Kuei-Hsien Chen

Abstract Atomically dispersed iron sites on nitrogen-doped carbon (Fe-NC) are the most active Pt-group-metal-free catalysts for oxygen reduction reaction (ORR). However, due to oxidative corrosion and the Fenton reaction, Fe-NC catalysts are insufficiently active and stable. Herein, w e demonstrated that the axial Cl-modified Fe-NC (Cl-Fe-NC) electrocatalyst is active and stable for the ORR in acidic conditions with high H2O2 tolerance. The Cl-Fe-NC exhibits excellent ORR activity, with a high half-wave potential (E1/2) of 0.82 V versus a reversible hydrogen electrode (RHE), comparable to Pt/C (E1/2 = 0.85 V versus RHE) and better than Fe-NC (E1/2 = 0.79 V versus RHE). X-ray absorption spectroscopy analysis confirms that chlorine is axially integrated into the FeN4. More interestingly, compared to Fe-NC, the Fenton reaction is markedly suppressed in Cl-Fe-NC. In situ electrochemical impedance spectroscopy reveals that Cl-Fe-NC provides efficient electron transfer and faster reaction kinetics than Fe-NC. Density functional theory calculations reveal that incorporating Cl into FeN4 can drive the electron density delocalization of the FeN4 site, leading to a moderate adsorption free energy of OH* (?GOH*), d-band center, and a high onset potential, and promotes the direct four-electron-transfer ORR with weak H2O2 binding ability compared to Cl-free FeN4, indicating superior intrinsic ORR activity.

Achieving lithium-ion batteries with both excellent electrochemical performance and cycling stability is a top priority for their real-world applications. This work reports high-performance and stable Cu2ZnSnS4 (CZTS) anode materials encapsulated by nitrogen-doped carbon (CZTS@N-C) for advanced lithium-ion battery application. Ex-situ X-ray photoelectron spectroscopy and transmission electron microscopy revealed that the nitrogen-doped carbon network features a more conducive solid-electrolyte interphase that enables lower charge-transfer resistance and fast Li+ diffusion kinetics with negligible initial irreversible capacity loss. As a result, the CZTS@N-C electrode delivers a significantly enhanced capacity of 710 mAh g−1 with 73% capacity retention after 220 cycles at a current rate of 0.5 mA g−1 and superior rate performance compared to that of unmodified CZTS. Additionally, the study sheds light on the fast lithiation dynamics chemistry of CZTS@N-C through kinetics analysis, explored by in-situ Raman, ex-situ X-ray absorption, and in-situ electrochemical impedance. This study provides a new approach for fabricating high-performance, durable conductive polymer-encapsulated low-cost transition-metal-sulfide anode materials.

Abstract Electrochemical reduction of oxygen into hydrogen peroxide in an acidic medium offers an energy-efficient and green H2O2 synthesis as an alternative to the energy-intensive anthraquinone process. Unfortunately, high overpotential, low production rates, and fierce competition from traditional four-electron reduction limit it. In this study, a metalloenzyme-like active structure is mimicked in carbon-based single-atom electrocatalysts for oxygen reduction to H2O2. Using a carbonization strategy, the primary electronic structure of the metal center with nitrogen and oxygen coordination is modulated, followed by epoxy oxygen functionalities close to the metal active sites. In an acidic medium, CoNOC active structures proceed with greater than 98% H2O2 selectivity (2e?/2H+) rather than CoNC active sites that are selective to H2O (4e?/4H+). Among all MNOC (M = Fe, Co, Mn, and Ni) single-atom electrocatalysts, the CoNOC is the most selective (> 98%) for H2O2 production, with a mass activity of 10 A g?1 at 0.60 V vs. RHE. X-ray absorption spectroscopy is used to identify the formation of unsymmetrical MNOC active structures. Experimental results are also compared to density functional theory calculations, which revealed that the structure-activity relationship of the epoxy-surrounded CoNOC active structure reaches optimum (?G*OOH) binding energies for high selectivity.

Abstract Earth-abundant commercial conductive carbon materials are ideal electrocatalyst supports but cannot be directly utilized for single-atom catalysts owing to the lack of anchoring sites. Therefore, we employed crosslink polymerization to modify the conductive carbon surface with Fe?Co dual-site electrocatalysts for oxygen reduction reaction (ORR). First, metal-coordinated polyurea (PU) aerogels were prepared using via crosslinked polymerization at ambient temperature. Then, carbon-supported, atomically dispersed Fe?Co dual-atom sites (FeCoNC/BP) were formed by high-temperatures pyrolysis with a nitrogen source. FTIR and 13C NMR measurements showed PU linkages, while 15N NMR revealed metal?nitrogen coordination in the PU gels. Asymmetric, N-coordinated, and isolated Fe?Co active structures were found after pyrolysis using XAS and STEM. In alkaline media, FeCoNC/BP exhibited excellent ORR activity, with a E1/2 of 0.93?V vs. RHE, higher than that of Pt/C (20?%) (0.90?V), FeNC/BP (0.88?V), and CoNC/BP (0.85?V). An accelerated durability test (ADT) on FeCoNC/BP indicated good durability over 35000 cycles. FeCoNC/BP also showed moderate ORR and ADT performance in acidic media. The macro/mesoporous N-doped carbon structures enhanced the mass transport properties of the dual Fe?Co active-sites. Therefore, modifying carbon supports with nonprecious metal catalysts may be a cost-effective-strategy for sustained electrochemical energy conversion.

