765 research outputs found
Second generation PEM fuel cells and the indirect reduction of oxygen
2015 is a breakthrough year for fuel cell technology
Effect of Temperature and Catholyte Concentration on the Performance of a Chemically Regenerative Fuel Cell POM-based catholytes for platinum-free polymer electrolyte fuel cells
Chemically regenerative redox cathode (CRRC) polymer electrolyte fuel cells (PEFCs) are attracting more interest as a platinum-free PEFC technology. These fuel cells utilise a liquid catalyst or catholyte, to perform the indirect reduction of oxygen, eliminating the major degradation mechanisms that plague PEFC durability. A key component of a CRRC PEFC system is the catholyte. This article reports a thorough study of the effect of catholyte concentration and temperature on CRRC PEFC system performance for H7PV4Mo8O40 and Na4H3PV4Mo8O40, two promising polyoxometalate (POM)-based catholytes. The results suggest 80ºC and a catholyte concentration of 0.3 M provide the optimum performance for both H7PV4Mo8O40 and Na4H3PV4Mo8O40 (for ambient pressure operation)
The Importance of Cell Compression Pressure for Flow Battery Performance
Compared to fuel cells, which possess similar cell architecture, flow batteries have poor performance. For example, conventional fuel cells can easily achieve current densities of 1.5 A cm-2 whereas the corresponding figure for the all vanadium flow battery (VFB) is an order of magnitude less, often less than 0.2 A cm 2 [1]. Consequently, relatively large flow battery cells are required for a given power, increasing the cost of the technology. There are a few noticeable exceptions to the relatively poor performance of flow batteries, including the work of Zawodzinski et al. who achieved current densities in excess of 0.8 A cm 2 with a VFB [2]. Most impressively, Weber and co-workers achieved current densities as high as 4 A cm 2 with a H2-Br2 flow battery [3]. In both cases, the researchers used fuel cell components and fuel cell assembly techniques to minimize the cell ohmic resistance, particularly the contact resistance between the cell parts (electrodes, bipolar plates and current collectors). Typically, fuel cells are assembled using compression pressures of above 8 bar to minimize contact resistance. In comparison, flow batteries use compression pressures less than 1 bar during cell assembly with carbon fibre felt electrodes; hence contact resistance values are relatively high. A number of studies have measured the effect of felt compression on battery performance [4-5], where the felt compression is increased from 0 to 30%, resulting in a decrease in cell resistance and a noticeable improvement in performance. This study builds on previous felt compression work by exploring a much wider range of electrode compression pressures in a VFB system
Varied effects of algal symbionts on transcription factor NF-κB in a sea anemone and a coral: possible roles in symbiosis and thermotolerance
Many cnidarians, including the reef-building corals, undergo symbiotic mutualisms with photosynthetic dinoflagellate algae of the family Symbiodiniaceae. These partnerships are sensitive to temperature extremes, which cause symbiont loss and increased coral mortality. Previous studies have implicated host immunity and specifically immunity transcription factor NF-κB as having a role in the maintenance of the cnidarian-algal symbiosis. Here we have further investigated a possible role for NF-κB in establishment and loss of symbiosis in various strains of the anemone Exaiptasia (Aiptasia) and in the coral Pocillopora damicornis. Our results show that NF-κB expression is reduced in Aiptasia larvae and adults that host certain algae strains. Treatment of Aiptasia larvae with a known symbiosis-promoting cytokine, transforming growth factor β, also led to decreased NF-κB expression. We also show that aposymbiotic Aiptasia (with high NF-κB expression) have increased survival following infection with the pathogenic bacterium Serratia marcescens as compared to symbiotic Aiptasia (low NF-κB expression). Furthermore, a P. damicornis coral colony hosting Durusdinium spp. (formerly clade D) symbionts had higher basal NF-κB expression and decreased heat-induced bleaching as compared to two individuals hosting Cladocopium spp. (formerly clade C) symbionts. Lastly, genome-wide gene expression profiling and genomic promoter analysis identified putative NF-κB target genes that may be involved in thermal bleaching, symbiont maintenance, and/or immune protection in P. damicornis. Our results provide further support for the hypothesis that modulation of NF-κB and immunity plays a role in some, but perhaps not all, cnidarian-Symbiodiniaceae partnerships as well as in resistance to pathogens and bleaching.Accepted manuscrip
The Roles of Extrinsic and Intrinsic Factors in the Freshwater Life-History Dynamics of a Migratory Salmonid
Key life-cycle transitions, such as metamorphosis or migration, can be altered by a variety of external factors, such as climate variation, strong species interactions, and management intervention, or modulated by density dependence. Given that these life-history transitions can influence population dynamics, understanding the simultaneous effects of intrinsic and extrinsic controls on life-history expression is particularly relevant for species of management or conservation importance. Here, we examined how life histories of steelhead (Oncorhynchus mykiss) are affected by weather, pink salmon abundance (Oncorhynchus gorbuscha), experimental nutrient addition, and density-dependent processes. We tested for impacts on the size of steelhead smolts (juveniles migrating to the sea), as well as their age and abundance across four decades in the Keogh River, British Columbia, Canada. Larger steelhead smolts were associated with warmer years and artificial nutrient addition. In addition, higher pink salmon abundance and artificial nutrient addition correlated with juvenile steelhead migrating at younger ages. While density dependence appeared to be the primary factor regulating the abundance of steelhead smolts, nutrient addition and temperature were positively and negatively associated with smolt production, respectively, prior to 1991, and pink salmon spawning abundance was positively associated with smolt production after 1990. Thus, this study provides evidence that the temporal dynamics of one species of salmon is linked to the juvenile life history of co-occurring steelhead. A complex interplay of species interactions, nutrient subsidies, density dependence, and climatic variation can control the life-history expression of species with complex life cycles
