Earth-abundant transition metal (Fe, Co, or Ni) and nitrogen-doped porous carbon electrocatalysts (M-N-C, where M denotes the metal) were synthesized from cheap precursors via silica-templated pyrolysis. The effect of the material composition and structure (i.e., porosity, nitrogen doping, metal identity, and oxygen functionalization) on the activity for the electrochemical CO2 reduction reaction (CO2RR) was investigated. The metal-free N-C exhibits a high selectivity but low activity for CO2RR. Incorporation of the Fe and Ni, but not Co, sites in the N-C material is able to significantly enhance the activity. The general selectivity order for CO2-to-CO conversion in water is found to be Ni > Fe ≫ Co with respect to the metal in M-N-C, while the activity follows Ni, Fe ≫ Co. Notably, the Ni-doped carbon exhibits a high selectivity with a faradaic efficiency of 93% for CO production. Tafel analysis shows a change of the rate-determining step as the metal overtakes the role of the nitrogen as the most active site. Recording the X-ray photoelectron spectra and extended X-ray absorption fine structure demonstrates that the metals are atomically dispersed in the carbon matrix, most likely coordinated to four nitrogen atoms and with carbon atoms serving as a second coordination shell. Presumably, the carbon atoms in the second coordination shell of the metal sites in M-N-C significantly affect the CO2RR activity because the opposite reactivity order is found for carbon supported metal meso-tetraphenylporphyrin complexes. From a better understanding of the relationship between the CO2RR activity and the material structure, it becomes possible to rationally design high-performance porous carbon electrocatalysts involving earth-abundant metals for CO2 valorization.
Bronze phase, TiO2(B), and anatase nanoparticles in various weight fractions and with different sizes have been synthesized by a very facile method and their electrochemical performances have been evaluated in Li-and Na-ion cells. The transition from a layered hydrogen-titanate precursor to TiO2(B)/anatase mixtures was monitored by in situ powder X-ray diffraction from room temperature to 800°C. Simple NaOH treatment of the precursor inhibited the transformation of precursor and TiO2(B) to anatase at elevated temperatures and allowed for preparation of larger TiO2(B) crystallites with extra high thermal stability.
The crystal structure, electronic structure, and transport properties of crystals with the nominal composition Nb 0.25 Bi 2 Se 3 are investigated. X-ray diffraction reveals that the as-grown crystals display phase segregation and contain major contributions of BiSe and the superconducting misfit layer compound (BiSe) 1.1 NbSe 2 . The inhomogeneous character of the samples is also reflected in the electronic structure and transport properties of different single crystals. Angle-resolved photoemission spectroscopy (ARPES) reveals an electronic structure that resembles poor-quality Bi 2 Se 3 with an ill-defined topological surface state. High-quality topological surface states are instead observed when using a highly focused beam size, i.e., nanoARPES. While the superconducting transition temperature is found to vary between 2.5 and 3.5 K, the majority of the bulk single crystals does not exhibit a zero-resistance state suggesting filamentary superconductivity in the materials. Susceptibility measurements of the system together with the temperature dependence of the coherence length extracted from the upper critical field are consistent with conventional BCS superconductivity of a type II superconductor.
Cu 2-δ Se is a cheap, nontoxic high performance thermoelectric material with extraordinary properties such as liquid-like phonons or a large enhancement of the thermopower at the phase transition between the low temperature β-phase and the super-ion conducting high temperature α-phase. Here, the nuclear-weighted X-ray maximum entropy method (NXMEM) is used to study disorder and ion migration in both the βand the α-phase based on the analysis of single crystal X-ray diffraction data. The NXMEM density calculated at different temperatures very convincingly shows an unbiased view of ion migration from copper-rich to copper-deficient layers eventually leading to the equal distribution in the cubic high temperature phase. This directly confirms that copper mobility and disordering is the driving force behind the peculiar phase transition. In the super-ionic phase, no density is observed in the octahedral holes even though it is the point of lowest density in the Se anion procrystal. Thus, Cu ions neither occupy this site at equilibrium nor use this site for migration. The NXMEM density suggests an ion migration pathway between 32f sites skirting around the octahedral cavity. In Cu 2 Se, the strong cation-cation repulsion leads to cooperative effects with highly complex equilibrium disorder and ion migration paths.Rationally designed improvements in the performance of functional materials are closely tied to a detailed understanding of the atomic arrangement. In an era of increasing demands for sustainable energy solutions, thermoelectric materials capable of interconverting heat and electricity have received great attention. Cu 2-δ Se is a cheap nontoxic material with a very high thermoelectric figure of merit. [1][2][3][4] Furthermore, it exhibits extraordinary physical properties. These include liquid-like phonons [2] or a highly peculiar thermopower enhancement prior to a super-ionic phase transition around 400 K. [1] The unusual transport behavior presumably is related to structural changes in the disordered low temperature β-Cu 2-δ Se crystal structure when the critical temperature is approached. Here, we use the model-independent maximum entropy method (MEM), [5] and
Materials with the structural motif of a square net have in recent years attracted attention for their often unexpected electronic properties. Here, we present low-temperature magnetotransport experiments on high-quality NdTe 3 single crystals with a carrier mobility of 6 × 10 4 cm 2 /V s at 2.25 K; an increase of 58% is observed below the Néel temperature. Shubnikov-de Haas oscillations up to 9 T exhibit anomalies in their amplitude which deviates from the conventional Lifshitz-Kosevich behavior below the magnetic ordering temperature and around 20 K. The latter, which has so far not been observed in any tritelluride, suggests a field-and temperature-induced electronic structure transition. We provide a thorough analysis of three different samples, highlighting the importance of sample quality for elucidating details in the transport behavior. Our results on NdTe 3 suggest that rare-earth tritellurides may display so-far obscured exotic electronic properties in addition to the well-studied high-temperature charge density wave anomalies.
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