Pohang Accelerator Laboratory

Transporter layers for greater stability Although perovskite solar cells (PSCs) can have power conversion efficiencies exceeding 20%, they can have limited stability under ultraviolet irradiation. This is in part because the mesoporous TiO 2 used as an electron-transporting layer can photocatalyze unwanted reactions in the perovskite layer. Shin et al. report a low-temperature colloidal method for depositing La-doped BaSnO 3 films as a replacement for TiO 2 to reduce such ultraviolet-induced damage. Solar cells retained over 90% of their initial performance after 1000 hours of full sun illumination. Science , this issue p. 167

It is expected that the market dominance of lithium-ion batteries will continue for at least another decade as there are currently no competing alternatives with the versatility of lithium-ion batteries for powering mobile and portable devices; and for buffering the fluctuating supply of intermittent energy sources such as wind and solar. While the pursuit of higher energy density and higher power density materials constitute the bulk of current interest, there is increasing interest in durable active battery materials that can be produced with minimum environmental impact. It is with these considerations that TiO2- and Sn-based anode materials are most interesting candidates for fulfilling future green energy storage materials. This review will focus on the recent developments of nanostructured TiO2 and Sn-based anode materials, including rutile, anatase, TiO2 (B), and coated TiO2, and pristine SnO2, and SnO2/C, Sn(M)/C composites.

Iron-nitrogen on carbon (Fe-N/C) catalysts have emerged as promising nonprecious metal catalysts (NPMCs) for oxygen reduction reaction (ORR) in energy conversion and storage devices. It has been widely suggested that an active site structure for Fe-N/C catalysts contains Fe-Nx coordination. However, the preparation of high-performance Fe-N/C catalysts mostly involves a high-temperature pyrolysis step, which generates not only catalytically active Fe-Nx sites, but also less active large iron-based particles. Herein, we report a general ???silica-protective-layer-assisted??? approach that can preferentially generate the catalytically active Fe-Nx sites in Fe-N/C catalysts while suppressing the formation of large Fe-based particles. The catalyst preparation consisted of an adsorption of iron porphyrin precursor on carbon nanotube (CNT), silica layer overcoating, high-temperature pyrolysis, and silica layer etching, which yielded CNTs coated with thin layer of porphyrinic carbon (CNT/PC) catalysts. Temperature-controlled in situ X-ray absorption spectroscopy during the preparation of CNT/PC catalyst revealed the coordination of silica layer to stabilize the Fe-N4 sites. The CNT/PC catalyst contained higher density of active Fe-Nx sites compared to the CNT/PC prepared without silica coating. The CNT/PC showed very high ORR activity and excellent stability in alkaline media. Importantly, an alkaline anion exchange membrane fuel cell (AEMFC) with a CNT/PC-based cathode exhibited record high current and power densities among NPMC-based AEMFCs. In addition, a CNT/PC-based cathode exhibited a high volumetric current density of 320 A cm-3 in acidic proton exchange membrane fuel cell. We further demonstrated the generality of this synthetic strategy to other carbon supports.

Large-scale carbon fixation requires high-volume chemicals production from carbon dioxide. Dry reforming of methane could provide an economically feasible route if coke- and sintering-resistant catalysts were developed. Here, we report a molybdenum-doped nickel nanocatalyst that is stabilized at the edges of a single-crystalline magnesium oxide (MgO) support and show quantitative production of synthesis gas from dry reforming of methane. The catalyst runs more than 850 hours of continuous operation under 60 liters per unit mass of catalyst per hour reactive gas flow with no detectable coking. Synchrotron studies also show no sintering and reveal that during activation, 2.9 nanometers as synthesized crystallites move to combine into stable 17-nanometer grains at the edges of MgO crystals above the Tammann temperature. Our findings enable an industrially and economically viable path for carbon reclamation, and the "Nanocatalysts On Single Crystal Edges" technique could lead to stable catalyst designs for many challenging reactions.

