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Novel Insights into Enhanced Stability of Li Rich Layered and High Voltage Olivine Phosphate Cathodes for Advanced Batteries through Surface Modification and Electron Structure Design
No full textThe design of cathode electrolyte interfaces in high energy density Li ion batteries is critical to protect the surface against undesirable oxygen release from the cathodes when batteries are charged to high voltage. However, the involvement of the engineered interface in the cationic and anionic redox reactions associated with de lithiation is often ignored, mostly due to the difficulty to separate these processes from chemical catalytic reactions at the cathode electrolyte interface. Here, a new electron energy band diagrams concept is developed that includes the examination of the electrochemical and ionization potentials evolution upon batteries cycling. The approach enables to forecast the intrinsic stability of the cathodes and discriminate the reaction pathways associated with interfacial electronic charge transfer mechanisms. Specifically, light is shed on the evolution of cationic and anionic redox in high energy density lithium rich 0.33Li2MnO3 0.67LiNi0.4Co0.2Mn0.4O2 HE NCM cathodes, particularly those that undergo surface modification through SO2 and NH3 double gas treatment to suppress the structural degradation. The chemical composition and energy distribution of the occupied and unoccupied electronic states at the different charging discharging states are quantitatively estimated by using advanced spectroscopy techniques, including operando Raman spectroscopy. The concept is successfully demonstrated in designing artificial interfaces for high voltage olivine structure cathodes enabling stable battery operation up to 5.1 V versus Li L
Studies of Atomic Layer Deposited Zn1 XSnXOY as an Alternative Buffer Layer for Cu2Zn Sn1 X ,GeX S4 Thin Film Solar Cells
No full textThin film solar cells based on Cu2Zn Sn1 amp; 8722;X,GeX S4 CZTGS absorbers are emerging technologies for solar energy conversion with low cost, nontoxic components. In this work, we investigated CZTGS based solar cells with varying Ge contents and an alternative buffer layer of Zn1 XSnXOY ZTO by atomic layer deposition. The results are compared to those of devices with the traditional CdS buffer layer. Overall, higher efficiency and open circuit voltage VOC are observed for the devices with ZTO compared to those with the CdS buffer layer. Moreover, the results show that the CZTGS ZTO device performance may further be improved by varying the ZTO properties band gap or thickness or by applying a surface treatment on the CZTGS absorber. An air annealing treatment of the chemically etched absorber surface improved the CZTGS device performance; however, nonetched devices show poor efficiency due to the presence of secondary phases GeO2 at the absorber buffer interface, shown by hard X ray photoelectron spectroscopy measurements, as previously reported for the full germanium Cu2ZnGeS4 based device
Significantly enhanced reversibility and mechanical stability in grain oriented MnNiGe based smart materials
No full textMaterials that undergo a magnetostructural transition MST usually exhibit fascinating magnetoresponsive properties, making them an important class of smart materials. However, practical application of this class of smart materials has been hindered by structural degradation as well as large irreversibility of the MST during consecutive thermal and field cycles. Here we report a significant improvement of the reversibility and mechanical stability in grain oriented MnNiGe based alloys that were fabricated using a directional solidification method. The preferred grain orientation enables synergistic deformations between neighboring grains during the MST, leading to a substantial reduction in the transition induced stress concentration. As a result, in situ and ex situ microscopic observations demonstrate a good mechanical stability of the textured alloys across the MST, in strong contrast to conventional MM X M, M Mn, Fe, Co, Ni; X Si, Ge materials with randomly oriented grains. The detailed transition stages of the MST have also been observed at the microscopic scale, and are reported for the first time in the MM X family. Besides, a low thermal hysteresis amp; 916;Thys of amp; 8764; 4 K was obtained in the textured alloys, which is the lowest amp; 916;Thys in the MM X material family. Textured MnNiGe based alloys show a large reversible isothermal entropy change amp; 8805; 35.9 Jkg amp; 8722;1K amp; 8722;1 in a 5 T field change, which is the highest among typical magnetocaloric materials. Consequently, this work provides a promising strategy for enhancing the cyclic stability of materials with a MST, which may boost their practical applications in solid state refrigerators, energy harvesters and high precision actuator
