Hyeri Kang’s research while affiliated with Chung-Ang University and other places

Alloying earth-abundant transition metals with iridium, one of the promising catalysts for the hydrogen evolution reaction (HER), is an effective way to reduce the usage of precious metal without sacrificing its catalytic activity. In particular, tungsten and early transition metals (Co, Fe, and Ni) are known to promote the inherent HER activity by modulating surface adsorption energy, but ternary alloy nanoparticles (NPs) of those elements and their universal synthetic strategy are not well-established yet. In this study, we synthesized a new iridium-based ternary alloy NP with tungsten and transition metals of cobalt, iron, and nickel (Ir-M-W, M = Co, Fe, and Ni) through colloidal synthesis. All the constituent metal elements were uniformly distributed over the particle, forming a homogeneous solid solution. The Ir-M-W NPs showed excellent catalytic activity and mass activity toward HER in an acidic electrolyte compared with the binary Ir-W NPs. In particular, the Ir-Co-W NPs exhibited the lowest overpotential (35.82 mV of overpotential to drive −10 mA cm⁻²) and the highest mass activity (3.98 A/mgIr) with 40 µg cm⁻² of catalyst loading. Such superior catalytic HER performance could be achieved by the modulation of the surface electronic structure induced by the inclusion of transition metal.

Among the various solution processes that utilize colloidal NPs, electrophoretic deposition (EPD) has recently emerged as an efficient technique for the fabrication of dense and robust NP films. However, the interaction between colloidal NPs and the organic solvent in EPD systems has not been fully investigated. In this study, we investigate the charge formation role of the solvent in EPD systems using hexane, toluene, and chloroform with different ratios of the solvents (10:0, 7:3, 5:5, 3:7, and 0:10). As the ratio of toluene to hexane in the solvent increases, the NP film becomes thicker and rougher. In contrast, increasing the ratio of chloroform to hexane significantly reduces the film thickness. A zeta potential is measured to determine the influence of the solvent on the EPD process. The zeta potential distribution of the NPs dispersed in toluene is broader than that of the NPs in chloroform, suggesting that the solvent might control the surface charge and influence the film thickness. These results indicate that the choice of the solvent can be a key factor in determining the morphology and processability of NP films fabricated via EPD.

The chemical transformation from zinc oxide (ZnO) to zinc sulphide (ZnS), using di-tert-butyl disulphide (TBDS) as a highly reactive sulphur precursor, is demonstrated herein. Through anion exchange, we investigate the phase and morphological changes associated with the nanoparticle (NP) transformation of ZnO to ZnS using TBDS. The Zn–O–S alloy was not formed through the anion exchange reaction, only the ZnO and ZnS phases were detected. The NPs were transformed from a solid sphere to a hollow structure, induced by the nanoscale Kirkendall effect. Even with the dramatic shape and phase changes occurring in the NPs, the Zn oxidation state remained as 2+ throughout the 2 h anion exchange reaction. In addition, trioctylphosphine (TOP), a soft base ligand, increased the anion exchange reaction rate, facilitating the reaction with TBDS. Furthermore, anion exchange with elemental sulphur required a longer reaction time (3 h) than that with TBDS (2 h). Consequently, this study offers not only insights into phase and morphological transformations by anion exchange, but also the advantages of utilizing TBDS as a sulphur precursor.

Colloidal nanoparticles (NPs) have been recently spotlighted as building blocks for various nanostructured devices. Their collective properties have been exhibited by arranging them on a substrate to form assembled NPs. In particular, electrophoretic deposition (EPD) is an emerging fabrication method for such nanostructured films. To maximize the benefits of this method, further studies are required to fully elucidate the key parameters that influence the NP deposition. Herein, two key parameters are examined, namely: (i) the aging of colloidal NPs and (ii) the charge formation by surface ligands. The aging of Cu2-xS NPs changes the charge states, thus leading to different NP deposition behaviors. The SEM images of NP films, dynamic light scattering, and zeta potential results demonstrated that the charge control and restoration of interparticle interactions for aged NPs were achieved via simple ligand engineering. The charge control of colloidal NPs was found to be more dominant than the influence of aging, which can alter the surface charges of the NPs. The present results thus reveal that the charge formation on the colloidal NPs, which depends on the surface ligands, is an important controllable parameter in EPD.

