7 research outputs found
Self-Assembled Multifunctional Hybrids: Toward Developing High-Performance Graphene-Based Architectures for Energy Storage Devices
The
prospect of developing multifunctional flexible three-dimensional
(3D) architectures based on integrative chemistry for lightweight,
foldable, yet robust, electronic components that can turn the many
promises of graphene-based devices into reality is an exciting direction
that has yet to be explored. Herein, inspired by nature, we demonstrate
that through a simple, yet novel solvophobic self-assembly processing
approach, nacre-mimicking, layer-by-layer grown, hybrid composite
materials (consisting of graphene oxide, carbon nanotubes, and conducting
polymers) can be made that can incorporate many of the exciting attributes
of graphene into real world materials. The as-produced, self-assembled
3D multifunctional architectures were found to be flexible, yet mechanically
robust and tough (Young’s modulus in excess of 26.1 GPa, tensile
strength of around 252 MPa, and toughness of 7.3 MJ m<sup>–3</sup>), and exhibited high native electrical conductivity (38700 S m<sup>–1</sup>) and unrivalled volumetric capacitance values (761
F cm<sup>–3</sup>) with excellent cyclability and rate performance
Boron-Doped Anatase TiO<sub>2</sub> as a High-Performance Anode Material for Sodium-Ion Batteries
Pristine and boron-doped
anatase TiO<sub>2</sub> were prepared via a facile sol–gel
method and the hydrothermal method for application as anode materials
in sodium-ion batteries (SIBs). The sol–gel method leads to
agglomerated TiO<sub>2</sub>, whereas the hydrothermal method is conducive
to the formation of highly crystalline and discrete nanoparticles.
The structure, morphology, and electrochemical properties were studied.
The crystal size of TiO<sub>2</sub> with boron doping is smaller than
that of the nondoped crystals, which indicates that the addition of
boron can inhibit the crystal growth. The electrochemical measurements
demonstrated that the reversible capacity of the B-doped TiO<sub>2</sub> is higher than that for the pristine sample. B-doping also effectively
enhances the rate performance. The capacity of the B-doped TiO<sub>2</sub> could reach 150 mAh/g at the high current rate of 2C and
the capacity decay is only about 8 mAh/g over 400 cycles. The remarkable
performance could be attributed to the lattice expansion resulting
from B doping and the shortened Li<sup>+</sup> diffusion distance
due to the nanosize. These results indicate that B-doped TiO<sub>2</sub> can be a good candidate for SIBs
Silicon/Mesoporous Carbon/Crystalline TiO<sub>2</sub> Nanoparticles for Highly Stable Lithium Storage
A core–shell–shell
heterostructure of Si nanoparticles
as the core with mesoporous carbon and crystalline TiO<sub>2</sub> as the double shells (Si@C@TiO<sub>2</sub>) is utilized as an anode
material for lithium-ion batteries, which could successfully tackle
the vital setbacks of Si anode materials, in terms of intrinsic low
conductivity, unstable solid–electrolyte interphase (SEI) films,
and serious volume variations. Combined with the high theoretical
capacity of the Si core (4200 mA h g<sup>–1</sup>), the double
shells can perfectly avoid direct contact of Si with electrolyte,
leading to stable SEI films and enhanced Coulombic efficiency. On
the other hand, the carbon inner shell is effective at improving the
overall conductivity of the Si-based electrode; the TiO<sub>2</sub> outer shell is expected to serve as a rigid layer to achieve high
structural stability and integrity of the core–shell–shell
structure. As a result, the elaborate Si@C@TiO<sub>2</sub> core–shell–shell
nanoparticles are proven to show excellent Li storage properties.
It delivers high reversible capacity of 1726 mA h g<sup>–1</sup> over 100 cycles, with outstanding cyclability of 1010 mA h g<sup>–1</sup> even after 710 cycles
Self-Assembled N/S Codoped Flexible Graphene Paper for High Performance Energy Storage and Oxygen Reduction Reaction
A novel flexible three-dimensional
(3D) architecture of nitrogen and sulfur codoped graphene has been
successfully synthesized via thermal treatment of a liquid crystalline
graphene oxide–doping agent composition, followed by a soft
self-assembly approach. The high temperature process turns the layer-by-layer
assembly into a high surface area macro- and nanoporous free-standing
material with different atomic configurations of graphene. The interconnected
3D network exhibits excellent charge capacitive performance of 305
F g<sup>–1</sup> (at 100 mV s<sup>–1</sup>), an unprecedented
volumetric capacitance of 188 F cm<sup>–3</sup> (at 1 A g<sup>–1</sup>), and outstanding energy density of 28.44 Wh kg<sup>–1</sup> as well as cycle life of 10 000 cycles as
a free-standing electrode for an aqueous electrolyte, symmetric supercapacitor
device. Moreover, the resulting nitrogen/sulfur doped graphene architecture
shows good electrocatalytic performance, long durability, and high
selectivity when they are used as metal-free catalyst for the oxygen
reduction reaction. This study demonstrates an efficient approach
for the development of multifunctional as well as flexible 3D architectures
for a series of heteroatom-doped graphene frameworks for modern energy
storage as well as energy source applications
Active-Site-Enriched Iron-Doped Nickel/Cobalt Hydroxide Nanosheets for Enhanced Oxygen Evolution Reaction
Highly
active, durable, and inexpensive nanostructured catalysts
are crucial for achieving efficient and economical electrochemical
water splitting. However, developing efficient approaches to further
improve the catalytic ability of the well-defined nanostructured catalysts
is still a big challenge. Herein, we report a facile and universal
cation-exchange process for synthesizing Fe-doped NiÂ(OH)<sub>2</sub> and CoÂ(OH)<sub>2</sub> nanosheets with enriched active sites toward
enhanced oxygen evolution reaction (OER). In comparison with typical
NiFe layered double hydroxide (LDH) nanosteets prepared by the conventional
one-pot method, Fe-doped NiÂ(OH)<sub>2</sub> nanosheets evolving from
NiÂ(OH)<sub>2</sub> via an Fe<sup>3+</sup>/Ni<sup>2+</sup> cation-exchange
process possess nanoporous surfaces with abundant defects. Accordingly,
Fe-doped NiÂ(OH)<sub>2</sub> nanosheets exhibit higher electrochemical
active surface area (ECSA) and improved surface wettability in comparison
to NiFe LDH nanosheets and deliver significantly enhanced catalytic
activity over NiFe LDH. Specifically, a low overpotential of only
245 mV is required to reach a current density of 10 mA cm<sup>–2</sup> for Ni<sub>0.83</sub>Fe<sub>0.17</sub>(OH)<sub>2</sub> nanosheets
with a low Tafel slope of 61 mV dec<sup>–1</sup>, which is
greatly decreased in comparison with those of NiFe LDH (310 mV and
78 mV dec<sup>–1</sup>). Additionally, this cation-exchange
process is successfully extended to prepare Fe-doped CoÂ(OH)<sub>2</sub> nanosheets with improved catalytic activity for oxygen evolution.