Three dimensional magnetic nanotextures with high order vorticity in soft magnetic wireframes
Get PDFAdditive nanotechnology enable curvilinear and three dimensional 3D magnetic architectures with tunable topology and functionalities surpassing their planar counterparts. Here, we experimentally reveal that 3D soft magnetic wireframe structures resemble compact manifolds and accommodate magnetic textures of high order vorticity determined by the Euler characteristic, amp; 967;. We demonstrate that self standing magnetic tetrapods homeomorphic to a sphere; amp; 967; amp; 8201; amp; 8201; amp; 8201;2 support six surface topological solitons, namely four vortices and two antivortices, with a total vorticity of amp; 8201; amp; 8201;2 equal to its Euler characteristic. Alternatively, wireframe structures with one loop homeomorphic to a torus; amp; 967; amp; 8201; amp; 8201;0 possess equal number of vortices and antivortices, which is relevant for spin wave splitters and 3D magnonics. Subsequent introduction of n holes into the wireframe geometry homeomorphic to an n torus; amp; 967; amp; 8201; lt; amp; 8201;0 enables the accommodation of a virtually unlimited number of antivortices, which suggests their usefulness for non conventional e.g., reservoir computation. Furthermore, complex stray field topologies around these objects are of interest for superconducting electronics, particle trapping and biomedical application
Operando X ray absorption spectroscopy of Fe N C catalysts based on carbon black and biomass derived support materials for the ORR
Get PDFIron nitrogen carbon Fe N C catalysts are among the most promising non platinum group metal catalysts for the oxygen reduction reaction ORR . Their activity and stability are considerably influenced by the structure of the C support. New biochar materials offer native heteroatom doping, making them a promising precursor for Fe N C catalysts. In this study, we apply operando X ray absorption spectroscopy at the Fe K edge to characterize the atomic Fe based active sites of a commercial Fe N C catalyst, a carbon black based catalyst as well as a novel biomass based Fe N C catalyst. We compare the density and the potential dependent nature of the FeNx type active sites during operation. Our results demonstrate that the novel biomass based catalyst exhibits a higher active site density compared to commercial and carbon black based Fe N C catalysts. Moreover, dynamic detection of the Fe K edge intensity during potential cycling reveals that their reversible iron redox potential is lower compared to that of conventional catalysts. Evaluation of the Fe K edge shift as well as of the extended X ray absorption fine structure EXAFS suggests hetero atom doping and iron under coordination as potential causes for the observed differences. These insights open the pathway to develop new optimization strategies for Fe N C catalysts based on biomass support material
A Time Domain Perspective on the Structural and Electronic Response in Epitaxial Ferroelectric Thin Films on Silicon
Get PDFThis operando study of epitaxial ferroelectric Pb Zr0.48Ti0.52 O3 capacitors on silicon substrates studies their structural response via synchrotron based time resolved X ray diffraction during hysteresis loop measurements in the 2 200 kHz range. At high frequencies, the polarization hysteresis loop is rounded and the classical butterfly like strain hysteresis acquires a flat dumbbell shape. We explain these observations from a time domain perspective The polarization and structural motion within the unit cell are coupled to the strain by the piezoelectric effect and limited by domain wall velocity. The solution of this coupled oscillator system is derived experimentally from the simultaneously measured electronic and structural data. The driving stress amp; 963;FE t is calculated as the product of the measured voltage U t and polarization P t . Unlike the electrical variables, amp; 963;FE t and amp; 951; t of the ferroelectric oscillate at twice the frequency of the applied electrical field. We model the measured frequency dependent phase shift between amp; 951; t and amp; 963;FE
Experimental determination of the sulfur K shell fundamental parameters employing the holistic approach
Get PDFSulfur and its compounds are both very abundant in the environment and very important for technological applications such as batteries. X ray spectroscopic characterizations of sulfur compounds for a quantitative determination of the present amount of sulfur often requires a good knowledge on the atomic fundamental parameters involved. These quantitatively describe the processes of photoionization and x ray fluorescence emission, making them crucial for x ray spectroscopy based quantifications. By employing the recently demonstrated holistic approach, the atomic fundamental parameters of the sulfur K shell were experimentally determined using the radiometrically calibrated instrumentation of the Physikalisch Technische Bundesanstalt. The transition probabilities of the main K shell fluorescence lines, the K shell fluorescence yield, the K shell Auger yield, the subshell photoionization cross section and fluorescence production cross section were determined by means of photon energy dependent x ray fluorescence and transmission measurements on a thin InS coated silicon nitride membrane. The results are also downloadable from Zenodo as plain tex
