Won Mo Seong

High-performance flexible perovskite solar cells exploiting Zn2SnO4 prepared in solution below 100 °C.

Fabricating inorganic–organic hybrid perovskite solar cells (PSCs) on plastic substrates broadens their scope for implementation in real systems by imparting portability, conformability and allowing high-throughput production, which is necessary for lowering costs. Here we report a new route to prepare highly dispersed Zn2SnO4 (ZSO) nanoparticles at low-temperature (<100 °C) for the development of high-performance flexible PSCs. The introduction of the ZSO film significantly improves transmittance of flexible polyethylene naphthalate/indium-doped tin oxide (PEN/ITO)-coated substrate from ∼75 to ∼90% over the entire range of wavelengths. The best performing flexible PSC, based on the ZSO and CH3NH3PbI3 layer, exhibits steady-state power conversion efficiency (PCE) of 14.85% under AM 1.5G 100 mW·cm−2 illumination. This renders ZSO a promising candidate as electron-conducting electrode for the highly efficient flexible PSC applications.

Niobium Doping Effects on TiO2 Mesoscopic Electron Transport Layer-Based Perovskite Solar Cells.

Perovskite solar cells (PSCs) are the most promising candidates as next-generation solar energy conversion systems. To design a highly efficient PSC, understanding electronic properties of mesoporous metal oxides is essential. Herein, we explore the effect of Nb doping of TiO2 on electronic structure and photovoltaic properties of PSCs. Light Nb doping (0.5 and 1.0 at %) increased the optical band gap slightly, but heavy doping (5.0 at %) distinctively decreased it. The relative Fermi level position of the conduction band is similar for the lightly Nb-doped TiO2 (NTO) and the undoped TiO2 whereas that of the heavy doped NTO decreased by as much as ∼0.3 eV. The lightly doped NTO-based PSCs exhibit 10 % higher efficiency than PSCs based on undoped TiO2 (from 12.2 % to 13.4 %) and 52 % higher than the PSCs utilizing heavy doped NTO (from 8.8 % to 13.4 %), which is attributed to fast electron injection/transport and preserved electron lifetime, verified by transient photocurrent decay and impedance studies.

Dissolution and ionization of sodium superoxide in sodium-oxygen batteries.

With the demand for high-energy-storage devices, the rechargeable metal–oxygen battery has attracted attention recently. Sodium–oxygen batteries have been regarded as the most promising candidates because of their lower-charge overpotential compared with that of lithium–oxygen system. However, conflicting observations with different discharge products have inhibited the understanding of precise reactions in the battery. Here we demonstrate that the competition between the electrochemical and chemical reactions in sodium–oxygen batteries leads to the dissolution and ionization of sodium superoxide, liberating superoxide anion and triggering the formation of sodium peroxide dihydrate (Na2O2·2H2O). On the formation of Na2O2·2H2O, the charge overpotential of sodium–oxygen cells significantly increases. This verification addresses the origin of conflicting discharge products and overpotentials observed in sodium–oxygen systems. Our proposed model provides guidelines to help direct the reactions in sodium–oxygen batteries to achieve high efficiency and rechargeability.

Crystallographically preferred oriented TiO2 nanotube arrays for efficient photovoltaic energy conversion

We describe the fabrication of crystallographically preferred oriented TiO2 anatase nanotube arrays (p-NTAs) and the characterization of their photovoltaic properties. The preferred orientation to the (004) plane of the TiO2 nanotube array (NTA) was carefully controlled by adjusting the water content in the anodizing electrolyte; ∼2 wt% of water yielded a p-NTA, whereas other contents of water yielded randomly oriented NTAs (r-NTAs). A structural analysis using X-ray diffraction and a high-resolution transmission electron microscope revealed that the p-NTA showed a preferred orientation along the [001] direction of the anatase crystal structure. When the NTAs were employed to dye-sensitized solar cells (DSSCs) as photoelectrodes, the p-NTA showed a similar electron lifetime to the r-NTA, which was an order of magnitude higher than that for a TiO2 nanoparticle (NP) film. Moreover, the p-NTA exhibited faster electron transport than the NP film, and even faster than the r-NTA. These properties resulted in a longer electron diffusion length of the p-NTA, compared to the r-NTA and NP film, thereby improving the charge collection property of the photoelectrode. The p-NTA exhibited superior photovoltaic energy conversion performance in the DSSC system, and showed a higher thickness for the optimal photovoltaic performance compared to the NP film, which were attributed to the excellent charge collection properties. Our results address that the crystallographic orientation of NTAs improves their charge transport properties, which can be applied to various optoelectronics, especially to solar-driven energy conversion devices.

High-efficiency and high-power rechargeable lithium–sulfur dioxide batteries exploiting conventional carbonate-based electrolytes

Shedding new light on conventional batteries sometimes inspires a chemistry adoptable for rechargeable batteries. Recently, the primary lithium-sulfur dioxide battery, which offers a high energy density and long shelf-life, is successfully renewed as a promising rechargeable system exhibiting small polarization and good reversibility. Here, we demonstrate for the first time that reversible operation of the lithium-sulfur dioxide battery is also possible by exploiting conventional carbonate-based electrolytes. Theoretical and experimental studies reveal that the sulfur dioxide electrochemistry is highly stable in carbonate-based electrolytes, enabling the reversible formation of lithium dithionite. The use of the carbonate-based electrolyte leads to a remarkable enhancement of power and reversibility; furthermore, the optimized lithium-sulfur dioxide battery with catalysts achieves outstanding cycle stability for over 450 cycles with 0.2 V polarization. This study highlights the potential promise of lithium-sulfur dioxide chemistry along with the viability of conventional carbonate-based electrolytes in metal-gas rechargeable systems.

