High-Quality Three-Dimensional Nanoporous Graphene

Soft Template-Assisted Fabrication of Mesoporous Graphenes for High-Performance Energy Storage Systems

Graphene is a promising active material for electric double layer supercapacitors (EDLCs) due to its high electric conductivity and lightweight nature. However, for practical uses as a power source of electronic devices, a porous structure is advantageous to maximize specific energy density. Here, we propose a facile fabrication approach of mesoporous graphene (m-G), in which self-assembled mesoporous structures of poly(styrene)-block-poly(2-vinylpyridine) copolymer (PS-b-P2VP) are exploited as both mesostructured catalytic template and a carbon source. Notably, the mesostructured catalytic template is sufficient to act as a rigid support without structural collapse, while PS-b-P2VP converts to graphene, generating m-G with a pore diameter of ca. 3.5 nm and high specific surface area of 186 m²/g. When the EDLCs were prepared using the obtained m-G and ionic liquids, excellent electrochemical behaviors were achieved even at high operation voltages (0 ∼ 3.5 V), including a large specific capacitance (130.2 F/g at 0.2 A/g), high-energy density of 55.4 W h/kg at power density of 350 W/kg, and excellent cycle stability (>10,000 cycles). This study demonstrates that m-G is a promising material for high-performance energy storage devices.

Dealloyed Nanoporous Materials for Rechargeable Post-Lithium Batteries

Nanoporous materials (NPMs) made by dealloying have been well recognized as multifunctional electrodes for lithium-ion batteries (LIBs). In recent years, there are ever-increasing demands on grid-scale energy storage devices composed by earth-abundant elements such as Na, K, Mg, Al, and Zn. Compared to LIBs, these electrochemical cells face critical challenges such as slow kinetics of redox reactions and structural instability owing to large ion size and/or multiple-electron process. Much interest has been focused on NPMs to address these issues with great success. This Minireview discusses the recent research progresses on these novel electrode materials in the emerging post-lithium batteries, including the rational-design of NPMs, structure-performance correlation in each battery system, and insights into future development of this rapidly growing field.

Synthesis and Electrochemical Study of Three-Dimensional Graphene-Based Nanomaterials for Energy Applications

Graphene is an attractive soft material for various applications due to its unique and exclusive properties. The processing and preservation of 2D graphene at large scales is challenging due to its inherent propensity for layer restacking. Three-dimensional graphene-based nanomaterials (3D-GNMs) preserve their structures while improving processability along with providing enhanced characteristics, which exhibit some notable advantages over 2D graphene. This feature article presents recent trends in the fabrication and characterization of 3D-GNMs toward the study of their morphologies, structures, functional groups, and chemical compositions using scanning electron microscopy, X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. Owing to the attractive properties of 3D-GNMs, which include high surface areas, porous structures, improved electrical conductivity, high mechanical strength, and robust structures, they have generated tremendous interest for various applications such as energy storage, sensors, and energy conversion. This article summarizes the most recent advances in electrochemical applications of 3D-GNMs, pertaining to energy storage, where they can serve as supercapacitor electrode materials and energy conversion as oxygen reduction reaction catalysts, along with an outlook.

Scalable synthesis of gyroid-inspired freestanding three-dimensional graphene architectures

Three-dimensional porous architectures of graphene are desirable for energy storage, catalysis, and sensing applications. Yet it has proven challenging to devise scalable methods capable of producing co-continuous architectures and well-defined, uniform pore and ligament sizes at length scales relevant to applications. This is further complicated by processing temperatures necessary for high quality graphene. Here, bicontinuous interfacially jammed emulsion gels (bijels) are formed and processed into sacrificial porous Ni scaffolds for chemical vapor deposition to produce freestanding three-dimensional turbostratic graphene (bi-3DG) monoliths with high specific surface area. Scanning electron microscopy (SEM) images show that the bi-3DG monoliths inherit the unique microstructural characteristics of their bijel parents. Processing of the Ni templates strongly influences the resultant bi-3DG structures, enabling the formation of stacked graphene flakes or fewer-layer continuous films. Despite the multilayer nature, Raman spectra exhibit no discernable defect peak and large relative intensity for the Raman 2D mode, which is a characteristic of turbostratic graphene. Moiré patterns, observed in scanning tunneling microscopy images, further confirm the presence of turbostratic graphene. Nanoindentation of macroscopic pillars reveals a Young's modulus of 30 MPa, one of the highest recorded for sp² carbon in a porous structure. Overall, this work highlights the utility of a scalable self-assembly method towards porous high quality graphene constructs with tunable, uniform, and co-continuous microstructure.

Chemical Dopants on Edge of Holey Graphene Accelerate Electrochemical Hydrogen Evolution Reaction

Carbon-based metal-free catalysts for the hydrogen evolution reaction (HER) are essential for the development of a sustainable hydrogen society. Identification of the active sites in heterogeneous catalysis is key for the rational design of low-cost and efficient catalysts. Here, by fabricating holey graphene with chemically dopants, the atomic-level mechanism for accelerating HER by chemical dopants is unveiled, through elemental mapping with atomistic characterizations, scanning electrochemical cell microscopy (SECCM), and density functional theory (DFT) calculations. It is found that the synergetic effects of two important factors-edge structure of graphene and nitrogen/phosphorous codoping-enhance HER activity. SECCM evidences that graphene edges with chemical dopants are electrochemically very active. Indeed, DFT calculation suggests that the pyridinic nitrogen atom could be the catalytically active sites. The HER activity is enhanced due to phosphorus dopants, because phosphorus dopants promote the charge accumulations on the catalytically active nitrogen atoms. These findings pave a path for engineering the edge structure of graphene in graphene-based catalysts.