High Spin State of a Ferrocene Electron Donor Revealed by Optical and X ray Transient Absorption Spectroscopy
Get PDFFerrocene is one of the most common electron donors, and mapping its ligand field excited states is critical to designing donor acceptor D A molecules with long lived charge transfer states. Although 3 d d states are commonly invoked in the photophysics of ferrocene complexes, mention of the high spin 5 d d state is scarce. Here, we provide clear evidence of 5 d d formation in a bimetallic D A molecule, ferrocenyl cobaltocenium hexafluorophosphate [FcCc]PF6 . Femtosecond optical transient absorption OTA spectroscopy reveals two distinct electronic excited states with 30 and 500 ps lifetimes. Using a combination of ultraviolet, visible, near infrared, and short wave infrared probe pulses, we capture the spectral features of these states over an ultrabroadband range spanning 320 to 2200 nm. Time dependent density functional theory DFT calculations of the lowest triplet and quintet states, both primarily Fe II d d in character, qualitatively agree with the experimental OTA spectra, allowing us to assign the 30 ps state as the 3 d d state and the 500 ps state as the high spin 5 d d state. To confirm the ferrocene centered high spin character of the 500 ps state, we performed X ray transient absorption XTA spectroscopy at the Fe and Co K edges. The Fe K edge XTA spectrum at 150 ps shows a red shift of the absorption edge that is consistent with an Fe II high spin state, as supported by ab initio calculations. The transient signal detected at the Co K edge is 50 weaker, confirming the ferrocene centered character of the excited state. Fitting of the transient extended X ray absorption fine structure region yields an Fe C bond length increase of 0.25 0.1 in the excited state, as expected for the high spin state based on DFT. Altogether, these results demonstrate that the high spin state of ferrocene should be considered when designing donor acceptor assemblies for photocatalysis and photovoltaic
Experimental testing of a Modified Active Disturbance Rejection Control for Microphonics Reduction in a 9 Cell TESLA Superconducting Cavity
Get PDFThis work shows the results obtained with a novel modified active disturbance rejection control MADRC algorithm when controlling the detuning of a 9 cell TESLA superconducting radio frequency cavity. This type of control is essential in SRF cavities with high loaded quality factors since they are extremely sensitive to detuning due to their extremely narrow bandwidth. The modification of the active disturbance rejection control is based on loop shaping techniques and confers ease of design and implementation, as well as an improved behavior in presence of delay. To begin with, a theoretical design of the controller based on an experimental transfer function of the cavity is carried out and its performance is compared with that of a proportional integral control. In this preliminary test, the simulation results indicate a remarkable performance superiority of the MADRC. Later, the controller is tested on a TESLA cavity validating both the MADRC algorithm and its design methodology, and achieving a peak detuning reduction of 63 , as well as an rms reduction of 80 with respect to the open loo
The Energy Level Alignment at the Buffer Cu In,Ga Se2 Thin Film Solar Cell Interface for CdS and GaOx
Get PDFSputter deposited GaOx i.e., oxygen deficient gallium oxide films are evaluated as a potential replacement for the standard CdS buffer layers in Cu In,Ga Se2 CIGSe based thin film photovoltaics. The energy level alignment at the GaOx CIGSe and CdS CIGSe interfaces are compared by means of direct and inverse photoemission. For the GaOx CIGSe a 0.04 0.07 eV i.e., a small spike like conduction band offset CBO and a amp; 8722;3.21 0.19 eV i.e., a large cliff like valence band offset VBO are found, which suggests a nearly ideal charge selective contact. The derived GaOx band gap of 4.80 0.25 eV confirms its utility as a highly transparent buffer layer. However, the GaOx with x derived to be 1.1 0.1 exhibits considerable presumably defect related occupied states above the valence band maximum. It is proposed that these states may increase charge carrier recombination and decrease open circuit voltage in respective devices; also explaining why solar cells with standard CdS buffer outperform devices with GaOx buffer, despite less ideal electronic interface properties CBO amp; 8722;0.18 0.07 eV, VBO amp; 8722;0.98 0.15 eV and the smaller CdS band gap of 2.35 0.22 e