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^–3), and exhibited high native electrical conductivity (38700 S m^–1) and unrivalled volumetric capacitance values (761 F cm^–3) with excellent cyclability and rate performance

Boron-Doped Anatase TiO₂ as a High-Performance Anode Material for Sodium-Ion Batteries

Pristine and boron-doped anatase TiO₂ 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 TiO₂, 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 TiO₂ 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 TiO₂ is higher than that for the pristine sample. B-doping also effectively enhances the rate performance. The capacity of the B-doped TiO₂ 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⁺ diffusion distance due to the nanosize. These results indicate that B-doped TiO₂ can be a good candidate for SIBs

Silicon/Mesoporous Carbon/Crystalline TiO₂ Nanoparticles for Highly Stable Lithium Storage

A core–shell–shell heterostructure of Si nanoparticles as the core with mesoporous carbon and crystalline TiO₂ as the double shells (Si@C@TiO₂) 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^–1), 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 TiO₂ 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@TiO₂ core–shell–shell nanoparticles are proven to show excellent Li storage properties. It delivers high reversible capacity of 1726 mA h g^–1 over 100 cycles, with outstanding cyclability of 1010 mA h g^–1 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^–1 (at 100 mV s^–1), an unprecedented volumetric capacitance of 188 F cm^–3 (at 1 A g^–1), and outstanding energy density of 28.44 Wh kg^–1 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 Ni­(OH)₂ and Co­(OH)₂ 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 Ni­(OH)₂ nanosheets evolving from Ni­(OH)₂ via an Fe³⁺/Ni²⁺ cation-exchange process possess nanoporous surfaces with abundant defects. Accordingly, Fe-doped Ni­(OH)₂ 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^–2 for Ni_0.83Fe_0.17(OH)₂ nanosheets with a low Tafel slope of 61 mV dec^–1, which is greatly decreased in comparison with those of NiFe LDH (310 mV and 78 mV dec^–1). Additionally, this cation-exchange process is successfully extended to prepare Fe-doped Co­(OH)₂ 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/cm²) 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_<i>x</i>(Fe_1/2Mn_1/2)O₂ 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_2/3(Fe_1/2Mn_1/2)­O₂ with a hexagonal structure and <i>P</i>6₃/<i>mmc</i> space group is considered a candidate sodium-ion battery cathode material due to its high capacity (∼190 mAh·g^–1) and energy density (∼520 mWh·g^–1), which are comparable to those of the commercial LiFePO₄ and LiMn₂O₄ 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_2/3(Fe_1/2Mn_1/2)­O₂ 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>6₃/<i>mmc</i> (P2-type at the open-circuit voltage) to <i>P</i>6₃ (OP4-type when fully charged) to <i>P</i>6₃/<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_2/3(Fe_1/2Mn_1/2)­O₂ 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_2/3(Fe_1/2Mn_1/2)­O₂ 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