As a renewable energy source to replace traditional fossil fuels, hydrogen is a promising sustainable energy carrier due to its eco-friendly nature and high energy density. Although Pt-based materials are widely used as catalysts for generating hydrogen through water electrolysis because of their high catalytic activity, Pt is expensive, and its scarcity hinders the commercialization of large-scale hydrogen production. The development of alternative low-cost materials with low hydrogen evolution reaction (HER) overpotential is the key to achieve low price and highly efficient hydrogen generators such as proton-exchange membrane water electrolysis (PEMWE). Among the numerous candidates of earth-abundant materials, transition metal-based phosphides (TMPs) are suitable for HER applications due to their low toxicity, high electrical conductivity, and catalytic activity. Especially, FeP is in the spotlight as a HER catalyst due to its high catalytic activity in solutions with a wide pH range, and this is a competitive material for large-scale commercial production of hydrogen, since Fe is one of the most abundant elements on earth. In this study, we synthesized the colloidal FeP nanoparticles (NPs) via a hot injection process, so that the pre-synthesized Fe NPs can be converted to FeP NPs through the phosphorization reactions by various phosphorus sources (trioctylphosphine, triphenylphosphite (TPP), tris(diethylamino)phosphine, and tri-n-butylphosphine). Different HER activities were induced by the phase transformation of NPs over the controlled reaction time for the NP samples synthesized with each phosphorus precursor. The FeP synthesized using TPP achieved excellent HER activity with an overpotential of 76 mV at 10 mA cm ⁻² in 0.5 M H 2 SO 4 as measured by drop casting the catalyst on Ti foil. In terms of the fabrication for the efficient HER electrode, the prepared FeP NPs were deposited on the carbon paper (CP) via electrophoretic deposition (EPD), which is a suitable method for the fabrication of NP films that can be used for various electronic applications. Different morphologies and catalytic activities of the FeP/CP samples were derived by controlling EPD parameters such as mixing ratio of solvents, NP concentration, and deposition voltage. The optimized EPD process lead to excellent HER activity of the FeP/CP with an overpotential of 38 mV and 91 mV at 10 mA cm ⁻² in 0.5 M H 2 SO 4 and 1 M KOH, respectively. The maximum activity of the catalyst was mainly attributed to the large active surface area and low charge transfer resistance due to the strong contact between the NPs and CP. A PEMWE single cell with the FeP/CP cathode achieved 1.2 A cm ⁻² at cell voltage of 2.0 V. This study of the EPD procedure for fabricating the NP film from the colloidal NP catalysts contributes to advance manufacturing of HER electrodes for efficient hydrogen generation through water electrolysis.

Development of highly efficient oxygen evolution reaction (OER) catalysts is necessary for commercial implementation of water electrolysis due to overcome slow kinetics of OER. Noble metal based electrocatalysts such as Ir and Ru demonstrate high performance for OER in acidic media. In contrast, the first-row transition metal oxides such as Ni, Co, and Fe oxides show the high catalytic activity and stability in alkaline media. In this work, the composition of bimetallic metal oxide NPs was tuned and their electrochemical performance was analyzed. Furthermore, the electrophoretic deposition (EPD) technique was utilized for the efficient fabrication of NP thin film electrode. The bimetallic transition metal oxide NPs were synthesized by using Ni(acac) 2 as Ni precursor and Fe(acac) 3 as Fe precursor, leading to the phase control of bimetallic Ni-Fe oxide NPs. As Fe precursor amount was gradually increased compared to that of Ni, the particle size of Ni-Fe oxide NPs was increased, and the shape of NPs was transformed from spherical to irregular shape. The magnetic property of Ni-Fe oxide NPs was also analyzed by vibrating sample magnetometer. The saturation magnetization (M s ) of bimetallic metal oxide was increased and the magnetic coercivity was decreased when the Fe composition was increased. For the electrochemical analysis for OER, the overpotential of bimetallic metal oxide NPs catalysts was highly influenced by the composition of bimetallic metal oxide NPs. Higher Fe contents than Ni in the NPs led to high performance as OER catalysts. The EPD technique was introduced to manufacture the uniform NP films with improved catalytic activity. The catalyst film formed through EPD technique was prepared under the applied voltage of 500 V for 10 minutes. The film of Ni-Fe oxide NPs was well covered on the substrate of the Ti foil. The overpotential of the bimetallic metal oxide NP film fabricated by EPD was decreased from 510 mV to 400 mV at 10 mA/cm ² compared to the films fabricated by drop casting. These results provide useful insights not only for understanding the catalytic activity of bimetallic metal oxide NPs in OER but also for suggesting the film fabrication method for enhancing the catalytic activity.