Low-Temperature Carbide-Mediated Growth of Bicontinuous Nitrogen-Doped Mesoporous Graphene as an Efficient Oxygen Reduction Electrocatalyst

Nitrogen-doped graphene exhibits high electrocatalytic activity toward the oxygen reduction reaction (ORR), which is essential for many renewable energy technologies. To maximize the catalytic efficiency, it is desirable to have both a high concentration of robust nitrogen dopants and a large accessible surface of the graphene electrodes for rapid access of oxygen to the active sites. Here, 3D bicontinuous nitrogen-doped mesoporous graphene synthesized by a low-temperature carbide-mediated graphene-growth method is reported. The mesoporous graphene has a mesoscale pore size of ≈25 nm and large specific surface area of 1015 m² g^-1 , which can effectively host and stabilize a high concentration of nitrogen dopants. Accordingly, it shows an excellent electrocatalytic activity toward the ORR with an efficient four-electron-dominated pathway and high durability in alkaline media. The synthesis route developed herein provides a new economic approach to synthesize bicontinuous porous graphene materials with tunable characteristic length, porosity, and chemical doping as high efficiency electrocatalysts for a wide range of electrochemical reactions.

Template-Free Synthesis of Nanoporous Nickel and Alloys as Binder-Free Current Collectors of Li Ion Batteries

This paper reports a versatile template-free method based on the hydrogen reduction of metallic salts for the synthesis of nanoporous Ni and alloys. The approach involves thermal decomposition and reduction of metallic precursors followed with metal cluster nucleation and ligament growth. Topological disordered porous architectures of metals with a controllable distribution of pore size and ligament size ranging from tens of nanometers to micrometers are synthesized. The reduction processes are scrutinized through X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The formation mechanism of the nanoporous metal is qualitatively explained. The as-prepared nanoporous Ni was tested as binder-free current collectors for nickel oxalate anodes of lithium ion batteries. The nanoporous Ni electrodes deliver enhanced reversible capacities and cyclic performances compared with commercial Ni foam. It is confirmed that this synthesis method has versatility not only because it is suitable for different types of metallic salts precursors but also for various other metals and alloys.

An ultrahigh volumetric capacitance of squeezable three-dimensional bicontinuous nanoporous graphene

Graphene with a large specific surface area and high conductivity has a large specific capacitance. However, its volumetric capacitance is usually very low because the restacking of 2D graphene sheets leads to the loss of the large ion-accessible surface area. Here we report squeezable bicontinuous nanoporous nitrogen-doped graphene, which is extremely flexible and can tolerate large volume contraction by mechanical compression without the face-to-face restacking occurring. The compressed nanoporous N-doped graphene with a large ion accessible surface area and high conductivity shows an ultrahigh volumetric capacitance of ∼300 F cm^-3 together with excellent cycling stability and high rate performance.

Synthesis and Electro-Magneto-Mechanical Properties of Graphene Aerogels Functionalized with Co-Fe-P Amorphous Alloys

Graphene aerogels (GAs) are functionalized with Fe-Co-P alloy using an electro-deposition method. The Fe-Co-P alloy coated on the graphene nanosheets is found to possess an amorphous structure and a nanoporous architecture of GAs. The electro-mechanical properties of GAs are significantly affected by the Fe-Co-P nanoparticles embedded inside GAs. The electro-mechanical responses of GA/Fe-Co-P nanoporous hybrid structures are sensitive to an applied magnetic field, demonstrating that they are promising for electro-magneto-mechanical applications. The light-weight, high-strength and nanoporous GAs functionalized with Fe-Co-P amorphous alloys are desirable sensors, actuators, and nano-electro-mechanical systems that could be controlled or manipulated by mechanical, electric and magnetic fields.

Recent advance in fabricating monolithic 3D porous graphene and their applications in biosensing and biofuel cells

Graphene shows great potential in biosensing and bioelectronics. To facilitate graphene's applications and enhance its performance, recently, three-dimensional (3D) graphene-based materials especially free-standing porous graphene with tunable pore size and void space, have attracted increasing attention for bio-related applications owing to their special features. 3D graphene usually shows the following merits such as an interconnected porous network, a high electronic conductivity, a large active surface area, good chemical/thermal stability and can be more easily handled compared with dispersed graphene sheets. With modified surface properties, graphene can also be bio-friendly. These properties make 3D graphene a perfect candidate as high-performance electrode materials in bioelectronics devices. In this review, we discuss recent advance in fabricating monolithic 3D graphene and their applications in biosensing and biofuel cells.

Micelle-template synthesis of nitrogen-doped mesoporous graphene as an efficient metal-free electrocatalyst for hydrogen production

Synthesis of mesoporous graphene materials by soft-template methods remains a great challenge, owing to the poor self-assembly capability of precursors and the severe agglomeration of graphene nanosheets. Herein, a micelle-template strategy to prepare porous graphene materials with controllable mesopores, high specific surface areas and large pore volumes is reported. By fine-tuning the synthesis parameters, the pore sizes of mesoporous graphene can be rationally controlled. Nitrogen heteroatom doping is found to remarkably render electrocatalytic properties towards hydrogen evolution reactions as a highly efficient metal-free catalyst. The synthesis strategy and the demonstration of highly efficient catalytic effect provide benchmarks for preparing well-defined mesoporous graphene materials for energy production applications.