The optical properties of carbon nanowall (CNW) films in the visible range have been studied and reported for the first time. Depending on the film structure, ultra-low total reflectance up to 0.13% can be reached, which makes the CNW films a promising candidate for the black body-like coating, and thus for a wide range of applications as a light absorber. We have estimated important trends in the optical property variation from sample to sample, and identified the presence of edge states and domain boundaries in carbon nanowalls as well as the film mass density variation as the key factors. Also we demonstrated that at much lower film thickness and density than for a carbon nanotube forest the CNWs yield one order higher specific light absorption.

SummaryThe often observed and still unexplained phenomenon of the growth of lithium peroxide crystal clusters during the discharge of Li–O2 cells is likely to happen because of self-assembling Li2O2 platelets that nucleate homogeneously right after the intermediate formation of superoxide ions by a single-electron oxygen reduction reaction (ORR). This feature limits the rechargeability of Li–O2 cells, but at the same time it can be beneficial for both capacity improvement and gain in recharge rate if a proper liquid phase mediator can be found.

One of the key problems hindering practical implementation of lithium−air batteries is caused by carbon cathode chemical instability leading to low energy efficiency and short cycle life. Titanium carbide (TiC) nanopowders are considered as an alternative cathode material; however, they are intrinsically reactive toward oxygen, and its stability is controlled totally by a surface overlayers. Using photoemission spectroscopy, we show that lithium−air battery discharge product, lithium peroxide (Li 2 O 2 ), easily oxidizes clean TiC surface. At the same time, TiC surface, which was treated by molecular oxygen under ambient conditions, shows much better stability in contact with Li 2 O 2 that can be explained by the presence of a surface layer containing a significant amount of elemental carbon in addition to oxides and oxycarbides. Nevertheless, such protective coatings produced by room temperature oxidation are not practically useful as one of its components, elemental carbon, is oxidized in the presence of lithium−air battery discharge intermediates. These results are of critical importance in understanding of TiC surface chemistry and in design of stable lithium−air battery electrodes. We postulate that dense, uniform, carbon-free titanium dioxide surface layers of 2−3 nm thickness on TiC will be a promising solution, and thus further efforts should be taken for developing synthetic protocols enabling preparation of TiO 2 /TiC core−shell structures.

A process of hydrothermal ageing of vanadia gels yields 10-20 nm thick and 90-100 nm wide nanobelts exceeding 10 microns in length. The diminished thickness and networking of anisotropic nanobelts lead to lithium intercalation capacities exceeding 450-500 mA h g À1 at a C/25 rate. The observed morphology features depend essentially on preparation conditions and allow to assume that this particular route results in a suitable morphology of nanobelts via chemical bond rearrangement in the course of olation and oxolation in the aged bulk gel. ''Unzipping'' of the layered structure of the precursor gel into single-crystalline nanobelts, and optimization of post-hydrothermal processing resulted in nanomaterials with enhanced electrochemical characteristics, making vanadia gels a precursor of choice for simple preparation of new battery nanomaterials.

The development of high specific energy Li−O 2 batteries faces a problem of poor cycling as a result of passivation of the positive electrode by both the discharge product (Li 2 O 2 ) and side products (Li 2 CO 3 , etc.). The latter are the result of oxidation of the electrode materials or electrolyte components primarily by discharge intermediate superoxide anions (O 2 − ) and, in less degree, by Li 2 O 2 . We report cyclic voltammetry studies of the electrode passivation in different relatively stable solvents. We found that slower passivation is observed for the electrolytes based on high donor number solvents or solvents with high viscosity. Moreover, such behavior is reproduced for three different electrode materials [glassy carbon (GC), TiC, and TiN] that pinpoints the primary role of different oxygen reduction reaction mechanisms (Li 2 O 2 surface deposition or solution growth) influenced by Li + solvation energy and solvent viscosity. The chemistry of interaction between LiO 2 /Li 2 O 2 and the electrode/solvent turns out to be less important. Additionally, we found that, for the electrode made of GC and TiN in all electrolyte solutions, the passivation by side products suppresses oxygen reduction after a certain number of cycles. In contrast, for TiC after several cycles, further passivation does not happen as a result of the formation of a thin and stable TiO 2 layer in high donor number solvents.