Kuei-Hsien Chen

Abstract A heterobimetallic corrole complex, comprising oxygen reduction reaction (ORR) active non-precious metals Co and Fe with a corrole-N4 center (PhFCC), is successfully synthesized and used to prepare a dual-atom molecular catalyst (DAMC) through subsequent low-temperature pyrolysis. This low-temperature pyrolyzed electrocatalyst exhibited impressive ORR performance, with onset potentials of 0.86 and 0.94 V, and half-wave potentials of 0.75 and 0.85 V, under acidic and basic conditions, respectively. During potential cycling, this DAMC displayed half-wave potential losses of only 25 and 5 mV under acidic and alkaline conditions after 3000 cycles, respectively, demonstrating its excellent stability. Single-cell Nafion-based proton exchange membrane fuel cell performance using this DAMC as the cathode catalyst showed a maximum power density of 225 mW cm−2, almost close to that of most metal–N4 macrocycle-based catalysts. The present study showed that preservation of the defined CoN4 structure along with the cocatalytic Fe–Cx site synergistically acted as a dual ORR active center to boost overall ORR performance. The development of DAMC from a heterobimetallic CoN4-macrocyclic system using low-temperature pyrolysis is also advantageous for practical applications.

The thickness of transition metal dichalcogenides (TMDs) plays a key role in enhancing their photocatalytic CO2 reduction activity. However, the optimum thickness of the layered TMDs that is required to achieve sufficient light absorption and excellent crystallinity has still not been definitively determined. In this work, ultra-thin molybdenum disulfide films (MoS2TF) with 25 nm thickness presented remarkable photocatalytic activity, and the product yield increased by about 2.3 times. The photocatalytic mechanism corresponding to the TMDs’ thickness was also proposed. This work demonstrates that the thickness optimization of TMDs provides a cogent direction for the design of high-performance photocatalysts.

Lithium metal anodes form a dendritic structure after cycling which causes an internal short circuit in flammable electrolytes and results in battery fires. Today's separators are insufficient for suppressing the formation of lithium dendrites. Herein, we report on the use of mesoporous silica thin films (MSTFs) with perpendicular nanochannels (pore size ∼5 nm) stacking on an anodic aluminum oxide (AAO) membrane as the MSTF⊥AAO separator for advancing Li metal batteries. The nanoporous MSTF⊥AAO separator with novel inorganic structures shows ultra-long term stability of Li plating/stripping in Li–Li cells at an ultra-high current density and capacity (10 mA cm−2 and 5 mA h cm−2). A significant improvement over the state-of-the-art separator is evaluated based on three performance indicators, e.g. cycle life, current density and capacity. In Li–Cu cells, the MSTF⊥AAO separator shows a coulombic efficiency of >99.9% at a current density of 10 mA cm−2 for more than 250 h of cycling. The separator gives improved rate capability in Li–LiFePO4 (LFP) batteries. The excellent performance of the MSTF⊥AAO separator is due to good wetting of electrolytes, straight nanopores with negative charges, uniform Li deposition and blocking the finest dendrite.

Most chemical vapor deposition methods for transition metal dichalcogenides use an extremely small amount of precursor to render large single-crystal flakes, which usually causes low coverage of the materials on the substrate. In this study, a self-capping vapor-liquid-solid reaction is proposed to fabricate large-grain, continuous MoS2 films. An intermediate liquid phase-Na2Mo2O7 is formed through a eutectic reaction of MoO3 and NaF, followed by being sulfurized into MoS2. The as-formed MoS2 seeds function as a capping layer that reduces the nucleation density and promotes lateral growth. By tuning the driving force of the reaction, large mono/bilayer (1.1 mm/200 μm) flakes or full-coverage films (with a record-high average grain size of 450 μm) can be grown on centimeter-scale substrates. The field-effect transistors fabricated from the full-coverage films show high mobility (33 and 49 cm2 V−1 s−1 for the mono and bilayer regions) and on/off ratio (1 ~ 5 × 108) across a 1.5 cm × 1.5 cm region.

Gas-phase photocatalytic reactions to convert carbon dioxide and water into oxygen and hydrocarbons are the foundation of life on earth. However, the efficiency of photosynthesis is relatively low (~1%), which leaves much room for artificial photosynthesis to reach the benchmark of the solar cells (>15%). In this work, carbon implanted SnS2 thin films (C–SnS2) were prepared to study photocatalytic activity and adsorbate-catalyst surface interactions during CO2 photoreduction. The electron density distribution in C–SnS2 and its contribution toward the photogenerated charge transfer process has been analyzed by the angle-dependent X-ray absorption near-edge structure (XANES) study. The C–SnS2 surface affinity toward the CO2 molecule was monitored by in-situ dark current and Raman spectroscopy measurements. By optimizing the dose during ion implantation, SnS2 thin film with 1 wt% carbon incorporation shows 108 times enhancement in the CO2 conversion efficiency and more than 89% product selectivity toward CH4 formation compared with the as-grown SnS2 without carbon incorporation. The improved photocatalytic activity can be ascribed to enhanced light harvesting, pronounced charge-transfer between SnS2 and carbon with improved carrier separation and the availability of highly active carbon sites that serve as favorable CO2 adsorption sites.