Graphitic carbon nitride (CN) has emerged as a highly promising material in the photocatalysis field. However, its bulk structure suffers from a lack of active sites, limiting its practical application. Herein, a boron-doped CN (BCN) was prepared by a green gas-blowing-assisted thermal polymerization and then subjected to different exfoliation processes in order to delaminate the layered structure and tune the surface-active sites. A thorough comparative study shows that thermal exfoliation creates unsaturated nitrogen sites and induces the formation of interconnected layers that act as an electron diffusion channel for better charge transport. Furthermore, the thermally exfoliated BCN is rich in structural disorders that serve as dissociation defects for photoinduced charge carriers with a low exciton binding energy of 27 meV. Experimental results supported by theoretical calculations show that the nitrogen adjacent to boron is activated by the surrounding surface amino groups and the perforated texture to serve as an active adsorption site towards CO2 and H2O. Consequently, the exfoliated BCN acts as an outstanding bifunctional photocatalyst towards CO2 reduction into CO (40.41 μmol g−1 h−1) and prominent hydrogen evolution (4740 μmol g−1 h−1, 12.2% apparent quantum yield (AQY)).

Ascertaining the function of in-plane intrinsic defects and edge atoms is necessary for developing efficient low-dimensional photocatalysts. We report the wireless photocatalytic CO2 reduction to CH4 over reconstructed edge atoms of monolayer 2H-WSe2 artificial leaves. Our first-principles calculations demonstrate that reconstructed and imperfect edge configurations enable CO2 binding to form linear and bent molecules. Experimental results show that the solar-to-fuel quantum efficiency is a reciprocal function of the flake size. It also indicates that the consumed electron rate per edge atom is two orders of magnitude larger than the in-plane intrinsic defects. Further, nanoscale redox mapping at the monolayer WSe2–liquid interface confirms that the edge is the most preferred region for charge transfer. Our results pave the way for designing a new class of monolayer transition metal dichalcogenides with reconstructed edges as a non-precious co-catalyst for wired or wireless hydrogen evolution or CO2 reduction reactions.

Employing direct Z-scheme semiconductor heterostructures in photocatalysis offers efficient charge carrier separation and isolation of both redox reactions, thus beneficial to reduce CO2 into solar fuels. Here, a ZnS/ZnIn2S4 heterostructure, comprising cubic ZnS nanocrystals on hexagonal ZnIn2S4 (ZIS) nanosheets, is successfully fabricated in a single-pot hydrothermal approach. The composite ZnS/ZnIn2S4 exhibits microstrain at its interface with an electric field favorable for Z-scheme. At an optimum ratio of Zn:In (~ 1:0.5), an excellent photochemical quantum efficiency of around 0.8% is reached, nearly 200-fold boost compared with pristine ZnS. Electronic levels and band alignments are deduced from ultraviolet photoemission spectroscopy and UV-Vis. Evidence of the direct Z-scheme and carrier dynamics is verified by photo-reduction experiment, along with photoluminescence (PL) and time-resolved PL. Finally, diffuse-reflectance infrared Fourier transformed spectroscopy explores the CO2 and related intermediate species adsorbed on the catalyst during the photocatalytic reaction. This microstrain-induced direct Z-scheme approach opens a new pathway for developing next-generation photocatalysts for CO2 reduction.

Bimetal oxides are promising materials in the field of energy storage due to their various oxidation states, synergistic interactions among multiple metal species, and stability. In this work, Co3V2O8 hollow spheres are synthesized by a two-step hydrothermal method: (i) synthesis of V2O5 spheres and (ii) partial replacement of V by Co through the Kirkendall effect. As an electrode, it shows an extrinsic pseudocapacitive charge-storage mechanism due to different oxidation states of V and Co ions. Because of the low crystallinity degree of the mesoporous wall and high accessible surface area of hollow spheres, the optimum Co3V2O8 electrode reaches a high specific capacitance of 2376F g−1 at a current density of 2 A g−1, which is more than two times higher than the top reported values, and a rate capability retention of ∼80% at 20 A g−1. Using Co3V2O8, activated carbon, and KOH as positive, negative electrodes, and electrolyte, respectively, a hybrid supercapacitor device presents maximum energy and power densities of 59.2 Wh kg−1 and 36.6 kW kg−1, respectively. Further, the aqueous supercapacitor device shows superior structural and electrochemical stabilities after 10,000 galvanostatic charge–discharge cycles because of the arrays of voids in the orthorhombic crystal structure of Co3V2O8 that can decrease the volume expansion/shrinkage during the intercalation/deintercalation processes. Our results provide a platform for exploring bimetallic Co and V-based oxides, hydroxides, and sulfides nanostructures as promising energy storage materials in the future.

