Wang Hay Kan

Bio-inspired phosphole-lipids: from highly fluorescent organogels to mechanically responsive FRET.

Sensitive gels: the amphiphilic features of phosphole-lipids lead to intriguing self-assembly properties and the formation of highly fluorescent organogels. Moreover, the dynamic structural features of the system make it possible to amplify the mechanochromic emission shifts (100 nm) in a donor-acceptor system through thermally and mechanically responsive fluorescence resonance energy transfer (FRET).

Trends in electrode development for next generation solid oxide fuel cells

High temperature electrochemical devices, such as solid oxide fuel cells (SOFCs), will play a vital role in the future green and sustainable energy industries due to direct utilization of carbon-based fuels and their ability to couple with renewable energies to convert by-products into valuable fuels using solid oxide electrolysis cells (SOECs). All-solid-state design provides a great opportunity toward the optimization of durability, cost, efficiency and robustness. Electrodes, one of the most important components that facilitate the electrochemical redox reactions, have been actively investigated for several decades to optimize a matrix of chemical composition, microstructure, and performance. Although some mixed ionic electronic conductors (MIECs) can provide electrochemically active surface with excellent chemical tolerance comparing to the composite electrodes made of conventional ceramic electrolyte and metal (cermet), their electrochemical activities may not be high enough to obtain a desirable power, even at moderate temperature operation. This shortage could be improved by engineering the microstructure of the electrodes, which control electrochemically active sites in SOFCs and SOECs. In this article, the current trends in electrode-engineering techniques for advanced SOFCs are reviewed.

Synthesis and properties of cholesteric click-phospholes.

Inspired by naturally occurring helical supramolecular architectures, a series of chiral conjugated phospholes with a cholesteryl pendant have been synthesized and characterized. The photophysically and electrochemically active conjugated phosphole species can act as dopants for the formation of chiral liquid crystals. The supramolecular structures were found to be tunable by careful choice of the conjugated headgroup as well as the rigidity of the linker of the new cholesteric phospholes.

Surface and bulk study of strontium-rich chromium ferrite oxide as a robust solid oxide fuel cell cathode

A novel Co-free cathode, La0.3Sr0.7Fe0.7Cr0.3O3−δ (LSFCr-3), exhibiting the desired combination of high electrical conductivity, physical and chemical stability, and electrocatalytic activity, was systematically investigated for SOFC applications. Its excellent performance is attributed primarily to the presence of Cr, which was found to be predominant in the 3+ and 4+ oxidation states in the LSFCr-3 bulk, thus likely maintaining a 6-fold coordination with oxygen anions. This, in turn, causes disorder in the oxygen vacancy sub-lattice, stabilized by the Fe ion–oxygen tetrahedra. However, on the surface of the LSFCr-3 oxide, Cr is primarily in the 6+ state, together with some Cr3+/Cr4+, even at 700 °C. Cr6+ can only be tetrahedrally coordinated by oxygen anions, resulting in a large concentration of oxygen vacancies on the LSFCr-3 surface, with a surface exchange coefficient and oxygen ionic conductivity of ca. 10−5 cm s−1 and ca. 10−2 S cm−1, respectively, at 700–800 °C. The use of LSFCr-3 as the cathode in a Ni–Ce0.8Sm0.2O2−δ (SDC) anode-supported single solid oxide fuel cell in 3% H2O–H2/air gave a maximum power density of 0.81 W cm2 at 750 °C, which is superior to that of similar cells in which La0.6Sr0.4Fe0.8Co0.2O3−δ, a previously well studied material, was used as the cathode.

Studies on polymorphic sequence during the formation of the 1:1 ordered perovskite-type BaCa(0.335)M(0.165)Nb(0.5)O(3-δ) (M = Mn, Fe, Co) using in situ and ex situ powder X-ray diffraction.

Here, we report a synthetic strategy to control the B-site ordering of the transition metal-doped perovskite-type oxides with the nominal formula of BaCa(0.335)M(0.165)Nb(0.5)O(3-δ) (M = Mn, Fe, Co). Variable temperature (in situ) and ex situ powder X-ray diffraction (PXRD), selected area electron diffraction (SAED), energy dispersive X-ray spectroscopy (EDX), Fourier transform infrared spectroscopy (FTIR), scanning/transmission electron microscopy (SEM/TEM), and thermogravimetic analysis (TGA) were used to understand the B-site ordering as a function of temperature. The present study shows that BaCa(0.335)M(0.165)Nb(0.5)O(3-δ) crystallizes in the B-site disordered primitive perovskite (space group s.g. Pm3̅m) at 900 °C in air, which can be converted into the B-site 1:2 ordered perovskite (s.g. P3̅m1) at 1200 °C and the B-site 1:1 ordered perovskite phase (s.g. Fm3̅m) at 1300 °C. However, the reverse reaction is not feasible when the temperature is reduced. FTIR revealed that no carbonate species were present in all three polymorphs. The chemical stability of the investigated perovskites in CO2 and H2 highly depends on the B-site cation ordering. For example, TGA confirmed that the B-site disordered primitive perovskite phase is more readily reduced in dry and wet 10% H2/90% N2 and is less stable in pure CO2 at elevated temperature, compared to the B-site 1:1 ordered perovskite-type phase of the same nominal composition.