Abstract The oxygen evolution reaction (OER) is a key reaction for many electrochemical devices. To date, many OER electrocatalysts function well in alkaline media, but exhibit poor performances in neutral and acidic media, especially the acidic stability. Herein, sodium‐decorated amorphous/crystalline RuO 2 with rich oxygen vacancies (a/c‐RuO 2 ) was developed as a pH‐universal OER electrocatalyst. The a/c‐RuO 2 shows remarkable resistance to acid corrosion and oxidation during OER, which leads to an extremely high catalytic stability, as confirmed by a negligible overpotential increase after continuously catalyzing OER for 60 h at pH=1. Besides, a/c‐RuO 2 also exhibits superior OER activities to commercial RuO 2 and most reported OER catalysts under all pH conditions. Theoretical calculations indicated that the introduction of Na dopant and oxygen vacancy in RuO 2 weakens the adsorption strength of the OER intermediates by engineering the d ‐band center, thereby lowering the energy barrier for OER.

New semi-crystalline photovoltaic polymers were synthesized and the optimized device exhibited 9.39% efficiency in a ∼300 nm thick single-cell device.

Doped up: The incorporation of Zn(2+) dopants in tetrahedral sites leads to the successful magnetism tuning of spinel metal ferrite nanoparticles (see picture). (Zn(0.4)Mn(0.6))Fe(2)O(4) nanoparticles exhibit the highest magnetization value among the metal ferrite nanoparticles. Such high magnetism results in the largest MRI contrast effects (r2=860 mm(-1) s(-1)) reported to date and also huge hyperthermic effects.

Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) along with hydrogen evolution reaction (HER) have been considered critical processes for electrochemical energy conversion and storage through metal‐air battery, fuel cell, and water electrolyzer technologies. Here, a new class of multifunctional electrocatalysts consisting of dominant metallic Ni or Co with small fraction of their oxides anchored onto nitrogen‐doped reduced graphene oxide (rGO) including Co‐CoO/N‐rGO and Ni‐NiO/N‐rGO are prepared via a pyrolysis of graphene oxide and cobalt or nickel salts. Ni‐NiO/N‐rGO shows the higher electrocatalytic activity for the OER in 0.1 m KOH with a low overpotential of 0.24 V at a current density of 10 mA cm −2 , which is superior to that of the commercial IrO 2 . In addition, it exhibits remarkable activity for the HER, demonstrating a low overpotential of 0.16 V at a current density of 20 mA cm −2 in 1.0 m KOH. Apart from similar HER activity to the Ni‐based catalyst, Co‐CoO/N‐rGO displays the higher activity for the ORR, comparable to Pt/C in zinc‐air batteries. This work provides a new avenue for the development of multifunctional electrocatalysts with optimal catalytic activity by varying transition metals (Ni or Co) for these highly demanded electrochemical energy technologies.

Rechargeable zinc-ion batteries (ZIBs) with high energy densities appear promising to meet the increasing demand for safe and sustainable energy storage devices. However, electrode research on this low-cost and green system are faced with stiff challenges of identifying materials that permit divalent ion-intercalation/deintercalation. Herein, we present layered-type LiV3O8 (LVO) as a prospective intercalation cathode for zinc-ion batteries (ZIBs) with high storage capacities. The detailed phase evolution study during Zn intercalation using electrochemistry, in situ XRD, and simulation techniques reveals the large presence of a single-phase domain that proceeds via a stoichiometric ZnLiV3O8 phase to reversible solid–solution ZnyLiV3O8 (y > 1) phase. The unique behavior, which is different from the reaction with lithium, contributes to high specific capacities of 172 mAh g–1 and amounts to 75% retention of the maximum capacity achieved in 65 cycles with 100% Coulombic efficiency at a current density of 133 mA g–1. The remarkable performance makes the development of this low-cost and safe battery technology very promising, and this study also offers opportunities to enhance the understanding on electrochemically induced metastable phases for energy storage applications.

