36 research outputs found
Metal-Free Nitrogen-Doped Carbon Foam Electrocatalysts for the Oxygen Reduction Reaction in Acid Solution
Metal-free, nitrogen-doped carbon foam is utilized as a model non-precious electrocatalyst system to investigate the role of nitrogen in the oxygen reduction reaction (ORR) in the absence of iron contamination. This graphene-like foam displays relatively high activity for the ORR in acid, despite being proven free from transition-metal impurities. The onset potential is 0.85 VRHE, the mass activity is 2.8 A/g at 0.6 VRHE, and the current density is â4.0 mA/cm2. The maximum electron transfer number is calculated to be 3.6, revealing that a 4-electron pathway is possible in nitrogen-doped carbon, even in the absence of transition-metal coordination sites. The excellent electrochemical activity is attributed to the large surface area (700 m2/g), improved conductivity after graphitization, and the relatively high proportion of tertiary (graphite-like) nitrogen
Alternative inflationary scenario due to compact extra dimensions
The main goal of this paper is to give an alternative interpretation of
space-like and time-like extra dimensions as a primary factor for inflation in
the early universe. We introduce the 5-dimensional perfect fluid and compare
the energy-momentum tensor for the bulk scalar field with space-like and
time-like extra dimensions. It is shown, that additional dimensions can imply
to negative pressure in the slow roll regime in the early higher-dimensional
world.Comment: 6 page
Patterned hydrophobic gas diffusion layers for enhanced water management in polymer electrolyte fuel cells
Flooding of the cathode due to water accumulation is one of the biggest limiting factors in the performance of polymer electrolyte fuel cells (PEFCs). This study therefore attempts to solve this issue by fabricating gas diffusion layers (GDLs) with differently patterned hydrophobic regions. The GDLs in three different patterns (triangular, diamond, and inverted-triangular) were prepared by brushing a Polytetrafluoroethylene (PTFE) solution onto commercial carbon papers through a mask and tested in PEFCs. The patterned GDLs results in superior performance in all cases compared to a uniformly PTFE-treated GDL. Notably, the oxygen transport resistance is significantly reduced, indicating that the water accumulation in the cathode is avoided. This is attributed to the patterned hydrophobicity gradient providing distinct pathways for water and oxygen. The GDL with triangular patterning displays the highest peak power density, due to the fact that the untreated less hydrophobic region is in direct contact with the cathode outlet in this case, facilitating the removal of excess liquid water. Overall, the study confirms that the GDLs with patterned hydrophobicity could be used to enhance the performance of commercial PEFC systems by facilitating water management, potentially leading to improved efficiency and durability
Limits on the gravity wave contribution to microwave anisotropies
We present limits on the fraction of large angle microwave anisotropies which
could come from tensor perturbations. We use the COBE results as well as
smaller scale CMB observations, measurements of galaxy correlations, abundances
of galaxy clusters, and Lyman alpha absorption cloud statistics. Our aim is to
provide conservative limits on the tensor-to-scalar ratio for standard
inflationary models. For power-law inflation, for example, we find T/S<0.52 at
95% confidence, with a similar constraint for phi^p potentials. However, for
models with tensor amplitude unrelated to the scalar spectral index it is still
currently possible to have T/S>1.Comment: 23 pages, 7 figures, accepted for publication in Phys. Rev. D.
Calculations extended to blue spectral index, Fig. 6 added, discussion of
results expande
Inflation with
We discuss various models of inflationary universe with . A
homogeneous universe with may appear due to creation of the
universe "from nothing" in the theories where the effective potential becomes
very steep at large , or in the theories where the inflaton field
nonminimally couples to gravity. Inflation with generally requires
intermediate first order phase transition with the bubble formation, and with a
second stage of inflation inside the bubble. It is possible to realize this
scenario in the context of a theory of one scalar field, but typically it
requires artificially bent effective potentials and/or nonminimal kinetic
terms. It is much easier to obtain an open universe in the models involving two
scalar fields. However, these models have their own specific problems. We
propose three different models of this type which can describe an open
homogeneous inflationary universe.Comment: 29 pages, LaTeX, parameters of one of the models are slightly
modifie
Cosmological Consequences of String-forming Open Inflation Models
We present a study of open inflation cosmological scenarios in which cosmic
strings form betwen the two inflationary epochs. It is shown that in these
models strings are stretched outside the horizon due to the inflationary
expansion but must necessarily re-enter the horizon before the epoch of equal
matter and radiation densities. We determine the power spectrum of cold dark
matter perturbations in these hybrid models, finding good agreement with
observations for values of and comparable
contributions from the active and passive sources to the CMB. Finally, we
briefly discuss other cosmological consequences of these models.Comment: 11 LaTeX pages with 3 eps figure
Gram-scale synthesis of alkoxide-derived nitrogen-doped carbon foam as a support for Fe-N-C electrocatalysts
Non-platinum group metal (non-PGM) catalysts for the oxygen reduction reaction (ORR) are set to reduce the cost of polymer electrolyte membrane fuel cells (PEFCs), by replacing platinum at the cathode. We previously developed unique nitrogen-doped carbon foams by template-free pyrolysis of alkoxide powders synthesized using a high temperature and high pressure solvothermal reaction. These were shown to be effective ORR electrocatalysts in alkaline media. Here, we present a new optimised synthesis protocol which is carried out at ambient temperature and pressure, enabling us to safely increase the batch size to 2 g, increase the yield by 60%, increase the specific surface area to 1866 m2/g, and control the nitrogen content (between 1.0 and 5.2 at%). These optimized nitrogen-doped carbon foams are then utilized as effective supports for Fe-N-C catalysts for the ORR in acid media, whilst multiphysics modelling is used to gain insight into the electrochemical performance. This work highlights the importance of the properties of the carbon support in the design of Pt-free electrocatalysts
Low-scale Quintessential Inflation
In quintessential inflationary model, the same master field that drives
inflation becomes, later on, the dynamical source of the (present) accelerated
expansion. Quintessential inflationary models require a curvature scale at the
end of inflation around in order to explain the large scale
fluctuations observed in the microwave sky. If the curvature scale at the end
of inflation is much smaller than , the large scale adiabatic
mode may be produced thanks to the relaxation of a scalar degree of freedom,
which will be generically denoted, according to the recent terminology, as the
curvaton field. The production of the adiabatic mode is analysed in detail in
the case of the minimal quintessential inflationary model originally proposed
by Peebles and Vilenkin.Comment: 25 pages; 5 figure
Inflationary perturbations from a potential with a step
We use a numerical code to compute the density perturbations generated during
an inflationary epoch which includes a spontaneous symmetry breaking phase
transition. A sharp step in the inflaton potential generates dependent
oscillations in the spectrum of primordial density perturbations. The amplitude
and extent in wavenumber of these oscillations depends on both the magnitude
and gradient of the step in the inflaton potential. We show that observations
of the cosmic microwave background anisotropy place strong constraints on the
step parameters.Comment: 6 pages, Revtex - v2. reference adde
Dynamics of the self-interacting chameleon cosmology
In this article we study the properties of the flat FRW chameleon cosmology
in which the cosmic expansion of the Universe is affected by the chameleon
field and dark energy. In particular, we perform a detailed examination of the
model in the light of numerical analysis. The results illustrate that the
interacting chameleon filed plays an important role in late time universe
acceleration and phantom crossing.Comment: 13 pages, 8 figures, to appear in Astrophysics and Space Sc