Effect of substitution of B-sites by Mn, Fe and Co in double perovskite-type Ba3CaNb2O9 on structure and electrical properties

A novel family of metal oxides of chemical formula Ba2Ca(1−x−y)M(x+z)Nb(1−z+y)O6−δ (M = Mn, Fe and Co) was developed as mixed ion and electronic conductors (MIECs) for application in solid oxide fuel cells (SOFCs). Investigated samples were synthesized by a ceramic method at 1400 °C in air and characterized using powder X-ray diffraction (PXRD), selected area electron diffraction (SAED), thermo-gravimetric analysis (TGA), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), dilatometry, chemical titration and electrochemical ac impedance spectroscopy (EIS). The Rietveld analysis of PXRD and SAED support the formation of ordered double perovskite-type structure with space group Fmm and chemical stability in 5000 ppm H2S/H2 at 600 °C. Iodometric titration shows the valences of Mn and Co were 2+ and 3+, while Fe shows 3+ and 4+ states. SEM reveals that the porosity of the samples is highly affected by dopants. The thermal expansion coefficients (TEC) of the as-prepared samples are very comparable to the oxide conducting Y-doped ZrO2. Ba2Ca0.79Co0.5Nb0.71O6−δ exhibits the total conductivity of 3.7 × 10−2 S cm−1 at 816 °C in air. The trend in the conductivity of Ba2Ca(1−x−y)M(x+z)Nb(1−z+y)O6−δ could be correlated to the reduction of the dopants and the subsequent loss of oxide ions at elevated temperatures.

Liquid crystalline 21,23-dithiaporphyrins

Meso-substituted 21,23-dithiaporhyrins were synthesized that self-assemble into columnar liquid crystalline phases. Furthermore, the porphyrins possess absorption profiles that span from 400 nm to 800 nm, undergo two reversible electrochemical redox reactions at approximately 465 and 670 mV vs. ferrocene/ferricenium and are electrochromic. Thin films of these dithiaporhyrins of approximately 60 nm thickness were formed by spin-casting methods from chloroform and were subjected to impedance spectroscopy. The pristine films showed very low conductivity at open circuit potentials (undoped); however, electrochemically p-doping resulted in stable films with four orders of magnitude higher conductivity.

Thermochemistry of Sr2Ce1–xPrxO4 (x = 0, 0.2, 0.5, 0.8, and 1): Variable-Temperature and -Atmosphere in-situ and ex-situ Powder X-ray Diffraction Studies and Their Physical Properties

A novel family of metal oxides with a chemical formula of Sr(2)Ce(1-x)Pr(x)O(4) (x = 0, 0.2, 0.5, 0.8, and 1) was developed as mixed oxide ion and electronic conductors for solid oxide fuel cells (SOFCs). All of the investigated samples were synthesized by the ceramic method at 1000 °C in air and characterized by powder X-ray diffraction (PXRD), selected area electron diffraction (SAED), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and electrochemical impedance spectroscopy (EIS). Ex-situ PXRD reveals that the Sr(2)PbO(4)-type Sr(2)CeO(4) decomposes readily into a mixture of perovskite-type SrCeO(3) and rock-salt-type SrO at 1400 °C in air. Surprisingly, the decomposed products are converted back to the original Sr(2)PbO(4)-type Sr(2)CeO(4) phase at 800 °C in air, as confirmed by in-situ PXRD. Thermal decomposition is highly suppressed in Sr(2)Ce(1-x)Pr(x)O(4) compounds for Pr > 0, suggesting that Pr improves the thermal stability of the compounds. Rietveld analysis of PXRD and SAED supported that both Pr and Ce ions are located on the 2a site in Pbam (space group no. 55). The electrical transport mechanism could be correlated to the reduction of Pr and/or Ce ions and subsequent loss of oxide ions at elevated temperatures, as shown by TGA and in-situ PXRD. Conductivity increases with Pr content in Sr(2)Ce(1-x)Pr(x)O(4). The highest total conductivity of 1.24 × 10(-1) S cm(-1) was observed for Sr(2)Ce(0.2)Pr(0.8)O(4) at 663 °C in air.

Determination of Fe oxidation states in the B-site ordered perovskite-type Ba2Ca0.67Fe0.33NbO6−δ at the surface (nano-scale) and bulk by variable temperature XPS and TGA and their impact on electrochemical catalysis

In this study, we analyzed the surface and bulk valences of Fe in the double perovskite-type Ba2Ca0.67Fe0.33NbO6−δ by both variable temperature X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA). The concentration of Fe2+ near-surface was found to be about 3 times higher than that in the bulk sample. The electrochemical performance of Ba2Ca0.67M0.33NbO6−δ (M = Mn, Fe and Co) was assessed by ac impedance spectroscopy using a Zr0.84Y0.16O1.92 (YSZ) supported half-cell. The area specific polarization resistances (ASR) of all samples were found to decrease with increasing temperature. We speculate that ASR for H2 oxidation are correlated with the higher concentrations of low valence Fe2+ species near-surface (nano-scale), as demonstrated for Ba2Ca0.67Fe0.33NbO6−δ.

Kinetics and thermodynamics of carbonation of a promising SOFC cathode material La0.5Ba0.5CoO3−δ (LBC)

This work examines the reactivity of a potentially promising SOFC cathode material, La0.5Ba0.5CoO3−δ (LBC), under CO2 atmosphere. Through a combination of different techniques – thermogravimetric analysis, ex situ XRD analyses of reacted products, and in situ XRD analysis – the reaction pathway and kinetics for LBC carbonation is investigated. The thermodynamic phase boundary for carbonate formation in the LBC–CO2 system has been generated and fundamental thermodynamic parameters – the standard state enthalpy (ΔH°) and the standard state entropy (ΔS°) – are determined.