Abstract Developing low-cost electrocatalysts to replace precious Ir-based materials is key for oxygen evolution reaction (OER). Here, we report atomically dispersed nickel coordinated with nitrogen and sulfur species in porous carbon nanosheets as an electrocatalyst exhibiting excellent activity and durability for OER with a low overpotential of 1.51 V at 10 mA cm −2 and a small Tafel slope of 45 mV dec −1 in alkaline media. Such electrocatalyst represents the best among all reported transition metal- and/or heteroatom-doped carbon electrocatalysts and is even superior to benchmark Ir/C. Theoretical and experimental results demonstrate that the well-dispersed molecular S|NiN x species act as active sites for catalyzing OER. The atomic structure of S|NiN x centers in the carbon matrix is clearly disclosed by aberration-corrected scanning transmission electron microscopy and synchrotron radiation X-ray absorption spectroscopy together with computational simulations. An integrated photoanode of nanocarbon on a Fe 2 O 3 nanosheet array enables highly active solar-driven oxygen production.

Ambipolar organic electronics offer great potential for simple and low-cost fabrication of complementary logic circuits on large-area and mechanically flexible substrates. Ambipolar transistors are ideal candidates for the simple and low-cost development of complementary logic circuits since they can operate as n-type and p-type transistors. Nevertheless, the experimental demonstration of ambipolar organic complementary circuits is limited to inverters. The control of the transistor polarity is crucial for proper circuit operation. Novel gating techniques enable to control the transistor polarity but result in dramatically reduced performances. Here we show high-performance non-planar ambipolar organic transistors with electrical control of the polarity and orders of magnitude higher performances with respect to state-of-art split-gate ambipolar transistors. Electrically reconfigurable complementary logic gates based on ambipolar organic transistors are experimentally demonstrated, thus opening up new opportunities for ambipolar organic complementary electronics.

Structural degradation of Ni-rich cathode materials (LiNi(x)M(1-x)O2; M = Mn, Co, and Al; x > 0.5) during cycling at both high voltage (>4.3 V) and high temperature (>50 °C) led to the continuous generation of microcracks in a secondary particle that consisted of aggregated micrometer-sized primary particles. These microcracks caused deterioration of the electrochemical properties by disconnecting the electrical pathway between the primary particles and creating thermal instability owing to oxygen evolution during phase transformation. Here, we report a new concept to overcome those problems of the Ni-rich cathode material via nanoscale surface treatment of the primary particles. The resultant primary particles' surfaces had a higher cobalt content and a cation-mixing phase (Fm3̅m) with nanoscale thickness in the LiNi0.6Co0.2Mn0.2O2 cathode, leading to mitigation of the microcracks by suppressing the structural change from a layered to rock-salt phase. Furthermore, the higher oxidation state of Mn(4+) at the surface minimized the oxygen evolution at high temperatures. This approach resulted in improved structural and thermal stability in the severe cycling-test environment at 60 °C between 3.0 and 4.45 V and at elevated temperatures, showing a rate capability that was comparable to that of the pristine sample.

Abstract Reversible nanostructured electrode materials are at the center of research relating to rechargeable lithium batteries, which require high power, high capacity, and high safety. The higher capacities and higher rate capabilities for the nanostructured electrode materials than for the bulk counterparts can be attributed to the higher surface area, which reduces the overpotential and allows faster reaction kinetics at the electrode surface. These electrochemical enhancements can lead to versatile potential applications of the batteries and can provide breakthroughs for the currently limited power suppliers of mobile electronics. This Feature Article describes recent research advances on nanostructured cathode and anode materials, such as metals, metal oxides, metal phosphides and LiCoO 2 , LiNi 1– x M x O 2 with zero‐, one‐, two‐, and three‐dimensional morphologies.