Nonnoble metal catalysts are low-cost alternatives to Pt for the oxygen reduction reactions (ORRs), which have been studied for various applications in electrocatalytic systems. Among them, transition metal complexes, characterized by a redox-active single-metal-atom with biomimetic ligands, such as pyrolyzed cobalt–nitrogen–carbon (Co–Nx/C), have attracted considerable attention. Therefore, we reported the ORR mechanism of pyrolyzed Vitamin B12 using operando X-ray absorption spectroscopy coupled with electrochemical impedance spectroscopy, which enables operando monitoring of the oxygen binding site on the metal center. Our results revealed the preferential adsorption of oxygen at the Co2+ center, with end-on coordination forming a Co2+-oxo species. Furthermore, the charge transfer mechanism between the catalyst and reactant enables further Co–O species formation. These experimental findings, corroborated with first-principle calculations, provide insight into metal active-site geometry and structural evolution during ORR, which could be used for developing material design strategies for high-performance electrocatalysts for fuel cell applications.

In addition to the Ge-vacancy control of GeTe, the antimony (Sb) substitution of GeTe for the improvement of thermoelectric performance is explored for Ge1−xSbxTe with x = 0.08–0.12. The concomitant carrier concentration (n) and the aliovalent Sb ion substitution led to an optimal doping level of x = 0.10 to show ZT ∼ 2.35 near ∼800 K, which is significantly higher than those single- and multi-element substitution studies of the GeTe system reported in the literature. In addition, Ge0.9Sb0.1Te demonstrates an impressively high power factor of ∼36 μW cm−1 K−2 and a low thermal conductivity of ∼1.1 W m−1 K−1 at 800 K. The enhanced ZT level for Ge0.9Sb0.1Te is explained through a systematic investigation of micro-structural change and strain analysis from room temperature to 800 K. A significant reduction of lattice thermal conductivity (κlat) is identified and explained by the Sb substitution-introduced strained and widened domain boundaries for the herringbone domain structure of Ge0.9Sb0.1Te. The Sb substitution created multiple forms of strain near the defect centre, the herringbone domain structure, and widened tensile/compressive domain boundaries to support phonon scattering that covers a wide frequency range of the phonon spectrum to reduce lattice thermal conductivity effectively.

A highly reproducible sample preparation method for pure GeTe in a rhombohedral structure without converting to the cubic structure up to ∼500 °C is reported to show control of the Ge-vacancy level and the corresponding herringbone-structured microdomains. The thermoelectric figure-of-merit (ZT) for GeTe powder could be raised from ∼0.8 to 1.37 at high temperature (HT) near ∼500 °C by tuning the Ge-vacancy level through the applied reversible in situ route, which made it highly controllable and reproducible. The enhanced ZT of GeTe was found to be strongly correlated with both its significantly increased Seebeck coefficient (∼161 μV K−1 at 500 °C) and reduced thermal conductivity (∼2.62 W m−1 K−1 at 500 °C) for a sample with nearly vacancy-free thicker herringbone-structured microdomains in the suppressed rhombohedral-to-cubic structure phase transformation. The microdomain and crystal structures were identified with HR-TEM (high-resolution transmission electron microscopy) and powder X-ray diffraction (XRD), while electron probe micro-analysis (EPMA) was used to confirm the stoichiometry changes of Ge : Te. Theoretical calculations for GeTe with various Ge-vacancy levels suggested that the Fermi level shifts toward the valence band as a function of increasing the Ge-vacancy level, which is consistent with the increased hole-type carrier concentration (n) and effective mass (m*) deduced from the Hall measurements. The uniquely prepared sample of a near-vacancy-free GeTe in a rhombohedral structure at high temperature favoured an enhanced Seebeck coefficient in view of the converging L- and Σ-bands of the heavy effective mass at the Fermi level, while the high density domain boundaries for the domain of low carrier density were shown to reduce the total thermal conductivity effectively.

Designing highly conducting metal–organic frameworks (MOFs) is currently a subject of great interest for their potential applications in diverse areas encompassing energy storage and generation. Herein, a strategic design in which a metal–sulfur plane is integrated within a MOF to achieve high electrical conductivity, is successfully demonstrated. The MOF {[Cu2(6-Hmna)(6-mn)]·NH4}n (1, 6-Hmna = 6-mercaptonicotinic acid, 6-mn = 6-mercaptonicotinate), consisting of a two dimensional (–Cu–S–)n plane, is synthesized from the reaction of Cu(NO3)2, and 6,6′-dithiodinicotinic acid via the in situ cleavage of an S–S bond under hydrothermal conditions. A single crystal of the MOF is found to have a low activation energy (6 meV), small bandgap (1.34 eV) and a highest electrical conductivity (10.96 S cm−1) among MOFs for single crystal measurements. This approach provides an ideal roadmap for producing highly conductive MOFs with great potential for applications in batteries, thermoelectric, supercapacitors and related areas.

