Kuei-Hsien Chen

Abstract An electrochemical capacitor configuration extends its operational potential window by leveraging diverse charge storage mechanisms on the positive and negative electrodes. Beyond harnessing capacitive, pseudocapacitive, or Faradaic energy storage mechanisms and enhancing electrochemical performance at high rates, achieving a balance of stored charge across electrodes poses a significant challenge over a wide range of charge?discharge currents or sweep rates. Consequently, fabricating hybrid and asymmetric supercapacitors demands precise electrochemical evaluations of electrode materials and the development of a reliable methodology. This work provides an overview of fundamental aspects related to charge-storage mechanisms and electrochemical methods, aiming to discern the contribution of each process. Subsequently, the electrochemical properties, including the working potential windows, rate capability profiles, and stabilities, of various families of electrode materials are explored. It is then demonstrated, how charge balancing between electrodes falters across a broad range of charge?discharge currents or sweep rates. Finally, a methodology for achieving charge balance in hybrid and asymmetric supercapacitors is proposed, outlining multiple conditions dependent on loaded mass and charge?discharge current. Two step-by-step tutorials and model examples for applying this methodology are also provided. The proposed methodology is anticipated to stimulate continued dialogue among researchers, fostering advancements in achieving stable and high-performance supercapacitor devices.

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.

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.

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.

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.

Reduction of CO2 to value-added hydrocarbons through artificial photosynthesis is one of the way to address the energy crisis and climate change issues. It is known that lowering the activation energy of CO2 molecules on the photocatalyst surface and key intermediates is crucial in photocatalytic CO2 reduction. Herein, we present phosphorus-implanted 20-nm SnS2 continuous thin film with sulfur vacancies (i.e., SV-SnS2:P where P substitutes on S sites). The fabrication process involves thermal evaporation, post-sulfurization, and ion implantation. Our gas-phase photocatalytic experiments show an enhanced and selective CO2 photoreduction to CH4 with a yield of 0.13 µmol cm−2 and selectivity of 92 % under solar-light irradiation for 4 h over an optimal ∼4.5 % P and ∼16 % SV. Experimental observations, conducted through X-ray absorption near edge, in situ near ambient pressure X-ray photoelectron, and in situ Fourier transform infrared spectroscopies, along with first-principle density functional theory calculations. Results reveal that P dopant is significantly affected by nearby SV via local charge density transfer from P to the nearest Sn and next-nearest S neighbor atoms, consequently, leads to the stabilization of *COOH and *CHO intermediates, where asterisks stand for P as the active site. Our results demonstrate how active site modulation, without using precious co-catalysts, plays a crucial role in intermediate stabilization in a wireless photocatalysis process.