Abstract Topological Weyl semimetal (TWS), a new state of quantum matter, has sparked enormous research interest recently. Possessing unique Weyl fermions in the bulk and Fermi arcs on the surface, TWSs offer a rare platform for realizing many exotic physical phenomena. TWSs can be classified into type-I that respect Lorentz symmetry and type-II that do not. Here, we directly visualize the electronic structure of MoTe 2 , a recently proposed type-II TWS. Using angle-resolved photoemission spectroscopy (ARPES), we unravel the unique surface Fermi arcs, in good agreement with our ab initio calculations that have nontrivial topological nature. Our work not only leads to new understandings of the unusual properties discovered in this family of compounds, but also allows for the further exploration of exotic properties and practical applications of type-II TWSs, as well as the interplay between superconductivity (MoTe 2 was discovered to be superconducting recently) and their topological order.

W-doping produced the two-phase (<italic>Fm</italic>3̄<italic>m</italic> and <italic>R</italic>3̄<italic>m</italic>) structure which improved the cycling and thermal stability of the Ni-rich layered cathodes.

The lack of high-efficient, low-cost, and durable bifunctional electrocatalysts that act simultaneously for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) is currently one of the major obstacles to commercializing the electrical rechargeability of zinc-air batteries. A nanocomposite CoO-NiO-NiCo bifunctional electrocatalyst supported by nitrogen-doped multiwall carbon nanotubes (NCNT/CoO-NiO-NiCo) exhibits excellent activity and stability for the ORR/OER in alkaline media. More importantly, real air cathodes made from the bifunctional NCNT/CoO-NiO-NiCo catalysts further demonstrated superior performance to state-of-the-art Pt/C or Pt/C+IrO2 catalysts in primary and rechargeable zinc-air batteries.

A superior cocatalytic behavior of reduced graphene oxide (rGO) was observed for the photocatalytic production of H2O2 in the TiO2-based system. The adsorption of phosphate on TiO2 enhanced the production of H2O2 up to a millimolar level. The in situ formation of cobalt phosphate on rGO/TiO2 enabled the photocatalytic production of H2O2 even in the absence of organic electron donors.

Femtosecond x-ray laser pulses were used to probe micrometer-sized water droplets that were cooled down to 227 kelvin in vacuum. Isothermal compressibility and correlation length were extracted from x-ray scattering at the low-momentum transfer region. The temperature dependence of these thermodynamic response and correlation functions shows maxima at 229 kelvin for water and 233 kelvin for heavy water. In addition, we observed that the liquids undergo the fastest growth of tetrahedral structures at similar temperatures. These observations point to the existence of a Widom line, defined as the locus of maximum correlation length emanating from a critical point at positive pressures in the deeply supercooled regime. The difference in the maximum value of the isothermal compressibility between the two isotopes shows the importance of nuclear quantum effects.

We have investigated the electronic structures of single- and double-layered MoS${}_{2}$, composing of heterojunction structures such as graphene, MoS${}_{2}$, and SiO${}_{2}$ and MoS${}_{2}$ and SiO${}_{2}$, using scanning photoelectron microscopy. Negative shifts of both core levels and valence bands toward the Fermi energy have been observed. In connection with first-principles calculations, we have confirmed that the direct gap of single-layer MoS${}_{2}$ is changed to an indirect gap by stacking additional layers via van der Waals interlayer interactions.

The development of a water oxidation catalyst has been a demanding challenge in realizing water splitting systems. The asymmetric geometry and flexible ligation of the biological Mn4CaO5 cluster are important properties for the function of photosystem II, and these properties can be applied to the design of new inorganic water oxidation catalysts. We identified a new crystal structure, Mn3(PO4)2·3H2O, that precipitates spontaneously in aqueous solution at room temperature and demonstrated its high catalytic performance under neutral conditions. The bulky phosphate polyhedron induces a less-ordered Mn geometry in Mn3(PO4)2·3H2O. Computational analysis indicated that the structural flexibility in Mn3(PO4)2·3H2O could stabilize the Jahn-Teller-distorted Mn(III) and thus facilitate Mn(II) oxidation. This study provides valuable insights into the interplay between atomic structure and catalytic activity.