Cheul-Ro Lee

Direct photoelectrochemical splitting of natural seawater offers a scalable route to green hydrogen but is constrained by an interfacial paradox: alkaline conditions that accelerate hydrogen evolution also induce Mg/Ca hydroxide scaling, rapidly blocking semiconductor surfaces. Here, we report a membrane-defined III–nitride photocathode that stabilizes the seawater interface and enables catalyst-free hydrogen evolution directly in Yellow Sea seawater. N-polar InGaN quantum pyramids integrated on vertical GaN nanowires provide broadband (UV–NIR) absorption and a composition-graded “quantum-sliding” band profile, funneling photogenerated electrons toward the pyramid apex and the catalytically active N-polar surface. A conformal graphene-oxide/polylactic-acid (GO/PLA) nano-membrane laminated on the nanowire forest serves as an ion-sieving transport layer, preserving water access while suppressing multivalent-ion-driven precipitation and electrode blockage. X-ray photoelectron spectroscopy quantifies heterointerfacial band offsets, while Raman spectroscopy and electron microscopy confirm conformal encapsulation and mitigated deposits after operation. The photocathode sustains ~0.5 mA cm−2 at −0.45 V versus RHE for over 200 h in untreated Yellow Sea seawater (pH 7) and reaches ~−16.51 mA cm−2 in Yellow Sea water containing 0.5 M NaOH, with onset potentials of 0.26–0.29 V versus RHE. This membrane–band co-engineering strategy decouples semiconductor photophysics from electrolyte complexity, enabling practical solar hydrogen production from unprocessed seawater.

A membrane-defined N-polar InGaN/GaN quantum pyramid photocathode, encapsulated by a selective GO/PLA nano-membrane, enables catalyst-free, long-term stable hydrogen evolution directly in untreated natural seawater.

Despite the considerable potential of AlGaN-based ultraviolet-B light-emitting diodes (UV-B LEDs) in various applications such as phototherapy, UV curing, plant growth, and analytical technology, their development is still ongoing due to low luminescence efficiency. In this study, we introduced a novel epitaxial growth mechanism to effectively control the height and thickness of AlGaN multiple wells (MWs) on AlGaN nanorod structures using horizontal reactor-based metal-organic chemical vapor deposition (MOCVD). By adjusting the H₂ carrier gas flow rate, we could control the growth boundary layer's thickness, successfully separating the AlGaN well and p-AlGaN layer from the substrate. Cathodoluminescence (CL) measurements confirmed the stability of the core-shell AlGaN quantum wells as a highly stable nonpolarized structure, with the wavelength peak remaining almost unchanged under various injection currents. Furthermore, transmission electron microscopy (TEM) provided clear evidence of differentiation, highlighting the distinct formation of the 275 nm AlGaN core and the 295 nm AlGaN shell structure. The developed AlGaN MW structure, characterized by these rectification features, not only demonstrated a significantly improved electroluminescence (EL) peak intensity but also exhibited a much lower leakage current compared to the conventional core-shell AlGaN structure. The newly proposed growth mechanism and advanced nonpolarized core-shell AlGaN structure are expected to serve as excellent alternatives for substantially enhancing the efficiency of the next generation of high-efficiency UV LEDs.

GaN‐based materials are the hottest research topic in UV photodetectors (PDs) because of their low operating voltage, small volume, long lifetime, high‐temperature resistance, and low energy consumption. However, there are still fundamental issues to be overcome, and the most important issue is to get a photoconductive gain. In this paper, the following new approaches are provided to innovatively improve the photoconductive gain of UV PDs in GaN‐based materials. First, the aspect ratio of the 1D GaN microwire (MW) structure is dramatically improved by analyzing the pulse growth mechanism using the metal‐organic vapor deposition system. Second, the comprehensive strain behavior in the MW epitaxial growth system is successfully analyzed. Third, the fabricated metal‐semiconductor‐metal‐based MW UV PD shows photoresponsivity and sensitivity of 28.365 A W⁻¹ and 93.16%, respectively, at the −2 V bias, which significantly outperforms the conventional structures in the UV region. Finally, a trap‐assisted Poole–Frenkel effect‐based energy bandgap mechanism, that allows the defect level formed by lattice mismatch between the substrate and GaN to be used as an electron carrier path, is newly defined. This study will present the direction of future UV PDs by providing a new MW structure based on GaN materials, a third‐generation semiconductor.