Performance Comparison of Protonic and Sodium Phosphomolybdovanadate Polyoxoanion Catholytes Within a Chemically Regenerative Redox Cathode Polymer Electrolyte Fuel Cell
The direct reduction of oxygen in conventional polymer electrolyte fuel cells (PEFCs) is seen by many researchers as a key challenge in PEFC development. Chemically regenerative redox cathode (CRRC) polymer electrolyte fuel cells offer an alternative approach via the indirect reduction of oxygen, improving durability and reducing cost. These systems substitute gaseous oxygen for a liquid catalyst that is reduced at the cathode then oxidised in a regeneration vessel via air bubbling. A key component of a CRRC system is the liquid catalyst or catholyte. To date, phosphomolybdovanadium polyoxometalates with empirical formula H3+nPVnMo12-nO40 have shown the most promise for CRRC PEFC systems. In this work, four catholyte formulations are studied and compared against each other. The catholytes vary in vanadium content, pH and counter ion, with empirical formulas H6PV3Mo9O40, H7PV4Mo8O40, Na3H3PV3Mo9O40 and Na4H3PV4Mo8O40. Thermodynamic properties, cell performance and regeneration rates are measured, generating new insights into how formulation chemistry affects the components of a CRRC system. The results include the best CRRC PEFC performance reported to date, with noticeable advantages over conventional PEFCs. The optimum catholyte formulation is then determined via steady state tests, the results of which will guide further optimization of the catholyte formulation
Are X-ray properties of loose groups different from those of compact groups?
We compare the X-ray properties of loose and compact galaxy groups, using a
combined sample of 42 groups. We find that we are unable to separate loose and
compact groups on the luminosity-temperature relation, the luminosity-velocity
dispersion relation or the velocity dispersion-temperature relation using
equally weighted errors. This suggests that the distinction between compact and
loose groups is not a fundamental one, and we argue that a more useful
distinction is that between X-ray bright and X-ray faint systems. Given their
similarity in X-ray properties, we combine the loose and compact subsamples to
derive relations based on the full sample. This provides the highest
statistical quality results to date on the way in which the correlations in
X-ray properties of low mass systems depart from those seen in rich clusters.Comment: 6 pages, 6 figures. Accepted for publication in MNRA
A novel ‘bottom-up’ synthesis of few- and multi-layer graphene platelets with partial oxidation via cavitation
The transient cavitation of diaromatic components such as 1-methylnaphthalene has been used to produce
graphene platelets in a ‘bottom-up’ synthesis via the high temperature (>5000 K) conditions that are generated
inside collapsing bubbles. Acoustic cavitation produced yields of 5.7×10−11 kgJ−1 at a production rate of
2.2×10−9 kgs−1. This can be improved by generating cavitation hydrodynamically, thus making commercial
scale production viable. Hydrodynamic cavitation produced platelets with larger lateral dimensions (≥2 μm)
than those formed by acoustic cavitation (10–200 nm). The partially oxidised nature of the platelets enables their
covalent chemical functionalisation, which was achieved by combining suitable molecules in the reaction
medium to affect a one-pot formation and functionalisation of graphen
X-ray modelling of galaxy cluster gas and mass profiles
We present a parametric analysis of the intracluster medium and gravitating
mass distribution of a statistical sample of 20 galaxy clusters using the
phenomenological cluster model of Ascasibar and Diego. We describe an effective
scheme for the estimation of errors on model parameters and derived quantities
using bootstrap resampling. We find that the model provides a good description
of the data in all cases and we quantify the mean fractional intrinsic scatter
about the best-fit density and temperature profiles, finding this to have
median values across the sample of 2 and 5 per cent, respectively. In addition,
we demonstrate good agreement between r500 determined directly from the model
and that estimated from a core-excluded global spectrum. We compare cool core
and non-cool core clusters in terms of the logarithmic slopes of their gas
density and temperature profiles and the distribution of model parameters and
conclude that the two categories are clearly separable. In particular, we
confirm the effectiveness of the logarithmic gradient of the gas density
profile measured at 0.04 r500 in differentiating between the two types of
cluster.Comment: 8 pages, 7 figures, accepted for publication in MNRA
Bayesian modelling of the cool core galaxy group NGC 4325
We present an X-ray analysis of the radio-quiet cool-core galaxy group NGC
4325 (z=0.026) based on Chandra and ROSAT observations. The Chandra data were
analysed using XSPEC deprojection, 2D spectral mapping and forward-fitting with
parametric models. Additionally, a Markov chain Monte Carlo method was used to
perform a joint Bayesian analysis of the Chandra and ROSAT data. The results of
the various analysis methods are compared, particularly those obtained by
forward-fitting and deprojection. The spectral mapping reveals the presence of
cool gas displaced up to 10 kpc from the group centre. The Chandra X-ray
surface brightness shows the group core to be highly disturbed, and indicates
the presence of two small X-ray cavities within 15 kpc of the group core. The
XSPEC deprojection analysis shows that the group has a particularly steep
entropy profile, suggesting that an AGN outburst may be about to occur. With
the evidence of prior AGN activity, but with no radio emission currently
observed, we suggest that the group in in a pre-outburst state, with the
cavities and displaced gas providing evidence of a previous, weak AGN outburst.Comment: 12 pages, 10 figures; accepted for publication in MNRA
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