On the performance of a hierarchically porous Ag2S-CuxS electrode in Li-ion batteries.

A new binder- and carbon-free electrode for lithium-ion batteries was prepared using a hierarchically porous Ag-based current collector. The latter was produced by applying the method of selective dissolution of the less noble metals from the Cu60Ag30Al10 master alloy tape. The current collector was reaction-coated with an electrochemically active Ag2S-CuxS coating. The metallic structure provided a mechanically stable conductive scaffold on the walls of which the Ag2S-CuxS skin material was directly deposited. The ordered porosity - hierarchical and directional - provided easy penetration of the liquid electrolyte as well as short Li+ ion diffusion paths. The as-prepared electrodes were tested in a half-cell configuration vs. Li/Li+ at various current rates to study the cycling and rate performances of the electrode. The first cycling capacity of ∼1250 mA h g-1 was measured at 0.4 A g-1 current rate. After a rapid decrease, a stable reversible capacity of ∼230 mA h g-1 was established at a current rate of 0.4 A g-1 (calculated vs. the weight of the incorporated sulphur). Excellent charge/discharge cycling and rate properties were observed for over 1000 cycles at higher rates of 1.0 and 2.0 A g-1, in the potential window of 0.15-2.8 V vs. Li/Li+. The observed cycling stability was ascribed to the mechanism of a "displacement" reaction with Li ions. Additional capacity is also available from alloying-dealloying with Ag (and Cu to some extent) and S redox reactions. These results open up a new opportunity for using a Cu-Ag alloy as the precursor for making electrodes for thin Li-ion and Li-S batteries with high cycling stability at relatively high current rates.

Microbial communities and organic biomarkers in a Proterozoic-analog sinkhole.

Little Salt Spring (Sarasota County, FL, USA) is a sinkhole with groundwater vents at ~77 m depth. The entire water column experiences sulfidic (~50 µM) conditions seasonally, resulting in a system poised between oxic and sulfidic conditions. Red pinnacle mats occupy the sediment-water interface in the sunlit upper basin of the sinkhole, and yielded 16S rRNA gene clones affiliated with Cyanobacteria, Chlorobi, and sulfate-reducing clades of Deltaproteobacteria. Nine bacteriochlorophyll e homologues and isorenieratene indicate contributions from Chlorobi, and abundant chlorophyll a and pheophytin a are consistent with the presence of Cyanobacteria. The red pinnacle mat contains hopanoids, including 2-methyl structures that have been interpreted as biomarkers for Cyanobacteria. A single sequence of hpnP, the gene required for methylation of hopanoids at the C-2 position, was recovered in both DNA and cDNA libraries from the red pinnacle mat. The hpnP sequence was most closely related to cyanobacterial hpnP sequences, implying that Cyanobacteria are a source of 2-methyl hopanoids present in the mat. The mats are capable of light-dependent primary productivity as evidenced by 13 C-bicarbonate photoassimilation. We also observed 13 C-bicarbonate photoassimilation in the presence of DCMU, an inhibitor of electron transfer to Photosystem II. Our results indicate that the mats carry out light-driven primary production in the absence of oxygen production-a mechanism that may have delayed the oxygenation of the Earth's oceans and atmosphere during the Proterozoic Eon. Furthermore, our observations of the production of 2-methyl hopanoids by Cyanobacteria under conditions of low oxygen and low light are consistent with the recovery of these structures from ancient black shales as well as their paucity in modern marine environments.

Lipid biomarkers in ooids from different locations and ages: evidence for a common bacterial flora.

Ooids are one of the common constituents of ancient carbonate rocks, yet the role that microbial communities may or may not play in their formation remains unresolved. To search for evidence of microbial activity in modern and Holocene ooids, samples collected from intertidal waters, beaches and outcrops in the Bahamas and in Shark Bay in Western Australia were examined for their contents of lipid biomarkers. Modern samples from Cat and Andros islands in the Bahamas and from Carbla Beach in Hamelin Pool, Western Australia, showed abundant and notably similar distributions of hydrocarbons, fatty acids (FAs) and alcohols. A large fraction of these lipids were bound into the carbonate matrix and only released on acid dissolution, which suggests that these lipids were being incorporated continuously during ooid growth. The distributions of hydrocarbons, and their disparate carbon isotopic signatures, were consistent with mixed input from cyanobacteria together with small and variable amounts of vascular plant leaf wax [C27 -C35 ; δ(13) C -25 to -32‰Vienna Pee Dee Belemnite (VPDB)]. The FAs comprised a complex mixture of C12 -C18 normal and branched short-chain compounds with the predominant straight-chain components attributable to bacteria and/or cyanobacteria. Branched FA, especially 10-MeC16 and 10-MeC17 , together with the prevalence of elemental sulfur in the extracts, indicate an origin from sulfate-reducing bacteria. The iso- and anteiso-FA were quite variable in their (13) C contents suggesting that they come from organisms with diverse physiologies. Hydrogen isotopic compositions provide further insight into this issue. FAs in each sample show disparate δD values consistent with inputs from autotrophs and heterotrophs. The most enigmatic lipid assemblage is an homologous series of long-chain (C24 -C32 ) FA with pronounced even carbon number preference. Typically, such long-chain FA are thought to come from land plant leaf wax, but in this case, their (13) C-enriched isotopic signatures compared to co-occurring n-alkanes (e.g., Hamelin Pool TLE FA C24 -C32 ; δ(13) C -20 to -24.2‰ VPDB; TLE n-alkanes δ(13) C -24.1 to -26.2 -‰VPDB) indicate a microbial origin, possibly sulfate-reducing bacteria. Lastly, we identified homohopanoic acid and bishomohopanol as the primary degradation products of bacterial hopanoids. The distributions of lipids isolated from Holocene oolites from the Rice Bay Formation of Cat Island, Bahamas were very similar to the beach ooids described above and, in total, these modern and fossil biomarker data lead us to hypothesize that ooids are colonized by a defined microbial community and that these microbes possibly mediate calcification.