Kuei-Hsien Chen

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.

Photocatalytic water splitting is a promising way to produce hydrogen fuel from solar energy. In this regard, the search for new photocatalytic materials that can efficiently split water into hydrogen is essential. Here, using first-principles simulations, we demonstrate that the dual-defective SnS2 (Ni-SnS2-VS), by both single-atom nickel doping and sulfur monovacancies, becomes a promising two-dimensional photocatalyst compared with SnS2. The Ni-SnS2-VS monolayer, in particular, exhibits a suitable band alignment that perfectly overcomes the redox potentials for overall water splitting. The dual-defective monolayer displays remarkable photocatalytic activity, a spatially separated carrier, a broadened optical absorption spectrum, and enhanced adsorption energy of H2O. Therefore, the dual-defective SnS2 monolayer can serve as an efficient photocatalyst for overall water splitting to produce hydrogen fuel. Furthermore, a novel dual-defect method can be an effective strategy to enhance the photocatalytic behavior of 2D materials; it may pave inroads in the development of solar-fuel generation.

Thermoelectric (TE) devices used to convert waste heat directly into electricity are highly desirable for alleviating the prevailing energy crisis and global climate-change issues. Among the various TE materials available, metal oxides exhibit high thermal and chemical stabilities in air, and are hence, preferred for use in many TE applications. However, most of them possess TE figures of merit (ZT) that are below the applicable value of 2, in the mid-temperature region (from 250 to 600 °C). In a previous work, the removal of a small amount of Bi from BiCuSeO was found to improve the ZT of BiCuSeO. In this work, we pursue another track and study the TE performance of BiCuTeO after the addition of up to 6% excess Bi. Bi1+xCuTeO (x = 0.00–0.06) samples were prepared by solid-state reactions, followed by hot-pressing to form pellets. By adding a stoichiometric excess of Bi into BiCuTeO, 16% enhancement in power factor was achieved at 450 °C. This enhancement can be attributed to the increase in the Seebeck coefficient because of the appearance of secondary phases. Detailed characterizations and discussions of the effect of the nominal excess Bi in BiCuTeO are presented in this paper. The findings of this study can be applied in the investigation of novel high-performance TE materials.

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.

BiCuTeO is a potential thermoelectric material owing to its low thermal conductivity and high carrier concentration. However, the thermoelectric performance of BiCuTeO is still below average and has much scope for improvement. In this study, we manipulated the nominal oxygen content in BiCuTeO and synthesized BiCuTeOx (x = 0.94–1.06) bulks by a solid-state reaction and pelletized them by a cold-press method. The power factor was enhanced by varying the nominal oxygen deficiency due to the increased Seebeck coefficient. The thermal conductivity was also reduced due to the decrease in lattice thermal conductivity owing to the small grain size generated by the optimal nominal oxygen content. Consequently, the ZT value was enhanced by ∼11% at 523 K for stoichiometric BiCuTeO0.94 compared to BiCuTeO. Thus, optimal oxygen manipulation in BiCuTeO can enhance the thermoelectric performance. This study can be applied to developing oxides with high thermoelectric performances.

Exploring electrocatalysts composed of earth-abundant elements for a highly efficient hydrogen evolution reaction (HER) is scientifically and technologically important for electrocatalytic water splitting. In this work, we report HER properties of acid treated pyrolyzed vitamin B12 supported on reduced graphene oxide (B12/G800A) that shows an extraordinarily enhanced catalytic activity with low overpotential (115 mV vs. RHE at 10 mA cm−2), which is better than that of most traditional nonprecious metal catalysts in acidic media. Stability tests through long-term potential cycles and at a constant current density confirm the exceptional durability of the catalyst. Notably, the B12/G800A catalyst exhibits extremely high turnover frequencies per cobalt site in acid, for example, 0.85 and 11.46 s−1 at overpotentials of 100 and 200 mV, respectively, which are higher than those reported for other scalable non-precious metal HER catalysts. Moreover, it has been conjectured that the covalency of Co–C and Co–N bonds affects HER activities by comparing the extended X-ray absorption fine structure (EXAFS) spectra of the B12/G800A. High-temperature treatment can modify the Co-corrin structure of B12 to form Co–C bonds along with Co–N, which broadens the band of cobalt, essentially lowering the d-band center from its Fermi level. The lower d-band center leads to a moderate hydrogen binding energy, which is favorable for hydrogen adsorption and desorption.

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.