This study reports an integrated device in which a lithium-ion battery (LIB) and Si solar cells are interconnected. The LIB is fabricated using the Li(Ni_0.65Co_0.15Mn_0.20)O₂ (NCM622) cathode and the Li₄Ti₅O₁₂ (LTO) anode. The surface and shape morphologies of the NCM and LTO powders were investigated by field emission scanning electron microscopy (FE-SEM). In addition, the structural properties were thoroughly examined by X-ray diffraction (XRD). Further, their electrochemical characterization was carried out on a potentiostat. The specific discharge capacity of the NCM cathode (half-cell) was 188.09 mAh/g at 0.1 C current density. In further experiments, the NCM-LTO full-cell has also shown an excellent specific capacity of 160 mAh/g at a high current density of 1 C. Additionally, the capacity retention was outstanding, with 99.63% at 1 C after 50 cycles. Moreover, to meet the charging voltage requirements of the NCM-LTO full-cell, six Si solar cells were connected in series. The open-circuit voltage (V_OC) and the short-circuit photocurrent density (J_SC) for the Si solar cells were 3.37 V and 5.42 mA/cm². The calculated fill factor (FF) and efficiency for the Si solar cells were 0.796 and 14.54%, respectively. Lastly, the integrated device has delivered a very high-power conversion-storage efficiency of 7.95%.

Commercial lithium-ion batteries using liquid electrolytes are still a safety hazard due to their poor chemical stability and other severe problems, such as electrolyte leakage and low thermal stability. To mitigate these critical issues, solid electrolytes are introduced. However, solid electrolytes have low ionic conductivity and inferior power density. This study reports the optimization of the synthesis of sodium superionic conductor-type Li_1.5Al_0.3Si_0.2Ti_1.7P_2.8O₁₂ (LASTP) solid electrolyte. The as-prepared powder was calcined at 650 °C, 700 °C, 750 °C, and 800 °C to optimize the synthesis conditions and yield high-quality LASTP powders. Later, LASTP was sintered at 950 °C, 1000 °C, 1050 °C, and 1100 °C to study the dependence of the relative density and ionic conductivity on the sintering temperature. Morphological changes were analyzed using field-emission scanning electron microscopy (FE-SEM), and structural changes were characterized using X-ray diffraction (XRD). Further, the ionic conductivity was measured using electrochemical impedance spectroscopy (EIS). Sintering at 1050 °C resulted in a high relative density and the highest ionic conductivity (9.455 × 10⁻⁴ S cm⁻¹). These findings corroborate with the activation energies that are calculated using the Arrhenius plot. Therefore, the as-synthesized superionic LASTP solid electrolytes can be used to design high-performance and safe all-solid-state batteries.

This study reports an integrated device of a lithium-ion battery (LIB) connected with Si solar cells. A Li(Ni_0.65Co_0.15Mn_0.20)O₂ (NCM) cathode and a graphite (G) anode were used to fabricate the lithium-ion battery (LIB). The surface and shape morphologies of NCM and graphite powder were characterized by field emission scanning electron microscopy (FE-SEM). The structural properties of NCM and graphite powder were determined by X-ray diffraction (XRD) analysis. XRD patterns of powders were well matched with those of JCPDS data. To investigate the electrochemical characteristics of NCM and graphite, cycling tests were performed after assembling the NCM-Li, the G-Li half-cell, and the NCM-G full-cell. The discharge capacity of the NCM cathode at 0.1C was 189.82 mAh/g⁻¹. The NCM-graphite full-cell showed 98.25% cycle retention at 1C after 50 cycles. To obtain enough charging voltage for the LIB connected with solar cells in an integrated device, eight single Si solar cells were connected in a series. The short-circuit photocurrent density for Si solar cells was 4.124 mA/cm². The fill factor and the open circuit voltage were 0.78 and 4.5 V, respectively. These Si solar cells showed a power conversion efficiency of 14.45%. The power conversion andstorage efficiency of the integrated device of the NCM battery and Si solar cells was 7.74%. Charging of the integrated device could be as effective as charging with a battery cycler.

We report an approach for improving the photoelectrochemical water splitting (PEC–WS) performance using GaN-nanowire (NW) bundles (GNW-BDLs) as the photoanode material. The bundles were formed by dipping GaN NWs with an average length of 648.3 nm and diameter of 35.2 nm into acetone. The effect thereof was that the upper sections of several NWs were agglomerated via van der Waals bonding to form a bundle structure. The current density and applied bias photon-to-current efficiency of the GNW-BDL photoanode in a 0.5 M H₂SO₄ electrolyte were measured to be 2.2 mA cm⁻² and 1.9%, respectively, at 0.6 V vs. reversible hydrogen electrode. These values were increased by 314% and 317% compared with those (0.7 mA cm⁻² and 0.6%) of the photoanode using pristine GaN NWs. In addition, the PEC–WS performance was observed at the potential of 0 V vs. Pt. This remarkable improvement is largely attributed to the reduction in the effect of the surface-trap states of GaN NWs on the migration of the photogenerated carriers to the electrolyte because of the formation of structures consisting of bundles of neighboring NWs.