K). With increasing MgO contents in the AlN-MgO composites, however, the thermal conductivity of the AlN-MgO composites decreased, from 33.3 to 14.9 W/mK, while their coefficient of thermal expansion generally increased, from 6.49×10-6 to 10.73×10-6 /K with increasing MgO content. The composite with an MgO content of 60 wt% exhibited the best mechanical properties overall. Thus, the composition and microstructure of AlN-MgO composites have a determining effect on their thermal and mechanical properties.

Plasma-enhanced atomic layer deposition (PEALD) of TiN thin films were investigated as an effective Se diffusion barrier layer for Cu (In, Ga) Se₂ (CIGS) solar cells. Before the deposition of TiN thin film on CIGS solar cells, a saturated growth rate of 0.67 Å/cycle was confirmed using tetrakis(dimethylamido)titanium (TDMAT) and N₂ plasma at 200 °C. Then, a Mo (≈30 nm)/PEALD-TiN (≈5 nm)/Mo (≈600 nm) back contact stack was fabricated to investigate the effects of PEALD-TiN thin films on the Se diffusion. After the selenization process, it was revealed that ≈5 nm-thick TiN thin films can effectively block Se diffusion and that only the top Mo layer prepared on the TiN thin films reacted with Se to form a MoSe₂ layer. Without the TiN diffusion barrier layer, however, Se continuously diffused along the grain boundaries of the entire Mo back contact electrode. Finally, the adoption of a TiN diffusion barrier layer improved the photovoltaic efficiency of the CIGS solar cell by approximately 10%.