Transition metal phosphides (TMPs) have recently emerged as promising hydrogen evolution reaction (HER) catalytic alternatives to platinum. Among them, molybdenum phosphide (MoP) has attracted extensive attention due to its high electrical conductivity, good stability, and Pt-like electronic structure; however, there is no systematic comparison of its different colloidal synthetic routes. This study systematically compares two colloidal synthetic methods, one-pot and two-step, for amorphous MoP and the associated morphological changes during their reaction time. The amorphous MoP nanoparticles synthesized via the two-step method within 4 h exhibited the highest HER performance with an overpotential of 177 mV in 0.50 M H2SO4 for a current density of −10 mA cm−2; this might be due to their highly developed Mo-P bondings revealed by X-ray photoelectron spectroscopy analysis. Thus, this work demonstrates that the HER catalytic performance of MoP can be significantly influenced by its synthetic method and reaction time.

Transition-metal phosphides have been investigated as promising materials for hydrogen evolution reaction (HER) catalysts because of their cost efficiency, high catalytic activity, and good stability. Here, we report the spherical iron phosphide nanoparticles (FeP NPs) doped with various transition metals (Mn, Co, and Ni) via a phosphorization process from Fe-based bimetallic NPs and characterize the changes in their HER activity as a result of doping with different elements. Electrochemical measurements indicated that the Co-FeP NPs exhibited an overpotential of 126 mV to achieve a 10 mA cm⁻², demonstrating higher activity than the FeP, Ni-FeP, and Mn-FeP NPs because of their large active surface area and fast charge transfer. X-ray photoelectron spectroscopy analysis showed that the Co-FeP NPs exhibit a significantly tuned chemical state through the presence of more metal–P bonds and fewer oxidized species compared with the other doped products. In addition, X-ray diffraction and X-ray absorption spectroscopy analysis revealed substitutional doping of Fe atoms by Co atoms in the FeP crystalline structure. This study provides new insights for the efficient synthesis of doped transition-metal phosphides and for designing nanocatalysts with enhanced catalytic activity.

Iron phosphide (FeP) is in the spotlight as a hydrogen evolution reaction (HER) catalyst due to its cost efficiency, good catalytic activity, and stability within a wide pH range. However, there is still a need to synthesize FeP nanoparticles (NPs) with systematic analysis to improve their catalytic activity. Herein, we report FeP NPs synthesized with various phosphorus sources (TOP, trioctylphosphine; TPP, triphenylphosphite; TEAP, tris(diethylamino)phosphine; and TBP, tri-n-butylphosphine) via phosphorization reaction. We clearly demonstrate that the HER activity of the catalyst based on the FeP phase is dependent on the choice of phosphorus source used in the colloidal NP synthesis. Among the samples, FeP NPs synthesized with TPP achieved the highest HER activity with an overpotential of 76 mV at 10 mA cm⁻² in 0.5 M H2SO4. Our results reveal a critical aspect of colloidal synthesis to achieve enhanced catalytic activity in the synthesis of transition metal phosphide NPs.