Owing to their unique chemistry and physical properties, metal-organic frameworks (MOFs) are an interesting class of materials which can be utilized for a wide array of applications. MOFs have been proposed to be used as catalysts for fuel cells, but their low intrinsic electronic conductivity hampered their utilization as is. In this work, we present the synthesis and application of MOF-based precious-metal-group-free (PGM-free) catalysts for oxygen reduction based on a unique metal-organic framework-carbon composite material. Benzene tricarboxylic acid-based MOFs were synthesized inside activated carbon (AC) with four different, first row transition metals: Mn, Fe, Co, and Cu. The MOFs@AC were analyzed electrochemically to measure their catalytic activity. Further physical and chemical characterization studies are performed to measure the material properties. The MOFs@AC are found to be conductive and active catalysts for the oxygen reduction reaction in an alkaline environment. Surprisingly, the Mn-MOF-based@AC exhibits the best performance with an onset potential of 0.9 V vs. RHE and the almost four-electron mechanism, as opposed to most other known PGM-free catalysts, which show Fe and Co as the most active metals.
Most of the recently discovered layered materials such as MoS 2 or MoSe 2 are n-type, while few materials, such as phosphorene, which suffers from rapid oxidation, are p-type. To form devices such as p−n junctions and heterojunctions, new p-type mono-/few-layers are needed. Here, we report a one-step synthesis of layered, crystalline, ptype copper sulfide by thermal annealing of a standard copper foil in an inert environment using chemical vapor deposition (CVD). Optical spectroscopies (photoluminescence and absorption) show definite correlating features around 2.5 eV. Surface photovoltage spectroscopy shows a photovoltage reduction around the same energy range, which would be expected from a bandgap of a p-type material, and p-type conductivity was also observed using a thermoelectric probe. TEM, XRD, and AFM showed that the synthesized material is layered and has a unique stoichiometry of Cu 9 S 5 . Using sonication and dropcasting, we succeeded to isolate few-layers and monolayers. We observed good bulk electrical conductivity and characterized the electrical conductivity of few-layer copper sulfide flakes using peak force tunneling atomic force microscopy (PF-TUNA). We observed an increase in conductivity for increasing number of layers. Given its conductivity and layered morphology, we tested the synthesized Cu 9 S 5 as an electrode for a Li-ion battery. The proposed bottom-up synthesis, which is simple and scalable, allows synthesizing bulk quantities of the p-type layered Cu 9 S 5 which can then be exfoliated (top-down) to deposit monolayer flakes on substrates. Combined with the progress achieved in the preparation of n-type layered materials, this p-type Cu 9 S 5 opens the door to the fabrication of 2D p−n heterojunctions.
In this paper, we studied the influence of LiAlO 2 coatings of 0.5, 1 and 2 nm thickness prepared by Atomic Layer Deposition onto LiNi 0.8 Co 0.15 Al 0.05 O 2 electrodes, on their electrochemical behavior at 30 and 60 • C. It was demonstrated that upon cycling, 2 nm LiAlO 2 coated electrodes displayed ∼3 times lower capacity fading and lower voltage hysteresis comparing to bare electrodes. We established a correlation among the thickness of the LiAlO 2 coating and parameters of the self-discharge processes at 30 and 60 • C. Significant results on the elevated temperature cycling and aging of bare and LiAlO 2 coated electrodes at 4.3 V were obtained and analyzed for the first time. By analyzing of X-ray diffraction patterns of bare and 2 nm coated LiNi 0.8 Co 0.15 Al 0.05 O 2 electrodes after cycling, we concluded that cycled materials preserved their original structure described by R-3m space group and no additional phases were detected. The lithium-ion batteries (LIB) market nowadays has expanded widely from cellular phones, computers and other electronic devices to the automotive industry. Positive electrodes for LIBs to be explored in electric vehicles are mainly based on lithiated transition-metal oxides comprising Ni, Co and Mn (LiNi y Co x Mn 1-y-x O 2 ) and designated as NCM.1,2 They have attracted much attention as promising materials and therefore many research projects have been dedicated to them in the past 20 years. [3][4][5][6][7][8][9][10][11][12][13] In studies of LiNi 1/3 Co 1/3 Mn 1/3 O 2 (NCM333) cathodes, it was demonstrated 4-6 that they provided a capacity of 200 mAh/g in the voltage range of 2.5-4.6 V, or ∼155 mAh/g if the cutoff voltage is limited by 4.3 V.7 A substantially improved high-voltage performance of NCM333 electrodes (up to 4.6 V) was achieved by introducing tris(2,2,2-trifluoroethyl) borate additive in the electrolyte solution.8 These authors suggest that the additive takes part in the formation of the solid-electrolyte interface by lowering and stabilization of the interfacial resistance. NCM333 and NCM424 materials were studied in our group from the viewpoint of their electrochemical behavior, aging mechanisms, thermal properties, and surface chemistry developed upon cycling in ethylene carbonate-dimethyl carbonate/LiPF 6 based solutions.9 LiNi y Co x Mn 1-y-x O 2 materials with higher Ni content (y > 5) are important due to high capacity that can be extracted by charging up to 4.3 V only. However, Ni-rich NCMs suffer from their low cycling stability especially for compositions with Ni ≥ 60%, severe capacity fading and impedance increase during cycling at elevated temperatures (e.g. 45• C). One of the effective ways to stabilize the structure of NCM materials, to increase cycling activity and to diminish the heat evolution of the electrodes in a charged 14 There is a consensus in the literature that the above drawbacks originate from the unstable Ni 4+ ions developed in the charge state (high anodic potential, most Li + extracted from NCA). These Ni-ions can be readily reduce...
Micro-patterning of a metal organic framework (MOF) from a solution of precursors is achieved by local laser heating, alleviating the need for pre-preparation and stabilization.
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