2 research outputs found
Tunable Order in Alginate/Graphene Biopolymer Nanocomposites
We
report on highly aligned graphene oxide or graphene sheets inside
an alginate matrix and their structure obtained for various compositions.
The order of the platelet particles with respect to one another has
been verified by environmental scanning electron microscopy (ESEM)
and 2-dimensional X-ray diffraction (2D XRD). The microscopic order
within the platelet particles has been analyzed by X-ray diffraction
(XRD) measurements in the Bragg–Brentano reflection configuration
as well as in Debye–Scherrer diffraction mode. The azimuthal
angle intensity profiles obtained from 2D XRD analysis have been fit
to Maier–Saupe and affine deformation model predictions, and
the affine deformation model proved to be the most reliable to quantify
the order parameter ⟨<i>P</i><sub>2</sub>⟩
values of graphene oxide/sodium alginate and graphene/calcium alginate
composites with different weight fractions of the filler. The ⟨<i>P</i><sub>2</sub>⟩ values for graphene oxide/sodium alginate
composites were found to show little dependence on the concentration
of graphene sheets above ∼10 wt %, with a maximum ⟨<i>P</i><sub>2</sub>⟩ value of 0.8 at 25 wt % graphene oxide
inside the sodium alginate matrix. The alignment of graphene sheets
inside the calcium alginate matrix has been observed to be lower,
with an average ⟨<i>P</i><sub>2</sub>⟩ value
of 0.7. We have not observed preferred orientation of graphene sheets
inside the barium alginate matrix. The formation of a highly aligned
graphene oxide/sodium alginate composite structure has been explained
by the affine deformation model, whereupon drying the developed yield
stress causes sheets to align in-plane with the polymer matrix. The
impaired orientation of graphene sheets inside the calcium alginate
matrix and absence of orientation in the barium alginate matrix have
been explained by the structure development in the polymer matrix
itself due to metal-ion-induced cross-linking
Use of Nano Seed Crystals To Control Peroxide Morphology in a Nonaqueous Li–O<sub>2</sub> Battery
The high theoretical energy density
of Li–O<sub>2</sub> batteries
as required for electrification of transport has pushed Li–O<sub>2</sub> research to the forefront. The poor cyclability of this system
due to incomplete Li<sub>2</sub>O<sub>2</sub> oxidation is one of
the major hurdles to be crossed if it is ever to deliver a high reversible
energy density. Here we present the use of nano seed crystallites
to control the size and morphology of the Li<sub>2</sub>O<sub>2</sub> crystals. The evolution of the Li<sub>2</sub>O<sub>2</sub> lattice
parameters during <i>operando</i> X-ray diffraction demonstrates
that the hexagonal NiO nanoparticles added to the activated carbon
electrode act as seed crystals for equiaxed growth of Li<sub>2</sub>O<sub>2</sub>, which is confirmed by scanning electron microscopy
energy-dispersive X-ray spectroscopy (SEM-EDX) elemental maps also
showing preferential formation of Li<sub>2</sub>O<sub>2</sub> on the
NiO surface. Even small amounts of NiO (∼5 wt %) particles
act as preferential sites for Li<sub>2</sub>O<sub>2</sub> nucleation,
effectively reducing the average size of the primary Li<sub>2</sub>O<sub>2</sub> crystallites and promoting crystalline growth. This
is supported by first principle calculations, which predict a low
interfacial energy for the formation of NiO–Li<sub>2</sub>O<sub>2</sub> interfaces. The eventual cell failure appears to be the consequence
of electrolyte side reactions, indicating the necessity of more stable
electrolytes. The demonstrated control of the Li<sub>2</sub>O<sub>2</sub> crystallite growth by the rational selection of appropriate
nano seed crystals appears to be a promising strategy to improve the
reversibility of Li–air electrodes