As a lead-free high-performance thermoelectric material, germanium telluride (GeTe) has recently been extensively studied for mid-temperature (500–800 K) applications. The carrier concentration and the thermal conductivity are reduced for vacancy-controlled GeTe compounds compared with pristine GeTe. We explored and optimized the Ge0.9−xSb0.1PxTe (x = 0.01–0.05) material's highest thermoelectric performance at elevated temperatures. Intrinsic Ge vacancy control and manipulation of Ge (+2) with Sb/P (+3) increased the charge contribution to power factor improvement to ∼42 µWcm−1 K−2 while minimizing the lattice thermal contribution to ∼0.4 W/mK. This resulted in an increase in thermoelectric performance of ∼2.4 @ 773 K for the Ge0.88Sb0.1P0.02Te sample. The inclusion of atomically disordered Sb/P ions considerably increases the scattering effects caused by the point defect, whereas stretched grain boundaries reveal the decreased lattice thermal contribution. The current work demonstrates the effectiveness of phosphorus as a co-dopant in increasing the average thermoelectric performance (ZTavg) value over the GeTe operating temperature range.

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A fine control of carriers in solids is the most essential thing while exploring any functionality. For a ternary skutterudite like CoSn1·5Te1.5−x, which has been recently recognized as a potential material for thermoelectric conversion, the dominant carrier could be either electrons or holes via chemically tuning the quaternary Sn2Te2 rings in the structure. Both theoretical calculation and different spectroscopic probes, such as X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) were employed to unveil the conduction type switching details. On the other hand, a Ni-for-Co substitution was applied to enhance electronic transport, and thereby the thermoelectric power factor. Thanks to the substantial cut-off of lattice thermal conductivity by the characteristic Sn2Te2 rings in the skutterudite structure, ultimately a 70-fold increase in the dimensionless figure-of-merit (zT) is achieved at 723 K with the nominal composition Co0·95Ni0·05Sn1·5Te1.5.

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Tin monosulfide (SnS) is a promising light-harvesting material for solar cell applications, owing to its potential for large-scale production, cost-effectiveness, eco-friendly source materials, and long-term stability. However, SnS crystallizes in an orthorhombic structure, which results in a highly anisotropic charge transport behavior. Tailoring the crystallographic orientation of the SnS absorber layer plays a critical role in the enhancement of the transfer of charge carriers and the power conversion efficiency (PCE). By controlling the substrate tilting angle and temperature ramp rate in vapor transport deposition, the crystal growth orientation was tuned to a preferred direction which significantly suppressed the unfavorable (040) crystallographic plane. Through the combination of these two approaches, the PCE could be increased from 0.11% to 2%. The effect of the tilting angle was numerically simulated to investigate its role in controlling the film uniformity and directing the film growth. In addition, the correlation between the texture coefficient of the (040) plane and the charge transport properties was determined by a combination of analytical methods such as device performance studies, electrochemical impedance spectroscopy, along with transient photovoltage, space-charge-limited current, and dark current measurements. These techniques were blended together to prove that the marked improvement in PCE can be ascribed to a reduced charge recombination (in both SnS bulk and interfaces) and an enhanced hole mobility.

Direct Z-scheme heterojunctions are widely used for photocatalytic water splitting and CO2 reduction due to facilitating well-separated photogenerated charge carriers and spatial isolation of redox reactions. Here, using a facile two-step hydrothermal and ion-exchange method, we uniformly decorate silver orthophosphate (i.e., Ag3PO4) quantum dots with an average characteristic size of ∼10 nm over tin(iv) sulphide (i.e., SnS2) nanosheets to form a 0D/2D heterojunction. The direct Z-scheme mechanism, i.e. charge transport for efficient electron (from SnS2) and hole (from Ag3PO4) recombination, is confirmed by the following experiments: (i) ultraviolet and X-ray photoelectron spectroscopies; (ii) photodeposition of Pt and PbO2 nanoparticles on reduction and oxidation sites, respectively; (iii) in situ X-ray photoelectron spectroscopy; and (iv) electron paramagnetic resonance spectroscopy. Owing to the photoreduction properties of Ag3PO4 with orthophosphate vacancies, Z-scheme charge carrier transfer, and efficient exciton dissociation, an optimized heterojunction shows a high CO2-to-CO reduction yield of 18.3 μmol g−1 h−1 with an illustrious selectivity of ∼95% under light illumination, which is about 3.0 and 47.8 times larger than that of Ag3PO4 and SnS2, respectively. The carbon source for the CO product is verified using a 13CO2 isotopic experiment. Moreover, by tracing the peak at ∼1190 cm−1 in the dark and under light irradiation, in situ diffuse reflectance infrared Fourier transform spectroscopy demonstrates that the CO2 reduction pathway goes through the COOH* intermediate.