Freestanding hierarchical porous carbon film derived from hybrid nanocellulose for high-power supercapacitors

Nanocellulose is a sustainable and eco-friendly nanomaterial derived from renewable biomass.In this study,we utilized the structural advantages of two types of nanocellulose and fabricated freestanding carbonized hybrid nanocellulose films as electrode materials for supercapacitors.The long cellulose nanofibrils (CNFs) formed a macroporous framework,and the short cellulose nanocrystals were assembled around the CNF framework and generated micro/mesopores.This two-level hierarchical porous structure was successfully preserved during carbonization because of a thin atomic layer deposited (ALD) Al2O3 conformal coating,which effectively prevented the aggregation of nanocellulose.These carbonized,partially graphitized nanocellulose fibers were interconnected,forming an integrated and highly conductive network with a large specific surface area of 1,244 m2·g-1.The two-level hierarchical porous structure facilitated fast ion transport in the film.When tested as an electrode material with a high mass loading of 4 mg·cm-2 for supercapacitors,the hierarchical porous carbon film derived from hybrid nanocellulose exhibited a specific capacitance of 170 F.g-1and extraordinary performance at high current densities.Even at a very high current of 50 A·g-1,it retained 65％ of its original specific capacitance,which makes it a promising electrode material for high-power applications.     

Growth, structure and properties of sputtered niobium oxide thin films

Niobium oxide (NbO_x) films were deposited by pulsed dc magnetron sputtering at different total gas pressures and oxygen flow rates. Various film properties were characterized by X-ray photoelectron spectroscopy, atomic force microscopy, variable angle spectroscopic ellipsometry and four point probe. It was found that oxygen flow rates required for preparing NbO, NbO₂ and Nb₂O₅ at a constant total pressure of 0.93 Pa were approximately 2, 4 and > 6 sccm, respectively. The results showed that the film properties, specifically composition can be significantly changed by the total gas pressure and the oxygen flow rate.     

Balancing the corrosion resistance and through-plane electrical conductivity of Cr coating via oxygen plasma treatment

Developing an electrically conductive and corrosion-resistant coating is essential for metal bipolar plates of polymer electrolyte membrane fuel cells(PEMFCs). Although enhanced corrosion resistance was seen for Cr coated stainless steel(Cr/SS) bipolar plates, they experience a quick decrease of through-plane electrical conductivity due to the formation of a porous and low-conductive corrosion product layer at the plate surface, thus leading to an increase in interfacial contact resistance(ICR). To tackle this issue, the multilayer Cr coatings were deposited using the magnetron sputtering with a remote inductively coupled oxygen plasma(O-ICP) in the present study. After the O-ICP treatment, a Cr oxide layer(Cr O*) is formed on the specimen surface. The Cr O*/Cr/SS has a remarkably lower stable corrosion rate(iss) than that of the native Cr oxides(Cr On/Cr/SS). Compared with Cr On/Cr/SS, the excellent performance of Cr O*/Cr/SS is attributed to a denser and thicker surface layer of Cr O* with Cr being oxidized to its highest valence state,Cr(VI). More importantly, the through-plane electrical conductivity of the specimens treated by the optimized O-ICP decreases much slowly than Cr On/Cr/SS and thus, the increament of ICR of Cr O*/Cr/SS after the potentiostatic polarization test is considerably smaller than that of Cr On/Cr/SS, which is benefited from the reduced issthat mitigates the deposition of corrosion products and hinders further oxidation of Cr coating. Therefore, Cr O*/Cr/SS proves to be a well balanced trade-off between corrosion resistance and through-plane electrical conductivity. The results of this study demonstrate that O-ICP treatment on a conductive metal coating is an effective strategy to improve the corrosion resistance and suppress the increase of ICR over the long-term polarization. The technique reported herein exhibits its promising potential application in preparing corrosion resistant and electrically conductive coatings on metal bipolar plates to be used in PEMFCs.     

Carbon nanosheets derived from reconstructed lignin for potassium and sodium storage with low voltage hysteresis

Lignin is the second most abundant and the only nature polymer rich in aromatic units. Although aromatic-unit-rich precursors often yield soft carbon after carbonization, the side chains in lignin crosslink with the aromatic units and form a rigid three-dimensional (3D) structure which eventually leads to hard carbons. Through a graphene oxide-catalyzed decomposition and repolymerization process, we successfully reconstructed lignin by partially tailoring the side chains. Compared to directly carbonized lignin, the carbonized reconstructed lignin possesses significantly fewer defects, 86% fewer oxygen-functionalities, 82% fewer micropores, and narrower interlayer space. These parameters can be tuned by the amount of catalysts (graphene oxide). When tested as anode for K-ion and Na-ion batteries, the carbonized reconstructed lignin delivers notably higher capacity at low-potential range (especially for Na-storage), shows much-improved performance at high current density, and most importantly, reduces voltage hysteresis between discharge and charge process by more than 50%, which is critical to the energy efficiency of the energy storage system. Our study reveals that the voltage hysteresis in K-storage is much severer than that in Na-storage for all samples. For practical K-ion battery applications, the voltage hysteresis deserves more attention in future electrode materials design and the reconstruct ion strategy introduced in this work provides potential low-cost solution.