Interface engineering of CdS/CZTS(Se) is an important aspect of improving the performance of buffer/absorber heterojunction combination. It has been demonstrated that the crossover phenomenon due to the interface recombination can be drastically eliminated by interface modification. Therefore, in-depth studies across the CdS/CZTS(Se) junction properties, as well as effective optimization processes, are very crucial for achieving high-efficiency CZTSSe solar cells. Here, we present a comprehensive study on the effects of soft-baking (SB) temperature on the junction properties and the corresponding optoelectronic and interface-structural properties. Based on in-depth photoemission studies corroborated with structural and composition analysis, we concluded that interdiffusion and intermixing of CZTSSe and CdS phases occurred on the Cu-poor surface of CZTSSe at elevated SB temperatures, and the interface dipole moments induced by electrostatic potential fluctuation were thus significantly eliminated. In contrast, with low SB temperature, the CdS/CZTSSe heterojunction revealed very sharp interface with very short interdiffusion, forming interface dipole moments and drastically deteriorating device performance. These post thermal treatments also significantly suppress defect energy level of interface measured by admittance spectroscopy from 294 to 109 meV due to CdS/CZTSSe interdiffusion. Meanwhile, the interdiffusion effects on the shift of valence band maximum, conduction band minimum and band offset across the heterojunction of thermally treated CdS/CZTSSe interface are spatially resolved at the atomic scale by measuring the local density of states with cross-sectional scanning tunneling microscopy and spectroscopy. A significant enhancement in the power conversion efficiency from 4.88% to 8.48% is achieved by a facile interface engineering process allowing a sufficient intermixing of CdS/Cd and CZTSSe/Se phases without detrimental recombination centers.

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The present investigation mainly addresses the open circuit voltage (Voc) issue in kesterite based Cu2ZnSn(S,Se)4 solar cells by simply introducing alkali metal fluoride nanolayers (~ several nm NaF, or LiF) to lower the work functions of the front ITO contacts without conventional hole-blocking ZnO layers. Kelvin probe measurements confirmed that the work function of the front ITO decreases from 4.82 to 3.39 and 3.65 eV for NaF and LiF, respectively, resulting in beneficial band alignment for electron collection and/or hole blocking on top electrodes. Moreover, a 10.4% power conversion efficiency (~ 11.5% in the cell effective area) CZTSSe cell with improved Voc of up to 90 mV has been attained. This demonstration may provide a new direction of further boosting the performance of copper chalcogenide based solar cells as well.

Photocatalytic formation of hydrocarbons using solar energy via artificial photosynthesis is a highly desirable renewable-energy source for replacing conventional fossil fuels. Using an l-cysteine-based hydrothermal process, here we synthesize a carbon-doped SnS2 (SnS2-C) metal dichalcogenide nanostructure, which exhibits a highly active and selective photocatalytic conversion of CO2 to hydrocarbons under visible-light. The interstitial carbon doping induced microstrain in the SnS2 lattice, resulting in different photophysical properties as compared with undoped SnS2. This SnS2-C photocatalyst significantly enhances the CO2 reduction activity under visible light, attaining a photochemical quantum efficiency of above 0.7%. The SnS2-C photocatalyst represents an important contribution towards high quantum efficiency artificial photosynthesis based on gas phase photocatalytic CO2 reduction under visible light, where the in situ carbon-doped SnS2 nanostructure improves the stability and the light harvesting and charge separation efficiency, and significantly enhances the photocatalytic activity.

Abstract The present investigation mainly addresses the open circuit voltage (Voc) issue in kesterites based Cu2ZnSn(S,Se)4 solar cells by simply introducing alkali metal fluoride nanolayers (  several nm NaF, or LiF) to lower the work functions of the front İTO\} contacts without conventional hole-blocking ZnO layers. Kelvin probe measurements confirmed that the work function of the front İTO\} decreases from 4.82 to 3.39 and 3.65 eV for NaF and LiF, respectively, resulting in beneficial band alignment for electron collection and/or hole blocking on top electrodes. Moreover, a 10.4% power conversion efficiency ( 11.5% in the cell effective area) \{CZTSSe\} cell with improved Voc of up to 90 mV has been attained. This demonstration may provide a new direction of further boosting the performance of copper chalcogenide based solar cells as well.

One of the key challenges in artificial photosynthesis is to design a photocatalyst that can bind and activate the CO2 molecule with the smallest possible activation energy and produce selective hydrocarbon products. In this contribution, a combined experimental and computational study on Ni-nanocluster loaded black TiO2 (Ni/TiO2[Vo]) with built-in dual active sites for selective photocatalytic CO2 conversion is reported. The findings reveal that the synergistic effects of deliberately induced Ni nanoclusters and oxygen vacancies provide (1) energetically stable CO2 binding sites with the lowest activation energy (0.08 eV), (2) highly reactive sites, (3) a fast electron transfer pathway, and (4) enhanced light harvesting by lowering the bandgap. The Ni/TiO2[Vo] photocatalyst has demonstrated highly selective and enhanced photocatalytic activity of more than 18 times higher solar fuel production than the commercial TiO2 (P-25). An insight into the mechanisms of interfacial charge transfer and product formation is explored.