Abstract  The N2 fixing bacterium Azotobacter vinelandii carries a molybdenum storage protein, referred to as MoSto, able to bind 25-fold more Mo than needed for maximum activity of its Mo nitrogenase. Here we have investigated a plausible role of MoSto as obligate intermediate in the pathway that provides Mo for the biosynthesis of nitrogenase iron-molybdenum cofactor (FeMo-co). The in vitro FeMo-co synthesis and insertion assay demonstrated that purified MoSto functions as Mo donor and that direct interaction with FeMo-co biosynthetic proteins stimulated Mo donation. The phenotype of an A. vinelandii strain lacking the MoSto subunit genes (ΔmosAB) was analyzed. Consistent with its role as storage protein, the ΔmosAB strain showed severe impairment to accumulate intracellular Mo and lower resilience than wild type to Mo starvation as demonstrated by decreased in vivo nitrogenase activity and competitive growth index. In addition, it was more sensitive than the wild type to diazotrophic growth inhibition by W. The ΔmosAB strain was found to readily derepress vnfDGK upon Mo step down, in contrast to the wild type that derepressed Vnf proteins only after prolonged Mo starvation. The ΔmosAB mutation was then introduced in a strain lacking V and Fe-only nitrogenase structural genes (Δvnf Δanf) to investigate possible compensations from these alternative systems. When grown in Mo-depleted medium, the ΔmosAB and mosAB+ strains showed low but similar nitrogenase activities regardless of the presence of Vnf proteins. This study highlights the selective advantage that MoSto confers to A. vinelandii in situations of metal limitation as those found in many soil ecosystems. Such a favorable trait should be included in the gene complement of future nitrogen fixing plants.

Abstract  In this work, VO2 thin films were deposited on Si wafers (onto (100) surface) by DC magnetron sputtering under different cathode bias voltages. The effects of substrate biasing on the structural and optical properties were investigated. The results show that the metal-insulator transition (MIT) temperature of VO2 thin films can be increased up to 14 K by applying a cathode bias voltage, compared to deposition conditions without any bias. The decrease in the transition efficiency and increase in the transition temperature are attributed to the enlarged grain size, increased defects, and the residual stress in the VO2 thin films induced by biasing. The optical transmittance measurements for different thickness films indicate an attenuation coefficient of 3.1 × 107 m-1 at 2000 nm or an extinction coefficient of 4.9 in the metal phase. The optical transmittance vs wavelength characteristics point to an indirect bandgap of 0.6 ± 0.5 eV and significant scattering in the bulk and/or at the interface.

Abstract  Cu and Al foils are commonly used in lithium-ion batteries as current collectors. They are inactive and do not participate in the charge-discharge reactions but take up resources and space within a cell. Herein, we demonstrate a proof of concept to turn the Cu and Al foils into active materials by constructing a Cu-Al full cell, with Cu undergoing stripping/deposition reactions at the cathode and Al alloying with lithium at the anode. Stable cycle performance is possible with the use of a highly concentrated electrolyte; a 3 V cell exhibits excellent cycle stability for more than 200 cycles in 6 M LiTFSI DMC electrolyte. Our Cu-Al battery can give a volumetric energy density in the range 79-156 Wh L-1 comparable to that of state-of-the-art all-vanadium redox-flow batteries. The use of inexpensive Cu and Al as active materials can also potentially reduce the cost of energy storage.