Polybenzimidazoles containing heterocyclic benzo[c]cinnoline structure are synthesized from 3,8-benzo[c]cinnoline dicarboxylic acid, terephthalic acid and 3,3′-diaminobenzidine. Their membranes are prepared by sol-gel process, involving the conversion of polymer solution in polyphosphoric acid to phosphoric acid. The acid doping levels of the as-prepared membranes increase as the contents of benzo[c]cinnoline increase, indicating good interaction between phosphoric acid and benzo[c]cinnoline structure. The as-prepared membranes with high acid doping levels might lead to the dissolution of membranes in phosphoric acid at temperature higher than 120 °C. A new method is proposed to adjust acid doping levels by immersing the as-prepared membranes in diluted phosphoric acid solutions of various concentrations. The adjusted membranes (acid doping levels around 30 PA RU−1) exhibit enhanced mechanical properties with tensile strength in the range of 4.1–5.2 MPa. The proton conductivity of adjusted membranes maintain at 0.15–0.17 S cm−1 at 160 °C under ambient atmosphere without humidification. The single cells based on the adjusted membranes exhibit open circuit voltages and peak power densities from 0.89 to 0.91 V and 691–1253 mW cm−2 at 160 °C, respectively. Compared to other polybenzimidazole membranes prepared by sol-gel process, the adjusted polybenzimidazoles show higher mechanical strength and better single cell performance.

Cr substituted tetrahedrites with the chemical formula Cu12−xCrxSb4S13 (x = 0.15, 0.25, 0.35, 0.5, 0.75, 1.0) have been synthesised for thermoelectric study. Cr substitutes at the Cu site to optimize the thermoelectric properties and achieve a higher figure of merit (zT). X-Ray diffraction (XRD) analysis revealed that the tetrahedrite is the major phase with minor impurity phases. Electron probe microanalysis (EPMA) shows the formation of tetrahedrite main phase with near stoichiometry and the presence of Cu3SbS4, CuSbS2 and Sb as secondary phases. X-ray photoelectron spectroscopy (XPS) shows the oxidation state of Cu, Sb and S as +1, +3 and −2, respectively, whereas for Cr, it could not be identified. Temperature-dependent magnetic susceptibility of sample x = 0.75 shows antiferromagnetic correlation originating from the Cr ion. The calculated effective magnetic moment of 2.83 μB per Cr atom indicates the presence of Cr+4 in this sample. The decrease in the electrical resistivity upon doping indicates the compensation of holes due to the substitution of Cr at the Cu site. But the x = 0.35 sample is not following the trend due to larger compensation of holes with an activation energy of 124.6 meV. The temperature-dependent behaviour of electrical resistivity shows the shift in the Fermi level from the valance band towards the band gap. The absolute Seebeck coefficient is positive throughout the temperature range and follows a similar trend as that of electrical resistivity, with the exception of the x = 0.35 sample. The electronic thermal conductivity reduces due to hole compensation caused by Cr substitution. Moreover, the substitution of Cr effectively reduces the lattice thermal conductivity due to point defect scattering of phonons. A maximum zT of 1.0 is achieved for sample x = 0.35 at 700 K.

BiCuSeO is a promising thermoelectric material having earth-abundant non-toxic constituents and favourable thermoelectric properties like ultra-low thermal conductivity. In this study, Mn+2 has been introduced at the Bi+3 site to increase hole concentration as well as Seebeck coefficient, through aliovalent doping and magnetic impurity incorporation respectively. Samples were prepared through two-step solid state synthesis with the composition Bi1-xMnxCuSeO (x = 0.0, 0.04, 0.06, 0.08, 0.10 and 0.12). X-ray diffraction patterns confirmed the tetragonal (space group: P4/nmm) crystal structure of BiCuSeO as well as phase purity of the samples. The Seebeck coefficient and electrical resistivity had a decreasing trend with increasing doping fraction owing to the generation of charge carriers. The samples with x = 0.04 and 0.06 showed temperature independent Seebeck coefficient above 523 K, which is a signature of small polaron hopping. While the Seebeck coefficient of the samples with x = 0.08, 0.10 and 0.12 increased above 523 K due to the combination of localized and extended states. The thermal conductivity was dominated by the lattice part of the thermal conductivity. As a result of moderate Seebeck coefficient and low electrical resistivity, the highest power factor of 0.284 mW m−1-K2 was obtained for the Bi0.92Mn0.08CuSeO at 773 K, leading to a maximum zT of 0.4 at 773.

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.

AbstractPorous α-Fe2O3 nanoparticles are directly grown on acid treated carbon cloth (ACC) using a simple hydrothermal method (denoted as ACC-α-Fe2O3) for employment as a flexible and wearable electrochemical electrode. The catalytic activity of ACC-α-Fe2O3 allowing the detection of dopamine (DA) is systematically investigated. The results showed that the ACC-α-Fe2O3 electrode exhibits impressive electrochemical sensitivity, stability and selectivity for the detection of DA. The detection limit determined with the amperometric method appears to be around 50nM with a linear range of 0.074–113μM. The impressive DA sensing ability of the as prepared ACC-α-Fe2O3 electrode is due to the good electrochemical behavior and high electroactive surface area (19.96cm2) of α-Fe2O3 nanoparticles anchored on the highly conductive ACC. It is worth noting that such remarkable sensing properties can be maintained even when the electrode is in a folded configuration.

