Dae-Hyun Nam

Tunable Sn structures in porosity-controlled carbon nanofibers for all-solid-state lithium-ion battery anodes

Volumetric expansion of active materials during lithium (Li) insertion is a critical drawback of Li-alloy anodes and a major bottleneck for their wide adoption in rechargeable batteries. Here, we report on a novel fabrication method of a tin (Sn) fully embedded one-dimensional (1D) carbon (C) matrix which results in minimal volumetric expansion. The 1D C matrix contributes to the buffer role and electron conduction path. This optimized Sn/C structure is enabled by confining the Sn nucleation site and minimizing the outward Sn diffusion originating from stress relaxation. The difference of thermal expansion coefficient between Sn and C derives the stress. The porosity of C nanofibers is a key parameter to modulate the Sn size and dispersion. It is controlled by stabilization and gas–solid reactions between CO (g), CO2 (g), and C nanofibers. The calcination under an Ar environment, which induced the lowest surface area and total pore volume (10.46 m2 g−1 and 0.0217 cm3 g−1), creates an ideal structure of 15 nm sized uniform Sn nanoparticle embedded C nanofibers. It displays a superior anode performance in all-solid-state Li-ion batteries with a capacity of 762 mA h g−1 and coulombic efficiency greater than 99.5% over 50 cycles. Our scheme provides a fundamental impact on anode materials of Li-ion batteries.

Designing thermal and electrochemical oxidation processes for δ-MnO2 nanofibers for high-performance electrochemical capacitors

To date, the phase of electrospun MnOx nanofibers (NFs) after thermal calcination has been limited to the low oxidation state of Mn (x < 2), which has resulted in insufficient specific capacitance. The organic contents in the as-spun MnOx NFs, which are essential for forming the NF structure, make it difficult to obtain the optimum phase (MnO2) to achieve high electrochemical performance. Herein, δ-MnO2 NFs, which were obtained by galvanostatic oxidation of thermally calcined MnOx NFs, were successfully fabricated while maintaining the 1-D nanoscale structure and inhibiting loss of the active materials. The galvanostatically oxidized Mn3O4 exhibited an outstanding performance of 380 F g−1 under a mass loading of 1.2 mg cm−2. The effect of galvanostatic oxidation was strongly dependent on the concentration and energetic stability of the Mn2+/3+ ions in the MnOx phases.

Enhanced conductivity of solution-processed indium tin oxide nanoparticle films by oxygen partial pressure controlled annealing

A highly conductive and transparent indium tin oxide (ITO) film was developed using a nanoparticle-based solution process through the control of oxygen partial pressure during annealing. At an oxygen partial pressure of 2.1 × 10−3 Torr, a maximum conductivity of 313 Ω−1 cm−1 was obtained: a great improvement over the conductivity of conventional ITO nanoparticle films (at this conductivity, the sheet resistance decreased to 30 Ω sq−1, and the transmittance reached 90%). By analyzing the electron concentration and mobility using Hall measurements, we determined that the main factor contributing to the enhanced conductivity is the increase in electron concentration that occurs due to the formation of oxygen vacancies under low oxygen partial pressures. However, if the oxygen partial pressure is too low, the removal of the organic ligands covering the ITO nanoparticles is incomplete, and the electron mobility is reduced. Microstructure control is also necessary for further improvement of the mobility.

Improving mechanical fatigue resistance by optimizing the nanoporous structure of inkjet-printed Ag electrodes for flexible devices

The development of highly conductive metallic electrodes with long-term reliability is in great demand for real industrialization of flexible electronics, which undergo repeated mechanical deformation during service. In the case of vacuum-deposited metallic electrodes, adequate conductivity is provided, but it degrades gradually during cyclic mechanical deformation. Here, we demonstrate a long-term reliable Ag electrode by inkjet printing. The electrical conductivity and the mechanical reliability during cyclic bending are investigated with respect to the nanoporous microstructure caused by post heat treatment, and are compared to those of evaporated Ag films of the same thickness. It is shown that there is an optimized nanoporous microstructure for inkjet-printed Ag films, which provides a high conductivity and improved reliability. It is argued that the nanoporous microstructure ensures connectivity within the particle network and at the same time reduces plastic deformation and the formation of fatigue damage. This concept provides a new guideline to develop an efficient method for highly conductive and reliable metallic electrodes for flexible electronics.