Amorphous Cobalt Phyllosilicate with Layered Crystalline Motifs as Water Oxidation Catalyst

The development of a high-performance oxygen evolution reaction (OER) catalyst is pivotal for the practical realization of a water-splitting system. Although an extensive search for OER catalysts has been performed in the past decades, cost-effective catalysts remain elusive. Herein, an amorphous cobalt phyllosilicate (ACP) with layered crystalline motif prepared by a room-temperature precipitation is introduced as a new OER catalyst; this material exhibits a remarkably low overpotential (η ≈ 367 mV for a current density of 10 mA cm-2 ). A structural investigation using X-ray absorption spectroscopy reveals that the amorphous structure contains layered motifs similar to the structure of CoOOH, which is demonstrated to be responsible for the OER catalysis based on density functional theory calculations. However, the calculations also reveal that the local environment of the active site in the layered crystalline motif in the ACP is significantly modulated by the silicate, leading to a substantial reduction of η of the OER compared with that of CoOOH. This work proposes amorphous phyllosilicates as a new group of efficient OER catalysts and suggests that tuning of the catalytic activity by introducing redox-inert groups may be a new unexplored avenue for the design of novel high-performance catalysts.

Nb-doped TiO2 air-electrode for advanced Li-air batteries

Abstract As new substrate materials to replace a conventional carbon substrate, TiO2 and Nb-doped TiO2 air-electrodes for Li-air batteries were investigated. Through a simple two-step process, we successfully synthesized anatase Nb-doped TiO2 nanoparticles and demonstrated the potential applicability of TiO2-based materials for use in Li-air battery electrode. An air-electrode with Nb-doped TiO2 nanoparticles could deliver a higher discharge capacity than a bare TiO2 electrode due to the enhanced conductivity, which implies the importance of facile electron transport during the discharge process.

Observation of anatase nanograins crystallizing from anodic amorphous TiO2 nanotubes

A mechanism underlying the appearance of a preferred orientation in anodized amorphous TiO2 nanotube arrays (NTAs) was studied. Transmission electron microscopic analyses of preferred-oriented nanotube arrays (p-NTAs) reveal that at an optimum water content (~2 wt%), large single crystalline domains oriented along grow from the outer wall to the inner wall to minimize the surface energy. In stark contrast, excessive water content (5 wt%) in the electrolyte leads to sporadic multiple nucleation of randomly oriented anatase crystallites in amorphous medium at the early stage of crystallization, which results in the formation of randomly oriented nanotube arrays (r-NTAs). During subsequent thermal annealing, multiple nucleation sites hindered the growth of the -oriented grains from the outer wall. When the water content in an ethylene glycol-based electrolyte is optimized by reducing the uncertainty of water content, the X-ray diffraction patterns of NTAs exhibited a 200 times increase in the intensity ratio of (004) to (200) peaks of the anatase phase. p-NTAs exhibit ~5 times lower electrical resistance than r-NTAs, which supports the idea that improving the preferred orientation of NTAs is a promising method for developing efficient electronic devices.

γ-Al2O3 nanospheres-directed synthesis of monodispersed BaAl2O4:Eu2+ nanosphere phosphors

Monodispersed BaAl2O4:Eu2+ nanospheres with 180 nm size were synthesized through forced hydrolysis using γ-Al2O3 nanospheres as a template followed by a subsequent heat treatment. The incorporation of a barium precursor onto an individual γ-Al2O3 template nanosphere was optimized by controlling the reaction time. The photoluminescence properties of the BaAl2O4:Eu2+ nanospheres were comparable to those of the bulk counterpart prepared at 1300 °C through a conventional solid-state reaction method.

Abnormal self-discharge in lithium-ion batteries

Lithium-ion batteries are expected to serve as a key technology for large-scale energy storage systems (ESSs), which will help satisfy recent increasing demands for renewable energy utilization. Besides their promising electrochemical performance, the low self-discharge rate (<5% of the stored capacity over 1 month) of lithium-ion batteries is one of their most significant advantages for ESSs. Herein, contrary to conventional belief, we report that the self-discharge of LIBs can be abnormally accelerated when the battery has been exposed even to a routine short-term thermal exposure. We demonstrate that this thermal ‘history’ in addition to the temperature itself is memorized in the battery and accelerates the self-discharge rate. The series of characterizations performed in our work reveal that the electrolyte salt acts as a strong oxidizing agent by vigorously damaging the surface of the cathode, producing an internal ‘parasitic’ lithium source that continuously supplies lithium for the self-discharge. Although it is widely known that battery operation at elevated temperature generally induces faster degradation of capacity over multiple cycles, the key finding here is that not only the operation temperature but also the ‘thermal history’ of the battery should be carefully considered because this history remains and continues to affect the self-discharge rate afterwards. The self-discharge of LIBs has remained largely neglected; however, our findings suggest that close attention must be paid to the self-discharge of LIBs applied to large-scale ESSs, which, unlike mobile electronic devices, will be exposed to various outdoor temperature conditions.