     

Evaluation of efficiency factors and internal resistance of thermoelectric materials

It is well known that the figure of merit (ZT) is unreliable in calculating the efficiency (?) of micro thermoelectric generators system level and unrealistic when comparing the performance of thermoelectric (TE) materials in the same metric units. To solve this problem, we have used COMSOL multiphysics to design a single leg of micro thermoelectric generators model for computing efficiency factors (? ) and internal resistance using TE materials' constants, such as electrical conductivity (σ ), TE conductivity (K ), and Seebeck coefficient (α ). The TE materials were placed between two copper electrodes, and the first data analyzed were the voltages per meter and electric currents per meter. The internal resistances were calculated by taking the ration of voltages to electric currents, and at the same time, the electric powers were calculated from the products of electric currents and voltages yielding power per unit area in μW cm^?2. The ? were calculated using changes in power (ΔP ), temperature gradient (ΔT ), and the surface area (A ). The obtained results showed that the TE materials with highest ? when the temperatures are between 375 and 550 K are n‐type SiGe and p‐type SiGe. When the temperatures are between 550 and 780 K, the TE materials with the highest ? are PbTe‐Pbl₂, PbTe‐CdTe, and PbTe‐SrTe‐Na. We noted that the ? obtained from eight TE materials in this work are within the range as those reported in the literature between 0.001 and 0.091 μW cm^?2 K^?2. The TE materials with high internal resistances such as PbS, PbTe, and PbSe have ? that is <0.0001 μW cm^?2 K^?2, and those with low internal resistances have ? in the range between 0.002 and 0.0091 μW cm^?2 K^?2. This work has shown that COMSOL multiphysics is a powerful computational tool that can be used to analyze internal resistances and ? of TE materials in the same temperature ranges. Copyright © 2016 John Wiley & Sons, Ltd.     

The effect of argon pressure, residual oxygen and exposure to air on the electrical and microstructural properties of sputtered chromium thin films

Chromium thin films were deposited on SiO₂/Si wafers using two sputtering systems with different levels of cleanliness, and at argon sputtering pressures varying between 0.13 and 0.93 Pa. Films from the two systems grown under identical sputtering conditions had significantly different resistivity values that are shown to be due to differences in residual oxygen in the chambers. Electrical transport measurements were conducted on the series of grown films to investigate the influence of argon pressure on film electrical resistivity. The films morphology, microstructure and composition were characterized using scanning electron microscopy and X-ray photoelectron spectroscopy. Significant differences were found in Cr thin films sputtered at different sputtering pressures; differences in resistivity performance and microstructure were noted. This change was shown to be due to the transition from porous structure to a denser microstructure. The Cr films sputtered at high pressure contained large quantities of oxygen when exposed to air. Some of the oxygen is added to the film during the deposition depending on the deposition rate and the base pressure of the sputtering system. The rest is incorporated into the film once it is exposed to air. The amount of oxygen added at this stage depends on the structure of the film and would be minimal for the films deposited at low sputtering pressures.     

GaSb-Ge pseudobinary phase diagram

The GaSb-Ge pseudobinary phase diagram has been determined by differential thermal analysis and confirmed by X-ray diffraction. The system is a simple eutectic with an invarient point at 41 ± 2 mole % Ge and 650 ± 5°C. The maximum solid solubilities of Ge in GaSb and GaSb in Ge are 2 ± 0.5 mole % and 6.5 ± 0.5 mole %, respectively, at the eutectic temperature.     

Effects of viscosity-dependent diffusion in the analysis of rotating disk electrode data

This paper demonstrates the application of a modified Levich equation for chemical systems with varying viscosity. A commonly used technique to analyze rotating disc electrode (RDE) experiments is to fit the data to the Levich equation assuming a constant effective diffusion coefficient which may be valid for conditions where the viscosity does not vary significantly (less than an order of magnitude). However, most diffusion coefficient models (e.g. Stokes–Einstein) show an inverse relationship with viscosity which consequently indicates that a constant effective diffusion coefficient may result in poorer model-to-data agreement. Here, data are presented for a series of RDE experiments for the electrodissolution of Cu in phosphoric acid, water and glycerin based baths. Viscosity changes of greater than one order of magnitude allow for testing the assumption of a constant effective diffusion coefficient. The collected data, as well as data published elsewhere, can be explained by a modified Levich equation which takes into account the viscosity dependence of the diffusion coefficient.