Defining a Materials Database for the Design of Copper Binary Alloy Catalysts for Electrochemical CO2 Conversion

While Cu electrodes are a versatile material in the electrochemical production of desired hydrocarbon fuels, Cu binary alloy electrodes are recently proposed to further tune reaction directionality and, more importantly, overcome the intrinsic limitation of scaling relations. Despite encouraging empirical demonstrations of various Cu-based metal alloy systems, the underlying principles of their outstanding performance are not fully addressed. In particular, possible phase segregation with concurrent composition changes, which is widely observed in the field of metallurgy, is not at all considered. Moreover, surface-exposed metals can easily form oxide species, which is another pivotal factor that determines overall catalytic properties. Here, the understanding of Cu binary alloy catalysts for CO2 reduction and recent progress in this field are discussed. From the viewpoint of the thermodynamic stability of the alloy system and elemental mixing, possible microstructures and naturally generated surface oxide species are proposed. These basic principles of material science can help to predict and understand metal alloy structure and, moreover, act as an inspiration for the development of new binary alloy catalysts to further improve CO2 conversion and, ultimately, achieve a carbon-neutral cycle.

Improvements of mechanical fatigue reliability of Cu interconnects on flexible substrates through MoTi alloy under-layer

The mechanical fatigue of Cu films and lines on flexible substrates was investigated, and an improvement in the structures through the use of a MoTi alloy under-layer was proposed. Fatigue reliability was decreased by 3-fold in lines compared with films in the tensile condition and by 6-fold in the compressive condition. Crack formation was observed to be more detrimental for lines than for films. With a MoTi under-layer, the fatigue limit was increased by 2 times that of a structure without MoTi in the tensile condition and by 15 times in the compressive bending condition. The suppression of delamination through the use of a MoTi under-layer improved the fatigue reliability under compressive bending.

Controlled Molybdenum Disulfide Assembly inside Carbon Nanofiber by Boudouard Reaction Inspired Selective Carbon Oxidation

Vertical stacking and lateral growth of molybdenum disulfide (MoS2 ) are controlled with remarkable precision, and MoS2 nanotubes are directly converted from nanofibers. Predictive synthesis is enabled by identifying the specific thermodynamic region where the Boudouard reaction becomes favored. It reveals how the chemical potential of each species in the MoSCO system can predict phase behaviors.

Synthetic Mechanism Discovery of Monophase Cuprous Oxide for Record High Photoelectrochemical Conversion of CO2 to Methanol in Water

Precise control of the oxidation state of transition-metal oxides, such as copper, is important for high selectivity of CO2 reduction in an aqueous condition to compete with the reduction of water. The phase of copper oxide nanofibers was controlled by predictive synthesis, which controls the nanoscale gas-solid reaction by considering thermodynamics and kinetics. The driving force of the phase transformation between the different oxidation states of copper oxide is calculated by comparing the Gibbs free energy of each of the oxidation states. From the calculation, the kinetically processable window for the fabrication of Cu2O in which monophase Cu2O can be fabricated in a reasonable reaction time scale is discovered. Herein, we report the monophase Cu2O nanofiber photocathode, which photoelectrochemically converted CO2 into methanol with over 90% selectivity in an aqueous electrolyte, and a hierarchical structure is developed to optimize the photoactivity and stability of the electrode. Our work suggests a rational design of the calcination strategy for precisely controlling the oxidation states of transition metals that can be applied to various applications in which the phase of the materials plays an important role.

Gaseous Nanocarving‐Mediated Carbon Framework with Spontaneous Metal Assembly for Structure‐Tunable Metal/Carbon Nanofibers

Vapor phase carbon (C)-reduction-based syntheses of C nanotubes and graphene, which are highly functional solid C nanomaterials, have received extensive attention in the field of materials science. This study suggests a revolutionary method for precisely controlling the C structures by oxidizing solid C nanomaterials into gaseous products in the opposite manner of the conventional approach. This gaseous nanocarving enables the modulation of inherent metal assembly in metal/C hybrid nanomaterials because of the promoted C oxidation at the metal/C interface, which produces inner pores inside C nanomaterials. This phenomenon is revealed by investigating the aspects of structure formation with selective C oxidation in the metal/C nanofibers, and density functional theory calculation. Interestingly, the tendency of C oxidation and calculated oxygen binding energy at the metal surface plane is coincident with the order Co > Ni > Cu > Pt. The customizable control of the structural factors of metal/C nanomaterials through thermodynamic-calculation-derived processing parameters is reported for the first time in this work. This approach can open a new class of gas-solid reaction-based synthetic routes that dramatically broaden the structure-design range of metal/C hybrid nanomaterials. It represents an advancement toward overcoming the limitations of intrinsic activities in various applications.