Germanium telluride (GeTe) is a very well known IV–VI group semiconducting material with the advantageous property of showing metallic conduction, which materializes from its superior carrier concentration (n) (high number of Ge vacancies). A systematic investigation into the thermoelectric properties (TEP) of GeTe was reported by way of carrier concentration (n) engineering. The present investigation focuses on studying the effects of doping (antimony – Sb) and co-doping (phosphorus – P) on the TEP of GeTe. In order to understand the system, we have prepared p-type GeTe and Ge0.9−xPxSb0.1Te (x = 0, 0.01, 0.03, or 0.05) samples via a non-equilibrium solid state melt quenching (MQ) process, followed by hot press consolidation. Temperature dependent synchrotron X-ray diffraction studies reveal a phase transition from rhombohedral to simple cubic in the Ge0.9−xPxSb0.1Te system at 573 K, which is clearly reflected in the TEP. Further high resolution transmission electron microscopy (HRTEM) studies reveal the pseudo-cubic nature of the sample. However, powder X-ray diffraction (PXRD) and field emission scanning electron microscopy (FESEM) images and energy dispersive X-ray spectroscopy (EDX) studies confirm the presence of germanium phosphide (GeP) in all P-doped samples. The presence of a secondary phase and point defects (Sb & P) enhanced the additional scattering effects in the system, which influenced the Seebeck coefficient and thermal conductivity of GeTe. A significant enhancement in the Seebeck coefficient (S) to ∼225 μV K−1 and a drastic reduction in thermal conductivity (κ) to ∼1.2 W mK−1 effectively enhanced the figure-of-merit (ZT) to ∼1.72 at 773 K for Ge0.87P0.03Sb0.1Te, which is a ∼3 fold increase for GeTe. Finally, P co-doped Ge0.9Sb0.1Te demonstrates an enhancement in ZT, making it a good candidate material for power generation applications.

Abstract Application of lasers is omnipresent in modern-day technology. However, preparation of a lasing device usually requires sophisticated design of the materials and is costly, which may limit the suitable choice of materials and the lasing wavelengths. Random lasers, on the other hand, can circumvent the aforementioned shortcomings with simpler fabrication process, lower processing cost, material flexibility for any lasing wavelengths with lower lasing threshold, providing a roadmap for the design of super-bright lighting, displays, Li-Fi, etc. In this work, ultralow-threshold random laser action from semiconductor nanoparticles assisted by a highly porous vertical-graphene-nanowalls (GNWs) network is demonstrated. The GNWs embedded by the nanomaterials produce a suitable cavity for trapping the optical photons with semiconductor nanomaterials acting as the gain medium. The observed laser action shows ultralow values of threshold energy density ≈10 nJ cm?2 due to the strong photon trapping within the GNWs. The threshold pump fluence can be further lowered to ≈1 nJ cm?2 by coating Ag/SiO2 upon the GNWs due to the combined effect of photon trapping and strong plasmonic enhancement. In view of the growing demand of functional materials and novel technologies, this work provides an important step toward realization of high-performance optoelectronic devices.

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.

Polyethersulfone ionomers containing quaternary ammoniums were prepared for the applications on alkaline anion exchange membrane (AAEM) fuel cells. The ionomers were synthesized from 2,2′-dimethyl-4,4′-biphenyldiol and bis(4-chlorophenyl) sulfone via nucleophilic substitution followed by bromination, quaternization and anion exchange reaction. The biphenyl structure in polymer main chain exhibited atropisomerism after bromination, leading to the anisochronous signals of geminal protons on bromomethyl groups in 1H NMR spectra. Model compounds were synthesized to confirm the atropisomerism by EI mass and 1H NMR spectra. The resonance peaks from five possible repeating units of brominated polyethersulfones in the 1H NMR spectra were identified and discussed in detail. The rotational barriers of biphenyl structures containing brominated methyl groups at 2 and 2′ positions were calculated by density functional theory. The properties of these polyethersulfone anion exchange membranes (AEMs) were characterized. Their IECs ranged from 0.81 to 1.75 mequiv/g. The corresponding water uptakes and dimensional changes were in the ranges of 19–42% and 12–38%, respectively. The tensile strength of an AEM (1.75MQAPES-OH) with an IEC of 1.75 mequiv/g remained 17 MPa even though the water uptake was 42%. The hydroxide conductivity of 1.75MQAPES-OH could reach 51.4 mS/cm at 98%RH and 80 °C. After alkaline stability test for 168 h, the AEMs degraded slightly in terms of their IECs and hydroxide conductivity.

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.

Production of hydrogen from water electrolysis has stimulated the search of sustainable electrocatalysts as possible alternatives. Recently, cobalt phosphide (CoP) and molybdenum phosphide (MoP) received great attention owing to their superior catalytic activity and stability towards the hydrogen evolution reaction (HER) which rivals platinum catalysts. In this study, we synthesize and study a series of catalysts based on hybrids of CoP and MoP with different Co/Mo ratio. The HER activity shows a volcano shape and reaches a maximum for Co/Mo = 1. Tafel analysis indicates a change in the dominating step of Volmer-Hyrovský mechanism. Interestingly, X-ray diffraction patterns confirmed a major ternary interstitial hexagonal CoMoP(2) crystal phase is formed which enhances the electrochemical activity.