Ni3Te2 has been reported as a highly efficient OER electrocatalyst with an overpotential of 180 mV at 10 mA cm−2 and also showing HER catalytic activity in alkaline medium.
The structural, elastic and electronic properties of two-dimensional (2D) titanium carbide/nitride based pristine (Tin+1Cn/Tin+1Nn) and functionalized MXenes (Tin+1CnT2/Tin+1NnT2, T stands for the terminal groups: -F, -O and -OH, n = 1, 2, 3) are investigated by density functional theory calculations. Carbide-based MXenes possess larger lattice constants and monolayer thicknesses than nitride-based MXenes. The in-plane Young's moduli of Tin+1Nn are larger than those of Tin+1Cn, whereas in both systems they decrease with the increase of the monolayer thickness.Cohesive energy calculations indicate that MXenes with a larger monolayer thickness have a better structural stability. Adsorption energy calculations imply that Tin+1Nn have stronger preference to adhere to the terminal groups, which suggests more active surfaces for nitridebased MXenes. More importantly, nearly free electron states are observed to exist outside the surfaces of -OH functionalized carbide/nitride based MXenes, especially in Tin+1Nn(OH)2, which provide almost perfect transmission channels without nuclear scattering for electron transport.The overall electrical conductivity of nitride-based MXenes is determined to be higher than that of carbide-based MXenes. The exceptional properties of titanium nitride-based MXenes, including strong surface adsorption, high elastic constant and Young's modulus, and good metallic conductivity, make them promising materials for catalysis and energy storage applications.
Elastocaloric cooling, which exploits the latent heat released and absorbed as stress-induced phase transformations are reversibly cycled in shape memory alloys, has recently emerged as a frontrunner in non-vapor-compression cooling technologies. The intrinsically high thermodynamic efficiency of elastocaloric materials is limited only by work hysteresis. Here, we report on creating high-performance low-hysteresis elastocaloric cooling materials via additive manufacturing of Titanium-Nickel (Ti-Ni) alloys. Contrary to established knowledge of the physical metallurgy of Ti-Ni alloys, intermetallic phases are found to be beneficial to elastocaloric performances when they are combined with the binary Ti-Ni compound in nanocomposite configurations. The resulting microstructure gives rise to quasi-linear stressstrain behaviors with extremely small hysteresis, leading to enhancement in the materials efficiency by a factor of five. Furthermore, despite being composed of more than 50% intermetallic phases, the reversible, repeatable elastocaloric performance of this material is shown to be stable over one million cycles. This result opens the door for direct implementation of additive manufacturing to elastocaloric cooling systems where versatile design strategy enables both topology optimization of heat exchangers as well as unique microstructural control of metallic refrigerants.One Sentence Summary: 3D printing produces highly efficient solid-state cooling nanocomposites with long fatigue life.
Designing high-efficiency electrocatalysts for water oxidation has become an increasingly important concept in the catalysis community due to its implications in clean energy generation and storage. In this respect transition-metal-doped mixed-metal selenides incorporating earth-abundant elements such as Ni and Fe have attracted attention due to their unexpectedly high electrocatalytic activity toward the oxygen evolution reaction (OER) with low overpotential in alkaline medium. In this article, quaternary mixed-metal selenide compositions incorporating Ni-Fe-Co were investigated through combinatorial electrodeposition by exploring the ternary phase diagram of Ni-Fe-Co systems. The OER electrocatalytic activity of the resultant quaternary and ternary mixed-metal selenide compositions was measured in order to systematically investigate the trend of catalytic activity as a function of catalyst composition. Accordingly, the composition(s) exhibiting the best catalytic efficiency for the quaternary Fe-Co-Ni mixed-metal selenide was identified. It was observed that the quaternary selenide outperformed the binary as well as the ternary metal selenides in this Ni-Fe-Co phase space. The elemental composition and relative abundance of the elements in the catalyst film was ascertained from energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). Mapping of the OER catalytic activity as a function of catalyst composition indicated that catalytic efficiency was more pronounced in the Fe-rich region with moderate amounts of Ni and trace amounts of Co doping, and the best performance was exhibited by (Ni0.25Fe0.68Co0.07)3Se4, which showed an overpotential of 230 mV (vs RHE) at 10 mA cm–2 with stability exceeding 8 h for continuous oxygen generation. It was also observed that typically the quaternary metal selenide composition was close to AB2Se4, which shows a spinel structure type. Electrochemical measurements along with density functional theory (DFT) calculations were performed to correlate the enhancement of catalytic activity toward the Fe-rich region with composition. First-principles DFT calculations were used to estimate the hydroxyl adsorption energy (E ads) on the surface of the mixed-metal selenides with varying compositions. This adsorption energy could be directly correlated to the onset of OER activity, and the results matched very well with the experimentally observed trend with respect to onset overpotential. The knowledge of the trend of catalytic activity as a function of composition will be very important for catalyst design through targeted material synthesis. This work represents an example of a systematic phase exploration for quaternary metal selenides and provides a strong foundation which can be expanded to study other mixed-metal selenide combinations.
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