The results suggest that this cation-exchange process should have
great potential in the rational design of defect-enriched catalysts
toward high-performance electrocatalysis
Reverse Microemulsion Synthesis of Sulfur/Graphene Composite for Lithium/Sulfur Batteries
Due to its high theoretical capacity,
high energy density, and easy availability, the lithium–sulfur
(Li–S) system is considered to be the most promising candidate
for electric and hybrid electric vehicle applications. Sulfur/carbon
cathode in Li–S batteries still suffers, however, from low
Coulombic efficiency and poor cycle life when sulfur loading and the
ratio of sulfur to carbon are high. Here, we address these challenges
by fabricating a sulfur/carboxylated–graphene composite using
a reverse (water-in-oil) microemulsion technique. The fabricated sulfur–graphene
composite cathode, which contains only 6 wt % graphene, can dramatically
improve the cycling stability as well as provide high capacity. The
electrochemical performance of the sulfur–graphene composite
is further enhanced after loading into a three-dimensional heteroatom-doped
(boron and nitrogen) carbon-cloth current collector. Even at high
sulfur loading (∼8 mg/cm<sup>2</sup>) on carbon cloth, this
composite showed 1256 mAh/g discharge capacity with more than 99%
capacity retention after 200 cycles
Interplay between Electrochemistry and Phase Evolution of the P2-type Na<sub><i>x</i></sub>(Fe<sub>1/2</sub>Mn<sub>1/2</sub>)O<sub>2</sub> Cathode for Use in Sodium-Ion Batteries
Sodium-ion
batteries are the next-generation in battery technology;
however, their commercial development is hampered by electrode performance.
The P2-type Na<sub>2/3</sub>(Fe<sub>1/2</sub>Mn<sub>1/2</sub>)ÂO<sub>2</sub> with a hexagonal structure and <i>P</i>6<sub>3</sub>/<i>mmc</i> space group is considered a candidate sodium-ion
battery cathode material due to its high capacity (∼190 mAh·g<sup>–1</sup>) and energy density (∼520 mWh·g<sup>–1</sup>), which are comparable to those of the commercial LiFePO<sub>4</sub> and LiMn<sub>2</sub>O<sub>4</sub> lithium-ion battery cathodes,
with previously unexplained poor cycling performance being the major
barrier to its commercial application. We use <i>operando</i> synchrotron X-ray powder diffraction to understand the origins of
the capacity fade of the Na<sub>2/3</sub>(Fe<sub>1/2</sub>Mn<sub>1/2</sub>)ÂO<sub>2</sub> material during cycling over the relatively wide 1.5–4.2
V (vs Na) window. We found a complex phase-evolution, involving transitions
from <i>P</i>6<sub>3</sub>/<i>mmc</i> (P2-type
at the open-circuit voltage) to <i>P</i>6<sub>3</sub> (OP4-type
when fully charged) to <i>P</i>6<sub>3</sub>/<i>mmc</i> (P2-type at 3.4–2.0 V) to <i>Cmcm</i> (P2-type
at 2.0–1.5 V) symmetry structures during the desodiation and
sodiation of the Na<sub>2/3</sub>(Fe<sub>1/2</sub>Mn<sub>1/2</sub>)ÂO<sub>2</sub> cathode. The associated large cell-volume changes
with the multiple two-phase reactions are likely to be responsible
for the poor cycling performance, clearly suggesting a 2.0–4.0
V window of operation as a strategy to improve cycling performance.
We demonstrated here that the P2-type Na<sub>2/3</sub>(Fe<sub>1/2</sub>Mn<sub>1/2</sub>)ÂO<sub>2</sub> cathode is able to deliver ∼25%
better cycling performance with the strategic operation window. This
significant improvement in cycling performance implies that by characterizing
the phase evolution and reaction mechanisms during battery function
we are able to propose these modifications to the conditions of battery
use that improve performance, highlighting the importance of the interplay
between structure and electrochemistry